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"
453 mode on systems where the run-time libraries distinguish between
454 binary and text files. If FILEHANDLE is an expression, the value is
455 taken as the name of the filehandle. Returns true on success,
458 DISCIPLINE can be either of C<:bytes> for "binary" mode or C<:crlf>
459 for "text" mode. If the DISCIPLINE is omitted, it defaults to
460 C<:bytes>. To mark FILEHANDLE as UTF-8, use C<:utf8>. For backward
461 compatibility C<binmode(FILEHANDLE)> also implicitly marks the
464 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
465 form C<:...>, are called I/O I<disciplines>. The C<open> pragma can
466 be used to establish default I/O disciplines. See L<open>.
468 The C<:raw> discipline is deprecated. (As opposed to what Camel III
469 said, it is not the inverse of C<:crlf>.) See L<perlrun> and the
470 discussion about the PERLIO environment variable.
472 In general, binmode() should be called after open() but before any I/O
473 is done on the filehandle. Calling binmode() will flush any possibly
474 pending buffered input or output data on the handle. The only
475 exception to this is the C<:encoding> discipline that changes
476 the default character encoding of the handle, see L<open>.
477 The C<:encoding> discipline sometimes needs to be called in
478 mid-stream, and it doesn't flush the stream.
480 On some systems (in general, DOS and Windows-based systems) binmode()
481 is necessary when you're not working with a text file. For the sake
482 of portability it is a good idea to always use it when appropriate,
483 and to never use it when it isn't appropriate.
485 In other words: regardless of platform, use binmode() on binary files
486 (like for example images), and do not use binmode() on text files.
488 The operating system, device drivers, C libraries, and Perl run-time
489 system all work together to let the programmer treat a single
490 character (C<\n>) as the line terminator, irrespective of the external
491 representation. On many operating systems, the native text file
492 representation matches the internal representation, but on some
493 platforms the external representation of C<\n> is made up of more than
496 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
497 character to end each line in the external representation of text (even
498 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
499 on Unix and most VMS files). Consequently binmode() has no effect on
500 these operating systems. In other systems like OS/2, DOS and the various
501 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
502 what's stored in text files are the two characters C<\cM\cJ>. That means
503 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
504 disk will be converted to C<\n> on input, and any C<\n> in your program
505 will be converted back to C<\cM\cJ> on output. This is what you want for
506 text files, but it can be disastrous for binary files.
508 Another consequence of using binmode() (on some systems) is that
509 special end-of-file markers will be seen as part of the data stream.
510 For systems from the Microsoft family this means that if your binary
511 data contains C<\cZ>, the I/O subsystem will regard it as the end of
512 the file, unless you use binmode().
514 binmode() is not only important for readline() and print() operations,
515 but also when using read(), seek(), sysread(), syswrite() and tell()
516 (see L<perlport> for more details). See the C<$/> and C<$\> variables
517 in L<perlvar> for how to manually set your input and output
518 line-termination sequences.
520 =item bless REF,CLASSNAME
524 This function tells the thingy referenced by REF that it is now an object
525 in the CLASSNAME package. If CLASSNAME is omitted, the current package
526 is used. Because a C<bless> is often the last thing in a constructor,
527 it returns the reference for convenience. Always use the two-argument
528 version if the function doing the blessing might be inherited by a
529 derived class. See L<perltoot> and L<perlobj> for more about the blessing
530 (and blessings) of objects.
532 Consider always blessing objects in CLASSNAMEs that are mixed case.
533 Namespaces with all lowercase names are considered reserved for
534 Perl pragmata. Builtin types have all uppercase names, so to prevent
535 confusion, you may wish to avoid such package names as well. Make sure
536 that CLASSNAME is a true value.
538 See L<perlmod/"Perl Modules">.
544 Returns the context of the current subroutine call. In scalar context,
545 returns the caller's package name if there is a caller, that is, if
546 we're in a subroutine or C<eval> or C<require>, and the undefined value
547 otherwise. In list context, returns
549 ($package, $filename, $line) = caller;
551 With EXPR, it returns some extra information that the debugger uses to
552 print a stack trace. The value of EXPR indicates how many call frames
553 to go back before the current one.
555 ($package, $filename, $line, $subroutine, $hasargs,
556 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
558 Here $subroutine may be C<(eval)> if the frame is not a subroutine
559 call, but an C<eval>. In such a case additional elements $evaltext and
560 C<$is_require> are set: C<$is_require> is true if the frame is created by a
561 C<require> or C<use> statement, $evaltext contains the text of the
562 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
563 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
564 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
565 frame.) $subroutine may also be C<(unknown)> if this particular
566 subroutine happens to have been deleted from the symbol table.
567 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
568 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
569 compiled with. The C<$hints> and C<$bitmask> values are subject to change
570 between versions of Perl, and are not meant for external use.
572 Furthermore, when called from within the DB package, caller returns more
573 detailed information: it sets the list variable C<@DB::args> to be the
574 arguments with which the subroutine was invoked.
576 Be aware that the optimizer might have optimized call frames away before
577 C<caller> had a chance to get the information. That means that C<caller(N)>
578 might not return information about the call frame you expect it do, for
579 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
580 previous time C<caller> was called.
584 Changes the working directory to EXPR, if possible. If EXPR is omitted,
585 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
586 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
587 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
588 neither is set, C<chdir> does nothing. It returns true upon success,
589 false otherwise. See the example under C<die>.
593 Changes the permissions of a list of files. The first element of the
594 list must be the numerical mode, which should probably be an octal
595 number, and which definitely should I<not> a string of octal digits:
596 C<0644> is okay, C<'0644'> is not. Returns the number of files
597 successfully changed. See also L</oct>, if all you have is a string.
599 $cnt = chmod 0755, 'foo', 'bar';
600 chmod 0755, @executables;
601 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
603 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
604 $mode = 0644; chmod $mode, 'foo'; # this is best
606 You can also import the symbolic C<S_I*> constants from the Fcntl
611 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
612 # This is identical to the chmod 0755 of the above example.
620 This safer version of L</chop> removes any trailing string
621 that corresponds to the current value of C<$/> (also known as
622 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
623 number of characters removed from all its arguments. It's often used to
624 remove the newline from the end of an input record when you're worried
625 that the final record may be missing its newline. When in paragraph
626 mode (C<$/ = "">), it removes all trailing newlines from the string.
627 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
628 a reference to an integer or the like, see L<perlvar>) chomp() won't
630 If VARIABLE is omitted, it chomps C<$_>. Example:
633 chomp; # avoid \n on last field
638 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
640 You can actually chomp anything that's an lvalue, including an assignment:
643 chomp($answer = <STDIN>);
645 If you chomp a list, each element is chomped, and the total number of
646 characters removed is returned.
654 Chops off the last character of a string and returns the character
655 chopped. It is much more efficient than C<s/.$//s> because it neither
656 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
657 If VARIABLE is a hash, it chops the hash's values, but not its keys.
659 You can actually chop anything that's an lvalue, including an assignment.
661 If you chop a list, each element is chopped. Only the value of the
662 last C<chop> is returned.
664 Note that C<chop> returns the last character. To return all but the last
665 character, use C<substr($string, 0, -1)>.
669 Changes the owner (and group) of a list of files. The first two
670 elements of the list must be the I<numeric> uid and gid, in that
671 order. A value of -1 in either position is interpreted by most
672 systems to leave that value unchanged. Returns the number of files
673 successfully changed.
675 $cnt = chown $uid, $gid, 'foo', 'bar';
676 chown $uid, $gid, @filenames;
678 Here's an example that looks up nonnumeric uids in the passwd file:
681 chomp($user = <STDIN>);
683 chomp($pattern = <STDIN>);
685 ($login,$pass,$uid,$gid) = getpwnam($user)
686 or die "$user not in passwd file";
688 @ary = glob($pattern); # expand filenames
689 chown $uid, $gid, @ary;
691 On most systems, you are not allowed to change the ownership of the
692 file unless you're the superuser, although you should be able to change
693 the group to any of your secondary groups. On insecure systems, these
694 restrictions may be relaxed, but this is not a portable assumption.
695 On POSIX systems, you can detect this condition this way:
697 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
698 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
704 Returns the character represented by that NUMBER in the character set.
705 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
706 chr(0x263a) is a Unicode smiley face. Note that characters from 127
707 to 255 (inclusive) are by default not encoded in Unicode for backward
708 compatibility reasons (but see L<encoding>).
710 For the reverse, use L</ord>.
711 See L<perlunicode> and L<encoding> for more about Unicode.
713 If NUMBER is omitted, uses C<$_>.
715 =item chroot FILENAME
719 This function works like the system call by the same name: it makes the
720 named directory the new root directory for all further pathnames that
721 begin with a C</> by your process and all its children. (It doesn't
722 change your current working directory, which is unaffected.) For security
723 reasons, this call is restricted to the superuser. If FILENAME is
724 omitted, does a C<chroot> to C<$_>.
726 =item close FILEHANDLE
730 Closes the file or pipe associated with the file handle, returning
731 true only if IO buffers are successfully flushed and closes the system
732 file descriptor. Closes the currently selected filehandle if the
735 You don't have to close FILEHANDLE if you are immediately going to do
736 another C<open> on it, because C<open> will close it for you. (See
737 C<open>.) However, an explicit C<close> on an input file resets the line
738 counter (C<$.>), while the implicit close done by C<open> does not.
740 If the file handle came from a piped open C<close> will additionally
741 return false if one of the other system calls involved fails or if the
742 program exits with non-zero status. (If the only problem was that the
743 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
744 also waits for the process executing on the pipe to complete, in case you
745 want to look at the output of the pipe afterwards, and
746 implicitly puts the exit status value of that command into C<$?>.
748 Prematurely closing the read end of a pipe (i.e. before the process
749 writing to it at the other end has closed it) will result in a
750 SIGPIPE being delivered to the writer. If the other end can't
751 handle that, be sure to read all the data before closing the pipe.
755 open(OUTPUT, '|sort >foo') # pipe to sort
756 or die "Can't start sort: $!";
757 #... # print stuff to output
758 close OUTPUT # wait for sort to finish
759 or warn $! ? "Error closing sort pipe: $!"
760 : "Exit status $? from sort";
761 open(INPUT, 'foo') # get sort's results
762 or die "Can't open 'foo' for input: $!";
764 FILEHANDLE may be an expression whose value can be used as an indirect
765 filehandle, usually the real filehandle name.
767 =item closedir DIRHANDLE
769 Closes a directory opened by C<opendir> and returns the success of that
772 DIRHANDLE may be an expression whose value can be used as an indirect
773 dirhandle, usually the real dirhandle name.
775 =item connect SOCKET,NAME
777 Attempts to connect to a remote socket, just as the connect system call
778 does. Returns true if it succeeded, false otherwise. NAME should be a
779 packed address of the appropriate type for the socket. See the examples in
780 L<perlipc/"Sockets: Client/Server Communication">.
784 Actually a flow control statement rather than a function. If there is a
785 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
786 C<foreach>), it is always executed just before the conditional is about to
787 be evaluated again, just like the third part of a C<for> loop in C. Thus
788 it can be used to increment a loop variable, even when the loop has been
789 continued via the C<next> statement (which is similar to the C C<continue>
792 C<last>, C<next>, or C<redo> may appear within a C<continue>
793 block. C<last> and C<redo> will behave as if they had been executed within
794 the main block. So will C<next>, but since it will execute a C<continue>
795 block, it may be more entertaining.
798 ### redo always comes here
801 ### next always comes here
803 # then back the top to re-check EXPR
805 ### last always comes here
807 Omitting the C<continue> section is semantically equivalent to using an
808 empty one, logically enough. In that case, C<next> goes directly back
809 to check the condition at the top of the loop.
815 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
816 takes cosine of C<$_>.
818 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
819 function, or use this relation:
821 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
823 =item crypt PLAINTEXT,SALT
825 Encrypts a string exactly like the crypt(3) function in the C library
826 (assuming that you actually have a version there that has not been
827 extirpated as a potential munition). This can prove useful for checking
828 the password file for lousy passwords, amongst other things. Only the
829 guys wearing white hats should do this.
831 Note that C<crypt> is intended to be a one-way function, much like
832 breaking eggs to make an omelette. There is no (known) corresponding
833 decrypt function (in other words, the crypt() is a one-way hash
834 function). As a result, this function isn't all that useful for
835 cryptography. (For that, see your nearby CPAN mirror.)
837 When verifying an existing encrypted string you should use the
838 encrypted text as the salt (like C<crypt($plain, $crypted) eq
839 $crypted>). This allows your code to work with the standard C<crypt>
840 and with more exotic implementations. In other words, do not assume
841 anything about the returned string itself, or how many bytes in
842 the encrypted string matter.
844 Traditionally the result is a string of 13 bytes: two first bytes of
845 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
846 the first eight bytes of the encrypted string mattered, but
847 alternative hashing schemes (like MD5), higher level security schemes
848 (like C2), and implementations on non-UNIX platforms may produce
851 When choosing a new salt create a random two character string whose
852 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
853 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
855 Here's an example that makes sure that whoever runs this program knows
858 $pwd = (getpwuid($<))[1];
862 chomp($word = <STDIN>);
866 if (crypt($word, $pwd) ne $pwd) {
872 Of course, typing in your own password to whoever asks you
875 The L<crypt> function is unsuitable for encrypting large quantities
876 of data, not least of all because you can't get the information
877 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
878 on your favorite CPAN mirror for a slew of potentially useful
881 If using crypt() on a Unicode string (which I<potentially> has
882 characters with codepoints above 255), Perl tries to make sense
883 of the situation by trying to downgrade (a copy of the string)
884 the string back to an eight-bit byte string before calling crypt()
885 (on that copy). If that works, good. If not, crypt() dies with
886 C<Wide character in crypt>.
890 [This function has been largely superseded by the C<untie> function.]
892 Breaks the binding between a DBM file and a hash.
894 =item dbmopen HASH,DBNAME,MASK
896 [This function has been largely superseded by the C<tie> function.]
898 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
899 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
900 argument is I<not> a filehandle, even though it looks like one). DBNAME
901 is the name of the database (without the F<.dir> or F<.pag> extension if
902 any). If the database does not exist, it is created with protection
903 specified by MASK (as modified by the C<umask>). If your system supports
904 only the older DBM functions, you may perform only one C<dbmopen> in your
905 program. In older versions of Perl, if your system had neither DBM nor
906 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
909 If you don't have write access to the DBM file, you can only read hash
910 variables, not set them. If you want to test whether you can write,
911 either use file tests or try setting a dummy hash entry inside an C<eval>,
912 which will trap the error.
914 Note that functions such as C<keys> and C<values> may return huge lists
915 when used on large DBM files. You may prefer to use the C<each>
916 function to iterate over large DBM files. Example:
918 # print out history file offsets
919 dbmopen(%HIST,'/usr/lib/news/history',0666);
920 while (($key,$val) = each %HIST) {
921 print $key, ' = ', unpack('L',$val), "\n";
925 See also L<AnyDBM_File> for a more general description of the pros and
926 cons of the various dbm approaches, as well as L<DB_File> for a particularly
929 You can control which DBM library you use by loading that library
930 before you call dbmopen():
933 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
934 or die "Can't open netscape history file: $!";
940 Returns a Boolean value telling whether EXPR has a value other than
941 the undefined value C<undef>. If EXPR is not present, C<$_> will be
944 Many operations return C<undef> to indicate failure, end of file,
945 system error, uninitialized variable, and other exceptional
946 conditions. This function allows you to distinguish C<undef> from
947 other values. (A simple Boolean test will not distinguish among
948 C<undef>, zero, the empty string, and C<"0">, which are all equally
949 false.) Note that since C<undef> is a valid scalar, its presence
950 doesn't I<necessarily> indicate an exceptional condition: C<pop>
951 returns C<undef> when its argument is an empty array, I<or> when the
952 element to return happens to be C<undef>.
954 You may also use C<defined(&func)> to check whether subroutine C<&func>
955 has ever been defined. The return value is unaffected by any forward
956 declarations of C<&foo>. Note that a subroutine which is not defined
957 may still be callable: its package may have an C<AUTOLOAD> method that
958 makes it spring into existence the first time that it is called -- see
961 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
962 used to report whether memory for that aggregate has ever been
963 allocated. This behavior may disappear in future versions of Perl.
964 You should instead use a simple test for size:
966 if (@an_array) { print "has array elements\n" }
967 if (%a_hash) { print "has hash members\n" }
969 When used on a hash element, it tells you whether the value is defined,
970 not whether the key exists in the hash. Use L</exists> for the latter
975 print if defined $switch{'D'};
976 print "$val\n" while defined($val = pop(@ary));
977 die "Can't readlink $sym: $!"
978 unless defined($value = readlink $sym);
979 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
980 $debugging = 0 unless defined $debugging;
982 Note: Many folks tend to overuse C<defined>, and then are surprised to
983 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
984 defined values. For example, if you say
988 The pattern match succeeds, and C<$1> is defined, despite the fact that it
989 matched "nothing". But it didn't really match nothing--rather, it
990 matched something that happened to be zero characters long. This is all
991 very above-board and honest. When a function returns an undefined value,
992 it's an admission that it couldn't give you an honest answer. So you
993 should use C<defined> only when you're questioning the integrity of what
994 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
997 See also L</undef>, L</exists>, L</ref>.
1001 Given an expression that specifies a hash element, array element, hash slice,
1002 or array slice, deletes the specified element(s) from the hash or array.
1003 In the case of an array, if the array elements happen to be at the end,
1004 the size of the array will shrink to the highest element that tests
1005 true for exists() (or 0 if no such element exists).
1007 Returns each element so deleted or the undefined value if there was no such
1008 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1009 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1010 from a C<tie>d hash or array may not necessarily return anything.
1012 Deleting an array element effectively returns that position of the array
1013 to its initial, uninitialized state. Subsequently testing for the same
1014 element with exists() will return false. Note that deleting array
1015 elements in the middle of an array will not shift the index of the ones
1016 after them down--use splice() for that. See L</exists>.
1018 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1020 foreach $key (keys %HASH) {
1024 foreach $index (0 .. $#ARRAY) {
1025 delete $ARRAY[$index];
1030 delete @HASH{keys %HASH};
1032 delete @ARRAY[0 .. $#ARRAY];
1034 But both of these are slower than just assigning the empty list
1035 or undefining %HASH or @ARRAY:
1037 %HASH = (); # completely empty %HASH
1038 undef %HASH; # forget %HASH ever existed
1040 @ARRAY = (); # completely empty @ARRAY
1041 undef @ARRAY; # forget @ARRAY ever existed
1043 Note that the EXPR can be arbitrarily complicated as long as the final
1044 operation is a hash element, array element, hash slice, or array slice
1047 delete $ref->[$x][$y]{$key};
1048 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1050 delete $ref->[$x][$y][$index];
1051 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1055 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1056 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1057 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1058 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1059 an C<eval(),> the error message is stuffed into C<$@> and the
1060 C<eval> is terminated with the undefined value. This makes
1061 C<die> the way to raise an exception.
1063 Equivalent examples:
1065 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1066 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1068 If the last element of LIST does not end in a newline, the current
1069 script line number and input line number (if any) are also printed,
1070 and a newline is supplied. Note that the "input line number" (also
1071 known as "chunk") is subject to whatever notion of "line" happens to
1072 be currently in effect, and is also available as the special variable
1073 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1075 Hint: sometimes appending C<", stopped"> to your message will cause it
1076 to make better sense when the string C<"at foo line 123"> is appended.
1077 Suppose you are running script "canasta".
1079 die "/etc/games is no good";
1080 die "/etc/games is no good, stopped";
1082 produce, respectively
1084 /etc/games is no good at canasta line 123.
1085 /etc/games is no good, stopped at canasta line 123.
1087 See also exit(), warn(), and the Carp module.
1089 If LIST is empty and C<$@> already contains a value (typically from a
1090 previous eval) that value is reused after appending C<"\t...propagated">.
1091 This is useful for propagating exceptions:
1094 die unless $@ =~ /Expected exception/;
1096 If LIST is empty and C<$@> contains an object reference that has a
1097 C<PROPAGATE> method, that method will be called with additional file
1098 and line number parameters. The return value replaces the value in
1099 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1102 If C<$@> is empty then the string C<"Died"> is used.
1104 die() can also be called with a reference argument. If this happens to be
1105 trapped within an eval(), $@ contains the reference. This behavior permits
1106 a more elaborate exception handling implementation using objects that
1107 maintain arbitrary state about the nature of the exception. Such a scheme
1108 is sometimes preferable to matching particular string values of $@ using
1109 regular expressions. Here's an example:
1111 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1113 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1114 # handle Some::Module::Exception
1117 # handle all other possible exceptions
1121 Because perl will stringify uncaught exception messages before displaying
1122 them, you may want to overload stringification operations on such custom
1123 exception objects. See L<overload> for details about that.
1125 You can arrange for a callback to be run just before the C<die>
1126 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1127 handler will be called with the error text and can change the error
1128 message, if it sees fit, by calling C<die> again. See
1129 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1130 L<"eval BLOCK"> for some examples. Although this feature was meant
1131 to be run only right before your program was to exit, this is not
1132 currently the case--the C<$SIG{__DIE__}> hook is currently called
1133 even inside eval()ed blocks/strings! If one wants the hook to do
1134 nothing in such situations, put
1138 as the first line of the handler (see L<perlvar/$^S>). Because
1139 this promotes strange action at a distance, this counterintuitive
1140 behavior may be fixed in a future release.
1144 Not really a function. Returns the value of the last command in the
1145 sequence of commands indicated by BLOCK. When modified by a loop
1146 modifier, executes the BLOCK once before testing the loop condition.
1147 (On other statements the loop modifiers test the conditional first.)
1149 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1150 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1151 See L<perlsyn> for alternative strategies.
1153 =item do SUBROUTINE(LIST)
1155 A deprecated form of subroutine call. See L<perlsub>.
1159 Uses the value of EXPR as a filename and executes the contents of the
1160 file as a Perl script. Its primary use is to include subroutines
1161 from a Perl subroutine library.
