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/$^S>). Because
1226 this promotes strange action at a distance, this counterintuitive
1227 behavior may be fixed in a future release.
1231 Not really a function. Returns the value of the last command in the
1232 sequence of commands indicated by BLOCK. When modified by the C<while> or
1233 C<until> loop modifier, executes the BLOCK once before testing the loop
1234 condition. (On other statements the loop modifiers test the conditional
1237 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1238 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1239 See L<perlsyn> for alternative strategies.
1241 =item do SUBROUTINE(LIST)
1243 This form of subroutine call is deprecated. See L<perlsub>.
1247 Uses the value of EXPR as a filename and executes the contents of the
1248 file as a Perl script.
1256 except that it's more efficient and concise, keeps track of the current
1257 filename for error messages, searches the @INC directories, and updates
1258 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1259 variables. It also differs in that code evaluated with C<do FILENAME>
1260 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1261 same, however, in that it does reparse the file every time you call it,
1262 so you probably don't want to do this inside a loop.
1264 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1265 error. If C<do> can read the file but cannot compile it, it
1266 returns undef and sets an error message in C<$@>. If the file is
1267 successfully compiled, C<do> returns the value of the last expression
1270 Note that inclusion of library modules is better done with the
1271 C<use> and C<require> operators, which also do automatic error checking
1272 and raise an exception if there's a problem.
1274 You might like to use C<do> to read in a program configuration
1275 file. Manual error checking can be done this way:
1277 # read in config files: system first, then user
1278 for $file ("/share/prog/defaults.rc",
1279 "$ENV{HOME}/.someprogrc")
1281 unless ($return = do $file) {
1282 warn "couldn't parse $file: $@" if $@;
1283 warn "couldn't do $file: $!" unless defined $return;
1284 warn "couldn't run $file" unless $return;
1292 This function causes an immediate core dump. See also the B<-u>
1293 command-line switch in L<perlrun>, which does the same thing.
1294 Primarily this is so that you can use the B<undump> program (not
1295 supplied) to turn your core dump into an executable binary after
1296 having initialized all your variables at the beginning of the
1297 program. When the new binary is executed it will begin by executing
1298 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1299 Think of it as a goto with an intervening core dump and reincarnation.
1300 If C<LABEL> is omitted, restarts the program from the top.
1302 B<WARNING>: Any files opened at the time of the dump will I<not>
1303 be open any more when the program is reincarnated, with possible
1304 resulting confusion on the part of Perl.
1306 This function is now largely obsolete, partly because it's very
1307 hard to convert a core file into an executable, and because the
1308 real compiler backends for generating portable bytecode and compilable
1309 C code have superseded it. That's why you should now invoke it as
1310 C<CORE::dump()>, if you don't want to be warned against a possible
1313 If you're looking to use L<dump> to speed up your program, consider
1314 generating bytecode or native C code as described in L<perlcc>. If
1315 you're just trying to accelerate a CGI script, consider using the
1316 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1317 You might also consider autoloading or selfloading, which at least
1318 make your program I<appear> to run faster.
1322 When called in list context, returns a 2-element list consisting of the
1323 key and value for the next element of a hash, so that you can iterate over
1324 it. When called in scalar context, returns only the key for the next
1325 element in the hash.
1327 Entries are returned in an apparently random order. The actual random
1328 order is subject to change in future versions of perl, but it is
1329 guaranteed to be in the same order as either the C<keys> or C<values>
1330 function would produce on the same (unmodified) hash. Since Perl
1331 5.8.1 the ordering is different even between different runs of Perl
1332 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1334 When the hash is entirely read, a null array is returned in list context
1335 (which when assigned produces a false (C<0>) value), and C<undef> in
1336 scalar context. The next call to C<each> after that will start iterating
1337 again. There is a single iterator for each hash, shared by all C<each>,
1338 C<keys>, and C<values> function calls in the program; it can be reset by
1339 reading all the elements from the hash, or by evaluating C<keys HASH> or
1340 C<values HASH>. If you add or delete elements of a hash while you're
1341 iterating over it, you may get entries skipped or duplicated, so
1342 don't. Exception: It is always safe to delete the item most recently
1343 returned by C<each()>, which means that the following code will work:
1345 while (($key, $value) = each %hash) {
1347 delete $hash{$key}; # This is safe
1350 The following prints out your environment like the printenv(1) program,
1351 only in a different order:
1353 while (($key,$value) = each %ENV) {
1354 print "$key=$value\n";
1357 See also C<keys>, C<values> and C<sort>.
1359 =item eof FILEHANDLE
1365 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1366 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1367 gives the real filehandle. (Note that this function actually
1368 reads a character and then C<ungetc>s it, so isn't very useful in an
1369 interactive context.) Do not read from a terminal file (or call
1370 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1371 as terminals may lose the end-of-file condition if you do.
1373 An C<eof> without an argument uses the last file read. Using C<eof()>
1374 with empty parentheses is very different. It refers to the pseudo file
1375 formed from the files listed on the command line and accessed via the
1376 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1377 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1378 used will cause C<@ARGV> to be examined to determine if input is
1379 available. Similarly, an C<eof()> after C<< <> >> has returned
1380 end-of-file will assume you are processing another C<@ARGV> list,
1381 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1382 see L<perlop/"I/O Operators">.
1384 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1385 detect the end of each file, C<eof()> will only detect the end of the
1386 last file. Examples:
1388 # reset line numbering on each input file
1390 next if /^\s*#/; # skip comments
1393 close ARGV if eof; # Not eof()!
1396 # insert dashes just before last line of last file
1398 if (eof()) { # check for end of last file
1399 print "--------------\n";
1402 last if eof(); # needed if we're reading from a terminal
1405 Practical hint: you almost never need to use C<eof> in Perl, because the
1406 input operators typically return C<undef> when they run out of data, or if
1415 In the first form, the return value of EXPR is parsed and executed as if it
1416 were a little Perl program. The value of the expression (which is itself
1417 determined within scalar context) is first parsed, and if there weren't any
1418 errors, executed in the lexical context of the current Perl program, so
1419 that any variable settings or subroutine and format definitions remain
1420 afterwards. Note that the value is parsed every time the C<eval> executes.
1421 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1422 delay parsing and subsequent execution of the text of EXPR until run time.
1424 In the second form, the code within the BLOCK is parsed only once--at the
1425 same time the code surrounding the C<eval> itself was parsed--and executed
1426 within the context of the current Perl program. This form is typically
1427 used to trap exceptions more efficiently than the first (see below), while
1428 also providing the benefit of checking the code within BLOCK at compile
1431 The final semicolon, if any, may be omitted from the value of EXPR or within
1434 In both forms, the value returned is the value of the last expression
1435 evaluated inside the mini-program; a return statement may be also used, just
1436 as with subroutines. The expression providing the return value is evaluated
1437 in void, scalar, or list context, depending on the context of the C<eval>
1438 itself. See L</wantarray> for more on how the evaluation context can be
1441 If there is a syntax error or runtime error, or a C<die> statement is
1442 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1443 error message. If there was no error, C<$@> is guaranteed to be a null
1444 string. Beware that using C<eval> neither silences perl from printing
1445 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1446 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1447 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1448 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1450 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1451 determining whether a particular feature (such as C<socket> or C<symlink>)
1452 is implemented. It is also Perl's exception trapping mechanism, where
1453 the die operator is used to raise exceptions.
1455 If the code to be executed doesn't vary, you may use the eval-BLOCK
1456 form to trap run-time errors without incurring the penalty of
1457 recompiling each time. The error, if any, is still returned in C<$@>.
1460 # make divide-by-zero nonfatal
1461 eval { $answer = $a / $b; }; warn $@ if $@;
1463 # same thing, but less efficient
1464 eval '$answer = $a / $b'; warn $@ if $@;
1466 # a compile-time error
1467 eval { $answer = }; # WRONG
1470 eval '$answer ='; # sets $@
1472 Using the C<eval{}> form as an exception trap in libraries does have some
1473 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1474 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1475 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1476 as shown in this example:
1478 # a very private exception trap for divide-by-zero
1479 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1482 This is especially significant, given that C<__DIE__> hooks can call
1483 C<die> again, which has the effect of changing their error messages:
1485 # __DIE__ hooks may modify error messages
1487 local $SIG{'__DIE__'} =
1488 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1489 eval { die "foo lives here" };
1490 print $@ if $@; # prints "bar lives here"
1493 Because this promotes action at a distance, this counterintuitive behavior
1494 may be fixed in a future release.
1496 With an C<eval>, you should be especially careful to remember what's
1497 being looked at when:
1503 eval { $x }; # CASE 4
1505 eval "\$$x++"; # CASE 5
1508 Cases 1 and 2 above behave identically: they run the code contained in
1509 the variable $x. (Although case 2 has misleading double quotes making
1510 the reader wonder what else might be happening (nothing is).) Cases 3
1511 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1512 does nothing but return the value of $x. (Case 4 is preferred for
1513 purely visual reasons, but it also has the advantage of compiling at
1514 compile-time instead of at run-time.) Case 5 is a place where
1515 normally you I<would> like to use double quotes, except that in this
1516 particular situation, you can just use symbolic references instead, as
1519 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1520 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1522 Note that as a very special case, an C<eval ''> executed within the C<DB>
1523 package doesn't see the usual surrounding lexical scope, but rather the
1524 scope of the first non-DB piece of code that called it. You don't normally
1525 need to worry about this unless you are writing a Perl debugger.
1529 =item exec PROGRAM LIST
1531 The C<exec> function executes a system command I<and never returns>--
1532 use C<system> instead of C<exec> if you want it to return. It fails and
1533 returns false only if the command does not exist I<and> it is executed
1534 directly instead of via your system's command shell (see below).
1536 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1537 warns you if there is a following statement which isn't C<die>, C<warn>,
1538 or C<exit> (if C<-w> is set - but you always do that). If you
1539 I<really> want to follow an C<exec> with some other statement, you
1540 can use one of these styles to avoid the warning:
1542 exec ('foo') or print STDERR "couldn't exec foo: $!";
1543 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1545 If there is more than one argument in LIST, or if LIST is an array
1546 with more than one value, calls execvp(3) with the arguments in LIST.
1547 If there is only one scalar argument or an array with one element in it,
1548 the argument is checked for shell metacharacters, and if there are any,
1549 the entire argument is passed to the system's command shell for parsing
1550 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1551 If there are no shell metacharacters in the argument, it is split into
1552 words and passed directly to C<execvp>, which is more efficient.
1555 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1556 exec "sort $outfile | uniq";
1558 If you don't really want to execute the first argument, but want to lie
1559 to the program you are executing about its own name, you can specify
1560 the program you actually want to run as an "indirect object" (without a
1561 comma) in front of the LIST. (This always forces interpretation of the
1562 LIST as a multivalued list, even if there is only a single scalar in
1565 $shell = '/bin/csh';
1566 exec $shell '-sh'; # pretend it's a login shell
1570 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1572 When the arguments get executed via the system shell, results will
1573 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1576 Using an indirect object with C<exec> or C<system> is also more
1577 secure. This usage (which also works fine with system()) forces
1578 interpretation of the arguments as a multivalued list, even if the
1579 list had just one argument. That way you're safe from the shell
1580 expanding wildcards or splitting up words with whitespace in them.
1582 @args = ( "echo surprise" );
1584 exec @args; # subject to shell escapes
1586 exec { $args[0] } @args; # safe even with one-arg list
1588 The first version, the one without the indirect object, ran the I<echo>
1589 program, passing it C<"surprise"> an argument. The second version
1590 didn't--it tried to run a program literally called I<"echo surprise">,
1591 didn't find it, and set C<$?> to a non-zero value indicating failure.
1593 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1594 output before the exec, but this may not be supported on some platforms
1595 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1596 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1597 open handles in order to avoid lost output.
1599 Note that C<exec> will not call your C<END> blocks, nor will it call
1600 any C<DESTROY> methods in your objects.
1604 Given an expression that specifies a hash element or array element,
1605 returns true if the specified element in the hash or array has ever
1606 been initialized, even if the corresponding value is undefined. The
1607 element is not autovivified if it doesn't exist.
1609 print "Exists\n" if exists $hash{$key};
1610 print "Defined\n" if defined $hash{$key};
1611 print "True\n" if $hash{$key};
1613 print "Exists\n" if exists $array[$index];
1614 print "Defined\n" if defined $array[$index];
1615 print "True\n" if $array[$index];
1617 A hash or array element can be true only if it's defined, and defined if
1618 it exists, but the reverse doesn't necessarily hold true.
1620 Given an expression that specifies the name of a subroutine,
1621 returns true if the specified subroutine has ever been declared, even
1622 if it is undefined. Mentioning a subroutine name for exists or defined
1623 does not count as declaring it. Note that a subroutine which does not
1624 exist may still be callable: its package may have an C<AUTOLOAD>
1625 method that makes it spring into existence the first time that it is
1626 called -- see L<perlsub>.
1628 print "Exists\n" if exists &subroutine;
1629 print "Defined\n" if defined &subroutine;
1631 Note that the EXPR can be arbitrarily complicated as long as the final
1632 operation is a hash or array key lookup or subroutine name:
1634 if (exists $ref->{A}->{B}->{$key}) { }
1635 if (exists $hash{A}{B}{$key}) { }
1637 if (exists $ref->{A}->{B}->[$ix]) { }
1638 if (exists $hash{A}{B}[$ix]) { }
1640 if (exists &{$ref->{A}{B}{$key}}) { }
1642 Although the deepest nested array or hash will not spring into existence
1643 just because its existence was tested, any intervening ones will.
1644 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1645 into existence due to the existence test for the $key element above.
1646 This happens anywhere the arrow operator is used, including even:
1649 if (exists $ref->{"Some key"}) { }
1650 print $ref; # prints HASH(0x80d3d5c)
1652 This surprising autovivification in what does not at first--or even
1653 second--glance appear to be an lvalue context may be fixed in a future
1656 Use of a subroutine call, rather than a subroutine name, as an argument
1657 to exists() is an error.
1660 exists &sub(); # Error
1666 Evaluates EXPR and exits immediately with that value. Example:
1669 exit 0 if $ans =~ /^[Xx]/;
1671 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1672 universally recognized values for EXPR are C<0> for success and C<1>
1673 for error; other values are subject to interpretation depending on the
1674 environment in which the Perl program is running. For example, exiting
1675 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1676 the mailer to return the item undelivered, but that's not true everywhere.
1678 Don't use C<exit> to abort a subroutine if there's any chance that
1679 someone might want to trap whatever error happened. Use C<die> instead,
1680 which can be trapped by an C<eval>.
1682 The exit() function does not always exit immediately. It calls any
1683 defined C<END> routines first, but these C<END> routines may not
1684 themselves abort the exit. Likewise any object destructors that need to
1685 be called are called before the real exit. If this is a problem, you
1686 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1687 See L<perlmod> for details.
1693 Returns I<e> (the natural logarithm base) to the power of EXPR.
1694 If EXPR is omitted, gives C<exp($_)>.
1696 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1698 Implements the fcntl(2) function. You'll probably have to say
1702 first to get the correct constant definitions. Argument processing and
1703 value return works just like C<ioctl> below.
1707 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1708 or die "can't fcntl F_GETFL: $!";
1710 You don't have to check for C<defined> on the return from C<fcntl>.
1711 Like C<ioctl>, it maps a C<0> return from the system call into
1712 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1713 in numeric context. It is also exempt from the normal B<-w> warnings
1714 on improper numeric conversions.
1716 Note that C<fcntl> will produce a fatal error if used on a machine that
1717 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1718 manpage to learn what functions are available on your system.
1720 Here's an example of setting a filehandle named C<REMOTE> to be
1721 non-blocking at the system level. You'll have to negotiate C<$|>
1722 on your own, though.
1724 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1726 $flags = fcntl(REMOTE, F_GETFL, 0)
1727 or die "Can't get flags for the socket: $!\n";
1729 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1730 or die "Can't set flags for the socket: $!\n";
1732 =item fileno FILEHANDLE
1734 Returns the file descriptor for a filehandle, or undefined if the
1735 filehandle is not open. This is mainly useful for constructing
1736 bitmaps for C<select> and low-level POSIX tty-handling operations.
1737 If FILEHANDLE is an expression, the value is taken as an indirect
1738 filehandle, generally its name.
1740 You can use this to find out whether two handles refer to the
1741 same underlying descriptor:
1743 if (fileno(THIS) == fileno(THAT)) {
1744 print "THIS and THAT are dups\n";
1747 (Filehandles connected to memory objects via new features of C<open> may
1748 return undefined even though they are open.)
1751 =item flock FILEHANDLE,OPERATION
1753 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1754 for success, false on failure. Produces a fatal error if used on a
1755 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1756 C<flock> is Perl's portable file locking interface, although it locks
1757 only entire files, not records.
1759 Two potentially non-obvious but traditional C<flock> semantics are
1760 that it waits indefinitely until the lock is granted, and that its locks
1761 B<merely advisory>. Such discretionary locks are more flexible, but offer
1762 fewer guarantees. This means that programs that do not also use C<flock>
1763 may modify files locked with C<flock>. See L<perlport>,
1764 your port's specific documentation, or your system-specific local manpages
1765 for details. It's best to assume traditional behavior if you're writing
1766 portable programs. (But if you're not, you should as always feel perfectly
1767 free to write for your own system's idiosyncrasies (sometimes called
1768 "features"). Slavish adherence to portability concerns shouldn't get
1769 in the way of your getting your job done.)
1771 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1772 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1773 you can use the symbolic names if you import them from the Fcntl module,
1774 either individually, or as a group using the ':flock' tag. LOCK_SH
1775 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1776 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1777 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1778 waiting for the lock (check the return status to see if you got it).
1780 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1781 before locking or unlocking it.
1783 Note that the emulation built with lockf(3) doesn't provide shared
1784 locks, and it requires that FILEHANDLE be open with write intent. These
1785 are the semantics that lockf(3) implements. Most if not all systems
1786 implement lockf(3) in terms of fcntl(2) locking, though, so the
1787 differing semantics shouldn't bite too many people.
1789 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1790 be open with read intent to use LOCK_SH and requires that it be open
1791 with write intent to use LOCK_EX.
1793 Note also that some versions of C<flock> cannot lock things over the
1794 network; you would need to use the more system-specific C<fcntl> for
1795 that. If you like you can force Perl to ignore your system's flock(2)
1796 function, and so provide its own fcntl(2)-based emulation, by passing
1797 the switch C<-Ud_flock> to the F<Configure> program when you configure
1800 Here's a mailbox appender for BSD systems.
1802 use Fcntl ':flock'; # import LOCK_* constants
1805 flock(MBOX,LOCK_EX);
1806 # and, in case someone appended
1807 # while we were waiting...
1812 flock(MBOX,LOCK_UN);
1815 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1816 or die "Can't open mailbox: $!";
1819 print MBOX $msg,"\n\n";
1822 On systems that support a real flock(), locks are inherited across fork()
1823 calls, whereas those that must resort to the more capricious fcntl()
1824 function lose the locks, making it harder to write servers.
1826 See also L<DB_File> for other flock() examples.
1830 Does a fork(2) system call to create a new process running the
1831 same program at the same point. It returns the child pid to the
1832 parent process, C<0> to the child process, or C<undef> if the fork is
1833 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1834 are shared, while everything else is copied. On most systems supporting
1835 fork(), great care has gone into making it extremely efficient (for
1836 example, using copy-on-write technology on data pages), making it the
1837 dominant paradigm for multitasking over the last few decades.
1839 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1840 output before forking the child process, but this may not be supported
1841 on some platforms (see L<perlport>). To be safe, you may need to set
1842 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1843 C<IO::Handle> on any open handles in order to avoid duplicate output.
1845 If you C<fork> without ever waiting on your children, you will
1846 accumulate zombies. On some systems, you can avoid this by setting
1847 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1848 forking and reaping moribund children.
1850 Note that if your forked child inherits system file descriptors like
1851 STDIN and STDOUT that are actually connected by a pipe or socket, even
1852 if you exit, then the remote server (such as, say, a CGI script or a
1853 backgrounded job launched from a remote shell) won't think you're done.
1854 You should reopen those to F</dev/null> if it's any issue.
1858 Declare a picture format for use by the C<write> function. For
1862 Test: @<<<<<<<< @||||| @>>>>>
1863 $str, $%, '$' . int($num)
1867 $num = $cost/$quantity;
1871 See L<perlform> for many details and examples.
1873 =item formline PICTURE,LIST
1875 This is an internal function used by C<format>s, though you may call it,
1876 too. It formats (see L<perlform>) a list of values according to the
1877 contents of PICTURE, placing the output into the format output
1878 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1879 Eventually, when a C<write> is done, the contents of
1880 C<$^A> are written to some filehandle. You could also read C<$^A>
1881 and then set C<$^A> back to C<"">. Note that a format typically
1882 does one C<formline> per line of form, but the C<formline> function itself
1883 doesn't care how many newlines are embedded in the PICTURE. This means
1884 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1885 You may therefore need to use multiple formlines to implement a single
1886 record format, just like the format compiler.
1888 Be careful if you put double quotes around the picture, because an C<@>
1889 character may be taken to mean the beginning of an array name.
1890 C<formline> always returns true. See L<perlform> for other examples.
1892 =item getc FILEHANDLE
1896 Returns the next character from the input file attached to FILEHANDLE,
1897 or the undefined value at end of file, or if there was an error (in
1898 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1899 STDIN. This is not particularly efficient. However, it cannot be
1900 used by itself to fetch single characters without waiting for the user
1901 to hit enter. For that, try something more like:
1904 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1907 system "stty", '-icanon', 'eol', "\001";
1913 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1916 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1920 Determination of whether $BSD_STYLE should be set
1921 is left as an exercise to the reader.
