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 $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 For more information see L<perlipc>.
436 Returns the arctangent of Y/X in the range -PI to PI.
438 For the tangent operation, you may use the C<Math::Trig::tan>
439 function, or use the familiar relation:
441 sub tan { sin($_[0]) / cos($_[0]) }
443 =item bind SOCKET,NAME
445 Binds a network address to a socket, just as the bind system call
446 does. Returns true if it succeeded, false otherwise. NAME should be a
447 packed address of the appropriate type for the socket. See the examples in
448 L<perlipc/"Sockets: Client/Server Communication">.
450 =item binmode FILEHANDLE, LAYER
452 =item binmode FILEHANDLE
454 Arranges for FILEHANDLE to be read or written in "binary" or "text"
455 mode on systems where the run-time libraries distinguish between
456 binary and text files. If FILEHANDLE is an expression, the value is
457 taken as the name of the filehandle. Returns true on success,
458 otherwise it returns C<undef> and sets C<$!> (errno).
460 On some systems (in general, DOS and Windows-based systems) binmode()
461 is necessary when you're not working with a text file. For the sake
462 of portability it is a good idea to always use it when appropriate,
463 and to never use it when it isn't appropriate. Also, people can
464 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
466 In other words: regardless of platform, use binmode() on binary data,
467 like for example images.
469 If LAYER is present it is a single string, but may contain multiple
470 directives. The directives alter the behaviour of the file handle.
471 When LAYER is present using binmode on text file makes sense.
473 If LAYER is omitted or specified as C<:raw> the filehandle is made
474 suitable for passing binary data. This includes turning off possible CRLF
475 translation and marking it as bytes (as opposed to Unicode characters).
476 Note that, despite what may be implied in I<"Programming Perl"> (the
477 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
478 -- other layers which would affect binary nature of the stream are
479 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
480 PERLIO environment variable.
482 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
483 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
484 establish default I/O layers. See L<open>.
486 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
487 in "Programming Perl, 3rd Edition". However, since the publishing of this
488 book, by many known as "Camel III", the consensus of the naming of this
489 functionality has moved from "discipline" to "layer". All documentation
490 of this version of Perl therefore refers to "layers" rather than to
491 "disciplines". Now back to the regularly scheduled documentation...>
493 To mark FILEHANDLE as UTF-8, use C<:utf8>.
495 In general, binmode() should be called after open() but before any I/O
496 is done on the filehandle. Calling binmode() will normally flush any
497 pending buffered output data (and perhaps pending input data) on the
498 handle. An exception to this is the C<:encoding> layer that
499 changes the default character encoding of the handle, see L<open>.
500 The C<:encoding> layer sometimes needs to be called in
501 mid-stream, and it doesn't flush the stream. The C<:encoding>
502 also implicitly pushes on top of itself the C<:utf8> layer because
503 internally Perl will operate on UTF-8 encoded Unicode characters.
505 The operating system, device drivers, C libraries, and Perl run-time
506 system all work together to let the programmer treat a single
507 character (C<\n>) as the line terminator, irrespective of the external
508 representation. On many operating systems, the native text file
509 representation matches the internal representation, but on some
510 platforms the external representation of C<\n> is made up of more than
513 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
514 character to end each line in the external representation of text (even
515 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
516 on Unix and most VMS files). In other systems like OS/2, DOS and the
517 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
518 but what's stored in text files are the two characters C<\cM\cJ>. That
519 means that, if you don't use binmode() on these systems, C<\cM\cJ>
520 sequences on disk will be converted to C<\n> on input, and any C<\n> in
521 your program will be converted back to C<\cM\cJ> on output. This is what
522 you want for text files, but it can be disastrous for binary files.
524 Another consequence of using binmode() (on some systems) is that
525 special end-of-file markers will be seen as part of the data stream.
526 For systems from the Microsoft family this means that if your binary
527 data contains C<\cZ>, the I/O subsystem will regard it as the end of
528 the file, unless you use binmode().
530 binmode() is not only important for readline() and print() operations,
531 but also when using read(), seek(), sysread(), syswrite() and tell()
532 (see L<perlport> for more details). See the C<$/> and C<$\> variables
533 in L<perlvar> for how to manually set your input and output
534 line-termination sequences.
536 =item bless REF,CLASSNAME
540 This function tells the thingy referenced by REF that it is now an object
541 in the CLASSNAME package. If CLASSNAME is omitted, the current package
542 is used. Because a C<bless> is often the last thing in a constructor,
543 it returns the reference for convenience. Always use the two-argument
544 version if the function doing the blessing might be inherited by a
545 derived class. See L<perltoot> and L<perlobj> for more about the blessing
546 (and blessings) of objects.
548 Consider always blessing objects in CLASSNAMEs that are mixed case.
549 Namespaces with all lowercase names are considered reserved for
550 Perl pragmata. Builtin types have all uppercase names, so to prevent
551 confusion, you may wish to avoid such package names as well. Make sure
552 that CLASSNAME is a true value.
554 See L<perlmod/"Perl Modules">.
560 Returns the context of the current subroutine call. In scalar context,
561 returns the caller's package name if there is a caller, that is, if
562 we're in a subroutine or C<eval> or C<require>, and the undefined value
563 otherwise. In list context, returns
565 ($package, $filename, $line) = caller;
567 With EXPR, it returns some extra information that the debugger uses to
568 print a stack trace. The value of EXPR indicates how many call frames
569 to go back before the current one.
571 ($package, $filename, $line, $subroutine, $hasargs,
572 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
574 Here $subroutine may be C<(eval)> if the frame is not a subroutine
575 call, but an C<eval>. In such a case additional elements $evaltext and
576 C<$is_require> are set: C<$is_require> is true if the frame is created by a
577 C<require> or C<use> statement, $evaltext contains the text of the
578 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
579 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
580 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
581 frame.) $subroutine may also be C<(unknown)> if this particular
582 subroutine happens to have been deleted from the symbol table.
583 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
584 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585 compiled with. The C<$hints> and C<$bitmask> values are subject to change
586 between versions of Perl, and are not meant for external use.
588 Furthermore, when called from within the DB package, caller returns more
589 detailed information: it sets the list variable C<@DB::args> to be the
590 arguments with which the subroutine was invoked.
592 Be aware that the optimizer might have optimized call frames away before
593 C<caller> had a chance to get the information. That means that C<caller(N)>
594 might not return information about the call frame you expect it do, for
595 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
596 previous time C<caller> was called.
600 Changes the working directory to EXPR, if possible. If EXPR is omitted,
601 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
602 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
603 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
604 neither is set, C<chdir> does nothing. It returns true upon success,
605 false otherwise. See the example under C<die>.
609 Changes the permissions of a list of files. The first element of the
610 list must be the numerical mode, which should probably be an octal
611 number, and which definitely should I<not> a string of octal digits:
612 C<0644> is okay, C<'0644'> is not. Returns the number of files
613 successfully changed. See also L</oct>, if all you have is a string.
615 $cnt = chmod 0755, 'foo', 'bar';
616 chmod 0755, @executables;
617 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
619 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
620 $mode = 0644; chmod $mode, 'foo'; # this is best
622 You can also import the symbolic C<S_I*> constants from the Fcntl
627 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
628 # This is identical to the chmod 0755 of the above example.
636 This safer version of L</chop> removes any trailing string
637 that corresponds to the current value of C<$/> (also known as
638 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
639 number of characters removed from all its arguments. It's often used to
640 remove the newline from the end of an input record when you're worried
641 that the final record may be missing its newline. When in paragraph
642 mode (C<$/ = "">), it removes all trailing newlines from the string.
643 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
644 a reference to an integer or the like, see L<perlvar>) chomp() won't
646 If VARIABLE is omitted, it chomps C<$_>. Example:
649 chomp; # avoid \n on last field
654 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
656 You can actually chomp anything that's an lvalue, including an assignment:
659 chomp($answer = <STDIN>);
661 If you chomp a list, each element is chomped, and the total number of
662 characters removed is returned.
664 Note that parentheses are necessary when you're chomping anything
665 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
666 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
667 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
668 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
677 Chops off the last character of a string and returns the character
678 chopped. It is much more efficient than C<s/.$//s> because it neither
679 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
680 If VARIABLE is a hash, it chops the hash's values, but not its keys.
682 You can actually chop anything that's an lvalue, including an assignment.
684 If you chop a list, each element is chopped. Only the value of the
685 last C<chop> is returned.
687 Note that C<chop> returns the last character. To return all but the last
688 character, use C<substr($string, 0, -1)>.
694 Changes the owner (and group) of a list of files. The first two
695 elements of the list must be the I<numeric> uid and gid, in that
696 order. A value of -1 in either position is interpreted by most
697 systems to leave that value unchanged. Returns the number of files
698 successfully changed.
700 $cnt = chown $uid, $gid, 'foo', 'bar';
701 chown $uid, $gid, @filenames;
703 Here's an example that looks up nonnumeric uids in the passwd file:
706 chomp($user = <STDIN>);
708 chomp($pattern = <STDIN>);
710 ($login,$pass,$uid,$gid) = getpwnam($user)
711 or die "$user not in passwd file";
713 @ary = glob($pattern); # expand filenames
714 chown $uid, $gid, @ary;
716 On most systems, you are not allowed to change the ownership of the
717 file unless you're the superuser, although you should be able to change
718 the group to any of your secondary groups. On insecure systems, these
719 restrictions may be relaxed, but this is not a portable assumption.
720 On POSIX systems, you can detect this condition this way:
722 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
723 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
729 Returns the character represented by that NUMBER in the character set.
730 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
731 chr(0x263a) is a Unicode smiley face. Note that characters from 128
732 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
733 backward compatibility reasons (but see L<encoding>).
735 If NUMBER is omitted, uses C<$_>.
737 For the reverse, use L</ord>.
739 Note that under the C<bytes> pragma the NUMBER is masked to
742 See L<perlunicode> and L<encoding> for more about Unicode.
744 =item chroot FILENAME
748 This function works like the system call by the same name: it makes the
749 named directory the new root directory for all further pathnames that
750 begin with a C</> by your process and all its children. (It doesn't
751 change your current working directory, which is unaffected.) For security
752 reasons, this call is restricted to the superuser. If FILENAME is
753 omitted, does a C<chroot> to C<$_>.
755 =item close FILEHANDLE
759 Closes the file or pipe associated with the file handle, returning
760 true only if IO buffers are successfully flushed and closes the system
761 file descriptor. Closes the currently selected filehandle if the
764 You don't have to close FILEHANDLE if you are immediately going to do
765 another C<open> on it, because C<open> will close it for you. (See
766 C<open>.) However, an explicit C<close> on an input file resets the line
767 counter (C<$.>), while the implicit close done by C<open> does not.
769 If the file handle came from a piped open C<close> will additionally
770 return false if one of the other system calls involved fails or if the
771 program exits with non-zero status. (If the only problem was that the
772 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
773 also waits for the process executing on the pipe to complete, in case you
774 want to look at the output of the pipe afterwards, and
775 implicitly puts the exit status value of that command into C<$?>.
777 Prematurely closing the read end of a pipe (i.e. before the process
778 writing to it at the other end has closed it) will result in a
779 SIGPIPE being delivered to the writer. If the other end can't
780 handle that, be sure to read all the data before closing the pipe.
784 open(OUTPUT, '|sort >foo') # pipe to sort
785 or die "Can't start sort: $!";
786 #... # print stuff to output
787 close OUTPUT # wait for sort to finish
788 or warn $! ? "Error closing sort pipe: $!"
789 : "Exit status $? from sort";
790 open(INPUT, 'foo') # get sort's results
791 or die "Can't open 'foo' for input: $!";
793 FILEHANDLE may be an expression whose value can be used as an indirect
794 filehandle, usually the real filehandle name.
796 =item closedir DIRHANDLE
798 Closes a directory opened by C<opendir> and returns the success of that
801 =item connect SOCKET,NAME
803 Attempts to connect to a remote socket, just as the connect system call
804 does. Returns true if it succeeded, false otherwise. NAME should be a
805 packed address of the appropriate type for the socket. See the examples in
806 L<perlipc/"Sockets: Client/Server Communication">.
810 Actually a flow control statement rather than a function. If there is a
811 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
812 C<foreach>), it is always executed just before the conditional is about to
813 be evaluated again, just like the third part of a C<for> loop in C. Thus
814 it can be used to increment a loop variable, even when the loop has been
815 continued via the C<next> statement (which is similar to the C C<continue>
818 C<last>, C<next>, or C<redo> may appear within a C<continue>
819 block. C<last> and C<redo> will behave as if they had been executed within
820 the main block. So will C<next>, but since it will execute a C<continue>
821 block, it may be more entertaining.
824 ### redo always comes here
827 ### next always comes here
829 # then back the top to re-check EXPR
831 ### last always comes here
833 Omitting the C<continue> section is semantically equivalent to using an
834 empty one, logically enough. In that case, C<next> goes directly back
835 to check the condition at the top of the loop.
841 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
842 takes cosine of C<$_>.
844 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
845 function, or use this relation:
847 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
849 =item crypt PLAINTEXT,SALT
851 Encrypts a string exactly like the crypt(3) function in the C library
852 (assuming that you actually have a version there that has not been
853 extirpated as a potential munition). This can prove useful for checking
854 the password file for lousy passwords, amongst other things. Only the
855 guys wearing white hats should do this.
857 Note that L<crypt|/crypt> is intended to be a one-way function, much like
858 breaking eggs to make an omelette. There is no (known) corresponding
859 decrypt function (in other words, the crypt() is a one-way hash
860 function). As a result, this function isn't all that useful for
861 cryptography. (For that, see your nearby CPAN mirror.)
863 When verifying an existing encrypted string you should use the
864 encrypted text as the salt (like C<crypt($plain, $crypted) eq
865 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
866 and with more exotic implementations. In other words, do not assume
867 anything about the returned string itself, or how many bytes in
868 the encrypted string matter.
870 Traditionally the result is a string of 13 bytes: two first bytes of
871 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
872 the first eight bytes of the encrypted string mattered, but
873 alternative hashing schemes (like MD5), higher level security schemes
874 (like C2), and implementations on non-UNIX platforms may produce
877 When choosing a new salt create a random two character string whose
878 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
879 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
880 characters is just a recommendation; the characters allowed in
881 the salt depend solely on your system's crypt library, and Perl can't
882 restrict what salts C<crypt()> accepts.
884 Here's an example that makes sure that whoever runs this program knows
887 $pwd = (getpwuid($<))[1];
891 chomp($word = <STDIN>);
895 if (crypt($word, $pwd) ne $pwd) {
901 Of course, typing in your own password to whoever asks you
904 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
905 of data, not least of all because you can't get the information
906 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
907 on your favorite CPAN mirror for a slew of potentially useful
910 If using crypt() on a Unicode string (which I<potentially> has
911 characters with codepoints above 255), Perl tries to make sense
912 of the situation by trying to downgrade (a copy of the string)
913 the string back to an eight-bit byte string before calling crypt()
914 (on that copy). If that works, good. If not, crypt() dies with
915 C<Wide character in crypt>.
919 [This function has been largely superseded by the C<untie> function.]
921 Breaks the binding between a DBM file and a hash.
923 =item dbmopen HASH,DBNAME,MASK
925 [This function has been largely superseded by the C<tie> function.]
927 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
928 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
929 argument is I<not> a filehandle, even though it looks like one). DBNAME
930 is the name of the database (without the F<.dir> or F<.pag> extension if
931 any). If the database does not exist, it is created with protection
932 specified by MASK (as modified by the C<umask>). If your system supports
933 only the older DBM functions, you may perform only one C<dbmopen> in your
934 program. In older versions of Perl, if your system had neither DBM nor
935 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
938 If you don't have write access to the DBM file, you can only read hash
939 variables, not set them. If you want to test whether you can write,
940 either use file tests or try setting a dummy hash entry inside an C<eval>,
941 which will trap the error.
943 Note that functions such as C<keys> and C<values> may return huge lists
944 when used on large DBM files. You may prefer to use the C<each>
945 function to iterate over large DBM files. Example:
947 # print out history file offsets
948 dbmopen(%HIST,'/usr/lib/news/history',0666);
949 while (($key,$val) = each %HIST) {
950 print $key, ' = ', unpack('L',$val), "\n";
954 See also L<AnyDBM_File> for a more general description of the pros and
955 cons of the various dbm approaches, as well as L<DB_File> for a particularly
958 You can control which DBM library you use by loading that library
959 before you call dbmopen():
962 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
963 or die "Can't open netscape history file: $!";
969 Returns a Boolean value telling whether EXPR has a value other than
970 the undefined value C<undef>. If EXPR is not present, C<$_> will be
973 Many operations return C<undef> to indicate failure, end of file,
974 system error, uninitialized variable, and other exceptional
975 conditions. This function allows you to distinguish C<undef> from
976 other values. (A simple Boolean test will not distinguish among
977 C<undef>, zero, the empty string, and C<"0">, which are all equally
978 false.) Note that since C<undef> is a valid scalar, its presence
979 doesn't I<necessarily> indicate an exceptional condition: C<pop>
980 returns C<undef> when its argument is an empty array, I<or> when the
981 element to return happens to be C<undef>.
983 You may also use C<defined(&func)> to check whether subroutine C<&func>
984 has ever been defined. The return value is unaffected by any forward
985 declarations of C<&func>. Note that a subroutine which is not defined
986 may still be callable: its package may have an C<AUTOLOAD> method that
987 makes it spring into existence the first time that it is called -- see
990 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
991 used to report whether memory for that aggregate has ever been
992 allocated. This behavior may disappear in future versions of Perl.
993 You should instead use a simple test for size:
995 if (@an_array) { print "has array elements\n" }
996 if (%a_hash) { print "has hash members\n" }
998 When used on a hash element, it tells you whether the value is defined,
999 not whether the key exists in the hash. Use L</exists> for the latter
1004 print if defined $switch{'D'};
1005 print "$val\n" while defined($val = pop(@ary));
1006 die "Can't readlink $sym: $!"
1007 unless defined($value = readlink $sym);
1008 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1009 $debugging = 0 unless defined $debugging;
1011 Note: Many folks tend to overuse C<defined>, and then are surprised to
1012 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1013 defined values. For example, if you say
1017 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1018 matched "nothing". But it didn't really match nothing--rather, it
1019 matched something that happened to be zero characters long. This is all
1020 very above-board and honest. When a function returns an undefined value,
1021 it's an admission that it couldn't give you an honest answer. So you
1022 should use C<defined> only when you're questioning the integrity of what
1023 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1026 See also L</undef>, L</exists>, L</ref>.
1030 Given an expression that specifies a hash element, array element, hash slice,
1031 or array slice, deletes the specified element(s) from the hash or array.
1032 In the case of an array, if the array elements happen to be at the end,
1033 the size of the array will shrink to the highest element that tests
1034 true for exists() (or 0 if no such element exists).
1036 Returns a list with the same number of elements as the number of elements
1037 for which deletion was attempted. Each element of that list consists of
1038 either the value of the element deleted, or the undefined value. In scalar
1039 context, this means that you get the value of the last element deleted (or
1040 the undefined value if that element did not exist).
1042 %hash = (foo => 11, bar => 22, baz => 33);
1043 $scalar = delete $hash{foo}; # $scalar is 11
1044 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1045 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1047 Deleting from C<%ENV> modifies the environment. Deleting from
1048 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1049 from a C<tie>d hash or array may not necessarily return anything.
1051 Deleting an array element effectively returns that position of the array
1052 to its initial, uninitialized state. Subsequently testing for the same
1053 element with exists() will return false. Note that deleting array
1054 elements in the middle of an array will not shift the index of the ones
1055 after them down--use splice() for that. See L</exists>.
1057 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1059 foreach $key (keys %HASH) {
1063 foreach $index (0 .. $#ARRAY) {
1064 delete $ARRAY[$index];
1069 delete @HASH{keys %HASH};
1071 delete @ARRAY[0 .. $#ARRAY];
1073 But both of these are slower than just assigning the empty list
1074 or undefining %HASH or @ARRAY:
1076 %HASH = (); # completely empty %HASH
1077 undef %HASH; # forget %HASH ever existed
1079 @ARRAY = (); # completely empty @ARRAY
1080 undef @ARRAY; # forget @ARRAY ever existed
1082 Note that the EXPR can be arbitrarily complicated as long as the final
1083 operation is a hash element, array element, hash slice, or array slice
1086 delete $ref->[$x][$y]{$key};
1087 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1089 delete $ref->[$x][$y][$index];
1090 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1094 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1095 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1096 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1097 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1098 an C<eval(),> the error message is stuffed into C<$@> and the
1099 C<eval> is terminated with the undefined value. This makes
1100 C<die> the way to raise an exception.
1102 Equivalent examples:
1104 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1105 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1107 If the last element of LIST does not end in a newline, the current
1108 script line number and input line number (if any) are also printed,
1109 and a newline is supplied. Note that the "input line number" (also
1110 known as "chunk") is subject to whatever notion of "line" happens to
1111 be currently in effect, and is also available as the special variable
1112 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1114 Hint: sometimes appending C<", stopped"> to your message will cause it
1115 to make better sense when the string C<"at foo line 123"> is appended.
1116 Suppose you are running script "canasta".
1118 die "/etc/games is no good";
1119 die "/etc/games is no good, stopped";
1121 produce, respectively
1123 /etc/games is no good at canasta line 123.
1124 /etc/games is no good, stopped at canasta line 123.
1126 See also exit(), warn(), and the Carp module.
1128 If LIST is empty and C<$@> already contains a value (typically from a
1129 previous eval) that value is reused after appending C<"\t...propagated">.
1130 This is useful for propagating exceptions:
1133 die unless $@ =~ /Expected exception/;
1135 If LIST is empty and C<$@> contains an object reference that has a
1136 C<PROPAGATE> method, that method will be called with additional file
1137 and line number parameters. The return value replaces the value in
1138 C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1141 If C<$@> is empty then the string C<"Died"> is used.
1143 die() can also be called with a reference argument. If this happens to be
1144 trapped within an eval(), $@ contains the reference. This behavior permits
1145 a more elaborate exception handling implementation using objects that
1146 maintain arbitrary state about the nature of the exception. Such a scheme
1147 is sometimes preferable to matching particular string values of $@ using
1148 regular expressions. Here's an example:
1150 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1152 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1153 # handle Some::Module::Exception
1156 # handle all other possible exceptions
1160 Because perl will stringify uncaught exception messages before displaying
1161 them, you may want to overload stringification operations on such custom
1162 exception objects. See L<overload> for details about that.
1164 You can arrange for a callback to be run just before the C<die>
1165 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1166 handler will be called with the error text and can change the error
1167 message, if it sees fit, by calling C<die> again. See
1168 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1169 L<"eval BLOCK"> for some examples. Although this feature was meant
1170 to be run only right before your program was to exit, this is not
1171 currently the case--the C<$SIG{__DIE__}> hook is currently called
1172 even inside eval()ed blocks/strings! If one wants the hook to do
1173 nothing in such situations, put
1177 as the first line of the handler (see L<perlvar/$^S>). Because
1178 this promotes strange action at a distance, this counterintuitive
1179 behavior may be fixed in a future release.
1183 Not really a function. Returns the value of the last command in the
1184 sequence of commands indicated by BLOCK. When modified by a loop
1185 modifier, executes the BLOCK once before testing the loop condition.
1186 (On other statements the loop modifiers test the conditional first.)
1188 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1189 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1190 See L<perlsyn> for alternative strategies.
