3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
434 Returns the arctangent of Y/X in the range -PI to PI.
436 For the tangent operation, you may use the C<Math::Trig::tan>
437 function, or use the familiar relation:
439 sub tan { sin($_[0]) / cos($_[0]) }
441 =item bind SOCKET,NAME
443 Binds a network address to a socket, just as the bind system call
444 does. Returns true if it succeeded, false otherwise. NAME should be a
445 packed address of the appropriate type for the socket. See the examples in
446 L<perlipc/"Sockets: Client/Server Communication">.
448 =item binmode FILEHANDLE, LAYER
450 =item binmode FILEHANDLE
452 Arranges for FILEHANDLE to be read or written in "binary" or "text"
453 mode on systems where the run-time libraries distinguish between
454 binary and text files. If FILEHANDLE is an expression, the value is
455 taken as the name of the filehandle. Returns true on success,
458 If LAYER is omitted or specified as C<:raw> the filehandle is made
459 suitable for passing binary data. This includes turning off possible CRLF
460 translation and marking it as bytes (as opposed to Unicode characters).
461 Note that as desipite what may be implied in I<"Programming Perl">
462 (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
463 -- other layers which would affect binary nature of the stream are
464 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
465 PERLIO environment variable.
467 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
468 in "Programming Perl, 3rd Edition". However, since the publishing of this
469 book, by many known as "Camel III", the consensus of the naming of this
470 functionality has moved from "discipline" to "layer". All documentation
471 of this version of Perl therefore refers to "layers" rather than to
472 "disciplines". Now back to the regularly scheduled documentation...>
474 On some systems (in general, DOS and Windows-based systems) binmode()
475 is necessary when you're not working with a text file. For the sake
476 of portability it is a good idea to always use it when appropriate,
477 and to never use it when it isn't appropriate.
479 In other words: regardless of platform, use binmode() on binary files
480 (like for example images).
482 If LAYER is present it is a single string, but may contain
483 multiple directives. The directives alter the behaviour of the
484 file handle. When LAYER is present using binmode on text
487 To mark FILEHANDLE as UTF-8, use C<:utf8>.
489 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
490 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
491 establish default I/O layers. See L<open>.
493 In general, binmode() should be called after open() but before any I/O
494 is done on the filehandle. Calling binmode() will normally flush any
495 pending buffered output data (and perhaps pending input data) on the
496 handle. An exception to this is the C<:encoding> layer that
497 changes the default character encoding of the handle, see L<open>.
498 The C<:encoding> layer sometimes needs to be called in
499 mid-stream, and it doesn't flush the stream.
501 The operating system, device drivers, C libraries, and Perl run-time
502 system all work together to let the programmer treat a single
503 character (C<\n>) as the line terminator, irrespective of the external
504 representation. On many operating systems, the native text file
505 representation matches the internal representation, but on some
506 platforms the external representation of C<\n> is made up of more than
509 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
510 character to end each line in the external representation of text (even
511 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
512 on Unix and most VMS files). In other systems like OS/2, DOS and the
513 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
514 but what's stored in text files are the two characters C<\cM\cJ>. That
515 means that, if you don't use binmode() on these systems, C<\cM\cJ>
516 sequences on disk will be converted to C<\n> on input, and any C<\n> in
517 your program will be converted back to C<\cM\cJ> on output. This is what
518 you want for text files, but it can be disastrous for binary files.
520 Another consequence of using binmode() (on some systems) is that
521 special end-of-file markers will be seen as part of the data stream.
522 For systems from the Microsoft family this means that if your binary
523 data contains C<\cZ>, the I/O subsystem will regard it as the end of
524 the file, unless you use binmode().
526 binmode() is not only important for readline() and print() operations,
527 but also when using read(), seek(), sysread(), syswrite() and tell()
528 (see L<perlport> for more details). See the C<$/> and C<$\> variables
529 in L<perlvar> for how to manually set your input and output
530 line-termination sequences.
532 =item bless REF,CLASSNAME
536 This function tells the thingy referenced by REF that it is now an object
537 in the CLASSNAME package. If CLASSNAME is omitted, the current package
538 is used. Because a C<bless> is often the last thing in a constructor,
539 it returns the reference for convenience. Always use the two-argument
540 version if the function doing the blessing might be inherited by a
541 derived class. See L<perltoot> and L<perlobj> for more about the blessing
542 (and blessings) of objects.
544 Consider always blessing objects in CLASSNAMEs that are mixed case.
545 Namespaces with all lowercase names are considered reserved for
546 Perl pragmata. Builtin types have all uppercase names, so to prevent
547 confusion, you may wish to avoid such package names as well. Make sure
548 that CLASSNAME is a true value.
550 See L<perlmod/"Perl Modules">.
556 Returns the context of the current subroutine call. In scalar context,
557 returns the caller's package name if there is a caller, that is, if
558 we're in a subroutine or C<eval> or C<require>, and the undefined value
559 otherwise. In list context, returns
561 ($package, $filename, $line) = caller;
563 With EXPR, it returns some extra information that the debugger uses to
564 print a stack trace. The value of EXPR indicates how many call frames
565 to go back before the current one.
567 ($package, $filename, $line, $subroutine, $hasargs,
568 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
570 Here $subroutine may be C<(eval)> if the frame is not a subroutine
571 call, but an C<eval>. In such a case additional elements $evaltext and
572 C<$is_require> are set: C<$is_require> is true if the frame is created by a
573 C<require> or C<use> statement, $evaltext contains the text of the
574 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
575 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
576 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
577 frame.) $subroutine may also be C<(unknown)> if this particular
578 subroutine happens to have been deleted from the symbol table.
579 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
580 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
581 compiled with. The C<$hints> and C<$bitmask> values are subject to change
582 between versions of Perl, and are not meant for external use.
584 Furthermore, when called from within the DB package, caller returns more
585 detailed information: it sets the list variable C<@DB::args> to be the
586 arguments with which the subroutine was invoked.
588 Be aware that the optimizer might have optimized call frames away before
589 C<caller> had a chance to get the information. That means that C<caller(N)>
590 might not return information about the call frame you expect it do, for
591 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
592 previous time C<caller> was called.
596 Changes the working directory to EXPR, if possible. If EXPR is omitted,
597 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
598 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
599 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
600 neither is set, C<chdir> does nothing. It returns true upon success,
601 false otherwise. See the example under C<die>.
605 Changes the permissions of a list of files. The first element of the
606 list must be the numerical mode, which should probably be an octal
607 number, and which definitely should I<not> a string of octal digits:
608 C<0644> is okay, C<'0644'> is not. Returns the number of files
609 successfully changed. See also L</oct>, if all you have is a string.
611 $cnt = chmod 0755, 'foo', 'bar';
612 chmod 0755, @executables;
613 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
615 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
616 $mode = 0644; chmod $mode, 'foo'; # this is best
618 You can also import the symbolic C<S_I*> constants from the Fcntl
623 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
624 # This is identical to the chmod 0755 of the above example.
632 This safer version of L</chop> removes any trailing string
633 that corresponds to the current value of C<$/> (also known as
634 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
635 number of characters removed from all its arguments. It's often used to
636 remove the newline from the end of an input record when you're worried
637 that the final record may be missing its newline. When in paragraph
638 mode (C<$/ = "">), it removes all trailing newlines from the string.
639 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
640 a reference to an integer or the like, see L<perlvar>) chomp() won't
642 If VARIABLE is omitted, it chomps C<$_>. Example:
645 chomp; # avoid \n on last field
650 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
652 You can actually chomp anything that's an lvalue, including an assignment:
655 chomp($answer = <STDIN>);
657 If you chomp a list, each element is chomped, and the total number of
658 characters removed is returned.
660 Note that parentheses are necessary when you're chomping anything
661 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
662 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
663 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
664 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
673 Chops off the last character of a string and returns the character
674 chopped. It is much more efficient than C<s/.$//s> because it neither
675 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
676 If VARIABLE is a hash, it chops the hash's values, but not its keys.
678 You can actually chop anything that's an lvalue, including an assignment.
680 If you chop a list, each element is chopped. Only the value of the
681 last C<chop> is returned.
683 Note that C<chop> returns the last character. To return all but the last
684 character, use C<substr($string, 0, -1)>.
690 Changes the owner (and group) of a list of files. The first two
691 elements of the list must be the I<numeric> uid and gid, in that
692 order. A value of -1 in either position is interpreted by most
693 systems to leave that value unchanged. Returns the number of files
694 successfully changed.
696 $cnt = chown $uid, $gid, 'foo', 'bar';
697 chown $uid, $gid, @filenames;
699 Here's an example that looks up nonnumeric uids in the passwd file:
702 chomp($user = <STDIN>);
704 chomp($pattern = <STDIN>);
706 ($login,$pass,$uid,$gid) = getpwnam($user)
707 or die "$user not in passwd file";
709 @ary = glob($pattern); # expand filenames
710 chown $uid, $gid, @ary;
712 On most systems, you are not allowed to change the ownership of the
713 file unless you're the superuser, although you should be able to change
714 the group to any of your secondary groups. On insecure systems, these
715 restrictions may be relaxed, but this is not a portable assumption.
716 On POSIX systems, you can detect this condition this way:
718 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
719 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
725 Returns the character represented by that NUMBER in the character set.
726 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
727 chr(0x263a) is a Unicode smiley face. Note that characters from 127
728 to 255 (inclusive) are by default not encoded in Unicode for backward
729 compatibility reasons (but see L<encoding>).
731 For the reverse, use L</ord>.
732 See L<perlunicode> and L<encoding> for more about Unicode.
734 If NUMBER is omitted, uses C<$_>.
736 =item chroot FILENAME
740 This function works like the system call by the same name: it makes the
741 named directory the new root directory for all further pathnames that
742 begin with a C</> by your process and all its children. (It doesn't
743 change your current working directory, which is unaffected.) For security
744 reasons, this call is restricted to the superuser. If FILENAME is
745 omitted, does a C<chroot> to C<$_>.
747 =item close FILEHANDLE
751 Closes the file or pipe associated with the file handle, returning
752 true only if IO buffers are successfully flushed and closes the system
753 file descriptor. Closes the currently selected filehandle if the
756 You don't have to close FILEHANDLE if you are immediately going to do
757 another C<open> on it, because C<open> will close it for you. (See
758 C<open>.) However, an explicit C<close> on an input file resets the line
759 counter (C<$.>), while the implicit close done by C<open> does not.
761 If the file handle came from a piped open C<close> will additionally
762 return false if one of the other system calls involved fails or if the
763 program exits with non-zero status. (If the only problem was that the
764 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
765 also waits for the process executing on the pipe to complete, in case you
766 want to look at the output of the pipe afterwards, and
767 implicitly puts the exit status value of that command into C<$?>.
769 Prematurely closing the read end of a pipe (i.e. before the process
770 writing to it at the other end has closed it) will result in a
771 SIGPIPE being delivered to the writer. If the other end can't
772 handle that, be sure to read all the data before closing the pipe.
776 open(OUTPUT, '|sort >foo') # pipe to sort
777 or die "Can't start sort: $!";
778 #... # print stuff to output
779 close OUTPUT # wait for sort to finish
780 or warn $! ? "Error closing sort pipe: $!"
781 : "Exit status $? from sort";
782 open(INPUT, 'foo') # get sort's results
783 or die "Can't open 'foo' for input: $!";
785 FILEHANDLE may be an expression whose value can be used as an indirect
786 filehandle, usually the real filehandle name.
788 =item closedir DIRHANDLE
790 Closes a directory opened by C<opendir> and returns the success of that
793 DIRHANDLE may be an expression whose value can be used as an indirect
794 dirhandle, usually the real dirhandle name.
796 =item connect SOCKET,NAME
798 Attempts to connect to a remote socket, just as the connect system call
799 does. Returns true if it succeeded, false otherwise. NAME should be a
800 packed address of the appropriate type for the socket. See the examples in
801 L<perlipc/"Sockets: Client/Server Communication">.
805 Actually a flow control statement rather than a function. If there is a
806 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
807 C<foreach>), it is always executed just before the conditional is about to
808 be evaluated again, just like the third part of a C<for> loop in C. Thus
809 it can be used to increment a loop variable, even when the loop has been
810 continued via the C<next> statement (which is similar to the C C<continue>
813 C<last>, C<next>, or C<redo> may appear within a C<continue>
814 block. C<last> and C<redo> will behave as if they had been executed within
815 the main block. So will C<next>, but since it will execute a C<continue>
816 block, it may be more entertaining.
819 ### redo always comes here
822 ### next always comes here
824 # then back the top to re-check EXPR
826 ### last always comes here
828 Omitting the C<continue> section is semantically equivalent to using an
829 empty one, logically enough. In that case, C<next> goes directly back
830 to check the condition at the top of the loop.
836 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
837 takes cosine of C<$_>.
839 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
840 function, or use this relation:
842 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
844 =item crypt PLAINTEXT,SALT
846 Encrypts a string exactly like the crypt(3) function in the C library
847 (assuming that you actually have a version there that has not been
848 extirpated as a potential munition). This can prove useful for checking
849 the password file for lousy passwords, amongst other things. Only the
850 guys wearing white hats should do this.
852 Note that C<crypt> is intended to be a one-way function, much like
853 breaking eggs to make an omelette. There is no (known) corresponding
854 decrypt function (in other words, the crypt() is a one-way hash
855 function). As a result, this function isn't all that useful for
856 cryptography. (For that, see your nearby CPAN mirror.)
858 When verifying an existing encrypted string you should use the
859 encrypted text as the salt (like C<crypt($plain, $crypted) eq
860 $crypted>). This allows your code to work with the standard C<crypt>
861 and with more exotic implementations. In other words, do not assume
862 anything about the returned string itself, or how many bytes in
863 the encrypted string matter.
865 Traditionally the result is a string of 13 bytes: two first bytes of
866 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
867 the first eight bytes of the encrypted string mattered, but
868 alternative hashing schemes (like MD5), higher level security schemes
869 (like C2), and implementations on non-UNIX platforms may produce
872 When choosing a new salt create a random two character string whose
873 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
874 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
876 Here's an example that makes sure that whoever runs this program knows
879 $pwd = (getpwuid($<))[1];
883 chomp($word = <STDIN>);
887 if (crypt($word, $pwd) ne $pwd) {
893 Of course, typing in your own password to whoever asks you
896 The L<crypt> function is unsuitable for encrypting large quantities
897 of data, not least of all because you can't get the information
898 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
899 on your favorite CPAN mirror for a slew of potentially useful
902 If using crypt() on a Unicode string (which I<potentially> has
903 characters with codepoints above 255), Perl tries to make sense
904 of the situation by trying to downgrade (a copy of the string)
905 the string back to an eight-bit byte string before calling crypt()
906 (on that copy). If that works, good. If not, crypt() dies with
907 C<Wide character in crypt>.
911 [This function has been largely superseded by the C<untie> function.]
913 Breaks the binding between a DBM file and a hash.
915 =item dbmopen HASH,DBNAME,MASK
917 [This function has been largely superseded by the C<tie> function.]
919 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
920 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
921 argument is I<not> a filehandle, even though it looks like one). DBNAME
922 is the name of the database (without the F<.dir> or F<.pag> extension if
923 any). If the database does not exist, it is created with protection
924 specified by MASK (as modified by the C<umask>). If your system supports
925 only the older DBM functions, you may perform only one C<dbmopen> in your
926 program. In older versions of Perl, if your system had neither DBM nor
927 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
930 If you don't have write access to the DBM file, you can only read hash
931 variables, not set them. If you want to test whether you can write,
932 either use file tests or try setting a dummy hash entry inside an C<eval>,
933 which will trap the error.
935 Note that functions such as C<keys> and C<values> may return huge lists
936 when used on large DBM files. You may prefer to use the C<each>
937 function to iterate over large DBM files. Example:
939 # print out history file offsets
940 dbmopen(%HIST,'/usr/lib/news/history',0666);
941 while (($key,$val) = each %HIST) {
942 print $key, ' = ', unpack('L',$val), "\n";
946 See also L<AnyDBM_File> for a more general description of the pros and
947 cons of the various dbm approaches, as well as L<DB_File> for a particularly
950 You can control which DBM library you use by loading that library
951 before you call dbmopen():
954 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
955 or die "Can't open netscape history file: $!";
961 Returns a Boolean value telling whether EXPR has a value other than
962 the undefined value C<undef>. If EXPR is not present, C<$_> will be
965 Many operations return C<undef> to indicate failure, end of file,
966 system error, uninitialized variable, and other exceptional
967 conditions. This function allows you to distinguish C<undef> from
968 other values. (A simple Boolean test will not distinguish among
969 C<undef>, zero, the empty string, and C<"0">, which are all equally
970 false.) Note that since C<undef> is a valid scalar, its presence
971 doesn't I<necessarily> indicate an exceptional condition: C<pop>
972 returns C<undef> when its argument is an empty array, I<or> when the
973 element to return happens to be C<undef>.
975 You may also use C<defined(&func)> to check whether subroutine C<&func>
976 has ever been defined. The return value is unaffected by any forward
977 declarations of C<&foo>. Note that a subroutine which is not defined
978 may still be callable: its package may have an C<AUTOLOAD> method that
979 makes it spring into existence the first time that it is called -- see
982 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
983 used to report whether memory for that aggregate has ever been
984 allocated. This behavior may disappear in future versions of Perl.
985 You should instead use a simple test for size:
987 if (@an_array) { print "has array elements\n" }
988 if (%a_hash) { print "has hash members\n" }
990 When used on a hash element, it tells you whether the value is defined,
991 not whether the key exists in the hash. Use L</exists> for the latter
996 print if defined $switch{'D'};
997 print "$val\n" while defined($val = pop(@ary));
998 die "Can't readlink $sym: $!"
999 unless defined($value = readlink $sym);
1000 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1001 $debugging = 0 unless defined $debugging;
1003 Note: Many folks tend to overuse C<defined>, and then are surprised to
1004 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1005 defined values. For example, if you say
1009 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1010 matched "nothing". But it didn't really match nothing--rather, it
1011 matched something that happened to be zero characters long. This is all
1012 very above-board and honest. When a function returns an undefined value,
1013 it's an admission that it couldn't give you an honest answer. So you
1014 should use C<defined> only when you're questioning the integrity of what
1015 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1018 See also L</undef>, L</exists>, L</ref>.
1022 Given an expression that specifies a hash element, array element, hash slice,
1023 or array slice, deletes the specified element(s) from the hash or array.
1024 In the case of an array, if the array elements happen to be at the end,
1025 the size of the array will shrink to the highest element that tests
1026 true for exists() (or 0 if no such element exists).
1028 Returns each element so deleted or the undefined value if there was no such
1029 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1030 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1031 from a C<tie>d hash or array may not necessarily return anything.
1033 Deleting an array element effectively returns that position of the array
1034 to its initial, uninitialized state. Subsequently testing for the same
1035 element with exists() will return false. Note that deleting array
1036 elements in the middle of an array will not shift the index of the ones
1037 after them down--use splice() for that. See L</exists>.
1039 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1041 foreach $key (keys %HASH) {
1045 foreach $index (0 .. $#ARRAY) {
1046 delete $ARRAY[$index];
1051 delete @HASH{keys %HASH};
1053 delete @ARRAY[0 .. $#ARRAY];
1055 But both of these are slower than just assigning the empty list
1056 or undefining %HASH or @ARRAY:
1058 %HASH = (); # completely empty %HASH
1059 undef %HASH; # forget %HASH ever existed
1061 @ARRAY = (); # completely empty @ARRAY
1062 undef @ARRAY; # forget @ARRAY ever existed
1064 Note that the EXPR can be arbitrarily complicated as long as the final
1065 operation is a hash element, array element, hash slice, or array slice
1068 delete $ref->[$x][$y]{$key};
1069 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1071 delete $ref->[$x][$y][$index];
1072 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1076 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1077 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1078 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1079 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1080 an C<eval(),> the error message is stuffed into C<$@> and the
1081 C<eval> is terminated with the undefined value. This makes
1082 C<die> the way to raise an exception.
1084 Equivalent examples:
1086 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1087 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1089 If the last element of LIST does not end in a newline, the current
1090 script line number and input line number (if any) are also printed,
1091 and a newline is supplied. Note that the "input line number" (also
1092 known as "chunk") is subject to whatever notion of "line" happens to
1093 be currently in effect, and is also available as the special variable
1094 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1096 Hint: sometimes appending C<", stopped"> to your message will cause it
1097 to make better sense when the string C<"at foo line 123"> is appended.
1098 Suppose you are running script "canasta".
1100 die "/etc/games is no good";
1101 die "/etc/games is no good, stopped";
1103 produce, respectively
1105 /etc/games is no good at canasta line 123.
1106 /etc/games is no good, stopped at canasta line 123.
1108 See also exit(), warn(), and the Carp module.
1110 If LIST is empty and C<$@> already contains a value (typically from a
1111 previous eval) that value is reused after appending C<"\t...propagated">.
1112 This is useful for propagating exceptions:
1115 die unless $@ =~ /Expected exception/;
1117 If LIST is empty and C<$@> contains an object reference that has a
1118 C<PROPAGATE> method, that method will be called with additional file
1119 and line number parameters. The return value replaces the value in
1120 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1123 If C<$@> is empty then the string C<"Died"> is used.
1125 die() can also be called with a reference argument. If this happens to be
1126 trapped within an eval(), $@ contains the reference. This behavior permits
1127 a more elaborate exception handling implementation using objects that
1128 maintain arbitrary state about the nature of the exception. Such a scheme
1129 is sometimes preferable to matching particular string values of $@ using
1130 regular expressions. Here's an example:
1132 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1134 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1135 # handle Some::Module::Exception
1138 # handle all other possible exceptions
1142 Because perl will stringify uncaught exception messages before displaying
1143 them, you may want to overload stringification operations on such custom
1144 exception objects. See L<overload> for details about that.
1146 You can arrange for a callback to be run just before the C<die>
1147 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1148 handler will be called with the error text and can change the error
1149 message, if it sees fit, by calling C<die> again. See
1150 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1151 L<"eval BLOCK"> for some examples. Although this feature was meant
1152 to be run only right before your program was to exit, this is not
1153 currently the case--the C<$SIG{__DIE__}> hook is currently called
1154 even inside eval()ed blocks/strings! If one wants the hook to do
1155 nothing in such situations, put
1159 as the first line of the handler (see L<perlvar/$^S>). Because
1160 this promotes strange action at a distance, this counterintuitive
1161 behavior may be fixed in a future release.
1165 Not really a function. Returns the value of the last command in the
1166 sequence of commands indicated by BLOCK. When modified by a loop
1167 modifier, executes the BLOCK once before testing the loop condition.
