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<umask>,
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<sockatmark>, 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<sockatmark>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
434 Returns the arctangent of Y/X in the range -PI to PI.
436 For the tangent operation, you may use the C<Math::Trig::tan>
437 function, or use the familiar relation:
439 sub tan { sin($_[0]) / cos($_[0]) }
441 =item bind SOCKET,NAME
443 Binds a network address to a socket, just as the bind system call
444 does. Returns true if it succeeded, false otherwise. NAME should be a
445 packed address of the appropriate type for the socket. See the examples in
446 L<perlipc/"Sockets: Client/Server Communication">.
448 =item binmode FILEHANDLE, DISCIPLINE
450 =item binmode FILEHANDLE
452 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
453 on systems where the run-time libraries distinguish between binary and
454 text files. If FILEHANDLE is an expression, the value is taken as the
455 name of the filehandle. DISCIPLINE can be either of C<:raw> for
456 binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
457 omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
458 failure. The C<:raw> are C<:clrf>, and any other directives of the
459 form C<:...>, are called I/O I<disciplines>.
461 The C<open> pragma can be used to establish default I/O disciplines.
464 In general, binmode() should be called after open() but before any I/O
465 is done on the filehandle. Calling binmode() will flush any possibly
466 pending buffered input or output data on the handle. The only
467 exception to this is the C<:encoding> discipline that changes
468 the default character encoding of the handle, see L<open>.
469 The C<:encoding> discipline sometimes needs to be called in
470 mid-stream, and it doesn't flush the stream.
472 On some systems binmode() is necessary when you're not working with a
473 text file. For the sake of portability it is a good idea to always use
474 it when appropriate, and to never use it when it isn't appropriate.
476 In other words: Regardless of platform, use binmode() on binary
477 files, and do not use binmode() on text files.
479 The operating system, device drivers, C libraries, and Perl run-time
480 system all work together to let the programmer treat a single
481 character (C<\n>) as the line terminator, irrespective of the external
482 representation. On many operating systems, the native text file
483 representation matches the internal representation, but on some
484 platforms the external representation of C<\n> is made up of more than
487 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
488 character to end each line in the external representation of text (even
489 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
490 on Unix and most VMS files). Consequently binmode() has no effect on
491 these operating systems. In other systems like OS/2, DOS and the various
492 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
493 what's stored in text files are the two characters C<\cM\cJ>. That means
494 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
495 disk will be converted to C<\n> on input, and any C<\n> in your program
496 will be converted back to C<\cM\cJ> on output. This is what you want for
497 text files, but it can be disastrous for binary files.
499 Another consequence of using binmode() (on some systems) is that
500 special end-of-file markers will be seen as part of the data stream.
501 For systems from the Microsoft family this means that if your binary
502 data contains C<\cZ>, the I/O subsystem will regard it as the end of
503 the file, unless you use binmode().
505 binmode() is not only important for readline() and print() operations,
506 but also when using read(), seek(), sysread(), syswrite() and tell()
507 (see L<perlport> for more details). See the C<$/> and C<$\> variables
508 in L<perlvar> for how to manually set your input and output
509 line-termination sequences.
511 =item bless REF,CLASSNAME
515 This function tells the thingy referenced by REF that it is now an object
516 in the CLASSNAME package. If CLASSNAME is omitted, the current package
517 is used. Because a C<bless> is often the last thing in a constructor,
518 it returns the reference for convenience. Always use the two-argument
519 version if the function doing the blessing might be inherited by a
520 derived class. See L<perltoot> and L<perlobj> for more about the blessing
521 (and blessings) of objects.
523 Consider always blessing objects in CLASSNAMEs that are mixed case.
524 Namespaces with all lowercase names are considered reserved for
525 Perl pragmata. Builtin types have all uppercase names, so to prevent
526 confusion, you may wish to avoid such package names as well. Make sure
527 that CLASSNAME is a true value.
529 See L<perlmod/"Perl Modules">.
535 Returns the context of the current subroutine call. In scalar context,
536 returns the caller's package name if there is a caller, that is, if
537 we're in a subroutine or C<eval> or C<require>, and the undefined value
538 otherwise. In list context, returns
540 ($package, $filename, $line) = caller;
542 With EXPR, it returns some extra information that the debugger uses to
543 print a stack trace. The value of EXPR indicates how many call frames
544 to go back before the current one.
546 ($package, $filename, $line, $subroutine, $hasargs,
547 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
549 Here $subroutine may be C<(eval)> if the frame is not a subroutine
550 call, but an C<eval>. In such a case additional elements $evaltext and
551 C<$is_require> are set: C<$is_require> is true if the frame is created by a
552 C<require> or C<use> statement, $evaltext contains the text of the
553 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
554 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
555 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
556 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
557 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
558 was compiled with. The C<$hints> and C<$bitmask> values are subject to
559 change between versions of Perl, and are not meant for external use.
561 Furthermore, when called from within the DB package, caller returns more
562 detailed information: it sets the list variable C<@DB::args> to be the
563 arguments with which the subroutine was invoked.
565 Be aware that the optimizer might have optimized call frames away before
566 C<caller> had a chance to get the information. That means that C<caller(N)>
567 might not return information about the call frame you expect it do, for
568 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
569 previous time C<caller> was called.
573 Changes the working directory to EXPR, if possible. If EXPR is omitted,
574 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
575 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
576 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
577 neither is set, C<chdir> does nothing. It returns true upon success,
578 false otherwise. See the example under C<die>.
582 Changes the permissions of a list of files. The first element of the
583 list must be the numerical mode, which should probably be an octal
584 number, and which definitely should I<not> a string of octal digits:
585 C<0644> is okay, C<'0644'> is not. Returns the number of files
586 successfully changed. See also L</oct>, if all you have is a string.
588 $cnt = chmod 0755, 'foo', 'bar';
589 chmod 0755, @executables;
590 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
592 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
593 $mode = 0644; chmod $mode, 'foo'; # this is best
595 You can also import the symbolic C<S_I*> constants from the Fcntl
600 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
601 # This is identical to the chmod 0755 of the above example.
609 This safer version of L</chop> removes any trailing string
610 that corresponds to the current value of C<$/> (also known as
611 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
612 number of characters removed from all its arguments. It's often used to
613 remove the newline from the end of an input record when you're worried
614 that the final record may be missing its newline. When in paragraph
615 mode (C<$/ = "">), it removes all trailing newlines from the string.
616 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
617 a reference to an integer or the like, see L<perlvar>) chomp() won't
619 If VARIABLE is omitted, it chomps C<$_>. Example:
622 chomp; # avoid \n on last field
627 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
629 You can actually chomp anything that's an lvalue, including an assignment:
632 chomp($answer = <STDIN>);
634 If you chomp a list, each element is chomped, and the total number of
635 characters removed is returned.
643 Chops off the last character of a string and returns the character
644 chopped. It is much more efficient than C<s/.$//s> because it neither
645 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
646 If VARIABLE is a hash, it chops the hash's values, but not its keys.
648 You can actually chop anything that's an lvalue, including an assignment.
650 If you chop a list, each element is chopped. Only the value of the
651 last C<chop> is returned.
653 Note that C<chop> returns the last character. To return all but the last
654 character, use C<substr($string, 0, -1)>.
658 Changes the owner (and group) of a list of files. The first two
659 elements of the list must be the I<numeric> uid and gid, in that
660 order. A value of -1 in either position is interpreted by most
661 systems to leave that value unchanged. Returns the number of files
662 successfully changed.
664 $cnt = chown $uid, $gid, 'foo', 'bar';
665 chown $uid, $gid, @filenames;
667 Here's an example that looks up nonnumeric uids in the passwd file:
670 chomp($user = <STDIN>);
672 chomp($pattern = <STDIN>);
674 ($login,$pass,$uid,$gid) = getpwnam($user)
675 or die "$user not in passwd file";
677 @ary = glob($pattern); # expand filenames
678 chown $uid, $gid, @ary;
680 On most systems, you are not allowed to change the ownership of the
681 file unless you're the superuser, although you should be able to change
682 the group to any of your secondary groups. On insecure systems, these
683 restrictions may be relaxed, but this is not a portable assumption.
684 On POSIX systems, you can detect this condition this way:
686 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
687 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
693 Returns the character represented by that NUMBER in the character set.
694 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
695 chr(0x263a) is a Unicode smiley face. Note that characters from 127
696 to 255 (inclusive) are by default not encoded in Unicode for backward
697 compatibility reasons (but see L<encoding>).
699 For the reverse, use L</ord>.
700 See L<perlunicode> and L<encoding> for more about Unicode.
702 If NUMBER is omitted, uses C<$_>.
704 =item chroot FILENAME
708 This function works like the system call by the same name: it makes the
709 named directory the new root directory for all further pathnames that
710 begin with a C</> by your process and all its children. (It doesn't
711 change your current working directory, which is unaffected.) For security
712 reasons, this call is restricted to the superuser. If FILENAME is
713 omitted, does a C<chroot> to C<$_>.
715 =item close FILEHANDLE
719 Closes the file or pipe associated with the file handle, returning
720 true only if IO buffers are successfully flushed and closes the system
721 file descriptor. Closes the currently selected filehandle if the
724 You don't have to close FILEHANDLE if you are immediately going to do
725 another C<open> on it, because C<open> will close it for you. (See
726 C<open>.) However, an explicit C<close> on an input file resets the line
727 counter (C<$.>), while the implicit close done by C<open> does not.
729 If the file handle came from a piped open C<close> will additionally
730 return false if one of the other system calls involved fails or if the
731 program exits with non-zero status. (If the only problem was that the
732 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
733 also waits for the process executing on the pipe to complete, in case you
734 want to look at the output of the pipe afterwards, and
735 implicitly puts the exit status value of that command into C<$?>.
737 Prematurely closing the read end of a pipe (i.e. before the process
738 writing to it at the other end has closed it) will result in a
739 SIGPIPE being delivered to the writer. If the other end can't
740 handle that, be sure to read all the data before closing the pipe.
744 open(OUTPUT, '|sort >foo') # pipe to sort
745 or die "Can't start sort: $!";
746 #... # print stuff to output
747 close OUTPUT # wait for sort to finish
748 or warn $! ? "Error closing sort pipe: $!"
749 : "Exit status $? from sort";
750 open(INPUT, 'foo') # get sort's results
751 or die "Can't open 'foo' for input: $!";
753 FILEHANDLE may be an expression whose value can be used as an indirect
754 filehandle, usually the real filehandle name.
756 =item closedir DIRHANDLE
758 Closes a directory opened by C<opendir> and returns the success of that
761 DIRHANDLE may be an expression whose value can be used as an indirect
762 dirhandle, usually the real dirhandle name.
764 =item connect SOCKET,NAME
766 Attempts to connect to a remote socket, just as the connect system call
767 does. Returns true if it succeeded, false otherwise. NAME should be a
768 packed address of the appropriate type for the socket. See the examples in
769 L<perlipc/"Sockets: Client/Server Communication">.
773 Actually a flow control statement rather than a function. If there is a
774 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
775 C<foreach>), it is always executed just before the conditional is about to
776 be evaluated again, just like the third part of a C<for> loop in C. Thus
777 it can be used to increment a loop variable, even when the loop has been
778 continued via the C<next> statement (which is similar to the C C<continue>
781 C<last>, C<next>, or C<redo> may appear within a C<continue>
782 block. C<last> and C<redo> will behave as if they had been executed within
783 the main block. So will C<next>, but since it will execute a C<continue>
784 block, it may be more entertaining.
787 ### redo always comes here
790 ### next always comes here
792 # then back the top to re-check EXPR
794 ### last always comes here
796 Omitting the C<continue> section is semantically equivalent to using an
797 empty one, logically enough. In that case, C<next> goes directly back
798 to check the condition at the top of the loop.
804 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
805 takes cosine of C<$_>.
807 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
808 function, or use this relation:
810 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
812 =item crypt PLAINTEXT,SALT
814 Encrypts a string exactly like the crypt(3) function in the C library
815 (assuming that you actually have a version there that has not been
816 extirpated as a potential munition). This can prove useful for checking
817 the password file for lousy passwords, amongst other things. Only the
818 guys wearing white hats should do this.
820 Note that C<crypt> is intended to be a one-way function, much like
821 breaking eggs to make an omelette. There is no (known) corresponding
822 decrypt function (in other words, the crypt() is a one-way hash
823 function). As a result, this function isn't all that useful for
824 cryptography. (For that, see your nearby CPAN mirror.)
826 When verifying an existing encrypted string you should use the
827 encrypted text as the salt (like C<crypt($plain, $crypted) eq
828 $crypted>). This allows your code to work with the standard C<crypt>
829 and with more exotic implementations. In other words, do not assume
830 anything about the returned string itself, or how many bytes in
831 the encrypted string matter.
833 Traditionally the result is a string of 13 bytes: two first bytes of
834 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
835 the first eight bytes of the encrypted string mattered, but
836 alternative hashing schemes (like MD5), higher level security schemes
837 (like C2), and implementations on non-UNIX platforms may produce
840 When choosing a new salt create a random two character string whose
841 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
842 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
844 Here's an example that makes sure that whoever runs this program knows
847 $pwd = (getpwuid($<))[1];
851 chomp($word = <STDIN>);
855 if (crypt($word, $pwd) ne $pwd) {
861 Of course, typing in your own password to whoever asks you
864 The L<crypt> function is unsuitable for encrypting large quantities
865 of data, not least of all because you can't get the information
866 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
867 on your favorite CPAN mirror for a slew of potentially useful
870 If using crypt() on a Unicode string (which I<potentially> has
871 characters with codepoints above 255), Perl tries to make sense
872 of the situation by trying to downgrade (a copy of the string)
873 the string back to an eight-bit byte string before calling crypt()
874 (on that copy). If that works, good. If not, crypt() dies with
875 C<Wide character in crypt>.
879 [This function has been largely superseded by the C<untie> function.]
881 Breaks the binding between a DBM file and a hash.
883 =item dbmopen HASH,DBNAME,MASK
885 [This function has been largely superseded by the C<tie> function.]
887 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
888 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
889 argument is I<not> a filehandle, even though it looks like one). DBNAME
890 is the name of the database (without the F<.dir> or F<.pag> extension if
891 any). If the database does not exist, it is created with protection
892 specified by MASK (as modified by the C<umask>). If your system supports
893 only the older DBM functions, you may perform only one C<dbmopen> in your
894 program. In older versions of Perl, if your system had neither DBM nor
895 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
898 If you don't have write access to the DBM file, you can only read hash
899 variables, not set them. If you want to test whether you can write,
900 either use file tests or try setting a dummy hash entry inside an C<eval>,
901 which will trap the error.
903 Note that functions such as C<keys> and C<values> may return huge lists
904 when used on large DBM files. You may prefer to use the C<each>
905 function to iterate over large DBM files. Example:
907 # print out history file offsets
908 dbmopen(%HIST,'/usr/lib/news/history',0666);
909 while (($key,$val) = each %HIST) {
910 print $key, ' = ', unpack('L',$val), "\n";
914 See also L<AnyDBM_File> for a more general description of the pros and
915 cons of the various dbm approaches, as well as L<DB_File> for a particularly
918 You can control which DBM library you use by loading that library
919 before you call dbmopen():
922 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
923 or die "Can't open netscape history file: $!";
929 Returns a Boolean value telling whether EXPR has a value other than
930 the undefined value C<undef>. If EXPR is not present, C<$_> will be
933 Many operations return C<undef> to indicate failure, end of file,
934 system error, uninitialized variable, and other exceptional
935 conditions. This function allows you to distinguish C<undef> from
936 other values. (A simple Boolean test will not distinguish among
937 C<undef>, zero, the empty string, and C<"0">, which are all equally
938 false.) Note that since C<undef> is a valid scalar, its presence
939 doesn't I<necessarily> indicate an exceptional condition: C<pop>
940 returns C<undef> when its argument is an empty array, I<or> when the
941 element to return happens to be C<undef>.
943 You may also use C<defined(&func)> to check whether subroutine C<&func>
944 has ever been defined. The return value is unaffected by any forward
945 declarations of C<&foo>. Note that a subroutine which is not defined
946 may still be callable: its package may have an C<AUTOLOAD> method that
947 makes it spring into existence the first time that it is called -- see
950 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
951 used to report whether memory for that aggregate has ever been
952 allocated. This behavior may disappear in future versions of Perl.
953 You should instead use a simple test for size:
955 if (@an_array) { print "has array elements\n" }
956 if (%a_hash) { print "has hash members\n" }
958 When used on a hash element, it tells you whether the value is defined,
959 not whether the key exists in the hash. Use L</exists> for the latter
964 print if defined $switch{'D'};
965 print "$val\n" while defined($val = pop(@ary));
966 die "Can't readlink $sym: $!"
967 unless defined($value = readlink $sym);
968 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
969 $debugging = 0 unless defined $debugging;
971 Note: Many folks tend to overuse C<defined>, and then are surprised to
972 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
973 defined values. For example, if you say
977 The pattern match succeeds, and C<$1> is defined, despite the fact that it
978 matched "nothing". But it didn't really match nothing--rather, it
979 matched something that happened to be zero characters long. This is all
980 very above-board and honest. When a function returns an undefined value,
981 it's an admission that it couldn't give you an honest answer. So you
982 should use C<defined> only when you're questioning the integrity of what
983 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
986 See also L</undef>, L</exists>, L</ref>.
990 Given an expression that specifies a hash element, array element, hash slice,
991 or array slice, deletes the specified element(s) from the hash or array.
992 In the case of an array, if the array elements happen to be at the end,
993 the size of the array will shrink to the highest element that tests
994 true for exists() (or 0 if no such element exists).
996 Returns each element so deleted or the undefined value if there was no such
997 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
998 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
999 from a C<tie>d hash or array may not necessarily return anything.
1001 Deleting an array element effectively returns that position of the array
1002 to its initial, uninitialized state. Subsequently testing for the same
1003 element with exists() will return false. Note that deleting array
1004 elements in the middle of an array will not shift the index of the ones
1005 after them down--use splice() for that. See L</exists>.
1007 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1009 foreach $key (keys %HASH) {
1013 foreach $index (0 .. $#ARRAY) {
1014 delete $ARRAY[$index];
1019 delete @HASH{keys %HASH};
1021 delete @ARRAY[0 .. $#ARRAY];
1023 But both of these are slower than just assigning the empty list
1024 or undefining %HASH or @ARRAY:
1026 %HASH = (); # completely empty %HASH
1027 undef %HASH; # forget %HASH ever existed
1029 @ARRAY = (); # completely empty @ARRAY
1030 undef @ARRAY; # forget @ARRAY ever existed
1032 Note that the EXPR can be arbitrarily complicated as long as the final
1033 operation is a hash element, array element, hash slice, or array slice
1036 delete $ref->[$x][$y]{$key};
1037 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1039 delete $ref->[$x][$y][$index];
1040 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1044 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1045 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1046 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1047 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1048 an C<eval(),> the error message is stuffed into C<$@> and the
1049 C<eval> is terminated with the undefined value. This makes
1050 C<die> the way to raise an exception.
1052 Equivalent examples:
1054 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1055 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1057 If the last element of LIST does not end in a newline, the current
1058 script line number and input line number (if any) are also printed,
1059 and a newline is supplied. Note that the "input line number" (also
1060 known as "chunk") is subject to whatever notion of "line" happens to
1061 be currently in effect, and is also available as the special variable
1062 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1064 Hint: sometimes appending C<", stopped"> to your message will cause it
1065 to make better sense when the string C<"at foo line 123"> is appended.
1066 Suppose you are running script "canasta".
1068 die "/etc/games is no good";
1069 die "/etc/games is no good, stopped";
1071 produce, respectively
1073 /etc/games is no good at canasta line 123.
1074 /etc/games is no good, stopped at canasta line 123.
1076 See also exit(), warn(), and the Carp module.
1078 If LIST is empty and C<$@> already contains a value (typically from a
1079 previous eval) that value is reused after appending C<"\t...propagated">.
1080 This is useful for propagating exceptions:
1083 die unless $@ =~ /Expected exception/;
1085 If LIST is empty and C<$@> contains an object reference that has a
1086 C<PROPAGATE> method, that method will be called with additional file
1087 and line number parameters. The return value replaces the value in
1088 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1091 If C<$@> is empty then the string C<"Died"> is used.
1093 die() can also be called with a reference argument. If this happens to be
1094 trapped within an eval(), $@ contains the reference. This behavior permits
1095 a more elaborate exception handling implementation using objects that
1096 maintain arbitrary state about the nature of the exception. Such a scheme
1097 is sometimes preferable to matching particular string values of $@ using
1098 regular expressions. Here's an example:
1100 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1102 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1103 # handle Some::Module::Exception
1106 # handle all other possible exceptions
1110 Because perl will stringify uncaught exception messages before displaying
1111 them, you may want to overload stringification operations on such custom
1112 exception objects. See L<overload> for details about that.
1114 You can arrange for a callback to be run just before the C<die>
1115 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1116 handler will be called with the error text and can change the error
1117 message, if it sees fit, by calling C<die> again. See
1118 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1119 L<"eval BLOCK"> for some examples. Although this feature was meant
1120 to be run only right before your program was to exit, this is not
1121 currently the case--the C<$SIG{__DIE__}> hook is currently called
1122 even inside eval()ed blocks/strings! If one wants the hook to do
1123 nothing in such situations, put
1127 as the first line of the handler (see L<perlvar/$^S>). Because
1128 this promotes strange action at a distance, this counterintuitive
1129 behavior may be fixed in a future release.
1133 Not really a function. Returns the value of the last command in the
1134 sequence of commands indicated by BLOCK. When modified by a loop
1135 modifier, executes the BLOCK once before testing the loop condition.
1136 (On other statements the loop modifiers test the conditional first.)
1138 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1139 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1140 See L<perlsyn> for alternative strategies.
1142 =item do SUBROUTINE(LIST)
1144 A deprecated form of subroutine call. See L<perlsub>.
1148 Uses the value of EXPR as a filename and executes the contents of the
1149 file as a Perl script. Its primary use is to include subroutines
1150 from a Perl subroutine library.