1923 The C<POSIX::getattr> function can do this more portably on
1924 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1925 module from your nearest CPAN site; details on CPAN can be found on
1930 This implements the C library function of the same name, which on most
1931 systems returns the current login from F</etc/utmp>, if any. If null,
1934 $login = getlogin || getpwuid($<) || "Kilroy";
1936 Do not consider C<getlogin> for authentication: it is not as
1937 secure as C<getpwuid>.
1939 =item getpeername SOCKET
1941 Returns the packed sockaddr address of other end of the SOCKET connection.
1944 $hersockaddr = getpeername(SOCK);
1945 ($port, $iaddr) = sockaddr_in($hersockaddr);
1946 $herhostname = gethostbyaddr($iaddr, AF_INET);
1947 $herstraddr = inet_ntoa($iaddr);
1951 Returns the current process group for the specified PID. Use
1952 a PID of C<0> to get the current process group for the
1953 current process. Will raise an exception if used on a machine that
1954 doesn't implement getpgrp(2). If PID is omitted, returns process
1955 group of current process. Note that the POSIX version of C<getpgrp>
1956 does not accept a PID argument, so only C<PID==0> is truly portable.
1960 Returns the process id of the parent process.
1962 Note for Linux users: on Linux, the C functions C<getpid()> and
1963 C<getppid()> return different values from different threads. In order to
1964 be portable, this behavior is not reflected by the perl-level function
1965 C<getppid()>, that returns a consistent value across threads. If you want
1966 to call the underlying C<getppid()>, you may use the CPAN module
1969 =item getpriority WHICH,WHO
1971 Returns the current priority for a process, a process group, or a user.
1972 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1973 machine that doesn't implement getpriority(2).
1979 =item gethostbyname NAME
1981 =item getnetbyname NAME
1983 =item getprotobyname NAME
1989 =item getservbyname NAME,PROTO
1991 =item gethostbyaddr ADDR,ADDRTYPE
1993 =item getnetbyaddr ADDR,ADDRTYPE
1995 =item getprotobynumber NUMBER
1997 =item getservbyport PORT,PROTO
2015 =item sethostent STAYOPEN
2017 =item setnetent STAYOPEN
2019 =item setprotoent STAYOPEN
2021 =item setservent STAYOPEN
2035 These routines perform the same functions as their counterparts in the
2036 system library. In list context, the return values from the
2037 various get routines are as follows:
2039 ($name,$passwd,$uid,$gid,
2040 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2041 ($name,$passwd,$gid,$members) = getgr*
2042 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2043 ($name,$aliases,$addrtype,$net) = getnet*
2044 ($name,$aliases,$proto) = getproto*
2045 ($name,$aliases,$port,$proto) = getserv*
2047 (If the entry doesn't exist you get a null list.)
2049 The exact meaning of the $gcos field varies but it usually contains
2050 the real name of the user (as opposed to the login name) and other
2051 information pertaining to the user. Beware, however, that in many
2052 system users are able to change this information and therefore it
2053 cannot be trusted and therefore the $gcos is tainted (see
2054 L<perlsec>). The $passwd and $shell, user's encrypted password and
2055 login shell, are also tainted, because of the same reason.
2057 In scalar context, you get the name, unless the function was a
2058 lookup by name, in which case you get the other thing, whatever it is.
2059 (If the entry doesn't exist you get the undefined value.) For example:
2061 $uid = getpwnam($name);
2062 $name = getpwuid($num);
2064 $gid = getgrnam($name);
2065 $name = getgrgid($num);
2069 In I<getpw*()> the fields $quota, $comment, and $expire are special
2070 cases in the sense that in many systems they are unsupported. If the
2071 $quota is unsupported, it is an empty scalar. If it is supported, it
2072 usually encodes the disk quota. If the $comment field is unsupported,
2073 it is an empty scalar. If it is supported it usually encodes some
2074 administrative comment about the user. In some systems the $quota
2075 field may be $change or $age, fields that have to do with password
2076 aging. In some systems the $comment field may be $class. The $expire
2077 field, if present, encodes the expiration period of the account or the
2078 password. For the availability and the exact meaning of these fields
2079 in your system, please consult your getpwnam(3) documentation and your
2080 F<pwd.h> file. You can also find out from within Perl what your
2081 $quota and $comment fields mean and whether you have the $expire field
2082 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2083 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2084 files are only supported if your vendor has implemented them in the
2085 intuitive fashion that calling the regular C library routines gets the
2086 shadow versions if you're running under privilege or if there exists
2087 the shadow(3) functions as found in System V (this includes Solaris
2088 and Linux.) Those systems that implement a proprietary shadow password
2089 facility are unlikely to be supported.
2091 The $members value returned by I<getgr*()> is a space separated list of
2092 the login names of the members of the group.
2094 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2095 C, it will be returned to you via C<$?> if the function call fails. The
2096 C<@addrs> value returned by a successful call is a list of the raw
2097 addresses returned by the corresponding system library call. In the
2098 Internet domain, each address is four bytes long and you can unpack it
2099 by saying something like:
2101 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2103 The Socket library makes this slightly easier:
2106 $iaddr = inet_aton("127.1"); # or whatever address
2107 $name = gethostbyaddr($iaddr, AF_INET);
2109 # or going the other way
2110 $straddr = inet_ntoa($iaddr);
2112 If you get tired of remembering which element of the return list
2113 contains which return value, by-name interfaces are provided
2114 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2115 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2116 and C<User::grent>. These override the normal built-ins, supplying
2117 versions that return objects with the appropriate names
2118 for each field. For example:
2122 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2124 Even though it looks like they're the same method calls (uid),
2125 they aren't, because a C<File::stat> object is different from
2126 a C<User::pwent> object.
2128 =item getsockname SOCKET
2130 Returns the packed sockaddr address of this end of the SOCKET connection,
2131 in case you don't know the address because you have several different
2132 IPs that the connection might have come in on.
2135 $mysockaddr = getsockname(SOCK);
2136 ($port, $myaddr) = sockaddr_in($mysockaddr);
2137 printf "Connect to %s [%s]\n",
2138 scalar gethostbyaddr($myaddr, AF_INET),
2141 =item getsockopt SOCKET,LEVEL,OPTNAME
2143 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2144 Options may exist at multiple protocol levels depending on the socket
2145 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2146 C<Socket> module) will exist. To query options at another level the
2147 protocol number of the appropriate protocol controlling the option
2148 should be supplied. For example, to indicate that an option is to be
2149 interpreted by the TCP protocol, LEVEL should be set to the protocol
2150 number of TCP, which you can get using getprotobyname.
2152 The call returns a packed string representing the requested socket option,
2153 or C<undef> if there is an error (the error reason will be in $!). What
2154 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2155 your system documentation for details. A very common case however is that
2156 the option is an integer, in which case the result will be a packed
2157 integer which you can decode using unpack with the C<i> (or C<I>) format.
2159 An example testing if Nagle's algorithm is turned on on a socket:
2161 use Socket qw(:all);
2163 defined(my $tcp = getprotobyname("tcp"))
2164 or die "Could not determine the protocol number for tcp";
2165 # my $tcp = IPPROTO_TCP; # Alternative
2166 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2167 or die "Could not query TCP_NODELAY socket option: $!";
2168 my $nodelay = unpack("I", $packed);
2169 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2176 In list context, returns a (possibly empty) list of filename expansions on
2177 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2178 scalar context, glob iterates through such filename expansions, returning
2179 undef when the list is exhausted. This is the internal function
2180 implementing the C<< <*.c> >> operator, but you can use it directly. If
2181 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2182 more detail in L<perlop/"I/O Operators">.
2184 Beginning with v5.6.0, this operator is implemented using the standard
2185 C<File::Glob> extension. See L<File::Glob> for details.
2191 Converts a time as returned by the time function to an 8-element list
2192 with the time localized for the standard Greenwich time zone.
2193 Typically used as follows:
2196 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2199 All list elements are numeric, and come straight out of the C `struct
2200 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2201 specified time. $mday is the day of the month, and $mon is the month
2202 itself, in the range C<0..11> with 0 indicating January and 11
2203 indicating December. $year is the number of years since 1900. That
2204 is, $year is C<123> in year 2023. $wday is the day of the week, with
2205 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2206 the year, in the range C<0..364> (or C<0..365> in leap years.)
2208 Note that the $year element is I<not> simply the last two digits of
2209 the year. If you assume it is then you create non-Y2K-compliant
2210 programs--and you wouldn't want to do that, would you?
2212 The proper way to get a complete 4-digit year is simply:
2216 And to get the last two digits of the year (e.g., '01' in 2001) do:
2218 $year = sprintf("%02d", $year % 100);
2220 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2222 In scalar context, C<gmtime()> returns the ctime(3) value:
2224 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2226 If you need local time instead of GMT use the L</localtime> builtin.
2227 See also the C<timegm> function provided by the C<Time::Local> module,
2228 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2230 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2231 instead a Perl builtin. To get somewhat similar but locale dependent date
2232 strings, see the example in L</localtime>.
2234 See L<perlport/gmtime> for portability concerns.
2242 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2243 execution there. It may not be used to go into any construct that
2244 requires initialization, such as a subroutine or a C<foreach> loop. It
2245 also can't be used to go into a construct that is optimized away,
2246 or to get out of a block or subroutine given to C<sort>.
2247 It can be used to go almost anywhere else within the dynamic scope,
2248 including out of subroutines, but it's usually better to use some other
2249 construct such as C<last> or C<die>. The author of Perl has never felt the
2250 need to use this form of C<goto> (in Perl, that is--C is another matter).
2251 (The difference being that C does not offer named loops combined with
2252 loop control. Perl does, and this replaces most structured uses of C<goto>
2253 in other languages.)
2255 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2256 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2257 necessarily recommended if you're optimizing for maintainability:
2259 goto ("FOO", "BAR", "GLARCH")[$i];
2261 The C<goto-&NAME> form is quite different from the other forms of
2262 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2263 doesn't have the stigma associated with other gotos. Instead, it
2264 exits the current subroutine (losing any changes set by local()) and
2265 immediately calls in its place the named subroutine using the current
2266 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2267 load another subroutine and then pretend that the other subroutine had
2268 been called in the first place (except that any modifications to C<@_>
2269 in the current subroutine are propagated to the other subroutine.)
2270 After the C<goto>, not even C<caller> will be able to tell that this
2271 routine was called first.
2273 NAME needn't be the name of a subroutine; it can be a scalar variable
2274 containing a code reference, or a block that evaluates to a code
2277 =item grep BLOCK LIST
2279 =item grep EXPR,LIST
2281 This is similar in spirit to, but not the same as, grep(1) and its
2282 relatives. In particular, it is not limited to using regular expressions.
2284 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2285 C<$_> to each element) and returns the list value consisting of those
2286 elements for which the expression evaluated to true. In scalar
2287 context, returns the number of times the expression was true.
2289 @foo = grep(!/^#/, @bar); # weed out comments
2293 @foo = grep {!/^#/} @bar; # weed out comments
2295 Note that C<$_> is an alias to the list value, so it can be used to
2296 modify the elements of the LIST. While this is useful and supported,
2297 it can cause bizarre results if the elements of LIST are not variables.
2298 Similarly, grep returns aliases into the original list, much as a for
2299 loop's index variable aliases the list elements. That is, modifying an
2300 element of a list returned by grep (for example, in a C<foreach>, C<map>
2301 or another C<grep>) actually modifies the element in the original list.
2302 This is usually something to be avoided when writing clear code.
2304 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2305 been declared with C<my $_>) then, in addition to being locally aliased to
2306 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2307 can't be seen from the outside, avoiding any potential side-effects.
2309 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2315 Interprets EXPR as a hex string and returns the corresponding value.
2316 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2317 L</oct>.) If EXPR is omitted, uses C<$_>.
2319 print hex '0xAf'; # prints '175'
2320 print hex 'aF'; # same
2322 Hex strings may only represent integers. Strings that would cause
2323 integer overflow trigger a warning. Leading whitespace is not stripped,
2324 unlike oct(). To present something as hex, look into L</printf>,
2325 L</sprintf>, or L</unpack>.
2329 There is no builtin C<import> function. It is just an ordinary
2330 method (subroutine) defined (or inherited) by modules that wish to export
2331 names to another module. The C<use> function calls the C<import> method
2332 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2334 =item index STR,SUBSTR,POSITION
2336 =item index STR,SUBSTR
2338 The index function searches for one string within another, but without
2339 the wildcard-like behavior of a full regular-expression pattern match.
2340 It returns the position of the first occurrence of SUBSTR in STR at
2341 or after POSITION. If POSITION is omitted, starts searching from the
2342 beginning of the string. The return value is based at C<0> (or whatever
2343 you've set the C<$[> variable to--but don't do that). If the substring
2344 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2350 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2351 You should not use this function for rounding: one because it truncates
2352 towards C<0>, and two because machine representations of floating point
2353 numbers can sometimes produce counterintuitive results. For example,
2354 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2355 because it's really more like -268.99999999999994315658 instead. Usually,
2356 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2357 functions will serve you better than will int().
2359 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2361 Implements the ioctl(2) function. You'll probably first have to say
2363 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
2365 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2366 exist or doesn't have the correct definitions you'll have to roll your
2367 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2368 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2369 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2370 written depending on the FUNCTION--a pointer to the string value of SCALAR
2371 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2372 has no string value but does have a numeric value, that value will be
2373 passed rather than a pointer to the string value. To guarantee this to be
2374 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2375 functions may be needed to manipulate the values of structures used by
2378 The return value of C<ioctl> (and C<fcntl>) is as follows:
2380 if OS returns: then Perl returns:
2382 0 string "0 but true"
2383 anything else that number
2385 Thus Perl returns true on success and false on failure, yet you can
2386 still easily determine the actual value returned by the operating
2389 $retval = ioctl(...) || -1;
2390 printf "System returned %d\n", $retval;
2392 The special string C<"0 but true"> is exempt from B<-w> complaints
2393 about improper numeric conversions.
2395 =item join EXPR,LIST
2397 Joins the separate strings of LIST into a single string with fields
2398 separated by the value of EXPR, and returns that new string. Example:
2400 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2402 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2403 first argument. Compare L</split>.
2407 Returns a list consisting of all the keys of the named hash.
2408 (In scalar context, returns the number of keys.)
2410 The keys are returned in an apparently random order. The actual
2411 random order is subject to change in future versions of perl, but it
2412 is guaranteed to be the same order as either the C<values> or C<each>
2413 function produces (given that the hash has not been modified). Since
2414 Perl 5.8.1 the ordering is different even between different runs of
2415 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2418 As a side effect, calling keys() resets the HASH's internal iterator
2419 (see L</each>). In particular, calling keys() in void context resets
2420 the iterator with no other overhead.
2422 Here is yet another way to print your environment:
2425 @values = values %ENV;
2427 print pop(@keys), '=', pop(@values), "\n";
2430 or how about sorted by key:
2432 foreach $key (sort(keys %ENV)) {
2433 print $key, '=', $ENV{$key}, "\n";
2436 The returned values are copies of the original keys in the hash, so
2437 modifying them will not affect the original hash. Compare L</values>.
2439 To sort a hash by value, you'll need to use a C<sort> function.
2440 Here's a descending numeric sort of a hash by its values:
2442 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2443 printf "%4d %s\n", $hash{$key}, $key;
2446 As an lvalue C<keys> allows you to increase the number of hash buckets
2447 allocated for the given hash. This can gain you a measure of efficiency if
2448 you know the hash is going to get big. (This is similar to pre-extending
2449 an array by assigning a larger number to $#array.) If you say
2453 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2454 in fact, since it rounds up to the next power of two. These
2455 buckets will be retained even if you do C<%hash = ()>, use C<undef
2456 %hash> if you want to free the storage while C<%hash> is still in scope.
2457 You can't shrink the number of buckets allocated for the hash using
2458 C<keys> in this way (but you needn't worry about doing this by accident,
2459 as trying has no effect).
2461 See also C<each>, C<values> and C<sort>.
2463 =item kill SIGNAL, LIST
2465 Sends a signal to a list of processes. Returns the number of
2466 processes successfully signaled (which is not necessarily the
2467 same as the number actually killed).
2469 $cnt = kill 1, $child1, $child2;
2472 If SIGNAL is zero, no signal is sent to the process. This is a
2473 useful way to check that a child process is alive and hasn't changed
2474 its UID. See L<perlport> for notes on the portability of this
2477 Unlike in the shell, if SIGNAL is negative, it kills
2478 process groups instead of processes. (On System V, a negative I<PROCESS>
2479 number will also kill process groups, but that's not portable.) That
2480 means you usually want to use positive not negative signals. You may also
2481 use a signal name in quotes.
2483 See L<perlipc/"Signals"> for more details.
2489 The C<last> command is like the C<break> statement in C (as used in
2490 loops); it immediately exits the loop in question. If the LABEL is
2491 omitted, the command refers to the innermost enclosing loop. The
2492 C<continue> block, if any, is not executed:
2494 LINE: while (<STDIN>) {
2495 last LINE if /^$/; # exit when done with header
2499 C<last> cannot be used to exit a block which returns a value such as
2500 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2501 a grep() or map() operation.
2503 Note that a block by itself is semantically identical to a loop
2504 that executes once. Thus C<last> can be used to effect an early
2505 exit out of such a block.
2507 See also L</continue> for an illustration of how C<last>, C<next>, and
2514 Returns a lowercased version of EXPR. This is the internal function
2515 implementing the C<\L> escape in double-quoted strings. Respects
2516 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2517 and L<perlunicode> for more details about locale and Unicode support.
2519 If EXPR is omitted, uses C<$_>.
2525 Returns the value of EXPR with the first character lowercased. This
2526 is the internal function implementing the C<\l> escape in
2527 double-quoted strings. Respects current LC_CTYPE locale if C<use
2528 locale> in force. See L<perllocale> and L<perlunicode> for more
2529 details about locale and Unicode support.
2531 If EXPR is omitted, uses C<$_>.
2537 Returns the length in I<characters> of the value of EXPR. If EXPR is
2538 omitted, returns length of C<$_>. Note that this cannot be used on
2539 an entire array or hash to find out how many elements these have.
2540 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2542 Note the I<characters>: if the EXPR is in Unicode, you will get the
2543 number of characters, not the number of bytes. To get the length
2544 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2546 =item link OLDFILE,NEWFILE
2548 Creates a new filename linked to the old filename. Returns true for
2549 success, false otherwise.
2551 =item listen SOCKET,QUEUESIZE
2553 Does the same thing that the listen system call does. Returns true if
2554 it succeeded, false otherwise. See the example in
2555 L<perlipc/"Sockets: Client/Server Communication">.
2559 You really probably want to be using C<my> instead, because C<local> isn't
2560 what most people think of as "local". See
2561 L<perlsub/"Private Variables via my()"> for details.
2563 A local modifies the listed variables to be local to the enclosing
2564 block, file, or eval. If more than one value is listed, the list must
2565 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2566 for details, including issues with tied arrays and hashes.
2568 =item localtime EXPR
2572 Converts a time as returned by the time function to a 9-element list
2573 with the time analyzed for the local time zone. Typically used as
2577 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2580 All list elements are numeric, and come straight out of the C `struct
2581 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2582 of the specified time.
2584 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2585 the range C<0..11> with 0 indicating January and 11 indicating December.
2586 This makes it easy to get a month name from a list:
2588 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2589 print "$abbr[$mon] $mday";
2590 # $mon=9, $mday=18 gives "Oct 18"
2592 C<$year> is the number of years since 1900, not just the last two digits
2593 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2594 to get a complete 4-digit year is simply:
2598 To get the last two digits of the year (e.g., '01' in 2001) do:
2600 $year = sprintf("%02d", $year % 100);
2602 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2603 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2604 (or C<0..365> in leap years.)
2606 C<$isdst> is true if the specified time occurs during Daylight Saving
2607 Time, false otherwise.
2609 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2611 In scalar context, C<localtime()> returns the ctime(3) value:
2613 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2615 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2616 instead of local time use the L</gmtime> builtin. See also the
2617 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2618 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2619 and mktime(3) functions.
2621 To get somewhat similar but locale dependent date strings, set up your
2622 locale environment variables appropriately (please see L<perllocale>) and
2625 use POSIX qw(strftime);
2626 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2627 # or for GMT formatted appropriately for your locale:
2628 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2630 Note that the C<%a> and C<%b>, the short forms of the day of the week
2631 and the month of the year, may not necessarily be three characters wide.
2633 See L<perlport/localtime> for portability concerns.
2637 This function places an advisory lock on a shared variable, or referenced
2638 object contained in I<THING> until the lock goes out of scope.
2640 lock() is a "weak keyword" : this means that if you've defined a function
2641 by this name (before any calls to it), that function will be called
2642 instead. (However, if you've said C<use threads>, lock() is always a
2643 keyword.) See L<threads>.
2649 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2650 returns log of C<$_>. To get the log of another base, use basic algebra:
2651 The base-N log of a number is equal to the natural log of that number
2652 divided by the natural log of N. For example:
2656 return log($n)/log(10);
2659 See also L</exp> for the inverse operation.
2665 Does the same thing as the C<stat> function (including setting the
2666 special C<_> filehandle) but stats a symbolic link instead of the file
2667 the symbolic link points to. If symbolic links are unimplemented on
2668 your system, a normal C<stat> is done. For much more detailed
2669 information, please see the documentation for C<stat>.