1169 except that it's more efficient and concise, keeps track of the current
1170 filename for error messages, searches the @INC libraries, and updates
1171 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1172 variables. It also differs in that code evaluated with C<do FILENAME>
1173 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1174 same, however, in that it does reparse the file every time you call it,
1175 so you probably don't want to do this inside a loop.
1177 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1178 error. If C<do> can read the file but cannot compile it, it
1179 returns undef and sets an error message in C<$@>. If the file is
1180 successfully compiled, C<do> returns the value of the last expression
1183 Note that inclusion of library modules is better done with the
1184 C<use> and C<require> operators, which also do automatic error checking
1185 and raise an exception if there's a problem.
1187 You might like to use C<do> to read in a program configuration
1188 file. Manual error checking can be done this way:
1190 # read in config files: system first, then user
1191 for $file ("/share/prog/defaults.rc",
1192 "$ENV{HOME}/.someprogrc")
1194 unless ($return = do $file) {
1195 warn "couldn't parse $file: $@" if $@;
1196 warn "couldn't do $file: $!" unless defined $return;
1197 warn "couldn't run $file" unless $return;
1205 This function causes an immediate core dump. See also the B<-u>
1206 command-line switch in L<perlrun>, which does the same thing.
1207 Primarily this is so that you can use the B<undump> program (not
1208 supplied) to turn your core dump into an executable binary after
1209 having initialized all your variables at the beginning of the
1210 program. When the new binary is executed it will begin by executing
1211 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1212 Think of it as a goto with an intervening core dump and reincarnation.
1213 If C<LABEL> is omitted, restarts the program from the top.
1215 B<WARNING>: Any files opened at the time of the dump will I<not>
1216 be open any more when the program is reincarnated, with possible
1217 resulting confusion on the part of Perl.
1219 This function is now largely obsolete, partly because it's very
1220 hard to convert a core file into an executable, and because the
1221 real compiler backends for generating portable bytecode and compilable
1222 C code have superseded it. That's why you should now invoke it as
1223 C<CORE::dump()>, if you don't want to be warned against a possible
1226 If you're looking to use L<dump> to speed up your program, consider
1227 generating bytecode or native C code as described in L<perlcc>. If
1228 you're just trying to accelerate a CGI script, consider using the
1229 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1230 You might also consider autoloading or selfloading, which at least
1231 make your program I<appear> to run faster.
1235 When called in list context, returns a 2-element list consisting of the
1236 key and value for the next element of a hash, so that you can iterate over
1237 it. When called in scalar context, returns only the key for the next
1238 element in the hash.
1240 Entries are returned in an apparently random order. The actual random
1241 order is subject to change in future versions of perl, but it is guaranteed
1242 to be in the same order as either the C<keys> or C<values> function
1243 would produce on the same (unmodified) hash.
1245 When the hash is entirely read, a null array is returned in list context
1246 (which when assigned produces a false (C<0>) value), and C<undef> in
1247 scalar context. The next call to C<each> after that will start iterating
1248 again. There is a single iterator for each hash, shared by all C<each>,
1249 C<keys>, and C<values> function calls in the program; it can be reset by
1250 reading all the elements from the hash, or by evaluating C<keys HASH> or
1251 C<values HASH>. If you add or delete elements of a hash while you're
1252 iterating over it, you may get entries skipped or duplicated, so
1253 don't. Exception: It is always safe to delete the item most recently
1254 returned by C<each()>, which means that the following code will work:
1256 while (($key, $value) = each %hash) {
1258 delete $hash{$key}; # This is safe
1261 The following prints out your environment like the printenv(1) program,
1262 only in a different order:
1264 while (($key,$value) = each %ENV) {
1265 print "$key=$value\n";
1268 See also C<keys>, C<values> and C<sort>.
1270 =item eof FILEHANDLE
1276 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1277 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1278 gives the real filehandle. (Note that this function actually
1279 reads a character and then C<ungetc>s it, so isn't very useful in an
1280 interactive context.) Do not read from a terminal file (or call
1281 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1282 as terminals may lose the end-of-file condition if you do.
1284 An C<eof> without an argument uses the last file read. Using C<eof()>
1285 with empty parentheses is very different. It refers to the pseudo file
1286 formed from the files listed on the command line and accessed via the
1287 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1288 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1289 used will cause C<@ARGV> to be examined to determine if input is
1290 available. Similarly, an C<eof()> after C<< <> >> has returned
1291 end-of-file will assume you are processing another C<@ARGV> list,
1292 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1293 see L<perlop/"I/O Operators">.
1295 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1296 detect the end of each file, C<eof()> will only detect the end of the
1297 last file. Examples:
1299 # reset line numbering on each input file
1301 next if /^\s*#/; # skip comments
1304 close ARGV if eof; # Not eof()!
1307 # insert dashes just before last line of last file
1309 if (eof()) { # check for end of current file
1310 print "--------------\n";
1311 close(ARGV); # close or last; is needed if we
1312 # are reading from the terminal
1317 Practical hint: you almost never need to use C<eof> in Perl, because the
1318 input operators typically return C<undef> when they run out of data, or if
1325 In the first form, the return value of EXPR is parsed and executed as if it
1326 were a little Perl program. The value of the expression (which is itself
1327 determined within scalar context) is first parsed, and if there weren't any
1328 errors, executed in the lexical context of the current Perl program, so
1329 that any variable settings or subroutine and format definitions remain
1330 afterwards. Note that the value is parsed every time the eval executes.
1331 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1332 delay parsing and subsequent execution of the text of EXPR until run time.
1334 In the second form, the code within the BLOCK is parsed only once--at the
1335 same time the code surrounding the eval itself was parsed--and executed
1336 within the context of the current Perl program. This form is typically
1337 used to trap exceptions more efficiently than the first (see below), while
1338 also providing the benefit of checking the code within BLOCK at compile
1341 The final semicolon, if any, may be omitted from the value of EXPR or within
1344 In both forms, the value returned is the value of the last expression
1345 evaluated inside the mini-program; a return statement may be also used, just
1346 as with subroutines. The expression providing the return value is evaluated
1347 in void, scalar, or list context, depending on the context of the eval itself.
1348 See L</wantarray> for more on how the evaluation context can be determined.
1350 If there is a syntax error or runtime error, or a C<die> statement is
1351 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1352 error message. If there was no error, C<$@> is guaranteed to be a null
1353 string. Beware that using C<eval> neither silences perl from printing
1354 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1355 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1356 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1357 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1359 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1360 determining whether a particular feature (such as C<socket> or C<symlink>)
1361 is implemented. It is also Perl's exception trapping mechanism, where
1362 the die operator is used to raise exceptions.
1364 If the code to be executed doesn't vary, you may use the eval-BLOCK
1365 form to trap run-time errors without incurring the penalty of
1366 recompiling each time. The error, if any, is still returned in C<$@>.
1369 # make divide-by-zero nonfatal
1370 eval { $answer = $a / $b; }; warn $@ if $@;
1372 # same thing, but less efficient
1373 eval '$answer = $a / $b'; warn $@ if $@;
1375 # a compile-time error
1376 eval { $answer = }; # WRONG
1379 eval '$answer ='; # sets $@
1381 Due to the current arguably broken state of C<__DIE__> hooks, when using
1382 the C<eval{}> form as an exception trap in libraries, you may wish not
1383 to trigger any C<__DIE__> hooks that user code may have installed.
1384 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1385 as shown in this example:
1387 # a very private exception trap for divide-by-zero
1388 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1391 This is especially significant, given that C<__DIE__> hooks can call
1392 C<die> again, which has the effect of changing their error messages:
1394 # __DIE__ hooks may modify error messages
1396 local $SIG{'__DIE__'} =
1397 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1398 eval { die "foo lives here" };
1399 print $@ if $@; # prints "bar lives here"
1402 Because this promotes action at a distance, this counterintuitive behavior
1403 may be fixed in a future release.
1405 With an C<eval>, you should be especially careful to remember what's
1406 being looked at when:
1412 eval { $x }; # CASE 4
1414 eval "\$$x++"; # CASE 5
1417 Cases 1 and 2 above behave identically: they run the code contained in
1418 the variable $x. (Although case 2 has misleading double quotes making
1419 the reader wonder what else might be happening (nothing is).) Cases 3
1420 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1421 does nothing but return the value of $x. (Case 4 is preferred for
1422 purely visual reasons, but it also has the advantage of compiling at
1423 compile-time instead of at run-time.) Case 5 is a place where
1424 normally you I<would> like to use double quotes, except that in this
1425 particular situation, you can just use symbolic references instead, as
1428 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1429 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1433 =item exec PROGRAM LIST
1435 The C<exec> function executes a system command I<and never returns>--
1436 use C<system> instead of C<exec> if you want it to return. It fails and
1437 returns false only if the command does not exist I<and> it is executed
1438 directly instead of via your system's command shell (see below).
1440 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1441 warns you if there is a following statement which isn't C<die>, C<warn>,
1442 or C<exit> (if C<-w> is set - but you always do that). If you
1443 I<really> want to follow an C<exec> with some other statement, you
1444 can use one of these styles to avoid the warning:
1446 exec ('foo') or print STDERR "couldn't exec foo: $!";
1447 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1449 If there is more than one argument in LIST, or if LIST is an array
1450 with more than one value, calls execvp(3) with the arguments in LIST.
1451 If there is only one scalar argument or an array with one element in it,
1452 the argument is checked for shell metacharacters, and if there are any,
1453 the entire argument is passed to the system's command shell for parsing
1454 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1455 If there are no shell metacharacters in the argument, it is split into
1456 words and passed directly to C<execvp>, which is more efficient.
1459 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1460 exec "sort $outfile | uniq";
1462 If you don't really want to execute the first argument, but want to lie
1463 to the program you are executing about its own name, you can specify
1464 the program you actually want to run as an "indirect object" (without a
1465 comma) in front of the LIST. (This always forces interpretation of the
1466 LIST as a multivalued list, even if there is only a single scalar in
1469 $shell = '/bin/csh';
1470 exec $shell '-sh'; # pretend it's a login shell
1474 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1476 When the arguments get executed via the system shell, results will
1477 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1480 Using an indirect object with C<exec> or C<system> is also more
1481 secure. This usage (which also works fine with system()) forces
1482 interpretation of the arguments as a multivalued list, even if the
1483 list had just one argument. That way you're safe from the shell
1484 expanding wildcards or splitting up words with whitespace in them.
1486 @args = ( "echo surprise" );
1488 exec @args; # subject to shell escapes
1490 exec { $args[0] } @args; # safe even with one-arg list
1492 The first version, the one without the indirect object, ran the I<echo>
1493 program, passing it C<"surprise"> an argument. The second version
1494 didn't--it tried to run a program literally called I<"echo surprise">,
1495 didn't find it, and set C<$?> to a non-zero value indicating failure.
1497 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1498 output before the exec, but this may not be supported on some platforms
1499 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1500 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1501 open handles in order to avoid lost output.
1503 Note that C<exec> will not call your C<END> blocks, nor will it call
1504 any C<DESTROY> methods in your objects.
1508 Given an expression that specifies a hash element or array element,
1509 returns true if the specified element in the hash or array has ever
1510 been initialized, even if the corresponding value is undefined. The
1511 element is not autovivified if it doesn't exist.
1513 print "Exists\n" if exists $hash{$key};
1514 print "Defined\n" if defined $hash{$key};
1515 print "True\n" if $hash{$key};
1517 print "Exists\n" if exists $array[$index];
1518 print "Defined\n" if defined $array[$index];
1519 print "True\n" if $array[$index];
1521 A hash or array element can be true only if it's defined, and defined if
1522 it exists, but the reverse doesn't necessarily hold true.
1524 Given an expression that specifies the name of a subroutine,
1525 returns true if the specified subroutine has ever been declared, even
1526 if it is undefined. Mentioning a subroutine name for exists or defined
1527 does not count as declaring it. Note that a subroutine which does not
1528 exist may still be callable: its package may have an C<AUTOLOAD>
1529 method that makes it spring into existence the first time that it is
1530 called -- see L<perlsub>.
1532 print "Exists\n" if exists &subroutine;
1533 print "Defined\n" if defined &subroutine;
1535 Note that the EXPR can be arbitrarily complicated as long as the final
1536 operation is a hash or array key lookup or subroutine name:
1538 if (exists $ref->{A}->{B}->{$key}) { }
1539 if (exists $hash{A}{B}{$key}) { }
1541 if (exists $ref->{A}->{B}->[$ix]) { }
1542 if (exists $hash{A}{B}[$ix]) { }
1544 if (exists &{$ref->{A}{B}{$key}}) { }
1546 Although the deepest nested array or hash will not spring into existence
1547 just because its existence was tested, any intervening ones will.
1548 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1549 into existence due to the existence test for the $key element above.
1550 This happens anywhere the arrow operator is used, including even:
1553 if (exists $ref->{"Some key"}) { }
1554 print $ref; # prints HASH(0x80d3d5c)
1556 This surprising autovivification in what does not at first--or even
1557 second--glance appear to be an lvalue context may be fixed in a future
1560 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1561 on how exists() acts when used on a pseudo-hash.
1563 Use of a subroutine call, rather than a subroutine name, as an argument
1564 to exists() is an error.
1567 exists &sub(); # Error
1571 Evaluates EXPR and exits immediately with that value. Example:
1574 exit 0 if $ans =~ /^[Xx]/;
1576 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1577 universally recognized values for EXPR are C<0> for success and C<1>
1578 for error; other values are subject to interpretation depending on the
1579 environment in which the Perl program is running. For example, exiting
1580 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1581 the mailer to return the item undelivered, but that's not true everywhere.
1583 Don't use C<exit> to abort a subroutine if there's any chance that
1584 someone might want to trap whatever error happened. Use C<die> instead,
1585 which can be trapped by an C<eval>.
1587 The exit() function does not always exit immediately. It calls any
1588 defined C<END> routines first, but these C<END> routines may not
1589 themselves abort the exit. Likewise any object destructors that need to
1590 be called are called before the real exit. If this is a problem, you
1591 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1592 See L<perlmod> for details.
1598 Returns I<e> (the natural logarithm base) to the power of EXPR.
1599 If EXPR is omitted, gives C<exp($_)>.
1601 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1603 Implements the fcntl(2) function. You'll probably have to say
1607 first to get the correct constant definitions. Argument processing and
1608 value return works just like C<ioctl> below.
1612 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1613 or die "can't fcntl F_GETFL: $!";
1615 You don't have to check for C<defined> on the return from C<fnctl>.
1616 Like C<ioctl>, it maps a C<0> return from the system call into
1617 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1618 in numeric context. It is also exempt from the normal B<-w> warnings
1619 on improper numeric conversions.
1621 Note that C<fcntl> will produce a fatal error if used on a machine that
1622 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1623 manpage to learn what functions are available on your system.
1625 =item fileno FILEHANDLE
1627 Returns the file descriptor for a filehandle, or undefined if the
1628 filehandle is not open. This is mainly useful for constructing
1629 bitmaps for C<select> and low-level POSIX tty-handling operations.
1630 If FILEHANDLE is an expression, the value is taken as an indirect
1631 filehandle, generally its name.
1633 You can use this to find out whether two handles refer to the
1634 same underlying descriptor:
1636 if (fileno(THIS) == fileno(THAT)) {
1637 print "THIS and THAT are dups\n";
1640 (Filehandles connected to memory objects via new features of C<open> may
1641 return undefined even though they are open.)
1644 =item flock FILEHANDLE,OPERATION
1646 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1647 for success, false on failure. Produces a fatal error if used on a
1648 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1649 C<flock> is Perl's portable file locking interface, although it locks
1650 only entire files, not records.
1652 Two potentially non-obvious but traditional C<flock> semantics are
1653 that it waits indefinitely until the lock is granted, and that its locks
1654 B<merely advisory>. Such discretionary locks are more flexible, but offer
1655 fewer guarantees. This means that files locked with C<flock> may be
1656 modified by programs that do not also use C<flock>. See L<perlport>,
1657 your port's specific documentation, or your system-specific local manpages
1658 for details. It's best to assume traditional behavior if you're writing
1659 portable programs. (But if you're not, you should as always feel perfectly
1660 free to write for your own system's idiosyncrasies (sometimes called
1661 "features"). Slavish adherence to portability concerns shouldn't get
1662 in the way of your getting your job done.)
1664 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1665 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1666 you can use the symbolic names if you import them from the Fcntl module,
1667 either individually, or as a group using the ':flock' tag. LOCK_SH
1668 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1669 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1670 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1671 waiting for the lock (check the return status to see if you got it).
1673 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1674 before locking or unlocking it.
1676 Note that the emulation built with lockf(3) doesn't provide shared
1677 locks, and it requires that FILEHANDLE be open with write intent. These
1678 are the semantics that lockf(3) implements. Most if not all systems
1679 implement lockf(3) in terms of fcntl(2) locking, though, so the
1680 differing semantics shouldn't bite too many people.
1682 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1683 be open with read intent to use LOCK_SH and requires that it be open
1684 with write intent to use LOCK_EX.
1686 Note also that some versions of C<flock> cannot lock things over the
1687 network; you would need to use the more system-specific C<fcntl> for
1688 that. If you like you can force Perl to ignore your system's flock(2)
1689 function, and so provide its own fcntl(2)-based emulation, by passing
1690 the switch C<-Ud_flock> to the F<Configure> program when you configure
1693 Here's a mailbox appender for BSD systems.
1695 use Fcntl ':flock'; # import LOCK_* constants
1698 flock(MBOX,LOCK_EX);
1699 # and, in case someone appended
1700 # while we were waiting...
1705 flock(MBOX,LOCK_UN);
1708 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1709 or die "Can't open mailbox: $!";
1712 print MBOX $msg,"\n\n";
1715 On systems that support a real flock(), locks are inherited across fork()
1716 calls, whereas those that must resort to the more capricious fcntl()
1717 function lose the locks, making it harder to write servers.
1719 See also L<DB_File> for other flock() examples.
1723 Does a fork(2) system call to create a new process running the
1724 same program at the same point. It returns the child pid to the
1725 parent process, C<0> to the child process, or C<undef> if the fork is
1726 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1727 are shared, while everything else is copied. On most systems supporting
1728 fork(), great care has gone into making it extremely efficient (for
1729 example, using copy-on-write technology on data pages), making it the
1730 dominant paradigm for multitasking over the last few decades.
1732 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1733 output before forking the child process, but this may not be supported
1734 on some platforms (see L<perlport>). To be safe, you may need to set
1735 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1736 C<IO::Handle> on any open handles in order to avoid duplicate output.
1738 If you C<fork> without ever waiting on your children, you will
1739 accumulate zombies. On some systems, you can avoid this by setting
1740 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1741 forking and reaping moribund children.
1743 Note that if your forked child inherits system file descriptors like
1744 STDIN and STDOUT that are actually connected by a pipe or socket, even
1745 if you exit, then the remote server (such as, say, a CGI script or a
1746 backgrounded job launched from a remote shell) won't think you're done.
1747 You should reopen those to F</dev/null> if it's any issue.
1751 Declare a picture format for use by the C<write> function. For
1755 Test: @<<<<<<<< @||||| @>>>>>
1756 $str, $%, '$' . int($num)
1760 $num = $cost/$quantity;
1764 See L<perlform> for many details and examples.
1766 =item formline PICTURE,LIST
1768 This is an internal function used by C<format>s, though you may call it,
1769 too. It formats (see L<perlform>) a list of values according to the
1770 contents of PICTURE, placing the output into the format output
1771 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1772 Eventually, when a C<write> is done, the contents of
1773 C<$^A> are written to some filehandle, but you could also read C<$^A>
1774 yourself and then set C<$^A> back to C<"">. Note that a format typically
1775 does one C<formline> per line of form, but the C<formline> function itself
1776 doesn't care how many newlines are embedded in the PICTURE. This means
1777 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1778 You may therefore need to use multiple formlines to implement a single
1779 record format, just like the format compiler.
1781 Be careful if you put double quotes around the picture, because an C<@>
1782 character may be taken to mean the beginning of an array name.
1783 C<formline> always returns true. See L<perlform> for other examples.
1785 =item getc FILEHANDLE
1789 Returns the next character from the input file attached to FILEHANDLE,
1790 or the undefined value at end of file, or if there was an error.
1791 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1792 efficient. However, it cannot be used by itself to fetch single
1793 characters without waiting for the user to hit enter. For that, try
1794 something more like:
1797 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1800 system "stty", '-icanon', 'eol', "\001";
1806 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1809 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1813 Determination of whether $BSD_STYLE should be set
1814 is left as an exercise to the reader.
1816 The C<POSIX::getattr> function can do this more portably on
1817 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1818 module from your nearest CPAN site; details on CPAN can be found on
1823 Implements the C library function of the same name, which on most
1824 systems returns the current login from F</etc/utmp>, if any. If null,
1827 $login = getlogin || getpwuid($<) || "Kilroy";
1829 Do not consider C<getlogin> for authentication: it is not as
1830 secure as C<getpwuid>.
1832 =item getpeername SOCKET
1834 Returns the packed sockaddr address of other end of the SOCKET connection.
1837 $hersockaddr = getpeername(SOCK);
1838 ($port, $iaddr) = sockaddr_in($hersockaddr);
1839 $herhostname = gethostbyaddr($iaddr, AF_INET);
1840 $herstraddr = inet_ntoa($iaddr);
1844 Returns the current process group for the specified PID. Use
1845 a PID of C<0> to get the current process group for the
1846 current process. Will raise an exception if used on a machine that
1847 doesn't implement getpgrp(2). If PID is omitted, returns process
1848 group of current process. Note that the POSIX version of C<getpgrp>
1849 does not accept a PID argument, so only C<PID==0> is truly portable.
1853 Returns the process id of the parent process.
1855 =item getpriority WHICH,WHO
1857 Returns the current priority for a process, a process group, or a user.
1858 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1859 machine that doesn't implement getpriority(2).
