3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 For more information see L<perlipc>.
436 Returns the arctangent of Y/X in the range -PI to PI.
438 For the tangent operation, you may use the C<Math::Trig::tan>
439 function, or use the familiar relation:
441 sub tan { sin($_[0]) / cos($_[0]) }
443 =item bind SOCKET,NAME
445 Binds a network address to a socket, just as the bind system call
446 does. Returns true if it succeeded, false otherwise. NAME should be a
447 packed address of the appropriate type for the socket. See the examples in
448 L<perlipc/"Sockets: Client/Server Communication">.
450 =item binmode FILEHANDLE, LAYER
452 =item binmode FILEHANDLE
454 Arranges for FILEHANDLE to be read or written in "binary" or "text"
455 mode on systems where the run-time libraries distinguish between
456 binary and text files. If FILEHANDLE is an expression, the value is
457 taken as the name of the filehandle. Returns true on success,
458 otherwise it returns C<undef> and sets C<$!> (errno).
460 On some systems (in general, DOS and Windows-based systems) binmode()
461 is necessary when you're not working with a text file. For the sake
462 of portability it is a good idea to always use it when appropriate,
463 and to never use it when it isn't appropriate. Also, people can
464 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
466 In other words: regardless of platform, use binmode() on binary data,
467 like for example images.
469 If LAYER is present it is a single string, but may contain multiple
470 directives. The directives alter the behaviour of the file handle.
471 When LAYER is present using binmode on text file makes sense.
473 If LAYER is omitted or specified as C<:raw> the filehandle is made
474 suitable for passing binary data. This includes turning off possible CRLF
475 translation and marking it as bytes (as opposed to Unicode characters).
476 Note that as despite what may be implied in I<"Programming Perl">
477 (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
478 -- other layers which would affect binary nature of the stream are
479 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
480 PERLIO environment variable.
482 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
483 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
484 establish default I/O layers. See L<open>.
486 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
487 in "Programming Perl, 3rd Edition". However, since the publishing of this
488 book, by many known as "Camel III", the consensus of the naming of this
489 functionality has moved from "discipline" to "layer". All documentation
490 of this version of Perl therefore refers to "layers" rather than to
491 "disciplines". Now back to the regularly scheduled documentation...>
493 To mark FILEHANDLE as UTF-8, use C<:utf8>.
495 In general, binmode() should be called after open() but before any I/O
496 is done on the filehandle. Calling binmode() will normally flush any
497 pending buffered output data (and perhaps pending input data) on the
498 handle. An exception to this is the C<:encoding> layer that
499 changes the default character encoding of the handle, see L<open>.
500 The C<:encoding> layer sometimes needs to be called in
501 mid-stream, and it doesn't flush the stream. The C<:encoding>
502 also implicitly pushes on top of itself the C<:utf8> layer because
503 internally Perl will operate on UTF-8 encoded Unicode characters.
505 The operating system, device drivers, C libraries, and Perl run-time
506 system all work together to let the programmer treat a single
507 character (C<\n>) as the line terminator, irrespective of the external
508 representation. On many operating systems, the native text file
509 representation matches the internal representation, but on some
510 platforms the external representation of C<\n> is made up of more than
513 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
514 character to end each line in the external representation of text (even
515 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
516 on Unix and most VMS files). In other systems like OS/2, DOS and the
517 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
518 but what's stored in text files are the two characters C<\cM\cJ>. That
519 means that, if you don't use binmode() on these systems, C<\cM\cJ>
520 sequences on disk will be converted to C<\n> on input, and any C<\n> in
521 your program will be converted back to C<\cM\cJ> on output. This is what
522 you want for text files, but it can be disastrous for binary files.
524 Another consequence of using binmode() (on some systems) is that
525 special end-of-file markers will be seen as part of the data stream.
526 For systems from the Microsoft family this means that if your binary
527 data contains C<\cZ>, the I/O subsystem will regard it as the end of
528 the file, unless you use binmode().
530 binmode() is not only important for readline() and print() operations,
531 but also when using read(), seek(), sysread(), syswrite() and tell()
532 (see L<perlport> for more details). See the C<$/> and C<$\> variables
533 in L<perlvar> for how to manually set your input and output
534 line-termination sequences.
536 =item bless REF,CLASSNAME
540 This function tells the thingy referenced by REF that it is now an object
541 in the CLASSNAME package. If CLASSNAME is omitted, the current package
542 is used. Because a C<bless> is often the last thing in a constructor,
543 it returns the reference for convenience. Always use the two-argument
544 version if the function doing the blessing might be inherited by a
545 derived class. See L<perltoot> and L<perlobj> for more about the blessing
546 (and blessings) of objects.
548 Consider always blessing objects in CLASSNAMEs that are mixed case.
549 Namespaces with all lowercase names are considered reserved for
550 Perl pragmata. Builtin types have all uppercase names, so to prevent
551 confusion, you may wish to avoid such package names as well. Make sure
552 that CLASSNAME is a true value.
554 See L<perlmod/"Perl Modules">.
560 Returns the context of the current subroutine call. In scalar context,
561 returns the caller's package name if there is a caller, that is, if
562 we're in a subroutine or C<eval> or C<require>, and the undefined value
563 otherwise. In list context, returns
565 ($package, $filename, $line) = caller;
567 With EXPR, it returns some extra information that the debugger uses to
568 print a stack trace. The value of EXPR indicates how many call frames
569 to go back before the current one.
571 ($package, $filename, $line, $subroutine, $hasargs,
572 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
574 Here $subroutine may be C<(eval)> if the frame is not a subroutine
575 call, but an C<eval>. In such a case additional elements $evaltext and
576 C<$is_require> are set: C<$is_require> is true if the frame is created by a
577 C<require> or C<use> statement, $evaltext contains the text of the
578 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
579 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
580 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
581 frame.) $subroutine may also be C<(unknown)> if this particular
582 subroutine happens to have been deleted from the symbol table.
583 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
584 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585 compiled with. The C<$hints> and C<$bitmask> values are subject to change
586 between versions of Perl, and are not meant for external use.
588 Furthermore, when called from within the DB package, caller returns more
589 detailed information: it sets the list variable C<@DB::args> to be the
590 arguments with which the subroutine was invoked.
592 Be aware that the optimizer might have optimized call frames away before
593 C<caller> had a chance to get the information. That means that C<caller(N)>
594 might not return information about the call frame you expect it do, for
595 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
596 previous time C<caller> was called.
600 Changes the working directory to EXPR, if possible. If EXPR is omitted,
601 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
602 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
603 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
604 neither is set, C<chdir> does nothing. It returns true upon success,
605 false otherwise. See the example under C<die>.
609 Changes the permissions of a list of files. The first element of the
610 list must be the numerical mode, which should probably be an octal
611 number, and which definitely should I<not> a string of octal digits:
612 C<0644> is okay, C<'0644'> is not. Returns the number of files
613 successfully changed. See also L</oct>, if all you have is a string.
615 $cnt = chmod 0755, 'foo', 'bar';
616 chmod 0755, @executables;
617 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
619 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
620 $mode = 0644; chmod $mode, 'foo'; # this is best
622 You can also import the symbolic C<S_I*> constants from the Fcntl
627 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
628 # This is identical to the chmod 0755 of the above example.
636 This safer version of L</chop> removes any trailing string
637 that corresponds to the current value of C<$/> (also known as
638 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
639 number of characters removed from all its arguments. It's often used to
640 remove the newline from the end of an input record when you're worried
641 that the final record may be missing its newline. When in paragraph
642 mode (C<$/ = "">), it removes all trailing newlines from the string.
643 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
644 a reference to an integer or the like, see L<perlvar>) chomp() won't
646 If VARIABLE is omitted, it chomps C<$_>. Example:
649 chomp; # avoid \n on last field
654 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
656 You can actually chomp anything that's an lvalue, including an assignment:
659 chomp($answer = <STDIN>);
661 If you chomp a list, each element is chomped, and the total number of
662 characters removed is returned.
664 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 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1590 on how exists() acts when used on a pseudo-hash.
1592 Use of a subroutine call, rather than a subroutine name, as an argument
1593 to exists() is an error.
1596 exists &sub(); # Error
1600 Evaluates EXPR and exits immediately with that value. Example:
1603 exit 0 if $ans =~ /^[Xx]/;
1605 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1606 universally recognized values for EXPR are C<0> for success and C<1>
1607 for error; other values are subject to interpretation depending on the
1608 environment in which the Perl program is running. For example, exiting
1609 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1610 the mailer to return the item undelivered, but that's not true everywhere.
1612 Don't use C<exit> to abort a subroutine if there's any chance that
1613 someone might want to trap whatever error happened. Use C<die> instead,
1614 which can be trapped by an C<eval>.
1616 The exit() function does not always exit immediately. It calls any
1617 defined C<END> routines first, but these C<END> routines may not
1618 themselves abort the exit. Likewise any object destructors that need to
1619 be called are called before the real exit. If this is a problem, you
1620 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1621 See L<perlmod> for details.
1627 Returns I<e> (the natural logarithm base) to the power of EXPR.
1628 If EXPR is omitted, gives C<exp($_)>.
1630 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1632 Implements the fcntl(2) function. You'll probably have to say
1636 first to get the correct constant definitions. Argument processing and
1637 value return works just like C<ioctl> below.
1641 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1642 or die "can't fcntl F_GETFL: $!";
1644 You don't have to check for C<defined> on the return from C<fnctl>.
1645 Like C<ioctl>, it maps a C<0> return from the system call into
1646 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1647 in numeric context. It is also exempt from the normal B<-w> warnings
1648 on improper numeric conversions.
1650 Note that C<fcntl> will produce a fatal error if used on a machine that
1651 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1652 manpage to learn what functions are available on your system.
1654 =item fileno FILEHANDLE
1656 Returns the file descriptor for a filehandle, or undefined if the
1657 filehandle is not open. This is mainly useful for constructing
1658 bitmaps for C<select> and low-level POSIX tty-handling operations.
1659 If FILEHANDLE is an expression, the value is taken as an indirect
1660 filehandle, generally its name.
1662 You can use this to find out whether two handles refer to the
1663 same underlying descriptor:
1665 if (fileno(THIS) == fileno(THAT)) {
1666 print "THIS and THAT are dups\n";
1669 (Filehandles connected to memory objects via new features of C<open> may
1670 return undefined even though they are open.)
1673 =item flock FILEHANDLE,OPERATION
1675 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1676 for success, false on failure. Produces a fatal error if used on a
1677 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1678 C<flock> is Perl's portable file locking interface, although it locks
1679 only entire files, not records.
1681 Two potentially non-obvious but traditional C<flock> semantics are
1682 that it waits indefinitely until the lock is granted, and that its locks
1683 B<merely advisory>. Such discretionary locks are more flexible, but offer
1684 fewer guarantees. This means that files locked with C<flock> may be
1685 modified by programs that do not also use C<flock>. See L<perlport>,
1686 your port's specific documentation, or your system-specific local manpages
1687 for details. It's best to assume traditional behavior if you're writing
1688 portable programs. (But if you're not, you should as always feel perfectly
1689 free to write for your own system's idiosyncrasies (sometimes called
1690 "features"). Slavish adherence to portability concerns shouldn't get
1691 in the way of your getting your job done.)
1693 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1694 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1695 you can use the symbolic names if you import them from the Fcntl module,
1696 either individually, or as a group using the ':flock' tag. LOCK_SH
1697 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1698 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1699 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1700 waiting for the lock (check the return status to see if you got it).
1702 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1703 before locking or unlocking it.
1705 Note that the emulation built with lockf(3) doesn't provide shared
1706 locks, and it requires that FILEHANDLE be open with write intent. These
1707 are the semantics that lockf(3) implements. Most if not all systems
1708 implement lockf(3) in terms of fcntl(2) locking, though, so the
1709 differing semantics shouldn't bite too many people.
1711 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1712 be open with read intent to use LOCK_SH and requires that it be open
1713 with write intent to use LOCK_EX.
1715 Note also that some versions of C<flock> cannot lock things over the
1716 network; you would need to use the more system-specific C<fcntl> for
1717 that. If you like you can force Perl to ignore your system's flock(2)
1718 function, and so provide its own fcntl(2)-based emulation, by passing
1719 the switch C<-Ud_flock> to the F<Configure> program when you configure
1722 Here's a mailbox appender for BSD systems.
1724 use Fcntl ':flock'; # import LOCK_* constants
1727 flock(MBOX,LOCK_EX);
1728 # and, in case someone appended
1729 # while we were waiting...
1734 flock(MBOX,LOCK_UN);
1737 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1738 or die "Can't open mailbox: $!";
1741 print MBOX $msg,"\n\n";
1744 On systems that support a real flock(), locks are inherited across fork()
1745 calls, whereas those that must resort to the more capricious fcntl()
1746 function lose the locks, making it harder to write servers.
1748 See also L<DB_File> for other flock() examples.
1752 Does a fork(2) system call to create a new process running the
1753 same program at the same point. It returns the child pid to the
1754 parent process, C<0> to the child process, or C<undef> if the fork is
1755 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1756 are shared, while everything else is copied. On most systems supporting
1757 fork(), great care has gone into making it extremely efficient (for
1758 example, using copy-on-write technology on data pages), making it the
1759 dominant paradigm for multitasking over the last few decades.
1761 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1762 output before forking the child process, but this may not be supported
1763 on some platforms (see L<perlport>). To be safe, you may need to set
1764 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1765 C<IO::Handle> on any open handles in order to avoid duplicate output.
1767 If you C<fork> without ever waiting on your children, you will
1768 accumulate zombies. On some systems, you can avoid this by setting
1769 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1770 forking and reaping moribund children.
1772 Note that if your forked child inherits system file descriptors like
1773 STDIN and STDOUT that are actually connected by a pipe or socket, even
1774 if you exit, then the remote server (such as, say, a CGI script or a
1775 backgrounded job launched from a remote shell) won't think you're done.
1776 You should reopen those to F</dev/null> if it's any issue.
1780 Declare a picture format for use by the C<write> function. For
1784 Test: @<<<<<<<< @||||| @>>>>>
1785 $str, $%, '$' . int($num)
1789 $num = $cost/$quantity;
1793 See L<perlform> for many details and examples.
1795 =item formline PICTURE,LIST
1797 This is an internal function used by C<format>s, though you may call it,
1798 too. It formats (see L<perlform>) a list of values according to the
1799 contents of PICTURE, placing the output into the format output
1800 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1801 Eventually, when a C<write> is done, the contents of
1802 C<$^A> are written to some filehandle, but you could also read C<$^A>
1803 yourself and then set C<$^A> back to C<"">. Note that a format typically
1804 does one C<formline> per line of form, but the C<formline> function itself
1805 doesn't care how many newlines are embedded in the PICTURE. This means
1806 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1807 You may therefore need to use multiple formlines to implement a single
1808 record format, just like the format compiler.
1810 Be careful if you put double quotes around the picture, because an C<@>
1811 character may be taken to mean the beginning of an array name.
1812 C<formline> always returns true. See L<perlform> for other examples.
1814 =item getc FILEHANDLE
1818 Returns the next character from the input file attached to FILEHANDLE,
1819 or the undefined value at end of file, or if there was an error (in
1820 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1821 STDIN. This is not particularly efficient. However, it cannot be
1822 used by itself to fetch single characters without waiting for the user
1823 to hit enter. For that, try something more like:
1826 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1829 system "stty", '-icanon', 'eol', "\001";
1835 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1838 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1842 Determination of whether $BSD_STYLE should be set
1843 is left as an exercise to the reader.
1845 The C<POSIX::getattr> function can do this more portably on
1846 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1847 module from your nearest CPAN site; details on CPAN can be found on
1852 Implements the C library function of the same name, which on most
1853 systems returns the current login from F</etc/utmp>, if any. If null,
1856 $login = getlogin || getpwuid($<) || "Kilroy";
1858 Do not consider C<getlogin> for authentication: it is not as
1859 secure as C<getpwuid>.
1861 =item getpeername SOCKET
1863 Returns the packed sockaddr address of other end of the SOCKET connection.
1866 $hersockaddr = getpeername(SOCK);
1867 ($port, $iaddr) = sockaddr_in($hersockaddr);
1868 $herhostname = gethostbyaddr($iaddr, AF_INET);
1869 $herstraddr = inet_ntoa($iaddr);
1873 Returns the current process group for the specified PID. Use
1874 a PID of C<0> to get the current process group for the
1875 current process. Will raise an exception if used on a machine that
1876 doesn't implement getpgrp(2). If PID is omitted, returns process
1877 group of current process. Note that the POSIX version of C<getpgrp>
1878 does not accept a PID argument, so only C<PID==0> is truly portable.
1882 Returns the process id of the parent process.
1884 Note for Linux users: on Linux, the C functions C<getpid()> and
1885 C<getppid()> return different values from different threads. In order to
1886 be portable, this behavior is not reflected by the perl-level function
1887 C<getppid()>, that returns a consistent value across threads. If you want
1888 to call the underlying C<getppid()>, you may use the CPAN module
1891 =item getpriority WHICH,WHO
1893 Returns the current priority for a process, a process group, or a user.
1894 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1895 machine that doesn't implement getpriority(2).
1901 =item gethostbyname NAME
1903 =item getnetbyname NAME
1905 =item getprotobyname NAME
1911 =item getservbyname NAME,PROTO
1913 =item gethostbyaddr ADDR,ADDRTYPE
1915 =item getnetbyaddr ADDR,ADDRTYPE
1917 =item getprotobynumber NUMBER
1919 =item getservbyport PORT,PROTO
1937 =item sethostent STAYOPEN
1939 =item setnetent STAYOPEN
1941 =item setprotoent STAYOPEN
1943 =item setservent STAYOPEN
1957 These routines perform the same functions as their counterparts in the
1958 system library. In list context, the return values from the
1959 various get routines are as follows:
1961 ($name,$passwd,$uid,$gid,
1962 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1963 ($name,$passwd,$gid,$members) = getgr*
1964 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1965 ($name,$aliases,$addrtype,$net) = getnet*
1966 ($name,$aliases,$proto) = getproto*
1967 ($name,$aliases,$port,$proto) = getserv*
1969 (If the entry doesn't exist you get a null list.)
1971 The exact meaning of the $gcos field varies but it usually contains
1972 the real name of the user (as opposed to the login name) and other
1973 information pertaining to the user. Beware, however, that in many
1974 system users are able to change this information and therefore it
1975 cannot be trusted and therefore the $gcos is tainted (see
1976 L<perlsec>). The $passwd and $shell, user's encrypted password and
1977 login shell, are also tainted, because of the same reason.
1979 In scalar context, you get the name, unless the function was a
1980 lookup by name, in which case you get the other thing, whatever it is.
1981 (If the entry doesn't exist you get the undefined value.) For example:
1983 $uid = getpwnam($name);
1984 $name = getpwuid($num);
1986 $gid = getgrnam($name);
1987 $name = getgrgid($num);
1991 In I<getpw*()> the fields $quota, $comment, and $expire are special
1992 cases in the sense that in many systems they are unsupported. If the
1993 $quota is unsupported, it is an empty scalar. If it is supported, it
1994 usually encodes the disk quota. If the $comment field is unsupported,
1995 it is an empty scalar. If it is supported it usually encodes some
1996 administrative comment about the user. In some systems the $quota
1997 field may be $change or $age, fields that have to do with password
1998 aging. In some systems the $comment field may be $class. The $expire
1999 field, if present, encodes the expiration period of the account or the
2000 password. For the availability and the exact meaning of these fields
2001 in your system, please consult your getpwnam(3) documentation and your
2002 F<pwd.h> file. You can also find out from within Perl what your
2003 $quota and $comment fields mean and whether you have the $expire field
2004 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2005 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2006 files are only supported if your vendor has implemented them in the
2007 intuitive fashion that calling the regular C library routines gets the
2008 shadow versions if you're running under privilege or if there exists
2009 the shadow(3) functions as found in System V ( this includes Solaris
2010 and Linux.) Those systems which implement a proprietary shadow password
2011 facility are unlikely to be supported.
