3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 For more information see L<perlipc>.
436 Returns the arctangent of Y/X in the range -PI to PI.
438 For the tangent operation, you may use the C<Math::Trig::tan>
439 function, or use the familiar relation:
441 sub tan { sin($_[0]) / cos($_[0]) }
443 =item bind SOCKET,NAME
445 Binds a network address to a socket, just as the bind system call
446 does. Returns true if it succeeded, false otherwise. NAME should be a
447 packed address of the appropriate type for the socket. See the examples in
448 L<perlipc/"Sockets: Client/Server Communication">.
450 =item binmode FILEHANDLE, LAYER
452 =item binmode FILEHANDLE
454 Arranges for FILEHANDLE to be read or written in "binary" or "text"
455 mode on systems where the run-time libraries distinguish between
456 binary and text files. If FILEHANDLE is an expression, the value is
457 taken as the name of the filehandle. Returns true on success,
458 otherwise it returns C<undef> and sets C<$!> (errno).
460 If LAYER is omitted or specified as C<:raw> the filehandle is made
461 suitable for passing binary data. This includes turning off possible CRLF
462 translation and marking it as bytes (as opposed to Unicode characters).
463 Note that as desipite what may be implied in I<"Programming Perl">
464 (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
465 -- other layers which would affect binary nature of the stream are
466 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
467 PERLIO environment variable.
469 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
470 in "Programming Perl, 3rd Edition". However, since the publishing of this
471 book, by many known as "Camel III", the consensus of the naming of this
472 functionality has moved from "discipline" to "layer". All documentation
473 of this version of Perl therefore refers to "layers" rather than to
474 "disciplines". Now back to the regularly scheduled documentation...>
476 On some systems (in general, DOS and Windows-based systems) binmode()
477 is necessary when you're not working with a text file. For the sake
478 of portability it is a good idea to always use it when appropriate,
479 and to never use it when it isn't appropriate.
481 In other words: regardless of platform, use binmode() on binary files
482 (like for example images).
484 If LAYER is present it is a single string, but may contain
485 multiple directives. The directives alter the behaviour of the
486 file handle. When LAYER is present using binmode on text
489 To mark FILEHANDLE as UTF-8, use C<:utf8>.
491 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
492 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
493 establish default I/O layers. See L<open>.
495 In general, binmode() should be called after open() but before any I/O
496 is done on the filehandle. Calling binmode() will normally flush any
497 pending buffered output data (and perhaps pending input data) on the
498 handle. An exception to this is the C<:encoding> layer that
499 changes the default character encoding of the handle, see L<open>.
500 The C<:encoding> layer sometimes needs to be called in
501 mid-stream, and it doesn't flush the stream. The C<:encoding>
502 also implicitly pushes on top of itself the C<:utf8> layer because
503 internally Perl will operate on UTF-8 encoded Unicode characters.
505 The operating system, device drivers, C libraries, and Perl run-time
506 system all work together to let the programmer treat a single
507 character (C<\n>) as the line terminator, irrespective of the external
508 representation. On many operating systems, the native text file
509 representation matches the internal representation, but on some
510 platforms the external representation of C<\n> is made up of more than
513 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
514 character to end each line in the external representation of text (even
515 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
516 on Unix and most VMS files). In other systems like OS/2, DOS and the
517 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
518 but what's stored in text files are the two characters C<\cM\cJ>. That
519 means that, if you don't use binmode() on these systems, C<\cM\cJ>
520 sequences on disk will be converted to C<\n> on input, and any C<\n> in
521 your program will be converted back to C<\cM\cJ> on output. This is what
522 you want for text files, but it can be disastrous for binary files.
524 Another consequence of using binmode() (on some systems) is that
525 special end-of-file markers will be seen as part of the data stream.
526 For systems from the Microsoft family this means that if your binary
527 data contains C<\cZ>, the I/O subsystem will regard it as the end of
528 the file, unless you use binmode().
530 binmode() is not only important for readline() and print() operations,
531 but also when using read(), seek(), sysread(), syswrite() and tell()
532 (see L<perlport> for more details). See the C<$/> and C<$\> variables
533 in L<perlvar> for how to manually set your input and output
534 line-termination sequences.
536 =item bless REF,CLASSNAME
540 This function tells the thingy referenced by REF that it is now an object
541 in the CLASSNAME package. If CLASSNAME is omitted, the current package
542 is used. Because a C<bless> is often the last thing in a constructor,
543 it returns the reference for convenience. Always use the two-argument
544 version if the function doing the blessing might be inherited by a
545 derived class. See L<perltoot> and L<perlobj> for more about the blessing
546 (and blessings) of objects.
548 Consider always blessing objects in CLASSNAMEs that are mixed case.
549 Namespaces with all lowercase names are considered reserved for
550 Perl pragmata. Builtin types have all uppercase names, so to prevent
551 confusion, you may wish to avoid such package names as well. Make sure
552 that CLASSNAME is a true value.
554 See L<perlmod/"Perl Modules">.
560 Returns the context of the current subroutine call. In scalar context,
561 returns the caller's package name if there is a caller, that is, if
562 we're in a subroutine or C<eval> or C<require>, and the undefined value
563 otherwise. In list context, returns
565 ($package, $filename, $line) = caller;
567 With EXPR, it returns some extra information that the debugger uses to
568 print a stack trace. The value of EXPR indicates how many call frames
569 to go back before the current one.
571 ($package, $filename, $line, $subroutine, $hasargs,
572 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
574 Here $subroutine may be C<(eval)> if the frame is not a subroutine
575 call, but an C<eval>. In such a case additional elements $evaltext and
576 C<$is_require> are set: C<$is_require> is true if the frame is created by a
577 C<require> or C<use> statement, $evaltext contains the text of the
578 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
579 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
580 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
581 frame.) $subroutine may also be C<(unknown)> if this particular
582 subroutine happens to have been deleted from the symbol table.
583 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
584 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585 compiled with. The C<$hints> and C<$bitmask> values are subject to change
586 between versions of Perl, and are not meant for external use.
588 Furthermore, when called from within the DB package, caller returns more
589 detailed information: it sets the list variable C<@DB::args> to be the
590 arguments with which the subroutine was invoked.
592 Be aware that the optimizer might have optimized call frames away before
593 C<caller> had a chance to get the information. That means that C<caller(N)>
594 might not return information about the call frame you expect it do, for
595 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
596 previous time C<caller> was called.
600 Changes the working directory to EXPR, if possible. If EXPR is omitted,
601 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
602 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
603 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
604 neither is set, C<chdir> does nothing. It returns true upon success,
605 false otherwise. See the example under C<die>.
609 Changes the permissions of a list of files. The first element of the
610 list must be the numerical mode, which should probably be an octal
611 number, and which definitely should I<not> a string of octal digits:
612 C<0644> is okay, C<'0644'> is not. Returns the number of files
613 successfully changed. See also L</oct>, if all you have is a string.
615 $cnt = chmod 0755, 'foo', 'bar';
616 chmod 0755, @executables;
617 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
619 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
620 $mode = 0644; chmod $mode, 'foo'; # this is best
622 You can also import the symbolic C<S_I*> constants from the Fcntl
627 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
628 # This is identical to the chmod 0755 of the above example.
636 This safer version of L</chop> removes any trailing string
637 that corresponds to the current value of C<$/> (also known as
638 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
639 number of characters removed from all its arguments. It's often used to
640 remove the newline from the end of an input record when you're worried
641 that the final record may be missing its newline. When in paragraph
642 mode (C<$/ = "">), it removes all trailing newlines from the string.
643 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
644 a reference to an integer or the like, see L<perlvar>) chomp() won't
646 If VARIABLE is omitted, it chomps C<$_>. Example:
649 chomp; # avoid \n on last field
654 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
656 You can actually chomp anything that's an lvalue, including an assignment:
659 chomp($answer = <STDIN>);
661 If you chomp a list, each element is chomped, and the total number of
662 characters removed is returned.
664 Note that parentheses are necessary when you're chomping anything
665 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
666 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
667 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
668 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
677 Chops off the last character of a string and returns the character
678 chopped. It is much more efficient than C<s/.$//s> because it neither
679 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
680 If VARIABLE is a hash, it chops the hash's values, but not its keys.
682 You can actually chop anything that's an lvalue, including an assignment.
684 If you chop a list, each element is chopped. Only the value of the
685 last C<chop> is returned.
687 Note that C<chop> returns the last character. To return all but the last
688 character, use C<substr($string, 0, -1)>.
694 Changes the owner (and group) of a list of files. The first two
695 elements of the list must be the I<numeric> uid and gid, in that
696 order. A value of -1 in either position is interpreted by most
697 systems to leave that value unchanged. Returns the number of files
698 successfully changed.
700 $cnt = chown $uid, $gid, 'foo', 'bar';
701 chown $uid, $gid, @filenames;
703 Here's an example that looks up nonnumeric uids in the passwd file:
706 chomp($user = <STDIN>);
708 chomp($pattern = <STDIN>);
710 ($login,$pass,$uid,$gid) = getpwnam($user)
711 or die "$user not in passwd file";
713 @ary = glob($pattern); # expand filenames
714 chown $uid, $gid, @ary;
716 On most systems, you are not allowed to change the ownership of the
717 file unless you're the superuser, although you should be able to change
718 the group to any of your secondary groups. On insecure systems, these
719 restrictions may be relaxed, but this is not a portable assumption.
720 On POSIX systems, you can detect this condition this way:
722 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
723 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
729 Returns the character represented by that NUMBER in the character set.
730 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
731 chr(0x263a) is a Unicode smiley face. Note that characters from 127
732 to 255 (inclusive) are by default not encoded in Unicode for backward
733 compatibility reasons (but see L<encoding>).
735 For the reverse, use L</ord>.
736 See L<perlunicode> and L<encoding> for more about Unicode.
738 If NUMBER is omitted, uses C<$_>.
740 =item chroot FILENAME
744 This function works like the system call by the same name: it makes the
745 named directory the new root directory for all further pathnames that
746 begin with a C</> by your process and all its children. (It doesn't
747 change your current working directory, which is unaffected.) For security
748 reasons, this call is restricted to the superuser. If FILENAME is
749 omitted, does a C<chroot> to C<$_>.
751 =item close FILEHANDLE
755 Closes the file or pipe associated with the file handle, returning
756 true only if IO buffers are successfully flushed and closes the system
757 file descriptor. Closes the currently selected filehandle if the
760 You don't have to close FILEHANDLE if you are immediately going to do
761 another C<open> on it, because C<open> will close it for you. (See
762 C<open>.) However, an explicit C<close> on an input file resets the line
763 counter (C<$.>), while the implicit close done by C<open> does not.
765 If the file handle came from a piped open C<close> will additionally
766 return false if one of the other system calls involved fails or if the
767 program exits with non-zero status. (If the only problem was that the
768 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
769 also waits for the process executing on the pipe to complete, in case you
770 want to look at the output of the pipe afterwards, and
771 implicitly puts the exit status value of that command into C<$?>.
773 Prematurely closing the read end of a pipe (i.e. before the process
774 writing to it at the other end has closed it) will result in a
775 SIGPIPE being delivered to the writer. If the other end can't
776 handle that, be sure to read all the data before closing the pipe.
780 open(OUTPUT, '|sort >foo') # pipe to sort
781 or die "Can't start sort: $!";
782 #... # print stuff to output
783 close OUTPUT # wait for sort to finish
784 or warn $! ? "Error closing sort pipe: $!"
785 : "Exit status $? from sort";
786 open(INPUT, 'foo') # get sort's results
787 or die "Can't open 'foo' for input: $!";
789 FILEHANDLE may be an expression whose value can be used as an indirect
790 filehandle, usually the real filehandle name.
792 =item closedir DIRHANDLE
794 Closes a directory opened by C<opendir> and returns the success of that
797 =item connect SOCKET,NAME
799 Attempts to connect to a remote socket, just as the connect system call
800 does. Returns true if it succeeded, false otherwise. NAME should be a
801 packed address of the appropriate type for the socket. See the examples in
802 L<perlipc/"Sockets: Client/Server Communication">.
806 Actually a flow control statement rather than a function. If there is a
807 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
808 C<foreach>), it is always executed just before the conditional is about to
809 be evaluated again, just like the third part of a C<for> loop in C. Thus
810 it can be used to increment a loop variable, even when the loop has been
811 continued via the C<next> statement (which is similar to the C C<continue>
814 C<last>, C<next>, or C<redo> may appear within a C<continue>
815 block. C<last> and C<redo> will behave as if they had been executed within
816 the main block. So will C<next>, but since it will execute a C<continue>
817 block, it may be more entertaining.
820 ### redo always comes here
823 ### next always comes here
825 # then back the top to re-check EXPR
827 ### last always comes here
829 Omitting the C<continue> section is semantically equivalent to using an
830 empty one, logically enough. In that case, C<next> goes directly back
831 to check the condition at the top of the loop.
837 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
838 takes cosine of C<$_>.
840 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
841 function, or use this relation:
843 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
845 =item crypt PLAINTEXT,SALT
847 Encrypts a string exactly like the crypt(3) function in the C library
848 (assuming that you actually have a version there that has not been
849 extirpated as a potential munition). This can prove useful for checking
850 the password file for lousy passwords, amongst other things. Only the
851 guys wearing white hats should do this.
853 Note that L<crypt|/crypt> is intended to be a one-way function, much like
854 breaking eggs to make an omelette. There is no (known) corresponding
855 decrypt function (in other words, the crypt() is a one-way hash
856 function). As a result, this function isn't all that useful for
857 cryptography. (For that, see your nearby CPAN mirror.)
859 When verifying an existing encrypted string you should use the
860 encrypted text as the salt (like C<crypt($plain, $crypted) eq
861 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
862 and with more exotic implementations. In other words, do not assume
863 anything about the returned string itself, or how many bytes in
864 the encrypted string matter.
866 Traditionally the result is a string of 13 bytes: two first bytes of
867 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
868 the first eight bytes of the encrypted string mattered, but
869 alternative hashing schemes (like MD5), higher level security schemes
870 (like C2), and implementations on non-UNIX platforms may produce
873 When choosing a new salt create a random two character string whose
874 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
875 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
876 characters is just a recommendation; the characters allowed in
877 the salt depend solely on your system's crypt library, and Perl can't
878 restrict what salts C<crypt()> accepts.
880 Here's an example that makes sure that whoever runs this program knows
883 $pwd = (getpwuid($<))[1];
887 chomp($word = <STDIN>);
891 if (crypt($word, $pwd) ne $pwd) {
897 Of course, typing in your own password to whoever asks you
900 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
901 of data, not least of all because you can't get the information
902 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
903 on your favorite CPAN mirror for a slew of potentially useful
906 If using crypt() on a Unicode string (which I<potentially> has
907 characters with codepoints above 255), Perl tries to make sense
908 of the situation by trying to downgrade (a copy of the string)
909 the string back to an eight-bit byte string before calling crypt()
910 (on that copy). If that works, good. If not, crypt() dies with
911 C<Wide character in crypt>.
915 [This function has been largely superseded by the C<untie> function.]
917 Breaks the binding between a DBM file and a hash.
919 =item dbmopen HASH,DBNAME,MASK
921 [This function has been largely superseded by the C<tie> function.]
923 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
924 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
925 argument is I<not> a filehandle, even though it looks like one). DBNAME
926 is the name of the database (without the F<.dir> or F<.pag> extension if
927 any). If the database does not exist, it is created with protection
928 specified by MASK (as modified by the C<umask>). If your system supports
929 only the older DBM functions, you may perform only one C<dbmopen> in your
930 program. In older versions of Perl, if your system had neither DBM nor
931 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
934 If you don't have write access to the DBM file, you can only read hash
935 variables, not set them. If you want to test whether you can write,
936 either use file tests or try setting a dummy hash entry inside an C<eval>,
937 which will trap the error.
939 Note that functions such as C<keys> and C<values> may return huge lists
940 when used on large DBM files. You may prefer to use the C<each>
941 function to iterate over large DBM files. Example:
943 # print out history file offsets
944 dbmopen(%HIST,'/usr/lib/news/history',0666);
945 while (($key,$val) = each %HIST) {
946 print $key, ' = ', unpack('L',$val), "\n";
950 See also L<AnyDBM_File> for a more general description of the pros and
951 cons of the various dbm approaches, as well as L<DB_File> for a particularly
954 You can control which DBM library you use by loading that library
955 before you call dbmopen():
958 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
959 or die "Can't open netscape history file: $!";
965 Returns a Boolean value telling whether EXPR has a value other than
966 the undefined value C<undef>. If EXPR is not present, C<$_> will be
969 Many operations return C<undef> to indicate failure, end of file,
970 system error, uninitialized variable, and other exceptional
971 conditions. This function allows you to distinguish C<undef> from
972 other values. (A simple Boolean test will not distinguish among
973 C<undef>, zero, the empty string, and C<"0">, which are all equally
974 false.) Note that since C<undef> is a valid scalar, its presence
975 doesn't I<necessarily> indicate an exceptional condition: C<pop>
976 returns C<undef> when its argument is an empty array, I<or> when the
977 element to return happens to be C<undef>.
979 You may also use C<defined(&func)> to check whether subroutine C<&func>
980 has ever been defined. The return value is unaffected by any forward
981 declarations of C<&func>. Note that a subroutine which is not defined
982 may still be callable: its package may have an C<AUTOLOAD> method that
983 makes it spring into existence the first time that it is called -- see
986 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
987 used to report whether memory for that aggregate has ever been
988 allocated. This behavior may disappear in future versions of Perl.
989 You should instead use a simple test for size:
991 if (@an_array) { print "has array elements\n" }
992 if (%a_hash) { print "has hash members\n" }
994 When used on a hash element, it tells you whether the value is defined,
995 not whether the key exists in the hash. Use L</exists> for the latter
1000 print if defined $switch{'D'};
1001 print "$val\n" while defined($val = pop(@ary));
1002 die "Can't readlink $sym: $!"
1003 unless defined($value = readlink $sym);
1004 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1005 $debugging = 0 unless defined $debugging;
1007 Note: Many folks tend to overuse C<defined>, and then are surprised to
1008 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1009 defined values. For example, if you say
1013 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1014 matched "nothing". But it didn't really match nothing--rather, it
1015 matched something that happened to be zero characters long. This is all
1016 very above-board and honest. When a function returns an undefined value,
1017 it's an admission that it couldn't give you an honest answer. So you
1018 should use C<defined> only when you're questioning the integrity of what
1019 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1022 See also L</undef>, L</exists>, L</ref>.
1026 Given an expression that specifies a hash element, array element, hash slice,
1027 or array slice, deletes the specified element(s) from the hash or array.
1028 In the case of an array, if the array elements happen to be at the end,
1029 the size of the array will shrink to the highest element that tests
1030 true for exists() (or 0 if no such element exists).
1032 Returns each element so deleted or the undefined value if there was no such
1033 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1034 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1035 from a C<tie>d hash or array may not necessarily return anything.
1037 Deleting an array element effectively returns that position of the array
1038 to its initial, uninitialized state. Subsequently testing for the same
1039 element with exists() will return false. Note that deleting array
1040 elements in the middle of an array will not shift the index of the ones
1041 after them down--use splice() for that. See L</exists>.
1043 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1045 foreach $key (keys %HASH) {
1049 foreach $index (0 .. $#ARRAY) {
1050 delete $ARRAY[$index];
1055 delete @HASH{keys %HASH};
1057 delete @ARRAY[0 .. $#ARRAY];
1059 But both of these are slower than just assigning the empty list
1060 or undefining %HASH or @ARRAY:
1062 %HASH = (); # completely empty %HASH
1063 undef %HASH; # forget %HASH ever existed
1065 @ARRAY = (); # completely empty @ARRAY
1066 undef @ARRAY; # forget @ARRAY ever existed
1068 Note that the EXPR can be arbitrarily complicated as long as the final
1069 operation is a hash element, array element, hash slice, or array slice
1072 delete $ref->[$x][$y]{$key};
1073 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1075 delete $ref->[$x][$y][$index];
1076 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1080 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1081 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1082 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1083 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1084 an C<eval(),> the error message is stuffed into C<$@> and the
1085 C<eval> is terminated with the undefined value. This makes
1086 C<die> the way to raise an exception.
1088 Equivalent examples:
1090 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1091 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1093 If the last element of LIST does not end in a newline, the current
1094 script line number and input line number (if any) are also printed,
1095 and a newline is supplied. Note that the "input line number" (also
1096 known as "chunk") is subject to whatever notion of "line" happens to
1097 be currently in effect, and is also available as the special variable
1098 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1100 Hint: sometimes appending C<", stopped"> to your message will cause it
1101 to make better sense when the string C<"at foo line 123"> is appended.
1102 Suppose you are running script "canasta".
1104 die "/etc/games is no good";
1105 die "/etc/games is no good, stopped";
1107 produce, respectively
1109 /etc/games is no good at canasta line 123.
1110 /etc/games is no good, stopped at canasta line 123.
1112 See also exit(), warn(), and the Carp module.
1114 If LIST is empty and C<$@> already contains a value (typically from a
1115 previous eval) that value is reused after appending C<"\t...propagated">.
1116 This is useful for propagating exceptions:
1119 die unless $@ =~ /Expected exception/;
1121 If LIST is empty and C<$@> contains an object reference that has a
1122 C<PROPAGATE> method, that method will be called with additional file
1123 and line number parameters. The return value replaces the value in
1124 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1127 If C<$@> is empty then the string C<"Died"> is used.
1129 die() can also be called with a reference argument. If this happens to be
1130 trapped within an eval(), $@ contains the reference. This behavior permits
1131 a more elaborate exception handling implementation using objects that
1132 maintain arbitrary state about the nature of the exception. Such a scheme
1133 is sometimes preferable to matching particular string values of $@ using
1134 regular expressions. Here's an example:
1136 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1138 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1139 # handle Some::Module::Exception
1142 # handle all other possible exceptions
1146 Because perl will stringify uncaught exception messages before displaying
1147 them, you may want to overload stringification operations on such custom
1148 exception objects. See L<overload> for details about that.
1150 You can arrange for a callback to be run just before the C<die>
1151 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1152 handler will be called with the error text and can change the error
1153 message, if it sees fit, by calling C<die> again. See
1154 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1155 L<"eval BLOCK"> for some examples. Although this feature was meant
1156 to be run only right before your program was to exit, this is not
1157 currently the case--the C<$SIG{__DIE__}> hook is currently called
1158 even inside eval()ed blocks/strings! If one wants the hook to do
1159 nothing in such situations, put
1163 as the first line of the handler (see L<perlvar/$^S>). Because
1164 this promotes strange action at a distance, this counterintuitive
1165 behavior may be fixed in a future release.
1169 Not really a function. Returns the value of the last command in the
1170 sequence of commands indicated by BLOCK. When modified by a loop
1171 modifier, executes the BLOCK once before testing the loop condition.
1172 (On other statements the loop modifiers test the conditional first.)
1174 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1175 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1176 See L<perlsyn> for alternative strategies.
