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 If the C<encoding> pragma is in scope then the lengths returned are
665 calculated from the length of C<$/> in Unicode characters, which is not
666 always the same as the length of C<$/> in the native encoding.
668 Note that parentheses are necessary when you're chomping anything
669 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
670 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
671 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
672 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
681 Chops off the last character of a string and returns the character
682 chopped. It is much more efficient than C<s/.$//s> because it neither
683 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
684 If VARIABLE is a hash, it chops the hash's values, but not its keys.
686 You can actually chop anything that's an lvalue, including an assignment.
688 If you chop a list, each element is chopped. Only the value of the
689 last C<chop> is returned.
691 Note that C<chop> returns the last character. To return all but the last
692 character, use C<substr($string, 0, -1)>.
698 Changes the owner (and group) of a list of files. The first two
699 elements of the list must be the I<numeric> uid and gid, in that
700 order. A value of -1 in either position is interpreted by most
701 systems to leave that value unchanged. Returns the number of files
702 successfully changed.
704 $cnt = chown $uid, $gid, 'foo', 'bar';
705 chown $uid, $gid, @filenames;
707 Here's an example that looks up nonnumeric uids in the passwd file:
710 chomp($user = <STDIN>);
712 chomp($pattern = <STDIN>);
714 ($login,$pass,$uid,$gid) = getpwnam($user)
715 or die "$user not in passwd file";
717 @ary = glob($pattern); # expand filenames
718 chown $uid, $gid, @ary;
720 On most systems, you are not allowed to change the ownership of the
721 file unless you're the superuser, although you should be able to change
722 the group to any of your secondary groups. On insecure systems, these
723 restrictions may be relaxed, but this is not a portable assumption.
724 On POSIX systems, you can detect this condition this way:
726 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
727 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
733 Returns the character represented by that NUMBER in the character set.
734 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
735 chr(0x263a) is a Unicode smiley face. Note that characters from 128
736 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
737 backward compatibility reasons (but see L<encoding>).
739 If NUMBER is omitted, uses C<$_>.
741 For the reverse, use L</ord>.
743 Note that under the C<bytes> pragma the NUMBER is masked to
746 See L<perlunicode> and L<encoding> for more about Unicode.
748 =item chroot FILENAME
752 This function works like the system call by the same name: it makes the
753 named directory the new root directory for all further pathnames that
754 begin with a C</> by your process and all its children. (It doesn't
755 change your current working directory, which is unaffected.) For security
756 reasons, this call is restricted to the superuser. If FILENAME is
757 omitted, does a C<chroot> to C<$_>.
759 =item close FILEHANDLE
763 Closes the file or pipe associated with the file handle, returning
764 true only if IO buffers are successfully flushed and closes the system
765 file descriptor. Closes the currently selected filehandle if the
768 You don't have to close FILEHANDLE if you are immediately going to do
769 another C<open> on it, because C<open> will close it for you. (See
770 C<open>.) However, an explicit C<close> on an input file resets the line
771 counter (C<$.>), while the implicit close done by C<open> does not.
773 If the file handle came from a piped open, C<close> will additionally
774 return false if one of the other system calls involved fails, or if the
775 program exits with non-zero status. (If the only problem was that the
776 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
777 also waits for the process executing on the pipe to complete, in case you
778 want to look at the output of the pipe afterwards, and
779 implicitly puts the exit status value of that command into C<$?>.
781 Prematurely closing the read end of a pipe (i.e. before the process
782 writing to it at the other end has closed it) will result in a
783 SIGPIPE being delivered to the writer. If the other end can't
784 handle that, be sure to read all the data before closing the pipe.
788 open(OUTPUT, '|sort >foo') # pipe to sort
789 or die "Can't start sort: $!";
790 #... # print stuff to output
791 close OUTPUT # wait for sort to finish
792 or warn $! ? "Error closing sort pipe: $!"
793 : "Exit status $? from sort";
794 open(INPUT, 'foo') # get sort's results
795 or die "Can't open 'foo' for input: $!";
797 FILEHANDLE may be an expression whose value can be used as an indirect
798 filehandle, usually the real filehandle name.
800 =item closedir DIRHANDLE
802 Closes a directory opened by C<opendir> and returns the success of that
805 =item connect SOCKET,NAME
807 Attempts to connect to a remote socket, just as the connect system call
808 does. Returns true if it succeeded, false otherwise. NAME should be a
809 packed address of the appropriate type for the socket. See the examples in
810 L<perlipc/"Sockets: Client/Server Communication">.
814 Actually a flow control statement rather than a function. If there is a
815 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
816 C<foreach>), it is always executed just before the conditional is about to
817 be evaluated again, just like the third part of a C<for> loop in C. Thus
818 it can be used to increment a loop variable, even when the loop has been
819 continued via the C<next> statement (which is similar to the C C<continue>
822 C<last>, C<next>, or C<redo> may appear within a C<continue>
823 block. C<last> and C<redo> will behave as if they had been executed within
824 the main block. So will C<next>, but since it will execute a C<continue>
825 block, it may be more entertaining.
828 ### redo always comes here
831 ### next always comes here
833 # then back the top to re-check EXPR
835 ### last always comes here
837 Omitting the C<continue> section is semantically equivalent to using an
838 empty one, logically enough. In that case, C<next> goes directly back
839 to check the condition at the top of the loop.
845 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
846 takes cosine of C<$_>.
848 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
849 function, or use this relation:
851 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
853 =item crypt PLAINTEXT,SALT
855 Encrypts a string exactly like the crypt(3) function in the C library
856 (assuming that you actually have a version there that has not been
857 extirpated as a potential munition). This can prove useful for checking
858 the password file for lousy passwords, amongst other things. Only the
859 guys wearing white hats should do this.
861 Note that L<crypt|/crypt> is intended to be a one-way function, much like
862 breaking eggs to make an omelette. There is no (known) corresponding
863 decrypt function (in other words, the crypt() is a one-way hash
864 function). As a result, this function isn't all that useful for
865 cryptography. (For that, see your nearby CPAN mirror.)
867 When verifying an existing encrypted string you should use the
868 encrypted text as the salt (like C<crypt($plain, $crypted) eq
869 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
870 and with more exotic implementations. In other words, do not assume
871 anything about the returned string itself, or how many bytes in
872 the encrypted string matter.
874 Traditionally the result is a string of 13 bytes: two first bytes of
875 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
876 the first eight bytes of the encrypted string mattered, but
877 alternative hashing schemes (like MD5), higher level security schemes
878 (like C2), and implementations on non-UNIX platforms may produce
881 When choosing a new salt create a random two character string whose
882 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
883 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
884 characters is just a recommendation; the characters allowed in
885 the salt depend solely on your system's crypt library, and Perl can't
886 restrict what salts C<crypt()> accepts.
888 Here's an example that makes sure that whoever runs this program knows
891 $pwd = (getpwuid($<))[1];
895 chomp($word = <STDIN>);
899 if (crypt($word, $pwd) ne $pwd) {
905 Of course, typing in your own password to whoever asks you
908 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
909 of data, not least of all because you can't get the information
910 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
911 on your favorite CPAN mirror for a slew of potentially useful
914 If using crypt() on a Unicode string (which I<potentially> has
915 characters with codepoints above 255), Perl tries to make sense
916 of the situation by trying to downgrade (a copy of the string)
917 the string back to an eight-bit byte string before calling crypt()
918 (on that copy). If that works, good. If not, crypt() dies with
919 C<Wide character in crypt>.
923 [This function has been largely superseded by the C<untie> function.]
925 Breaks the binding between a DBM file and a hash.
927 =item dbmopen HASH,DBNAME,MASK
929 [This function has been largely superseded by the C<tie> function.]
931 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
932 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
933 argument is I<not> a filehandle, even though it looks like one). DBNAME
934 is the name of the database (without the F<.dir> or F<.pag> extension if
935 any). If the database does not exist, it is created with protection
936 specified by MASK (as modified by the C<umask>). If your system supports
937 only the older DBM functions, you may perform only one C<dbmopen> in your
938 program. In older versions of Perl, if your system had neither DBM nor
939 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
942 If you don't have write access to the DBM file, you can only read hash
943 variables, not set them. If you want to test whether you can write,
944 either use file tests or try setting a dummy hash entry inside an C<eval>,
945 which will trap the error.
947 Note that functions such as C<keys> and C<values> may return huge lists
948 when used on large DBM files. You may prefer to use the C<each>
949 function to iterate over large DBM files. Example:
951 # print out history file offsets
952 dbmopen(%HIST,'/usr/lib/news/history',0666);
953 while (($key,$val) = each %HIST) {
954 print $key, ' = ', unpack('L',$val), "\n";
958 See also L<AnyDBM_File> for a more general description of the pros and
959 cons of the various dbm approaches, as well as L<DB_File> for a particularly
962 You can control which DBM library you use by loading that library
963 before you call dbmopen():
966 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
967 or die "Can't open netscape history file: $!";
973 Returns a Boolean value telling whether EXPR has a value other than
974 the undefined value C<undef>. If EXPR is not present, C<$_> will be
977 Many operations return C<undef> to indicate failure, end of file,
978 system error, uninitialized variable, and other exceptional
979 conditions. This function allows you to distinguish C<undef> from
980 other values. (A simple Boolean test will not distinguish among
981 C<undef>, zero, the empty string, and C<"0">, which are all equally
982 false.) Note that since C<undef> is a valid scalar, its presence
983 doesn't I<necessarily> indicate an exceptional condition: C<pop>
984 returns C<undef> when its argument is an empty array, I<or> when the
985 element to return happens to be C<undef>.
987 You may also use C<defined(&func)> to check whether subroutine C<&func>
988 has ever been defined. The return value is unaffected by any forward
989 declarations of C<&func>. Note that a subroutine which is not defined
990 may still be callable: its package may have an C<AUTOLOAD> method that
991 makes it spring into existence the first time that it is called -- see
994 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
995 used to report whether memory for that aggregate has ever been
996 allocated. This behavior may disappear in future versions of Perl.
997 You should instead use a simple test for size:
999 if (@an_array) { print "has array elements\n" }
1000 if (%a_hash) { print "has hash members\n" }
1002 When used on a hash element, it tells you whether the value is defined,
1003 not whether the key exists in the hash. Use L</exists> for the latter
1008 print if defined $switch{'D'};
1009 print "$val\n" while defined($val = pop(@ary));
1010 die "Can't readlink $sym: $!"
1011 unless defined($value = readlink $sym);
1012 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1013 $debugging = 0 unless defined $debugging;
1015 Note: Many folks tend to overuse C<defined>, and then are surprised to
1016 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1017 defined values. For example, if you say
1021 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1022 matched "nothing". But it didn't really match nothing--rather, it
1023 matched something that happened to be zero characters long. This is all
1024 very above-board and honest. When a function returns an undefined value,
1025 it's an admission that it couldn't give you an honest answer. So you
1026 should use C<defined> only when you're questioning the integrity of what
1027 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1030 See also L</undef>, L</exists>, L</ref>.
1034 Given an expression that specifies a hash element, array element, hash slice,
1035 or array slice, deletes the specified element(s) from the hash or array.
1036 In the case of an array, if the array elements happen to be at the end,
1037 the size of the array will shrink to the highest element that tests
1038 true for exists() (or 0 if no such element exists).
1040 Returns a list with the same number of elements as the number of elements
1041 for which deletion was attempted. Each element of that list consists of
1042 either the value of the element deleted, or the undefined value. In scalar
1043 context, this means that you get the value of the last element deleted (or
1044 the undefined value if that element did not exist).
1046 %hash = (foo => 11, bar => 22, baz => 33);
1047 $scalar = delete $hash{foo}; # $scalar is 11
1048 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1049 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1051 Deleting from C<%ENV> modifies the environment. Deleting from
1052 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1053 from a C<tie>d hash or array may not necessarily return anything.
1055 Deleting an array element effectively returns that position of the array
1056 to its initial, uninitialized state. Subsequently testing for the same
1057 element with exists() will return false. Note that deleting array
1058 elements in the middle of an array will not shift the index of the ones
1059 after them down--use splice() for that. See L</exists>.
1061 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1063 foreach $key (keys %HASH) {
1067 foreach $index (0 .. $#ARRAY) {
1068 delete $ARRAY[$index];
1073 delete @HASH{keys %HASH};
1075 delete @ARRAY[0 .. $#ARRAY];
1077 But both of these are slower than just assigning the empty list
1078 or undefining %HASH or @ARRAY:
1080 %HASH = (); # completely empty %HASH
1081 undef %HASH; # forget %HASH ever existed
1083 @ARRAY = (); # completely empty @ARRAY
1084 undef @ARRAY; # forget @ARRAY ever existed
1086 Note that the EXPR can be arbitrarily complicated as long as the final
1087 operation is a hash element, array element, hash slice, or array slice
1090 delete $ref->[$x][$y]{$key};
1091 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1093 delete $ref->[$x][$y][$index];
1094 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1098 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1099 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1100 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1101 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1102 an C<eval(),> the error message is stuffed into C<$@> and the
1103 C<eval> is terminated with the undefined value. This makes
1104 C<die> the way to raise an exception.
1106 Equivalent examples:
1108 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1109 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1111 If the last element of LIST does not end in a newline, the current
1112 script line number and input line number (if any) are also printed,
1113 and a newline is supplied. Note that the "input line number" (also
1114 known as "chunk") is subject to whatever notion of "line" happens to
1115 be currently in effect, and is also available as the special variable
1116 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1118 Hint: sometimes appending C<", stopped"> to your message will cause it
1119 to make better sense when the string C<"at foo line 123"> is appended.
1120 Suppose you are running script "canasta".
1122 die "/etc/games is no good";
1123 die "/etc/games is no good, stopped";
1125 produce, respectively
1127 /etc/games is no good at canasta line 123.
1128 /etc/games is no good, stopped at canasta line 123.
1130 See also exit(), warn(), and the Carp module.
1132 If LIST is empty and C<$@> already contains a value (typically from a
1133 previous eval) that value is reused after appending C<"\t...propagated">.
1134 This is useful for propagating exceptions:
1137 die unless $@ =~ /Expected exception/;
1139 If LIST is empty and C<$@> contains an object reference that has a
1140 C<PROPAGATE> method, that method will be called with additional file
1141 and line number parameters. The return value replaces the value in
1142 C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1145 If C<$@> is empty then the string C<"Died"> is used.
1147 die() can also be called with a reference argument. If this happens to be
1148 trapped within an eval(), $@ contains the reference. This behavior permits
1149 a more elaborate exception handling implementation using objects that
1150 maintain arbitrary state about the nature of the exception. Such a scheme
1151 is sometimes preferable to matching particular string values of $@ using
1152 regular expressions. Here's an example:
1154 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1156 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1157 # handle Some::Module::Exception
1160 # handle all other possible exceptions
1164 Because perl will stringify uncaught exception messages before displaying
1165 them, you may want to overload stringification operations on such custom
1166 exception objects. See L<overload> for details about that.
1168 You can arrange for a callback to be run just before the C<die>
1169 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1170 handler will be called with the error text and can change the error
1171 message, if it sees fit, by calling C<die> again. See
1172 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1173 L<"eval BLOCK"> for some examples. Although this feature was meant
1174 to be run only right before your program was to exit, this is not
1175 currently the case--the C<$SIG{__DIE__}> hook is currently called
1176 even inside eval()ed blocks/strings! If one wants the hook to do
1177 nothing in such situations, put
1181 as the first line of the handler (see L<perlvar/$^S>). Because
1182 this promotes strange action at a distance, this counterintuitive
1183 behavior may be fixed in a future release.
1187 Not really a function. Returns the value of the last command in the
1188 sequence of commands indicated by BLOCK. When modified by a loop
1189 modifier, executes the BLOCK once before testing the loop condition.
1190 (On other statements the loop modifiers test the conditional first.)
1192 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1193 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1194 See L<perlsyn> for alternative strategies.
1196 =item do SUBROUTINE(LIST)
1198 A deprecated form of subroutine call. See L<perlsub>.
1202 Uses the value of EXPR as a filename and executes the contents of the
1203 file as a Perl script. Its primary use is to include subroutines
1204 from a Perl subroutine library.
1212 except that it's more efficient and concise, keeps track of the current
1213 filename for error messages, searches the @INC libraries, and updates
1214 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1215 variables. It also differs in that code evaluated with C<do FILENAME>
1216 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1217 same, however, in that it does reparse the file every time you call it,
1218 so you probably don't want to do this inside a loop.
1220 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1221 error. If C<do> can read the file but cannot compile it, it
1222 returns undef and sets an error message in C<$@>. If the file is
1223 successfully compiled, C<do> returns the value of the last expression
1226 Note that inclusion of library modules is better done with the
1227 C<use> and C<require> operators, which also do automatic error checking
1228 and raise an exception if there's a problem.
1230 You might like to use C<do> to read in a program configuration
1231 file. Manual error checking can be done this way:
1233 # read in config files: system first, then user
1234 for $file ("/share/prog/defaults.rc",
1235 "$ENV{HOME}/.someprogrc")
1237 unless ($return = do $file) {
1238 warn "couldn't parse $file: $@" if $@;
1239 warn "couldn't do $file: $!" unless defined $return;
1240 warn "couldn't run $file" unless $return;
1248 This function causes an immediate core dump. See also the B<-u>
1249 command-line switch in L<perlrun>, which does the same thing.
1250 Primarily this is so that you can use the B<undump> program (not
1251 supplied) to turn your core dump into an executable binary after
1252 having initialized all your variables at the beginning of the
1253 program. When the new binary is executed it will begin by executing
1254 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1255 Think of it as a goto with an intervening core dump and reincarnation.
1256 If C<LABEL> is omitted, restarts the program from the top.
1258 B<WARNING>: Any files opened at the time of the dump will I<not>
1259 be open any more when the program is reincarnated, with possible
1260 resulting confusion on the part of Perl.
1262 This function is now largely obsolete, partly because it's very
1263 hard to convert a core file into an executable, and because the
1264 real compiler backends for generating portable bytecode and compilable
1265 C code have superseded it. That's why you should now invoke it as
1266 C<CORE::dump()>, if you don't want to be warned against a possible
1269 If you're looking to use L<dump> to speed up your program, consider
1270 generating bytecode or native C code as described in L<perlcc>. If
1271 you're just trying to accelerate a CGI script, consider using the
1272 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1273 You might also consider autoloading or selfloading, which at least
1274 make your program I<appear> to run faster.
1278 When called in list context, returns a 2-element list consisting of the
1279 key and value for the next element of a hash, so that you can iterate over
1280 it. When called in scalar context, returns only the key for the next
1281 element in the hash.
1283 Entries are returned in an apparently random order. The actual random
1284 order is subject to change in future versions of perl, but it is
1285 guaranteed to be in the same order as either the C<keys> or C<values>
1286 function would produce on the same (unmodified) hash. Since Perl
1287 5.8.1 the ordering is different even between different runs of Perl
1288 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1290 When the hash is entirely read, a null array is returned in list context
1291 (which when assigned produces a false (C<0>) value), and C<undef> in
1292 scalar context. The next call to C<each> after that will start iterating
1293 again. There is a single iterator for each hash, shared by all C<each>,
1294 C<keys>, and C<values> function calls in the program; it can be reset by
1295 reading all the elements from the hash, or by evaluating C<keys HASH> or
1296 C<values HASH>. If you add or delete elements of a hash while you're
1297 iterating over it, you may get entries skipped or duplicated, so
1298 don't. Exception: It is always safe to delete the item most recently
1299 returned by C<each()>, which means that the following code will work:
1301 while (($key, $value) = each %hash) {
1303 delete $hash{$key}; # This is safe
1306 The following prints out your environment like the printenv(1) program,
1307 only in a different order:
1309 while (($key,$value) = each %ENV) {
1310 print "$key=$value\n";
1313 See also C<keys>, C<values> and C<sort>.
1315 =item eof FILEHANDLE
1321 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1322 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1323 gives the real filehandle. (Note that this function actually
1324 reads a character and then C<ungetc>s it, so isn't very useful in an
1325 interactive context.) Do not read from a terminal file (or call
1326 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1327 as terminals may lose the end-of-file condition if you do.
1329 An C<eof> without an argument uses the last file read. Using C<eof()>
1330 with empty parentheses is very different. It refers to the pseudo file
1331 formed from the files listed on the command line and accessed via the
1332 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1333 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1334 used will cause C<@ARGV> to be examined to determine if input is
1335 available. Similarly, an C<eof()> after C<< <> >> has returned
1336 end-of-file will assume you are processing another C<@ARGV> list,
1337 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1338 see L<perlop/"I/O Operators">.
1340 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1341 detect the end of each file, C<eof()> will only detect the end of the
1342 last file. Examples:
1344 # reset line numbering on each input file
1346 next if /^\s*#/; # skip comments
1349 close ARGV if eof; # Not eof()!
1352 # insert dashes just before last line of last file
1354 if (eof()) { # check for end of last file
1355 print "--------------\n";
1358 last if eof(); # needed if we're reading from a terminal
1361 Practical hint: you almost never need to use C<eof> in Perl, because the
1362 input operators typically return C<undef> when they run out of data, or if
1369 In the first form, the return value of EXPR is parsed and executed as if it
1370 were a little Perl program. The value of the expression (which is itself
1371 determined within scalar context) is first parsed, and if there weren't any
1372 errors, executed in the lexical context of the current Perl program, so
1373 that any variable settings or subroutine and format definitions remain
1374 afterwards. Note that the value is parsed every time the eval executes.
1375 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1376 delay parsing and subsequent execution of the text of EXPR until run time.
1378 In the second form, the code within the BLOCK is parsed only once--at the
1379 same time the code surrounding the eval itself was parsed--and executed
1380 within the context of the current Perl program. This form is typically
1381 used to trap exceptions more efficiently than the first (see below), while
1382 also providing the benefit of checking the code within BLOCK at compile
1385 The final semicolon, if any, may be omitted from the value of EXPR or within
1388 In both forms, the value returned is the value of the last expression
1389 evaluated inside the mini-program; a return statement may be also used, just
1390 as with subroutines. The expression providing the return value is evaluated
1391 in void, scalar, or list context, depending on the context of the eval itself.
1392 See L</wantarray> for more on how the evaluation context can be determined.
1394 If there is a syntax error or runtime error, or a C<die> statement is
1395 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1396 error message. If there was no error, C<$@> is guaranteed to be a null
1397 string. Beware that using C<eval> neither silences perl from printing
1398 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1399 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1400 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1401 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1403 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1404 determining whether a particular feature (such as C<socket> or C<symlink>)
1405 is implemented. It is also Perl's exception trapping mechanism, where
1406 the die operator is used to raise exceptions.
