3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
369 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370 test operators, in a way that C<-f -w -x $file> is equivalent to
371 C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372 the return value of C<-f $file> as an argument to another filetest
373 operator, no special magic will happen.)
379 Returns the absolute value of its argument.
380 If VALUE is omitted, uses C<$_>.
382 =item accept NEWSOCKET,GENERICSOCKET
384 Accepts an incoming socket connect, just as the accept(2) system call
385 does. Returns the packed address if it succeeded, false otherwise.
386 See the example in L<perlipc/"Sockets: Client/Server Communication">.
388 On systems that support a close-on-exec flag on files, the flag will
389 be set for the newly opened file descriptor, as determined by the
390 value of $^F. See L<perlvar/$^F>.
396 Arranges to have a SIGALRM delivered to this process after the
397 specified number of wallclock seconds have elapsed. If SECONDS is not
398 specified, the value stored in C<$_> is used. (On some machines,
399 unfortunately, the elapsed time may be up to one second less or more
400 than you specified because of how seconds are counted, and process
401 scheduling may delay the delivery of the signal even further.)
403 Only one timer may be counting at once. Each call disables the
404 previous timer, and an argument of C<0> may be supplied to cancel the
405 previous timer without starting a new one. The returned value is the
406 amount of time remaining on the previous timer.
408 For delays of finer granularity than one second, you may use Perl's
409 four-argument version of select() leaving the first three arguments
410 undefined, or you might be able to use the C<syscall> interface to
411 access setitimer(2) if your system supports it. The Time::HiRes
412 module (from CPAN, and starting from Perl 5.8 part of the standard
413 distribution) may also prove useful.
415 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416 (C<sleep> may be internally implemented in your system with C<alarm>)
418 If you want to use C<alarm> to time out a system call you need to use an
419 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
420 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
421 restart system calls on some systems. Using C<eval>/C<die> always works,
422 modulo the caveats given in L<perlipc/"Signals">.
425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
427 $nread = sysread SOCKET, $buffer, $size;
431 die unless $@ eq "alarm\n"; # propagate unexpected errors
438 For more information see L<perlipc>.
442 Returns the arctangent of Y/X in the range -PI to PI.
444 For the tangent operation, you may use the C<Math::Trig::tan>
445 function, or use the familiar relation:
447 sub tan { sin($_[0]) / cos($_[0]) }
449 =item bind SOCKET,NAME
451 Binds a network address to a socket, just as the bind system call
452 does. Returns true if it succeeded, false otherwise. NAME should be a
453 packed address of the appropriate type for the socket. See the examples in
454 L<perlipc/"Sockets: Client/Server Communication">.
456 =item binmode FILEHANDLE, LAYER
458 =item binmode FILEHANDLE
460 Arranges for FILEHANDLE to be read or written in "binary" or "text"
461 mode on systems where the run-time libraries distinguish between
462 binary and text files. If FILEHANDLE is an expression, the value is
463 taken as the name of the filehandle. Returns true on success,
464 otherwise it returns C<undef> and sets C<$!> (errno).
466 On some systems (in general, DOS and Windows-based systems) binmode()
467 is necessary when you're not working with a text file. For the sake
468 of portability it is a good idea to always use it when appropriate,
469 and to never use it when it isn't appropriate. Also, people can
470 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
472 In other words: regardless of platform, use binmode() on binary data,
473 like for example images.
475 If LAYER is present it is a single string, but may contain multiple
476 directives. The directives alter the behaviour of the file handle.
477 When LAYER is present using binmode on text file makes sense.
479 If LAYER is omitted or specified as C<:raw> the filehandle is made
480 suitable for passing binary data. This includes turning off possible CRLF
481 translation and marking it as bytes (as opposed to Unicode characters).
482 Note that, despite what may be implied in I<"Programming Perl"> (the
483 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
484 -- other layers which would affect binary nature of the stream are
485 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486 PERLIO environment variable.
488 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490 establish default I/O layers. See L<open>.
492 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493 in "Programming Perl, 3rd Edition". However, since the publishing of this
494 book, by many known as "Camel III", the consensus of the naming of this
495 functionality has moved from "discipline" to "layer". All documentation
496 of this version of Perl therefore refers to "layers" rather than to
497 "disciplines". Now back to the regularly scheduled documentation...>
499 To mark FILEHANDLE as UTF-8, use C<:utf8>.
501 In general, binmode() should be called after open() but before any I/O
502 is done on the filehandle. Calling binmode() will normally flush any
503 pending buffered output data (and perhaps pending input data) on the
504 handle. An exception to this is the C<:encoding> layer that
505 changes the default character encoding of the handle, see L<open>.
506 The C<:encoding> layer sometimes needs to be called in
507 mid-stream, and it doesn't flush the stream. The C<:encoding>
508 also implicitly pushes on top of itself the C<:utf8> layer because
509 internally Perl will operate on UTF-8 encoded Unicode characters.
511 The operating system, device drivers, C libraries, and Perl run-time
512 system all work together to let the programmer treat a single
513 character (C<\n>) as the line terminator, irrespective of the external
514 representation. On many operating systems, the native text file
515 representation matches the internal representation, but on some
516 platforms the external representation of C<\n> is made up of more than
519 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520 character to end each line in the external representation of text (even
521 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
522 on Unix and most VMS files). In other systems like OS/2, DOS and the
523 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524 but what's stored in text files are the two characters C<\cM\cJ>. That
525 means that, if you don't use binmode() on these systems, C<\cM\cJ>
526 sequences on disk will be converted to C<\n> on input, and any C<\n> in
527 your program will be converted back to C<\cM\cJ> on output. This is what
528 you want for text files, but it can be disastrous for binary files.
530 Another consequence of using binmode() (on some systems) is that
531 special end-of-file markers will be seen as part of the data stream.
532 For systems from the Microsoft family this means that if your binary
533 data contains C<\cZ>, the I/O subsystem will regard it as the end of
534 the file, unless you use binmode().
536 binmode() is not only important for readline() and print() operations,
537 but also when using read(), seek(), sysread(), syswrite() and tell()
538 (see L<perlport> for more details). See the C<$/> and C<$\> variables
539 in L<perlvar> for how to manually set your input and output
540 line-termination sequences.
542 =item bless REF,CLASSNAME
546 This function tells the thingy referenced by REF that it is now an object
547 in the CLASSNAME package. If CLASSNAME is omitted, the current package
548 is used. Because a C<bless> is often the last thing in a constructor,
549 it returns the reference for convenience. Always use the two-argument
550 version if the function doing the blessing might be inherited by a
551 derived class. See L<perltoot> and L<perlobj> for more about the blessing
552 (and blessings) of objects.
554 Consider always blessing objects in CLASSNAMEs that are mixed case.
555 Namespaces with all lowercase names are considered reserved for
556 Perl pragmata. Builtin types have all uppercase names, so to prevent
557 confusion, you may wish to avoid such package names as well. Make sure
558 that CLASSNAME is a true value.
560 See L<perlmod/"Perl Modules">.
566 Returns the context of the current subroutine call. In scalar context,
567 returns the caller's package name if there is a caller, that is, if
568 we're in a subroutine or C<eval> or C<require>, and the undefined value
569 otherwise. In list context, returns
571 ($package, $filename, $line) = caller;
573 With EXPR, it returns some extra information that the debugger uses to
574 print a stack trace. The value of EXPR indicates how many call frames
575 to go back before the current one.
577 ($package, $filename, $line, $subroutine, $hasargs,
578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
580 Here $subroutine may be C<(eval)> if the frame is not a subroutine
581 call, but an C<eval>. In such a case additional elements $evaltext and
582 C<$is_require> are set: C<$is_require> is true if the frame is created by a
583 C<require> or C<use> statement, $evaltext contains the text of the
584 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
585 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
586 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587 frame.) $subroutine may also be C<(unknown)> if this particular
588 subroutine happens to have been deleted from the symbol table.
589 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591 compiled with. The C<$hints> and C<$bitmask> values are subject to change
592 between versions of Perl, and are not meant for external use.
594 Furthermore, when called from within the DB package, caller returns more
595 detailed information: it sets the list variable C<@DB::args> to be the
596 arguments with which the subroutine was invoked.
598 Be aware that the optimizer might have optimized call frames away before
599 C<caller> had a chance to get the information. That means that C<caller(N)>
600 might not return information about the call frame you expect it do, for
601 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
602 previous time C<caller> was called.
606 Changes the working directory to EXPR, if possible. If EXPR is omitted,
607 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
608 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
609 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
610 neither is set, C<chdir> does nothing. It returns true upon success,
611 false otherwise. See the example under C<die>.
615 Changes the permissions of a list of files. The first element of the
616 list must be the numerical mode, which should probably be an octal
617 number, and which definitely should I<not> a string of octal digits:
618 C<0644> is okay, C<'0644'> is not. Returns the number of files
619 successfully changed. See also L</oct>, if all you have is a string.
621 $cnt = chmod 0755, 'foo', 'bar';
622 chmod 0755, @executables;
623 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
625 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
626 $mode = 0644; chmod $mode, 'foo'; # this is best
628 You can also import the symbolic C<S_I*> constants from the Fcntl
633 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
634 # This is identical to the chmod 0755 of the above example.
642 This safer version of L</chop> removes any trailing string
643 that corresponds to the current value of C<$/> (also known as
644 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
645 number of characters removed from all its arguments. It's often used to
646 remove the newline from the end of an input record when you're worried
647 that the final record may be missing its newline. When in paragraph
648 mode (C<$/ = "">), it removes all trailing newlines from the string.
649 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
650 a reference to an integer or the like, see L<perlvar>) chomp() won't
652 If VARIABLE is omitted, it chomps C<$_>. Example:
655 chomp; # avoid \n on last field
660 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
662 You can actually chomp anything that's an lvalue, including an assignment:
665 chomp($answer = <STDIN>);
667 If you chomp a list, each element is chomped, and the total number of
668 characters removed is returned.
670 If the C<encoding> pragma is in scope then the lengths returned are
671 calculated from the length of C<$/> in Unicode characters, which is not
672 always the same as the length of C<$/> in the native encoding.
674 Note that parentheses are necessary when you're chomping anything
675 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
676 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
677 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
678 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
687 Chops off the last character of a string and returns the character
688 chopped. It is much more efficient than C<s/.$//s> because it neither
689 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
690 If VARIABLE is a hash, it chops the hash's values, but not its keys.
692 You can actually chop anything that's an lvalue, including an assignment.
694 If you chop a list, each element is chopped. Only the value of the
695 last C<chop> is returned.
697 Note that C<chop> returns the last character. To return all but the last
698 character, use C<substr($string, 0, -1)>.
704 Changes the owner (and group) of a list of files. The first two
705 elements of the list must be the I<numeric> uid and gid, in that
706 order. A value of -1 in either position is interpreted by most
707 systems to leave that value unchanged. Returns the number of files
708 successfully changed.
710 $cnt = chown $uid, $gid, 'foo', 'bar';
711 chown $uid, $gid, @filenames;
713 Here's an example that looks up nonnumeric uids in the passwd file:
716 chomp($user = <STDIN>);
718 chomp($pattern = <STDIN>);
720 ($login,$pass,$uid,$gid) = getpwnam($user)
721 or die "$user not in passwd file";
723 @ary = glob($pattern); # expand filenames
724 chown $uid, $gid, @ary;
726 On most systems, you are not allowed to change the ownership of the
727 file unless you're the superuser, although you should be able to change
728 the group to any of your secondary groups. On insecure systems, these
729 restrictions may be relaxed, but this is not a portable assumption.
730 On POSIX systems, you can detect this condition this way:
732 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
733 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
739 Returns the character represented by that NUMBER in the character set.
740 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
741 chr(0x263a) is a Unicode smiley face. Note that characters from 128
742 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743 backward compatibility reasons (but see L<encoding>).
745 If NUMBER is omitted, uses C<$_>.
747 For the reverse, use L</ord>.
749 Note that under the C<bytes> pragma the NUMBER is masked to
752 See L<perlunicode> and L<encoding> for more about Unicode.
754 =item chroot FILENAME
758 This function works like the system call by the same name: it makes the
759 named directory the new root directory for all further pathnames that
760 begin with a C</> by your process and all its children. (It doesn't
761 change your current working directory, which is unaffected.) For security
762 reasons, this call is restricted to the superuser. If FILENAME is
763 omitted, does a C<chroot> to C<$_>.
765 =item close FILEHANDLE
769 Closes the file or pipe associated with the file handle, returning
770 true only if IO buffers are successfully flushed and closes the system
771 file descriptor. Closes the currently selected filehandle if the
774 You don't have to close FILEHANDLE if you are immediately going to do
775 another C<open> on it, because C<open> will close it for you. (See
776 C<open>.) However, an explicit C<close> on an input file resets the line
777 counter (C<$.>), while the implicit close done by C<open> does not.
779 If the file handle came from a piped open, C<close> will additionally
780 return false if one of the other system calls involved fails, or if the
781 program exits with non-zero status. (If the only problem was that the
782 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
783 also waits for the process executing on the pipe to complete, in case you
784 want to look at the output of the pipe afterwards, and
785 implicitly puts the exit status value of that command into C<$?>.
787 Prematurely closing the read end of a pipe (i.e. before the process
788 writing to it at the other end has closed it) will result in a
789 SIGPIPE being delivered to the writer. If the other end can't
790 handle that, be sure to read all the data before closing the pipe.
794 open(OUTPUT, '|sort >foo') # pipe to sort
795 or die "Can't start sort: $!";
796 #... # print stuff to output
797 close OUTPUT # wait for sort to finish
798 or warn $! ? "Error closing sort pipe: $!"
799 : "Exit status $? from sort";
800 open(INPUT, 'foo') # get sort's results
801 or die "Can't open 'foo' for input: $!";
803 FILEHANDLE may be an expression whose value can be used as an indirect
804 filehandle, usually the real filehandle name.
806 =item closedir DIRHANDLE
808 Closes a directory opened by C<opendir> and returns the success of that
811 =item connect SOCKET,NAME
813 Attempts to connect to a remote socket, just as the connect system call
814 does. Returns true if it succeeded, false otherwise. NAME should be a
815 packed address of the appropriate type for the socket. See the examples in
816 L<perlipc/"Sockets: Client/Server Communication">.
820 Actually a flow control statement rather than a function. If there is a
821 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
822 C<foreach>), it is always executed just before the conditional is about to
823 be evaluated again, just like the third part of a C<for> loop in C. Thus
824 it can be used to increment a loop variable, even when the loop has been
825 continued via the C<next> statement (which is similar to the C C<continue>
828 C<last>, C<next>, or C<redo> may appear within a C<continue>
829 block. C<last> and C<redo> will behave as if they had been executed within
830 the main block. So will C<next>, but since it will execute a C<continue>
831 block, it may be more entertaining.
834 ### redo always comes here
837 ### next always comes here
839 # then back the top to re-check EXPR
841 ### last always comes here
843 Omitting the C<continue> section is semantically equivalent to using an
844 empty one, logically enough. In that case, C<next> goes directly back
845 to check the condition at the top of the loop.
851 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
852 takes cosine of C<$_>.
854 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
855 function, or use this relation:
857 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
859 =item crypt PLAINTEXT,SALT
861 Encrypts a string exactly like the crypt(3) function in the C library
862 (assuming that you actually have a version there that has not been
863 extirpated as a potential munition). This can prove useful for checking
864 the password file for lousy passwords, amongst other things. Only the
865 guys wearing white hats should do this.
867 Note that L<crypt|/crypt> is intended to be a one-way function, much like
868 breaking eggs to make an omelette. There is no (known) corresponding
869 decrypt function (in other words, the crypt() is a one-way hash
870 function). As a result, this function isn't all that useful for
871 cryptography. (For that, see your nearby CPAN mirror.)
873 When verifying an existing encrypted string you should use the
874 encrypted text as the salt (like C<crypt($plain, $crypted) eq
875 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
876 and with more exotic implementations. In other words, do not assume
877 anything about the returned string itself, or how many bytes in
878 the encrypted string matter.
880 Traditionally the result is a string of 13 bytes: two first bytes of
881 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
882 the first eight bytes of the encrypted string mattered, but
883 alternative hashing schemes (like MD5), higher level security schemes
884 (like C2), and implementations on non-UNIX platforms may produce
887 When choosing a new salt create a random two character string whose
888 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
889 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
890 characters is just a recommendation; the characters allowed in
891 the salt depend solely on your system's crypt library, and Perl can't
892 restrict what salts C<crypt()> accepts.
894 Here's an example that makes sure that whoever runs this program knows
897 $pwd = (getpwuid($<))[1];
901 chomp($word = <STDIN>);
905 if (crypt($word, $pwd) ne $pwd) {
911 Of course, typing in your own password to whoever asks you
914 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
915 of data, not least of all because you can't get the information
916 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
917 on your favorite CPAN mirror for a slew of potentially useful
920 If using crypt() on a Unicode string (which I<potentially> has
921 characters with codepoints above 255), Perl tries to make sense
922 of the situation by trying to downgrade (a copy of the string)
923 the string back to an eight-bit byte string before calling crypt()
924 (on that copy). If that works, good. If not, crypt() dies with
925 C<Wide character in crypt>.
929 [This function has been largely superseded by the C<untie> function.]
931 Breaks the binding between a DBM file and a hash.
933 =item dbmopen HASH,DBNAME,MASK
935 [This function has been largely superseded by the C<tie> function.]
937 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
938 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
939 argument is I<not> a filehandle, even though it looks like one). DBNAME
940 is the name of the database (without the F<.dir> or F<.pag> extension if
941 any). If the database does not exist, it is created with protection
942 specified by MASK (as modified by the C<umask>). If your system supports
943 only the older DBM functions, you may perform only one C<dbmopen> in your
944 program. In older versions of Perl, if your system had neither DBM nor
945 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
948 If you don't have write access to the DBM file, you can only read hash
949 variables, not set them. If you want to test whether you can write,
950 either use file tests or try setting a dummy hash entry inside an C<eval>,
951 which will trap the error.
953 Note that functions such as C<keys> and C<values> may return huge lists
954 when used on large DBM files. You may prefer to use the C<each>
955 function to iterate over large DBM files. Example:
957 # print out history file offsets
958 dbmopen(%HIST,'/usr/lib/news/history',0666);
959 while (($key,$val) = each %HIST) {
960 print $key, ' = ', unpack('L',$val), "\n";
964 See also L<AnyDBM_File> for a more general description of the pros and
965 cons of the various dbm approaches, as well as L<DB_File> for a particularly
968 You can control which DBM library you use by loading that library
969 before you call dbmopen():
972 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
973 or die "Can't open netscape history file: $!";
979 Returns a Boolean value telling whether EXPR has a value other than
980 the undefined value C<undef>. If EXPR is not present, C<$_> will be
983 Many operations return C<undef> to indicate failure, end of file,
984 system error, uninitialized variable, and other exceptional
985 conditions. This function allows you to distinguish C<undef> from
986 other values. (A simple Boolean test will not distinguish among
987 C<undef>, zero, the empty string, and C<"0">, which are all equally
988 false.) Note that since C<undef> is a valid scalar, its presence
989 doesn't I<necessarily> indicate an exceptional condition: C<pop>
990 returns C<undef> when its argument is an empty array, I<or> when the
991 element to return happens to be C<undef>.
993 You may also use C<defined(&func)> to check whether subroutine C<&func>
994 has ever been defined. The return value is unaffected by any forward
995 declarations of C<&func>. Note that a subroutine which is not defined
996 may still be callable: its package may have an C<AUTOLOAD> method that
997 makes it spring into existence the first time that it is called -- see
1000 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1001 used to report whether memory for that aggregate has ever been
1002 allocated. This behavior may disappear in future versions of Perl.
1003 You should instead use a simple test for size:
1005 if (@an_array) { print "has array elements\n" }
1006 if (%a_hash) { print "has hash members\n" }
1008 When used on a hash element, it tells you whether the value is defined,
1009 not whether the key exists in the hash. Use L</exists> for the latter
1014 print if defined $switch{'D'};
1015 print "$val\n" while defined($val = pop(@ary));
1016 die "Can't readlink $sym: $!"
1017 unless defined($value = readlink $sym);
1018 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1019 $debugging = 0 unless defined $debugging;
1021 Note: Many folks tend to overuse C<defined>, and then are surprised to
1022 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1023 defined values. For example, if you say
1027 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1028 matched "nothing". But it didn't really match nothing--rather, it
1029 matched something that happened to be zero characters long. This is all
1030 very above-board and honest. When a function returns an undefined value,
1031 it's an admission that it couldn't give you an honest answer. So you
1032 should use C<defined> only when you're questioning the integrity of what
1033 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1036 See also L</undef>, L</exists>, L</ref>.
1040 Given an expression that specifies a hash element, array element, hash slice,
1041 or array slice, deletes the specified element(s) from the hash or array.
1042 In the case of an array, if the array elements happen to be at the end,
1043 the size of the array will shrink to the highest element that tests
1044 true for exists() (or 0 if no such element exists).
1046 Returns a list with the same number of elements as the number of elements
1047 for which deletion was attempted. Each element of that list consists of
1048 either the value of the element deleted, or the undefined value. In scalar
1049 context, this means that you get the value of the last element deleted (or
1050 the undefined value if that element did not exist).
1052 %hash = (foo => 11, bar => 22, baz => 33);
1053 $scalar = delete $hash{foo}; # $scalar is 11
1054 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1055 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1057 Deleting from C<%ENV> modifies the environment. Deleting from
1058 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1059 from a C<tie>d hash or array may not necessarily return anything.
1061 Deleting an array element effectively returns that position of the array
1062 to its initial, uninitialized state. Subsequently testing for the same
1063 element with exists() will return false. Note that deleting array
1064 elements in the middle of an array will not shift the index of the ones
1065 after them down--use splice() for that. See L</exists>.
1067 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1069 foreach $key (keys %HASH) {
1073 foreach $index (0 .. $#ARRAY) {
1074 delete $ARRAY[$index];
1079 delete @HASH{keys %HASH};
1081 delete @ARRAY[0 .. $#ARRAY];
1083 But both of these are slower than just assigning the empty list
1084 or undefining %HASH or @ARRAY:
1086 %HASH = (); # completely empty %HASH
1087 undef %HASH; # forget %HASH ever existed
1089 @ARRAY = (); # completely empty @ARRAY
1090 undef @ARRAY; # forget @ARRAY ever existed
1092 Note that the EXPR can be arbitrarily complicated as long as the final
1093 operation is a hash element, array element, hash slice, or array slice
1096 delete $ref->[$x][$y]{$key};
1097 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1099 delete $ref->[$x][$y][$index];
1100 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1104 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1105 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1106 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1107 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1108 an C<eval(),> the error message is stuffed into C<$@> and the
1109 C<eval> is terminated with the undefined value. This makes
1110 C<die> the way to raise an exception.
1112 Equivalent examples:
1114 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1115 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1117 If the last element of LIST does not end in a newline, the current
1118 script line number and input line number (if any) are also printed,
1119 and a newline is supplied. Note that the "input line number" (also
1120 known as "chunk") is subject to whatever notion of "line" happens to
1121 be currently in effect, and is also available as the special variable
1122 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1124 Hint: sometimes appending C<", stopped"> to your message will cause it
1125 to make better sense when the string C<"at foo line 123"> is appended.
1126 Suppose you are running script "canasta".
1128 die "/etc/games is no good";
1129 die "/etc/games is no good, stopped";
1131 produce, respectively
1133 /etc/games is no good at canasta line 123.
1134 /etc/games is no good, stopped at canasta line 123.
1136 See also exit(), warn(), and the Carp module.
1138 If LIST is empty and C<$@> already contains a value (typically from a
1139 previous eval) that value is reused after appending C<"\t...propagated">.