1158 except that it's more efficient and concise, keeps track of the current
1159 filename for error messages, searches the @INC libraries, and updates
1160 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1161 variables. It also differs in that code evaluated with C<do FILENAME>
1162 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1163 same, however, in that it does reparse the file every time you call it,
1164 so you probably don't want to do this inside a loop.
1166 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1167 error. If C<do> can read the file but cannot compile it, it
1168 returns undef and sets an error message in C<$@>. If the file is
1169 successfully compiled, C<do> returns the value of the last expression
1172 Note that inclusion of library modules is better done with the
1173 C<use> and C<require> operators, which also do automatic error checking
1174 and raise an exception if there's a problem.
1176 You might like to use C<do> to read in a program configuration
1177 file. Manual error checking can be done this way:
1179 # read in config files: system first, then user
1180 for $file ("/share/prog/defaults.rc",
1181 "$ENV{HOME}/.someprogrc")
1183 unless ($return = do $file) {
1184 warn "couldn't parse $file: $@" if $@;
1185 warn "couldn't do $file: $!" unless defined $return;
1186 warn "couldn't run $file" unless $return;
1194 This function causes an immediate core dump. See also the B<-u>
1195 command-line switch in L<perlrun>, which does the same thing.
1196 Primarily this is so that you can use the B<undump> program (not
1197 supplied) to turn your core dump into an executable binary after
1198 having initialized all your variables at the beginning of the
1199 program. When the new binary is executed it will begin by executing
1200 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1201 Think of it as a goto with an intervening core dump and reincarnation.
1202 If C<LABEL> is omitted, restarts the program from the top.
1204 B<WARNING>: Any files opened at the time of the dump will I<not>
1205 be open any more when the program is reincarnated, with possible
1206 resulting confusion on the part of Perl.
1208 This function is now largely obsolete, partly because it's very
1209 hard to convert a core file into an executable, and because the
1210 real compiler backends for generating portable bytecode and compilable
1211 C code have superseded it. That's why you should now invoke it as
1212 C<CORE::dump()>, if you don't want to be warned against a possible
1215 If you're looking to use L<dump> to speed up your program, consider
1216 generating bytecode or native C code as described in L<perlcc>. If
1217 you're just trying to accelerate a CGI script, consider using the
1218 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1219 You might also consider autoloading or selfloading, which at least
1220 make your program I<appear> to run faster.
1224 When called in list context, returns a 2-element list consisting of the
1225 key and value for the next element of a hash, so that you can iterate over
1226 it. When called in scalar context, returns only the key for the next
1227 element in the hash.
1229 Entries are returned in an apparently random order. The actual random
1230 order is subject to change in future versions of perl, but it is guaranteed
1231 to be in the same order as either the C<keys> or C<values> function
1232 would produce on the same (unmodified) hash.
1234 When the hash is entirely read, a null array is returned in list context
1235 (which when assigned produces a false (C<0>) value), and C<undef> in
1236 scalar context. The next call to C<each> after that will start iterating
1237 again. There is a single iterator for each hash, shared by all C<each>,
1238 C<keys>, and C<values> function calls in the program; it can be reset by
1239 reading all the elements from the hash, or by evaluating C<keys HASH> or
1240 C<values HASH>. If you add or delete elements of a hash while you're
1241 iterating over it, you may get entries skipped or duplicated, so
1242 don't. Exception: It is always safe to delete the item most recently
1243 returned by C<each()>, which means that the following code will work:
1245 while (($key, $value) = each %hash) {
1247 delete $hash{$key}; # This is safe
1250 The following prints out your environment like the printenv(1) program,
1251 only in a different order:
1253 while (($key,$value) = each %ENV) {
1254 print "$key=$value\n";
1257 See also C<keys>, C<values> and C<sort>.
1259 =item eof FILEHANDLE
1265 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1266 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1267 gives the real filehandle. (Note that this function actually
1268 reads a character and then C<ungetc>s it, so isn't very useful in an
1269 interactive context.) Do not read from a terminal file (or call
1270 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1271 as terminals may lose the end-of-file condition if you do.
1273 An C<eof> without an argument uses the last file read. Using C<eof()>
1274 with empty parentheses is very different. It refers to the pseudo file
1275 formed from the files listed on the command line and accessed via the
1276 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1277 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1278 used will cause C<@ARGV> to be examined to determine if input is
1279 available. Similarly, an C<eof()> after C<< <> >> has returned
1280 end-of-file will assume you are processing another C<@ARGV> list,
1281 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1282 see L<perlop/"I/O Operators">.
1284 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1285 detect the end of each file, C<eof()> will only detect the end of the
1286 last file. Examples:
1288 # reset line numbering on each input file
1290 next if /^\s*#/; # skip comments
1293 close ARGV if eof; # Not eof()!
1296 # insert dashes just before last line of last file
1298 if (eof()) { # check for end of current file
1299 print "--------------\n";
1300 close(ARGV); # close or last; is needed if we
1301 # are reading from the terminal
1306 Practical hint: you almost never need to use C<eof> in Perl, because the
1307 input operators typically return C<undef> when they run out of data, or if
1314 In the first form, the return value of EXPR is parsed and executed as if it
1315 were a little Perl program. The value of the expression (which is itself
1316 determined within scalar context) is first parsed, and if there weren't any
1317 errors, executed in the lexical context of the current Perl program, so
1318 that any variable settings or subroutine and format definitions remain
1319 afterwards. Note that the value is parsed every time the eval executes.
1320 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1321 delay parsing and subsequent execution of the text of EXPR until run time.
1323 In the second form, the code within the BLOCK is parsed only once--at the
1324 same time the code surrounding the eval itself was parsed--and executed
1325 within the context of the current Perl program. This form is typically
1326 used to trap exceptions more efficiently than the first (see below), while
1327 also providing the benefit of checking the code within BLOCK at compile
1330 The final semicolon, if any, may be omitted from the value of EXPR or within
1333 In both forms, the value returned is the value of the last expression
1334 evaluated inside the mini-program; a return statement may be also used, just
1335 as with subroutines. The expression providing the return value is evaluated
1336 in void, scalar, or list context, depending on the context of the eval itself.
1337 See L</wantarray> for more on how the evaluation context can be determined.
1339 If there is a syntax error or runtime error, or a C<die> statement is
1340 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1341 error message. If there was no error, C<$@> is guaranteed to be a null
1342 string. Beware that using C<eval> neither silences perl from printing
1343 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1344 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1345 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1346 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1348 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1349 determining whether a particular feature (such as C<socket> or C<symlink>)
1350 is implemented. It is also Perl's exception trapping mechanism, where
1351 the die operator is used to raise exceptions.
1353 If the code to be executed doesn't vary, you may use the eval-BLOCK
1354 form to trap run-time errors without incurring the penalty of
1355 recompiling each time. The error, if any, is still returned in C<$@>.
1358 # make divide-by-zero nonfatal
1359 eval { $answer = $a / $b; }; warn $@ if $@;
1361 # same thing, but less efficient
1362 eval '$answer = $a / $b'; warn $@ if $@;
1364 # a compile-time error
1365 eval { $answer = }; # WRONG
1368 eval '$answer ='; # sets $@
1370 Due to the current arguably broken state of C<__DIE__> hooks, when using
1371 the C<eval{}> form as an exception trap in libraries, you may wish not
1372 to trigger any C<__DIE__> hooks that user code may have installed.
1373 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1374 as shown in this example:
1376 # a very private exception trap for divide-by-zero
1377 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1380 This is especially significant, given that C<__DIE__> hooks can call
1381 C<die> again, which has the effect of changing their error messages:
1383 # __DIE__ hooks may modify error messages
1385 local $SIG{'__DIE__'} =
1386 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1387 eval { die "foo lives here" };
1388 print $@ if $@; # prints "bar lives here"
1391 Because this promotes action at a distance, this counterintuitive behavior
1392 may be fixed in a future release.
1394 With an C<eval>, you should be especially careful to remember what's
1395 being looked at when:
1401 eval { $x }; # CASE 4
1403 eval "\$$x++"; # CASE 5
1406 Cases 1 and 2 above behave identically: they run the code contained in
1407 the variable $x. (Although case 2 has misleading double quotes making
1408 the reader wonder what else might be happening (nothing is).) Cases 3
1409 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1410 does nothing but return the value of $x. (Case 4 is preferred for
1411 purely visual reasons, but it also has the advantage of compiling at
1412 compile-time instead of at run-time.) Case 5 is a place where
1413 normally you I<would> like to use double quotes, except that in this
1414 particular situation, you can just use symbolic references instead, as
1417 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1418 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1422 =item exec PROGRAM LIST
1424 The C<exec> function executes a system command I<and never returns>--
1425 use C<system> instead of C<exec> if you want it to return. It fails and
1426 returns false only if the command does not exist I<and> it is executed
1427 directly instead of via your system's command shell (see below).
1429 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1430 warns you if there is a following statement which isn't C<die>, C<warn>,
1431 or C<exit> (if C<-w> is set - but you always do that). If you
1432 I<really> want to follow an C<exec> with some other statement, you
1433 can use one of these styles to avoid the warning:
1435 exec ('foo') or print STDERR "couldn't exec foo: $!";
1436 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1438 If there is more than one argument in LIST, or if LIST is an array
1439 with more than one value, calls execvp(3) with the arguments in LIST.
1440 If there is only one scalar argument or an array with one element in it,
1441 the argument is checked for shell metacharacters, and if there are any,
1442 the entire argument is passed to the system's command shell for parsing
1443 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1444 If there are no shell metacharacters in the argument, it is split into
1445 words and passed directly to C<execvp>, which is more efficient.
1448 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1449 exec "sort $outfile | uniq";
1451 If you don't really want to execute the first argument, but want to lie
1452 to the program you are executing about its own name, you can specify
1453 the program you actually want to run as an "indirect object" (without a
1454 comma) in front of the LIST. (This always forces interpretation of the
1455 LIST as a multivalued list, even if there is only a single scalar in
1458 $shell = '/bin/csh';
1459 exec $shell '-sh'; # pretend it's a login shell
1463 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1465 When the arguments get executed via the system shell, results will
1466 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1469 Using an indirect object with C<exec> or C<system> is also more
1470 secure. This usage (which also works fine with system()) forces
1471 interpretation of the arguments as a multivalued list, even if the
1472 list had just one argument. That way you're safe from the shell
1473 expanding wildcards or splitting up words with whitespace in them.
1475 @args = ( "echo surprise" );
1477 exec @args; # subject to shell escapes
1479 exec { $args[0] } @args; # safe even with one-arg list
1481 The first version, the one without the indirect object, ran the I<echo>
1482 program, passing it C<"surprise"> an argument. The second version
1483 didn't--it tried to run a program literally called I<"echo surprise">,
1484 didn't find it, and set C<$?> to a non-zero value indicating failure.
1486 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1487 output before the exec, but this may not be supported on some platforms
1488 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1489 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1490 open handles in order to avoid lost output.
1492 Note that C<exec> will not call your C<END> blocks, nor will it call
1493 any C<DESTROY> methods in your objects.
1497 Given an expression that specifies a hash element or array element,
1498 returns true if the specified element in the hash or array has ever
1499 been initialized, even if the corresponding value is undefined. The
1500 element is not autovivified if it doesn't exist.
1502 print "Exists\n" if exists $hash{$key};
1503 print "Defined\n" if defined $hash{$key};
1504 print "True\n" if $hash{$key};
1506 print "Exists\n" if exists $array[$index];
1507 print "Defined\n" if defined $array[$index];
1508 print "True\n" if $array[$index];
1510 A hash or array element can be true only if it's defined, and defined if
1511 it exists, but the reverse doesn't necessarily hold true.
1513 Given an expression that specifies the name of a subroutine,
1514 returns true if the specified subroutine has ever been declared, even
1515 if it is undefined. Mentioning a subroutine name for exists or defined
1516 does not count as declaring it. Note that a subroutine which does not
1517 exist may still be callable: its package may have an C<AUTOLOAD>
1518 method that makes it spring into existence the first time that it is
1519 called -- see L<perlsub>.
1521 print "Exists\n" if exists &subroutine;
1522 print "Defined\n" if defined &subroutine;
1524 Note that the EXPR can be arbitrarily complicated as long as the final
1525 operation is a hash or array key lookup or subroutine name:
1527 if (exists $ref->{A}->{B}->{$key}) { }
1528 if (exists $hash{A}{B}{$key}) { }
1530 if (exists $ref->{A}->{B}->[$ix]) { }
1531 if (exists $hash{A}{B}[$ix]) { }
1533 if (exists &{$ref->{A}{B}{$key}}) { }
1535 Although the deepest nested array or hash will not spring into existence
1536 just because its existence was tested, any intervening ones will.
1537 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1538 into existence due to the existence test for the $key element above.
1539 This happens anywhere the arrow operator is used, including even:
1542 if (exists $ref->{"Some key"}) { }
1543 print $ref; # prints HASH(0x80d3d5c)
1545 This surprising autovivification in what does not at first--or even
1546 second--glance appear to be an lvalue context may be fixed in a future
1549 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1550 on how exists() acts when used on a pseudo-hash.
1552 Use of a subroutine call, rather than a subroutine name, as an argument
1553 to exists() is an error.
1556 exists &sub(); # Error
1560 Evaluates EXPR and exits immediately with that value. Example:
1563 exit 0 if $ans =~ /^[Xx]/;
1565 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1566 universally recognized values for EXPR are C<0> for success and C<1>
1567 for error; other values are subject to interpretation depending on the
1568 environment in which the Perl program is running. For example, exiting
1569 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1570 the mailer to return the item undelivered, but that's not true everywhere.
1572 Don't use C<exit> to abort a subroutine if there's any chance that
1573 someone might want to trap whatever error happened. Use C<die> instead,
1574 which can be trapped by an C<eval>.
1576 The exit() function does not always exit immediately. It calls any
1577 defined C<END> routines first, but these C<END> routines may not
1578 themselves abort the exit. Likewise any object destructors that need to
1579 be called are called before the real exit. If this is a problem, you
1580 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1581 See L<perlmod> for details.
1587 Returns I<e> (the natural logarithm base) to the power of EXPR.
1588 If EXPR is omitted, gives C<exp($_)>.
1590 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1592 Implements the fcntl(2) function. You'll probably have to say
1596 first to get the correct constant definitions. Argument processing and
1597 value return works just like C<ioctl> below.
1601 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1602 or die "can't fcntl F_GETFL: $!";
1604 You don't have to check for C<defined> on the return from C<fnctl>.
1605 Like C<ioctl>, it maps a C<0> return from the system call into
1606 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1607 in numeric context. It is also exempt from the normal B<-w> warnings
1608 on improper numeric conversions.
1610 Note that C<fcntl> will produce a fatal error if used on a machine that
1611 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1612 manpage to learn what functions are available on your system.
1614 =item fileno FILEHANDLE
1616 Returns the file descriptor for a filehandle, or undefined if the
1617 filehandle is not open. This is mainly useful for constructing
1618 bitmaps for C<select> and low-level POSIX tty-handling operations.
1619 If FILEHANDLE is an expression, the value is taken as an indirect
1620 filehandle, generally its name.
1622 You can use this to find out whether two handles refer to the
1623 same underlying descriptor:
1625 if (fileno(THIS) == fileno(THAT)) {
1626 print "THIS and THAT are dups\n";
1629 (Filehandles connected to memory objects via new features of C<open> may
1630 return undefined even though they are open.)
1633 =item flock FILEHANDLE,OPERATION
1635 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1636 for success, false on failure. Produces a fatal error if used on a
1637 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1638 C<flock> is Perl's portable file locking interface, although it locks
1639 only entire files, not records.
1641 Two potentially non-obvious but traditional C<flock> semantics are
1642 that it waits indefinitely until the lock is granted, and that its locks
1643 B<merely advisory>. Such discretionary locks are more flexible, but offer
1644 fewer guarantees. This means that files locked with C<flock> may be
1645 modified by programs that do not also use C<flock>. See L<perlport>,
1646 your port's specific documentation, or your system-specific local manpages
1647 for details. It's best to assume traditional behavior if you're writing
1648 portable programs. (But if you're not, you should as always feel perfectly
1649 free to write for your own system's idiosyncrasies (sometimes called
1650 "features"). Slavish adherence to portability concerns shouldn't get
1651 in the way of your getting your job done.)
1653 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1654 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1655 you can use the symbolic names if you import them from the Fcntl module,
1656 either individually, or as a group using the ':flock' tag. LOCK_SH
1657 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1658 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1659 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1660 waiting for the lock (check the return status to see if you got it).
1662 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1663 before locking or unlocking it.
1665 Note that the emulation built with lockf(3) doesn't provide shared
1666 locks, and it requires that FILEHANDLE be open with write intent. These
1667 are the semantics that lockf(3) implements. Most if not all systems
1668 implement lockf(3) in terms of fcntl(2) locking, though, so the
1669 differing semantics shouldn't bite too many people.
1671 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1672 be open with read intent to use LOCK_SH and requires that it be open
1673 with write intent to use LOCK_EX.
1675 Note also that some versions of C<flock> cannot lock things over the
1676 network; you would need to use the more system-specific C<fcntl> for
1677 that. If you like you can force Perl to ignore your system's flock(2)
1678 function, and so provide its own fcntl(2)-based emulation, by passing
1679 the switch C<-Ud_flock> to the F<Configure> program when you configure
1682 Here's a mailbox appender for BSD systems.
1684 use Fcntl ':flock'; # import LOCK_* constants
1687 flock(MBOX,LOCK_EX);
1688 # and, in case someone appended
1689 # while we were waiting...
1694 flock(MBOX,LOCK_UN);
1697 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1698 or die "Can't open mailbox: $!";
1701 print MBOX $msg,"\n\n";
1704 On systems that support a real flock(), locks are inherited across fork()
1705 calls, whereas those that must resort to the more capricious fcntl()
1706 function lose the locks, making it harder to write servers.
1708 See also L<DB_File> for other flock() examples.
1712 Does a fork(2) system call to create a new process running the
1713 same program at the same point. It returns the child pid to the
1714 parent process, C<0> to the child process, or C<undef> if the fork is
1715 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1716 are shared, while everything else is copied. On most systems supporting
1717 fork(), great care has gone into making it extremely efficient (for
1718 example, using copy-on-write technology on data pages), making it the
1719 dominant paradigm for multitasking over the last few decades.
1721 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1722 output before forking the child process, but this may not be supported
1723 on some platforms (see L<perlport>). To be safe, you may need to set
1724 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1725 C<IO::Handle> on any open handles in order to avoid duplicate output.
1727 If you C<fork> without ever waiting on your children, you will
1728 accumulate zombies. On some systems, you can avoid this by setting
1729 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1730 forking and reaping moribund children.
1732 Note that if your forked child inherits system file descriptors like
1733 STDIN and STDOUT that are actually connected by a pipe or socket, even
1734 if you exit, then the remote server (such as, say, a CGI script or a
1735 backgrounded job launched from a remote shell) won't think you're done.
1736 You should reopen those to F</dev/null> if it's any issue.
1740 Declare a picture format for use by the C<write> function. For
1744 Test: @<<<<<<<< @||||| @>>>>>
1745 $str, $%, '$' . int($num)
1749 $num = $cost/$quantity;
1753 See L<perlform> for many details and examples.
1755 =item formline PICTURE,LIST
1757 This is an internal function used by C<format>s, though you may call it,
1758 too. It formats (see L<perlform>) a list of values according to the
1759 contents of PICTURE, placing the output into the format output
1760 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1761 Eventually, when a C<write> is done, the contents of
1762 C<$^A> are written to some filehandle, but you could also read C<$^A>
1763 yourself and then set C<$^A> back to C<"">. Note that a format typically
1764 does one C<formline> per line of form, but the C<formline> function itself
1765 doesn't care how many newlines are embedded in the PICTURE. This means
1766 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1767 You may therefore need to use multiple formlines to implement a single
1768 record format, just like the format compiler.
1770 Be careful if you put double quotes around the picture, because an C<@>
1771 character may be taken to mean the beginning of an array name.
1772 C<formline> always returns true. See L<perlform> for other examples.
1774 =item getc FILEHANDLE
1778 Returns the next character from the input file attached to FILEHANDLE,
1779 or the undefined value at end of file, or if there was an error.
1780 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1781 efficient. However, it cannot be used by itself to fetch single
1782 characters without waiting for the user to hit enter. For that, try
1783 something more like:
1786 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1789 system "stty", '-icanon', 'eol', "\001";
1795 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1798 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1802 Determination of whether $BSD_STYLE should be set
1803 is left as an exercise to the reader.
1805 The C<POSIX::getattr> function can do this more portably on
1806 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1807 module from your nearest CPAN site; details on CPAN can be found on
1812 Implements the C library function of the same name, which on most
1813 systems returns the current login from F</etc/utmp>, if any. If null,
1816 $login = getlogin || getpwuid($<) || "Kilroy";
1818 Do not consider C<getlogin> for authentication: it is not as
1819 secure as C<getpwuid>.
1821 =item getpeername SOCKET
1823 Returns the packed sockaddr address of other end of the SOCKET connection.
1826 $hersockaddr = getpeername(SOCK);
1827 ($port, $iaddr) = sockaddr_in($hersockaddr);
1828 $herhostname = gethostbyaddr($iaddr, AF_INET);
1829 $herstraddr = inet_ntoa($iaddr);
1833 Returns the current process group for the specified PID. Use
1834 a PID of C<0> to get the current process group for the
1835 current process. Will raise an exception if used on a machine that
1836 doesn't implement getpgrp(2). If PID is omitted, returns process
1837 group of current process. Note that the POSIX version of C<getpgrp>
1838 does not accept a PID argument, so only C<PID==0> is truly portable.
1842 Returns the process id of the parent process.
1844 =item getpriority WHICH,WHO
1846 Returns the current priority for a process, a process group, or a user.
1847 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1848 machine that doesn't implement getpriority(2).
