3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
434 Returns the arctangent of Y/X in the range -PI to PI.
436 For the tangent operation, you may use the C<Math::Trig::tan>
437 function, or use the familiar relation:
439 sub tan { sin($_[0]) / cos($_[0]) }
441 =item bind SOCKET,NAME
443 Binds a network address to a socket, just as the bind system call
444 does. Returns true if it succeeded, false otherwise. NAME should be a
445 packed address of the appropriate type for the socket. See the examples in
446 L<perlipc/"Sockets: Client/Server Communication">.
448 =item binmode FILEHANDLE, DISCIPLINE
450 =item binmode FILEHANDLE
452 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
453 on systems where the run-time libraries distinguish between binary and
454 text files. If FILEHANDLE is an expression, the value is taken as the
455 name of the filehandle.
457 DISCIPLINE can be either of C<:raw> for binary mode or C<:crlf> for
458 "text" mode. If the DISCIPLINE is omitted, it defaults to C<:raw>.
459 Returns true on success, C<undef> on failure. To mark FILEHANDLE as
460 UTF-8, use C<:utf8>, and to mark it as bytes, use C<:bytes>.
462 The C<:raw> are C<:clrf>, and any other directives of the form
463 C<:...>, are called I/O I<disciplines>. The C<open> pragma can be
464 used to establish default I/O disciplines. See L<open>.
466 In general, binmode() should be called after open() but before any I/O
467 is done on the filehandle. Calling binmode() will flush any possibly
468 pending buffered input or output data on the handle. The only
469 exception to this is the C<:encoding> discipline that changes
470 the default character encoding of the handle, see L<open>.
471 The C<:encoding> discipline sometimes needs to be called in
472 mid-stream, and it doesn't flush the stream.
474 On some systems binmode() is necessary when you're not working with a
475 text file. For the sake of portability it is a good idea to always use
476 it when appropriate, and to never use it when it isn't appropriate.
478 In other words: Regardless of platform, use binmode() on binary
479 files, and do not use binmode() on text files.
481 The operating system, device drivers, C libraries, and Perl run-time
482 system all work together to let the programmer treat a single
483 character (C<\n>) as the line terminator, irrespective of the external
484 representation. On many operating systems, the native text file
485 representation matches the internal representation, but on some
486 platforms the external representation of C<\n> is made up of more than
489 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
490 character to end each line in the external representation of text (even
491 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
492 on Unix and most VMS files). Consequently binmode() has no effect on
493 these operating systems. In other systems like OS/2, DOS and the various
494 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
495 what's stored in text files are the two characters C<\cM\cJ>. That means
496 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
497 disk will be converted to C<\n> on input, and any C<\n> in your program
498 will be converted back to C<\cM\cJ> on output. This is what you want for
499 text files, but it can be disastrous for binary files.
501 Another consequence of using binmode() (on some systems) is that
502 special end-of-file markers will be seen as part of the data stream.
503 For systems from the Microsoft family this means that if your binary
504 data contains C<\cZ>, the I/O subsystem will regard it as the end of
505 the file, unless you use binmode().
507 binmode() is not only important for readline() and print() operations,
508 but also when using read(), seek(), sysread(), syswrite() and tell()
509 (see L<perlport> for more details). See the C<$/> and C<$\> variables
510 in L<perlvar> for how to manually set your input and output
511 line-termination sequences.
513 =item bless REF,CLASSNAME
517 This function tells the thingy referenced by REF that it is now an object
518 in the CLASSNAME package. If CLASSNAME is omitted, the current package
519 is used. Because a C<bless> is often the last thing in a constructor,
520 it returns the reference for convenience. Always use the two-argument
521 version if the function doing the blessing might be inherited by a
522 derived class. See L<perltoot> and L<perlobj> for more about the blessing
523 (and blessings) of objects.
525 Consider always blessing objects in CLASSNAMEs that are mixed case.
526 Namespaces with all lowercase names are considered reserved for
527 Perl pragmata. Builtin types have all uppercase names, so to prevent
528 confusion, you may wish to avoid such package names as well. Make sure
529 that CLASSNAME is a true value.
531 See L<perlmod/"Perl Modules">.
537 Returns the context of the current subroutine call. In scalar context,
538 returns the caller's package name if there is a caller, that is, if
539 we're in a subroutine or C<eval> or C<require>, and the undefined value
540 otherwise. In list context, returns
542 ($package, $filename, $line) = caller;
544 With EXPR, it returns some extra information that the debugger uses to
545 print a stack trace. The value of EXPR indicates how many call frames
546 to go back before the current one.
548 ($package, $filename, $line, $subroutine, $hasargs,
549 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
551 Here $subroutine may be C<(eval)> if the frame is not a subroutine
552 call, but an C<eval>. In such a case additional elements $evaltext and
553 C<$is_require> are set: C<$is_require> is true if the frame is created by a
554 C<require> or C<use> statement, $evaltext contains the text of the
555 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
556 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
557 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
558 frame.) $subroutine may also be C<(unknown)> if this particular
559 subroutine happens to have been deleted from the symbol table.
560 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
561 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
562 compiled with. The C<$hints> and C<$bitmask> values are subject to change
563 between versions of Perl, and are not meant for external use.
565 Furthermore, when called from within the DB package, caller returns more
566 detailed information: it sets the list variable C<@DB::args> to be the
567 arguments with which the subroutine was invoked.
569 Be aware that the optimizer might have optimized call frames away before
570 C<caller> had a chance to get the information. That means that C<caller(N)>
571 might not return information about the call frame you expect it do, for
572 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
573 previous time C<caller> was called.
577 Changes the working directory to EXPR, if possible. If EXPR is omitted,
578 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
579 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
580 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
581 neither is set, C<chdir> does nothing. It returns true upon success,
582 false otherwise. See the example under C<die>.
586 Changes the permissions of a list of files. The first element of the
587 list must be the numerical mode, which should probably be an octal
588 number, and which definitely should I<not> a string of octal digits:
589 C<0644> is okay, C<'0644'> is not. Returns the number of files
590 successfully changed. See also L</oct>, if all you have is a string.
592 $cnt = chmod 0755, 'foo', 'bar';
593 chmod 0755, @executables;
594 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
596 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
597 $mode = 0644; chmod $mode, 'foo'; # this is best
599 You can also import the symbolic C<S_I*> constants from the Fcntl
604 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
605 # This is identical to the chmod 0755 of the above example.
613 This safer version of L</chop> removes any trailing string
614 that corresponds to the current value of C<$/> (also known as
615 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
616 number of characters removed from all its arguments. It's often used to
617 remove the newline from the end of an input record when you're worried
618 that the final record may be missing its newline. When in paragraph
619 mode (C<$/ = "">), it removes all trailing newlines from the string.
620 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
621 a reference to an integer or the like, see L<perlvar>) chomp() won't
623 If VARIABLE is omitted, it chomps C<$_>. Example:
626 chomp; # avoid \n on last field
631 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
633 You can actually chomp anything that's an lvalue, including an assignment:
636 chomp($answer = <STDIN>);
638 If you chomp a list, each element is chomped, and the total number of
639 characters removed is returned.
647 Chops off the last character of a string and returns the character
648 chopped. It is much more efficient than C<s/.$//s> because it neither
649 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
650 If VARIABLE is a hash, it chops the hash's values, but not its keys.
652 You can actually chop anything that's an lvalue, including an assignment.
654 If you chop a list, each element is chopped. Only the value of the
655 last C<chop> is returned.
657 Note that C<chop> returns the last character. To return all but the last
658 character, use C<substr($string, 0, -1)>.
662 Changes the owner (and group) of a list of files. The first two
663 elements of the list must be the I<numeric> uid and gid, in that
664 order. A value of -1 in either position is interpreted by most
665 systems to leave that value unchanged. Returns the number of files
666 successfully changed.
668 $cnt = chown $uid, $gid, 'foo', 'bar';
669 chown $uid, $gid, @filenames;
671 Here's an example that looks up nonnumeric uids in the passwd file:
674 chomp($user = <STDIN>);
676 chomp($pattern = <STDIN>);
678 ($login,$pass,$uid,$gid) = getpwnam($user)
679 or die "$user not in passwd file";
681 @ary = glob($pattern); # expand filenames
682 chown $uid, $gid, @ary;
684 On most systems, you are not allowed to change the ownership of the
685 file unless you're the superuser, although you should be able to change
686 the group to any of your secondary groups. On insecure systems, these
687 restrictions may be relaxed, but this is not a portable assumption.
688 On POSIX systems, you can detect this condition this way:
690 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
691 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
697 Returns the character represented by that NUMBER in the character set.
698 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
699 chr(0x263a) is a Unicode smiley face. Note that characters from 127
700 to 255 (inclusive) are by default not encoded in Unicode for backward
701 compatibility reasons (but see L<encoding>).
703 For the reverse, use L</ord>.
704 See L<perlunicode> and L<encoding> for more about Unicode.
706 If NUMBER is omitted, uses C<$_>.
708 =item chroot FILENAME
712 This function works like the system call by the same name: it makes the
713 named directory the new root directory for all further pathnames that
714 begin with a C</> by your process and all its children. (It doesn't
715 change your current working directory, which is unaffected.) For security
716 reasons, this call is restricted to the superuser. If FILENAME is
717 omitted, does a C<chroot> to C<$_>.
719 =item close FILEHANDLE
723 Closes the file or pipe associated with the file handle, returning
724 true only if IO buffers are successfully flushed and closes the system
725 file descriptor. Closes the currently selected filehandle if the
728 You don't have to close FILEHANDLE if you are immediately going to do
729 another C<open> on it, because C<open> will close it for you. (See
730 C<open>.) However, an explicit C<close> on an input file resets the line
731 counter (C<$.>), while the implicit close done by C<open> does not.
733 If the file handle came from a piped open C<close> will additionally
734 return false if one of the other system calls involved fails or if the
735 program exits with non-zero status. (If the only problem was that the
736 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
737 also waits for the process executing on the pipe to complete, in case you
738 want to look at the output of the pipe afterwards, and
739 implicitly puts the exit status value of that command into C<$?>.
741 Prematurely closing the read end of a pipe (i.e. before the process
742 writing to it at the other end has closed it) will result in a
743 SIGPIPE being delivered to the writer. If the other end can't
744 handle that, be sure to read all the data before closing the pipe.
748 open(OUTPUT, '|sort >foo') # pipe to sort
749 or die "Can't start sort: $!";
750 #... # print stuff to output
751 close OUTPUT # wait for sort to finish
752 or warn $! ? "Error closing sort pipe: $!"
753 : "Exit status $? from sort";
754 open(INPUT, 'foo') # get sort's results
755 or die "Can't open 'foo' for input: $!";
757 FILEHANDLE may be an expression whose value can be used as an indirect
758 filehandle, usually the real filehandle name.
760 =item closedir DIRHANDLE
762 Closes a directory opened by C<opendir> and returns the success of that
765 DIRHANDLE may be an expression whose value can be used as an indirect
766 dirhandle, usually the real dirhandle name.
768 =item connect SOCKET,NAME
770 Attempts to connect to a remote socket, just as the connect system call
771 does. Returns true if it succeeded, false otherwise. NAME should be a
772 packed address of the appropriate type for the socket. See the examples in
773 L<perlipc/"Sockets: Client/Server Communication">.
777 Actually a flow control statement rather than a function. If there is a
778 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
779 C<foreach>), it is always executed just before the conditional is about to
780 be evaluated again, just like the third part of a C<for> loop in C. Thus
781 it can be used to increment a loop variable, even when the loop has been
782 continued via the C<next> statement (which is similar to the C C<continue>
785 C<last>, C<next>, or C<redo> may appear within a C<continue>
786 block. C<last> and C<redo> will behave as if they had been executed within
787 the main block. So will C<next>, but since it will execute a C<continue>
788 block, it may be more entertaining.
791 ### redo always comes here
794 ### next always comes here
796 # then back the top to re-check EXPR
798 ### last always comes here
800 Omitting the C<continue> section is semantically equivalent to using an
801 empty one, logically enough. In that case, C<next> goes directly back
802 to check the condition at the top of the loop.
808 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
809 takes cosine of C<$_>.
811 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
812 function, or use this relation:
814 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
816 =item crypt PLAINTEXT,SALT
818 Encrypts a string exactly like the crypt(3) function in the C library
819 (assuming that you actually have a version there that has not been
820 extirpated as a potential munition). This can prove useful for checking
821 the password file for lousy passwords, amongst other things. Only the
822 guys wearing white hats should do this.
824 Note that C<crypt> is intended to be a one-way function, much like
825 breaking eggs to make an omelette. There is no (known) corresponding
826 decrypt function (in other words, the crypt() is a one-way hash
827 function). As a result, this function isn't all that useful for
828 cryptography. (For that, see your nearby CPAN mirror.)
830 When verifying an existing encrypted string you should use the
831 encrypted text as the salt (like C<crypt($plain, $crypted) eq
832 $crypted>). This allows your code to work with the standard C<crypt>
833 and with more exotic implementations. In other words, do not assume
834 anything about the returned string itself, or how many bytes in
835 the encrypted string matter.
837 Traditionally the result is a string of 13 bytes: two first bytes of
838 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
839 the first eight bytes of the encrypted string mattered, but
840 alternative hashing schemes (like MD5), higher level security schemes
841 (like C2), and implementations on non-UNIX platforms may produce
844 When choosing a new salt create a random two character string whose
845 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
846 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
848 Here's an example that makes sure that whoever runs this program knows
851 $pwd = (getpwuid($<))[1];
855 chomp($word = <STDIN>);
859 if (crypt($word, $pwd) ne $pwd) {
865 Of course, typing in your own password to whoever asks you
868 The L<crypt> function is unsuitable for encrypting large quantities
869 of data, not least of all because you can't get the information
870 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
871 on your favorite CPAN mirror for a slew of potentially useful
874 If using crypt() on a Unicode string (which I<potentially> has
875 characters with codepoints above 255), Perl tries to make sense
876 of the situation by trying to downgrade (a copy of the string)
877 the string back to an eight-bit byte string before calling crypt()
878 (on that copy). If that works, good. If not, crypt() dies with
879 C<Wide character in crypt>.
883 [This function has been largely superseded by the C<untie> function.]
885 Breaks the binding between a DBM file and a hash.
887 =item dbmopen HASH,DBNAME,MASK
889 [This function has been largely superseded by the C<tie> function.]
891 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
892 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
893 argument is I<not> a filehandle, even though it looks like one). DBNAME
894 is the name of the database (without the F<.dir> or F<.pag> extension if
895 any). If the database does not exist, it is created with protection
896 specified by MASK (as modified by the C<umask>). If your system supports
897 only the older DBM functions, you may perform only one C<dbmopen> in your
898 program. In older versions of Perl, if your system had neither DBM nor
899 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
902 If you don't have write access to the DBM file, you can only read hash
903 variables, not set them. If you want to test whether you can write,
904 either use file tests or try setting a dummy hash entry inside an C<eval>,
905 which will trap the error.
907 Note that functions such as C<keys> and C<values> may return huge lists
908 when used on large DBM files. You may prefer to use the C<each>
909 function to iterate over large DBM files. Example:
911 # print out history file offsets
912 dbmopen(%HIST,'/usr/lib/news/history',0666);
913 while (($key,$val) = each %HIST) {
914 print $key, ' = ', unpack('L',$val), "\n";
918 See also L<AnyDBM_File> for a more general description of the pros and
919 cons of the various dbm approaches, as well as L<DB_File> for a particularly
922 You can control which DBM library you use by loading that library
923 before you call dbmopen():
926 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
927 or die "Can't open netscape history file: $!";
933 Returns a Boolean value telling whether EXPR has a value other than
934 the undefined value C<undef>. If EXPR is not present, C<$_> will be
937 Many operations return C<undef> to indicate failure, end of file,
938 system error, uninitialized variable, and other exceptional
939 conditions. This function allows you to distinguish C<undef> from
940 other values. (A simple Boolean test will not distinguish among
941 C<undef>, zero, the empty string, and C<"0">, which are all equally
942 false.) Note that since C<undef> is a valid scalar, its presence
943 doesn't I<necessarily> indicate an exceptional condition: C<pop>
944 returns C<undef> when its argument is an empty array, I<or> when the
945 element to return happens to be C<undef>.
947 You may also use C<defined(&func)> to check whether subroutine C<&func>
948 has ever been defined. The return value is unaffected by any forward
949 declarations of C<&foo>. Note that a subroutine which is not defined
950 may still be callable: its package may have an C<AUTOLOAD> method that
951 makes it spring into existence the first time that it is called -- see
954 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
955 used to report whether memory for that aggregate has ever been
956 allocated. This behavior may disappear in future versions of Perl.
957 You should instead use a simple test for size:
959 if (@an_array) { print "has array elements\n" }
960 if (%a_hash) { print "has hash members\n" }
962 When used on a hash element, it tells you whether the value is defined,
963 not whether the key exists in the hash. Use L</exists> for the latter
968 print if defined $switch{'D'};
969 print "$val\n" while defined($val = pop(@ary));
970 die "Can't readlink $sym: $!"
971 unless defined($value = readlink $sym);
972 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
973 $debugging = 0 unless defined $debugging;
975 Note: Many folks tend to overuse C<defined>, and then are surprised to
976 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
977 defined values. For example, if you say
981 The pattern match succeeds, and C<$1> is defined, despite the fact that it
982 matched "nothing". But it didn't really match nothing--rather, it
983 matched something that happened to be zero characters long. This is all
984 very above-board and honest. When a function returns an undefined value,
985 it's an admission that it couldn't give you an honest answer. So you
986 should use C<defined> only when you're questioning the integrity of what
987 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
990 See also L</undef>, L</exists>, L</ref>.
994 Given an expression that specifies a hash element, array element, hash slice,
995 or array slice, deletes the specified element(s) from the hash or array.
996 In the case of an array, if the array elements happen to be at the end,
997 the size of the array will shrink to the highest element that tests
998 true for exists() (or 0 if no such element exists).
1000 Returns each element so deleted or the undefined value if there was no such
1001 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1002 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1003 from a C<tie>d hash or array may not necessarily return anything.
1005 Deleting an array element effectively returns that position of the array
1006 to its initial, uninitialized state. Subsequently testing for the same
1007 element with exists() will return false. Note that deleting array
1008 elements in the middle of an array will not shift the index of the ones
1009 after them down--use splice() for that. See L</exists>.
1011 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1013 foreach $key (keys %HASH) {
1017 foreach $index (0 .. $#ARRAY) {
1018 delete $ARRAY[$index];
1023 delete @HASH{keys %HASH};
1025 delete @ARRAY[0 .. $#ARRAY];
1027 But both of these are slower than just assigning the empty list
1028 or undefining %HASH or @ARRAY:
1030 %HASH = (); # completely empty %HASH
1031 undef %HASH; # forget %HASH ever existed
1033 @ARRAY = (); # completely empty @ARRAY
1034 undef @ARRAY; # forget @ARRAY ever existed
1036 Note that the EXPR can be arbitrarily complicated as long as the final
1037 operation is a hash element, array element, hash slice, or array slice
1040 delete $ref->[$x][$y]{$key};
1041 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1043 delete $ref->[$x][$y][$index];
1044 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1048 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1049 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1050 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1051 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1052 an C<eval(),> the error message is stuffed into C<$@> and the
1053 C<eval> is terminated with the undefined value. This makes
1054 C<die> the way to raise an exception.
1056 Equivalent examples:
1058 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1059 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1061 If the last element of LIST does not end in a newline, the current
1062 script line number and input line number (if any) are also printed,
1063 and a newline is supplied. Note that the "input line number" (also
1064 known as "chunk") is subject to whatever notion of "line" happens to
1065 be currently in effect, and is also available as the special variable
1066 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1068 Hint: sometimes appending C<", stopped"> to your message will cause it
1069 to make better sense when the string C<"at foo line 123"> is appended.
1070 Suppose you are running script "canasta".
1072 die "/etc/games is no good";
1073 die "/etc/games is no good, stopped";
1075 produce, respectively
1077 /etc/games is no good at canasta line 123.
1078 /etc/games is no good, stopped at canasta line 123.
1080 See also exit(), warn(), and the Carp module.
1082 If LIST is empty and C<$@> already contains a value (typically from a
1083 previous eval) that value is reused after appending C<"\t...propagated">.
1084 This is useful for propagating exceptions:
1087 die unless $@ =~ /Expected exception/;
1089 If LIST is empty and C<$@> contains an object reference that has a
1090 C<PROPAGATE> method, that method will be called with additional file
1091 and line number parameters. The return value replaces the value in
1092 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1095 If C<$@> is empty then the string C<"Died"> is used.
1097 die() can also be called with a reference argument. If this happens to be
1098 trapped within an eval(), $@ contains the reference. This behavior permits
1099 a more elaborate exception handling implementation using objects that
1100 maintain arbitrary state about the nature of the exception. Such a scheme
1101 is sometimes preferable to matching particular string values of $@ using
1102 regular expressions. Here's an example:
1104 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1106 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1107 # handle Some::Module::Exception
1110 # handle all other possible exceptions
1114 Because perl will stringify uncaught exception messages before displaying
1115 them, you may want to overload stringification operations on such custom
1116 exception objects. See L<overload> for details about that.
1118 You can arrange for a callback to be run just before the C<die>
1119 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1120 handler will be called with the error text and can change the error
1121 message, if it sees fit, by calling C<die> again. See
1122 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1123 L<"eval BLOCK"> for some examples. Although this feature was meant
1124 to be run only right before your program was to exit, this is not
1125 currently the case--the C<$SIG{__DIE__}> hook is currently called
1126 even inside eval()ed blocks/strings! If one wants the hook to do
1127 nothing in such situations, put
1131 as the first line of the handler (see L<perlvar/$^S>). Because
1132 this promotes strange action at a distance, this counterintuitive
1133 behavior may be fixed in a future release.