1168 (On other statements the loop modifiers test the conditional first.)
1170 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1171 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1172 See L<perlsyn> for alternative strategies.
1174 =item do SUBROUTINE(LIST)
1176 A deprecated form of subroutine call. See L<perlsub>.
1180 Uses the value of EXPR as a filename and executes the contents of the
1181 file as a Perl script. Its primary use is to include subroutines
1182 from a Perl subroutine library.
1190 except that it's more efficient and concise, keeps track of the current
1191 filename for error messages, searches the @INC libraries, and updates
1192 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1193 variables. It also differs in that code evaluated with C<do FILENAME>
1194 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1195 same, however, in that it does reparse the file every time you call it,
1196 so you probably don't want to do this inside a loop.
1198 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1199 error. If C<do> can read the file but cannot compile it, it
1200 returns undef and sets an error message in C<$@>. If the file is
1201 successfully compiled, C<do> returns the value of the last expression
1204 Note that inclusion of library modules is better done with the
1205 C<use> and C<require> operators, which also do automatic error checking
1206 and raise an exception if there's a problem.
1208 You might like to use C<do> to read in a program configuration
1209 file. Manual error checking can be done this way:
1211 # read in config files: system first, then user
1212 for $file ("/share/prog/defaults.rc",
1213 "$ENV{HOME}/.someprogrc")
1215 unless ($return = do $file) {
1216 warn "couldn't parse $file: $@" if $@;
1217 warn "couldn't do $file: $!" unless defined $return;
1218 warn "couldn't run $file" unless $return;
1226 This function causes an immediate core dump. See also the B<-u>
1227 command-line switch in L<perlrun>, which does the same thing.
1228 Primarily this is so that you can use the B<undump> program (not
1229 supplied) to turn your core dump into an executable binary after
1230 having initialized all your variables at the beginning of the
1231 program. When the new binary is executed it will begin by executing
1232 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1233 Think of it as a goto with an intervening core dump and reincarnation.
1234 If C<LABEL> is omitted, restarts the program from the top.
1236 B<WARNING>: Any files opened at the time of the dump will I<not>
1237 be open any more when the program is reincarnated, with possible
1238 resulting confusion on the part of Perl.
1240 This function is now largely obsolete, partly because it's very
1241 hard to convert a core file into an executable, and because the
1242 real compiler backends for generating portable bytecode and compilable
1243 C code have superseded it. That's why you should now invoke it as
1244 C<CORE::dump()>, if you don't want to be warned against a possible
1247 If you're looking to use L<dump> to speed up your program, consider
1248 generating bytecode or native C code as described in L<perlcc>. If
1249 you're just trying to accelerate a CGI script, consider using the
1250 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1251 You might also consider autoloading or selfloading, which at least
1252 make your program I<appear> to run faster.
1256 When called in list context, returns a 2-element list consisting of the
1257 key and value for the next element of a hash, so that you can iterate over
1258 it. When called in scalar context, returns only the key for the next
1259 element in the hash.
1261 Entries are returned in an apparently random order. The actual random
1262 order is subject to change in future versions of perl, but it is guaranteed
1263 to be in the same order as either the C<keys> or C<values> function
1264 would produce on the same (unmodified) hash.
1266 When the hash is entirely read, a null array is returned in list context
1267 (which when assigned produces a false (C<0>) value), and C<undef> in
1268 scalar context. The next call to C<each> after that will start iterating
1269 again. There is a single iterator for each hash, shared by all C<each>,
1270 C<keys>, and C<values> function calls in the program; it can be reset by
1271 reading all the elements from the hash, or by evaluating C<keys HASH> or
1272 C<values HASH>. If you add or delete elements of a hash while you're
1273 iterating over it, you may get entries skipped or duplicated, so
1274 don't. Exception: It is always safe to delete the item most recently
1275 returned by C<each()>, which means that the following code will work:
1277 while (($key, $value) = each %hash) {
1279 delete $hash{$key}; # This is safe
1282 The following prints out your environment like the printenv(1) program,
1283 only in a different order:
1285 while (($key,$value) = each %ENV) {
1286 print "$key=$value\n";
1289 See also C<keys>, C<values> and C<sort>.
1291 =item eof FILEHANDLE
1297 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1298 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1299 gives the real filehandle. (Note that this function actually
1300 reads a character and then C<ungetc>s it, so isn't very useful in an
1301 interactive context.) Do not read from a terminal file (or call
1302 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1303 as terminals may lose the end-of-file condition if you do.
1305 An C<eof> without an argument uses the last file read. Using C<eof()>
1306 with empty parentheses is very different. It refers to the pseudo file
1307 formed from the files listed on the command line and accessed via the
1308 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1309 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1310 used will cause C<@ARGV> to be examined to determine if input is
1311 available. Similarly, an C<eof()> after C<< <> >> has returned
1312 end-of-file will assume you are processing another C<@ARGV> list,
1313 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1314 see L<perlop/"I/O Operators">.
1316 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1317 detect the end of each file, C<eof()> will only detect the end of the
1318 last file. Examples:
1320 # reset line numbering on each input file
1322 next if /^\s*#/; # skip comments
1325 close ARGV if eof; # Not eof()!
1328 # insert dashes just before last line of last file
1330 if (eof()) { # check for end of current file
1331 print "--------------\n";
1332 close(ARGV); # close or last; is needed if we
1333 # are reading from the terminal
1338 Practical hint: you almost never need to use C<eof> in Perl, because the
1339 input operators typically return C<undef> when they run out of data, or if
1346 In the first form, the return value of EXPR is parsed and executed as if it
1347 were a little Perl program. The value of the expression (which is itself
1348 determined within scalar context) is first parsed, and if there weren't any
1349 errors, executed in the lexical context of the current Perl program, so
1350 that any variable settings or subroutine and format definitions remain
1351 afterwards. Note that the value is parsed every time the eval executes.
1352 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1353 delay parsing and subsequent execution of the text of EXPR until run time.
1355 In the second form, the code within the BLOCK is parsed only once--at the
1356 same time the code surrounding the eval itself was parsed--and executed
1357 within the context of the current Perl program. This form is typically
1358 used to trap exceptions more efficiently than the first (see below), while
1359 also providing the benefit of checking the code within BLOCK at compile
1362 The final semicolon, if any, may be omitted from the value of EXPR or within
1365 In both forms, the value returned is the value of the last expression
1366 evaluated inside the mini-program; a return statement may be also used, just
1367 as with subroutines. The expression providing the return value is evaluated
1368 in void, scalar, or list context, depending on the context of the eval itself.
1369 See L</wantarray> for more on how the evaluation context can be determined.
1371 If there is a syntax error or runtime error, or a C<die> statement is
1372 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1373 error message. If there was no error, C<$@> is guaranteed to be a null
1374 string. Beware that using C<eval> neither silences perl from printing
1375 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1376 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1377 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1378 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1380 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1381 determining whether a particular feature (such as C<socket> or C<symlink>)
1382 is implemented. It is also Perl's exception trapping mechanism, where
1383 the die operator is used to raise exceptions.
1385 If the code to be executed doesn't vary, you may use the eval-BLOCK
1386 form to trap run-time errors without incurring the penalty of
1387 recompiling each time. The error, if any, is still returned in C<$@>.
1390 # make divide-by-zero nonfatal
1391 eval { $answer = $a / $b; }; warn $@ if $@;
1393 # same thing, but less efficient
1394 eval '$answer = $a / $b'; warn $@ if $@;
1396 # a compile-time error
1397 eval { $answer = }; # WRONG
1400 eval '$answer ='; # sets $@
1402 Due to the current arguably broken state of C<__DIE__> hooks, when using
1403 the C<eval{}> form as an exception trap in libraries, you may wish not
1404 to trigger any C<__DIE__> hooks that user code may have installed.
1405 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1406 as shown in this example:
1408 # a very private exception trap for divide-by-zero
1409 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1412 This is especially significant, given that C<__DIE__> hooks can call
1413 C<die> again, which has the effect of changing their error messages:
1415 # __DIE__ hooks may modify error messages
1417 local $SIG{'__DIE__'} =
1418 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1419 eval { die "foo lives here" };
1420 print $@ if $@; # prints "bar lives here"
1423 Because this promotes action at a distance, this counterintuitive behavior
1424 may be fixed in a future release.
1426 With an C<eval>, you should be especially careful to remember what's
1427 being looked at when:
1433 eval { $x }; # CASE 4
1435 eval "\$$x++"; # CASE 5
1438 Cases 1 and 2 above behave identically: they run the code contained in
1439 the variable $x. (Although case 2 has misleading double quotes making
1440 the reader wonder what else might be happening (nothing is).) Cases 3
1441 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1442 does nothing but return the value of $x. (Case 4 is preferred for
1443 purely visual reasons, but it also has the advantage of compiling at
1444 compile-time instead of at run-time.) Case 5 is a place where
1445 normally you I<would> like to use double quotes, except that in this
1446 particular situation, you can just use symbolic references instead, as
1449 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1450 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1454 =item exec PROGRAM LIST
1456 The C<exec> function executes a system command I<and never returns>--
1457 use C<system> instead of C<exec> if you want it to return. It fails and
1458 returns false only if the command does not exist I<and> it is executed
1459 directly instead of via your system's command shell (see below).
1461 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1462 warns you if there is a following statement which isn't C<die>, C<warn>,
1463 or C<exit> (if C<-w> is set - but you always do that). If you
1464 I<really> want to follow an C<exec> with some other statement, you
1465 can use one of these styles to avoid the warning:
1467 exec ('foo') or print STDERR "couldn't exec foo: $!";
1468 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1470 If there is more than one argument in LIST, or if LIST is an array
1471 with more than one value, calls execvp(3) with the arguments in LIST.
1472 If there is only one scalar argument or an array with one element in it,
1473 the argument is checked for shell metacharacters, and if there are any,
1474 the entire argument is passed to the system's command shell for parsing
1475 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1476 If there are no shell metacharacters in the argument, it is split into
1477 words and passed directly to C<execvp>, which is more efficient.
1480 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1481 exec "sort $outfile | uniq";
1483 If you don't really want to execute the first argument, but want to lie
1484 to the program you are executing about its own name, you can specify
1485 the program you actually want to run as an "indirect object" (without a
1486 comma) in front of the LIST. (This always forces interpretation of the
1487 LIST as a multivalued list, even if there is only a single scalar in
1490 $shell = '/bin/csh';
1491 exec $shell '-sh'; # pretend it's a login shell
1495 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1497 When the arguments get executed via the system shell, results will
1498 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1501 Using an indirect object with C<exec> or C<system> is also more
1502 secure. This usage (which also works fine with system()) forces
1503 interpretation of the arguments as a multivalued list, even if the
1504 list had just one argument. That way you're safe from the shell
1505 expanding wildcards or splitting up words with whitespace in them.
1507 @args = ( "echo surprise" );
1509 exec @args; # subject to shell escapes
1511 exec { $args[0] } @args; # safe even with one-arg list
1513 The first version, the one without the indirect object, ran the I<echo>
1514 program, passing it C<"surprise"> an argument. The second version
1515 didn't--it tried to run a program literally called I<"echo surprise">,
1516 didn't find it, and set C<$?> to a non-zero value indicating failure.
1518 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1519 output before the exec, but this may not be supported on some platforms
1520 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1521 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1522 open handles in order to avoid lost output.
1524 Note that C<exec> will not call your C<END> blocks, nor will it call
1525 any C<DESTROY> methods in your objects.
1529 Given an expression that specifies a hash element or array element,
1530 returns true if the specified element in the hash or array has ever
1531 been initialized, even if the corresponding value is undefined. The
1532 element is not autovivified if it doesn't exist.
1534 print "Exists\n" if exists $hash{$key};
1535 print "Defined\n" if defined $hash{$key};
1536 print "True\n" if $hash{$key};
1538 print "Exists\n" if exists $array[$index];
1539 print "Defined\n" if defined $array[$index];
1540 print "True\n" if $array[$index];
1542 A hash or array element can be true only if it's defined, and defined if
1543 it exists, but the reverse doesn't necessarily hold true.
1545 Given an expression that specifies the name of a subroutine,
1546 returns true if the specified subroutine has ever been declared, even
1547 if it is undefined. Mentioning a subroutine name for exists or defined
1548 does not count as declaring it. Note that a subroutine which does not
1549 exist may still be callable: its package may have an C<AUTOLOAD>
1550 method that makes it spring into existence the first time that it is
1551 called -- see L<perlsub>.
1553 print "Exists\n" if exists &subroutine;
1554 print "Defined\n" if defined &subroutine;
1556 Note that the EXPR can be arbitrarily complicated as long as the final
1557 operation is a hash or array key lookup or subroutine name:
1559 if (exists $ref->{A}->{B}->{$key}) { }
1560 if (exists $hash{A}{B}{$key}) { }
1562 if (exists $ref->{A}->{B}->[$ix]) { }
1563 if (exists $hash{A}{B}[$ix]) { }
1565 if (exists &{$ref->{A}{B}{$key}}) { }
1567 Although the deepest nested array or hash will not spring into existence
1568 just because its existence was tested, any intervening ones will.
1569 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1570 into existence due to the existence test for the $key element above.
1571 This happens anywhere the arrow operator is used, including even:
1574 if (exists $ref->{"Some key"}) { }
1575 print $ref; # prints HASH(0x80d3d5c)
1577 This surprising autovivification in what does not at first--or even
1578 second--glance appear to be an lvalue context may be fixed in a future
1581 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1582 on how exists() acts when used on a pseudo-hash.
1584 Use of a subroutine call, rather than a subroutine name, as an argument
1585 to exists() is an error.
1588 exists &sub(); # Error
1592 Evaluates EXPR and exits immediately with that value. Example:
1595 exit 0 if $ans =~ /^[Xx]/;
1597 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1598 universally recognized values for EXPR are C<0> for success and C<1>
1599 for error; other values are subject to interpretation depending on the
1600 environment in which the Perl program is running. For example, exiting
1601 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1602 the mailer to return the item undelivered, but that's not true everywhere.
1604 Don't use C<exit> to abort a subroutine if there's any chance that
1605 someone might want to trap whatever error happened. Use C<die> instead,
1606 which can be trapped by an C<eval>.
1608 The exit() function does not always exit immediately. It calls any
1609 defined C<END> routines first, but these C<END> routines may not
1610 themselves abort the exit. Likewise any object destructors that need to
1611 be called are called before the real exit. If this is a problem, you
1612 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1613 See L<perlmod> for details.
1619 Returns I<e> (the natural logarithm base) to the power of EXPR.
1620 If EXPR is omitted, gives C<exp($_)>.
1622 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1624 Implements the fcntl(2) function. You'll probably have to say
1628 first to get the correct constant definitions. Argument processing and
1629 value return works just like C<ioctl> below.
1633 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1634 or die "can't fcntl F_GETFL: $!";
1636 You don't have to check for C<defined> on the return from C<fnctl>.
1637 Like C<ioctl>, it maps a C<0> return from the system call into
1638 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1639 in numeric context. It is also exempt from the normal B<-w> warnings
1640 on improper numeric conversions.
1642 Note that C<fcntl> will produce a fatal error if used on a machine that
1643 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1644 manpage to learn what functions are available on your system.
1646 =item fileno FILEHANDLE
1648 Returns the file descriptor for a filehandle, or undefined if the
1649 filehandle is not open. This is mainly useful for constructing
1650 bitmaps for C<select> and low-level POSIX tty-handling operations.
1651 If FILEHANDLE is an expression, the value is taken as an indirect
1652 filehandle, generally its name.
1654 You can use this to find out whether two handles refer to the
1655 same underlying descriptor:
1657 if (fileno(THIS) == fileno(THAT)) {
1658 print "THIS and THAT are dups\n";
1661 (Filehandles connected to memory objects via new features of C<open> may
1662 return undefined even though they are open.)
1665 =item flock FILEHANDLE,OPERATION
1667 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1668 for success, false on failure. Produces a fatal error if used on a
1669 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1670 C<flock> is Perl's portable file locking interface, although it locks
1671 only entire files, not records.
1673 Two potentially non-obvious but traditional C<flock> semantics are
1674 that it waits indefinitely until the lock is granted, and that its locks
1675 B<merely advisory>. Such discretionary locks are more flexible, but offer
1676 fewer guarantees. This means that files locked with C<flock> may be
1677 modified by programs that do not also use C<flock>. See L<perlport>,
1678 your port's specific documentation, or your system-specific local manpages
1679 for details. It's best to assume traditional behavior if you're writing
1680 portable programs. (But if you're not, you should as always feel perfectly
1681 free to write for your own system's idiosyncrasies (sometimes called
1682 "features"). Slavish adherence to portability concerns shouldn't get
1683 in the way of your getting your job done.)
1685 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1686 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1687 you can use the symbolic names if you import them from the Fcntl module,
1688 either individually, or as a group using the ':flock' tag. LOCK_SH
1689 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1690 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1691 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1692 waiting for the lock (check the return status to see if you got it).
1694 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1695 before locking or unlocking it.
1697 Note that the emulation built with lockf(3) doesn't provide shared
1698 locks, and it requires that FILEHANDLE be open with write intent. These
1699 are the semantics that lockf(3) implements. Most if not all systems
1700 implement lockf(3) in terms of fcntl(2) locking, though, so the
1701 differing semantics shouldn't bite too many people.
1703 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1704 be open with read intent to use LOCK_SH and requires that it be open
1705 with write intent to use LOCK_EX.
1707 Note also that some versions of C<flock> cannot lock things over the
1708 network; you would need to use the more system-specific C<fcntl> for
1709 that. If you like you can force Perl to ignore your system's flock(2)
1710 function, and so provide its own fcntl(2)-based emulation, by passing
1711 the switch C<-Ud_flock> to the F<Configure> program when you configure
1714 Here's a mailbox appender for BSD systems.
1716 use Fcntl ':flock'; # import LOCK_* constants
1719 flock(MBOX,LOCK_EX);
1720 # and, in case someone appended
1721 # while we were waiting...
1726 flock(MBOX,LOCK_UN);
1729 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1730 or die "Can't open mailbox: $!";
1733 print MBOX $msg,"\n\n";
1736 On systems that support a real flock(), locks are inherited across fork()
1737 calls, whereas those that must resort to the more capricious fcntl()
1738 function lose the locks, making it harder to write servers.
1740 See also L<DB_File> for other flock() examples.
1744 Does a fork(2) system call to create a new process running the
1745 same program at the same point. It returns the child pid to the
1746 parent process, C<0> to the child process, or C<undef> if the fork is
1747 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1748 are shared, while everything else is copied. On most systems supporting
1749 fork(), great care has gone into making it extremely efficient (for
1750 example, using copy-on-write technology on data pages), making it the
1751 dominant paradigm for multitasking over the last few decades.
1753 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1754 output before forking the child process, but this may not be supported
1755 on some platforms (see L<perlport>). To be safe, you may need to set
1756 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1757 C<IO::Handle> on any open handles in order to avoid duplicate output.
1759 If you C<fork> without ever waiting on your children, you will
1760 accumulate zombies. On some systems, you can avoid this by setting
1761 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1762 forking and reaping moribund children.
1764 Note that if your forked child inherits system file descriptors like
1765 STDIN and STDOUT that are actually connected by a pipe or socket, even
1766 if you exit, then the remote server (such as, say, a CGI script or a
1767 backgrounded job launched from a remote shell) won't think you're done.
1768 You should reopen those to F</dev/null> if it's any issue.
1772 Declare a picture format for use by the C<write> function. For
1776 Test: @<<<<<<<< @||||| @>>>>>
1777 $str, $%, '$' . int($num)
1781 $num = $cost/$quantity;
1785 See L<perlform> for many details and examples.
1787 =item formline PICTURE,LIST
1789 This is an internal function used by C<format>s, though you may call it,
1790 too. It formats (see L<perlform>) a list of values according to the
1791 contents of PICTURE, placing the output into the format output
1792 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1793 Eventually, when a C<write> is done, the contents of
1794 C<$^A> are written to some filehandle, but you could also read C<$^A>
1795 yourself and then set C<$^A> back to C<"">. Note that a format typically
1796 does one C<formline> per line of form, but the C<formline> function itself
1797 doesn't care how many newlines are embedded in the PICTURE. This means
1798 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1799 You may therefore need to use multiple formlines to implement a single
1800 record format, just like the format compiler.
1802 Be careful if you put double quotes around the picture, because an C<@>
1803 character may be taken to mean the beginning of an array name.
1804 C<formline> always returns true. See L<perlform> for other examples.
1806 =item getc FILEHANDLE
1810 Returns the next character from the input file attached to FILEHANDLE,
1811 or the undefined value at end of file, or if there was an error.
1812 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1813 efficient. However, it cannot be used by itself to fetch single
1814 characters without waiting for the user to hit enter. For that, try
1815 something more like:
1818 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1821 system "stty", '-icanon', 'eol', "\001";
1827 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1830 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1834 Determination of whether $BSD_STYLE should be set
1835 is left as an exercise to the reader.
1837 The C<POSIX::getattr> function can do this more portably on
1838 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1839 module from your nearest CPAN site; details on CPAN can be found on
1844 Implements the C library function of the same name, which on most
1845 systems returns the current login from F</etc/utmp>, if any. If null,
1848 $login = getlogin || getpwuid($<) || "Kilroy";
1850 Do not consider C<getlogin> for authentication: it is not as
1851 secure as C<getpwuid>.
1853 =item getpeername SOCKET
1855 Returns the packed sockaddr address of other end of the SOCKET connection.
1858 $hersockaddr = getpeername(SOCK);
1859 ($port, $iaddr) = sockaddr_in($hersockaddr);
1860 $herhostname = gethostbyaddr($iaddr, AF_INET);
1861 $herstraddr = inet_ntoa($iaddr);
1865 Returns the current process group for the specified PID. Use
1866 a PID of C<0> to get the current process group for the
1867 current process. Will raise an exception if used on a machine that
1868 doesn't implement getpgrp(2). If PID is omitted, returns process
1869 group of current process. Note that the POSIX version of C<getpgrp>
1870 does not accept a PID argument, so only C<PID==0> is truly portable.
1874 Returns the process id of the parent process.
1876 =item getpriority WHICH,WHO
1878 Returns the current priority for a process, a process group, or a user.
1879 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1880 machine that doesn't implement getpriority(2).