1865 =item gethostbyname NAME
1867 =item getnetbyname NAME
1869 =item getprotobyname NAME
1875 =item getservbyname NAME,PROTO
1877 =item gethostbyaddr ADDR,ADDRTYPE
1879 =item getnetbyaddr ADDR,ADDRTYPE
1881 =item getprotobynumber NUMBER
1883 =item getservbyport PORT,PROTO
1901 =item sethostent STAYOPEN
1903 =item setnetent STAYOPEN
1905 =item setprotoent STAYOPEN
1907 =item setservent STAYOPEN
1921 These routines perform the same functions as their counterparts in the
1922 system library. In list context, the return values from the
1923 various get routines are as follows:
1925 ($name,$passwd,$uid,$gid,
1926 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1927 ($name,$passwd,$gid,$members) = getgr*
1928 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1929 ($name,$aliases,$addrtype,$net) = getnet*
1930 ($name,$aliases,$proto) = getproto*
1931 ($name,$aliases,$port,$proto) = getserv*
1933 (If the entry doesn't exist you get a null list.)
1935 The exact meaning of the $gcos field varies but it usually contains
1936 the real name of the user (as opposed to the login name) and other
1937 information pertaining to the user. Beware, however, that in many
1938 system users are able to change this information and therefore it
1939 cannot be trusted and therefore the $gcos is tainted (see
1940 L<perlsec>). The $passwd and $shell, user's encrypted password and
1941 login shell, are also tainted, because of the same reason.
1943 In scalar context, you get the name, unless the function was a
1944 lookup by name, in which case you get the other thing, whatever it is.
1945 (If the entry doesn't exist you get the undefined value.) For example:
1947 $uid = getpwnam($name);
1948 $name = getpwuid($num);
1950 $gid = getgrnam($name);
1951 $name = getgrgid($num;
1955 In I<getpw*()> the fields $quota, $comment, and $expire are special
1956 cases in the sense that in many systems they are unsupported. If the
1957 $quota is unsupported, it is an empty scalar. If it is supported, it
1958 usually encodes the disk quota. If the $comment field is unsupported,
1959 it is an empty scalar. If it is supported it usually encodes some
1960 administrative comment about the user. In some systems the $quota
1961 field may be $change or $age, fields that have to do with password
1962 aging. In some systems the $comment field may be $class. The $expire
1963 field, if present, encodes the expiration period of the account or the
1964 password. For the availability and the exact meaning of these fields
1965 in your system, please consult your getpwnam(3) documentation and your
1966 F<pwd.h> file. You can also find out from within Perl what your
1967 $quota and $comment fields mean and whether you have the $expire field
1968 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1969 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1970 files are only supported if your vendor has implemented them in the
1971 intuitive fashion that calling the regular C library routines gets the
1972 shadow versions if you're running under privilege or if there exists
1973 the shadow(3) functions as found in System V ( this includes Solaris
1974 and Linux.) Those systems which implement a proprietary shadow password
1975 facility are unlikely to be supported.
1977 The $members value returned by I<getgr*()> is a space separated list of
1978 the login names of the members of the group.
1980 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1981 C, it will be returned to you via C<$?> if the function call fails. The
1982 C<@addrs> value returned by a successful call is a list of the raw
1983 addresses returned by the corresponding system library call. In the
1984 Internet domain, each address is four bytes long and you can unpack it
1985 by saying something like:
1987 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1989 The Socket library makes this slightly easier:
1992 $iaddr = inet_aton("127.1"); # or whatever address
1993 $name = gethostbyaddr($iaddr, AF_INET);
1995 # or going the other way
1996 $straddr = inet_ntoa($iaddr);
1998 If you get tired of remembering which element of the return list
1999 contains which return value, by-name interfaces are provided
2000 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2001 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2002 and C<User::grent>. These override the normal built-ins, supplying
2003 versions that return objects with the appropriate names
2004 for each field. For example:
2008 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2010 Even though it looks like they're the same method calls (uid),
2011 they aren't, because a C<File::stat> object is different from
2012 a C<User::pwent> object.
2014 =item getsockname SOCKET
2016 Returns the packed sockaddr address of this end of the SOCKET connection,
2017 in case you don't know the address because you have several different
2018 IPs that the connection might have come in on.
2021 $mysockaddr = getsockname(SOCK);
2022 ($port, $myaddr) = sockaddr_in($mysockaddr);
2023 printf "Connect to %s [%s]\n",
2024 scalar gethostbyaddr($myaddr, AF_INET),
2027 =item getsockopt SOCKET,LEVEL,OPTNAME
2029 Returns the socket option requested, or undef if there is an error.
2035 Returns the value of EXPR with filename expansions such as the
2036 standard Unix shell F</bin/csh> would do. This is the internal function
2037 implementing the C<< <*.c> >> operator, but you can use it directly.
2038 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2039 discussed in more detail in L<perlop/"I/O Operators">.
2041 Beginning with v5.6.0, this operator is implemented using the standard
2042 C<File::Glob> extension. See L<File::Glob> for details.
2046 Converts a time as returned by the time function to an 8-element list
2047 with the time localized for the standard Greenwich time zone.
2048 Typically used as follows:
2051 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2054 All list elements are numeric, and come straight out of the C `struct
2055 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2056 specified time. $mday is the day of the month, and $mon is the month
2057 itself, in the range C<0..11> with 0 indicating January and 11
2058 indicating December. $year is the number of years since 1900. That
2059 is, $year is C<123> in year 2023. $wday is the day of the week, with
2060 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2061 the year, in the range C<0..364> (or C<0..365> in leap years.)
2063 Note that the $year element is I<not> simply the last two digits of
2064 the year. If you assume it is, then you create non-Y2K-compliant
2065 programs--and you wouldn't want to do that, would you?
2067 The proper way to get a complete 4-digit year is simply:
2071 And to get the last two digits of the year (e.g., '01' in 2001) do:
2073 $year = sprintf("%02d", $year % 100);
2075 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2077 In scalar context, C<gmtime()> returns the ctime(3) value:
2079 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2081 Also see the C<timegm> function provided by the C<Time::Local> module,
2082 and the strftime(3) function available via the POSIX module.
2084 This scalar value is B<not> locale dependent (see L<perllocale>), but
2085 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2086 strftime(3) and mktime(3) functions available via the POSIX module. To
2087 get somewhat similar but locale dependent date strings, set up your
2088 locale environment variables appropriately (please see L<perllocale>)
2089 and try for example:
2091 use POSIX qw(strftime);
2092 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2094 Note that the C<%a> and C<%b> escapes, which represent the short forms
2095 of the day of the week and the month of the year, may not necessarily
2096 be three characters wide in all locales.
2104 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2105 execution there. It may not be used to go into any construct that
2106 requires initialization, such as a subroutine or a C<foreach> loop. It
2107 also can't be used to go into a construct that is optimized away,
2108 or to get out of a block or subroutine given to C<sort>.
2109 It can be used to go almost anywhere else within the dynamic scope,
2110 including out of subroutines, but it's usually better to use some other
2111 construct such as C<last> or C<die>. The author of Perl has never felt the
2112 need to use this form of C<goto> (in Perl, that is--C is another matter).
2113 (The difference being that C does not offer named loops combined with
2114 loop control. Perl does, and this replaces most structured uses of C<goto>
2115 in other languages.)
2117 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2118 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2119 necessarily recommended if you're optimizing for maintainability:
2121 goto ("FOO", "BAR", "GLARCH")[$i];
2123 The C<goto-&NAME> form is quite different from the other forms of
2124 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2125 doesn't have the stigma associated with other gotos. Instead, it
2126 exits the current subroutine (losing any changes set by local()) and
2127 immediately calls in its place the named subroutine using the current
2128 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2129 load another subroutine and then pretend that the other subroutine had
2130 been called in the first place (except that any modifications to C<@_>
2131 in the current subroutine are propagated to the other subroutine.)
2132 After the C<goto>, not even C<caller> will be able to tell that this
2133 routine was called first.
2135 NAME needn't be the name of a subroutine; it can be a scalar variable
2136 containing a code reference, or a block which evaluates to a code
2139 =item grep BLOCK LIST
2141 =item grep EXPR,LIST
2143 This is similar in spirit to, but not the same as, grep(1) and its
2144 relatives. In particular, it is not limited to using regular expressions.
2146 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2147 C<$_> to each element) and returns the list value consisting of those
2148 elements for which the expression evaluated to true. In scalar
2149 context, returns the number of times the expression was true.
2151 @foo = grep(!/^#/, @bar); # weed out comments
2155 @foo = grep {!/^#/} @bar; # weed out comments
2157 Note that C<$_> is an alias to the list value, so it can be used to
2158 modify the elements of the LIST. While this is useful and supported,
2159 it can cause bizarre results if the elements of LIST are not variables.
2160 Similarly, grep returns aliases into the original list, much as a for
2161 loop's index variable aliases the list elements. That is, modifying an
2162 element of a list returned by grep (for example, in a C<foreach>, C<map>
2163 or another C<grep>) actually modifies the element in the original list.
2164 This is usually something to be avoided when writing clear code.
2166 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2172 Interprets EXPR as a hex string and returns the corresponding value.
2173 (To convert strings that might start with either 0, 0x, or 0b, see
2174 L</oct>.) If EXPR is omitted, uses C<$_>.
2176 print hex '0xAf'; # prints '175'
2177 print hex 'aF'; # same
2179 Hex strings may only represent integers. Strings that would cause
2180 integer overflow trigger a warning. Leading whitespace is not stripped,
2185 There is no builtin C<import> function. It is just an ordinary
2186 method (subroutine) defined (or inherited) by modules that wish to export
2187 names to another module. The C<use> function calls the C<import> method
2188 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2190 =item index STR,SUBSTR,POSITION
2192 =item index STR,SUBSTR
2194 The index function searches for one string within another, but without
2195 the wildcard-like behavior of a full regular-expression pattern match.
2196 It returns the position of the first occurrence of SUBSTR in STR at
2197 or after POSITION. If POSITION is omitted, starts searching from the
2198 beginning of the string. The return value is based at C<0> (or whatever
2199 you've set the C<$[> variable to--but don't do that). If the substring
2200 is not found, returns one less than the base, ordinarily C<-1>.
2206 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2207 You should not use this function for rounding: one because it truncates
2208 towards C<0>, and two because machine representations of floating point
2209 numbers can sometimes produce counterintuitive results. For example,
2210 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2211 because it's really more like -268.99999999999994315658 instead. Usually,
2212 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2213 functions will serve you better than will int().
2215 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2217 Implements the ioctl(2) function. You'll probably first have to say
2219 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2221 to get the correct function definitions. If F<ioctl.ph> doesn't
2222 exist or doesn't have the correct definitions you'll have to roll your
2223 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2224 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2225 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2226 written depending on the FUNCTION--a pointer to the string value of SCALAR
2227 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2228 has no string value but does have a numeric value, that value will be
2229 passed rather than a pointer to the string value. To guarantee this to be
2230 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2231 functions may be needed to manipulate the values of structures used by
2234 The return value of C<ioctl> (and C<fcntl>) is as follows:
2236 if OS returns: then Perl returns:
2238 0 string "0 but true"
2239 anything else that number
2241 Thus Perl returns true on success and false on failure, yet you can
2242 still easily determine the actual value returned by the operating
2245 $retval = ioctl(...) || -1;
2246 printf "System returned %d\n", $retval;
2248 The special string "C<0> but true" is exempt from B<-w> complaints
2249 about improper numeric conversions.
2251 Here's an example of setting a filehandle named C<REMOTE> to be
2252 non-blocking at the system level. You'll have to negotiate C<$|>
2253 on your own, though.
2255 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2257 $flags = fcntl(REMOTE, F_GETFL, 0)
2258 or die "Can't get flags for the socket: $!\n";
2260 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2261 or die "Can't set flags for the socket: $!\n";
2263 =item join EXPR,LIST
2265 Joins the separate strings of LIST into a single string with fields
2266 separated by the value of EXPR, and returns that new string. Example:
2268 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2270 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2271 first argument. Compare L</split>.
2275 Returns a list consisting of all the keys of the named hash. (In
2276 scalar context, returns the number of keys.) The keys are returned in
2277 an apparently random order. The actual random order is subject to
2278 change in future versions of perl, but it is guaranteed to be the same
2279 order as either the C<values> or C<each> function produces (given
2280 that the hash has not been modified). As a side effect, it resets
2283 Here is yet another way to print your environment:
2286 @values = values %ENV;
2288 print pop(@keys), '=', pop(@values), "\n";
2291 or how about sorted by key:
2293 foreach $key (sort(keys %ENV)) {
2294 print $key, '=', $ENV{$key}, "\n";
2297 The returned values are copies of the original keys in the hash, so
2298 modifying them will not affect the original hash. Compare L</values>.
2300 To sort a hash by value, you'll need to use a C<sort> function.
2301 Here's a descending numeric sort of a hash by its values:
2303 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2304 printf "%4d %s\n", $hash{$key}, $key;
2307 As an lvalue C<keys> allows you to increase the number of hash buckets
2308 allocated for the given hash. This can gain you a measure of efficiency if
2309 you know the hash is going to get big. (This is similar to pre-extending
2310 an array by assigning a larger number to $#array.) If you say
2314 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2315 in fact, since it rounds up to the next power of two. These
2316 buckets will be retained even if you do C<%hash = ()>, use C<undef
2317 %hash> if you want to free the storage while C<%hash> is still in scope.
2318 You can't shrink the number of buckets allocated for the hash using
2319 C<keys> in this way (but you needn't worry about doing this by accident,
2320 as trying has no effect).
2322 See also C<each>, C<values> and C<sort>.
2324 =item kill SIGNAL, LIST
2326 Sends a signal to a list of processes. Returns the number of
2327 processes successfully signaled (which is not necessarily the
2328 same as the number actually killed).
2330 $cnt = kill 1, $child1, $child2;
2333 If SIGNAL is zero, no signal is sent to the process. This is a
2334 useful way to check that the process is alive and hasn't changed
2335 its UID. See L<perlport> for notes on the portability of this
2338 Unlike in the shell, if SIGNAL is negative, it kills
2339 process groups instead of processes. (On System V, a negative I<PROCESS>
2340 number will also kill process groups, but that's not portable.) That
2341 means you usually want to use positive not negative signals. You may also
2342 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2348 The C<last> command is like the C<break> statement in C (as used in
2349 loops); it immediately exits the loop in question. If the LABEL is
2350 omitted, the command refers to the innermost enclosing loop. The
2351 C<continue> block, if any, is not executed:
2353 LINE: while (<STDIN>) {
2354 last LINE if /^$/; # exit when done with header
2358 C<last> cannot be used to exit a block which returns a value such as
2359 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2360 a grep() or map() operation.
2362 Note that a block by itself is semantically identical to a loop
2363 that executes once. Thus C<last> can be used to effect an early
2364 exit out of such a block.
2366 See also L</continue> for an illustration of how C<last>, C<next>, and
2373 Returns a lowercased version of EXPR. This is the internal function
2374 implementing the C<\L> escape in double-quoted strings. Respects
2375 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2376 and L<perlunicode> for more details about locale and Unicode support.
2378 If EXPR is omitted, uses C<$_>.
2384 Returns the value of EXPR with the first character lowercased. This
2385 is the internal function implementing the C<\l> escape in
2386 double-quoted strings. Respects current LC_CTYPE locale if C<use
2387 locale> in force. See L<perllocale> and L<perlunicode> for more
2388 details about locale and Unicode support.
2390 If EXPR is omitted, uses C<$_>.
2396 Returns the length in characters of the value of EXPR. If EXPR is
2397 omitted, returns length of C<$_>. Note that this cannot be used on
2398 an entire array or hash to find out how many elements these have.
2399 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2401 =item link OLDFILE,NEWFILE
2403 Creates a new filename linked to the old filename. Returns true for
2404 success, false otherwise.
2406 =item listen SOCKET,QUEUESIZE
2408 Does the same thing that the listen system call does. Returns true if
2409 it succeeded, false otherwise. See the example in
2410 L<perlipc/"Sockets: Client/Server Communication">.
2414 You really probably want to be using C<my> instead, because C<local> isn't
2415 what most people think of as "local". See
2416 L<perlsub/"Private Variables via my()"> for details.
2418 A local modifies the listed variables to be local to the enclosing
2419 block, file, or eval. If more than one value is listed, the list must
2420 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2421 for details, including issues with tied arrays and hashes.
2423 =item localtime EXPR
2425 Converts a time as returned by the time function to a 9-element list
2426 with the time analyzed for the local time zone. Typically used as
2430 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2433 All list elements are numeric, and come straight out of the C `struct
2434 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2435 specified time. $mday is the day of the month, and $mon is the month
2436 itself, in the range C<0..11> with 0 indicating January and 11
2437 indicating December. $year is the number of years since 1900. That
2438 is, $year is C<123> in year 2023. $wday is the day of the week, with
2439 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2440 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2441 is true if the specified time occurs during daylight savings time,
2444 Note that the $year element is I<not> simply the last two digits of
2445 the year. If you assume it is, then you create non-Y2K-compliant
2446 programs--and you wouldn't want to do that, would you?
2448 The proper way to get a complete 4-digit year is simply:
2452 And to get the last two digits of the year (e.g., '01' in 2001) do:
2454 $year = sprintf("%02d", $year % 100);
2456 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2458 In scalar context, C<localtime()> returns the ctime(3) value:
2460 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2462 This scalar value is B<not> locale dependent, see L<perllocale>, but
2463 instead a Perl builtin. Also see the C<Time::Local> module
2464 (to convert the second, minutes, hours, ... back to seconds since the
2465 stroke of midnight the 1st of January 1970, the value returned by
2466 time()), and the strftime(3) and mktime(3) functions available via the
2467 POSIX module. To get somewhat similar but locale dependent date
2468 strings, set up your locale environment variables appropriately
2469 (please see L<perllocale>) and try for example:
2471 use POSIX qw(strftime);
2472 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2474 Note that the C<%a> and C<%b>, the short forms of the day of the week
2475 and the month of the year, may not necessarily be three characters wide.
2479 This function places an advisory lock on a shared variable, or referenced
2480 object contained in I<THING> until the lock goes out of scope.
2482 lock() is a "weak keyword" : this means that if you've defined a function
2483 by this name (before any calls to it), that function will be called
2484 instead. (However, if you've said C<use threads>, lock() is always a
2485 keyword.) See L<threads>.
2491 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2492 returns log of C<$_>. To get the log of another base, use basic algebra:
2493 The base-N log of a number is equal to the natural log of that number
2494 divided by the natural log of N. For example:
2498 return log($n)/log(10);
2501 See also L</exp> for the inverse operation.
2507 Does the same thing as the C<stat> function (including setting the
2508 special C<_> filehandle) but stats a symbolic link instead of the file
2509 the symbolic link points to. If symbolic links are unimplemented on
2510 your system, a normal C<stat> is done.
2512 If EXPR is omitted, stats C<$_>.
2516 The match operator. See L<perlop>.
2518 =item map BLOCK LIST
2522 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2523 C<$_> to each element) and returns the list value composed of the
2524 results of each such evaluation. In scalar context, returns the
2525 total number of elements so generated. Evaluates BLOCK or EXPR in
2526 list context, so each element of LIST may produce zero, one, or
2527 more elements in the returned value.
2529 @chars = map(chr, @nums);
2531 translates a list of numbers to the corresponding characters. And
2533 %hash = map { getkey($_) => $_ } @array;
2535 is just a funny way to write
2538 foreach $_ (@array) {
2539 $hash{getkey($_)} = $_;
2542 Note that C<$_> is an alias to the list value, so it can be used to
2543 modify the elements of the LIST. While this is useful and supported,
2544 it can cause bizarre results if the elements of LIST are not variables.
2545 Using a regular C<foreach> loop for this purpose would be clearer in
2546 most cases. See also L</grep> for an array composed of those items of
2547 the original list for which the BLOCK or EXPR evaluates to true.
2549 C<{> starts both hash references and blocks, so C<map { ...> could be either
2550 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2551 ahead for the closing C<}> it has to take a guess at which its dealing with
2552 based what it finds just after the C<{>. Usually it gets it right, but if it
2553 doesn't it won't realize something is wrong until it gets to the C<}> and
2554 encounters the missing (or unexpected) comma. The syntax error will be
2555 reported close to the C<}> but you'll need to change something near the C<{>
2556 such as using a unary C<+> to give perl some help:
2558 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2559 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2560 %hash = map { ("\L$_", 1) } @array # this also works
2561 %hash = map { lc($_), 1 } @array # as does this.
2562 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2564 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2566 or to force an anon hash constructor use C<+{>
2568 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2570 and you get list of anonymous hashes each with only 1 entry.
2572 =item mkdir FILENAME,MASK
2574 =item mkdir FILENAME
2576 Creates the directory specified by FILENAME, with permissions
2577 specified by MASK (as modified by C<umask>). If it succeeds it
2578 returns true, otherwise it returns false and sets C<$!> (errno).
2579 If omitted, MASK defaults to 0777.
2581 In general, it is better to create directories with permissive MASK,
2582 and let the user modify that with their C<umask>, than it is to supply
2583 a restrictive MASK and give the user no way to be more permissive.
2584 The exceptions to this rule are when the file or directory should be
2585 kept private (mail files, for instance). The perlfunc(1) entry on
2586 C<umask> discusses the choice of MASK in more detail.
2588 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2589 number of trailing slashes. Some operating and filesystems do not get
2590 this right, so Perl automatically removes all trailing slashes to keep
2593 =item msgctl ID,CMD,ARG
2595 Calls the System V IPC function msgctl(2). You'll probably have to say
2599 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2600 then ARG must be a variable which will hold the returned C<msqid_ds>
2601 structure. Returns like C<ioctl>: the undefined value for error,
2602 C<"0 but true"> for zero, or the actual return value otherwise. See also
2603 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2605 =item msgget KEY,FLAGS
2607 Calls the System V IPC function msgget(2). Returns the message queue
2608 id, or the undefined value if there is an error. See also
2609 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2611 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2613 Calls the System V IPC function msgrcv to receive a message from
2614 message queue ID into variable VAR with a maximum message size of
2615 SIZE. Note that when a message is received, the message type as a
2616 native long integer will be the first thing in VAR, followed by the
2617 actual message. This packing may be opened with C<unpack("l! a*")>.
