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<gethostent>,
228 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
249 =item I<-X> FILEHANDLE
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 $@ eq "alarm\n"; # propagate unexpected errors
434 Returns the arctangent of Y/X in the range -PI to PI.
436 For the tangent operation, you may use the C<Math::Trig::tan>
437 function, or use the familiar relation:
439 sub tan { sin($_[0]) / cos($_[0]) }
441 =item bind SOCKET,NAME
443 Binds a network address to a socket, just as the bind system call
444 does. Returns true if it succeeded, false otherwise. NAME should be a
445 packed address of the appropriate type for the socket. See the examples in
446 L<perlipc/"Sockets: Client/Server Communication">.
448 =item binmode FILEHANDLE, DISCIPLINE
450 =item binmode FILEHANDLE
452 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
453 on systems where the run-time libraries distinguish between binary and
454 text files. If FILEHANDLE is an expression, the value is taken as the
455 name of the filehandle.
457 DISCIPLINE can be either of C<:raw> for binary mode or C<:crlf> for
458 "text" mode. If the DISCIPLINE is omitted, it defaults to C<:raw>.
459 Returns true on success, C<undef> on failure. To mark FILEHANDLE as
460 UTF-8, use C<:utf8>, and to mark the as bytes, use C<:bytes>.
462 The C<:raw> are C<:clrf>, and any other directives of the form
463 C<:...>, are called I/O I<disciplines>. The C<open> pragma can be
464 used to establish default I/O disciplines. See L<open>.
466 In general, binmode() should be called after open() but before any I/O
467 is done on the filehandle. Calling binmode() will flush any possibly
468 pending buffered input or output data on the handle. The only
469 exception to this is the C<:encoding> discipline that changes
470 the default character encoding of the handle, see L<open>.
471 The C<:encoding> discipline sometimes needs to be called in
472 mid-stream, and it doesn't flush the stream.
474 On some systems binmode() is necessary when you're not working with a
475 text file. For the sake of portability it is a good idea to always use
476 it when appropriate, and to never use it when it isn't appropriate.
478 In other words: Regardless of platform, use binmode() on binary
479 files, and do not use binmode() on text files.
481 The operating system, device drivers, C libraries, and Perl run-time
482 system all work together to let the programmer treat a single
483 character (C<\n>) as the line terminator, irrespective of the external
484 representation. On many operating systems, the native text file
485 representation matches the internal representation, but on some
486 platforms the external representation of C<\n> is made up of more than
489 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
490 character to end each line in the external representation of text (even
491 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
492 on Unix and most VMS files). Consequently binmode() has no effect on
493 these operating systems. In other systems like OS/2, DOS and the various
494 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
495 what's stored in text files are the two characters C<\cM\cJ>. That means
496 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
497 disk will be converted to C<\n> on input, and any C<\n> in your program
498 will be converted back to C<\cM\cJ> on output. This is what you want for
499 text files, but it can be disastrous for binary files.
501 Another consequence of using binmode() (on some systems) is that
502 special end-of-file markers will be seen as part of the data stream.
503 For systems from the Microsoft family this means that if your binary
504 data contains C<\cZ>, the I/O subsystem will regard it as the end of
505 the file, unless you use binmode().
507 binmode() is not only important for readline() and print() operations,
508 but also when using read(), seek(), sysread(), syswrite() and tell()
509 (see L<perlport> for more details). See the C<$/> and C<$\> variables
510 in L<perlvar> for how to manually set your input and output
511 line-termination sequences.
513 =item bless REF,CLASSNAME
517 This function tells the thingy referenced by REF that it is now an object
518 in the CLASSNAME package. If CLASSNAME is omitted, the current package
519 is used. Because a C<bless> is often the last thing in a constructor,
520 it returns the reference for convenience. Always use the two-argument
521 version if the function doing the blessing might be inherited by a
522 derived class. See L<perltoot> and L<perlobj> for more about the blessing
523 (and blessings) of objects.
525 Consider always blessing objects in CLASSNAMEs that are mixed case.
526 Namespaces with all lowercase names are considered reserved for
527 Perl pragmata. Builtin types have all uppercase names, so to prevent
528 confusion, you may wish to avoid such package names as well. Make sure
529 that CLASSNAME is a true value.
531 See L<perlmod/"Perl Modules">.
537 Returns the context of the current subroutine call. In scalar context,
538 returns the caller's package name if there is a caller, that is, if
539 we're in a subroutine or C<eval> or C<require>, and the undefined value
540 otherwise. In list context, returns
542 ($package, $filename, $line) = caller;
544 With EXPR, it returns some extra information that the debugger uses to
545 print a stack trace. The value of EXPR indicates how many call frames
546 to go back before the current one.
548 ($package, $filename, $line, $subroutine, $hasargs,
549 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
551 Here $subroutine may be C<(eval)> if the frame is not a subroutine
552 call, but an C<eval>. In such a case additional elements $evaltext and
553 C<$is_require> are set: C<$is_require> is true if the frame is created by a
554 C<require> or C<use> statement, $evaltext contains the text of the
555 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
556 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
557 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
558 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
559 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
560 was compiled with. The C<$hints> and C<$bitmask> values are subject to
561 change between versions of Perl, and are not meant for external use.
563 Furthermore, when called from within the DB package, caller returns more
564 detailed information: it sets the list variable C<@DB::args> to be the
565 arguments with which the subroutine was invoked.
567 Be aware that the optimizer might have optimized call frames away before
568 C<caller> had a chance to get the information. That means that C<caller(N)>
569 might not return information about the call frame you expect it do, for
570 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
571 previous time C<caller> was called.
575 Changes the working directory to EXPR, if possible. If EXPR is omitted,
576 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
577 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
578 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
579 neither is set, C<chdir> does nothing. It returns true upon success,
580 false otherwise. See the example under C<die>.
584 Changes the permissions of a list of files. The first element of the
585 list must be the numerical mode, which should probably be an octal
586 number, and which definitely should I<not> a string of octal digits:
587 C<0644> is okay, C<'0644'> is not. Returns the number of files
588 successfully changed. See also L</oct>, if all you have is a string.
590 $cnt = chmod 0755, 'foo', 'bar';
591 chmod 0755, @executables;
592 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
594 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
595 $mode = 0644; chmod $mode, 'foo'; # this is best
597 You can also import the symbolic C<S_I*> constants from the Fcntl
602 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
603 # This is identical to the chmod 0755 of the above example.
611 This safer version of L</chop> removes any trailing string
612 that corresponds to the current value of C<$/> (also known as
613 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
614 number of characters removed from all its arguments. It's often used to
615 remove the newline from the end of an input record when you're worried
616 that the final record may be missing its newline. When in paragraph
617 mode (C<$/ = "">), it removes all trailing newlines from the string.
618 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
619 a reference to an integer or the like, see L<perlvar>) chomp() won't
621 If VARIABLE is omitted, it chomps C<$_>. Example:
624 chomp; # avoid \n on last field
629 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
631 You can actually chomp anything that's an lvalue, including an assignment:
634 chomp($answer = <STDIN>);
636 If you chomp a list, each element is chomped, and the total number of
637 characters removed is returned.
645 Chops off the last character of a string and returns the character
646 chopped. It is much more efficient than C<s/.$//s> because it neither
647 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
648 If VARIABLE is a hash, it chops the hash's values, but not its keys.
650 You can actually chop anything that's an lvalue, including an assignment.
652 If you chop a list, each element is chopped. Only the value of the
653 last C<chop> is returned.
655 Note that C<chop> returns the last character. To return all but the last
656 character, use C<substr($string, 0, -1)>.
660 Changes the owner (and group) of a list of files. The first two
661 elements of the list must be the I<numeric> uid and gid, in that
662 order. A value of -1 in either position is interpreted by most
663 systems to leave that value unchanged. Returns the number of files
664 successfully changed.
666 $cnt = chown $uid, $gid, 'foo', 'bar';
667 chown $uid, $gid, @filenames;
669 Here's an example that looks up nonnumeric uids in the passwd file:
672 chomp($user = <STDIN>);
674 chomp($pattern = <STDIN>);
676 ($login,$pass,$uid,$gid) = getpwnam($user)
677 or die "$user not in passwd file";
679 @ary = glob($pattern); # expand filenames
680 chown $uid, $gid, @ary;
682 On most systems, you are not allowed to change the ownership of the
683 file unless you're the superuser, although you should be able to change
684 the group to any of your secondary groups. On insecure systems, these
685 restrictions may be relaxed, but this is not a portable assumption.
686 On POSIX systems, you can detect this condition this way:
688 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
689 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
695 Returns the character represented by that NUMBER in the character set.
696 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
697 chr(0x263a) is a Unicode smiley face. Note that characters from 127
698 to 255 (inclusive) are by default not encoded in Unicode for backward
699 compatibility reasons (but see L<encoding>).
701 For the reverse, use L</ord>.
702 See L<perlunicode> and L<encoding> for more about Unicode.
704 If NUMBER is omitted, uses C<$_>.
706 =item chroot FILENAME
710 This function works like the system call by the same name: it makes the
711 named directory the new root directory for all further pathnames that
712 begin with a C</> by your process and all its children. (It doesn't
713 change your current working directory, which is unaffected.) For security
714 reasons, this call is restricted to the superuser. If FILENAME is
715 omitted, does a C<chroot> to C<$_>.
717 =item close FILEHANDLE
721 Closes the file or pipe associated with the file handle, returning
722 true only if IO buffers are successfully flushed and closes the system
723 file descriptor. Closes the currently selected filehandle if the
726 You don't have to close FILEHANDLE if you are immediately going to do
727 another C<open> on it, because C<open> will close it for you. (See
728 C<open>.) However, an explicit C<close> on an input file resets the line
729 counter (C<$.>), while the implicit close done by C<open> does not.
731 If the file handle came from a piped open C<close> will additionally
732 return false if one of the other system calls involved fails or if the
733 program exits with non-zero status. (If the only problem was that the
734 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
735 also waits for the process executing on the pipe to complete, in case you
736 want to look at the output of the pipe afterwards, and
737 implicitly puts the exit status value of that command into C<$?>.
739 Prematurely closing the read end of a pipe (i.e. before the process
740 writing to it at the other end has closed it) will result in a
741 SIGPIPE being delivered to the writer. If the other end can't
742 handle that, be sure to read all the data before closing the pipe.
746 open(OUTPUT, '|sort >foo') # pipe to sort
747 or die "Can't start sort: $!";
748 #... # print stuff to output
749 close OUTPUT # wait for sort to finish
750 or warn $! ? "Error closing sort pipe: $!"
751 : "Exit status $? from sort";
752 open(INPUT, 'foo') # get sort's results
753 or die "Can't open 'foo' for input: $!";
755 FILEHANDLE may be an expression whose value can be used as an indirect
756 filehandle, usually the real filehandle name.
758 =item closedir DIRHANDLE
760 Closes a directory opened by C<opendir> and returns the success of that
763 DIRHANDLE may be an expression whose value can be used as an indirect
764 dirhandle, usually the real dirhandle name.
766 =item connect SOCKET,NAME
768 Attempts to connect to a remote socket, just as the connect system call
769 does. Returns true if it succeeded, false otherwise. NAME should be a
770 packed address of the appropriate type for the socket. See the examples in
771 L<perlipc/"Sockets: Client/Server Communication">.
775 Actually a flow control statement rather than a function. If there is a
776 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
777 C<foreach>), it is always executed just before the conditional is about to
778 be evaluated again, just like the third part of a C<for> loop in C. Thus
779 it can be used to increment a loop variable, even when the loop has been
780 continued via the C<next> statement (which is similar to the C C<continue>
783 C<last>, C<next>, or C<redo> may appear within a C<continue>
784 block. C<last> and C<redo> will behave as if they had been executed within
785 the main block. So will C<next>, but since it will execute a C<continue>
786 block, it may be more entertaining.
789 ### redo always comes here
792 ### next always comes here
794 # then back the top to re-check EXPR
796 ### last always comes here
798 Omitting the C<continue> section is semantically equivalent to using an
799 empty one, logically enough. In that case, C<next> goes directly back
800 to check the condition at the top of the loop.
806 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
807 takes cosine of C<$_>.
809 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
810 function, or use this relation:
812 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
814 =item crypt PLAINTEXT,SALT
816 Encrypts a string exactly like the crypt(3) function in the C library
817 (assuming that you actually have a version there that has not been
818 extirpated as a potential munition). This can prove useful for checking
819 the password file for lousy passwords, amongst other things. Only the
820 guys wearing white hats should do this.
822 Note that C<crypt> is intended to be a one-way function, much like
823 breaking eggs to make an omelette. There is no (known) corresponding
824 decrypt function (in other words, the crypt() is a one-way hash
825 function). As a result, this function isn't all that useful for
826 cryptography. (For that, see your nearby CPAN mirror.)
828 When verifying an existing encrypted string you should use the
829 encrypted text as the salt (like C<crypt($plain, $crypted) eq
830 $crypted>). This allows your code to work with the standard C<crypt>
831 and with more exotic implementations. In other words, do not assume
832 anything about the returned string itself, or how many bytes in
833 the encrypted string matter.
835 Traditionally the result is a string of 13 bytes: two first bytes of
836 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
837 the first eight bytes of the encrypted string mattered, but
838 alternative hashing schemes (like MD5), higher level security schemes
839 (like C2), and implementations on non-UNIX platforms may produce
842 When choosing a new salt create a random two character string whose
843 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
844 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
846 Here's an example that makes sure that whoever runs this program knows
849 $pwd = (getpwuid($<))[1];
853 chomp($word = <STDIN>);
857 if (crypt($word, $pwd) ne $pwd) {
863 Of course, typing in your own password to whoever asks you
866 The L<crypt> function is unsuitable for encrypting large quantities
867 of data, not least of all because you can't get the information
868 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
869 on your favorite CPAN mirror for a slew of potentially useful
872 If using crypt() on a Unicode string (which I<potentially> has
873 characters with codepoints above 255), Perl tries to make sense
874 of the situation by trying to downgrade (a copy of the string)
875 the string back to an eight-bit byte string before calling crypt()
876 (on that copy). If that works, good. If not, crypt() dies with
877 C<Wide character in crypt>.
881 [This function has been largely superseded by the C<untie> function.]
883 Breaks the binding between a DBM file and a hash.
885 =item dbmopen HASH,DBNAME,MASK
887 [This function has been largely superseded by the C<tie> function.]
889 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
890 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
891 argument is I<not> a filehandle, even though it looks like one). DBNAME
892 is the name of the database (without the F<.dir> or F<.pag> extension if
893 any). If the database does not exist, it is created with protection
894 specified by MASK (as modified by the C<umask>). If your system supports
895 only the older DBM functions, you may perform only one C<dbmopen> in your
896 program. In older versions of Perl, if your system had neither DBM nor
897 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
900 If you don't have write access to the DBM file, you can only read hash
901 variables, not set them. If you want to test whether you can write,
902 either use file tests or try setting a dummy hash entry inside an C<eval>,
903 which will trap the error.
905 Note that functions such as C<keys> and C<values> may return huge lists
906 when used on large DBM files. You may prefer to use the C<each>
907 function to iterate over large DBM files. Example:
909 # print out history file offsets
910 dbmopen(%HIST,'/usr/lib/news/history',0666);
911 while (($key,$val) = each %HIST) {
912 print $key, ' = ', unpack('L',$val), "\n";
916 See also L<AnyDBM_File> for a more general description of the pros and
917 cons of the various dbm approaches, as well as L<DB_File> for a particularly
920 You can control which DBM library you use by loading that library
921 before you call dbmopen():
924 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
925 or die "Can't open netscape history file: $!";
931 Returns a Boolean value telling whether EXPR has a value other than
932 the undefined value C<undef>. If EXPR is not present, C<$_> will be
935 Many operations return C<undef> to indicate failure, end of file,
936 system error, uninitialized variable, and other exceptional
937 conditions. This function allows you to distinguish C<undef> from
938 other values. (A simple Boolean test will not distinguish among
939 C<undef>, zero, the empty string, and C<"0">, which are all equally
940 false.) Note that since C<undef> is a valid scalar, its presence
941 doesn't I<necessarily> indicate an exceptional condition: C<pop>
942 returns C<undef> when its argument is an empty array, I<or> when the
943 element to return happens to be C<undef>.
945 You may also use C<defined(&func)> to check whether subroutine C<&func>
946 has ever been defined. The return value is unaffected by any forward
947 declarations of C<&foo>. Note that a subroutine which is not defined
948 may still be callable: its package may have an C<AUTOLOAD> method that
949 makes it spring into existence the first time that it is called -- see
952 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
953 used to report whether memory for that aggregate has ever been
954 allocated. This behavior may disappear in future versions of Perl.
955 You should instead use a simple test for size:
957 if (@an_array) { print "has array elements\n" }
958 if (%a_hash) { print "has hash members\n" }
960 When used on a hash element, it tells you whether the value is defined,
961 not whether the key exists in the hash. Use L</exists> for the latter
966 print if defined $switch{'D'};
967 print "$val\n" while defined($val = pop(@ary));
968 die "Can't readlink $sym: $!"
969 unless defined($value = readlink $sym);
970 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
971 $debugging = 0 unless defined $debugging;
973 Note: Many folks tend to overuse C<defined>, and then are surprised to
974 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
975 defined values. For example, if you say
979 The pattern match succeeds, and C<$1> is defined, despite the fact that it
980 matched "nothing". But it didn't really match nothing--rather, it
981 matched something that happened to be zero characters long. This is all
982 very above-board and honest. When a function returns an undefined value,
983 it's an admission that it couldn't give you an honest answer. So you
984 should use C<defined> only when you're questioning the integrity of what
985 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
988 See also L</undef>, L</exists>, L</ref>.
992 Given an expression that specifies a hash element, array element, hash slice,
993 or array slice, deletes the specified element(s) from the hash or array.
994 In the case of an array, if the array elements happen to be at the end,
995 the size of the array will shrink to the highest element that tests
996 true for exists() (or 0 if no such element exists).
998 Returns each element so deleted or the undefined value if there was no such
999 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1000 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1001 from a C<tie>d hash or array may not necessarily return anything.
1003 Deleting an array element effectively returns that position of the array
1004 to its initial, uninitialized state. Subsequently testing for the same
1005 element with exists() will return false. Note that deleting array
1006 elements in the middle of an array will not shift the index of the ones
1007 after them down--use splice() for that. See L</exists>.
1009 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1011 foreach $key (keys %HASH) {
1015 foreach $index (0 .. $#ARRAY) {
1016 delete $ARRAY[$index];
1021 delete @HASH{keys %HASH};
1023 delete @ARRAY[0 .. $#ARRAY];
1025 But both of these are slower than just assigning the empty list
1026 or undefining %HASH or @ARRAY:
1028 %HASH = (); # completely empty %HASH
1029 undef %HASH; # forget %HASH ever existed
1031 @ARRAY = (); # completely empty @ARRAY
1032 undef @ARRAY; # forget @ARRAY ever existed
1034 Note that the EXPR can be arbitrarily complicated as long as the final
1035 operation is a hash element, array element, hash slice, or array slice
1038 delete $ref->[$x][$y]{$key};
1039 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1041 delete $ref->[$x][$y][$index];
1042 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1046 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1047 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1048 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1049 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1050 an C<eval(),> the error message is stuffed into C<$@> and the
1051 C<eval> is terminated with the undefined value. This makes
1052 C<die> the way to raise an exception.
1054 Equivalent examples:
1056 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1057 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1059 If the last element of LIST does not end in a newline, the current
1060 script line number and input line number (if any) are also printed,
1061 and a newline is supplied. Note that the "input line number" (also
1062 known as "chunk") is subject to whatever notion of "line" happens to
1063 be currently in effect, and is also available as the special variable
1064 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1066 Hint: sometimes appending C<", stopped"> to your message will cause it
1067 to make better sense when the string C<"at foo line 123"> is appended.
1068 Suppose you are running script "canasta".
1070 die "/etc/games is no good";
1071 die "/etc/games is no good, stopped";
1073 produce, respectively
1075 /etc/games is no good at canasta line 123.
1076 /etc/games is no good, stopped at canasta line 123.
1078 See also exit(), warn(), and the Carp module.
1080 If LIST is empty and C<$@> already contains a value (typically from a
1081 previous eval) that value is reused after appending C<"\t...propagated">.
1082 This is useful for propagating exceptions:
1085 die unless $@ =~ /Expected exception/;
1087 If LIST is empty and C<$@> contains an object reference that has a
1088 C<PROPAGATE> method, that method will be called with additional file
1089 and line number parameters. The return value replaces the value in
1090 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1093 If C<$@> is empty then the string C<"Died"> is used.
1095 die() can also be called with a reference argument. If this happens to be
1096 trapped within an eval(), $@ contains the reference. This behavior permits
1097 a more elaborate exception handling implementation using objects that
1098 maintain arbitrary state about the nature of the exception. Such a scheme
1099 is sometimes preferable to matching particular string values of $@ using
1100 regular expressions. Here's an example:
1102 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1104 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1105 # handle Some::Module::Exception
1108 # handle all other possible exceptions
1112 Because perl will stringify uncaught exception messages before displaying
1113 them, you may want to overload stringification operations on such custom
1114 exception objects. See L<overload> for details about that.
1116 You can arrange for a callback to be run just before the C<die>
1117 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1118 handler will be called with the error text and can change the error
1119 message, if it sees fit, by calling C<die> again. See
1120 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1121 L<"eval BLOCK"> for some examples. Although this feature was meant
1122 to be run only right before your program was to exit, this is not
1123 currently the case--the C<$SIG{__DIE__}> hook is currently called
1124 even inside eval()ed blocks/strings! If one wants the hook to do
1125 nothing in such situations, put
1129 as the first line of the handler (see L<perlvar/$^S>). Because
1130 this promotes strange action at a distance, this counterintuitive
1131 behavior may be fixed in a future release.