2013 The $members value returned by I<getgr*()> is a space separated list of
2014 the login names of the members of the group.
2016 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2017 C, it will be returned to you via C<$?> if the function call fails. The
2018 C<@addrs> value returned by a successful call is a list of the raw
2019 addresses returned by the corresponding system library call. In the
2020 Internet domain, each address is four bytes long and you can unpack it
2021 by saying something like:
2023 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2025 The Socket library makes this slightly easier:
2028 $iaddr = inet_aton("127.1"); # or whatever address
2029 $name = gethostbyaddr($iaddr, AF_INET);
2031 # or going the other way
2032 $straddr = inet_ntoa($iaddr);
2034 If you get tired of remembering which element of the return list
2035 contains which return value, by-name interfaces are provided
2036 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2037 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2038 and C<User::grent>. These override the normal built-ins, supplying
2039 versions that return objects with the appropriate names
2040 for each field. For example:
2044 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2046 Even though it looks like they're the same method calls (uid),
2047 they aren't, because a C<File::stat> object is different from
2048 a C<User::pwent> object.
2050 =item getsockname SOCKET
2052 Returns the packed sockaddr address of this end of the SOCKET connection,
2053 in case you don't know the address because you have several different
2054 IPs that the connection might have come in on.
2057 $mysockaddr = getsockname(SOCK);
2058 ($port, $myaddr) = sockaddr_in($mysockaddr);
2059 printf "Connect to %s [%s]\n",
2060 scalar gethostbyaddr($myaddr, AF_INET),
2063 =item getsockopt SOCKET,LEVEL,OPTNAME
2065 Returns the socket option requested, or undef if there is an error.
2071 In list context, returns a (possibly empty) list of filename expansions on
2072 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2073 scalar context, glob iterates through such filename expansions, returning
2074 undef when the list is exhausted. This is the internal function
2075 implementing the C<< <*.c> >> operator, but you can use it directly. If
2076 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2077 more detail in L<perlop/"I/O Operators">.
2079 Beginning with v5.6.0, this operator is implemented using the standard
2080 C<File::Glob> extension. See L<File::Glob> for details.
2084 Converts a time as returned by the time function to an 8-element list
2085 with the time localized for the standard Greenwich time zone.
2086 Typically used as follows:
2089 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2092 All list elements are numeric, and come straight out of the C `struct
2093 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2094 specified time. $mday is the day of the month, and $mon is the month
2095 itself, in the range C<0..11> with 0 indicating January and 11
2096 indicating December. $year is the number of years since 1900. That
2097 is, $year is C<123> in year 2023. $wday is the day of the week, with
2098 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2099 the year, in the range C<0..364> (or C<0..365> in leap years.)
2101 Note that the $year element is I<not> simply the last two digits of
2102 the year. If you assume it is, then you create non-Y2K-compliant
2103 programs--and you wouldn't want to do that, would you?
2105 The proper way to get a complete 4-digit year is simply:
2109 And to get the last two digits of the year (e.g., '01' in 2001) do:
2111 $year = sprintf("%02d", $year % 100);
2113 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2115 In scalar context, C<gmtime()> returns the ctime(3) value:
2117 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2119 Also see the C<timegm> function provided by the C<Time::Local> module,
2120 and the strftime(3) function available via the POSIX module.
2122 This scalar value is B<not> locale dependent (see L<perllocale>), but
2123 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2124 strftime(3) and mktime(3) functions available via the POSIX module. To
2125 get somewhat similar but locale dependent date strings, set up your
2126 locale environment variables appropriately (please see L<perllocale>)
2127 and try for example:
2129 use POSIX qw(strftime);
2130 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2132 Note that the C<%a> and C<%b> escapes, which represent the short forms
2133 of the day of the week and the month of the year, may not necessarily
2134 be three characters wide in all locales.
2142 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2143 execution there. It may not be used to go into any construct that
2144 requires initialization, such as a subroutine or a C<foreach> loop. It
2145 also can't be used to go into a construct that is optimized away,
2146 or to get out of a block or subroutine given to C<sort>.
2147 It can be used to go almost anywhere else within the dynamic scope,
2148 including out of subroutines, but it's usually better to use some other
2149 construct such as C<last> or C<die>. The author of Perl has never felt the
2150 need to use this form of C<goto> (in Perl, that is--C is another matter).
2151 (The difference being that C does not offer named loops combined with
2152 loop control. Perl does, and this replaces most structured uses of C<goto>
2153 in other languages.)
2155 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2156 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2157 necessarily recommended if you're optimizing for maintainability:
2159 goto ("FOO", "BAR", "GLARCH")[$i];
2161 The C<goto-&NAME> form is quite different from the other forms of
2162 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2163 doesn't have the stigma associated with other gotos. Instead, it
2164 exits the current subroutine (losing any changes set by local()) and
2165 immediately calls in its place the named subroutine using the current
2166 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2167 load another subroutine and then pretend that the other subroutine had
2168 been called in the first place (except that any modifications to C<@_>
2169 in the current subroutine are propagated to the other subroutine.)
2170 After the C<goto>, not even C<caller> will be able to tell that this
2171 routine was called first.
2173 NAME needn't be the name of a subroutine; it can be a scalar variable
2174 containing a code reference, or a block which evaluates to a code
2177 =item grep BLOCK LIST
2179 =item grep EXPR,LIST
2181 This is similar in spirit to, but not the same as, grep(1) and its
2182 relatives. In particular, it is not limited to using regular expressions.
2184 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2185 C<$_> to each element) and returns the list value consisting of those
2186 elements for which the expression evaluated to true. In scalar
2187 context, returns the number of times the expression was true.
2189 @foo = grep(!/^#/, @bar); # weed out comments
2193 @foo = grep {!/^#/} @bar; # weed out comments
2195 Note that C<$_> is an alias to the list value, so it can be used to
2196 modify the elements of the LIST. While this is useful and supported,
2197 it can cause bizarre results if the elements of LIST are not variables.
2198 Similarly, grep returns aliases into the original list, much as a for
2199 loop's index variable aliases the list elements. That is, modifying an
2200 element of a list returned by grep (for example, in a C<foreach>, C<map>
2201 or another C<grep>) actually modifies the element in the original list.
2202 This is usually something to be avoided when writing clear code.
2204 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2210 Interprets EXPR as a hex string and returns the corresponding value.
2211 (To convert strings that might start with either 0, 0x, or 0b, see
2212 L</oct>.) If EXPR is omitted, uses C<$_>.
2214 print hex '0xAf'; # prints '175'
2215 print hex 'aF'; # same
2217 Hex strings may only represent integers. Strings that would cause
2218 integer overflow trigger a warning. Leading whitespace is not stripped,
2223 There is no builtin C<import> function. It is just an ordinary
2224 method (subroutine) defined (or inherited) by modules that wish to export
2225 names to another module. The C<use> function calls the C<import> method
2226 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2228 =item index STR,SUBSTR,POSITION
2230 =item index STR,SUBSTR
2232 The index function searches for one string within another, but without
2233 the wildcard-like behavior of a full regular-expression pattern match.
2234 It returns the position of the first occurrence of SUBSTR in STR at
2235 or after POSITION. If POSITION is omitted, starts searching from the
2236 beginning of the string. The return value is based at C<0> (or whatever
2237 you've set the C<$[> variable to--but don't do that). If the substring
2238 is not found, returns one less than the base, ordinarily C<-1>.
2244 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2245 You should not use this function for rounding: one because it truncates
2246 towards C<0>, and two because machine representations of floating point
2247 numbers can sometimes produce counterintuitive results. For example,
2248 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2249 because it's really more like -268.99999999999994315658 instead. Usually,
2250 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2251 functions will serve you better than will int().
2253 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2255 Implements the ioctl(2) function. You'll probably first have to say
2257 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2259 to get the correct function definitions. If F<ioctl.ph> doesn't
2260 exist or doesn't have the correct definitions you'll have to roll your
2261 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2262 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2263 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2264 written depending on the FUNCTION--a pointer to the string value of SCALAR
2265 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2266 has no string value but does have a numeric value, that value will be
2267 passed rather than a pointer to the string value. To guarantee this to be
2268 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2269 functions may be needed to manipulate the values of structures used by
2272 The return value of C<ioctl> (and C<fcntl>) is as follows:
2274 if OS returns: then Perl returns:
2276 0 string "0 but true"
2277 anything else that number
2279 Thus Perl returns true on success and false on failure, yet you can
2280 still easily determine the actual value returned by the operating
2283 $retval = ioctl(...) || -1;
2284 printf "System returned %d\n", $retval;
2286 The special string "C<0> but true" is exempt from B<-w> complaints
2287 about improper numeric conversions.
2289 Here's an example of setting a filehandle named C<REMOTE> to be
2290 non-blocking at the system level. You'll have to negotiate C<$|>
2291 on your own, though.
2293 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2295 $flags = fcntl(REMOTE, F_GETFL, 0)
2296 or die "Can't get flags for the socket: $!\n";
2298 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2299 or die "Can't set flags for the socket: $!\n";
2301 =item join EXPR,LIST
2303 Joins the separate strings of LIST into a single string with fields
2304 separated by the value of EXPR, and returns that new string. Example:
2306 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2308 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2309 first argument. Compare L</split>.
2313 Returns a list consisting of all the keys of the named hash. (In
2314 scalar context, returns the number of keys.) The keys are returned in
2315 an apparently random order. The actual random order is subject to
2316 change in future versions of perl, but it is guaranteed to be the same
2317 order as either the C<values> or C<each> function produces (given
2318 that the hash has not been modified). As a side effect, it resets
2321 Here is yet another way to print your environment:
2324 @values = values %ENV;
2326 print pop(@keys), '=', pop(@values), "\n";
2329 or how about sorted by key:
2331 foreach $key (sort(keys %ENV)) {
2332 print $key, '=', $ENV{$key}, "\n";
2335 The returned values are copies of the original keys in the hash, so
2336 modifying them will not affect the original hash. Compare L</values>.
2338 To sort a hash by value, you'll need to use a C<sort> function.
2339 Here's a descending numeric sort of a hash by its values:
2341 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2342 printf "%4d %s\n", $hash{$key}, $key;
2345 As an lvalue C<keys> allows you to increase the number of hash buckets
2346 allocated for the given hash. This can gain you a measure of efficiency if
2347 you know the hash is going to get big. (This is similar to pre-extending
2348 an array by assigning a larger number to $#array.) If you say
2352 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2353 in fact, since it rounds up to the next power of two. These
2354 buckets will be retained even if you do C<%hash = ()>, use C<undef
2355 %hash> if you want to free the storage while C<%hash> is still in scope.
2356 You can't shrink the number of buckets allocated for the hash using
2357 C<keys> in this way (but you needn't worry about doing this by accident,
2358 as trying has no effect).
2360 See also C<each>, C<values> and C<sort>.
2362 =item kill SIGNAL, LIST
2364 Sends a signal to a list of processes. Returns the number of
2365 processes successfully signaled (which is not necessarily the
2366 same as the number actually killed).
2368 $cnt = kill 1, $child1, $child2;
2371 If SIGNAL is zero, no signal is sent to the process. This is a
2372 useful way to check that the process is alive and hasn't changed
2373 its UID. See L<perlport> for notes on the portability of this
2376 Unlike in the shell, if SIGNAL is negative, it kills
2377 process groups instead of processes. (On System V, a negative I<PROCESS>
2378 number will also kill process groups, but that's not portable.) That
2379 means you usually want to use positive not negative signals. You may also
2380 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2386 The C<last> command is like the C<break> statement in C (as used in
2387 loops); it immediately exits the loop in question. If the LABEL is
2388 omitted, the command refers to the innermost enclosing loop. The
2389 C<continue> block, if any, is not executed:
2391 LINE: while (<STDIN>) {
2392 last LINE if /^$/; # exit when done with header
2396 C<last> cannot be used to exit a block which returns a value such as
2397 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2398 a grep() or map() operation.
2400 Note that a block by itself is semantically identical to a loop
2401 that executes once. Thus C<last> can be used to effect an early
2402 exit out of such a block.
2404 See also L</continue> for an illustration of how C<last>, C<next>, and
2411 Returns a lowercased version of EXPR. This is the internal function
2412 implementing the C<\L> escape in double-quoted strings. Respects
2413 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2414 and L<perlunicode> for more details about locale and Unicode support.
2416 If EXPR is omitted, uses C<$_>.
2422 Returns the value of EXPR with the first character lowercased. This
2423 is the internal function implementing the C<\l> escape in
2424 double-quoted strings. Respects current LC_CTYPE locale if C<use
2425 locale> in force. See L<perllocale> and L<perlunicode> for more
2426 details about locale and Unicode support.
2428 If EXPR is omitted, uses C<$_>.
2434 Returns the length in characters of the value of EXPR. If EXPR is
2435 omitted, returns length of C<$_>. Note that this cannot be used on
2436 an entire array or hash to find out how many elements these have.
2437 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2439 =item link OLDFILE,NEWFILE
2441 Creates a new filename linked to the old filename. Returns true for
2442 success, false otherwise.
2444 =item listen SOCKET,QUEUESIZE
2446 Does the same thing that the listen system call does. Returns true if
2447 it succeeded, false otherwise. See the example in
2448 L<perlipc/"Sockets: Client/Server Communication">.
2452 You really probably want to be using C<my> instead, because C<local> isn't
2453 what most people think of as "local". See
2454 L<perlsub/"Private Variables via my()"> for details.
2456 A local modifies the listed variables to be local to the enclosing
2457 block, file, or eval. If more than one value is listed, the list must
2458 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2459 for details, including issues with tied arrays and hashes.
2461 =item localtime EXPR
2463 Converts a time as returned by the time function to a 9-element list
2464 with the time analyzed for the local time zone. Typically used as
2468 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2471 All list elements are numeric, and come straight out of the C `struct
2472 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2473 specified time. $mday is the day of the month, and $mon is the month
2474 itself, in the range C<0..11> with 0 indicating January and 11
2475 indicating December. $year is the number of years since 1900. That
2476 is, $year is C<123> in year 2023. $wday is the day of the week, with
2477 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2478 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2479 is true if the specified time occurs during daylight savings time,
2482 Note that the $year element is I<not> simply the last two digits of
2483 the year. If you assume it is, then you create non-Y2K-compliant
2484 programs--and you wouldn't want to do that, would you?
2486 The proper way to get a complete 4-digit year is simply:
2490 And to get the last two digits of the year (e.g., '01' in 2001) do:
2492 $year = sprintf("%02d", $year % 100);
2494 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2496 In scalar context, C<localtime()> returns the ctime(3) value:
2498 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2500 This scalar value is B<not> locale dependent, see L<perllocale>, but
2501 instead a Perl builtin. Also see the C<Time::Local> module
2502 (to convert the second, minutes, hours, ... back to seconds since the
2503 stroke of midnight the 1st of January 1970, the value returned by
2504 time()), and the strftime(3) and mktime(3) functions available via the
2505 POSIX module. To get somewhat similar but locale dependent date
2506 strings, set up your locale environment variables appropriately
2507 (please see L<perllocale>) and try for example:
2509 use POSIX qw(strftime);
2510 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2512 Note that the C<%a> and C<%b>, the short forms of the day of the week
2513 and the month of the year, may not necessarily be three characters wide.
2517 This function places an advisory lock on a shared variable, or referenced
2518 object contained in I<THING> until the lock goes out of scope.
2520 lock() is a "weak keyword" : this means that if you've defined a function
2521 by this name (before any calls to it), that function will be called
2522 instead. (However, if you've said C<use threads>, lock() is always a
2523 keyword.) See L<threads>.
2529 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2530 returns log of C<$_>. To get the log of another base, use basic algebra:
2531 The base-N log of a number is equal to the natural log of that number
2532 divided by the natural log of N. For example:
2536 return log($n)/log(10);
2539 See also L</exp> for the inverse operation.
2545 Does the same thing as the C<stat> function (including setting the
2546 special C<_> filehandle) but stats a symbolic link instead of the file
2547 the symbolic link points to. If symbolic links are unimplemented on
2548 your system, a normal C<stat> is done. For much more detailed
2549 information, please see the documentation for L</stat>.
2551 If EXPR is omitted, stats C<$_>.
2555 The match operator. See L<perlop>.
2557 =item map BLOCK LIST
2561 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2562 C<$_> to each element) and returns the list value composed of the
2563 results of each such evaluation. In scalar context, returns the
2564 total number of elements so generated. Evaluates BLOCK or EXPR in
2565 list context, so each element of LIST may produce zero, one, or
2566 more elements in the returned value.
2568 @chars = map(chr, @nums);
2570 translates a list of numbers to the corresponding characters. And
2572 %hash = map { getkey($_) => $_ } @array;
2574 is just a funny way to write
2577 foreach $_ (@array) {
2578 $hash{getkey($_)} = $_;
2581 Note that C<$_> is an alias to the list value, so it can be used to
2582 modify the elements of the LIST. While this is useful and supported,
2583 it can cause bizarre results if the elements of LIST are not variables.
2584 Using a regular C<foreach> loop for this purpose would be clearer in
2585 most cases. See also L</grep> for an array composed of those items of
2586 the original list for which the BLOCK or EXPR evaluates to true.
2588 C<{> starts both hash references and blocks, so C<map { ...> could be either
2589 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2590 ahead for the closing C<}> it has to take a guess at which its dealing with
2591 based what it finds just after the C<{>. Usually it gets it right, but if it
2592 doesn't it won't realize something is wrong until it gets to the C<}> and
2593 encounters the missing (or unexpected) comma. The syntax error will be
2594 reported close to the C<}> but you'll need to change something near the C<{>
2595 such as using a unary C<+> to give perl some help:
2597 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2598 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2599 %hash = map { ("\L$_", 1) } @array # this also works
2600 %hash = map { lc($_), 1 } @array # as does this.
2601 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2603 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2605 or to force an anon hash constructor use C<+{>
2607 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2609 and you get list of anonymous hashes each with only 1 entry.
2611 =item mkdir FILENAME,MASK
2613 =item mkdir FILENAME
2615 Creates the directory specified by FILENAME, with permissions
2616 specified by MASK (as modified by C<umask>). If it succeeds it
2617 returns true, otherwise it returns false and sets C<$!> (errno).
2618 If omitted, MASK defaults to 0777.
2620 In general, it is better to create directories with permissive MASK,
2621 and let the user modify that with their C<umask>, than it is to supply
2622 a restrictive MASK and give the user no way to be more permissive.
2623 The exceptions to this rule are when the file or directory should be
2624 kept private (mail files, for instance). The perlfunc(1) entry on
2625 C<umask> discusses the choice of MASK in more detail.
2627 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2628 number of trailing slashes. Some operating and filesystems do not get
2629 this right, so Perl automatically removes all trailing slashes to keep
2632 =item msgctl ID,CMD,ARG
2634 Calls the System V IPC function msgctl(2). You'll probably have to say
2638 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2639 then ARG must be a variable which will hold the returned C<msqid_ds>
2640 structure. Returns like C<ioctl>: the undefined value for error,
2641 C<"0 but true"> for zero, or the actual return value otherwise. See also
2642 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2644 =item msgget KEY,FLAGS
2646 Calls the System V IPC function msgget(2). Returns the message queue
2647 id, or the undefined value if there is an error. See also
2648 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2650 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2652 Calls the System V IPC function msgrcv to receive a message from
2653 message queue ID into variable VAR with a maximum message size of
2654 SIZE. Note that when a message is received, the message type as a
2655 native long integer will be the first thing in VAR, followed by the
2656 actual message. This packing may be opened with C<unpack("l! a*")>.