1140 This is useful for propagating exceptions:
1143 die unless $@ =~ /Expected exception/;
1145 If LIST is empty and C<$@> contains an object reference that has a
1146 C<PROPAGATE> method, that method will be called with additional file
1147 and line number parameters. The return value replaces the value in
1148 C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1151 If C<$@> is empty then the string C<"Died"> is used.
1153 die() can also be called with a reference argument. If this happens to be
1154 trapped within an eval(), $@ contains the reference. This behavior permits
1155 a more elaborate exception handling implementation using objects that
1156 maintain arbitrary state about the nature of the exception. Such a scheme
1157 is sometimes preferable to matching particular string values of $@ using
1158 regular expressions. Here's an example:
1160 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1162 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1163 # handle Some::Module::Exception
1166 # handle all other possible exceptions
1170 Because perl will stringify uncaught exception messages before displaying
1171 them, you may want to overload stringification operations on such custom
1172 exception objects. See L<overload> for details about that.
1174 You can arrange for a callback to be run just before the C<die>
1175 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1176 handler will be called with the error text and can change the error
1177 message, if it sees fit, by calling C<die> again. See
1178 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1179 L<"eval BLOCK"> for some examples. Although this feature was meant
1180 to be run only right before your program was to exit, this is not
1181 currently the case--the C<$SIG{__DIE__}> hook is currently called
1182 even inside eval()ed blocks/strings! If one wants the hook to do
1183 nothing in such situations, put
1187 as the first line of the handler (see L<perlvar/$^S>). Because
1188 this promotes strange action at a distance, this counterintuitive
1189 behavior may be fixed in a future release.
1193 Not really a function. Returns the value of the last command in the
1194 sequence of commands indicated by BLOCK. When modified by a loop
1195 modifier, executes the BLOCK once before testing the loop condition.
1196 (On other statements the loop modifiers test the conditional first.)
1198 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1199 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1200 See L<perlsyn> for alternative strategies.
1202 =item do SUBROUTINE(LIST)
1204 A deprecated form of subroutine call. See L<perlsub>.
1208 Uses the value of EXPR as a filename and executes the contents of the
1209 file as a Perl script. Its primary use is to include subroutines
1210 from a Perl subroutine library.
1218 except that it's more efficient and concise, keeps track of the current
1219 filename for error messages, searches the @INC libraries, and updates
1220 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1221 variables. It also differs in that code evaluated with C<do FILENAME>
1222 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1223 same, however, in that it does reparse the file every time you call it,
1224 so you probably don't want to do this inside a loop.
1226 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1227 error. If C<do> can read the file but cannot compile it, it
1228 returns undef and sets an error message in C<$@>. If the file is
1229 successfully compiled, C<do> returns the value of the last expression
1232 Note that inclusion of library modules is better done with the
1233 C<use> and C<require> operators, which also do automatic error checking
1234 and raise an exception if there's a problem.
1236 You might like to use C<do> to read in a program configuration
1237 file. Manual error checking can be done this way:
1239 # read in config files: system first, then user
1240 for $file ("/share/prog/defaults.rc",
1241 "$ENV{HOME}/.someprogrc")
1243 unless ($return = do $file) {
1244 warn "couldn't parse $file: $@" if $@;
1245 warn "couldn't do $file: $!" unless defined $return;
1246 warn "couldn't run $file" unless $return;
1254 This function causes an immediate core dump. See also the B<-u>
1255 command-line switch in L<perlrun>, which does the same thing.
1256 Primarily this is so that you can use the B<undump> program (not
1257 supplied) to turn your core dump into an executable binary after
1258 having initialized all your variables at the beginning of the
1259 program. When the new binary is executed it will begin by executing
1260 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1261 Think of it as a goto with an intervening core dump and reincarnation.
1262 If C<LABEL> is omitted, restarts the program from the top.
1264 B<WARNING>: Any files opened at the time of the dump will I<not>
1265 be open any more when the program is reincarnated, with possible
1266 resulting confusion on the part of Perl.
1268 This function is now largely obsolete, partly because it's very
1269 hard to convert a core file into an executable, and because the
1270 real compiler backends for generating portable bytecode and compilable
1271 C code have superseded it. That's why you should now invoke it as
1272 C<CORE::dump()>, if you don't want to be warned against a possible
1275 If you're looking to use L<dump> to speed up your program, consider
1276 generating bytecode or native C code as described in L<perlcc>. If
1277 you're just trying to accelerate a CGI script, consider using the
1278 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1279 You might also consider autoloading or selfloading, which at least
1280 make your program I<appear> to run faster.
1284 When called in list context, returns a 2-element list consisting of the
1285 key and value for the next element of a hash, so that you can iterate over
1286 it. When called in scalar context, returns only the key for the next
1287 element in the hash.
1289 Entries are returned in an apparently random order. The actual random
1290 order is subject to change in future versions of perl, but it is
1291 guaranteed to be in the same order as either the C<keys> or C<values>
1292 function would produce on the same (unmodified) hash. Since Perl
1293 5.8.1 the ordering is different even between different runs of Perl
1294 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1296 When the hash is entirely read, a null array is returned in list context
1297 (which when assigned produces a false (C<0>) value), and C<undef> in
1298 scalar context. The next call to C<each> after that will start iterating
1299 again. There is a single iterator for each hash, shared by all C<each>,
1300 C<keys>, and C<values> function calls in the program; it can be reset by
1301 reading all the elements from the hash, or by evaluating C<keys HASH> or
1302 C<values HASH>. If you add or delete elements of a hash while you're
1303 iterating over it, you may get entries skipped or duplicated, so
1304 don't. Exception: It is always safe to delete the item most recently
1305 returned by C<each()>, which means that the following code will work:
1307 while (($key, $value) = each %hash) {
1309 delete $hash{$key}; # This is safe
1312 The following prints out your environment like the printenv(1) program,
1313 only in a different order:
1315 while (($key,$value) = each %ENV) {
1316 print "$key=$value\n";
1319 See also C<keys>, C<values> and C<sort>.
1321 =item eof FILEHANDLE
1327 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1328 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1329 gives the real filehandle. (Note that this function actually
1330 reads a character and then C<ungetc>s it, so isn't very useful in an
1331 interactive context.) Do not read from a terminal file (or call
1332 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1333 as terminals may lose the end-of-file condition if you do.
1335 An C<eof> without an argument uses the last file read. Using C<eof()>
1336 with empty parentheses is very different. It refers to the pseudo file
1337 formed from the files listed on the command line and accessed via the
1338 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1339 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1340 used will cause C<@ARGV> to be examined to determine if input is
1341 available. Similarly, an C<eof()> after C<< <> >> has returned
1342 end-of-file will assume you are processing another C<@ARGV> list,
1343 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1344 see L<perlop/"I/O Operators">.
1346 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1347 detect the end of each file, C<eof()> will only detect the end of the
1348 last file. Examples:
1350 # reset line numbering on each input file
1352 next if /^\s*#/; # skip comments
1355 close ARGV if eof; # Not eof()!
1358 # insert dashes just before last line of last file
1360 if (eof()) { # check for end of last file
1361 print "--------------\n";
1364 last if eof(); # needed if we're reading from a terminal
1367 Practical hint: you almost never need to use C<eof> in Perl, because the
1368 input operators typically return C<undef> when they run out of data, or if
1375 In the first form, the return value of EXPR is parsed and executed as if it
1376 were a little Perl program. The value of the expression (which is itself
1377 determined within scalar context) is first parsed, and if there weren't any
1378 errors, executed in the lexical context of the current Perl program, so
1379 that any variable settings or subroutine and format definitions remain
1380 afterwards. Note that the value is parsed every time the eval executes.
1381 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1382 delay parsing and subsequent execution of the text of EXPR until run time.
1384 In the second form, the code within the BLOCK is parsed only once--at the
1385 same time the code surrounding the eval itself was parsed--and executed
1386 within the context of the current Perl program. This form is typically
1387 used to trap exceptions more efficiently than the first (see below), while
1388 also providing the benefit of checking the code within BLOCK at compile
1391 The final semicolon, if any, may be omitted from the value of EXPR or within
1394 In both forms, the value returned is the value of the last expression
1395 evaluated inside the mini-program; a return statement may be also used, just
1396 as with subroutines. The expression providing the return value is evaluated
1397 in void, scalar, or list context, depending on the context of the eval itself.
1398 See L</wantarray> for more on how the evaluation context can be determined.
1400 If there is a syntax error or runtime error, or a C<die> statement is
1401 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1402 error message. If there was no error, C<$@> is guaranteed to be a null
1403 string. Beware that using C<eval> neither silences perl from printing
1404 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1405 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1406 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1407 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1409 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1410 determining whether a particular feature (such as C<socket> or C<symlink>)
1411 is implemented. It is also Perl's exception trapping mechanism, where
1412 the die operator is used to raise exceptions.
1414 If the code to be executed doesn't vary, you may use the eval-BLOCK
1415 form to trap run-time errors without incurring the penalty of
1416 recompiling each time. The error, if any, is still returned in C<$@>.
1419 # make divide-by-zero nonfatal
1420 eval { $answer = $a / $b; }; warn $@ if $@;
1422 # same thing, but less efficient
1423 eval '$answer = $a / $b'; warn $@ if $@;
1425 # a compile-time error
1426 eval { $answer = }; # WRONG
1429 eval '$answer ='; # sets $@
1431 Due to the current arguably broken state of C<__DIE__> hooks, when using
1432 the C<eval{}> form as an exception trap in libraries, you may wish not
1433 to trigger any C<__DIE__> hooks that user code may have installed.
1434 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1435 as shown in this example:
1437 # a very private exception trap for divide-by-zero
1438 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1441 This is especially significant, given that C<__DIE__> hooks can call
1442 C<die> again, which has the effect of changing their error messages:
1444 # __DIE__ hooks may modify error messages
1446 local $SIG{'__DIE__'} =
1447 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1448 eval { die "foo lives here" };
1449 print $@ if $@; # prints "bar lives here"
1452 Because this promotes action at a distance, this counterintuitive behavior
1453 may be fixed in a future release.
1455 With an C<eval>, you should be especially careful to remember what's
1456 being looked at when:
1462 eval { $x }; # CASE 4
1464 eval "\$$x++"; # CASE 5
1467 Cases 1 and 2 above behave identically: they run the code contained in
1468 the variable $x. (Although case 2 has misleading double quotes making
1469 the reader wonder what else might be happening (nothing is).) Cases 3
1470 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1471 does nothing but return the value of $x. (Case 4 is preferred for
1472 purely visual reasons, but it also has the advantage of compiling at
1473 compile-time instead of at run-time.) Case 5 is a place where
1474 normally you I<would> like to use double quotes, except that in this
1475 particular situation, you can just use symbolic references instead, as
1478 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1479 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1481 Note that as a very special case, an C<eval ''> executed within the C<DB>
1482 package doesn't see the usual surrounding lexical scope, but rather the
1483 scope of the first non-DB piece of code that called it. You don't normally
1484 need to worry about this unless you are writing a Perl debugger.
1488 =item exec PROGRAM LIST
1490 The C<exec> function executes a system command I<and never returns>--
1491 use C<system> instead of C<exec> if you want it to return. It fails and
1492 returns false only if the command does not exist I<and> it is executed
1493 directly instead of via your system's command shell (see below).
1495 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1496 warns you if there is a following statement which isn't C<die>, C<warn>,
1497 or C<exit> (if C<-w> is set - but you always do that). If you
1498 I<really> want to follow an C<exec> with some other statement, you
1499 can use one of these styles to avoid the warning:
1501 exec ('foo') or print STDERR "couldn't exec foo: $!";
1502 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1504 If there is more than one argument in LIST, or if LIST is an array
1505 with more than one value, calls execvp(3) with the arguments in LIST.
1506 If there is only one scalar argument or an array with one element in it,
1507 the argument is checked for shell metacharacters, and if there are any,
1508 the entire argument is passed to the system's command shell for parsing
1509 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1510 If there are no shell metacharacters in the argument, it is split into
1511 words and passed directly to C<execvp>, which is more efficient.
1514 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1515 exec "sort $outfile | uniq";
1517 If you don't really want to execute the first argument, but want to lie
1518 to the program you are executing about its own name, you can specify
1519 the program you actually want to run as an "indirect object" (without a
1520 comma) in front of the LIST. (This always forces interpretation of the
1521 LIST as a multivalued list, even if there is only a single scalar in
1524 $shell = '/bin/csh';
1525 exec $shell '-sh'; # pretend it's a login shell
1529 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1531 When the arguments get executed via the system shell, results will
1532 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1535 Using an indirect object with C<exec> or C<system> is also more
1536 secure. This usage (which also works fine with system()) forces
1537 interpretation of the arguments as a multivalued list, even if the
1538 list had just one argument. That way you're safe from the shell
1539 expanding wildcards or splitting up words with whitespace in them.
1541 @args = ( "echo surprise" );
1543 exec @args; # subject to shell escapes
1545 exec { $args[0] } @args; # safe even with one-arg list
1547 The first version, the one without the indirect object, ran the I<echo>
1548 program, passing it C<"surprise"> an argument. The second version
1549 didn't--it tried to run a program literally called I<"echo surprise">,
1550 didn't find it, and set C<$?> to a non-zero value indicating failure.
1552 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1553 output before the exec, but this may not be supported on some platforms
1554 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1555 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1556 open handles in order to avoid lost output.
1558 Note that C<exec> will not call your C<END> blocks, nor will it call
1559 any C<DESTROY> methods in your objects.
1563 Given an expression that specifies a hash element or array element,
1564 returns true if the specified element in the hash or array has ever
1565 been initialized, even if the corresponding value is undefined. The
1566 element is not autovivified if it doesn't exist.
1568 print "Exists\n" if exists $hash{$key};
1569 print "Defined\n" if defined $hash{$key};
1570 print "True\n" if $hash{$key};
1572 print "Exists\n" if exists $array[$index];
1573 print "Defined\n" if defined $array[$index];
1574 print "True\n" if $array[$index];
1576 A hash or array element can be true only if it's defined, and defined if
1577 it exists, but the reverse doesn't necessarily hold true.
1579 Given an expression that specifies the name of a subroutine,
1580 returns true if the specified subroutine has ever been declared, even
1581 if it is undefined. Mentioning a subroutine name for exists or defined
1582 does not count as declaring it. Note that a subroutine which does not
1583 exist may still be callable: its package may have an C<AUTOLOAD>
1584 method that makes it spring into existence the first time that it is
1585 called -- see L<perlsub>.
1587 print "Exists\n" if exists &subroutine;
1588 print "Defined\n" if defined &subroutine;
1590 Note that the EXPR can be arbitrarily complicated as long as the final
1591 operation is a hash or array key lookup or subroutine name:
1593 if (exists $ref->{A}->{B}->{$key}) { }
1594 if (exists $hash{A}{B}{$key}) { }
1596 if (exists $ref->{A}->{B}->[$ix]) { }
1597 if (exists $hash{A}{B}[$ix]) { }
1599 if (exists &{$ref->{A}{B}{$key}}) { }
1601 Although the deepest nested array or hash will not spring into existence
1602 just because its existence was tested, any intervening ones will.
1603 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1604 into existence due to the existence test for the $key element above.
1605 This happens anywhere the arrow operator is used, including even:
1608 if (exists $ref->{"Some key"}) { }
1609 print $ref; # prints HASH(0x80d3d5c)
1611 This surprising autovivification in what does not at first--or even
1612 second--glance appear to be an lvalue context may be fixed in a future
1615 Use of a subroutine call, rather than a subroutine name, as an argument
1616 to exists() is an error.
1619 exists &sub(); # Error
1623 Evaluates EXPR and exits immediately with that value. Example:
1626 exit 0 if $ans =~ /^[Xx]/;
1628 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1629 universally recognized values for EXPR are C<0> for success and C<1>
1630 for error; other values are subject to interpretation depending on the
1631 environment in which the Perl program is running. For example, exiting
1632 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1633 the mailer to return the item undelivered, but that's not true everywhere.
1635 Don't use C<exit> to abort a subroutine if there's any chance that
1636 someone might want to trap whatever error happened. Use C<die> instead,
1637 which can be trapped by an C<eval>.
1639 The exit() function does not always exit immediately. It calls any
1640 defined C<END> routines first, but these C<END> routines may not
1641 themselves abort the exit. Likewise any object destructors that need to
1642 be called are called before the real exit. If this is a problem, you
1643 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1644 See L<perlmod> for details.
1650 Returns I<e> (the natural logarithm base) to the power of EXPR.
1651 If EXPR is omitted, gives C<exp($_)>.
1653 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1655 Implements the fcntl(2) function. You'll probably have to say
1659 first to get the correct constant definitions. Argument processing and
1660 value return works just like C<ioctl> below.
1664 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1665 or die "can't fcntl F_GETFL: $!";
1667 You don't have to check for C<defined> on the return from C<fcntl>.
1668 Like C<ioctl>, it maps a C<0> return from the system call into
1669 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1670 in numeric context. It is also exempt from the normal B<-w> warnings
1671 on improper numeric conversions.
1673 Note that C<fcntl> will produce a fatal error if used on a machine that
1674 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1675 manpage to learn what functions are available on your system.
1677 Here's an example of setting a filehandle named C<REMOTE> to be
1678 non-blocking at the system level. You'll have to negotiate C<$|>
1679 on your own, though.
1681 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1683 $flags = fcntl(REMOTE, F_GETFL, 0)
1684 or die "Can't get flags for the socket: $!\n";
1686 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1687 or die "Can't set flags for the socket: $!\n";
1689 =item fileno FILEHANDLE
1691 Returns the file descriptor for a filehandle, or undefined if the
1692 filehandle is not open. This is mainly useful for constructing
1693 bitmaps for C<select> and low-level POSIX tty-handling operations.
1694 If FILEHANDLE is an expression, the value is taken as an indirect
1695 filehandle, generally its name.
1697 You can use this to find out whether two handles refer to the
1698 same underlying descriptor:
1700 if (fileno(THIS) == fileno(THAT)) {
1701 print "THIS and THAT are dups\n";
1704 (Filehandles connected to memory objects via new features of C<open> may
1705 return undefined even though they are open.)
1708 =item flock FILEHANDLE,OPERATION
1710 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1711 for success, false on failure. Produces a fatal error if used on a
1712 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1713 C<flock> is Perl's portable file locking interface, although it locks
1714 only entire files, not records.
1716 Two potentially non-obvious but traditional C<flock> semantics are
1717 that it waits indefinitely until the lock is granted, and that its locks
1718 B<merely advisory>. Such discretionary locks are more flexible, but offer
1719 fewer guarantees. This means that files locked with C<flock> may be
1720 modified by programs that do not also use C<flock>. See L<perlport>,
1721 your port's specific documentation, or your system-specific local manpages
1722 for details. It's best to assume traditional behavior if you're writing
1723 portable programs. (But if you're not, you should as always feel perfectly
1724 free to write for your own system's idiosyncrasies (sometimes called
1725 "features"). Slavish adherence to portability concerns shouldn't get
1726 in the way of your getting your job done.)
1728 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1729 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1730 you can use the symbolic names if you import them from the Fcntl module,
1731 either individually, or as a group using the ':flock' tag. LOCK_SH
1732 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1733 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1734 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1735 waiting for the lock (check the return status to see if you got it).
1737 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1738 before locking or unlocking it.
1740 Note that the emulation built with lockf(3) doesn't provide shared
1741 locks, and it requires that FILEHANDLE be open with write intent. These
1742 are the semantics that lockf(3) implements. Most if not all systems
1743 implement lockf(3) in terms of fcntl(2) locking, though, so the
1744 differing semantics shouldn't bite too many people.
1746 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1747 be open with read intent to use LOCK_SH and requires that it be open
1748 with write intent to use LOCK_EX.
1750 Note also that some versions of C<flock> cannot lock things over the
1751 network; you would need to use the more system-specific C<fcntl> for
1752 that. If you like you can force Perl to ignore your system's flock(2)
1753 function, and so provide its own fcntl(2)-based emulation, by passing
1754 the switch C<-Ud_flock> to the F<Configure> program when you configure
1757 Here's a mailbox appender for BSD systems.
1759 use Fcntl ':flock'; # import LOCK_* constants
1762 flock(MBOX,LOCK_EX);
1763 # and, in case someone appended
1764 # while we were waiting...
1769 flock(MBOX,LOCK_UN);
1772 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1773 or die "Can't open mailbox: $!";
1776 print MBOX $msg,"\n\n";
1779 On systems that support a real flock(), locks are inherited across fork()
1780 calls, whereas those that must resort to the more capricious fcntl()
1781 function lose the locks, making it harder to write servers.
1783 See also L<DB_File> for other flock() examples.
1787 Does a fork(2) system call to create a new process running the
1788 same program at the same point. It returns the child pid to the
1789 parent process, C<0> to the child process, or C<undef> if the fork is
1790 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1791 are shared, while everything else is copied. On most systems supporting
1792 fork(), great care has gone into making it extremely efficient (for
1793 example, using copy-on-write technology on data pages), making it the
1794 dominant paradigm for multitasking over the last few decades.
1796 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1797 output before forking the child process, but this may not be supported
1798 on some platforms (see L<perlport>). To be safe, you may need to set
1799 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1800 C<IO::Handle> on any open handles in order to avoid duplicate output.
1802 If you C<fork> without ever waiting on your children, you will
1803 accumulate zombies. On some systems, you can avoid this by setting
1804 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1805 forking and reaping moribund children.
1807 Note that if your forked child inherits system file descriptors like
1808 STDIN and STDOUT that are actually connected by a pipe or socket, even
1809 if you exit, then the remote server (such as, say, a CGI script or a
1810 backgrounded job launched from a remote shell) won't think you're done.
1811 You should reopen those to F</dev/null> if it's any issue.
1815 Declare a picture format for use by the C<write> function. For
1819 Test: @<<<<<<<< @||||| @>>>>>
1820 $str, $%, '$' . int($num)
1824 $num = $cost/$quantity;
1828 See L<perlform> for many details and examples.
1830 =item formline PICTURE,LIST
1832 This is an internal function used by C<format>s, though you may call it,
1833 too. It formats (see L<perlform>) a list of values according to the
1834 contents of PICTURE, placing the output into the format output
1835 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1836 Eventually, when a C<write> is done, the contents of
1837 C<$^A> are written to some filehandle, but you could also read C<$^A>
1838 yourself and then set C<$^A> back to C<"">. Note that a format typically
1839 does one C<formline> per line of form, but the C<formline> function itself
1840 doesn't care how many newlines are embedded in the PICTURE. This means
1841 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1842 You may therefore need to use multiple formlines to implement a single
1843 record format, just like the format compiler.
1845 Be careful if you put double quotes around the picture, because an C<@>
1846 character may be taken to mean the beginning of an array name.
1847 C<formline> always returns true. See L<perlform> for other examples.
1849 =item getc FILEHANDLE
1853 Returns the next character from the input file attached to FILEHANDLE,
1854 or the undefined value at end of file, or if there was an error (in
1855 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1856 STDIN. This is not particularly efficient. However, it cannot be
1857 used by itself to fetch single characters without waiting for the user
1858 to hit enter. For that, try something more like:
1861 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1864 system "stty", '-icanon', 'eol', "\001";
1870 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1873 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1877 Determination of whether $BSD_STYLE should be set
1878 is left as an exercise to the reader.
1880 The C<POSIX::getattr> function can do this more portably on
1881 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1882 module from your nearest CPAN site; details on CPAN can be found on
1887 Implements the C library function of the same name, which on most
1888 systems returns the current login from F</etc/utmp>, if any. If null,
1891 $login = getlogin || getpwuid($<) || "Kilroy";
1893 Do not consider C<getlogin> for authentication: it is not as
1894 secure as C<getpwuid>.