2671 If EXPR is omitted, stats C<$_>.
2675 The match operator. See L<perlop>.
2677 =item map BLOCK LIST
2681 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2682 C<$_> to each element) and returns the list value composed of the
2683 results of each such evaluation. In scalar context, returns the
2684 total number of elements so generated. Evaluates BLOCK or EXPR in
2685 list context, so each element of LIST may produce zero, one, or
2686 more elements in the returned value.
2688 @chars = map(chr, @nums);
2690 translates a list of numbers to the corresponding characters. And
2692 %hash = map { getkey($_) => $_ } @array;
2694 is just a funny way to write
2697 foreach $_ (@array) {
2698 $hash{getkey($_)} = $_;
2701 Note that C<$_> is an alias to the list value, so it can be used to
2702 modify the elements of the LIST. While this is useful and supported,
2703 it can cause bizarre results if the elements of LIST are not variables.
2704 Using a regular C<foreach> loop for this purpose would be clearer in
2705 most cases. See also L</grep> for an array composed of those items of
2706 the original list for which the BLOCK or EXPR evaluates to true.
2708 If C<$_> is lexical in the scope where the C<map> appears (because it has
2709 been declared with C<my $_>) then, in addition to being locally aliased to
2710 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2711 can't be seen from the outside, avoiding any potential side-effects.
2713 C<{> starts both hash references and blocks, so C<map { ...> could be either
2714 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2715 ahead for the closing C<}> it has to take a guess at which its dealing with
2716 based what it finds just after the C<{>. Usually it gets it right, but if it
2717 doesn't it won't realize something is wrong until it gets to the C<}> and
2718 encounters the missing (or unexpected) comma. The syntax error will be
2719 reported close to the C<}> but you'll need to change something near the C<{>
2720 such as using a unary C<+> to give perl some help:
2722 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2723 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2724 %hash = map { ("\L$_", 1) } @array # this also works
2725 %hash = map { lc($_), 1 } @array # as does this.
2726 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2728 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2730 or to force an anon hash constructor use C<+{>
2732 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2734 and you get list of anonymous hashes each with only 1 entry.
2736 =item mkdir FILENAME,MASK
2738 =item mkdir FILENAME
2742 Creates the directory specified by FILENAME, with permissions
2743 specified by MASK (as modified by C<umask>). If it succeeds it
2744 returns true, otherwise it returns false and sets C<$!> (errno).
2745 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2748 In general, it is better to create directories with permissive MASK,
2749 and let the user modify that with their C<umask>, than it is to supply
2750 a restrictive MASK and give the user no way to be more permissive.
2751 The exceptions to this rule are when the file or directory should be
2752 kept private (mail files, for instance). The perlfunc(1) entry on
2753 C<umask> discusses the choice of MASK in more detail.
2755 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2756 number of trailing slashes. Some operating and filesystems do not get
2757 this right, so Perl automatically removes all trailing slashes to keep
2760 =item msgctl ID,CMD,ARG
2762 Calls the System V IPC function msgctl(2). You'll probably have to say
2766 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2767 then ARG must be a variable that will hold the returned C<msqid_ds>
2768 structure. Returns like C<ioctl>: the undefined value for error,
2769 C<"0 but true"> for zero, or the actual return value otherwise. See also
2770 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2772 =item msgget KEY,FLAGS
2774 Calls the System V IPC function msgget(2). Returns the message queue
2775 id, or the undefined value if there is an error. See also
2776 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2778 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2780 Calls the System V IPC function msgrcv to receive a message from
2781 message queue ID into variable VAR with a maximum message size of
2782 SIZE. Note that when a message is received, the message type as a
2783 native long integer will be the first thing in VAR, followed by the
2784 actual message. This packing may be opened with C<unpack("l! a*")>.
2785 Taints the variable. Returns true if successful, or false if there is
2786 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2787 C<IPC::SysV::Msg> documentation.
2789 =item msgsnd ID,MSG,FLAGS
2791 Calls the System V IPC function msgsnd to send the message MSG to the
2792 message queue ID. MSG must begin with the native long integer message
2793 type, and be followed by the length of the actual message, and finally
2794 the message itself. This kind of packing can be achieved with
2795 C<pack("l! a*", $type, $message)>. Returns true if successful,
2796 or false if there is an error. See also C<IPC::SysV>
2797 and C<IPC::SysV::Msg> documentation.
2803 =item my EXPR : ATTRS
2805 =item my TYPE EXPR : ATTRS
2807 A C<my> declares the listed variables to be local (lexically) to the
2808 enclosing block, file, or C<eval>. If more than one value is listed,
2809 the list must be placed in parentheses.
2811 The exact semantics and interface of TYPE and ATTRS are still
2812 evolving. TYPE is currently bound to the use of C<fields> pragma,
2813 and attributes are handled using the C<attributes> pragma, or starting
2814 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2815 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2816 L<attributes>, and L<Attribute::Handlers>.
2822 The C<next> command is like the C<continue> statement in C; it starts
2823 the next iteration of the loop:
2825 LINE: while (<STDIN>) {
2826 next LINE if /^#/; # discard comments
2830 Note that if there were a C<continue> block on the above, it would get
2831 executed even on discarded lines. If the LABEL is omitted, the command
2832 refers to the innermost enclosing loop.
2834 C<next> cannot be used to exit a block which returns a value such as
2835 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2836 a grep() or map() operation.
2838 Note that a block by itself is semantically identical to a loop
2839 that executes once. Thus C<next> will exit such a block early.
2841 See also L</continue> for an illustration of how C<last>, C<next>, and
2844 =item no Module VERSION LIST
2846 =item no Module VERSION
2848 =item no Module LIST
2852 See the C<use> function, of which C<no> is the opposite.
2858 Interprets EXPR as an octal string and returns the corresponding
2859 value. (If EXPR happens to start off with C<0x>, interprets it as a
2860 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2861 binary string. Leading whitespace is ignored in all three cases.)
2862 The following will handle decimal, binary, octal, and hex in the standard
2865 $val = oct($val) if $val =~ /^0/;
2867 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2868 in octal), use sprintf() or printf():
2870 $perms = (stat("filename"))[2] & 07777;
2871 $oct_perms = sprintf "%lo", $perms;
2873 The oct() function is commonly used when a string such as C<644> needs
2874 to be converted into a file mode, for example. (Although perl will
2875 automatically convert strings into numbers as needed, this automatic
2876 conversion assumes base 10.)
2878 =item open FILEHANDLE,EXPR
2880 =item open FILEHANDLE,MODE,EXPR
2882 =item open FILEHANDLE,MODE,EXPR,LIST
2884 =item open FILEHANDLE,MODE,REFERENCE
2886 =item open FILEHANDLE
2888 Opens the file whose filename is given by EXPR, and associates it with
2891 (The following is a comprehensive reference to open(): for a gentler
2892 introduction you may consider L<perlopentut>.)
2894 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2895 the variable is assigned a reference to a new anonymous filehandle,
2896 otherwise if FILEHANDLE is an expression, its value is used as the name of
2897 the real filehandle wanted. (This is considered a symbolic reference, so
2898 C<use strict 'refs'> should I<not> be in effect.)
2900 If EXPR is omitted, the scalar variable of the same name as the
2901 FILEHANDLE contains the filename. (Note that lexical variables--those
2902 declared with C<my>--will not work for this purpose; so if you're
2903 using C<my>, specify EXPR in your call to open.)
2905 If three or more arguments are specified then the mode of opening and
2906 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2907 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2908 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2909 the file is opened for appending, again being created if necessary.
2911 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2912 indicate that you want both read and write access to the file; thus
2913 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2914 '+>' >> mode would clobber the file first. You can't usually use
2915 either read-write mode for updating textfiles, since they have
2916 variable length records. See the B<-i> switch in L<perlrun> for a
2917 better approach. The file is created with permissions of C<0666>
2918 modified by the process' C<umask> value.
2920 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2921 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2923 In the 2-arguments (and 1-argument) form of the call the mode and
2924 filename should be concatenated (in this order), possibly separated by
2925 spaces. It is possible to omit the mode in these forms if the mode is
2928 If the filename begins with C<'|'>, the filename is interpreted as a
2929 command to which output is to be piped, and if the filename ends with a
2930 C<'|'>, the filename is interpreted as a command which pipes output to
2931 us. See L<perlipc/"Using open() for IPC">
2932 for more examples of this. (You are not allowed to C<open> to a command
2933 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2934 and L<perlipc/"Bidirectional Communication with Another Process">
2937 For three or more arguments if MODE is C<'|-'>, the filename is
2938 interpreted as a command to which output is to be piped, and if MODE
2939 is C<'-|'>, the filename is interpreted as a command which pipes
2940 output to us. In the 2-arguments (and 1-argument) form one should
2941 replace dash (C<'-'>) with the command.
2942 See L<perlipc/"Using open() for IPC"> for more examples of this.
2943 (You are not allowed to C<open> to a command that pipes both in I<and>
2944 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2945 L<perlipc/"Bidirectional Communication"> for alternatives.)
2947 In the three-or-more argument form of pipe opens, if LIST is specified
2948 (extra arguments after the command name) then LIST becomes arguments
2949 to the command invoked if the platform supports it. The meaning of
2950 C<open> with more than three arguments for non-pipe modes is not yet
2951 specified. Experimental "layers" may give extra LIST arguments
2954 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2955 and opening C<< '>-' >> opens STDOUT.
2957 You may use the three-argument form of open to specify IO "layers"
2958 (sometimes also referred to as "disciplines") to be applied to the handle
2959 that affect how the input and output are processed (see L<open> and
2960 L<PerlIO> for more details). For example
2962 open(FH, "<:utf8", "file")
2964 will open the UTF-8 encoded file containing Unicode characters,
2965 see L<perluniintro>. (Note that if layers are specified in the
2966 three-arg form then default layers set by the C<open> pragma are
2969 Open returns nonzero upon success, the undefined value otherwise. If
2970 the C<open> involved a pipe, the return value happens to be the pid of
2973 If you're running Perl on a system that distinguishes between text
2974 files and binary files, then you should check out L</binmode> for tips
2975 for dealing with this. The key distinction between systems that need
2976 C<binmode> and those that don't is their text file formats. Systems
2977 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2978 character, and which encode that character in C as C<"\n">, do not
2979 need C<binmode>. The rest need it.
2981 When opening a file, it's usually a bad idea to continue normal execution
2982 if the request failed, so C<open> is frequently used in connection with
2983 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2984 where you want to make a nicely formatted error message (but there are
2985 modules that can help with that problem)) you should always check
2986 the return value from opening a file. The infrequent exception is when
2987 working with an unopened filehandle is actually what you want to do.
2989 As a special case the 3-arg form with a read/write mode and the third
2990 argument being C<undef>:
2992 open(TMP, "+>", undef) or die ...
2994 opens a filehandle to an anonymous temporary file. Also using "+<"
2995 works for symmetry, but you really should consider writing something
2996 to the temporary file first. You will need to seek() to do the
2999 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3000 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3001 "in memory" files held in Perl scalars via:
3003 open($fh, '>', \$variable) || ..
3005 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3006 file, you have to close it first:
3009 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3014 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3015 while (<ARTICLE>) {...
3017 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3018 # if the open fails, output is discarded
3020 open(DBASE, '+<', 'dbase.mine') # open for update
3021 or die "Can't open 'dbase.mine' for update: $!";
3023 open(DBASE, '+<dbase.mine') # ditto
3024 or die "Can't open 'dbase.mine' for update: $!";
3026 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3027 or die "Can't start caesar: $!";
3029 open(ARTICLE, "caesar <$article |") # ditto
3030 or die "Can't start caesar: $!";
3032 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3033 or die "Can't start sort: $!";
3036 open(MEMORY,'>', \$var)
3037 or die "Can't open memory file: $!";
3038 print MEMORY "foo!\n"; # output will end up in $var
3040 # process argument list of files along with any includes
3042 foreach $file (@ARGV) {
3043 process($file, 'fh00');
3047 my($filename, $input) = @_;
3048 $input++; # this is a string increment
3049 unless (open($input, $filename)) {
3050 print STDERR "Can't open $filename: $!\n";
3055 while (<$input>) { # note use of indirection
3056 if (/^#include "(.*)"/) {
3057 process($1, $input);
3064 See L<perliol> for detailed info on PerlIO.
3066 You may also, in the Bourne shell tradition, specify an EXPR beginning
3067 with C<< '>&' >>, in which case the rest of the string is interpreted
3068 as the name of a filehandle (or file descriptor, if numeric) to be
3069 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3070 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3071 The mode you specify should match the mode of the original filehandle.
3072 (Duping a filehandle does not take into account any existing contents
3073 of IO buffers.) If you use the 3-arg form then you can pass either a
3074 number, the name of a filehandle or the normal "reference to a glob".
3076 Here is a script that saves, redirects, and restores C<STDOUT> and
3077 C<STDERR> using various methods:
3080 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3081 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3083 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3084 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3086 select STDERR; $| = 1; # make unbuffered
3087 select STDOUT; $| = 1; # make unbuffered
3089 print STDOUT "stdout 1\n"; # this works for
3090 print STDERR "stderr 1\n"; # subprocesses too
3092 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3093 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3095 print STDOUT "stdout 2\n";
3096 print STDERR "stderr 2\n";
3098 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3099 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3100 that file descriptor (and not call L<dup(2)>); this is more
3101 parsimonious of file descriptors. For example:
3103 # open for input, reusing the fileno of $fd
3104 open(FILEHANDLE, "<&=$fd")
3108 open(FILEHANDLE, "<&=", $fd)
3112 # open for append, using the fileno of OLDFH
3113 open(FH, ">>&=", OLDFH)
3117 open(FH, ">>&=OLDFH")
3119 Being parsimonious on filehandles is also useful (besides being
3120 parsimonious) for example when something is dependent on file
3121 descriptors, like for example locking using flock(). If you do just
3122 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3123 descriptor as B, and therefore flock(A) will not flock(B), and vice
3124 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3125 the same file descriptor.
3127 Note that if you are using Perls older than 5.8.0, Perl will be using
3128 the standard C libraries' fdopen() to implement the "=" functionality.
3129 On many UNIX systems fdopen() fails when file descriptors exceed a
3130 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3131 most often the default.
3133 You can see whether Perl has been compiled with PerlIO or not by
3134 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3135 is C<define>, you have PerlIO, otherwise you don't.
3137 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3138 with 2-arguments (or 1-argument) form of open(), then
3139 there is an implicit fork done, and the return value of open is the pid
3140 of the child within the parent process, and C<0> within the child
3141 process. (Use C<defined($pid)> to determine whether the open was successful.)
3142 The filehandle behaves normally for the parent, but i/o to that
3143 filehandle is piped from/to the STDOUT/STDIN of the child process.
3144 In the child process the filehandle isn't opened--i/o happens from/to
3145 the new STDOUT or STDIN. Typically this is used like the normal
3146 piped open when you want to exercise more control over just how the
3147 pipe command gets executed, such as when you are running setuid, and
3148 don't want to have to scan shell commands for metacharacters.
3149 The following triples are more or less equivalent:
3151 open(FOO, "|tr '[a-z]' '[A-Z]'");
3152 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3153 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3154 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3156 open(FOO, "cat -n '$file'|");
3157 open(FOO, '-|', "cat -n '$file'");
3158 open(FOO, '-|') || exec 'cat', '-n', $file;
3159 open(FOO, '-|', "cat", '-n', $file);
3161 The last example in each block shows the pipe as "list form", which is
3162 not yet supported on all platforms. A good rule of thumb is that if
3163 your platform has true C<fork()> (in other words, if your platform is
3164 UNIX) you can use the list form.
3166 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3168 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3169 output before any operation that may do a fork, but this may not be
3170 supported on some platforms (see L<perlport>). To be safe, you may need
3171 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3172 of C<IO::Handle> on any open handles.
3174 On systems that support a close-on-exec flag on files, the flag will
3175 be set for the newly opened file descriptor as determined by the value
3176 of $^F. See L<perlvar/$^F>.
3178 Closing any piped filehandle causes the parent process to wait for the
3179 child to finish, and returns the status value in C<$?> and
3180 C<${^CHILD_ERROR_NATIVE}>.
3182 The filename passed to 2-argument (or 1-argument) form of open() will
3183 have leading and trailing whitespace deleted, and the normal
3184 redirection characters honored. This property, known as "magic open",
3185 can often be used to good effect. A user could specify a filename of
3186 F<"rsh cat file |">, or you could change certain filenames as needed:
3188 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3189 open(FH, $filename) or die "Can't open $filename: $!";
3191 Use 3-argument form to open a file with arbitrary weird characters in it,
3193 open(FOO, '<', $file);
3195 otherwise it's necessary to protect any leading and trailing whitespace:
3197 $file =~ s#^(\s)#./$1#;
3198 open(FOO, "< $file\0");
3200 (this may not work on some bizarre filesystems). One should
3201 conscientiously choose between the I<magic> and 3-arguments form
3206 will allow the user to specify an argument of the form C<"rsh cat file |">,
3207 but will not work on a filename which happens to have a trailing space, while
3209 open IN, '<', $ARGV[0];
3211 will have exactly the opposite restrictions.
3213 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3214 should use the C<sysopen> function, which involves no such magic (but
3215 may use subtly different filemodes than Perl open(), which is mapped
3216 to C fopen()). This is
3217 another way to protect your filenames from interpretation. For example:
3220 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3221 or die "sysopen $path: $!";
3222 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3223 print HANDLE "stuff $$\n";
3225 print "File contains: ", <HANDLE>;
3227 Using the constructor from the C<IO::Handle> package (or one of its
3228 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3229 filehandles that have the scope of whatever variables hold references to
3230 them, and automatically close whenever and however you leave that scope:
3234 sub read_myfile_munged {
3236 my $handle = new IO::File;
3237 open($handle, "myfile") or die "myfile: $!";
3239 or return (); # Automatically closed here.
3240 mung $first or die "mung failed"; # Or here.
3241 return $first, <$handle> if $ALL; # Or here.
3245 See L</seek> for some details about mixing reading and writing.
3247 =item opendir DIRHANDLE,EXPR
3249 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3250 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3251 DIRHANDLE may be an expression whose value can be used as an indirect
3252 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3253 scalar variable (or array or hash element), the variable is assigned a
3254 reference to a new anonymous dirhandle.
3255 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3261 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3262 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3265 For the reverse, see L</chr>.
3266 See L<perlunicode> and L<encoding> for more about Unicode.
3272 =item our EXPR : ATTRS
3274 =item our TYPE EXPR : ATTRS
3276 C<our> associates a simple name with a package variable in the current
3277 package for use within the current scope. When C<use strict 'vars'> is in
3278 effect, C<our> lets you use declared global variables without qualifying
3279 them with package names, within the lexical scope of the C<our> declaration.
3280 In this way C<our> differs from C<use vars>, which is package scoped.
3282 Unlike C<my>, which both allocates storage for a variable and associates
3283 a simple name with that storage for use within the current scope, C<our>
3284 associates a simple name with a package variable in the current package,
3285 for use within the current scope. In other words, C<our> has the same
3286 scoping rules as C<my>, but does not necessarily create a
3289 If more than one value is listed, the list must be placed
3295 An C<our> declaration declares a global variable that will be visible
3296 across its entire lexical scope, even across package boundaries. The
3297 package in which the variable is entered is determined at the point
3298 of the declaration, not at the point of use. This means the following
3302 our $bar; # declares $Foo::bar for rest of lexical scope
3306 print $bar; # prints 20, as it refers to $Foo::bar
3308 Multiple C<our> declarations with the same name in the same lexical
3309 scope are allowed if they are in different packages. If they happen
3310 to be in the same package, Perl will emit warnings if you have asked
3311 for them, just like multiple C<my> declarations. Unlike a second
3312 C<my> declaration, which will bind the name to a fresh variable, a
3313 second C<our> declaration in the same package, in the same scope, is
3318 our $bar; # declares $Foo::bar for rest of lexical scope
3322 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3323 print $bar; # prints 30
3325 our $bar; # emits warning but has no other effect
3326 print $bar; # still prints 30
3328 An C<our> declaration may also have a list of attributes associated
3331 The exact semantics and interface of TYPE and ATTRS are still
3332 evolving. TYPE is currently bound to the use of C<fields> pragma,
3333 and attributes are handled using the C<attributes> pragma, or starting
3334 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3335 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3336 L<attributes>, and L<Attribute::Handlers>.
3338 The only currently recognized C<our()> attribute is C<unique> which
3339 indicates that a single copy of the global is to be used by all
3340 interpreters should the program happen to be running in a
3341 multi-interpreter environment. (The default behaviour would be for
3342 each interpreter to have its own copy of the global.) Examples:
3344 our @EXPORT : unique = qw(foo);
3345 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3346 our $VERSION : unique = "1.00";
3348 Note that this attribute also has the effect of making the global
3349 readonly when the first new interpreter is cloned (for example,
3350 when the first new thread is created).
3352 Multi-interpreter environments can come to being either through the
3353 fork() emulation on Windows platforms, or by embedding perl in a
3354 multi-threaded application. The C<unique> attribute does nothing in
3355 all other environments.
