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 Commas should separate elements of the LIST.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your Perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
369 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370 test operators, in a way that C<-f -w -x $file> is equivalent to
371 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
372 the return value of C<-f $file> as an argument to another filetest
373 operator, no special magic will happen.)
379 Returns the absolute value of its argument.
380 If VALUE is omitted, uses C<$_>.
382 =item accept NEWSOCKET,GENERICSOCKET
384 Accepts an incoming socket connect, just as the accept(2) system call
385 does. Returns the packed address if it succeeded, false otherwise.
386 See the example in L<perlipc/"Sockets: Client/Server Communication">.
388 On systems that support a close-on-exec flag on files, the flag will
389 be set for the newly opened file descriptor, as determined by the
390 value of $^F. See L<perlvar/$^F>.
396 Arranges to have a SIGALRM delivered to this process after the
397 specified number of wallclock seconds has elapsed. If SECONDS is not
398 specified, the value stored in C<$_> is used. (On some machines,
399 unfortunately, the elapsed time may be up to one second less or more
400 than you specified because of how seconds are counted, and process
401 scheduling may delay the delivery of the signal even further.)
403 Only one timer may be counting at once. Each call disables the
404 previous timer, and an argument of C<0> may be supplied to cancel the
405 previous timer without starting a new one. The returned value is the
406 amount of time remaining on the previous timer.
408 For delays of finer granularity than one second, you may use Perl's
409 four-argument version of select() leaving the first three arguments
410 undefined, or you might be able to use the C<syscall> interface to
411 access setitimer(2) if your system supports it. The Time::HiRes
412 module (from CPAN, and starting from Perl 5.8 part of the standard
413 distribution) may also prove useful.
415 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416 (C<sleep> may be internally implemented in your system with C<alarm>)
418 If you want to use C<alarm> to time out a system call you need to use an
419 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
420 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
421 restart system calls on some systems. Using C<eval>/C<die> always works,
422 modulo the caveats given in L<perlipc/"Signals">.
425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
427 $nread = sysread SOCKET, $buffer, $size;
431 die unless $@ eq "alarm\n"; # propagate unexpected errors
438 For more information see L<perlipc>.
442 Returns the arctangent of Y/X in the range -PI to PI.
444 For the tangent operation, you may use the C<Math::Trig::tan>
445 function, or use the familiar relation:
447 sub tan { sin($_[0]) / cos($_[0]) }
449 =item bind SOCKET,NAME
451 Binds a network address to a socket, just as the bind system call
452 does. Returns true if it succeeded, false otherwise. NAME should be a
453 packed address of the appropriate type for the socket. See the examples in
454 L<perlipc/"Sockets: Client/Server Communication">.
456 =item binmode FILEHANDLE, LAYER
458 =item binmode FILEHANDLE
460 Arranges for FILEHANDLE to be read or written in "binary" or "text"
461 mode on systems where the run-time libraries distinguish between
462 binary and text files. If FILEHANDLE is an expression, the value is
463 taken as the name of the filehandle. Returns true on success,
464 otherwise it returns C<undef> and sets C<$!> (errno).
466 On some systems (in general, DOS and Windows-based systems) binmode()
467 is necessary when you're not working with a text file. For the sake
468 of portability it is a good idea to always use it when appropriate,
469 and to never use it when it isn't appropriate. Also, people can
470 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
472 In other words: regardless of platform, use binmode() on binary data,
473 like for example images.
475 If LAYER is present it is a single string, but may contain multiple
476 directives. The directives alter the behaviour of the file handle.
477 When LAYER is present using binmode on text file makes sense.
479 If LAYER is omitted or specified as C<:raw> the filehandle is made
480 suitable for passing binary data. This includes turning off possible CRLF
481 translation and marking it as bytes (as opposed to Unicode characters).
482 Note that, despite what may be implied in I<"Programming Perl"> (the
483 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
484 -- other layers which would affect binary nature of the stream are
485 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486 PERLIO environment variable.
488 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490 establish default I/O layers. See L<open>.
492 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493 in "Programming Perl, 3rd Edition". However, since the publishing of this
494 book, by many known as "Camel III", the consensus of the naming of this
495 functionality has moved from "discipline" to "layer". All documentation
496 of this version of Perl therefore refers to "layers" rather than to
497 "disciplines". Now back to the regularly scheduled documentation...>
499 To mark FILEHANDLE as UTF-8, use C<:utf8>.
501 In general, binmode() should be called after open() but before any I/O
502 is done on the filehandle. Calling binmode() will normally flush any
503 pending buffered output data (and perhaps pending input data) on the
504 handle. An exception to this is the C<:encoding> layer that
505 changes the default character encoding of the handle, see L<open>.
506 The C<:encoding> layer sometimes needs to be called in
507 mid-stream, and it doesn't flush the stream. The C<:encoding>
508 also implicitly pushes on top of itself the C<:utf8> layer because
509 internally Perl will operate on UTF-8 encoded Unicode characters.
511 The operating system, device drivers, C libraries, and Perl run-time
512 system all work together to let the programmer treat a single
513 character (C<\n>) as the line terminator, irrespective of the external
514 representation. On many operating systems, the native text file
515 representation matches the internal representation, but on some
516 platforms the external representation of C<\n> is made up of more than
519 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520 character to end each line in the external representation of text (even
521 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
522 on Unix and most VMS files). In other systems like OS/2, DOS and the
523 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524 but what's stored in text files are the two characters C<\cM\cJ>. That
525 means that, if you don't use binmode() on these systems, C<\cM\cJ>
526 sequences on disk will be converted to C<\n> on input, and any C<\n> in
527 your program will be converted back to C<\cM\cJ> on output. This is what
528 you want for text files, but it can be disastrous for binary files.
530 Another consequence of using binmode() (on some systems) is that
531 special end-of-file markers will be seen as part of the data stream.
532 For systems from the Microsoft family this means that if your binary
533 data contains C<\cZ>, the I/O subsystem will regard it as the end of
534 the file, unless you use binmode().
536 binmode() is not only important for readline() and print() operations,
537 but also when using read(), seek(), sysread(), syswrite() and tell()
538 (see L<perlport> for more details). See the C<$/> and C<$\> variables
539 in L<perlvar> for how to manually set your input and output
540 line-termination sequences.
542 =item bless REF,CLASSNAME
546 This function tells the thingy referenced by REF that it is now an object
547 in the CLASSNAME package. If CLASSNAME is omitted, the current package
548 is used. Because a C<bless> is often the last thing in a constructor,
549 it returns the reference for convenience. Always use the two-argument
550 version if a derived class might inherit the function doing the blessing.
551 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
554 Consider always blessing objects in CLASSNAMEs that are mixed case.
555 Namespaces with all lowercase names are considered reserved for
556 Perl pragmata. Builtin types have all uppercase names. To prevent
557 confusion, you may wish to avoid such package names as well. Make sure
558 that CLASSNAME is a true value.
560 See L<perlmod/"Perl Modules">.
566 Returns the context of the current subroutine call. In scalar context,
567 returns the caller's package name if there is a caller, that is, if
568 we're in a subroutine or C<eval> or C<require>, and the undefined value
569 otherwise. In list context, returns
571 ($package, $filename, $line) = caller;
573 With EXPR, it returns some extra information that the debugger uses to
574 print a stack trace. The value of EXPR indicates how many call frames
575 to go back before the current one.
577 ($package, $filename, $line, $subroutine, $hasargs,
578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
580 Here $subroutine may be C<(eval)> if the frame is not a subroutine
581 call, but an C<eval>. In such a case additional elements $evaltext and
582 C<$is_require> are set: C<$is_require> is true if the frame is created by a
583 C<require> or C<use> statement, $evaltext contains the text of the
584 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
585 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
586 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587 frame.) $subroutine may also be C<(unknown)> if this particular
588 subroutine happens to have been deleted from the symbol table.
589 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591 compiled with. The C<$hints> and C<$bitmask> values are subject to change
592 between versions of Perl, and are not meant for external use.
594 Furthermore, when called from within the DB package, caller returns more
595 detailed information: it sets the list variable C<@DB::args> to be the
596 arguments with which the subroutine was invoked.
598 Be aware that the optimizer might have optimized call frames away before
599 C<caller> had a chance to get the information. That means that C<caller(N)>
600 might not return information about the call frame you expect it do, for
601 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
602 previous time C<caller> was called.
606 =item chdir FILEHANDLE
608 =item chdir DIRHANDLE
612 Changes the working directory to EXPR, if possible. If EXPR is omitted,
613 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
614 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
615 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
616 neither is set, C<chdir> does nothing. It returns true upon success,
617 false otherwise. See the example under C<die>.
619 On systems that support fchdir, you might pass a file handle or
620 directory handle as argument. On systems that don't support fchdir,
621 passing handles produces a fatal error at run time.
625 Changes the permissions of a list of files. The first element of the
626 list must be the numerical mode, which should probably be an octal
627 number, and which definitely should I<not> be a string of octal digits:
628 C<0644> is okay, C<'0644'> is not. Returns the number of files
629 successfully changed. See also L</oct>, if all you have is a string.
631 $cnt = chmod 0755, 'foo', 'bar';
632 chmod 0755, @executables;
633 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
635 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
636 $mode = 0644; chmod $mode, 'foo'; # this is best
638 On systems that support fchmod, you might pass file handles among the
639 files. On systems that don't support fchmod, passing file handles
640 produces a fatal error at run time.
642 open(my $fh, "<", "foo");
643 my $perm = (stat $fh)[2] & 07777;
644 chmod($perm | 0600, $fh);
646 You can also import the symbolic C<S_I*> constants from the Fcntl
651 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
652 # This is identical to the chmod 0755 of the above example.
660 This safer version of L</chop> removes any trailing string
661 that corresponds to the current value of C<$/> (also known as
662 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
663 number of characters removed from all its arguments. It's often used to
664 remove the newline from the end of an input record when you're worried
665 that the final record may be missing its newline. When in paragraph
666 mode (C<$/ = "">), it removes all trailing newlines from the string.
667 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
668 a reference to an integer or the like, see L<perlvar>) chomp() won't
670 If VARIABLE is omitted, it chomps C<$_>. Example:
673 chomp; # avoid \n on last field
678 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
680 You can actually chomp anything that's an lvalue, including an assignment:
683 chomp($answer = <STDIN>);
685 If you chomp a list, each element is chomped, and the total number of
686 characters removed is returned.
688 If the C<encoding> pragma is in scope then the lengths returned are
689 calculated from the length of C<$/> in Unicode characters, which is not
690 always the same as the length of C<$/> in the native encoding.
692 Note that parentheses are necessary when you're chomping anything
693 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
694 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
695 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
696 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
705 Chops off the last character of a string and returns the character
706 chopped. It is much more efficient than C<s/.$//s> because it neither
707 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
708 If VARIABLE is a hash, it chops the hash's values, but not its keys.
710 You can actually chop anything that's an lvalue, including an assignment.
712 If you chop a list, each element is chopped. Only the value of the
713 last C<chop> is returned.
715 Note that C<chop> returns the last character. To return all but the last
716 character, use C<substr($string, 0, -1)>.
722 Changes the owner (and group) of a list of files. The first two
723 elements of the list must be the I<numeric> uid and gid, in that
724 order. A value of -1 in either position is interpreted by most
725 systems to leave that value unchanged. Returns the number of files
726 successfully changed.
728 $cnt = chown $uid, $gid, 'foo', 'bar';
729 chown $uid, $gid, @filenames;
731 On systems that support fchown, you might pass file handles among the
732 files. On systems that don't support fchown, passing file handles
733 produces a fatal error at run time.
735 Here's an example that looks up nonnumeric uids in the passwd file:
738 chomp($user = <STDIN>);
740 chomp($pattern = <STDIN>);
742 ($login,$pass,$uid,$gid) = getpwnam($user)
743 or die "$user not in passwd file";
745 @ary = glob($pattern); # expand filenames
746 chown $uid, $gid, @ary;
748 On most systems, you are not allowed to change the ownership of the
749 file unless you're the superuser, although you should be able to change
750 the group to any of your secondary groups. On insecure systems, these
751 restrictions may be relaxed, but this is not a portable assumption.
752 On POSIX systems, you can detect this condition this way:
754 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
755 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
761 Returns the character represented by that NUMBER in the character set.
762 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
763 chr(0x263a) is a Unicode smiley face. Note that characters from 128
764 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
765 backward compatibility reasons (but see L<encoding>).
767 Negative values give the Unicode replacement character (chr(0xfffd)),
768 except under the L</bytes> pragma, where low eight bits of the value
769 (truncated to an integer) are used.
771 If NUMBER is omitted, uses C<$_>.
773 For the reverse, use L</ord>.
775 Note that under the C<bytes> pragma the NUMBER is masked to
778 See L<perlunicode> and L<encoding> for more about Unicode.
780 =item chroot FILENAME
784 This function works like the system call by the same name: it makes the
785 named directory the new root directory for all further pathnames that
786 begin with a C</> by your process and all its children. (It doesn't
787 change your current working directory, which is unaffected.) For security
788 reasons, this call is restricted to the superuser. If FILENAME is
789 omitted, does a C<chroot> to C<$_>.
791 =item close FILEHANDLE
795 Closes the file or pipe associated with the file handle, returning
796 true only if IO buffers are successfully flushed and closes the system
797 file descriptor. Closes the currently selected filehandle if the
800 You don't have to close FILEHANDLE if you are immediately going to do
801 another C<open> on it, because C<open> will close it for you. (See
802 C<open>.) However, an explicit C<close> on an input file resets the line
803 counter (C<$.>), while the implicit close done by C<open> does not.
805 If the file handle came from a piped open, C<close> will additionally
806 return false if one of the other system calls involved fails, or if the
807 program exits with non-zero status. (If the only problem was that the
808 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
809 also waits for the process executing on the pipe to complete, in case you
810 want to look at the output of the pipe afterwards, and
811 implicitly puts the exit status value of that command into C<$?> and
812 C<${^CHILD_ERROR_NATIVE}>.
814 Prematurely closing the read end of a pipe (i.e. before the process
815 writing to it at the other end has closed it) will result in a
816 SIGPIPE being delivered to the writer. If the other end can't
817 handle that, be sure to read all the data before closing the pipe.
821 open(OUTPUT, '|sort >foo') # pipe to sort
822 or die "Can't start sort: $!";
823 #... # print stuff to output
824 close OUTPUT # wait for sort to finish
825 or warn $! ? "Error closing sort pipe: $!"
826 : "Exit status $? from sort";
827 open(INPUT, 'foo') # get sort's results
828 or die "Can't open 'foo' for input: $!";
830 FILEHANDLE may be an expression whose value can be used as an indirect
831 filehandle, usually the real filehandle name.
833 =item closedir DIRHANDLE
835 Closes a directory opened by C<opendir> and returns the success of that
838 =item connect SOCKET,NAME
840 Attempts to connect to a remote socket, just as the connect system call
841 does. Returns true if it succeeded, false otherwise. NAME should be a
842 packed address of the appropriate type for the socket. See the examples in
843 L<perlipc/"Sockets: Client/Server Communication">.
847 C<continue> is actually a flow control statement rather than a function. If
848 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
849 C<foreach>), it is always executed just before the conditional is about to
850 be evaluated again, just like the third part of a C<for> loop in C. Thus
851 it can be used to increment a loop variable, even when the loop has been
852 continued via the C<next> statement (which is similar to the C C<continue>
855 C<last>, C<next>, or C<redo> may appear within a C<continue>
856 block. C<last> and C<redo> will behave as if they had been executed within
857 the main block. So will C<next>, but since it will execute a C<continue>
858 block, it may be more entertaining.
861 ### redo always comes here
864 ### next always comes here
866 # then back the top to re-check EXPR
868 ### last always comes here
870 Omitting the C<continue> section is semantically equivalent to using an
871 empty one, logically enough. In that case, C<next> goes directly back
872 to check the condition at the top of the loop.
878 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
879 takes cosine of C<$_>.
881 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
882 function, or use this relation:
884 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
886 =item crypt PLAINTEXT,SALT
888 Creates a digest string exactly like the crypt(3) function in the C
889 library (assuming that you actually have a version there that has not
890 been extirpated as a potential munitions).
892 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
893 into a short string, called a digest, which is returned. The same
894 PLAINTEXT and SALT will always return the same string, but there is no
895 (known) way to get the original PLAINTEXT from the hash. Small
896 changes in the PLAINTEXT or SALT will result in large changes in the
899 There is no decrypt function. This function isn't all that useful for
900 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
901 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
902 primarily used to check if two pieces of text are the same without
903 having to transmit or store the text itself. An example is checking
904 if a correct password is given. The digest of the password is stored,
905 not the password itself. The user types in a password that is
906 crypt()'d with the same salt as the stored digest. If the two digests
907 match the password is correct.
909 When verifying an existing digest string you should use the digest as
910 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
911 to create the digest is visible as part of the digest. This ensures
912 crypt() will hash the new string with the same salt as the digest.
913 This allows your code to work with the standard L<crypt|/crypt> and
914 with more exotic implementations. In other words, do not assume
915 anything about the returned string itself, or how many bytes in the
918 Traditionally the result is a string of 13 bytes: two first bytes of
919 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
920 the first eight bytes of the digest string mattered, but alternative
921 hashing schemes (like MD5), higher level security schemes (like C2),
922 and implementations on non-UNIX platforms may produce different
925 When choosing a new salt create a random two character string whose
926 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
927 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
928 characters is just a recommendation; the characters allowed in
929 the salt depend solely on your system's crypt library, and Perl can't
930 restrict what salts C<crypt()> accepts.
932 Here's an example that makes sure that whoever runs this program knows
935 $pwd = (getpwuid($<))[1];
939 chomp($word = <STDIN>);
943 if (crypt($word, $pwd) ne $pwd) {
949 Of course, typing in your own password to whoever asks you
952 The L<crypt|/crypt> function is unsuitable for hashing large quantities
953 of data, not least of all because you can't get the information
954 back. Look at the L<Digest> module for more robust algorithms.
956 If using crypt() on a Unicode string (which I<potentially> has
957 characters with codepoints above 255), Perl tries to make sense
958 of the situation by trying to downgrade (a copy of the string)
959 the string back to an eight-bit byte string before calling crypt()
960 (on that copy). If that works, good. If not, crypt() dies with
961 C<Wide character in crypt>.
965 [This function has been largely superseded by the C<untie> function.]
967 Breaks the binding between a DBM file and a hash.
969 =item dbmopen HASH,DBNAME,MASK
971 [This function has been largely superseded by the C<tie> function.]
973 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
974 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
975 argument is I<not> a filehandle, even though it looks like one). DBNAME
976 is the name of the database (without the F<.dir> or F<.pag> extension if
977 any). If the database does not exist, it is created with protection
978 specified by MASK (as modified by the C<umask>). If your system supports
979 only the older DBM functions, you may perform only one C<dbmopen> in your
980 program. In older versions of Perl, if your system had neither DBM nor
981 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
984 If you don't have write access to the DBM file, you can only read hash
985 variables, not set them. If you want to test whether you can write,
986 either use file tests or try setting a dummy hash entry inside an C<eval>,
987 which will trap the error.
989 Note that functions such as C<keys> and C<values> may return huge lists
990 when used on large DBM files. You may prefer to use the C<each>
991 function to iterate over large DBM files. Example:
993 # print out history file offsets
994 dbmopen(%HIST,'/usr/lib/news/history',0666);
995 while (($key,$val) = each %HIST) {
996 print $key, ' = ', unpack('L',$val), "\n";
1000 See also L<AnyDBM_File> for a more general description of the pros and
1001 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1002 rich implementation.
1004 You can control which DBM library you use by loading that library
1005 before you call dbmopen():
1008 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1009 or die "Can't open netscape history file: $!";
1015 Returns a Boolean value telling whether EXPR has a value other than
1016 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1019 Many operations return C<undef> to indicate failure, end of file,
1020 system error, uninitialized variable, and other exceptional
1021 conditions. This function allows you to distinguish C<undef> from
1022 other values. (A simple Boolean test will not distinguish among
1023 C<undef>, zero, the empty string, and C<"0">, which are all equally
1024 false.) Note that since C<undef> is a valid scalar, its presence
1025 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1026 returns C<undef> when its argument is an empty array, I<or> when the
1027 element to return happens to be C<undef>.
1029 You may also use C<defined(&func)> to check whether subroutine C<&func>
1030 has ever been defined. The return value is unaffected by any forward
1031 declarations of C<&func>. Note that a subroutine which is not defined
1032 may still be callable: its package may have an C<AUTOLOAD> method that
1033 makes it spring into existence the first time that it is called -- see
1036 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1037 used to report whether memory for that aggregate has ever been
1038 allocated. This behavior may disappear in future versions of Perl.
1039 You should instead use a simple test for size:
1041 if (@an_array) { print "has array elements\n" }
1042 if (%a_hash) { print "has hash members\n" }
1044 When used on a hash element, it tells you whether the value is defined,
1045 not whether the key exists in the hash. Use L</exists> for the latter
1050 print if defined $switch{'D'};
1051 print "$val\n" while defined($val = pop(@ary));
1052 die "Can't readlink $sym: $!"
1053 unless defined($value = readlink $sym);
1054 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1055 $debugging = 0 unless defined $debugging;
1057 Note: Many folks tend to overuse C<defined>, and then are surprised to
1058 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1059 defined values. For example, if you say
1063 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1064 matched "nothing". It didn't really fail to match anything. Rather, it
1065 matched something that happened to be zero characters long. This is all
1066 very above-board and honest. When a function returns an undefined value,
1067 it's an admission that it couldn't give you an honest answer. So you
1068 should use C<defined> only when you're questioning the integrity of what
1069 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1072 See also L</undef>, L</exists>, L</ref>.
1076 Given an expression that specifies a hash element, array element, hash slice,
1077 or array slice, deletes the specified element(s) from the hash or array.
1078 In the case of an array, if the array elements happen to be at the end,
1079 the size of the array will shrink to the highest element that tests
1080 true for exists() (or 0 if no such element exists).
1082 Returns a list with the same number of elements as the number of elements
1083 for which deletion was attempted. Each element of that list consists of
1084 either the value of the element deleted, or the undefined value. In scalar
1085 context, this means that you get the value of the last element deleted (or
1086 the undefined value if that element did not exist).
1088 %hash = (foo => 11, bar => 22, baz => 33);
1089 $scalar = delete $hash{foo}; # $scalar is 11
1090 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1091 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1093 Deleting from C<%ENV> modifies the environment. Deleting from
1094 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1095 from a C<tie>d hash or array may not necessarily return anything.
1097 Deleting an array element effectively returns that position of the array
1098 to its initial, uninitialized state. Subsequently testing for the same
1099 element with exists() will return false. Also, deleting array elements
1100 in the middle of an array will not shift the index of the elements
1101 after them down. Use splice() for that. See L</exists>.
1103 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1105 foreach $key (keys %HASH) {
1109 foreach $index (0 .. $#ARRAY) {
1110 delete $ARRAY[$index];
1115 delete @HASH{keys %HASH};
1117 delete @ARRAY[0 .. $#ARRAY];
1119 But both of these are slower than just assigning the empty list
1120 or undefining %HASH or @ARRAY:
1122 %HASH = (); # completely empty %HASH
1123 undef %HASH; # forget %HASH ever existed
1125 @ARRAY = (); # completely empty @ARRAY
1126 undef @ARRAY; # forget @ARRAY ever existed
1128 Note that the EXPR can be arbitrarily complicated as long as the final
1129 operation is a hash element, array element, hash slice, or array slice
1132 delete $ref->[$x][$y]{$key};
1133 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1135 delete $ref->[$x][$y][$index];
1136 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1140 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1141 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1142 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1143 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1144 an C<eval(),> the error message is stuffed into C<$@> and the
1145 C<eval> is terminated with the undefined value. This makes
1146 C<die> the way to raise an exception.
1148 Equivalent examples:
1150 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1151 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1153 If the last element of LIST does not end in a newline, the current
1154 script line number and input line number (if any) are also printed,
1155 and a newline is supplied. Note that the "input line number" (also
1156 known as "chunk") is subject to whatever notion of "line" happens to
1157 be currently in effect, and is also available as the special variable
1158 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1160 Hint: sometimes appending C<", stopped"> to your message will cause it
1161 to make better sense when the string C<"at foo line 123"> is appended.
1162 Suppose you are running script "canasta".
1164 die "/etc/games is no good";
1165 die "/etc/games is no good, stopped";
1167 produce, respectively
1169 /etc/games is no good at canasta line 123.
1170 /etc/games is no good, stopped at canasta line 123.
1172 See also exit(), warn(), and the Carp module.
1174 If LIST is empty and C<$@> already contains a value (typically from a
1175 previous eval) that value is reused after appending C<"\t...propagated">.
