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 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
729 Returns the character represented by that NUMBER in the character set.
730 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
731 chr(0x263a) is a Unicode smiley face. Note that characters from 127
732 to 255 (inclusive) are by default not encoded in Unicode for backward
733 compatibility reasons (but see L<encoding>).
735 If NUMBER is omitted, uses C<$_>.
737 For the reverse, use L</ord>.
739 Note that under the C<bytes> pragma the NUMBER is masked to
742 See L<perlunicode> and L<encoding> for more about Unicode.
744 =item chroot FILENAME
748 This function works like the system call by the same name: it makes the
749 named directory the new root directory for all further pathnames that
750 begin with a C</> by your process and all its children. (It doesn't
751 change your current working directory, which is unaffected.) For security
752 reasons, this call is restricted to the superuser. If FILENAME is
753 omitted, does a C<chroot> to C<$_>.
755 =item close FILEHANDLE
759 Closes the file or pipe associated with the file handle, returning
760 true only if IO buffers are successfully flushed and closes the system
761 file descriptor. Closes the currently selected filehandle if the
764 You don't have to close FILEHANDLE if you are immediately going to do
765 another C<open> on it, because C<open> will close it for you. (See
766 C<open>.) However, an explicit C<close> on an input file resets the line
767 counter (C<$.>), while the implicit close done by C<open> does not.
769 If the file handle came from a piped open C<close> will additionally
770 return false if one of the other system calls involved fails or if the
771 program exits with non-zero status. (If the only problem was that the
772 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
773 also waits for the process executing on the pipe to complete, in case you
774 want to look at the output of the pipe afterwards, and
775 implicitly puts the exit status value of that command into C<$?>.
777 Prematurely closing the read end of a pipe (i.e. before the process
778 writing to it at the other end has closed it) will result in a
779 SIGPIPE being delivered to the writer. If the other end can't
780 handle that, be sure to read all the data before closing the pipe.
784 open(OUTPUT, '|sort >foo') # pipe to sort
785 or die "Can't start sort: $!";
786 #... # print stuff to output
787 close OUTPUT # wait for sort to finish
788 or warn $! ? "Error closing sort pipe: $!"
789 : "Exit status $? from sort";
790 open(INPUT, 'foo') # get sort's results
791 or die "Can't open 'foo' for input: $!";
793 FILEHANDLE may be an expression whose value can be used as an indirect
794 filehandle, usually the real filehandle name.
796 =item closedir DIRHANDLE
798 Closes a directory opened by C<opendir> and returns the success of that
801 =item connect SOCKET,NAME
803 Attempts to connect to a remote socket, just as the connect system call
804 does. Returns true if it succeeded, false otherwise. NAME should be a
805 packed address of the appropriate type for the socket. See the examples in
806 L<perlipc/"Sockets: Client/Server Communication">.
810 Actually a flow control statement rather than a function. If there is a
811 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
812 C<foreach>), it is always executed just before the conditional is about to
813 be evaluated again, just like the third part of a C<for> loop in C. Thus
814 it can be used to increment a loop variable, even when the loop has been
815 continued via the C<next> statement (which is similar to the C C<continue>
818 C<last>, C<next>, or C<redo> may appear within a C<continue>
819 block. C<last> and C<redo> will behave as if they had been executed within
820 the main block. So will C<next>, but since it will execute a C<continue>
821 block, it may be more entertaining.
824 ### redo always comes here
827 ### next always comes here
829 # then back the top to re-check EXPR
831 ### last always comes here
833 Omitting the C<continue> section is semantically equivalent to using an
834 empty one, logically enough. In that case, C<next> goes directly back
835 to check the condition at the top of the loop.
841 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
842 takes cosine of C<$_>.
844 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
845 function, or use this relation:
847 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
849 =item crypt PLAINTEXT,SALT
851 Encrypts a string exactly like the crypt(3) function in the C library
852 (assuming that you actually have a version there that has not been
853 extirpated as a potential munition). This can prove useful for checking
854 the password file for lousy passwords, amongst other things. Only the
855 guys wearing white hats should do this.
857 Note that L<crypt|/crypt> is intended to be a one-way function, much like
858 breaking eggs to make an omelette. There is no (known) corresponding
859 decrypt function (in other words, the crypt() is a one-way hash
860 function). As a result, this function isn't all that useful for
861 cryptography. (For that, see your nearby CPAN mirror.)
863 When verifying an existing encrypted string you should use the
864 encrypted text as the salt (like C<crypt($plain, $crypted) eq
865 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
866 and with more exotic implementations. In other words, do not assume
867 anything about the returned string itself, or how many bytes in
868 the encrypted string matter.
870 Traditionally the result is a string of 13 bytes: two first bytes of
871 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
872 the first eight bytes of the encrypted string mattered, but
873 alternative hashing schemes (like MD5), higher level security schemes
874 (like C2), and implementations on non-UNIX platforms may produce
877 When choosing a new salt create a random two character string whose
878 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
879 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
880 characters is just a recommendation; the characters allowed in
881 the salt depend solely on your system's crypt library, and Perl can't
882 restrict what salts C<crypt()> accepts.
884 Here's an example that makes sure that whoever runs this program knows
887 $pwd = (getpwuid($<))[1];
891 chomp($word = <STDIN>);
895 if (crypt($word, $pwd) ne $pwd) {
901 Of course, typing in your own password to whoever asks you
904 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
905 of data, not least of all because you can't get the information
906 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
907 on your favorite CPAN mirror for a slew of potentially useful
910 If using crypt() on a Unicode string (which I<potentially> has
911 characters with codepoints above 255), Perl tries to make sense
912 of the situation by trying to downgrade (a copy of the string)
913 the string back to an eight-bit byte string before calling crypt()
914 (on that copy). If that works, good. If not, crypt() dies with
915 C<Wide character in crypt>.
919 [This function has been largely superseded by the C<untie> function.]
921 Breaks the binding between a DBM file and a hash.
923 =item dbmopen HASH,DBNAME,MASK
925 [This function has been largely superseded by the C<tie> function.]
927 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
928 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
929 argument is I<not> a filehandle, even though it looks like one). DBNAME
930 is the name of the database (without the F<.dir> or F<.pag> extension if
931 any). If the database does not exist, it is created with protection
932 specified by MASK (as modified by the C<umask>). If your system supports
933 only the older DBM functions, you may perform only one C<dbmopen> in your
934 program. In older versions of Perl, if your system had neither DBM nor
935 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
938 If you don't have write access to the DBM file, you can only read hash
939 variables, not set them. If you want to test whether you can write,
940 either use file tests or try setting a dummy hash entry inside an C<eval>,
941 which will trap the error.
943 Note that functions such as C<keys> and C<values> may return huge lists
944 when used on large DBM files. You may prefer to use the C<each>
945 function to iterate over large DBM files. Example:
947 # print out history file offsets
948 dbmopen(%HIST,'/usr/lib/news/history',0666);
949 while (($key,$val) = each %HIST) {
950 print $key, ' = ', unpack('L',$val), "\n";
954 See also L<AnyDBM_File> for a more general description of the pros and
955 cons of the various dbm approaches, as well as L<DB_File> for a particularly
958 You can control which DBM library you use by loading that library
959 before you call dbmopen():
962 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
963 or die "Can't open netscape history file: $!";
969 Returns a Boolean value telling whether EXPR has a value other than
970 the undefined value C<undef>. If EXPR is not present, C<$_> will be
973 Many operations return C<undef> to indicate failure, end of file,
974 system error, uninitialized variable, and other exceptional
975 conditions. This function allows you to distinguish C<undef> from
976 other values. (A simple Boolean test will not distinguish among
977 C<undef>, zero, the empty string, and C<"0">, which are all equally
978 false.) Note that since C<undef> is a valid scalar, its presence
979 doesn't I<necessarily> indicate an exceptional condition: C<pop>
980 returns C<undef> when its argument is an empty array, I<or> when the
981 element to return happens to be C<undef>.
983 You may also use C<defined(&func)> to check whether subroutine C<&func>
984 has ever been defined. The return value is unaffected by any forward
985 declarations of C<&func>. Note that a subroutine which is not defined
986 may still be callable: its package may have an C<AUTOLOAD> method that
987 makes it spring into existence the first time that it is called -- see
990 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
991 used to report whether memory for that aggregate has ever been
992 allocated. This behavior may disappear in future versions of Perl.
993 You should instead use a simple test for size:
995 if (@an_array) { print "has array elements\n" }
996 if (%a_hash) { print "has hash members\n" }
998 When used on a hash element, it tells you whether the value is defined,
999 not whether the key exists in the hash. Use L</exists> for the latter
1004 print if defined $switch{'D'};
1005 print "$val\n" while defined($val = pop(@ary));
1006 die "Can't readlink $sym: $!"
1007 unless defined($value = readlink $sym);
1008 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1009 $debugging = 0 unless defined $debugging;
1011 Note: Many folks tend to overuse C<defined>, and then are surprised to
1012 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1013 defined values. For example, if you say
1017 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1018 matched "nothing". But it didn't really match nothing--rather, it
1019 matched something that happened to be zero characters long. This is all
1020 very above-board and honest. When a function returns an undefined value,
1021 it's an admission that it couldn't give you an honest answer. So you
1022 should use C<defined> only when you're questioning the integrity of what
1023 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1026 See also L</undef>, L</exists>, L</ref>.
1030 Given an expression that specifies a hash element, array element, hash slice,
1031 or array slice, deletes the specified element(s) from the hash or array.
1032 In the case of an array, if the array elements happen to be at the end,
1033 the size of the array will shrink to the highest element that tests
1034 true for exists() (or 0 if no such element exists).
1036 Returns each element so deleted or the undefined value if there was no such
1037 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1038 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1039 from a C<tie>d hash or array may not necessarily return anything.
1041 Deleting an array element effectively returns that position of the array
1042 to its initial, uninitialized state. Subsequently testing for the same
1043 element with exists() will return false. Note that deleting array
1044 elements in the middle of an array will not shift the index of the ones
1045 after them down--use splice() for that. See L</exists>.
1047 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1049 foreach $key (keys %HASH) {
1053 foreach $index (0 .. $#ARRAY) {
1054 delete $ARRAY[$index];
1059 delete @HASH{keys %HASH};
1061 delete @ARRAY[0 .. $#ARRAY];
1063 But both of these are slower than just assigning the empty list
1064 or undefining %HASH or @ARRAY:
1066 %HASH = (); # completely empty %HASH
1067 undef %HASH; # forget %HASH ever existed
1069 @ARRAY = (); # completely empty @ARRAY
1070 undef @ARRAY; # forget @ARRAY ever existed
1072 Note that the EXPR can be arbitrarily complicated as long as the final
1073 operation is a hash element, array element, hash slice, or array slice
1076 delete $ref->[$x][$y]{$key};
1077 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1079 delete $ref->[$x][$y][$index];
1080 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1084 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1085 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1086 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1087 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1088 an C<eval(),> the error message is stuffed into C<$@> and the
1089 C<eval> is terminated with the undefined value. This makes
1090 C<die> the way to raise an exception.
1092 Equivalent examples:
1094 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1095 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1097 If the last element of LIST does not end in a newline, the current
1098 script line number and input line number (if any) are also printed,
1099 and a newline is supplied. Note that the "input line number" (also
1100 known as "chunk") is subject to whatever notion of "line" happens to
1101 be currently in effect, and is also available as the special variable
1102 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1104 Hint: sometimes appending C<", stopped"> to your message will cause it
1105 to make better sense when the string C<"at foo line 123"> is appended.
1106 Suppose you are running script "canasta".
1108 die "/etc/games is no good";
1109 die "/etc/games is no good, stopped";
1111 produce, respectively
1113 /etc/games is no good at canasta line 123.
1114 /etc/games is no good, stopped at canasta line 123.
1116 See also exit(), warn(), and the Carp module.
1118 If LIST is empty and C<$@> already contains a value (typically from a
1119 previous eval) that value is reused after appending C<"\t...propagated">.
1120 This is useful for propagating exceptions:
1123 die unless $@ =~ /Expected exception/;
1125 If LIST is empty and C<$@> contains an object reference that has a
1126 C<PROPAGATE> method, that method will be called with additional file
1127 and line number parameters. The return value replaces the value in
1128 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1131 If C<$@> is empty then the string C<"Died"> is used.
1133 die() can also be called with a reference argument. If this happens to be
1134 trapped within an eval(), $@ contains the reference. This behavior permits
1135 a more elaborate exception handling implementation using objects that
1136 maintain arbitrary state about the nature of the exception. Such a scheme
1137 is sometimes preferable to matching particular string values of $@ using
1138 regular expressions. Here's an example:
1140 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1142 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1143 # handle Some::Module::Exception
1146 # handle all other possible exceptions
1150 Because perl will stringify uncaught exception messages before displaying
1151 them, you may want to overload stringification operations on such custom
1152 exception objects. See L<overload> for details about that.
1154 You can arrange for a callback to be run just before the C<die>
1155 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1156 handler will be called with the error text and can change the error
1157 message, if it sees fit, by calling C<die> again. See
1158 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1159 L<"eval BLOCK"> for some examples. Although this feature was meant
1160 to be run only right before your program was to exit, this is not
1161 currently the case--the C<$SIG{__DIE__}> hook is currently called
1162 even inside eval()ed blocks/strings! If one wants the hook to do
1163 nothing in such situations, put
1167 as the first line of the handler (see L<perlvar/$^S>). Because
1168 this promotes strange action at a distance, this counterintuitive
1169 behavior may be fixed in a future release.
1173 Not really a function. Returns the value of the last command in the
1174 sequence of commands indicated by BLOCK. When modified by a loop
1175 modifier, executes the BLOCK once before testing the loop condition.
1176 (On other statements the loop modifiers test the conditional first.)
1178 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1179 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1180 See L<perlsyn> for alternative strategies.
1182 =item do SUBROUTINE(LIST)
1184 A deprecated form of subroutine call. See L<perlsub>.
1188 Uses the value of EXPR as a filename and executes the contents of the
1189 file as a Perl script. Its primary use is to include subroutines
1190 from a Perl subroutine library.
1198 except that it's more efficient and concise, keeps track of the current
1199 filename for error messages, searches the @INC libraries, and updates
1200 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1201 variables. It also differs in that code evaluated with C<do FILENAME>
1202 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1203 same, however, in that it does reparse the file every time you call it,
1204 so you probably don't want to do this inside a loop.
1206 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1207 error. If C<do> can read the file but cannot compile it, it
1208 returns undef and sets an error message in C<$@>. If the file is
1209 successfully compiled, C<do> returns the value of the last expression
1212 Note that inclusion of library modules is better done with the
1213 C<use> and C<require> operators, which also do automatic error checking
1214 and raise an exception if there's a problem.
1216 You might like to use C<do> to read in a program configuration
1217 file. Manual error checking can be done this way:
1219 # read in config files: system first, then user
1220 for $file ("/share/prog/defaults.rc",
1221 "$ENV{HOME}/.someprogrc")
1223 unless ($return = do $file) {
1224 warn "couldn't parse $file: $@" if $@;
1225 warn "couldn't do $file: $!" unless defined $return;
1226 warn "couldn't run $file" unless $return;
1234 This function causes an immediate core dump. See also the B<-u>
1235 command-line switch in L<perlrun>, which does the same thing.
1236 Primarily this is so that you can use the B<undump> program (not
1237 supplied) to turn your core dump into an executable binary after
1238 having initialized all your variables at the beginning of the
1239 program. When the new binary is executed it will begin by executing
1240 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1241 Think of it as a goto with an intervening core dump and reincarnation.
1242 If C<LABEL> is omitted, restarts the program from the top.
1244 B<WARNING>: Any files opened at the time of the dump will I<not>
1245 be open any more when the program is reincarnated, with possible
1246 resulting confusion on the part of Perl.
1248 This function is now largely obsolete, partly because it's very
1249 hard to convert a core file into an executable, and because the
1250 real compiler backends for generating portable bytecode and compilable
1251 C code have superseded it. That's why you should now invoke it as
1252 C<CORE::dump()>, if you don't want to be warned against a possible
1255 If you're looking to use L<dump> to speed up your program, consider
1256 generating bytecode or native C code as described in L<perlcc>. If
1257 you're just trying to accelerate a CGI script, consider using the
1258 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1259 You might also consider autoloading or selfloading, which at least
1260 make your program I<appear> to run faster.
1264 When called in list context, returns a 2-element list consisting of the
1265 key and value for the next element of a hash, so that you can iterate over
1266 it. When called in scalar context, returns only the key for the next
1267 element in the hash.
1269 Entries are returned in an apparently random order. The actual random
1270 order is subject to change in future versions of perl, but it is guaranteed
1271 to be in the same order as either the C<keys> or C<values> function
1272 would produce on the same (unmodified) hash.
1274 When the hash is entirely read, a null array is returned in list context
1275 (which when assigned produces a false (C<0>) value), and C<undef> in
1276 scalar context. The next call to C<each> after that will start iterating
1277 again. There is a single iterator for each hash, shared by all C<each>,
1278 C<keys>, and C<values> function calls in the program; it can be reset by
1279 reading all the elements from the hash, or by evaluating C<keys HASH> or
1280 C<values HASH>. If you add or delete elements of a hash while you're
1281 iterating over it, you may get entries skipped or duplicated, so
1282 don't. Exception: It is always safe to delete the item most recently
1283 returned by C<each()>, which means that the following code will work:
1285 while (($key, $value) = each %hash) {
1287 delete $hash{$key}; # This is safe
1290 The following prints out your environment like the printenv(1) program,
1291 only in a different order:
1293 while (($key,$value) = each %ENV) {
1294 print "$key=$value\n";
1297 See also C<keys>, C<values> and C<sort>.
1299 =item eof FILEHANDLE
1305 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1306 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1307 gives the real filehandle. (Note that this function actually
1308 reads a character and then C<ungetc>s it, so isn't very useful in an
1309 interactive context.) Do not read from a terminal file (or call
1310 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1311 as terminals may lose the end-of-file condition if you do.
1313 An C<eof> without an argument uses the last file read. Using C<eof()>
1314 with empty parentheses is very different. It refers to the pseudo file
1315 formed from the files listed on the command line and accessed via the
1316 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1317 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1318 used will cause C<@ARGV> to be examined to determine if input is
1319 available. Similarly, an C<eof()> after C<< <> >> has returned
1320 end-of-file will assume you are processing another C<@ARGV> list,
1321 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1322 see L<perlop/"I/O Operators">.
1324 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1325 detect the end of each file, C<eof()> will only detect the end of the
1326 last file. Examples:
1328 # reset line numbering on each input file
1330 next if /^\s*#/; # skip comments
1333 close ARGV if eof; # Not eof()!
1336 # insert dashes just before last line of last file
1338 if (eof()) { # check for end of last file
1339 print "--------------\n";
1342 last if eof(); # needed if we're reading from a terminal
1345 Practical hint: you almost never need to use C<eof> in Perl, because the
1346 input operators typically return C<undef> when they run out of data, or if
1353 In the first form, the return value of EXPR is parsed and executed as if it
1354 were a little Perl program. The value of the expression (which is itself
1355 determined within scalar context) is first parsed, and if there weren't any
1356 errors, executed in the lexical context of the current Perl program, so
1357 that any variable settings or subroutine and format definitions remain
1358 afterwards. Note that the value is parsed every time the eval executes.
1359 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1360 delay parsing and subsequent execution of the text of EXPR until run time.
1362 In the second form, the code within the BLOCK is parsed only once--at the
1363 same time the code surrounding the eval itself was parsed--and executed
1364 within the context of the current Perl program. This form is typically
1365 used to trap exceptions more efficiently than the first (see below), while
1366 also providing the benefit of checking the code within BLOCK at compile
1369 The final semicolon, if any, may be omitted from the value of EXPR or within
1372 In both forms, the value returned is the value of the last expression
1373 evaluated inside the mini-program; a return statement may be also used, just
1374 as with subroutines. The expression providing the return value is evaluated
1375 in void, scalar, or list context, depending on the context of the eval itself.
1376 See L</wantarray> for more on how the evaluation context can be determined.
1378 If there is a syntax error or runtime error, or a C<die> statement is
1379 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1380 error message. If there was no error, C<$@> is guaranteed to be a null
1381 string. Beware that using C<eval> neither silences perl from printing
1382 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1383 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1384 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1385 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1387 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1388 determining whether a particular feature (such as C<socket> or C<symlink>)
1389 is implemented. It is also Perl's exception trapping mechanism, where
1390 the die operator is used to raise exceptions.
1392 If the code to be executed doesn't vary, you may use the eval-BLOCK
1393 form to trap run-time errors without incurring the penalty of
1394 recompiling each time. The error, if any, is still returned in C<$@>.
1397 # make divide-by-zero nonfatal
1398 eval { $answer = $a / $b; }; warn $@ if $@;
1400 # same thing, but less efficient
1401 eval '$answer = $a / $b'; warn $@ if $@;
1403 # a compile-time error
1404 eval { $answer = }; # WRONG
1407 eval '$answer ='; # sets $@
1409 Due to the current arguably broken state of C<__DIE__> hooks, when using
1410 the C<eval{}> form as an exception trap in libraries, you may wish not
1411 to trigger any C<__DIE__> hooks that user code may have installed.
1412 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1413 as shown in this example:
1415 # a very private exception trap for divide-by-zero
1416 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1419 This is especially significant, given that C<__DIE__> hooks can call
1420 C<die> again, which has the effect of changing their error messages:
1422 # __DIE__ hooks may modify error messages
1424 local $SIG{'__DIE__'} =
1425 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1426 eval { die "foo lives here" };
1427 print $@ if $@; # prints "bar lives here"
1430 Because this promotes action at a distance, this counterintuitive behavior
1431 may be fixed in a future release.