1178 =item do SUBROUTINE(LIST)
1180 A deprecated form of subroutine call. See L<perlsub>.
1184 Uses the value of EXPR as a filename and executes the contents of the
1185 file as a Perl script. Its primary use is to include subroutines
1186 from a Perl subroutine library.
1194 except that it's more efficient and concise, keeps track of the current
1195 filename for error messages, searches the @INC libraries, and updates
1196 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1197 variables. It also differs in that code evaluated with C<do FILENAME>
1198 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1199 same, however, in that it does reparse the file every time you call it,
1200 so you probably don't want to do this inside a loop.
1202 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1203 error. If C<do> can read the file but cannot compile it, it
1204 returns undef and sets an error message in C<$@>. If the file is
1205 successfully compiled, C<do> returns the value of the last expression
1208 Note that inclusion of library modules is better done with the
1209 C<use> and C<require> operators, which also do automatic error checking
1210 and raise an exception if there's a problem.
1212 You might like to use C<do> to read in a program configuration
1213 file. Manual error checking can be done this way:
1215 # read in config files: system first, then user
1216 for $file ("/share/prog/defaults.rc",
1217 "$ENV{HOME}/.someprogrc")
1219 unless ($return = do $file) {
1220 warn "couldn't parse $file: $@" if $@;
1221 warn "couldn't do $file: $!" unless defined $return;
1222 warn "couldn't run $file" unless $return;
1230 This function causes an immediate core dump. See also the B<-u>
1231 command-line switch in L<perlrun>, which does the same thing.
1232 Primarily this is so that you can use the B<undump> program (not
1233 supplied) to turn your core dump into an executable binary after
1234 having initialized all your variables at the beginning of the
1235 program. When the new binary is executed it will begin by executing
1236 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1237 Think of it as a goto with an intervening core dump and reincarnation.
1238 If C<LABEL> is omitted, restarts the program from the top.
1240 B<WARNING>: Any files opened at the time of the dump will I<not>
1241 be open any more when the program is reincarnated, with possible
1242 resulting confusion on the part of Perl.
1244 This function is now largely obsolete, partly because it's very
1245 hard to convert a core file into an executable, and because the
1246 real compiler backends for generating portable bytecode and compilable
1247 C code have superseded it. That's why you should now invoke it as
1248 C<CORE::dump()>, if you don't want to be warned against a possible
1251 If you're looking to use L<dump> to speed up your program, consider
1252 generating bytecode or native C code as described in L<perlcc>. If
1253 you're just trying to accelerate a CGI script, consider using the
1254 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1255 You might also consider autoloading or selfloading, which at least
1256 make your program I<appear> to run faster.
1260 When called in list context, returns a 2-element list consisting of the
1261 key and value for the next element of a hash, so that you can iterate over
1262 it. When called in scalar context, returns only the key for the next
1263 element in the hash.
1265 Entries are returned in an apparently random order. The actual random
1266 order is subject to change in future versions of perl, but it is guaranteed
1267 to be in the same order as either the C<keys> or C<values> function
1268 would produce on the same (unmodified) hash.
1270 When the hash is entirely read, a null array is returned in list context
1271 (which when assigned produces a false (C<0>) value), and C<undef> in
1272 scalar context. The next call to C<each> after that will start iterating
1273 again. There is a single iterator for each hash, shared by all C<each>,
1274 C<keys>, and C<values> function calls in the program; it can be reset by
1275 reading all the elements from the hash, or by evaluating C<keys HASH> or
1276 C<values HASH>. If you add or delete elements of a hash while you're
1277 iterating over it, you may get entries skipped or duplicated, so
1278 don't. Exception: It is always safe to delete the item most recently
1279 returned by C<each()>, which means that the following code will work:
1281 while (($key, $value) = each %hash) {
1283 delete $hash{$key}; # This is safe
1286 The following prints out your environment like the printenv(1) program,
1287 only in a different order:
1289 while (($key,$value) = each %ENV) {
1290 print "$key=$value\n";
1293 See also C<keys>, C<values> and C<sort>.
1295 =item eof FILEHANDLE
1301 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1302 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1303 gives the real filehandle. (Note that this function actually
1304 reads a character and then C<ungetc>s it, so isn't very useful in an
1305 interactive context.) Do not read from a terminal file (or call
1306 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1307 as terminals may lose the end-of-file condition if you do.
1309 An C<eof> without an argument uses the last file read. Using C<eof()>
1310 with empty parentheses is very different. It refers to the pseudo file
1311 formed from the files listed on the command line and accessed via the
1312 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1313 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1314 used will cause C<@ARGV> to be examined to determine if input is
1315 available. Similarly, an C<eof()> after C<< <> >> has returned
1316 end-of-file will assume you are processing another C<@ARGV> list,
1317 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1318 see L<perlop/"I/O Operators">.
1320 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1321 detect the end of each file, C<eof()> will only detect the end of the
1322 last file. Examples:
1324 # reset line numbering on each input file
1326 next if /^\s*#/; # skip comments
1329 close ARGV if eof; # Not eof()!
1332 # insert dashes just before last line of last file
1334 if (eof()) { # check for end of last file
1335 print "--------------\n";
1338 last if eof(); # needed if we're reading from a terminal
1341 Practical hint: you almost never need to use C<eof> in Perl, because the
1342 input operators typically return C<undef> when they run out of data, or if
1349 In the first form, the return value of EXPR is parsed and executed as if it
1350 were a little Perl program. The value of the expression (which is itself
1351 determined within scalar context) is first parsed, and if there weren't any
1352 errors, executed in the lexical context of the current Perl program, so
1353 that any variable settings or subroutine and format definitions remain
1354 afterwards. Note that the value is parsed every time the eval executes.
1355 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1356 delay parsing and subsequent execution of the text of EXPR until run time.
1358 In the second form, the code within the BLOCK is parsed only once--at the
1359 same time the code surrounding the eval itself was parsed--and executed
1360 within the context of the current Perl program. This form is typically
1361 used to trap exceptions more efficiently than the first (see below), while
1362 also providing the benefit of checking the code within BLOCK at compile
1365 The final semicolon, if any, may be omitted from the value of EXPR or within
1368 In both forms, the value returned is the value of the last expression
1369 evaluated inside the mini-program; a return statement may be also used, just
1370 as with subroutines. The expression providing the return value is evaluated
1371 in void, scalar, or list context, depending on the context of the eval itself.
1372 See L</wantarray> for more on how the evaluation context can be determined.
1374 If there is a syntax error or runtime error, or a C<die> statement is
1375 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1376 error message. If there was no error, C<$@> is guaranteed to be a null
1377 string. Beware that using C<eval> neither silences perl from printing
1378 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1379 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1380 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1381 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1383 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1384 determining whether a particular feature (such as C<socket> or C<symlink>)
1385 is implemented. It is also Perl's exception trapping mechanism, where
1386 the die operator is used to raise exceptions.
1388 If the code to be executed doesn't vary, you may use the eval-BLOCK
1389 form to trap run-time errors without incurring the penalty of
1390 recompiling each time. The error, if any, is still returned in C<$@>.
1393 # make divide-by-zero nonfatal
1394 eval { $answer = $a / $b; }; warn $@ if $@;
1396 # same thing, but less efficient
1397 eval '$answer = $a / $b'; warn $@ if $@;
1399 # a compile-time error
1400 eval { $answer = }; # WRONG
1403 eval '$answer ='; # sets $@
1405 Due to the current arguably broken state of C<__DIE__> hooks, when using
1406 the C<eval{}> form as an exception trap in libraries, you may wish not
1407 to trigger any C<__DIE__> hooks that user code may have installed.
1408 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1409 as shown in this example:
1411 # a very private exception trap for divide-by-zero
1412 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1415 This is especially significant, given that C<__DIE__> hooks can call
1416 C<die> again, which has the effect of changing their error messages:
1418 # __DIE__ hooks may modify error messages
1420 local $SIG{'__DIE__'} =
1421 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1422 eval { die "foo lives here" };
1423 print $@ if $@; # prints "bar lives here"
1426 Because this promotes action at a distance, this counterintuitive behavior
1427 may be fixed in a future release.
1429 With an C<eval>, you should be especially careful to remember what's
1430 being looked at when:
1436 eval { $x }; # CASE 4
1438 eval "\$$x++"; # CASE 5
1441 Cases 1 and 2 above behave identically: they run the code contained in
1442 the variable $x. (Although case 2 has misleading double quotes making
1443 the reader wonder what else might be happening (nothing is).) Cases 3
1444 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1445 does nothing but return the value of $x. (Case 4 is preferred for
1446 purely visual reasons, but it also has the advantage of compiling at
1447 compile-time instead of at run-time.) Case 5 is a place where
1448 normally you I<would> like to use double quotes, except that in this
1449 particular situation, you can just use symbolic references instead, as
1452 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1453 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1455 Note that as a very special case, an C<eval ''> executed within the C<DB>
1456 package doesn't see the usual surrounding lexical scope, but rather the
1457 scope of the first non-DB piece of code that called it. You don't normally
1458 need to worry about this unless you are writing a Perl debugger.
1462 =item exec PROGRAM LIST
1464 The C<exec> function executes a system command I<and never returns>--
1465 use C<system> instead of C<exec> if you want it to return. It fails and
1466 returns false only if the command does not exist I<and> it is executed
1467 directly instead of via your system's command shell (see below).
1469 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1470 warns you if there is a following statement which isn't C<die>, C<warn>,
1471 or C<exit> (if C<-w> is set - but you always do that). If you
1472 I<really> want to follow an C<exec> with some other statement, you
1473 can use one of these styles to avoid the warning:
1475 exec ('foo') or print STDERR "couldn't exec foo: $!";
1476 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1478 If there is more than one argument in LIST, or if LIST is an array
1479 with more than one value, calls execvp(3) with the arguments in LIST.
1480 If there is only one scalar argument or an array with one element in it,
1481 the argument is checked for shell metacharacters, and if there are any,
1482 the entire argument is passed to the system's command shell for parsing
1483 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1484 If there are no shell metacharacters in the argument, it is split into
1485 words and passed directly to C<execvp>, which is more efficient.
1488 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1489 exec "sort $outfile | uniq";
1491 If you don't really want to execute the first argument, but want to lie
1492 to the program you are executing about its own name, you can specify
1493 the program you actually want to run as an "indirect object" (without a
1494 comma) in front of the LIST. (This always forces interpretation of the
1495 LIST as a multivalued list, even if there is only a single scalar in
1498 $shell = '/bin/csh';
1499 exec $shell '-sh'; # pretend it's a login shell
1503 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1505 When the arguments get executed via the system shell, results will
1506 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1509 Using an indirect object with C<exec> or C<system> is also more
1510 secure. This usage (which also works fine with system()) forces
1511 interpretation of the arguments as a multivalued list, even if the
1512 list had just one argument. That way you're safe from the shell
1513 expanding wildcards or splitting up words with whitespace in them.
1515 @args = ( "echo surprise" );
1517 exec @args; # subject to shell escapes
1519 exec { $args[0] } @args; # safe even with one-arg list
1521 The first version, the one without the indirect object, ran the I<echo>
1522 program, passing it C<"surprise"> an argument. The second version
1523 didn't--it tried to run a program literally called I<"echo surprise">,
1524 didn't find it, and set C<$?> to a non-zero value indicating failure.
1526 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1527 output before the exec, but this may not be supported on some platforms
1528 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1529 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1530 open handles in order to avoid lost output.
1532 Note that C<exec> will not call your C<END> blocks, nor will it call
1533 any C<DESTROY> methods in your objects.
1537 Given an expression that specifies a hash element or array element,
1538 returns true if the specified element in the hash or array has ever
1539 been initialized, even if the corresponding value is undefined. The
1540 element is not autovivified if it doesn't exist.
1542 print "Exists\n" if exists $hash{$key};
1543 print "Defined\n" if defined $hash{$key};
1544 print "True\n" if $hash{$key};
1546 print "Exists\n" if exists $array[$index];
1547 print "Defined\n" if defined $array[$index];
1548 print "True\n" if $array[$index];
1550 A hash or array element can be true only if it's defined, and defined if
1551 it exists, but the reverse doesn't necessarily hold true.
1553 Given an expression that specifies the name of a subroutine,
1554 returns true if the specified subroutine has ever been declared, even
1555 if it is undefined. Mentioning a subroutine name for exists or defined
1556 does not count as declaring it. Note that a subroutine which does not
1557 exist may still be callable: its package may have an C<AUTOLOAD>
1558 method that makes it spring into existence the first time that it is
1559 called -- see L<perlsub>.
1561 print "Exists\n" if exists &subroutine;
1562 print "Defined\n" if defined &subroutine;
1564 Note that the EXPR can be arbitrarily complicated as long as the final
1565 operation is a hash or array key lookup or subroutine name:
1567 if (exists $ref->{A}->{B}->{$key}) { }
1568 if (exists $hash{A}{B}{$key}) { }
1570 if (exists $ref->{A}->{B}->[$ix]) { }
1571 if (exists $hash{A}{B}[$ix]) { }
1573 if (exists &{$ref->{A}{B}{$key}}) { }
1575 Although the deepest nested array or hash will not spring into existence
1576 just because its existence was tested, any intervening ones will.
1577 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1578 into existence due to the existence test for the $key element above.
1579 This happens anywhere the arrow operator is used, including even:
1582 if (exists $ref->{"Some key"}) { }
1583 print $ref; # prints HASH(0x80d3d5c)
1585 This surprising autovivification in what does not at first--or even
1586 second--glance appear to be an lvalue context may be fixed in a future
1589 Use of a subroutine call, rather than a subroutine name, as an argument
1590 to exists() is an error.
1593 exists &sub(); # Error
1597 Evaluates EXPR and exits immediately with that value. Example:
1600 exit 0 if $ans =~ /^[Xx]/;
1602 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1603 universally recognized values for EXPR are C<0> for success and C<1>
1604 for error; other values are subject to interpretation depending on the
1605 environment in which the Perl program is running. For example, exiting
1606 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1607 the mailer to return the item undelivered, but that's not true everywhere.
1609 Don't use C<exit> to abort a subroutine if there's any chance that
1610 someone might want to trap whatever error happened. Use C<die> instead,
1611 which can be trapped by an C<eval>.
1613 The exit() function does not always exit immediately. It calls any
1614 defined C<END> routines first, but these C<END> routines may not
1615 themselves abort the exit. Likewise any object destructors that need to
1616 be called are called before the real exit. If this is a problem, you
1617 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1618 See L<perlmod> for details.
1624 Returns I<e> (the natural logarithm base) to the power of EXPR.
1625 If EXPR is omitted, gives C<exp($_)>.
1627 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1629 Implements the fcntl(2) function. You'll probably have to say
1633 first to get the correct constant definitions. Argument processing and
1634 value return works just like C<ioctl> below.
1638 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1639 or die "can't fcntl F_GETFL: $!";
1641 You don't have to check for C<defined> on the return from C<fnctl>.
1642 Like C<ioctl>, it maps a C<0> return from the system call into
1643 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1644 in numeric context. It is also exempt from the normal B<-w> warnings
1645 on improper numeric conversions.
1647 Note that C<fcntl> will produce a fatal error if used on a machine that
1648 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1649 manpage to learn what functions are available on your system.
1651 =item fileno FILEHANDLE
1653 Returns the file descriptor for a filehandle, or undefined if the
1654 filehandle is not open. This is mainly useful for constructing
1655 bitmaps for C<select> and low-level POSIX tty-handling operations.
1656 If FILEHANDLE is an expression, the value is taken as an indirect
1657 filehandle, generally its name.
1659 You can use this to find out whether two handles refer to the
1660 same underlying descriptor:
1662 if (fileno(THIS) == fileno(THAT)) {
1663 print "THIS and THAT are dups\n";
1666 (Filehandles connected to memory objects via new features of C<open> may
1667 return undefined even though they are open.)
1670 =item flock FILEHANDLE,OPERATION
1672 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1673 for success, false on failure. Produces a fatal error if used on a
1674 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1675 C<flock> is Perl's portable file locking interface, although it locks
1676 only entire files, not records.
1678 Two potentially non-obvious but traditional C<flock> semantics are
1679 that it waits indefinitely until the lock is granted, and that its locks
1680 B<merely advisory>. Such discretionary locks are more flexible, but offer
1681 fewer guarantees. This means that files locked with C<flock> may be
1682 modified by programs that do not also use C<flock>. See L<perlport>,
1683 your port's specific documentation, or your system-specific local manpages
1684 for details. It's best to assume traditional behavior if you're writing
1685 portable programs. (But if you're not, you should as always feel perfectly
1686 free to write for your own system's idiosyncrasies (sometimes called
1687 "features"). Slavish adherence to portability concerns shouldn't get
1688 in the way of your getting your job done.)
1690 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1691 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1692 you can use the symbolic names if you import them from the Fcntl module,
1693 either individually, or as a group using the ':flock' tag. LOCK_SH
1694 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1695 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1696 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1697 waiting for the lock (check the return status to see if you got it).
1699 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1700 before locking or unlocking it.
1702 Note that the emulation built with lockf(3) doesn't provide shared
1703 locks, and it requires that FILEHANDLE be open with write intent. These
1704 are the semantics that lockf(3) implements. Most if not all systems
1705 implement lockf(3) in terms of fcntl(2) locking, though, so the
1706 differing semantics shouldn't bite too many people.
1708 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1709 be open with read intent to use LOCK_SH and requires that it be open
1710 with write intent to use LOCK_EX.
1712 Note also that some versions of C<flock> cannot lock things over the
1713 network; you would need to use the more system-specific C<fcntl> for
1714 that. If you like you can force Perl to ignore your system's flock(2)
1715 function, and so provide its own fcntl(2)-based emulation, by passing
1716 the switch C<-Ud_flock> to the F<Configure> program when you configure
1719 Here's a mailbox appender for BSD systems.
1721 use Fcntl ':flock'; # import LOCK_* constants
1724 flock(MBOX,LOCK_EX);
1725 # and, in case someone appended
1726 # while we were waiting...
1731 flock(MBOX,LOCK_UN);
1734 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1735 or die "Can't open mailbox: $!";
1738 print MBOX $msg,"\n\n";
1741 On systems that support a real flock(), locks are inherited across fork()
1742 calls, whereas those that must resort to the more capricious fcntl()
1743 function lose the locks, making it harder to write servers.
1745 See also L<DB_File> for other flock() examples.
1749 Does a fork(2) system call to create a new process running the
1750 same program at the same point. It returns the child pid to the
1751 parent process, C<0> to the child process, or C<undef> if the fork is
1752 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1753 are shared, while everything else is copied. On most systems supporting
1754 fork(), great care has gone into making it extremely efficient (for
1755 example, using copy-on-write technology on data pages), making it the
1756 dominant paradigm for multitasking over the last few decades.
1758 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1759 output before forking the child process, but this may not be supported
1760 on some platforms (see L<perlport>). To be safe, you may need to set
1761 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1762 C<IO::Handle> on any open handles in order to avoid duplicate output.
1764 If you C<fork> without ever waiting on your children, you will
1765 accumulate zombies. On some systems, you can avoid this by setting
1766 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1767 forking and reaping moribund children.
1769 Note that if your forked child inherits system file descriptors like
1770 STDIN and STDOUT that are actually connected by a pipe or socket, even
1771 if you exit, then the remote server (such as, say, a CGI script or a
1772 backgrounded job launched from a remote shell) won't think you're done.
1773 You should reopen those to F</dev/null> if it's any issue.
1777 Declare a picture format for use by the C<write> function. For
1781 Test: @<<<<<<<< @||||| @>>>>>
1782 $str, $%, '$' . int($num)
1786 $num = $cost/$quantity;
1790 See L<perlform> for many details and examples.
1792 =item formline PICTURE,LIST
1794 This is an internal function used by C<format>s, though you may call it,
1795 too. It formats (see L<perlform>) a list of values according to the
1796 contents of PICTURE, placing the output into the format output
1797 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1798 Eventually, when a C<write> is done, the contents of
1799 C<$^A> are written to some filehandle, but you could also read C<$^A>
1800 yourself and then set C<$^A> back to C<"">. Note that a format typically
1801 does one C<formline> per line of form, but the C<formline> function itself
1802 doesn't care how many newlines are embedded in the PICTURE. This means
1803 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1804 You may therefore need to use multiple formlines to implement a single
1805 record format, just like the format compiler.
1807 Be careful if you put double quotes around the picture, because an C<@>
1808 character may be taken to mean the beginning of an array name.
1809 C<formline> always returns true. See L<perlform> for other examples.
1811 =item getc FILEHANDLE
1815 Returns the next character from the input file attached to FILEHANDLE,
1816 or the undefined value at end of file, or if there was an error (in
1817 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1818 STDIN. This is not particularly efficient. However, it cannot be
1819 used by itself to fetch single characters without waiting for the user
1820 to hit enter. For that, try something more like:
1823 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1826 system "stty", '-icanon', 'eol', "\001";
1832 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1835 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1839 Determination of whether $BSD_STYLE should be set
1840 is left as an exercise to the reader.
1842 The C<POSIX::getattr> function can do this more portably on
1843 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1844 module from your nearest CPAN site; details on CPAN can be found on
1849 Implements the C library function of the same name, which on most
1850 systems returns the current login from F</etc/utmp>, if any. If null,
1853 $login = getlogin || getpwuid($<) || "Kilroy";
1855 Do not consider C<getlogin> for authentication: it is not as
1856 secure as C<getpwuid>.
1858 =item getpeername SOCKET
1860 Returns the packed sockaddr address of other end of the SOCKET connection.
1863 $hersockaddr = getpeername(SOCK);
1864 ($port, $iaddr) = sockaddr_in($hersockaddr);
1865 $herhostname = gethostbyaddr($iaddr, AF_INET);
1866 $herstraddr = inet_ntoa($iaddr);
1870 Returns the current process group for the specified PID. Use
1871 a PID of C<0> to get the current process group for the
1872 current process. Will raise an exception if used on a machine that
1873 doesn't implement getpgrp(2). If PID is omitted, returns process
1874 group of current process. Note that the POSIX version of C<getpgrp>
1875 does not accept a PID argument, so only C<PID==0> is truly portable.
1879 Returns the process id of the parent process.
1881 Note for Linux users: on Linux, the C functions C<getpid()> and
1882 C<getppid()> return different values from different threads. In order to
1883 be portable, this behavior is not reflected by the perl-level function
1884 C<getppid()>, that returns a consistent value across threads. If you want
1885 to call the underlying C<getppid()>, you may use the CPAN module
1888 =item getpriority WHICH,WHO
1890 Returns the current priority for a process, a process group, or a user.
1891 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1892 machine that doesn't implement getpriority(2).
1898 =item gethostbyname NAME
1900 =item getnetbyname NAME
1902 =item getprotobyname NAME
1908 =item getservbyname NAME,PROTO
1910 =item gethostbyaddr ADDR,ADDRTYPE
1912 =item getnetbyaddr ADDR,ADDRTYPE
1914 =item getprotobynumber NUMBER
1916 =item getservbyport PORT,PROTO
1934 =item sethostent STAYOPEN
1936 =item setnetent STAYOPEN
1938 =item setprotoent STAYOPEN
1940 =item setservent STAYOPEN
1954 These routines perform the same functions as their counterparts in the
1955 system library. In list context, the return values from the
1956 various get routines are as follows:
1958 ($name,$passwd,$uid,$gid,
1959 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1960 ($name,$passwd,$gid,$members) = getgr*
1961 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1962 ($name,$aliases,$addrtype,$net) = getnet*
1963 ($name,$aliases,$proto) = getproto*
1964 ($name,$aliases,$port,$proto) = getserv*
1966 (If the entry doesn't exist you get a null list.)