1408 If the code to be executed doesn't vary, you may use the eval-BLOCK
1409 form to trap run-time errors without incurring the penalty of
1410 recompiling each time. The error, if any, is still returned in C<$@>.
1413 # make divide-by-zero nonfatal
1414 eval { $answer = $a / $b; }; warn $@ if $@;
1416 # same thing, but less efficient
1417 eval '$answer = $a / $b'; warn $@ if $@;
1419 # a compile-time error
1420 eval { $answer = }; # WRONG
1423 eval '$answer ='; # sets $@
1425 Due to the current arguably broken state of C<__DIE__> hooks, when using
1426 the C<eval{}> form as an exception trap in libraries, you may wish not
1427 to trigger any C<__DIE__> hooks that user code may have installed.
1428 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1429 as shown in this example:
1431 # a very private exception trap for divide-by-zero
1432 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1435 This is especially significant, given that C<__DIE__> hooks can call
1436 C<die> again, which has the effect of changing their error messages:
1438 # __DIE__ hooks may modify error messages
1440 local $SIG{'__DIE__'} =
1441 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1442 eval { die "foo lives here" };
1443 print $@ if $@; # prints "bar lives here"
1446 Because this promotes action at a distance, this counterintuitive behavior
1447 may be fixed in a future release.
1449 With an C<eval>, you should be especially careful to remember what's
1450 being looked at when:
1456 eval { $x }; # CASE 4
1458 eval "\$$x++"; # CASE 5
1461 Cases 1 and 2 above behave identically: they run the code contained in
1462 the variable $x. (Although case 2 has misleading double quotes making
1463 the reader wonder what else might be happening (nothing is).) Cases 3
1464 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1465 does nothing but return the value of $x. (Case 4 is preferred for
1466 purely visual reasons, but it also has the advantage of compiling at
1467 compile-time instead of at run-time.) Case 5 is a place where
1468 normally you I<would> like to use double quotes, except that in this
1469 particular situation, you can just use symbolic references instead, as
1472 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1473 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1475 Note that as a very special case, an C<eval ''> executed within the C<DB>
1476 package doesn't see the usual surrounding lexical scope, but rather the
1477 scope of the first non-DB piece of code that called it. You don't normally
1478 need to worry about this unless you are writing a Perl debugger.
1482 =item exec PROGRAM LIST
1484 The C<exec> function executes a system command I<and never returns>--
1485 use C<system> instead of C<exec> if you want it to return. It fails and
1486 returns false only if the command does not exist I<and> it is executed
1487 directly instead of via your system's command shell (see below).
1489 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1490 warns you if there is a following statement which isn't C<die>, C<warn>,
1491 or C<exit> (if C<-w> is set - but you always do that). If you
1492 I<really> want to follow an C<exec> with some other statement, you
1493 can use one of these styles to avoid the warning:
1495 exec ('foo') or print STDERR "couldn't exec foo: $!";
1496 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1498 If there is more than one argument in LIST, or if LIST is an array
1499 with more than one value, calls execvp(3) with the arguments in LIST.
1500 If there is only one scalar argument or an array with one element in it,
1501 the argument is checked for shell metacharacters, and if there are any,
1502 the entire argument is passed to the system's command shell for parsing
1503 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1504 If there are no shell metacharacters in the argument, it is split into
1505 words and passed directly to C<execvp>, which is more efficient.
1508 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1509 exec "sort $outfile | uniq";
1511 If you don't really want to execute the first argument, but want to lie
1512 to the program you are executing about its own name, you can specify
1513 the program you actually want to run as an "indirect object" (without a
1514 comma) in front of the LIST. (This always forces interpretation of the
1515 LIST as a multivalued list, even if there is only a single scalar in
1518 $shell = '/bin/csh';
1519 exec $shell '-sh'; # pretend it's a login shell
1523 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1525 When the arguments get executed via the system shell, results will
1526 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1529 Using an indirect object with C<exec> or C<system> is also more
1530 secure. This usage (which also works fine with system()) forces
1531 interpretation of the arguments as a multivalued list, even if the
1532 list had just one argument. That way you're safe from the shell
1533 expanding wildcards or splitting up words with whitespace in them.
1535 @args = ( "echo surprise" );
1537 exec @args; # subject to shell escapes
1539 exec { $args[0] } @args; # safe even with one-arg list
1541 The first version, the one without the indirect object, ran the I<echo>
1542 program, passing it C<"surprise"> an argument. The second version
1543 didn't--it tried to run a program literally called I<"echo surprise">,
1544 didn't find it, and set C<$?> to a non-zero value indicating failure.
1546 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1547 output before the exec, but this may not be supported on some platforms
1548 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1549 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1550 open handles in order to avoid lost output.
1552 Note that C<exec> will not call your C<END> blocks, nor will it call
1553 any C<DESTROY> methods in your objects.
1557 Given an expression that specifies a hash element or array element,
1558 returns true if the specified element in the hash or array has ever
1559 been initialized, even if the corresponding value is undefined. The
1560 element is not autovivified if it doesn't exist.
1562 print "Exists\n" if exists $hash{$key};
1563 print "Defined\n" if defined $hash{$key};
1564 print "True\n" if $hash{$key};
1566 print "Exists\n" if exists $array[$index];
1567 print "Defined\n" if defined $array[$index];
1568 print "True\n" if $array[$index];
1570 A hash or array element can be true only if it's defined, and defined if
1571 it exists, but the reverse doesn't necessarily hold true.
1573 Given an expression that specifies the name of a subroutine,
1574 returns true if the specified subroutine has ever been declared, even
1575 if it is undefined. Mentioning a subroutine name for exists or defined
1576 does not count as declaring it. Note that a subroutine which does not
1577 exist may still be callable: its package may have an C<AUTOLOAD>
1578 method that makes it spring into existence the first time that it is
1579 called -- see L<perlsub>.
1581 print "Exists\n" if exists &subroutine;
1582 print "Defined\n" if defined &subroutine;
1584 Note that the EXPR can be arbitrarily complicated as long as the final
1585 operation is a hash or array key lookup or subroutine name:
1587 if (exists $ref->{A}->{B}->{$key}) { }
1588 if (exists $hash{A}{B}{$key}) { }
1590 if (exists $ref->{A}->{B}->[$ix]) { }
1591 if (exists $hash{A}{B}[$ix]) { }
1593 if (exists &{$ref->{A}{B}{$key}}) { }
1595 Although the deepest nested array or hash will not spring into existence
1596 just because its existence was tested, any intervening ones will.
1597 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1598 into existence due to the existence test for the $key element above.
1599 This happens anywhere the arrow operator is used, including even:
1602 if (exists $ref->{"Some key"}) { }
1603 print $ref; # prints HASH(0x80d3d5c)
1605 This surprising autovivification in what does not at first--or even
1606 second--glance appear to be an lvalue context may be fixed in a future
1609 Use of a subroutine call, rather than a subroutine name, as an argument
1610 to exists() is an error.
1613 exists &sub(); # Error
1617 Evaluates EXPR and exits immediately with that value. Example:
1620 exit 0 if $ans =~ /^[Xx]/;
1622 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1623 universally recognized values for EXPR are C<0> for success and C<1>
1624 for error; other values are subject to interpretation depending on the
1625 environment in which the Perl program is running. For example, exiting
1626 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1627 the mailer to return the item undelivered, but that's not true everywhere.
1629 Don't use C<exit> to abort a subroutine if there's any chance that
1630 someone might want to trap whatever error happened. Use C<die> instead,
1631 which can be trapped by an C<eval>.
1633 The exit() function does not always exit immediately. It calls any
1634 defined C<END> routines first, but these C<END> routines may not
1635 themselves abort the exit. Likewise any object destructors that need to
1636 be called are called before the real exit. If this is a problem, you
1637 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1638 See L<perlmod> for details.
1644 Returns I<e> (the natural logarithm base) to the power of EXPR.
1645 If EXPR is omitted, gives C<exp($_)>.
1647 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1649 Implements the fcntl(2) function. You'll probably have to say
1653 first to get the correct constant definitions. Argument processing and
1654 value return works just like C<ioctl> below.
1658 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1659 or die "can't fcntl F_GETFL: $!";
1661 You don't have to check for C<defined> on the return from C<fcntl>.
1662 Like C<ioctl>, it maps a C<0> return from the system call into
1663 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1664 in numeric context. It is also exempt from the normal B<-w> warnings
1665 on improper numeric conversions.
1667 Note that C<fcntl> will produce a fatal error if used on a machine that
1668 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1669 manpage to learn what functions are available on your system.
1671 Here's an example of setting a filehandle named C<REMOTE> to be
1672 non-blocking at the system level. You'll have to negotiate C<$|>
1673 on your own, though.
1675 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1677 $flags = fcntl(REMOTE, F_GETFL, 0)
1678 or die "Can't get flags for the socket: $!\n";
1680 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1681 or die "Can't set flags for the socket: $!\n";
1683 =item fileno FILEHANDLE
1685 Returns the file descriptor for a filehandle, or undefined if the
1686 filehandle is not open. This is mainly useful for constructing
1687 bitmaps for C<select> and low-level POSIX tty-handling operations.
1688 If FILEHANDLE is an expression, the value is taken as an indirect
1689 filehandle, generally its name.
1691 You can use this to find out whether two handles refer to the
1692 same underlying descriptor:
1694 if (fileno(THIS) == fileno(THAT)) {
1695 print "THIS and THAT are dups\n";
1698 (Filehandles connected to memory objects via new features of C<open> may
1699 return undefined even though they are open.)
1702 =item flock FILEHANDLE,OPERATION
1704 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1705 for success, false on failure. Produces a fatal error if used on a
1706 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1707 C<flock> is Perl's portable file locking interface, although it locks
1708 only entire files, not records.
1710 Two potentially non-obvious but traditional C<flock> semantics are
1711 that it waits indefinitely until the lock is granted, and that its locks
1712 B<merely advisory>. Such discretionary locks are more flexible, but offer
1713 fewer guarantees. This means that files locked with C<flock> may be
1714 modified by programs that do not also use C<flock>. See L<perlport>,
1715 your port's specific documentation, or your system-specific local manpages
1716 for details. It's best to assume traditional behavior if you're writing
1717 portable programs. (But if you're not, you should as always feel perfectly
1718 free to write for your own system's idiosyncrasies (sometimes called
1719 "features"). Slavish adherence to portability concerns shouldn't get
1720 in the way of your getting your job done.)
1722 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1723 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1724 you can use the symbolic names if you import them from the Fcntl module,
1725 either individually, or as a group using the ':flock' tag. LOCK_SH
1726 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1727 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1728 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1729 waiting for the lock (check the return status to see if you got it).
1731 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1732 before locking or unlocking it.
1734 Note that the emulation built with lockf(3) doesn't provide shared
1735 locks, and it requires that FILEHANDLE be open with write intent. These
1736 are the semantics that lockf(3) implements. Most if not all systems
1737 implement lockf(3) in terms of fcntl(2) locking, though, so the
1738 differing semantics shouldn't bite too many people.
1740 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1741 be open with read intent to use LOCK_SH and requires that it be open
1742 with write intent to use LOCK_EX.
1744 Note also that some versions of C<flock> cannot lock things over the
1745 network; you would need to use the more system-specific C<fcntl> for
1746 that. If you like you can force Perl to ignore your system's flock(2)
1747 function, and so provide its own fcntl(2)-based emulation, by passing
1748 the switch C<-Ud_flock> to the F<Configure> program when you configure
1751 Here's a mailbox appender for BSD systems.
1753 use Fcntl ':flock'; # import LOCK_* constants
1756 flock(MBOX,LOCK_EX);
1757 # and, in case someone appended
1758 # while we were waiting...
1763 flock(MBOX,LOCK_UN);
1766 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1767 or die "Can't open mailbox: $!";
1770 print MBOX $msg,"\n\n";
1773 On systems that support a real flock(), locks are inherited across fork()
1774 calls, whereas those that must resort to the more capricious fcntl()
1775 function lose the locks, making it harder to write servers.
1777 See also L<DB_File> for other flock() examples.
1781 Does a fork(2) system call to create a new process running the
1782 same program at the same point. It returns the child pid to the
1783 parent process, C<0> to the child process, or C<undef> if the fork is
1784 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1785 are shared, while everything else is copied. On most systems supporting
1786 fork(), great care has gone into making it extremely efficient (for
1787 example, using copy-on-write technology on data pages), making it the
1788 dominant paradigm for multitasking over the last few decades.
1790 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1791 output before forking the child process, but this may not be supported
1792 on some platforms (see L<perlport>). To be safe, you may need to set
1793 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1794 C<IO::Handle> on any open handles in order to avoid duplicate output.
1796 If you C<fork> without ever waiting on your children, you will
1797 accumulate zombies. On some systems, you can avoid this by setting
1798 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1799 forking and reaping moribund children.
1801 Note that if your forked child inherits system file descriptors like
1802 STDIN and STDOUT that are actually connected by a pipe or socket, even
1803 if you exit, then the remote server (such as, say, a CGI script or a
1804 backgrounded job launched from a remote shell) won't think you're done.
1805 You should reopen those to F</dev/null> if it's any issue.
1809 Declare a picture format for use by the C<write> function. For
1813 Test: @<<<<<<<< @||||| @>>>>>
1814 $str, $%, '$' . int($num)
1818 $num = $cost/$quantity;
1822 See L<perlform> for many details and examples.
1824 =item formline PICTURE,LIST
1826 This is an internal function used by C<format>s, though you may call it,
1827 too. It formats (see L<perlform>) a list of values according to the
1828 contents of PICTURE, placing the output into the format output
1829 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1830 Eventually, when a C<write> is done, the contents of
1831 C<$^A> are written to some filehandle, but you could also read C<$^A>
1832 yourself and then set C<$^A> back to C<"">. Note that a format typically
1833 does one C<formline> per line of form, but the C<formline> function itself
1834 doesn't care how many newlines are embedded in the PICTURE. This means
1835 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1836 You may therefore need to use multiple formlines to implement a single
1837 record format, just like the format compiler.
1839 Be careful if you put double quotes around the picture, because an C<@>
1840 character may be taken to mean the beginning of an array name.
1841 C<formline> always returns true. See L<perlform> for other examples.
1843 =item getc FILEHANDLE
1847 Returns the next character from the input file attached to FILEHANDLE,
1848 or the undefined value at end of file, or if there was an error (in
1849 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1850 STDIN. This is not particularly efficient. However, it cannot be
1851 used by itself to fetch single characters without waiting for the user
1852 to hit enter. For that, try something more like:
1855 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1858 system "stty", '-icanon', 'eol', "\001";
1864 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1867 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1871 Determination of whether $BSD_STYLE should be set
1872 is left as an exercise to the reader.
1874 The C<POSIX::getattr> function can do this more portably on
1875 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1876 module from your nearest CPAN site; details on CPAN can be found on
1881 Implements the C library function of the same name, which on most
1882 systems returns the current login from F</etc/utmp>, if any. If null,
1885 $login = getlogin || getpwuid($<) || "Kilroy";
1887 Do not consider C<getlogin> for authentication: it is not as
1888 secure as C<getpwuid>.
1890 =item getpeername SOCKET
1892 Returns the packed sockaddr address of other end of the SOCKET connection.
1895 $hersockaddr = getpeername(SOCK);
1896 ($port, $iaddr) = sockaddr_in($hersockaddr);
1897 $herhostname = gethostbyaddr($iaddr, AF_INET);
1898 $herstraddr = inet_ntoa($iaddr);
1902 Returns the current process group for the specified PID. Use
1903 a PID of C<0> to get the current process group for the
1904 current process. Will raise an exception if used on a machine that
1905 doesn't implement getpgrp(2). If PID is omitted, returns process
1906 group of current process. Note that the POSIX version of C<getpgrp>
1907 does not accept a PID argument, so only C<PID==0> is truly portable.
1911 Returns the process id of the parent process.
1913 Note for Linux users: on Linux, the C functions C<getpid()> and
1914 C<getppid()> return different values from different threads. In order to
1915 be portable, this behavior is not reflected by the perl-level function
1916 C<getppid()>, that returns a consistent value across threads. If you want
1917 to call the underlying C<getppid()>, you may use the CPAN module
1920 =item getpriority WHICH,WHO
1922 Returns the current priority for a process, a process group, or a user.
1923 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1924 machine that doesn't implement getpriority(2).
1930 =item gethostbyname NAME
1932 =item getnetbyname NAME
1934 =item getprotobyname NAME
1940 =item getservbyname NAME,PROTO
1942 =item gethostbyaddr ADDR,ADDRTYPE
1944 =item getnetbyaddr ADDR,ADDRTYPE
1946 =item getprotobynumber NUMBER
1948 =item getservbyport PORT,PROTO
1966 =item sethostent STAYOPEN
1968 =item setnetent STAYOPEN
1970 =item setprotoent STAYOPEN
1972 =item setservent STAYOPEN
1986 These routines perform the same functions as their counterparts in the
1987 system library. In list context, the return values from the
1988 various get routines are as follows:
1990 ($name,$passwd,$uid,$gid,
1991 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1992 ($name,$passwd,$gid,$members) = getgr*
1993 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1994 ($name,$aliases,$addrtype,$net) = getnet*
1995 ($name,$aliases,$proto) = getproto*
1996 ($name,$aliases,$port,$proto) = getserv*
1998 (If the entry doesn't exist you get a null list.)
2000 The exact meaning of the $gcos field varies but it usually contains
2001 the real name of the user (as opposed to the login name) and other
2002 information pertaining to the user. Beware, however, that in many
2003 system users are able to change this information and therefore it
2004 cannot be trusted and therefore the $gcos is tainted (see
2005 L<perlsec>). The $passwd and $shell, user's encrypted password and
2006 login shell, are also tainted, because of the same reason.
2008 In scalar context, you get the name, unless the function was a
2009 lookup by name, in which case you get the other thing, whatever it is.
2010 (If the entry doesn't exist you get the undefined value.) For example:
2012 $uid = getpwnam($name);
2013 $name = getpwuid($num);
2015 $gid = getgrnam($name);
2016 $name = getgrgid($num);
2020 In I<getpw*()> the fields $quota, $comment, and $expire are special
2021 cases in the sense that in many systems they are unsupported. If the
2022 $quota is unsupported, it is an empty scalar. If it is supported, it
2023 usually encodes the disk quota. If the $comment field is unsupported,
2024 it is an empty scalar. If it is supported it usually encodes some
2025 administrative comment about the user. In some systems the $quota
2026 field may be $change or $age, fields that have to do with password
2027 aging. In some systems the $comment field may be $class. The $expire
2028 field, if present, encodes the expiration period of the account or the
2029 password. For the availability and the exact meaning of these fields
2030 in your system, please consult your getpwnam(3) documentation and your
2031 F<pwd.h> file. You can also find out from within Perl what your
2032 $quota and $comment fields mean and whether you have the $expire field
2033 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2034 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2035 files are only supported if your vendor has implemented them in the
2036 intuitive fashion that calling the regular C library routines gets the
2037 shadow versions if you're running under privilege or if there exists
2038 the shadow(3) functions as found in System V ( this includes Solaris
2039 and Linux.) Those systems which implement a proprietary shadow password
2040 facility are unlikely to be supported.
2042 The $members value returned by I<getgr*()> is a space separated list of
2043 the login names of the members of the group.
2045 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2046 C, it will be returned to you via C<$?> if the function call fails. The
2047 C<@addrs> value returned by a successful call is a list of the raw
2048 addresses returned by the corresponding system library call. In the
2049 Internet domain, each address is four bytes long and you can unpack it
2050 by saying something like:
2052 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2054 The Socket library makes this slightly easier:
2057 $iaddr = inet_aton("127.1"); # or whatever address
2058 $name = gethostbyaddr($iaddr, AF_INET);
2060 # or going the other way
2061 $straddr = inet_ntoa($iaddr);
2063 If you get tired of remembering which element of the return list
2064 contains which return value, by-name interfaces are provided
2065 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2066 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2067 and C<User::grent>. These override the normal built-ins, supplying
2068 versions that return objects with the appropriate names
2069 for each field. For example:
2073 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2075 Even though it looks like they're the same method calls (uid),
2076 they aren't, because a C<File::stat> object is different from
2077 a C<User::pwent> object.
2079 =item getsockname SOCKET
2081 Returns the packed sockaddr address of this end of the SOCKET connection,
2082 in case you don't know the address because you have several different
2083 IPs that the connection might have come in on.
2086 $mysockaddr = getsockname(SOCK);
2087 ($port, $myaddr) = sockaddr_in($mysockaddr);
2088 printf "Connect to %s [%s]\n",
2089 scalar gethostbyaddr($myaddr, AF_INET),
2092 =item getsockopt SOCKET,LEVEL,OPTNAME
2094 Returns the socket option requested, or undef if there is an error.
2100 In list context, returns a (possibly empty) list of filename expansions on
2101 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2102 scalar context, glob iterates through such filename expansions, returning
2103 undef when the list is exhausted. This is the internal function
2104 implementing the C<< <*.c> >> operator, but you can use it directly. If
2105 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2106 more detail in L<perlop/"I/O Operators">.
2108 Beginning with v5.6.0, this operator is implemented using the standard
2109 C<File::Glob> extension. See L<File::Glob> for details.
2113 Converts a time as returned by the time function to an 8-element list
2114 with the time localized for the standard Greenwich time zone.
2115 Typically used as follows:
2118 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2121 All list elements are numeric, and come straight out of the C `struct
2122 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2123 specified time. $mday is the day of the month, and $mon is the month
2124 itself, in the range C<0..11> with 0 indicating January and 11
2125 indicating December. $year is the number of years since 1900. That
2126 is, $year is C<123> in year 2023. $wday is the day of the week, with
2127 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2128 the year, in the range C<0..364> (or C<0..365> in leap years.)
2130 Note that the $year element is I<not> simply the last two digits of
2131 the year. If you assume it is, then you create non-Y2K-compliant
2132 programs--and you wouldn't want to do that, would you?
2134 The proper way to get a complete 4-digit year is simply:
2138 And to get the last two digits of the year (e.g., '01' in 2001) do:
2140 $year = sprintf("%02d", $year % 100);
2142 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2144 In scalar context, C<gmtime()> returns the ctime(3) value:
2146 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2148 Also see the C<timegm> function provided by the C<Time::Local> module,
2149 and the strftime(3) function available via the POSIX module.