1433 With an C<eval>, you should be especially careful to remember what's
1434 being looked at when:
1440 eval { $x }; # CASE 4
1442 eval "\$$x++"; # CASE 5
1445 Cases 1 and 2 above behave identically: they run the code contained in
1446 the variable $x. (Although case 2 has misleading double quotes making
1447 the reader wonder what else might be happening (nothing is).) Cases 3
1448 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1449 does nothing but return the value of $x. (Case 4 is preferred for
1450 purely visual reasons, but it also has the advantage of compiling at
1451 compile-time instead of at run-time.) Case 5 is a place where
1452 normally you I<would> like to use double quotes, except that in this
1453 particular situation, you can just use symbolic references instead, as
1456 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1457 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1459 Note that as a very special case, an C<eval ''> executed within the C<DB>
1460 package doesn't see the usual surrounding lexical scope, but rather the
1461 scope of the first non-DB piece of code that called it. You don't normally
1462 need to worry about this unless you are writing a Perl debugger.
1466 =item exec PROGRAM LIST
1468 The C<exec> function executes a system command I<and never returns>--
1469 use C<system> instead of C<exec> if you want it to return. It fails and
1470 returns false only if the command does not exist I<and> it is executed
1471 directly instead of via your system's command shell (see below).
1473 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1474 warns you if there is a following statement which isn't C<die>, C<warn>,
1475 or C<exit> (if C<-w> is set - but you always do that). If you
1476 I<really> want to follow an C<exec> with some other statement, you
1477 can use one of these styles to avoid the warning:
1479 exec ('foo') or print STDERR "couldn't exec foo: $!";
1480 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1482 If there is more than one argument in LIST, or if LIST is an array
1483 with more than one value, calls execvp(3) with the arguments in LIST.
1484 If there is only one scalar argument or an array with one element in it,
1485 the argument is checked for shell metacharacters, and if there are any,
1486 the entire argument is passed to the system's command shell for parsing
1487 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1488 If there are no shell metacharacters in the argument, it is split into
1489 words and passed directly to C<execvp>, which is more efficient.
1492 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1493 exec "sort $outfile | uniq";
1495 If you don't really want to execute the first argument, but want to lie
1496 to the program you are executing about its own name, you can specify
1497 the program you actually want to run as an "indirect object" (without a
1498 comma) in front of the LIST. (This always forces interpretation of the
1499 LIST as a multivalued list, even if there is only a single scalar in
1502 $shell = '/bin/csh';
1503 exec $shell '-sh'; # pretend it's a login shell
1507 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1509 When the arguments get executed via the system shell, results will
1510 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1513 Using an indirect object with C<exec> or C<system> is also more
1514 secure. This usage (which also works fine with system()) forces
1515 interpretation of the arguments as a multivalued list, even if the
1516 list had just one argument. That way you're safe from the shell
1517 expanding wildcards or splitting up words with whitespace in them.
1519 @args = ( "echo surprise" );
1521 exec @args; # subject to shell escapes
1523 exec { $args[0] } @args; # safe even with one-arg list
1525 The first version, the one without the indirect object, ran the I<echo>
1526 program, passing it C<"surprise"> an argument. The second version
1527 didn't--it tried to run a program literally called I<"echo surprise">,
1528 didn't find it, and set C<$?> to a non-zero value indicating failure.
1530 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1531 output before the exec, but this may not be supported on some platforms
1532 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1533 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1534 open handles in order to avoid lost output.
1536 Note that C<exec> will not call your C<END> blocks, nor will it call
1537 any C<DESTROY> methods in your objects.
1541 Given an expression that specifies a hash element or array element,
1542 returns true if the specified element in the hash or array has ever
1543 been initialized, even if the corresponding value is undefined. The
1544 element is not autovivified if it doesn't exist.
1546 print "Exists\n" if exists $hash{$key};
1547 print "Defined\n" if defined $hash{$key};
1548 print "True\n" if $hash{$key};
1550 print "Exists\n" if exists $array[$index];
1551 print "Defined\n" if defined $array[$index];
1552 print "True\n" if $array[$index];
1554 A hash or array element can be true only if it's defined, and defined if
1555 it exists, but the reverse doesn't necessarily hold true.
1557 Given an expression that specifies the name of a subroutine,
1558 returns true if the specified subroutine has ever been declared, even
1559 if it is undefined. Mentioning a subroutine name for exists or defined
1560 does not count as declaring it. Note that a subroutine which does not
1561 exist may still be callable: its package may have an C<AUTOLOAD>
1562 method that makes it spring into existence the first time that it is
1563 called -- see L<perlsub>.
1565 print "Exists\n" if exists &subroutine;
1566 print "Defined\n" if defined &subroutine;
1568 Note that the EXPR can be arbitrarily complicated as long as the final
1569 operation is a hash or array key lookup or subroutine name:
1571 if (exists $ref->{A}->{B}->{$key}) { }
1572 if (exists $hash{A}{B}{$key}) { }
1574 if (exists $ref->{A}->{B}->[$ix]) { }
1575 if (exists $hash{A}{B}[$ix]) { }
1577 if (exists &{$ref->{A}{B}{$key}}) { }
1579 Although the deepest nested array or hash will not spring into existence
1580 just because its existence was tested, any intervening ones will.
1581 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1582 into existence due to the existence test for the $key element above.
1583 This happens anywhere the arrow operator is used, including even:
1586 if (exists $ref->{"Some key"}) { }
1587 print $ref; # prints HASH(0x80d3d5c)
1589 This surprising autovivification in what does not at first--or even
1590 second--glance appear to be an lvalue context may be fixed in a future
1593 Use of a subroutine call, rather than a subroutine name, as an argument
1594 to exists() is an error.
1597 exists &sub(); # Error
1601 Evaluates EXPR and exits immediately with that value. Example:
1604 exit 0 if $ans =~ /^[Xx]/;
1606 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1607 universally recognized values for EXPR are C<0> for success and C<1>
1608 for error; other values are subject to interpretation depending on the
1609 environment in which the Perl program is running. For example, exiting
1610 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1611 the mailer to return the item undelivered, but that's not true everywhere.
1613 Don't use C<exit> to abort a subroutine if there's any chance that
1614 someone might want to trap whatever error happened. Use C<die> instead,
1615 which can be trapped by an C<eval>.
1617 The exit() function does not always exit immediately. It calls any
1618 defined C<END> routines first, but these C<END> routines may not
1619 themselves abort the exit. Likewise any object destructors that need to
1620 be called are called before the real exit. If this is a problem, you
1621 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1622 See L<perlmod> for details.
1628 Returns I<e> (the natural logarithm base) to the power of EXPR.
1629 If EXPR is omitted, gives C<exp($_)>.
1631 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1633 Implements the fcntl(2) function. You'll probably have to say
1637 first to get the correct constant definitions. Argument processing and
1638 value return works just like C<ioctl> below.
1642 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1643 or die "can't fcntl F_GETFL: $!";
1645 You don't have to check for C<defined> on the return from C<fnctl>.
1646 Like C<ioctl>, it maps a C<0> return from the system call into
1647 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1648 in numeric context. It is also exempt from the normal B<-w> warnings
1649 on improper numeric conversions.
1651 Note that C<fcntl> will produce a fatal error if used on a machine that
1652 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1653 manpage to learn what functions are available on your system.
1655 =item fileno FILEHANDLE
1657 Returns the file descriptor for a filehandle, or undefined if the
1658 filehandle is not open. This is mainly useful for constructing
1659 bitmaps for C<select> and low-level POSIX tty-handling operations.
1660 If FILEHANDLE is an expression, the value is taken as an indirect
1661 filehandle, generally its name.
1663 You can use this to find out whether two handles refer to the
1664 same underlying descriptor:
1666 if (fileno(THIS) == fileno(THAT)) {
1667 print "THIS and THAT are dups\n";
1670 (Filehandles connected to memory objects via new features of C<open> may
1671 return undefined even though they are open.)
1674 =item flock FILEHANDLE,OPERATION
1676 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1677 for success, false on failure. Produces a fatal error if used on a
1678 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1679 C<flock> is Perl's portable file locking interface, although it locks
1680 only entire files, not records.
1682 Two potentially non-obvious but traditional C<flock> semantics are
1683 that it waits indefinitely until the lock is granted, and that its locks
1684 B<merely advisory>. Such discretionary locks are more flexible, but offer
1685 fewer guarantees. This means that files locked with C<flock> may be
1686 modified by programs that do not also use C<flock>. See L<perlport>,
1687 your port's specific documentation, or your system-specific local manpages
1688 for details. It's best to assume traditional behavior if you're writing
1689 portable programs. (But if you're not, you should as always feel perfectly
1690 free to write for your own system's idiosyncrasies (sometimes called
1691 "features"). Slavish adherence to portability concerns shouldn't get
1692 in the way of your getting your job done.)
1694 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1695 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1696 you can use the symbolic names if you import them from the Fcntl module,
1697 either individually, or as a group using the ':flock' tag. LOCK_SH
1698 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1699 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1700 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1701 waiting for the lock (check the return status to see if you got it).
1703 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1704 before locking or unlocking it.
1706 Note that the emulation built with lockf(3) doesn't provide shared
1707 locks, and it requires that FILEHANDLE be open with write intent. These
1708 are the semantics that lockf(3) implements. Most if not all systems
1709 implement lockf(3) in terms of fcntl(2) locking, though, so the
1710 differing semantics shouldn't bite too many people.
1712 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1713 be open with read intent to use LOCK_SH and requires that it be open
1714 with write intent to use LOCK_EX.
1716 Note also that some versions of C<flock> cannot lock things over the
1717 network; you would need to use the more system-specific C<fcntl> for
1718 that. If you like you can force Perl to ignore your system's flock(2)
1719 function, and so provide its own fcntl(2)-based emulation, by passing
1720 the switch C<-Ud_flock> to the F<Configure> program when you configure
1723 Here's a mailbox appender for BSD systems.
1725 use Fcntl ':flock'; # import LOCK_* constants
1728 flock(MBOX,LOCK_EX);
1729 # and, in case someone appended
1730 # while we were waiting...
1735 flock(MBOX,LOCK_UN);
1738 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1739 or die "Can't open mailbox: $!";
1742 print MBOX $msg,"\n\n";
1745 On systems that support a real flock(), locks are inherited across fork()
1746 calls, whereas those that must resort to the more capricious fcntl()
1747 function lose the locks, making it harder to write servers.
1749 See also L<DB_File> for other flock() examples.
1753 Does a fork(2) system call to create a new process running the
1754 same program at the same point. It returns the child pid to the
1755 parent process, C<0> to the child process, or C<undef> if the fork is
1756 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1757 are shared, while everything else is copied. On most systems supporting
1758 fork(), great care has gone into making it extremely efficient (for
1759 example, using copy-on-write technology on data pages), making it the
1760 dominant paradigm for multitasking over the last few decades.
1762 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1763 output before forking the child process, but this may not be supported
1764 on some platforms (see L<perlport>). To be safe, you may need to set
1765 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1766 C<IO::Handle> on any open handles in order to avoid duplicate output.
1768 If you C<fork> without ever waiting on your children, you will
1769 accumulate zombies. On some systems, you can avoid this by setting
1770 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1771 forking and reaping moribund children.
1773 Note that if your forked child inherits system file descriptors like
1774 STDIN and STDOUT that are actually connected by a pipe or socket, even
1775 if you exit, then the remote server (such as, say, a CGI script or a
1776 backgrounded job launched from a remote shell) won't think you're done.
1777 You should reopen those to F</dev/null> if it's any issue.
1781 Declare a picture format for use by the C<write> function. For
1785 Test: @<<<<<<<< @||||| @>>>>>
1786 $str, $%, '$' . int($num)
1790 $num = $cost/$quantity;
1794 See L<perlform> for many details and examples.
1796 =item formline PICTURE,LIST
1798 This is an internal function used by C<format>s, though you may call it,
1799 too. It formats (see L<perlform>) a list of values according to the
1800 contents of PICTURE, placing the output into the format output
1801 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1802 Eventually, when a C<write> is done, the contents of
1803 C<$^A> are written to some filehandle, but you could also read C<$^A>
1804 yourself and then set C<$^A> back to C<"">. Note that a format typically
1805 does one C<formline> per line of form, but the C<formline> function itself
1806 doesn't care how many newlines are embedded in the PICTURE. This means
1807 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1808 You may therefore need to use multiple formlines to implement a single
1809 record format, just like the format compiler.
1811 Be careful if you put double quotes around the picture, because an C<@>
1812 character may be taken to mean the beginning of an array name.
1813 C<formline> always returns true. See L<perlform> for other examples.
1815 =item getc FILEHANDLE
1819 Returns the next character from the input file attached to FILEHANDLE,
1820 or the undefined value at end of file, or if there was an error (in
1821 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1822 STDIN. This is not particularly efficient. However, it cannot be
1823 used by itself to fetch single characters without waiting for the user
1824 to hit enter. For that, try something more like:
1827 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1830 system "stty", '-icanon', 'eol', "\001";
1836 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1839 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1843 Determination of whether $BSD_STYLE should be set
1844 is left as an exercise to the reader.
1846 The C<POSIX::getattr> function can do this more portably on
1847 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1848 module from your nearest CPAN site; details on CPAN can be found on
1853 Implements the C library function of the same name, which on most
1854 systems returns the current login from F</etc/utmp>, if any. If null,
1857 $login = getlogin || getpwuid($<) || "Kilroy";
1859 Do not consider C<getlogin> for authentication: it is not as
1860 secure as C<getpwuid>.
1862 =item getpeername SOCKET
1864 Returns the packed sockaddr address of other end of the SOCKET connection.
1867 $hersockaddr = getpeername(SOCK);
1868 ($port, $iaddr) = sockaddr_in($hersockaddr);
1869 $herhostname = gethostbyaddr($iaddr, AF_INET);
1870 $herstraddr = inet_ntoa($iaddr);
1874 Returns the current process group for the specified PID. Use
1875 a PID of C<0> to get the current process group for the
1876 current process. Will raise an exception if used on a machine that
1877 doesn't implement getpgrp(2). If PID is omitted, returns process
1878 group of current process. Note that the POSIX version of C<getpgrp>
1879 does not accept a PID argument, so only C<PID==0> is truly portable.
1883 Returns the process id of the parent process.
1885 Note for Linux users: on Linux, the C functions C<getpid()> and
1886 C<getppid()> return different values from different threads. In order to
1887 be portable, this behavior is not reflected by the perl-level function
1888 C<getppid()>, that returns a consistent value across threads. If you want
1889 to call the underlying C<getppid()>, you may use the CPAN module
1892 =item getpriority WHICH,WHO
1894 Returns the current priority for a process, a process group, or a user.
1895 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1896 machine that doesn't implement getpriority(2).
1902 =item gethostbyname NAME
1904 =item getnetbyname NAME
1906 =item getprotobyname NAME
1912 =item getservbyname NAME,PROTO
1914 =item gethostbyaddr ADDR,ADDRTYPE
1916 =item getnetbyaddr ADDR,ADDRTYPE
1918 =item getprotobynumber NUMBER
1920 =item getservbyport PORT,PROTO
1938 =item sethostent STAYOPEN
1940 =item setnetent STAYOPEN
1942 =item setprotoent STAYOPEN
1944 =item setservent STAYOPEN
1958 These routines perform the same functions as their counterparts in the
1959 system library. In list context, the return values from the
1960 various get routines are as follows:
1962 ($name,$passwd,$uid,$gid,
1963 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1964 ($name,$passwd,$gid,$members) = getgr*
1965 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1966 ($name,$aliases,$addrtype,$net) = getnet*
1967 ($name,$aliases,$proto) = getproto*
1968 ($name,$aliases,$port,$proto) = getserv*
1970 (If the entry doesn't exist you get a null list.)
1972 The exact meaning of the $gcos field varies but it usually contains
1973 the real name of the user (as opposed to the login name) and other
1974 information pertaining to the user. Beware, however, that in many
1975 system users are able to change this information and therefore it
1976 cannot be trusted and therefore the $gcos is tainted (see
1977 L<perlsec>). The $passwd and $shell, user's encrypted password and
1978 login shell, are also tainted, because of the same reason.
1980 In scalar context, you get the name, unless the function was a
1981 lookup by name, in which case you get the other thing, whatever it is.
1982 (If the entry doesn't exist you get the undefined value.) For example:
1984 $uid = getpwnam($name);
1985 $name = getpwuid($num);
1987 $gid = getgrnam($name);
1988 $name = getgrgid($num);
1992 In I<getpw*()> the fields $quota, $comment, and $expire are special
1993 cases in the sense that in many systems they are unsupported. If the
1994 $quota is unsupported, it is an empty scalar. If it is supported, it
1995 usually encodes the disk quota. If the $comment field is unsupported,
1996 it is an empty scalar. If it is supported it usually encodes some
1997 administrative comment about the user. In some systems the $quota
1998 field may be $change or $age, fields that have to do with password
1999 aging. In some systems the $comment field may be $class. The $expire
2000 field, if present, encodes the expiration period of the account or the
2001 password. For the availability and the exact meaning of these fields
2002 in your system, please consult your getpwnam(3) documentation and your
2003 F<pwd.h> file. You can also find out from within Perl what your
2004 $quota and $comment fields mean and whether you have the $expire field
2005 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2006 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2007 files are only supported if your vendor has implemented them in the
2008 intuitive fashion that calling the regular C library routines gets the
2009 shadow versions if you're running under privilege or if there exists
2010 the shadow(3) functions as found in System V ( this includes Solaris
2011 and Linux.) Those systems which implement a proprietary shadow password
2012 facility are unlikely to be supported.
2014 The $members value returned by I<getgr*()> is a space separated list of
2015 the login names of the members of the group.
2017 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2018 C, it will be returned to you via C<$?> if the function call fails. The
2019 C<@addrs> value returned by a successful call is a list of the raw
2020 addresses returned by the corresponding system library call. In the
2021 Internet domain, each address is four bytes long and you can unpack it
2022 by saying something like:
2024 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2026 The Socket library makes this slightly easier:
2029 $iaddr = inet_aton("127.1"); # or whatever address
2030 $name = gethostbyaddr($iaddr, AF_INET);
2032 # or going the other way
2033 $straddr = inet_ntoa($iaddr);
2035 If you get tired of remembering which element of the return list
2036 contains which return value, by-name interfaces are provided
2037 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2038 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2039 and C<User::grent>. These override the normal built-ins, supplying
2040 versions that return objects with the appropriate names
2041 for each field. For example:
2045 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2047 Even though it looks like they're the same method calls (uid),
2048 they aren't, because a C<File::stat> object is different from
2049 a C<User::pwent> object.
2051 =item getsockname SOCKET
2053 Returns the packed sockaddr address of this end of the SOCKET connection,
2054 in case you don't know the address because you have several different
2055 IPs that the connection might have come in on.
2058 $mysockaddr = getsockname(SOCK);
2059 ($port, $myaddr) = sockaddr_in($mysockaddr);
2060 printf "Connect to %s [%s]\n",
2061 scalar gethostbyaddr($myaddr, AF_INET),
2064 =item getsockopt SOCKET,LEVEL,OPTNAME
2066 Returns the socket option requested, or undef if there is an error.
2072 In list context, returns a (possibly empty) list of filename expansions on
2073 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2074 scalar context, glob iterates through such filename expansions, returning
2075 undef when the list is exhausted. This is the internal function
2076 implementing the C<< <*.c> >> operator, but you can use it directly. If
2077 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2078 more detail in L<perlop/"I/O Operators">.
2080 Beginning with v5.6.0, this operator is implemented using the standard
2081 C<File::Glob> extension. See L<File::Glob> for details.
2085 Converts a time as returned by the time function to an 8-element list
2086 with the time localized for the standard Greenwich time zone.
2087 Typically used as follows:
2090 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2093 All list elements are numeric, and come straight out of the C `struct
2094 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2095 specified time. $mday is the day of the month, and $mon is the month
2096 itself, in the range C<0..11> with 0 indicating January and 11
2097 indicating December. $year is the number of years since 1900. That
2098 is, $year is C<123> in year 2023. $wday is the day of the week, with
2099 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2100 the year, in the range C<0..364> (or C<0..365> in leap years.)
2102 Note that the $year element is I<not> simply the last two digits of
2103 the year. If you assume it is, then you create non-Y2K-compliant
2104 programs--and you wouldn't want to do that, would you?
2106 The proper way to get a complete 4-digit year is simply:
2110 And to get the last two digits of the year (e.g., '01' in 2001) do:
2112 $year = sprintf("%02d", $year % 100);
2114 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2116 In scalar context, C<gmtime()> returns the ctime(3) value:
2118 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2120 Also see the C<timegm> function provided by the C<Time::Local> module,
2121 and the strftime(3) function available via the POSIX module.
2123 This scalar value is B<not> locale dependent (see L<perllocale>), but
2124 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2125 strftime(3) and mktime(3) functions available via the POSIX module. To
2126 get somewhat similar but locale dependent date strings, set up your
2127 locale environment variables appropriately (please see L<perllocale>)
2128 and try for example:
2130 use POSIX qw(strftime);
2131 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2133 Note that the C<%a> and C<%b> escapes, which represent the short forms
2134 of the day of the week and the month of the year, may not necessarily
2135 be three characters wide in all locales.
2143 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2144 execution there. It may not be used to go into any construct that
2145 requires initialization, such as a subroutine or a C<foreach> loop. It
2146 also can't be used to go into a construct that is optimized away,
2147 or to get out of a block or subroutine given to C<sort>.
2148 It can be used to go almost anywhere else within the dynamic scope,
2149 including out of subroutines, but it's usually better to use some other
2150 construct such as C<last> or C<die>. The author of Perl has never felt the
2151 need to use this form of C<goto> (in Perl, that is--C is another matter).
2152 (The difference being that C does not offer named loops combined with
2153 loop control. Perl does, and this replaces most structured uses of C<goto>
2154 in other languages.)
