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 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2234 been declared with C<my $_>) then, in addition the be locally aliased to
2235 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2236 can't be seen from the outside, avoiding any potential side-effects.
2238 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2244 Interprets EXPR as a hex string and returns the corresponding value.
2245 (To convert strings that might start with either 0, 0x, or 0b, see
2246 L</oct>.) If EXPR is omitted, uses C<$_>.
2248 print hex '0xAf'; # prints '175'
2249 print hex 'aF'; # same
2251 Hex strings may only represent integers. Strings that would cause
2252 integer overflow trigger a warning. Leading whitespace is not stripped,
2257 There is no builtin C<import> function. It is just an ordinary
2258 method (subroutine) defined (or inherited) by modules that wish to export
2259 names to another module. The C<use> function calls the C<import> method
2260 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2262 =item index STR,SUBSTR,POSITION
2264 =item index STR,SUBSTR
2266 The index function searches for one string within another, but without
2267 the wildcard-like behavior of a full regular-expression pattern match.
2268 It returns the position of the first occurrence of SUBSTR in STR at
2269 or after POSITION. If POSITION is omitted, starts searching from the
2270 beginning of the string. The return value is based at C<0> (or whatever
2271 you've set the C<$[> variable to--but don't do that). If the substring
2272 is not found, returns one less than the base, ordinarily C<-1>.
2278 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2279 You should not use this function for rounding: one because it truncates
2280 towards C<0>, and two because machine representations of floating point
2281 numbers can sometimes produce counterintuitive results. For example,
2282 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2283 because it's really more like -268.99999999999994315658 instead. Usually,
2284 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2285 functions will serve you better than will int().
2287 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2289 Implements the ioctl(2) function. You'll probably first have to say
2291 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2293 to get the correct function definitions. If F<ioctl.ph> doesn't
2294 exist or doesn't have the correct definitions you'll have to roll your
2295 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2296 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2297 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2298 written depending on the FUNCTION--a pointer to the string value of SCALAR
2299 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2300 has no string value but does have a numeric value, that value will be
2301 passed rather than a pointer to the string value. To guarantee this to be
2302 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2303 functions may be needed to manipulate the values of structures used by
2306 The return value of C<ioctl> (and C<fcntl>) is as follows:
2308 if OS returns: then Perl returns:
2310 0 string "0 but true"
2311 anything else that number
2313 Thus Perl returns true on success and false on failure, yet you can
2314 still easily determine the actual value returned by the operating
2317 $retval = ioctl(...) || -1;
2318 printf "System returned %d\n", $retval;
2320 The special string C<"0 but true"> is exempt from B<-w> complaints
2321 about improper numeric conversions.
2323 =item join EXPR,LIST
2325 Joins the separate strings of LIST into a single string with fields
2326 separated by the value of EXPR, and returns that new string. Example:
2328 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2330 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2331 first argument. Compare L</split>.
2335 Returns a list consisting of all the keys of the named hash.
2336 (In scalar context, returns the number of keys.)
2338 The keys are returned in an apparently random order. The actual
2339 random order is subject to change in future versions of perl, but it
2340 is guaranteed to be the same order as either the C<values> or C<each>
2341 function produces (given that the hash has not been modified). Since
2342 Perl 5.8.1 the ordering is different even between different runs of
2343 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2346 As a side effect, calling keys() resets the HASH's internal iterator,
2347 see L</each>. (In particular, calling keys() in void context resets
2348 the iterator with no other overhead.)
2350 Here is yet another way to print your environment:
2353 @values = values %ENV;
2355 print pop(@keys), '=', pop(@values), "\n";
2358 or how about sorted by key:
2360 foreach $key (sort(keys %ENV)) {
2361 print $key, '=', $ENV{$key}, "\n";
2364 The returned values are copies of the original keys in the hash, so
2365 modifying them will not affect the original hash. Compare L</values>.
2367 To sort a hash by value, you'll need to use a C<sort> function.
2368 Here's a descending numeric sort of a hash by its values:
2370 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2371 printf "%4d %s\n", $hash{$key}, $key;
2374 As an lvalue C<keys> allows you to increase the number of hash buckets
2375 allocated for the given hash. This can gain you a measure of efficiency if
2376 you know the hash is going to get big. (This is similar to pre-extending
2377 an array by assigning a larger number to $#array.) If you say
2381 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2382 in fact, since it rounds up to the next power of two. These
2383 buckets will be retained even if you do C<%hash = ()>, use C<undef
2384 %hash> if you want to free the storage while C<%hash> is still in scope.
2385 You can't shrink the number of buckets allocated for the hash using
2386 C<keys> in this way (but you needn't worry about doing this by accident,
2387 as trying has no effect).
2389 See also C<each>, C<values> and C<sort>.
2391 =item kill SIGNAL, LIST
2393 Sends a signal to a list of processes. Returns the number of
2394 processes successfully signaled (which is not necessarily the
2395 same as the number actually killed).
2397 $cnt = kill 1, $child1, $child2;
2400 If SIGNAL is zero, no signal is sent to the process. This is a
2401 useful way to check that a child process is alive and hasn't changed
2402 its UID. See L<perlport> for notes on the portability of this
2405 Unlike in the shell, if SIGNAL is negative, it kills
2406 process groups instead of processes. (On System V, a negative I<PROCESS>
2407 number will also kill process groups, but that's not portable.) That
2408 means you usually want to use positive not negative signals. You may also
2409 use a signal name in quotes.
2411 See L<perlipc/"Signals"> for more details.
2417 The C<last> command is like the C<break> statement in C (as used in
2418 loops); it immediately exits the loop in question. If the LABEL is
2419 omitted, the command refers to the innermost enclosing loop. The
2420 C<continue> block, if any, is not executed:
2422 LINE: while (<STDIN>) {
2423 last LINE if /^$/; # exit when done with header
2427 C<last> cannot be used to exit a block which returns a value such as
2428 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2429 a grep() or map() operation.
2431 Note that a block by itself is semantically identical to a loop
2432 that executes once. Thus C<last> can be used to effect an early
2433 exit out of such a block.
2435 See also L</continue> for an illustration of how C<last>, C<next>, and
2442 Returns a lowercased version of EXPR. This is the internal function
2443 implementing the C<\L> escape in double-quoted strings. Respects
2444 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2445 and L<perlunicode> for more details about locale and Unicode support.
2447 If EXPR is omitted, uses C<$_>.
2453 Returns the value of EXPR with the first character lowercased. This
2454 is the internal function implementing the C<\l> escape in
2455 double-quoted strings. Respects current LC_CTYPE locale if C<use
2456 locale> in force. See L<perllocale> and L<perlunicode> for more
2457 details about locale and Unicode support.
2459 If EXPR is omitted, uses C<$_>.
2465 Returns the length in I<characters> of the value of EXPR. If EXPR is
2466 omitted, returns length of C<$_>. Note that this cannot be used on
2467 an entire array or hash to find out how many elements these have.
2468 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2470 Note the I<characters>: if the EXPR is in Unicode, you will get the
2471 number of characters, not the number of bytes. To get the length
2472 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2474 =item link OLDFILE,NEWFILE
2476 Creates a new filename linked to the old filename. Returns true for
2477 success, false otherwise.
2479 =item listen SOCKET,QUEUESIZE
2481 Does the same thing that the listen system call does. Returns true if
2482 it succeeded, false otherwise. See the example in
2483 L<perlipc/"Sockets: Client/Server Communication">.
2487 You really probably want to be using C<my> instead, because C<local> isn't
2488 what most people think of as "local". See
2489 L<perlsub/"Private Variables via my()"> for details.
2491 A local modifies the listed variables to be local to the enclosing
2492 block, file, or eval. If more than one value is listed, the list must
2493 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2494 for details, including issues with tied arrays and hashes.
2496 =item localtime EXPR
2498 Converts a time as returned by the time function to a 9-element list
2499 with the time analyzed for the local time zone. Typically used as
2503 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2506 All list elements are numeric, and come straight out of the C `struct
2507 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2508 specified time. $mday is the day of the month, and $mon is the month
2509 itself, in the range C<0..11> with 0 indicating January and 11
2510 indicating December. $year is the number of years since 1900. That
2511 is, $year is C<123> in year 2023. $wday is the day of the week, with
2512 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2513 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2514 is true if the specified time occurs during daylight savings time,
2517 Note that the $year element is I<not> simply the last two digits of
2518 the year. If you assume it is, then you create non-Y2K-compliant
2519 programs--and you wouldn't want to do that, would you?
2521 The proper way to get a complete 4-digit year is simply:
2525 And to get the last two digits of the year (e.g., '01' in 2001) do:
2527 $year = sprintf("%02d", $year % 100);
2529 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2531 In scalar context, C<localtime()> returns the ctime(3) value:
2533 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2535 This scalar value is B<not> locale dependent, see L<perllocale>, but
2536 instead a Perl builtin. Also see the C<Time::Local> module
2537 (to convert the second, minutes, hours, ... back to seconds since the
2538 stroke of midnight the 1st of January 1970, the value returned by
2539 time()), and the strftime(3) and mktime(3) functions available via the
2540 POSIX module. To get somewhat similar but locale dependent date
2541 strings, set up your locale environment variables appropriately
2542 (please see L<perllocale>) and try for example:
2544 use POSIX qw(strftime);
2545 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2547 Note that the C<%a> and C<%b>, the short forms of the day of the week
2548 and the month of the year, may not necessarily be three characters wide.
2552 This function places an advisory lock on a shared variable, or referenced
2553 object contained in I<THING> until the lock goes out of scope.
2555 lock() is a "weak keyword" : this means that if you've defined a function
2556 by this name (before any calls to it), that function will be called
2557 instead. (However, if you've said C<use threads>, lock() is always a
2558 keyword.) See L<threads>.
2564 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2565 returns log of C<$_>. To get the log of another base, use basic algebra:
2566 The base-N log of a number is equal to the natural log of that number
2567 divided by the natural log of N. For example:
2571 return log($n)/log(10);
2574 See also L</exp> for the inverse operation.
2580 Does the same thing as the C<stat> function (including setting the
2581 special C<_> filehandle) but stats a symbolic link instead of the file
2582 the symbolic link points to. If symbolic links are unimplemented on
2583 your system, a normal C<stat> is done. For much more detailed
2584 information, please see the documentation for C<stat>.
2586 If EXPR is omitted, stats C<$_>.
2590 The match operator. See L<perlop>.
2592 =item map BLOCK LIST
2596 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2597 C<$_> to each element) and returns the list value composed of the
2598 results of each such evaluation. In scalar context, returns the
2599 total number of elements so generated. Evaluates BLOCK or EXPR in
2600 list context, so each element of LIST may produce zero, one, or
2601 more elements in the returned value.
2603 @chars = map(chr, @nums);
2605 translates a list of numbers to the corresponding characters. And
2607 %hash = map { getkey($_) => $_ } @array;
2609 is just a funny way to write
2612 foreach $_ (@array) {
2613 $hash{getkey($_)} = $_;
2616 Note that C<$_> is an alias to the list value, so it can be used to
2617 modify the elements of the LIST. While this is useful and supported,
2618 it can cause bizarre results if the elements of LIST are not variables.
2619 Using a regular C<foreach> loop for this purpose would be clearer in
2620 most cases. See also L</grep> for an array composed of those items of
2621 the original list for which the BLOCK or EXPR evaluates to true.
2623 If C<$_> is lexical in the scope where the C<map> appears (because it has
2624 been declared with C<my $_>) then, in addition the be locally aliased to
2625 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2626 can't be seen from the outside, avoiding any potential side-effects.
2628 C<{> starts both hash references and blocks, so C<map { ...> could be either
2629 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2630 ahead for the closing C<}> it has to take a guess at which its dealing with
2631 based what it finds just after the C<{>. Usually it gets it right, but if it
2632 doesn't it won't realize something is wrong until it gets to the C<}> and
2633 encounters the missing (or unexpected) comma. The syntax error will be
2634 reported close to the C<}> but you'll need to change something near the C<{>
2635 such as using a unary C<+> to give perl some help:
2637 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2638 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2639 %hash = map { ("\L$_", 1) } @array # this also works
2640 %hash = map { lc($_), 1 } @array # as does this.
2641 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2643 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2645 or to force an anon hash constructor use C<+{>
2647 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2649 and you get list of anonymous hashes each with only 1 entry.
2651 =item mkdir FILENAME,MASK
2653 =item mkdir FILENAME
2655 Creates the directory specified by FILENAME, with permissions
2656 specified by MASK (as modified by C<umask>). If it succeeds it
2657 returns true, otherwise it returns false and sets C<$!> (errno).
2658 If omitted, MASK defaults to 0777.
2660 In general, it is better to create directories with permissive MASK,
2661 and let the user modify that with their C<umask>, than it is to supply
2662 a restrictive MASK and give the user no way to be more permissive.
2663 The exceptions to this rule are when the file or directory should be
2664 kept private (mail files, for instance). The perlfunc(1) entry on
2665 C<umask> discusses the choice of MASK in more detail.
2667 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2668 number of trailing slashes. Some operating and filesystems do not get
2669 this right, so Perl automatically removes all trailing slashes to keep
2672 =item msgctl ID,CMD,ARG
2674 Calls the System V IPC function msgctl(2). You'll probably have to say
2678 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2679 then ARG must be a variable which will hold the returned C<msqid_ds>
2680 structure. Returns like C<ioctl>: the undefined value for error,
2681 C<"0 but true"> for zero, or the actual return value otherwise. See also
2682 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2684 =item msgget KEY,FLAGS
2686 Calls the System V IPC function msgget(2). Returns the message queue
2687 id, or the undefined value if there is an error. See also
2688 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2690 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2692 Calls the System V IPC function msgrcv to receive a message from
2693 message queue ID into variable VAR with a maximum message size of
2694 SIZE. Note that when a message is received, the message type as a
2695 native long integer will be the first thing in VAR, followed by the
2696 actual message. This packing may be opened with C<unpack("l! a*")>.
2697 Taints the variable. Returns true if successful, or false if there is
2698 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2699 C<IPC::SysV::Msg> documentation.
2701 =item msgsnd ID,MSG,FLAGS
2703 Calls the System V IPC function msgsnd to send the message MSG to the
2704 message queue ID. MSG must begin with the native long integer message
2705 type, and be followed by the length of the actual message, and finally
2706 the message itself. This kind of packing can be achieved with
2707 C<pack("l! a*", $type, $message)>. Returns true if successful,
2708 or false if there is an error. See also C<IPC::SysV>
2709 and C<IPC::SysV::Msg> documentation.
2715 =item my EXPR : ATTRS
2717 =item my TYPE EXPR : ATTRS
2719 A C<my> declares the listed variables to be local (lexically) to the
2720 enclosing block, file, or C<eval>. If more than one value is listed,
2721 the list must be placed in parentheses.
2723 The exact semantics and interface of TYPE and ATTRS are still
2724 evolving. TYPE is currently bound to the use of C<fields> pragma,
2725 and attributes are handled using the C<attributes> pragma, or starting
2726 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2727 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2728 L<attributes>, and L<Attribute::Handlers>.
2734 The C<next> command is like the C<continue> statement in C; it starts
2735 the next iteration of the loop:
2737 LINE: while (<STDIN>) {
2738 next LINE if /^#/; # discard comments
2742 Note that if there were a C<continue> block on the above, it would get
2743 executed even on discarded lines. If the LABEL is omitted, the command
2744 refers to the innermost enclosing loop.
2746 C<next> cannot be used to exit a block which returns a value such as
2747 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2748 a grep() or map() operation.
2750 Note that a block by itself is semantically identical to a loop
2751 that executes once. Thus C<next> will exit such a block early.
2753 See also L</continue> for an illustration of how C<last>, C<next>, and
2756 =item no Module VERSION LIST
2758 =item no Module VERSION
2760 =item no Module LIST
2764 See the C<use> function, of which C<no> is the opposite.
2770 Interprets EXPR as an octal string and returns the corresponding
2771 value. (If EXPR happens to start off with C<0x>, interprets it as a
2772 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2773 binary string. Leading whitespace is ignored in all three cases.)
2774 The following will handle decimal, binary, octal, and hex in the standard
2777 $val = oct($val) if $val =~ /^0/;
2779 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2780 in octal), use sprintf() or printf():
2782 $perms = (stat("filename"))[2] & 07777;
2783 $oct_perms = sprintf "%lo", $perms;
2785 The oct() function is commonly used when a string such as C<644> needs
2786 to be converted into a file mode, for example. (Although perl will
2787 automatically convert strings into numbers as needed, this automatic
2788 conversion assumes base 10.)