1137 Not really a function. Returns the value of the last command in the
1138 sequence of commands indicated by BLOCK. When modified by a loop
1139 modifier, executes the BLOCK once before testing the loop condition.
1140 (On other statements the loop modifiers test the conditional first.)
1142 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1143 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1144 See L<perlsyn> for alternative strategies.
1146 =item do SUBROUTINE(LIST)
1148 A deprecated form of subroutine call. See L<perlsub>.
1152 Uses the value of EXPR as a filename and executes the contents of the
1153 file as a Perl script. Its primary use is to include subroutines
1154 from a Perl subroutine library.
1162 except that it's more efficient and concise, keeps track of the current
1163 filename for error messages, searches the @INC libraries, and updates
1164 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1165 variables. It also differs in that code evaluated with C<do FILENAME>
1166 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1167 same, however, in that it does reparse the file every time you call it,
1168 so you probably don't want to do this inside a loop.
1170 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1171 error. If C<do> can read the file but cannot compile it, it
1172 returns undef and sets an error message in C<$@>. If the file is
1173 successfully compiled, C<do> returns the value of the last expression
1176 Note that inclusion of library modules is better done with the
1177 C<use> and C<require> operators, which also do automatic error checking
1178 and raise an exception if there's a problem.
1180 You might like to use C<do> to read in a program configuration
1181 file. Manual error checking can be done this way:
1183 # read in config files: system first, then user
1184 for $file ("/share/prog/defaults.rc",
1185 "$ENV{HOME}/.someprogrc")
1187 unless ($return = do $file) {
1188 warn "couldn't parse $file: $@" if $@;
1189 warn "couldn't do $file: $!" unless defined $return;
1190 warn "couldn't run $file" unless $return;
1198 This function causes an immediate core dump. See also the B<-u>
1199 command-line switch in L<perlrun>, which does the same thing.
1200 Primarily this is so that you can use the B<undump> program (not
1201 supplied) to turn your core dump into an executable binary after
1202 having initialized all your variables at the beginning of the
1203 program. When the new binary is executed it will begin by executing
1204 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1205 Think of it as a goto with an intervening core dump and reincarnation.
1206 If C<LABEL> is omitted, restarts the program from the top.
1208 B<WARNING>: Any files opened at the time of the dump will I<not>
1209 be open any more when the program is reincarnated, with possible
1210 resulting confusion on the part of Perl.
1212 This function is now largely obsolete, partly because it's very
1213 hard to convert a core file into an executable, and because the
1214 real compiler backends for generating portable bytecode and compilable
1215 C code have superseded it. That's why you should now invoke it as
1216 C<CORE::dump()>, if you don't want to be warned against a possible
1219 If you're looking to use L<dump> to speed up your program, consider
1220 generating bytecode or native C code as described in L<perlcc>. If
1221 you're just trying to accelerate a CGI script, consider using the
1222 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1223 You might also consider autoloading or selfloading, which at least
1224 make your program I<appear> to run faster.
1228 When called in list context, returns a 2-element list consisting of the
1229 key and value for the next element of a hash, so that you can iterate over
1230 it. When called in scalar context, returns only the key for the next
1231 element in the hash.
1233 Entries are returned in an apparently random order. The actual random
1234 order is subject to change in future versions of perl, but it is guaranteed
1235 to be in the same order as either the C<keys> or C<values> function
1236 would produce on the same (unmodified) hash.
1238 When the hash is entirely read, a null array is returned in list context
1239 (which when assigned produces a false (C<0>) value), and C<undef> in
1240 scalar context. The next call to C<each> after that will start iterating
1241 again. There is a single iterator for each hash, shared by all C<each>,
1242 C<keys>, and C<values> function calls in the program; it can be reset by
1243 reading all the elements from the hash, or by evaluating C<keys HASH> or
1244 C<values HASH>. If you add or delete elements of a hash while you're
1245 iterating over it, you may get entries skipped or duplicated, so
1246 don't. Exception: It is always safe to delete the item most recently
1247 returned by C<each()>, which means that the following code will work:
1249 while (($key, $value) = each %hash) {
1251 delete $hash{$key}; # This is safe
1254 The following prints out your environment like the printenv(1) program,
1255 only in a different order:
1257 while (($key,$value) = each %ENV) {
1258 print "$key=$value\n";
1261 See also C<keys>, C<values> and C<sort>.
1263 =item eof FILEHANDLE
1269 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1270 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1271 gives the real filehandle. (Note that this function actually
1272 reads a character and then C<ungetc>s it, so isn't very useful in an
1273 interactive context.) Do not read from a terminal file (or call
1274 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1275 as terminals may lose the end-of-file condition if you do.
1277 An C<eof> without an argument uses the last file read. Using C<eof()>
1278 with empty parentheses is very different. It refers to the pseudo file
1279 formed from the files listed on the command line and accessed via the
1280 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1281 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1282 used will cause C<@ARGV> to be examined to determine if input is
1283 available. Similarly, an C<eof()> after C<< <> >> has returned
1284 end-of-file will assume you are processing another C<@ARGV> list,
1285 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1286 see L<perlop/"I/O Operators">.
1288 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1289 detect the end of each file, C<eof()> will only detect the end of the
1290 last file. Examples:
1292 # reset line numbering on each input file
1294 next if /^\s*#/; # skip comments
1297 close ARGV if eof; # Not eof()!
1300 # insert dashes just before last line of last file
1302 if (eof()) { # check for end of current file
1303 print "--------------\n";
1304 close(ARGV); # close or last; is needed if we
1305 # are reading from the terminal
1310 Practical hint: you almost never need to use C<eof> in Perl, because the
1311 input operators typically return C<undef> when they run out of data, or if
1318 In the first form, the return value of EXPR is parsed and executed as if it
1319 were a little Perl program. The value of the expression (which is itself
1320 determined within scalar context) is first parsed, and if there weren't any
1321 errors, executed in the lexical context of the current Perl program, so
1322 that any variable settings or subroutine and format definitions remain
1323 afterwards. Note that the value is parsed every time the eval executes.
1324 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1325 delay parsing and subsequent execution of the text of EXPR until run time.
1327 In the second form, the code within the BLOCK is parsed only once--at the
1328 same time the code surrounding the eval itself was parsed--and executed
1329 within the context of the current Perl program. This form is typically
1330 used to trap exceptions more efficiently than the first (see below), while
1331 also providing the benefit of checking the code within BLOCK at compile
1334 The final semicolon, if any, may be omitted from the value of EXPR or within
1337 In both forms, the value returned is the value of the last expression
1338 evaluated inside the mini-program; a return statement may be also used, just
1339 as with subroutines. The expression providing the return value is evaluated
1340 in void, scalar, or list context, depending on the context of the eval itself.
1341 See L</wantarray> for more on how the evaluation context can be determined.
1343 If there is a syntax error or runtime error, or a C<die> statement is
1344 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1345 error message. If there was no error, C<$@> is guaranteed to be a null
1346 string. Beware that using C<eval> neither silences perl from printing
1347 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1348 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1349 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1350 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1352 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1353 determining whether a particular feature (such as C<socket> or C<symlink>)
1354 is implemented. It is also Perl's exception trapping mechanism, where
1355 the die operator is used to raise exceptions.
1357 If the code to be executed doesn't vary, you may use the eval-BLOCK
1358 form to trap run-time errors without incurring the penalty of
1359 recompiling each time. The error, if any, is still returned in C<$@>.
1362 # make divide-by-zero nonfatal
1363 eval { $answer = $a / $b; }; warn $@ if $@;
1365 # same thing, but less efficient
1366 eval '$answer = $a / $b'; warn $@ if $@;
1368 # a compile-time error
1369 eval { $answer = }; # WRONG
1372 eval '$answer ='; # sets $@
1374 Due to the current arguably broken state of C<__DIE__> hooks, when using
1375 the C<eval{}> form as an exception trap in libraries, you may wish not
1376 to trigger any C<__DIE__> hooks that user code may have installed.
1377 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1378 as shown in this example:
1380 # a very private exception trap for divide-by-zero
1381 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1384 This is especially significant, given that C<__DIE__> hooks can call
1385 C<die> again, which has the effect of changing their error messages:
1387 # __DIE__ hooks may modify error messages
1389 local $SIG{'__DIE__'} =
1390 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1391 eval { die "foo lives here" };
1392 print $@ if $@; # prints "bar lives here"
1395 Because this promotes action at a distance, this counterintuitive behavior
1396 may be fixed in a future release.
1398 With an C<eval>, you should be especially careful to remember what's
1399 being looked at when:
1405 eval { $x }; # CASE 4
1407 eval "\$$x++"; # CASE 5
1410 Cases 1 and 2 above behave identically: they run the code contained in
1411 the variable $x. (Although case 2 has misleading double quotes making
1412 the reader wonder what else might be happening (nothing is).) Cases 3
1413 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1414 does nothing but return the value of $x. (Case 4 is preferred for
1415 purely visual reasons, but it also has the advantage of compiling at
1416 compile-time instead of at run-time.) Case 5 is a place where
1417 normally you I<would> like to use double quotes, except that in this
1418 particular situation, you can just use symbolic references instead, as
1421 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1422 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1426 =item exec PROGRAM LIST
1428 The C<exec> function executes a system command I<and never returns>--
1429 use C<system> instead of C<exec> if you want it to return. It fails and
1430 returns false only if the command does not exist I<and> it is executed
1431 directly instead of via your system's command shell (see below).
1433 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1434 warns you if there is a following statement which isn't C<die>, C<warn>,
1435 or C<exit> (if C<-w> is set - but you always do that). If you
1436 I<really> want to follow an C<exec> with some other statement, you
1437 can use one of these styles to avoid the warning:
1439 exec ('foo') or print STDERR "couldn't exec foo: $!";
1440 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1442 If there is more than one argument in LIST, or if LIST is an array
1443 with more than one value, calls execvp(3) with the arguments in LIST.
1444 If there is only one scalar argument or an array with one element in it,
1445 the argument is checked for shell metacharacters, and if there are any,
1446 the entire argument is passed to the system's command shell for parsing
1447 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1448 If there are no shell metacharacters in the argument, it is split into
1449 words and passed directly to C<execvp>, which is more efficient.
1452 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1453 exec "sort $outfile | uniq";
1455 If you don't really want to execute the first argument, but want to lie
1456 to the program you are executing about its own name, you can specify
1457 the program you actually want to run as an "indirect object" (without a
1458 comma) in front of the LIST. (This always forces interpretation of the
1459 LIST as a multivalued list, even if there is only a single scalar in
1462 $shell = '/bin/csh';
1463 exec $shell '-sh'; # pretend it's a login shell
1467 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1469 When the arguments get executed via the system shell, results will
1470 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1473 Using an indirect object with C<exec> or C<system> is also more
1474 secure. This usage (which also works fine with system()) forces
1475 interpretation of the arguments as a multivalued list, even if the
1476 list had just one argument. That way you're safe from the shell
1477 expanding wildcards or splitting up words with whitespace in them.
1479 @args = ( "echo surprise" );
1481 exec @args; # subject to shell escapes
1483 exec { $args[0] } @args; # safe even with one-arg list
1485 The first version, the one without the indirect object, ran the I<echo>
1486 program, passing it C<"surprise"> an argument. The second version
1487 didn't--it tried to run a program literally called I<"echo surprise">,
1488 didn't find it, and set C<$?> to a non-zero value indicating failure.
1490 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1491 output before the exec, but this may not be supported on some platforms
1492 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1493 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1494 open handles in order to avoid lost output.
1496 Note that C<exec> will not call your C<END> blocks, nor will it call
1497 any C<DESTROY> methods in your objects.
1501 Given an expression that specifies a hash element or array element,
1502 returns true if the specified element in the hash or array has ever
1503 been initialized, even if the corresponding value is undefined. The
1504 element is not autovivified if it doesn't exist.
1506 print "Exists\n" if exists $hash{$key};
1507 print "Defined\n" if defined $hash{$key};
1508 print "True\n" if $hash{$key};
1510 print "Exists\n" if exists $array[$index];
1511 print "Defined\n" if defined $array[$index];
1512 print "True\n" if $array[$index];
1514 A hash or array element can be true only if it's defined, and defined if
1515 it exists, but the reverse doesn't necessarily hold true.
1517 Given an expression that specifies the name of a subroutine,
1518 returns true if the specified subroutine has ever been declared, even
1519 if it is undefined. Mentioning a subroutine name for exists or defined
1520 does not count as declaring it. Note that a subroutine which does not
1521 exist may still be callable: its package may have an C<AUTOLOAD>
1522 method that makes it spring into existence the first time that it is
1523 called -- see L<perlsub>.
1525 print "Exists\n" if exists &subroutine;
1526 print "Defined\n" if defined &subroutine;
1528 Note that the EXPR can be arbitrarily complicated as long as the final
1529 operation is a hash or array key lookup or subroutine name:
1531 if (exists $ref->{A}->{B}->{$key}) { }
1532 if (exists $hash{A}{B}{$key}) { }
1534 if (exists $ref->{A}->{B}->[$ix]) { }
1535 if (exists $hash{A}{B}[$ix]) { }
1537 if (exists &{$ref->{A}{B}{$key}}) { }
1539 Although the deepest nested array or hash will not spring into existence
1540 just because its existence was tested, any intervening ones will.
1541 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1542 into existence due to the existence test for the $key element above.
1543 This happens anywhere the arrow operator is used, including even:
1546 if (exists $ref->{"Some key"}) { }
1547 print $ref; # prints HASH(0x80d3d5c)
1549 This surprising autovivification in what does not at first--or even
1550 second--glance appear to be an lvalue context may be fixed in a future
1553 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1554 on how exists() acts when used on a pseudo-hash.
1556 Use of a subroutine call, rather than a subroutine name, as an argument
1557 to exists() is an error.
1560 exists &sub(); # Error
1564 Evaluates EXPR and exits immediately with that value. Example:
1567 exit 0 if $ans =~ /^[Xx]/;
1569 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1570 universally recognized values for EXPR are C<0> for success and C<1>
1571 for error; other values are subject to interpretation depending on the
1572 environment in which the Perl program is running. For example, exiting
1573 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1574 the mailer to return the item undelivered, but that's not true everywhere.
1576 Don't use C<exit> to abort a subroutine if there's any chance that
1577 someone might want to trap whatever error happened. Use C<die> instead,
1578 which can be trapped by an C<eval>.
1580 The exit() function does not always exit immediately. It calls any
1581 defined C<END> routines first, but these C<END> routines may not
1582 themselves abort the exit. Likewise any object destructors that need to
1583 be called are called before the real exit. If this is a problem, you
1584 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1585 See L<perlmod> for details.
1591 Returns I<e> (the natural logarithm base) to the power of EXPR.
1592 If EXPR is omitted, gives C<exp($_)>.
1594 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1596 Implements the fcntl(2) function. You'll probably have to say
1600 first to get the correct constant definitions. Argument processing and
1601 value return works just like C<ioctl> below.
1605 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1606 or die "can't fcntl F_GETFL: $!";
1608 You don't have to check for C<defined> on the return from C<fnctl>.
1609 Like C<ioctl>, it maps a C<0> return from the system call into
1610 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1611 in numeric context. It is also exempt from the normal B<-w> warnings
1612 on improper numeric conversions.
1614 Note that C<fcntl> will produce a fatal error if used on a machine that
1615 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1616 manpage to learn what functions are available on your system.
1618 =item fileno FILEHANDLE
1620 Returns the file descriptor for a filehandle, or undefined if the
1621 filehandle is not open. This is mainly useful for constructing
1622 bitmaps for C<select> and low-level POSIX tty-handling operations.
1623 If FILEHANDLE is an expression, the value is taken as an indirect
1624 filehandle, generally its name.
1626 You can use this to find out whether two handles refer to the
1627 same underlying descriptor:
1629 if (fileno(THIS) == fileno(THAT)) {
1630 print "THIS and THAT are dups\n";
1633 (Filehandles connected to memory objects via new features of C<open> may
1634 return undefined even though they are open.)
1637 =item flock FILEHANDLE,OPERATION
1639 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1640 for success, false on failure. Produces a fatal error if used on a
1641 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1642 C<flock> is Perl's portable file locking interface, although it locks
1643 only entire files, not records.
1645 Two potentially non-obvious but traditional C<flock> semantics are
1646 that it waits indefinitely until the lock is granted, and that its locks
1647 B<merely advisory>. Such discretionary locks are more flexible, but offer
1648 fewer guarantees. This means that files locked with C<flock> may be
1649 modified by programs that do not also use C<flock>. See L<perlport>,
1650 your port's specific documentation, or your system-specific local manpages
1651 for details. It's best to assume traditional behavior if you're writing
1652 portable programs. (But if you're not, you should as always feel perfectly
1653 free to write for your own system's idiosyncrasies (sometimes called
1654 "features"). Slavish adherence to portability concerns shouldn't get
1655 in the way of your getting your job done.)
1657 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1658 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1659 you can use the symbolic names if you import them from the Fcntl module,
1660 either individually, or as a group using the ':flock' tag. LOCK_SH
1661 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1662 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1663 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1664 waiting for the lock (check the return status to see if you got it).
1666 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1667 before locking or unlocking it.
1669 Note that the emulation built with lockf(3) doesn't provide shared
1670 locks, and it requires that FILEHANDLE be open with write intent. These
1671 are the semantics that lockf(3) implements. Most if not all systems
1672 implement lockf(3) in terms of fcntl(2) locking, though, so the
1673 differing semantics shouldn't bite too many people.
1675 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1676 be open with read intent to use LOCK_SH and requires that it be open
1677 with write intent to use LOCK_EX.
1679 Note also that some versions of C<flock> cannot lock things over the
1680 network; you would need to use the more system-specific C<fcntl> for
1681 that. If you like you can force Perl to ignore your system's flock(2)
1682 function, and so provide its own fcntl(2)-based emulation, by passing
1683 the switch C<-Ud_flock> to the F<Configure> program when you configure
1686 Here's a mailbox appender for BSD systems.
1688 use Fcntl ':flock'; # import LOCK_* constants
1691 flock(MBOX,LOCK_EX);
1692 # and, in case someone appended
1693 # while we were waiting...
1698 flock(MBOX,LOCK_UN);
1701 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1702 or die "Can't open mailbox: $!";
1705 print MBOX $msg,"\n\n";
1708 On systems that support a real flock(), locks are inherited across fork()
1709 calls, whereas those that must resort to the more capricious fcntl()
1710 function lose the locks, making it harder to write servers.
1712 See also L<DB_File> for other flock() examples.
1716 Does a fork(2) system call to create a new process running the
1717 same program at the same point. It returns the child pid to the
1718 parent process, C<0> to the child process, or C<undef> if the fork is
1719 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1720 are shared, while everything else is copied. On most systems supporting
1721 fork(), great care has gone into making it extremely efficient (for
1722 example, using copy-on-write technology on data pages), making it the
1723 dominant paradigm for multitasking over the last few decades.
1725 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1726 output before forking the child process, but this may not be supported
1727 on some platforms (see L<perlport>). To be safe, you may need to set
1728 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1729 C<IO::Handle> on any open handles in order to avoid duplicate output.
1731 If you C<fork> without ever waiting on your children, you will
1732 accumulate zombies. On some systems, you can avoid this by setting
1733 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1734 forking and reaping moribund children.
1736 Note that if your forked child inherits system file descriptors like
1737 STDIN and STDOUT that are actually connected by a pipe or socket, even
1738 if you exit, then the remote server (such as, say, a CGI script or a
1739 backgrounded job launched from a remote shell) won't think you're done.
1740 You should reopen those to F</dev/null> if it's any issue.
1744 Declare a picture format for use by the C<write> function. For
1748 Test: @<<<<<<<< @||||| @>>>>>
1749 $str, $%, '$' . int($num)
1753 $num = $cost/$quantity;
1757 See L<perlform> for many details and examples.
1759 =item formline PICTURE,LIST
1761 This is an internal function used by C<format>s, though you may call it,
1762 too. It formats (see L<perlform>) a list of values according to the
1763 contents of PICTURE, placing the output into the format output
1764 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1765 Eventually, when a C<write> is done, the contents of
1766 C<$^A> are written to some filehandle, but you could also read C<$^A>
1767 yourself and then set C<$^A> back to C<"">. Note that a format typically
1768 does one C<formline> per line of form, but the C<formline> function itself
1769 doesn't care how many newlines are embedded in the PICTURE. This means
1770 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1771 You may therefore need to use multiple formlines to implement a single
1772 record format, just like the format compiler.
1774 Be careful if you put double quotes around the picture, because an C<@>
1775 character may be taken to mean the beginning of an array name.
1776 C<formline> always returns true. See L<perlform> for other examples.
1778 =item getc FILEHANDLE
1782 Returns the next character from the input file attached to FILEHANDLE,
1783 or the undefined value at end of file, or if there was an error.
1784 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1785 efficient. However, it cannot be used by itself to fetch single
1786 characters without waiting for the user to hit enter. For that, try
1787 something more like:
1790 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1793 system "stty", '-icanon', 'eol', "\001";
1799 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1802 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1806 Determination of whether $BSD_STYLE should be set
1807 is left as an exercise to the reader.
1809 The C<POSIX::getattr> function can do this more portably on
1810 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1811 module from your nearest CPAN site; details on CPAN can be found on
1816 Implements the C library function of the same name, which on most
1817 systems returns the current login from F</etc/utmp>, if any. If null,
1820 $login = getlogin || getpwuid($<) || "Kilroy";
1822 Do not consider C<getlogin> for authentication: it is not as
1823 secure as C<getpwuid>.