1176 This is useful for propagating exceptions:
1179 die unless $@ =~ /Expected exception/;
1181 If LIST is empty and C<$@> contains an object reference that has a
1182 C<PROPAGATE> method, that method will be called with additional file
1183 and line number parameters. The return value replaces the value in
1184 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1187 If C<$@> is empty then the string C<"Died"> is used.
1189 die() can also be called with a reference argument. If this happens to be
1190 trapped within an eval(), $@ contains the reference. This behavior permits
1191 a more elaborate exception handling implementation using objects that
1192 maintain arbitrary state about the nature of the exception. Such a scheme
1193 is sometimes preferable to matching particular string values of $@ using
1194 regular expressions. Here's an example:
1196 use Scalar::Util 'blessed';
1198 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1200 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1201 # handle Some::Module::Exception
1204 # handle all other possible exceptions
1208 Because perl will stringify uncaught exception messages before displaying
1209 them, you may want to overload stringification operations on such custom
1210 exception objects. See L<overload> for details about that.
1212 You can arrange for a callback to be run just before the C<die>
1213 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1214 handler will be called with the error text and can change the error
1215 message, if it sees fit, by calling C<die> again. See
1216 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1217 L<"eval BLOCK"> for some examples. Although this feature was
1218 to be run only right before your program was to exit, this is not
1219 currently the case--the C<$SIG{__DIE__}> hook is currently called
1220 even inside eval()ed blocks/strings! If one wants the hook to do
1221 nothing in such situations, put
1225 as the first line of the handler (see L<perlvar/$^S>). Because
1226 this promotes strange action at a distance, this counterintuitive
1227 behavior may be fixed in a future release.
1231 Not really a function. Returns the value of the last command in the
1232 sequence of commands indicated by BLOCK. When modified by a loop
1233 modifier, executes the BLOCK once before testing the loop condition.
1234 (On other statements the loop modifiers test the conditional first.)
1236 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1237 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1238 See L<perlsyn> for alternative strategies.
1240 =item do SUBROUTINE(LIST)
1242 This form of subroutine call is deprecated. See L<perlsub>.
1246 Uses the value of EXPR as a filename and executes the contents of the
1247 file as a Perl script.
1255 except that it's more efficient and concise, keeps track of the current
1256 filename for error messages, searches the @INC directories, and updates
1257 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1258 variables. It also differs in that code evaluated with C<do FILENAME>
1259 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1260 same, however, in that it does reparse the file every time you call it,
1261 so you probably don't want to do this inside a loop.
1263 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1264 error. If C<do> can read the file but cannot compile it, it
1265 returns undef and sets an error message in C<$@>. If the file is
1266 successfully compiled, C<do> returns the value of the last expression
1269 Note that inclusion of library modules is better done with the
1270 C<use> and C<require> operators, which also do automatic error checking
1271 and raise an exception if there's a problem.
1273 You might like to use C<do> to read in a program configuration
1274 file. Manual error checking can be done this way:
1276 # read in config files: system first, then user
1277 for $file ("/share/prog/defaults.rc",
1278 "$ENV{HOME}/.someprogrc")
1280 unless ($return = do $file) {
1281 warn "couldn't parse $file: $@" if $@;
1282 warn "couldn't do $file: $!" unless defined $return;
1283 warn "couldn't run $file" unless $return;
1291 This function causes an immediate core dump. See also the B<-u>
1292 command-line switch in L<perlrun>, which does the same thing.
1293 Primarily this is so that you can use the B<undump> program (not
1294 supplied) to turn your core dump into an executable binary after
1295 having initialized all your variables at the beginning of the
1296 program. When the new binary is executed it will begin by executing
1297 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1298 Think of it as a goto with an intervening core dump and reincarnation.
1299 If C<LABEL> is omitted, restarts the program from the top.
1301 B<WARNING>: Any files opened at the time of the dump will I<not>
1302 be open any more when the program is reincarnated, with possible
1303 resulting confusion on the part of Perl.
1305 This function is now largely obsolete, partly because it's very
1306 hard to convert a core file into an executable, and because the
1307 real compiler backends for generating portable bytecode and compilable
1308 C code have superseded it. That's why you should now invoke it as
1309 C<CORE::dump()>, if you don't want to be warned against a possible
1312 If you're looking to use L<dump> to speed up your program, consider
1313 generating bytecode or native C code as described in L<perlcc>. If
1314 you're just trying to accelerate a CGI script, consider using the
1315 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1316 You might also consider autoloading or selfloading, which at least
1317 make your program I<appear> to run faster.
1321 When called in list context, returns a 2-element list consisting of the
1322 key and value for the next element of a hash, so that you can iterate over
1323 it. When called in scalar context, returns only the key for the next
1324 element in the hash.
1326 Entries are returned in an apparently random order. The actual random
1327 order is subject to change in future versions of perl, but it is
1328 guaranteed to be in the same order as either the C<keys> or C<values>
1329 function would produce on the same (unmodified) hash. Since Perl
1330 5.8.1 the ordering is different even between different runs of Perl
1331 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1333 When the hash is entirely read, a null array is returned in list context
1334 (which when assigned produces a false (C<0>) value), and C<undef> in
1335 scalar context. The next call to C<each> after that will start iterating
1336 again. There is a single iterator for each hash, shared by all C<each>,
1337 C<keys>, and C<values> function calls in the program; it can be reset by
1338 reading all the elements from the hash, or by evaluating C<keys HASH> or
1339 C<values HASH>. If you add or delete elements of a hash while you're
1340 iterating over it, you may get entries skipped or duplicated, so
1341 don't. Exception: It is always safe to delete the item most recently
1342 returned by C<each()>, which means that the following code will work:
1344 while (($key, $value) = each %hash) {
1346 delete $hash{$key}; # This is safe
1349 The following prints out your environment like the printenv(1) program,
1350 only in a different order:
1352 while (($key,$value) = each %ENV) {
1353 print "$key=$value\n";
1356 See also C<keys>, C<values> and C<sort>.
1358 =item eof FILEHANDLE
1364 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1365 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1366 gives the real filehandle. (Note that this function actually
1367 reads a character and then C<ungetc>s it, so isn't very useful in an
1368 interactive context.) Do not read from a terminal file (or call
1369 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1370 as terminals may lose the end-of-file condition if you do.
1372 An C<eof> without an argument uses the last file read. Using C<eof()>
1373 with empty parentheses is very different. It refers to the pseudo file
1374 formed from the files listed on the command line and accessed via the
1375 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1376 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1377 used will cause C<@ARGV> to be examined to determine if input is
1378 available. Similarly, an C<eof()> after C<< <> >> has returned
1379 end-of-file will assume you are processing another C<@ARGV> list,
1380 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1381 see L<perlop/"I/O Operators">.
1383 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1384 detect the end of each file, C<eof()> will only detect the end of the
1385 last file. Examples:
1387 # reset line numbering on each input file
1389 next if /^\s*#/; # skip comments
1392 close ARGV if eof; # Not eof()!
1395 # insert dashes just before last line of last file
1397 if (eof()) { # check for end of last file
1398 print "--------------\n";
1401 last if eof(); # needed if we're reading from a terminal
1404 Practical hint: you almost never need to use C<eof> in Perl, because the
1405 input operators typically return C<undef> when they run out of data, or if
1414 In the first form, the return value of EXPR is parsed and executed as if it
1415 were a little Perl program. The value of the expression (which is itself
1416 determined within scalar context) is first parsed, and if there weren't any
1417 errors, executed in the lexical context of the current Perl program, so
1418 that any variable settings or subroutine and format definitions remain
1419 afterwards. Note that the value is parsed every time the C<eval> executes.
1420 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1421 delay parsing and subsequent execution of the text of EXPR until run time.
1423 In the second form, the code within the BLOCK is parsed only once--at the
1424 same time the code surrounding the C<eval> itself was parsed--and executed
1425 within the context of the current Perl program. This form is typically
1426 used to trap exceptions more efficiently than the first (see below), while
1427 also providing the benefit of checking the code within BLOCK at compile
1430 The final semicolon, if any, may be omitted from the value of EXPR or within
1433 In both forms, the value returned is the value of the last expression
1434 evaluated inside the mini-program; a return statement may be also used, just
1435 as with subroutines. The expression providing the return value is evaluated
1436 in void, scalar, or list context, depending on the context of the C<eval>
1437 itself. See L</wantarray> for more on how the evaluation context can be
1440 If there is a syntax error or runtime error, or a C<die> statement is
1441 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1442 error message. If there was no error, C<$@> is guaranteed to be a null
1443 string. Beware that using C<eval> neither silences perl from printing
1444 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1445 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1446 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1447 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1449 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1450 determining whether a particular feature (such as C<socket> or C<symlink>)
1451 is implemented. It is also Perl's exception trapping mechanism, where
1452 the die operator is used to raise exceptions.
1454 If the code to be executed doesn't vary, you may use the eval-BLOCK
1455 form to trap run-time errors without incurring the penalty of
1456 recompiling each time. The error, if any, is still returned in C<$@>.
1459 # make divide-by-zero nonfatal
1460 eval { $answer = $a / $b; }; warn $@ if $@;
1462 # same thing, but less efficient
1463 eval '$answer = $a / $b'; warn $@ if $@;
1465 # a compile-time error
1466 eval { $answer = }; # WRONG
1469 eval '$answer ='; # sets $@
1471 Using the C<eval{}> form as an exception trap in libraries does have some
1472 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1473 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1474 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1475 as shown in this example:
1477 # a very private exception trap for divide-by-zero
1478 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1481 This is especially significant, given that C<__DIE__> hooks can call
1482 C<die> again, which has the effect of changing their error messages:
1484 # __DIE__ hooks may modify error messages
1486 local $SIG{'__DIE__'} =
1487 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1488 eval { die "foo lives here" };
1489 print $@ if $@; # prints "bar lives here"
1492 Because this promotes action at a distance, this counterintuitive behavior
1493 may be fixed in a future release.
1495 With an C<eval>, you should be especially careful to remember what's
1496 being looked at when:
1502 eval { $x }; # CASE 4
1504 eval "\$$x++"; # CASE 5
1507 Cases 1 and 2 above behave identically: they run the code contained in
1508 the variable $x. (Although case 2 has misleading double quotes making
1509 the reader wonder what else might be happening (nothing is).) Cases 3
1510 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1511 does nothing but return the value of $x. (Case 4 is preferred for
1512 purely visual reasons, but it also has the advantage of compiling at
1513 compile-time instead of at run-time.) Case 5 is a place where
1514 normally you I<would> like to use double quotes, except that in this
1515 particular situation, you can just use symbolic references instead, as
1518 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1519 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1521 Note that as a very special case, an C<eval ''> executed within the C<DB>
1522 package doesn't see the usual surrounding lexical scope, but rather the
1523 scope of the first non-DB piece of code that called it. You don't normally
1524 need to worry about this unless you are writing a Perl debugger.
1528 =item exec PROGRAM LIST
1530 The C<exec> function executes a system command I<and never returns>--
1531 use C<system> instead of C<exec> if you want it to return. It fails and
1532 returns false only if the command does not exist I<and> it is executed
1533 directly instead of via your system's command shell (see below).
1535 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1536 warns you if there is a following statement which isn't C<die>, C<warn>,
1537 or C<exit> (if C<-w> is set - but you always do that). If you
1538 I<really> want to follow an C<exec> with some other statement, you
1539 can use one of these styles to avoid the warning:
1541 exec ('foo') or print STDERR "couldn't exec foo: $!";
1542 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1544 If there is more than one argument in LIST, or if LIST is an array
1545 with more than one value, calls execvp(3) with the arguments in LIST.
1546 If there is only one scalar argument or an array with one element in it,
1547 the argument is checked for shell metacharacters, and if there are any,
1548 the entire argument is passed to the system's command shell for parsing
1549 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1550 If there are no shell metacharacters in the argument, it is split into
1551 words and passed directly to C<execvp>, which is more efficient.
1554 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1555 exec "sort $outfile | uniq";
1557 If you don't really want to execute the first argument, but want to lie
1558 to the program you are executing about its own name, you can specify
1559 the program you actually want to run as an "indirect object" (without a
1560 comma) in front of the LIST. (This always forces interpretation of the
1561 LIST as a multivalued list, even if there is only a single scalar in
1564 $shell = '/bin/csh';
1565 exec $shell '-sh'; # pretend it's a login shell
1569 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1571 When the arguments get executed via the system shell, results will
1572 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1575 Using an indirect object with C<exec> or C<system> is also more
1576 secure. This usage (which also works fine with system()) forces
1577 interpretation of the arguments as a multivalued list, even if the
1578 list had just one argument. That way you're safe from the shell
1579 expanding wildcards or splitting up words with whitespace in them.
1581 @args = ( "echo surprise" );
1583 exec @args; # subject to shell escapes
1585 exec { $args[0] } @args; # safe even with one-arg list
1587 The first version, the one without the indirect object, ran the I<echo>
1588 program, passing it C<"surprise"> an argument. The second version
1589 didn't--it tried to run a program literally called I<"echo surprise">,
1590 didn't find it, and set C<$?> to a non-zero value indicating failure.
1592 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1593 output before the exec, but this may not be supported on some platforms
1594 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1595 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1596 open handles in order to avoid lost output.
1598 Note that C<exec> will not call your C<END> blocks, nor will it call
1599 any C<DESTROY> methods in your objects.
1603 Given an expression that specifies a hash element or array element,
1604 returns true if the specified element in the hash or array has ever
1605 been initialized, even if the corresponding value is undefined. The
1606 element is not autovivified if it doesn't exist.
1608 print "Exists\n" if exists $hash{$key};
1609 print "Defined\n" if defined $hash{$key};
1610 print "True\n" if $hash{$key};
1612 print "Exists\n" if exists $array[$index];
1613 print "Defined\n" if defined $array[$index];
1614 print "True\n" if $array[$index];
1616 A hash or array element can be true only if it's defined, and defined if
1617 it exists, but the reverse doesn't necessarily hold true.
1619 Given an expression that specifies the name of a subroutine,
1620 returns true if the specified subroutine has ever been declared, even
1621 if it is undefined. Mentioning a subroutine name for exists or defined
1622 does not count as declaring it. Note that a subroutine which does not
1623 exist may still be callable: its package may have an C<AUTOLOAD>
1624 method that makes it spring into existence the first time that it is
1625 called -- see L<perlsub>.
1627 print "Exists\n" if exists &subroutine;
1628 print "Defined\n" if defined &subroutine;
1630 Note that the EXPR can be arbitrarily complicated as long as the final
1631 operation is a hash or array key lookup or subroutine name:
1633 if (exists $ref->{A}->{B}->{$key}) { }
1634 if (exists $hash{A}{B}{$key}) { }
1636 if (exists $ref->{A}->{B}->[$ix]) { }
1637 if (exists $hash{A}{B}[$ix]) { }
1639 if (exists &{$ref->{A}{B}{$key}}) { }
1641 Although the deepest nested array or hash will not spring into existence
1642 just because its existence was tested, any intervening ones will.
1643 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1644 into existence due to the existence test for the $key element above.
1645 This happens anywhere the arrow operator is used, including even:
1648 if (exists $ref->{"Some key"}) { }
1649 print $ref; # prints HASH(0x80d3d5c)
1651 This surprising autovivification in what does not at first--or even
1652 second--glance appear to be an lvalue context may be fixed in a future
1655 Use of a subroutine call, rather than a subroutine name, as an argument
1656 to exists() is an error.
1659 exists &sub(); # Error
1665 Evaluates EXPR and exits immediately with that value. Example:
1668 exit 0 if $ans =~ /^[Xx]/;
1670 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1671 universally recognized values for EXPR are C<0> for success and C<1>
1672 for error; other values are subject to interpretation depending on the
1673 environment in which the Perl program is running. For example, exiting
1674 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1675 the mailer to return the item undelivered, but that's not true everywhere.
1677 Don't use C<exit> to abort a subroutine if there's any chance that
1678 someone might want to trap whatever error happened. Use C<die> instead,
1679 which can be trapped by an C<eval>.
1681 The exit() function does not always exit immediately. It calls any
1682 defined C<END> routines first, but these C<END> routines may not
1683 themselves abort the exit. Likewise any object destructors that need to
1684 be called are called before the real exit. If this is a problem, you
1685 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1686 See L<perlmod> for details.
1692 Returns I<e> (the natural logarithm base) to the power of EXPR.
1693 If EXPR is omitted, gives C<exp($_)>.
1695 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1697 Implements the fcntl(2) function. You'll probably have to say
1701 first to get the correct constant definitions. Argument processing and
1702 value return works just like C<ioctl> below.
1706 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1707 or die "can't fcntl F_GETFL: $!";
1709 You don't have to check for C<defined> on the return from C<fcntl>.
1710 Like C<ioctl>, it maps a C<0> return from the system call into
1711 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1712 in numeric context. It is also exempt from the normal B<-w> warnings
1713 on improper numeric conversions.
1715 Note that C<fcntl> will produce a fatal error if used on a machine that
1716 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1717 manpage to learn what functions are available on your system.
1719 Here's an example of setting a filehandle named C<REMOTE> to be
1720 non-blocking at the system level. You'll have to negotiate C<$|>
1721 on your own, though.
1723 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1725 $flags = fcntl(REMOTE, F_GETFL, 0)
1726 or die "Can't get flags for the socket: $!\n";
1728 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1729 or die "Can't set flags for the socket: $!\n";
1731 =item fileno FILEHANDLE
1733 Returns the file descriptor for a filehandle, or undefined if the
1734 filehandle is not open. This is mainly useful for constructing
1735 bitmaps for C<select> and low-level POSIX tty-handling operations.
1736 If FILEHANDLE is an expression, the value is taken as an indirect
1737 filehandle, generally its name.
1739 You can use this to find out whether two handles refer to the
1740 same underlying descriptor:
1742 if (fileno(THIS) == fileno(THAT)) {
1743 print "THIS and THAT are dups\n";
1746 (Filehandles connected to memory objects via new features of C<open> may
1747 return undefined even though they are open.)
1750 =item flock FILEHANDLE,OPERATION
1752 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1753 for success, false on failure. Produces a fatal error if used on a
1754 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1755 C<flock> is Perl's portable file locking interface, although it locks
1756 only entire files, not records.
1758 Two potentially non-obvious but traditional C<flock> semantics are
1759 that it waits indefinitely until the lock is granted, and that its locks
1760 B<merely advisory>. Such discretionary locks are more flexible, but offer
1761 fewer guarantees. This means that programs that do not also use C<flock>
1762 may modify files locked with C<flock>. See L<perlport>,
1763 your port's specific documentation, or your system-specific local manpages
1764 for details. It's best to assume traditional behavior if you're writing
1765 portable programs. (But if you're not, you should as always feel perfectly
1766 free to write for your own system's idiosyncrasies (sometimes called
1767 "features"). Slavish adherence to portability concerns shouldn't get
1768 in the way of your getting your job done.)
1770 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1771 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1772 you can use the symbolic names if you import them from the Fcntl module,
1773 either individually, or as a group using the ':flock' tag. LOCK_SH
1774 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1775 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1776 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1777 waiting for the lock (check the return status to see if you got it).
1779 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1780 before locking or unlocking it.
1782 Note that the emulation built with lockf(3) doesn't provide shared
1783 locks, and it requires that FILEHANDLE be open with write intent. These
1784 are the semantics that lockf(3) implements. Most if not all systems
1785 implement lockf(3) in terms of fcntl(2) locking, though, so the
1786 differing semantics shouldn't bite too many people.
1788 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1789 be open with read intent to use LOCK_SH and requires that it be open
1790 with write intent to use LOCK_EX.
1792 Note also that some versions of C<flock> cannot lock things over the
1793 network; you would need to use the more system-specific C<fcntl> for
1794 that. If you like you can force Perl to ignore your system's flock(2)
1795 function, and so provide its own fcntl(2)-based emulation, by passing
1796 the switch C<-Ud_flock> to the F<Configure> program when you configure
1799 Here's a mailbox appender for BSD systems.
1801 use Fcntl ':flock'; # import LOCK_* constants
1804 flock(MBOX,LOCK_EX);
1805 # and, in case someone appended
1806 # while we were waiting...
1811 flock(MBOX,LOCK_UN);
1814 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1815 or die "Can't open mailbox: $!";
1818 print MBOX $msg,"\n\n";
1821 On systems that support a real flock(), locks are inherited across fork()
1822 calls, whereas those that must resort to the more capricious fcntl()
1823 function lose the locks, making it harder to write servers.
1825 See also L<DB_File> for other flock() examples.
1829 Does a fork(2) system call to create a new process running the
1830 same program at the same point. It returns the child pid to the
1831 parent process, C<0> to the child process, or C<undef> if the fork is
1832 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1833 are shared, while everything else is copied. On most systems supporting
1834 fork(), great care has gone into making it extremely efficient (for
1835 example, using copy-on-write technology on data pages), making it the
1836 dominant paradigm for multitasking over the last few decades.
1838 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1839 output before forking the child process, but this may not be supported
1840 on some platforms (see L<perlport>). To be safe, you may need to set
1841 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1842 C<IO::Handle> on any open handles in order to avoid duplicate output.
1844 If you C<fork> without ever waiting on your children, you will
1845 accumulate zombies. On some systems, you can avoid this by setting
1846 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1847 forking and reaping moribund children.
1849 Note that if your forked child inherits system file descriptors like
1850 STDIN and STDOUT that are actually connected by a pipe or socket, even
1851 if you exit, then the remote server (such as, say, a CGI script or a
1852 backgrounded job launched from a remote shell) won't think you're done.
1853 You should reopen those to F</dev/null> if it's any issue.
1857 Declare a picture format for use by the C<write> function. For
1861 Test: @<<<<<<<< @||||| @>>>>>
1862 $str, $%, '$' . int($num)
1866 $num = $cost/$quantity;
1870 See L<perlform> for many details and examples.
1872 =item formline PICTURE,LIST
1874 This is an internal function used by C<format>s, though you may call it,
1875 too. It formats (see L<perlform>) a list of values according to the
1876 contents of PICTURE, placing the output into the format output
1877 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1878 Eventually, when a C<write> is done, the contents of
1879 C<$^A> are written to some filehandle. You could also read C<$^A>
1880 and then set C<$^A> back to C<"">. Note that a format typically
1881 does one C<formline> per line of form, but the C<formline> function itself
1882 doesn't care how many newlines are embedded in the PICTURE. This means
1883 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1884 You may therefore need to use multiple formlines to implement a single
1885 record format, just like the format compiler.
1887 Be careful if you put double quotes around the picture, because an C<@>
1888 character may be taken to mean the beginning of an array name.
1889 C<formline> always returns true. See L<perlform> for other examples.
1891 =item getc FILEHANDLE
1895 Returns the next character from the input file attached to FILEHANDLE,
1896 or the undefined value at end of file, or if there was an error (in
1897 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1898 STDIN. This is not particularly efficient. However, it cannot be
1899 used by itself to fetch single characters without waiting for the user
1900 to hit enter. For that, try something more like:
1903 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1906 system "stty", '-icanon', 'eol', "\001";
1912 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1915 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1919 Determination of whether $BSD_STYLE should be set
1920 is left as an exercise to the reader.
1922 The C<POSIX::getattr> function can do this more portably on
1923 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1924 module from your nearest CPAN site; details on CPAN can be found on
1929 This implements the C library function of the same name, which on most
1930 systems returns the current login from F</etc/utmp>, if any. If null,
1933 $login = getlogin || getpwuid($<) || "Kilroy";
1935 Do not consider C<getlogin> for authentication: it is not as
1936 secure as C<getpwuid>.
1938 =item getpeername SOCKET
1940 Returns the packed sockaddr address of other end of the SOCKET connection.
1943 $hersockaddr = getpeername(SOCK);
1944 ($port, $iaddr) = sockaddr_in($hersockaddr);
1945 $herhostname = gethostbyaddr($iaddr, AF_INET);
1946 $herstraddr = inet_ntoa($iaddr);
1950 Returns the current process group for the specified PID. Use
1951 a PID of C<0> to get the current process group for the
1952 current process. Will raise an exception if used on a machine that
1953 doesn't implement getpgrp(2). If PID is omitted, returns process
1954 group of current process. Note that the POSIX version of C<getpgrp>
1955 does not accept a PID argument, so only C<PID==0> is truly portable.
1959 Returns the process id of the parent process.
1961 Note for Linux users: on Linux, the C functions C<getpid()> and
1962 C<getppid()> return different values from different threads. In order to
1963 be portable, this behavior is not reflected by the perl-level function
1964 C<getppid()>, that returns a consistent value across threads. If you want
1965 to call the underlying C<getppid()>, you may use the CPAN module
1968 =item getpriority WHICH,WHO
1970 Returns the current priority for a process, a process group, or a user.
1971 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1972 machine that doesn't implement getpriority(2).