1854 =item gethostbyname NAME
1856 =item getnetbyname NAME
1858 =item getprotobyname NAME
1864 =item getservbyname NAME,PROTO
1866 =item gethostbyaddr ADDR,ADDRTYPE
1868 =item getnetbyaddr ADDR,ADDRTYPE
1870 =item getprotobynumber NUMBER
1872 =item getservbyport PORT,PROTO
1890 =item sethostent STAYOPEN
1892 =item setnetent STAYOPEN
1894 =item setprotoent STAYOPEN
1896 =item setservent STAYOPEN
1910 These routines perform the same functions as their counterparts in the
1911 system library. In list context, the return values from the
1912 various get routines are as follows:
1914 ($name,$passwd,$uid,$gid,
1915 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1916 ($name,$passwd,$gid,$members) = getgr*
1917 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1918 ($name,$aliases,$addrtype,$net) = getnet*
1919 ($name,$aliases,$proto) = getproto*
1920 ($name,$aliases,$port,$proto) = getserv*
1922 (If the entry doesn't exist you get a null list.)
1924 The exact meaning of the $gcos field varies but it usually contains
1925 the real name of the user (as opposed to the login name) and other
1926 information pertaining to the user. Beware, however, that in many
1927 system users are able to change this information and therefore it
1928 cannot be trusted and therefore the $gcos is tainted (see
1929 L<perlsec>). The $passwd and $shell, user's encrypted password and
1930 login shell, are also tainted, because of the same reason.
1932 In scalar context, you get the name, unless the function was a
1933 lookup by name, in which case you get the other thing, whatever it is.
1934 (If the entry doesn't exist you get the undefined value.) For example:
1936 $uid = getpwnam($name);
1937 $name = getpwuid($num);
1939 $gid = getgrnam($name);
1940 $name = getgrgid($num;
1944 In I<getpw*()> the fields $quota, $comment, and $expire are special
1945 cases in the sense that in many systems they are unsupported. If the
1946 $quota is unsupported, it is an empty scalar. If it is supported, it
1947 usually encodes the disk quota. If the $comment field is unsupported,
1948 it is an empty scalar. If it is supported it usually encodes some
1949 administrative comment about the user. In some systems the $quota
1950 field may be $change or $age, fields that have to do with password
1951 aging. In some systems the $comment field may be $class. The $expire
1952 field, if present, encodes the expiration period of the account or the
1953 password. For the availability and the exact meaning of these fields
1954 in your system, please consult your getpwnam(3) documentation and your
1955 F<pwd.h> file. You can also find out from within Perl what your
1956 $quota and $comment fields mean and whether you have the $expire field
1957 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1958 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1959 files are only supported if your vendor has implemented them in the
1960 intuitive fashion that calling the regular C library routines gets the
1961 shadow versions if you're running under privilege or if there exists
1962 the shadow(3) functions as found in System V ( this includes Solaris
1963 and Linux.) Those systems which implement a proprietary shadow password
1964 facility are unlikely to be supported.
1966 The $members value returned by I<getgr*()> is a space separated list of
1967 the login names of the members of the group.
1969 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1970 C, it will be returned to you via C<$?> if the function call fails. The
1971 C<@addrs> value returned by a successful call is a list of the raw
1972 addresses returned by the corresponding system library call. In the
1973 Internet domain, each address is four bytes long and you can unpack it
1974 by saying something like:
1976 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1978 The Socket library makes this slightly easier:
1981 $iaddr = inet_aton("127.1"); # or whatever address
1982 $name = gethostbyaddr($iaddr, AF_INET);
1984 # or going the other way
1985 $straddr = inet_ntoa($iaddr);
1987 If you get tired of remembering which element of the return list
1988 contains which return value, by-name interfaces are provided
1989 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1990 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1991 and C<User::grent>. These override the normal built-ins, supplying
1992 versions that return objects with the appropriate names
1993 for each field. For example:
1997 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1999 Even though it looks like they're the same method calls (uid),
2000 they aren't, because a C<File::stat> object is different from
2001 a C<User::pwent> object.
2003 =item getsockname SOCKET
2005 Returns the packed sockaddr address of this end of the SOCKET connection,
2006 in case you don't know the address because you have several different
2007 IPs that the connection might have come in on.
2010 $mysockaddr = getsockname(SOCK);
2011 ($port, $myaddr) = sockaddr_in($mysockaddr);
2012 printf "Connect to %s [%s]\n",
2013 scalar gethostbyaddr($myaddr, AF_INET),
2016 =item getsockopt SOCKET,LEVEL,OPTNAME
2018 Returns the socket option requested, or undef if there is an error.
2024 Returns the value of EXPR with filename expansions such as the
2025 standard Unix shell F</bin/csh> would do. This is the internal function
2026 implementing the C<< <*.c> >> operator, but you can use it directly.
2027 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2028 discussed in more detail in L<perlop/"I/O Operators">.
2030 Beginning with v5.6.0, this operator is implemented using the standard
2031 C<File::Glob> extension. See L<File::Glob> for details.
2035 Converts a time as returned by the time function to an 8-element list
2036 with the time localized for the standard Greenwich time zone.
2037 Typically used as follows:
2040 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2043 All list elements are numeric, and come straight out of the C `struct
2044 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2045 specified time. $mday is the day of the month, and $mon is the month
2046 itself, in the range C<0..11> with 0 indicating January and 11
2047 indicating December. $year is the number of years since 1900. That
2048 is, $year is C<123> in year 2023. $wday is the day of the week, with
2049 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2050 the year, in the range C<0..364> (or C<0..365> in leap years.)
2052 Note that the $year element is I<not> simply the last two digits of
2053 the year. If you assume it is, then you create non-Y2K-compliant
2054 programs--and you wouldn't want to do that, would you?
2056 The proper way to get a complete 4-digit year is simply:
2060 And to get the last two digits of the year (e.g., '01' in 2001) do:
2062 $year = sprintf("%02d", $year % 100);
2064 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2066 In scalar context, C<gmtime()> returns the ctime(3) value:
2068 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2070 Also see the C<timegm> function provided by the C<Time::Local> module,
2071 and the strftime(3) function available via the POSIX module.
2073 This scalar value is B<not> locale dependent (see L<perllocale>), but
2074 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2075 strftime(3) and mktime(3) functions available via the POSIX module. To
2076 get somewhat similar but locale dependent date strings, set up your
2077 locale environment variables appropriately (please see L<perllocale>)
2078 and try for example:
2080 use POSIX qw(strftime);
2081 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2083 Note that the C<%a> and C<%b> escapes, which represent the short forms
2084 of the day of the week and the month of the year, may not necessarily
2085 be three characters wide in all locales.
2093 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2094 execution there. It may not be used to go into any construct that
2095 requires initialization, such as a subroutine or a C<foreach> loop. It
2096 also can't be used to go into a construct that is optimized away,
2097 or to get out of a block or subroutine given to C<sort>.
2098 It can be used to go almost anywhere else within the dynamic scope,
2099 including out of subroutines, but it's usually better to use some other
2100 construct such as C<last> or C<die>. The author of Perl has never felt the
2101 need to use this form of C<goto> (in Perl, that is--C is another matter).
2102 (The difference being that C does not offer named loops combined with
2103 loop control. Perl does, and this replaces most structured uses of C<goto>
2104 in other languages.)
2106 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2107 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2108 necessarily recommended if you're optimizing for maintainability:
2110 goto ("FOO", "BAR", "GLARCH")[$i];
2112 The C<goto-&NAME> form is quite different from the other forms of
2113 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2114 doesn't have the stigma associated with other gotos. Instead, it
2115 exits the current subroutine (losing any changes set by local()) and
2116 immediately calls in its place the named subroutine using the current
2117 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2118 load another subroutine and then pretend that the other subroutine had
2119 been called in the first place (except that any modifications to C<@_>
2120 in the current subroutine are propagated to the other subroutine.)
2121 After the C<goto>, not even C<caller> will be able to tell that this
2122 routine was called first.
2124 NAME needn't be the name of a subroutine; it can be a scalar variable
2125 containing a code reference, or a block which evaluates to a code
2128 =item grep BLOCK LIST
2130 =item grep EXPR,LIST
2132 This is similar in spirit to, but not the same as, grep(1) and its
2133 relatives. In particular, it is not limited to using regular expressions.
2135 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2136 C<$_> to each element) and returns the list value consisting of those
2137 elements for which the expression evaluated to true. In scalar
2138 context, returns the number of times the expression was true.
2140 @foo = grep(!/^#/, @bar); # weed out comments
2144 @foo = grep {!/^#/} @bar; # weed out comments
2146 Note that C<$_> is an alias to the list value, so it can be used to
2147 modify the elements of the LIST. While this is useful and supported,
2148 it can cause bizarre results if the elements of LIST are not variables.
2149 Similarly, grep returns aliases into the original list, much as a for
2150 loop's index variable aliases the list elements. That is, modifying an
2151 element of a list returned by grep (for example, in a C<foreach>, C<map>
2152 or another C<grep>) actually modifies the element in the original list.
2153 This is usually something to be avoided when writing clear code.
2155 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2161 Interprets EXPR as a hex string and returns the corresponding value.
2162 (To convert strings that might start with either 0, 0x, or 0b, see
2163 L</oct>.) If EXPR is omitted, uses C<$_>.
2165 print hex '0xAf'; # prints '175'
2166 print hex 'aF'; # same
2168 Hex strings may only represent integers. Strings that would cause
2169 integer overflow trigger a warning. Leading whitespace is not stripped,
2174 There is no builtin C<import> function. It is just an ordinary
2175 method (subroutine) defined (or inherited) by modules that wish to export
2176 names to another module. The C<use> function calls the C<import> method
2177 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2179 =item index STR,SUBSTR,POSITION
2181 =item index STR,SUBSTR
2183 The index function searches for one string within another, but without
2184 the wildcard-like behavior of a full regular-expression pattern match.
2185 It returns the position of the first occurrence of SUBSTR in STR at
2186 or after POSITION. If POSITION is omitted, starts searching from the
2187 beginning of the string. The return value is based at C<0> (or whatever
2188 you've set the C<$[> variable to--but don't do that). If the substring
2189 is not found, returns one less than the base, ordinarily C<-1>.
2195 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2196 You should not use this function for rounding: one because it truncates
2197 towards C<0>, and two because machine representations of floating point
2198 numbers can sometimes produce counterintuitive results. For example,
2199 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2200 because it's really more like -268.99999999999994315658 instead. Usually,
2201 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2202 functions will serve you better than will int().
2204 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2206 Implements the ioctl(2) function. You'll probably first have to say
2208 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2210 to get the correct function definitions. If F<ioctl.ph> doesn't
2211 exist or doesn't have the correct definitions you'll have to roll your
2212 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2213 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2214 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2215 written depending on the FUNCTION--a pointer to the string value of SCALAR
2216 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2217 has no string value but does have a numeric value, that value will be
2218 passed rather than a pointer to the string value. To guarantee this to be
2219 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2220 functions may be needed to manipulate the values of structures used by
2223 The return value of C<ioctl> (and C<fcntl>) is as follows:
2225 if OS returns: then Perl returns:
2227 0 string "0 but true"
2228 anything else that number
2230 Thus Perl returns true on success and false on failure, yet you can
2231 still easily determine the actual value returned by the operating
2234 $retval = ioctl(...) || -1;
2235 printf "System returned %d\n", $retval;
2237 The special string "C<0> but true" is exempt from B<-w> complaints
2238 about improper numeric conversions.
2240 Here's an example of setting a filehandle named C<REMOTE> to be
2241 non-blocking at the system level. You'll have to negotiate C<$|>
2242 on your own, though.
2244 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2246 $flags = fcntl(REMOTE, F_GETFL, 0)
2247 or die "Can't get flags for the socket: $!\n";
2249 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2250 or die "Can't set flags for the socket: $!\n";
2252 =item join EXPR,LIST
2254 Joins the separate strings of LIST into a single string with fields
2255 separated by the value of EXPR, and returns that new string. Example:
2257 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2259 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2260 first argument. Compare L</split>.
2264 Returns a list consisting of all the keys of the named hash. (In
2265 scalar context, returns the number of keys.) The keys are returned in
2266 an apparently random order. The actual random order is subject to
2267 change in future versions of perl, but it is guaranteed to be the same
2268 order as either the C<values> or C<each> function produces (given
2269 that the hash has not been modified). As a side effect, it resets
2272 Here is yet another way to print your environment:
2275 @values = values %ENV;
2277 print pop(@keys), '=', pop(@values), "\n";
2280 or how about sorted by key:
2282 foreach $key (sort(keys %ENV)) {
2283 print $key, '=', $ENV{$key}, "\n";
2286 The returned values are copies of the original keys in the hash, so
2287 modifying them will not affect the original hash. Compare L</values>.
2289 To sort a hash by value, you'll need to use a C<sort> function.
2290 Here's a descending numeric sort of a hash by its values:
2292 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2293 printf "%4d %s\n", $hash{$key}, $key;
2296 As an lvalue C<keys> allows you to increase the number of hash buckets
2297 allocated for the given hash. This can gain you a measure of efficiency if
2298 you know the hash is going to get big. (This is similar to pre-extending
2299 an array by assigning a larger number to $#array.) If you say
2303 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2304 in fact, since it rounds up to the next power of two. These
2305 buckets will be retained even if you do C<%hash = ()>, use C<undef
2306 %hash> if you want to free the storage while C<%hash> is still in scope.
2307 You can't shrink the number of buckets allocated for the hash using
2308 C<keys> in this way (but you needn't worry about doing this by accident,
2309 as trying has no effect).
2311 See also C<each>, C<values> and C<sort>.
2313 =item kill SIGNAL, LIST
2315 Sends a signal to a list of processes. Returns the number of
2316 processes successfully signaled (which is not necessarily the
2317 same as the number actually killed).
2319 $cnt = kill 1, $child1, $child2;
2322 If SIGNAL is zero, no signal is sent to the process. This is a
2323 useful way to check that the process is alive and hasn't changed
2324 its UID. See L<perlport> for notes on the portability of this
2327 Unlike in the shell, if SIGNAL is negative, it kills
2328 process groups instead of processes. (On System V, a negative I<PROCESS>
2329 number will also kill process groups, but that's not portable.) That
2330 means you usually want to use positive not negative signals. You may also
2331 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2337 The C<last> command is like the C<break> statement in C (as used in
2338 loops); it immediately exits the loop in question. If the LABEL is
2339 omitted, the command refers to the innermost enclosing loop. The
2340 C<continue> block, if any, is not executed:
2342 LINE: while (<STDIN>) {
2343 last LINE if /^$/; # exit when done with header
2347 C<last> cannot be used to exit a block which returns a value such as
2348 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2349 a grep() or map() operation.
2351 Note that a block by itself is semantically identical to a loop
2352 that executes once. Thus C<last> can be used to effect an early
2353 exit out of such a block.
2355 See also L</continue> for an illustration of how C<last>, C<next>, and
2362 Returns a lowercased version of EXPR. This is the internal function
2363 implementing the C<\L> escape in double-quoted strings. Respects
2364 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2365 and L<perlunicode> for more details about locale and Unicode support.
2367 If EXPR is omitted, uses C<$_>.
2373 Returns the value of EXPR with the first character lowercased. This
2374 is the internal function implementing the C<\l> escape in
2375 double-quoted strings. Respects current LC_CTYPE locale if C<use
2376 locale> in force. See L<perllocale> and L<perlunicode> for more
2377 details about locale and Unicode support.
2379 If EXPR is omitted, uses C<$_>.
2385 Returns the length in characters of the value of EXPR. If EXPR is
2386 omitted, returns length of C<$_>. Note that this cannot be used on
2387 an entire array or hash to find out how many elements these have.
2388 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2390 =item link OLDFILE,NEWFILE
2392 Creates a new filename linked to the old filename. Returns true for
2393 success, false otherwise.
2395 =item listen SOCKET,QUEUESIZE
2397 Does the same thing that the listen system call does. Returns true if
2398 it succeeded, false otherwise. See the example in
2399 L<perlipc/"Sockets: Client/Server Communication">.
2403 You really probably want to be using C<my> instead, because C<local> isn't
2404 what most people think of as "local". See
2405 L<perlsub/"Private Variables via my()"> for details.
2407 A local modifies the listed variables to be local to the enclosing
2408 block, file, or eval. If more than one value is listed, the list must
2409 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2410 for details, including issues with tied arrays and hashes.
2412 =item localtime EXPR
2414 Converts a time as returned by the time function to a 9-element list
2415 with the time analyzed for the local time zone. Typically used as
2419 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2422 All list elements are numeric, and come straight out of the C `struct
2423 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2424 specified time. $mday is the day of the month, and $mon is the month
2425 itself, in the range C<0..11> with 0 indicating January and 11
2426 indicating December. $year is the number of years since 1900. That
2427 is, $year is C<123> in year 2023. $wday is the day of the week, with
2428 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2429 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2430 is true if the specified time occurs during daylight savings time,
2433 Note that the $year element is I<not> simply the last two digits of
2434 the year. If you assume it is, then you create non-Y2K-compliant
2435 programs--and you wouldn't want to do that, would you?
2437 The proper way to get a complete 4-digit year is simply:
2441 And to get the last two digits of the year (e.g., '01' in 2001) do:
2443 $year = sprintf("%02d", $year % 100);
2445 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2447 In scalar context, C<localtime()> returns the ctime(3) value:
2449 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2451 This scalar value is B<not> locale dependent, see L<perllocale>, but
2452 instead a Perl builtin. Also see the C<Time::Local> module
2453 (to convert the second, minutes, hours, ... back to seconds since the
2454 stroke of midnight the 1st of January 1970, the value returned by
2455 time()), and the strftime(3) and mktime(3) functions available via the
2456 POSIX module. To get somewhat similar but locale dependent date
2457 strings, set up your locale environment variables appropriately
2458 (please see L<perllocale>) and try for example:
2460 use POSIX qw(strftime);
2461 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2463 Note that the C<%a> and C<%b>, the short forms of the day of the week
2464 and the month of the year, may not necessarily be three characters wide.
2468 This function places an advisory lock on a variable, subroutine,
2469 or referenced object contained in I<THING> until the lock goes out
2470 of scope. This is a built-in function only if your version of Perl
2471 was built with threading enabled, and if you've said C<use Thread>.
2472 Otherwise a user-defined function by this name will be called.
2479 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2480 returns log of C<$_>. To get the log of another base, use basic algebra:
2481 The base-N log of a number is equal to the natural log of that number
2482 divided by the natural log of N. For example:
2486 return log($n)/log(10);
2489 See also L</exp> for the inverse operation.
2495 Does the same thing as the C<stat> function (including setting the
2496 special C<_> filehandle) but stats a symbolic link instead of the file
2497 the symbolic link points to. If symbolic links are unimplemented on
2498 your system, a normal C<stat> is done.
2500 If EXPR is omitted, stats C<$_>.
2504 The match operator. See L<perlop>.
2506 =item map BLOCK LIST
2510 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2511 C<$_> to each element) and returns the list value composed of the
2512 results of each such evaluation. In scalar context, returns the
2513 total number of elements so generated. Evaluates BLOCK or EXPR in
2514 list context, so each element of LIST may produce zero, one, or
2515 more elements in the returned value.
2517 @chars = map(chr, @nums);
2519 translates a list of numbers to the corresponding characters. And
2521 %hash = map { getkey($_) => $_ } @array;
2523 is just a funny way to write
2526 foreach $_ (@array) {
2527 $hash{getkey($_)} = $_;
2530 Note that C<$_> is an alias to the list value, so it can be used to
2531 modify the elements of the LIST. While this is useful and supported,
2532 it can cause bizarre results if the elements of LIST are not variables.
2533 Using a regular C<foreach> loop for this purpose would be clearer in
2534 most cases. See also L</grep> for an array composed of those items of
2535 the original list for which the BLOCK or EXPR evaluates to true.
2537 C<{> starts both hash references and blocks, so C<map { ...> could be either
2538 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2539 ahead for the closing C<}> it has to take a guess at which its dealing with
2540 based what it finds just after the C<{>. Usually it gets it right, but if it
2541 doesn't it won't realize something is wrong until it gets to the C<}> and
2542 encounters the missing (or unexpected) comma. The syntax error will be
2543 reported close to the C<}> but you'll need to change something near the C<{>
2544 such as using a unary C<+> to give perl some help:
2546 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2547 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2548 %hash = map { ("\L$_", 1) } @array # this also works
2549 %hash = map { lc($_), 1 } @array # as does this.
2550 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2552 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2554 or to force an anon hash constructor use C<+{>
2556 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2558 and you get list of anonymous hashes each with only 1 entry.
2560 =item mkdir FILENAME,MASK
2562 =item mkdir FILENAME
2564 Creates the directory specified by FILENAME, with permissions
2565 specified by MASK (as modified by C<umask>). If it succeeds it
2566 returns true, otherwise it returns false and sets C<$!> (errno).
2567 If omitted, MASK defaults to 0777.
2569 In general, it is better to create directories with permissive MASK,
2570 and let the user modify that with their C<umask>, than it is to supply
2571 a restrictive MASK and give the user no way to be more permissive.
2572 The exceptions to this rule are when the file or directory should be
2573 kept private (mail files, for instance). The perlfunc(1) entry on
2574 C<umask> discusses the choice of MASK in more detail.
2576 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2577 number of trailing slashes. Some operating and filesystems do not get
2578 this right, so Perl automatically removes all trailing slashes to keep
2581 =item msgctl ID,CMD,ARG
2583 Calls the System V IPC function msgctl(2). You'll probably have to say
2587 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2588 then ARG must be a variable which will hold the returned C<msqid_ds>
2589 structure. Returns like C<ioctl>: the undefined value for error,
2590 C<"0 but true"> for zero, or the actual return value otherwise. See also
2591 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2593 =item msgget KEY,FLAGS
2595 Calls the System V IPC function msgget(2). Returns the message queue
2596 id, or the undefined value if there is an error. See also
2597 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2599 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2601 Calls the System V IPC function msgrcv to receive a message from
2602 message queue ID into variable VAR with a maximum message size of
2603 SIZE. Note that when a message is received, the message type as a
2604 native long integer will be the first thing in VAR, followed by the
2605 actual message. This packing may be opened with C<unpack("l! a*")>.