2156 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2157 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2158 necessarily recommended if you're optimizing for maintainability:
2160 goto ("FOO", "BAR", "GLARCH")[$i];
2162 The C<goto-&NAME> form is quite different from the other forms of
2163 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2164 doesn't have the stigma associated with other gotos. Instead, it
2165 exits the current subroutine (losing any changes set by local()) and
2166 immediately calls in its place the named subroutine using the current
2167 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2168 load another subroutine and then pretend that the other subroutine had
2169 been called in the first place (except that any modifications to C<@_>
2170 in the current subroutine are propagated to the other subroutine.)
2171 After the C<goto>, not even C<caller> will be able to tell that this
2172 routine was called first.
2174 NAME needn't be the name of a subroutine; it can be a scalar variable
2175 containing a code reference, or a block which evaluates to a code
2178 =item grep BLOCK LIST
2180 =item grep EXPR,LIST
2182 This is similar in spirit to, but not the same as, grep(1) and its
2183 relatives. In particular, it is not limited to using regular expressions.
2185 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2186 C<$_> to each element) and returns the list value consisting of those
2187 elements for which the expression evaluated to true. In scalar
2188 context, returns the number of times the expression was true.
2190 @foo = grep(!/^#/, @bar); # weed out comments
2194 @foo = grep {!/^#/} @bar; # weed out comments
2196 Note that C<$_> is an alias to the list value, so it can be used to
2197 modify the elements of the LIST. While this is useful and supported,
2198 it can cause bizarre results if the elements of LIST are not variables.
2199 Similarly, grep returns aliases into the original list, much as a for
2200 loop's index variable aliases the list elements. That is, modifying an
2201 element of a list returned by grep (for example, in a C<foreach>, C<map>
2202 or another C<grep>) actually modifies the element in the original list.
2203 This is usually something to be avoided when writing clear code.
2205 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2211 Interprets EXPR as a hex string and returns the corresponding value.
2212 (To convert strings that might start with either 0, 0x, or 0b, see
2213 L</oct>.) If EXPR is omitted, uses C<$_>.
2215 print hex '0xAf'; # prints '175'
2216 print hex 'aF'; # same
2218 Hex strings may only represent integers. Strings that would cause
2219 integer overflow trigger a warning. Leading whitespace is not stripped,
2224 There is no builtin C<import> function. It is just an ordinary
2225 method (subroutine) defined (or inherited) by modules that wish to export
2226 names to another module. The C<use> function calls the C<import> method
2227 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2229 =item index STR,SUBSTR,POSITION
2231 =item index STR,SUBSTR
2233 The index function searches for one string within another, but without
2234 the wildcard-like behavior of a full regular-expression pattern match.
2235 It returns the position of the first occurrence of SUBSTR in STR at
2236 or after POSITION. If POSITION is omitted, starts searching from the
2237 beginning of the string. The return value is based at C<0> (or whatever
2238 you've set the C<$[> variable to--but don't do that). If the substring
2239 is not found, returns one less than the base, ordinarily C<-1>.
2245 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2246 You should not use this function for rounding: one because it truncates
2247 towards C<0>, and two because machine representations of floating point
2248 numbers can sometimes produce counterintuitive results. For example,
2249 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2250 because it's really more like -268.99999999999994315658 instead. Usually,
2251 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2252 functions will serve you better than will int().
2254 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2256 Implements the ioctl(2) function. You'll probably first have to say
2258 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2260 to get the correct function definitions. If F<ioctl.ph> doesn't
2261 exist or doesn't have the correct definitions you'll have to roll your
2262 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2263 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2264 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2265 written depending on the FUNCTION--a pointer to the string value of SCALAR
2266 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2267 has no string value but does have a numeric value, that value will be
2268 passed rather than a pointer to the string value. To guarantee this to be
2269 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2270 functions may be needed to manipulate the values of structures used by
2273 The return value of C<ioctl> (and C<fcntl>) is as follows:
2275 if OS returns: then Perl returns:
2277 0 string "0 but true"
2278 anything else that number
2280 Thus Perl returns true on success and false on failure, yet you can
2281 still easily determine the actual value returned by the operating
2284 $retval = ioctl(...) || -1;
2285 printf "System returned %d\n", $retval;
2287 The special string "C<0> but true" is exempt from B<-w> complaints
2288 about improper numeric conversions.
2290 Here's an example of setting a filehandle named C<REMOTE> to be
2291 non-blocking at the system level. You'll have to negotiate C<$|>
2292 on your own, though.
2294 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2296 $flags = fcntl(REMOTE, F_GETFL, 0)
2297 or die "Can't get flags for the socket: $!\n";
2299 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2300 or die "Can't set flags for the socket: $!\n";
2302 =item join EXPR,LIST
2304 Joins the separate strings of LIST into a single string with fields
2305 separated by the value of EXPR, and returns that new string. Example:
2307 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2309 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2310 first argument. Compare L</split>.
2314 Returns a list consisting of all the keys of the named hash. (In
2315 scalar context, returns the number of keys.) The keys are returned in
2316 an apparently random order. The actual random order is subject to
2317 change in future versions of perl, but it is guaranteed to be the same
2318 order as either the C<values> or C<each> function produces (given
2319 that the hash has not been modified). As a side effect, it resets
2322 Here is yet another way to print your environment:
2325 @values = values %ENV;
2327 print pop(@keys), '=', pop(@values), "\n";
2330 or how about sorted by key:
2332 foreach $key (sort(keys %ENV)) {
2333 print $key, '=', $ENV{$key}, "\n";
2336 The returned values are copies of the original keys in the hash, so
2337 modifying them will not affect the original hash. Compare L</values>.
2339 To sort a hash by value, you'll need to use a C<sort> function.
2340 Here's a descending numeric sort of a hash by its values:
2342 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2343 printf "%4d %s\n", $hash{$key}, $key;
2346 As an lvalue C<keys> allows you to increase the number of hash buckets
2347 allocated for the given hash. This can gain you a measure of efficiency if
2348 you know the hash is going to get big. (This is similar to pre-extending
2349 an array by assigning a larger number to $#array.) If you say
2353 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2354 in fact, since it rounds up to the next power of two. These
2355 buckets will be retained even if you do C<%hash = ()>, use C<undef
2356 %hash> if you want to free the storage while C<%hash> is still in scope.
2357 You can't shrink the number of buckets allocated for the hash using
2358 C<keys> in this way (but you needn't worry about doing this by accident,
2359 as trying has no effect).
2361 See also C<each>, C<values> and C<sort>.
2363 =item kill SIGNAL, LIST
2365 Sends a signal to a list of processes. Returns the number of
2366 processes successfully signaled (which is not necessarily the
2367 same as the number actually killed).
2369 $cnt = kill 1, $child1, $child2;
2372 If SIGNAL is zero, no signal is sent to the process. This is a
2373 useful way to check that the process is alive and hasn't changed
2374 its UID. See L<perlport> for notes on the portability of this
2377 Unlike in the shell, if SIGNAL is negative, it kills
2378 process groups instead of processes. (On System V, a negative I<PROCESS>
2379 number will also kill process groups, but that's not portable.) That
2380 means you usually want to use positive not negative signals. You may also
2381 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2387 The C<last> command is like the C<break> statement in C (as used in
2388 loops); it immediately exits the loop in question. If the LABEL is
2389 omitted, the command refers to the innermost enclosing loop. The
2390 C<continue> block, if any, is not executed:
2392 LINE: while (<STDIN>) {
2393 last LINE if /^$/; # exit when done with header
2397 C<last> cannot be used to exit a block which returns a value such as
2398 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2399 a grep() or map() operation.
2401 Note that a block by itself is semantically identical to a loop
2402 that executes once. Thus C<last> can be used to effect an early
2403 exit out of such a block.
2405 See also L</continue> for an illustration of how C<last>, C<next>, and
2412 Returns a lowercased version of EXPR. This is the internal function
2413 implementing the C<\L> escape in double-quoted strings. Respects
2414 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2415 and L<perlunicode> for more details about locale and Unicode support.
2417 If EXPR is omitted, uses C<$_>.
2423 Returns the value of EXPR with the first character lowercased. This
2424 is the internal function implementing the C<\l> escape in
2425 double-quoted strings. Respects current LC_CTYPE locale if C<use
2426 locale> in force. See L<perllocale> and L<perlunicode> for more
2427 details about locale and Unicode support.
2429 If EXPR is omitted, uses C<$_>.
2435 Returns the length in I<characters> of the value of EXPR. If EXPR is
2436 omitted, returns length of C<$_>. Note that this cannot be used on
2437 an entire array or hash to find out how many elements these have.
2438 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2440 Note the I<characters>: if the EXPR is in Unicode, you will get the
2441 number of characters, not the number of bytes. To get the length
2442 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2444 =item link OLDFILE,NEWFILE
2446 Creates a new filename linked to the old filename. Returns true for
2447 success, false otherwise.
2449 =item listen SOCKET,QUEUESIZE
2451 Does the same thing that the listen system call does. Returns true if
2452 it succeeded, false otherwise. See the example in
2453 L<perlipc/"Sockets: Client/Server Communication">.
2457 You really probably want to be using C<my> instead, because C<local> isn't
2458 what most people think of as "local". See
2459 L<perlsub/"Private Variables via my()"> for details.
2461 A local modifies the listed variables to be local to the enclosing
2462 block, file, or eval. If more than one value is listed, the list must
2463 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2464 for details, including issues with tied arrays and hashes.
2466 =item localtime EXPR
2468 Converts a time as returned by the time function to a 9-element list
2469 with the time analyzed for the local time zone. Typically used as
2473 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2476 All list elements are numeric, and come straight out of the C `struct
2477 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2478 specified time. $mday is the day of the month, and $mon is the month
2479 itself, in the range C<0..11> with 0 indicating January and 11
2480 indicating December. $year is the number of years since 1900. That
2481 is, $year is C<123> in year 2023. $wday is the day of the week, with
2482 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2483 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2484 is true if the specified time occurs during daylight savings time,
2487 Note that the $year element is I<not> simply the last two digits of
2488 the year. If you assume it is, then you create non-Y2K-compliant
2489 programs--and you wouldn't want to do that, would you?
2491 The proper way to get a complete 4-digit year is simply:
2495 And to get the last two digits of the year (e.g., '01' in 2001) do:
2497 $year = sprintf("%02d", $year % 100);
2499 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2501 In scalar context, C<localtime()> returns the ctime(3) value:
2503 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2505 This scalar value is B<not> locale dependent, see L<perllocale>, but
2506 instead a Perl builtin. Also see the C<Time::Local> module
2507 (to convert the second, minutes, hours, ... back to seconds since the
2508 stroke of midnight the 1st of January 1970, the value returned by
2509 time()), and the strftime(3) and mktime(3) functions available via the
2510 POSIX module. To get somewhat similar but locale dependent date
2511 strings, set up your locale environment variables appropriately
2512 (please see L<perllocale>) and try for example:
2514 use POSIX qw(strftime);
2515 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2517 Note that the C<%a> and C<%b>, the short forms of the day of the week
2518 and the month of the year, may not necessarily be three characters wide.
2522 This function places an advisory lock on a shared variable, or referenced
2523 object contained in I<THING> until the lock goes out of scope.
2525 lock() is a "weak keyword" : this means that if you've defined a function
2526 by this name (before any calls to it), that function will be called
2527 instead. (However, if you've said C<use threads>, lock() is always a
2528 keyword.) See L<threads>.
2534 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2535 returns log of C<$_>. To get the log of another base, use basic algebra:
2536 The base-N log of a number is equal to the natural log of that number
2537 divided by the natural log of N. For example:
2541 return log($n)/log(10);
2544 See also L</exp> for the inverse operation.
2550 Does the same thing as the C<stat> function (including setting the
2551 special C<_> filehandle) but stats a symbolic link instead of the file
2552 the symbolic link points to. If symbolic links are unimplemented on
2553 your system, a normal C<stat> is done. For much more detailed
2554 information, please see the documentation for C<stat>.
2556 If EXPR is omitted, stats C<$_>.
2560 The match operator. See L<perlop>.
2562 =item map BLOCK LIST
2566 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2567 C<$_> to each element) and returns the list value composed of the
2568 results of each such evaluation. In scalar context, returns the
2569 total number of elements so generated. Evaluates BLOCK or EXPR in
2570 list context, so each element of LIST may produce zero, one, or
2571 more elements in the returned value.
2573 @chars = map(chr, @nums);
2575 translates a list of numbers to the corresponding characters. And
2577 %hash = map { getkey($_) => $_ } @array;
2579 is just a funny way to write
2582 foreach $_ (@array) {
2583 $hash{getkey($_)} = $_;
2586 Note that C<$_> is an alias to the list value, so it can be used to
2587 modify the elements of the LIST. While this is useful and supported,
2588 it can cause bizarre results if the elements of LIST are not variables.
2589 Using a regular C<foreach> loop for this purpose would be clearer in
2590 most cases. See also L</grep> for an array composed of those items of
2591 the original list for which the BLOCK or EXPR evaluates to true.
2593 C<{> starts both hash references and blocks, so C<map { ...> could be either
2594 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2595 ahead for the closing C<}> it has to take a guess at which its dealing with
2596 based what it finds just after the C<{>. Usually it gets it right, but if it
2597 doesn't it won't realize something is wrong until it gets to the C<}> and
2598 encounters the missing (or unexpected) comma. The syntax error will be
2599 reported close to the C<}> but you'll need to change something near the C<{>
2600 such as using a unary C<+> to give perl some help:
2602 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2603 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2604 %hash = map { ("\L$_", 1) } @array # this also works
2605 %hash = map { lc($_), 1 } @array # as does this.
2606 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2608 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2610 or to force an anon hash constructor use C<+{>
2612 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2614 and you get list of anonymous hashes each with only 1 entry.
2616 =item mkdir FILENAME,MASK
2618 =item mkdir FILENAME
2620 Creates the directory specified by FILENAME, with permissions
2621 specified by MASK (as modified by C<umask>). If it succeeds it
2622 returns true, otherwise it returns false and sets C<$!> (errno).
2623 If omitted, MASK defaults to 0777.
2625 In general, it is better to create directories with permissive MASK,
2626 and let the user modify that with their C<umask>, than it is to supply
2627 a restrictive MASK and give the user no way to be more permissive.
2628 The exceptions to this rule are when the file or directory should be
2629 kept private (mail files, for instance). The perlfunc(1) entry on
2630 C<umask> discusses the choice of MASK in more detail.
2632 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2633 number of trailing slashes. Some operating and filesystems do not get
2634 this right, so Perl automatically removes all trailing slashes to keep
2637 =item msgctl ID,CMD,ARG
2639 Calls the System V IPC function msgctl(2). You'll probably have to say
2643 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2644 then ARG must be a variable which will hold the returned C<msqid_ds>
2645 structure. Returns like C<ioctl>: the undefined value for error,
2646 C<"0 but true"> for zero, or the actual return value otherwise. See also
2647 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2649 =item msgget KEY,FLAGS
2651 Calls the System V IPC function msgget(2). Returns the message queue
2652 id, or the undefined value if there is an error. See also
2653 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2655 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2657 Calls the System V IPC function msgrcv to receive a message from
2658 message queue ID into variable VAR with a maximum message size of
2659 SIZE. Note that when a message is received, the message type as a
2660 native long integer will be the first thing in VAR, followed by the
2661 actual message. This packing may be opened with C<unpack("l! a*")>.
2662 Taints the variable. Returns true if successful, or false if there is
2663 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2664 C<IPC::SysV::Msg> documentation.
2666 =item msgsnd ID,MSG,FLAGS
2668 Calls the System V IPC function msgsnd to send the message MSG to the
2669 message queue ID. MSG must begin with the native long integer message
2670 type, and be followed by the length of the actual message, and finally
2671 the message itself. This kind of packing can be achieved with
2672 C<pack("l! a*", $type, $message)>. Returns true if successful,
2673 or false if there is an error. See also C<IPC::SysV>
2674 and C<IPC::SysV::Msg> documentation.
2680 =item my EXPR : ATTRS
2682 =item my TYPE EXPR : ATTRS
2684 A C<my> declares the listed variables to be local (lexically) to the
2685 enclosing block, file, or C<eval>. If more than one value is listed,
2686 the list must be placed in parentheses.
2688 The exact semantics and interface of TYPE and ATTRS are still
2689 evolving. TYPE is currently bound to the use of C<fields> pragma,
2690 and attributes are handled using the C<attributes> pragma, or starting
2691 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2692 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2693 L<attributes>, and L<Attribute::Handlers>.
2699 The C<next> command is like the C<continue> statement in C; it starts
2700 the next iteration of the loop:
2702 LINE: while (<STDIN>) {
2703 next LINE if /^#/; # discard comments
2707 Note that if there were a C<continue> block on the above, it would get
2708 executed even on discarded lines. If the LABEL is omitted, the command
2709 refers to the innermost enclosing loop.
2711 C<next> cannot be used to exit a block which returns a value such as
2712 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2713 a grep() or map() operation.
2715 Note that a block by itself is semantically identical to a loop
2716 that executes once. Thus C<next> will exit such a block early.
2718 See also L</continue> for an illustration of how C<last>, C<next>, and
2721 =item no Module VERSION LIST
2723 =item no Module VERSION
2725 =item no Module LIST
2729 See the C<use> function, which C<no> is the opposite of.
2735 Interprets EXPR as an octal string and returns the corresponding
2736 value. (If EXPR happens to start off with C<0x>, interprets it as a
2737 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2738 binary string. Leading whitespace is ignored in all three cases.)
2739 The following will handle decimal, binary, octal, and hex in the standard
2742 $val = oct($val) if $val =~ /^0/;
2744 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2745 in octal), use sprintf() or printf():
2747 $perms = (stat("filename"))[2] & 07777;
2748 $oct_perms = sprintf "%lo", $perms;
2750 The oct() function is commonly used when a string such as C<644> needs
2751 to be converted into a file mode, for example. (Although perl will
2752 automatically convert strings into numbers as needed, this automatic
2753 conversion assumes base 10.)
2755 =item open FILEHANDLE,EXPR
2757 =item open FILEHANDLE,MODE,EXPR
2759 =item open FILEHANDLE,MODE,EXPR,LIST
2761 =item open FILEHANDLE,MODE,REFERENCE
2763 =item open FILEHANDLE
2765 Opens the file whose filename is given by EXPR, and associates it with
2768 (The following is a comprehensive reference to open(): for a gentler
2769 introduction you may consider L<perlopentut>.)
2771 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2772 the variable is assigned a reference to a new anonymous filehandle,
2773 otherwise if FILEHANDLE is an expression, its value is used as the name of
2774 the real filehandle wanted. (This is considered a symbolic reference, so
2775 C<use strict 'refs'> should I<not> be in effect.)
2777 If EXPR is omitted, the scalar variable of the same name as the
2778 FILEHANDLE contains the filename. (Note that lexical variables--those
2779 declared with C<my>--will not work for this purpose; so if you're
2780 using C<my>, specify EXPR in your call to open.)
2782 If three or more arguments are specified then the mode of opening and
2783 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2784 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2785 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2786 the file is opened for appending, again being created if necessary.
2788 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2789 indicate that you want both read and write access to the file; thus
2790 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2791 '+>' >> mode would clobber the file first. You can't usually use
2792 either read-write mode for updating textfiles, since they have
2793 variable length records. See the B<-i> switch in L<perlrun> for a
2794 better approach. The file is created with permissions of C<0666>
2795 modified by the process' C<umask> value.
2797 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2798 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2800 In the 2-arguments (and 1-argument) form of the call the mode and
2801 filename should be concatenated (in this order), possibly separated by
2802 spaces. It is possible to omit the mode in these forms if the mode is
2805 If the filename begins with C<'|'>, the filename is interpreted as a
2806 command to which output is to be piped, and if the filename ends with a
2807 C<'|'>, the filename is interpreted as a command which pipes output to
2808 us. See L<perlipc/"Using open() for IPC">
2809 for more examples of this. (You are not allowed to C<open> to a command
2810 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2811 and L<perlipc/"Bidirectional Communication with Another Process">
2814 For three or more arguments if MODE is C<'|-'>, the filename is
2815 interpreted as a command to which output is to be piped, and if MODE
2816 is C<'-|'>, the filename is interpreted as a command which pipes
2817 output to us. In the 2-arguments (and 1-argument) form one should
2818 replace dash (C<'-'>) with the command.
2819 See L<perlipc/"Using open() for IPC"> for more examples of this.
2820 (You are not allowed to C<open> to a command that pipes both in I<and>
2821 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2822 L<perlipc/"Bidirectional Communication"> for alternatives.)
2824 In the three-or-more argument form of pipe opens, if LIST is specified
2825 (extra arguments after the command name) then LIST becomes arguments
2826 to the command invoked if the platform supports it. The meaning of
2827 C<open> with more than three arguments for non-pipe modes is not yet
2828 specified. Experimental "layers" may give extra LIST arguments
2831 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2832 and opening C<< '>-' >> opens STDOUT.
2834 You may use the three-argument form of open to specify IO "layers"
2835 (sometimes also referred to as "disciplines") to be applied to the handle
2836 that affect how the input and output are processed (see L<open> and
2837 L<PerlIO> for more details). For example
2839 open(FH, "<:utf8", "file")
2841 will open the UTF-8 encoded file containing Unicode characters,
2842 see L<perluniintro>. (Note that if layers are specified in the
2843 three-arg form then default layers set by the C<open> pragma are
2846 Open returns nonzero upon success, the undefined value otherwise. If
2847 the C<open> involved a pipe, the return value happens to be the pid of
2850 If you're running Perl on a system that distinguishes between text
2851 files and binary files, then you should check out L</binmode> for tips
2852 for dealing with this. The key distinction between systems that need
2853 C<binmode> and those that don't is their text file formats. Systems
2854 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2855 character, and which encode that character in C as C<"\n">, do not
2856 need C<binmode>. The rest need it.
2858 When opening a file, it's usually a bad idea to continue normal execution
2859 if the request failed, so C<open> is frequently used in connection with
2860 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2861 where you want to make a nicely formatted error message (but there are
2862 modules that can help with that problem)) you should always check
2863 the return value from opening a file. The infrequent exception is when
2864 working with an unopened filehandle is actually what you want to do.