3357 Warning: the current implementation of this attribute operates on the
3358 typeglob associated with the variable; this means that C<our $x : unique>
3359 also has the effect of C<our @x : unique; our %x : unique>. This may be
3362 =item pack TEMPLATE,LIST
3364 Takes a LIST of values and converts it into a string using the rules
3365 given by the TEMPLATE. The resulting string is the concatenation of
3366 the converted values. Typically, each converted value looks
3367 like its machine-level representation. For example, on 32-bit machines
3368 an integer may be represented by a sequence of 4 bytes that will be
3369 converted to a sequence of 4 characters.
3371 The TEMPLATE is a sequence of characters that give the order and type
3372 of values, as follows:
3374 a A string with arbitrary binary data, will be null padded.
3375 A A text (ASCII) string, will be space padded.
3376 Z A null terminated (ASCIZ) string, will be null padded.
3378 b A bit string (ascending bit order inside each byte, like vec()).
3379 B A bit string (descending bit order inside each byte).
3380 h A hex string (low nybble first).
3381 H A hex string (high nybble first).
3383 c A signed char (8-bit) value.
3384 C An unsigned C char (octet) even under Unicode. Should normally not
3385 be used. See U and W instead.
3386 W An unsigned char value (can be greater than 255).
3388 s A signed short (16-bit) value.
3389 S An unsigned short value.
3391 l A signed long (32-bit) value.
3392 L An unsigned long value.
3394 q A signed quad (64-bit) value.
3395 Q An unsigned quad value.
3396 (Quads are available only if your system supports 64-bit
3397 integer values _and_ if Perl has been compiled to support those.
3398 Causes a fatal error otherwise.)
3400 i A signed integer value.
3401 I A unsigned integer value.
3402 (This 'integer' is _at_least_ 32 bits wide. Its exact
3403 size depends on what a local C compiler calls 'int'.)
3405 n An unsigned short (16-bit) in "network" (big-endian) order.
3406 N An unsigned long (32-bit) in "network" (big-endian) order.
3407 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3408 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3410 j A Perl internal signed integer value (IV).
3411 J A Perl internal unsigned integer value (UV).
3413 f A single-precision float in the native format.
3414 d A double-precision float in the native format.
3416 F A Perl internal floating point value (NV) in the native format
3417 D A long double-precision float in the native format.
3418 (Long doubles are available only if your system supports long
3419 double values _and_ if Perl has been compiled to support those.
3420 Causes a fatal error otherwise.)
3422 p A pointer to a null-terminated string.
3423 P A pointer to a structure (fixed-length string).
3425 u A uuencoded string.
3426 U A Unicode character number. Encodes to UTF-8 internally
3427 (or UTF-EBCDIC in EBCDIC platforms).
3429 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3430 details). Its bytes represent an unsigned integer in base 128,
3431 most significant digit first, with as few digits as possible. Bit
3432 eight (the high bit) is set on each byte except the last.
3436 @ Null fill or truncate to absolute position, counted from the
3437 start of the innermost ()-group.
3438 . Null fill or truncate to absolute position specified by value.
3439 ( Start of a ()-group.
3441 One or more of the modifiers below may optionally follow some letters in the
3442 TEMPLATE (the second column lists the letters for which the modifier is
3445 ! sSlLiI Forces native (short, long, int) sizes instead
3446 of fixed (16-/32-bit) sizes.
3448 xX Make x and X act as alignment commands.
3450 nNvV Treat integers as signed instead of unsigned.
3452 @. Specify position as byte offset in the internal
3453 representation of the packed string. Efficient but
3456 > sSiIlLqQ Force big-endian byte-order on the type.
3457 jJfFdDpP (The "big end" touches the construct.)
3459 < sSiIlLqQ Force little-endian byte-order on the type.
3460 jJfFdDpP (The "little end" touches the construct.)
3462 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3463 in which case they force a certain byte-order on all components of
3464 that group, including subgroups.
3466 The following rules apply:
3472 Each letter may optionally be followed by a number giving a repeat
3473 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3474 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3475 that many values from the LIST. A C<*> for the repeat count means to
3476 use however many items are left, except for C<@>, C<x>, C<X>, where it
3477 is equivalent to C<0>, for <.> where it means relative to string start
3478 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3479 A numeric repeat count may optionally be enclosed in brackets, as in
3480 C<pack 'C[80]', @arr>.
3482 One can replace the numeric repeat count by a template enclosed in brackets;
3483 then the packed length of this template in bytes is used as a count.
3484 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3485 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3486 If the template in brackets contains alignment commands (such as C<x![d]>),
3487 its packed length is calculated as if the start of the template has the maximal
3490 When used with C<Z>, C<*> results in the addition of a trailing null
3491 byte (so the packed result will be one longer than the byte C<length>
3494 When used with C<@>, the repeat count represents an offset from the start
3495 of the innermost () group.
3497 When used with C<.>, the repeat count is used to determine the starting
3498 position from where the value offset is calculated. If the repeat count
3499 is 0, it's relative to the current position. If the repeat count is C<*>,
3500 the offset is relative to the start of the packed string. And if its an
3501 integer C<n> the offset is relative to the start of the n-th innermost
3502 () group (or the start of the string if C<n> is bigger then the group
3505 The repeat count for C<u> is interpreted as the maximal number of bytes
3506 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3507 count should not be more than 65.
3511 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3512 string of length count, padding with nulls or spaces as necessary. When
3513 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3514 after the first null, and C<a> returns data verbatim.
3516 If the value-to-pack is too long, it is truncated. If too long and an
3517 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3518 by a null byte. Thus C<Z> always packs a trailing null (except when the
3523 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3524 Each character of the input field of pack() generates 1 bit of the result.
3525 Each result bit is based on the least-significant bit of the corresponding
3526 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3527 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3529 Starting from the beginning of the input string of pack(), each 8-tuple
3530 of characters is converted to 1 character of output. With format C<b>
3531 the first character of the 8-tuple determines the least-significant bit of a
3532 character, and with format C<B> it determines the most-significant bit of
3535 If the length of the input string is not exactly divisible by 8, the
3536 remainder is packed as if the input string were padded by null characters
3537 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3539 If the input string of pack() is longer than needed, extra characters are
3540 ignored. A C<*> for the repeat count of pack() means to use all the
3541 characters of the input field. On unpack()ing the bits are converted to a
3542 string of C<"0">s and C<"1">s.
3546 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3547 representable as hexadecimal digits, 0-9a-f) long.
3549 Each character of the input field of pack() generates 4 bits of the result.
3550 For non-alphabetical characters the result is based on the 4 least-significant
3551 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3552 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3553 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3554 is compatible with the usual hexadecimal digits, so that C<"a"> and
3555 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3556 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3558 Starting from the beginning of the input string of pack(), each pair
3559 of characters is converted to 1 character of output. With format C<h> the
3560 first character of the pair determines the least-significant nybble of the
3561 output character, and with format C<H> it determines the most-significant
3564 If the length of the input string is not even, it behaves as if padded
3565 by a null character at the end. Similarly, during unpack()ing the "extra"
3566 nybbles are ignored.
3568 If the input string of pack() is longer than needed, extra characters are
3570 A C<*> for the repeat count of pack() means to use all the characters of
3571 the input field. On unpack()ing the nybbles are converted to a string
3572 of hexadecimal digits.
3576 The C<p> type packs a pointer to a null-terminated string. You are
3577 responsible for ensuring the string is not a temporary value (which can
3578 potentially get deallocated before you get around to using the packed result).
3579 The C<P> type packs a pointer to a structure of the size indicated by the
3580 length. A NULL pointer is created if the corresponding value for C<p> or
3581 C<P> is C<undef>, similarly for unpack().
3583 If your system has a strange pointer size (i.e. a pointer is neither as
3584 big as an int nor as big as a long), it may not be possible to pack or
3585 unpack pointers in big- or little-endian byte order. Attempting to do
3586 so will result in a fatal error.
3590 The C</> template character allows packing and unpacking of a sequence of
3591 items where the packed structure contains a packed item count followed by
3592 the packed items themselves.
3593 You write I<length-item>C</>I<sequence-item>.
3595 The I<length-item> can be any C<pack> template letter, and describes
3596 how the length value is packed. The ones likely to be of most use are
3597 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3598 SNMP) and C<N> (for Sun XDR).
3600 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3601 the minimum of that and the number of available items is used as argument
3602 for the I<length-item>. If it has no repeat count or uses a '*', the number
3603 of available items is used. For C<unpack> the repeat count is always obtained
3604 by decoding the packed item count, and the I<sequence-item> must not have a
3607 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3608 the I<length-item> is a string length, not a number of strings. If there is
3609 an explicit repeat count for pack, the packed string will be adjusted to that
3612 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3613 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3614 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3615 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3617 The I<length-item> is not returned explicitly from C<unpack>.
3619 Adding a count to the I<length-item> letter is unlikely to do anything
3620 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3621 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3622 which Perl does not regard as legal in numeric strings.
3626 The integer types C<s>, C<S>, C<l>, and C<L> may be
3627 followed by a C<!> modifier to signify native shorts or
3628 longs--as you can see from above for example a bare C<l> does mean
3629 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3630 may be larger. This is an issue mainly in 64-bit platforms. You can
3631 see whether using C<!> makes any difference by
3633 print length(pack("s")), " ", length(pack("s!")), "\n";
3634 print length(pack("l")), " ", length(pack("l!")), "\n";
3636 C<i!> and C<I!> also work but only because of completeness;
3637 they are identical to C<i> and C<I>.
3639 The actual sizes (in bytes) of native shorts, ints, longs, and long
3640 longs on the platform where Perl was built are also available via
3644 print $Config{shortsize}, "\n";
3645 print $Config{intsize}, "\n";
3646 print $Config{longsize}, "\n";
3647 print $Config{longlongsize}, "\n";
3649 (The C<$Config{longlongsize}> will be undefined if your system does
3650 not support long longs.)
3654 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3655 are inherently non-portable between processors and operating systems
3656 because they obey the native byteorder and endianness. For example a
3657 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3658 (arranged in and handled by the CPU registers) into bytes as
3660 0x12 0x34 0x56 0x78 # big-endian
3661 0x78 0x56 0x34 0x12 # little-endian
3663 Basically, the Intel and VAX CPUs are little-endian, while everybody
3664 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3665 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3666 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3669 The names `big-endian' and `little-endian' are comic references to
3670 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3671 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3672 the egg-eating habits of the Lilliputians.
3674 Some systems may have even weirder byte orders such as
3679 You can see your system's preference with
3681 print join(" ", map { sprintf "%#02x", $_ }
3682 unpack("W*",pack("L",0x12345678))), "\n";
3684 The byteorder on the platform where Perl was built is also available
3688 print $Config{byteorder}, "\n";
3690 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3691 and C<'87654321'> are big-endian.
3693 If you want portable packed integers you can either use the formats
3694 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3695 modifiers. These modifiers are only available as of perl 5.9.2.
3696 See also L<perlport>.
3700 All integer and floating point formats as well as C<p> and C<P> and
3701 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3702 to force big- or little- endian byte-order, respectively.
3703 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3704 signed integers, 64-bit integers and floating point values. However,
3705 there are some things to keep in mind.
3707 Exchanging signed integers between different platforms only works
3708 if all platforms store them in the same format. Most platforms store
3709 signed integers in two's complement, so usually this is not an issue.
3711 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3712 formats on big- or little-endian machines. Otherwise, attempting to
3713 do so will result in a fatal error.
3715 Forcing big- or little-endian byte-order on floating point values for
3716 data exchange can only work if all platforms are using the same
3717 binary representation (e.g. IEEE floating point format). Even if all
3718 platforms are using IEEE, there may be subtle differences. Being able
3719 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3720 but also very dangerous if you don't know exactly what you're doing.
3721 It is definitely not a general way to portably store floating point
3724 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3725 all types inside the group that accept the byte-order modifiers,
3726 including all subgroups. It will silently be ignored for all other
3727 types. You are not allowed to override the byte-order within a group
3728 that already has a byte-order modifier suffix.
3732 Real numbers (floats and doubles) are in the native machine format only;
3733 due to the multiplicity of floating formats around, and the lack of a
3734 standard "network" representation, no facility for interchange has been
3735 made. This means that packed floating point data written on one machine
3736 may not be readable on another - even if both use IEEE floating point
3737 arithmetic (as the endian-ness of the memory representation is not part
3738 of the IEEE spec). See also L<perlport>.
3740 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3741 modifiers to force big- or little-endian byte-order on floating point values.
3743 Note that Perl uses doubles (or long doubles, if configured) internally for
3744 all numeric calculation, and converting from double into float and thence back
3745 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3746 will not in general equal $foo).
3750 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3751 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3752 where the packed string is processed in its UTF-8-encoded Unicode form on
3753 a byte by byte basis. Character mode is the default unless the format string
3754 starts with an C<U>. You can switch mode at any moment with an explicit
3755 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3756 or until the end of the ()-group in which it was entered.
3760 You must yourself do any alignment or padding by inserting for example
3761 enough C<'x'>es while packing. There is no way to pack() and unpack()
3762 could know where the characters are going to or coming from. Therefore
3763 C<pack> (and C<unpack>) handle their output and input as flat
3764 sequences of characters.
3768 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3769 take a repeat count, both as postfix, and for unpack() also via the C</>
3770 template character. Within each repetition of a group, positioning with
3771 C<@> starts again at 0. Therefore, the result of
3773 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3775 is the string "\0a\0\0bc".
3779 C<x> and C<X> accept C<!> modifier. In this case they act as
3780 alignment commands: they jump forward/back to the closest position
3781 aligned at a multiple of C<count> characters. For example, to pack() or
3782 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3783 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3784 aligned on the double's size.
3786 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3787 both result in no-ops.
3791 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3792 will represent signed 16-/32-bit integers in big-/little-endian order.
3793 This is only portable if all platforms sharing the packed data use the
3794 same binary representation for signed integers (e.g. all platforms are
3795 using two's complement representation).
3799 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3800 White space may be used to separate pack codes from each other, but
3801 modifiers and a repeat count must follow immediately.
3805 If TEMPLATE requires more arguments to pack() than actually given, pack()
3806 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3807 to pack() than actually given, extra arguments are ignored.
3813 $foo = pack("WWWW",65,66,67,68);
3815 $foo = pack("W4",65,66,67,68);
3817 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3818 # same thing with Unicode circled letters.
3819 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3820 # same thing with Unicode circled letters. You don't get the UTF-8
3821 # bytes because the U at the start of the format caused a switch to
3822 # U0-mode, so the UTF-8 bytes get joined into characters
3823 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3824 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3825 # This is the UTF-8 encoding of the string in the previous example
3827 $foo = pack("ccxxcc",65,66,67,68);
3830 # note: the above examples featuring "W" and "c" are true
3831 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3832 # and UTF-8. In EBCDIC the first example would be
3833 # $foo = pack("WWWW",193,194,195,196);
3835 $foo = pack("s2",1,2);
3836 # "\1\0\2\0" on little-endian
3837 # "\0\1\0\2" on big-endian
3839 $foo = pack("a4","abcd","x","y","z");
3842 $foo = pack("aaaa","abcd","x","y","z");
3845 $foo = pack("a14","abcdefg");
3846 # "abcdefg\0\0\0\0\0\0\0"
3848 $foo = pack("i9pl", gmtime);
3849 # a real struct tm (on my system anyway)
3851 $utmp_template = "Z8 Z8 Z16 L";
3852 $utmp = pack($utmp_template, @utmp1);
3853 # a struct utmp (BSDish)
3855 @utmp2 = unpack($utmp_template, $utmp);
3856 # "@utmp1" eq "@utmp2"
3859 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3862 $foo = pack('sx2l', 12, 34);
3863 # short 12, two zero bytes padding, long 34
3864 $bar = pack('s@4l', 12, 34);
3865 # short 12, zero fill to position 4, long 34
3867 $baz = pack('s.l', 12, 4, 34);
3868 # short 12, zero fill to position 4, long 34
3870 $foo = pack('nN', 42, 4711);
3871 # pack big-endian 16- and 32-bit unsigned integers
3872 $foo = pack('S>L>', 42, 4711);
3874 $foo = pack('s<l<', -42, 4711);
3875 # pack little-endian 16- and 32-bit signed integers
3876 $foo = pack('(sl)<', -42, 4711);
3879 The same template may generally also be used in unpack().
3881 =item package NAMESPACE
3885 Declares the compilation unit as being in the given namespace. The scope
3886 of the package declaration is from the declaration itself through the end
3887 of the enclosing block, file, or eval (the same as the C<my> operator).
3888 All further unqualified dynamic identifiers will be in this namespace.
3889 A package statement affects only dynamic variables--including those
3890 you've used C<local> on--but I<not> lexical variables, which are created
3891 with C<my>. Typically it would be the first declaration in a file to
3892 be included by the C<require> or C<use> operator. You can switch into a
3893 package in more than one place; it merely influences which symbol table
3894 is used by the compiler for the rest of that block. You can refer to
3895 variables and filehandles in other packages by prefixing the identifier
3896 with the package name and a double colon: C<$Package::Variable>.
3897 If the package name is null, the C<main> package as assumed. That is,
3898 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3899 still seen in older code).
3901 If NAMESPACE is omitted, then there is no current package, and all
3902 identifiers must be fully qualified or lexicals. However, you are
3903 strongly advised not to make use of this feature. Its use can cause
3904 unexpected behaviour, even crashing some versions of Perl. It is
3905 deprecated, and will be removed from a future release.
3907 See L<perlmod/"Packages"> for more information about packages, modules,
3908 and classes. See L<perlsub> for other scoping issues.
3910 =item pipe READHANDLE,WRITEHANDLE
3912 Opens a pair of connected pipes like the corresponding system call.
3913 Note that if you set up a loop of piped processes, deadlock can occur
3914 unless you are very careful. In addition, note that Perl's pipes use
3915 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3916 after each command, depending on the application.
3918 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3919 for examples of such things.
3921 On systems that support a close-on-exec flag on files, the flag will be set
3922 for the newly opened file descriptors as determined by the value of $^F.
3929 Pops and returns the last value of the array, shortening the array by
3930 one element. Has an effect similar to
3934 If there are no elements in the array, returns the undefined value
3935 (although this may happen at other times as well). If ARRAY is
3936 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3937 array in subroutines, just like C<shift>.
3943 Returns the offset of where the last C<m//g> search left off for the variable
3944 in question (C<$_> is used when the variable is not specified). Note that
3945 0 is a valid match offset. C<undef> indicates that the search position
3946 is reset (usually due to match failure, but can also be because no match has
3947 yet been performed on the scalar). C<pos> directly accesses the location used
3948 by the regexp engine to store the offset, so assigning to C<pos> will change
3949 that offset, and so will also influence the C<\G> zero-width assertion in
3950 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
3951 the return from C<pos> won't change either in this case. See L<perlre> and
3954 =item print FILEHANDLE LIST
3960 Prints a string or a list of strings. Returns true if successful.
3961 FILEHANDLE may be a scalar variable name, in which case the variable
3962 contains the name of or a reference to the filehandle, thus introducing
3963 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3964 the next token is a term, it may be misinterpreted as an operator
3965 unless you interpose a C<+> or put parentheses around the arguments.)
3966 If FILEHANDLE is omitted, prints by default to standard output (or
3967 to the last selected output channel--see L</select>). If LIST is
3968 also omitted, prints C<$_> to the currently selected output channel.
3969 To set the default output channel to something other than STDOUT
3970 use the select operation. The current value of C<$,> (if any) is
3971 printed between each LIST item. The current value of C<$\> (if
3972 any) is printed after the entire LIST has been printed. Because
3973 print takes a LIST, anything in the LIST is evaluated in list
3974 context, and any subroutine that you call will have one or more of
3975 its expressions evaluated in list context. Also be careful not to
3976 follow the print keyword with a left parenthesis unless you want
3977 the corresponding right parenthesis to terminate the arguments to
3978 the print--interpose a C<+> or put parentheses around all the
3981 Note that if you're storing FILEHANDLEs in an array, or if you're using
3982 any other expression more complex than a scalar variable to retrieve it,
3983 you will have to use a block returning the filehandle value instead:
3985 print { $files[$i] } "stuff\n";
3986 print { $OK ? STDOUT : STDERR } "stuff\n";
3988 =item printf FILEHANDLE FORMAT, LIST
3990 =item printf FORMAT, LIST
3992 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3993 (the output record separator) is not appended. The first argument
3994 of the list will be interpreted as the C<printf> format. See C<sprintf>
3995 for an explanation of the format argument. If C<use locale> is in effect,
3996 the character used for the decimal point in formatted real numbers is
3997 affected by the LC_NUMERIC locale. See L<perllocale>.
3999 Don't fall into the trap of using a C<printf> when a simple
4000 C<print> would do. The C<print> is more efficient and less
4003 =item prototype FUNCTION
4005 Returns the prototype of a function as a string (or C<undef> if the
4006 function has no prototype). FUNCTION is a reference to, or the name of,
4007 the function whose prototype you want to retrieve.
4009 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4010 name for Perl builtin. If the builtin is not I<overridable> (such as
4011 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4012 C<system>) returns C<undef> because the builtin does not really behave
4013 like a Perl function. Otherwise, the string describing the equivalent
4014 prototype is returned.
4016 =item push ARRAY,LIST
4018 Treats ARRAY as a stack, and pushes the values of LIST
4019 onto the end of ARRAY. The length of ARRAY increases by the length of
4020 LIST. Has the same effect as
4023 $ARRAY[++$#ARRAY] = $value;
4026 but is more efficient. Returns the new number of elements in the array.