1192 =item do SUBROUTINE(LIST)
1194 A deprecated form of subroutine call. See L<perlsub>.
1198 Uses the value of EXPR as a filename and executes the contents of the
1199 file as a Perl script. Its primary use is to include subroutines
1200 from a Perl subroutine library.
1208 except that it's more efficient and concise, keeps track of the current
1209 filename for error messages, searches the @INC libraries, and updates
1210 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1211 variables. It also differs in that code evaluated with C<do FILENAME>
1212 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1213 same, however, in that it does reparse the file every time you call it,
1214 so you probably don't want to do this inside a loop.
1216 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1217 error. If C<do> can read the file but cannot compile it, it
1218 returns undef and sets an error message in C<$@>. If the file is
1219 successfully compiled, C<do> returns the value of the last expression
1222 Note that inclusion of library modules is better done with the
1223 C<use> and C<require> operators, which also do automatic error checking
1224 and raise an exception if there's a problem.
1226 You might like to use C<do> to read in a program configuration
1227 file. Manual error checking can be done this way:
1229 # read in config files: system first, then user
1230 for $file ("/share/prog/defaults.rc",
1231 "$ENV{HOME}/.someprogrc")
1233 unless ($return = do $file) {
1234 warn "couldn't parse $file: $@" if $@;
1235 warn "couldn't do $file: $!" unless defined $return;
1236 warn "couldn't run $file" unless $return;
1244 This function causes an immediate core dump. See also the B<-u>
1245 command-line switch in L<perlrun>, which does the same thing.
1246 Primarily this is so that you can use the B<undump> program (not
1247 supplied) to turn your core dump into an executable binary after
1248 having initialized all your variables at the beginning of the
1249 program. When the new binary is executed it will begin by executing
1250 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1251 Think of it as a goto with an intervening core dump and reincarnation.
1252 If C<LABEL> is omitted, restarts the program from the top.
1254 B<WARNING>: Any files opened at the time of the dump will I<not>
1255 be open any more when the program is reincarnated, with possible
1256 resulting confusion on the part of Perl.
1258 This function is now largely obsolete, partly because it's very
1259 hard to convert a core file into an executable, and because the
1260 real compiler backends for generating portable bytecode and compilable
1261 C code have superseded it. That's why you should now invoke it as
1262 C<CORE::dump()>, if you don't want to be warned against a possible
1265 If you're looking to use L<dump> to speed up your program, consider
1266 generating bytecode or native C code as described in L<perlcc>. If
1267 you're just trying to accelerate a CGI script, consider using the
1268 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1269 You might also consider autoloading or selfloading, which at least
1270 make your program I<appear> to run faster.
1274 When called in list context, returns a 2-element list consisting of the
1275 key and value for the next element of a hash, so that you can iterate over
1276 it. When called in scalar context, returns only the key for the next
1277 element in the hash.
1279 Entries are returned in an apparently random order. The actual random
1280 order is subject to change in future versions of perl, but it is
1281 guaranteed to be in the same order as either the C<keys> or C<values>
1282 function would produce on the same (unmodified) hash. Since Perl
1283 5.8.1 the ordering is different even between different runs of Perl
1284 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1286 When the hash is entirely read, a null array is returned in list context
1287 (which when assigned produces a false (C<0>) value), and C<undef> in
1288 scalar context. The next call to C<each> after that will start iterating
1289 again. There is a single iterator for each hash, shared by all C<each>,
1290 C<keys>, and C<values> function calls in the program; it can be reset by
1291 reading all the elements from the hash, or by evaluating C<keys HASH> or
1292 C<values HASH>. If you add or delete elements of a hash while you're
1293 iterating over it, you may get entries skipped or duplicated, so
1294 don't. Exception: It is always safe to delete the item most recently
1295 returned by C<each()>, which means that the following code will work:
1297 while (($key, $value) = each %hash) {
1299 delete $hash{$key}; # This is safe
1302 The following prints out your environment like the printenv(1) program,
1303 only in a different order:
1305 while (($key,$value) = each %ENV) {
1306 print "$key=$value\n";
1309 See also C<keys>, C<values> and C<sort>.
1311 =item eof FILEHANDLE
1317 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1318 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1319 gives the real filehandle. (Note that this function actually
1320 reads a character and then C<ungetc>s it, so isn't very useful in an
1321 interactive context.) Do not read from a terminal file (or call
1322 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1323 as terminals may lose the end-of-file condition if you do.
1325 An C<eof> without an argument uses the last file read. Using C<eof()>
1326 with empty parentheses is very different. It refers to the pseudo file
1327 formed from the files listed on the command line and accessed via the
1328 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1329 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1330 used will cause C<@ARGV> to be examined to determine if input is
1331 available. Similarly, an C<eof()> after C<< <> >> has returned
1332 end-of-file will assume you are processing another C<@ARGV> list,
1333 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1334 see L<perlop/"I/O Operators">.
1336 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1337 detect the end of each file, C<eof()> will only detect the end of the
1338 last file. Examples:
1340 # reset line numbering on each input file
1342 next if /^\s*#/; # skip comments
1345 close ARGV if eof; # Not eof()!
1348 # insert dashes just before last line of last file
1350 if (eof()) { # check for end of last file
1351 print "--------------\n";
1354 last if eof(); # needed if we're reading from a terminal
1357 Practical hint: you almost never need to use C<eof> in Perl, because the
1358 input operators typically return C<undef> when they run out of data, or if
1365 In the first form, the return value of EXPR is parsed and executed as if it
1366 were a little Perl program. The value of the expression (which is itself
1367 determined within scalar context) is first parsed, and if there weren't any
1368 errors, executed in the lexical context of the current Perl program, so
1369 that any variable settings or subroutine and format definitions remain
1370 afterwards. Note that the value is parsed every time the eval executes.
1371 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1372 delay parsing and subsequent execution of the text of EXPR until run time.
1374 In the second form, the code within the BLOCK is parsed only once--at the
1375 same time the code surrounding the eval itself was parsed--and executed
1376 within the context of the current Perl program. This form is typically
1377 used to trap exceptions more efficiently than the first (see below), while
1378 also providing the benefit of checking the code within BLOCK at compile
1381 The final semicolon, if any, may be omitted from the value of EXPR or within
1384 In both forms, the value returned is the value of the last expression
1385 evaluated inside the mini-program; a return statement may be also used, just
1386 as with subroutines. The expression providing the return value is evaluated
1387 in void, scalar, or list context, depending on the context of the eval itself.
1388 See L</wantarray> for more on how the evaluation context can be determined.
1390 If there is a syntax error or runtime error, or a C<die> statement is
1391 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1392 error message. If there was no error, C<$@> is guaranteed to be a null
1393 string. Beware that using C<eval> neither silences perl from printing
1394 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1395 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1396 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1397 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1399 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1400 determining whether a particular feature (such as C<socket> or C<symlink>)
1401 is implemented. It is also Perl's exception trapping mechanism, where
1402 the die operator is used to raise exceptions.
1404 If the code to be executed doesn't vary, you may use the eval-BLOCK
1405 form to trap run-time errors without incurring the penalty of
1406 recompiling each time. The error, if any, is still returned in C<$@>.
1409 # make divide-by-zero nonfatal
1410 eval { $answer = $a / $b; }; warn $@ if $@;
1412 # same thing, but less efficient
1413 eval '$answer = $a / $b'; warn $@ if $@;
1415 # a compile-time error
1416 eval { $answer = }; # WRONG
1419 eval '$answer ='; # sets $@
1421 Due to the current arguably broken state of C<__DIE__> hooks, when using
1422 the C<eval{}> form as an exception trap in libraries, you may wish not
1423 to trigger any C<__DIE__> hooks that user code may have installed.
1424 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1425 as shown in this example:
1427 # a very private exception trap for divide-by-zero
1428 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1431 This is especially significant, given that C<__DIE__> hooks can call
1432 C<die> again, which has the effect of changing their error messages:
1434 # __DIE__ hooks may modify error messages
1436 local $SIG{'__DIE__'} =
1437 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1438 eval { die "foo lives here" };
1439 print $@ if $@; # prints "bar lives here"
1442 Because this promotes action at a distance, this counterintuitive behavior
1443 may be fixed in a future release.
1445 With an C<eval>, you should be especially careful to remember what's
1446 being looked at when:
1452 eval { $x }; # CASE 4
1454 eval "\$$x++"; # CASE 5
1457 Cases 1 and 2 above behave identically: they run the code contained in
1458 the variable $x. (Although case 2 has misleading double quotes making
1459 the reader wonder what else might be happening (nothing is).) Cases 3
1460 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1461 does nothing but return the value of $x. (Case 4 is preferred for
1462 purely visual reasons, but it also has the advantage of compiling at
1463 compile-time instead of at run-time.) Case 5 is a place where
1464 normally you I<would> like to use double quotes, except that in this
1465 particular situation, you can just use symbolic references instead, as
1468 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1469 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1471 Note that as a very special case, an C<eval ''> executed within the C<DB>
1472 package doesn't see the usual surrounding lexical scope, but rather the
1473 scope of the first non-DB piece of code that called it. You don't normally
1474 need to worry about this unless you are writing a Perl debugger.
1478 =item exec PROGRAM LIST
1480 The C<exec> function executes a system command I<and never returns>--
1481 use C<system> instead of C<exec> if you want it to return. It fails and
1482 returns false only if the command does not exist I<and> it is executed
1483 directly instead of via your system's command shell (see below).
1485 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1486 warns you if there is a following statement which isn't C<die>, C<warn>,
1487 or C<exit> (if C<-w> is set - but you always do that). If you
1488 I<really> want to follow an C<exec> with some other statement, you
1489 can use one of these styles to avoid the warning:
1491 exec ('foo') or print STDERR "couldn't exec foo: $!";
1492 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1494 If there is more than one argument in LIST, or if LIST is an array
1495 with more than one value, calls execvp(3) with the arguments in LIST.
1496 If there is only one scalar argument or an array with one element in it,
1497 the argument is checked for shell metacharacters, and if there are any,
1498 the entire argument is passed to the system's command shell for parsing
1499 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1500 If there are no shell metacharacters in the argument, it is split into
1501 words and passed directly to C<execvp>, which is more efficient.
1504 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1505 exec "sort $outfile | uniq";
1507 If you don't really want to execute the first argument, but want to lie
1508 to the program you are executing about its own name, you can specify
1509 the program you actually want to run as an "indirect object" (without a
1510 comma) in front of the LIST. (This always forces interpretation of the
1511 LIST as a multivalued list, even if there is only a single scalar in
1514 $shell = '/bin/csh';
1515 exec $shell '-sh'; # pretend it's a login shell
1519 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1521 When the arguments get executed via the system shell, results will
1522 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1525 Using an indirect object with C<exec> or C<system> is also more
1526 secure. This usage (which also works fine with system()) forces
1527 interpretation of the arguments as a multivalued list, even if the
1528 list had just one argument. That way you're safe from the shell
1529 expanding wildcards or splitting up words with whitespace in them.
1531 @args = ( "echo surprise" );
1533 exec @args; # subject to shell escapes
1535 exec { $args[0] } @args; # safe even with one-arg list
1537 The first version, the one without the indirect object, ran the I<echo>
1538 program, passing it C<"surprise"> an argument. The second version
1539 didn't--it tried to run a program literally called I<"echo surprise">,
1540 didn't find it, and set C<$?> to a non-zero value indicating failure.
1542 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1543 output before the exec, but this may not be supported on some platforms
1544 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1545 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1546 open handles in order to avoid lost output.
1548 Note that C<exec> will not call your C<END> blocks, nor will it call
1549 any C<DESTROY> methods in your objects.
1553 Given an expression that specifies a hash element or array element,
1554 returns true if the specified element in the hash or array has ever
1555 been initialized, even if the corresponding value is undefined. The
1556 element is not autovivified if it doesn't exist.
1558 print "Exists\n" if exists $hash{$key};
1559 print "Defined\n" if defined $hash{$key};
1560 print "True\n" if $hash{$key};
1562 print "Exists\n" if exists $array[$index];
1563 print "Defined\n" if defined $array[$index];
1564 print "True\n" if $array[$index];
1566 A hash or array element can be true only if it's defined, and defined if
1567 it exists, but the reverse doesn't necessarily hold true.
1569 Given an expression that specifies the name of a subroutine,
1570 returns true if the specified subroutine has ever been declared, even
1571 if it is undefined. Mentioning a subroutine name for exists or defined
1572 does not count as declaring it. Note that a subroutine which does not
1573 exist may still be callable: its package may have an C<AUTOLOAD>
1574 method that makes it spring into existence the first time that it is
1575 called -- see L<perlsub>.
1577 print "Exists\n" if exists &subroutine;
1578 print "Defined\n" if defined &subroutine;
1580 Note that the EXPR can be arbitrarily complicated as long as the final
1581 operation is a hash or array key lookup or subroutine name:
1583 if (exists $ref->{A}->{B}->{$key}) { }
1584 if (exists $hash{A}{B}{$key}) { }
1586 if (exists $ref->{A}->{B}->[$ix]) { }
1587 if (exists $hash{A}{B}[$ix]) { }
1589 if (exists &{$ref->{A}{B}{$key}}) { }
1591 Although the deepest nested array or hash will not spring into existence
1592 just because its existence was tested, any intervening ones will.
1593 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1594 into existence due to the existence test for the $key element above.
1595 This happens anywhere the arrow operator is used, including even:
1598 if (exists $ref->{"Some key"}) { }
1599 print $ref; # prints HASH(0x80d3d5c)
1601 This surprising autovivification in what does not at first--or even
1602 second--glance appear to be an lvalue context may be fixed in a future
1605 Use of a subroutine call, rather than a subroutine name, as an argument
1606 to exists() is an error.
1609 exists &sub(); # Error
1613 Evaluates EXPR and exits immediately with that value. Example:
1616 exit 0 if $ans =~ /^[Xx]/;
1618 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1619 universally recognized values for EXPR are C<0> for success and C<1>
1620 for error; other values are subject to interpretation depending on the
1621 environment in which the Perl program is running. For example, exiting
1622 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1623 the mailer to return the item undelivered, but that's not true everywhere.
1625 Don't use C<exit> to abort a subroutine if there's any chance that
1626 someone might want to trap whatever error happened. Use C<die> instead,
1627 which can be trapped by an C<eval>.
1629 The exit() function does not always exit immediately. It calls any
1630 defined C<END> routines first, but these C<END> routines may not
1631 themselves abort the exit. Likewise any object destructors that need to
1632 be called are called before the real exit. If this is a problem, you
1633 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1634 See L<perlmod> for details.
1640 Returns I<e> (the natural logarithm base) to the power of EXPR.
1641 If EXPR is omitted, gives C<exp($_)>.
1643 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1645 Implements the fcntl(2) function. You'll probably have to say
1649 first to get the correct constant definitions. Argument processing and
1650 value return works just like C<ioctl> below.
1654 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1655 or die "can't fcntl F_GETFL: $!";
1657 You don't have to check for C<defined> on the return from C<fcntl>.
1658 Like C<ioctl>, it maps a C<0> return from the system call into
1659 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1660 in numeric context. It is also exempt from the normal B<-w> warnings
1661 on improper numeric conversions.
1663 Note that C<fcntl> will produce a fatal error if used on a machine that
1664 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1665 manpage to learn what functions are available on your system.
1667 Here's an example of setting a filehandle named C<REMOTE> to be
1668 non-blocking at the system level. You'll have to negotiate C<$|>
1669 on your own, though.
1671 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1673 $flags = fcntl(REMOTE, F_GETFL, 0)
1674 or die "Can't get flags for the socket: $!\n";
1676 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1677 or die "Can't set flags for the socket: $!\n";
1679 =item fileno FILEHANDLE
1681 Returns the file descriptor for a filehandle, or undefined if the
1682 filehandle is not open. This is mainly useful for constructing
1683 bitmaps for C<select> and low-level POSIX tty-handling operations.
1684 If FILEHANDLE is an expression, the value is taken as an indirect
1685 filehandle, generally its name.
1687 You can use this to find out whether two handles refer to the
1688 same underlying descriptor:
1690 if (fileno(THIS) == fileno(THAT)) {
1691 print "THIS and THAT are dups\n";
1694 (Filehandles connected to memory objects via new features of C<open> may
1695 return undefined even though they are open.)
1698 =item flock FILEHANDLE,OPERATION
1700 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1701 for success, false on failure. Produces a fatal error if used on a
1702 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1703 C<flock> is Perl's portable file locking interface, although it locks
1704 only entire files, not records.
1706 Two potentially non-obvious but traditional C<flock> semantics are
1707 that it waits indefinitely until the lock is granted, and that its locks
1708 B<merely advisory>. Such discretionary locks are more flexible, but offer
1709 fewer guarantees. This means that files locked with C<flock> may be
1710 modified by programs that do not also use C<flock>. See L<perlport>,
1711 your port's specific documentation, or your system-specific local manpages
1712 for details. It's best to assume traditional behavior if you're writing
1713 portable programs. (But if you're not, you should as always feel perfectly
1714 free to write for your own system's idiosyncrasies (sometimes called
1715 "features"). Slavish adherence to portability concerns shouldn't get
1716 in the way of your getting your job done.)
1718 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1719 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1720 you can use the symbolic names if you import them from the Fcntl module,
1721 either individually, or as a group using the ':flock' tag. LOCK_SH
1722 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1723 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1724 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1725 waiting for the lock (check the return status to see if you got it).
1727 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1728 before locking or unlocking it.
1730 Note that the emulation built with lockf(3) doesn't provide shared
1731 locks, and it requires that FILEHANDLE be open with write intent. These
1732 are the semantics that lockf(3) implements. Most if not all systems
1733 implement lockf(3) in terms of fcntl(2) locking, though, so the
1734 differing semantics shouldn't bite too many people.
1736 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1737 be open with read intent to use LOCK_SH and requires that it be open
1738 with write intent to use LOCK_EX.
1740 Note also that some versions of C<flock> cannot lock things over the
1741 network; you would need to use the more system-specific C<fcntl> for
1742 that. If you like you can force Perl to ignore your system's flock(2)
1743 function, and so provide its own fcntl(2)-based emulation, by passing
1744 the switch C<-Ud_flock> to the F<Configure> program when you configure
1747 Here's a mailbox appender for BSD systems.
1749 use Fcntl ':flock'; # import LOCK_* constants
1752 flock(MBOX,LOCK_EX);
1753 # and, in case someone appended
1754 # while we were waiting...
1759 flock(MBOX,LOCK_UN);
1762 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1763 or die "Can't open mailbox: $!";
1766 print MBOX $msg,"\n\n";
1769 On systems that support a real flock(), locks are inherited across fork()
1770 calls, whereas those that must resort to the more capricious fcntl()
1771 function lose the locks, making it harder to write servers.
1773 See also L<DB_File> for other flock() examples.
1777 Does a fork(2) system call to create a new process running the
1778 same program at the same point. It returns the child pid to the
1779 parent process, C<0> to the child process, or C<undef> if the fork is
1780 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1781 are shared, while everything else is copied. On most systems supporting
1782 fork(), great care has gone into making it extremely efficient (for
1783 example, using copy-on-write technology on data pages), making it the
1784 dominant paradigm for multitasking over the last few decades.
1786 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1787 output before forking the child process, but this may not be supported
1788 on some platforms (see L<perlport>). To be safe, you may need to set
1789 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1790 C<IO::Handle> on any open handles in order to avoid duplicate output.
1792 If you C<fork> without ever waiting on your children, you will
1793 accumulate zombies. On some systems, you can avoid this by setting
1794 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1795 forking and reaping moribund children.
1797 Note that if your forked child inherits system file descriptors like
1798 STDIN and STDOUT that are actually connected by a pipe or socket, even
1799 if you exit, then the remote server (such as, say, a CGI script or a
1800 backgrounded job launched from a remote shell) won't think you're done.
1801 You should reopen those to F</dev/null> if it's any issue.
1805 Declare a picture format for use by the C<write> function. For
1809 Test: @<<<<<<<< @||||| @>>>>>
1810 $str, $%, '$' . int($num)
1814 $num = $cost/$quantity;
1818 See L<perlform> for many details and examples.
1820 =item formline PICTURE,LIST
1822 This is an internal function used by C<format>s, though you may call it,
1823 too. It formats (see L<perlform>) a list of values according to the
1824 contents of PICTURE, placing the output into the format output
1825 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1826 Eventually, when a C<write> is done, the contents of
1827 C<$^A> are written to some filehandle, but you could also read C<$^A>
1828 yourself and then set C<$^A> back to C<"">. Note that a format typically
1829 does one C<formline> per line of form, but the C<formline> function itself
1830 doesn't care how many newlines are embedded in the PICTURE. This means
1831 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1832 You may therefore need to use multiple formlines to implement a single
1833 record format, just like the format compiler.
1835 Be careful if you put double quotes around the picture, because an C<@>
1836 character may be taken to mean the beginning of an array name.
1837 C<formline> always returns true. See L<perlform> for other examples.
1839 =item getc FILEHANDLE
1843 Returns the next character from the input file attached to FILEHANDLE,
1844 or the undefined value at end of file, or if there was an error (in
1845 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1846 STDIN. This is not particularly efficient. However, it cannot be
1847 used by itself to fetch single characters without waiting for the user
1848 to hit enter. For that, try something more like:
1851 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1854 system "stty", '-icanon', 'eol', "\001";
1860 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1863 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1867 Determination of whether $BSD_STYLE should be set
1868 is left as an exercise to the reader.
1870 The C<POSIX::getattr> function can do this more portably on
1871 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1872 module from your nearest CPAN site; details on CPAN can be found on
1877 Implements the C library function of the same name, which on most
1878 systems returns the current login from F</etc/utmp>, if any. If null,
1881 $login = getlogin || getpwuid($<) || "Kilroy";
1883 Do not consider C<getlogin> for authentication: it is not as
1884 secure as C<getpwuid>.
1886 =item getpeername SOCKET
1888 Returns the packed sockaddr address of other end of the SOCKET connection.
1891 $hersockaddr = getpeername(SOCK);
1892 ($port, $iaddr) = sockaddr_in($hersockaddr);
1893 $herhostname = gethostbyaddr($iaddr, AF_INET);
1894 $herstraddr = inet_ntoa($iaddr);
1898 Returns the current process group for the specified PID. Use
1899 a PID of C<0> to get the current process group for the
1900 current process. Will raise an exception if used on a machine that
1901 doesn't implement getpgrp(2). If PID is omitted, returns process
1902 group of current process. Note that the POSIX version of C<getpgrp>
1903 does not accept a PID argument, so only C<PID==0> is truly portable.
1907 Returns the process id of the parent process.
1909 Note for Linux users: on Linux, the C functions C<getpid()> and
1910 C<getppid()> return different values from different threads. In order to
1911 be portable, this behavior is not reflected by the perl-level function
1912 C<getppid()>, that returns a consistent value across threads. If you want
1913 to call the underlying C<getppid()>, you may use the CPAN module
1916 =item getpriority WHICH,WHO
1918 Returns the current priority for a process, a process group, or a user.
1919 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1920 machine that doesn't implement getpriority(2).