2866 As a special case the 3 arg form with a read/write mode and the third
2867 argument being C<undef>:
2869 open(TMP, "+>", undef) or die ...
2871 opens a filehandle to an anonymous temporary file. Also using "+<"
2872 works for symmetry, but you really should consider writing something
2873 to the temporary file first. You will need to seek() to do the
2876 File handles can be opened to "in memory" files held in Perl scalars via:
2878 open($fh, '>', \$variable) || ..
2880 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2881 file, you have to close it first:
2884 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2889 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2890 while (<ARTICLE>) {...
2892 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2893 # if the open fails, output is discarded
2895 open(DBASE, '+<', 'dbase.mine') # open for update
2896 or die "Can't open 'dbase.mine' for update: $!";
2898 open(DBASE, '+<dbase.mine') # ditto
2899 or die "Can't open 'dbase.mine' for update: $!";
2901 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2902 or die "Can't start caesar: $!";
2904 open(ARTICLE, "caesar <$article |") # ditto
2905 or die "Can't start caesar: $!";
2907 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2908 or die "Can't start sort: $!";
2911 open(MEMORY,'>', \$var)
2912 or die "Can't open memory file: $!";
2913 print MEMORY "foo!\n"; # output will end up in $var
2915 # process argument list of files along with any includes
2917 foreach $file (@ARGV) {
2918 process($file, 'fh00');
2922 my($filename, $input) = @_;
2923 $input++; # this is a string increment
2924 unless (open($input, $filename)) {
2925 print STDERR "Can't open $filename: $!\n";
2930 while (<$input>) { # note use of indirection
2931 if (/^#include "(.*)"/) {
2932 process($1, $input);
2939 You may also, in the Bourne shell tradition, specify an EXPR beginning
2940 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2941 name of a filehandle (or file descriptor, if numeric) to be
2942 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2943 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2944 mode you specify should match the mode of the original filehandle.
2945 (Duping a filehandle does not take into account any existing contents of
2946 IO buffers.) If you use the 3 arg form then you can pass either a number,
2947 the name of a filehandle or the normal "reference to a glob".
2949 Here is a script that saves, redirects, and restores C<STDOUT> and
2950 C<STDERR> using various methods:
2953 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2954 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2956 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2957 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2959 select STDERR; $| = 1; # make unbuffered
2960 select STDOUT; $| = 1; # make unbuffered
2962 print STDOUT "stdout 1\n"; # this works for
2963 print STDERR "stderr 1\n"; # subprocesses too
2968 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2969 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2971 print STDOUT "stdout 2\n";
2972 print STDERR "stderr 2\n";
2974 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2975 do an equivalent of C's C<fdopen> of that file descriptor; this is
2976 more parsimonious of file descriptors. For example:
2978 open(FILEHANDLE, "<&=$fd")
2982 open(FILEHANDLE, "<&=", $fd)
2984 Note that if Perl is using the standard C libraries' fdopen() then on
2985 many UNIX systems, fdopen() is known to fail when file descriptors
2986 exceed a certain value, typically 255. If you need more file
2987 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2989 You can see whether Perl has been compiled with PerlIO or not by
2990 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2991 is C<define>, you have PerlIO, otherwise you don't.
2993 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2994 with 2-arguments (or 1-argument) form of open(), then
2995 there is an implicit fork done, and the return value of open is the pid
2996 of the child within the parent process, and C<0> within the child
2997 process. (Use C<defined($pid)> to determine whether the open was successful.)
2998 The filehandle behaves normally for the parent, but i/o to that
2999 filehandle is piped from/to the STDOUT/STDIN of the child process.
3000 In the child process the filehandle isn't opened--i/o happens from/to
3001 the new STDOUT or STDIN. Typically this is used like the normal
3002 piped open when you want to exercise more control over just how the
3003 pipe command gets executed, such as when you are running setuid, and
3004 don't want to have to scan shell commands for metacharacters.
3005 The following triples are more or less equivalent:
3007 open(FOO, "|tr '[a-z]' '[A-Z]'");
3008 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3009 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3010 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3012 open(FOO, "cat -n '$file'|");
3013 open(FOO, '-|', "cat -n '$file'");
3014 open(FOO, '-|') || exec 'cat', '-n', $file;
3015 open(FOO, '-|', "cat", '-n', $file);
3017 The last example in each block shows the pipe as "list form", which is
3018 not yet supported on all platforms. A good rule of thumb is that if
3019 your platform has true C<fork()> (in other words, if your platform is
3020 UNIX) you can use the list form.
3022 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3024 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3025 output before any operation that may do a fork, but this may not be
3026 supported on some platforms (see L<perlport>). To be safe, you may need
3027 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3028 of C<IO::Handle> on any open handles.
3030 On systems that support a close-on-exec flag on files, the flag will
3031 be set for the newly opened file descriptor as determined by the value
3032 of $^F. See L<perlvar/$^F>.
3034 Closing any piped filehandle causes the parent process to wait for the
3035 child to finish, and returns the status value in C<$?>.
3037 The filename passed to 2-argument (or 1-argument) form of open() will
3038 have leading and trailing whitespace deleted, and the normal
3039 redirection characters honored. This property, known as "magic open",
3040 can often be used to good effect. A user could specify a filename of
3041 F<"rsh cat file |">, or you could change certain filenames as needed:
3043 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3044 open(FH, $filename) or die "Can't open $filename: $!";
3046 Use 3-argument form to open a file with arbitrary weird characters in it,
3048 open(FOO, '<', $file);
3050 otherwise it's necessary to protect any leading and trailing whitespace:
3052 $file =~ s#^(\s)#./$1#;
3053 open(FOO, "< $file\0");
3055 (this may not work on some bizarre filesystems). One should
3056 conscientiously choose between the I<magic> and 3-arguments form
3061 will allow the user to specify an argument of the form C<"rsh cat file |">,
3062 but will not work on a filename which happens to have a trailing space, while
3064 open IN, '<', $ARGV[0];
3066 will have exactly the opposite restrictions.
3068 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3069 should use the C<sysopen> function, which involves no such magic (but
3070 may use subtly different filemodes than Perl open(), which is mapped
3071 to C fopen()). This is
3072 another way to protect your filenames from interpretation. For example:
3075 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3076 or die "sysopen $path: $!";
3077 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3078 print HANDLE "stuff $$\n";
3080 print "File contains: ", <HANDLE>;
3082 Using the constructor from the C<IO::Handle> package (or one of its
3083 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3084 filehandles that have the scope of whatever variables hold references to
3085 them, and automatically close whenever and however you leave that scope:
3089 sub read_myfile_munged {
3091 my $handle = new IO::File;
3092 open($handle, "myfile") or die "myfile: $!";
3094 or return (); # Automatically closed here.
3095 mung $first or die "mung failed"; # Or here.
3096 return $first, <$handle> if $ALL; # Or here.
3100 See L</seek> for some details about mixing reading and writing.
3102 =item opendir DIRHANDLE,EXPR
3104 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3105 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3106 DIRHANDLE may be an expression whose value can be used as an indirect
3107 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3108 scalar variable (or array or hash element), the variable is assigned a
3109 reference to a new anonymous dirhandle.
3110 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3116 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3117 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3120 For the reverse, see L</chr>.
3121 See L<perlunicode> and L<encoding> for more about Unicode.
3127 =item our EXPR : ATTRS
3129 =item our TYPE EXPR : ATTRS
3131 An C<our> declares the listed variables to be valid globals within
3132 the enclosing block, file, or C<eval>. That is, it has the same
3133 scoping rules as a "my" declaration, but does not create a local
3134 variable. If more than one value is listed, the list must be placed
3135 in parentheses. The C<our> declaration has no semantic effect unless
3136 "use strict vars" is in effect, in which case it lets you use the
3137 declared global variable without qualifying it with a package name.
3138 (But only within the lexical scope of the C<our> declaration. In this
3139 it differs from "use vars", which is package scoped.)
3141 An C<our> declaration declares a global variable that will be visible
3142 across its entire lexical scope, even across package boundaries. The
3143 package in which the variable is entered is determined at the point
3144 of the declaration, not at the point of use. This means the following
3148 our $bar; # declares $Foo::bar for rest of lexical scope
3152 print $bar; # prints 20
3154 Multiple C<our> declarations in the same lexical scope are allowed
3155 if they are in different packages. If they happened to be in the same
3156 package, Perl will emit warnings if you have asked for them.
3160 our $bar; # declares $Foo::bar for rest of lexical scope
3164 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3165 print $bar; # prints 30
3167 our $bar; # emits warning
3169 An C<our> declaration may also have a list of attributes associated
3172 The exact semantics and interface of TYPE and ATTRS are still
3173 evolving. TYPE is currently bound to the use of C<fields> pragma,
3174 and attributes are handled using the C<attributes> pragma, or starting
3175 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3176 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3177 L<attributes>, and L<Attribute::Handlers>.
3179 The only currently recognized C<our()> attribute is C<unique> which
3180 indicates that a single copy of the global is to be used by all
3181 interpreters should the program happen to be running in a
3182 multi-interpreter environment. (The default behaviour would be for
3183 each interpreter to have its own copy of the global.) Examples:
3185 our @EXPORT : unique = qw(foo);
3186 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3187 our $VERSION : unique = "1.00";
3189 Note that this attribute also has the effect of making the global
3190 readonly when the first new interpreter is cloned (for example,
3191 when the first new thread is created).
3193 Multi-interpreter environments can come to being either through the
3194 fork() emulation on Windows platforms, or by embedding perl in a
3195 multi-threaded application. The C<unique> attribute does nothing in
3196 all other environments.
3198 =item pack TEMPLATE,LIST
3200 Takes a LIST of values and converts it into a string using the rules
3201 given by the TEMPLATE. The resulting string is the concatenation of
3202 the converted values. Typically, each converted value looks
3203 like its machine-level representation. For example, on 32-bit machines
3204 a converted integer may be represented by a sequence of 4 bytes.
3206 The TEMPLATE is a sequence of characters that give the order and type
3207 of values, as follows:
3209 a A string with arbitrary binary data, will be null padded.
3210 A A text (ASCII) string, will be space padded.
3211 Z A null terminated (ASCIZ) string, will be null padded.
3213 b A bit string (ascending bit order inside each byte, like vec()).
3214 B A bit string (descending bit order inside each byte).
3215 h A hex string (low nybble first).
3216 H A hex string (high nybble first).
3218 c A signed char value.
3219 C An unsigned char value. Only does bytes. See U for Unicode.
3221 s A signed short value.
3222 S An unsigned short value.
3223 (This 'short' is _exactly_ 16 bits, which may differ from
3224 what a local C compiler calls 'short'. If you want
3225 native-length shorts, use the '!' suffix.)
3227 i A signed integer value.
3228 I An unsigned integer value.
3229 (This 'integer' is _at_least_ 32 bits wide. Its exact
3230 size depends on what a local C compiler calls 'int',
3231 and may even be larger than the 'long' described in
3234 l A signed long value.
3235 L An unsigned long value.
3236 (This 'long' is _exactly_ 32 bits, which may differ from
3237 what a local C compiler calls 'long'. If you want
3238 native-length longs, use the '!' suffix.)
3240 n An unsigned short in "network" (big-endian) order.
3241 N An unsigned long in "network" (big-endian) order.
3242 v An unsigned short in "VAX" (little-endian) order.
3243 V An unsigned long in "VAX" (little-endian) order.
3244 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3245 _exactly_ 32 bits, respectively.)
3247 q A signed quad (64-bit) value.
3248 Q An unsigned quad value.
3249 (Quads are available only if your system supports 64-bit
3250 integer values _and_ if Perl has been compiled to support those.
3251 Causes a fatal error otherwise.)
3253 j A signed integer value (a Perl internal integer, IV).
3254 J An unsigned integer value (a Perl internal unsigned integer, UV).
3256 f A single-precision float in the native format.
3257 d A double-precision float in the native format.
3259 F A floating point value in the native native format
3260 (a Perl internal floating point value, NV).
3261 D A long double-precision float in the native format.
3262 (Long doubles are available only if your system supports long
3263 double values _and_ if Perl has been compiled to support those.
3264 Causes a fatal error otherwise.)
3266 p A pointer to a null-terminated string.
3267 P A pointer to a structure (fixed-length string).
3269 u A uuencoded string.
3270 U A Unicode character number. Encodes to UTF-8 internally
3271 (or UTF-EBCDIC in EBCDIC platforms).
3273 w A BER compressed integer. Its bytes represent an unsigned
3274 integer in base 128, most significant digit first, with as
3275 few digits as possible. Bit eight (the high bit) is set
3276 on each byte except the last.
3280 @ Null fill to absolute position, counted from the start of
3281 the innermost ()-group.
3282 ( Start of a ()-group.
3284 The following rules apply:
3290 Each letter may optionally be followed by a number giving a repeat
3291 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3292 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3293 many values from the LIST. A C<*> for the repeat count means to use
3294 however many items are left, except for C<@>, C<x>, C<X>, where it is
3295 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3296 is the same). A numeric repeat count may optionally be enclosed in
3297 brackets, as in C<pack 'C[80]', @arr>.
3299 One can replace the numeric repeat count by a template enclosed in brackets;
3300 then the packed length of this template in bytes is used as a count.
3301 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3302 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3303 If the template in brackets contains alignment commands (such as C<x![d]>),
3304 its packed length is calculated as if the start of the template has the maximal
3307 When used with C<Z>, C<*> results in the addition of a trailing null
3308 byte (so the packed result will be one longer than the byte C<length>
3311 The repeat count for C<u> is interpreted as the maximal number of bytes
3312 to encode per line of output, with 0 and 1 replaced by 45.
3316 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3317 string of length count, padding with nulls or spaces as necessary. When
3318 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3319 after the first null, and C<a> returns data verbatim. When packing,
3320 C<a>, and C<Z> are equivalent.
3322 If the value-to-pack is too long, it is truncated. If too long and an
3323 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3324 by a null byte. Thus C<Z> always packs a trailing null byte under
3329 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3330 Each byte of the input field of pack() generates 1 bit of the result.
3331 Each result bit is based on the least-significant bit of the corresponding
3332 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3333 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3335 Starting from the beginning of the input string of pack(), each 8-tuple
3336 of bytes is converted to 1 byte of output. With format C<b>
3337 the first byte of the 8-tuple determines the least-significant bit of a
3338 byte, and with format C<B> it determines the most-significant bit of
3341 If the length of the input string is not exactly divisible by 8, the
3342 remainder is packed as if the input string were padded by null bytes
3343 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3345 If the input string of pack() is longer than needed, extra bytes are ignored.
3346 A C<*> for the repeat count of pack() means to use all the bytes of
3347 the input field. On unpack()ing the bits are converted to a string
3348 of C<"0">s and C<"1">s.
3352 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3353 representable as hexadecimal digits, 0-9a-f) long.
3355 Each byte of the input field of pack() generates 4 bits of the result.
3356 For non-alphabetical bytes the result is based on the 4 least-significant
3357 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3358 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3359 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3360 is compatible with the usual hexadecimal digits, so that C<"a"> and
3361 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3362 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3364 Starting from the beginning of the input string of pack(), each pair
3365 of bytes is converted to 1 byte of output. With format C<h> the
3366 first byte of the pair determines the least-significant nybble of the
3367 output byte, and with format C<H> it determines the most-significant
3370 If the length of the input string is not even, it behaves as if padded
3371 by a null byte at the end. Similarly, during unpack()ing the "extra"
3372 nybbles are ignored.
3374 If the input string of pack() is longer than needed, extra bytes are ignored.
3375 A C<*> for the repeat count of pack() means to use all the bytes of
3376 the input field. On unpack()ing the bits are converted to a string
3377 of hexadecimal digits.
3381 The C<p> type packs a pointer to a null-terminated string. You are
3382 responsible for ensuring the string is not a temporary value (which can
3383 potentially get deallocated before you get around to using the packed result).
3384 The C<P> type packs a pointer to a structure of the size indicated by the
3385 length. A NULL pointer is created if the corresponding value for C<p> or
3386 C<P> is C<undef>, similarly for unpack().
3390 The C</> template character allows packing and unpacking of strings where
3391 the packed structure contains a byte count followed by the string itself.
3392 You write I<length-item>C</>I<string-item>.
3394 The I<length-item> can be any C<pack> template letter, and describes
3395 how the length value is packed. The ones likely to be of most use are
3396 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3397 SNMP) and C<N> (for Sun XDR).
3399 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3400 C<"Z*">. For C<unpack> the length of the string is obtained from the
3401 I<length-item>, but if you put in the '*' it will be ignored. For all other
3402 codes, C<unpack> applies the length value to the next item, which must not
3403 have a repeat count.
3405 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3406 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3407 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3409 The I<length-item> is not returned explicitly from C<unpack>.
3411 Adding a count to the I<length-item> letter is unlikely to do anything
3412 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3413 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3414 which Perl does not regard as legal in numeric strings.
3418 The integer types C<s>, C<S>, C<l>, and C<L> may be
3419 immediately followed by a C<!> suffix to signify native shorts or
3420 longs--as you can see from above for example a bare C<l> does mean
3421 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3422 may be larger. This is an issue mainly in 64-bit platforms. You can
3423 see whether using C<!> makes any difference by
3425 print length(pack("s")), " ", length(pack("s!")), "\n";
3426 print length(pack("l")), " ", length(pack("l!")), "\n";
3428 C<i!> and C<I!> also work but only because of completeness;
3429 they are identical to C<i> and C<I>.
3431 The actual sizes (in bytes) of native shorts, ints, longs, and long
3432 longs on the platform where Perl was built are also available via
3436 print $Config{shortsize}, "\n";
3437 print $Config{intsize}, "\n";
3438 print $Config{longsize}, "\n";
3439 print $Config{longlongsize}, "\n";
3441 (The C<$Config{longlongsize}> will be undefined if your system does
3442 not support long longs.)
3446 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3447 are inherently non-portable between processors and operating systems
3448 because they obey the native byteorder and endianness. For example a
3449 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3450 (arranged in and handled by the CPU registers) into bytes as
3452 0x12 0x34 0x56 0x78 # big-endian
3453 0x78 0x56 0x34 0x12 # little-endian
3455 Basically, the Intel and VAX CPUs are little-endian, while everybody
3456 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3457 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3458 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3461 The names `big-endian' and `little-endian' are comic references to
3462 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3463 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3464 the egg-eating habits of the Lilliputians.
3466 Some systems may have even weirder byte orders such as
3471 You can see your system's preference with
3473 print join(" ", map { sprintf "%#02x", $_ }
3474 unpack("C*",pack("L",0x12345678))), "\n";
3476 The byteorder on the platform where Perl was built is also available
3480 print $Config{byteorder}, "\n";
3482 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3483 and C<'87654321'> are big-endian.
3485 If you want portable packed integers use the formats C<n>, C<N>,
3486 C<v>, and C<V>, their byte endianness and size are known.
3487 See also L<perlport>.
3491 Real numbers (floats and doubles) are in the native machine format only;
3492 due to the multiplicity of floating formats around, and the lack of a
3493 standard "network" representation, no facility for interchange has been
3494 made. This means that packed floating point data written on one machine
3495 may not be readable on another - even if both use IEEE floating point
3496 arithmetic (as the endian-ness of the memory representation is not part
3497 of the IEEE spec). See also L<perlport>.
3499 Note that Perl uses doubles internally for all numeric calculation, and
3500 converting from double into float and thence back to double again will
3501 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3506 If the pattern begins with a C<U>, the resulting string will be treated
3507 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3508 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3509 characters. If you don't want this to happen, you can begin your pattern
3510 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3511 string, and then follow this with a C<U*> somewhere in your pattern.
3515 You must yourself do any alignment or padding by inserting for example
3516 enough C<'x'>es while packing. There is no way to pack() and unpack()
3517 could know where the bytes are going to or coming from. Therefore
3518 C<pack> (and C<unpack>) handle their output and input as flat
3523 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3524 take a repeat count, both as postfix, and for unpack() also via the C</>
3525 template character. Within each repetition of a group, positioning with
3526 C<@> starts again at 0. Therefore, the result of
3528 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3530 is the string "\0a\0\0bc".
3535 C<x> and C<X> accept C<!> modifier. In this case they act as
3536 alignment commands: they jump forward/back to the closest position
3537 aligned at a multiple of C<count> bytes. For example, to pack() or
3538 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3539 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3540 aligned on the double's size.
3542 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3543 both result in no-ops.
3547 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3548 White space may be used to separate pack codes from each other, but
3549 a C<!> modifier and a repeat count must follow immediately.
3553 If TEMPLATE requires more arguments to pack() than actually given, pack()
3554 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3555 to pack() than actually given, extra arguments are ignored.
3561 $foo = pack("CCCC",65,66,67,68);
3563 $foo = pack("C4",65,66,67,68);
3565 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3566 # same thing with Unicode circled letters
3568 $foo = pack("ccxxcc",65,66,67,68);
3571 # note: the above examples featuring "C" and "c" are true
3572 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3573 # and UTF-8. In EBCDIC the first example would be
3574 # $foo = pack("CCCC",193,194,195,196);
3576 $foo = pack("s2",1,2);
3577 # "\1\0\2\0" on little-endian
3578 # "\0\1\0\2" on big-endian
3580 $foo = pack("a4","abcd","x","y","z");
3583 $foo = pack("aaaa","abcd","x","y","z");
3586 $foo = pack("a14","abcdefg");
3587 # "abcdefg\0\0\0\0\0\0\0"
3589 $foo = pack("i9pl", gmtime);
3590 # a real struct tm (on my system anyway)
3592 $utmp_template = "Z8 Z8 Z16 L";
3593 $utmp = pack($utmp_template, @utmp1);
3594 # a struct utmp (BSDish)
3596 @utmp2 = unpack($utmp_template, $utmp);
3597 # "@utmp1" eq "@utmp2"
3600 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3603 $foo = pack('sx2l', 12, 34);
3604 # short 12, two zero bytes padding, long 34
3605 $bar = pack('s@4l', 12, 34);
3606 # short 12, zero fill to position 4, long 34
3609 The same template may generally also be used in unpack().
3611 =item package NAMESPACE
3615 Declares the compilation unit as being in the given namespace. The scope
3616 of the package declaration is from the declaration itself through the end
3617 of the enclosing block, file, or eval (the same as the C<my> operator).