1896 =item getpeername SOCKET
1898 Returns the packed sockaddr address of other end of the SOCKET connection.
1901 $hersockaddr = getpeername(SOCK);
1902 ($port, $iaddr) = sockaddr_in($hersockaddr);
1903 $herhostname = gethostbyaddr($iaddr, AF_INET);
1904 $herstraddr = inet_ntoa($iaddr);
1908 Returns the current process group for the specified PID. Use
1909 a PID of C<0> to get the current process group for the
1910 current process. Will raise an exception if used on a machine that
1911 doesn't implement getpgrp(2). If PID is omitted, returns process
1912 group of current process. Note that the POSIX version of C<getpgrp>
1913 does not accept a PID argument, so only C<PID==0> is truly portable.
1917 Returns the process id of the parent process.
1919 Note for Linux users: on Linux, the C functions C<getpid()> and
1920 C<getppid()> return different values from different threads. In order to
1921 be portable, this behavior is not reflected by the perl-level function
1922 C<getppid()>, that returns a consistent value across threads. If you want
1923 to call the underlying C<getppid()>, you may use the CPAN module
1926 =item getpriority WHICH,WHO
1928 Returns the current priority for a process, a process group, or a user.
1929 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1930 machine that doesn't implement getpriority(2).
1936 =item gethostbyname NAME
1938 =item getnetbyname NAME
1940 =item getprotobyname NAME
1946 =item getservbyname NAME,PROTO
1948 =item gethostbyaddr ADDR,ADDRTYPE
1950 =item getnetbyaddr ADDR,ADDRTYPE
1952 =item getprotobynumber NUMBER
1954 =item getservbyport PORT,PROTO
1972 =item sethostent STAYOPEN
1974 =item setnetent STAYOPEN
1976 =item setprotoent STAYOPEN
1978 =item setservent STAYOPEN
1992 These routines perform the same functions as their counterparts in the
1993 system library. In list context, the return values from the
1994 various get routines are as follows:
1996 ($name,$passwd,$uid,$gid,
1997 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1998 ($name,$passwd,$gid,$members) = getgr*
1999 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2000 ($name,$aliases,$addrtype,$net) = getnet*
2001 ($name,$aliases,$proto) = getproto*
2002 ($name,$aliases,$port,$proto) = getserv*
2004 (If the entry doesn't exist you get a null list.)
2006 The exact meaning of the $gcos field varies but it usually contains
2007 the real name of the user (as opposed to the login name) and other
2008 information pertaining to the user. Beware, however, that in many
2009 system users are able to change this information and therefore it
2010 cannot be trusted and therefore the $gcos is tainted (see
2011 L<perlsec>). The $passwd and $shell, user's encrypted password and
2012 login shell, are also tainted, because of the same reason.
2014 In scalar context, you get the name, unless the function was a
2015 lookup by name, in which case you get the other thing, whatever it is.
2016 (If the entry doesn't exist you get the undefined value.) For example:
2018 $uid = getpwnam($name);
2019 $name = getpwuid($num);
2021 $gid = getgrnam($name);
2022 $name = getgrgid($num);
2026 In I<getpw*()> the fields $quota, $comment, and $expire are special
2027 cases in the sense that in many systems they are unsupported. If the
2028 $quota is unsupported, it is an empty scalar. If it is supported, it
2029 usually encodes the disk quota. If the $comment field is unsupported,
2030 it is an empty scalar. If it is supported it usually encodes some
2031 administrative comment about the user. In some systems the $quota
2032 field may be $change or $age, fields that have to do with password
2033 aging. In some systems the $comment field may be $class. The $expire
2034 field, if present, encodes the expiration period of the account or the
2035 password. For the availability and the exact meaning of these fields
2036 in your system, please consult your getpwnam(3) documentation and your
2037 F<pwd.h> file. You can also find out from within Perl what your
2038 $quota and $comment fields mean and whether you have the $expire field
2039 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2040 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2041 files are only supported if your vendor has implemented them in the
2042 intuitive fashion that calling the regular C library routines gets the
2043 shadow versions if you're running under privilege or if there exists
2044 the shadow(3) functions as found in System V ( this includes Solaris
2045 and Linux.) Those systems which implement a proprietary shadow password
2046 facility are unlikely to be supported.
2048 The $members value returned by I<getgr*()> is a space separated list of
2049 the login names of the members of the group.
2051 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2052 C, it will be returned to you via C<$?> if the function call fails. The
2053 C<@addrs> value returned by a successful call is a list of the raw
2054 addresses returned by the corresponding system library call. In the
2055 Internet domain, each address is four bytes long and you can unpack it
2056 by saying something like:
2058 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2060 The Socket library makes this slightly easier:
2063 $iaddr = inet_aton("127.1"); # or whatever address
2064 $name = gethostbyaddr($iaddr, AF_INET);
2066 # or going the other way
2067 $straddr = inet_ntoa($iaddr);
2069 If you get tired of remembering which element of the return list
2070 contains which return value, by-name interfaces are provided
2071 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2072 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2073 and C<User::grent>. These override the normal built-ins, supplying
2074 versions that return objects with the appropriate names
2075 for each field. For example:
2079 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2081 Even though it looks like they're the same method calls (uid),
2082 they aren't, because a C<File::stat> object is different from
2083 a C<User::pwent> object.
2085 =item getsockname SOCKET
2087 Returns the packed sockaddr address of this end of the SOCKET connection,
2088 in case you don't know the address because you have several different
2089 IPs that the connection might have come in on.
2092 $mysockaddr = getsockname(SOCK);
2093 ($port, $myaddr) = sockaddr_in($mysockaddr);
2094 printf "Connect to %s [%s]\n",
2095 scalar gethostbyaddr($myaddr, AF_INET),
2098 =item getsockopt SOCKET,LEVEL,OPTNAME
2100 Returns the socket option requested, or undef if there is an error.
2106 In list context, returns a (possibly empty) list of filename expansions on
2107 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2108 scalar context, glob iterates through such filename expansions, returning
2109 undef when the list is exhausted. This is the internal function
2110 implementing the C<< <*.c> >> operator, but you can use it directly. If
2111 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2112 more detail in L<perlop/"I/O Operators">.
2114 Beginning with v5.6.0, this operator is implemented using the standard
2115 C<File::Glob> extension. See L<File::Glob> for details.
2119 Converts a time as returned by the time function to an 8-element list
2120 with the time localized for the standard Greenwich time zone.
2121 Typically used as follows:
2124 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2127 All list elements are numeric, and come straight out of the C `struct
2128 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2129 specified time. $mday is the day of the month, and $mon is the month
2130 itself, in the range C<0..11> with 0 indicating January and 11
2131 indicating December. $year is the number of years since 1900. That
2132 is, $year is C<123> in year 2023. $wday is the day of the week, with
2133 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2134 the year, in the range C<0..364> (or C<0..365> in leap years.)
2136 Note that the $year element is I<not> simply the last two digits of
2137 the year. If you assume it is, then you create non-Y2K-compliant
2138 programs--and you wouldn't want to do that, would you?
2140 The proper way to get a complete 4-digit year is simply:
2144 And to get the last two digits of the year (e.g., '01' in 2001) do:
2146 $year = sprintf("%02d", $year % 100);
2148 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2150 In scalar context, C<gmtime()> returns the ctime(3) value:
2152 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2154 Also see the C<timegm> function provided by the C<Time::Local> module,
2155 and the strftime(3) function available via the POSIX module.
2157 This scalar value is B<not> locale dependent (see L<perllocale>), but
2158 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2159 strftime(3) and mktime(3) functions available via the POSIX module. To
2160 get somewhat similar but locale dependent date strings, set up your
2161 locale environment variables appropriately (please see L<perllocale>)
2162 and try for example:
2164 use POSIX qw(strftime);
2165 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2167 Note that the C<%a> and C<%b> escapes, which represent the short forms
2168 of the day of the week and the month of the year, may not necessarily
2169 be three characters wide in all locales.
2177 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2178 execution there. It may not be used to go into any construct that
2179 requires initialization, such as a subroutine or a C<foreach> loop. It
2180 also can't be used to go into a construct that is optimized away,
2181 or to get out of a block or subroutine given to C<sort>.
2182 It can be used to go almost anywhere else within the dynamic scope,
2183 including out of subroutines, but it's usually better to use some other
2184 construct such as C<last> or C<die>. The author of Perl has never felt the
2185 need to use this form of C<goto> (in Perl, that is--C is another matter).
2186 (The difference being that C does not offer named loops combined with
2187 loop control. Perl does, and this replaces most structured uses of C<goto>
2188 in other languages.)
2190 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2191 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2192 necessarily recommended if you're optimizing for maintainability:
2194 goto ("FOO", "BAR", "GLARCH")[$i];
2196 The C<goto-&NAME> form is quite different from the other forms of
2197 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2198 doesn't have the stigma associated with other gotos. Instead, it
2199 exits the current subroutine (losing any changes set by local()) and
2200 immediately calls in its place the named subroutine using the current
2201 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2202 load another subroutine and then pretend that the other subroutine had
2203 been called in the first place (except that any modifications to C<@_>
2204 in the current subroutine are propagated to the other subroutine.)
2205 After the C<goto>, not even C<caller> will be able to tell that this
2206 routine was called first.
2208 NAME needn't be the name of a subroutine; it can be a scalar variable
2209 containing a code reference, or a block which evaluates to a code
2212 =item grep BLOCK LIST
2214 =item grep EXPR,LIST
2216 This is similar in spirit to, but not the same as, grep(1) and its
2217 relatives. In particular, it is not limited to using regular expressions.
2219 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2220 C<$_> to each element) and returns the list value consisting of those
2221 elements for which the expression evaluated to true. In scalar
2222 context, returns the number of times the expression was true.
2224 @foo = grep(!/^#/, @bar); # weed out comments
2228 @foo = grep {!/^#/} @bar; # weed out comments
2230 Note that C<$_> is an alias to the list value, so it can be used to
2231 modify the elements of the LIST. While this is useful and supported,
2232 it can cause bizarre results if the elements of LIST are not variables.
2233 Similarly, grep returns aliases into the original list, much as a for
2234 loop's index variable aliases the list elements. That is, modifying an
2235 element of a list returned by grep (for example, in a C<foreach>, C<map>
2236 or another C<grep>) actually modifies the element in the original list.
2237 This is usually something to be avoided when writing clear code.
2239 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2240 been declared with C<my $_>) then, in addition the be locally aliased to
2241 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2242 can't be seen from the outside, avoiding any potential side-effects.
2244 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2250 Interprets EXPR as a hex string and returns the corresponding value.
2251 (To convert strings that might start with either 0, 0x, or 0b, see
2252 L</oct>.) If EXPR is omitted, uses C<$_>.
2254 print hex '0xAf'; # prints '175'
2255 print hex 'aF'; # same
2257 Hex strings may only represent integers. Strings that would cause
2258 integer overflow trigger a warning. Leading whitespace is not stripped,
2263 There is no builtin C<import> function. It is just an ordinary
2264 method (subroutine) defined (or inherited) by modules that wish to export
2265 names to another module. The C<use> function calls the C<import> method
2266 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2268 =item index STR,SUBSTR,POSITION
2270 =item index STR,SUBSTR
2272 The index function searches for one string within another, but without
2273 the wildcard-like behavior of a full regular-expression pattern match.
2274 It returns the position of the first occurrence of SUBSTR in STR at
2275 or after POSITION. If POSITION is omitted, starts searching from the
2276 beginning of the string. The return value is based at C<0> (or whatever
2277 you've set the C<$[> variable to--but don't do that). If the substring
2278 is not found, returns one less than the base, ordinarily C<-1>.
2284 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2285 You should not use this function for rounding: one because it truncates
2286 towards C<0>, and two because machine representations of floating point
2287 numbers can sometimes produce counterintuitive results. For example,
2288 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2289 because it's really more like -268.99999999999994315658 instead. Usually,
2290 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2291 functions will serve you better than will int().
2293 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2295 Implements the ioctl(2) function. You'll probably first have to say
2297 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2299 to get the correct function definitions. If F<ioctl.ph> doesn't
2300 exist or doesn't have the correct definitions you'll have to roll your
2301 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2302 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2303 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2304 written depending on the FUNCTION--a pointer to the string value of SCALAR
2305 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2306 has no string value but does have a numeric value, that value will be
2307 passed rather than a pointer to the string value. To guarantee this to be
2308 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2309 functions may be needed to manipulate the values of structures used by
2312 The return value of C<ioctl> (and C<fcntl>) is as follows:
2314 if OS returns: then Perl returns:
2316 0 string "0 but true"
2317 anything else that number
2319 Thus Perl returns true on success and false on failure, yet you can
2320 still easily determine the actual value returned by the operating
2323 $retval = ioctl(...) || -1;
2324 printf "System returned %d\n", $retval;
2326 The special string C<"0 but true"> is exempt from B<-w> complaints
2327 about improper numeric conversions.
2329 =item join EXPR,LIST
2331 Joins the separate strings of LIST into a single string with fields
2332 separated by the value of EXPR, and returns that new string. Example:
2334 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2336 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2337 first argument. Compare L</split>.
2341 Returns a list consisting of all the keys of the named hash.
2342 (In scalar context, returns the number of keys.)
2344 The keys are returned in an apparently random order. The actual
2345 random order is subject to change in future versions of perl, but it
2346 is guaranteed to be the same order as either the C<values> or C<each>
2347 function produces (given that the hash has not been modified). Since
2348 Perl 5.8.1 the ordering is different even between different runs of
2349 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2352 As a side effect, calling keys() resets the HASH's internal iterator,
2353 see L</each>. (In particular, calling keys() in void context resets
2354 the iterator with no other overhead.)
2356 Here is yet another way to print your environment:
2359 @values = values %ENV;
2361 print pop(@keys), '=', pop(@values), "\n";
2364 or how about sorted by key:
2366 foreach $key (sort(keys %ENV)) {
2367 print $key, '=', $ENV{$key}, "\n";
2370 The returned values are copies of the original keys in the hash, so
2371 modifying them will not affect the original hash. Compare L</values>.
2373 To sort a hash by value, you'll need to use a C<sort> function.
2374 Here's a descending numeric sort of a hash by its values:
2376 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2377 printf "%4d %s\n", $hash{$key}, $key;
2380 As an lvalue C<keys> allows you to increase the number of hash buckets
2381 allocated for the given hash. This can gain you a measure of efficiency if
2382 you know the hash is going to get big. (This is similar to pre-extending
2383 an array by assigning a larger number to $#array.) If you say
2387 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2388 in fact, since it rounds up to the next power of two. These
2389 buckets will be retained even if you do C<%hash = ()>, use C<undef
2390 %hash> if you want to free the storage while C<%hash> is still in scope.
2391 You can't shrink the number of buckets allocated for the hash using
2392 C<keys> in this way (but you needn't worry about doing this by accident,
2393 as trying has no effect).
2395 See also C<each>, C<values> and C<sort>.
2397 =item kill SIGNAL, LIST
2399 Sends a signal to a list of processes. Returns the number of
2400 processes successfully signaled (which is not necessarily the
2401 same as the number actually killed).
2403 $cnt = kill 1, $child1, $child2;
2406 If SIGNAL is zero, no signal is sent to the process. This is a
2407 useful way to check that a child process is alive and hasn't changed
2408 its UID. See L<perlport> for notes on the portability of this
2411 Unlike in the shell, if SIGNAL is negative, it kills
2412 process groups instead of processes. (On System V, a negative I<PROCESS>
2413 number will also kill process groups, but that's not portable.) That
2414 means you usually want to use positive not negative signals. You may also
2415 use a signal name in quotes.
2417 See L<perlipc/"Signals"> for more details.
2423 The C<last> command is like the C<break> statement in C (as used in
2424 loops); it immediately exits the loop in question. If the LABEL is
2425 omitted, the command refers to the innermost enclosing loop. The
2426 C<continue> block, if any, is not executed:
2428 LINE: while (<STDIN>) {
2429 last LINE if /^$/; # exit when done with header
2433 C<last> cannot be used to exit a block which returns a value such as
2434 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2435 a grep() or map() operation.
2437 Note that a block by itself is semantically identical to a loop
2438 that executes once. Thus C<last> can be used to effect an early
2439 exit out of such a block.
2441 See also L</continue> for an illustration of how C<last>, C<next>, and
2448 Returns a lowercased version of EXPR. This is the internal function
2449 implementing the C<\L> escape in double-quoted strings. Respects
2450 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2451 and L<perlunicode> for more details about locale and Unicode support.
2453 If EXPR is omitted, uses C<$_>.
2459 Returns the value of EXPR with the first character lowercased. This
2460 is the internal function implementing the C<\l> escape in
2461 double-quoted strings. Respects current LC_CTYPE locale if C<use
2462 locale> in force. See L<perllocale> and L<perlunicode> for more
2463 details about locale and Unicode support.
2465 If EXPR is omitted, uses C<$_>.
2471 Returns the length in I<characters> of the value of EXPR. If EXPR is
2472 omitted, returns length of C<$_>. Note that this cannot be used on
2473 an entire array or hash to find out how many elements these have.
2474 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2476 Note the I<characters>: if the EXPR is in Unicode, you will get the
2477 number of characters, not the number of bytes. To get the length
2478 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2480 =item link OLDFILE,NEWFILE
2482 Creates a new filename linked to the old filename. Returns true for
2483 success, false otherwise.
2485 =item listen SOCKET,QUEUESIZE
2487 Does the same thing that the listen system call does. Returns true if
2488 it succeeded, false otherwise. See the example in
2489 L<perlipc/"Sockets: Client/Server Communication">.
2493 You really probably want to be using C<my> instead, because C<local> isn't
2494 what most people think of as "local". See
2495 L<perlsub/"Private Variables via my()"> for details.
2497 A local modifies the listed variables to be local to the enclosing
2498 block, file, or eval. If more than one value is listed, the list must
2499 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2500 for details, including issues with tied arrays and hashes.
2502 =item localtime EXPR
2504 Converts a time as returned by the time function to a 9-element list
2505 with the time analyzed for the local time zone. Typically used as
2509 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2512 All list elements are numeric, and come straight out of the C `struct
2513 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2514 specified time. $mday is the day of the month, and $mon is the month
2515 itself, in the range C<0..11> with 0 indicating January and 11
2516 indicating December. $year is the number of years since 1900. That
2517 is, $year is C<123> in year 2023. $wday is the day of the week, with
2518 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2519 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2520 is true if the specified time occurs during daylight savings time,
2523 Note that the $year element is I<not> simply the last two digits of
2524 the year. If you assume it is, then you create non-Y2K-compliant
2525 programs--and you wouldn't want to do that, would you?
2527 The proper way to get a complete 4-digit year is simply:
2531 And to get the last two digits of the year (e.g., '01' in 2001) do:
2533 $year = sprintf("%02d", $year % 100);
2535 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2537 In scalar context, C<localtime()> returns the ctime(3) value:
2539 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2541 This scalar value is B<not> locale dependent, see L<perllocale>, but
2542 instead a Perl builtin. Also see the C<Time::Local> module
2543 (to convert the second, minutes, hours, ... back to seconds since the
2544 stroke of midnight the 1st of January 1970, the value returned by
2545 time()), and the strftime(3) and mktime(3) functions available via the
2546 POSIX module. To get somewhat similar but locale dependent date
2547 strings, set up your locale environment variables appropriately
2548 (please see L<perllocale>) and try for example:
2550 use POSIX qw(strftime);
2551 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2553 Note that the C<%a> and C<%b>, the short forms of the day of the week
2554 and the month of the year, may not necessarily be three characters wide.
2558 This function places an advisory lock on a shared variable, or referenced
2559 object contained in I<THING> until the lock goes out of scope.
2561 lock() is a "weak keyword" : this means that if you've defined a function
2562 by this name (before any calls to it), that function will be called
2563 instead. (However, if you've said C<use threads>, lock() is always a
2564 keyword.) See L<threads>.
2570 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2571 returns log of C<$_>. To get the log of another base, use basic algebra:
2572 The base-N log of a number is equal to the natural log of that number
2573 divided by the natural log of N. For example:
2577 return log($n)/log(10);
2580 See also L</exp> for the inverse operation.
2586 Does the same thing as the C<stat> function (including setting the
2587 special C<_> filehandle) but stats a symbolic link instead of the file
2588 the symbolic link points to. If symbolic links are unimplemented on
2589 your system, a normal C<stat> is done. For much more detailed
2590 information, please see the documentation for C<stat>.
2592 If EXPR is omitted, stats C<$_>.
2596 The match operator. See L<perlop>.
2598 =item map BLOCK LIST
2602 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2603 C<$_> to each element) and returns the list value composed of the
2604 results of each such evaluation. In scalar context, returns the
2605 total number of elements so generated. Evaluates BLOCK or EXPR in
2606 list context, so each element of LIST may produce zero, one, or
2607 more elements in the returned value.
2609 @chars = map(chr, @nums);
2611 translates a list of numbers to the corresponding characters. And
2613 %hash = map { getkey($_) => $_ } @array;
2615 is just a funny way to write
2618 foreach $_ (@array) {
2619 $hash{getkey($_)} = $_;
2622 Note that C<$_> is an alias to the list value, so it can be used to
2623 modify the elements of the LIST. While this is useful and supported,
2624 it can cause bizarre results if the elements of LIST are not variables.
2625 Using a regular C<foreach> loop for this purpose would be clearer in
2626 most cases. See also L</grep> for an array composed of those items of
2627 the original list for which the BLOCK or EXPR evaluates to true.
2629 If C<$_> is lexical in the scope where the C<map> appears (because it has
2630 been declared with C<my $_>) then, in addition the be locally aliased to
2631 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2632 can't be seen from the outside, avoiding any potential side-effects.
2634 C<{> starts both hash references and blocks, so C<map { ...> could be either
2635 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2636 ahead for the closing C<}> it has to take a guess at which its dealing with
2637 based what it finds just after the C<{>. Usually it gets it right, but if it
2638 doesn't it won't realize something is wrong until it gets to the C<}> and
2639 encounters the missing (or unexpected) comma. The syntax error will be
2640 reported close to the C<}> but you'll need to change something near the C<{>
2641 such as using a unary C<+> to give perl some help:
2643 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2644 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2645 %hash = map { ("\L$_", 1) } @array # this also works
2646 %hash = map { lc($_), 1 } @array # as does this.
2647 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2649 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2651 or to force an anon hash constructor use C<+{>
2653 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2655 and you get list of anonymous hashes each with only 1 entry.
2657 =item mkdir FILENAME,MASK
2659 =item mkdir FILENAME
2661 Creates the directory specified by FILENAME, with permissions
2662 specified by MASK (as modified by C<umask>). If it succeeds it
2663 returns true, otherwise it returns false and sets C<$!> (errno).
2664 If omitted, MASK defaults to 0777.
2666 In general, it is better to create directories with permissive MASK,
2667 and let the user modify that with their C<umask>, than it is to supply
2668 a restrictive MASK and give the user no way to be more permissive.
2669 The exceptions to this rule are when the file or directory should be
2670 kept private (mail files, for instance). The perlfunc(1) entry on
2671 C<umask> discusses the choice of MASK in more detail.
2673 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2674 number of trailing slashes. Some operating and filesystems do not get
2675 this right, so Perl automatically removes all trailing slashes to keep
2678 =item msgctl ID,CMD,ARG
2680 Calls the System V IPC function msgctl(2). You'll probably have to say
2684 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2685 then ARG must be a variable which will hold the returned C<msqid_ds>
2686 structure. Returns like C<ioctl>: the undefined value for error,
2687 C<"0 but true"> for zero, or the actual return value otherwise. See also
2688 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2690 =item msgget KEY,FLAGS
2692 Calls the System V IPC function msgget(2). Returns the message queue
2693 id, or the undefined value if there is an error. See also
2694 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2696 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2698 Calls the System V IPC function msgrcv to receive a message from
2699 message queue ID into variable VAR with a maximum message size of
2700 SIZE. Note that when a message is received, the message type as a
2701 native long integer will be the first thing in VAR, followed by the
2702 actual message. This packing may be opened with C<unpack("l! a*")>.