1978 =item gethostbyname NAME
1980 =item getnetbyname NAME
1982 =item getprotobyname NAME
1988 =item getservbyname NAME,PROTO
1990 =item gethostbyaddr ADDR,ADDRTYPE
1992 =item getnetbyaddr ADDR,ADDRTYPE
1994 =item getprotobynumber NUMBER
1996 =item getservbyport PORT,PROTO
2014 =item sethostent STAYOPEN
2016 =item setnetent STAYOPEN
2018 =item setprotoent STAYOPEN
2020 =item setservent STAYOPEN
2034 These routines perform the same functions as their counterparts in the
2035 system library. In list context, the return values from the
2036 various get routines are as follows:
2038 ($name,$passwd,$uid,$gid,
2039 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2040 ($name,$passwd,$gid,$members) = getgr*
2041 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2042 ($name,$aliases,$addrtype,$net) = getnet*
2043 ($name,$aliases,$proto) = getproto*
2044 ($name,$aliases,$port,$proto) = getserv*
2046 (If the entry doesn't exist you get a null list.)
2048 The exact meaning of the $gcos field varies but it usually contains
2049 the real name of the user (as opposed to the login name) and other
2050 information pertaining to the user. Beware, however, that in many
2051 system users are able to change this information and therefore it
2052 cannot be trusted and therefore the $gcos is tainted (see
2053 L<perlsec>). The $passwd and $shell, user's encrypted password and
2054 login shell, are also tainted, because of the same reason.
2056 In scalar context, you get the name, unless the function was a
2057 lookup by name, in which case you get the other thing, whatever it is.
2058 (If the entry doesn't exist you get the undefined value.) For example:
2060 $uid = getpwnam($name);
2061 $name = getpwuid($num);
2063 $gid = getgrnam($name);
2064 $name = getgrgid($num);
2068 In I<getpw*()> the fields $quota, $comment, and $expire are special
2069 cases in the sense that in many systems they are unsupported. If the
2070 $quota is unsupported, it is an empty scalar. If it is supported, it
2071 usually encodes the disk quota. If the $comment field is unsupported,
2072 it is an empty scalar. If it is supported it usually encodes some
2073 administrative comment about the user. In some systems the $quota
2074 field may be $change or $age, fields that have to do with password
2075 aging. In some systems the $comment field may be $class. The $expire
2076 field, if present, encodes the expiration period of the account or the
2077 password. For the availability and the exact meaning of these fields
2078 in your system, please consult your getpwnam(3) documentation and your
2079 F<pwd.h> file. You can also find out from within Perl what your
2080 $quota and $comment fields mean and whether you have the $expire field
2081 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2082 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2083 files are only supported if your vendor has implemented them in the
2084 intuitive fashion that calling the regular C library routines gets the
2085 shadow versions if you're running under privilege or if there exists
2086 the shadow(3) functions as found in System V (this includes Solaris
2087 and Linux.) Those systems that implement a proprietary shadow password
2088 facility are unlikely to be supported.
2090 The $members value returned by I<getgr*()> is a space separated list of
2091 the login names of the members of the group.
2093 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2094 C, it will be returned to you via C<$?> if the function call fails. The
2095 C<@addrs> value returned by a successful call is a list of the raw
2096 addresses returned by the corresponding system library call. In the
2097 Internet domain, each address is four bytes long and you can unpack it
2098 by saying something like:
2100 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2102 The Socket library makes this slightly easier:
2105 $iaddr = inet_aton("127.1"); # or whatever address
2106 $name = gethostbyaddr($iaddr, AF_INET);
2108 # or going the other way
2109 $straddr = inet_ntoa($iaddr);
2111 If you get tired of remembering which element of the return list
2112 contains which return value, by-name interfaces are provided
2113 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2114 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2115 and C<User::grent>. These override the normal built-ins, supplying
2116 versions that return objects with the appropriate names
2117 for each field. For example:
2121 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2123 Even though it looks like they're the same method calls (uid),
2124 they aren't, because a C<File::stat> object is different from
2125 a C<User::pwent> object.
2127 =item getsockname SOCKET
2129 Returns the packed sockaddr address of this end of the SOCKET connection,
2130 in case you don't know the address because you have several different
2131 IPs that the connection might have come in on.
2134 $mysockaddr = getsockname(SOCK);
2135 ($port, $myaddr) = sockaddr_in($mysockaddr);
2136 printf "Connect to %s [%s]\n",
2137 scalar gethostbyaddr($myaddr, AF_INET),
2140 =item getsockopt SOCKET,LEVEL,OPTNAME
2142 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2143 Options may exist at multiple protocol levels depending on the socket
2144 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2145 C<Socket> module) will exist. To query options at another level the
2146 protocol number of the appropriate protocol controlling the option
2147 should be supplied. For example, to indicate that an option is to be
2148 interpreted by the TCP protocol, LEVEL should be set to the protocol
2149 number of TCP, which you can get using getprotobyname.
2151 The call returns a packed string representing the requested socket option,
2152 or C<undef> if there is an error (the error reason will be in $!). What
2153 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2154 your system documentation for details. A very common case however is that
2155 the option is an integer, in which case the result will be a packed
2156 integer which you can decode using unpack with the C<i> (or C<I>) format.
2158 An example testing if Nagle's algorithm is turned on on a socket:
2160 use Socket qw(:all);
2162 defined(my $tcp = getprotobyname("tcp"))
2163 or die "Could not determine the protocol number for tcp";
2164 # my $tcp = IPPROTO_TCP; # Alternative
2165 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2166 or die "Could not query TCP_NODELAY socket option: $!";
2167 my $nodelay = unpack("I", $packed);
2168 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2175 In list context, returns a (possibly empty) list of filename expansions on
2176 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2177 scalar context, glob iterates through such filename expansions, returning
2178 undef when the list is exhausted. This is the internal function
2179 implementing the C<< <*.c> >> operator, but you can use it directly. If
2180 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2181 more detail in L<perlop/"I/O Operators">.
2183 Beginning with v5.6.0, this operator is implemented using the standard
2184 C<File::Glob> extension. See L<File::Glob> for details.
2190 Converts a time as returned by the time function to an 8-element list
2191 with the time localized for the standard Greenwich time zone.
2192 Typically used as follows:
2195 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2198 All list elements are numeric, and come straight out of the C `struct
2199 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2200 specified time. $mday is the day of the month, and $mon is the month
2201 itself, in the range C<0..11> with 0 indicating January and 11
2202 indicating December. $year is the number of years since 1900. That
2203 is, $year is C<123> in year 2023. $wday is the day of the week, with
2204 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2205 the year, in the range C<0..364> (or C<0..365> in leap years.)
2207 Note that the $year element is I<not> simply the last two digits of
2208 the year. If you assume it is then you create non-Y2K-compliant
2209 programs--and you wouldn't want to do that, would you?
2211 The proper way to get a complete 4-digit year is simply:
2215 And to get the last two digits of the year (e.g., '01' in 2001) do:
2217 $year = sprintf("%02d", $year % 100);
2219 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2221 In scalar context, C<gmtime()> returns the ctime(3) value:
2223 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2225 If you need local time instead of GMT use the L</localtime> builtin.
2226 See also the C<timegm> function provided by the C<Time::Local> module,
2227 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2229 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2230 instead a Perl builtin. To get somewhat similar but locale dependent date
2231 strings, see the example in L</localtime>.
2233 See L<perlport/gmtime> for portability concerns.
2241 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2242 execution there. It may not be used to go into any construct that
2243 requires initialization, such as a subroutine or a C<foreach> loop. It
2244 also can't be used to go into a construct that is optimized away,
2245 or to get out of a block or subroutine given to C<sort>.
2246 It can be used to go almost anywhere else within the dynamic scope,
2247 including out of subroutines, but it's usually better to use some other
2248 construct such as C<last> or C<die>. The author of Perl has never felt the
2249 need to use this form of C<goto> (in Perl, that is--C is another matter).
2250 (The difference being that C does not offer named loops combined with
2251 loop control. Perl does, and this replaces most structured uses of C<goto>
2252 in other languages.)
2254 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2255 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2256 necessarily recommended if you're optimizing for maintainability:
2258 goto ("FOO", "BAR", "GLARCH")[$i];
2260 The C<goto-&NAME> form is quite different from the other forms of
2261 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2262 doesn't have the stigma associated with other gotos. Instead, it
2263 exits the current subroutine (losing any changes set by local()) and
2264 immediately calls in its place the named subroutine using the current
2265 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2266 load another subroutine and then pretend that the other subroutine had
2267 been called in the first place (except that any modifications to C<@_>
2268 in the current subroutine are propagated to the other subroutine.)
2269 After the C<goto>, not even C<caller> will be able to tell that this
2270 routine was called first.
2272 NAME needn't be the name of a subroutine; it can be a scalar variable
2273 containing a code reference, or a block that evaluates to a code
2276 =item grep BLOCK LIST
2278 =item grep EXPR,LIST
2280 This is similar in spirit to, but not the same as, grep(1) and its
2281 relatives. In particular, it is not limited to using regular expressions.
2283 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2284 C<$_> to each element) and returns the list value consisting of those
2285 elements for which the expression evaluated to true. In scalar
2286 context, returns the number of times the expression was true.
2288 @foo = grep(!/^#/, @bar); # weed out comments
2292 @foo = grep {!/^#/} @bar; # weed out comments
2294 Note that C<$_> is an alias to the list value, so it can be used to
2295 modify the elements of the LIST. While this is useful and supported,
2296 it can cause bizarre results if the elements of LIST are not variables.
2297 Similarly, grep returns aliases into the original list, much as a for
2298 loop's index variable aliases the list elements. That is, modifying an
2299 element of a list returned by grep (for example, in a C<foreach>, C<map>
2300 or another C<grep>) actually modifies the element in the original list.
2301 This is usually something to be avoided when writing clear code.
2303 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2304 been declared with C<my $_>) then, in addition to being locally aliased to
2305 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2306 can't be seen from the outside, avoiding any potential side-effects.
2308 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2314 Interprets EXPR as a hex string and returns the corresponding value.
2315 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2316 L</oct>.) If EXPR is omitted, uses C<$_>.
2318 print hex '0xAf'; # prints '175'
2319 print hex 'aF'; # same
2321 Hex strings may only represent integers. Strings that would cause
2322 integer overflow trigger a warning. Leading whitespace is not stripped,
2323 unlike oct(). To present something as hex, look into L</printf>,
2324 L</sprintf>, or L</unpack>.
2328 There is no builtin C<import> function. It is just an ordinary
2329 method (subroutine) defined (or inherited) by modules that wish to export
2330 names to another module. The C<use> function calls the C<import> method
2331 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2333 =item index STR,SUBSTR,POSITION
2335 =item index STR,SUBSTR
2337 The index function searches for one string within another, but without
2338 the wildcard-like behavior of a full regular-expression pattern match.
2339 It returns the position of the first occurrence of SUBSTR in STR at
2340 or after POSITION. If POSITION is omitted, starts searching from the
2341 beginning of the string. The return value is based at C<0> (or whatever
2342 you've set the C<$[> variable to--but don't do that). If the substring
2343 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2349 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2350 You should not use this function for rounding: one because it truncates
2351 towards C<0>, and two because machine representations of floating point
2352 numbers can sometimes produce counterintuitive results. For example,
2353 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2354 because it's really more like -268.99999999999994315658 instead. Usually,
2355 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2356 functions will serve you better than will int().
2358 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2360 Implements the ioctl(2) function. You'll probably first have to say
2362 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
2364 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2365 exist or doesn't have the correct definitions you'll have to roll your
2366 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2367 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2368 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2369 written depending on the FUNCTION--a pointer to the string value of SCALAR
2370 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2371 has no string value but does have a numeric value, that value will be
2372 passed rather than a pointer to the string value. To guarantee this to be
2373 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2374 functions may be needed to manipulate the values of structures used by
2377 The return value of C<ioctl> (and C<fcntl>) is as follows:
2379 if OS returns: then Perl returns:
2381 0 string "0 but true"
2382 anything else that number
2384 Thus Perl returns true on success and false on failure, yet you can
2385 still easily determine the actual value returned by the operating
2388 $retval = ioctl(...) || -1;
2389 printf "System returned %d\n", $retval;
2391 The special string C<"0 but true"> is exempt from B<-w> complaints
2392 about improper numeric conversions.
2394 =item join EXPR,LIST
2396 Joins the separate strings of LIST into a single string with fields
2397 separated by the value of EXPR, and returns that new string. Example:
2399 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2401 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2402 first argument. Compare L</split>.
2406 Returns a list consisting of all the keys of the named hash.
2407 (In scalar context, returns the number of keys.)
2409 The keys are returned in an apparently random order. The actual
2410 random order is subject to change in future versions of perl, but it
2411 is guaranteed to be the same order as either the C<values> or C<each>
2412 function produces (given that the hash has not been modified). Since
2413 Perl 5.8.1 the ordering is different even between different runs of
2414 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2417 As a side effect, calling keys() resets the HASH's internal iterator
2418 (see L</each>). In particular, calling keys() in void context resets
2419 the iterator with no other overhead.
2421 Here is yet another way to print your environment:
2424 @values = values %ENV;
2426 print pop(@keys), '=', pop(@values), "\n";
2429 or how about sorted by key:
2431 foreach $key (sort(keys %ENV)) {
2432 print $key, '=', $ENV{$key}, "\n";
2435 The returned values are copies of the original keys in the hash, so
2436 modifying them will not affect the original hash. Compare L</values>.
2438 To sort a hash by value, you'll need to use a C<sort> function.
2439 Here's a descending numeric sort of a hash by its values:
2441 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2442 printf "%4d %s\n", $hash{$key}, $key;
2445 As an lvalue C<keys> allows you to increase the number of hash buckets
2446 allocated for the given hash. This can gain you a measure of efficiency if
2447 you know the hash is going to get big. (This is similar to pre-extending
2448 an array by assigning a larger number to $#array.) If you say
2452 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2453 in fact, since it rounds up to the next power of two. These
2454 buckets will be retained even if you do C<%hash = ()>, use C<undef
2455 %hash> if you want to free the storage while C<%hash> is still in scope.
2456 You can't shrink the number of buckets allocated for the hash using
2457 C<keys> in this way (but you needn't worry about doing this by accident,
2458 as trying has no effect).
2460 See also C<each>, C<values> and C<sort>.
2462 =item kill SIGNAL, LIST
2464 Sends a signal to a list of processes. Returns the number of
2465 processes successfully signaled (which is not necessarily the
2466 same as the number actually killed).
2468 $cnt = kill 1, $child1, $child2;
2471 If SIGNAL is zero, no signal is sent to the process. This is a
2472 useful way to check that a child process is alive and hasn't changed
2473 its UID. See L<perlport> for notes on the portability of this
2476 Unlike in the shell, if SIGNAL is negative, it kills
2477 process groups instead of processes. (On System V, a negative I<PROCESS>
2478 number will also kill process groups, but that's not portable.) That
2479 means you usually want to use positive not negative signals. You may also
2480 use a signal name in quotes.
2482 See L<perlipc/"Signals"> for more details.
2488 The C<last> command is like the C<break> statement in C (as used in
2489 loops); it immediately exits the loop in question. If the LABEL is
2490 omitted, the command refers to the innermost enclosing loop. The
2491 C<continue> block, if any, is not executed:
2493 LINE: while (<STDIN>) {
2494 last LINE if /^$/; # exit when done with header
2498 C<last> cannot be used to exit a block which returns a value such as
2499 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2500 a grep() or map() operation.
2502 Note that a block by itself is semantically identical to a loop
2503 that executes once. Thus C<last> can be used to effect an early
2504 exit out of such a block.
2506 See also L</continue> for an illustration of how C<last>, C<next>, and
2513 Returns a lowercased version of EXPR. This is the internal function
2514 implementing the C<\L> escape in double-quoted strings. Respects
2515 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2516 and L<perlunicode> for more details about locale and Unicode support.
2518 If EXPR is omitted, uses C<$_>.
2524 Returns the value of EXPR with the first character lowercased. This
2525 is the internal function implementing the C<\l> escape in
2526 double-quoted strings. Respects current LC_CTYPE locale if C<use
2527 locale> in force. See L<perllocale> and L<perlunicode> for more
2528 details about locale and Unicode support.
2530 If EXPR is omitted, uses C<$_>.
2536 Returns the length in I<characters> of the value of EXPR. If EXPR is
2537 omitted, returns length of C<$_>. Note that this cannot be used on
2538 an entire array or hash to find out how many elements these have.
2539 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2541 Note the I<characters>: if the EXPR is in Unicode, you will get the
2542 number of characters, not the number of bytes. To get the length
2543 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2545 =item link OLDFILE,NEWFILE
2547 Creates a new filename linked to the old filename. Returns true for
2548 success, false otherwise.
2550 =item listen SOCKET,QUEUESIZE
2552 Does the same thing that the listen system call does. Returns true if
2553 it succeeded, false otherwise. See the example in
2554 L<perlipc/"Sockets: Client/Server Communication">.
2558 You really probably want to be using C<my> instead, because C<local> isn't
2559 what most people think of as "local". See
2560 L<perlsub/"Private Variables via my()"> for details.
2562 A local modifies the listed variables to be local to the enclosing
2563 block, file, or eval. If more than one value is listed, the list must
2564 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2565 for details, including issues with tied arrays and hashes.
2567 =item localtime EXPR
2571 Converts a time as returned by the time function to a 9-element list
2572 with the time analyzed for the local time zone. Typically used as
2576 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2579 All list elements are numeric, and come straight out of the C `struct
2580 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2581 of the specified time.
2583 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2584 the range C<0..11> with 0 indicating January and 11 indicating December.
2585 This makes it easy to get a month name from a list:
2587 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2588 print "$abbr[$mon] $mday";
2589 # $mon=9, $mday=18 gives "Oct 18"
2591 C<$year> is the number of years since 1900, not just the last two digits
2592 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2593 to get a complete 4-digit year is simply:
2597 To get the last two digits of the year (e.g., '01' in 2001) do:
2599 $year = sprintf("%02d", $year % 100);
2601 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2602 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2603 (or C<0..365> in leap years.)
2605 C<$isdst> is true if the specified time occurs during Daylight Saving
2606 Time, false otherwise.
2608 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2610 In scalar context, C<localtime()> returns the ctime(3) value:
2612 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2614 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2615 instead of local time use the L</gmtime> builtin. See also the
2616 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2617 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2618 and mktime(3) functions.
2620 To get somewhat similar but locale dependent date strings, set up your
2621 locale environment variables appropriately (please see L<perllocale>) and
2624 use POSIX qw(strftime);
2625 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2626 # or for GMT formatted appropriately for your locale:
2627 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2629 Note that the C<%a> and C<%b>, the short forms of the day of the week
2630 and the month of the year, may not necessarily be three characters wide.
2632 See L<perlport/localtime> for portability concerns.
2636 This function places an advisory lock on a shared variable, or referenced
2637 object contained in I<THING> until the lock goes out of scope.
2639 lock() is a "weak keyword" : this means that if you've defined a function
2640 by this name (before any calls to it), that function will be called
2641 instead. (However, if you've said C<use threads>, lock() is always a
2642 keyword.) See L<threads>.
2648 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2649 returns log of C<$_>. To get the log of another base, use basic algebra:
2650 The base-N log of a number is equal to the natural log of that number
2651 divided by the natural log of N. For example:
2655 return log($n)/log(10);
2658 See also L</exp> for the inverse operation.
2664 Does the same thing as the C<stat> function (including setting the
2665 special C<_> filehandle) but stats a symbolic link instead of the file
2666 the symbolic link points to. If symbolic links are unimplemented on
2667 your system, a normal C<stat> is done. For much more detailed
2668 information, please see the documentation for C<stat>.
2670 If EXPR is omitted, stats C<$_>.
2674 The match operator. See L<perlop>.
2676 =item map BLOCK LIST
2680 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2681 C<$_> to each element) and returns the list value composed of the
2682 results of each such evaluation. In scalar context, returns the
2683 total number of elements so generated. Evaluates BLOCK or EXPR in
2684 list context, so each element of LIST may produce zero, one, or
2685 more elements in the returned value.
2687 @chars = map(chr, @nums);
2689 translates a list of numbers to the corresponding characters. And
2691 %hash = map { getkey($_) => $_ } @array;
2693 is just a funny way to write
2696 foreach $_ (@array) {
2697 $hash{getkey($_)} = $_;
2700 Note that C<$_> is an alias to the list value, so it can be used to
2701 modify the elements of the LIST. While this is useful and supported,
2702 it can cause bizarre results if the elements of LIST are not variables.
2703 Using a regular C<foreach> loop for this purpose would be clearer in
2704 most cases. See also L</grep> for an array composed of those items of
2705 the original list for which the BLOCK or EXPR evaluates to true.
2707 If C<$_> is lexical in the scope where the C<map> appears (because it has
2708 been declared with C<my $_>) then, in addition to being locally aliased to
2709 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2710 can't be seen from the outside, avoiding any potential side-effects.
2712 C<{> starts both hash references and blocks, so C<map { ...> could be either
2713 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2714 ahead for the closing C<}> it has to take a guess at which its dealing with
2715 based what it finds just after the C<{>. Usually it gets it right, but if it
2716 doesn't it won't realize something is wrong until it gets to the C<}> and
2717 encounters the missing (or unexpected) comma. The syntax error will be
2718 reported close to the C<}> but you'll need to change something near the C<{>
2719 such as using a unary C<+> to give perl some help:
2721 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2722 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2723 %hash = map { ("\L$_", 1) } @array # this also works
2724 %hash = map { lc($_), 1 } @array # as does this.
2725 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2727 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2729 or to force an anon hash constructor use C<+{>
2731 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2733 and you get list of anonymous hashes each with only 1 entry.
2735 =item mkdir FILENAME,MASK
2737 =item mkdir FILENAME
2741 Creates the directory specified by FILENAME, with permissions
2742 specified by MASK (as modified by C<umask>). If it succeeds it
2743 returns true, otherwise it returns false and sets C<$!> (errno).
2744 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2747 In general, it is better to create directories with permissive MASK,
2748 and let the user modify that with their C<umask>, than it is to supply
2749 a restrictive MASK and give the user no way to be more permissive.
2750 The exceptions to this rule are when the file or directory should be
2751 kept private (mail files, for instance). The perlfunc(1) entry on
2752 C<umask> discusses the choice of MASK in more detail.
2754 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2755 number of trailing slashes. Some operating and filesystems do not get
2756 this right, so Perl automatically removes all trailing slashes to keep
2759 =item msgctl ID,CMD,ARG
2761 Calls the System V IPC function msgctl(2). You'll probably have to say
2765 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2766 then ARG must be a variable that will hold the returned C<msqid_ds>
2767 structure. Returns like C<ioctl>: the undefined value for error,
2768 C<"0 but true"> for zero, or the actual return value otherwise. See also
2769 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2771 =item msgget KEY,FLAGS
2773 Calls the System V IPC function msgget(2). Returns the message queue
2774 id, or the undefined value if there is an error. See also
2775 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2777 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2779 Calls the System V IPC function msgrcv to receive a message from
2780 message queue ID into variable VAR with a maximum message size of
2781 SIZE. Note that when a message is received, the message type as a
2782 native long integer will be the first thing in VAR, followed by the
2783 actual message. This packing may be opened with C<unpack("l! a*")>.
2784 Taints the variable. Returns true if successful, or false if there is
2785 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2786 C<IPC::SysV::Msg> documentation.
2788 =item msgsnd ID,MSG,FLAGS
2790 Calls the System V IPC function msgsnd to send the message MSG to the
2791 message queue ID. MSG must begin with the native long integer message
2792 type, and be followed by the length of the actual message, and finally
2793 the message itself. This kind of packing can be achieved with
2794 C<pack("l! a*", $type, $message)>. Returns true if successful,
2795 or false if there is an error. See also C<IPC::SysV>
2796 and C<IPC::SysV::Msg> documentation.
2802 =item my EXPR : ATTRS
2804 =item my TYPE EXPR : ATTRS
2806 A C<my> declares the listed variables to be local (lexically) to the
2807 enclosing block, file, or C<eval>. If more than one value is listed,
2808 the list must be placed in parentheses.
2810 The exact semantics and interface of TYPE and ATTRS are still
2811 evolving. TYPE is currently bound to the use of C<fields> pragma,
2812 and attributes are handled using the C<attributes> pragma, or starting
2813 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2814 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2815 L<attributes>, and L<Attribute::Handlers>.