2606 Taints the variable. Returns true if successful, or false if there is
2607 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2608 C<IPC::SysV::Msg> documentation.
2610 =item msgsnd ID,MSG,FLAGS
2612 Calls the System V IPC function msgsnd to send the message MSG to the
2613 message queue ID. MSG must begin with the native long integer message
2614 type, and be followed by the length of the actual message, and finally
2615 the message itself. This kind of packing can be achieved with
2616 C<pack("l! a*", $type, $message)>. Returns true if successful,
2617 or false if there is an error. See also C<IPC::SysV>
2618 and C<IPC::SysV::Msg> documentation.
2622 =item my EXPR : ATTRIBUTES
2624 A C<my> declares the listed variables to be local (lexically) to the
2625 enclosing block, file, or C<eval>. If
2626 more than one value is listed, the list must be placed in parentheses. See
2627 L<perlsub/"Private Variables via my()"> for details.
2633 The C<next> command is like the C<continue> statement in C; it starts
2634 the next iteration of the loop:
2636 LINE: while (<STDIN>) {
2637 next LINE if /^#/; # discard comments
2641 Note that if there were a C<continue> block on the above, it would get
2642 executed even on discarded lines. If the LABEL is omitted, the command
2643 refers to the innermost enclosing loop.
2645 C<next> cannot be used to exit a block which returns a value such as
2646 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2647 a grep() or map() operation.
2649 Note that a block by itself is semantically identical to a loop
2650 that executes once. Thus C<next> will exit such a block early.
2652 See also L</continue> for an illustration of how C<last>, C<next>, and
2655 =item no Module VERSION LIST
2657 =item no Module VERSION
2659 =item no Module LIST
2663 See the L</use> function, which C<no> is the opposite of.
2669 Interprets EXPR as an octal string and returns the corresponding
2670 value. (If EXPR happens to start off with C<0x>, interprets it as a
2671 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2672 binary string. Leading whitespace is ignored in all three cases.)
2673 The following will handle decimal, binary, octal, and hex in the standard
2676 $val = oct($val) if $val =~ /^0/;
2678 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2679 in octal), use sprintf() or printf():
2681 $perms = (stat("filename"))[2] & 07777;
2682 $oct_perms = sprintf "%lo", $perms;
2684 The oct() function is commonly used when a string such as C<644> needs
2685 to be converted into a file mode, for example. (Although perl will
2686 automatically convert strings into numbers as needed, this automatic
2687 conversion assumes base 10.)
2689 =item open FILEHANDLE,EXPR
2691 =item open FILEHANDLE,MODE,EXPR
2693 =item open FILEHANDLE,MODE,EXPR,LIST
2695 =item open FILEHANDLE,MODE,REFERENCE
2697 =item open FILEHANDLE
2699 Opens the file whose filename is given by EXPR, and associates it with
2702 (The following is a comprehensive reference to open(): for a gentler
2703 introduction you may consider L<perlopentut>.)
2705 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2706 assigned a reference to a new anonymous filehandle, otherwise if
2707 FILEHANDLE is an expression, its value is used as the name of the real
2708 filehandle wanted. (This is considered a symbolic reference, so C<use
2709 strict 'refs'> should I<not> be in effect.)
2711 If EXPR is omitted, the scalar variable of the same name as the
2712 FILEHANDLE contains the filename. (Note that lexical variables--those
2713 declared with C<my>--will not work for this purpose; so if you're
2714 using C<my>, specify EXPR in your call to open.)
2716 If three or more arguments are specified then the mode of opening and
2717 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2718 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2719 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2720 the file is opened for appending, again being created if necessary.
2722 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2723 indicate that you want both read and write access to the file; thus
2724 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2725 '+>' >> mode would clobber the file first. You can't usually use
2726 either read-write mode for updating textfiles, since they have
2727 variable length records. See the B<-i> switch in L<perlrun> for a
2728 better approach. The file is created with permissions of C<0666>
2729 modified by the process' C<umask> value.
2731 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2732 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2734 In the 2-arguments (and 1-argument) form of the call the mode and
2735 filename should be concatenated (in this order), possibly separated by
2736 spaces. It is possible to omit the mode in these forms if the mode is
2739 If the filename begins with C<'|'>, the filename is interpreted as a
2740 command to which output is to be piped, and if the filename ends with a
2741 C<'|'>, the filename is interpreted as a command which pipes output to
2742 us. See L<perlipc/"Using open() for IPC">
2743 for more examples of this. (You are not allowed to C<open> to a command
2744 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2745 and L<perlipc/"Bidirectional Communication with Another Process">
2748 For three or more arguments if MODE is C<'|-'>, the filename is
2749 interpreted as a command to which output is to be piped, and if MODE
2750 is C<'-|'>, the filename is interpreted as a command which pipes
2751 output to us. In the 2-arguments (and 1-argument) form one should
2752 replace dash (C<'-'>) with the command.
2753 See L<perlipc/"Using open() for IPC"> for more examples of this.
2754 (You are not allowed to C<open> to a command that pipes both in I<and>
2755 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2756 L<perlipc/"Bidirectional Communication"> for alternatives.)
2758 In the three-or-more argument form of pipe opens, if LIST is specified
2759 (extra arguments after the command name) then LIST becomes arguments
2760 to the command invoked if the platform supports it. The meaning of
2761 C<open> with more than three arguments for non-pipe modes is not yet
2762 specified. Experimental "layers" may give extra LIST arguments
2765 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2766 and opening C<< '>-' >> opens STDOUT.
2768 You may use the three-argument form of open to specify
2769 I<I/O disciplines> that affect how the input and output
2770 are processed: see L</binmode> and L<open>. For example
2772 open(FH, "<:utf8", "file")
2774 will open the UTF-8 encoded file containing Unicode characters,
2775 see L<perluniintro>.
2777 Open returns nonzero upon success, the undefined value otherwise. If
2778 the C<open> involved a pipe, the return value happens to be the pid of
2781 If you're running Perl on a system that distinguishes between text
2782 files and binary files, then you should check out L</binmode> for tips
2783 for dealing with this. The key distinction between systems that need
2784 C<binmode> and those that don't is their text file formats. Systems
2785 like Unix, MacOS, and Plan9, which delimit lines with a single
2786 character, and which encode that character in C as C<"\n">, do not
2787 need C<binmode>. The rest need it.
2789 In the three argument form MODE may also contain a list of IO "layers"
2790 (see L<open> and L<PerlIO> for more details) to be applied to the
2791 handle. This can be used to achieve the effect of C<binmode> as well
2792 as more complex behaviours.
2794 When opening a file, it's usually a bad idea to continue normal execution
2795 if the request failed, so C<open> is frequently used in connection with
2796 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2797 where you want to make a nicely formatted error message (but there are
2798 modules that can help with that problem)) you should always check
2799 the return value from opening a file. The infrequent exception is when
2800 working with an unopened filehandle is actually what you want to do.
2802 As a special case the 3 arg form with a read/write mode and the third
2803 argument being C<undef>:
2805 open(TMP, "+>", undef) or die ...
2807 opens a filehandle to an anonymous temporary file.
2809 File handles can be opened to "in memory" files held in Perl scalars via:
2811 open($fh,'>', \$variable) || ..
2816 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2817 while (<ARTICLE>) {...
2819 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2820 # if the open fails, output is discarded
2822 open(DBASE, '+<', 'dbase.mine') # open for update
2823 or die "Can't open 'dbase.mine' for update: $!";
2825 open(DBASE, '+<dbase.mine') # ditto
2826 or die "Can't open 'dbase.mine' for update: $!";
2828 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2829 or die "Can't start caesar: $!";
2831 open(ARTICLE, "caesar <$article |") # ditto
2832 or die "Can't start caesar: $!";
2834 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2835 or die "Can't start sort: $!";
2838 open(MEMORY,'>', \$var)
2839 or die "Can't open memory file: $!";
2840 print MEMORY "foo!\n"; # output will end up in $var
2842 # process argument list of files along with any includes
2844 foreach $file (@ARGV) {
2845 process($file, 'fh00');
2849 my($filename, $input) = @_;
2850 $input++; # this is a string increment
2851 unless (open($input, $filename)) {
2852 print STDERR "Can't open $filename: $!\n";
2857 while (<$input>) { # note use of indirection
2858 if (/^#include "(.*)"/) {
2859 process($1, $input);
2866 You may also, in the Bourne shell tradition, specify an EXPR beginning
2867 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2868 name of a filehandle (or file descriptor, if numeric) to be
2869 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2870 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2871 mode you specify should match the mode of the original filehandle.
2872 (Duping a filehandle does not take into account any existing contents of
2873 IO buffers.) If you use the 3 arg form then you can pass either a number,
2874 the name of a filehandle or the normal "reference to a glob".
2876 Here is a script that saves, redirects, and restores C<STDOUT> and
2877 C<STDERR> using various methods:
2880 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2881 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2883 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2884 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2886 select STDERR; $| = 1; # make unbuffered
2887 select STDOUT; $| = 1; # make unbuffered
2889 print STDOUT "stdout 1\n"; # this works for
2890 print STDERR "stderr 1\n"; # subprocesses too
2895 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2896 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2898 print STDOUT "stdout 2\n";
2899 print STDERR "stderr 2\n";
2901 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2902 do an equivalent of C's C<fdopen> of that file descriptor; this is
2903 more parsimonious of file descriptors. For example:
2905 open(FILEHANDLE, "<&=$fd")
2909 open(FILEHANDLE, "<&=", $fd)
2911 Note that if Perl is using the standard C libraries' fdopen() then on
2912 many UNIX systems, fdopen() is known to fail when file descriptors
2913 exceed a certain value, typically 255. If you need more file
2914 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2916 You can see whether Perl has been compiled with PerlIO or not by
2917 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2918 is C<define>, you have PerlIO, otherwise you don't.
2920 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2921 with 2-arguments (or 1-argument) form of open(), then
2922 there is an implicit fork done, and the return value of open is the pid
2923 of the child within the parent process, and C<0> within the child
2924 process. (Use C<defined($pid)> to determine whether the open was successful.)
2925 The filehandle behaves normally for the parent, but i/o to that
2926 filehandle is piped from/to the STDOUT/STDIN of the child process.
2927 In the child process the filehandle isn't opened--i/o happens from/to
2928 the new STDOUT or STDIN. Typically this is used like the normal
2929 piped open when you want to exercise more control over just how the
2930 pipe command gets executed, such as when you are running setuid, and
2931 don't want to have to scan shell commands for metacharacters.
2932 The following triples are more or less equivalent:
2934 open(FOO, "|tr '[a-z]' '[A-Z]'");
2935 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2936 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2937 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2939 open(FOO, "cat -n '$file'|");
2940 open(FOO, '-|', "cat -n '$file'");
2941 open(FOO, '-|') || exec 'cat', '-n', $file;
2942 open(FOO, '-|', "cat", '-n', $file);
2944 The last example in each block shows the pipe as "list form", which is
2945 not yet supported on all platforms.
2947 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2949 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2950 output before any operation that may do a fork, but this may not be
2951 supported on some platforms (see L<perlport>). To be safe, you may need
2952 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2953 of C<IO::Handle> on any open handles.
2955 On systems that support a close-on-exec flag on files, the flag will
2956 be set for the newly opened file descriptor as determined by the value
2957 of $^F. See L<perlvar/$^F>.
2959 Closing any piped filehandle causes the parent process to wait for the
2960 child to finish, and returns the status value in C<$?>.
2962 The filename passed to 2-argument (or 1-argument) form of open() will
2963 have leading and trailing whitespace deleted, and the normal
2964 redirection characters honored. This property, known as "magic open",
2965 can often be used to good effect. A user could specify a filename of
2966 F<"rsh cat file |">, or you could change certain filenames as needed:
2968 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2969 open(FH, $filename) or die "Can't open $filename: $!";
2971 Use 3-argument form to open a file with arbitrary weird characters in it,
2973 open(FOO, '<', $file);
2975 otherwise it's necessary to protect any leading and trailing whitespace:
2977 $file =~ s#^(\s)#./$1#;
2978 open(FOO, "< $file\0");
2980 (this may not work on some bizarre filesystems). One should
2981 conscientiously choose between the I<magic> and 3-arguments form
2986 will allow the user to specify an argument of the form C<"rsh cat file |">,
2987 but will not work on a filename which happens to have a trailing space, while
2989 open IN, '<', $ARGV[0];
2991 will have exactly the opposite restrictions.
2993 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2994 should use the C<sysopen> function, which involves no such magic (but
2995 may use subtly different filemodes than Perl open(), which is mapped
2996 to C fopen()). This is
2997 another way to protect your filenames from interpretation. For example:
3000 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3001 or die "sysopen $path: $!";
3002 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3003 print HANDLE "stuff $$\n";
3005 print "File contains: ", <HANDLE>;
3007 Using the constructor from the C<IO::Handle> package (or one of its
3008 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3009 filehandles that have the scope of whatever variables hold references to
3010 them, and automatically close whenever and however you leave that scope:
3014 sub read_myfile_munged {
3016 my $handle = new IO::File;
3017 open($handle, "myfile") or die "myfile: $!";
3019 or return (); # Automatically closed here.
3020 mung $first or die "mung failed"; # Or here.
3021 return $first, <$handle> if $ALL; # Or here.
3025 See L</seek> for some details about mixing reading and writing.
3027 =item opendir DIRHANDLE,MODE,EXPR
3029 =item opendir DIRHANDLE,EXPR
3031 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3032 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3033 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3035 In three-argument form the middle argument may be C<:utf8> to force
3036 the filenames returned by readdir() to be in UTF-8 encoding of Unicode.
3037 This naturally works only if your filesystem returns UTF-8 filenames.
3038 The encoding pragma affects this feature, see L<encoding>.
3044 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3045 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3048 For the reverse, see L</chr>.
3049 See L<perlunicode> and L<encoding> for more about Unicode.
3053 =item our EXPR : ATTRIBUTES
3055 An C<our> declares the listed variables to be valid globals within
3056 the enclosing block, file, or C<eval>. That is, it has the same
3057 scoping rules as a "my" declaration, but does not create a local
3058 variable. If more than one value is listed, the list must be placed
3059 in parentheses. The C<our> declaration has no semantic effect unless
3060 "use strict vars" is in effect, in which case it lets you use the
3061 declared global variable without qualifying it with a package name.
3062 (But only within the lexical scope of the C<our> declaration. In this
3063 it differs from "use vars", which is package scoped.)
3065 An C<our> declaration declares a global variable that will be visible
3066 across its entire lexical scope, even across package boundaries. The
3067 package in which the variable is entered is determined at the point
3068 of the declaration, not at the point of use. This means the following
3072 our $bar; # declares $Foo::bar for rest of lexical scope
3076 print $bar; # prints 20
3078 Multiple C<our> declarations in the same lexical scope are allowed
3079 if they are in different packages. If they happened to be in the same
3080 package, Perl will emit warnings if you have asked for them.
3084 our $bar; # declares $Foo::bar for rest of lexical scope
3088 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3089 print $bar; # prints 30
3091 our $bar; # emits warning
3093 An C<our> declaration may also have a list of attributes associated
3094 with it. B<WARNING>: This is an experimental feature that may be
3095 changed or removed in future releases of Perl. It should not be
3098 The only currently recognized attribute is C<unique> which indicates
3099 that a single copy of the global is to be used by all interpreters
3100 should the program happen to be running in a multi-interpreter
3101 environment. (The default behaviour would be for each interpreter to
3102 have its own copy of the global.) In such an environment, this
3103 attribute also has the effect of making the global readonly.
3106 our @EXPORT : unique = qw(foo);
3107 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3108 our $VERSION : unique = "1.00";
3110 Multi-interpreter environments can come to being either through the
3111 fork() emulation on Windows platforms, or by embedding perl in a
3112 multi-threaded application. The C<unique> attribute does nothing in
3113 all other environments.
3115 =item pack TEMPLATE,LIST
3117 Takes a LIST of values and converts it into a string using the rules
3118 given by the TEMPLATE. The resulting string is the concatenation of
3119 the converted values. Typically, each converted value looks
3120 like its machine-level representation. For example, on 32-bit machines
3121 a converted integer may be represented by a sequence of 4 bytes.
3123 The TEMPLATE is a sequence of characters that give the order and type
3124 of values, as follows:
3126 a A string with arbitrary binary data, will be null padded.
3127 A A text (ASCII) string, will be space padded.
3128 Z A null terminated (ASCIZ) string, will be null padded.
3130 b A bit string (ascending bit order inside each byte, like vec()).
3131 B A bit string (descending bit order inside each byte).
3132 h A hex string (low nybble first).
3133 H A hex string (high nybble first).
3135 c A signed char value.
3136 C An unsigned char value. Only does bytes. See U for Unicode.
3138 s A signed short value.
3139 S An unsigned short value.
3140 (This 'short' is _exactly_ 16 bits, which may differ from
3141 what a local C compiler calls 'short'. If you want
3142 native-length shorts, use the '!' suffix.)
3144 i A signed integer value.
3145 I An unsigned integer value.
3146 (This 'integer' is _at_least_ 32 bits wide. Its exact
3147 size depends on what a local C compiler calls 'int',
3148 and may even be larger than the 'long' described in
3151 l A signed long value.
3152 L An unsigned long value.
3153 (This 'long' is _exactly_ 32 bits, which may differ from
3154 what a local C compiler calls 'long'. If you want
3155 native-length longs, use the '!' suffix.)
3157 n An unsigned short in "network" (big-endian) order.
3158 N An unsigned long in "network" (big-endian) order.
3159 v An unsigned short in "VAX" (little-endian) order.
3160 V An unsigned long in "VAX" (little-endian) order.
3161 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3162 _exactly_ 32 bits, respectively.)
3164 q A signed quad (64-bit) value.
3165 Q An unsigned quad value.
3166 (Quads are available only if your system supports 64-bit
3167 integer values _and_ if Perl has been compiled to support those.
3168 Causes a fatal error otherwise.)
3170 j A signed integer value (a Perl internal integer, IV).
3171 J An unsigned integer value (a Perl internal unsigned integer, UV).
3173 f A single-precision float in the native format.
3174 d A double-precision float in the native format.
3176 F A floating point value in the native native format
3177 (a Perl internal floating point value, NV).
3178 D A long double-precision float in the native format.
3179 (Long doubles are available only if your system supports long
3180 double values _and_ if Perl has been compiled to support those.
3181 Causes a fatal error otherwise.)
3183 p A pointer to a null-terminated string.
3184 P A pointer to a structure (fixed-length string).
3186 u A uuencoded string.
3187 U A Unicode character number. Encodes to UTF-8 internally
3188 (or UTF-EBCDIC in EBCDIC platforms).
3190 w A BER compressed integer. Its bytes represent an unsigned
3191 integer in base 128, most significant digit first, with as
3192 few digits as possible. Bit eight (the high bit) is set
3193 on each byte except the last.
3197 @ Null fill to absolute position.
3198 ( Start of a ()-group.
3200 The following rules apply:
3206 Each letter may optionally be followed by a number giving a repeat
3207 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3208 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3209 many values from the LIST. A C<*> for the repeat count means to use
3210 however many items are left, except for C<@>, C<x>, C<X>, where it is
3211 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3212 is the same). A numeric repeat count may optionally be enclosed in
3213 brackets, as in C<pack 'C[80]', @arr>.
3215 One can replace the numeric repeat count by a template enclosed in brackets;
3216 then the packed length of this template in bytes is used as a count.
3217 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3218 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3219 If the template in brackets contains alignment commands (such as C<x![d]>),
3220 its packed length is calculated as if the start of the template has the maximal
3223 When used with C<Z>, C<*> results in the addition of a trailing null
3224 byte (so the packed result will be one longer than the byte C<length>
3227 The repeat count for C<u> is interpreted as the maximal number of bytes
3228 to encode per line of output, with 0 and 1 replaced by 45.
3232 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3233 string of length count, padding with nulls or spaces as necessary. When
3234 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3235 after the first null, and C<a> returns data verbatim. When packing,
3236 C<a>, and C<Z> are equivalent.
3238 If the value-to-pack is too long, it is truncated. If too long and an
3239 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3240 by a null byte. Thus C<Z> always packs a trailing null byte under
3245 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3246 Each byte of the input field of pack() generates 1 bit of the result.
3247 Each result bit is based on the least-significant bit of the corresponding
3248 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3249 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3251 Starting from the beginning of the input string of pack(), each 8-tuple
3252 of bytes is converted to 1 byte of output. With format C<b>
3253 the first byte of the 8-tuple determines the least-significant bit of a
3254 byte, and with format C<B> it determines the most-significant bit of
3257 If the length of the input string is not exactly divisible by 8, the
3258 remainder is packed as if the input string were padded by null bytes
3259 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3261 If the input string of pack() is longer than needed, extra bytes are ignored.
3262 A C<*> for the repeat count of pack() means to use all the bytes of
3263 the input field. On unpack()ing the bits are converted to a string
3264 of C<"0">s and C<"1">s.
3268 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3269 representable as hexadecimal digits, 0-9a-f) long.
3271 Each byte of the input field of pack() generates 4 bits of the result.
3272 For non-alphabetical bytes the result is based on the 4 least-significant
3273 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3274 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3275 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3276 is compatible with the usual hexadecimal digits, so that C<"a"> and
3277 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3278 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3280 Starting from the beginning of the input string of pack(), each pair
3281 of bytes is converted to 1 byte of output. With format C<h> the
3282 first byte of the pair determines the least-significant nybble of the
3283 output byte, and with format C<H> it determines the most-significant
3286 If the length of the input string is not even, it behaves as if padded
3287 by a null byte at the end. Similarly, during unpack()ing the "extra"
3288 nybbles are ignored.
