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<gethostent>,
228 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
249 =item I<-X> FILEHANDLE
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 $@ eq "alarm\n"; # propagate unexpected errors
434 Returns the arctangent of Y/X in the range -PI to PI.
436 For the tangent operation, you may use the C<Math::Trig::tan>
437 function, or use the familiar relation:
439 sub tan { sin($_[0]) / cos($_[0]) }
441 =item bind SOCKET,NAME
443 Binds a network address to a socket, just as the bind system call
444 does. Returns true if it succeeded, false otherwise. NAME should be a
445 packed address of the appropriate type for the socket. See the examples in
446 L<perlipc/"Sockets: Client/Server Communication">.
448 =item binmode FILEHANDLE, DISCIPLINE
450 =item binmode FILEHANDLE
452 Arranges for FILEHANDLE to be read or written in "binary" or "text"
453 mode on systems where the run-time libraries distinguish between
454 binary and text files. If FILEHANDLE is an expression, the value is
455 taken as the name of the filehandle. Returns true on success,
458 If DISCIPLINE is ommited the filehandle is made suitable for
459 passing binary data. This includes turning off CRLF translation
460 and marking it as bytes.
462 On some systems (in general, DOS and Windows-based systems) binmode()
463 is necessary when you're not working with a text file. For the sake
464 of portability it is a good idea to always use it when appropriate,
465 and to never use it when it isn't appropriate.
467 In other words: regardless of platform, use binmode() on binary files
468 (like for example images).
470 If DISCIPLINE is present it is a single string, but may contain
471 multiple directives. The directives alter the behaviour of the
472 file handle. When DISCIPLINE is present using binmode on text
475 To mark FILEHANDLE as UTF-8, use C<:utf8>.
477 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
478 form C<:...>, are called I/O I<disciplines>. The normal implementation
479 of disciplines in perl5.8 and later is in terms of I<layers>. See
480 L<PerlIO>. (There is typically a one-to-one correspondence between
481 layers and disiplines.) The C<open> pragma can be used to establish
482 default I/O disciplines. See L<open>.
484 The C<:raw> discipline is deprecated. (As opposed to what Camel III
485 said, it is not the inverse of C<:crlf>.) See L<PerlIO>, L<perlrun>
486 and the discussion about the PERLIO environment variable.
488 In general, binmode() should be called after open() but before any I/O
489 is done on the filehandle. Calling binmode() will normally flush any
490 pending buffered output data (and perhaps pending input data) on the
491 handle. An exception to this is the C<:encoding> discipline that
492 changes the default character encoding of the handle, see L<open>.
493 The C<:encoding> discipline sometimes needs to be called in
494 mid-stream, and it doesn't flush the stream.
496 The operating system, device drivers, C libraries, and Perl run-time
497 system all work together to let the programmer treat a single
498 character (C<\n>) as the line terminator, irrespective of the external
499 representation. On many operating systems, the native text file
500 representation matches the internal representation, but on some
501 platforms the external representation of C<\n> is made up of more than
504 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
505 character to end each line in the external representation of text (even
506 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
507 on Unix and most VMS files). In other systems like OS/2, DOS and the
508 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
509 but what's stored in text files are the two characters C<\cM\cJ>. That
510 means that, if you don't use binmode() on these systems, C<\cM\cJ>
511 sequences on disk will be converted to C<\n> on input, and any C<\n> in
512 your program will be converted back to C<\cM\cJ> on output. This is what
513 you want for text files, but it can be disastrous for binary files.
515 Another consequence of using binmode() (on some systems) is that
516 special end-of-file markers will be seen as part of the data stream.
517 For systems from the Microsoft family this means that if your binary
518 data contains C<\cZ>, the I/O subsystem will regard it as the end of
519 the file, unless you use binmode().
521 binmode() is not only important for readline() and print() operations,
522 but also when using read(), seek(), sysread(), syswrite() and tell()
523 (see L<perlport> for more details). See the C<$/> and C<$\> variables
524 in L<perlvar> for how to manually set your input and output
525 line-termination sequences.
527 =item bless REF,CLASSNAME
531 This function tells the thingy referenced by REF that it is now an object
532 in the CLASSNAME package. If CLASSNAME is omitted, the current package
533 is used. Because a C<bless> is often the last thing in a constructor,
534 it returns the reference for convenience. Always use the two-argument
535 version if the function doing the blessing might be inherited by a
536 derived class. See L<perltoot> and L<perlobj> for more about the blessing
537 (and blessings) of objects.
539 Consider always blessing objects in CLASSNAMEs that are mixed case.
540 Namespaces with all lowercase names are considered reserved for
541 Perl pragmata. Builtin types have all uppercase names, so to prevent
542 confusion, you may wish to avoid such package names as well. Make sure
543 that CLASSNAME is a true value.
545 See L<perlmod/"Perl Modules">.
551 Returns the context of the current subroutine call. In scalar context,
552 returns the caller's package name if there is a caller, that is, if
553 we're in a subroutine or C<eval> or C<require>, and the undefined value
554 otherwise. In list context, returns
556 ($package, $filename, $line) = caller;
558 With EXPR, it returns some extra information that the debugger uses to
559 print a stack trace. The value of EXPR indicates how many call frames
560 to go back before the current one.
562 ($package, $filename, $line, $subroutine, $hasargs,
563 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
565 Here $subroutine may be C<(eval)> if the frame is not a subroutine
566 call, but an C<eval>. In such a case additional elements $evaltext and
567 C<$is_require> are set: C<$is_require> is true if the frame is created by a
568 C<require> or C<use> statement, $evaltext contains the text of the
569 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
570 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
571 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
572 frame.) $subroutine may also be C<(unknown)> if this particular
573 subroutine happens to have been deleted from the symbol table.
574 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
575 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
576 compiled with. The C<$hints> and C<$bitmask> values are subject to change
577 between versions of Perl, and are not meant for external use.
579 Furthermore, when called from within the DB package, caller returns more
580 detailed information: it sets the list variable C<@DB::args> to be the
581 arguments with which the subroutine was invoked.
583 Be aware that the optimizer might have optimized call frames away before
584 C<caller> had a chance to get the information. That means that C<caller(N)>
585 might not return information about the call frame you expect it do, for
586 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
587 previous time C<caller> was called.
591 Changes the working directory to EXPR, if possible. If EXPR is omitted,
592 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
593 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
594 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
595 neither is set, C<chdir> does nothing. It returns true upon success,
596 false otherwise. See the example under C<die>.
600 Changes the permissions of a list of files. The first element of the
601 list must be the numerical mode, which should probably be an octal
602 number, and which definitely should I<not> a string of octal digits:
603 C<0644> is okay, C<'0644'> is not. Returns the number of files
604 successfully changed. See also L</oct>, if all you have is a string.
606 $cnt = chmod 0755, 'foo', 'bar';
607 chmod 0755, @executables;
608 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
610 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
611 $mode = 0644; chmod $mode, 'foo'; # this is best
613 You can also import the symbolic C<S_I*> constants from the Fcntl
618 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
619 # This is identical to the chmod 0755 of the above example.
627 This safer version of L</chop> removes any trailing string
628 that corresponds to the current value of C<$/> (also known as
629 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
630 number of characters removed from all its arguments. It's often used to
631 remove the newline from the end of an input record when you're worried
632 that the final record may be missing its newline. When in paragraph
633 mode (C<$/ = "">), it removes all trailing newlines from the string.
634 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
635 a reference to an integer or the like, see L<perlvar>) chomp() won't
637 If VARIABLE is omitted, it chomps C<$_>. Example:
640 chomp; # avoid \n on last field
645 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
647 You can actually chomp anything that's an lvalue, including an assignment:
650 chomp($answer = <STDIN>);
652 If you chomp a list, each element is chomped, and the total number of
653 characters removed is returned.
661 Chops off the last character of a string and returns the character
662 chopped. It is much more efficient than C<s/.$//s> because it neither
663 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
664 If VARIABLE is a hash, it chops the hash's values, but not its keys.
666 You can actually chop anything that's an lvalue, including an assignment.
668 If you chop a list, each element is chopped. Only the value of the
669 last C<chop> is returned.
671 Note that C<chop> returns the last character. To return all but the last
672 character, use C<substr($string, 0, -1)>.
676 Changes the owner (and group) of a list of files. The first two
677 elements of the list must be the I<numeric> uid and gid, in that
678 order. A value of -1 in either position is interpreted by most
679 systems to leave that value unchanged. Returns the number of files
680 successfully changed.
682 $cnt = chown $uid, $gid, 'foo', 'bar';
683 chown $uid, $gid, @filenames;
685 Here's an example that looks up nonnumeric uids in the passwd file:
688 chomp($user = <STDIN>);
690 chomp($pattern = <STDIN>);
692 ($login,$pass,$uid,$gid) = getpwnam($user)
693 or die "$user not in passwd file";
695 @ary = glob($pattern); # expand filenames
696 chown $uid, $gid, @ary;
698 On most systems, you are not allowed to change the ownership of the
699 file unless you're the superuser, although you should be able to change
700 the group to any of your secondary groups. On insecure systems, these
701 restrictions may be relaxed, but this is not a portable assumption.
702 On POSIX systems, you can detect this condition this way:
704 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
705 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
711 Returns the character represented by that NUMBER in the character set.
712 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
713 chr(0x263a) is a Unicode smiley face. Note that characters from 127
714 to 255 (inclusive) are by default not encoded in Unicode for backward
715 compatibility reasons (but see L<encoding>).
717 For the reverse, use L</ord>.
718 See L<perlunicode> and L<encoding> for more about Unicode.
720 If NUMBER is omitted, uses C<$_>.
722 =item chroot FILENAME
726 This function works like the system call by the same name: it makes the
727 named directory the new root directory for all further pathnames that
728 begin with a C</> by your process and all its children. (It doesn't
729 change your current working directory, which is unaffected.) For security
730 reasons, this call is restricted to the superuser. If FILENAME is
731 omitted, does a C<chroot> to C<$_>.
733 =item close FILEHANDLE
737 Closes the file or pipe associated with the file handle, returning
738 true only if IO buffers are successfully flushed and closes the system
739 file descriptor. Closes the currently selected filehandle if the
742 You don't have to close FILEHANDLE if you are immediately going to do
743 another C<open> on it, because C<open> will close it for you. (See
744 C<open>.) However, an explicit C<close> on an input file resets the line
745 counter (C<$.>), while the implicit close done by C<open> does not.
747 If the file handle came from a piped open C<close> will additionally
748 return false if one of the other system calls involved fails or if the
749 program exits with non-zero status. (If the only problem was that the
750 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
751 also waits for the process executing on the pipe to complete, in case you
752 want to look at the output of the pipe afterwards, and
753 implicitly puts the exit status value of that command into C<$?>.
755 Prematurely closing the read end of a pipe (i.e. before the process
756 writing to it at the other end has closed it) will result in a
757 SIGPIPE being delivered to the writer. If the other end can't
758 handle that, be sure to read all the data before closing the pipe.
762 open(OUTPUT, '|sort >foo') # pipe to sort
763 or die "Can't start sort: $!";
764 #... # print stuff to output
765 close OUTPUT # wait for sort to finish
766 or warn $! ? "Error closing sort pipe: $!"
767 : "Exit status $? from sort";
768 open(INPUT, 'foo') # get sort's results
769 or die "Can't open 'foo' for input: $!";
771 FILEHANDLE may be an expression whose value can be used as an indirect
772 filehandle, usually the real filehandle name.
774 =item closedir DIRHANDLE
776 Closes a directory opened by C<opendir> and returns the success of that
779 DIRHANDLE may be an expression whose value can be used as an indirect
780 dirhandle, usually the real dirhandle name.