2821 The C<next> command is like the C<continue> statement in C; it starts
2822 the next iteration of the loop:
2824 LINE: while (<STDIN>) {
2825 next LINE if /^#/; # discard comments
2829 Note that if there were a C<continue> block on the above, it would get
2830 executed even on discarded lines. If the LABEL is omitted, the command
2831 refers to the innermost enclosing loop.
2833 C<next> cannot be used to exit a block which returns a value such as
2834 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2835 a grep() or map() operation.
2837 Note that a block by itself is semantically identical to a loop
2838 that executes once. Thus C<next> will exit such a block early.
2840 See also L</continue> for an illustration of how C<last>, C<next>, and
2843 =item no Module VERSION LIST
2845 =item no Module VERSION
2847 =item no Module LIST
2851 See the C<use> function, of which C<no> is the opposite.
2857 Interprets EXPR as an octal string and returns the corresponding
2858 value. (If EXPR happens to start off with C<0x>, interprets it as a
2859 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2860 binary string. Leading whitespace is ignored in all three cases.)
2861 The following will handle decimal, binary, octal, and hex in the standard
2864 $val = oct($val) if $val =~ /^0/;
2866 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2867 in octal), use sprintf() or printf():
2869 $perms = (stat("filename"))[2] & 07777;
2870 $oct_perms = sprintf "%lo", $perms;
2872 The oct() function is commonly used when a string such as C<644> needs
2873 to be converted into a file mode, for example. (Although perl will
2874 automatically convert strings into numbers as needed, this automatic
2875 conversion assumes base 10.)
2877 =item open FILEHANDLE,EXPR
2879 =item open FILEHANDLE,MODE,EXPR
2881 =item open FILEHANDLE,MODE,EXPR,LIST
2883 =item open FILEHANDLE,MODE,REFERENCE
2885 =item open FILEHANDLE
2887 Opens the file whose filename is given by EXPR, and associates it with
2890 (The following is a comprehensive reference to open(): for a gentler
2891 introduction you may consider L<perlopentut>.)
2893 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2894 the variable is assigned a reference to a new anonymous filehandle,
2895 otherwise if FILEHANDLE is an expression, its value is used as the name of
2896 the real filehandle wanted. (This is considered a symbolic reference, so
2897 C<use strict 'refs'> should I<not> be in effect.)
2899 If EXPR is omitted, the scalar variable of the same name as the
2900 FILEHANDLE contains the filename. (Note that lexical variables--those
2901 declared with C<my>--will not work for this purpose; so if you're
2902 using C<my>, specify EXPR in your call to open.)
2904 If three or more arguments are specified then the mode of opening and
2905 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2906 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2907 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2908 the file is opened for appending, again being created if necessary.
2910 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2911 indicate that you want both read and write access to the file; thus
2912 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2913 '+>' >> mode would clobber the file first. You can't usually use
2914 either read-write mode for updating textfiles, since they have
2915 variable length records. See the B<-i> switch in L<perlrun> for a
2916 better approach. The file is created with permissions of C<0666>
2917 modified by the process' C<umask> value.
2919 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2920 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2922 In the 2-arguments (and 1-argument) form of the call the mode and
2923 filename should be concatenated (in this order), possibly separated by
2924 spaces. It is possible to omit the mode in these forms if the mode is
2927 If the filename begins with C<'|'>, the filename is interpreted as a
2928 command to which output is to be piped, and if the filename ends with a
2929 C<'|'>, the filename is interpreted as a command which pipes output to
2930 us. See L<perlipc/"Using open() for IPC">
2931 for more examples of this. (You are not allowed to C<open> to a command
2932 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2933 and L<perlipc/"Bidirectional Communication with Another Process">
2936 For three or more arguments if MODE is C<'|-'>, the filename is
2937 interpreted as a command to which output is to be piped, and if MODE
2938 is C<'-|'>, the filename is interpreted as a command which pipes
2939 output to us. In the 2-arguments (and 1-argument) form one should
2940 replace dash (C<'-'>) with the command.
2941 See L<perlipc/"Using open() for IPC"> for more examples of this.
2942 (You are not allowed to C<open> to a command that pipes both in I<and>
2943 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2944 L<perlipc/"Bidirectional Communication"> for alternatives.)
2946 In the three-or-more argument form of pipe opens, if LIST is specified
2947 (extra arguments after the command name) then LIST becomes arguments
2948 to the command invoked if the platform supports it. The meaning of
2949 C<open> with more than three arguments for non-pipe modes is not yet
2950 specified. Experimental "layers" may give extra LIST arguments
2953 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2954 and opening C<< '>-' >> opens STDOUT.
2956 You may use the three-argument form of open to specify IO "layers"
2957 (sometimes also referred to as "disciplines") to be applied to the handle
2958 that affect how the input and output are processed (see L<open> and
2959 L<PerlIO> for more details). For example
2961 open(FH, "<:utf8", "file")
2963 will open the UTF-8 encoded file containing Unicode characters,
2964 see L<perluniintro>. (Note that if layers are specified in the
2965 three-arg form then default layers set by the C<open> pragma are
2968 Open returns nonzero upon success, the undefined value otherwise. If
2969 the C<open> involved a pipe, the return value happens to be the pid of
2972 If you're running Perl on a system that distinguishes between text
2973 files and binary files, then you should check out L</binmode> for tips
2974 for dealing with this. The key distinction between systems that need
2975 C<binmode> and those that don't is their text file formats. Systems
2976 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2977 character, and which encode that character in C as C<"\n">, do not
2978 need C<binmode>. The rest need it.
2980 When opening a file, it's usually a bad idea to continue normal execution
2981 if the request failed, so C<open> is frequently used in connection with
2982 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2983 where you want to make a nicely formatted error message (but there are
2984 modules that can help with that problem)) you should always check
2985 the return value from opening a file. The infrequent exception is when
2986 working with an unopened filehandle is actually what you want to do.
2988 As a special case the 3-arg form with a read/write mode and the third
2989 argument being C<undef>:
2991 open(TMP, "+>", undef) or die ...
2993 opens a filehandle to an anonymous temporary file. Also using "+<"
2994 works for symmetry, but you really should consider writing something
2995 to the temporary file first. You will need to seek() to do the
2998 Since v5.8.0, perl has built using PerlIO by default. Unless you've
2999 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3000 "in memory" files held in Perl scalars via:
3002 open($fh, '>', \$variable) || ..
3004 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3005 file, you have to close it first:
3008 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3013 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3014 while (<ARTICLE>) {...
3016 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3017 # if the open fails, output is discarded
3019 open(DBASE, '+<', 'dbase.mine') # open for update
3020 or die "Can't open 'dbase.mine' for update: $!";
3022 open(DBASE, '+<dbase.mine') # ditto
3023 or die "Can't open 'dbase.mine' for update: $!";
3025 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3026 or die "Can't start caesar: $!";
3028 open(ARTICLE, "caesar <$article |") # ditto
3029 or die "Can't start caesar: $!";
3031 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3032 or die "Can't start sort: $!";
3035 open(MEMORY,'>', \$var)
3036 or die "Can't open memory file: $!";
3037 print MEMORY "foo!\n"; # output will end up in $var
3039 # process argument list of files along with any includes
3041 foreach $file (@ARGV) {
3042 process($file, 'fh00');
3046 my($filename, $input) = @_;
3047 $input++; # this is a string increment
3048 unless (open($input, $filename)) {
3049 print STDERR "Can't open $filename: $!\n";
3054 while (<$input>) { # note use of indirection
3055 if (/^#include "(.*)"/) {
3056 process($1, $input);
3063 See L<perliol> for detailed info on PerlIO.
3065 You may also, in the Bourne shell tradition, specify an EXPR beginning
3066 with C<< '>&' >>, in which case the rest of the string is interpreted
3067 as the name of a filehandle (or file descriptor, if numeric) to be
3068 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3069 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3070 The mode you specify should match the mode of the original filehandle.
3071 (Duping a filehandle does not take into account any existing contents
3072 of IO buffers.) If you use the 3-arg form then you can pass either a
3073 number, the name of a filehandle or the normal "reference to a glob".
3075 Here is a script that saves, redirects, and restores C<STDOUT> and
3076 C<STDERR> using various methods:
3079 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3080 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3082 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3083 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3085 select STDERR; $| = 1; # make unbuffered
3086 select STDOUT; $| = 1; # make unbuffered
3088 print STDOUT "stdout 1\n"; # this works for
3089 print STDERR "stderr 1\n"; # subprocesses too
3091 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3092 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3094 print STDOUT "stdout 2\n";
3095 print STDERR "stderr 2\n";
3097 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3098 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3099 that file descriptor (and not call L<dup(2)>); this is more
3100 parsimonious of file descriptors. For example:
3102 # open for input, reusing the fileno of $fd
3103 open(FILEHANDLE, "<&=$fd")
3107 open(FILEHANDLE, "<&=", $fd)
3111 # open for append, using the fileno of OLDFH
3112 open(FH, ">>&=", OLDFH)
3116 open(FH, ">>&=OLDFH")
3118 Being parsimonious on filehandles is also useful (besides being
3119 parsimonious) for example when something is dependent on file
3120 descriptors, like for example locking using flock(). If you do just
3121 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3122 descriptor as B, and therefore flock(A) will not flock(B), and vice
3123 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3124 the same file descriptor.
3126 Note that if you are using Perls older than 5.8.0, Perl will be using
3127 the standard C libraries' fdopen() to implement the "=" functionality.
3128 On many UNIX systems fdopen() fails when file descriptors exceed a
3129 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3130 most often the default.
3132 You can see whether Perl has been compiled with PerlIO or not by
3133 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3134 is C<define>, you have PerlIO, otherwise you don't.
3136 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3137 with 2-arguments (or 1-argument) form of open(), then
3138 there is an implicit fork done, and the return value of open is the pid
3139 of the child within the parent process, and C<0> within the child
3140 process. (Use C<defined($pid)> to determine whether the open was successful.)
3141 The filehandle behaves normally for the parent, but i/o to that
3142 filehandle is piped from/to the STDOUT/STDIN of the child process.
3143 In the child process the filehandle isn't opened--i/o happens from/to
3144 the new STDOUT or STDIN. Typically this is used like the normal
3145 piped open when you want to exercise more control over just how the
3146 pipe command gets executed, such as when you are running setuid, and
3147 don't want to have to scan shell commands for metacharacters.
3148 The following triples are more or less equivalent:
3150 open(FOO, "|tr '[a-z]' '[A-Z]'");
3151 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3152 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3153 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3155 open(FOO, "cat -n '$file'|");
3156 open(FOO, '-|', "cat -n '$file'");
3157 open(FOO, '-|') || exec 'cat', '-n', $file;
3158 open(FOO, '-|', "cat", '-n', $file);
3160 The last example in each block shows the pipe as "list form", which is
3161 not yet supported on all platforms. A good rule of thumb is that if
3162 your platform has true C<fork()> (in other words, if your platform is
3163 UNIX) you can use the list form.
3165 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3167 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3168 output before any operation that may do a fork, but this may not be
3169 supported on some platforms (see L<perlport>). To be safe, you may need
3170 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3171 of C<IO::Handle> on any open handles.
3173 On systems that support a close-on-exec flag on files, the flag will
3174 be set for the newly opened file descriptor as determined by the value
3175 of $^F. See L<perlvar/$^F>.
3177 Closing any piped filehandle causes the parent process to wait for the
3178 child to finish, and returns the status value in C<$?> and
3179 C<${^CHILD_ERROR_NATIVE}>.
3181 The filename passed to 2-argument (or 1-argument) form of open() will
3182 have leading and trailing whitespace deleted, and the normal
3183 redirection characters honored. This property, known as "magic open",
3184 can often be used to good effect. A user could specify a filename of
3185 F<"rsh cat file |">, or you could change certain filenames as needed:
3187 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3188 open(FH, $filename) or die "Can't open $filename: $!";
3190 Use 3-argument form to open a file with arbitrary weird characters in it,
3192 open(FOO, '<', $file);
3194 otherwise it's necessary to protect any leading and trailing whitespace:
3196 $file =~ s#^(\s)#./$1#;
3197 open(FOO, "< $file\0");
3199 (this may not work on some bizarre filesystems). One should
3200 conscientiously choose between the I<magic> and 3-arguments form
3205 will allow the user to specify an argument of the form C<"rsh cat file |">,
3206 but will not work on a filename which happens to have a trailing space, while
3208 open IN, '<', $ARGV[0];
3210 will have exactly the opposite restrictions.
3212 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3213 should use the C<sysopen> function, which involves no such magic (but
3214 may use subtly different filemodes than Perl open(), which is mapped
3215 to C fopen()). This is
3216 another way to protect your filenames from interpretation. For example:
3219 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3220 or die "sysopen $path: $!";
3221 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3222 print HANDLE "stuff $$\n";
3224 print "File contains: ", <HANDLE>;
3226 Using the constructor from the C<IO::Handle> package (or one of its
3227 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3228 filehandles that have the scope of whatever variables hold references to
3229 them, and automatically close whenever and however you leave that scope:
3233 sub read_myfile_munged {
3235 my $handle = new IO::File;
3236 open($handle, "myfile") or die "myfile: $!";
3238 or return (); # Automatically closed here.
3239 mung $first or die "mung failed"; # Or here.
3240 return $first, <$handle> if $ALL; # Or here.
3244 See L</seek> for some details about mixing reading and writing.
3246 =item opendir DIRHANDLE,EXPR
3248 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3249 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3250 DIRHANDLE may be an expression whose value can be used as an indirect
3251 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3252 scalar variable (or array or hash element), the variable is assigned a
3253 reference to a new anonymous dirhandle.
3254 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3260 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3261 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3264 For the reverse, see L</chr>.
3265 See L<perlunicode> and L<encoding> for more about Unicode.
3271 =item our EXPR : ATTRS
3273 =item our TYPE EXPR : ATTRS
3275 C<our> associates a simple name with a package variable in the current
3276 package for use within the current scope. When C<use strict 'vars'> is in
3277 effect, C<our> lets you use declared global variables without qualifying
3278 them with package names, within the lexical scope of the C<our> declaration.
3279 In this way C<our> differs from C<use vars>, which is package scoped.
3281 Unlike C<my>, which both allocates storage for a variable and associates
3282 a simple name with that storage for use within the current scope, C<our>
3283 associates a simple name with a package variable in the current package,
3284 for use within the current scope. In other words, C<our> has the same
3285 scoping rules as C<my>, but does not necessarily create a
3288 If more than one value is listed, the list must be placed
3294 An C<our> declaration declares a global variable that will be visible
3295 across its entire lexical scope, even across package boundaries. The
3296 package in which the variable is entered is determined at the point
3297 of the declaration, not at the point of use. This means the following
3301 our $bar; # declares $Foo::bar for rest of lexical scope
3305 print $bar; # prints 20, as it refers to $Foo::bar
3307 Multiple C<our> declarations with the same name in the same lexical
3308 scope are allowed if they are in different packages. If they happen
3309 to be in the same package, Perl will emit warnings if you have asked
3310 for them, just like multiple C<my> declarations. Unlike a second
3311 C<my> declaration, which will bind the name to a fresh variable, a
3312 second C<our> declaration in the same package, in the same scope, is
3317 our $bar; # declares $Foo::bar for rest of lexical scope
3321 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3322 print $bar; # prints 30
3324 our $bar; # emits warning but has no other effect
3325 print $bar; # still prints 30
3327 An C<our> declaration may also have a list of attributes associated
3330 The exact semantics and interface of TYPE and ATTRS are still
3331 evolving. TYPE is currently bound to the use of C<fields> pragma,
3332 and attributes are handled using the C<attributes> pragma, or starting
3333 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3334 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3335 L<attributes>, and L<Attribute::Handlers>.
3337 The only currently recognized C<our()> attribute is C<unique> which
3338 indicates that a single copy of the global is to be used by all
3339 interpreters should the program happen to be running in a
3340 multi-interpreter environment. (The default behaviour would be for
3341 each interpreter to have its own copy of the global.) Examples:
3343 our @EXPORT : unique = qw(foo);
3344 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3345 our $VERSION : unique = "1.00";
3347 Note that this attribute also has the effect of making the global
3348 readonly when the first new interpreter is cloned (for example,
3349 when the first new thread is created).
3351 Multi-interpreter environments can come to being either through the
3352 fork() emulation on Windows platforms, or by embedding perl in a
3353 multi-threaded application. The C<unique> attribute does nothing in
3354 all other environments.
3356 Warning: the current implementation of this attribute operates on the
3357 typeglob associated with the variable; this means that C<our $x : unique>
3358 also has the effect of C<our @x : unique; our %x : unique>. This may be
3361 =item pack TEMPLATE,LIST
3363 Takes a LIST of values and converts it into a string using the rules
3364 given by the TEMPLATE. The resulting string is the concatenation of
3365 the converted values. Typically, each converted value looks
3366 like its machine-level representation. For example, on 32-bit machines
3367 an integer may be represented by a sequence of 4 bytes that will be
3368 converted to a sequence of 4 characters.
3370 The TEMPLATE is a sequence of characters that give the order and type
3371 of values, as follows:
3373 a A string with arbitrary binary data, will be null padded.
3374 A A text (ASCII) string, will be space padded.
3375 Z A null terminated (ASCIZ) string, will be null padded.
3377 b A bit string (ascending bit order inside each byte, like vec()).
3378 B A bit string (descending bit order inside each byte).
3379 h A hex string (low nybble first).
3380 H A hex string (high nybble first).
3382 c A signed char (8-bit) value.
3383 C An unsigned C char (octet) even under Unicode. Should normally not
3384 be used. See U and W instead.
3385 W An unsigned char value (can be greater than 255).
3387 s A signed short (16-bit) value.
3388 S An unsigned short value.
3390 l A signed long (32-bit) value.
3391 L An unsigned long value.
3393 q A signed quad (64-bit) value.
3394 Q An unsigned quad value.
3395 (Quads are available only if your system supports 64-bit
3396 integer values _and_ if Perl has been compiled to support those.
3397 Causes a fatal error otherwise.)
3399 i A signed integer value.
3400 I A unsigned integer value.
3401 (This 'integer' is _at_least_ 32 bits wide. Its exact
3402 size depends on what a local C compiler calls 'int'.)
3404 n An unsigned short (16-bit) in "network" (big-endian) order.
3405 N An unsigned long (32-bit) in "network" (big-endian) order.
3406 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3407 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3409 j A Perl internal signed integer value (IV).
3410 J A Perl internal unsigned integer value (UV).
3412 f A single-precision float in the native format.
3413 d A double-precision float in the native format.
3415 F A Perl internal floating point value (NV) in the native format
3416 D A long double-precision float in the native format.
3417 (Long doubles are available only if your system supports long
3418 double values _and_ if Perl has been compiled to support those.
3419 Causes a fatal error otherwise.)
3421 p A pointer to a null-terminated string.
3422 P A pointer to a structure (fixed-length string).
3424 u A uuencoded string.
3425 U A Unicode character number. Encodes to UTF-8 internally
3426 (or UTF-EBCDIC in EBCDIC platforms).
3428 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3429 details). Its bytes represent an unsigned integer in base 128,
3430 most significant digit first, with as few digits as possible. Bit
3431 eight (the high bit) is set on each byte except the last.
3435 @ Null fill or truncate to absolute position, counted from the
3436 start of the innermost ()-group.
3437 . Null fill or truncate to absolute position specified by value.
3438 ( Start of a ()-group.
3440 One or more of the modifiers below may optionally follow some letters in the
3441 TEMPLATE (the second column lists the letters for which the modifier is
3444 ! sSlLiI Forces native (short, long, int) sizes instead
3445 of fixed (16-/32-bit) sizes.
3447 xX Make x and X act as alignment commands.
3449 nNvV Treat integers as signed instead of unsigned.
3451 @. Specify position as byte offset in the internal
3452 representation of the packed string. Efficient but
3455 > sSiIlLqQ Force big-endian byte-order on the type.
3456 jJfFdDpP (The "big end" touches the construct.)
3458 < sSiIlLqQ Force little-endian byte-order on the type.
3459 jJfFdDpP (The "little end" touches the construct.)
3461 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3462 in which case they force a certain byte-order on all components of
3463 that group, including subgroups.
3465 The following rules apply:
3471 Each letter may optionally be followed by a number giving a repeat
3472 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3473 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3474 that many values from the LIST. A C<*> for the repeat count means to
3475 use however many items are left, except for C<@>, C<x>, C<X>, where it
3476 is equivalent to C<0>, for <.> where it means relative to string start
3477 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3478 A numeric repeat count may optionally be enclosed in brackets, as in
3479 C<pack 'C[80]', @arr>.
3481 One can replace the numeric repeat count by a template enclosed in brackets;
3482 then the packed length of this template in bytes is used as a count.
3483 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3484 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3485 If the template in brackets contains alignment commands (such as C<x![d]>),
3486 its packed length is calculated as if the start of the template has the maximal
3489 When used with C<Z>, C<*> results in the addition of a trailing null
3490 byte (so the packed result will be one longer than the byte C<length>
3493 When used with C<@>, the repeat count represents an offset from the start
3494 of the innermost () group.
3496 When used with C<.>, the repeat count is used to determine the starting
3497 position from where the value offset is calculated. If the repeat count
3498 is 0, it's relative to the current position. If the repeat count is C<*>,
3499 the offset is relative to the start of the packed string. And if its an
3500 integer C<n> the offset is relative to the start of the n-th innermost
3501 () group (or the start of the string if C<n> is bigger then the group
3504 The repeat count for C<u> is interpreted as the maximal number of bytes
3505 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3506 count should not be more than 65.
3510 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3511 string of length count, padding with nulls or spaces as necessary. When
3512 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3513 after the first null, and C<a> returns data verbatim.
3515 If the value-to-pack is too long, it is truncated. If too long and an
3516 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3517 by a null byte. Thus C<Z> always packs a trailing null (except when the
3522 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3523 Each character of the input field of pack() generates 1 bit of the result.
3524 Each result bit is based on the least-significant bit of the corresponding
3525 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3526 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3528 Starting from the beginning of the input string of pack(), each 8-tuple
3529 of characters is converted to 1 character of output. With format C<b>
3530 the first character of the 8-tuple determines the least-significant bit of a
3531 character, and with format C<B> it determines the most-significant bit of
3534 If the length of the input string is not exactly divisible by 8, the
3535 remainder is packed as if the input string were padded by null characters
3536 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3538 If the input string of pack() is longer than needed, extra characters are
3539 ignored. A C<*> for the repeat count of pack() means to use all the
3540 characters of the input field. On unpack()ing the bits are converted to a
3541 string of C<"0">s and C<"1">s.
3545 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3546 representable as hexadecimal digits, 0-9a-f) long.
3548 Each character of the input field of pack() generates 4 bits of the result.
3549 For non-alphabetical characters the result is based on the 4 least-significant
3550 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3551 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3552 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3553 is compatible with the usual hexadecimal digits, so that C<"a"> and
3554 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3555 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3557 Starting from the beginning of the input string of pack(), each pair
3558 of characters is converted to 1 character of output. With format C<h> the
3559 first character of the pair determines the least-significant nybble of the
3560 output character, and with format C<H> it determines the most-significant
3563 If the length of the input string is not even, it behaves as if padded
3564 by a null character at the end. Similarly, during unpack()ing the "extra"
3565 nybbles are ignored.
3567 If the input string of pack() is longer than needed, extra characters are
3569 A C<*> for the repeat count of pack() means to use all the characters of
3570 the input field. On unpack()ing the nybbles are converted to a string
3571 of hexadecimal digits.
3575 The C<p> type packs a pointer to a null-terminated string. You are
3576 responsible for ensuring the string is not a temporary value (which can
3577 potentially get deallocated before you get around to using the packed result).
3578 The C<P> type packs a pointer to a structure of the size indicated by the
3579 length. A NULL pointer is created if the corresponding value for C<p> or
3580 C<P> is C<undef>, similarly for unpack().
3582 If your system has a strange pointer size (i.e. a pointer is neither as
3583 big as an int nor as big as a long), it may not be possible to pack or
3584 unpack pointers in big- or little-endian byte order. Attempting to do
3585 so will result in a fatal error.
3589 The C</> template character allows packing and unpacking of a sequence of
3590 items where the packed structure contains a packed item count followed by
3591 the packed items themselves.
3592 You write I<length-item>C</>I<sequence-item>.
3594 The I<length-item> can be any C<pack> template letter, and describes
3595 how the length value is packed. The ones likely to be of most use are
3596 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3597 SNMP) and C<N> (for Sun XDR).