3290 If the input string of pack() is longer than needed, extra bytes are ignored.
3291 A C<*> for the repeat count of pack() means to use all the bytes of
3292 the input field. On unpack()ing the bits are converted to a string
3293 of hexadecimal digits.
3297 The C<p> type packs a pointer to a null-terminated string. You are
3298 responsible for ensuring the string is not a temporary value (which can
3299 potentially get deallocated before you get around to using the packed result).
3300 The C<P> type packs a pointer to a structure of the size indicated by the
3301 length. A NULL pointer is created if the corresponding value for C<p> or
3302 C<P> is C<undef>, similarly for unpack().
3306 The C</> template character allows packing and unpacking of strings where
3307 the packed structure contains a byte count followed by the string itself.
3308 You write I<length-item>C</>I<string-item>.
3310 The I<length-item> can be any C<pack> template letter, and describes
3311 how the length value is packed. The ones likely to be of most use are
3312 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3313 SNMP) and C<N> (for Sun XDR).
3315 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3316 For C<unpack> the length of the string is obtained from the I<length-item>,
3317 but if you put in the '*' it will be ignored.
3319 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3320 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3321 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3323 The I<length-item> is not returned explicitly from C<unpack>.
3325 Adding a count to the I<length-item> letter is unlikely to do anything
3326 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3327 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3328 which Perl does not regard as legal in numeric strings.
3332 The integer types C<s>, C<S>, C<l>, and C<L> may be
3333 immediately followed by a C<!> suffix to signify native shorts or
3334 longs--as you can see from above for example a bare C<l> does mean
3335 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3336 may be larger. This is an issue mainly in 64-bit platforms. You can
3337 see whether using C<!> makes any difference by
3339 print length(pack("s")), " ", length(pack("s!")), "\n";
3340 print length(pack("l")), " ", length(pack("l!")), "\n";
3342 C<i!> and C<I!> also work but only because of completeness;
3343 they are identical to C<i> and C<I>.
3345 The actual sizes (in bytes) of native shorts, ints, longs, and long
3346 longs on the platform where Perl was built are also available via
3350 print $Config{shortsize}, "\n";
3351 print $Config{intsize}, "\n";
3352 print $Config{longsize}, "\n";
3353 print $Config{longlongsize}, "\n";
3355 (The C<$Config{longlongsize}> will be undefine if your system does
3356 not support long longs.)
3360 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3361 are inherently non-portable between processors and operating systems
3362 because they obey the native byteorder and endianness. For example a
3363 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3364 (arranged in and handled by the CPU registers) into bytes as
3366 0x12 0x34 0x56 0x78 # big-endian
3367 0x78 0x56 0x34 0x12 # little-endian
3369 Basically, the Intel and VAX CPUs are little-endian, while everybody
3370 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3371 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3372 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3375 The names `big-endian' and `little-endian' are comic references to
3376 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3377 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3378 the egg-eating habits of the Lilliputians.
3380 Some systems may have even weirder byte orders such as
3385 You can see your system's preference with
3387 print join(" ", map { sprintf "%#02x", $_ }
3388 unpack("C*",pack("L",0x12345678))), "\n";
3390 The byteorder on the platform where Perl was built is also available
3394 print $Config{byteorder}, "\n";
3396 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3397 and C<'87654321'> are big-endian.
3399 If you want portable packed integers use the formats C<n>, C<N>,
3400 C<v>, and C<V>, their byte endianness and size are known.
3401 See also L<perlport>.
3405 Real numbers (floats and doubles) are in the native machine format only;
3406 due to the multiplicity of floating formats around, and the lack of a
3407 standard "network" representation, no facility for interchange has been
3408 made. This means that packed floating point data written on one machine
3409 may not be readable on another - even if both use IEEE floating point
3410 arithmetic (as the endian-ness of the memory representation is not part
3411 of the IEEE spec). See also L<perlport>.
3413 Note that Perl uses doubles internally for all numeric calculation, and
3414 converting from double into float and thence back to double again will
3415 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3420 If the pattern begins with a C<U>, the resulting string will be treated
3421 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3422 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3423 characters. If you don't want this to happen, you can begin your pattern
3424 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3425 string, and then follow this with a C<U*> somewhere in your pattern.
3429 You must yourself do any alignment or padding by inserting for example
3430 enough C<'x'>es while packing. There is no way to pack() and unpack()
3431 could know where the bytes are going to or coming from. Therefore
3432 C<pack> (and C<unpack>) handle their output and input as flat
3437 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3438 take a repeat count, both as postfix, and via the C</> template
3443 C<x> and C<X> accept C<!> modifier. In this case they act as
3444 alignment commands: they jump forward/back to the closest position
3445 aligned at a multiple of C<count> bytes. For example, to pack() or
3446 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3447 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3448 aligned on the double's size.
3450 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3451 both result in no-ops.
3455 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3459 If TEMPLATE requires more arguments to pack() than actually given, pack()
3460 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3461 to pack() than actually given, extra arguments are ignored.
3467 $foo = pack("CCCC",65,66,67,68);
3469 $foo = pack("C4",65,66,67,68);
3471 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3472 # same thing with Unicode circled letters
3474 $foo = pack("ccxxcc",65,66,67,68);
3477 # note: the above examples featuring "C" and "c" are true
3478 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3479 # and UTF-8. In EBCDIC the first example would be
3480 # $foo = pack("CCCC",193,194,195,196);
3482 $foo = pack("s2",1,2);
3483 # "\1\0\2\0" on little-endian
3484 # "\0\1\0\2" on big-endian
3486 $foo = pack("a4","abcd","x","y","z");
3489 $foo = pack("aaaa","abcd","x","y","z");
3492 $foo = pack("a14","abcdefg");
3493 # "abcdefg\0\0\0\0\0\0\0"
3495 $foo = pack("i9pl", gmtime);
3496 # a real struct tm (on my system anyway)
3498 $utmp_template = "Z8 Z8 Z16 L";
3499 $utmp = pack($utmp_template, @utmp1);
3500 # a struct utmp (BSDish)
3502 @utmp2 = unpack($utmp_template, $utmp);
3503 # "@utmp1" eq "@utmp2"
3506 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3509 $foo = pack('sx2l', 12, 34);
3510 # short 12, two zero bytes padding, long 34
3511 $bar = pack('s@4l', 12, 34);
3512 # short 12, zero fill to position 4, long 34
3515 The same template may generally also be used in unpack().
3517 =item package NAMESPACE
3521 Declares the compilation unit as being in the given namespace. The scope
3522 of the package declaration is from the declaration itself through the end
3523 of the enclosing block, file, or eval (the same as the C<my> operator).
3524 All further unqualified dynamic identifiers will be in this namespace.
3525 A package statement affects only dynamic variables--including those
3526 you've used C<local> on--but I<not> lexical variables, which are created
3527 with C<my>. Typically it would be the first declaration in a file to
3528 be included by the C<require> or C<use> operator. You can switch into a
3529 package in more than one place; it merely influences which symbol table
3530 is used by the compiler for the rest of that block. You can refer to
3531 variables and filehandles in other packages by prefixing the identifier
3532 with the package name and a double colon: C<$Package::Variable>.
3533 If the package name is null, the C<main> package as assumed. That is,
3534 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3535 still seen in older code).
3537 If NAMESPACE is omitted, then there is no current package, and all
3538 identifiers must be fully qualified or lexicals. However, you are
3539 strongly advised not to make use of this feature. Its use can cause
3540 unexpected behaviour, even crashing some versions of Perl. It is
3541 deprecated, and will be removed from a future release.
3543 See L<perlmod/"Packages"> for more information about packages, modules,
3544 and classes. See L<perlsub> for other scoping issues.
3546 =item pipe READHANDLE,WRITEHANDLE
3548 Opens a pair of connected pipes like the corresponding system call.
3549 Note that if you set up a loop of piped processes, deadlock can occur
3550 unless you are very careful. In addition, note that Perl's pipes use
3551 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3552 after each command, depending on the application.
3554 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3555 for examples of such things.
3557 On systems that support a close-on-exec flag on files, the flag will be set
3558 for the newly opened file descriptors as determined by the value of $^F.
3565 Pops and returns the last value of the array, shortening the array by
3566 one element. Has an effect similar to
3570 If there are no elements in the array, returns the undefined value
3571 (although this may happen at other times as well). If ARRAY is
3572 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3573 array in subroutines, just like C<shift>.
3579 Returns the offset of where the last C<m//g> search left off for the variable
3580 in question (C<$_> is used when the variable is not specified). May be
3581 modified to change that offset. Such modification will also influence
3582 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3585 =item print FILEHANDLE LIST
3591 Prints a string or a list of strings. Returns true if successful.
3592 FILEHANDLE may be a scalar variable name, in which case the variable
3593 contains the name of or a reference to the filehandle, thus introducing
3594 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3595 the next token is a term, it may be misinterpreted as an operator
3596 unless you interpose a C<+> or put parentheses around the arguments.)
3597 If FILEHANDLE is omitted, prints by default to standard output (or
3598 to the last selected output channel--see L</select>). If LIST is
3599 also omitted, prints C<$_> to the currently selected output channel.
3600 To set the default output channel to something other than STDOUT
3601 use the select operation. The current value of C<$,> (if any) is
3602 printed between each LIST item. The current value of C<$\> (if
3603 any) is printed after the entire LIST has been printed. Because
3604 print takes a LIST, anything in the LIST is evaluated in list
3605 context, and any subroutine that you call will have one or more of
3606 its expressions evaluated in list context. Also be careful not to
3607 follow the print keyword with a left parenthesis unless you want
3608 the corresponding right parenthesis to terminate the arguments to
3609 the print--interpose a C<+> or put parentheses around all the
3612 Note that if you're storing FILEHANDLES in an array or other expression,
3613 you will have to use a block returning its value instead:
3615 print { $files[$i] } "stuff\n";
3616 print { $OK ? STDOUT : STDERR } "stuff\n";
3618 =item printf FILEHANDLE FORMAT, LIST
3620 =item printf FORMAT, LIST
3622 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3623 (the output record separator) is not appended. The first argument
3624 of the list will be interpreted as the C<printf> format. See C<sprintf>
3625 for an explanation of the format argument. If C<use locale> is in effect,
3626 the character used for the decimal point in formatted real numbers is
3627 affected by the LC_NUMERIC locale. See L<perllocale>.
3629 Don't fall into the trap of using a C<printf> when a simple
3630 C<print> would do. The C<print> is more efficient and less
3633 =item prototype FUNCTION
3635 Returns the prototype of a function as a string (or C<undef> if the
3636 function has no prototype). FUNCTION is a reference to, or the name of,
3637 the function whose prototype you want to retrieve.
3639 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3640 name for Perl builtin. If the builtin is not I<overridable> (such as
3641 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3642 C<system>) returns C<undef> because the builtin does not really behave
3643 like a Perl function. Otherwise, the string describing the equivalent
3644 prototype is returned.
3646 =item push ARRAY,LIST
3648 Treats ARRAY as a stack, and pushes the values of LIST
3649 onto the end of ARRAY. The length of ARRAY increases by the length of
3650 LIST. Has the same effect as
3653 $ARRAY[++$#ARRAY] = $value;
3656 but is more efficient. Returns the new number of elements in the array.
3668 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3670 =item quotemeta EXPR
3674 Returns the value of EXPR with all non-"word"
3675 characters backslashed. (That is, all characters not matching
3676 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3677 returned string, regardless of any locale settings.)
3678 This is the internal function implementing
3679 the C<\Q> escape in double-quoted strings.
3681 If EXPR is omitted, uses C<$_>.
3687 Returns a random fractional number greater than or equal to C<0> and less
3688 than the value of EXPR. (EXPR should be positive.) If EXPR is
3689 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3690 unless C<srand> has already been called. See also C<srand>.
3692 Apply C<int()> to the value returned by C<rand()> if you want random
3693 integers instead of random fractional numbers. For example,
3697 returns a random integer between C<0> and C<9>, inclusive.
3699 (Note: If your rand function consistently returns numbers that are too
3700 large or too small, then your version of Perl was probably compiled
3701 with the wrong number of RANDBITS.)
3703 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3705 =item read FILEHANDLE,SCALAR,LENGTH
3707 Attempts to read LENGTH I<characters> of data into variable SCALAR
3708 from the specified FILEHANDLE. Returns the number of characters
3709 actually read, C<0> at end of file, or undef if there was an error.
3710 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3711 needs growing, the new bytes will be zero bytes. An OFFSET may be
3712 specified to place the read data into some other place in SCALAR than
3713 the beginning. The call is actually implemented in terms of either
3714 Perl's or system's fread() call. To get a true read(2) system call,
3717 Note the I<characters>: depending on the status of the filehandle,
3718 either (8-bit) bytes or characters are read. By default all
3719 filehandles operate on bytes, but for example if the filehandle has
3720 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3721 pragma, L<open>), the I/O will operate on characters, not bytes.
3723 =item readdir DIRHANDLE
3725 Returns the next directory entry for a directory opened by C<opendir>.
3726 If used in list context, returns all the rest of the entries in the
3727 directory. If there are no more entries, returns an undefined value in
3728 scalar context or a null list in list context.
3730 If you're planning to filetest the return values out of a C<readdir>, you'd
3731 better prepend the directory in question. Otherwise, because we didn't
3732 C<chdir> there, it would have been testing the wrong file.
3734 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3735 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3738 In some filesystems it is possible to return UTF-8 encoded filenames.
3739 To get readdir() to return such filenames, you must use C<:utf8> with
3740 the three-argument form of opendir(), see L</opendir>.
3744 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3745 context, each call reads and returns the next line, until end-of-file is
3746 reached, whereupon the subsequent call returns undef. In list context,
3747 reads until end-of-file is reached and returns a list of lines. Note that
3748 the notion of "line" used here is however you may have defined it
3749 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3751 When C<$/> is set to C<undef>, when readline() is in scalar
3752 context (i.e. file slurp mode), and when an empty file is read, it
3753 returns C<''> the first time, followed by C<undef> subsequently.
3755 This is the internal function implementing the C<< <EXPR> >>
3756 operator, but you can use it directly. The C<< <EXPR> >>
3757 operator is discussed in more detail in L<perlop/"I/O Operators">.
3760 $line = readline(*STDIN); # same thing
3766 Returns the value of a symbolic link, if symbolic links are
3767 implemented. If not, gives a fatal error. If there is some system
3768 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3769 omitted, uses C<$_>.
3773 EXPR is executed as a system command.
3774 The collected standard output of the command is returned.
3775 In scalar context, it comes back as a single (potentially
3776 multi-line) string. In list context, returns a list of lines
3777 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3778 This is the internal function implementing the C<qx/EXPR/>
3779 operator, but you can use it directly. The C<qx/EXPR/>
3780 operator is discussed in more detail in L<perlop/"I/O Operators">.
3782 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3784 Receives a message on a socket. Attempts to receive LENGTH characters
3785 of data into variable SCALAR from the specified SOCKET filehandle.
3786 SCALAR will be grown or shrunk to the length actually read. Takes the
3787 same flags as the system call of the same name. Returns the address
3788 of the sender if SOCKET's protocol supports this; returns an empty
3789 string otherwise. If there's an error, returns the undefined value.
3790 This call is actually implemented in terms of recvfrom(2) system call.
3791 See L<perlipc/"UDP: Message Passing"> for examples.
3793 Note the I<characters>: depending on the status of the socket, either
3794 (8-bit) bytes or characters are received. By default all sockets
3795 operate on bytes, but for example if the socket has been changed using
3796 binmode() to operate with the C<:utf8> discipline (see the C<open>
3797 pragma, L<open>), the I/O will operate on characters, not bytes.
3803 The C<redo> command restarts the loop block without evaluating the
3804 conditional again. The C<continue> block, if any, is not executed. If
3805 the LABEL is omitted, the command refers to the innermost enclosing
3806 loop. This command is normally used by programs that want to lie to
3807 themselves about what was just input:
3809 # a simpleminded Pascal comment stripper
3810 # (warning: assumes no { or } in strings)
3811 LINE: while (<STDIN>) {
3812 while (s|({.*}.*){.*}|$1 |) {}
3817 if (/}/) { # end of comment?
3826 C<redo> cannot be used to retry a block which returns a value such as
3827 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3828 a grep() or map() operation.
3830 Note that a block by itself is semantically identical to a loop
3831 that executes once. Thus C<redo> inside such a block will effectively
3832 turn it into a looping construct.
3834 See also L</continue> for an illustration of how C<last>, C<next>, and
3841 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3842 is not specified, C<$_> will be used. The value returned depends on the
3843 type of thing the reference is a reference to.
3844 Builtin types include:
3854 If the referenced object has been blessed into a package, then that package
3855 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3857 if (ref($r) eq "HASH") {
3858 print "r is a reference to a hash.\n";
3861 print "r is not a reference at all.\n";
3863 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3864 print "r is a reference to something that isa hash.\n";
3867 See also L<perlref>.
3869 =item rename OLDNAME,NEWNAME
3871 Changes the name of a file; an existing file NEWNAME will be
3872 clobbered. Returns true for success, false otherwise.
3874 Behavior of this function varies wildly depending on your system
3875 implementation. For example, it will usually not work across file system
3876 boundaries, even though the system I<mv> command sometimes compensates
3877 for this. Other restrictions include whether it works on directories,
3878 open files, or pre-existing files. Check L<perlport> and either the
3879 rename(2) manpage or equivalent system documentation for details.
3881 =item require VERSION
3887 Demands a version of Perl specified by VERSION, or demands some semantics
3888 specified by EXPR or by C<$_> if EXPR is not supplied.
3890 VERSION may be either a numeric argument such as 5.006, which will be
3891 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3892 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3893 VERSION is greater than the version of the current Perl interpreter.
3894 Compare with L</use>, which can do a similar check at compile time.
3896 Specifying VERSION as a literal of the form v5.6.1 should generally be
3897 avoided, because it leads to misleading error messages under earlier
3898 versions of Perl which do not support this syntax. The equivalent numeric
3899 version should be used instead.
3901 require v5.6.1; # run time version check
3902 require 5.6.1; # ditto
3903 require 5.006_001; # ditto; preferred for backwards compatibility
3905 Otherwise, demands that a library file be included if it hasn't already
3906 been included. The file is included via the do-FILE mechanism, which is
3907 essentially just a variety of C<eval>. Has semantics similar to the following
3912 return 1 if $INC{$filename};
3913 my($realfilename,$result);
3915 foreach $prefix (@INC) {
3916 $realfilename = "$prefix/$filename";
3917 if (-f $realfilename) {
3918 $INC{$filename} = $realfilename;
3919 $result = do $realfilename;
3923 die "Can't find $filename in \@INC";
3925 delete $INC{$filename} if $@ || !$result;
3927 die "$filename did not return true value" unless $result;
3931 Note that the file will not be included twice under the same specified
3932 name. The file must return true as the last statement to indicate
3933 successful execution of any initialization code, so it's customary to
3934 end such a file with C<1;> unless you're sure it'll return true
3935 otherwise. But it's better just to put the C<1;>, in case you add more
3938 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3939 replaces "F<::>" with "F</>" in the filename for you,
3940 to make it easy to load standard modules. This form of loading of
3941 modules does not risk altering your namespace.
3943 In other words, if you try this:
3945 require Foo::Bar; # a splendid bareword
3947 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3948 directories specified in the C<@INC> array.
3950 But if you try this:
3952 $class = 'Foo::Bar';
3953 require $class; # $class is not a bareword
3955 require "Foo::Bar"; # not a bareword because of the ""
3957 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3958 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3960 eval "require $class";
3962 You can also insert hooks into the import facility, by putting directly
3963 Perl code into the @INC array. There are three forms of hooks: subroutine
3964 references, array references and blessed objects.
3966 Subroutine references are the simplest case. When the inclusion system
3967 walks through @INC and encounters a subroutine, this subroutine gets
3968 called with two parameters, the first being a reference to itself, and the
3969 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3970 subroutine should return C<undef> or a filehandle, from which the file to
3971 include will be read. If C<undef> is returned, C<require> will look at
3972 the remaining elements of @INC.
3974 If the hook is an array reference, its first element must be a subroutine
3975 reference. This subroutine is called as above, but the first parameter is
3976 the array reference. This enables to pass indirectly some arguments to
3979 In other words, you can write:
3981 push @INC, \&my_sub;
3983 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3989 push @INC, [ \&my_sub, $x, $y, ... ];
3991 my ($arrayref, $filename) = @_;
3992 # Retrieve $x, $y, ...
3993 my @parameters = @$arrayref[1..$#$arrayref];
3997 If the hook is an object, it must provide an INC method, that will be
3998 called as above, the first parameter being the object itself. (Note that
3999 you must fully qualify the sub's name, as it is always forced into package
4000 C<main>.) Here is a typical code layout:
4006 my ($self, $filename) = @_;
4010 # In the main program
4011 push @INC, new Foo(...);
4013 Note that these hooks are also permitted to set the %INC entry
4014 corresponding to the files they have loaded. See L<perlvar/%INC>.
4016 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4022 Generally used in a C<continue> block at the end of a loop to clear
4023 variables and reset C<??> searches so that they work again. The
4024 expression is interpreted as a list of single characters (hyphens
4025 allowed for ranges). All variables and arrays beginning with one of
4026 those letters are reset to their pristine state. If the expression is
4027 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4028 only variables or searches in the current package. Always returns
4031 reset 'X'; # reset all X variables
4032 reset 'a-z'; # reset lower case variables
4033 reset; # just reset ?one-time? searches
4035 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4036 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4037 variables--lexical variables are unaffected, but they clean themselves
4038 up on scope exit anyway, so you'll probably want to use them instead.
4045 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4046 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4047 context, depending on how the return value will be used, and the context
4048 may vary from one execution to the next (see C<wantarray>). If no EXPR
4049 is given, returns an empty list in list context, the undefined value in
4050 scalar context, and (of course) nothing at all in a void context.