4038 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4040 =item quotemeta EXPR
4044 Returns the value of EXPR with all non-"word"
4045 characters backslashed. (That is, all characters not matching
4046 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4047 returned string, regardless of any locale settings.)
4048 This is the internal function implementing
4049 the C<\Q> escape in double-quoted strings.
4051 If EXPR is omitted, uses C<$_>.
4057 Returns a random fractional number greater than or equal to C<0> and less
4058 than the value of EXPR. (EXPR should be positive.) If EXPR is
4059 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4060 also special-cased as C<1> - this has not been documented before perl 5.8.0
4061 and is subject to change in future versions of perl. Automatically calls
4062 C<srand> unless C<srand> has already been called. See also C<srand>.
4064 Apply C<int()> to the value returned by C<rand()> if you want random
4065 integers instead of random fractional numbers. For example,
4069 returns a random integer between C<0> and C<9>, inclusive.
4071 (Note: If your rand function consistently returns numbers that are too
4072 large or too small, then your version of Perl was probably compiled
4073 with the wrong number of RANDBITS.)
4075 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4077 =item read FILEHANDLE,SCALAR,LENGTH
4079 Attempts to read LENGTH I<characters> of data into variable SCALAR
4080 from the specified FILEHANDLE. Returns the number of characters
4081 actually read, C<0> at end of file, or undef if there was an error (in
4082 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4083 so that the last character actually read is the last character of the
4084 scalar after the read.
4086 An OFFSET may be specified to place the read data at some place in the
4087 string other than the beginning. A negative OFFSET specifies
4088 placement at that many characters counting backwards from the end of
4089 the string. A positive OFFSET greater than the length of SCALAR
4090 results in the string being padded to the required size with C<"\0">
4091 bytes before the result of the read is appended.
4093 The call is actually implemented in terms of either Perl's or system's
4094 fread() call. To get a true read(2) system call, see C<sysread>.
4096 Note the I<characters>: depending on the status of the filehandle,
4097 either (8-bit) bytes or characters are read. By default all
4098 filehandles operate on bytes, but for example if the filehandle has
4099 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4100 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4101 characters, not bytes. Similarly for the C<:encoding> pragma:
4102 in that case pretty much any characters can be read.
4104 =item readdir DIRHANDLE
4106 Returns the next directory entry for a directory opened by C<opendir>.
4107 If used in list context, returns all the rest of the entries in the
4108 directory. If there are no more entries, returns an undefined value in
4109 scalar context or a null list in list context.
4111 If you're planning to filetest the return values out of a C<readdir>, you'd
4112 better prepend the directory in question. Otherwise, because we didn't
4113 C<chdir> there, it would have been testing the wrong file.
4115 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4116 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4121 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4122 context, each call reads and returns the next line, until end-of-file is
4123 reached, whereupon the subsequent call returns undef. In list context,
4124 reads until end-of-file is reached and returns a list of lines. Note that
4125 the notion of "line" used here is however you may have defined it
4126 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4128 When C<$/> is set to C<undef>, when readline() is in scalar
4129 context (i.e. file slurp mode), and when an empty file is read, it
4130 returns C<''> the first time, followed by C<undef> subsequently.
4132 This is the internal function implementing the C<< <EXPR> >>
4133 operator, but you can use it directly. The C<< <EXPR> >>
4134 operator is discussed in more detail in L<perlop/"I/O Operators">.
4137 $line = readline(*STDIN); # same thing
4139 If readline encounters an operating system error, C<$!> will be set with the
4140 corresponding error message. It can be helpful to check C<$!> when you are
4141 reading from filehandles you don't trust, such as a tty or a socket. The
4142 following example uses the operator form of C<readline>, and takes the necessary
4143 steps to ensure that C<readline> was successful.
4147 unless (defined( $line = <> )) {
4158 Returns the value of a symbolic link, if symbolic links are
4159 implemented. If not, gives a fatal error. If there is some system
4160 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4161 omitted, uses C<$_>.
4165 EXPR is executed as a system command.
4166 The collected standard output of the command is returned.
4167 In scalar context, it comes back as a single (potentially
4168 multi-line) string. In list context, returns a list of lines
4169 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4170 This is the internal function implementing the C<qx/EXPR/>
4171 operator, but you can use it directly. The C<qx/EXPR/>
4172 operator is discussed in more detail in L<perlop/"I/O Operators">.
4174 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4176 Receives a message on a socket. Attempts to receive LENGTH characters
4177 of data into variable SCALAR from the specified SOCKET filehandle.
4178 SCALAR will be grown or shrunk to the length actually read. Takes the
4179 same flags as the system call of the same name. Returns the address
4180 of the sender if SOCKET's protocol supports this; returns an empty
4181 string otherwise. If there's an error, returns the undefined value.
4182 This call is actually implemented in terms of recvfrom(2) system call.
4183 See L<perlipc/"UDP: Message Passing"> for examples.
4185 Note the I<characters>: depending on the status of the socket, either
4186 (8-bit) bytes or characters are received. By default all sockets
4187 operate on bytes, but for example if the socket has been changed using
4188 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4189 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4190 characters, not bytes. Similarly for the C<:encoding> pragma:
4191 in that case pretty much any characters can be read.
4197 The C<redo> command restarts the loop block without evaluating the
4198 conditional again. The C<continue> block, if any, is not executed. If
4199 the LABEL is omitted, the command refers to the innermost enclosing
4200 loop. Programs that want to lie to themselves about what was just input
4201 normally use this command:
4203 # a simpleminded Pascal comment stripper
4204 # (warning: assumes no { or } in strings)
4205 LINE: while (<STDIN>) {
4206 while (s|({.*}.*){.*}|$1 |) {}
4211 if (/}/) { # end of comment?
4220 C<redo> cannot be used to retry a block which returns a value such as
4221 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4222 a grep() or map() operation.
4224 Note that a block by itself is semantically identical to a loop
4225 that executes once. Thus C<redo> inside such a block will effectively
4226 turn it into a looping construct.
4228 See also L</continue> for an illustration of how C<last>, C<next>, and
4235 Returns a non-empty string if EXPR is a reference, the empty
4236 string otherwise. If EXPR
4237 is not specified, C<$_> will be used. The value returned depends on the
4238 type of thing the reference is a reference to.
4239 Builtin types include:
4249 If the referenced object has been blessed into a package, then that package
4250 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4252 if (ref($r) eq "HASH") {
4253 print "r is a reference to a hash.\n";
4256 print "r is not a reference at all.\n";
4259 See also L<perlref>.
4261 =item rename OLDNAME,NEWNAME
4263 Changes the name of a file; an existing file NEWNAME will be
4264 clobbered. Returns true for success, false otherwise.
4266 Behavior of this function varies wildly depending on your system
4267 implementation. For example, it will usually not work across file system
4268 boundaries, even though the system I<mv> command sometimes compensates
4269 for this. Other restrictions include whether it works on directories,
4270 open files, or pre-existing files. Check L<perlport> and either the
4271 rename(2) manpage or equivalent system documentation for details.
4273 =item require VERSION
4279 Demands a version of Perl specified by VERSION, or demands some semantics
4280 specified by EXPR or by C<$_> if EXPR is not supplied.
4282 VERSION may be either a numeric argument such as 5.006, which will be
4283 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4284 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4285 VERSION is greater than the version of the current Perl interpreter.
4286 Compare with L</use>, which can do a similar check at compile time.
4288 Specifying VERSION as a literal of the form v5.6.1 should generally be
4289 avoided, because it leads to misleading error messages under earlier
4290 versions of Perl that do not support this syntax. The equivalent numeric
4291 version should be used instead.
4293 require v5.6.1; # run time version check
4294 require 5.6.1; # ditto
4295 require 5.006_001; # ditto; preferred for backwards compatibility
4297 Otherwise, C<ref> demands that a library file be included if it hasn't already
4298 been included. The file is included via the do-FILE mechanism, which is
4299 essentially just a variety of C<eval>. Has semantics similar to the
4300 following subroutine:
4303 my ($filename) = @_;
4304 if (exists $INC{$filename}) {
4305 return 1 if $INC{$filename};
4306 die "Compilation failed in require";
4308 my ($realfilename,$result);
4310 foreach $prefix (@INC) {
4311 $realfilename = "$prefix/$filename";
4312 if (-f $realfilename) {
4313 $INC{$filename} = $realfilename;
4314 $result = do $realfilename;
4318 die "Can't find $filename in \@INC";
4321 $INC{$filename} = undef;
4323 } elsif (!$result) {
4324 delete $INC{$filename};
4325 die "$filename did not return true value";
4331 Note that the file will not be included twice under the same specified
4334 The file must return true as the last statement to indicate
4335 successful execution of any initialization code, so it's customary to
4336 end such a file with C<1;> unless you're sure it'll return true
4337 otherwise. But it's better just to put the C<1;>, in case you add more
4340 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4341 replaces "F<::>" with "F</>" in the filename for you,
4342 to make it easy to load standard modules. This form of loading of
4343 modules does not risk altering your namespace.
4345 In other words, if you try this:
4347 require Foo::Bar; # a splendid bareword
4349 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4350 directories specified in the C<@INC> array.
4352 But if you try this:
4354 $class = 'Foo::Bar';
4355 require $class; # $class is not a bareword
4357 require "Foo::Bar"; # not a bareword because of the ""
4359 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4360 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4362 eval "require $class";
4364 Now that you understand how C<require> looks for files in the case of
4365 a bareword argument, there is a little extra functionality going on
4366 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4367 it will first look for a filename with a "F<.pmc>" extension. A file
4368 with this extension is assumed to be Perl bytecode generated by
4369 L<B::Bytecode|B::Bytecode>. If this file is found, and its modification
4370 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4371 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4373 You can also insert hooks into the import facility, by putting directly
4374 Perl code into the @INC array. There are three forms of hooks: subroutine
4375 references, array references and blessed objects.
4377 Subroutine references are the simplest case. When the inclusion system
4378 walks through @INC and encounters a subroutine, this subroutine gets
4379 called with two parameters, the first being a reference to itself, and the
4380 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4381 subroutine should return C<undef> or a filehandle, from which the file to
4382 include will be read. If C<undef> is returned, C<require> will look at
4383 the remaining elements of @INC.
4385 If the hook is an array reference, its first element must be a subroutine
4386 reference. This subroutine is called as above, but the first parameter is
4387 the array reference. This enables to pass indirectly some arguments to
4390 In other words, you can write:
4392 push @INC, \&my_sub;
4394 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4400 push @INC, [ \&my_sub, $x, $y, ... ];
4402 my ($arrayref, $filename) = @_;
4403 # Retrieve $x, $y, ...
4404 my @parameters = @$arrayref[1..$#$arrayref];
4408 If the hook is an object, it must provide an INC method that will be
4409 called as above, the first parameter being the object itself. (Note that
4410 you must fully qualify the sub's name, as it is always forced into package
4411 C<main>.) Here is a typical code layout:
4417 my ($self, $filename) = @_;
4421 # In the main program
4422 push @INC, new Foo(...);
4424 Note that these hooks are also permitted to set the %INC entry
4425 corresponding to the files they have loaded. See L<perlvar/%INC>.
4427 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4433 Generally used in a C<continue> block at the end of a loop to clear
4434 variables and reset C<??> searches so that they work again. The
4435 expression is interpreted as a list of single characters (hyphens
4436 allowed for ranges). All variables and arrays beginning with one of
4437 those letters are reset to their pristine state. If the expression is
4438 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4439 only variables or searches in the current package. Always returns
4442 reset 'X'; # reset all X variables
4443 reset 'a-z'; # reset lower case variables
4444 reset; # just reset ?one-time? searches
4446 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4447 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4448 variables--lexical variables are unaffected, but they clean themselves
4449 up on scope exit anyway, so you'll probably want to use them instead.
4456 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4457 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4458 context, depending on how the return value will be used, and the context
4459 may vary from one execution to the next (see C<wantarray>). If no EXPR
4460 is given, returns an empty list in list context, the undefined value in
4461 scalar context, and (of course) nothing at all in a void context.
4463 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4464 or do FILE will automatically return the value of the last expression
4469 In list context, returns a list value consisting of the elements
4470 of LIST in the opposite order. In scalar context, concatenates the
4471 elements of LIST and returns a string value with all characters
4472 in the opposite order.
4474 print reverse <>; # line tac, last line first
4476 undef $/; # for efficiency of <>
4477 print scalar reverse <>; # character tac, last line tsrif
4479 Used without arguments in scalar context, reverse() reverses C<$_>.
4481 This operator is also handy for inverting a hash, although there are some
4482 caveats. If a value is duplicated in the original hash, only one of those
4483 can be represented as a key in the inverted hash. Also, this has to
4484 unwind one hash and build a whole new one, which may take some time
4485 on a large hash, such as from a DBM file.
4487 %by_name = reverse %by_address; # Invert the hash
4489 =item rewinddir DIRHANDLE
4491 Sets the current position to the beginning of the directory for the
4492 C<readdir> routine on DIRHANDLE.
4494 =item rindex STR,SUBSTR,POSITION
4496 =item rindex STR,SUBSTR
4498 Works just like index() except that it returns the position of the LAST
4499 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4500 last occurrence at or before that position.
4502 =item rmdir FILENAME
4506 Deletes the directory specified by FILENAME if that directory is
4507 empty. If it succeeds it returns true, otherwise it returns false and
4508 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4512 The substitution operator. See L<perlop>.
4516 Forces EXPR to be interpreted in scalar context and returns the value
4519 @counts = ( scalar @a, scalar @b, scalar @c );
4521 There is no equivalent operator to force an expression to
4522 be interpolated in list context because in practice, this is never
4523 needed. If you really wanted to do so, however, you could use
4524 the construction C<@{[ (some expression) ]}>, but usually a simple
4525 C<(some expression)> suffices.
4527 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4528 parenthesized list, this behaves as a scalar comma expression, evaluating
4529 all but the last element in void context and returning the final element
4530 evaluated in scalar context. This is seldom what you want.
4532 The following single statement:
4534 print uc(scalar(&foo,$bar)),$baz;
4536 is the moral equivalent of these two:
4539 print(uc($bar),$baz);
4541 See L<perlop> for more details on unary operators and the comma operator.
4543 =item seek FILEHANDLE,POSITION,WHENCE
4545 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4546 FILEHANDLE may be an expression whose value gives the name of the
4547 filehandle. The values for WHENCE are C<0> to set the new position
4548 I<in bytes> to POSITION, C<1> to set it to the current position plus
4549 POSITION, and C<2> to set it to EOF plus POSITION (typically
4550 negative). For WHENCE you may use the constants C<SEEK_SET>,
4551 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4552 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4555 Note the I<in bytes>: even if the filehandle has been set to
4556 operate on characters (for example by using the C<:utf8> open
4557 layer), tell() will return byte offsets, not character offsets
4558 (because implementing that would render seek() and tell() rather slow).
4560 If you want to position file for C<sysread> or C<syswrite>, don't use
4561 C<seek>--buffering makes its effect on the file's system position
4562 unpredictable and non-portable. Use C<sysseek> instead.
4564 Due to the rules and rigors of ANSI C, on some systems you have to do a
4565 seek whenever you switch between reading and writing. Amongst other
4566 things, this may have the effect of calling stdio's clearerr(3).
4567 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4571 This is also useful for applications emulating C<tail -f>. Once you hit
4572 EOF on your read, and then sleep for a while, you might have to stick in a
4573 seek() to reset things. The C<seek> doesn't change the current position,
4574 but it I<does> clear the end-of-file condition on the handle, so that the
4575 next C<< <FILE> >> makes Perl try again to read something. We hope.
4577 If that doesn't work (some IO implementations are particularly
4578 cantankerous), then you may need something more like this:
4581 for ($curpos = tell(FILE); $_ = <FILE>;
4582 $curpos = tell(FILE)) {
4583 # search for some stuff and put it into files
4585 sleep($for_a_while);
4586 seek(FILE, $curpos, 0);
4589 =item seekdir DIRHANDLE,POS
4591 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4592 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4593 about possible directory compaction as the corresponding system library
4596 =item select FILEHANDLE
4600 Returns the currently selected filehandle. Sets the current default
4601 filehandle for output, if FILEHANDLE is supplied. This has two
4602 effects: first, a C<write> or a C<print> without a filehandle will
4603 default to this FILEHANDLE. Second, references to variables related to
4604 output will refer to this output channel. For example, if you have to
4605 set the top of form format for more than one output channel, you might
4613 FILEHANDLE may be an expression whose value gives the name of the
4614 actual filehandle. Thus:
4616 $oldfh = select(STDERR); $| = 1; select($oldfh);
4618 Some programmers may prefer to think of filehandles as objects with
4619 methods, preferring to write the last example as:
4622 STDERR->autoflush(1);
4624 =item select RBITS,WBITS,EBITS,TIMEOUT
4626 This calls the select(2) system call with the bit masks specified, which
4627 can be constructed using C<fileno> and C<vec>, along these lines:
4629 $rin = $win = $ein = '';
4630 vec($rin,fileno(STDIN),1) = 1;
4631 vec($win,fileno(STDOUT),1) = 1;
4634 If you want to select on many filehandles you might wish to write a
4638 my(@fhlist) = split(' ',$_[0]);
4641 vec($bits,fileno($_),1) = 1;
4645 $rin = fhbits('STDIN TTY SOCK');
4649 ($nfound,$timeleft) =
4650 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4652 or to block until something becomes ready just do this
4654 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4656 Most systems do not bother to return anything useful in $timeleft, so
4657 calling select() in scalar context just returns $nfound.
4659 Any of the bit masks can also be undef. The timeout, if specified, is
4660 in seconds, which may be fractional. Note: not all implementations are
4661 capable of returning the $timeleft. If not, they always return
4662 $timeleft equal to the supplied $timeout.
4664 You can effect a sleep of 250 milliseconds this way:
4666 select(undef, undef, undef, 0.25);
4668 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4669 is implementation-dependent. See also L<perlport> for notes on the
4670 portability of C<select>.
4672 On error, C<select> returns C<undef> and sets C<$!>.
4674 Note: on some Unixes, the select(2) system call may report a socket file
4675 descriptor as "ready for reading", when actually no data is available,
4676 thus a subsequent read blocks. It can be avoided using always the
4677 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4680 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4681 or <FH>) with C<select>, except as permitted by POSIX, and even
4682 then only on POSIX systems. You have to use C<sysread> instead.
4684 =item semctl ID,SEMNUM,CMD,ARG
4686 Calls the System V IPC function C<semctl>. You'll probably have to say
4690 first to get the correct constant definitions. If CMD is IPC_STAT or
4691 GETALL, then ARG must be a variable that will hold the returned
4692 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4693 the undefined value for error, "C<0 but true>" for zero, or the actual
4694 return value otherwise. The ARG must consist of a vector of native
4695 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4696 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4699 =item semget KEY,NSEMS,FLAGS
4701 Calls the System V IPC function semget. Returns the semaphore id, or
4702 the undefined value if there is an error. See also
4703 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4706 =item semop KEY,OPSTRING
4708 Calls the System V IPC function semop to perform semaphore operations
4709 such as signalling and waiting. OPSTRING must be a packed array of
4710 semop structures. Each semop structure can be generated with
4711 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4712 implies the number of semaphore operations. Returns true if
4713 successful, or false if there is an error. As an example, the
4714 following code waits on semaphore $semnum of semaphore id $semid:
4716 $semop = pack("s!3", $semnum, -1, 0);
4717 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4719 To signal the semaphore, replace C<-1> with C<1>. See also
4720 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4723 =item send SOCKET,MSG,FLAGS,TO
4725 =item send SOCKET,MSG,FLAGS
4727 Sends a message on a socket. Attempts to send the scalar MSG to the
4728 SOCKET filehandle. Takes the same flags as the system call of the
4729 same name. On unconnected sockets you must specify a destination to
4730 send TO, in which case it does a C C<sendto>. Returns the number of
4731 characters sent, or the undefined value if there is an error. The C
4732 system call sendmsg(2) is currently unimplemented. See
4733 L<perlipc/"UDP: Message Passing"> for examples.
4735 Note the I<characters>: depending on the status of the socket, either
4736 (8-bit) bytes or characters are sent. By default all sockets operate
4737 on bytes, but for example if the socket has been changed using
4738 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4739 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4740 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4741 in that case pretty much any characters can be sent.
4743 =item setpgrp PID,PGRP
4745 Sets the current process group for the specified PID, C<0> for the current
4746 process. Will produce a fatal error if used on a machine that doesn't
4747 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4748 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4749 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4752 =item setpriority WHICH,WHO,PRIORITY
4754 Sets the current priority for a process, a process group, or a user.
4755 (See setpriority(2).) Will produce a fatal error if used on a machine
4756 that doesn't implement setpriority(2).
4758 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4760 Sets the socket option requested. Returns undefined if there is an
4761 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4768 Shifts the first value of the array off and returns it, shortening the
4769 array by 1 and moving everything down. If there are no elements in the
4770 array, returns the undefined value. If ARRAY is omitted, shifts the
4771 C<@_> array within the lexical scope of subroutines and formats, and the
4772 C<@ARGV> array outside of a subroutine and also within the lexical scopes
4773 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
4774 and C<END {}> constructs.
4776 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4777 same thing to the left end of an array that C<pop> and C<push> do to the
4780 =item shmctl ID,CMD,ARG
4782 Calls the System V IPC function shmctl. You'll probably have to say
4786 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4787 then ARG must be a variable that will hold the returned C<shmid_ds>
4788 structure. Returns like ioctl: the undefined value for error, "C<0> but
4789 true" for zero, or the actual return value otherwise.