1825 =item getpeername SOCKET
1827 Returns the packed sockaddr address of other end of the SOCKET connection.
1830 $hersockaddr = getpeername(SOCK);
1831 ($port, $iaddr) = sockaddr_in($hersockaddr);
1832 $herhostname = gethostbyaddr($iaddr, AF_INET);
1833 $herstraddr = inet_ntoa($iaddr);
1837 Returns the current process group for the specified PID. Use
1838 a PID of C<0> to get the current process group for the
1839 current process. Will raise an exception if used on a machine that
1840 doesn't implement getpgrp(2). If PID is omitted, returns process
1841 group of current process. Note that the POSIX version of C<getpgrp>
1842 does not accept a PID argument, so only C<PID==0> is truly portable.
1846 Returns the process id of the parent process.
1848 =item getpriority WHICH,WHO
1850 Returns the current priority for a process, a process group, or a user.
1851 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1852 machine that doesn't implement getpriority(2).
1858 =item gethostbyname NAME
1860 =item getnetbyname NAME
1862 =item getprotobyname NAME
1868 =item getservbyname NAME,PROTO
1870 =item gethostbyaddr ADDR,ADDRTYPE
1872 =item getnetbyaddr ADDR,ADDRTYPE
1874 =item getprotobynumber NUMBER
1876 =item getservbyport PORT,PROTO
1894 =item sethostent STAYOPEN
1896 =item setnetent STAYOPEN
1898 =item setprotoent STAYOPEN
1900 =item setservent STAYOPEN
1914 These routines perform the same functions as their counterparts in the
1915 system library. In list context, the return values from the
1916 various get routines are as follows:
1918 ($name,$passwd,$uid,$gid,
1919 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1920 ($name,$passwd,$gid,$members) = getgr*
1921 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1922 ($name,$aliases,$addrtype,$net) = getnet*
1923 ($name,$aliases,$proto) = getproto*
1924 ($name,$aliases,$port,$proto) = getserv*
1926 (If the entry doesn't exist you get a null list.)
1928 The exact meaning of the $gcos field varies but it usually contains
1929 the real name of the user (as opposed to the login name) and other
1930 information pertaining to the user. Beware, however, that in many
1931 system users are able to change this information and therefore it
1932 cannot be trusted and therefore the $gcos is tainted (see
1933 L<perlsec>). The $passwd and $shell, user's encrypted password and
1934 login shell, are also tainted, because of the same reason.
1936 In scalar context, you get the name, unless the function was a
1937 lookup by name, in which case you get the other thing, whatever it is.
1938 (If the entry doesn't exist you get the undefined value.) For example:
1940 $uid = getpwnam($name);
1941 $name = getpwuid($num);
1943 $gid = getgrnam($name);
1944 $name = getgrgid($num;
1948 In I<getpw*()> the fields $quota, $comment, and $expire are special
1949 cases in the sense that in many systems they are unsupported. If the
1950 $quota is unsupported, it is an empty scalar. If it is supported, it
1951 usually encodes the disk quota. If the $comment field is unsupported,
1952 it is an empty scalar. If it is supported it usually encodes some
1953 administrative comment about the user. In some systems the $quota
1954 field may be $change or $age, fields that have to do with password
1955 aging. In some systems the $comment field may be $class. The $expire
1956 field, if present, encodes the expiration period of the account or the
1957 password. For the availability and the exact meaning of these fields
1958 in your system, please consult your getpwnam(3) documentation and your
1959 F<pwd.h> file. You can also find out from within Perl what your
1960 $quota and $comment fields mean and whether you have the $expire field
1961 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1962 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1963 files are only supported if your vendor has implemented them in the
1964 intuitive fashion that calling the regular C library routines gets the
1965 shadow versions if you're running under privilege or if there exists
1966 the shadow(3) functions as found in System V ( this includes Solaris
1967 and Linux.) Those systems which implement a proprietary shadow password
1968 facility are unlikely to be supported.
1970 The $members value returned by I<getgr*()> is a space separated list of
1971 the login names of the members of the group.
1973 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1974 C, it will be returned to you via C<$?> if the function call fails. The
1975 C<@addrs> value returned by a successful call is a list of the raw
1976 addresses returned by the corresponding system library call. In the
1977 Internet domain, each address is four bytes long and you can unpack it
1978 by saying something like:
1980 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1982 The Socket library makes this slightly easier:
1985 $iaddr = inet_aton("127.1"); # or whatever address
1986 $name = gethostbyaddr($iaddr, AF_INET);
1988 # or going the other way
1989 $straddr = inet_ntoa($iaddr);
1991 If you get tired of remembering which element of the return list
1992 contains which return value, by-name interfaces are provided
1993 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1994 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1995 and C<User::grent>. These override the normal built-ins, supplying
1996 versions that return objects with the appropriate names
1997 for each field. For example:
2001 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2003 Even though it looks like they're the same method calls (uid),
2004 they aren't, because a C<File::stat> object is different from
2005 a C<User::pwent> object.
2007 =item getsockname SOCKET
2009 Returns the packed sockaddr address of this end of the SOCKET connection,
2010 in case you don't know the address because you have several different
2011 IPs that the connection might have come in on.
2014 $mysockaddr = getsockname(SOCK);
2015 ($port, $myaddr) = sockaddr_in($mysockaddr);
2016 printf "Connect to %s [%s]\n",
2017 scalar gethostbyaddr($myaddr, AF_INET),
2020 =item getsockopt SOCKET,LEVEL,OPTNAME
2022 Returns the socket option requested, or undef if there is an error.
2028 Returns the value of EXPR with filename expansions such as the
2029 standard Unix shell F</bin/csh> would do. This is the internal function
2030 implementing the C<< <*.c> >> operator, but you can use it directly.
2031 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2032 discussed in more detail in L<perlop/"I/O Operators">.
2034 Beginning with v5.6.0, this operator is implemented using the standard
2035 C<File::Glob> extension. See L<File::Glob> for details.
2039 Converts a time as returned by the time function to an 8-element list
2040 with the time localized for the standard Greenwich time zone.
2041 Typically used as follows:
2044 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2047 All list elements are numeric, and come straight out of the C `struct
2048 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2049 specified time. $mday is the day of the month, and $mon is the month
2050 itself, in the range C<0..11> with 0 indicating January and 11
2051 indicating December. $year is the number of years since 1900. That
2052 is, $year is C<123> in year 2023. $wday is the day of the week, with
2053 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2054 the year, in the range C<0..364> (or C<0..365> in leap years.)
2056 Note that the $year element is I<not> simply the last two digits of
2057 the year. If you assume it is, then you create non-Y2K-compliant
2058 programs--and you wouldn't want to do that, would you?
2060 The proper way to get a complete 4-digit year is simply:
2064 And to get the last two digits of the year (e.g., '01' in 2001) do:
2066 $year = sprintf("%02d", $year % 100);
2068 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2070 In scalar context, C<gmtime()> returns the ctime(3) value:
2072 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2074 Also see the C<timegm> function provided by the C<Time::Local> module,
2075 and the strftime(3) function available via the POSIX module.
2077 This scalar value is B<not> locale dependent (see L<perllocale>), but
2078 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2079 strftime(3) and mktime(3) functions available via the POSIX module. To
2080 get somewhat similar but locale dependent date strings, set up your
2081 locale environment variables appropriately (please see L<perllocale>)
2082 and try for example:
2084 use POSIX qw(strftime);
2085 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2087 Note that the C<%a> and C<%b> escapes, which represent the short forms
2088 of the day of the week and the month of the year, may not necessarily
2089 be three characters wide in all locales.
2097 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2098 execution there. It may not be used to go into any construct that
2099 requires initialization, such as a subroutine or a C<foreach> loop. It
2100 also can't be used to go into a construct that is optimized away,
2101 or to get out of a block or subroutine given to C<sort>.
2102 It can be used to go almost anywhere else within the dynamic scope,
2103 including out of subroutines, but it's usually better to use some other
2104 construct such as C<last> or C<die>. The author of Perl has never felt the
2105 need to use this form of C<goto> (in Perl, that is--C is another matter).
2106 (The difference being that C does not offer named loops combined with
2107 loop control. Perl does, and this replaces most structured uses of C<goto>
2108 in other languages.)
2110 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2111 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2112 necessarily recommended if you're optimizing for maintainability:
2114 goto ("FOO", "BAR", "GLARCH")[$i];
2116 The C<goto-&NAME> form is quite different from the other forms of
2117 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2118 doesn't have the stigma associated with other gotos. Instead, it
2119 exits the current subroutine (losing any changes set by local()) and
2120 immediately calls in its place the named subroutine using the current
2121 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2122 load another subroutine and then pretend that the other subroutine had
2123 been called in the first place (except that any modifications to C<@_>
2124 in the current subroutine are propagated to the other subroutine.)
2125 After the C<goto>, not even C<caller> will be able to tell that this
2126 routine was called first.
2128 NAME needn't be the name of a subroutine; it can be a scalar variable
2129 containing a code reference, or a block which evaluates to a code
2132 =item grep BLOCK LIST
2134 =item grep EXPR,LIST
2136 This is similar in spirit to, but not the same as, grep(1) and its
2137 relatives. In particular, it is not limited to using regular expressions.
2139 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2140 C<$_> to each element) and returns the list value consisting of those
2141 elements for which the expression evaluated to true. In scalar
2142 context, returns the number of times the expression was true.
2144 @foo = grep(!/^#/, @bar); # weed out comments
2148 @foo = grep {!/^#/} @bar; # weed out comments
2150 Note that C<$_> is an alias to the list value, so it can be used to
2151 modify the elements of the LIST. While this is useful and supported,
2152 it can cause bizarre results if the elements of LIST are not variables.
2153 Similarly, grep returns aliases into the original list, much as a for
2154 loop's index variable aliases the list elements. That is, modifying an
2155 element of a list returned by grep (for example, in a C<foreach>, C<map>
2156 or another C<grep>) actually modifies the element in the original list.
2157 This is usually something to be avoided when writing clear code.
2159 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2165 Interprets EXPR as a hex string and returns the corresponding value.
2166 (To convert strings that might start with either 0, 0x, or 0b, see
2167 L</oct>.) If EXPR is omitted, uses C<$_>.
2169 print hex '0xAf'; # prints '175'
2170 print hex 'aF'; # same
2172 Hex strings may only represent integers. Strings that would cause
2173 integer overflow trigger a warning. Leading whitespace is not stripped,
2178 There is no builtin C<import> function. It is just an ordinary
2179 method (subroutine) defined (or inherited) by modules that wish to export
2180 names to another module. The C<use> function calls the C<import> method
2181 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2183 =item index STR,SUBSTR,POSITION
2185 =item index STR,SUBSTR
2187 The index function searches for one string within another, but without
2188 the wildcard-like behavior of a full regular-expression pattern match.
2189 It returns the position of the first occurrence of SUBSTR in STR at
2190 or after POSITION. If POSITION is omitted, starts searching from the
2191 beginning of the string. The return value is based at C<0> (or whatever
2192 you've set the C<$[> variable to--but don't do that). If the substring
2193 is not found, returns one less than the base, ordinarily C<-1>.
2199 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2200 You should not use this function for rounding: one because it truncates
2201 towards C<0>, and two because machine representations of floating point
2202 numbers can sometimes produce counterintuitive results. For example,
2203 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2204 because it's really more like -268.99999999999994315658 instead. Usually,
2205 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2206 functions will serve you better than will int().
2208 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2210 Implements the ioctl(2) function. You'll probably first have to say
2212 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2214 to get the correct function definitions. If F<ioctl.ph> doesn't
2215 exist or doesn't have the correct definitions you'll have to roll your
2216 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2217 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2218 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2219 written depending on the FUNCTION--a pointer to the string value of SCALAR
2220 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2221 has no string value but does have a numeric value, that value will be
2222 passed rather than a pointer to the string value. To guarantee this to be
2223 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2224 functions may be needed to manipulate the values of structures used by
2227 The return value of C<ioctl> (and C<fcntl>) is as follows:
2229 if OS returns: then Perl returns:
2231 0 string "0 but true"
2232 anything else that number
2234 Thus Perl returns true on success and false on failure, yet you can
2235 still easily determine the actual value returned by the operating
2238 $retval = ioctl(...) || -1;
2239 printf "System returned %d\n", $retval;
2241 The special string "C<0> but true" is exempt from B<-w> complaints
2242 about improper numeric conversions.
2244 Here's an example of setting a filehandle named C<REMOTE> to be
2245 non-blocking at the system level. You'll have to negotiate C<$|>
2246 on your own, though.
2248 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2250 $flags = fcntl(REMOTE, F_GETFL, 0)
2251 or die "Can't get flags for the socket: $!\n";
2253 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2254 or die "Can't set flags for the socket: $!\n";
2256 =item join EXPR,LIST
2258 Joins the separate strings of LIST into a single string with fields
2259 separated by the value of EXPR, and returns that new string. Example:
2261 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2263 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2264 first argument. Compare L</split>.
2268 Returns a list consisting of all the keys of the named hash. (In
2269 scalar context, returns the number of keys.) The keys are returned in
2270 an apparently random order. The actual random order is subject to
2271 change in future versions of perl, but it is guaranteed to be the same
2272 order as either the C<values> or C<each> function produces (given
2273 that the hash has not been modified). As a side effect, it resets
2276 Here is yet another way to print your environment:
2279 @values = values %ENV;
2281 print pop(@keys), '=', pop(@values), "\n";
2284 or how about sorted by key:
2286 foreach $key (sort(keys %ENV)) {
2287 print $key, '=', $ENV{$key}, "\n";
2290 The returned values are copies of the original keys in the hash, so
2291 modifying them will not affect the original hash. Compare L</values>.
2293 To sort a hash by value, you'll need to use a C<sort> function.
2294 Here's a descending numeric sort of a hash by its values:
2296 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2297 printf "%4d %s\n", $hash{$key}, $key;
2300 As an lvalue C<keys> allows you to increase the number of hash buckets
2301 allocated for the given hash. This can gain you a measure of efficiency if
2302 you know the hash is going to get big. (This is similar to pre-extending
2303 an array by assigning a larger number to $#array.) If you say
2307 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2308 in fact, since it rounds up to the next power of two. These
2309 buckets will be retained even if you do C<%hash = ()>, use C<undef
2310 %hash> if you want to free the storage while C<%hash> is still in scope.
2311 You can't shrink the number of buckets allocated for the hash using
2312 C<keys> in this way (but you needn't worry about doing this by accident,
2313 as trying has no effect).
2315 See also C<each>, C<values> and C<sort>.
2317 =item kill SIGNAL, LIST
2319 Sends a signal to a list of processes. Returns the number of
2320 processes successfully signaled (which is not necessarily the
2321 same as the number actually killed).
2323 $cnt = kill 1, $child1, $child2;
2326 If SIGNAL is zero, no signal is sent to the process. This is a
2327 useful way to check that the process is alive and hasn't changed
2328 its UID. See L<perlport> for notes on the portability of this
2331 Unlike in the shell, if SIGNAL is negative, it kills
2332 process groups instead of processes. (On System V, a negative I<PROCESS>
2333 number will also kill process groups, but that's not portable.) That
2334 means you usually want to use positive not negative signals. You may also
2335 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2341 The C<last> command is like the C<break> statement in C (as used in
2342 loops); it immediately exits the loop in question. If the LABEL is
2343 omitted, the command refers to the innermost enclosing loop. The
2344 C<continue> block, if any, is not executed:
2346 LINE: while (<STDIN>) {
2347 last LINE if /^$/; # exit when done with header
2351 C<last> cannot be used to exit a block which returns a value such as
2352 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2353 a grep() or map() operation.
2355 Note that a block by itself is semantically identical to a loop
2356 that executes once. Thus C<last> can be used to effect an early
2357 exit out of such a block.
2359 See also L</continue> for an illustration of how C<last>, C<next>, and
2366 Returns a lowercased version of EXPR. This is the internal function
2367 implementing the C<\L> escape in double-quoted strings. Respects
2368 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2369 and L<perlunicode> for more details about locale and Unicode support.
2371 If EXPR is omitted, uses C<$_>.
2377 Returns the value of EXPR with the first character lowercased. This
2378 is the internal function implementing the C<\l> escape in
2379 double-quoted strings. Respects current LC_CTYPE locale if C<use
2380 locale> in force. See L<perllocale> and L<perlunicode> for more
2381 details about locale and Unicode support.
2383 If EXPR is omitted, uses C<$_>.
2389 Returns the length in characters of the value of EXPR. If EXPR is
2390 omitted, returns length of C<$_>. Note that this cannot be used on
2391 an entire array or hash to find out how many elements these have.
2392 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2394 =item link OLDFILE,NEWFILE
2396 Creates a new filename linked to the old filename. Returns true for
2397 success, false otherwise.
2399 =item listen SOCKET,QUEUESIZE
2401 Does the same thing that the listen system call does. Returns true if
2402 it succeeded, false otherwise. See the example in
2403 L<perlipc/"Sockets: Client/Server Communication">.
2407 You really probably want to be using C<my> instead, because C<local> isn't
2408 what most people think of as "local". See
2409 L<perlsub/"Private Variables via my()"> for details.
2411 A local modifies the listed variables to be local to the enclosing
2412 block, file, or eval. If more than one value is listed, the list must
2413 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2414 for details, including issues with tied arrays and hashes.
2416 =item localtime EXPR
2418 Converts a time as returned by the time function to a 9-element list
2419 with the time analyzed for the local time zone. Typically used as
2423 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2426 All list elements are numeric, and come straight out of the C `struct
2427 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2428 specified time. $mday is the day of the month, and $mon is the month
2429 itself, in the range C<0..11> with 0 indicating January and 11
2430 indicating December. $year is the number of years since 1900. That
2431 is, $year is C<123> in year 2023. $wday is the day of the week, with
2432 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2433 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2434 is true if the specified time occurs during daylight savings time,
2437 Note that the $year element is I<not> simply the last two digits of
2438 the year. If you assume it is, then you create non-Y2K-compliant
2439 programs--and you wouldn't want to do that, would you?
2441 The proper way to get a complete 4-digit year is simply:
2445 And to get the last two digits of the year (e.g., '01' in 2001) do:
2447 $year = sprintf("%02d", $year % 100);
2449 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2451 In scalar context, C<localtime()> returns the ctime(3) value:
2453 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2455 This scalar value is B<not> locale dependent, see L<perllocale>, but
2456 instead a Perl builtin. Also see the C<Time::Local> module
2457 (to convert the second, minutes, hours, ... back to seconds since the
2458 stroke of midnight the 1st of January 1970, the value returned by
2459 time()), and the strftime(3) and mktime(3) functions available via the
2460 POSIX module. To get somewhat similar but locale dependent date
2461 strings, set up your locale environment variables appropriately
2462 (please see L<perllocale>) and try for example:
2464 use POSIX qw(strftime);
2465 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2467 Note that the C<%a> and C<%b>, the short forms of the day of the week
2468 and the month of the year, may not necessarily be three characters wide.
2472 This function places an advisory lock on a variable, subroutine,
2473 or referenced object contained in I<THING> until the lock goes out
2476 lock() is a "weak keyword" : this means that if you've defined a function
2477 by this name (before any calls to it), that function will be called
2478 instead. (However, if you've said C<use Thread>, lock() is always a
2479 keyword.) See L<Thread>.
2485 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2486 returns log of C<$_>. To get the log of another base, use basic algebra:
2487 The base-N log of a number is equal to the natural log of that number
2488 divided by the natural log of N. For example:
2492 return log($n)/log(10);
2495 See also L</exp> for the inverse operation.
2501 Does the same thing as the C<stat> function (including setting the
2502 special C<_> filehandle) but stats a symbolic link instead of the file
2503 the symbolic link points to. If symbolic links are unimplemented on
2504 your system, a normal C<stat> is done.
2506 If EXPR is omitted, stats C<$_>.
2510 The match operator. See L<perlop>.
2512 =item map BLOCK LIST
2516 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2517 C<$_> to each element) and returns the list value composed of the
2518 results of each such evaluation. In scalar context, returns the
2519 total number of elements so generated. Evaluates BLOCK or EXPR in
2520 list context, so each element of LIST may produce zero, one, or
2521 more elements in the returned value.
2523 @chars = map(chr, @nums);
2525 translates a list of numbers to the corresponding characters. And
2527 %hash = map { getkey($_) => $_ } @array;
2529 is just a funny way to write
2532 foreach $_ (@array) {
2533 $hash{getkey($_)} = $_;
2536 Note that C<$_> is an alias to the list value, so it can be used to
2537 modify the elements of the LIST. While this is useful and supported,
2538 it can cause bizarre results if the elements of LIST are not variables.
2539 Using a regular C<foreach> loop for this purpose would be clearer in
2540 most cases. See also L</grep> for an array composed of those items of
2541 the original list for which the BLOCK or EXPR evaluates to true.
2543 C<{> starts both hash references and blocks, so C<map { ...> could be either
2544 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2545 ahead for the closing C<}> it has to take a guess at which its dealing with
2546 based what it finds just after the C<{>. Usually it gets it right, but if it
2547 doesn't it won't realize something is wrong until it gets to the C<}> and
2548 encounters the missing (or unexpected) comma. The syntax error will be
2549 reported close to the C<}> but you'll need to change something near the C<{>
2550 such as using a unary C<+> to give perl some help:
2552 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2553 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2554 %hash = map { ("\L$_", 1) } @array # this also works
2555 %hash = map { lc($_), 1 } @array # as does this.
2556 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2558 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2560 or to force an anon hash constructor use C<+{>
2562 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2564 and you get list of anonymous hashes each with only 1 entry.
2566 =item mkdir FILENAME,MASK
2568 =item mkdir FILENAME
2570 Creates the directory specified by FILENAME, with permissions
2571 specified by MASK (as modified by C<umask>). If it succeeds it
2572 returns true, otherwise it returns false and sets C<$!> (errno).
2573 If omitted, MASK defaults to 0777.
2575 In general, it is better to create directories with permissive MASK,
2576 and let the user modify that with their C<umask>, than it is to supply
2577 a restrictive MASK and give the user no way to be more permissive.