2657 Taints the variable. Returns true if successful, or false if there is
2658 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2659 C<IPC::SysV::Msg> documentation.
2661 =item msgsnd ID,MSG,FLAGS
2663 Calls the System V IPC function msgsnd to send the message MSG to the
2664 message queue ID. MSG must begin with the native long integer message
2665 type, and be followed by the length of the actual message, and finally
2666 the message itself. This kind of packing can be achieved with
2667 C<pack("l! a*", $type, $message)>. Returns true if successful,
2668 or false if there is an error. See also C<IPC::SysV>
2669 and C<IPC::SysV::Msg> documentation.
2675 =item my EXPR : ATTRS
2677 =item my TYPE EXPR : ATTRS
2679 A C<my> declares the listed variables to be local (lexically) to the
2680 enclosing block, file, or C<eval>. If more than one value is listed,
2681 the list must be placed in parentheses.
2683 The exact semantics and interface of TYPE and ATTRS are still
2684 evolving. TYPE is currently bound to the use of C<fields> pragma,
2685 and attributes are handled using the C<attributes> pragma, or starting
2686 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2687 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2688 L<attributes>, and L<Attribute::Handlers>.
2694 The C<next> command is like the C<continue> statement in C; it starts
2695 the next iteration of the loop:
2697 LINE: while (<STDIN>) {
2698 next LINE if /^#/; # discard comments
2702 Note that if there were a C<continue> block on the above, it would get
2703 executed even on discarded lines. If the LABEL is omitted, the command
2704 refers to the innermost enclosing loop.
2706 C<next> cannot be used to exit a block which returns a value such as
2707 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2708 a grep() or map() operation.
2710 Note that a block by itself is semantically identical to a loop
2711 that executes once. Thus C<next> will exit such a block early.
2713 See also L</continue> for an illustration of how C<last>, C<next>, and
2716 =item no Module VERSION LIST
2718 =item no Module VERSION
2720 =item no Module LIST
2724 See the C<use> function, which C<no> is the opposite of.
2730 Interprets EXPR as an octal string and returns the corresponding
2731 value. (If EXPR happens to start off with C<0x>, interprets it as a
2732 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2733 binary string. Leading whitespace is ignored in all three cases.)
2734 The following will handle decimal, binary, octal, and hex in the standard
2737 $val = oct($val) if $val =~ /^0/;
2739 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2740 in octal), use sprintf() or printf():
2742 $perms = (stat("filename"))[2] & 07777;
2743 $oct_perms = sprintf "%lo", $perms;
2745 The oct() function is commonly used when a string such as C<644> needs
2746 to be converted into a file mode, for example. (Although perl will
2747 automatically convert strings into numbers as needed, this automatic
2748 conversion assumes base 10.)
2750 =item open FILEHANDLE,EXPR
2752 =item open FILEHANDLE,MODE,EXPR
2754 =item open FILEHANDLE,MODE,EXPR,LIST
2756 =item open FILEHANDLE,MODE,REFERENCE
2758 =item open FILEHANDLE
2760 Opens the file whose filename is given by EXPR, and associates it with
2763 (The following is a comprehensive reference to open(): for a gentler
2764 introduction you may consider L<perlopentut>.)
2766 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2767 the variable is assigned a reference to a new anonymous filehandle,
2768 otherwise if FILEHANDLE is an expression, its value is used as the name of
2769 the real filehandle wanted. (This is considered a symbolic reference, so
2770 C<use strict 'refs'> should I<not> be in effect.)
2772 If EXPR is omitted, the scalar variable of the same name as the
2773 FILEHANDLE contains the filename. (Note that lexical variables--those
2774 declared with C<my>--will not work for this purpose; so if you're
2775 using C<my>, specify EXPR in your call to open.)
2777 If three or more arguments are specified then the mode of opening and
2778 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2779 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2780 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2781 the file is opened for appending, again being created if necessary.
2783 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2784 indicate that you want both read and write access to the file; thus
2785 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2786 '+>' >> mode would clobber the file first. You can't usually use
2787 either read-write mode for updating textfiles, since they have
2788 variable length records. See the B<-i> switch in L<perlrun> for a
2789 better approach. The file is created with permissions of C<0666>
2790 modified by the process' C<umask> value.
2792 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2793 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2795 In the 2-arguments (and 1-argument) form of the call the mode and
2796 filename should be concatenated (in this order), possibly separated by
2797 spaces. It is possible to omit the mode in these forms if the mode is
2800 If the filename begins with C<'|'>, the filename is interpreted as a
2801 command to which output is to be piped, and if the filename ends with a
2802 C<'|'>, the filename is interpreted as a command which pipes output to
2803 us. See L<perlipc/"Using open() for IPC">
2804 for more examples of this. (You are not allowed to C<open> to a command
2805 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2806 and L<perlipc/"Bidirectional Communication with Another Process">
2809 For three or more arguments if MODE is C<'|-'>, the filename is
2810 interpreted as a command to which output is to be piped, and if MODE
2811 is C<'-|'>, the filename is interpreted as a command which pipes
2812 output to us. In the 2-arguments (and 1-argument) form one should
2813 replace dash (C<'-'>) with the command.
2814 See L<perlipc/"Using open() for IPC"> for more examples of this.
2815 (You are not allowed to C<open> to a command that pipes both in I<and>
2816 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2817 L<perlipc/"Bidirectional Communication"> for alternatives.)
2819 In the three-or-more argument form of pipe opens, if LIST is specified
2820 (extra arguments after the command name) then LIST becomes arguments
2821 to the command invoked if the platform supports it. The meaning of
2822 C<open> with more than three arguments for non-pipe modes is not yet
2823 specified. Experimental "layers" may give extra LIST arguments
2826 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2827 and opening C<< '>-' >> opens STDOUT.
2829 You may use the three-argument form of open to specify IO "layers"
2830 (sometimes also referred to as "disciplines") to be applied to the handle
2831 that affect how the input and output are processed (see L<open> and
2832 L<PerlIO> for more details). For example
2834 open(FH, "<:utf8", "file")
2836 will open the UTF-8 encoded file containing Unicode characters,
2837 see L<perluniintro>. (Note that if layers are specified in the
2838 three-arg form then default layers set by the C<open> pragma are
2841 Open returns nonzero upon success, the undefined value otherwise. If
2842 the C<open> involved a pipe, the return value happens to be the pid of
2845 If you're running Perl on a system that distinguishes between text
2846 files and binary files, then you should check out L</binmode> for tips
2847 for dealing with this. The key distinction between systems that need
2848 C<binmode> and those that don't is their text file formats. Systems
2849 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2850 character, and which encode that character in C as C<"\n">, do not
2851 need C<binmode>. The rest need it.
2853 When opening a file, it's usually a bad idea to continue normal execution
2854 if the request failed, so C<open> is frequently used in connection with
2855 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2856 where you want to make a nicely formatted error message (but there are
2857 modules that can help with that problem)) you should always check
2858 the return value from opening a file. The infrequent exception is when
2859 working with an unopened filehandle is actually what you want to do.
2861 As a special case the 3 arg form with a read/write mode and the third
2862 argument being C<undef>:
2864 open(TMP, "+>", undef) or die ...
2866 opens a filehandle to an anonymous temporary file. Also using "+<"
2867 works for symmetry, but you really should consider writing something
2868 to the temporary file first. You will need to seek() to do the
2871 File handles can be opened to "in memory" files held in Perl scalars via:
2873 open($fh, '>', \$variable) || ..
2875 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2876 file, you have to close it first:
2879 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2884 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2885 while (<ARTICLE>) {...
2887 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2888 # if the open fails, output is discarded
2890 open(DBASE, '+<', 'dbase.mine') # open for update
2891 or die "Can't open 'dbase.mine' for update: $!";
2893 open(DBASE, '+<dbase.mine') # ditto
2894 or die "Can't open 'dbase.mine' for update: $!";
2896 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2897 or die "Can't start caesar: $!";
2899 open(ARTICLE, "caesar <$article |") # ditto
2900 or die "Can't start caesar: $!";
2902 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2903 or die "Can't start sort: $!";
2906 open(MEMORY,'>', \$var)
2907 or die "Can't open memory file: $!";
2908 print MEMORY "foo!\n"; # output will end up in $var
2910 # process argument list of files along with any includes
2912 foreach $file (@ARGV) {
2913 process($file, 'fh00');
2917 my($filename, $input) = @_;
2918 $input++; # this is a string increment
2919 unless (open($input, $filename)) {
2920 print STDERR "Can't open $filename: $!\n";
2925 while (<$input>) { # note use of indirection
2926 if (/^#include "(.*)"/) {
2927 process($1, $input);
2934 You may also, in the Bourne shell tradition, specify an EXPR beginning
2935 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2936 name of a filehandle (or file descriptor, if numeric) to be
2937 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2938 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2939 mode you specify should match the mode of the original filehandle.
2940 (Duping a filehandle does not take into account any existing contents of
2941 IO buffers.) If you use the 3 arg form then you can pass either a number,
2942 the name of a filehandle or the normal "reference to a glob".
2944 Here is a script that saves, redirects, and restores C<STDOUT> and
2945 C<STDERR> using various methods:
2948 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2949 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2951 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2952 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2954 select STDERR; $| = 1; # make unbuffered
2955 select STDOUT; $| = 1; # make unbuffered
2957 print STDOUT "stdout 1\n"; # this works for
2958 print STDERR "stderr 1\n"; # subprocesses too
2963 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2964 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2966 print STDOUT "stdout 2\n";
2967 print STDERR "stderr 2\n";
2969 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2970 do an equivalent of C's C<fdopen> of that file descriptor; this is
2971 more parsimonious of file descriptors. For example:
2973 open(FILEHANDLE, "<&=$fd")
2977 open(FILEHANDLE, "<&=", $fd)
2979 Note that if Perl is using the standard C libraries' fdopen() then on
2980 many UNIX systems, fdopen() is known to fail when file descriptors
2981 exceed a certain value, typically 255. If you need more file
2982 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2984 You can see whether Perl has been compiled with PerlIO or not by
2985 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2986 is C<define>, you have PerlIO, otherwise you don't.
2988 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2989 with 2-arguments (or 1-argument) form of open(), then
2990 there is an implicit fork done, and the return value of open is the pid
2991 of the child within the parent process, and C<0> within the child
2992 process. (Use C<defined($pid)> to determine whether the open was successful.)
2993 The filehandle behaves normally for the parent, but i/o to that
2994 filehandle is piped from/to the STDOUT/STDIN of the child process.
2995 In the child process the filehandle isn't opened--i/o happens from/to
2996 the new STDOUT or STDIN. Typically this is used like the normal
2997 piped open when you want to exercise more control over just how the
2998 pipe command gets executed, such as when you are running setuid, and
2999 don't want to have to scan shell commands for metacharacters.
3000 The following triples are more or less equivalent:
3002 open(FOO, "|tr '[a-z]' '[A-Z]'");
3003 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3004 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3005 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3007 open(FOO, "cat -n '$file'|");
3008 open(FOO, '-|', "cat -n '$file'");
3009 open(FOO, '-|') || exec 'cat', '-n', $file;
3010 open(FOO, '-|', "cat", '-n', $file);
3012 The last example in each block shows the pipe as "list form", which is
3013 not yet supported on all platforms. A good rule of thumb is that if
3014 your platform has true C<fork()> (in other words, if your platform is
3015 UNIX) you can use the list form.
3017 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3019 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3020 output before any operation that may do a fork, but this may not be
3021 supported on some platforms (see L<perlport>). To be safe, you may need
3022 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3023 of C<IO::Handle> on any open handles.
3025 On systems that support a close-on-exec flag on files, the flag will
3026 be set for the newly opened file descriptor as determined by the value
3027 of $^F. See L<perlvar/$^F>.
3029 Closing any piped filehandle causes the parent process to wait for the
3030 child to finish, and returns the status value in C<$?>.
3032 The filename passed to 2-argument (or 1-argument) form of open() will
3033 have leading and trailing whitespace deleted, and the normal
3034 redirection characters honored. This property, known as "magic open",
3035 can often be used to good effect. A user could specify a filename of
3036 F<"rsh cat file |">, or you could change certain filenames as needed:
3038 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3039 open(FH, $filename) or die "Can't open $filename: $!";
3041 Use 3-argument form to open a file with arbitrary weird characters in it,
3043 open(FOO, '<', $file);
3045 otherwise it's necessary to protect any leading and trailing whitespace:
3047 $file =~ s#^(\s)#./$1#;
3048 open(FOO, "< $file\0");
3050 (this may not work on some bizarre filesystems). One should
3051 conscientiously choose between the I<magic> and 3-arguments form
3056 will allow the user to specify an argument of the form C<"rsh cat file |">,
3057 but will not work on a filename which happens to have a trailing space, while
3059 open IN, '<', $ARGV[0];
3061 will have exactly the opposite restrictions.
3063 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3064 should use the C<sysopen> function, which involves no such magic (but
3065 may use subtly different filemodes than Perl open(), which is mapped
3066 to C fopen()). This is
3067 another way to protect your filenames from interpretation. For example:
3070 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3071 or die "sysopen $path: $!";
3072 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3073 print HANDLE "stuff $$\n";
3075 print "File contains: ", <HANDLE>;
3077 Using the constructor from the C<IO::Handle> package (or one of its
3078 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3079 filehandles that have the scope of whatever variables hold references to
3080 them, and automatically close whenever and however you leave that scope:
3084 sub read_myfile_munged {
3086 my $handle = new IO::File;
3087 open($handle, "myfile") or die "myfile: $!";
3089 or return (); # Automatically closed here.
3090 mung $first or die "mung failed"; # Or here.
3091 return $first, <$handle> if $ALL; # Or here.
3095 See L</seek> for some details about mixing reading and writing.
3097 =item opendir DIRHANDLE,EXPR
3099 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3100 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3101 DIRHANDLE may be an expression whose value can be used as an indirect
3102 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3103 scalar variable (or array or hash element), the variable is assigned a
3104 reference to a new anonymous dirhandle.
3105 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3111 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3112 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3115 For the reverse, see L</chr>.
3116 See L<perlunicode> and L<encoding> for more about Unicode.
3122 =item our EXPR : ATTRS
3124 =item our TYPE EXPR : ATTRS
3126 An C<our> declares the listed variables to be valid globals within
3127 the enclosing block, file, or C<eval>. That is, it has the same
3128 scoping rules as a "my" declaration, but does not create a local
3129 variable. If more than one value is listed, the list must be placed
3130 in parentheses. The C<our> declaration has no semantic effect unless
3131 "use strict vars" is in effect, in which case it lets you use the
3132 declared global variable without qualifying it with a package name.
3133 (But only within the lexical scope of the C<our> declaration. In this
3134 it differs from "use vars", which is package scoped.)
3136 An C<our> declaration declares a global variable that will be visible
3137 across its entire lexical scope, even across package boundaries. The
3138 package in which the variable is entered is determined at the point
3139 of the declaration, not at the point of use. This means the following
3143 our $bar; # declares $Foo::bar for rest of lexical scope
3147 print $bar; # prints 20
3149 Multiple C<our> declarations in the same lexical scope are allowed
3150 if they are in different packages. If they happened to be in the same
3151 package, Perl will emit warnings if you have asked for them.
3155 our $bar; # declares $Foo::bar for rest of lexical scope
3159 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3160 print $bar; # prints 30
3162 our $bar; # emits warning
3164 An C<our> declaration may also have a list of attributes associated
3167 The exact semantics and interface of TYPE and ATTRS are still
3168 evolving. TYPE is currently bound to the use of C<fields> pragma,
3169 and attributes are handled using the C<attributes> pragma, or starting
3170 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3171 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3172 L<attributes>, and L<Attribute::Handlers>.
3174 The only currently recognized C<our()> attribute is C<unique> which
3175 indicates that a single copy of the global is to be used by all
3176 interpreters should the program happen to be running in a
3177 multi-interpreter environment. (The default behaviour would be for
3178 each interpreter to have its own copy of the global.) Examples:
3180 our @EXPORT : unique = qw(foo);
3181 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3182 our $VERSION : unique = "1.00";
3184 Note that this attribute also has the effect of making the global
3185 readonly when the first new interpreter is cloned (for example,
3186 when the first new thread is created).
3188 Multi-interpreter environments can come to being either through the
3189 fork() emulation on Windows platforms, or by embedding perl in a
3190 multi-threaded application. The C<unique> attribute does nothing in
3191 all other environments.
3193 =item pack TEMPLATE,LIST
3195 Takes a LIST of values and converts it into a string using the rules
3196 given by the TEMPLATE. The resulting string is the concatenation of
3197 the converted values. Typically, each converted value looks
3198 like its machine-level representation. For example, on 32-bit machines
3199 a converted integer may be represented by a sequence of 4 bytes.
3201 The TEMPLATE is a sequence of characters that give the order and type
3202 of values, as follows:
3204 a A string with arbitrary binary data, will be null padded.
3205 A A text (ASCII) string, will be space padded.
3206 Z A null terminated (ASCIZ) string, will be null padded.
3208 b A bit string (ascending bit order inside each byte, like vec()).
3209 B A bit string (descending bit order inside each byte).
3210 h A hex string (low nybble first).
3211 H A hex string (high nybble first).
3213 c A signed char value.
3214 C An unsigned char value. Only does bytes. See U for Unicode.
3216 s A signed short value.
3217 S An unsigned short value.
3218 (This 'short' is _exactly_ 16 bits, which may differ from
3219 what a local C compiler calls 'short'. If you want
3220 native-length shorts, use the '!' suffix.)
3222 i A signed integer value.
3223 I An unsigned integer value.
3224 (This 'integer' is _at_least_ 32 bits wide. Its exact
3225 size depends on what a local C compiler calls 'int',
3226 and may even be larger than the 'long' described in
3229 l A signed long value.
3230 L An unsigned long value.
3231 (This 'long' is _exactly_ 32 bits, which may differ from
3232 what a local C compiler calls 'long'. If you want
3233 native-length longs, use the '!' suffix.)
3235 n An unsigned short in "network" (big-endian) order.
3236 N An unsigned long in "network" (big-endian) order.
3237 v An unsigned short in "VAX" (little-endian) order.
3238 V An unsigned long in "VAX" (little-endian) order.
3239 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3240 _exactly_ 32 bits, respectively.)
3242 q A signed quad (64-bit) value.
3243 Q An unsigned quad value.
3244 (Quads are available only if your system supports 64-bit
3245 integer values _and_ if Perl has been compiled to support those.
3246 Causes a fatal error otherwise.)
3248 j A signed integer value (a Perl internal integer, IV).
3249 J An unsigned integer value (a Perl internal unsigned integer, UV).
3251 f A single-precision float in the native format.
3252 d A double-precision float in the native format.
3254 F A floating point value in the native native format
3255 (a Perl internal floating point value, NV).
3256 D A long double-precision float in the native format.
3257 (Long doubles are available only if your system supports long
3258 double values _and_ if Perl has been compiled to support those.