4790 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4792 =item shmget KEY,SIZE,FLAGS
4794 Calls the System V IPC function shmget. Returns the shared memory
4795 segment id, or the undefined value if there is an error.
4796 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4798 =item shmread ID,VAR,POS,SIZE
4800 =item shmwrite ID,STRING,POS,SIZE
4802 Reads or writes the System V shared memory segment ID starting at
4803 position POS for size SIZE by attaching to it, copying in/out, and
4804 detaching from it. When reading, VAR must be a variable that will
4805 hold the data read. When writing, if STRING is too long, only SIZE
4806 bytes are used; if STRING is too short, nulls are written to fill out
4807 SIZE bytes. Return true if successful, or false if there is an error.
4808 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4809 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4811 =item shutdown SOCKET,HOW
4813 Shuts down a socket connection in the manner indicated by HOW, which
4814 has the same interpretation as in the system call of the same name.
4816 shutdown(SOCKET, 0); # I/we have stopped reading data
4817 shutdown(SOCKET, 1); # I/we have stopped writing data
4818 shutdown(SOCKET, 2); # I/we have stopped using this socket
4820 This is useful with sockets when you want to tell the other
4821 side you're done writing but not done reading, or vice versa.
4822 It's also a more insistent form of close because it also
4823 disables the file descriptor in any forked copies in other
4830 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4831 returns sine of C<$_>.
4833 For the inverse sine operation, you may use the C<Math::Trig::asin>
4834 function, or use this relation:
4836 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4842 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4843 May be interrupted if the process receives a signal such as C<SIGALRM>.
4844 Returns the number of seconds actually slept. You probably cannot
4845 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4848 On some older systems, it may sleep up to a full second less than what
4849 you requested, depending on how it counts seconds. Most modern systems
4850 always sleep the full amount. They may appear to sleep longer than that,
4851 however, because your process might not be scheduled right away in a
4852 busy multitasking system.
4854 For delays of finer granularity than one second, you may use Perl's
4855 C<syscall> interface to access setitimer(2) if your system supports
4856 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4857 and starting from Perl 5.8 part of the standard distribution) may also
4860 See also the POSIX module's C<pause> function.
4862 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4864 Opens a socket of the specified kind and attaches it to filehandle
4865 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4866 the system call of the same name. You should C<use Socket> first
4867 to get the proper definitions imported. See the examples in
4868 L<perlipc/"Sockets: Client/Server Communication">.
4870 On systems that support a close-on-exec flag on files, the flag will
4871 be set for the newly opened file descriptor, as determined by the
4872 value of $^F. See L<perlvar/$^F>.
4874 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4876 Creates an unnamed pair of sockets in the specified domain, of the
4877 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4878 for the system call of the same name. If unimplemented, yields a fatal
4879 error. Returns true if successful.
4881 On systems that support a close-on-exec flag on files, the flag will
4882 be set for the newly opened file descriptors, as determined by the value
4883 of $^F. See L<perlvar/$^F>.
4885 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4886 to C<pipe(Rdr, Wtr)> is essentially:
4889 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4890 shutdown(Rdr, 1); # no more writing for reader
4891 shutdown(Wtr, 0); # no more reading for writer
4893 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4894 emulate socketpair using IP sockets to localhost if your system implements
4895 sockets but not socketpair.
4897 =item sort SUBNAME LIST
4899 =item sort BLOCK LIST
4903 In list context, this sorts the LIST and returns the sorted list value.
4904 In scalar context, the behaviour of C<sort()> is undefined.
4906 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4907 order. If SUBNAME is specified, it gives the name of a subroutine
4908 that returns an integer less than, equal to, or greater than C<0>,
4909 depending on how the elements of the list are to be ordered. (The C<<
4910 <=> >> and C<cmp> operators are extremely useful in such routines.)
4911 SUBNAME may be a scalar variable name (unsubscripted), in which case
4912 the value provides the name of (or a reference to) the actual
4913 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4914 an anonymous, in-line sort subroutine.
4916 If the subroutine's prototype is C<($$)>, the elements to be compared
4917 are passed by reference in C<@_>, as for a normal subroutine. This is
4918 slower than unprototyped subroutines, where the elements to be
4919 compared are passed into the subroutine
4920 as the package global variables $a and $b (see example below). Note that
4921 in the latter case, it is usually counter-productive to declare $a and
4924 In either case, the subroutine may not be recursive. The values to be
4925 compared are always passed by reference and should not be modified.
4927 You also cannot exit out of the sort block or subroutine using any of the
4928 loop control operators described in L<perlsyn> or with C<goto>.
4930 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4931 current collation locale. See L<perllocale>.
4933 sort() returns aliases into the original list, much as a for loop's index
4934 variable aliases the list elements. That is, modifying an element of a
4935 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
4936 actually modifies the element in the original list. This is usually
4937 something to be avoided when writing clear code.
4939 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4940 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4941 preserves the input order of elements that compare equal. Although
4942 quicksort's run time is O(NlogN) when averaged over all arrays of
4943 length N, the time can be O(N**2), I<quadratic> behavior, for some
4944 inputs.) In 5.7, the quicksort implementation was replaced with
4945 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
4946 But benchmarks indicated that for some inputs, on some platforms,
4947 the original quicksort was faster. 5.8 has a sort pragma for
4948 limited control of the sort. Its rather blunt control of the
4949 underlying algorithm may not persist into future Perls, but the
4950 ability to characterize the input or output in implementation
4951 independent ways quite probably will. See L<sort>.
4956 @articles = sort @files;
4958 # same thing, but with explicit sort routine
4959 @articles = sort {$a cmp $b} @files;
4961 # now case-insensitively
4962 @articles = sort {uc($a) cmp uc($b)} @files;
4964 # same thing in reversed order
4965 @articles = sort {$b cmp $a} @files;
4967 # sort numerically ascending
4968 @articles = sort {$a <=> $b} @files;
4970 # sort numerically descending
4971 @articles = sort {$b <=> $a} @files;
4973 # this sorts the %age hash by value instead of key
4974 # using an in-line function
4975 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4977 # sort using explicit subroutine name
4979 $age{$a} <=> $age{$b}; # presuming numeric
4981 @sortedclass = sort byage @class;
4983 sub backwards { $b cmp $a }
4984 @harry = qw(dog cat x Cain Abel);
4985 @george = qw(gone chased yz Punished Axed);
4987 # prints AbelCaincatdogx
4988 print sort backwards @harry;
4989 # prints xdogcatCainAbel
4990 print sort @george, 'to', @harry;
4991 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4993 # inefficiently sort by descending numeric compare using
4994 # the first integer after the first = sign, or the
4995 # whole record case-insensitively otherwise
4998 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5003 # same thing, but much more efficiently;
5004 # we'll build auxiliary indices instead
5008 push @nums, /=(\d+)/;
5013 $nums[$b] <=> $nums[$a]
5015 $caps[$a] cmp $caps[$b]
5019 # same thing, but without any temps
5020 @new = map { $_->[0] }
5021 sort { $b->[1] <=> $a->[1]
5024 } map { [$_, /=(\d+)/, uc($_)] } @old;
5026 # using a prototype allows you to use any comparison subroutine
5027 # as a sort subroutine (including other package's subroutines)
5029 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5032 @new = sort other::backwards @old;
5034 # guarantee stability, regardless of algorithm
5036 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5038 # force use of mergesort (not portable outside Perl 5.8)
5039 use sort '_mergesort'; # note discouraging _
5040 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5042 If you're using strict, you I<must not> declare $a
5043 and $b as lexicals. They are package globals. That means
5044 if you're in the C<main> package and type
5046 @articles = sort {$b <=> $a} @files;
5048 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5049 but if you're in the C<FooPack> package, it's the same as typing
5051 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5053 The comparison function is required to behave. If it returns
5054 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5055 sometimes saying the opposite, for example) the results are not
5058 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5059 (not-a-number), and because C<sort> will trigger a fatal error unless the
5060 result of a comparison is defined, when sorting with a comparison function
5061 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5062 The following example takes advantage of the fact that C<NaN != NaN> to
5063 eliminate any C<NaN>s from the input.
5065 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5067 =item splice ARRAY,OFFSET,LENGTH,LIST
5069 =item splice ARRAY,OFFSET,LENGTH
5071 =item splice ARRAY,OFFSET
5075 Removes the elements designated by OFFSET and LENGTH from an array, and
5076 replaces them with the elements of LIST, if any. In list context,
5077 returns the elements removed from the array. In scalar context,
5078 returns the last element removed, or C<undef> if no elements are
5079 removed. The array grows or shrinks as necessary.
5080 If OFFSET is negative then it starts that far from the end of the array.
5081 If LENGTH is omitted, removes everything from OFFSET onward.
5082 If LENGTH is negative, removes the elements from OFFSET onward
5083 except for -LENGTH elements at the end of the array.
5084 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5085 past the end of the array, perl issues a warning, and splices at the
5088 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5090 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5091 pop(@a) splice(@a,-1)
5092 shift(@a) splice(@a,0,1)
5093 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5094 $a[$i] = $y splice(@a,$i,1,$y)
5096 Example, assuming array lengths are passed before arrays:
5098 sub aeq { # compare two list values
5099 my(@a) = splice(@_,0,shift);
5100 my(@b) = splice(@_,0,shift);
5101 return 0 unless @a == @b; # same len?
5103 return 0 if pop(@a) ne pop(@b);
5107 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5109 =item split /PATTERN/,EXPR,LIMIT
5111 =item split /PATTERN/,EXPR
5113 =item split /PATTERN/
5117 Splits the string EXPR into a list of strings and returns that list. By
5118 default, empty leading fields are preserved, and empty trailing ones are
5119 deleted. (If all fields are empty, they are considered to be trailing.)
5121 In scalar context, returns the number of fields found and splits into
5122 the C<@_> array. Use of split in scalar context is deprecated, however,
5123 because it clobbers your subroutine arguments.
5125 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5126 splits on whitespace (after skipping any leading whitespace). Anything
5127 matching PATTERN is taken to be a delimiter separating the fields. (Note
5128 that the delimiter may be longer than one character.)
5130 If LIMIT is specified and positive, it represents the maximum number
5131 of fields the EXPR will be split into, though the actual number of
5132 fields returned depends on the number of times PATTERN matches within
5133 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5134 stripped (which potential users of C<pop> would do well to remember).
5135 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5136 had been specified. Note that splitting an EXPR that evaluates to the
5137 empty string always returns the empty list, regardless of the LIMIT
5140 A pattern matching the null string (not to be confused with
5141 a null pattern C<//>, which is just one member of the set of patterns
5142 matching a null string) will split the value of EXPR into separate
5143 characters at each point it matches that way. For example:
5145 print join(':', split(/ */, 'hi there'));
5147 produces the output 'h:i:t:h:e:r:e'.
5149 As a special case for C<split>, using the empty pattern C<//> specifically
5150 matches only the null string, and is not be confused with the regular use
5151 of C<//> to mean "the last successful pattern match". So, for C<split>,
5154 print join(':', split(//, 'hi there'));
5156 produces the output 'h:i: :t:h:e:r:e'.
5158 Empty leading (or trailing) fields are produced when there are positive
5159 width matches at the beginning (or end) of the string; a zero-width match
5160 at the beginning (or end) of the string does not produce an empty field.
5163 print join(':', split(/(?=\w)/, 'hi there!'));
5165 produces the output 'h:i :t:h:e:r:e!'.
5167 The LIMIT parameter can be used to split a line partially
5169 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5171 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5172 a LIMIT one larger than the number of variables in the list, to avoid
5173 unnecessary work. For the list above LIMIT would have been 4 by
5174 default. In time critical applications it behooves you not to split
5175 into more fields than you really need.
5177 If the PATTERN contains parentheses, additional list elements are
5178 created from each matching substring in the delimiter.
5180 split(/([,-])/, "1-10,20", 3);
5182 produces the list value
5184 (1, '-', 10, ',', 20)
5186 If you had the entire header of a normal Unix email message in $header,
5187 you could split it up into fields and their values this way:
5189 $header =~ s/\n\s+/ /g; # fix continuation lines
5190 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5192 The pattern C</PATTERN/> may be replaced with an expression to specify
5193 patterns that vary at runtime. (To do runtime compilation only once,
5194 use C</$variable/o>.)
5196 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5197 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5198 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5199 will give you as many null initial fields as there are leading spaces.
5200 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5201 whitespace produces a null first field. A C<split> with no arguments
5202 really does a S<C<split(' ', $_)>> internally.
5204 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5209 open(PASSWD, '/etc/passwd');
5212 ($login, $passwd, $uid, $gid,
5213 $gcos, $home, $shell) = split(/:/);
5217 As with regular pattern matching, any capturing parentheses that are not
5218 matched in a C<split()> will be set to C<undef> when returned:
5220 @fields = split /(A)|B/, "1A2B3";
5221 # @fields is (1, 'A', 2, undef, 3)
5223 =item sprintf FORMAT, LIST
5225 Returns a string formatted by the usual C<printf> conventions of the C
5226 library function C<sprintf>. See below for more details
5227 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5228 the general principles.
5232 # Format number with up to 8 leading zeroes
5233 $result = sprintf("%08d", $number);
5235 # Round number to 3 digits after decimal point
5236 $rounded = sprintf("%.3f", $number);
5238 Perl does its own C<sprintf> formatting--it emulates the C
5239 function C<sprintf>, but it doesn't use it (except for floating-point
5240 numbers, and even then only the standard modifiers are allowed). As a
5241 result, any non-standard extensions in your local C<sprintf> are not
5242 available from Perl.
5244 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5245 pass it an array as your first argument. The array is given scalar context,
5246 and instead of using the 0th element of the array as the format, Perl will
5247 use the count of elements in the array as the format, which is almost never
5250 Perl's C<sprintf> permits the following universally-known conversions:
5253 %c a character with the given number
5255 %d a signed integer, in decimal
5256 %u an unsigned integer, in decimal
5257 %o an unsigned integer, in octal
5258 %x an unsigned integer, in hexadecimal
5259 %e a floating-point number, in scientific notation
5260 %f a floating-point number, in fixed decimal notation
5261 %g a floating-point number, in %e or %f notation
5263 In addition, Perl permits the following widely-supported conversions:
5265 %X like %x, but using upper-case letters
5266 %E like %e, but using an upper-case "E"
5267 %G like %g, but with an upper-case "E" (if applicable)
5268 %b an unsigned integer, in binary
5269 %p a pointer (outputs the Perl value's address in hexadecimal)
5270 %n special: *stores* the number of characters output so far
5271 into the next variable in the parameter list
5273 Finally, for backward (and we do mean "backward") compatibility, Perl
5274 permits these unnecessary but widely-supported conversions:
5277 %D a synonym for %ld
5278 %U a synonym for %lu
5279 %O a synonym for %lo
5282 Note that the number of exponent digits in the scientific notation produced
5283 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5284 exponent less than 100 is system-dependent: it may be three or less
5285 (zero-padded as necessary). In other words, 1.23 times ten to the
5286 99th may be either "1.23e99" or "1.23e099".
5288 Between the C<%> and the format letter, you may specify a number of
5289 additional attributes controlling the interpretation of the format.
5290 In order, these are:
5294 =item format parameter index
5296 An explicit format parameter index, such as C<2$>. By default sprintf
5297 will format the next unused argument in the list, but this allows you
5298 to take the arguments out of order, e.g.:
5300 printf '%2$d %1$d', 12, 34; # prints "34 12"
5301 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5306 space prefix positive number with a space
5307 + prefix positive number with a plus sign
5308 - left-justify within the field
5309 0 use zeros, not spaces, to right-justify
5310 # prefix non-zero octal with "0", non-zero hex with "0x",
5311 non-zero binary with "0b"
5315 printf '<% d>', 12; # prints "< 12>"
5316 printf '<%+d>', 12; # prints "<+12>"
5317 printf '<%6s>', 12; # prints "< 12>"
5318 printf '<%-6s>', 12; # prints "<12 >"
5319 printf '<%06s>', 12; # prints "<000012>"
5320 printf '<%#x>', 12; # prints "<0xc>"
5324 The vector flag C<v>, optionally specifying the join string to use.
5325 This flag tells perl to interpret the supplied string as a vector
5326 of integers, one for each character in the string, separated by
5327 a given string (a dot C<.> by default). This can be useful for
5328 displaying ordinal values of characters in arbitrary strings:
5330 printf "version is v%vd\n", $^V; # Perl's version
5332 Put an asterisk C<*> before the C<v> to override the string to
5333 use to separate the numbers:
5335 printf "address is %*vX\n", ":", $addr; # IPv6 address
5336 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5338 You can also explicitly specify the argument number to use for
5339 the join string using e.g. C<*2$v>:
5341 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5343 =item (minimum) width
5345 Arguments are usually formatted to be only as wide as required to
5346 display the given value. You can override the width by putting
5347 a number here, or get the width from the next argument (with C<*>)
5348 or from a specified argument (with e.g. C<*2$>):
5350 printf '<%s>', "a"; # prints "<a>"
5351 printf '<%6s>', "a"; # prints "< a>"
5352 printf '<%*s>', 6, "a"; # prints "< a>"
5353 printf '<%*2$s>', "a", 6; # prints "< a>"
5354 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5356 If a field width obtained through C<*> is negative, it has the same
5357 effect as the C<-> flag: left-justification.
5359 =item precision, or maximum width
5361 You can specify a precision (for numeric conversions) or a maximum
5362 width (for string conversions) by specifying a C<.> followed by a number.
5363 For floating point formats, with the exception of 'g' and 'G', this specifies
5364 the number of decimal places to show (the default being 6), e.g.:
5366 # these examples are subject to system-specific variation
5367 printf '<%f>', 1; # prints "<1.000000>"
5368 printf '<%.1f>', 1; # prints "<1.0>"
5369 printf '<%.0f>', 1; # prints "<1>"
5370 printf '<%e>', 10; # prints "<1.000000e+01>"
5371 printf '<%.1e>', 10; # prints "<1.0e+01>"
5373 For 'g' and 'G', this specifies the maximum number of digits to show,
5374 including prior to the decimal point as well as after it, e.g.:
5376 # these examples are subject to system-specific variation
5377 printf '<%g>', 1; # prints "<1>"
5378 printf '<%.10g>', 1; # prints "<1>"
5379 printf '<%g>', 100; # prints "<100>"
5380 printf '<%.1g>', 100; # prints "<1e+02>"
5381 printf '<%.2g>', 100.01; # prints "<1e+02>"
5382 printf '<%.5g>', 100.01; # prints "<100.01>"
5383 printf '<%.4g>', 100.01; # prints "<100>"
5385 For integer conversions, specifying a precision implies that the
5386 output of the number itself should be zero-padded to this width:
5388 printf '<%.6x>', 1; # prints "<000001>"
5389 printf '<%#.6x>', 1; # prints "<0x000001>"
5390 printf '<%-10.6x>', 1; # prints "<000001 >"
5392 For string conversions, specifying a precision truncates the string
5393 to fit in the specified width:
5395 printf '<%.5s>', "truncated"; # prints "<trunc>"
5396 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5398 You can also get the precision from the next argument using C<.*>:
5400 printf '<%.6x>', 1; # prints "<000001>"
5401 printf '<%.*x>', 6, 1; # prints "<000001>"
5403 You cannot currently get the precision from a specified number,
5404 but it is intended that this will be possible in the future using
5407 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5411 For numeric conversions, you can specify the size to interpret the
5412 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5413 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5414 whatever the default integer size is on your platform (usually 32 or 64
5415 bits), but you can override this to use instead one of the standard C types,
5416 as supported by the compiler used to build Perl:
5418 l interpret integer as C type "long" or "unsigned long"
5419 h interpret integer as C type "short" or "unsigned short"
5420 q, L or ll interpret integer as C type "long long", "unsigned long long".
5421 or "quads" (typically 64-bit integers)
5423 The last will produce errors if Perl does not understand "quads" in your
5424 installation. (This requires that either the platform natively supports quads
5425 or Perl was specifically compiled to support quads.) You can find out
5426 whether your Perl supports quads via L<Config>:
5429 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5432 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5433 to be the default floating point size on your platform (double or long double),
5434 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5435 platform supports them. You can find out whether your Perl supports long
5436 doubles via L<Config>:
5439 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5441 You can find out whether Perl considers 'long double' to be the default
5442 floating point size to use on your platform via L<Config>:
5445 ($Config{uselongdouble} eq 'define') &&
5446 print "long doubles by default\n";
5448 It can also be the case that long doubles and doubles are the same thing:
5451 ($Config{doublesize} == $Config{longdblsize}) &&
5452 print "doubles are long doubles\n";
5454 The size specifier C<V> has no effect for Perl code, but it is supported
5455 for compatibility with XS code; it means 'use the standard size for
5456 a Perl integer (or floating-point number)', which is already the
5457 default for Perl code.
5459 =item order of arguments
5461 Normally, sprintf takes the next unused argument as the value to
5462 format for each format specification. If the format specification
5463 uses C<*> to require additional arguments, these are consumed from
5464 the argument list in the order in which they appear in the format
5465 specification I<before> the value to format. Where an argument is
5466 specified using an explicit index, this does not affect the normal
5467 order for the arguments (even when the explicitly specified index
5468 would have been the next argument in any case).