782 =item connect SOCKET,NAME
784 Attempts to connect to a remote socket, just as the connect system call
785 does. Returns true if it succeeded, false otherwise. NAME should be a
786 packed address of the appropriate type for the socket. See the examples in
787 L<perlipc/"Sockets: Client/Server Communication">.
791 Actually a flow control statement rather than a function. If there is a
792 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
793 C<foreach>), it is always executed just before the conditional is about to
794 be evaluated again, just like the third part of a C<for> loop in C. Thus
795 it can be used to increment a loop variable, even when the loop has been
796 continued via the C<next> statement (which is similar to the C C<continue>
799 C<last>, C<next>, or C<redo> may appear within a C<continue>
800 block. C<last> and C<redo> will behave as if they had been executed within
801 the main block. So will C<next>, but since it will execute a C<continue>
802 block, it may be more entertaining.
805 ### redo always comes here
808 ### next always comes here
810 # then back the top to re-check EXPR
812 ### last always comes here
814 Omitting the C<continue> section is semantically equivalent to using an
815 empty one, logically enough. In that case, C<next> goes directly back
816 to check the condition at the top of the loop.
822 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
823 takes cosine of C<$_>.
825 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
826 function, or use this relation:
828 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
830 =item crypt PLAINTEXT,SALT
832 Encrypts a string exactly like the crypt(3) function in the C library
833 (assuming that you actually have a version there that has not been
834 extirpated as a potential munition). This can prove useful for checking
835 the password file for lousy passwords, amongst other things. Only the
836 guys wearing white hats should do this.
838 Note that C<crypt> is intended to be a one-way function, much like
839 breaking eggs to make an omelette. There is no (known) corresponding
840 decrypt function (in other words, the crypt() is a one-way hash
841 function). As a result, this function isn't all that useful for
842 cryptography. (For that, see your nearby CPAN mirror.)
844 When verifying an existing encrypted string you should use the
845 encrypted text as the salt (like C<crypt($plain, $crypted) eq
846 $crypted>). This allows your code to work with the standard C<crypt>
847 and with more exotic implementations. In other words, do not assume
848 anything about the returned string itself, or how many bytes in
849 the encrypted string matter.
851 Traditionally the result is a string of 13 bytes: two first bytes of
852 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
853 the first eight bytes of the encrypted string mattered, but
854 alternative hashing schemes (like MD5), higher level security schemes
855 (like C2), and implementations on non-UNIX platforms may produce
858 When choosing a new salt create a random two character string whose
859 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
860 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
862 Here's an example that makes sure that whoever runs this program knows
865 $pwd = (getpwuid($<))[1];
869 chomp($word = <STDIN>);
873 if (crypt($word, $pwd) ne $pwd) {
879 Of course, typing in your own password to whoever asks you
882 The L<crypt> function is unsuitable for encrypting large quantities
883 of data, not least of all because you can't get the information
884 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
885 on your favorite CPAN mirror for a slew of potentially useful
888 If using crypt() on a Unicode string (which I<potentially> has
889 characters with codepoints above 255), Perl tries to make sense
890 of the situation by trying to downgrade (a copy of the string)
891 the string back to an eight-bit byte string before calling crypt()
892 (on that copy). If that works, good. If not, crypt() dies with
893 C<Wide character in crypt>.
897 [This function has been largely superseded by the C<untie> function.]
899 Breaks the binding between a DBM file and a hash.
901 =item dbmopen HASH,DBNAME,MASK
903 [This function has been largely superseded by the C<tie> function.]
905 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
906 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
907 argument is I<not> a filehandle, even though it looks like one). DBNAME
908 is the name of the database (without the F<.dir> or F<.pag> extension if
909 any). If the database does not exist, it is created with protection
910 specified by MASK (as modified by the C<umask>). If your system supports
911 only the older DBM functions, you may perform only one C<dbmopen> in your
912 program. In older versions of Perl, if your system had neither DBM nor
913 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
916 If you don't have write access to the DBM file, you can only read hash
917 variables, not set them. If you want to test whether you can write,
918 either use file tests or try setting a dummy hash entry inside an C<eval>,
919 which will trap the error.
921 Note that functions such as C<keys> and C<values> may return huge lists
922 when used on large DBM files. You may prefer to use the C<each>
923 function to iterate over large DBM files. Example:
925 # print out history file offsets
926 dbmopen(%HIST,'/usr/lib/news/history',0666);
927 while (($key,$val) = each %HIST) {
928 print $key, ' = ', unpack('L',$val), "\n";
932 See also L<AnyDBM_File> for a more general description of the pros and
933 cons of the various dbm approaches, as well as L<DB_File> for a particularly
936 You can control which DBM library you use by loading that library
937 before you call dbmopen():
940 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
941 or die "Can't open netscape history file: $!";
947 Returns a Boolean value telling whether EXPR has a value other than
948 the undefined value C<undef>. If EXPR is not present, C<$_> will be
951 Many operations return C<undef> to indicate failure, end of file,
952 system error, uninitialized variable, and other exceptional
953 conditions. This function allows you to distinguish C<undef> from
954 other values. (A simple Boolean test will not distinguish among
955 C<undef>, zero, the empty string, and C<"0">, which are all equally
956 false.) Note that since C<undef> is a valid scalar, its presence
957 doesn't I<necessarily> indicate an exceptional condition: C<pop>
958 returns C<undef> when its argument is an empty array, I<or> when the
959 element to return happens to be C<undef>.
961 You may also use C<defined(&func)> to check whether subroutine C<&func>
962 has ever been defined. The return value is unaffected by any forward
963 declarations of C<&foo>. Note that a subroutine which is not defined
964 may still be callable: its package may have an C<AUTOLOAD> method that
965 makes it spring into existence the first time that it is called -- see
968 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
969 used to report whether memory for that aggregate has ever been
970 allocated. This behavior may disappear in future versions of Perl.
971 You should instead use a simple test for size:
973 if (@an_array) { print "has array elements\n" }
974 if (%a_hash) { print "has hash members\n" }
976 When used on a hash element, it tells you whether the value is defined,
977 not whether the key exists in the hash. Use L</exists> for the latter
982 print if defined $switch{'D'};
983 print "$val\n" while defined($val = pop(@ary));
984 die "Can't readlink $sym: $!"
985 unless defined($value = readlink $sym);
986 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
987 $debugging = 0 unless defined $debugging;
989 Note: Many folks tend to overuse C<defined>, and then are surprised to
990 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
991 defined values. For example, if you say
995 The pattern match succeeds, and C<$1> is defined, despite the fact that it
996 matched "nothing". But it didn't really match nothing--rather, it
997 matched something that happened to be zero characters long. This is all
998 very above-board and honest. When a function returns an undefined value,
999 it's an admission that it couldn't give you an honest answer. So you
1000 should use C<defined> only when you're questioning the integrity of what
1001 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1004 See also L</undef>, L</exists>, L</ref>.
1008 Given an expression that specifies a hash element, array element, hash slice,
1009 or array slice, deletes the specified element(s) from the hash or array.
1010 In the case of an array, if the array elements happen to be at the end,
1011 the size of the array will shrink to the highest element that tests
1012 true for exists() (or 0 if no such element exists).
1014 Returns each element so deleted or the undefined value if there was no such
1015 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1016 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1017 from a C<tie>d hash or array may not necessarily return anything.
1019 Deleting an array element effectively returns that position of the array
1020 to its initial, uninitialized state. Subsequently testing for the same
1021 element with exists() will return false. Note that deleting array
1022 elements in the middle of an array will not shift the index of the ones
1023 after them down--use splice() for that. See L</exists>.
1025 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1027 foreach $key (keys %HASH) {
1031 foreach $index (0 .. $#ARRAY) {
1032 delete $ARRAY[$index];
1037 delete @HASH{keys %HASH};
1039 delete @ARRAY[0 .. $#ARRAY];
1041 But both of these are slower than just assigning the empty list
1042 or undefining %HASH or @ARRAY:
1044 %HASH = (); # completely empty %HASH
1045 undef %HASH; # forget %HASH ever existed
1047 @ARRAY = (); # completely empty @ARRAY
1048 undef @ARRAY; # forget @ARRAY ever existed
1050 Note that the EXPR can be arbitrarily complicated as long as the final
1051 operation is a hash element, array element, hash slice, or array slice
1054 delete $ref->[$x][$y]{$key};
1055 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1057 delete $ref->[$x][$y][$index];
1058 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1062 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1063 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1064 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1065 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1066 an C<eval(),> the error message is stuffed into C<$@> and the
1067 C<eval> is terminated with the undefined value. This makes
1068 C<die> the way to raise an exception.
1070 Equivalent examples:
1072 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1073 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1075 If the last element of LIST does not end in a newline, the current
1076 script line number and input line number (if any) are also printed,
1077 and a newline is supplied. Note that the "input line number" (also
1078 known as "chunk") is subject to whatever notion of "line" happens to
1079 be currently in effect, and is also available as the special variable
1080 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1082 Hint: sometimes appending C<", stopped"> to your message will cause it
1083 to make better sense when the string C<"at foo line 123"> is appended.
1084 Suppose you are running script "canasta".
1086 die "/etc/games is no good";
1087 die "/etc/games is no good, stopped";
1089 produce, respectively
1091 /etc/games is no good at canasta line 123.
1092 /etc/games is no good, stopped at canasta line 123.
1094 See also exit(), warn(), and the Carp module.
1096 If LIST is empty and C<$@> already contains a value (typically from a
1097 previous eval) that value is reused after appending C<"\t...propagated">.
1098 This is useful for propagating exceptions:
1101 die unless $@ =~ /Expected exception/;
1103 If LIST is empty and C<$@> contains an object reference that has a
1104 C<PROPAGATE> method, that method will be called with additional file
1105 and line number parameters. The return value replaces the value in
1106 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1109 If C<$@> is empty then the string C<"Died"> is used.
1111 die() can also be called with a reference argument. If this happens to be
1112 trapped within an eval(), $@ contains the reference. This behavior permits
1113 a more elaborate exception handling implementation using objects that
1114 maintain arbitrary state about the nature of the exception. Such a scheme
1115 is sometimes preferable to matching particular string values of $@ using
1116 regular expressions. Here's an example:
1118 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1120 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1121 # handle Some::Module::Exception
1124 # handle all other possible exceptions
1128 Because perl will stringify uncaught exception messages before displaying
1129 them, you may want to overload stringification operations on such custom
1130 exception objects. See L<overload> for details about that.
1132 You can arrange for a callback to be run just before the C<die>
1133 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1134 handler will be called with the error text and can change the error
1135 message, if it sees fit, by calling C<die> again. See
1136 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1137 L<"eval BLOCK"> for some examples. Although this feature was meant
1138 to be run only right before your program was to exit, this is not
1139 currently the case--the C<$SIG{__DIE__}> hook is currently called
1140 even inside eval()ed blocks/strings! If one wants the hook to do
1141 nothing in such situations, put
1145 as the first line of the handler (see L<perlvar/$^S>). Because
1146 this promotes strange action at a distance, this counterintuitive
1147 behavior may be fixed in a future release.
1151 Not really a function. Returns the value of the last command in the
1152 sequence of commands indicated by BLOCK. When modified by a loop
1153 modifier, executes the BLOCK once before testing the loop condition.
1154 (On other statements the loop modifiers test the conditional first.)
1156 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1157 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1158 See L<perlsyn> for alternative strategies.
1160 =item do SUBROUTINE(LIST)
1162 A deprecated form of subroutine call. See L<perlsub>.
1166 Uses the value of EXPR as a filename and executes the contents of the
1167 file as a Perl script. Its primary use is to include subroutines
1168 from a Perl subroutine library.