1886 =item gethostbyname NAME
1888 =item getnetbyname NAME
1890 =item getprotobyname NAME
1896 =item getservbyname NAME,PROTO
1898 =item gethostbyaddr ADDR,ADDRTYPE
1900 =item getnetbyaddr ADDR,ADDRTYPE
1902 =item getprotobynumber NUMBER
1904 =item getservbyport PORT,PROTO
1922 =item sethostent STAYOPEN
1924 =item setnetent STAYOPEN
1926 =item setprotoent STAYOPEN
1928 =item setservent STAYOPEN
1942 These routines perform the same functions as their counterparts in the
1943 system library. In list context, the return values from the
1944 various get routines are as follows:
1946 ($name,$passwd,$uid,$gid,
1947 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1948 ($name,$passwd,$gid,$members) = getgr*
1949 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1950 ($name,$aliases,$addrtype,$net) = getnet*
1951 ($name,$aliases,$proto) = getproto*
1952 ($name,$aliases,$port,$proto) = getserv*
1954 (If the entry doesn't exist you get a null list.)
1956 The exact meaning of the $gcos field varies but it usually contains
1957 the real name of the user (as opposed to the login name) and other
1958 information pertaining to the user. Beware, however, that in many
1959 system users are able to change this information and therefore it
1960 cannot be trusted and therefore the $gcos is tainted (see
1961 L<perlsec>). The $passwd and $shell, user's encrypted password and
1962 login shell, are also tainted, because of the same reason.
1964 In scalar context, you get the name, unless the function was a
1965 lookup by name, in which case you get the other thing, whatever it is.
1966 (If the entry doesn't exist you get the undefined value.) For example:
1968 $uid = getpwnam($name);
1969 $name = getpwuid($num);
1971 $gid = getgrnam($name);
1972 $name = getgrgid($num;
1976 In I<getpw*()> the fields $quota, $comment, and $expire are special
1977 cases in the sense that in many systems they are unsupported. If the
1978 $quota is unsupported, it is an empty scalar. If it is supported, it
1979 usually encodes the disk quota. If the $comment field is unsupported,
1980 it is an empty scalar. If it is supported it usually encodes some
1981 administrative comment about the user. In some systems the $quota
1982 field may be $change or $age, fields that have to do with password
1983 aging. In some systems the $comment field may be $class. The $expire
1984 field, if present, encodes the expiration period of the account or the
1985 password. For the availability and the exact meaning of these fields
1986 in your system, please consult your getpwnam(3) documentation and your
1987 F<pwd.h> file. You can also find out from within Perl what your
1988 $quota and $comment fields mean and whether you have the $expire field
1989 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1990 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1991 files are only supported if your vendor has implemented them in the
1992 intuitive fashion that calling the regular C library routines gets the
1993 shadow versions if you're running under privilege or if there exists
1994 the shadow(3) functions as found in System V ( this includes Solaris
1995 and Linux.) Those systems which implement a proprietary shadow password
1996 facility are unlikely to be supported.
1998 The $members value returned by I<getgr*()> is a space separated list of
1999 the login names of the members of the group.
2001 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2002 C, it will be returned to you via C<$?> if the function call fails. The
2003 C<@addrs> value returned by a successful call is a list of the raw
2004 addresses returned by the corresponding system library call. In the
2005 Internet domain, each address is four bytes long and you can unpack it
2006 by saying something like:
2008 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2010 The Socket library makes this slightly easier:
2013 $iaddr = inet_aton("127.1"); # or whatever address
2014 $name = gethostbyaddr($iaddr, AF_INET);
2016 # or going the other way
2017 $straddr = inet_ntoa($iaddr);
2019 If you get tired of remembering which element of the return list
2020 contains which return value, by-name interfaces are provided
2021 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2022 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2023 and C<User::grent>. These override the normal built-ins, supplying
2024 versions that return objects with the appropriate names
2025 for each field. For example:
2029 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2031 Even though it looks like they're the same method calls (uid),
2032 they aren't, because a C<File::stat> object is different from
2033 a C<User::pwent> object.
2035 =item getsockname SOCKET
2037 Returns the packed sockaddr address of this end of the SOCKET connection,
2038 in case you don't know the address because you have several different
2039 IPs that the connection might have come in on.
2042 $mysockaddr = getsockname(SOCK);
2043 ($port, $myaddr) = sockaddr_in($mysockaddr);
2044 printf "Connect to %s [%s]\n",
2045 scalar gethostbyaddr($myaddr, AF_INET),
2048 =item getsockopt SOCKET,LEVEL,OPTNAME
2050 Returns the socket option requested, or undef if there is an error.
2056 In list context, returns a (possibly empty) list of filename expansions on
2057 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2058 scalar context, glob iterates through such filename expansions, returning
2059 undef when the list is exhausted. This is the internal function
2060 implementing the C<< <*.c> >> operator, but you can use it directly. If
2061 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2062 more detail in L<perlop/"I/O Operators">.
2064 Beginning with v5.6.0, this operator is implemented using the standard
2065 C<File::Glob> extension. See L<File::Glob> for details.
2069 Converts a time as returned by the time function to an 8-element list
2070 with the time localized for the standard Greenwich time zone.
2071 Typically used as follows:
2074 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2077 All list elements are numeric, and come straight out of the C `struct
2078 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2079 specified time. $mday is the day of the month, and $mon is the month
2080 itself, in the range C<0..11> with 0 indicating January and 11
2081 indicating December. $year is the number of years since 1900. That
2082 is, $year is C<123> in year 2023. $wday is the day of the week, with
2083 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2084 the year, in the range C<0..364> (or C<0..365> in leap years.)
2086 Note that the $year element is I<not> simply the last two digits of
2087 the year. If you assume it is, then you create non-Y2K-compliant
2088 programs--and you wouldn't want to do that, would you?
2090 The proper way to get a complete 4-digit year is simply:
2094 And to get the last two digits of the year (e.g., '01' in 2001) do:
2096 $year = sprintf("%02d", $year % 100);
2098 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2100 In scalar context, C<gmtime()> returns the ctime(3) value:
2102 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2104 Also see the C<timegm> function provided by the C<Time::Local> module,
2105 and the strftime(3) function available via the POSIX module.
2107 This scalar value is B<not> locale dependent (see L<perllocale>), but
2108 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2109 strftime(3) and mktime(3) functions available via the POSIX module. To
2110 get somewhat similar but locale dependent date strings, set up your
2111 locale environment variables appropriately (please see L<perllocale>)
2112 and try for example:
2114 use POSIX qw(strftime);
2115 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2117 Note that the C<%a> and C<%b> escapes, which represent the short forms
2118 of the day of the week and the month of the year, may not necessarily
2119 be three characters wide in all locales.
2127 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2128 execution there. It may not be used to go into any construct that
2129 requires initialization, such as a subroutine or a C<foreach> loop. It
2130 also can't be used to go into a construct that is optimized away,
2131 or to get out of a block or subroutine given to C<sort>.
2132 It can be used to go almost anywhere else within the dynamic scope,
2133 including out of subroutines, but it's usually better to use some other
2134 construct such as C<last> or C<die>. The author of Perl has never felt the
2135 need to use this form of C<goto> (in Perl, that is--C is another matter).
2136 (The difference being that C does not offer named loops combined with
2137 loop control. Perl does, and this replaces most structured uses of C<goto>
2138 in other languages.)
2140 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2141 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2142 necessarily recommended if you're optimizing for maintainability:
2144 goto ("FOO", "BAR", "GLARCH")[$i];
2146 The C<goto-&NAME> form is quite different from the other forms of
2147 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2148 doesn't have the stigma associated with other gotos. Instead, it
2149 exits the current subroutine (losing any changes set by local()) and
2150 immediately calls in its place the named subroutine using the current
2151 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2152 load another subroutine and then pretend that the other subroutine had
2153 been called in the first place (except that any modifications to C<@_>
2154 in the current subroutine are propagated to the other subroutine.)
2155 After the C<goto>, not even C<caller> will be able to tell that this
2156 routine was called first.
2158 NAME needn't be the name of a subroutine; it can be a scalar variable
2159 containing a code reference, or a block which evaluates to a code
2162 =item grep BLOCK LIST
2164 =item grep EXPR,LIST
2166 This is similar in spirit to, but not the same as, grep(1) and its
2167 relatives. In particular, it is not limited to using regular expressions.
2169 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2170 C<$_> to each element) and returns the list value consisting of those
2171 elements for which the expression evaluated to true. In scalar
2172 context, returns the number of times the expression was true.
2174 @foo = grep(!/^#/, @bar); # weed out comments
2178 @foo = grep {!/^#/} @bar; # weed out comments
2180 Note that C<$_> is an alias to the list value, so it can be used to
2181 modify the elements of the LIST. While this is useful and supported,
2182 it can cause bizarre results if the elements of LIST are not variables.
2183 Similarly, grep returns aliases into the original list, much as a for
2184 loop's index variable aliases the list elements. That is, modifying an
2185 element of a list returned by grep (for example, in a C<foreach>, C<map>
2186 or another C<grep>) actually modifies the element in the original list.
2187 This is usually something to be avoided when writing clear code.
2189 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2195 Interprets EXPR as a hex string and returns the corresponding value.
2196 (To convert strings that might start with either 0, 0x, or 0b, see
2197 L</oct>.) If EXPR is omitted, uses C<$_>.
2199 print hex '0xAf'; # prints '175'
2200 print hex 'aF'; # same
2202 Hex strings may only represent integers. Strings that would cause
2203 integer overflow trigger a warning. Leading whitespace is not stripped,
2208 There is no builtin C<import> function. It is just an ordinary
2209 method (subroutine) defined (or inherited) by modules that wish to export
2210 names to another module. The C<use> function calls the C<import> method
2211 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2213 =item index STR,SUBSTR,POSITION
2215 =item index STR,SUBSTR
2217 The index function searches for one string within another, but without
2218 the wildcard-like behavior of a full regular-expression pattern match.
2219 It returns the position of the first occurrence of SUBSTR in STR at
2220 or after POSITION. If POSITION is omitted, starts searching from the
2221 beginning of the string. The return value is based at C<0> (or whatever
2222 you've set the C<$[> variable to--but don't do that). If the substring
2223 is not found, returns one less than the base, ordinarily C<-1>.
2229 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2230 You should not use this function for rounding: one because it truncates
2231 towards C<0>, and two because machine representations of floating point
2232 numbers can sometimes produce counterintuitive results. For example,
2233 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2234 because it's really more like -268.99999999999994315658 instead. Usually,
2235 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2236 functions will serve you better than will int().
2238 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2240 Implements the ioctl(2) function. You'll probably first have to say
2242 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2244 to get the correct function definitions. If F<ioctl.ph> doesn't
2245 exist or doesn't have the correct definitions you'll have to roll your
2246 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2247 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2248 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2249 written depending on the FUNCTION--a pointer to the string value of SCALAR
2250 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2251 has no string value but does have a numeric value, that value will be
2252 passed rather than a pointer to the string value. To guarantee this to be
2253 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2254 functions may be needed to manipulate the values of structures used by
2257 The return value of C<ioctl> (and C<fcntl>) is as follows:
2259 if OS returns: then Perl returns:
2261 0 string "0 but true"
2262 anything else that number
2264 Thus Perl returns true on success and false on failure, yet you can
2265 still easily determine the actual value returned by the operating
2268 $retval = ioctl(...) || -1;
2269 printf "System returned %d\n", $retval;
2271 The special string "C<0> but true" is exempt from B<-w> complaints
2272 about improper numeric conversions.
2274 Here's an example of setting a filehandle named C<REMOTE> to be
2275 non-blocking at the system level. You'll have to negotiate C<$|>
2276 on your own, though.
2278 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2280 $flags = fcntl(REMOTE, F_GETFL, 0)
2281 or die "Can't get flags for the socket: $!\n";
2283 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2284 or die "Can't set flags for the socket: $!\n";
2286 =item join EXPR,LIST
2288 Joins the separate strings of LIST into a single string with fields
2289 separated by the value of EXPR, and returns that new string. Example:
2291 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2293 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2294 first argument. Compare L</split>.
2298 Returns a list consisting of all the keys of the named hash. (In
2299 scalar context, returns the number of keys.) The keys are returned in
2300 an apparently random order. The actual random order is subject to
2301 change in future versions of perl, but it is guaranteed to be the same
2302 order as either the C<values> or C<each> function produces (given
2303 that the hash has not been modified). As a side effect, it resets
2306 Here is yet another way to print your environment:
2309 @values = values %ENV;
2311 print pop(@keys), '=', pop(@values), "\n";
2314 or how about sorted by key:
2316 foreach $key (sort(keys %ENV)) {
2317 print $key, '=', $ENV{$key}, "\n";
2320 The returned values are copies of the original keys in the hash, so
2321 modifying them will not affect the original hash. Compare L</values>.
2323 To sort a hash by value, you'll need to use a C<sort> function.
2324 Here's a descending numeric sort of a hash by its values:
2326 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2327 printf "%4d %s\n", $hash{$key}, $key;
2330 As an lvalue C<keys> allows you to increase the number of hash buckets
2331 allocated for the given hash. This can gain you a measure of efficiency if
2332 you know the hash is going to get big. (This is similar to pre-extending
2333 an array by assigning a larger number to $#array.) If you say
2337 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2338 in fact, since it rounds up to the next power of two. These
2339 buckets will be retained even if you do C<%hash = ()>, use C<undef
2340 %hash> if you want to free the storage while C<%hash> is still in scope.
2341 You can't shrink the number of buckets allocated for the hash using
2342 C<keys> in this way (but you needn't worry about doing this by accident,
2343 as trying has no effect).
2345 See also C<each>, C<values> and C<sort>.
2347 =item kill SIGNAL, LIST
2349 Sends a signal to a list of processes. Returns the number of
2350 processes successfully signaled (which is not necessarily the
2351 same as the number actually killed).
2353 $cnt = kill 1, $child1, $child2;
2356 If SIGNAL is zero, no signal is sent to the process. This is a
2357 useful way to check that the process is alive and hasn't changed
2358 its UID. See L<perlport> for notes on the portability of this
2361 Unlike in the shell, if SIGNAL is negative, it kills
2362 process groups instead of processes. (On System V, a negative I<PROCESS>
2363 number will also kill process groups, but that's not portable.) That
2364 means you usually want to use positive not negative signals. You may also
2365 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2371 The C<last> command is like the C<break> statement in C (as used in
2372 loops); it immediately exits the loop in question. If the LABEL is
2373 omitted, the command refers to the innermost enclosing loop. The
2374 C<continue> block, if any, is not executed:
2376 LINE: while (<STDIN>) {
2377 last LINE if /^$/; # exit when done with header
2381 C<last> cannot be used to exit a block which returns a value such as
2382 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2383 a grep() or map() operation.
2385 Note that a block by itself is semantically identical to a loop
2386 that executes once. Thus C<last> can be used to effect an early
2387 exit out of such a block.
2389 See also L</continue> for an illustration of how C<last>, C<next>, and
2396 Returns a lowercased version of EXPR. This is the internal function
2397 implementing the C<\L> escape in double-quoted strings. Respects
2398 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2399 and L<perlunicode> for more details about locale and Unicode support.
2401 If EXPR is omitted, uses C<$_>.
2407 Returns the value of EXPR with the first character lowercased. This
2408 is the internal function implementing the C<\l> escape in
2409 double-quoted strings. Respects current LC_CTYPE locale if C<use
2410 locale> in force. See L<perllocale> and L<perlunicode> for more
2411 details about locale and Unicode support.
2413 If EXPR is omitted, uses C<$_>.
2419 Returns the length in characters of the value of EXPR. If EXPR is
2420 omitted, returns length of C<$_>. Note that this cannot be used on
2421 an entire array or hash to find out how many elements these have.
2422 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2424 =item link OLDFILE,NEWFILE
2426 Creates a new filename linked to the old filename. Returns true for
2427 success, false otherwise.
2429 =item listen SOCKET,QUEUESIZE
2431 Does the same thing that the listen system call does. Returns true if
2432 it succeeded, false otherwise. See the example in
2433 L<perlipc/"Sockets: Client/Server Communication">.
2437 You really probably want to be using C<my> instead, because C<local> isn't
2438 what most people think of as "local". See
2439 L<perlsub/"Private Variables via my()"> for details.
2441 A local modifies the listed variables to be local to the enclosing
2442 block, file, or eval. If more than one value is listed, the list must
2443 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2444 for details, including issues with tied arrays and hashes.
2446 =item localtime EXPR
2448 Converts a time as returned by the time function to a 9-element list
2449 with the time analyzed for the local time zone. Typically used as
2453 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2456 All list elements are numeric, and come straight out of the C `struct
2457 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2458 specified time. $mday is the day of the month, and $mon is the month
2459 itself, in the range C<0..11> with 0 indicating January and 11
2460 indicating December. $year is the number of years since 1900. That
2461 is, $year is C<123> in year 2023. $wday is the day of the week, with
2462 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2463 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2464 is true if the specified time occurs during daylight savings time,
2467 Note that the $year element is I<not> simply the last two digits of
2468 the year. If you assume it is, then you create non-Y2K-compliant
2469 programs--and you wouldn't want to do that, would you?
2471 The proper way to get a complete 4-digit year is simply:
2475 And to get the last two digits of the year (e.g., '01' in 2001) do:
2477 $year = sprintf("%02d", $year % 100);
2479 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2481 In scalar context, C<localtime()> returns the ctime(3) value:
2483 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2485 This scalar value is B<not> locale dependent, see L<perllocale>, but
2486 instead a Perl builtin. Also see the C<Time::Local> module
2487 (to convert the second, minutes, hours, ... back to seconds since the
2488 stroke of midnight the 1st of January 1970, the value returned by
2489 time()), and the strftime(3) and mktime(3) functions available via the
2490 POSIX module. To get somewhat similar but locale dependent date
2491 strings, set up your locale environment variables appropriately
2492 (please see L<perllocale>) and try for example:
2494 use POSIX qw(strftime);
2495 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2497 Note that the C<%a> and C<%b>, the short forms of the day of the week
2498 and the month of the year, may not necessarily be three characters wide.
2502 This function places an advisory lock on a shared variable, or referenced
2503 object contained in I<THING> until the lock goes out of scope.
2505 lock() is a "weak keyword" : this means that if you've defined a function
2506 by this name (before any calls to it), that function will be called
2507 instead. (However, if you've said C<use threads>, lock() is always a
2508 keyword.) See L<threads>.
2514 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2515 returns log of C<$_>. To get the log of another base, use basic algebra:
2516 The base-N log of a number is equal to the natural log of that number
2517 divided by the natural log of N. For example:
2521 return log($n)/log(10);
2524 See also L</exp> for the inverse operation.
2530 Does the same thing as the C<stat> function (including setting the
2531 special C<_> filehandle) but stats a symbolic link instead of the file
2532 the symbolic link points to. If symbolic links are unimplemented on
2533 your system, a normal C<stat> is done.
2535 If EXPR is omitted, stats C<$_>.
2539 The match operator. See L<perlop>.
2541 =item map BLOCK LIST
2545 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2546 C<$_> to each element) and returns the list value composed of the
2547 results of each such evaluation. In scalar context, returns the
2548 total number of elements so generated. Evaluates BLOCK or EXPR in
2549 list context, so each element of LIST may produce zero, one, or
2550 more elements in the returned value.
2552 @chars = map(chr, @nums);
2554 translates a list of numbers to the corresponding characters. And
2556 %hash = map { getkey($_) => $_ } @array;
2558 is just a funny way to write
2561 foreach $_ (@array) {
2562 $hash{getkey($_)} = $_;
2565 Note that C<$_> is an alias to the list value, so it can be used to
2566 modify the elements of the LIST. While this is useful and supported,
2567 it can cause bizarre results if the elements of LIST are not variables.
2568 Using a regular C<foreach> loop for this purpose would be clearer in
2569 most cases. See also L</grep> for an array composed of those items of
2570 the original list for which the BLOCK or EXPR evaluates to true.
2572 C<{> starts both hash references and blocks, so C<map { ...> could be either
2573 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2574 ahead for the closing C<}> it has to take a guess at which its dealing with
2575 based what it finds just after the C<{>. Usually it gets it right, but if it
2576 doesn't it won't realize something is wrong until it gets to the C<}> and
2577 encounters the missing (or unexpected) comma. The syntax error will be
2578 reported close to the C<}> but you'll need to change something near the C<{>
2579 such as using a unary C<+> to give perl some help:
2581 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2582 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2583 %hash = map { ("\L$_", 1) } @array # this also works
2584 %hash = map { lc($_), 1 } @array # as does this.
2585 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2587 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2589 or to force an anon hash constructor use C<+{>
2591 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2593 and you get list of anonymous hashes each with only 1 entry.
2595 =item mkdir FILENAME,MASK
2597 =item mkdir FILENAME
2599 Creates the directory specified by FILENAME, with permissions
2600 specified by MASK (as modified by C<umask>). If it succeeds it
2601 returns true, otherwise it returns false and sets C<$!> (errno).
2602 If omitted, MASK defaults to 0777.
2604 In general, it is better to create directories with permissive MASK,
2605 and let the user modify that with their C<umask>, than it is to supply
2606 a restrictive MASK and give the user no way to be more permissive.
2607 The exceptions to this rule are when the file or directory should be
2608 kept private (mail files, for instance). The perlfunc(1) entry on
2609 C<umask> discusses the choice of MASK in more detail.
2611 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2612 number of trailing slashes. Some operating and filesystems do not get
2613 this right, so Perl automatically removes all trailing slashes to keep
2616 =item msgctl ID,CMD,ARG
2618 Calls the System V IPC function msgctl(2). You'll probably have to say
2622 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2623 then ARG must be a variable which will hold the returned C<msqid_ds>
2624 structure. Returns like C<ioctl>: the undefined value for error,
2625 C<"0 but true"> for zero, or the actual return value otherwise. See also
2626 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2628 =item msgget KEY,FLAGS
2630 Calls the System V IPC function msgget(2). Returns the message queue
2631 id, or the undefined value if there is an error. See also
2632 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2634 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2636 Calls the System V IPC function msgrcv to receive a message from
2637 message queue ID into variable VAR with a maximum message size of
2638 SIZE. Note that when a message is received, the message type as a
2639 native long integer will be the first thing in VAR, followed by the
2640 actual message. This packing may be opened with C<unpack("l! a*")>.
2641 Taints the variable. Returns true if successful, or false if there is
2642 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2643 C<IPC::SysV::Msg> documentation.
2645 =item msgsnd ID,MSG,FLAGS
2647 Calls the System V IPC function msgsnd to send the message MSG to the
2648 message queue ID. MSG must begin with the native long integer message
2649 type, and be followed by the length of the actual message, and finally
2650 the message itself. This kind of packing can be achieved with
2651 C<pack("l! a*", $type, $message)>. Returns true if successful,
2652 or false if there is an error. See also C<IPC::SysV>
2653 and C<IPC::SysV::Msg> documentation.