2578 The exceptions to this rule are when the file or directory should be
2579 kept private (mail files, for instance). The perlfunc(1) entry on
2580 C<umask> discusses the choice of MASK in more detail.
2582 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2583 number of trailing slashes. Some operating and filesystems do not get
2584 this right, so Perl automatically removes all trailing slashes to keep
2587 =item msgctl ID,CMD,ARG
2589 Calls the System V IPC function msgctl(2). You'll probably have to say
2593 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2594 then ARG must be a variable which will hold the returned C<msqid_ds>
2595 structure. Returns like C<ioctl>: the undefined value for error,
2596 C<"0 but true"> for zero, or the actual return value otherwise. See also
2597 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2599 =item msgget KEY,FLAGS
2601 Calls the System V IPC function msgget(2). Returns the message queue
2602 id, or the undefined value if there is an error. See also
2603 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2605 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2607 Calls the System V IPC function msgrcv to receive a message from
2608 message queue ID into variable VAR with a maximum message size of
2609 SIZE. Note that when a message is received, the message type as a
2610 native long integer will be the first thing in VAR, followed by the
2611 actual message. This packing may be opened with C<unpack("l! a*")>.
2612 Taints the variable. Returns true if successful, or false if there is
2613 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2614 C<IPC::SysV::Msg> documentation.
2616 =item msgsnd ID,MSG,FLAGS
2618 Calls the System V IPC function msgsnd to send the message MSG to the
2619 message queue ID. MSG must begin with the native long integer message
2620 type, and be followed by the length of the actual message, and finally
2621 the message itself. This kind of packing can be achieved with
2622 C<pack("l! a*", $type, $message)>. Returns true if successful,
2623 or false if there is an error. See also C<IPC::SysV>
2624 and C<IPC::SysV::Msg> documentation.
2628 =item my EXPR : ATTRIBUTES
2630 A C<my> declares the listed variables to be local (lexically) to the
2631 enclosing block, file, or C<eval>. If
2632 more than one value is listed, the list must be placed in parentheses. See
2633 L<perlsub/"Private Variables via my()"> for details.
2639 The C<next> command is like the C<continue> statement in C; it starts
2640 the next iteration of the loop:
2642 LINE: while (<STDIN>) {
2643 next LINE if /^#/; # discard comments
2647 Note that if there were a C<continue> block on the above, it would get
2648 executed even on discarded lines. If the LABEL is omitted, the command
2649 refers to the innermost enclosing loop.
2651 C<next> cannot be used to exit a block which returns a value such as
2652 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2653 a grep() or map() operation.
2655 Note that a block by itself is semantically identical to a loop
2656 that executes once. Thus C<next> will exit such a block early.
2658 See also L</continue> for an illustration of how C<last>, C<next>, and
2661 =item no Module VERSION LIST
2663 =item no Module VERSION
2665 =item no Module LIST
2669 See the L</use> function, which C<no> is the opposite of.
2675 Interprets EXPR as an octal string and returns the corresponding
2676 value. (If EXPR happens to start off with C<0x>, interprets it as a
2677 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2678 binary string. Leading whitespace is ignored in all three cases.)
2679 The following will handle decimal, binary, octal, and hex in the standard
2682 $val = oct($val) if $val =~ /^0/;
2684 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2685 in octal), use sprintf() or printf():
2687 $perms = (stat("filename"))[2] & 07777;
2688 $oct_perms = sprintf "%lo", $perms;
2690 The oct() function is commonly used when a string such as C<644> needs
2691 to be converted into a file mode, for example. (Although perl will
2692 automatically convert strings into numbers as needed, this automatic
2693 conversion assumes base 10.)
2695 =item open FILEHANDLE,EXPR
2697 =item open FILEHANDLE,MODE,EXPR
2699 =item open FILEHANDLE,MODE,EXPR,LIST
2701 =item open FILEHANDLE,MODE,REFERENCE
2703 =item open FILEHANDLE
2705 Opens the file whose filename is given by EXPR, and associates it with
2708 (The following is a comprehensive reference to open(): for a gentler
2709 introduction you may consider L<perlopentut>.)
2711 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2712 assigned a reference to a new anonymous filehandle, otherwise if
2713 FILEHANDLE is an expression, its value is used as the name of the real
2714 filehandle wanted. (This is considered a symbolic reference, so C<use
2715 strict 'refs'> should I<not> be in effect.)
2717 If EXPR is omitted, the scalar variable of the same name as the
2718 FILEHANDLE contains the filename. (Note that lexical variables--those
2719 declared with C<my>--will not work for this purpose; so if you're
2720 using C<my>, specify EXPR in your call to open.)
2722 If three or more arguments are specified then the mode of opening and
2723 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2724 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2725 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2726 the file is opened for appending, again being created if necessary.
2728 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2729 indicate that you want both read and write access to the file; thus
2730 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2731 '+>' >> mode would clobber the file first. You can't usually use
2732 either read-write mode for updating textfiles, since they have
2733 variable length records. See the B<-i> switch in L<perlrun> for a
2734 better approach. The file is created with permissions of C<0666>
2735 modified by the process' C<umask> value.
2737 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2738 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2740 In the 2-arguments (and 1-argument) form of the call the mode and
2741 filename should be concatenated (in this order), possibly separated by
2742 spaces. It is possible to omit the mode in these forms if the mode is
2745 If the filename begins with C<'|'>, the filename is interpreted as a
2746 command to which output is to be piped, and if the filename ends with a
2747 C<'|'>, the filename is interpreted as a command which pipes output to
2748 us. See L<perlipc/"Using open() for IPC">
2749 for more examples of this. (You are not allowed to C<open> to a command
2750 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2751 and L<perlipc/"Bidirectional Communication with Another Process">
2754 For three or more arguments if MODE is C<'|-'>, the filename is
2755 interpreted as a command to which output is to be piped, and if MODE
2756 is C<'-|'>, the filename is interpreted as a command which pipes
2757 output to us. In the 2-arguments (and 1-argument) form one should
2758 replace dash (C<'-'>) with the command.
2759 See L<perlipc/"Using open() for IPC"> for more examples of this.
2760 (You are not allowed to C<open> to a command that pipes both in I<and>
2761 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2762 L<perlipc/"Bidirectional Communication"> for alternatives.)
2764 In the three-or-more argument form of pipe opens, if LIST is specified
2765 (extra arguments after the command name) then LIST becomes arguments
2766 to the command invoked if the platform supports it. The meaning of
2767 C<open> with more than three arguments for non-pipe modes is not yet
2768 specified. Experimental "layers" may give extra LIST arguments
2771 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2772 and opening C<< '>-' >> opens STDOUT.
2774 You may use the three-argument form of open to specify
2775 I<I/O disciplines> that affect how the input and output
2776 are processed: see L</binmode> and L<open>. For example
2778 open(FH, "<:utf8", "file")
2780 will open the UTF-8 encoded file containing Unicode characters,
2781 see L<perluniintro>.
2783 Open returns nonzero upon success, the undefined value otherwise. If
2784 the C<open> involved a pipe, the return value happens to be the pid of
2787 If you're running Perl on a system that distinguishes between text
2788 files and binary files, then you should check out L</binmode> for tips
2789 for dealing with this. The key distinction between systems that need
2790 C<binmode> and those that don't is their text file formats. Systems
2791 like Unix, MacOS, and Plan9, which delimit lines with a single
2792 character, and which encode that character in C as C<"\n">, do not
2793 need C<binmode>. The rest need it.
2795 In the three argument form MODE may also contain a list of IO "layers"
2796 (see L<open> and L<PerlIO> for more details) to be applied to the
2797 handle. This can be used to achieve the effect of C<binmode> as well
2798 as more complex behaviours.
2800 When opening a file, it's usually a bad idea to continue normal execution
2801 if the request failed, so C<open> is frequently used in connection with
2802 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2803 where you want to make a nicely formatted error message (but there are
2804 modules that can help with that problem)) you should always check
2805 the return value from opening a file. The infrequent exception is when
2806 working with an unopened filehandle is actually what you want to do.
2808 As a special case the 3 arg form with a read/write mode and the third
2809 argument being C<undef>:
2811 open(TMP, "+>", undef) or die ...
2813 opens a filehandle to an anonymous temporary file.
2815 File handles can be opened to "in memory" files held in Perl scalars via:
2817 open($fh, '>', \$variable) || ..
2819 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2820 file, you have to close it first:
2823 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2828 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2829 while (<ARTICLE>) {...
2831 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2832 # if the open fails, output is discarded
2834 open(DBASE, '+<', 'dbase.mine') # open for update
2835 or die "Can't open 'dbase.mine' for update: $!";
2837 open(DBASE, '+<dbase.mine') # ditto
2838 or die "Can't open 'dbase.mine' for update: $!";
2840 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2841 or die "Can't start caesar: $!";
2843 open(ARTICLE, "caesar <$article |") # ditto
2844 or die "Can't start caesar: $!";
2846 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2847 or die "Can't start sort: $!";
2850 open(MEMORY,'>', \$var)
2851 or die "Can't open memory file: $!";
2852 print MEMORY "foo!\n"; # output will end up in $var
2854 # process argument list of files along with any includes
2856 foreach $file (@ARGV) {
2857 process($file, 'fh00');
2861 my($filename, $input) = @_;
2862 $input++; # this is a string increment
2863 unless (open($input, $filename)) {
2864 print STDERR "Can't open $filename: $!\n";
2869 while (<$input>) { # note use of indirection
2870 if (/^#include "(.*)"/) {
2871 process($1, $input);
2878 You may also, in the Bourne shell tradition, specify an EXPR beginning
2879 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2880 name of a filehandle (or file descriptor, if numeric) to be
2881 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2882 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2883 mode you specify should match the mode of the original filehandle.
2884 (Duping a filehandle does not take into account any existing contents of
2885 IO buffers.) If you use the 3 arg form then you can pass either a number,
2886 the name of a filehandle or the normal "reference to a glob".
2888 Here is a script that saves, redirects, and restores C<STDOUT> and
2889 C<STDERR> using various methods:
2892 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2893 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2895 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2896 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2898 select STDERR; $| = 1; # make unbuffered
2899 select STDOUT; $| = 1; # make unbuffered
2901 print STDOUT "stdout 1\n"; # this works for
2902 print STDERR "stderr 1\n"; # subprocesses too
2907 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2908 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2910 print STDOUT "stdout 2\n";
2911 print STDERR "stderr 2\n";
2913 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2914 do an equivalent of C's C<fdopen> of that file descriptor; this is
2915 more parsimonious of file descriptors. For example:
2917 open(FILEHANDLE, "<&=$fd")
2921 open(FILEHANDLE, "<&=", $fd)
2923 Note that if Perl is using the standard C libraries' fdopen() then on
2924 many UNIX systems, fdopen() is known to fail when file descriptors
2925 exceed a certain value, typically 255. If you need more file
2926 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2928 You can see whether Perl has been compiled with PerlIO or not by
2929 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2930 is C<define>, you have PerlIO, otherwise you don't.
2932 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2933 with 2-arguments (or 1-argument) form of open(), then
2934 there is an implicit fork done, and the return value of open is the pid
2935 of the child within the parent process, and C<0> within the child
2936 process. (Use C<defined($pid)> to determine whether the open was successful.)
2937 The filehandle behaves normally for the parent, but i/o to that
2938 filehandle is piped from/to the STDOUT/STDIN of the child process.
2939 In the child process the filehandle isn't opened--i/o happens from/to
2940 the new STDOUT or STDIN. Typically this is used like the normal
2941 piped open when you want to exercise more control over just how the
2942 pipe command gets executed, such as when you are running setuid, and
2943 don't want to have to scan shell commands for metacharacters.
2944 The following triples are more or less equivalent:
2946 open(FOO, "|tr '[a-z]' '[A-Z]'");
2947 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2948 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2949 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2951 open(FOO, "cat -n '$file'|");
2952 open(FOO, '-|', "cat -n '$file'");
2953 open(FOO, '-|') || exec 'cat', '-n', $file;
2954 open(FOO, '-|', "cat", '-n', $file);
2956 The last example in each block shows the pipe as "list form", which is
2957 not yet supported on all platforms.
2959 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2961 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2962 output before any operation that may do a fork, but this may not be
2963 supported on some platforms (see L<perlport>). To be safe, you may need
2964 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2965 of C<IO::Handle> on any open handles.
2967 On systems that support a close-on-exec flag on files, the flag will
2968 be set for the newly opened file descriptor as determined by the value
2969 of $^F. See L<perlvar/$^F>.
2971 Closing any piped filehandle causes the parent process to wait for the
2972 child to finish, and returns the status value in C<$?>.
2974 The filename passed to 2-argument (or 1-argument) form of open() will
2975 have leading and trailing whitespace deleted, and the normal
2976 redirection characters honored. This property, known as "magic open",
2977 can often be used to good effect. A user could specify a filename of
2978 F<"rsh cat file |">, or you could change certain filenames as needed:
2980 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2981 open(FH, $filename) or die "Can't open $filename: $!";
2983 Use 3-argument form to open a file with arbitrary weird characters in it,
2985 open(FOO, '<', $file);
2987 otherwise it's necessary to protect any leading and trailing whitespace:
2989 $file =~ s#^(\s)#./$1#;
2990 open(FOO, "< $file\0");
2992 (this may not work on some bizarre filesystems). One should
2993 conscientiously choose between the I<magic> and 3-arguments form
2998 will allow the user to specify an argument of the form C<"rsh cat file |">,
2999 but will not work on a filename which happens to have a trailing space, while
3001 open IN, '<', $ARGV[0];
3003 will have exactly the opposite restrictions.
3005 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3006 should use the C<sysopen> function, which involves no such magic (but
3007 may use subtly different filemodes than Perl open(), which is mapped
3008 to C fopen()). This is
3009 another way to protect your filenames from interpretation. For example:
3012 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3013 or die "sysopen $path: $!";
3014 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3015 print HANDLE "stuff $$\n";
3017 print "File contains: ", <HANDLE>;
3019 Using the constructor from the C<IO::Handle> package (or one of its
3020 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3021 filehandles that have the scope of whatever variables hold references to
3022 them, and automatically close whenever and however you leave that scope:
3026 sub read_myfile_munged {
3028 my $handle = new IO::File;
3029 open($handle, "myfile") or die "myfile: $!";
3031 or return (); # Automatically closed here.
3032 mung $first or die "mung failed"; # Or here.
3033 return $first, <$handle> if $ALL; # Or here.
3037 See L</seek> for some details about mixing reading and writing.
3039 =item opendir DIRHANDLE,EXPR
3041 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3042 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3043 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3049 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3050 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3053 For the reverse, see L</chr>.
3054 See L<perlunicode> and L<encoding> for more about Unicode.
3058 =item our EXPR : ATTRIBUTES
3060 An C<our> declares the listed variables to be valid globals within
3061 the enclosing block, file, or C<eval>. That is, it has the same
3062 scoping rules as a "my" declaration, but does not create a local
3063 variable. If more than one value is listed, the list must be placed
3064 in parentheses. The C<our> declaration has no semantic effect unless
3065 "use strict vars" is in effect, in which case it lets you use the
3066 declared global variable without qualifying it with a package name.
3067 (But only within the lexical scope of the C<our> declaration. In this
3068 it differs from "use vars", which is package scoped.)
3070 An C<our> declaration declares a global variable that will be visible
3071 across its entire lexical scope, even across package boundaries. The
3072 package in which the variable is entered is determined at the point
3073 of the declaration, not at the point of use. This means the following
3077 our $bar; # declares $Foo::bar for rest of lexical scope
3081 print $bar; # prints 20
3083 Multiple C<our> declarations in the same lexical scope are allowed
3084 if they are in different packages. If they happened to be in the same
3085 package, Perl will emit warnings if you have asked for them.
3089 our $bar; # declares $Foo::bar for rest of lexical scope
3093 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3094 print $bar; # prints 30
3096 our $bar; # emits warning
3098 An C<our> declaration may also have a list of attributes associated
3099 with it. B<WARNING>: This is an experimental feature that may be
3100 changed or removed in future releases of Perl. It should not be
3103 The only currently recognized attribute is C<unique> which indicates
3104 that a single copy of the global is to be used by all interpreters
3105 should the program happen to be running in a multi-interpreter
3106 environment. (The default behaviour would be for each interpreter to
3107 have its own copy of the global.) In such an environment, this
3108 attribute also has the effect of making the global readonly.
3111 our @EXPORT : unique = qw(foo);
3112 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3113 our $VERSION : unique = "1.00";
3115 Multi-interpreter environments can come to being either through the
3116 fork() emulation on Windows platforms, or by embedding perl in a
3117 multi-threaded application. The C<unique> attribute does nothing in
3118 all other environments.
3120 =item pack TEMPLATE,LIST
3122 Takes a LIST of values and converts it into a string using the rules
3123 given by the TEMPLATE. The resulting string is the concatenation of
3124 the converted values. Typically, each converted value looks
3125 like its machine-level representation. For example, on 32-bit machines
3126 a converted integer may be represented by a sequence of 4 bytes.
3128 The TEMPLATE is a sequence of characters that give the order and type
3129 of values, as follows:
3131 a A string with arbitrary binary data, will be null padded.
3132 A A text (ASCII) string, will be space padded.
3133 Z A null terminated (ASCIZ) string, will be null padded.
3135 b A bit string (ascending bit order inside each byte, like vec()).
3136 B A bit string (descending bit order inside each byte).
3137 h A hex string (low nybble first).
3138 H A hex string (high nybble first).
3140 c A signed char value.
3141 C An unsigned char value. Only does bytes. See U for Unicode.
3143 s A signed short value.
3144 S An unsigned short value.
3145 (This 'short' is _exactly_ 16 bits, which may differ from
3146 what a local C compiler calls 'short'. If you want
3147 native-length shorts, use the '!' suffix.)
3149 i A signed integer value.
3150 I An unsigned integer value.
3151 (This 'integer' is _at_least_ 32 bits wide. Its exact
3152 size depends on what a local C compiler calls 'int',
3153 and may even be larger than the 'long' described in
3156 l A signed long value.
3157 L An unsigned long value.
3158 (This 'long' is _exactly_ 32 bits, which may differ from
3159 what a local C compiler calls 'long'. If you want
3160 native-length longs, use the '!' suffix.)
3162 n An unsigned short in "network" (big-endian) order.
3163 N An unsigned long in "network" (big-endian) order.
3164 v An unsigned short in "VAX" (little-endian) order.
3165 V An unsigned long in "VAX" (little-endian) order.
3166 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3167 _exactly_ 32 bits, respectively.)
3169 q A signed quad (64-bit) value.
3170 Q An unsigned quad value.
3171 (Quads are available only if your system supports 64-bit
3172 integer values _and_ if Perl has been compiled to support those.
3173 Causes a fatal error otherwise.)
3175 j A signed integer value (a Perl internal integer, IV).
3176 J An unsigned integer value (a Perl internal unsigned integer, UV).
3178 f A single-precision float in the native format.
3179 d A double-precision float in the native format.
3181 F A floating point value in the native native format
3182 (a Perl internal floating point value, NV).
3183 D A long double-precision float in the native format.
3184 (Long doubles are available only if your system supports long
3185 double values _and_ if Perl has been compiled to support those.
3186 Causes a fatal error otherwise.)
3188 p A pointer to a null-terminated string.
3189 P A pointer to a structure (fixed-length string).
3191 u A uuencoded string.
3192 U A Unicode character number. Encodes to UTF-8 internally
3193 (or UTF-EBCDIC in EBCDIC platforms).
3195 w A BER compressed integer. Its bytes represent an unsigned
3196 integer in base 128, most significant digit first, with as
3197 few digits as possible. Bit eight (the high bit) is set
3198 on each byte except the last.
3202 @ Null fill to absolute position.
3203 ( Start of a ()-group.
3205 The following rules apply:
3211 Each letter may optionally be followed by a number giving a repeat
3212 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3213 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3214 many values from the LIST. A C<*> for the repeat count means to use
3215 however many items are left, except for C<@>, C<x>, C<X>, where it is
3216 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3217 is the same). A numeric repeat count may optionally be enclosed in
3218 brackets, as in C<pack 'C[80]', @arr>.
3220 One can replace the numeric repeat count by a template enclosed in brackets;
3221 then the packed length of this template in bytes is used as a count.
3222 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3223 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3224 If the template in brackets contains alignment commands (such as C<x![d]>),
3225 its packed length is calculated as if the start of the template has the maximal
3228 When used with C<Z>, C<*> results in the addition of a trailing null
3229 byte (so the packed result will be one longer than the byte C<length>
3232 The repeat count for C<u> is interpreted as the maximal number of bytes
3233 to encode per line of output, with 0 and 1 replaced by 45.
3237 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3238 string of length count, padding with nulls or spaces as necessary. When
3239 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3240 after the first null, and C<a> returns data verbatim. When packing,
3241 C<a>, and C<Z> are equivalent.
3243 If the value-to-pack is too long, it is truncated. If too long and an
3244 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3245 by a null byte. Thus C<Z> always packs a trailing null byte under
3250 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3251 Each byte of the input field of pack() generates 1 bit of the result.
3252 Each result bit is based on the least-significant bit of the corresponding
3253 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3254 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3256 Starting from the beginning of the input string of pack(), each 8-tuple
3257 of bytes is converted to 1 byte of output. With format C<b>
3258 the first byte of the 8-tuple determines the least-significant bit of a
3259 byte, and with format C<B> it determines the most-significant bit of
3262 If the length of the input string is not exactly divisible by 8, the
3263 remainder is packed as if the input string were padded by null bytes
3264 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3266 If the input string of pack() is longer than needed, extra bytes are ignored.
3267 A C<*> for the repeat count of pack() means to use all the bytes of
3268 the input field. On unpack()ing the bits are converted to a string
3269 of C<"0">s and C<"1">s.
3273 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3274 representable as hexadecimal digits, 0-9a-f) long.