1176 except that it's more efficient and concise, keeps track of the current
1177 filename for error messages, searches the @INC libraries, and updates
1178 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1179 variables. It also differs in that code evaluated with C<do FILENAME>
1180 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1181 same, however, in that it does reparse the file every time you call it,
1182 so you probably don't want to do this inside a loop.
1184 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1185 error. If C<do> can read the file but cannot compile it, it
1186 returns undef and sets an error message in C<$@>. If the file is
1187 successfully compiled, C<do> returns the value of the last expression
1190 Note that inclusion of library modules is better done with the
1191 C<use> and C<require> operators, which also do automatic error checking
1192 and raise an exception if there's a problem.
1194 You might like to use C<do> to read in a program configuration
1195 file. Manual error checking can be done this way:
1197 # read in config files: system first, then user
1198 for $file ("/share/prog/defaults.rc",
1199 "$ENV{HOME}/.someprogrc")
1201 unless ($return = do $file) {
1202 warn "couldn't parse $file: $@" if $@;
1203 warn "couldn't do $file: $!" unless defined $return;
1204 warn "couldn't run $file" unless $return;
1212 This function causes an immediate core dump. See also the B<-u>
1213 command-line switch in L<perlrun>, which does the same thing.
1214 Primarily this is so that you can use the B<undump> program (not
1215 supplied) to turn your core dump into an executable binary after
1216 having initialized all your variables at the beginning of the
1217 program. When the new binary is executed it will begin by executing
1218 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1219 Think of it as a goto with an intervening core dump and reincarnation.
1220 If C<LABEL> is omitted, restarts the program from the top.
1222 B<WARNING>: Any files opened at the time of the dump will I<not>
1223 be open any more when the program is reincarnated, with possible
1224 resulting confusion on the part of Perl.
1226 This function is now largely obsolete, partly because it's very
1227 hard to convert a core file into an executable, and because the
1228 real compiler backends for generating portable bytecode and compilable
1229 C code have superseded it. That's why you should now invoke it as
1230 C<CORE::dump()>, if you don't want to be warned against a possible
1233 If you're looking to use L<dump> to speed up your program, consider
1234 generating bytecode or native C code as described in L<perlcc>. If
1235 you're just trying to accelerate a CGI script, consider using the
1236 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1237 You might also consider autoloading or selfloading, which at least
1238 make your program I<appear> to run faster.
1242 When called in list context, returns a 2-element list consisting of the
1243 key and value for the next element of a hash, so that you can iterate over
1244 it. When called in scalar context, returns only the key for the next
1245 element in the hash.
1247 Entries are returned in an apparently random order. The actual random
1248 order is subject to change in future versions of perl, but it is guaranteed
1249 to be in the same order as either the C<keys> or C<values> function
1250 would produce on the same (unmodified) hash.
1252 When the hash is entirely read, a null array is returned in list context
1253 (which when assigned produces a false (C<0>) value), and C<undef> in
1254 scalar context. The next call to C<each> after that will start iterating
1255 again. There is a single iterator for each hash, shared by all C<each>,
1256 C<keys>, and C<values> function calls in the program; it can be reset by
1257 reading all the elements from the hash, or by evaluating C<keys HASH> or
1258 C<values HASH>. If you add or delete elements of a hash while you're
1259 iterating over it, you may get entries skipped or duplicated, so
1260 don't. Exception: It is always safe to delete the item most recently
1261 returned by C<each()>, which means that the following code will work:
1263 while (($key, $value) = each %hash) {
1265 delete $hash{$key}; # This is safe
1268 The following prints out your environment like the printenv(1) program,
1269 only in a different order:
1271 while (($key,$value) = each %ENV) {
1272 print "$key=$value\n";
1275 See also C<keys>, C<values> and C<sort>.
1277 =item eof FILEHANDLE
1283 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1284 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1285 gives the real filehandle. (Note that this function actually
1286 reads a character and then C<ungetc>s it, so isn't very useful in an
1287 interactive context.) Do not read from a terminal file (or call
1288 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1289 as terminals may lose the end-of-file condition if you do.
1291 An C<eof> without an argument uses the last file read. Using C<eof()>
1292 with empty parentheses is very different. It refers to the pseudo file
1293 formed from the files listed on the command line and accessed via the
1294 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1295 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1296 used will cause C<@ARGV> to be examined to determine if input is
1297 available. Similarly, an C<eof()> after C<< <> >> has returned
1298 end-of-file will assume you are processing another C<@ARGV> list,
1299 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1300 see L<perlop/"I/O Operators">.
1302 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1303 detect the end of each file, C<eof()> will only detect the end of the
1304 last file. Examples:
1306 # reset line numbering on each input file
1308 next if /^\s*#/; # skip comments
1311 close ARGV if eof; # Not eof()!
1314 # insert dashes just before last line of last file
1316 if (eof()) { # check for end of current file
1317 print "--------------\n";
1318 close(ARGV); # close or last; is needed if we
1319 # are reading from the terminal
1324 Practical hint: you almost never need to use C<eof> in Perl, because the
1325 input operators typically return C<undef> when they run out of data, or if
1332 In the first form, the return value of EXPR is parsed and executed as if it
1333 were a little Perl program. The value of the expression (which is itself
1334 determined within scalar context) is first parsed, and if there weren't any
1335 errors, executed in the lexical context of the current Perl program, so
1336 that any variable settings or subroutine and format definitions remain
1337 afterwards. Note that the value is parsed every time the eval executes.
1338 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1339 delay parsing and subsequent execution of the text of EXPR until run time.
1341 In the second form, the code within the BLOCK is parsed only once--at the
1342 same time the code surrounding the eval itself was parsed--and executed
1343 within the context of the current Perl program. This form is typically
1344 used to trap exceptions more efficiently than the first (see below), while
1345 also providing the benefit of checking the code within BLOCK at compile
1348 The final semicolon, if any, may be omitted from the value of EXPR or within
1351 In both forms, the value returned is the value of the last expression
1352 evaluated inside the mini-program; a return statement may be also used, just
1353 as with subroutines. The expression providing the return value is evaluated
1354 in void, scalar, or list context, depending on the context of the eval itself.
1355 See L</wantarray> for more on how the evaluation context can be determined.
1357 If there is a syntax error or runtime error, or a C<die> statement is
1358 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1359 error message. If there was no error, C<$@> is guaranteed to be a null
1360 string. Beware that using C<eval> neither silences perl from printing
1361 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1362 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1363 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1364 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1366 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1367 determining whether a particular feature (such as C<socket> or C<symlink>)
1368 is implemented. It is also Perl's exception trapping mechanism, where
1369 the die operator is used to raise exceptions.
1371 If the code to be executed doesn't vary, you may use the eval-BLOCK
1372 form to trap run-time errors without incurring the penalty of
1373 recompiling each time. The error, if any, is still returned in C<$@>.
1376 # make divide-by-zero nonfatal
1377 eval { $answer = $a / $b; }; warn $@ if $@;
1379 # same thing, but less efficient
1380 eval '$answer = $a / $b'; warn $@ if $@;
1382 # a compile-time error
1383 eval { $answer = }; # WRONG
1386 eval '$answer ='; # sets $@
1388 Due to the current arguably broken state of C<__DIE__> hooks, when using
1389 the C<eval{}> form as an exception trap in libraries, you may wish not
1390 to trigger any C<__DIE__> hooks that user code may have installed.
1391 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1392 as shown in this example:
1394 # a very private exception trap for divide-by-zero
1395 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1398 This is especially significant, given that C<__DIE__> hooks can call
1399 C<die> again, which has the effect of changing their error messages:
1401 # __DIE__ hooks may modify error messages
1403 local $SIG{'__DIE__'} =
1404 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1405 eval { die "foo lives here" };
1406 print $@ if $@; # prints "bar lives here"
1409 Because this promotes action at a distance, this counterintuitive behavior
1410 may be fixed in a future release.
1412 With an C<eval>, you should be especially careful to remember what's
1413 being looked at when:
1419 eval { $x }; # CASE 4
1421 eval "\$$x++"; # CASE 5
1424 Cases 1 and 2 above behave identically: they run the code contained in
1425 the variable $x. (Although case 2 has misleading double quotes making
1426 the reader wonder what else might be happening (nothing is).) Cases 3
1427 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1428 does nothing but return the value of $x. (Case 4 is preferred for
1429 purely visual reasons, but it also has the advantage of compiling at
1430 compile-time instead of at run-time.) Case 5 is a place where
1431 normally you I<would> like to use double quotes, except that in this
1432 particular situation, you can just use symbolic references instead, as
1435 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1436 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1440 =item exec PROGRAM LIST
1442 The C<exec> function executes a system command I<and never returns>--
1443 use C<system> instead of C<exec> if you want it to return. It fails and
1444 returns false only if the command does not exist I<and> it is executed
1445 directly instead of via your system's command shell (see below).
1447 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1448 warns you if there is a following statement which isn't C<die>, C<warn>,
1449 or C<exit> (if C<-w> is set - but you always do that). If you
1450 I<really> want to follow an C<exec> with some other statement, you
1451 can use one of these styles to avoid the warning:
1453 exec ('foo') or print STDERR "couldn't exec foo: $!";
1454 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1456 If there is more than one argument in LIST, or if LIST is an array
1457 with more than one value, calls execvp(3) with the arguments in LIST.
1458 If there is only one scalar argument or an array with one element in it,
1459 the argument is checked for shell metacharacters, and if there are any,
1460 the entire argument is passed to the system's command shell for parsing
1461 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1462 If there are no shell metacharacters in the argument, it is split into
1463 words and passed directly to C<execvp>, which is more efficient.
1466 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1467 exec "sort $outfile | uniq";
1469 If you don't really want to execute the first argument, but want to lie
1470 to the program you are executing about its own name, you can specify
1471 the program you actually want to run as an "indirect object" (without a
1472 comma) in front of the LIST. (This always forces interpretation of the
1473 LIST as a multivalued list, even if there is only a single scalar in
1476 $shell = '/bin/csh';
1477 exec $shell '-sh'; # pretend it's a login shell
1481 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1483 When the arguments get executed via the system shell, results will
1484 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1487 Using an indirect object with C<exec> or C<system> is also more
1488 secure. This usage (which also works fine with system()) forces
1489 interpretation of the arguments as a multivalued list, even if the
1490 list had just one argument. That way you're safe from the shell
1491 expanding wildcards or splitting up words with whitespace in them.
1493 @args = ( "echo surprise" );
1495 exec @args; # subject to shell escapes
1497 exec { $args[0] } @args; # safe even with one-arg list
1499 The first version, the one without the indirect object, ran the I<echo>
1500 program, passing it C<"surprise"> an argument. The second version
1501 didn't--it tried to run a program literally called I<"echo surprise">,
1502 didn't find it, and set C<$?> to a non-zero value indicating failure.
1504 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1505 output before the exec, but this may not be supported on some platforms
1506 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1507 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1508 open handles in order to avoid lost output.
1510 Note that C<exec> will not call your C<END> blocks, nor will it call
1511 any C<DESTROY> methods in your objects.
1515 Given an expression that specifies a hash element or array element,
1516 returns true if the specified element in the hash or array has ever
1517 been initialized, even if the corresponding value is undefined. The
1518 element is not autovivified if it doesn't exist.
1520 print "Exists\n" if exists $hash{$key};
1521 print "Defined\n" if defined $hash{$key};
1522 print "True\n" if $hash{$key};
1524 print "Exists\n" if exists $array[$index];
1525 print "Defined\n" if defined $array[$index];
1526 print "True\n" if $array[$index];
1528 A hash or array element can be true only if it's defined, and defined if
1529 it exists, but the reverse doesn't necessarily hold true.