2790 =item open FILEHANDLE,EXPR
2792 =item open FILEHANDLE,MODE,EXPR
2794 =item open FILEHANDLE,MODE,EXPR,LIST
2796 =item open FILEHANDLE,MODE,REFERENCE
2798 =item open FILEHANDLE
2800 Opens the file whose filename is given by EXPR, and associates it with
2803 (The following is a comprehensive reference to open(): for a gentler
2804 introduction you may consider L<perlopentut>.)
2806 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2807 the variable is assigned a reference to a new anonymous filehandle,
2808 otherwise if FILEHANDLE is an expression, its value is used as the name of
2809 the real filehandle wanted. (This is considered a symbolic reference, so
2810 C<use strict 'refs'> should I<not> be in effect.)
2812 If EXPR is omitted, the scalar variable of the same name as the
2813 FILEHANDLE contains the filename. (Note that lexical variables--those
2814 declared with C<my>--will not work for this purpose; so if you're
2815 using C<my>, specify EXPR in your call to open.)
2817 If three or more arguments are specified then the mode of opening and
2818 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2819 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2820 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2821 the file is opened for appending, again being created if necessary.
2823 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2824 indicate that you want both read and write access to the file; thus
2825 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2826 '+>' >> mode would clobber the file first. You can't usually use
2827 either read-write mode for updating textfiles, since they have
2828 variable length records. See the B<-i> switch in L<perlrun> for a
2829 better approach. The file is created with permissions of C<0666>
2830 modified by the process' C<umask> value.
2832 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2833 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2835 In the 2-arguments (and 1-argument) form of the call the mode and
2836 filename should be concatenated (in this order), possibly separated by
2837 spaces. It is possible to omit the mode in these forms if the mode is
2840 If the filename begins with C<'|'>, the filename is interpreted as a
2841 command to which output is to be piped, and if the filename ends with a
2842 C<'|'>, the filename is interpreted as a command which pipes output to
2843 us. See L<perlipc/"Using open() for IPC">
2844 for more examples of this. (You are not allowed to C<open> to a command
2845 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2846 and L<perlipc/"Bidirectional Communication with Another Process">
2849 For three or more arguments if MODE is C<'|-'>, the filename is
2850 interpreted as a command to which output is to be piped, and if MODE
2851 is C<'-|'>, the filename is interpreted as a command which pipes
2852 output to us. In the 2-arguments (and 1-argument) form one should
2853 replace dash (C<'-'>) with the command.
2854 See L<perlipc/"Using open() for IPC"> for more examples of this.
2855 (You are not allowed to C<open> to a command that pipes both in I<and>
2856 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2857 L<perlipc/"Bidirectional Communication"> for alternatives.)
2859 In the three-or-more argument form of pipe opens, if LIST is specified
2860 (extra arguments after the command name) then LIST becomes arguments
2861 to the command invoked if the platform supports it. The meaning of
2862 C<open> with more than three arguments for non-pipe modes is not yet
2863 specified. Experimental "layers" may give extra LIST arguments
2866 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2867 and opening C<< '>-' >> opens STDOUT.
2869 You may use the three-argument form of open to specify IO "layers"
2870 (sometimes also referred to as "disciplines") to be applied to the handle
2871 that affect how the input and output are processed (see L<open> and
2872 L<PerlIO> for more details). For example
2874 open(FH, "<:utf8", "file")
2876 will open the UTF-8 encoded file containing Unicode characters,
2877 see L<perluniintro>. (Note that if layers are specified in the
2878 three-arg form then default layers set by the C<open> pragma are
2881 Open returns nonzero upon success, the undefined value otherwise. If
2882 the C<open> involved a pipe, the return value happens to be the pid of
2885 If you're running Perl on a system that distinguishes between text
2886 files and binary files, then you should check out L</binmode> for tips
2887 for dealing with this. The key distinction between systems that need
2888 C<binmode> and those that don't is their text file formats. Systems
2889 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2890 character, and which encode that character in C as C<"\n">, do not
2891 need C<binmode>. The rest need it.
2893 When opening a file, it's usually a bad idea to continue normal execution
2894 if the request failed, so C<open> is frequently used in connection with
2895 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2896 where you want to make a nicely formatted error message (but there are
2897 modules that can help with that problem)) you should always check
2898 the return value from opening a file. The infrequent exception is when
2899 working with an unopened filehandle is actually what you want to do.
2901 As a special case the 3 arg form with a read/write mode and the third
2902 argument being C<undef>:
2904 open(TMP, "+>", undef) or die ...
2906 opens a filehandle to an anonymous temporary file. Also using "+<"
2907 works for symmetry, but you really should consider writing something
2908 to the temporary file first. You will need to seek() to do the
2911 File handles can be opened to "in memory" files held in Perl scalars via:
2913 open($fh, '>', \$variable) || ..
2915 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2916 file, you have to close it first:
2919 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2924 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2925 while (<ARTICLE>) {...
2927 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2928 # if the open fails, output is discarded
2930 open(DBASE, '+<', 'dbase.mine') # open for update
2931 or die "Can't open 'dbase.mine' for update: $!";
2933 open(DBASE, '+<dbase.mine') # ditto
2934 or die "Can't open 'dbase.mine' for update: $!";
2936 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2937 or die "Can't start caesar: $!";
2939 open(ARTICLE, "caesar <$article |") # ditto
2940 or die "Can't start caesar: $!";
2942 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
2943 or die "Can't start sort: $!";
2946 open(MEMORY,'>', \$var)
2947 or die "Can't open memory file: $!";
2948 print MEMORY "foo!\n"; # output will end up in $var
2950 # process argument list of files along with any includes
2952 foreach $file (@ARGV) {
2953 process($file, 'fh00');
2957 my($filename, $input) = @_;
2958 $input++; # this is a string increment
2959 unless (open($input, $filename)) {
2960 print STDERR "Can't open $filename: $!\n";
2965 while (<$input>) { # note use of indirection
2966 if (/^#include "(.*)"/) {
2967 process($1, $input);
2974 You may also, in the Bourne shell tradition, specify an EXPR beginning
2975 with C<< '>&' >>, in which case the rest of the string is interpreted
2976 as the name of a filehandle (or file descriptor, if numeric) to be
2977 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2978 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2979 The mode you specify should match the mode of the original filehandle.
2980 (Duping a filehandle does not take into account any existing contents
2981 of IO buffers.) If you use the 3 arg form then you can pass either a
2982 number, the name of a filehandle or the normal "reference to a glob".
2984 Here is a script that saves, redirects, and restores C<STDOUT> and
2985 C<STDERR> using various methods:
2988 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2989 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2991 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2992 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2994 select STDERR; $| = 1; # make unbuffered
2995 select STDOUT; $| = 1; # make unbuffered
2997 print STDOUT "stdout 1\n"; # this works for
2998 print STDERR "stderr 1\n"; # subprocesses too
3000 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3001 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3003 print STDOUT "stdout 2\n";
3004 print STDERR "stderr 2\n";
3006 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3007 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3008 that file descriptor (and not call L<dup(2)>); this is more
3009 parsimonious of file descriptors. For example:
3011 # open for input, reusing the fileno of $fd
3012 open(FILEHANDLE, "<&=$fd")
3016 open(FILEHANDLE, "<&=", $fd)
3020 # open for append, using the fileno of OLDFH
3021 open(FH, ">>&=", OLDFH)
3025 open(FH, ">>&=OLDFH")
3027 Being parsimonious on filehandles is also useful (besides being
3028 parsimonious) for example when something is dependent on file
3029 descriptors, like for example locking using flock(). If you do just
3030 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3031 descriptor as B, and therefore flock(A) will not flock(B), and vice
3032 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3033 the same file descriptor.
3035 Note that if you are using Perls older than 5.8.0, Perl will be using
3036 the standard C libraries' fdopen() to implement the "=" functionality.
3037 On many UNIX systems fdopen() fails when file descriptors exceed a
3038 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3039 most often the default.
3041 You can see whether Perl has been compiled with PerlIO or not by
3042 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3043 is C<define>, you have PerlIO, otherwise you don't.
3045 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3046 with 2-arguments (or 1-argument) form of open(), then
3047 there is an implicit fork done, and the return value of open is the pid
3048 of the child within the parent process, and C<0> within the child
3049 process. (Use C<defined($pid)> to determine whether the open was successful.)
3050 The filehandle behaves normally for the parent, but i/o to that
3051 filehandle is piped from/to the STDOUT/STDIN of the child process.
3052 In the child process the filehandle isn't opened--i/o happens from/to
3053 the new STDOUT or STDIN. Typically this is used like the normal
3054 piped open when you want to exercise more control over just how the
3055 pipe command gets executed, such as when you are running setuid, and
3056 don't want to have to scan shell commands for metacharacters.
3057 The following triples are more or less equivalent:
3059 open(FOO, "|tr '[a-z]' '[A-Z]'");
3060 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3061 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3062 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3064 open(FOO, "cat -n '$file'|");
3065 open(FOO, '-|', "cat -n '$file'");
3066 open(FOO, '-|') || exec 'cat', '-n', $file;
3067 open(FOO, '-|', "cat", '-n', $file);
3069 The last example in each block shows the pipe as "list form", which is
3070 not yet supported on all platforms. A good rule of thumb is that if
3071 your platform has true C<fork()> (in other words, if your platform is
3072 UNIX) you can use the list form.
3074 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3076 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3077 output before any operation that may do a fork, but this may not be
3078 supported on some platforms (see L<perlport>). To be safe, you may need
3079 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3080 of C<IO::Handle> on any open handles.
3082 On systems that support a close-on-exec flag on files, the flag will
3083 be set for the newly opened file descriptor as determined by the value
3084 of $^F. See L<perlvar/$^F>.
3086 Closing any piped filehandle causes the parent process to wait for the
3087 child to finish, and returns the status value in C<$?>.
3089 The filename passed to 2-argument (or 1-argument) form of open() will
3090 have leading and trailing whitespace deleted, and the normal
3091 redirection characters honored. This property, known as "magic open",
3092 can often be used to good effect. A user could specify a filename of
3093 F<"rsh cat file |">, or you could change certain filenames as needed:
3095 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3096 open(FH, $filename) or die "Can't open $filename: $!";
3098 Use 3-argument form to open a file with arbitrary weird characters in it,
3100 open(FOO, '<', $file);
3102 otherwise it's necessary to protect any leading and trailing whitespace:
3104 $file =~ s#^(\s)#./$1#;
3105 open(FOO, "< $file\0");
3107 (this may not work on some bizarre filesystems). One should
3108 conscientiously choose between the I<magic> and 3-arguments form
3113 will allow the user to specify an argument of the form C<"rsh cat file |">,
3114 but will not work on a filename which happens to have a trailing space, while
3116 open IN, '<', $ARGV[0];
3118 will have exactly the opposite restrictions.
3120 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3121 should use the C<sysopen> function, which involves no such magic (but
3122 may use subtly different filemodes than Perl open(), which is mapped
3123 to C fopen()). This is
3124 another way to protect your filenames from interpretation. For example:
3127 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3128 or die "sysopen $path: $!";
3129 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3130 print HANDLE "stuff $$\n";
3132 print "File contains: ", <HANDLE>;
3134 Using the constructor from the C<IO::Handle> package (or one of its
3135 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3136 filehandles that have the scope of whatever variables hold references to
3137 them, and automatically close whenever and however you leave that scope:
3141 sub read_myfile_munged {
3143 my $handle = new IO::File;
3144 open($handle, "myfile") or die "myfile: $!";
3146 or return (); # Automatically closed here.
3147 mung $first or die "mung failed"; # Or here.
3148 return $first, <$handle> if $ALL; # Or here.
3152 See L</seek> for some details about mixing reading and writing.
3154 =item opendir DIRHANDLE,EXPR
3156 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3157 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3158 DIRHANDLE may be an expression whose value can be used as an indirect
3159 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3160 scalar variable (or array or hash element), the variable is assigned a
3161 reference to a new anonymous dirhandle.
3162 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3168 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3169 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3172 For the reverse, see L</chr>.
3173 See L<perlunicode> and L<encoding> for more about Unicode.
3179 =item our EXPR : ATTRS
3181 =item our TYPE EXPR : ATTRS
3183 An C<our> declares the listed variables to be valid globals within
3184 the enclosing block, file, or C<eval>. That is, it has the same
3185 scoping rules as a "my" declaration, but does not create a local
3186 variable. If more than one value is listed, the list must be placed
3187 in parentheses. The C<our> declaration has no semantic effect unless
3188 "use strict vars" is in effect, in which case it lets you use the
3189 declared global variable without qualifying it with a package name.
3190 (But only within the lexical scope of the C<our> declaration. In this
3191 it differs from "use vars", which is package scoped.)
3193 An C<our> declaration declares a global variable that will be visible
3194 across its entire lexical scope, even across package boundaries. The
3195 package in which the variable is entered is determined at the point
3196 of the declaration, not at the point of use. This means the following
3200 our $bar; # declares $Foo::bar for rest of lexical scope
3204 print $bar; # prints 20
3206 Multiple C<our> declarations in the same lexical scope are allowed
3207 if they are in different packages. If they happened to be in the same
3208 package, Perl will emit warnings if you have asked for them.
3212 our $bar; # declares $Foo::bar for rest of lexical scope
3216 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3217 print $bar; # prints 30
3219 our $bar; # emits warning
3221 An C<our> declaration may also have a list of attributes associated
3224 The exact semantics and interface of TYPE and ATTRS are still
3225 evolving. TYPE is currently bound to the use of C<fields> pragma,
3226 and attributes are handled using the C<attributes> pragma, or starting
3227 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3228 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3229 L<attributes>, and L<Attribute::Handlers>.
3231 The only currently recognized C<our()> attribute is C<unique> which
3232 indicates that a single copy of the global is to be used by all
3233 interpreters should the program happen to be running in a
3234 multi-interpreter environment. (The default behaviour would be for
3235 each interpreter to have its own copy of the global.) Examples:
3237 our @EXPORT : unique = qw(foo);
3238 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3239 our $VERSION : unique = "1.00";
3241 Note that this attribute also has the effect of making the global
3242 readonly when the first new interpreter is cloned (for example,
3243 when the first new thread is created).
3245 Multi-interpreter environments can come to being either through the
3246 fork() emulation on Windows platforms, or by embedding perl in a
3247 multi-threaded application. The C<unique> attribute does nothing in
3248 all other environments.
3250 Warning: the current implementation of this attribute operates on the
3251 typeglob associated with the variable; this means that C<our $x : unique>
3252 also has the effect of C<our @x : unique; our %x : unique>. This may be
3255 =item pack TEMPLATE,LIST
3257 Takes a LIST of values and converts it into a string using the rules
3258 given by the TEMPLATE. The resulting string is the concatenation of
3259 the converted values. Typically, each converted value looks
3260 like its machine-level representation. For example, on 32-bit machines
3261 a converted integer may be represented by a sequence of 4 bytes.
3263 The TEMPLATE is a sequence of characters that give the order and type
3264 of values, as follows:
3266 a A string with arbitrary binary data, will be null padded.
3267 A A text (ASCII) string, will be space padded.
3268 Z A null terminated (ASCIZ) string, will be null padded.
3270 b A bit string (ascending bit order inside each byte, like vec()).
3271 B A bit string (descending bit order inside each byte).
3272 h A hex string (low nybble first).
3273 H A hex string (high nybble first).
3275 c A signed char value.
3276 C An unsigned char value. Only does bytes. See U for Unicode.
3278 s A signed short value.
3279 S An unsigned short value.
3280 (This 'short' is _exactly_ 16 bits, which may differ from
3281 what a local C compiler calls 'short'. If you want
3282 native-length shorts, use the '!' suffix.)
3284 i A signed integer value.
3285 I An unsigned integer value.
3286 (This 'integer' is _at_least_ 32 bits wide. Its exact
3287 size depends on what a local C compiler calls 'int',
3288 and may even be larger than the 'long' described in
3291 l A signed long value.
3292 L An unsigned long value.
3293 (This 'long' is _exactly_ 32 bits, which may differ from
3294 what a local C compiler calls 'long'. If you want
3295 native-length longs, use the '!' suffix.)
3297 n An unsigned short in "network" (big-endian) order.
3298 N An unsigned long in "network" (big-endian) order.
3299 v An unsigned short in "VAX" (little-endian) order.
3300 V An unsigned long in "VAX" (little-endian) order.
3301 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3302 _exactly_ 32 bits, respectively.)
3304 q A signed quad (64-bit) value.
3305 Q An unsigned quad value.
3306 (Quads are available only if your system supports 64-bit
3307 integer values _and_ if Perl has been compiled to support those.
3308 Causes a fatal error otherwise.)
3310 j A signed integer value (a Perl internal integer, IV).
3311 J An unsigned integer value (a Perl internal unsigned integer, UV).
3313 f A single-precision float in the native format.