2151 This scalar value is B<not> locale dependent (see L<perllocale>), but
2152 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2153 strftime(3) and mktime(3) functions available via the POSIX module. To
2154 get somewhat similar but locale dependent date strings, set up your
2155 locale environment variables appropriately (please see L<perllocale>)
2156 and try for example:
2158 use POSIX qw(strftime);
2159 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2161 Note that the C<%a> and C<%b> escapes, which represent the short forms
2162 of the day of the week and the month of the year, may not necessarily
2163 be three characters wide in all locales.
2171 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2172 execution there. It may not be used to go into any construct that
2173 requires initialization, such as a subroutine or a C<foreach> loop. It
2174 also can't be used to go into a construct that is optimized away,
2175 or to get out of a block or subroutine given to C<sort>.
2176 It can be used to go almost anywhere else within the dynamic scope,
2177 including out of subroutines, but it's usually better to use some other
2178 construct such as C<last> or C<die>. The author of Perl has never felt the
2179 need to use this form of C<goto> (in Perl, that is--C is another matter).
2180 (The difference being that C does not offer named loops combined with
2181 loop control. Perl does, and this replaces most structured uses of C<goto>
2182 in other languages.)
2184 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2185 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2186 necessarily recommended if you're optimizing for maintainability:
2188 goto ("FOO", "BAR", "GLARCH")[$i];
2190 The C<goto-&NAME> form is quite different from the other forms of
2191 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2192 doesn't have the stigma associated with other gotos. Instead, it
2193 exits the current subroutine (losing any changes set by local()) and
2194 immediately calls in its place the named subroutine using the current
2195 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2196 load another subroutine and then pretend that the other subroutine had
2197 been called in the first place (except that any modifications to C<@_>
2198 in the current subroutine are propagated to the other subroutine.)
2199 After the C<goto>, not even C<caller> will be able to tell that this
2200 routine was called first.
2202 NAME needn't be the name of a subroutine; it can be a scalar variable
2203 containing a code reference, or a block which evaluates to a code
2206 =item grep BLOCK LIST
2208 =item grep EXPR,LIST
2210 This is similar in spirit to, but not the same as, grep(1) and its
2211 relatives. In particular, it is not limited to using regular expressions.
2213 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2214 C<$_> to each element) and returns the list value consisting of those
2215 elements for which the expression evaluated to true. In scalar
2216 context, returns the number of times the expression was true.
2218 @foo = grep(!/^#/, @bar); # weed out comments
2222 @foo = grep {!/^#/} @bar; # weed out comments
2224 Note that C<$_> is an alias to the list value, so it can be used to
2225 modify the elements of the LIST. While this is useful and supported,
2226 it can cause bizarre results if the elements of LIST are not variables.
2227 Similarly, grep returns aliases into the original list, much as a for
2228 loop's index variable aliases the list elements. That is, modifying an
2229 element of a list returned by grep (for example, in a C<foreach>, C<map>
2230 or another C<grep>) actually modifies the element in the original list.
2231 This is usually something to be avoided when writing clear code.
2233 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2239 Interprets EXPR as a hex string and returns the corresponding value.
2240 (To convert strings that might start with either 0, 0x, or 0b, see
2241 L</oct>.) If EXPR is omitted, uses C<$_>.
2243 print hex '0xAf'; # prints '175'
2244 print hex 'aF'; # same
2246 Hex strings may only represent integers. Strings that would cause
2247 integer overflow trigger a warning. Leading whitespace is not stripped,
2252 There is no builtin C<import> function. It is just an ordinary
2253 method (subroutine) defined (or inherited) by modules that wish to export
2254 names to another module. The C<use> function calls the C<import> method
2255 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2257 =item index STR,SUBSTR,POSITION
2259 =item index STR,SUBSTR
2261 The index function searches for one string within another, but without
2262 the wildcard-like behavior of a full regular-expression pattern match.
2263 It returns the position of the first occurrence of SUBSTR in STR at
2264 or after POSITION. If POSITION is omitted, starts searching from the
2265 beginning of the string. The return value is based at C<0> (or whatever
2266 you've set the C<$[> variable to--but don't do that). If the substring
2267 is not found, returns one less than the base, ordinarily C<-1>.
2273 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2274 You should not use this function for rounding: one because it truncates
2275 towards C<0>, and two because machine representations of floating point
2276 numbers can sometimes produce counterintuitive results. For example,
2277 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2278 because it's really more like -268.99999999999994315658 instead. Usually,
2279 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2280 functions will serve you better than will int().
2282 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2284 Implements the ioctl(2) function. You'll probably first have to say
2286 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2288 to get the correct function definitions. If F<ioctl.ph> doesn't
2289 exist or doesn't have the correct definitions you'll have to roll your
2290 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2291 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2292 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2293 written depending on the FUNCTION--a pointer to the string value of SCALAR
2294 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2295 has no string value but does have a numeric value, that value will be
2296 passed rather than a pointer to the string value. To guarantee this to be
2297 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2298 functions may be needed to manipulate the values of structures used by
2301 The return value of C<ioctl> (and C<fcntl>) is as follows:
2303 if OS returns: then Perl returns:
2305 0 string "0 but true"
2306 anything else that number
2308 Thus Perl returns true on success and false on failure, yet you can
2309 still easily determine the actual value returned by the operating
2312 $retval = ioctl(...) || -1;
2313 printf "System returned %d\n", $retval;
2315 The special string C<"0 but true"> is exempt from B<-w> complaints
2316 about improper numeric conversions.
2318 =item join EXPR,LIST
2320 Joins the separate strings of LIST into a single string with fields
2321 separated by the value of EXPR, and returns that new string. Example:
2323 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2325 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2326 first argument. Compare L</split>.
2330 Returns a list consisting of all the keys of the named hash.
2331 (In scalar context, returns the number of keys.)
2333 The keys are returned in an apparently random order. The actual
2334 random order is subject to change in future versions of perl, but it
2335 is guaranteed to be the same order as either the C<values> or C<each>
2336 function produces (given that the hash has not been modified). Since
2337 Perl 5.8.1 the ordering is different even between different runs of
2338 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2341 As a side effect, calling keys() resets the HASH's internal iterator,
2342 see L</each>. (In particular, calling keys() in void context resets
2343 the iterator with no other overhead.)
2345 Here is yet another way to print your environment:
2348 @values = values %ENV;
2350 print pop(@keys), '=', pop(@values), "\n";
2353 or how about sorted by key:
2355 foreach $key (sort(keys %ENV)) {
2356 print $key, '=', $ENV{$key}, "\n";
2359 The returned values are copies of the original keys in the hash, so
2360 modifying them will not affect the original hash. Compare L</values>.
2362 To sort a hash by value, you'll need to use a C<sort> function.
2363 Here's a descending numeric sort of a hash by its values:
2365 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2366 printf "%4d %s\n", $hash{$key}, $key;
2369 As an lvalue C<keys> allows you to increase the number of hash buckets
2370 allocated for the given hash. This can gain you a measure of efficiency if
2371 you know the hash is going to get big. (This is similar to pre-extending
2372 an array by assigning a larger number to $#array.) If you say
2376 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2377 in fact, since it rounds up to the next power of two. These
2378 buckets will be retained even if you do C<%hash = ()>, use C<undef
2379 %hash> if you want to free the storage while C<%hash> is still in scope.
2380 You can't shrink the number of buckets allocated for the hash using
2381 C<keys> in this way (but you needn't worry about doing this by accident,
2382 as trying has no effect).
2384 See also C<each>, C<values> and C<sort>.
2386 =item kill SIGNAL, LIST
2388 Sends a signal to a list of processes. Returns the number of
2389 processes successfully signaled (which is not necessarily the
2390 same as the number actually killed).
2392 $cnt = kill 1, $child1, $child2;
2395 If SIGNAL is zero, no signal is sent to the process. This is a
2396 useful way to check that a child process is alive and hasn't changed
2397 its UID. See L<perlport> for notes on the portability of this
2400 Unlike in the shell, if SIGNAL is negative, it kills
2401 process groups instead of processes. (On System V, a negative I<PROCESS>
2402 number will also kill process groups, but that's not portable.) That
2403 means you usually want to use positive not negative signals. You may also
2404 use a signal name in quotes.
2406 See L<perlipc/"Signals"> for more details.
2412 The C<last> command is like the C<break> statement in C (as used in
2413 loops); it immediately exits the loop in question. If the LABEL is
2414 omitted, the command refers to the innermost enclosing loop. The
2415 C<continue> block, if any, is not executed:
2417 LINE: while (<STDIN>) {
2418 last LINE if /^$/; # exit when done with header
2422 C<last> cannot be used to exit a block which returns a value such as
2423 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2424 a grep() or map() operation.
2426 Note that a block by itself is semantically identical to a loop
2427 that executes once. Thus C<last> can be used to effect an early
2428 exit out of such a block.
2430 See also L</continue> for an illustration of how C<last>, C<next>, and
2437 Returns a lowercased version of EXPR. This is the internal function
2438 implementing the C<\L> escape in double-quoted strings. Respects
2439 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2440 and L<perlunicode> for more details about locale and Unicode support.
2442 If EXPR is omitted, uses C<$_>.
2448 Returns the value of EXPR with the first character lowercased. This
2449 is the internal function implementing the C<\l> escape in
2450 double-quoted strings. Respects current LC_CTYPE locale if C<use
2451 locale> in force. See L<perllocale> and L<perlunicode> for more
2452 details about locale and Unicode support.
2454 If EXPR is omitted, uses C<$_>.
2460 Returns the length in I<characters> of the value of EXPR. If EXPR is
2461 omitted, returns length of C<$_>. Note that this cannot be used on
2462 an entire array or hash to find out how many elements these have.
2463 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2465 Note the I<characters>: if the EXPR is in Unicode, you will get the
2466 number of characters, not the number of bytes. To get the length
2467 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2469 =item link OLDFILE,NEWFILE
2471 Creates a new filename linked to the old filename. Returns true for
2472 success, false otherwise.
2474 =item listen SOCKET,QUEUESIZE
2476 Does the same thing that the listen system call does. Returns true if
2477 it succeeded, false otherwise. See the example in
2478 L<perlipc/"Sockets: Client/Server Communication">.
2482 You really probably want to be using C<my> instead, because C<local> isn't
2483 what most people think of as "local". See
2484 L<perlsub/"Private Variables via my()"> for details.
2486 A local modifies the listed variables to be local to the enclosing
2487 block, file, or eval. If more than one value is listed, the list must
2488 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2489 for details, including issues with tied arrays and hashes.
2491 =item localtime EXPR
2493 Converts a time as returned by the time function to a 9-element list
2494 with the time analyzed for the local time zone. Typically used as
2498 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2501 All list elements are numeric, and come straight out of the C `struct
2502 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2503 specified time. $mday is the day of the month, and $mon is the month
2504 itself, in the range C<0..11> with 0 indicating January and 11
2505 indicating December. $year is the number of years since 1900. That
2506 is, $year is C<123> in year 2023. $wday is the day of the week, with
2507 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2508 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2509 is true if the specified time occurs during daylight savings time,
2512 Note that the $year element is I<not> simply the last two digits of
2513 the year. If you assume it is, then you create non-Y2K-compliant
2514 programs--and you wouldn't want to do that, would you?
2516 The proper way to get a complete 4-digit year is simply:
2520 And to get the last two digits of the year (e.g., '01' in 2001) do:
2522 $year = sprintf("%02d", $year % 100);
2524 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2526 In scalar context, C<localtime()> returns the ctime(3) value:
2528 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2530 This scalar value is B<not> locale dependent, see L<perllocale>, but
2531 instead a Perl builtin. Also see the C<Time::Local> module
2532 (to convert the second, minutes, hours, ... back to seconds since the
2533 stroke of midnight the 1st of January 1970, the value returned by
2534 time()), and the strftime(3) and mktime(3) functions available via the
2535 POSIX module. To get somewhat similar but locale dependent date
2536 strings, set up your locale environment variables appropriately
2537 (please see L<perllocale>) and try for example:
2539 use POSIX qw(strftime);
2540 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2542 Note that the C<%a> and C<%b>, the short forms of the day of the week
2543 and the month of the year, may not necessarily be three characters wide.
2547 This function places an advisory lock on a shared variable, or referenced
2548 object contained in I<THING> until the lock goes out of scope.
2550 lock() is a "weak keyword" : this means that if you've defined a function
2551 by this name (before any calls to it), that function will be called
2552 instead. (However, if you've said C<use threads>, lock() is always a
2553 keyword.) See L<threads>.
2559 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2560 returns log of C<$_>. To get the log of another base, use basic algebra:
2561 The base-N log of a number is equal to the natural log of that number
2562 divided by the natural log of N. For example:
2566 return log($n)/log(10);
2569 See also L</exp> for the inverse operation.
2575 Does the same thing as the C<stat> function (including setting the
2576 special C<_> filehandle) but stats a symbolic link instead of the file
2577 the symbolic link points to. If symbolic links are unimplemented on
2578 your system, a normal C<stat> is done. For much more detailed
2579 information, please see the documentation for C<stat>.
2581 If EXPR is omitted, stats C<$_>.
2585 The match operator. See L<perlop>.
2587 =item map BLOCK LIST
2591 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2592 C<$_> to each element) and returns the list value composed of the
2593 results of each such evaluation. In scalar context, returns the
2594 total number of elements so generated. Evaluates BLOCK or EXPR in
2595 list context, so each element of LIST may produce zero, one, or
2596 more elements in the returned value.
2598 @chars = map(chr, @nums);
2600 translates a list of numbers to the corresponding characters. And
2602 %hash = map { getkey($_) => $_ } @array;
2604 is just a funny way to write
2607 foreach $_ (@array) {
2608 $hash{getkey($_)} = $_;
2611 Note that C<$_> is an alias to the list value, so it can be used to
2612 modify the elements of the LIST. While this is useful and supported,
2613 it can cause bizarre results if the elements of LIST are not variables.
2614 Using a regular C<foreach> loop for this purpose would be clearer in
2615 most cases. See also L</grep> for an array composed of those items of
2616 the original list for which the BLOCK or EXPR evaluates to true.
2618 C<{> starts both hash references and blocks, so C<map { ...> could be either
2619 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2620 ahead for the closing C<}> it has to take a guess at which its dealing with
2621 based what it finds just after the C<{>. Usually it gets it right, but if it
2622 doesn't it won't realize something is wrong until it gets to the C<}> and
2623 encounters the missing (or unexpected) comma. The syntax error will be
2624 reported close to the C<}> but you'll need to change something near the C<{>
2625 such as using a unary C<+> to give perl some help:
2627 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2628 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2629 %hash = map { ("\L$_", 1) } @array # this also works
2630 %hash = map { lc($_), 1 } @array # as does this.
2631 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2633 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2635 or to force an anon hash constructor use C<+{>
2637 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2639 and you get list of anonymous hashes each with only 1 entry.
2641 =item mkdir FILENAME,MASK
2643 =item mkdir FILENAME
2645 Creates the directory specified by FILENAME, with permissions
2646 specified by MASK (as modified by C<umask>). If it succeeds it
2647 returns true, otherwise it returns false and sets C<$!> (errno).
2648 If omitted, MASK defaults to 0777.
2650 In general, it is better to create directories with permissive MASK,
2651 and let the user modify that with their C<umask>, than it is to supply
2652 a restrictive MASK and give the user no way to be more permissive.
2653 The exceptions to this rule are when the file or directory should be
2654 kept private (mail files, for instance). The perlfunc(1) entry on
2655 C<umask> discusses the choice of MASK in more detail.
2657 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2658 number of trailing slashes. Some operating and filesystems do not get
2659 this right, so Perl automatically removes all trailing slashes to keep
2662 =item msgctl ID,CMD,ARG
2664 Calls the System V IPC function msgctl(2). You'll probably have to say
2668 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2669 then ARG must be a variable which will hold the returned C<msqid_ds>
2670 structure. Returns like C<ioctl>: the undefined value for error,
2671 C<"0 but true"> for zero, or the actual return value otherwise. See also
2672 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2674 =item msgget KEY,FLAGS
2676 Calls the System V IPC function msgget(2). Returns the message queue
2677 id, or the undefined value if there is an error. See also
2678 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2680 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2682 Calls the System V IPC function msgrcv to receive a message from
2683 message queue ID into variable VAR with a maximum message size of
2684 SIZE. Note that when a message is received, the message type as a
2685 native long integer will be the first thing in VAR, followed by the
2686 actual message. This packing may be opened with C<unpack("l! a*")>.
2687 Taints the variable. Returns true if successful, or false if there is
2688 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2689 C<IPC::SysV::Msg> documentation.
2691 =item msgsnd ID,MSG,FLAGS
2693 Calls the System V IPC function msgsnd to send the message MSG to the
2694 message queue ID. MSG must begin with the native long integer message
2695 type, and be followed by the length of the actual message, and finally
2696 the message itself. This kind of packing can be achieved with
2697 C<pack("l! a*", $type, $message)>. Returns true if successful,
2698 or false if there is an error. See also C<IPC::SysV>
2699 and C<IPC::SysV::Msg> documentation.
2705 =item my EXPR : ATTRS
2707 =item my TYPE EXPR : ATTRS
2709 A C<my> declares the listed variables to be local (lexically) to the
2710 enclosing block, file, or C<eval>. If more than one value is listed,
2711 the list must be placed in parentheses.
2713 The exact semantics and interface of TYPE and ATTRS are still
2714 evolving. TYPE is currently bound to the use of C<fields> pragma,
2715 and attributes are handled using the C<attributes> pragma, or starting
2716 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2717 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2718 L<attributes>, and L<Attribute::Handlers>.
2724 The C<next> command is like the C<continue> statement in C; it starts
2725 the next iteration of the loop:
2727 LINE: while (<STDIN>) {
2728 next LINE if /^#/; # discard comments
2732 Note that if there were a C<continue> block on the above, it would get
2733 executed even on discarded lines. If the LABEL is omitted, the command
2734 refers to the innermost enclosing loop.
2736 C<next> cannot be used to exit a block which returns a value such as
2737 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2738 a grep() or map() operation.
2740 Note that a block by itself is semantically identical to a loop
2741 that executes once. Thus C<next> will exit such a block early.
2743 See also L</continue> for an illustration of how C<last>, C<next>, and
2746 =item no Module VERSION LIST
2748 =item no Module VERSION
2750 =item no Module LIST
2754 See the C<use> function, of which C<no> is the opposite.
2760 Interprets EXPR as an octal string and returns the corresponding
2761 value. (If EXPR happens to start off with C<0x>, interprets it as a
2762 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2763 binary string. Leading whitespace is ignored in all three cases.)
2764 The following will handle decimal, binary, octal, and hex in the standard
2767 $val = oct($val) if $val =~ /^0/;
2769 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2770 in octal), use sprintf() or printf():
2772 $perms = (stat("filename"))[2] & 07777;
2773 $oct_perms = sprintf "%lo", $perms;
2775 The oct() function is commonly used when a string such as C<644> needs
2776 to be converted into a file mode, for example. (Although perl will
2777 automatically convert strings into numbers as needed, this automatic
2778 conversion assumes base 10.)
2780 =item open FILEHANDLE,EXPR
2782 =item open FILEHANDLE,MODE,EXPR
2784 =item open FILEHANDLE,MODE,EXPR,LIST
2786 =item open FILEHANDLE,MODE,REFERENCE
2788 =item open FILEHANDLE
2790 Opens the file whose filename is given by EXPR, and associates it with
2793 (The following is a comprehensive reference to open(): for a gentler
2794 introduction you may consider L<perlopentut>.)
2796 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2797 the variable is assigned a reference to a new anonymous filehandle,
2798 otherwise if FILEHANDLE is an expression, its value is used as the name of
2799 the real filehandle wanted. (This is considered a symbolic reference, so
2800 C<use strict 'refs'> should I<not> be in effect.)
2802 If EXPR is omitted, the scalar variable of the same name as the
2803 FILEHANDLE contains the filename. (Note that lexical variables--those
2804 declared with C<my>--will not work for this purpose; so if you're
2805 using C<my>, specify EXPR in your call to open.)
2807 If three or more arguments are specified then the mode of opening and
2808 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2809 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2810 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2811 the file is opened for appending, again being created if necessary.
2813 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2814 indicate that you want both read and write access to the file; thus
2815 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2816 '+>' >> mode would clobber the file first. You can't usually use
2817 either read-write mode for updating textfiles, since they have
2818 variable length records. See the B<-i> switch in L<perlrun> for a
2819 better approach. The file is created with permissions of C<0666>
2820 modified by the process' C<umask> value.
2822 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2823 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2825 In the 2-arguments (and 1-argument) form of the call the mode and
2826 filename should be concatenated (in this order), possibly separated by
2827 spaces. It is possible to omit the mode in these forms if the mode is
2830 If the filename begins with C<'|'>, the filename is interpreted as a
2831 command to which output is to be piped, and if the filename ends with a
2832 C<'|'>, the filename is interpreted as a command which pipes output to
2833 us. See L<perlipc/"Using open() for IPC">
2834 for more examples of this. (You are not allowed to C<open> to a command
2835 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2836 and L<perlipc/"Bidirectional Communication with Another Process">
2839 For three or more arguments if MODE is C<'|-'>, the filename is
2840 interpreted as a command to which output is to be piped, and if MODE
2841 is C<'-|'>, the filename is interpreted as a command which pipes
2842 output to us. In the 2-arguments (and 1-argument) form one should
2843 replace dash (C<'-'>) with the command.
2844 See L<perlipc/"Using open() for IPC"> for more examples of this.
2845 (You are not allowed to C<open> to a command that pipes both in I<and>
2846 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2847 L<perlipc/"Bidirectional Communication"> for alternatives.)
2849 In the three-or-more argument form of pipe opens, if LIST is specified
2850 (extra arguments after the command name) then LIST becomes arguments
2851 to the command invoked if the platform supports it. The meaning of
2852 C<open> with more than three arguments for non-pipe modes is not yet
2853 specified. Experimental "layers" may give extra LIST arguments
2856 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2857 and opening C<< '>-' >> opens STDOUT.
2859 You may use the three-argument form of open to specify IO "layers"
2860 (sometimes also referred to as "disciplines") to be applied to the handle
2861 that affect how the input and output are processed (see L<open> and
2862 L<PerlIO> for more details). For example
2864 open(FH, "<:utf8", "file")
2866 will open the UTF-8 encoded file containing Unicode characters,
2867 see L<perluniintro>. (Note that if layers are specified in the
2868 three-arg form then default layers set by the C<open> pragma are
2871 Open returns nonzero upon success, the undefined value otherwise. If
2872 the C<open> involved a pipe, the return value happens to be the pid of
2875 If you're running Perl on a system that distinguishes between text
2876 files and binary files, then you should check out L</binmode> for tips
2877 for dealing with this. The key distinction between systems that need
2878 C<binmode> and those that don't is their text file formats. Systems
2879 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2880 character, and which encode that character in C as C<"\n">, do not
2881 need C<binmode>. The rest need it.