1531 Given an expression that specifies the name of a subroutine,
1532 returns true if the specified subroutine has ever been declared, even
1533 if it is undefined. Mentioning a subroutine name for exists or defined
1534 does not count as declaring it. Note that a subroutine which does not
1535 exist may still be callable: its package may have an C<AUTOLOAD>
1536 method that makes it spring into existence the first time that it is
1537 called -- see L<perlsub>.
1539 print "Exists\n" if exists &subroutine;
1540 print "Defined\n" if defined &subroutine;
1542 Note that the EXPR can be arbitrarily complicated as long as the final
1543 operation is a hash or array key lookup or subroutine name:
1545 if (exists $ref->{A}->{B}->{$key}) { }
1546 if (exists $hash{A}{B}{$key}) { }
1548 if (exists $ref->{A}->{B}->[$ix]) { }
1549 if (exists $hash{A}{B}[$ix]) { }
1551 if (exists &{$ref->{A}{B}{$key}}) { }
1553 Although the deepest nested array or hash will not spring into existence
1554 just because its existence was tested, any intervening ones will.
1555 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1556 into existence due to the existence test for the $key element above.
1557 This happens anywhere the arrow operator is used, including even:
1560 if (exists $ref->{"Some key"}) { }
1561 print $ref; # prints HASH(0x80d3d5c)
1563 This surprising autovivification in what does not at first--or even
1564 second--glance appear to be an lvalue context may be fixed in a future
1567 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1568 on how exists() acts when used on a pseudo-hash.
1570 Use of a subroutine call, rather than a subroutine name, as an argument
1571 to exists() is an error.
1574 exists &sub(); # Error
1578 Evaluates EXPR and exits immediately with that value. Example:
1581 exit 0 if $ans =~ /^[Xx]/;
1583 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1584 universally recognized values for EXPR are C<0> for success and C<1>
1585 for error; other values are subject to interpretation depending on the
1586 environment in which the Perl program is running. For example, exiting
1587 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1588 the mailer to return the item undelivered, but that's not true everywhere.
1590 Don't use C<exit> to abort a subroutine if there's any chance that
1591 someone might want to trap whatever error happened. Use C<die> instead,
1592 which can be trapped by an C<eval>.
1594 The exit() function does not always exit immediately. It calls any
1595 defined C<END> routines first, but these C<END> routines may not
1596 themselves abort the exit. Likewise any object destructors that need to
1597 be called are called before the real exit. If this is a problem, you
1598 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1599 See L<perlmod> for details.
1605 Returns I<e> (the natural logarithm base) to the power of EXPR.
1606 If EXPR is omitted, gives C<exp($_)>.
1608 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1610 Implements the fcntl(2) function. You'll probably have to say
1614 first to get the correct constant definitions. Argument processing and
1615 value return works just like C<ioctl> below.
1619 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1620 or die "can't fcntl F_GETFL: $!";
1622 You don't have to check for C<defined> on the return from C<fnctl>.
1623 Like C<ioctl>, it maps a C<0> return from the system call into
1624 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1625 in numeric context. It is also exempt from the normal B<-w> warnings
1626 on improper numeric conversions.
1628 Note that C<fcntl> will produce a fatal error if used on a machine that
1629 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1630 manpage to learn what functions are available on your system.
1632 =item fileno FILEHANDLE
1634 Returns the file descriptor for a filehandle, or undefined if the
1635 filehandle is not open. This is mainly useful for constructing
1636 bitmaps for C<select> and low-level POSIX tty-handling operations.
1637 If FILEHANDLE is an expression, the value is taken as an indirect
1638 filehandle, generally its name.
1640 You can use this to find out whether two handles refer to the
1641 same underlying descriptor:
1643 if (fileno(THIS) == fileno(THAT)) {
1644 print "THIS and THAT are dups\n";
1647 (Filehandles connected to memory objects via new features of C<open> may
1648 return undefined even though they are open.)
1651 =item flock FILEHANDLE,OPERATION
1653 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1654 for success, false on failure. Produces a fatal error if used on a
1655 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1656 C<flock> is Perl's portable file locking interface, although it locks
1657 only entire files, not records.
1659 Two potentially non-obvious but traditional C<flock> semantics are
1660 that it waits indefinitely until the lock is granted, and that its locks
1661 B<merely advisory>. Such discretionary locks are more flexible, but offer
1662 fewer guarantees. This means that files locked with C<flock> may be
1663 modified by programs that do not also use C<flock>. See L<perlport>,
1664 your port's specific documentation, or your system-specific local manpages
1665 for details. It's best to assume traditional behavior if you're writing
1666 portable programs. (But if you're not, you should as always feel perfectly
1667 free to write for your own system's idiosyncrasies (sometimes called
1668 "features"). Slavish adherence to portability concerns shouldn't get
1669 in the way of your getting your job done.)
1671 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1672 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1673 you can use the symbolic names if you import them from the Fcntl module,
1674 either individually, or as a group using the ':flock' tag. LOCK_SH
1675 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1676 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1677 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1678 waiting for the lock (check the return status to see if you got it).
1680 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1681 before locking or unlocking it.
1683 Note that the emulation built with lockf(3) doesn't provide shared
1684 locks, and it requires that FILEHANDLE be open with write intent. These
1685 are the semantics that lockf(3) implements. Most if not all systems
1686 implement lockf(3) in terms of fcntl(2) locking, though, so the
1687 differing semantics shouldn't bite too many people.
1689 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1690 be open with read intent to use LOCK_SH and requires that it be open
1691 with write intent to use LOCK_EX.
1693 Note also that some versions of C<flock> cannot lock things over the
1694 network; you would need to use the more system-specific C<fcntl> for
1695 that. If you like you can force Perl to ignore your system's flock(2)
1696 function, and so provide its own fcntl(2)-based emulation, by passing
1697 the switch C<-Ud_flock> to the F<Configure> program when you configure
1700 Here's a mailbox appender for BSD systems.
1702 use Fcntl ':flock'; # import LOCK_* constants
1705 flock(MBOX,LOCK_EX);
1706 # and, in case someone appended
1707 # while we were waiting...
1712 flock(MBOX,LOCK_UN);
1715 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1716 or die "Can't open mailbox: $!";
1719 print MBOX $msg,"\n\n";
1722 On systems that support a real flock(), locks are inherited across fork()
1723 calls, whereas those that must resort to the more capricious fcntl()
1724 function lose the locks, making it harder to write servers.
1726 See also L<DB_File> for other flock() examples.
1730 Does a fork(2) system call to create a new process running the
1731 same program at the same point. It returns the child pid to the
1732 parent process, C<0> to the child process, or C<undef> if the fork is
1733 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1734 are shared, while everything else is copied. On most systems supporting
1735 fork(), great care has gone into making it extremely efficient (for
1736 example, using copy-on-write technology on data pages), making it the
1737 dominant paradigm for multitasking over the last few decades.
1739 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1740 output before forking the child process, but this may not be supported
1741 on some platforms (see L<perlport>). To be safe, you may need to set
1742 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1743 C<IO::Handle> on any open handles in order to avoid duplicate output.
1745 If you C<fork> without ever waiting on your children, you will
1746 accumulate zombies. On some systems, you can avoid this by setting
1747 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1748 forking and reaping moribund children.
1750 Note that if your forked child inherits system file descriptors like
1751 STDIN and STDOUT that are actually connected by a pipe or socket, even
1752 if you exit, then the remote server (such as, say, a CGI script or a
1753 backgrounded job launched from a remote shell) won't think you're done.
1754 You should reopen those to F</dev/null> if it's any issue.
1758 Declare a picture format for use by the C<write> function. For
1762 Test: @<<<<<<<< @||||| @>>>>>
1763 $str, $%, '$' . int($num)
1767 $num = $cost/$quantity;
1771 See L<perlform> for many details and examples.
1773 =item formline PICTURE,LIST
1775 This is an internal function used by C<format>s, though you may call it,
1776 too. It formats (see L<perlform>) a list of values according to the
1777 contents of PICTURE, placing the output into the format output
1778 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1779 Eventually, when a C<write> is done, the contents of
1780 C<$^A> are written to some filehandle, but you could also read C<$^A>
1781 yourself and then set C<$^A> back to C<"">. Note that a format typically
1782 does one C<formline> per line of form, but the C<formline> function itself
1783 doesn't care how many newlines are embedded in the PICTURE. This means
1784 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1785 You may therefore need to use multiple formlines to implement a single
1786 record format, just like the format compiler.
1788 Be careful if you put double quotes around the picture, because an C<@>
1789 character may be taken to mean the beginning of an array name.
1790 C<formline> always returns true. See L<perlform> for other examples.
1792 =item getc FILEHANDLE
1796 Returns the next character from the input file attached to FILEHANDLE,
1797 or the undefined value at end of file, or if there was an error.
1798 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1799 efficient. However, it cannot be used by itself to fetch single
1800 characters without waiting for the user to hit enter. For that, try
1801 something more like:
1804 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1807 system "stty", '-icanon', 'eol', "\001";
1813 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1816 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1820 Determination of whether $BSD_STYLE should be set
1821 is left as an exercise to the reader.
1823 The C<POSIX::getattr> function can do this more portably on
1824 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1825 module from your nearest CPAN site; details on CPAN can be found on
1830 Implements the C library function of the same name, which on most
1831 systems returns the current login from F</etc/utmp>, if any. If null,
1834 $login = getlogin || getpwuid($<) || "Kilroy";
1836 Do not consider C<getlogin> for authentication: it is not as
1837 secure as C<getpwuid>.
1839 =item getpeername SOCKET
1841 Returns the packed sockaddr address of other end of the SOCKET connection.
1844 $hersockaddr = getpeername(SOCK);
1845 ($port, $iaddr) = sockaddr_in($hersockaddr);
1846 $herhostname = gethostbyaddr($iaddr, AF_INET);
1847 $herstraddr = inet_ntoa($iaddr);
1851 Returns the current process group for the specified PID. Use
1852 a PID of C<0> to get the current process group for the
1853 current process. Will raise an exception if used on a machine that
1854 doesn't implement getpgrp(2). If PID is omitted, returns process
1855 group of current process. Note that the POSIX version of C<getpgrp>
1856 does not accept a PID argument, so only C<PID==0> is truly portable.
1860 Returns the process id of the parent process.
1862 =item getpriority WHICH,WHO
1864 Returns the current priority for a process, a process group, or a user.
1865 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1866 machine that doesn't implement getpriority(2).
1872 =item gethostbyname NAME
1874 =item getnetbyname NAME
1876 =item getprotobyname NAME
1882 =item getservbyname NAME,PROTO
1884 =item gethostbyaddr ADDR,ADDRTYPE
1886 =item getnetbyaddr ADDR,ADDRTYPE
1888 =item getprotobynumber NUMBER
1890 =item getservbyport PORT,PROTO
1908 =item sethostent STAYOPEN
1910 =item setnetent STAYOPEN
1912 =item setprotoent STAYOPEN
1914 =item setservent STAYOPEN
1928 These routines perform the same functions as their counterparts in the
1929 system library. In list context, the return values from the
1930 various get routines are as follows:
1932 ($name,$passwd,$uid,$gid,
1933 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1934 ($name,$passwd,$gid,$members) = getgr*
1935 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1936 ($name,$aliases,$addrtype,$net) = getnet*
1937 ($name,$aliases,$proto) = getproto*
1938 ($name,$aliases,$port,$proto) = getserv*
1940 (If the entry doesn't exist you get a null list.)
1942 The exact meaning of the $gcos field varies but it usually contains
1943 the real name of the user (as opposed to the login name) and other
1944 information pertaining to the user. Beware, however, that in many
1945 system users are able to change this information and therefore it
1946 cannot be trusted and therefore the $gcos is tainted (see
1947 L<perlsec>). The $passwd and $shell, user's encrypted password and
1948 login shell, are also tainted, because of the same reason.