2618 Taints the variable. Returns true if successful, or false if there is
2619 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2620 C<IPC::SysV::Msg> documentation.
2622 =item msgsnd ID,MSG,FLAGS
2624 Calls the System V IPC function msgsnd to send the message MSG to the
2625 message queue ID. MSG must begin with the native long integer message
2626 type, and be followed by the length of the actual message, and finally
2627 the message itself. This kind of packing can be achieved with
2628 C<pack("l! a*", $type, $message)>. Returns true if successful,
2629 or false if there is an error. See also C<IPC::SysV>
2630 and C<IPC::SysV::Msg> documentation.
2636 =item my EXPR : ATTRS
2638 =item my TYPE EXPR : ATTRS
2640 A C<my> declares the listed variables to be local (lexically) to the
2641 enclosing block, file, or C<eval>. If more than one value is listed,
2642 the list must be placed in parentheses.
2644 The exact semantics and interface of TYPE and ATTRS are still
2645 evolving. TYPE is currently bound to the use of C<fields> pragma,
2646 and attributes are handled using the C<attributes> pragma, or starting
2647 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2648 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2649 L<attributes>, and L<Attribute::Handlers>.
2655 The C<next> command is like the C<continue> statement in C; it starts
2656 the next iteration of the loop:
2658 LINE: while (<STDIN>) {
2659 next LINE if /^#/; # discard comments
2663 Note that if there were a C<continue> block on the above, it would get
2664 executed even on discarded lines. If the LABEL is omitted, the command
2665 refers to the innermost enclosing loop.
2667 C<next> cannot be used to exit a block which returns a value such as
2668 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2669 a grep() or map() operation.
2671 Note that a block by itself is semantically identical to a loop
2672 that executes once. Thus C<next> will exit such a block early.
2674 See also L</continue> for an illustration of how C<last>, C<next>, and
2677 =item no Module VERSION LIST
2679 =item no Module VERSION
2681 =item no Module LIST
2685 See the L</use> function, which C<no> is the opposite of.
2691 Interprets EXPR as an octal string and returns the corresponding
2692 value. (If EXPR happens to start off with C<0x>, interprets it as a
2693 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2694 binary string. Leading whitespace is ignored in all three cases.)
2695 The following will handle decimal, binary, octal, and hex in the standard
2698 $val = oct($val) if $val =~ /^0/;
2700 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2701 in octal), use sprintf() or printf():
2703 $perms = (stat("filename"))[2] & 07777;
2704 $oct_perms = sprintf "%lo", $perms;
2706 The oct() function is commonly used when a string such as C<644> needs
2707 to be converted into a file mode, for example. (Although perl will
2708 automatically convert strings into numbers as needed, this automatic
2709 conversion assumes base 10.)
2711 =item open FILEHANDLE,EXPR
2713 =item open FILEHANDLE,MODE,EXPR
2715 =item open FILEHANDLE,MODE,EXPR,LIST
2717 =item open FILEHANDLE,MODE,REFERENCE
2719 =item open FILEHANDLE
2721 Opens the file whose filename is given by EXPR, and associates it with
2724 (The following is a comprehensive reference to open(): for a gentler
2725 introduction you may consider L<perlopentut>.)
2727 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2728 assigned a reference to a new anonymous filehandle, otherwise if
2729 FILEHANDLE is an expression, its value is used as the name of the real
2730 filehandle wanted. (This is considered a symbolic reference, so C<use
2731 strict 'refs'> should I<not> be in effect.)
2733 If EXPR is omitted, the scalar variable of the same name as the
2734 FILEHANDLE contains the filename. (Note that lexical variables--those
2735 declared with C<my>--will not work for this purpose; so if you're
2736 using C<my>, specify EXPR in your call to open.)
2738 If three or more arguments are specified then the mode of opening and
2739 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2740 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2741 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2742 the file is opened for appending, again being created if necessary.
2744 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2745 indicate that you want both read and write access to the file; thus
2746 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2747 '+>' >> mode would clobber the file first. You can't usually use
2748 either read-write mode for updating textfiles, since they have
2749 variable length records. See the B<-i> switch in L<perlrun> for a
2750 better approach. The file is created with permissions of C<0666>
2751 modified by the process' C<umask> value.
2753 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2754 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2756 In the 2-arguments (and 1-argument) form of the call the mode and
2757 filename should be concatenated (in this order), possibly separated by
2758 spaces. It is possible to omit the mode in these forms if the mode is
2761 If the filename begins with C<'|'>, the filename is interpreted as a
2762 command to which output is to be piped, and if the filename ends with a
2763 C<'|'>, the filename is interpreted as a command which pipes output to
2764 us. See L<perlipc/"Using open() for IPC">
2765 for more examples of this. (You are not allowed to C<open> to a command
2766 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2767 and L<perlipc/"Bidirectional Communication with Another Process">
2770 For three or more arguments if MODE is C<'|-'>, the filename is
2771 interpreted as a command to which output is to be piped, and if MODE
2772 is C<'-|'>, the filename is interpreted as a command which pipes
2773 output to us. In the 2-arguments (and 1-argument) form one should
2774 replace dash (C<'-'>) with the command.
2775 See L<perlipc/"Using open() for IPC"> for more examples of this.
2776 (You are not allowed to C<open> to a command that pipes both in I<and>
2777 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2778 L<perlipc/"Bidirectional Communication"> for alternatives.)
2780 In the three-or-more argument form of pipe opens, if LIST is specified
2781 (extra arguments after the command name) then LIST becomes arguments
2782 to the command invoked if the platform supports it. The meaning of
2783 C<open> with more than three arguments for non-pipe modes is not yet
2784 specified. Experimental "layers" may give extra LIST arguments
2787 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2788 and opening C<< '>-' >> opens STDOUT.
2790 You may use the three-argument form of open to specify
2791 I<I/O disciplines> that affect how the input and output
2792 are processed: see L</binmode> and L<open>. For example
2794 open(FH, "<:utf8", "file")
2796 will open the UTF-8 encoded file containing Unicode characters,
2797 see L<perluniintro>.
2799 Open returns nonzero upon success, the undefined value otherwise. If
2800 the C<open> involved a pipe, the return value happens to be the pid of
2803 If you're running Perl on a system that distinguishes between text
2804 files and binary files, then you should check out L</binmode> for tips
2805 for dealing with this. The key distinction between systems that need
2806 C<binmode> and those that don't is their text file formats. Systems
2807 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2808 character, and which encode that character in C as C<"\n">, do not
2809 need C<binmode>. The rest need it.
2811 In the three argument form MODE may also contain a list of IO "layers"
2812 (see L<open> and L<PerlIO> for more details) to be applied to the
2813 handle. This can be used to achieve the effect of C<binmode> as well
2814 as more complex behaviours.
2816 When opening a file, it's usually a bad idea to continue normal execution
2817 if the request failed, so C<open> is frequently used in connection with
2818 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2819 where you want to make a nicely formatted error message (but there are
2820 modules that can help with that problem)) you should always check
2821 the return value from opening a file. The infrequent exception is when
2822 working with an unopened filehandle is actually what you want to do.
2824 As a special case the 3 arg form with a read/write mode and the third
2825 argument being C<undef>:
2827 open(TMP, "+>", undef) or die ...
2829 opens a filehandle to an anonymous temporary file.
2831 File handles can be opened to "in memory" files held in Perl scalars via:
2833 open($fh, '>', \$variable) || ..
2835 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2836 file, you have to close it first:
2839 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2844 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2845 while (<ARTICLE>) {...
2847 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2848 # if the open fails, output is discarded
2850 open(DBASE, '+<', 'dbase.mine') # open for update
2851 or die "Can't open 'dbase.mine' for update: $!";
2853 open(DBASE, '+<dbase.mine') # ditto
2854 or die "Can't open 'dbase.mine' for update: $!";
2856 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2857 or die "Can't start caesar: $!";
2859 open(ARTICLE, "caesar <$article |") # ditto
2860 or die "Can't start caesar: $!";
2862 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2863 or die "Can't start sort: $!";
2866 open(MEMORY,'>', \$var)
2867 or die "Can't open memory file: $!";
2868 print MEMORY "foo!\n"; # output will end up in $var
2870 # process argument list of files along with any includes
2872 foreach $file (@ARGV) {
2873 process($file, 'fh00');
2877 my($filename, $input) = @_;
2878 $input++; # this is a string increment
2879 unless (open($input, $filename)) {
2880 print STDERR "Can't open $filename: $!\n";
2885 while (<$input>) { # note use of indirection
2886 if (/^#include "(.*)"/) {
2887 process($1, $input);
2894 You may also, in the Bourne shell tradition, specify an EXPR beginning
2895 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2896 name of a filehandle (or file descriptor, if numeric) to be
2897 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2898 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2899 mode you specify should match the mode of the original filehandle.
2900 (Duping a filehandle does not take into account any existing contents of
2901 IO buffers.) If you use the 3 arg form then you can pass either a number,
2902 the name of a filehandle or the normal "reference to a glob".
2904 Here is a script that saves, redirects, and restores C<STDOUT> and
2905 C<STDERR> using various methods:
2908 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2909 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2911 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2912 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2914 select STDERR; $| = 1; # make unbuffered
2915 select STDOUT; $| = 1; # make unbuffered
2917 print STDOUT "stdout 1\n"; # this works for
2918 print STDERR "stderr 1\n"; # subprocesses too
2923 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2924 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2926 print STDOUT "stdout 2\n";
2927 print STDERR "stderr 2\n";
2929 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2930 do an equivalent of C's C<fdopen> of that file descriptor; this is
2931 more parsimonious of file descriptors. For example:
2933 open(FILEHANDLE, "<&=$fd")
2937 open(FILEHANDLE, "<&=", $fd)
2939 Note that if Perl is using the standard C libraries' fdopen() then on
2940 many UNIX systems, fdopen() is known to fail when file descriptors
2941 exceed a certain value, typically 255. If you need more file
2942 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2944 You can see whether Perl has been compiled with PerlIO or not by
2945 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2946 is C<define>, you have PerlIO, otherwise you don't.
2948 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2949 with 2-arguments (or 1-argument) form of open(), then
2950 there is an implicit fork done, and the return value of open is the pid
2951 of the child within the parent process, and C<0> within the child
2952 process. (Use C<defined($pid)> to determine whether the open was successful.)
2953 The filehandle behaves normally for the parent, but i/o to that
2954 filehandle is piped from/to the STDOUT/STDIN of the child process.
2955 In the child process the filehandle isn't opened--i/o happens from/to
2956 the new STDOUT or STDIN. Typically this is used like the normal
2957 piped open when you want to exercise more control over just how the
2958 pipe command gets executed, such as when you are running setuid, and
2959 don't want to have to scan shell commands for metacharacters.
2960 The following triples are more or less equivalent:
2962 open(FOO, "|tr '[a-z]' '[A-Z]'");
2963 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2964 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2965 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2967 open(FOO, "cat -n '$file'|");
2968 open(FOO, '-|', "cat -n '$file'");
2969 open(FOO, '-|') || exec 'cat', '-n', $file;
2970 open(FOO, '-|', "cat", '-n', $file);
2972 The last example in each block shows the pipe as "list form", which is
2973 not yet supported on all platforms. A good rule of thumb is that if
2974 your platform has true C<fork()> (in other words, if your platform is
2975 UNIX) you can use the list form.
2977 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2979 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2980 output before any operation that may do a fork, but this may not be
2981 supported on some platforms (see L<perlport>). To be safe, you may need
2982 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2983 of C<IO::Handle> on any open handles.
2985 On systems that support a close-on-exec flag on files, the flag will
2986 be set for the newly opened file descriptor as determined by the value
2987 of $^F. See L<perlvar/$^F>.
2989 Closing any piped filehandle causes the parent process to wait for the
2990 child to finish, and returns the status value in C<$?>.
2992 The filename passed to 2-argument (or 1-argument) form of open() will
2993 have leading and trailing whitespace deleted, and the normal
2994 redirection characters honored. This property, known as "magic open",
2995 can often be used to good effect. A user could specify a filename of
2996 F<"rsh cat file |">, or you could change certain filenames as needed:
2998 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2999 open(FH, $filename) or die "Can't open $filename: $!";
3001 Use 3-argument form to open a file with arbitrary weird characters in it,
3003 open(FOO, '<', $file);
3005 otherwise it's necessary to protect any leading and trailing whitespace:
3007 $file =~ s#^(\s)#./$1#;
3008 open(FOO, "< $file\0");
3010 (this may not work on some bizarre filesystems). One should
3011 conscientiously choose between the I<magic> and 3-arguments form
3016 will allow the user to specify an argument of the form C<"rsh cat file |">,
3017 but will not work on a filename which happens to have a trailing space, while
3019 open IN, '<', $ARGV[0];
3021 will have exactly the opposite restrictions.
3023 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3024 should use the C<sysopen> function, which involves no such magic (but
3025 may use subtly different filemodes than Perl open(), which is mapped
3026 to C fopen()). This is
3027 another way to protect your filenames from interpretation. For example:
3030 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3031 or die "sysopen $path: $!";
3032 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3033 print HANDLE "stuff $$\n";
3035 print "File contains: ", <HANDLE>;
3037 Using the constructor from the C<IO::Handle> package (or one of its
3038 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3039 filehandles that have the scope of whatever variables hold references to
3040 them, and automatically close whenever and however you leave that scope:
3044 sub read_myfile_munged {
3046 my $handle = new IO::File;
3047 open($handle, "myfile") or die "myfile: $!";
3049 or return (); # Automatically closed here.
3050 mung $first or die "mung failed"; # Or here.
3051 return $first, <$handle> if $ALL; # Or here.
3055 See L</seek> for some details about mixing reading and writing.
3057 =item opendir DIRHANDLE,EXPR
3059 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3060 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3061 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3067 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3068 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3071 For the reverse, see L</chr>.
3072 See L<perlunicode> and L<encoding> for more about Unicode.
3078 =item our EXPR : ATTRS
3080 =item our TYPE EXPR : ATTRS
3082 An C<our> declares the listed variables to be valid globals within
3083 the enclosing block, file, or C<eval>. That is, it has the same
3084 scoping rules as a "my" declaration, but does not create a local
3085 variable. If more than one value is listed, the list must be placed
3086 in parentheses. The C<our> declaration has no semantic effect unless
3087 "use strict vars" is in effect, in which case it lets you use the
3088 declared global variable without qualifying it with a package name.
3089 (But only within the lexical scope of the C<our> declaration. In this
3090 it differs from "use vars", which is package scoped.)
3092 An C<our> declaration declares a global variable that will be visible
3093 across its entire lexical scope, even across package boundaries. The
3094 package in which the variable is entered is determined at the point
3095 of the declaration, not at the point of use. This means the following
3099 our $bar; # declares $Foo::bar for rest of lexical scope
3103 print $bar; # prints 20
3105 Multiple C<our> declarations in the same lexical scope are allowed
3106 if they are in different packages. If they happened to be in the same
3107 package, Perl will emit warnings if you have asked for them.
3111 our $bar; # declares $Foo::bar for rest of lexical scope
3115 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3116 print $bar; # prints 30
3118 our $bar; # emits warning
3120 An C<our> declaration may also have a list of attributes associated
3123 The exact semantics and interface of TYPE and ATTRS are still
3124 evolving. TYPE is currently bound to the use of C<fields> pragma,
3125 and attributes are handled using the C<attributes> pragma, or starting
3126 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3127 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3128 L<attributes>, and L<Attribute::Handlers>.
3130 The only currently recognized C<our()> attribute is C<unique> which
3131 indicates that a single copy of the global is to be used by all
3132 interpreters should the program happen to be running in a
3133 multi-interpreter environment. (The default behaviour would be for
3134 each interpreter to have its own copy of the global.) Examples:
3136 our @EXPORT : unique = qw(foo);
3137 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3138 our $VERSION : unique = "1.00";
3140 Note that this attribute also has the effect of making the global
3141 readonly when the first new interpreter is cloned (for example,
3142 when the first new thread is created).
3144 Multi-interpreter environments can come to being either through the
3145 fork() emulation on Windows platforms, or by embedding perl in a
3146 multi-threaded application. The C<unique> attribute does nothing in
3147 all other environments.
3149 =item pack TEMPLATE,LIST
3151 Takes a LIST of values and converts it into a string using the rules
3152 given by the TEMPLATE. The resulting string is the concatenation of
3153 the converted values. Typically, each converted value looks
3154 like its machine-level representation. For example, on 32-bit machines
3155 a converted integer may be represented by a sequence of 4 bytes.
3157 The TEMPLATE is a sequence of characters that give the order and type
3158 of values, as follows:
3160 a A string with arbitrary binary data, will be null padded.
3161 A A text (ASCII) string, will be space padded.
3162 Z A null terminated (ASCIZ) string, will be null padded.
3164 b A bit string (ascending bit order inside each byte, like vec()).
3165 B A bit string (descending bit order inside each byte).
3166 h A hex string (low nybble first).
3167 H A hex string (high nybble first).
3169 c A signed char value.
3170 C An unsigned char value. Only does bytes. See U for Unicode.
3172 s A signed short value.
3173 S An unsigned short value.
3174 (This 'short' is _exactly_ 16 bits, which may differ from
3175 what a local C compiler calls 'short'. If you want
3176 native-length shorts, use the '!' suffix.)
3178 i A signed integer value.
3179 I An unsigned integer value.
3180 (This 'integer' is _at_least_ 32 bits wide. Its exact
3181 size depends on what a local C compiler calls 'int',
3182 and may even be larger than the 'long' described in
3185 l A signed long value.
3186 L An unsigned long value.
3187 (This 'long' is _exactly_ 32 bits, which may differ from
3188 what a local C compiler calls 'long'. If you want
3189 native-length longs, use the '!' suffix.)
3191 n An unsigned short in "network" (big-endian) order.
3192 N An unsigned long in "network" (big-endian) order.
3193 v An unsigned short in "VAX" (little-endian) order.
3194 V An unsigned long in "VAX" (little-endian) order.
3195 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3196 _exactly_ 32 bits, respectively.)
3198 q A signed quad (64-bit) value.
3199 Q An unsigned quad value.
3200 (Quads are available only if your system supports 64-bit
3201 integer values _and_ if Perl has been compiled to support those.
3202 Causes a fatal error otherwise.)
3204 j A signed integer value (a Perl internal integer, IV).
3205 J An unsigned integer value (a Perl internal unsigned integer, UV).
3207 f A single-precision float in the native format.
3208 d A double-precision float in the native format.
3210 F A floating point value in the native native format
3211 (a Perl internal floating point value, NV).
3212 D A long double-precision float in the native format.
3213 (Long doubles are available only if your system supports long
3214 double values _and_ if Perl has been compiled to support those.
3215 Causes a fatal error otherwise.)
3217 p A pointer to a null-terminated string.
3218 P A pointer to a structure (fixed-length string).
3220 u A uuencoded string.
3221 U A Unicode character number. Encodes to UTF-8 internally
3222 (or UTF-EBCDIC in EBCDIC platforms).
3224 w A BER compressed integer. Its bytes represent an unsigned
3225 integer in base 128, most significant digit first, with as
3226 few digits as possible. Bit eight (the high bit) is set
3227 on each byte except the last.
3231 @ Null fill to absolute position.
3232 ( Start of a ()-group.
3234 The following rules apply:
3240 Each letter may optionally be followed by a number giving a repeat
3241 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3242 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3243 many values from the LIST. A C<*> for the repeat count means to use
3244 however many items are left, except for C<@>, C<x>, C<X>, where it is
3245 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3246 is the same). A numeric repeat count may optionally be enclosed in
3247 brackets, as in C<pack 'C[80]', @arr>.
3249 One can replace the numeric repeat count by a template enclosed in brackets;
3250 then the packed length of this template in bytes is used as a count.
3251 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3252 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3253 If the template in brackets contains alignment commands (such as C<x![d]>),
3254 its packed length is calculated as if the start of the template has the maximal
3257 When used with C<Z>, C<*> results in the addition of a trailing null
3258 byte (so the packed result will be one longer than the byte C<length>
3261 The repeat count for C<u> is interpreted as the maximal number of bytes
3262 to encode per line of output, with 0 and 1 replaced by 45.
3266 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3267 string of length count, padding with nulls or spaces as necessary. When
3268 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3269 after the first null, and C<a> returns data verbatim. When packing,
3270 C<a>, and C<Z> are equivalent.
3272 If the value-to-pack is too long, it is truncated. If too long and an
3273 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3274 by a null byte. Thus C<Z> always packs a trailing null byte under
3279 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3280 Each byte of the input field of pack() generates 1 bit of the result.
3281 Each result bit is based on the least-significant bit of the corresponding
3282 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3283 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3285 Starting from the beginning of the input string of pack(), each 8-tuple
3286 of bytes is converted to 1 byte of output. With format C<b>
3287 the first byte of the 8-tuple determines the least-significant bit of a
3288 byte, and with format C<B> it determines the most-significant bit of
3291 If the length of the input string is not exactly divisible by 8, the
3292 remainder is packed as if the input string were padded by null bytes
3293 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3295 If the input string of pack() is longer than needed, extra bytes are ignored.
3296 A C<*> for the repeat count of pack() means to use all the bytes of
3297 the input field. On unpack()ing the bits are converted to a string
3298 of C<"0">s and C<"1">s.
3302 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3303 representable as hexadecimal digits, 0-9a-f) long.