2703 Taints the variable. Returns true if successful, or false if there is
2704 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2705 C<IPC::SysV::Msg> documentation.
2707 =item msgsnd ID,MSG,FLAGS
2709 Calls the System V IPC function msgsnd to send the message MSG to the
2710 message queue ID. MSG must begin with the native long integer message
2711 type, and be followed by the length of the actual message, and finally
2712 the message itself. This kind of packing can be achieved with
2713 C<pack("l! a*", $type, $message)>. Returns true if successful,
2714 or false if there is an error. See also C<IPC::SysV>
2715 and C<IPC::SysV::Msg> documentation.
2721 =item my EXPR : ATTRS
2723 =item my TYPE EXPR : ATTRS
2725 A C<my> declares the listed variables to be local (lexically) to the
2726 enclosing block, file, or C<eval>. If more than one value is listed,
2727 the list must be placed in parentheses.
2729 The exact semantics and interface of TYPE and ATTRS are still
2730 evolving. TYPE is currently bound to the use of C<fields> pragma,
2731 and attributes are handled using the C<attributes> pragma, or starting
2732 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2733 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2734 L<attributes>, and L<Attribute::Handlers>.
2740 The C<next> command is like the C<continue> statement in C; it starts
2741 the next iteration of the loop:
2743 LINE: while (<STDIN>) {
2744 next LINE if /^#/; # discard comments
2748 Note that if there were a C<continue> block on the above, it would get
2749 executed even on discarded lines. If the LABEL is omitted, the command
2750 refers to the innermost enclosing loop.
2752 C<next> cannot be used to exit a block which returns a value such as
2753 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2754 a grep() or map() operation.
2756 Note that a block by itself is semantically identical to a loop
2757 that executes once. Thus C<next> will exit such a block early.
2759 See also L</continue> for an illustration of how C<last>, C<next>, and
2762 =item no Module VERSION LIST
2764 =item no Module VERSION
2766 =item no Module LIST
2770 See the C<use> function, of which C<no> is the opposite.
2776 Interprets EXPR as an octal string and returns the corresponding
2777 value. (If EXPR happens to start off with C<0x>, interprets it as a
2778 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2779 binary string. Leading whitespace is ignored in all three cases.)
2780 The following will handle decimal, binary, octal, and hex in the standard
2783 $val = oct($val) if $val =~ /^0/;
2785 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2786 in octal), use sprintf() or printf():
2788 $perms = (stat("filename"))[2] & 07777;
2789 $oct_perms = sprintf "%lo", $perms;
2791 The oct() function is commonly used when a string such as C<644> needs
2792 to be converted into a file mode, for example. (Although perl will
2793 automatically convert strings into numbers as needed, this automatic
2794 conversion assumes base 10.)
2796 =item open FILEHANDLE,EXPR
2798 =item open FILEHANDLE,MODE,EXPR
2800 =item open FILEHANDLE,MODE,EXPR,LIST
2802 =item open FILEHANDLE,MODE,REFERENCE
2804 =item open FILEHANDLE
2806 Opens the file whose filename is given by EXPR, and associates it with
2809 (The following is a comprehensive reference to open(): for a gentler
2810 introduction you may consider L<perlopentut>.)
2812 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2813 the variable is assigned a reference to a new anonymous filehandle,
2814 otherwise if FILEHANDLE is an expression, its value is used as the name of
2815 the real filehandle wanted. (This is considered a symbolic reference, so
2816 C<use strict 'refs'> should I<not> be in effect.)
2818 If EXPR is omitted, the scalar variable of the same name as the
2819 FILEHANDLE contains the filename. (Note that lexical variables--those
2820 declared with C<my>--will not work for this purpose; so if you're
2821 using C<my>, specify EXPR in your call to open.)
2823 If three or more arguments are specified then the mode of opening and
2824 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2825 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2826 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2827 the file is opened for appending, again being created if necessary.
2829 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2830 indicate that you want both read and write access to the file; thus
2831 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2832 '+>' >> mode would clobber the file first. You can't usually use
2833 either read-write mode for updating textfiles, since they have
2834 variable length records. See the B<-i> switch in L<perlrun> for a
2835 better approach. The file is created with permissions of C<0666>
2836 modified by the process' C<umask> value.
2838 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2839 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2841 In the 2-arguments (and 1-argument) form of the call the mode and
2842 filename should be concatenated (in this order), possibly separated by
2843 spaces. It is possible to omit the mode in these forms if the mode is
2846 If the filename begins with C<'|'>, the filename is interpreted as a
2847 command to which output is to be piped, and if the filename ends with a
2848 C<'|'>, the filename is interpreted as a command which pipes output to
2849 us. See L<perlipc/"Using open() for IPC">
2850 for more examples of this. (You are not allowed to C<open> to a command
2851 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2852 and L<perlipc/"Bidirectional Communication with Another Process">
2855 For three or more arguments if MODE is C<'|-'>, the filename is
2856 interpreted as a command to which output is to be piped, and if MODE
2857 is C<'-|'>, the filename is interpreted as a command which pipes
2858 output to us. In the 2-arguments (and 1-argument) form one should
2859 replace dash (C<'-'>) with the command.
2860 See L<perlipc/"Using open() for IPC"> for more examples of this.
2861 (You are not allowed to C<open> to a command that pipes both in I<and>
2862 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2863 L<perlipc/"Bidirectional Communication"> for alternatives.)
2865 In the three-or-more argument form of pipe opens, if LIST is specified
2866 (extra arguments after the command name) then LIST becomes arguments
2867 to the command invoked if the platform supports it. The meaning of
2868 C<open> with more than three arguments for non-pipe modes is not yet
2869 specified. Experimental "layers" may give extra LIST arguments
2872 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2873 and opening C<< '>-' >> opens STDOUT.
2875 You may use the three-argument form of open to specify IO "layers"
2876 (sometimes also referred to as "disciplines") to be applied to the handle
2877 that affect how the input and output are processed (see L<open> and
2878 L<PerlIO> for more details). For example
2880 open(FH, "<:utf8", "file")
2882 will open the UTF-8 encoded file containing Unicode characters,
2883 see L<perluniintro>. (Note that if layers are specified in the
2884 three-arg form then default layers set by the C<open> pragma are
2887 Open returns nonzero upon success, the undefined value otherwise. If
2888 the C<open> involved a pipe, the return value happens to be the pid of
2891 If you're running Perl on a system that distinguishes between text
2892 files and binary files, then you should check out L</binmode> for tips
2893 for dealing with this. The key distinction between systems that need
2894 C<binmode> and those that don't is their text file formats. Systems
2895 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2896 character, and which encode that character in C as C<"\n">, do not
2897 need C<binmode>. The rest need it.
2899 When opening a file, it's usually a bad idea to continue normal execution
2900 if the request failed, so C<open> is frequently used in connection with
2901 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2902 where you want to make a nicely formatted error message (but there are
2903 modules that can help with that problem)) you should always check
2904 the return value from opening a file. The infrequent exception is when
2905 working with an unopened filehandle is actually what you want to do.
2907 As a special case the 3 arg form with a read/write mode and the third
2908 argument being C<undef>:
2910 open(TMP, "+>", undef) or die ...
2912 opens a filehandle to an anonymous temporary file. Also using "+<"
2913 works for symmetry, but you really should consider writing something
2914 to the temporary file first. You will need to seek() to do the
2917 File handles can be opened to "in memory" files held in Perl scalars via:
2919 open($fh, '>', \$variable) || ..
2921 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2922 file, you have to close it first:
2925 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2930 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2931 while (<ARTICLE>) {...
2933 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2934 # if the open fails, output is discarded
2936 open(DBASE, '+<', 'dbase.mine') # open for update
2937 or die "Can't open 'dbase.mine' for update: $!";
2939 open(DBASE, '+<dbase.mine') # ditto
2940 or die "Can't open 'dbase.mine' for update: $!";
2942 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2943 or die "Can't start caesar: $!";
2945 open(ARTICLE, "caesar <$article |") # ditto
2946 or die "Can't start caesar: $!";
2948 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
2949 or die "Can't start sort: $!";
2952 open(MEMORY,'>', \$var)
2953 or die "Can't open memory file: $!";
2954 print MEMORY "foo!\n"; # output will end up in $var
2956 # process argument list of files along with any includes
2958 foreach $file (@ARGV) {
2959 process($file, 'fh00');
2963 my($filename, $input) = @_;
2964 $input++; # this is a string increment
2965 unless (open($input, $filename)) {
2966 print STDERR "Can't open $filename: $!\n";
2971 while (<$input>) { # note use of indirection
2972 if (/^#include "(.*)"/) {
2973 process($1, $input);
2980 You may also, in the Bourne shell tradition, specify an EXPR beginning
2981 with C<< '>&' >>, in which case the rest of the string is interpreted
2982 as the name of a filehandle (or file descriptor, if numeric) to be
2983 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2984 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2985 The mode you specify should match the mode of the original filehandle.
2986 (Duping a filehandle does not take into account any existing contents
2987 of IO buffers.) If you use the 3 arg form then you can pass either a
2988 number, the name of a filehandle or the normal "reference to a glob".
2990 Here is a script that saves, redirects, and restores C<STDOUT> and
2991 C<STDERR> using various methods:
2994 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2995 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2997 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2998 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3000 select STDERR; $| = 1; # make unbuffered
3001 select STDOUT; $| = 1; # make unbuffered
3003 print STDOUT "stdout 1\n"; # this works for
3004 print STDERR "stderr 1\n"; # subprocesses too
3006 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3007 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3009 print STDOUT "stdout 2\n";
3010 print STDERR "stderr 2\n";
3012 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3013 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3014 that file descriptor (and not call L<dup(2)>); this is more
3015 parsimonious of file descriptors. For example:
3017 # open for input, reusing the fileno of $fd
3018 open(FILEHANDLE, "<&=$fd")
3022 open(FILEHANDLE, "<&=", $fd)
3026 # open for append, using the fileno of OLDFH
3027 open(FH, ">>&=", OLDFH)
3031 open(FH, ">>&=OLDFH")
3033 Being parsimonious on filehandles is also useful (besides being
3034 parsimonious) for example when something is dependent on file
3035 descriptors, like for example locking using flock(). If you do just
3036 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3037 descriptor as B, and therefore flock(A) will not flock(B), and vice
3038 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3039 the same file descriptor.
3041 Note that if you are using Perls older than 5.8.0, Perl will be using
3042 the standard C libraries' fdopen() to implement the "=" functionality.
3043 On many UNIX systems fdopen() fails when file descriptors exceed a
3044 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3045 most often the default.
3047 You can see whether Perl has been compiled with PerlIO or not by
3048 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3049 is C<define>, you have PerlIO, otherwise you don't.
3051 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3052 with 2-arguments (or 1-argument) form of open(), then
3053 there is an implicit fork done, and the return value of open is the pid
3054 of the child within the parent process, and C<0> within the child
3055 process. (Use C<defined($pid)> to determine whether the open was successful.)
3056 The filehandle behaves normally for the parent, but i/o to that
3057 filehandle is piped from/to the STDOUT/STDIN of the child process.
3058 In the child process the filehandle isn't opened--i/o happens from/to
3059 the new STDOUT or STDIN. Typically this is used like the normal
3060 piped open when you want to exercise more control over just how the
3061 pipe command gets executed, such as when you are running setuid, and
3062 don't want to have to scan shell commands for metacharacters.
3063 The following triples are more or less equivalent:
3065 open(FOO, "|tr '[a-z]' '[A-Z]'");
3066 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3067 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3068 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3070 open(FOO, "cat -n '$file'|");
3071 open(FOO, '-|', "cat -n '$file'");
3072 open(FOO, '-|') || exec 'cat', '-n', $file;
3073 open(FOO, '-|', "cat", '-n', $file);
3075 The last example in each block shows the pipe as "list form", which is
3076 not yet supported on all platforms. A good rule of thumb is that if
3077 your platform has true C<fork()> (in other words, if your platform is
3078 UNIX) you can use the list form.
3080 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3082 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3083 output before any operation that may do a fork, but this may not be
3084 supported on some platforms (see L<perlport>). To be safe, you may need
3085 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3086 of C<IO::Handle> on any open handles.
3088 On systems that support a close-on-exec flag on files, the flag will
3089 be set for the newly opened file descriptor as determined by the value
3090 of $^F. See L<perlvar/$^F>.
3092 Closing any piped filehandle causes the parent process to wait for the
3093 child to finish, and returns the status value in C<$?>.
3095 The filename passed to 2-argument (or 1-argument) form of open() will
3096 have leading and trailing whitespace deleted, and the normal
3097 redirection characters honored. This property, known as "magic open",
3098 can often be used to good effect. A user could specify a filename of
3099 F<"rsh cat file |">, or you could change certain filenames as needed:
3101 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3102 open(FH, $filename) or die "Can't open $filename: $!";
3104 Use 3-argument form to open a file with arbitrary weird characters in it,
3106 open(FOO, '<', $file);
3108 otherwise it's necessary to protect any leading and trailing whitespace:
3110 $file =~ s#^(\s)#./$1#;
3111 open(FOO, "< $file\0");
3113 (this may not work on some bizarre filesystems). One should
3114 conscientiously choose between the I<magic> and 3-arguments form
3119 will allow the user to specify an argument of the form C<"rsh cat file |">,
3120 but will not work on a filename which happens to have a trailing space, while
3122 open IN, '<', $ARGV[0];
3124 will have exactly the opposite restrictions.
3126 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3127 should use the C<sysopen> function, which involves no such magic (but
3128 may use subtly different filemodes than Perl open(), which is mapped
3129 to C fopen()). This is
3130 another way to protect your filenames from interpretation. For example:
3133 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3134 or die "sysopen $path: $!";
3135 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3136 print HANDLE "stuff $$\n";
3138 print "File contains: ", <HANDLE>;
3140 Using the constructor from the C<IO::Handle> package (or one of its
3141 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3142 filehandles that have the scope of whatever variables hold references to
3143 them, and automatically close whenever and however you leave that scope:
3147 sub read_myfile_munged {
3149 my $handle = new IO::File;
3150 open($handle, "myfile") or die "myfile: $!";
3152 or return (); # Automatically closed here.
3153 mung $first or die "mung failed"; # Or here.
3154 return $first, <$handle> if $ALL; # Or here.
3158 See L</seek> for some details about mixing reading and writing.
3160 =item opendir DIRHANDLE,EXPR
3162 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3163 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3164 DIRHANDLE may be an expression whose value can be used as an indirect
3165 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3166 scalar variable (or array or hash element), the variable is assigned a
3167 reference to a new anonymous dirhandle.
3168 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3174 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3175 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3178 For the reverse, see L</chr>.
3179 See L<perlunicode> and L<encoding> for more about Unicode.
3185 =item our EXPR : ATTRS
3187 =item our TYPE EXPR : ATTRS
3189 An C<our> declares the listed variables to be valid globals within
3190 the enclosing block, file, or C<eval>. That is, it has the same
3191 scoping rules as a "my" declaration, but does not create a local
3192 variable. If more than one value is listed, the list must be placed
3193 in parentheses. The C<our> declaration has no semantic effect unless
3194 "use strict vars" is in effect, in which case it lets you use the
3195 declared global variable without qualifying it with a package name.
3196 (But only within the lexical scope of the C<our> declaration. In this
3197 it differs from "use vars", which is package scoped.)
3199 An C<our> declaration declares a global variable that will be visible
3200 across its entire lexical scope, even across package boundaries. The
3201 package in which the variable is entered is determined at the point
3202 of the declaration, not at the point of use. This means the following
3206 our $bar; # declares $Foo::bar for rest of lexical scope
3210 print $bar; # prints 20
3212 Multiple C<our> declarations in the same lexical scope are allowed
3213 if they are in different packages. If they happened to be in the same
3214 package, Perl will emit warnings if you have asked for them.
3218 our $bar; # declares $Foo::bar for rest of lexical scope
3222 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3223 print $bar; # prints 30
3225 our $bar; # emits warning
3227 An C<our> declaration may also have a list of attributes associated
3230 The exact semantics and interface of TYPE and ATTRS are still
3231 evolving. TYPE is currently bound to the use of C<fields> pragma,
3232 and attributes are handled using the C<attributes> pragma, or starting
3233 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3234 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3235 L<attributes>, and L<Attribute::Handlers>.
3237 The only currently recognized C<our()> attribute is C<unique> which
3238 indicates that a single copy of the global is to be used by all
3239 interpreters should the program happen to be running in a
3240 multi-interpreter environment. (The default behaviour would be for
3241 each interpreter to have its own copy of the global.) Examples:
3243 our @EXPORT : unique = qw(foo);
3244 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3245 our $VERSION : unique = "1.00";
3247 Note that this attribute also has the effect of making the global
3248 readonly when the first new interpreter is cloned (for example,
3249 when the first new thread is created).
3251 Multi-interpreter environments can come to being either through the
3252 fork() emulation on Windows platforms, or by embedding perl in a
3253 multi-threaded application. The C<unique> attribute does nothing in
3254 all other environments.
3256 Warning: the current implementation of this attribute operates on the
3257 typeglob associated with the variable; this means that C<our $x : unique>
3258 also has the effect of C<our @x : unique; our %x : unique>. This may be
3261 =item pack TEMPLATE,LIST
3263 Takes a LIST of values and converts it into a string using the rules
3264 given by the TEMPLATE. The resulting string is the concatenation of
3265 the converted values. Typically, each converted value looks
3266 like its machine-level representation. For example, on 32-bit machines
3267 a converted integer may be represented by a sequence of 4 bytes.
3269 The TEMPLATE is a sequence of characters that give the order and type
3270 of values, as follows:
3272 a A string with arbitrary binary data, will be null padded.
3273 A A text (ASCII) string, will be space padded.
3274 Z A null terminated (ASCIZ) string, will be null padded.
3276 b A bit string (ascending bit order inside each byte, like vec()).
3277 B A bit string (descending bit order inside each byte).
3278 h A hex string (low nybble first).
3279 H A hex string (high nybble first).
3281 c A signed char value.
3282 C An unsigned char value. Only does bytes. See U for Unicode.
3284 s A signed short value.
3285 S An unsigned short value.
3286 (This 'short' is _exactly_ 16 bits, which may differ from
3287 what a local C compiler calls 'short'. If you want
3288 native-length shorts, use the '!' suffix.)
3290 i A signed integer value.
3291 I An unsigned integer value.
3292 (This 'integer' is _at_least_ 32 bits wide. Its exact
3293 size depends on what a local C compiler calls 'int',
3294 and may even be larger than the 'long' described in
3297 l A signed long value.
3298 L An unsigned long value.
3299 (This 'long' is _exactly_ 32 bits, which may differ from
3300 what a local C compiler calls 'long'. If you want
3301 native-length longs, use the '!' suffix.)
3303 n An unsigned short in "network" (big-endian) order.
3304 N An unsigned long in "network" (big-endian) order.
3305 v An unsigned short in "VAX" (little-endian) order.
3306 V An unsigned long in "VAX" (little-endian) order.
3307 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3308 _exactly_ 32 bits, respectively.)
3310 q A signed quad (64-bit) value.
3311 Q An unsigned quad value.
3312 (Quads are available only if your system supports 64-bit
3313 integer values _and_ if Perl has been compiled to support those.
3314 Causes a fatal error otherwise.)
3316 j A signed integer value (a Perl internal integer, IV).
3317 J An unsigned integer value (a Perl internal unsigned integer, UV).
3319 f A single-precision float in the native format.
3320 d A double-precision float in the native format.
3322 F A floating point value in the native native format
3323 (a Perl internal floating point value, NV).
3324 D A long double-precision float in the native format.
3325 (Long doubles are available only if your system supports long
3326 double values _and_ if Perl has been compiled to support those.
3327 Causes a fatal error otherwise.)
3329 p A pointer to a null-terminated string.
3330 P A pointer to a structure (fixed-length string).
3332 u A uuencoded string.
3333 U A Unicode character number. Encodes to UTF-8 internally
3334 (or UTF-EBCDIC in EBCDIC platforms).
3336 w A BER compressed integer. Its bytes represent an unsigned
3337 integer in base 128, most significant digit first, with as
3338 few digits as possible. Bit eight (the high bit) is set
3339 on each byte except the last.
3343 @ Null fill to absolute position, counted from the start of
3344 the innermost ()-group.
3345 ( Start of a ()-group.
3347 The following rules apply:
3353 Each letter may optionally be followed by a number giving a repeat
3354 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3355 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3356 many values from the LIST. A C<*> for the repeat count means to use
3357 however many items are left, except for C<@>, C<x>, C<X>, where it is
3358 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3359 is the same). A numeric repeat count may optionally be enclosed in
3360 brackets, as in C<pack 'C[80]', @arr>.
3362 One can replace the numeric repeat count by a template enclosed in brackets;
3363 then the packed length of this template in bytes is used as a count.
3364 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3365 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3366 If the template in brackets contains alignment commands (such as C<x![d]>),
3367 its packed length is calculated as if the start of the template has the maximal
3370 When used with C<Z>, C<*> results in the addition of a trailing null
3371 byte (so the packed result will be one longer than the byte C<length>
3374 The repeat count for C<u> is interpreted as the maximal number of bytes
3375 to encode per line of output, with 0 and 1 replaced by 45.
3379 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3380 string of length count, padding with nulls or spaces as necessary. When
3381 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3382 after the first null, and C<a> returns data verbatim. When packing,
3383 C<a>, and C<Z> are equivalent.
3385 If the value-to-pack is too long, it is truncated. If too long and an
3386 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3387 by a null byte. Thus C<Z> always packs a trailing null byte under
3392 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3393 Each byte of the input field of pack() generates 1 bit of the result.
3394 Each result bit is based on the least-significant bit of the corresponding
3395 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3396 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3398 Starting from the beginning of the input string of pack(), each 8-tuple
3399 of bytes is converted to 1 byte of output. With format C<b>
3400 the first byte of the 8-tuple determines the least-significant bit of a
3401 byte, and with format C<B> it determines the most-significant bit of
3404 If the length of the input string is not exactly divisible by 8, the
3405 remainder is packed as if the input string were padded by null bytes
3406 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3408 If the input string of pack() is longer than needed, extra bytes are ignored.
3409 A C<*> for the repeat count of pack() means to use all the bytes of
3410 the input field. On unpack()ing the bits are converted to a string
3411 of C<"0">s and C<"1">s.
3415 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3416 representable as hexadecimal digits, 0-9a-f) long.
3418 Each byte of the input field of pack() generates 4 bits of the result.
3419 For non-alphabetical bytes the result is based on the 4 least-significant
3420 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3421 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3422 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3423 is compatible with the usual hexadecimal digits, so that C<"a"> and
3424 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3425 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3427 Starting from the beginning of the input string of pack(), each pair
3428 of bytes is converted to 1 byte of output. With format C<h> the
3429 first byte of the pair determines the least-significant nybble of the
3430 output byte, and with format C<H> it determines the most-significant
3433 If the length of the input string is not even, it behaves as if padded
3434 by a null byte at the end. Similarly, during unpack()ing the "extra"
3435 nybbles are ignored.
3437 If the input string of pack() is longer than needed, extra bytes are ignored.
3438 A C<*> for the repeat count of pack() means to use all the bytes of
3439 the input field. On unpack()ing the bits are converted to a string
3440 of hexadecimal digits.
3444 The C<p> type packs a pointer to a null-terminated string. You are
3445 responsible for ensuring the string is not a temporary value (which can
3446 potentially get deallocated before you get around to using the packed result).
3447 The C<P> type packs a pointer to a structure of the size indicated by the
3448 length. A NULL pointer is created if the corresponding value for C<p> or
3449 C<P> is C<undef>, similarly for unpack().
3453 The C</> template character allows packing and unpacking of strings where
3454 the packed structure contains a byte count followed by the string itself.