2659 =item my EXPR : ATTRS
2661 =item my TYPE EXPR : ATTRS
2663 A C<my> declares the listed variables to be local (lexically) to the
2664 enclosing block, file, or C<eval>. If more than one value is listed,
2665 the list must be placed in parentheses.
2667 The exact semantics and interface of TYPE and ATTRS are still
2668 evolving. TYPE is currently bound to the use of C<fields> pragma,
2669 and attributes are handled using the C<attributes> pragma, or starting
2670 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2671 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2672 L<attributes>, and L<Attribute::Handlers>.
2678 The C<next> command is like the C<continue> statement in C; it starts
2679 the next iteration of the loop:
2681 LINE: while (<STDIN>) {
2682 next LINE if /^#/; # discard comments
2686 Note that if there were a C<continue> block on the above, it would get
2687 executed even on discarded lines. If the LABEL is omitted, the command
2688 refers to the innermost enclosing loop.
2690 C<next> cannot be used to exit a block which returns a value such as
2691 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2692 a grep() or map() operation.
2694 Note that a block by itself is semantically identical to a loop
2695 that executes once. Thus C<next> will exit such a block early.
2697 See also L</continue> for an illustration of how C<last>, C<next>, and
2700 =item no Module VERSION LIST
2702 =item no Module VERSION
2704 =item no Module LIST
2708 See the L</use> function, which C<no> is the opposite of.
2714 Interprets EXPR as an octal string and returns the corresponding
2715 value. (If EXPR happens to start off with C<0x>, interprets it as a
2716 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2717 binary string. Leading whitespace is ignored in all three cases.)
2718 The following will handle decimal, binary, octal, and hex in the standard
2721 $val = oct($val) if $val =~ /^0/;
2723 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2724 in octal), use sprintf() or printf():
2726 $perms = (stat("filename"))[2] & 07777;
2727 $oct_perms = sprintf "%lo", $perms;
2729 The oct() function is commonly used when a string such as C<644> needs
2730 to be converted into a file mode, for example. (Although perl will
2731 automatically convert strings into numbers as needed, this automatic
2732 conversion assumes base 10.)
2734 =item open FILEHANDLE,EXPR
2736 =item open FILEHANDLE,MODE,EXPR
2738 =item open FILEHANDLE,MODE,EXPR,LIST
2740 =item open FILEHANDLE,MODE,REFERENCE
2742 =item open FILEHANDLE
2744 Opens the file whose filename is given by EXPR, and associates it with
2747 (The following is a comprehensive reference to open(): for a gentler
2748 introduction you may consider L<perlopentut>.)
2750 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2751 assigned a reference to a new anonymous filehandle, otherwise if
2752 FILEHANDLE is an expression, its value is used as the name of the real
2753 filehandle wanted. (This is considered a symbolic reference, so C<use
2754 strict 'refs'> should I<not> be in effect.)
2756 If EXPR is omitted, the scalar variable of the same name as the
2757 FILEHANDLE contains the filename. (Note that lexical variables--those
2758 declared with C<my>--will not work for this purpose; so if you're
2759 using C<my>, specify EXPR in your call to open.)
2761 If three or more arguments are specified then the mode of opening and
2762 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2763 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2764 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2765 the file is opened for appending, again being created if necessary.
2767 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2768 indicate that you want both read and write access to the file; thus
2769 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2770 '+>' >> mode would clobber the file first. You can't usually use
2771 either read-write mode for updating textfiles, since they have
2772 variable length records. See the B<-i> switch in L<perlrun> for a
2773 better approach. The file is created with permissions of C<0666>
2774 modified by the process' C<umask> value.
2776 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2777 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2779 In the 2-arguments (and 1-argument) form of the call the mode and
2780 filename should be concatenated (in this order), possibly separated by
2781 spaces. It is possible to omit the mode in these forms if the mode is
2784 If the filename begins with C<'|'>, the filename is interpreted as a
2785 command to which output is to be piped, and if the filename ends with a
2786 C<'|'>, the filename is interpreted as a command which pipes output to
2787 us. See L<perlipc/"Using open() for IPC">
2788 for more examples of this. (You are not allowed to C<open> to a command
2789 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2790 and L<perlipc/"Bidirectional Communication with Another Process">
2793 For three or more arguments if MODE is C<'|-'>, the filename is
2794 interpreted as a command to which output is to be piped, and if MODE
2795 is C<'-|'>, the filename is interpreted as a command which pipes
2796 output to us. In the 2-arguments (and 1-argument) form one should
2797 replace dash (C<'-'>) with the command.
2798 See L<perlipc/"Using open() for IPC"> for more examples of this.
2799 (You are not allowed to C<open> to a command that pipes both in I<and>
2800 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2801 L<perlipc/"Bidirectional Communication"> for alternatives.)
2803 In the three-or-more argument form of pipe opens, if LIST is specified
2804 (extra arguments after the command name) then LIST becomes arguments
2805 to the command invoked if the platform supports it. The meaning of
2806 C<open> with more than three arguments for non-pipe modes is not yet
2807 specified. Experimental "layers" may give extra LIST arguments
2810 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2811 and opening C<< '>-' >> opens STDOUT.
2813 You may use the three-argument form of open to specify IO "layers"
2814 (sometimes also referred to as "disciplines") to be applied to the handle
2815 that affect how the input and output are processed (see L<open> and
2816 L<PerlIO> for more details). For example
2818 open(FH, "<:utf8", "file")
2820 will open the UTF-8 encoded file containing Unicode characters,
2821 see L<perluniintro>. (Note that if layers are specified in the
2822 three-arg form then default layers set by the C<open> pragma are
2825 Open returns nonzero upon success, the undefined value otherwise. If
2826 the C<open> involved a pipe, the return value happens to be the pid of
2829 If you're running Perl on a system that distinguishes between text
2830 files and binary files, then you should check out L</binmode> for tips
2831 for dealing with this. The key distinction between systems that need
2832 C<binmode> and those that don't is their text file formats. Systems
2833 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2834 character, and which encode that character in C as C<"\n">, do not
2835 need C<binmode>. The rest need it.
2837 When opening a file, it's usually a bad idea to continue normal execution
2838 if the request failed, so C<open> is frequently used in connection with
2839 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2840 where you want to make a nicely formatted error message (but there are
2841 modules that can help with that problem)) you should always check
2842 the return value from opening a file. The infrequent exception is when
2843 working with an unopened filehandle is actually what you want to do.
2845 As a special case the 3 arg form with a read/write mode and the third
2846 argument being C<undef>:
2848 open(TMP, "+>", undef) or die ...
2850 opens a filehandle to an anonymous temporary file.
2852 File handles can be opened to "in memory" files held in Perl scalars via:
2854 open($fh, '>', \$variable) || ..
2856 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2857 file, you have to close it first:
2860 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2865 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2866 while (<ARTICLE>) {...
2868 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2869 # if the open fails, output is discarded
2871 open(DBASE, '+<', 'dbase.mine') # open for update
2872 or die "Can't open 'dbase.mine' for update: $!";
2874 open(DBASE, '+<dbase.mine') # ditto
2875 or die "Can't open 'dbase.mine' for update: $!";
2877 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2878 or die "Can't start caesar: $!";
2880 open(ARTICLE, "caesar <$article |") # ditto
2881 or die "Can't start caesar: $!";
2883 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2884 or die "Can't start sort: $!";
2887 open(MEMORY,'>', \$var)
2888 or die "Can't open memory file: $!";
2889 print MEMORY "foo!\n"; # output will end up in $var
2891 # process argument list of files along with any includes
2893 foreach $file (@ARGV) {
2894 process($file, 'fh00');
2898 my($filename, $input) = @_;
2899 $input++; # this is a string increment
2900 unless (open($input, $filename)) {
2901 print STDERR "Can't open $filename: $!\n";
2906 while (<$input>) { # note use of indirection
2907 if (/^#include "(.*)"/) {
2908 process($1, $input);
2915 You may also, in the Bourne shell tradition, specify an EXPR beginning
2916 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2917 name of a filehandle (or file descriptor, if numeric) to be
2918 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2919 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2920 mode you specify should match the mode of the original filehandle.
2921 (Duping a filehandle does not take into account any existing contents of
2922 IO buffers.) If you use the 3 arg form then you can pass either a number,
2923 the name of a filehandle or the normal "reference to a glob".
2925 Here is a script that saves, redirects, and restores C<STDOUT> and
2926 C<STDERR> using various methods:
2929 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2930 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2932 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2933 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2935 select STDERR; $| = 1; # make unbuffered
2936 select STDOUT; $| = 1; # make unbuffered
2938 print STDOUT "stdout 1\n"; # this works for
2939 print STDERR "stderr 1\n"; # subprocesses too
2944 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2945 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2947 print STDOUT "stdout 2\n";
2948 print STDERR "stderr 2\n";
2950 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2951 do an equivalent of C's C<fdopen> of that file descriptor; this is
2952 more parsimonious of file descriptors. For example:
2954 open(FILEHANDLE, "<&=$fd")
2958 open(FILEHANDLE, "<&=", $fd)
2960 Note that if Perl is using the standard C libraries' fdopen() then on
2961 many UNIX systems, fdopen() is known to fail when file descriptors
2962 exceed a certain value, typically 255. If you need more file
2963 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2965 You can see whether Perl has been compiled with PerlIO or not by
2966 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2967 is C<define>, you have PerlIO, otherwise you don't.
2969 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2970 with 2-arguments (or 1-argument) form of open(), then
2971 there is an implicit fork done, and the return value of open is the pid
2972 of the child within the parent process, and C<0> within the child
2973 process. (Use C<defined($pid)> to determine whether the open was successful.)
2974 The filehandle behaves normally for the parent, but i/o to that
2975 filehandle is piped from/to the STDOUT/STDIN of the child process.
2976 In the child process the filehandle isn't opened--i/o happens from/to
2977 the new STDOUT or STDIN. Typically this is used like the normal
2978 piped open when you want to exercise more control over just how the
2979 pipe command gets executed, such as when you are running setuid, and
2980 don't want to have to scan shell commands for metacharacters.
2981 The following triples are more or less equivalent:
2983 open(FOO, "|tr '[a-z]' '[A-Z]'");
2984 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2985 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2986 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2988 open(FOO, "cat -n '$file'|");
2989 open(FOO, '-|', "cat -n '$file'");
2990 open(FOO, '-|') || exec 'cat', '-n', $file;
2991 open(FOO, '-|', "cat", '-n', $file);
2993 The last example in each block shows the pipe as "list form", which is
2994 not yet supported on all platforms. A good rule of thumb is that if
2995 your platform has true C<fork()> (in other words, if your platform is
2996 UNIX) you can use the list form.
2998 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3000 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3001 output before any operation that may do a fork, but this may not be
3002 supported on some platforms (see L<perlport>). To be safe, you may need
3003 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3004 of C<IO::Handle> on any open handles.
3006 On systems that support a close-on-exec flag on files, the flag will
3007 be set for the newly opened file descriptor as determined by the value
3008 of $^F. See L<perlvar/$^F>.
3010 Closing any piped filehandle causes the parent process to wait for the
3011 child to finish, and returns the status value in C<$?>.
3013 The filename passed to 2-argument (or 1-argument) form of open() will
3014 have leading and trailing whitespace deleted, and the normal
3015 redirection characters honored. This property, known as "magic open",
3016 can often be used to good effect. A user could specify a filename of
3017 F<"rsh cat file |">, or you could change certain filenames as needed:
3019 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3020 open(FH, $filename) or die "Can't open $filename: $!";
3022 Use 3-argument form to open a file with arbitrary weird characters in it,
3024 open(FOO, '<', $file);
3026 otherwise it's necessary to protect any leading and trailing whitespace:
3028 $file =~ s#^(\s)#./$1#;
3029 open(FOO, "< $file\0");
3031 (this may not work on some bizarre filesystems). One should
3032 conscientiously choose between the I<magic> and 3-arguments form
3037 will allow the user to specify an argument of the form C<"rsh cat file |">,
3038 but will not work on a filename which happens to have a trailing space, while
3040 open IN, '<', $ARGV[0];
3042 will have exactly the opposite restrictions.
3044 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3045 should use the C<sysopen> function, which involves no such magic (but
3046 may use subtly different filemodes than Perl open(), which is mapped
3047 to C fopen()). This is
3048 another way to protect your filenames from interpretation. For example:
3051 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3052 or die "sysopen $path: $!";
3053 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3054 print HANDLE "stuff $$\n";
3056 print "File contains: ", <HANDLE>;
3058 Using the constructor from the C<IO::Handle> package (or one of its
3059 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3060 filehandles that have the scope of whatever variables hold references to
3061 them, and automatically close whenever and however you leave that scope:
3065 sub read_myfile_munged {
3067 my $handle = new IO::File;
3068 open($handle, "myfile") or die "myfile: $!";
3070 or return (); # Automatically closed here.
3071 mung $first or die "mung failed"; # Or here.
3072 return $first, <$handle> if $ALL; # Or here.
3076 See L</seek> for some details about mixing reading and writing.
3078 =item opendir DIRHANDLE,EXPR
3080 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3081 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3082 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3088 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3089 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3092 For the reverse, see L</chr>.
3093 See L<perlunicode> and L<encoding> for more about Unicode.
3099 =item our EXPR : ATTRS
3101 =item our TYPE EXPR : ATTRS
3103 An C<our> declares the listed variables to be valid globals within
3104 the enclosing block, file, or C<eval>. That is, it has the same
3105 scoping rules as a "my" declaration, but does not create a local
3106 variable. If more than one value is listed, the list must be placed
3107 in parentheses. The C<our> declaration has no semantic effect unless
3108 "use strict vars" is in effect, in which case it lets you use the
3109 declared global variable without qualifying it with a package name.
3110 (But only within the lexical scope of the C<our> declaration. In this
3111 it differs from "use vars", which is package scoped.)
3113 An C<our> declaration declares a global variable that will be visible
3114 across its entire lexical scope, even across package boundaries. The
3115 package in which the variable is entered is determined at the point
3116 of the declaration, not at the point of use. This means the following
3120 our $bar; # declares $Foo::bar for rest of lexical scope
3124 print $bar; # prints 20
3126 Multiple C<our> declarations in the same lexical scope are allowed
3127 if they are in different packages. If they happened to be in the same
3128 package, Perl will emit warnings if you have asked for them.
3132 our $bar; # declares $Foo::bar for rest of lexical scope
3136 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3137 print $bar; # prints 30
3139 our $bar; # emits warning
3141 An C<our> declaration may also have a list of attributes associated
3144 The exact semantics and interface of TYPE and ATTRS are still
3145 evolving. TYPE is currently bound to the use of C<fields> pragma,
3146 and attributes are handled using the C<attributes> pragma, or starting
3147 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3148 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3149 L<attributes>, and L<Attribute::Handlers>.
3151 The only currently recognized C<our()> attribute is C<unique> which
3152 indicates that a single copy of the global is to be used by all
3153 interpreters should the program happen to be running in a
3154 multi-interpreter environment. (The default behaviour would be for
3155 each interpreter to have its own copy of the global.) Examples:
3157 our @EXPORT : unique = qw(foo);
3158 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3159 our $VERSION : unique = "1.00";
3161 Note that this attribute also has the effect of making the global
3162 readonly when the first new interpreter is cloned (for example,
3163 when the first new thread is created).
3165 Multi-interpreter environments can come to being either through the
3166 fork() emulation on Windows platforms, or by embedding perl in a
3167 multi-threaded application. The C<unique> attribute does nothing in
3168 all other environments.
3170 =item pack TEMPLATE,LIST
3172 Takes a LIST of values and converts it into a string using the rules
3173 given by the TEMPLATE. The resulting string is the concatenation of
3174 the converted values. Typically, each converted value looks
3175 like its machine-level representation. For example, on 32-bit machines
3176 a converted integer may be represented by a sequence of 4 bytes.
3178 The TEMPLATE is a sequence of characters that give the order and type
3179 of values, as follows:
3181 a A string with arbitrary binary data, will be null padded.
3182 A A text (ASCII) string, will be space padded.
3183 Z A null terminated (ASCIZ) string, will be null padded.
3185 b A bit string (ascending bit order inside each byte, like vec()).
3186 B A bit string (descending bit order inside each byte).
3187 h A hex string (low nybble first).
3188 H A hex string (high nybble first).
3190 c A signed char value.
3191 C An unsigned char value. Only does bytes. See U for Unicode.
3193 s A signed short value.
3194 S An unsigned short value.
3195 (This 'short' is _exactly_ 16 bits, which may differ from
3196 what a local C compiler calls 'short'. If you want
3197 native-length shorts, use the '!' suffix.)
3199 i A signed integer value.
3200 I An unsigned integer value.
3201 (This 'integer' is _at_least_ 32 bits wide. Its exact
3202 size depends on what a local C compiler calls 'int',
3203 and may even be larger than the 'long' described in
3206 l A signed long value.
3207 L An unsigned long value.
3208 (This 'long' is _exactly_ 32 bits, which may differ from
3209 what a local C compiler calls 'long'. If you want
3210 native-length longs, use the '!' suffix.)
3212 n An unsigned short in "network" (big-endian) order.
3213 N An unsigned long in "network" (big-endian) order.
3214 v An unsigned short in "VAX" (little-endian) order.
3215 V An unsigned long in "VAX" (little-endian) order.
3216 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3217 _exactly_ 32 bits, respectively.)
3219 q A signed quad (64-bit) value.
3220 Q An unsigned quad value.
3221 (Quads are available only if your system supports 64-bit
3222 integer values _and_ if Perl has been compiled to support those.
3223 Causes a fatal error otherwise.)
3225 j A signed integer value (a Perl internal integer, IV).
3226 J An unsigned integer value (a Perl internal unsigned integer, UV).
3228 f A single-precision float in the native format.
3229 d A double-precision float in the native format.
3231 F A floating point value in the native native format
3232 (a Perl internal floating point value, NV).
3233 D A long double-precision float in the native format.
3234 (Long doubles are available only if your system supports long
3235 double values _and_ if Perl has been compiled to support those.
3236 Causes a fatal error otherwise.)
3238 p A pointer to a null-terminated string.
3239 P A pointer to a structure (fixed-length string).
3241 u A uuencoded string.
3242 U A Unicode character number. Encodes to UTF-8 internally
3243 (or UTF-EBCDIC in EBCDIC platforms).
3245 w A BER compressed integer. Its bytes represent an unsigned
3246 integer in base 128, most significant digit first, with as
3247 few digits as possible. Bit eight (the high bit) is set
3248 on each byte except the last.
3252 @ Null fill to absolute position.
3253 ( Start of a ()-group.
3255 The following rules apply:
3261 Each letter may optionally be followed by a number giving a repeat
3262 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3263 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3264 many values from the LIST. A C<*> for the repeat count means to use
3265 however many items are left, except for C<@>, C<x>, C<X>, where it is
3266 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3267 is the same). A numeric repeat count may optionally be enclosed in
3268 brackets, as in C<pack 'C[80]', @arr>.
3270 One can replace the numeric repeat count by a template enclosed in brackets;
3271 then the packed length of this template in bytes is used as a count.
3272 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3273 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3274 If the template in brackets contains alignment commands (such as C<x![d]>),
3275 its packed length is calculated as if the start of the template has the maximal
3278 When used with C<Z>, C<*> results in the addition of a trailing null
3279 byte (so the packed result will be one longer than the byte C<length>
3282 The repeat count for C<u> is interpreted as the maximal number of bytes
3283 to encode per line of output, with 0 and 1 replaced by 45.
3287 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3288 string of length count, padding with nulls or spaces as necessary. When
3289 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3290 after the first null, and C<a> returns data verbatim. When packing,
3291 C<a>, and C<Z> are equivalent.
3293 If the value-to-pack is too long, it is truncated. If too long and an
3294 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3295 by a null byte. Thus C<Z> always packs a trailing null byte under
3300 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3301 Each byte of the input field of pack() generates 1 bit of the result.
3302 Each result bit is based on the least-significant bit of the corresponding
3303 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3304 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3306 Starting from the beginning of the input string of pack(), each 8-tuple
3307 of bytes is converted to 1 byte of output. With format C<b>
3308 the first byte of the 8-tuple determines the least-significant bit of a
3309 byte, and with format C<B> it determines the most-significant bit of
3312 If the length of the input string is not exactly divisible by 8, the
3313 remainder is packed as if the input string were padded by null bytes
3314 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3316 If the input string of pack() is longer than needed, extra bytes are ignored.
3317 A C<*> for the repeat count of pack() means to use all the bytes of
3318 the input field. On unpack()ing the bits are converted to a string
3319 of C<"0">s and C<"1">s.
3323 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3324 representable as hexadecimal digits, 0-9a-f) long.
3326 Each byte of the input field of pack() generates 4 bits of the result.
3327 For non-alphabetical bytes the result is based on the 4 least-significant
3328 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3329 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3330 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3331 is compatible with the usual hexadecimal digits, so that C<"a"> and
3332 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3333 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3335 Starting from the beginning of the input string of pack(), each pair
3336 of bytes is converted to 1 byte of output. With format C<h> the
3337 first byte of the pair determines the least-significant nybble of the
3338 output byte, and with format C<H> it determines the most-significant
3341 If the length of the input string is not even, it behaves as if padded
3342 by a null byte at the end. Similarly, during unpack()ing the "extra"
3343 nybbles are ignored.
3345 If the input string of pack() is longer than needed, extra bytes are ignored.
3346 A C<*> for the repeat count of pack() means to use all the bytes of
3347 the input field. On unpack()ing the bits are converted to a string
3348 of hexadecimal digits.
3352 The C<p> type packs a pointer to a null-terminated string. You are
3353 responsible for ensuring the string is not a temporary value (which can
3354 potentially get deallocated before you get around to using the packed result).
3355 The C<P> type packs a pointer to a structure of the size indicated by the
3356 length. A NULL pointer is created if the corresponding value for C<p> or
3357 C<P> is C<undef>, similarly for unpack().
3361 The C</> template character allows packing and unpacking of strings where
3362 the packed structure contains a byte count followed by the string itself.
3363 You write I<length-item>C</>I<string-item>.
3365 The I<length-item> can be any C<pack> template letter, and describes
3366 how the length value is packed. The ones likely to be of most use are
3367 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3368 SNMP) and C<N> (for Sun XDR).
3370 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3371 For C<unpack> the length of the string is obtained from the I<length-item>,
3372 but if you put in the '*' it will be ignored.
3374 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3375 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3376 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3378 The I<length-item> is not returned explicitly from C<unpack>.