1926 =item gethostbyname NAME
1928 =item getnetbyname NAME
1930 =item getprotobyname NAME
1936 =item getservbyname NAME,PROTO
1938 =item gethostbyaddr ADDR,ADDRTYPE
1940 =item getnetbyaddr ADDR,ADDRTYPE
1942 =item getprotobynumber NUMBER
1944 =item getservbyport PORT,PROTO
1962 =item sethostent STAYOPEN
1964 =item setnetent STAYOPEN
1966 =item setprotoent STAYOPEN
1968 =item setservent STAYOPEN
1982 These routines perform the same functions as their counterparts in the
1983 system library. In list context, the return values from the
1984 various get routines are as follows:
1986 ($name,$passwd,$uid,$gid,
1987 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1988 ($name,$passwd,$gid,$members) = getgr*
1989 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1990 ($name,$aliases,$addrtype,$net) = getnet*
1991 ($name,$aliases,$proto) = getproto*
1992 ($name,$aliases,$port,$proto) = getserv*
1994 (If the entry doesn't exist you get a null list.)
1996 The exact meaning of the $gcos field varies but it usually contains
1997 the real name of the user (as opposed to the login name) and other
1998 information pertaining to the user. Beware, however, that in many
1999 system users are able to change this information and therefore it
2000 cannot be trusted and therefore the $gcos is tainted (see
2001 L<perlsec>). The $passwd and $shell, user's encrypted password and
2002 login shell, are also tainted, because of the same reason.
2004 In scalar context, you get the name, unless the function was a
2005 lookup by name, in which case you get the other thing, whatever it is.
2006 (If the entry doesn't exist you get the undefined value.) For example:
2008 $uid = getpwnam($name);
2009 $name = getpwuid($num);
2011 $gid = getgrnam($name);
2012 $name = getgrgid($num);
2016 In I<getpw*()> the fields $quota, $comment, and $expire are special
2017 cases in the sense that in many systems they are unsupported. If the
2018 $quota is unsupported, it is an empty scalar. If it is supported, it
2019 usually encodes the disk quota. If the $comment field is unsupported,
2020 it is an empty scalar. If it is supported it usually encodes some
2021 administrative comment about the user. In some systems the $quota
2022 field may be $change or $age, fields that have to do with password
2023 aging. In some systems the $comment field may be $class. The $expire
2024 field, if present, encodes the expiration period of the account or the
2025 password. For the availability and the exact meaning of these fields
2026 in your system, please consult your getpwnam(3) documentation and your
2027 F<pwd.h> file. You can also find out from within Perl what your
2028 $quota and $comment fields mean and whether you have the $expire field
2029 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2030 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2031 files are only supported if your vendor has implemented them in the
2032 intuitive fashion that calling the regular C library routines gets the
2033 shadow versions if you're running under privilege or if there exists
2034 the shadow(3) functions as found in System V ( this includes Solaris
2035 and Linux.) Those systems which implement a proprietary shadow password
2036 facility are unlikely to be supported.
2038 The $members value returned by I<getgr*()> is a space separated list of
2039 the login names of the members of the group.
2041 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2042 C, it will be returned to you via C<$?> if the function call fails. The
2043 C<@addrs> value returned by a successful call is a list of the raw
2044 addresses returned by the corresponding system library call. In the
2045 Internet domain, each address is four bytes long and you can unpack it
2046 by saying something like:
2048 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2050 The Socket library makes this slightly easier:
2053 $iaddr = inet_aton("127.1"); # or whatever address
2054 $name = gethostbyaddr($iaddr, AF_INET);
2056 # or going the other way
2057 $straddr = inet_ntoa($iaddr);
2059 If you get tired of remembering which element of the return list
2060 contains which return value, by-name interfaces are provided
2061 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2062 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2063 and C<User::grent>. These override the normal built-ins, supplying
2064 versions that return objects with the appropriate names
2065 for each field. For example:
2069 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2071 Even though it looks like they're the same method calls (uid),
2072 they aren't, because a C<File::stat> object is different from
2073 a C<User::pwent> object.
2075 =item getsockname SOCKET
2077 Returns the packed sockaddr address of this end of the SOCKET connection,
2078 in case you don't know the address because you have several different
2079 IPs that the connection might have come in on.
2082 $mysockaddr = getsockname(SOCK);
2083 ($port, $myaddr) = sockaddr_in($mysockaddr);
2084 printf "Connect to %s [%s]\n",
2085 scalar gethostbyaddr($myaddr, AF_INET),
2088 =item getsockopt SOCKET,LEVEL,OPTNAME
2090 Returns the socket option requested, or undef if there is an error.
2096 In list context, returns a (possibly empty) list of filename expansions on
2097 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2098 scalar context, glob iterates through such filename expansions, returning
2099 undef when the list is exhausted. This is the internal function
2100 implementing the C<< <*.c> >> operator, but you can use it directly. If
2101 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2102 more detail in L<perlop/"I/O Operators">.
2104 Beginning with v5.6.0, this operator is implemented using the standard
2105 C<File::Glob> extension. See L<File::Glob> for details.
2109 Converts a time as returned by the time function to an 8-element list
2110 with the time localized for the standard Greenwich time zone.
2111 Typically used as follows:
2114 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2117 All list elements are numeric, and come straight out of the C `struct
2118 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2119 specified time. $mday is the day of the month, and $mon is the month
2120 itself, in the range C<0..11> with 0 indicating January and 11
2121 indicating December. $year is the number of years since 1900. That
2122 is, $year is C<123> in year 2023. $wday is the day of the week, with
2123 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2124 the year, in the range C<0..364> (or C<0..365> in leap years.)
2126 Note that the $year element is I<not> simply the last two digits of
2127 the year. If you assume it is, then you create non-Y2K-compliant
2128 programs--and you wouldn't want to do that, would you?
2130 The proper way to get a complete 4-digit year is simply:
2134 And to get the last two digits of the year (e.g., '01' in 2001) do:
2136 $year = sprintf("%02d", $year % 100);
2138 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2140 In scalar context, C<gmtime()> returns the ctime(3) value:
2142 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2144 Also see the C<timegm> function provided by the C<Time::Local> module,
2145 and the strftime(3) function available via the POSIX module.
2147 This scalar value is B<not> locale dependent (see L<perllocale>), but
2148 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2149 strftime(3) and mktime(3) functions available via the POSIX module. To
2150 get somewhat similar but locale dependent date strings, set up your
2151 locale environment variables appropriately (please see L<perllocale>)
2152 and try for example:
2154 use POSIX qw(strftime);
2155 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2157 Note that the C<%a> and C<%b> escapes, which represent the short forms
2158 of the day of the week and the month of the year, may not necessarily
2159 be three characters wide in all locales.
2167 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2168 execution there. It may not be used to go into any construct that
2169 requires initialization, such as a subroutine or a C<foreach> loop. It
2170 also can't be used to go into a construct that is optimized away,
2171 or to get out of a block or subroutine given to C<sort>.
2172 It can be used to go almost anywhere else within the dynamic scope,
2173 including out of subroutines, but it's usually better to use some other
2174 construct such as C<last> or C<die>. The author of Perl has never felt the
2175 need to use this form of C<goto> (in Perl, that is--C is another matter).
2176 (The difference being that C does not offer named loops combined with
2177 loop control. Perl does, and this replaces most structured uses of C<goto>
2178 in other languages.)
2180 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2181 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2182 necessarily recommended if you're optimizing for maintainability:
2184 goto ("FOO", "BAR", "GLARCH")[$i];
2186 The C<goto-&NAME> form is quite different from the other forms of
2187 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2188 doesn't have the stigma associated with other gotos. Instead, it
2189 exits the current subroutine (losing any changes set by local()) and
2190 immediately calls in its place the named subroutine using the current
2191 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2192 load another subroutine and then pretend that the other subroutine had
2193 been called in the first place (except that any modifications to C<@_>
2194 in the current subroutine are propagated to the other subroutine.)
2195 After the C<goto>, not even C<caller> will be able to tell that this
2196 routine was called first.
2198 NAME needn't be the name of a subroutine; it can be a scalar variable
2199 containing a code reference, or a block which evaluates to a code
2202 =item grep BLOCK LIST
2204 =item grep EXPR,LIST
2206 This is similar in spirit to, but not the same as, grep(1) and its
2207 relatives. In particular, it is not limited to using regular expressions.
2209 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2210 C<$_> to each element) and returns the list value consisting of those
2211 elements for which the expression evaluated to true. In scalar
2212 context, returns the number of times the expression was true.
2214 @foo = grep(!/^#/, @bar); # weed out comments
2218 @foo = grep {!/^#/} @bar; # weed out comments
2220 Note that C<$_> is an alias to the list value, so it can be used to
2221 modify the elements of the LIST. While this is useful and supported,
2222 it can cause bizarre results if the elements of LIST are not variables.
2223 Similarly, grep returns aliases into the original list, much as a for
2224 loop's index variable aliases the list elements. That is, modifying an
2225 element of a list returned by grep (for example, in a C<foreach>, C<map>
2226 or another C<grep>) actually modifies the element in the original list.
2227 This is usually something to be avoided when writing clear code.
2229 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2235 Interprets EXPR as a hex string and returns the corresponding value.
2236 (To convert strings that might start with either 0, 0x, or 0b, see
2237 L</oct>.) If EXPR is omitted, uses C<$_>.
2239 print hex '0xAf'; # prints '175'
2240 print hex 'aF'; # same
2242 Hex strings may only represent integers. Strings that would cause
2243 integer overflow trigger a warning. Leading whitespace is not stripped,
2248 There is no builtin C<import> function. It is just an ordinary
2249 method (subroutine) defined (or inherited) by modules that wish to export
2250 names to another module. The C<use> function calls the C<import> method
2251 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2253 =item index STR,SUBSTR,POSITION
2255 =item index STR,SUBSTR
2257 The index function searches for one string within another, but without
2258 the wildcard-like behavior of a full regular-expression pattern match.
2259 It returns the position of the first occurrence of SUBSTR in STR at
2260 or after POSITION. If POSITION is omitted, starts searching from the
2261 beginning of the string. The return value is based at C<0> (or whatever
2262 you've set the C<$[> variable to--but don't do that). If the substring
2263 is not found, returns one less than the base, ordinarily C<-1>.
2269 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2270 You should not use this function for rounding: one because it truncates
2271 towards C<0>, and two because machine representations of floating point
2272 numbers can sometimes produce counterintuitive results. For example,
2273 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2274 because it's really more like -268.99999999999994315658 instead. Usually,
2275 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2276 functions will serve you better than will int().
2278 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2280 Implements the ioctl(2) function. You'll probably first have to say
2282 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2284 to get the correct function definitions. If F<ioctl.ph> doesn't
2285 exist or doesn't have the correct definitions you'll have to roll your
2286 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2287 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2288 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2289 written depending on the FUNCTION--a pointer to the string value of SCALAR
2290 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2291 has no string value but does have a numeric value, that value will be
2292 passed rather than a pointer to the string value. To guarantee this to be
2293 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2294 functions may be needed to manipulate the values of structures used by
2297 The return value of C<ioctl> (and C<fcntl>) is as follows:
2299 if OS returns: then Perl returns:
2301 0 string "0 but true"
2302 anything else that number
2304 Thus Perl returns true on success and false on failure, yet you can
2305 still easily determine the actual value returned by the operating
2308 $retval = ioctl(...) || -1;
2309 printf "System returned %d\n", $retval;
2311 The special string C<"0 but true"> is exempt from B<-w> complaints
2312 about improper numeric conversions.
2314 =item join EXPR,LIST
2316 Joins the separate strings of LIST into a single string with fields
2317 separated by the value of EXPR, and returns that new string. Example:
2319 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2321 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2322 first argument. Compare L</split>.
2326 Returns a list consisting of all the keys of the named hash.
2327 (In scalar context, returns the number of keys.)
2329 The keys are returned in an apparently random order. The actual
2330 random order is subject to change in future versions of perl, but it
2331 is guaranteed to be the same order as either the C<values> or C<each>
2332 function produces (given that the hash has not been modified). Since
2333 Perl 5.8.1 the ordering is different even between different runs of
2334 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2337 As a side effect, calling keys() resets the HASH's internal iterator,
2338 see L</each>. (In particular, calling keys() in void context resets
2339 the iterator with no other overhead.)
2341 Here is yet another way to print your environment:
2344 @values = values %ENV;
2346 print pop(@keys), '=', pop(@values), "\n";
2349 or how about sorted by key:
2351 foreach $key (sort(keys %ENV)) {
2352 print $key, '=', $ENV{$key}, "\n";
2355 The returned values are copies of the original keys in the hash, so
2356 modifying them will not affect the original hash. Compare L</values>.
2358 To sort a hash by value, you'll need to use a C<sort> function.
2359 Here's a descending numeric sort of a hash by its values:
2361 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2362 printf "%4d %s\n", $hash{$key}, $key;
2365 As an lvalue C<keys> allows you to increase the number of hash buckets
2366 allocated for the given hash. This can gain you a measure of efficiency if
2367 you know the hash is going to get big. (This is similar to pre-extending
2368 an array by assigning a larger number to $#array.) If you say
2372 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2373 in fact, since it rounds up to the next power of two. These
2374 buckets will be retained even if you do C<%hash = ()>, use C<undef
2375 %hash> if you want to free the storage while C<%hash> is still in scope.
2376 You can't shrink the number of buckets allocated for the hash using
2377 C<keys> in this way (but you needn't worry about doing this by accident,
2378 as trying has no effect).
2380 See also C<each>, C<values> and C<sort>.
2382 =item kill SIGNAL, LIST
2384 Sends a signal to a list of processes. Returns the number of
2385 processes successfully signaled (which is not necessarily the
2386 same as the number actually killed).
2388 $cnt = kill 1, $child1, $child2;
2391 If SIGNAL is zero, no signal is sent to the process. This is a
2392 useful way to check that a child process is alive and hasn't changed
2393 its UID. See L<perlport> for notes on the portability of this
2396 Unlike in the shell, if SIGNAL is negative, it kills
2397 process groups instead of processes. (On System V, a negative I<PROCESS>
2398 number will also kill process groups, but that's not portable.) That
2399 means you usually want to use positive not negative signals. You may also
2400 use a signal name in quotes.
2402 See L<perlipc/"Signals"> for more details.
2408 The C<last> command is like the C<break> statement in C (as used in
2409 loops); it immediately exits the loop in question. If the LABEL is
2410 omitted, the command refers to the innermost enclosing loop. The
2411 C<continue> block, if any, is not executed:
2413 LINE: while (<STDIN>) {
2414 last LINE if /^$/; # exit when done with header
2418 C<last> cannot be used to exit a block which returns a value such as
2419 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2420 a grep() or map() operation.
2422 Note that a block by itself is semantically identical to a loop
2423 that executes once. Thus C<last> can be used to effect an early
2424 exit out of such a block.
2426 See also L</continue> for an illustration of how C<last>, C<next>, and
2433 Returns a lowercased version of EXPR. This is the internal function
2434 implementing the C<\L> escape in double-quoted strings. Respects
2435 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2436 and L<perlunicode> for more details about locale and Unicode support.
2438 If EXPR is omitted, uses C<$_>.
2444 Returns the value of EXPR with the first character lowercased. This
2445 is the internal function implementing the C<\l> escape in
2446 double-quoted strings. Respects current LC_CTYPE locale if C<use
2447 locale> in force. See L<perllocale> and L<perlunicode> for more
2448 details about locale and Unicode support.
2450 If EXPR is omitted, uses C<$_>.
2456 Returns the length in I<characters> of the value of EXPR. If EXPR is
2457 omitted, returns length of C<$_>. Note that this cannot be used on
2458 an entire array or hash to find out how many elements these have.
2459 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2461 Note the I<characters>: if the EXPR is in Unicode, you will get the
2462 number of characters, not the number of bytes. To get the length
2463 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2465 =item link OLDFILE,NEWFILE
2467 Creates a new filename linked to the old filename. Returns true for
2468 success, false otherwise.
2470 =item listen SOCKET,QUEUESIZE
2472 Does the same thing that the listen system call does. Returns true if
2473 it succeeded, false otherwise. See the example in
2474 L<perlipc/"Sockets: Client/Server Communication">.
2478 You really probably want to be using C<my> instead, because C<local> isn't
2479 what most people think of as "local". See
2480 L<perlsub/"Private Variables via my()"> for details.
2482 A local modifies the listed variables to be local to the enclosing
2483 block, file, or eval. If more than one value is listed, the list must
2484 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2485 for details, including issues with tied arrays and hashes.
2487 =item localtime EXPR
2489 Converts a time as returned by the time function to a 9-element list
2490 with the time analyzed for the local time zone. Typically used as
2494 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2497 All list elements are numeric, and come straight out of the C `struct
2498 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2499 specified time. $mday is the day of the month, and $mon is the month
2500 itself, in the range C<0..11> with 0 indicating January and 11
2501 indicating December. $year is the number of years since 1900. That
2502 is, $year is C<123> in year 2023. $wday is the day of the week, with
2503 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2504 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2505 is true if the specified time occurs during daylight savings time,
2508 Note that the $year element is I<not> simply the last two digits of
2509 the year. If you assume it is, then you create non-Y2K-compliant
2510 programs--and you wouldn't want to do that, would you?
2512 The proper way to get a complete 4-digit year is simply:
2516 And to get the last two digits of the year (e.g., '01' in 2001) do:
2518 $year = sprintf("%02d", $year % 100);
2520 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2522 In scalar context, C<localtime()> returns the ctime(3) value:
2524 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2526 This scalar value is B<not> locale dependent, see L<perllocale>, but
2527 instead a Perl builtin. Also see the C<Time::Local> module
2528 (to convert the second, minutes, hours, ... back to seconds since the
2529 stroke of midnight the 1st of January 1970, the value returned by
2530 time()), and the strftime(3) and mktime(3) functions available via the
2531 POSIX module. To get somewhat similar but locale dependent date
2532 strings, set up your locale environment variables appropriately
2533 (please see L<perllocale>) and try for example:
2535 use POSIX qw(strftime);
2536 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2538 Note that the C<%a> and C<%b>, the short forms of the day of the week
2539 and the month of the year, may not necessarily be three characters wide.
2543 This function places an advisory lock on a shared variable, or referenced
2544 object contained in I<THING> until the lock goes out of scope.
2546 lock() is a "weak keyword" : this means that if you've defined a function
2547 by this name (before any calls to it), that function will be called
2548 instead. (However, if you've said C<use threads>, lock() is always a
2549 keyword.) See L<threads>.
2555 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2556 returns log of C<$_>. To get the log of another base, use basic algebra:
2557 The base-N log of a number is equal to the natural log of that number
2558 divided by the natural log of N. For example:
2562 return log($n)/log(10);
2565 See also L</exp> for the inverse operation.
2571 Does the same thing as the C<stat> function (including setting the
2572 special C<_> filehandle) but stats a symbolic link instead of the file
2573 the symbolic link points to. If symbolic links are unimplemented on
2574 your system, a normal C<stat> is done. For much more detailed
2575 information, please see the documentation for C<stat>.
2577 If EXPR is omitted, stats C<$_>.
2581 The match operator. See L<perlop>.
2583 =item map BLOCK LIST
2587 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2588 C<$_> to each element) and returns the list value composed of the
2589 results of each such evaluation. In scalar context, returns the
2590 total number of elements so generated. Evaluates BLOCK or EXPR in
2591 list context, so each element of LIST may produce zero, one, or
2592 more elements in the returned value.
2594 @chars = map(chr, @nums);
2596 translates a list of numbers to the corresponding characters. And
2598 %hash = map { getkey($_) => $_ } @array;
2600 is just a funny way to write
2603 foreach $_ (@array) {
2604 $hash{getkey($_)} = $_;
2607 Note that C<$_> is an alias to the list value, so it can be used to
2608 modify the elements of the LIST. While this is useful and supported,
2609 it can cause bizarre results if the elements of LIST are not variables.
2610 Using a regular C<foreach> loop for this purpose would be clearer in
2611 most cases. See also L</grep> for an array composed of those items of
2612 the original list for which the BLOCK or EXPR evaluates to true.
2614 C<{> starts both hash references and blocks, so C<map { ...> could be either
2615 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2616 ahead for the closing C<}> it has to take a guess at which its dealing with
2617 based what it finds just after the C<{>. Usually it gets it right, but if it
2618 doesn't it won't realize something is wrong until it gets to the C<}> and
2619 encounters the missing (or unexpected) comma. The syntax error will be
2620 reported close to the C<}> but you'll need to change something near the C<{>
2621 such as using a unary C<+> to give perl some help:
2623 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2624 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2625 %hash = map { ("\L$_", 1) } @array # this also works
2626 %hash = map { lc($_), 1 } @array # as does this.
2627 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2629 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2631 or to force an anon hash constructor use C<+{>
2633 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2635 and you get list of anonymous hashes each with only 1 entry.
2637 =item mkdir FILENAME,MASK
2639 =item mkdir FILENAME
2641 Creates the directory specified by FILENAME, with permissions
2642 specified by MASK (as modified by C<umask>). If it succeeds it
2643 returns true, otherwise it returns false and sets C<$!> (errno).
2644 If omitted, MASK defaults to 0777.
2646 In general, it is better to create directories with permissive MASK,
2647 and let the user modify that with their C<umask>, than it is to supply
2648 a restrictive MASK and give the user no way to be more permissive.
2649 The exceptions to this rule are when the file or directory should be
2650 kept private (mail files, for instance). The perlfunc(1) entry on
2651 C<umask> discusses the choice of MASK in more detail.
2653 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2654 number of trailing slashes. Some operating and filesystems do not get
2655 this right, so Perl automatically removes all trailing slashes to keep
2658 =item msgctl ID,CMD,ARG
2660 Calls the System V IPC function msgctl(2). You'll probably have to say
2664 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2665 then ARG must be a variable which will hold the returned C<msqid_ds>
2666 structure. Returns like C<ioctl>: the undefined value for error,
2667 C<"0 but true"> for zero, or the actual return value otherwise. See also
2668 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2670 =item msgget KEY,FLAGS
2672 Calls the System V IPC function msgget(2). Returns the message queue
2673 id, or the undefined value if there is an error. See also
2674 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2676 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2678 Calls the System V IPC function msgrcv to receive a message from
2679 message queue ID into variable VAR with a maximum message size of
2680 SIZE. Note that when a message is received, the message type as a
2681 native long integer will be the first thing in VAR, followed by the
2682 actual message. This packing may be opened with C<unpack("l! a*")>.
2683 Taints the variable. Returns true if successful, or false if there is
2684 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2685 C<IPC::SysV::Msg> documentation.
2687 =item msgsnd ID,MSG,FLAGS
2689 Calls the System V IPC function msgsnd to send the message MSG to the
2690 message queue ID. MSG must begin with the native long integer message
2691 type, and be followed by the length of the actual message, and finally
2692 the message itself. This kind of packing can be achieved with
2693 C<pack("l! a*", $type, $message)>. Returns true if successful,
2694 or false if there is an error. See also C<IPC::SysV>
2695 and C<IPC::SysV::Msg> documentation.
2701 =item my EXPR : ATTRS
2703 =item my TYPE EXPR : ATTRS
2705 A C<my> declares the listed variables to be local (lexically) to the
2706 enclosing block, file, or C<eval>. If more than one value is listed,
2707 the list must be placed in parentheses.
2709 The exact semantics and interface of TYPE and ATTRS are still
2710 evolving. TYPE is currently bound to the use of C<fields> pragma,
2711 and attributes are handled using the C<attributes> pragma, or starting
2712 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2713 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2714 L<attributes>, and L<Attribute::Handlers>.
2720 The C<next> command is like the C<continue> statement in C; it starts
2721 the next iteration of the loop:
2723 LINE: while (<STDIN>) {
2724 next LINE if /^#/; # discard comments
2728 Note that if there were a C<continue> block on the above, it would get
2729 executed even on discarded lines. If the LABEL is omitted, the command
2730 refers to the innermost enclosing loop.