3455 You write I<length-item>C</>I<string-item>.
3457 The I<length-item> can be any C<pack> template letter, and describes
3458 how the length value is packed. The ones likely to be of most use are
3459 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3460 SNMP) and C<N> (for Sun XDR).
3462 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3463 C<"Z*">. For C<unpack> the length of the string is obtained from the
3464 I<length-item>, but if you put in the '*' it will be ignored. For all other
3465 codes, C<unpack> applies the length value to the next item, which must not
3466 have a repeat count.
3468 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3469 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3470 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3472 The I<length-item> is not returned explicitly from C<unpack>.
3474 Adding a count to the I<length-item> letter is unlikely to do anything
3475 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3476 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3477 which Perl does not regard as legal in numeric strings.
3481 The integer types C<s>, C<S>, C<l>, and C<L> may be
3482 immediately followed by a C<!> suffix to signify native shorts or
3483 longs--as you can see from above for example a bare C<l> does mean
3484 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3485 may be larger. This is an issue mainly in 64-bit platforms. You can
3486 see whether using C<!> makes any difference by
3488 print length(pack("s")), " ", length(pack("s!")), "\n";
3489 print length(pack("l")), " ", length(pack("l!")), "\n";
3491 C<i!> and C<I!> also work but only because of completeness;
3492 they are identical to C<i> and C<I>.
3494 The actual sizes (in bytes) of native shorts, ints, longs, and long
3495 longs on the platform where Perl was built are also available via
3499 print $Config{shortsize}, "\n";
3500 print $Config{intsize}, "\n";
3501 print $Config{longsize}, "\n";
3502 print $Config{longlongsize}, "\n";
3504 (The C<$Config{longlongsize}> will be undefined if your system does
3505 not support long longs.)
3509 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3510 are inherently non-portable between processors and operating systems
3511 because they obey the native byteorder and endianness. For example a
3512 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3513 (arranged in and handled by the CPU registers) into bytes as
3515 0x12 0x34 0x56 0x78 # big-endian
3516 0x78 0x56 0x34 0x12 # little-endian
3518 Basically, the Intel and VAX CPUs are little-endian, while everybody
3519 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3520 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3521 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3524 The names `big-endian' and `little-endian' are comic references to
3525 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3526 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3527 the egg-eating habits of the Lilliputians.
3529 Some systems may have even weirder byte orders such as
3534 You can see your system's preference with
3536 print join(" ", map { sprintf "%#02x", $_ }
3537 unpack("C*",pack("L",0x12345678))), "\n";
3539 The byteorder on the platform where Perl was built is also available
3543 print $Config{byteorder}, "\n";
3545 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3546 and C<'87654321'> are big-endian.
3548 If you want portable packed integers use the formats C<n>, C<N>,
3549 C<v>, and C<V>, their byte endianness and size are known.
3550 See also L<perlport>.
3554 Real numbers (floats and doubles) are in the native machine format only;
3555 due to the multiplicity of floating formats around, and the lack of a
3556 standard "network" representation, no facility for interchange has been
3557 made. This means that packed floating point data written on one machine
3558 may not be readable on another - even if both use IEEE floating point
3559 arithmetic (as the endian-ness of the memory representation is not part
3560 of the IEEE spec). See also L<perlport>.
3562 Note that Perl uses doubles internally for all numeric calculation, and
3563 converting from double into float and thence back to double again will
3564 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3569 If the pattern begins with a C<U>, the resulting string will be
3570 treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
3571 string with an initial C<U0>, and the bytes that follow will be
3572 interpreted as Unicode characters. If you don't want this to happen,
3573 you can begin your pattern with C<C0> (or anything else) to force Perl
3574 not to UTF-8 encode your string, and then follow this with a C<U*>
3575 somewhere in your pattern.
3579 You must yourself do any alignment or padding by inserting for example
3580 enough C<'x'>es while packing. There is no way to pack() and unpack()
3581 could know where the bytes are going to or coming from. Therefore
3582 C<pack> (and C<unpack>) handle their output and input as flat
3587 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3588 take a repeat count, both as postfix, and for unpack() also via the C</>
3589 template character. Within each repetition of a group, positioning with
3590 C<@> starts again at 0. Therefore, the result of
3592 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3594 is the string "\0a\0\0bc".
3599 C<x> and C<X> accept C<!> modifier. In this case they act as
3600 alignment commands: they jump forward/back to the closest position
3601 aligned at a multiple of C<count> bytes. For example, to pack() or
3602 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3603 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3604 aligned on the double's size.
3606 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3607 both result in no-ops.
3611 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3612 White space may be used to separate pack codes from each other, but
3613 a C<!> modifier and a repeat count must follow immediately.
3617 If TEMPLATE requires more arguments to pack() than actually given, pack()
3618 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3619 to pack() than actually given, extra arguments are ignored.
3625 $foo = pack("CCCC",65,66,67,68);
3627 $foo = pack("C4",65,66,67,68);
3629 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3630 # same thing with Unicode circled letters
3632 $foo = pack("ccxxcc",65,66,67,68);
3635 # note: the above examples featuring "C" and "c" are true
3636 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3637 # and UTF-8. In EBCDIC the first example would be
3638 # $foo = pack("CCCC",193,194,195,196);
3640 $foo = pack("s2",1,2);
3641 # "\1\0\2\0" on little-endian
3642 # "\0\1\0\2" on big-endian
3644 $foo = pack("a4","abcd","x","y","z");
3647 $foo = pack("aaaa","abcd","x","y","z");
3650 $foo = pack("a14","abcdefg");
3651 # "abcdefg\0\0\0\0\0\0\0"
3653 $foo = pack("i9pl", gmtime);
3654 # a real struct tm (on my system anyway)
3656 $utmp_template = "Z8 Z8 Z16 L";
3657 $utmp = pack($utmp_template, @utmp1);
3658 # a struct utmp (BSDish)
3660 @utmp2 = unpack($utmp_template, $utmp);
3661 # "@utmp1" eq "@utmp2"
3664 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3667 $foo = pack('sx2l', 12, 34);
3668 # short 12, two zero bytes padding, long 34
3669 $bar = pack('s@4l', 12, 34);
3670 # short 12, zero fill to position 4, long 34
3673 The same template may generally also be used in unpack().
3675 =item package NAMESPACE
3679 Declares the compilation unit as being in the given namespace. The scope
3680 of the package declaration is from the declaration itself through the end
3681 of the enclosing block, file, or eval (the same as the C<my> operator).
3682 All further unqualified dynamic identifiers will be in this namespace.
3683 A package statement affects only dynamic variables--including those
3684 you've used C<local> on--but I<not> lexical variables, which are created
3685 with C<my>. Typically it would be the first declaration in a file to
3686 be included by the C<require> or C<use> operator. You can switch into a
3687 package in more than one place; it merely influences which symbol table
3688 is used by the compiler for the rest of that block. You can refer to
3689 variables and filehandles in other packages by prefixing the identifier
3690 with the package name and a double colon: C<$Package::Variable>.
3691 If the package name is null, the C<main> package as assumed. That is,
3692 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3693 still seen in older code).
3695 If NAMESPACE is omitted, then there is no current package, and all
3696 identifiers must be fully qualified or lexicals. However, you are
3697 strongly advised not to make use of this feature. Its use can cause
3698 unexpected behaviour, even crashing some versions of Perl. It is
3699 deprecated, and will be removed from a future release.
3701 See L<perlmod/"Packages"> for more information about packages, modules,
3702 and classes. See L<perlsub> for other scoping issues.
3704 =item pipe READHANDLE,WRITEHANDLE
3706 Opens a pair of connected pipes like the corresponding system call.
3707 Note that if you set up a loop of piped processes, deadlock can occur
3708 unless you are very careful. In addition, note that Perl's pipes use
3709 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3710 after each command, depending on the application.
3712 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3713 for examples of such things.
3715 On systems that support a close-on-exec flag on files, the flag will be set
3716 for the newly opened file descriptors as determined by the value of $^F.
3723 Pops and returns the last value of the array, shortening the array by
3724 one element. Has an effect similar to
3728 If there are no elements in the array, returns the undefined value
3729 (although this may happen at other times as well). If ARRAY is
3730 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3731 array in subroutines, just like C<shift>.
3737 Returns the offset of where the last C<m//g> search left off for the variable
3738 in question (C<$_> is used when the variable is not specified). May be
3739 modified to change that offset. Such modification will also influence
3740 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3743 =item print FILEHANDLE LIST
3749 Prints a string or a list of strings. Returns true if successful.
3750 FILEHANDLE may be a scalar variable name, in which case the variable
3751 contains the name of or a reference to the filehandle, thus introducing
3752 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3753 the next token is a term, it may be misinterpreted as an operator
3754 unless you interpose a C<+> or put parentheses around the arguments.)
3755 If FILEHANDLE is omitted, prints by default to standard output (or
3756 to the last selected output channel--see L</select>). If LIST is
3757 also omitted, prints C<$_> to the currently selected output channel.
3758 To set the default output channel to something other than STDOUT
3759 use the select operation. The current value of C<$,> (if any) is
3760 printed between each LIST item. The current value of C<$\> (if
3761 any) is printed after the entire LIST has been printed. Because
3762 print takes a LIST, anything in the LIST is evaluated in list
3763 context, and any subroutine that you call will have one or more of
3764 its expressions evaluated in list context. Also be careful not to
3765 follow the print keyword with a left parenthesis unless you want
3766 the corresponding right parenthesis to terminate the arguments to
3767 the print--interpose a C<+> or put parentheses around all the
3770 Note that if you're storing FILEHANDLES in an array or other expression,
3771 you will have to use a block returning its value instead:
3773 print { $files[$i] } "stuff\n";
3774 print { $OK ? STDOUT : STDERR } "stuff\n";
3776 =item printf FILEHANDLE FORMAT, LIST
3778 =item printf FORMAT, LIST
3780 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3781 (the output record separator) is not appended. The first argument
3782 of the list will be interpreted as the C<printf> format. See C<sprintf>
3783 for an explanation of the format argument. If C<use locale> is in effect,
3784 the character used for the decimal point in formatted real numbers is
3785 affected by the LC_NUMERIC locale. See L<perllocale>.
3787 Don't fall into the trap of using a C<printf> when a simple
3788 C<print> would do. The C<print> is more efficient and less
3791 =item prototype FUNCTION
3793 Returns the prototype of a function as a string (or C<undef> if the
3794 function has no prototype). FUNCTION is a reference to, or the name of,
3795 the function whose prototype you want to retrieve.
3797 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3798 name for Perl builtin. If the builtin is not I<overridable> (such as
3799 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3800 C<system>) returns C<undef> because the builtin does not really behave
3801 like a Perl function. Otherwise, the string describing the equivalent
3802 prototype is returned.
3804 =item push ARRAY,LIST
3806 Treats ARRAY as a stack, and pushes the values of LIST
3807 onto the end of ARRAY. The length of ARRAY increases by the length of
3808 LIST. Has the same effect as
3811 $ARRAY[++$#ARRAY] = $value;
3814 but is more efficient. Returns the new number of elements in the array.
3826 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3828 =item quotemeta EXPR
3832 Returns the value of EXPR with all non-"word"
3833 characters backslashed. (That is, all characters not matching
3834 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3835 returned string, regardless of any locale settings.)
3836 This is the internal function implementing
3837 the C<\Q> escape in double-quoted strings.
3839 If EXPR is omitted, uses C<$_>.
3845 Returns a random fractional number greater than or equal to C<0> and less
3846 than the value of EXPR. (EXPR should be positive.) If EXPR is
3847 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3848 also special-cased as C<1> - this has not been documented before perl 5.8.0
3849 and is subject to change in future versions of perl. Automatically calls
3850 C<srand> unless C<srand> has already been called. See also C<srand>.
3852 Apply C<int()> to the value returned by C<rand()> if you want random
3853 integers instead of random fractional numbers. For example,
3857 returns a random integer between C<0> and C<9>, inclusive.
3859 (Note: If your rand function consistently returns numbers that are too
3860 large or too small, then your version of Perl was probably compiled
3861 with the wrong number of RANDBITS.)
3863 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3865 =item read FILEHANDLE,SCALAR,LENGTH
3867 Attempts to read LENGTH I<characters> of data into variable SCALAR
3868 from the specified FILEHANDLE. Returns the number of characters
3869 actually read, C<0> at end of file, or undef if there was an error (in
3870 the latter case C<$!> is also set). SCALAR will be grown or shrunk
3871 so that the last character actually read is the last character of the
3872 scalar after the read.
3874 An OFFSET may be specified to place the read data at some place in the
3875 string other than the beginning. A negative OFFSET specifies
3876 placement at that many characters counting backwards from the end of
3877 the string. A positive OFFSET greater than the length of SCALAR
3878 results in the string being padded to the required size with C<"\0">
3879 bytes before the result of the read is appended.
3881 The call is actually implemented in terms of either Perl's or system's
3882 fread() call. To get a true read(2) system call, see C<sysread>.
3884 Note the I<characters>: depending on the status of the filehandle,
3885 either (8-bit) bytes or characters are read. By default all
3886 filehandles operate on bytes, but for example if the filehandle has
3887 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3888 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3889 characters, not bytes. Similarly for the C<:encoding> pragma:
3890 in that case pretty much any characters can be read.
3892 =item readdir DIRHANDLE
3894 Returns the next directory entry for a directory opened by C<opendir>.
3895 If used in list context, returns all the rest of the entries in the
3896 directory. If there are no more entries, returns an undefined value in
3897 scalar context or a null list in list context.
3899 If you're planning to filetest the return values out of a C<readdir>, you'd
3900 better prepend the directory in question. Otherwise, because we didn't
3901 C<chdir> there, it would have been testing the wrong file.
3903 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3904 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3909 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3910 context, each call reads and returns the next line, until end-of-file is
3911 reached, whereupon the subsequent call returns undef. In list context,
3912 reads until end-of-file is reached and returns a list of lines. Note that
3913 the notion of "line" used here is however you may have defined it
3914 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3916 When C<$/> is set to C<undef>, when readline() is in scalar
3917 context (i.e. file slurp mode), and when an empty file is read, it
3918 returns C<''> the first time, followed by C<undef> subsequently.
3920 This is the internal function implementing the C<< <EXPR> >>
3921 operator, but you can use it directly. The C<< <EXPR> >>
3922 operator is discussed in more detail in L<perlop/"I/O Operators">.
3925 $line = readline(*STDIN); # same thing
3927 If readline encounters an operating system error, C<$!> will be set with the
3928 corresponding error message. It can be helpful to check C<$!> when you are
3929 reading from filehandles you don't trust, such as a tty or a socket. The
3930 following example uses the operator form of C<readline>, and takes the necessary
3931 steps to ensure that C<readline> was successful.
3935 unless (defined( $line = <> )) {
3946 Returns the value of a symbolic link, if symbolic links are
3947 implemented. If not, gives a fatal error. If there is some system
3948 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3949 omitted, uses C<$_>.
3953 EXPR is executed as a system command.
3954 The collected standard output of the command is returned.
3955 In scalar context, it comes back as a single (potentially
3956 multi-line) string. In list context, returns a list of lines
3957 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3958 This is the internal function implementing the C<qx/EXPR/>
3959 operator, but you can use it directly. The C<qx/EXPR/>
3960 operator is discussed in more detail in L<perlop/"I/O Operators">.
3962 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3964 Receives a message on a socket. Attempts to receive LENGTH characters
3965 of data into variable SCALAR from the specified SOCKET filehandle.
3966 SCALAR will be grown or shrunk to the length actually read. Takes the
3967 same flags as the system call of the same name. Returns the address
3968 of the sender if SOCKET's protocol supports this; returns an empty
3969 string otherwise. If there's an error, returns the undefined value.
3970 This call is actually implemented in terms of recvfrom(2) system call.
3971 See L<perlipc/"UDP: Message Passing"> for examples.
3973 Note the I<characters>: depending on the status of the socket, either
3974 (8-bit) bytes or characters are received. By default all sockets
3975 operate on bytes, but for example if the socket has been changed using
3976 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3977 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3978 characters, not bytes. Similarly for the C<:encoding> pragma:
3979 in that case pretty much any characters can be read.
3985 The C<redo> command restarts the loop block without evaluating the
3986 conditional again. The C<continue> block, if any, is not executed. If
3987 the LABEL is omitted, the command refers to the innermost enclosing
3988 loop. This command is normally used by programs that want to lie to
3989 themselves about what was just input:
3991 # a simpleminded Pascal comment stripper
3992 # (warning: assumes no { or } in strings)
3993 LINE: while (<STDIN>) {
3994 while (s|({.*}.*){.*}|$1 |) {}
3999 if (/}/) { # end of comment?
4008 C<redo> cannot be used to retry a block which returns a value such as
4009 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4010 a grep() or map() operation.
4012 Note that a block by itself is semantically identical to a loop
4013 that executes once. Thus C<redo> inside such a block will effectively
4014 turn it into a looping construct.
4016 See also L</continue> for an illustration of how C<last>, C<next>, and
4023 Returns a non-empty string if EXPR is a reference, the empty
4024 string otherwise. If EXPR
4025 is not specified, C<$_> will be used. The value returned depends on the
4026 type of thing the reference is a reference to.
4027 Builtin types include:
4037 If the referenced object has been blessed into a package, then that package
4038 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4040 if (ref($r) eq "HASH") {
4041 print "r is a reference to a hash.\n";
4044 print "r is not a reference at all.\n";
4046 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
4047 print "r is a reference to something that isa hash.\n";
4050 See also L<perlref>.
4052 =item rename OLDNAME,NEWNAME
4054 Changes the name of a file; an existing file NEWNAME will be
4055 clobbered. Returns true for success, false otherwise.
4057 Behavior of this function varies wildly depending on your system
4058 implementation. For example, it will usually not work across file system
4059 boundaries, even though the system I<mv> command sometimes compensates
4060 for this. Other restrictions include whether it works on directories,
4061 open files, or pre-existing files. Check L<perlport> and either the
4062 rename(2) manpage or equivalent system documentation for details.
4064 =item require VERSION
4070 Demands a version of Perl specified by VERSION, or demands some semantics
4071 specified by EXPR or by C<$_> if EXPR is not supplied.
4073 VERSION may be either a numeric argument such as 5.006, which will be
4074 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4075 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4076 VERSION is greater than the version of the current Perl interpreter.
4077 Compare with L</use>, which can do a similar check at compile time.
4079 Specifying VERSION as a literal of the form v5.6.1 should generally be
4080 avoided, because it leads to misleading error messages under earlier
4081 versions of Perl which do not support this syntax. The equivalent numeric
4082 version should be used instead.
4084 require v5.6.1; # run time version check
4085 require 5.6.1; # ditto
4086 require 5.006_001; # ditto; preferred for backwards compatibility
4088 Otherwise, demands that a library file be included if it hasn't already
4089 been included. The file is included via the do-FILE mechanism, which is
4090 essentially just a variety of C<eval>. Has semantics similar to the
4091 following subroutine:
4094 my ($filename) = @_;
4095 if (exists $INC{$filename}) {
4096 return 1 if $INC{$filename};
4097 die "Compilation failed in require";
4099 my ($realfilename,$result);
4101 foreach $prefix (@INC) {
4102 $realfilename = "$prefix/$filename";
4103 if (-f $realfilename) {
4104 $INC{$filename} = $realfilename;
4105 $result = do $realfilename;
4109 die "Can't find $filename in \@INC";
4112 $INC{$filename} = undef;
4114 } elsif (!$result) {
4115 delete $INC{$filename};
4116 die "$filename did not return true value";
4122 Note that the file will not be included twice under the same specified
4125 The file must return true as the last statement to indicate
4126 successful execution of any initialization code, so it's customary to
4127 end such a file with C<1;> unless you're sure it'll return true
4128 otherwise. But it's better just to put the C<1;>, in case you add more
4131 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4132 replaces "F<::>" with "F</>" in the filename for you,
4133 to make it easy to load standard modules. This form of loading of
4134 modules does not risk altering your namespace.
4136 In other words, if you try this:
4138 require Foo::Bar; # a splendid bareword
4140 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4141 directories specified in the C<@INC> array.
4143 But if you try this:
4145 $class = 'Foo::Bar';
4146 require $class; # $class is not a bareword
4148 require "Foo::Bar"; # not a bareword because of the ""
4150 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4151 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4153 eval "require $class";
4155 Now that you understand how C<require> looks for files in the case of
4156 a bareword argument, there is a little extra functionality going on
4157 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4158 it will first look for a filename with a "F<.pmc>" extension. A file
4159 with this extension is assumed to be Perl bytecode generated by
4160 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4161 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4162 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4164 You can also insert hooks into the import facility, by putting directly
4165 Perl code into the @INC array. There are three forms of hooks: subroutine
4166 references, array references and blessed objects.
4168 Subroutine references are the simplest case. When the inclusion system
4169 walks through @INC and encounters a subroutine, this subroutine gets
4170 called with two parameters, the first being a reference to itself, and the
4171 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4172 subroutine should return C<undef> or a filehandle, from which the file to
4173 include will be read. If C<undef> is returned, C<require> will look at
4174 the remaining elements of @INC.
4176 If the hook is an array reference, its first element must be a subroutine
4177 reference. This subroutine is called as above, but the first parameter is
4178 the array reference. This enables to pass indirectly some arguments to
4181 In other words, you can write:
4183 push @INC, \&my_sub;
4185 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4191 push @INC, [ \&my_sub, $x, $y, ... ];
4193 my ($arrayref, $filename) = @_;
4194 # Retrieve $x, $y, ...
4195 my @parameters = @$arrayref[1..$#$arrayref];
4199 If the hook is an object, it must provide an INC method, that will be
4200 called as above, the first parameter being the object itself. (Note that
4201 you must fully qualify the sub's name, as it is always forced into package
4202 C<main>.) Here is a typical code layout:
4208 my ($self, $filename) = @_;
4212 # In the main program
4213 push @INC, new Foo(...);
4215 Note that these hooks are also permitted to set the %INC entry
4216 corresponding to the files they have loaded. See L<perlvar/%INC>.
4218 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4224 Generally used in a C<continue> block at the end of a loop to clear
4225 variables and reset C<??> searches so that they work again. The
4226 expression is interpreted as a list of single characters (hyphens
4227 allowed for ranges). All variables and arrays beginning with one of
4228 those letters are reset to their pristine state. If the expression is
4229 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4230 only variables or searches in the current package. Always returns
4233 reset 'X'; # reset all X variables
4234 reset 'a-z'; # reset lower case variables
4235 reset; # just reset ?one-time? searches
4237 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4238 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4239 variables--lexical variables are unaffected, but they clean themselves
4240 up on scope exit anyway, so you'll probably want to use them instead.
4247 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4248 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4249 context, depending on how the return value will be used, and the context
4250 may vary from one execution to the next (see C<wantarray>). If no EXPR
4251 is given, returns an empty list in list context, the undefined value in
4252 scalar context, and (of course) nothing at all in a void context.
4254 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4255 or do FILE will automatically return the value of the last expression
4260 In list context, returns a list value consisting of the elements
4261 of LIST in the opposite order. In scalar context, concatenates the
4262 elements of LIST and returns a string value with all characters
4263 in the opposite order.
4265 print reverse <>; # line tac, last line first
4267 undef $/; # for efficiency of <>
4268 print scalar reverse <>; # character tac, last line tsrif
4270 Used without arguments in scalar context, reverse() reverses C<$_>.
4272 This operator is also handy for inverting a hash, although there are some
4273 caveats. If a value is duplicated in the original hash, only one of those
4274 can be represented as a key in the inverted hash. Also, this has to
4275 unwind one hash and build a whole new one, which may take some time
4276 on a large hash, such as from a DBM file.
4278 %by_name = reverse %by_address; # Invert the hash
4280 =item rewinddir DIRHANDLE
4282 Sets the current position to the beginning of the directory for the
4283 C<readdir> routine on DIRHANDLE.
4285 =item rindex STR,SUBSTR,POSITION
4287 =item rindex STR,SUBSTR
4289 Works just like index() except that it returns the position of the LAST
4290 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4291 last occurrence at or before that position.