3599 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3600 the minimum of that and the number of available items is used as argument
3601 for the I<length-item>. If it has no repeat count or uses a '*', the number
3602 of available items is used. For C<unpack> the repeat count is always obtained
3603 by decoding the packed item count, and the I<sequence-item> must not have a
3606 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3607 the I<length-item> is a string length, not a number of strings. If there is
3608 an explicit repeat count for pack, the packed string will be adjusted to that
3611 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3612 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3613 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3614 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3616 The I<length-item> is not returned explicitly from C<unpack>.
3618 Adding a count to the I<length-item> letter is unlikely to do anything
3619 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3620 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3621 which Perl does not regard as legal in numeric strings.
3625 The integer types C<s>, C<S>, C<l>, and C<L> may be
3626 followed by a C<!> modifier to signify native shorts or
3627 longs--as you can see from above for example a bare C<l> does mean
3628 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3629 may be larger. This is an issue mainly in 64-bit platforms. You can
3630 see whether using C<!> makes any difference by
3632 print length(pack("s")), " ", length(pack("s!")), "\n";
3633 print length(pack("l")), " ", length(pack("l!")), "\n";
3635 C<i!> and C<I!> also work but only because of completeness;
3636 they are identical to C<i> and C<I>.
3638 The actual sizes (in bytes) of native shorts, ints, longs, and long
3639 longs on the platform where Perl was built are also available via
3643 print $Config{shortsize}, "\n";
3644 print $Config{intsize}, "\n";
3645 print $Config{longsize}, "\n";
3646 print $Config{longlongsize}, "\n";
3648 (The C<$Config{longlongsize}> will be undefined if your system does
3649 not support long longs.)
3653 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3654 are inherently non-portable between processors and operating systems
3655 because they obey the native byteorder and endianness. For example a
3656 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3657 (arranged in and handled by the CPU registers) into bytes as
3659 0x12 0x34 0x56 0x78 # big-endian
3660 0x78 0x56 0x34 0x12 # little-endian
3662 Basically, the Intel and VAX CPUs are little-endian, while everybody
3663 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3664 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3665 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3668 The names `big-endian' and `little-endian' are comic references to
3669 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3670 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3671 the egg-eating habits of the Lilliputians.
3673 Some systems may have even weirder byte orders such as
3678 You can see your system's preference with
3680 print join(" ", map { sprintf "%#02x", $_ }
3681 unpack("W*",pack("L",0x12345678))), "\n";
3683 The byteorder on the platform where Perl was built is also available
3687 print $Config{byteorder}, "\n";
3689 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3690 and C<'87654321'> are big-endian.
3692 If you want portable packed integers you can either use the formats
3693 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3694 modifiers. These modifiers are only available as of perl 5.9.2.
3695 See also L<perlport>.
3699 All integer and floating point formats as well as C<p> and C<P> and
3700 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3701 to force big- or little- endian byte-order, respectively.
3702 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3703 signed integers, 64-bit integers and floating point values. However,
3704 there are some things to keep in mind.
3706 Exchanging signed integers between different platforms only works
3707 if all platforms store them in the same format. Most platforms store
3708 signed integers in two's complement, so usually this is not an issue.
3710 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3711 formats on big- or little-endian machines. Otherwise, attempting to
3712 do so will result in a fatal error.
3714 Forcing big- or little-endian byte-order on floating point values for
3715 data exchange can only work if all platforms are using the same
3716 binary representation (e.g. IEEE floating point format). Even if all
3717 platforms are using IEEE, there may be subtle differences. Being able
3718 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3719 but also very dangerous if you don't know exactly what you're doing.
3720 It is definitely not a general way to portably store floating point
3723 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3724 all types inside the group that accept the byte-order modifiers,
3725 including all subgroups. It will silently be ignored for all other
3726 types. You are not allowed to override the byte-order within a group
3727 that already has a byte-order modifier suffix.
3731 Real numbers (floats and doubles) are in the native machine format only;
3732 due to the multiplicity of floating formats around, and the lack of a
3733 standard "network" representation, no facility for interchange has been
3734 made. This means that packed floating point data written on one machine
3735 may not be readable on another - even if both use IEEE floating point
3736 arithmetic (as the endian-ness of the memory representation is not part
3737 of the IEEE spec). See also L<perlport>.
3739 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3740 modifiers to force big- or little-endian byte-order on floating point values.
3742 Note that Perl uses doubles (or long doubles, if configured) internally for
3743 all numeric calculation, and converting from double into float and thence back
3744 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3745 will not in general equal $foo).
3749 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3750 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3751 where the packed string is processed in its UTF-8-encoded Unicode form on
3752 a byte by byte basis. Character mode is the default unless the format string
3753 starts with an C<U>. You can switch mode at any moment with an explicit
3754 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3755 or until the end of the ()-group in which it was entered.
3759 You must yourself do any alignment or padding by inserting for example
3760 enough C<'x'>es while packing. There is no way to pack() and unpack()
3761 could know where the characters are going to or coming from. Therefore
3762 C<pack> (and C<unpack>) handle their output and input as flat
3763 sequences of characters.
3767 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3768 take a repeat count, both as postfix, and for unpack() also via the C</>
3769 template character. Within each repetition of a group, positioning with
3770 C<@> starts again at 0. Therefore, the result of
3772 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3774 is the string "\0a\0\0bc".
3778 C<x> and C<X> accept C<!> modifier. In this case they act as
3779 alignment commands: they jump forward/back to the closest position
3780 aligned at a multiple of C<count> characters. For example, to pack() or
3781 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3782 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3783 aligned on the double's size.
3785 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3786 both result in no-ops.
3790 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3791 will represent signed 16-/32-bit integers in big-/little-endian order.
3792 This is only portable if all platforms sharing the packed data use the
3793 same binary representation for signed integers (e.g. all platforms are
3794 using two's complement representation).
3798 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3799 White space may be used to separate pack codes from each other, but
3800 modifiers and a repeat count must follow immediately.
3804 If TEMPLATE requires more arguments to pack() than actually given, pack()
3805 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3806 to pack() than actually given, extra arguments are ignored.
3812 $foo = pack("WWWW",65,66,67,68);
3814 $foo = pack("W4",65,66,67,68);
3816 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3817 # same thing with Unicode circled letters.
3818 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3819 # same thing with Unicode circled letters. You don't get the UTF-8
3820 # bytes because the U at the start of the format caused a switch to
3821 # U0-mode, so the UTF-8 bytes get joined into characters
3822 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3823 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3824 # This is the UTF-8 encoding of the string in the previous example
3826 $foo = pack("ccxxcc",65,66,67,68);
3829 # note: the above examples featuring "W" and "c" are true
3830 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3831 # and UTF-8. In EBCDIC the first example would be
3832 # $foo = pack("WWWW",193,194,195,196);
3834 $foo = pack("s2",1,2);
3835 # "\1\0\2\0" on little-endian
3836 # "\0\1\0\2" on big-endian
3838 $foo = pack("a4","abcd","x","y","z");
3841 $foo = pack("aaaa","abcd","x","y","z");
3844 $foo = pack("a14","abcdefg");
3845 # "abcdefg\0\0\0\0\0\0\0"
3847 $foo = pack("i9pl", gmtime);
3848 # a real struct tm (on my system anyway)
3850 $utmp_template = "Z8 Z8 Z16 L";
3851 $utmp = pack($utmp_template, @utmp1);
3852 # a struct utmp (BSDish)
3854 @utmp2 = unpack($utmp_template, $utmp);
3855 # "@utmp1" eq "@utmp2"
3858 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3861 $foo = pack('sx2l', 12, 34);
3862 # short 12, two zero bytes padding, long 34
3863 $bar = pack('s@4l', 12, 34);
3864 # short 12, zero fill to position 4, long 34
3866 $baz = pack('s.l', 12, 4, 34);
3867 # short 12, zero fill to position 4, long 34
3869 $foo = pack('nN', 42, 4711);
3870 # pack big-endian 16- and 32-bit unsigned integers
3871 $foo = pack('S>L>', 42, 4711);
3873 $foo = pack('s<l<', -42, 4711);
3874 # pack little-endian 16- and 32-bit signed integers
3875 $foo = pack('(sl)<', -42, 4711);
3878 The same template may generally also be used in unpack().
3880 =item package NAMESPACE
3884 Declares the compilation unit as being in the given namespace. The scope
3885 of the package declaration is from the declaration itself through the end
3886 of the enclosing block, file, or eval (the same as the C<my> operator).
3887 All further unqualified dynamic identifiers will be in this namespace.
3888 A package statement affects only dynamic variables--including those
3889 you've used C<local> on--but I<not> lexical variables, which are created
3890 with C<my>. Typically it would be the first declaration in a file to
3891 be included by the C<require> or C<use> operator. You can switch into a
3892 package in more than one place; it merely influences which symbol table
3893 is used by the compiler for the rest of that block. You can refer to
3894 variables and filehandles in other packages by prefixing the identifier
3895 with the package name and a double colon: C<$Package::Variable>.
3896 If the package name is null, the C<main> package as assumed. That is,
3897 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3898 still seen in older code).
3900 If NAMESPACE is omitted, then there is no current package, and all
3901 identifiers must be fully qualified or lexicals. However, you are
3902 strongly advised not to make use of this feature. Its use can cause
3903 unexpected behaviour, even crashing some versions of Perl. It is
3904 deprecated, and will be removed from a future release.
3906 See L<perlmod/"Packages"> for more information about packages, modules,
3907 and classes. See L<perlsub> for other scoping issues.
3909 =item pipe READHANDLE,WRITEHANDLE
3911 Opens a pair of connected pipes like the corresponding system call.
3912 Note that if you set up a loop of piped processes, deadlock can occur
3913 unless you are very careful. In addition, note that Perl's pipes use
3914 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3915 after each command, depending on the application.
3917 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3918 for examples of such things.
3920 On systems that support a close-on-exec flag on files, the flag will be set
3921 for the newly opened file descriptors as determined by the value of $^F.
3928 Pops and returns the last value of the array, shortening the array by
3929 one element. Has an effect similar to
3933 If there are no elements in the array, returns the undefined value
3934 (although this may happen at other times as well). If ARRAY is
3935 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3936 array in subroutines, just like C<shift>.
3942 Returns the offset of where the last C<m//g> search left off for the variable
3943 in question (C<$_> is used when the variable is not specified). Note that
3944 0 is a valid match offset. C<undef> indicates that the search position
3945 is reset (usually due to match failure, but can also be because no match has
3946 yet been performed on the scalar). C<pos> directly accesses the location used
3947 by the regexp engine to store the offset, so assigning to C<pos> will change
3948 that offset, and so will also influence the C<\G> zero-width assertion in
3949 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
3950 the return from C<pos> won't change either in this case. See L<perlre> and
3953 =item print FILEHANDLE LIST
3959 Prints a string or a list of strings. Returns true if successful.
3960 FILEHANDLE may be a scalar variable name, in which case the variable
3961 contains the name of or a reference to the filehandle, thus introducing
3962 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3963 the next token is a term, it may be misinterpreted as an operator
3964 unless you interpose a C<+> or put parentheses around the arguments.)
3965 If FILEHANDLE is omitted, prints by default to standard output (or
3966 to the last selected output channel--see L</select>). If LIST is
3967 also omitted, prints C<$_> to the currently selected output channel.
3968 To set the default output channel to something other than STDOUT
3969 use the select operation. The current value of C<$,> (if any) is
3970 printed between each LIST item. The current value of C<$\> (if
3971 any) is printed after the entire LIST has been printed. Because
3972 print takes a LIST, anything in the LIST is evaluated in list
3973 context, and any subroutine that you call will have one or more of
3974 its expressions evaluated in list context. Also be careful not to
3975 follow the print keyword with a left parenthesis unless you want
3976 the corresponding right parenthesis to terminate the arguments to
3977 the print--interpose a C<+> or put parentheses around all the
3980 Note that if you're storing FILEHANDLEs in an array, or if you're using
3981 any other expression more complex than a scalar variable to retrieve it,
3982 you will have to use a block returning the filehandle value instead:
3984 print { $files[$i] } "stuff\n";
3985 print { $OK ? STDOUT : STDERR } "stuff\n";
3987 =item printf FILEHANDLE FORMAT, LIST
3989 =item printf FORMAT, LIST
3991 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3992 (the output record separator) is not appended. The first argument
3993 of the list will be interpreted as the C<printf> format. See C<sprintf>
3994 for an explanation of the format argument. If C<use locale> is in effect,
3995 the character used for the decimal point in formatted real numbers is
3996 affected by the LC_NUMERIC locale. See L<perllocale>.
3998 Don't fall into the trap of using a C<printf> when a simple
3999 C<print> would do. The C<print> is more efficient and less
4002 =item prototype FUNCTION
4004 Returns the prototype of a function as a string (or C<undef> if the
4005 function has no prototype). FUNCTION is a reference to, or the name of,
4006 the function whose prototype you want to retrieve.
4008 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4009 name for Perl builtin. If the builtin is not I<overridable> (such as
4010 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4011 C<system>) returns C<undef> because the builtin does not really behave
4012 like a Perl function. Otherwise, the string describing the equivalent
4013 prototype is returned.
4015 =item push ARRAY,LIST
4017 Treats ARRAY as a stack, and pushes the values of LIST
4018 onto the end of ARRAY. The length of ARRAY increases by the length of
4019 LIST. Has the same effect as
4022 $ARRAY[++$#ARRAY] = $value;
4025 but is more efficient. Returns the new number of elements in the array.
4037 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4039 =item quotemeta EXPR
4043 Returns the value of EXPR with all non-"word"
4044 characters backslashed. (That is, all characters not matching
4045 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4046 returned string, regardless of any locale settings.)
4047 This is the internal function implementing
4048 the C<\Q> escape in double-quoted strings.
4050 If EXPR is omitted, uses C<$_>.
4056 Returns a random fractional number greater than or equal to C<0> and less
4057 than the value of EXPR. (EXPR should be positive.) If EXPR is
4058 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4059 also special-cased as C<1> - this has not been documented before perl 5.8.0
4060 and is subject to change in future versions of perl. Automatically calls
4061 C<srand> unless C<srand> has already been called. See also C<srand>.
4063 Apply C<int()> to the value returned by C<rand()> if you want random
4064 integers instead of random fractional numbers. For example,
4068 returns a random integer between C<0> and C<9>, inclusive.
4070 (Note: If your rand function consistently returns numbers that are too
4071 large or too small, then your version of Perl was probably compiled
4072 with the wrong number of RANDBITS.)
4074 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4076 =item read FILEHANDLE,SCALAR,LENGTH
4078 Attempts to read LENGTH I<characters> of data into variable SCALAR
4079 from the specified FILEHANDLE. Returns the number of characters
4080 actually read, C<0> at end of file, or undef if there was an error (in
4081 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4082 so that the last character actually read is the last character of the
4083 scalar after the read.
4085 An OFFSET may be specified to place the read data at some place in the
4086 string other than the beginning. A negative OFFSET specifies
4087 placement at that many characters counting backwards from the end of
4088 the string. A positive OFFSET greater than the length of SCALAR
4089 results in the string being padded to the required size with C<"\0">
4090 bytes before the result of the read is appended.
4092 The call is actually implemented in terms of either Perl's or system's
4093 fread() call. To get a true read(2) system call, see C<sysread>.
4095 Note the I<characters>: depending on the status of the filehandle,
4096 either (8-bit) bytes or characters are read. By default all
4097 filehandles operate on bytes, but for example if the filehandle has
4098 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4099 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4100 characters, not bytes. Similarly for the C<:encoding> pragma:
4101 in that case pretty much any characters can be read.
4103 =item readdir DIRHANDLE
4105 Returns the next directory entry for a directory opened by C<opendir>.
4106 If used in list context, returns all the rest of the entries in the
4107 directory. If there are no more entries, returns an undefined value in
4108 scalar context or a null list in list context.
4110 If you're planning to filetest the return values out of a C<readdir>, you'd
4111 better prepend the directory in question. Otherwise, because we didn't
4112 C<chdir> there, it would have been testing the wrong file.
4114 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4115 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4120 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4121 context, each call reads and returns the next line, until end-of-file is
4122 reached, whereupon the subsequent call returns undef. In list context,
4123 reads until end-of-file is reached and returns a list of lines. Note that
4124 the notion of "line" used here is however you may have defined it
4125 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4127 When C<$/> is set to C<undef>, when readline() is in scalar
4128 context (i.e. file slurp mode), and when an empty file is read, it
4129 returns C<''> the first time, followed by C<undef> subsequently.
4131 This is the internal function implementing the C<< <EXPR> >>
4132 operator, but you can use it directly. The C<< <EXPR> >>
4133 operator is discussed in more detail in L<perlop/"I/O Operators">.
4136 $line = readline(*STDIN); # same thing
4138 If readline encounters an operating system error, C<$!> will be set with the
4139 corresponding error message. It can be helpful to check C<$!> when you are
4140 reading from filehandles you don't trust, such as a tty or a socket. The
4141 following example uses the operator form of C<readline>, and takes the necessary
4142 steps to ensure that C<readline> was successful.
4146 unless (defined( $line = <> )) {
4157 Returns the value of a symbolic link, if symbolic links are
4158 implemented. If not, gives a fatal error. If there is some system
4159 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4160 omitted, uses C<$_>.
4164 EXPR is executed as a system command.
4165 The collected standard output of the command is returned.
4166 In scalar context, it comes back as a single (potentially
4167 multi-line) string. In list context, returns a list of lines
4168 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4169 This is the internal function implementing the C<qx/EXPR/>
4170 operator, but you can use it directly. The C<qx/EXPR/>
4171 operator is discussed in more detail in L<perlop/"I/O Operators">.
4173 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4175 Receives a message on a socket. Attempts to receive LENGTH characters
4176 of data into variable SCALAR from the specified SOCKET filehandle.
4177 SCALAR will be grown or shrunk to the length actually read. Takes the
4178 same flags as the system call of the same name. Returns the address
4179 of the sender if SOCKET's protocol supports this; returns an empty
4180 string otherwise. If there's an error, returns the undefined value.
4181 This call is actually implemented in terms of recvfrom(2) system call.
4182 See L<perlipc/"UDP: Message Passing"> for examples.
4184 Note the I<characters>: depending on the status of the socket, either
4185 (8-bit) bytes or characters are received. By default all sockets
4186 operate on bytes, but for example if the socket has been changed using
4187 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4188 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4189 characters, not bytes. Similarly for the C<:encoding> pragma:
4190 in that case pretty much any characters can be read.
4196 The C<redo> command restarts the loop block without evaluating the
4197 conditional again. The C<continue> block, if any, is not executed. If
4198 the LABEL is omitted, the command refers to the innermost enclosing
4199 loop. Programs that want to lie to themselves about what was just input
4200 normally use this command:
4202 # a simpleminded Pascal comment stripper
4203 # (warning: assumes no { or } in strings)
4204 LINE: while (<STDIN>) {
4205 while (s|({.*}.*){.*}|$1 |) {}
4210 if (/}/) { # end of comment?
4219 C<redo> cannot be used to retry a block which returns a value such as
4220 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4221 a grep() or map() operation.
4223 Note that a block by itself is semantically identical to a loop
4224 that executes once. Thus C<redo> inside such a block will effectively
4225 turn it into a looping construct.
4227 See also L</continue> for an illustration of how C<last>, C<next>, and
4234 Returns a non-empty string if EXPR is a reference, the empty
4235 string otherwise. If EXPR
4236 is not specified, C<$_> will be used. The value returned depends on the
4237 type of thing the reference is a reference to.
4238 Builtin types include:
4248 If the referenced object has been blessed into a package, then that package
4249 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4251 if (ref($r) eq "HASH") {
4252 print "r is a reference to a hash.\n";
4255 print "r is not a reference at all.\n";
4258 See also L<perlref>.
4260 =item rename OLDNAME,NEWNAME
4262 Changes the name of a file; an existing file NEWNAME will be
4263 clobbered. Returns true for success, false otherwise.
4265 Behavior of this function varies wildly depending on your system
4266 implementation. For example, it will usually not work across file system
4267 boundaries, even though the system I<mv> command sometimes compensates
4268 for this. Other restrictions include whether it works on directories,
4269 open files, or pre-existing files. Check L<perlport> and either the
4270 rename(2) manpage or equivalent system documentation for details.
4272 =item require VERSION
4278 Demands a version of Perl specified by VERSION, or demands some semantics
4279 specified by EXPR or by C<$_> if EXPR is not supplied.
4281 VERSION may be either a numeric argument such as 5.006, which will be
4282 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4283 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4284 VERSION is greater than the version of the current Perl interpreter.
4285 Compare with L</use>, which can do a similar check at compile time.
4287 Specifying VERSION as a literal of the form v5.6.1 should generally be
4288 avoided, because it leads to misleading error messages under earlier
4289 versions of Perl that do not support this syntax. The equivalent numeric
4290 version should be used instead.
4292 require v5.6.1; # run time version check
4293 require 5.6.1; # ditto
4294 require 5.006_001; # ditto; preferred for backwards compatibility
4296 Otherwise, C<ref> demands that a library file be included if it hasn't already
4297 been included. The file is included via the do-FILE mechanism, which is
4298 essentially just a variety of C<eval>. Has semantics similar to the
4299 following subroutine:
4302 my ($filename) = @_;
4303 if (exists $INC{$filename}) {
4304 return 1 if $INC{$filename};
4305 die "Compilation failed in require";
4307 my ($realfilename,$result);
4309 foreach $prefix (@INC) {
4310 $realfilename = "$prefix/$filename";
4311 if (-f $realfilename) {
4312 $INC{$filename} = $realfilename;
4313 $result = do $realfilename;
4317 die "Can't find $filename in \@INC";
4320 $INC{$filename} = undef;
4322 } elsif (!$result) {
4323 delete $INC{$filename};
4324 die "$filename did not return true value";
4330 Note that the file will not be included twice under the same specified
4333 The file must return true as the last statement to indicate
4334 successful execution of any initialization code, so it's customary to
4335 end such a file with C<1;> unless you're sure it'll return true
4336 otherwise. But it's better just to put the C<1;>, in case you add more
4339 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4340 replaces "F<::>" with "F</>" in the filename for you,
4341 to make it easy to load standard modules. This form of loading of
4342 modules does not risk altering your namespace.
4344 In other words, if you try this:
4346 require Foo::Bar; # a splendid bareword
4348 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4349 directories specified in the C<@INC> array.
4351 But if you try this:
4353 $class = 'Foo::Bar';
4354 require $class; # $class is not a bareword
4356 require "Foo::Bar"; # not a bareword because of the ""
4358 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4359 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4361 eval "require $class";
4363 Now that you understand how C<require> looks for files in the case of
4364 a bareword argument, there is a little extra functionality going on
4365 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4366 it will first look for a filename with a "F<.pmc>" extension. A file
4367 with this extension is assumed to be Perl bytecode generated by
4368 L<B::Bytecode|B::Bytecode>. If this file is found, and its modification
4369 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4370 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4372 You can also insert hooks into the import facility, by putting directly
4373 Perl code into the @INC array. There are three forms of hooks: subroutine
4374 references, array references and blessed objects.
4376 Subroutine references are the simplest case. When the inclusion system
4377 walks through @INC and encounters a subroutine, this subroutine gets
4378 called with two parameters, the first being a reference to itself, and the
4379 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4380 subroutine should return C<undef> or a filehandle, from which the file to
4381 include will be read. If C<undef> is returned, C<require> will look at
4382 the remaining elements of @INC.
4384 If the hook is an array reference, its first element must be a subroutine
4385 reference. This subroutine is called as above, but the first parameter is
4386 the array reference. This enables to pass indirectly some arguments to
4389 In other words, you can write:
4391 push @INC, \&my_sub;
4393 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4399 push @INC, [ \&my_sub, $x, $y, ... ];
4401 my ($arrayref, $filename) = @_;
4402 # Retrieve $x, $y, ...
4403 my @parameters = @$arrayref[1..$#$arrayref];
4407 If the hook is an object, it must provide an INC method that will be
4408 called as above, the first parameter being the object itself. (Note that
4409 you must fully qualify the sub's name, as it is always forced into package
4410 C<main>.) Here is a typical code layout:
4416 my ($self, $filename) = @_;
4420 # In the main program
4421 push @INC, new Foo(...);
4423 Note that these hooks are also permitted to set the %INC entry
4424 corresponding to the files they have loaded. See L<perlvar/%INC>.
4426 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4432 Generally used in a C<continue> block at the end of a loop to clear
4433 variables and reset C<??> searches so that they work again. The
4434 expression is interpreted as a list of single characters (hyphens
4435 allowed for ranges). All variables and arrays beginning with one of
4436 those letters are reset to their pristine state. If the expression is
4437 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4438 only variables or searches in the current package. Always returns
4441 reset 'X'; # reset all X variables
4442 reset 'a-z'; # reset lower case variables
4443 reset; # just reset ?one-time? searches
4445 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4446 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4447 variables--lexical variables are unaffected, but they clean themselves
4448 up on scope exit anyway, so you'll probably want to use them instead.