3314 d A double-precision float in the native format.
3316 F A floating point value in the native native format
3317 (a Perl internal floating point value, NV).
3318 D A long double-precision float in the native format.
3319 (Long doubles are available only if your system supports long
3320 double values _and_ if Perl has been compiled to support those.
3321 Causes a fatal error otherwise.)
3323 p A pointer to a null-terminated string.
3324 P A pointer to a structure (fixed-length string).
3326 u A uuencoded string.
3327 U A Unicode character number. Encodes to UTF-8 internally
3328 (or UTF-EBCDIC in EBCDIC platforms).
3330 w A BER compressed integer. Its bytes represent an unsigned
3331 integer in base 128, most significant digit first, with as
3332 few digits as possible. Bit eight (the high bit) is set
3333 on each byte except the last.
3337 @ Null fill to absolute position, counted from the start of
3338 the innermost ()-group.
3339 ( Start of a ()-group.
3341 The following rules apply:
3347 Each letter may optionally be followed by a number giving a repeat
3348 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3349 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3350 many values from the LIST. A C<*> for the repeat count means to use
3351 however many items are left, except for C<@>, C<x>, C<X>, where it is
3352 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3353 is the same). A numeric repeat count may optionally be enclosed in
3354 brackets, as in C<pack 'C[80]', @arr>.
3356 One can replace the numeric repeat count by a template enclosed in brackets;
3357 then the packed length of this template in bytes is used as a count.
3358 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3359 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3360 If the template in brackets contains alignment commands (such as C<x![d]>),
3361 its packed length is calculated as if the start of the template has the maximal
3364 When used with C<Z>, C<*> results in the addition of a trailing null
3365 byte (so the packed result will be one longer than the byte C<length>
3368 The repeat count for C<u> is interpreted as the maximal number of bytes
3369 to encode per line of output, with 0 and 1 replaced by 45.
3373 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3374 string of length count, padding with nulls or spaces as necessary. When
3375 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3376 after the first null, and C<a> returns data verbatim. When packing,
3377 C<a>, and C<Z> are equivalent.
3379 If the value-to-pack is too long, it is truncated. If too long and an
3380 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3381 by a null byte. Thus C<Z> always packs a trailing null byte under
3386 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3387 Each byte of the input field of pack() generates 1 bit of the result.
3388 Each result bit is based on the least-significant bit of the corresponding
3389 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3390 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3392 Starting from the beginning of the input string of pack(), each 8-tuple
3393 of bytes is converted to 1 byte of output. With format C<b>
3394 the first byte of the 8-tuple determines the least-significant bit of a
3395 byte, and with format C<B> it determines the most-significant bit of
3398 If the length of the input string is not exactly divisible by 8, the
3399 remainder is packed as if the input string were padded by null bytes
3400 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3402 If the input string of pack() is longer than needed, extra bytes are ignored.
3403 A C<*> for the repeat count of pack() means to use all the bytes of
3404 the input field. On unpack()ing the bits are converted to a string
3405 of C<"0">s and C<"1">s.
3409 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3410 representable as hexadecimal digits, 0-9a-f) long.
3412 Each byte of the input field of pack() generates 4 bits of the result.
3413 For non-alphabetical bytes the result is based on the 4 least-significant
3414 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3415 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3416 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3417 is compatible with the usual hexadecimal digits, so that C<"a"> and
3418 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3419 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3421 Starting from the beginning of the input string of pack(), each pair
3422 of bytes is converted to 1 byte of output. With format C<h> the
3423 first byte of the pair determines the least-significant nybble of the
3424 output byte, and with format C<H> it determines the most-significant
3427 If the length of the input string is not even, it behaves as if padded
3428 by a null byte at the end. Similarly, during unpack()ing the "extra"
3429 nybbles are ignored.
3431 If the input string of pack() is longer than needed, extra bytes are ignored.
3432 A C<*> for the repeat count of pack() means to use all the bytes of
3433 the input field. On unpack()ing the bits are converted to a string
3434 of hexadecimal digits.
3438 The C<p> type packs a pointer to a null-terminated string. You are
3439 responsible for ensuring the string is not a temporary value (which can
3440 potentially get deallocated before you get around to using the packed result).
3441 The C<P> type packs a pointer to a structure of the size indicated by the
3442 length. A NULL pointer is created if the corresponding value for C<p> or
3443 C<P> is C<undef>, similarly for unpack().
3447 The C</> template character allows packing and unpacking of strings where
3448 the packed structure contains a byte count followed by the string itself.
3449 You write I<length-item>C</>I<string-item>.
3451 The I<length-item> can be any C<pack> template letter, and describes
3452 how the length value is packed. The ones likely to be of most use are
3453 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3454 SNMP) and C<N> (for Sun XDR).
3456 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3457 C<"Z*">. For C<unpack> the length of the string is obtained from the
3458 I<length-item>, but if you put in the '*' it will be ignored. For all other
3459 codes, C<unpack> applies the length value to the next item, which must not
3460 have a repeat count.
3462 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3463 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3464 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3466 The I<length-item> is not returned explicitly from C<unpack>.
3468 Adding a count to the I<length-item> letter is unlikely to do anything
3469 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3470 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3471 which Perl does not regard as legal in numeric strings.
3475 The integer types C<s>, C<S>, C<l>, and C<L> may be
3476 immediately followed by a C<!> suffix to signify native shorts or
3477 longs--as you can see from above for example a bare C<l> does mean
3478 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3479 may be larger. This is an issue mainly in 64-bit platforms. You can
3480 see whether using C<!> makes any difference by
3482 print length(pack("s")), " ", length(pack("s!")), "\n";
3483 print length(pack("l")), " ", length(pack("l!")), "\n";
3485 C<i!> and C<I!> also work but only because of completeness;
3486 they are identical to C<i> and C<I>.
3488 The actual sizes (in bytes) of native shorts, ints, longs, and long
3489 longs on the platform where Perl was built are also available via
3493 print $Config{shortsize}, "\n";
3494 print $Config{intsize}, "\n";
3495 print $Config{longsize}, "\n";
3496 print $Config{longlongsize}, "\n";
3498 (The C<$Config{longlongsize}> will be undefined if your system does
3499 not support long longs.)
3503 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3504 are inherently non-portable between processors and operating systems
3505 because they obey the native byteorder and endianness. For example a
3506 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3507 (arranged in and handled by the CPU registers) into bytes as
3509 0x12 0x34 0x56 0x78 # big-endian
3510 0x78 0x56 0x34 0x12 # little-endian
3512 Basically, the Intel and VAX CPUs are little-endian, while everybody
3513 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3514 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3515 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3518 The names `big-endian' and `little-endian' are comic references to
3519 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3520 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3521 the egg-eating habits of the Lilliputians.
3523 Some systems may have even weirder byte orders such as
3528 You can see your system's preference with
3530 print join(" ", map { sprintf "%#02x", $_ }
3531 unpack("C*",pack("L",0x12345678))), "\n";
3533 The byteorder on the platform where Perl was built is also available
3537 print $Config{byteorder}, "\n";
3539 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3540 and C<'87654321'> are big-endian.
3542 If you want portable packed integers use the formats C<n>, C<N>,
3543 C<v>, and C<V>, their byte endianness and size are known.
3544 See also L<perlport>.
3548 Real numbers (floats and doubles) are in the native machine format only;
3549 due to the multiplicity of floating formats around, and the lack of a
3550 standard "network" representation, no facility for interchange has been
3551 made. This means that packed floating point data written on one machine
3552 may not be readable on another - even if both use IEEE floating point
3553 arithmetic (as the endian-ness of the memory representation is not part
3554 of the IEEE spec). See also L<perlport>.
3556 Note that Perl uses doubles internally for all numeric calculation, and
3557 converting from double into float and thence back to double again will
3558 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3563 If the pattern begins with a C<U>, the resulting string will be
3564 treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
3565 string with an initial C<U0>, and the bytes that follow will be
3566 interpreted as Unicode characters. If you don't want this to happen,
3567 you can begin your pattern with C<C0> (or anything else) to force Perl
3568 not to UTF-8 encode your string, and then follow this with a C<U*>
3569 somewhere in your pattern.
3573 You must yourself do any alignment or padding by inserting for example
3574 enough C<'x'>es while packing. There is no way to pack() and unpack()
3575 could know where the bytes are going to or coming from. Therefore
3576 C<pack> (and C<unpack>) handle their output and input as flat
3581 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3582 take a repeat count, both as postfix, and for unpack() also via the C</>
3583 template character. Within each repetition of a group, positioning with
3584 C<@> starts again at 0. Therefore, the result of
3586 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3588 is the string "\0a\0\0bc".
3593 C<x> and C<X> accept C<!> modifier. In this case they act as
3594 alignment commands: they jump forward/back to the closest position
3595 aligned at a multiple of C<count> bytes. For example, to pack() or
3596 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3597 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3598 aligned on the double's size.
3600 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3601 both result in no-ops.
3605 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3606 White space may be used to separate pack codes from each other, but
3607 a C<!> modifier and a repeat count must follow immediately.
3611 If TEMPLATE requires more arguments to pack() than actually given, pack()
3612 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3613 to pack() than actually given, extra arguments are ignored.
3619 $foo = pack("CCCC",65,66,67,68);
3621 $foo = pack("C4",65,66,67,68);
3623 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3624 # same thing with Unicode circled letters
3626 $foo = pack("ccxxcc",65,66,67,68);
3629 # note: the above examples featuring "C" and "c" are true
3630 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3631 # and UTF-8. In EBCDIC the first example would be
3632 # $foo = pack("CCCC",193,194,195,196);
3634 $foo = pack("s2",1,2);
3635 # "\1\0\2\0" on little-endian
3636 # "\0\1\0\2" on big-endian
3638 $foo = pack("a4","abcd","x","y","z");
3641 $foo = pack("aaaa","abcd","x","y","z");
3644 $foo = pack("a14","abcdefg");
3645 # "abcdefg\0\0\0\0\0\0\0"
3647 $foo = pack("i9pl", gmtime);
3648 # a real struct tm (on my system anyway)
3650 $utmp_template = "Z8 Z8 Z16 L";
3651 $utmp = pack($utmp_template, @utmp1);
3652 # a struct utmp (BSDish)
3654 @utmp2 = unpack($utmp_template, $utmp);
3655 # "@utmp1" eq "@utmp2"
3658 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3661 $foo = pack('sx2l', 12, 34);
3662 # short 12, two zero bytes padding, long 34
3663 $bar = pack('s@4l', 12, 34);
3664 # short 12, zero fill to position 4, long 34
3667 The same template may generally also be used in unpack().
3669 =item package NAMESPACE
3673 Declares the compilation unit as being in the given namespace. The scope
3674 of the package declaration is from the declaration itself through the end
3675 of the enclosing block, file, or eval (the same as the C<my> operator).
3676 All further unqualified dynamic identifiers will be in this namespace.
3677 A package statement affects only dynamic variables--including those
3678 you've used C<local> on--but I<not> lexical variables, which are created
3679 with C<my>. Typically it would be the first declaration in a file to
3680 be included by the C<require> or C<use> operator. You can switch into a
3681 package in more than one place; it merely influences which symbol table
3682 is used by the compiler for the rest of that block. You can refer to
3683 variables and filehandles in other packages by prefixing the identifier
3684 with the package name and a double colon: C<$Package::Variable>.
3685 If the package name is null, the C<main> package as assumed. That is,
3686 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3687 still seen in older code).
3689 If NAMESPACE is omitted, then there is no current package, and all
3690 identifiers must be fully qualified or lexicals. However, you are
3691 strongly advised not to make use of this feature. Its use can cause
3692 unexpected behaviour, even crashing some versions of Perl. It is
3693 deprecated, and will be removed from a future release.
3695 See L<perlmod/"Packages"> for more information about packages, modules,
3696 and classes. See L<perlsub> for other scoping issues.
3698 =item pipe READHANDLE,WRITEHANDLE
3700 Opens a pair of connected pipes like the corresponding system call.
3701 Note that if you set up a loop of piped processes, deadlock can occur
3702 unless you are very careful. In addition, note that Perl's pipes use
3703 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3704 after each command, depending on the application.
3706 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3707 for examples of such things.
3709 On systems that support a close-on-exec flag on files, the flag will be set
3710 for the newly opened file descriptors as determined by the value of $^F.
3717 Pops and returns the last value of the array, shortening the array by
3718 one element. Has an effect similar to
3722 If there are no elements in the array, returns the undefined value
3723 (although this may happen at other times as well). If ARRAY is
3724 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3725 array in subroutines, just like C<shift>.
3731 Returns the offset of where the last C<m//g> search left off for the variable
3732 in question (C<$_> is used when the variable is not specified). May be
3733 modified to change that offset. Such modification will also influence
3734 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3737 =item print FILEHANDLE LIST
3743 Prints a string or a list of strings. Returns true if successful.
3744 FILEHANDLE may be a scalar variable name, in which case the variable
3745 contains the name of or a reference to the filehandle, thus introducing
3746 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3747 the next token is a term, it may be misinterpreted as an operator
3748 unless you interpose a C<+> or put parentheses around the arguments.)
3749 If FILEHANDLE is omitted, prints by default to standard output (or
3750 to the last selected output channel--see L</select>). If LIST is
3751 also omitted, prints C<$_> to the currently selected output channel.
3752 To set the default output channel to something other than STDOUT
3753 use the select operation. The current value of C<$,> (if any) is
3754 printed between each LIST item. The current value of C<$\> (if
3755 any) is printed after the entire LIST has been printed. Because
3756 print takes a LIST, anything in the LIST is evaluated in list
3757 context, and any subroutine that you call will have one or more of
3758 its expressions evaluated in list context. Also be careful not to
3759 follow the print keyword with a left parenthesis unless you want
3760 the corresponding right parenthesis to terminate the arguments to
3761 the print--interpose a C<+> or put parentheses around all the
3764 Note that if you're storing FILEHANDLES in an array or other expression,
3765 you will have to use a block returning its value instead:
3767 print { $files[$i] } "stuff\n";
3768 print { $OK ? STDOUT : STDERR } "stuff\n";
3770 =item printf FILEHANDLE FORMAT, LIST
3772 =item printf FORMAT, LIST
3774 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3775 (the output record separator) is not appended. The first argument
3776 of the list will be interpreted as the C<printf> format. See C<sprintf>
3777 for an explanation of the format argument. If C<use locale> is in effect,
3778 the character used for the decimal point in formatted real numbers is
3779 affected by the LC_NUMERIC locale. See L<perllocale>.
3781 Don't fall into the trap of using a C<printf> when a simple
3782 C<print> would do. The C<print> is more efficient and less
3785 =item prototype FUNCTION
3787 Returns the prototype of a function as a string (or C<undef> if the
3788 function has no prototype). FUNCTION is a reference to, or the name of,
3789 the function whose prototype you want to retrieve.
3791 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3792 name for Perl builtin. If the builtin is not I<overridable> (such as
3793 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3794 C<system>) returns C<undef> because the builtin does not really behave
3795 like a Perl function. Otherwise, the string describing the equivalent
3796 prototype is returned.
3798 =item push ARRAY,LIST
3800 Treats ARRAY as a stack, and pushes the values of LIST
3801 onto the end of ARRAY. The length of ARRAY increases by the length of
3802 LIST. Has the same effect as
3805 $ARRAY[++$#ARRAY] = $value;
3808 but is more efficient. Returns the new number of elements in the array.
3820 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3822 =item quotemeta EXPR
3826 Returns the value of EXPR with all non-"word"
3827 characters backslashed. (That is, all characters not matching
3828 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3829 returned string, regardless of any locale settings.)
3830 This is the internal function implementing
3831 the C<\Q> escape in double-quoted strings.
3833 If EXPR is omitted, uses C<$_>.
3839 Returns a random fractional number greater than or equal to C<0> and less
3840 than the value of EXPR. (EXPR should be positive.) If EXPR is
3841 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3842 also special-cased as C<1> - this has not been documented before perl 5.8.0
3843 and is subject to change in future versions of perl. Automatically calls
3844 C<srand> unless C<srand> has already been called. See also C<srand>.
3846 Apply C<int()> to the value returned by C<rand()> if you want random
3847 integers instead of random fractional numbers. For example,
3851 returns a random integer between C<0> and C<9>, inclusive.
3853 (Note: If your rand function consistently returns numbers that are too
3854 large or too small, then your version of Perl was probably compiled
3855 with the wrong number of RANDBITS.)
3857 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3859 =item read FILEHANDLE,SCALAR,LENGTH
3861 Attempts to read LENGTH I<characters> of data into variable SCALAR
3862 from the specified FILEHANDLE. Returns the number of characters
3863 actually read, C<0> at end of file, or undef if there was an error (in
3864 the latter case C<$!> is also set). SCALAR will be grown or shrunk
3865 so that the last character actually read is the last character of the
3866 scalar after the read.