3276 Each byte of the input field of pack() generates 4 bits of the result.
3277 For non-alphabetical bytes the result is based on the 4 least-significant
3278 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3279 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3280 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3281 is compatible with the usual hexadecimal digits, so that C<"a"> and
3282 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3283 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3285 Starting from the beginning of the input string of pack(), each pair
3286 of bytes is converted to 1 byte of output. With format C<h> the
3287 first byte of the pair determines the least-significant nybble of the
3288 output byte, and with format C<H> it determines the most-significant
3291 If the length of the input string is not even, it behaves as if padded
3292 by a null byte at the end. Similarly, during unpack()ing the "extra"
3293 nybbles are ignored.
3295 If the input string of pack() is longer than needed, extra bytes are ignored.
3296 A C<*> for the repeat count of pack() means to use all the bytes of
3297 the input field. On unpack()ing the bits are converted to a string
3298 of hexadecimal digits.
3302 The C<p> type packs a pointer to a null-terminated string. You are
3303 responsible for ensuring the string is not a temporary value (which can
3304 potentially get deallocated before you get around to using the packed result).
3305 The C<P> type packs a pointer to a structure of the size indicated by the
3306 length. A NULL pointer is created if the corresponding value for C<p> or
3307 C<P> is C<undef>, similarly for unpack().
3311 The C</> template character allows packing and unpacking of strings where
3312 the packed structure contains a byte count followed by the string itself.
3313 You write I<length-item>C</>I<string-item>.
3315 The I<length-item> can be any C<pack> template letter, and describes
3316 how the length value is packed. The ones likely to be of most use are
3317 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3318 SNMP) and C<N> (for Sun XDR).
3320 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3321 For C<unpack> the length of the string is obtained from the I<length-item>,
3322 but if you put in the '*' it will be ignored.
3324 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3325 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3326 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3328 The I<length-item> is not returned explicitly from C<unpack>.
3330 Adding a count to the I<length-item> letter is unlikely to do anything
3331 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3332 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3333 which Perl does not regard as legal in numeric strings.
3337 The integer types C<s>, C<S>, C<l>, and C<L> may be
3338 immediately followed by a C<!> suffix to signify native shorts or
3339 longs--as you can see from above for example a bare C<l> does mean
3340 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3341 may be larger. This is an issue mainly in 64-bit platforms. You can
3342 see whether using C<!> makes any difference by
3344 print length(pack("s")), " ", length(pack("s!")), "\n";
3345 print length(pack("l")), " ", length(pack("l!")), "\n";
3347 C<i!> and C<I!> also work but only because of completeness;
3348 they are identical to C<i> and C<I>.
3350 The actual sizes (in bytes) of native shorts, ints, longs, and long
3351 longs on the platform where Perl was built are also available via
3355 print $Config{shortsize}, "\n";
3356 print $Config{intsize}, "\n";
3357 print $Config{longsize}, "\n";
3358 print $Config{longlongsize}, "\n";
3360 (The C<$Config{longlongsize}> will be undefine if your system does
3361 not support long longs.)
3365 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3366 are inherently non-portable between processors and operating systems
3367 because they obey the native byteorder and endianness. For example a
3368 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3369 (arranged in and handled by the CPU registers) into bytes as
3371 0x12 0x34 0x56 0x78 # big-endian
3372 0x78 0x56 0x34 0x12 # little-endian
3374 Basically, the Intel and VAX CPUs are little-endian, while everybody
3375 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3376 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3377 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3380 The names `big-endian' and `little-endian' are comic references to
3381 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3382 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3383 the egg-eating habits of the Lilliputians.
3385 Some systems may have even weirder byte orders such as
3390 You can see your system's preference with
3392 print join(" ", map { sprintf "%#02x", $_ }
3393 unpack("C*",pack("L",0x12345678))), "\n";
3395 The byteorder on the platform where Perl was built is also available
3399 print $Config{byteorder}, "\n";
3401 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3402 and C<'87654321'> are big-endian.
3404 If you want portable packed integers use the formats C<n>, C<N>,
3405 C<v>, and C<V>, their byte endianness and size are known.
3406 See also L<perlport>.
3410 Real numbers (floats and doubles) are in the native machine format only;
3411 due to the multiplicity of floating formats around, and the lack of a
3412 standard "network" representation, no facility for interchange has been
3413 made. This means that packed floating point data written on one machine
3414 may not be readable on another - even if both use IEEE floating point
3415 arithmetic (as the endian-ness of the memory representation is not part
3416 of the IEEE spec). See also L<perlport>.
3418 Note that Perl uses doubles internally for all numeric calculation, and
3419 converting from double into float and thence back to double again will
3420 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3425 If the pattern begins with a C<U>, the resulting string will be treated
3426 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3427 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3428 characters. If you don't want this to happen, you can begin your pattern
3429 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3430 string, and then follow this with a C<U*> somewhere in your pattern.
3434 You must yourself do any alignment or padding by inserting for example
3435 enough C<'x'>es while packing. There is no way to pack() and unpack()
3436 could know where the bytes are going to or coming from. Therefore
3437 C<pack> (and C<unpack>) handle their output and input as flat
3442 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3443 take a repeat count, both as postfix, and via the C</> template
3448 C<x> and C<X> accept C<!> modifier. In this case they act as
3449 alignment commands: they jump forward/back to the closest position
3450 aligned at a multiple of C<count> bytes. For example, to pack() or
3451 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3452 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3453 aligned on the double's size.
3455 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3456 both result in no-ops.
3460 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3464 If TEMPLATE requires more arguments to pack() than actually given, pack()
3465 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3466 to pack() than actually given, extra arguments are ignored.
3472 $foo = pack("CCCC",65,66,67,68);
3474 $foo = pack("C4",65,66,67,68);
3476 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3477 # same thing with Unicode circled letters
3479 $foo = pack("ccxxcc",65,66,67,68);
3482 # note: the above examples featuring "C" and "c" are true
3483 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3484 # and UTF-8. In EBCDIC the first example would be
3485 # $foo = pack("CCCC",193,194,195,196);
3487 $foo = pack("s2",1,2);
3488 # "\1\0\2\0" on little-endian
3489 # "\0\1\0\2" on big-endian
3491 $foo = pack("a4","abcd","x","y","z");
3494 $foo = pack("aaaa","abcd","x","y","z");
3497 $foo = pack("a14","abcdefg");
3498 # "abcdefg\0\0\0\0\0\0\0"
3500 $foo = pack("i9pl", gmtime);
3501 # a real struct tm (on my system anyway)
3503 $utmp_template = "Z8 Z8 Z16 L";
3504 $utmp = pack($utmp_template, @utmp1);
3505 # a struct utmp (BSDish)
3507 @utmp2 = unpack($utmp_template, $utmp);
3508 # "@utmp1" eq "@utmp2"
3511 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3514 $foo = pack('sx2l', 12, 34);
3515 # short 12, two zero bytes padding, long 34
3516 $bar = pack('s@4l', 12, 34);
3517 # short 12, zero fill to position 4, long 34
3520 The same template may generally also be used in unpack().
3522 =item package NAMESPACE
3526 Declares the compilation unit as being in the given namespace. The scope
3527 of the package declaration is from the declaration itself through the end
3528 of the enclosing block, file, or eval (the same as the C<my> operator).
3529 All further unqualified dynamic identifiers will be in this namespace.
3530 A package statement affects only dynamic variables--including those
3531 you've used C<local> on--but I<not> lexical variables, which are created
3532 with C<my>. Typically it would be the first declaration in a file to
3533 be included by the C<require> or C<use> operator. You can switch into a
3534 package in more than one place; it merely influences which symbol table
3535 is used by the compiler for the rest of that block. You can refer to
3536 variables and filehandles in other packages by prefixing the identifier
3537 with the package name and a double colon: C<$Package::Variable>.
3538 If the package name is null, the C<main> package as assumed. That is,
3539 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3540 still seen in older code).
3542 If NAMESPACE is omitted, then there is no current package, and all
3543 identifiers must be fully qualified or lexicals. However, you are
3544 strongly advised not to make use of this feature. Its use can cause
3545 unexpected behaviour, even crashing some versions of Perl. It is
3546 deprecated, and will be removed from a future release.
3548 See L<perlmod/"Packages"> for more information about packages, modules,
3549 and classes. See L<perlsub> for other scoping issues.
3551 =item pipe READHANDLE,WRITEHANDLE
3553 Opens a pair of connected pipes like the corresponding system call.
3554 Note that if you set up a loop of piped processes, deadlock can occur
3555 unless you are very careful. In addition, note that Perl's pipes use
3556 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3557 after each command, depending on the application.
3559 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3560 for examples of such things.
3562 On systems that support a close-on-exec flag on files, the flag will be set
3563 for the newly opened file descriptors as determined by the value of $^F.
3570 Pops and returns the last value of the array, shortening the array by
3571 one element. Has an effect similar to
3575 If there are no elements in the array, returns the undefined value
3576 (although this may happen at other times as well). If ARRAY is
3577 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3578 array in subroutines, just like C<shift>.
3584 Returns the offset of where the last C<m//g> search left off for the variable
3585 in question (C<$_> is used when the variable is not specified). May be
3586 modified to change that offset. Such modification will also influence
3587 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3590 =item print FILEHANDLE LIST
3596 Prints a string or a list of strings. Returns true if successful.
3597 FILEHANDLE may be a scalar variable name, in which case the variable
3598 contains the name of or a reference to the filehandle, thus introducing
3599 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3600 the next token is a term, it may be misinterpreted as an operator
3601 unless you interpose a C<+> or put parentheses around the arguments.)
3602 If FILEHANDLE is omitted, prints by default to standard output (or
3603 to the last selected output channel--see L</select>). If LIST is
3604 also omitted, prints C<$_> to the currently selected output channel.
3605 To set the default output channel to something other than STDOUT
3606 use the select operation. The current value of C<$,> (if any) is
3607 printed between each LIST item. The current value of C<$\> (if
3608 any) is printed after the entire LIST has been printed. Because
3609 print takes a LIST, anything in the LIST is evaluated in list
3610 context, and any subroutine that you call will have one or more of
3611 its expressions evaluated in list context. Also be careful not to
3612 follow the print keyword with a left parenthesis unless you want
3613 the corresponding right parenthesis to terminate the arguments to
3614 the print--interpose a C<+> or put parentheses around all the
3617 Note that if you're storing FILEHANDLES in an array or other expression,
3618 you will have to use a block returning its value instead:
3620 print { $files[$i] } "stuff\n";
3621 print { $OK ? STDOUT : STDERR } "stuff\n";
3623 =item printf FILEHANDLE FORMAT, LIST
3625 =item printf FORMAT, LIST
3627 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3628 (the output record separator) is not appended. The first argument
3629 of the list will be interpreted as the C<printf> format. See C<sprintf>
3630 for an explanation of the format argument. If C<use locale> is in effect,
3631 the character used for the decimal point in formatted real numbers is
3632 affected by the LC_NUMERIC locale. See L<perllocale>.
3634 Don't fall into the trap of using a C<printf> when a simple
3635 C<print> would do. The C<print> is more efficient and less
3638 =item prototype FUNCTION
3640 Returns the prototype of a function as a string (or C<undef> if the
3641 function has no prototype). FUNCTION is a reference to, or the name of,
3642 the function whose prototype you want to retrieve.
3644 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3645 name for Perl builtin. If the builtin is not I<overridable> (such as
3646 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3647 C<system>) returns C<undef> because the builtin does not really behave
3648 like a Perl function. Otherwise, the string describing the equivalent
3649 prototype is returned.
3651 =item push ARRAY,LIST
3653 Treats ARRAY as a stack, and pushes the values of LIST
3654 onto the end of ARRAY. The length of ARRAY increases by the length of
3655 LIST. Has the same effect as
3658 $ARRAY[++$#ARRAY] = $value;
3661 but is more efficient. Returns the new number of elements in the array.
3673 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3675 =item quotemeta EXPR
3679 Returns the value of EXPR with all non-"word"
3680 characters backslashed. (That is, all characters not matching
3681 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3682 returned string, regardless of any locale settings.)
3683 This is the internal function implementing
3684 the C<\Q> escape in double-quoted strings.
3686 If EXPR is omitted, uses C<$_>.
3692 Returns a random fractional number greater than or equal to C<0> and less
3693 than the value of EXPR. (EXPR should be positive.) If EXPR is
3694 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3695 unless C<srand> has already been called. See also C<srand>.
3697 Apply C<int()> to the value returned by C<rand()> if you want random
3698 integers instead of random fractional numbers. For example,
3702 returns a random integer between C<0> and C<9>, inclusive.
3704 (Note: If your rand function consistently returns numbers that are too
3705 large or too small, then your version of Perl was probably compiled
3706 with the wrong number of RANDBITS.)
3708 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3710 =item read FILEHANDLE,SCALAR,LENGTH
3712 Attempts to read LENGTH I<characters> of data into variable SCALAR
3713 from the specified FILEHANDLE. Returns the number of characters
3714 actually read, C<0> at end of file, or undef if there was an error.
3715 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3716 needs growing, the new bytes will be zero bytes. An OFFSET may be
3717 specified to place the read data into some other place in SCALAR than
3718 the beginning. The call is actually implemented in terms of either
3719 Perl's or system's fread() call. To get a true read(2) system call,
3722 Note the I<characters>: depending on the status of the filehandle,
3723 either (8-bit) bytes or characters are read. By default all
3724 filehandles operate on bytes, but for example if the filehandle has
3725 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3726 pragma, L<open>), the I/O will operate on characters, not bytes.
3728 =item readdir DIRHANDLE
3730 Returns the next directory entry for a directory opened by C<opendir>.
3731 If used in list context, returns all the rest of the entries in the
3732 directory. If there are no more entries, returns an undefined value in
3733 scalar context or a null list in list context.
3735 If you're planning to filetest the return values out of a C<readdir>, you'd
3736 better prepend the directory in question. Otherwise, because we didn't
3737 C<chdir> there, it would have been testing the wrong file.
3739 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3740 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3745 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3746 context, each call reads and returns the next line, until end-of-file is
3747 reached, whereupon the subsequent call returns undef. In list context,
3748 reads until end-of-file is reached and returns a list of lines. Note that
3749 the notion of "line" used here is however you may have defined it
3750 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3752 When C<$/> is set to C<undef>, when readline() is in scalar
3753 context (i.e. file slurp mode), and when an empty file is read, it
3754 returns C<''> the first time, followed by C<undef> subsequently.
3756 This is the internal function implementing the C<< <EXPR> >>
3757 operator, but you can use it directly. The C<< <EXPR> >>
3758 operator is discussed in more detail in L<perlop/"I/O Operators">.
3761 $line = readline(*STDIN); # same thing
3767 Returns the value of a symbolic link, if symbolic links are
3768 implemented. If not, gives a fatal error. If there is some system
3769 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3770 omitted, uses C<$_>.
3774 EXPR is executed as a system command.
3775 The collected standard output of the command is returned.
3776 In scalar context, it comes back as a single (potentially
3777 multi-line) string. In list context, returns a list of lines
3778 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3779 This is the internal function implementing the C<qx/EXPR/>
3780 operator, but you can use it directly. The C<qx/EXPR/>
3781 operator is discussed in more detail in L<perlop/"I/O Operators">.
3783 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3785 Receives a message on a socket. Attempts to receive LENGTH characters
3786 of data into variable SCALAR from the specified SOCKET filehandle.
3787 SCALAR will be grown or shrunk to the length actually read. Takes the
3788 same flags as the system call of the same name. Returns the address
3789 of the sender if SOCKET's protocol supports this; returns an empty
3790 string otherwise. If there's an error, returns the undefined value.
3791 This call is actually implemented in terms of recvfrom(2) system call.
3792 See L<perlipc/"UDP: Message Passing"> for examples.
3794 Note the I<characters>: depending on the status of the socket, either
3795 (8-bit) bytes or characters are received. By default all sockets
3796 operate on bytes, but for example if the socket has been changed using
3797 binmode() to operate with the C<:utf8> discipline (see the C<open>
3798 pragma, L<open>), the I/O will operate on characters, not bytes.
3804 The C<redo> command restarts the loop block without evaluating the
3805 conditional again. The C<continue> block, if any, is not executed. If
3806 the LABEL is omitted, the command refers to the innermost enclosing
3807 loop. This command is normally used by programs that want to lie to
3808 themselves about what was just input:
3810 # a simpleminded Pascal comment stripper
3811 # (warning: assumes no { or } in strings)
3812 LINE: while (<STDIN>) {
3813 while (s|({.*}.*){.*}|$1 |) {}
3818 if (/}/) { # end of comment?
3827 C<redo> cannot be used to retry a block which returns a value such as
3828 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3829 a grep() or map() operation.
3831 Note that a block by itself is semantically identical to a loop
3832 that executes once. Thus C<redo> inside such a block will effectively
3833 turn it into a looping construct.
3835 See also L</continue> for an illustration of how C<last>, C<next>, and
3842 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3843 is not specified, C<$_> will be used. The value returned depends on the
3844 type of thing the reference is a reference to.
3845 Builtin types include:
3855 If the referenced object has been blessed into a package, then that package
3856 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3858 if (ref($r) eq "HASH") {
3859 print "r is a reference to a hash.\n";
3862 print "r is not a reference at all.\n";
3864 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3865 print "r is a reference to something that isa hash.\n";
3868 See also L<perlref>.
3870 =item rename OLDNAME,NEWNAME
3872 Changes the name of a file; an existing file NEWNAME will be
3873 clobbered. Returns true for success, false otherwise.
3875 Behavior of this function varies wildly depending on your system
3876 implementation. For example, it will usually not work across file system
3877 boundaries, even though the system I<mv> command sometimes compensates
3878 for this. Other restrictions include whether it works on directories,
3879 open files, or pre-existing files. Check L<perlport> and either the
3880 rename(2) manpage or equivalent system documentation for details.
3882 =item require VERSION
3888 Demands a version of Perl specified by VERSION, or demands some semantics
3889 specified by EXPR or by C<$_> if EXPR is not supplied.
3891 VERSION may be either a numeric argument such as 5.006, which will be
3892 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3893 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3894 VERSION is greater than the version of the current Perl interpreter.
3895 Compare with L</use>, which can do a similar check at compile time.
3897 Specifying VERSION as a literal of the form v5.6.1 should generally be
3898 avoided, because it leads to misleading error messages under earlier
3899 versions of Perl which do not support this syntax. The equivalent numeric
3900 version should be used instead.
3902 require v5.6.1; # run time version check
3903 require 5.6.1; # ditto
3904 require 5.006_001; # ditto; preferred for backwards compatibility
3906 Otherwise, demands that a library file be included if it hasn't already
3907 been included. The file is included via the do-FILE mechanism, which is
3908 essentially just a variety of C<eval>. Has semantics similar to the following
3913 return 1 if $INC{$filename};
3914 my($realfilename,$result);
3916 foreach $prefix (@INC) {
3917 $realfilename = "$prefix/$filename";
3918 if (-f $realfilename) {
3919 $INC{$filename} = $realfilename;
3920 $result = do $realfilename;
3924 die "Can't find $filename in \@INC";
3926 delete $INC{$filename} if $@ || !$result;
3928 die "$filename did not return true value" unless $result;
3932 Note that the file will not be included twice under the same specified
3933 name. The file must return true as the last statement to indicate
3934 successful execution of any initialization code, so it's customary to
3935 end such a file with C<1;> unless you're sure it'll return true
3936 otherwise. But it's better just to put the C<1;>, in case you add more
3939 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3940 replaces "F<::>" with "F</>" in the filename for you,
3941 to make it easy to load standard modules. This form of loading of
3942 modules does not risk altering your namespace.
3944 In other words, if you try this:
3946 require Foo::Bar; # a splendid bareword
3948 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3949 directories specified in the C<@INC> array.
3951 But if you try this:
3953 $class = 'Foo::Bar';
3954 require $class; # $class is not a bareword
3956 require "Foo::Bar"; # not a bareword because of the ""
3958 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3959 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3961 eval "require $class";
3963 You can also insert hooks into the import facility, by putting directly
3964 Perl code into the @INC array. There are three forms of hooks: subroutine
3965 references, array references and blessed objects.
3967 Subroutine references are the simplest case. When the inclusion system
3968 walks through @INC and encounters a subroutine, this subroutine gets
3969 called with two parameters, the first being a reference to itself, and the
3970 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3971 subroutine should return C<undef> or a filehandle, from which the file to
3972 include will be read. If C<undef> is returned, C<require> will look at
3973 the remaining elements of @INC.
3975 If the hook is an array reference, its first element must be a subroutine
3976 reference. This subroutine is called as above, but the first parameter is
3977 the array reference. This enables to pass indirectly some arguments to
3980 In other words, you can write:
3982 push @INC, \&my_sub;
3984 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3990 push @INC, [ \&my_sub, $x, $y, ... ];
3992 my ($arrayref, $filename) = @_;
3993 # Retrieve $x, $y, ...
3994 my @parameters = @$arrayref[1..$#$arrayref];
3998 If the hook is an object, it must provide an INC method, that will be
3999 called as above, the first parameter being the object itself. (Note that
4000 you must fully qualify the sub's name, as it is always forced into package
4001 C<main>.) Here is a typical code layout:
4007 my ($self, $filename) = @_;
4011 # In the main program
4012 push @INC, new Foo(...);
4014 Note that these hooks are also permitted to set the %INC entry
4015 corresponding to the files they have loaded. See L<perlvar/%INC>.
4017 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4023 Generally used in a C<continue> block at the end of a loop to clear
4024 variables and reset C<??> searches so that they work again. The
4025 expression is interpreted as a list of single characters (hyphens
4026 allowed for ranges). All variables and arrays beginning with one of
4027 those letters are reset to their pristine state. If the expression is
4028 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4029 only variables or searches in the current package. Always returns
4032 reset 'X'; # reset all X variables
4033 reset 'a-z'; # reset lower case variables
4034 reset; # just reset ?one-time? searches
4036 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4037 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4038 variables--lexical variables are unaffected, but they clean themselves
4039 up on scope exit anyway, so you'll probably want to use them instead.