3618 All further unqualified dynamic identifiers will be in this namespace.
3619 A package statement affects only dynamic variables--including those
3620 you've used C<local> on--but I<not> lexical variables, which are created
3621 with C<my>. Typically it would be the first declaration in a file to
3622 be included by the C<require> or C<use> operator. You can switch into a
3623 package in more than one place; it merely influences which symbol table
3624 is used by the compiler for the rest of that block. You can refer to
3625 variables and filehandles in other packages by prefixing the identifier
3626 with the package name and a double colon: C<$Package::Variable>.
3627 If the package name is null, the C<main> package as assumed. That is,
3628 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3629 still seen in older code).
3631 If NAMESPACE is omitted, then there is no current package, and all
3632 identifiers must be fully qualified or lexicals. However, you are
3633 strongly advised not to make use of this feature. Its use can cause
3634 unexpected behaviour, even crashing some versions of Perl. It is
3635 deprecated, and will be removed from a future release.
3637 See L<perlmod/"Packages"> for more information about packages, modules,
3638 and classes. See L<perlsub> for other scoping issues.
3640 =item pipe READHANDLE,WRITEHANDLE
3642 Opens a pair of connected pipes like the corresponding system call.
3643 Note that if you set up a loop of piped processes, deadlock can occur
3644 unless you are very careful. In addition, note that Perl's pipes use
3645 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3646 after each command, depending on the application.
3648 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3649 for examples of such things.
3651 On systems that support a close-on-exec flag on files, the flag will be set
3652 for the newly opened file descriptors as determined by the value of $^F.
3659 Pops and returns the last value of the array, shortening the array by
3660 one element. Has an effect similar to
3664 If there are no elements in the array, returns the undefined value
3665 (although this may happen at other times as well). If ARRAY is
3666 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3667 array in subroutines, just like C<shift>.
3673 Returns the offset of where the last C<m//g> search left off for the variable
3674 in question (C<$_> is used when the variable is not specified). May be
3675 modified to change that offset. Such modification will also influence
3676 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3679 =item print FILEHANDLE LIST
3685 Prints a string or a list of strings. Returns true if successful.
3686 FILEHANDLE may be a scalar variable name, in which case the variable
3687 contains the name of or a reference to the filehandle, thus introducing
3688 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3689 the next token is a term, it may be misinterpreted as an operator
3690 unless you interpose a C<+> or put parentheses around the arguments.)
3691 If FILEHANDLE is omitted, prints by default to standard output (or
3692 to the last selected output channel--see L</select>). If LIST is
3693 also omitted, prints C<$_> to the currently selected output channel.
3694 To set the default output channel to something other than STDOUT
3695 use the select operation. The current value of C<$,> (if any) is
3696 printed between each LIST item. The current value of C<$\> (if
3697 any) is printed after the entire LIST has been printed. Because
3698 print takes a LIST, anything in the LIST is evaluated in list
3699 context, and any subroutine that you call will have one or more of
3700 its expressions evaluated in list context. Also be careful not to
3701 follow the print keyword with a left parenthesis unless you want
3702 the corresponding right parenthesis to terminate the arguments to
3703 the print--interpose a C<+> or put parentheses around all the
3706 Note that if you're storing FILEHANDLES in an array or other expression,
3707 you will have to use a block returning its value instead:
3709 print { $files[$i] } "stuff\n";
3710 print { $OK ? STDOUT : STDERR } "stuff\n";
3712 =item printf FILEHANDLE FORMAT, LIST
3714 =item printf FORMAT, LIST
3716 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3717 (the output record separator) is not appended. The first argument
3718 of the list will be interpreted as the C<printf> format. See C<sprintf>
3719 for an explanation of the format argument. If C<use locale> is in effect,
3720 the character used for the decimal point in formatted real numbers is
3721 affected by the LC_NUMERIC locale. See L<perllocale>.
3723 Don't fall into the trap of using a C<printf> when a simple
3724 C<print> would do. The C<print> is more efficient and less
3727 =item prototype FUNCTION
3729 Returns the prototype of a function as a string (or C<undef> if the
3730 function has no prototype). FUNCTION is a reference to, or the name of,
3731 the function whose prototype you want to retrieve.
3733 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3734 name for Perl builtin. If the builtin is not I<overridable> (such as
3735 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3736 C<system>) returns C<undef> because the builtin does not really behave
3737 like a Perl function. Otherwise, the string describing the equivalent
3738 prototype is returned.
3740 =item push ARRAY,LIST
3742 Treats ARRAY as a stack, and pushes the values of LIST
3743 onto the end of ARRAY. The length of ARRAY increases by the length of
3744 LIST. Has the same effect as
3747 $ARRAY[++$#ARRAY] = $value;
3750 but is more efficient. Returns the new number of elements in the array.
3762 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3764 =item quotemeta EXPR
3768 Returns the value of EXPR with all non-"word"
3769 characters backslashed. (That is, all characters not matching
3770 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3771 returned string, regardless of any locale settings.)
3772 This is the internal function implementing
3773 the C<\Q> escape in double-quoted strings.
3775 If EXPR is omitted, uses C<$_>.
3781 Returns a random fractional number greater than or equal to C<0> and less
3782 than the value of EXPR. (EXPR should be positive.) If EXPR is
3783 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3784 also special-cased as C<1> - this has not been documented before perl 5.8.0
3785 and is subject to change in future versions of perl. Automatically calls
3786 C<srand> unless C<srand> has already been called. See also C<srand>.
3788 Apply C<int()> to the value returned by C<rand()> if you want random
3789 integers instead of random fractional numbers. For example,
3793 returns a random integer between C<0> and C<9>, inclusive.
3795 (Note: If your rand function consistently returns numbers that are too
3796 large or too small, then your version of Perl was probably compiled
3797 with the wrong number of RANDBITS.)
3799 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3801 =item read FILEHANDLE,SCALAR,LENGTH
3803 Attempts to read LENGTH I<characters> of data into variable SCALAR
3804 from the specified FILEHANDLE. Returns the number of characters
3805 actually read, C<0> at end of file, or undef if there was an error (in
3806 the latter case C<$!> is also set). SCALAR will be grown or shrunk to
3807 the length actually read. If SCALAR needs growing, the new bytes will
3808 be zero bytes. An OFFSET may be specified to place the read data into
3809 some other place in SCALAR than the beginning. The call is actually
3810 implemented in terms of either Perl's or system's fread() call. To
3811 get a true read(2) system call, see C<sysread>.
3813 Note the I<characters>: depending on the status of the filehandle,
3814 either (8-bit) bytes or characters are read. By default all
3815 filehandles operate on bytes, but for example if the filehandle has
3816 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3817 pragma, L<open>), the I/O will operate on characters, not bytes.
3819 =item readdir DIRHANDLE
3821 Returns the next directory entry for a directory opened by C<opendir>.
3822 If used in list context, returns all the rest of the entries in the
3823 directory. If there are no more entries, returns an undefined value in
3824 scalar context or a null list in list context.
3826 If you're planning to filetest the return values out of a C<readdir>, you'd
3827 better prepend the directory in question. Otherwise, because we didn't
3828 C<chdir> there, it would have been testing the wrong file.
3830 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3831 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3836 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3837 context, each call reads and returns the next line, until end-of-file is
3838 reached, whereupon the subsequent call returns undef. In list context,
3839 reads until end-of-file is reached and returns a list of lines. Note that
3840 the notion of "line" used here is however you may have defined it
3841 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3843 When C<$/> is set to C<undef>, when readline() is in scalar
3844 context (i.e. file slurp mode), and when an empty file is read, it
3845 returns C<''> the first time, followed by C<undef> subsequently.
3847 This is the internal function implementing the C<< <EXPR> >>
3848 operator, but you can use it directly. The C<< <EXPR> >>
3849 operator is discussed in more detail in L<perlop/"I/O Operators">.
3852 $line = readline(*STDIN); # same thing
3854 If readline encounters an operating system error, C<$!> will be set with the
3855 corresponding error message. It can be helpful to check C<$!> when you are
3856 reading from filehandles you don't trust, such as a tty or a socket. The
3857 following example uses the operator form of C<readline>, and takes the necessary
3858 steps to ensure that C<readline> was successful.
3862 unless (defined( $line = <> )) {
3873 Returns the value of a symbolic link, if symbolic links are
3874 implemented. If not, gives a fatal error. If there is some system
3875 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3876 omitted, uses C<$_>.
3880 EXPR is executed as a system command.
3881 The collected standard output of the command is returned.
3882 In scalar context, it comes back as a single (potentially
3883 multi-line) string. In list context, returns a list of lines
3884 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3885 This is the internal function implementing the C<qx/EXPR/>
3886 operator, but you can use it directly. The C<qx/EXPR/>
3887 operator is discussed in more detail in L<perlop/"I/O Operators">.
3889 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3891 Receives a message on a socket. Attempts to receive LENGTH characters
3892 of data into variable SCALAR from the specified SOCKET filehandle.
3893 SCALAR will be grown or shrunk to the length actually read. Takes the
3894 same flags as the system call of the same name. Returns the address
3895 of the sender if SOCKET's protocol supports this; returns an empty
3896 string otherwise. If there's an error, returns the undefined value.
3897 This call is actually implemented in terms of recvfrom(2) system call.
3898 See L<perlipc/"UDP: Message Passing"> for examples.
3900 Note the I<characters>: depending on the status of the socket, either
3901 (8-bit) bytes or characters are received. By default all sockets
3902 operate on bytes, but for example if the socket has been changed using
3903 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3904 pragma, L<open>), the I/O will operate on characters, not bytes.
3910 The C<redo> command restarts the loop block without evaluating the
3911 conditional again. The C<continue> block, if any, is not executed. If
3912 the LABEL is omitted, the command refers to the innermost enclosing
3913 loop. This command is normally used by programs that want to lie to
3914 themselves about what was just input:
3916 # a simpleminded Pascal comment stripper
3917 # (warning: assumes no { or } in strings)
3918 LINE: while (<STDIN>) {
3919 while (s|({.*}.*){.*}|$1 |) {}
3924 if (/}/) { # end of comment?
3933 C<redo> cannot be used to retry a block which returns a value such as
3934 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3935 a grep() or map() operation.
3937 Note that a block by itself is semantically identical to a loop
3938 that executes once. Thus C<redo> inside such a block will effectively
3939 turn it into a looping construct.
3941 See also L</continue> for an illustration of how C<last>, C<next>, and
3948 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3949 is not specified, C<$_> will be used. The value returned depends on the
3950 type of thing the reference is a reference to.
3951 Builtin types include:
3961 If the referenced object has been blessed into a package, then that package
3962 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3964 if (ref($r) eq "HASH") {
3965 print "r is a reference to a hash.\n";
3968 print "r is not a reference at all.\n";
3970 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3971 print "r is a reference to something that isa hash.\n";
3974 See also L<perlref>.
3976 =item rename OLDNAME,NEWNAME
3978 Changes the name of a file; an existing file NEWNAME will be
3979 clobbered. Returns true for success, false otherwise.
3981 Behavior of this function varies wildly depending on your system
3982 implementation. For example, it will usually not work across file system
3983 boundaries, even though the system I<mv> command sometimes compensates
3984 for this. Other restrictions include whether it works on directories,
3985 open files, or pre-existing files. Check L<perlport> and either the
3986 rename(2) manpage or equivalent system documentation for details.
3988 =item require VERSION
3994 Demands a version of Perl specified by VERSION, or demands some semantics
3995 specified by EXPR or by C<$_> if EXPR is not supplied.
3997 VERSION may be either a numeric argument such as 5.006, which will be
3998 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3999 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4000 VERSION is greater than the version of the current Perl interpreter.
4001 Compare with L</use>, which can do a similar check at compile time.
4003 Specifying VERSION as a literal of the form v5.6.1 should generally be
4004 avoided, because it leads to misleading error messages under earlier
4005 versions of Perl which do not support this syntax. The equivalent numeric
4006 version should be used instead.
4008 require v5.6.1; # run time version check
4009 require 5.6.1; # ditto
4010 require 5.006_001; # ditto; preferred for backwards compatibility
4012 Otherwise, demands that a library file be included if it hasn't already
4013 been included. The file is included via the do-FILE mechanism, which is
4014 essentially just a variety of C<eval>. Has semantics similar to the following
4019 return 1 if $INC{$filename};
4020 my($realfilename,$result);
4022 foreach $prefix (@INC) {
4023 $realfilename = "$prefix/$filename";
4024 if (-f $realfilename) {
4025 $INC{$filename} = $realfilename;
4026 $result = do $realfilename;
4030 die "Can't find $filename in \@INC";
4032 delete $INC{$filename} if $@ || !$result;
4034 die "$filename did not return true value" unless $result;
4038 Note that the file will not be included twice under the same specified
4039 name. The file must return true as the last statement to indicate
4040 successful execution of any initialization code, so it's customary to
4041 end such a file with C<1;> unless you're sure it'll return true
4042 otherwise. But it's better just to put the C<1;>, in case you add more
4045 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4046 replaces "F<::>" with "F</>" in the filename for you,
4047 to make it easy to load standard modules. This form of loading of
4048 modules does not risk altering your namespace.
4050 In other words, if you try this:
4052 require Foo::Bar; # a splendid bareword
4054 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4055 directories specified in the C<@INC> array.
4057 But if you try this:
4059 $class = 'Foo::Bar';
4060 require $class; # $class is not a bareword
4062 require "Foo::Bar"; # not a bareword because of the ""
4064 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4065 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4067 eval "require $class";
4069 Now that you understand how C<require> looks for files in the case of
4070 a bareword argument, there is a little extra functionality going on
4071 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4072 it will first look for a filename with a "F<.pmc>" extension. A file
4073 with this extension is assumed to be Perl bytecode generated by
4074 L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
4075 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4076 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4078 You can also insert hooks into the import facility, by putting directly
4079 Perl code into the @INC array. There are three forms of hooks: subroutine
4080 references, array references and blessed objects.
4082 Subroutine references are the simplest case. When the inclusion system
4083 walks through @INC and encounters a subroutine, this subroutine gets
4084 called with two parameters, the first being a reference to itself, and the
4085 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4086 subroutine should return C<undef> or a filehandle, from which the file to
4087 include will be read. If C<undef> is returned, C<require> will look at
4088 the remaining elements of @INC.
4090 If the hook is an array reference, its first element must be a subroutine
4091 reference. This subroutine is called as above, but the first parameter is
4092 the array reference. This enables to pass indirectly some arguments to
4095 In other words, you can write:
4097 push @INC, \&my_sub;
4099 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4105 push @INC, [ \&my_sub, $x, $y, ... ];
4107 my ($arrayref, $filename) = @_;
4108 # Retrieve $x, $y, ...
4109 my @parameters = @$arrayref[1..$#$arrayref];
4113 If the hook is an object, it must provide an INC method, that will be
4114 called as above, the first parameter being the object itself. (Note that
4115 you must fully qualify the sub's name, as it is always forced into package
4116 C<main>.) Here is a typical code layout:
4122 my ($self, $filename) = @_;
4126 # In the main program
4127 push @INC, new Foo(...);
4129 Note that these hooks are also permitted to set the %INC entry
4130 corresponding to the files they have loaded. See L<perlvar/%INC>.
4132 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4138 Generally used in a C<continue> block at the end of a loop to clear
4139 variables and reset C<??> searches so that they work again. The
4140 expression is interpreted as a list of single characters (hyphens
4141 allowed for ranges). All variables and arrays beginning with one of
4142 those letters are reset to their pristine state. If the expression is
4143 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4144 only variables or searches in the current package. Always returns
4147 reset 'X'; # reset all X variables
4148 reset 'a-z'; # reset lower case variables
4149 reset; # just reset ?one-time? searches
4151 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4152 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4153 variables--lexical variables are unaffected, but they clean themselves
4154 up on scope exit anyway, so you'll probably want to use them instead.
4161 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4162 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4163 context, depending on how the return value will be used, and the context
4164 may vary from one execution to the next (see C<wantarray>). If no EXPR
4165 is given, returns an empty list in list context, the undefined value in
4166 scalar context, and (of course) nothing at all in a void context.
4168 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4169 or do FILE will automatically return the value of the last expression
4174 In list context, returns a list value consisting of the elements
4175 of LIST in the opposite order. In scalar context, concatenates the
4176 elements of LIST and returns a string value with all characters
4177 in the opposite order.
4179 print reverse <>; # line tac, last line first
4181 undef $/; # for efficiency of <>
4182 print scalar reverse <>; # character tac, last line tsrif
4184 This operator is also handy for inverting a hash, although there are some
4185 caveats. If a value is duplicated in the original hash, only one of those
4186 can be represented as a key in the inverted hash. Also, this has to
4187 unwind one hash and build a whole new one, which may take some time
4188 on a large hash, such as from a DBM file.
4190 %by_name = reverse %by_address; # Invert the hash
4192 =item rewinddir DIRHANDLE
4194 Sets the current position to the beginning of the directory for the
4195 C<readdir> routine on DIRHANDLE.
4197 =item rindex STR,SUBSTR,POSITION
4199 =item rindex STR,SUBSTR
4201 Works just like index() except that it returns the position of the LAST
4202 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4203 last occurrence at or before that position.
4205 =item rmdir FILENAME
4209 Deletes the directory specified by FILENAME if that directory is
4210 empty. If it succeeds it returns true, otherwise it returns false and
4211 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4215 The substitution operator. See L<perlop>.
4219 Forces EXPR to be interpreted in scalar context and returns the value
4222 @counts = ( scalar @a, scalar @b, scalar @c );
4224 There is no equivalent operator to force an expression to
4225 be interpolated in list context because in practice, this is never
4226 needed. If you really wanted to do so, however, you could use
4227 the construction C<@{[ (some expression) ]}>, but usually a simple
4228 C<(some expression)> suffices.
4230 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4231 parenthesized list, this behaves as a scalar comma expression, evaluating
4232 all but the last element in void context and returning the final element
4233 evaluated in scalar context. This is seldom what you want.
4235 The following single statement:
4237 print uc(scalar(&foo,$bar)),$baz;
4239 is the moral equivalent of these two:
4242 print(uc($bar),$baz);
4244 See L<perlop> for more details on unary operators and the comma operator.
4246 =item seek FILEHANDLE,POSITION,WHENCE
4248 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4249 FILEHANDLE may be an expression whose value gives the name of the
4250 filehandle. The values for WHENCE are C<0> to set the new position
4251 I<in bytes> to POSITION, C<1> to set it to the current position plus
4252 POSITION, and C<2> to set it to EOF plus POSITION (typically
4253 negative). For WHENCE you may use the constants C<SEEK_SET>,
4254 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4255 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4258 Note the I<in bytes>: even if the filehandle has been set to
4259 operate on characters (for example by using the C<:utf8> open
4260 layer), tell() will return byte offsets, not character offsets
4261 (because implementing that would render seek() and tell() rather slow).
4263 If you want to position file for C<sysread> or C<syswrite>, don't use
4264 C<seek>--buffering makes its effect on the file's system position
4265 unpredictable and non-portable. Use C<sysseek> instead.
4267 Due to the rules and rigors of ANSI C, on some systems you have to do a
4268 seek whenever you switch between reading and writing. Amongst other
4269 things, this may have the effect of calling stdio's clearerr(3).
4270 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4274 This is also useful for applications emulating C<tail -f>. Once you hit
4275 EOF on your read, and then sleep for a while, you might have to stick in a
4276 seek() to reset things. The C<seek> doesn't change the current position,
4277 but it I<does> clear the end-of-file condition on the handle, so that the
4278 next C<< <FILE> >> makes Perl try again to read something. We hope.
4280 If that doesn't work (some IO implementations are particularly
4281 cantankerous), then you may need something more like this:
4284 for ($curpos = tell(FILE); $_ = <FILE>;
4285 $curpos = tell(FILE)) {
4286 # search for some stuff and put it into files
4288 sleep($for_a_while);
4289 seek(FILE, $curpos, 0);
4292 =item seekdir DIRHANDLE,POS
4294 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4295 must be a value returned by C<telldir>. Has the same caveats about
4296 possible directory compaction as the corresponding system library
4299 =item select FILEHANDLE
4303 Returns the currently selected filehandle. Sets the current default
4304 filehandle for output, if FILEHANDLE is supplied. This has two
4305 effects: first, a C<write> or a C<print> without a filehandle will
4306 default to this FILEHANDLE. Second, references to variables related to
4307 output will refer to this output channel. For example, if you have to
4308 set the top of form format for more than one output channel, you might
4316 FILEHANDLE may be an expression whose value gives the name of the
4317 actual filehandle. Thus:
4319 $oldfh = select(STDERR); $| = 1; select($oldfh);
4321 Some programmers may prefer to think of filehandles as objects with
4322 methods, preferring to write the last example as:
4325 STDERR->autoflush(1);
4327 =item select RBITS,WBITS,EBITS,TIMEOUT
4329 This calls the select(2) system call with the bit masks specified, which
4330 can be constructed using C<fileno> and C<vec>, along these lines:
4332 $rin = $win = $ein = '';
4333 vec($rin,fileno(STDIN),1) = 1;
4334 vec($win,fileno(STDOUT),1) = 1;
4337 If you want to select on many filehandles you might wish to write a
4341 my(@fhlist) = split(' ',$_[0]);
4344 vec($bits,fileno($_),1) = 1;
4348 $rin = fhbits('STDIN TTY SOCK');
4352 ($nfound,$timeleft) =
4353 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4355 or to block until something becomes ready just do this
4357 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4359 Most systems do not bother to return anything useful in $timeleft, so
4360 calling select() in scalar context just returns $nfound.