3868 An OFFSET may be specified to place the read data at some place in the
3869 string other than the beginning. A negative OFFSET specifies
3870 placement at that many characters counting backwards from the end of
3871 the string. A positive OFFSET greater than the length of SCALAR
3872 results in the string being padded to the required size with C<"\0">
3873 bytes before the result of the read is appended.
3875 The call is actually implemented in terms of either Perl's or system's
3876 fread() call. To get a true read(2) system call, see C<sysread>.
3878 Note the I<characters>: depending on the status of the filehandle,
3879 either (8-bit) bytes or characters are read. By default all
3880 filehandles operate on bytes, but for example if the filehandle has
3881 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3882 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3883 characters, not bytes. Similarly for the C<:encoding> pragma:
3884 in that case pretty much any characters can be read.
3886 =item readdir DIRHANDLE
3888 Returns the next directory entry for a directory opened by C<opendir>.
3889 If used in list context, returns all the rest of the entries in the
3890 directory. If there are no more entries, returns an undefined value in
3891 scalar context or a null list in list context.
3893 If you're planning to filetest the return values out of a C<readdir>, you'd
3894 better prepend the directory in question. Otherwise, because we didn't
3895 C<chdir> there, it would have been testing the wrong file.
3897 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3898 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3903 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3904 context, each call reads and returns the next line, until end-of-file is
3905 reached, whereupon the subsequent call returns undef. In list context,
3906 reads until end-of-file is reached and returns a list of lines. Note that
3907 the notion of "line" used here is however you may have defined it
3908 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3910 When C<$/> is set to C<undef>, when readline() is in scalar
3911 context (i.e. file slurp mode), and when an empty file is read, it
3912 returns C<''> the first time, followed by C<undef> subsequently.
3914 This is the internal function implementing the C<< <EXPR> >>
3915 operator, but you can use it directly. The C<< <EXPR> >>
3916 operator is discussed in more detail in L<perlop/"I/O Operators">.
3919 $line = readline(*STDIN); # same thing
3921 If readline encounters an operating system error, C<$!> will be set with the
3922 corresponding error message. It can be helpful to check C<$!> when you are
3923 reading from filehandles you don't trust, such as a tty or a socket. The
3924 following example uses the operator form of C<readline>, and takes the necessary
3925 steps to ensure that C<readline> was successful.
3929 unless (defined( $line = <> )) {
3940 Returns the value of a symbolic link, if symbolic links are
3941 implemented. If not, gives a fatal error. If there is some system
3942 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3943 omitted, uses C<$_>.
3947 EXPR is executed as a system command.
3948 The collected standard output of the command is returned.
3949 In scalar context, it comes back as a single (potentially
3950 multi-line) string. In list context, returns a list of lines
3951 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3952 This is the internal function implementing the C<qx/EXPR/>
3953 operator, but you can use it directly. The C<qx/EXPR/>
3954 operator is discussed in more detail in L<perlop/"I/O Operators">.
3956 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3958 Receives a message on a socket. Attempts to receive LENGTH characters
3959 of data into variable SCALAR from the specified SOCKET filehandle.
3960 SCALAR will be grown or shrunk to the length actually read. Takes the
3961 same flags as the system call of the same name. Returns the address
3962 of the sender if SOCKET's protocol supports this; returns an empty
3963 string otherwise. If there's an error, returns the undefined value.
3964 This call is actually implemented in terms of recvfrom(2) system call.
3965 See L<perlipc/"UDP: Message Passing"> for examples.
3967 Note the I<characters>: depending on the status of the socket, either
3968 (8-bit) bytes or characters are received. By default all sockets
3969 operate on bytes, but for example if the socket has been changed using
3970 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3971 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
3972 characters, not bytes. Similarly for the C<:encoding> pragma:
3973 in that case pretty much any characters can be read.
3979 The C<redo> command restarts the loop block without evaluating the
3980 conditional again. The C<continue> block, if any, is not executed. If
3981 the LABEL is omitted, the command refers to the innermost enclosing
3982 loop. This command is normally used by programs that want to lie to
3983 themselves about what was just input:
3985 # a simpleminded Pascal comment stripper
3986 # (warning: assumes no { or } in strings)
3987 LINE: while (<STDIN>) {
3988 while (s|({.*}.*){.*}|$1 |) {}
3993 if (/}/) { # end of comment?
4002 C<redo> cannot be used to retry a block which returns a value such as
4003 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4004 a grep() or map() operation.
4006 Note that a block by itself is semantically identical to a loop
4007 that executes once. Thus C<redo> inside such a block will effectively
4008 turn it into a looping construct.
4010 See also L</continue> for an illustration of how C<last>, C<next>, and
4017 Returns a non-empty string if EXPR is a reference, the empty
4018 string otherwise. If EXPR
4019 is not specified, C<$_> will be used. The value returned depends on the
4020 type of thing the reference is a reference to.
4021 Builtin types include:
4031 If the referenced object has been blessed into a package, then that package
4032 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4034 if (ref($r) eq "HASH") {
4035 print "r is a reference to a hash.\n";
4038 print "r is not a reference at all.\n";
4040 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
4041 print "r is a reference to something that isa hash.\n";
4044 See also L<perlref>.
4046 =item rename OLDNAME,NEWNAME
4048 Changes the name of a file; an existing file NEWNAME will be
4049 clobbered. Returns true for success, false otherwise.
4051 Behavior of this function varies wildly depending on your system
4052 implementation. For example, it will usually not work across file system
4053 boundaries, even though the system I<mv> command sometimes compensates
4054 for this. Other restrictions include whether it works on directories,
4055 open files, or pre-existing files. Check L<perlport> and either the
4056 rename(2) manpage or equivalent system documentation for details.
4058 =item require VERSION
4064 Demands a version of Perl specified by VERSION, or demands some semantics
4065 specified by EXPR or by C<$_> if EXPR is not supplied.
4067 VERSION may be either a numeric argument such as 5.006, which will be
4068 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4069 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4070 VERSION is greater than the version of the current Perl interpreter.
4071 Compare with L</use>, which can do a similar check at compile time.
4073 Specifying VERSION as a literal of the form v5.6.1 should generally be
4074 avoided, because it leads to misleading error messages under earlier
4075 versions of Perl which do not support this syntax. The equivalent numeric
4076 version should be used instead.
4078 require v5.6.1; # run time version check
4079 require 5.6.1; # ditto
4080 require 5.006_001; # ditto; preferred for backwards compatibility
4082 Otherwise, demands that a library file be included if it hasn't already
4083 been included. The file is included via the do-FILE mechanism, which is
4084 essentially just a variety of C<eval>. Has semantics similar to the
4085 following subroutine:
4088 my ($filename) = @_;
4089 if (exists $INC{$filename}) {
4090 return 1 if $INC{$filename};
4091 die "Compilation failed in require";
4093 my ($realfilename,$result);
4095 foreach $prefix (@INC) {
4096 $realfilename = "$prefix/$filename";
4097 if (-f $realfilename) {
4098 $INC{$filename} = $realfilename;
4099 $result = do $realfilename;
4103 die "Can't find $filename in \@INC";
4106 $INC{$filename} = undef;
4108 } elsif (!$result) {
4109 delete $INC{$filename};
4110 die "$filename did not return true value";
4116 Note that the file will not be included twice under the same specified
4119 The file must return true as the last statement to indicate
4120 successful execution of any initialization code, so it's customary to
4121 end such a file with C<1;> unless you're sure it'll return true
4122 otherwise. But it's better just to put the C<1;>, in case you add more
4125 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4126 replaces "F<::>" with "F</>" in the filename for you,
4127 to make it easy to load standard modules. This form of loading of
4128 modules does not risk altering your namespace.
4130 In other words, if you try this:
4132 require Foo::Bar; # a splendid bareword
4134 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4135 directories specified in the C<@INC> array.
4137 But if you try this:
4139 $class = 'Foo::Bar';
4140 require $class; # $class is not a bareword
4142 require "Foo::Bar"; # not a bareword because of the ""
4144 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4145 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4147 eval "require $class";
4149 Now that you understand how C<require> looks for files in the case of
4150 a bareword argument, there is a little extra functionality going on
4151 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4152 it will first look for a filename with a "F<.pmc>" extension. A file
4153 with this extension is assumed to be Perl bytecode generated by
4154 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4155 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4156 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4158 You can also insert hooks into the import facility, by putting directly
4159 Perl code into the @INC array. There are three forms of hooks: subroutine
4160 references, array references and blessed objects.
4162 Subroutine references are the simplest case. When the inclusion system
4163 walks through @INC and encounters a subroutine, this subroutine gets
4164 called with two parameters, the first being a reference to itself, and the
4165 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4166 subroutine should return C<undef> or a filehandle, from which the file to
4167 include will be read. If C<undef> is returned, C<require> will look at
4168 the remaining elements of @INC.
4170 If the hook is an array reference, its first element must be a subroutine
4171 reference. This subroutine is called as above, but the first parameter is
4172 the array reference. This enables to pass indirectly some arguments to
4175 In other words, you can write:
4177 push @INC, \&my_sub;
4179 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4185 push @INC, [ \&my_sub, $x, $y, ... ];
4187 my ($arrayref, $filename) = @_;
4188 # Retrieve $x, $y, ...
4189 my @parameters = @$arrayref[1..$#$arrayref];
4193 If the hook is an object, it must provide an INC method, that will be
4194 called as above, the first parameter being the object itself. (Note that
4195 you must fully qualify the sub's name, as it is always forced into package
4196 C<main>.) Here is a typical code layout:
4202 my ($self, $filename) = @_;
4206 # In the main program
4207 push @INC, new Foo(...);
4209 Note that these hooks are also permitted to set the %INC entry
4210 corresponding to the files they have loaded. See L<perlvar/%INC>.
4212 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4218 Generally used in a C<continue> block at the end of a loop to clear
4219 variables and reset C<??> searches so that they work again. The
4220 expression is interpreted as a list of single characters (hyphens
4221 allowed for ranges). All variables and arrays beginning with one of
4222 those letters are reset to their pristine state. If the expression is
4223 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4224 only variables or searches in the current package. Always returns
4227 reset 'X'; # reset all X variables
4228 reset 'a-z'; # reset lower case variables
4229 reset; # just reset ?one-time? searches
4231 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4232 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4233 variables--lexical variables are unaffected, but they clean themselves
4234 up on scope exit anyway, so you'll probably want to use them instead.
4241 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4242 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4243 context, depending on how the return value will be used, and the context
4244 may vary from one execution to the next (see C<wantarray>). If no EXPR
4245 is given, returns an empty list in list context, the undefined value in
4246 scalar context, and (of course) nothing at all in a void context.
4248 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4249 or do FILE will automatically return the value of the last expression
4254 In list context, returns a list value consisting of the elements
4255 of LIST in the opposite order. In scalar context, concatenates the
4256 elements of LIST and returns a string value with all characters
4257 in the opposite order.
4259 print reverse <>; # line tac, last line first
4261 undef $/; # for efficiency of <>
4262 print scalar reverse <>; # character tac, last line tsrif
4264 Used without arguments in scalar context, reverse() reverses C<$_>.
4266 This operator is also handy for inverting a hash, although there are some
4267 caveats. If a value is duplicated in the original hash, only one of those
4268 can be represented as a key in the inverted hash. Also, this has to
4269 unwind one hash and build a whole new one, which may take some time
4270 on a large hash, such as from a DBM file.
4272 %by_name = reverse %by_address; # Invert the hash
4274 =item rewinddir DIRHANDLE
4276 Sets the current position to the beginning of the directory for the
4277 C<readdir> routine on DIRHANDLE.
4279 =item rindex STR,SUBSTR,POSITION
4281 =item rindex STR,SUBSTR
4283 Works just like index() except that it returns the position of the LAST
4284 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4285 last occurrence at or before that position.
4287 =item rmdir FILENAME
4291 Deletes the directory specified by FILENAME if that directory is
4292 empty. If it succeeds it returns true, otherwise it returns false and
4293 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4297 The substitution operator. See L<perlop>.
4301 Forces EXPR to be interpreted in scalar context and returns the value
4304 @counts = ( scalar @a, scalar @b, scalar @c );
4306 There is no equivalent operator to force an expression to
4307 be interpolated in list context because in practice, this is never
4308 needed. If you really wanted to do so, however, you could use
4309 the construction C<@{[ (some expression) ]}>, but usually a simple
4310 C<(some expression)> suffices.
4312 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4313 parenthesized list, this behaves as a scalar comma expression, evaluating
4314 all but the last element in void context and returning the final element
4315 evaluated in scalar context. This is seldom what you want.
4317 The following single statement:
4319 print uc(scalar(&foo,$bar)),$baz;
4321 is the moral equivalent of these two:
4324 print(uc($bar),$baz);
4326 See L<perlop> for more details on unary operators and the comma operator.
4328 =item seek FILEHANDLE,POSITION,WHENCE
4330 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4331 FILEHANDLE may be an expression whose value gives the name of the
4332 filehandle. The values for WHENCE are C<0> to set the new position
4333 I<in bytes> to POSITION, C<1> to set it to the current position plus
4334 POSITION, and C<2> to set it to EOF plus POSITION (typically
4335 negative). For WHENCE you may use the constants C<SEEK_SET>,
4336 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4337 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4340 Note the I<in bytes>: even if the filehandle has been set to
4341 operate on characters (for example by using the C<:utf8> open
4342 layer), tell() will return byte offsets, not character offsets
4343 (because implementing that would render seek() and tell() rather slow).
4345 If you want to position file for C<sysread> or C<syswrite>, don't use
4346 C<seek>--buffering makes its effect on the file's system position
4347 unpredictable and non-portable. Use C<sysseek> instead.
4349 Due to the rules and rigors of ANSI C, on some systems you have to do a
4350 seek whenever you switch between reading and writing. Amongst other
4351 things, this may have the effect of calling stdio's clearerr(3).
4352 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4356 This is also useful for applications emulating C<tail -f>. Once you hit
4357 EOF on your read, and then sleep for a while, you might have to stick in a
4358 seek() to reset things. The C<seek> doesn't change the current position,
4359 but it I<does> clear the end-of-file condition on the handle, so that the
4360 next C<< <FILE> >> makes Perl try again to read something. We hope.
4362 If that doesn't work (some IO implementations are particularly
4363 cantankerous), then you may need something more like this:
4366 for ($curpos = tell(FILE); $_ = <FILE>;
4367 $curpos = tell(FILE)) {
4368 # search for some stuff and put it into files
4370 sleep($for_a_while);
4371 seek(FILE, $curpos, 0);
4374 =item seekdir DIRHANDLE,POS
4376 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4377 must be a value returned by C<telldir>. Has the same caveats about
4378 possible directory compaction as the corresponding system library
4381 =item select FILEHANDLE
4385 Returns the currently selected filehandle. Sets the current default
4386 filehandle for output, if FILEHANDLE is supplied. This has two
4387 effects: first, a C<write> or a C<print> without a filehandle will
4388 default to this FILEHANDLE. Second, references to variables related to
4389 output will refer to this output channel. For example, if you have to
4390 set the top of form format for more than one output channel, you might
4398 FILEHANDLE may be an expression whose value gives the name of the
4399 actual filehandle. Thus:
4401 $oldfh = select(STDERR); $| = 1; select($oldfh);
4403 Some programmers may prefer to think of filehandles as objects with
4404 methods, preferring to write the last example as:
4407 STDERR->autoflush(1);
4409 =item select RBITS,WBITS,EBITS,TIMEOUT
4411 This calls the select(2) system call with the bit masks specified, which
4412 can be constructed using C<fileno> and C<vec>, along these lines:
4414 $rin = $win = $ein = '';
4415 vec($rin,fileno(STDIN),1) = 1;
4416 vec($win,fileno(STDOUT),1) = 1;
4419 If you want to select on many filehandles you might wish to write a
4423 my(@fhlist) = split(' ',$_[0]);
4426 vec($bits,fileno($_),1) = 1;
4430 $rin = fhbits('STDIN TTY SOCK');
4434 ($nfound,$timeleft) =
4435 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4437 or to block until something becomes ready just do this
4439 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4441 Most systems do not bother to return anything useful in $timeleft, so
4442 calling select() in scalar context just returns $nfound.
4444 Any of the bit masks can also be undef. The timeout, if specified, is
4445 in seconds, which may be fractional. Note: not all implementations are
4446 capable of returning the $timeleft. If not, they always return
4447 $timeleft equal to the supplied $timeout.
4449 You can effect a sleep of 250 milliseconds this way:
4451 select(undef, undef, undef, 0.25);
4453 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4454 is implementation-dependent.