2883 When opening a file, it's usually a bad idea to continue normal execution
2884 if the request failed, so C<open> is frequently used in connection with
2885 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2886 where you want to make a nicely formatted error message (but there are
2887 modules that can help with that problem)) you should always check
2888 the return value from opening a file. The infrequent exception is when
2889 working with an unopened filehandle is actually what you want to do.
2891 As a special case the 3 arg form with a read/write mode and the third
2892 argument being C<undef>:
2894 open(TMP, "+>", undef) or die ...
2896 opens a filehandle to an anonymous temporary file. Also using "+<"
2897 works for symmetry, but you really should consider writing something
2898 to the temporary file first. You will need to seek() to do the
2901 File handles can be opened to "in memory" files held in Perl scalars via:
2903 open($fh, '>', \$variable) || ..
2905 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2906 file, you have to close it first:
2909 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2914 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2915 while (<ARTICLE>) {...
2917 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2918 # if the open fails, output is discarded
2920 open(DBASE, '+<', 'dbase.mine') # open for update
2921 or die "Can't open 'dbase.mine' for update: $!";
2923 open(DBASE, '+<dbase.mine') # ditto
2924 or die "Can't open 'dbase.mine' for update: $!";
2926 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2927 or die "Can't start caesar: $!";
2929 open(ARTICLE, "caesar <$article |") # ditto
2930 or die "Can't start caesar: $!";
2932 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2933 or die "Can't start sort: $!";
2936 open(MEMORY,'>', \$var)
2937 or die "Can't open memory file: $!";
2938 print MEMORY "foo!\n"; # output will end up in $var
2940 # process argument list of files along with any includes
2942 foreach $file (@ARGV) {
2943 process($file, 'fh00');
2947 my($filename, $input) = @_;
2948 $input++; # this is a string increment
2949 unless (open($input, $filename)) {
2950 print STDERR "Can't open $filename: $!\n";
2955 while (<$input>) { # note use of indirection
2956 if (/^#include "(.*)"/) {
2957 process($1, $input);
2964 You may also, in the Bourne shell tradition, specify an EXPR beginning
2965 with C<< '>&' >>, in which case the rest of the string is interpreted
2966 as the name of a filehandle (or file descriptor, if numeric) to be
2967 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2968 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2969 The mode you specify should match the mode of the original filehandle.
2970 (Duping a filehandle does not take into account any existing contents
2971 of IO buffers.) If you use the 3 arg form then you can pass either a
2972 number, the name of a filehandle or the normal "reference to a glob".
2974 Here is a script that saves, redirects, and restores C<STDOUT> and
2975 C<STDERR> using various methods:
2978 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2979 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2981 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2982 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2984 select STDERR; $| = 1; # make unbuffered
2985 select STDOUT; $| = 1; # make unbuffered
2987 print STDOUT "stdout 1\n"; # this works for
2988 print STDERR "stderr 1\n"; # subprocesses too
2990 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2991 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2993 print STDOUT "stdout 2\n";
2994 print STDERR "stderr 2\n";
2996 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
2997 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
2998 that file descriptor (and not call L<dup(2)>); this is more
2999 parsimonious of file descriptors. For example:
3001 # open for input, reusing the fileno of $fd
3002 open(FILEHANDLE, "<&=$fd")
3006 open(FILEHANDLE, "<&=", $fd)
3010 # open for append, using the fileno of OLDFH
3011 open(FH, ">>&=", OLDFH)
3015 open(FH, ">>&=OLDFH")
3017 Being parsimonious on filehandles is also useful (besides being
3018 parsimonious) for example when something is dependent on file
3019 descriptors, like for example locking using flock(). If you do just
3020 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3021 descriptor as B, and therefore flock(A) will not flock(B), and vice
3022 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3023 the same file descriptor.
3025 Note that if you are using Perls older than 5.8.0, Perl will be using
3026 the standard C libraries' fdopen() to implement the "=" functionality.
3027 On many UNIX systems fdopen() fails when file descriptors exceed a
3028 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3029 most often the default.
3031 You can see whether Perl has been compiled with PerlIO or not by
3032 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3033 is C<define>, you have PerlIO, otherwise you don't.
3035 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3036 with 2-arguments (or 1-argument) form of open(), then
3037 there is an implicit fork done, and the return value of open is the pid
3038 of the child within the parent process, and C<0> within the child
3039 process. (Use C<defined($pid)> to determine whether the open was successful.)
3040 The filehandle behaves normally for the parent, but i/o to that
3041 filehandle is piped from/to the STDOUT/STDIN of the child process.
3042 In the child process the filehandle isn't opened--i/o happens from/to
3043 the new STDOUT or STDIN. Typically this is used like the normal
3044 piped open when you want to exercise more control over just how the
3045 pipe command gets executed, such as when you are running setuid, and
3046 don't want to have to scan shell commands for metacharacters.
3047 The following triples are more or less equivalent:
3049 open(FOO, "|tr '[a-z]' '[A-Z]'");
3050 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3051 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3052 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3054 open(FOO, "cat -n '$file'|");
3055 open(FOO, '-|', "cat -n '$file'");
3056 open(FOO, '-|') || exec 'cat', '-n', $file;
3057 open(FOO, '-|', "cat", '-n', $file);
3059 The last example in each block shows the pipe as "list form", which is
3060 not yet supported on all platforms. A good rule of thumb is that if
3061 your platform has true C<fork()> (in other words, if your platform is
3062 UNIX) you can use the list form.
3064 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3066 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3067 output before any operation that may do a fork, but this may not be
3068 supported on some platforms (see L<perlport>). To be safe, you may need
3069 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3070 of C<IO::Handle> on any open handles.
3072 On systems that support a close-on-exec flag on files, the flag will
3073 be set for the newly opened file descriptor as determined by the value
3074 of $^F. See L<perlvar/$^F>.
3076 Closing any piped filehandle causes the parent process to wait for the
3077 child to finish, and returns the status value in C<$?>.
3079 The filename passed to 2-argument (or 1-argument) form of open() will
3080 have leading and trailing whitespace deleted, and the normal
3081 redirection characters honored. This property, known as "magic open",
3082 can often be used to good effect. A user could specify a filename of
3083 F<"rsh cat file |">, or you could change certain filenames as needed:
3085 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3086 open(FH, $filename) or die "Can't open $filename: $!";
3088 Use 3-argument form to open a file with arbitrary weird characters in it,
3090 open(FOO, '<', $file);
3092 otherwise it's necessary to protect any leading and trailing whitespace:
3094 $file =~ s#^(\s)#./$1#;
3095 open(FOO, "< $file\0");
3097 (this may not work on some bizarre filesystems). One should
3098 conscientiously choose between the I<magic> and 3-arguments form
3103 will allow the user to specify an argument of the form C<"rsh cat file |">,
3104 but will not work on a filename which happens to have a trailing space, while
3106 open IN, '<', $ARGV[0];
3108 will have exactly the opposite restrictions.
3110 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3111 should use the C<sysopen> function, which involves no such magic (but
3112 may use subtly different filemodes than Perl open(), which is mapped
3113 to C fopen()). This is
3114 another way to protect your filenames from interpretation. For example:
3117 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3118 or die "sysopen $path: $!";
3119 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3120 print HANDLE "stuff $$\n";
3122 print "File contains: ", <HANDLE>;
3124 Using the constructor from the C<IO::Handle> package (or one of its
3125 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3126 filehandles that have the scope of whatever variables hold references to
3127 them, and automatically close whenever and however you leave that scope:
3131 sub read_myfile_munged {
3133 my $handle = new IO::File;
3134 open($handle, "myfile") or die "myfile: $!";
3136 or return (); # Automatically closed here.
3137 mung $first or die "mung failed"; # Or here.
3138 return $first, <$handle> if $ALL; # Or here.
3142 See L</seek> for some details about mixing reading and writing.
3144 =item opendir DIRHANDLE,EXPR
3146 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3147 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3148 DIRHANDLE may be an expression whose value can be used as an indirect
3149 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3150 scalar variable (or array or hash element), the variable is assigned a
3151 reference to a new anonymous dirhandle.
3152 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3158 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3159 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3162 For the reverse, see L</chr>.
3163 See L<perlunicode> and L<encoding> for more about Unicode.
3169 =item our EXPR : ATTRS
3171 =item our TYPE EXPR : ATTRS
3173 An C<our> declares the listed variables to be valid globals within
3174 the enclosing block, file, or C<eval>. That is, it has the same
3175 scoping rules as a "my" declaration, but does not create a local
3176 variable. If more than one value is listed, the list must be placed
3177 in parentheses. The C<our> declaration has no semantic effect unless
3178 "use strict vars" is in effect, in which case it lets you use the
3179 declared global variable without qualifying it with a package name.
3180 (But only within the lexical scope of the C<our> declaration. In this
3181 it differs from "use vars", which is package scoped.)
3183 An C<our> declaration declares a global variable that will be visible
3184 across its entire lexical scope, even across package boundaries. The
3185 package in which the variable is entered is determined at the point
3186 of the declaration, not at the point of use. This means the following
3190 our $bar; # declares $Foo::bar for rest of lexical scope
3194 print $bar; # prints 20
3196 Multiple C<our> declarations in the same lexical scope are allowed
3197 if they are in different packages. If they happened to be in the same
3198 package, Perl will emit warnings if you have asked for them.
3202 our $bar; # declares $Foo::bar for rest of lexical scope
3206 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3207 print $bar; # prints 30
3209 our $bar; # emits warning
3211 An C<our> declaration may also have a list of attributes associated
3214 The exact semantics and interface of TYPE and ATTRS are still
3215 evolving. TYPE is currently bound to the use of C<fields> pragma,
3216 and attributes are handled using the C<attributes> pragma, or starting
3217 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3218 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3219 L<attributes>, and L<Attribute::Handlers>.
3221 The only currently recognized C<our()> attribute is C<unique> which
3222 indicates that a single copy of the global is to be used by all
3223 interpreters should the program happen to be running in a
3224 multi-interpreter environment. (The default behaviour would be for
3225 each interpreter to have its own copy of the global.) Examples:
3227 our @EXPORT : unique = qw(foo);
3228 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3229 our $VERSION : unique = "1.00";
3231 Note that this attribute also has the effect of making the global
3232 readonly when the first new interpreter is cloned (for example,
3233 when the first new thread is created).
3235 Multi-interpreter environments can come to being either through the
3236 fork() emulation on Windows platforms, or by embedding perl in a
3237 multi-threaded application. The C<unique> attribute does nothing in
3238 all other environments.
3240 Warning: the current implementation of this attribute operates on the
3241 typeglob associated with the variable; this means that C<our $x : unique>
3242 also has the effect of C<our @x : unique; our %x : unique>. This may be
3245 =item pack TEMPLATE,LIST
3247 Takes a LIST of values and converts it into a string using the rules
3248 given by the TEMPLATE. The resulting string is the concatenation of
3249 the converted values. Typically, each converted value looks
3250 like its machine-level representation. For example, on 32-bit machines
3251 a converted integer may be represented by a sequence of 4 bytes.
3253 The TEMPLATE is a sequence of characters that give the order and type
3254 of values, as follows:
3256 a A string with arbitrary binary data, will be null padded.
3257 A A text (ASCII) string, will be space padded.
3258 Z A null terminated (ASCIZ) string, will be null padded.
3260 b A bit string (ascending bit order inside each byte, like vec()).
3261 B A bit string (descending bit order inside each byte).
3262 h A hex string (low nybble first).
3263 H A hex string (high nybble first).
3265 c A signed char value.
3266 C An unsigned char value. Only does bytes. See U for Unicode.
3268 s A signed short value.
3269 S An unsigned short value.
3270 (This 'short' is _exactly_ 16 bits, which may differ from
3271 what a local C compiler calls 'short'. If you want
3272 native-length shorts, use the '!' suffix.)
3274 i A signed integer value.
3275 I An unsigned integer value.
3276 (This 'integer' is _at_least_ 32 bits wide. Its exact
3277 size depends on what a local C compiler calls 'int',
3278 and may even be larger than the 'long' described in
3281 l A signed long value.
3282 L An unsigned long value.
3283 (This 'long' is _exactly_ 32 bits, which may differ from
3284 what a local C compiler calls 'long'. If you want
3285 native-length longs, use the '!' suffix.)
3287 n An unsigned short in "network" (big-endian) order.
3288 N An unsigned long in "network" (big-endian) order.
3289 v An unsigned short in "VAX" (little-endian) order.
3290 V An unsigned long in "VAX" (little-endian) order.
3291 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3292 _exactly_ 32 bits, respectively.)
3294 q A signed quad (64-bit) value.
3295 Q An unsigned quad value.
3296 (Quads are available only if your system supports 64-bit
3297 integer values _and_ if Perl has been compiled to support those.
3298 Causes a fatal error otherwise.)
3300 j A signed integer value (a Perl internal integer, IV).
3301 J An unsigned integer value (a Perl internal unsigned integer, UV).
3303 f A single-precision float in the native format.
3304 d A double-precision float in the native format.
3306 F A floating point value in the native native format
3307 (a Perl internal floating point value, NV).
3308 D A long double-precision float in the native format.
3309 (Long doubles are available only if your system supports long
3310 double values _and_ if Perl has been compiled to support those.
3311 Causes a fatal error otherwise.)
3313 p A pointer to a null-terminated string.
3314 P A pointer to a structure (fixed-length string).
3316 u A uuencoded string.
3317 U A Unicode character number. Encodes to UTF-8 internally
3318 (or UTF-EBCDIC in EBCDIC platforms).
3320 w A BER compressed integer. Its bytes represent an unsigned
3321 integer in base 128, most significant digit first, with as
3322 few digits as possible. Bit eight (the high bit) is set
3323 on each byte except the last.
3327 @ Null fill to absolute position, counted from the start of
3328 the innermost ()-group.
3329 ( Start of a ()-group.
3331 The following rules apply:
3337 Each letter may optionally be followed by a number giving a repeat
3338 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3339 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3340 many values from the LIST. A C<*> for the repeat count means to use
3341 however many items are left, except for C<@>, C<x>, C<X>, where it is
3342 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3343 is the same). A numeric repeat count may optionally be enclosed in
3344 brackets, as in C<pack 'C[80]', @arr>.
3346 One can replace the numeric repeat count by a template enclosed in brackets;
3347 then the packed length of this template in bytes is used as a count.
3348 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3349 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3350 If the template in brackets contains alignment commands (such as C<x![d]>),
3351 its packed length is calculated as if the start of the template has the maximal
3354 When used with C<Z>, C<*> results in the addition of a trailing null
3355 byte (so the packed result will be one longer than the byte C<length>
3358 The repeat count for C<u> is interpreted as the maximal number of bytes
3359 to encode per line of output, with 0 and 1 replaced by 45.
3363 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3364 string of length count, padding with nulls or spaces as necessary. When
3365 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3366 after the first null, and C<a> returns data verbatim. When packing,
3367 C<a>, and C<Z> are equivalent.
3369 If the value-to-pack is too long, it is truncated. If too long and an
3370 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3371 by a null byte. Thus C<Z> always packs a trailing null byte under
3376 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3377 Each byte of the input field of pack() generates 1 bit of the result.
3378 Each result bit is based on the least-significant bit of the corresponding
3379 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3380 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3382 Starting from the beginning of the input string of pack(), each 8-tuple
3383 of bytes is converted to 1 byte of output. With format C<b>
3384 the first byte of the 8-tuple determines the least-significant bit of a
3385 byte, and with format C<B> it determines the most-significant bit of
3388 If the length of the input string is not exactly divisible by 8, the
3389 remainder is packed as if the input string were padded by null bytes
3390 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3392 If the input string of pack() is longer than needed, extra bytes are ignored.
3393 A C<*> for the repeat count of pack() means to use all the bytes of
3394 the input field. On unpack()ing the bits are converted to a string
3395 of C<"0">s and C<"1">s.
3399 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3400 representable as hexadecimal digits, 0-9a-f) long.
3402 Each byte of the input field of pack() generates 4 bits of the result.
3403 For non-alphabetical bytes the result is based on the 4 least-significant
3404 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3405 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3406 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3407 is compatible with the usual hexadecimal digits, so that C<"a"> and
3408 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3409 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3411 Starting from the beginning of the input string of pack(), each pair
3412 of bytes is converted to 1 byte of output. With format C<h> the
3413 first byte of the pair determines the least-significant nybble of the
3414 output byte, and with format C<H> it determines the most-significant
3417 If the length of the input string is not even, it behaves as if padded
3418 by a null byte at the end. Similarly, during unpack()ing the "extra"
3419 nybbles are ignored.
3421 If the input string of pack() is longer than needed, extra bytes are ignored.
3422 A C<*> for the repeat count of pack() means to use all the bytes of
3423 the input field. On unpack()ing the bits are converted to a string
3424 of hexadecimal digits.
3428 The C<p> type packs a pointer to a null-terminated string. You are
3429 responsible for ensuring the string is not a temporary value (which can
3430 potentially get deallocated before you get around to using the packed result).
3431 The C<P> type packs a pointer to a structure of the size indicated by the
3432 length. A NULL pointer is created if the corresponding value for C<p> or
3433 C<P> is C<undef>, similarly for unpack().
3437 The C</> template character allows packing and unpacking of strings where
3438 the packed structure contains a byte count followed by the string itself.
3439 You write I<length-item>C</>I<string-item>.
3441 The I<length-item> can be any C<pack> template letter, and describes
3442 how the length value is packed. The ones likely to be of most use are
3443 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3444 SNMP) and C<N> (for Sun XDR).
3446 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3447 C<"Z*">. For C<unpack> the length of the string is obtained from the
3448 I<length-item>, but if you put in the '*' it will be ignored. For all other
3449 codes, C<unpack> applies the length value to the next item, which must not
3450 have a repeat count.
3452 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3453 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3454 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3456 The I<length-item> is not returned explicitly from C<unpack>.
3458 Adding a count to the I<length-item> letter is unlikely to do anything
3459 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3460 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3461 which Perl does not regard as legal in numeric strings.
3465 The integer types C<s>, C<S>, C<l>, and C<L> may be
3466 immediately followed by a C<!> suffix to signify native shorts or
3467 longs--as you can see from above for example a bare C<l> does mean
3468 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3469 may be larger. This is an issue mainly in 64-bit platforms. You can
3470 see whether using C<!> makes any difference by
3472 print length(pack("s")), " ", length(pack("s!")), "\n";
3473 print length(pack("l")), " ", length(pack("l!")), "\n";
3475 C<i!> and C<I!> also work but only because of completeness;
3476 they are identical to C<i> and C<I>.
3478 The actual sizes (in bytes) of native shorts, ints, longs, and long
3479 longs on the platform where Perl was built are also available via
3483 print $Config{shortsize}, "\n";
3484 print $Config{intsize}, "\n";
3485 print $Config{longsize}, "\n";
3486 print $Config{longlongsize}, "\n";
3488 (The C<$Config{longlongsize}> will be undefined if your system does
3489 not support long longs.)
3493 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3494 are inherently non-portable between processors and operating systems
3495 because they obey the native byteorder and endianness. For example a
3496 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3497 (arranged in and handled by the CPU registers) into bytes as
3499 0x12 0x34 0x56 0x78 # big-endian
3500 0x78 0x56 0x34 0x12 # little-endian
3502 Basically, the Intel and VAX CPUs are little-endian, while everybody
3503 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3504 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3505 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3508 The names `big-endian' and `little-endian' are comic references to
3509 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3510 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3511 the egg-eating habits of the Lilliputians.
3513 Some systems may have even weirder byte orders such as
3518 You can see your system's preference with
3520 print join(" ", map { sprintf "%#02x", $_ }
3521 unpack("C*",pack("L",0x12345678))), "\n";
3523 The byteorder on the platform where Perl was built is also available
3527 print $Config{byteorder}, "\n";
3529 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3530 and C<'87654321'> are big-endian.
3532 If you want portable packed integers use the formats C<n>, C<N>,
3533 C<v>, and C<V>, their byte endianness and size are known.
3534 See also L<perlport>.
3538 Real numbers (floats and doubles) are in the native machine format only;
3539 due to the multiplicity of floating formats around, and the lack of a
3540 standard "network" representation, no facility for interchange has been
3541 made. This means that packed floating point data written on one machine
3542 may not be readable on another - even if both use IEEE floating point
3543 arithmetic (as the endian-ness of the memory representation is not part
3544 of the IEEE spec). See also L<perlport>.
3546 Note that Perl uses doubles internally for all numeric calculation, and
3547 converting from double into float and thence back to double again will
3548 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3553 If the pattern begins with a C<U>, the resulting string will be
3554 treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
3555 string with an initial C<U0>, and the bytes that follow will be
3556 interpreted as Unicode characters. If you don't want this to happen,
3557 you can begin your pattern with C<C0> (or anything else) to force Perl
3558 not to UTF-8 encode your string, and then follow this with a C<U*>
3559 somewhere in your pattern.
3563 You must yourself do any alignment or padding by inserting for example
3564 enough C<'x'>es while packing. There is no way to pack() and unpack()
3565 could know where the bytes are going to or coming from. Therefore
3566 C<pack> (and C<unpack>) handle their output and input as flat
3571 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3572 take a repeat count, both as postfix, and for unpack() also via the C</>
3573 template character. Within each repetition of a group, positioning with
3574 C<@> starts again at 0. Therefore, the result of
3576 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3578 is the string "\0a\0\0bc".
3583 C<x> and C<X> accept C<!> modifier. In this case they act as
3584 alignment commands: they jump forward/back to the closest position
3585 aligned at a multiple of C<count> bytes. For example, to pack() or
3586 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3587 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3588 aligned on the double's size.
3590 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3591 both result in no-ops.
3595 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3596 White space may be used to separate pack codes from each other, but
3597 a C<!> modifier and a repeat count must follow immediately.
3601 If TEMPLATE requires more arguments to pack() than actually given, pack()
3602 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3603 to pack() than actually given, extra arguments are ignored.