3380 Adding a count to the I<length-item> letter is unlikely to do anything
3381 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3382 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3383 which Perl does not regard as legal in numeric strings.
3387 The integer types C<s>, C<S>, C<l>, and C<L> may be
3388 immediately followed by a C<!> suffix to signify native shorts or
3389 longs--as you can see from above for example a bare C<l> does mean
3390 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3391 may be larger. This is an issue mainly in 64-bit platforms. You can
3392 see whether using C<!> makes any difference by
3394 print length(pack("s")), " ", length(pack("s!")), "\n";
3395 print length(pack("l")), " ", length(pack("l!")), "\n";
3397 C<i!> and C<I!> also work but only because of completeness;
3398 they are identical to C<i> and C<I>.
3400 The actual sizes (in bytes) of native shorts, ints, longs, and long
3401 longs on the platform where Perl was built are also available via
3405 print $Config{shortsize}, "\n";
3406 print $Config{intsize}, "\n";
3407 print $Config{longsize}, "\n";
3408 print $Config{longlongsize}, "\n";
3410 (The C<$Config{longlongsize}> will be undefine if your system does
3411 not support long longs.)
3415 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3416 are inherently non-portable between processors and operating systems
3417 because they obey the native byteorder and endianness. For example a
3418 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3419 (arranged in and handled by the CPU registers) into bytes as
3421 0x12 0x34 0x56 0x78 # big-endian
3422 0x78 0x56 0x34 0x12 # little-endian
3424 Basically, the Intel and VAX CPUs are little-endian, while everybody
3425 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3426 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3427 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3430 The names `big-endian' and `little-endian' are comic references to
3431 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3432 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3433 the egg-eating habits of the Lilliputians.
3435 Some systems may have even weirder byte orders such as
3440 You can see your system's preference with
3442 print join(" ", map { sprintf "%#02x", $_ }
3443 unpack("C*",pack("L",0x12345678))), "\n";
3445 The byteorder on the platform where Perl was built is also available
3449 print $Config{byteorder}, "\n";
3451 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3452 and C<'87654321'> are big-endian.
3454 If you want portable packed integers use the formats C<n>, C<N>,
3455 C<v>, and C<V>, their byte endianness and size are known.
3456 See also L<perlport>.
3460 Real numbers (floats and doubles) are in the native machine format only;
3461 due to the multiplicity of floating formats around, and the lack of a
3462 standard "network" representation, no facility for interchange has been
3463 made. This means that packed floating point data written on one machine
3464 may not be readable on another - even if both use IEEE floating point
3465 arithmetic (as the endian-ness of the memory representation is not part
3466 of the IEEE spec). See also L<perlport>.
3468 Note that Perl uses doubles internally for all numeric calculation, and
3469 converting from double into float and thence back to double again will
3470 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3475 If the pattern begins with a C<U>, the resulting string will be treated
3476 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3477 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3478 characters. If you don't want this to happen, you can begin your pattern
3479 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3480 string, and then follow this with a C<U*> somewhere in your pattern.
3484 You must yourself do any alignment or padding by inserting for example
3485 enough C<'x'>es while packing. There is no way to pack() and unpack()
3486 could know where the bytes are going to or coming from. Therefore
3487 C<pack> (and C<unpack>) handle their output and input as flat
3492 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3493 take a repeat count, both as postfix, and via the C</> template
3498 C<x> and C<X> accept C<!> modifier. In this case they act as
3499 alignment commands: they jump forward/back to the closest position
3500 aligned at a multiple of C<count> bytes. For example, to pack() or
3501 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3502 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3503 aligned on the double's size.
3505 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3506 both result in no-ops.
3510 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3514 If TEMPLATE requires more arguments to pack() than actually given, pack()
3515 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3516 to pack() than actually given, extra arguments are ignored.
3522 $foo = pack("CCCC",65,66,67,68);
3524 $foo = pack("C4",65,66,67,68);
3526 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3527 # same thing with Unicode circled letters
3529 $foo = pack("ccxxcc",65,66,67,68);
3532 # note: the above examples featuring "C" and "c" are true
3533 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3534 # and UTF-8. In EBCDIC the first example would be
3535 # $foo = pack("CCCC",193,194,195,196);
3537 $foo = pack("s2",1,2);
3538 # "\1\0\2\0" on little-endian
3539 # "\0\1\0\2" on big-endian
3541 $foo = pack("a4","abcd","x","y","z");
3544 $foo = pack("aaaa","abcd","x","y","z");
3547 $foo = pack("a14","abcdefg");
3548 # "abcdefg\0\0\0\0\0\0\0"
3550 $foo = pack("i9pl", gmtime);
3551 # a real struct tm (on my system anyway)
3553 $utmp_template = "Z8 Z8 Z16 L";
3554 $utmp = pack($utmp_template, @utmp1);
3555 # a struct utmp (BSDish)
3557 @utmp2 = unpack($utmp_template, $utmp);
3558 # "@utmp1" eq "@utmp2"
3561 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3564 $foo = pack('sx2l', 12, 34);
3565 # short 12, two zero bytes padding, long 34
3566 $bar = pack('s@4l', 12, 34);
3567 # short 12, zero fill to position 4, long 34
3570 The same template may generally also be used in unpack().
3572 =item package NAMESPACE
3576 Declares the compilation unit as being in the given namespace. The scope
3577 of the package declaration is from the declaration itself through the end
3578 of the enclosing block, file, or eval (the same as the C<my> operator).
3579 All further unqualified dynamic identifiers will be in this namespace.
3580 A package statement affects only dynamic variables--including those
3581 you've used C<local> on--but I<not> lexical variables, which are created
3582 with C<my>. Typically it would be the first declaration in a file to
3583 be included by the C<require> or C<use> operator. You can switch into a
3584 package in more than one place; it merely influences which symbol table
3585 is used by the compiler for the rest of that block. You can refer to
3586 variables and filehandles in other packages by prefixing the identifier
3587 with the package name and a double colon: C<$Package::Variable>.
3588 If the package name is null, the C<main> package as assumed. That is,
3589 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3590 still seen in older code).
3592 If NAMESPACE is omitted, then there is no current package, and all
3593 identifiers must be fully qualified or lexicals. However, you are
3594 strongly advised not to make use of this feature. Its use can cause
3595 unexpected behaviour, even crashing some versions of Perl. It is
3596 deprecated, and will be removed from a future release.
3598 See L<perlmod/"Packages"> for more information about packages, modules,
3599 and classes. See L<perlsub> for other scoping issues.
3601 =item pipe READHANDLE,WRITEHANDLE
3603 Opens a pair of connected pipes like the corresponding system call.
3604 Note that if you set up a loop of piped processes, deadlock can occur
3605 unless you are very careful. In addition, note that Perl's pipes use
3606 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3607 after each command, depending on the application.
3609 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3610 for examples of such things.
3612 On systems that support a close-on-exec flag on files, the flag will be set
3613 for the newly opened file descriptors as determined by the value of $^F.
3620 Pops and returns the last value of the array, shortening the array by
3621 one element. Has an effect similar to
3625 If there are no elements in the array, returns the undefined value
3626 (although this may happen at other times as well). If ARRAY is
3627 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3628 array in subroutines, just like C<shift>.
3634 Returns the offset of where the last C<m//g> search left off for the variable
3635 in question (C<$_> is used when the variable is not specified). May be
3636 modified to change that offset. Such modification will also influence
3637 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3640 =item print FILEHANDLE LIST
3646 Prints a string or a list of strings. Returns true if successful.
3647 FILEHANDLE may be a scalar variable name, in which case the variable
3648 contains the name of or a reference to the filehandle, thus introducing
3649 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3650 the next token is a term, it may be misinterpreted as an operator
3651 unless you interpose a C<+> or put parentheses around the arguments.)
3652 If FILEHANDLE is omitted, prints by default to standard output (or
3653 to the last selected output channel--see L</select>). If LIST is
3654 also omitted, prints C<$_> to the currently selected output channel.
3655 To set the default output channel to something other than STDOUT
3656 use the select operation. The current value of C<$,> (if any) is
3657 printed between each LIST item. The current value of C<$\> (if
3658 any) is printed after the entire LIST has been printed. Because
3659 print takes a LIST, anything in the LIST is evaluated in list
3660 context, and any subroutine that you call will have one or more of
3661 its expressions evaluated in list context. Also be careful not to
3662 follow the print keyword with a left parenthesis unless you want
3663 the corresponding right parenthesis to terminate the arguments to
3664 the print--interpose a C<+> or put parentheses around all the
3667 Note that if you're storing FILEHANDLES in an array or other expression,
3668 you will have to use a block returning its value instead:
3670 print { $files[$i] } "stuff\n";
3671 print { $OK ? STDOUT : STDERR } "stuff\n";
3673 =item printf FILEHANDLE FORMAT, LIST
3675 =item printf FORMAT, LIST
3677 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3678 (the output record separator) is not appended. The first argument
3679 of the list will be interpreted as the C<printf> format. See C<sprintf>
3680 for an explanation of the format argument. If C<use locale> is in effect,
3681 the character used for the decimal point in formatted real numbers is
3682 affected by the LC_NUMERIC locale. See L<perllocale>.
3684 Don't fall into the trap of using a C<printf> when a simple
3685 C<print> would do. The C<print> is more efficient and less
3688 =item prototype FUNCTION
3690 Returns the prototype of a function as a string (or C<undef> if the
3691 function has no prototype). FUNCTION is a reference to, or the name of,
3692 the function whose prototype you want to retrieve.
3694 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3695 name for Perl builtin. If the builtin is not I<overridable> (such as
3696 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3697 C<system>) returns C<undef> because the builtin does not really behave
3698 like a Perl function. Otherwise, the string describing the equivalent
3699 prototype is returned.
3701 =item push ARRAY,LIST
3703 Treats ARRAY as a stack, and pushes the values of LIST
3704 onto the end of ARRAY. The length of ARRAY increases by the length of
3705 LIST. Has the same effect as
3708 $ARRAY[++$#ARRAY] = $value;
3711 but is more efficient. Returns the new number of elements in the array.
3723 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3725 =item quotemeta EXPR
3729 Returns the value of EXPR with all non-"word"
3730 characters backslashed. (That is, all characters not matching
3731 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3732 returned string, regardless of any locale settings.)
3733 This is the internal function implementing
3734 the C<\Q> escape in double-quoted strings.
3736 If EXPR is omitted, uses C<$_>.
3742 Returns a random fractional number greater than or equal to C<0> and less
3743 than the value of EXPR. (EXPR should be positive.) If EXPR is
3744 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3745 also special-cased as C<1> - this has not been documented before perl 5.8.0
3746 and is subject to change in future versions of perl. Automatically calls
3747 C<srand> unless C<srand> has already been called. See also C<srand>.
3749 Apply C<int()> to the value returned by C<rand()> if you want random
3750 integers instead of random fractional numbers. For example,
3754 returns a random integer between C<0> and C<9>, inclusive.
3756 (Note: If your rand function consistently returns numbers that are too
3757 large or too small, then your version of Perl was probably compiled
3758 with the wrong number of RANDBITS.)
3760 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3762 =item read FILEHANDLE,SCALAR,LENGTH
3764 Attempts to read LENGTH I<characters> of data into variable SCALAR
3765 from the specified FILEHANDLE. Returns the number of characters
3766 actually read, C<0> at end of file, or undef if there was an error.
3767 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3768 needs growing, the new bytes will be zero bytes. An OFFSET may be
3769 specified to place the read data into some other place in SCALAR than
3770 the beginning. The call is actually implemented in terms of either
3771 Perl's or system's fread() call. To get a true read(2) system call,
3774 Note the I<characters>: depending on the status of the filehandle,
3775 either (8-bit) bytes or characters are read. By default all
3776 filehandles operate on bytes, but for example if the filehandle has
3777 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3778 pragma, L<open>), the I/O will operate on characters, not bytes.
3780 =item readdir DIRHANDLE
3782 Returns the next directory entry for a directory opened by C<opendir>.
3783 If used in list context, returns all the rest of the entries in the
3784 directory. If there are no more entries, returns an undefined value in
3785 scalar context or a null list in list context.
3787 If you're planning to filetest the return values out of a C<readdir>, you'd
3788 better prepend the directory in question. Otherwise, because we didn't
3789 C<chdir> there, it would have been testing the wrong file.
3791 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3792 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3797 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3798 context, each call reads and returns the next line, until end-of-file is
3799 reached, whereupon the subsequent call returns undef. In list context,
3800 reads until end-of-file is reached and returns a list of lines. Note that
3801 the notion of "line" used here is however you may have defined it
3802 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3804 When C<$/> is set to C<undef>, when readline() is in scalar
3805 context (i.e. file slurp mode), and when an empty file is read, it
3806 returns C<''> the first time, followed by C<undef> subsequently.
3808 This is the internal function implementing the C<< <EXPR> >>
3809 operator, but you can use it directly. The C<< <EXPR> >>
3810 operator is discussed in more detail in L<perlop/"I/O Operators">.
3813 $line = readline(*STDIN); # same thing
3819 Returns the value of a symbolic link, if symbolic links are
3820 implemented. If not, gives a fatal error. If there is some system
3821 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3822 omitted, uses C<$_>.
3826 EXPR is executed as a system command.
3827 The collected standard output of the command is returned.
3828 In scalar context, it comes back as a single (potentially
3829 multi-line) string. In list context, returns a list of lines
3830 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3831 This is the internal function implementing the C<qx/EXPR/>
3832 operator, but you can use it directly. The C<qx/EXPR/>
3833 operator is discussed in more detail in L<perlop/"I/O Operators">.
3835 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3837 Receives a message on a socket. Attempts to receive LENGTH characters
3838 of data into variable SCALAR from the specified SOCKET filehandle.
3839 SCALAR will be grown or shrunk to the length actually read. Takes the
3840 same flags as the system call of the same name. Returns the address
3841 of the sender if SOCKET's protocol supports this; returns an empty
3842 string otherwise. If there's an error, returns the undefined value.
3843 This call is actually implemented in terms of recvfrom(2) system call.
3844 See L<perlipc/"UDP: Message Passing"> for examples.
3846 Note the I<characters>: depending on the status of the socket, either
3847 (8-bit) bytes or characters are received. By default all sockets
3848 operate on bytes, but for example if the socket has been changed using
3849 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3850 pragma, L<open>), the I/O will operate on characters, not bytes.
3856 The C<redo> command restarts the loop block without evaluating the
3857 conditional again. The C<continue> block, if any, is not executed. If
3858 the LABEL is omitted, the command refers to the innermost enclosing
3859 loop. This command is normally used by programs that want to lie to
3860 themselves about what was just input:
3862 # a simpleminded Pascal comment stripper
3863 # (warning: assumes no { or } in strings)
3864 LINE: while (<STDIN>) {
3865 while (s|({.*}.*){.*}|$1 |) {}
3870 if (/}/) { # end of comment?
3879 C<redo> cannot be used to retry a block which returns a value such as
3880 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3881 a grep() or map() operation.
3883 Note that a block by itself is semantically identical to a loop
3884 that executes once. Thus C<redo> inside such a block will effectively
3885 turn it into a looping construct.
3887 See also L</continue> for an illustration of how C<last>, C<next>, and
3894 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3895 is not specified, C<$_> will be used. The value returned depends on the
3896 type of thing the reference is a reference to.
3897 Builtin types include:
3907 If the referenced object has been blessed into a package, then that package
3908 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3910 if (ref($r) eq "HASH") {
3911 print "r is a reference to a hash.\n";
3914 print "r is not a reference at all.\n";
3916 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3917 print "r is a reference to something that isa hash.\n";
3920 See also L<perlref>.
3922 =item rename OLDNAME,NEWNAME
3924 Changes the name of a file; an existing file NEWNAME will be
3925 clobbered. Returns true for success, false otherwise.
3927 Behavior of this function varies wildly depending on your system
3928 implementation. For example, it will usually not work across file system
3929 boundaries, even though the system I<mv> command sometimes compensates
3930 for this. Other restrictions include whether it works on directories,
3931 open files, or pre-existing files. Check L<perlport> and either the
3932 rename(2) manpage or equivalent system documentation for details.
3934 =item require VERSION
3940 Demands a version of Perl specified by VERSION, or demands some semantics
3941 specified by EXPR or by C<$_> if EXPR is not supplied.
3943 VERSION may be either a numeric argument such as 5.006, which will be
3944 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3945 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3946 VERSION is greater than the version of the current Perl interpreter.
3947 Compare with L</use>, which can do a similar check at compile time.
3949 Specifying VERSION as a literal of the form v5.6.1 should generally be
3950 avoided, because it leads to misleading error messages under earlier
3951 versions of Perl which do not support this syntax. The equivalent numeric
3952 version should be used instead.
3954 require v5.6.1; # run time version check
3955 require 5.6.1; # ditto
3956 require 5.006_001; # ditto; preferred for backwards compatibility
3958 Otherwise, demands that a library file be included if it hasn't already
3959 been included. The file is included via the do-FILE mechanism, which is
3960 essentially just a variety of C<eval>. Has semantics similar to the following
3965 return 1 if $INC{$filename};
3966 my($realfilename,$result);
3968 foreach $prefix (@INC) {
3969 $realfilename = "$prefix/$filename";
3970 if (-f $realfilename) {
3971 $INC{$filename} = $realfilename;
3972 $result = do $realfilename;
3976 die "Can't find $filename in \@INC";
3978 delete $INC{$filename} if $@ || !$result;
3980 die "$filename did not return true value" unless $result;
3984 Note that the file will not be included twice under the same specified
3985 name. The file must return true as the last statement to indicate
3986 successful execution of any initialization code, so it's customary to
3987 end such a file with C<1;> unless you're sure it'll return true
3988 otherwise. But it's better just to put the C<1;>, in case you add more
3991 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3992 replaces "F<::>" with "F</>" in the filename for you,
3993 to make it easy to load standard modules. This form of loading of
3994 modules does not risk altering your namespace.
3996 In other words, if you try this:
3998 require Foo::Bar; # a splendid bareword
4000 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4001 directories specified in the C<@INC> array.
4003 But if you try this:
4005 $class = 'Foo::Bar';
4006 require $class; # $class is not a bareword
4008 require "Foo::Bar"; # not a bareword because of the ""
4010 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4011 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4013 eval "require $class";
4015 You can also insert hooks into the import facility, by putting directly
4016 Perl code into the @INC array. There are three forms of hooks: subroutine
4017 references, array references and blessed objects.
4019 Subroutine references are the simplest case. When the inclusion system
4020 walks through @INC and encounters a subroutine, this subroutine gets
4021 called with two parameters, the first being a reference to itself, and the
4022 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4023 subroutine should return C<undef> or a filehandle, from which the file to
4024 include will be read. If C<undef> is returned, C<require> will look at
4025 the remaining elements of @INC.
4027 If the hook is an array reference, its first element must be a subroutine
4028 reference. This subroutine is called as above, but the first parameter is
4029 the array reference. This enables to pass indirectly some arguments to
4032 In other words, you can write:
4034 push @INC, \&my_sub;
4036 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4042 push @INC, [ \&my_sub, $x, $y, ... ];
4044 my ($arrayref, $filename) = @_;
4045 # Retrieve $x, $y, ...
4046 my @parameters = @$arrayref[1..$#$arrayref];
4050 If the hook is an object, it must provide an INC method, that will be
4051 called as above, the first parameter being the object itself. (Note that
4052 you must fully qualify the sub's name, as it is always forced into package
4053 C<main>.) Here is a typical code layout:
4059 my ($self, $filename) = @_;
4063 # In the main program
4064 push @INC, new Foo(...);
4066 Note that these hooks are also permitted to set the %INC entry
4067 corresponding to the files they have loaded. See L<perlvar/%INC>.
4069 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4075 Generally used in a C<continue> block at the end of a loop to clear
4076 variables and reset C<??> searches so that they work again. The
4077 expression is interpreted as a list of single characters (hyphens
4078 allowed for ranges). All variables and arrays beginning with one of
4079 those letters are reset to their pristine state. If the expression is
4080 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4081 only variables or searches in the current package. Always returns
4084 reset 'X'; # reset all X variables
4085 reset 'a-z'; # reset lower case variables
4086 reset; # just reset ?one-time? searches
4088 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4089 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4090 variables--lexical variables are unaffected, but they clean themselves
4091 up on scope exit anyway, so you'll probably want to use them instead.
4098 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4099 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4100 context, depending on how the return value will be used, and the context
4101 may vary from one execution to the next (see C<wantarray>). If no EXPR
4102 is given, returns an empty list in list context, the undefined value in
4103 scalar context, and (of course) nothing at all in a void context.
4105 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4106 or do FILE will automatically return the value of the last expression
4111 In list context, returns a list value consisting of the elements
4112 of LIST in the opposite order. In scalar context, concatenates the
4113 elements of LIST and returns a string value with all characters
4114 in the opposite order.
4116 print reverse <>; # line tac, last line first
4118 undef $/; # for efficiency of <>
4119 print scalar reverse <>; # character tac, last line tsrif
4121 This operator is also handy for inverting a hash, although there are some
4122 caveats. If a value is duplicated in the original hash, only one of those
4123 can be represented as a key in the inverted hash. Also, this has to
4124 unwind one hash and build a whole new one, which may take some time
4125 on a large hash, such as from a DBM file.
4127 %by_name = reverse %by_address; # Invert the hash
4129 =item rewinddir DIRHANDLE
4131 Sets the current position to the beginning of the directory for the
4132 C<readdir> routine on DIRHANDLE.
4134 =item rindex STR,SUBSTR,POSITION
4136 =item rindex STR,SUBSTR
4138 Works just like index() except that it returns the position of the LAST
4139 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4140 last occurrence at or before that position.
4142 =item rmdir FILENAME
4146 Deletes the directory specified by FILENAME if that directory is empty. If it
4147 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4148 FILENAME is omitted, uses C<$_>.
4152 The substitution operator. See L<perlop>.
4156 Forces EXPR to be interpreted in scalar context and returns the value
4159 @counts = ( scalar @a, scalar @b, scalar @c );
4161 There is no equivalent operator to force an expression to
4162 be interpolated in list context because in practice, this is never
4163 needed. If you really wanted to do so, however, you could use
4164 the construction C<@{[ (some expression) ]}>, but usually a simple
4165 C<(some expression)> suffices.
4167 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4168 parenthesized list, this behaves as a scalar comma expression, evaluating
4169 all but the last element in void context and returning the final element
4170 evaluated in scalar context. This is seldom what you want.