4293 =item rmdir FILENAME
4297 Deletes the directory specified by FILENAME if that directory is
4298 empty. If it succeeds it returns true, otherwise it returns false and
4299 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4303 The substitution operator. See L<perlop>.
4307 Forces EXPR to be interpreted in scalar context and returns the value
4310 @counts = ( scalar @a, scalar @b, scalar @c );
4312 There is no equivalent operator to force an expression to
4313 be interpolated in list context because in practice, this is never
4314 needed. If you really wanted to do so, however, you could use
4315 the construction C<@{[ (some expression) ]}>, but usually a simple
4316 C<(some expression)> suffices.
4318 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4319 parenthesized list, this behaves as a scalar comma expression, evaluating
4320 all but the last element in void context and returning the final element
4321 evaluated in scalar context. This is seldom what you want.
4323 The following single statement:
4325 print uc(scalar(&foo,$bar)),$baz;
4327 is the moral equivalent of these two:
4330 print(uc($bar),$baz);
4332 See L<perlop> for more details on unary operators and the comma operator.
4334 =item seek FILEHANDLE,POSITION,WHENCE
4336 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4337 FILEHANDLE may be an expression whose value gives the name of the
4338 filehandle. The values for WHENCE are C<0> to set the new position
4339 I<in bytes> to POSITION, C<1> to set it to the current position plus
4340 POSITION, and C<2> to set it to EOF plus POSITION (typically
4341 negative). For WHENCE you may use the constants C<SEEK_SET>,
4342 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4343 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4346 Note the I<in bytes>: even if the filehandle has been set to
4347 operate on characters (for example by using the C<:utf8> open
4348 layer), tell() will return byte offsets, not character offsets
4349 (because implementing that would render seek() and tell() rather slow).
4351 If you want to position file for C<sysread> or C<syswrite>, don't use
4352 C<seek>--buffering makes its effect on the file's system position
4353 unpredictable and non-portable. Use C<sysseek> instead.
4355 Due to the rules and rigors of ANSI C, on some systems you have to do a
4356 seek whenever you switch between reading and writing. Amongst other
4357 things, this may have the effect of calling stdio's clearerr(3).
4358 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4362 This is also useful for applications emulating C<tail -f>. Once you hit
4363 EOF on your read, and then sleep for a while, you might have to stick in a
4364 seek() to reset things. The C<seek> doesn't change the current position,
4365 but it I<does> clear the end-of-file condition on the handle, so that the
4366 next C<< <FILE> >> makes Perl try again to read something. We hope.
4368 If that doesn't work (some IO implementations are particularly
4369 cantankerous), then you may need something more like this:
4372 for ($curpos = tell(FILE); $_ = <FILE>;
4373 $curpos = tell(FILE)) {
4374 # search for some stuff and put it into files
4376 sleep($for_a_while);
4377 seek(FILE, $curpos, 0);
4380 =item seekdir DIRHANDLE,POS
4382 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4383 must be a value returned by C<telldir>. Has the same caveats about
4384 possible directory compaction as the corresponding system library
4387 =item select FILEHANDLE
4391 Returns the currently selected filehandle. Sets the current default
4392 filehandle for output, if FILEHANDLE is supplied. This has two
4393 effects: first, a C<write> or a C<print> without a filehandle will
4394 default to this FILEHANDLE. Second, references to variables related to
4395 output will refer to this output channel. For example, if you have to
4396 set the top of form format for more than one output channel, you might
4404 FILEHANDLE may be an expression whose value gives the name of the
4405 actual filehandle. Thus:
4407 $oldfh = select(STDERR); $| = 1; select($oldfh);
4409 Some programmers may prefer to think of filehandles as objects with
4410 methods, preferring to write the last example as:
4413 STDERR->autoflush(1);
4415 =item select RBITS,WBITS,EBITS,TIMEOUT
4417 This calls the select(2) system call with the bit masks specified, which
4418 can be constructed using C<fileno> and C<vec>, along these lines:
4420 $rin = $win = $ein = '';
4421 vec($rin,fileno(STDIN),1) = 1;
4422 vec($win,fileno(STDOUT),1) = 1;
4425 If you want to select on many filehandles you might wish to write a
4429 my(@fhlist) = split(' ',$_[0]);
4432 vec($bits,fileno($_),1) = 1;
4436 $rin = fhbits('STDIN TTY SOCK');
4440 ($nfound,$timeleft) =
4441 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4443 or to block until something becomes ready just do this
4445 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4447 Most systems do not bother to return anything useful in $timeleft, so
4448 calling select() in scalar context just returns $nfound.
4450 Any of the bit masks can also be undef. The timeout, if specified, is
4451 in seconds, which may be fractional. Note: not all implementations are
4452 capable of returning the $timeleft. If not, they always return
4453 $timeleft equal to the supplied $timeout.
4455 You can effect a sleep of 250 milliseconds this way:
4457 select(undef, undef, undef, 0.25);
4459 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4460 is implementation-dependent.
4462 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4463 or <FH>) with C<select>, except as permitted by POSIX, and even
4464 then only on POSIX systems. You have to use C<sysread> instead.
4466 =item semctl ID,SEMNUM,CMD,ARG
4468 Calls the System V IPC function C<semctl>. You'll probably have to say
4472 first to get the correct constant definitions. If CMD is IPC_STAT or
4473 GETALL, then ARG must be a variable which will hold the returned
4474 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4475 the undefined value for error, "C<0 but true>" for zero, or the actual
4476 return value otherwise. The ARG must consist of a vector of native
4477 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4478 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4481 =item semget KEY,NSEMS,FLAGS
4483 Calls the System V IPC function semget. Returns the semaphore id, or
4484 the undefined value if there is an error. See also
4485 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4488 =item semop KEY,OPSTRING
4490 Calls the System V IPC function semop to perform semaphore operations
4491 such as signalling and waiting. OPSTRING must be a packed array of
4492 semop structures. Each semop structure can be generated with
4493 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4494 operations is implied by the length of OPSTRING. Returns true if
4495 successful, or false if there is an error. As an example, the
4496 following code waits on semaphore $semnum of semaphore id $semid:
4498 $semop = pack("s!3", $semnum, -1, 0);
4499 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4501 To signal the semaphore, replace C<-1> with C<1>. See also
4502 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4505 =item send SOCKET,MSG,FLAGS,TO
4507 =item send SOCKET,MSG,FLAGS
4509 Sends a message on a socket. Attempts to send the scalar MSG to the
4510 SOCKET filehandle. Takes the same flags as the system call of the
4511 same name. On unconnected sockets you must specify a destination to
4512 send TO, in which case it does a C C<sendto>. Returns the number of
4513 characters sent, or the undefined value if there is an error. The C
4514 system call sendmsg(2) is currently unimplemented. See
4515 L<perlipc/"UDP: Message Passing"> for examples.
4517 Note the I<characters>: depending on the status of the socket, either
4518 (8-bit) bytes or characters are sent. By default all sockets operate
4519 on bytes, but for example if the socket has been changed using
4520 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4521 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4522 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4523 in that case pretty much any characters can be sent.
4525 =item setpgrp PID,PGRP
4527 Sets the current process group for the specified PID, C<0> for the current
4528 process. Will produce a fatal error if used on a machine that doesn't
4529 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4530 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4531 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4534 =item setpriority WHICH,WHO,PRIORITY
4536 Sets the current priority for a process, a process group, or a user.
4537 (See setpriority(2).) Will produce a fatal error if used on a machine
4538 that doesn't implement setpriority(2).
4540 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4542 Sets the socket option requested. Returns undefined if there is an
4543 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4550 Shifts the first value of the array off and returns it, shortening the
4551 array by 1 and moving everything down. If there are no elements in the
4552 array, returns the undefined value. If ARRAY is omitted, shifts the
4553 C<@_> array within the lexical scope of subroutines and formats, and the
4554 C<@ARGV> array at file scopes or within the lexical scopes established by
4555 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4558 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4559 same thing to the left end of an array that C<pop> and C<push> do to the
4562 =item shmctl ID,CMD,ARG
4564 Calls the System V IPC function shmctl. You'll probably have to say
4568 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4569 then ARG must be a variable which will hold the returned C<shmid_ds>
4570 structure. Returns like ioctl: the undefined value for error, "C<0> but
4571 true" for zero, or the actual return value otherwise.
4572 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4574 =item shmget KEY,SIZE,FLAGS
4576 Calls the System V IPC function shmget. Returns the shared memory
4577 segment id, or the undefined value if there is an error.
4578 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4580 =item shmread ID,VAR,POS,SIZE
4582 =item shmwrite ID,STRING,POS,SIZE
4584 Reads or writes the System V shared memory segment ID starting at
4585 position POS for size SIZE by attaching to it, copying in/out, and
4586 detaching from it. When reading, VAR must be a variable that will
4587 hold the data read. When writing, if STRING is too long, only SIZE
4588 bytes are used; if STRING is too short, nulls are written to fill out
4589 SIZE bytes. Return true if successful, or false if there is an error.
4590 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4591 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4593 =item shutdown SOCKET,HOW
4595 Shuts down a socket connection in the manner indicated by HOW, which
4596 has the same interpretation as in the system call of the same name.
4598 shutdown(SOCKET, 0); # I/we have stopped reading data
4599 shutdown(SOCKET, 1); # I/we have stopped writing data
4600 shutdown(SOCKET, 2); # I/we have stopped using this socket
4602 This is useful with sockets when you want to tell the other
4603 side you're done writing but not done reading, or vice versa.
4604 It's also a more insistent form of close because it also
4605 disables the file descriptor in any forked copies in other
4612 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4613 returns sine of C<$_>.
4615 For the inverse sine operation, you may use the C<Math::Trig::asin>
4616 function, or use this relation:
4618 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4624 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4625 May be interrupted if the process receives a signal such as C<SIGALRM>.
4626 Returns the number of seconds actually slept. You probably cannot
4627 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4630 On some older systems, it may sleep up to a full second less than what
4631 you requested, depending on how it counts seconds. Most modern systems
4632 always sleep the full amount. They may appear to sleep longer than that,
4633 however, because your process might not be scheduled right away in a
4634 busy multitasking system.
4636 For delays of finer granularity than one second, you may use Perl's
4637 C<syscall> interface to access setitimer(2) if your system supports
4638 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4639 and starting from Perl 5.8 part of the standard distribution) may also
4642 See also the POSIX module's C<pause> function.
4644 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4646 Opens a socket of the specified kind and attaches it to filehandle
4647 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4648 the system call of the same name. You should C<use Socket> first
4649 to get the proper definitions imported. See the examples in
4650 L<perlipc/"Sockets: Client/Server Communication">.
4652 On systems that support a close-on-exec flag on files, the flag will
4653 be set for the newly opened file descriptor, as determined by the
4654 value of $^F. See L<perlvar/$^F>.
4656 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4658 Creates an unnamed pair of sockets in the specified domain, of the
4659 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4660 for the system call of the same name. If unimplemented, yields a fatal
4661 error. Returns true if successful.
4663 On systems that support a close-on-exec flag on files, the flag will
4664 be set for the newly opened file descriptors, as determined by the value
4665 of $^F. See L<perlvar/$^F>.
4667 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4668 to C<pipe(Rdr, Wtr)> is essentially:
4671 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4672 shutdown(Rdr, 1); # no more writing for reader
4673 shutdown(Wtr, 0); # no more reading for writer
4675 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4676 emulate socketpair using IP sockets to localhost if your system implements
4677 sockets but not socketpair.
4679 =item sort SUBNAME LIST
4681 =item sort BLOCK LIST
4685 In list context, this sorts the LIST and returns the sorted list value.
4686 In scalar context, the behaviour of C<sort()> is undefined.
4688 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4689 order. If SUBNAME is specified, it gives the name of a subroutine
4690 that returns an integer less than, equal to, or greater than C<0>,
4691 depending on how the elements of the list are to be ordered. (The C<<
4692 <=> >> and C<cmp> operators are extremely useful in such routines.)
4693 SUBNAME may be a scalar variable name (unsubscripted), in which case
4694 the value provides the name of (or a reference to) the actual
4695 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4696 an anonymous, in-line sort subroutine.
4698 If the subroutine's prototype is C<($$)>, the elements to be compared
4699 are passed by reference in C<@_>, as for a normal subroutine. This is
4700 slower than unprototyped subroutines, where the elements to be
4701 compared are passed into the subroutine
4702 as the package global variables $a and $b (see example below). Note that
4703 in the latter case, it is usually counter-productive to declare $a and
4706 In either case, the subroutine may not be recursive. The values to be
4707 compared are always passed by reference, so don't modify them.
4709 You also cannot exit out of the sort block or subroutine using any of the
4710 loop control operators described in L<perlsyn> or with C<goto>.
4712 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4713 current collation locale. See L<perllocale>.
4715 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4716 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4717 preserves the input order of elements that compare equal. Although
4718 quicksort's run time is O(NlogN) when averaged over all arrays of
4719 length N, the time can be O(N**2), I<quadratic> behavior, for some
4720 inputs.) In 5.7, the quicksort implementation was replaced with
4721 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4722 But benchmarks indicated that for some inputs, on some platforms,
4723 the original quicksort was faster. 5.8 has a sort pragma for
4724 limited control of the sort. Its rather blunt control of the
4725 underlying algorithm may not persist into future perls, but the
4726 ability to characterize the input or output in implementation
4727 independent ways quite probably will. See L<sort>.
4732 @articles = sort @files;
4734 # same thing, but with explicit sort routine
4735 @articles = sort {$a cmp $b} @files;
4737 # now case-insensitively
4738 @articles = sort {uc($a) cmp uc($b)} @files;
4740 # same thing in reversed order
4741 @articles = sort {$b cmp $a} @files;
4743 # sort numerically ascending
4744 @articles = sort {$a <=> $b} @files;
4746 # sort numerically descending
4747 @articles = sort {$b <=> $a} @files;
4749 # this sorts the %age hash by value instead of key
4750 # using an in-line function
4751 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4753 # sort using explicit subroutine name
4755 $age{$a} <=> $age{$b}; # presuming numeric
4757 @sortedclass = sort byage @class;
4759 sub backwards { $b cmp $a }
4760 @harry = qw(dog cat x Cain Abel);
4761 @george = qw(gone chased yz Punished Axed);
4763 # prints AbelCaincatdogx
4764 print sort backwards @harry;
4765 # prints xdogcatCainAbel
4766 print sort @george, 'to', @harry;
4767 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4769 # inefficiently sort by descending numeric compare using
4770 # the first integer after the first = sign, or the
4771 # whole record case-insensitively otherwise
4774 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4779 # same thing, but much more efficiently;
4780 # we'll build auxiliary indices instead
4784 push @nums, /=(\d+)/;
4789 $nums[$b] <=> $nums[$a]
4791 $caps[$a] cmp $caps[$b]
4795 # same thing, but without any temps
4796 @new = map { $_->[0] }
4797 sort { $b->[1] <=> $a->[1]
4800 } map { [$_, /=(\d+)/, uc($_)] } @old;
4802 # using a prototype allows you to use any comparison subroutine
4803 # as a sort subroutine (including other package's subroutines)
4805 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4808 @new = sort other::backwards @old;
4810 # guarantee stability, regardless of algorithm
4812 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4814 # force use of mergesort (not portable outside Perl 5.8)
4815 use sort '_mergesort'; # note discouraging _
4816 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4818 If you're using strict, you I<must not> declare $a
4819 and $b as lexicals. They are package globals. That means
4820 if you're in the C<main> package and type
4822 @articles = sort {$b <=> $a} @files;
4824 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4825 but if you're in the C<FooPack> package, it's the same as typing
4827 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4829 The comparison function is required to behave. If it returns
4830 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4831 sometimes saying the opposite, for example) the results are not
4834 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
4835 (not-a-number), and because C<sort> will trigger a fatal error unless the
4836 result of a comparison is defined, when sorting with a comparison function
4837 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
4838 The following example takes advantage of the fact that C<NaN != NaN> to
4839 eliminate any C<NaN>s from the input.
4841 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
4843 =item splice ARRAY,OFFSET,LENGTH,LIST
4845 =item splice ARRAY,OFFSET,LENGTH
4847 =item splice ARRAY,OFFSET
4851 Removes the elements designated by OFFSET and LENGTH from an array, and
4852 replaces them with the elements of LIST, if any. In list context,
4853 returns the elements removed from the array. In scalar context,
4854 returns the last element removed, or C<undef> if no elements are
4855 removed. The array grows or shrinks as necessary.
4856 If OFFSET is negative then it starts that far from the end of the array.
4857 If LENGTH is omitted, removes everything from OFFSET onward.
4858 If LENGTH is negative, removes the elements from OFFSET onward
4859 except for -LENGTH elements at the end of the array.
4860 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4861 past the end of the array, perl issues a warning, and splices at the
4864 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4866 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4867 pop(@a) splice(@a,-1)
4868 shift(@a) splice(@a,0,1)
4869 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4870 $a[$i] = $y splice(@a,$i,1,$y)
4872 Example, assuming array lengths are passed before arrays:
4874 sub aeq { # compare two list values
4875 my(@a) = splice(@_,0,shift);
4876 my(@b) = splice(@_,0,shift);
4877 return 0 unless @a == @b; # same len?
4879 return 0 if pop(@a) ne pop(@b);
4883 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4885 =item split /PATTERN/,EXPR,LIMIT
4887 =item split /PATTERN/,EXPR
4889 =item split /PATTERN/
4893 Splits the string EXPR into a list of strings and returns that list. By
4894 default, empty leading fields are preserved, and empty trailing ones are
4897 In scalar context, returns the number of fields found and splits into
4898 the C<@_> array. Use of split in scalar context is deprecated, however,
4899 because it clobbers your subroutine arguments.
4901 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4902 splits on whitespace (after skipping any leading whitespace). Anything
4903 matching PATTERN is taken to be a delimiter separating the fields. (Note
4904 that the delimiter may be longer than one character.)
4906 If LIMIT is specified and positive, it represents the maximum number
4907 of fields the EXPR will be split into, though the actual number of
4908 fields returned depends on the number of times PATTERN matches within
4909 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4910 stripped (which potential users of C<pop> would do well to remember).
4911 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4912 had been specified. Note that splitting an EXPR that evaluates to the
4913 empty string always returns the empty list, regardless of the LIMIT
4916 A pattern matching the null string (not to be confused with
4917 a null pattern C<//>, which is just one member of the set of patterns
4918 matching a null string) will split the value of EXPR into separate
4919 characters at each point it matches that way. For example:
4921 print join(':', split(/ */, 'hi there'));
4923 produces the output 'h:i:t:h:e:r:e'.
4925 Using the empty pattern C<//> specifically matches the null string, and is
4926 not be confused with the use of C<//> to mean "the last successful pattern
4929 Empty leading (or trailing) fields are produced when there are positive width
4930 matches at the beginning (or end) of the string; a zero-width match at the
4931 beginning (or end) of the string does not produce an empty field. For
4934 print join(':', split(/(?=\w)/, 'hi there!'));
4936 produces the output 'h:i :t:h:e:r:e!'.
4938 The LIMIT parameter can be used to split a line partially
4940 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4942 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
4943 a LIMIT one larger than the number of variables in the list, to avoid
4944 unnecessary work. For the list above LIMIT would have been 4 by
4945 default. In time critical applications it behooves you not to split
4946 into more fields than you really need.
4948 If the PATTERN contains parentheses, additional list elements are
4949 created from each matching substring in the delimiter.
4951 split(/([,-])/, "1-10,20", 3);
4953 produces the list value
4955 (1, '-', 10, ',', 20)
4957 If you had the entire header of a normal Unix email message in $header,
4958 you could split it up into fields and their values this way:
4960 $header =~ s/\n\s+/ /g; # fix continuation lines
4961 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4963 The pattern C</PATTERN/> may be replaced with an expression to specify
4964 patterns that vary at runtime. (To do runtime compilation only once,
4965 use C</$variable/o>.)
4967 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4968 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4969 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4970 will give you as many null initial fields as there are leading spaces.
4971 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4972 whitespace produces a null first field. A C<split> with no arguments
4973 really does a S<C<split(' ', $_)>> internally.
4975 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4980 open(PASSWD, '/etc/passwd');
4983 ($login, $passwd, $uid, $gid,
4984 $gcos, $home, $shell) = split(/:/);
4988 As with regular pattern matching, any capturing parentheses that are not
4989 matched in a C<split()> will be set to C<undef> when returned:
4991 @fields = split /(A)|B/, "1A2B3";
4992 # @fields is (1, 'A', 2, undef, 3)
4994 =item sprintf FORMAT, LIST
4996 Returns a string formatted by the usual C<printf> conventions of the C
4997 library function C<sprintf>. See below for more details
4998 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4999 the general principles.
5003 # Format number with up to 8 leading zeroes
5004 $result = sprintf("%08d", $number);
5006 # Round number to 3 digits after decimal point
5007 $rounded = sprintf("%.3f", $number);
5009 Perl does its own C<sprintf> formatting--it emulates the C
5010 function C<sprintf>, but it doesn't use it (except for floating-point
5011 numbers, and even then only the standard modifiers are allowed). As a
5012 result, any non-standard extensions in your local C<sprintf> are not
5013 available from Perl.
5015 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5016 pass it an array as your first argument. The array is given scalar context,
5017 and instead of using the 0th element of the array as the format, Perl will
5018 use the count of elements in the array as the format, which is almost never
5021 Perl's C<sprintf> permits the following universally-known conversions:
5024 %c a character with the given number
5026 %d a signed integer, in decimal
5027 %u an unsigned integer, in decimal
5028 %o an unsigned integer, in octal
5029 %x an unsigned integer, in hexadecimal
5030 %e a floating-point number, in scientific notation
5031 %f a floating-point number, in fixed decimal notation
5032 %g a floating-point number, in %e or %f notation
5034 In addition, Perl permits the following widely-supported conversions:
5036 %X like %x, but using upper-case letters
5037 %E like %e, but using an upper-case "E"
5038 %G like %g, but with an upper-case "E" (if applicable)
5039 %b an unsigned integer, in binary
5040 %p a pointer (outputs the Perl value's address in hexadecimal)
5041 %n special: *stores* the number of characters output so far
5042 into the next variable in the parameter list
5044 Finally, for backward (and we do mean "backward") compatibility, Perl
5045 permits these unnecessary but widely-supported conversions:
5048 %D a synonym for %ld
5049 %U a synonym for %lu
5050 %O a synonym for %lo
5053 Note that the number of exponent digits in the scientific notation produced
5054 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5055 exponent less than 100 is system-dependent: it may be three or less
5056 (zero-padded as necessary). In other words, 1.23 times ten to the
5057 99th may be either "1.23e99" or "1.23e099".
5059 Between the C<%> and the format letter, you may specify a number of
5060 additional attributes controlling the interpretation of the format.