1135 Not really a function. Returns the value of the last command in the
1136 sequence of commands indicated by BLOCK. When modified by a loop
1137 modifier, executes the BLOCK once before testing the loop condition.
1138 (On other statements the loop modifiers test the conditional first.)
1140 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1141 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1142 See L<perlsyn> for alternative strategies.
1144 =item do SUBROUTINE(LIST)
1146 A deprecated form of subroutine call. See L<perlsub>.
1150 Uses the value of EXPR as a filename and executes the contents of the
1151 file as a Perl script. Its primary use is to include subroutines
1152 from a Perl subroutine library.
1160 except that it's more efficient and concise, keeps track of the current
1161 filename for error messages, searches the @INC libraries, and updates
1162 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1163 variables. It also differs in that code evaluated with C<do FILENAME>
1164 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1165 same, however, in that it does reparse the file every time you call it,
1166 so you probably don't want to do this inside a loop.
1168 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1169 error. If C<do> can read the file but cannot compile it, it
1170 returns undef and sets an error message in C<$@>. If the file is
1171 successfully compiled, C<do> returns the value of the last expression
1174 Note that inclusion of library modules is better done with the
1175 C<use> and C<require> operators, which also do automatic error checking
1176 and raise an exception if there's a problem.
1178 You might like to use C<do> to read in a program configuration
1179 file. Manual error checking can be done this way:
1181 # read in config files: system first, then user
1182 for $file ("/share/prog/defaults.rc",
1183 "$ENV{HOME}/.someprogrc")
1185 unless ($return = do $file) {
1186 warn "couldn't parse $file: $@" if $@;
1187 warn "couldn't do $file: $!" unless defined $return;
1188 warn "couldn't run $file" unless $return;
1196 This function causes an immediate core dump. See also the B<-u>
1197 command-line switch in L<perlrun>, which does the same thing.
1198 Primarily this is so that you can use the B<undump> program (not
1199 supplied) to turn your core dump into an executable binary after
1200 having initialized all your variables at the beginning of the
1201 program. When the new binary is executed it will begin by executing
1202 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1203 Think of it as a goto with an intervening core dump and reincarnation.
1204 If C<LABEL> is omitted, restarts the program from the top.
1206 B<WARNING>: Any files opened at the time of the dump will I<not>
1207 be open any more when the program is reincarnated, with possible
1208 resulting confusion on the part of Perl.
1210 This function is now largely obsolete, partly because it's very
1211 hard to convert a core file into an executable, and because the
1212 real compiler backends for generating portable bytecode and compilable
1213 C code have superseded it. That's why you should now invoke it as
1214 C<CORE::dump()>, if you don't want to be warned against a possible
1217 If you're looking to use L<dump> to speed up your program, consider
1218 generating bytecode or native C code as described in L<perlcc>. If
1219 you're just trying to accelerate a CGI script, consider using the
1220 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1221 You might also consider autoloading or selfloading, which at least
1222 make your program I<appear> to run faster.
1226 When called in list context, returns a 2-element list consisting of the
1227 key and value for the next element of a hash, so that you can iterate over
1228 it. When called in scalar context, returns only the key for the next
1229 element in the hash.
1231 Entries are returned in an apparently random order. The actual random
1232 order is subject to change in future versions of perl, but it is guaranteed
1233 to be in the same order as either the C<keys> or C<values> function
1234 would produce on the same (unmodified) hash.
1236 When the hash is entirely read, a null array is returned in list context
1237 (which when assigned produces a false (C<0>) value), and C<undef> in
1238 scalar context. The next call to C<each> after that will start iterating
1239 again. There is a single iterator for each hash, shared by all C<each>,
1240 C<keys>, and C<values> function calls in the program; it can be reset by
1241 reading all the elements from the hash, or by evaluating C<keys HASH> or
1242 C<values HASH>. If you add or delete elements of a hash while you're
1243 iterating over it, you may get entries skipped or duplicated, so
1244 don't. Exception: It is always safe to delete the item most recently
1245 returned by C<each()>, which means that the following code will work:
1247 while (($key, $value) = each %hash) {
1249 delete $hash{$key}; # This is safe
1252 The following prints out your environment like the printenv(1) program,
1253 only in a different order:
1255 while (($key,$value) = each %ENV) {
1256 print "$key=$value\n";
1259 See also C<keys>, C<values> and C<sort>.
1261 =item eof FILEHANDLE
1267 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1268 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1269 gives the real filehandle. (Note that this function actually
1270 reads a character and then C<ungetc>s it, so isn't very useful in an
1271 interactive context.) Do not read from a terminal file (or call
1272 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1273 as terminals may lose the end-of-file condition if you do.
1275 An C<eof> without an argument uses the last file read. Using C<eof()>
1276 with empty parentheses is very different. It refers to the pseudo file
1277 formed from the files listed on the command line and accessed via the
1278 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1279 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1280 used will cause C<@ARGV> to be examined to determine if input is
1281 available. Similarly, an C<eof()> after C<< <> >> has returned
1282 end-of-file will assume you are processing another C<@ARGV> list,
1283 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1284 see L<perlop/"I/O Operators">.
1286 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1287 detect the end of each file, C<eof()> will only detect the end of the
1288 last file. Examples:
1290 # reset line numbering on each input file
1292 next if /^\s*#/; # skip comments
1295 close ARGV if eof; # Not eof()!
1298 # insert dashes just before last line of last file
1300 if (eof()) { # check for end of current file
1301 print "--------------\n";
1302 close(ARGV); # close or last; is needed if we
1303 # are reading from the terminal
1308 Practical hint: you almost never need to use C<eof> in Perl, because the
1309 input operators typically return C<undef> when they run out of data, or if
1316 In the first form, the return value of EXPR is parsed and executed as if it
1317 were a little Perl program. The value of the expression (which is itself
1318 determined within scalar context) is first parsed, and if there weren't any
1319 errors, executed in the lexical context of the current Perl program, so
1320 that any variable settings or subroutine and format definitions remain
1321 afterwards. Note that the value is parsed every time the eval executes.
1322 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1323 delay parsing and subsequent execution of the text of EXPR until run time.
1325 In the second form, the code within the BLOCK is parsed only once--at the
1326 same time the code surrounding the eval itself was parsed--and executed
1327 within the context of the current Perl program. This form is typically
1328 used to trap exceptions more efficiently than the first (see below), while
1329 also providing the benefit of checking the code within BLOCK at compile
1332 The final semicolon, if any, may be omitted from the value of EXPR or within
1335 In both forms, the value returned is the value of the last expression
1336 evaluated inside the mini-program; a return statement may be also used, just
1337 as with subroutines. The expression providing the return value is evaluated
1338 in void, scalar, or list context, depending on the context of the eval itself.
1339 See L</wantarray> for more on how the evaluation context can be determined.
1341 If there is a syntax error or runtime error, or a C<die> statement is
1342 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1343 error message. If there was no error, C<$@> is guaranteed to be a null
1344 string. Beware that using C<eval> neither silences perl from printing
1345 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1346 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1347 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1348 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1350 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1351 determining whether a particular feature (such as C<socket> or C<symlink>)
1352 is implemented. It is also Perl's exception trapping mechanism, where
1353 the die operator is used to raise exceptions.
1355 If the code to be executed doesn't vary, you may use the eval-BLOCK
1356 form to trap run-time errors without incurring the penalty of
1357 recompiling each time. The error, if any, is still returned in C<$@>.
1360 # make divide-by-zero nonfatal
1361 eval { $answer = $a / $b; }; warn $@ if $@;
1363 # same thing, but less efficient
1364 eval '$answer = $a / $b'; warn $@ if $@;
1366 # a compile-time error
1367 eval { $answer = }; # WRONG
1370 eval '$answer ='; # sets $@
1372 Due to the current arguably broken state of C<__DIE__> hooks, when using
1373 the C<eval{}> form as an exception trap in libraries, you may wish not
1374 to trigger any C<__DIE__> hooks that user code may have installed.
1375 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1376 as shown in this example:
1378 # a very private exception trap for divide-by-zero
1379 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1382 This is especially significant, given that C<__DIE__> hooks can call
1383 C<die> again, which has the effect of changing their error messages:
1385 # __DIE__ hooks may modify error messages
1387 local $SIG{'__DIE__'} =
1388 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1389 eval { die "foo lives here" };
1390 print $@ if $@; # prints "bar lives here"
1393 Because this promotes action at a distance, this counterintuitive behavior
1394 may be fixed in a future release.
1396 With an C<eval>, you should be especially careful to remember what's
1397 being looked at when:
1403 eval { $x }; # CASE 4
1405 eval "\$$x++"; # CASE 5
1408 Cases 1 and 2 above behave identically: they run the code contained in
1409 the variable $x. (Although case 2 has misleading double quotes making
1410 the reader wonder what else might be happening (nothing is).) Cases 3
1411 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1412 does nothing but return the value of $x. (Case 4 is preferred for
1413 purely visual reasons, but it also has the advantage of compiling at
1414 compile-time instead of at run-time.) Case 5 is a place where
1415 normally you I<would> like to use double quotes, except that in this
1416 particular situation, you can just use symbolic references instead, as
1419 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1420 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1424 =item exec PROGRAM LIST
1426 The C<exec> function executes a system command I<and never returns>--
1427 use C<system> instead of C<exec> if you want it to return. It fails and
1428 returns false only if the command does not exist I<and> it is executed
1429 directly instead of via your system's command shell (see below).
1431 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1432 warns you if there is a following statement which isn't C<die>, C<warn>,
1433 or C<exit> (if C<-w> is set - but you always do that). If you
1434 I<really> want to follow an C<exec> with some other statement, you
1435 can use one of these styles to avoid the warning:
1437 exec ('foo') or print STDERR "couldn't exec foo: $!";
1438 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1440 If there is more than one argument in LIST, or if LIST is an array
1441 with more than one value, calls execvp(3) with the arguments in LIST.
1442 If there is only one scalar argument or an array with one element in it,
1443 the argument is checked for shell metacharacters, and if there are any,
1444 the entire argument is passed to the system's command shell for parsing
1445 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1446 If there are no shell metacharacters in the argument, it is split into
1447 words and passed directly to C<execvp>, which is more efficient.
1450 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1451 exec "sort $outfile | uniq";
1453 If you don't really want to execute the first argument, but want to lie
1454 to the program you are executing about its own name, you can specify
1455 the program you actually want to run as an "indirect object" (without a
1456 comma) in front of the LIST. (This always forces interpretation of the
1457 LIST as a multivalued list, even if there is only a single scalar in
1460 $shell = '/bin/csh';
1461 exec $shell '-sh'; # pretend it's a login shell
1465 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1467 When the arguments get executed via the system shell, results will
1468 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1471 Using an indirect object with C<exec> or C<system> is also more
1472 secure. This usage (which also works fine with system()) forces
1473 interpretation of the arguments as a multivalued list, even if the
1474 list had just one argument. That way you're safe from the shell
1475 expanding wildcards or splitting up words with whitespace in them.
1477 @args = ( "echo surprise" );
1479 exec @args; # subject to shell escapes
1481 exec { $args[0] } @args; # safe even with one-arg list
1483 The first version, the one without the indirect object, ran the I<echo>
1484 program, passing it C<"surprise"> an argument. The second version
1485 didn't--it tried to run a program literally called I<"echo surprise">,
1486 didn't find it, and set C<$?> to a non-zero value indicating failure.
1488 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1489 output before the exec, but this may not be supported on some platforms
1490 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1491 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1492 open handles in order to avoid lost output.
1494 Note that C<exec> will not call your C<END> blocks, nor will it call
1495 any C<DESTROY> methods in your objects.
1499 Given an expression that specifies a hash element or array element,
1500 returns true if the specified element in the hash or array has ever
1501 been initialized, even if the corresponding value is undefined. The
1502 element is not autovivified if it doesn't exist.
1504 print "Exists\n" if exists $hash{$key};
1505 print "Defined\n" if defined $hash{$key};
1506 print "True\n" if $hash{$key};
1508 print "Exists\n" if exists $array[$index];
1509 print "Defined\n" if defined $array[$index];
1510 print "True\n" if $array[$index];
1512 A hash or array element can be true only if it's defined, and defined if
1513 it exists, but the reverse doesn't necessarily hold true.
1515 Given an expression that specifies the name of a subroutine,
1516 returns true if the specified subroutine has ever been declared, even
1517 if it is undefined. Mentioning a subroutine name for exists or defined
1518 does not count as declaring it. Note that a subroutine which does not
1519 exist may still be callable: its package may have an C<AUTOLOAD>
1520 method that makes it spring into existence the first time that it is
1521 called -- see L<perlsub>.
1523 print "Exists\n" if exists &subroutine;
1524 print "Defined\n" if defined &subroutine;
1526 Note that the EXPR can be arbitrarily complicated as long as the final
1527 operation is a hash or array key lookup or subroutine name:
1529 if (exists $ref->{A}->{B}->{$key}) { }
1530 if (exists $hash{A}{B}{$key}) { }
1532 if (exists $ref->{A}->{B}->[$ix]) { }
1533 if (exists $hash{A}{B}[$ix]) { }
1535 if (exists &{$ref->{A}{B}{$key}}) { }
1537 Although the deepest nested array or hash will not spring into existence
1538 just because its existence was tested, any intervening ones will.
1539 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1540 into existence due to the existence test for the $key element above.
1541 This happens anywhere the arrow operator is used, including even:
1544 if (exists $ref->{"Some key"}) { }
1545 print $ref; # prints HASH(0x80d3d5c)
1547 This surprising autovivification in what does not at first--or even
1548 second--glance appear to be an lvalue context may be fixed in a future
1551 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1552 on how exists() acts when used on a pseudo-hash.
1554 Use of a subroutine call, rather than a subroutine name, as an argument
1555 to exists() is an error.
1558 exists &sub(); # Error
1562 Evaluates EXPR and exits immediately with that value. Example:
1565 exit 0 if $ans =~ /^[Xx]/;
1567 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1568 universally recognized values for EXPR are C<0> for success and C<1>
1569 for error; other values are subject to interpretation depending on the
1570 environment in which the Perl program is running. For example, exiting
1571 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1572 the mailer to return the item undelivered, but that's not true everywhere.
1574 Don't use C<exit> to abort a subroutine if there's any chance that
1575 someone might want to trap whatever error happened. Use C<die> instead,
1576 which can be trapped by an C<eval>.
1578 The exit() function does not always exit immediately. It calls any
1579 defined C<END> routines first, but these C<END> routines may not
1580 themselves abort the exit. Likewise any object destructors that need to
1581 be called are called before the real exit. If this is a problem, you
1582 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1583 See L<perlmod> for details.
1589 Returns I<e> (the natural logarithm base) to the power of EXPR.
1590 If EXPR is omitted, gives C<exp($_)>.
1592 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1594 Implements the fcntl(2) function. You'll probably have to say
1598 first to get the correct constant definitions. Argument processing and
1599 value return works just like C<ioctl> below.
1603 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1604 or die "can't fcntl F_GETFL: $!";
1606 You don't have to check for C<defined> on the return from C<fnctl>.
1607 Like C<ioctl>, it maps a C<0> return from the system call into
1608 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1609 in numeric context. It is also exempt from the normal B<-w> warnings
1610 on improper numeric conversions.
1612 Note that C<fcntl> will produce a fatal error if used on a machine that
1613 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1614 manpage to learn what functions are available on your system.
1616 =item fileno FILEHANDLE
1618 Returns the file descriptor for a filehandle, or undefined if the
1619 filehandle is not open. This is mainly useful for constructing
1620 bitmaps for C<select> and low-level POSIX tty-handling operations.
1621 If FILEHANDLE is an expression, the value is taken as an indirect
1622 filehandle, generally its name.
1624 You can use this to find out whether two handles refer to the
1625 same underlying descriptor:
1627 if (fileno(THIS) == fileno(THAT)) {
1628 print "THIS and THAT are dups\n";
1631 (Filehandles connected to memory objects via new features of C<open> may
1632 return undefined even though they are open.)
1635 =item flock FILEHANDLE,OPERATION
1637 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1638 for success, false on failure. Produces a fatal error if used on a
1639 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1640 C<flock> is Perl's portable file locking interface, although it locks
1641 only entire files, not records.
1643 Two potentially non-obvious but traditional C<flock> semantics are
1644 that it waits indefinitely until the lock is granted, and that its locks
1645 B<merely advisory>. Such discretionary locks are more flexible, but offer
1646 fewer guarantees. This means that files locked with C<flock> may be
1647 modified by programs that do not also use C<flock>. See L<perlport>,
1648 your port's specific documentation, or your system-specific local manpages
1649 for details. It's best to assume traditional behavior if you're writing
1650 portable programs. (But if you're not, you should as always feel perfectly
1651 free to write for your own system's idiosyncrasies (sometimes called
1652 "features"). Slavish adherence to portability concerns shouldn't get
1653 in the way of your getting your job done.)
1655 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1656 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1657 you can use the symbolic names if you import them from the Fcntl module,
1658 either individually, or as a group using the ':flock' tag. LOCK_SH
1659 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1660 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1661 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1662 waiting for the lock (check the return status to see if you got it).
1664 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1665 before locking or unlocking it.
1667 Note that the emulation built with lockf(3) doesn't provide shared
1668 locks, and it requires that FILEHANDLE be open with write intent. These
1669 are the semantics that lockf(3) implements. Most if not all systems
1670 implement lockf(3) in terms of fcntl(2) locking, though, so the
1671 differing semantics shouldn't bite too many people.
1673 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1674 be open with read intent to use LOCK_SH and requires that it be open
1675 with write intent to use LOCK_EX.
1677 Note also that some versions of C<flock> cannot lock things over the
1678 network; you would need to use the more system-specific C<fcntl> for
1679 that. If you like you can force Perl to ignore your system's flock(2)
1680 function, and so provide its own fcntl(2)-based emulation, by passing
1681 the switch C<-Ud_flock> to the F<Configure> program when you configure
1684 Here's a mailbox appender for BSD systems.
1686 use Fcntl ':flock'; # import LOCK_* constants
1689 flock(MBOX,LOCK_EX);
1690 # and, in case someone appended
1691 # while we were waiting...
1696 flock(MBOX,LOCK_UN);
1699 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1700 or die "Can't open mailbox: $!";
1703 print MBOX $msg,"\n\n";
1706 On systems that support a real flock(), locks are inherited across fork()
1707 calls, whereas those that must resort to the more capricious fcntl()
1708 function lose the locks, making it harder to write servers.
1710 See also L<DB_File> for other flock() examples.
1714 Does a fork(2) system call to create a new process running the
1715 same program at the same point. It returns the child pid to the
1716 parent process, C<0> to the child process, or C<undef> if the fork is
1717 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1718 are shared, while everything else is copied. On most systems supporting
1719 fork(), great care has gone into making it extremely efficient (for
1720 example, using copy-on-write technology on data pages), making it the
1721 dominant paradigm for multitasking over the last few decades.
1723 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1724 output before forking the child process, but this may not be supported
1725 on some platforms (see L<perlport>). To be safe, you may need to set
1726 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1727 C<IO::Handle> on any open handles in order to avoid duplicate output.
1729 If you C<fork> without ever waiting on your children, you will
1730 accumulate zombies. On some systems, you can avoid this by setting
1731 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1732 forking and reaping moribund children.
1734 Note that if your forked child inherits system file descriptors like
1735 STDIN and STDOUT that are actually connected by a pipe or socket, even
1736 if you exit, then the remote server (such as, say, a CGI script or a
1737 backgrounded job launched from a remote shell) won't think you're done.
1738 You should reopen those to F</dev/null> if it's any issue.
1742 Declare a picture format for use by the C<write> function. For
1746 Test: @<<<<<<<< @||||| @>>>>>
1747 $str, $%, '$' . int($num)
1751 $num = $cost/$quantity;
1755 See L<perlform> for many details and examples.
1757 =item formline PICTURE,LIST
1759 This is an internal function used by C<format>s, though you may call it,
1760 too. It formats (see L<perlform>) a list of values according to the
1761 contents of PICTURE, placing the output into the format output
1762 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1763 Eventually, when a C<write> is done, the contents of
1764 C<$^A> are written to some filehandle, but you could also read C<$^A>
1765 yourself and then set C<$^A> back to C<"">. Note that a format typically
1766 does one C<formline> per line of form, but the C<formline> function itself
1767 doesn't care how many newlines are embedded in the PICTURE. This means
1768 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1769 You may therefore need to use multiple formlines to implement a single
1770 record format, just like the format compiler.
1772 Be careful if you put double quotes around the picture, because an C<@>
1773 character may be taken to mean the beginning of an array name.
1774 C<formline> always returns true. See L<perlform> for other examples.
1776 =item getc FILEHANDLE
1780 Returns the next character from the input file attached to FILEHANDLE,
1781 or the undefined value at end of file, or if there was an error.
1782 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1783 efficient. However, it cannot be used by itself to fetch single
1784 characters without waiting for the user to hit enter. For that, try
1785 something more like:
1788 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1791 system "stty", '-icanon', 'eol', "\001";
1797 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1800 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1804 Determination of whether $BSD_STYLE should be set
1805 is left as an exercise to the reader.
1807 The C<POSIX::getattr> function can do this more portably on
1808 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1809 module from your nearest CPAN site; details on CPAN can be found on
1814 Implements the C library function of the same name, which on most
1815 systems returns the current login from F</etc/utmp>, if any. If null,
1818 $login = getlogin || getpwuid($<) || "Kilroy";
1820 Do not consider C<getlogin> for authentication: it is not as
1821 secure as C<getpwuid>.