3609 $foo = pack("CCCC",65,66,67,68);
3611 $foo = pack("C4",65,66,67,68);
3613 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3614 # same thing with Unicode circled letters
3616 $foo = pack("ccxxcc",65,66,67,68);
3619 # note: the above examples featuring "C" and "c" are true
3620 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3621 # and UTF-8. In EBCDIC the first example would be
3622 # $foo = pack("CCCC",193,194,195,196);
3624 $foo = pack("s2",1,2);
3625 # "\1\0\2\0" on little-endian
3626 # "\0\1\0\2" on big-endian
3628 $foo = pack("a4","abcd","x","y","z");
3631 $foo = pack("aaaa","abcd","x","y","z");
3634 $foo = pack("a14","abcdefg");
3635 # "abcdefg\0\0\0\0\0\0\0"
3637 $foo = pack("i9pl", gmtime);
3638 # a real struct tm (on my system anyway)
3640 $utmp_template = "Z8 Z8 Z16 L";
3641 $utmp = pack($utmp_template, @utmp1);
3642 # a struct utmp (BSDish)
3644 @utmp2 = unpack($utmp_template, $utmp);
3645 # "@utmp1" eq "@utmp2"
3648 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3651 $foo = pack('sx2l', 12, 34);
3652 # short 12, two zero bytes padding, long 34
3653 $bar = pack('s@4l', 12, 34);
3654 # short 12, zero fill to position 4, long 34
3657 The same template may generally also be used in unpack().
3659 =item package NAMESPACE
3663 Declares the compilation unit as being in the given namespace. The scope
3664 of the package declaration is from the declaration itself through the end
3665 of the enclosing block, file, or eval (the same as the C<my> operator).
3666 All further unqualified dynamic identifiers will be in this namespace.
3667 A package statement affects only dynamic variables--including those
3668 you've used C<local> on--but I<not> lexical variables, which are created
3669 with C<my>. Typically it would be the first declaration in a file to
3670 be included by the C<require> or C<use> operator. You can switch into a
3671 package in more than one place; it merely influences which symbol table
3672 is used by the compiler for the rest of that block. You can refer to
3673 variables and filehandles in other packages by prefixing the identifier
3674 with the package name and a double colon: C<$Package::Variable>.
3675 If the package name is null, the C<main> package as assumed. That is,
3676 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3677 still seen in older code).
3679 If NAMESPACE is omitted, then there is no current package, and all
3680 identifiers must be fully qualified or lexicals. However, you are
3681 strongly advised not to make use of this feature. Its use can cause
3682 unexpected behaviour, even crashing some versions of Perl. It is
3683 deprecated, and will be removed from a future release.
3685 See L<perlmod/"Packages"> for more information about packages, modules,
3686 and classes. See L<perlsub> for other scoping issues.
3688 =item pipe READHANDLE,WRITEHANDLE
3690 Opens a pair of connected pipes like the corresponding system call.
3691 Note that if you set up a loop of piped processes, deadlock can occur
3692 unless you are very careful. In addition, note that Perl's pipes use
3693 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3694 after each command, depending on the application.
3696 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3697 for examples of such things.
3699 On systems that support a close-on-exec flag on files, the flag will be set
3700 for the newly opened file descriptors as determined by the value of $^F.
3707 Pops and returns the last value of the array, shortening the array by
3708 one element. Has an effect similar to
3712 If there are no elements in the array, returns the undefined value
3713 (although this may happen at other times as well). If ARRAY is
3714 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3715 array in subroutines, just like C<shift>.
3721 Returns the offset of where the last C<m//g> search left off for the variable
3722 in question (C<$_> is used when the variable is not specified). May be
3723 modified to change that offset. Such modification will also influence
3724 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3727 =item print FILEHANDLE LIST
3733 Prints a string or a list of strings. Returns true if successful.
3734 FILEHANDLE may be a scalar variable name, in which case the variable
3735 contains the name of or a reference to the filehandle, thus introducing
3736 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3737 the next token is a term, it may be misinterpreted as an operator
3738 unless you interpose a C<+> or put parentheses around the arguments.)
3739 If FILEHANDLE is omitted, prints by default to standard output (or
3740 to the last selected output channel--see L</select>). If LIST is
3741 also omitted, prints C<$_> to the currently selected output channel.
3742 To set the default output channel to something other than STDOUT
3743 use the select operation. The current value of C<$,> (if any) is
3744 printed between each LIST item. The current value of C<$\> (if
3745 any) is printed after the entire LIST has been printed. Because
3746 print takes a LIST, anything in the LIST is evaluated in list
3747 context, and any subroutine that you call will have one or more of
3748 its expressions evaluated in list context. Also be careful not to
3749 follow the print keyword with a left parenthesis unless you want
3750 the corresponding right parenthesis to terminate the arguments to
3751 the print--interpose a C<+> or put parentheses around all the
3754 Note that if you're storing FILEHANDLES in an array or other expression,
3755 you will have to use a block returning its value instead:
3757 print { $files[$i] } "stuff\n";
3758 print { $OK ? STDOUT : STDERR } "stuff\n";
3760 =item printf FILEHANDLE FORMAT, LIST
3762 =item printf FORMAT, LIST
3764 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3765 (the output record separator) is not appended. The first argument
3766 of the list will be interpreted as the C<printf> format. See C<sprintf>
3767 for an explanation of the format argument. If C<use locale> is in effect,
3768 the character used for the decimal point in formatted real numbers is
3769 affected by the LC_NUMERIC locale. See L<perllocale>.
3771 Don't fall into the trap of using a C<printf> when a simple
3772 C<print> would do. The C<print> is more efficient and less
3775 =item prototype FUNCTION
3777 Returns the prototype of a function as a string (or C<undef> if the
3778 function has no prototype). FUNCTION is a reference to, or the name of,
3779 the function whose prototype you want to retrieve.
3781 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3782 name for Perl builtin. If the builtin is not I<overridable> (such as
3783 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3784 C<system>) returns C<undef> because the builtin does not really behave
3785 like a Perl function. Otherwise, the string describing the equivalent
3786 prototype is returned.
3788 =item push ARRAY,LIST
3790 Treats ARRAY as a stack, and pushes the values of LIST
3791 onto the end of ARRAY. The length of ARRAY increases by the length of
3792 LIST. Has the same effect as
3795 $ARRAY[++$#ARRAY] = $value;
3798 but is more efficient. Returns the new number of elements in the array.
3810 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3812 =item quotemeta EXPR
3816 Returns the value of EXPR with all non-"word"
3817 characters backslashed. (That is, all characters not matching
3818 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3819 returned string, regardless of any locale settings.)
3820 This is the internal function implementing
3821 the C<\Q> escape in double-quoted strings.
3823 If EXPR is omitted, uses C<$_>.
3829 Returns a random fractional number greater than or equal to C<0> and less
3830 than the value of EXPR. (EXPR should be positive.) If EXPR is
3831 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3832 also special-cased as C<1> - this has not been documented before perl 5.8.0
3833 and is subject to change in future versions of perl. Automatically calls
3834 C<srand> unless C<srand> has already been called. See also C<srand>.
3836 Apply C<int()> to the value returned by C<rand()> if you want random
3837 integers instead of random fractional numbers. For example,
3841 returns a random integer between C<0> and C<9>, inclusive.
3843 (Note: If your rand function consistently returns numbers that are too
3844 large or too small, then your version of Perl was probably compiled
3845 with the wrong number of RANDBITS.)
3847 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3849 =item read FILEHANDLE,SCALAR,LENGTH
3851 Attempts to read LENGTH I<characters> of data into variable SCALAR
3852 from the specified FILEHANDLE. Returns the number of characters
3853 actually read, C<0> at end of file, or undef if there was an error (in
3854 the latter case C<$!> is also set). SCALAR will be grown or shrunk
3855 so that the last character actually read is the last character of the
3856 scalar after the read.
3858 An OFFSET may be specified to place the read data at some place in the
3859 string other than the beginning. A negative OFFSET specifies
3860 placement at that many characters counting backwards from the end of
3861 the string. A positive OFFSET greater than the length of SCALAR
3862 results in the string being padded to the required size with C<"\0">
3863 bytes before the result of the read is appended.
3865 The call is actually implemented in terms of either Perl's or system's
3866 fread() call. To get a true read(2) system call, see C<sysread>.
3868 Note the I<characters>: depending on the status of the filehandle,
3869 either (8-bit) bytes or characters are read. By default all
3870 filehandles operate on bytes, but for example if the filehandle has
3871 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3872 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3873 characters, not bytes. Similarly for the C<:encoding> pragma:
3874 in that case pretty much any characters can be read.
3876 =item readdir DIRHANDLE
3878 Returns the next directory entry for a directory opened by C<opendir>.
3879 If used in list context, returns all the rest of the entries in the
3880 directory. If there are no more entries, returns an undefined value in
3881 scalar context or a null list in list context.
3883 If you're planning to filetest the return values out of a C<readdir>, you'd
3884 better prepend the directory in question. Otherwise, because we didn't
3885 C<chdir> there, it would have been testing the wrong file.
3887 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3888 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3893 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3894 context, each call reads and returns the next line, until end-of-file is
3895 reached, whereupon the subsequent call returns undef. In list context,
3896 reads until end-of-file is reached and returns a list of lines. Note that
3897 the notion of "line" used here is however you may have defined it
3898 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3900 When C<$/> is set to C<undef>, when readline() is in scalar
3901 context (i.e. file slurp mode), and when an empty file is read, it
3902 returns C<''> the first time, followed by C<undef> subsequently.
3904 This is the internal function implementing the C<< <EXPR> >>
3905 operator, but you can use it directly. The C<< <EXPR> >>
3906 operator is discussed in more detail in L<perlop/"I/O Operators">.
3909 $line = readline(*STDIN); # same thing
3911 If readline encounters an operating system error, C<$!> will be set with the
3912 corresponding error message. It can be helpful to check C<$!> when you are
3913 reading from filehandles you don't trust, such as a tty or a socket. The
3914 following example uses the operator form of C<readline>, and takes the necessary
3915 steps to ensure that C<readline> was successful.
3919 unless (defined( $line = <> )) {
3930 Returns the value of a symbolic link, if symbolic links are
3931 implemented. If not, gives a fatal error. If there is some system
3932 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3933 omitted, uses C<$_>.
3937 EXPR is executed as a system command.
3938 The collected standard output of the command is returned.
3939 In scalar context, it comes back as a single (potentially
3940 multi-line) string. In list context, returns a list of lines
3941 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3942 This is the internal function implementing the C<qx/EXPR/>
3943 operator, but you can use it directly. The C<qx/EXPR/>
3944 operator is discussed in more detail in L<perlop/"I/O Operators">.
3946 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3948 Receives a message on a socket. Attempts to receive LENGTH characters
3949 of data into variable SCALAR from the specified SOCKET filehandle.
3950 SCALAR will be grown or shrunk to the length actually read. Takes the
3951 same flags as the system call of the same name. Returns the address
3952 of the sender if SOCKET's protocol supports this; returns an empty
3953 string otherwise. If there's an error, returns the undefined value.
3954 This call is actually implemented in terms of recvfrom(2) system call.
3955 See L<perlipc/"UDP: Message Passing"> for examples.
3957 Note the I<characters>: depending on the status of the socket, either
3958 (8-bit) bytes or characters are received. By default all sockets
3959 operate on bytes, but for example if the socket has been changed using
3960 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3961 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3962 characters, not bytes. Similarly for the C<:encoding> pragma:
3963 in that case pretty much any characters can be read.
3969 The C<redo> command restarts the loop block without evaluating the
3970 conditional again. The C<continue> block, if any, is not executed. If
3971 the LABEL is omitted, the command refers to the innermost enclosing
3972 loop. This command is normally used by programs that want to lie to
3973 themselves about what was just input:
3975 # a simpleminded Pascal comment stripper
3976 # (warning: assumes no { or } in strings)
3977 LINE: while (<STDIN>) {
3978 while (s|({.*}.*){.*}|$1 |) {}
3983 if (/}/) { # end of comment?
3992 C<redo> cannot be used to retry a block which returns a value such as
3993 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3994 a grep() or map() operation.
3996 Note that a block by itself is semantically identical to a loop
3997 that executes once. Thus C<redo> inside such a block will effectively
3998 turn it into a looping construct.
4000 See also L</continue> for an illustration of how C<last>, C<next>, and
4007 Returns a non-empty string if EXPR is a reference, the empty
4008 string otherwise. If EXPR
4009 is not specified, C<$_> will be used. The value returned depends on the
4010 type of thing the reference is a reference to.
4011 Builtin types include:
4021 If the referenced object has been blessed into a package, then that package
4022 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4024 if (ref($r) eq "HASH") {
4025 print "r is a reference to a hash.\n";
4028 print "r is not a reference at all.\n";
4030 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
4031 print "r is a reference to something that isa hash.\n";
4034 See also L<perlref>.
4036 =item rename OLDNAME,NEWNAME
4038 Changes the name of a file; an existing file NEWNAME will be
4039 clobbered. Returns true for success, false otherwise.
4041 Behavior of this function varies wildly depending on your system
4042 implementation. For example, it will usually not work across file system
4043 boundaries, even though the system I<mv> command sometimes compensates
4044 for this. Other restrictions include whether it works on directories,
4045 open files, or pre-existing files. Check L<perlport> and either the
4046 rename(2) manpage or equivalent system documentation for details.
4048 =item require VERSION
4054 Demands a version of Perl specified by VERSION, or demands some semantics
4055 specified by EXPR or by C<$_> if EXPR is not supplied.
4057 VERSION may be either a numeric argument such as 5.006, which will be
4058 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4059 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4060 VERSION is greater than the version of the current Perl interpreter.
4061 Compare with L</use>, which can do a similar check at compile time.
4063 Specifying VERSION as a literal of the form v5.6.1 should generally be
4064 avoided, because it leads to misleading error messages under earlier
4065 versions of Perl which do not support this syntax. The equivalent numeric
4066 version should be used instead.
4068 require v5.6.1; # run time version check
4069 require 5.6.1; # ditto
4070 require 5.006_001; # ditto; preferred for backwards compatibility
4072 Otherwise, demands that a library file be included if it hasn't already
4073 been included. The file is included via the do-FILE mechanism, which is
4074 essentially just a variety of C<eval>. Has semantics similar to the
4075 following subroutine:
4078 my ($filename) = @_;
4079 if (exists $INC{$filename}) {
4080 return 1 if $INC{$filename};
4081 die "Compilation failed in require";
4083 my ($realfilename,$result);
4085 foreach $prefix (@INC) {
4086 $realfilename = "$prefix/$filename";
4087 if (-f $realfilename) {
4088 $INC{$filename} = $realfilename;
4089 $result = do $realfilename;
4093 die "Can't find $filename in \@INC";
4096 $INC{$filename} = undef;
4098 } elsif (!$result) {
4099 delete $INC{$filename};
4100 die "$filename did not return true value";
4106 Note that the file will not be included twice under the same specified
4109 The file must return true as the last statement to indicate
4110 successful execution of any initialization code, so it's customary to
4111 end such a file with C<1;> unless you're sure it'll return true
4112 otherwise. But it's better just to put the C<1;>, in case you add more
4115 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4116 replaces "F<::>" with "F</>" in the filename for you,
4117 to make it easy to load standard modules. This form of loading of
4118 modules does not risk altering your namespace.
4120 In other words, if you try this:
4122 require Foo::Bar; # a splendid bareword
4124 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4125 directories specified in the C<@INC> array.
4127 But if you try this:
4129 $class = 'Foo::Bar';
4130 require $class; # $class is not a bareword
4132 require "Foo::Bar"; # not a bareword because of the ""
4134 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4135 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4137 eval "require $class";
4139 Now that you understand how C<require> looks for files in the case of
4140 a bareword argument, there is a little extra functionality going on
4141 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4142 it will first look for a filename with a "F<.pmc>" extension. A file
4143 with this extension is assumed to be Perl bytecode generated by
4144 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4145 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4146 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4148 You can also insert hooks into the import facility, by putting directly
4149 Perl code into the @INC array. There are three forms of hooks: subroutine
4150 references, array references and blessed objects.
4152 Subroutine references are the simplest case. When the inclusion system
4153 walks through @INC and encounters a subroutine, this subroutine gets
4154 called with two parameters, the first being a reference to itself, and the
4155 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4156 subroutine should return C<undef> or a filehandle, from which the file to
4157 include will be read. If C<undef> is returned, C<require> will look at
4158 the remaining elements of @INC.
4160 If the hook is an array reference, its first element must be a subroutine
4161 reference. This subroutine is called as above, but the first parameter is
4162 the array reference. This enables to pass indirectly some arguments to
4165 In other words, you can write:
4167 push @INC, \&my_sub;
4169 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4175 push @INC, [ \&my_sub, $x, $y, ... ];
4177 my ($arrayref, $filename) = @_;
4178 # Retrieve $x, $y, ...
4179 my @parameters = @$arrayref[1..$#$arrayref];
4183 If the hook is an object, it must provide an INC method, that will be
4184 called as above, the first parameter being the object itself. (Note that
4185 you must fully qualify the sub's name, as it is always forced into package
4186 C<main>.) Here is a typical code layout:
4192 my ($self, $filename) = @_;
4196 # In the main program
4197 push @INC, new Foo(...);
4199 Note that these hooks are also permitted to set the %INC entry
4200 corresponding to the files they have loaded. See L<perlvar/%INC>.
4202 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4208 Generally used in a C<continue> block at the end of a loop to clear
4209 variables and reset C<??> searches so that they work again. The
4210 expression is interpreted as a list of single characters (hyphens
4211 allowed for ranges). All variables and arrays beginning with one of
4212 those letters are reset to their pristine state. If the expression is
4213 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4214 only variables or searches in the current package. Always returns
4217 reset 'X'; # reset all X variables
4218 reset 'a-z'; # reset lower case variables
4219 reset; # just reset ?one-time? searches
4221 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4222 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4223 variables--lexical variables are unaffected, but they clean themselves
4224 up on scope exit anyway, so you'll probably want to use them instead.
4231 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4232 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4233 context, depending on how the return value will be used, and the context
4234 may vary from one execution to the next (see C<wantarray>). If no EXPR
4235 is given, returns an empty list in list context, the undefined value in
4236 scalar context, and (of course) nothing at all in a void context.
4238 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4239 or do FILE will automatically return the value of the last expression
4244 In list context, returns a list value consisting of the elements
4245 of LIST in the opposite order. In scalar context, concatenates the
4246 elements of LIST and returns a string value with all characters
4247 in the opposite order.
4249 print reverse <>; # line tac, last line first
4251 undef $/; # for efficiency of <>
4252 print scalar reverse <>; # character tac, last line tsrif
4254 Used without arguments in scalar context, reverse() reverses C<$_>.
4256 This operator is also handy for inverting a hash, although there are some
4257 caveats. If a value is duplicated in the original hash, only one of those
4258 can be represented as a key in the inverted hash. Also, this has to
4259 unwind one hash and build a whole new one, which may take some time
4260 on a large hash, such as from a DBM file.
4262 %by_name = reverse %by_address; # Invert the hash
4264 =item rewinddir DIRHANDLE
4266 Sets the current position to the beginning of the directory for the
4267 C<readdir> routine on DIRHANDLE.
4269 =item rindex STR,SUBSTR,POSITION
4271 =item rindex STR,SUBSTR
4273 Works just like index() except that it returns the position of the LAST
4274 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4275 last occurrence at or before that position.
4277 =item rmdir FILENAME
4281 Deletes the directory specified by FILENAME if that directory is
4282 empty. If it succeeds it returns true, otherwise it returns false and
4283 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4287 The substitution operator. See L<perlop>.
4291 Forces EXPR to be interpreted in scalar context and returns the value
4294 @counts = ( scalar @a, scalar @b, scalar @c );
4296 There is no equivalent operator to force an expression to
4297 be interpolated in list context because in practice, this is never
4298 needed. If you really wanted to do so, however, you could use
4299 the construction C<@{[ (some expression) ]}>, but usually a simple
4300 C<(some expression)> suffices.
4302 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4303 parenthesized list, this behaves as a scalar comma expression, evaluating
4304 all but the last element in void context and returning the final element
4305 evaluated in scalar context. This is seldom what you want.
4307 The following single statement:
4309 print uc(scalar(&foo,$bar)),$baz;
4311 is the moral equivalent of these two:
4314 print(uc($bar),$baz);
4316 See L<perlop> for more details on unary operators and the comma operator.
4318 =item seek FILEHANDLE,POSITION,WHENCE
4320 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4321 FILEHANDLE may be an expression whose value gives the name of the
4322 filehandle. The values for WHENCE are C<0> to set the new position
4323 I<in bytes> to POSITION, C<1> to set it to the current position plus
4324 POSITION, and C<2> to set it to EOF plus POSITION (typically
4325 negative). For WHENCE you may use the constants C<SEEK_SET>,
4326 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4327 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4330 Note the I<in bytes>: even if the filehandle has been set to
4331 operate on characters (for example by using the C<:utf8> open
4332 layer), tell() will return byte offsets, not character offsets
4333 (because implementing that would render seek() and tell() rather slow).
4335 If you want to position file for C<sysread> or C<syswrite>, don't use
4336 C<seek>--buffering makes its effect on the file's system position
4337 unpredictable and non-portable. Use C<sysseek> instead.
4339 Due to the rules and rigors of ANSI C, on some systems you have to do a
4340 seek whenever you switch between reading and writing. Amongst other
4341 things, this may have the effect of calling stdio's clearerr(3).
4342 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4346 This is also useful for applications emulating C<tail -f>. Once you hit
4347 EOF on your read, and then sleep for a while, you might have to stick in a
4348 seek() to reset things. The C<seek> doesn't change the current position,
4349 but it I<does> clear the end-of-file condition on the handle, so that the
4350 next C<< <FILE> >> makes Perl try again to read something. We hope.
4352 If that doesn't work (some IO implementations are particularly
4353 cantankerous), then you may need something more like this:
4356 for ($curpos = tell(FILE); $_ = <FILE>;
4357 $curpos = tell(FILE)) {
4358 # search for some stuff and put it into files
4360 sleep($for_a_while);
4361 seek(FILE, $curpos, 0);
4364 =item seekdir DIRHANDLE,POS
4366 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4367 must be a value returned by C<telldir>. Has the same caveats about
4368 possible directory compaction as the corresponding system library
4371 =item select FILEHANDLE
4375 Returns the currently selected filehandle. Sets the current default
4376 filehandle for output, if FILEHANDLE is supplied. This has two
4377 effects: first, a C<write> or a C<print> without a filehandle will
4378 default to this FILEHANDLE. Second, references to variables related to
4379 output will refer to this output channel. For example, if you have to
4380 set the top of form format for more than one output channel, you might
4388 FILEHANDLE may be an expression whose value gives the name of the
4389 actual filehandle. Thus:
4391 $oldfh = select(STDERR); $| = 1; select($oldfh);
4393 Some programmers may prefer to think of filehandles as objects with
4394 methods, preferring to write the last example as:
4397 STDERR->autoflush(1);
4399 =item select RBITS,WBITS,EBITS,TIMEOUT
4401 This calls the select(2) system call with the bit masks specified, which
4402 can be constructed using C<fileno> and C<vec>, along these lines:
4404 $rin = $win = $ein = '';
4405 vec($rin,fileno(STDIN),1) = 1;
4406 vec($win,fileno(STDOUT),1) = 1;
4409 If you want to select on many filehandles you might wish to write a
4413 my(@fhlist) = split(' ',$_[0]);
4416 vec($bits,fileno($_),1) = 1;
4420 $rin = fhbits('STDIN TTY SOCK');
4424 ($nfound,$timeleft) =
4425 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4427 or to block until something becomes ready just do this
4429 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4431 Most systems do not bother to return anything useful in $timeleft, so
4432 calling select() in scalar context just returns $nfound.