4362 Any of the bit masks can also be undef. The timeout, if specified, is
4363 in seconds, which may be fractional. Note: not all implementations are
4364 capable of returning the $timeleft. If not, they always return
4365 $timeleft equal to the supplied $timeout.
4367 You can effect a sleep of 250 milliseconds this way:
4369 select(undef, undef, undef, 0.25);
4371 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4372 is implementation-dependent.
4374 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4375 or <FH>) with C<select>, except as permitted by POSIX, and even
4376 then only on POSIX systems. You have to use C<sysread> instead.
4378 =item semctl ID,SEMNUM,CMD,ARG
4380 Calls the System V IPC function C<semctl>. You'll probably have to say
4384 first to get the correct constant definitions. If CMD is IPC_STAT or
4385 GETALL, then ARG must be a variable which will hold the returned
4386 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4387 the undefined value for error, "C<0 but true>" for zero, or the actual
4388 return value otherwise. The ARG must consist of a vector of native
4389 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4390 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4393 =item semget KEY,NSEMS,FLAGS
4395 Calls the System V IPC function semget. Returns the semaphore id, or
4396 the undefined value if there is an error. See also
4397 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4400 =item semop KEY,OPSTRING
4402 Calls the System V IPC function semop to perform semaphore operations
4403 such as signalling and waiting. OPSTRING must be a packed array of
4404 semop structures. Each semop structure can be generated with
4405 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4406 operations is implied by the length of OPSTRING. Returns true if
4407 successful, or false if there is an error. As an example, the
4408 following code waits on semaphore $semnum of semaphore id $semid:
4410 $semop = pack("s!3", $semnum, -1, 0);
4411 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4413 To signal the semaphore, replace C<-1> with C<1>. See also
4414 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4417 =item send SOCKET,MSG,FLAGS,TO
4419 =item send SOCKET,MSG,FLAGS
4421 Sends a message on a socket. Attempts to send the scalar MSG to the
4422 SOCKET filehandle. Takes the same flags as the system call of the
4423 same name. On unconnected sockets you must specify a destination to
4424 send TO, in which case it does a C C<sendto>. Returns the number of
4425 characters sent, or the undefined value if there is an error. The C
4426 system call sendmsg(2) is currently unimplemented. See
4427 L<perlipc/"UDP: Message Passing"> for examples.
4429 Note the I<characters>: depending on the status of the socket, either
4430 (8-bit) bytes or characters are sent. By default all sockets operate
4431 on bytes, but for example if the socket has been changed using
4432 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4433 the C<open> pragma, L<open>), the I/O will operate on characters, not
4436 =item setpgrp PID,PGRP
4438 Sets the current process group for the specified PID, C<0> for the current
4439 process. Will produce a fatal error if used on a machine that doesn't
4440 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4441 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4442 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4445 =item setpriority WHICH,WHO,PRIORITY
4447 Sets the current priority for a process, a process group, or a user.
4448 (See setpriority(2).) Will produce a fatal error if used on a machine
4449 that doesn't implement setpriority(2).
4451 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4453 Sets the socket option requested. Returns undefined if there is an
4454 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4461 Shifts the first value of the array off and returns it, shortening the
4462 array by 1 and moving everything down. If there are no elements in the
4463 array, returns the undefined value. If ARRAY is omitted, shifts the
4464 C<@_> array within the lexical scope of subroutines and formats, and the
4465 C<@ARGV> array at file scopes or within the lexical scopes established by
4466 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4469 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4470 same thing to the left end of an array that C<pop> and C<push> do to the
4473 =item shmctl ID,CMD,ARG
4475 Calls the System V IPC function shmctl. You'll probably have to say
4479 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4480 then ARG must be a variable which will hold the returned C<shmid_ds>
4481 structure. Returns like ioctl: the undefined value for error, "C<0> but
4482 true" for zero, or the actual return value otherwise.
4483 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4485 =item shmget KEY,SIZE,FLAGS
4487 Calls the System V IPC function shmget. Returns the shared memory
4488 segment id, or the undefined value if there is an error.
4489 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4491 =item shmread ID,VAR,POS,SIZE
4493 =item shmwrite ID,STRING,POS,SIZE
4495 Reads or writes the System V shared memory segment ID starting at
4496 position POS for size SIZE by attaching to it, copying in/out, and
4497 detaching from it. When reading, VAR must be a variable that will
4498 hold the data read. When writing, if STRING is too long, only SIZE
4499 bytes are used; if STRING is too short, nulls are written to fill out
4500 SIZE bytes. Return true if successful, or false if there is an error.
4501 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4502 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4504 =item shutdown SOCKET,HOW
4506 Shuts down a socket connection in the manner indicated by HOW, which
4507 has the same interpretation as in the system call of the same name.
4509 shutdown(SOCKET, 0); # I/we have stopped reading data
4510 shutdown(SOCKET, 1); # I/we have stopped writing data
4511 shutdown(SOCKET, 2); # I/we have stopped using this socket
4513 This is useful with sockets when you want to tell the other
4514 side you're done writing but not done reading, or vice versa.
4515 It's also a more insistent form of close because it also
4516 disables the file descriptor in any forked copies in other
4523 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4524 returns sine of C<$_>.
4526 For the inverse sine operation, you may use the C<Math::Trig::asin>
4527 function, or use this relation:
4529 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4535 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4536 May be interrupted if the process receives a signal such as C<SIGALRM>.
4537 Returns the number of seconds actually slept. You probably cannot
4538 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4541 On some older systems, it may sleep up to a full second less than what
4542 you requested, depending on how it counts seconds. Most modern systems
4543 always sleep the full amount. They may appear to sleep longer than that,
4544 however, because your process might not be scheduled right away in a
4545 busy multitasking system.
4547 For delays of finer granularity than one second, you may use Perl's
4548 C<syscall> interface to access setitimer(2) if your system supports
4549 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4550 and starting from Perl 5.8 part of the standard distribution) may also
4553 See also the POSIX module's C<pause> function.
4555 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4557 Opens a socket of the specified kind and attaches it to filehandle
4558 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4559 the system call of the same name. You should C<use Socket> first
4560 to get the proper definitions imported. See the examples in
4561 L<perlipc/"Sockets: Client/Server Communication">.
4563 On systems that support a close-on-exec flag on files, the flag will
4564 be set for the newly opened file descriptor, as determined by the
4565 value of $^F. See L<perlvar/$^F>.
4567 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4569 Creates an unnamed pair of sockets in the specified domain, of the
4570 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4571 for the system call of the same name. If unimplemented, yields a fatal
4572 error. Returns true if successful.
4574 On systems that support a close-on-exec flag on files, the flag will
4575 be set for the newly opened file descriptors, as determined by the value
4576 of $^F. See L<perlvar/$^F>.
4578 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4579 to C<pipe(Rdr, Wtr)> is essentially:
4582 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4583 shutdown(Rdr, 1); # no more writing for reader
4584 shutdown(Wtr, 0); # no more reading for writer
4586 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4587 emulate socketpair using IP sockets to localhost if your system implements
4588 sockets but not socketpair.
4590 =item sort SUBNAME LIST
4592 =item sort BLOCK LIST
4596 In list context, this sorts the LIST and returns the sorted list value.
4597 In scalar context, the behaviour of C<sort()> is undefined.
4599 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4600 order. If SUBNAME is specified, it gives the name of a subroutine
4601 that returns an integer less than, equal to, or greater than C<0>,
4602 depending on how the elements of the list are to be ordered. (The C<<
4603 <=> >> and C<cmp> operators are extremely useful in such routines.)
4604 SUBNAME may be a scalar variable name (unsubscripted), in which case
4605 the value provides the name of (or a reference to) the actual
4606 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4607 an anonymous, in-line sort subroutine.
4609 If the subroutine's prototype is C<($$)>, the elements to be compared
4610 are passed by reference in C<@_>, as for a normal subroutine. This is
4611 slower than unprototyped subroutines, where the elements to be
4612 compared are passed into the subroutine
4613 as the package global variables $a and $b (see example below). Note that
4614 in the latter case, it is usually counter-productive to declare $a and
4617 In either case, the subroutine may not be recursive. The values to be
4618 compared are always passed by reference, so don't modify them.
4620 You also cannot exit out of the sort block or subroutine using any of the
4621 loop control operators described in L<perlsyn> or with C<goto>.
4623 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4624 current collation locale. See L<perllocale>.
4626 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4627 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4628 preserves the input order of elements that compare equal. Although
4629 quicksort's run time is O(NlogN) when averaged over all arrays of
4630 length N, the time can be O(N**2), I<quadratic> behavior, for some
4631 inputs.) In 5.7, the quicksort implementation was replaced with
4632 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4633 But benchmarks indicated that for some inputs, on some platforms,
4634 the original quicksort was faster. 5.8 has a sort pragma for
4635 limited control of the sort. Its rather blunt control of the
4636 underlying algorithm may not persist into future perls, but the
4637 ability to characterize the input or output in implementation
4638 independent ways quite probably will. See L<sort>.
4643 @articles = sort @files;
4645 # same thing, but with explicit sort routine
4646 @articles = sort {$a cmp $b} @files;
4648 # now case-insensitively
4649 @articles = sort {uc($a) cmp uc($b)} @files;
4651 # same thing in reversed order
4652 @articles = sort {$b cmp $a} @files;
4654 # sort numerically ascending
4655 @articles = sort {$a <=> $b} @files;
4657 # sort numerically descending
4658 @articles = sort {$b <=> $a} @files;
4660 # this sorts the %age hash by value instead of key
4661 # using an in-line function
4662 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4664 # sort using explicit subroutine name
4666 $age{$a} <=> $age{$b}; # presuming numeric
4668 @sortedclass = sort byage @class;
4670 sub backwards { $b cmp $a }
4671 @harry = qw(dog cat x Cain Abel);
4672 @george = qw(gone chased yz Punished Axed);
4674 # prints AbelCaincatdogx
4675 print sort backwards @harry;
4676 # prints xdogcatCainAbel
4677 print sort @george, 'to', @harry;
4678 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4680 # inefficiently sort by descending numeric compare using
4681 # the first integer after the first = sign, or the
4682 # whole record case-insensitively otherwise
4685 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4690 # same thing, but much more efficiently;
4691 # we'll build auxiliary indices instead
4695 push @nums, /=(\d+)/;
4700 $nums[$b] <=> $nums[$a]
4702 $caps[$a] cmp $caps[$b]
4706 # same thing, but without any temps
4707 @new = map { $_->[0] }
4708 sort { $b->[1] <=> $a->[1]
4711 } map { [$_, /=(\d+)/, uc($_)] } @old;
4713 # using a prototype allows you to use any comparison subroutine
4714 # as a sort subroutine (including other package's subroutines)
4716 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4719 @new = sort other::backwards @old;
4721 # guarantee stability, regardless of algorithm
4723 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4725 # force use of mergesort (not portable outside Perl 5.8)
4726 use sort '_mergesort'; # note discouraging _
4727 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4729 If you're using strict, you I<must not> declare $a
4730 and $b as lexicals. They are package globals. That means
4731 if you're in the C<main> package and type
4733 @articles = sort {$b <=> $a} @files;
4735 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4736 but if you're in the C<FooPack> package, it's the same as typing
4738 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4740 The comparison function is required to behave. If it returns
4741 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4742 sometimes saying the opposite, for example) the results are not
4745 =item splice ARRAY,OFFSET,LENGTH,LIST
4747 =item splice ARRAY,OFFSET,LENGTH
4749 =item splice ARRAY,OFFSET
4753 Removes the elements designated by OFFSET and LENGTH from an array, and
4754 replaces them with the elements of LIST, if any. In list context,
4755 returns the elements removed from the array. In scalar context,
4756 returns the last element removed, or C<undef> if no elements are
4757 removed. The array grows or shrinks as necessary.
4758 If OFFSET is negative then it starts that far from the end of the array.
4759 If LENGTH is omitted, removes everything from OFFSET onward.
4760 If LENGTH is negative, removes the elements from OFFSET onward
4761 except for -LENGTH elements at the end of the array.
4762 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4763 past the end of the array, perl issues a warning, and splices at the
4766 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4768 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4769 pop(@a) splice(@a,-1)
4770 shift(@a) splice(@a,0,1)
4771 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4772 $a[$i] = $y splice(@a,$i,1,$y)
4774 Example, assuming array lengths are passed before arrays:
4776 sub aeq { # compare two list values
4777 my(@a) = splice(@_,0,shift);
4778 my(@b) = splice(@_,0,shift);
4779 return 0 unless @a == @b; # same len?
4781 return 0 if pop(@a) ne pop(@b);
4785 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4787 =item split /PATTERN/,EXPR,LIMIT
4789 =item split /PATTERN/,EXPR
4791 =item split /PATTERN/
4795 Splits a string into a list of strings and returns that list. By default,
4796 empty leading fields are preserved, and empty trailing ones are deleted.
4798 In scalar context, returns the number of fields found and splits into
4799 the C<@_> array. Use of split in scalar context is deprecated, however,
4800 because it clobbers your subroutine arguments.
4802 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4803 splits on whitespace (after skipping any leading whitespace). Anything
4804 matching PATTERN is taken to be a delimiter separating the fields. (Note
4805 that the delimiter may be longer than one character.)
4807 If LIMIT is specified and positive, it represents the maximum number
4808 of fields the EXPR will be split into, though the actual number of
4809 fields returned depends on the number of times PATTERN matches within
4810 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4811 stripped (which potential users of C<pop> would do well to remember).
4812 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4813 had been specified. Note that splitting an EXPR that evaluates to the
4814 empty string always returns the empty list, regardless of the LIMIT
4817 A pattern matching the null string (not to be confused with
4818 a null pattern C<//>, which is just one member of the set of patterns
4819 matching a null string) will split the value of EXPR into separate
4820 characters at each point it matches that way. For example:
4822 print join(':', split(/ */, 'hi there'));
4824 produces the output 'h:i:t:h:e:r:e'.
4826 Using the empty pattern C<//> specifically matches the null string, and is
4827 not be confused with the use of C<//> to mean "the last successful pattern
4830 Empty leading (or trailing) fields are produced when there are positive width
4831 matches at the beginning (or end) of the string; a zero-width match at the
4832 beginning (or end) of the string does not produce an empty field. For
4835 print join(':', split(/(?=\w)/, 'hi there!'));
4837 produces the output 'h:i :t:h:e:r:e!'.
4839 The LIMIT parameter can be used to split a line partially
4841 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4843 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4844 one larger than the number of variables in the list, to avoid
4845 unnecessary work. For the list above LIMIT would have been 4 by
4846 default. In time critical applications it behooves you not to split
4847 into more fields than you really need.
4849 If the PATTERN contains parentheses, additional list elements are
4850 created from each matching substring in the delimiter.
4852 split(/([,-])/, "1-10,20", 3);
4854 produces the list value
4856 (1, '-', 10, ',', 20)
4858 If you had the entire header of a normal Unix email message in $header,
4859 you could split it up into fields and their values this way:
4861 $header =~ s/\n\s+/ /g; # fix continuation lines
4862 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4864 The pattern C</PATTERN/> may be replaced with an expression to specify
4865 patterns that vary at runtime. (To do runtime compilation only once,
4866 use C</$variable/o>.)
4868 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4869 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4870 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4871 will give you as many null initial fields as there are leading spaces.
4872 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4873 whitespace produces a null first field. A C<split> with no arguments
4874 really does a S<C<split(' ', $_)>> internally.
4876 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4881 open(PASSWD, '/etc/passwd');
4884 ($login, $passwd, $uid, $gid,
4885 $gcos, $home, $shell) = split(/:/);
4889 As with regular pattern matching, any capturing parentheses that are not
4890 matched in a C<split()> will be set to C<undef> when returned:
4892 @fields = split /(A)|B/, "1A2B3";
4893 # @fields is (1, 'A', 2, undef, 3)
4895 =item sprintf FORMAT, LIST
4897 Returns a string formatted by the usual C<printf> conventions of the C
4898 library function C<sprintf>. See below for more details
4899 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4900 the general principles.
4904 # Format number with up to 8 leading zeroes
4905 $result = sprintf("%08d", $number);
4907 # Round number to 3 digits after decimal point
4908 $rounded = sprintf("%.3f", $number);
4910 Perl does its own C<sprintf> formatting--it emulates the C
4911 function C<sprintf>, but it doesn't use it (except for floating-point
4912 numbers, and even then only the standard modifiers are allowed). As a
4913 result, any non-standard extensions in your local C<sprintf> are not
4914 available from Perl.
4916 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4917 pass it an array as your first argument. The array is given scalar context,
4918 and instead of using the 0th element of the array as the format, Perl will
4919 use the count of elements in the array as the format, which is almost never
4922 Perl's C<sprintf> permits the following universally-known conversions:
4925 %c a character with the given number
4927 %d a signed integer, in decimal
4928 %u an unsigned integer, in decimal
4929 %o an unsigned integer, in octal
4930 %x an unsigned integer, in hexadecimal
4931 %e a floating-point number, in scientific notation
4932 %f a floating-point number, in fixed decimal notation
4933 %g a floating-point number, in %e or %f notation
4935 In addition, Perl permits the following widely-supported conversions:
4937 %X like %x, but using upper-case letters
4938 %E like %e, but using an upper-case "E"
4939 %G like %g, but with an upper-case "E" (if applicable)
4940 %b an unsigned integer, in binary
4941 %p a pointer (outputs the Perl value's address in hexadecimal)
4942 %n special: *stores* the number of characters output so far
4943 into the next variable in the parameter list
4945 Finally, for backward (and we do mean "backward") compatibility, Perl
4946 permits these unnecessary but widely-supported conversions:
4949 %D a synonym for %ld
4950 %U a synonym for %lu
4951 %O a synonym for %lo
4954 Note that the number of exponent digits in the scientific notation produced
4955 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4956 exponent less than 100 is system-dependent: it may be three or less
4957 (zero-padded as necessary). In other words, 1.23 times ten to the
4958 99th may be either "1.23e99" or "1.23e099".
4960 Between the C<%> and the format letter, you may specify a number of
4961 additional attributes controlling the interpretation of the format.
4962 In order, these are:
4966 =item format parameter index
4968 An explicit format parameter index, such as C<2$>. By default sprintf
4969 will format the next unused argument in the list, but this allows you
4970 to take the arguments out of order. Eg:
4972 printf '%2$d %1$d', 12, 34; # prints "34 12"
4973 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4978 space prefix positive number with a space
4979 + prefix positive number with a plus sign
4980 - left-justify within the field
4981 0 use zeros, not spaces, to right-justify
4982 # prefix non-zero octal with "0", non-zero hex with "0x",
4983 non-zero binary with "0b"
4987 printf '<% d>', 12; # prints "< 12>"
4988 printf '<%+d>', 12; # prints "<+12>"
4989 printf '<%6s>', 12; # prints "< 12>"
4990 printf '<%-6s>', 12; # prints "<12 >"
4991 printf '<%06s>', 12; # prints "<000012>"
4992 printf '<%#x>', 12; # prints "<0xc>"
4996 The vector flag C<v>, optionally specifying the join string to use.
4997 This flag tells perl to interpret the supplied string as a vector
4998 of integers, one for each character in the string, separated by
4999 a given string (a dot C<.> by default). This can be useful for
5000 displaying ordinal values of characters in arbitrary strings:
5002 printf "version is v%vd\n", $^V; # Perl's version
5004 Put an asterisk C<*> before the C<v> to override the string to
5005 use to separate the numbers:
5007 printf "address is %*vX\n", ":", $addr; # IPv6 address
5008 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5010 You can also explicitly specify the argument number to use for
5011 the join string using eg C<*2$v>:
5013 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5015 =item (minimum) width
5017 Arguments are usually formatted to be only as wide as required to
5018 display the given value. You can override the width by putting
5019 a number here, or get the width from the next argument (with C<*>)
5020 or from a specified argument (with eg C<*2$>):
5022 printf '<%s>', "a"; # prints "<a>"
5023 printf '<%6s>', "a"; # prints "< a>"
5024 printf '<%*s>', 6, "a"; # prints "< a>"
5025 printf '<%*2$s>', "a", 6; # prints "< a>"
5026 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5028 If a field width obtained through C<*> is negative, it has the same
5029 effect as the C<-> flag: left-justification.
5031 =item precision, or maximum width
5033 You can specify a precision (for numeric conversions) or a maximum
5034 width (for string conversions) by specifying a C<.> followed by a number.
5035 For floating point formats, with the exception of 'g' and 'G', this specifies
5036 the number of decimal places to show (the default being 6), eg:
5038 # these examples are subject to system-specific variation
5039 printf '<%f>', 1; # prints "<1.000000>"
5040 printf '<%.1f>', 1; # prints "<1.0>"
5041 printf '<%.0f>', 1; # prints "<1>"
5042 printf '<%e>', 10; # prints "<1.000000e+01>"
5043 printf '<%.1e>', 10; # prints "<1.0e+01>"
5045 For 'g' and 'G', this specifies the maximum number of digits to show,
5046 including prior to the decimal point as well as after it, eg:
5048 # these examples are subject to system-specific variation
5049 printf '<%g>', 1; # prints "<1>"
5050 printf '<%.10g>', 1; # prints "<1>"
5051 printf '<%g>', 100; # prints "<100>"
5052 printf '<%.1g>', 100; # prints "<1e+02>"
5053 printf '<%.2g>', 100.01; # prints "<1e+02>"
5054 printf '<%.5g>', 100.01; # prints "<100.01>"
5055 printf '<%.4g>', 100.01; # prints "<100>"
5057 For integer conversions, specifying a precision implies that the
5058 output of the number itself should be zero-padded to this width:
5060 printf '<%.6x>', 1; # prints "<000001>"
5061 printf '<%#.6x>', 1; # prints "<0x000001>"
5062 printf '<%-10.6x>', 1; # prints "<000001 >"
5064 For string conversions, specifying a precision truncates the string
5065 to fit in the specified width:
5067 printf '<%.5s>', "truncated"; # prints "<trunc>"
5068 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5070 You can also get the precision from the next argument using C<.*>:
5072 printf '<%.6x>', 1; # prints "<000001>"
5073 printf '<%.*x>', 6, 1; # prints "<000001>"
5075 You cannot currently get the precision from a specified number,
5076 but it is intended that this will be possible in the future using
5079 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5083 For numeric conversions, you can specify the size to interpret the
5084 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5085 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5086 whatever the default integer size is on your platform (usually 32 or 64
5087 bits), but you can override this to use instead one of the standard C types,
5088 as supported by the compiler used to build Perl:
5090 l interpret integer as C type "long" or "unsigned long"
5091 h interpret integer as C type "short" or "unsigned short"
5092 q, L or ll interpret integer as C type "long long", "unsigned long long".