4052 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4053 or do FILE will automatically return the value of the last expression
4058 In list context, returns a list value consisting of the elements
4059 of LIST in the opposite order. In scalar context, concatenates the
4060 elements of LIST and returns a string value with all characters
4061 in the opposite order.
4063 print reverse <>; # line tac, last line first
4065 undef $/; # for efficiency of <>
4066 print scalar reverse <>; # character tac, last line tsrif
4068 This operator is also handy for inverting a hash, although there are some
4069 caveats. If a value is duplicated in the original hash, only one of those
4070 can be represented as a key in the inverted hash. Also, this has to
4071 unwind one hash and build a whole new one, which may take some time
4072 on a large hash, such as from a DBM file.
4074 %by_name = reverse %by_address; # Invert the hash
4076 =item rewinddir DIRHANDLE
4078 Sets the current position to the beginning of the directory for the
4079 C<readdir> routine on DIRHANDLE.
4081 =item rindex STR,SUBSTR,POSITION
4083 =item rindex STR,SUBSTR
4085 Works just like index() except that it returns the position of the LAST
4086 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4087 last occurrence at or before that position.
4089 =item rmdir FILENAME
4093 Deletes the directory specified by FILENAME if that directory is empty. If it
4094 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4095 FILENAME is omitted, uses C<$_>.
4099 The substitution operator. See L<perlop>.
4103 Forces EXPR to be interpreted in scalar context and returns the value
4106 @counts = ( scalar @a, scalar @b, scalar @c );
4108 There is no equivalent operator to force an expression to
4109 be interpolated in list context because in practice, this is never
4110 needed. If you really wanted to do so, however, you could use
4111 the construction C<@{[ (some expression) ]}>, but usually a simple
4112 C<(some expression)> suffices.
4114 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4115 parenthesized list, this behaves as a scalar comma expression, evaluating
4116 all but the last element in void context and returning the final element
4117 evaluated in scalar context. This is seldom what you want.
4119 The following single statement:
4121 print uc(scalar(&foo,$bar)),$baz;
4123 is the moral equivalent of these two:
4126 print(uc($bar),$baz);
4128 See L<perlop> for more details on unary operators and the comma operator.
4130 =item seek FILEHANDLE,POSITION,WHENCE
4132 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4133 FILEHANDLE may be an expression whose value gives the name of the
4134 filehandle. The values for WHENCE are C<0> to set the new position
4135 I<in bytes> to POSITION, C<1> to set it to the current position plus
4136 POSITION, and C<2> to set it to EOF plus POSITION (typically
4137 negative). For WHENCE you may use the constants C<SEEK_SET>,
4138 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4139 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4142 Note the I<in bytes>: even if the filehandle has been set to
4143 operate on characters (for example by using the C<:utf8> open
4144 discipline), tell() will return byte offsets, not character offsets
4145 (because implementing that would render seek() and tell() rather slow).
4147 If you want to position file for C<sysread> or C<syswrite>, don't use
4148 C<seek>--buffering makes its effect on the file's system position
4149 unpredictable and non-portable. Use C<sysseek> instead.
4151 Due to the rules and rigors of ANSI C, on some systems you have to do a
4152 seek whenever you switch between reading and writing. Amongst other
4153 things, this may have the effect of calling stdio's clearerr(3).
4154 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4158 This is also useful for applications emulating C<tail -f>. Once you hit
4159 EOF on your read, and then sleep for a while, you might have to stick in a
4160 seek() to reset things. The C<seek> doesn't change the current position,
4161 but it I<does> clear the end-of-file condition on the handle, so that the
4162 next C<< <FILE> >> makes Perl try again to read something. We hope.
4164 If that doesn't work (some IO implementations are particularly
4165 cantankerous), then you may need something more like this:
4168 for ($curpos = tell(FILE); $_ = <FILE>;
4169 $curpos = tell(FILE)) {
4170 # search for some stuff and put it into files
4172 sleep($for_a_while);
4173 seek(FILE, $curpos, 0);
4176 =item seekdir DIRHANDLE,POS
4178 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4179 must be a value returned by C<telldir>. Has the same caveats about
4180 possible directory compaction as the corresponding system library
4183 =item select FILEHANDLE
4187 Returns the currently selected filehandle. Sets the current default
4188 filehandle for output, if FILEHANDLE is supplied. This has two
4189 effects: first, a C<write> or a C<print> without a filehandle will
4190 default to this FILEHANDLE. Second, references to variables related to
4191 output will refer to this output channel. For example, if you have to
4192 set the top of form format for more than one output channel, you might
4200 FILEHANDLE may be an expression whose value gives the name of the
4201 actual filehandle. Thus:
4203 $oldfh = select(STDERR); $| = 1; select($oldfh);
4205 Some programmers may prefer to think of filehandles as objects with
4206 methods, preferring to write the last example as:
4209 STDERR->autoflush(1);
4211 =item select RBITS,WBITS,EBITS,TIMEOUT
4213 This calls the select(2) system call with the bit masks specified, which
4214 can be constructed using C<fileno> and C<vec>, along these lines:
4216 $rin = $win = $ein = '';
4217 vec($rin,fileno(STDIN),1) = 1;
4218 vec($win,fileno(STDOUT),1) = 1;
4221 If you want to select on many filehandles you might wish to write a
4225 my(@fhlist) = split(' ',$_[0]);
4228 vec($bits,fileno($_),1) = 1;
4232 $rin = fhbits('STDIN TTY SOCK');
4236 ($nfound,$timeleft) =
4237 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4239 or to block until something becomes ready just do this
4241 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4243 Most systems do not bother to return anything useful in $timeleft, so
4244 calling select() in scalar context just returns $nfound.
4246 Any of the bit masks can also be undef. The timeout, if specified, is
4247 in seconds, which may be fractional. Note: not all implementations are
4248 capable of returning the $timeleft. If not, they always return
4249 $timeleft equal to the supplied $timeout.
4251 You can effect a sleep of 250 milliseconds this way:
4253 select(undef, undef, undef, 0.25);
4255 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4256 or <FH>) with C<select>, except as permitted by POSIX, and even
4257 then only on POSIX systems. You have to use C<sysread> instead.
4259 =item semctl ID,SEMNUM,CMD,ARG
4261 Calls the System V IPC function C<semctl>. You'll probably have to say
4265 first to get the correct constant definitions. If CMD is IPC_STAT or
4266 GETALL, then ARG must be a variable which will hold the returned
4267 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4268 the undefined value for error, "C<0 but true>" for zero, or the actual
4269 return value otherwise. The ARG must consist of a vector of native
4270 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4271 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4274 =item semget KEY,NSEMS,FLAGS
4276 Calls the System V IPC function semget. Returns the semaphore id, or
4277 the undefined value if there is an error. See also
4278 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4281 =item semop KEY,OPSTRING
4283 Calls the System V IPC function semop to perform semaphore operations
4284 such as signalling and waiting. OPSTRING must be a packed array of
4285 semop structures. Each semop structure can be generated with
4286 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4287 operations is implied by the length of OPSTRING. Returns true if
4288 successful, or false if there is an error. As an example, the
4289 following code waits on semaphore $semnum of semaphore id $semid:
4291 $semop = pack("s!3", $semnum, -1, 0);
4292 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4294 To signal the semaphore, replace C<-1> with C<1>. See also
4295 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4298 =item send SOCKET,MSG,FLAGS,TO
4300 =item send SOCKET,MSG,FLAGS
4302 Sends a message on a socket. Attempts to send the scalar MSG to the
4303 SOCKET filehandle. Takes the same flags as the system call of the
4304 same name. On unconnected sockets you must specify a destination to
4305 send TO, in which case it does a C C<sendto>. Returns the number of
4306 characters sent, or the undefined value if there is an error. The C
4307 system call sendmsg(2) is currently unimplemented. See
4308 L<perlipc/"UDP: Message Passing"> for examples.
4310 Note the I<characters>: depending on the status of the socket, either
4311 (8-bit) bytes or characters are sent. By default all sockets operate
4312 on bytes, but for example if the socket has been changed using
4313 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4314 the C<open> pragma, L<open>), the I/O will operate on characters, not
4317 =item setpgrp PID,PGRP
4319 Sets the current process group for the specified PID, C<0> for the current
4320 process. Will produce a fatal error if used on a machine that doesn't
4321 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4322 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4323 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4326 =item setpriority WHICH,WHO,PRIORITY
4328 Sets the current priority for a process, a process group, or a user.
4329 (See setpriority(2).) Will produce a fatal error if used on a machine
4330 that doesn't implement setpriority(2).
4332 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4334 Sets the socket option requested. Returns undefined if there is an
4335 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4342 Shifts the first value of the array off and returns it, shortening the
4343 array by 1 and moving everything down. If there are no elements in the
4344 array, returns the undefined value. If ARRAY is omitted, shifts the
4345 C<@_> array within the lexical scope of subroutines and formats, and the
4346 C<@ARGV> array at file scopes or within the lexical scopes established by
4347 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4350 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4351 same thing to the left end of an array that C<pop> and C<push> do to the
4354 =item shmctl ID,CMD,ARG
4356 Calls the System V IPC function shmctl. You'll probably have to say
4360 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4361 then ARG must be a variable which will hold the returned C<shmid_ds>
4362 structure. Returns like ioctl: the undefined value for error, "C<0> but
4363 true" for zero, or the actual return value otherwise.
4364 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4366 =item shmget KEY,SIZE,FLAGS
4368 Calls the System V IPC function shmget. Returns the shared memory
4369 segment id, or the undefined value if there is an error.
4370 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4372 =item shmread ID,VAR,POS,SIZE
4374 =item shmwrite ID,STRING,POS,SIZE
4376 Reads or writes the System V shared memory segment ID starting at
4377 position POS for size SIZE by attaching to it, copying in/out, and
4378 detaching from it. When reading, VAR must be a variable that will
4379 hold the data read. When writing, if STRING is too long, only SIZE
4380 bytes are used; if STRING is too short, nulls are written to fill out
4381 SIZE bytes. Return true if successful, or false if there is an error.
4382 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4383 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4385 =item shutdown SOCKET,HOW
4387 Shuts down a socket connection in the manner indicated by HOW, which
4388 has the same interpretation as in the system call of the same name.
4390 shutdown(SOCKET, 0); # I/we have stopped reading data
4391 shutdown(SOCKET, 1); # I/we have stopped writing data
4392 shutdown(SOCKET, 2); # I/we have stopped using this socket
4394 This is useful with sockets when you want to tell the other
4395 side you're done writing but not done reading, or vice versa.
4396 It's also a more insistent form of close because it also
4397 disables the file descriptor in any forked copies in other
4404 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4405 returns sine of C<$_>.
4407 For the inverse sine operation, you may use the C<Math::Trig::asin>
4408 function, or use this relation:
4410 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4416 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4417 May be interrupted if the process receives a signal such as C<SIGALRM>.
4418 Returns the number of seconds actually slept. You probably cannot
4419 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4422 On some older systems, it may sleep up to a full second less than what
4423 you requested, depending on how it counts seconds. Most modern systems
4424 always sleep the full amount. They may appear to sleep longer than that,
4425 however, because your process might not be scheduled right away in a
4426 busy multitasking system.
4428 For delays of finer granularity than one second, you may use Perl's
4429 C<syscall> interface to access setitimer(2) if your system supports
4430 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4431 and starting from Perl 5.8 part of the standard distribution) may also
4434 See also the POSIX module's C<pause> function.
4436 =item sockatmark SOCKET
4438 Returns true if the socket is positioned at the out-of-band mark
4439 (also known as the urgent data mark), false otherwise. Use right
4440 after reading from the socket.
4442 Not available directly, one has to import the function from
4443 the IO::Socket extension
4445 use IO::Socket 'sockatmark';
4447 Even this doesn't guarantee that sockatmark() really is available,
4448 though, because sockatmark() is a relatively recent addition to
4449 the family of socket functions. If it is unavailable, attempt to
4452 IO::Socket::atmark not implemented on this architecture ...
4454 See also L<IO::Socket>.
4456 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4458 Opens a socket of the specified kind and attaches it to filehandle
4459 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4460 the system call of the same name. You should C<use Socket> first
4461 to get the proper definitions imported. See the examples in
4462 L<perlipc/"Sockets: Client/Server Communication">.
4464 On systems that support a close-on-exec flag on files, the flag will
4465 be set for the newly opened file descriptor, as determined by the
4466 value of $^F. See L<perlvar/$^F>.
4468 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4470 Creates an unnamed pair of sockets in the specified domain, of the
4471 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4472 for the system call of the same name. If unimplemented, yields a fatal
4473 error. Returns true if successful.
4475 On systems that support a close-on-exec flag on files, the flag will
4476 be set for the newly opened file descriptors, as determined by the value
4477 of $^F. See L<perlvar/$^F>.
4479 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4480 to C<pipe(Rdr, Wtr)> is essentially:
4483 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4484 shutdown(Rdr, 1); # no more writing for reader
4485 shutdown(Wtr, 0); # no more reading for writer
4487 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4488 emulate socketpair using IP sockets to localhost if your system implements
4489 sockets but not socketpair.
4491 =item sort SUBNAME LIST
4493 =item sort BLOCK LIST
4497 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4498 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4499 specified, it gives the name of a subroutine that returns an integer
4500 less than, equal to, or greater than C<0>, depending on how the elements
4501 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4502 operators are extremely useful in such routines.) SUBNAME may be a
4503 scalar variable name (unsubscripted), in which case the value provides
4504 the name of (or a reference to) the actual subroutine to use. In place
4505 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4508 If the subroutine's prototype is C<($$)>, the elements to be compared
4509 are passed by reference in C<@_>, as for a normal subroutine. This is
4510 slower than unprototyped subroutines, where the elements to be
4511 compared are passed into the subroutine
4512 as the package global variables $a and $b (see example below). Note that
4513 in the latter case, it is usually counter-productive to declare $a and
4516 In either case, the subroutine may not be recursive. The values to be
4517 compared are always passed by reference, so don't modify them.
4519 You also cannot exit out of the sort block or subroutine using any of the
4520 loop control operators described in L<perlsyn> or with C<goto>.
4522 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4523 current collation locale. See L<perllocale>.
4525 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4526 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4527 preserves the input order of elements that compare equal. Although
4528 quicksort's run time is O(NlogN) when averaged over all arrays of
4529 length N, the time can be O(N**2), I<quadratic> behavior, for some
4530 inputs.) In 5.7, the quicksort implementation was replaced with
4531 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4532 But benchmarks indicated that for some inputs, on some platforms,
4533 the original quicksort was faster. 5.8 has a sort pragma for
4534 limited control of the sort. Its rather blunt control of the
4535 underlying algorithm may not persist into future perls, but the
4536 ability to characterize the input or output in implementation
4537 independent ways quite probably will. See L</use>.
4542 @articles = sort @files;
4544 # same thing, but with explicit sort routine
4545 @articles = sort {$a cmp $b} @files;
4547 # now case-insensitively
4548 @articles = sort {uc($a) cmp uc($b)} @files;
4550 # same thing in reversed order
4551 @articles = sort {$b cmp $a} @files;
4553 # sort numerically ascending
4554 @articles = sort {$a <=> $b} @files;
4556 # sort numerically descending
4557 @articles = sort {$b <=> $a} @files;
4559 # this sorts the %age hash by value instead of key
4560 # using an in-line function
4561 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4563 # sort using explicit subroutine name
4565 $age{$a} <=> $age{$b}; # presuming numeric
4567 @sortedclass = sort byage @class;
4569 sub backwards { $b cmp $a }
4570 @harry = qw(dog cat x Cain Abel);
4571 @george = qw(gone chased yz Punished Axed);
4573 # prints AbelCaincatdogx
4574 print sort backwards @harry;
4575 # prints xdogcatCainAbel
4576 print sort @george, 'to', @harry;
4577 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4579 # inefficiently sort by descending numeric compare using
4580 # the first integer after the first = sign, or the
4581 # whole record case-insensitively otherwise
4584 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4589 # same thing, but much more efficiently;
4590 # we'll build auxiliary indices instead
4594 push @nums, /=(\d+)/;
4599 $nums[$b] <=> $nums[$a]
4601 $caps[$a] cmp $caps[$b]
4605 # same thing, but without any temps
4606 @new = map { $_->[0] }
4607 sort { $b->[1] <=> $a->[1]
4610 } map { [$_, /=(\d+)/, uc($_)] } @old;
4612 # using a prototype allows you to use any comparison subroutine
4613 # as a sort subroutine (including other package's subroutines)
4615 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4618 @new = sort other::backwards @old;
4620 # guarantee stability, regardless of algorithm
4622 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4624 # force use of mergesort (not portable outside Perl 5.8)
4625 use sort '_mergesort'; # note discouraging _
4626 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4628 # Similar to the previous example, but demand stability as well
4629 # Because of the way quicksort is "stabilized", this combination
4631 use sort qw( _quicksort stable );
4632 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4634 If you're using strict, you I<must not> declare $a
4635 and $b as lexicals. They are package globals. That means
4636 if you're in the C<main> package and type
4638 @articles = sort {$b <=> $a} @files;
4640 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4641 but if you're in the C<FooPack> package, it's the same as typing
4643 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4645 The comparison function is required to behave. If it returns
4646 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4647 sometimes saying the opposite, for example) the results are not
4650 =item splice ARRAY,OFFSET,LENGTH,LIST
4652 =item splice ARRAY,OFFSET,LENGTH
4654 =item splice ARRAY,OFFSET
4658 Removes the elements designated by OFFSET and LENGTH from an array, and
4659 replaces them with the elements of LIST, if any. In list context,
4660 returns the elements removed from the array. In scalar context,
4661 returns the last element removed, or C<undef> if no elements are
4662 removed. The array grows or shrinks as necessary.
4663 If OFFSET is negative then it starts that far from the end of the array.
4664 If LENGTH is omitted, removes everything from OFFSET onward.
4665 If LENGTH is negative, removes the elements from OFFSET onward
4666 except for -LENGTH elements at the end of the array.
4667 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4668 past the end of the array, perl issues a warning, and splices at the
4671 The following equivalences hold (assuming C<$[ == 0>):
4673 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4674 pop(@a) splice(@a,-1)
4675 shift(@a) splice(@a,0,1)
4676 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4677 $a[$x] = $y splice(@a,$x,1,$y)
4679 Example, assuming array lengths are passed before arrays:
4681 sub aeq { # compare two list values
4682 my(@a) = splice(@_,0,shift);
4683 my(@b) = splice(@_,0,shift);
4684 return 0 unless @a == @b; # same len?
4686 return 0 if pop(@a) ne pop(@b);
4690 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4692 =item split /PATTERN/,EXPR,LIMIT
4694 =item split /PATTERN/,EXPR
4696 =item split /PATTERN/
4700 Splits a string into a list of strings and returns that list. By default,
4701 empty leading fields are preserved, and empty trailing ones are deleted.
4703 In scalar context, returns the number of fields found and splits into
4704 the C<@_> array. Use of split in scalar context is deprecated, however,
4705 because it clobbers your subroutine arguments.
4707 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4708 splits on whitespace (after skipping any leading whitespace). Anything
4709 matching PATTERN is taken to be a delimiter separating the fields. (Note
4710 that the delimiter may be longer than one character.)
4712 If LIMIT is specified and positive, it represents the maximum number
4713 of fields the EXPR will be split into, though the actual number of
4714 fields returned depends on the number of times PATTERN matches within
4715 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4716 stripped (which potential users of C<pop> would do well to remember).
4717 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4718 had been specified. Note that splitting an EXPR that evaluates to the
4719 empty string always returns the empty list, regardless of the LIMIT
4722 A pattern matching the null string (not to be confused with
4723 a null pattern C<//>, which is just one member of the set of patterns
4724 matching a null string) will split the value of EXPR into separate
4725 characters at each point it matches that way. For example:
4727 print join(':', split(/ */, 'hi there'));
4729 produces the output 'h:i:t:h:e:r:e'.
4731 Using the empty pattern C<//> specifically matches the null string, and is
4732 not be confused with the use of C<//> to mean "the last successful pattern
4735 Empty leading (or trailing) fields are produced when there are positive width
4736 matches at the beginning (or end) of the string; a zero-width match at the
4737 beginning (or end) of the string does not produce an empty field. For
4740 print join(':', split(/(?=\w)/, 'hi there!'));
4742 produces the output 'h:i :t:h:e:r:e!'.
4744 The LIMIT parameter can be used to split a line partially
4746 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4748 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4749 one larger than the number of variables in the list, to avoid
4750 unnecessary work. For the list above LIMIT would have been 4 by
4751 default. In time critical applications it behooves you not to split
4752 into more fields than you really need.
4754 If the PATTERN contains parentheses, additional list elements are
4755 created from each matching substring in the delimiter.
4757 split(/([,-])/, "1-10,20", 3);
4759 produces the list value
4761 (1, '-', 10, ',', 20)
4763 If you had the entire header of a normal Unix email message in $header,
4764 you could split it up into fields and their values this way:
4766 $header =~ s/\n\s+/ /g; # fix continuation lines
4767 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4769 The pattern C</PATTERN/> may be replaced with an expression to specify
4770 patterns that vary at runtime. (To do runtime compilation only once,
4771 use C</$variable/o>.)
4773 As a special case, specifying a PATTERN of space (C<' '>) will split on
4774 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4775 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4776 will give you as many null initial fields as there are leading spaces.
4777 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4778 whitespace produces a null first field. A C<split> with no arguments
4779 really does a C<split(' ', $_)> internally.
4781 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4786 open(PASSWD, '/etc/passwd');
4789 ($login, $passwd, $uid, $gid,
4790 $gcos, $home, $shell) = split(/:/);
4794 As with regular pattern matching, any capturing parentheses that are not
4795 matched in a C<split()> will be set to C<undef> when returned:
4797 @fields = split /(A)|B/, "1A2B3";
4798 # @fields is (1, 'A', 2, undef, 3)
4800 =item sprintf FORMAT, LIST
4802 Returns a string formatted by the usual C<printf> conventions of the C
4803 library function C<sprintf>. See below for more details
4804 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4805 the general principles.