4456 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4457 or <FH>) with C<select>, except as permitted by POSIX, and even
4458 then only on POSIX systems. You have to use C<sysread> instead.
4460 =item semctl ID,SEMNUM,CMD,ARG
4462 Calls the System V IPC function C<semctl>. You'll probably have to say
4466 first to get the correct constant definitions. If CMD is IPC_STAT or
4467 GETALL, then ARG must be a variable which will hold the returned
4468 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4469 the undefined value for error, "C<0 but true>" for zero, or the actual
4470 return value otherwise. The ARG must consist of a vector of native
4471 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4472 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4475 =item semget KEY,NSEMS,FLAGS
4477 Calls the System V IPC function semget. Returns the semaphore id, or
4478 the undefined value if there is an error. See also
4479 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4482 =item semop KEY,OPSTRING
4484 Calls the System V IPC function semop to perform semaphore operations
4485 such as signalling and waiting. OPSTRING must be a packed array of
4486 semop structures. Each semop structure can be generated with
4487 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4488 operations is implied by the length of OPSTRING. Returns true if
4489 successful, or false if there is an error. As an example, the
4490 following code waits on semaphore $semnum of semaphore id $semid:
4492 $semop = pack("s!3", $semnum, -1, 0);
4493 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4495 To signal the semaphore, replace C<-1> with C<1>. See also
4496 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4499 =item send SOCKET,MSG,FLAGS,TO
4501 =item send SOCKET,MSG,FLAGS
4503 Sends a message on a socket. Attempts to send the scalar MSG to the
4504 SOCKET filehandle. Takes the same flags as the system call of the
4505 same name. On unconnected sockets you must specify a destination to
4506 send TO, in which case it does a C C<sendto>. Returns the number of
4507 characters sent, or the undefined value if there is an error. The C
4508 system call sendmsg(2) is currently unimplemented. See
4509 L<perlipc/"UDP: Message Passing"> for examples.
4511 Note the I<characters>: depending on the status of the socket, either
4512 (8-bit) bytes or characters are sent. By default all sockets operate
4513 on bytes, but for example if the socket has been changed using
4514 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4515 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4516 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4517 in that case pretty much any characters can be sent.
4519 =item setpgrp PID,PGRP
4521 Sets the current process group for the specified PID, C<0> for the current
4522 process. Will produce a fatal error if used on a machine that doesn't
4523 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4524 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4525 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4528 =item setpriority WHICH,WHO,PRIORITY
4530 Sets the current priority for a process, a process group, or a user.
4531 (See setpriority(2).) Will produce a fatal error if used on a machine
4532 that doesn't implement setpriority(2).
4534 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4536 Sets the socket option requested. Returns undefined if there is an
4537 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4544 Shifts the first value of the array off and returns it, shortening the
4545 array by 1 and moving everything down. If there are no elements in the
4546 array, returns the undefined value. If ARRAY is omitted, shifts the
4547 C<@_> array within the lexical scope of subroutines and formats, and the
4548 C<@ARGV> array at file scopes or within the lexical scopes established by
4549 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4552 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4553 same thing to the left end of an array that C<pop> and C<push> do to the
4556 =item shmctl ID,CMD,ARG
4558 Calls the System V IPC function shmctl. You'll probably have to say
4562 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4563 then ARG must be a variable which will hold the returned C<shmid_ds>
4564 structure. Returns like ioctl: the undefined value for error, "C<0> but
4565 true" for zero, or the actual return value otherwise.
4566 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4568 =item shmget KEY,SIZE,FLAGS
4570 Calls the System V IPC function shmget. Returns the shared memory
4571 segment id, or the undefined value if there is an error.
4572 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4574 =item shmread ID,VAR,POS,SIZE
4576 =item shmwrite ID,STRING,POS,SIZE
4578 Reads or writes the System V shared memory segment ID starting at
4579 position POS for size SIZE by attaching to it, copying in/out, and
4580 detaching from it. When reading, VAR must be a variable that will
4581 hold the data read. When writing, if STRING is too long, only SIZE
4582 bytes are used; if STRING is too short, nulls are written to fill out
4583 SIZE bytes. Return true if successful, or false if there is an error.
4584 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4585 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4587 =item shutdown SOCKET,HOW
4589 Shuts down a socket connection in the manner indicated by HOW, which
4590 has the same interpretation as in the system call of the same name.
4592 shutdown(SOCKET, 0); # I/we have stopped reading data
4593 shutdown(SOCKET, 1); # I/we have stopped writing data
4594 shutdown(SOCKET, 2); # I/we have stopped using this socket
4596 This is useful with sockets when you want to tell the other
4597 side you're done writing but not done reading, or vice versa.
4598 It's also a more insistent form of close because it also
4599 disables the file descriptor in any forked copies in other
4606 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4607 returns sine of C<$_>.
4609 For the inverse sine operation, you may use the C<Math::Trig::asin>
4610 function, or use this relation:
4612 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4618 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4619 May be interrupted if the process receives a signal such as C<SIGALRM>.
4620 Returns the number of seconds actually slept. You probably cannot
4621 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4624 On some older systems, it may sleep up to a full second less than what
4625 you requested, depending on how it counts seconds. Most modern systems
4626 always sleep the full amount. They may appear to sleep longer than that,
4627 however, because your process might not be scheduled right away in a
4628 busy multitasking system.
4630 For delays of finer granularity than one second, you may use Perl's
4631 C<syscall> interface to access setitimer(2) if your system supports
4632 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4633 and starting from Perl 5.8 part of the standard distribution) may also
4636 See also the POSIX module's C<pause> function.
4638 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4640 Opens a socket of the specified kind and attaches it to filehandle
4641 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4642 the system call of the same name. You should C<use Socket> first
4643 to get the proper definitions imported. See the examples in
4644 L<perlipc/"Sockets: Client/Server Communication">.
4646 On systems that support a close-on-exec flag on files, the flag will
4647 be set for the newly opened file descriptor, as determined by the
4648 value of $^F. See L<perlvar/$^F>.
4650 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4652 Creates an unnamed pair of sockets in the specified domain, of the
4653 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4654 for the system call of the same name. If unimplemented, yields a fatal
4655 error. Returns true if successful.
4657 On systems that support a close-on-exec flag on files, the flag will
4658 be set for the newly opened file descriptors, as determined by the value
4659 of $^F. See L<perlvar/$^F>.
4661 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4662 to C<pipe(Rdr, Wtr)> is essentially:
4665 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4666 shutdown(Rdr, 1); # no more writing for reader
4667 shutdown(Wtr, 0); # no more reading for writer
4669 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4670 emulate socketpair using IP sockets to localhost if your system implements
4671 sockets but not socketpair.
4673 =item sort SUBNAME LIST
4675 =item sort BLOCK LIST
4679 In list context, this sorts the LIST and returns the sorted list value.
4680 In scalar context, the behaviour of C<sort()> is undefined.
4682 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4683 order. If SUBNAME is specified, it gives the name of a subroutine
4684 that returns an integer less than, equal to, or greater than C<0>,
4685 depending on how the elements of the list are to be ordered. (The C<<
4686 <=> >> and C<cmp> operators are extremely useful in such routines.)
4687 SUBNAME may be a scalar variable name (unsubscripted), in which case
4688 the value provides the name of (or a reference to) the actual
4689 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4690 an anonymous, in-line sort subroutine.
4692 If the subroutine's prototype is C<($$)>, the elements to be compared
4693 are passed by reference in C<@_>, as for a normal subroutine. This is
4694 slower than unprototyped subroutines, where the elements to be
4695 compared are passed into the subroutine
4696 as the package global variables $a and $b (see example below). Note that
4697 in the latter case, it is usually counter-productive to declare $a and
4700 In either case, the subroutine may not be recursive. The values to be
4701 compared are always passed by reference, so don't modify them.
4703 You also cannot exit out of the sort block or subroutine using any of the
4704 loop control operators described in L<perlsyn> or with C<goto>.
4706 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4707 current collation locale. See L<perllocale>.
4709 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4710 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4711 preserves the input order of elements that compare equal. Although
4712 quicksort's run time is O(NlogN) when averaged over all arrays of
4713 length N, the time can be O(N**2), I<quadratic> behavior, for some
4714 inputs.) In 5.7, the quicksort implementation was replaced with
4715 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4716 But benchmarks indicated that for some inputs, on some platforms,
4717 the original quicksort was faster. 5.8 has a sort pragma for
4718 limited control of the sort. Its rather blunt control of the
4719 underlying algorithm may not persist into future perls, but the
4720 ability to characterize the input or output in implementation
4721 independent ways quite probably will. See L<sort>.
4726 @articles = sort @files;
4728 # same thing, but with explicit sort routine
4729 @articles = sort {$a cmp $b} @files;
4731 # now case-insensitively
4732 @articles = sort {uc($a) cmp uc($b)} @files;
4734 # same thing in reversed order
4735 @articles = sort {$b cmp $a} @files;
4737 # sort numerically ascending
4738 @articles = sort {$a <=> $b} @files;
4740 # sort numerically descending
4741 @articles = sort {$b <=> $a} @files;
4743 # this sorts the %age hash by value instead of key
4744 # using an in-line function
4745 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4747 # sort using explicit subroutine name
4749 $age{$a} <=> $age{$b}; # presuming numeric
4751 @sortedclass = sort byage @class;
4753 sub backwards { $b cmp $a }
4754 @harry = qw(dog cat x Cain Abel);
4755 @george = qw(gone chased yz Punished Axed);
4757 # prints AbelCaincatdogx
4758 print sort backwards @harry;
4759 # prints xdogcatCainAbel
4760 print sort @george, 'to', @harry;
4761 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4763 # inefficiently sort by descending numeric compare using
4764 # the first integer after the first = sign, or the
4765 # whole record case-insensitively otherwise
4768 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4773 # same thing, but much more efficiently;
4774 # we'll build auxiliary indices instead
4778 push @nums, /=(\d+)/;
4783 $nums[$b] <=> $nums[$a]
4785 $caps[$a] cmp $caps[$b]
4789 # same thing, but without any temps
4790 @new = map { $_->[0] }
4791 sort { $b->[1] <=> $a->[1]
4794 } map { [$_, /=(\d+)/, uc($_)] } @old;
4796 # using a prototype allows you to use any comparison subroutine
4797 # as a sort subroutine (including other package's subroutines)
4799 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4802 @new = sort other::backwards @old;
4804 # guarantee stability, regardless of algorithm
4806 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4808 # force use of mergesort (not portable outside Perl 5.8)
4809 use sort '_mergesort'; # note discouraging _
4810 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4812 If you're using strict, you I<must not> declare $a
4813 and $b as lexicals. They are package globals. That means
4814 if you're in the C<main> package and type
4816 @articles = sort {$b <=> $a} @files;
4818 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4819 but if you're in the C<FooPack> package, it's the same as typing
4821 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4823 The comparison function is required to behave. If it returns
4824 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4825 sometimes saying the opposite, for example) the results are not
4828 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
4829 (not-a-number), and because C<sort> will trigger a fatal error unless the
4830 result of a comparison is defined, when sorting with a comparison function
4831 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
4832 The following example takes advantage of the fact that C<NaN != NaN> to
4833 eliminate any C<NaN>s from the input.
4835 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
4837 =item splice ARRAY,OFFSET,LENGTH,LIST
4839 =item splice ARRAY,OFFSET,LENGTH
4841 =item splice ARRAY,OFFSET
4845 Removes the elements designated by OFFSET and LENGTH from an array, and
4846 replaces them with the elements of LIST, if any. In list context,
4847 returns the elements removed from the array. In scalar context,
4848 returns the last element removed, or C<undef> if no elements are
4849 removed. The array grows or shrinks as necessary.
4850 If OFFSET is negative then it starts that far from the end of the array.
4851 If LENGTH is omitted, removes everything from OFFSET onward.
4852 If LENGTH is negative, removes the elements from OFFSET onward
4853 except for -LENGTH elements at the end of the array.
4854 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4855 past the end of the array, perl issues a warning, and splices at the
4858 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4860 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4861 pop(@a) splice(@a,-1)
4862 shift(@a) splice(@a,0,1)
4863 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4864 $a[$i] = $y splice(@a,$i,1,$y)
4866 Example, assuming array lengths are passed before arrays:
4868 sub aeq { # compare two list values
4869 my(@a) = splice(@_,0,shift);
4870 my(@b) = splice(@_,0,shift);
4871 return 0 unless @a == @b; # same len?
4873 return 0 if pop(@a) ne pop(@b);
4877 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4879 =item split /PATTERN/,EXPR,LIMIT
4881 =item split /PATTERN/,EXPR
4883 =item split /PATTERN/
4887 Splits a string into a list of strings and returns that list. By default,
4888 empty leading fields are preserved, and empty trailing ones are deleted.
4890 In scalar context, returns the number of fields found and splits into
4891 the C<@_> array. Use of split in scalar context is deprecated, however,
4892 because it clobbers your subroutine arguments.
4894 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4895 splits on whitespace (after skipping any leading whitespace). Anything
4896 matching PATTERN is taken to be a delimiter separating the fields. (Note
4897 that the delimiter may be longer than one character.)
4899 If LIMIT is specified and positive, it represents the maximum number
4900 of fields the EXPR will be split into, though the actual number of
4901 fields returned depends on the number of times PATTERN matches within
4902 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4903 stripped (which potential users of C<pop> would do well to remember).
4904 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4905 had been specified. Note that splitting an EXPR that evaluates to the
4906 empty string always returns the empty list, regardless of the LIMIT
4909 A pattern matching the null string (not to be confused with
4910 a null pattern C<//>, which is just one member of the set of patterns
4911 matching a null string) will split the value of EXPR into separate
4912 characters at each point it matches that way. For example:
4914 print join(':', split(/ */, 'hi there'));
4916 produces the output 'h:i:t:h:e:r:e'.
4918 Using the empty pattern C<//> specifically matches the null string, and is
4919 not be confused with the use of C<//> to mean "the last successful pattern
4922 Empty leading (or trailing) fields are produced when there are positive width
4923 matches at the beginning (or end) of the string; a zero-width match at the
4924 beginning (or end) of the string does not produce an empty field. For
4927 print join(':', split(/(?=\w)/, 'hi there!'));
4929 produces the output 'h:i :t:h:e:r:e!'.
4931 The LIMIT parameter can be used to split a line partially
4933 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4935 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
4936 a LIMIT one larger than the number of variables in the list, to avoid
4937 unnecessary work. For the list above LIMIT would have been 4 by
4938 default. In time critical applications it behooves you not to split
4939 into more fields than you really need.
4941 If the PATTERN contains parentheses, additional list elements are
4942 created from each matching substring in the delimiter.
4944 split(/([,-])/, "1-10,20", 3);
4946 produces the list value
4948 (1, '-', 10, ',', 20)
4950 If you had the entire header of a normal Unix email message in $header,
4951 you could split it up into fields and their values this way:
4953 $header =~ s/\n\s+/ /g; # fix continuation lines
4954 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4956 The pattern C</PATTERN/> may be replaced with an expression to specify
4957 patterns that vary at runtime. (To do runtime compilation only once,
4958 use C</$variable/o>.)
4960 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4961 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4962 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4963 will give you as many null initial fields as there are leading spaces.
4964 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4965 whitespace produces a null first field. A C<split> with no arguments
4966 really does a S<C<split(' ', $_)>> internally.
4968 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4973 open(PASSWD, '/etc/passwd');
4976 ($login, $passwd, $uid, $gid,
4977 $gcos, $home, $shell) = split(/:/);
4981 As with regular pattern matching, any capturing parentheses that are not
4982 matched in a C<split()> will be set to C<undef> when returned:
4984 @fields = split /(A)|B/, "1A2B3";
4985 # @fields is (1, 'A', 2, undef, 3)
4987 =item sprintf FORMAT, LIST
4989 Returns a string formatted by the usual C<printf> conventions of the C
4990 library function C<sprintf>. See below for more details
4991 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4992 the general principles.
4996 # Format number with up to 8 leading zeroes
4997 $result = sprintf("%08d", $number);
4999 # Round number to 3 digits after decimal point
5000 $rounded = sprintf("%.3f", $number);
5002 Perl does its own C<sprintf> formatting--it emulates the C
5003 function C<sprintf>, but it doesn't use it (except for floating-point
5004 numbers, and even then only the standard modifiers are allowed). As a
5005 result, any non-standard extensions in your local C<sprintf> are not
5006 available from Perl.