5061 In order, these are:
5065 =item format parameter index
5067 An explicit format parameter index, such as C<2$>. By default sprintf
5068 will format the next unused argument in the list, but this allows you
5069 to take the arguments out of order. Eg:
5071 printf '%2$d %1$d', 12, 34; # prints "34 12"
5072 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5077 space prefix positive number with a space
5078 + prefix positive number with a plus sign
5079 - left-justify within the field
5080 0 use zeros, not spaces, to right-justify
5081 # prefix non-zero octal with "0", non-zero hex with "0x",
5082 non-zero binary with "0b"
5086 printf '<% d>', 12; # prints "< 12>"
5087 printf '<%+d>', 12; # prints "<+12>"
5088 printf '<%6s>', 12; # prints "< 12>"
5089 printf '<%-6s>', 12; # prints "<12 >"
5090 printf '<%06s>', 12; # prints "<000012>"
5091 printf '<%#x>', 12; # prints "<0xc>"
5095 The vector flag C<v>, optionally specifying the join string to use.
5096 This flag tells perl to interpret the supplied string as a vector
5097 of integers, one for each character in the string, separated by
5098 a given string (a dot C<.> by default). This can be useful for
5099 displaying ordinal values of characters in arbitrary strings:
5101 printf "version is v%vd\n", $^V; # Perl's version
5103 Put an asterisk C<*> before the C<v> to override the string to
5104 use to separate the numbers:
5106 printf "address is %*vX\n", ":", $addr; # IPv6 address
5107 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5109 You can also explicitly specify the argument number to use for
5110 the join string using eg C<*2$v>:
5112 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5114 =item (minimum) width
5116 Arguments are usually formatted to be only as wide as required to
5117 display the given value. You can override the width by putting
5118 a number here, or get the width from the next argument (with C<*>)
5119 or from a specified argument (with eg C<*2$>):
5121 printf '<%s>', "a"; # prints "<a>"
5122 printf '<%6s>', "a"; # prints "< a>"
5123 printf '<%*s>', 6, "a"; # prints "< a>"
5124 printf '<%*2$s>', "a", 6; # prints "< a>"
5125 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5127 If a field width obtained through C<*> is negative, it has the same
5128 effect as the C<-> flag: left-justification.
5130 =item precision, or maximum width
5132 You can specify a precision (for numeric conversions) or a maximum
5133 width (for string conversions) by specifying a C<.> followed by a number.
5134 For floating point formats, with the exception of 'g' and 'G', this specifies
5135 the number of decimal places to show (the default being 6), eg:
5137 # these examples are subject to system-specific variation
5138 printf '<%f>', 1; # prints "<1.000000>"
5139 printf '<%.1f>', 1; # prints "<1.0>"
5140 printf '<%.0f>', 1; # prints "<1>"
5141 printf '<%e>', 10; # prints "<1.000000e+01>"
5142 printf '<%.1e>', 10; # prints "<1.0e+01>"
5144 For 'g' and 'G', this specifies the maximum number of digits to show,
5145 including prior to the decimal point as well as after it, eg:
5147 # these examples are subject to system-specific variation
5148 printf '<%g>', 1; # prints "<1>"
5149 printf '<%.10g>', 1; # prints "<1>"
5150 printf '<%g>', 100; # prints "<100>"
5151 printf '<%.1g>', 100; # prints "<1e+02>"
5152 printf '<%.2g>', 100.01; # prints "<1e+02>"
5153 printf '<%.5g>', 100.01; # prints "<100.01>"
5154 printf '<%.4g>', 100.01; # prints "<100>"
5156 For integer conversions, specifying a precision implies that the
5157 output of the number itself should be zero-padded to this width:
5159 printf '<%.6x>', 1; # prints "<000001>"
5160 printf '<%#.6x>', 1; # prints "<0x000001>"
5161 printf '<%-10.6x>', 1; # prints "<000001 >"
5163 For string conversions, specifying a precision truncates the string
5164 to fit in the specified width:
5166 printf '<%.5s>', "truncated"; # prints "<trunc>"
5167 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5169 You can also get the precision from the next argument using C<.*>:
5171 printf '<%.6x>', 1; # prints "<000001>"
5172 printf '<%.*x>', 6, 1; # prints "<000001>"
5174 You cannot currently get the precision from a specified number,
5175 but it is intended that this will be possible in the future using
5178 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5182 For numeric conversions, you can specify the size to interpret the
5183 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5184 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5185 whatever the default integer size is on your platform (usually 32 or 64
5186 bits), but you can override this to use instead one of the standard C types,
5187 as supported by the compiler used to build Perl:
5189 l interpret integer as C type "long" or "unsigned long"
5190 h interpret integer as C type "short" or "unsigned short"
5191 q, L or ll interpret integer as C type "long long", "unsigned long long".
5192 or "quads" (typically 64-bit integers)
5194 The last will produce errors if Perl does not understand "quads" in your
5195 installation. (This requires that either the platform natively supports quads
5196 or Perl was specifically compiled to support quads.) You can find out
5197 whether your Perl supports quads via L<Config>:
5200 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5203 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5204 to be the default floating point size on your platform (double or long double),
5205 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5206 platform supports them. You can find out whether your Perl supports long
5207 doubles via L<Config>:
5210 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5212 You can find out whether Perl considers 'long double' to be the default
5213 floating point size to use on your platform via L<Config>:
5216 ($Config{uselongdouble} eq 'define') &&
5217 print "long doubles by default\n";
5219 It can also be the case that long doubles and doubles are the same thing:
5222 ($Config{doublesize} == $Config{longdblsize}) &&
5223 print "doubles are long doubles\n";
5225 The size specifier C<V> has no effect for Perl code, but it is supported
5226 for compatibility with XS code; it means 'use the standard size for
5227 a Perl integer (or floating-point number)', which is already the
5228 default for Perl code.
5230 =item order of arguments
5232 Normally, sprintf takes the next unused argument as the value to
5233 format for each format specification. If the format specification
5234 uses C<*> to require additional arguments, these are consumed from
5235 the argument list in the order in which they appear in the format
5236 specification I<before> the value to format. Where an argument is
5237 specified using an explicit index, this does not affect the normal
5238 order for the arguments (even when the explicitly specified index
5239 would have been the next argument in any case).
5243 printf '<%*.*s>', $a, $b, $c;
5245 would use C<$a> for the width, C<$b> for the precision and C<$c>
5246 as the value to format, while:
5248 print '<%*1$.*s>', $a, $b;
5250 would use C<$a> for the width and the precision, and C<$b> as the
5253 Here are some more examples - beware that when using an explicit
5254 index, the C<$> may need to be escaped:
5256 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5257 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5258 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5259 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5263 If C<use locale> is in effect, the character used for the decimal
5264 point in formatted real numbers is affected by the LC_NUMERIC locale.
5271 Return the square root of EXPR. If EXPR is omitted, returns square
5272 root of C<$_>. Only works on non-negative operands, unless you've
5273 loaded the standard Math::Complex module.
5276 print sqrt(-2); # prints 1.4142135623731i
5282 Sets the random number seed for the C<rand> operator.
5284 The point of the function is to "seed" the C<rand> function so that
5285 C<rand> can produce a different sequence each time you run your
5288 If srand() is not called explicitly, it is called implicitly at the
5289 first use of the C<rand> operator. However, this was not the case in
5290 versions of Perl before 5.004, so if your script will run under older
5291 Perl versions, it should call C<srand>.
5293 Most programs won't even call srand() at all, except those that
5294 need a cryptographically-strong starting point rather than the
5295 generally acceptable default, which is based on time of day,
5296 process ID, and memory allocation, or the F</dev/urandom> device,
5299 You can call srand($seed) with the same $seed to reproduce the
5300 I<same> sequence from rand(), but this is usually reserved for
5301 generating predictable results for testing or debugging.
5302 Otherwise, don't call srand() more than once in your program.
5304 Do B<not> call srand() (i.e. without an argument) more than once in
5305 a script. The internal state of the random number generator should
5306 contain more entropy than can be provided by any seed, so calling
5307 srand() again actually I<loses> randomness.
5309 Most implementations of C<srand> take an integer and will silently
5310 truncate decimal numbers. This means C<srand(42)> will usually
5311 produce the same results as C<srand(42.1)>. To be safe, always pass
5312 C<srand> an integer.
5314 In versions of Perl prior to 5.004 the default seed was just the
5315 current C<time>. This isn't a particularly good seed, so many old
5316 programs supply their own seed value (often C<time ^ $$> or C<time ^
5317 ($$ + ($$ << 15))>), but that isn't necessary any more.
5319 Note that you need something much more random than the default seed for
5320 cryptographic purposes. Checksumming the compressed output of one or more
5321 rapidly changing operating system status programs is the usual method. For
5324 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5326 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5329 Frequently called programs (like CGI scripts) that simply use
5333 for a seed can fall prey to the mathematical property that
5337 one-third of the time. So don't do that.
5339 =item stat FILEHANDLE
5345 Returns a 13-element list giving the status info for a file, either
5346 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5347 it stats C<$_>. Returns a null list if the stat fails. Typically used
5350 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5351 $atime,$mtime,$ctime,$blksize,$blocks)
5354 Not all fields are supported on all filesystem types. Here are the
5355 meanings of the fields:
5357 0 dev device number of filesystem
5359 2 mode file mode (type and permissions)
5360 3 nlink number of (hard) links to the file
5361 4 uid numeric user ID of file's owner
5362 5 gid numeric group ID of file's owner
5363 6 rdev the device identifier (special files only)
5364 7 size total size of file, in bytes
5365 8 atime last access time in seconds since the epoch
5366 9 mtime last modify time in seconds since the epoch
5367 10 ctime inode change time in seconds since the epoch (*)
5368 11 blksize preferred block size for file system I/O
5369 12 blocks actual number of blocks allocated
5371 (The epoch was at 00:00 January 1, 1970 GMT.)
5373 (*) The ctime field is non-portable. In particular, you cannot expect
5374 it to be a "creation time", see L<perlport/"Files and Filesystems">
5377 If C<stat> is passed the special filehandle consisting of an underline, no
5378 stat is done, but the current contents of the stat structure from the
5379 last C<stat>, C<lstat>, or filetest are returned. Example:
5381 if (-x $file && (($d) = stat(_)) && $d < 0) {
5382 print "$file is executable NFS file\n";
5385 (This works on machines only for which the device number is negative
5388 Because the mode contains both the file type and its permissions, you
5389 should mask off the file type portion and (s)printf using a C<"%o">
5390 if you want to see the real permissions.
5392 $mode = (stat($filename))[2];
5393 printf "Permissions are %04o\n", $mode & 07777;
5395 In scalar context, C<stat> returns a boolean value indicating success
5396 or failure, and, if successful, sets the information associated with
5397 the special filehandle C<_>.
5399 The File::stat module provides a convenient, by-name access mechanism:
5402 $sb = stat($filename);
5403 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5404 $filename, $sb->size, $sb->mode & 07777,
5405 scalar localtime $sb->mtime;
5407 You can import symbolic mode constants (C<S_IF*>) and functions
5408 (C<S_IS*>) from the Fcntl module:
5412 $mode = (stat($filename))[2];
5414 $user_rwx = ($mode & S_IRWXU) >> 6;
5415 $group_read = ($mode & S_IRGRP) >> 3;
5416 $other_execute = $mode & S_IXOTH;
5418 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5420 $is_setuid = $mode & S_ISUID;
5421 $is_setgid = S_ISDIR($mode);
5423 You could write the last two using the C<-u> and C<-d> operators.
5424 The commonly available C<S_IF*> constants are
5426 # Permissions: read, write, execute, for user, group, others.
5428 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5429 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5430 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5432 # Setuid/Setgid/Stickiness/SaveText.
5433 # Note that the exact meaning of these is system dependent.
5435 S_ISUID S_ISGID S_ISVTX S_ISTXT
5437 # File types. Not necessarily all are available on your system.
5439 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5441 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5443 S_IREAD S_IWRITE S_IEXEC
5445 and the C<S_IF*> functions are
5447 S_IMODE($mode) the part of $mode containing the permission bits
5448 and the setuid/setgid/sticky bits
5450 S_IFMT($mode) the part of $mode containing the file type
5451 which can be bit-anded with e.g. S_IFREG
5452 or with the following functions
5454 # The operators -f, -d, -l, -b, -c, -p, and -S.
5456 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5457 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5459 # No direct -X operator counterpart, but for the first one
5460 # the -g operator is often equivalent. The ENFMT stands for
5461 # record flocking enforcement, a platform-dependent feature.
5463 S_ISENFMT($mode) S_ISWHT($mode)
5465 See your native chmod(2) and stat(2) documentation for more details
5466 about the C<S_*> constants. To get status info for a symbolic link
5467 instead of the target file behind the link, use the C<lstat> function.
5473 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5474 doing many pattern matches on the string before it is next modified.
5475 This may or may not save time, depending on the nature and number of
5476 patterns you are searching on, and on the distribution of character
5477 frequencies in the string to be searched--you probably want to compare
5478 run times with and without it to see which runs faster. Those loops
5479 which scan for many short constant strings (including the constant
5480 parts of more complex patterns) will benefit most. You may have only
5481 one C<study> active at a time--if you study a different scalar the first
5482 is "unstudied". (The way C<study> works is this: a linked list of every
5483 character in the string to be searched is made, so we know, for
5484 example, where all the C<'k'> characters are. From each search string,
5485 the rarest character is selected, based on some static frequency tables
5486 constructed from some C programs and English text. Only those places
5487 that contain this "rarest" character are examined.)
5489 For example, here is a loop that inserts index producing entries
5490 before any line containing a certain pattern:
5494 print ".IX foo\n" if /\bfoo\b/;
5495 print ".IX bar\n" if /\bbar\b/;
5496 print ".IX blurfl\n" if /\bblurfl\b/;
5501 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5502 will be looked at, because C<f> is rarer than C<o>. In general, this is
5503 a big win except in pathological cases. The only question is whether
5504 it saves you more time than it took to build the linked list in the
5507 Note that if you have to look for strings that you don't know till
5508 runtime, you can build an entire loop as a string and C<eval> that to
5509 avoid recompiling all your patterns all the time. Together with
5510 undefining C<$/> to input entire files as one record, this can be very
5511 fast, often faster than specialized programs like fgrep(1). The following
5512 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5513 out the names of those files that contain a match:
5515 $search = 'while (<>) { study;';
5516 foreach $word (@words) {
5517 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5522 eval $search; # this screams
5523 $/ = "\n"; # put back to normal input delimiter
5524 foreach $file (sort keys(%seen)) {
5528 =item sub NAME BLOCK
5530 =item sub NAME (PROTO) BLOCK
5532 =item sub NAME : ATTRS BLOCK
5534 =item sub NAME (PROTO) : ATTRS BLOCK
5536 This is subroutine definition, not a real function I<per se>.
5537 Without a BLOCK it's just a forward declaration. Without a NAME,
5538 it's an anonymous function declaration, and does actually return
5539 a value: the CODE ref of the closure you just created.
5541 See L<perlsub> and L<perlref> for details about subroutines and
5542 references, and L<attributes> and L<Attribute::Handlers> for more
5543 information about attributes.
5545 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5547 =item substr EXPR,OFFSET,LENGTH
5549 =item substr EXPR,OFFSET
5551 Extracts a substring out of EXPR and returns it. First character is at
5552 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5553 If OFFSET is negative (or more precisely, less than C<$[>), starts
5554 that far from the end of the string. If LENGTH is omitted, returns
5555 everything to the end of the string. If LENGTH is negative, leaves that
5556 many characters off the end of the string.
5558 You can use the substr() function as an lvalue, in which case EXPR
5559 must itself be an lvalue. If you assign something shorter than LENGTH,
5560 the string will shrink, and if you assign something longer than LENGTH,
5561 the string will grow to accommodate it. To keep the string the same
5562 length you may need to pad or chop your value using C<sprintf>.
5564 If OFFSET and LENGTH specify a substring that is partly outside the
5565 string, only the part within the string is returned. If the substring
5566 is beyond either end of the string, substr() returns the undefined
5567 value and produces a warning. When used as an lvalue, specifying a
5568 substring that is entirely outside the string is a fatal error.
5569 Here's an example showing the behavior for boundary cases:
5572 substr($name, 4) = 'dy'; # $name is now 'freddy'
5573 my $null = substr $name, 6, 2; # returns '' (no warning)
5574 my $oops = substr $name, 7; # returns undef, with warning
5575 substr($name, 7) = 'gap'; # fatal error
5577 An alternative to using substr() as an lvalue is to specify the
5578 replacement string as the 4th argument. This allows you to replace
5579 parts of the EXPR and return what was there before in one operation,
5580 just as you can with splice().
5582 Note that the lvalue returned by by the 3-arg version of substr() acts as
5583 a 'magic bullet'; each time it is assigned to, it remembers which part
5584 of the original string is being modified; for example:
5587 for (substr($x,1,2)) {
5588 $_ = 'a'; print $x,"\n"; # prints 1a4
5589 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
5591 $_ = 'pq'; print $x,"\n"; # prints 5pq9
5595 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
5598 =item symlink OLDFILE,NEWFILE
5600 Creates a new filename symbolically linked to the old filename.
5601 Returns C<1> for success, C<0> otherwise. On systems that don't support
5602 symbolic links, produces a fatal error at run time. To check for that,
5605 $symlink_exists = eval { symlink("",""); 1 };
5607 =item syscall NUMBER, LIST
5609 Calls the system call specified as the first element of the list,
5610 passing the remaining elements as arguments to the system call. If
5611 unimplemented, produces a fatal error. The arguments are interpreted
5612 as follows: if a given argument is numeric, the argument is passed as
5613 an int. If not, the pointer to the string value is passed. You are
5614 responsible to make sure a string is pre-extended long enough to
5615 receive any result that might be written into a string. You can't use a
5616 string literal (or other read-only string) as an argument to C<syscall>
5617 because Perl has to assume that any string pointer might be written
5619 integer arguments are not literals and have never been interpreted in a
5620 numeric context, you may need to add C<0> to them to force them to look
5621 like numbers. This emulates the C<syswrite> function (or vice versa):
5623 require 'syscall.ph'; # may need to run h2ph
5625 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5627 Note that Perl supports passing of up to only 14 arguments to your system call,
5628 which in practice should usually suffice.
5630 Syscall returns whatever value returned by the system call it calls.
5631 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5632 Note that some system calls can legitimately return C<-1>. The proper
5633 way to handle such calls is to assign C<$!=0;> before the call and
5634 check the value of C<$!> if syscall returns C<-1>.
5636 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5637 number of the read end of the pipe it creates. There is no way
5638 to retrieve the file number of the other end. You can avoid this
5639 problem by using C<pipe> instead.
5641 =item sysopen FILEHANDLE,FILENAME,MODE
5643 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5645 Opens the file whose filename is given by FILENAME, and associates it
5646 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5647 the name of the real filehandle wanted. This function calls the
5648 underlying operating system's C<open> function with the parameters
5649 FILENAME, MODE, PERMS.
5651 The possible values and flag bits of the MODE parameter are
5652 system-dependent; they are available via the standard module C<Fcntl>.
5653 See the documentation of your operating system's C<open> to see which
5654 values and flag bits are available. You may combine several flags
5655 using the C<|>-operator.
5657 Some of the most common values are C<O_RDONLY> for opening the file in
5658 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5659 and C<O_RDWR> for opening the file in read-write mode, and.
5661 For historical reasons, some values work on almost every system
5662 supported by perl: zero means read-only, one means write-only, and two
5663 means read/write. We know that these values do I<not> work under
5664 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5665 use them in new code.
5667 If the file named by FILENAME does not exist and the C<open> call creates
5668 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5669 PERMS specifies the permissions of the newly created file. If you omit
5670 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5671 These permission values need to be in octal, and are modified by your
5672 process's current C<umask>.
5674 In many systems the C<O_EXCL> flag is available for opening files in
5675 exclusive mode. This is B<not> locking: exclusiveness means here that
5676 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5679 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5681 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5682 that takes away the user's option to have a more permissive umask.
5683 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5686 Note that C<sysopen> depends on the fdopen() C library function.
5687 On many UNIX systems, fdopen() is known to fail when file descriptors
5688 exceed a certain value, typically 255. If you need more file
5689 descriptors than that, consider rebuilding Perl to use the C<sfio>
5690 library, or perhaps using the POSIX::open() function.
5692 See L<perlopentut> for a kinder, gentler explanation of opening files.
5694 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5696 =item sysread FILEHANDLE,SCALAR,LENGTH
5698 Attempts to read LENGTH bytes of data into variable SCALAR from the
5699 specified FILEHANDLE, using the system call read(2). It bypasses
5700 buffered IO, so mixing this with other kinds of reads, C<print>,
5701 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5702 perlio or stdio layers usually buffers data. Returns the number of
5703 bytes actually read, C<0> at end of file, or undef if there was an
5704 error (in the latter case C<$!> is also set). SCALAR will be grown or
5705 shrunk so that the last byte actually read is the last byte of the
5706 scalar after the read.
5708 An OFFSET may be specified to place the read data at some place in the
5709 string other than the beginning. A negative OFFSET specifies
5710 placement at that many characters counting backwards from the end of
5711 the string. A positive OFFSET greater than the length of SCALAR
5712 results in the string being padded to the required size with C<"\0">
5713 bytes before the result of the read is appended.
5715 There is no syseof() function, which is ok, since eof() doesn't work
5716 very well on device files (like ttys) anyway. Use sysread() and check
5717 for a return value for 0 to decide whether you're done.
5719 Note that if the filehandle has been marked as C<:utf8> Unicode
5720 characters are read instead of bytes (the LENGTH, OFFSET, and the
5721 return value of sysread() are in Unicode characters).
5722 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5723 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5725 =item sysseek FILEHANDLE,POSITION,WHENCE
5727 Sets FILEHANDLE's system position in bytes using the system call
5728 lseek(2). FILEHANDLE may be an expression whose value gives the name
5729 of the filehandle. The values for WHENCE are C<0> to set the new
5730 position to POSITION, C<1> to set the it to the current position plus
5731 POSITION, and C<2> to set it to EOF plus POSITION (typically
5734 Note the I<in bytes>: even if the filehandle has been set to operate
5735 on characters (for example by using the C<:utf8> I/O layer), tell()
5736 will return byte offsets, not character offsets (because implementing
5737 that would render sysseek() very slow).
5739 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5740 than C<sysread>, for example >< or read()) C<print>, C<write>,
5741 C<seek>, C<tell>, or C<eof> may cause confusion.
5743 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5744 and C<SEEK_END> (start of the file, current position, end of the file)
5745 from the Fcntl module. Use of the constants is also more portable
5746 than relying on 0, 1, and 2. For example to define a "systell" function:
5748 use Fcntl 'SEEK_CUR';
5749 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5751 Returns the new position, or the undefined value on failure. A position
5752 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5753 true on success and false on failure, yet you can still easily determine
5758 =item system PROGRAM LIST
5760 Does exactly the same thing as C<exec LIST>, except that a fork is
5761 done first, and the parent process waits for the child process to
5762 complete. Note that argument processing varies depending on the
5763 number of arguments. If there is more than one argument in LIST,
5764 or if LIST is an array with more than one value, starts the program
5765 given by the first element of the list with arguments given by the
5766 rest of the list. If there is only one scalar argument, the argument
5767 is checked for shell metacharacters, and if there are any, the
5768 entire argument is passed to the system's command shell for parsing
5769 (this is C</bin/sh -c> on Unix platforms, but varies on other
5770 platforms). If there are no shell metacharacters in the argument,
5771 it is split into words and passed directly to C<execvp>, which is
5774 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5775 output before any operation that may do a fork, but this may not be
5776 supported on some platforms (see L<perlport>). To be safe, you may need
5777 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5778 of C<IO::Handle> on any open handles.
5780 The return value is the exit status of the program as returned by the
5781 C<wait> call. To get the actual exit value shift right by eight (see below).
5782 See also L</exec>. This is I<not> what you want to use to capture
5783 the output from a command, for that you should use merely backticks or
5784 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5785 indicates a failure to start the program (inspect $! for the reason).
5787 Like C<exec>, C<system> allows you to lie to a program about its name if
5788 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5790 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
5791 C<system>, if you expect your program to terminate on receipt of these
5792 signals you will need to arrange to do so yourself based on the return
5795 @args = ("command", "arg1", "arg2");
5797 or die "system @args failed: $?"
5799 You can check all the failure possibilities by inspecting
5803 print "failed to execute: $!\n";
5806 printf "child died with signal %d, %s coredump\n",
5807 ($? & 127), ($? & 128) ? 'with' : 'without';
5810 printf "child exited with value %d\n", $? >> 8;
5813 or more portably by using the W*() calls of the POSIX extension;
5814 see L<perlport> for more information.