3305 Each byte of the input field of pack() generates 4 bits of the result.
3306 For non-alphabetical bytes the result is based on the 4 least-significant
3307 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3308 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3309 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3310 is compatible with the usual hexadecimal digits, so that C<"a"> and
3311 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3312 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3314 Starting from the beginning of the input string of pack(), each pair
3315 of bytes is converted to 1 byte of output. With format C<h> the
3316 first byte of the pair determines the least-significant nybble of the
3317 output byte, and with format C<H> it determines the most-significant
3320 If the length of the input string is not even, it behaves as if padded
3321 by a null byte at the end. Similarly, during unpack()ing the "extra"
3322 nybbles are ignored.
3324 If the input string of pack() is longer than needed, extra bytes are ignored.
3325 A C<*> for the repeat count of pack() means to use all the bytes of
3326 the input field. On unpack()ing the bits are converted to a string
3327 of hexadecimal digits.
3331 The C<p> type packs a pointer to a null-terminated string. You are
3332 responsible for ensuring the string is not a temporary value (which can
3333 potentially get deallocated before you get around to using the packed result).
3334 The C<P> type packs a pointer to a structure of the size indicated by the
3335 length. A NULL pointer is created if the corresponding value for C<p> or
3336 C<P> is C<undef>, similarly for unpack().
3340 The C</> template character allows packing and unpacking of strings where
3341 the packed structure contains a byte count followed by the string itself.
3342 You write I<length-item>C</>I<string-item>.
3344 The I<length-item> can be any C<pack> template letter, and describes
3345 how the length value is packed. The ones likely to be of most use are
3346 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3347 SNMP) and C<N> (for Sun XDR).
3349 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3350 For C<unpack> the length of the string is obtained from the I<length-item>,
3351 but if you put in the '*' it will be ignored.
3353 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3354 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3355 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3357 The I<length-item> is not returned explicitly from C<unpack>.
3359 Adding a count to the I<length-item> letter is unlikely to do anything
3360 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3361 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3362 which Perl does not regard as legal in numeric strings.
3366 The integer types C<s>, C<S>, C<l>, and C<L> may be
3367 immediately followed by a C<!> suffix to signify native shorts or
3368 longs--as you can see from above for example a bare C<l> does mean
3369 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3370 may be larger. This is an issue mainly in 64-bit platforms. You can
3371 see whether using C<!> makes any difference by
3373 print length(pack("s")), " ", length(pack("s!")), "\n";
3374 print length(pack("l")), " ", length(pack("l!")), "\n";
3376 C<i!> and C<I!> also work but only because of completeness;
3377 they are identical to C<i> and C<I>.
3379 The actual sizes (in bytes) of native shorts, ints, longs, and long
3380 longs on the platform where Perl was built are also available via
3384 print $Config{shortsize}, "\n";
3385 print $Config{intsize}, "\n";
3386 print $Config{longsize}, "\n";
3387 print $Config{longlongsize}, "\n";
3389 (The C<$Config{longlongsize}> will be undefine if your system does
3390 not support long longs.)
3394 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3395 are inherently non-portable between processors and operating systems
3396 because they obey the native byteorder and endianness. For example a
3397 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3398 (arranged in and handled by the CPU registers) into bytes as
3400 0x12 0x34 0x56 0x78 # big-endian
3401 0x78 0x56 0x34 0x12 # little-endian
3403 Basically, the Intel and VAX CPUs are little-endian, while everybody
3404 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3405 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3406 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3409 The names `big-endian' and `little-endian' are comic references to
3410 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3411 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3412 the egg-eating habits of the Lilliputians.
3414 Some systems may have even weirder byte orders such as
3419 You can see your system's preference with
3421 print join(" ", map { sprintf "%#02x", $_ }
3422 unpack("C*",pack("L",0x12345678))), "\n";
3424 The byteorder on the platform where Perl was built is also available
3428 print $Config{byteorder}, "\n";
3430 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3431 and C<'87654321'> are big-endian.
3433 If you want portable packed integers use the formats C<n>, C<N>,
3434 C<v>, and C<V>, their byte endianness and size are known.
3435 See also L<perlport>.
3439 Real numbers (floats and doubles) are in the native machine format only;
3440 due to the multiplicity of floating formats around, and the lack of a
3441 standard "network" representation, no facility for interchange has been
3442 made. This means that packed floating point data written on one machine
3443 may not be readable on another - even if both use IEEE floating point
3444 arithmetic (as the endian-ness of the memory representation is not part
3445 of the IEEE spec). See also L<perlport>.
3447 Note that Perl uses doubles internally for all numeric calculation, and
3448 converting from double into float and thence back to double again will
3449 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3454 If the pattern begins with a C<U>, the resulting string will be treated
3455 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3456 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3457 characters. If you don't want this to happen, you can begin your pattern
3458 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3459 string, and then follow this with a C<U*> somewhere in your pattern.
3463 You must yourself do any alignment or padding by inserting for example
3464 enough C<'x'>es while packing. There is no way to pack() and unpack()
3465 could know where the bytes are going to or coming from. Therefore
3466 C<pack> (and C<unpack>) handle their output and input as flat
3471 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3472 take a repeat count, both as postfix, and via the C</> template
3477 C<x> and C<X> accept C<!> modifier. In this case they act as
3478 alignment commands: they jump forward/back to the closest position
3479 aligned at a multiple of C<count> bytes. For example, to pack() or
3480 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3481 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3482 aligned on the double's size.
3484 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3485 both result in no-ops.
3489 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3493 If TEMPLATE requires more arguments to pack() than actually given, pack()
3494 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3495 to pack() than actually given, extra arguments are ignored.
3501 $foo = pack("CCCC",65,66,67,68);
3503 $foo = pack("C4",65,66,67,68);
3505 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3506 # same thing with Unicode circled letters
3508 $foo = pack("ccxxcc",65,66,67,68);
3511 # note: the above examples featuring "C" and "c" are true
3512 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3513 # and UTF-8. In EBCDIC the first example would be
3514 # $foo = pack("CCCC",193,194,195,196);
3516 $foo = pack("s2",1,2);
3517 # "\1\0\2\0" on little-endian
3518 # "\0\1\0\2" on big-endian
3520 $foo = pack("a4","abcd","x","y","z");
3523 $foo = pack("aaaa","abcd","x","y","z");
3526 $foo = pack("a14","abcdefg");
3527 # "abcdefg\0\0\0\0\0\0\0"
3529 $foo = pack("i9pl", gmtime);
3530 # a real struct tm (on my system anyway)
3532 $utmp_template = "Z8 Z8 Z16 L";
3533 $utmp = pack($utmp_template, @utmp1);
3534 # a struct utmp (BSDish)
3536 @utmp2 = unpack($utmp_template, $utmp);
3537 # "@utmp1" eq "@utmp2"
3540 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3543 $foo = pack('sx2l', 12, 34);
3544 # short 12, two zero bytes padding, long 34
3545 $bar = pack('s@4l', 12, 34);
3546 # short 12, zero fill to position 4, long 34
3549 The same template may generally also be used in unpack().
3551 =item package NAMESPACE
3555 Declares the compilation unit as being in the given namespace. The scope
3556 of the package declaration is from the declaration itself through the end
3557 of the enclosing block, file, or eval (the same as the C<my> operator).
3558 All further unqualified dynamic identifiers will be in this namespace.
3559 A package statement affects only dynamic variables--including those
3560 you've used C<local> on--but I<not> lexical variables, which are created
3561 with C<my>. Typically it would be the first declaration in a file to
3562 be included by the C<require> or C<use> operator. You can switch into a
3563 package in more than one place; it merely influences which symbol table
3564 is used by the compiler for the rest of that block. You can refer to
3565 variables and filehandles in other packages by prefixing the identifier
3566 with the package name and a double colon: C<$Package::Variable>.
3567 If the package name is null, the C<main> package as assumed. That is,
3568 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3569 still seen in older code).
3571 If NAMESPACE is omitted, then there is no current package, and all
3572 identifiers must be fully qualified or lexicals. However, you are
3573 strongly advised not to make use of this feature. Its use can cause
3574 unexpected behaviour, even crashing some versions of Perl. It is
3575 deprecated, and will be removed from a future release.
3577 See L<perlmod/"Packages"> for more information about packages, modules,
3578 and classes. See L<perlsub> for other scoping issues.
3580 =item pipe READHANDLE,WRITEHANDLE
3582 Opens a pair of connected pipes like the corresponding system call.
3583 Note that if you set up a loop of piped processes, deadlock can occur
3584 unless you are very careful. In addition, note that Perl's pipes use
3585 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3586 after each command, depending on the application.
3588 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3589 for examples of such things.
3591 On systems that support a close-on-exec flag on files, the flag will be set
3592 for the newly opened file descriptors as determined by the value of $^F.
3599 Pops and returns the last value of the array, shortening the array by
3600 one element. Has an effect similar to
3604 If there are no elements in the array, returns the undefined value
3605 (although this may happen at other times as well). If ARRAY is
3606 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3607 array in subroutines, just like C<shift>.
3613 Returns the offset of where the last C<m//g> search left off for the variable
3614 in question (C<$_> is used when the variable is not specified). May be
3615 modified to change that offset. Such modification will also influence
3616 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3619 =item print FILEHANDLE LIST
3625 Prints a string or a list of strings. Returns true if successful.
3626 FILEHANDLE may be a scalar variable name, in which case the variable
3627 contains the name of or a reference to the filehandle, thus introducing
3628 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3629 the next token is a term, it may be misinterpreted as an operator
3630 unless you interpose a C<+> or put parentheses around the arguments.)
3631 If FILEHANDLE is omitted, prints by default to standard output (or
3632 to the last selected output channel--see L</select>). If LIST is
3633 also omitted, prints C<$_> to the currently selected output channel.
3634 To set the default output channel to something other than STDOUT
3635 use the select operation. The current value of C<$,> (if any) is
3636 printed between each LIST item. The current value of C<$\> (if
3637 any) is printed after the entire LIST has been printed. Because
3638 print takes a LIST, anything in the LIST is evaluated in list
3639 context, and any subroutine that you call will have one or more of
3640 its expressions evaluated in list context. Also be careful not to
3641 follow the print keyword with a left parenthesis unless you want
3642 the corresponding right parenthesis to terminate the arguments to
3643 the print--interpose a C<+> or put parentheses around all the
3646 Note that if you're storing FILEHANDLES in an array or other expression,
3647 you will have to use a block returning its value instead:
3649 print { $files[$i] } "stuff\n";
3650 print { $OK ? STDOUT : STDERR } "stuff\n";
3652 =item printf FILEHANDLE FORMAT, LIST
3654 =item printf FORMAT, LIST
3656 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3657 (the output record separator) is not appended. The first argument
3658 of the list will be interpreted as the C<printf> format. See C<sprintf>
3659 for an explanation of the format argument. If C<use locale> is in effect,
3660 the character used for the decimal point in formatted real numbers is
3661 affected by the LC_NUMERIC locale. See L<perllocale>.
3663 Don't fall into the trap of using a C<printf> when a simple
3664 C<print> would do. The C<print> is more efficient and less
3667 =item prototype FUNCTION
3669 Returns the prototype of a function as a string (or C<undef> if the
3670 function has no prototype). FUNCTION is a reference to, or the name of,
3671 the function whose prototype you want to retrieve.
3673 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3674 name for Perl builtin. If the builtin is not I<overridable> (such as
3675 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3676 C<system>) returns C<undef> because the builtin does not really behave
3677 like a Perl function. Otherwise, the string describing the equivalent
3678 prototype is returned.
3680 =item push ARRAY,LIST
3682 Treats ARRAY as a stack, and pushes the values of LIST
3683 onto the end of ARRAY. The length of ARRAY increases by the length of
3684 LIST. Has the same effect as
3687 $ARRAY[++$#ARRAY] = $value;
3690 but is more efficient. Returns the new number of elements in the array.
3702 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3704 =item quotemeta EXPR
3708 Returns the value of EXPR with all non-"word"
3709 characters backslashed. (That is, all characters not matching
3710 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3711 returned string, regardless of any locale settings.)
3712 This is the internal function implementing
3713 the C<\Q> escape in double-quoted strings.
3715 If EXPR is omitted, uses C<$_>.
3721 Returns a random fractional number greater than or equal to C<0> and less
3722 than the value of EXPR. (EXPR should be positive.) If EXPR is
3723 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3724 unless C<srand> has already been called. See also C<srand>.
3726 Apply C<int()> to the value returned by C<rand()> if you want random
3727 integers instead of random fractional numbers. For example,
3731 returns a random integer between C<0> and C<9>, inclusive.
3733 (Note: If your rand function consistently returns numbers that are too
3734 large or too small, then your version of Perl was probably compiled
3735 with the wrong number of RANDBITS.)
3737 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3739 =item read FILEHANDLE,SCALAR,LENGTH
3741 Attempts to read LENGTH I<characters> of data into variable SCALAR
3742 from the specified FILEHANDLE. Returns the number of characters
3743 actually read, C<0> at end of file, or undef if there was an error.
3744 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3745 needs growing, the new bytes will be zero bytes. An OFFSET may be
3746 specified to place the read data into some other place in SCALAR than
3747 the beginning. The call is actually implemented in terms of either
3748 Perl's or system's fread() call. To get a true read(2) system call,
3751 Note the I<characters>: depending on the status of the filehandle,
3752 either (8-bit) bytes or characters are read. By default all
3753 filehandles operate on bytes, but for example if the filehandle has
3754 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3755 pragma, L<open>), the I/O will operate on characters, not bytes.
3757 =item readdir DIRHANDLE
3759 Returns the next directory entry for a directory opened by C<opendir>.
3760 If used in list context, returns all the rest of the entries in the
3761 directory. If there are no more entries, returns an undefined value in
3762 scalar context or a null list in list context.
3764 If you're planning to filetest the return values out of a C<readdir>, you'd
3765 better prepend the directory in question. Otherwise, because we didn't
3766 C<chdir> there, it would have been testing the wrong file.
3768 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3769 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3774 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3775 context, each call reads and returns the next line, until end-of-file is
3776 reached, whereupon the subsequent call returns undef. In list context,
3777 reads until end-of-file is reached and returns a list of lines. Note that
3778 the notion of "line" used here is however you may have defined it
3779 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3781 When C<$/> is set to C<undef>, when readline() is in scalar
3782 context (i.e. file slurp mode), and when an empty file is read, it
3783 returns C<''> the first time, followed by C<undef> subsequently.
3785 This is the internal function implementing the C<< <EXPR> >>
3786 operator, but you can use it directly. The C<< <EXPR> >>
3787 operator is discussed in more detail in L<perlop/"I/O Operators">.
3790 $line = readline(*STDIN); # same thing
3796 Returns the value of a symbolic link, if symbolic links are
3797 implemented. If not, gives a fatal error. If there is some system
3798 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3799 omitted, uses C<$_>.
3803 EXPR is executed as a system command.
3804 The collected standard output of the command is returned.
3805 In scalar context, it comes back as a single (potentially
3806 multi-line) string. In list context, returns a list of lines
3807 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3808 This is the internal function implementing the C<qx/EXPR/>
3809 operator, but you can use it directly. The C<qx/EXPR/>
3810 operator is discussed in more detail in L<perlop/"I/O Operators">.
3812 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3814 Receives a message on a socket. Attempts to receive LENGTH characters
3815 of data into variable SCALAR from the specified SOCKET filehandle.
3816 SCALAR will be grown or shrunk to the length actually read. Takes the
3817 same flags as the system call of the same name. Returns the address
3818 of the sender if SOCKET's protocol supports this; returns an empty
3819 string otherwise. If there's an error, returns the undefined value.
3820 This call is actually implemented in terms of recvfrom(2) system call.
3821 See L<perlipc/"UDP: Message Passing"> for examples.
3823 Note the I<characters>: depending on the status of the socket, either
3824 (8-bit) bytes or characters are received. By default all sockets
3825 operate on bytes, but for example if the socket has been changed using
3826 binmode() to operate with the C<:utf8> discipline (see the C<open>
3827 pragma, L<open>), the I/O will operate on characters, not bytes.
3833 The C<redo> command restarts the loop block without evaluating the
3834 conditional again. The C<continue> block, if any, is not executed. If
3835 the LABEL is omitted, the command refers to the innermost enclosing
3836 loop. This command is normally used by programs that want to lie to
3837 themselves about what was just input:
3839 # a simpleminded Pascal comment stripper
3840 # (warning: assumes no { or } in strings)
3841 LINE: while (<STDIN>) {
3842 while (s|({.*}.*){.*}|$1 |) {}
3847 if (/}/) { # end of comment?
3856 C<redo> cannot be used to retry a block which returns a value such as
3857 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3858 a grep() or map() operation.
3860 Note that a block by itself is semantically identical to a loop
3861 that executes once. Thus C<redo> inside such a block will effectively
3862 turn it into a looping construct.
3864 See also L</continue> for an illustration of how C<last>, C<next>, and
3871 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3872 is not specified, C<$_> will be used. The value returned depends on the
3873 type of thing the reference is a reference to.
3874 Builtin types include:
3884 If the referenced object has been blessed into a package, then that package
3885 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3887 if (ref($r) eq "HASH") {
3888 print "r is a reference to a hash.\n";
3891 print "r is not a reference at all.\n";
3893 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3894 print "r is a reference to something that isa hash.\n";
3897 See also L<perlref>.
3899 =item rename OLDNAME,NEWNAME
3901 Changes the name of a file; an existing file NEWNAME will be
3902 clobbered. Returns true for success, false otherwise.
3904 Behavior of this function varies wildly depending on your system
3905 implementation. For example, it will usually not work across file system
3906 boundaries, even though the system I<mv> command sometimes compensates
3907 for this. Other restrictions include whether it works on directories,
3908 open files, or pre-existing files. Check L<perlport> and either the
3909 rename(2) manpage or equivalent system documentation for details.
3911 =item require VERSION
3917 Demands a version of Perl specified by VERSION, or demands some semantics
3918 specified by EXPR or by C<$_> if EXPR is not supplied.
3920 VERSION may be either a numeric argument such as 5.006, which will be
3921 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3922 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3923 VERSION is greater than the version of the current Perl interpreter.
3924 Compare with L</use>, which can do a similar check at compile time.
3926 Specifying VERSION as a literal of the form v5.6.1 should generally be
3927 avoided, because it leads to misleading error messages under earlier
3928 versions of Perl which do not support this syntax. The equivalent numeric
3929 version should be used instead.
3931 require v5.6.1; # run time version check
3932 require 5.6.1; # ditto
3933 require 5.006_001; # ditto; preferred for backwards compatibility
3935 Otherwise, demands that a library file be included if it hasn't already
3936 been included. The file is included via the do-FILE mechanism, which is
3937 essentially just a variety of C<eval>. Has semantics similar to the following
3942 return 1 if $INC{$filename};
3943 my($realfilename,$result);
3945 foreach $prefix (@INC) {
3946 $realfilename = "$prefix/$filename";
3947 if (-f $realfilename) {
3948 $INC{$filename} = $realfilename;
3949 $result = do $realfilename;
3953 die "Can't find $filename in \@INC";
3955 delete $INC{$filename} if $@ || !$result;
3957 die "$filename did not return true value" unless $result;
3961 Note that the file will not be included twice under the same specified
3962 name. The file must return true as the last statement to indicate
3963 successful execution of any initialization code, so it's customary to
3964 end such a file with C<1;> unless you're sure it'll return true
3965 otherwise. But it's better just to put the C<1;>, in case you add more
3968 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3969 replaces "F<::>" with "F</>" in the filename for you,
3970 to make it easy to load standard modules. This form of loading of
3971 modules does not risk altering your namespace.
3973 In other words, if you try this:
3975 require Foo::Bar; # a splendid bareword
3977 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3978 directories specified in the C<@INC> array.
3980 But if you try this:
3982 $class = 'Foo::Bar';
3983 require $class; # $class is not a bareword
3985 require "Foo::Bar"; # not a bareword because of the ""
3987 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3988 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3990 eval "require $class";
3992 You can also insert hooks into the import facility, by putting directly
3993 Perl code into the @INC array. There are three forms of hooks: subroutine
3994 references, array references and blessed objects.
3996 Subroutine references are the simplest case. When the inclusion system
3997 walks through @INC and encounters a subroutine, this subroutine gets
3998 called with two parameters, the first being a reference to itself, and the
3999 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4000 subroutine should return C<undef> or a filehandle, from which the file to
4001 include will be read. If C<undef> is returned, C<require> will look at
4002 the remaining elements of @INC.
4004 If the hook is an array reference, its first element must be a subroutine
4005 reference. This subroutine is called as above, but the first parameter is
4006 the array reference. This enables to pass indirectly some arguments to
4009 In other words, you can write:
4011 push @INC, \&my_sub;
4013 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4019 push @INC, [ \&my_sub, $x, $y, ... ];
4021 my ($arrayref, $filename) = @_;
4022 # Retrieve $x, $y, ...
4023 my @parameters = @$arrayref[1..$#$arrayref];
4027 If the hook is an object, it must provide an INC method, that will be
4028 called as above, the first parameter being the object itself. (Note that
4029 you must fully qualify the sub's name, as it is always forced into package
4030 C<main>.) Here is a typical code layout:
4036 my ($self, $filename) = @_;
4040 # In the main program
4041 push @INC, new Foo(...);
4043 Note that these hooks are also permitted to set the %INC entry
4044 corresponding to the files they have loaded. See L<perlvar/%INC>.
4046 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4052 Generally used in a C<continue> block at the end of a loop to clear
4053 variables and reset C<??> searches so that they work again. The
4054 expression is interpreted as a list of single characters (hyphens
4055 allowed for ranges). All variables and arrays beginning with one of
4056 those letters are reset to their pristine state. If the expression is
4057 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4058 only variables or searches in the current package. Always returns
4061 reset 'X'; # reset all X variables
4062 reset 'a-z'; # reset lower case variables
4063 reset; # just reset ?one-time? searches
4065 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4066 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4067 variables--lexical variables are unaffected, but they clean themselves
4068 up on scope exit anyway, so you'll probably want to use them instead.
4075 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4076 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4077 context, depending on how the return value will be used, and the context
4078 may vary from one execution to the next (see C<wantarray>). If no EXPR
4079 is given, returns an empty list in list context, the undefined value in
4080 scalar context, and (of course) nothing at all in a void context.