2732 C<next> cannot be used to exit a block which returns a value such as
2733 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2734 a grep() or map() operation.
2736 Note that a block by itself is semantically identical to a loop
2737 that executes once. Thus C<next> will exit such a block early.
2739 See also L</continue> for an illustration of how C<last>, C<next>, and
2742 =item no Module VERSION LIST
2744 =item no Module VERSION
2746 =item no Module LIST
2750 See the C<use> function, of which C<no> is the opposite.
2756 Interprets EXPR as an octal string and returns the corresponding
2757 value. (If EXPR happens to start off with C<0x>, interprets it as a
2758 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2759 binary string. Leading whitespace is ignored in all three cases.)
2760 The following will handle decimal, binary, octal, and hex in the standard
2763 $val = oct($val) if $val =~ /^0/;
2765 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2766 in octal), use sprintf() or printf():
2768 $perms = (stat("filename"))[2] & 07777;
2769 $oct_perms = sprintf "%lo", $perms;
2771 The oct() function is commonly used when a string such as C<644> needs
2772 to be converted into a file mode, for example. (Although perl will
2773 automatically convert strings into numbers as needed, this automatic
2774 conversion assumes base 10.)
2776 =item open FILEHANDLE,EXPR
2778 =item open FILEHANDLE,MODE,EXPR
2780 =item open FILEHANDLE,MODE,EXPR,LIST
2782 =item open FILEHANDLE,MODE,REFERENCE
2784 =item open FILEHANDLE
2786 Opens the file whose filename is given by EXPR, and associates it with
2789 (The following is a comprehensive reference to open(): for a gentler
2790 introduction you may consider L<perlopentut>.)
2792 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2793 the variable is assigned a reference to a new anonymous filehandle,
2794 otherwise if FILEHANDLE is an expression, its value is used as the name of
2795 the real filehandle wanted. (This is considered a symbolic reference, so
2796 C<use strict 'refs'> should I<not> be in effect.)
2798 If EXPR is omitted, the scalar variable of the same name as the
2799 FILEHANDLE contains the filename. (Note that lexical variables--those
2800 declared with C<my>--will not work for this purpose; so if you're
2801 using C<my>, specify EXPR in your call to open.)
2803 If three or more arguments are specified then the mode of opening and
2804 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2805 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2806 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2807 the file is opened for appending, again being created if necessary.
2809 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2810 indicate that you want both read and write access to the file; thus
2811 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2812 '+>' >> mode would clobber the file first. You can't usually use
2813 either read-write mode for updating textfiles, since they have
2814 variable length records. See the B<-i> switch in L<perlrun> for a
2815 better approach. The file is created with permissions of C<0666>
2816 modified by the process' C<umask> value.
2818 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2819 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2821 In the 2-arguments (and 1-argument) form of the call the mode and
2822 filename should be concatenated (in this order), possibly separated by
2823 spaces. It is possible to omit the mode in these forms if the mode is
2826 If the filename begins with C<'|'>, the filename is interpreted as a
2827 command to which output is to be piped, and if the filename ends with a
2828 C<'|'>, the filename is interpreted as a command which pipes output to
2829 us. See L<perlipc/"Using open() for IPC">
2830 for more examples of this. (You are not allowed to C<open> to a command
2831 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2832 and L<perlipc/"Bidirectional Communication with Another Process">
2835 For three or more arguments if MODE is C<'|-'>, the filename is
2836 interpreted as a command to which output is to be piped, and if MODE
2837 is C<'-|'>, the filename is interpreted as a command which pipes
2838 output to us. In the 2-arguments (and 1-argument) form one should
2839 replace dash (C<'-'>) with the command.
2840 See L<perlipc/"Using open() for IPC"> for more examples of this.
2841 (You are not allowed to C<open> to a command that pipes both in I<and>
2842 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2843 L<perlipc/"Bidirectional Communication"> for alternatives.)
2845 In the three-or-more argument form of pipe opens, if LIST is specified
2846 (extra arguments after the command name) then LIST becomes arguments
2847 to the command invoked if the platform supports it. The meaning of
2848 C<open> with more than three arguments for non-pipe modes is not yet
2849 specified. Experimental "layers" may give extra LIST arguments
2852 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2853 and opening C<< '>-' >> opens STDOUT.
2855 You may use the three-argument form of open to specify IO "layers"
2856 (sometimes also referred to as "disciplines") to be applied to the handle
2857 that affect how the input and output are processed (see L<open> and
2858 L<PerlIO> for more details). For example
2860 open(FH, "<:utf8", "file")
2862 will open the UTF-8 encoded file containing Unicode characters,
2863 see L<perluniintro>. (Note that if layers are specified in the
2864 three-arg form then default layers set by the C<open> pragma are
2867 Open returns nonzero upon success, the undefined value otherwise. If
2868 the C<open> involved a pipe, the return value happens to be the pid of
2871 If you're running Perl on a system that distinguishes between text
2872 files and binary files, then you should check out L</binmode> for tips
2873 for dealing with this. The key distinction between systems that need
2874 C<binmode> and those that don't is their text file formats. Systems
2875 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2876 character, and which encode that character in C as C<"\n">, do not
2877 need C<binmode>. The rest need it.
2879 When opening a file, it's usually a bad idea to continue normal execution
2880 if the request failed, so C<open> is frequently used in connection with
2881 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2882 where you want to make a nicely formatted error message (but there are
2883 modules that can help with that problem)) you should always check
2884 the return value from opening a file. The infrequent exception is when
2885 working with an unopened filehandle is actually what you want to do.
2887 As a special case the 3 arg form with a read/write mode and the third
2888 argument being C<undef>:
2890 open(TMP, "+>", undef) or die ...
2892 opens a filehandle to an anonymous temporary file. Also using "+<"
2893 works for symmetry, but you really should consider writing something
2894 to the temporary file first. You will need to seek() to do the
2897 File handles can be opened to "in memory" files held in Perl scalars via:
2899 open($fh, '>', \$variable) || ..
2901 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2902 file, you have to close it first:
2905 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2910 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2911 while (<ARTICLE>) {...
2913 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2914 # if the open fails, output is discarded
2916 open(DBASE, '+<', 'dbase.mine') # open for update
2917 or die "Can't open 'dbase.mine' for update: $!";
2919 open(DBASE, '+<dbase.mine') # ditto
2920 or die "Can't open 'dbase.mine' for update: $!";
2922 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2923 or die "Can't start caesar: $!";
2925 open(ARTICLE, "caesar <$article |") # ditto
2926 or die "Can't start caesar: $!";
2928 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2929 or die "Can't start sort: $!";
2932 open(MEMORY,'>', \$var)
2933 or die "Can't open memory file: $!";
2934 print MEMORY "foo!\n"; # output will end up in $var
2936 # process argument list of files along with any includes
2938 foreach $file (@ARGV) {
2939 process($file, 'fh00');
2943 my($filename, $input) = @_;
2944 $input++; # this is a string increment
2945 unless (open($input, $filename)) {
2946 print STDERR "Can't open $filename: $!\n";
2951 while (<$input>) { # note use of indirection
2952 if (/^#include "(.*)"/) {
2953 process($1, $input);
2960 You may also, in the Bourne shell tradition, specify an EXPR beginning
2961 with C<< '>&' >>, in which case the rest of the string is interpreted
2962 as the name of a filehandle (or file descriptor, if numeric) to be
2963 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2964 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2965 The mode you specify should match the mode of the original filehandle.
2966 (Duping a filehandle does not take into account any existing contents
2967 of IO buffers.) If you use the 3 arg form then you can pass either a
2968 number, the name of a filehandle or the normal "reference to a glob".
2970 Here is a script that saves, redirects, and restores C<STDOUT> and
2971 C<STDERR> using various methods:
2974 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2975 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2977 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2978 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2980 select STDERR; $| = 1; # make unbuffered
2981 select STDOUT; $| = 1; # make unbuffered
2983 print STDOUT "stdout 1\n"; # this works for
2984 print STDERR "stderr 1\n"; # subprocesses too
2989 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2990 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2992 print STDOUT "stdout 2\n";
2993 print STDERR "stderr 2\n";
2995 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
2996 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
2997 that file descriptor (and not call L<dup(2)>); this is more
2998 parsimonious of file descriptors. For example:
3000 # open for input, reusing the fileno of $fd
3001 open(FILEHANDLE, "<&=$fd")
3005 open(FILEHANDLE, "<&=", $fd)
3009 # open for append, using the fileno of OLDFH
3010 open(FH, ">>&=", OLDFH)
3014 open(FH, ">>&=OLDFH")
3016 Being parsimonious on filehandles is also useful (besides being
3017 parsimonious) for example when something is dependent on file
3018 descriptors, like for example locking using flock(). If you do just
3019 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3020 descriptor as B, and therefore flock(A) will not flock(B), and vice
3021 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3022 the same file descriptor.
3024 Note that if you are using Perls older than 5.8.0, Perl will be using
3025 the standard C libraries' fdopen() to implement the "=" functionality.
3026 On many UNIX systems fdopen() fails when file descriptors exceed a
3027 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3028 most often the default.
3030 You can see whether Perl has been compiled with PerlIO or not by
3031 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3032 is C<define>, you have PerlIO, otherwise you don't.
3034 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3035 with 2-arguments (or 1-argument) form of open(), then
3036 there is an implicit fork done, and the return value of open is the pid
3037 of the child within the parent process, and C<0> within the child
3038 process. (Use C<defined($pid)> to determine whether the open was successful.)
3039 The filehandle behaves normally for the parent, but i/o to that
3040 filehandle is piped from/to the STDOUT/STDIN of the child process.
3041 In the child process the filehandle isn't opened--i/o happens from/to
3042 the new STDOUT or STDIN. Typically this is used like the normal
3043 piped open when you want to exercise more control over just how the
3044 pipe command gets executed, such as when you are running setuid, and
3045 don't want to have to scan shell commands for metacharacters.
3046 The following triples are more or less equivalent:
3048 open(FOO, "|tr '[a-z]' '[A-Z]'");
3049 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3050 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3051 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3053 open(FOO, "cat -n '$file'|");
3054 open(FOO, '-|', "cat -n '$file'");
3055 open(FOO, '-|') || exec 'cat', '-n', $file;
3056 open(FOO, '-|', "cat", '-n', $file);
3058 The last example in each block shows the pipe as "list form", which is
3059 not yet supported on all platforms. A good rule of thumb is that if
3060 your platform has true C<fork()> (in other words, if your platform is
3061 UNIX) you can use the list form.
3063 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3065 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3066 output before any operation that may do a fork, but this may not be
3067 supported on some platforms (see L<perlport>). To be safe, you may need
3068 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3069 of C<IO::Handle> on any open handles.
3071 On systems that support a close-on-exec flag on files, the flag will
3072 be set for the newly opened file descriptor as determined by the value
3073 of $^F. See L<perlvar/$^F>.
3075 Closing any piped filehandle causes the parent process to wait for the
3076 child to finish, and returns the status value in C<$?>.
3078 The filename passed to 2-argument (or 1-argument) form of open() will
3079 have leading and trailing whitespace deleted, and the normal
3080 redirection characters honored. This property, known as "magic open",
3081 can often be used to good effect. A user could specify a filename of
3082 F<"rsh cat file |">, or you could change certain filenames as needed:
3084 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3085 open(FH, $filename) or die "Can't open $filename: $!";
3087 Use 3-argument form to open a file with arbitrary weird characters in it,
3089 open(FOO, '<', $file);
3091 otherwise it's necessary to protect any leading and trailing whitespace:
3093 $file =~ s#^(\s)#./$1#;
3094 open(FOO, "< $file\0");
3096 (this may not work on some bizarre filesystems). One should
3097 conscientiously choose between the I<magic> and 3-arguments form
3102 will allow the user to specify an argument of the form C<"rsh cat file |">,
3103 but will not work on a filename which happens to have a trailing space, while
3105 open IN, '<', $ARGV[0];
3107 will have exactly the opposite restrictions.
3109 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3110 should use the C<sysopen> function, which involves no such magic (but
3111 may use subtly different filemodes than Perl open(), which is mapped
3112 to C fopen()). This is
3113 another way to protect your filenames from interpretation. For example:
3116 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3117 or die "sysopen $path: $!";
3118 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3119 print HANDLE "stuff $$\n";
3121 print "File contains: ", <HANDLE>;
3123 Using the constructor from the C<IO::Handle> package (or one of its
3124 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3125 filehandles that have the scope of whatever variables hold references to
3126 them, and automatically close whenever and however you leave that scope:
3130 sub read_myfile_munged {
3132 my $handle = new IO::File;
3133 open($handle, "myfile") or die "myfile: $!";
3135 or return (); # Automatically closed here.
3136 mung $first or die "mung failed"; # Or here.
3137 return $first, <$handle> if $ALL; # Or here.
3141 See L</seek> for some details about mixing reading and writing.
3143 =item opendir DIRHANDLE,EXPR
3145 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3146 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3147 DIRHANDLE may be an expression whose value can be used as an indirect
3148 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3149 scalar variable (or array or hash element), the variable is assigned a
3150 reference to a new anonymous dirhandle.
3151 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3157 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3158 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3161 For the reverse, see L</chr>.
3162 See L<perlunicode> and L<encoding> for more about Unicode.
3168 =item our EXPR : ATTRS
3170 =item our TYPE EXPR : ATTRS
3172 An C<our> declares the listed variables to be valid globals within
3173 the enclosing block, file, or C<eval>. That is, it has the same
3174 scoping rules as a "my" declaration, but does not create a local
3175 variable. If more than one value is listed, the list must be placed
3176 in parentheses. The C<our> declaration has no semantic effect unless
3177 "use strict vars" is in effect, in which case it lets you use the
3178 declared global variable without qualifying it with a package name.
3179 (But only within the lexical scope of the C<our> declaration. In this
3180 it differs from "use vars", which is package scoped.)
3182 An C<our> declaration declares a global variable that will be visible
3183 across its entire lexical scope, even across package boundaries. The
3184 package in which the variable is entered is determined at the point
3185 of the declaration, not at the point of use. This means the following
3189 our $bar; # declares $Foo::bar for rest of lexical scope
3193 print $bar; # prints 20
3195 Multiple C<our> declarations in the same lexical scope are allowed
3196 if they are in different packages. If they happened to be in the same
3197 package, Perl will emit warnings if you have asked for them.
3201 our $bar; # declares $Foo::bar for rest of lexical scope
3205 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3206 print $bar; # prints 30
3208 our $bar; # emits warning
3210 An C<our> declaration may also have a list of attributes associated
3213 The exact semantics and interface of TYPE and ATTRS are still
3214 evolving. TYPE is currently bound to the use of C<fields> pragma,
3215 and attributes are handled using the C<attributes> pragma, or starting
3216 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3217 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3218 L<attributes>, and L<Attribute::Handlers>.
3220 The only currently recognized C<our()> attribute is C<unique> which
3221 indicates that a single copy of the global is to be used by all
3222 interpreters should the program happen to be running in a
3223 multi-interpreter environment. (The default behaviour would be for
3224 each interpreter to have its own copy of the global.) Examples:
3226 our @EXPORT : unique = qw(foo);
3227 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3228 our $VERSION : unique = "1.00";
3230 Note that this attribute also has the effect of making the global
3231 readonly when the first new interpreter is cloned (for example,
3232 when the first new thread is created).
3234 Multi-interpreter environments can come to being either through the
3235 fork() emulation on Windows platforms, or by embedding perl in a
3236 multi-threaded application. The C<unique> attribute does nothing in
3237 all other environments.
3239 Warning: the current implementation of this attribute operates on the
3240 typeglob associated with the variable; this means that C<our $x : unique>
3241 also has the effect of C<our @x : unique; our %x : unique>. This may be
3244 =item pack TEMPLATE,LIST
3246 Takes a LIST of values and converts it into a string using the rules
3247 given by the TEMPLATE. The resulting string is the concatenation of
3248 the converted values. Typically, each converted value looks
3249 like its machine-level representation. For example, on 32-bit machines
3250 a converted integer may be represented by a sequence of 4 bytes.
3252 The TEMPLATE is a sequence of characters that give the order and type
3253 of values, as follows:
3255 a A string with arbitrary binary data, will be null padded.
3256 A A text (ASCII) string, will be space padded.
3257 Z A null terminated (ASCIZ) string, will be null padded.
3259 b A bit string (ascending bit order inside each byte, like vec()).
3260 B A bit string (descending bit order inside each byte).
3261 h A hex string (low nybble first).
3262 H A hex string (high nybble first).
3264 c A signed char value.
3265 C An unsigned char value. Only does bytes. See U for Unicode.
3267 s A signed short value.
3268 S An unsigned short value.
3269 (This 'short' is _exactly_ 16 bits, which may differ from
3270 what a local C compiler calls 'short'. If you want
3271 native-length shorts, use the '!' suffix.)
3273 i A signed integer value.
3274 I An unsigned integer value.
3275 (This 'integer' is _at_least_ 32 bits wide. Its exact
3276 size depends on what a local C compiler calls 'int',
3277 and may even be larger than the 'long' described in
3280 l A signed long value.
3281 L An unsigned long value.
3282 (This 'long' is _exactly_ 32 bits, which may differ from
3283 what a local C compiler calls 'long'. If you want
3284 native-length longs, use the '!' suffix.)
3286 n An unsigned short in "network" (big-endian) order.
3287 N An unsigned long in "network" (big-endian) order.
3288 v An unsigned short in "VAX" (little-endian) order.
3289 V An unsigned long in "VAX" (little-endian) order.
3290 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3291 _exactly_ 32 bits, respectively.)
3293 q A signed quad (64-bit) value.
3294 Q An unsigned quad value.
3295 (Quads are available only if your system supports 64-bit
3296 integer values _and_ if Perl has been compiled to support those.
3297 Causes a fatal error otherwise.)
3299 j A signed integer value (a Perl internal integer, IV).
3300 J An unsigned integer value (a Perl internal unsigned integer, UV).
3302 f A single-precision float in the native format.
3303 d A double-precision float in the native format.
3305 F A floating point value in the native native format
3306 (a Perl internal floating point value, NV).
3307 D A long double-precision float in the native format.
3308 (Long doubles are available only if your system supports long
3309 double values _and_ if Perl has been compiled to support those.
3310 Causes a fatal error otherwise.)
3312 p A pointer to a null-terminated string.
3313 P A pointer to a structure (fixed-length string).
3315 u A uuencoded string.
3316 U A Unicode character number. Encodes to UTF-8 internally
3317 (or UTF-EBCDIC in EBCDIC platforms).
3319 w A BER compressed integer. Its bytes represent an unsigned
3320 integer in base 128, most significant digit first, with as
3321 few digits as possible. Bit eight (the high bit) is set
3322 on each byte except the last.
3326 @ Null fill to absolute position, counted from the start of
3327 the innermost ()-group.
3328 ( Start of a ()-group.
3330 The following rules apply:
3336 Each letter may optionally be followed by a number giving a repeat
3337 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3338 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3339 many values from the LIST. A C<*> for the repeat count means to use
3340 however many items are left, except for C<@>, C<x>, C<X>, where it is
3341 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3342 is the same). A numeric repeat count may optionally be enclosed in
3343 brackets, as in C<pack 'C[80]', @arr>.
3345 One can replace the numeric repeat count by a template enclosed in brackets;
3346 then the packed length of this template in bytes is used as a count.
3347 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3348 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3349 If the template in brackets contains alignment commands (such as C<x![d]>),
3350 its packed length is calculated as if the start of the template has the maximal
3353 When used with C<Z>, C<*> results in the addition of a trailing null
3354 byte (so the packed result will be one longer than the byte C<length>
3357 The repeat count for C<u> is interpreted as the maximal number of bytes
3358 to encode per line of output, with 0 and 1 replaced by 45.
3362 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3363 string of length count, padding with nulls or spaces as necessary. When
3364 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3365 after the first null, and C<a> returns data verbatim. When packing,
3366 C<a>, and C<Z> are equivalent.
3368 If the value-to-pack is too long, it is truncated. If too long and an
3369 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3370 by a null byte. Thus C<Z> always packs a trailing null byte under
3375 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3376 Each byte of the input field of pack() generates 1 bit of the result.
3377 Each result bit is based on the least-significant bit of the corresponding
3378 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3379 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3381 Starting from the beginning of the input string of pack(), each 8-tuple
3382 of bytes is converted to 1 byte of output. With format C<b>
3383 the first byte of the 8-tuple determines the least-significant bit of a
3384 byte, and with format C<B> it determines the most-significant bit of
3387 If the length of the input string is not exactly divisible by 8, the
3388 remainder is packed as if the input string were padded by null bytes
3389 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3391 If the input string of pack() is longer than needed, extra bytes are ignored.
3392 A C<*> for the repeat count of pack() means to use all the bytes of
3393 the input field. On unpack()ing the bits are converted to a string
3394 of C<"0">s and C<"1">s.
3398 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3399 representable as hexadecimal digits, 0-9a-f) long.
3401 Each byte of the input field of pack() generates 4 bits of the result.
3402 For non-alphabetical bytes the result is based on the 4 least-significant
3403 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3404 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3405 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3406 is compatible with the usual hexadecimal digits, so that C<"a"> and
3407 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3408 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3410 Starting from the beginning of the input string of pack(), each pair
3411 of bytes is converted to 1 byte of output. With format C<h> the
3412 first byte of the pair determines the least-significant nybble of the
3413 output byte, and with format C<H> it determines the most-significant
3416 If the length of the input string is not even, it behaves as if padded
3417 by a null byte at the end. Similarly, during unpack()ing the "extra"
3418 nybbles are ignored.
3420 If the input string of pack() is longer than needed, extra bytes are ignored.
3421 A C<*> for the repeat count of pack() means to use all the bytes of
3422 the input field. On unpack()ing the bits are converted to a string
3423 of hexadecimal digits.
3427 The C<p> type packs a pointer to a null-terminated string. You are
3428 responsible for ensuring the string is not a temporary value (which can
3429 potentially get deallocated before you get around to using the packed result).
3430 The C<P> type packs a pointer to a structure of the size indicated by the
3431 length. A NULL pointer is created if the corresponding value for C<p> or
3432 C<P> is C<undef>, similarly for unpack().
3436 The C</> template character allows packing and unpacking of strings where
3437 the packed structure contains a byte count followed by the string itself.
3438 You write I<length-item>C</>I<string-item>.
3440 The I<length-item> can be any C<pack> template letter, and describes
3441 how the length value is packed. The ones likely to be of most use are
3442 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3443 SNMP) and C<N> (for Sun XDR).
3445 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3446 C<"Z*">. For C<unpack> the length of the string is obtained from the
3447 I<length-item>, but if you put in the '*' it will be ignored. For all other
3448 codes, C<unpack> applies the length value to the next item, which must not
3449 have a repeat count.
3451 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3452 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3453 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3455 The I<length-item> is not returned explicitly from C<unpack>.
3457 Adding a count to the I<length-item> letter is unlikely to do anything
3458 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3459 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3460 which Perl does not regard as legal in numeric strings.
3464 The integer types C<s>, C<S>, C<l>, and C<L> may be
3465 immediately followed by a C<!> suffix to signify native shorts or
3466 longs--as you can see from above for example a bare C<l> does mean
3467 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3468 may be larger. This is an issue mainly in 64-bit platforms. You can
3469 see whether using C<!> makes any difference by
3471 print length(pack("s")), " ", length(pack("s!")), "\n";
3472 print length(pack("l")), " ", length(pack("l!")), "\n";
3474 C<i!> and C<I!> also work but only because of completeness;
3475 they are identical to C<i> and C<I>.