4434 Any of the bit masks can also be undef. The timeout, if specified, is
4435 in seconds, which may be fractional. Note: not all implementations are
4436 capable of returning the $timeleft. If not, they always return
4437 $timeleft equal to the supplied $timeout.
4439 You can effect a sleep of 250 milliseconds this way:
4441 select(undef, undef, undef, 0.25);
4443 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4444 is implementation-dependent.
4446 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4447 or <FH>) with C<select>, except as permitted by POSIX, and even
4448 then only on POSIX systems. You have to use C<sysread> instead.
4450 =item semctl ID,SEMNUM,CMD,ARG
4452 Calls the System V IPC function C<semctl>. You'll probably have to say
4456 first to get the correct constant definitions. If CMD is IPC_STAT or
4457 GETALL, then ARG must be a variable which will hold the returned
4458 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4459 the undefined value for error, "C<0 but true>" for zero, or the actual
4460 return value otherwise. The ARG must consist of a vector of native
4461 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4462 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4465 =item semget KEY,NSEMS,FLAGS
4467 Calls the System V IPC function semget. Returns the semaphore id, or
4468 the undefined value if there is an error. See also
4469 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4472 =item semop KEY,OPSTRING
4474 Calls the System V IPC function semop to perform semaphore operations
4475 such as signalling and waiting. OPSTRING must be a packed array of
4476 semop structures. Each semop structure can be generated with
4477 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4478 operations is implied by the length of OPSTRING. Returns true if
4479 successful, or false if there is an error. As an example, the
4480 following code waits on semaphore $semnum of semaphore id $semid:
4482 $semop = pack("s!3", $semnum, -1, 0);
4483 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4485 To signal the semaphore, replace C<-1> with C<1>. See also
4486 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4489 =item send SOCKET,MSG,FLAGS,TO
4491 =item send SOCKET,MSG,FLAGS
4493 Sends a message on a socket. Attempts to send the scalar MSG to the
4494 SOCKET filehandle. Takes the same flags as the system call of the
4495 same name. On unconnected sockets you must specify a destination to
4496 send TO, in which case it does a C C<sendto>. Returns the number of
4497 characters sent, or the undefined value if there is an error. The C
4498 system call sendmsg(2) is currently unimplemented. See
4499 L<perlipc/"UDP: Message Passing"> for examples.
4501 Note the I<characters>: depending on the status of the socket, either
4502 (8-bit) bytes or characters are sent. By default all sockets operate
4503 on bytes, but for example if the socket has been changed using
4504 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4505 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4506 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4507 in that case pretty much any characters can be sent.
4509 =item setpgrp PID,PGRP
4511 Sets the current process group for the specified PID, C<0> for the current
4512 process. Will produce a fatal error if used on a machine that doesn't
4513 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4514 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4515 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4518 =item setpriority WHICH,WHO,PRIORITY
4520 Sets the current priority for a process, a process group, or a user.
4521 (See setpriority(2).) Will produce a fatal error if used on a machine
4522 that doesn't implement setpriority(2).
4524 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4526 Sets the socket option requested. Returns undefined if there is an
4527 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4534 Shifts the first value of the array off and returns it, shortening the
4535 array by 1 and moving everything down. If there are no elements in the
4536 array, returns the undefined value. If ARRAY is omitted, shifts the
4537 C<@_> array within the lexical scope of subroutines and formats, and the
4538 C<@ARGV> array at file scopes or within the lexical scopes established by
4539 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4542 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4543 same thing to the left end of an array that C<pop> and C<push> do to the
4546 =item shmctl ID,CMD,ARG
4548 Calls the System V IPC function shmctl. You'll probably have to say
4552 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4553 then ARG must be a variable which will hold the returned C<shmid_ds>
4554 structure. Returns like ioctl: the undefined value for error, "C<0> but
4555 true" for zero, or the actual return value otherwise.
4556 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4558 =item shmget KEY,SIZE,FLAGS
4560 Calls the System V IPC function shmget. Returns the shared memory
4561 segment id, or the undefined value if there is an error.
4562 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4564 =item shmread ID,VAR,POS,SIZE
4566 =item shmwrite ID,STRING,POS,SIZE
4568 Reads or writes the System V shared memory segment ID starting at
4569 position POS for size SIZE by attaching to it, copying in/out, and
4570 detaching from it. When reading, VAR must be a variable that will
4571 hold the data read. When writing, if STRING is too long, only SIZE
4572 bytes are used; if STRING is too short, nulls are written to fill out
4573 SIZE bytes. Return true if successful, or false if there is an error.
4574 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4575 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4577 =item shutdown SOCKET,HOW
4579 Shuts down a socket connection in the manner indicated by HOW, which
4580 has the same interpretation as in the system call of the same name.
4582 shutdown(SOCKET, 0); # I/we have stopped reading data
4583 shutdown(SOCKET, 1); # I/we have stopped writing data
4584 shutdown(SOCKET, 2); # I/we have stopped using this socket
4586 This is useful with sockets when you want to tell the other
4587 side you're done writing but not done reading, or vice versa.
4588 It's also a more insistent form of close because it also
4589 disables the file descriptor in any forked copies in other
4596 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4597 returns sine of C<$_>.
4599 For the inverse sine operation, you may use the C<Math::Trig::asin>
4600 function, or use this relation:
4602 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4608 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4609 May be interrupted if the process receives a signal such as C<SIGALRM>.
4610 Returns the number of seconds actually slept. You probably cannot
4611 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4614 On some older systems, it may sleep up to a full second less than what
4615 you requested, depending on how it counts seconds. Most modern systems
4616 always sleep the full amount. They may appear to sleep longer than that,
4617 however, because your process might not be scheduled right away in a
4618 busy multitasking system.
4620 For delays of finer granularity than one second, you may use Perl's
4621 C<syscall> interface to access setitimer(2) if your system supports
4622 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4623 and starting from Perl 5.8 part of the standard distribution) may also
4626 See also the POSIX module's C<pause> function.
4628 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4630 Opens a socket of the specified kind and attaches it to filehandle
4631 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4632 the system call of the same name. You should C<use Socket> first
4633 to get the proper definitions imported. See the examples in
4634 L<perlipc/"Sockets: Client/Server Communication">.
4636 On systems that support a close-on-exec flag on files, the flag will
4637 be set for the newly opened file descriptor, as determined by the
4638 value of $^F. See L<perlvar/$^F>.
4640 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4642 Creates an unnamed pair of sockets in the specified domain, of the
4643 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4644 for the system call of the same name. If unimplemented, yields a fatal
4645 error. Returns true if successful.
4647 On systems that support a close-on-exec flag on files, the flag will
4648 be set for the newly opened file descriptors, as determined by the value
4649 of $^F. See L<perlvar/$^F>.
4651 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4652 to C<pipe(Rdr, Wtr)> is essentially:
4655 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4656 shutdown(Rdr, 1); # no more writing for reader
4657 shutdown(Wtr, 0); # no more reading for writer
4659 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4660 emulate socketpair using IP sockets to localhost if your system implements
4661 sockets but not socketpair.
4663 =item sort SUBNAME LIST
4665 =item sort BLOCK LIST
4669 In list context, this sorts the LIST and returns the sorted list value.
4670 In scalar context, the behaviour of C<sort()> is undefined.
4672 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4673 order. If SUBNAME is specified, it gives the name of a subroutine
4674 that returns an integer less than, equal to, or greater than C<0>,
4675 depending on how the elements of the list are to be ordered. (The C<<
4676 <=> >> and C<cmp> operators are extremely useful in such routines.)
4677 SUBNAME may be a scalar variable name (unsubscripted), in which case
4678 the value provides the name of (or a reference to) the actual
4679 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4680 an anonymous, in-line sort subroutine.
4682 If the subroutine's prototype is C<($$)>, the elements to be compared
4683 are passed by reference in C<@_>, as for a normal subroutine. This is
4684 slower than unprototyped subroutines, where the elements to be
4685 compared are passed into the subroutine
4686 as the package global variables $a and $b (see example below). Note that
4687 in the latter case, it is usually counter-productive to declare $a and
4690 In either case, the subroutine may not be recursive. The values to be
4691 compared are always passed by reference, so don't modify them.
4693 You also cannot exit out of the sort block or subroutine using any of the
4694 loop control operators described in L<perlsyn> or with C<goto>.
4696 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4697 current collation locale. See L<perllocale>.
4699 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4700 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4701 preserves the input order of elements that compare equal. Although
4702 quicksort's run time is O(NlogN) when averaged over all arrays of
4703 length N, the time can be O(N**2), I<quadratic> behavior, for some
4704 inputs.) In 5.7, the quicksort implementation was replaced with
4705 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4706 But benchmarks indicated that for some inputs, on some platforms,
4707 the original quicksort was faster. 5.8 has a sort pragma for
4708 limited control of the sort. Its rather blunt control of the
4709 underlying algorithm may not persist into future perls, but the
4710 ability to characterize the input or output in implementation
4711 independent ways quite probably will. See L<sort>.
4716 @articles = sort @files;
4718 # same thing, but with explicit sort routine
4719 @articles = sort {$a cmp $b} @files;
4721 # now case-insensitively
4722 @articles = sort {uc($a) cmp uc($b)} @files;
4724 # same thing in reversed order
4725 @articles = sort {$b cmp $a} @files;
4727 # sort numerically ascending
4728 @articles = sort {$a <=> $b} @files;
4730 # sort numerically descending
4731 @articles = sort {$b <=> $a} @files;
4733 # this sorts the %age hash by value instead of key
4734 # using an in-line function
4735 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4737 # sort using explicit subroutine name
4739 $age{$a} <=> $age{$b}; # presuming numeric
4741 @sortedclass = sort byage @class;
4743 sub backwards { $b cmp $a }
4744 @harry = qw(dog cat x Cain Abel);
4745 @george = qw(gone chased yz Punished Axed);
4747 # prints AbelCaincatdogx
4748 print sort backwards @harry;
4749 # prints xdogcatCainAbel
4750 print sort @george, 'to', @harry;
4751 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4753 # inefficiently sort by descending numeric compare using
4754 # the first integer after the first = sign, or the
4755 # whole record case-insensitively otherwise
4758 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4763 # same thing, but much more efficiently;
4764 # we'll build auxiliary indices instead
4768 push @nums, /=(\d+)/;
4773 $nums[$b] <=> $nums[$a]
4775 $caps[$a] cmp $caps[$b]
4779 # same thing, but without any temps
4780 @new = map { $_->[0] }
4781 sort { $b->[1] <=> $a->[1]
4784 } map { [$_, /=(\d+)/, uc($_)] } @old;
4786 # using a prototype allows you to use any comparison subroutine
4787 # as a sort subroutine (including other package's subroutines)
4789 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4792 @new = sort other::backwards @old;
4794 # guarantee stability, regardless of algorithm
4796 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4798 # force use of mergesort (not portable outside Perl 5.8)
4799 use sort '_mergesort'; # note discouraging _
4800 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4802 If you're using strict, you I<must not> declare $a
4803 and $b as lexicals. They are package globals. That means
4804 if you're in the C<main> package and type
4806 @articles = sort {$b <=> $a} @files;
4808 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4809 but if you're in the C<FooPack> package, it's the same as typing
4811 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4813 The comparison function is required to behave. If it returns
4814 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4815 sometimes saying the opposite, for example) the results are not
4818 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
4819 (not-a-number), and because C<sort> will trigger a fatal error unless the
4820 result of a comparison is defined, when sorting with a comparison function
4821 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
4822 The following example takes advantage of the fact that C<NaN != NaN> to
4823 eliminate any C<NaN>s from the input.
4825 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
4827 =item splice ARRAY,OFFSET,LENGTH,LIST
4829 =item splice ARRAY,OFFSET,LENGTH
4831 =item splice ARRAY,OFFSET
4835 Removes the elements designated by OFFSET and LENGTH from an array, and
4836 replaces them with the elements of LIST, if any. In list context,
4837 returns the elements removed from the array. In scalar context,
4838 returns the last element removed, or C<undef> if no elements are
4839 removed. The array grows or shrinks as necessary.
4840 If OFFSET is negative then it starts that far from the end of the array.
4841 If LENGTH is omitted, removes everything from OFFSET onward.
4842 If LENGTH is negative, removes the elements from OFFSET onward
4843 except for -LENGTH elements at the end of the array.
4844 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4845 past the end of the array, perl issues a warning, and splices at the
4848 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4850 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4851 pop(@a) splice(@a,-1)
4852 shift(@a) splice(@a,0,1)
4853 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4854 $a[$i] = $y splice(@a,$i,1,$y)
4856 Example, assuming array lengths are passed before arrays:
4858 sub aeq { # compare two list values
4859 my(@a) = splice(@_,0,shift);
4860 my(@b) = splice(@_,0,shift);
4861 return 0 unless @a == @b; # same len?
4863 return 0 if pop(@a) ne pop(@b);
4867 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4869 =item split /PATTERN/,EXPR,LIMIT
4871 =item split /PATTERN/,EXPR
4873 =item split /PATTERN/
4877 Splits a string into a list of strings and returns that list. By default,
4878 empty leading fields are preserved, and empty trailing ones are deleted.
4880 In scalar context, returns the number of fields found and splits into
4881 the C<@_> array. Use of split in scalar context is deprecated, however,
4882 because it clobbers your subroutine arguments.
4884 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4885 splits on whitespace (after skipping any leading whitespace). Anything
4886 matching PATTERN is taken to be a delimiter separating the fields. (Note
4887 that the delimiter may be longer than one character.)
4889 If LIMIT is specified and positive, it represents the maximum number
4890 of fields the EXPR will be split into, though the actual number of
4891 fields returned depends on the number of times PATTERN matches within
4892 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4893 stripped (which potential users of C<pop> would do well to remember).
4894 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4895 had been specified. Note that splitting an EXPR that evaluates to the
4896 empty string always returns the empty list, regardless of the LIMIT
4899 A pattern matching the null string (not to be confused with
4900 a null pattern C<//>, which is just one member of the set of patterns
4901 matching a null string) will split the value of EXPR into separate
4902 characters at each point it matches that way. For example:
4904 print join(':', split(/ */, 'hi there'));
4906 produces the output 'h:i:t:h:e:r:e'.
4908 Using the empty pattern C<//> specifically matches the null string, and is
4909 not be confused with the use of C<//> to mean "the last successful pattern
4912 Empty leading (or trailing) fields are produced when there are positive width
4913 matches at the beginning (or end) of the string; a zero-width match at the
4914 beginning (or end) of the string does not produce an empty field. For
4917 print join(':', split(/(?=\w)/, 'hi there!'));
4919 produces the output 'h:i :t:h:e:r:e!'.
4921 The LIMIT parameter can be used to split a line partially
4923 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4925 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
4926 a LIMIT one larger than the number of variables in the list, to avoid
4927 unnecessary work. For the list above LIMIT would have been 4 by
4928 default. In time critical applications it behooves you not to split
4929 into more fields than you really need.
4931 If the PATTERN contains parentheses, additional list elements are
4932 created from each matching substring in the delimiter.
4934 split(/([,-])/, "1-10,20", 3);
4936 produces the list value
4938 (1, '-', 10, ',', 20)
4940 If you had the entire header of a normal Unix email message in $header,
4941 you could split it up into fields and their values this way:
4943 $header =~ s/\n\s+/ /g; # fix continuation lines
4944 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4946 The pattern C</PATTERN/> may be replaced with an expression to specify
4947 patterns that vary at runtime. (To do runtime compilation only once,
4948 use C</$variable/o>.)
4950 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4951 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4952 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4953 will give you as many null initial fields as there are leading spaces.
4954 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4955 whitespace produces a null first field. A C<split> with no arguments
4956 really does a S<C<split(' ', $_)>> internally.
4958 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4963 open(PASSWD, '/etc/passwd');
4966 ($login, $passwd, $uid, $gid,
4967 $gcos, $home, $shell) = split(/:/);
4971 As with regular pattern matching, any capturing parentheses that are not
4972 matched in a C<split()> will be set to C<undef> when returned:
4974 @fields = split /(A)|B/, "1A2B3";
4975 # @fields is (1, 'A', 2, undef, 3)
4977 =item sprintf FORMAT, LIST
4979 Returns a string formatted by the usual C<printf> conventions of the C
4980 library function C<sprintf>. See below for more details
4981 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4982 the general principles.
4986 # Format number with up to 8 leading zeroes
4987 $result = sprintf("%08d", $number);
4989 # Round number to 3 digits after decimal point
4990 $rounded = sprintf("%.3f", $number);
4992 Perl does its own C<sprintf> formatting--it emulates the C
4993 function C<sprintf>, but it doesn't use it (except for floating-point
4994 numbers, and even then only the standard modifiers are allowed). As a
4995 result, any non-standard extensions in your local C<sprintf> are not
4996 available from Perl.
4998 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4999 pass it an array as your first argument. The array is given scalar context,
5000 and instead of using the 0th element of the array as the format, Perl will
5001 use the count of elements in the array as the format, which is almost never
5004 Perl's C<sprintf> permits the following universally-known conversions:
5007 %c a character with the given number
5009 %d a signed integer, in decimal
5010 %u an unsigned integer, in decimal
5011 %o an unsigned integer, in octal
5012 %x an unsigned integer, in hexadecimal
5013 %e a floating-point number, in scientific notation
5014 %f a floating-point number, in fixed decimal notation
5015 %g a floating-point number, in %e or %f notation
5017 In addition, Perl permits the following widely-supported conversions:
5019 %X like %x, but using upper-case letters
5020 %E like %e, but using an upper-case "E"
5021 %G like %g, but with an upper-case "E" (if applicable)
5022 %b an unsigned integer, in binary
5023 %p a pointer (outputs the Perl value's address in hexadecimal)
5024 %n special: *stores* the number of characters output so far
5025 into the next variable in the parameter list
5027 Finally, for backward (and we do mean "backward") compatibility, Perl
5028 permits these unnecessary but widely-supported conversions:
5031 %D a synonym for %ld
5032 %U a synonym for %lu
5033 %O a synonym for %lo
5036 Note that the number of exponent digits in the scientific notation produced
5037 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5038 exponent less than 100 is system-dependent: it may be three or less
5039 (zero-padded as necessary). In other words, 1.23 times ten to the
5040 99th may be either "1.23e99" or "1.23e099".
5042 Between the C<%> and the format letter, you may specify a number of
5043 additional attributes controlling the interpretation of the format.
5044 In order, these are:
5048 =item format parameter index
5050 An explicit format parameter index, such as C<2$>. By default sprintf
5051 will format the next unused argument in the list, but this allows you
5052 to take the arguments out of order. Eg:
5054 printf '%2$d %1$d', 12, 34; # prints "34 12"
5055 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5060 space prefix positive number with a space
5061 + prefix positive number with a plus sign
5062 - left-justify within the field
5063 0 use zeros, not spaces, to right-justify
5064 # prefix non-zero octal with "0", non-zero hex with "0x",
5065 non-zero binary with "0b"
5069 printf '<% d>', 12; # prints "< 12>"
5070 printf '<%+d>', 12; # prints "<+12>"
5071 printf '<%6s>', 12; # prints "< 12>"
5072 printf '<%-6s>', 12; # prints "<12 >"
5073 printf '<%06s>', 12; # prints "<000012>"
5074 printf '<%#x>', 12; # prints "<0xc>"
5078 The vector flag C<v>, optionally specifying the join string to use.
5079 This flag tells perl to interpret the supplied string as a vector
5080 of integers, one for each character in the string, separated by
5081 a given string (a dot C<.> by default). This can be useful for
5082 displaying ordinal values of characters in arbitrary strings:
5084 printf "version is v%vd\n", $^V; # Perl's version
5086 Put an asterisk C<*> before the C<v> to override the string to
5087 use to separate the numbers:
5089 printf "address is %*vX\n", ":", $addr; # IPv6 address
5090 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5092 You can also explicitly specify the argument number to use for
5093 the join string using eg C<*2$v>:
5095 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5097 =item (minimum) width
5099 Arguments are usually formatted to be only as wide as required to
5100 display the given value. You can override the width by putting
5101 a number here, or get the width from the next argument (with C<*>)
5102 or from a specified argument (with eg C<*2$>):
5104 printf '<%s>', "a"; # prints "<a>"
5105 printf '<%6s>', "a"; # prints "< a>"
5106 printf '<%*s>', 6, "a"; # prints "< a>"
5107 printf '<%*2$s>', "a", 6; # prints "< a>"
5108 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5110 If a field width obtained through C<*> is negative, it has the same
5111 effect as the C<-> flag: left-justification.
5113 =item precision, or maximum width
5115 You can specify a precision (for numeric conversions) or a maximum
5116 width (for string conversions) by specifying a C<.> followed by a number.