5008 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5009 pass it an array as your first argument. The array is given scalar context,
5010 and instead of using the 0th element of the array as the format, Perl will
5011 use the count of elements in the array as the format, which is almost never
5014 Perl's C<sprintf> permits the following universally-known conversions:
5017 %c a character with the given number
5019 %d a signed integer, in decimal
5020 %u an unsigned integer, in decimal
5021 %o an unsigned integer, in octal
5022 %x an unsigned integer, in hexadecimal
5023 %e a floating-point number, in scientific notation
5024 %f a floating-point number, in fixed decimal notation
5025 %g a floating-point number, in %e or %f notation
5027 In addition, Perl permits the following widely-supported conversions:
5029 %X like %x, but using upper-case letters
5030 %E like %e, but using an upper-case "E"
5031 %G like %g, but with an upper-case "E" (if applicable)
5032 %b an unsigned integer, in binary
5033 %p a pointer (outputs the Perl value's address in hexadecimal)
5034 %n special: *stores* the number of characters output so far
5035 into the next variable in the parameter list
5037 Finally, for backward (and we do mean "backward") compatibility, Perl
5038 permits these unnecessary but widely-supported conversions:
5041 %D a synonym for %ld
5042 %U a synonym for %lu
5043 %O a synonym for %lo
5046 Note that the number of exponent digits in the scientific notation produced
5047 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5048 exponent less than 100 is system-dependent: it may be three or less
5049 (zero-padded as necessary). In other words, 1.23 times ten to the
5050 99th may be either "1.23e99" or "1.23e099".
5052 Between the C<%> and the format letter, you may specify a number of
5053 additional attributes controlling the interpretation of the format.
5054 In order, these are:
5058 =item format parameter index
5060 An explicit format parameter index, such as C<2$>. By default sprintf
5061 will format the next unused argument in the list, but this allows you
5062 to take the arguments out of order. Eg:
5064 printf '%2$d %1$d', 12, 34; # prints "34 12"
5065 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5070 space prefix positive number with a space
5071 + prefix positive number with a plus sign
5072 - left-justify within the field
5073 0 use zeros, not spaces, to right-justify
5074 # prefix non-zero octal with "0", non-zero hex with "0x",
5075 non-zero binary with "0b"
5079 printf '<% d>', 12; # prints "< 12>"
5080 printf '<%+d>', 12; # prints "<+12>"
5081 printf '<%6s>', 12; # prints "< 12>"
5082 printf '<%-6s>', 12; # prints "<12 >"
5083 printf '<%06s>', 12; # prints "<000012>"
5084 printf '<%#x>', 12; # prints "<0xc>"
5088 The vector flag C<v>, optionally specifying the join string to use.
5089 This flag tells perl to interpret the supplied string as a vector
5090 of integers, one for each character in the string, separated by
5091 a given string (a dot C<.> by default). This can be useful for
5092 displaying ordinal values of characters in arbitrary strings:
5094 printf "version is v%vd\n", $^V; # Perl's version
5096 Put an asterisk C<*> before the C<v> to override the string to
5097 use to separate the numbers:
5099 printf "address is %*vX\n", ":", $addr; # IPv6 address
5100 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5102 You can also explicitly specify the argument number to use for
5103 the join string using eg C<*2$v>:
5105 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5107 =item (minimum) width
5109 Arguments are usually formatted to be only as wide as required to
5110 display the given value. You can override the width by putting
5111 a number here, or get the width from the next argument (with C<*>)
5112 or from a specified argument (with eg C<*2$>):
5114 printf '<%s>', "a"; # prints "<a>"
5115 printf '<%6s>', "a"; # prints "< a>"
5116 printf '<%*s>', 6, "a"; # prints "< a>"
5117 printf '<%*2$s>', "a", 6; # prints "< a>"
5118 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5120 If a field width obtained through C<*> is negative, it has the same
5121 effect as the C<-> flag: left-justification.
5123 =item precision, or maximum width
5125 You can specify a precision (for numeric conversions) or a maximum
5126 width (for string conversions) by specifying a C<.> followed by a number.
5127 For floating point formats, with the exception of 'g' and 'G', this specifies
5128 the number of decimal places to show (the default being 6), eg:
5130 # these examples are subject to system-specific variation
5131 printf '<%f>', 1; # prints "<1.000000>"
5132 printf '<%.1f>', 1; # prints "<1.0>"
5133 printf '<%.0f>', 1; # prints "<1>"
5134 printf '<%e>', 10; # prints "<1.000000e+01>"
5135 printf '<%.1e>', 10; # prints "<1.0e+01>"
5137 For 'g' and 'G', this specifies the maximum number of digits to show,
5138 including prior to the decimal point as well as after it, eg:
5140 # these examples are subject to system-specific variation
5141 printf '<%g>', 1; # prints "<1>"
5142 printf '<%.10g>', 1; # prints "<1>"
5143 printf '<%g>', 100; # prints "<100>"
5144 printf '<%.1g>', 100; # prints "<1e+02>"
5145 printf '<%.2g>', 100.01; # prints "<1e+02>"
5146 printf '<%.5g>', 100.01; # prints "<100.01>"
5147 printf '<%.4g>', 100.01; # prints "<100>"
5149 For integer conversions, specifying a precision implies that the
5150 output of the number itself should be zero-padded to this width:
5152 printf '<%.6x>', 1; # prints "<000001>"
5153 printf '<%#.6x>', 1; # prints "<0x000001>"
5154 printf '<%-10.6x>', 1; # prints "<000001 >"
5156 For string conversions, specifying a precision truncates the string
5157 to fit in the specified width:
5159 printf '<%.5s>', "truncated"; # prints "<trunc>"
5160 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5162 You can also get the precision from the next argument using C<.*>:
5164 printf '<%.6x>', 1; # prints "<000001>"
5165 printf '<%.*x>', 6, 1; # prints "<000001>"
5167 You cannot currently get the precision from a specified number,
5168 but it is intended that this will be possible in the future using
5171 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5175 For numeric conversions, you can specify the size to interpret the
5176 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5177 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5178 whatever the default integer size is on your platform (usually 32 or 64
5179 bits), but you can override this to use instead one of the standard C types,
5180 as supported by the compiler used to build Perl:
5182 l interpret integer as C type "long" or "unsigned long"
5183 h interpret integer as C type "short" or "unsigned short"
5184 q, L or ll interpret integer as C type "long long", "unsigned long long".
5185 or "quads" (typically 64-bit integers)
5187 The last will produce errors if Perl does not understand "quads" in your
5188 installation. (This requires that either the platform natively supports quads
5189 or Perl was specifically compiled to support quads.) You can find out
5190 whether your Perl supports quads via L<Config>:
5193 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5196 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5197 to be the default floating point size on your platform (double or long double),
5198 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5199 platform supports them. You can find out whether your Perl supports long
5200 doubles via L<Config>:
5203 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5205 You can find out whether Perl considers 'long double' to be the default
5206 floating point size to use on your platform via L<Config>:
5209 ($Config{uselongdouble} eq 'define') &&
5210 print "long doubles by default\n";
5212 It can also be the case that long doubles and doubles are the same thing:
5215 ($Config{doublesize} == $Config{longdblsize}) &&
5216 print "doubles are long doubles\n";
5218 The size specifier C<V> has no effect for Perl code, but it is supported
5219 for compatibility with XS code; it means 'use the standard size for
5220 a Perl integer (or floating-point number)', which is already the
5221 default for Perl code.
5223 =item order of arguments
5225 Normally, sprintf takes the next unused argument as the value to
5226 format for each format specification. If the format specification
5227 uses C<*> to require additional arguments, these are consumed from
5228 the argument list in the order in which they appear in the format
5229 specification I<before> the value to format. Where an argument is
5230 specified using an explicit index, this does not affect the normal
5231 order for the arguments (even when the explicitly specified index
5232 would have been the next argument in any case).
5236 printf '<%*.*s>', $a, $b, $c;
5238 would use C<$a> for the width, C<$b> for the precision and C<$c>
5239 as the value to format, while:
5241 print '<%*1$.*s>', $a, $b;
5243 would use C<$a> for the width and the precision, and C<$b> as the
5246 Here are some more examples - beware that when using an explicit
5247 index, the C<$> may need to be escaped:
5249 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5250 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5251 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5252 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5256 If C<use locale> is in effect, the character used for the decimal
5257 point in formatted real numbers is affected by the LC_NUMERIC locale.
5264 Return the square root of EXPR. If EXPR is omitted, returns square
5265 root of C<$_>. Only works on non-negative operands, unless you've
5266 loaded the standard Math::Complex module.
5269 print sqrt(-2); # prints 1.4142135623731i
5275 Sets the random number seed for the C<rand> operator.
5277 The point of the function is to "seed" the C<rand> function so that
5278 C<rand> can produce a different sequence each time you run your
5281 If srand() is not called explicitly, it is called implicitly at the
5282 first use of the C<rand> operator. However, this was not the case in
5283 versions of Perl before 5.004, so if your script will run under older
5284 Perl versions, it should call C<srand>.
5286 Most programs won't even call srand() at all, except those that
5287 need a cryptographically-strong starting point rather than the
5288 generally acceptable default, which is based on time of day,
5289 process ID, and memory allocation, or the F</dev/urandom> device,
5292 You can call srand($seed) with the same $seed to reproduce the
5293 I<same> sequence from rand(), but this is usually reserved for
5294 generating predictable results for testing or debugging.
5295 Otherwise, don't call srand() more than once in your program.
5297 Do B<not> call srand() (i.e. without an argument) more than once in
5298 a script. The internal state of the random number generator should
5299 contain more entropy than can be provided by any seed, so calling
5300 srand() again actually I<loses> randomness.
5302 Most implementations of C<srand> take an integer and will silently
5303 truncate decimal numbers. This means C<srand(42)> will usually
5304 produce the same results as C<srand(42.1)>. To be safe, always pass
5305 C<srand> an integer.
5307 In versions of Perl prior to 5.004 the default seed was just the
5308 current C<time>. This isn't a particularly good seed, so many old
5309 programs supply their own seed value (often C<time ^ $$> or C<time ^
5310 ($$ + ($$ << 15))>), but that isn't necessary any more.
5312 Note that you need something much more random than the default seed for
5313 cryptographic purposes. Checksumming the compressed output of one or more
5314 rapidly changing operating system status programs is the usual method. For
5317 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5319 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5322 Frequently called programs (like CGI scripts) that simply use
5326 for a seed can fall prey to the mathematical property that
5330 one-third of the time. So don't do that.
5332 =item stat FILEHANDLE
5338 Returns a 13-element list giving the status info for a file, either
5339 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5340 it stats C<$_>. Returns a null list if the stat fails. Typically used
5343 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5344 $atime,$mtime,$ctime,$blksize,$blocks)
5347 Not all fields are supported on all filesystem types. Here are the
5348 meaning of the fields:
5350 0 dev device number of filesystem
5352 2 mode file mode (type and permissions)
5353 3 nlink number of (hard) links to the file
5354 4 uid numeric user ID of file's owner
5355 5 gid numeric group ID of file's owner
5356 6 rdev the device identifier (special files only)
5357 7 size total size of file, in bytes
5358 8 atime last access time in seconds since the epoch
5359 9 mtime last modify time in seconds since the epoch
5360 10 ctime inode change time in seconds since the epoch (*)
5361 11 blksize preferred block size for file system I/O
5362 12 blocks actual number of blocks allocated
5364 (The epoch was at 00:00 January 1, 1970 GMT.)
5366 (*) The ctime field is non-portable, in particular you cannot expect
5367 it to be a "creation time", see L<perlport/"Files and Filesystems">
5370 If stat is passed the special filehandle consisting of an underline, no
5371 stat is done, but the current contents of the stat structure from the
5372 last stat or filetest are returned. Example:
5374 if (-x $file && (($d) = stat(_)) && $d < 0) {
5375 print "$file is executable NFS file\n";
5378 (This works on machines only for which the device number is negative
5381 Because the mode contains both the file type and its permissions, you
5382 should mask off the file type portion and (s)printf using a C<"%o">
5383 if you want to see the real permissions.
5385 $mode = (stat($filename))[2];
5386 printf "Permissions are %04o\n", $mode & 07777;
5388 In scalar context, C<stat> returns a boolean value indicating success
5389 or failure, and, if successful, sets the information associated with
5390 the special filehandle C<_>.
5392 The File::stat module provides a convenient, by-name access mechanism:
5395 $sb = stat($filename);
5396 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5397 $filename, $sb->size, $sb->mode & 07777,
5398 scalar localtime $sb->mtime;
5400 You can import symbolic mode constants (C<S_IF*>) and functions
5401 (C<S_IS*>) from the Fcntl module:
5405 $mode = (stat($filename))[2];
5407 $user_rwx = ($mode & S_IRWXU) >> 6;
5408 $group_read = ($mode & S_IRGRP) >> 3;
5409 $other_execute = $mode & S_IXOTH;
5411 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5413 $is_setuid = $mode & S_ISUID;
5414 $is_setgid = S_ISDIR($mode);
5416 You could write the last two using the C<-u> and C<-d> operators.
5417 The commonly available S_IF* constants are
5419 # Permissions: read, write, execute, for user, group, others.
5421 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5422 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5423 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5425 # Setuid/Setgid/Stickiness/SaveText.
5426 # Note that the exact meaning of these is system dependent.
5428 S_ISUID S_ISGID S_ISVTX S_ISTXT
5430 # File types. Not necessarily all are available on your system.
5432 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5434 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5436 S_IREAD S_IWRITE S_IEXEC
5438 and the S_IF* functions are
5440 S_IMODE($mode) the part of $mode containing the permission bits
5441 and the setuid/setgid/sticky bits
5443 S_IFMT($mode) the part of $mode containing the file type
5444 which can be bit-anded with e.g. S_IFREG
5445 or with the following functions
5447 # The operators -f, -d, -l, -b, -c, -p, and -s.
5449 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5450 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5452 # No direct -X operator counterpart, but for the first one
5453 # the -g operator is often equivalent. The ENFMT stands for
5454 # record flocking enforcement, a platform-dependent feature.
5456 S_ISENFMT($mode) S_ISWHT($mode)
5458 See your native chmod(2) and stat(2) documentation for more details
5459 about the S_* constants. To get status info for a symbolic link
5460 instead of the target file behind the link, use the C<lstat> function.
5466 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5467 doing many pattern matches on the string before it is next modified.
5468 This may or may not save time, depending on the nature and number of
5469 patterns you are searching on, and on the distribution of character
5470 frequencies in the string to be searched--you probably want to compare
5471 run times with and without it to see which runs faster. Those loops
5472 which scan for many short constant strings (including the constant
5473 parts of more complex patterns) will benefit most. You may have only
5474 one C<study> active at a time--if you study a different scalar the first
5475 is "unstudied". (The way C<study> works is this: a linked list of every
5476 character in the string to be searched is made, so we know, for
5477 example, where all the C<'k'> characters are. From each search string,
5478 the rarest character is selected, based on some static frequency tables
5479 constructed from some C programs and English text. Only those places
5480 that contain this "rarest" character are examined.)
5482 For example, here is a loop that inserts index producing entries
5483 before any line containing a certain pattern:
5487 print ".IX foo\n" if /\bfoo\b/;
5488 print ".IX bar\n" if /\bbar\b/;
5489 print ".IX blurfl\n" if /\bblurfl\b/;
5494 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5495 will be looked at, because C<f> is rarer than C<o>. In general, this is
5496 a big win except in pathological cases. The only question is whether
5497 it saves you more time than it took to build the linked list in the
5500 Note that if you have to look for strings that you don't know till
5501 runtime, you can build an entire loop as a string and C<eval> that to
5502 avoid recompiling all your patterns all the time. Together with
5503 undefining C<$/> to input entire files as one record, this can be very
5504 fast, often faster than specialized programs like fgrep(1). The following
5505 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5506 out the names of those files that contain a match:
5508 $search = 'while (<>) { study;';
5509 foreach $word (@words) {
5510 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5515 eval $search; # this screams
5516 $/ = "\n"; # put back to normal input delimiter
5517 foreach $file (sort keys(%seen)) {
5521 =item sub NAME BLOCK
5523 =item sub NAME (PROTO) BLOCK
5525 =item sub NAME : ATTRS BLOCK
5527 =item sub NAME (PROTO) : ATTRS BLOCK
5529 This is subroutine definition, not a real function I<per se>.
5530 Without a BLOCK it's just a forward declaration. Without a NAME,
5531 it's an anonymous function declaration, and does actually return
5532 a value: the CODE ref of the closure you just created.
5534 See L<perlsub> and L<perlref> for details about subroutines and
5535 references, and L<attributes> and L<Attribute::Handlers> for more
5536 information about attributes.
5538 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5540 =item substr EXPR,OFFSET,LENGTH
5542 =item substr EXPR,OFFSET
5544 Extracts a substring out of EXPR and returns it. First character is at
5545 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5546 If OFFSET is negative (or more precisely, less than C<$[>), starts
5547 that far from the end of the string. If LENGTH is omitted, returns
5548 everything to the end of the string. If LENGTH is negative, leaves that
5549 many characters off the end of the string.