1823 =item getpeername SOCKET
1825 Returns the packed sockaddr address of other end of the SOCKET connection.
1828 $hersockaddr = getpeername(SOCK);
1829 ($port, $iaddr) = sockaddr_in($hersockaddr);
1830 $herhostname = gethostbyaddr($iaddr, AF_INET);
1831 $herstraddr = inet_ntoa($iaddr);
1835 Returns the current process group for the specified PID. Use
1836 a PID of C<0> to get the current process group for the
1837 current process. Will raise an exception if used on a machine that
1838 doesn't implement getpgrp(2). If PID is omitted, returns process
1839 group of current process. Note that the POSIX version of C<getpgrp>
1840 does not accept a PID argument, so only C<PID==0> is truly portable.
1844 Returns the process id of the parent process.
1846 =item getpriority WHICH,WHO
1848 Returns the current priority for a process, a process group, or a user.
1849 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1850 machine that doesn't implement getpriority(2).
1856 =item gethostbyname NAME
1858 =item getnetbyname NAME
1860 =item getprotobyname NAME
1866 =item getservbyname NAME,PROTO
1868 =item gethostbyaddr ADDR,ADDRTYPE
1870 =item getnetbyaddr ADDR,ADDRTYPE
1872 =item getprotobynumber NUMBER
1874 =item getservbyport PORT,PROTO
1892 =item sethostent STAYOPEN
1894 =item setnetent STAYOPEN
1896 =item setprotoent STAYOPEN
1898 =item setservent STAYOPEN
1912 These routines perform the same functions as their counterparts in the
1913 system library. In list context, the return values from the
1914 various get routines are as follows:
1916 ($name,$passwd,$uid,$gid,
1917 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1918 ($name,$passwd,$gid,$members) = getgr*
1919 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1920 ($name,$aliases,$addrtype,$net) = getnet*
1921 ($name,$aliases,$proto) = getproto*
1922 ($name,$aliases,$port,$proto) = getserv*
1924 (If the entry doesn't exist you get a null list.)
1926 The exact meaning of the $gcos field varies but it usually contains
1927 the real name of the user (as opposed to the login name) and other
1928 information pertaining to the user. Beware, however, that in many
1929 system users are able to change this information and therefore it
1930 cannot be trusted and therefore the $gcos is tainted (see
1931 L<perlsec>). The $passwd and $shell, user's encrypted password and
1932 login shell, are also tainted, because of the same reason.
1934 In scalar context, you get the name, unless the function was a
1935 lookup by name, in which case you get the other thing, whatever it is.
1936 (If the entry doesn't exist you get the undefined value.) For example:
1938 $uid = getpwnam($name);
1939 $name = getpwuid($num);
1941 $gid = getgrnam($name);
1942 $name = getgrgid($num;
1946 In I<getpw*()> the fields $quota, $comment, and $expire are special
1947 cases in the sense that in many systems they are unsupported. If the
1948 $quota is unsupported, it is an empty scalar. If it is supported, it
1949 usually encodes the disk quota. If the $comment field is unsupported,
1950 it is an empty scalar. If it is supported it usually encodes some
1951 administrative comment about the user. In some systems the $quota
1952 field may be $change or $age, fields that have to do with password
1953 aging. In some systems the $comment field may be $class. The $expire
1954 field, if present, encodes the expiration period of the account or the
1955 password. For the availability and the exact meaning of these fields
1956 in your system, please consult your getpwnam(3) documentation and your
1957 F<pwd.h> file. You can also find out from within Perl what your
1958 $quota and $comment fields mean and whether you have the $expire field
1959 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1960 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1961 files are only supported if your vendor has implemented them in the
1962 intuitive fashion that calling the regular C library routines gets the
1963 shadow versions if you're running under privilege or if there exists
1964 the shadow(3) functions as found in System V ( this includes Solaris
1965 and Linux.) Those systems which implement a proprietary shadow password
1966 facility are unlikely to be supported.
1968 The $members value returned by I<getgr*()> is a space separated list of
1969 the login names of the members of the group.
1971 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1972 C, it will be returned to you via C<$?> if the function call fails. The
1973 C<@addrs> value returned by a successful call is a list of the raw
1974 addresses returned by the corresponding system library call. In the
1975 Internet domain, each address is four bytes long and you can unpack it
1976 by saying something like:
1978 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1980 The Socket library makes this slightly easier:
1983 $iaddr = inet_aton("127.1"); # or whatever address
1984 $name = gethostbyaddr($iaddr, AF_INET);
1986 # or going the other way
1987 $straddr = inet_ntoa($iaddr);
1989 If you get tired of remembering which element of the return list
1990 contains which return value, by-name interfaces are provided
1991 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1992 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1993 and C<User::grent>. These override the normal built-ins, supplying
1994 versions that return objects with the appropriate names
1995 for each field. For example:
1999 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2001 Even though it looks like they're the same method calls (uid),
2002 they aren't, because a C<File::stat> object is different from
2003 a C<User::pwent> object.
2005 =item getsockname SOCKET
2007 Returns the packed sockaddr address of this end of the SOCKET connection,
2008 in case you don't know the address because you have several different
2009 IPs that the connection might have come in on.
2012 $mysockaddr = getsockname(SOCK);
2013 ($port, $myaddr) = sockaddr_in($mysockaddr);
2014 printf "Connect to %s [%s]\n",
2015 scalar gethostbyaddr($myaddr, AF_INET),
2018 =item getsockopt SOCKET,LEVEL,OPTNAME
2020 Returns the socket option requested, or undef if there is an error.
2026 Returns the value of EXPR with filename expansions such as the
2027 standard Unix shell F</bin/csh> would do. This is the internal function
2028 implementing the C<< <*.c> >> operator, but you can use it directly.
2029 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2030 discussed in more detail in L<perlop/"I/O Operators">.
2032 Beginning with v5.6.0, this operator is implemented using the standard
2033 C<File::Glob> extension. See L<File::Glob> for details.
2037 Converts a time as returned by the time function to an 8-element list
2038 with the time localized for the standard Greenwich time zone.
2039 Typically used as follows:
2042 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2045 All list elements are numeric, and come straight out of the C `struct
2046 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2047 specified time. $mday is the day of the month, and $mon is the month
2048 itself, in the range C<0..11> with 0 indicating January and 11
2049 indicating December. $year is the number of years since 1900. That
2050 is, $year is C<123> in year 2023. $wday is the day of the week, with
2051 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2052 the year, in the range C<0..364> (or C<0..365> in leap years.)
2054 Note that the $year element is I<not> simply the last two digits of
2055 the year. If you assume it is, then you create non-Y2K-compliant
2056 programs--and you wouldn't want to do that, would you?
2058 The proper way to get a complete 4-digit year is simply:
2062 And to get the last two digits of the year (e.g., '01' in 2001) do:
2064 $year = sprintf("%02d", $year % 100);
2066 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2068 In scalar context, C<gmtime()> returns the ctime(3) value:
2070 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2072 Also see the C<timegm> function provided by the C<Time::Local> module,
2073 and the strftime(3) function available via the POSIX module.
2075 This scalar value is B<not> locale dependent (see L<perllocale>), but
2076 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2077 strftime(3) and mktime(3) functions available via the POSIX module. To
2078 get somewhat similar but locale dependent date strings, set up your
2079 locale environment variables appropriately (please see L<perllocale>)
2080 and try for example:
2082 use POSIX qw(strftime);
2083 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2085 Note that the C<%a> and C<%b> escapes, which represent the short forms
2086 of the day of the week and the month of the year, may not necessarily
2087 be three characters wide in all locales.
2095 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2096 execution there. It may not be used to go into any construct that
2097 requires initialization, such as a subroutine or a C<foreach> loop. It
2098 also can't be used to go into a construct that is optimized away,
2099 or to get out of a block or subroutine given to C<sort>.
2100 It can be used to go almost anywhere else within the dynamic scope,
2101 including out of subroutines, but it's usually better to use some other
2102 construct such as C<last> or C<die>. The author of Perl has never felt the
2103 need to use this form of C<goto> (in Perl, that is--C is another matter).
2104 (The difference being that C does not offer named loops combined with
2105 loop control. Perl does, and this replaces most structured uses of C<goto>
2106 in other languages.)
2108 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2109 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2110 necessarily recommended if you're optimizing for maintainability:
2112 goto ("FOO", "BAR", "GLARCH")[$i];
2114 The C<goto-&NAME> form is quite different from the other forms of
2115 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2116 doesn't have the stigma associated with other gotos. Instead, it
2117 exits the current subroutine (losing any changes set by local()) and
2118 immediately calls in its place the named subroutine using the current
2119 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2120 load another subroutine and then pretend that the other subroutine had
2121 been called in the first place (except that any modifications to C<@_>
2122 in the current subroutine are propagated to the other subroutine.)
2123 After the C<goto>, not even C<caller> will be able to tell that this
2124 routine was called first.
2126 NAME needn't be the name of a subroutine; it can be a scalar variable
2127 containing a code reference, or a block which evaluates to a code
2130 =item grep BLOCK LIST
2132 =item grep EXPR,LIST
2134 This is similar in spirit to, but not the same as, grep(1) and its
2135 relatives. In particular, it is not limited to using regular expressions.
2137 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2138 C<$_> to each element) and returns the list value consisting of those
2139 elements for which the expression evaluated to true. In scalar
2140 context, returns the number of times the expression was true.
2142 @foo = grep(!/^#/, @bar); # weed out comments
2146 @foo = grep {!/^#/} @bar; # weed out comments
2148 Note that C<$_> is an alias to the list value, so it can be used to
2149 modify the elements of the LIST. While this is useful and supported,
2150 it can cause bizarre results if the elements of LIST are not variables.
2151 Similarly, grep returns aliases into the original list, much as a for
2152 loop's index variable aliases the list elements. That is, modifying an
2153 element of a list returned by grep (for example, in a C<foreach>, C<map>
2154 or another C<grep>) actually modifies the element in the original list.
2155 This is usually something to be avoided when writing clear code.
2157 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2163 Interprets EXPR as a hex string and returns the corresponding value.
2164 (To convert strings that might start with either 0, 0x, or 0b, see
2165 L</oct>.) If EXPR is omitted, uses C<$_>.
2167 print hex '0xAf'; # prints '175'
2168 print hex 'aF'; # same
2170 Hex strings may only represent integers. Strings that would cause
2171 integer overflow trigger a warning. Leading whitespace is not stripped,
2176 There is no builtin C<import> function. It is just an ordinary
2177 method (subroutine) defined (or inherited) by modules that wish to export
2178 names to another module. The C<use> function calls the C<import> method
2179 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2181 =item index STR,SUBSTR,POSITION
2183 =item index STR,SUBSTR
2185 The index function searches for one string within another, but without
2186 the wildcard-like behavior of a full regular-expression pattern match.
2187 It returns the position of the first occurrence of SUBSTR in STR at
2188 or after POSITION. If POSITION is omitted, starts searching from the
2189 beginning of the string. The return value is based at C<0> (or whatever
2190 you've set the C<$[> variable to--but don't do that). If the substring
2191 is not found, returns one less than the base, ordinarily C<-1>.
2197 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2198 You should not use this function for rounding: one because it truncates
2199 towards C<0>, and two because machine representations of floating point
2200 numbers can sometimes produce counterintuitive results. For example,
2201 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2202 because it's really more like -268.99999999999994315658 instead. Usually,
2203 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2204 functions will serve you better than will int().
2206 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2208 Implements the ioctl(2) function. You'll probably first have to say
2210 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2212 to get the correct function definitions. If F<ioctl.ph> doesn't
2213 exist or doesn't have the correct definitions you'll have to roll your
2214 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2215 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2216 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2217 written depending on the FUNCTION--a pointer to the string value of SCALAR
2218 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2219 has no string value but does have a numeric value, that value will be
2220 passed rather than a pointer to the string value. To guarantee this to be
2221 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2222 functions may be needed to manipulate the values of structures used by
2225 The return value of C<ioctl> (and C<fcntl>) is as follows:
2227 if OS returns: then Perl returns:
2229 0 string "0 but true"
2230 anything else that number
2232 Thus Perl returns true on success and false on failure, yet you can
2233 still easily determine the actual value returned by the operating
2236 $retval = ioctl(...) || -1;
2237 printf "System returned %d\n", $retval;
2239 The special string "C<0> but true" is exempt from B<-w> complaints
2240 about improper numeric conversions.
2242 Here's an example of setting a filehandle named C<REMOTE> to be
2243 non-blocking at the system level. You'll have to negotiate C<$|>
2244 on your own, though.
2246 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2248 $flags = fcntl(REMOTE, F_GETFL, 0)
2249 or die "Can't get flags for the socket: $!\n";
2251 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2252 or die "Can't set flags for the socket: $!\n";
2254 =item join EXPR,LIST
2256 Joins the separate strings of LIST into a single string with fields
2257 separated by the value of EXPR, and returns that new string. Example:
2259 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2261 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2262 first argument. Compare L</split>.
2266 Returns a list consisting of all the keys of the named hash. (In
2267 scalar context, returns the number of keys.) The keys are returned in
2268 an apparently random order. The actual random order is subject to
2269 change in future versions of perl, but it is guaranteed to be the same
2270 order as either the C<values> or C<each> function produces (given
2271 that the hash has not been modified). As a side effect, it resets
2274 Here is yet another way to print your environment:
2277 @values = values %ENV;
2279 print pop(@keys), '=', pop(@values), "\n";
2282 or how about sorted by key:
2284 foreach $key (sort(keys %ENV)) {
2285 print $key, '=', $ENV{$key}, "\n";
2288 The returned values are copies of the original keys in the hash, so
2289 modifying them will not affect the original hash. Compare L</values>.
2291 To sort a hash by value, you'll need to use a C<sort> function.
2292 Here's a descending numeric sort of a hash by its values:
2294 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2295 printf "%4d %s\n", $hash{$key}, $key;
2298 As an lvalue C<keys> allows you to increase the number of hash buckets
2299 allocated for the given hash. This can gain you a measure of efficiency if
2300 you know the hash is going to get big. (This is similar to pre-extending
2301 an array by assigning a larger number to $#array.) If you say
2305 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2306 in fact, since it rounds up to the next power of two. These
2307 buckets will be retained even if you do C<%hash = ()>, use C<undef
2308 %hash> if you want to free the storage while C<%hash> is still in scope.
2309 You can't shrink the number of buckets allocated for the hash using
2310 C<keys> in this way (but you needn't worry about doing this by accident,
2311 as trying has no effect).
2313 See also C<each>, C<values> and C<sort>.
2315 =item kill SIGNAL, LIST
2317 Sends a signal to a list of processes. Returns the number of
2318 processes successfully signaled (which is not necessarily the
2319 same as the number actually killed).
2321 $cnt = kill 1, $child1, $child2;
2324 If SIGNAL is zero, no signal is sent to the process. This is a
2325 useful way to check that the process is alive and hasn't changed
2326 its UID. See L<perlport> for notes on the portability of this
2329 Unlike in the shell, if SIGNAL is negative, it kills
2330 process groups instead of processes. (On System V, a negative I<PROCESS>
2331 number will also kill process groups, but that's not portable.) That
2332 means you usually want to use positive not negative signals. You may also
2333 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2339 The C<last> command is like the C<break> statement in C (as used in
2340 loops); it immediately exits the loop in question. If the LABEL is
2341 omitted, the command refers to the innermost enclosing loop. The
2342 C<continue> block, if any, is not executed:
2344 LINE: while (<STDIN>) {
2345 last LINE if /^$/; # exit when done with header
2349 C<last> cannot be used to exit a block which returns a value such as
2350 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2351 a grep() or map() operation.
2353 Note that a block by itself is semantically identical to a loop
2354 that executes once. Thus C<last> can be used to effect an early
2355 exit out of such a block.
2357 See also L</continue> for an illustration of how C<last>, C<next>, and
2364 Returns a lowercased version of EXPR. This is the internal function
2365 implementing the C<\L> escape in double-quoted strings. Respects
2366 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2367 and L<perlunicode> for more details about locale and Unicode support.
2369 If EXPR is omitted, uses C<$_>.
2375 Returns the value of EXPR with the first character lowercased. This
2376 is the internal function implementing the C<\l> escape in
2377 double-quoted strings. Respects current LC_CTYPE locale if C<use
2378 locale> in force. See L<perllocale> and L<perlunicode> for more
2379 details about locale and Unicode support.
2381 If EXPR is omitted, uses C<$_>.
2387 Returns the length in characters of the value of EXPR. If EXPR is
2388 omitted, returns length of C<$_>. Note that this cannot be used on
2389 an entire array or hash to find out how many elements these have.
2390 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2392 =item link OLDFILE,NEWFILE
2394 Creates a new filename linked to the old filename. Returns true for
2395 success, false otherwise.
2397 =item listen SOCKET,QUEUESIZE
2399 Does the same thing that the listen system call does. Returns true if
2400 it succeeded, false otherwise. See the example in
2401 L<perlipc/"Sockets: Client/Server Communication">.
2405 You really probably want to be using C<my> instead, because C<local> isn't
2406 what most people think of as "local". See
2407 L<perlsub/"Private Variables via my()"> for details.
2409 A local modifies the listed variables to be local to the enclosing
2410 block, file, or eval. If more than one value is listed, the list must
2411 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2412 for details, including issues with tied arrays and hashes.
2414 =item localtime EXPR
2416 Converts a time as returned by the time function to a 9-element list
2417 with the time analyzed for the local time zone. Typically used as
2421 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2424 All list elements are numeric, and come straight out of the C `struct
2425 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2426 specified time. $mday is the day of the month, and $mon is the month
2427 itself, in the range C<0..11> with 0 indicating January and 11
2428 indicating December. $year is the number of years since 1900. That
2429 is, $year is C<123> in year 2023. $wday is the day of the week, with
2430 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2431 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2432 is true if the specified time occurs during daylight savings time,
2435 Note that the $year element is I<not> simply the last two digits of
2436 the year. If you assume it is, then you create non-Y2K-compliant
2437 programs--and you wouldn't want to do that, would you?
2439 The proper way to get a complete 4-digit year is simply:
2443 And to get the last two digits of the year (e.g., '01' in 2001) do:
2445 $year = sprintf("%02d", $year % 100);
2447 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2449 In scalar context, C<localtime()> returns the ctime(3) value:
2451 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2453 This scalar value is B<not> locale dependent, see L<perllocale>, but
2454 instead a Perl builtin. Also see the C<Time::Local> module
2455 (to convert the second, minutes, hours, ... back to seconds since the
2456 stroke of midnight the 1st of January 1970, the value returned by
2457 time()), and the strftime(3) and mktime(3) functions available via the
2458 POSIX module. To get somewhat similar but locale dependent date
2459 strings, set up your locale environment variables appropriately
2460 (please see L<perllocale>) and try for example:
2462 use POSIX qw(strftime);
2463 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2465 Note that the C<%a> and C<%b>, the short forms of the day of the week
2466 and the month of the year, may not necessarily be three characters wide.
2470 This function places an advisory lock on a variable, subroutine,
2471 or referenced object contained in I<THING> until the lock goes out
2472 of scope. This is a built-in function only if your version of Perl
2473 was built with threading enabled, and if you've said C<use Thread>.
2474 Otherwise a user-defined function by this name will be called.
2481 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2482 returns log of C<$_>. To get the log of another base, use basic algebra:
2483 The base-N log of a number is equal to the natural log of that number
2484 divided by the natural log of N. For example:
2488 return log($n)/log(10);
2491 See also L</exp> for the inverse operation.
2497 Does the same thing as the C<stat> function (including setting the
2498 special C<_> filehandle) but stats a symbolic link instead of the file
2499 the symbolic link points to. If symbolic links are unimplemented on
2500 your system, a normal C<stat> is done.
2502 If EXPR is omitted, stats C<$_>.
2506 The match operator. See L<perlop>.
2508 =item map BLOCK LIST
2512 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2513 C<$_> to each element) and returns the list value composed of the
2514 results of each such evaluation. In scalar context, returns the
2515 total number of elements so generated. Evaluates BLOCK or EXPR in
2516 list context, so each element of LIST may produce zero, one, or
2517 more elements in the returned value.
2519 @chars = map(chr, @nums);
2521 translates a list of numbers to the corresponding characters. And
2523 %hash = map { getkey($_) => $_ } @array;
2525 is just a funny way to write
2528 foreach $_ (@array) {
2529 $hash{getkey($_)} = $_;
2532 Note that C<$_> is an alias to the list value, so it can be used to
2533 modify the elements of the LIST. While this is useful and supported,
2534 it can cause bizarre results if the elements of LIST are not variables.
2535 Using a regular C<foreach> loop for this purpose would be clearer in
2536 most cases. See also L</grep> for an array composed of those items of
2537 the original list for which the BLOCK or EXPR evaluates to true.
2539 C<{> starts both hash references and blocks, so C<map { ...> could be either
2540 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2541 ahead for the closing C<}> it has to take a guess at which its dealing with
2542 based what it finds just after the C<{>. Usually it gets it right, but if it
2543 doesn't it won't realize something is wrong until it gets to the C<}> and
2544 encounters the missing (or unexpected) comma. The syntax error will be
2545 reported close to the C<}> but you'll need to change something near the C<{>
2546 such as using a unary C<+> to give perl some help:
2548 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2549 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2550 %hash = map { ("\L$_", 1) } @array # this also works
2551 %hash = map { lc($_), 1 } @array # as does this.
2552 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2554 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2556 or to force an anon hash constructor use C<+{>
2558 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2560 and you get list of anonymous hashes each with only 1 entry.
2562 =item mkdir FILENAME,MASK
2564 =item mkdir FILENAME
2566 Creates the directory specified by FILENAME, with permissions
2567 specified by MASK (as modified by C<umask>). If it succeeds it
2568 returns true, otherwise it returns false and sets C<$!> (errno).
2569 If omitted, MASK defaults to 0777.
2571 In general, it is better to create directories with permissive MASK,
2572 and let the user modify that with their C<umask>, than it is to supply
2573 a restrictive MASK and give the user no way to be more permissive.