5117 For floating point formats, with the exception of 'g' and 'G', this specifies
5118 the number of decimal places to show (the default being 6), eg:
5120 # these examples are subject to system-specific variation
5121 printf '<%f>', 1; # prints "<1.000000>"
5122 printf '<%.1f>', 1; # prints "<1.0>"
5123 printf '<%.0f>', 1; # prints "<1>"
5124 printf '<%e>', 10; # prints "<1.000000e+01>"
5125 printf '<%.1e>', 10; # prints "<1.0e+01>"
5127 For 'g' and 'G', this specifies the maximum number of digits to show,
5128 including prior to the decimal point as well as after it, eg:
5130 # these examples are subject to system-specific variation
5131 printf '<%g>', 1; # prints "<1>"
5132 printf '<%.10g>', 1; # prints "<1>"
5133 printf '<%g>', 100; # prints "<100>"
5134 printf '<%.1g>', 100; # prints "<1e+02>"
5135 printf '<%.2g>', 100.01; # prints "<1e+02>"
5136 printf '<%.5g>', 100.01; # prints "<100.01>"
5137 printf '<%.4g>', 100.01; # prints "<100>"
5139 For integer conversions, specifying a precision implies that the
5140 output of the number itself should be zero-padded to this width:
5142 printf '<%.6x>', 1; # prints "<000001>"
5143 printf '<%#.6x>', 1; # prints "<0x000001>"
5144 printf '<%-10.6x>', 1; # prints "<000001 >"
5146 For string conversions, specifying a precision truncates the string
5147 to fit in the specified width:
5149 printf '<%.5s>', "truncated"; # prints "<trunc>"
5150 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5152 You can also get the precision from the next argument using C<.*>:
5154 printf '<%.6x>', 1; # prints "<000001>"
5155 printf '<%.*x>', 6, 1; # prints "<000001>"
5157 You cannot currently get the precision from a specified number,
5158 but it is intended that this will be possible in the future using
5161 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5165 For numeric conversions, you can specify the size to interpret the
5166 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5167 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5168 whatever the default integer size is on your platform (usually 32 or 64
5169 bits), but you can override this to use instead one of the standard C types,
5170 as supported by the compiler used to build Perl:
5172 l interpret integer as C type "long" or "unsigned long"
5173 h interpret integer as C type "short" or "unsigned short"
5174 q, L or ll interpret integer as C type "long long", "unsigned long long".
5175 or "quads" (typically 64-bit integers)
5177 The last will produce errors if Perl does not understand "quads" in your
5178 installation. (This requires that either the platform natively supports quads
5179 or Perl was specifically compiled to support quads.) You can find out
5180 whether your Perl supports quads via L<Config>:
5183 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5186 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5187 to be the default floating point size on your platform (double or long double),
5188 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5189 platform supports them. You can find out whether your Perl supports long
5190 doubles via L<Config>:
5193 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5195 You can find out whether Perl considers 'long double' to be the default
5196 floating point size to use on your platform via L<Config>:
5199 ($Config{uselongdouble} eq 'define') &&
5200 print "long doubles by default\n";
5202 It can also be the case that long doubles and doubles are the same thing:
5205 ($Config{doublesize} == $Config{longdblsize}) &&
5206 print "doubles are long doubles\n";
5208 The size specifier C<V> has no effect for Perl code, but it is supported
5209 for compatibility with XS code; it means 'use the standard size for
5210 a Perl integer (or floating-point number)', which is already the
5211 default for Perl code.
5213 =item order of arguments
5215 Normally, sprintf takes the next unused argument as the value to
5216 format for each format specification. If the format specification
5217 uses C<*> to require additional arguments, these are consumed from
5218 the argument list in the order in which they appear in the format
5219 specification I<before> the value to format. Where an argument is
5220 specified using an explicit index, this does not affect the normal
5221 order for the arguments (even when the explicitly specified index
5222 would have been the next argument in any case).
5226 printf '<%*.*s>', $a, $b, $c;
5228 would use C<$a> for the width, C<$b> for the precision and C<$c>
5229 as the value to format, while:
5231 print '<%*1$.*s>', $a, $b;
5233 would use C<$a> for the width and the precision, and C<$b> as the
5236 Here are some more examples - beware that when using an explicit
5237 index, the C<$> may need to be escaped:
5239 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5240 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5241 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5242 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5246 If C<use locale> is in effect, the character used for the decimal
5247 point in formatted real numbers is affected by the LC_NUMERIC locale.
5254 Return the square root of EXPR. If EXPR is omitted, returns square
5255 root of C<$_>. Only works on non-negative operands, unless you've
5256 loaded the standard Math::Complex module.
5259 print sqrt(-2); # prints 1.4142135623731i
5265 Sets the random number seed for the C<rand> operator.
5267 The point of the function is to "seed" the C<rand> function so that
5268 C<rand> can produce a different sequence each time you run your
5271 If srand() is not called explicitly, it is called implicitly at the
5272 first use of the C<rand> operator. However, this was not the case in
5273 versions of Perl before 5.004, so if your script will run under older
5274 Perl versions, it should call C<srand>.
5276 Most programs won't even call srand() at all, except those that
5277 need a cryptographically-strong starting point rather than the
5278 generally acceptable default, which is based on time of day,
5279 process ID, and memory allocation, or the F</dev/urandom> device,
5282 You can call srand($seed) with the same $seed to reproduce the
5283 I<same> sequence from rand(), but this is usually reserved for
5284 generating predictable results for testing or debugging.
5285 Otherwise, don't call srand() more than once in your program.
5287 Do B<not> call srand() (i.e. without an argument) more than once in
5288 a script. The internal state of the random number generator should
5289 contain more entropy than can be provided by any seed, so calling
5290 srand() again actually I<loses> randomness.
5292 Most implementations of C<srand> take an integer and will silently
5293 truncate decimal numbers. This means C<srand(42)> will usually
5294 produce the same results as C<srand(42.1)>. To be safe, always pass
5295 C<srand> an integer.
5297 In versions of Perl prior to 5.004 the default seed was just the
5298 current C<time>. This isn't a particularly good seed, so many old
5299 programs supply their own seed value (often C<time ^ $$> or C<time ^
5300 ($$ + ($$ << 15))>), but that isn't necessary any more.
5302 Note that you need something much more random than the default seed for
5303 cryptographic purposes. Checksumming the compressed output of one or more
5304 rapidly changing operating system status programs is the usual method. For
5307 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5309 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5312 Frequently called programs (like CGI scripts) that simply use
5316 for a seed can fall prey to the mathematical property that
5320 one-third of the time. So don't do that.
5322 =item stat FILEHANDLE
5328 Returns a 13-element list giving the status info for a file, either
5329 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5330 it stats C<$_>. Returns a null list if the stat fails. Typically used
5333 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5334 $atime,$mtime,$ctime,$blksize,$blocks)
5337 Not all fields are supported on all filesystem types. Here are the
5338 meaning of the fields:
5340 0 dev device number of filesystem
5342 2 mode file mode (type and permissions)
5343 3 nlink number of (hard) links to the file
5344 4 uid numeric user ID of file's owner
5345 5 gid numeric group ID of file's owner
5346 6 rdev the device identifier (special files only)
5347 7 size total size of file, in bytes
5348 8 atime last access time in seconds since the epoch
5349 9 mtime last modify time in seconds since the epoch
5350 10 ctime inode change time in seconds since the epoch (*)
5351 11 blksize preferred block size for file system I/O
5352 12 blocks actual number of blocks allocated
5354 (The epoch was at 00:00 January 1, 1970 GMT.)
5356 (*) The ctime field is non-portable, in particular you cannot expect
5357 it to be a "creation time", see L<perlport/"Files and Filesystems">
5360 If stat is passed the special filehandle consisting of an underline, no
5361 stat is done, but the current contents of the stat structure from the
5362 last stat or filetest are returned. Example:
5364 if (-x $file && (($d) = stat(_)) && $d < 0) {
5365 print "$file is executable NFS file\n";
5368 (This works on machines only for which the device number is negative
5371 Because the mode contains both the file type and its permissions, you
5372 should mask off the file type portion and (s)printf using a C<"%o">
5373 if you want to see the real permissions.
5375 $mode = (stat($filename))[2];
5376 printf "Permissions are %04o\n", $mode & 07777;
5378 In scalar context, C<stat> returns a boolean value indicating success
5379 or failure, and, if successful, sets the information associated with
5380 the special filehandle C<_>.
5382 The File::stat module provides a convenient, by-name access mechanism:
5385 $sb = stat($filename);
5386 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5387 $filename, $sb->size, $sb->mode & 07777,
5388 scalar localtime $sb->mtime;
5390 You can import symbolic mode constants (C<S_IF*>) and functions
5391 (C<S_IS*>) from the Fcntl module:
5395 $mode = (stat($filename))[2];
5397 $user_rwx = ($mode & S_IRWXU) >> 6;
5398 $group_read = ($mode & S_IRGRP) >> 3;
5399 $other_execute = $mode & S_IXOTH;
5401 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5403 $is_setuid = $mode & S_ISUID;
5404 $is_setgid = S_ISDIR($mode);
5406 You could write the last two using the C<-u> and C<-d> operators.
5407 The commonly available S_IF* constants are
5409 # Permissions: read, write, execute, for user, group, others.
5411 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5412 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5413 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5415 # Setuid/Setgid/Stickiness/SaveText.
5416 # Note that the exact meaning of these is system dependent.
5418 S_ISUID S_ISGID S_ISVTX S_ISTXT
5420 # File types. Not necessarily all are available on your system.
5422 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5424 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5426 S_IREAD S_IWRITE S_IEXEC
5428 and the S_IF* functions are
5430 S_IMODE($mode) the part of $mode containing the permission bits
5431 and the setuid/setgid/sticky bits
5433 S_IFMT($mode) the part of $mode containing the file type
5434 which can be bit-anded with e.g. S_IFREG
5435 or with the following functions
5437 # The operators -f, -d, -l, -b, -c, -p, and -s.
5439 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5440 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5442 # No direct -X operator counterpart, but for the first one
5443 # the -g operator is often equivalent. The ENFMT stands for
5444 # record flocking enforcement, a platform-dependent feature.
5446 S_ISENFMT($mode) S_ISWHT($mode)
5448 See your native chmod(2) and stat(2) documentation for more details
5449 about the S_* constants. To get status info for a symbolic link
5450 instead of the target file behind the link, use the C<lstat> function.
5456 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5457 doing many pattern matches on the string before it is next modified.
5458 This may or may not save time, depending on the nature and number of
5459 patterns you are searching on, and on the distribution of character
5460 frequencies in the string to be searched--you probably want to compare
5461 run times with and without it to see which runs faster. Those loops
5462 which scan for many short constant strings (including the constant
5463 parts of more complex patterns) will benefit most. You may have only
5464 one C<study> active at a time--if you study a different scalar the first
5465 is "unstudied". (The way C<study> works is this: a linked list of every
5466 character in the string to be searched is made, so we know, for
5467 example, where all the C<'k'> characters are. From each search string,
5468 the rarest character is selected, based on some static frequency tables
5469 constructed from some C programs and English text. Only those places
5470 that contain this "rarest" character are examined.)
5472 For example, here is a loop that inserts index producing entries
5473 before any line containing a certain pattern:
5477 print ".IX foo\n" if /\bfoo\b/;
5478 print ".IX bar\n" if /\bbar\b/;
5479 print ".IX blurfl\n" if /\bblurfl\b/;
5484 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5485 will be looked at, because C<f> is rarer than C<o>. In general, this is
5486 a big win except in pathological cases. The only question is whether
5487 it saves you more time than it took to build the linked list in the
5490 Note that if you have to look for strings that you don't know till
5491 runtime, you can build an entire loop as a string and C<eval> that to
5492 avoid recompiling all your patterns all the time. Together with
5493 undefining C<$/> to input entire files as one record, this can be very
5494 fast, often faster than specialized programs like fgrep(1). The following
5495 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5496 out the names of those files that contain a match:
5498 $search = 'while (<>) { study;';
5499 foreach $word (@words) {
5500 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5505 eval $search; # this screams
5506 $/ = "\n"; # put back to normal input delimiter
5507 foreach $file (sort keys(%seen)) {
5511 =item sub NAME BLOCK
5513 =item sub NAME (PROTO) BLOCK
5515 =item sub NAME : ATTRS BLOCK
5517 =item sub NAME (PROTO) : ATTRS BLOCK
5519 This is subroutine definition, not a real function I<per se>.
5520 Without a BLOCK it's just a forward declaration. Without a NAME,
5521 it's an anonymous function declaration, and does actually return
5522 a value: the CODE ref of the closure you just created.
5524 See L<perlsub> and L<perlref> for details about subroutines and
5525 references, and L<attributes> and L<Attribute::Handlers> for more
5526 information about attributes.
5528 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5530 =item substr EXPR,OFFSET,LENGTH
5532 =item substr EXPR,OFFSET
5534 Extracts a substring out of EXPR and returns it. First character is at
5535 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5536 If OFFSET is negative (or more precisely, less than C<$[>), starts
5537 that far from the end of the string. If LENGTH is omitted, returns
5538 everything to the end of the string. If LENGTH is negative, leaves that
5539 many characters off the end of the string.
5541 You can use the substr() function as an lvalue, in which case EXPR
5542 must itself be an lvalue. If you assign something shorter than LENGTH,
5543 the string will shrink, and if you assign something longer than LENGTH,
5544 the string will grow to accommodate it. To keep the string the same
5545 length you may need to pad or chop your value using C<sprintf>.
5547 If OFFSET and LENGTH specify a substring that is partly outside the
5548 string, only the part within the string is returned. If the substring
5549 is beyond either end of the string, substr() returns the undefined
5550 value and produces a warning. When used as an lvalue, specifying a
5551 substring that is entirely outside the string is a fatal error.
5552 Here's an example showing the behavior for boundary cases:
5555 substr($name, 4) = 'dy'; # $name is now 'freddy'
5556 my $null = substr $name, 6, 2; # returns '' (no warning)
5557 my $oops = substr $name, 7; # returns undef, with warning
5558 substr($name, 7) = 'gap'; # fatal error
5560 An alternative to using substr() as an lvalue is to specify the
5561 replacement string as the 4th argument. This allows you to replace
5562 parts of the EXPR and return what was there before in one operation,
5563 just as you can with splice().
5565 If the lvalue returned by substr is used after the EXPR is changed in
5566 any way, the behaviour may not be as expected and is subject to change.
5567 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5568 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5569 substring assignment, and then the substr is used again), or where a
5570 substr() is aliased via a C<foreach> loop or passed as a parameter or
5571 a reference to it is taken and then the alias, parameter, or deref'd
5572 reference either is used after the original EXPR has been changed or
5573 is assigned to and then used a second time.
5575 =item symlink OLDFILE,NEWFILE
5577 Creates a new filename symbolically linked to the old filename.
5578 Returns C<1> for success, C<0> otherwise. On systems that don't support
5579 symbolic links, produces a fatal error at run time. To check for that,
5582 $symlink_exists = eval { symlink("",""); 1 };
5584 =item syscall NUMBER, LIST
5586 Calls the system call specified as the first element of the list,
5587 passing the remaining elements as arguments to the system call. If
5588 unimplemented, produces a fatal error. The arguments are interpreted
5589 as follows: if a given argument is numeric, the argument is passed as
5590 an int. If not, the pointer to the string value is passed. You are
5591 responsible to make sure a string is pre-extended long enough to
5592 receive any result that might be written into a string. You can't use a
5593 string literal (or other read-only string) as an argument to C<syscall>
5594 because Perl has to assume that any string pointer might be written
5596 integer arguments are not literals and have never been interpreted in a
5597 numeric context, you may need to add C<0> to them to force them to look
5598 like numbers. This emulates the C<syswrite> function (or vice versa):
5600 require 'syscall.ph'; # may need to run h2ph
5602 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5604 Note that Perl supports passing of up to only 14 arguments to your system call,
5605 which in practice should usually suffice.
5607 Syscall returns whatever value returned by the system call it calls.
5608 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5609 Note that some system calls can legitimately return C<-1>. The proper
5610 way to handle such calls is to assign C<$!=0;> before the call and
5611 check the value of C<$!> if syscall returns C<-1>.
5613 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5614 number of the read end of the pipe it creates. There is no way
5615 to retrieve the file number of the other end. You can avoid this
5616 problem by using C<pipe> instead.
5618 =item sysopen FILEHANDLE,FILENAME,MODE
5620 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5622 Opens the file whose filename is given by FILENAME, and associates it
5623 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5624 the name of the real filehandle wanted. This function calls the
5625 underlying operating system's C<open> function with the parameters
5626 FILENAME, MODE, PERMS.
5628 The possible values and flag bits of the MODE parameter are
5629 system-dependent; they are available via the standard module C<Fcntl>.
5630 See the documentation of your operating system's C<open> to see which
5631 values and flag bits are available. You may combine several flags
5632 using the C<|>-operator.
5634 Some of the most common values are C<O_RDONLY> for opening the file in
5635 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5636 and C<O_RDWR> for opening the file in read-write mode, and.
5638 For historical reasons, some values work on almost every system
5639 supported by perl: zero means read-only, one means write-only, and two
5640 means read/write. We know that these values do I<not> work under
5641 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5642 use them in new code.
5644 If the file named by FILENAME does not exist and the C<open> call creates
5645 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5646 PERMS specifies the permissions of the newly created file. If you omit
5647 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5648 These permission values need to be in octal, and are modified by your
5649 process's current C<umask>.
5651 In many systems the C<O_EXCL> flag is available for opening files in
5652 exclusive mode. This is B<not> locking: exclusiveness means here that
5653 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5656 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5658 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5659 that takes away the user's option to have a more permissive umask.
5660 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5663 Note that C<sysopen> depends on the fdopen() C library function.
5664 On many UNIX systems, fdopen() is known to fail when file descriptors
5665 exceed a certain value, typically 255. If you need more file
5666 descriptors than that, consider rebuilding Perl to use the C<sfio>
5667 library, or perhaps using the POSIX::open() function.
5669 See L<perlopentut> for a kinder, gentler explanation of opening files.
5671 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5673 =item sysread FILEHANDLE,SCALAR,LENGTH
5675 Attempts to read LENGTH bytes of data into variable SCALAR from the
5676 specified FILEHANDLE, using the system call read(2). It bypasses
5677 buffered IO, so mixing this with other kinds of reads, C<print>,
5678 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5679 perlio or stdio layers usually buffers data. Returns the number of
5680 bytes actually read, C<0> at end of file, or undef if there was an
5681 error (in the latter case C<$!> is also set). SCALAR will be grown or
5682 shrunk so that the last byte actually read is the last byte of the
5683 scalar after the read.
5685 An OFFSET may be specified to place the read data at some place in the
5686 string other than the beginning. A negative OFFSET specifies
5687 placement at that many characters counting backwards from the end of
5688 the string. A positive OFFSET greater than the length of SCALAR
5689 results in the string being padded to the required size with C<"\0">
5690 bytes before the result of the read is appended.
5692 There is no syseof() function, which is ok, since eof() doesn't work
5693 very well on device files (like ttys) anyway. Use sysread() and check
5694 for a return value for 0 to decide whether you're done.
5696 Note that if the filehandle has been marked as C<:utf8> Unicode
5697 characters are read instead of bytes (the LENGTH, OFFSET, and the
5698 return value of sysread() are in Unicode characters).
5699 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5700 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5702 =item sysseek FILEHANDLE,POSITION,WHENCE
5704 Sets FILEHANDLE's system position in bytes using the system call
5705 lseek(2). FILEHANDLE may be an expression whose value gives the name
5706 of the filehandle. The values for WHENCE are C<0> to set the new
5707 position to POSITION, C<1> to set the it to the current position plus
5708 POSITION, and C<2> to set it to EOF plus POSITION (typically
5711 Note the I<in bytes>: even if the filehandle has been set to operate
5712 on characters (for example by using the C<:utf8> I/O layer), tell()
5713 will return byte offsets, not character offsets (because implementing
5714 that would render sysseek() very slow).
5716 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5717 than C<sysread>, for example >< or read()) C<print>, C<write>,
5718 C<seek>, C<tell>, or C<eof> may cause confusion.
5720 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5721 and C<SEEK_END> (start of the file, current position, end of the file)
5722 from the Fcntl module. Use of the constants is also more portable
5723 than relying on 0, 1, and 2. For example to define a "systell" function:
5725 use Fcntl 'SEEK_CUR';
5726 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5728 Returns the new position, or the undefined value on failure. A position
5729 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5730 true on success and false on failure, yet you can still easily determine
5735 =item system PROGRAM LIST
5737 Does exactly the same thing as C<exec LIST>, except that a fork is
5738 done first, and the parent process waits for the child process to
5739 complete. Note that argument processing varies depending on the
5740 number of arguments. If there is more than one argument in LIST,
5741 or if LIST is an array with more than one value, starts the program
5742 given by the first element of the list with arguments given by the
5743 rest of the list. If there is only one scalar argument, the argument
5744 is checked for shell metacharacters, and if there are any, the
5745 entire argument is passed to the system's command shell for parsing
5746 (this is C</bin/sh -c> on Unix platforms, but varies on other
5747 platforms). If there are no shell metacharacters in the argument,
5748 it is split into words and passed directly to C<execvp>, which is
5751 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5752 output before any operation that may do a fork, but this may not be
5753 supported on some platforms (see L<perlport>). To be safe, you may need
5754 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5755 of C<IO::Handle> on any open handles.
5757 The return value is the exit status of the program as returned by the
5758 C<wait> call. To get the actual exit value shift right by eight (see below).
5759 See also L</exec>. This is I<not> what you want to use to capture
5760 the output from a command, for that you should use merely backticks or
5761 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5762 indicates a failure to start the program (inspect $! for the reason).
5764 Like C<exec>, C<system> allows you to lie to a program about its name if
5765 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5767 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
5768 C<system>, if you expect your program to terminate on receipt of these
5769 signals you will need to arrange to do so yourself based on the return
5772 @args = ("command", "arg1", "arg2");
5774 or die "system @args failed: $?"
5776 You can check all the failure possibilities by inspecting
5780 print "failed to execute: $!\n";
5783 printf "child died with signal %d, %s coredump\n",
5784 ($? & 127), ($? & 128) ? 'with' : 'without';
5787 printf "child exited with value %d\n", $? >> 8;
5790 or more portably by using the W*() calls of the POSIX extension;
5791 see L<perlport> for more information.
5793 When the arguments get executed via the system shell, results
5794 and return codes will be subject to its quirks and capabilities.
5795 See L<perlop/"`STRING`"> and L</exec> for details.
5797 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5799 =item syswrite FILEHANDLE,SCALAR,LENGTH
5801 =item syswrite FILEHANDLE,SCALAR
5803 Attempts to write LENGTH bytes of data from variable SCALAR to the
5804 specified FILEHANDLE, using the system call write(2). If LENGTH is
5805 not specified, writes whole SCALAR. It bypasses buffered IO, so
5806 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5807 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5808 stdio layers usually buffers data. Returns the number of bytes
5809 actually written, or C<undef> if there was an error (in this case the
5810 errno variable C<$!> is also set). If the LENGTH is greater than the
5811 available data in the SCALAR after the OFFSET, only as much data as is
5812 available will be written.
5814 An OFFSET may be specified to write the data from some part of the
5815 string other than the beginning. A negative OFFSET specifies writing
5816 that many characters counting backwards from the end of the string.
5817 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5819 Note that if the filehandle has been marked as C<:utf8>, Unicode
5820 characters are written instead of bytes (the LENGTH, OFFSET, and the
5821 return value of syswrite() are in UTF-8 encoded Unicode characters).