4455 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4456 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4457 context, depending on how the return value will be used, and the context
4458 may vary from one execution to the next (see C<wantarray>). If no EXPR
4459 is given, returns an empty list in list context, the undefined value in
4460 scalar context, and (of course) nothing at all in a void context.
4462 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4463 or do FILE will automatically return the value of the last expression
4468 In list context, returns a list value consisting of the elements
4469 of LIST in the opposite order. In scalar context, concatenates the
4470 elements of LIST and returns a string value with all characters
4471 in the opposite order.
4473 print reverse <>; # line tac, last line first
4475 undef $/; # for efficiency of <>
4476 print scalar reverse <>; # character tac, last line tsrif
4478 Used without arguments in scalar context, reverse() reverses C<$_>.
4480 This operator is also handy for inverting a hash, although there are some
4481 caveats. If a value is duplicated in the original hash, only one of those
4482 can be represented as a key in the inverted hash. Also, this has to
4483 unwind one hash and build a whole new one, which may take some time
4484 on a large hash, such as from a DBM file.
4486 %by_name = reverse %by_address; # Invert the hash
4488 =item rewinddir DIRHANDLE
4490 Sets the current position to the beginning of the directory for the
4491 C<readdir> routine on DIRHANDLE.
4493 =item rindex STR,SUBSTR,POSITION
4495 =item rindex STR,SUBSTR
4497 Works just like index() except that it returns the position of the LAST
4498 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4499 last occurrence at or before that position.
4501 =item rmdir FILENAME
4505 Deletes the directory specified by FILENAME if that directory is
4506 empty. If it succeeds it returns true, otherwise it returns false and
4507 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4511 The substitution operator. See L<perlop>.
4515 Forces EXPR to be interpreted in scalar context and returns the value
4518 @counts = ( scalar @a, scalar @b, scalar @c );
4520 There is no equivalent operator to force an expression to
4521 be interpolated in list context because in practice, this is never
4522 needed. If you really wanted to do so, however, you could use
4523 the construction C<@{[ (some expression) ]}>, but usually a simple
4524 C<(some expression)> suffices.
4526 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4527 parenthesized list, this behaves as a scalar comma expression, evaluating
4528 all but the last element in void context and returning the final element
4529 evaluated in scalar context. This is seldom what you want.
4531 The following single statement:
4533 print uc(scalar(&foo,$bar)),$baz;
4535 is the moral equivalent of these two:
4538 print(uc($bar),$baz);
4540 See L<perlop> for more details on unary operators and the comma operator.
4542 =item seek FILEHANDLE,POSITION,WHENCE
4544 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4545 FILEHANDLE may be an expression whose value gives the name of the
4546 filehandle. The values for WHENCE are C<0> to set the new position
4547 I<in bytes> to POSITION, C<1> to set it to the current position plus
4548 POSITION, and C<2> to set it to EOF plus POSITION (typically
4549 negative). For WHENCE you may use the constants C<SEEK_SET>,
4550 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4551 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4554 Note the I<in bytes>: even if the filehandle has been set to
4555 operate on characters (for example by using the C<:utf8> open
4556 layer), tell() will return byte offsets, not character offsets
4557 (because implementing that would render seek() and tell() rather slow).
4559 If you want to position file for C<sysread> or C<syswrite>, don't use
4560 C<seek>--buffering makes its effect on the file's system position
4561 unpredictable and non-portable. Use C<sysseek> instead.
4563 Due to the rules and rigors of ANSI C, on some systems you have to do a
4564 seek whenever you switch between reading and writing. Amongst other
4565 things, this may have the effect of calling stdio's clearerr(3).
4566 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4570 This is also useful for applications emulating C<tail -f>. Once you hit
4571 EOF on your read, and then sleep for a while, you might have to stick in a
4572 seek() to reset things. The C<seek> doesn't change the current position,
4573 but it I<does> clear the end-of-file condition on the handle, so that the
4574 next C<< <FILE> >> makes Perl try again to read something. We hope.
4576 If that doesn't work (some IO implementations are particularly
4577 cantankerous), then you may need something more like this:
4580 for ($curpos = tell(FILE); $_ = <FILE>;
4581 $curpos = tell(FILE)) {
4582 # search for some stuff and put it into files
4584 sleep($for_a_while);
4585 seek(FILE, $curpos, 0);
4588 =item seekdir DIRHANDLE,POS
4590 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4591 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4592 about possible directory compaction as the corresponding system library
4595 =item select FILEHANDLE
4599 Returns the currently selected filehandle. Sets the current default
4600 filehandle for output, if FILEHANDLE is supplied. This has two
4601 effects: first, a C<write> or a C<print> without a filehandle will
4602 default to this FILEHANDLE. Second, references to variables related to
4603 output will refer to this output channel. For example, if you have to
4604 set the top of form format for more than one output channel, you might
4612 FILEHANDLE may be an expression whose value gives the name of the
4613 actual filehandle. Thus:
4615 $oldfh = select(STDERR); $| = 1; select($oldfh);
4617 Some programmers may prefer to think of filehandles as objects with
4618 methods, preferring to write the last example as:
4621 STDERR->autoflush(1);
4623 =item select RBITS,WBITS,EBITS,TIMEOUT
4625 This calls the select(2) system call with the bit masks specified, which
4626 can be constructed using C<fileno> and C<vec>, along these lines:
4628 $rin = $win = $ein = '';
4629 vec($rin,fileno(STDIN),1) = 1;
4630 vec($win,fileno(STDOUT),1) = 1;
4633 If you want to select on many filehandles you might wish to write a
4637 my(@fhlist) = split(' ',$_[0]);
4640 vec($bits,fileno($_),1) = 1;
4644 $rin = fhbits('STDIN TTY SOCK');
4648 ($nfound,$timeleft) =
4649 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4651 or to block until something becomes ready just do this
4653 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4655 Most systems do not bother to return anything useful in $timeleft, so
4656 calling select() in scalar context just returns $nfound.
4658 Any of the bit masks can also be undef. The timeout, if specified, is
4659 in seconds, which may be fractional. Note: not all implementations are
4660 capable of returning the $timeleft. If not, they always return
4661 $timeleft equal to the supplied $timeout.
4663 You can effect a sleep of 250 milliseconds this way:
4665 select(undef, undef, undef, 0.25);
4667 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4668 is implementation-dependent. See also L<perlport> for notes on the
4669 portability of C<select>.
4671 On error, C<select> returns C<undef> and sets C<$!>.
4673 Note: on some Unixes, the select(2) system call may report a socket file
4674 descriptor as "ready for reading", when actually no data is available,
4675 thus a subsequent read blocks. It can be avoided using always the
4676 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4679 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4680 or <FH>) with C<select>, except as permitted by POSIX, and even
4681 then only on POSIX systems. You have to use C<sysread> instead.
4683 =item semctl ID,SEMNUM,CMD,ARG
4685 Calls the System V IPC function C<semctl>. You'll probably have to say
4689 first to get the correct constant definitions. If CMD is IPC_STAT or
4690 GETALL, then ARG must be a variable that will hold the returned
4691 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4692 the undefined value for error, "C<0 but true>" for zero, or the actual
4693 return value otherwise. The ARG must consist of a vector of native
4694 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4695 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4698 =item semget KEY,NSEMS,FLAGS
4700 Calls the System V IPC function semget. Returns the semaphore id, or
4701 the undefined value if there is an error. See also
4702 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4705 =item semop KEY,OPSTRING
4707 Calls the System V IPC function semop to perform semaphore operations
4708 such as signalling and waiting. OPSTRING must be a packed array of
4709 semop structures. Each semop structure can be generated with
4710 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4711 implies the number of semaphore operations. Returns true if
4712 successful, or false if there is an error. As an example, the
4713 following code waits on semaphore $semnum of semaphore id $semid:
4715 $semop = pack("s!3", $semnum, -1, 0);
4716 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4718 To signal the semaphore, replace C<-1> with C<1>. See also
4719 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4722 =item send SOCKET,MSG,FLAGS,TO
4724 =item send SOCKET,MSG,FLAGS
4726 Sends a message on a socket. Attempts to send the scalar MSG to the
4727 SOCKET filehandle. Takes the same flags as the system call of the
4728 same name. On unconnected sockets you must specify a destination to
4729 send TO, in which case it does a C C<sendto>. Returns the number of
4730 characters sent, or the undefined value if there is an error. The C
4731 system call sendmsg(2) is currently unimplemented. See
4732 L<perlipc/"UDP: Message Passing"> for examples.
4734 Note the I<characters>: depending on the status of the socket, either
4735 (8-bit) bytes or characters are sent. By default all sockets operate
4736 on bytes, but for example if the socket has been changed using
4737 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4738 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4739 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4740 in that case pretty much any characters can be sent.
4742 =item setpgrp PID,PGRP
4744 Sets the current process group for the specified PID, C<0> for the current
4745 process. Will produce a fatal error if used on a machine that doesn't
4746 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4747 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4748 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4751 =item setpriority WHICH,WHO,PRIORITY
4753 Sets the current priority for a process, a process group, or a user.
4754 (See setpriority(2).) Will produce a fatal error if used on a machine
4755 that doesn't implement setpriority(2).
4757 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4759 Sets the socket option requested. Returns undefined if there is an
4760 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4767 Shifts the first value of the array off and returns it, shortening the
4768 array by 1 and moving everything down. If there are no elements in the
4769 array, returns the undefined value. If ARRAY is omitted, shifts the
4770 C<@_> array within the lexical scope of subroutines and formats, and the
4771 C<@ARGV> array outside of a subroutine and also within the lexical scopes
4772 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
4773 and C<END {}> constructs.
4775 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4776 same thing to the left end of an array that C<pop> and C<push> do to the
4779 =item shmctl ID,CMD,ARG
4781 Calls the System V IPC function shmctl. You'll probably have to say
4785 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4786 then ARG must be a variable that will hold the returned C<shmid_ds>
4787 structure. Returns like ioctl: the undefined value for error, "C<0> but
4788 true" for zero, or the actual return value otherwise.
4789 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4791 =item shmget KEY,SIZE,FLAGS
4793 Calls the System V IPC function shmget. Returns the shared memory
4794 segment id, or the undefined value if there is an error.
4795 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4797 =item shmread ID,VAR,POS,SIZE
4799 =item shmwrite ID,STRING,POS,SIZE
4801 Reads or writes the System V shared memory segment ID starting at
4802 position POS for size SIZE by attaching to it, copying in/out, and
4803 detaching from it. When reading, VAR must be a variable that will
4804 hold the data read. When writing, if STRING is too long, only SIZE
4805 bytes are used; if STRING is too short, nulls are written to fill out
4806 SIZE bytes. Return true if successful, or false if there is an error.
4807 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4808 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4810 =item shutdown SOCKET,HOW
4812 Shuts down a socket connection in the manner indicated by HOW, which
4813 has the same interpretation as in the system call of the same name.
4815 shutdown(SOCKET, 0); # I/we have stopped reading data
4816 shutdown(SOCKET, 1); # I/we have stopped writing data
4817 shutdown(SOCKET, 2); # I/we have stopped using this socket
4819 This is useful with sockets when you want to tell the other
4820 side you're done writing but not done reading, or vice versa.
4821 It's also a more insistent form of close because it also
4822 disables the file descriptor in any forked copies in other
4829 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4830 returns sine of C<$_>.
4832 For the inverse sine operation, you may use the C<Math::Trig::asin>
4833 function, or use this relation:
4835 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4841 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4842 May be interrupted if the process receives a signal such as C<SIGALRM>.
4843 Returns the number of seconds actually slept. You probably cannot
4844 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4847 On some older systems, it may sleep up to a full second less than what
4848 you requested, depending on how it counts seconds. Most modern systems
4849 always sleep the full amount. They may appear to sleep longer than that,
4850 however, because your process might not be scheduled right away in a
4851 busy multitasking system.
4853 For delays of finer granularity than one second, you may use Perl's
4854 C<syscall> interface to access setitimer(2) if your system supports
4855 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4856 and starting from Perl 5.8 part of the standard distribution) may also
4859 See also the POSIX module's C<pause> function.
4861 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4863 Opens a socket of the specified kind and attaches it to filehandle
4864 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4865 the system call of the same name. You should C<use Socket> first
4866 to get the proper definitions imported. See the examples in
4867 L<perlipc/"Sockets: Client/Server Communication">.
4869 On systems that support a close-on-exec flag on files, the flag will
4870 be set for the newly opened file descriptor, as determined by the
4871 value of $^F. See L<perlvar/$^F>.
4873 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4875 Creates an unnamed pair of sockets in the specified domain, of the
4876 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4877 for the system call of the same name. If unimplemented, yields a fatal
4878 error. Returns true if successful.
4880 On systems that support a close-on-exec flag on files, the flag will
4881 be set for the newly opened file descriptors, as determined by the value
4882 of $^F. See L<perlvar/$^F>.
4884 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4885 to C<pipe(Rdr, Wtr)> is essentially:
4888 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4889 shutdown(Rdr, 1); # no more writing for reader
4890 shutdown(Wtr, 0); # no more reading for writer
4892 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4893 emulate socketpair using IP sockets to localhost if your system implements
4894 sockets but not socketpair.
4896 =item sort SUBNAME LIST
4898 =item sort BLOCK LIST
4902 In list context, this sorts the LIST and returns the sorted list value.
4903 In scalar context, the behaviour of C<sort()> is undefined.
4905 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4906 order. If SUBNAME is specified, it gives the name of a subroutine
4907 that returns an integer less than, equal to, or greater than C<0>,
4908 depending on how the elements of the list are to be ordered. (The C<<
4909 <=> >> and C<cmp> operators are extremely useful in such routines.)
4910 SUBNAME may be a scalar variable name (unsubscripted), in which case
4911 the value provides the name of (or a reference to) the actual
4912 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4913 an anonymous, in-line sort subroutine.
4915 If the subroutine's prototype is C<($$)>, the elements to be compared
4916 are passed by reference in C<@_>, as for a normal subroutine. This is
4917 slower than unprototyped subroutines, where the elements to be
4918 compared are passed into the subroutine
4919 as the package global variables $a and $b (see example below). Note that
4920 in the latter case, it is usually counter-productive to declare $a and
4923 In either case, the subroutine may not be recursive. The values to be
4924 compared are always passed by reference and should not be modified.
4926 You also cannot exit out of the sort block or subroutine using any of the
4927 loop control operators described in L<perlsyn> or with C<goto>.
4929 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4930 current collation locale. See L<perllocale>.
4932 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4933 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4934 preserves the input order of elements that compare equal. Although
4935 quicksort's run time is O(NlogN) when averaged over all arrays of
4936 length N, the time can be O(N**2), I<quadratic> behavior, for some
4937 inputs.) In 5.7, the quicksort implementation was replaced with
4938 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
4939 But benchmarks indicated that for some inputs, on some platforms,
4940 the original quicksort was faster. 5.8 has a sort pragma for
4941 limited control of the sort. Its rather blunt control of the
4942 underlying algorithm may not persist into future Perls, but the
4943 ability to characterize the input or output in implementation
4944 independent ways quite probably will. See L<sort>.
4949 @articles = sort @files;
4951 # same thing, but with explicit sort routine
4952 @articles = sort {$a cmp $b} @files;
4954 # now case-insensitively
4955 @articles = sort {uc($a) cmp uc($b)} @files;
4957 # same thing in reversed order
4958 @articles = sort {$b cmp $a} @files;
4960 # sort numerically ascending
4961 @articles = sort {$a <=> $b} @files;
4963 # sort numerically descending
4964 @articles = sort {$b <=> $a} @files;
4966 # this sorts the %age hash by value instead of key
4967 # using an in-line function
4968 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4970 # sort using explicit subroutine name
4972 $age{$a} <=> $age{$b}; # presuming numeric
4974 @sortedclass = sort byage @class;
4976 sub backwards { $b cmp $a }
4977 @harry = qw(dog cat x Cain Abel);
4978 @george = qw(gone chased yz Punished Axed);
4980 # prints AbelCaincatdogx
4981 print sort backwards @harry;
4982 # prints xdogcatCainAbel
4983 print sort @george, 'to', @harry;
4984 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4986 # inefficiently sort by descending numeric compare using
4987 # the first integer after the first = sign, or the
4988 # whole record case-insensitively otherwise
4991 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4996 # same thing, but much more efficiently;
4997 # we'll build auxiliary indices instead
5001 push @nums, /=(\d+)/;
5006 $nums[$b] <=> $nums[$a]
5008 $caps[$a] cmp $caps[$b]
5012 # same thing, but without any temps
5013 @new = map { $_->[0] }
5014 sort { $b->[1] <=> $a->[1]
5017 } map { [$_, /=(\d+)/, uc($_)] } @old;
5019 # using a prototype allows you to use any comparison subroutine
5020 # as a sort subroutine (including other package's subroutines)
5022 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5025 @new = sort other::backwards @old;
5027 # guarantee stability, regardless of algorithm
5029 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5031 # force use of mergesort (not portable outside Perl 5.8)
5032 use sort '_mergesort'; # note discouraging _
5033 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5035 If you're using strict, you I<must not> declare $a
5036 and $b as lexicals. They are package globals. That means
5037 if you're in the C<main> package and type
5039 @articles = sort {$b <=> $a} @files;
5041 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5042 but if you're in the C<FooPack> package, it's the same as typing
5044 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5046 The comparison function is required to behave. If it returns
5047 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5048 sometimes saying the opposite, for example) the results are not
5051 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5052 (not-a-number), and because C<sort> will trigger a fatal error unless the
5053 result of a comparison is defined, when sorting with a comparison function
5054 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5055 The following example takes advantage of the fact that C<NaN != NaN> to
5056 eliminate any C<NaN>s from the input.
5058 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5060 =item splice ARRAY,OFFSET,LENGTH,LIST
5062 =item splice ARRAY,OFFSET,LENGTH
5064 =item splice ARRAY,OFFSET
5068 Removes the elements designated by OFFSET and LENGTH from an array, and
5069 replaces them with the elements of LIST, if any. In list context,
5070 returns the elements removed from the array. In scalar context,
5071 returns the last element removed, or C<undef> if no elements are
5072 removed. The array grows or shrinks as necessary.
5073 If OFFSET is negative then it starts that far from the end of the array.
5074 If LENGTH is omitted, removes everything from OFFSET onward.
5075 If LENGTH is negative, removes the elements from OFFSET onward
5076 except for -LENGTH elements at the end of the array.
5077 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5078 past the end of the array, perl issues a warning, and splices at the
5081 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5083 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5084 pop(@a) splice(@a,-1)
5085 shift(@a) splice(@a,0,1)
5086 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5087 $a[$i] = $y splice(@a,$i,1,$y)
5089 Example, assuming array lengths are passed before arrays:
5091 sub aeq { # compare two list values
5092 my(@a) = splice(@_,0,shift);
5093 my(@b) = splice(@_,0,shift);
5094 return 0 unless @a == @b; # same len?
5096 return 0 if pop(@a) ne pop(@b);
5100 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5102 =item split /PATTERN/,EXPR,LIMIT
5104 =item split /PATTERN/,EXPR
5106 =item split /PATTERN/
5110 Splits the string EXPR into a list of strings and returns that list. By
5111 default, empty leading fields are preserved, and empty trailing ones are
5112 deleted. (If all fields are empty, they are considered to be trailing.)
5114 In scalar context, returns the number of fields found and splits into
5115 the C<@_> array. Use of split in scalar context is deprecated, however,
5116 because it clobbers your subroutine arguments.
5118 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5119 splits on whitespace (after skipping any leading whitespace). Anything
5120 matching PATTERN is taken to be a delimiter separating the fields. (Note
5121 that the delimiter may be longer than one character.)
5123 If LIMIT is specified and positive, it represents the maximum number
5124 of fields the EXPR will be split into, though the actual number of
5125 fields returned depends on the number of times PATTERN matches within
5126 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5127 stripped (which potential users of C<pop> would do well to remember).
5128 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5129 had been specified. Note that splitting an EXPR that evaluates to the
5130 empty string always returns the empty list, regardless of the LIMIT
5133 A pattern matching the null string (not to be confused with
5134 a null pattern C<//>, which is just one member of the set of patterns
5135 matching a null string) will split the value of EXPR into separate
5136 characters at each point it matches that way. For example:
5138 print join(':', split(/ */, 'hi there'));
5140 produces the output 'h:i:t:h:e:r:e'.
5142 As a special case for C<split>, using the empty pattern C<//> specifically
5143 matches only the null string, and is not be confused with the regular use
5144 of C<//> to mean "the last successful pattern match". So, for C<split>,
5147 print join(':', split(//, 'hi there'));
5149 produces the output 'h:i: :t:h:e:r:e'.
5151 Empty leading (or trailing) fields are produced when there are positive
5152 width matches at the beginning (or end) of the string; a zero-width match
5153 at the beginning (or end) of the string does not produce an empty field.
5156 print join(':', split(/(?=\w)/, 'hi there!'));
5158 produces the output 'h:i :t:h:e:r:e!'.
5160 The LIMIT parameter can be used to split a line partially
5162 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5164 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5165 a LIMIT one larger than the number of variables in the list, to avoid
5166 unnecessary work. For the list above LIMIT would have been 4 by
5167 default. In time critical applications it behooves you not to split
5168 into more fields than you really need.
5170 If the PATTERN contains parentheses, additional list elements are
5171 created from each matching substring in the delimiter.
5173 split(/([,-])/, "1-10,20", 3);
5175 produces the list value
5177 (1, '-', 10, ',', 20)
5179 If you had the entire header of a normal Unix email message in $header,
5180 you could split it up into fields and their values this way:
5182 $header =~ s/\n\s+/ /g; # fix continuation lines
5183 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5185 The pattern C</PATTERN/> may be replaced with an expression to specify
5186 patterns that vary at runtime. (To do runtime compilation only once,
5187 use C</$variable/o>.)
5189 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5190 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5191 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5192 will give you as many null initial fields as there are leading spaces.
5193 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5194 whitespace produces a null first field. A C<split> with no arguments
5195 really does a S<C<split(' ', $_)>> internally.
5197 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5202 open(PASSWD, '/etc/passwd');
5205 ($login, $passwd, $uid, $gid,
5206 $gcos, $home, $shell) = split(/:/);
5210 As with regular pattern matching, any capturing parentheses that are not
5211 matched in a C<split()> will be set to C<undef> when returned:
5213 @fields = split /(A)|B/, "1A2B3";
5214 # @fields is (1, 'A', 2, undef, 3)
5216 =item sprintf FORMAT, LIST
5218 Returns a string formatted by the usual C<printf> conventions of the C
5219 library function C<sprintf>. See below for more details
5220 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5221 the general principles.
5225 # Format number with up to 8 leading zeroes
5226 $result = sprintf("%08d", $number);
5228 # Round number to 3 digits after decimal point
5229 $rounded = sprintf("%.3f", $number);
5231 Perl does its own C<sprintf> formatting--it emulates the C
5232 function C<sprintf>, but it doesn't use it (except for floating-point
5233 numbers, and even then only the standard modifiers are allowed). As a
5234 result, any non-standard extensions in your local C<sprintf> are not
5235 available from Perl.
5237 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5238 pass it an array as your first argument. The array is given scalar context,
5239 and instead of using the 0th element of the array as the format, Perl will
5240 use the count of elements in the array as the format, which is almost never
5243 Perl's C<sprintf> permits the following universally-known conversions:
5246 %c a character with the given number
5248 %d a signed integer, in decimal
5249 %u an unsigned integer, in decimal
5250 %o an unsigned integer, in octal
5251 %x an unsigned integer, in hexadecimal
5252 %e a floating-point number, in scientific notation
5253 %f a floating-point number, in fixed decimal notation
5254 %g a floating-point number, in %e or %f notation
5256 In addition, Perl permits the following widely-supported conversions:
5258 %X like %x, but using upper-case letters
5259 %E like %e, but using an upper-case "E"
5260 %G like %g, but with an upper-case "E" (if applicable)
5261 %b an unsigned integer, in binary
5262 %p a pointer (outputs the Perl value's address in hexadecimal)
5263 %n special: *stores* the number of characters output so far
5264 into the next variable in the parameter list
5266 Finally, for backward (and we do mean "backward") compatibility, Perl
5267 permits these unnecessary but widely-supported conversions:
5270 %D a synonym for %ld
5271 %U a synonym for %lu
5272 %O a synonym for %lo
5275 Note that the number of exponent digits in the scientific notation produced
5276 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5277 exponent less than 100 is system-dependent: it may be three or less
5278 (zero-padded as necessary). In other words, 1.23 times ten to the
5279 99th may be either "1.23e99" or "1.23e099".