3477 The actual sizes (in bytes) of native shorts, ints, longs, and long
3478 longs on the platform where Perl was built are also available via
3482 print $Config{shortsize}, "\n";
3483 print $Config{intsize}, "\n";
3484 print $Config{longsize}, "\n";
3485 print $Config{longlongsize}, "\n";
3487 (The C<$Config{longlongsize}> will be undefined if your system does
3488 not support long longs.)
3492 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3493 are inherently non-portable between processors and operating systems
3494 because they obey the native byteorder and endianness. For example a
3495 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3496 (arranged in and handled by the CPU registers) into bytes as
3498 0x12 0x34 0x56 0x78 # big-endian
3499 0x78 0x56 0x34 0x12 # little-endian
3501 Basically, the Intel and VAX CPUs are little-endian, while everybody
3502 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3503 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3504 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3507 The names `big-endian' and `little-endian' are comic references to
3508 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3509 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3510 the egg-eating habits of the Lilliputians.
3512 Some systems may have even weirder byte orders such as
3517 You can see your system's preference with
3519 print join(" ", map { sprintf "%#02x", $_ }
3520 unpack("C*",pack("L",0x12345678))), "\n";
3522 The byteorder on the platform where Perl was built is also available
3526 print $Config{byteorder}, "\n";
3528 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3529 and C<'87654321'> are big-endian.
3531 If you want portable packed integers use the formats C<n>, C<N>,
3532 C<v>, and C<V>, their byte endianness and size are known.
3533 See also L<perlport>.
3537 Real numbers (floats and doubles) are in the native machine format only;
3538 due to the multiplicity of floating formats around, and the lack of a
3539 standard "network" representation, no facility for interchange has been
3540 made. This means that packed floating point data written on one machine
3541 may not be readable on another - even if both use IEEE floating point
3542 arithmetic (as the endian-ness of the memory representation is not part
3543 of the IEEE spec). See also L<perlport>.
3545 Note that Perl uses doubles internally for all numeric calculation, and
3546 converting from double into float and thence back to double again will
3547 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3552 If the pattern begins with a C<U>, the resulting string will be
3553 treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
3554 string with an initial C<U0>, and the bytes that follow will be
3555 interpreted as Unicode characters. If you don't want this to happen,
3556 you can begin your pattern with C<C0> (or anything else) to force Perl
3557 not to UTF-8 encode your string, and then follow this with a C<U*>
3558 somewhere in your pattern.
3562 You must yourself do any alignment or padding by inserting for example
3563 enough C<'x'>es while packing. There is no way to pack() and unpack()
3564 could know where the bytes are going to or coming from. Therefore
3565 C<pack> (and C<unpack>) handle their output and input as flat
3570 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3571 take a repeat count, both as postfix, and for unpack() also via the C</>
3572 template character. Within each repetition of a group, positioning with
3573 C<@> starts again at 0. Therefore, the result of
3575 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3577 is the string "\0a\0\0bc".
3582 C<x> and C<X> accept C<!> modifier. In this case they act as
3583 alignment commands: they jump forward/back to the closest position
3584 aligned at a multiple of C<count> bytes. For example, to pack() or
3585 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3586 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3587 aligned on the double's size.
3589 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3590 both result in no-ops.
3594 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3595 White space may be used to separate pack codes from each other, but
3596 a C<!> modifier and a repeat count must follow immediately.
3600 If TEMPLATE requires more arguments to pack() than actually given, pack()
3601 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3602 to pack() than actually given, extra arguments are ignored.
3608 $foo = pack("CCCC",65,66,67,68);
3610 $foo = pack("C4",65,66,67,68);
3612 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3613 # same thing with Unicode circled letters
3615 $foo = pack("ccxxcc",65,66,67,68);
3618 # note: the above examples featuring "C" and "c" are true
3619 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3620 # and UTF-8. In EBCDIC the first example would be
3621 # $foo = pack("CCCC",193,194,195,196);
3623 $foo = pack("s2",1,2);
3624 # "\1\0\2\0" on little-endian
3625 # "\0\1\0\2" on big-endian
3627 $foo = pack("a4","abcd","x","y","z");
3630 $foo = pack("aaaa","abcd","x","y","z");
3633 $foo = pack("a14","abcdefg");
3634 # "abcdefg\0\0\0\0\0\0\0"
3636 $foo = pack("i9pl", gmtime);
3637 # a real struct tm (on my system anyway)
3639 $utmp_template = "Z8 Z8 Z16 L";
3640 $utmp = pack($utmp_template, @utmp1);
3641 # a struct utmp (BSDish)
3643 @utmp2 = unpack($utmp_template, $utmp);
3644 # "@utmp1" eq "@utmp2"
3647 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3650 $foo = pack('sx2l', 12, 34);
3651 # short 12, two zero bytes padding, long 34
3652 $bar = pack('s@4l', 12, 34);
3653 # short 12, zero fill to position 4, long 34
3656 The same template may generally also be used in unpack().
3658 =item package NAMESPACE
3662 Declares the compilation unit as being in the given namespace. The scope
3663 of the package declaration is from the declaration itself through the end
3664 of the enclosing block, file, or eval (the same as the C<my> operator).
3665 All further unqualified dynamic identifiers will be in this namespace.
3666 A package statement affects only dynamic variables--including those
3667 you've used C<local> on--but I<not> lexical variables, which are created
3668 with C<my>. Typically it would be the first declaration in a file to
3669 be included by the C<require> or C<use> operator. You can switch into a
3670 package in more than one place; it merely influences which symbol table
3671 is used by the compiler for the rest of that block. You can refer to
3672 variables and filehandles in other packages by prefixing the identifier
3673 with the package name and a double colon: C<$Package::Variable>.
3674 If the package name is null, the C<main> package as assumed. That is,
3675 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3676 still seen in older code).
3678 If NAMESPACE is omitted, then there is no current package, and all
3679 identifiers must be fully qualified or lexicals. However, you are
3680 strongly advised not to make use of this feature. Its use can cause
3681 unexpected behaviour, even crashing some versions of Perl. It is
3682 deprecated, and will be removed from a future release.
3684 See L<perlmod/"Packages"> for more information about packages, modules,
3685 and classes. See L<perlsub> for other scoping issues.
3687 =item pipe READHANDLE,WRITEHANDLE
3689 Opens a pair of connected pipes like the corresponding system call.
3690 Note that if you set up a loop of piped processes, deadlock can occur
3691 unless you are very careful. In addition, note that Perl's pipes use
3692 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3693 after each command, depending on the application.
3695 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3696 for examples of such things.
3698 On systems that support a close-on-exec flag on files, the flag will be set
3699 for the newly opened file descriptors as determined by the value of $^F.
3706 Pops and returns the last value of the array, shortening the array by
3707 one element. Has an effect similar to
3711 If there are no elements in the array, returns the undefined value
3712 (although this may happen at other times as well). If ARRAY is
3713 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3714 array in subroutines, just like C<shift>.
3720 Returns the offset of where the last C<m//g> search left off for the variable
3721 in question (C<$_> is used when the variable is not specified). May be
3722 modified to change that offset. Such modification will also influence
3723 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3726 =item print FILEHANDLE LIST
3732 Prints a string or a list of strings. Returns true if successful.
3733 FILEHANDLE may be a scalar variable name, in which case the variable
3734 contains the name of or a reference to the filehandle, thus introducing
3735 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3736 the next token is a term, it may be misinterpreted as an operator
3737 unless you interpose a C<+> or put parentheses around the arguments.)
3738 If FILEHANDLE is omitted, prints by default to standard output (or
3739 to the last selected output channel--see L</select>). If LIST is
3740 also omitted, prints C<$_> to the currently selected output channel.
3741 To set the default output channel to something other than STDOUT
3742 use the select operation. The current value of C<$,> (if any) is
3743 printed between each LIST item. The current value of C<$\> (if
3744 any) is printed after the entire LIST has been printed. Because
3745 print takes a LIST, anything in the LIST is evaluated in list
3746 context, and any subroutine that you call will have one or more of
3747 its expressions evaluated in list context. Also be careful not to
3748 follow the print keyword with a left parenthesis unless you want
3749 the corresponding right parenthesis to terminate the arguments to
3750 the print--interpose a C<+> or put parentheses around all the
3753 Note that if you're storing FILEHANDLES in an array or other expression,
3754 you will have to use a block returning its value instead:
3756 print { $files[$i] } "stuff\n";
3757 print { $OK ? STDOUT : STDERR } "stuff\n";
3759 =item printf FILEHANDLE FORMAT, LIST
3761 =item printf FORMAT, LIST
3763 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3764 (the output record separator) is not appended. The first argument
3765 of the list will be interpreted as the C<printf> format. See C<sprintf>
3766 for an explanation of the format argument. If C<use locale> is in effect,
3767 the character used for the decimal point in formatted real numbers is
3768 affected by the LC_NUMERIC locale. See L<perllocale>.
3770 Don't fall into the trap of using a C<printf> when a simple
3771 C<print> would do. The C<print> is more efficient and less
3774 =item prototype FUNCTION
3776 Returns the prototype of a function as a string (or C<undef> if the
3777 function has no prototype). FUNCTION is a reference to, or the name of,
3778 the function whose prototype you want to retrieve.
3780 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3781 name for Perl builtin. If the builtin is not I<overridable> (such as
3782 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3783 C<system>) returns C<undef> because the builtin does not really behave
3784 like a Perl function. Otherwise, the string describing the equivalent
3785 prototype is returned.
3787 =item push ARRAY,LIST
3789 Treats ARRAY as a stack, and pushes the values of LIST
3790 onto the end of ARRAY. The length of ARRAY increases by the length of
3791 LIST. Has the same effect as
3794 $ARRAY[++$#ARRAY] = $value;
3797 but is more efficient. Returns the new number of elements in the array.
3809 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3811 =item quotemeta EXPR
3815 Returns the value of EXPR with all non-"word"
3816 characters backslashed. (That is, all characters not matching
3817 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3818 returned string, regardless of any locale settings.)
3819 This is the internal function implementing
3820 the C<\Q> escape in double-quoted strings.
3822 If EXPR is omitted, uses C<$_>.
3828 Returns a random fractional number greater than or equal to C<0> and less
3829 than the value of EXPR. (EXPR should be positive.) If EXPR is
3830 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3831 also special-cased as C<1> - this has not been documented before perl 5.8.0
3832 and is subject to change in future versions of perl. Automatically calls
3833 C<srand> unless C<srand> has already been called. See also C<srand>.
3835 Apply C<int()> to the value returned by C<rand()> if you want random
3836 integers instead of random fractional numbers. For example,
3840 returns a random integer between C<0> and C<9>, inclusive.
3842 (Note: If your rand function consistently returns numbers that are too
3843 large or too small, then your version of Perl was probably compiled
3844 with the wrong number of RANDBITS.)
3846 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3848 =item read FILEHANDLE,SCALAR,LENGTH
3850 Attempts to read LENGTH I<characters> of data into variable SCALAR
3851 from the specified FILEHANDLE. Returns the number of characters
3852 actually read, C<0> at end of file, or undef if there was an error (in
3853 the latter case C<$!> is also set). SCALAR will be grown or shrunk
3854 so that the last character actually read is the last character of the
3855 scalar after the read.
3857 An OFFSET may be specified to place the read data at some place in the
3858 string other than the beginning. A negative OFFSET specifies
3859 placement at that many characters counting backwards from the end of
3860 the string. A positive OFFSET greater than the length of SCALAR
3861 results in the string being padded to the required size with C<"\0">
3862 bytes before the result of the read is appended.
3864 The call is actually implemented in terms of either Perl's or system's
3865 fread() call. To get a true read(2) system call, see C<sysread>.
3867 Note the I<characters>: depending on the status of the filehandle,
3868 either (8-bit) bytes or characters are read. By default all
3869 filehandles operate on bytes, but for example if the filehandle has
3870 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3871 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3872 characters, not bytes. Similarly for the C<:encoding> pragma:
3873 in that case pretty much any characters can be read.
3875 =item readdir DIRHANDLE
3877 Returns the next directory entry for a directory opened by C<opendir>.
3878 If used in list context, returns all the rest of the entries in the
3879 directory. If there are no more entries, returns an undefined value in
3880 scalar context or a null list in list context.
3882 If you're planning to filetest the return values out of a C<readdir>, you'd
3883 better prepend the directory in question. Otherwise, because we didn't
3884 C<chdir> there, it would have been testing the wrong file.
3886 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3887 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3892 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3893 context, each call reads and returns the next line, until end-of-file is
3894 reached, whereupon the subsequent call returns undef. In list context,
3895 reads until end-of-file is reached and returns a list of lines. Note that
3896 the notion of "line" used here is however you may have defined it
3897 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3899 When C<$/> is set to C<undef>, when readline() is in scalar
3900 context (i.e. file slurp mode), and when an empty file is read, it
3901 returns C<''> the first time, followed by C<undef> subsequently.
3903 This is the internal function implementing the C<< <EXPR> >>
3904 operator, but you can use it directly. The C<< <EXPR> >>
3905 operator is discussed in more detail in L<perlop/"I/O Operators">.
3908 $line = readline(*STDIN); # same thing
3910 If readline encounters an operating system error, C<$!> will be set with the
3911 corresponding error message. It can be helpful to check C<$!> when you are
3912 reading from filehandles you don't trust, such as a tty or a socket. The
3913 following example uses the operator form of C<readline>, and takes the necessary
3914 steps to ensure that C<readline> was successful.
3918 unless (defined( $line = <> )) {
3929 Returns the value of a symbolic link, if symbolic links are
3930 implemented. If not, gives a fatal error. If there is some system
3931 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3932 omitted, uses C<$_>.
3936 EXPR is executed as a system command.
3937 The collected standard output of the command is returned.
3938 In scalar context, it comes back as a single (potentially
3939 multi-line) string. In list context, returns a list of lines
3940 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3941 This is the internal function implementing the C<qx/EXPR/>
3942 operator, but you can use it directly. The C<qx/EXPR/>
3943 operator is discussed in more detail in L<perlop/"I/O Operators">.
3945 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3947 Receives a message on a socket. Attempts to receive LENGTH characters
3948 of data into variable SCALAR from the specified SOCKET filehandle.
3949 SCALAR will be grown or shrunk to the length actually read. Takes the
3950 same flags as the system call of the same name. Returns the address
3951 of the sender if SOCKET's protocol supports this; returns an empty
3952 string otherwise. If there's an error, returns the undefined value.
3953 This call is actually implemented in terms of recvfrom(2) system call.
3954 See L<perlipc/"UDP: Message Passing"> for examples.
3956 Note the I<characters>: depending on the status of the socket, either
3957 (8-bit) bytes or characters are received. By default all sockets
3958 operate on bytes, but for example if the socket has been changed using
3959 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3960 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3961 characters, not bytes. Similarly for the C<:encoding> pragma:
3962 in that case pretty much any characters can be read.
3968 The C<redo> command restarts the loop block without evaluating the
3969 conditional again. The C<continue> block, if any, is not executed. If
3970 the LABEL is omitted, the command refers to the innermost enclosing
3971 loop. This command is normally used by programs that want to lie to
3972 themselves about what was just input:
3974 # a simpleminded Pascal comment stripper
3975 # (warning: assumes no { or } in strings)
3976 LINE: while (<STDIN>) {
3977 while (s|({.*}.*){.*}|$1 |) {}
3982 if (/}/) { # end of comment?
3991 C<redo> cannot be used to retry a block which returns a value such as
3992 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3993 a grep() or map() operation.
3995 Note that a block by itself is semantically identical to a loop
3996 that executes once. Thus C<redo> inside such a block will effectively
3997 turn it into a looping construct.
3999 See also L</continue> for an illustration of how C<last>, C<next>, and
4006 Returns a non-empty string if EXPR is a reference, the empty
4007 string otherwise. If EXPR
4008 is not specified, C<$_> will be used. The value returned depends on the
4009 type of thing the reference is a reference to.
4010 Builtin types include:
4020 If the referenced object has been blessed into a package, then that package
4021 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4023 if (ref($r) eq "HASH") {
4024 print "r is a reference to a hash.\n";
4027 print "r is not a reference at all.\n";
4029 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
4030 print "r is a reference to something that isa hash.\n";
4033 See also L<perlref>.
4035 =item rename OLDNAME,NEWNAME
4037 Changes the name of a file; an existing file NEWNAME will be
4038 clobbered. Returns true for success, false otherwise.
4040 Behavior of this function varies wildly depending on your system
4041 implementation. For example, it will usually not work across file system
4042 boundaries, even though the system I<mv> command sometimes compensates
4043 for this. Other restrictions include whether it works on directories,
4044 open files, or pre-existing files. Check L<perlport> and either the
4045 rename(2) manpage or equivalent system documentation for details.
4047 =item require VERSION
4053 Demands a version of Perl specified by VERSION, or demands some semantics
4054 specified by EXPR or by C<$_> if EXPR is not supplied.
4056 VERSION may be either a numeric argument such as 5.006, which will be
4057 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4058 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4059 VERSION is greater than the version of the current Perl interpreter.
4060 Compare with L</use>, which can do a similar check at compile time.
4062 Specifying VERSION as a literal of the form v5.6.1 should generally be
4063 avoided, because it leads to misleading error messages under earlier
4064 versions of Perl which do not support this syntax. The equivalent numeric
4065 version should be used instead.
4067 require v5.6.1; # run time version check
4068 require 5.6.1; # ditto
4069 require 5.006_001; # ditto; preferred for backwards compatibility
4071 Otherwise, demands that a library file be included if it hasn't already
4072 been included. The file is included via the do-FILE mechanism, which is
4073 essentially just a variety of C<eval>. Has semantics similar to the
4074 following subroutine:
4077 my ($filename) = @_;
4078 if (exists $INC{$filename}) {
4079 return 1 if $INC{$filename};
4080 die "Compilation failed in require";
4082 my ($realfilename,$result);
4084 foreach $prefix (@INC) {
4085 $realfilename = "$prefix/$filename";
4086 if (-f $realfilename) {
4087 $INC{$filename} = $realfilename;
4088 $result = do $realfilename;
4092 die "Can't find $filename in \@INC";
4095 $INC{$filename} = undef;
4097 } elsif (!$result) {
4098 delete $INC{$filename};
4099 die "$filename did not return true value";
4105 Note that the file will not be included twice under the same specified
4108 The file must return true as the last statement to indicate
4109 successful execution of any initialization code, so it's customary to
4110 end such a file with C<1;> unless you're sure it'll return true
4111 otherwise. But it's better just to put the C<1;>, in case you add more
4114 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4115 replaces "F<::>" with "F</>" in the filename for you,
4116 to make it easy to load standard modules. This form of loading of
4117 modules does not risk altering your namespace.
4119 In other words, if you try this:
4121 require Foo::Bar; # a splendid bareword
4123 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4124 directories specified in the C<@INC> array.
4126 But if you try this:
4128 $class = 'Foo::Bar';
4129 require $class; # $class is not a bareword
4131 require "Foo::Bar"; # not a bareword because of the ""
4133 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4134 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4136 eval "require $class";
4138 Now that you understand how C<require> looks for files in the case of
4139 a bareword argument, there is a little extra functionality going on
4140 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4141 it will first look for a filename with a "F<.pmc>" extension. A file
4142 with this extension is assumed to be Perl bytecode generated by
4143 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4144 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4145 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4147 You can also insert hooks into the import facility, by putting directly
4148 Perl code into the @INC array. There are three forms of hooks: subroutine
4149 references, array references and blessed objects.
4151 Subroutine references are the simplest case. When the inclusion system
4152 walks through @INC and encounters a subroutine, this subroutine gets
4153 called with two parameters, the first being a reference to itself, and the
4154 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4155 subroutine should return C<undef> or a filehandle, from which the file to
4156 include will be read. If C<undef> is returned, C<require> will look at
4157 the remaining elements of @INC.
4159 If the hook is an array reference, its first element must be a subroutine
4160 reference. This subroutine is called as above, but the first parameter is
4161 the array reference. This enables to pass indirectly some arguments to
4164 In other words, you can write:
4166 push @INC, \&my_sub;
4168 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4174 push @INC, [ \&my_sub, $x, $y, ... ];
4176 my ($arrayref, $filename) = @_;
4177 # Retrieve $x, $y, ...
4178 my @parameters = @$arrayref[1..$#$arrayref];
4182 If the hook is an object, it must provide an INC method, that will be
4183 called as above, the first parameter being the object itself. (Note that
4184 you must fully qualify the sub's name, as it is always forced into package
4185 C<main>.) Here is a typical code layout:
4191 my ($self, $filename) = @_;
4195 # In the main program
4196 push @INC, new Foo(...);
4198 Note that these hooks are also permitted to set the %INC entry
4199 corresponding to the files they have loaded. See L<perlvar/%INC>.
4201 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4207 Generally used in a C<continue> block at the end of a loop to clear
4208 variables and reset C<??> searches so that they work again. The
4209 expression is interpreted as a list of single characters (hyphens
4210 allowed for ranges). All variables and arrays beginning with one of
4211 those letters are reset to their pristine state. If the expression is
4212 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4213 only variables or searches in the current package. Always returns
4216 reset 'X'; # reset all X variables
4217 reset 'a-z'; # reset lower case variables
4218 reset; # just reset ?one-time? searches
4220 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4221 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4222 variables--lexical variables are unaffected, but they clean themselves
4223 up on scope exit anyway, so you'll probably want to use them instead.
4230 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4231 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4232 context, depending on how the return value will be used, and the context
4233 may vary from one execution to the next (see C<wantarray>). If no EXPR
4234 is given, returns an empty list in list context, the undefined value in
4235 scalar context, and (of course) nothing at all in a void context.
4237 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4238 or do FILE will automatically return the value of the last expression
4243 In list context, returns a list value consisting of the elements
4244 of LIST in the opposite order. In scalar context, concatenates the
4245 elements of LIST and returns a string value with all characters
4246 in the opposite order.
4248 print reverse <>; # line tac, last line first
4250 undef $/; # for efficiency of <>
4251 print scalar reverse <>; # character tac, last line tsrif
4253 This operator is also handy for inverting a hash, although there are some
4254 caveats. If a value is duplicated in the original hash, only one of those
4255 can be represented as a key in the inverted hash. Also, this has to
4256 unwind one hash and build a whole new one, which may take some time
4257 on a large hash, such as from a DBM file.
4259 %by_name = reverse %by_address; # Invert the hash
4261 =item rewinddir DIRHANDLE
4263 Sets the current position to the beginning of the directory for the
4264 C<readdir> routine on DIRHANDLE.
4266 =item rindex STR,SUBSTR,POSITION
4268 =item rindex STR,SUBSTR
4270 Works just like index() except that it returns the position of the LAST
4271 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4272 last occurrence at or before that position.
4274 =item rmdir FILENAME
4278 Deletes the directory specified by FILENAME if that directory is
4279 empty. If it succeeds it returns true, otherwise it returns false and
4280 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4284 The substitution operator. See L<perlop>.
4288 Forces EXPR to be interpreted in scalar context and returns the value
4291 @counts = ( scalar @a, scalar @b, scalar @c );
4293 There is no equivalent operator to force an expression to
4294 be interpolated in list context because in practice, this is never
4295 needed. If you really wanted to do so, however, you could use
4296 the construction C<@{[ (some expression) ]}>, but usually a simple
4297 C<(some expression)> suffices.
4299 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4300 parenthesized list, this behaves as a scalar comma expression, evaluating
4301 all but the last element in void context and returning the final element
4302 evaluated in scalar context. This is seldom what you want.