1968 The exact meaning of the $gcos field varies but it usually contains
1969 the real name of the user (as opposed to the login name) and other
1970 information pertaining to the user. Beware, however, that in many
1971 system users are able to change this information and therefore it
1972 cannot be trusted and therefore the $gcos is tainted (see
1973 L<perlsec>). The $passwd and $shell, user's encrypted password and
1974 login shell, are also tainted, because of the same reason.
1976 In scalar context, you get the name, unless the function was a
1977 lookup by name, in which case you get the other thing, whatever it is.
1978 (If the entry doesn't exist you get the undefined value.) For example:
1980 $uid = getpwnam($name);
1981 $name = getpwuid($num);
1983 $gid = getgrnam($name);
1984 $name = getgrgid($num);
1988 In I<getpw*()> the fields $quota, $comment, and $expire are special
1989 cases in the sense that in many systems they are unsupported. If the
1990 $quota is unsupported, it is an empty scalar. If it is supported, it
1991 usually encodes the disk quota. If the $comment field is unsupported,
1992 it is an empty scalar. If it is supported it usually encodes some
1993 administrative comment about the user. In some systems the $quota
1994 field may be $change or $age, fields that have to do with password
1995 aging. In some systems the $comment field may be $class. The $expire
1996 field, if present, encodes the expiration period of the account or the
1997 password. For the availability and the exact meaning of these fields
1998 in your system, please consult your getpwnam(3) documentation and your
1999 F<pwd.h> file. You can also find out from within Perl what your
2000 $quota and $comment fields mean and whether you have the $expire field
2001 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2002 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2003 files are only supported if your vendor has implemented them in the
2004 intuitive fashion that calling the regular C library routines gets the
2005 shadow versions if you're running under privilege or if there exists
2006 the shadow(3) functions as found in System V ( this includes Solaris
2007 and Linux.) Those systems which implement a proprietary shadow password
2008 facility are unlikely to be supported.
2010 The $members value returned by I<getgr*()> is a space separated list of
2011 the login names of the members of the group.
2013 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2014 C, it will be returned to you via C<$?> if the function call fails. The
2015 C<@addrs> value returned by a successful call is a list of the raw
2016 addresses returned by the corresponding system library call. In the
2017 Internet domain, each address is four bytes long and you can unpack it
2018 by saying something like:
2020 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2022 The Socket library makes this slightly easier:
2025 $iaddr = inet_aton("127.1"); # or whatever address
2026 $name = gethostbyaddr($iaddr, AF_INET);
2028 # or going the other way
2029 $straddr = inet_ntoa($iaddr);
2031 If you get tired of remembering which element of the return list
2032 contains which return value, by-name interfaces are provided
2033 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2034 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2035 and C<User::grent>. These override the normal built-ins, supplying
2036 versions that return objects with the appropriate names
2037 for each field. For example:
2041 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2043 Even though it looks like they're the same method calls (uid),
2044 they aren't, because a C<File::stat> object is different from
2045 a C<User::pwent> object.
2047 =item getsockname SOCKET
2049 Returns the packed sockaddr address of this end of the SOCKET connection,
2050 in case you don't know the address because you have several different
2051 IPs that the connection might have come in on.
2054 $mysockaddr = getsockname(SOCK);
2055 ($port, $myaddr) = sockaddr_in($mysockaddr);
2056 printf "Connect to %s [%s]\n",
2057 scalar gethostbyaddr($myaddr, AF_INET),
2060 =item getsockopt SOCKET,LEVEL,OPTNAME
2062 Returns the socket option requested, or undef if there is an error.
2068 In list context, returns a (possibly empty) list of filename expansions on
2069 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2070 scalar context, glob iterates through such filename expansions, returning
2071 undef when the list is exhausted. This is the internal function
2072 implementing the C<< <*.c> >> operator, but you can use it directly. If
2073 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2074 more detail in L<perlop/"I/O Operators">.
2076 Beginning with v5.6.0, this operator is implemented using the standard
2077 C<File::Glob> extension. See L<File::Glob> for details.
2081 Converts a time as returned by the time function to an 8-element list
2082 with the time localized for the standard Greenwich time zone.
2083 Typically used as follows:
2086 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2089 All list elements are numeric, and come straight out of the C `struct
2090 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2091 specified time. $mday is the day of the month, and $mon is the month
2092 itself, in the range C<0..11> with 0 indicating January and 11
2093 indicating December. $year is the number of years since 1900. That
2094 is, $year is C<123> in year 2023. $wday is the day of the week, with
2095 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2096 the year, in the range C<0..364> (or C<0..365> in leap years.)
2098 Note that the $year element is I<not> simply the last two digits of
2099 the year. If you assume it is, then you create non-Y2K-compliant
2100 programs--and you wouldn't want to do that, would you?
2102 The proper way to get a complete 4-digit year is simply:
2106 And to get the last two digits of the year (e.g., '01' in 2001) do:
2108 $year = sprintf("%02d", $year % 100);
2110 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2112 In scalar context, C<gmtime()> returns the ctime(3) value:
2114 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2116 Also see the C<timegm> function provided by the C<Time::Local> module,
2117 and the strftime(3) function available via the POSIX module.
2119 This scalar value is B<not> locale dependent (see L<perllocale>), but
2120 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2121 strftime(3) and mktime(3) functions available via the POSIX module. To
2122 get somewhat similar but locale dependent date strings, set up your
2123 locale environment variables appropriately (please see L<perllocale>)
2124 and try for example:
2126 use POSIX qw(strftime);
2127 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2129 Note that the C<%a> and C<%b> escapes, which represent the short forms
2130 of the day of the week and the month of the year, may not necessarily
2131 be three characters wide in all locales.
2139 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2140 execution there. It may not be used to go into any construct that
2141 requires initialization, such as a subroutine or a C<foreach> loop. It
2142 also can't be used to go into a construct that is optimized away,
2143 or to get out of a block or subroutine given to C<sort>.
2144 It can be used to go almost anywhere else within the dynamic scope,
2145 including out of subroutines, but it's usually better to use some other
2146 construct such as C<last> or C<die>. The author of Perl has never felt the
2147 need to use this form of C<goto> (in Perl, that is--C is another matter).
2148 (The difference being that C does not offer named loops combined with
2149 loop control. Perl does, and this replaces most structured uses of C<goto>
2150 in other languages.)
2152 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2153 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2154 necessarily recommended if you're optimizing for maintainability:
2156 goto ("FOO", "BAR", "GLARCH")[$i];
2158 The C<goto-&NAME> form is quite different from the other forms of
2159 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2160 doesn't have the stigma associated with other gotos. Instead, it
2161 exits the current subroutine (losing any changes set by local()) and
2162 immediately calls in its place the named subroutine using the current
2163 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2164 load another subroutine and then pretend that the other subroutine had
2165 been called in the first place (except that any modifications to C<@_>
2166 in the current subroutine are propagated to the other subroutine.)
2167 After the C<goto>, not even C<caller> will be able to tell that this
2168 routine was called first.
2170 NAME needn't be the name of a subroutine; it can be a scalar variable
2171 containing a code reference, or a block which evaluates to a code
2174 =item grep BLOCK LIST
2176 =item grep EXPR,LIST
2178 This is similar in spirit to, but not the same as, grep(1) and its
2179 relatives. In particular, it is not limited to using regular expressions.
2181 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2182 C<$_> to each element) and returns the list value consisting of those
2183 elements for which the expression evaluated to true. In scalar
2184 context, returns the number of times the expression was true.
2186 @foo = grep(!/^#/, @bar); # weed out comments
2190 @foo = grep {!/^#/} @bar; # weed out comments
2192 Note that C<$_> is an alias to the list value, so it can be used to
2193 modify the elements of the LIST. While this is useful and supported,
2194 it can cause bizarre results if the elements of LIST are not variables.
2195 Similarly, grep returns aliases into the original list, much as a for
2196 loop's index variable aliases the list elements. That is, modifying an
2197 element of a list returned by grep (for example, in a C<foreach>, C<map>
2198 or another C<grep>) actually modifies the element in the original list.
2199 This is usually something to be avoided when writing clear code.
2201 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2207 Interprets EXPR as a hex string and returns the corresponding value.
2208 (To convert strings that might start with either 0, 0x, or 0b, see
2209 L</oct>.) If EXPR is omitted, uses C<$_>.
2211 print hex '0xAf'; # prints '175'
2212 print hex 'aF'; # same
2214 Hex strings may only represent integers. Strings that would cause
2215 integer overflow trigger a warning. Leading whitespace is not stripped,
2220 There is no builtin C<import> function. It is just an ordinary
2221 method (subroutine) defined (or inherited) by modules that wish to export
2222 names to another module. The C<use> function calls the C<import> method
2223 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2225 =item index STR,SUBSTR,POSITION
2227 =item index STR,SUBSTR
2229 The index function searches for one string within another, but without
2230 the wildcard-like behavior of a full regular-expression pattern match.
2231 It returns the position of the first occurrence of SUBSTR in STR at
2232 or after POSITION. If POSITION is omitted, starts searching from the
2233 beginning of the string. The return value is based at C<0> (or whatever
2234 you've set the C<$[> variable to--but don't do that). If the substring
2235 is not found, returns one less than the base, ordinarily C<-1>.
2241 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2242 You should not use this function for rounding: one because it truncates
2243 towards C<0>, and two because machine representations of floating point
2244 numbers can sometimes produce counterintuitive results. For example,
2245 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2246 because it's really more like -268.99999999999994315658 instead. Usually,
2247 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2248 functions will serve you better than will int().
2250 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2252 Implements the ioctl(2) function. You'll probably first have to say
2254 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2256 to get the correct function definitions. If F<ioctl.ph> doesn't
2257 exist or doesn't have the correct definitions you'll have to roll your
2258 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2259 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2260 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2261 written depending on the FUNCTION--a pointer to the string value of SCALAR
2262 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2263 has no string value but does have a numeric value, that value will be
2264 passed rather than a pointer to the string value. To guarantee this to be
2265 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2266 functions may be needed to manipulate the values of structures used by
2269 The return value of C<ioctl> (and C<fcntl>) is as follows:
2271 if OS returns: then Perl returns:
2273 0 string "0 but true"
2274 anything else that number
2276 Thus Perl returns true on success and false on failure, yet you can
2277 still easily determine the actual value returned by the operating
2280 $retval = ioctl(...) || -1;
2281 printf "System returned %d\n", $retval;
2283 The special string "C<0> but true" is exempt from B<-w> complaints
2284 about improper numeric conversions.
2286 Here's an example of setting a filehandle named C<REMOTE> to be
2287 non-blocking at the system level. You'll have to negotiate C<$|>
2288 on your own, though.
2290 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2292 $flags = fcntl(REMOTE, F_GETFL, 0)
2293 or die "Can't get flags for the socket: $!\n";
2295 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2296 or die "Can't set flags for the socket: $!\n";
2298 =item join EXPR,LIST
2300 Joins the separate strings of LIST into a single string with fields
2301 separated by the value of EXPR, and returns that new string. Example:
2303 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2305 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2306 first argument. Compare L</split>.
2310 Returns a list consisting of all the keys of the named hash. (In
2311 scalar context, returns the number of keys.) The keys are returned in
2312 an apparently random order. The actual random order is subject to
2313 change in future versions of perl, but it is guaranteed to be the same
2314 order as either the C<values> or C<each> function produces (given
2315 that the hash has not been modified). As a side effect, it resets
2318 Here is yet another way to print your environment:
2321 @values = values %ENV;
2323 print pop(@keys), '=', pop(@values), "\n";
2326 or how about sorted by key:
2328 foreach $key (sort(keys %ENV)) {
2329 print $key, '=', $ENV{$key}, "\n";
2332 The returned values are copies of the original keys in the hash, so
2333 modifying them will not affect the original hash. Compare L</values>.
2335 To sort a hash by value, you'll need to use a C<sort> function.
2336 Here's a descending numeric sort of a hash by its values:
2338 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2339 printf "%4d %s\n", $hash{$key}, $key;
2342 As an lvalue C<keys> allows you to increase the number of hash buckets
2343 allocated for the given hash. This can gain you a measure of efficiency if
2344 you know the hash is going to get big. (This is similar to pre-extending
2345 an array by assigning a larger number to $#array.) If you say
2349 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2350 in fact, since it rounds up to the next power of two. These
2351 buckets will be retained even if you do C<%hash = ()>, use C<undef
2352 %hash> if you want to free the storage while C<%hash> is still in scope.
2353 You can't shrink the number of buckets allocated for the hash using
2354 C<keys> in this way (but you needn't worry about doing this by accident,
2355 as trying has no effect).
2357 See also C<each>, C<values> and C<sort>.
2359 =item kill SIGNAL, LIST
2361 Sends a signal to a list of processes. Returns the number of
2362 processes successfully signaled (which is not necessarily the
2363 same as the number actually killed).
2365 $cnt = kill 1, $child1, $child2;
2368 If SIGNAL is zero, no signal is sent to the process. This is a
2369 useful way to check that the process is alive and hasn't changed
2370 its UID. See L<perlport> for notes on the portability of this
2373 Unlike in the shell, if SIGNAL is negative, it kills
2374 process groups instead of processes. (On System V, a negative I<PROCESS>
2375 number will also kill process groups, but that's not portable.) That
2376 means you usually want to use positive not negative signals. You may also
2377 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2383 The C<last> command is like the C<break> statement in C (as used in
2384 loops); it immediately exits the loop in question. If the LABEL is
2385 omitted, the command refers to the innermost enclosing loop. The
2386 C<continue> block, if any, is not executed:
2388 LINE: while (<STDIN>) {
2389 last LINE if /^$/; # exit when done with header
2393 C<last> cannot be used to exit a block which returns a value such as
2394 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2395 a grep() or map() operation.
2397 Note that a block by itself is semantically identical to a loop
2398 that executes once. Thus C<last> can be used to effect an early
2399 exit out of such a block.
2401 See also L</continue> for an illustration of how C<last>, C<next>, and
2408 Returns a lowercased version of EXPR. This is the internal function
2409 implementing the C<\L> escape in double-quoted strings. Respects
2410 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2411 and L<perlunicode> for more details about locale and Unicode support.
2413 If EXPR is omitted, uses C<$_>.
2419 Returns the value of EXPR with the first character lowercased. This
2420 is the internal function implementing the C<\l> escape in
2421 double-quoted strings. Respects current LC_CTYPE locale if C<use
2422 locale> in force. See L<perllocale> and L<perlunicode> for more
2423 details about locale and Unicode support.
2425 If EXPR is omitted, uses C<$_>.
2431 Returns the length in characters of the value of EXPR. If EXPR is
2432 omitted, returns length of C<$_>. Note that this cannot be used on
2433 an entire array or hash to find out how many elements these have.
2434 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2436 =item link OLDFILE,NEWFILE
2438 Creates a new filename linked to the old filename. Returns true for
2439 success, false otherwise.
2441 =item listen SOCKET,QUEUESIZE
2443 Does the same thing that the listen system call does. Returns true if
2444 it succeeded, false otherwise. See the example in
2445 L<perlipc/"Sockets: Client/Server Communication">.
2449 You really probably want to be using C<my> instead, because C<local> isn't
2450 what most people think of as "local". See
2451 L<perlsub/"Private Variables via my()"> for details.
2453 A local modifies the listed variables to be local to the enclosing
2454 block, file, or eval. If more than one value is listed, the list must
2455 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2456 for details, including issues with tied arrays and hashes.
2458 =item localtime EXPR
2460 Converts a time as returned by the time function to a 9-element list
2461 with the time analyzed for the local time zone. Typically used as
2465 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2468 All list elements are numeric, and come straight out of the C `struct
2469 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2470 specified time. $mday is the day of the month, and $mon is the month
2471 itself, in the range C<0..11> with 0 indicating January and 11
2472 indicating December. $year is the number of years since 1900. That
2473 is, $year is C<123> in year 2023. $wday is the day of the week, with
2474 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2475 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2476 is true if the specified time occurs during daylight savings time,
2479 Note that the $year element is I<not> simply the last two digits of
2480 the year. If you assume it is, then you create non-Y2K-compliant
2481 programs--and you wouldn't want to do that, would you?
2483 The proper way to get a complete 4-digit year is simply:
2487 And to get the last two digits of the year (e.g., '01' in 2001) do:
2489 $year = sprintf("%02d", $year % 100);
2491 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2493 In scalar context, C<localtime()> returns the ctime(3) value:
2495 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2497 This scalar value is B<not> locale dependent, see L<perllocale>, but
2498 instead a Perl builtin. Also see the C<Time::Local> module
2499 (to convert the second, minutes, hours, ... back to seconds since the
2500 stroke of midnight the 1st of January 1970, the value returned by
2501 time()), and the strftime(3) and mktime(3) functions available via the
2502 POSIX module. To get somewhat similar but locale dependent date
2503 strings, set up your locale environment variables appropriately
2504 (please see L<perllocale>) and try for example:
2506 use POSIX qw(strftime);
2507 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2509 Note that the C<%a> and C<%b>, the short forms of the day of the week
2510 and the month of the year, may not necessarily be three characters wide.
2514 This function places an advisory lock on a shared variable, or referenced
2515 object contained in I<THING> until the lock goes out of scope.
2517 lock() is a "weak keyword" : this means that if you've defined a function
2518 by this name (before any calls to it), that function will be called
2519 instead. (However, if you've said C<use threads>, lock() is always a
2520 keyword.) See L<threads>.
2526 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2527 returns log of C<$_>. To get the log of another base, use basic algebra:
2528 The base-N log of a number is equal to the natural log of that number
2529 divided by the natural log of N. For example:
2533 return log($n)/log(10);
2536 See also L</exp> for the inverse operation.
2542 Does the same thing as the C<stat> function (including setting the
2543 special C<_> filehandle) but stats a symbolic link instead of the file
2544 the symbolic link points to. If symbolic links are unimplemented on
2545 your system, a normal C<stat> is done. For much more detailed
2546 information, please see the documentation for C<stat>.
2548 If EXPR is omitted, stats C<$_>.
2552 The match operator. See L<perlop>.
2554 =item map BLOCK LIST
2558 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2559 C<$_> to each element) and returns the list value composed of the
2560 results of each such evaluation. In scalar context, returns the
2561 total number of elements so generated. Evaluates BLOCK or EXPR in
2562 list context, so each element of LIST may produce zero, one, or
2563 more elements in the returned value.
2565 @chars = map(chr, @nums);
2567 translates a list of numbers to the corresponding characters. And
2569 %hash = map { getkey($_) => $_ } @array;
2571 is just a funny way to write
2574 foreach $_ (@array) {
2575 $hash{getkey($_)} = $_;
2578 Note that C<$_> is an alias to the list value, so it can be used to
2579 modify the elements of the LIST. While this is useful and supported,
2580 it can cause bizarre results if the elements of LIST are not variables.
2581 Using a regular C<foreach> loop for this purpose would be clearer in
2582 most cases. See also L</grep> for an array composed of those items of
2583 the original list for which the BLOCK or EXPR evaluates to true.
2585 C<{> starts both hash references and blocks, so C<map { ...> could be either
2586 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2587 ahead for the closing C<}> it has to take a guess at which its dealing with
2588 based what it finds just after the C<{>. Usually it gets it right, but if it
2589 doesn't it won't realize something is wrong until it gets to the C<}> and
2590 encounters the missing (or unexpected) comma. The syntax error will be
2591 reported close to the C<}> but you'll need to change something near the C<{>
2592 such as using a unary C<+> to give perl some help:
2594 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2595 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2596 %hash = map { ("\L$_", 1) } @array # this also works
2597 %hash = map { lc($_), 1 } @array # as does this.
2598 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2600 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2602 or to force an anon hash constructor use C<+{>
2604 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2606 and you get list of anonymous hashes each with only 1 entry.
2608 =item mkdir FILENAME,MASK
2610 =item mkdir FILENAME
2612 Creates the directory specified by FILENAME, with permissions
2613 specified by MASK (as modified by C<umask>). If it succeeds it
2614 returns true, otherwise it returns false and sets C<$!> (errno).
2615 If omitted, MASK defaults to 0777.
2617 In general, it is better to create directories with permissive MASK,
2618 and let the user modify that with their C<umask>, than it is to supply
2619 a restrictive MASK and give the user no way to be more permissive.
2620 The exceptions to this rule are when the file or directory should be
2621 kept private (mail files, for instance). The perlfunc(1) entry on
2622 C<umask> discusses the choice of MASK in more detail.
2624 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2625 number of trailing slashes. Some operating and filesystems do not get
2626 this right, so Perl automatically removes all trailing slashes to keep
2629 =item msgctl ID,CMD,ARG
2631 Calls the System V IPC function msgctl(2). You'll probably have to say
2635 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2636 then ARG must be a variable which will hold the returned C<msqid_ds>
2637 structure. Returns like C<ioctl>: the undefined value for error,
2638 C<"0 but true"> for zero, or the actual return value otherwise. See also
2639 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2641 =item msgget KEY,FLAGS
2643 Calls the System V IPC function msgget(2). Returns the message queue
2644 id, or the undefined value if there is an error. See also
2645 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2647 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2649 Calls the System V IPC function msgrcv to receive a message from
2650 message queue ID into variable VAR with a maximum message size of
2651 SIZE. Note that when a message is received, the message type as a
2652 native long integer will be the first thing in VAR, followed by the
2653 actual message. This packing may be opened with C<unpack("l! a*")>.
2654 Taints the variable. Returns true if successful, or false if there is
2655 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2656 C<IPC::SysV::Msg> documentation.
2658 =item msgsnd ID,MSG,FLAGS
2660 Calls the System V IPC function msgsnd to send the message MSG to the
2661 message queue ID. MSG must begin with the native long integer message
2662 type, and be followed by the length of the actual message, and finally
2663 the message itself. This kind of packing can be achieved with
2664 C<pack("l! a*", $type, $message)>. Returns true if successful,
2665 or false if there is an error. See also C<IPC::SysV>
2666 and C<IPC::SysV::Msg> documentation.
2672 =item my EXPR : ATTRS
2674 =item my TYPE EXPR : ATTRS
2676 A C<my> declares the listed variables to be local (lexically) to the
2677 enclosing block, file, or C<eval>. If more than one value is listed,
2678 the list must be placed in parentheses.
2680 The exact semantics and interface of TYPE and ATTRS are still
2681 evolving. TYPE is currently bound to the use of C<fields> pragma,
2682 and attributes are handled using the C<attributes> pragma, or starting
2683 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2684 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2685 L<attributes>, and L<Attribute::Handlers>.