5281 Between the C<%> and the format letter, you may specify a number of
5282 additional attributes controlling the interpretation of the format.
5283 In order, these are:
5287 =item format parameter index
5289 An explicit format parameter index, such as C<2$>. By default sprintf
5290 will format the next unused argument in the list, but this allows you
5291 to take the arguments out of order, e.g.:
5293 printf '%2$d %1$d', 12, 34; # prints "34 12"
5294 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5299 space prefix positive number with a space
5300 + prefix positive number with a plus sign
5301 - left-justify within the field
5302 0 use zeros, not spaces, to right-justify
5303 # prefix non-zero octal with "0", non-zero hex with "0x",
5304 non-zero binary with "0b"
5308 printf '<% d>', 12; # prints "< 12>"
5309 printf '<%+d>', 12; # prints "<+12>"
5310 printf '<%6s>', 12; # prints "< 12>"
5311 printf '<%-6s>', 12; # prints "<12 >"
5312 printf '<%06s>', 12; # prints "<000012>"
5313 printf '<%#x>', 12; # prints "<0xc>"
5317 The vector flag C<v>, optionally specifying the join string to use.
5318 This flag tells perl to interpret the supplied string as a vector
5319 of integers, one for each character in the string, separated by
5320 a given string (a dot C<.> by default). This can be useful for
5321 displaying ordinal values of characters in arbitrary strings:
5323 printf "version is v%vd\n", $^V; # Perl's version
5325 Put an asterisk C<*> before the C<v> to override the string to
5326 use to separate the numbers:
5328 printf "address is %*vX\n", ":", $addr; # IPv6 address
5329 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5331 You can also explicitly specify the argument number to use for
5332 the join string using e.g. C<*2$v>:
5334 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5336 =item (minimum) width
5338 Arguments are usually formatted to be only as wide as required to
5339 display the given value. You can override the width by putting
5340 a number here, or get the width from the next argument (with C<*>)
5341 or from a specified argument (with e.g. C<*2$>):
5343 printf '<%s>', "a"; # prints "<a>"
5344 printf '<%6s>', "a"; # prints "< a>"
5345 printf '<%*s>', 6, "a"; # prints "< a>"
5346 printf '<%*2$s>', "a", 6; # prints "< a>"
5347 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5349 If a field width obtained through C<*> is negative, it has the same
5350 effect as the C<-> flag: left-justification.
5352 =item precision, or maximum width
5354 You can specify a precision (for numeric conversions) or a maximum
5355 width (for string conversions) by specifying a C<.> followed by a number.
5356 For floating point formats, with the exception of 'g' and 'G', this specifies
5357 the number of decimal places to show (the default being 6), e.g.:
5359 # these examples are subject to system-specific variation
5360 printf '<%f>', 1; # prints "<1.000000>"
5361 printf '<%.1f>', 1; # prints "<1.0>"
5362 printf '<%.0f>', 1; # prints "<1>"
5363 printf '<%e>', 10; # prints "<1.000000e+01>"
5364 printf '<%.1e>', 10; # prints "<1.0e+01>"
5366 For 'g' and 'G', this specifies the maximum number of digits to show,
5367 including prior to the decimal point as well as after it, e.g.:
5369 # these examples are subject to system-specific variation
5370 printf '<%g>', 1; # prints "<1>"
5371 printf '<%.10g>', 1; # prints "<1>"
5372 printf '<%g>', 100; # prints "<100>"
5373 printf '<%.1g>', 100; # prints "<1e+02>"
5374 printf '<%.2g>', 100.01; # prints "<1e+02>"
5375 printf '<%.5g>', 100.01; # prints "<100.01>"
5376 printf '<%.4g>', 100.01; # prints "<100>"
5378 For integer conversions, specifying a precision implies that the
5379 output of the number itself should be zero-padded to this width:
5381 printf '<%.6x>', 1; # prints "<000001>"
5382 printf '<%#.6x>', 1; # prints "<0x000001>"
5383 printf '<%-10.6x>', 1; # prints "<000001 >"
5385 For string conversions, specifying a precision truncates the string
5386 to fit in the specified width:
5388 printf '<%.5s>', "truncated"; # prints "<trunc>"
5389 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5391 You can also get the precision from the next argument using C<.*>:
5393 printf '<%.6x>', 1; # prints "<000001>"
5394 printf '<%.*x>', 6, 1; # prints "<000001>"
5396 You cannot currently get the precision from a specified number,
5397 but it is intended that this will be possible in the future using
5400 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5404 For numeric conversions, you can specify the size to interpret the
5405 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5406 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5407 whatever the default integer size is on your platform (usually 32 or 64
5408 bits), but you can override this to use instead one of the standard C types,
5409 as supported by the compiler used to build Perl:
5411 l interpret integer as C type "long" or "unsigned long"
5412 h interpret integer as C type "short" or "unsigned short"
5413 q, L or ll interpret integer as C type "long long", "unsigned long long".
5414 or "quads" (typically 64-bit integers)
5416 The last will produce errors if Perl does not understand "quads" in your
5417 installation. (This requires that either the platform natively supports quads
5418 or Perl was specifically compiled to support quads.) You can find out
5419 whether your Perl supports quads via L<Config>:
5422 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5425 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5426 to be the default floating point size on your platform (double or long double),
5427 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5428 platform supports them. You can find out whether your Perl supports long
5429 doubles via L<Config>:
5432 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5434 You can find out whether Perl considers 'long double' to be the default
5435 floating point size to use on your platform via L<Config>:
5438 ($Config{uselongdouble} eq 'define') &&
5439 print "long doubles by default\n";
5441 It can also be the case that long doubles and doubles are the same thing:
5444 ($Config{doublesize} == $Config{longdblsize}) &&
5445 print "doubles are long doubles\n";
5447 The size specifier C<V> has no effect for Perl code, but it is supported
5448 for compatibility with XS code; it means 'use the standard size for
5449 a Perl integer (or floating-point number)', which is already the
5450 default for Perl code.
5452 =item order of arguments
5454 Normally, sprintf takes the next unused argument as the value to
5455 format for each format specification. If the format specification
5456 uses C<*> to require additional arguments, these are consumed from
5457 the argument list in the order in which they appear in the format
5458 specification I<before> the value to format. Where an argument is
5459 specified using an explicit index, this does not affect the normal
5460 order for the arguments (even when the explicitly specified index
5461 would have been the next argument in any case).
5465 printf '<%*.*s>', $a, $b, $c;
5467 would use C<$a> for the width, C<$b> for the precision and C<$c>
5468 as the value to format, while:
5470 print '<%*1$.*s>', $a, $b;
5472 would use C<$a> for the width and the precision, and C<$b> as the
5475 Here are some more examples - beware that when using an explicit
5476 index, the C<$> may need to be escaped:
5478 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5479 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5480 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5481 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5485 If C<use locale> is in effect, the character used for the decimal
5486 point in formatted real numbers is affected by the LC_NUMERIC locale.
5493 Return the square root of EXPR. If EXPR is omitted, returns square
5494 root of C<$_>. Only works on non-negative operands, unless you've
5495 loaded the standard Math::Complex module.
5498 print sqrt(-2); # prints 1.4142135623731i
5504 Sets the random number seed for the C<rand> operator.
5506 The point of the function is to "seed" the C<rand> function so that
5507 C<rand> can produce a different sequence each time you run your
5510 If srand() is not called explicitly, it is called implicitly at the
5511 first use of the C<rand> operator. However, this was not the case in
5512 versions of Perl before 5.004, so if your script will run under older
5513 Perl versions, it should call C<srand>.
5515 Most programs won't even call srand() at all, except those that
5516 need a cryptographically-strong starting point rather than the
5517 generally acceptable default, which is based on time of day,
5518 process ID, and memory allocation, or the F</dev/urandom> device,
5521 You can call srand($seed) with the same $seed to reproduce the
5522 I<same> sequence from rand(), but this is usually reserved for
5523 generating predictable results for testing or debugging.
5524 Otherwise, don't call srand() more than once in your program.
5526 Do B<not> call srand() (i.e. without an argument) more than once in
5527 a script. The internal state of the random number generator should
5528 contain more entropy than can be provided by any seed, so calling
5529 srand() again actually I<loses> randomness.
5531 Most implementations of C<srand> take an integer and will silently
5532 truncate decimal numbers. This means C<srand(42)> will usually
5533 produce the same results as C<srand(42.1)>. To be safe, always pass
5534 C<srand> an integer.
5536 In versions of Perl prior to 5.004 the default seed was just the
5537 current C<time>. This isn't a particularly good seed, so many old
5538 programs supply their own seed value (often C<time ^ $$> or C<time ^
5539 ($$ + ($$ << 15))>), but that isn't necessary any more.
5541 For cryptographic purposes, however, you need something much more random
5542 than the default seed. Checksumming the compressed output of one or more
5543 rapidly changing operating system status programs is the usual method. For
5546 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5548 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5551 Frequently called programs (like CGI scripts) that simply use
5555 for a seed can fall prey to the mathematical property that
5559 one-third of the time. So don't do that.
5561 =item stat FILEHANDLE
5567 Returns a 13-element list giving the status info for a file, either
5568 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5569 it stats C<$_>. Returns a null list if the stat fails. Typically used
5572 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5573 $atime,$mtime,$ctime,$blksize,$blocks)
5576 Not all fields are supported on all filesystem types. Here are the
5577 meanings of the fields:
5579 0 dev device number of filesystem
5581 2 mode file mode (type and permissions)
5582 3 nlink number of (hard) links to the file
5583 4 uid numeric user ID of file's owner
5584 5 gid numeric group ID of file's owner
5585 6 rdev the device identifier (special files only)
5586 7 size total size of file, in bytes
5587 8 atime last access time in seconds since the epoch
5588 9 mtime last modify time in seconds since the epoch
5589 10 ctime inode change time in seconds since the epoch (*)
5590 11 blksize preferred block size for file system I/O
5591 12 blocks actual number of blocks allocated
5593 (The epoch was at 00:00 January 1, 1970 GMT.)
5595 (*) Not all fields are supported on all filesystem types. Notably, the
5596 ctime field is non-portable. In particular, you cannot expect it to be a
5597 "creation time", see L<perlport/"Files and Filesystems"> for details.
5599 If C<stat> is passed the special filehandle consisting of an underline, no
5600 stat is done, but the current contents of the stat structure from the
5601 last C<stat>, C<lstat>, or filetest are returned. Example:
5603 if (-x $file && (($d) = stat(_)) && $d < 0) {
5604 print "$file is executable NFS file\n";
5607 (This works on machines only for which the device number is negative
5610 Because the mode contains both the file type and its permissions, you
5611 should mask off the file type portion and (s)printf using a C<"%o">
5612 if you want to see the real permissions.
5614 $mode = (stat($filename))[2];
5615 printf "Permissions are %04o\n", $mode & 07777;
5617 In scalar context, C<stat> returns a boolean value indicating success
5618 or failure, and, if successful, sets the information associated with
5619 the special filehandle C<_>.
5621 The File::stat module provides a convenient, by-name access mechanism:
5624 $sb = stat($filename);
5625 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5626 $filename, $sb->size, $sb->mode & 07777,
5627 scalar localtime $sb->mtime;
5629 You can import symbolic mode constants (C<S_IF*>) and functions
5630 (C<S_IS*>) from the Fcntl module:
5634 $mode = (stat($filename))[2];
5636 $user_rwx = ($mode & S_IRWXU) >> 6;
5637 $group_read = ($mode & S_IRGRP) >> 3;
5638 $other_execute = $mode & S_IXOTH;
5640 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5642 $is_setuid = $mode & S_ISUID;
5643 $is_setgid = S_ISDIR($mode);
5645 You could write the last two using the C<-u> and C<-d> operators.
5646 The commonly available C<S_IF*> constants are
5648 # Permissions: read, write, execute, for user, group, others.
5650 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5651 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5652 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5654 # Setuid/Setgid/Stickiness/SaveText.
5655 # Note that the exact meaning of these is system dependent.
5657 S_ISUID S_ISGID S_ISVTX S_ISTXT
5659 # File types. Not necessarily all are available on your system.
5661 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5663 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5665 S_IREAD S_IWRITE S_IEXEC
5667 and the C<S_IF*> functions are
5669 S_IMODE($mode) the part of $mode containing the permission bits
5670 and the setuid/setgid/sticky bits
5672 S_IFMT($mode) the part of $mode containing the file type
5673 which can be bit-anded with e.g. S_IFREG
5674 or with the following functions
5676 # The operators -f, -d, -l, -b, -c, -p, and -S.
5678 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5679 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5681 # No direct -X operator counterpart, but for the first one
5682 # the -g operator is often equivalent. The ENFMT stands for
5683 # record flocking enforcement, a platform-dependent feature.
5685 S_ISENFMT($mode) S_ISWHT($mode)
5687 See your native chmod(2) and stat(2) documentation for more details
5688 about the C<S_*> constants. To get status info for a symbolic link
5689 instead of the target file behind the link, use the C<lstat> function.
5695 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5696 doing many pattern matches on the string before it is next modified.
5697 This may or may not save time, depending on the nature and number of
5698 patterns you are searching on, and on the distribution of character
5699 frequencies in the string to be searched--you probably want to compare
5700 run times with and without it to see which runs faster. Those loops
5701 that scan for many short constant strings (including the constant
5702 parts of more complex patterns) will benefit most. You may have only
5703 one C<study> active at a time--if you study a different scalar the first
5704 is "unstudied". (The way C<study> works is this: a linked list of every
5705 character in the string to be searched is made, so we know, for
5706 example, where all the C<'k'> characters are. From each search string,
5707 the rarest character is selected, based on some static frequency tables
5708 constructed from some C programs and English text. Only those places
5709 that contain this "rarest" character are examined.)
5711 For example, here is a loop that inserts index producing entries
5712 before any line containing a certain pattern:
5716 print ".IX foo\n" if /\bfoo\b/;
5717 print ".IX bar\n" if /\bbar\b/;
5718 print ".IX blurfl\n" if /\bblurfl\b/;
5723 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5724 will be looked at, because C<f> is rarer than C<o>. In general, this is
5725 a big win except in pathological cases. The only question is whether
5726 it saves you more time than it took to build the linked list in the
5729 Note that if you have to look for strings that you don't know till
5730 runtime, you can build an entire loop as a string and C<eval> that to
5731 avoid recompiling all your patterns all the time. Together with
5732 undefining C<$/> to input entire files as one record, this can be very
5733 fast, often faster than specialized programs like fgrep(1). The following
5734 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5735 out the names of those files that contain a match:
5737 $search = 'while (<>) { study;';
5738 foreach $word (@words) {
5739 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5744 eval $search; # this screams
5745 $/ = "\n"; # put back to normal input delimiter
5746 foreach $file (sort keys(%seen)) {
5750 =item sub NAME BLOCK
5752 =item sub NAME (PROTO) BLOCK
5754 =item sub NAME : ATTRS BLOCK
5756 =item sub NAME (PROTO) : ATTRS BLOCK
5758 This is subroutine definition, not a real function I<per se>.
5759 Without a BLOCK it's just a forward declaration. Without a NAME,
5760 it's an anonymous function declaration, and does actually return
5761 a value: the CODE ref of the closure you just created.
5763 See L<perlsub> and L<perlref> for details about subroutines and
5764 references, and L<attributes> and L<Attribute::Handlers> for more
5765 information about attributes.
5767 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5769 =item substr EXPR,OFFSET,LENGTH
5771 =item substr EXPR,OFFSET
5773 Extracts a substring out of EXPR and returns it. First character is at
5774 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5775 If OFFSET is negative (or more precisely, less than C<$[>), starts
5776 that far from the end of the string. If LENGTH is omitted, returns
5777 everything to the end of the string. If LENGTH is negative, leaves that
5778 many characters off the end of the string.
5780 You can use the substr() function as an lvalue, in which case EXPR
5781 must itself be an lvalue. If you assign something shorter than LENGTH,
5782 the string will shrink, and if you assign something longer than LENGTH,
5783 the string will grow to accommodate it. To keep the string the same
5784 length you may need to pad or chop your value using C<sprintf>.
5786 If OFFSET and LENGTH specify a substring that is partly outside the
5787 string, only the part within the string is returned. If the substring
5788 is beyond either end of the string, substr() returns the undefined
5789 value and produces a warning. When used as an lvalue, specifying a
5790 substring that is entirely outside the string is a fatal error.
5791 Here's an example showing the behavior for boundary cases:
5794 substr($name, 4) = 'dy'; # $name is now 'freddy'
5795 my $null = substr $name, 6, 2; # returns '' (no warning)
5796 my $oops = substr $name, 7; # returns undef, with warning
5797 substr($name, 7) = 'gap'; # fatal error
5799 An alternative to using substr() as an lvalue is to specify the
5800 replacement string as the 4th argument. This allows you to replace
5801 parts of the EXPR and return what was there before in one operation,
5802 just as you can with splice().
5804 Note that the lvalue returned by the 3-arg version of substr() acts as
5805 a 'magic bullet'; each time it is assigned to, it remembers which part
5806 of the original string is being modified; for example:
5809 for (substr($x,1,2)) {
5810 $_ = 'a'; print $x,"\n"; # prints 1a4
5811 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
5813 $_ = 'pq'; print $x,"\n"; # prints 5pq9
5817 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
5820 =item symlink OLDFILE,NEWFILE
5822 Creates a new filename symbolically linked to the old filename.
5823 Returns C<1> for success, C<0> otherwise. On systems that don't support
5824 symbolic links, produces a fatal error at run time. To check for that,
5827 $symlink_exists = eval { symlink("",""); 1 };
5829 =item syscall NUMBER, LIST
5831 Calls the system call specified as the first element of the list,
5832 passing the remaining elements as arguments to the system call. If
5833 unimplemented, produces a fatal error. The arguments are interpreted
5834 as follows: if a given argument is numeric, the argument is passed as
5835 an int. If not, the pointer to the string value is passed. You are
5836 responsible to make sure a string is pre-extended long enough to
5837 receive any result that might be written into a string. You can't use a
5838 string literal (or other read-only string) as an argument to C<syscall>
5839 because Perl has to assume that any string pointer might be written
5841 integer arguments are not literals and have never been interpreted in a
5842 numeric context, you may need to add C<0> to them to force them to look
5843 like numbers. This emulates the C<syswrite> function (or vice versa):
5845 require 'syscall.ph'; # may need to run h2ph
5847 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5849 Note that Perl supports passing of up to only 14 arguments to your system call,
5850 which in practice should usually suffice.
5852 Syscall returns whatever value returned by the system call it calls.
5853 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5854 Note that some system calls can legitimately return C<-1>. The proper
5855 way to handle such calls is to assign C<$!=0;> before the call and
5856 check the value of C<$!> if syscall returns C<-1>.
5858 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5859 number of the read end of the pipe it creates. There is no way
5860 to retrieve the file number of the other end. You can avoid this
5861 problem by using C<pipe> instead.
5863 =item sysopen FILEHANDLE,FILENAME,MODE
5865 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5867 Opens the file whose filename is given by FILENAME, and associates it
5868 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5869 the name of the real filehandle wanted. This function calls the
5870 underlying operating system's C<open> function with the parameters
5871 FILENAME, MODE, PERMS.
5873 The possible values and flag bits of the MODE parameter are
5874 system-dependent; they are available via the standard module C<Fcntl>.
5875 See the documentation of your operating system's C<open> to see which
5876 values and flag bits are available. You may combine several flags
5877 using the C<|>-operator.
5879 Some of the most common values are C<O_RDONLY> for opening the file in
5880 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5881 and C<O_RDWR> for opening the file in read-write mode.
5883 For historical reasons, some values work on almost every system
5884 supported by perl: zero means read-only, one means write-only, and two
5885 means read/write. We know that these values do I<not> work under
5886 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5887 use them in new code.
5889 If the file named by FILENAME does not exist and the C<open> call creates
5890 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5891 PERMS specifies the permissions of the newly created file. If you omit
5892 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5893 These permission values need to be in octal, and are modified by your
5894 process's current C<umask>.
5896 In many systems the C<O_EXCL> flag is available for opening files in
5897 exclusive mode. This is B<not> locking: exclusiveness means here that
5898 if the file already exists, sysopen() fails. C<O_EXCL> may not work
5899 on network filesystems, and has no effect unless the C<O_CREAT> flag
5900 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
5901 being opened if it is a symbolic link. It does not protect against
5902 symbolic links in the file's path.
5904 Sometimes you may want to truncate an already-existing file. This
5905 can be done using the C<O_TRUNC> flag. The behavior of
5906 C<O_TRUNC> with C<O_RDONLY> is undefined.
5908 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5909 that takes away the user's option to have a more permissive umask.
5910 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5913 Note that C<sysopen> depends on the fdopen() C library function.
5914 On many UNIX systems, fdopen() is known to fail when file descriptors
5915 exceed a certain value, typically 255. If you need more file
5916 descriptors than that, consider rebuilding Perl to use the C<sfio>
5917 library, or perhaps using the POSIX::open() function.
5919 See L<perlopentut> for a kinder, gentler explanation of opening files.
5921 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5923 =item sysread FILEHANDLE,SCALAR,LENGTH
5925 Attempts to read LENGTH bytes of data into variable SCALAR from the
5926 specified FILEHANDLE, using the system call read(2). It bypasses
5927 buffered IO, so mixing this with other kinds of reads, C<print>,
5928 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5929 perlio or stdio layers usually buffers data. Returns the number of
5930 bytes actually read, C<0> at end of file, or undef if there was an
5931 error (in the latter case C<$!> is also set). SCALAR will be grown or
5932 shrunk so that the last byte actually read is the last byte of the
5933 scalar after the read.
5935 An OFFSET may be specified to place the read data at some place in the
5936 string other than the beginning. A negative OFFSET specifies
5937 placement at that many characters counting backwards from the end of
5938 the string. A positive OFFSET greater than the length of SCALAR
5939 results in the string being padded to the required size with C<"\0">
5940 bytes before the result of the read is appended.
5942 There is no syseof() function, which is ok, since eof() doesn't work
5943 very well on device files (like ttys) anyway. Use sysread() and check
5944 for a return value for 0 to decide whether you're done.
5946 Note that if the filehandle has been marked as C<:utf8> Unicode
5947 characters are read instead of bytes (the LENGTH, OFFSET, and the
5948 return value of sysread() are in Unicode characters).
5949 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5950 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5952 =item sysseek FILEHANDLE,POSITION,WHENCE
5954 Sets FILEHANDLE's system position in bytes using the system call
5955 lseek(2). FILEHANDLE may be an expression whose value gives the name
5956 of the filehandle. The values for WHENCE are C<0> to set the new
5957 position to POSITION, C<1> to set the it to the current position plus
5958 POSITION, and C<2> to set it to EOF plus POSITION (typically
5961 Note the I<in bytes>: even if the filehandle has been set to operate
5962 on characters (for example by using the C<:utf8> I/O layer), tell()
5963 will return byte offsets, not character offsets (because implementing
5964 that would render sysseek() very slow).
5966 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5967 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
5968 C<seek>, C<tell>, or C<eof> may cause confusion.
5970 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5971 and C<SEEK_END> (start of the file, current position, end of the file)
5972 from the Fcntl module. Use of the constants is also more portable
5973 than relying on 0, 1, and 2. For example to define a "systell" function:
5975 use Fcntl 'SEEK_CUR';
5976 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5978 Returns the new position, or the undefined value on failure. A position
5979 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5980 true on success and false on failure, yet you can still easily determine
5985 =item system PROGRAM LIST
5987 Does exactly the same thing as C<exec LIST>, except that a fork is
5988 done first, and the parent process waits for the child process to
5989 complete. Note that argument processing varies depending on the
5990 number of arguments. If there is more than one argument in LIST,
5991 or if LIST is an array with more than one value, starts the program
5992 given by the first element of the list with arguments given by the
5993 rest of the list. If there is only one scalar argument, the argument
5994 is checked for shell metacharacters, and if there are any, the
5995 entire argument is passed to the system's command shell for parsing
5996 (this is C</bin/sh -c> on Unix platforms, but varies on other
5997 platforms). If there are no shell metacharacters in the argument,
5998 it is split into words and passed directly to C<execvp>, which is
6001 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6002 output before any operation that may do a fork, but this may not be
6003 supported on some platforms (see L<perlport>). To be safe, you may need
6004 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6005 of C<IO::Handle> on any open handles.
6007 The return value is the exit status of the program as returned by the
6008 C<wait> call. To get the actual exit value, shift right by eight (see below).
6009 See also L</exec>. This is I<not> what you want to use to capture
6010 the output from a command, for that you should use merely backticks or
6011 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6012 indicates a failure to start the program (inspect $! for the reason).