4809 # Format number with up to 8 leading zeroes
4810 $result = sprintf("%08d", $number);
4812 # Round number to 3 digits after decimal point
4813 $rounded = sprintf("%.3f", $number);
4815 Perl does its own C<sprintf> formatting--it emulates the C
4816 function C<sprintf>, but it doesn't use it (except for floating-point
4817 numbers, and even then only the standard modifiers are allowed). As a
4818 result, any non-standard extensions in your local C<sprintf> are not
4819 available from Perl.
4821 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4822 pass it an array as your first argument. The array is given scalar context,
4823 and instead of using the 0th element of the array as the format, Perl will
4824 use the count of elements in the array as the format, which is almost never
4827 Perl's C<sprintf> permits the following universally-known conversions:
4830 %c a character with the given number
4832 %d a signed integer, in decimal
4833 %u an unsigned integer, in decimal
4834 %o an unsigned integer, in octal
4835 %x an unsigned integer, in hexadecimal
4836 %e a floating-point number, in scientific notation
4837 %f a floating-point number, in fixed decimal notation
4838 %g a floating-point number, in %e or %f notation
4840 In addition, Perl permits the following widely-supported conversions:
4842 %X like %x, but using upper-case letters
4843 %E like %e, but using an upper-case "E"
4844 %G like %g, but with an upper-case "E" (if applicable)
4845 %b an unsigned integer, in binary
4846 %p a pointer (outputs the Perl value's address in hexadecimal)
4847 %n special: *stores* the number of characters output so far
4848 into the next variable in the parameter list
4850 Finally, for backward (and we do mean "backward") compatibility, Perl
4851 permits these unnecessary but widely-supported conversions:
4854 %D a synonym for %ld
4855 %U a synonym for %lu
4856 %O a synonym for %lo
4859 Note that the number of exponent digits in the scientific notation by
4860 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4861 exponent less than 100 is system-dependent: it may be three or less
4862 (zero-padded as necessary). In other words, 1.23 times ten to the
4863 99th may be either "1.23e99" or "1.23e099".
4865 Perl permits the following universally-known flags between the C<%>
4866 and the conversion letter:
4868 space prefix positive number with a space
4869 + prefix positive number with a plus sign
4870 - left-justify within the field
4871 0 use zeros, not spaces, to right-justify
4872 # prefix non-zero octal with "0", non-zero hex with "0x"
4873 number minimum field width
4874 .number "precision": digits after decimal point for
4875 floating-point, max length for string, minimum length
4877 l interpret integer as C type "long" or "unsigned long"
4878 h interpret integer as C type "short" or "unsigned short"
4879 If no flags, interpret integer as C type "int" or "unsigned"
4881 Perl supports parameter ordering, in other words, fetching the
4882 parameters in some explicitly specified "random" ordering as opposed
4883 to the default implicit sequential ordering. The syntax is, instead
4884 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4885 where the I<digits> is the wanted index, from one upwards. For example:
4887 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4888 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4890 Note that using the reordering syntax does not interfere with the usual
4891 implicit sequential fetching of the parameters:
4893 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4894 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4895 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4896 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4897 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4899 There are also two Perl-specific flags:
4901 V interpret integer as Perl's standard integer type
4902 v interpret string as a vector of integers, output as
4903 numbers separated either by dots, or by an arbitrary
4904 string received from the argument list when the flag
4907 Where a number would appear in the flags, an asterisk (C<*>) may be
4908 used instead, in which case Perl uses the next item in the parameter
4909 list as the given number (that is, as the field width or precision).
4910 If a field width obtained through C<*> is negative, it has the same
4911 effect as the C<-> flag: left-justification.
4913 The C<v> flag is useful for displaying ordinal values of characters
4914 in arbitrary strings:
4916 printf "version is v%vd\n", $^V; # Perl's version
4917 printf "address is %*vX\n", ":", $addr; # IPv6 address
4918 printf "bits are %*vb\n", " ", $bits; # random bitstring
4920 If C<use locale> is in effect, the character used for the decimal
4921 point in formatted real numbers is affected by the LC_NUMERIC locale.
4924 If Perl understands "quads" (64-bit integers) (this requires
4925 either that the platform natively support quads or that Perl
4926 be specifically compiled to support quads), the characters
4930 print quads, and they may optionally be preceded by
4938 You can find out whether your Perl supports quads via L<Config>:
4941 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4944 If Perl understands "long doubles" (this requires that the platform
4945 support long doubles), the flags
4949 may optionally be preceded by
4957 You can find out whether your Perl supports long doubles via L<Config>:
4960 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4966 Return the square root of EXPR. If EXPR is omitted, returns square
4967 root of C<$_>. Only works on non-negative operands, unless you've
4968 loaded the standard Math::Complex module.
4971 print sqrt(-2); # prints 1.4142135623731i
4977 Sets the random number seed for the C<rand> operator.
4979 The point of the function is to "seed" the C<rand> function so that
4980 C<rand> can produce a different sequence each time you run your
4983 If srand() is not called explicitly, it is called implicitly at the
4984 first use of the C<rand> operator. However, this was not the case in
4985 versions of Perl before 5.004, so if your script will run under older
4986 Perl versions, it should call C<srand>.
4988 Most programs won't even call srand() at all, except those that
4989 need a cryptographically-strong starting point rather than the
4990 generally acceptable default, which is based on time of day,
4991 process ID, and memory allocation, or the F</dev/urandom> device,
4994 You can call srand($seed) with the same $seed to reproduce the
4995 I<same> sequence from rand(), but this is usually reserved for
4996 generating predictable results for testing or debugging.
4997 Otherwise, don't call srand() more than once in your program.
4999 Do B<not> call srand() (i.e. without an argument) more than once in
5000 a script. The internal state of the random number generator should
5001 contain more entropy than can be provided by any seed, so calling
5002 srand() again actually I<loses> randomness.
5004 Most implementations of C<srand> take an integer and will silently
5005 truncate decimal numbers. This means C<srand(42)> will usually
5006 produce the same results as C<srand(42.1)>. To be safe, always pass
5007 C<srand> an integer.
5009 In versions of Perl prior to 5.004 the default seed was just the
5010 current C<time>. This isn't a particularly good seed, so many old
5011 programs supply their own seed value (often C<time ^ $$> or C<time ^
5012 ($$ + ($$ << 15))>), but that isn't necessary any more.
5014 Note that you need something much more random than the default seed for
5015 cryptographic purposes. Checksumming the compressed output of one or more
5016 rapidly changing operating system status programs is the usual method. For
5019 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5021 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5024 Frequently called programs (like CGI scripts) that simply use
5028 for a seed can fall prey to the mathematical property that
5032 one-third of the time. So don't do that.
5034 =item stat FILEHANDLE
5040 Returns a 13-element list giving the status info for a file, either
5041 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5042 it stats C<$_>. Returns a null list if the stat fails. Typically used
5045 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5046 $atime,$mtime,$ctime,$blksize,$blocks)
5049 Not all fields are supported on all filesystem types. Here are the
5050 meaning of the fields:
5052 0 dev device number of filesystem
5054 2 mode file mode (type and permissions)
5055 3 nlink number of (hard) links to the file
5056 4 uid numeric user ID of file's owner
5057 5 gid numeric group ID of file's owner
5058 6 rdev the device identifier (special files only)
5059 7 size total size of file, in bytes
5060 8 atime last access time in seconds since the epoch
5061 9 mtime last modify time in seconds since the epoch
5062 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5063 11 blksize preferred block size for file system I/O
5064 12 blocks actual number of blocks allocated
5066 (The epoch was at 00:00 January 1, 1970 GMT.)
5068 If stat is passed the special filehandle consisting of an underline, no
5069 stat is done, but the current contents of the stat structure from the
5070 last stat or filetest are returned. Example:
5072 if (-x $file && (($d) = stat(_)) && $d < 0) {
5073 print "$file is executable NFS file\n";
5076 (This works on machines only for which the device number is negative
5079 Because the mode contains both the file type and its permissions, you
5080 should mask off the file type portion and (s)printf using a C<"%o">
5081 if you want to see the real permissions.
5083 $mode = (stat($filename))[2];
5084 printf "Permissions are %04o\n", $mode & 07777;
5086 In scalar context, C<stat> returns a boolean value indicating success
5087 or failure, and, if successful, sets the information associated with
5088 the special filehandle C<_>.
5090 The File::stat module provides a convenient, by-name access mechanism:
5093 $sb = stat($filename);
5094 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5095 $filename, $sb->size, $sb->mode & 07777,
5096 scalar localtime $sb->mtime;
5098 You can import symbolic mode constants (C<S_IF*>) and functions
5099 (C<S_IS*>) from the Fcntl module:
5103 $mode = (stat($filename))[2];
5105 $user_rwx = ($mode & S_IRWXU) >> 6;
5106 $group_read = ($mode & S_IRGRP) >> 3;
5107 $other_execute = $mode & S_IXOTH;
5109 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5111 $is_setuid = $mode & S_ISUID;
5112 $is_setgid = S_ISDIR($mode);
5114 You could write the last two using the C<-u> and C<-d> operators.
5115 The commonly available S_IF* constants are
5117 # Permissions: read, write, execute, for user, group, others.
5119 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5120 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5121 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5123 # Setuid/Setgid/Stickiness.
5125 S_ISUID S_ISGID S_ISVTX S_ISTXT
5127 # File types. Not necessarily all are available on your system.
5129 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5131 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5133 S_IREAD S_IWRITE S_IEXEC
5135 and the S_IF* functions are
5137 S_IFMODE($mode) the part of $mode containing the permission bits
5138 and the setuid/setgid/sticky bits
5140 S_IFMT($mode) the part of $mode containing the file type
5141 which can be bit-anded with e.g. S_IFREG
5142 or with the following functions
5144 # The operators -f, -d, -l, -b, -c, -p, and -s.
5146 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5147 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5149 # No direct -X operator counterpart, but for the first one
5150 # the -g operator is often equivalent. The ENFMT stands for
5151 # record flocking enforcement, a platform-dependent feature.
5153 S_ISENFMT($mode) S_ISWHT($mode)
5155 See your native chmod(2) and stat(2) documentation for more details
5156 about the S_* constants.
5162 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5163 doing many pattern matches on the string before it is next modified.
5164 This may or may not save time, depending on the nature and number of
5165 patterns you are searching on, and on the distribution of character
5166 frequencies in the string to be searched--you probably want to compare
5167 run times with and without it to see which runs faster. Those loops
5168 which scan for many short constant strings (including the constant
5169 parts of more complex patterns) will benefit most. You may have only
5170 one C<study> active at a time--if you study a different scalar the first
5171 is "unstudied". (The way C<study> works is this: a linked list of every
5172 character in the string to be searched is made, so we know, for
5173 example, where all the C<'k'> characters are. From each search string,
5174 the rarest character is selected, based on some static frequency tables
5175 constructed from some C programs and English text. Only those places
5176 that contain this "rarest" character are examined.)
5178 For example, here is a loop that inserts index producing entries
5179 before any line containing a certain pattern:
5183 print ".IX foo\n" if /\bfoo\b/;
5184 print ".IX bar\n" if /\bbar\b/;
5185 print ".IX blurfl\n" if /\bblurfl\b/;
5190 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5191 will be looked at, because C<f> is rarer than C<o>. In general, this is
5192 a big win except in pathological cases. The only question is whether
5193 it saves you more time than it took to build the linked list in the
5196 Note that if you have to look for strings that you don't know till
5197 runtime, you can build an entire loop as a string and C<eval> that to
5198 avoid recompiling all your patterns all the time. Together with
5199 undefining C<$/> to input entire files as one record, this can be very
5200 fast, often faster than specialized programs like fgrep(1). The following
5201 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5202 out the names of those files that contain a match:
5204 $search = 'while (<>) { study;';
5205 foreach $word (@words) {
5206 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5211 eval $search; # this screams
5212 $/ = "\n"; # put back to normal input delimiter
5213 foreach $file (sort keys(%seen)) {
5221 =item sub NAME BLOCK
5223 This is subroutine definition, not a real function I<per se>. With just a
5224 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5225 Without a NAME, it's an anonymous function declaration, and does actually
5226 return a value: the CODE ref of the closure you just created. See L<perlsub>
5227 and L<perlref> for details.
5229 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5231 =item substr EXPR,OFFSET,LENGTH
5233 =item substr EXPR,OFFSET
5235 Extracts a substring out of EXPR and returns it. First character is at
5236 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5237 If OFFSET is negative (or more precisely, less than C<$[>), starts
5238 that far from the end of the string. If LENGTH is omitted, returns
5239 everything to the end of the string. If LENGTH is negative, leaves that
5240 many characters off the end of the string.
5242 You can use the substr() function as an lvalue, in which case EXPR
5243 must itself be an lvalue. If you assign something shorter than LENGTH,
5244 the string will shrink, and if you assign something longer than LENGTH,
5245 the string will grow to accommodate it. To keep the string the same
5246 length you may need to pad or chop your value using C<sprintf>.
5248 If OFFSET and LENGTH specify a substring that is partly outside the
5249 string, only the part within the string is returned. If the substring
5250 is beyond either end of the string, substr() returns the undefined
5251 value and produces a warning. When used as an lvalue, specifying a
5252 substring that is entirely outside the string is a fatal error.
5253 Here's an example showing the behavior for boundary cases:
5256 substr($name, 4) = 'dy'; # $name is now 'freddy'
5257 my $null = substr $name, 6, 2; # returns '' (no warning)
5258 my $oops = substr $name, 7; # returns undef, with warning
5259 substr($name, 7) = 'gap'; # fatal error
5261 An alternative to using substr() as an lvalue is to specify the
5262 replacement string as the 4th argument. This allows you to replace
5263 parts of the EXPR and return what was there before in one operation,
5264 just as you can with splice().
5266 =item symlink OLDFILE,NEWFILE
5268 Creates a new filename symbolically linked to the old filename.
5269 Returns C<1> for success, C<0> otherwise. On systems that don't support
5270 symbolic links, produces a fatal error at run time. To check for that,
5273 $symlink_exists = eval { symlink("",""); 1 };
5277 Calls the system call specified as the first element of the list,
5278 passing the remaining elements as arguments to the system call. If
5279 unimplemented, produces a fatal error. The arguments are interpreted
5280 as follows: if a given argument is numeric, the argument is passed as
5281 an int. If not, the pointer to the string value is passed. You are
5282 responsible to make sure a string is pre-extended long enough to
5283 receive any result that might be written into a string. You can't use a
5284 string literal (or other read-only string) as an argument to C<syscall>
5285 because Perl has to assume that any string pointer might be written
5287 integer arguments are not literals and have never been interpreted in a
5288 numeric context, you may need to add C<0> to them to force them to look
5289 like numbers. This emulates the C<syswrite> function (or vice versa):
5291 require 'syscall.ph'; # may need to run h2ph
5293 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5295 Note that Perl supports passing of up to only 14 arguments to your system call,
5296 which in practice should usually suffice.
5298 Syscall returns whatever value returned by the system call it calls.
5299 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5300 Note that some system calls can legitimately return C<-1>. The proper
5301 way to handle such calls is to assign C<$!=0;> before the call and
5302 check the value of C<$!> if syscall returns C<-1>.
5304 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5305 number of the read end of the pipe it creates. There is no way
5306 to retrieve the file number of the other end. You can avoid this
5307 problem by using C<pipe> instead.
5309 =item sysopen FILEHANDLE,FILENAME,MODE
5311 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5313 Opens the file whose filename is given by FILENAME, and associates it
5314 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5315 the name of the real filehandle wanted. This function calls the
5316 underlying operating system's C<open> function with the parameters
5317 FILENAME, MODE, PERMS.
5319 The possible values and flag bits of the MODE parameter are
5320 system-dependent; they are available via the standard module C<Fcntl>.
5321 See the documentation of your operating system's C<open> to see which
5322 values and flag bits are available. You may combine several flags
5323 using the C<|>-operator.
5325 Some of the most common values are C<O_RDONLY> for opening the file in
5326 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5327 and C<O_RDWR> for opening the file in read-write mode, and.
5329 For historical reasons, some values work on almost every system
5330 supported by perl: zero means read-only, one means write-only, and two
5331 means read/write. We know that these values do I<not> work under
5332 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5333 use them in new code.
5335 If the file named by FILENAME does not exist and the C<open> call creates
5336 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5337 PERMS specifies the permissions of the newly created file. If you omit
5338 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5339 These permission values need to be in octal, and are modified by your
5340 process's current C<umask>.
5342 In many systems the C<O_EXCL> flag is available for opening files in
5343 exclusive mode. This is B<not> locking: exclusiveness means here that
5344 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5347 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5349 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5350 that takes away the user's option to have a more permissive umask.
5351 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5354 Note that C<sysopen> depends on the fdopen() C library function.
5355 On many UNIX systems, fdopen() is known to fail when file descriptors
5356 exceed a certain value, typically 255. If you need more file
5357 descriptors than that, consider rebuilding Perl to use the C<sfio>
5358 library, or perhaps using the POSIX::open() function.
5360 See L<perlopentut> for a kinder, gentler explanation of opening files.
5362 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5364 =item sysread FILEHANDLE,SCALAR,LENGTH
5366 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5367 the specified FILEHANDLE, using the system call read(2). It bypasses
5368 buffered IO, so mixing this with other kinds of reads, C<print>,
5369 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5370 stdio usually buffers data. Returns the number of characters actually
5371 read, C<0> at end of file, or undef if there was an error. SCALAR
5372 will be grown or shrunk so that the last byte actually read is the
5373 last byte of the scalar after the read.
5375 Note the I<characters>: depending on the status of the filehandle,
5376 either (8-bit) bytes or characters are read. By default all
5377 filehandles operate on bytes, but for example if the filehandle has
5378 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5379 pragma, L<open>), the I/O will operate on characters, not bytes.
5381 An OFFSET may be specified to place the read data at some place in the
5382 string other than the beginning. A negative OFFSET specifies
5383 placement at that many characters counting backwards from the end of
5384 the string. A positive OFFSET greater than the length of SCALAR
5385 results in the string being padded to the required size with C<"\0">
5386 bytes before the result of the read is appended.
5388 There is no syseof() function, which is ok, since eof() doesn't work
5389 very well on device files (like ttys) anyway. Use sysread() and check
5390 for a return value for 0 to decide whether you're done.
5392 =item sysseek FILEHANDLE,POSITION,WHENCE
5394 Sets FILEHANDLE's system position I<in bytes> using the system call
5395 lseek(2). FILEHANDLE may be an expression whose value gives the name
5396 of the filehandle. The values for WHENCE are C<0> to set the new
5397 position to POSITION, C<1> to set the it to the current position plus
5398 POSITION, and C<2> to set it to EOF plus POSITION (typically
5401 Note the I<in bytes>: even if the filehandle has been set to operate
5402 on characters (for example by using the C<:utf8> discipline), tell()
5403 will return byte offsets, not character offsets (because implementing
5404 that would render sysseek() very slow).
5406 sysseek() bypasses normal buffered io, so mixing this with reads (other
5407 than C<sysread>, for example >< or read()) C<print>, C<write>,
5408 C<seek>, C<tell>, or C<eof> may cause confusion.
5410 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5411 and C<SEEK_END> (start of the file, current position, end of the file)
5412 from the Fcntl module. Use of the constants is also more portable
5413 than relying on 0, 1, and 2. For example to define a "systell" function:
5415 use Fnctl 'SEEK_CUR';
5416 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5418 Returns the new position, or the undefined value on failure. A position
5419 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5420 true on success and false on failure, yet you can still easily determine
5425 =item system PROGRAM LIST
5427 Does exactly the same thing as C<exec LIST>, except that a fork is
5428 done first, and the parent process waits for the child process to
5429 complete. Note that argument processing varies depending on the
5430 number of arguments. If there is more than one argument in LIST,
5431 or if LIST is an array with more than one value, starts the program
5432 given by the first element of the list with arguments given by the
5433 rest of the list. If there is only one scalar argument, the argument
5434 is checked for shell metacharacters, and if there are any, the
5435 entire argument is passed to the system's command shell for parsing
5436 (this is C</bin/sh -c> on Unix platforms, but varies on other
5437 platforms). If there are no shell metacharacters in the argument,
5438 it is split into words and passed directly to C<execvp>, which is
5441 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5442 output before any operation that may do a fork, but this may not be
5443 supported on some platforms (see L<perlport>). To be safe, you may need
5444 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5445 of C<IO::Handle> on any open handles.
5447 The return value is the exit status of the program as returned by the
5448 C<wait> call. To get the actual exit value shift right by eight (see below).
5449 See also L</exec>. This is I<not> what you want to use to capture
5450 the output from a command, for that you should use merely backticks or
5451 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5452 indicates a failure to start the program (inspect $! for the reason).
5454 Like C<exec>, C<system> allows you to lie to a program about its name if
5455 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5457 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5458 killing the program they're running doesn't actually interrupt
5461 @args = ("command", "arg1", "arg2");
5463 or die "system @args failed: $?"
5465 You can check all the failure possibilities by inspecting
5468 $exit_value = $? >> 8;
5469 $signal_num = $? & 127;
5470 $dumped_core = $? & 128;
5472 or more portably by using the W*() calls of the POSIX extension;
5473 see L<perlport> for more information.
5475 When the arguments get executed via the system shell, results
5476 and return codes will be subject to its quirks and capabilities.
5477 See L<perlop/"`STRING`"> and L</exec> for details.
5479 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5481 =item syswrite FILEHANDLE,SCALAR,LENGTH
5483 =item syswrite FILEHANDLE,SCALAR
5485 Attempts to write LENGTH characters of data from variable SCALAR to
5486 the specified FILEHANDLE, using the system call write(2). If LENGTH
5487 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5488 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5489 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5490 buffers data. Returns the number of characters actually written, or
5491 C<undef> if there was an error. If the LENGTH is greater than the
5492 available data in the SCALAR after the OFFSET, only as much data as is
5493 available will be written.