4082 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4083 or do FILE will automatically return the value of the last expression
4088 In list context, returns a list value consisting of the elements
4089 of LIST in the opposite order. In scalar context, concatenates the
4090 elements of LIST and returns a string value with all characters
4091 in the opposite order.
4093 print reverse <>; # line tac, last line first
4095 undef $/; # for efficiency of <>
4096 print scalar reverse <>; # character tac, last line tsrif
4098 This operator is also handy for inverting a hash, although there are some
4099 caveats. If a value is duplicated in the original hash, only one of those
4100 can be represented as a key in the inverted hash. Also, this has to
4101 unwind one hash and build a whole new one, which may take some time
4102 on a large hash, such as from a DBM file.
4104 %by_name = reverse %by_address; # Invert the hash
4106 =item rewinddir DIRHANDLE
4108 Sets the current position to the beginning of the directory for the
4109 C<readdir> routine on DIRHANDLE.
4111 =item rindex STR,SUBSTR,POSITION
4113 =item rindex STR,SUBSTR
4115 Works just like index() except that it returns the position of the LAST
4116 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4117 last occurrence at or before that position.
4119 =item rmdir FILENAME
4123 Deletes the directory specified by FILENAME if that directory is empty. If it
4124 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4125 FILENAME is omitted, uses C<$_>.
4129 The substitution operator. See L<perlop>.
4133 Forces EXPR to be interpreted in scalar context and returns the value
4136 @counts = ( scalar @a, scalar @b, scalar @c );
4138 There is no equivalent operator to force an expression to
4139 be interpolated in list context because in practice, this is never
4140 needed. If you really wanted to do so, however, you could use
4141 the construction C<@{[ (some expression) ]}>, but usually a simple
4142 C<(some expression)> suffices.
4144 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4145 parenthesized list, this behaves as a scalar comma expression, evaluating
4146 all but the last element in void context and returning the final element
4147 evaluated in scalar context. This is seldom what you want.
4149 The following single statement:
4151 print uc(scalar(&foo,$bar)),$baz;
4153 is the moral equivalent of these two:
4156 print(uc($bar),$baz);
4158 See L<perlop> for more details on unary operators and the comma operator.
4160 =item seek FILEHANDLE,POSITION,WHENCE
4162 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4163 FILEHANDLE may be an expression whose value gives the name of the
4164 filehandle. The values for WHENCE are C<0> to set the new position
4165 I<in bytes> to POSITION, C<1> to set it to the current position plus
4166 POSITION, and C<2> to set it to EOF plus POSITION (typically
4167 negative). For WHENCE you may use the constants C<SEEK_SET>,
4168 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4169 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4172 Note the I<in bytes>: even if the filehandle has been set to
4173 operate on characters (for example by using the C<:utf8> open
4174 discipline), tell() will return byte offsets, not character offsets
4175 (because implementing that would render seek() and tell() rather slow).
4177 If you want to position file for C<sysread> or C<syswrite>, don't use
4178 C<seek>--buffering makes its effect on the file's system position
4179 unpredictable and non-portable. Use C<sysseek> instead.
4181 Due to the rules and rigors of ANSI C, on some systems you have to do a
4182 seek whenever you switch between reading and writing. Amongst other
4183 things, this may have the effect of calling stdio's clearerr(3).
4184 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4188 This is also useful for applications emulating C<tail -f>. Once you hit
4189 EOF on your read, and then sleep for a while, you might have to stick in a
4190 seek() to reset things. The C<seek> doesn't change the current position,
4191 but it I<does> clear the end-of-file condition on the handle, so that the
4192 next C<< <FILE> >> makes Perl try again to read something. We hope.
4194 If that doesn't work (some IO implementations are particularly
4195 cantankerous), then you may need something more like this:
4198 for ($curpos = tell(FILE); $_ = <FILE>;
4199 $curpos = tell(FILE)) {
4200 # search for some stuff and put it into files
4202 sleep($for_a_while);
4203 seek(FILE, $curpos, 0);
4206 =item seekdir DIRHANDLE,POS
4208 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4209 must be a value returned by C<telldir>. Has the same caveats about
4210 possible directory compaction as the corresponding system library
4213 =item select FILEHANDLE
4217 Returns the currently selected filehandle. Sets the current default
4218 filehandle for output, if FILEHANDLE is supplied. This has two
4219 effects: first, a C<write> or a C<print> without a filehandle will
4220 default to this FILEHANDLE. Second, references to variables related to
4221 output will refer to this output channel. For example, if you have to
4222 set the top of form format for more than one output channel, you might
4230 FILEHANDLE may be an expression whose value gives the name of the
4231 actual filehandle. Thus:
4233 $oldfh = select(STDERR); $| = 1; select($oldfh);
4235 Some programmers may prefer to think of filehandles as objects with
4236 methods, preferring to write the last example as:
4239 STDERR->autoflush(1);
4241 =item select RBITS,WBITS,EBITS,TIMEOUT
4243 This calls the select(2) system call with the bit masks specified, which
4244 can be constructed using C<fileno> and C<vec>, along these lines:
4246 $rin = $win = $ein = '';
4247 vec($rin,fileno(STDIN),1) = 1;
4248 vec($win,fileno(STDOUT),1) = 1;
4251 If you want to select on many filehandles you might wish to write a
4255 my(@fhlist) = split(' ',$_[0]);
4258 vec($bits,fileno($_),1) = 1;
4262 $rin = fhbits('STDIN TTY SOCK');
4266 ($nfound,$timeleft) =
4267 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4269 or to block until something becomes ready just do this
4271 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4273 Most systems do not bother to return anything useful in $timeleft, so
4274 calling select() in scalar context just returns $nfound.
4276 Any of the bit masks can also be undef. The timeout, if specified, is
4277 in seconds, which may be fractional. Note: not all implementations are
4278 capable of returning the $timeleft. If not, they always return
4279 $timeleft equal to the supplied $timeout.
4281 You can effect a sleep of 250 milliseconds this way:
4283 select(undef, undef, undef, 0.25);
4285 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4286 is implementation-dependent.
4288 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4289 or <FH>) with C<select>, except as permitted by POSIX, and even
4290 then only on POSIX systems. You have to use C<sysread> instead.
4292 =item semctl ID,SEMNUM,CMD,ARG
4294 Calls the System V IPC function C<semctl>. You'll probably have to say
4298 first to get the correct constant definitions. If CMD is IPC_STAT or
4299 GETALL, then ARG must be a variable which will hold the returned
4300 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4301 the undefined value for error, "C<0 but true>" for zero, or the actual
4302 return value otherwise. The ARG must consist of a vector of native
4303 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4304 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4307 =item semget KEY,NSEMS,FLAGS
4309 Calls the System V IPC function semget. Returns the semaphore id, or
4310 the undefined value if there is an error. See also
4311 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4314 =item semop KEY,OPSTRING
4316 Calls the System V IPC function semop to perform semaphore operations
4317 such as signalling and waiting. OPSTRING must be a packed array of
4318 semop structures. Each semop structure can be generated with
4319 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4320 operations is implied by the length of OPSTRING. Returns true if
4321 successful, or false if there is an error. As an example, the
4322 following code waits on semaphore $semnum of semaphore id $semid:
4324 $semop = pack("s!3", $semnum, -1, 0);
4325 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4327 To signal the semaphore, replace C<-1> with C<1>. See also
4328 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4331 =item send SOCKET,MSG,FLAGS,TO
4333 =item send SOCKET,MSG,FLAGS
4335 Sends a message on a socket. Attempts to send the scalar MSG to the
4336 SOCKET filehandle. Takes the same flags as the system call of the
4337 same name. On unconnected sockets you must specify a destination to
4338 send TO, in which case it does a C C<sendto>. Returns the number of
4339 characters sent, or the undefined value if there is an error. The C
4340 system call sendmsg(2) is currently unimplemented. See
4341 L<perlipc/"UDP: Message Passing"> for examples.
4343 Note the I<characters>: depending on the status of the socket, either
4344 (8-bit) bytes or characters are sent. By default all sockets operate
4345 on bytes, but for example if the socket has been changed using
4346 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4347 the C<open> pragma, L<open>), the I/O will operate on characters, not
4350 =item setpgrp PID,PGRP
4352 Sets the current process group for the specified PID, C<0> for the current
4353 process. Will produce a fatal error if used on a machine that doesn't
4354 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4355 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4356 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4359 =item setpriority WHICH,WHO,PRIORITY
4361 Sets the current priority for a process, a process group, or a user.
4362 (See setpriority(2).) Will produce a fatal error if used on a machine
4363 that doesn't implement setpriority(2).
4365 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4367 Sets the socket option requested. Returns undefined if there is an
4368 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4375 Shifts the first value of the array off and returns it, shortening the
4376 array by 1 and moving everything down. If there are no elements in the
4377 array, returns the undefined value. If ARRAY is omitted, shifts the
4378 C<@_> array within the lexical scope of subroutines and formats, and the
4379 C<@ARGV> array at file scopes or within the lexical scopes established by
4380 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4383 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4384 same thing to the left end of an array that C<pop> and C<push> do to the
4387 =item shmctl ID,CMD,ARG
4389 Calls the System V IPC function shmctl. You'll probably have to say
4393 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4394 then ARG must be a variable which will hold the returned C<shmid_ds>
4395 structure. Returns like ioctl: the undefined value for error, "C<0> but
4396 true" for zero, or the actual return value otherwise.
4397 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4399 =item shmget KEY,SIZE,FLAGS
4401 Calls the System V IPC function shmget. Returns the shared memory
4402 segment id, or the undefined value if there is an error.
4403 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4405 =item shmread ID,VAR,POS,SIZE
4407 =item shmwrite ID,STRING,POS,SIZE
4409 Reads or writes the System V shared memory segment ID starting at
4410 position POS for size SIZE by attaching to it, copying in/out, and
4411 detaching from it. When reading, VAR must be a variable that will
4412 hold the data read. When writing, if STRING is too long, only SIZE
4413 bytes are used; if STRING is too short, nulls are written to fill out
4414 SIZE bytes. Return true if successful, or false if there is an error.
4415 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4416 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4418 =item shutdown SOCKET,HOW
4420 Shuts down a socket connection in the manner indicated by HOW, which
4421 has the same interpretation as in the system call of the same name.
4423 shutdown(SOCKET, 0); # I/we have stopped reading data
4424 shutdown(SOCKET, 1); # I/we have stopped writing data
4425 shutdown(SOCKET, 2); # I/we have stopped using this socket
4427 This is useful with sockets when you want to tell the other
4428 side you're done writing but not done reading, or vice versa.
4429 It's also a more insistent form of close because it also
4430 disables the file descriptor in any forked copies in other
4437 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4438 returns sine of C<$_>.
4440 For the inverse sine operation, you may use the C<Math::Trig::asin>
4441 function, or use this relation:
4443 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4449 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4450 May be interrupted if the process receives a signal such as C<SIGALRM>.
4451 Returns the number of seconds actually slept. You probably cannot
4452 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4455 On some older systems, it may sleep up to a full second less than what
4456 you requested, depending on how it counts seconds. Most modern systems
4457 always sleep the full amount. They may appear to sleep longer than that,
4458 however, because your process might not be scheduled right away in a
4459 busy multitasking system.
4461 For delays of finer granularity than one second, you may use Perl's
4462 C<syscall> interface to access setitimer(2) if your system supports
4463 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4464 and starting from Perl 5.8 part of the standard distribution) may also
4467 See also the POSIX module's C<pause> function.
4469 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4471 Opens a socket of the specified kind and attaches it to filehandle
4472 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4473 the system call of the same name. You should C<use Socket> first
4474 to get the proper definitions imported. See the examples in
4475 L<perlipc/"Sockets: Client/Server Communication">.
4477 On systems that support a close-on-exec flag on files, the flag will
4478 be set for the newly opened file descriptor, as determined by the
4479 value of $^F. See L<perlvar/$^F>.
4481 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4483 Creates an unnamed pair of sockets in the specified domain, of the
4484 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4485 for the system call of the same name. If unimplemented, yields a fatal
4486 error. Returns true if successful.
4488 On systems that support a close-on-exec flag on files, the flag will
4489 be set for the newly opened file descriptors, as determined by the value
4490 of $^F. See L<perlvar/$^F>.
4492 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4493 to C<pipe(Rdr, Wtr)> is essentially:
4496 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4497 shutdown(Rdr, 1); # no more writing for reader
4498 shutdown(Wtr, 0); # no more reading for writer
4500 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4501 emulate socketpair using IP sockets to localhost if your system implements
4502 sockets but not socketpair.
4504 =item sort SUBNAME LIST
4506 =item sort BLOCK LIST
4510 In list context, this sorts the LIST and returns the sorted list value.
4511 In scalar context, the behaviour of C<sort()> is undefined.
4513 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4514 order. If SUBNAME is specified, it gives the name of a subroutine
4515 that returns an integer less than, equal to, or greater than C<0>,
4516 depending on how the elements of the list are to be ordered. (The C<<
4517 <=> >> and C<cmp> operators are extremely useful in such routines.)
4518 SUBNAME may be a scalar variable name (unsubscripted), in which case
4519 the value provides the name of (or a reference to) the actual
4520 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4521 an anonymous, in-line sort subroutine.
4523 If the subroutine's prototype is C<($$)>, the elements to be compared
4524 are passed by reference in C<@_>, as for a normal subroutine. This is
4525 slower than unprototyped subroutines, where the elements to be
4526 compared are passed into the subroutine
4527 as the package global variables $a and $b (see example below). Note that
4528 in the latter case, it is usually counter-productive to declare $a and
4531 In either case, the subroutine may not be recursive. The values to be
4532 compared are always passed by reference, so don't modify them.
4534 You also cannot exit out of the sort block or subroutine using any of the
4535 loop control operators described in L<perlsyn> or with C<goto>.
4537 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4538 current collation locale. See L<perllocale>.
4540 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4541 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4542 preserves the input order of elements that compare equal. Although
4543 quicksort's run time is O(NlogN) when averaged over all arrays of
4544 length N, the time can be O(N**2), I<quadratic> behavior, for some
4545 inputs.) In 5.7, the quicksort implementation was replaced with
4546 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4547 But benchmarks indicated that for some inputs, on some platforms,
4548 the original quicksort was faster. 5.8 has a sort pragma for
4549 limited control of the sort. Its rather blunt control of the
4550 underlying algorithm may not persist into future perls, but the
4551 ability to characterize the input or output in implementation
4552 independent ways quite probably will. See L</use>.
4557 @articles = sort @files;
4559 # same thing, but with explicit sort routine
4560 @articles = sort {$a cmp $b} @files;
4562 # now case-insensitively
4563 @articles = sort {uc($a) cmp uc($b)} @files;
4565 # same thing in reversed order
4566 @articles = sort {$b cmp $a} @files;
4568 # sort numerically ascending
4569 @articles = sort {$a <=> $b} @files;
4571 # sort numerically descending
4572 @articles = sort {$b <=> $a} @files;
4574 # this sorts the %age hash by value instead of key
4575 # using an in-line function
4576 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4578 # sort using explicit subroutine name
4580 $age{$a} <=> $age{$b}; # presuming numeric
4582 @sortedclass = sort byage @class;
4584 sub backwards { $b cmp $a }
4585 @harry = qw(dog cat x Cain Abel);
4586 @george = qw(gone chased yz Punished Axed);
4588 # prints AbelCaincatdogx
4589 print sort backwards @harry;
4590 # prints xdogcatCainAbel
4591 print sort @george, 'to', @harry;
4592 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4594 # inefficiently sort by descending numeric compare using
4595 # the first integer after the first = sign, or the
4596 # whole record case-insensitively otherwise
4599 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4604 # same thing, but much more efficiently;
4605 # we'll build auxiliary indices instead
4609 push @nums, /=(\d+)/;
4614 $nums[$b] <=> $nums[$a]
4616 $caps[$a] cmp $caps[$b]
4620 # same thing, but without any temps
4621 @new = map { $_->[0] }
4622 sort { $b->[1] <=> $a->[1]
4625 } map { [$_, /=(\d+)/, uc($_)] } @old;
4627 # using a prototype allows you to use any comparison subroutine
4628 # as a sort subroutine (including other package's subroutines)
4630 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4633 @new = sort other::backwards @old;
4635 # guarantee stability, regardless of algorithm
4637 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4639 # force use of mergesort (not portable outside Perl 5.8)
4640 use sort '_mergesort'; # note discouraging _
4641 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4643 If you're using strict, you I<must not> declare $a
4644 and $b as lexicals. They are package globals. That means
4645 if you're in the C<main> package and type
4647 @articles = sort {$b <=> $a} @files;
4649 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4650 but if you're in the C<FooPack> package, it's the same as typing
4652 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4654 The comparison function is required to behave. If it returns
4655 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4656 sometimes saying the opposite, for example) the results are not
4659 =item splice ARRAY,OFFSET,LENGTH,LIST
4661 =item splice ARRAY,OFFSET,LENGTH
4663 =item splice ARRAY,OFFSET
4667 Removes the elements designated by OFFSET and LENGTH from an array, and
4668 replaces them with the elements of LIST, if any. In list context,
4669 returns the elements removed from the array. In scalar context,
4670 returns the last element removed, or C<undef> if no elements are
4671 removed. The array grows or shrinks as necessary.
4672 If OFFSET is negative then it starts that far from the end of the array.
4673 If LENGTH is omitted, removes everything from OFFSET onward.
4674 If LENGTH is negative, removes the elements from OFFSET onward
4675 except for -LENGTH elements at the end of the array.
4676 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4677 past the end of the array, perl issues a warning, and splices at the
4680 The following equivalences hold (assuming C<$[ == 0>):
4682 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4683 pop(@a) splice(@a,-1)
4684 shift(@a) splice(@a,0,1)
4685 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4686 $a[$x] = $y splice(@a,$x,1,$y)
4688 Example, assuming array lengths are passed before arrays:
4690 sub aeq { # compare two list values
4691 my(@a) = splice(@_,0,shift);
4692 my(@b) = splice(@_,0,shift);
4693 return 0 unless @a == @b; # same len?
4695 return 0 if pop(@a) ne pop(@b);
4699 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4701 =item split /PATTERN/,EXPR,LIMIT
4703 =item split /PATTERN/,EXPR
4705 =item split /PATTERN/
4709 Splits a string into a list of strings and returns that list. By default,
4710 empty leading fields are preserved, and empty trailing ones are deleted.
4712 In scalar context, returns the number of fields found and splits into
4713 the C<@_> array. Use of split in scalar context is deprecated, however,
4714 because it clobbers your subroutine arguments.
4716 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4717 splits on whitespace (after skipping any leading whitespace). Anything
4718 matching PATTERN is taken to be a delimiter separating the fields. (Note
4719 that the delimiter may be longer than one character.)
4721 If LIMIT is specified and positive, it represents the maximum number
4722 of fields the EXPR will be split into, though the actual number of
4723 fields returned depends on the number of times PATTERN matches within
4724 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4725 stripped (which potential users of C<pop> would do well to remember).
4726 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4727 had been specified. Note that splitting an EXPR that evaluates to the
4728 empty string always returns the empty list, regardless of the LIMIT
4731 A pattern matching the null string (not to be confused with
4732 a null pattern C<//>, which is just one member of the set of patterns
4733 matching a null string) will split the value of EXPR into separate
4734 characters at each point it matches that way. For example:
4736 print join(':', split(/ */, 'hi there'));
4738 produces the output 'h:i:t:h:e:r:e'.
4740 Using the empty pattern C<//> specifically matches the null string, and is
4741 not be confused with the use of C<//> to mean "the last successful pattern
4744 Empty leading (or trailing) fields are produced when there are positive width
4745 matches at the beginning (or end) of the string; a zero-width match at the
4746 beginning (or end) of the string does not produce an empty field. For
4749 print join(':', split(/(?=\w)/, 'hi there!'));
4751 produces the output 'h:i :t:h:e:r:e!'.
4753 The LIMIT parameter can be used to split a line partially
4755 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4757 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4758 one larger than the number of variables in the list, to avoid
4759 unnecessary work. For the list above LIMIT would have been 4 by
4760 default. In time critical applications it behooves you not to split
4761 into more fields than you really need.
4763 If the PATTERN contains parentheses, additional list elements are
4764 created from each matching substring in the delimiter.
4766 split(/([,-])/, "1-10,20", 3);
4768 produces the list value
4770 (1, '-', 10, ',', 20)
4772 If you had the entire header of a normal Unix email message in $header,
4773 you could split it up into fields and their values this way:
4775 $header =~ s/\n\s+/ /g; # fix continuation lines
4776 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4778 The pattern C</PATTERN/> may be replaced with an expression to specify
4779 patterns that vary at runtime. (To do runtime compilation only once,
4780 use C</$variable/o>.)
4782 As a special case, specifying a PATTERN of space (C<' '>) will split on
4783 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4784 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4785 will give you as many null initial fields as there are leading spaces.
4786 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4787 whitespace produces a null first field. A C<split> with no arguments
4788 really does a C<split(' ', $_)> internally.
4790 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4795 open(PASSWD, '/etc/passwd');
4798 ($login, $passwd, $uid, $gid,
4799 $gcos, $home, $shell) = split(/:/);
4803 As with regular pattern matching, any capturing parentheses that are not
4804 matched in a C<split()> will be set to C<undef> when returned:
4806 @fields = split /(A)|B/, "1A2B3";
4807 # @fields is (1, 'A', 2, undef, 3)
4809 =item sprintf FORMAT, LIST
4811 Returns a string formatted by the usual C<printf> conventions of the C
4812 library function C<sprintf>. See below for more details
4813 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4814 the general principles.