4046 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4047 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4048 context, depending on how the return value will be used, and the context
4049 may vary from one execution to the next (see C<wantarray>). If no EXPR
4050 is given, returns an empty list in list context, the undefined value in
4051 scalar context, and (of course) nothing at all in a void context.
4053 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4054 or do FILE will automatically return the value of the last expression
4059 In list context, returns a list value consisting of the elements
4060 of LIST in the opposite order. In scalar context, concatenates the
4061 elements of LIST and returns a string value with all characters
4062 in the opposite order.
4064 print reverse <>; # line tac, last line first
4066 undef $/; # for efficiency of <>
4067 print scalar reverse <>; # character tac, last line tsrif
4069 This operator is also handy for inverting a hash, although there are some
4070 caveats. If a value is duplicated in the original hash, only one of those
4071 can be represented as a key in the inverted hash. Also, this has to
4072 unwind one hash and build a whole new one, which may take some time
4073 on a large hash, such as from a DBM file.
4075 %by_name = reverse %by_address; # Invert the hash
4077 =item rewinddir DIRHANDLE
4079 Sets the current position to the beginning of the directory for the
4080 C<readdir> routine on DIRHANDLE.
4082 =item rindex STR,SUBSTR,POSITION
4084 =item rindex STR,SUBSTR
4086 Works just like index() except that it returns the position of the LAST
4087 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4088 last occurrence at or before that position.
4090 =item rmdir FILENAME
4094 Deletes the directory specified by FILENAME if that directory is empty. If it
4095 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4096 FILENAME is omitted, uses C<$_>.
4100 The substitution operator. See L<perlop>.
4104 Forces EXPR to be interpreted in scalar context and returns the value
4107 @counts = ( scalar @a, scalar @b, scalar @c );
4109 There is no equivalent operator to force an expression to
4110 be interpolated in list context because in practice, this is never
4111 needed. If you really wanted to do so, however, you could use
4112 the construction C<@{[ (some expression) ]}>, but usually a simple
4113 C<(some expression)> suffices.
4115 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4116 parenthesized list, this behaves as a scalar comma expression, evaluating
4117 all but the last element in void context and returning the final element
4118 evaluated in scalar context. This is seldom what you want.
4120 The following single statement:
4122 print uc(scalar(&foo,$bar)),$baz;
4124 is the moral equivalent of these two:
4127 print(uc($bar),$baz);
4129 See L<perlop> for more details on unary operators and the comma operator.
4131 =item seek FILEHANDLE,POSITION,WHENCE
4133 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4134 FILEHANDLE may be an expression whose value gives the name of the
4135 filehandle. The values for WHENCE are C<0> to set the new position
4136 I<in bytes> to POSITION, C<1> to set it to the current position plus
4137 POSITION, and C<2> to set it to EOF plus POSITION (typically
4138 negative). For WHENCE you may use the constants C<SEEK_SET>,
4139 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4140 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4143 Note the I<in bytes>: even if the filehandle has been set to
4144 operate on characters (for example by using the C<:utf8> open
4145 discipline), tell() will return byte offsets, not character offsets
4146 (because implementing that would render seek() and tell() rather slow).
4148 If you want to position file for C<sysread> or C<syswrite>, don't use
4149 C<seek>--buffering makes its effect on the file's system position
4150 unpredictable and non-portable. Use C<sysseek> instead.
4152 Due to the rules and rigors of ANSI C, on some systems you have to do a
4153 seek whenever you switch between reading and writing. Amongst other
4154 things, this may have the effect of calling stdio's clearerr(3).
4155 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4159 This is also useful for applications emulating C<tail -f>. Once you hit
4160 EOF on your read, and then sleep for a while, you might have to stick in a
4161 seek() to reset things. The C<seek> doesn't change the current position,
4162 but it I<does> clear the end-of-file condition on the handle, so that the
4163 next C<< <FILE> >> makes Perl try again to read something. We hope.
4165 If that doesn't work (some IO implementations are particularly
4166 cantankerous), then you may need something more like this:
4169 for ($curpos = tell(FILE); $_ = <FILE>;
4170 $curpos = tell(FILE)) {
4171 # search for some stuff and put it into files
4173 sleep($for_a_while);
4174 seek(FILE, $curpos, 0);
4177 =item seekdir DIRHANDLE,POS
4179 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4180 must be a value returned by C<telldir>. Has the same caveats about
4181 possible directory compaction as the corresponding system library
4184 =item select FILEHANDLE
4188 Returns the currently selected filehandle. Sets the current default
4189 filehandle for output, if FILEHANDLE is supplied. This has two
4190 effects: first, a C<write> or a C<print> without a filehandle will
4191 default to this FILEHANDLE. Second, references to variables related to
4192 output will refer to this output channel. For example, if you have to
4193 set the top of form format for more than one output channel, you might
4201 FILEHANDLE may be an expression whose value gives the name of the
4202 actual filehandle. Thus:
4204 $oldfh = select(STDERR); $| = 1; select($oldfh);
4206 Some programmers may prefer to think of filehandles as objects with
4207 methods, preferring to write the last example as:
4210 STDERR->autoflush(1);
4212 =item select RBITS,WBITS,EBITS,TIMEOUT
4214 This calls the select(2) system call with the bit masks specified, which
4215 can be constructed using C<fileno> and C<vec>, along these lines:
4217 $rin = $win = $ein = '';
4218 vec($rin,fileno(STDIN),1) = 1;
4219 vec($win,fileno(STDOUT),1) = 1;
4222 If you want to select on many filehandles you might wish to write a
4226 my(@fhlist) = split(' ',$_[0]);
4229 vec($bits,fileno($_),1) = 1;
4233 $rin = fhbits('STDIN TTY SOCK');
4237 ($nfound,$timeleft) =
4238 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4240 or to block until something becomes ready just do this
4242 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4244 Most systems do not bother to return anything useful in $timeleft, so
4245 calling select() in scalar context just returns $nfound.
4247 Any of the bit masks can also be undef. The timeout, if specified, is
4248 in seconds, which may be fractional. Note: not all implementations are
4249 capable of returning the $timeleft. If not, they always return
4250 $timeleft equal to the supplied $timeout.
4252 You can effect a sleep of 250 milliseconds this way:
4254 select(undef, undef, undef, 0.25);
4256 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4257 is implementation-dependent.
4259 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4260 or <FH>) with C<select>, except as permitted by POSIX, and even
4261 then only on POSIX systems. You have to use C<sysread> instead.
4263 =item semctl ID,SEMNUM,CMD,ARG
4265 Calls the System V IPC function C<semctl>. You'll probably have to say
4269 first to get the correct constant definitions. If CMD is IPC_STAT or
4270 GETALL, then ARG must be a variable which will hold the returned
4271 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4272 the undefined value for error, "C<0 but true>" for zero, or the actual
4273 return value otherwise. The ARG must consist of a vector of native
4274 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4275 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4278 =item semget KEY,NSEMS,FLAGS
4280 Calls the System V IPC function semget. Returns the semaphore id, or
4281 the undefined value if there is an error. See also
4282 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4285 =item semop KEY,OPSTRING
4287 Calls the System V IPC function semop to perform semaphore operations
4288 such as signalling and waiting. OPSTRING must be a packed array of
4289 semop structures. Each semop structure can be generated with
4290 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4291 operations is implied by the length of OPSTRING. Returns true if
4292 successful, or false if there is an error. As an example, the
4293 following code waits on semaphore $semnum of semaphore id $semid:
4295 $semop = pack("s!3", $semnum, -1, 0);
4296 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4298 To signal the semaphore, replace C<-1> with C<1>. See also
4299 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4302 =item send SOCKET,MSG,FLAGS,TO
4304 =item send SOCKET,MSG,FLAGS
4306 Sends a message on a socket. Attempts to send the scalar MSG to the
4307 SOCKET filehandle. Takes the same flags as the system call of the
4308 same name. On unconnected sockets you must specify a destination to
4309 send TO, in which case it does a C C<sendto>. Returns the number of
4310 characters sent, or the undefined value if there is an error. The C
4311 system call sendmsg(2) is currently unimplemented. See
4312 L<perlipc/"UDP: Message Passing"> for examples.
4314 Note the I<characters>: depending on the status of the socket, either
4315 (8-bit) bytes or characters are sent. By default all sockets operate
4316 on bytes, but for example if the socket has been changed using
4317 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4318 the C<open> pragma, L<open>), the I/O will operate on characters, not
4321 =item setpgrp PID,PGRP
4323 Sets the current process group for the specified PID, C<0> for the current
4324 process. Will produce a fatal error if used on a machine that doesn't
4325 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4326 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4327 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4330 =item setpriority WHICH,WHO,PRIORITY
4332 Sets the current priority for a process, a process group, or a user.
4333 (See setpriority(2).) Will produce a fatal error if used on a machine
4334 that doesn't implement setpriority(2).
4336 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4338 Sets the socket option requested. Returns undefined if there is an
4339 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4346 Shifts the first value of the array off and returns it, shortening the
4347 array by 1 and moving everything down. If there are no elements in the
4348 array, returns the undefined value. If ARRAY is omitted, shifts the
4349 C<@_> array within the lexical scope of subroutines and formats, and the
4350 C<@ARGV> array at file scopes or within the lexical scopes established by
4351 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4354 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4355 same thing to the left end of an array that C<pop> and C<push> do to the
4358 =item shmctl ID,CMD,ARG
4360 Calls the System V IPC function shmctl. You'll probably have to say
4364 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4365 then ARG must be a variable which will hold the returned C<shmid_ds>
4366 structure. Returns like ioctl: the undefined value for error, "C<0> but
4367 true" for zero, or the actual return value otherwise.
4368 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4370 =item shmget KEY,SIZE,FLAGS
4372 Calls the System V IPC function shmget. Returns the shared memory
4373 segment id, or the undefined value if there is an error.
4374 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4376 =item shmread ID,VAR,POS,SIZE
4378 =item shmwrite ID,STRING,POS,SIZE
4380 Reads or writes the System V shared memory segment ID starting at
4381 position POS for size SIZE by attaching to it, copying in/out, and
4382 detaching from it. When reading, VAR must be a variable that will
4383 hold the data read. When writing, if STRING is too long, only SIZE
4384 bytes are used; if STRING is too short, nulls are written to fill out
4385 SIZE bytes. Return true if successful, or false if there is an error.
4386 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4387 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4389 =item shutdown SOCKET,HOW
4391 Shuts down a socket connection in the manner indicated by HOW, which
4392 has the same interpretation as in the system call of the same name.
4394 shutdown(SOCKET, 0); # I/we have stopped reading data
4395 shutdown(SOCKET, 1); # I/we have stopped writing data
4396 shutdown(SOCKET, 2); # I/we have stopped using this socket
4398 This is useful with sockets when you want to tell the other
4399 side you're done writing but not done reading, or vice versa.
4400 It's also a more insistent form of close because it also
4401 disables the file descriptor in any forked copies in other
4408 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4409 returns sine of C<$_>.
4411 For the inverse sine operation, you may use the C<Math::Trig::asin>
4412 function, or use this relation:
4414 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4420 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4421 May be interrupted if the process receives a signal such as C<SIGALRM>.
4422 Returns the number of seconds actually slept. You probably cannot
4423 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4426 On some older systems, it may sleep up to a full second less than what
4427 you requested, depending on how it counts seconds. Most modern systems
4428 always sleep the full amount. They may appear to sleep longer than that,
4429 however, because your process might not be scheduled right away in a
4430 busy multitasking system.
4432 For delays of finer granularity than one second, you may use Perl's
4433 C<syscall> interface to access setitimer(2) if your system supports
4434 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4435 and starting from Perl 5.8 part of the standard distribution) may also
4438 See also the POSIX module's C<pause> function.
4440 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4442 Opens a socket of the specified kind and attaches it to filehandle
4443 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4444 the system call of the same name. You should C<use Socket> first
4445 to get the proper definitions imported. See the examples in
4446 L<perlipc/"Sockets: Client/Server Communication">.
4448 On systems that support a close-on-exec flag on files, the flag will
4449 be set for the newly opened file descriptor, as determined by the
4450 value of $^F. See L<perlvar/$^F>.
4452 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4454 Creates an unnamed pair of sockets in the specified domain, of the
4455 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4456 for the system call of the same name. If unimplemented, yields a fatal
4457 error. Returns true if successful.
4459 On systems that support a close-on-exec flag on files, the flag will
4460 be set for the newly opened file descriptors, as determined by the value
4461 of $^F. See L<perlvar/$^F>.
4463 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4464 to C<pipe(Rdr, Wtr)> is essentially:
4467 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4468 shutdown(Rdr, 1); # no more writing for reader
4469 shutdown(Wtr, 0); # no more reading for writer
4471 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4472 emulate socketpair using IP sockets to localhost if your system implements
4473 sockets but not socketpair.
4475 =item sort SUBNAME LIST
4477 =item sort BLOCK LIST
4481 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4482 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4483 specified, it gives the name of a subroutine that returns an integer
4484 less than, equal to, or greater than C<0>, depending on how the elements
4485 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4486 operators are extremely useful in such routines.) SUBNAME may be a
4487 scalar variable name (unsubscripted), in which case the value provides
4488 the name of (or a reference to) the actual subroutine to use. In place
4489 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4492 If the subroutine's prototype is C<($$)>, the elements to be compared
4493 are passed by reference in C<@_>, as for a normal subroutine. This is
4494 slower than unprototyped subroutines, where the elements to be
4495 compared are passed into the subroutine
4496 as the package global variables $a and $b (see example below). Note that
4497 in the latter case, it is usually counter-productive to declare $a and
4500 In either case, the subroutine may not be recursive. The values to be
4501 compared are always passed by reference, so don't modify them.
4503 You also cannot exit out of the sort block or subroutine using any of the
4504 loop control operators described in L<perlsyn> or with C<goto>.
4506 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4507 current collation locale. See L<perllocale>.
4509 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4510 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4511 preserves the input order of elements that compare equal. Although
4512 quicksort's run time is O(NlogN) when averaged over all arrays of
4513 length N, the time can be O(N**2), I<quadratic> behavior, for some
4514 inputs.) In 5.7, the quicksort implementation was replaced with
4515 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4516 But benchmarks indicated that for some inputs, on some platforms,
4517 the original quicksort was faster. 5.8 has a sort pragma for
4518 limited control of the sort. Its rather blunt control of the
4519 underlying algorithm may not persist into future perls, but the
4520 ability to characterize the input or output in implementation
4521 independent ways quite probably will. See L</use>.
4526 @articles = sort @files;
4528 # same thing, but with explicit sort routine
4529 @articles = sort {$a cmp $b} @files;
4531 # now case-insensitively
4532 @articles = sort {uc($a) cmp uc($b)} @files;
4534 # same thing in reversed order
4535 @articles = sort {$b cmp $a} @files;
4537 # sort numerically ascending
4538 @articles = sort {$a <=> $b} @files;
4540 # sort numerically descending
4541 @articles = sort {$b <=> $a} @files;
4543 # this sorts the %age hash by value instead of key
4544 # using an in-line function
4545 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4547 # sort using explicit subroutine name
4549 $age{$a} <=> $age{$b}; # presuming numeric
4551 @sortedclass = sort byage @class;
4553 sub backwards { $b cmp $a }
4554 @harry = qw(dog cat x Cain Abel);
4555 @george = qw(gone chased yz Punished Axed);
4557 # prints AbelCaincatdogx
4558 print sort backwards @harry;
4559 # prints xdogcatCainAbel
4560 print sort @george, 'to', @harry;
4561 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4563 # inefficiently sort by descending numeric compare using
4564 # the first integer after the first = sign, or the
4565 # whole record case-insensitively otherwise
4568 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4573 # same thing, but much more efficiently;
4574 # we'll build auxiliary indices instead
4578 push @nums, /=(\d+)/;
4583 $nums[$b] <=> $nums[$a]
4585 $caps[$a] cmp $caps[$b]
4589 # same thing, but without any temps
4590 @new = map { $_->[0] }
4591 sort { $b->[1] <=> $a->[1]
4594 } map { [$_, /=(\d+)/, uc($_)] } @old;
4596 # using a prototype allows you to use any comparison subroutine
4597 # as a sort subroutine (including other package's subroutines)
4599 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4602 @new = sort other::backwards @old;
4604 # guarantee stability, regardless of algorithm
4606 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4608 # force use of mergesort (not portable outside Perl 5.8)
4609 use sort '_mergesort'; # note discouraging _
4610 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4612 If you're using strict, you I<must not> declare $a
4613 and $b as lexicals. They are package globals. That means
4614 if you're in the C<main> package and type
4616 @articles = sort {$b <=> $a} @files;
4618 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4619 but if you're in the C<FooPack> package, it's the same as typing
4621 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4623 The comparison function is required to behave. If it returns
4624 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4625 sometimes saying the opposite, for example) the results are not
4628 =item splice ARRAY,OFFSET,LENGTH,LIST
4630 =item splice ARRAY,OFFSET,LENGTH
4632 =item splice ARRAY,OFFSET
4636 Removes the elements designated by OFFSET and LENGTH from an array, and
4637 replaces them with the elements of LIST, if any. In list context,
4638 returns the elements removed from the array. In scalar context,
4639 returns the last element removed, or C<undef> if no elements are
4640 removed. The array grows or shrinks as necessary.
4641 If OFFSET is negative then it starts that far from the end of the array.
4642 If LENGTH is omitted, removes everything from OFFSET onward.
4643 If LENGTH is negative, removes the elements from OFFSET onward
4644 except for -LENGTH elements at the end of the array.
4645 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4646 past the end of the array, perl issues a warning, and splices at the
4649 The following equivalences hold (assuming C<$[ == 0>):
4651 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4652 pop(@a) splice(@a,-1)
4653 shift(@a) splice(@a,0,1)
4654 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4655 $a[$x] = $y splice(@a,$x,1,$y)
4657 Example, assuming array lengths are passed before arrays:
4659 sub aeq { # compare two list values
4660 my(@a) = splice(@_,0,shift);
4661 my(@b) = splice(@_,0,shift);
4662 return 0 unless @a == @b; # same len?
4664 return 0 if pop(@a) ne pop(@b);
4668 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4670 =item split /PATTERN/,EXPR,LIMIT
4672 =item split /PATTERN/,EXPR
4674 =item split /PATTERN/
4678 Splits a string into a list of strings and returns that list. By default,
4679 empty leading fields are preserved, and empty trailing ones are deleted.
4681 In scalar context, returns the number of fields found and splits into
4682 the C<@_> array. Use of split in scalar context is deprecated, however,
4683 because it clobbers your subroutine arguments.
4685 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4686 splits on whitespace (after skipping any leading whitespace). Anything
4687 matching PATTERN is taken to be a delimiter separating the fields. (Note
4688 that the delimiter may be longer than one character.)
4690 If LIMIT is specified and positive, it represents the maximum number
4691 of fields the EXPR will be split into, though the actual number of
4692 fields returned depends on the number of times PATTERN matches within
4693 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4694 stripped (which potential users of C<pop> would do well to remember).
4695 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4696 had been specified. Note that splitting an EXPR that evaluates to the
4697 empty string always returns the empty list, regardless of the LIMIT
4700 A pattern matching the null string (not to be confused with
4701 a null pattern C<//>, which is just one member of the set of patterns
4702 matching a null string) will split the value of EXPR into separate
4703 characters at each point it matches that way. For example:
4705 print join(':', split(/ */, 'hi there'));
4707 produces the output 'h:i:t:h:e:r:e'.
4709 Using the empty pattern C<//> specifically matches the null string, and is
4710 not be confused with the use of C<//> to mean "the last successful pattern
4713 Empty leading (or trailing) fields are produced when there are positive width
4714 matches at the beginning (or end) of the string; a zero-width match at the
4715 beginning (or end) of the string does not produce an empty field. For
4718 print join(':', split(/(?=\w)/, 'hi there!'));
4720 produces the output 'h:i :t:h:e:r:e!'.
4722 The LIMIT parameter can be used to split a line partially
4724 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4726 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4727 one larger than the number of variables in the list, to avoid
4728 unnecessary work. For the list above LIMIT would have been 4 by
4729 default. In time critical applications it behooves you not to split
4730 into more fields than you really need.
4732 If the PATTERN contains parentheses, additional list elements are
4733 created from each matching substring in the delimiter.
4735 split(/([,-])/, "1-10,20", 3);
4737 produces the list value
4739 (1, '-', 10, ',', 20)
4741 If you had the entire header of a normal Unix email message in $header,
4742 you could split it up into fields and their values this way:
4744 $header =~ s/\n\s+/ /g; # fix continuation lines
4745 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4747 The pattern C</PATTERN/> may be replaced with an expression to specify
4748 patterns that vary at runtime. (To do runtime compilation only once,
4749 use C</$variable/o>.)
4751 As a special case, specifying a PATTERN of space (C<' '>) will split on
4752 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4753 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4754 will give you as many null initial fields as there are leading spaces.
4755 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4756 whitespace produces a null first field. A C<split> with no arguments
4757 really does a C<split(' ', $_)> internally.
4759 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4764 open(PASSWD, '/etc/passwd');
4767 ($login, $passwd, $uid, $gid,
4768 $gcos, $home, $shell) = split(/:/);
4772 As with regular pattern matching, any capturing parentheses that are not
4773 matched in a C<split()> will be set to C<undef> when returned:
4775 @fields = split /(A)|B/, "1A2B3";
4776 # @fields is (1, 'A', 2, undef, 3)
4778 =item sprintf FORMAT, LIST
4780 Returns a string formatted by the usual C<printf> conventions of the C
4781 library function C<sprintf>. See below for more details
4782 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4783 the general principles.