5093 or "quads" (typically 64-bit integers)
5095 The last will produce errors if Perl does not understand "quads" in your
5096 installation. (This requires that either the platform natively supports quads
5097 or Perl was specifically compiled to support quads.) You can find out
5098 whether your Perl supports quads via L<Config>:
5101 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5104 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5105 to be the default floating point size on your platform (double or long double),
5106 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5107 platform supports them. You can find out whether your Perl supports long
5108 doubles via L<Config>:
5111 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5113 You can find out whether Perl considers 'long double' to be the default
5114 floating point size to use on your platform via L<Config>:
5117 ($Config{uselongdouble} eq 'define') &&
5118 print "long doubles by default\n";
5120 It can also be the case that long doubles and doubles are the same thing:
5123 ($Config{doublesize} == $Config{longdblsize}) &&
5124 print "doubles are long doubles\n";
5126 The size specifier C<V> has no effect for Perl code, but it is supported
5127 for compatibility with XS code; it means 'use the standard size for
5128 a Perl integer (or floating-point number)', which is already the
5129 default for Perl code.
5131 =item order of arguments
5133 Normally, sprintf takes the next unused argument as the value to
5134 format for each format specification. If the format specification
5135 uses C<*> to require additional arguments, these are consumed from
5136 the argument list in the order in which they appear in the format
5137 specification I<before> the value to format. Where an argument is
5138 specified using an explicit index, this does not affect the normal
5139 order for the arguments (even when the explicitly specified index
5140 would have been the next argument in any case).
5144 printf '<%*.*s>', $a, $b, $c;
5146 would use C<$a> for the width, C<$b> for the precision and C<$c>
5147 as the value to format, while:
5149 print '<%*1$.*s>', $a, $b;
5151 would use C<$a> for the width and the precision, and C<$b> as the
5154 Here are some more examples - beware that when using an explicit
5155 index, the C<$> may need to be escaped:
5157 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5158 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5159 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5160 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5164 If C<use locale> is in effect, the character used for the decimal
5165 point in formatted real numbers is affected by the LC_NUMERIC locale.
5172 Return the square root of EXPR. If EXPR is omitted, returns square
5173 root of C<$_>. Only works on non-negative operands, unless you've
5174 loaded the standard Math::Complex module.
5177 print sqrt(-2); # prints 1.4142135623731i
5183 Sets the random number seed for the C<rand> operator.
5185 The point of the function is to "seed" the C<rand> function so that
5186 C<rand> can produce a different sequence each time you run your
5189 If srand() is not called explicitly, it is called implicitly at the
5190 first use of the C<rand> operator. However, this was not the case in
5191 versions of Perl before 5.004, so if your script will run under older
5192 Perl versions, it should call C<srand>.
5194 Most programs won't even call srand() at all, except those that
5195 need a cryptographically-strong starting point rather than the
5196 generally acceptable default, which is based on time of day,
5197 process ID, and memory allocation, or the F</dev/urandom> device,
5200 You can call srand($seed) with the same $seed to reproduce the
5201 I<same> sequence from rand(), but this is usually reserved for
5202 generating predictable results for testing or debugging.
5203 Otherwise, don't call srand() more than once in your program.
5205 Do B<not> call srand() (i.e. without an argument) more than once in
5206 a script. The internal state of the random number generator should
5207 contain more entropy than can be provided by any seed, so calling
5208 srand() again actually I<loses> randomness.
5210 Most implementations of C<srand> take an integer and will silently
5211 truncate decimal numbers. This means C<srand(42)> will usually
5212 produce the same results as C<srand(42.1)>. To be safe, always pass
5213 C<srand> an integer.
5215 In versions of Perl prior to 5.004 the default seed was just the
5216 current C<time>. This isn't a particularly good seed, so many old
5217 programs supply their own seed value (often C<time ^ $$> or C<time ^
5218 ($$ + ($$ << 15))>), but that isn't necessary any more.
5220 Note that you need something much more random than the default seed for
5221 cryptographic purposes. Checksumming the compressed output of one or more
5222 rapidly changing operating system status programs is the usual method. For
5225 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5227 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5230 Frequently called programs (like CGI scripts) that simply use
5234 for a seed can fall prey to the mathematical property that
5238 one-third of the time. So don't do that.
5240 =item stat FILEHANDLE
5246 Returns a 13-element list giving the status info for a file, either
5247 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5248 it stats C<$_>. Returns a null list if the stat fails. Typically used
5251 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5252 $atime,$mtime,$ctime,$blksize,$blocks)
5255 Not all fields are supported on all filesystem types. Here are the
5256 meaning of the fields:
5258 0 dev device number of filesystem
5260 2 mode file mode (type and permissions)
5261 3 nlink number of (hard) links to the file
5262 4 uid numeric user ID of file's owner
5263 5 gid numeric group ID of file's owner
5264 6 rdev the device identifier (special files only)
5265 7 size total size of file, in bytes
5266 8 atime last access time in seconds since the epoch
5267 9 mtime last modify time in seconds since the epoch
5268 10 ctime inode change time in seconds since the epoch (*)
5269 11 blksize preferred block size for file system I/O
5270 12 blocks actual number of blocks allocated
5272 (The epoch was at 00:00 January 1, 1970 GMT.)
5274 (*) The ctime field is non-portable, in particular you cannot expect
5275 it to be a "creation time", see L<perlport/"Files and Filesystems">
5278 If stat is passed the special filehandle consisting of an underline, no
5279 stat is done, but the current contents of the stat structure from the
5280 last stat or filetest are returned. Example:
5282 if (-x $file && (($d) = stat(_)) && $d < 0) {
5283 print "$file is executable NFS file\n";
5286 (This works on machines only for which the device number is negative
5289 Because the mode contains both the file type and its permissions, you
5290 should mask off the file type portion and (s)printf using a C<"%o">
5291 if you want to see the real permissions.
5293 $mode = (stat($filename))[2];
5294 printf "Permissions are %04o\n", $mode & 07777;
5296 In scalar context, C<stat> returns a boolean value indicating success
5297 or failure, and, if successful, sets the information associated with
5298 the special filehandle C<_>.
5300 The File::stat module provides a convenient, by-name access mechanism:
5303 $sb = stat($filename);
5304 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5305 $filename, $sb->size, $sb->mode & 07777,
5306 scalar localtime $sb->mtime;
5308 You can import symbolic mode constants (C<S_IF*>) and functions
5309 (C<S_IS*>) from the Fcntl module:
5313 $mode = (stat($filename))[2];
5315 $user_rwx = ($mode & S_IRWXU) >> 6;
5316 $group_read = ($mode & S_IRGRP) >> 3;
5317 $other_execute = $mode & S_IXOTH;
5319 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5321 $is_setuid = $mode & S_ISUID;
5322 $is_setgid = S_ISDIR($mode);
5324 You could write the last two using the C<-u> and C<-d> operators.
5325 The commonly available S_IF* constants are
5327 # Permissions: read, write, execute, for user, group, others.
5329 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5330 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5331 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5333 # Setuid/Setgid/Stickiness/SaveText.
5334 # Note that the exact meaning of these is system dependent.
5336 S_ISUID S_ISGID S_ISVTX S_ISTXT
5338 # File types. Not necessarily all are available on your system.
5340 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5342 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5344 S_IREAD S_IWRITE S_IEXEC
5346 and the S_IF* functions are
5348 S_IMODE($mode) the part of $mode containing the permission bits
5349 and the setuid/setgid/sticky bits
5351 S_IFMT($mode) the part of $mode containing the file type
5352 which can be bit-anded with e.g. S_IFREG
5353 or with the following functions
5355 # The operators -f, -d, -l, -b, -c, -p, and -s.
5357 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5358 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5360 # No direct -X operator counterpart, but for the first one
5361 # the -g operator is often equivalent. The ENFMT stands for
5362 # record flocking enforcement, a platform-dependent feature.
5364 S_ISENFMT($mode) S_ISWHT($mode)
5366 See your native chmod(2) and stat(2) documentation for more details
5367 about the S_* constants. To get status info for a symbolic link
5368 instead of the target file behind the link, use the C<lstat> function.
5374 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5375 doing many pattern matches on the string before it is next modified.
5376 This may or may not save time, depending on the nature and number of
5377 patterns you are searching on, and on the distribution of character
5378 frequencies in the string to be searched--you probably want to compare
5379 run times with and without it to see which runs faster. Those loops
5380 which scan for many short constant strings (including the constant
5381 parts of more complex patterns) will benefit most. You may have only
5382 one C<study> active at a time--if you study a different scalar the first
5383 is "unstudied". (The way C<study> works is this: a linked list of every
5384 character in the string to be searched is made, so we know, for
5385 example, where all the C<'k'> characters are. From each search string,
5386 the rarest character is selected, based on some static frequency tables
5387 constructed from some C programs and English text. Only those places
5388 that contain this "rarest" character are examined.)
5390 For example, here is a loop that inserts index producing entries
5391 before any line containing a certain pattern:
5395 print ".IX foo\n" if /\bfoo\b/;
5396 print ".IX bar\n" if /\bbar\b/;
5397 print ".IX blurfl\n" if /\bblurfl\b/;
5402 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5403 will be looked at, because C<f> is rarer than C<o>. In general, this is
5404 a big win except in pathological cases. The only question is whether
5405 it saves you more time than it took to build the linked list in the
5408 Note that if you have to look for strings that you don't know till
5409 runtime, you can build an entire loop as a string and C<eval> that to
5410 avoid recompiling all your patterns all the time. Together with
5411 undefining C<$/> to input entire files as one record, this can be very
5412 fast, often faster than specialized programs like fgrep(1). The following
5413 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5414 out the names of those files that contain a match:
5416 $search = 'while (<>) { study;';
5417 foreach $word (@words) {
5418 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5423 eval $search; # this screams
5424 $/ = "\n"; # put back to normal input delimiter
5425 foreach $file (sort keys(%seen)) {
5429 =item sub NAME BLOCK
5431 =item sub NAME (PROTO) BLOCK
5433 =item sub NAME : ATTRS BLOCK
5435 =item sub NAME (PROTO) : ATTRS BLOCK
5437 This is subroutine definition, not a real function I<per se>.
5438 Without a BLOCK it's just a forward declaration. Without a NAME,
5439 it's an anonymous function declaration, and does actually return
5440 a value: the CODE ref of the closure you just created.
5442 See L<perlsub> and L<perlref> for details about subroutines and
5443 references, and L<attributes> and L<Attribute::Handlers> for more
5444 information about attributes.
5446 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5448 =item substr EXPR,OFFSET,LENGTH
5450 =item substr EXPR,OFFSET
5452 Extracts a substring out of EXPR and returns it. First character is at
5453 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5454 If OFFSET is negative (or more precisely, less than C<$[>), starts
5455 that far from the end of the string. If LENGTH is omitted, returns
5456 everything to the end of the string. If LENGTH is negative, leaves that
5457 many characters off the end of the string.
5459 You can use the substr() function as an lvalue, in which case EXPR
5460 must itself be an lvalue. If you assign something shorter than LENGTH,
5461 the string will shrink, and if you assign something longer than LENGTH,
5462 the string will grow to accommodate it. To keep the string the same
5463 length you may need to pad or chop your value using C<sprintf>.
5465 If OFFSET and LENGTH specify a substring that is partly outside the
5466 string, only the part within the string is returned. If the substring
5467 is beyond either end of the string, substr() returns the undefined
5468 value and produces a warning. When used as an lvalue, specifying a
5469 substring that is entirely outside the string is a fatal error.
5470 Here's an example showing the behavior for boundary cases:
5473 substr($name, 4) = 'dy'; # $name is now 'freddy'
5474 my $null = substr $name, 6, 2; # returns '' (no warning)
5475 my $oops = substr $name, 7; # returns undef, with warning
5476 substr($name, 7) = 'gap'; # fatal error
5478 An alternative to using substr() as an lvalue is to specify the
5479 replacement string as the 4th argument. This allows you to replace
5480 parts of the EXPR and return what was there before in one operation,
5481 just as you can with splice().
5483 If the lvalue returned by substr is used after the EXPR is changed in
5484 any way, the behaviour may not be as expected and is subject to change.
5485 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5486 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5487 substring assignment, and then the substr is used again), or where a
5488 substr() is aliased via a C<foreach> loop or passed as a parameter or
5489 a reference to it is taken and then the alias, parameter, or deref'd
5490 reference either is used after the original EXPR has been changed or
5491 is assigned to and then used a second time.
5493 =item symlink OLDFILE,NEWFILE
5495 Creates a new filename symbolically linked to the old filename.
5496 Returns C<1> for success, C<0> otherwise. On systems that don't support
5497 symbolic links, produces a fatal error at run time. To check for that,
5500 $symlink_exists = eval { symlink("",""); 1 };
5504 Calls the system call specified as the first element of the list,
5505 passing the remaining elements as arguments to the system call. If
5506 unimplemented, produces a fatal error. The arguments are interpreted
5507 as follows: if a given argument is numeric, the argument is passed as
5508 an int. If not, the pointer to the string value is passed. You are
5509 responsible to make sure a string is pre-extended long enough to
5510 receive any result that might be written into a string. You can't use a
5511 string literal (or other read-only string) as an argument to C<syscall>
5512 because Perl has to assume that any string pointer might be written
5514 integer arguments are not literals and have never been interpreted in a
5515 numeric context, you may need to add C<0> to them to force them to look
5516 like numbers. This emulates the C<syswrite> function (or vice versa):
5518 require 'syscall.ph'; # may need to run h2ph
5520 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5522 Note that Perl supports passing of up to only 14 arguments to your system call,
5523 which in practice should usually suffice.
5525 Syscall returns whatever value returned by the system call it calls.
5526 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5527 Note that some system calls can legitimately return C<-1>. The proper
5528 way to handle such calls is to assign C<$!=0;> before the call and
5529 check the value of C<$!> if syscall returns C<-1>.
5531 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5532 number of the read end of the pipe it creates. There is no way
5533 to retrieve the file number of the other end. You can avoid this
5534 problem by using C<pipe> instead.
5536 =item sysopen FILEHANDLE,FILENAME,MODE
5538 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5540 Opens the file whose filename is given by FILENAME, and associates it
5541 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5542 the name of the real filehandle wanted. This function calls the
5543 underlying operating system's C<open> function with the parameters
5544 FILENAME, MODE, PERMS.
5546 The possible values and flag bits of the MODE parameter are
5547 system-dependent; they are available via the standard module C<Fcntl>.
5548 See the documentation of your operating system's C<open> to see which
5549 values and flag bits are available. You may combine several flags
5550 using the C<|>-operator.
5552 Some of the most common values are C<O_RDONLY> for opening the file in
5553 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5554 and C<O_RDWR> for opening the file in read-write mode, and.
5556 For historical reasons, some values work on almost every system
5557 supported by perl: zero means read-only, one means write-only, and two
5558 means read/write. We know that these values do I<not> work under
5559 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5560 use them in new code.
5562 If the file named by FILENAME does not exist and the C<open> call creates
5563 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5564 PERMS specifies the permissions of the newly created file. If you omit
5565 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5566 These permission values need to be in octal, and are modified by your
5567 process's current C<umask>.
5569 In many systems the C<O_EXCL> flag is available for opening files in
5570 exclusive mode. This is B<not> locking: exclusiveness means here that
5571 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5574 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5576 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5577 that takes away the user's option to have a more permissive umask.
5578 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5581 Note that C<sysopen> depends on the fdopen() C library function.
5582 On many UNIX systems, fdopen() is known to fail when file descriptors
5583 exceed a certain value, typically 255. If you need more file
5584 descriptors than that, consider rebuilding Perl to use the C<sfio>
5585 library, or perhaps using the POSIX::open() function.
5587 See L<perlopentut> for a kinder, gentler explanation of opening files.
5589 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5591 =item sysread FILEHANDLE,SCALAR,LENGTH
5593 Attempts to read LENGTH bytes of data into variable SCALAR from the
5594 specified FILEHANDLE, using the system call read(2). It bypasses
5595 buffered IO, so mixing this with other kinds of reads, C<print>,
5596 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
5597 perlio or stdio layers usually buffers data. Returns the number of
5598 bytes actually read, C<0> at end of file, or undef if there was an
5599 error (in the latter case C<$!> is also set). SCALAR will be grown or
5600 shrunk so that the last byte actually read is the last byte of the
5601 scalar after the read.
5603 An OFFSET may be specified to place the read data at some place in the
5604 string other than the beginning. A negative OFFSET specifies
5605 placement at that many characters counting backwards from the end of
5606 the string. A positive OFFSET greater than the length of SCALAR
5607 results in the string being padded to the required size with C<"\0">
5608 bytes before the result of the read is appended.
5610 There is no syseof() function, which is ok, since eof() doesn't work
5611 very well on device files (like ttys) anyway. Use sysread() and check
5612 for a return value for 0 to decide whether you're done.
5614 Note that if the filehandle has been marked as C<:utf8> Unicode
5615 characters are read instead of bytes (the LENGTH, OFFSET, and the
5616 return value of sysread() are in Unicode characters).
5617 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5618 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5620 =item sysseek FILEHANDLE,POSITION,WHENCE
5622 Sets FILEHANDLE's system position in bytes using the system call
5623 lseek(2). FILEHANDLE may be an expression whose value gives the name
5624 of the filehandle. The values for WHENCE are C<0> to set the new
5625 position to POSITION, C<1> to set the it to the current position plus
5626 POSITION, and C<2> to set it to EOF plus POSITION (typically
5629 Note the I<in bytes>: even if the filehandle has been set to operate
5630 on characters (for example by using the C<:utf8> I/O layer), tell()
5631 will return byte offsets, not character offsets (because implementing
5632 that would render sysseek() very slow).
5634 sysseek() bypasses normal buffered IO, so mixing this with reads (other
5635 than C<sysread>, for example >< or read()) C<print>, C<write>,
5636 C<seek>, C<tell>, or C<eof> may cause confusion.
5638 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5639 and C<SEEK_END> (start of the file, current position, end of the file)
5640 from the Fcntl module. Use of the constants is also more portable
5641 than relying on 0, 1, and 2. For example to define a "systell" function:
5643 use Fnctl 'SEEK_CUR';
5644 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5646 Returns the new position, or the undefined value on failure. A position
5647 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5648 true on success and false on failure, yet you can still easily determine
5653 =item system PROGRAM LIST
5655 Does exactly the same thing as C<exec LIST>, except that a fork is
5656 done first, and the parent process waits for the child process to
5657 complete. Note that argument processing varies depending on the
5658 number of arguments. If there is more than one argument in LIST,
5659 or if LIST is an array with more than one value, starts the program
5660 given by the first element of the list with arguments given by the
5661 rest of the list. If there is only one scalar argument, the argument
5662 is checked for shell metacharacters, and if there are any, the
5663 entire argument is passed to the system's command shell for parsing
5664 (this is C</bin/sh -c> on Unix platforms, but varies on other
5665 platforms). If there are no shell metacharacters in the argument,
5666 it is split into words and passed directly to C<execvp>, which is
5669 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5670 output before any operation that may do a fork, but this may not be
5671 supported on some platforms (see L<perlport>). To be safe, you may need
5672 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5673 of C<IO::Handle> on any open handles.
5675 The return value is the exit status of the program as returned by the
5676 C<wait> call. To get the actual exit value shift right by eight (see below).
5677 See also L</exec>. This is I<not> what you want to use to capture
5678 the output from a command, for that you should use merely backticks or
5679 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5680 indicates a failure to start the program (inspect $! for the reason).
5682 Like C<exec>, C<system> allows you to lie to a program about its name if
5683 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5685 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5686 killing the program they're running doesn't actually interrupt
5689 @args = ("command", "arg1", "arg2");
5691 or die "system @args failed: $?"
5693 You can check all the failure possibilities by inspecting
5696 $exit_value = $? >> 8;
5697 $signal_num = $? & 127;
5698 $dumped_core = $? & 128;
5700 or more portably by using the W*() calls of the POSIX extension;
5701 see L<perlport> for more information.
5703 When the arguments get executed via the system shell, results
5704 and return codes will be subject to its quirks and capabilities.
5705 See L<perlop/"`STRING`"> and L</exec> for details.
5707 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5709 =item syswrite FILEHANDLE,SCALAR,LENGTH
5711 =item syswrite FILEHANDLE,SCALAR
5713 Attempts to write LENGTH bytes of data from variable SCALAR to the
5714 specified FILEHANDLE, using the system call write(2). If LENGTH is
5715 not specified, writes whole SCALAR. It bypasses buffered IO, so
5716 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5717 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
5718 stdio layers usually buffers data. Returns the number of bytes
5719 actually written, or C<undef> if there was an error (in this case the
5720 errno variable C<$!> is also set). If the LENGTH is greater than the
5721 available data in the SCALAR after the OFFSET, only as much data as is
5722 available will be written.
5724 An OFFSET may be specified to write the data from some part of the
5725 string other than the beginning. A negative OFFSET specifies writing
5726 that many characters counting backwards from the end of the string.
5727 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5729 Note that if the filehandle has been marked as C<:utf8>,
5730 Unicode characters are written instead of bytes (the LENGTH, OFFSET,
5731 and the return value of syswrite() are in Unicode characters).