2691 The C<next> command is like the C<continue> statement in C; it starts
2692 the next iteration of the loop:
2694 LINE: while (<STDIN>) {
2695 next LINE if /^#/; # discard comments
2699 Note that if there were a C<continue> block on the above, it would get
2700 executed even on discarded lines. If the LABEL is omitted, the command
2701 refers to the innermost enclosing loop.
2703 C<next> cannot be used to exit a block which returns a value such as
2704 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2705 a grep() or map() operation.
2707 Note that a block by itself is semantically identical to a loop
2708 that executes once. Thus C<next> will exit such a block early.
2710 See also L</continue> for an illustration of how C<last>, C<next>, and
2713 =item no Module VERSION LIST
2715 =item no Module VERSION
2717 =item no Module LIST
2721 See the C<use> function, which C<no> is the opposite of.
2727 Interprets EXPR as an octal string and returns the corresponding
2728 value. (If EXPR happens to start off with C<0x>, interprets it as a
2729 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2730 binary string. Leading whitespace is ignored in all three cases.)
2731 The following will handle decimal, binary, octal, and hex in the standard
2734 $val = oct($val) if $val =~ /^0/;
2736 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2737 in octal), use sprintf() or printf():
2739 $perms = (stat("filename"))[2] & 07777;
2740 $oct_perms = sprintf "%lo", $perms;
2742 The oct() function is commonly used when a string such as C<644> needs
2743 to be converted into a file mode, for example. (Although perl will
2744 automatically convert strings into numbers as needed, this automatic
2745 conversion assumes base 10.)
2747 =item open FILEHANDLE,EXPR
2749 =item open FILEHANDLE,MODE,EXPR
2751 =item open FILEHANDLE,MODE,EXPR,LIST
2753 =item open FILEHANDLE,MODE,REFERENCE
2755 =item open FILEHANDLE
2757 Opens the file whose filename is given by EXPR, and associates it with
2760 (The following is a comprehensive reference to open(): for a gentler
2761 introduction you may consider L<perlopentut>.)
2763 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2764 the variable is assigned a reference to a new anonymous filehandle,
2765 otherwise if FILEHANDLE is an expression, its value is used as the name of
2766 the real filehandle wanted. (This is considered a symbolic reference, so
2767 C<use strict 'refs'> should I<not> be in effect.)
2769 If EXPR is omitted, the scalar variable of the same name as the
2770 FILEHANDLE contains the filename. (Note that lexical variables--those
2771 declared with C<my>--will not work for this purpose; so if you're
2772 using C<my>, specify EXPR in your call to open.)
2774 If three or more arguments are specified then the mode of opening and
2775 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2776 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2777 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2778 the file is opened for appending, again being created if necessary.
2780 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2781 indicate that you want both read and write access to the file; thus
2782 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2783 '+>' >> mode would clobber the file first. You can't usually use
2784 either read-write mode for updating textfiles, since they have
2785 variable length records. See the B<-i> switch in L<perlrun> for a
2786 better approach. The file is created with permissions of C<0666>
2787 modified by the process' C<umask> value.
2789 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2790 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2792 In the 2-arguments (and 1-argument) form of the call the mode and
2793 filename should be concatenated (in this order), possibly separated by
2794 spaces. It is possible to omit the mode in these forms if the mode is
2797 If the filename begins with C<'|'>, the filename is interpreted as a
2798 command to which output is to be piped, and if the filename ends with a
2799 C<'|'>, the filename is interpreted as a command which pipes output to
2800 us. See L<perlipc/"Using open() for IPC">
2801 for more examples of this. (You are not allowed to C<open> to a command
2802 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2803 and L<perlipc/"Bidirectional Communication with Another Process">
2806 For three or more arguments if MODE is C<'|-'>, the filename is
2807 interpreted as a command to which output is to be piped, and if MODE
2808 is C<'-|'>, the filename is interpreted as a command which pipes
2809 output to us. In the 2-arguments (and 1-argument) form one should
2810 replace dash (C<'-'>) with the command.
2811 See L<perlipc/"Using open() for IPC"> for more examples of this.
2812 (You are not allowed to C<open> to a command that pipes both in I<and>
2813 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2814 L<perlipc/"Bidirectional Communication"> for alternatives.)
2816 In the three-or-more argument form of pipe opens, if LIST is specified
2817 (extra arguments after the command name) then LIST becomes arguments
2818 to the command invoked if the platform supports it. The meaning of
2819 C<open> with more than three arguments for non-pipe modes is not yet
2820 specified. Experimental "layers" may give extra LIST arguments
2823 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2824 and opening C<< '>-' >> opens STDOUT.
2826 You may use the three-argument form of open to specify IO "layers"
2827 (sometimes also referred to as "disciplines") to be applied to the handle
2828 that affect how the input and output are processed (see L<open> and
2829 L<PerlIO> for more details). For example
2831 open(FH, "<:utf8", "file")
2833 will open the UTF-8 encoded file containing Unicode characters,
2834 see L<perluniintro>. (Note that if layers are specified in the
2835 three-arg form then default layers set by the C<open> pragma are
2838 Open returns nonzero upon success, the undefined value otherwise. If
2839 the C<open> involved a pipe, the return value happens to be the pid of
2842 If you're running Perl on a system that distinguishes between text
2843 files and binary files, then you should check out L</binmode> for tips
2844 for dealing with this. The key distinction between systems that need
2845 C<binmode> and those that don't is their text file formats. Systems
2846 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2847 character, and which encode that character in C as C<"\n">, do not
2848 need C<binmode>. The rest need it.
2850 When opening a file, it's usually a bad idea to continue normal execution
2851 if the request failed, so C<open> is frequently used in connection with
2852 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2853 where you want to make a nicely formatted error message (but there are
2854 modules that can help with that problem)) you should always check
2855 the return value from opening a file. The infrequent exception is when
2856 working with an unopened filehandle is actually what you want to do.
2858 As a special case the 3 arg form with a read/write mode and the third
2859 argument being C<undef>:
2861 open(TMP, "+>", undef) or die ...
2863 opens a filehandle to an anonymous temporary file. Also using "+<"
2864 works for symmetry, but you really should consider writing something
2865 to the temporary file first. You will need to seek() to do the
2866 reading. Starting from Perl 5.8.1 the temporary files are created
2867 using the File::Temp module for greater portability, in Perl 5.8.0 the
2868 mkstemp() system call (which has known bugs in some platforms) was used.
2870 File handles can be opened to "in memory" files held in Perl scalars via:
2872 open($fh, '>', \$variable) || ..
2874 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2875 file, you have to close it first:
2878 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2883 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2884 while (<ARTICLE>) {...
2886 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2887 # if the open fails, output is discarded
2889 open(DBASE, '+<', 'dbase.mine') # open for update
2890 or die "Can't open 'dbase.mine' for update: $!";
2892 open(DBASE, '+<dbase.mine') # ditto
2893 or die "Can't open 'dbase.mine' for update: $!";
2895 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2896 or die "Can't start caesar: $!";
2898 open(ARTICLE, "caesar <$article |") # ditto
2899 or die "Can't start caesar: $!";
2901 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2902 or die "Can't start sort: $!";
2905 open(MEMORY,'>', \$var)
2906 or die "Can't open memory file: $!";
2907 print MEMORY "foo!\n"; # output will end up in $var
2909 # process argument list of files along with any includes
2911 foreach $file (@ARGV) {
2912 process($file, 'fh00');
2916 my($filename, $input) = @_;
2917 $input++; # this is a string increment
2918 unless (open($input, $filename)) {
2919 print STDERR "Can't open $filename: $!\n";
2924 while (<$input>) { # note use of indirection
2925 if (/^#include "(.*)"/) {
2926 process($1, $input);
2933 You may also, in the Bourne shell tradition, specify an EXPR beginning
2934 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2935 name of a filehandle (or file descriptor, if numeric) to be
2936 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2937 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2938 mode you specify should match the mode of the original filehandle.
2939 (Duping a filehandle does not take into account any existing contents of
2940 IO buffers.) If you use the 3 arg form then you can pass either a number,
2941 the name of a filehandle or the normal "reference to a glob".
2943 Here is a script that saves, redirects, and restores C<STDOUT> and
2944 C<STDERR> using various methods:
2947 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2948 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2950 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2951 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2953 select STDERR; $| = 1; # make unbuffered
2954 select STDOUT; $| = 1; # make unbuffered
2956 print STDOUT "stdout 1\n"; # this works for
2957 print STDERR "stderr 1\n"; # subprocesses too
2962 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2963 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2965 print STDOUT "stdout 2\n";
2966 print STDERR "stderr 2\n";
2968 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2969 do an equivalent of C's C<fdopen> of that file descriptor; this is
2970 more parsimonious of file descriptors. For example:
2972 open(FILEHANDLE, "<&=$fd")
2976 open(FILEHANDLE, "<&=", $fd)
2978 Note that if Perl is using the standard C libraries' fdopen() then on
2979 many UNIX systems, fdopen() is known to fail when file descriptors
2980 exceed a certain value, typically 255. If you need more file
2981 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2983 You can see whether Perl has been compiled with PerlIO or not by
2984 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2985 is C<define>, you have PerlIO, otherwise you don't.
2987 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2988 with 2-arguments (or 1-argument) form of open(), then
2989 there is an implicit fork done, and the return value of open is the pid
2990 of the child within the parent process, and C<0> within the child
2991 process. (Use C<defined($pid)> to determine whether the open was successful.)
2992 The filehandle behaves normally for the parent, but i/o to that
2993 filehandle is piped from/to the STDOUT/STDIN of the child process.
2994 In the child process the filehandle isn't opened--i/o happens from/to
2995 the new STDOUT or STDIN. Typically this is used like the normal
2996 piped open when you want to exercise more control over just how the
2997 pipe command gets executed, such as when you are running setuid, and
2998 don't want to have to scan shell commands for metacharacters.
2999 The following triples are more or less equivalent:
3001 open(FOO, "|tr '[a-z]' '[A-Z]'");
3002 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3003 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3004 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3006 open(FOO, "cat -n '$file'|");
3007 open(FOO, '-|', "cat -n '$file'");
3008 open(FOO, '-|') || exec 'cat', '-n', $file;
3009 open(FOO, '-|', "cat", '-n', $file);
3011 The last example in each block shows the pipe as "list form", which is
3012 not yet supported on all platforms. A good rule of thumb is that if
3013 your platform has true C<fork()> (in other words, if your platform is
3014 UNIX) you can use the list form.
3016 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3018 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3019 output before any operation that may do a fork, but this may not be
3020 supported on some platforms (see L<perlport>). To be safe, you may need
3021 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3022 of C<IO::Handle> on any open handles.
3024 On systems that support a close-on-exec flag on files, the flag will
3025 be set for the newly opened file descriptor as determined by the value
3026 of $^F. See L<perlvar/$^F>.
3028 Closing any piped filehandle causes the parent process to wait for the
3029 child to finish, and returns the status value in C<$?>.
3031 The filename passed to 2-argument (or 1-argument) form of open() will
3032 have leading and trailing whitespace deleted, and the normal
3033 redirection characters honored. This property, known as "magic open",
3034 can often be used to good effect. A user could specify a filename of
3035 F<"rsh cat file |">, or you could change certain filenames as needed:
3037 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3038 open(FH, $filename) or die "Can't open $filename: $!";
3040 Use 3-argument form to open a file with arbitrary weird characters in it,
3042 open(FOO, '<', $file);
3044 otherwise it's necessary to protect any leading and trailing whitespace:
3046 $file =~ s#^(\s)#./$1#;
3047 open(FOO, "< $file\0");
3049 (this may not work on some bizarre filesystems). One should
3050 conscientiously choose between the I<magic> and 3-arguments form
3055 will allow the user to specify an argument of the form C<"rsh cat file |">,
3056 but will not work on a filename which happens to have a trailing space, while
3058 open IN, '<', $ARGV[0];
3060 will have exactly the opposite restrictions.
3062 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3063 should use the C<sysopen> function, which involves no such magic (but
3064 may use subtly different filemodes than Perl open(), which is mapped
3065 to C fopen()). This is
3066 another way to protect your filenames from interpretation. For example:
3069 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3070 or die "sysopen $path: $!";
3071 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3072 print HANDLE "stuff $$\n";
3074 print "File contains: ", <HANDLE>;
3076 Using the constructor from the C<IO::Handle> package (or one of its
3077 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3078 filehandles that have the scope of whatever variables hold references to
3079 them, and automatically close whenever and however you leave that scope:
3083 sub read_myfile_munged {
3085 my $handle = new IO::File;
3086 open($handle, "myfile") or die "myfile: $!";
3088 or return (); # Automatically closed here.
3089 mung $first or die "mung failed"; # Or here.
3090 return $first, <$handle> if $ALL; # Or here.
3094 See L</seek> for some details about mixing reading and writing.
3096 =item opendir DIRHANDLE,EXPR
3098 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3099 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3100 DIRHANDLE may be an expression whose value can be used as an indirect
3101 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3102 scalar variable (or array or hash element), the variable is assigned a
3103 reference to a new anonymous dirhandle.
3104 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3110 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3111 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3114 For the reverse, see L</chr>.
3115 See L<perlunicode> and L<encoding> for more about Unicode.
3121 =item our EXPR : ATTRS
3123 =item our TYPE EXPR : ATTRS
3125 An C<our> declares the listed variables to be valid globals within
3126 the enclosing block, file, or C<eval>. That is, it has the same
3127 scoping rules as a "my" declaration, but does not create a local
3128 variable. If more than one value is listed, the list must be placed
3129 in parentheses. The C<our> declaration has no semantic effect unless
3130 "use strict vars" is in effect, in which case it lets you use the
3131 declared global variable without qualifying it with a package name.
3132 (But only within the lexical scope of the C<our> declaration. In this
3133 it differs from "use vars", which is package scoped.)
3135 An C<our> declaration declares a global variable that will be visible
3136 across its entire lexical scope, even across package boundaries. The
3137 package in which the variable is entered is determined at the point
3138 of the declaration, not at the point of use. This means the following
3142 our $bar; # declares $Foo::bar for rest of lexical scope
3146 print $bar; # prints 20
3148 Multiple C<our> declarations in the same lexical scope are allowed
3149 if they are in different packages. If they happened to be in the same
3150 package, Perl will emit warnings if you have asked for them.
3154 our $bar; # declares $Foo::bar for rest of lexical scope
3158 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3159 print $bar; # prints 30
3161 our $bar; # emits warning
3163 An C<our> declaration may also have a list of attributes associated
3166 The exact semantics and interface of TYPE and ATTRS are still
3167 evolving. TYPE is currently bound to the use of C<fields> pragma,
3168 and attributes are handled using the C<attributes> pragma, or starting
3169 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3170 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3171 L<attributes>, and L<Attribute::Handlers>.
3173 The only currently recognized C<our()> attribute is C<unique> which
3174 indicates that a single copy of the global is to be used by all
3175 interpreters should the program happen to be running in a
3176 multi-interpreter environment. (The default behaviour would be for
3177 each interpreter to have its own copy of the global.) Examples:
3179 our @EXPORT : unique = qw(foo);
3180 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3181 our $VERSION : unique = "1.00";
3183 Note that this attribute also has the effect of making the global
3184 readonly when the first new interpreter is cloned (for example,
3185 when the first new thread is created).
3187 Multi-interpreter environments can come to being either through the
3188 fork() emulation on Windows platforms, or by embedding perl in a
3189 multi-threaded application. The C<unique> attribute does nothing in
3190 all other environments.
3192 =item pack TEMPLATE,LIST
3194 Takes a LIST of values and converts it into a string using the rules
3195 given by the TEMPLATE. The resulting string is the concatenation of
3196 the converted values. Typically, each converted value looks
3197 like its machine-level representation. For example, on 32-bit machines
3198 a converted integer may be represented by a sequence of 4 bytes.
3200 The TEMPLATE is a sequence of characters that give the order and type
3201 of values, as follows:
3203 a A string with arbitrary binary data, will be null padded.
3204 A A text (ASCII) string, will be space padded.
3205 Z A null terminated (ASCIZ) string, will be null padded.
3207 b A bit string (ascending bit order inside each byte, like vec()).
3208 B A bit string (descending bit order inside each byte).
3209 h A hex string (low nybble first).
3210 H A hex string (high nybble first).
3212 c A signed char value.
3213 C An unsigned char value. Only does bytes. See U for Unicode.
3215 s A signed short value.
3216 S An unsigned short value.
3217 (This 'short' is _exactly_ 16 bits, which may differ from
3218 what a local C compiler calls 'short'. If you want
3219 native-length shorts, use the '!' suffix.)
3221 i A signed integer value.
3222 I An unsigned integer value.
3223 (This 'integer' is _at_least_ 32 bits wide. Its exact
3224 size depends on what a local C compiler calls 'int',
3225 and may even be larger than the 'long' described in
3228 l A signed long value.
3229 L An unsigned long value.
3230 (This 'long' is _exactly_ 32 bits, which may differ from
3231 what a local C compiler calls 'long'. If you want
3232 native-length longs, use the '!' suffix.)
3234 n An unsigned short in "network" (big-endian) order.
3235 N An unsigned long in "network" (big-endian) order.
3236 v An unsigned short in "VAX" (little-endian) order.
3237 V An unsigned long in "VAX" (little-endian) order.
3238 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3239 _exactly_ 32 bits, respectively.)
3241 q A signed quad (64-bit) value.
3242 Q An unsigned quad value.
3243 (Quads are available only if your system supports 64-bit
3244 integer values _and_ if Perl has been compiled to support those.
3245 Causes a fatal error otherwise.)
3247 j A signed integer value (a Perl internal integer, IV).
3248 J An unsigned integer value (a Perl internal unsigned integer, UV).
3250 f A single-precision float in the native format.
3251 d A double-precision float in the native format.
3253 F A floating point value in the native native format
3254 (a Perl internal floating point value, NV).
3255 D A long double-precision float in the native format.
3256 (Long doubles are available only if your system supports long
3257 double values _and_ if Perl has been compiled to support those.
3258 Causes a fatal error otherwise.)
3260 p A pointer to a null-terminated string.
3261 P A pointer to a structure (fixed-length string).
3263 u A uuencoded string.
3264 U A Unicode character number. Encodes to UTF-8 internally
3265 (or UTF-EBCDIC in EBCDIC platforms).
3267 w A BER compressed integer. Its bytes represent an unsigned
3268 integer in base 128, most significant digit first, with as
3269 few digits as possible. Bit eight (the high bit) is set
3270 on each byte except the last.
3274 @ Null fill to absolute position, counted from the start of
3275 the innermost ()-group.
3276 ( Start of a ()-group.
3278 The following rules apply:
3284 Each letter may optionally be followed by a number giving a repeat
3285 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3286 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3287 many values from the LIST. A C<*> for the repeat count means to use
3288 however many items are left, except for C<@>, C<x>, C<X>, where it is
3289 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3290 is the same). A numeric repeat count may optionally be enclosed in
3291 brackets, as in C<pack 'C[80]', @arr>.
3293 One can replace the numeric repeat count by a template enclosed in brackets;
3294 then the packed length of this template in bytes is used as a count.
3295 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3296 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3297 If the template in brackets contains alignment commands (such as C<x![d]>),
3298 its packed length is calculated as if the start of the template has the maximal
3301 When used with C<Z>, C<*> results in the addition of a trailing null
3302 byte (so the packed result will be one longer than the byte C<length>
3305 The repeat count for C<u> is interpreted as the maximal number of bytes
3306 to encode per line of output, with 0 and 1 replaced by 45.
3310 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3311 string of length count, padding with nulls or spaces as necessary. When
3312 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3313 after the first null, and C<a> returns data verbatim. When packing,
3314 C<a>, and C<Z> are equivalent.
3316 If the value-to-pack is too long, it is truncated. If too long and an
3317 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3318 by a null byte. Thus C<Z> always packs a trailing null byte under
3323 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3324 Each byte of the input field of pack() generates 1 bit of the result.
3325 Each result bit is based on the least-significant bit of the corresponding
3326 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3327 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3329 Starting from the beginning of the input string of pack(), each 8-tuple
3330 of bytes is converted to 1 byte of output. With format C<b>
3331 the first byte of the 8-tuple determines the least-significant bit of a
3332 byte, and with format C<B> it determines the most-significant bit of
3335 If the length of the input string is not exactly divisible by 8, the
3336 remainder is packed as if the input string were padded by null bytes
3337 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3339 If the input string of pack() is longer than needed, extra bytes are ignored.
3340 A C<*> for the repeat count of pack() means to use all the bytes of
3341 the input field. On unpack()ing the bits are converted to a string
3342 of C<"0">s and C<"1">s.
3346 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3347 representable as hexadecimal digits, 0-9a-f) long.
3349 Each byte of the input field of pack() generates 4 bits of the result.
3350 For non-alphabetical bytes the result is based on the 4 least-significant
3351 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3352 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3353 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3354 is compatible with the usual hexadecimal digits, so that C<"a"> and
3355 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3356 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3358 Starting from the beginning of the input string of pack(), each pair
3359 of bytes is converted to 1 byte of output. With format C<h> the
3360 first byte of the pair determines the least-significant nybble of the
3361 output byte, and with format C<H> it determines the most-significant
3364 If the length of the input string is not even, it behaves as if padded
3365 by a null byte at the end. Similarly, during unpack()ing the "extra"
3366 nybbles are ignored.
3368 If the input string of pack() is longer than needed, extra bytes are ignored.
3369 A C<*> for the repeat count of pack() means to use all the bytes of
3370 the input field. On unpack()ing the bits are converted to a string
3371 of hexadecimal digits.
3375 The C<p> type packs a pointer to a null-terminated string. You are
3376 responsible for ensuring the string is not a temporary value (which can
3377 potentially get deallocated before you get around to using the packed result).
3378 The C<P> type packs a pointer to a structure of the size indicated by the
3379 length. A NULL pointer is created if the corresponding value for C<p> or
3380 C<P> is C<undef>, similarly for unpack().
3384 The C</> template character allows packing and unpacking of strings where
3385 the packed structure contains a byte count followed by the string itself.
3386 You write I<length-item>C</>I<string-item>.
3388 The I<length-item> can be any C<pack> template letter, and describes
3389 how the length value is packed. The ones likely to be of most use are
3390 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3391 SNMP) and C<N> (for Sun XDR).
3393 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3394 C<"Z*">. For C<unpack> the length of the string is obtained from the
3395 I<length-item>, but if you put in the '*' it will be ignored. For all other
3396 codes, C<unpack> applies the length value to the next item, which must not
3397 have a repeat count.
3399 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3400 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3401 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3403 The I<length-item> is not returned explicitly from C<unpack>.
3405 Adding a count to the I<length-item> letter is unlikely to do anything
3406 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3407 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3408 which Perl does not regard as legal in numeric strings.
3412 The integer types C<s>, C<S>, C<l>, and C<L> may be
3413 immediately followed by a C<!> suffix to signify native shorts or
3414 longs--as you can see from above for example a bare C<l> does mean
3415 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3416 may be larger. This is an issue mainly in 64-bit platforms. You can
3417 see whether using C<!> makes any difference by
3419 print length(pack("s")), " ", length(pack("s!")), "\n";
3420 print length(pack("l")), " ", length(pack("l!")), "\n";
3422 C<i!> and C<I!> also work but only because of completeness;
3423 they are identical to C<i> and C<I>.