4818 # Format number with up to 8 leading zeroes
4819 $result = sprintf("%08d", $number);
4821 # Round number to 3 digits after decimal point
4822 $rounded = sprintf("%.3f", $number);
4824 Perl does its own C<sprintf> formatting--it emulates the C
4825 function C<sprintf>, but it doesn't use it (except for floating-point
4826 numbers, and even then only the standard modifiers are allowed). As a
4827 result, any non-standard extensions in your local C<sprintf> are not
4828 available from Perl.
4830 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4831 pass it an array as your first argument. The array is given scalar context,
4832 and instead of using the 0th element of the array as the format, Perl will
4833 use the count of elements in the array as the format, which is almost never
4836 Perl's C<sprintf> permits the following universally-known conversions:
4839 %c a character with the given number
4841 %d a signed integer, in decimal
4842 %u an unsigned integer, in decimal
4843 %o an unsigned integer, in octal
4844 %x an unsigned integer, in hexadecimal
4845 %e a floating-point number, in scientific notation
4846 %f a floating-point number, in fixed decimal notation
4847 %g a floating-point number, in %e or %f notation
4849 In addition, Perl permits the following widely-supported conversions:
4851 %X like %x, but using upper-case letters
4852 %E like %e, but using an upper-case "E"
4853 %G like %g, but with an upper-case "E" (if applicable)
4854 %b an unsigned integer, in binary
4855 %p a pointer (outputs the Perl value's address in hexadecimal)
4856 %n special: *stores* the number of characters output so far
4857 into the next variable in the parameter list
4859 Finally, for backward (and we do mean "backward") compatibility, Perl
4860 permits these unnecessary but widely-supported conversions:
4863 %D a synonym for %ld
4864 %U a synonym for %lu
4865 %O a synonym for %lo
4868 Note that the number of exponent digits in the scientific notation by
4869 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4870 exponent less than 100 is system-dependent: it may be three or less
4871 (zero-padded as necessary). In other words, 1.23 times ten to the
4872 99th may be either "1.23e99" or "1.23e099".
4874 Perl permits the following universally-known flags between the C<%>
4875 and the conversion letter:
4877 space prefix positive number with a space
4878 + prefix positive number with a plus sign
4879 - left-justify within the field
4880 0 use zeros, not spaces, to right-justify
4881 # prefix non-zero octal with "0", non-zero hex with "0x"
4882 number minimum field width
4883 .number "precision": digits after decimal point for
4884 floating-point, max length for string, minimum length
4886 l interpret integer as C type "long" or "unsigned long"
4887 h interpret integer as C type "short" or "unsigned short"
4888 If no flags, interpret integer as C type "int" or "unsigned"
4890 Perl supports parameter ordering, in other words, fetching the
4891 parameters in some explicitly specified "random" ordering as opposed
4892 to the default implicit sequential ordering. The syntax is, instead
4893 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4894 where the I<digits> is the wanted index, from one upwards. For example:
4896 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4897 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4899 Note that using the reordering syntax does not interfere with the usual
4900 implicit sequential fetching of the parameters:
4902 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4903 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4904 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4905 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4906 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4908 There are also two Perl-specific flags:
4910 V interpret integer as Perl's standard integer type
4911 v interpret string as a vector of integers, output as
4912 numbers separated either by dots, or by an arbitrary
4913 string received from the argument list when the flag
4916 Where a number would appear in the flags, an asterisk (C<*>) may be
4917 used instead, in which case Perl uses the next item in the parameter
4918 list as the given number (that is, as the field width or precision).
4919 If a field width obtained through C<*> is negative, it has the same
4920 effect as the C<-> flag: left-justification.
4922 The C<v> flag is useful for displaying ordinal values of characters
4923 in arbitrary strings:
4925 printf "version is v%vd\n", $^V; # Perl's version
4926 printf "address is %*vX\n", ":", $addr; # IPv6 address
4927 printf "bits are %*vb\n", " ", $bits; # random bitstring
4929 If C<use locale> is in effect, the character used for the decimal
4930 point in formatted real numbers is affected by the LC_NUMERIC locale.
4933 If Perl understands "quads" (64-bit integers) (this requires
4934 either that the platform natively support quads or that Perl
4935 be specifically compiled to support quads), the characters
4939 print quads, and they may optionally be preceded by
4947 You can find out whether your Perl supports quads via L<Config>:
4950 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4953 If Perl understands "long doubles" (this requires that the platform
4954 support long doubles), the flags
4958 may optionally be preceded by
4966 You can find out whether your Perl supports long doubles via L<Config>:
4969 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4975 Return the square root of EXPR. If EXPR is omitted, returns square
4976 root of C<$_>. Only works on non-negative operands, unless you've
4977 loaded the standard Math::Complex module.
4980 print sqrt(-2); # prints 1.4142135623731i
4986 Sets the random number seed for the C<rand> operator.
4988 The point of the function is to "seed" the C<rand> function so that
4989 C<rand> can produce a different sequence each time you run your
4992 If srand() is not called explicitly, it is called implicitly at the
4993 first use of the C<rand> operator. However, this was not the case in
4994 versions of Perl before 5.004, so if your script will run under older
4995 Perl versions, it should call C<srand>.
4997 Most programs won't even call srand() at all, except those that
4998 need a cryptographically-strong starting point rather than the
4999 generally acceptable default, which is based on time of day,
5000 process ID, and memory allocation, or the F</dev/urandom> device,
5003 You can call srand($seed) with the same $seed to reproduce the
5004 I<same> sequence from rand(), but this is usually reserved for
5005 generating predictable results for testing or debugging.
5006 Otherwise, don't call srand() more than once in your program.
5008 Do B<not> call srand() (i.e. without an argument) more than once in
5009 a script. The internal state of the random number generator should
5010 contain more entropy than can be provided by any seed, so calling
5011 srand() again actually I<loses> randomness.
5013 Most implementations of C<srand> take an integer and will silently
5014 truncate decimal numbers. This means C<srand(42)> will usually
5015 produce the same results as C<srand(42.1)>. To be safe, always pass
5016 C<srand> an integer.
5018 In versions of Perl prior to 5.004 the default seed was just the
5019 current C<time>. This isn't a particularly good seed, so many old
5020 programs supply their own seed value (often C<time ^ $$> or C<time ^
5021 ($$ + ($$ << 15))>), but that isn't necessary any more.
5023 Note that you need something much more random than the default seed for
5024 cryptographic purposes. Checksumming the compressed output of one or more
5025 rapidly changing operating system status programs is the usual method. For
5028 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5030 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5033 Frequently called programs (like CGI scripts) that simply use
5037 for a seed can fall prey to the mathematical property that
5041 one-third of the time. So don't do that.
5043 =item stat FILEHANDLE
5049 Returns a 13-element list giving the status info for a file, either
5050 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5051 it stats C<$_>. Returns a null list if the stat fails. Typically used
5054 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5055 $atime,$mtime,$ctime,$blksize,$blocks)
5058 Not all fields are supported on all filesystem types. Here are the
5059 meaning of the fields:
5061 0 dev device number of filesystem
5063 2 mode file mode (type and permissions)
5064 3 nlink number of (hard) links to the file
5065 4 uid numeric user ID of file's owner
5066 5 gid numeric group ID of file's owner
5067 6 rdev the device identifier (special files only)
5068 7 size total size of file, in bytes
5069 8 atime last access time in seconds since the epoch
5070 9 mtime last modify time in seconds since the epoch
5071 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5072 11 blksize preferred block size for file system I/O
5073 12 blocks actual number of blocks allocated
5075 (The epoch was at 00:00 January 1, 1970 GMT.)
5077 If stat is passed the special filehandle consisting of an underline, no
5078 stat is done, but the current contents of the stat structure from the
5079 last stat or filetest are returned. Example:
5081 if (-x $file && (($d) = stat(_)) && $d < 0) {
5082 print "$file is executable NFS file\n";
5085 (This works on machines only for which the device number is negative
5088 Because the mode contains both the file type and its permissions, you
5089 should mask off the file type portion and (s)printf using a C<"%o">
5090 if you want to see the real permissions.
5092 $mode = (stat($filename))[2];
5093 printf "Permissions are %04o\n", $mode & 07777;
5095 In scalar context, C<stat> returns a boolean value indicating success
5096 or failure, and, if successful, sets the information associated with
5097 the special filehandle C<_>.
5099 The File::stat module provides a convenient, by-name access mechanism:
5102 $sb = stat($filename);
5103 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5104 $filename, $sb->size, $sb->mode & 07777,
5105 scalar localtime $sb->mtime;
5107 You can import symbolic mode constants (C<S_IF*>) and functions
5108 (C<S_IS*>) from the Fcntl module:
5112 $mode = (stat($filename))[2];
5114 $user_rwx = ($mode & S_IRWXU) >> 6;
5115 $group_read = ($mode & S_IRGRP) >> 3;
5116 $other_execute = $mode & S_IXOTH;
5118 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5120 $is_setuid = $mode & S_ISUID;
5121 $is_setgid = S_ISDIR($mode);
5123 You could write the last two using the C<-u> and C<-d> operators.
5124 The commonly available S_IF* constants are
5126 # Permissions: read, write, execute, for user, group, others.
5128 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5129 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5130 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5132 # Setuid/Setgid/Stickiness.
5134 S_ISUID S_ISGID S_ISVTX S_ISTXT
5136 # File types. Not necessarily all are available on your system.
5138 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5140 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5142 S_IREAD S_IWRITE S_IEXEC
5144 and the S_IF* functions are
5146 S_IFMODE($mode) the part of $mode containing the permission bits
5147 and the setuid/setgid/sticky bits
5149 S_IFMT($mode) the part of $mode containing the file type
5150 which can be bit-anded with e.g. S_IFREG
5151 or with the following functions
5153 # The operators -f, -d, -l, -b, -c, -p, and -s.
5155 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5156 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5158 # No direct -X operator counterpart, but for the first one
5159 # the -g operator is often equivalent. The ENFMT stands for
5160 # record flocking enforcement, a platform-dependent feature.
5162 S_ISENFMT($mode) S_ISWHT($mode)
5164 See your native chmod(2) and stat(2) documentation for more details
5165 about the S_* constants.
5171 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5172 doing many pattern matches on the string before it is next modified.
5173 This may or may not save time, depending on the nature and number of
5174 patterns you are searching on, and on the distribution of character
5175 frequencies in the string to be searched--you probably want to compare
5176 run times with and without it to see which runs faster. Those loops
5177 which scan for many short constant strings (including the constant
5178 parts of more complex patterns) will benefit most. You may have only
5179 one C<study> active at a time--if you study a different scalar the first
5180 is "unstudied". (The way C<study> works is this: a linked list of every
5181 character in the string to be searched is made, so we know, for
5182 example, where all the C<'k'> characters are. From each search string,
5183 the rarest character is selected, based on some static frequency tables
5184 constructed from some C programs and English text. Only those places
5185 that contain this "rarest" character are examined.)
5187 For example, here is a loop that inserts index producing entries
5188 before any line containing a certain pattern:
5192 print ".IX foo\n" if /\bfoo\b/;
5193 print ".IX bar\n" if /\bbar\b/;
5194 print ".IX blurfl\n" if /\bblurfl\b/;
5199 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5200 will be looked at, because C<f> is rarer than C<o>. In general, this is
5201 a big win except in pathological cases. The only question is whether
5202 it saves you more time than it took to build the linked list in the
5205 Note that if you have to look for strings that you don't know till
5206 runtime, you can build an entire loop as a string and C<eval> that to
5207 avoid recompiling all your patterns all the time. Together with
5208 undefining C<$/> to input entire files as one record, this can be very
5209 fast, often faster than specialized programs like fgrep(1). The following
5210 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5211 out the names of those files that contain a match:
5213 $search = 'while (<>) { study;';
5214 foreach $word (@words) {
5215 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5220 eval $search; # this screams
5221 $/ = "\n"; # put back to normal input delimiter
5222 foreach $file (sort keys(%seen)) {
5226 =item sub NAME BLOCK
5228 =item sub NAME (PROTO) BLOCK
5230 =item sub NAME : ATTRS BLOCK
5232 =item sub NAME (PROTO) : ATTRS BLOCK
5234 This is subroutine definition, not a real function I<per se>.
5235 Without a BLOCK it's just a forward declaration. Without a NAME,
5236 it's an anonymous function declaration, and does actually return
5237 a value: the CODE ref of the closure you just created.
5239 See L<perlsub> and L<perlref> for details about subroutines and
5240 references, and L<attributes> and L<Attribute::Handlers> for more
5241 information about attributes.
5243 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5245 =item substr EXPR,OFFSET,LENGTH
5247 =item substr EXPR,OFFSET
5249 Extracts a substring out of EXPR and returns it. First character is at
5250 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5251 If OFFSET is negative (or more precisely, less than C<$[>), starts
5252 that far from the end of the string. If LENGTH is omitted, returns
5253 everything to the end of the string. If LENGTH is negative, leaves that
5254 many characters off the end of the string.
5256 You can use the substr() function as an lvalue, in which case EXPR
5257 must itself be an lvalue. If you assign something shorter than LENGTH,
5258 the string will shrink, and if you assign something longer than LENGTH,
5259 the string will grow to accommodate it. To keep the string the same
5260 length you may need to pad or chop your value using C<sprintf>.
5262 If OFFSET and LENGTH specify a substring that is partly outside the
5263 string, only the part within the string is returned. If the substring
5264 is beyond either end of the string, substr() returns the undefined
5265 value and produces a warning. When used as an lvalue, specifying a
5266 substring that is entirely outside the string is a fatal error.
5267 Here's an example showing the behavior for boundary cases:
5270 substr($name, 4) = 'dy'; # $name is now 'freddy'
5271 my $null = substr $name, 6, 2; # returns '' (no warning)
5272 my $oops = substr $name, 7; # returns undef, with warning
5273 substr($name, 7) = 'gap'; # fatal error
5275 An alternative to using substr() as an lvalue is to specify the
5276 replacement string as the 4th argument. This allows you to replace
5277 parts of the EXPR and return what was there before in one operation,
5278 just as you can with splice().
5280 =item symlink OLDFILE,NEWFILE
5282 Creates a new filename symbolically linked to the old filename.
5283 Returns C<1> for success, C<0> otherwise. On systems that don't support
5284 symbolic links, produces a fatal error at run time. To check for that,
5287 $symlink_exists = eval { symlink("",""); 1 };
5291 Calls the system call specified as the first element of the list,
5292 passing the remaining elements as arguments to the system call. If
5293 unimplemented, produces a fatal error. The arguments are interpreted
5294 as follows: if a given argument is numeric, the argument is passed as
5295 an int. If not, the pointer to the string value is passed. You are
5296 responsible to make sure a string is pre-extended long enough to
5297 receive any result that might be written into a string. You can't use a
5298 string literal (or other read-only string) as an argument to C<syscall>
5299 because Perl has to assume that any string pointer might be written
5301 integer arguments are not literals and have never been interpreted in a
5302 numeric context, you may need to add C<0> to them to force them to look
5303 like numbers. This emulates the C<syswrite> function (or vice versa):
5305 require 'syscall.ph'; # may need to run h2ph
5307 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5309 Note that Perl supports passing of up to only 14 arguments to your system call,
5310 which in practice should usually suffice.
5312 Syscall returns whatever value returned by the system call it calls.
5313 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5314 Note that some system calls can legitimately return C<-1>. The proper
5315 way to handle such calls is to assign C<$!=0;> before the call and
5316 check the value of C<$!> if syscall returns C<-1>.
5318 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5319 number of the read end of the pipe it creates. There is no way
5320 to retrieve the file number of the other end. You can avoid this
5321 problem by using C<pipe> instead.
5323 =item sysopen FILEHANDLE,FILENAME,MODE
5325 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5327 Opens the file whose filename is given by FILENAME, and associates it
5328 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5329 the name of the real filehandle wanted. This function calls the
5330 underlying operating system's C<open> function with the parameters
5331 FILENAME, MODE, PERMS.
5333 The possible values and flag bits of the MODE parameter are
5334 system-dependent; they are available via the standard module C<Fcntl>.
5335 See the documentation of your operating system's C<open> to see which
5336 values and flag bits are available. You may combine several flags
5337 using the C<|>-operator.
5339 Some of the most common values are C<O_RDONLY> for opening the file in
5340 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5341 and C<O_RDWR> for opening the file in read-write mode, and.
5343 For historical reasons, some values work on almost every system
5344 supported by perl: zero means read-only, one means write-only, and two
5345 means read/write. We know that these values do I<not> work under
5346 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5347 use them in new code.
5349 If the file named by FILENAME does not exist and the C<open> call creates
5350 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5351 PERMS specifies the permissions of the newly created file. If you omit
5352 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5353 These permission values need to be in octal, and are modified by your
5354 process's current C<umask>.
5356 In many systems the C<O_EXCL> flag is available for opening files in
5357 exclusive mode. This is B<not> locking: exclusiveness means here that
5358 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5361 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5363 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5364 that takes away the user's option to have a more permissive umask.
5365 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5368 Note that C<sysopen> depends on the fdopen() C library function.
5369 On many UNIX systems, fdopen() is known to fail when file descriptors
5370 exceed a certain value, typically 255. If you need more file
5371 descriptors than that, consider rebuilding Perl to use the C<sfio>
5372 library, or perhaps using the POSIX::open() function.
5374 See L<perlopentut> for a kinder, gentler explanation of opening files.
5376 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5378 =item sysread FILEHANDLE,SCALAR,LENGTH
5380 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5381 the specified FILEHANDLE, using the system call read(2). It bypasses
5382 buffered IO, so mixing this with other kinds of reads, C<print>,
5383 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5384 stdio usually buffers data. Returns the number of characters actually
5385 read, C<0> at end of file, or undef if there was an error. SCALAR
5386 will be grown or shrunk so that the last byte actually read is the
5387 last byte of the scalar after the read.
5389 Note the I<characters>: depending on the status of the filehandle,
5390 either (8-bit) bytes or characters are read. By default all
5391 filehandles operate on bytes, but for example if the filehandle has
5392 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5393 pragma, L<open>), the I/O will operate on characters, not bytes.
5395 An OFFSET may be specified to place the read data at some place in the
5396 string other than the beginning. A negative OFFSET specifies
5397 placement at that many characters counting backwards from the end of
5398 the string. A positive OFFSET greater than the length of SCALAR
5399 results in the string being padded to the required size with C<"\0">
5400 bytes before the result of the read is appended.
5402 There is no syseof() function, which is ok, since eof() doesn't work
5403 very well on device files (like ttys) anyway. Use sysread() and check
5404 for a return value for 0 to decide whether you're done.
5406 =item sysseek FILEHANDLE,POSITION,WHENCE
5408 Sets FILEHANDLE's system position I<in bytes> using the system call
5409 lseek(2). FILEHANDLE may be an expression whose value gives the name
5410 of the filehandle. The values for WHENCE are C<0> to set the new
5411 position to POSITION, C<1> to set the it to the current position plus
5412 POSITION, and C<2> to set it to EOF plus POSITION (typically
5415 Note the I<in bytes>: even if the filehandle has been set to operate
5416 on characters (for example by using the C<:utf8> discipline), tell()
5417 will return byte offsets, not character offsets (because implementing
5418 that would render sysseek() very slow).
5420 sysseek() bypasses normal buffered io, so mixing this with reads (other
5421 than C<sysread>, for example >< or read()) C<print>, C<write>,
5422 C<seek>, C<tell>, or C<eof> may cause confusion.
5424 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5425 and C<SEEK_END> (start of the file, current position, end of the file)
5426 from the Fcntl module. Use of the constants is also more portable
5427 than relying on 0, 1, and 2. For example to define a "systell" function:
5429 use Fnctl 'SEEK_CUR';
5430 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5432 Returns the new position, or the undefined value on failure. A position
5433 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5434 true on success and false on failure, yet you can still easily determine
5439 =item system PROGRAM LIST
5441 Does exactly the same thing as C<exec LIST>, except that a fork is
5442 done first, and the parent process waits for the child process to
5443 complete. Note that argument processing varies depending on the
5444 number of arguments. If there is more than one argument in LIST,
5445 or if LIST is an array with more than one value, starts the program
5446 given by the first element of the list with arguments given by the
5447 rest of the list. If there is only one scalar argument, the argument
5448 is checked for shell metacharacters, and if there are any, the
5449 entire argument is passed to the system's command shell for parsing
5450 (this is C</bin/sh -c> on Unix platforms, but varies on other
5451 platforms). If there are no shell metacharacters in the argument,
5452 it is split into words and passed directly to C<execvp>, which is
5455 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5456 output before any operation that may do a fork, but this may not be
5457 supported on some platforms (see L<perlport>). To be safe, you may need
5458 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5459 of C<IO::Handle> on any open handles.
5461 The return value is the exit status of the program as returned by the
5462 C<wait> call. To get the actual exit value shift right by eight (see below).
5463 See also L</exec>. This is I<not> what you want to use to capture
5464 the output from a command, for that you should use merely backticks or
5465 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5466 indicates a failure to start the program (inspect $! for the reason).
5468 Like C<exec>, C<system> allows you to lie to a program about its name if
5469 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5471 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5472 killing the program they're running doesn't actually interrupt
5475 @args = ("command", "arg1", "arg2");
5477 or die "system @args failed: $?"
5479 You can check all the failure possibilities by inspecting
5482 $exit_value = $? >> 8;
5483 $signal_num = $? & 127;
5484 $dumped_core = $? & 128;
5486 or more portably by using the W*() calls of the POSIX extension;
5487 see L<perlport> for more information.
5489 When the arguments get executed via the system shell, results
5490 and return codes will be subject to its quirks and capabilities.
5491 See L<perlop/"`STRING`"> and L</exec> for details.
5493 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5495 =item syswrite FILEHANDLE,SCALAR,LENGTH
5497 =item syswrite FILEHANDLE,SCALAR
5499 Attempts to write LENGTH characters of data from variable SCALAR to
5500 the specified FILEHANDLE, using the system call write(2). If LENGTH
5501 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5502 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5503 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5504 buffers data. Returns the number of characters actually written, or
5505 C<undef> if there was an error. If the LENGTH is greater than the
5506 available data in the SCALAR after the OFFSET, only as much data as is
5507 available will be written.