5495 An OFFSET may be specified to write the data from some part of the
5496 string other than the beginning. A negative OFFSET specifies writing
5497 that many characters counting backwards from the end of the string.
5498 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5500 Note the I<characters>: depending on the status of the filehandle,
5501 either (8-bit) bytes or characters are written. By default all
5502 filehandles operate on bytes, but for example if the filehandle has
5503 been opened with the C<:utf8> discipline (see L</open>, and the open
5504 pragma, L<open>), the I/O will operate on characters, not bytes.
5506 =item tell FILEHANDLE
5510 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5511 error. FILEHANDLE may be an expression whose value gives the name of
5512 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5515 Note the I<in bytes>: even if the filehandle has been set to
5516 operate on characters (for example by using the C<:utf8> open
5517 discipline), tell() will return byte offsets, not character offsets
5518 (because that would render seek() and tell() rather slow).
5520 The return value of tell() for the standard streams like the STDIN
5521 depends on the operating system: it may return -1 or something else.
5522 tell() on pipes, fifos, and sockets usually returns -1.
5524 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5526 Do not use tell() on a filehandle that has been opened using
5527 sysopen(), use sysseek() for that as described above. Why? Because
5528 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5529 buffered filehandles. sysseek() make sense only on the first kind,
5530 tell() only makes sense on the second kind.
5532 =item telldir DIRHANDLE
5534 Returns the current position of the C<readdir> routines on DIRHANDLE.
5535 Value may be given to C<seekdir> to access a particular location in a
5536 directory. Has the same caveats about possible directory compaction as
5537 the corresponding system library routine.
5539 =item tie VARIABLE,CLASSNAME,LIST
5541 This function binds a variable to a package class that will provide the
5542 implementation for the variable. VARIABLE is the name of the variable
5543 to be enchanted. CLASSNAME is the name of a class implementing objects
5544 of correct type. Any additional arguments are passed to the C<new>
5545 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5546 or C<TIEHASH>). Typically these are arguments such as might be passed
5547 to the C<dbm_open()> function of C. The object returned by the C<new>
5548 method is also returned by the C<tie> function, which would be useful
5549 if you want to access other methods in CLASSNAME.
5551 Note that functions such as C<keys> and C<values> may return huge lists
5552 when used on large objects, like DBM files. You may prefer to use the
5553 C<each> function to iterate over such. Example:
5555 # print out history file offsets
5557 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5558 while (($key,$val) = each %HIST) {
5559 print $key, ' = ', unpack('L',$val), "\n";
5563 A class implementing a hash should have the following methods:
5565 TIEHASH classname, LIST
5567 STORE this, key, value
5572 NEXTKEY this, lastkey
5576 A class implementing an ordinary array should have the following methods:
5578 TIEARRAY classname, LIST
5580 STORE this, key, value
5582 STORESIZE this, count
5588 SPLICE this, offset, length, LIST
5593 A class implementing a file handle should have the following methods:
5595 TIEHANDLE classname, LIST
5596 READ this, scalar, length, offset
5599 WRITE this, scalar, length, offset
5601 PRINTF this, format, LIST
5605 SEEK this, position, whence
5607 OPEN this, mode, LIST
5612 A class implementing a scalar should have the following methods:
5614 TIESCALAR classname, LIST
5620 Not all methods indicated above need be implemented. See L<perltie>,
5621 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5623 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5624 for you--you need to do that explicitly yourself. See L<DB_File>
5625 or the F<Config> module for interesting C<tie> implementations.
5627 For further details see L<perltie>, L<"tied VARIABLE">.
5631 Returns a reference to the object underlying VARIABLE (the same value
5632 that was originally returned by the C<tie> call that bound the variable
5633 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5638 Returns the number of non-leap seconds since whatever time the system
5639 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5640 and 00:00:00 UTC, January 1, 1970 for most other systems).
5641 Suitable for feeding to C<gmtime> and C<localtime>.
5643 For measuring time in better granularity than one second,
5644 you may use either the Time::HiRes module from CPAN, or
5645 if you have gettimeofday(2), you may be able to use the
5646 C<syscall> interface of Perl, see L<perlfaq8> for details.
5650 Returns a four-element list giving the user and system times, in
5651 seconds, for this process and the children of this process.
5653 ($user,$system,$cuser,$csystem) = times;
5655 In scalar context, C<times> returns C<$user>.
5659 The transliteration operator. Same as C<y///>. See L<perlop>.
5661 =item truncate FILEHANDLE,LENGTH
5663 =item truncate EXPR,LENGTH
5665 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5666 specified length. Produces a fatal error if truncate isn't implemented
5667 on your system. Returns true if successful, the undefined value
5674 Returns an uppercased version of EXPR. This is the internal function
5675 implementing the C<\U> escape in double-quoted strings. Respects
5676 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5677 and L<perlunicode> for more details about locale and Unicode support.
5678 It does not attempt to do titlecase mapping on initial letters. See
5679 C<ucfirst> for that.
5681 If EXPR is omitted, uses C<$_>.
5687 Returns the value of EXPR with the first character in uppercase
5688 (titlecase in Unicode). This is the internal function implementing
5689 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5690 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5691 for more details about locale and Unicode support.
5693 If EXPR is omitted, uses C<$_>.
5699 Sets the umask for the process to EXPR and returns the previous value.
5700 If EXPR is omitted, merely returns the current umask.
5702 The Unix permission C<rwxr-x---> is represented as three sets of three
5703 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5704 and isn't one of the digits). The C<umask> value is such a number
5705 representing disabled permissions bits. The permission (or "mode")
5706 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5707 even if you tell C<sysopen> to create a file with permissions C<0777>,
5708 if your umask is C<0022> then the file will actually be created with
5709 permissions C<0755>. If your C<umask> were C<0027> (group can't
5710 write; others can't read, write, or execute), then passing
5711 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5714 Here's some advice: supply a creation mode of C<0666> for regular
5715 files (in C<sysopen>) and one of C<0777> for directories (in
5716 C<mkdir>) and executable files. This gives users the freedom of
5717 choice: if they want protected files, they might choose process umasks
5718 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5719 Programs should rarely if ever make policy decisions better left to
5720 the user. The exception to this is when writing files that should be
5721 kept private: mail files, web browser cookies, I<.rhosts> files, and
5724 If umask(2) is not implemented on your system and you are trying to
5725 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5726 fatal error at run time. If umask(2) is not implemented and you are
5727 not trying to restrict access for yourself, returns C<undef>.
5729 Remember that a umask is a number, usually given in octal; it is I<not> a
5730 string of octal digits. See also L</oct>, if all you have is a string.
5736 Undefines the value of EXPR, which must be an lvalue. Use only on a
5737 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5738 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5739 will probably not do what you expect on most predefined variables or
5740 DBM list values, so don't do that; see L<delete>.) Always returns the
5741 undefined value. You can omit the EXPR, in which case nothing is
5742 undefined, but you still get an undefined value that you could, for
5743 instance, return from a subroutine, assign to a variable or pass as a
5744 parameter. Examples:
5747 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5751 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5752 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5753 select undef, undef, undef, 0.25;
5754 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5756 Note that this is a unary operator, not a list operator.
5762 Deletes a list of files. Returns the number of files successfully
5765 $cnt = unlink 'a', 'b', 'c';
5769 Note: C<unlink> will not delete directories unless you are superuser and
5770 the B<-U> flag is supplied to Perl. Even if these conditions are
5771 met, be warned that unlinking a directory can inflict damage on your
5772 filesystem. Use C<rmdir> instead.
5774 If LIST is omitted, uses C<$_>.
5776 =item unpack TEMPLATE,EXPR
5778 C<unpack> does the reverse of C<pack>: it takes a string
5779 and expands it out into a list of values.
5780 (In scalar context, it returns merely the first value produced.)
5782 The string is broken into chunks described by the TEMPLATE. Each chunk
5783 is converted separately to a value. Typically, either the string is a result
5784 of C<pack>, or the bytes of the string represent a C structure of some
5787 The TEMPLATE has the same format as in the C<pack> function.
5788 Here's a subroutine that does substring:
5791 my($what,$where,$howmuch) = @_;
5792 unpack("x$where a$howmuch", $what);
5797 sub ordinal { unpack("c",$_[0]); } # same as ord()
5799 In addition to fields allowed in pack(), you may prefix a field with
5800 a %<number> to indicate that
5801 you want a <number>-bit checksum of the items instead of the items
5802 themselves. Default is a 16-bit checksum. Checksum is calculated by
5803 summing numeric values of expanded values (for string fields the sum of
5804 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5806 For example, the following
5807 computes the same number as the System V sum program:
5811 unpack("%32C*",<>) % 65535;
5814 The following efficiently counts the number of set bits in a bit vector:
5816 $setbits = unpack("%32b*", $selectmask);
5818 The C<p> and C<P> formats should be used with care. Since Perl
5819 has no way of checking whether the value passed to C<unpack()>
5820 corresponds to a valid memory location, passing a pointer value that's
5821 not known to be valid is likely to have disastrous consequences.
5823 If the repeat count of a field is larger than what the remainder of
5824 the input string allows, repeat count is decreased. If the input string
5825 is longer than one described by the TEMPLATE, the rest is ignored.
5827 See L</pack> for more examples and notes.
5829 =item untie VARIABLE
5831 Breaks the binding between a variable and a package. (See C<tie>.)
5833 =item unshift ARRAY,LIST
5835 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5836 depending on how you look at it. Prepends list to the front of the
5837 array, and returns the new number of elements in the array.
5839 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5841 Note the LIST is prepended whole, not one element at a time, so the
5842 prepended elements stay in the same order. Use C<reverse> to do the
5845 =item use Module VERSION LIST
5847 =item use Module VERSION
5849 =item use Module LIST
5855 Imports some semantics into the current package from the named module,
5856 generally by aliasing certain subroutine or variable names into your
5857 package. It is exactly equivalent to
5859 BEGIN { require Module; import Module LIST; }
5861 except that Module I<must> be a bareword.
5863 VERSION may be either a numeric argument such as 5.006, which will be
5864 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5865 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5866 greater than the version of the current Perl interpreter; Perl will not
5867 attempt to parse the rest of the file. Compare with L</require>, which can
5868 do a similar check at run time.
5870 Specifying VERSION as a literal of the form v5.6.1 should generally be
5871 avoided, because it leads to misleading error messages under earlier
5872 versions of Perl which do not support this syntax. The equivalent numeric
5873 version should be used instead.
5875 use v5.6.1; # compile time version check
5877 use 5.006_001; # ditto; preferred for backwards compatibility
5879 This is often useful if you need to check the current Perl version before
5880 C<use>ing library modules that have changed in incompatible ways from
5881 older versions of Perl. (We try not to do this more than we have to.)
5883 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5884 C<require> makes sure the module is loaded into memory if it hasn't been
5885 yet. The C<import> is not a builtin--it's just an ordinary static method
5886 call into the C<Module> package to tell the module to import the list of
5887 features back into the current package. The module can implement its
5888 C<import> method any way it likes, though most modules just choose to
5889 derive their C<import> method via inheritance from the C<Exporter> class that
5890 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5891 method can be found then the call is skipped.
5893 If you do not want to call the package's C<import> method (for instance,
5894 to stop your namespace from being altered), explicitly supply the empty list:
5898 That is exactly equivalent to
5900 BEGIN { require Module }
5902 If the VERSION argument is present between Module and LIST, then the
5903 C<use> will call the VERSION method in class Module with the given
5904 version as an argument. The default VERSION method, inherited from
5905 the UNIVERSAL class, croaks if the given version is larger than the
5906 value of the variable C<$Module::VERSION>.
5908 Again, there is a distinction between omitting LIST (C<import> called
5909 with no arguments) and an explicit empty LIST C<()> (C<import> not
5910 called). Note that there is no comma after VERSION!
5912 Because this is a wide-open interface, pragmas (compiler directives)
5913 are also implemented this way. Currently implemented pragmas are:
5918 use sigtrap qw(SEGV BUS);
5919 use strict qw(subs vars refs);
5920 use subs qw(afunc blurfl);
5921 use warnings qw(all);
5922 use sort qw(stable _quicksort _mergesort);
5924 Some of these pseudo-modules import semantics into the current
5925 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5926 which import symbols into the current package (which are effective
5927 through the end of the file).
5929 There's a corresponding C<no> command that unimports meanings imported
5930 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5936 If no C<unimport> method can be found the call fails with a fatal error.
5938 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5939 for the C<-M> and C<-m> command-line options to perl that give C<use>
5940 functionality from the command-line.
5944 Changes the access and modification times on each file of a list of
5945 files. The first two elements of the list must be the NUMERICAL access
5946 and modification times, in that order. Returns the number of files
5947 successfully changed. The inode change time of each file is set
5948 to the current time. This code has the same effect as the C<touch>
5949 command if the files already exist:
5953 utime $now, $now, @ARGV;
5955 If the first two elements of the list are C<undef>, then the utime(2)
5956 function in the C library will be called with a null second argument.
5957 On most systems, this will set the file's access and modification
5958 times to the current time. (i.e. equivalent to the example above.)
5960 utime undef, undef, @ARGV;
5964 Returns a list consisting of all the values of the named hash. (In a
5965 scalar context, returns the number of values.) The values are
5966 returned in an apparently random order. The actual random order is
5967 subject to change in future versions of perl, but it is guaranteed to
5968 be the same order as either the C<keys> or C<each> function would
5969 produce on the same (unmodified) hash.
5971 Note that the values are not copied, which means modifying them will
5972 modify the contents of the hash:
5974 for (values %hash) { s/foo/bar/g } # modifies %hash values
5975 for (@hash{keys %hash}) { s/foo/bar/g } # same
5977 As a side effect, calling values() resets the HASH's internal iterator.
5978 See also C<keys>, C<each>, and C<sort>.
5980 =item vec EXPR,OFFSET,BITS
5982 Treats the string in EXPR as a bit vector made up of elements of
5983 width BITS, and returns the value of the element specified by OFFSET
5984 as an unsigned integer. BITS therefore specifies the number of bits
5985 that are reserved for each element in the bit vector. This must
5986 be a power of two from 1 to 32 (or 64, if your platform supports
5989 If BITS is 8, "elements" coincide with bytes of the input string.
5991 If BITS is 16 or more, bytes of the input string are grouped into chunks
5992 of size BITS/8, and each group is converted to a number as with
5993 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5994 for BITS==64). See L<"pack"> for details.
5996 If bits is 4 or less, the string is broken into bytes, then the bits
5997 of each byte are broken into 8/BITS groups. Bits of a byte are
5998 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5999 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6000 breaking the single input byte C<chr(0x36)> into two groups gives a list
6001 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6003 C<vec> may also be assigned to, in which case parentheses are needed
6004 to give the expression the correct precedence as in
6006 vec($image, $max_x * $x + $y, 8) = 3;
6008 If the selected element is outside the string, the value 0 is returned.
6009 If an element off the end of the string is written to, Perl will first
6010 extend the string with sufficiently many zero bytes. It is an error
6011 to try to write off the beginning of the string (i.e. negative OFFSET).
6013 The string should not contain any character with the value > 255 (which
6014 can only happen if you're using UTF8 encoding). If it does, it will be
6015 treated as something which is not UTF8 encoded. When the C<vec> was
6016 assigned to, other parts of your program will also no longer consider the
6017 string to be UTF8 encoded. In other words, if you do have such characters
6018 in your string, vec() will operate on the actual byte string, and not the
6019 conceptual character string.
6021 Strings created with C<vec> can also be manipulated with the logical
6022 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6023 vector operation is desired when both operands are strings.
6024 See L<perlop/"Bitwise String Operators">.
6026 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6027 The comments show the string after each step. Note that this code works
6028 in the same way on big-endian or little-endian machines.
6031 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6033 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6034 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6036 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6037 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6038 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6039 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6040 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6041 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6043 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6044 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6045 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6048 To transform a bit vector into a string or list of 0's and 1's, use these:
6050 $bits = unpack("b*", $vector);
6051 @bits = split(//, unpack("b*", $vector));
6053 If you know the exact length in bits, it can be used in place of the C<*>.
6055 Here is an example to illustrate how the bits actually fall in place:
6061 unpack("V",$_) 01234567890123456789012345678901
6062 ------------------------------------------------------------------
6067 for ($shift=0; $shift < $width; ++$shift) {
6068 for ($off=0; $off < 32/$width; ++$off) {
6069 $str = pack("B*", "0"x32);
6070 $bits = (1<<$shift);
6071 vec($str, $off, $width) = $bits;
6072 $res = unpack("b*",$str);
6073 $val = unpack("V", $str);
6080 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6081 $off, $width, $bits, $val, $res
6085 Regardless of the machine architecture on which it is run, the above
6086 example should print the following table:
6089 unpack("V",$_) 01234567890123456789012345678901
6090 ------------------------------------------------------------------
6091 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6092 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6093 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6094 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6095 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6096 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6097 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6098 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6099 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6100 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6101 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6102 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6103 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6104 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6105 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6106 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6107 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6108 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6109 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6110 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6111 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6112 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6113 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6114 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6115 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6116 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6117 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6118 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6119 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6120 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6121 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6122 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6123 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6124 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6125 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6126 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6127 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6128 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6129 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6130 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6131 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6132 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6133 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6134 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6135 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6136 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6137 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6138 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6139 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6140 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6141 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6142 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6143 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6144 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6145 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6146 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6147 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6148 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6149 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6150 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6151 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6152 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6153 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6154 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6155 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6156 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6157 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6158 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6159 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6160 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6161 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6162 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6163 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6164 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6165 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6166 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6167 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6168 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6169 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6170 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6171 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6172 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6173 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6174 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6175 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6176 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6177 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6178 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6179 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6180 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6181 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6182 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6183 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6184 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6185 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6186 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6187 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6188 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6189 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6190 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6191 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6192 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6193 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6194 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6195 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6196 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6197 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6198 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6199 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6200 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6201 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6202 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6203 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6204 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6205 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6206 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6207 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6208 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6209 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6210 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6211 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6212 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6213 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6214 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6215 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6216 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6217 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6218 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6222 Behaves like the wait(2) system call on your system: it waits for a child
6223 process to terminate and returns the pid of the deceased process, or
6224 C<-1> if there are no child processes. The status is returned in C<$?>.
6225 Note that a return value of C<-1> could mean that child processes are
6226 being automatically reaped, as described in L<perlipc>.
6228 =item waitpid PID,FLAGS
6230 Waits for a particular child process to terminate and returns the pid of
6231 the deceased process, or C<-1> if there is no such child process. On some
6232 systems, a value of 0 indicates that there are processes still running.
6233 The status is returned in C<$?>. If you say
6235 use POSIX ":sys_wait_h";
6238 $kid = waitpid(-1, WNOHANG);
6241 then you can do a non-blocking wait for all pending zombie processes.
6242 Non-blocking wait is available on machines supporting either the
6243 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6244 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6245 system call by remembering the status values of processes that have
6246 exited but have not been harvested by the Perl script yet.)
6248 Note that on some systems, a return value of C<-1> could mean that child
6249 processes are being automatically reaped. See L<perlipc> for details,
6250 and for other examples.
6254 Returns true if the context of the currently executing subroutine is
6255 looking for a list value. Returns false if the context is looking
6256 for a scalar. Returns the undefined value if the context is looking
6257 for no value (void context).
6259 return unless defined wantarray; # don't bother doing more
6260 my @a = complex_calculation();
6261 return wantarray ? @a : "@a";
6263 This function should have been named wantlist() instead.
6267 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6270 If LIST is empty and C<$@> already contains a value (typically from a
6271 previous eval) that value is used after appending C<"\t...caught">
6272 to C<$@>. This is useful for staying almost, but not entirely similar to
6275 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6277 No message is printed if there is a C<$SIG{__WARN__}> handler
6278 installed. It is the handler's responsibility to deal with the message
6279 as it sees fit (like, for instance, converting it into a C<die>). Most
6280 handlers must therefore make arrangements to actually display the
6281 warnings that they are not prepared to deal with, by calling C<warn>
6282 again in the handler. Note that this is quite safe and will not
6283 produce an endless loop, since C<__WARN__> hooks are not called from
6286 You will find this behavior is slightly different from that of
6287 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6288 instead call C<die> again to change it).
6290 Using a C<__WARN__> handler provides a powerful way to silence all
6291 warnings (even the so-called mandatory ones). An example:
6293 # wipe out *all* compile-time warnings
6294 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6296 my $foo = 20; # no warning about duplicate my $foo,
6297 # but hey, you asked for it!
6298 # no compile-time or run-time warnings before here
6301 # run-time warnings enabled after here
6302 warn "\$foo is alive and $foo!"; # does show up
6304 See L<perlvar> for details on setting C<%SIG> entries, and for more
6305 examples. See the Carp module for other kinds of warnings using its
6306 carp() and cluck() functions.
6308 =item write FILEHANDLE
6314 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6315 using the format associated with that file. By default the format for
6316 a file is the one having the same name as the filehandle, but the
6317 format for the current output channel (see the C<select> function) may be set
6318 explicitly by assigning the name of the format to the C<$~> variable.
6320 Top of form processing is handled automatically: if there is
6321 insufficient room on the current page for the formatted record, the
6322 page is advanced by writing a form feed, a special top-of-page format
6323 is used to format the new page header, and then the record is written.
6324 By default the top-of-page format is the name of the filehandle with
6325 "_TOP" appended, but it may be dynamically set to the format of your
6326 choice by assigning the name to the C<$^> variable while the filehandle is
6327 selected. The number of lines remaining on the current page is in
6328 variable C<$->, which can be set to C<0> to force a new page.
6330 If FILEHANDLE is unspecified, output goes to the current default output
6331 channel, which starts out as STDOUT but may be changed by the
6332 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6333 is evaluated and the resulting string is used to look up the name of
6334 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6336 Note that write is I<not> the opposite of C<read>. Unfortunately.
6340 The transliteration operator. Same as C<tr///>. See L<perlop>.