3425 The actual sizes (in bytes) of native shorts, ints, longs, and long
3426 longs on the platform where Perl was built are also available via
3430 print $Config{shortsize}, "\n";
3431 print $Config{intsize}, "\n";
3432 print $Config{longsize}, "\n";
3433 print $Config{longlongsize}, "\n";
3435 (The C<$Config{longlongsize}> will be undefined if your system does
3436 not support long longs.)
3440 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3441 are inherently non-portable between processors and operating systems
3442 because they obey the native byteorder and endianness. For example a
3443 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3444 (arranged in and handled by the CPU registers) into bytes as
3446 0x12 0x34 0x56 0x78 # big-endian
3447 0x78 0x56 0x34 0x12 # little-endian
3449 Basically, the Intel and VAX CPUs are little-endian, while everybody
3450 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3451 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3452 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3455 The names `big-endian' and `little-endian' are comic references to
3456 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3457 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3458 the egg-eating habits of the Lilliputians.
3460 Some systems may have even weirder byte orders such as
3465 You can see your system's preference with
3467 print join(" ", map { sprintf "%#02x", $_ }
3468 unpack("C*",pack("L",0x12345678))), "\n";
3470 The byteorder on the platform where Perl was built is also available
3474 print $Config{byteorder}, "\n";
3476 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3477 and C<'87654321'> are big-endian.
3479 If you want portable packed integers use the formats C<n>, C<N>,
3480 C<v>, and C<V>, their byte endianness and size are known.
3481 See also L<perlport>.
3485 Real numbers (floats and doubles) are in the native machine format only;
3486 due to the multiplicity of floating formats around, and the lack of a
3487 standard "network" representation, no facility for interchange has been
3488 made. This means that packed floating point data written on one machine
3489 may not be readable on another - even if both use IEEE floating point
3490 arithmetic (as the endian-ness of the memory representation is not part
3491 of the IEEE spec). See also L<perlport>.
3493 Note that Perl uses doubles internally for all numeric calculation, and
3494 converting from double into float and thence back to double again will
3495 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3500 If the pattern begins with a C<U>, the resulting string will be treated
3501 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3502 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3503 characters. If you don't want this to happen, you can begin your pattern
3504 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3505 string, and then follow this with a C<U*> somewhere in your pattern.
3509 You must yourself do any alignment or padding by inserting for example
3510 enough C<'x'>es while packing. There is no way to pack() and unpack()
3511 could know where the bytes are going to or coming from. Therefore
3512 C<pack> (and C<unpack>) handle their output and input as flat
3517 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3518 take a repeat count, both as postfix, and for unpack() also via the C</>
3519 template character. Within each repetition of a group, positioning with
3520 C<@> starts again at 0. Therefore, the result of
3522 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3524 is the string "\0a\0\0bc".
3529 C<x> and C<X> accept C<!> modifier. In this case they act as
3530 alignment commands: they jump forward/back to the closest position
3531 aligned at a multiple of C<count> bytes. For example, to pack() or
3532 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3533 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3534 aligned on the double's size.
3536 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3537 both result in no-ops.
3541 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3542 White space may be used to separate pack codes from each other, but
3543 a C<!> modifier and a repeat count must follow immediately.
3547 If TEMPLATE requires more arguments to pack() than actually given, pack()
3548 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3549 to pack() than actually given, extra arguments are ignored.
3555 $foo = pack("CCCC",65,66,67,68);
3557 $foo = pack("C4",65,66,67,68);
3559 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3560 # same thing with Unicode circled letters
3562 $foo = pack("ccxxcc",65,66,67,68);
3565 # note: the above examples featuring "C" and "c" are true
3566 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3567 # and UTF-8. In EBCDIC the first example would be
3568 # $foo = pack("CCCC",193,194,195,196);
3570 $foo = pack("s2",1,2);
3571 # "\1\0\2\0" on little-endian
3572 # "\0\1\0\2" on big-endian
3574 $foo = pack("a4","abcd","x","y","z");
3577 $foo = pack("aaaa","abcd","x","y","z");
3580 $foo = pack("a14","abcdefg");
3581 # "abcdefg\0\0\0\0\0\0\0"
3583 $foo = pack("i9pl", gmtime);
3584 # a real struct tm (on my system anyway)
3586 $utmp_template = "Z8 Z8 Z16 L";
3587 $utmp = pack($utmp_template, @utmp1);
3588 # a struct utmp (BSDish)
3590 @utmp2 = unpack($utmp_template, $utmp);
3591 # "@utmp1" eq "@utmp2"
3594 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3597 $foo = pack('sx2l', 12, 34);
3598 # short 12, two zero bytes padding, long 34
3599 $bar = pack('s@4l', 12, 34);
3600 # short 12, zero fill to position 4, long 34
3603 The same template may generally also be used in unpack().
3605 =item package NAMESPACE
3609 Declares the compilation unit as being in the given namespace. The scope
3610 of the package declaration is from the declaration itself through the end
3611 of the enclosing block, file, or eval (the same as the C<my> operator).
3612 All further unqualified dynamic identifiers will be in this namespace.
3613 A package statement affects only dynamic variables--including those
3614 you've used C<local> on--but I<not> lexical variables, which are created
3615 with C<my>. Typically it would be the first declaration in a file to
3616 be included by the C<require> or C<use> operator. You can switch into a
3617 package in more than one place; it merely influences which symbol table
3618 is used by the compiler for the rest of that block. You can refer to
3619 variables and filehandles in other packages by prefixing the identifier
3620 with the package name and a double colon: C<$Package::Variable>.
3621 If the package name is null, the C<main> package as assumed. That is,
3622 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3623 still seen in older code).
3625 If NAMESPACE is omitted, then there is no current package, and all
3626 identifiers must be fully qualified or lexicals. However, you are
3627 strongly advised not to make use of this feature. Its use can cause
3628 unexpected behaviour, even crashing some versions of Perl. It is
3629 deprecated, and will be removed from a future release.
3631 See L<perlmod/"Packages"> for more information about packages, modules,
3632 and classes. See L<perlsub> for other scoping issues.
3634 =item pipe READHANDLE,WRITEHANDLE
3636 Opens a pair of connected pipes like the corresponding system call.
3637 Note that if you set up a loop of piped processes, deadlock can occur
3638 unless you are very careful. In addition, note that Perl's pipes use
3639 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3640 after each command, depending on the application.
3642 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3643 for examples of such things.
3645 On systems that support a close-on-exec flag on files, the flag will be set
3646 for the newly opened file descriptors as determined by the value of $^F.
3653 Pops and returns the last value of the array, shortening the array by
3654 one element. Has an effect similar to
3658 If there are no elements in the array, returns the undefined value
3659 (although this may happen at other times as well). If ARRAY is
3660 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3661 array in subroutines, just like C<shift>.
3667 Returns the offset of where the last C<m//g> search left off for the variable
3668 in question (C<$_> is used when the variable is not specified). May be
3669 modified to change that offset. Such modification will also influence
3670 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3673 =item print FILEHANDLE LIST
3679 Prints a string or a list of strings. Returns true if successful.
3680 FILEHANDLE may be a scalar variable name, in which case the variable
3681 contains the name of or a reference to the filehandle, thus introducing
3682 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3683 the next token is a term, it may be misinterpreted as an operator
3684 unless you interpose a C<+> or put parentheses around the arguments.)
3685 If FILEHANDLE is omitted, prints by default to standard output (or
3686 to the last selected output channel--see L</select>). If LIST is
3687 also omitted, prints C<$_> to the currently selected output channel.
3688 To set the default output channel to something other than STDOUT
3689 use the select operation. The current value of C<$,> (if any) is
3690 printed between each LIST item. The current value of C<$\> (if
3691 any) is printed after the entire LIST has been printed. Because
3692 print takes a LIST, anything in the LIST is evaluated in list
3693 context, and any subroutine that you call will have one or more of
3694 its expressions evaluated in list context. Also be careful not to
3695 follow the print keyword with a left parenthesis unless you want
3696 the corresponding right parenthesis to terminate the arguments to
3697 the print--interpose a C<+> or put parentheses around all the
3700 Note that if you're storing FILEHANDLES in an array or other expression,
3701 you will have to use a block returning its value instead:
3703 print { $files[$i] } "stuff\n";
3704 print { $OK ? STDOUT : STDERR } "stuff\n";
3706 =item printf FILEHANDLE FORMAT, LIST
3708 =item printf FORMAT, LIST
3710 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3711 (the output record separator) is not appended. The first argument
3712 of the list will be interpreted as the C<printf> format. See C<sprintf>
3713 for an explanation of the format argument. If C<use locale> is in effect,
3714 the character used for the decimal point in formatted real numbers is
3715 affected by the LC_NUMERIC locale. See L<perllocale>.
3717 Don't fall into the trap of using a C<printf> when a simple
3718 C<print> would do. The C<print> is more efficient and less
3721 =item prototype FUNCTION
3723 Returns the prototype of a function as a string (or C<undef> if the
3724 function has no prototype). FUNCTION is a reference to, or the name of,
3725 the function whose prototype you want to retrieve.
3727 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3728 name for Perl builtin. If the builtin is not I<overridable> (such as
3729 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3730 C<system>) returns C<undef> because the builtin does not really behave
3731 like a Perl function. Otherwise, the string describing the equivalent
3732 prototype is returned.
3734 =item push ARRAY,LIST
3736 Treats ARRAY as a stack, and pushes the values of LIST
3737 onto the end of ARRAY. The length of ARRAY increases by the length of
3738 LIST. Has the same effect as
3741 $ARRAY[++$#ARRAY] = $value;
3744 but is more efficient. Returns the new number of elements in the array.
3756 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3758 =item quotemeta EXPR
3762 Returns the value of EXPR with all non-"word"
3763 characters backslashed. (That is, all characters not matching
3764 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3765 returned string, regardless of any locale settings.)
3766 This is the internal function implementing
3767 the C<\Q> escape in double-quoted strings.
3769 If EXPR is omitted, uses C<$_>.
3775 Returns a random fractional number greater than or equal to C<0> and less
3776 than the value of EXPR. (EXPR should be positive.) If EXPR is
3777 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3778 also special-cased as C<1> - this has not been documented before perl 5.8.0
3779 and is subject to change in future versions of perl. Automatically calls
3780 C<srand> unless C<srand> has already been called. See also C<srand>.
3782 Apply C<int()> to the value returned by C<rand()> if you want random
3783 integers instead of random fractional numbers. For example,
3787 returns a random integer between C<0> and C<9>, inclusive.
3789 (Note: If your rand function consistently returns numbers that are too
3790 large or too small, then your version of Perl was probably compiled
3791 with the wrong number of RANDBITS.)
3793 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3795 =item read FILEHANDLE,SCALAR,LENGTH
3797 Attempts to read LENGTH I<characters> of data into variable SCALAR
3798 from the specified FILEHANDLE. Returns the number of characters
3799 actually read, C<0> at end of file, or undef if there was an error (in
3800 the latter case C<$!> is also set). SCALAR will be grown or shrunk to
3801 the length actually read. If SCALAR needs growing, the new bytes will
3802 be zero bytes. An OFFSET may be specified to place the read data into
3803 some other place in SCALAR than the beginning. The call is actually
3804 implemented in terms of either Perl's or system's fread() call. To
3805 get a true read(2) system call, see C<sysread>.
3807 Note the I<characters>: depending on the status of the filehandle,
3808 either (8-bit) bytes or characters are read. By default all
3809 filehandles operate on bytes, but for example if the filehandle has
3810 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3811 pragma, L<open>), the I/O will operate on characters, not bytes.
3813 =item readdir DIRHANDLE
3815 Returns the next directory entry for a directory opened by C<opendir>.
3816 If used in list context, returns all the rest of the entries in the
3817 directory. If there are no more entries, returns an undefined value in
3818 scalar context or a null list in list context.
3820 If you're planning to filetest the return values out of a C<readdir>, you'd
3821 better prepend the directory in question. Otherwise, because we didn't
3822 C<chdir> there, it would have been testing the wrong file.
3824 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3825 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3830 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3831 context, each call reads and returns the next line, until end-of-file is
3832 reached, whereupon the subsequent call returns undef. In list context,
3833 reads until end-of-file is reached and returns a list of lines. Note that
3834 the notion of "line" used here is however you may have defined it
3835 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3837 When C<$/> is set to C<undef>, when readline() is in scalar
3838 context (i.e. file slurp mode), and when an empty file is read, it
3839 returns C<''> the first time, followed by C<undef> subsequently.
3841 This is the internal function implementing the C<< <EXPR> >>
3842 operator, but you can use it directly. The C<< <EXPR> >>
3843 operator is discussed in more detail in L<perlop/"I/O Operators">.
3846 $line = readline(*STDIN); # same thing
3848 If readline encounters an operating system error, C<$!> will be set with the
3849 corresponding error message. It can be helpful to check C<$!> when you are
3850 reading from filehandles you don't trust, such as a tty or a socket. The
3851 following example uses the operator form of C<readline>, and takes the necessary
3852 steps to ensure that C<readline> was successful.
3856 unless (defined( $line = <> )) {
3867 Returns the value of a symbolic link, if symbolic links are
3868 implemented. If not, gives a fatal error. If there is some system
3869 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3870 omitted, uses C<$_>.
3874 EXPR is executed as a system command.
3875 The collected standard output of the command is returned.
3876 In scalar context, it comes back as a single (potentially
3877 multi-line) string. In list context, returns a list of lines
3878 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3879 This is the internal function implementing the C<qx/EXPR/>
3880 operator, but you can use it directly. The C<qx/EXPR/>
3881 operator is discussed in more detail in L<perlop/"I/O Operators">.
3883 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3885 Receives a message on a socket. Attempts to receive LENGTH characters
3886 of data into variable SCALAR from the specified SOCKET filehandle.
3887 SCALAR will be grown or shrunk to the length actually read. Takes the
3888 same flags as the system call of the same name. Returns the address
3889 of the sender if SOCKET's protocol supports this; returns an empty
3890 string otherwise. If there's an error, returns the undefined value.
3891 This call is actually implemented in terms of recvfrom(2) system call.
3892 See L<perlipc/"UDP: Message Passing"> for examples.
3894 Note the I<characters>: depending on the status of the socket, either
3895 (8-bit) bytes or characters are received. By default all sockets
3896 operate on bytes, but for example if the socket has been changed using
3897 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3898 pragma, L<open>), the I/O will operate on characters, not bytes.
3904 The C<redo> command restarts the loop block without evaluating the
3905 conditional again. The C<continue> block, if any, is not executed. If
3906 the LABEL is omitted, the command refers to the innermost enclosing
3907 loop. This command is normally used by programs that want to lie to
3908 themselves about what was just input:
3910 # a simpleminded Pascal comment stripper
3911 # (warning: assumes no { or } in strings)
3912 LINE: while (<STDIN>) {
3913 while (s|({.*}.*){.*}|$1 |) {}
3918 if (/}/) { # end of comment?
3927 C<redo> cannot be used to retry a block which returns a value such as
3928 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3929 a grep() or map() operation.
3931 Note that a block by itself is semantically identical to a loop
3932 that executes once. Thus C<redo> inside such a block will effectively
3933 turn it into a looping construct.
3935 See also L</continue> for an illustration of how C<last>, C<next>, and
3942 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3943 is not specified, C<$_> will be used. The value returned depends on the
3944 type of thing the reference is a reference to.
3945 Builtin types include:
3955 If the referenced object has been blessed into a package, then that package
3956 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3958 if (ref($r) eq "HASH") {
3959 print "r is a reference to a hash.\n";
3962 print "r is not a reference at all.\n";
3964 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3965 print "r is a reference to something that isa hash.\n";
3968 See also L<perlref>.
3970 =item rename OLDNAME,NEWNAME
3972 Changes the name of a file; an existing file NEWNAME will be
3973 clobbered. Returns true for success, false otherwise.
3975 Behavior of this function varies wildly depending on your system
3976 implementation. For example, it will usually not work across file system
3977 boundaries, even though the system I<mv> command sometimes compensates
3978 for this. Other restrictions include whether it works on directories,
3979 open files, or pre-existing files. Check L<perlport> and either the
3980 rename(2) manpage or equivalent system documentation for details.
3982 =item require VERSION
3988 Demands a version of Perl specified by VERSION, or demands some semantics
3989 specified by EXPR or by C<$_> if EXPR is not supplied.
3991 VERSION may be either a numeric argument such as 5.006, which will be
3992 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3993 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3994 VERSION is greater than the version of the current Perl interpreter.
3995 Compare with L</use>, which can do a similar check at compile time.
3997 Specifying VERSION as a literal of the form v5.6.1 should generally be
3998 avoided, because it leads to misleading error messages under earlier
3999 versions of Perl which do not support this syntax. The equivalent numeric
4000 version should be used instead.
4002 require v5.6.1; # run time version check
4003 require 5.6.1; # ditto
4004 require 5.006_001; # ditto; preferred for backwards compatibility
4006 Otherwise, demands that a library file be included if it hasn't already
4007 been included. The file is included via the do-FILE mechanism, which is
4008 essentially just a variety of C<eval>. Has semantics similar to the following
4013 return 1 if $INC{$filename};
4014 my($realfilename,$result);
4016 foreach $prefix (@INC) {
4017 $realfilename = "$prefix/$filename";
4018 if (-f $realfilename) {
4019 $INC{$filename} = $realfilename;
4020 $result = do $realfilename;
4024 die "Can't find $filename in \@INC";
4026 delete $INC{$filename} if $@ || !$result;
4028 die "$filename did not return true value" unless $result;
4032 Note that the file will not be included twice under the same specified
4033 name. The file must return true as the last statement to indicate
4034 successful execution of any initialization code, so it's customary to
4035 end such a file with C<1;> unless you're sure it'll return true
4036 otherwise. But it's better just to put the C<1;>, in case you add more
4039 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4040 replaces "F<::>" with "F</>" in the filename for you,
4041 to make it easy to load standard modules. This form of loading of
4042 modules does not risk altering your namespace.
4044 In other words, if you try this:
4046 require Foo::Bar; # a splendid bareword
4048 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4049 directories specified in the C<@INC> array.
4051 But if you try this:
4053 $class = 'Foo::Bar';
4054 require $class; # $class is not a bareword
4056 require "Foo::Bar"; # not a bareword because of the ""
4058 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4059 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4061 eval "require $class";
4063 Now that you understand how C<require> looks for files in the case of
4064 a bareword argument, there is a little extra functionality going on
4065 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4066 it will first look for a filename with a "F<.pmc>" extension. A file
4067 with this extension is assumed to be Perl bytecode generated by
4068 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4069 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4070 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4072 You can also insert hooks into the import facility, by putting directly
4073 Perl code into the @INC array. There are three forms of hooks: subroutine
4074 references, array references and blessed objects.
4076 Subroutine references are the simplest case. When the inclusion system
4077 walks through @INC and encounters a subroutine, this subroutine gets
4078 called with two parameters, the first being a reference to itself, and the
4079 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4080 subroutine should return C<undef> or a filehandle, from which the file to
4081 include will be read. If C<undef> is returned, C<require> will look at
4082 the remaining elements of @INC.
4084 If the hook is an array reference, its first element must be a subroutine
4085 reference. This subroutine is called as above, but the first parameter is
4086 the array reference. This enables to pass indirectly some arguments to
4089 In other words, you can write:
4091 push @INC, \&my_sub;
4093 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4099 push @INC, [ \&my_sub, $x, $y, ... ];
4101 my ($arrayref, $filename) = @_;
4102 # Retrieve $x, $y, ...
4103 my @parameters = @$arrayref[1..$#$arrayref];
4107 If the hook is an object, it must provide an INC method, that will be
4108 called as above, the first parameter being the object itself. (Note that
4109 you must fully qualify the sub's name, as it is always forced into package
4110 C<main>.) Here is a typical code layout:
4116 my ($self, $filename) = @_;
4120 # In the main program
4121 push @INC, new Foo(...);
4123 Note that these hooks are also permitted to set the %INC entry
4124 corresponding to the files they have loaded. See L<perlvar/%INC>.
4126 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4132 Generally used in a C<continue> block at the end of a loop to clear
4133 variables and reset C<??> searches so that they work again. The
4134 expression is interpreted as a list of single characters (hyphens
4135 allowed for ranges). All variables and arrays beginning with one of
4136 those letters are reset to their pristine state. If the expression is
4137 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4138 only variables or searches in the current package. Always returns
4141 reset 'X'; # reset all X variables
4142 reset 'a-z'; # reset lower case variables
4143 reset; # just reset ?one-time? searches
4145 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4146 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4147 variables--lexical variables are unaffected, but they clean themselves
4148 up on scope exit anyway, so you'll probably want to use them instead.
4155 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4156 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4157 context, depending on how the return value will be used, and the context
4158 may vary from one execution to the next (see C<wantarray>). If no EXPR
4159 is given, returns an empty list in list context, the undefined value in
4160 scalar context, and (of course) nothing at all in a void context.
4162 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4163 or do FILE will automatically return the value of the last expression
4168 In list context, returns a list value consisting of the elements
4169 of LIST in the opposite order. In scalar context, concatenates the
4170 elements of LIST and returns a string value with all characters
4171 in the opposite order.
4173 print reverse <>; # line tac, last line first
4175 undef $/; # for efficiency of <>
4176 print scalar reverse <>; # character tac, last line tsrif
4178 This operator is also handy for inverting a hash, although there are some
4179 caveats. If a value is duplicated in the original hash, only one of those
4180 can be represented as a key in the inverted hash. Also, this has to
4181 unwind one hash and build a whole new one, which may take some time
4182 on a large hash, such as from a DBM file.
4184 %by_name = reverse %by_address; # Invert the hash
4186 =item rewinddir DIRHANDLE
4188 Sets the current position to the beginning of the directory for the
4189 C<readdir> routine on DIRHANDLE.
4191 =item rindex STR,SUBSTR,POSITION
4193 =item rindex STR,SUBSTR
4195 Works just like index() except that it returns the position of the LAST
4196 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4197 last occurrence at or before that position.
4199 =item rmdir FILENAME
4203 Deletes the directory specified by FILENAME if that directory is empty. If it
4204 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4205 FILENAME is omitted, uses C<$_>.
4209 The substitution operator. See L<perlop>.
4213 Forces EXPR to be interpreted in scalar context and returns the value
4216 @counts = ( scalar @a, scalar @b, scalar @c );
4218 There is no equivalent operator to force an expression to
4219 be interpolated in list context because in practice, this is never
4220 needed. If you really wanted to do so, however, you could use
4221 the construction C<@{[ (some expression) ]}>, but usually a simple
4222 C<(some expression)> suffices.
4224 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4225 parenthesized list, this behaves as a scalar comma expression, evaluating
4226 all but the last element in void context and returning the final element
4227 evaluated in scalar context. This is seldom what you want.