2574 The exceptions to this rule are when the file or directory should be
2575 kept private (mail files, for instance). The perlfunc(1) entry on
2576 C<umask> discusses the choice of MASK in more detail.
2578 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2579 number of trailing slashes. Some operating and filesystems do not get
2580 this right, so Perl automatically removes all trailing slashes to keep
2583 =item msgctl ID,CMD,ARG
2585 Calls the System V IPC function msgctl(2). You'll probably have to say
2589 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2590 then ARG must be a variable which will hold the returned C<msqid_ds>
2591 structure. Returns like C<ioctl>: the undefined value for error,
2592 C<"0 but true"> for zero, or the actual return value otherwise. See also
2593 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2595 =item msgget KEY,FLAGS
2597 Calls the System V IPC function msgget(2). Returns the message queue
2598 id, or the undefined value if there is an error. See also
2599 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2601 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2603 Calls the System V IPC function msgrcv to receive a message from
2604 message queue ID into variable VAR with a maximum message size of
2605 SIZE. Note that when a message is received, the message type as a
2606 native long integer will be the first thing in VAR, followed by the
2607 actual message. This packing may be opened with C<unpack("l! a*")>.
2608 Taints the variable. Returns true if successful, or false if there is
2609 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2610 C<IPC::SysV::Msg> documentation.
2612 =item msgsnd ID,MSG,FLAGS
2614 Calls the System V IPC function msgsnd to send the message MSG to the
2615 message queue ID. MSG must begin with the native long integer message
2616 type, and be followed by the length of the actual message, and finally
2617 the message itself. This kind of packing can be achieved with
2618 C<pack("l! a*", $type, $message)>. Returns true if successful,
2619 or false if there is an error. See also C<IPC::SysV>
2620 and C<IPC::SysV::Msg> documentation.
2624 =item my EXPR : ATTRIBUTES
2626 A C<my> declares the listed variables to be local (lexically) to the
2627 enclosing block, file, or C<eval>. If
2628 more than one value is listed, the list must be placed in parentheses. See
2629 L<perlsub/"Private Variables via my()"> for details.
2635 The C<next> command is like the C<continue> statement in C; it starts
2636 the next iteration of the loop:
2638 LINE: while (<STDIN>) {
2639 next LINE if /^#/; # discard comments
2643 Note that if there were a C<continue> block on the above, it would get
2644 executed even on discarded lines. If the LABEL is omitted, the command
2645 refers to the innermost enclosing loop.
2647 C<next> cannot be used to exit a block which returns a value such as
2648 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2649 a grep() or map() operation.
2651 Note that a block by itself is semantically identical to a loop
2652 that executes once. Thus C<next> will exit such a block early.
2654 See also L</continue> for an illustration of how C<last>, C<next>, and
2657 =item no Module VERSION LIST
2659 =item no Module VERSION
2661 =item no Module LIST
2665 See the L</use> function, which C<no> is the opposite of.
2671 Interprets EXPR as an octal string and returns the corresponding
2672 value. (If EXPR happens to start off with C<0x>, interprets it as a
2673 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2674 binary string. Leading whitespace is ignored in all three cases.)
2675 The following will handle decimal, binary, octal, and hex in the standard
2678 $val = oct($val) if $val =~ /^0/;
2680 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2681 in octal), use sprintf() or printf():
2683 $perms = (stat("filename"))[2] & 07777;
2684 $oct_perms = sprintf "%lo", $perms;
2686 The oct() function is commonly used when a string such as C<644> needs
2687 to be converted into a file mode, for example. (Although perl will
2688 automatically convert strings into numbers as needed, this automatic
2689 conversion assumes base 10.)
2691 =item open FILEHANDLE,EXPR
2693 =item open FILEHANDLE,MODE,EXPR
2695 =item open FILEHANDLE,MODE,EXPR,LIST
2697 =item open FILEHANDLE,MODE,REFERENCE
2699 =item open FILEHANDLE
2701 Opens the file whose filename is given by EXPR, and associates it with
2704 (The following is a comprehensive reference to open(): for a gentler
2705 introduction you may consider L<perlopentut>.)
2707 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2708 assigned a reference to a new anonymous filehandle, otherwise if
2709 FILEHANDLE is an expression, its value is used as the name of the real
2710 filehandle wanted. (This is considered a symbolic reference, so C<use
2711 strict 'refs'> should I<not> be in effect.)
2713 If EXPR is omitted, the scalar variable of the same name as the
2714 FILEHANDLE contains the filename. (Note that lexical variables--those
2715 declared with C<my>--will not work for this purpose; so if you're
2716 using C<my>, specify EXPR in your call to open.)
2718 If three or more arguments are specified then the mode of opening and
2719 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2720 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2721 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2722 the file is opened for appending, again being created if necessary.
2724 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2725 indicate that you want both read and write access to the file; thus
2726 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2727 '+>' >> mode would clobber the file first. You can't usually use
2728 either read-write mode for updating textfiles, since they have
2729 variable length records. See the B<-i> switch in L<perlrun> for a
2730 better approach. The file is created with permissions of C<0666>
2731 modified by the process' C<umask> value.
2733 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2734 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2736 In the 2-arguments (and 1-argument) form of the call the mode and
2737 filename should be concatenated (in this order), possibly separated by
2738 spaces. It is possible to omit the mode in these forms if the mode is
2741 If the filename begins with C<'|'>, the filename is interpreted as a
2742 command to which output is to be piped, and if the filename ends with a
2743 C<'|'>, the filename is interpreted as a command which pipes output to
2744 us. See L<perlipc/"Using open() for IPC">
2745 for more examples of this. (You are not allowed to C<open> to a command
2746 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2747 and L<perlipc/"Bidirectional Communication with Another Process">
2750 For three or more arguments if MODE is C<'|-'>, the filename is
2751 interpreted as a command to which output is to be piped, and if MODE
2752 is C<'-|'>, the filename is interpreted as a command which pipes
2753 output to us. In the 2-arguments (and 1-argument) form one should
2754 replace dash (C<'-'>) with the command.
2755 See L<perlipc/"Using open() for IPC"> for more examples of this.
2756 (You are not allowed to C<open> to a command that pipes both in I<and>
2757 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2758 L<perlipc/"Bidirectional Communication"> for alternatives.)
2760 In the three-or-more argument form of pipe opens, if LIST is specified
2761 (extra arguments after the command name) then LIST becomes arguments
2762 to the command invoked if the platform supports it. The meaning of
2763 C<open> with more than three arguments for non-pipe modes is not yet
2764 specified. Experimental "layers" may give extra LIST arguments
2767 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2768 and opening C<< '>-' >> opens STDOUT.
2770 You may use the three-argument form of open to specify
2771 I<I/O disciplines> that affect how the input and output
2772 are processed: see L</binmode> and L<open>. For example
2774 open(FH, "<:utf8", "file")
2776 will open the UTF-8 encoded file containing Unicode characters,
2777 see L<perluniintro>.
2779 Open returns nonzero upon success, the undefined value otherwise. If
2780 the C<open> involved a pipe, the return value happens to be the pid of
2783 If you're running Perl on a system that distinguishes between text
2784 files and binary files, then you should check out L</binmode> for tips
2785 for dealing with this. The key distinction between systems that need
2786 C<binmode> and those that don't is their text file formats. Systems
2787 like Unix, MacOS, and Plan9, which delimit lines with a single
2788 character, and which encode that character in C as C<"\n">, do not
2789 need C<binmode>. The rest need it.
2791 In the three argument form MODE may also contain a list of IO "layers"
2792 (see L<open> and L<PerlIO> for more details) to be applied to the
2793 handle. This can be used to achieve the effect of C<binmode> as well
2794 as more complex behaviours.
2796 When opening a file, it's usually a bad idea to continue normal execution
2797 if the request failed, so C<open> is frequently used in connection with
2798 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2799 where you want to make a nicely formatted error message (but there are
2800 modules that can help with that problem)) you should always check
2801 the return value from opening a file. The infrequent exception is when
2802 working with an unopened filehandle is actually what you want to do.
2804 As a special case the 3 arg form with a read/write mode and the third
2805 argument being C<undef>:
2807 open(TMP, "+>", undef) or die ...
2809 opens a filehandle to an anonymous temporary file.
2811 File handles can be opened to "in memory" files held in Perl scalars via:
2813 open($fh, '>', \$variable) || ..
2815 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2816 file, you have to close it first:
2819 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2824 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2825 while (<ARTICLE>) {...
2827 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2828 # if the open fails, output is discarded
2830 open(DBASE, '+<', 'dbase.mine') # open for update
2831 or die "Can't open 'dbase.mine' for update: $!";
2833 open(DBASE, '+<dbase.mine') # ditto
2834 or die "Can't open 'dbase.mine' for update: $!";
2836 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2837 or die "Can't start caesar: $!";
2839 open(ARTICLE, "caesar <$article |") # ditto
2840 or die "Can't start caesar: $!";
2842 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2843 or die "Can't start sort: $!";
2846 open(MEMORY,'>', \$var)
2847 or die "Can't open memory file: $!";
2848 print MEMORY "foo!\n"; # output will end up in $var
2850 # process argument list of files along with any includes
2852 foreach $file (@ARGV) {
2853 process($file, 'fh00');
2857 my($filename, $input) = @_;
2858 $input++; # this is a string increment
2859 unless (open($input, $filename)) {
2860 print STDERR "Can't open $filename: $!\n";
2865 while (<$input>) { # note use of indirection
2866 if (/^#include "(.*)"/) {
2867 process($1, $input);
2874 You may also, in the Bourne shell tradition, specify an EXPR beginning
2875 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2876 name of a filehandle (or file descriptor, if numeric) to be
2877 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2878 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2879 mode you specify should match the mode of the original filehandle.
2880 (Duping a filehandle does not take into account any existing contents of
2881 IO buffers.) If you use the 3 arg form then you can pass either a number,
2882 the name of a filehandle or the normal "reference to a glob".
2884 Here is a script that saves, redirects, and restores C<STDOUT> and
2885 C<STDERR> using various methods:
2888 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2889 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2891 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2892 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2894 select STDERR; $| = 1; # make unbuffered
2895 select STDOUT; $| = 1; # make unbuffered
2897 print STDOUT "stdout 1\n"; # this works for
2898 print STDERR "stderr 1\n"; # subprocesses too
2903 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2904 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2906 print STDOUT "stdout 2\n";
2907 print STDERR "stderr 2\n";
2909 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2910 do an equivalent of C's C<fdopen> of that file descriptor; this is
2911 more parsimonious of file descriptors. For example:
2913 open(FILEHANDLE, "<&=$fd")
2917 open(FILEHANDLE, "<&=", $fd)
2919 Note that if Perl is using the standard C libraries' fdopen() then on
2920 many UNIX systems, fdopen() is known to fail when file descriptors
2921 exceed a certain value, typically 255. If you need more file
2922 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2924 You can see whether Perl has been compiled with PerlIO or not by
2925 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2926 is C<define>, you have PerlIO, otherwise you don't.
2928 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2929 with 2-arguments (or 1-argument) form of open(), then
2930 there is an implicit fork done, and the return value of open is the pid
2931 of the child within the parent process, and C<0> within the child
2932 process. (Use C<defined($pid)> to determine whether the open was successful.)
2933 The filehandle behaves normally for the parent, but i/o to that
2934 filehandle is piped from/to the STDOUT/STDIN of the child process.
2935 In the child process the filehandle isn't opened--i/o happens from/to
2936 the new STDOUT or STDIN. Typically this is used like the normal
2937 piped open when you want to exercise more control over just how the
2938 pipe command gets executed, such as when you are running setuid, and
2939 don't want to have to scan shell commands for metacharacters.
2940 The following triples are more or less equivalent:
2942 open(FOO, "|tr '[a-z]' '[A-Z]'");
2943 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2944 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2945 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2947 open(FOO, "cat -n '$file'|");
2948 open(FOO, '-|', "cat -n '$file'");
2949 open(FOO, '-|') || exec 'cat', '-n', $file;
2950 open(FOO, '-|', "cat", '-n', $file);
2952 The last example in each block shows the pipe as "list form", which is
2953 not yet supported on all platforms.
2955 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2957 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2958 output before any operation that may do a fork, but this may not be
2959 supported on some platforms (see L<perlport>). To be safe, you may need
2960 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2961 of C<IO::Handle> on any open handles.
2963 On systems that support a close-on-exec flag on files, the flag will
2964 be set for the newly opened file descriptor as determined by the value
2965 of $^F. See L<perlvar/$^F>.
2967 Closing any piped filehandle causes the parent process to wait for the
2968 child to finish, and returns the status value in C<$?>.
2970 The filename passed to 2-argument (or 1-argument) form of open() will
2971 have leading and trailing whitespace deleted, and the normal
2972 redirection characters honored. This property, known as "magic open",
2973 can often be used to good effect. A user could specify a filename of
2974 F<"rsh cat file |">, or you could change certain filenames as needed:
2976 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2977 open(FH, $filename) or die "Can't open $filename: $!";
2979 Use 3-argument form to open a file with arbitrary weird characters in it,
2981 open(FOO, '<', $file);
2983 otherwise it's necessary to protect any leading and trailing whitespace:
2985 $file =~ s#^(\s)#./$1#;
2986 open(FOO, "< $file\0");
2988 (this may not work on some bizarre filesystems). One should
2989 conscientiously choose between the I<magic> and 3-arguments form
2994 will allow the user to specify an argument of the form C<"rsh cat file |">,
2995 but will not work on a filename which happens to have a trailing space, while
2997 open IN, '<', $ARGV[0];
2999 will have exactly the opposite restrictions.
3001 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3002 should use the C<sysopen> function, which involves no such magic (but
3003 may use subtly different filemodes than Perl open(), which is mapped
3004 to C fopen()). This is
3005 another way to protect your filenames from interpretation. For example:
3008 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3009 or die "sysopen $path: $!";
3010 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3011 print HANDLE "stuff $$\n";
3013 print "File contains: ", <HANDLE>;
3015 Using the constructor from the C<IO::Handle> package (or one of its
3016 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3017 filehandles that have the scope of whatever variables hold references to
3018 them, and automatically close whenever and however you leave that scope:
3022 sub read_myfile_munged {
3024 my $handle = new IO::File;
3025 open($handle, "myfile") or die "myfile: $!";
3027 or return (); # Automatically closed here.
3028 mung $first or die "mung failed"; # Or here.
3029 return $first, <$handle> if $ALL; # Or here.
3033 See L</seek> for some details about mixing reading and writing.
3035 =item opendir DIRHANDLE,EXPR
3037 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3038 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3039 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3045 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3046 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3049 For the reverse, see L</chr>.
3050 See L<perlunicode> and L<encoding> for more about Unicode.
3054 =item our EXPR : ATTRIBUTES
3056 An C<our> declares the listed variables to be valid globals within
3057 the enclosing block, file, or C<eval>. That is, it has the same
3058 scoping rules as a "my" declaration, but does not create a local
3059 variable. If more than one value is listed, the list must be placed
3060 in parentheses. The C<our> declaration has no semantic effect unless
3061 "use strict vars" is in effect, in which case it lets you use the
3062 declared global variable without qualifying it with a package name.
3063 (But only within the lexical scope of the C<our> declaration. In this
3064 it differs from "use vars", which is package scoped.)
3066 An C<our> declaration declares a global variable that will be visible
3067 across its entire lexical scope, even across package boundaries. The
3068 package in which the variable is entered is determined at the point
3069 of the declaration, not at the point of use. This means the following
3073 our $bar; # declares $Foo::bar for rest of lexical scope
3077 print $bar; # prints 20
3079 Multiple C<our> declarations in the same lexical scope are allowed
3080 if they are in different packages. If they happened to be in the same
3081 package, Perl will emit warnings if you have asked for them.
3085 our $bar; # declares $Foo::bar for rest of lexical scope
3089 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3090 print $bar; # prints 30
3092 our $bar; # emits warning
3094 An C<our> declaration may also have a list of attributes associated
3095 with it. B<WARNING>: This is an experimental feature that may be
3096 changed or removed in future releases of Perl. It should not be
3099 The only currently recognized attribute is C<unique> which indicates
3100 that a single copy of the global is to be used by all interpreters
3101 should the program happen to be running in a multi-interpreter
3102 environment. (The default behaviour would be for each interpreter to
3103 have its own copy of the global.) In such an environment, this
3104 attribute also has the effect of making the global readonly.
3107 our @EXPORT : unique = qw(foo);
3108 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3109 our $VERSION : unique = "1.00";
3111 Multi-interpreter environments can come to being either through the
3112 fork() emulation on Windows platforms, or by embedding perl in a
3113 multi-threaded application. The C<unique> attribute does nothing in
3114 all other environments.
3116 =item pack TEMPLATE,LIST
3118 Takes a LIST of values and converts it into a string using the rules
3119 given by the TEMPLATE. The resulting string is the concatenation of
3120 the converted values. Typically, each converted value looks
3121 like its machine-level representation. For example, on 32-bit machines
3122 a converted integer may be represented by a sequence of 4 bytes.
3124 The TEMPLATE is a sequence of characters that give the order and type
3125 of values, as follows:
3127 a A string with arbitrary binary data, will be null padded.
3128 A A text (ASCII) string, will be space padded.
3129 Z A null terminated (ASCIZ) string, will be null padded.
3131 b A bit string (ascending bit order inside each byte, like vec()).
3132 B A bit string (descending bit order inside each byte).
3133 h A hex string (low nybble first).
3134 H A hex string (high nybble first).
3136 c A signed char value.
3137 C An unsigned char value. Only does bytes. See U for Unicode.
3139 s A signed short value.
3140 S An unsigned short value.
3141 (This 'short' is _exactly_ 16 bits, which may differ from
3142 what a local C compiler calls 'short'. If you want
3143 native-length shorts, use the '!' suffix.)
3145 i A signed integer value.
3146 I An unsigned integer value.
3147 (This 'integer' is _at_least_ 32 bits wide. Its exact
3148 size depends on what a local C compiler calls 'int',
3149 and may even be larger than the 'long' described in
3152 l A signed long value.
3153 L An unsigned long value.
3154 (This 'long' is _exactly_ 32 bits, which may differ from
3155 what a local C compiler calls 'long'. If you want
3156 native-length longs, use the '!' suffix.)
3158 n An unsigned short in "network" (big-endian) order.
3159 N An unsigned long in "network" (big-endian) order.
3160 v An unsigned short in "VAX" (little-endian) order.
3161 V An unsigned long in "VAX" (little-endian) order.
3162 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3163 _exactly_ 32 bits, respectively.)
3165 q A signed quad (64-bit) value.
3166 Q An unsigned quad value.
3167 (Quads are available only if your system supports 64-bit
3168 integer values _and_ if Perl has been compiled to support those.
3169 Causes a fatal error otherwise.)
3171 j A signed integer value (a Perl internal integer, IV).
3172 J An unsigned integer value (a Perl internal unsigned integer, UV).
3174 f A single-precision float in the native format.
3175 d A double-precision float in the native format.
3177 F A floating point value in the native native format
3178 (a Perl internal floating point value, NV).
3179 D A long double-precision float in the native format.
3180 (Long doubles are available only if your system supports long
3181 double values _and_ if Perl has been compiled to support those.
3182 Causes a fatal error otherwise.)
3184 p A pointer to a null-terminated string.
3185 P A pointer to a structure (fixed-length string).
3187 u A uuencoded string.
3188 U A Unicode character number. Encodes to UTF-8 internally
3189 (or UTF-EBCDIC in EBCDIC platforms).
3191 w A BER compressed integer. Its bytes represent an unsigned
3192 integer in base 128, most significant digit first, with as
3193 few digits as possible. Bit eight (the high bit) is set
3194 on each byte except the last.
3198 @ Null fill to absolute position.
3199 ( Start of a ()-group.
3201 The following rules apply:
3207 Each letter may optionally be followed by a number giving a repeat
3208 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3209 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3210 many values from the LIST. A C<*> for the repeat count means to use
3211 however many items are left, except for C<@>, C<x>, C<X>, where it is
3212 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3213 is the same). A numeric repeat count may optionally be enclosed in
3214 brackets, as in C<pack 'C[80]', @arr>.
3216 One can replace the numeric repeat count by a template enclosed in brackets;
3217 then the packed length of this template in bytes is used as a count.
3218 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3219 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3220 If the template in brackets contains alignment commands (such as C<x![d]>),
3221 its packed length is calculated as if the start of the template has the maximal
3224 When used with C<Z>, C<*> results in the addition of a trailing null
3225 byte (so the packed result will be one longer than the byte C<length>
3228 The repeat count for C<u> is interpreted as the maximal number of bytes
3229 to encode per line of output, with 0 and 1 replaced by 45.
3233 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3234 string of length count, padding with nulls or spaces as necessary. When
3235 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3236 after the first null, and C<a> returns data verbatim. When packing,
3237 C<a>, and C<Z> are equivalent.
3239 If the value-to-pack is too long, it is truncated. If too long and an
3240 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3241 by a null byte. Thus C<Z> always packs a trailing null byte under
3246 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3247 Each byte of the input field of pack() generates 1 bit of the result.
3248 Each result bit is based on the least-significant bit of the corresponding
3249 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3250 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3252 Starting from the beginning of the input string of pack(), each 8-tuple
3253 of bytes is converted to 1 byte of output. With format C<b>
3254 the first byte of the 8-tuple determines the least-significant bit of a
3255 byte, and with format C<B> it determines the most-significant bit of
3258 If the length of the input string is not exactly divisible by 8, the
3259 remainder is packed as if the input string were padded by null bytes
3260 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3262 If the input string of pack() is longer than needed, extra bytes are ignored.
3263 A C<*> for the repeat count of pack() means to use all the bytes of
3264 the input field. On unpack()ing the bits are converted to a string
3265 of C<"0">s and C<"1">s.
3269 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3270 representable as hexadecimal digits, 0-9a-f) long.