5732 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
5733 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
5735 =item tell FILEHANDLE
5739 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5740 error. FILEHANDLE may be an expression whose value gives the name of
5741 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5744 Note the I<in bytes>: even if the filehandle has been set to
5745 operate on characters (for example by using the C<:utf8> open
5746 layer), tell() will return byte offsets, not character offsets
5747 (because that would render seek() and tell() rather slow).
5749 The return value of tell() for the standard streams like the STDIN
5750 depends on the operating system: it may return -1 or something else.
5751 tell() on pipes, fifos, and sockets usually returns -1.
5753 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5755 Do not use tell() on a filehandle that has been opened using
5756 sysopen(), use sysseek() for that as described above. Why? Because
5757 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5758 buffered filehandles. sysseek() make sense only on the first kind,
5759 tell() only makes sense on the second kind.
5761 =item telldir DIRHANDLE
5763 Returns the current position of the C<readdir> routines on DIRHANDLE.
5764 Value may be given to C<seekdir> to access a particular location in a
5765 directory. Has the same caveats about possible directory compaction as
5766 the corresponding system library routine.
5768 =item tie VARIABLE,CLASSNAME,LIST
5770 This function binds a variable to a package class that will provide the
5771 implementation for the variable. VARIABLE is the name of the variable
5772 to be enchanted. CLASSNAME is the name of a class implementing objects
5773 of correct type. Any additional arguments are passed to the C<new>
5774 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5775 or C<TIEHASH>). Typically these are arguments such as might be passed
5776 to the C<dbm_open()> function of C. The object returned by the C<new>
5777 method is also returned by the C<tie> function, which would be useful
5778 if you want to access other methods in CLASSNAME.
5780 Note that functions such as C<keys> and C<values> may return huge lists
5781 when used on large objects, like DBM files. You may prefer to use the
5782 C<each> function to iterate over such. Example:
5784 # print out history file offsets
5786 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5787 while (($key,$val) = each %HIST) {
5788 print $key, ' = ', unpack('L',$val), "\n";
5792 A class implementing a hash should have the following methods:
5794 TIEHASH classname, LIST
5796 STORE this, key, value
5801 NEXTKEY this, lastkey
5805 A class implementing an ordinary array should have the following methods:
5807 TIEARRAY classname, LIST
5809 STORE this, key, value
5811 STORESIZE this, count
5817 SPLICE this, offset, length, LIST
5822 A class implementing a file handle should have the following methods:
5824 TIEHANDLE classname, LIST
5825 READ this, scalar, length, offset
5828 WRITE this, scalar, length, offset
5830 PRINTF this, format, LIST
5834 SEEK this, position, whence
5836 OPEN this, mode, LIST
5841 A class implementing a scalar should have the following methods:
5843 TIESCALAR classname, LIST
5849 Not all methods indicated above need be implemented. See L<perltie>,
5850 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5852 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5853 for you--you need to do that explicitly yourself. See L<DB_File>
5854 or the F<Config> module for interesting C<tie> implementations.
5856 For further details see L<perltie>, L<"tied VARIABLE">.
5860 Returns a reference to the object underlying VARIABLE (the same value
5861 that was originally returned by the C<tie> call that bound the variable
5862 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5867 Returns the number of non-leap seconds since whatever time the system
5868 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5869 and 00:00:00 UTC, January 1, 1970 for most other systems).
5870 Suitable for feeding to C<gmtime> and C<localtime>.
5872 For measuring time in better granularity than one second,
5873 you may use either the Time::HiRes module (from CPAN, and starting from
5874 Perl 5.8 part of the standard distribution), or if you have
5875 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5876 See L<perlfaq8> for details.
5880 Returns a four-element list giving the user and system times, in
5881 seconds, for this process and the children of this process.
5883 ($user,$system,$cuser,$csystem) = times;
5885 In scalar context, C<times> returns C<$user>.
5889 The transliteration operator. Same as C<y///>. See L<perlop>.
5891 =item truncate FILEHANDLE,LENGTH
5893 =item truncate EXPR,LENGTH
5895 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5896 specified length. Produces a fatal error if truncate isn't implemented
5897 on your system. Returns true if successful, the undefined value
5900 The behavior is undefined if LENGTH is greater than the length of the
5907 Returns an uppercased version of EXPR. This is the internal function
5908 implementing the C<\U> escape in double-quoted strings. Respects
5909 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5910 and L<perlunicode> for more details about locale and Unicode support.
5911 It does not attempt to do titlecase mapping on initial letters. See
5912 C<ucfirst> for that.
5914 If EXPR is omitted, uses C<$_>.
5920 Returns the value of EXPR with the first character in uppercase
5921 (titlecase in Unicode). This is the internal function implementing
5922 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5923 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5924 for more details about locale and Unicode support.
5926 If EXPR is omitted, uses C<$_>.
5932 Sets the umask for the process to EXPR and returns the previous value.
5933 If EXPR is omitted, merely returns the current umask.
5935 The Unix permission C<rwxr-x---> is represented as three sets of three
5936 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5937 and isn't one of the digits). The C<umask> value is such a number
5938 representing disabled permissions bits. The permission (or "mode")
5939 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5940 even if you tell C<sysopen> to create a file with permissions C<0777>,
5941 if your umask is C<0022> then the file will actually be created with
5942 permissions C<0755>. If your C<umask> were C<0027> (group can't
5943 write; others can't read, write, or execute), then passing
5944 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5947 Here's some advice: supply a creation mode of C<0666> for regular
5948 files (in C<sysopen>) and one of C<0777> for directories (in
5949 C<mkdir>) and executable files. This gives users the freedom of
5950 choice: if they want protected files, they might choose process umasks
5951 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5952 Programs should rarely if ever make policy decisions better left to
5953 the user. The exception to this is when writing files that should be
5954 kept private: mail files, web browser cookies, I<.rhosts> files, and
5957 If umask(2) is not implemented on your system and you are trying to
5958 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5959 fatal error at run time. If umask(2) is not implemented and you are
5960 not trying to restrict access for yourself, returns C<undef>.
5962 Remember that a umask is a number, usually given in octal; it is I<not> a
5963 string of octal digits. See also L</oct>, if all you have is a string.
5969 Undefines the value of EXPR, which must be an lvalue. Use only on a
5970 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5971 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5972 will probably not do what you expect on most predefined variables or
5973 DBM list values, so don't do that; see L<delete>.) Always returns the
5974 undefined value. You can omit the EXPR, in which case nothing is
5975 undefined, but you still get an undefined value that you could, for
5976 instance, return from a subroutine, assign to a variable or pass as a
5977 parameter. Examples:
5980 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5984 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5985 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5986 select undef, undef, undef, 0.25;
5987 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5989 Note that this is a unary operator, not a list operator.
5995 Deletes a list of files. Returns the number of files successfully
5998 $cnt = unlink 'a', 'b', 'c';
6002 Note: C<unlink> will not delete directories unless you are superuser and
6003 the B<-U> flag is supplied to Perl. Even if these conditions are
6004 met, be warned that unlinking a directory can inflict damage on your
6005 filesystem. Use C<rmdir> instead.
6007 If LIST is omitted, uses C<$_>.
6009 =item unpack TEMPLATE,EXPR
6011 =item unpack TEMPLATE
6013 C<unpack> does the reverse of C<pack>: it takes a string
6014 and expands it out into a list of values.
6015 (In scalar context, it returns merely the first value produced.)
6017 If EXPR is omitted, unpacks the C<$_> string.
6019 The string is broken into chunks described by the TEMPLATE. Each chunk
6020 is converted separately to a value. Typically, either the string is a result
6021 of C<pack>, or the bytes of the string represent a C structure of some
6024 The TEMPLATE has the same format as in the C<pack> function.
6025 Here's a subroutine that does substring:
6028 my($what,$where,$howmuch) = @_;
6029 unpack("x$where a$howmuch", $what);
6034 sub ordinal { unpack("c",$_[0]); } # same as ord()
6036 In addition to fields allowed in pack(), you may prefix a field with
6037 a %<number> to indicate that
6038 you want a <number>-bit checksum of the items instead of the items
6039 themselves. Default is a 16-bit checksum. Checksum is calculated by
6040 summing numeric values of expanded values (for string fields the sum of
6041 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6043 For example, the following
6044 computes the same number as the System V sum program:
6048 unpack("%32C*",<>) % 65535;
6051 The following efficiently counts the number of set bits in a bit vector:
6053 $setbits = unpack("%32b*", $selectmask);
6055 The C<p> and C<P> formats should be used with care. Since Perl
6056 has no way of checking whether the value passed to C<unpack()>
6057 corresponds to a valid memory location, passing a pointer value that's
6058 not known to be valid is likely to have disastrous consequences.
6060 If there are more pack codes or if the repeat count of a field or a group
6061 is larger than what the remainder of the input string allows, the result
6062 is not well defined: in some cases, the repeat count is decreased, or
6063 C<unpack()> will produce null strings or zeroes, or terminate with an
6064 error. If the input string is longer than one described by the TEMPLATE,
6065 the rest is ignored.
6067 See L</pack> for more examples and notes.
6069 =item untie VARIABLE
6071 Breaks the binding between a variable and a package. (See C<tie>.)
6072 Has no effect if the variable is not tied.
6074 =item unshift ARRAY,LIST
6076 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6077 depending on how you look at it. Prepends list to the front of the
6078 array, and returns the new number of elements in the array.
6080 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6082 Note the LIST is prepended whole, not one element at a time, so the
6083 prepended elements stay in the same order. Use C<reverse> to do the
6086 =item use Module VERSION LIST
6088 =item use Module VERSION
6090 =item use Module LIST
6096 Imports some semantics into the current package from the named module,
6097 generally by aliasing certain subroutine or variable names into your
6098 package. It is exactly equivalent to
6100 BEGIN { require Module; import Module LIST; }
6102 except that Module I<must> be a bareword.
6104 VERSION may be either a numeric argument such as 5.006, which will be
6105 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6106 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6107 greater than the version of the current Perl interpreter; Perl will not
6108 attempt to parse the rest of the file. Compare with L</require>, which can
6109 do a similar check at run time.
6111 Specifying VERSION as a literal of the form v5.6.1 should generally be
6112 avoided, because it leads to misleading error messages under earlier
6113 versions of Perl which do not support this syntax. The equivalent numeric
6114 version should be used instead.
6116 use v5.6.1; # compile time version check
6118 use 5.006_001; # ditto; preferred for backwards compatibility
6120 This is often useful if you need to check the current Perl version before
6121 C<use>ing library modules that have changed in incompatible ways from
6122 older versions of Perl. (We try not to do this more than we have to.)
6124 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6125 C<require> makes sure the module is loaded into memory if it hasn't been
6126 yet. The C<import> is not a builtin--it's just an ordinary static method
6127 call into the C<Module> package to tell the module to import the list of
6128 features back into the current package. The module can implement its
6129 C<import> method any way it likes, though most modules just choose to
6130 derive their C<import> method via inheritance from the C<Exporter> class that
6131 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6132 method can be found then the call is skipped.
6134 If you do not want to call the package's C<import> method (for instance,
6135 to stop your namespace from being altered), explicitly supply the empty list:
6139 That is exactly equivalent to
6141 BEGIN { require Module }
6143 If the VERSION argument is present between Module and LIST, then the
6144 C<use> will call the VERSION method in class Module with the given
6145 version as an argument. The default VERSION method, inherited from
6146 the UNIVERSAL class, croaks if the given version is larger than the
6147 value of the variable C<$Module::VERSION>.
6149 Again, there is a distinction between omitting LIST (C<import> called
6150 with no arguments) and an explicit empty LIST C<()> (C<import> not
6151 called). Note that there is no comma after VERSION!
6153 Because this is a wide-open interface, pragmas (compiler directives)
6154 are also implemented this way. Currently implemented pragmas are:
6159 use sigtrap qw(SEGV BUS);
6160 use strict qw(subs vars refs);
6161 use subs qw(afunc blurfl);
6162 use warnings qw(all);
6163 use sort qw(stable _quicksort _mergesort);
6165 Some of these pseudo-modules import semantics into the current
6166 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6167 which import symbols into the current package (which are effective
6168 through the end of the file).
6170 There's a corresponding C<no> command that unimports meanings imported
6171 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6177 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6178 for the C<-M> and C<-m> command-line options to perl that give C<use>
6179 functionality from the command-line.
6183 Changes the access and modification times on each file of a list of
6184 files. The first two elements of the list must be the NUMERICAL access
6185 and modification times, in that order. Returns the number of files
6186 successfully changed. The inode change time of each file is set
6187 to the current time. For example, this code has the same effect as the
6188 Unix touch(1) command when the files I<already exist>.
6192 utime $now, $now, @ARGV;
6194 B<Note:> Under NFS, touch(1) uses the time of the NFS server, not
6195 the time of the local machine. If there is a time synchronization
6196 problem, the NFS server and local machine will have different times.
6198 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6199 the utime(2) function in the C library will be called with a null second
6200 argument. On most systems, this will set the file's access and
6201 modification times to the current time (i.e. equivalent to the example
6204 utime undef, undef, @ARGV;
6208 Returns a list consisting of all the values of the named hash. (In a
6209 scalar context, returns the number of values.) The values are
6210 returned in an apparently random order. The actual random order is
6211 subject to change in future versions of perl, but it is guaranteed to
6212 be the same order as either the C<keys> or C<each> function would
6213 produce on the same (unmodified) hash.
6215 Note that the values are not copied, which means modifying them will
6216 modify the contents of the hash:
6218 for (values %hash) { s/foo/bar/g } # modifies %hash values
6219 for (@hash{keys %hash}) { s/foo/bar/g } # same
6221 As a side effect, calling values() resets the HASH's internal iterator.
6222 See also C<keys>, C<each>, and C<sort>.
6224 =item vec EXPR,OFFSET,BITS
6226 Treats the string in EXPR as a bit vector made up of elements of
6227 width BITS, and returns the value of the element specified by OFFSET
6228 as an unsigned integer. BITS therefore specifies the number of bits
6229 that are reserved for each element in the bit vector. This must
6230 be a power of two from 1 to 32 (or 64, if your platform supports
6233 If BITS is 8, "elements" coincide with bytes of the input string.
6235 If BITS is 16 or more, bytes of the input string are grouped into chunks
6236 of size BITS/8, and each group is converted to a number as with
6237 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6238 for BITS==64). See L<"pack"> for details.
6240 If bits is 4 or less, the string is broken into bytes, then the bits
6241 of each byte are broken into 8/BITS groups. Bits of a byte are
6242 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6243 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6244 breaking the single input byte C<chr(0x36)> into two groups gives a list
6245 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6247 C<vec> may also be assigned to, in which case parentheses are needed
6248 to give the expression the correct precedence as in
6250 vec($image, $max_x * $x + $y, 8) = 3;
6252 If the selected element is outside the string, the value 0 is returned.
6253 If an element off the end of the string is written to, Perl will first
6254 extend the string with sufficiently many zero bytes. It is an error
6255 to try to write off the beginning of the string (i.e. negative OFFSET).
6257 The string should not contain any character with the value > 255 (which
6258 can only happen if you're using UTF8 encoding). If it does, it will be
6259 treated as something which is not UTF8 encoded. When the C<vec> was
6260 assigned to, other parts of your program will also no longer consider the
6261 string to be UTF8 encoded. In other words, if you do have such characters
6262 in your string, vec() will operate on the actual byte string, and not the
6263 conceptual character string.
6265 Strings created with C<vec> can also be manipulated with the logical
6266 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6267 vector operation is desired when both operands are strings.
6268 See L<perlop/"Bitwise String Operators">.
6270 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6271 The comments show the string after each step. Note that this code works
6272 in the same way on big-endian or little-endian machines.
6275 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6277 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6278 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6280 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6281 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6282 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6283 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6284 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6285 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6287 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6288 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6289 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6292 To transform a bit vector into a string or list of 0's and 1's, use these:
6294 $bits = unpack("b*", $vector);
6295 @bits = split(//, unpack("b*", $vector));
6297 If you know the exact length in bits, it can be used in place of the C<*>.
6299 Here is an example to illustrate how the bits actually fall in place:
6305 unpack("V",$_) 01234567890123456789012345678901
6306 ------------------------------------------------------------------
6311 for ($shift=0; $shift < $width; ++$shift) {
6312 for ($off=0; $off < 32/$width; ++$off) {
6313 $str = pack("B*", "0"x32);
6314 $bits = (1<<$shift);
6315 vec($str, $off, $width) = $bits;
6316 $res = unpack("b*",$str);
6317 $val = unpack("V", $str);
6324 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6325 $off, $width, $bits, $val, $res
6329 Regardless of the machine architecture on which it is run, the above
6330 example should print the following table:
6333 unpack("V",$_) 01234567890123456789012345678901
6334 ------------------------------------------------------------------
6335 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6336 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6337 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6338 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6339 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6340 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6341 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6342 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6343 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6344 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6345 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6346 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6347 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6348 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6349 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6350 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6351 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6352 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6353 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6354 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6355 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6356 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6357 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6358 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6359 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6360 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6361 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6362 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6363 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6364 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6365 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6366 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6367 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6368 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6369 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6370 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6371 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6372 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6373 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6374 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6375 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6376 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6377 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6378 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6379 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6380 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6381 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6382 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6383 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6384 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6385 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6386 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6387 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6388 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6389 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6390 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6391 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6392 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6393 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6394 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6395 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6396 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6397 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6398 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6399 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6400 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6401 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6402 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6403 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6404 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6405 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6406 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6407 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6408 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6409 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6410 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6411 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6412 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6413 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6414 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6415 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6416 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6417 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6418 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6419 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6420 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6421 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6422 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6423 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6424 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6425 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6426 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6427 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6428 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6429 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6430 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6431 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6432 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6433 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6434 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6435 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6436 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6437 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6438 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6439 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6440 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6441 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6442 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6443 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6444 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6445 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6446 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6447 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6448 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6449 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6450 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6451 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6452 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6453 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6454 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6455 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6456 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6457 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6458 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6459 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6460 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6461 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6462 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6466 Behaves like the wait(2) system call on your system: it waits for a child
6467 process to terminate and returns the pid of the deceased process, or
6468 C<-1> if there are no child processes. The status is returned in C<$?>.
6469 Note that a return value of C<-1> could mean that child processes are
6470 being automatically reaped, as described in L<perlipc>.
6472 =item waitpid PID,FLAGS
6474 Waits for a particular child process to terminate and returns the pid of
6475 the deceased process, or C<-1> if there is no such child process. On some
6476 systems, a value of 0 indicates that there are processes still running.
6477 The status is returned in C<$?>. If you say
6479 use POSIX ":sys_wait_h";
6482 $kid = waitpid(-1, WNOHANG);
6485 then you can do a non-blocking wait for all pending zombie processes.
6486 Non-blocking wait is available on machines supporting either the
6487 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6488 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6489 system call by remembering the status values of processes that have
6490 exited but have not been harvested by the Perl script yet.)
6492 Note that on some systems, a return value of C<-1> could mean that child
6493 processes are being automatically reaped. See L<perlipc> for details,
6494 and for other examples.
6498 Returns true if the context of the currently executing subroutine is
6499 looking for a list value. Returns false if the context is looking
6500 for a scalar. Returns the undefined value if the context is looking
6501 for no value (void context).
6503 return unless defined wantarray; # don't bother doing more
6504 my @a = complex_calculation();
6505 return wantarray ? @a : "@a";
6507 This function should have been named wantlist() instead.
6511 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6514 If LIST is empty and C<$@> already contains a value (typically from a
6515 previous eval) that value is used after appending C<"\t...caught">
6516 to C<$@>. This is useful for staying almost, but not entirely similar to
6519 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6521 No message is printed if there is a C<$SIG{__WARN__}> handler
6522 installed. It is the handler's responsibility to deal with the message
6523 as it sees fit (like, for instance, converting it into a C<die>). Most
6524 handlers must therefore make arrangements to actually display the
6525 warnings that they are not prepared to deal with, by calling C<warn>
6526 again in the handler. Note that this is quite safe and will not
6527 produce an endless loop, since C<__WARN__> hooks are not called from
6530 You will find this behavior is slightly different from that of
6531 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6532 instead call C<die> again to change it).
6534 Using a C<__WARN__> handler provides a powerful way to silence all
6535 warnings (even the so-called mandatory ones). An example:
6537 # wipe out *all* compile-time warnings
6538 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6540 my $foo = 20; # no warning about duplicate my $foo,
6541 # but hey, you asked for it!
6542 # no compile-time or run-time warnings before here
6545 # run-time warnings enabled after here
6546 warn "\$foo is alive and $foo!"; # does show up
6548 See L<perlvar> for details on setting C<%SIG> entries, and for more
6549 examples. See the Carp module for other kinds of warnings using its
6550 carp() and cluck() functions.
6552 =item write FILEHANDLE
6558 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6559 using the format associated with that file. By default the format for
6560 a file is the one having the same name as the filehandle, but the
6561 format for the current output channel (see the C<select> function) may be set
6562 explicitly by assigning the name of the format to the C<$~> variable.
6564 Top of form processing is handled automatically: if there is
6565 insufficient room on the current page for the formatted record, the
6566 page is advanced by writing a form feed, a special top-of-page format
6567 is used to format the new page header, and then the record is written.
6568 By default the top-of-page format is the name of the filehandle with
6569 "_TOP" appended, but it may be dynamically set to the format of your
6570 choice by assigning the name to the C<$^> variable while the filehandle is
6571 selected. The number of lines remaining on the current page is in
6572 variable C<$->, which can be set to C<0> to force a new page.
6574 If FILEHANDLE is unspecified, output goes to the current default output
6575 channel, which starts out as STDOUT but may be changed by the
6576 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6577 is evaluated and the resulting string is used to look up the name of
6578 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6580 Note that write is I<not> the opposite of C<read>. Unfortunately.
6584 The transliteration operator. Same as C<tr///>. See L<perlop>.