4229 The following single statement:
4231 print uc(scalar(&foo,$bar)),$baz;
4233 is the moral equivalent of these two:
4236 print(uc($bar),$baz);
4238 See L<perlop> for more details on unary operators and the comma operator.
4240 =item seek FILEHANDLE,POSITION,WHENCE
4242 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4243 FILEHANDLE may be an expression whose value gives the name of the
4244 filehandle. The values for WHENCE are C<0> to set the new position
4245 I<in bytes> to POSITION, C<1> to set it to the current position plus
4246 POSITION, and C<2> to set it to EOF plus POSITION (typically
4247 negative). For WHENCE you may use the constants C<SEEK_SET>,
4248 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4249 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4252 Note the I<in bytes>: even if the filehandle has been set to
4253 operate on characters (for example by using the C<:utf8> open
4254 layer), tell() will return byte offsets, not character offsets
4255 (because implementing that would render seek() and tell() rather slow).
4257 If you want to position file for C<sysread> or C<syswrite>, don't use
4258 C<seek>--buffering makes its effect on the file's system position
4259 unpredictable and non-portable. Use C<sysseek> instead.
4261 Due to the rules and rigors of ANSI C, on some systems you have to do a
4262 seek whenever you switch between reading and writing. Amongst other
4263 things, this may have the effect of calling stdio's clearerr(3).
4264 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4268 This is also useful for applications emulating C<tail -f>. Once you hit
4269 EOF on your read, and then sleep for a while, you might have to stick in a
4270 seek() to reset things. The C<seek> doesn't change the current position,
4271 but it I<does> clear the end-of-file condition on the handle, so that the
4272 next C<< <FILE> >> makes Perl try again to read something. We hope.
4274 If that doesn't work (some IO implementations are particularly
4275 cantankerous), then you may need something more like this:
4278 for ($curpos = tell(FILE); $_ = <FILE>;
4279 $curpos = tell(FILE)) {
4280 # search for some stuff and put it into files
4282 sleep($for_a_while);
4283 seek(FILE, $curpos, 0);
4286 =item seekdir DIRHANDLE,POS
4288 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4289 must be a value returned by C<telldir>. Has the same caveats about
4290 possible directory compaction as the corresponding system library
4293 =item select FILEHANDLE
4297 Returns the currently selected filehandle. Sets the current default
4298 filehandle for output, if FILEHANDLE is supplied. This has two
4299 effects: first, a C<write> or a C<print> without a filehandle will
4300 default to this FILEHANDLE. Second, references to variables related to
4301 output will refer to this output channel. For example, if you have to
4302 set the top of form format for more than one output channel, you might
4310 FILEHANDLE may be an expression whose value gives the name of the
4311 actual filehandle. Thus:
4313 $oldfh = select(STDERR); $| = 1; select($oldfh);
4315 Some programmers may prefer to think of filehandles as objects with
4316 methods, preferring to write the last example as:
4319 STDERR->autoflush(1);
4321 =item select RBITS,WBITS,EBITS,TIMEOUT
4323 This calls the select(2) system call with the bit masks specified, which
4324 can be constructed using C<fileno> and C<vec>, along these lines:
4326 $rin = $win = $ein = '';
4327 vec($rin,fileno(STDIN),1) = 1;
4328 vec($win,fileno(STDOUT),1) = 1;
4331 If you want to select on many filehandles you might wish to write a
4335 my(@fhlist) = split(' ',$_[0]);
4338 vec($bits,fileno($_),1) = 1;
4342 $rin = fhbits('STDIN TTY SOCK');
4346 ($nfound,$timeleft) =
4347 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4349 or to block until something becomes ready just do this
4351 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4353 Most systems do not bother to return anything useful in $timeleft, so
4354 calling select() in scalar context just returns $nfound.
4356 Any of the bit masks can also be undef. The timeout, if specified, is
4357 in seconds, which may be fractional. Note: not all implementations are
4358 capable of returning the $timeleft. If not, they always return
4359 $timeleft equal to the supplied $timeout.
4361 You can effect a sleep of 250 milliseconds this way:
4363 select(undef, undef, undef, 0.25);
4365 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4366 is implementation-dependent.
4368 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4369 or <FH>) with C<select>, except as permitted by POSIX, and even
4370 then only on POSIX systems. You have to use C<sysread> instead.
4372 =item semctl ID,SEMNUM,CMD,ARG
4374 Calls the System V IPC function C<semctl>. You'll probably have to say
4378 first to get the correct constant definitions. If CMD is IPC_STAT or
4379 GETALL, then ARG must be a variable which will hold the returned
4380 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4381 the undefined value for error, "C<0 but true>" for zero, or the actual
4382 return value otherwise. The ARG must consist of a vector of native
4383 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4384 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4387 =item semget KEY,NSEMS,FLAGS
4389 Calls the System V IPC function semget. Returns the semaphore id, or
4390 the undefined value if there is an error. See also
4391 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4394 =item semop KEY,OPSTRING
4396 Calls the System V IPC function semop to perform semaphore operations
4397 such as signalling and waiting. OPSTRING must be a packed array of
4398 semop structures. Each semop structure can be generated with
4399 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4400 operations is implied by the length of OPSTRING. Returns true if
4401 successful, or false if there is an error. As an example, the
4402 following code waits on semaphore $semnum of semaphore id $semid:
4404 $semop = pack("s!3", $semnum, -1, 0);
4405 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4407 To signal the semaphore, replace C<-1> with C<1>. See also
4408 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4411 =item send SOCKET,MSG,FLAGS,TO
4413 =item send SOCKET,MSG,FLAGS
4415 Sends a message on a socket. Attempts to send the scalar MSG to the
4416 SOCKET filehandle. Takes the same flags as the system call of the
4417 same name. On unconnected sockets you must specify a destination to
4418 send TO, in which case it does a C C<sendto>. Returns the number of
4419 characters sent, or the undefined value if there is an error. The C
4420 system call sendmsg(2) is currently unimplemented. See
4421 L<perlipc/"UDP: Message Passing"> for examples.
4423 Note the I<characters>: depending on the status of the socket, either
4424 (8-bit) bytes or characters are sent. By default all sockets operate
4425 on bytes, but for example if the socket has been changed using
4426 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4427 the C<open> pragma, L<open>), the I/O will operate on characters, not
4430 =item setpgrp PID,PGRP
4432 Sets the current process group for the specified PID, C<0> for the current
4433 process. Will produce a fatal error if used on a machine that doesn't
4434 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4435 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4436 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4439 =item setpriority WHICH,WHO,PRIORITY
4441 Sets the current priority for a process, a process group, or a user.
4442 (See setpriority(2).) Will produce a fatal error if used on a machine
4443 that doesn't implement setpriority(2).
4445 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4447 Sets the socket option requested. Returns undefined if there is an
4448 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4455 Shifts the first value of the array off and returns it, shortening the
4456 array by 1 and moving everything down. If there are no elements in the
4457 array, returns the undefined value. If ARRAY is omitted, shifts the
4458 C<@_> array within the lexical scope of subroutines and formats, and the
4459 C<@ARGV> array at file scopes or within the lexical scopes established by
4460 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4463 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4464 same thing to the left end of an array that C<pop> and C<push> do to the
4467 =item shmctl ID,CMD,ARG
4469 Calls the System V IPC function shmctl. You'll probably have to say
4473 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4474 then ARG must be a variable which will hold the returned C<shmid_ds>
4475 structure. Returns like ioctl: the undefined value for error, "C<0> but
4476 true" for zero, or the actual return value otherwise.
4477 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4479 =item shmget KEY,SIZE,FLAGS
4481 Calls the System V IPC function shmget. Returns the shared memory
4482 segment id, or the undefined value if there is an error.
4483 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4485 =item shmread ID,VAR,POS,SIZE
4487 =item shmwrite ID,STRING,POS,SIZE
4489 Reads or writes the System V shared memory segment ID starting at
4490 position POS for size SIZE by attaching to it, copying in/out, and
4491 detaching from it. When reading, VAR must be a variable that will
4492 hold the data read. When writing, if STRING is too long, only SIZE
4493 bytes are used; if STRING is too short, nulls are written to fill out
4494 SIZE bytes. Return true if successful, or false if there is an error.
4495 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4496 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4498 =item shutdown SOCKET,HOW
4500 Shuts down a socket connection in the manner indicated by HOW, which
4501 has the same interpretation as in the system call of the same name.
4503 shutdown(SOCKET, 0); # I/we have stopped reading data
4504 shutdown(SOCKET, 1); # I/we have stopped writing data
4505 shutdown(SOCKET, 2); # I/we have stopped using this socket
4507 This is useful with sockets when you want to tell the other
4508 side you're done writing but not done reading, or vice versa.
4509 It's also a more insistent form of close because it also
4510 disables the file descriptor in any forked copies in other
4517 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4518 returns sine of C<$_>.
4520 For the inverse sine operation, you may use the C<Math::Trig::asin>
4521 function, or use this relation:
4523 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4529 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4530 May be interrupted if the process receives a signal such as C<SIGALRM>.
4531 Returns the number of seconds actually slept. You probably cannot
4532 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4535 On some older systems, it may sleep up to a full second less than what
4536 you requested, depending on how it counts seconds. Most modern systems
4537 always sleep the full amount. They may appear to sleep longer than that,
4538 however, because your process might not be scheduled right away in a
4539 busy multitasking system.
4541 For delays of finer granularity than one second, you may use Perl's
4542 C<syscall> interface to access setitimer(2) if your system supports
4543 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4544 and starting from Perl 5.8 part of the standard distribution) may also
4547 See also the POSIX module's C<pause> function.
4549 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4551 Opens a socket of the specified kind and attaches it to filehandle
4552 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4553 the system call of the same name. You should C<use Socket> first
4554 to get the proper definitions imported. See the examples in
4555 L<perlipc/"Sockets: Client/Server Communication">.
4557 On systems that support a close-on-exec flag on files, the flag will
4558 be set for the newly opened file descriptor, as determined by the
4559 value of $^F. See L<perlvar/$^F>.
4561 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4563 Creates an unnamed pair of sockets in the specified domain, of the
4564 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4565 for the system call of the same name. If unimplemented, yields a fatal
4566 error. Returns true if successful.
4568 On systems that support a close-on-exec flag on files, the flag will
4569 be set for the newly opened file descriptors, as determined by the value
4570 of $^F. See L<perlvar/$^F>.
4572 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4573 to C<pipe(Rdr, Wtr)> is essentially:
4576 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4577 shutdown(Rdr, 1); # no more writing for reader
4578 shutdown(Wtr, 0); # no more reading for writer
4580 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4581 emulate socketpair using IP sockets to localhost if your system implements
4582 sockets but not socketpair.
4584 =item sort SUBNAME LIST
4586 =item sort BLOCK LIST
4590 In list context, this sorts the LIST and returns the sorted list value.
4591 In scalar context, the behaviour of C<sort()> is undefined.
4593 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4594 order. If SUBNAME is specified, it gives the name of a subroutine
4595 that returns an integer less than, equal to, or greater than C<0>,
4596 depending on how the elements of the list are to be ordered. (The C<<
4597 <=> >> and C<cmp> operators are extremely useful in such routines.)
4598 SUBNAME may be a scalar variable name (unsubscripted), in which case
4599 the value provides the name of (or a reference to) the actual
4600 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4601 an anonymous, in-line sort subroutine.
4603 If the subroutine's prototype is C<($$)>, the elements to be compared
4604 are passed by reference in C<@_>, as for a normal subroutine. This is
4605 slower than unprototyped subroutines, where the elements to be
4606 compared are passed into the subroutine
4607 as the package global variables $a and $b (see example below). Note that
4608 in the latter case, it is usually counter-productive to declare $a and
4611 In either case, the subroutine may not be recursive. The values to be
4612 compared are always passed by reference, so don't modify them.
4614 You also cannot exit out of the sort block or subroutine using any of the
4615 loop control operators described in L<perlsyn> or with C<goto>.
4617 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4618 current collation locale. See L<perllocale>.
4620 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4621 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4622 preserves the input order of elements that compare equal. Although
4623 quicksort's run time is O(NlogN) when averaged over all arrays of
4624 length N, the time can be O(N**2), I<quadratic> behavior, for some
4625 inputs.) In 5.7, the quicksort implementation was replaced with
4626 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4627 But benchmarks indicated that for some inputs, on some platforms,
4628 the original quicksort was faster. 5.8 has a sort pragma for
4629 limited control of the sort. Its rather blunt control of the
4630 underlying algorithm may not persist into future perls, but the
4631 ability to characterize the input or output in implementation
4632 independent ways quite probably will. See L<sort>.
4637 @articles = sort @files;
4639 # same thing, but with explicit sort routine
4640 @articles = sort {$a cmp $b} @files;
4642 # now case-insensitively
4643 @articles = sort {uc($a) cmp uc($b)} @files;
4645 # same thing in reversed order
4646 @articles = sort {$b cmp $a} @files;
4648 # sort numerically ascending
4649 @articles = sort {$a <=> $b} @files;
4651 # sort numerically descending
4652 @articles = sort {$b <=> $a} @files;
4654 # this sorts the %age hash by value instead of key
4655 # using an in-line function
4656 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4658 # sort using explicit subroutine name
4660 $age{$a} <=> $age{$b}; # presuming numeric
4662 @sortedclass = sort byage @class;
4664 sub backwards { $b cmp $a }
4665 @harry = qw(dog cat x Cain Abel);
4666 @george = qw(gone chased yz Punished Axed);
4668 # prints AbelCaincatdogx
4669 print sort backwards @harry;
4670 # prints xdogcatCainAbel
4671 print sort @george, 'to', @harry;
4672 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4674 # inefficiently sort by descending numeric compare using
4675 # the first integer after the first = sign, or the
4676 # whole record case-insensitively otherwise
4679 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4684 # same thing, but much more efficiently;
4685 # we'll build auxiliary indices instead
4689 push @nums, /=(\d+)/;
4694 $nums[$b] <=> $nums[$a]
4696 $caps[$a] cmp $caps[$b]
4700 # same thing, but without any temps
4701 @new = map { $_->[0] }
4702 sort { $b->[1] <=> $a->[1]
4705 } map { [$_, /=(\d+)/, uc($_)] } @old;
4707 # using a prototype allows you to use any comparison subroutine
4708 # as a sort subroutine (including other package's subroutines)
4710 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4713 @new = sort other::backwards @old;
4715 # guarantee stability, regardless of algorithm
4717 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4719 # force use of mergesort (not portable outside Perl 5.8)
4720 use sort '_mergesort'; # note discouraging _
4721 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4723 If you're using strict, you I<must not> declare $a
4724 and $b as lexicals. They are package globals. That means
4725 if you're in the C<main> package and type
4727 @articles = sort {$b <=> $a} @files;
4729 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4730 but if you're in the C<FooPack> package, it's the same as typing
4732 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4734 The comparison function is required to behave. If it returns
4735 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4736 sometimes saying the opposite, for example) the results are not
4739 =item splice ARRAY,OFFSET,LENGTH,LIST
4741 =item splice ARRAY,OFFSET,LENGTH
4743 =item splice ARRAY,OFFSET
4747 Removes the elements designated by OFFSET and LENGTH from an array, and
4748 replaces them with the elements of LIST, if any. In list context,
4749 returns the elements removed from the array. In scalar context,
4750 returns the last element removed, or C<undef> if no elements are
4751 removed. The array grows or shrinks as necessary.
4752 If OFFSET is negative then it starts that far from the end of the array.
4753 If LENGTH is omitted, removes everything from OFFSET onward.
4754 If LENGTH is negative, removes the elements from OFFSET onward
4755 except for -LENGTH elements at the end of the array.
4756 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4757 past the end of the array, perl issues a warning, and splices at the
4760 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4762 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4763 pop(@a) splice(@a,-1)
4764 shift(@a) splice(@a,0,1)
4765 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4766 $a[$i] = $y splice(@a,$i,1,$y)
4768 Example, assuming array lengths are passed before arrays:
4770 sub aeq { # compare two list values
4771 my(@a) = splice(@_,0,shift);
4772 my(@b) = splice(@_,0,shift);
4773 return 0 unless @a == @b; # same len?
4775 return 0 if pop(@a) ne pop(@b);
4779 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4781 =item split /PATTERN/,EXPR,LIMIT
4783 =item split /PATTERN/,EXPR
4785 =item split /PATTERN/
4789 Splits a string into a list of strings and returns that list. By default,
4790 empty leading fields are preserved, and empty trailing ones are deleted.
4792 In scalar context, returns the number of fields found and splits into
4793 the C<@_> array. Use of split in scalar context is deprecated, however,
4794 because it clobbers your subroutine arguments.
4796 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4797 splits on whitespace (after skipping any leading whitespace). Anything
4798 matching PATTERN is taken to be a delimiter separating the fields. (Note
4799 that the delimiter may be longer than one character.)
4801 If LIMIT is specified and positive, it represents the maximum number
4802 of fields the EXPR will be split into, though the actual number of
4803 fields returned depends on the number of times PATTERN matches within
4804 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4805 stripped (which potential users of C<pop> would do well to remember).
4806 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4807 had been specified. Note that splitting an EXPR that evaluates to the
4808 empty string always returns the empty list, regardless of the LIMIT
4811 A pattern matching the null string (not to be confused with
4812 a null pattern C<//>, which is just one member of the set of patterns
4813 matching a null string) will split the value of EXPR into separate
4814 characters at each point it matches that way. For example:
4816 print join(':', split(/ */, 'hi there'));
4818 produces the output 'h:i:t:h:e:r:e'.
4820 Using the empty pattern C<//> specifically matches the null string, and is
4821 not be confused with the use of C<//> to mean "the last successful pattern
4824 Empty leading (or trailing) fields are produced when there are positive width
4825 matches at the beginning (or end) of the string; a zero-width match at the
4826 beginning (or end) of the string does not produce an empty field. For
4829 print join(':', split(/(?=\w)/, 'hi there!'));
4831 produces the output 'h:i :t:h:e:r:e!'.
4833 The LIMIT parameter can be used to split a line partially
4835 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4837 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4838 one larger than the number of variables in the list, to avoid
4839 unnecessary work. For the list above LIMIT would have been 4 by
4840 default. In time critical applications it behooves you not to split
4841 into more fields than you really need.
4843 If the PATTERN contains parentheses, additional list elements are
4844 created from each matching substring in the delimiter.
4846 split(/([,-])/, "1-10,20", 3);
4848 produces the list value
4850 (1, '-', 10, ',', 20)
4852 If you had the entire header of a normal Unix email message in $header,
4853 you could split it up into fields and their values this way:
4855 $header =~ s/\n\s+/ /g; # fix continuation lines
4856 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4858 The pattern C</PATTERN/> may be replaced with an expression to specify
4859 patterns that vary at runtime. (To do runtime compilation only once,
4860 use C</$variable/o>.)
4862 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4863 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4864 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4865 will give you as many null initial fields as there are leading spaces.
4866 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4867 whitespace produces a null first field. A C<split> with no arguments
4868 really does a S<C<split(' ', $_)>> internally.
4870 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4875 open(PASSWD, '/etc/passwd');
4878 ($login, $passwd, $uid, $gid,
4879 $gcos, $home, $shell) = split(/:/);
4883 As with regular pattern matching, any capturing parentheses that are not
4884 matched in a C<split()> will be set to C<undef> when returned:
4886 @fields = split /(A)|B/, "1A2B3";
4887 # @fields is (1, 'A', 2, undef, 3)
4889 =item sprintf FORMAT, LIST
4891 Returns a string formatted by the usual C<printf> conventions of the C
4892 library function C<sprintf>. See below for more details
4893 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4894 the general principles.
4898 # Format number with up to 8 leading zeroes
4899 $result = sprintf("%08d", $number);
4901 # Round number to 3 digits after decimal point
4902 $rounded = sprintf("%.3f", $number);
4904 Perl does its own C<sprintf> formatting--it emulates the C
4905 function C<sprintf>, but it doesn't use it (except for floating-point
4906 numbers, and even then only the standard modifiers are allowed). As a
4907 result, any non-standard extensions in your local C<sprintf> are not
4908 available from Perl.
4910 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4911 pass it an array as your first argument. The array is given scalar context,
4912 and instead of using the 0th element of the array as the format, Perl will
4913 use the count of elements in the array as the format, which is almost never
4916 Perl's C<sprintf> permits the following universally-known conversions:
4919 %c a character with the given number
4921 %d a signed integer, in decimal
4922 %u an unsigned integer, in decimal
4923 %o an unsigned integer, in octal
4924 %x an unsigned integer, in hexadecimal
4925 %e a floating-point number, in scientific notation
4926 %f a floating-point number, in fixed decimal notation
4927 %g a floating-point number, in %e or %f notation
4929 In addition, Perl permits the following widely-supported conversions:
4931 %X like %x, but using upper-case letters
4932 %E like %e, but using an upper-case "E"
4933 %G like %g, but with an upper-case "E" (if applicable)
4934 %b an unsigned integer, in binary
4935 %p a pointer (outputs the Perl value's address in hexadecimal)
4936 %n special: *stores* the number of characters output so far
4937 into the next variable in the parameter list
4939 Finally, for backward (and we do mean "backward") compatibility, Perl
4940 permits these unnecessary but widely-supported conversions:
4943 %D a synonym for %ld
4944 %U a synonym for %lu
4945 %O a synonym for %lo
4948 Note that the number of exponent digits in the scientific notation produced
4949 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4950 exponent less than 100 is system-dependent: it may be three or less
4951 (zero-padded as necessary). In other words, 1.23 times ten to the
4952 99th may be either "1.23e99" or "1.23e099".
4954 Between the C<%> and the format letter, you may specify a number of
4955 additional attributes controlling the interpretation of the format.
4956 In order, these are:
4960 =item format parameter index
4962 An explicit format parameter index, such as C<2$>. By default sprintf
4963 will format the next unused argument in the list, but this allows you
4964 to take the arguments out of order. Eg:
4966 printf '%2$d %1$d', 12, 34; # prints "34 12"
4967 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4972 space prefix positive number with a space
4973 + prefix positive number with a plus sign
4974 - left-justify within the field
4975 0 use zeros, not spaces, to right-justify
4976 # prefix non-zero octal with "0", non-zero hex with "0x",
4977 non-zero binary with "0b"
4981 printf '<% d>', 12; # prints "< 12>"
4982 printf '<%+d>', 12; # prints "<+12>"
4983 printf '<%6s>', 12; # prints "< 12>"
4984 printf '<%-6s>', 12; # prints "<12 >"
4985 printf '<%06s>', 12; # prints "<000012>"
4986 printf '<%#x>', 12; # prints "<0xc>"
4990 The vector flag C<v>, optionally specifying the join string to use.
4991 This flag tells perl to interpret the supplied string as a vector
4992 of integers, one for each character in the string, separated by
4993 a given string (a dot C<.> by default). This can be useful for
4994 displaying ordinal values of characters in arbitrary strings:
4996 printf "version is v%vd\n", $^V; # Perl's version
4998 Put an asterisk C<*> before the C<v> to override the string to
4999 use to separate the numbers:
5001 printf "address is %*vX\n", ":", $addr; # IPv6 address
5002 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5004 You can also explicitly specify the argument number to use for
5005 the join string using eg C<*2$v>:
5007 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5009 =item (minimum) width
5011 Arguments are usually formatted to be only as wide as required to
5012 display the given value. You can override the width by putting
5013 a number here, or get the width from the next argument (with C<*>)
5014 or from a specified argument (with eg C<*2$>):
5016 printf '<%s>', "a"; # prints "<a>"
5017 printf '<%6s>', "a"; # prints "< a>"
5018 printf '<%*s>', 6, "a"; # prints "< a>"
5019 printf '<%*2$s>', "a", 6; # prints "< a>"
5020 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5022 If a field width obtained through C<*> is negative, it has the same
5023 effect as the C<-> flag: left-justification.