4172 The following single statement:
4174 print uc(scalar(&foo,$bar)),$baz;
4176 is the moral equivalent of these two:
4179 print(uc($bar),$baz);
4181 See L<perlop> for more details on unary operators and the comma operator.
4183 =item seek FILEHANDLE,POSITION,WHENCE
4185 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4186 FILEHANDLE may be an expression whose value gives the name of the
4187 filehandle. The values for WHENCE are C<0> to set the new position
4188 I<in bytes> to POSITION, C<1> to set it to the current position plus
4189 POSITION, and C<2> to set it to EOF plus POSITION (typically
4190 negative). For WHENCE you may use the constants C<SEEK_SET>,
4191 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4192 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4195 Note the I<in bytes>: even if the filehandle has been set to
4196 operate on characters (for example by using the C<:utf8> open
4197 layer), tell() will return byte offsets, not character offsets
4198 (because implementing that would render seek() and tell() rather slow).
4200 If you want to position file for C<sysread> or C<syswrite>, don't use
4201 C<seek>--buffering makes its effect on the file's system position
4202 unpredictable and non-portable. Use C<sysseek> instead.
4204 Due to the rules and rigors of ANSI C, on some systems you have to do a
4205 seek whenever you switch between reading and writing. Amongst other
4206 things, this may have the effect of calling stdio's clearerr(3).
4207 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4211 This is also useful for applications emulating C<tail -f>. Once you hit
4212 EOF on your read, and then sleep for a while, you might have to stick in a
4213 seek() to reset things. The C<seek> doesn't change the current position,
4214 but it I<does> clear the end-of-file condition on the handle, so that the
4215 next C<< <FILE> >> makes Perl try again to read something. We hope.
4217 If that doesn't work (some IO implementations are particularly
4218 cantankerous), then you may need something more like this:
4221 for ($curpos = tell(FILE); $_ = <FILE>;
4222 $curpos = tell(FILE)) {
4223 # search for some stuff and put it into files
4225 sleep($for_a_while);
4226 seek(FILE, $curpos, 0);
4229 =item seekdir DIRHANDLE,POS
4231 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4232 must be a value returned by C<telldir>. Has the same caveats about
4233 possible directory compaction as the corresponding system library
4236 =item select FILEHANDLE
4240 Returns the currently selected filehandle. Sets the current default
4241 filehandle for output, if FILEHANDLE is supplied. This has two
4242 effects: first, a C<write> or a C<print> without a filehandle will
4243 default to this FILEHANDLE. Second, references to variables related to
4244 output will refer to this output channel. For example, if you have to
4245 set the top of form format for more than one output channel, you might
4253 FILEHANDLE may be an expression whose value gives the name of the
4254 actual filehandle. Thus:
4256 $oldfh = select(STDERR); $| = 1; select($oldfh);
4258 Some programmers may prefer to think of filehandles as objects with
4259 methods, preferring to write the last example as:
4262 STDERR->autoflush(1);
4264 =item select RBITS,WBITS,EBITS,TIMEOUT
4266 This calls the select(2) system call with the bit masks specified, which
4267 can be constructed using C<fileno> and C<vec>, along these lines:
4269 $rin = $win = $ein = '';
4270 vec($rin,fileno(STDIN),1) = 1;
4271 vec($win,fileno(STDOUT),1) = 1;
4274 If you want to select on many filehandles you might wish to write a
4278 my(@fhlist) = split(' ',$_[0]);
4281 vec($bits,fileno($_),1) = 1;
4285 $rin = fhbits('STDIN TTY SOCK');
4289 ($nfound,$timeleft) =
4290 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4292 or to block until something becomes ready just do this
4294 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4296 Most systems do not bother to return anything useful in $timeleft, so
4297 calling select() in scalar context just returns $nfound.
4299 Any of the bit masks can also be undef. The timeout, if specified, is
4300 in seconds, which may be fractional. Note: not all implementations are
4301 capable of returning the $timeleft. If not, they always return
4302 $timeleft equal to the supplied $timeout.
4304 You can effect a sleep of 250 milliseconds this way:
4306 select(undef, undef, undef, 0.25);
4308 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4309 is implementation-dependent.
4311 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4312 or <FH>) with C<select>, except as permitted by POSIX, and even
4313 then only on POSIX systems. You have to use C<sysread> instead.
4315 =item semctl ID,SEMNUM,CMD,ARG
4317 Calls the System V IPC function C<semctl>. You'll probably have to say
4321 first to get the correct constant definitions. If CMD is IPC_STAT or
4322 GETALL, then ARG must be a variable which will hold the returned
4323 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4324 the undefined value for error, "C<0 but true>" for zero, or the actual
4325 return value otherwise. The ARG must consist of a vector of native
4326 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4327 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4330 =item semget KEY,NSEMS,FLAGS
4332 Calls the System V IPC function semget. Returns the semaphore id, or
4333 the undefined value if there is an error. See also
4334 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4337 =item semop KEY,OPSTRING
4339 Calls the System V IPC function semop to perform semaphore operations
4340 such as signalling and waiting. OPSTRING must be a packed array of
4341 semop structures. Each semop structure can be generated with
4342 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4343 operations is implied by the length of OPSTRING. Returns true if
4344 successful, or false if there is an error. As an example, the
4345 following code waits on semaphore $semnum of semaphore id $semid:
4347 $semop = pack("s!3", $semnum, -1, 0);
4348 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4350 To signal the semaphore, replace C<-1> with C<1>. See also
4351 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4354 =item send SOCKET,MSG,FLAGS,TO
4356 =item send SOCKET,MSG,FLAGS
4358 Sends a message on a socket. Attempts to send the scalar MSG to the
4359 SOCKET filehandle. Takes the same flags as the system call of the
4360 same name. On unconnected sockets you must specify a destination to
4361 send TO, in which case it does a C C<sendto>. Returns the number of
4362 characters sent, or the undefined value if there is an error. The C
4363 system call sendmsg(2) is currently unimplemented. See
4364 L<perlipc/"UDP: Message Passing"> for examples.
4366 Note the I<characters>: depending on the status of the socket, either
4367 (8-bit) bytes or characters are sent. By default all sockets operate
4368 on bytes, but for example if the socket has been changed using
4369 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4370 the C<open> pragma, L<open>), the I/O will operate on characters, not
4373 =item setpgrp PID,PGRP
4375 Sets the current process group for the specified PID, C<0> for the current
4376 process. Will produce a fatal error if used on a machine that doesn't
4377 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4378 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4379 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4382 =item setpriority WHICH,WHO,PRIORITY
4384 Sets the current priority for a process, a process group, or a user.
4385 (See setpriority(2).) Will produce a fatal error if used on a machine
4386 that doesn't implement setpriority(2).
4388 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4390 Sets the socket option requested. Returns undefined if there is an
4391 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4398 Shifts the first value of the array off and returns it, shortening the
4399 array by 1 and moving everything down. If there are no elements in the
4400 array, returns the undefined value. If ARRAY is omitted, shifts the
4401 C<@_> array within the lexical scope of subroutines and formats, and the
4402 C<@ARGV> array at file scopes or within the lexical scopes established by
4403 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4406 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4407 same thing to the left end of an array that C<pop> and C<push> do to the
4410 =item shmctl ID,CMD,ARG
4412 Calls the System V IPC function shmctl. You'll probably have to say
4416 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4417 then ARG must be a variable which will hold the returned C<shmid_ds>
4418 structure. Returns like ioctl: the undefined value for error, "C<0> but
4419 true" for zero, or the actual return value otherwise.
4420 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4422 =item shmget KEY,SIZE,FLAGS
4424 Calls the System V IPC function shmget. Returns the shared memory
4425 segment id, or the undefined value if there is an error.
4426 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4428 =item shmread ID,VAR,POS,SIZE
4430 =item shmwrite ID,STRING,POS,SIZE
4432 Reads or writes the System V shared memory segment ID starting at
4433 position POS for size SIZE by attaching to it, copying in/out, and
4434 detaching from it. When reading, VAR must be a variable that will
4435 hold the data read. When writing, if STRING is too long, only SIZE
4436 bytes are used; if STRING is too short, nulls are written to fill out
4437 SIZE bytes. Return true if successful, or false if there is an error.
4438 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4439 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4441 =item shutdown SOCKET,HOW
4443 Shuts down a socket connection in the manner indicated by HOW, which
4444 has the same interpretation as in the system call of the same name.
4446 shutdown(SOCKET, 0); # I/we have stopped reading data
4447 shutdown(SOCKET, 1); # I/we have stopped writing data
4448 shutdown(SOCKET, 2); # I/we have stopped using this socket
4450 This is useful with sockets when you want to tell the other
4451 side you're done writing but not done reading, or vice versa.
4452 It's also a more insistent form of close because it also
4453 disables the file descriptor in any forked copies in other
4460 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4461 returns sine of C<$_>.
4463 For the inverse sine operation, you may use the C<Math::Trig::asin>
4464 function, or use this relation:
4466 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4472 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4473 May be interrupted if the process receives a signal such as C<SIGALRM>.
4474 Returns the number of seconds actually slept. You probably cannot
4475 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4478 On some older systems, it may sleep up to a full second less than what
4479 you requested, depending on how it counts seconds. Most modern systems
4480 always sleep the full amount. They may appear to sleep longer than that,
4481 however, because your process might not be scheduled right away in a
4482 busy multitasking system.
4484 For delays of finer granularity than one second, you may use Perl's
4485 C<syscall> interface to access setitimer(2) if your system supports
4486 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4487 and starting from Perl 5.8 part of the standard distribution) may also
4490 See also the POSIX module's C<pause> function.
4492 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4494 Opens a socket of the specified kind and attaches it to filehandle
4495 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4496 the system call of the same name. You should C<use Socket> first
4497 to get the proper definitions imported. See the examples in
4498 L<perlipc/"Sockets: Client/Server Communication">.
4500 On systems that support a close-on-exec flag on files, the flag will
4501 be set for the newly opened file descriptor, as determined by the
4502 value of $^F. See L<perlvar/$^F>.
4504 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4506 Creates an unnamed pair of sockets in the specified domain, of the
4507 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4508 for the system call of the same name. If unimplemented, yields a fatal
4509 error. Returns true if successful.
4511 On systems that support a close-on-exec flag on files, the flag will
4512 be set for the newly opened file descriptors, as determined by the value
4513 of $^F. See L<perlvar/$^F>.
4515 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4516 to C<pipe(Rdr, Wtr)> is essentially:
4519 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4520 shutdown(Rdr, 1); # no more writing for reader
4521 shutdown(Wtr, 0); # no more reading for writer
4523 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4524 emulate socketpair using IP sockets to localhost if your system implements
4525 sockets but not socketpair.
4527 =item sort SUBNAME LIST
4529 =item sort BLOCK LIST
4533 In list context, this sorts the LIST and returns the sorted list value.
4534 In scalar context, the behaviour of C<sort()> is undefined.
4536 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4537 order. If SUBNAME is specified, it gives the name of a subroutine
4538 that returns an integer less than, equal to, or greater than C<0>,
4539 depending on how the elements of the list are to be ordered. (The C<<
4540 <=> >> and C<cmp> operators are extremely useful in such routines.)
4541 SUBNAME may be a scalar variable name (unsubscripted), in which case
4542 the value provides the name of (or a reference to) the actual
4543 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4544 an anonymous, in-line sort subroutine.
4546 If the subroutine's prototype is C<($$)>, the elements to be compared
4547 are passed by reference in C<@_>, as for a normal subroutine. This is
4548 slower than unprototyped subroutines, where the elements to be
4549 compared are passed into the subroutine
4550 as the package global variables $a and $b (see example below). Note that
4551 in the latter case, it is usually counter-productive to declare $a and
4554 In either case, the subroutine may not be recursive. The values to be
4555 compared are always passed by reference, so don't modify them.
4557 You also cannot exit out of the sort block or subroutine using any of the
4558 loop control operators described in L<perlsyn> or with C<goto>.
4560 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4561 current collation locale. See L<perllocale>.
4563 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4564 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4565 preserves the input order of elements that compare equal. Although
4566 quicksort's run time is O(NlogN) when averaged over all arrays of
4567 length N, the time can be O(N**2), I<quadratic> behavior, for some
4568 inputs.) In 5.7, the quicksort implementation was replaced with
4569 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4570 But benchmarks indicated that for some inputs, on some platforms,
4571 the original quicksort was faster. 5.8 has a sort pragma for
4572 limited control of the sort. Its rather blunt control of the
4573 underlying algorithm may not persist into future perls, but the
4574 ability to characterize the input or output in implementation
4575 independent ways quite probably will. See L<sort>.
4580 @articles = sort @files;
4582 # same thing, but with explicit sort routine
4583 @articles = sort {$a cmp $b} @files;
4585 # now case-insensitively
4586 @articles = sort {uc($a) cmp uc($b)} @files;
4588 # same thing in reversed order
4589 @articles = sort {$b cmp $a} @files;
4591 # sort numerically ascending
4592 @articles = sort {$a <=> $b} @files;
4594 # sort numerically descending
4595 @articles = sort {$b <=> $a} @files;
4597 # this sorts the %age hash by value instead of key
4598 # using an in-line function
4599 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4601 # sort using explicit subroutine name
4603 $age{$a} <=> $age{$b}; # presuming numeric
4605 @sortedclass = sort byage @class;
4607 sub backwards { $b cmp $a }
4608 @harry = qw(dog cat x Cain Abel);
4609 @george = qw(gone chased yz Punished Axed);
4611 # prints AbelCaincatdogx
4612 print sort backwards @harry;
4613 # prints xdogcatCainAbel
4614 print sort @george, 'to', @harry;
4615 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4617 # inefficiently sort by descending numeric compare using
4618 # the first integer after the first = sign, or the
4619 # whole record case-insensitively otherwise
4622 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4627 # same thing, but much more efficiently;
4628 # we'll build auxiliary indices instead
4632 push @nums, /=(\d+)/;
4637 $nums[$b] <=> $nums[$a]
4639 $caps[$a] cmp $caps[$b]
4643 # same thing, but without any temps
4644 @new = map { $_->[0] }
4645 sort { $b->[1] <=> $a->[1]
4648 } map { [$_, /=(\d+)/, uc($_)] } @old;
4650 # using a prototype allows you to use any comparison subroutine
4651 # as a sort subroutine (including other package's subroutines)
4653 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4656 @new = sort other::backwards @old;
4658 # guarantee stability, regardless of algorithm
4660 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4662 # force use of mergesort (not portable outside Perl 5.8)
4663 use sort '_mergesort'; # note discouraging _
4664 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4666 If you're using strict, you I<must not> declare $a
4667 and $b as lexicals. They are package globals. That means
4668 if you're in the C<main> package and type
4670 @articles = sort {$b <=> $a} @files;
4672 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4673 but if you're in the C<FooPack> package, it's the same as typing
4675 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4677 The comparison function is required to behave. If it returns
4678 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4679 sometimes saying the opposite, for example) the results are not
4682 =item splice ARRAY,OFFSET,LENGTH,LIST
4684 =item splice ARRAY,OFFSET,LENGTH
4686 =item splice ARRAY,OFFSET
4690 Removes the elements designated by OFFSET and LENGTH from an array, and
4691 replaces them with the elements of LIST, if any. In list context,
4692 returns the elements removed from the array. In scalar context,
4693 returns the last element removed, or C<undef> if no elements are
4694 removed. The array grows or shrinks as necessary.
4695 If OFFSET is negative then it starts that far from the end of the array.
4696 If LENGTH is omitted, removes everything from OFFSET onward.
4697 If LENGTH is negative, removes the elements from OFFSET onward
4698 except for -LENGTH elements at the end of the array.
4699 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4700 past the end of the array, perl issues a warning, and splices at the
4703 The following equivalences hold (assuming C<$[ == 0>):
4705 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4706 pop(@a) splice(@a,-1)
4707 shift(@a) splice(@a,0,1)
4708 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4709 $a[$x] = $y splice(@a,$x,1,$y)
4711 Example, assuming array lengths are passed before arrays:
4713 sub aeq { # compare two list values
4714 my(@a) = splice(@_,0,shift);
4715 my(@b) = splice(@_,0,shift);
4716 return 0 unless @a == @b; # same len?
4718 return 0 if pop(@a) ne pop(@b);
4722 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4724 =item split /PATTERN/,EXPR,LIMIT
4726 =item split /PATTERN/,EXPR
4728 =item split /PATTERN/
4732 Splits a string into a list of strings and returns that list. By default,
4733 empty leading fields are preserved, and empty trailing ones are deleted.
4735 In scalar context, returns the number of fields found and splits into
4736 the C<@_> array. Use of split in scalar context is deprecated, however,
4737 because it clobbers your subroutine arguments.
4739 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4740 splits on whitespace (after skipping any leading whitespace). Anything
4741 matching PATTERN is taken to be a delimiter separating the fields. (Note
4742 that the delimiter may be longer than one character.)
4744 If LIMIT is specified and positive, it represents the maximum number
4745 of fields the EXPR will be split into, though the actual number of
4746 fields returned depends on the number of times PATTERN matches within
4747 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4748 stripped (which potential users of C<pop> would do well to remember).
4749 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4750 had been specified. Note that splitting an EXPR that evaluates to the
4751 empty string always returns the empty list, regardless of the LIMIT
4754 A pattern matching the null string (not to be confused with
4755 a null pattern C<//>, which is just one member of the set of patterns
4756 matching a null string) will split the value of EXPR into separate
4757 characters at each point it matches that way. For example:
4759 print join(':', split(/ */, 'hi there'));
4761 produces the output 'h:i:t:h:e:r:e'.
4763 Using the empty pattern C<//> specifically matches the null string, and is
4764 not be confused with the use of C<//> to mean "the last successful pattern
4767 Empty leading (or trailing) fields are produced when there are positive width
4768 matches at the beginning (or end) of the string; a zero-width match at the
4769 beginning (or end) of the string does not produce an empty field. For
4772 print join(':', split(/(?=\w)/, 'hi there!'));
4774 produces the output 'h:i :t:h:e:r:e!'.
4776 The LIMIT parameter can be used to split a line partially
4778 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4780 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4781 one larger than the number of variables in the list, to avoid
4782 unnecessary work. For the list above LIMIT would have been 4 by
4783 default. In time critical applications it behooves you not to split
4784 into more fields than you really need.
4786 If the PATTERN contains parentheses, additional list elements are
4787 created from each matching substring in the delimiter.
4789 split(/([,-])/, "1-10,20", 3);
4791 produces the list value
4793 (1, '-', 10, ',', 20)
4795 If you had the entire header of a normal Unix email message in $header,
4796 you could split it up into fields and their values this way:
4798 $header =~ s/\n\s+/ /g; # fix continuation lines
4799 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4801 The pattern C</PATTERN/> may be replaced with an expression to specify
4802 patterns that vary at runtime. (To do runtime compilation only once,
4803 use C</$variable/o>.)
4805 As a special case, specifying a PATTERN of space (C<' '>) will split on
4806 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4807 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4808 will give you as many null initial fields as there are leading spaces.
4809 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4810 whitespace produces a null first field. A C<split> with no arguments
4811 really does a C<split(' ', $_)> internally.
4813 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4818 open(PASSWD, '/etc/passwd');
4821 ($login, $passwd, $uid, $gid,
4822 $gcos, $home, $shell) = split(/:/);
4826 As with regular pattern matching, any capturing parentheses that are not
4827 matched in a C<split()> will be set to C<undef> when returned:
4829 @fields = split /(A)|B/, "1A2B3";
4830 # @fields is (1, 'A', 2, undef, 3)
4832 =item sprintf FORMAT, LIST
4834 Returns a string formatted by the usual C<printf> conventions of the C
4835 library function C<sprintf>. See below for more details
4836 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4837 the general principles.
4841 # Format number with up to 8 leading zeroes
4842 $result = sprintf("%08d", $number);
4844 # Round number to 3 digits after decimal point
4845 $rounded = sprintf("%.3f", $number);
4847 Perl does its own C<sprintf> formatting--it emulates the C
4848 function C<sprintf>, but it doesn't use it (except for floating-point
4849 numbers, and even then only the standard modifiers are allowed). As a
4850 result, any non-standard extensions in your local C<sprintf> are not
4851 available from Perl.
4853 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4854 pass it an array as your first argument. The array is given scalar context,
4855 and instead of using the 0th element of the array as the format, Perl will
4856 use the count of elements in the array as the format, which is almost never
4859 Perl's C<sprintf> permits the following universally-known conversions:
4862 %c a character with the given number
4864 %d a signed integer, in decimal
4865 %u an unsigned integer, in decimal
4866 %o an unsigned integer, in octal
4867 %x an unsigned integer, in hexadecimal
4868 %e a floating-point number, in scientific notation
4869 %f a floating-point number, in fixed decimal notation
4870 %g a floating-point number, in %e or %f notation
4872 In addition, Perl permits the following widely-supported conversions:
4874 %X like %x, but using upper-case letters
4875 %E like %e, but using an upper-case "E"
4876 %G like %g, but with an upper-case "E" (if applicable)
4877 %b an unsigned integer, in binary
4878 %p a pointer (outputs the Perl value's address in hexadecimal)
4879 %n special: *stores* the number of characters output so far
4880 into the next variable in the parameter list
4882 Finally, for backward (and we do mean "backward") compatibility, Perl
4883 permits these unnecessary but widely-supported conversions:
4886 %D a synonym for %ld
4887 %U a synonym for %lu
4888 %O a synonym for %lo
4891 Note that the number of exponent digits in the scientific notation produced
4892 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4893 exponent less than 100 is system-dependent: it may be three or less
4894 (zero-padded as necessary). In other words, 1.23 times ten to the
4895 99th may be either "1.23e99" or "1.23e099".
4897 Between the C<%> and the format letter, you may specify a number of
4898 additional attributes controlling the interpretation of the format.