5822 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5823 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5825 =item tell FILEHANDLE
5829 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5830 error. FILEHANDLE may be an expression whose value gives the name of
5831 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5834 Note the I<in bytes>: even if the filehandle has been set to
5835 operate on characters (for example by using the C<:utf8> open
5836 layer), tell() will return byte offsets, not character offsets
5837 (because that would render seek() and tell() rather slow).
5839 The return value of tell() for the standard streams like the STDIN
5840 depends on the operating system: it may return -1 or something else.
5841 tell() on pipes, fifos, and sockets usually returns -1.
5843 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5845 Do not use tell() on a filehandle that has been opened using
5846 sysopen(), use sysseek() for that as described above. Why? Because
5847 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5848 buffered filehandles. sysseek() make sense only on the first kind,
5849 tell() only makes sense on the second kind.
5851 =item telldir DIRHANDLE
5853 Returns the current position of the C<readdir> routines on DIRHANDLE.
5854 Value may be given to C<seekdir> to access a particular location in a
5855 directory. Has the same caveats about possible directory compaction as
5856 the corresponding system library routine.
5858 =item tie VARIABLE,CLASSNAME,LIST
5860 This function binds a variable to a package class that will provide the
5861 implementation for the variable. VARIABLE is the name of the variable
5862 to be enchanted. CLASSNAME is the name of a class implementing objects
5863 of correct type. Any additional arguments are passed to the C<new>
5864 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5865 or C<TIEHASH>). Typically these are arguments such as might be passed
5866 to the C<dbm_open()> function of C. The object returned by the C<new>
5867 method is also returned by the C<tie> function, which would be useful
5868 if you want to access other methods in CLASSNAME.
5870 Note that functions such as C<keys> and C<values> may return huge lists
5871 when used on large objects, like DBM files. You may prefer to use the
5872 C<each> function to iterate over such. Example:
5874 # print out history file offsets
5876 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5877 while (($key,$val) = each %HIST) {
5878 print $key, ' = ', unpack('L',$val), "\n";
5882 A class implementing a hash should have the following methods:
5884 TIEHASH classname, LIST
5886 STORE this, key, value
5891 NEXTKEY this, lastkey
5896 A class implementing an ordinary array should have the following methods:
5898 TIEARRAY classname, LIST
5900 STORE this, key, value
5902 STORESIZE this, count
5908 SPLICE this, offset, length, LIST
5913 A class implementing a file handle should have the following methods:
5915 TIEHANDLE classname, LIST
5916 READ this, scalar, length, offset
5919 WRITE this, scalar, length, offset
5921 PRINTF this, format, LIST
5925 SEEK this, position, whence
5927 OPEN this, mode, LIST
5932 A class implementing a scalar should have the following methods:
5934 TIESCALAR classname, LIST
5940 Not all methods indicated above need be implemented. See L<perltie>,
5941 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5943 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5944 for you--you need to do that explicitly yourself. See L<DB_File>
5945 or the F<Config> module for interesting C<tie> implementations.
5947 For further details see L<perltie>, L<"tied VARIABLE">.
5951 Returns a reference to the object underlying VARIABLE (the same value
5952 that was originally returned by the C<tie> call that bound the variable
5953 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5958 Returns the number of non-leap seconds since whatever time the system
5959 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5960 and 00:00:00 UTC, January 1, 1970 for most other systems).
5961 Suitable for feeding to C<gmtime> and C<localtime>.
5963 For measuring time in better granularity than one second,
5964 you may use either the Time::HiRes module (from CPAN, and starting from
5965 Perl 5.8 part of the standard distribution), or if you have
5966 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5967 See L<perlfaq8> for details.
5971 Returns a four-element list giving the user and system times, in
5972 seconds, for this process and the children of this process.
5974 ($user,$system,$cuser,$csystem) = times;
5976 In scalar context, C<times> returns C<$user>.
5980 The transliteration operator. Same as C<y///>. See L<perlop>.
5982 =item truncate FILEHANDLE,LENGTH
5984 =item truncate EXPR,LENGTH
5986 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5987 specified length. Produces a fatal error if truncate isn't implemented
5988 on your system. Returns true if successful, the undefined value
5991 The behavior is undefined if LENGTH is greater than the length of the
5998 Returns an uppercased version of EXPR. This is the internal function
5999 implementing the C<\U> escape in double-quoted strings. Respects
6000 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6001 and L<perlunicode> for more details about locale and Unicode support.
6002 It does not attempt to do titlecase mapping on initial letters. See
6003 C<ucfirst> for that.
6005 If EXPR is omitted, uses C<$_>.
6011 Returns the value of EXPR with the first character in uppercase
6012 (titlecase in Unicode). This is the internal function implementing
6013 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6014 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6015 for more details about locale and Unicode support.
6017 If EXPR is omitted, uses C<$_>.
6023 Sets the umask for the process to EXPR and returns the previous value.
6024 If EXPR is omitted, merely returns the current umask.
6026 The Unix permission C<rwxr-x---> is represented as three sets of three
6027 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6028 and isn't one of the digits). The C<umask> value is such a number
6029 representing disabled permissions bits. The permission (or "mode")
6030 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6031 even if you tell C<sysopen> to create a file with permissions C<0777>,
6032 if your umask is C<0022> then the file will actually be created with
6033 permissions C<0755>. If your C<umask> were C<0027> (group can't
6034 write; others can't read, write, or execute), then passing
6035 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6038 Here's some advice: supply a creation mode of C<0666> for regular
6039 files (in C<sysopen>) and one of C<0777> for directories (in
6040 C<mkdir>) and executable files. This gives users the freedom of
6041 choice: if they want protected files, they might choose process umasks
6042 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6043 Programs should rarely if ever make policy decisions better left to
6044 the user. The exception to this is when writing files that should be
6045 kept private: mail files, web browser cookies, I<.rhosts> files, and
6048 If umask(2) is not implemented on your system and you are trying to
6049 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6050 fatal error at run time. If umask(2) is not implemented and you are
6051 not trying to restrict access for yourself, returns C<undef>.
6053 Remember that a umask is a number, usually given in octal; it is I<not> a
6054 string of octal digits. See also L</oct>, if all you have is a string.
6060 Undefines the value of EXPR, which must be an lvalue. Use only on a
6061 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6062 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6063 will probably not do what you expect on most predefined variables or
6064 DBM list values, so don't do that; see L<delete>.) Always returns the
6065 undefined value. You can omit the EXPR, in which case nothing is
6066 undefined, but you still get an undefined value that you could, for
6067 instance, return from a subroutine, assign to a variable or pass as a
6068 parameter. Examples:
6071 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6075 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6076 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6077 select undef, undef, undef, 0.25;
6078 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6080 Note that this is a unary operator, not a list operator.
6086 Deletes a list of files. Returns the number of files successfully
6089 $cnt = unlink 'a', 'b', 'c';
6093 Note: C<unlink> will not delete directories unless you are superuser and
6094 the B<-U> flag is supplied to Perl. Even if these conditions are
6095 met, be warned that unlinking a directory can inflict damage on your
6096 filesystem. Use C<rmdir> instead.
6098 If LIST is omitted, uses C<$_>.
6100 =item unpack TEMPLATE,EXPR
6102 =item unpack TEMPLATE
6104 C<unpack> does the reverse of C<pack>: it takes a string
6105 and expands it out into a list of values.
6106 (In scalar context, it returns merely the first value produced.)
6108 If EXPR is omitted, unpacks the C<$_> string.
6110 The string is broken into chunks described by the TEMPLATE. Each chunk
6111 is converted separately to a value. Typically, either the string is a result
6112 of C<pack>, or the bytes of the string represent a C structure of some
6115 The TEMPLATE has the same format as in the C<pack> function.
6116 Here's a subroutine that does substring:
6119 my($what,$where,$howmuch) = @_;
6120 unpack("x$where a$howmuch", $what);
6125 sub ordinal { unpack("c",$_[0]); } # same as ord()
6127 In addition to fields allowed in pack(), you may prefix a field with
6128 a %<number> to indicate that
6129 you want a <number>-bit checksum of the items instead of the items
6130 themselves. Default is a 16-bit checksum. Checksum is calculated by
6131 summing numeric values of expanded values (for string fields the sum of
6132 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6134 For example, the following
6135 computes the same number as the System V sum program:
6139 unpack("%32C*",<>) % 65535;
6142 The following efficiently counts the number of set bits in a bit vector:
6144 $setbits = unpack("%32b*", $selectmask);
6146 The C<p> and C<P> formats should be used with care. Since Perl
6147 has no way of checking whether the value passed to C<unpack()>
6148 corresponds to a valid memory location, passing a pointer value that's
6149 not known to be valid is likely to have disastrous consequences.
6151 If there are more pack codes or if the repeat count of a field or a group
6152 is larger than what the remainder of the input string allows, the result
6153 is not well defined: in some cases, the repeat count is decreased, or
6154 C<unpack()> will produce null strings or zeroes, or terminate with an
6155 error. If the input string is longer than one described by the TEMPLATE,
6156 the rest is ignored.
6158 See L</pack> for more examples and notes.
6160 =item untie VARIABLE
6162 Breaks the binding between a variable and a package. (See C<tie>.)
6163 Has no effect if the variable is not tied.
6165 =item unshift ARRAY,LIST
6167 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6168 depending on how you look at it. Prepends list to the front of the
6169 array, and returns the new number of elements in the array.
6171 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6173 Note the LIST is prepended whole, not one element at a time, so the
6174 prepended elements stay in the same order. Use C<reverse> to do the
6177 =item use Module VERSION LIST
6179 =item use Module VERSION
6181 =item use Module LIST
6187 Imports some semantics into the current package from the named module,
6188 generally by aliasing certain subroutine or variable names into your
6189 package. It is exactly equivalent to
6191 BEGIN { require Module; import Module LIST; }
6193 except that Module I<must> be a bareword.
6195 VERSION may be either a numeric argument such as 5.006, which will be
6196 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6197 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6198 greater than the version of the current Perl interpreter; Perl will not
6199 attempt to parse the rest of the file. Compare with L</require>, which can
6200 do a similar check at run time.
6202 Specifying VERSION as a literal of the form v5.6.1 should generally be
6203 avoided, because it leads to misleading error messages under earlier
6204 versions of Perl which do not support this syntax. The equivalent numeric
6205 version should be used instead.
6207 use v5.6.1; # compile time version check
6209 use 5.006_001; # ditto; preferred for backwards compatibility
6211 This is often useful if you need to check the current Perl version before
6212 C<use>ing library modules that have changed in incompatible ways from
6213 older versions of Perl. (We try not to do this more than we have to.)
6215 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6216 C<require> makes sure the module is loaded into memory if it hasn't been
6217 yet. The C<import> is not a builtin--it's just an ordinary static method
6218 call into the C<Module> package to tell the module to import the list of
6219 features back into the current package. The module can implement its
6220 C<import> method any way it likes, though most modules just choose to
6221 derive their C<import> method via inheritance from the C<Exporter> class that
6222 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6223 method can be found then the call is skipped, even if there is an AUTOLOAD
6226 If you do not want to call the package's C<import> method (for instance,
6227 to stop your namespace from being altered), explicitly supply the empty list:
6231 That is exactly equivalent to
6233 BEGIN { require Module }
6235 If the VERSION argument is present between Module and LIST, then the
6236 C<use> will call the VERSION method in class Module with the given
6237 version as an argument. The default VERSION method, inherited from
6238 the UNIVERSAL class, croaks if the given version is larger than the
6239 value of the variable C<$Module::VERSION>.
6241 Again, there is a distinction between omitting LIST (C<import> called
6242 with no arguments) and an explicit empty LIST C<()> (C<import> not
6243 called). Note that there is no comma after VERSION!
6245 Because this is a wide-open interface, pragmas (compiler directives)
6246 are also implemented this way. Currently implemented pragmas are:
6251 use sigtrap qw(SEGV BUS);
6252 use strict qw(subs vars refs);
6253 use subs qw(afunc blurfl);
6254 use warnings qw(all);
6255 use sort qw(stable _quicksort _mergesort);
6257 Some of these pseudo-modules import semantics into the current
6258 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6259 which import symbols into the current package (which are effective
6260 through the end of the file).
6262 There's a corresponding C<no> command that unimports meanings imported
6263 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6264 It behaves exactly as C<import> does with respect to VERSION, an
6265 omitted LIST, empty LIST, or no unimport method being found.
6271 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6272 for the C<-M> and C<-m> command-line options to perl that give C<use>
6273 functionality from the command-line.
6277 Changes the access and modification times on each file of a list of
6278 files. The first two elements of the list must be the NUMERICAL access
6279 and modification times, in that order. Returns the number of files
6280 successfully changed. The inode change time of each file is set
6281 to the current time. For example, this code has the same effect as the
6282 Unix touch(1) command when the files I<already exist>.
6285 $atime = $mtime = time;
6286 utime $atime, $mtime, @ARGV;
6288 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6289 the utime(2) function in the C library will be called with a null second
6290 argument. On most systems, this will set the file's access and
6291 modification times to the current time (i.e. equivalent to the example
6294 utime undef, undef, @ARGV;
6296 Under NFS this will use the time of the NFS server, not the time of
6297 the local machine. If there is a time synchronization problem, the
6298 NFS server and local machine will have different times. The Unix
6299 touch(1) command will in fact normally use this form instead of the
6300 one shown in the first example.
6302 Note that only passing one of the first two elements as C<undef> will
6303 be equivalent of passing it as 0 and will not have the same effect as
6304 described when they are both C<undef>. This case will also trigger an
6305 uninitialized warning.
6309 Returns a list consisting of all the values of the named hash.
6310 (In a scalar context, returns the number of values.)
6312 The values are returned in an apparently random order. The actual
6313 random order is subject to change in future versions of perl, but it
6314 is guaranteed to be the same order as either the C<keys> or C<each>
6315 function would produce on the same (unmodified) hash. Since Perl
6316 5.8.1 the ordering is different even between different runs of Perl
6317 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6319 As a side effect, calling values() resets the HASH's internal iterator,
6320 see L</each>. (In particular, calling values() in void context resets
6321 the iterator with no other overhead.)
6323 Note that the values are not copied, which means modifying them will
6324 modify the contents of the hash:
6326 for (values %hash) { s/foo/bar/g } # modifies %hash values
6327 for (@hash{keys %hash}) { s/foo/bar/g } # same
6329 See also C<keys>, C<each>, and C<sort>.
6331 =item vec EXPR,OFFSET,BITS
6333 Treats the string in EXPR as a bit vector made up of elements of
6334 width BITS, and returns the value of the element specified by OFFSET
6335 as an unsigned integer. BITS therefore specifies the number of bits
6336 that are reserved for each element in the bit vector. This must
6337 be a power of two from 1 to 32 (or 64, if your platform supports
6340 If BITS is 8, "elements" coincide with bytes of the input string.
6342 If BITS is 16 or more, bytes of the input string are grouped into chunks
6343 of size BITS/8, and each group is converted to a number as with
6344 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6345 for BITS==64). See L<"pack"> for details.
6347 If bits is 4 or less, the string is broken into bytes, then the bits
6348 of each byte are broken into 8/BITS groups. Bits of a byte are
6349 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6350 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6351 breaking the single input byte C<chr(0x36)> into two groups gives a list
6352 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6354 C<vec> may also be assigned to, in which case parentheses are needed
6355 to give the expression the correct precedence as in
6357 vec($image, $max_x * $x + $y, 8) = 3;
6359 If the selected element is outside the string, the value 0 is returned.
6360 If an element off the end of the string is written to, Perl will first
6361 extend the string with sufficiently many zero bytes. It is an error
6362 to try to write off the beginning of the string (i.e. negative OFFSET).
6364 The string should not contain any character with the value > 255 (which
6365 can only happen if you're using UTF-8 encoding). If it does, it will be
6366 treated as something which is not UTF-8 encoded. When the C<vec> was
6367 assigned to, other parts of your program will also no longer consider the
6368 string to be UTF-8 encoded. In other words, if you do have such characters
6369 in your string, vec() will operate on the actual byte string, and not the
6370 conceptual character string.
6372 Strings created with C<vec> can also be manipulated with the logical
6373 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6374 vector operation is desired when both operands are strings.
6375 See L<perlop/"Bitwise String Operators">.
6377 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6378 The comments show the string after each step. Note that this code works
6379 in the same way on big-endian or little-endian machines.
6382 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6384 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6385 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6387 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6388 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6389 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6390 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6391 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6392 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6394 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6395 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6396 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6399 To transform a bit vector into a string or list of 0's and 1's, use these:
6401 $bits = unpack("b*", $vector);
6402 @bits = split(//, unpack("b*", $vector));
6404 If you know the exact length in bits, it can be used in place of the C<*>.
6406 Here is an example to illustrate how the bits actually fall in place:
6412 unpack("V",$_) 01234567890123456789012345678901
6413 ------------------------------------------------------------------
6418 for ($shift=0; $shift < $width; ++$shift) {
6419 for ($off=0; $off < 32/$width; ++$off) {
6420 $str = pack("B*", "0"x32);
6421 $bits = (1<<$shift);
6422 vec($str, $off, $width) = $bits;
6423 $res = unpack("b*",$str);
6424 $val = unpack("V", $str);
6431 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6432 $off, $width, $bits, $val, $res
6436 Regardless of the machine architecture on which it is run, the above
6437 example should print the following table:
6440 unpack("V",$_) 01234567890123456789012345678901
6441 ------------------------------------------------------------------
6442 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6443 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6444 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6445 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6446 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6447 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6448 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6449 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6450 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6451 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6452 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6453 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6454 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6455 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6456 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6457 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6458 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6459 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6460 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6461 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6462 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6463 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6464 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6465 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6466 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6467 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6468 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6469 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6470 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6471 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6472 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6473 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6474 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6475 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6476 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6477 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6478 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6479 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6480 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6481 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6482 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6483 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6484 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6485 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6486 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6487 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6488 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6489 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6490 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6491 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6492 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6493 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6494 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6495 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6496 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6497 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6498 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6499 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6500 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6501 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6502 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6503 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6504 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6505 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6506 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6507 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6508 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6509 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6510 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6511 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6512 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6513 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6514 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6515 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6516 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6517 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6518 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6519 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6520 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6521 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6522 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6523 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6524 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6525 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6526 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6527 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6528 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6529 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6530 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6531 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6532 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6533 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6534 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6535 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6536 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6537 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6538 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6539 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6540 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6541 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6542 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6543 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6544 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6545 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6546 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6547 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6548 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6549 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6550 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6551 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6552 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6553 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6554 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6555 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6556 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6557 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6558 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6559 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6560 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6561 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6562 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6563 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6564 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6565 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6566 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6567 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6568 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6569 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6573 Behaves like the wait(2) system call on your system: it waits for a child
6574 process to terminate and returns the pid of the deceased process, or
6575 C<-1> if there are no child processes. The status is returned in C<$?>.
6576 Note that a return value of C<-1> could mean that child processes are
6577 being automatically reaped, as described in L<perlipc>.
6579 =item waitpid PID,FLAGS
6581 Waits for a particular child process to terminate and returns the pid of
6582 the deceased process, or C<-1> if there is no such child process. On some
6583 systems, a value of 0 indicates that there are processes still running.
6584 The status is returned in C<$?>. If you say
6586 use POSIX ":sys_wait_h";
6589 $kid = waitpid(-1, WNOHANG);
6592 then you can do a non-blocking wait for all pending zombie processes.
6593 Non-blocking wait is available on machines supporting either the
6594 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6595 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6596 system call by remembering the status values of processes that have
6597 exited but have not been harvested by the Perl script yet.)
6599 Note that on some systems, a return value of C<-1> could mean that child
6600 processes are being automatically reaped. See L<perlipc> for details,
6601 and for other examples.
6605 Returns true if the context of the currently executing subroutine is
6606 looking for a list value. Returns false if the context is looking
6607 for a scalar. Returns the undefined value if the context is looking
6608 for no value (void context).
6610 return unless defined wantarray; # don't bother doing more
6611 my @a = complex_calculation();
6612 return wantarray ? @a : "@a";
6614 This function should have been named wantlist() instead.
6618 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6621 If LIST is empty and C<$@> already contains a value (typically from a
6622 previous eval) that value is used after appending C<"\t...caught">
6623 to C<$@>. This is useful for staying almost, but not entirely similar to
6626 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6628 No message is printed if there is a C<$SIG{__WARN__}> handler
6629 installed. It is the handler's responsibility to deal with the message
6630 as it sees fit (like, for instance, converting it into a C<die>). Most
6631 handlers must therefore make arrangements to actually display the
6632 warnings that they are not prepared to deal with, by calling C<warn>
6633 again in the handler. Note that this is quite safe and will not
6634 produce an endless loop, since C<__WARN__> hooks are not called from
6637 You will find this behavior is slightly different from that of
6638 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6639 instead call C<die> again to change it).
6641 Using a C<__WARN__> handler provides a powerful way to silence all
6642 warnings (even the so-called mandatory ones). An example:
6644 # wipe out *all* compile-time warnings
6645 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6647 my $foo = 20; # no warning about duplicate my $foo,
6648 # but hey, you asked for it!
6649 # no compile-time or run-time warnings before here
6652 # run-time warnings enabled after here
6653 warn "\$foo is alive and $foo!"; # does show up
6655 See L<perlvar> for details on setting C<%SIG> entries, and for more
6656 examples. See the Carp module for other kinds of warnings using its
6657 carp() and cluck() functions.
6659 =item write FILEHANDLE
6665 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6666 using the format associated with that file. By default the format for
6667 a file is the one having the same name as the filehandle, but the
6668 format for the current output channel (see the C<select> function) may be set
6669 explicitly by assigning the name of the format to the C<$~> variable.
6671 Top of form processing is handled automatically: if there is
6672 insufficient room on the current page for the formatted record, the
6673 page is advanced by writing a form feed, a special top-of-page format
6674 is used to format the new page header, and then the record is written.
6675 By default the top-of-page format is the name of the filehandle with
6676 "_TOP" appended, but it may be dynamically set to the format of your
6677 choice by assigning the name to the C<$^> variable while the filehandle is
6678 selected. The number of lines remaining on the current page is in
6679 variable C<$->, which can be set to C<0> to force a new page.
6681 If FILEHANDLE is unspecified, output goes to the current default output
6682 channel, which starts out as STDOUT but may be changed by the
6683 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6684 is evaluated and the resulting string is used to look up the name of
6685 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6687 Note that write is I<not> the opposite of C<read>. Unfortunately.
6691 The transliteration operator. Same as C<tr///>. See L<perlop>.