5025 =item precision, or maximum width
5027 You can specify a precision (for numeric conversions) or a maximum
5028 width (for string conversions) by specifying a C<.> followed by a number.
5029 For floating point formats, with the exception of 'g' and 'G', this specifies
5030 the number of decimal places to show (the default being 6), eg:
5032 # these examples are subject to system-specific variation
5033 printf '<%f>', 1; # prints "<1.000000>"
5034 printf '<%.1f>', 1; # prints "<1.0>"
5035 printf '<%.0f>', 1; # prints "<1>"
5036 printf '<%e>', 10; # prints "<1.000000e+01>"
5037 printf '<%.1e>', 10; # prints "<1.0e+01>"
5039 For 'g' and 'G', this specifies the maximum number of digits to show,
5040 including prior to the decimal point as well as after it, eg:
5042 # these examples are subject to system-specific variation
5043 printf '<%g>', 1; # prints "<1>"
5044 printf '<%.10g>', 1; # prints "<1>"
5045 printf '<%g>', 100; # prints "<100>"
5046 printf '<%.1g>', 100; # prints "<1e+02>"
5047 printf '<%.2g>', 100.01; # prints "<1e+02>"
5048 printf '<%.5g>', 100.01; # prints "<100.01>"
5049 printf '<%.4g>', 100.01; # prints "<100>"
5051 For integer conversions, specifying a precision implies that the
5052 output of the number itself should be zero-padded to this width:
5054 printf '<%.6x>', 1; # prints "<000001>"
5055 printf '<%#.6x>', 1; # prints "<0x000001>"
5056 printf '<%-10.6x>', 1; # prints "<000001 >"
5058 For string conversions, specifying a precision truncates the string
5059 to fit in the specified width:
5061 printf '<%.5s>', "truncated"; # prints "<trunc>"
5062 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5064 You can also get the precision from the next argument using C<.*>:
5066 printf '<%.6x>', 1; # prints "<000001>"
5067 printf '<%.*x>', 6, 1; # prints "<000001>"
5069 You cannot currently get the precision from a specified number,
5070 but it is intended that this will be possible in the future using
5073 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5077 For numeric conversions, you can specify the size to interpret the
5078 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5079 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5080 whatever the default integer size is on your platform (usually 32 or 64
5081 bits), but you can override this to use instead one of the standard C types,
5082 as supported by the compiler used to build Perl:
5084 l interpret integer as C type "long" or "unsigned long"
5085 h interpret integer as C type "short" or "unsigned short"
5086 q, L or ll interpret integer as C type "long long", "unsigned long long".
5087 or "quads" (typically 64-bit integers)
5089 The last will produce errors if Perl does not understand "quads" in your
5090 installation. (This requires that either the platform natively supports quads
5091 or Perl was specifically compiled to support quads.) You can find out
5092 whether your Perl supports quads via L<Config>:
5095 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5098 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5099 to be the default floating point size on your platform (double or long double),
5100 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5101 platform supports them. You can find out whether your Perl supports long
5102 doubles via L<Config>:
5105 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5107 You can find out whether Perl considers 'long double' to be the default
5108 floating point size to use on your platform via L<Config>:
5111 ($Config{uselongdouble} eq 'define') &&
5112 print "long doubles by default\n";
5114 It can also be the case that long doubles and doubles are the same thing:
5117 ($Config{doublesize} == $Config{longdblsize}) &&
5118 print "doubles are long doubles\n";
5120 The size specifier C<V> has no effect for Perl code, but it is supported
5121 for compatibility with XS code; it means 'use the standard size for
5122 a Perl integer (or floating-point number)', which is already the
5123 default for Perl code.
5125 =item order of arguments
5127 Normally, sprintf takes the next unused argument as the value to
5128 format for each format specification. If the format specification
5129 uses C<*> to require additional arguments, these are consumed from
5130 the argument list in the order in which they appear in the format
5131 specification I<before> the value to format. Where an argument is
5132 specified using an explicit index, this does not affect the normal
5133 order for the arguments (even when the explicitly specified index
5134 would have been the next argument in any case).
5138 printf '<%*.*s>', $a, $b, $c;
5140 would use C<$a> for the width, C<$b> for the precision and C<$c>
5141 as the value to format, while:
5143 print '<%*1$.*s>', $a, $b;
5145 would use C<$a> for the width and the precision, and C<$b> as the
5148 Here are some more examples - beware that when using an explicit
5149 index, the C<$> may need to be escaped:
5151 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5152 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5153 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5154 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5158 If C<use locale> is in effect, the character used for the decimal
5159 point in formatted real numbers is affected by the LC_NUMERIC locale.
5166 Return the square root of EXPR. If EXPR is omitted, returns square
5167 root of C<$_>. Only works on non-negative operands, unless you've
5168 loaded the standard Math::Complex module.
5171 print sqrt(-2); # prints 1.4142135623731i
5177 Sets the random number seed for the C<rand> operator.
5179 The point of the function is to "seed" the C<rand> function so that
5180 C<rand> can produce a different sequence each time you run your
5183 If srand() is not called explicitly, it is called implicitly at the
5184 first use of the C<rand> operator. However, this was not the case in
5185 versions of Perl before 5.004, so if your script will run under older
5186 Perl versions, it should call C<srand>.
5188 Most programs won't even call srand() at all, except those that
5189 need a cryptographically-strong starting point rather than the
5190 generally acceptable default, which is based on time of day,
5191 process ID, and memory allocation, or the F</dev/urandom> device,
5194 You can call srand($seed) with the same $seed to reproduce the
5195 I<same> sequence from rand(), but this is usually reserved for
5196 generating predictable results for testing or debugging.
5197 Otherwise, don't call srand() more than once in your program.
5199 Do B<not> call srand() (i.e. without an argument) more than once in
5200 a script. The internal state of the random number generator should
5201 contain more entropy than can be provided by any seed, so calling
5202 srand() again actually I<loses> randomness.
5204 Most implementations of C<srand> take an integer and will silently
5205 truncate decimal numbers. This means C<srand(42)> will usually
5206 produce the same results as C<srand(42.1)>. To be safe, always pass
5207 C<srand> an integer.
5209 In versions of Perl prior to 5.004 the default seed was just the
5210 current C<time>. This isn't a particularly good seed, so many old
5211 programs supply their own seed value (often C<time ^ $$> or C<time ^
5212 ($$ + ($$ << 15))>), but that isn't necessary any more.
5214 Note that you need something much more random than the default seed for
5215 cryptographic purposes. Checksumming the compressed output of one or more
5216 rapidly changing operating system status programs is the usual method. For
5219 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5221 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5224 Frequently called programs (like CGI scripts) that simply use
5228 for a seed can fall prey to the mathematical property that
5232 one-third of the time. So don't do that.
5234 =item stat FILEHANDLE
5240 Returns a 13-element list giving the status info for a file, either
5241 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5242 it stats C<$_>. Returns a null list if the stat fails. Typically used
5245 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5246 $atime,$mtime,$ctime,$blksize,$blocks)
5249 Not all fields are supported on all filesystem types. Here are the
5250 meaning of the fields:
5252 0 dev device number of filesystem
5254 2 mode file mode (type and permissions)
5255 3 nlink number of (hard) links to the file
5256 4 uid numeric user ID of file's owner
5257 5 gid numeric group ID of file's owner
5258 6 rdev the device identifier (special files only)
5259 7 size total size of file, in bytes
5260 8 atime last access time in seconds since the epoch
5261 9 mtime last modify time in seconds since the epoch
5262 10 ctime inode change time in seconds since the epoch (*)
5263 11 blksize preferred block size for file system I/O
5264 12 blocks actual number of blocks allocated
5266 (The epoch was at 00:00 January 1, 1970 GMT.)
5268 (*) The ctime field is non-portable, in particular you cannot expect
5269 it to be a "creation time", see L<perlport/"Files and Filesystems">
5272 If stat is passed the special filehandle consisting of an underline, no
5273 stat is done, but the current contents of the stat structure from the
5274 last stat or filetest are returned. Example:
5276 if (-x $file && (($d) = stat(_)) && $d < 0) {
5277 print "$file is executable NFS file\n";
5280 (This works on machines only for which the device number is negative
5283 Because the mode contains both the file type and its permissions, you
5284 should mask off the file type portion and (s)printf using a C<"%o">
5285 if you want to see the real permissions.
5287 $mode = (stat($filename))[2];
5288 printf "Permissions are %04o\n", $mode & 07777;
5290 In scalar context, C<stat> returns a boolean value indicating success
5291 or failure, and, if successful, sets the information associated with
5292 the special filehandle C<_>.
5294 The File::stat module provides a convenient, by-name access mechanism:
5297 $sb = stat($filename);
5298 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5299 $filename, $sb->size, $sb->mode & 07777,
5300 scalar localtime $sb->mtime;
5302 You can import symbolic mode constants (C<S_IF*>) and functions
5303 (C<S_IS*>) from the Fcntl module:
5307 $mode = (stat($filename))[2];
5309 $user_rwx = ($mode & S_IRWXU) >> 6;
5310 $group_read = ($mode & S_IRGRP) >> 3;
5311 $other_execute = $mode & S_IXOTH;
5313 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5315 $is_setuid = $mode & S_ISUID;
5316 $is_setgid = S_ISDIR($mode);
5318 You could write the last two using the C<-u> and C<-d> operators.
5319 The commonly available S_IF* constants are
5321 # Permissions: read, write, execute, for user, group, others.
5323 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5324 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5325 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5327 # Setuid/Setgid/Stickiness/SaveText.
5328 # Note that the exact meaning of these is system dependent.
5330 S_ISUID S_ISGID S_ISVTX S_ISTXT
5332 # File types. Not necessarily all are available on your system.
5334 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5336 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5338 S_IREAD S_IWRITE S_IEXEC
5340 and the S_IF* functions are
5342 S_IMODE($mode) the part of $mode containing the permission bits
5343 and the setuid/setgid/sticky bits
5345 S_IFMT($mode) the part of $mode containing the file type
5346 which can be bit-anded with e.g. S_IFREG
5347 or with the following functions
5349 # The operators -f, -d, -l, -b, -c, -p, and -s.
5351 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5352 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5354 # No direct -X operator counterpart, but for the first one
5355 # the -g operator is often equivalent. The ENFMT stands for
5356 # record flocking enforcement, a platform-dependent feature.
5358 S_ISENFMT($mode) S_ISWHT($mode)
5360 See your native chmod(2) and stat(2) documentation for more details
5361 about the S_* constants. To get status info for a symbolic link
5362 instead of the target file behind the link, use the C<lstat> function.
5368 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5369 doing many pattern matches on the string before it is next modified.
5370 This may or may not save time, depending on the nature and number of
5371 patterns you are searching on, and on the distribution of character
5372 frequencies in the string to be searched--you probably want to compare
5373 run times with and without it to see which runs faster. Those loops
5374 which scan for many short constant strings (including the constant
5375 parts of more complex patterns) will benefit most. You may have only
5376 one C<study> active at a time--if you study a different scalar the first
5377 is "unstudied". (The way C<study> works is this: a linked list of every
5378 character in the string to be searched is made, so we know, for
5379 example, where all the C<'k'> characters are. From each search string,
5380 the rarest character is selected, based on some static frequency tables
5381 constructed from some C programs and English text. Only those places
5382 that contain this "rarest" character are examined.)
5384 For example, here is a loop that inserts index producing entries
5385 before any line containing a certain pattern:
5389 print ".IX foo\n" if /\bfoo\b/;
5390 print ".IX bar\n" if /\bbar\b/;
5391 print ".IX blurfl\n" if /\bblurfl\b/;
5396 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5397 will be looked at, because C<f> is rarer than C<o>. In general, this is
5398 a big win except in pathological cases. The only question is whether
5399 it saves you more time than it took to build the linked list in the
5402 Note that if you have to look for strings that you don't know till
5403 runtime, you can build an entire loop as a string and C<eval> that to
5404 avoid recompiling all your patterns all the time. Together with
5405 undefining C<$/> to input entire files as one record, this can be very
5406 fast, often faster than specialized programs like fgrep(1). The following
5407 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5408 out the names of those files that contain a match:
5410 $search = 'while (<>) { study;';
5411 foreach $word (@words) {
5412 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5417 eval $search; # this screams
5418 $/ = "\n"; # put back to normal input delimiter
5419 foreach $file (sort keys(%seen)) {
5423 =item sub NAME BLOCK
5425 =item sub NAME (PROTO) BLOCK
5427 =item sub NAME : ATTRS BLOCK
5429 =item sub NAME (PROTO) : ATTRS BLOCK
5431 This is subroutine definition, not a real function I<per se>.
5432 Without a BLOCK it's just a forward declaration. Without a NAME,
5433 it's an anonymous function declaration, and does actually return
5434 a value: the CODE ref of the closure you just created.
5436 See L<perlsub> and L<perlref> for details about subroutines and
5437 references, and L<attributes> and L<Attribute::Handlers> for more
5438 information about attributes.
5440 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5442 =item substr EXPR,OFFSET,LENGTH
5444 =item substr EXPR,OFFSET
5446 Extracts a substring out of EXPR and returns it. First character is at
5447 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5448 If OFFSET is negative (or more precisely, less than C<$[>), starts
5449 that far from the end of the string. If LENGTH is omitted, returns
5450 everything to the end of the string. If LENGTH is negative, leaves that
5451 many characters off the end of the string.
5453 You can use the substr() function as an lvalue, in which case EXPR
5454 must itself be an lvalue. If you assign something shorter than LENGTH,
5455 the string will shrink, and if you assign something longer than LENGTH,
5456 the string will grow to accommodate it. To keep the string the same
5457 length you may need to pad or chop your value using C<sprintf>.
5459 If OFFSET and LENGTH specify a substring that is partly outside the
5460 string, only the part within the string is returned. If the substring
5461 is beyond either end of the string, substr() returns the undefined
5462 value and produces a warning. When used as an lvalue, specifying a
5463 substring that is entirely outside the string is a fatal error.
5464 Here's an example showing the behavior for boundary cases:
5467 substr($name, 4) = 'dy'; # $name is now 'freddy'
5468 my $null = substr $name, 6, 2; # returns '' (no warning)
5469 my $oops = substr $name, 7; # returns undef, with warning
5470 substr($name, 7) = 'gap'; # fatal error
5472 An alternative to using substr() as an lvalue is to specify the
5473 replacement string as the 4th argument. This allows you to replace
5474 parts of the EXPR and return what was there before in one operation,
5475 just as you can with splice().
5477 If the lvalue returned by substr is used after the EXPR is changed in
5478 any way, the behaviour may not be as expected and is subject to change.
5479 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5480 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5481 substring assignment, and then the substr is used again), or where a
5482 substr() is aliased via a C<foreach> loop or passed as a parameter or
5483 a reference to it is taken and then the alias, parameter, or deref'd
5484 reference either is used after the original EXPR has been changed or
5485 is assigned to and then used a second time.
5487 =item symlink OLDFILE,NEWFILE
5489 Creates a new filename symbolically linked to the old filename.
5490 Returns C<1> for success, C<0> otherwise. On systems that don't support
5491 symbolic links, produces a fatal error at run time. To check for that,
5494 $symlink_exists = eval { symlink("",""); 1 };
5498 Calls the system call specified as the first element of the list,
5499 passing the remaining elements as arguments to the system call. If
5500 unimplemented, produces a fatal error. The arguments are interpreted
5501 as follows: if a given argument is numeric, the argument is passed as
5502 an int. If not, the pointer to the string value is passed. You are
5503 responsible to make sure a string is pre-extended long enough to
5504 receive any result that might be written into a string. You can't use a
5505 string literal (or other read-only string) as an argument to C<syscall>
5506 because Perl has to assume that any string pointer might be written
5508 integer arguments are not literals and have never been interpreted in a
5509 numeric context, you may need to add C<0> to them to force them to look
5510 like numbers. This emulates the C<syswrite> function (or vice versa):
5512 require 'syscall.ph'; # may need to run h2ph
5514 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5516 Note that Perl supports passing of up to only 14 arguments to your system call,
5517 which in practice should usually suffice.
5519 Syscall returns whatever value returned by the system call it calls.
5520 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5521 Note that some system calls can legitimately return C<-1>. The proper
5522 way to handle such calls is to assign C<$!=0;> before the call and
5523 check the value of C<$!> if syscall returns C<-1>.
5525 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5526 number of the read end of the pipe it creates. There is no way
5527 to retrieve the file number of the other end. You can avoid this
5528 problem by using C<pipe> instead.
5530 =item sysopen FILEHANDLE,FILENAME,MODE
5532 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5534 Opens the file whose filename is given by FILENAME, and associates it
5535 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5536 the name of the real filehandle wanted. This function calls the
5537 underlying operating system's C<open> function with the parameters
5538 FILENAME, MODE, PERMS.
5540 The possible values and flag bits of the MODE parameter are
5541 system-dependent; they are available via the standard module C<Fcntl>.
5542 See the documentation of your operating system's C<open> to see which
5543 values and flag bits are available. You may combine several flags
5544 using the C<|>-operator.
5546 Some of the most common values are C<O_RDONLY> for opening the file in
5547 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5548 and C<O_RDWR> for opening the file in read-write mode, and.
5550 For historical reasons, some values work on almost every system
5551 supported by perl: zero means read-only, one means write-only, and two
5552 means read/write. We know that these values do I<not> work under
5553 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5554 use them in new code.
5556 If the file named by FILENAME does not exist and the C<open> call creates
5557 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5558 PERMS specifies the permissions of the newly created file. If you omit
5559 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5560 These permission values need to be in octal, and are modified by your
5561 process's current C<umask>.
5563 In many systems the C<O_EXCL> flag is available for opening files in
5564 exclusive mode. This is B<not> locking: exclusiveness means here that
5565 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5568 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5570 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5571 that takes away the user's option to have a more permissive umask.
5572 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5575 Note that C<sysopen> depends on the fdopen() C library function.
5576 On many UNIX systems, fdopen() is known to fail when file descriptors
5577 exceed a certain value, typically 255. If you need more file
5578 descriptors than that, consider rebuilding Perl to use the C<sfio>
5579 library, or perhaps using the POSIX::open() function.
5581 See L<perlopentut> for a kinder, gentler explanation of opening files.
5583 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5585 =item sysread FILEHANDLE,SCALAR,LENGTH
5587 Attempts to read LENGTH bytes of data into variable SCALAR from the
5588 specified FILEHANDLE, using the system call read(2). It bypasses
5589 buffered IO, so mixing this with other kinds of reads, C<print>,
5590 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5591 perlio or stdio layers usually buffers data. Returns the number of
5592 bytes actually read, C<0> at end of file, or undef if there was an
5593 error (in the latter case C<$!> is also set). SCALAR will be grown or
5594 shrunk so that the last byte actually read is the last byte of the
5595 scalar after the read.
5597 An OFFSET may be specified to place the read data at some place in the
5598 string other than the beginning. A negative OFFSET specifies
5599 placement at that many characters counting backwards from the end of
5600 the string. A positive OFFSET greater than the length of SCALAR
5601 results in the string being padded to the required size with C<"\0">
5602 bytes before the result of the read is appended.
5604 There is no syseof() function, which is ok, since eof() doesn't work
5605 very well on device files (like ttys) anyway. Use sysread() and check
5606 for a return value for 0 to decide whether you're done.
5608 Note that if the filehandle has been marked as C<:utf8> Unicode
5609 characters are read instead of bytes (the LENGTH, OFFSET, and the
5610 return value of sysread() are in Unicode characters).
5611 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5612 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5614 =item sysseek FILEHANDLE,POSITION,WHENCE
5616 Sets FILEHANDLE's system position in bytes using the system call
5617 lseek(2). FILEHANDLE may be an expression whose value gives the name
5618 of the filehandle. The values for WHENCE are C<0> to set the new
5619 position to POSITION, C<1> to set the it to the current position plus
5620 POSITION, and C<2> to set it to EOF plus POSITION (typically
5623 Note the I<in bytes>: even if the filehandle has been set to operate
5624 on characters (for example by using the C<:utf8> I/O layer), tell()
5625 will return byte offsets, not character offsets (because implementing
5626 that would render sysseek() very slow).
5628 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5629 than C<sysread>, for example >< or read()) C<print>, C<write>,
5630 C<seek>, C<tell>, or C<eof> may cause confusion.
5632 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5633 and C<SEEK_END> (start of the file, current position, end of the file)
5634 from the Fcntl module. Use of the constants is also more portable
5635 than relying on 0, 1, and 2. For example to define a "systell" function:
5637 use Fnctl 'SEEK_CUR';
5638 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5640 Returns the new position, or the undefined value on failure. A position
5641 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5642 true on success and false on failure, yet you can still easily determine
5647 =item system PROGRAM LIST
5649 Does exactly the same thing as C<exec LIST>, except that a fork is
5650 done first, and the parent process waits for the child process to
5651 complete. Note that argument processing varies depending on the
5652 number of arguments. If there is more than one argument in LIST,
5653 or if LIST is an array with more than one value, starts the program
5654 given by the first element of the list with arguments given by the
5655 rest of the list. If there is only one scalar argument, the argument
5656 is checked for shell metacharacters, and if there are any, the
5657 entire argument is passed to the system's command shell for parsing
5658 (this is C</bin/sh -c> on Unix platforms, but varies on other
5659 platforms). If there are no shell metacharacters in the argument,
5660 it is split into words and passed directly to C<execvp>, which is
5663 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5664 output before any operation that may do a fork, but this may not be
5665 supported on some platforms (see L<perlport>). To be safe, you may need
5666 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5667 of C<IO::Handle> on any open handles.
5669 The return value is the exit status of the program as returned by the
5670 C<wait> call. To get the actual exit value shift right by eight (see below).
5671 See also L</exec>. This is I<not> what you want to use to capture
5672 the output from a command, for that you should use merely backticks or
5673 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5674 indicates a failure to start the program (inspect $! for the reason).
5676 Like C<exec>, C<system> allows you to lie to a program about its name if
5677 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5679 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5680 killing the program they're running doesn't actually interrupt
5683 @args = ("command", "arg1", "arg2");
5685 or die "system @args failed: $?"
5687 You can check all the failure possibilities by inspecting
5690 $exit_value = $? >> 8;
5691 $signal_num = $? & 127;
5692 $dumped_core = $? & 128;
5694 or more portably by using the W*() calls of the POSIX extension;
5695 see L<perlport> for more information.