4304 The following single statement:
4306 print uc(scalar(&foo,$bar)),$baz;
4308 is the moral equivalent of these two:
4311 print(uc($bar),$baz);
4313 See L<perlop> for more details on unary operators and the comma operator.
4315 =item seek FILEHANDLE,POSITION,WHENCE
4317 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4318 FILEHANDLE may be an expression whose value gives the name of the
4319 filehandle. The values for WHENCE are C<0> to set the new position
4320 I<in bytes> to POSITION, C<1> to set it to the current position plus
4321 POSITION, and C<2> to set it to EOF plus POSITION (typically
4322 negative). For WHENCE you may use the constants C<SEEK_SET>,
4323 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4324 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4327 Note the I<in bytes>: even if the filehandle has been set to
4328 operate on characters (for example by using the C<:utf8> open
4329 layer), tell() will return byte offsets, not character offsets
4330 (because implementing that would render seek() and tell() rather slow).
4332 If you want to position file for C<sysread> or C<syswrite>, don't use
4333 C<seek>--buffering makes its effect on the file's system position
4334 unpredictable and non-portable. Use C<sysseek> instead.
4336 Due to the rules and rigors of ANSI C, on some systems you have to do a
4337 seek whenever you switch between reading and writing. Amongst other
4338 things, this may have the effect of calling stdio's clearerr(3).
4339 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4343 This is also useful for applications emulating C<tail -f>. Once you hit
4344 EOF on your read, and then sleep for a while, you might have to stick in a
4345 seek() to reset things. The C<seek> doesn't change the current position,
4346 but it I<does> clear the end-of-file condition on the handle, so that the
4347 next C<< <FILE> >> makes Perl try again to read something. We hope.
4349 If that doesn't work (some IO implementations are particularly
4350 cantankerous), then you may need something more like this:
4353 for ($curpos = tell(FILE); $_ = <FILE>;
4354 $curpos = tell(FILE)) {
4355 # search for some stuff and put it into files
4357 sleep($for_a_while);
4358 seek(FILE, $curpos, 0);
4361 =item seekdir DIRHANDLE,POS
4363 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4364 must be a value returned by C<telldir>. Has the same caveats about
4365 possible directory compaction as the corresponding system library
4368 =item select FILEHANDLE
4372 Returns the currently selected filehandle. Sets the current default
4373 filehandle for output, if FILEHANDLE is supplied. This has two
4374 effects: first, a C<write> or a C<print> without a filehandle will
4375 default to this FILEHANDLE. Second, references to variables related to
4376 output will refer to this output channel. For example, if you have to
4377 set the top of form format for more than one output channel, you might
4385 FILEHANDLE may be an expression whose value gives the name of the
4386 actual filehandle. Thus:
4388 $oldfh = select(STDERR); $| = 1; select($oldfh);
4390 Some programmers may prefer to think of filehandles as objects with
4391 methods, preferring to write the last example as:
4394 STDERR->autoflush(1);
4396 =item select RBITS,WBITS,EBITS,TIMEOUT
4398 This calls the select(2) system call with the bit masks specified, which
4399 can be constructed using C<fileno> and C<vec>, along these lines:
4401 $rin = $win = $ein = '';
4402 vec($rin,fileno(STDIN),1) = 1;
4403 vec($win,fileno(STDOUT),1) = 1;
4406 If you want to select on many filehandles you might wish to write a
4410 my(@fhlist) = split(' ',$_[0]);
4413 vec($bits,fileno($_),1) = 1;
4417 $rin = fhbits('STDIN TTY SOCK');
4421 ($nfound,$timeleft) =
4422 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4424 or to block until something becomes ready just do this
4426 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4428 Most systems do not bother to return anything useful in $timeleft, so
4429 calling select() in scalar context just returns $nfound.
4431 Any of the bit masks can also be undef. The timeout, if specified, is
4432 in seconds, which may be fractional. Note: not all implementations are
4433 capable of returning the $timeleft. If not, they always return
4434 $timeleft equal to the supplied $timeout.
4436 You can effect a sleep of 250 milliseconds this way:
4438 select(undef, undef, undef, 0.25);
4440 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4441 is implementation-dependent.
4443 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4444 or <FH>) with C<select>, except as permitted by POSIX, and even
4445 then only on POSIX systems. You have to use C<sysread> instead.
4447 =item semctl ID,SEMNUM,CMD,ARG
4449 Calls the System V IPC function C<semctl>. You'll probably have to say
4453 first to get the correct constant definitions. If CMD is IPC_STAT or
4454 GETALL, then ARG must be a variable which will hold the returned
4455 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4456 the undefined value for error, "C<0 but true>" for zero, or the actual
4457 return value otherwise. The ARG must consist of a vector of native
4458 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4459 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4462 =item semget KEY,NSEMS,FLAGS
4464 Calls the System V IPC function semget. Returns the semaphore id, or
4465 the undefined value if there is an error. See also
4466 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4469 =item semop KEY,OPSTRING
4471 Calls the System V IPC function semop to perform semaphore operations
4472 such as signalling and waiting. OPSTRING must be a packed array of
4473 semop structures. Each semop structure can be generated with
4474 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4475 operations is implied by the length of OPSTRING. Returns true if
4476 successful, or false if there is an error. As an example, the
4477 following code waits on semaphore $semnum of semaphore id $semid:
4479 $semop = pack("s!3", $semnum, -1, 0);
4480 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4482 To signal the semaphore, replace C<-1> with C<1>. See also
4483 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4486 =item send SOCKET,MSG,FLAGS,TO
4488 =item send SOCKET,MSG,FLAGS
4490 Sends a message on a socket. Attempts to send the scalar MSG to the
4491 SOCKET filehandle. Takes the same flags as the system call of the
4492 same name. On unconnected sockets you must specify a destination to
4493 send TO, in which case it does a C C<sendto>. Returns the number of
4494 characters sent, or the undefined value if there is an error. The C
4495 system call sendmsg(2) is currently unimplemented. See
4496 L<perlipc/"UDP: Message Passing"> for examples.
4498 Note the I<characters>: depending on the status of the socket, either
4499 (8-bit) bytes or characters are sent. By default all sockets operate
4500 on bytes, but for example if the socket has been changed using
4501 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4502 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4503 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4504 in that case pretty much any characters can be sent.
4506 =item setpgrp PID,PGRP
4508 Sets the current process group for the specified PID, C<0> for the current
4509 process. Will produce a fatal error if used on a machine that doesn't
4510 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4511 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4512 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4515 =item setpriority WHICH,WHO,PRIORITY
4517 Sets the current priority for a process, a process group, or a user.
4518 (See setpriority(2).) Will produce a fatal error if used on a machine
4519 that doesn't implement setpriority(2).
4521 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4523 Sets the socket option requested. Returns undefined if there is an
4524 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4531 Shifts the first value of the array off and returns it, shortening the
4532 array by 1 and moving everything down. If there are no elements in the
4533 array, returns the undefined value. If ARRAY is omitted, shifts the
4534 C<@_> array within the lexical scope of subroutines and formats, and the
4535 C<@ARGV> array at file scopes or within the lexical scopes established by
4536 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4539 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4540 same thing to the left end of an array that C<pop> and C<push> do to the
4543 =item shmctl ID,CMD,ARG
4545 Calls the System V IPC function shmctl. You'll probably have to say
4549 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4550 then ARG must be a variable which will hold the returned C<shmid_ds>
4551 structure. Returns like ioctl: the undefined value for error, "C<0> but
4552 true" for zero, or the actual return value otherwise.
4553 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4555 =item shmget KEY,SIZE,FLAGS
4557 Calls the System V IPC function shmget. Returns the shared memory
4558 segment id, or the undefined value if there is an error.
4559 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4561 =item shmread ID,VAR,POS,SIZE
4563 =item shmwrite ID,STRING,POS,SIZE
4565 Reads or writes the System V shared memory segment ID starting at
4566 position POS for size SIZE by attaching to it, copying in/out, and
4567 detaching from it. When reading, VAR must be a variable that will
4568 hold the data read. When writing, if STRING is too long, only SIZE
4569 bytes are used; if STRING is too short, nulls are written to fill out
4570 SIZE bytes. Return true if successful, or false if there is an error.
4571 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4572 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4574 =item shutdown SOCKET,HOW
4576 Shuts down a socket connection in the manner indicated by HOW, which
4577 has the same interpretation as in the system call of the same name.
4579 shutdown(SOCKET, 0); # I/we have stopped reading data
4580 shutdown(SOCKET, 1); # I/we have stopped writing data
4581 shutdown(SOCKET, 2); # I/we have stopped using this socket
4583 This is useful with sockets when you want to tell the other
4584 side you're done writing but not done reading, or vice versa.
4585 It's also a more insistent form of close because it also
4586 disables the file descriptor in any forked copies in other
4593 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4594 returns sine of C<$_>.
4596 For the inverse sine operation, you may use the C<Math::Trig::asin>
4597 function, or use this relation:
4599 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4605 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4606 May be interrupted if the process receives a signal such as C<SIGALRM>.
4607 Returns the number of seconds actually slept. You probably cannot
4608 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4611 On some older systems, it may sleep up to a full second less than what
4612 you requested, depending on how it counts seconds. Most modern systems
4613 always sleep the full amount. They may appear to sleep longer than that,
4614 however, because your process might not be scheduled right away in a
4615 busy multitasking system.
4617 For delays of finer granularity than one second, you may use Perl's
4618 C<syscall> interface to access setitimer(2) if your system supports
4619 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4620 and starting from Perl 5.8 part of the standard distribution) may also
4623 See also the POSIX module's C<pause> function.
4625 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4627 Opens a socket of the specified kind and attaches it to filehandle
4628 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4629 the system call of the same name. You should C<use Socket> first
4630 to get the proper definitions imported. See the examples in
4631 L<perlipc/"Sockets: Client/Server Communication">.
4633 On systems that support a close-on-exec flag on files, the flag will
4634 be set for the newly opened file descriptor, as determined by the
4635 value of $^F. See L<perlvar/$^F>.
4637 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4639 Creates an unnamed pair of sockets in the specified domain, of the
4640 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4641 for the system call of the same name. If unimplemented, yields a fatal
4642 error. Returns true if successful.
4644 On systems that support a close-on-exec flag on files, the flag will
4645 be set for the newly opened file descriptors, as determined by the value
4646 of $^F. See L<perlvar/$^F>.
4648 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4649 to C<pipe(Rdr, Wtr)> is essentially:
4652 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4653 shutdown(Rdr, 1); # no more writing for reader
4654 shutdown(Wtr, 0); # no more reading for writer
4656 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4657 emulate socketpair using IP sockets to localhost if your system implements
4658 sockets but not socketpair.
4660 =item sort SUBNAME LIST
4662 =item sort BLOCK LIST
4666 In list context, this sorts the LIST and returns the sorted list value.
4667 In scalar context, the behaviour of C<sort()> is undefined.
4669 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4670 order. If SUBNAME is specified, it gives the name of a subroutine
4671 that returns an integer less than, equal to, or greater than C<0>,
4672 depending on how the elements of the list are to be ordered. (The C<<
4673 <=> >> and C<cmp> operators are extremely useful in such routines.)
4674 SUBNAME may be a scalar variable name (unsubscripted), in which case
4675 the value provides the name of (or a reference to) the actual
4676 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4677 an anonymous, in-line sort subroutine.
4679 If the subroutine's prototype is C<($$)>, the elements to be compared
4680 are passed by reference in C<@_>, as for a normal subroutine. This is
4681 slower than unprototyped subroutines, where the elements to be
4682 compared are passed into the subroutine
4683 as the package global variables $a and $b (see example below). Note that
4684 in the latter case, it is usually counter-productive to declare $a and
4687 In either case, the subroutine may not be recursive. The values to be
4688 compared are always passed by reference, so don't modify them.
4690 You also cannot exit out of the sort block or subroutine using any of the
4691 loop control operators described in L<perlsyn> or with C<goto>.
4693 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4694 current collation locale. See L<perllocale>.
4696 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4697 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4698 preserves the input order of elements that compare equal. Although
4699 quicksort's run time is O(NlogN) when averaged over all arrays of
4700 length N, the time can be O(N**2), I<quadratic> behavior, for some
4701 inputs.) In 5.7, the quicksort implementation was replaced with
4702 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4703 But benchmarks indicated that for some inputs, on some platforms,
4704 the original quicksort was faster. 5.8 has a sort pragma for
4705 limited control of the sort. Its rather blunt control of the
4706 underlying algorithm may not persist into future perls, but the
4707 ability to characterize the input or output in implementation
4708 independent ways quite probably will. See L<sort>.
4713 @articles = sort @files;
4715 # same thing, but with explicit sort routine
4716 @articles = sort {$a cmp $b} @files;
4718 # now case-insensitively
4719 @articles = sort {uc($a) cmp uc($b)} @files;
4721 # same thing in reversed order
4722 @articles = sort {$b cmp $a} @files;
4724 # sort numerically ascending
4725 @articles = sort {$a <=> $b} @files;
4727 # sort numerically descending
4728 @articles = sort {$b <=> $a} @files;
4730 # this sorts the %age hash by value instead of key
4731 # using an in-line function
4732 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4734 # sort using explicit subroutine name
4736 $age{$a} <=> $age{$b}; # presuming numeric
4738 @sortedclass = sort byage @class;
4740 sub backwards { $b cmp $a }
4741 @harry = qw(dog cat x Cain Abel);
4742 @george = qw(gone chased yz Punished Axed);
4744 # prints AbelCaincatdogx
4745 print sort backwards @harry;
4746 # prints xdogcatCainAbel
4747 print sort @george, 'to', @harry;
4748 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4750 # inefficiently sort by descending numeric compare using
4751 # the first integer after the first = sign, or the
4752 # whole record case-insensitively otherwise
4755 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4760 # same thing, but much more efficiently;
4761 # we'll build auxiliary indices instead
4765 push @nums, /=(\d+)/;
4770 $nums[$b] <=> $nums[$a]
4772 $caps[$a] cmp $caps[$b]
4776 # same thing, but without any temps
4777 @new = map { $_->[0] }
4778 sort { $b->[1] <=> $a->[1]
4781 } map { [$_, /=(\d+)/, uc($_)] } @old;
4783 # using a prototype allows you to use any comparison subroutine
4784 # as a sort subroutine (including other package's subroutines)
4786 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4789 @new = sort other::backwards @old;
4791 # guarantee stability, regardless of algorithm
4793 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4795 # force use of mergesort (not portable outside Perl 5.8)
4796 use sort '_mergesort'; # note discouraging _
4797 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4799 If you're using strict, you I<must not> declare $a
4800 and $b as lexicals. They are package globals. That means
4801 if you're in the C<main> package and type
4803 @articles = sort {$b <=> $a} @files;
4805 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4806 but if you're in the C<FooPack> package, it's the same as typing
4808 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4810 The comparison function is required to behave. If it returns
4811 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4812 sometimes saying the opposite, for example) the results are not
4815 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
4816 (not-a-number), and because C<sort> will trigger a fatal error unless the
4817 result of a comparison is defined, when sorting with a comparison function
4818 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
4819 The following example takes advantage of the fact that C<NaN != NaN> to
4820 eliminate any C<NaN>s from the input.
4822 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
4824 =item splice ARRAY,OFFSET,LENGTH,LIST
4826 =item splice ARRAY,OFFSET,LENGTH
4828 =item splice ARRAY,OFFSET
4832 Removes the elements designated by OFFSET and LENGTH from an array, and
4833 replaces them with the elements of LIST, if any. In list context,
4834 returns the elements removed from the array. In scalar context,
4835 returns the last element removed, or C<undef> if no elements are
4836 removed. The array grows or shrinks as necessary.
4837 If OFFSET is negative then it starts that far from the end of the array.
4838 If LENGTH is omitted, removes everything from OFFSET onward.
4839 If LENGTH is negative, removes the elements from OFFSET onward
4840 except for -LENGTH elements at the end of the array.
4841 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4842 past the end of the array, perl issues a warning, and splices at the
4845 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4847 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4848 pop(@a) splice(@a,-1)
4849 shift(@a) splice(@a,0,1)
4850 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4851 $a[$i] = $y splice(@a,$i,1,$y)
4853 Example, assuming array lengths are passed before arrays:
4855 sub aeq { # compare two list values
4856 my(@a) = splice(@_,0,shift);
4857 my(@b) = splice(@_,0,shift);
4858 return 0 unless @a == @b; # same len?
4860 return 0 if pop(@a) ne pop(@b);
4864 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4866 =item split /PATTERN/,EXPR,LIMIT
4868 =item split /PATTERN/,EXPR
4870 =item split /PATTERN/
4874 Splits a string into a list of strings and returns that list. By default,
4875 empty leading fields are preserved, and empty trailing ones are deleted.
4877 In scalar context, returns the number of fields found and splits into
4878 the C<@_> array. Use of split in scalar context is deprecated, however,
4879 because it clobbers your subroutine arguments.
4881 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4882 splits on whitespace (after skipping any leading whitespace). Anything
4883 matching PATTERN is taken to be a delimiter separating the fields. (Note
4884 that the delimiter may be longer than one character.)
4886 If LIMIT is specified and positive, it represents the maximum number
4887 of fields the EXPR will be split into, though the actual number of
4888 fields returned depends on the number of times PATTERN matches within
4889 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4890 stripped (which potential users of C<pop> would do well to remember).
4891 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4892 had been specified. Note that splitting an EXPR that evaluates to the
4893 empty string always returns the empty list, regardless of the LIMIT
4896 A pattern matching the null string (not to be confused with
4897 a null pattern C<//>, which is just one member of the set of patterns
4898 matching a null string) will split the value of EXPR into separate
4899 characters at each point it matches that way. For example:
4901 print join(':', split(/ */, 'hi there'));
4903 produces the output 'h:i:t:h:e:r:e'.
4905 Using the empty pattern C<//> specifically matches the null string, and is
4906 not be confused with the use of C<//> to mean "the last successful pattern
4909 Empty leading (or trailing) fields are produced when there are positive width
4910 matches at the beginning (or end) of the string; a zero-width match at the
4911 beginning (or end) of the string does not produce an empty field. For
4914 print join(':', split(/(?=\w)/, 'hi there!'));
4916 produces the output 'h:i :t:h:e:r:e!'.
4918 The LIMIT parameter can be used to split a line partially
4920 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4922 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
4923 a LIMIT one larger than the number of variables in the list, to avoid
4924 unnecessary work. For the list above LIMIT would have been 4 by
4925 default. In time critical applications it behooves you not to split
4926 into more fields than you really need.
4928 If the PATTERN contains parentheses, additional list elements are
4929 created from each matching substring in the delimiter.
4931 split(/([,-])/, "1-10,20", 3);
4933 produces the list value
4935 (1, '-', 10, ',', 20)
4937 If you had the entire header of a normal Unix email message in $header,
4938 you could split it up into fields and their values this way:
4940 $header =~ s/\n\s+/ /g; # fix continuation lines
4941 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4943 The pattern C</PATTERN/> may be replaced with an expression to specify
4944 patterns that vary at runtime. (To do runtime compilation only once,
4945 use C</$variable/o>.)
4947 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4948 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4949 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4950 will give you as many null initial fields as there are leading spaces.
4951 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4952 whitespace produces a null first field. A C<split> with no arguments
4953 really does a S<C<split(' ', $_)>> internally.
4955 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4960 open(PASSWD, '/etc/passwd');
4963 ($login, $passwd, $uid, $gid,
4964 $gcos, $home, $shell) = split(/:/);
4968 As with regular pattern matching, any capturing parentheses that are not
4969 matched in a C<split()> will be set to C<undef> when returned:
4971 @fields = split /(A)|B/, "1A2B3";
4972 # @fields is (1, 'A', 2, undef, 3)
4974 =item sprintf FORMAT, LIST
4976 Returns a string formatted by the usual C<printf> conventions of the C
4977 library function C<sprintf>. See below for more details
4978 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4979 the general principles.
4983 # Format number with up to 8 leading zeroes
4984 $result = sprintf("%08d", $number);
4986 # Round number to 3 digits after decimal point
4987 $rounded = sprintf("%.3f", $number);
4989 Perl does its own C<sprintf> formatting--it emulates the C
4990 function C<sprintf>, but it doesn't use it (except for floating-point
4991 numbers, and even then only the standard modifiers are allowed). As a
4992 result, any non-standard extensions in your local C<sprintf> are not
4993 available from Perl.
4995 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4996 pass it an array as your first argument. The array is given scalar context,
4997 and instead of using the 0th element of the array as the format, Perl will
4998 use the count of elements in the array as the format, which is almost never
5001 Perl's C<sprintf> permits the following universally-known conversions:
5004 %c a character with the given number
5006 %d a signed integer, in decimal
5007 %u an unsigned integer, in decimal
5008 %o an unsigned integer, in octal
5009 %x an unsigned integer, in hexadecimal
5010 %e a floating-point number, in scientific notation
5011 %f a floating-point number, in fixed decimal notation
5012 %g a floating-point number, in %e or %f notation
5014 In addition, Perl permits the following widely-supported conversions:
5016 %X like %x, but using upper-case letters
5017 %E like %e, but using an upper-case "E"
5018 %G like %g, but with an upper-case "E" (if applicable)
5019 %b an unsigned integer, in binary
5020 %p a pointer (outputs the Perl value's address in hexadecimal)
5021 %n special: *stores* the number of characters output so far
5022 into the next variable in the parameter list
5024 Finally, for backward (and we do mean "backward") compatibility, Perl
5025 permits these unnecessary but widely-supported conversions:
5028 %D a synonym for %ld
5029 %U a synonym for %lu
5030 %O a synonym for %lo
5033 Note that the number of exponent digits in the scientific notation produced
5034 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5035 exponent less than 100 is system-dependent: it may be three or less
5036 (zero-padded as necessary). In other words, 1.23 times ten to the
5037 99th may be either "1.23e99" or "1.23e099".
5039 Between the C<%> and the format letter, you may specify a number of
5040 additional attributes controlling the interpretation of the format.
5041 In order, these are:
5045 =item format parameter index
5047 An explicit format parameter index, such as C<2$>. By default sprintf
5048 will format the next unused argument in the list, but this allows you
5049 to take the arguments out of order. Eg:
5051 printf '%2$d %1$d', 12, 34; # prints "34 12"
5052 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5057 space prefix positive number with a space
5058 + prefix positive number with a plus sign
5059 - left-justify within the field
5060 0 use zeros, not spaces, to right-justify
5061 # prefix non-zero octal with "0", non-zero hex with "0x",
5062 non-zero binary with "0b"
5066 printf '<% d>', 12; # prints "< 12>"
5067 printf '<%+d>', 12; # prints "<+12>"
5068 printf '<%6s>', 12; # prints "< 12>"
5069 printf '<%-6s>', 12; # prints "<12 >"
5070 printf '<%06s>', 12; # prints "<000012>"
5071 printf '<%#x>', 12; # prints "<0xc>"
5075 The vector flag C<v>, optionally specifying the join string to use.
5076 This flag tells perl to interpret the supplied string as a vector
5077 of integers, one for each character in the string, separated by
5078 a given string (a dot C<.> by default). This can be useful for
5079 displaying ordinal values of characters in arbitrary strings:
5081 printf "version is v%vd\n", $^V; # Perl's version
5083 Put an asterisk C<*> before the C<v> to override the string to
5084 use to separate the numbers:
5086 printf "address is %*vX\n", ":", $addr; # IPv6 address
5087 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5089 You can also explicitly specify the argument number to use for
5090 the join string using eg C<*2$v>:
5092 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5094 =item (minimum) width
5096 Arguments are usually formatted to be only as wide as required to
5097 display the given value. You can override the width by putting
5098 a number here, or get the width from the next argument (with C<*>)
5099 or from a specified argument (with eg C<*2$>):
5101 printf '<%s>', "a"; # prints "<a>"
5102 printf '<%6s>', "a"; # prints "< a>"
5103 printf '<%*s>', 6, "a"; # prints "< a>"
5104 printf '<%*2$s>', "a", 6; # prints "< a>"
5105 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5107 If a field width obtained through C<*> is negative, it has the same
5108 effect as the C<-> flag: left-justification.