3259 Causes a fatal error otherwise.)
3261 p A pointer to a null-terminated string.
3262 P A pointer to a structure (fixed-length string).
3264 u A uuencoded string.
3265 U A Unicode character number. Encodes to UTF-8 internally
3266 (or UTF-EBCDIC in EBCDIC platforms).
3268 w A BER compressed integer. Its bytes represent an unsigned
3269 integer in base 128, most significant digit first, with as
3270 few digits as possible. Bit eight (the high bit) is set
3271 on each byte except the last.
3275 @ Null fill to absolute position, counted from the start of
3276 the innermost ()-group.
3277 ( Start of a ()-group.
3279 The following rules apply:
3285 Each letter may optionally be followed by a number giving a repeat
3286 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3287 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3288 many values from the LIST. A C<*> for the repeat count means to use
3289 however many items are left, except for C<@>, C<x>, C<X>, where it is
3290 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3291 is the same). A numeric repeat count may optionally be enclosed in
3292 brackets, as in C<pack 'C[80]', @arr>.
3294 One can replace the numeric repeat count by a template enclosed in brackets;
3295 then the packed length of this template in bytes is used as a count.
3296 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3297 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3298 If the template in brackets contains alignment commands (such as C<x![d]>),
3299 its packed length is calculated as if the start of the template has the maximal
3302 When used with C<Z>, C<*> results in the addition of a trailing null
3303 byte (so the packed result will be one longer than the byte C<length>
3306 The repeat count for C<u> is interpreted as the maximal number of bytes
3307 to encode per line of output, with 0 and 1 replaced by 45.
3311 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3312 string of length count, padding with nulls or spaces as necessary. When
3313 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3314 after the first null, and C<a> returns data verbatim. When packing,
3315 C<a>, and C<Z> are equivalent.
3317 If the value-to-pack is too long, it is truncated. If too long and an
3318 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3319 by a null byte. Thus C<Z> always packs a trailing null byte under
3324 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3325 Each byte of the input field of pack() generates 1 bit of the result.
3326 Each result bit is based on the least-significant bit of the corresponding
3327 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3328 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3330 Starting from the beginning of the input string of pack(), each 8-tuple
3331 of bytes is converted to 1 byte of output. With format C<b>
3332 the first byte of the 8-tuple determines the least-significant bit of a
3333 byte, and with format C<B> it determines the most-significant bit of
3336 If the length of the input string is not exactly divisible by 8, the
3337 remainder is packed as if the input string were padded by null bytes
3338 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3340 If the input string of pack() is longer than needed, extra bytes are ignored.
3341 A C<*> for the repeat count of pack() means to use all the bytes of
3342 the input field. On unpack()ing the bits are converted to a string
3343 of C<"0">s and C<"1">s.
3347 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3348 representable as hexadecimal digits, 0-9a-f) long.
3350 Each byte of the input field of pack() generates 4 bits of the result.
3351 For non-alphabetical bytes the result is based on the 4 least-significant
3352 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3353 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3354 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3355 is compatible with the usual hexadecimal digits, so that C<"a"> and
3356 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3357 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3359 Starting from the beginning of the input string of pack(), each pair
3360 of bytes is converted to 1 byte of output. With format C<h> the
3361 first byte of the pair determines the least-significant nybble of the
3362 output byte, and with format C<H> it determines the most-significant
3365 If the length of the input string is not even, it behaves as if padded
3366 by a null byte at the end. Similarly, during unpack()ing the "extra"
3367 nybbles are ignored.
3369 If the input string of pack() is longer than needed, extra bytes are ignored.
3370 A C<*> for the repeat count of pack() means to use all the bytes of
3371 the input field. On unpack()ing the bits are converted to a string
3372 of hexadecimal digits.
3376 The C<p> type packs a pointer to a null-terminated string. You are
3377 responsible for ensuring the string is not a temporary value (which can
3378 potentially get deallocated before you get around to using the packed result).
3379 The C<P> type packs a pointer to a structure of the size indicated by the
3380 length. A NULL pointer is created if the corresponding value for C<p> or
3381 C<P> is C<undef>, similarly for unpack().
3385 The C</> template character allows packing and unpacking of strings where
3386 the packed structure contains a byte count followed by the string itself.
3387 You write I<length-item>C</>I<string-item>.
3389 The I<length-item> can be any C<pack> template letter, and describes
3390 how the length value is packed. The ones likely to be of most use are
3391 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3392 SNMP) and C<N> (for Sun XDR).
3394 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3395 C<"Z*">. For C<unpack> the length of the string is obtained from the
3396 I<length-item>, but if you put in the '*' it will be ignored. For all other
3397 codes, C<unpack> applies the length value to the next item, which must not
3398 have a repeat count.
3400 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3401 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3402 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3404 The I<length-item> is not returned explicitly from C<unpack>.
3406 Adding a count to the I<length-item> letter is unlikely to do anything
3407 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3408 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3409 which Perl does not regard as legal in numeric strings.
3413 The integer types C<s>, C<S>, C<l>, and C<L> may be
3414 immediately followed by a C<!> suffix to signify native shorts or
3415 longs--as you can see from above for example a bare C<l> does mean
3416 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3417 may be larger. This is an issue mainly in 64-bit platforms. You can
3418 see whether using C<!> makes any difference by
3420 print length(pack("s")), " ", length(pack("s!")), "\n";
3421 print length(pack("l")), " ", length(pack("l!")), "\n";
3423 C<i!> and C<I!> also work but only because of completeness;
3424 they are identical to C<i> and C<I>.
3426 The actual sizes (in bytes) of native shorts, ints, longs, and long
3427 longs on the platform where Perl was built are also available via
3431 print $Config{shortsize}, "\n";
3432 print $Config{intsize}, "\n";
3433 print $Config{longsize}, "\n";
3434 print $Config{longlongsize}, "\n";
3436 (The C<$Config{longlongsize}> will be undefined if your system does
3437 not support long longs.)
3441 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3442 are inherently non-portable between processors and operating systems
3443 because they obey the native byteorder and endianness. For example a
3444 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3445 (arranged in and handled by the CPU registers) into bytes as
3447 0x12 0x34 0x56 0x78 # big-endian
3448 0x78 0x56 0x34 0x12 # little-endian
3450 Basically, the Intel and VAX CPUs are little-endian, while everybody
3451 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3452 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3453 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3456 The names `big-endian' and `little-endian' are comic references to
3457 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3458 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3459 the egg-eating habits of the Lilliputians.
3461 Some systems may have even weirder byte orders such as
3466 You can see your system's preference with
3468 print join(" ", map { sprintf "%#02x", $_ }
3469 unpack("C*",pack("L",0x12345678))), "\n";
3471 The byteorder on the platform where Perl was built is also available
3475 print $Config{byteorder}, "\n";
3477 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3478 and C<'87654321'> are big-endian.
3480 If you want portable packed integers use the formats C<n>, C<N>,
3481 C<v>, and C<V>, their byte endianness and size are known.
3482 See also L<perlport>.
3486 Real numbers (floats and doubles) are in the native machine format only;
3487 due to the multiplicity of floating formats around, and the lack of a
3488 standard "network" representation, no facility for interchange has been
3489 made. This means that packed floating point data written on one machine
3490 may not be readable on another - even if both use IEEE floating point
3491 arithmetic (as the endian-ness of the memory representation is not part
3492 of the IEEE spec). See also L<perlport>.
3494 Note that Perl uses doubles internally for all numeric calculation, and
3495 converting from double into float and thence back to double again will
3496 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3501 If the pattern begins with a C<U>, the resulting string will be treated
3502 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3503 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3504 characters. If you don't want this to happen, you can begin your pattern
3505 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3506 string, and then follow this with a C<U*> somewhere in your pattern.
3510 You must yourself do any alignment or padding by inserting for example
3511 enough C<'x'>es while packing. There is no way to pack() and unpack()
3512 could know where the bytes are going to or coming from. Therefore
3513 C<pack> (and C<unpack>) handle their output and input as flat
3518 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3519 take a repeat count, both as postfix, and for unpack() also via the C</>
3520 template character. Within each repetition of a group, positioning with
3521 C<@> starts again at 0. Therefore, the result of
3523 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3525 is the string "\0a\0\0bc".
3530 C<x> and C<X> accept C<!> modifier. In this case they act as
3531 alignment commands: they jump forward/back to the closest position
3532 aligned at a multiple of C<count> bytes. For example, to pack() or
3533 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3534 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3535 aligned on the double's size.
3537 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3538 both result in no-ops.
3542 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3543 White space may be used to separate pack codes from each other, but
3544 a C<!> modifier and a repeat count must follow immediately.
3548 If TEMPLATE requires more arguments to pack() than actually given, pack()
3549 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3550 to pack() than actually given, extra arguments are ignored.
3556 $foo = pack("CCCC",65,66,67,68);
3558 $foo = pack("C4",65,66,67,68);
3560 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3561 # same thing with Unicode circled letters
3563 $foo = pack("ccxxcc",65,66,67,68);
3566 # note: the above examples featuring "C" and "c" are true
3567 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3568 # and UTF-8. In EBCDIC the first example would be
3569 # $foo = pack("CCCC",193,194,195,196);
3571 $foo = pack("s2",1,2);
3572 # "\1\0\2\0" on little-endian
3573 # "\0\1\0\2" on big-endian
3575 $foo = pack("a4","abcd","x","y","z");
3578 $foo = pack("aaaa","abcd","x","y","z");
3581 $foo = pack("a14","abcdefg");
3582 # "abcdefg\0\0\0\0\0\0\0"
3584 $foo = pack("i9pl", gmtime);
3585 # a real struct tm (on my system anyway)
3587 $utmp_template = "Z8 Z8 Z16 L";
3588 $utmp = pack($utmp_template, @utmp1);
3589 # a struct utmp (BSDish)
3591 @utmp2 = unpack($utmp_template, $utmp);
3592 # "@utmp1" eq "@utmp2"
3595 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3598 $foo = pack('sx2l', 12, 34);
3599 # short 12, two zero bytes padding, long 34
3600 $bar = pack('s@4l', 12, 34);
3601 # short 12, zero fill to position 4, long 34
3604 The same template may generally also be used in unpack().
3606 =item package NAMESPACE
3610 Declares the compilation unit as being in the given namespace. The scope
3611 of the package declaration is from the declaration itself through the end
3612 of the enclosing block, file, or eval (the same as the C<my> operator).
3613 All further unqualified dynamic identifiers will be in this namespace.
3614 A package statement affects only dynamic variables--including those
3615 you've used C<local> on--but I<not> lexical variables, which are created
3616 with C<my>. Typically it would be the first declaration in a file to
3617 be included by the C<require> or C<use> operator. You can switch into a
3618 package in more than one place; it merely influences which symbol table
3619 is used by the compiler for the rest of that block. You can refer to
3620 variables and filehandles in other packages by prefixing the identifier
3621 with the package name and a double colon: C<$Package::Variable>.
3622 If the package name is null, the C<main> package as assumed. That is,
3623 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3624 still seen in older code).
3626 If NAMESPACE is omitted, then there is no current package, and all
3627 identifiers must be fully qualified or lexicals. However, you are
3628 strongly advised not to make use of this feature. Its use can cause
3629 unexpected behaviour, even crashing some versions of Perl. It is
3630 deprecated, and will be removed from a future release.
3632 See L<perlmod/"Packages"> for more information about packages, modules,
3633 and classes. See L<perlsub> for other scoping issues.
3635 =item pipe READHANDLE,WRITEHANDLE
3637 Opens a pair of connected pipes like the corresponding system call.
3638 Note that if you set up a loop of piped processes, deadlock can occur
3639 unless you are very careful. In addition, note that Perl's pipes use
3640 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3641 after each command, depending on the application.
3643 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3644 for examples of such things.
3646 On systems that support a close-on-exec flag on files, the flag will be set
3647 for the newly opened file descriptors as determined by the value of $^F.
3654 Pops and returns the last value of the array, shortening the array by
3655 one element. Has an effect similar to
3659 If there are no elements in the array, returns the undefined value
3660 (although this may happen at other times as well). If ARRAY is
3661 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3662 array in subroutines, just like C<shift>.
3668 Returns the offset of where the last C<m//g> search left off for the variable
3669 in question (C<$_> is used when the variable is not specified). May be
3670 modified to change that offset. Such modification will also influence
3671 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3674 =item print FILEHANDLE LIST
3680 Prints a string or a list of strings. Returns true if successful.
3681 FILEHANDLE may be a scalar variable name, in which case the variable
3682 contains the name of or a reference to the filehandle, thus introducing
3683 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3684 the next token is a term, it may be misinterpreted as an operator
3685 unless you interpose a C<+> or put parentheses around the arguments.)
3686 If FILEHANDLE is omitted, prints by default to standard output (or
3687 to the last selected output channel--see L</select>). If LIST is
3688 also omitted, prints C<$_> to the currently selected output channel.
3689 To set the default output channel to something other than STDOUT
3690 use the select operation. The current value of C<$,> (if any) is
3691 printed between each LIST item. The current value of C<$\> (if
3692 any) is printed after the entire LIST has been printed. Because
3693 print takes a LIST, anything in the LIST is evaluated in list
3694 context, and any subroutine that you call will have one or more of
3695 its expressions evaluated in list context. Also be careful not to
3696 follow the print keyword with a left parenthesis unless you want
3697 the corresponding right parenthesis to terminate the arguments to
3698 the print--interpose a C<+> or put parentheses around all the
3701 Note that if you're storing FILEHANDLES in an array or other expression,
3702 you will have to use a block returning its value instead:
3704 print { $files[$i] } "stuff\n";
3705 print { $OK ? STDOUT : STDERR } "stuff\n";
3707 =item printf FILEHANDLE FORMAT, LIST
3709 =item printf FORMAT, LIST
3711 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3712 (the output record separator) is not appended. The first argument
3713 of the list will be interpreted as the C<printf> format. See C<sprintf>
3714 for an explanation of the format argument. If C<use locale> is in effect,
3715 the character used for the decimal point in formatted real numbers is
3716 affected by the LC_NUMERIC locale. See L<perllocale>.
3718 Don't fall into the trap of using a C<printf> when a simple
3719 C<print> would do. The C<print> is more efficient and less
3722 =item prototype FUNCTION
3724 Returns the prototype of a function as a string (or C<undef> if the
3725 function has no prototype). FUNCTION is a reference to, or the name of,
3726 the function whose prototype you want to retrieve.
3728 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3729 name for Perl builtin. If the builtin is not I<overridable> (such as
3730 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3731 C<system>) returns C<undef> because the builtin does not really behave
3732 like a Perl function. Otherwise, the string describing the equivalent
3733 prototype is returned.
3735 =item push ARRAY,LIST
3737 Treats ARRAY as a stack, and pushes the values of LIST
3738 onto the end of ARRAY. The length of ARRAY increases by the length of
3739 LIST. Has the same effect as
3742 $ARRAY[++$#ARRAY] = $value;
3745 but is more efficient. Returns the new number of elements in the array.
3757 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3759 =item quotemeta EXPR
3763 Returns the value of EXPR with all non-"word"
3764 characters backslashed. (That is, all characters not matching
3765 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3766 returned string, regardless of any locale settings.)
3767 This is the internal function implementing
3768 the C<\Q> escape in double-quoted strings.
3770 If EXPR is omitted, uses C<$_>.
3776 Returns a random fractional number greater than or equal to C<0> and less
3777 than the value of EXPR. (EXPR should be positive.) If EXPR is
3778 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3779 also special-cased as C<1> - this has not been documented before perl 5.8.0
3780 and is subject to change in future versions of perl. Automatically calls
3781 C<srand> unless C<srand> has already been called. See also C<srand>.
3783 Apply C<int()> to the value returned by C<rand()> if you want random
3784 integers instead of random fractional numbers. For example,
3788 returns a random integer between C<0> and C<9>, inclusive.
3790 (Note: If your rand function consistently returns numbers that are too
3791 large or too small, then your version of Perl was probably compiled
3792 with the wrong number of RANDBITS.)
3794 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3796 =item read FILEHANDLE,SCALAR,LENGTH
3798 Attempts to read LENGTH I<characters> of data into variable SCALAR
3799 from the specified FILEHANDLE. Returns the number of characters
3800 actually read, C<0> at end of file, or undef if there was an error (in
3801 the latter case C<$!> is also set). SCALAR will be grown or shrunk to
3802 the length actually read. If SCALAR needs growing, the new bytes will
3803 be zero bytes. An OFFSET may be specified to place the read data into
3804 some other place in SCALAR than the beginning. The call is actually
3805 implemented in terms of either Perl's or system's fread() call. To
3806 get a true read(2) system call, see C<sysread>.
3808 Note the I<characters>: depending on the status of the filehandle,
3809 either (8-bit) bytes or characters are read. By default all
3810 filehandles operate on bytes, but for example if the filehandle has
3811 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3812 pragma, L<open>), the I/O will operate on characters, not bytes.
3814 =item readdir DIRHANDLE
3816 Returns the next directory entry for a directory opened by C<opendir>.
3817 If used in list context, returns all the rest of the entries in the
3818 directory. If there are no more entries, returns an undefined value in
3819 scalar context or a null list in list context.
3821 If you're planning to filetest the return values out of a C<readdir>, you'd
3822 better prepend the directory in question. Otherwise, because we didn't
3823 C<chdir> there, it would have been testing the wrong file.
3825 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3826 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3831 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3832 context, each call reads and returns the next line, until end-of-file is
3833 reached, whereupon the subsequent call returns undef. In list context,
3834 reads until end-of-file is reached and returns a list of lines. Note that
3835 the notion of "line" used here is however you may have defined it
3836 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3838 When C<$/> is set to C<undef>, when readline() is in scalar
3839 context (i.e. file slurp mode), and when an empty file is read, it
3840 returns C<''> the first time, followed by C<undef> subsequently.
3842 This is the internal function implementing the C<< <EXPR> >>
3843 operator, but you can use it directly. The C<< <EXPR> >>
3844 operator is discussed in more detail in L<perlop/"I/O Operators">.
3847 $line = readline(*STDIN); # same thing
3849 If readline encounters an operating system error, C<$!> will be set with the
3850 corresponding error message. It can be helpful to check C<$!> when you are
3851 reading from filehandles you don't trust, such as a tty or a socket. The
3852 following example uses the operator form of C<readline>, and takes the necessary
3853 steps to ensure that C<readline> was successful.
3857 unless (defined( $line = <> )) {
3868 Returns the value of a symbolic link, if symbolic links are
3869 implemented. If not, gives a fatal error. If there is some system
3870 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3871 omitted, uses C<$_>.
3875 EXPR is executed as a system command.
3876 The collected standard output of the command is returned.
3877 In scalar context, it comes back as a single (potentially
3878 multi-line) string. In list context, returns a list of lines
3879 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3880 This is the internal function implementing the C<qx/EXPR/>
3881 operator, but you can use it directly. The C<qx/EXPR/>
3882 operator is discussed in more detail in L<perlop/"I/O Operators">.
3884 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3886 Receives a message on a socket. Attempts to receive LENGTH characters
3887 of data into variable SCALAR from the specified SOCKET filehandle.
3888 SCALAR will be grown or shrunk to the length actually read. Takes the
3889 same flags as the system call of the same name. Returns the address
3890 of the sender if SOCKET's protocol supports this; returns an empty
3891 string otherwise. If there's an error, returns the undefined value.