1950 In scalar context, you get the name, unless the function was a
1951 lookup by name, in which case you get the other thing, whatever it is.
1952 (If the entry doesn't exist you get the undefined value.) For example:
1954 $uid = getpwnam($name);
1955 $name = getpwuid($num);
1957 $gid = getgrnam($name);
1958 $name = getgrgid($num;
1962 In I<getpw*()> the fields $quota, $comment, and $expire are special
1963 cases in the sense that in many systems they are unsupported. If the
1964 $quota is unsupported, it is an empty scalar. If it is supported, it
1965 usually encodes the disk quota. If the $comment field is unsupported,
1966 it is an empty scalar. If it is supported it usually encodes some
1967 administrative comment about the user. In some systems the $quota
1968 field may be $change or $age, fields that have to do with password
1969 aging. In some systems the $comment field may be $class. The $expire
1970 field, if present, encodes the expiration period of the account or the
1971 password. For the availability and the exact meaning of these fields
1972 in your system, please consult your getpwnam(3) documentation and your
1973 F<pwd.h> file. You can also find out from within Perl what your
1974 $quota and $comment fields mean and whether you have the $expire field
1975 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1976 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1977 files are only supported if your vendor has implemented them in the
1978 intuitive fashion that calling the regular C library routines gets the
1979 shadow versions if you're running under privilege or if there exists
1980 the shadow(3) functions as found in System V ( this includes Solaris
1981 and Linux.) Those systems which implement a proprietary shadow password
1982 facility are unlikely to be supported.
1984 The $members value returned by I<getgr*()> is a space separated list of
1985 the login names of the members of the group.
1987 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1988 C, it will be returned to you via C<$?> if the function call fails. The
1989 C<@addrs> value returned by a successful call is a list of the raw
1990 addresses returned by the corresponding system library call. In the
1991 Internet domain, each address is four bytes long and you can unpack it
1992 by saying something like:
1994 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1996 The Socket library makes this slightly easier:
1999 $iaddr = inet_aton("127.1"); # or whatever address
2000 $name = gethostbyaddr($iaddr, AF_INET);
2002 # or going the other way
2003 $straddr = inet_ntoa($iaddr);
2005 If you get tired of remembering which element of the return list
2006 contains which return value, by-name interfaces are provided
2007 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2008 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2009 and C<User::grent>. These override the normal built-ins, supplying
2010 versions that return objects with the appropriate names
2011 for each field. For example:
2015 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2017 Even though it looks like they're the same method calls (uid),
2018 they aren't, because a C<File::stat> object is different from
2019 a C<User::pwent> object.
2021 =item getsockname SOCKET
2023 Returns the packed sockaddr address of this end of the SOCKET connection,
2024 in case you don't know the address because you have several different
2025 IPs that the connection might have come in on.
2028 $mysockaddr = getsockname(SOCK);
2029 ($port, $myaddr) = sockaddr_in($mysockaddr);
2030 printf "Connect to %s [%s]\n",
2031 scalar gethostbyaddr($myaddr, AF_INET),
2034 =item getsockopt SOCKET,LEVEL,OPTNAME
2036 Returns the socket option requested, or undef if there is an error.
2042 Returns the value of EXPR with filename expansions such as the
2043 standard Unix shell F</bin/csh> would do. This is the internal function
2044 implementing the C<< <*.c> >> operator, but you can use it directly.
2045 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2046 discussed in more detail in L<perlop/"I/O Operators">.
2048 Beginning with v5.6.0, this operator is implemented using the standard
2049 C<File::Glob> extension. See L<File::Glob> for details.
2053 Converts a time as returned by the time function to an 8-element list
2054 with the time localized for the standard Greenwich time zone.
2055 Typically used as follows:
2058 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2061 All list elements are numeric, and come straight out of the C `struct
2062 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2063 specified time. $mday is the day of the month, and $mon is the month
2064 itself, in the range C<0..11> with 0 indicating January and 11
2065 indicating December. $year is the number of years since 1900. That
2066 is, $year is C<123> in year 2023. $wday is the day of the week, with
2067 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2068 the year, in the range C<0..364> (or C<0..365> in leap years.)
2070 Note that the $year element is I<not> simply the last two digits of
2071 the year. If you assume it is, then you create non-Y2K-compliant
2072 programs--and you wouldn't want to do that, would you?
2074 The proper way to get a complete 4-digit year is simply:
2078 And to get the last two digits of the year (e.g., '01' in 2001) do:
2080 $year = sprintf("%02d", $year % 100);
2082 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2084 In scalar context, C<gmtime()> returns the ctime(3) value:
2086 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2088 Also see the C<timegm> function provided by the C<Time::Local> module,
2089 and the strftime(3) function available via the POSIX module.
2091 This scalar value is B<not> locale dependent (see L<perllocale>), but
2092 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2093 strftime(3) and mktime(3) functions available via the POSIX module. To
2094 get somewhat similar but locale dependent date strings, set up your
2095 locale environment variables appropriately (please see L<perllocale>)
2096 and try for example:
2098 use POSIX qw(strftime);
2099 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2101 Note that the C<%a> and C<%b> escapes, which represent the short forms
2102 of the day of the week and the month of the year, may not necessarily
2103 be three characters wide in all locales.
2111 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2112 execution there. It may not be used to go into any construct that
2113 requires initialization, such as a subroutine or a C<foreach> loop. It
2114 also can't be used to go into a construct that is optimized away,
2115 or to get out of a block or subroutine given to C<sort>.
2116 It can be used to go almost anywhere else within the dynamic scope,
2117 including out of subroutines, but it's usually better to use some other
2118 construct such as C<last> or C<die>. The author of Perl has never felt the
2119 need to use this form of C<goto> (in Perl, that is--C is another matter).
2120 (The difference being that C does not offer named loops combined with
2121 loop control. Perl does, and this replaces most structured uses of C<goto>
2122 in other languages.)
2124 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2125 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2126 necessarily recommended if you're optimizing for maintainability:
2128 goto ("FOO", "BAR", "GLARCH")[$i];
2130 The C<goto-&NAME> form is quite different from the other forms of
2131 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2132 doesn't have the stigma associated with other gotos. Instead, it
2133 exits the current subroutine (losing any changes set by local()) and
2134 immediately calls in its place the named subroutine using the current
2135 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2136 load another subroutine and then pretend that the other subroutine had
2137 been called in the first place (except that any modifications to C<@_>
2138 in the current subroutine are propagated to the other subroutine.)
2139 After the C<goto>, not even C<caller> will be able to tell that this
2140 routine was called first.
2142 NAME needn't be the name of a subroutine; it can be a scalar variable
2143 containing a code reference, or a block which evaluates to a code
2146 =item grep BLOCK LIST
2148 =item grep EXPR,LIST
2150 This is similar in spirit to, but not the same as, grep(1) and its
2151 relatives. In particular, it is not limited to using regular expressions.
2153 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2154 C<$_> to each element) and returns the list value consisting of those
2155 elements for which the expression evaluated to true. In scalar
2156 context, returns the number of times the expression was true.
2158 @foo = grep(!/^#/, @bar); # weed out comments
2162 @foo = grep {!/^#/} @bar; # weed out comments
2164 Note that C<$_> is an alias to the list value, so it can be used to
2165 modify the elements of the LIST. While this is useful and supported,
2166 it can cause bizarre results if the elements of LIST are not variables.
2167 Similarly, grep returns aliases into the original list, much as a for
2168 loop's index variable aliases the list elements. That is, modifying an
2169 element of a list returned by grep (for example, in a C<foreach>, C<map>
2170 or another C<grep>) actually modifies the element in the original list.
2171 This is usually something to be avoided when writing clear code.
2173 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2179 Interprets EXPR as a hex string and returns the corresponding value.
2180 (To convert strings that might start with either 0, 0x, or 0b, see
2181 L</oct>.) If EXPR is omitted, uses C<$_>.
2183 print hex '0xAf'; # prints '175'
2184 print hex 'aF'; # same
2186 Hex strings may only represent integers. Strings that would cause
2187 integer overflow trigger a warning. Leading whitespace is not stripped,
2192 There is no builtin C<import> function. It is just an ordinary
2193 method (subroutine) defined (or inherited) by modules that wish to export
2194 names to another module. The C<use> function calls the C<import> method
2195 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2197 =item index STR,SUBSTR,POSITION
2199 =item index STR,SUBSTR
2201 The index function searches for one string within another, but without
2202 the wildcard-like behavior of a full regular-expression pattern match.
2203 It returns the position of the first occurrence of SUBSTR in STR at
2204 or after POSITION. If POSITION is omitted, starts searching from the
2205 beginning of the string. The return value is based at C<0> (or whatever
2206 you've set the C<$[> variable to--but don't do that). If the substring
2207 is not found, returns one less than the base, ordinarily C<-1>.
2213 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2214 You should not use this function for rounding: one because it truncates
2215 towards C<0>, and two because machine representations of floating point
2216 numbers can sometimes produce counterintuitive results. For example,
2217 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2218 because it's really more like -268.99999999999994315658 instead. Usually,
2219 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2220 functions will serve you better than will int().
2222 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2224 Implements the ioctl(2) function. You'll probably first have to say
2226 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2228 to get the correct function definitions. If F<ioctl.ph> doesn't
2229 exist or doesn't have the correct definitions you'll have to roll your
2230 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2231 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2232 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2233 written depending on the FUNCTION--a pointer to the string value of SCALAR
2234 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2235 has no string value but does have a numeric value, that value will be
2236 passed rather than a pointer to the string value. To guarantee this to be
2237 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2238 functions may be needed to manipulate the values of structures used by
2241 The return value of C<ioctl> (and C<fcntl>) is as follows:
2243 if OS returns: then Perl returns:
2245 0 string "0 but true"
2246 anything else that number
2248 Thus Perl returns true on success and false on failure, yet you can
2249 still easily determine the actual value returned by the operating
2252 $retval = ioctl(...) || -1;
2253 printf "System returned %d\n", $retval;
2255 The special string "C<0> but true" is exempt from B<-w> complaints
2256 about improper numeric conversions.
2258 Here's an example of setting a filehandle named C<REMOTE> to be
2259 non-blocking at the system level. You'll have to negotiate C<$|>
2260 on your own, though.
2262 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2264 $flags = fcntl(REMOTE, F_GETFL, 0)
2265 or die "Can't get flags for the socket: $!\n";
2267 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2268 or die "Can't set flags for the socket: $!\n";
2270 =item join EXPR,LIST
2272 Joins the separate strings of LIST into a single string with fields
2273 separated by the value of EXPR, and returns that new string. Example:
2275 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2277 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2278 first argument. Compare L</split>.
2282 Returns a list consisting of all the keys of the named hash. (In
2283 scalar context, returns the number of keys.) The keys are returned in
2284 an apparently random order. The actual random order is subject to
2285 change in future versions of perl, but it is guaranteed to be the same
2286 order as either the C<values> or C<each> function produces (given
2287 that the hash has not been modified). As a side effect, it resets
2290 Here is yet another way to print your environment:
2293 @values = values %ENV;
2295 print pop(@keys), '=', pop(@values), "\n";
2298 or how about sorted by key:
2300 foreach $key (sort(keys %ENV)) {
2301 print $key, '=', $ENV{$key}, "\n";
2304 The returned values are copies of the original keys in the hash, so
2305 modifying them will not affect the original hash. Compare L</values>.