5472 printf '<%*.*s>', $a, $b, $c;
5474 would use C<$a> for the width, C<$b> for the precision and C<$c>
5475 as the value to format, while:
5477 print '<%*1$.*s>', $a, $b;
5479 would use C<$a> for the width and the precision, and C<$b> as the
5482 Here are some more examples - beware that when using an explicit
5483 index, the C<$> may need to be escaped:
5485 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5486 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5487 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5488 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5492 If C<use locale> is in effect, the character used for the decimal
5493 point in formatted real numbers is affected by the LC_NUMERIC locale.
5500 Return the square root of EXPR. If EXPR is omitted, returns square
5501 root of C<$_>. Only works on non-negative operands, unless you've
5502 loaded the standard Math::Complex module.
5505 print sqrt(-2); # prints 1.4142135623731i
5511 Sets the random number seed for the C<rand> operator.
5513 The point of the function is to "seed" the C<rand> function so that
5514 C<rand> can produce a different sequence each time you run your
5517 If srand() is not called explicitly, it is called implicitly at the
5518 first use of the C<rand> operator. However, this was not the case in
5519 versions of Perl before 5.004, so if your script will run under older
5520 Perl versions, it should call C<srand>.
5522 Most programs won't even call srand() at all, except those that
5523 need a cryptographically-strong starting point rather than the
5524 generally acceptable default, which is based on time of day,
5525 process ID, and memory allocation, or the F</dev/urandom> device,
5528 You can call srand($seed) with the same $seed to reproduce the
5529 I<same> sequence from rand(), but this is usually reserved for
5530 generating predictable results for testing or debugging.
5531 Otherwise, don't call srand() more than once in your program.
5533 Do B<not> call srand() (i.e. without an argument) more than once in
5534 a script. The internal state of the random number generator should
5535 contain more entropy than can be provided by any seed, so calling
5536 srand() again actually I<loses> randomness.
5538 Most implementations of C<srand> take an integer and will silently
5539 truncate decimal numbers. This means C<srand(42)> will usually
5540 produce the same results as C<srand(42.1)>. To be safe, always pass
5541 C<srand> an integer.
5543 In versions of Perl prior to 5.004 the default seed was just the
5544 current C<time>. This isn't a particularly good seed, so many old
5545 programs supply their own seed value (often C<time ^ $$> or C<time ^
5546 ($$ + ($$ << 15))>), but that isn't necessary any more.
5548 For cryptographic purposes, however, you need something much more random
5549 than the default seed. Checksumming the compressed output of one or more
5550 rapidly changing operating system status programs is the usual method. For
5553 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5555 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5558 Frequently called programs (like CGI scripts) that simply use
5562 for a seed can fall prey to the mathematical property that
5566 one-third of the time. So don't do that.
5568 =item stat FILEHANDLE
5574 Returns a 13-element list giving the status info for a file, either
5575 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5576 it stats C<$_>. Returns a null list if the stat fails. Typically used
5579 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5580 $atime,$mtime,$ctime,$blksize,$blocks)
5583 Not all fields are supported on all filesystem types. Here are the
5584 meanings of the fields:
5586 0 dev device number of filesystem
5588 2 mode file mode (type and permissions)
5589 3 nlink number of (hard) links to the file
5590 4 uid numeric user ID of file's owner
5591 5 gid numeric group ID of file's owner
5592 6 rdev the device identifier (special files only)
5593 7 size total size of file, in bytes
5594 8 atime last access time in seconds since the epoch
5595 9 mtime last modify time in seconds since the epoch
5596 10 ctime inode change time in seconds since the epoch (*)
5597 11 blksize preferred block size for file system I/O
5598 12 blocks actual number of blocks allocated
5600 (The epoch was at 00:00 January 1, 1970 GMT.)
5602 (*) Not all fields are supported on all filesystem types. Notably, the
5603 ctime field is non-portable. In particular, you cannot expect it to be a
5604 "creation time", see L<perlport/"Files and Filesystems"> for details.
5606 If C<stat> is passed the special filehandle consisting of an underline, no
5607 stat is done, but the current contents of the stat structure from the
5608 last C<stat>, C<lstat>, or filetest are returned. Example:
5610 if (-x $file && (($d) = stat(_)) && $d < 0) {
5611 print "$file is executable NFS file\n";
5614 (This works on machines only for which the device number is negative
5617 Because the mode contains both the file type and its permissions, you
5618 should mask off the file type portion and (s)printf using a C<"%o">
5619 if you want to see the real permissions.
5621 $mode = (stat($filename))[2];
5622 printf "Permissions are %04o\n", $mode & 07777;
5624 In scalar context, C<stat> returns a boolean value indicating success
5625 or failure, and, if successful, sets the information associated with
5626 the special filehandle C<_>.
5628 The File::stat module provides a convenient, by-name access mechanism:
5631 $sb = stat($filename);
5632 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5633 $filename, $sb->size, $sb->mode & 07777,
5634 scalar localtime $sb->mtime;
5636 You can import symbolic mode constants (C<S_IF*>) and functions
5637 (C<S_IS*>) from the Fcntl module:
5641 $mode = (stat($filename))[2];
5643 $user_rwx = ($mode & S_IRWXU) >> 6;
5644 $group_read = ($mode & S_IRGRP) >> 3;
5645 $other_execute = $mode & S_IXOTH;
5647 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5649 $is_setuid = $mode & S_ISUID;
5650 $is_setgid = S_ISDIR($mode);
5652 You could write the last two using the C<-u> and C<-d> operators.
5653 The commonly available C<S_IF*> constants are
5655 # Permissions: read, write, execute, for user, group, others.
5657 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5658 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5659 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5661 # Setuid/Setgid/Stickiness/SaveText.
5662 # Note that the exact meaning of these is system dependent.
5664 S_ISUID S_ISGID S_ISVTX S_ISTXT
5666 # File types. Not necessarily all are available on your system.
5668 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5670 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5672 S_IREAD S_IWRITE S_IEXEC
5674 and the C<S_IF*> functions are
5676 S_IMODE($mode) the part of $mode containing the permission bits
5677 and the setuid/setgid/sticky bits
5679 S_IFMT($mode) the part of $mode containing the file type
5680 which can be bit-anded with e.g. S_IFREG
5681 or with the following functions
5683 # The operators -f, -d, -l, -b, -c, -p, and -S.
5685 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5686 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5688 # No direct -X operator counterpart, but for the first one
5689 # the -g operator is often equivalent. The ENFMT stands for
5690 # record flocking enforcement, a platform-dependent feature.
5692 S_ISENFMT($mode) S_ISWHT($mode)
5694 See your native chmod(2) and stat(2) documentation for more details
5695 about the C<S_*> constants. To get status info for a symbolic link
5696 instead of the target file behind the link, use the C<lstat> function.
5702 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5703 doing many pattern matches on the string before it is next modified.
5704 This may or may not save time, depending on the nature and number of
5705 patterns you are searching on, and on the distribution of character
5706 frequencies in the string to be searched--you probably want to compare
5707 run times with and without it to see which runs faster. Those loops
5708 that scan for many short constant strings (including the constant
5709 parts of more complex patterns) will benefit most. You may have only
5710 one C<study> active at a time--if you study a different scalar the first
5711 is "unstudied". (The way C<study> works is this: a linked list of every
5712 character in the string to be searched is made, so we know, for
5713 example, where all the C<'k'> characters are. From each search string,
5714 the rarest character is selected, based on some static frequency tables
5715 constructed from some C programs and English text. Only those places
5716 that contain this "rarest" character are examined.)
5718 For example, here is a loop that inserts index producing entries
5719 before any line containing a certain pattern:
5723 print ".IX foo\n" if /\bfoo\b/;
5724 print ".IX bar\n" if /\bbar\b/;
5725 print ".IX blurfl\n" if /\bblurfl\b/;
5730 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5731 will be looked at, because C<f> is rarer than C<o>. In general, this is
5732 a big win except in pathological cases. The only question is whether
5733 it saves you more time than it took to build the linked list in the
5736 Note that if you have to look for strings that you don't know till
5737 runtime, you can build an entire loop as a string and C<eval> that to
5738 avoid recompiling all your patterns all the time. Together with
5739 undefining C<$/> to input entire files as one record, this can be very
5740 fast, often faster than specialized programs like fgrep(1). The following
5741 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5742 out the names of those files that contain a match:
5744 $search = 'while (<>) { study;';
5745 foreach $word (@words) {
5746 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5751 eval $search; # this screams
5752 $/ = "\n"; # put back to normal input delimiter
5753 foreach $file (sort keys(%seen)) {
5757 =item sub NAME BLOCK
5759 =item sub NAME (PROTO) BLOCK
5761 =item sub NAME : ATTRS BLOCK
5763 =item sub NAME (PROTO) : ATTRS BLOCK
5765 This is subroutine definition, not a real function I<per se>.
5766 Without a BLOCK it's just a forward declaration. Without a NAME,
5767 it's an anonymous function declaration, and does actually return
5768 a value: the CODE ref of the closure you just created.
5770 See L<perlsub> and L<perlref> for details about subroutines and
5771 references, and L<attributes> and L<Attribute::Handlers> for more
5772 information about attributes.
5774 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5776 =item substr EXPR,OFFSET,LENGTH
5778 =item substr EXPR,OFFSET
5780 Extracts a substring out of EXPR and returns it. First character is at
5781 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5782 If OFFSET is negative (or more precisely, less than C<$[>), starts
5783 that far from the end of the string. If LENGTH is omitted, returns
5784 everything to the end of the string. If LENGTH is negative, leaves that
5785 many characters off the end of the string.
5787 You can use the substr() function as an lvalue, in which case EXPR
5788 must itself be an lvalue. If you assign something shorter than LENGTH,
5789 the string will shrink, and if you assign something longer than LENGTH,
5790 the string will grow to accommodate it. To keep the string the same
5791 length you may need to pad or chop your value using C<sprintf>.
5793 If OFFSET and LENGTH specify a substring that is partly outside the
5794 string, only the part within the string is returned. If the substring
5795 is beyond either end of the string, substr() returns the undefined
5796 value and produces a warning. When used as an lvalue, specifying a
5797 substring that is entirely outside the string is a fatal error.
5798 Here's an example showing the behavior for boundary cases:
5801 substr($name, 4) = 'dy'; # $name is now 'freddy'
5802 my $null = substr $name, 6, 2; # returns '' (no warning)
5803 my $oops = substr $name, 7; # returns undef, with warning
5804 substr($name, 7) = 'gap'; # fatal error
5806 An alternative to using substr() as an lvalue is to specify the
5807 replacement string as the 4th argument. This allows you to replace
5808 parts of the EXPR and return what was there before in one operation,
5809 just as you can with splice().
5811 Note that the lvalue returned by the 3-arg version of substr() acts as
5812 a 'magic bullet'; each time it is assigned to, it remembers which part
5813 of the original string is being modified; for example:
5816 for (substr($x,1,2)) {
5817 $_ = 'a'; print $x,"\n"; # prints 1a4
5818 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
5820 $_ = 'pq'; print $x,"\n"; # prints 5pq9
5824 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
5827 =item symlink OLDFILE,NEWFILE
5829 Creates a new filename symbolically linked to the old filename.
5830 Returns C<1> for success, C<0> otherwise. On systems that don't support
5831 symbolic links, produces a fatal error at run time. To check for that,
5834 $symlink_exists = eval { symlink("",""); 1 };
5836 =item syscall NUMBER, LIST
5838 Calls the system call specified as the first element of the list,
5839 passing the remaining elements as arguments to the system call. If
5840 unimplemented, produces a fatal error. The arguments are interpreted
5841 as follows: if a given argument is numeric, the argument is passed as
5842 an int. If not, the pointer to the string value is passed. You are
5843 responsible to make sure a string is pre-extended long enough to
5844 receive any result that might be written into a string. You can't use a
5845 string literal (or other read-only string) as an argument to C<syscall>
5846 because Perl has to assume that any string pointer might be written
5848 integer arguments are not literals and have never been interpreted in a
5849 numeric context, you may need to add C<0> to them to force them to look
5850 like numbers. This emulates the C<syswrite> function (or vice versa):
5852 require 'syscall.ph'; # may need to run h2ph
5854 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5856 Note that Perl supports passing of up to only 14 arguments to your system call,
5857 which in practice should usually suffice.
5859 Syscall returns whatever value returned by the system call it calls.
5860 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5861 Note that some system calls can legitimately return C<-1>. The proper
5862 way to handle such calls is to assign C<$!=0;> before the call and
5863 check the value of C<$!> if syscall returns C<-1>.
5865 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5866 number of the read end of the pipe it creates. There is no way
5867 to retrieve the file number of the other end. You can avoid this
5868 problem by using C<pipe> instead.
5870 =item sysopen FILEHANDLE,FILENAME,MODE
5872 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5874 Opens the file whose filename is given by FILENAME, and associates it
5875 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5876 the name of the real filehandle wanted. This function calls the
5877 underlying operating system's C<open> function with the parameters
5878 FILENAME, MODE, PERMS.
5880 The possible values and flag bits of the MODE parameter are
5881 system-dependent; they are available via the standard module C<Fcntl>.
5882 See the documentation of your operating system's C<open> to see which
5883 values and flag bits are available. You may combine several flags
5884 using the C<|>-operator.
5886 Some of the most common values are C<O_RDONLY> for opening the file in
5887 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5888 and C<O_RDWR> for opening the file in read-write mode.
5890 For historical reasons, some values work on almost every system
5891 supported by perl: zero means read-only, one means write-only, and two
5892 means read/write. We know that these values do I<not> work under
5893 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5894 use them in new code.
5896 If the file named by FILENAME does not exist and the C<open> call creates
5897 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5898 PERMS specifies the permissions of the newly created file. If you omit
5899 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5900 These permission values need to be in octal, and are modified by your
5901 process's current C<umask>.
5903 In many systems the C<O_EXCL> flag is available for opening files in
5904 exclusive mode. This is B<not> locking: exclusiveness means here that
5905 if the file already exists, sysopen() fails. C<O_EXCL> may not work
5906 on network filesystems, and has no effect unless the C<O_CREAT> flag
5907 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
5908 being opened if it is a symbolic link. It does not protect against
5909 symbolic links in the file's path.
5911 Sometimes you may want to truncate an already-existing file. This
5912 can be done using the C<O_TRUNC> flag. The behavior of
5913 C<O_TRUNC> with C<O_RDONLY> is undefined.
5915 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5916 that takes away the user's option to have a more permissive umask.
5917 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5920 Note that C<sysopen> depends on the fdopen() C library function.
5921 On many UNIX systems, fdopen() is known to fail when file descriptors
5922 exceed a certain value, typically 255. If you need more file
5923 descriptors than that, consider rebuilding Perl to use the C<sfio>
5924 library, or perhaps using the POSIX::open() function.
5926 See L<perlopentut> for a kinder, gentler explanation of opening files.
5928 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5930 =item sysread FILEHANDLE,SCALAR,LENGTH
5932 Attempts to read LENGTH bytes of data into variable SCALAR from the
5933 specified FILEHANDLE, using the system call read(2). It bypasses
5934 buffered IO, so mixing this with other kinds of reads, C<print>,
5935 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5936 perlio or stdio layers usually buffers data. Returns the number of
5937 bytes actually read, C<0> at end of file, or undef if there was an
5938 error (in the latter case C<$!> is also set). SCALAR will be grown or
5939 shrunk so that the last byte actually read is the last byte of the
5940 scalar after the read.
5942 An OFFSET may be specified to place the read data at some place in the
5943 string other than the beginning. A negative OFFSET specifies
5944 placement at that many characters counting backwards from the end of
5945 the string. A positive OFFSET greater than the length of SCALAR
5946 results in the string being padded to the required size with C<"\0">
5947 bytes before the result of the read is appended.
5949 There is no syseof() function, which is ok, since eof() doesn't work
5950 very well on device files (like ttys) anyway. Use sysread() and check
5951 for a return value for 0 to decide whether you're done.
5953 Note that if the filehandle has been marked as C<:utf8> Unicode
5954 characters are read instead of bytes (the LENGTH, OFFSET, and the
5955 return value of sysread() are in Unicode characters).
5956 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5957 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5959 =item sysseek FILEHANDLE,POSITION,WHENCE
5961 Sets FILEHANDLE's system position in bytes using the system call
5962 lseek(2). FILEHANDLE may be an expression whose value gives the name
5963 of the filehandle. The values for WHENCE are C<0> to set the new
5964 position to POSITION, C<1> to set the it to the current position plus
5965 POSITION, and C<2> to set it to EOF plus POSITION (typically
5968 Note the I<in bytes>: even if the filehandle has been set to operate
5969 on characters (for example by using the C<:utf8> I/O layer), tell()
5970 will return byte offsets, not character offsets (because implementing
5971 that would render sysseek() very slow).
5973 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5974 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
5975 C<seek>, C<tell>, or C<eof> may cause confusion.
5977 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5978 and C<SEEK_END> (start of the file, current position, end of the file)
5979 from the Fcntl module. Use of the constants is also more portable
5980 than relying on 0, 1, and 2. For example to define a "systell" function:
5982 use Fcntl 'SEEK_CUR';
5983 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5985 Returns the new position, or the undefined value on failure. A position
5986 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5987 true on success and false on failure, yet you can still easily determine
5992 =item system PROGRAM LIST
5994 Does exactly the same thing as C<exec LIST>, except that a fork is
5995 done first, and the parent process waits for the child process to
5996 complete. Note that argument processing varies depending on the
5997 number of arguments. If there is more than one argument in LIST,
5998 or if LIST is an array with more than one value, starts the program
5999 given by the first element of the list with arguments given by the
6000 rest of the list. If there is only one scalar argument, the argument
6001 is checked for shell metacharacters, and if there are any, the
6002 entire argument is passed to the system's command shell for parsing
6003 (this is C</bin/sh -c> on Unix platforms, but varies on other
6004 platforms). If there are no shell metacharacters in the argument,
6005 it is split into words and passed directly to C<execvp>, which is
6008 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6009 output before any operation that may do a fork, but this may not be
6010 supported on some platforms (see L<perlport>). To be safe, you may need
6011 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6012 of C<IO::Handle> on any open handles.
6014 The return value is the exit status of the program as returned by the
6015 C<wait> call. To get the actual exit value, shift right by eight (see below).
6016 See also L</exec>. This is I<not> what you want to use to capture
6017 the output from a command, for that you should use merely backticks or
6018 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6019 indicates a failure to start the program (inspect $! for the reason).
6021 Like C<exec>, C<system> allows you to lie to a program about its name if
6022 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6024 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6025 C<system>, if you expect your program to terminate on receipt of these
6026 signals you will need to arrange to do so yourself based on the return
6029 @args = ("command", "arg1", "arg2");
6031 or die "system @args failed: $?"
6033 You can check all the failure possibilities by inspecting
6037 print "failed to execute: $!\n";
6040 printf "child died with signal %d, %s coredump\n",
6041 ($? & 127), ($? & 128) ? 'with' : 'without';
6044 printf "child exited with value %d\n", $? >> 8;
6047 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6048 with the W*() calls of the POSIX extension.
6050 When the arguments get executed via the system shell, results
6051 and return codes will be subject to its quirks and capabilities.
6052 See L<perlop/"`STRING`"> and L</exec> for details.
6054 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6056 =item syswrite FILEHANDLE,SCALAR,LENGTH
6058 =item syswrite FILEHANDLE,SCALAR
6060 Attempts to write LENGTH bytes of data from variable SCALAR to the
6061 specified FILEHANDLE, using the system call write(2). If LENGTH is
6062 not specified, writes whole SCALAR. It bypasses buffered IO, so
6063 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6064 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6065 stdio layers usually buffers data. Returns the number of bytes
6066 actually written, or C<undef> if there was an error (in this case the
6067 errno variable C<$!> is also set). If the LENGTH is greater than the
6068 available data in the SCALAR after the OFFSET, only as much data as is
6069 available will be written.
6071 An OFFSET may be specified to write the data from some part of the
6072 string other than the beginning. A negative OFFSET specifies writing
6073 that many characters counting backwards from the end of the string.
6074 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6076 Note that if the filehandle has been marked as C<:utf8>, Unicode
6077 characters are written instead of bytes (the LENGTH, OFFSET, and the
6078 return value of syswrite() are in UTF-8 encoded Unicode characters).
6079 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6080 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6082 =item tell FILEHANDLE
6086 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6087 error. FILEHANDLE may be an expression whose value gives the name of
6088 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6091 Note the I<in bytes>: even if the filehandle has been set to
6092 operate on characters (for example by using the C<:utf8> open
6093 layer), tell() will return byte offsets, not character offsets
6094 (because that would render seek() and tell() rather slow).
6096 The return value of tell() for the standard streams like the STDIN
6097 depends on the operating system: it may return -1 or something else.
6098 tell() on pipes, fifos, and sockets usually returns -1.
6100 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6102 Do not use tell() (or other buffered I/O operations) on a file handle
6103 that has been manipulated by sysread(), syswrite() or sysseek().
6104 Those functions ignore the buffering, while tell() does not.
6106 =item telldir DIRHANDLE
6108 Returns the current position of the C<readdir> routines on DIRHANDLE.
6109 Value may be given to C<seekdir> to access a particular location in a
6110 directory. C<telldir> has the same caveats about possible directory
6111 compaction as the corresponding system library routine.