3892 This call is actually implemented in terms of recvfrom(2) system call.
3893 See L<perlipc/"UDP: Message Passing"> for examples.
3895 Note the I<characters>: depending on the status of the socket, either
3896 (8-bit) bytes or characters are received. By default all sockets
3897 operate on bytes, but for example if the socket has been changed using
3898 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3899 pragma, L<open>), the I/O will operate on characters, not bytes.
3905 The C<redo> command restarts the loop block without evaluating the
3906 conditional again. The C<continue> block, if any, is not executed. If
3907 the LABEL is omitted, the command refers to the innermost enclosing
3908 loop. This command is normally used by programs that want to lie to
3909 themselves about what was just input:
3911 # a simpleminded Pascal comment stripper
3912 # (warning: assumes no { or } in strings)
3913 LINE: while (<STDIN>) {
3914 while (s|({.*}.*){.*}|$1 |) {}
3919 if (/}/) { # end of comment?
3928 C<redo> cannot be used to retry a block which returns a value such as
3929 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3930 a grep() or map() operation.
3932 Note that a block by itself is semantically identical to a loop
3933 that executes once. Thus C<redo> inside such a block will effectively
3934 turn it into a looping construct.
3936 See also L</continue> for an illustration of how C<last>, C<next>, and
3943 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3944 is not specified, C<$_> will be used. The value returned depends on the
3945 type of thing the reference is a reference to.
3946 Builtin types include:
3956 If the referenced object has been blessed into a package, then that package
3957 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3959 if (ref($r) eq "HASH") {
3960 print "r is a reference to a hash.\n";
3963 print "r is not a reference at all.\n";
3965 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3966 print "r is a reference to something that isa hash.\n";
3969 See also L<perlref>.
3971 =item rename OLDNAME,NEWNAME
3973 Changes the name of a file; an existing file NEWNAME will be
3974 clobbered. Returns true for success, false otherwise.
3976 Behavior of this function varies wildly depending on your system
3977 implementation. For example, it will usually not work across file system
3978 boundaries, even though the system I<mv> command sometimes compensates
3979 for this. Other restrictions include whether it works on directories,
3980 open files, or pre-existing files. Check L<perlport> and either the
3981 rename(2) manpage or equivalent system documentation for details.
3983 =item require VERSION
3989 Demands a version of Perl specified by VERSION, or demands some semantics
3990 specified by EXPR or by C<$_> if EXPR is not supplied.
3992 VERSION may be either a numeric argument such as 5.006, which will be
3993 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3994 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3995 VERSION is greater than the version of the current Perl interpreter.
3996 Compare with L</use>, which can do a similar check at compile time.
3998 Specifying VERSION as a literal of the form v5.6.1 should generally be
3999 avoided, because it leads to misleading error messages under earlier
4000 versions of Perl which do not support this syntax. The equivalent numeric
4001 version should be used instead.
4003 require v5.6.1; # run time version check
4004 require 5.6.1; # ditto
4005 require 5.006_001; # ditto; preferred for backwards compatibility
4007 Otherwise, demands that a library file be included if it hasn't already
4008 been included. The file is included via the do-FILE mechanism, which is
4009 essentially just a variety of C<eval>. Has semantics similar to the following
4014 return 1 if $INC{$filename};
4015 my($realfilename,$result);
4017 foreach $prefix (@INC) {
4018 $realfilename = "$prefix/$filename";
4019 if (-f $realfilename) {
4020 $INC{$filename} = $realfilename;
4021 $result = do $realfilename;
4025 die "Can't find $filename in \@INC";
4027 delete $INC{$filename} if $@ || !$result;
4029 die "$filename did not return true value" unless $result;
4033 Note that the file will not be included twice under the same specified
4034 name. The file must return true as the last statement to indicate
4035 successful execution of any initialization code, so it's customary to
4036 end such a file with C<1;> unless you're sure it'll return true
4037 otherwise. But it's better just to put the C<1;>, in case you add more
4040 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4041 replaces "F<::>" with "F</>" in the filename for you,
4042 to make it easy to load standard modules. This form of loading of
4043 modules does not risk altering your namespace.
4045 In other words, if you try this:
4047 require Foo::Bar; # a splendid bareword
4049 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4050 directories specified in the C<@INC> array.
4052 But if you try this:
4054 $class = 'Foo::Bar';
4055 require $class; # $class is not a bareword
4057 require "Foo::Bar"; # not a bareword because of the ""
4059 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4060 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4062 eval "require $class";
4064 Now that you understand how C<require> looks for files in the case of
4065 a bareword argument, there is a little extra functionality going on
4066 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4067 it will first look for a filename with a "F<.pmc>" extension. A file
4068 with this extension is assumed to be Perl bytecode generated by
4069 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4070 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4071 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4073 You can also insert hooks into the import facility, by putting directly
4074 Perl code into the @INC array. There are three forms of hooks: subroutine
4075 references, array references and blessed objects.
4077 Subroutine references are the simplest case. When the inclusion system
4078 walks through @INC and encounters a subroutine, this subroutine gets
4079 called with two parameters, the first being a reference to itself, and the
4080 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4081 subroutine should return C<undef> or a filehandle, from which the file to
4082 include will be read. If C<undef> is returned, C<require> will look at
4083 the remaining elements of @INC.
4085 If the hook is an array reference, its first element must be a subroutine
4086 reference. This subroutine is called as above, but the first parameter is
4087 the array reference. This enables to pass indirectly some arguments to
4090 In other words, you can write:
4092 push @INC, \&my_sub;
4094 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4100 push @INC, [ \&my_sub, $x, $y, ... ];
4102 my ($arrayref, $filename) = @_;
4103 # Retrieve $x, $y, ...
4104 my @parameters = @$arrayref[1..$#$arrayref];
4108 If the hook is an object, it must provide an INC method, that will be
4109 called as above, the first parameter being the object itself. (Note that
4110 you must fully qualify the sub's name, as it is always forced into package
4111 C<main>.) Here is a typical code layout:
4117 my ($self, $filename) = @_;
4121 # In the main program
4122 push @INC, new Foo(...);
4124 Note that these hooks are also permitted to set the %INC entry
4125 corresponding to the files they have loaded. See L<perlvar/%INC>.
4127 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4133 Generally used in a C<continue> block at the end of a loop to clear
4134 variables and reset C<??> searches so that they work again. The
4135 expression is interpreted as a list of single characters (hyphens
4136 allowed for ranges). All variables and arrays beginning with one of
4137 those letters are reset to their pristine state. If the expression is
4138 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4139 only variables or searches in the current package. Always returns
4142 reset 'X'; # reset all X variables
4143 reset 'a-z'; # reset lower case variables
4144 reset; # just reset ?one-time? searches
4146 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4147 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4148 variables--lexical variables are unaffected, but they clean themselves
4149 up on scope exit anyway, so you'll probably want to use them instead.
4156 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4157 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4158 context, depending on how the return value will be used, and the context
4159 may vary from one execution to the next (see C<wantarray>). If no EXPR
4160 is given, returns an empty list in list context, the undefined value in
4161 scalar context, and (of course) nothing at all in a void context.
4163 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4164 or do FILE will automatically return the value of the last expression
4169 In list context, returns a list value consisting of the elements
4170 of LIST in the opposite order. In scalar context, concatenates the
4171 elements of LIST and returns a string value with all characters
4172 in the opposite order.
4174 print reverse <>; # line tac, last line first
4176 undef $/; # for efficiency of <>
4177 print scalar reverse <>; # character tac, last line tsrif
4179 This operator is also handy for inverting a hash, although there are some
4180 caveats. If a value is duplicated in the original hash, only one of those
4181 can be represented as a key in the inverted hash. Also, this has to
4182 unwind one hash and build a whole new one, which may take some time
4183 on a large hash, such as from a DBM file.
4185 %by_name = reverse %by_address; # Invert the hash
4187 =item rewinddir DIRHANDLE
4189 Sets the current position to the beginning of the directory for the
4190 C<readdir> routine on DIRHANDLE.
4192 =item rindex STR,SUBSTR,POSITION
4194 =item rindex STR,SUBSTR
4196 Works just like index() except that it returns the position of the LAST
4197 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4198 last occurrence at or before that position.
4200 =item rmdir FILENAME
4204 Deletes the directory specified by FILENAME if that directory is empty. If it
4205 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4206 FILENAME is omitted, uses C<$_>.
4210 The substitution operator. See L<perlop>.
4214 Forces EXPR to be interpreted in scalar context and returns the value
4217 @counts = ( scalar @a, scalar @b, scalar @c );
4219 There is no equivalent operator to force an expression to
4220 be interpolated in list context because in practice, this is never
4221 needed. If you really wanted to do so, however, you could use
4222 the construction C<@{[ (some expression) ]}>, but usually a simple
4223 C<(some expression)> suffices.
4225 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4226 parenthesized list, this behaves as a scalar comma expression, evaluating
4227 all but the last element in void context and returning the final element
4228 evaluated in scalar context. This is seldom what you want.
4230 The following single statement:
4232 print uc(scalar(&foo,$bar)),$baz;
4234 is the moral equivalent of these two:
4237 print(uc($bar),$baz);
4239 See L<perlop> for more details on unary operators and the comma operator.
4241 =item seek FILEHANDLE,POSITION,WHENCE
4243 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4244 FILEHANDLE may be an expression whose value gives the name of the
4245 filehandle. The values for WHENCE are C<0> to set the new position
4246 I<in bytes> to POSITION, C<1> to set it to the current position plus
4247 POSITION, and C<2> to set it to EOF plus POSITION (typically
4248 negative). For WHENCE you may use the constants C<SEEK_SET>,
4249 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4250 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4253 Note the I<in bytes>: even if the filehandle has been set to
4254 operate on characters (for example by using the C<:utf8> open
4255 layer), tell() will return byte offsets, not character offsets
4256 (because implementing that would render seek() and tell() rather slow).
4258 If you want to position file for C<sysread> or C<syswrite>, don't use
4259 C<seek>--buffering makes its effect on the file's system position
4260 unpredictable and non-portable. Use C<sysseek> instead.
4262 Due to the rules and rigors of ANSI C, on some systems you have to do a
4263 seek whenever you switch between reading and writing. Amongst other
4264 things, this may have the effect of calling stdio's clearerr(3).
4265 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4269 This is also useful for applications emulating C<tail -f>. Once you hit
4270 EOF on your read, and then sleep for a while, you might have to stick in a
4271 seek() to reset things. The C<seek> doesn't change the current position,
4272 but it I<does> clear the end-of-file condition on the handle, so that the
4273 next C<< <FILE> >> makes Perl try again to read something. We hope.
4275 If that doesn't work (some IO implementations are particularly
4276 cantankerous), then you may need something more like this:
4279 for ($curpos = tell(FILE); $_ = <FILE>;
4280 $curpos = tell(FILE)) {
4281 # search for some stuff and put it into files
4283 sleep($for_a_while);
4284 seek(FILE, $curpos, 0);
4287 =item seekdir DIRHANDLE,POS
4289 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4290 must be a value returned by C<telldir>. Has the same caveats about
4291 possible directory compaction as the corresponding system library
4294 =item select FILEHANDLE
4298 Returns the currently selected filehandle. Sets the current default
4299 filehandle for output, if FILEHANDLE is supplied. This has two
4300 effects: first, a C<write> or a C<print> without a filehandle will
4301 default to this FILEHANDLE. Second, references to variables related to
4302 output will refer to this output channel. For example, if you have to
4303 set the top of form format for more than one output channel, you might
4311 FILEHANDLE may be an expression whose value gives the name of the
4312 actual filehandle. Thus:
4314 $oldfh = select(STDERR); $| = 1; select($oldfh);
4316 Some programmers may prefer to think of filehandles as objects with
4317 methods, preferring to write the last example as:
4320 STDERR->autoflush(1);
4322 =item select RBITS,WBITS,EBITS,TIMEOUT
4324 This calls the select(2) system call with the bit masks specified, which
4325 can be constructed using C<fileno> and C<vec>, along these lines:
4327 $rin = $win = $ein = '';
4328 vec($rin,fileno(STDIN),1) = 1;
4329 vec($win,fileno(STDOUT),1) = 1;
4332 If you want to select on many filehandles you might wish to write a
4336 my(@fhlist) = split(' ',$_[0]);
4339 vec($bits,fileno($_),1) = 1;
4343 $rin = fhbits('STDIN TTY SOCK');
4347 ($nfound,$timeleft) =
4348 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4350 or to block until something becomes ready just do this
4352 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4354 Most systems do not bother to return anything useful in $timeleft, so
4355 calling select() in scalar context just returns $nfound.
4357 Any of the bit masks can also be undef. The timeout, if specified, is
4358 in seconds, which may be fractional. Note: not all implementations are
4359 capable of returning the $timeleft. If not, they always return
4360 $timeleft equal to the supplied $timeout.
4362 You can effect a sleep of 250 milliseconds this way:
4364 select(undef, undef, undef, 0.25);
4366 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4367 is implementation-dependent.
4369 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4370 or <FH>) with C<select>, except as permitted by POSIX, and even
4371 then only on POSIX systems. You have to use C<sysread> instead.
4373 =item semctl ID,SEMNUM,CMD,ARG
4375 Calls the System V IPC function C<semctl>. You'll probably have to say
4379 first to get the correct constant definitions. If CMD is IPC_STAT or
4380 GETALL, then ARG must be a variable which will hold the returned
4381 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4382 the undefined value for error, "C<0 but true>" for zero, or the actual
4383 return value otherwise. The ARG must consist of a vector of native
4384 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4385 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4388 =item semget KEY,NSEMS,FLAGS
4390 Calls the System V IPC function semget. Returns the semaphore id, or
4391 the undefined value if there is an error. See also
4392 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4395 =item semop KEY,OPSTRING
4397 Calls the System V IPC function semop to perform semaphore operations
4398 such as signalling and waiting. OPSTRING must be a packed array of
4399 semop structures. Each semop structure can be generated with
4400 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4401 operations is implied by the length of OPSTRING. Returns true if
4402 successful, or false if there is an error. As an example, the
4403 following code waits on semaphore $semnum of semaphore id $semid:
4405 $semop = pack("s!3", $semnum, -1, 0);
4406 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4408 To signal the semaphore, replace C<-1> with C<1>. See also
4409 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4412 =item send SOCKET,MSG,FLAGS,TO
4414 =item send SOCKET,MSG,FLAGS
4416 Sends a message on a socket. Attempts to send the scalar MSG to the
4417 SOCKET filehandle. Takes the same flags as the system call of the
4418 same name. On unconnected sockets you must specify a destination to
4419 send TO, in which case it does a C C<sendto>. Returns the number of
4420 characters sent, or the undefined value if there is an error. The C
4421 system call sendmsg(2) is currently unimplemented. See
4422 L<perlipc/"UDP: Message Passing"> for examples.
4424 Note the I<characters>: depending on the status of the socket, either
4425 (8-bit) bytes or characters are sent. By default all sockets operate
4426 on bytes, but for example if the socket has been changed using
4427 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4428 the C<open> pragma, L<open>), the I/O will operate on characters, not
4431 =item setpgrp PID,PGRP
4433 Sets the current process group for the specified PID, C<0> for the current
4434 process. Will produce a fatal error if used on a machine that doesn't
4435 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4436 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4437 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4440 =item setpriority WHICH,WHO,PRIORITY
4442 Sets the current priority for a process, a process group, or a user.
4443 (See setpriority(2).) Will produce a fatal error if used on a machine
4444 that doesn't implement setpriority(2).
4446 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4448 Sets the socket option requested. Returns undefined if there is an
4449 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4456 Shifts the first value of the array off and returns it, shortening the
4457 array by 1 and moving everything down. If there are no elements in the
4458 array, returns the undefined value. If ARRAY is omitted, shifts the
4459 C<@_> array within the lexical scope of subroutines and formats, and the
4460 C<@ARGV> array at file scopes or within the lexical scopes established by
4461 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4464 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4465 same thing to the left end of an array that C<pop> and C<push> do to the
4468 =item shmctl ID,CMD,ARG
4470 Calls the System V IPC function shmctl. You'll probably have to say
4474 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4475 then ARG must be a variable which will hold the returned C<shmid_ds>
4476 structure. Returns like ioctl: the undefined value for error, "C<0> but
4477 true" for zero, or the actual return value otherwise.
4478 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4480 =item shmget KEY,SIZE,FLAGS
4482 Calls the System V IPC function shmget. Returns the shared memory
4483 segment id, or the undefined value if there is an error.
4484 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4486 =item shmread ID,VAR,POS,SIZE
4488 =item shmwrite ID,STRING,POS,SIZE
4490 Reads or writes the System V shared memory segment ID starting at
4491 position POS for size SIZE by attaching to it, copying in/out, and
4492 detaching from it. When reading, VAR must be a variable that will
4493 hold the data read. When writing, if STRING is too long, only SIZE
4494 bytes are used; if STRING is too short, nulls are written to fill out
4495 SIZE bytes. Return true if successful, or false if there is an error.
4496 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4497 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4499 =item shutdown SOCKET,HOW
4501 Shuts down a socket connection in the manner indicated by HOW, which
4502 has the same interpretation as in the system call of the same name.
4504 shutdown(SOCKET, 0); # I/we have stopped reading data
4505 shutdown(SOCKET, 1); # I/we have stopped writing data
4506 shutdown(SOCKET, 2); # I/we have stopped using this socket
4508 This is useful with sockets when you want to tell the other
4509 side you're done writing but not done reading, or vice versa.
4510 It's also a more insistent form of close because it also
4511 disables the file descriptor in any forked copies in other
4518 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4519 returns sine of C<$_>.
4521 For the inverse sine operation, you may use the C<Math::Trig::asin>
4522 function, or use this relation:
4524 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4530 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4531 May be interrupted if the process receives a signal such as C<SIGALRM>.
4532 Returns the number of seconds actually slept. You probably cannot
4533 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4536 On some older systems, it may sleep up to a full second less than what
4537 you requested, depending on how it counts seconds. Most modern systems
4538 always sleep the full amount. They may appear to sleep longer than that,
4539 however, because your process might not be scheduled right away in a
4540 busy multitasking system.
4542 For delays of finer granularity than one second, you may use Perl's
4543 C<syscall> interface to access setitimer(2) if your system supports
4544 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4545 and starting from Perl 5.8 part of the standard distribution) may also
4548 See also the POSIX module's C<pause> function.
4550 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4552 Opens a socket of the specified kind and attaches it to filehandle
4553 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4554 the system call of the same name. You should C<use Socket> first
4555 to get the proper definitions imported. See the examples in
4556 L<perlipc/"Sockets: Client/Server Communication">.
4558 On systems that support a close-on-exec flag on files, the flag will
4559 be set for the newly opened file descriptor, as determined by the
4560 value of $^F. See L<perlvar/$^F>.
4562 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4564 Creates an unnamed pair of sockets in the specified domain, of the
4565 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4566 for the system call of the same name. If unimplemented, yields a fatal
4567 error. Returns true if successful.
4569 On systems that support a close-on-exec flag on files, the flag will
4570 be set for the newly opened file descriptors, as determined by the value
4571 of $^F. See L<perlvar/$^F>.
4573 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4574 to C<pipe(Rdr, Wtr)> is essentially:
4577 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4578 shutdown(Rdr, 1); # no more writing for reader
4579 shutdown(Wtr, 0); # no more reading for writer
4581 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4582 emulate socketpair using IP sockets to localhost if your system implements
4583 sockets but not socketpair.