5110 =item precision, or maximum width
5112 You can specify a precision (for numeric conversions) or a maximum
5113 width (for string conversions) by specifying a C<.> followed by a number.
5114 For floating point formats, with the exception of 'g' and 'G', this specifies
5115 the number of decimal places to show (the default being 6), eg:
5117 # these examples are subject to system-specific variation
5118 printf '<%f>', 1; # prints "<1.000000>"
5119 printf '<%.1f>', 1; # prints "<1.0>"
5120 printf '<%.0f>', 1; # prints "<1>"
5121 printf '<%e>', 10; # prints "<1.000000e+01>"
5122 printf '<%.1e>', 10; # prints "<1.0e+01>"
5124 For 'g' and 'G', this specifies the maximum number of digits to show,
5125 including prior to the decimal point as well as after it, eg:
5127 # these examples are subject to system-specific variation
5128 printf '<%g>', 1; # prints "<1>"
5129 printf '<%.10g>', 1; # prints "<1>"
5130 printf '<%g>', 100; # prints "<100>"
5131 printf '<%.1g>', 100; # prints "<1e+02>"
5132 printf '<%.2g>', 100.01; # prints "<1e+02>"
5133 printf '<%.5g>', 100.01; # prints "<100.01>"
5134 printf '<%.4g>', 100.01; # prints "<100>"
5136 For integer conversions, specifying a precision implies that the
5137 output of the number itself should be zero-padded to this width:
5139 printf '<%.6x>', 1; # prints "<000001>"
5140 printf '<%#.6x>', 1; # prints "<0x000001>"
5141 printf '<%-10.6x>', 1; # prints "<000001 >"
5143 For string conversions, specifying a precision truncates the string
5144 to fit in the specified width:
5146 printf '<%.5s>', "truncated"; # prints "<trunc>"
5147 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5149 You can also get the precision from the next argument using C<.*>:
5151 printf '<%.6x>', 1; # prints "<000001>"
5152 printf '<%.*x>', 6, 1; # prints "<000001>"
5154 You cannot currently get the precision from a specified number,
5155 but it is intended that this will be possible in the future using
5158 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5162 For numeric conversions, you can specify the size to interpret the
5163 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5164 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5165 whatever the default integer size is on your platform (usually 32 or 64
5166 bits), but you can override this to use instead one of the standard C types,
5167 as supported by the compiler used to build Perl:
5169 l interpret integer as C type "long" or "unsigned long"
5170 h interpret integer as C type "short" or "unsigned short"
5171 q, L or ll interpret integer as C type "long long", "unsigned long long".
5172 or "quads" (typically 64-bit integers)
5174 The last will produce errors if Perl does not understand "quads" in your
5175 installation. (This requires that either the platform natively supports quads
5176 or Perl was specifically compiled to support quads.) You can find out
5177 whether your Perl supports quads via L<Config>:
5180 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5183 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5184 to be the default floating point size on your platform (double or long double),
5185 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5186 platform supports them. You can find out whether your Perl supports long
5187 doubles via L<Config>:
5190 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5192 You can find out whether Perl considers 'long double' to be the default
5193 floating point size to use on your platform via L<Config>:
5196 ($Config{uselongdouble} eq 'define') &&
5197 print "long doubles by default\n";
5199 It can also be the case that long doubles and doubles are the same thing:
5202 ($Config{doublesize} == $Config{longdblsize}) &&
5203 print "doubles are long doubles\n";
5205 The size specifier C<V> has no effect for Perl code, but it is supported
5206 for compatibility with XS code; it means 'use the standard size for
5207 a Perl integer (or floating-point number)', which is already the
5208 default for Perl code.
5210 =item order of arguments
5212 Normally, sprintf takes the next unused argument as the value to
5213 format for each format specification. If the format specification
5214 uses C<*> to require additional arguments, these are consumed from
5215 the argument list in the order in which they appear in the format
5216 specification I<before> the value to format. Where an argument is
5217 specified using an explicit index, this does not affect the normal
5218 order for the arguments (even when the explicitly specified index
5219 would have been the next argument in any case).
5223 printf '<%*.*s>', $a, $b, $c;
5225 would use C<$a> for the width, C<$b> for the precision and C<$c>
5226 as the value to format, while:
5228 print '<%*1$.*s>', $a, $b;
5230 would use C<$a> for the width and the precision, and C<$b> as the
5233 Here are some more examples - beware that when using an explicit
5234 index, the C<$> may need to be escaped:
5236 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5237 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5238 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5239 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5243 If C<use locale> is in effect, the character used for the decimal
5244 point in formatted real numbers is affected by the LC_NUMERIC locale.
5251 Return the square root of EXPR. If EXPR is omitted, returns square
5252 root of C<$_>. Only works on non-negative operands, unless you've
5253 loaded the standard Math::Complex module.
5256 print sqrt(-2); # prints 1.4142135623731i
5262 Sets the random number seed for the C<rand> operator.
5264 The point of the function is to "seed" the C<rand> function so that
5265 C<rand> can produce a different sequence each time you run your
5268 If srand() is not called explicitly, it is called implicitly at the
5269 first use of the C<rand> operator. However, this was not the case in
5270 versions of Perl before 5.004, so if your script will run under older
5271 Perl versions, it should call C<srand>.
5273 Most programs won't even call srand() at all, except those that
5274 need a cryptographically-strong starting point rather than the
5275 generally acceptable default, which is based on time of day,
5276 process ID, and memory allocation, or the F</dev/urandom> device,
5279 You can call srand($seed) with the same $seed to reproduce the
5280 I<same> sequence from rand(), but this is usually reserved for
5281 generating predictable results for testing or debugging.
5282 Otherwise, don't call srand() more than once in your program.
5284 Do B<not> call srand() (i.e. without an argument) more than once in
5285 a script. The internal state of the random number generator should
5286 contain more entropy than can be provided by any seed, so calling
5287 srand() again actually I<loses> randomness.
5289 Most implementations of C<srand> take an integer and will silently
5290 truncate decimal numbers. This means C<srand(42)> will usually
5291 produce the same results as C<srand(42.1)>. To be safe, always pass
5292 C<srand> an integer.
5294 In versions of Perl prior to 5.004 the default seed was just the
5295 current C<time>. This isn't a particularly good seed, so many old
5296 programs supply their own seed value (often C<time ^ $$> or C<time ^
5297 ($$ + ($$ << 15))>), but that isn't necessary any more.
5299 Note that you need something much more random than the default seed for
5300 cryptographic purposes. Checksumming the compressed output of one or more
5301 rapidly changing operating system status programs is the usual method. For
5304 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5306 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5309 Frequently called programs (like CGI scripts) that simply use
5313 for a seed can fall prey to the mathematical property that
5317 one-third of the time. So don't do that.
5319 =item stat FILEHANDLE
5325 Returns a 13-element list giving the status info for a file, either
5326 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5327 it stats C<$_>. Returns a null list if the stat fails. Typically used
5330 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5331 $atime,$mtime,$ctime,$blksize,$blocks)
5334 Not all fields are supported on all filesystem types. Here are the
5335 meaning of the fields:
5337 0 dev device number of filesystem
5339 2 mode file mode (type and permissions)
5340 3 nlink number of (hard) links to the file
5341 4 uid numeric user ID of file's owner
5342 5 gid numeric group ID of file's owner
5343 6 rdev the device identifier (special files only)
5344 7 size total size of file, in bytes
5345 8 atime last access time in seconds since the epoch
5346 9 mtime last modify time in seconds since the epoch
5347 10 ctime inode change time in seconds since the epoch (*)
5348 11 blksize preferred block size for file system I/O
5349 12 blocks actual number of blocks allocated
5351 (The epoch was at 00:00 January 1, 1970 GMT.)
5353 (*) The ctime field is non-portable, in particular you cannot expect
5354 it to be a "creation time", see L<perlport/"Files and Filesystems">
5357 If stat is passed the special filehandle consisting of an underline, no
5358 stat is done, but the current contents of the stat structure from the
5359 last stat or filetest are returned. Example:
5361 if (-x $file && (($d) = stat(_)) && $d < 0) {
5362 print "$file is executable NFS file\n";
5365 (This works on machines only for which the device number is negative
5368 Because the mode contains both the file type and its permissions, you
5369 should mask off the file type portion and (s)printf using a C<"%o">
5370 if you want to see the real permissions.
5372 $mode = (stat($filename))[2];
5373 printf "Permissions are %04o\n", $mode & 07777;
5375 In scalar context, C<stat> returns a boolean value indicating success
5376 or failure, and, if successful, sets the information associated with
5377 the special filehandle C<_>.
5379 The File::stat module provides a convenient, by-name access mechanism:
5382 $sb = stat($filename);
5383 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5384 $filename, $sb->size, $sb->mode & 07777,
5385 scalar localtime $sb->mtime;
5387 You can import symbolic mode constants (C<S_IF*>) and functions
5388 (C<S_IS*>) from the Fcntl module:
5392 $mode = (stat($filename))[2];
5394 $user_rwx = ($mode & S_IRWXU) >> 6;
5395 $group_read = ($mode & S_IRGRP) >> 3;
5396 $other_execute = $mode & S_IXOTH;
5398 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5400 $is_setuid = $mode & S_ISUID;
5401 $is_setgid = S_ISDIR($mode);
5403 You could write the last two using the C<-u> and C<-d> operators.
5404 The commonly available S_IF* constants are
5406 # Permissions: read, write, execute, for user, group, others.
5408 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5409 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5410 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5412 # Setuid/Setgid/Stickiness/SaveText.
5413 # Note that the exact meaning of these is system dependent.
5415 S_ISUID S_ISGID S_ISVTX S_ISTXT
5417 # File types. Not necessarily all are available on your system.
5419 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5421 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5423 S_IREAD S_IWRITE S_IEXEC
5425 and the S_IF* functions are
5427 S_IMODE($mode) the part of $mode containing the permission bits
5428 and the setuid/setgid/sticky bits
5430 S_IFMT($mode) the part of $mode containing the file type
5431 which can be bit-anded with e.g. S_IFREG
5432 or with the following functions
5434 # The operators -f, -d, -l, -b, -c, -p, and -s.
5436 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5437 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5439 # No direct -X operator counterpart, but for the first one
5440 # the -g operator is often equivalent. The ENFMT stands for
5441 # record flocking enforcement, a platform-dependent feature.
5443 S_ISENFMT($mode) S_ISWHT($mode)
5445 See your native chmod(2) and stat(2) documentation for more details
5446 about the S_* constants. To get status info for a symbolic link
5447 instead of the target file behind the link, use the C<lstat> function.
5453 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5454 doing many pattern matches on the string before it is next modified.
5455 This may or may not save time, depending on the nature and number of
5456 patterns you are searching on, and on the distribution of character
5457 frequencies in the string to be searched--you probably want to compare
5458 run times with and without it to see which runs faster. Those loops
5459 which scan for many short constant strings (including the constant
5460 parts of more complex patterns) will benefit most. You may have only
5461 one C<study> active at a time--if you study a different scalar the first
5462 is "unstudied". (The way C<study> works is this: a linked list of every
5463 character in the string to be searched is made, so we know, for
5464 example, where all the C<'k'> characters are. From each search string,
5465 the rarest character is selected, based on some static frequency tables
5466 constructed from some C programs and English text. Only those places
5467 that contain this "rarest" character are examined.)
5469 For example, here is a loop that inserts index producing entries
5470 before any line containing a certain pattern:
5474 print ".IX foo\n" if /\bfoo\b/;
5475 print ".IX bar\n" if /\bbar\b/;
5476 print ".IX blurfl\n" if /\bblurfl\b/;
5481 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5482 will be looked at, because C<f> is rarer than C<o>. In general, this is
5483 a big win except in pathological cases. The only question is whether
5484 it saves you more time than it took to build the linked list in the
5487 Note that if you have to look for strings that you don't know till
5488 runtime, you can build an entire loop as a string and C<eval> that to
5489 avoid recompiling all your patterns all the time. Together with
5490 undefining C<$/> to input entire files as one record, this can be very
5491 fast, often faster than specialized programs like fgrep(1). The following
5492 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5493 out the names of those files that contain a match:
5495 $search = 'while (<>) { study;';
5496 foreach $word (@words) {
5497 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5502 eval $search; # this screams
5503 $/ = "\n"; # put back to normal input delimiter
5504 foreach $file (sort keys(%seen)) {
5508 =item sub NAME BLOCK
5510 =item sub NAME (PROTO) BLOCK
5512 =item sub NAME : ATTRS BLOCK
5514 =item sub NAME (PROTO) : ATTRS BLOCK
5516 This is subroutine definition, not a real function I<per se>.
5517 Without a BLOCK it's just a forward declaration. Without a NAME,
5518 it's an anonymous function declaration, and does actually return
5519 a value: the CODE ref of the closure you just created.
5521 See L<perlsub> and L<perlref> for details about subroutines and
5522 references, and L<attributes> and L<Attribute::Handlers> for more
5523 information about attributes.
5525 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5527 =item substr EXPR,OFFSET,LENGTH
5529 =item substr EXPR,OFFSET
5531 Extracts a substring out of EXPR and returns it. First character is at
5532 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5533 If OFFSET is negative (or more precisely, less than C<$[>), starts
5534 that far from the end of the string. If LENGTH is omitted, returns
5535 everything to the end of the string. If LENGTH is negative, leaves that
5536 many characters off the end of the string.
5538 You can use the substr() function as an lvalue, in which case EXPR
5539 must itself be an lvalue. If you assign something shorter than LENGTH,
5540 the string will shrink, and if you assign something longer than LENGTH,
5541 the string will grow to accommodate it. To keep the string the same
5542 length you may need to pad or chop your value using C<sprintf>.
5544 If OFFSET and LENGTH specify a substring that is partly outside the
5545 string, only the part within the string is returned. If the substring
5546 is beyond either end of the string, substr() returns the undefined
5547 value and produces a warning. When used as an lvalue, specifying a
5548 substring that is entirely outside the string is a fatal error.
5549 Here's an example showing the behavior for boundary cases:
5552 substr($name, 4) = 'dy'; # $name is now 'freddy'
5553 my $null = substr $name, 6, 2; # returns '' (no warning)
5554 my $oops = substr $name, 7; # returns undef, with warning
5555 substr($name, 7) = 'gap'; # fatal error
5557 An alternative to using substr() as an lvalue is to specify the
5558 replacement string as the 4th argument. This allows you to replace
5559 parts of the EXPR and return what was there before in one operation,
5560 just as you can with splice().
5562 If the lvalue returned by substr is used after the EXPR is changed in
5563 any way, the behaviour may not be as expected and is subject to change.
5564 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5565 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5566 substring assignment, and then the substr is used again), or where a
5567 substr() is aliased via a C<foreach> loop or passed as a parameter or
5568 a reference to it is taken and then the alias, parameter, or deref'd
5569 reference either is used after the original EXPR has been changed or
5570 is assigned to and then used a second time.
5572 =item symlink OLDFILE,NEWFILE
5574 Creates a new filename symbolically linked to the old filename.
5575 Returns C<1> for success, C<0> otherwise. On systems that don't support
5576 symbolic links, produces a fatal error at run time. To check for that,
5579 $symlink_exists = eval { symlink("",""); 1 };
5581 =item syscall NUMBER, LIST
5583 Calls the system call specified as the first element of the list,
5584 passing the remaining elements as arguments to the system call. If
5585 unimplemented, produces a fatal error. The arguments are interpreted
5586 as follows: if a given argument is numeric, the argument is passed as
5587 an int. If not, the pointer to the string value is passed. You are
5588 responsible to make sure a string is pre-extended long enough to
5589 receive any result that might be written into a string. You can't use a
5590 string literal (or other read-only string) as an argument to C<syscall>
5591 because Perl has to assume that any string pointer might be written
5593 integer arguments are not literals and have never been interpreted in a
5594 numeric context, you may need to add C<0> to them to force them to look
5595 like numbers. This emulates the C<syswrite> function (or vice versa):
5597 require 'syscall.ph'; # may need to run h2ph
5599 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5601 Note that Perl supports passing of up to only 14 arguments to your system call,
5602 which in practice should usually suffice.
5604 Syscall returns whatever value returned by the system call it calls.
5605 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5606 Note that some system calls can legitimately return C<-1>. The proper
5607 way to handle such calls is to assign C<$!=0;> before the call and
5608 check the value of C<$!> if syscall returns C<-1>.
5610 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5611 number of the read end of the pipe it creates. There is no way
5612 to retrieve the file number of the other end. You can avoid this
5613 problem by using C<pipe> instead.
5615 =item sysopen FILEHANDLE,FILENAME,MODE
5617 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5619 Opens the file whose filename is given by FILENAME, and associates it
5620 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5621 the name of the real filehandle wanted. This function calls the
5622 underlying operating system's C<open> function with the parameters
5623 FILENAME, MODE, PERMS.
5625 The possible values and flag bits of the MODE parameter are
5626 system-dependent; they are available via the standard module C<Fcntl>.
5627 See the documentation of your operating system's C<open> to see which
5628 values and flag bits are available. You may combine several flags
5629 using the C<|>-operator.
5631 Some of the most common values are C<O_RDONLY> for opening the file in
5632 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5633 and C<O_RDWR> for opening the file in read-write mode, and.
5635 For historical reasons, some values work on almost every system
5636 supported by perl: zero means read-only, one means write-only, and two
5637 means read/write. We know that these values do I<not> work under
5638 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5639 use them in new code.
5641 If the file named by FILENAME does not exist and the C<open> call creates
5642 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5643 PERMS specifies the permissions of the newly created file. If you omit
5644 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5645 These permission values need to be in octal, and are modified by your
5646 process's current C<umask>.
5648 In many systems the C<O_EXCL> flag is available for opening files in
5649 exclusive mode. This is B<not> locking: exclusiveness means here that
5650 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5653 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5655 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5656 that takes away the user's option to have a more permissive umask.
5657 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5660 Note that C<sysopen> depends on the fdopen() C library function.
5661 On many UNIX systems, fdopen() is known to fail when file descriptors
5662 exceed a certain value, typically 255. If you need more file
5663 descriptors than that, consider rebuilding Perl to use the C<sfio>
5664 library, or perhaps using the POSIX::open() function.
5666 See L<perlopentut> for a kinder, gentler explanation of opening files.
5668 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5670 =item sysread FILEHANDLE,SCALAR,LENGTH
5672 Attempts to read LENGTH bytes of data into variable SCALAR from the
5673 specified FILEHANDLE, using the system call read(2). It bypasses
5674 buffered IO, so mixing this with other kinds of reads, C<print>,
5675 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5676 perlio or stdio layers usually buffers data. Returns the number of
5677 bytes actually read, C<0> at end of file, or undef if there was an
5678 error (in the latter case C<$!> is also set). SCALAR will be grown or
5679 shrunk so that the last byte actually read is the last byte of the
5680 scalar after the read.
5682 An OFFSET may be specified to place the read data at some place in the
5683 string other than the beginning. A negative OFFSET specifies
5684 placement at that many characters counting backwards from the end of
5685 the string. A positive OFFSET greater than the length of SCALAR
5686 results in the string being padded to the required size with C<"\0">
5687 bytes before the result of the read is appended.
5689 There is no syseof() function, which is ok, since eof() doesn't work
5690 very well on device files (like ttys) anyway. Use sysread() and check
5691 for a return value for 0 to decide whether you're done.
5693 Note that if the filehandle has been marked as C<:utf8> Unicode
5694 characters are read instead of bytes (the LENGTH, OFFSET, and the
5695 return value of sysread() are in Unicode characters).
5696 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5697 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5699 =item sysseek FILEHANDLE,POSITION,WHENCE
5701 Sets FILEHANDLE's system position in bytes using the system call
5702 lseek(2). FILEHANDLE may be an expression whose value gives the name
5703 of the filehandle. The values for WHENCE are C<0> to set the new
5704 position to POSITION, C<1> to set the it to the current position plus
5705 POSITION, and C<2> to set it to EOF plus POSITION (typically
5708 Note the I<in bytes>: even if the filehandle has been set to operate
5709 on characters (for example by using the C<:utf8> I/O layer), tell()
5710 will return byte offsets, not character offsets (because implementing
5711 that would render sysseek() very slow).
5713 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5714 than C<sysread>, for example >< or read()) C<print>, C<write>,
5715 C<seek>, C<tell>, or C<eof> may cause confusion.
5717 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5718 and C<SEEK_END> (start of the file, current position, end of the file)
5719 from the Fcntl module. Use of the constants is also more portable
5720 than relying on 0, 1, and 2. For example to define a "systell" function:
5722 use Fcntl 'SEEK_CUR';
5723 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5725 Returns the new position, or the undefined value on failure. A position
5726 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5727 true on success and false on failure, yet you can still easily determine
5732 =item system PROGRAM LIST
5734 Does exactly the same thing as C<exec LIST>, except that a fork is
5735 done first, and the parent process waits for the child process to
5736 complete. Note that argument processing varies depending on the
5737 number of arguments. If there is more than one argument in LIST,
5738 or if LIST is an array with more than one value, starts the program
5739 given by the first element of the list with arguments given by the
5740 rest of the list. If there is only one scalar argument, the argument
5741 is checked for shell metacharacters, and if there are any, the
5742 entire argument is passed to the system's command shell for parsing
5743 (this is C</bin/sh -c> on Unix platforms, but varies on other
5744 platforms). If there are no shell metacharacters in the argument,
5745 it is split into words and passed directly to C<execvp>, which is
5748 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5749 output before any operation that may do a fork, but this may not be
5750 supported on some platforms (see L<perlport>). To be safe, you may need
5751 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5752 of C<IO::Handle> on any open handles.
5754 The return value is the exit status of the program as returned by the
5755 C<wait> call. To get the actual exit value shift right by eight (see below).
5756 See also L</exec>. This is I<not> what you want to use to capture
5757 the output from a command, for that you should use merely backticks or
5758 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5759 indicates a failure to start the program (inspect $! for the reason).
5761 Like C<exec>, C<system> allows you to lie to a program about its name if
5762 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5764 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5765 killing the program they're running doesn't actually interrupt
5768 @args = ("command", "arg1", "arg2");
5770 or die "system @args failed: $?"
5772 You can check all the failure possibilities by inspecting
5776 print "failed to execute: $!\n";
5779 printf "child died with signal %d, %s coredump\n",
5780 ($? & 127), ($? & 128) ? 'with' : 'without';
5783 printf "child exited with value %d\n", $? >> 8;
5787 or more portably by using the W*() calls of the POSIX extension;
5788 see L<perlport> for more information.
5790 When the arguments get executed via the system shell, results
5791 and return codes will be subject to its quirks and capabilities.