4899 In order, these are:
4903 =item format parameter index
4905 An explicit format parameter index, such as C<2$>. By default sprintf
4906 will format the next unused argument in the list, but this allows you
4907 to take the arguments out of order. Eg:
4909 printf '%2$d %1$d', 12, 34; # prints "34 12"
4910 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4915 space prefix positive number with a space
4916 + prefix positive number with a plus sign
4917 - left-justify within the field
4918 0 use zeros, not spaces, to right-justify
4919 # prefix non-zero octal with "0", non-zero hex with "0x",
4920 non-zero binary with "0b"
4924 printf '<% d>', 12; # prints "< 12>"
4925 printf '<%+d>', 12; # prints "<+12>"
4926 printf '<%6s>', 12; # prints "< 12>"
4927 printf '<%-6s>', 12; # prints "<12 >"
4928 printf '<%06s>', 12; # prints "<000012>"
4929 printf '<%#x>', 12; # prints "<0xc>"
4933 The vector flag C<v>, optionally specifying the join string to use.
4934 This flag tells perl to interpret the supplied string as a vector
4935 of integers, one for each character in the string, separated by
4936 a given string (a dot C<.> by default). This can be useful for
4937 displaying ordinal values of characters in arbitrary strings:
4939 printf "version is v%vd\n", $^V; # Perl's version
4941 Put an asterisk C<*> before the C<v> to override the string to
4942 use to separate the numbers:
4944 printf "address is %*vX\n", ":", $addr; # IPv6 address
4945 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
4947 You can also explicitly specify the argument number to use for
4948 the join string using eg C<*2$v>:
4950 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
4952 =item (minimum) width
4954 Arguments are usually formatted to be only as wide as required to
4955 display the given value. You can override the width by putting
4956 a number here, or get the width from the next argument (with C<*>)
4957 or from a specified argument (with eg C<*2$>):
4959 printf '<%s>', "a"; # prints "<a>"
4960 printf '<%6s>', "a"; # prints "< a>"
4961 printf '<%*s>', 6, "a"; # prints "< a>"
4962 printf '<%*2$s>', "a", 6; # prints "< a>"
4963 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
4965 If a field width obtained through C<*> is negative, it has the same
4966 effect as the C<-> flag: left-justification.
4968 =item precision, or maximum width
4970 You can specify a precision (for numeric conversions) or a maximum
4971 width (for string conversions) by specifying a C<.> followed by a number.
4972 For floating point formats, this specifies the number of decimal places
4973 to show (the default being 6), eg:
4975 # these examples are subject to system-specific variation
4976 printf '<%f>', 1; # prints "<1.000000>"
4977 printf '<%.1f>', 1; # prints "<1.0>"
4978 printf '<%.0f>', 1; # prints "<1>"
4979 printf '<%e>', 10; # prints "<1.000000e+01>"
4980 printf '<%.1e>', 10; # prints "<1.0e+01>"
4982 For integer conversions, specifying a precision implies that the
4983 output of the number itself should be zero-padded to this width:
4985 printf '<%.6x>', 1; # prints "<000001>"
4986 printf '<%#.6x>', 1; # prints "<0x000001>"
4987 printf '<%-10.6x>', 1; # prints "<000001 >"
4989 For string conversions, specifying a precision truncates the string
4990 to fit in the specified width:
4992 printf '<%.5s>', "truncated"; # prints "<trunc>"
4993 printf '<%10.5s>', "truncated"; # prints "< trunc>"
4995 You can also get the precision from the next argument using C<.*>:
4997 printf '<%.6x>', 1; # prints "<000001>"
4998 printf '<%.*x>', 6, 1; # prints "<000001>"
5000 You cannot currently get the precision from a specified number,
5001 but it is intended that this will be possible in the future using
5004 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5008 For numeric conversions, you can specify the size to interpret the
5009 number as using C<l>, C<h>, C<V>, C<q>, C<L> or C<ll>. For integer
5010 conversions, numbers are usually assumed to be whatever the default
5011 integer size is on your platform (usually 32 or 64 bits), but you
5012 can override this to use instead one of the standard C types, as
5013 supported by the compiler used to build Perl:
5015 l interpret integer as C type "long" or "unsigned long"
5016 h interpret integer as C type "short" or "unsigned short"
5017 q, L or ll interpret integer as C type "long long" or "unsigned long long"
5018 (if your platform supports such a type, else it is an error)
5020 For floating point conversions, numbers are usually assumed to be
5021 the default floating point size on your platform (double or long double),
5022 but you can force 'long double' with C<q>, C<L> or C<ll> if your
5023 platform supports them.
5025 The size specifier 'V' has no effect for Perl code, but it supported
5026 for compatibility with XS code; it means 'use the standard size for
5027 a Perl integer (or floating-point number)', which is already the
5028 default for Perl code.
5030 =item order of arguments
5032 Normally, sprintf takes the next unused argument as the value to
5033 format for each format specification. If the format specification
5034 uses C<*> to require additional arguments, these are consumed from
5035 the argument list in the order in which they appear in the format
5036 specification I<before> the value to format. Where an argument is
5037 specified using an explicit index, this does not affect the normal
5038 order for the arguments (even when the explicitly specified index
5039 would have been the next argument in any case).
5043 printf '<%*.*s>', $a, $b, $c;
5045 would use C<$a> for the width, C<$b> for the precision and C<$c>
5046 as the value to format, while:
5048 print '<%*1$.*s>', $a, $b;
5050 would use C<$a> for the width and the precision, and C<$b> as the
5053 Here are some more examples - beware that when using an explicit
5054 index, the C<$> may need to be escaped:
5056 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5057 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5058 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5059 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5063 If C<use locale> is in effect, the character used for the decimal
5064 point in formatted real numbers is affected by the LC_NUMERIC locale.
5067 If Perl understands "quads" (64-bit integers) (this requires
5068 either that the platform natively support quads or that Perl
5069 be specifically compiled to support quads), the characters
5073 print quads, and they may optionally be preceded by
5081 You can find out whether your Perl supports quads via L<Config>:
5084 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
5087 If Perl understands "long doubles" (this requires that the platform
5088 support long doubles), the flags
5092 may optionally be preceded by
5100 You can find out whether your Perl supports long doubles via L<Config>:
5103 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5109 Return the square root of EXPR. If EXPR is omitted, returns square
5110 root of C<$_>. Only works on non-negative operands, unless you've
5111 loaded the standard Math::Complex module.
5114 print sqrt(-2); # prints 1.4142135623731i
5120 Sets the random number seed for the C<rand> operator.
5122 The point of the function is to "seed" the C<rand> function so that
5123 C<rand> can produce a different sequence each time you run your
5126 If srand() is not called explicitly, it is called implicitly at the
5127 first use of the C<rand> operator. However, this was not the case in
5128 versions of Perl before 5.004, so if your script will run under older
5129 Perl versions, it should call C<srand>.
5131 Most programs won't even call srand() at all, except those that
5132 need a cryptographically-strong starting point rather than the
5133 generally acceptable default, which is based on time of day,
5134 process ID, and memory allocation, or the F</dev/urandom> device,
5137 You can call srand($seed) with the same $seed to reproduce the
5138 I<same> sequence from rand(), but this is usually reserved for
5139 generating predictable results for testing or debugging.
5140 Otherwise, don't call srand() more than once in your program.
5142 Do B<not> call srand() (i.e. without an argument) more than once in
5143 a script. The internal state of the random number generator should
5144 contain more entropy than can be provided by any seed, so calling
5145 srand() again actually I<loses> randomness.
5147 Most implementations of C<srand> take an integer and will silently
5148 truncate decimal numbers. This means C<srand(42)> will usually
5149 produce the same results as C<srand(42.1)>. To be safe, always pass
5150 C<srand> an integer.
5152 In versions of Perl prior to 5.004 the default seed was just the
5153 current C<time>. This isn't a particularly good seed, so many old
5154 programs supply their own seed value (often C<time ^ $$> or C<time ^
5155 ($$ + ($$ << 15))>), but that isn't necessary any more.
5157 Note that you need something much more random than the default seed for
5158 cryptographic purposes. Checksumming the compressed output of one or more
5159 rapidly changing operating system status programs is the usual method. For
5162 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5164 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5167 Frequently called programs (like CGI scripts) that simply use
5171 for a seed can fall prey to the mathematical property that
5175 one-third of the time. So don't do that.
5177 =item stat FILEHANDLE
5183 Returns a 13-element list giving the status info for a file, either
5184 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5185 it stats C<$_>. Returns a null list if the stat fails. Typically used
5188 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5189 $atime,$mtime,$ctime,$blksize,$blocks)
5192 Not all fields are supported on all filesystem types. Here are the
5193 meaning of the fields:
5195 0 dev device number of filesystem
5197 2 mode file mode (type and permissions)
5198 3 nlink number of (hard) links to the file
5199 4 uid numeric user ID of file's owner
5200 5 gid numeric group ID of file's owner
5201 6 rdev the device identifier (special files only)
5202 7 size total size of file, in bytes
5203 8 atime last access time in seconds since the epoch
5204 9 mtime last modify time in seconds since the epoch
5205 10 ctime inode change time in seconds since the epoch (*)
5206 11 blksize preferred block size for file system I/O
5207 12 blocks actual number of blocks allocated
5209 (The epoch was at 00:00 January 1, 1970 GMT.)
5211 (*) The ctime field is non-portable, in particular you cannot expect
5212 it to be a "creation time", see L<perlport/"Files and Filesystems">
5215 If stat is passed the special filehandle consisting of an underline, no
5216 stat is done, but the current contents of the stat structure from the
5217 last stat or filetest are returned. Example:
5219 if (-x $file && (($d) = stat(_)) && $d < 0) {
5220 print "$file is executable NFS file\n";
5223 (This works on machines only for which the device number is negative
5226 Because the mode contains both the file type and its permissions, you
5227 should mask off the file type portion and (s)printf using a C<"%o">
5228 if you want to see the real permissions.
5230 $mode = (stat($filename))[2];
5231 printf "Permissions are %04o\n", $mode & 07777;
5233 In scalar context, C<stat> returns a boolean value indicating success
5234 or failure, and, if successful, sets the information associated with
5235 the special filehandle C<_>.
5237 The File::stat module provides a convenient, by-name access mechanism:
5240 $sb = stat($filename);
5241 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5242 $filename, $sb->size, $sb->mode & 07777,
5243 scalar localtime $sb->mtime;
5245 You can import symbolic mode constants (C<S_IF*>) and functions
5246 (C<S_IS*>) from the Fcntl module:
5250 $mode = (stat($filename))[2];
5252 $user_rwx = ($mode & S_IRWXU) >> 6;
5253 $group_read = ($mode & S_IRGRP) >> 3;
5254 $other_execute = $mode & S_IXOTH;
5256 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5258 $is_setuid = $mode & S_ISUID;
5259 $is_setgid = S_ISDIR($mode);
5261 You could write the last two using the C<-u> and C<-d> operators.
5262 The commonly available S_IF* constants are
5264 # Permissions: read, write, execute, for user, group, others.
5266 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5267 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5268 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5270 # Setuid/Setgid/Stickiness.
5272 S_ISUID S_ISGID S_ISVTX S_ISTXT
5274 # File types. Not necessarily all are available on your system.
5276 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5278 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5280 S_IREAD S_IWRITE S_IEXEC
5282 and the S_IF* functions are
5284 S_IFMODE($mode) the part of $mode containing the permission bits
5285 and the setuid/setgid/sticky bits
5287 S_IFMT($mode) the part of $mode containing the file type
5288 which can be bit-anded with e.g. S_IFREG
5289 or with the following functions
5291 # The operators -f, -d, -l, -b, -c, -p, and -s.
5293 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5294 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5296 # No direct -X operator counterpart, but for the first one
5297 # the -g operator is often equivalent. The ENFMT stands for
5298 # record flocking enforcement, a platform-dependent feature.
5300 S_ISENFMT($mode) S_ISWHT($mode)
5302 See your native chmod(2) and stat(2) documentation for more details
5303 about the S_* constants.
5309 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5310 doing many pattern matches on the string before it is next modified.
5311 This may or may not save time, depending on the nature and number of
5312 patterns you are searching on, and on the distribution of character
5313 frequencies in the string to be searched--you probably want to compare
5314 run times with and without it to see which runs faster. Those loops
5315 which scan for many short constant strings (including the constant
5316 parts of more complex patterns) will benefit most. You may have only
5317 one C<study> active at a time--if you study a different scalar the first
5318 is "unstudied". (The way C<study> works is this: a linked list of every
5319 character in the string to be searched is made, so we know, for
5320 example, where all the C<'k'> characters are. From each search string,
5321 the rarest character is selected, based on some static frequency tables
5322 constructed from some C programs and English text. Only those places
5323 that contain this "rarest" character are examined.)
5325 For example, here is a loop that inserts index producing entries
5326 before any line containing a certain pattern:
5330 print ".IX foo\n" if /\bfoo\b/;
5331 print ".IX bar\n" if /\bbar\b/;
5332 print ".IX blurfl\n" if /\bblurfl\b/;
5337 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5338 will be looked at, because C<f> is rarer than C<o>. In general, this is
5339 a big win except in pathological cases. The only question is whether
5340 it saves you more time than it took to build the linked list in the
5343 Note that if you have to look for strings that you don't know till
5344 runtime, you can build an entire loop as a string and C<eval> that to
5345 avoid recompiling all your patterns all the time. Together with
5346 undefining C<$/> to input entire files as one record, this can be very
5347 fast, often faster than specialized programs like fgrep(1). The following
5348 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5349 out the names of those files that contain a match:
5351 $search = 'while (<>) { study;';
5352 foreach $word (@words) {
5353 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5358 eval $search; # this screams
5359 $/ = "\n"; # put back to normal input delimiter
5360 foreach $file (sort keys(%seen)) {
5364 =item sub NAME BLOCK
5366 =item sub NAME (PROTO) BLOCK
5368 =item sub NAME : ATTRS BLOCK
5370 =item sub NAME (PROTO) : ATTRS BLOCK
5372 This is subroutine definition, not a real function I<per se>.
5373 Without a BLOCK it's just a forward declaration. Without a NAME,
5374 it's an anonymous function declaration, and does actually return
5375 a value: the CODE ref of the closure you just created.
5377 See L<perlsub> and L<perlref> for details about subroutines and
5378 references, and L<attributes> and L<Attribute::Handlers> for more
5379 information about attributes.
5381 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5383 =item substr EXPR,OFFSET,LENGTH
5385 =item substr EXPR,OFFSET
5387 Extracts a substring out of EXPR and returns it. First character is at
5388 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5389 If OFFSET is negative (or more precisely, less than C<$[>), starts
5390 that far from the end of the string. If LENGTH is omitted, returns
5391 everything to the end of the string. If LENGTH is negative, leaves that
5392 many characters off the end of the string.
5394 You can use the substr() function as an lvalue, in which case EXPR
5395 must itself be an lvalue. If you assign something shorter than LENGTH,
5396 the string will shrink, and if you assign something longer than LENGTH,
5397 the string will grow to accommodate it. To keep the string the same
5398 length you may need to pad or chop your value using C<sprintf>.
5400 If OFFSET and LENGTH specify a substring that is partly outside the
5401 string, only the part within the string is returned. If the substring
5402 is beyond either end of the string, substr() returns the undefined
5403 value and produces a warning. When used as an lvalue, specifying a
5404 substring that is entirely outside the string is a fatal error.
5405 Here's an example showing the behavior for boundary cases:
5408 substr($name, 4) = 'dy'; # $name is now 'freddy'
5409 my $null = substr $name, 6, 2; # returns '' (no warning)
5410 my $oops = substr $name, 7; # returns undef, with warning
5411 substr($name, 7) = 'gap'; # fatal error
5413 An alternative to using substr() as an lvalue is to specify the
5414 replacement string as the 4th argument. This allows you to replace
5415 parts of the EXPR and return what was there before in one operation,
5416 just as you can with splice().
5418 =item symlink OLDFILE,NEWFILE
5420 Creates a new filename symbolically linked to the old filename.
5421 Returns C<1> for success, C<0> otherwise. On systems that don't support
5422 symbolic links, produces a fatal error at run time. To check for that,
5425 $symlink_exists = eval { symlink("",""); 1 };
5429 Calls the system call specified as the first element of the list,
5430 passing the remaining elements as arguments to the system call. If
5431 unimplemented, produces a fatal error. The arguments are interpreted
5432 as follows: if a given argument is numeric, the argument is passed as
5433 an int. If not, the pointer to the string value is passed. You are
5434 responsible to make sure a string is pre-extended long enough to
5435 receive any result that might be written into a string. You can't use a
5436 string literal (or other read-only string) as an argument to C<syscall>
5437 because Perl has to assume that any string pointer might be written
5439 integer arguments are not literals and have never been interpreted in a
5440 numeric context, you may need to add C<0> to them to force them to look
5441 like numbers. This emulates the C<syswrite> function (or vice versa):
5443 require 'syscall.ph'; # may need to run h2ph
5445 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5447 Note that Perl supports passing of up to only 14 arguments to your system call,
5448 which in practice should usually suffice.
5450 Syscall returns whatever value returned by the system call it calls.
5451 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5452 Note that some system calls can legitimately return C<-1>. The proper
5453 way to handle such calls is to assign C<$!=0;> before the call and
5454 check the value of C<$!> if syscall returns C<-1>.
5456 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5457 number of the read end of the pipe it creates. There is no way
5458 to retrieve the file number of the other end. You can avoid this
5459 problem by using C<pipe> instead.
5461 =item sysopen FILEHANDLE,FILENAME,MODE
5463 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5465 Opens the file whose filename is given by FILENAME, and associates it
5466 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5467 the name of the real filehandle wanted. This function calls the
5468 underlying operating system's C<open> function with the parameters
5469 FILENAME, MODE, PERMS.
5471 The possible values and flag bits of the MODE parameter are
5472 system-dependent; they are available via the standard module C<Fcntl>.
5473 See the documentation of your operating system's C<open> to see which
5474 values and flag bits are available. You may combine several flags
5475 using the C<|>-operator.
5477 Some of the most common values are C<O_RDONLY> for opening the file in
5478 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5479 and C<O_RDWR> for opening the file in read-write mode, and.
5481 For historical reasons, some values work on almost every system
5482 supported by perl: zero means read-only, one means write-only, and two
5483 means read/write. We know that these values do I<not> work under
5484 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5485 use them in new code.
5487 If the file named by FILENAME does not exist and the C<open> call creates
5488 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5489 PERMS specifies the permissions of the newly created file. If you omit
5490 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5491 These permission values need to be in octal, and are modified by your
5492 process's current C<umask>.
5494 In many systems the C<O_EXCL> flag is available for opening files in
5495 exclusive mode. This is B<not> locking: exclusiveness means here that
5496 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5499 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5501 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5502 that takes away the user's option to have a more permissive umask.
5503 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5506 Note that C<sysopen> depends on the fdopen() C library function.
5507 On many UNIX systems, fdopen() is known to fail when file descriptors
5508 exceed a certain value, typically 255. If you need more file
5509 descriptors than that, consider rebuilding Perl to use the C<sfio>
5510 library, or perhaps using the POSIX::open() function.
5512 See L<perlopentut> for a kinder, gentler explanation of opening files.
5514 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5516 =item sysread FILEHANDLE,SCALAR,LENGTH
5518 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5519 the specified FILEHANDLE, using the system call read(2). It bypasses
5520 buffered IO, so mixing this with other kinds of reads, C<print>,
5521 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5522 stdio usually buffers data. Returns the number of characters actually
5523 read, C<0> at end of file, or undef if there was an error. SCALAR
5524 will be grown or shrunk so that the last byte actually read is the
5525 last byte of the scalar after the read.
5527 Note the I<characters>: depending on the status of the filehandle,
5528 either (8-bit) bytes or characters are read. By default all
5529 filehandles operate on bytes, but for example if the filehandle has
5530 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5531 pragma, L<open>), the I/O will operate on characters, not bytes.
5533 An OFFSET may be specified to place the read data at some place in the
5534 string other than the beginning. A negative OFFSET specifies
5535 placement at that many characters counting backwards from the end of
5536 the string. A positive OFFSET greater than the length of SCALAR
5537 results in the string being padded to the required size with C<"\0">
5538 bytes before the result of the read is appended.
5540 There is no syseof() function, which is ok, since eof() doesn't work
5541 very well on device files (like ttys) anyway. Use sysread() and check
5542 for a return value for 0 to decide whether you're done.
5544 =item sysseek FILEHANDLE,POSITION,WHENCE
5546 Sets FILEHANDLE's system position I<in bytes> using the system call
5547 lseek(2). FILEHANDLE may be an expression whose value gives the name
5548 of the filehandle. The values for WHENCE are C<0> to set the new
5549 position to POSITION, C<1> to set the it to the current position plus
5550 POSITION, and C<2> to set it to EOF plus POSITION (typically
5553 Note the I<in bytes>: even if the filehandle has been set to operate
5554 on characters (for example by using the C<:utf8> I/O layer), tell()
5555 will return byte offsets, not character offsets (because implementing
5556 that would render sysseek() very slow).
5558 sysseek() bypasses normal buffered io, so mixing this with reads (other
5559 than C<sysread>, for example >< or read()) C<print>, C<write>,
5560 C<seek>, C<tell>, or C<eof> may cause confusion.
5562 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5563 and C<SEEK_END> (start of the file, current position, end of the file)
5564 from the Fcntl module. Use of the constants is also more portable
5565 than relying on 0, 1, and 2. For example to define a "systell" function:
5567 use Fnctl 'SEEK_CUR';
5568 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5570 Returns the new position, or the undefined value on failure. A position
5571 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5572 true on success and false on failure, yet you can still easily determine
5577 =item system PROGRAM LIST
5579 Does exactly the same thing as C<exec LIST>, except that a fork is
5580 done first, and the parent process waits for the child process to
5581 complete. Note that argument processing varies depending on the
5582 number of arguments. If there is more than one argument in LIST,
5583 or if LIST is an array with more than one value, starts the program
5584 given by the first element of the list with arguments given by the
5585 rest of the list. If there is only one scalar argument, the argument
5586 is checked for shell metacharacters, and if there are any, the
5587 entire argument is passed to the system's command shell for parsing
5588 (this is C</bin/sh -c> on Unix platforms, but varies on other
5589 platforms). If there are no shell metacharacters in the argument,
5590 it is split into words and passed directly to C<execvp>, which is
5593 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5594 output before any operation that may do a fork, but this may not be
5595 supported on some platforms (see L<perlport>). To be safe, you may need
5596 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5597 of C<IO::Handle> on any open handles.
5599 The return value is the exit status of the program as returned by the
5600 C<wait> call. To get the actual exit value shift right by eight (see below).
5601 See also L</exec>. This is I<not> what you want to use to capture
5602 the output from a command, for that you should use merely backticks or
5603 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5604 indicates a failure to start the program (inspect $! for the reason).
5606 Like C<exec>, C<system> allows you to lie to a program about its name if
5607 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5609 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5610 killing the program they're running doesn't actually interrupt
5613 @args = ("command", "arg1", "arg2");
5615 or die "system @args failed: $?"
5617 You can check all the failure possibilities by inspecting
5620 $exit_value = $? >> 8;
5621 $signal_num = $? & 127;
5622 $dumped_core = $? & 128;
5624 or more portably by using the W*() calls of the POSIX extension;
5625 see L<perlport> for more information.