5697 When the arguments get executed via the system shell, results
5698 and return codes will be subject to its quirks and capabilities.
5699 See L<perlop/"`STRING`"> and L</exec> for details.
5701 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5703 =item syswrite FILEHANDLE,SCALAR,LENGTH
5705 =item syswrite FILEHANDLE,SCALAR
5707 Attempts to write LENGTH bytes of data from variable SCALAR to the
5708 specified FILEHANDLE, using the system call write(2). If LENGTH is
5709 not specified, writes whole SCALAR. It bypasses buffered IO, so
5710 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5711 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5712 stdio layers usually buffers data. Returns the number of bytes
5713 actually written, or C<undef> if there was an error (in this case the
5714 errno variable C<$!> is also set). If the LENGTH is greater than the
5715 available data in the SCALAR after the OFFSET, only as much data as is
5716 available will be written.
5718 An OFFSET may be specified to write the data from some part of the
5719 string other than the beginning. A negative OFFSET specifies writing
5720 that many characters counting backwards from the end of the string.
5721 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5723 Note that if the filehandle has been marked as C<:utf8>,
5724 Unicode characters are written instead of bytes (the LENGTH, OFFSET,
5725 and the return value of syswrite() are in Unicode characters).
5726 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5727 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5729 =item tell FILEHANDLE
5733 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5734 error. FILEHANDLE may be an expression whose value gives the name of
5735 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5738 Note the I<in bytes>: even if the filehandle has been set to
5739 operate on characters (for example by using the C<:utf8> open
5740 layer), tell() will return byte offsets, not character offsets
5741 (because that would render seek() and tell() rather slow).
5743 The return value of tell() for the standard streams like the STDIN
5744 depends on the operating system: it may return -1 or something else.
5745 tell() on pipes, fifos, and sockets usually returns -1.
5747 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5749 Do not use tell() on a filehandle that has been opened using
5750 sysopen(), use sysseek() for that as described above. Why? Because
5751 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5752 buffered filehandles. sysseek() make sense only on the first kind,
5753 tell() only makes sense on the second kind.
5755 =item telldir DIRHANDLE
5757 Returns the current position of the C<readdir> routines on DIRHANDLE.
5758 Value may be given to C<seekdir> to access a particular location in a
5759 directory. Has the same caveats about possible directory compaction as
5760 the corresponding system library routine.
5762 =item tie VARIABLE,CLASSNAME,LIST
5764 This function binds a variable to a package class that will provide the
5765 implementation for the variable. VARIABLE is the name of the variable
5766 to be enchanted. CLASSNAME is the name of a class implementing objects
5767 of correct type. Any additional arguments are passed to the C<new>
5768 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5769 or C<TIEHASH>). Typically these are arguments such as might be passed
5770 to the C<dbm_open()> function of C. The object returned by the C<new>
5771 method is also returned by the C<tie> function, which would be useful
5772 if you want to access other methods in CLASSNAME.
5774 Note that functions such as C<keys> and C<values> may return huge lists
5775 when used on large objects, like DBM files. You may prefer to use the
5776 C<each> function to iterate over such. Example:
5778 # print out history file offsets
5780 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5781 while (($key,$val) = each %HIST) {
5782 print $key, ' = ', unpack('L',$val), "\n";
5786 A class implementing a hash should have the following methods:
5788 TIEHASH classname, LIST
5790 STORE this, key, value
5795 NEXTKEY this, lastkey
5799 A class implementing an ordinary array should have the following methods:
5801 TIEARRAY classname, LIST
5803 STORE this, key, value
5805 STORESIZE this, count
5811 SPLICE this, offset, length, LIST
5816 A class implementing a file handle should have the following methods:
5818 TIEHANDLE classname, LIST
5819 READ this, scalar, length, offset
5822 WRITE this, scalar, length, offset
5824 PRINTF this, format, LIST
5828 SEEK this, position, whence
5830 OPEN this, mode, LIST
5835 A class implementing a scalar should have the following methods:
5837 TIESCALAR classname, LIST
5843 Not all methods indicated above need be implemented. See L<perltie>,
5844 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5846 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5847 for you--you need to do that explicitly yourself. See L<DB_File>
5848 or the F<Config> module for interesting C<tie> implementations.
5850 For further details see L<perltie>, L<"tied VARIABLE">.
5854 Returns a reference to the object underlying VARIABLE (the same value
5855 that was originally returned by the C<tie> call that bound the variable
5856 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5861 Returns the number of non-leap seconds since whatever time the system
5862 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5863 and 00:00:00 UTC, January 1, 1970 for most other systems).
5864 Suitable for feeding to C<gmtime> and C<localtime>.
5866 For measuring time in better granularity than one second,
5867 you may use either the Time::HiRes module (from CPAN, and starting from
5868 Perl 5.8 part of the standard distribution), or if you have
5869 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5870 See L<perlfaq8> for details.
5874 Returns a four-element list giving the user and system times, in
5875 seconds, for this process and the children of this process.
5877 ($user,$system,$cuser,$csystem) = times;
5879 In scalar context, C<times> returns C<$user>.
5883 The transliteration operator. Same as C<y///>. See L<perlop>.
5885 =item truncate FILEHANDLE,LENGTH
5887 =item truncate EXPR,LENGTH
5889 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5890 specified length. Produces a fatal error if truncate isn't implemented
5891 on your system. Returns true if successful, the undefined value
5894 The behavior is undefined if LENGTH is greater than the length of the
5901 Returns an uppercased version of EXPR. This is the internal function
5902 implementing the C<\U> escape in double-quoted strings. Respects
5903 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5904 and L<perlunicode> for more details about locale and Unicode support.
5905 It does not attempt to do titlecase mapping on initial letters. See
5906 C<ucfirst> for that.
5908 If EXPR is omitted, uses C<$_>.
5914 Returns the value of EXPR with the first character in uppercase
5915 (titlecase in Unicode). This is the internal function implementing
5916 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5917 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5918 for more details about locale and Unicode support.
5920 If EXPR is omitted, uses C<$_>.
5926 Sets the umask for the process to EXPR and returns the previous value.
5927 If EXPR is omitted, merely returns the current umask.
5929 The Unix permission C<rwxr-x---> is represented as three sets of three
5930 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5931 and isn't one of the digits). The C<umask> value is such a number
5932 representing disabled permissions bits. The permission (or "mode")
5933 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5934 even if you tell C<sysopen> to create a file with permissions C<0777>,
5935 if your umask is C<0022> then the file will actually be created with
5936 permissions C<0755>. If your C<umask> were C<0027> (group can't
5937 write; others can't read, write, or execute), then passing
5938 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5941 Here's some advice: supply a creation mode of C<0666> for regular
5942 files (in C<sysopen>) and one of C<0777> for directories (in
5943 C<mkdir>) and executable files. This gives users the freedom of
5944 choice: if they want protected files, they might choose process umasks
5945 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5946 Programs should rarely if ever make policy decisions better left to
5947 the user. The exception to this is when writing files that should be
5948 kept private: mail files, web browser cookies, I<.rhosts> files, and
5951 If umask(2) is not implemented on your system and you are trying to
5952 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5953 fatal error at run time. If umask(2) is not implemented and you are
5954 not trying to restrict access for yourself, returns C<undef>.
5956 Remember that a umask is a number, usually given in octal; it is I<not> a
5957 string of octal digits. See also L</oct>, if all you have is a string.
5963 Undefines the value of EXPR, which must be an lvalue. Use only on a
5964 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5965 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5966 will probably not do what you expect on most predefined variables or
5967 DBM list values, so don't do that; see L<delete>.) Always returns the
5968 undefined value. You can omit the EXPR, in which case nothing is
5969 undefined, but you still get an undefined value that you could, for
5970 instance, return from a subroutine, assign to a variable or pass as a
5971 parameter. Examples:
5974 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5978 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5979 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5980 select undef, undef, undef, 0.25;
5981 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5983 Note that this is a unary operator, not a list operator.
5989 Deletes a list of files. Returns the number of files successfully
5992 $cnt = unlink 'a', 'b', 'c';
5996 Note: C<unlink> will not delete directories unless you are superuser and
5997 the B<-U> flag is supplied to Perl. Even if these conditions are
5998 met, be warned that unlinking a directory can inflict damage on your
5999 filesystem. Use C<rmdir> instead.
6001 If LIST is omitted, uses C<$_>.
6003 =item unpack TEMPLATE,EXPR
6005 =item unpack TEMPLATE
6007 C<unpack> does the reverse of C<pack>: it takes a string
6008 and expands it out into a list of values.
6009 (In scalar context, it returns merely the first value produced.)
6011 If EXPR is omitted, unpacks the C<$_> string.
6013 The string is broken into chunks described by the TEMPLATE. Each chunk
6014 is converted separately to a value. Typically, either the string is a result
6015 of C<pack>, or the bytes of the string represent a C structure of some
6018 The TEMPLATE has the same format as in the C<pack> function.
6019 Here's a subroutine that does substring:
6022 my($what,$where,$howmuch) = @_;
6023 unpack("x$where a$howmuch", $what);
6028 sub ordinal { unpack("c",$_[0]); } # same as ord()
6030 In addition to fields allowed in pack(), you may prefix a field with
6031 a %<number> to indicate that
6032 you want a <number>-bit checksum of the items instead of the items
6033 themselves. Default is a 16-bit checksum. Checksum is calculated by
6034 summing numeric values of expanded values (for string fields the sum of
6035 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6037 For example, the following
6038 computes the same number as the System V sum program:
6042 unpack("%32C*",<>) % 65535;
6045 The following efficiently counts the number of set bits in a bit vector:
6047 $setbits = unpack("%32b*", $selectmask);
6049 The C<p> and C<P> formats should be used with care. Since Perl
6050 has no way of checking whether the value passed to C<unpack()>
6051 corresponds to a valid memory location, passing a pointer value that's
6052 not known to be valid is likely to have disastrous consequences.
6054 If there are more pack codes or if the repeat count of a field or a group
6055 is larger than what the remainder of the input string allows, the result
6056 is not well defined: in some cases, the repeat count is decreased, or
6057 C<unpack()> will produce null strings or zeroes, or terminate with an
6058 error. If the input string is longer than one described by the TEMPLATE,
6059 the rest is ignored.
6061 See L</pack> for more examples and notes.
6063 =item untie VARIABLE
6065 Breaks the binding between a variable and a package. (See C<tie>.)
6066 Has no effect if the variable is not tied.
6068 =item unshift ARRAY,LIST
6070 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6071 depending on how you look at it. Prepends list to the front of the
6072 array, and returns the new number of elements in the array.
6074 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6076 Note the LIST is prepended whole, not one element at a time, so the
6077 prepended elements stay in the same order. Use C<reverse> to do the
6080 =item use Module VERSION LIST
6082 =item use Module VERSION
6084 =item use Module LIST
6090 Imports some semantics into the current package from the named module,
6091 generally by aliasing certain subroutine or variable names into your
6092 package. It is exactly equivalent to
6094 BEGIN { require Module; import Module LIST; }
6096 except that Module I<must> be a bareword.
6098 VERSION may be either a numeric argument such as 5.006, which will be
6099 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6100 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6101 greater than the version of the current Perl interpreter; Perl will not
6102 attempt to parse the rest of the file. Compare with L</require>, which can
6103 do a similar check at run time.
6105 Specifying VERSION as a literal of the form v5.6.1 should generally be
6106 avoided, because it leads to misleading error messages under earlier
6107 versions of Perl which do not support this syntax. The equivalent numeric
6108 version should be used instead.
6110 use v5.6.1; # compile time version check
6112 use 5.006_001; # ditto; preferred for backwards compatibility
6114 This is often useful if you need to check the current Perl version before
6115 C<use>ing library modules that have changed in incompatible ways from
6116 older versions of Perl. (We try not to do this more than we have to.)
6118 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6119 C<require> makes sure the module is loaded into memory if it hasn't been
6120 yet. The C<import> is not a builtin--it's just an ordinary static method
6121 call into the C<Module> package to tell the module to import the list of
6122 features back into the current package. The module can implement its
6123 C<import> method any way it likes, though most modules just choose to
6124 derive their C<import> method via inheritance from the C<Exporter> class that
6125 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6126 method can be found then the call is skipped.
6128 If you do not want to call the package's C<import> method (for instance,
6129 to stop your namespace from being altered), explicitly supply the empty list:
6133 That is exactly equivalent to
6135 BEGIN { require Module }
6137 If the VERSION argument is present between Module and LIST, then the
6138 C<use> will call the VERSION method in class Module with the given
6139 version as an argument. The default VERSION method, inherited from
6140 the UNIVERSAL class, croaks if the given version is larger than the
6141 value of the variable C<$Module::VERSION>.
6143 Again, there is a distinction between omitting LIST (C<import> called
6144 with no arguments) and an explicit empty LIST C<()> (C<import> not
6145 called). Note that there is no comma after VERSION!
6147 Because this is a wide-open interface, pragmas (compiler directives)
6148 are also implemented this way. Currently implemented pragmas are:
6153 use sigtrap qw(SEGV BUS);
6154 use strict qw(subs vars refs);
6155 use subs qw(afunc blurfl);
6156 use warnings qw(all);
6157 use sort qw(stable _quicksort _mergesort);
6159 Some of these pseudo-modules import semantics into the current
6160 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6161 which import symbols into the current package (which are effective
6162 through the end of the file).
6164 There's a corresponding C<no> command that unimports meanings imported
6165 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6171 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6172 for the C<-M> and C<-m> command-line options to perl that give C<use>
6173 functionality from the command-line.
6177 Changes the access and modification times on each file of a list of
6178 files. The first two elements of the list must be the NUMERICAL access
6179 and modification times, in that order. Returns the number of files
6180 successfully changed. The inode change time of each file is set
6181 to the current time. For example, this code has the same effect as the
6182 Unix touch(1) command when the files I<already exist>.
6186 utime $now, $now, @ARGV;
6188 B<Note:> Under NFS, touch(1) uses the time of the NFS server, not
6189 the time of the local machine. If there is a time synchronization
6190 problem, the NFS server and local machine will have different times.
6192 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6193 the utime(2) function in the C library will be called with a null second
6194 argument. On most systems, this will set the file's access and
6195 modification times to the current time (i.e. equivalent to the example
6198 utime undef, undef, @ARGV;
6202 Returns a list consisting of all the values of the named hash. (In a
6203 scalar context, returns the number of values.) The values are
6204 returned in an apparently random order. The actual random order is
6205 subject to change in future versions of perl, but it is guaranteed to
6206 be the same order as either the C<keys> or C<each> function would
6207 produce on the same (unmodified) hash.
6209 Note that the values are not copied, which means modifying them will
6210 modify the contents of the hash:
6212 for (values %hash) { s/foo/bar/g } # modifies %hash values
6213 for (@hash{keys %hash}) { s/foo/bar/g } # same
6215 As a side effect, calling values() resets the HASH's internal iterator.
6216 See also C<keys>, C<each>, and C<sort>.
6218 =item vec EXPR,OFFSET,BITS
6220 Treats the string in EXPR as a bit vector made up of elements of
6221 width BITS, and returns the value of the element specified by OFFSET
6222 as an unsigned integer. BITS therefore specifies the number of bits
6223 that are reserved for each element in the bit vector. This must
6224 be a power of two from 1 to 32 (or 64, if your platform supports
6227 If BITS is 8, "elements" coincide with bytes of the input string.
6229 If BITS is 16 or more, bytes of the input string are grouped into chunks
6230 of size BITS/8, and each group is converted to a number as with
6231 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6232 for BITS==64). See L<"pack"> for details.
6234 If bits is 4 or less, the string is broken into bytes, then the bits
6235 of each byte are broken into 8/BITS groups. Bits of a byte are
6236 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6237 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6238 breaking the single input byte C<chr(0x36)> into two groups gives a list
6239 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6241 C<vec> may also be assigned to, in which case parentheses are needed
6242 to give the expression the correct precedence as in
6244 vec($image, $max_x * $x + $y, 8) = 3;
6246 If the selected element is outside the string, the value 0 is returned.
6247 If an element off the end of the string is written to, Perl will first
6248 extend the string with sufficiently many zero bytes. It is an error
6249 to try to write off the beginning of the string (i.e. negative OFFSET).
6251 The string should not contain any character with the value > 255 (which
6252 can only happen if you're using UTF8 encoding). If it does, it will be
6253 treated as something which is not UTF8 encoded. When the C<vec> was
6254 assigned to, other parts of your program will also no longer consider the
6255 string to be UTF8 encoded. In other words, if you do have such characters
6256 in your string, vec() will operate on the actual byte string, and not the
6257 conceptual character string.
6259 Strings created with C<vec> can also be manipulated with the logical
6260 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6261 vector operation is desired when both operands are strings.
6262 See L<perlop/"Bitwise String Operators">.
6264 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6265 The comments show the string after each step. Note that this code works
6266 in the same way on big-endian or little-endian machines.
6269 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6271 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6272 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6274 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6275 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6276 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6277 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6278 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6279 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6281 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6282 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6283 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6286 To transform a bit vector into a string or list of 0's and 1's, use these:
6288 $bits = unpack("b*", $vector);
6289 @bits = split(//, unpack("b*", $vector));
6291 If you know the exact length in bits, it can be used in place of the C<*>.
6293 Here is an example to illustrate how the bits actually fall in place:
6299 unpack("V",$_) 01234567890123456789012345678901
6300 ------------------------------------------------------------------
6305 for ($shift=0; $shift < $width; ++$shift) {
6306 for ($off=0; $off < 32/$width; ++$off) {
6307 $str = pack("B*", "0"x32);
6308 $bits = (1<<$shift);
6309 vec($str, $off, $width) = $bits;
6310 $res = unpack("b*",$str);
6311 $val = unpack("V", $str);
6318 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6319 $off, $width, $bits, $val, $res
6323 Regardless of the machine architecture on which it is run, the above
6324 example should print the following table:
6327 unpack("V",$_) 01234567890123456789012345678901
6328 ------------------------------------------------------------------
6329 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6330 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6331 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6332 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6333 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6334 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6335 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6336 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6337 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6338 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6339 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6340 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6341 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6342 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6343 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6344 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6345 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6346 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6347 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6348 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6349 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6350 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6351 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6352 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6353 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6354 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6355 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6356 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6357 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6358 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6359 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6360 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6361 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6362 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6363 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6364 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6365 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6366 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6367 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6368 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6369 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6370 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6371 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6372 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6373 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6374 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6375 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6376 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6377 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6378 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6379 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6380 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6381 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6382 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6383 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6384 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6385 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6386 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6387 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6388 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6389 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6390 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6391 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6392 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6393 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6394 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6395 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6396 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6397 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6398 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6399 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6400 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6401 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6402 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6403 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6404 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6405 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6406 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6407 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6408 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6409 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6410 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6411 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6412 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6413 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6414 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6415 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6416 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6417 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6418 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6419 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6420 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6421 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6422 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6423 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6424 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6425 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6426 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6427 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6428 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6429 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6430 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6431 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6432 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6433 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6434 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6435 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6436 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6437 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6438 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6439 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6440 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6441 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6442 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6443 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6444 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6445 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6446 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6447 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6448 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6449 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6450 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6451 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6452 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6453 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6454 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6455 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6456 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6460 Behaves like the wait(2) system call on your system: it waits for a child
6461 process to terminate and returns the pid of the deceased process, or
6462 C<-1> if there are no child processes. The status is returned in C<$?>.
6463 Note that a return value of C<-1> could mean that child processes are
6464 being automatically reaped, as described in L<perlipc>.
6466 =item waitpid PID,FLAGS
6468 Waits for a particular child process to terminate and returns the pid of
6469 the deceased process, or C<-1> if there is no such child process. On some
6470 systems, a value of 0 indicates that there are processes still running.
6471 The status is returned in C<$?>. If you say
6473 use POSIX ":sys_wait_h";
6476 $kid = waitpid(-1, WNOHANG);
6479 then you can do a non-blocking wait for all pending zombie processes.
6480 Non-blocking wait is available on machines supporting either the
6481 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6482 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6483 system call by remembering the status values of processes that have
6484 exited but have not been harvested by the Perl script yet.)
6486 Note that on some systems, a return value of C<-1> could mean that child
6487 processes are being automatically reaped. See L<perlipc> for details,
6488 and for other examples.
6492 Returns true if the context of the currently executing subroutine is
6493 looking for a list value. Returns false if the context is looking
6494 for a scalar. Returns the undefined value if the context is looking
6495 for no value (void context).
6497 return unless defined wantarray; # don't bother doing more
6498 my @a = complex_calculation();
6499 return wantarray ? @a : "@a";
6501 This function should have been named wantlist() instead.
6505 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6508 If LIST is empty and C<$@> already contains a value (typically from a
6509 previous eval) that value is used after appending C<"\t...caught">
6510 to C<$@>. This is useful for staying almost, but not entirely similar to
6513 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6515 No message is printed if there is a C<$SIG{__WARN__}> handler
6516 installed. It is the handler's responsibility to deal with the message
6517 as it sees fit (like, for instance, converting it into a C<die>). Most
6518 handlers must therefore make arrangements to actually display the
6519 warnings that they are not prepared to deal with, by calling C<warn>
6520 again in the handler. Note that this is quite safe and will not
6521 produce an endless loop, since C<__WARN__> hooks are not called from
6524 You will find this behavior is slightly different from that of
6525 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6526 instead call C<die> again to change it).
6528 Using a C<__WARN__> handler provides a powerful way to silence all
6529 warnings (even the so-called mandatory ones). An example:
6531 # wipe out *all* compile-time warnings
6532 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6534 my $foo = 20; # no warning about duplicate my $foo,
6535 # but hey, you asked for it!
6536 # no compile-time or run-time warnings before here
6539 # run-time warnings enabled after here
6540 warn "\$foo is alive and $foo!"; # does show up
6542 See L<perlvar> for details on setting C<%SIG> entries, and for more
6543 examples. See the Carp module for other kinds of warnings using its
6544 carp() and cluck() functions.
6546 =item write FILEHANDLE
6552 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6553 using the format associated with that file. By default the format for
6554 a file is the one having the same name as the filehandle, but the
6555 format for the current output channel (see the C<select> function) may be set
6556 explicitly by assigning the name of the format to the C<$~> variable.
6558 Top of form processing is handled automatically: if there is
6559 insufficient room on the current page for the formatted record, the
6560 page is advanced by writing a form feed, a special top-of-page format
6561 is used to format the new page header, and then the record is written.
6562 By default the top-of-page format is the name of the filehandle with
6563 "_TOP" appended, but it may be dynamically set to the format of your
6564 choice by assigning the name to the C<$^> variable while the filehandle is
6565 selected. The number of lines remaining on the current page is in
6566 variable C<$->, which can be set to C<0> to force a new page.
6568 If FILEHANDLE is unspecified, output goes to the current default output
6569 channel, which starts out as STDOUT but may be changed by the
6570 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6571 is evaluated and the resulting string is used to look up the name of
6572 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6574 Note that write is I<not> the opposite of C<read>. Unfortunately.
6578 The transliteration operator. Same as C<tr///>. See L<perlop>.