4585 =item sort SUBNAME LIST
4587 =item sort BLOCK LIST
4591 In list context, this sorts the LIST and returns the sorted list value.
4592 In scalar context, the behaviour of C<sort()> is undefined.
4594 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4595 order. If SUBNAME is specified, it gives the name of a subroutine
4596 that returns an integer less than, equal to, or greater than C<0>,
4597 depending on how the elements of the list are to be ordered. (The C<<
4598 <=> >> and C<cmp> operators are extremely useful in such routines.)
4599 SUBNAME may be a scalar variable name (unsubscripted), in which case
4600 the value provides the name of (or a reference to) the actual
4601 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4602 an anonymous, in-line sort subroutine.
4604 If the subroutine's prototype is C<($$)>, the elements to be compared
4605 are passed by reference in C<@_>, as for a normal subroutine. This is
4606 slower than unprototyped subroutines, where the elements to be
4607 compared are passed into the subroutine
4608 as the package global variables $a and $b (see example below). Note that
4609 in the latter case, it is usually counter-productive to declare $a and
4612 In either case, the subroutine may not be recursive. The values to be
4613 compared are always passed by reference, so don't modify them.
4615 You also cannot exit out of the sort block or subroutine using any of the
4616 loop control operators described in L<perlsyn> or with C<goto>.
4618 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4619 current collation locale. See L<perllocale>.
4621 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4622 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4623 preserves the input order of elements that compare equal. Although
4624 quicksort's run time is O(NlogN) when averaged over all arrays of
4625 length N, the time can be O(N**2), I<quadratic> behavior, for some
4626 inputs.) In 5.7, the quicksort implementation was replaced with
4627 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4628 But benchmarks indicated that for some inputs, on some platforms,
4629 the original quicksort was faster. 5.8 has a sort pragma for
4630 limited control of the sort. Its rather blunt control of the
4631 underlying algorithm may not persist into future perls, but the
4632 ability to characterize the input or output in implementation
4633 independent ways quite probably will. See L<sort>.
4638 @articles = sort @files;
4640 # same thing, but with explicit sort routine
4641 @articles = sort {$a cmp $b} @files;
4643 # now case-insensitively
4644 @articles = sort {uc($a) cmp uc($b)} @files;
4646 # same thing in reversed order
4647 @articles = sort {$b cmp $a} @files;
4649 # sort numerically ascending
4650 @articles = sort {$a <=> $b} @files;
4652 # sort numerically descending
4653 @articles = sort {$b <=> $a} @files;
4655 # this sorts the %age hash by value instead of key
4656 # using an in-line function
4657 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4659 # sort using explicit subroutine name
4661 $age{$a} <=> $age{$b}; # presuming numeric
4663 @sortedclass = sort byage @class;
4665 sub backwards { $b cmp $a }
4666 @harry = qw(dog cat x Cain Abel);
4667 @george = qw(gone chased yz Punished Axed);
4669 # prints AbelCaincatdogx
4670 print sort backwards @harry;
4671 # prints xdogcatCainAbel
4672 print sort @george, 'to', @harry;
4673 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4675 # inefficiently sort by descending numeric compare using
4676 # the first integer after the first = sign, or the
4677 # whole record case-insensitively otherwise
4680 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4685 # same thing, but much more efficiently;
4686 # we'll build auxiliary indices instead
4690 push @nums, /=(\d+)/;
4695 $nums[$b] <=> $nums[$a]
4697 $caps[$a] cmp $caps[$b]
4701 # same thing, but without any temps
4702 @new = map { $_->[0] }
4703 sort { $b->[1] <=> $a->[1]
4706 } map { [$_, /=(\d+)/, uc($_)] } @old;
4708 # using a prototype allows you to use any comparison subroutine
4709 # as a sort subroutine (including other package's subroutines)
4711 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4714 @new = sort other::backwards @old;
4716 # guarantee stability, regardless of algorithm
4718 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4720 # force use of mergesort (not portable outside Perl 5.8)
4721 use sort '_mergesort'; # note discouraging _
4722 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4724 If you're using strict, you I<must not> declare $a
4725 and $b as lexicals. They are package globals. That means
4726 if you're in the C<main> package and type
4728 @articles = sort {$b <=> $a} @files;
4730 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4731 but if you're in the C<FooPack> package, it's the same as typing
4733 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4735 The comparison function is required to behave. If it returns
4736 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4737 sometimes saying the opposite, for example) the results are not
4740 =item splice ARRAY,OFFSET,LENGTH,LIST
4742 =item splice ARRAY,OFFSET,LENGTH
4744 =item splice ARRAY,OFFSET
4748 Removes the elements designated by OFFSET and LENGTH from an array, and
4749 replaces them with the elements of LIST, if any. In list context,
4750 returns the elements removed from the array. In scalar context,
4751 returns the last element removed, or C<undef> if no elements are
4752 removed. The array grows or shrinks as necessary.
4753 If OFFSET is negative then it starts that far from the end of the array.
4754 If LENGTH is omitted, removes everything from OFFSET onward.
4755 If LENGTH is negative, removes the elements from OFFSET onward
4756 except for -LENGTH elements at the end of the array.
4757 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4758 past the end of the array, perl issues a warning, and splices at the
4761 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4763 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4764 pop(@a) splice(@a,-1)
4765 shift(@a) splice(@a,0,1)
4766 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4767 $a[$i] = $y splice(@a,$i,1,$y)
4769 Example, assuming array lengths are passed before arrays:
4771 sub aeq { # compare two list values
4772 my(@a) = splice(@_,0,shift);
4773 my(@b) = splice(@_,0,shift);
4774 return 0 unless @a == @b; # same len?
4776 return 0 if pop(@a) ne pop(@b);
4780 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4782 =item split /PATTERN/,EXPR,LIMIT
4784 =item split /PATTERN/,EXPR
4786 =item split /PATTERN/
4790 Splits a string into a list of strings and returns that list. By default,
4791 empty leading fields are preserved, and empty trailing ones are deleted.
4793 In scalar context, returns the number of fields found and splits into
4794 the C<@_> array. Use of split in scalar context is deprecated, however,
4795 because it clobbers your subroutine arguments.
4797 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4798 splits on whitespace (after skipping any leading whitespace). Anything
4799 matching PATTERN is taken to be a delimiter separating the fields. (Note
4800 that the delimiter may be longer than one character.)
4802 If LIMIT is specified and positive, it represents the maximum number
4803 of fields the EXPR will be split into, though the actual number of
4804 fields returned depends on the number of times PATTERN matches within
4805 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4806 stripped (which potential users of C<pop> would do well to remember).
4807 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4808 had been specified. Note that splitting an EXPR that evaluates to the
4809 empty string always returns the empty list, regardless of the LIMIT
4812 A pattern matching the null string (not to be confused with
4813 a null pattern C<//>, which is just one member of the set of patterns
4814 matching a null string) will split the value of EXPR into separate
4815 characters at each point it matches that way. For example:
4817 print join(':', split(/ */, 'hi there'));
4819 produces the output 'h:i:t:h:e:r:e'.
4821 Using the empty pattern C<//> specifically matches the null string, and is
4822 not be confused with the use of C<//> to mean "the last successful pattern
4825 Empty leading (or trailing) fields are produced when there are positive width
4826 matches at the beginning (or end) of the string; a zero-width match at the
4827 beginning (or end) of the string does not produce an empty field. For
4830 print join(':', split(/(?=\w)/, 'hi there!'));
4832 produces the output 'h:i :t:h:e:r:e!'.
4834 The LIMIT parameter can be used to split a line partially
4836 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4838 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4839 one larger than the number of variables in the list, to avoid
4840 unnecessary work. For the list above LIMIT would have been 4 by
4841 default. In time critical applications it behooves you not to split
4842 into more fields than you really need.
4844 If the PATTERN contains parentheses, additional list elements are
4845 created from each matching substring in the delimiter.
4847 split(/([,-])/, "1-10,20", 3);
4849 produces the list value
4851 (1, '-', 10, ',', 20)
4853 If you had the entire header of a normal Unix email message in $header,
4854 you could split it up into fields and their values this way:
4856 $header =~ s/\n\s+/ /g; # fix continuation lines
4857 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4859 The pattern C</PATTERN/> may be replaced with an expression to specify
4860 patterns that vary at runtime. (To do runtime compilation only once,
4861 use C</$variable/o>.)
4863 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4864 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4865 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4866 will give you as many null initial fields as there are leading spaces.
4867 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4868 whitespace produces a null first field. A C<split> with no arguments
4869 really does a S<C<split(' ', $_)>> internally.
4871 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4876 open(PASSWD, '/etc/passwd');
4879 ($login, $passwd, $uid, $gid,
4880 $gcos, $home, $shell) = split(/:/);
4884 As with regular pattern matching, any capturing parentheses that are not
4885 matched in a C<split()> will be set to C<undef> when returned:
4887 @fields = split /(A)|B/, "1A2B3";
4888 # @fields is (1, 'A', 2, undef, 3)
4890 =item sprintf FORMAT, LIST
4892 Returns a string formatted by the usual C<printf> conventions of the C
4893 library function C<sprintf>. See below for more details
4894 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4895 the general principles.
4899 # Format number with up to 8 leading zeroes
4900 $result = sprintf("%08d", $number);
4902 # Round number to 3 digits after decimal point
4903 $rounded = sprintf("%.3f", $number);
4905 Perl does its own C<sprintf> formatting--it emulates the C
4906 function C<sprintf>, but it doesn't use it (except for floating-point
4907 numbers, and even then only the standard modifiers are allowed). As a
4908 result, any non-standard extensions in your local C<sprintf> are not
4909 available from Perl.
4911 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4912 pass it an array as your first argument. The array is given scalar context,
4913 and instead of using the 0th element of the array as the format, Perl will
4914 use the count of elements in the array as the format, which is almost never
4917 Perl's C<sprintf> permits the following universally-known conversions:
4920 %c a character with the given number
4922 %d a signed integer, in decimal
4923 %u an unsigned integer, in decimal
4924 %o an unsigned integer, in octal
4925 %x an unsigned integer, in hexadecimal
4926 %e a floating-point number, in scientific notation
4927 %f a floating-point number, in fixed decimal notation
4928 %g a floating-point number, in %e or %f notation
4930 In addition, Perl permits the following widely-supported conversions:
4932 %X like %x, but using upper-case letters
4933 %E like %e, but using an upper-case "E"
4934 %G like %g, but with an upper-case "E" (if applicable)
4935 %b an unsigned integer, in binary
4936 %p a pointer (outputs the Perl value's address in hexadecimal)
4937 %n special: *stores* the number of characters output so far
4938 into the next variable in the parameter list
4940 Finally, for backward (and we do mean "backward") compatibility, Perl
4941 permits these unnecessary but widely-supported conversions:
4944 %D a synonym for %ld
4945 %U a synonym for %lu
4946 %O a synonym for %lo
4949 Note that the number of exponent digits in the scientific notation produced
4950 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4951 exponent less than 100 is system-dependent: it may be three or less
4952 (zero-padded as necessary). In other words, 1.23 times ten to the
4953 99th may be either "1.23e99" or "1.23e099".
4955 Between the C<%> and the format letter, you may specify a number of
4956 additional attributes controlling the interpretation of the format.
4957 In order, these are:
4961 =item format parameter index
4963 An explicit format parameter index, such as C<2$>. By default sprintf
4964 will format the next unused argument in the list, but this allows you
4965 to take the arguments out of order. Eg:
4967 printf '%2$d %1$d', 12, 34; # prints "34 12"
4968 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4973 space prefix positive number with a space
4974 + prefix positive number with a plus sign
4975 - left-justify within the field
4976 0 use zeros, not spaces, to right-justify
4977 # prefix non-zero octal with "0", non-zero hex with "0x",
4978 non-zero binary with "0b"
4982 printf '<% d>', 12; # prints "< 12>"
4983 printf '<%+d>', 12; # prints "<+12>"
4984 printf '<%6s>', 12; # prints "< 12>"
4985 printf '<%-6s>', 12; # prints "<12 >"
4986 printf '<%06s>', 12; # prints "<000012>"
4987 printf '<%#x>', 12; # prints "<0xc>"
4991 The vector flag C<v>, optionally specifying the join string to use.
4992 This flag tells perl to interpret the supplied string as a vector
4993 of integers, one for each character in the string, separated by
4994 a given string (a dot C<.> by default). This can be useful for
4995 displaying ordinal values of characters in arbitrary strings:
4997 printf "version is v%vd\n", $^V; # Perl's version
4999 Put an asterisk C<*> before the C<v> to override the string to
5000 use to separate the numbers:
5002 printf "address is %*vX\n", ":", $addr; # IPv6 address
5003 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5005 You can also explicitly specify the argument number to use for
5006 the join string using eg C<*2$v>:
5008 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5010 =item (minimum) width
5012 Arguments are usually formatted to be only as wide as required to
5013 display the given value. You can override the width by putting
5014 a number here, or get the width from the next argument (with C<*>)
5015 or from a specified argument (with eg C<*2$>):
5017 printf '<%s>', "a"; # prints "<a>"
5018 printf '<%6s>', "a"; # prints "< a>"
5019 printf '<%*s>', 6, "a"; # prints "< a>"
5020 printf '<%*2$s>', "a", 6; # prints "< a>"
5021 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5023 If a field width obtained through C<*> is negative, it has the same
5024 effect as the C<-> flag: left-justification.
5026 =item precision, or maximum width
5028 You can specify a precision (for numeric conversions) or a maximum
5029 width (for string conversions) by specifying a C<.> followed by a number.
5030 For floating point formats, with the exception of 'g' and 'G', this specifies
5031 the number of decimal places to show (the default being 6), eg:
5033 # these examples are subject to system-specific variation
5034 printf '<%f>', 1; # prints "<1.000000>"
5035 printf '<%.1f>', 1; # prints "<1.0>"
5036 printf '<%.0f>', 1; # prints "<1>"
5037 printf '<%e>', 10; # prints "<1.000000e+01>"
5038 printf '<%.1e>', 10; # prints "<1.0e+01>"
5040 For 'g' and 'G', this specifies the maximum number of digits to show,
5041 including prior to the decimal point as well as after it, eg:
5043 # these examples are subject to system-specific variation
5044 printf '<%g>', 1; # prints "<1>"
5045 printf '<%.10g>', 1; # prints "<1>"
5046 printf '<%g>', 100; # prints "<100>"
5047 printf '<%.1g>', 100; # prints "<1e+02>"
5048 printf '<%.2g>', 100.01; # prints "<1e+02>"
5049 printf '<%.5g>', 100.01; # prints "<100.01>"
5050 printf '<%.4g>', 100.01; # prints "<100>"
5052 For integer conversions, specifying a precision implies that the
5053 output of the number itself should be zero-padded to this width:
5055 printf '<%.6x>', 1; # prints "<000001>"
5056 printf '<%#.6x>', 1; # prints "<0x000001>"
5057 printf '<%-10.6x>', 1; # prints "<000001 >"
5059 For string conversions, specifying a precision truncates the string
5060 to fit in the specified width:
5062 printf '<%.5s>', "truncated"; # prints "<trunc>"
5063 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5065 You can also get the precision from the next argument using C<.*>:
5067 printf '<%.6x>', 1; # prints "<000001>"
5068 printf '<%.*x>', 6, 1; # prints "<000001>"
5070 You cannot currently get the precision from a specified number,
5071 but it is intended that this will be possible in the future using
5074 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5078 For numeric conversions, you can specify the size to interpret the
5079 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5080 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5081 whatever the default integer size is on your platform (usually 32 or 64
5082 bits), but you can override this to use instead one of the standard C types,
5083 as supported by the compiler used to build Perl:
5085 l interpret integer as C type "long" or "unsigned long"
5086 h interpret integer as C type "short" or "unsigned short"
5087 q, L or ll interpret integer as C type "long long", "unsigned long long".
5088 or "quads" (typically 64-bit integers)
5090 The last will produce errors if Perl does not understand "quads" in your
5091 installation. (This requires that either the platform natively supports quads
5092 or Perl was specifically compiled to support quads.) You can find out
5093 whether your Perl supports quads via L<Config>:
5096 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5099 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5100 to be the default floating point size on your platform (double or long double),
5101 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5102 platform supports them. You can find out whether your Perl supports long
5103 doubles via L<Config>:
5106 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5108 You can find out whether Perl considers 'long double' to be the default
5109 floating point size to use on your platform via L<Config>:
5112 ($Config{uselongdouble} eq 'define') &&
5113 print "long doubles by default\n";
5115 It can also be the case that long doubles and doubles are the same thing:
5118 ($Config{doublesize} == $Config{longdblsize}) &&
5119 print "doubles are long doubles\n";
5121 The size specifier C<V> has no effect for Perl code, but it is supported
5122 for compatibility with XS code; it means 'use the standard size for
5123 a Perl integer (or floating-point number)', which is already the
5124 default for Perl code.
5126 =item order of arguments
5128 Normally, sprintf takes the next unused argument as the value to
5129 format for each format specification. If the format specification
5130 uses C<*> to require additional arguments, these are consumed from
5131 the argument list in the order in which they appear in the format
5132 specification I<before> the value to format. Where an argument is
5133 specified using an explicit index, this does not affect the normal
5134 order for the arguments (even when the explicitly specified index
5135 would have been the next argument in any case).
5139 printf '<%*.*s>', $a, $b, $c;
5141 would use C<$a> for the width, C<$b> for the precision and C<$c>
5142 as the value to format, while:
5144 print '<%*1$.*s>', $a, $b;
5146 would use C<$a> for the width and the precision, and C<$b> as the
5149 Here are some more examples - beware that when using an explicit
5150 index, the C<$> may need to be escaped:
5152 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5153 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5154 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5155 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5159 If C<use locale> is in effect, the character used for the decimal
5160 point in formatted real numbers is affected by the LC_NUMERIC locale.
5167 Return the square root of EXPR. If EXPR is omitted, returns square
5168 root of C<$_>. Only works on non-negative operands, unless you've
5169 loaded the standard Math::Complex module.
5172 print sqrt(-2); # prints 1.4142135623731i
5178 Sets the random number seed for the C<rand> operator.
5180 The point of the function is to "seed" the C<rand> function so that
5181 C<rand> can produce a different sequence each time you run your
5184 If srand() is not called explicitly, it is called implicitly at the
5185 first use of the C<rand> operator. However, this was not the case in
5186 versions of Perl before 5.004, so if your script will run under older
5187 Perl versions, it should call C<srand>.
5189 Most programs won't even call srand() at all, except those that
5190 need a cryptographically-strong starting point rather than the
5191 generally acceptable default, which is based on time of day,
5192 process ID, and memory allocation, or the F</dev/urandom> device,
5195 You can call srand($seed) with the same $seed to reproduce the
5196 I<same> sequence from rand(), but this is usually reserved for
5197 generating predictable results for testing or debugging.
5198 Otherwise, don't call srand() more than once in your program.
5200 Do B<not> call srand() (i.e. without an argument) more than once in
5201 a script. The internal state of the random number generator should
5202 contain more entropy than can be provided by any seed, so calling
5203 srand() again actually I<loses> randomness.
5205 Most implementations of C<srand> take an integer and will silently
5206 truncate decimal numbers. This means C<srand(42)> will usually
5207 produce the same results as C<srand(42.1)>. To be safe, always pass
5208 C<srand> an integer.
5210 In versions of Perl prior to 5.004 the default seed was just the
5211 current C<time>. This isn't a particularly good seed, so many old
5212 programs supply their own seed value (often C<time ^ $$> or C<time ^
5213 ($$ + ($$ << 15))>), but that isn't necessary any more.