3272 Each byte of the input field of pack() generates 4 bits of the result.
3273 For non-alphabetical bytes the result is based on the 4 least-significant
3274 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3275 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3276 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3277 is compatible with the usual hexadecimal digits, so that C<"a"> and
3278 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3279 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3281 Starting from the beginning of the input string of pack(), each pair
3282 of bytes is converted to 1 byte of output. With format C<h> the
3283 first byte of the pair determines the least-significant nybble of the
3284 output byte, and with format C<H> it determines the most-significant
3287 If the length of the input string is not even, it behaves as if padded
3288 by a null byte at the end. Similarly, during unpack()ing the "extra"
3289 nybbles are ignored.
3291 If the input string of pack() is longer than needed, extra bytes are ignored.
3292 A C<*> for the repeat count of pack() means to use all the bytes of
3293 the input field. On unpack()ing the bits are converted to a string
3294 of hexadecimal digits.
3298 The C<p> type packs a pointer to a null-terminated string. You are
3299 responsible for ensuring the string is not a temporary value (which can
3300 potentially get deallocated before you get around to using the packed result).
3301 The C<P> type packs a pointer to a structure of the size indicated by the
3302 length. A NULL pointer is created if the corresponding value for C<p> or
3303 C<P> is C<undef>, similarly for unpack().
3307 The C</> template character allows packing and unpacking of strings where
3308 the packed structure contains a byte count followed by the string itself.
3309 You write I<length-item>C</>I<string-item>.
3311 The I<length-item> can be any C<pack> template letter, and describes
3312 how the length value is packed. The ones likely to be of most use are
3313 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3314 SNMP) and C<N> (for Sun XDR).
3316 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3317 For C<unpack> the length of the string is obtained from the I<length-item>,
3318 but if you put in the '*' it will be ignored.
3320 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3321 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3322 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3324 The I<length-item> is not returned explicitly from C<unpack>.
3326 Adding a count to the I<length-item> letter is unlikely to do anything
3327 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3328 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3329 which Perl does not regard as legal in numeric strings.
3333 The integer types C<s>, C<S>, C<l>, and C<L> may be
3334 immediately followed by a C<!> suffix to signify native shorts or
3335 longs--as you can see from above for example a bare C<l> does mean
3336 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3337 may be larger. This is an issue mainly in 64-bit platforms. You can
3338 see whether using C<!> makes any difference by
3340 print length(pack("s")), " ", length(pack("s!")), "\n";
3341 print length(pack("l")), " ", length(pack("l!")), "\n";
3343 C<i!> and C<I!> also work but only because of completeness;
3344 they are identical to C<i> and C<I>.
3346 The actual sizes (in bytes) of native shorts, ints, longs, and long
3347 longs on the platform where Perl was built are also available via
3351 print $Config{shortsize}, "\n";
3352 print $Config{intsize}, "\n";
3353 print $Config{longsize}, "\n";
3354 print $Config{longlongsize}, "\n";
3356 (The C<$Config{longlongsize}> will be undefine if your system does
3357 not support long longs.)
3361 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3362 are inherently non-portable between processors and operating systems
3363 because they obey the native byteorder and endianness. For example a
3364 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3365 (arranged in and handled by the CPU registers) into bytes as
3367 0x12 0x34 0x56 0x78 # big-endian
3368 0x78 0x56 0x34 0x12 # little-endian
3370 Basically, the Intel and VAX CPUs are little-endian, while everybody
3371 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3372 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3373 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3376 The names `big-endian' and `little-endian' are comic references to
3377 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3378 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3379 the egg-eating habits of the Lilliputians.
3381 Some systems may have even weirder byte orders such as
3386 You can see your system's preference with
3388 print join(" ", map { sprintf "%#02x", $_ }
3389 unpack("C*",pack("L",0x12345678))), "\n";
3391 The byteorder on the platform where Perl was built is also available
3395 print $Config{byteorder}, "\n";
3397 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3398 and C<'87654321'> are big-endian.
3400 If you want portable packed integers use the formats C<n>, C<N>,
3401 C<v>, and C<V>, their byte endianness and size are known.
3402 See also L<perlport>.
3406 Real numbers (floats and doubles) are in the native machine format only;
3407 due to the multiplicity of floating formats around, and the lack of a
3408 standard "network" representation, no facility for interchange has been
3409 made. This means that packed floating point data written on one machine
3410 may not be readable on another - even if both use IEEE floating point
3411 arithmetic (as the endian-ness of the memory representation is not part
3412 of the IEEE spec). See also L<perlport>.
3414 Note that Perl uses doubles internally for all numeric calculation, and
3415 converting from double into float and thence back to double again will
3416 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3421 If the pattern begins with a C<U>, the resulting string will be treated
3422 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3423 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3424 characters. If you don't want this to happen, you can begin your pattern
3425 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3426 string, and then follow this with a C<U*> somewhere in your pattern.
3430 You must yourself do any alignment or padding by inserting for example
3431 enough C<'x'>es while packing. There is no way to pack() and unpack()
3432 could know where the bytes are going to or coming from. Therefore
3433 C<pack> (and C<unpack>) handle their output and input as flat
3438 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3439 take a repeat count, both as postfix, and via the C</> template
3444 C<x> and C<X> accept C<!> modifier. In this case they act as
3445 alignment commands: they jump forward/back to the closest position
3446 aligned at a multiple of C<count> bytes. For example, to pack() or
3447 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3448 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3449 aligned on the double's size.
3451 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3452 both result in no-ops.
3456 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3460 If TEMPLATE requires more arguments to pack() than actually given, pack()
3461 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3462 to pack() than actually given, extra arguments are ignored.
3468 $foo = pack("CCCC",65,66,67,68);
3470 $foo = pack("C4",65,66,67,68);
3472 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3473 # same thing with Unicode circled letters
3475 $foo = pack("ccxxcc",65,66,67,68);
3478 # note: the above examples featuring "C" and "c" are true
3479 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3480 # and UTF-8. In EBCDIC the first example would be
3481 # $foo = pack("CCCC",193,194,195,196);
3483 $foo = pack("s2",1,2);
3484 # "\1\0\2\0" on little-endian
3485 # "\0\1\0\2" on big-endian
3487 $foo = pack("a4","abcd","x","y","z");
3490 $foo = pack("aaaa","abcd","x","y","z");
3493 $foo = pack("a14","abcdefg");
3494 # "abcdefg\0\0\0\0\0\0\0"
3496 $foo = pack("i9pl", gmtime);
3497 # a real struct tm (on my system anyway)
3499 $utmp_template = "Z8 Z8 Z16 L";
3500 $utmp = pack($utmp_template, @utmp1);
3501 # a struct utmp (BSDish)
3503 @utmp2 = unpack($utmp_template, $utmp);
3504 # "@utmp1" eq "@utmp2"
3507 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3510 $foo = pack('sx2l', 12, 34);
3511 # short 12, two zero bytes padding, long 34
3512 $bar = pack('s@4l', 12, 34);
3513 # short 12, zero fill to position 4, long 34
3516 The same template may generally also be used in unpack().
3518 =item package NAMESPACE
3522 Declares the compilation unit as being in the given namespace. The scope
3523 of the package declaration is from the declaration itself through the end
3524 of the enclosing block, file, or eval (the same as the C<my> operator).
3525 All further unqualified dynamic identifiers will be in this namespace.
3526 A package statement affects only dynamic variables--including those
3527 you've used C<local> on--but I<not> lexical variables, which are created
3528 with C<my>. Typically it would be the first declaration in a file to
3529 be included by the C<require> or C<use> operator. You can switch into a
3530 package in more than one place; it merely influences which symbol table
3531 is used by the compiler for the rest of that block. You can refer to
3532 variables and filehandles in other packages by prefixing the identifier
3533 with the package name and a double colon: C<$Package::Variable>.
3534 If the package name is null, the C<main> package as assumed. That is,
3535 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3536 still seen in older code).
3538 If NAMESPACE is omitted, then there is no current package, and all
3539 identifiers must be fully qualified or lexicals. However, you are
3540 strongly advised not to make use of this feature. Its use can cause
3541 unexpected behaviour, even crashing some versions of Perl. It is
3542 deprecated, and will be removed from a future release.
3544 See L<perlmod/"Packages"> for more information about packages, modules,
3545 and classes. See L<perlsub> for other scoping issues.
3547 =item pipe READHANDLE,WRITEHANDLE
3549 Opens a pair of connected pipes like the corresponding system call.
3550 Note that if you set up a loop of piped processes, deadlock can occur
3551 unless you are very careful. In addition, note that Perl's pipes use
3552 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3553 after each command, depending on the application.
3555 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3556 for examples of such things.
3558 On systems that support a close-on-exec flag on files, the flag will be set
3559 for the newly opened file descriptors as determined by the value of $^F.
3566 Pops and returns the last value of the array, shortening the array by
3567 one element. Has an effect similar to
3571 If there are no elements in the array, returns the undefined value
3572 (although this may happen at other times as well). If ARRAY is
3573 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3574 array in subroutines, just like C<shift>.
3580 Returns the offset of where the last C<m//g> search left off for the variable
3581 in question (C<$_> is used when the variable is not specified). May be
3582 modified to change that offset. Such modification will also influence
3583 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3586 =item print FILEHANDLE LIST
3592 Prints a string or a list of strings. Returns true if successful.
3593 FILEHANDLE may be a scalar variable name, in which case the variable
3594 contains the name of or a reference to the filehandle, thus introducing
3595 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3596 the next token is a term, it may be misinterpreted as an operator
3597 unless you interpose a C<+> or put parentheses around the arguments.)
3598 If FILEHANDLE is omitted, prints by default to standard output (or
3599 to the last selected output channel--see L</select>). If LIST is
3600 also omitted, prints C<$_> to the currently selected output channel.
3601 To set the default output channel to something other than STDOUT
3602 use the select operation. The current value of C<$,> (if any) is
3603 printed between each LIST item. The current value of C<$\> (if
3604 any) is printed after the entire LIST has been printed. Because
3605 print takes a LIST, anything in the LIST is evaluated in list
3606 context, and any subroutine that you call will have one or more of
3607 its expressions evaluated in list context. Also be careful not to
3608 follow the print keyword with a left parenthesis unless you want
3609 the corresponding right parenthesis to terminate the arguments to
3610 the print--interpose a C<+> or put parentheses around all the
3613 Note that if you're storing FILEHANDLES in an array or other expression,
3614 you will have to use a block returning its value instead:
3616 print { $files[$i] } "stuff\n";
3617 print { $OK ? STDOUT : STDERR } "stuff\n";
3619 =item printf FILEHANDLE FORMAT, LIST
3621 =item printf FORMAT, LIST
3623 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3624 (the output record separator) is not appended. The first argument
3625 of the list will be interpreted as the C<printf> format. See C<sprintf>
3626 for an explanation of the format argument. If C<use locale> is in effect,
3627 the character used for the decimal point in formatted real numbers is
3628 affected by the LC_NUMERIC locale. See L<perllocale>.
3630 Don't fall into the trap of using a C<printf> when a simple
3631 C<print> would do. The C<print> is more efficient and less
3634 =item prototype FUNCTION
3636 Returns the prototype of a function as a string (or C<undef> if the
3637 function has no prototype). FUNCTION is a reference to, or the name of,
3638 the function whose prototype you want to retrieve.
3640 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3641 name for Perl builtin. If the builtin is not I<overridable> (such as
3642 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3643 C<system>) returns C<undef> because the builtin does not really behave
3644 like a Perl function. Otherwise, the string describing the equivalent
3645 prototype is returned.
3647 =item push ARRAY,LIST
3649 Treats ARRAY as a stack, and pushes the values of LIST
3650 onto the end of ARRAY. The length of ARRAY increases by the length of
3651 LIST. Has the same effect as
3654 $ARRAY[++$#ARRAY] = $value;
3657 but is more efficient. Returns the new number of elements in the array.
3669 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3671 =item quotemeta EXPR
3675 Returns the value of EXPR with all non-"word"
3676 characters backslashed. (That is, all characters not matching
3677 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3678 returned string, regardless of any locale settings.)
3679 This is the internal function implementing
3680 the C<\Q> escape in double-quoted strings.
3682 If EXPR is omitted, uses C<$_>.
3688 Returns a random fractional number greater than or equal to C<0> and less
3689 than the value of EXPR. (EXPR should be positive.) If EXPR is
3690 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3691 unless C<srand> has already been called. See also C<srand>.
3693 Apply C<int()> to the value returned by C<rand()> if you want random
3694 integers instead of random fractional numbers. For example,
3698 returns a random integer between C<0> and C<9>, inclusive.
3700 (Note: If your rand function consistently returns numbers that are too
3701 large or too small, then your version of Perl was probably compiled
3702 with the wrong number of RANDBITS.)
3704 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3706 =item read FILEHANDLE,SCALAR,LENGTH
3708 Attempts to read LENGTH I<characters> of data into variable SCALAR
3709 from the specified FILEHANDLE. Returns the number of characters
3710 actually read, C<0> at end of file, or undef if there was an error.
3711 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3712 needs growing, the new bytes will be zero bytes. An OFFSET may be
3713 specified to place the read data into some other place in SCALAR than
3714 the beginning. The call is actually implemented in terms of either
3715 Perl's or system's fread() call. To get a true read(2) system call,
3718 Note the I<characters>: depending on the status of the filehandle,
3719 either (8-bit) bytes or characters are read. By default all
3720 filehandles operate on bytes, but for example if the filehandle has
3721 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3722 pragma, L<open>), the I/O will operate on characters, not bytes.
3724 =item readdir DIRHANDLE
3726 Returns the next directory entry for a directory opened by C<opendir>.
3727 If used in list context, returns all the rest of the entries in the
3728 directory. If there are no more entries, returns an undefined value in
3729 scalar context or a null list in list context.
3731 If you're planning to filetest the return values out of a C<readdir>, you'd
3732 better prepend the directory in question. Otherwise, because we didn't
3733 C<chdir> there, it would have been testing the wrong file.
3735 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3736 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3741 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3742 context, each call reads and returns the next line, until end-of-file is
3743 reached, whereupon the subsequent call returns undef. In list context,
3744 reads until end-of-file is reached and returns a list of lines. Note that
3745 the notion of "line" used here is however you may have defined it
3746 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3748 When C<$/> is set to C<undef>, when readline() is in scalar
3749 context (i.e. file slurp mode), and when an empty file is read, it
3750 returns C<''> the first time, followed by C<undef> subsequently.
3752 This is the internal function implementing the C<< <EXPR> >>
3753 operator, but you can use it directly. The C<< <EXPR> >>
3754 operator is discussed in more detail in L<perlop/"I/O Operators">.
3757 $line = readline(*STDIN); # same thing
3763 Returns the value of a symbolic link, if symbolic links are
3764 implemented. If not, gives a fatal error. If there is some system
3765 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3766 omitted, uses C<$_>.
3770 EXPR is executed as a system command.
3771 The collected standard output of the command is returned.
3772 In scalar context, it comes back as a single (potentially
3773 multi-line) string. In list context, returns a list of lines
3774 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3775 This is the internal function implementing the C<qx/EXPR/>
3776 operator, but you can use it directly. The C<qx/EXPR/>
3777 operator is discussed in more detail in L<perlop/"I/O Operators">.
3779 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3781 Receives a message on a socket. Attempts to receive LENGTH characters
3782 of data into variable SCALAR from the specified SOCKET filehandle.
3783 SCALAR will be grown or shrunk to the length actually read. Takes the
3784 same flags as the system call of the same name. Returns the address
3785 of the sender if SOCKET's protocol supports this; returns an empty
3786 string otherwise. If there's an error, returns the undefined value.
3787 This call is actually implemented in terms of recvfrom(2) system call.
3788 See L<perlipc/"UDP: Message Passing"> for examples.
3790 Note the I<characters>: depending on the status of the socket, either
3791 (8-bit) bytes or characters are received. By default all sockets
3792 operate on bytes, but for example if the socket has been changed using
3793 binmode() to operate with the C<:utf8> discipline (see the C<open>
3794 pragma, L<open>), the I/O will operate on characters, not bytes.
3800 The C<redo> command restarts the loop block without evaluating the
3801 conditional again. The C<continue> block, if any, is not executed. If
3802 the LABEL is omitted, the command refers to the innermost enclosing
3803 loop. This command is normally used by programs that want to lie to
3804 themselves about what was just input:
3806 # a simpleminded Pascal comment stripper
3807 # (warning: assumes no { or } in strings)
3808 LINE: while (<STDIN>) {
3809 while (s|({.*}.*){.*}|$1 |) {}
3814 if (/}/) { # end of comment?
3823 C<redo> cannot be used to retry a block which returns a value such as
3824 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3825 a grep() or map() operation.
3827 Note that a block by itself is semantically identical to a loop
3828 that executes once. Thus C<redo> inside such a block will effectively
3829 turn it into a looping construct.
3831 See also L</continue> for an illustration of how C<last>, C<next>, and
3838 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3839 is not specified, C<$_> will be used. The value returned depends on the
3840 type of thing the reference is a reference to.
3841 Builtin types include:
3851 If the referenced object has been blessed into a package, then that package
3852 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3854 if (ref($r) eq "HASH") {
3855 print "r is a reference to a hash.\n";
3858 print "r is not a reference at all.\n";
3860 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3861 print "r is a reference to something that isa hash.\n";
3864 See also L<perlref>.
3866 =item rename OLDNAME,NEWNAME
3868 Changes the name of a file; an existing file NEWNAME will be
3869 clobbered. Returns true for success, false otherwise.
3871 Behavior of this function varies wildly depending on your system
3872 implementation. For example, it will usually not work across file system
3873 boundaries, even though the system I<mv> command sometimes compensates
3874 for this. Other restrictions include whether it works on directories,
3875 open files, or pre-existing files. Check L<perlport> and either the
3876 rename(2) manpage or equivalent system documentation for details.
3878 =item require VERSION
3884 Demands a version of Perl specified by VERSION, or demands some semantics
3885 specified by EXPR or by C<$_> if EXPR is not supplied.
3887 VERSION may be either a numeric argument such as 5.006, which will be
3888 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3889 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3890 VERSION is greater than the version of the current Perl interpreter.
3891 Compare with L</use>, which can do a similar check at compile time.
3893 Specifying VERSION as a literal of the form v5.6.1 should generally be
3894 avoided, because it leads to misleading error messages under earlier
3895 versions of Perl which do not support this syntax. The equivalent numeric
3896 version should be used instead.
3898 require v5.6.1; # run time version check
3899 require 5.6.1; # ditto
3900 require 5.006_001; # ditto; preferred for backwards compatibility
3902 Otherwise, demands that a library file be included if it hasn't already
3903 been included. The file is included via the do-FILE mechanism, which is
3904 essentially just a variety of C<eval>. Has semantics similar to the following
3909 return 1 if $INC{$filename};
3910 my($realfilename,$result);
3912 foreach $prefix (@INC) {
3913 $realfilename = "$prefix/$filename";
3914 if (-f $realfilename) {
3915 $INC{$filename} = $realfilename;
3916 $result = do $realfilename;
3920 die "Can't find $filename in \@INC";
3922 delete $INC{$filename} if $@ || !$result;
3924 die "$filename did not return true value" unless $result;
3928 Note that the file will not be included twice under the same specified
3929 name. The file must return true as the last statement to indicate
3930 successful execution of any initialization code, so it's customary to
3931 end such a file with C<1;> unless you're sure it'll return true
3932 otherwise. But it's better just to put the C<1;>, in case you add more
3935 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3936 replaces "F<::>" with "F</>" in the filename for you,
3937 to make it easy to load standard modules. This form of loading of
3938 modules does not risk altering your namespace.
3940 In other words, if you try this:
3942 require Foo::Bar; # a splendid bareword
3944 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3945 directories specified in the C<@INC> array.
3947 But if you try this:
3949 $class = 'Foo::Bar';
3950 require $class; # $class is not a bareword
3952 require "Foo::Bar"; # not a bareword because of the ""
3954 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3955 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3957 eval "require $class";
3959 You can also insert hooks into the import facility, by putting directly
3960 Perl code into the @INC array. There are three forms of hooks: subroutine
3961 references, array references and blessed objects.
3963 Subroutine references are the simplest case. When the inclusion system
3964 walks through @INC and encounters a subroutine, this subroutine gets
3965 called with two parameters, the first being a reference to itself, and the
3966 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3967 subroutine should return C<undef> or a filehandle, from which the file to
3968 include will be read. If C<undef> is returned, C<require> will look at
3969 the remaining elements of @INC.
3971 If the hook is an array reference, its first element must be a subroutine
3972 reference. This subroutine is called as above, but the first parameter is
3973 the array reference. This enables to pass indirectly some arguments to
3976 In other words, you can write:
3978 push @INC, \&my_sub;
3980 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3986 push @INC, [ \&my_sub, $x, $y, ... ];
3988 my ($arrayref, $filename) = @_;
3989 # Retrieve $x, $y, ...
3990 my @parameters = @$arrayref[1..$#$arrayref];
3994 If the hook is an object, it must provide an INC method, that will be
3995 called as above, the first parameter being the object itself. (Note that
3996 you must fully qualify the sub's name, as it is always forced into package
3997 C<main>.) Here is a typical code layout:
4003 my ($self, $filename) = @_;
4007 # In the main program
4008 push @INC, new Foo(...);
4010 Note that these hooks are also permitted to set the %INC entry
4011 corresponding to the files they have loaded. See L<perlvar/%INC>.
4013 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4019 Generally used in a C<continue> block at the end of a loop to clear
4020 variables and reset C<??> searches so that they work again. The
4021 expression is interpreted as a list of single characters (hyphens
4022 allowed for ranges). All variables and arrays beginning with one of
4023 those letters are reset to their pristine state. If the expression is
4024 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4025 only variables or searches in the current package. Always returns
4028 reset 'X'; # reset all X variables
4029 reset 'a-z'; # reset lower case variables
4030 reset; # just reset ?one-time? searches
4032 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4033 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4034 variables--lexical variables are unaffected, but they clean themselves
4035 up on scope exit anyway, so you'll probably want to use them instead.