5627 When the arguments get executed via the system shell, results
5628 and return codes will be subject to its quirks and capabilities.
5629 See L<perlop/"`STRING`"> and L</exec> for details.
5631 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5633 =item syswrite FILEHANDLE,SCALAR,LENGTH
5635 =item syswrite FILEHANDLE,SCALAR
5637 Attempts to write LENGTH characters of data from variable SCALAR to
5638 the specified FILEHANDLE, using the system call write(2). If LENGTH
5639 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5640 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5641 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5642 buffers data. Returns the number of characters actually written, or
5643 C<undef> if there was an error. If the LENGTH is greater than the
5644 available data in the SCALAR after the OFFSET, only as much data as is
5645 available will be written.
5647 An OFFSET may be specified to write the data from some part of the
5648 string other than the beginning. A negative OFFSET specifies writing
5649 that many characters counting backwards from the end of the string.
5650 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5652 Note the I<characters>: depending on the status of the filehandle,
5653 either (8-bit) bytes or characters are written. By default all
5654 filehandles operate on bytes, but for example if the filehandle has
5655 been opened with the C<:utf8> I/O layer (see L</open>, and the open
5656 pragma, L<open>), the I/O will operate on characters, not bytes.
5658 =item tell FILEHANDLE
5662 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5663 error. FILEHANDLE may be an expression whose value gives the name of
5664 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5667 Note the I<in bytes>: even if the filehandle has been set to
5668 operate on characters (for example by using the C<:utf8> open
5669 layer), tell() will return byte offsets, not character offsets
5670 (because that would render seek() and tell() rather slow).
5672 The return value of tell() for the standard streams like the STDIN
5673 depends on the operating system: it may return -1 or something else.
5674 tell() on pipes, fifos, and sockets usually returns -1.
5676 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5678 Do not use tell() on a filehandle that has been opened using
5679 sysopen(), use sysseek() for that as described above. Why? Because
5680 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5681 buffered filehandles. sysseek() make sense only on the first kind,
5682 tell() only makes sense on the second kind.
5684 =item telldir DIRHANDLE
5686 Returns the current position of the C<readdir> routines on DIRHANDLE.
5687 Value may be given to C<seekdir> to access a particular location in a
5688 directory. Has the same caveats about possible directory compaction as
5689 the corresponding system library routine.
5691 =item tie VARIABLE,CLASSNAME,LIST
5693 This function binds a variable to a package class that will provide the
5694 implementation for the variable. VARIABLE is the name of the variable
5695 to be enchanted. CLASSNAME is the name of a class implementing objects
5696 of correct type. Any additional arguments are passed to the C<new>
5697 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5698 or C<TIEHASH>). Typically these are arguments such as might be passed
5699 to the C<dbm_open()> function of C. The object returned by the C<new>
5700 method is also returned by the C<tie> function, which would be useful
5701 if you want to access other methods in CLASSNAME.
5703 Note that functions such as C<keys> and C<values> may return huge lists
5704 when used on large objects, like DBM files. You may prefer to use the
5705 C<each> function to iterate over such. Example:
5707 # print out history file offsets
5709 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5710 while (($key,$val) = each %HIST) {
5711 print $key, ' = ', unpack('L',$val), "\n";
5715 A class implementing a hash should have the following methods:
5717 TIEHASH classname, LIST
5719 STORE this, key, value
5724 NEXTKEY this, lastkey
5728 A class implementing an ordinary array should have the following methods:
5730 TIEARRAY classname, LIST
5732 STORE this, key, value
5734 STORESIZE this, count
5740 SPLICE this, offset, length, LIST
5745 A class implementing a file handle should have the following methods:
5747 TIEHANDLE classname, LIST
5748 READ this, scalar, length, offset
5751 WRITE this, scalar, length, offset
5753 PRINTF this, format, LIST
5757 SEEK this, position, whence
5759 OPEN this, mode, LIST
5764 A class implementing a scalar should have the following methods:
5766 TIESCALAR classname, LIST
5772 Not all methods indicated above need be implemented. See L<perltie>,
5773 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5775 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5776 for you--you need to do that explicitly yourself. See L<DB_File>
5777 or the F<Config> module for interesting C<tie> implementations.
5779 For further details see L<perltie>, L<"tied VARIABLE">.
5783 Returns a reference to the object underlying VARIABLE (the same value
5784 that was originally returned by the C<tie> call that bound the variable
5785 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5790 Returns the number of non-leap seconds since whatever time the system
5791 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5792 and 00:00:00 UTC, January 1, 1970 for most other systems).
5793 Suitable for feeding to C<gmtime> and C<localtime>.
5795 For measuring time in better granularity than one second,
5796 you may use either the Time::HiRes module from CPAN, or
5797 if you have gettimeofday(2), you may be able to use the
5798 C<syscall> interface of Perl, see L<perlfaq8> for details.
5802 Returns a four-element list giving the user and system times, in
5803 seconds, for this process and the children of this process.
5805 ($user,$system,$cuser,$csystem) = times;
5807 In scalar context, C<times> returns C<$user>.
5811 The transliteration operator. Same as C<y///>. See L<perlop>.
5813 =item truncate FILEHANDLE,LENGTH
5815 =item truncate EXPR,LENGTH
5817 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5818 specified length. Produces a fatal error if truncate isn't implemented
5819 on your system. Returns true if successful, the undefined value
5822 The behavior is undefined if LENGTH is greater than the length of the
5829 Returns an uppercased version of EXPR. This is the internal function
5830 implementing the C<\U> escape in double-quoted strings. Respects
5831 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5832 and L<perlunicode> for more details about locale and Unicode support.
5833 It does not attempt to do titlecase mapping on initial letters. See
5834 C<ucfirst> for that.
5836 If EXPR is omitted, uses C<$_>.
5842 Returns the value of EXPR with the first character in uppercase
5843 (titlecase in Unicode). This is the internal function implementing
5844 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5845 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5846 for more details about locale and Unicode support.
5848 If EXPR is omitted, uses C<$_>.
5854 Sets the umask for the process to EXPR and returns the previous value.
5855 If EXPR is omitted, merely returns the current umask.
5857 The Unix permission C<rwxr-x---> is represented as three sets of three
5858 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5859 and isn't one of the digits). The C<umask> value is such a number
5860 representing disabled permissions bits. The permission (or "mode")
5861 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5862 even if you tell C<sysopen> to create a file with permissions C<0777>,
5863 if your umask is C<0022> then the file will actually be created with
5864 permissions C<0755>. If your C<umask> were C<0027> (group can't
5865 write; others can't read, write, or execute), then passing
5866 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5869 Here's some advice: supply a creation mode of C<0666> for regular
5870 files (in C<sysopen>) and one of C<0777> for directories (in
5871 C<mkdir>) and executable files. This gives users the freedom of
5872 choice: if they want protected files, they might choose process umasks
5873 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5874 Programs should rarely if ever make policy decisions better left to
5875 the user. The exception to this is when writing files that should be
5876 kept private: mail files, web browser cookies, I<.rhosts> files, and
5879 If umask(2) is not implemented on your system and you are trying to
5880 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5881 fatal error at run time. If umask(2) is not implemented and you are
5882 not trying to restrict access for yourself, returns C<undef>.
5884 Remember that a umask is a number, usually given in octal; it is I<not> a
5885 string of octal digits. See also L</oct>, if all you have is a string.
5891 Undefines the value of EXPR, which must be an lvalue. Use only on a
5892 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5893 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5894 will probably not do what you expect on most predefined variables or
5895 DBM list values, so don't do that; see L<delete>.) Always returns the
5896 undefined value. You can omit the EXPR, in which case nothing is
5897 undefined, but you still get an undefined value that you could, for
5898 instance, return from a subroutine, assign to a variable or pass as a
5899 parameter. Examples:
5902 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5906 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5907 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5908 select undef, undef, undef, 0.25;
5909 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5911 Note that this is a unary operator, not a list operator.
5917 Deletes a list of files. Returns the number of files successfully
5920 $cnt = unlink 'a', 'b', 'c';
5924 Note: C<unlink> will not delete directories unless you are superuser and
5925 the B<-U> flag is supplied to Perl. Even if these conditions are
5926 met, be warned that unlinking a directory can inflict damage on your
5927 filesystem. Use C<rmdir> instead.
5929 If LIST is omitted, uses C<$_>.
5931 =item unpack TEMPLATE,EXPR
5933 C<unpack> does the reverse of C<pack>: it takes a string
5934 and expands it out into a list of values.
5935 (In scalar context, it returns merely the first value produced.)
5937 The string is broken into chunks described by the TEMPLATE. Each chunk
5938 is converted separately to a value. Typically, either the string is a result
5939 of C<pack>, or the bytes of the string represent a C structure of some
5942 The TEMPLATE has the same format as in the C<pack> function.
5943 Here's a subroutine that does substring:
5946 my($what,$where,$howmuch) = @_;
5947 unpack("x$where a$howmuch", $what);
5952 sub ordinal { unpack("c",$_[0]); } # same as ord()
5954 In addition to fields allowed in pack(), you may prefix a field with
5955 a %<number> to indicate that
5956 you want a <number>-bit checksum of the items instead of the items
5957 themselves. Default is a 16-bit checksum. Checksum is calculated by
5958 summing numeric values of expanded values (for string fields the sum of
5959 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5961 For example, the following
5962 computes the same number as the System V sum program:
5966 unpack("%32C*",<>) % 65535;
5969 The following efficiently counts the number of set bits in a bit vector:
5971 $setbits = unpack("%32b*", $selectmask);
5973 The C<p> and C<P> formats should be used with care. Since Perl
5974 has no way of checking whether the value passed to C<unpack()>
5975 corresponds to a valid memory location, passing a pointer value that's
5976 not known to be valid is likely to have disastrous consequences.
5978 If the repeat count of a field is larger than what the remainder of
5979 the input string allows, repeat count is decreased. If the input string
5980 is longer than one described by the TEMPLATE, the rest is ignored.
5982 See L</pack> for more examples and notes.
5984 =item untie VARIABLE
5986 Breaks the binding between a variable and a package. (See C<tie>.)
5987 Has no effect if the variable is not tied.
5989 =item unshift ARRAY,LIST
5991 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5992 depending on how you look at it. Prepends list to the front of the
5993 array, and returns the new number of elements in the array.
5995 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5997 Note the LIST is prepended whole, not one element at a time, so the
5998 prepended elements stay in the same order. Use C<reverse> to do the
6001 =item use Module VERSION LIST
6003 =item use Module VERSION
6005 =item use Module LIST
6011 Imports some semantics into the current package from the named module,
6012 generally by aliasing certain subroutine or variable names into your
6013 package. It is exactly equivalent to
6015 BEGIN { require Module; import Module LIST; }
6017 except that Module I<must> be a bareword.
6019 VERSION may be either a numeric argument such as 5.006, which will be
6020 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6021 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6022 greater than the version of the current Perl interpreter; Perl will not
6023 attempt to parse the rest of the file. Compare with L</require>, which can
6024 do a similar check at run time.
6026 Specifying VERSION as a literal of the form v5.6.1 should generally be
6027 avoided, because it leads to misleading error messages under earlier
6028 versions of Perl which do not support this syntax. The equivalent numeric
6029 version should be used instead.
6031 use v5.6.1; # compile time version check
6033 use 5.006_001; # ditto; preferred for backwards compatibility
6035 This is often useful if you need to check the current Perl version before
6036 C<use>ing library modules that have changed in incompatible ways from
6037 older versions of Perl. (We try not to do this more than we have to.)
6039 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6040 C<require> makes sure the module is loaded into memory if it hasn't been
6041 yet. The C<import> is not a builtin--it's just an ordinary static method
6042 call into the C<Module> package to tell the module to import the list of
6043 features back into the current package. The module can implement its
6044 C<import> method any way it likes, though most modules just choose to
6045 derive their C<import> method via inheritance from the C<Exporter> class that
6046 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6047 method can be found then the call is skipped.
6049 If you do not want to call the package's C<import> method (for instance,
6050 to stop your namespace from being altered), explicitly supply the empty list:
6054 That is exactly equivalent to
6056 BEGIN { require Module }
6058 If the VERSION argument is present between Module and LIST, then the
6059 C<use> will call the VERSION method in class Module with the given
6060 version as an argument. The default VERSION method, inherited from
6061 the UNIVERSAL class, croaks if the given version is larger than the
6062 value of the variable C<$Module::VERSION>.
6064 Again, there is a distinction between omitting LIST (C<import> called
6065 with no arguments) and an explicit empty LIST C<()> (C<import> not
6066 called). Note that there is no comma after VERSION!
6068 Because this is a wide-open interface, pragmas (compiler directives)
6069 are also implemented this way. Currently implemented pragmas are:
6074 use sigtrap qw(SEGV BUS);
6075 use strict qw(subs vars refs);
6076 use subs qw(afunc blurfl);
6077 use warnings qw(all);
6078 use sort qw(stable _quicksort _mergesort);
6080 Some of these pseudo-modules import semantics into the current
6081 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6082 which import symbols into the current package (which are effective
6083 through the end of the file).
6085 There's a corresponding C<no> command that unimports meanings imported
6086 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6092 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6093 for the C<-M> and C<-m> command-line options to perl that give C<use>
6094 functionality from the command-line.
6098 Changes the access and modification times on each file of a list of
6099 files. The first two elements of the list must be the NUMERICAL access
6100 and modification times, in that order. Returns the number of files
6101 successfully changed. The inode change time of each file is set
6102 to the current time. This code has the same effect as the C<touch>
6103 command if the files already exist:
6107 utime $now, $now, @ARGV;
6109 If the first two elements of the list are C<undef>, then the utime(2)
6110 function in the C library will be called with a null second argument.
6111 On most systems, this will set the file's access and modification
6112 times to the current time. (i.e. equivalent to the example above.)
6114 utime undef, undef, @ARGV;
6118 Returns a list consisting of all the values of the named hash. (In a
6119 scalar context, returns the number of values.) The values are
6120 returned in an apparently random order. The actual random order is
6121 subject to change in future versions of perl, but it is guaranteed to
6122 be the same order as either the C<keys> or C<each> function would
6123 produce on the same (unmodified) hash.
6125 Note that the values are not copied, which means modifying them will
6126 modify the contents of the hash:
6128 for (values %hash) { s/foo/bar/g } # modifies %hash values
6129 for (@hash{keys %hash}) { s/foo/bar/g } # same
6131 As a side effect, calling values() resets the HASH's internal iterator.
6132 See also C<keys>, C<each>, and C<sort>.
6134 =item vec EXPR,OFFSET,BITS
6136 Treats the string in EXPR as a bit vector made up of elements of
6137 width BITS, and returns the value of the element specified by OFFSET
6138 as an unsigned integer. BITS therefore specifies the number of bits
6139 that are reserved for each element in the bit vector. This must
6140 be a power of two from 1 to 32 (or 64, if your platform supports
6143 If BITS is 8, "elements" coincide with bytes of the input string.
6145 If BITS is 16 or more, bytes of the input string are grouped into chunks
6146 of size BITS/8, and each group is converted to a number as with
6147 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6148 for BITS==64). See L<"pack"> for details.
6150 If bits is 4 or less, the string is broken into bytes, then the bits
6151 of each byte are broken into 8/BITS groups. Bits of a byte are
6152 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6153 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6154 breaking the single input byte C<chr(0x36)> into two groups gives a list
6155 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6157 C<vec> may also be assigned to, in which case parentheses are needed
6158 to give the expression the correct precedence as in
6160 vec($image, $max_x * $x + $y, 8) = 3;
6162 If the selected element is outside the string, the value 0 is returned.
6163 If an element off the end of the string is written to, Perl will first
6164 extend the string with sufficiently many zero bytes. It is an error
6165 to try to write off the beginning of the string (i.e. negative OFFSET).
6167 The string should not contain any character with the value > 255 (which
6168 can only happen if you're using UTF8 encoding). If it does, it will be
6169 treated as something which is not UTF8 encoded. When the C<vec> was
6170 assigned to, other parts of your program will also no longer consider the
6171 string to be UTF8 encoded. In other words, if you do have such characters
6172 in your string, vec() will operate on the actual byte string, and not the
6173 conceptual character string.
6175 Strings created with C<vec> can also be manipulated with the logical
6176 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6177 vector operation is desired when both operands are strings.
6178 See L<perlop/"Bitwise String Operators">.
6180 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6181 The comments show the string after each step. Note that this code works
6182 in the same way on big-endian or little-endian machines.
6185 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6187 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6188 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6190 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6191 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6192 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6193 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6194 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6195 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6197 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6198 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6199 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6202 To transform a bit vector into a string or list of 0's and 1's, use these:
6204 $bits = unpack("b*", $vector);
6205 @bits = split(//, unpack("b*", $vector));
6207 If you know the exact length in bits, it can be used in place of the C<*>.
6209 Here is an example to illustrate how the bits actually fall in place:
6215 unpack("V",$_) 01234567890123456789012345678901
6216 ------------------------------------------------------------------
6221 for ($shift=0; $shift < $width; ++$shift) {
6222 for ($off=0; $off < 32/$width; ++$off) {
6223 $str = pack("B*", "0"x32);
6224 $bits = (1<<$shift);
6225 vec($str, $off, $width) = $bits;
6226 $res = unpack("b*",$str);
6227 $val = unpack("V", $str);
6234 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6235 $off, $width, $bits, $val, $res
6239 Regardless of the machine architecture on which it is run, the above
6240 example should print the following table:
6243 unpack("V",$_) 01234567890123456789012345678901
6244 ------------------------------------------------------------------
6245 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6246 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6247 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6248 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6249 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6250 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6251 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6252 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6253 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6254 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6255 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6256 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6257 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6258 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6259 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6260 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6261 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6262 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6263 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6264 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6265 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6266 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6267 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6268 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6269 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6270 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6271 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6272 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6273 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6274 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6275 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6276 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6277 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6278 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6279 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6280 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6281 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6282 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6283 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6284 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6285 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6286 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6287 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6288 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6289 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6290 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6291 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6292 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6293 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6294 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6295 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6296 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6297 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6298 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6299 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6300 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6301 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6302 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6303 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6304 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6305 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6306 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6307 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6308 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6309 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6310 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6311 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6312 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6313 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6314 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6315 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6316 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6317 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6318 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6319 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6320 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6321 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6322 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6323 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6324 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6325 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6326 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6327 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6328 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6329 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6330 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6331 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6332 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6333 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6334 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6335 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6336 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6337 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6338 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6339 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6340 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6341 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6342 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6343 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6344 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6345 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6346 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6347 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6348 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6349 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6350 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6351 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6352 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6353 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6354 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6355 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6356 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6357 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6358 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6359 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6360 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6361 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6362 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6363 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6364 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6365 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6366 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6367 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6368 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6369 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6370 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6371 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6372 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6376 Behaves like the wait(2) system call on your system: it waits for a child
6377 process to terminate and returns the pid of the deceased process, or
6378 C<-1> if there are no child processes. The status is returned in C<$?>.
6379 Note that a return value of C<-1> could mean that child processes are
6380 being automatically reaped, as described in L<perlipc>.
6382 =item waitpid PID,FLAGS
6384 Waits for a particular child process to terminate and returns the pid of
6385 the deceased process, or C<-1> if there is no such child process. On some
6386 systems, a value of 0 indicates that there are processes still running.
6387 The status is returned in C<$?>. If you say
6389 use POSIX ":sys_wait_h";
6392 $kid = waitpid(-1, WNOHANG);
6395 then you can do a non-blocking wait for all pending zombie processes.
6396 Non-blocking wait is available on machines supporting either the
6397 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6398 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6399 system call by remembering the status values of processes that have
6400 exited but have not been harvested by the Perl script yet.)
6402 Note that on some systems, a return value of C<-1> could mean that child
6403 processes are being automatically reaped. See L<perlipc> for details,
6404 and for other examples.
6408 Returns true if the context of the currently executing subroutine is
6409 looking for a list value. Returns false if the context is looking
6410 for a scalar. Returns the undefined value if the context is looking
6411 for no value (void context).
6413 return unless defined wantarray; # don't bother doing more
6414 my @a = complex_calculation();
6415 return wantarray ? @a : "@a";
6417 This function should have been named wantlist() instead.
6421 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6424 If LIST is empty and C<$@> already contains a value (typically from a
6425 previous eval) that value is used after appending C<"\t...caught">
6426 to C<$@>. This is useful for staying almost, but not entirely similar to
6429 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6431 No message is printed if there is a C<$SIG{__WARN__}> handler
6432 installed. It is the handler's responsibility to deal with the message
6433 as it sees fit (like, for instance, converting it into a C<die>). Most
6434 handlers must therefore make arrangements to actually display the
6435 warnings that they are not prepared to deal with, by calling C<warn>
6436 again in the handler. Note that this is quite safe and will not
6437 produce an endless loop, since C<__WARN__> hooks are not called from
6440 You will find this behavior is slightly different from that of
6441 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6442 instead call C<die> again to change it).
6444 Using a C<__WARN__> handler provides a powerful way to silence all
6445 warnings (even the so-called mandatory ones). An example:
6447 # wipe out *all* compile-time warnings
6448 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6450 my $foo = 20; # no warning about duplicate my $foo,
6451 # but hey, you asked for it!
6452 # no compile-time or run-time warnings before here
6455 # run-time warnings enabled after here
6456 warn "\$foo is alive and $foo!"; # does show up
6458 See L<perlvar> for details on setting C<%SIG> entries, and for more
6459 examples. See the Carp module for other kinds of warnings using its
6460 carp() and cluck() functions.
6462 =item write FILEHANDLE
6468 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6469 using the format associated with that file. By default the format for
6470 a file is the one having the same name as the filehandle, but the
6471 format for the current output channel (see the C<select> function) may be set
6472 explicitly by assigning the name of the format to the C<$~> variable.
6474 Top of form processing is handled automatically: if there is
6475 insufficient room on the current page for the formatted record, the
6476 page is advanced by writing a form feed, a special top-of-page format
6477 is used to format the new page header, and then the record is written.
6478 By default the top-of-page format is the name of the filehandle with
6479 "_TOP" appended, but it may be dynamically set to the format of your
6480 choice by assigning the name to the C<$^> variable while the filehandle is
6481 selected. The number of lines remaining on the current page is in
6482 variable C<$->, which can be set to C<0> to force a new page.
6484 If FILEHANDLE is unspecified, output goes to the current default output
6485 channel, which starts out as STDOUT but may be changed by the
6486 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6487 is evaluated and the resulting string is used to look up the name of
6488 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6490 Note that write is I<not> the opposite of C<read>. Unfortunately.
6494 The transliteration operator. Same as C<tr///>. See L<perlop>.