5551 You can use the substr() function as an lvalue, in which case EXPR
5552 must itself be an lvalue. If you assign something shorter than LENGTH,
5553 the string will shrink, and if you assign something longer than LENGTH,
5554 the string will grow to accommodate it. To keep the string the same
5555 length you may need to pad or chop your value using C<sprintf>.
5557 If OFFSET and LENGTH specify a substring that is partly outside the
5558 string, only the part within the string is returned. If the substring
5559 is beyond either end of the string, substr() returns the undefined
5560 value and produces a warning. When used as an lvalue, specifying a
5561 substring that is entirely outside the string is a fatal error.
5562 Here's an example showing the behavior for boundary cases:
5565 substr($name, 4) = 'dy'; # $name is now 'freddy'
5566 my $null = substr $name, 6, 2; # returns '' (no warning)
5567 my $oops = substr $name, 7; # returns undef, with warning
5568 substr($name, 7) = 'gap'; # fatal error
5570 An alternative to using substr() as an lvalue is to specify the
5571 replacement string as the 4th argument. This allows you to replace
5572 parts of the EXPR and return what was there before in one operation,
5573 just as you can with splice().
5575 If the lvalue returned by substr is used after the EXPR is changed in
5576 any way, the behaviour may not be as expected and is subject to change.
5577 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5578 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5579 substring assignment, and then the substr is used again), or where a
5580 substr() is aliased via a C<foreach> loop or passed as a parameter or
5581 a reference to it is taken and then the alias, parameter, or deref'd
5582 reference either is used after the original EXPR has been changed or
5583 is assigned to and then used a second time.
5585 =item symlink OLDFILE,NEWFILE
5587 Creates a new filename symbolically linked to the old filename.
5588 Returns C<1> for success, C<0> otherwise. On systems that don't support
5589 symbolic links, produces a fatal error at run time. To check for that,
5592 $symlink_exists = eval { symlink("",""); 1 };
5594 =item syscall NUMBER, LIST
5596 Calls the system call specified as the first element of the list,
5597 passing the remaining elements as arguments to the system call. If
5598 unimplemented, produces a fatal error. The arguments are interpreted
5599 as follows: if a given argument is numeric, the argument is passed as
5600 an int. If not, the pointer to the string value is passed. You are
5601 responsible to make sure a string is pre-extended long enough to
5602 receive any result that might be written into a string. You can't use a
5603 string literal (or other read-only string) as an argument to C<syscall>
5604 because Perl has to assume that any string pointer might be written
5606 integer arguments are not literals and have never been interpreted in a
5607 numeric context, you may need to add C<0> to them to force them to look
5608 like numbers. This emulates the C<syswrite> function (or vice versa):
5610 require 'syscall.ph'; # may need to run h2ph
5612 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5614 Note that Perl supports passing of up to only 14 arguments to your system call,
5615 which in practice should usually suffice.
5617 Syscall returns whatever value returned by the system call it calls.
5618 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5619 Note that some system calls can legitimately return C<-1>. The proper
5620 way to handle such calls is to assign C<$!=0;> before the call and
5621 check the value of C<$!> if syscall returns C<-1>.
5623 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5624 number of the read end of the pipe it creates. There is no way
5625 to retrieve the file number of the other end. You can avoid this
5626 problem by using C<pipe> instead.
5628 =item sysopen FILEHANDLE,FILENAME,MODE
5630 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5632 Opens the file whose filename is given by FILENAME, and associates it
5633 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5634 the name of the real filehandle wanted. This function calls the
5635 underlying operating system's C<open> function with the parameters
5636 FILENAME, MODE, PERMS.
5638 The possible values and flag bits of the MODE parameter are
5639 system-dependent; they are available via the standard module C<Fcntl>.
5640 See the documentation of your operating system's C<open> to see which
5641 values and flag bits are available. You may combine several flags
5642 using the C<|>-operator.
5644 Some of the most common values are C<O_RDONLY> for opening the file in
5645 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5646 and C<O_RDWR> for opening the file in read-write mode, and.
5648 For historical reasons, some values work on almost every system
5649 supported by perl: zero means read-only, one means write-only, and two
5650 means read/write. We know that these values do I<not> work under
5651 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5652 use them in new code.
5654 If the file named by FILENAME does not exist and the C<open> call creates
5655 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5656 PERMS specifies the permissions of the newly created file. If you omit
5657 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5658 These permission values need to be in octal, and are modified by your
5659 process's current C<umask>.
5661 In many systems the C<O_EXCL> flag is available for opening files in
5662 exclusive mode. This is B<not> locking: exclusiveness means here that
5663 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5666 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5668 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5669 that takes away the user's option to have a more permissive umask.
5670 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5673 Note that C<sysopen> depends on the fdopen() C library function.
5674 On many UNIX systems, fdopen() is known to fail when file descriptors
5675 exceed a certain value, typically 255. If you need more file
5676 descriptors than that, consider rebuilding Perl to use the C<sfio>
5677 library, or perhaps using the POSIX::open() function.
5679 See L<perlopentut> for a kinder, gentler explanation of opening files.
5681 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5683 =item sysread FILEHANDLE,SCALAR,LENGTH
5685 Attempts to read LENGTH bytes of data into variable SCALAR from the
5686 specified FILEHANDLE, using the system call read(2). It bypasses
5687 buffered IO, so mixing this with other kinds of reads, C<print>,
5688 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5689 perlio or stdio layers usually buffers data. Returns the number of
5690 bytes actually read, C<0> at end of file, or undef if there was an
5691 error (in the latter case C<$!> is also set). SCALAR will be grown or
5692 shrunk so that the last byte actually read is the last byte of the
5693 scalar after the read.
5695 An OFFSET may be specified to place the read data at some place in the
5696 string other than the beginning. A negative OFFSET specifies
5697 placement at that many characters counting backwards from the end of
5698 the string. A positive OFFSET greater than the length of SCALAR
5699 results in the string being padded to the required size with C<"\0">
5700 bytes before the result of the read is appended.
5702 There is no syseof() function, which is ok, since eof() doesn't work
5703 very well on device files (like ttys) anyway. Use sysread() and check
5704 for a return value for 0 to decide whether you're done.
5706 Note that if the filehandle has been marked as C<:utf8> Unicode
5707 characters are read instead of bytes (the LENGTH, OFFSET, and the
5708 return value of sysread() are in Unicode characters).
5709 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5710 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5712 =item sysseek FILEHANDLE,POSITION,WHENCE
5714 Sets FILEHANDLE's system position in bytes using the system call
5715 lseek(2). FILEHANDLE may be an expression whose value gives the name
5716 of the filehandle. The values for WHENCE are C<0> to set the new
5717 position to POSITION, C<1> to set the it to the current position plus
5718 POSITION, and C<2> to set it to EOF plus POSITION (typically
5721 Note the I<in bytes>: even if the filehandle has been set to operate
5722 on characters (for example by using the C<:utf8> I/O layer), tell()
5723 will return byte offsets, not character offsets (because implementing
5724 that would render sysseek() very slow).
5726 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5727 than C<sysread>, for example >< or read()) C<print>, C<write>,
5728 C<seek>, C<tell>, or C<eof> may cause confusion.
5730 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5731 and C<SEEK_END> (start of the file, current position, end of the file)
5732 from the Fcntl module. Use of the constants is also more portable
5733 than relying on 0, 1, and 2. For example to define a "systell" function:
5735 use Fcntl 'SEEK_CUR';
5736 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5738 Returns the new position, or the undefined value on failure. A position
5739 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5740 true on success and false on failure, yet you can still easily determine
5745 =item system PROGRAM LIST
5747 Does exactly the same thing as C<exec LIST>, except that a fork is
5748 done first, and the parent process waits for the child process to
5749 complete. Note that argument processing varies depending on the
5750 number of arguments. If there is more than one argument in LIST,
5751 or if LIST is an array with more than one value, starts the program
5752 given by the first element of the list with arguments given by the
5753 rest of the list. If there is only one scalar argument, the argument
5754 is checked for shell metacharacters, and if there are any, the
5755 entire argument is passed to the system's command shell for parsing
5756 (this is C</bin/sh -c> on Unix platforms, but varies on other
5757 platforms). If there are no shell metacharacters in the argument,
5758 it is split into words and passed directly to C<execvp>, which is
5761 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5762 output before any operation that may do a fork, but this may not be
5763 supported on some platforms (see L<perlport>). To be safe, you may need
5764 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5765 of C<IO::Handle> on any open handles.
5767 The return value is the exit status of the program as returned by the
5768 C<wait> call. To get the actual exit value shift right by eight (see below).
5769 See also L</exec>. This is I<not> what you want to use to capture
5770 the output from a command, for that you should use merely backticks or
5771 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5772 indicates a failure to start the program (inspect $! for the reason).
5774 Like C<exec>, C<system> allows you to lie to a program about its name if
5775 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5777 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
5778 C<system>, if you expect your program to terminate on receipt of these
5779 signals you will need to arrange to do so yourself based on the return
5782 @args = ("command", "arg1", "arg2");
5784 or die "system @args failed: $?"
5786 You can check all the failure possibilities by inspecting
5790 print "failed to execute: $!\n";
5793 printf "child died with signal %d, %s coredump\n",
5794 ($? & 127), ($? & 128) ? 'with' : 'without';
5797 printf "child exited with value %d\n", $? >> 8;
5800 or more portably by using the W*() calls of the POSIX extension;
5801 see L<perlport> for more information.
5803 When the arguments get executed via the system shell, results
5804 and return codes will be subject to its quirks and capabilities.
5805 See L<perlop/"`STRING`"> and L</exec> for details.
5807 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5809 =item syswrite FILEHANDLE,SCALAR,LENGTH
5811 =item syswrite FILEHANDLE,SCALAR
5813 Attempts to write LENGTH bytes of data from variable SCALAR to the
5814 specified FILEHANDLE, using the system call write(2). If LENGTH is
5815 not specified, writes whole SCALAR. It bypasses buffered IO, so
5816 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5817 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5818 stdio layers usually buffers data. Returns the number of bytes
5819 actually written, or C<undef> if there was an error (in this case the
5820 errno variable C<$!> is also set). If the LENGTH is greater than the
5821 available data in the SCALAR after the OFFSET, only as much data as is
5822 available will be written.
5824 An OFFSET may be specified to write the data from some part of the
5825 string other than the beginning. A negative OFFSET specifies writing
5826 that many characters counting backwards from the end of the string.
5827 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5829 Note that if the filehandle has been marked as C<:utf8>, Unicode
5830 characters are written instead of bytes (the LENGTH, OFFSET, and the
5831 return value of syswrite() are in UTF-8 encoded Unicode characters).
5832 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5833 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5835 =item tell FILEHANDLE
5839 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5840 error. FILEHANDLE may be an expression whose value gives the name of
5841 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5844 Note the I<in bytes>: even if the filehandle has been set to
5845 operate on characters (for example by using the C<:utf8> open
5846 layer), tell() will return byte offsets, not character offsets
5847 (because that would render seek() and tell() rather slow).
5849 The return value of tell() for the standard streams like the STDIN
5850 depends on the operating system: it may return -1 or something else.
5851 tell() on pipes, fifos, and sockets usually returns -1.
5853 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5855 Do not use tell() on a filehandle that has been opened using
5856 sysopen(), use sysseek() for that as described above. Why? Because
5857 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5858 buffered filehandles. sysseek() make sense only on the first kind,
5859 tell() only makes sense on the second kind.
5861 =item telldir DIRHANDLE
5863 Returns the current position of the C<readdir> routines on DIRHANDLE.
5864 Value may be given to C<seekdir> to access a particular location in a
5865 directory. Has the same caveats about possible directory compaction as
5866 the corresponding system library routine.
5868 =item tie VARIABLE,CLASSNAME,LIST
5870 This function binds a variable to a package class that will provide the
5871 implementation for the variable. VARIABLE is the name of the variable
5872 to be enchanted. CLASSNAME is the name of a class implementing objects
5873 of correct type. Any additional arguments are passed to the C<new>
5874 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5875 or C<TIEHASH>). Typically these are arguments such as might be passed
5876 to the C<dbm_open()> function of C. The object returned by the C<new>
5877 method is also returned by the C<tie> function, which would be useful
5878 if you want to access other methods in CLASSNAME.
5880 Note that functions such as C<keys> and C<values> may return huge lists
5881 when used on large objects, like DBM files. You may prefer to use the
5882 C<each> function to iterate over such. Example:
5884 # print out history file offsets
5886 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5887 while (($key,$val) = each %HIST) {
5888 print $key, ' = ', unpack('L',$val), "\n";
5892 A class implementing a hash should have the following methods:
5894 TIEHASH classname, LIST
5896 STORE this, key, value
5901 NEXTKEY this, lastkey
5906 A class implementing an ordinary array should have the following methods:
5908 TIEARRAY classname, LIST
5910 STORE this, key, value
5912 STORESIZE this, count
5918 SPLICE this, offset, length, LIST
5923 A class implementing a file handle should have the following methods:
5925 TIEHANDLE classname, LIST
5926 READ this, scalar, length, offset
5929 WRITE this, scalar, length, offset
5931 PRINTF this, format, LIST
5935 SEEK this, position, whence
5937 OPEN this, mode, LIST
5942 A class implementing a scalar should have the following methods:
5944 TIESCALAR classname, LIST
5950 Not all methods indicated above need be implemented. See L<perltie>,
5951 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5953 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5954 for you--you need to do that explicitly yourself. See L<DB_File>
5955 or the F<Config> module for interesting C<tie> implementations.
5957 For further details see L<perltie>, L<"tied VARIABLE">.
5961 Returns a reference to the object underlying VARIABLE (the same value
5962 that was originally returned by the C<tie> call that bound the variable
5963 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5968 Returns the number of non-leap seconds since whatever time the system
5969 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5970 and 00:00:00 UTC, January 1, 1970 for most other systems).
5971 Suitable for feeding to C<gmtime> and C<localtime>.
5973 For measuring time in better granularity than one second,
5974 you may use either the Time::HiRes module (from CPAN, and starting from
5975 Perl 5.8 part of the standard distribution), or if you have
5976 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5977 See L<perlfaq8> for details.
5981 Returns a four-element list giving the user and system times, in
5982 seconds, for this process and the children of this process.
5984 ($user,$system,$cuser,$csystem) = times;
5986 In scalar context, C<times> returns C<$user>.
5990 The transliteration operator. Same as C<y///>. See L<perlop>.
5992 =item truncate FILEHANDLE,LENGTH
5994 =item truncate EXPR,LENGTH
5996 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5997 specified length. Produces a fatal error if truncate isn't implemented
5998 on your system. Returns true if successful, the undefined value
6001 The behavior is undefined if LENGTH is greater than the length of the
6008 Returns an uppercased version of EXPR. This is the internal function
6009 implementing the C<\U> escape in double-quoted strings. Respects
6010 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6011 and L<perlunicode> for more details about locale and Unicode support.
6012 It does not attempt to do titlecase mapping on initial letters. See
6013 C<ucfirst> for that.
6015 If EXPR is omitted, uses C<$_>.
6021 Returns the value of EXPR with the first character in uppercase
6022 (titlecase in Unicode). This is the internal function implementing
6023 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6024 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6025 for more details about locale and Unicode support.
6027 If EXPR is omitted, uses C<$_>.
6033 Sets the umask for the process to EXPR and returns the previous value.
6034 If EXPR is omitted, merely returns the current umask.
6036 The Unix permission C<rwxr-x---> is represented as three sets of three
6037 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6038 and isn't one of the digits). The C<umask> value is such a number
6039 representing disabled permissions bits. The permission (or "mode")
6040 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6041 even if you tell C<sysopen> to create a file with permissions C<0777>,
6042 if your umask is C<0022> then the file will actually be created with
6043 permissions C<0755>. If your C<umask> were C<0027> (group can't
6044 write; others can't read, write, or execute), then passing
6045 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6048 Here's some advice: supply a creation mode of C<0666> for regular
6049 files (in C<sysopen>) and one of C<0777> for directories (in
6050 C<mkdir>) and executable files. This gives users the freedom of
6051 choice: if they want protected files, they might choose process umasks
6052 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6053 Programs should rarely if ever make policy decisions better left to
6054 the user. The exception to this is when writing files that should be
6055 kept private: mail files, web browser cookies, I<.rhosts> files, and
6058 If umask(2) is not implemented on your system and you are trying to
6059 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6060 fatal error at run time. If umask(2) is not implemented and you are
6061 not trying to restrict access for yourself, returns C<undef>.