4787 # Format number with up to 8 leading zeroes
4788 $result = sprintf("%08d", $number);
4790 # Round number to 3 digits after decimal point
4791 $rounded = sprintf("%.3f", $number);
4793 Perl does its own C<sprintf> formatting--it emulates the C
4794 function C<sprintf>, but it doesn't use it (except for floating-point
4795 numbers, and even then only the standard modifiers are allowed). As a
4796 result, any non-standard extensions in your local C<sprintf> are not
4797 available from Perl.
4799 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4800 pass it an array as your first argument. The array is given scalar context,
4801 and instead of using the 0th element of the array as the format, Perl will
4802 use the count of elements in the array as the format, which is almost never
4805 Perl's C<sprintf> permits the following universally-known conversions:
4808 %c a character with the given number
4810 %d a signed integer, in decimal
4811 %u an unsigned integer, in decimal
4812 %o an unsigned integer, in octal
4813 %x an unsigned integer, in hexadecimal
4814 %e a floating-point number, in scientific notation
4815 %f a floating-point number, in fixed decimal notation
4816 %g a floating-point number, in %e or %f notation
4818 In addition, Perl permits the following widely-supported conversions:
4820 %X like %x, but using upper-case letters
4821 %E like %e, but using an upper-case "E"
4822 %G like %g, but with an upper-case "E" (if applicable)
4823 %b an unsigned integer, in binary
4824 %p a pointer (outputs the Perl value's address in hexadecimal)
4825 %n special: *stores* the number of characters output so far
4826 into the next variable in the parameter list
4828 Finally, for backward (and we do mean "backward") compatibility, Perl
4829 permits these unnecessary but widely-supported conversions:
4832 %D a synonym for %ld
4833 %U a synonym for %lu
4834 %O a synonym for %lo
4837 Note that the number of exponent digits in the scientific notation by
4838 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4839 exponent less than 100 is system-dependent: it may be three or less
4840 (zero-padded as necessary). In other words, 1.23 times ten to the
4841 99th may be either "1.23e99" or "1.23e099".
4843 Perl permits the following universally-known flags between the C<%>
4844 and the conversion letter:
4846 space prefix positive number with a space
4847 + prefix positive number with a plus sign
4848 - left-justify within the field
4849 0 use zeros, not spaces, to right-justify
4850 # prefix non-zero octal with "0", non-zero hex with "0x"
4851 number minimum field width
4852 .number "precision": digits after decimal point for
4853 floating-point, max length for string, minimum length
4855 l interpret integer as C type "long" or "unsigned long"
4856 h interpret integer as C type "short" or "unsigned short"
4857 If no flags, interpret integer as C type "int" or "unsigned"
4859 Perl supports parameter ordering, in other words, fetching the
4860 parameters in some explicitly specified "random" ordering as opposed
4861 to the default implicit sequential ordering. The syntax is, instead
4862 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4863 where the I<digits> is the wanted index, from one upwards. For example:
4865 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4866 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4868 Note that using the reordering syntax does not interfere with the usual
4869 implicit sequential fetching of the parameters:
4871 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4872 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4873 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4874 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4875 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4877 There are also two Perl-specific flags:
4879 V interpret integer as Perl's standard integer type
4880 v interpret string as a vector of integers, output as
4881 numbers separated either by dots, or by an arbitrary
4882 string received from the argument list when the flag
4885 Where a number would appear in the flags, an asterisk (C<*>) may be
4886 used instead, in which case Perl uses the next item in the parameter
4887 list as the given number (that is, as the field width or precision).
4888 If a field width obtained through C<*> is negative, it has the same
4889 effect as the C<-> flag: left-justification.
4891 The C<v> flag is useful for displaying ordinal values of characters
4892 in arbitrary strings:
4894 printf "version is v%vd\n", $^V; # Perl's version
4895 printf "address is %*vX\n", ":", $addr; # IPv6 address
4896 printf "bits are %*vb\n", " ", $bits; # random bitstring
4898 If C<use locale> is in effect, the character used for the decimal
4899 point in formatted real numbers is affected by the LC_NUMERIC locale.
4902 If Perl understands "quads" (64-bit integers) (this requires
4903 either that the platform natively support quads or that Perl
4904 be specifically compiled to support quads), the characters
4908 print quads, and they may optionally be preceded by
4916 You can find out whether your Perl supports quads via L<Config>:
4919 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4922 If Perl understands "long doubles" (this requires that the platform
4923 support long doubles), the flags
4927 may optionally be preceded by
4935 You can find out whether your Perl supports long doubles via L<Config>:
4938 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4944 Return the square root of EXPR. If EXPR is omitted, returns square
4945 root of C<$_>. Only works on non-negative operands, unless you've
4946 loaded the standard Math::Complex module.
4949 print sqrt(-2); # prints 1.4142135623731i
4955 Sets the random number seed for the C<rand> operator.
4957 The point of the function is to "seed" the C<rand> function so that
4958 C<rand> can produce a different sequence each time you run your
4961 If srand() is not called explicitly, it is called implicitly at the
4962 first use of the C<rand> operator. However, this was not the case in
4963 versions of Perl before 5.004, so if your script will run under older
4964 Perl versions, it should call C<srand>.
4966 Most programs won't even call srand() at all, except those that
4967 need a cryptographically-strong starting point rather than the
4968 generally acceptable default, which is based on time of day,
4969 process ID, and memory allocation, or the F</dev/urandom> device,
4972 You can call srand($seed) with the same $seed to reproduce the
4973 I<same> sequence from rand(), but this is usually reserved for
4974 generating predictable results for testing or debugging.
4975 Otherwise, don't call srand() more than once in your program.
4977 Do B<not> call srand() (i.e. without an argument) more than once in
4978 a script. The internal state of the random number generator should
4979 contain more entropy than can be provided by any seed, so calling
4980 srand() again actually I<loses> randomness.
4982 Most implementations of C<srand> take an integer and will silently
4983 truncate decimal numbers. This means C<srand(42)> will usually
4984 produce the same results as C<srand(42.1)>. To be safe, always pass
4985 C<srand> an integer.
4987 In versions of Perl prior to 5.004 the default seed was just the
4988 current C<time>. This isn't a particularly good seed, so many old
4989 programs supply their own seed value (often C<time ^ $$> or C<time ^
4990 ($$ + ($$ << 15))>), but that isn't necessary any more.
4992 Note that you need something much more random than the default seed for
4993 cryptographic purposes. Checksumming the compressed output of one or more
4994 rapidly changing operating system status programs is the usual method. For
4997 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4999 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5002 Frequently called programs (like CGI scripts) that simply use
5006 for a seed can fall prey to the mathematical property that
5010 one-third of the time. So don't do that.
5012 =item stat FILEHANDLE
5018 Returns a 13-element list giving the status info for a file, either
5019 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5020 it stats C<$_>. Returns a null list if the stat fails. Typically used
5023 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5024 $atime,$mtime,$ctime,$blksize,$blocks)
5027 Not all fields are supported on all filesystem types. Here are the
5028 meaning of the fields:
5030 0 dev device number of filesystem
5032 2 mode file mode (type and permissions)
5033 3 nlink number of (hard) links to the file
5034 4 uid numeric user ID of file's owner
5035 5 gid numeric group ID of file's owner
5036 6 rdev the device identifier (special files only)
5037 7 size total size of file, in bytes
5038 8 atime last access time in seconds since the epoch
5039 9 mtime last modify time in seconds since the epoch
5040 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5041 11 blksize preferred block size for file system I/O
5042 12 blocks actual number of blocks allocated
5044 (The epoch was at 00:00 January 1, 1970 GMT.)
5046 If stat is passed the special filehandle consisting of an underline, no
5047 stat is done, but the current contents of the stat structure from the
5048 last stat or filetest are returned. Example:
5050 if (-x $file && (($d) = stat(_)) && $d < 0) {
5051 print "$file is executable NFS file\n";
5054 (This works on machines only for which the device number is negative
5057 Because the mode contains both the file type and its permissions, you
5058 should mask off the file type portion and (s)printf using a C<"%o">
5059 if you want to see the real permissions.
5061 $mode = (stat($filename))[2];
5062 printf "Permissions are %04o\n", $mode & 07777;
5064 In scalar context, C<stat> returns a boolean value indicating success
5065 or failure, and, if successful, sets the information associated with
5066 the special filehandle C<_>.
5068 The File::stat module provides a convenient, by-name access mechanism:
5071 $sb = stat($filename);
5072 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5073 $filename, $sb->size, $sb->mode & 07777,
5074 scalar localtime $sb->mtime;
5076 You can import symbolic mode constants (C<S_IF*>) and functions
5077 (C<S_IS*>) from the Fcntl module:
5081 $mode = (stat($filename))[2];
5083 $user_rwx = ($mode & S_IRWXU) >> 6;
5084 $group_read = ($mode & S_IRGRP) >> 3;
5085 $other_execute = $mode & S_IXOTH;
5087 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5089 $is_setuid = $mode & S_ISUID;
5090 $is_setgid = S_ISDIR($mode);
5092 You could write the last two using the C<-u> and C<-d> operators.
5093 The commonly available S_IF* constants are
5095 # Permissions: read, write, execute, for user, group, others.
5097 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5098 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5099 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5101 # Setuid/Setgid/Stickiness.
5103 S_ISUID S_ISGID S_ISVTX S_ISTXT
5105 # File types. Not necessarily all are available on your system.
5107 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5109 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5111 S_IREAD S_IWRITE S_IEXEC
5113 and the S_IF* functions are
5115 S_IFMODE($mode) the part of $mode containing the permission bits
5116 and the setuid/setgid/sticky bits
5118 S_IFMT($mode) the part of $mode containing the file type
5119 which can be bit-anded with e.g. S_IFREG
5120 or with the following functions
5122 # The operators -f, -d, -l, -b, -c, -p, and -s.
5124 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5125 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5127 # No direct -X operator counterpart, but for the first one
5128 # the -g operator is often equivalent. The ENFMT stands for
5129 # record flocking enforcement, a platform-dependent feature.
5131 S_ISENFMT($mode) S_ISWHT($mode)
5133 See your native chmod(2) and stat(2) documentation for more details
5134 about the S_* constants.
5140 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5141 doing many pattern matches on the string before it is next modified.
5142 This may or may not save time, depending on the nature and number of
5143 patterns you are searching on, and on the distribution of character
5144 frequencies in the string to be searched--you probably want to compare
5145 run times with and without it to see which runs faster. Those loops
5146 which scan for many short constant strings (including the constant
5147 parts of more complex patterns) will benefit most. You may have only
5148 one C<study> active at a time--if you study a different scalar the first
5149 is "unstudied". (The way C<study> works is this: a linked list of every
5150 character in the string to be searched is made, so we know, for
5151 example, where all the C<'k'> characters are. From each search string,
5152 the rarest character is selected, based on some static frequency tables
5153 constructed from some C programs and English text. Only those places
5154 that contain this "rarest" character are examined.)
5156 For example, here is a loop that inserts index producing entries
5157 before any line containing a certain pattern:
5161 print ".IX foo\n" if /\bfoo\b/;
5162 print ".IX bar\n" if /\bbar\b/;
5163 print ".IX blurfl\n" if /\bblurfl\b/;
5168 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5169 will be looked at, because C<f> is rarer than C<o>. In general, this is
5170 a big win except in pathological cases. The only question is whether
5171 it saves you more time than it took to build the linked list in the
5174 Note that if you have to look for strings that you don't know till
5175 runtime, you can build an entire loop as a string and C<eval> that to
5176 avoid recompiling all your patterns all the time. Together with
5177 undefining C<$/> to input entire files as one record, this can be very
5178 fast, often faster than specialized programs like fgrep(1). The following
5179 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5180 out the names of those files that contain a match:
5182 $search = 'while (<>) { study;';
5183 foreach $word (@words) {
5184 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5189 eval $search; # this screams
5190 $/ = "\n"; # put back to normal input delimiter
5191 foreach $file (sort keys(%seen)) {
5199 =item sub NAME BLOCK
5201 This is subroutine definition, not a real function I<per se>. With just a
5202 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5203 Without a NAME, it's an anonymous function declaration, and does actually
5204 return a value: the CODE ref of the closure you just created. See L<perlsub>
5205 and L<perlref> for details.
5207 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5209 =item substr EXPR,OFFSET,LENGTH
5211 =item substr EXPR,OFFSET
5213 Extracts a substring out of EXPR and returns it. First character is at
5214 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5215 If OFFSET is negative (or more precisely, less than C<$[>), starts
5216 that far from the end of the string. If LENGTH is omitted, returns
5217 everything to the end of the string. If LENGTH is negative, leaves that
5218 many characters off the end of the string.
5220 You can use the substr() function as an lvalue, in which case EXPR
5221 must itself be an lvalue. If you assign something shorter than LENGTH,
5222 the string will shrink, and if you assign something longer than LENGTH,
5223 the string will grow to accommodate it. To keep the string the same
5224 length you may need to pad or chop your value using C<sprintf>.
5226 If OFFSET and LENGTH specify a substring that is partly outside the
5227 string, only the part within the string is returned. If the substring
5228 is beyond either end of the string, substr() returns the undefined
5229 value and produces a warning. When used as an lvalue, specifying a
5230 substring that is entirely outside the string is a fatal error.
5231 Here's an example showing the behavior for boundary cases:
5234 substr($name, 4) = 'dy'; # $name is now 'freddy'
5235 my $null = substr $name, 6, 2; # returns '' (no warning)
5236 my $oops = substr $name, 7; # returns undef, with warning
5237 substr($name, 7) = 'gap'; # fatal error
5239 An alternative to using substr() as an lvalue is to specify the
5240 replacement string as the 4th argument. This allows you to replace
5241 parts of the EXPR and return what was there before in one operation,
5242 just as you can with splice().
5244 =item symlink OLDFILE,NEWFILE
5246 Creates a new filename symbolically linked to the old filename.
5247 Returns C<1> for success, C<0> otherwise. On systems that don't support
5248 symbolic links, produces a fatal error at run time. To check for that,
5251 $symlink_exists = eval { symlink("",""); 1 };
5255 Calls the system call specified as the first element of the list,
5256 passing the remaining elements as arguments to the system call. If
5257 unimplemented, produces a fatal error. The arguments are interpreted
5258 as follows: if a given argument is numeric, the argument is passed as
5259 an int. If not, the pointer to the string value is passed. You are
5260 responsible to make sure a string is pre-extended long enough to
5261 receive any result that might be written into a string. You can't use a
5262 string literal (or other read-only string) as an argument to C<syscall>
5263 because Perl has to assume that any string pointer might be written
5265 integer arguments are not literals and have never been interpreted in a
5266 numeric context, you may need to add C<0> to them to force them to look
5267 like numbers. This emulates the C<syswrite> function (or vice versa):
5269 require 'syscall.ph'; # may need to run h2ph
5271 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5273 Note that Perl supports passing of up to only 14 arguments to your system call,
5274 which in practice should usually suffice.
5276 Syscall returns whatever value returned by the system call it calls.
5277 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5278 Note that some system calls can legitimately return C<-1>. The proper
5279 way to handle such calls is to assign C<$!=0;> before the call and
5280 check the value of C<$!> if syscall returns C<-1>.
5282 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5283 number of the read end of the pipe it creates. There is no way
5284 to retrieve the file number of the other end. You can avoid this
5285 problem by using C<pipe> instead.
5287 =item sysopen FILEHANDLE,FILENAME,MODE
5289 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5291 Opens the file whose filename is given by FILENAME, and associates it
5292 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5293 the name of the real filehandle wanted. This function calls the
5294 underlying operating system's C<open> function with the parameters
5295 FILENAME, MODE, PERMS.
5297 The possible values and flag bits of the MODE parameter are
5298 system-dependent; they are available via the standard module C<Fcntl>.
5299 See the documentation of your operating system's C<open> to see which
5300 values and flag bits are available. You may combine several flags
5301 using the C<|>-operator.
5303 Some of the most common values are C<O_RDONLY> for opening the file in
5304 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5305 and C<O_RDWR> for opening the file in read-write mode, and.
5307 For historical reasons, some values work on almost every system
5308 supported by perl: zero means read-only, one means write-only, and two
5309 means read/write. We know that these values do I<not> work under
5310 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5311 use them in new code.
5313 If the file named by FILENAME does not exist and the C<open> call creates
5314 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5315 PERMS specifies the permissions of the newly created file. If you omit
5316 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5317 These permission values need to be in octal, and are modified by your
5318 process's current C<umask>.
5320 In many systems the C<O_EXCL> flag is available for opening files in
5321 exclusive mode. This is B<not> locking: exclusiveness means here that
5322 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5325 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5327 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5328 that takes away the user's option to have a more permissive umask.
5329 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5332 Note that C<sysopen> depends on the fdopen() C library function.
5333 On many UNIX systems, fdopen() is known to fail when file descriptors
5334 exceed a certain value, typically 255. If you need more file
5335 descriptors than that, consider rebuilding Perl to use the C<sfio>
5336 library, or perhaps using the POSIX::open() function.
5338 See L<perlopentut> for a kinder, gentler explanation of opening files.
5340 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5342 =item sysread FILEHANDLE,SCALAR,LENGTH
5344 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5345 the specified FILEHANDLE, using the system call read(2). It bypasses
5346 buffered IO, so mixing this with other kinds of reads, C<print>,
5347 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5348 stdio usually buffers data. Returns the number of characters actually
5349 read, C<0> at end of file, or undef if there was an error. SCALAR
5350 will be grown or shrunk so that the last byte actually read is the
5351 last byte of the scalar after the read.
5353 Note the I<characters>: depending on the status of the filehandle,
5354 either (8-bit) bytes or characters are read. By default all
5355 filehandles operate on bytes, but for example if the filehandle has
5356 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5357 pragma, L<open>), the I/O will operate on characters, not bytes.
5359 An OFFSET may be specified to place the read data at some place in the
5360 string other than the beginning. A negative OFFSET specifies
5361 placement at that many characters counting backwards from the end of
5362 the string. A positive OFFSET greater than the length of SCALAR
5363 results in the string being padded to the required size with C<"\0">
5364 bytes before the result of the read is appended.
5366 There is no syseof() function, which is ok, since eof() doesn't work
5367 very well on device files (like ttys) anyway. Use sysread() and check
5368 for a return value for 0 to decide whether you're done.
5370 =item sysseek FILEHANDLE,POSITION,WHENCE
5372 Sets FILEHANDLE's system position I<in bytes> using the system call
5373 lseek(2). FILEHANDLE may be an expression whose value gives the name
5374 of the filehandle. The values for WHENCE are C<0> to set the new
5375 position to POSITION, C<1> to set the it to the current position plus
5376 POSITION, and C<2> to set it to EOF plus POSITION (typically
5379 Note the I<in bytes>: even if the filehandle has been set to operate
5380 on characters (for example by using the C<:utf8> discipline), tell()
5381 will return byte offsets, not character offsets (because implementing
5382 that would render sysseek() very slow).
5384 sysseek() bypasses normal buffered io, so mixing this with reads (other
5385 than C<sysread>, for example >< or read()) C<print>, C<write>,
5386 C<seek>, C<tell>, or C<eof> may cause confusion.
5388 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5389 and C<SEEK_END> (start of the file, current position, end of the file)
5390 from the Fcntl module. Use of the constants is also more portable
5391 than relying on 0, 1, and 2. For example to define a "systell" function:
5393 use Fnctl 'SEEK_CUR';
5394 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5396 Returns the new position, or the undefined value on failure. A position
5397 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5398 true on success and false on failure, yet you can still easily determine
5403 =item system PROGRAM LIST
5405 Does exactly the same thing as C<exec LIST>, except that a fork is
5406 done first, and the parent process waits for the child process to
5407 complete. Note that argument processing varies depending on the
5408 number of arguments. If there is more than one argument in LIST,
5409 or if LIST is an array with more than one value, starts the program
5410 given by the first element of the list with arguments given by the
5411 rest of the list. If there is only one scalar argument, the argument
5412 is checked for shell metacharacters, and if there are any, the
5413 entire argument is passed to the system's command shell for parsing
5414 (this is C</bin/sh -c> on Unix platforms, but varies on other
5415 platforms). If there are no shell metacharacters in the argument,
5416 it is split into words and passed directly to C<execvp>, which is
5419 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5420 output before any operation that may do a fork, but this may not be
5421 supported on some platforms (see L<perlport>). To be safe, you may need
5422 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5423 of C<IO::Handle> on any open handles.
5425 The return value is the exit status of the program as returned by the
5426 C<wait> call. To get the actual exit value shift right by eight (see below).
5427 See also L</exec>. This is I<not> what you want to use to capture
5428 the output from a command, for that you should use merely backticks or
5429 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5430 indicates a failure to start the program (inspect $! for the reason).
5432 Like C<exec>, C<system> allows you to lie to a program about its name if
5433 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5435 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5436 killing the program they're running doesn't actually interrupt
5439 @args = ("command", "arg1", "arg2");
5441 or die "system @args failed: $?"
5443 You can check all the failure possibilities by inspecting
5446 $exit_value = $? >> 8;
5447 $signal_num = $? & 127;
5448 $dumped_core = $? & 128;
5450 or more portably by using the W*() calls of the POSIX extension;
5451 see L<perlport> for more information.
5453 When the arguments get executed via the system shell, results
5454 and return codes will be subject to its quirks and capabilities.
5455 See L<perlop/"`STRING`"> and L</exec> for details.
5457 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5459 =item syswrite FILEHANDLE,SCALAR,LENGTH
5461 =item syswrite FILEHANDLE,SCALAR
5463 Attempts to write LENGTH characters of data from variable SCALAR to
5464 the specified FILEHANDLE, using the system call write(2). If LENGTH
5465 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5466 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5467 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5468 buffers data. Returns the number of characters actually written, or
5469 C<undef> if there was an error. If the LENGTH is greater than the
5470 available data in the SCALAR after the OFFSET, only as much data as is
5471 available will be written.