5215 Note that you need something much more random than the default seed for
5216 cryptographic purposes. Checksumming the compressed output of one or more
5217 rapidly changing operating system status programs is the usual method. For
5220 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5222 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5225 Frequently called programs (like CGI scripts) that simply use
5229 for a seed can fall prey to the mathematical property that
5233 one-third of the time. So don't do that.
5235 =item stat FILEHANDLE
5241 Returns a 13-element list giving the status info for a file, either
5242 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5243 it stats C<$_>. Returns a null list if the stat fails. Typically used
5246 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5247 $atime,$mtime,$ctime,$blksize,$blocks)
5250 Not all fields are supported on all filesystem types. Here are the
5251 meaning of the fields:
5253 0 dev device number of filesystem
5255 2 mode file mode (type and permissions)
5256 3 nlink number of (hard) links to the file
5257 4 uid numeric user ID of file's owner
5258 5 gid numeric group ID of file's owner
5259 6 rdev the device identifier (special files only)
5260 7 size total size of file, in bytes
5261 8 atime last access time in seconds since the epoch
5262 9 mtime last modify time in seconds since the epoch
5263 10 ctime inode change time in seconds since the epoch (*)
5264 11 blksize preferred block size for file system I/O
5265 12 blocks actual number of blocks allocated
5267 (The epoch was at 00:00 January 1, 1970 GMT.)
5269 (*) The ctime field is non-portable, in particular you cannot expect
5270 it to be a "creation time", see L<perlport/"Files and Filesystems">
5273 If stat is passed the special filehandle consisting of an underline, no
5274 stat is done, but the current contents of the stat structure from the
5275 last stat or filetest are returned. Example:
5277 if (-x $file && (($d) = stat(_)) && $d < 0) {
5278 print "$file is executable NFS file\n";
5281 (This works on machines only for which the device number is negative
5284 Because the mode contains both the file type and its permissions, you
5285 should mask off the file type portion and (s)printf using a C<"%o">
5286 if you want to see the real permissions.
5288 $mode = (stat($filename))[2];
5289 printf "Permissions are %04o\n", $mode & 07777;
5291 In scalar context, C<stat> returns a boolean value indicating success
5292 or failure, and, if successful, sets the information associated with
5293 the special filehandle C<_>.
5295 The File::stat module provides a convenient, by-name access mechanism:
5298 $sb = stat($filename);
5299 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5300 $filename, $sb->size, $sb->mode & 07777,
5301 scalar localtime $sb->mtime;
5303 You can import symbolic mode constants (C<S_IF*>) and functions
5304 (C<S_IS*>) from the Fcntl module:
5308 $mode = (stat($filename))[2];
5310 $user_rwx = ($mode & S_IRWXU) >> 6;
5311 $group_read = ($mode & S_IRGRP) >> 3;
5312 $other_execute = $mode & S_IXOTH;
5314 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5316 $is_setuid = $mode & S_ISUID;
5317 $is_setgid = S_ISDIR($mode);
5319 You could write the last two using the C<-u> and C<-d> operators.
5320 The commonly available S_IF* constants are
5322 # Permissions: read, write, execute, for user, group, others.
5324 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5325 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5326 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5328 # Setuid/Setgid/Stickiness/SaveText.
5329 # Note that the exact meaning of these is system dependent.
5331 S_ISUID S_ISGID S_ISVTX S_ISTXT
5333 # File types. Not necessarily all are available on your system.
5335 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5337 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5339 S_IREAD S_IWRITE S_IEXEC
5341 and the S_IF* functions are
5343 S_IMODE($mode) the part of $mode containing the permission bits
5344 and the setuid/setgid/sticky bits
5346 S_IFMT($mode) the part of $mode containing the file type
5347 which can be bit-anded with e.g. S_IFREG
5348 or with the following functions
5350 # The operators -f, -d, -l, -b, -c, -p, and -s.
5352 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5353 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5355 # No direct -X operator counterpart, but for the first one
5356 # the -g operator is often equivalent. The ENFMT stands for
5357 # record flocking enforcement, a platform-dependent feature.
5359 S_ISENFMT($mode) S_ISWHT($mode)
5361 See your native chmod(2) and stat(2) documentation for more details
5362 about the S_* constants.
5364 To get status info for a symbolic link instead of the target file
5365 behind the link, use the C<lstat> function, see L</stat>.
5371 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5372 doing many pattern matches on the string before it is next modified.
5373 This may or may not save time, depending on the nature and number of
5374 patterns you are searching on, and on the distribution of character
5375 frequencies in the string to be searched--you probably want to compare
5376 run times with and without it to see which runs faster. Those loops
5377 which scan for many short constant strings (including the constant
5378 parts of more complex patterns) will benefit most. You may have only
5379 one C<study> active at a time--if you study a different scalar the first
5380 is "unstudied". (The way C<study> works is this: a linked list of every
5381 character in the string to be searched is made, so we know, for
5382 example, where all the C<'k'> characters are. From each search string,
5383 the rarest character is selected, based on some static frequency tables
5384 constructed from some C programs and English text. Only those places
5385 that contain this "rarest" character are examined.)
5387 For example, here is a loop that inserts index producing entries
5388 before any line containing a certain pattern:
5392 print ".IX foo\n" if /\bfoo\b/;
5393 print ".IX bar\n" if /\bbar\b/;
5394 print ".IX blurfl\n" if /\bblurfl\b/;
5399 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5400 will be looked at, because C<f> is rarer than C<o>. In general, this is
5401 a big win except in pathological cases. The only question is whether
5402 it saves you more time than it took to build the linked list in the
5405 Note that if you have to look for strings that you don't know till
5406 runtime, you can build an entire loop as a string and C<eval> that to
5407 avoid recompiling all your patterns all the time. Together with
5408 undefining C<$/> to input entire files as one record, this can be very
5409 fast, often faster than specialized programs like fgrep(1). The following
5410 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5411 out the names of those files that contain a match:
5413 $search = 'while (<>) { study;';
5414 foreach $word (@words) {
5415 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5420 eval $search; # this screams
5421 $/ = "\n"; # put back to normal input delimiter
5422 foreach $file (sort keys(%seen)) {
5426 =item sub NAME BLOCK
5428 =item sub NAME (PROTO) BLOCK
5430 =item sub NAME : ATTRS BLOCK
5432 =item sub NAME (PROTO) : ATTRS BLOCK
5434 This is subroutine definition, not a real function I<per se>.
5435 Without a BLOCK it's just a forward declaration. Without a NAME,
5436 it's an anonymous function declaration, and does actually return
5437 a value: the CODE ref of the closure you just created.
5439 See L<perlsub> and L<perlref> for details about subroutines and
5440 references, and L<attributes> and L<Attribute::Handlers> for more
5441 information about attributes.
5443 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5445 =item substr EXPR,OFFSET,LENGTH
5447 =item substr EXPR,OFFSET
5449 Extracts a substring out of EXPR and returns it. First character is at
5450 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5451 If OFFSET is negative (or more precisely, less than C<$[>), starts
5452 that far from the end of the string. If LENGTH is omitted, returns
5453 everything to the end of the string. If LENGTH is negative, leaves that
5454 many characters off the end of the string.
5456 You can use the substr() function as an lvalue, in which case EXPR
5457 must itself be an lvalue. If you assign something shorter than LENGTH,
5458 the string will shrink, and if you assign something longer than LENGTH,
5459 the string will grow to accommodate it. To keep the string the same
5460 length you may need to pad or chop your value using C<sprintf>.
5462 If OFFSET and LENGTH specify a substring that is partly outside the
5463 string, only the part within the string is returned. If the substring
5464 is beyond either end of the string, substr() returns the undefined
5465 value and produces a warning. When used as an lvalue, specifying a
5466 substring that is entirely outside the string is a fatal error.
5467 Here's an example showing the behavior for boundary cases:
5470 substr($name, 4) = 'dy'; # $name is now 'freddy'
5471 my $null = substr $name, 6, 2; # returns '' (no warning)
5472 my $oops = substr $name, 7; # returns undef, with warning
5473 substr($name, 7) = 'gap'; # fatal error
5475 An alternative to using substr() as an lvalue is to specify the
5476 replacement string as the 4th argument. This allows you to replace
5477 parts of the EXPR and return what was there before in one operation,
5478 just as you can with splice().
5480 If the lvalue returned by substr is used after the EXPR is changed in
5481 any way, the behaviour may not be as expected and is subject to change.
5482 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5483 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5484 substring assignment, and then the substr is used again), or where a
5485 substr() is aliased via a C<foreach> loop or passed as a parameter or
5486 a reference to it is taken and then the alias, parameter, or deref'd
5487 reference either is used after the original EXPR has been changed or
5488 is assigned to and then used a second time.
5490 =item symlink OLDFILE,NEWFILE
5492 Creates a new filename symbolically linked to the old filename.
5493 Returns C<1> for success, C<0> otherwise. On systems that don't support
5494 symbolic links, produces a fatal error at run time. To check for that,
5497 $symlink_exists = eval { symlink("",""); 1 };
5501 Calls the system call specified as the first element of the list,
5502 passing the remaining elements as arguments to the system call. If
5503 unimplemented, produces a fatal error. The arguments are interpreted
5504 as follows: if a given argument is numeric, the argument is passed as
5505 an int. If not, the pointer to the string value is passed. You are
5506 responsible to make sure a string is pre-extended long enough to
5507 receive any result that might be written into a string. You can't use a
5508 string literal (or other read-only string) as an argument to C<syscall>
5509 because Perl has to assume that any string pointer might be written
5511 integer arguments are not literals and have never been interpreted in a
5512 numeric context, you may need to add C<0> to them to force them to look
5513 like numbers. This emulates the C<syswrite> function (or vice versa):
5515 require 'syscall.ph'; # may need to run h2ph
5517 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5519 Note that Perl supports passing of up to only 14 arguments to your system call,
5520 which in practice should usually suffice.
5522 Syscall returns whatever value returned by the system call it calls.
5523 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5524 Note that some system calls can legitimately return C<-1>. The proper
5525 way to handle such calls is to assign C<$!=0;> before the call and
5526 check the value of C<$!> if syscall returns C<-1>.
5528 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5529 number of the read end of the pipe it creates. There is no way
5530 to retrieve the file number of the other end. You can avoid this
5531 problem by using C<pipe> instead.
5533 =item sysopen FILEHANDLE,FILENAME,MODE
5535 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5537 Opens the file whose filename is given by FILENAME, and associates it
5538 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5539 the name of the real filehandle wanted. This function calls the
5540 underlying operating system's C<open> function with the parameters
5541 FILENAME, MODE, PERMS.
5543 The possible values and flag bits of the MODE parameter are
5544 system-dependent; they are available via the standard module C<Fcntl>.
5545 See the documentation of your operating system's C<open> to see which
5546 values and flag bits are available. You may combine several flags
5547 using the C<|>-operator.
5549 Some of the most common values are C<O_RDONLY> for opening the file in
5550 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5551 and C<O_RDWR> for opening the file in read-write mode, and.
5553 For historical reasons, some values work on almost every system
5554 supported by perl: zero means read-only, one means write-only, and two
5555 means read/write. We know that these values do I<not> work under
5556 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5557 use them in new code.
5559 If the file named by FILENAME does not exist and the C<open> call creates
5560 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5561 PERMS specifies the permissions of the newly created file. If you omit
5562 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5563 These permission values need to be in octal, and are modified by your
5564 process's current C<umask>.
5566 In many systems the C<O_EXCL> flag is available for opening files in
5567 exclusive mode. This is B<not> locking: exclusiveness means here that
5568 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5571 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5573 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5574 that takes away the user's option to have a more permissive umask.
5575 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5578 Note that C<sysopen> depends on the fdopen() C library function.
5579 On many UNIX systems, fdopen() is known to fail when file descriptors
5580 exceed a certain value, typically 255. If you need more file
5581 descriptors than that, consider rebuilding Perl to use the C<sfio>
5582 library, or perhaps using the POSIX::open() function.
5584 See L<perlopentut> for a kinder, gentler explanation of opening files.
5586 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5588 =item sysread FILEHANDLE,SCALAR,LENGTH
5590 Attempts to read LENGTH bytes of data into variable SCALAR from the
5591 specified FILEHANDLE, using the system call read(2). It bypasses
5592 buffered IO, so mixing this with other kinds of reads, C<print>,
5593 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5594 perlio or stdio layers usually buffers data. Returns the number of
5595 bytes actually read, C<0> at end of file, or undef if there was an
5596 error (in the latter case C<$!> is also set). SCALAR will be grown or
5597 shrunk so that the last byte actually read is the last byte of the
5598 scalar after the read.
5600 An OFFSET may be specified to place the read data at some place in the
5601 string other than the beginning. A negative OFFSET specifies
5602 placement at that many characters counting backwards from the end of
5603 the string. A positive OFFSET greater than the length of SCALAR
5604 results in the string being padded to the required size with C<"\0">
5605 bytes before the result of the read is appended.
5607 There is no syseof() function, which is ok, since eof() doesn't work
5608 very well on device files (like ttys) anyway. Use sysread() and check
5609 for a return value for 0 to decide whether you're done.
5611 Note that if the filehandle has been marked as C<:utf8> Unicode
5612 characters are read instead of bytes (the LENGTH, OFFSET, and the
5613 return value of sysread() are in Unicode characters).
5614 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5615 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5617 =item sysseek FILEHANDLE,POSITION,WHENCE
5619 Sets FILEHANDLE's system position in bytes using the system call
5620 lseek(2). FILEHANDLE may be an expression whose value gives the name
5621 of the filehandle. The values for WHENCE are C<0> to set the new
5622 position to POSITION, C<1> to set the it to the current position plus
5623 POSITION, and C<2> to set it to EOF plus POSITION (typically
5626 Note the I<in bytes>: even if the filehandle has been set to operate
5627 on characters (for example by using the C<:utf8> I/O layer), tell()
5628 will return byte offsets, not character offsets (because implementing
5629 that would render sysseek() very slow).
5631 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5632 than C<sysread>, for example >< or read()) C<print>, C<write>,
5633 C<seek>, C<tell>, or C<eof> may cause confusion.
5635 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5636 and C<SEEK_END> (start of the file, current position, end of the file)
5637 from the Fcntl module. Use of the constants is also more portable
5638 than relying on 0, 1, and 2. For example to define a "systell" function:
5640 use Fnctl 'SEEK_CUR';
5641 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5643 Returns the new position, or the undefined value on failure. A position
5644 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5645 true on success and false on failure, yet you can still easily determine
5650 =item system PROGRAM LIST
5652 Does exactly the same thing as C<exec LIST>, except that a fork is
5653 done first, and the parent process waits for the child process to
5654 complete. Note that argument processing varies depending on the
5655 number of arguments. If there is more than one argument in LIST,
5656 or if LIST is an array with more than one value, starts the program
5657 given by the first element of the list with arguments given by the
5658 rest of the list. If there is only one scalar argument, the argument
5659 is checked for shell metacharacters, and if there are any, the
5660 entire argument is passed to the system's command shell for parsing
5661 (this is C</bin/sh -c> on Unix platforms, but varies on other
5662 platforms). If there are no shell metacharacters in the argument,
5663 it is split into words and passed directly to C<execvp>, which is
5666 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5667 output before any operation that may do a fork, but this may not be
5668 supported on some platforms (see L<perlport>). To be safe, you may need
5669 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5670 of C<IO::Handle> on any open handles.
5672 The return value is the exit status of the program as returned by the
5673 C<wait> call. To get the actual exit value shift right by eight (see below).
5674 See also L</exec>. This is I<not> what you want to use to capture
5675 the output from a command, for that you should use merely backticks or
5676 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5677 indicates a failure to start the program (inspect $! for the reason).
5679 Like C<exec>, C<system> allows you to lie to a program about its name if
5680 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5682 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5683 killing the program they're running doesn't actually interrupt
5686 @args = ("command", "arg1", "arg2");
5688 or die "system @args failed: $?"
5690 You can check all the failure possibilities by inspecting
5693 $exit_value = $? >> 8;
5694 $signal_num = $? & 127;
5695 $dumped_core = $? & 128;
5697 or more portably by using the W*() calls of the POSIX extension;
5698 see L<perlport> for more information.
5700 When the arguments get executed via the system shell, results
5701 and return codes will be subject to its quirks and capabilities.
5702 See L<perlop/"`STRING`"> and L</exec> for details.
5704 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5706 =item syswrite FILEHANDLE,SCALAR,LENGTH
5708 =item syswrite FILEHANDLE,SCALAR
5710 Attempts to write LENGTH bytes of data from variable SCALAR to the
5711 specified FILEHANDLE, using the system call write(2). If LENGTH is
5712 not specified, writes whole SCALAR. It bypasses buffered IO, so
5713 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5714 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5715 stdio layers usually buffers data. Returns the number of bytes
5716 actually written, or C<undef> if there was an error (in this case the
5717 errno variable C<$!> is also set). If the LENGTH is greater than the
5718 available data in the SCALAR after the OFFSET, only as much data as is
5719 available will be written.
5721 An OFFSET may be specified to write the data from some part of the
5722 string other than the beginning. A negative OFFSET specifies writing
5723 that many characters counting backwards from the end of the string.
5724 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5726 Note that if the filehandle has been marked as C<:utf8>,
5727 Unicode characters are written instead of bytes (the LENGTH, OFFSET,
5728 and the return value of syswrite() are in Unicode characters).
5729 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5730 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5732 =item tell FILEHANDLE
5736 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5737 error. FILEHANDLE may be an expression whose value gives the name of
5738 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5741 Note the I<in bytes>: even if the filehandle has been set to
5742 operate on characters (for example by using the C<:utf8> open
5743 layer), tell() will return byte offsets, not character offsets
5744 (because that would render seek() and tell() rather slow).
5746 The return value of tell() for the standard streams like the STDIN
5747 depends on the operating system: it may return -1 or something else.
5748 tell() on pipes, fifos, and sockets usually returns -1.
5750 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5752 Do not use tell() on a filehandle that has been opened using
5753 sysopen(), use sysseek() for that as described above. Why? Because
5754 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5755 buffered filehandles. sysseek() make sense only on the first kind,
5756 tell() only makes sense on the second kind.
5758 =item telldir DIRHANDLE
5760 Returns the current position of the C<readdir> routines on DIRHANDLE.
5761 Value may be given to C<seekdir> to access a particular location in a
5762 directory. Has the same caveats about possible directory compaction as
5763 the corresponding system library routine.
5765 =item tie VARIABLE,CLASSNAME,LIST
5767 This function binds a variable to a package class that will provide the
5768 implementation for the variable. VARIABLE is the name of the variable
5769 to be enchanted. CLASSNAME is the name of a class implementing objects
5770 of correct type. Any additional arguments are passed to the C<new>
5771 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5772 or C<TIEHASH>). Typically these are arguments such as might be passed
5773 to the C<dbm_open()> function of C. The object returned by the C<new>
5774 method is also returned by the C<tie> function, which would be useful
5775 if you want to access other methods in CLASSNAME.
5777 Note that functions such as C<keys> and C<values> may return huge lists
5778 when used on large objects, like DBM files. You may prefer to use the
5779 C<each> function to iterate over such. Example:
5781 # print out history file offsets
5783 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5784 while (($key,$val) = each %HIST) {
5785 print $key, ' = ', unpack('L',$val), "\n";
5789 A class implementing a hash should have the following methods:
5791 TIEHASH classname, LIST
5793 STORE this, key, value
5798 NEXTKEY this, lastkey
5802 A class implementing an ordinary array should have the following methods:
5804 TIEARRAY classname, LIST
5806 STORE this, key, value
5808 STORESIZE this, count
5814 SPLICE this, offset, length, LIST
5819 A class implementing a file handle should have the following methods:
5821 TIEHANDLE classname, LIST
5822 READ this, scalar, length, offset
5825 WRITE this, scalar, length, offset
5827 PRINTF this, format, LIST
5831 SEEK this, position, whence
5833 OPEN this, mode, LIST
5838 A class implementing a scalar should have the following methods:
5840 TIESCALAR classname, LIST
5846 Not all methods indicated above need be implemented. See L<perltie>,
5847 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5849 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5850 for you--you need to do that explicitly yourself. See L<DB_File>
5851 or the F<Config> module for interesting C<tie> implementations.