6113 =item tie VARIABLE,CLASSNAME,LIST
6115 This function binds a variable to a package class that will provide the
6116 implementation for the variable. VARIABLE is the name of the variable
6117 to be enchanted. CLASSNAME is the name of a class implementing objects
6118 of correct type. Any additional arguments are passed to the C<new>
6119 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6120 or C<TIEHASH>). Typically these are arguments such as might be passed
6121 to the C<dbm_open()> function of C. The object returned by the C<new>
6122 method is also returned by the C<tie> function, which would be useful
6123 if you want to access other methods in CLASSNAME.
6125 Note that functions such as C<keys> and C<values> may return huge lists
6126 when used on large objects, like DBM files. You may prefer to use the
6127 C<each> function to iterate over such. Example:
6129 # print out history file offsets
6131 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6132 while (($key,$val) = each %HIST) {
6133 print $key, ' = ', unpack('L',$val), "\n";
6137 A class implementing a hash should have the following methods:
6139 TIEHASH classname, LIST
6141 STORE this, key, value
6146 NEXTKEY this, lastkey
6151 A class implementing an ordinary array should have the following methods:
6153 TIEARRAY classname, LIST
6155 STORE this, key, value
6157 STORESIZE this, count
6163 SPLICE this, offset, length, LIST
6168 A class implementing a file handle should have the following methods:
6170 TIEHANDLE classname, LIST
6171 READ this, scalar, length, offset
6174 WRITE this, scalar, length, offset
6176 PRINTF this, format, LIST
6180 SEEK this, position, whence
6182 OPEN this, mode, LIST
6187 A class implementing a scalar should have the following methods:
6189 TIESCALAR classname, LIST
6195 Not all methods indicated above need be implemented. See L<perltie>,
6196 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6198 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6199 for you--you need to do that explicitly yourself. See L<DB_File>
6200 or the F<Config> module for interesting C<tie> implementations.
6202 For further details see L<perltie>, L<"tied VARIABLE">.
6206 Returns a reference to the object underlying VARIABLE (the same value
6207 that was originally returned by the C<tie> call that bound the variable
6208 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6213 Returns the number of non-leap seconds since whatever time the system
6214 considers to be the epoch, suitable for feeding to C<gmtime> and
6215 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6216 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6217 1904 in the current local time zone for its epoch.
6219 For measuring time in better granularity than one second,
6220 you may use either the Time::HiRes module (from CPAN, and starting from
6221 Perl 5.8 part of the standard distribution), or if you have
6222 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6223 See L<perlfaq8> for details.
6227 Returns a four-element list giving the user and system times, in
6228 seconds, for this process and the children of this process.
6230 ($user,$system,$cuser,$csystem) = times;
6232 In scalar context, C<times> returns C<$user>.
6236 The transliteration operator. Same as C<y///>. See L<perlop>.
6238 =item truncate FILEHANDLE,LENGTH
6240 =item truncate EXPR,LENGTH
6242 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6243 specified length. Produces a fatal error if truncate isn't implemented
6244 on your system. Returns true if successful, the undefined value
6247 The behavior is undefined if LENGTH is greater than the length of the
6254 Returns an uppercased version of EXPR. This is the internal function
6255 implementing the C<\U> escape in double-quoted strings. Respects
6256 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6257 and L<perlunicode> for more details about locale and Unicode support.
6258 It does not attempt to do titlecase mapping on initial letters. See
6259 C<ucfirst> for that.
6261 If EXPR is omitted, uses C<$_>.
6267 Returns the value of EXPR with the first character in uppercase
6268 (titlecase in Unicode). This is the internal function implementing
6269 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6270 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6271 for more details about locale and Unicode support.
6273 If EXPR is omitted, uses C<$_>.
6279 Sets the umask for the process to EXPR and returns the previous value.
6280 If EXPR is omitted, merely returns the current umask.
6282 The Unix permission C<rwxr-x---> is represented as three sets of three
6283 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6284 and isn't one of the digits). The C<umask> value is such a number
6285 representing disabled permissions bits. The permission (or "mode")
6286 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6287 even if you tell C<sysopen> to create a file with permissions C<0777>,
6288 if your umask is C<0022> then the file will actually be created with
6289 permissions C<0755>. If your C<umask> were C<0027> (group can't
6290 write; others can't read, write, or execute), then passing
6291 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6294 Here's some advice: supply a creation mode of C<0666> for regular
6295 files (in C<sysopen>) and one of C<0777> for directories (in
6296 C<mkdir>) and executable files. This gives users the freedom of
6297 choice: if they want protected files, they might choose process umasks
6298 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6299 Programs should rarely if ever make policy decisions better left to
6300 the user. The exception to this is when writing files that should be
6301 kept private: mail files, web browser cookies, I<.rhosts> files, and
6304 If umask(2) is not implemented on your system and you are trying to
6305 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6306 fatal error at run time. If umask(2) is not implemented and you are
6307 not trying to restrict access for yourself, returns C<undef>.
6309 Remember that a umask is a number, usually given in octal; it is I<not> a
6310 string of octal digits. See also L</oct>, if all you have is a string.
6316 Undefines the value of EXPR, which must be an lvalue. Use only on a
6317 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6318 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6319 will probably not do what you expect on most predefined variables or
6320 DBM list values, so don't do that; see L<delete>.) Always returns the
6321 undefined value. You can omit the EXPR, in which case nothing is
6322 undefined, but you still get an undefined value that you could, for
6323 instance, return from a subroutine, assign to a variable or pass as a
6324 parameter. Examples:
6327 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6331 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6332 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6333 select undef, undef, undef, 0.25;
6334 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6336 Note that this is a unary operator, not a list operator.
6342 Deletes a list of files. Returns the number of files successfully
6345 $cnt = unlink 'a', 'b', 'c';
6349 Note: C<unlink> will not delete directories unless you are superuser and
6350 the B<-U> flag is supplied to Perl. Even if these conditions are
6351 met, be warned that unlinking a directory can inflict damage on your
6352 filesystem. Use C<rmdir> instead.
6354 If LIST is omitted, uses C<$_>.
6356 =item unpack TEMPLATE,EXPR
6358 =item unpack TEMPLATE
6360 C<unpack> does the reverse of C<pack>: it takes a string
6361 and expands it out into a list of values.
6362 (In scalar context, it returns merely the first value produced.)
6364 If EXPR is omitted, unpacks the C<$_> string.
6366 The string is broken into chunks described by the TEMPLATE. Each chunk
6367 is converted separately to a value. Typically, either the string is a result
6368 of C<pack>, or the characters of the string represent a C structure of some
6371 The TEMPLATE has the same format as in the C<pack> function.
6372 Here's a subroutine that does substring:
6375 my($what,$where,$howmuch) = @_;
6376 unpack("x$where a$howmuch", $what);
6381 sub ordinal { unpack("W",$_[0]); } # same as ord()
6383 In addition to fields allowed in pack(), you may prefix a field with
6384 a %<number> to indicate that
6385 you want a <number>-bit checksum of the items instead of the items
6386 themselves. Default is a 16-bit checksum. Checksum is calculated by
6387 summing numeric values of expanded values (for string fields the sum of
6388 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6390 For example, the following
6391 computes the same number as the System V sum program:
6395 unpack("%32W*",<>) % 65535;
6398 The following efficiently counts the number of set bits in a bit vector:
6400 $setbits = unpack("%32b*", $selectmask);
6402 The C<p> and C<P> formats should be used with care. Since Perl
6403 has no way of checking whether the value passed to C<unpack()>
6404 corresponds to a valid memory location, passing a pointer value that's
6405 not known to be valid is likely to have disastrous consequences.
6407 If there are more pack codes or if the repeat count of a field or a group
6408 is larger than what the remainder of the input string allows, the result
6409 is not well defined: in some cases, the repeat count is decreased, or
6410 C<unpack()> will produce null strings or zeroes, or terminate with an
6411 error. If the input string is longer than one described by the TEMPLATE,
6412 the rest is ignored.
6414 See L</pack> for more examples and notes.
6416 =item untie VARIABLE
6418 Breaks the binding between a variable and a package. (See C<tie>.)
6419 Has no effect if the variable is not tied.
6421 =item unshift ARRAY,LIST
6423 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6424 depending on how you look at it. Prepends list to the front of the
6425 array, and returns the new number of elements in the array.
6427 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6429 Note the LIST is prepended whole, not one element at a time, so the
6430 prepended elements stay in the same order. Use C<reverse> to do the
6433 =item use Module VERSION LIST
6435 =item use Module VERSION
6437 =item use Module LIST
6443 Imports some semantics into the current package from the named module,
6444 generally by aliasing certain subroutine or variable names into your
6445 package. It is exactly equivalent to
6447 BEGIN { require Module; import Module LIST; }
6449 except that Module I<must> be a bareword.
6451 VERSION may be either a numeric argument such as 5.006, which will be
6452 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6453 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6454 greater than the version of the current Perl interpreter; Perl will not
6455 attempt to parse the rest of the file. Compare with L</require>, which can
6456 do a similar check at run time.
6458 Specifying VERSION as a literal of the form v5.6.1 should generally be
6459 avoided, because it leads to misleading error messages under earlier
6460 versions of Perl that do not support this syntax. The equivalent numeric
6461 version should be used instead.
6463 use v5.6.1; # compile time version check
6465 use 5.006_001; # ditto; preferred for backwards compatibility
6467 This is often useful if you need to check the current Perl version before
6468 C<use>ing library modules that have changed in incompatible ways from
6469 older versions of Perl. (We try not to do this more than we have to.)
6471 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6472 C<require> makes sure the module is loaded into memory if it hasn't been
6473 yet. The C<import> is not a builtin--it's just an ordinary static method
6474 call into the C<Module> package to tell the module to import the list of
6475 features back into the current package. The module can implement its
6476 C<import> method any way it likes, though most modules just choose to
6477 derive their C<import> method via inheritance from the C<Exporter> class that
6478 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6479 method can be found then the call is skipped, even if there is an AUTOLOAD
6482 If you do not want to call the package's C<import> method (for instance,
6483 to stop your namespace from being altered), explicitly supply the empty list:
6487 That is exactly equivalent to
6489 BEGIN { require Module }
6491 If the VERSION argument is present between Module and LIST, then the
6492 C<use> will call the VERSION method in class Module with the given
6493 version as an argument. The default VERSION method, inherited from
6494 the UNIVERSAL class, croaks if the given version is larger than the
6495 value of the variable C<$Module::VERSION>.
6497 Again, there is a distinction between omitting LIST (C<import> called
6498 with no arguments) and an explicit empty LIST C<()> (C<import> not
6499 called). Note that there is no comma after VERSION!
6501 Because this is a wide-open interface, pragmas (compiler directives)
6502 are also implemented this way. Currently implemented pragmas are:
6507 use sigtrap qw(SEGV BUS);
6508 use strict qw(subs vars refs);
6509 use subs qw(afunc blurfl);
6510 use warnings qw(all);
6511 use sort qw(stable _quicksort _mergesort);
6513 Some of these pseudo-modules import semantics into the current
6514 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6515 which import symbols into the current package (which are effective
6516 through the end of the file).
6518 There's a corresponding C<no> command that unimports meanings imported
6519 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6520 It behaves exactly as C<import> does with respect to VERSION, an
6521 omitted LIST, empty LIST, or no unimport method being found.
6527 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6528 for the C<-M> and C<-m> command-line options to perl that give C<use>
6529 functionality from the command-line.
6533 Changes the access and modification times on each file of a list of
6534 files. The first two elements of the list must be the NUMERICAL access
6535 and modification times, in that order. Returns the number of files
6536 successfully changed. The inode change time of each file is set
6537 to the current time. For example, this code has the same effect as the
6538 Unix touch(1) command when the files I<already exist> and belong to
6539 the user running the program:
6542 $atime = $mtime = time;
6543 utime $atime, $mtime, @ARGV;
6545 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6546 the utime(2) function in the C library will be called with a null second
6547 argument. On most systems, this will set the file's access and
6548 modification times to the current time (i.e. equivalent to the example
6549 above) and will even work on other users' files where you have write
6552 utime undef, undef, @ARGV;
6554 Under NFS this will use the time of the NFS server, not the time of
6555 the local machine. If there is a time synchronization problem, the
6556 NFS server and local machine will have different times. The Unix
6557 touch(1) command will in fact normally use this form instead of the
6558 one shown in the first example.
6560 Note that only passing one of the first two elements as C<undef> will
6561 be equivalent of passing it as 0 and will not have the same effect as
6562 described when they are both C<undef>. This case will also trigger an
6563 uninitialized warning.
6567 Returns a list consisting of all the values of the named hash.
6568 (In a scalar context, returns the number of values.)
6570 The values are returned in an apparently random order. The actual
6571 random order is subject to change in future versions of perl, but it
6572 is guaranteed to be the same order as either the C<keys> or C<each>
6573 function would produce on the same (unmodified) hash. Since Perl
6574 5.8.1 the ordering is different even between different runs of Perl
6575 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6577 As a side effect, calling values() resets the HASH's internal iterator,
6578 see L</each>. (In particular, calling values() in void context resets
6579 the iterator with no other overhead.)
6581 Note that the values are not copied, which means modifying them will
6582 modify the contents of the hash:
6584 for (values %hash) { s/foo/bar/g } # modifies %hash values
6585 for (@hash{keys %hash}) { s/foo/bar/g } # same
6587 See also C<keys>, C<each>, and C<sort>.
6589 =item vec EXPR,OFFSET,BITS
6591 Treats the string in EXPR as a bit vector made up of elements of
6592 width BITS, and returns the value of the element specified by OFFSET
6593 as an unsigned integer. BITS therefore specifies the number of bits
6594 that are reserved for each element in the bit vector. This must
6595 be a power of two from 1 to 32 (or 64, if your platform supports
6598 If BITS is 8, "elements" coincide with bytes of the input string.
6600 If BITS is 16 or more, bytes of the input string are grouped into chunks
6601 of size BITS/8, and each group is converted to a number as with
6602 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6603 for BITS==64). See L<"pack"> for details.
6605 If bits is 4 or less, the string is broken into bytes, then the bits
6606 of each byte are broken into 8/BITS groups. Bits of a byte are
6607 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6608 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6609 breaking the single input byte C<chr(0x36)> into two groups gives a list
6610 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6612 C<vec> may also be assigned to, in which case parentheses are needed
6613 to give the expression the correct precedence as in
6615 vec($image, $max_x * $x + $y, 8) = 3;
6617 If the selected element is outside the string, the value 0 is returned.
6618 If an element off the end of the string is written to, Perl will first
6619 extend the string with sufficiently many zero bytes. It is an error
6620 to try to write off the beginning of the string (i.e. negative OFFSET).
6622 The string should not contain any character with the value > 255 (which
6623 can only happen if you're using UTF-8 encoding). If it does, it will be
6624 treated as something that is not UTF-8 encoded. When the C<vec> was
6625 assigned to, other parts of your program will also no longer consider the
6626 string to be UTF-8 encoded. In other words, if you do have such characters
6627 in your string, vec() will operate on the actual byte string, and not the
6628 conceptual character string.
6630 Strings created with C<vec> can also be manipulated with the logical
6631 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6632 vector operation is desired when both operands are strings.
6633 See L<perlop/"Bitwise String Operators">.
6635 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6636 The comments show the string after each step. Note that this code works
6637 in the same way on big-endian or little-endian machines.
6640 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6642 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6643 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6645 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6646 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6647 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6648 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6649 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6650 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6652 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6653 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6654 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6657 To transform a bit vector into a string or list of 0's and 1's, use these:
6659 $bits = unpack("b*", $vector);
6660 @bits = split(//, unpack("b*", $vector));
6662 If you know the exact length in bits, it can be used in place of the C<*>.
6664 Here is an example to illustrate how the bits actually fall in place:
6670 unpack("V",$_) 01234567890123456789012345678901
6671 ------------------------------------------------------------------
6676 for ($shift=0; $shift < $width; ++$shift) {
6677 for ($off=0; $off < 32/$width; ++$off) {
6678 $str = pack("B*", "0"x32);
6679 $bits = (1<<$shift);
6680 vec($str, $off, $width) = $bits;
6681 $res = unpack("b*",$str);
6682 $val = unpack("V", $str);
6689 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6690 $off, $width, $bits, $val, $res
6694 Regardless of the machine architecture on which it is run, the above
6695 example should print the following table:
6698 unpack("V",$_) 01234567890123456789012345678901
6699 ------------------------------------------------------------------
6700 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6701 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6702 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6703 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6704 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6705 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6706 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6707 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6708 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6709 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6710 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6711 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6712 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6713 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6714 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6715 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6716 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6717 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6718 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6719 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6720 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6721 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6722 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6723 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6724 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6725 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6726 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6727 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6728 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6729 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6730 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6731 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6732 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6733 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6734 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6735 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6736 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6737 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6738 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6739 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6740 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6741 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6742 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6743 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6744 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6745 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6746 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6747 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6748 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6749 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6750 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6751 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6752 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6753 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6754 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6755 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6756 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6757 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6758 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6759 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6760 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6761 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6762 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6763 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6764 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6765 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6766 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6767 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6768 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6769 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6770 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6771 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6772 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6773 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6774 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6775 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6776 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6777 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6778 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6779 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6780 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6781 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6782 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6783 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6784 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6785 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6786 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6787 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6788 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6789 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6790 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6791 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6792 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6793 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6794 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6795 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6796 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6797 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6798 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6799 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6800 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6801 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6802 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6803 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6804 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6805 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6806 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6807 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6808 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6809 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6810 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6811 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6812 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6813 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6814 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6815 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6816 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6817 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6818 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6819 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6820 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6821 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6822 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6823 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6824 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6825 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6826 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6827 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6831 Behaves like the wait(2) system call on your system: it waits for a child
6832 process to terminate and returns the pid of the deceased process, or
6833 C<-1> if there are no child processes. The status is returned in C<$?>
6834 and C<{^CHILD_ERROR_NATIVE}>.
6835 Note that a return value of C<-1> could mean that child processes are
6836 being automatically reaped, as described in L<perlipc>.
6838 =item waitpid PID,FLAGS
6840 Waits for a particular child process to terminate and returns the pid of
6841 the deceased process, or C<-1> if there is no such child process. On some
6842 systems, a value of 0 indicates that there are processes still running.
6843 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
6845 use POSIX ":sys_wait_h";
6848 $kid = waitpid(-1, WNOHANG);
6851 then you can do a non-blocking wait for all pending zombie processes.
6852 Non-blocking wait is available on machines supporting either the
6853 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6854 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6855 system call by remembering the status values of processes that have
6856 exited but have not been harvested by the Perl script yet.)
6858 Note that on some systems, a return value of C<-1> could mean that child
6859 processes are being automatically reaped. See L<perlipc> for details,
6860 and for other examples.
6864 Returns true if the context of the currently executing subroutine or
6865 C<eval> is looking for a list value. Returns false if the context is
6866 looking for a scalar. Returns the undefined value if the context is
6867 looking for no value (void context).
6869 return unless defined wantarray; # don't bother doing more
6870 my @a = complex_calculation();
6871 return wantarray ? @a : "@a";
6873 C<wantarray()>'s result is unspecified in the top level of a file,
6874 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
6877 This function should have been named wantlist() instead.
6881 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6884 If LIST is empty and C<$@> already contains a value (typically from a
6885 previous eval) that value is used after appending C<"\t...caught">
6886 to C<$@>. This is useful for staying almost, but not entirely similar to
6889 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6891 No message is printed if there is a C<$SIG{__WARN__}> handler
6892 installed. It is the handler's responsibility to deal with the message
6893 as it sees fit (like, for instance, converting it into a C<die>). Most
6894 handlers must therefore make arrangements to actually display the
6895 warnings that they are not prepared to deal with, by calling C<warn>
6896 again in the handler. Note that this is quite safe and will not
6897 produce an endless loop, since C<__WARN__> hooks are not called from
6900 You will find this behavior is slightly different from that of
6901 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6902 instead call C<die> again to change it).
6904 Using a C<__WARN__> handler provides a powerful way to silence all
6905 warnings (even the so-called mandatory ones). An example:
6907 # wipe out *all* compile-time warnings
6908 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6910 my $foo = 20; # no warning about duplicate my $foo,
6911 # but hey, you asked for it!
6912 # no compile-time or run-time warnings before here
6915 # run-time warnings enabled after here
6916 warn "\$foo is alive and $foo!"; # does show up
6918 See L<perlvar> for details on setting C<%SIG> entries, and for more
6919 examples. See the Carp module for other kinds of warnings using its
6920 carp() and cluck() functions.
6922 =item write FILEHANDLE
6928 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6929 using the format associated with that file. By default the format for
6930 a file is the one having the same name as the filehandle, but the
6931 format for the current output channel (see the C<select> function) may be set
6932 explicitly by assigning the name of the format to the C<$~> variable.
6934 Top of form processing is handled automatically: if there is
6935 insufficient room on the current page for the formatted record, the
6936 page is advanced by writing a form feed, a special top-of-page format
6937 is used to format the new page header, and then the record is written.
6938 By default the top-of-page format is the name of the filehandle with
6939 "_TOP" appended, but it may be dynamically set to the format of your
6940 choice by assigning the name to the C<$^> variable while the filehandle is
6941 selected. The number of lines remaining on the current page is in
6942 variable C<$->, which can be set to C<0> to force a new page.
6944 If FILEHANDLE is unspecified, output goes to the current default output
6945 channel, which starts out as STDOUT but may be changed by the
6946 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6947 is evaluated and the resulting string is used to look up the name of
6948 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6950 Note that write is I<not> the opposite of C<read>. Unfortunately.
6954 The transliteration operator. Same as C<tr///>. See L<perlop>.