6014 Like C<exec>, C<system> allows you to lie to a program about its name if
6015 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6017 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6018 C<system>, if you expect your program to terminate on receipt of these
6019 signals you will need to arrange to do so yourself based on the return
6022 @args = ("command", "arg1", "arg2");
6024 or die "system @args failed: $?"
6026 You can check all the failure possibilities by inspecting
6030 print "failed to execute: $!\n";
6033 printf "child died with signal %d, %s coredump\n",
6034 ($? & 127), ($? & 128) ? 'with' : 'without';
6037 printf "child exited with value %d\n", $? >> 8;
6040 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6041 with the W*() calls of the POSIX extension.
6043 When the arguments get executed via the system shell, results
6044 and return codes will be subject to its quirks and capabilities.
6045 See L<perlop/"`STRING`"> and L</exec> for details.
6047 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6049 =item syswrite FILEHANDLE,SCALAR,LENGTH
6051 =item syswrite FILEHANDLE,SCALAR
6053 Attempts to write LENGTH bytes of data from variable SCALAR to the
6054 specified FILEHANDLE, using the system call write(2). If LENGTH is
6055 not specified, writes whole SCALAR. It bypasses buffered IO, so
6056 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6057 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6058 stdio layers usually buffers data. Returns the number of bytes
6059 actually written, or C<undef> if there was an error (in this case the
6060 errno variable C<$!> is also set). If the LENGTH is greater than the
6061 available data in the SCALAR after the OFFSET, only as much data as is
6062 available will be written.
6064 An OFFSET may be specified to write the data from some part of the
6065 string other than the beginning. A negative OFFSET specifies writing
6066 that many characters counting backwards from the end of the string.
6067 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6069 Note that if the filehandle has been marked as C<:utf8>, Unicode
6070 characters are written instead of bytes (the LENGTH, OFFSET, and the
6071 return value of syswrite() are in UTF-8 encoded Unicode characters).
6072 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6073 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6075 =item tell FILEHANDLE
6079 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6080 error. FILEHANDLE may be an expression whose value gives the name of
6081 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6084 Note the I<in bytes>: even if the filehandle has been set to
6085 operate on characters (for example by using the C<:utf8> open
6086 layer), tell() will return byte offsets, not character offsets
6087 (because that would render seek() and tell() rather slow).
6089 The return value of tell() for the standard streams like the STDIN
6090 depends on the operating system: it may return -1 or something else.
6091 tell() on pipes, fifos, and sockets usually returns -1.
6093 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6095 Do not use tell() (or other buffered I/O operations) on a file handle
6096 that has been manipulated by sysread(), syswrite() or sysseek().
6097 Those functions ignore the buffering, while tell() does not.
6099 =item telldir DIRHANDLE
6101 Returns the current position of the C<readdir> routines on DIRHANDLE.
6102 Value may be given to C<seekdir> to access a particular location in a
6103 directory. C<telldir> has the same caveats about possible directory
6104 compaction as the corresponding system library routine.
6106 =item tie VARIABLE,CLASSNAME,LIST
6108 This function binds a variable to a package class that will provide the
6109 implementation for the variable. VARIABLE is the name of the variable
6110 to be enchanted. CLASSNAME is the name of a class implementing objects
6111 of correct type. Any additional arguments are passed to the C<new>
6112 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6113 or C<TIEHASH>). Typically these are arguments such as might be passed
6114 to the C<dbm_open()> function of C. The object returned by the C<new>
6115 method is also returned by the C<tie> function, which would be useful
6116 if you want to access other methods in CLASSNAME.
6118 Note that functions such as C<keys> and C<values> may return huge lists
6119 when used on large objects, like DBM files. You may prefer to use the
6120 C<each> function to iterate over such. Example:
6122 # print out history file offsets
6124 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6125 while (($key,$val) = each %HIST) {
6126 print $key, ' = ', unpack('L',$val), "\n";
6130 A class implementing a hash should have the following methods:
6132 TIEHASH classname, LIST
6134 STORE this, key, value
6139 NEXTKEY this, lastkey
6144 A class implementing an ordinary array should have the following methods:
6146 TIEARRAY classname, LIST
6148 STORE this, key, value
6150 STORESIZE this, count
6156 SPLICE this, offset, length, LIST
6161 A class implementing a file handle should have the following methods:
6163 TIEHANDLE classname, LIST
6164 READ this, scalar, length, offset
6167 WRITE this, scalar, length, offset
6169 PRINTF this, format, LIST
6173 SEEK this, position, whence
6175 OPEN this, mode, LIST
6180 A class implementing a scalar should have the following methods:
6182 TIESCALAR classname, LIST
6188 Not all methods indicated above need be implemented. See L<perltie>,
6189 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6191 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6192 for you--you need to do that explicitly yourself. See L<DB_File>
6193 or the F<Config> module for interesting C<tie> implementations.
6195 For further details see L<perltie>, L<"tied VARIABLE">.
6199 Returns a reference to the object underlying VARIABLE (the same value
6200 that was originally returned by the C<tie> call that bound the variable
6201 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6206 Returns the number of non-leap seconds since whatever time the system
6207 considers to be the epoch, suitable for feeding to C<gmtime> and
6208 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6209 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6210 1904 in the current local time zone for its epoch.
6212 For measuring time in better granularity than one second,
6213 you may use either the Time::HiRes module (from CPAN, and starting from
6214 Perl 5.8 part of the standard distribution), or if you have
6215 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6216 See L<perlfaq8> for details.
6220 Returns a four-element list giving the user and system times, in
6221 seconds, for this process and the children of this process.
6223 ($user,$system,$cuser,$csystem) = times;
6225 In scalar context, C<times> returns C<$user>.
6229 The transliteration operator. Same as C<y///>. See L<perlop>.
6231 =item truncate FILEHANDLE,LENGTH
6233 =item truncate EXPR,LENGTH
6235 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6236 specified length. Produces a fatal error if truncate isn't implemented
6237 on your system. Returns true if successful, the undefined value
6240 The behavior is undefined if LENGTH is greater than the length of the
6247 Returns an uppercased version of EXPR. This is the internal function
6248 implementing the C<\U> escape in double-quoted strings. Respects
6249 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6250 and L<perlunicode> for more details about locale and Unicode support.
6251 It does not attempt to do titlecase mapping on initial letters. See
6252 C<ucfirst> for that.
6254 If EXPR is omitted, uses C<$_>.
6260 Returns the value of EXPR with the first character in uppercase
6261 (titlecase in Unicode). This is the internal function implementing
6262 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6263 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6264 for more details about locale and Unicode support.
6266 If EXPR is omitted, uses C<$_>.
6272 Sets the umask for the process to EXPR and returns the previous value.
6273 If EXPR is omitted, merely returns the current umask.
6275 The Unix permission C<rwxr-x---> is represented as three sets of three
6276 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6277 and isn't one of the digits). The C<umask> value is such a number
6278 representing disabled permissions bits. The permission (or "mode")
6279 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6280 even if you tell C<sysopen> to create a file with permissions C<0777>,
6281 if your umask is C<0022> then the file will actually be created with
6282 permissions C<0755>. If your C<umask> were C<0027> (group can't
6283 write; others can't read, write, or execute), then passing
6284 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6287 Here's some advice: supply a creation mode of C<0666> for regular
6288 files (in C<sysopen>) and one of C<0777> for directories (in
6289 C<mkdir>) and executable files. This gives users the freedom of
6290 choice: if they want protected files, they might choose process umasks
6291 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6292 Programs should rarely if ever make policy decisions better left to
6293 the user. The exception to this is when writing files that should be
6294 kept private: mail files, web browser cookies, I<.rhosts> files, and
6297 If umask(2) is not implemented on your system and you are trying to
6298 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6299 fatal error at run time. If umask(2) is not implemented and you are
6300 not trying to restrict access for yourself, returns C<undef>.
6302 Remember that a umask is a number, usually given in octal; it is I<not> a
6303 string of octal digits. See also L</oct>, if all you have is a string.
6309 Undefines the value of EXPR, which must be an lvalue. Use only on a
6310 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6311 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6312 will probably not do what you expect on most predefined variables or
6313 DBM list values, so don't do that; see L<delete>.) Always returns the
6314 undefined value. You can omit the EXPR, in which case nothing is
6315 undefined, but you still get an undefined value that you could, for
6316 instance, return from a subroutine, assign to a variable or pass as a
6317 parameter. Examples:
6320 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6324 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6325 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6326 select undef, undef, undef, 0.25;
6327 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6329 Note that this is a unary operator, not a list operator.
6335 Deletes a list of files. Returns the number of files successfully
6338 $cnt = unlink 'a', 'b', 'c';
6342 Note: C<unlink> will not delete directories unless you are superuser and
6343 the B<-U> flag is supplied to Perl. Even if these conditions are
6344 met, be warned that unlinking a directory can inflict damage on your
6345 filesystem. Use C<rmdir> instead.
6347 If LIST is omitted, uses C<$_>.
6349 =item unpack TEMPLATE,EXPR
6351 =item unpack TEMPLATE
6353 C<unpack> does the reverse of C<pack>: it takes a string
6354 and expands it out into a list of values.
6355 (In scalar context, it returns merely the first value produced.)
6357 If EXPR is omitted, unpacks the C<$_> string.
6359 The string is broken into chunks described by the TEMPLATE. Each chunk
6360 is converted separately to a value. Typically, either the string is a result
6361 of C<pack>, or the characters of the string represent a C structure of some
6364 The TEMPLATE has the same format as in the C<pack> function.
6365 Here's a subroutine that does substring:
6368 my($what,$where,$howmuch) = @_;
6369 unpack("x$where a$howmuch", $what);
6374 sub ordinal { unpack("W",$_[0]); } # same as ord()
6376 In addition to fields allowed in pack(), you may prefix a field with
6377 a %<number> to indicate that
6378 you want a <number>-bit checksum of the items instead of the items
6379 themselves. Default is a 16-bit checksum. Checksum is calculated by
6380 summing numeric values of expanded values (for string fields the sum of
6381 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6383 For example, the following
6384 computes the same number as the System V sum program:
6388 unpack("%32W*",<>) % 65535;
6391 The following efficiently counts the number of set bits in a bit vector:
6393 $setbits = unpack("%32b*", $selectmask);
6395 The C<p> and C<P> formats should be used with care. Since Perl
6396 has no way of checking whether the value passed to C<unpack()>
6397 corresponds to a valid memory location, passing a pointer value that's
6398 not known to be valid is likely to have disastrous consequences.
6400 If there are more pack codes or if the repeat count of a field or a group
6401 is larger than what the remainder of the input string allows, the result
6402 is not well defined: in some cases, the repeat count is decreased, or
6403 C<unpack()> will produce null strings or zeroes, or terminate with an
6404 error. If the input string is longer than one described by the TEMPLATE,
6405 the rest is ignored.
6407 See L</pack> for more examples and notes.
6409 =item untie VARIABLE
6411 Breaks the binding between a variable and a package. (See C<tie>.)
6412 Has no effect if the variable is not tied.
6414 =item unshift ARRAY,LIST
6416 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6417 depending on how you look at it. Prepends list to the front of the
6418 array, and returns the new number of elements in the array.
6420 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6422 Note the LIST is prepended whole, not one element at a time, so the
6423 prepended elements stay in the same order. Use C<reverse> to do the
6426 =item use Module VERSION LIST
6428 =item use Module VERSION
6430 =item use Module LIST
6436 Imports some semantics into the current package from the named module,
6437 generally by aliasing certain subroutine or variable names into your
6438 package. It is exactly equivalent to
6440 BEGIN { require Module; import Module LIST; }
6442 except that Module I<must> be a bareword.
6444 VERSION may be either a numeric argument such as 5.006, which will be
6445 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6446 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6447 greater than the version of the current Perl interpreter; Perl will not
6448 attempt to parse the rest of the file. Compare with L</require>, which can
6449 do a similar check at run time.
6451 Specifying VERSION as a literal of the form v5.6.1 should generally be
6452 avoided, because it leads to misleading error messages under earlier
6453 versions of Perl that do not support this syntax. The equivalent numeric
6454 version should be used instead.
6456 use v5.6.1; # compile time version check
6458 use 5.006_001; # ditto; preferred for backwards compatibility
6460 This is often useful if you need to check the current Perl version before
6461 C<use>ing library modules that have changed in incompatible ways from
6462 older versions of Perl. (We try not to do this more than we have to.)
6464 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6465 C<require> makes sure the module is loaded into memory if it hasn't been
6466 yet. The C<import> is not a builtin--it's just an ordinary static method
6467 call into the C<Module> package to tell the module to import the list of
6468 features back into the current package. The module can implement its
6469 C<import> method any way it likes, though most modules just choose to
6470 derive their C<import> method via inheritance from the C<Exporter> class that
6471 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6472 method can be found then the call is skipped, even if there is an AUTOLOAD
6475 If you do not want to call the package's C<import> method (for instance,
6476 to stop your namespace from being altered), explicitly supply the empty list:
6480 That is exactly equivalent to
6482 BEGIN { require Module }
6484 If the VERSION argument is present between Module and LIST, then the
6485 C<use> will call the VERSION method in class Module with the given
6486 version as an argument. The default VERSION method, inherited from
6487 the UNIVERSAL class, croaks if the given version is larger than the
6488 value of the variable C<$Module::VERSION>.
6490 Again, there is a distinction between omitting LIST (C<import> called
6491 with no arguments) and an explicit empty LIST C<()> (C<import> not
6492 called). Note that there is no comma after VERSION!
6494 Because this is a wide-open interface, pragmas (compiler directives)
6495 are also implemented this way. Currently implemented pragmas are:
6500 use sigtrap qw(SEGV BUS);
6501 use strict qw(subs vars refs);
6502 use subs qw(afunc blurfl);
6503 use warnings qw(all);
6504 use sort qw(stable _quicksort _mergesort);
6506 Some of these pseudo-modules import semantics into the current
6507 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6508 which import symbols into the current package (which are effective
6509 through the end of the file).
6511 There's a corresponding C<no> command that unimports meanings imported
6512 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6513 It behaves exactly as C<import> does with respect to VERSION, an
6514 omitted LIST, empty LIST, or no unimport method being found.
6520 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6521 for the C<-M> and C<-m> command-line options to perl that give C<use>
6522 functionality from the command-line.
6526 Changes the access and modification times on each file of a list of
6527 files. The first two elements of the list must be the NUMERICAL access
6528 and modification times, in that order. Returns the number of files
6529 successfully changed. The inode change time of each file is set
6530 to the current time. For example, this code has the same effect as the
6531 Unix touch(1) command when the files I<already exist> and belong to
6532 the user running the program:
6535 $atime = $mtime = time;
6536 utime $atime, $mtime, @ARGV;
6538 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6539 the utime(2) function in the C library will be called with a null second
6540 argument. On most systems, this will set the file's access and
6541 modification times to the current time (i.e. equivalent to the example
6542 above) and will even work on other users' files where you have write
6545 utime undef, undef, @ARGV;
6547 Under NFS this will use the time of the NFS server, not the time of
6548 the local machine. If there is a time synchronization problem, the
6549 NFS server and local machine will have different times. The Unix
6550 touch(1) command will in fact normally use this form instead of the
6551 one shown in the first example.
6553 Note that only passing one of the first two elements as C<undef> will
6554 be equivalent of passing it as 0 and will not have the same effect as
6555 described when they are both C<undef>. This case will also trigger an
6556 uninitialized warning.
6560 Returns a list consisting of all the values of the named hash.
6561 (In a scalar context, returns the number of values.)
6563 The values are returned in an apparently random order. The actual
6564 random order is subject to change in future versions of perl, but it
6565 is guaranteed to be the same order as either the C<keys> or C<each>
6566 function would produce on the same (unmodified) hash. Since Perl
6567 5.8.1 the ordering is different even between different runs of Perl
6568 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6570 As a side effect, calling values() resets the HASH's internal iterator,
6571 see L</each>. (In particular, calling values() in void context resets
6572 the iterator with no other overhead.)
6574 Note that the values are not copied, which means modifying them will
6575 modify the contents of the hash:
6577 for (values %hash) { s/foo/bar/g } # modifies %hash values
6578 for (@hash{keys %hash}) { s/foo/bar/g } # same
6580 See also C<keys>, C<each>, and C<sort>.
6582 =item vec EXPR,OFFSET,BITS
6584 Treats the string in EXPR as a bit vector made up of elements of
6585 width BITS, and returns the value of the element specified by OFFSET
6586 as an unsigned integer. BITS therefore specifies the number of bits
6587 that are reserved for each element in the bit vector. This must
6588 be a power of two from 1 to 32 (or 64, if your platform supports
6591 If BITS is 8, "elements" coincide with bytes of the input string.
6593 If BITS is 16 or more, bytes of the input string are grouped into chunks
6594 of size BITS/8, and each group is converted to a number as with
6595 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6596 for BITS==64). See L<"pack"> for details.
6598 If bits is 4 or less, the string is broken into bytes, then the bits
6599 of each byte are broken into 8/BITS groups. Bits of a byte are
6600 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6601 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6602 breaking the single input byte C<chr(0x36)> into two groups gives a list
6603 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6605 C<vec> may also be assigned to, in which case parentheses are needed
6606 to give the expression the correct precedence as in
6608 vec($image, $max_x * $x + $y, 8) = 3;
6610 If the selected element is outside the string, the value 0 is returned.
6611 If an element off the end of the string is written to, Perl will first
6612 extend the string with sufficiently many zero bytes. It is an error
6613 to try to write off the beginning of the string (i.e. negative OFFSET).
6615 The string should not contain any character with the value > 255 (which
6616 can only happen if you're using UTF-8 encoding). If it does, it will be
6617 treated as something that is not UTF-8 encoded. When the C<vec> was
6618 assigned to, other parts of your program will also no longer consider the
6619 string to be UTF-8 encoded. In other words, if you do have such characters
6620 in your string, vec() will operate on the actual byte string, and not the
6621 conceptual character string.
6623 Strings created with C<vec> can also be manipulated with the logical
6624 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6625 vector operation is desired when both operands are strings.
6626 See L<perlop/"Bitwise String Operators">.
6628 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6629 The comments show the string after each step. Note that this code works
6630 in the same way on big-endian or little-endian machines.
6633 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6635 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6636 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6638 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6639 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6640 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6641 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6642 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6643 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6645 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6646 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6647 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6650 To transform a bit vector into a string or list of 0's and 1's, use these:
6652 $bits = unpack("b*", $vector);
6653 @bits = split(//, unpack("b*", $vector));
6655 If you know the exact length in bits, it can be used in place of the C<*>.
6657 Here is an example to illustrate how the bits actually fall in place:
6663 unpack("V",$_) 01234567890123456789012345678901
6664 ------------------------------------------------------------------
6669 for ($shift=0; $shift < $width; ++$shift) {
6670 for ($off=0; $off < 32/$width; ++$off) {
6671 $str = pack("B*", "0"x32);
6672 $bits = (1<<$shift);
6673 vec($str, $off, $width) = $bits;
6674 $res = unpack("b*",$str);
6675 $val = unpack("V", $str);
6682 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6683 $off, $width, $bits, $val, $res
6687 Regardless of the machine architecture on which it is run, the above
6688 example should print the following table:
6691 unpack("V",$_) 01234567890123456789012345678901
6692 ------------------------------------------------------------------
6693 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6694 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6695 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6696 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6697 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6698 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6699 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6700 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6701 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6702 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6703 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6704 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6705 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6706 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6707 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6708 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6709 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6710 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6711 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6712 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6713 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6714 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6715 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6716 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6717 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6718 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6719 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6720 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6721 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6722 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6723 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6724 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6725 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6726 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6727 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6728 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6729 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6730 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6731 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6732 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6733 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6734 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6735 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6736 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6737 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6738 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6739 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6740 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6741 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6742 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6743 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6744 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6745 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6746 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6747 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6748 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6749 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6750 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6751 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6752 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6753 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6754 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6755 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6756 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6757 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6758 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6759 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6760 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6761 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6762 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6763 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6764 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6765 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6766 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6767 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6768 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6769 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6770 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6771 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6772 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6773 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6774 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6775 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6776 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6777 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6778 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6779 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6780 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6781 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6782 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6783 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6784 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6785 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6786 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6787 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6788 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6789 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6790 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6791 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6792 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6793 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6794 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6795 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6796 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6797 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6798 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6799 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6800 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6801 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6802 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6803 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6804 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6805 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6806 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6807 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6808 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6809 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6810 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6811 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6812 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6813 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6814 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6815 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6816 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6817 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6818 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6819 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6820 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6824 Behaves like the wait(2) system call on your system: it waits for a child
6825 process to terminate and returns the pid of the deceased process, or
6826 C<-1> if there are no child processes. The status is returned in C<$?>
6827 and C<{^CHILD_ERROR_NATIVE}>.
6828 Note that a return value of C<-1> could mean that child processes are
6829 being automatically reaped, as described in L<perlipc>.
6831 =item waitpid PID,FLAGS
6833 Waits for a particular child process to terminate and returns the pid of
6834 the deceased process, or C<-1> if there is no such child process. On some
6835 systems, a value of 0 indicates that there are processes still running.
6836 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
6838 use POSIX ":sys_wait_h";
6841 $kid = waitpid(-1, WNOHANG);
6844 then you can do a non-blocking wait for all pending zombie processes.
6845 Non-blocking wait is available on machines supporting either the
6846 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6847 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6848 system call by remembering the status values of processes that have
6849 exited but have not been harvested by the Perl script yet.)
6851 Note that on some systems, a return value of C<-1> could mean that child
6852 processes are being automatically reaped. See L<perlipc> for details,
6853 and for other examples.
6857 Returns true if the context of the currently executing subroutine or
6858 C<eval> is looking for a list value. Returns false if the context is
6859 looking for a scalar. Returns the undefined value if the context is
6860 looking for no value (void context).
6862 return unless defined wantarray; # don't bother doing more
6863 my @a = complex_calculation();
6864 return wantarray ? @a : "@a";
6866 C<wantarray()>'s result is unspecified in the top level of a file,
6867 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
6870 This function should have been named wantlist() instead.
6874 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6877 If LIST is empty and C<$@> already contains a value (typically from a
6878 previous eval) that value is used after appending C<"\t...caught">
6879 to C<$@>. This is useful for staying almost, but not entirely similar to
6882 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6884 No message is printed if there is a C<$SIG{__WARN__}> handler
6885 installed. It is the handler's responsibility to deal with the message
6886 as it sees fit (like, for instance, converting it into a C<die>). Most
6887 handlers must therefore make arrangements to actually display the
6888 warnings that they are not prepared to deal with, by calling C<warn>
6889 again in the handler. Note that this is quite safe and will not
6890 produce an endless loop, since C<__WARN__> hooks are not called from
6893 You will find this behavior is slightly different from that of
6894 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6895 instead call C<die> again to change it).
6897 Using a C<__WARN__> handler provides a powerful way to silence all
6898 warnings (even the so-called mandatory ones). An example:
6900 # wipe out *all* compile-time warnings
6901 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6903 my $foo = 20; # no warning about duplicate my $foo,
6904 # but hey, you asked for it!
6905 # no compile-time or run-time warnings before here
6908 # run-time warnings enabled after here
6909 warn "\$foo is alive and $foo!"; # does show up
6911 See L<perlvar> for details on setting C<%SIG> entries, and for more
6912 examples. See the Carp module for other kinds of warnings using its
6913 carp() and cluck() functions.
6915 =item write FILEHANDLE
6921 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6922 using the format associated with that file. By default the format for
6923 a file is the one having the same name as the filehandle, but the
6924 format for the current output channel (see the C<select> function) may be set
6925 explicitly by assigning the name of the format to the C<$~> variable.
6927 Top of form processing is handled automatically: if there is
6928 insufficient room on the current page for the formatted record, the
6929 page is advanced by writing a form feed, a special top-of-page format
6930 is used to format the new page header, and then the record is written.
6931 By default the top-of-page format is the name of the filehandle with
6932 "_TOP" appended, but it may be dynamically set to the format of your
6933 choice by assigning the name to the C<$^> variable while the filehandle is
6934 selected. The number of lines remaining on the current page is in
6935 variable C<$->, which can be set to C<0> to force a new page.
6937 If FILEHANDLE is unspecified, output goes to the current default output
6938 channel, which starts out as STDOUT but may be changed by the
6939 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6940 is evaluated and the resulting string is used to look up the name of
6941 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6943 Note that write is I<not> the opposite of C<read>. Unfortunately.
6947 The transliteration operator. Same as C<tr///>. See L<perlop>.