6063 Remember that a umask is a number, usually given in octal; it is I<not> a
6064 string of octal digits. See also L</oct>, if all you have is a string.
6070 Undefines the value of EXPR, which must be an lvalue. Use only on a
6071 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6072 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6073 will probably not do what you expect on most predefined variables or
6074 DBM list values, so don't do that; see L<delete>.) Always returns the
6075 undefined value. You can omit the EXPR, in which case nothing is
6076 undefined, but you still get an undefined value that you could, for
6077 instance, return from a subroutine, assign to a variable or pass as a
6078 parameter. Examples:
6081 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6085 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6086 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6087 select undef, undef, undef, 0.25;
6088 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6090 Note that this is a unary operator, not a list operator.
6096 Deletes a list of files. Returns the number of files successfully
6099 $cnt = unlink 'a', 'b', 'c';
6103 Note: C<unlink> will not delete directories unless you are superuser and
6104 the B<-U> flag is supplied to Perl. Even if these conditions are
6105 met, be warned that unlinking a directory can inflict damage on your
6106 filesystem. Use C<rmdir> instead.
6108 If LIST is omitted, uses C<$_>.
6110 =item unpack TEMPLATE,EXPR
6112 =item unpack TEMPLATE
6114 C<unpack> does the reverse of C<pack>: it takes a string
6115 and expands it out into a list of values.
6116 (In scalar context, it returns merely the first value produced.)
6118 If EXPR is omitted, unpacks the C<$_> string.
6120 The string is broken into chunks described by the TEMPLATE. Each chunk
6121 is converted separately to a value. Typically, either the string is a result
6122 of C<pack>, or the bytes of the string represent a C structure of some
6125 The TEMPLATE has the same format as in the C<pack> function.
6126 Here's a subroutine that does substring:
6129 my($what,$where,$howmuch) = @_;
6130 unpack("x$where a$howmuch", $what);
6135 sub ordinal { unpack("c",$_[0]); } # same as ord()
6137 In addition to fields allowed in pack(), you may prefix a field with
6138 a %<number> to indicate that
6139 you want a <number>-bit checksum of the items instead of the items
6140 themselves. Default is a 16-bit checksum. Checksum is calculated by
6141 summing numeric values of expanded values (for string fields the sum of
6142 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6144 For example, the following
6145 computes the same number as the System V sum program:
6149 unpack("%32C*",<>) % 65535;
6152 The following efficiently counts the number of set bits in a bit vector:
6154 $setbits = unpack("%32b*", $selectmask);
6156 The C<p> and C<P> formats should be used with care. Since Perl
6157 has no way of checking whether the value passed to C<unpack()>
6158 corresponds to a valid memory location, passing a pointer value that's
6159 not known to be valid is likely to have disastrous consequences.
6161 If there are more pack codes or if the repeat count of a field or a group
6162 is larger than what the remainder of the input string allows, the result
6163 is not well defined: in some cases, the repeat count is decreased, or
6164 C<unpack()> will produce null strings or zeroes, or terminate with an
6165 error. If the input string is longer than one described by the TEMPLATE,
6166 the rest is ignored.
6168 See L</pack> for more examples and notes.
6170 =item untie VARIABLE
6172 Breaks the binding between a variable and a package. (See C<tie>.)
6173 Has no effect if the variable is not tied.
6175 =item unshift ARRAY,LIST
6177 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6178 depending on how you look at it. Prepends list to the front of the
6179 array, and returns the new number of elements in the array.
6181 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6183 Note the LIST is prepended whole, not one element at a time, so the
6184 prepended elements stay in the same order. Use C<reverse> to do the
6187 =item use Module VERSION LIST
6189 =item use Module VERSION
6191 =item use Module LIST
6197 Imports some semantics into the current package from the named module,
6198 generally by aliasing certain subroutine or variable names into your
6199 package. It is exactly equivalent to
6201 BEGIN { require Module; import Module LIST; }
6203 except that Module I<must> be a bareword.
6205 VERSION may be either a numeric argument such as 5.006, which will be
6206 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6207 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6208 greater than the version of the current Perl interpreter; Perl will not
6209 attempt to parse the rest of the file. Compare with L</require>, which can
6210 do a similar check at run time.
6212 Specifying VERSION as a literal of the form v5.6.1 should generally be
6213 avoided, because it leads to misleading error messages under earlier
6214 versions of Perl which do not support this syntax. The equivalent numeric
6215 version should be used instead.
6217 use v5.6.1; # compile time version check
6219 use 5.006_001; # ditto; preferred for backwards compatibility
6221 This is often useful if you need to check the current Perl version before
6222 C<use>ing library modules that have changed in incompatible ways from
6223 older versions of Perl. (We try not to do this more than we have to.)
6225 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6226 C<require> makes sure the module is loaded into memory if it hasn't been
6227 yet. The C<import> is not a builtin--it's just an ordinary static method
6228 call into the C<Module> package to tell the module to import the list of
6229 features back into the current package. The module can implement its
6230 C<import> method any way it likes, though most modules just choose to
6231 derive their C<import> method via inheritance from the C<Exporter> class that
6232 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6233 method can be found then the call is skipped, even if there is an AUTOLOAD
6236 If you do not want to call the package's C<import> method (for instance,
6237 to stop your namespace from being altered), explicitly supply the empty list:
6241 That is exactly equivalent to
6243 BEGIN { require Module }
6245 If the VERSION argument is present between Module and LIST, then the
6246 C<use> will call the VERSION method in class Module with the given
6247 version as an argument. The default VERSION method, inherited from
6248 the UNIVERSAL class, croaks if the given version is larger than the
6249 value of the variable C<$Module::VERSION>.
6251 Again, there is a distinction between omitting LIST (C<import> called
6252 with no arguments) and an explicit empty LIST C<()> (C<import> not
6253 called). Note that there is no comma after VERSION!
6255 Because this is a wide-open interface, pragmas (compiler directives)
6256 are also implemented this way. Currently implemented pragmas are:
6261 use sigtrap qw(SEGV BUS);
6262 use strict qw(subs vars refs);
6263 use subs qw(afunc blurfl);
6264 use warnings qw(all);
6265 use sort qw(stable _quicksort _mergesort);
6267 Some of these pseudo-modules import semantics into the current
6268 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6269 which import symbols into the current package (which are effective
6270 through the end of the file).
6272 There's a corresponding C<no> command that unimports meanings imported
6273 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6274 It behaves exactly as C<import> does with respect to VERSION, an
6275 omitted LIST, empty LIST, or no unimport method being found.
6281 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6282 for the C<-M> and C<-m> command-line options to perl that give C<use>
6283 functionality from the command-line.
6287 Changes the access and modification times on each file of a list of
6288 files. The first two elements of the list must be the NUMERICAL access
6289 and modification times, in that order. Returns the number of files
6290 successfully changed. The inode change time of each file is set
6291 to the current time. For example, this code has the same effect as the
6292 Unix touch(1) command when the files I<already exist>.
6295 $atime = $mtime = time;
6296 utime $atime, $mtime, @ARGV;
6298 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6299 the utime(2) function in the C library will be called with a null second
6300 argument. On most systems, this will set the file's access and
6301 modification times to the current time (i.e. equivalent to the example
6304 utime undef, undef, @ARGV;
6306 Under NFS this will use the time of the NFS server, not the time of
6307 the local machine. If there is a time synchronization problem, the
6308 NFS server and local machine will have different times. The Unix
6309 touch(1) command will in fact normally use this form instead of the
6310 one shown in the first example.
6312 Note that only passing one of the first two elements as C<undef> will
6313 be equivalent of passing it as 0 and will not have the same effect as
6314 described when they are both C<undef>. This case will also trigger an
6315 uninitialized warning.
6319 Returns a list consisting of all the values of the named hash.
6320 (In a scalar context, returns the number of values.)
6322 The values are returned in an apparently random order. The actual
6323 random order is subject to change in future versions of perl, but it
6324 is guaranteed to be the same order as either the C<keys> or C<each>
6325 function would produce on the same (unmodified) hash. Since Perl
6326 5.8.1 the ordering is different even between different runs of Perl
6327 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6329 As a side effect, calling values() resets the HASH's internal iterator,
6330 see L</each>. (In particular, calling values() in void context resets
6331 the iterator with no other overhead.)
6333 Note that the values are not copied, which means modifying them will
6334 modify the contents of the hash:
6336 for (values %hash) { s/foo/bar/g } # modifies %hash values
6337 for (@hash{keys %hash}) { s/foo/bar/g } # same
6339 See also C<keys>, C<each>, and C<sort>.
6341 =item vec EXPR,OFFSET,BITS
6343 Treats the string in EXPR as a bit vector made up of elements of
6344 width BITS, and returns the value of the element specified by OFFSET
6345 as an unsigned integer. BITS therefore specifies the number of bits
6346 that are reserved for each element in the bit vector. This must
6347 be a power of two from 1 to 32 (or 64, if your platform supports
6350 If BITS is 8, "elements" coincide with bytes of the input string.
6352 If BITS is 16 or more, bytes of the input string are grouped into chunks
6353 of size BITS/8, and each group is converted to a number as with
6354 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6355 for BITS==64). See L<"pack"> for details.
6357 If bits is 4 or less, the string is broken into bytes, then the bits
6358 of each byte are broken into 8/BITS groups. Bits of a byte are
6359 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6360 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6361 breaking the single input byte C<chr(0x36)> into two groups gives a list
6362 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6364 C<vec> may also be assigned to, in which case parentheses are needed
6365 to give the expression the correct precedence as in
6367 vec($image, $max_x * $x + $y, 8) = 3;
6369 If the selected element is outside the string, the value 0 is returned.
6370 If an element off the end of the string is written to, Perl will first
6371 extend the string with sufficiently many zero bytes. It is an error
6372 to try to write off the beginning of the string (i.e. negative OFFSET).
6374 The string should not contain any character with the value > 255 (which
6375 can only happen if you're using UTF-8 encoding). If it does, it will be
6376 treated as something which is not UTF-8 encoded. When the C<vec> was
6377 assigned to, other parts of your program will also no longer consider the
6378 string to be UTF-8 encoded. In other words, if you do have such characters
6379 in your string, vec() will operate on the actual byte string, and not the
6380 conceptual character string.
6382 Strings created with C<vec> can also be manipulated with the logical
6383 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6384 vector operation is desired when both operands are strings.
6385 See L<perlop/"Bitwise String Operators">.
6387 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6388 The comments show the string after each step. Note that this code works
6389 in the same way on big-endian or little-endian machines.
6392 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6394 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6395 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6397 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6398 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6399 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6400 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6401 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6402 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6404 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6405 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6406 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6409 To transform a bit vector into a string or list of 0's and 1's, use these:
6411 $bits = unpack("b*", $vector);
6412 @bits = split(//, unpack("b*", $vector));
6414 If you know the exact length in bits, it can be used in place of the C<*>.
6416 Here is an example to illustrate how the bits actually fall in place:
6422 unpack("V",$_) 01234567890123456789012345678901
6423 ------------------------------------------------------------------
6428 for ($shift=0; $shift < $width; ++$shift) {
6429 for ($off=0; $off < 32/$width; ++$off) {
6430 $str = pack("B*", "0"x32);
6431 $bits = (1<<$shift);
6432 vec($str, $off, $width) = $bits;
6433 $res = unpack("b*",$str);
6434 $val = unpack("V", $str);
6441 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6442 $off, $width, $bits, $val, $res
6446 Regardless of the machine architecture on which it is run, the above
6447 example should print the following table:
6450 unpack("V",$_) 01234567890123456789012345678901
6451 ------------------------------------------------------------------
6452 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6453 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6454 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6455 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6456 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6457 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6458 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6459 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6460 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6461 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6462 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6463 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6464 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6465 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6466 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6467 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6468 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6469 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6470 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6471 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6472 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6473 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6474 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6475 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6476 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6477 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6478 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6479 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6480 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6481 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6482 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6483 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6484 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6485 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6486 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6487 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6488 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6489 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6490 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6491 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6492 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6493 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6494 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6495 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6496 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6497 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6498 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6499 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6500 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6501 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6502 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6503 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6504 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6505 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6506 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6507 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6508 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6509 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6510 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6511 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6512 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6513 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6514 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6515 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6516 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6517 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6518 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6519 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6520 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6521 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6522 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6523 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6524 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6525 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6526 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6527 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6528 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6529 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6530 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6531 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6532 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6533 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6534 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6535 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6536 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6537 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6538 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6539 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6540 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6541 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6542 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6543 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6544 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6545 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6546 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6547 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6548 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6549 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6550 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6551 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6552 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6553 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6554 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6555 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6556 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6557 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6558 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6559 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6560 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6561 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6562 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6563 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6564 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6565 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6566 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6567 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6568 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6569 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6570 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6571 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6572 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6573 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6574 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6575 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6576 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6577 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6578 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6579 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6583 Behaves like the wait(2) system call on your system: it waits for a child
6584 process to terminate and returns the pid of the deceased process, or
6585 C<-1> if there are no child processes. The status is returned in C<$?>.
6586 Note that a return value of C<-1> could mean that child processes are
6587 being automatically reaped, as described in L<perlipc>.
6589 =item waitpid PID,FLAGS
6591 Waits for a particular child process to terminate and returns the pid of
6592 the deceased process, or C<-1> if there is no such child process. On some
6593 systems, a value of 0 indicates that there are processes still running.
6594 The status is returned in C<$?>. If you say
6596 use POSIX ":sys_wait_h";
6599 $kid = waitpid(-1, WNOHANG);
6602 then you can do a non-blocking wait for all pending zombie processes.
6603 Non-blocking wait is available on machines supporting either the
6604 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6605 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6606 system call by remembering the status values of processes that have
6607 exited but have not been harvested by the Perl script yet.)
6609 Note that on some systems, a return value of C<-1> could mean that child
6610 processes are being automatically reaped. See L<perlipc> for details,
6611 and for other examples.
6615 Returns true if the context of the currently executing subroutine is
6616 looking for a list value. Returns false if the context is looking
6617 for a scalar. Returns the undefined value if the context is looking
6618 for no value (void context).
6620 return unless defined wantarray; # don't bother doing more
6621 my @a = complex_calculation();
6622 return wantarray ? @a : "@a";
6624 This function should have been named wantlist() instead.
6628 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6631 If LIST is empty and C<$@> already contains a value (typically from a
6632 previous eval) that value is used after appending C<"\t...caught">
6633 to C<$@>. This is useful for staying almost, but not entirely similar to
6636 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6638 No message is printed if there is a C<$SIG{__WARN__}> handler
6639 installed. It is the handler's responsibility to deal with the message
6640 as it sees fit (like, for instance, converting it into a C<die>). Most
6641 handlers must therefore make arrangements to actually display the
6642 warnings that they are not prepared to deal with, by calling C<warn>
6643 again in the handler. Note that this is quite safe and will not
6644 produce an endless loop, since C<__WARN__> hooks are not called from
6647 You will find this behavior is slightly different from that of
6648 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6649 instead call C<die> again to change it).
6651 Using a C<__WARN__> handler provides a powerful way to silence all
6652 warnings (even the so-called mandatory ones). An example:
6654 # wipe out *all* compile-time warnings
6655 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6657 my $foo = 20; # no warning about duplicate my $foo,
6658 # but hey, you asked for it!
6659 # no compile-time or run-time warnings before here
6662 # run-time warnings enabled after here
6663 warn "\$foo is alive and $foo!"; # does show up
6665 See L<perlvar> for details on setting C<%SIG> entries, and for more
6666 examples. See the Carp module for other kinds of warnings using its
6667 carp() and cluck() functions.
6669 =item write FILEHANDLE
6675 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6676 using the format associated with that file. By default the format for
6677 a file is the one having the same name as the filehandle, but the
6678 format for the current output channel (see the C<select> function) may be set
6679 explicitly by assigning the name of the format to the C<$~> variable.
6681 Top of form processing is handled automatically: if there is
6682 insufficient room on the current page for the formatted record, the
6683 page is advanced by writing a form feed, a special top-of-page format
6684 is used to format the new page header, and then the record is written.
6685 By default the top-of-page format is the name of the filehandle with
6686 "_TOP" appended, but it may be dynamically set to the format of your
6687 choice by assigning the name to the C<$^> variable while the filehandle is
6688 selected. The number of lines remaining on the current page is in
6689 variable C<$->, which can be set to C<0> to force a new page.
6691 If FILEHANDLE is unspecified, output goes to the current default output
6692 channel, which starts out as STDOUT but may be changed by the
6693 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6694 is evaluated and the resulting string is used to look up the name of
6695 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6697 Note that write is I<not> the opposite of C<read>. Unfortunately.
6701 The transliteration operator. Same as C<tr///>. See L<perlop>.