2307 To sort a hash by value, you'll need to use a C<sort> function.
2308 Here's a descending numeric sort of a hash by its values:
2310 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2311 printf "%4d %s\n", $hash{$key}, $key;
2314 As an lvalue C<keys> allows you to increase the number of hash buckets
2315 allocated for the given hash. This can gain you a measure of efficiency if
2316 you know the hash is going to get big. (This is similar to pre-extending
2317 an array by assigning a larger number to $#array.) If you say
2321 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2322 in fact, since it rounds up to the next power of two. These
2323 buckets will be retained even if you do C<%hash = ()>, use C<undef
2324 %hash> if you want to free the storage while C<%hash> is still in scope.
2325 You can't shrink the number of buckets allocated for the hash using
2326 C<keys> in this way (but you needn't worry about doing this by accident,
2327 as trying has no effect).
2329 See also C<each>, C<values> and C<sort>.
2331 =item kill SIGNAL, LIST
2333 Sends a signal to a list of processes. Returns the number of
2334 processes successfully signaled (which is not necessarily the
2335 same as the number actually killed).
2337 $cnt = kill 1, $child1, $child2;
2340 If SIGNAL is zero, no signal is sent to the process. This is a
2341 useful way to check that the process is alive and hasn't changed
2342 its UID. See L<perlport> for notes on the portability of this
2345 Unlike in the shell, if SIGNAL is negative, it kills
2346 process groups instead of processes. (On System V, a negative I<PROCESS>
2347 number will also kill process groups, but that's not portable.) That
2348 means you usually want to use positive not negative signals. You may also
2349 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2355 The C<last> command is like the C<break> statement in C (as used in
2356 loops); it immediately exits the loop in question. If the LABEL is
2357 omitted, the command refers to the innermost enclosing loop. The
2358 C<continue> block, if any, is not executed:
2360 LINE: while (<STDIN>) {
2361 last LINE if /^$/; # exit when done with header
2365 C<last> cannot be used to exit a block which returns a value such as
2366 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2367 a grep() or map() operation.
2369 Note that a block by itself is semantically identical to a loop
2370 that executes once. Thus C<last> can be used to effect an early
2371 exit out of such a block.
2373 See also L</continue> for an illustration of how C<last>, C<next>, and
2380 Returns a lowercased version of EXPR. This is the internal function
2381 implementing the C<\L> escape in double-quoted strings. Respects
2382 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2383 and L<perlunicode> for more details about locale and Unicode support.
2385 If EXPR is omitted, uses C<$_>.
2391 Returns the value of EXPR with the first character lowercased. This
2392 is the internal function implementing the C<\l> escape in
2393 double-quoted strings. Respects current LC_CTYPE locale if C<use
2394 locale> in force. See L<perllocale> and L<perlunicode> for more
2395 details about locale and Unicode support.
2397 If EXPR is omitted, uses C<$_>.
2403 Returns the length in characters of the value of EXPR. If EXPR is
2404 omitted, returns length of C<$_>. Note that this cannot be used on
2405 an entire array or hash to find out how many elements these have.
2406 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2408 =item link OLDFILE,NEWFILE
2410 Creates a new filename linked to the old filename. Returns true for
2411 success, false otherwise.
2413 =item listen SOCKET,QUEUESIZE
2415 Does the same thing that the listen system call does. Returns true if
2416 it succeeded, false otherwise. See the example in
2417 L<perlipc/"Sockets: Client/Server Communication">.
2421 You really probably want to be using C<my> instead, because C<local> isn't
2422 what most people think of as "local". See
2423 L<perlsub/"Private Variables via my()"> for details.
2425 A local modifies the listed variables to be local to the enclosing
2426 block, file, or eval. If more than one value is listed, the list must
2427 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2428 for details, including issues with tied arrays and hashes.
2430 =item localtime EXPR
2432 Converts a time as returned by the time function to a 9-element list
2433 with the time analyzed for the local time zone. Typically used as
2437 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2440 All list elements are numeric, and come straight out of the C `struct
2441 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2442 specified time. $mday is the day of the month, and $mon is the month
2443 itself, in the range C<0..11> with 0 indicating January and 11
2444 indicating December. $year is the number of years since 1900. That
2445 is, $year is C<123> in year 2023. $wday is the day of the week, with
2446 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2447 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2448 is true if the specified time occurs during daylight savings time,
2451 Note that the $year element is I<not> simply the last two digits of
2452 the year. If you assume it is, then you create non-Y2K-compliant
2453 programs--and you wouldn't want to do that, would you?
2455 The proper way to get a complete 4-digit year is simply:
2459 And to get the last two digits of the year (e.g., '01' in 2001) do:
2461 $year = sprintf("%02d", $year % 100);
2463 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2465 In scalar context, C<localtime()> returns the ctime(3) value:
2467 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2469 This scalar value is B<not> locale dependent, see L<perllocale>, but
2470 instead a Perl builtin. Also see the C<Time::Local> module
2471 (to convert the second, minutes, hours, ... back to seconds since the
2472 stroke of midnight the 1st of January 1970, the value returned by
2473 time()), and the strftime(3) and mktime(3) functions available via the
2474 POSIX module. To get somewhat similar but locale dependent date
2475 strings, set up your locale environment variables appropriately
2476 (please see L<perllocale>) and try for example:
2478 use POSIX qw(strftime);
2479 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2481 Note that the C<%a> and C<%b>, the short forms of the day of the week
2482 and the month of the year, may not necessarily be three characters wide.
2486 This function places an advisory lock on a shared variable, or referenced
2487 object contained in I<THING> until the lock goes out of scope.
2489 lock() is a "weak keyword" : this means that if you've defined a function
2490 by this name (before any calls to it), that function will be called
2491 instead. (However, if you've said C<use threads>, lock() is always a
2492 keyword.) See L<threads>.
2498 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2499 returns log of C<$_>. To get the log of another base, use basic algebra:
2500 The base-N log of a number is equal to the natural log of that number
2501 divided by the natural log of N. For example:
2505 return log($n)/log(10);
2508 See also L</exp> for the inverse operation.
2514 Does the same thing as the C<stat> function (including setting the
2515 special C<_> filehandle) but stats a symbolic link instead of the file
2516 the symbolic link points to. If symbolic links are unimplemented on
2517 your system, a normal C<stat> is done.
2519 If EXPR is omitted, stats C<$_>.
2523 The match operator. See L<perlop>.
2525 =item map BLOCK LIST
2529 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2530 C<$_> to each element) and returns the list value composed of the
2531 results of each such evaluation. In scalar context, returns the
2532 total number of elements so generated. Evaluates BLOCK or EXPR in
2533 list context, so each element of LIST may produce zero, one, or
2534 more elements in the returned value.
2536 @chars = map(chr, @nums);
2538 translates a list of numbers to the corresponding characters. And
2540 %hash = map { getkey($_) => $_ } @array;
2542 is just a funny way to write
2545 foreach $_ (@array) {
2546 $hash{getkey($_)} = $_;
2549 Note that C<$_> is an alias to the list value, so it can be used to
2550 modify the elements of the LIST. While this is useful and supported,
2551 it can cause bizarre results if the elements of LIST are not variables.
2552 Using a regular C<foreach> loop for this purpose would be clearer in
2553 most cases. See also L</grep> for an array composed of those items of
2554 the original list for which the BLOCK or EXPR evaluates to true.
2556 C<{> starts both hash references and blocks, so C<map { ...> could be either
2557 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2558 ahead for the closing C<}> it has to take a guess at which its dealing with
2559 based what it finds just after the C<{>. Usually it gets it right, but if it
2560 doesn't it won't realize something is wrong until it gets to the C<}> and
2561 encounters the missing (or unexpected) comma. The syntax error will be
2562 reported close to the C<}> but you'll need to change something near the C<{>
2563 such as using a unary C<+> to give perl some help:
2565 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2566 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2567 %hash = map { ("\L$_", 1) } @array # this also works
2568 %hash = map { lc($_), 1 } @array # as does this.
2569 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2571 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2573 or to force an anon hash constructor use C<+{>
2575 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2577 and you get list of anonymous hashes each with only 1 entry.
2579 =item mkdir FILENAME,MASK
2581 =item mkdir FILENAME
2583 Creates the directory specified by FILENAME, with permissions
2584 specified by MASK (as modified by C<umask>). If it succeeds it
2585 returns true, otherwise it returns false and sets C<$!> (errno).
2586 If omitted, MASK defaults to 0777.
2588 In general, it is better to create directories with permissive MASK,
2589 and let the user modify that with their C<umask>, than it is to supply
2590 a restrictive MASK and give the user no way to be more permissive.
2591 The exceptions to this rule are when the file or directory should be
2592 kept private (mail files, for instance). The perlfunc(1) entry on
2593 C<umask> discusses the choice of MASK in more detail.
2595 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2596 number of trailing slashes. Some operating and filesystems do not get
2597 this right, so Perl automatically removes all trailing slashes to keep
2600 =item msgctl ID,CMD,ARG
2602 Calls the System V IPC function msgctl(2). You'll probably have to say
2606 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2607 then ARG must be a variable which will hold the returned C<msqid_ds>
2608 structure. Returns like C<ioctl>: the undefined value for error,
2609 C<"0 but true"> for zero, or the actual return value otherwise. See also
2610 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2612 =item msgget KEY,FLAGS
2614 Calls the System V IPC function msgget(2). Returns the message queue
2615 id, or the undefined value if there is an error. See also
2616 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2618 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2620 Calls the System V IPC function msgrcv to receive a message from
2621 message queue ID into variable VAR with a maximum message size of
2622 SIZE. Note that when a message is received, the message type as a
2623 native long integer will be the first thing in VAR, followed by the
2624 actual message. This packing may be opened with C<unpack("l! a*")>.
2625 Taints the variable. Returns true if successful, or false if there is
2626 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2627 C<IPC::SysV::Msg> documentation.
2629 =item msgsnd ID,MSG,FLAGS
2631 Calls the System V IPC function msgsnd to send the message MSG to the
2632 message queue ID. MSG must begin with the native long integer message
2633 type, and be followed by the length of the actual message, and finally
2634 the message itself. This kind of packing can be achieved with
2635 C<pack("l! a*", $type, $message)>. Returns true if successful,
2636 or false if there is an error. See also C<IPC::SysV>
2637 and C<IPC::SysV::Msg> documentation.
2643 =item my EXPR : ATTRS
2645 =item my TYPE EXPR : ATTRS
2647 A C<my> declares the listed variables to be local (lexically) to the
2648 enclosing block, file, or C<eval>. If more than one value is listed,
2649 the list must be placed in parentheses.
2651 The exact semantics and interface of TYPE and ATTRS are still
2652 evolving. TYPE is currently bound to the use of C<fields> pragma,
2653 and attributes are handled using the C<attributes> pragma, or starting
2654 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2655 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2656 L<attributes>, and L<Attribute::Handlers>.
2662 The C<next> command is like the C<continue> statement in C; it starts
2663 the next iteration of the loop:
2665 LINE: while (<STDIN>) {
2666 next LINE if /^#/; # discard comments
2670 Note that if there were a C<continue> block on the above, it would get
2671 executed even on discarded lines. If the LABEL is omitted, the command
2672 refers to the innermost enclosing loop.
2674 C<next> cannot be used to exit a block which returns a value such as
2675 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2676 a grep() or map() operation.
2678 Note that a block by itself is semantically identical to a loop
2679 that executes once. Thus C<next> will exit such a block early.
2681 See also L</continue> for an illustration of how C<last>, C<next>, and
2684 =item no Module VERSION LIST
2686 =item no Module VERSION
2688 =item no Module LIST
2692 See the L</use> function, which C<no> is the opposite of.
2698 Interprets EXPR as an octal string and returns the corresponding
2699 value. (If EXPR happens to start off with C<0x>, interprets it as a
2700 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2701 binary string. Leading whitespace is ignored in all three cases.)