5509 An OFFSET may be specified to write the data from some part of the
5510 string other than the beginning. A negative OFFSET specifies writing
5511 that many characters counting backwards from the end of the string.
5512 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5514 Note the I<characters>: depending on the status of the filehandle,
5515 either (8-bit) bytes or characters are written. By default all
5516 filehandles operate on bytes, but for example if the filehandle has
5517 been opened with the C<:utf8> discipline (see L</open>, and the open
5518 pragma, L<open>), the I/O will operate on characters, not bytes.
5520 =item tell FILEHANDLE
5524 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5525 error. FILEHANDLE may be an expression whose value gives the name of
5526 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5529 Note the I<in bytes>: even if the filehandle has been set to
5530 operate on characters (for example by using the C<:utf8> open
5531 discipline), tell() will return byte offsets, not character offsets
5532 (because that would render seek() and tell() rather slow).
5534 The return value of tell() for the standard streams like the STDIN
5535 depends on the operating system: it may return -1 or something else.
5536 tell() on pipes, fifos, and sockets usually returns -1.
5538 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5540 Do not use tell() on a filehandle that has been opened using
5541 sysopen(), use sysseek() for that as described above. Why? Because
5542 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5543 buffered filehandles. sysseek() make sense only on the first kind,
5544 tell() only makes sense on the second kind.
5546 =item telldir DIRHANDLE
5548 Returns the current position of the C<readdir> routines on DIRHANDLE.
5549 Value may be given to C<seekdir> to access a particular location in a
5550 directory. Has the same caveats about possible directory compaction as
5551 the corresponding system library routine.
5553 =item tie VARIABLE,CLASSNAME,LIST
5555 This function binds a variable to a package class that will provide the
5556 implementation for the variable. VARIABLE is the name of the variable
5557 to be enchanted. CLASSNAME is the name of a class implementing objects
5558 of correct type. Any additional arguments are passed to the C<new>
5559 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5560 or C<TIEHASH>). Typically these are arguments such as might be passed
5561 to the C<dbm_open()> function of C. The object returned by the C<new>
5562 method is also returned by the C<tie> function, which would be useful
5563 if you want to access other methods in CLASSNAME.
5565 Note that functions such as C<keys> and C<values> may return huge lists
5566 when used on large objects, like DBM files. You may prefer to use the
5567 C<each> function to iterate over such. Example:
5569 # print out history file offsets
5571 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5572 while (($key,$val) = each %HIST) {
5573 print $key, ' = ', unpack('L',$val), "\n";
5577 A class implementing a hash should have the following methods:
5579 TIEHASH classname, LIST
5581 STORE this, key, value
5586 NEXTKEY this, lastkey
5590 A class implementing an ordinary array should have the following methods:
5592 TIEARRAY classname, LIST
5594 STORE this, key, value
5596 STORESIZE this, count
5602 SPLICE this, offset, length, LIST
5607 A class implementing a file handle should have the following methods:
5609 TIEHANDLE classname, LIST
5610 READ this, scalar, length, offset
5613 WRITE this, scalar, length, offset
5615 PRINTF this, format, LIST
5619 SEEK this, position, whence
5621 OPEN this, mode, LIST
5626 A class implementing a scalar should have the following methods:
5628 TIESCALAR classname, LIST
5634 Not all methods indicated above need be implemented. See L<perltie>,
5635 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5637 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5638 for you--you need to do that explicitly yourself. See L<DB_File>
5639 or the F<Config> module for interesting C<tie> implementations.
5641 For further details see L<perltie>, L<"tied VARIABLE">.
5645 Returns a reference to the object underlying VARIABLE (the same value
5646 that was originally returned by the C<tie> call that bound the variable
5647 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5652 Returns the number of non-leap seconds since whatever time the system
5653 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5654 and 00:00:00 UTC, January 1, 1970 for most other systems).
5655 Suitable for feeding to C<gmtime> and C<localtime>.
5657 For measuring time in better granularity than one second,
5658 you may use either the Time::HiRes module from CPAN, or
5659 if you have gettimeofday(2), you may be able to use the
5660 C<syscall> interface of Perl, see L<perlfaq8> for details.
5664 Returns a four-element list giving the user and system times, in
5665 seconds, for this process and the children of this process.
5667 ($user,$system,$cuser,$csystem) = times;
5669 In scalar context, C<times> returns C<$user>.
5673 The transliteration operator. Same as C<y///>. See L<perlop>.
5675 =item truncate FILEHANDLE,LENGTH
5677 =item truncate EXPR,LENGTH
5679 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5680 specified length. Produces a fatal error if truncate isn't implemented
5681 on your system. Returns true if successful, the undefined value
5684 The behavior is undefined if LENGTH is greater than the length of the
5691 Returns an uppercased version of EXPR. This is the internal function
5692 implementing the C<\U> escape in double-quoted strings. Respects
5693 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5694 and L<perlunicode> for more details about locale and Unicode support.
5695 It does not attempt to do titlecase mapping on initial letters. See
5696 C<ucfirst> for that.
5698 If EXPR is omitted, uses C<$_>.
5704 Returns the value of EXPR with the first character in uppercase
5705 (titlecase in Unicode). This is the internal function implementing
5706 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5707 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5708 for more details about locale and Unicode support.
5710 If EXPR is omitted, uses C<$_>.
5716 Sets the umask for the process to EXPR and returns the previous value.
5717 If EXPR is omitted, merely returns the current umask.
5719 The Unix permission C<rwxr-x---> is represented as three sets of three
5720 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5721 and isn't one of the digits). The C<umask> value is such a number
5722 representing disabled permissions bits. The permission (or "mode")
5723 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5724 even if you tell C<sysopen> to create a file with permissions C<0777>,
5725 if your umask is C<0022> then the file will actually be created with
5726 permissions C<0755>. If your C<umask> were C<0027> (group can't
5727 write; others can't read, write, or execute), then passing
5728 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5731 Here's some advice: supply a creation mode of C<0666> for regular
5732 files (in C<sysopen>) and one of C<0777> for directories (in
5733 C<mkdir>) and executable files. This gives users the freedom of
5734 choice: if they want protected files, they might choose process umasks
5735 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5736 Programs should rarely if ever make policy decisions better left to
5737 the user. The exception to this is when writing files that should be
5738 kept private: mail files, web browser cookies, I<.rhosts> files, and
5741 If umask(2) is not implemented on your system and you are trying to
5742 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5743 fatal error at run time. If umask(2) is not implemented and you are
5744 not trying to restrict access for yourself, returns C<undef>.
5746 Remember that a umask is a number, usually given in octal; it is I<not> a
5747 string of octal digits. See also L</oct>, if all you have is a string.
5753 Undefines the value of EXPR, which must be an lvalue. Use only on a
5754 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5755 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5756 will probably not do what you expect on most predefined variables or
5757 DBM list values, so don't do that; see L<delete>.) Always returns the
5758 undefined value. You can omit the EXPR, in which case nothing is
5759 undefined, but you still get an undefined value that you could, for
5760 instance, return from a subroutine, assign to a variable or pass as a
5761 parameter. Examples:
5764 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5768 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5769 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5770 select undef, undef, undef, 0.25;
5771 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5773 Note that this is a unary operator, not a list operator.
5779 Deletes a list of files. Returns the number of files successfully
5782 $cnt = unlink 'a', 'b', 'c';
5786 Note: C<unlink> will not delete directories unless you are superuser and
5787 the B<-U> flag is supplied to Perl. Even if these conditions are
5788 met, be warned that unlinking a directory can inflict damage on your
5789 filesystem. Use C<rmdir> instead.
5791 If LIST is omitted, uses C<$_>.
5793 =item unpack TEMPLATE,EXPR
5795 C<unpack> does the reverse of C<pack>: it takes a string
5796 and expands it out into a list of values.
5797 (In scalar context, it returns merely the first value produced.)
5799 The string is broken into chunks described by the TEMPLATE. Each chunk
5800 is converted separately to a value. Typically, either the string is a result
5801 of C<pack>, or the bytes of the string represent a C structure of some
5804 The TEMPLATE has the same format as in the C<pack> function.
5805 Here's a subroutine that does substring:
5808 my($what,$where,$howmuch) = @_;
5809 unpack("x$where a$howmuch", $what);
5814 sub ordinal { unpack("c",$_[0]); } # same as ord()
5816 In addition to fields allowed in pack(), you may prefix a field with
5817 a %<number> to indicate that
5818 you want a <number>-bit checksum of the items instead of the items
5819 themselves. Default is a 16-bit checksum. Checksum is calculated by
5820 summing numeric values of expanded values (for string fields the sum of
5821 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5823 For example, the following
5824 computes the same number as the System V sum program:
5828 unpack("%32C*",<>) % 65535;
5831 The following efficiently counts the number of set bits in a bit vector:
5833 $setbits = unpack("%32b*", $selectmask);
5835 The C<p> and C<P> formats should be used with care. Since Perl
5836 has no way of checking whether the value passed to C<unpack()>
5837 corresponds to a valid memory location, passing a pointer value that's
5838 not known to be valid is likely to have disastrous consequences.
5840 If the repeat count of a field is larger than what the remainder of
5841 the input string allows, repeat count is decreased. If the input string
5842 is longer than one described by the TEMPLATE, the rest is ignored.
5844 See L</pack> for more examples and notes.
5846 =item untie VARIABLE
5848 Breaks the binding between a variable and a package. (See C<tie>.)
5850 =item unshift ARRAY,LIST
5852 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5853 depending on how you look at it. Prepends list to the front of the
5854 array, and returns the new number of elements in the array.
5856 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5858 Note the LIST is prepended whole, not one element at a time, so the
5859 prepended elements stay in the same order. Use C<reverse> to do the
5862 =item use Module VERSION LIST
5864 =item use Module VERSION
5866 =item use Module LIST
5872 Imports some semantics into the current package from the named module,
5873 generally by aliasing certain subroutine or variable names into your
5874 package. It is exactly equivalent to
5876 BEGIN { require Module; import Module LIST; }
5878 except that Module I<must> be a bareword.
5880 VERSION may be either a numeric argument such as 5.006, which will be
5881 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5882 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5883 greater than the version of the current Perl interpreter; Perl will not
5884 attempt to parse the rest of the file. Compare with L</require>, which can
5885 do a similar check at run time.
5887 Specifying VERSION as a literal of the form v5.6.1 should generally be
5888 avoided, because it leads to misleading error messages under earlier
5889 versions of Perl which do not support this syntax. The equivalent numeric
5890 version should be used instead.
5892 use v5.6.1; # compile time version check
5894 use 5.006_001; # ditto; preferred for backwards compatibility
5896 This is often useful if you need to check the current Perl version before
5897 C<use>ing library modules that have changed in incompatible ways from
5898 older versions of Perl. (We try not to do this more than we have to.)
5900 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5901 C<require> makes sure the module is loaded into memory if it hasn't been
5902 yet. The C<import> is not a builtin--it's just an ordinary static method
5903 call into the C<Module> package to tell the module to import the list of
5904 features back into the current package. The module can implement its
5905 C<import> method any way it likes, though most modules just choose to
5906 derive their C<import> method via inheritance from the C<Exporter> class that
5907 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5908 method can be found then the call is skipped.
5910 If you do not want to call the package's C<import> method (for instance,
5911 to stop your namespace from being altered), explicitly supply the empty list:
5915 That is exactly equivalent to
5917 BEGIN { require Module }
5919 If the VERSION argument is present between Module and LIST, then the
5920 C<use> will call the VERSION method in class Module with the given
5921 version as an argument. The default VERSION method, inherited from
5922 the UNIVERSAL class, croaks if the given version is larger than the
5923 value of the variable C<$Module::VERSION>.
5925 Again, there is a distinction between omitting LIST (C<import> called
5926 with no arguments) and an explicit empty LIST C<()> (C<import> not
5927 called). Note that there is no comma after VERSION!
5929 Because this is a wide-open interface, pragmas (compiler directives)
5930 are also implemented this way. Currently implemented pragmas are:
5935 use sigtrap qw(SEGV BUS);
5936 use strict qw(subs vars refs);
5937 use subs qw(afunc blurfl);
5938 use warnings qw(all);
5939 use sort qw(stable _quicksort _mergesort);
5941 Some of these pseudo-modules import semantics into the current
5942 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5943 which import symbols into the current package (which are effective
5944 through the end of the file).
5946 There's a corresponding C<no> command that unimports meanings imported
5947 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5953 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5954 for the C<-M> and C<-m> command-line options to perl that give C<use>
5955 functionality from the command-line.
5959 Changes the access and modification times on each file of a list of
5960 files. The first two elements of the list must be the NUMERICAL access
5961 and modification times, in that order. Returns the number of files
5962 successfully changed. The inode change time of each file is set
5963 to the current time. This code has the same effect as the C<touch>
5964 command if the files already exist:
5968 utime $now, $now, @ARGV;
5970 If the first two elements of the list are C<undef>, then the utime(2)
5971 function in the C library will be called with a null second argument.
5972 On most systems, this will set the file's access and modification
5973 times to the current time. (i.e. equivalent to the example above.)
5975 utime undef, undef, @ARGV;
5979 Returns a list consisting of all the values of the named hash. (In a
5980 scalar context, returns the number of values.) The values are
5981 returned in an apparently random order. The actual random order is
5982 subject to change in future versions of perl, but it is guaranteed to
5983 be the same order as either the C<keys> or C<each> function would
5984 produce on the same (unmodified) hash.
5986 Note that the values are not copied, which means modifying them will
5987 modify the contents of the hash:
5989 for (values %hash) { s/foo/bar/g } # modifies %hash values
5990 for (@hash{keys %hash}) { s/foo/bar/g } # same
5992 As a side effect, calling values() resets the HASH's internal iterator.
5993 See also C<keys>, C<each>, and C<sort>.
5995 =item vec EXPR,OFFSET,BITS
5997 Treats the string in EXPR as a bit vector made up of elements of
5998 width BITS, and returns the value of the element specified by OFFSET
5999 as an unsigned integer. BITS therefore specifies the number of bits
6000 that are reserved for each element in the bit vector. This must
6001 be a power of two from 1 to 32 (or 64, if your platform supports
6004 If BITS is 8, "elements" coincide with bytes of the input string.
6006 If BITS is 16 or more, bytes of the input string are grouped into chunks
6007 of size BITS/8, and each group is converted to a number as with
6008 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6009 for BITS==64). See L<"pack"> for details.
6011 If bits is 4 or less, the string is broken into bytes, then the bits
6012 of each byte are broken into 8/BITS groups. Bits of a byte are
6013 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6014 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6015 breaking the single input byte C<chr(0x36)> into two groups gives a list
6016 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6018 C<vec> may also be assigned to, in which case parentheses are needed
6019 to give the expression the correct precedence as in
6021 vec($image, $max_x * $x + $y, 8) = 3;
6023 If the selected element is outside the string, the value 0 is returned.
6024 If an element off the end of the string is written to, Perl will first
6025 extend the string with sufficiently many zero bytes. It is an error
6026 to try to write off the beginning of the string (i.e. negative OFFSET).
6028 The string should not contain any character with the value > 255 (which
6029 can only happen if you're using UTF8 encoding). If it does, it will be
6030 treated as something which is not UTF8 encoded. When the C<vec> was
6031 assigned to, other parts of your program will also no longer consider the
6032 string to be UTF8 encoded. In other words, if you do have such characters
6033 in your string, vec() will operate on the actual byte string, and not the
6034 conceptual character string.
6036 Strings created with C<vec> can also be manipulated with the logical
6037 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6038 vector operation is desired when both operands are strings.
6039 See L<perlop/"Bitwise String Operators">.
6041 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6042 The comments show the string after each step. Note that this code works
6043 in the same way on big-endian or little-endian machines.
6046 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6048 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6049 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6051 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6052 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6053 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6054 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6055 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6056 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6058 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6059 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6060 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6063 To transform a bit vector into a string or list of 0's and 1's, use these:
6065 $bits = unpack("b*", $vector);
6066 @bits = split(//, unpack("b*", $vector));
6068 If you know the exact length in bits, it can be used in place of the C<*>.
6070 Here is an example to illustrate how the bits actually fall in place:
6076 unpack("V",$_) 01234567890123456789012345678901
6077 ------------------------------------------------------------------
6082 for ($shift=0; $shift < $width; ++$shift) {
6083 for ($off=0; $off < 32/$width; ++$off) {
6084 $str = pack("B*", "0"x32);
6085 $bits = (1<<$shift);
6086 vec($str, $off, $width) = $bits;
6087 $res = unpack("b*",$str);
6088 $val = unpack("V", $str);
6095 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6096 $off, $width, $bits, $val, $res
6100 Regardless of the machine architecture on which it is run, the above
6101 example should print the following table:
6104 unpack("V",$_) 01234567890123456789012345678901
6105 ------------------------------------------------------------------
6106 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6107 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6108 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6109 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6110 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6111 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6112 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6113 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6114 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6115 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6116 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6117 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6118 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6119 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6120 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6121 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6122 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6123 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6124 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6125 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6126 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6127 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6128 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6129 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6130 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6131 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6132 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6133 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6134 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6135 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6136 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6137 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6138 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6139 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6140 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6141 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6142 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6143 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6144 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6145 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6146 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6147 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6148 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6149 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6150 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6151 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6152 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6153 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6154 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6155 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6156 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6157 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6158 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6159 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6160 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6161 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6162 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6163 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6164 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6165 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6166 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6167 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6168 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6169 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6170 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6171 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6172 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6173 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6174 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6175 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6176 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6177 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6178 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6179 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6180 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6181 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6182 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6183 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6184 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6185 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6186 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6187 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6188 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6189 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6190 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6191 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6192 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6193 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6194 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6195 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6196 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6197 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6198 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6199 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6200 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6201 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6202 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6203 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6204 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6205 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6206 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6207 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6208 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6209 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6210 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6211 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6212 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6213 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6214 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6215 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6216 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6217 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6218 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6219 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6220 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6221 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6222 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6223 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6224 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6225 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6226 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6227 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6228 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6229 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6230 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6231 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6232 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6233 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6237 Behaves like the wait(2) system call on your system: it waits for a child
6238 process to terminate and returns the pid of the deceased process, or
6239 C<-1> if there are no child processes. The status is returned in C<$?>.
6240 Note that a return value of C<-1> could mean that child processes are
6241 being automatically reaped, as described in L<perlipc>.
6243 =item waitpid PID,FLAGS
6245 Waits for a particular child process to terminate and returns the pid of
6246 the deceased process, or C<-1> if there is no such child process. On some
6247 systems, a value of 0 indicates that there are processes still running.
6248 The status is returned in C<$?>. If you say
6250 use POSIX ":sys_wait_h";
6253 $kid = waitpid(-1, WNOHANG);
6256 then you can do a non-blocking wait for all pending zombie processes.
6257 Non-blocking wait is available on machines supporting either the
6258 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6259 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6260 system call by remembering the status values of processes that have
6261 exited but have not been harvested by the Perl script yet.)
6263 Note that on some systems, a return value of C<-1> could mean that child
6264 processes are being automatically reaped. See L<perlipc> for details,
6265 and for other examples.
6269 Returns true if the context of the currently executing subroutine is
6270 looking for a list value. Returns false if the context is looking
6271 for a scalar. Returns the undefined value if the context is looking
6272 for no value (void context).
6274 return unless defined wantarray; # don't bother doing more
6275 my @a = complex_calculation();
6276 return wantarray ? @a : "@a";
6278 This function should have been named wantlist() instead.
6282 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6285 If LIST is empty and C<$@> already contains a value (typically from a
6286 previous eval) that value is used after appending C<"\t...caught">
6287 to C<$@>. This is useful for staying almost, but not entirely similar to
6290 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6292 No message is printed if there is a C<$SIG{__WARN__}> handler
6293 installed. It is the handler's responsibility to deal with the message
6294 as it sees fit (like, for instance, converting it into a C<die>). Most
6295 handlers must therefore make arrangements to actually display the
6296 warnings that they are not prepared to deal with, by calling C<warn>
6297 again in the handler. Note that this is quite safe and will not
6298 produce an endless loop, since C<__WARN__> hooks are not called from
6301 You will find this behavior is slightly different from that of
6302 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6303 instead call C<die> again to change it).
6305 Using a C<__WARN__> handler provides a powerful way to silence all
6306 warnings (even the so-called mandatory ones). An example:
6308 # wipe out *all* compile-time warnings
6309 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6311 my $foo = 20; # no warning about duplicate my $foo,
6312 # but hey, you asked for it!
6313 # no compile-time or run-time warnings before here
6316 # run-time warnings enabled after here
6317 warn "\$foo is alive and $foo!"; # does show up
6319 See L<perlvar> for details on setting C<%SIG> entries, and for more
6320 examples. See the Carp module for other kinds of warnings using its
6321 carp() and cluck() functions.
6323 =item write FILEHANDLE
6329 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6330 using the format associated with that file. By default the format for
6331 a file is the one having the same name as the filehandle, but the
6332 format for the current output channel (see the C<select> function) may be set
6333 explicitly by assigning the name of the format to the C<$~> variable.
6335 Top of form processing is handled automatically: if there is
6336 insufficient room on the current page for the formatted record, the
6337 page is advanced by writing a form feed, a special top-of-page format
6338 is used to format the new page header, and then the record is written.
6339 By default the top-of-page format is the name of the filehandle with
6340 "_TOP" appended, but it may be dynamically set to the format of your
6341 choice by assigning the name to the C<$^> variable while the filehandle is
6342 selected. The number of lines remaining on the current page is in
6343 variable C<$->, which can be set to C<0> to force a new page.
6345 If FILEHANDLE is unspecified, output goes to the current default output
6346 channel, which starts out as STDOUT but may be changed by the
6347 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6348 is evaluated and the resulting string is used to look up the name of
6349 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6351 Note that write is I<not> the opposite of C<read>. Unfortunately.
6355 The transliteration operator. Same as C<tr///>. See L<perlop>.