4042 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4043 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4044 context, depending on how the return value will be used, and the context
4045 may vary from one execution to the next (see C<wantarray>). If no EXPR
4046 is given, returns an empty list in list context, the undefined value in
4047 scalar context, and (of course) nothing at all in a void context.
4049 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4050 or do FILE will automatically return the value of the last expression
4055 In list context, returns a list value consisting of the elements
4056 of LIST in the opposite order. In scalar context, concatenates the
4057 elements of LIST and returns a string value with all characters
4058 in the opposite order.
4060 print reverse <>; # line tac, last line first
4062 undef $/; # for efficiency of <>
4063 print scalar reverse <>; # character tac, last line tsrif
4065 This operator is also handy for inverting a hash, although there are some
4066 caveats. If a value is duplicated in the original hash, only one of those
4067 can be represented as a key in the inverted hash. Also, this has to
4068 unwind one hash and build a whole new one, which may take some time
4069 on a large hash, such as from a DBM file.
4071 %by_name = reverse %by_address; # Invert the hash
4073 =item rewinddir DIRHANDLE
4075 Sets the current position to the beginning of the directory for the
4076 C<readdir> routine on DIRHANDLE.
4078 =item rindex STR,SUBSTR,POSITION
4080 =item rindex STR,SUBSTR
4082 Works just like index() except that it returns the position of the LAST
4083 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4084 last occurrence at or before that position.
4086 =item rmdir FILENAME
4090 Deletes the directory specified by FILENAME if that directory is empty. If it
4091 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4092 FILENAME is omitted, uses C<$_>.
4096 The substitution operator. See L<perlop>.
4100 Forces EXPR to be interpreted in scalar context and returns the value
4103 @counts = ( scalar @a, scalar @b, scalar @c );
4105 There is no equivalent operator to force an expression to
4106 be interpolated in list context because in practice, this is never
4107 needed. If you really wanted to do so, however, you could use
4108 the construction C<@{[ (some expression) ]}>, but usually a simple
4109 C<(some expression)> suffices.
4111 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4112 parenthesized list, this behaves as a scalar comma expression, evaluating
4113 all but the last element in void context and returning the final element
4114 evaluated in scalar context. This is seldom what you want.
4116 The following single statement:
4118 print uc(scalar(&foo,$bar)),$baz;
4120 is the moral equivalent of these two:
4123 print(uc($bar),$baz);
4125 See L<perlop> for more details on unary operators and the comma operator.
4127 =item seek FILEHANDLE,POSITION,WHENCE
4129 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4130 FILEHANDLE may be an expression whose value gives the name of the
4131 filehandle. The values for WHENCE are C<0> to set the new position
4132 I<in bytes> to POSITION, C<1> to set it to the current position plus
4133 POSITION, and C<2> to set it to EOF plus POSITION (typically
4134 negative). For WHENCE you may use the constants C<SEEK_SET>,
4135 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4136 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4139 Note the I<in bytes>: even if the filehandle has been set to
4140 operate on characters (for example by using the C<:utf8> open
4141 discipline), tell() will return byte offsets, not character offsets
4142 (because implementing that would render seek() and tell() rather slow).
4144 If you want to position file for C<sysread> or C<syswrite>, don't use
4145 C<seek>--buffering makes its effect on the file's system position
4146 unpredictable and non-portable. Use C<sysseek> instead.
4148 Due to the rules and rigors of ANSI C, on some systems you have to do a
4149 seek whenever you switch between reading and writing. Amongst other
4150 things, this may have the effect of calling stdio's clearerr(3).
4151 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4155 This is also useful for applications emulating C<tail -f>. Once you hit
4156 EOF on your read, and then sleep for a while, you might have to stick in a
4157 seek() to reset things. The C<seek> doesn't change the current position,
4158 but it I<does> clear the end-of-file condition on the handle, so that the
4159 next C<< <FILE> >> makes Perl try again to read something. We hope.
4161 If that doesn't work (some IO implementations are particularly
4162 cantankerous), then you may need something more like this:
4165 for ($curpos = tell(FILE); $_ = <FILE>;
4166 $curpos = tell(FILE)) {
4167 # search for some stuff and put it into files
4169 sleep($for_a_while);
4170 seek(FILE, $curpos, 0);
4173 =item seekdir DIRHANDLE,POS
4175 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4176 must be a value returned by C<telldir>. Has the same caveats about
4177 possible directory compaction as the corresponding system library
4180 =item select FILEHANDLE
4184 Returns the currently selected filehandle. Sets the current default
4185 filehandle for output, if FILEHANDLE is supplied. This has two
4186 effects: first, a C<write> or a C<print> without a filehandle will
4187 default to this FILEHANDLE. Second, references to variables related to
4188 output will refer to this output channel. For example, if you have to
4189 set the top of form format for more than one output channel, you might
4197 FILEHANDLE may be an expression whose value gives the name of the
4198 actual filehandle. Thus:
4200 $oldfh = select(STDERR); $| = 1; select($oldfh);
4202 Some programmers may prefer to think of filehandles as objects with
4203 methods, preferring to write the last example as:
4206 STDERR->autoflush(1);
4208 =item select RBITS,WBITS,EBITS,TIMEOUT
4210 This calls the select(2) system call with the bit masks specified, which
4211 can be constructed using C<fileno> and C<vec>, along these lines:
4213 $rin = $win = $ein = '';
4214 vec($rin,fileno(STDIN),1) = 1;
4215 vec($win,fileno(STDOUT),1) = 1;
4218 If you want to select on many filehandles you might wish to write a
4222 my(@fhlist) = split(' ',$_[0]);
4225 vec($bits,fileno($_),1) = 1;
4229 $rin = fhbits('STDIN TTY SOCK');
4233 ($nfound,$timeleft) =
4234 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4236 or to block until something becomes ready just do this
4238 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4240 Most systems do not bother to return anything useful in $timeleft, so
4241 calling select() in scalar context just returns $nfound.
4243 Any of the bit masks can also be undef. The timeout, if specified, is
4244 in seconds, which may be fractional. Note: not all implementations are
4245 capable of returning the $timeleft. If not, they always return
4246 $timeleft equal to the supplied $timeout.
4248 You can effect a sleep of 250 milliseconds this way:
4250 select(undef, undef, undef, 0.25);
4252 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4253 or <FH>) with C<select>, except as permitted by POSIX, and even
4254 then only on POSIX systems. You have to use C<sysread> instead.
4256 =item semctl ID,SEMNUM,CMD,ARG
4258 Calls the System V IPC function C<semctl>. You'll probably have to say
4262 first to get the correct constant definitions. If CMD is IPC_STAT or
4263 GETALL, then ARG must be a variable which will hold the returned
4264 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4265 the undefined value for error, "C<0 but true>" for zero, or the actual
4266 return value otherwise. The ARG must consist of a vector of native
4267 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4268 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4271 =item semget KEY,NSEMS,FLAGS
4273 Calls the System V IPC function semget. Returns the semaphore id, or
4274 the undefined value if there is an error. See also
4275 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4278 =item semop KEY,OPSTRING
4280 Calls the System V IPC function semop to perform semaphore operations
4281 such as signalling and waiting. OPSTRING must be a packed array of
4282 semop structures. Each semop structure can be generated with
4283 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4284 operations is implied by the length of OPSTRING. Returns true if
4285 successful, or false if there is an error. As an example, the
4286 following code waits on semaphore $semnum of semaphore id $semid:
4288 $semop = pack("s!3", $semnum, -1, 0);
4289 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4291 To signal the semaphore, replace C<-1> with C<1>. See also
4292 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4295 =item send SOCKET,MSG,FLAGS,TO
4297 =item send SOCKET,MSG,FLAGS
4299 Sends a message on a socket. Attempts to send the scalar MSG to the
4300 SOCKET filehandle. Takes the same flags as the system call of the
4301 same name. On unconnected sockets you must specify a destination to
4302 send TO, in which case it does a C C<sendto>. Returns the number of
4303 characters sent, or the undefined value if there is an error. The C
4304 system call sendmsg(2) is currently unimplemented. See
4305 L<perlipc/"UDP: Message Passing"> for examples.
4307 Note the I<characters>: depending on the status of the socket, either
4308 (8-bit) bytes or characters are sent. By default all sockets operate
4309 on bytes, but for example if the socket has been changed using
4310 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4311 the C<open> pragma, L<open>), the I/O will operate on characters, not
4314 =item setpgrp PID,PGRP
4316 Sets the current process group for the specified PID, C<0> for the current
4317 process. Will produce a fatal error if used on a machine that doesn't
4318 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4319 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4320 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4323 =item setpriority WHICH,WHO,PRIORITY
4325 Sets the current priority for a process, a process group, or a user.
4326 (See setpriority(2).) Will produce a fatal error if used on a machine
4327 that doesn't implement setpriority(2).
4329 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4331 Sets the socket option requested. Returns undefined if there is an
4332 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4339 Shifts the first value of the array off and returns it, shortening the
4340 array by 1 and moving everything down. If there are no elements in the
4341 array, returns the undefined value. If ARRAY is omitted, shifts the
4342 C<@_> array within the lexical scope of subroutines and formats, and the
4343 C<@ARGV> array at file scopes or within the lexical scopes established by
4344 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4347 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4348 same thing to the left end of an array that C<pop> and C<push> do to the
4351 =item shmctl ID,CMD,ARG
4353 Calls the System V IPC function shmctl. You'll probably have to say
4357 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4358 then ARG must be a variable which will hold the returned C<shmid_ds>
4359 structure. Returns like ioctl: the undefined value for error, "C<0> but
4360 true" for zero, or the actual return value otherwise.
4361 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4363 =item shmget KEY,SIZE,FLAGS
4365 Calls the System V IPC function shmget. Returns the shared memory
4366 segment id, or the undefined value if there is an error.
4367 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4369 =item shmread ID,VAR,POS,SIZE
4371 =item shmwrite ID,STRING,POS,SIZE
4373 Reads or writes the System V shared memory segment ID starting at
4374 position POS for size SIZE by attaching to it, copying in/out, and
4375 detaching from it. When reading, VAR must be a variable that will
4376 hold the data read. When writing, if STRING is too long, only SIZE
4377 bytes are used; if STRING is too short, nulls are written to fill out
4378 SIZE bytes. Return true if successful, or false if there is an error.
4379 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4380 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4382 =item shutdown SOCKET,HOW
4384 Shuts down a socket connection in the manner indicated by HOW, which
4385 has the same interpretation as in the system call of the same name.
4387 shutdown(SOCKET, 0); # I/we have stopped reading data
4388 shutdown(SOCKET, 1); # I/we have stopped writing data
4389 shutdown(SOCKET, 2); # I/we have stopped using this socket
4391 This is useful with sockets when you want to tell the other
4392 side you're done writing but not done reading, or vice versa.
4393 It's also a more insistent form of close because it also
4394 disables the file descriptor in any forked copies in other
4401 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4402 returns sine of C<$_>.
4404 For the inverse sine operation, you may use the C<Math::Trig::asin>
4405 function, or use this relation:
4407 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4413 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4414 May be interrupted if the process receives a signal such as C<SIGALRM>.
4415 Returns the number of seconds actually slept. You probably cannot
4416 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4419 On some older systems, it may sleep up to a full second less than what
4420 you requested, depending on how it counts seconds. Most modern systems
4421 always sleep the full amount. They may appear to sleep longer than that,
4422 however, because your process might not be scheduled right away in a
4423 busy multitasking system.
4425 For delays of finer granularity than one second, you may use Perl's
4426 C<syscall> interface to access setitimer(2) if your system supports
4427 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4428 and starting from Perl 5.8 part of the standard distribution) may also
4431 See also the POSIX module's C<pause> function.
4433 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4435 Opens a socket of the specified kind and attaches it to filehandle
4436 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4437 the system call of the same name. You should C<use Socket> first
4438 to get the proper definitions imported. See the examples in
4439 L<perlipc/"Sockets: Client/Server Communication">.
4441 On systems that support a close-on-exec flag on files, the flag will
4442 be set for the newly opened file descriptor, as determined by the
4443 value of $^F. See L<perlvar/$^F>.
4445 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4447 Creates an unnamed pair of sockets in the specified domain, of the
4448 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4449 for the system call of the same name. If unimplemented, yields a fatal
4450 error. Returns true if successful.
4452 On systems that support a close-on-exec flag on files, the flag will
4453 be set for the newly opened file descriptors, as determined by the value
4454 of $^F. See L<perlvar/$^F>.
4456 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4457 to C<pipe(Rdr, Wtr)> is essentially:
4460 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4461 shutdown(Rdr, 1); # no more writing for reader
4462 shutdown(Wtr, 0); # no more reading for writer
4464 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4465 emulate socketpair using IP sockets to localhost if your system implements
4466 sockets but not socketpair.
4468 =item sort SUBNAME LIST
4470 =item sort BLOCK LIST
4474 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4475 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4476 specified, it gives the name of a subroutine that returns an integer
4477 less than, equal to, or greater than C<0>, depending on how the elements
4478 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4479 operators are extremely useful in such routines.) SUBNAME may be a
4480 scalar variable name (unsubscripted), in which case the value provides
4481 the name of (or a reference to) the actual subroutine to use. In place
4482 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4485 If the subroutine's prototype is C<($$)>, the elements to be compared
4486 are passed by reference in C<@_>, as for a normal subroutine. This is
4487 slower than unprototyped subroutines, where the elements to be
4488 compared are passed into the subroutine
4489 as the package global variables $a and $b (see example below). Note that
4490 in the latter case, it is usually counter-productive to declare $a and
4493 In either case, the subroutine may not be recursive. The values to be
4494 compared are always passed by reference, so don't modify them.
4496 You also cannot exit out of the sort block or subroutine using any of the
4497 loop control operators described in L<perlsyn> or with C<goto>.
4499 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4500 current collation locale. See L<perllocale>.
4502 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4503 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4504 preserves the input order of elements that compare equal. Although
4505 quicksort's run time is O(NlogN) when averaged over all arrays of
4506 length N, the time can be O(N**2), I<quadratic> behavior, for some
4507 inputs.) In 5.7, the quicksort implementation was replaced with
4508 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4509 But benchmarks indicated that for some inputs, on some platforms,
4510 the original quicksort was faster. 5.8 has a sort pragma for
4511 limited control of the sort. Its rather blunt control of the
4512 underlying algorithm may not persist into future perls, but the
4513 ability to characterize the input or output in implementation
4514 independent ways quite probably will. See L</use>.
4519 @articles = sort @files;
4521 # same thing, but with explicit sort routine
4522 @articles = sort {$a cmp $b} @files;
4524 # now case-insensitively
4525 @articles = sort {uc($a) cmp uc($b)} @files;
4527 # same thing in reversed order
4528 @articles = sort {$b cmp $a} @files;
4530 # sort numerically ascending
4531 @articles = sort {$a <=> $b} @files;
4533 # sort numerically descending
4534 @articles = sort {$b <=> $a} @files;
4536 # this sorts the %age hash by value instead of key
4537 # using an in-line function
4538 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4540 # sort using explicit subroutine name
4542 $age{$a} <=> $age{$b}; # presuming numeric
4544 @sortedclass = sort byage @class;
4546 sub backwards { $b cmp $a }
4547 @harry = qw(dog cat x Cain Abel);
4548 @george = qw(gone chased yz Punished Axed);
4550 # prints AbelCaincatdogx
4551 print sort backwards @harry;
4552 # prints xdogcatCainAbel
4553 print sort @george, 'to', @harry;
4554 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4556 # inefficiently sort by descending numeric compare using
4557 # the first integer after the first = sign, or the
4558 # whole record case-insensitively otherwise
4561 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4566 # same thing, but much more efficiently;
4567 # we'll build auxiliary indices instead
4571 push @nums, /=(\d+)/;
4576 $nums[$b] <=> $nums[$a]
4578 $caps[$a] cmp $caps[$b]
4582 # same thing, but without any temps
4583 @new = map { $_->[0] }
4584 sort { $b->[1] <=> $a->[1]
4587 } map { [$_, /=(\d+)/, uc($_)] } @old;
4589 # using a prototype allows you to use any comparison subroutine
4590 # as a sort subroutine (including other package's subroutines)
4592 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4595 @new = sort other::backwards @old;
4597 # guarantee stability, regardless of algorithm
4599 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4601 # force use of mergesort (not portable outside Perl 5.8)
4602 use sort '_mergesort'; # note discouraging _
4603 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4605 If you're using strict, you I<must not> declare $a
4606 and $b as lexicals. They are package globals. That means
4607 if you're in the C<main> package and type
4609 @articles = sort {$b <=> $a} @files;
4611 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4612 but if you're in the C<FooPack> package, it's the same as typing
4614 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4616 The comparison function is required to behave. If it returns
4617 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4618 sometimes saying the opposite, for example) the results are not
4621 =item splice ARRAY,OFFSET,LENGTH,LIST
4623 =item splice ARRAY,OFFSET,LENGTH
4625 =item splice ARRAY,OFFSET
4629 Removes the elements designated by OFFSET and LENGTH from an array, and
4630 replaces them with the elements of LIST, if any. In list context,
4631 returns the elements removed from the array. In scalar context,
4632 returns the last element removed, or C<undef> if no elements are
4633 removed. The array grows or shrinks as necessary.
4634 If OFFSET is negative then it starts that far from the end of the array.
4635 If LENGTH is omitted, removes everything from OFFSET onward.
4636 If LENGTH is negative, removes the elements from OFFSET onward
4637 except for -LENGTH elements at the end of the array.
4638 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4639 past the end of the array, perl issues a warning, and splices at the
4642 The following equivalences hold (assuming C<$[ == 0>):
4644 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4645 pop(@a) splice(@a,-1)
4646 shift(@a) splice(@a,0,1)
4647 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4648 $a[$x] = $y splice(@a,$x,1,$y)
4650 Example, assuming array lengths are passed before arrays:
4652 sub aeq { # compare two list values
4653 my(@a) = splice(@_,0,shift);
4654 my(@b) = splice(@_,0,shift);
4655 return 0 unless @a == @b; # same len?
4657 return 0 if pop(@a) ne pop(@b);
4661 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4663 =item split /PATTERN/,EXPR,LIMIT
4665 =item split /PATTERN/,EXPR
4667 =item split /PATTERN/
4671 Splits a string into a list of strings and returns that list. By default,
4672 empty leading fields are preserved, and empty trailing ones are deleted.
4674 In scalar context, returns the number of fields found and splits into
4675 the C<@_> array. Use of split in scalar context is deprecated, however,
4676 because it clobbers your subroutine arguments.
4678 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4679 splits on whitespace (after skipping any leading whitespace). Anything
4680 matching PATTERN is taken to be a delimiter separating the fields. (Note
4681 that the delimiter may be longer than one character.)
4683 If LIMIT is specified and positive, it represents the maximum number
4684 of fields the EXPR will be split into, though the actual number of
4685 fields returned depends on the number of times PATTERN matches within
4686 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4687 stripped (which potential users of C<pop> would do well to remember).
4688 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4689 had been specified. Note that splitting an EXPR that evaluates to the
4690 empty string always returns the empty list, regardless of the LIMIT
4693 A pattern matching the null string (not to be confused with
4694 a null pattern C<//>, which is just one member of the set of patterns
4695 matching a null string) will split the value of EXPR into separate
4696 characters at each point it matches that way. For example:
4698 print join(':', split(/ */, 'hi there'));
4700 produces the output 'h:i:t:h:e:r:e'.
4702 Using the empty pattern C<//> specifically matches the null string, and is
4703 not be confused with the use of C<//> to mean "the last successful pattern
4706 Empty leading (or trailing) fields are produced when there are positive width
4707 matches at the beginning (or end) of the string; a zero-width match at the
4708 beginning (or end) of the string does not produce an empty field. For
4711 print join(':', split(/(?=\w)/, 'hi there!'));
4713 produces the output 'h:i :t:h:e:r:e!'.
4715 The LIMIT parameter can be used to split a line partially
4717 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4719 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4720 one larger than the number of variables in the list, to avoid
4721 unnecessary work. For the list above LIMIT would have been 4 by
4722 default. In time critical applications it behooves you not to split
4723 into more fields than you really need.
4725 If the PATTERN contains parentheses, additional list elements are
4726 created from each matching substring in the delimiter.
4728 split(/([,-])/, "1-10,20", 3);
4730 produces the list value
4732 (1, '-', 10, ',', 20)
4734 If you had the entire header of a normal Unix email message in $header,
4735 you could split it up into fields and their values this way:
4737 $header =~ s/\n\s+/ /g; # fix continuation lines
4738 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4740 The pattern C</PATTERN/> may be replaced with an expression to specify
4741 patterns that vary at runtime. (To do runtime compilation only once,
4742 use C</$variable/o>.)
4744 As a special case, specifying a PATTERN of space (C<' '>) will split on
4745 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4746 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4747 will give you as many null initial fields as there are leading spaces.
4748 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4749 whitespace produces a null first field. A C<split> with no arguments
4750 really does a C<split(' ', $_)> internally.
4752 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4757 open(PASSWD, '/etc/passwd');
4760 ($login, $passwd, $uid, $gid,
4761 $gcos, $home, $shell) = split(/:/);
4765 As with regular pattern matching, any capturing parentheses that are not
4766 matched in a C<split()> will be set to C<undef> when returned:
4768 @fields = split /(A)|B/, "1A2B3";
4769 # @fields is (1, 'A', 2, undef, 3)
4771 =item sprintf FORMAT, LIST
4773 Returns a string formatted by the usual C<printf> conventions of the C
4774 library function C<sprintf>. See below for more details
4775 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4776 the general principles.