5792 See L<perlop/"`STRING`"> and L</exec> for details.
5794 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5796 =item syswrite FILEHANDLE,SCALAR,LENGTH
5798 =item syswrite FILEHANDLE,SCALAR
5800 Attempts to write LENGTH bytes of data from variable SCALAR to the
5801 specified FILEHANDLE, using the system call write(2). If LENGTH is
5802 not specified, writes whole SCALAR. It bypasses buffered IO, so
5803 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5804 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5805 stdio layers usually buffers data. Returns the number of bytes
5806 actually written, or C<undef> if there was an error (in this case the
5807 errno variable C<$!> is also set). If the LENGTH is greater than the
5808 available data in the SCALAR after the OFFSET, only as much data as is
5809 available will be written.
5811 An OFFSET may be specified to write the data from some part of the
5812 string other than the beginning. A negative OFFSET specifies writing
5813 that many characters counting backwards from the end of the string.
5814 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5816 Note that if the filehandle has been marked as C<:utf8>, Unicode
5817 characters are written instead of bytes (the LENGTH, OFFSET, and the
5818 return value of syswrite() are in UTF-8 encoded Unicode characters).
5819 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5820 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5822 =item tell FILEHANDLE
5826 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5827 error. FILEHANDLE may be an expression whose value gives the name of
5828 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5831 Note the I<in bytes>: even if the filehandle has been set to
5832 operate on characters (for example by using the C<:utf8> open
5833 layer), tell() will return byte offsets, not character offsets
5834 (because that would render seek() and tell() rather slow).
5836 The return value of tell() for the standard streams like the STDIN
5837 depends on the operating system: it may return -1 or something else.
5838 tell() on pipes, fifos, and sockets usually returns -1.
5840 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5842 Do not use tell() on a filehandle that has been opened using
5843 sysopen(), use sysseek() for that as described above. Why? Because
5844 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5845 buffered filehandles. sysseek() make sense only on the first kind,
5846 tell() only makes sense on the second kind.
5848 =item telldir DIRHANDLE
5850 Returns the current position of the C<readdir> routines on DIRHANDLE.
5851 Value may be given to C<seekdir> to access a particular location in a
5852 directory. Has the same caveats about possible directory compaction as
5853 the corresponding system library routine.
5855 =item tie VARIABLE,CLASSNAME,LIST
5857 This function binds a variable to a package class that will provide the
5858 implementation for the variable. VARIABLE is the name of the variable
5859 to be enchanted. CLASSNAME is the name of a class implementing objects
5860 of correct type. Any additional arguments are passed to the C<new>
5861 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5862 or C<TIEHASH>). Typically these are arguments such as might be passed
5863 to the C<dbm_open()> function of C. The object returned by the C<new>
5864 method is also returned by the C<tie> function, which would be useful
5865 if you want to access other methods in CLASSNAME.
5867 Note that functions such as C<keys> and C<values> may return huge lists
5868 when used on large objects, like DBM files. You may prefer to use the
5869 C<each> function to iterate over such. Example:
5871 # print out history file offsets
5873 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5874 while (($key,$val) = each %HIST) {
5875 print $key, ' = ', unpack('L',$val), "\n";
5879 A class implementing a hash should have the following methods:
5881 TIEHASH classname, LIST
5883 STORE this, key, value
5888 NEXTKEY this, lastkey
5893 A class implementing an ordinary array should have the following methods:
5895 TIEARRAY classname, LIST
5897 STORE this, key, value
5899 STORESIZE this, count
5905 SPLICE this, offset, length, LIST
5910 A class implementing a file handle should have the following methods:
5912 TIEHANDLE classname, LIST
5913 READ this, scalar, length, offset
5916 WRITE this, scalar, length, offset
5918 PRINTF this, format, LIST
5922 SEEK this, position, whence
5924 OPEN this, mode, LIST
5929 A class implementing a scalar should have the following methods:
5931 TIESCALAR classname, LIST
5937 Not all methods indicated above need be implemented. See L<perltie>,
5938 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5940 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5941 for you--you need to do that explicitly yourself. See L<DB_File>
5942 or the F<Config> module for interesting C<tie> implementations.
5944 For further details see L<perltie>, L<"tied VARIABLE">.
5948 Returns a reference to the object underlying VARIABLE (the same value
5949 that was originally returned by the C<tie> call that bound the variable
5950 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5955 Returns the number of non-leap seconds since whatever time the system
5956 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5957 and 00:00:00 UTC, January 1, 1970 for most other systems).
5958 Suitable for feeding to C<gmtime> and C<localtime>.
5960 For measuring time in better granularity than one second,
5961 you may use either the Time::HiRes module (from CPAN, and starting from
5962 Perl 5.8 part of the standard distribution), or if you have
5963 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5964 See L<perlfaq8> for details.
5968 Returns a four-element list giving the user and system times, in
5969 seconds, for this process and the children of this process.
5971 ($user,$system,$cuser,$csystem) = times;
5973 In scalar context, C<times> returns C<$user>.
5977 The transliteration operator. Same as C<y///>. See L<perlop>.
5979 =item truncate FILEHANDLE,LENGTH
5981 =item truncate EXPR,LENGTH
5983 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5984 specified length. Produces a fatal error if truncate isn't implemented
5985 on your system. Returns true if successful, the undefined value
5988 The behavior is undefined if LENGTH is greater than the length of the
5995 Returns an uppercased version of EXPR. This is the internal function
5996 implementing the C<\U> escape in double-quoted strings. Respects
5997 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5998 and L<perlunicode> for more details about locale and Unicode support.
5999 It does not attempt to do titlecase mapping on initial letters. See
6000 C<ucfirst> for that.
6002 If EXPR is omitted, uses C<$_>.
6008 Returns the value of EXPR with the first character in uppercase
6009 (titlecase in Unicode). This is the internal function implementing
6010 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6011 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6012 for more details about locale and Unicode support.
6014 If EXPR is omitted, uses C<$_>.
6020 Sets the umask for the process to EXPR and returns the previous value.
6021 If EXPR is omitted, merely returns the current umask.
6023 The Unix permission C<rwxr-x---> is represented as three sets of three
6024 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6025 and isn't one of the digits). The C<umask> value is such a number
6026 representing disabled permissions bits. The permission (or "mode")
6027 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6028 even if you tell C<sysopen> to create a file with permissions C<0777>,
6029 if your umask is C<0022> then the file will actually be created with
6030 permissions C<0755>. If your C<umask> were C<0027> (group can't
6031 write; others can't read, write, or execute), then passing
6032 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6035 Here's some advice: supply a creation mode of C<0666> for regular
6036 files (in C<sysopen>) and one of C<0777> for directories (in
6037 C<mkdir>) and executable files. This gives users the freedom of
6038 choice: if they want protected files, they might choose process umasks
6039 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6040 Programs should rarely if ever make policy decisions better left to
6041 the user. The exception to this is when writing files that should be
6042 kept private: mail files, web browser cookies, I<.rhosts> files, and
6045 If umask(2) is not implemented on your system and you are trying to
6046 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6047 fatal error at run time. If umask(2) is not implemented and you are
6048 not trying to restrict access for yourself, returns C<undef>.
6050 Remember that a umask is a number, usually given in octal; it is I<not> a
6051 string of octal digits. See also L</oct>, if all you have is a string.
6057 Undefines the value of EXPR, which must be an lvalue. Use only on a
6058 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6059 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6060 will probably not do what you expect on most predefined variables or
6061 DBM list values, so don't do that; see L<delete>.) Always returns the
6062 undefined value. You can omit the EXPR, in which case nothing is
6063 undefined, but you still get an undefined value that you could, for
6064 instance, return from a subroutine, assign to a variable or pass as a
6065 parameter. Examples:
6068 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6072 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6073 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6074 select undef, undef, undef, 0.25;
6075 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6077 Note that this is a unary operator, not a list operator.
6083 Deletes a list of files. Returns the number of files successfully
6086 $cnt = unlink 'a', 'b', 'c';
6090 Note: C<unlink> will not delete directories unless you are superuser and
6091 the B<-U> flag is supplied to Perl. Even if these conditions are
6092 met, be warned that unlinking a directory can inflict damage on your
6093 filesystem. Use C<rmdir> instead.
6095 If LIST is omitted, uses C<$_>.
6097 =item unpack TEMPLATE,EXPR
6099 =item unpack TEMPLATE
6101 C<unpack> does the reverse of C<pack>: it takes a string
6102 and expands it out into a list of values.
6103 (In scalar context, it returns merely the first value produced.)
6105 If EXPR is omitted, unpacks the C<$_> string.
6107 The string is broken into chunks described by the TEMPLATE. Each chunk
6108 is converted separately to a value. Typically, either the string is a result
6109 of C<pack>, or the bytes of the string represent a C structure of some
6112 The TEMPLATE has the same format as in the C<pack> function.
6113 Here's a subroutine that does substring:
6116 my($what,$where,$howmuch) = @_;
6117 unpack("x$where a$howmuch", $what);
6122 sub ordinal { unpack("c",$_[0]); } # same as ord()
6124 In addition to fields allowed in pack(), you may prefix a field with
6125 a %<number> to indicate that
6126 you want a <number>-bit checksum of the items instead of the items
6127 themselves. Default is a 16-bit checksum. Checksum is calculated by
6128 summing numeric values of expanded values (for string fields the sum of
6129 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6131 For example, the following
6132 computes the same number as the System V sum program:
6136 unpack("%32C*",<>) % 65535;
6139 The following efficiently counts the number of set bits in a bit vector:
6141 $setbits = unpack("%32b*", $selectmask);
6143 The C<p> and C<P> formats should be used with care. Since Perl
6144 has no way of checking whether the value passed to C<unpack()>
6145 corresponds to a valid memory location, passing a pointer value that's
6146 not known to be valid is likely to have disastrous consequences.
6148 If there are more pack codes or if the repeat count of a field or a group
6149 is larger than what the remainder of the input string allows, the result
6150 is not well defined: in some cases, the repeat count is decreased, or
6151 C<unpack()> will produce null strings or zeroes, or terminate with an
6152 error. If the input string is longer than one described by the TEMPLATE,
6153 the rest is ignored.
6155 See L</pack> for more examples and notes.
6157 =item untie VARIABLE
6159 Breaks the binding between a variable and a package. (See C<tie>.)
6160 Has no effect if the variable is not tied.
6162 =item unshift ARRAY,LIST
6164 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6165 depending on how you look at it. Prepends list to the front of the
6166 array, and returns the new number of elements in the array.
6168 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6170 Note the LIST is prepended whole, not one element at a time, so the
6171 prepended elements stay in the same order. Use C<reverse> to do the
6174 =item use Module VERSION LIST
6176 =item use Module VERSION
6178 =item use Module LIST
6184 Imports some semantics into the current package from the named module,
6185 generally by aliasing certain subroutine or variable names into your
6186 package. It is exactly equivalent to
6188 BEGIN { require Module; import Module LIST; }
6190 except that Module I<must> be a bareword.
6192 VERSION may be either a numeric argument such as 5.006, which will be
6193 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6194 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6195 greater than the version of the current Perl interpreter; Perl will not
6196 attempt to parse the rest of the file. Compare with L</require>, which can
6197 do a similar check at run time.
6199 Specifying VERSION as a literal of the form v5.6.1 should generally be
6200 avoided, because it leads to misleading error messages under earlier
6201 versions of Perl which do not support this syntax. The equivalent numeric
6202 version should be used instead.
6204 use v5.6.1; # compile time version check
6206 use 5.006_001; # ditto; preferred for backwards compatibility
6208 This is often useful if you need to check the current Perl version before
6209 C<use>ing library modules that have changed in incompatible ways from
6210 older versions of Perl. (We try not to do this more than we have to.)
6212 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6213 C<require> makes sure the module is loaded into memory if it hasn't been
6214 yet. The C<import> is not a builtin--it's just an ordinary static method
6215 call into the C<Module> package to tell the module to import the list of
6216 features back into the current package. The module can implement its
6217 C<import> method any way it likes, though most modules just choose to
6218 derive their C<import> method via inheritance from the C<Exporter> class that
6219 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6220 method can be found then the call is skipped, even if there is an AUTOLOAD
6223 If you do not want to call the package's C<import> method (for instance,
6224 to stop your namespace from being altered), explicitly supply the empty list:
6228 That is exactly equivalent to
6230 BEGIN { require Module }
6232 If the VERSION argument is present between Module and LIST, then the
6233 C<use> will call the VERSION method in class Module with the given
6234 version as an argument. The default VERSION method, inherited from
6235 the UNIVERSAL class, croaks if the given version is larger than the
6236 value of the variable C<$Module::VERSION>.
6238 Again, there is a distinction between omitting LIST (C<import> called
6239 with no arguments) and an explicit empty LIST C<()> (C<import> not
6240 called). Note that there is no comma after VERSION!
6242 Because this is a wide-open interface, pragmas (compiler directives)
6243 are also implemented this way. Currently implemented pragmas are:
6248 use sigtrap qw(SEGV BUS);
6249 use strict qw(subs vars refs);
6250 use subs qw(afunc blurfl);
6251 use warnings qw(all);
6252 use sort qw(stable _quicksort _mergesort);
6254 Some of these pseudo-modules import semantics into the current
6255 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6256 which import symbols into the current package (which are effective
6257 through the end of the file).
6259 There's a corresponding C<no> command that unimports meanings imported
6260 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6261 It behaves exactly as C<import> does with respect to VERSION, an
6262 omitted LIST, empty LIST, or no unimport method being found.
6268 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6269 for the C<-M> and C<-m> command-line options to perl that give C<use>
6270 functionality from the command-line.
6274 Changes the access and modification times on each file of a list of
6275 files. The first two elements of the list must be the NUMERICAL access
6276 and modification times, in that order. Returns the number of files
6277 successfully changed. The inode change time of each file is set
6278 to the current time. For example, this code has the same effect as the
6279 Unix touch(1) command when the files I<already exist>.
6282 $atime = $mtime = time;
6283 utime $atime, $mtime, @ARGV;
6285 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6286 the utime(2) function in the C library will be called with a null second
6287 argument. On most systems, this will set the file's access and
6288 modification times to the current time (i.e. equivalent to the example
6291 utime undef, undef, @ARGV;
6293 Under NFS this will use the time of the NFS server, not the time of
6294 the local machine. If there is a time synchronization problem, the
6295 NFS server and local machine will have different times. The Unix
6296 touch(1) command will in fact normally use this form instead of the
6297 one shown in the first example.
6299 Note that only passing one of the first two elements as C<undef> will
6300 be equivalent of passing it as 0 and will not have the same effect as
6301 described when they are both C<undef>. This case will also trigger an
6302 uninitialized warning.
6306 Returns a list consisting of all the values of the named hash.
6307 (In a scalar context, returns the number of values.)
6309 The values are returned in an apparently random order. The actual
6310 random order is subject to change in future versions of perl, but it
6311 is guaranteed to be the same order as either the C<keys> or C<each>
6312 function would produce on the same (unmodified) hash. Since Perl
6313 5.8.1 the ordering is different even between different runs of Perl
6314 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6316 As a side effect, calling values() resets the HASH's internal iterator,
6317 see L</each>. (In particular, calling values() in void context resets
6318 the iterator with no other overhead.)
6320 Note that the values are not copied, which means modifying them will
6321 modify the contents of the hash:
6323 for (values %hash) { s/foo/bar/g } # modifies %hash values
6324 for (@hash{keys %hash}) { s/foo/bar/g } # same
6326 See also C<keys>, C<each>, and C<sort>.
6328 =item vec EXPR,OFFSET,BITS
6330 Treats the string in EXPR as a bit vector made up of elements of
6331 width BITS, and returns the value of the element specified by OFFSET
6332 as an unsigned integer. BITS therefore specifies the number of bits
6333 that are reserved for each element in the bit vector. This must
6334 be a power of two from 1 to 32 (or 64, if your platform supports
6337 If BITS is 8, "elements" coincide with bytes of the input string.
6339 If BITS is 16 or more, bytes of the input string are grouped into chunks
6340 of size BITS/8, and each group is converted to a number as with
6341 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6342 for BITS==64). See L<"pack"> for details.
6344 If bits is 4 or less, the string is broken into bytes, then the bits
6345 of each byte are broken into 8/BITS groups. Bits of a byte are
6346 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6347 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6348 breaking the single input byte C<chr(0x36)> into two groups gives a list
6349 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6351 C<vec> may also be assigned to, in which case parentheses are needed
6352 to give the expression the correct precedence as in
6354 vec($image, $max_x * $x + $y, 8) = 3;
6356 If the selected element is outside the string, the value 0 is returned.
6357 If an element off the end of the string is written to, Perl will first
6358 extend the string with sufficiently many zero bytes. It is an error
6359 to try to write off the beginning of the string (i.e. negative OFFSET).
6361 The string should not contain any character with the value > 255 (which
6362 can only happen if you're using UTF-8 encoding). If it does, it will be
6363 treated as something which is not UTF-8 encoded. When the C<vec> was
6364 assigned to, other parts of your program will also no longer consider the
6365 string to be UTF-8 encoded. In other words, if you do have such characters
6366 in your string, vec() will operate on the actual byte string, and not the
6367 conceptual character string.
6369 Strings created with C<vec> can also be manipulated with the logical
6370 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6371 vector operation is desired when both operands are strings.
6372 See L<perlop/"Bitwise String Operators">.
6374 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6375 The comments show the string after each step. Note that this code works
6376 in the same way on big-endian or little-endian machines.
6379 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6381 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6382 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6384 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6385 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6386 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6387 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6388 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6389 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6391 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6392 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6393 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6396 To transform a bit vector into a string or list of 0's and 1's, use these:
6398 $bits = unpack("b*", $vector);
6399 @bits = split(//, unpack("b*", $vector));
6401 If you know the exact length in bits, it can be used in place of the C<*>.
6403 Here is an example to illustrate how the bits actually fall in place:
6409 unpack("V",$_) 01234567890123456789012345678901
6410 ------------------------------------------------------------------
6415 for ($shift=0; $shift < $width; ++$shift) {
6416 for ($off=0; $off < 32/$width; ++$off) {
6417 $str = pack("B*", "0"x32);
6418 $bits = (1<<$shift);
6419 vec($str, $off, $width) = $bits;
6420 $res = unpack("b*",$str);
6421 $val = unpack("V", $str);
6428 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6429 $off, $width, $bits, $val, $res
6433 Regardless of the machine architecture on which it is run, the above
6434 example should print the following table:
6437 unpack("V",$_) 01234567890123456789012345678901
6438 ------------------------------------------------------------------
6439 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6440 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6441 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6442 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6443 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6444 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6445 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6446 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6447 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6448 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6449 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6450 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6451 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6452 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6453 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6454 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6455 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6456 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6457 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6458 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6459 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6460 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6461 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6462 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6463 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6464 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6465 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6466 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6467 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6468 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6469 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6470 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6471 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6472 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6473 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6474 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6475 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6476 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6477 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6478 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6479 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6480 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6481 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6482 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6483 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6484 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6485 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6486 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6487 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6488 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6489 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6490 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6491 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6492 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6493 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6494 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6495 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6496 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6497 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6498 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6499 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6500 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6501 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6502 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6503 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6504 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6505 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6506 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6507 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6508 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6509 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6510 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6511 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6512 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6513 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6514 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6515 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6516 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6517 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6518 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6519 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6520 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6521 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6522 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6523 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6524 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6525 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6526 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6527 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6528 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6529 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6530 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6531 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6532 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6533 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6534 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6535 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6536 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6537 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6538 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6539 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6540 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6541 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6542 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6543 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6544 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6545 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6546 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6547 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6548 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6549 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6550 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6551 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6552 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6553 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6554 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6555 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6556 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6557 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6558 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6559 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6560 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6561 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6562 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6563 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6564 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6565 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6566 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6570 Behaves like the wait(2) system call on your system: it waits for a child
6571 process to terminate and returns the pid of the deceased process, or
6572 C<-1> if there are no child processes. The status is returned in C<$?>.
6573 Note that a return value of C<-1> could mean that child processes are
6574 being automatically reaped, as described in L<perlipc>.
6576 =item waitpid PID,FLAGS
6578 Waits for a particular child process to terminate and returns the pid of
6579 the deceased process, or C<-1> if there is no such child process. On some
6580 systems, a value of 0 indicates that there are processes still running.
6581 The status is returned in C<$?>. If you say
6583 use POSIX ":sys_wait_h";
6586 $kid = waitpid(-1, WNOHANG);
6589 then you can do a non-blocking wait for all pending zombie processes.
6590 Non-blocking wait is available on machines supporting either the
6591 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6592 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6593 system call by remembering the status values of processes that have
6594 exited but have not been harvested by the Perl script yet.)
6596 Note that on some systems, a return value of C<-1> could mean that child
6597 processes are being automatically reaped. See L<perlipc> for details,
6598 and for other examples.
6602 Returns true if the context of the currently executing subroutine is
6603 looking for a list value. Returns false if the context is looking
6604 for a scalar. Returns the undefined value if the context is looking
6605 for no value (void context).
6607 return unless defined wantarray; # don't bother doing more
6608 my @a = complex_calculation();
6609 return wantarray ? @a : "@a";
6611 This function should have been named wantlist() instead.
6615 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6618 If LIST is empty and C<$@> already contains a value (typically from a
6619 previous eval) that value is used after appending C<"\t...caught">
6620 to C<$@>. This is useful for staying almost, but not entirely similar to
6623 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6625 No message is printed if there is a C<$SIG{__WARN__}> handler
6626 installed. It is the handler's responsibility to deal with the message
6627 as it sees fit (like, for instance, converting it into a C<die>). Most
6628 handlers must therefore make arrangements to actually display the
6629 warnings that they are not prepared to deal with, by calling C<warn>
6630 again in the handler. Note that this is quite safe and will not
6631 produce an endless loop, since C<__WARN__> hooks are not called from
6634 You will find this behavior is slightly different from that of
6635 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6636 instead call C<die> again to change it).
6638 Using a C<__WARN__> handler provides a powerful way to silence all
6639 warnings (even the so-called mandatory ones). An example:
6641 # wipe out *all* compile-time warnings
6642 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6644 my $foo = 20; # no warning about duplicate my $foo,
6645 # but hey, you asked for it!
6646 # no compile-time or run-time warnings before here
6649 # run-time warnings enabled after here
6650 warn "\$foo is alive and $foo!"; # does show up
6652 See L<perlvar> for details on setting C<%SIG> entries, and for more
6653 examples. See the Carp module for other kinds of warnings using its
6654 carp() and cluck() functions.
6656 =item write FILEHANDLE
6662 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6663 using the format associated with that file. By default the format for
6664 a file is the one having the same name as the filehandle, but the
6665 format for the current output channel (see the C<select> function) may be set
6666 explicitly by assigning the name of the format to the C<$~> variable.
6668 Top of form processing is handled automatically: if there is
6669 insufficient room on the current page for the formatted record, the
6670 page is advanced by writing a form feed, a special top-of-page format
6671 is used to format the new page header, and then the record is written.
6672 By default the top-of-page format is the name of the filehandle with
6673 "_TOP" appended, but it may be dynamically set to the format of your
6674 choice by assigning the name to the C<$^> variable while the filehandle is
6675 selected. The number of lines remaining on the current page is in
6676 variable C<$->, which can be set to C<0> to force a new page.
6678 If FILEHANDLE is unspecified, output goes to the current default output
6679 channel, which starts out as STDOUT but may be changed by the
6680 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6681 is evaluated and the resulting string is used to look up the name of
6682 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6684 Note that write is I<not> the opposite of C<read>. Unfortunately.
6688 The transliteration operator. Same as C<tr///>. See L<perlop>.