5473 An OFFSET may be specified to write the data from some part of the
5474 string other than the beginning. A negative OFFSET specifies writing
5475 that many characters counting backwards from the end of the string.
5476 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5478 Note the I<characters>: depending on the status of the filehandle,
5479 either (8-bit) bytes or characters are written. By default all
5480 filehandles operate on bytes, but for example if the filehandle has
5481 been opened with the C<:utf8> discipline (see L</open>, and the open
5482 pragma, L<open>), the I/O will operate on characters, not bytes.
5484 =item tell FILEHANDLE
5488 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5489 error. FILEHANDLE may be an expression whose value gives the name of
5490 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5493 Note the I<in bytes>: even if the filehandle has been set to
5494 operate on characters (for example by using the C<:utf8> open
5495 discipline), tell() will return byte offsets, not character offsets
5496 (because that would render seek() and tell() rather slow).
5498 The return value of tell() for the standard streams like the STDIN
5499 depends on the operating system: it may return -1 or something else.
5500 tell() on pipes, fifos, and sockets usually returns -1.
5502 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5504 Do not use tell() on a filehandle that has been opened using
5505 sysopen(), use sysseek() for that as described above. Why? Because
5506 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5507 buffered filehandles. sysseek() make sense only on the first kind,
5508 tell() only makes sense on the second kind.
5510 =item telldir DIRHANDLE
5512 Returns the current position of the C<readdir> routines on DIRHANDLE.
5513 Value may be given to C<seekdir> to access a particular location in a
5514 directory. Has the same caveats about possible directory compaction as
5515 the corresponding system library routine.
5517 =item tie VARIABLE,CLASSNAME,LIST
5519 This function binds a variable to a package class that will provide the
5520 implementation for the variable. VARIABLE is the name of the variable
5521 to be enchanted. CLASSNAME is the name of a class implementing objects
5522 of correct type. Any additional arguments are passed to the C<new>
5523 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5524 or C<TIEHASH>). Typically these are arguments such as might be passed
5525 to the C<dbm_open()> function of C. The object returned by the C<new>
5526 method is also returned by the C<tie> function, which would be useful
5527 if you want to access other methods in CLASSNAME.
5529 Note that functions such as C<keys> and C<values> may return huge lists
5530 when used on large objects, like DBM files. You may prefer to use the
5531 C<each> function to iterate over such. Example:
5533 # print out history file offsets
5535 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5536 while (($key,$val) = each %HIST) {
5537 print $key, ' = ', unpack('L',$val), "\n";
5541 A class implementing a hash should have the following methods:
5543 TIEHASH classname, LIST
5545 STORE this, key, value
5550 NEXTKEY this, lastkey
5554 A class implementing an ordinary array should have the following methods:
5556 TIEARRAY classname, LIST
5558 STORE this, key, value
5560 STORESIZE this, count
5566 SPLICE this, offset, length, LIST
5571 A class implementing a file handle should have the following methods:
5573 TIEHANDLE classname, LIST
5574 READ this, scalar, length, offset
5577 WRITE this, scalar, length, offset
5579 PRINTF this, format, LIST
5583 SEEK this, position, whence
5585 OPEN this, mode, LIST
5590 A class implementing a scalar should have the following methods:
5592 TIESCALAR classname, LIST
5598 Not all methods indicated above need be implemented. See L<perltie>,
5599 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5601 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5602 for you--you need to do that explicitly yourself. See L<DB_File>
5603 or the F<Config> module for interesting C<tie> implementations.
5605 For further details see L<perltie>, L<"tied VARIABLE">.
5609 Returns a reference to the object underlying VARIABLE (the same value
5610 that was originally returned by the C<tie> call that bound the variable
5611 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5616 Returns the number of non-leap seconds since whatever time the system
5617 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5618 and 00:00:00 UTC, January 1, 1970 for most other systems).
5619 Suitable for feeding to C<gmtime> and C<localtime>.
5621 For measuring time in better granularity than one second,
5622 you may use either the Time::HiRes module from CPAN, or
5623 if you have gettimeofday(2), you may be able to use the
5624 C<syscall> interface of Perl, see L<perlfaq8> for details.
5628 Returns a four-element list giving the user and system times, in
5629 seconds, for this process and the children of this process.
5631 ($user,$system,$cuser,$csystem) = times;
5633 In scalar context, C<times> returns C<$user>.
5637 The transliteration operator. Same as C<y///>. See L<perlop>.
5639 =item truncate FILEHANDLE,LENGTH
5641 =item truncate EXPR,LENGTH
5643 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5644 specified length. Produces a fatal error if truncate isn't implemented
5645 on your system. Returns true if successful, the undefined value
5648 The behavior is undefined if LENGTH is greater than the length of the
5655 Returns an uppercased version of EXPR. This is the internal function
5656 implementing the C<\U> escape in double-quoted strings. Respects
5657 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5658 and L<perlunicode> for more details about locale and Unicode support.
5659 It does not attempt to do titlecase mapping on initial letters. See
5660 C<ucfirst> for that.
5662 If EXPR is omitted, uses C<$_>.
5668 Returns the value of EXPR with the first character in uppercase
5669 (titlecase in Unicode). This is the internal function implementing
5670 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5671 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5672 for more details about locale and Unicode support.
5674 If EXPR is omitted, uses C<$_>.
5680 Sets the umask for the process to EXPR and returns the previous value.
5681 If EXPR is omitted, merely returns the current umask.
5683 The Unix permission C<rwxr-x---> is represented as three sets of three
5684 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5685 and isn't one of the digits). The C<umask> value is such a number
5686 representing disabled permissions bits. The permission (or "mode")
5687 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5688 even if you tell C<sysopen> to create a file with permissions C<0777>,
5689 if your umask is C<0022> then the file will actually be created with
5690 permissions C<0755>. If your C<umask> were C<0027> (group can't
5691 write; others can't read, write, or execute), then passing
5692 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5695 Here's some advice: supply a creation mode of C<0666> for regular
5696 files (in C<sysopen>) and one of C<0777> for directories (in
5697 C<mkdir>) and executable files. This gives users the freedom of
5698 choice: if they want protected files, they might choose process umasks
5699 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5700 Programs should rarely if ever make policy decisions better left to
5701 the user. The exception to this is when writing files that should be
5702 kept private: mail files, web browser cookies, I<.rhosts> files, and
5705 If umask(2) is not implemented on your system and you are trying to
5706 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5707 fatal error at run time. If umask(2) is not implemented and you are
5708 not trying to restrict access for yourself, returns C<undef>.
5710 Remember that a umask is a number, usually given in octal; it is I<not> a
5711 string of octal digits. See also L</oct>, if all you have is a string.
5717 Undefines the value of EXPR, which must be an lvalue. Use only on a
5718 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5719 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5720 will probably not do what you expect on most predefined variables or
5721 DBM list values, so don't do that; see L<delete>.) Always returns the
5722 undefined value. You can omit the EXPR, in which case nothing is
5723 undefined, but you still get an undefined value that you could, for
5724 instance, return from a subroutine, assign to a variable or pass as a
5725 parameter. Examples:
5728 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5732 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5733 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5734 select undef, undef, undef, 0.25;
5735 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5737 Note that this is a unary operator, not a list operator.
5743 Deletes a list of files. Returns the number of files successfully
5746 $cnt = unlink 'a', 'b', 'c';
5750 Note: C<unlink> will not delete directories unless you are superuser and
5751 the B<-U> flag is supplied to Perl. Even if these conditions are
5752 met, be warned that unlinking a directory can inflict damage on your
5753 filesystem. Use C<rmdir> instead.
5755 If LIST is omitted, uses C<$_>.
5757 =item unpack TEMPLATE,EXPR
5759 C<unpack> does the reverse of C<pack>: it takes a string
5760 and expands it out into a list of values.
5761 (In scalar context, it returns merely the first value produced.)
5763 The string is broken into chunks described by the TEMPLATE. Each chunk
5764 is converted separately to a value. Typically, either the string is a result
5765 of C<pack>, or the bytes of the string represent a C structure of some
5768 The TEMPLATE has the same format as in the C<pack> function.
5769 Here's a subroutine that does substring:
5772 my($what,$where,$howmuch) = @_;
5773 unpack("x$where a$howmuch", $what);
5778 sub ordinal { unpack("c",$_[0]); } # same as ord()
5780 In addition to fields allowed in pack(), you may prefix a field with
5781 a %<number> to indicate that
5782 you want a <number>-bit checksum of the items instead of the items
5783 themselves. Default is a 16-bit checksum. Checksum is calculated by
5784 summing numeric values of expanded values (for string fields the sum of
5785 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5787 For example, the following
5788 computes the same number as the System V sum program:
5792 unpack("%32C*",<>) % 65535;
5795 The following efficiently counts the number of set bits in a bit vector:
5797 $setbits = unpack("%32b*", $selectmask);
5799 The C<p> and C<P> formats should be used with care. Since Perl
5800 has no way of checking whether the value passed to C<unpack()>
5801 corresponds to a valid memory location, passing a pointer value that's
5802 not known to be valid is likely to have disastrous consequences.
5804 If the repeat count of a field is larger than what the remainder of
5805 the input string allows, repeat count is decreased. If the input string
5806 is longer than one described by the TEMPLATE, the rest is ignored.
5808 See L</pack> for more examples and notes.
5810 =item untie VARIABLE
5812 Breaks the binding between a variable and a package. (See C<tie>.)
5814 =item unshift ARRAY,LIST
5816 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5817 depending on how you look at it. Prepends list to the front of the
5818 array, and returns the new number of elements in the array.
5820 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5822 Note the LIST is prepended whole, not one element at a time, so the
5823 prepended elements stay in the same order. Use C<reverse> to do the
5826 =item use Module VERSION LIST
5828 =item use Module VERSION
5830 =item use Module LIST
5836 Imports some semantics into the current package from the named module,
5837 generally by aliasing certain subroutine or variable names into your
5838 package. It is exactly equivalent to
5840 BEGIN { require Module; import Module LIST; }
5842 except that Module I<must> be a bareword.
5844 VERSION may be either a numeric argument such as 5.006, which will be
5845 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5846 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5847 greater than the version of the current Perl interpreter; Perl will not
5848 attempt to parse the rest of the file. Compare with L</require>, which can
5849 do a similar check at run time.
5851 Specifying VERSION as a literal of the form v5.6.1 should generally be
5852 avoided, because it leads to misleading error messages under earlier
5853 versions of Perl which do not support this syntax. The equivalent numeric
5854 version should be used instead.
5856 use v5.6.1; # compile time version check
5858 use 5.006_001; # ditto; preferred for backwards compatibility
5860 This is often useful if you need to check the current Perl version before
5861 C<use>ing library modules that have changed in incompatible ways from
5862 older versions of Perl. (We try not to do this more than we have to.)
5864 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5865 C<require> makes sure the module is loaded into memory if it hasn't been
5866 yet. The C<import> is not a builtin--it's just an ordinary static method
5867 call into the C<Module> package to tell the module to import the list of
5868 features back into the current package. The module can implement its
5869 C<import> method any way it likes, though most modules just choose to
5870 derive their C<import> method via inheritance from the C<Exporter> class that
5871 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5872 method can be found then the call is skipped.
5874 If you do not want to call the package's C<import> method (for instance,
5875 to stop your namespace from being altered), explicitly supply the empty list:
5879 That is exactly equivalent to
5881 BEGIN { require Module }
5883 If the VERSION argument is present between Module and LIST, then the
5884 C<use> will call the VERSION method in class Module with the given
5885 version as an argument. The default VERSION method, inherited from
5886 the UNIVERSAL class, croaks if the given version is larger than the
5887 value of the variable C<$Module::VERSION>.
5889 Again, there is a distinction between omitting LIST (C<import> called
5890 with no arguments) and an explicit empty LIST C<()> (C<import> not
5891 called). Note that there is no comma after VERSION!
5893 Because this is a wide-open interface, pragmas (compiler directives)
5894 are also implemented this way. Currently implemented pragmas are:
5899 use sigtrap qw(SEGV BUS);
5900 use strict qw(subs vars refs);
5901 use subs qw(afunc blurfl);
5902 use warnings qw(all);
5903 use sort qw(stable _quicksort _mergesort);
5905 Some of these pseudo-modules import semantics into the current
5906 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5907 which import symbols into the current package (which are effective
5908 through the end of the file).
5910 There's a corresponding C<no> command that unimports meanings imported
5911 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5917 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5918 for the C<-M> and C<-m> command-line options to perl that give C<use>
5919 functionality from the command-line.
5923 Changes the access and modification times on each file of a list of
5924 files. The first two elements of the list must be the NUMERICAL access
5925 and modification times, in that order. Returns the number of files
5926 successfully changed. The inode change time of each file is set
5927 to the current time. This code has the same effect as the C<touch>
5928 command if the files already exist:
5932 utime $now, $now, @ARGV;
5934 If the first two elements of the list are C<undef>, then the utime(2)
5935 function in the C library will be called with a null second argument.
5936 On most systems, this will set the file's access and modification
5937 times to the current time. (i.e. equivalent to the example above.)
5939 utime undef, undef, @ARGV;
5943 Returns a list consisting of all the values of the named hash. (In a
5944 scalar context, returns the number of values.) The values are
5945 returned in an apparently random order. The actual random order is
5946 subject to change in future versions of perl, but it is guaranteed to
5947 be the same order as either the C<keys> or C<each> function would
5948 produce on the same (unmodified) hash.
5950 Note that the values are not copied, which means modifying them will
5951 modify the contents of the hash:
5953 for (values %hash) { s/foo/bar/g } # modifies %hash values
5954 for (@hash{keys %hash}) { s/foo/bar/g } # same
5956 As a side effect, calling values() resets the HASH's internal iterator.
5957 See also C<keys>, C<each>, and C<sort>.
5959 =item vec EXPR,OFFSET,BITS
5961 Treats the string in EXPR as a bit vector made up of elements of
5962 width BITS, and returns the value of the element specified by OFFSET
5963 as an unsigned integer. BITS therefore specifies the number of bits
5964 that are reserved for each element in the bit vector. This must
5965 be a power of two from 1 to 32 (or 64, if your platform supports
5968 If BITS is 8, "elements" coincide with bytes of the input string.
5970 If BITS is 16 or more, bytes of the input string are grouped into chunks
5971 of size BITS/8, and each group is converted to a number as with
5972 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5973 for BITS==64). See L<"pack"> for details.
5975 If bits is 4 or less, the string is broken into bytes, then the bits
5976 of each byte are broken into 8/BITS groups. Bits of a byte are
5977 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5978 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5979 breaking the single input byte C<chr(0x36)> into two groups gives a list
5980 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5982 C<vec> may also be assigned to, in which case parentheses are needed
5983 to give the expression the correct precedence as in
5985 vec($image, $max_x * $x + $y, 8) = 3;
5987 If the selected element is outside the string, the value 0 is returned.
5988 If an element off the end of the string is written to, Perl will first
5989 extend the string with sufficiently many zero bytes. It is an error
5990 to try to write off the beginning of the string (i.e. negative OFFSET).
5992 The string should not contain any character with the value > 255 (which
5993 can only happen if you're using UTF8 encoding). If it does, it will be
5994 treated as something which is not UTF8 encoded. When the C<vec> was
5995 assigned to, other parts of your program will also no longer consider the
5996 string to be UTF8 encoded. In other words, if you do have such characters
5997 in your string, vec() will operate on the actual byte string, and not the
5998 conceptual character string.
6000 Strings created with C<vec> can also be manipulated with the logical
6001 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6002 vector operation is desired when both operands are strings.
6003 See L<perlop/"Bitwise String Operators">.
6005 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6006 The comments show the string after each step. Note that this code works
6007 in the same way on big-endian or little-endian machines.
6010 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6012 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6013 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6015 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6016 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6017 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6018 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6019 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6020 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6022 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6023 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6024 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6027 To transform a bit vector into a string or list of 0's and 1's, use these:
6029 $bits = unpack("b*", $vector);
6030 @bits = split(//, unpack("b*", $vector));
6032 If you know the exact length in bits, it can be used in place of the C<*>.
6034 Here is an example to illustrate how the bits actually fall in place:
6040 unpack("V",$_) 01234567890123456789012345678901
6041 ------------------------------------------------------------------
6046 for ($shift=0; $shift < $width; ++$shift) {
6047 for ($off=0; $off < 32/$width; ++$off) {
6048 $str = pack("B*", "0"x32);
6049 $bits = (1<<$shift);
6050 vec($str, $off, $width) = $bits;
6051 $res = unpack("b*",$str);
6052 $val = unpack("V", $str);
6059 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6060 $off, $width, $bits, $val, $res
6064 Regardless of the machine architecture on which it is run, the above
6065 example should print the following table:
6068 unpack("V",$_) 01234567890123456789012345678901
6069 ------------------------------------------------------------------
6070 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6071 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6072 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6073 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6074 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6075 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6076 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6077 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6078 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6079 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6080 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6081 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6082 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6083 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6084 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6085 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6086 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6087 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6088 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6089 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6090 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6091 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6092 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6093 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6094 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6095 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6096 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6097 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6098 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6099 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6100 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6101 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6102 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6103 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6104 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6105 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6106 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6107 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6108 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6109 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6110 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6111 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6112 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6113 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6114 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6115 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6116 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6117 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6118 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6119 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6120 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6121 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6122 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6123 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6124 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6125 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6126 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6127 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6128 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6129 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6130 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6131 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6132 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6133 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6134 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6135 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6136 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6137 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6138 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6139 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6140 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6141 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6142 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6143 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6144 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6145 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6146 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6147 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6148 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6149 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6150 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6151 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6152 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6153 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6154 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6155 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6156 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6157 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6158 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6159 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6160 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6161 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6162 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6163 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6164 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6165 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6166 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6167 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6168 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6169 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6170 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6171 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6172 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6173 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6174 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6175 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6176 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6177 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6178 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6179 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6180 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6181 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6182 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6183 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6184 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6185 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6186 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6187 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6188 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6189 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6190 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6191 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6192 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6193 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6194 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6195 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6196 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6197 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6201 Behaves like the wait(2) system call on your system: it waits for a child
6202 process to terminate and returns the pid of the deceased process, or
6203 C<-1> if there are no child processes. The status is returned in C<$?>.
6204 Note that a return value of C<-1> could mean that child processes are
6205 being automatically reaped, as described in L<perlipc>.
6207 =item waitpid PID,FLAGS
6209 Waits for a particular child process to terminate and returns the pid of
6210 the deceased process, or C<-1> if there is no such child process. On some
6211 systems, a value of 0 indicates that there are processes still running.
6212 The status is returned in C<$?>. If you say
6214 use POSIX ":sys_wait_h";
6217 $kid = waitpid(-1, WNOHANG);
6220 then you can do a non-blocking wait for all pending zombie processes.
6221 Non-blocking wait is available on machines supporting either the
6222 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6223 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6224 system call by remembering the status values of processes that have
6225 exited but have not been harvested by the Perl script yet.)
6227 Note that on some systems, a return value of C<-1> could mean that child
6228 processes are being automatically reaped. See L<perlipc> for details,
6229 and for other examples.
6233 Returns true if the context of the currently executing subroutine is
6234 looking for a list value. Returns false if the context is looking
6235 for a scalar. Returns the undefined value if the context is looking
6236 for no value (void context).
6238 return unless defined wantarray; # don't bother doing more
6239 my @a = complex_calculation();
6240 return wantarray ? @a : "@a";
6242 This function should have been named wantlist() instead.
6246 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6249 If LIST is empty and C<$@> already contains a value (typically from a
6250 previous eval) that value is used after appending C<"\t...caught">
6251 to C<$@>. This is useful for staying almost, but not entirely similar to
6254 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6256 No message is printed if there is a C<$SIG{__WARN__}> handler
6257 installed. It is the handler's responsibility to deal with the message
6258 as it sees fit (like, for instance, converting it into a C<die>). Most
6259 handlers must therefore make arrangements to actually display the
6260 warnings that they are not prepared to deal with, by calling C<warn>
6261 again in the handler. Note that this is quite safe and will not
6262 produce an endless loop, since C<__WARN__> hooks are not called from
6265 You will find this behavior is slightly different from that of
6266 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6267 instead call C<die> again to change it).
6269 Using a C<__WARN__> handler provides a powerful way to silence all
6270 warnings (even the so-called mandatory ones). An example:
6272 # wipe out *all* compile-time warnings
6273 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6275 my $foo = 20; # no warning about duplicate my $foo,
6276 # but hey, you asked for it!
6277 # no compile-time or run-time warnings before here
6280 # run-time warnings enabled after here
6281 warn "\$foo is alive and $foo!"; # does show up
6283 See L<perlvar> for details on setting C<%SIG> entries, and for more
6284 examples. See the Carp module for other kinds of warnings using its
6285 carp() and cluck() functions.
6287 =item write FILEHANDLE
6293 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6294 using the format associated with that file. By default the format for
6295 a file is the one having the same name as the filehandle, but the
6296 format for the current output channel (see the C<select> function) may be set
6297 explicitly by assigning the name of the format to the C<$~> variable.
6299 Top of form processing is handled automatically: if there is
6300 insufficient room on the current page for the formatted record, the
6301 page is advanced by writing a form feed, a special top-of-page format
6302 is used to format the new page header, and then the record is written.
6303 By default the top-of-page format is the name of the filehandle with
6304 "_TOP" appended, but it may be dynamically set to the format of your
6305 choice by assigning the name to the C<$^> variable while the filehandle is
6306 selected. The number of lines remaining on the current page is in
6307 variable C<$->, which can be set to C<0> to force a new page.
6309 If FILEHANDLE is unspecified, output goes to the current default output
6310 channel, which starts out as STDOUT but may be changed by the
6311 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6312 is evaluated and the resulting string is used to look up the name of
6313 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6315 Note that write is I<not> the opposite of C<read>. Unfortunately.
6319 The transliteration operator. Same as C<tr///>. See L<perlop>.