4780 # Format number with up to 8 leading zeroes
4781 $result = sprintf("%08d", $number);
4783 # Round number to 3 digits after decimal point
4784 $rounded = sprintf("%.3f", $number);
4786 Perl does its own C<sprintf> formatting--it emulates the C
4787 function C<sprintf>, but it doesn't use it (except for floating-point
4788 numbers, and even then only the standard modifiers are allowed). As a
4789 result, any non-standard extensions in your local C<sprintf> are not
4790 available from Perl.
4792 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4793 pass it an array as your first argument. The array is given scalar context,
4794 and instead of using the 0th element of the array as the format, Perl will
4795 use the count of elements in the array as the format, which is almost never
4798 Perl's C<sprintf> permits the following universally-known conversions:
4801 %c a character with the given number
4803 %d a signed integer, in decimal
4804 %u an unsigned integer, in decimal
4805 %o an unsigned integer, in octal
4806 %x an unsigned integer, in hexadecimal
4807 %e a floating-point number, in scientific notation
4808 %f a floating-point number, in fixed decimal notation
4809 %g a floating-point number, in %e or %f notation
4811 In addition, Perl permits the following widely-supported conversions:
4813 %X like %x, but using upper-case letters
4814 %E like %e, but using an upper-case "E"
4815 %G like %g, but with an upper-case "E" (if applicable)
4816 %b an unsigned integer, in binary
4817 %p a pointer (outputs the Perl value's address in hexadecimal)
4818 %n special: *stores* the number of characters output so far
4819 into the next variable in the parameter list
4821 Finally, for backward (and we do mean "backward") compatibility, Perl
4822 permits these unnecessary but widely-supported conversions:
4825 %D a synonym for %ld
4826 %U a synonym for %lu
4827 %O a synonym for %lo
4830 Note that the number of exponent digits in the scientific notation by
4831 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4832 exponent less than 100 is system-dependent: it may be three or less
4833 (zero-padded as necessary). In other words, 1.23 times ten to the
4834 99th may be either "1.23e99" or "1.23e099".
4836 Perl permits the following universally-known flags between the C<%>
4837 and the conversion letter:
4839 space prefix positive number with a space
4840 + prefix positive number with a plus sign
4841 - left-justify within the field
4842 0 use zeros, not spaces, to right-justify
4843 # prefix non-zero octal with "0", non-zero hex with "0x"
4844 number minimum field width
4845 .number "precision": digits after decimal point for
4846 floating-point, max length for string, minimum length
4848 l interpret integer as C type "long" or "unsigned long"
4849 h interpret integer as C type "short" or "unsigned short"
4850 If no flags, interpret integer as C type "int" or "unsigned"
4852 Perl supports parameter ordering, in other words, fetching the
4853 parameters in some explicitly specified "random" ordering as opposed
4854 to the default implicit sequential ordering. The syntax is, instead
4855 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4856 where the I<digits> is the wanted index, from one upwards. For example:
4858 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4859 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4861 Note that using the reordering syntax does not interfere with the usual
4862 implicit sequential fetching of the parameters:
4864 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4865 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4866 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4867 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4868 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4870 There are also two Perl-specific flags:
4872 V interpret integer as Perl's standard integer type
4873 v interpret string as a vector of integers, output as
4874 numbers separated either by dots, or by an arbitrary
4875 string received from the argument list when the flag
4878 Where a number would appear in the flags, an asterisk (C<*>) may be
4879 used instead, in which case Perl uses the next item in the parameter
4880 list as the given number (that is, as the field width or precision).
4881 If a field width obtained through C<*> is negative, it has the same
4882 effect as the C<-> flag: left-justification.
4884 The C<v> flag is useful for displaying ordinal values of characters
4885 in arbitrary strings:
4887 printf "version is v%vd\n", $^V; # Perl's version
4888 printf "address is %*vX\n", ":", $addr; # IPv6 address
4889 printf "bits are %*vb\n", " ", $bits; # random bitstring
4891 If C<use locale> is in effect, the character used for the decimal
4892 point in formatted real numbers is affected by the LC_NUMERIC locale.
4895 If Perl understands "quads" (64-bit integers) (this requires
4896 either that the platform natively support quads or that Perl
4897 be specifically compiled to support quads), the characters
4901 print quads, and they may optionally be preceded by
4909 You can find out whether your Perl supports quads via L<Config>:
4912 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4915 If Perl understands "long doubles" (this requires that the platform
4916 support long doubles), the flags
4920 may optionally be preceded by
4928 You can find out whether your Perl supports long doubles via L<Config>:
4931 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4937 Return the square root of EXPR. If EXPR is omitted, returns square
4938 root of C<$_>. Only works on non-negative operands, unless you've
4939 loaded the standard Math::Complex module.
4942 print sqrt(-2); # prints 1.4142135623731i
4948 Sets the random number seed for the C<rand> operator.
4950 The point of the function is to "seed" the C<rand> function so that
4951 C<rand> can produce a different sequence each time you run your
4954 If srand() is not called explicitly, it is called implicitly at the
4955 first use of the C<rand> operator. However, this was not the case in
4956 versions of Perl before 5.004, so if your script will run under older
4957 Perl versions, it should call C<srand>.
4959 Most programs won't even call srand() at all, except those that
4960 need a cryptographically-strong starting point rather than the
4961 generally acceptable default, which is based on time of day,
4962 process ID, and memory allocation, or the F</dev/urandom> device,
4965 You can call srand($seed) with the same $seed to reproduce the
4966 I<same> sequence from rand(), but this is usually reserved for
4967 generating predictable results for testing or debugging.
4968 Otherwise, don't call srand() more than once in your program.
4970 Do B<not> call srand() (i.e. without an argument) more than once in
4971 a script. The internal state of the random number generator should
4972 contain more entropy than can be provided by any seed, so calling
4973 srand() again actually I<loses> randomness.
4975 Most implementations of C<srand> take an integer and will silently
4976 truncate decimal numbers. This means C<srand(42)> will usually
4977 produce the same results as C<srand(42.1)>. To be safe, always pass
4978 C<srand> an integer.
4980 In versions of Perl prior to 5.004 the default seed was just the
4981 current C<time>. This isn't a particularly good seed, so many old
4982 programs supply their own seed value (often C<time ^ $$> or C<time ^
4983 ($$ + ($$ << 15))>), but that isn't necessary any more.
4985 Note that you need something much more random than the default seed for
4986 cryptographic purposes. Checksumming the compressed output of one or more
4987 rapidly changing operating system status programs is the usual method. For
4990 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4992 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4995 Frequently called programs (like CGI scripts) that simply use
4999 for a seed can fall prey to the mathematical property that
5003 one-third of the time. So don't do that.
5005 =item stat FILEHANDLE
5011 Returns a 13-element list giving the status info for a file, either
5012 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5013 it stats C<$_>. Returns a null list if the stat fails. Typically used
5016 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5017 $atime,$mtime,$ctime,$blksize,$blocks)
5020 Not all fields are supported on all filesystem types. Here are the
5021 meaning of the fields:
5023 0 dev device number of filesystem
5025 2 mode file mode (type and permissions)
5026 3 nlink number of (hard) links to the file
5027 4 uid numeric user ID of file's owner
5028 5 gid numeric group ID of file's owner
5029 6 rdev the device identifier (special files only)
5030 7 size total size of file, in bytes
5031 8 atime last access time in seconds since the epoch
5032 9 mtime last modify time in seconds since the epoch
5033 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5034 11 blksize preferred block size for file system I/O
5035 12 blocks actual number of blocks allocated
5037 (The epoch was at 00:00 January 1, 1970 GMT.)
5039 If stat is passed the special filehandle consisting of an underline, no
5040 stat is done, but the current contents of the stat structure from the
5041 last stat or filetest are returned. Example:
5043 if (-x $file && (($d) = stat(_)) && $d < 0) {
5044 print "$file is executable NFS file\n";
5047 (This works on machines only for which the device number is negative
5050 Because the mode contains both the file type and its permissions, you
5051 should mask off the file type portion and (s)printf using a C<"%o">
5052 if you want to see the real permissions.
5054 $mode = (stat($filename))[2];
5055 printf "Permissions are %04o\n", $mode & 07777;
5057 In scalar context, C<stat> returns a boolean value indicating success
5058 or failure, and, if successful, sets the information associated with
5059 the special filehandle C<_>.
5061 The File::stat module provides a convenient, by-name access mechanism:
5064 $sb = stat($filename);
5065 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5066 $filename, $sb->size, $sb->mode & 07777,
5067 scalar localtime $sb->mtime;
5069 You can import symbolic mode constants (C<S_IF*>) and functions
5070 (C<S_IS*>) from the Fcntl module:
5074 $mode = (stat($filename))[2];
5076 $user_rwx = ($mode & S_IRWXU) >> 6;
5077 $group_read = ($mode & S_IRGRP) >> 3;
5078 $other_execute = $mode & S_IXOTH;
5080 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5082 $is_setuid = $mode & S_ISUID;
5083 $is_setgid = S_ISDIR($mode);
5085 You could write the last two using the C<-u> and C<-d> operators.
5086 The commonly available S_IF* constants are
5088 # Permissions: read, write, execute, for user, group, others.
5090 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5091 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5092 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5094 # Setuid/Setgid/Stickiness.
5096 S_ISUID S_ISGID S_ISVTX S_ISTXT
5098 # File types. Not necessarily all are available on your system.
5100 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5102 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5104 S_IREAD S_IWRITE S_IEXEC
5106 and the S_IF* functions are
5108 S_IFMODE($mode) the part of $mode containing the permission bits
5109 and the setuid/setgid/sticky bits
5111 S_IFMT($mode) the part of $mode containing the file type
5112 which can be bit-anded with e.g. S_IFREG
5113 or with the following functions
5115 # The operators -f, -d, -l, -b, -c, -p, and -s.
5117 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5118 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5120 # No direct -X operator counterpart, but for the first one
5121 # the -g operator is often equivalent. The ENFMT stands for
5122 # record flocking enforcement, a platform-dependent feature.
5124 S_ISENFMT($mode) S_ISWHT($mode)
5126 See your native chmod(2) and stat(2) documentation for more details
5127 about the S_* constants.
5133 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5134 doing many pattern matches on the string before it is next modified.
5135 This may or may not save time, depending on the nature and number of
5136 patterns you are searching on, and on the distribution of character
5137 frequencies in the string to be searched--you probably want to compare
5138 run times with and without it to see which runs faster. Those loops
5139 which scan for many short constant strings (including the constant
5140 parts of more complex patterns) will benefit most. You may have only
5141 one C<study> active at a time--if you study a different scalar the first
5142 is "unstudied". (The way C<study> works is this: a linked list of every
5143 character in the string to be searched is made, so we know, for
5144 example, where all the C<'k'> characters are. From each search string,
5145 the rarest character is selected, based on some static frequency tables
5146 constructed from some C programs and English text. Only those places
5147 that contain this "rarest" character are examined.)
5149 For example, here is a loop that inserts index producing entries
5150 before any line containing a certain pattern:
5154 print ".IX foo\n" if /\bfoo\b/;
5155 print ".IX bar\n" if /\bbar\b/;
5156 print ".IX blurfl\n" if /\bblurfl\b/;
5161 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5162 will be looked at, because C<f> is rarer than C<o>. In general, this is
5163 a big win except in pathological cases. The only question is whether
5164 it saves you more time than it took to build the linked list in the
5167 Note that if you have to look for strings that you don't know till
5168 runtime, you can build an entire loop as a string and C<eval> that to
5169 avoid recompiling all your patterns all the time. Together with
5170 undefining C<$/> to input entire files as one record, this can be very
5171 fast, often faster than specialized programs like fgrep(1). The following
5172 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5173 out the names of those files that contain a match:
5175 $search = 'while (<>) { study;';
5176 foreach $word (@words) {
5177 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5182 eval $search; # this screams
5183 $/ = "\n"; # put back to normal input delimiter
5184 foreach $file (sort keys(%seen)) {
5192 =item sub NAME BLOCK
5194 This is subroutine definition, not a real function I<per se>. With just a
5195 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5196 Without a NAME, it's an anonymous function declaration, and does actually
5197 return a value: the CODE ref of the closure you just created. See L<perlsub>
5198 and L<perlref> for details.
5200 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5202 =item substr EXPR,OFFSET,LENGTH
5204 =item substr EXPR,OFFSET
5206 Extracts a substring out of EXPR and returns it. First character is at
5207 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5208 If OFFSET is negative (or more precisely, less than C<$[>), starts
5209 that far from the end of the string. If LENGTH is omitted, returns
5210 everything to the end of the string. If LENGTH is negative, leaves that
5211 many characters off the end of the string.
5213 You can use the substr() function as an lvalue, in which case EXPR
5214 must itself be an lvalue. If you assign something shorter than LENGTH,
5215 the string will shrink, and if you assign something longer than LENGTH,
5216 the string will grow to accommodate it. To keep the string the same
5217 length you may need to pad or chop your value using C<sprintf>.
5219 If OFFSET and LENGTH specify a substring that is partly outside the
5220 string, only the part within the string is returned. If the substring
5221 is beyond either end of the string, substr() returns the undefined
5222 value and produces a warning. When used as an lvalue, specifying a
5223 substring that is entirely outside the string is a fatal error.
5224 Here's an example showing the behavior for boundary cases:
5227 substr($name, 4) = 'dy'; # $name is now 'freddy'
5228 my $null = substr $name, 6, 2; # returns '' (no warning)
5229 my $oops = substr $name, 7; # returns undef, with warning
5230 substr($name, 7) = 'gap'; # fatal error
5232 An alternative to using substr() as an lvalue is to specify the
5233 replacement string as the 4th argument. This allows you to replace
5234 parts of the EXPR and return what was there before in one operation,
5235 just as you can with splice().
5237 =item symlink OLDFILE,NEWFILE
5239 Creates a new filename symbolically linked to the old filename.
5240 Returns C<1> for success, C<0> otherwise. On systems that don't support
5241 symbolic links, produces a fatal error at run time. To check for that,
5244 $symlink_exists = eval { symlink("",""); 1 };
5248 Calls the system call specified as the first element of the list,
5249 passing the remaining elements as arguments to the system call. If
5250 unimplemented, produces a fatal error. The arguments are interpreted
5251 as follows: if a given argument is numeric, the argument is passed as
5252 an int. If not, the pointer to the string value is passed. You are
5253 responsible to make sure a string is pre-extended long enough to
5254 receive any result that might be written into a string. You can't use a
5255 string literal (or other read-only string) as an argument to C<syscall>
5256 because Perl has to assume that any string pointer might be written
5258 integer arguments are not literals and have never been interpreted in a
5259 numeric context, you may need to add C<0> to them to force them to look
5260 like numbers. This emulates the C<syswrite> function (or vice versa):
5262 require 'syscall.ph'; # may need to run h2ph
5264 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5266 Note that Perl supports passing of up to only 14 arguments to your system call,
5267 which in practice should usually suffice.
5269 Syscall returns whatever value returned by the system call it calls.
5270 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5271 Note that some system calls can legitimately return C<-1>. The proper
5272 way to handle such calls is to assign C<$!=0;> before the call and
5273 check the value of C<$!> if syscall returns C<-1>.
5275 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5276 number of the read end of the pipe it creates. There is no way
5277 to retrieve the file number of the other end. You can avoid this
5278 problem by using C<pipe> instead.
5280 =item sysopen FILEHANDLE,FILENAME,MODE
5282 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5284 Opens the file whose filename is given by FILENAME, and associates it
5285 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5286 the name of the real filehandle wanted. This function calls the
5287 underlying operating system's C<open> function with the parameters
5288 FILENAME, MODE, PERMS.
5290 The possible values and flag bits of the MODE parameter are
5291 system-dependent; they are available via the standard module C<Fcntl>.
5292 See the documentation of your operating system's C<open> to see which
5293 values and flag bits are available. You may combine several flags
5294 using the C<|>-operator.
5296 Some of the most common values are C<O_RDONLY> for opening the file in
5297 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5298 and C<O_RDWR> for opening the file in read-write mode, and.
5300 For historical reasons, some values work on almost every system
5301 supported by perl: zero means read-only, one means write-only, and two
5302 means read/write. We know that these values do I<not> work under
5303 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5304 use them in new code.
5306 If the file named by FILENAME does not exist and the C<open> call creates
5307 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5308 PERMS specifies the permissions of the newly created file. If you omit
5309 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5310 These permission values need to be in octal, and are modified by your
5311 process's current C<umask>.
5313 In many systems the C<O_EXCL> flag is available for opening files in
5314 exclusive mode. This is B<not> locking: exclusiveness means here that
5315 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5318 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5320 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5321 that takes away the user's option to have a more permissive umask.
5322 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5325 Note that C<sysopen> depends on the fdopen() C library function.
5326 On many UNIX systems, fdopen() is known to fail when file descriptors
5327 exceed a certain value, typically 255. If you need more file
5328 descriptors than that, consider rebuilding Perl to use the C<sfio>
5329 library, or perhaps using the POSIX::open() function.
5331 See L<perlopentut> for a kinder, gentler explanation of opening files.
5333 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5335 =item sysread FILEHANDLE,SCALAR,LENGTH
5337 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5338 the specified FILEHANDLE, using the system call read(2). It bypasses
5339 buffered IO, so mixing this with other kinds of reads, C<print>,
5340 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5341 stdio usually buffers data. Returns the number of characters actually
5342 read, C<0> at end of file, or undef if there was an error. SCALAR
5343 will be grown or shrunk so that the last byte actually read is the
5344 last byte of the scalar after the read.
5346 Note the I<characters>: depending on the status of the filehandle,
5347 either (8-bit) bytes or characters are read. By default all
5348 filehandles operate on bytes, but for example if the filehandle has
5349 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5350 pragma, L<open>), the I/O will operate on characters, not bytes.
5352 An OFFSET may be specified to place the read data at some place in the
5353 string other than the beginning. A negative OFFSET specifies
5354 placement at that many characters counting backwards from the end of
5355 the string. A positive OFFSET greater than the length of SCALAR
5356 results in the string being padded to the required size with C<"\0">
5357 bytes before the result of the read is appended.
5359 There is no syseof() function, which is ok, since eof() doesn't work
5360 very well on device files (like ttys) anyway. Use sysread() and check
5361 for a return value for 0 to decide whether you're done.
5363 =item sysseek FILEHANDLE,POSITION,WHENCE
5365 Sets FILEHANDLE's system position I<in bytes> using the system call
5366 lseek(2). FILEHANDLE may be an expression whose value gives the name
5367 of the filehandle. The values for WHENCE are C<0> to set the new
5368 position to POSITION, C<1> to set the it to the current position plus
5369 POSITION, and C<2> to set it to EOF plus POSITION (typically
5372 Note the I<in bytes>: even if the filehandle has been set to operate
5373 on characters (for example by using the C<:utf8> discipline), tell()
5374 will return byte offsets, not character offsets (because implementing
5375 that would render sysseek() very slow).
5377 sysseek() bypasses normal buffered io, so mixing this with reads (other
5378 than C<sysread>, for example >< or read()) C<print>, C<write>,
5379 C<seek>, C<tell>, or C<eof> may cause confusion.
5381 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5382 and C<SEEK_END> (start of the file, current position, end of the file)
5383 from the Fcntl module. Use of the constants is also more portable
5384 than relying on 0, 1, and 2. For example to define a "systell" function:
5386 use Fnctl 'SEEK_CUR';
5387 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5389 Returns the new position, or the undefined value on failure. A position
5390 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5391 true on success and false on failure, yet you can still easily determine
5396 =item system PROGRAM LIST
5398 Does exactly the same thing as C<exec LIST>, except that a fork is
5399 done first, and the parent process waits for the child process to
5400 complete. Note that argument processing varies depending on the
5401 number of arguments. If there is more than one argument in LIST,
5402 or if LIST is an array with more than one value, starts the program
5403 given by the first element of the list with arguments given by the
5404 rest of the list. If there is only one scalar argument, the argument
5405 is checked for shell metacharacters, and if there are any, the
5406 entire argument is passed to the system's command shell for parsing
5407 (this is C</bin/sh -c> on Unix platforms, but varies on other
5408 platforms). If there are no shell metacharacters in the argument,
5409 it is split into words and passed directly to C<execvp>, which is
5412 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5413 output before any operation that may do a fork, but this may not be
5414 supported on some platforms (see L<perlport>). To be safe, you may need
5415 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5416 of C<IO::Handle> on any open handles.
5418 The return value is the exit status of the program as returned by the
5419 C<wait> call. To get the actual exit value shift right by eight (see below).
5420 See also L</exec>. This is I<not> what you want to use to capture
5421 the output from a command, for that you should use merely backticks or
5422 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5423 indicates a failure to start the program (inspect $! for the reason).
5425 Like C<exec>, C<system> allows you to lie to a program about its name if
5426 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5428 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5429 killing the program they're running doesn't actually interrupt
5432 @args = ("command", "arg1", "arg2");
5434 or die "system @args failed: $?"
5436 You can check all the failure possibilities by inspecting
5439 $exit_value = $? >> 8;
5440 $signal_num = $? & 127;
5441 $dumped_core = $? & 128;
5443 or more portably by using the W*() calls of the POSIX extension;
5444 see L<perlport> for more information.
5446 When the arguments get executed via the system shell, results
5447 and return codes will be subject to its quirks and capabilities.
5448 See L<perlop/"`STRING`"> and L</exec> for details.
5450 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5452 =item syswrite FILEHANDLE,SCALAR,LENGTH
5454 =item syswrite FILEHANDLE,SCALAR
5456 Attempts to write LENGTH characters of data from variable SCALAR to
5457 the specified FILEHANDLE, using the system call write(2). If LENGTH
5458 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5459 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5460 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5461 buffers data. Returns the number of characters actually written, or
5462 C<undef> if there was an error. If the LENGTH is greater than the
5463 available data in the SCALAR after the OFFSET, only as much data as is
5464 available will be written.
5466 An OFFSET may be specified to write the data from some part of the
5467 string other than the beginning. A negative OFFSET specifies writing
5468 that many characters counting backwards from the end of the string.