2702 The following will handle decimal, binary, octal, and hex in the standard
2705 $val = oct($val) if $val =~ /^0/;
2707 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2708 in octal), use sprintf() or printf():
2710 $perms = (stat("filename"))[2] & 07777;
2711 $oct_perms = sprintf "%lo", $perms;
2713 The oct() function is commonly used when a string such as C<644> needs
2714 to be converted into a file mode, for example. (Although perl will
2715 automatically convert strings into numbers as needed, this automatic
2716 conversion assumes base 10.)
2718 =item open FILEHANDLE,EXPR
2720 =item open FILEHANDLE,MODE,EXPR
2722 =item open FILEHANDLE,MODE,EXPR,LIST
2724 =item open FILEHANDLE,MODE,REFERENCE
2726 =item open FILEHANDLE
2728 Opens the file whose filename is given by EXPR, and associates it with
2731 (The following is a comprehensive reference to open(): for a gentler
2732 introduction you may consider L<perlopentut>.)
2734 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2735 assigned a reference to a new anonymous filehandle, otherwise if
2736 FILEHANDLE is an expression, its value is used as the name of the real
2737 filehandle wanted. (This is considered a symbolic reference, so C<use
2738 strict 'refs'> should I<not> be in effect.)
2740 If EXPR is omitted, the scalar variable of the same name as the
2741 FILEHANDLE contains the filename. (Note that lexical variables--those
2742 declared with C<my>--will not work for this purpose; so if you're
2743 using C<my>, specify EXPR in your call to open.)
2745 If three or more arguments are specified then the mode of opening and
2746 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2747 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2748 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2749 the file is opened for appending, again being created if necessary.
2751 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2752 indicate that you want both read and write access to the file; thus
2753 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2754 '+>' >> mode would clobber the file first. You can't usually use
2755 either read-write mode for updating textfiles, since they have
2756 variable length records. See the B<-i> switch in L<perlrun> for a
2757 better approach. The file is created with permissions of C<0666>
2758 modified by the process' C<umask> value.
2760 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2761 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2763 In the 2-arguments (and 1-argument) form of the call the mode and
2764 filename should be concatenated (in this order), possibly separated by
2765 spaces. It is possible to omit the mode in these forms if the mode is
2768 If the filename begins with C<'|'>, the filename is interpreted as a
2769 command to which output is to be piped, and if the filename ends with a
2770 C<'|'>, the filename is interpreted as a command which pipes output to
2771 us. See L<perlipc/"Using open() for IPC">
2772 for more examples of this. (You are not allowed to C<open> to a command
2773 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2774 and L<perlipc/"Bidirectional Communication with Another Process">
2777 For three or more arguments if MODE is C<'|-'>, the filename is
2778 interpreted as a command to which output is to be piped, and if MODE
2779 is C<'-|'>, the filename is interpreted as a command which pipes
2780 output to us. In the 2-arguments (and 1-argument) form one should
2781 replace dash (C<'-'>) with the command.
2782 See L<perlipc/"Using open() for IPC"> for more examples of this.
2783 (You are not allowed to C<open> to a command that pipes both in I<and>
2784 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2785 L<perlipc/"Bidirectional Communication"> for alternatives.)
2787 In the three-or-more argument form of pipe opens, if LIST is specified
2788 (extra arguments after the command name) then LIST becomes arguments
2789 to the command invoked if the platform supports it. The meaning of
2790 C<open> with more than three arguments for non-pipe modes is not yet
2791 specified. Experimental "layers" may give extra LIST arguments
2794 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2795 and opening C<< '>-' >> opens STDOUT.
2797 You may use the three-argument form of open to specify
2798 I<I/O disciplines> or IO "layers" to be applied to the handle that affect how the input and output
2799 are processed: (see L<open> and L<PerlIO> for more details).
2802 open(FH, "<:utf8", "file")
2804 will open the UTF-8 encoded file containing Unicode characters,
2805 see L<perluniintro>. (Note that if disciplines are specified in the
2806 three-arg form then default disciplines set by the C<open> pragma are
2809 Open returns nonzero upon success, the undefined value otherwise. If
2810 the C<open> involved a pipe, the return value happens to be the pid of
2813 If you're running Perl on a system that distinguishes between text
2814 files and binary files, then you should check out L</binmode> for tips
2815 for dealing with this. The key distinction between systems that need
2816 C<binmode> and those that don't is their text file formats. Systems
2817 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2818 character, and which encode that character in C as C<"\n">, do not
2819 need C<binmode>. The rest need it.
2821 When opening a file, it's usually a bad idea to continue normal execution
2822 if the request failed, so C<open> is frequently used in connection with
2823 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2824 where you want to make a nicely formatted error message (but there are
2825 modules that can help with that problem)) you should always check
2826 the return value from opening a file. The infrequent exception is when
2827 working with an unopened filehandle is actually what you want to do.
2829 As a special case the 3 arg form with a read/write mode and the third
2830 argument being C<undef>:
2832 open(TMP, "+>", undef) or die ...
2834 opens a filehandle to an anonymous temporary file.
2836 File handles can be opened to "in memory" files held in Perl scalars via:
2838 open($fh, '>', \$variable) || ..
2840 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2841 file, you have to close it first:
2844 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2849 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2850 while (<ARTICLE>) {...
2852 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2853 # if the open fails, output is discarded
2855 open(DBASE, '+<', 'dbase.mine') # open for update
2856 or die "Can't open 'dbase.mine' for update: $!";
2858 open(DBASE, '+<dbase.mine') # ditto
2859 or die "Can't open 'dbase.mine' for update: $!";
2861 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2862 or die "Can't start caesar: $!";
2864 open(ARTICLE, "caesar <$article |") # ditto
2865 or die "Can't start caesar: $!";
2867 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2868 or die "Can't start sort: $!";
2871 open(MEMORY,'>', \$var)
2872 or die "Can't open memory file: $!";
2873 print MEMORY "foo!\n"; # output will end up in $var
2875 # process argument list of files along with any includes
2877 foreach $file (@ARGV) {
2878 process($file, 'fh00');
2882 my($filename, $input) = @_;
2883 $input++; # this is a string increment
2884 unless (open($input, $filename)) {
2885 print STDERR "Can't open $filename: $!\n";
2890 while (<$input>) { # note use of indirection
2891 if (/^#include "(.*)"/) {
2892 process($1, $input);
2899 You may also, in the Bourne shell tradition, specify an EXPR beginning
2900 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2901 name of a filehandle (or file descriptor, if numeric) to be
2902 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2903 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2904 mode you specify should match the mode of the original filehandle.
2905 (Duping a filehandle does not take into account any existing contents of
2906 IO buffers.) If you use the 3 arg form then you can pass either a number,
2907 the name of a filehandle or the normal "reference to a glob".
2909 Here is a script that saves, redirects, and restores C<STDOUT> and
2910 C<STDERR> using various methods:
2913 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2914 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2916 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2917 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2919 select STDERR; $| = 1; # make unbuffered
2920 select STDOUT; $| = 1; # make unbuffered
2922 print STDOUT "stdout 1\n"; # this works for
2923 print STDERR "stderr 1\n"; # subprocesses too
2928 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2929 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2931 print STDOUT "stdout 2\n";
2932 print STDERR "stderr 2\n";
2934 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2935 do an equivalent of C's C<fdopen> of that file descriptor; this is
2936 more parsimonious of file descriptors. For example:
2938 open(FILEHANDLE, "<&=$fd")
2942 open(FILEHANDLE, "<&=", $fd)
2944 Note that if Perl is using the standard C libraries' fdopen() then on
2945 many UNIX systems, fdopen() is known to fail when file descriptors
2946 exceed a certain value, typically 255. If you need more file
2947 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2949 You can see whether Perl has been compiled with PerlIO or not by
2950 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2951 is C<define>, you have PerlIO, otherwise you don't.
2953 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2954 with 2-arguments (or 1-argument) form of open(), then
2955 there is an implicit fork done, and the return value of open is the pid
2956 of the child within the parent process, and C<0> within the child
2957 process. (Use C<defined($pid)> to determine whether the open was successful.)
2958 The filehandle behaves normally for the parent, but i/o to that
2959 filehandle is piped from/to the STDOUT/STDIN of the child process.
2960 In the child process the filehandle isn't opened--i/o happens from/to
2961 the new STDOUT or STDIN. Typically this is used like the normal
2962 piped open when you want to exercise more control over just how the
2963 pipe command gets executed, such as when you are running setuid, and
2964 don't want to have to scan shell commands for metacharacters.
2965 The following triples are more or less equivalent:
2967 open(FOO, "|tr '[a-z]' '[A-Z]'");
2968 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2969 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2970 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2972 open(FOO, "cat -n '$file'|");
2973 open(FOO, '-|', "cat -n '$file'");
2974 open(FOO, '-|') || exec 'cat', '-n', $file;
2975 open(FOO, '-|', "cat", '-n', $file);
2977 The last example in each block shows the pipe as "list form", which is
2978 not yet supported on all platforms. A good rule of thumb is that if
2979 your platform has true C<fork()> (in other words, if your platform is
2980 UNIX) you can use the list form.
2982 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2984 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2985 output before any operation that may do a fork, but this may not be
2986 supported on some platforms (see L<perlport>). To be safe, you may need
2987 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2988 of C<IO::Handle> on any open handles.
2990 On systems that support a close-on-exec flag on files, the flag will
2991 be set for the newly opened file descriptor as determined by the value
2992 of $^F. See L<perlvar/$^F>.
2994 Closing any piped filehandle causes the parent process to wait for the
2995 child to finish, and returns the status value in C<$?>.
2997 The filename passed to 2-argument (or 1-argument) form of open() will
2998 have leading and trailing whitespace deleted, and the normal
2999 redirection characters honored. This property, known as "magic open",
3000 can often be used to good effect. A user could specify a filename of
3001 F<"rsh cat file |">, or you could change certain filenames as needed:
3003 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3004 open(FH, $filename) or die "Can't open $filename: $!";
3006 Use 3-argument form to open a file with arbitrary weird characters in it,
3008 open(FOO, '<', $file);
3010 otherwise it's necessary to protect any leading and trailing whitespace:
3012 $file =~ s#^(\s)#./$1#;
3013 open(FOO, "< $file\0");
3015 (this may not work on some bizarre filesystems). One should
3016 conscientiously choose between the I<magic> and 3-arguments form
3021 will allow the user to specify an argument of the form C<"rsh cat file |">,
3022 but will not work on a filename which happens to have a trailing space, while
3024 open IN, '<', $ARGV[0];
3026 will have exactly the opposite restrictions.
3028 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3029 should use the C<sysopen> function, which involves no such magic (but
3030 may use subtly different filemodes than Perl open(), which is mapped
3031 to C fopen()). This is
3032 another way to protect your filenames from interpretation. For example:
3035 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3036 or die "sysopen $path: $!";
3037 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3038 print HANDLE "stuff $$\n";
3040 print "File contains: ", <HANDLE>;
3042 Using the constructor from the C<IO::Handle> package (or one of its
3043 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3044 filehandles that have the scope of whatever variables hold references to
3045 them, and automatically close whenever and however you leave that scope:
3049 sub read_myfile_munged {
3051 my $handle = new IO::File;
3052 open($handle, "myfile") or die "myfile: $!";
3054 or return (); # Automatically closed here.
3055 mung $first or die "mung failed"; # Or here.
3056 return $first, <$handle> if $ALL; # Or here.
3060 See L</seek> for some details about mixing reading and writing.
3062 =item opendir DIRHANDLE,EXPR
3064 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3065 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3066 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3072 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3073 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3076 For the reverse, see L</chr>.