5816 When the arguments get executed via the system shell, results
5817 and return codes will be subject to its quirks and capabilities.
5818 See L<perlop/"`STRING`"> and L</exec> for details.
5820 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5822 =item syswrite FILEHANDLE,SCALAR,LENGTH
5824 =item syswrite FILEHANDLE,SCALAR
5826 Attempts to write LENGTH bytes of data from variable SCALAR to the
5827 specified FILEHANDLE, using the system call write(2). If LENGTH is
5828 not specified, writes whole SCALAR. It bypasses buffered IO, so
5829 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5830 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5831 stdio layers usually buffers data. Returns the number of bytes
5832 actually written, or C<undef> if there was an error (in this case the
5833 errno variable C<$!> is also set). If the LENGTH is greater than the
5834 available data in the SCALAR after the OFFSET, only as much data as is
5835 available will be written.
5837 An OFFSET may be specified to write the data from some part of the
5838 string other than the beginning. A negative OFFSET specifies writing
5839 that many characters counting backwards from the end of the string.
5840 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5842 Note that if the filehandle has been marked as C<:utf8>, Unicode
5843 characters are written instead of bytes (the LENGTH, OFFSET, and the
5844 return value of syswrite() are in UTF-8 encoded Unicode characters).
5845 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5846 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5848 =item tell FILEHANDLE
5852 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5853 error. FILEHANDLE may be an expression whose value gives the name of
5854 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5857 Note the I<in bytes>: even if the filehandle has been set to
5858 operate on characters (for example by using the C<:utf8> open
5859 layer), tell() will return byte offsets, not character offsets
5860 (because that would render seek() and tell() rather slow).
5862 The return value of tell() for the standard streams like the STDIN
5863 depends on the operating system: it may return -1 or something else.
5864 tell() on pipes, fifos, and sockets usually returns -1.
5866 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5868 Do not use tell() on a filehandle that has been opened using
5869 sysopen(), use sysseek() for that as described above. Why? Because
5870 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5871 buffered filehandles. sysseek() make sense only on the first kind,
5872 tell() only makes sense on the second kind.
5874 =item telldir DIRHANDLE
5876 Returns the current position of the C<readdir> routines on DIRHANDLE.
5877 Value may be given to C<seekdir> to access a particular location in a
5878 directory. Has the same caveats about possible directory compaction as
5879 the corresponding system library routine.
5881 =item tie VARIABLE,CLASSNAME,LIST
5883 This function binds a variable to a package class that will provide the
5884 implementation for the variable. VARIABLE is the name of the variable
5885 to be enchanted. CLASSNAME is the name of a class implementing objects
5886 of correct type. Any additional arguments are passed to the C<new>
5887 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5888 or C<TIEHASH>). Typically these are arguments such as might be passed
5889 to the C<dbm_open()> function of C. The object returned by the C<new>
5890 method is also returned by the C<tie> function, which would be useful
5891 if you want to access other methods in CLASSNAME.
5893 Note that functions such as C<keys> and C<values> may return huge lists
5894 when used on large objects, like DBM files. You may prefer to use the
5895 C<each> function to iterate over such. Example:
5897 # print out history file offsets
5899 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5900 while (($key,$val) = each %HIST) {
5901 print $key, ' = ', unpack('L',$val), "\n";
5905 A class implementing a hash should have the following methods:
5907 TIEHASH classname, LIST
5909 STORE this, key, value
5914 NEXTKEY this, lastkey
5919 A class implementing an ordinary array should have the following methods:
5921 TIEARRAY classname, LIST
5923 STORE this, key, value
5925 STORESIZE this, count
5931 SPLICE this, offset, length, LIST
5936 A class implementing a file handle should have the following methods:
5938 TIEHANDLE classname, LIST
5939 READ this, scalar, length, offset
5942 WRITE this, scalar, length, offset
5944 PRINTF this, format, LIST
5948 SEEK this, position, whence
5950 OPEN this, mode, LIST
5955 A class implementing a scalar should have the following methods:
5957 TIESCALAR classname, LIST
5963 Not all methods indicated above need be implemented. See L<perltie>,
5964 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5966 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5967 for you--you need to do that explicitly yourself. See L<DB_File>
5968 or the F<Config> module for interesting C<tie> implementations.
5970 For further details see L<perltie>, L<"tied VARIABLE">.
5974 Returns a reference to the object underlying VARIABLE (the same value
5975 that was originally returned by the C<tie> call that bound the variable
5976 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5981 Returns the number of non-leap seconds since whatever time the system
5982 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5983 and 00:00:00 UTC, January 1, 1970 for most other systems).
5984 Suitable for feeding to C<gmtime> and C<localtime>.
5986 For measuring time in better granularity than one second,
5987 you may use either the Time::HiRes module (from CPAN, and starting from
5988 Perl 5.8 part of the standard distribution), or if you have
5989 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5990 See L<perlfaq8> for details.
5994 Returns a four-element list giving the user and system times, in
5995 seconds, for this process and the children of this process.
5997 ($user,$system,$cuser,$csystem) = times;
5999 In scalar context, C<times> returns C<$user>.
6003 The transliteration operator. Same as C<y///>. See L<perlop>.
6005 =item truncate FILEHANDLE,LENGTH
6007 =item truncate EXPR,LENGTH
6009 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6010 specified length. Produces a fatal error if truncate isn't implemented
6011 on your system. Returns true if successful, the undefined value
6014 The behavior is undefined if LENGTH is greater than the length of the
6021 Returns an uppercased version of EXPR. This is the internal function
6022 implementing the C<\U> escape in double-quoted strings. Respects
6023 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6024 and L<perlunicode> for more details about locale and Unicode support.
6025 It does not attempt to do titlecase mapping on initial letters. See
6026 C<ucfirst> for that.
6028 If EXPR is omitted, uses C<$_>.
6034 Returns the value of EXPR with the first character in uppercase
6035 (titlecase in Unicode). This is the internal function implementing
6036 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6037 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6038 for more details about locale and Unicode support.
6040 If EXPR is omitted, uses C<$_>.
6046 Sets the umask for the process to EXPR and returns the previous value.
6047 If EXPR is omitted, merely returns the current umask.
6049 The Unix permission C<rwxr-x---> is represented as three sets of three
6050 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6051 and isn't one of the digits). The C<umask> value is such a number
6052 representing disabled permissions bits. The permission (or "mode")
6053 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6054 even if you tell C<sysopen> to create a file with permissions C<0777>,
6055 if your umask is C<0022> then the file will actually be created with
6056 permissions C<0755>. If your C<umask> were C<0027> (group can't
6057 write; others can't read, write, or execute), then passing
6058 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6061 Here's some advice: supply a creation mode of C<0666> for regular
6062 files (in C<sysopen>) and one of C<0777> for directories (in
6063 C<mkdir>) and executable files. This gives users the freedom of
6064 choice: if they want protected files, they might choose process umasks
6065 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6066 Programs should rarely if ever make policy decisions better left to
6067 the user. The exception to this is when writing files that should be
6068 kept private: mail files, web browser cookies, I<.rhosts> files, and
6071 If umask(2) is not implemented on your system and you are trying to
6072 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6073 fatal error at run time. If umask(2) is not implemented and you are
6074 not trying to restrict access for yourself, returns C<undef>.
6076 Remember that a umask is a number, usually given in octal; it is I<not> a
6077 string of octal digits. See also L</oct>, if all you have is a string.
6083 Undefines the value of EXPR, which must be an lvalue. Use only on a
6084 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6085 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6086 will probably not do what you expect on most predefined variables or
6087 DBM list values, so don't do that; see L<delete>.) Always returns the
6088 undefined value. You can omit the EXPR, in which case nothing is
6089 undefined, but you still get an undefined value that you could, for
6090 instance, return from a subroutine, assign to a variable or pass as a
6091 parameter. Examples:
6094 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6098 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6099 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6100 select undef, undef, undef, 0.25;
6101 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6103 Note that this is a unary operator, not a list operator.
6109 Deletes a list of files. Returns the number of files successfully
6112 $cnt = unlink 'a', 'b', 'c';
6116 Note: C<unlink> will not delete directories unless you are superuser and
6117 the B<-U> flag is supplied to Perl. Even if these conditions are
6118 met, be warned that unlinking a directory can inflict damage on your
6119 filesystem. Use C<rmdir> instead.
6121 If LIST is omitted, uses C<$_>.
6123 =item unpack TEMPLATE,EXPR
6125 =item unpack TEMPLATE
6127 C<unpack> does the reverse of C<pack>: it takes a string
6128 and expands it out into a list of values.
6129 (In scalar context, it returns merely the first value produced.)
6131 If EXPR is omitted, unpacks the C<$_> string.
6133 The string is broken into chunks described by the TEMPLATE. Each chunk
6134 is converted separately to a value. Typically, either the string is a result
6135 of C<pack>, or the bytes of the string represent a C structure of some
6138 The TEMPLATE has the same format as in the C<pack> function.
6139 Here's a subroutine that does substring:
6142 my($what,$where,$howmuch) = @_;
6143 unpack("x$where a$howmuch", $what);
6148 sub ordinal { unpack("c",$_[0]); } # same as ord()
6150 In addition to fields allowed in pack(), you may prefix a field with
6151 a %<number> to indicate that
6152 you want a <number>-bit checksum of the items instead of the items
6153 themselves. Default is a 16-bit checksum. Checksum is calculated by
6154 summing numeric values of expanded values (for string fields the sum of
6155 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6157 For example, the following
6158 computes the same number as the System V sum program:
6162 unpack("%32C*",<>) % 65535;
6165 The following efficiently counts the number of set bits in a bit vector:
6167 $setbits = unpack("%32b*", $selectmask);
6169 The C<p> and C<P> formats should be used with care. Since Perl
6170 has no way of checking whether the value passed to C<unpack()>
6171 corresponds to a valid memory location, passing a pointer value that's
6172 not known to be valid is likely to have disastrous consequences.
6174 If there are more pack codes or if the repeat count of a field or a group
6175 is larger than what the remainder of the input string allows, the result
6176 is not well defined: in some cases, the repeat count is decreased, or
6177 C<unpack()> will produce null strings or zeroes, or terminate with an
6178 error. If the input string is longer than one described by the TEMPLATE,
6179 the rest is ignored.
6181 See L</pack> for more examples and notes.
6183 =item untie VARIABLE
6185 Breaks the binding between a variable and a package. (See C<tie>.)
6186 Has no effect if the variable is not tied.
6188 =item unshift ARRAY,LIST
6190 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6191 depending on how you look at it. Prepends list to the front of the
6192 array, and returns the new number of elements in the array.
6194 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6196 Note the LIST is prepended whole, not one element at a time, so the
6197 prepended elements stay in the same order. Use C<reverse> to do the
6200 =item use Module VERSION LIST
6202 =item use Module VERSION
6204 =item use Module LIST
6210 Imports some semantics into the current package from the named module,
6211 generally by aliasing certain subroutine or variable names into your
6212 package. It is exactly equivalent to
6214 BEGIN { require Module; import Module LIST; }
6216 except that Module I<must> be a bareword.
6218 VERSION may be either a numeric argument such as 5.006, which will be
6219 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6220 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6221 greater than the version of the current Perl interpreter; Perl will not
6222 attempt to parse the rest of the file. Compare with L</require>, which can
6223 do a similar check at run time.
6225 Specifying VERSION as a literal of the form v5.6.1 should generally be
6226 avoided, because it leads to misleading error messages under earlier
6227 versions of Perl which do not support this syntax. The equivalent numeric
6228 version should be used instead.
6230 use v5.6.1; # compile time version check
6232 use 5.006_001; # ditto; preferred for backwards compatibility
6234 This is often useful if you need to check the current Perl version before
6235 C<use>ing library modules that have changed in incompatible ways from
6236 older versions of Perl. (We try not to do this more than we have to.)
6238 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6239 C<require> makes sure the module is loaded into memory if it hasn't been
6240 yet. The C<import> is not a builtin--it's just an ordinary static method
6241 call into the C<Module> package to tell the module to import the list of
6242 features back into the current package. The module can implement its
6243 C<import> method any way it likes, though most modules just choose to
6244 derive their C<import> method via inheritance from the C<Exporter> class that
6245 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6246 method can be found then the call is skipped, even if there is an AUTOLOAD
6249 If you do not want to call the package's C<import> method (for instance,
6250 to stop your namespace from being altered), explicitly supply the empty list:
6254 That is exactly equivalent to
6256 BEGIN { require Module }
6258 If the VERSION argument is present between Module and LIST, then the
6259 C<use> will call the VERSION method in class Module with the given
6260 version as an argument. The default VERSION method, inherited from
6261 the UNIVERSAL class, croaks if the given version is larger than the
6262 value of the variable C<$Module::VERSION>.
6264 Again, there is a distinction between omitting LIST (C<import> called
6265 with no arguments) and an explicit empty LIST C<()> (C<import> not
6266 called). Note that there is no comma after VERSION!
6268 Because this is a wide-open interface, pragmas (compiler directives)
6269 are also implemented this way. Currently implemented pragmas are:
6274 use sigtrap qw(SEGV BUS);
6275 use strict qw(subs vars refs);
6276 use subs qw(afunc blurfl);
6277 use warnings qw(all);
6278 use sort qw(stable _quicksort _mergesort);
6280 Some of these pseudo-modules import semantics into the current
6281 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6282 which import symbols into the current package (which are effective
6283 through the end of the file).
6285 There's a corresponding C<no> command that unimports meanings imported
6286 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6287 It behaves exactly as C<import> does with respect to VERSION, an
6288 omitted LIST, empty LIST, or no unimport method being found.
6294 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6295 for the C<-M> and C<-m> command-line options to perl that give C<use>
6296 functionality from the command-line.
6300 Changes the access and modification times on each file of a list of
6301 files. The first two elements of the list must be the NUMERICAL access
6302 and modification times, in that order. Returns the number of files
6303 successfully changed. The inode change time of each file is set
6304 to the current time. For example, this code has the same effect as the
6305 Unix touch(1) command when the files I<already exist>.
6308 $atime = $mtime = time;
6309 utime $atime, $mtime, @ARGV;
6311 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6312 the utime(2) function in the C library will be called with a null second
6313 argument. On most systems, this will set the file's access and
6314 modification times to the current time (i.e. equivalent to the example
6317 utime undef, undef, @ARGV;
6319 Under NFS this will use the time of the NFS server, not the time of
6320 the local machine. If there is a time synchronization problem, the
6321 NFS server and local machine will have different times. The Unix
6322 touch(1) command will in fact normally use this form instead of the
6323 one shown in the first example.
6325 Note that only passing one of the first two elements as C<undef> will
6326 be equivalent of passing it as 0 and will not have the same effect as
6327 described when they are both C<undef>. This case will also trigger an
6328 uninitialized warning.
6332 Returns a list consisting of all the values of the named hash.
6333 (In a scalar context, returns the number of values.)
6335 The values are returned in an apparently random order. The actual
6336 random order is subject to change in future versions of perl, but it
6337 is guaranteed to be the same order as either the C<keys> or C<each>
6338 function would produce on the same (unmodified) hash. Since Perl
6339 5.8.1 the ordering is different even between different runs of Perl
6340 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6342 As a side effect, calling values() resets the HASH's internal iterator,
6343 see L</each>. (In particular, calling values() in void context resets
6344 the iterator with no other overhead.)
6346 Note that the values are not copied, which means modifying them will
6347 modify the contents of the hash:
6349 for (values %hash) { s/foo/bar/g } # modifies %hash values
6350 for (@hash{keys %hash}) { s/foo/bar/g } # same
6352 See also C<keys>, C<each>, and C<sort>.
6354 =item vec EXPR,OFFSET,BITS
6356 Treats the string in EXPR as a bit vector made up of elements of
6357 width BITS, and returns the value of the element specified by OFFSET
6358 as an unsigned integer. BITS therefore specifies the number of bits
6359 that are reserved for each element in the bit vector. This must
6360 be a power of two from 1 to 32 (or 64, if your platform supports
6363 If BITS is 8, "elements" coincide with bytes of the input string.
6365 If BITS is 16 or more, bytes of the input string are grouped into chunks
6366 of size BITS/8, and each group is converted to a number as with
6367 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6368 for BITS==64). See L<"pack"> for details.
6370 If bits is 4 or less, the string is broken into bytes, then the bits
6371 of each byte are broken into 8/BITS groups. Bits of a byte are
6372 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6373 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6374 breaking the single input byte C<chr(0x36)> into two groups gives a list
6375 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6377 C<vec> may also be assigned to, in which case parentheses are needed
6378 to give the expression the correct precedence as in
6380 vec($image, $max_x * $x + $y, 8) = 3;
6382 If the selected element is outside the string, the value 0 is returned.
6383 If an element off the end of the string is written to, Perl will first
6384 extend the string with sufficiently many zero bytes. It is an error
6385 to try to write off the beginning of the string (i.e. negative OFFSET).
6387 The string should not contain any character with the value > 255 (which
6388 can only happen if you're using UTF-8 encoding). If it does, it will be
6389 treated as something which is not UTF-8 encoded. When the C<vec> was
6390 assigned to, other parts of your program will also no longer consider the
6391 string to be UTF-8 encoded. In other words, if you do have such characters
6392 in your string, vec() will operate on the actual byte string, and not the
6393 conceptual character string.
6395 Strings created with C<vec> can also be manipulated with the logical
6396 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6397 vector operation is desired when both operands are strings.
6398 See L<perlop/"Bitwise String Operators">.
6400 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6401 The comments show the string after each step. Note that this code works
6402 in the same way on big-endian or little-endian machines.
6405 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6407 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6408 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6410 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6411 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6412 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6413 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6414 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6415 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6417 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6418 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6419 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6422 To transform a bit vector into a string or list of 0's and 1's, use these:
6424 $bits = unpack("b*", $vector);
6425 @bits = split(//, unpack("b*", $vector));
6427 If you know the exact length in bits, it can be used in place of the C<*>.
6429 Here is an example to illustrate how the bits actually fall in place:
6435 unpack("V",$_) 01234567890123456789012345678901
6436 ------------------------------------------------------------------
6441 for ($shift=0; $shift < $width; ++$shift) {
6442 for ($off=0; $off < 32/$width; ++$off) {
6443 $str = pack("B*", "0"x32);
6444 $bits = (1<<$shift);
6445 vec($str, $off, $width) = $bits;
6446 $res = unpack("b*",$str);
6447 $val = unpack("V", $str);
6454 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6455 $off, $width, $bits, $val, $res
6459 Regardless of the machine architecture on which it is run, the above
6460 example should print the following table:
6463 unpack("V",$_) 01234567890123456789012345678901
6464 ------------------------------------------------------------------
6465 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6466 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6467 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6468 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6469 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6470 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6471 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6472 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6473 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6474 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6475 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6476 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6477 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6478 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6479 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6480 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6481 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6482 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6483 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6484 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6485 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6486 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6487 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6488 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6489 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6490 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6491 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6492 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6493 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6494 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6495 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6496 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6497 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6498 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6499 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6500 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6501 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6502 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6503 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6504 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6505 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6506 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6507 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6508 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6509 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6510 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6511 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6512 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6513 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6514 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6515 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6516 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6517 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6518 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6519 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6520 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6521 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6522 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6523 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6524 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6525 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6526 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6527 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6528 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6529 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6530 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6531 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6532 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6533 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6534 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6535 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6536 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6537 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6538 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6539 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6540 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6541 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6542 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6543 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6544 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6545 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6546 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6547 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6548 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6549 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6550 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6551 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6552 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6553 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6554 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6555 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6556 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6557 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6558 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6559 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6560 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6561 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6562 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6563 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6564 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6565 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6566 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6567 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6568 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6569 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6570 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6571 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6572 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6573 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6574 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6575 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6576 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6577 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6578 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6579 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6580 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6581 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6582 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6583 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6584 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6585 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6586 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6587 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6588 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6589 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6590 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6591 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6592 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6596 Behaves like the wait(2) system call on your system: it waits for a child
6597 process to terminate and returns the pid of the deceased process, or
6598 C<-1> if there are no child processes. The status is returned in C<$?>.
6599 Note that a return value of C<-1> could mean that child processes are
6600 being automatically reaped, as described in L<perlipc>.
6602 =item waitpid PID,FLAGS
6604 Waits for a particular child process to terminate and returns the pid of
6605 the deceased process, or C<-1> if there is no such child process. On some
6606 systems, a value of 0 indicates that there are processes still running.
6607 The status is returned in C<$?>. If you say
6609 use POSIX ":sys_wait_h";
6612 $kid = waitpid(-1, WNOHANG);
6615 then you can do a non-blocking wait for all pending zombie processes.
6616 Non-blocking wait is available on machines supporting either the
6617 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6618 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6619 system call by remembering the status values of processes that have
6620 exited but have not been harvested by the Perl script yet.)
6622 Note that on some systems, a return value of C<-1> could mean that child
6623 processes are being automatically reaped. See L<perlipc> for details,
6624 and for other examples.
6628 Returns true if the context of the currently executing subroutine or
6629 eval() block is looking for a list value. Returns false if the context is
6630 looking for a scalar. Returns the undefined value if the context is
6631 looking for no value (void context).
6633 return unless defined wantarray; # don't bother doing more
6634 my @a = complex_calculation();
6635 return wantarray ? @a : "@a";
6637 This function should have been named wantlist() instead.
6641 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6644 If LIST is empty and C<$@> already contains a value (typically from a
6645 previous eval) that value is used after appending C<"\t...caught">
6646 to C<$@>. This is useful for staying almost, but not entirely similar to
6649 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6651 No message is printed if there is a C<$SIG{__WARN__}> handler
6652 installed. It is the handler's responsibility to deal with the message
6653 as it sees fit (like, for instance, converting it into a C<die>). Most
6654 handlers must therefore make arrangements to actually display the
6655 warnings that they are not prepared to deal with, by calling C<warn>
6656 again in the handler. Note that this is quite safe and will not
6657 produce an endless loop, since C<__WARN__> hooks are not called from
6660 You will find this behavior is slightly different from that of
6661 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6662 instead call C<die> again to change it).
6664 Using a C<__WARN__> handler provides a powerful way to silence all
6665 warnings (even the so-called mandatory ones). An example:
6667 # wipe out *all* compile-time warnings
6668 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6670 my $foo = 20; # no warning about duplicate my $foo,
6671 # but hey, you asked for it!
6672 # no compile-time or run-time warnings before here
6675 # run-time warnings enabled after here
6676 warn "\$foo is alive and $foo!"; # does show up
6678 See L<perlvar> for details on setting C<%SIG> entries, and for more
6679 examples. See the Carp module for other kinds of warnings using its
6680 carp() and cluck() functions.
6682 =item write FILEHANDLE
6688 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6689 using the format associated with that file. By default the format for
6690 a file is the one having the same name as the filehandle, but the
6691 format for the current output channel (see the C<select> function) may be set
6692 explicitly by assigning the name of the format to the C<$~> variable.
6694 Top of form processing is handled automatically: if there is
6695 insufficient room on the current page for the formatted record, the
6696 page is advanced by writing a form feed, a special top-of-page format
6697 is used to format the new page header, and then the record is written.
6698 By default the top-of-page format is the name of the filehandle with
6699 "_TOP" appended, but it may be dynamically set to the format of your
6700 choice by assigning the name to the C<$^> variable while the filehandle is
6701 selected. The number of lines remaining on the current page is in
6702 variable C<$->, which can be set to C<0> to force a new page.
6704 If FILEHANDLE is unspecified, output goes to the current default output
6705 channel, which starts out as STDOUT but may be changed by the
6706 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6707 is evaluated and the resulting string is used to look up the name of
6708 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6710 Note that write is I<not> the opposite of C<read>. Unfortunately.
6714 The transliteration operator. Same as C<tr///>. See L<perlop>.