5469 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5471 Note the I<characters>: depending on the status of the filehandle,
5472 either (8-bit) bytes or characters are written. By default all
5473 filehandles operate on bytes, but for example if the filehandle has
5474 been opened with the C<:utf8> discipline (see L</open>, and the open
5475 pragma, L<open>), the I/O will operate on characters, not bytes.
5477 =item tell FILEHANDLE
5481 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5482 error. FILEHANDLE may be an expression whose value gives the name of
5483 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5486 Note the I<in bytes>: even if the filehandle has been set to
5487 operate on characters (for example by using the C<:utf8> open
5488 discipline), tell() will return byte offsets, not character offsets
5489 (because that would render seek() and tell() rather slow).
5491 The return value of tell() for the standard streams like the STDIN
5492 depends on the operating system: it may return -1 or something else.
5493 tell() on pipes, fifos, and sockets usually returns -1.
5495 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5497 Do not use tell() on a filehandle that has been opened using
5498 sysopen(), use sysseek() for that as described above. Why? Because
5499 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5500 buffered filehandles. sysseek() make sense only on the first kind,
5501 tell() only makes sense on the second kind.
5503 =item telldir DIRHANDLE
5505 Returns the current position of the C<readdir> routines on DIRHANDLE.
5506 Value may be given to C<seekdir> to access a particular location in a
5507 directory. Has the same caveats about possible directory compaction as
5508 the corresponding system library routine.
5510 =item tie VARIABLE,CLASSNAME,LIST
5512 This function binds a variable to a package class that will provide the
5513 implementation for the variable. VARIABLE is the name of the variable
5514 to be enchanted. CLASSNAME is the name of a class implementing objects
5515 of correct type. Any additional arguments are passed to the C<new>
5516 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5517 or C<TIEHASH>). Typically these are arguments such as might be passed
5518 to the C<dbm_open()> function of C. The object returned by the C<new>
5519 method is also returned by the C<tie> function, which would be useful
5520 if you want to access other methods in CLASSNAME.
5522 Note that functions such as C<keys> and C<values> may return huge lists
5523 when used on large objects, like DBM files. You may prefer to use the
5524 C<each> function to iterate over such. Example:
5526 # print out history file offsets
5528 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5529 while (($key,$val) = each %HIST) {
5530 print $key, ' = ', unpack('L',$val), "\n";
5534 A class implementing a hash should have the following methods:
5536 TIEHASH classname, LIST
5538 STORE this, key, value
5543 NEXTKEY this, lastkey
5547 A class implementing an ordinary array should have the following methods:
5549 TIEARRAY classname, LIST
5551 STORE this, key, value
5553 STORESIZE this, count
5559 SPLICE this, offset, length, LIST
5564 A class implementing a file handle should have the following methods:
5566 TIEHANDLE classname, LIST
5567 READ this, scalar, length, offset
5570 WRITE this, scalar, length, offset
5572 PRINTF this, format, LIST
5576 SEEK this, position, whence
5578 OPEN this, mode, LIST
5583 A class implementing a scalar should have the following methods:
5585 TIESCALAR classname, LIST
5591 Not all methods indicated above need be implemented. See L<perltie>,
5592 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5594 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5595 for you--you need to do that explicitly yourself. See L<DB_File>
5596 or the F<Config> module for interesting C<tie> implementations.
5598 For further details see L<perltie>, L<"tied VARIABLE">.
5602 Returns a reference to the object underlying VARIABLE (the same value
5603 that was originally returned by the C<tie> call that bound the variable
5604 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5609 Returns the number of non-leap seconds since whatever time the system
5610 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5611 and 00:00:00 UTC, January 1, 1970 for most other systems).
5612 Suitable for feeding to C<gmtime> and C<localtime>.
5614 For measuring time in better granularity than one second,
5615 you may use either the Time::HiRes module from CPAN, or
5616 if you have gettimeofday(2), you may be able to use the
5617 C<syscall> interface of Perl, see L<perlfaq8> for details.
5621 Returns a four-element list giving the user and system times, in
5622 seconds, for this process and the children of this process.
5624 ($user,$system,$cuser,$csystem) = times;
5626 In scalar context, C<times> returns C<$user>.
5630 The transliteration operator. Same as C<y///>. See L<perlop>.
5632 =item truncate FILEHANDLE,LENGTH
5634 =item truncate EXPR,LENGTH
5636 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5637 specified length. Produces a fatal error if truncate isn't implemented
5638 on your system. Returns true if successful, the undefined value
5641 The behavior is undefined if LENGTH is greater than the length of the
5648 Returns an uppercased version of EXPR. This is the internal function
5649 implementing the C<\U> escape in double-quoted strings. Respects
5650 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5651 and L<perlunicode> for more details about locale and Unicode support.
5652 It does not attempt to do titlecase mapping on initial letters. See
5653 C<ucfirst> for that.
5655 If EXPR is omitted, uses C<$_>.
5661 Returns the value of EXPR with the first character in uppercase
5662 (titlecase in Unicode). This is the internal function implementing
5663 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5664 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5665 for more details about locale and Unicode support.
5667 If EXPR is omitted, uses C<$_>.
5673 Sets the umask for the process to EXPR and returns the previous value.
5674 If EXPR is omitted, merely returns the current umask.
5676 The Unix permission C<rwxr-x---> is represented as three sets of three
5677 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5678 and isn't one of the digits). The C<umask> value is such a number
5679 representing disabled permissions bits. The permission (or "mode")
5680 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5681 even if you tell C<sysopen> to create a file with permissions C<0777>,
5682 if your umask is C<0022> then the file will actually be created with
5683 permissions C<0755>. If your C<umask> were C<0027> (group can't
5684 write; others can't read, write, or execute), then passing
5685 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5688 Here's some advice: supply a creation mode of C<0666> for regular
5689 files (in C<sysopen>) and one of C<0777> for directories (in
5690 C<mkdir>) and executable files. This gives users the freedom of
5691 choice: if they want protected files, they might choose process umasks
5692 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5693 Programs should rarely if ever make policy decisions better left to
5694 the user. The exception to this is when writing files that should be
5695 kept private: mail files, web browser cookies, I<.rhosts> files, and
5698 If umask(2) is not implemented on your system and you are trying to
5699 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5700 fatal error at run time. If umask(2) is not implemented and you are
5701 not trying to restrict access for yourself, returns C<undef>.
5703 Remember that a umask is a number, usually given in octal; it is I<not> a
5704 string of octal digits. See also L</oct>, if all you have is a string.
5710 Undefines the value of EXPR, which must be an lvalue. Use only on a
5711 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5712 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5713 will probably not do what you expect on most predefined variables or
5714 DBM list values, so don't do that; see L<delete>.) Always returns the
5715 undefined value. You can omit the EXPR, in which case nothing is
5716 undefined, but you still get an undefined value that you could, for
5717 instance, return from a subroutine, assign to a variable or pass as a
5718 parameter. Examples:
5721 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5725 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5726 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5727 select undef, undef, undef, 0.25;
5728 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5730 Note that this is a unary operator, not a list operator.
5736 Deletes a list of files. Returns the number of files successfully
5739 $cnt = unlink 'a', 'b', 'c';
5743 Note: C<unlink> will not delete directories unless you are superuser and
5744 the B<-U> flag is supplied to Perl. Even if these conditions are
5745 met, be warned that unlinking a directory can inflict damage on your
5746 filesystem. Use C<rmdir> instead.
5748 If LIST is omitted, uses C<$_>.
5750 =item unpack TEMPLATE,EXPR
5752 C<unpack> does the reverse of C<pack>: it takes a string
5753 and expands it out into a list of values.
5754 (In scalar context, it returns merely the first value produced.)
5756 The string is broken into chunks described by the TEMPLATE. Each chunk
5757 is converted separately to a value. Typically, either the string is a result
5758 of C<pack>, or the bytes of the string represent a C structure of some
5761 The TEMPLATE has the same format as in the C<pack> function.
5762 Here's a subroutine that does substring:
5765 my($what,$where,$howmuch) = @_;
5766 unpack("x$where a$howmuch", $what);
5771 sub ordinal { unpack("c",$_[0]); } # same as ord()
5773 In addition to fields allowed in pack(), you may prefix a field with
5774 a %<number> to indicate that
5775 you want a <number>-bit checksum of the items instead of the items
5776 themselves. Default is a 16-bit checksum. Checksum is calculated by
5777 summing numeric values of expanded values (for string fields the sum of
5778 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5780 For example, the following
5781 computes the same number as the System V sum program:
5785 unpack("%32C*",<>) % 65535;
5788 The following efficiently counts the number of set bits in a bit vector:
5790 $setbits = unpack("%32b*", $selectmask);
5792 The C<p> and C<P> formats should be used with care. Since Perl
5793 has no way of checking whether the value passed to C<unpack()>
5794 corresponds to a valid memory location, passing a pointer value that's
5795 not known to be valid is likely to have disastrous consequences.
5797 If the repeat count of a field is larger than what the remainder of
5798 the input string allows, repeat count is decreased. If the input string
5799 is longer than one described by the TEMPLATE, the rest is ignored.
5801 See L</pack> for more examples and notes.
5803 =item untie VARIABLE
5805 Breaks the binding between a variable and a package. (See C<tie>.)
5807 =item unshift ARRAY,LIST
5809 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5810 depending on how you look at it. Prepends list to the front of the
5811 array, and returns the new number of elements in the array.
5813 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5815 Note the LIST is prepended whole, not one element at a time, so the
5816 prepended elements stay in the same order. Use C<reverse> to do the
5819 =item use Module VERSION LIST
5821 =item use Module VERSION
5823 =item use Module LIST
5829 Imports some semantics into the current package from the named module,
5830 generally by aliasing certain subroutine or variable names into your
5831 package. It is exactly equivalent to
5833 BEGIN { require Module; import Module LIST; }
5835 except that Module I<must> be a bareword.
5837 VERSION may be either a numeric argument such as 5.006, which will be
5838 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5839 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5840 greater than the version of the current Perl interpreter; Perl will not
5841 attempt to parse the rest of the file. Compare with L</require>, which can
5842 do a similar check at run time.
5844 Specifying VERSION as a literal of the form v5.6.1 should generally be
5845 avoided, because it leads to misleading error messages under earlier
5846 versions of Perl which do not support this syntax. The equivalent numeric
5847 version should be used instead.
5849 use v5.6.1; # compile time version check
5851 use 5.006_001; # ditto; preferred for backwards compatibility
5853 This is often useful if you need to check the current Perl version before
5854 C<use>ing library modules that have changed in incompatible ways from
5855 older versions of Perl. (We try not to do this more than we have to.)
5857 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5858 C<require> makes sure the module is loaded into memory if it hasn't been
5859 yet. The C<import> is not a builtin--it's just an ordinary static method
5860 call into the C<Module> package to tell the module to import the list of
5861 features back into the current package. The module can implement its
5862 C<import> method any way it likes, though most modules just choose to
5863 derive their C<import> method via inheritance from the C<Exporter> class that
5864 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5865 method can be found then the call is skipped.
5867 If you do not want to call the package's C<import> method (for instance,
5868 to stop your namespace from being altered), explicitly supply the empty list:
5872 That is exactly equivalent to
5874 BEGIN { require Module }
5876 If the VERSION argument is present between Module and LIST, then the
5877 C<use> will call the VERSION method in class Module with the given
5878 version as an argument. The default VERSION method, inherited from
5879 the UNIVERSAL class, croaks if the given version is larger than the
5880 value of the variable C<$Module::VERSION>.
5882 Again, there is a distinction between omitting LIST (C<import> called
5883 with no arguments) and an explicit empty LIST C<()> (C<import> not
5884 called). Note that there is no comma after VERSION!
5886 Because this is a wide-open interface, pragmas (compiler directives)
5887 are also implemented this way. Currently implemented pragmas are:
5892 use sigtrap qw(SEGV BUS);
5893 use strict qw(subs vars refs);
5894 use subs qw(afunc blurfl);
5895 use warnings qw(all);
5896 use sort qw(stable _quicksort _mergesort);
5898 Some of these pseudo-modules import semantics into the current
5899 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5900 which import symbols into the current package (which are effective
5901 through the end of the file).
5903 There's a corresponding C<no> command that unimports meanings imported
5904 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5910 If no C<unimport> method can be found the call fails with a fatal error.
5912 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5913 for the C<-M> and C<-m> command-line options to perl that give C<use>
5914 functionality from the command-line.
5918 Changes the access and modification times on each file of a list of
5919 files. The first two elements of the list must be the NUMERICAL access
5920 and modification times, in that order. Returns the number of files
5921 successfully changed. The inode change time of each file is set
5922 to the current time. This code has the same effect as the C<touch>
5923 command if the files already exist:
5927 utime $now, $now, @ARGV;
5929 If the first two elements of the list are C<undef>, then the utime(2)
5930 function in the C library will be called with a null second argument.
5931 On most systems, this will set the file's access and modification
5932 times to the current time. (i.e. equivalent to the example above.)
5934 utime undef, undef, @ARGV;
5938 Returns a list consisting of all the values of the named hash. (In a
5939 scalar context, returns the number of values.) The values are
5940 returned in an apparently random order. The actual random order is
5941 subject to change in future versions of perl, but it is guaranteed to
5942 be the same order as either the C<keys> or C<each> function would
5943 produce on the same (unmodified) hash.
5945 Note that the values are not copied, which means modifying them will
5946 modify the contents of the hash:
5948 for (values %hash) { s/foo/bar/g } # modifies %hash values
5949 for (@hash{keys %hash}) { s/foo/bar/g } # same
5951 As a side effect, calling values() resets the HASH's internal iterator.
5952 See also C<keys>, C<each>, and C<sort>.
5954 =item vec EXPR,OFFSET,BITS
5956 Treats the string in EXPR as a bit vector made up of elements of
5957 width BITS, and returns the value of the element specified by OFFSET
5958 as an unsigned integer. BITS therefore specifies the number of bits
5959 that are reserved for each element in the bit vector. This must
5960 be a power of two from 1 to 32 (or 64, if your platform supports
5963 If BITS is 8, "elements" coincide with bytes of the input string.
5965 If BITS is 16 or more, bytes of the input string are grouped into chunks
5966 of size BITS/8, and each group is converted to a number as with
5967 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5968 for BITS==64). See L<"pack"> for details.
5970 If bits is 4 or less, the string is broken into bytes, then the bits
5971 of each byte are broken into 8/BITS groups. Bits of a byte are
5972 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5973 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5974 breaking the single input byte C<chr(0x36)> into two groups gives a list
5975 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5977 C<vec> may also be assigned to, in which case parentheses are needed
5978 to give the expression the correct precedence as in
5980 vec($image, $max_x * $x + $y, 8) = 3;
5982 If the selected element is outside the string, the value 0 is returned.
5983 If an element off the end of the string is written to, Perl will first
5984 extend the string with sufficiently many zero bytes. It is an error
5985 to try to write off the beginning of the string (i.e. negative OFFSET).
5987 The string should not contain any character with the value > 255 (which
5988 can only happen if you're using UTF8 encoding). If it does, it will be
5989 treated as something which is not UTF8 encoded. When the C<vec> was
5990 assigned to, other parts of your program will also no longer consider the
5991 string to be UTF8 encoded. In other words, if you do have such characters
5992 in your string, vec() will operate on the actual byte string, and not the
5993 conceptual character string.
5995 Strings created with C<vec> can also be manipulated with the logical
5996 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5997 vector operation is desired when both operands are strings.
5998 See L<perlop/"Bitwise String Operators">.
6000 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6001 The comments show the string after each step. Note that this code works
6002 in the same way on big-endian or little-endian machines.
6005 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6007 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6008 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6010 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6011 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6012 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6013 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6014 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6015 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6017 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6018 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6019 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6022 To transform a bit vector into a string or list of 0's and 1's, use these:
6024 $bits = unpack("b*", $vector);
6025 @bits = split(//, unpack("b*", $vector));
6027 If you know the exact length in bits, it can be used in place of the C<*>.
6029 Here is an example to illustrate how the bits actually fall in place:
6035 unpack("V",$_) 01234567890123456789012345678901
6036 ------------------------------------------------------------------
6041 for ($shift=0; $shift < $width; ++$shift) {
6042 for ($off=0; $off < 32/$width; ++$off) {
6043 $str = pack("B*", "0"x32);
6044 $bits = (1<<$shift);
6045 vec($str, $off, $width) = $bits;
6046 $res = unpack("b*",$str);
6047 $val = unpack("V", $str);
6054 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6055 $off, $width, $bits, $val, $res
6059 Regardless of the machine architecture on which it is run, the above
6060 example should print the following table:
6063 unpack("V",$_) 01234567890123456789012345678901
6064 ------------------------------------------------------------------
6065 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6066 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6067 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6068 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6069 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6070 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6071 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6072 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6073 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6074 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6075 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6076 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6077 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6078 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6079 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6080 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6081 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6082 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6083 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6084 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6085 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6086 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6087 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6088 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6089 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6090 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6091 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6092 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6093 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6094 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6095 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6096 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6097 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6098 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6099 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6100 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6101 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6102 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6103 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6104 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6105 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6106 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6107 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6108 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6109 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6110 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6111 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6112 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6113 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6114 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6115 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6116 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6117 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6118 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6119 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6120 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6121 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6122 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6123 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6124 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6125 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6126 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6127 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6128 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6129 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6130 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6131 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6132 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6133 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6134 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6135 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6136 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6137 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6138 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6139 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6140 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6141 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6142 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6143 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6144 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6145 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6146 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6147 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6148 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6149 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6150 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6151 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6152 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6153 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6154 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6155 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6156 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6157 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6158 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6159 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6160 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6161 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6162 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6163 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6164 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6165 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6166 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6167 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6168 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6169 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6170 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6171 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6172 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6173 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6174 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6175 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6176 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6177 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6178 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6179 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6180 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6181 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6182 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6183 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6184 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6185 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6186 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6187 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6188 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6189 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6190 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6191 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6192 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6196 Behaves like the wait(2) system call on your system: it waits for a child
6197 process to terminate and returns the pid of the deceased process, or
6198 C<-1> if there are no child processes. The status is returned in C<$?>.
6199 Note that a return value of C<-1> could mean that child processes are
6200 being automatically reaped, as described in L<perlipc>.
6202 =item waitpid PID,FLAGS
6204 Waits for a particular child process to terminate and returns the pid of
6205 the deceased process, or C<-1> if there is no such child process. On some
6206 systems, a value of 0 indicates that there are processes still running.
6207 The status is returned in C<$?>. If you say
6209 use POSIX ":sys_wait_h";
6212 $kid = waitpid(-1, WNOHANG);
6215 then you can do a non-blocking wait for all pending zombie processes.
6216 Non-blocking wait is available on machines supporting either the
6217 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6218 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6219 system call by remembering the status values of processes that have
6220 exited but have not been harvested by the Perl script yet.)
6222 Note that on some systems, a return value of C<-1> could mean that child
6223 processes are being automatically reaped. See L<perlipc> for details,
6224 and for other examples.
6228 Returns true if the context of the currently executing subroutine is
6229 looking for a list value. Returns false if the context is looking
6230 for a scalar. Returns the undefined value if the context is looking
6231 for no value (void context).
6233 return unless defined wantarray; # don't bother doing more
6234 my @a = complex_calculation();
6235 return wantarray ? @a : "@a";
6237 This function should have been named wantlist() instead.
6241 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6244 If LIST is empty and C<$@> already contains a value (typically from a
6245 previous eval) that value is used after appending C<"\t...caught">
6246 to C<$@>. This is useful for staying almost, but not entirely similar to
6249 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6251 No message is printed if there is a C<$SIG{__WARN__}> handler
6252 installed. It is the handler's responsibility to deal with the message
6253 as it sees fit (like, for instance, converting it into a C<die>). Most
6254 handlers must therefore make arrangements to actually display the
6255 warnings that they are not prepared to deal with, by calling C<warn>
6256 again in the handler. Note that this is quite safe and will not
6257 produce an endless loop, since C<__WARN__> hooks are not called from
6260 You will find this behavior is slightly different from that of
6261 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6262 instead call C<die> again to change it).
6264 Using a C<__WARN__> handler provides a powerful way to silence all
6265 warnings (even the so-called mandatory ones). An example:
6267 # wipe out *all* compile-time warnings
6268 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6270 my $foo = 20; # no warning about duplicate my $foo,
6271 # but hey, you asked for it!
6272 # no compile-time or run-time warnings before here
6275 # run-time warnings enabled after here
6276 warn "\$foo is alive and $foo!"; # does show up
6278 See L<perlvar> for details on setting C<%SIG> entries, and for more
6279 examples. See the Carp module for other kinds of warnings using its
6280 carp() and cluck() functions.
6282 =item write FILEHANDLE
6288 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6289 using the format associated with that file. By default the format for
6290 a file is the one having the same name as the filehandle, but the
6291 format for the current output channel (see the C<select> function) may be set
6292 explicitly by assigning the name of the format to the C<$~> variable.
6294 Top of form processing is handled automatically: if there is
6295 insufficient room on the current page for the formatted record, the
6296 page is advanced by writing a form feed, a special top-of-page format
6297 is used to format the new page header, and then the record is written.
6298 By default the top-of-page format is the name of the filehandle with
6299 "_TOP" appended, but it may be dynamically set to the format of your
6300 choice by assigning the name to the C<$^> variable while the filehandle is
6301 selected. The number of lines remaining on the current page is in
6302 variable C<$->, which can be set to C<0> to force a new page.
6304 If FILEHANDLE is unspecified, output goes to the current default output
6305 channel, which starts out as STDOUT but may be changed by the
6306 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6307 is evaluated and the resulting string is used to look up the name of
6308 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6310 Note that write is I<not> the opposite of C<read>. Unfortunately.
6314 The transliteration operator. Same as C<tr///>. See L<perlop>.