5853 For further details see L<perltie>, L<"tied VARIABLE">.
5857 Returns a reference to the object underlying VARIABLE (the same value
5858 that was originally returned by the C<tie> call that bound the variable
5859 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5864 Returns the number of non-leap seconds since whatever time the system
5865 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5866 and 00:00:00 UTC, January 1, 1970 for most other systems).
5867 Suitable for feeding to C<gmtime> and C<localtime>.
5869 For measuring time in better granularity than one second,
5870 you may use either the Time::HiRes module (from CPAN, and starting from
5871 Perl 5.8 part of the standard distribution), or if you have
5872 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5873 See L<perlfaq8> for details.
5877 Returns a four-element list giving the user and system times, in
5878 seconds, for this process and the children of this process.
5880 ($user,$system,$cuser,$csystem) = times;
5882 In scalar context, C<times> returns C<$user>.
5886 The transliteration operator. Same as C<y///>. See L<perlop>.
5888 =item truncate FILEHANDLE,LENGTH
5890 =item truncate EXPR,LENGTH
5892 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5893 specified length. Produces a fatal error if truncate isn't implemented
5894 on your system. Returns true if successful, the undefined value
5897 The behavior is undefined if LENGTH is greater than the length of the
5904 Returns an uppercased version of EXPR. This is the internal function
5905 implementing the C<\U> escape in double-quoted strings. Respects
5906 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5907 and L<perlunicode> for more details about locale and Unicode support.
5908 It does not attempt to do titlecase mapping on initial letters. See
5909 C<ucfirst> for that.
5911 If EXPR is omitted, uses C<$_>.
5917 Returns the value of EXPR with the first character in uppercase
5918 (titlecase in Unicode). This is the internal function implementing
5919 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5920 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5921 for more details about locale and Unicode support.
5923 If EXPR is omitted, uses C<$_>.
5929 Sets the umask for the process to EXPR and returns the previous value.
5930 If EXPR is omitted, merely returns the current umask.
5932 The Unix permission C<rwxr-x---> is represented as three sets of three
5933 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5934 and isn't one of the digits). The C<umask> value is such a number
5935 representing disabled permissions bits. The permission (or "mode")
5936 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5937 even if you tell C<sysopen> to create a file with permissions C<0777>,
5938 if your umask is C<0022> then the file will actually be created with
5939 permissions C<0755>. If your C<umask> were C<0027> (group can't
5940 write; others can't read, write, or execute), then passing
5941 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5944 Here's some advice: supply a creation mode of C<0666> for regular
5945 files (in C<sysopen>) and one of C<0777> for directories (in
5946 C<mkdir>) and executable files. This gives users the freedom of
5947 choice: if they want protected files, they might choose process umasks
5948 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5949 Programs should rarely if ever make policy decisions better left to
5950 the user. The exception to this is when writing files that should be
5951 kept private: mail files, web browser cookies, I<.rhosts> files, and
5954 If umask(2) is not implemented on your system and you are trying to
5955 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5956 fatal error at run time. If umask(2) is not implemented and you are
5957 not trying to restrict access for yourself, returns C<undef>.
5959 Remember that a umask is a number, usually given in octal; it is I<not> a
5960 string of octal digits. See also L</oct>, if all you have is a string.
5966 Undefines the value of EXPR, which must be an lvalue. Use only on a
5967 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5968 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5969 will probably not do what you expect on most predefined variables or
5970 DBM list values, so don't do that; see L<delete>.) Always returns the
5971 undefined value. You can omit the EXPR, in which case nothing is
5972 undefined, but you still get an undefined value that you could, for
5973 instance, return from a subroutine, assign to a variable or pass as a
5974 parameter. Examples:
5977 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5981 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5982 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5983 select undef, undef, undef, 0.25;
5984 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5986 Note that this is a unary operator, not a list operator.
5992 Deletes a list of files. Returns the number of files successfully
5995 $cnt = unlink 'a', 'b', 'c';
5999 Note: C<unlink> will not delete directories unless you are superuser and
6000 the B<-U> flag is supplied to Perl. Even if these conditions are
6001 met, be warned that unlinking a directory can inflict damage on your
6002 filesystem. Use C<rmdir> instead.
6004 If LIST is omitted, uses C<$_>.
6006 =item unpack TEMPLATE,EXPR
6008 C<unpack> does the reverse of C<pack>: it takes a string
6009 and expands it out into a list of values.
6010 (In scalar context, it returns merely the first value produced.)
6012 The string is broken into chunks described by the TEMPLATE. Each chunk
6013 is converted separately to a value. Typically, either the string is a result
6014 of C<pack>, or the bytes of the string represent a C structure of some
6017 The TEMPLATE has the same format as in the C<pack> function.
6018 Here's a subroutine that does substring:
6021 my($what,$where,$howmuch) = @_;
6022 unpack("x$where a$howmuch", $what);
6027 sub ordinal { unpack("c",$_[0]); } # same as ord()
6029 In addition to fields allowed in pack(), you may prefix a field with
6030 a %<number> to indicate that
6031 you want a <number>-bit checksum of the items instead of the items
6032 themselves. Default is a 16-bit checksum. Checksum is calculated by
6033 summing numeric values of expanded values (for string fields the sum of
6034 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6036 For example, the following
6037 computes the same number as the System V sum program:
6041 unpack("%32C*",<>) % 65535;
6044 The following efficiently counts the number of set bits in a bit vector:
6046 $setbits = unpack("%32b*", $selectmask);
6048 The C<p> and C<P> formats should be used with care. Since Perl
6049 has no way of checking whether the value passed to C<unpack()>
6050 corresponds to a valid memory location, passing a pointer value that's
6051 not known to be valid is likely to have disastrous consequences.
6053 If there are more pack codes or if the repeat count of a field or a group
6054 is larger than what the remainder of the input string allows, the result
6055 is not well defined: in some cases, the repeat count is decreased, or
6056 C<unpack()> will produce null strings or zeroes, or terminate with an
6057 error. If the input string is longer than one described by the TEMPLATE,
6058 the rest is ignored.
6060 See L</pack> for more examples and notes.
6062 =item untie VARIABLE
6064 Breaks the binding between a variable and a package. (See C<tie>.)
6065 Has no effect if the variable is not tied.
6067 =item unshift ARRAY,LIST
6069 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6070 depending on how you look at it. Prepends list to the front of the
6071 array, and returns the new number of elements in the array.
6073 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6075 Note the LIST is prepended whole, not one element at a time, so the
6076 prepended elements stay in the same order. Use C<reverse> to do the
6079 =item use Module VERSION LIST
6081 =item use Module VERSION
6083 =item use Module LIST
6089 Imports some semantics into the current package from the named module,
6090 generally by aliasing certain subroutine or variable names into your
6091 package. It is exactly equivalent to
6093 BEGIN { require Module; import Module LIST; }
6095 except that Module I<must> be a bareword.
6097 VERSION may be either a numeric argument such as 5.006, which will be
6098 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6099 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6100 greater than the version of the current Perl interpreter; Perl will not
6101 attempt to parse the rest of the file. Compare with L</require>, which can
6102 do a similar check at run time.
6104 Specifying VERSION as a literal of the form v5.6.1 should generally be
6105 avoided, because it leads to misleading error messages under earlier
6106 versions of Perl which do not support this syntax. The equivalent numeric
6107 version should be used instead.
6109 use v5.6.1; # compile time version check
6111 use 5.006_001; # ditto; preferred for backwards compatibility
6113 This is often useful if you need to check the current Perl version before
6114 C<use>ing library modules that have changed in incompatible ways from
6115 older versions of Perl. (We try not to do this more than we have to.)
6117 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6118 C<require> makes sure the module is loaded into memory if it hasn't been
6119 yet. The C<import> is not a builtin--it's just an ordinary static method
6120 call into the C<Module> package to tell the module to import the list of
6121 features back into the current package. The module can implement its
6122 C<import> method any way it likes, though most modules just choose to
6123 derive their C<import> method via inheritance from the C<Exporter> class that
6124 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6125 method can be found then the call is skipped.
6127 If you do not want to call the package's C<import> method (for instance,
6128 to stop your namespace from being altered), explicitly supply the empty list:
6132 That is exactly equivalent to
6134 BEGIN { require Module }
6136 If the VERSION argument is present between Module and LIST, then the
6137 C<use> will call the VERSION method in class Module with the given
6138 version as an argument. The default VERSION method, inherited from
6139 the UNIVERSAL class, croaks if the given version is larger than the
6140 value of the variable C<$Module::VERSION>.
6142 Again, there is a distinction between omitting LIST (C<import> called
6143 with no arguments) and an explicit empty LIST C<()> (C<import> not
6144 called). Note that there is no comma after VERSION!
6146 Because this is a wide-open interface, pragmas (compiler directives)
6147 are also implemented this way. Currently implemented pragmas are:
6152 use sigtrap qw(SEGV BUS);
6153 use strict qw(subs vars refs);
6154 use subs qw(afunc blurfl);
6155 use warnings qw(all);
6156 use sort qw(stable _quicksort _mergesort);
6158 Some of these pseudo-modules import semantics into the current
6159 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6160 which import symbols into the current package (which are effective
6161 through the end of the file).
6163 There's a corresponding C<no> command that unimports meanings imported
6164 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6170 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6171 for the C<-M> and C<-m> command-line options to perl that give C<use>
6172 functionality from the command-line.
6176 Changes the access and modification times on each file of a list of
6177 files. The first two elements of the list must be the NUMERICAL access
6178 and modification times, in that order. Returns the number of files
6179 successfully changed. The inode change time of each file is set
6180 to the current time. For example, this code has the same effect as the
6181 Unix touch(1) command when the files I<already exist>.
6185 utime $now, $now, @ARGV;
6187 B<Note:> Under NFS, touch(1) uses the time of the NFS server, not
6188 the time of the local machine. If there is a time synchronization
6189 problem, the NFS server and local machine will have different times.
6191 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6192 the utime(2) function in the C library will be called with a null second
6193 argument. On most systems, this will set the file's access and
6194 modification times to the current time (i.e. equivalent to the example
6197 utime undef, undef, @ARGV;
6201 Returns a list consisting of all the values of the named hash. (In a
6202 scalar context, returns the number of values.) The values are
6203 returned in an apparently random order. The actual random order is
6204 subject to change in future versions of perl, but it is guaranteed to
6205 be the same order as either the C<keys> or C<each> function would
6206 produce on the same (unmodified) hash.
6208 Note that the values are not copied, which means modifying them will
6209 modify the contents of the hash:
6211 for (values %hash) { s/foo/bar/g } # modifies %hash values
6212 for (@hash{keys %hash}) { s/foo/bar/g } # same
6214 As a side effect, calling values() resets the HASH's internal iterator.
6215 See also C<keys>, C<each>, and C<sort>.
6217 =item vec EXPR,OFFSET,BITS
6219 Treats the string in EXPR as a bit vector made up of elements of
6220 width BITS, and returns the value of the element specified by OFFSET
6221 as an unsigned integer. BITS therefore specifies the number of bits
6222 that are reserved for each element in the bit vector. This must
6223 be a power of two from 1 to 32 (or 64, if your platform supports
6226 If BITS is 8, "elements" coincide with bytes of the input string.
6228 If BITS is 16 or more, bytes of the input string are grouped into chunks
6229 of size BITS/8, and each group is converted to a number as with
6230 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6231 for BITS==64). See L<"pack"> for details.
6233 If bits is 4 or less, the string is broken into bytes, then the bits
6234 of each byte are broken into 8/BITS groups. Bits of a byte are
6235 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6236 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6237 breaking the single input byte C<chr(0x36)> into two groups gives a list
6238 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6240 C<vec> may also be assigned to, in which case parentheses are needed
6241 to give the expression the correct precedence as in
6243 vec($image, $max_x * $x + $y, 8) = 3;
6245 If the selected element is outside the string, the value 0 is returned.
6246 If an element off the end of the string is written to, Perl will first
6247 extend the string with sufficiently many zero bytes. It is an error
6248 to try to write off the beginning of the string (i.e. negative OFFSET).
6250 The string should not contain any character with the value > 255 (which
6251 can only happen if you're using UTF8 encoding). If it does, it will be
6252 treated as something which is not UTF8 encoded. When the C<vec> was
6253 assigned to, other parts of your program will also no longer consider the
6254 string to be UTF8 encoded. In other words, if you do have such characters
6255 in your string, vec() will operate on the actual byte string, and not the
6256 conceptual character string.
6258 Strings created with C<vec> can also be manipulated with the logical
6259 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6260 vector operation is desired when both operands are strings.
6261 See L<perlop/"Bitwise String Operators">.
6263 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6264 The comments show the string after each step. Note that this code works
6265 in the same way on big-endian or little-endian machines.
6268 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6270 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6271 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6273 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6274 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6275 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6276 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6277 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6278 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6280 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6281 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6282 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6285 To transform a bit vector into a string or list of 0's and 1's, use these:
6287 $bits = unpack("b*", $vector);
6288 @bits = split(//, unpack("b*", $vector));
6290 If you know the exact length in bits, it can be used in place of the C<*>.
6292 Here is an example to illustrate how the bits actually fall in place:
6298 unpack("V",$_) 01234567890123456789012345678901
6299 ------------------------------------------------------------------
6304 for ($shift=0; $shift < $width; ++$shift) {
6305 for ($off=0; $off < 32/$width; ++$off) {
6306 $str = pack("B*", "0"x32);
6307 $bits = (1<<$shift);
6308 vec($str, $off, $width) = $bits;
6309 $res = unpack("b*",$str);
6310 $val = unpack("V", $str);
6317 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6318 $off, $width, $bits, $val, $res
6322 Regardless of the machine architecture on which it is run, the above
6323 example should print the following table:
6326 unpack("V",$_) 01234567890123456789012345678901
6327 ------------------------------------------------------------------
6328 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6329 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6330 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6331 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6332 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6333 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6334 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6335 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6336 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6337 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6338 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6339 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6340 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6341 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6342 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6343 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6344 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6345 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6346 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6347 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6348 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6349 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6350 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6351 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6352 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6353 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6354 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6355 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6356 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6357 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6358 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6359 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6360 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6361 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6362 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6363 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6364 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6365 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6366 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6367 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6368 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6369 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6370 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6371 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6372 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6373 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6374 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6375 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6376 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6377 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6378 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6379 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6380 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6381 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6382 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6383 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6384 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6385 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6386 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6387 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6388 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6389 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6390 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6391 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6392 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6393 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6394 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6395 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6396 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6397 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6398 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6399 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6400 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6401 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6402 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6403 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6404 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6405 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6406 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6407 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6408 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6409 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6410 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6411 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6412 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6413 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6414 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6415 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6416 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6417 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6418 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6419 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6420 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6421 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6422 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6423 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6424 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6425 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6426 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6427 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6428 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6429 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6430 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6431 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6432 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6433 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6434 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6435 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6436 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6437 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6438 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6439 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6440 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6441 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6442 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6443 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6444 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6445 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6446 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6447 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6448 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6449 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6450 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6451 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6452 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6453 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6454 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6455 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6459 Behaves like the wait(2) system call on your system: it waits for a child
6460 process to terminate and returns the pid of the deceased process, or
6461 C<-1> if there are no child processes. The status is returned in C<$?>.
6462 Note that a return value of C<-1> could mean that child processes are
6463 being automatically reaped, as described in L<perlipc>.
6465 =item waitpid PID,FLAGS
6467 Waits for a particular child process to terminate and returns the pid of
6468 the deceased process, or C<-1> if there is no such child process. On some
6469 systems, a value of 0 indicates that there are processes still running.
6470 The status is returned in C<$?>. If you say
6472 use POSIX ":sys_wait_h";
6475 $kid = waitpid(-1, WNOHANG);
6478 then you can do a non-blocking wait for all pending zombie processes.
6479 Non-blocking wait is available on machines supporting either the
6480 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6481 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6482 system call by remembering the status values of processes that have
6483 exited but have not been harvested by the Perl script yet.)
6485 Note that on some systems, a return value of C<-1> could mean that child
6486 processes are being automatically reaped. See L<perlipc> for details,
6487 and for other examples.
6491 Returns true if the context of the currently executing subroutine is
6492 looking for a list value. Returns false if the context is looking
6493 for a scalar. Returns the undefined value if the context is looking
6494 for no value (void context).
6496 return unless defined wantarray; # don't bother doing more
6497 my @a = complex_calculation();
6498 return wantarray ? @a : "@a";
6500 This function should have been named wantlist() instead.
6504 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6507 If LIST is empty and C<$@> already contains a value (typically from a
6508 previous eval) that value is used after appending C<"\t...caught">
6509 to C<$@>. This is useful for staying almost, but not entirely similar to
6512 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6514 No message is printed if there is a C<$SIG{__WARN__}> handler
6515 installed. It is the handler's responsibility to deal with the message
6516 as it sees fit (like, for instance, converting it into a C<die>). Most
6517 handlers must therefore make arrangements to actually display the
6518 warnings that they are not prepared to deal with, by calling C<warn>
6519 again in the handler. Note that this is quite safe and will not
6520 produce an endless loop, since C<__WARN__> hooks are not called from
6523 You will find this behavior is slightly different from that of
6524 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6525 instead call C<die> again to change it).
6527 Using a C<__WARN__> handler provides a powerful way to silence all
6528 warnings (even the so-called mandatory ones). An example:
6530 # wipe out *all* compile-time warnings
6531 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6533 my $foo = 20; # no warning about duplicate my $foo,
6534 # but hey, you asked for it!
6535 # no compile-time or run-time warnings before here
6538 # run-time warnings enabled after here
6539 warn "\$foo is alive and $foo!"; # does show up
6541 See L<perlvar> for details on setting C<%SIG> entries, and for more
6542 examples. See the Carp module for other kinds of warnings using its
6543 carp() and cluck() functions.
6545 =item write FILEHANDLE
6551 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6552 using the format associated with that file. By default the format for
6553 a file is the one having the same name as the filehandle, but the
6554 format for the current output channel (see the C<select> function) may be set
6555 explicitly by assigning the name of the format to the C<$~> variable.
6557 Top of form processing is handled automatically: if there is
6558 insufficient room on the current page for the formatted record, the
6559 page is advanced by writing a form feed, a special top-of-page format
6560 is used to format the new page header, and then the record is written.
6561 By default the top-of-page format is the name of the filehandle with
6562 "_TOP" appended, but it may be dynamically set to the format of your
6563 choice by assigning the name to the C<$^> variable while the filehandle is
6564 selected. The number of lines remaining on the current page is in
6565 variable C<$->, which can be set to C<0> to force a new page.
6567 If FILEHANDLE is unspecified, output goes to the current default output
6568 channel, which starts out as STDOUT but may be changed by the
6569 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6570 is evaluated and the resulting string is used to look up the name of
6571 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6573 Note that write is I<not> the opposite of C<read>. Unfortunately.
6577 The transliteration operator. Same as C<tr///>. See L<perlop>.