3077 See L<perlunicode> and L<encoding> for more about Unicode.
3083 =item our EXPR : ATTRS
3085 =item our TYPE EXPR : ATTRS
3087 An C<our> declares the listed variables to be valid globals within
3088 the enclosing block, file, or C<eval>. That is, it has the same
3089 scoping rules as a "my" declaration, but does not create a local
3090 variable. If more than one value is listed, the list must be placed
3091 in parentheses. The C<our> declaration has no semantic effect unless
3092 "use strict vars" is in effect, in which case it lets you use the
3093 declared global variable without qualifying it with a package name.
3094 (But only within the lexical scope of the C<our> declaration. In this
3095 it differs from "use vars", which is package scoped.)
3097 An C<our> declaration declares a global variable that will be visible
3098 across its entire lexical scope, even across package boundaries. The
3099 package in which the variable is entered is determined at the point
3100 of the declaration, not at the point of use. This means the following
3104 our $bar; # declares $Foo::bar for rest of lexical scope
3108 print $bar; # prints 20
3110 Multiple C<our> declarations in the same lexical scope are allowed
3111 if they are in different packages. If they happened to be in the same
3112 package, Perl will emit warnings if you have asked for them.
3116 our $bar; # declares $Foo::bar for rest of lexical scope
3120 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3121 print $bar; # prints 30
3123 our $bar; # emits warning
3125 An C<our> declaration may also have a list of attributes associated
3128 The exact semantics and interface of TYPE and ATTRS are still
3129 evolving. TYPE is currently bound to the use of C<fields> pragma,
3130 and attributes are handled using the C<attributes> pragma, or starting
3131 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3132 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3133 L<attributes>, and L<Attribute::Handlers>.
3135 The only currently recognized C<our()> attribute is C<unique> which
3136 indicates that a single copy of the global is to be used by all
3137 interpreters should the program happen to be running in a
3138 multi-interpreter environment. (The default behaviour would be for
3139 each interpreter to have its own copy of the global.) Examples:
3141 our @EXPORT : unique = qw(foo);
3142 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3143 our $VERSION : unique = "1.00";
3145 Note that this attribute also has the effect of making the global
3146 readonly when the first new interpreter is cloned (for example,
3147 when the first new thread is created).
3149 Multi-interpreter environments can come to being either through the
3150 fork() emulation on Windows platforms, or by embedding perl in a
3151 multi-threaded application. The C<unique> attribute does nothing in
3152 all other environments.
3154 =item pack TEMPLATE,LIST
3156 Takes a LIST of values and converts it into a string using the rules
3157 given by the TEMPLATE. The resulting string is the concatenation of
3158 the converted values. Typically, each converted value looks
3159 like its machine-level representation. For example, on 32-bit machines
3160 a converted integer may be represented by a sequence of 4 bytes.
3162 The TEMPLATE is a sequence of characters that give the order and type
3163 of values, as follows:
3165 a A string with arbitrary binary data, will be null padded.
3166 A A text (ASCII) string, will be space padded.
3167 Z A null terminated (ASCIZ) string, will be null padded.
3169 b A bit string (ascending bit order inside each byte, like vec()).
3170 B A bit string (descending bit order inside each byte).
3171 h A hex string (low nybble first).
3172 H A hex string (high nybble first).
3174 c A signed char value.
3175 C An unsigned char value. Only does bytes. See U for Unicode.
3177 s A signed short value.
3178 S An unsigned short value.
3179 (This 'short' is _exactly_ 16 bits, which may differ from
3180 what a local C compiler calls 'short'. If you want
3181 native-length shorts, use the '!' suffix.)
3183 i A signed integer value.
3184 I An unsigned integer value.
3185 (This 'integer' is _at_least_ 32 bits wide. Its exact
3186 size depends on what a local C compiler calls 'int',
3187 and may even be larger than the 'long' described in
3190 l A signed long value.
3191 L An unsigned long value.
3192 (This 'long' is _exactly_ 32 bits, which may differ from
3193 what a local C compiler calls 'long'. If you want
3194 native-length longs, use the '!' suffix.)
3196 n An unsigned short in "network" (big-endian) order.
3197 N An unsigned long in "network" (big-endian) order.
3198 v An unsigned short in "VAX" (little-endian) order.
3199 V An unsigned long in "VAX" (little-endian) order.
3200 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3201 _exactly_ 32 bits, respectively.)
3203 q A signed quad (64-bit) value.
3204 Q An unsigned quad value.
3205 (Quads are available only if your system supports 64-bit
3206 integer values _and_ if Perl has been compiled to support those.
3207 Causes a fatal error otherwise.)
3209 j A signed integer value (a Perl internal integer, IV).
3210 J An unsigned integer value (a Perl internal unsigned integer, UV).
3212 f A single-precision float in the native format.
3213 d A double-precision float in the native format.
3215 F A floating point value in the native native format
3216 (a Perl internal floating point value, NV).
3217 D A long double-precision float in the native format.
3218 (Long doubles are available only if your system supports long
3219 double values _and_ if Perl has been compiled to support those.
3220 Causes a fatal error otherwise.)
3222 p A pointer to a null-terminated string.
3223 P A pointer to a structure (fixed-length string).
3225 u A uuencoded string.
3226 U A Unicode character number. Encodes to UTF-8 internally
3227 (or UTF-EBCDIC in EBCDIC platforms).
3229 w A BER compressed integer. Its bytes represent an unsigned
3230 integer in base 128, most significant digit first, with as
3231 few digits as possible. Bit eight (the high bit) is set
3232 on each byte except the last.
3236 @ Null fill to absolute position.
3237 ( Start of a ()-group.
3239 The following rules apply:
3245 Each letter may optionally be followed by a number giving a repeat
3246 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3247 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3248 many values from the LIST. A C<*> for the repeat count means to use
3249 however many items are left, except for C<@>, C<x>, C<X>, where it is
3250 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3251 is the same). A numeric repeat count may optionally be enclosed in
3252 brackets, as in C<pack 'C[80]', @arr>.
3254 One can replace the numeric repeat count by a template enclosed in brackets;
3255 then the packed length of this template in bytes is used as a count.
3256 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3257 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3258 If the template in brackets contains alignment commands (such as C<x![d]>),
3259 its packed length is calculated as if the start of the template has the maximal
3262 When used with C<Z>, C<*> results in the addition of a trailing null
3263 byte (so the packed result will be one longer than the byte C<length>
3266 The repeat count for C<u> is interpreted as the maximal number of bytes
3267 to encode per line of output, with 0 and 1 replaced by 45.
3271 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3272 string of length count, padding with nulls or spaces as necessary. When
3273 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3274 after the first null, and C<a> returns data verbatim. When packing,
3275 C<a>, and C<Z> are equivalent.
3277 If the value-to-pack is too long, it is truncated. If too long and an
3278 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3279 by a null byte. Thus C<Z> always packs a trailing null byte under
3284 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3285 Each byte of the input field of pack() generates 1 bit of the result.
3286 Each result bit is based on the least-significant bit of the corresponding
3287 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3288 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3290 Starting from the beginning of the input string of pack(), each 8-tuple
3291 of bytes is converted to 1 byte of output. With format C<b>
3292 the first byte of the 8-tuple determines the least-significant bit of a
3293 byte, and with format C<B> it determines the most-significant bit of
3296 If the length of the input string is not exactly divisible by 8, the
3297 remainder is packed as if the input string were padded by null bytes
3298 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3300 If the input string of pack() is longer than needed, extra bytes are ignored.
3301 A C<*> for the repeat count of pack() means to use all the bytes of
3302 the input field. On unpack()ing the bits are converted to a string
3303 of C<"0">s and C<"1">s.
3307 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3308 representable as hexadecimal digits, 0-9a-f) long.
3310 Each byte of the input field of pack() generates 4 bits of the result.
3311 For non-alphabetical bytes the result is based on the 4 least-significant
3312 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3313 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3314 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3315 is compatible with the usual hexadecimal digits, so that C<"a"> and
3316 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3317 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3319 Starting from the beginning of the input string of pack(), each pair
3320 of bytes is converted to 1 byte of output. With format C<h> the
3321 first byte of the pair determines the least-significant nybble of the
3322 output byte, and with format C<H> it determines the most-significant
3325 If the length of the input string is not even, it behaves as if padded
3326 by a null byte at the end. Similarly, during unpack()ing the "extra"
3327 nybbles are ignored.
3329 If the input string of pack() is longer than needed, extra bytes are ignored.
3330 A C<*> for the repeat count of pack() means to use all the bytes of
3331 the input field. On unpack()ing the bits are converted to a string
3332 of hexadecimal digits.
3336 The C<p> type packs a pointer to a null-terminated string. You are
3337 responsible for ensuring the string is not a temporary value (which can
3338 potentially get deallocated before you get around to using the packed result).
3339 The C<P> type packs a pointer to a structure of the size indicated by the
3340 length. A NULL pointer is created if the corresponding value for C<p> or
3341 C<P> is C<undef>, similarly for unpack().
3345 The C</> template character allows packing and unpacking of strings where
3346 the packed structure contains a byte count followed by the string itself.
3347 You write I<length-item>C</>I<string-item>.
3349 The I<length-item> can be any C<pack> template letter, and describes
3350 how the length value is packed. The ones likely to be of most use are
3351 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3352 SNMP) and C<N> (for Sun XDR).
3354 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3355 For C<unpack> the length of the string is obtained from the I<length-item>,
3356 but if you put in the '*' it will be ignored.
3358 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3359 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3360 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3362 The I<length-item> is not returned explicitly from C<unpack>.
3364 Adding a count to the I<length-item> letter is unlikely to do anything
3365 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3366 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3367 which Perl does not regard as legal in numeric strings.
3371 The integer types C<s>, C<S>, C<l>, and C<L> may be
3372 immediately followed by a C<!> suffix to signify native shorts or
3373 longs--as you can see from above for example a bare C<l> does mean
3374 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3375 may be larger. This is an issue mainly in 64-bit platforms. You can
3376 see whether using C<!> makes any difference by
3378 print length(pack("s")), " ", length(pack("s!")), "\n";
3379 print length(pack("l")), " ", length(pack("l!")), "\n";
3381 C<i!> and C<I!> also work but only because of completeness;
3382 they are identical to C<i> and C<I>.
3384 The actual sizes (in bytes) of native shorts, ints, longs, and long
3385 longs on the platform where Perl was built are also available via
3389 print $Config{shortsize}, "\n";
3390 print $Config{intsize}, "\n";
3391 print $Config{longsize}, "\n";
3392 print $Config{longlongsize}, "\n";
3394 (The C<$Config{longlongsize}> will be undefine if your system does
3395 not support long longs.)
3399 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3400 are inherently non-portable between processors and operating systems
3401 because they obey the native byteorder and endianness. For example a
3402 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3403 (arranged in and handled by the CPU registers) into bytes as
3405 0x12 0x34 0x56 0x78 # big-endian
3406 0x78 0x56 0x34 0x12 # little-endian
3408 Basically, the Intel and VAX CPUs are little-endian, while everybody
3409 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3410 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3411 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3414 The names `big-endian' and `little-endian' are comic references to
3415 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3416 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3417 the egg-eating habits of the Lilliputians.
3419 Some systems may have even weirder byte orders such as
3424 You can see your system's preference with
3426 print join(" ", map { sprintf "%#02x", $_ }
3427 unpack("C*",pack("L",0x12345678))), "\n";
3429 The byteorder on the platform where Perl was built is also available
3433 print $Config{byteorder}, "\n";
3435 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3436 and C<'87654321'> are big-endian.
3438 If you want portable packed integers use the formats C<n>, C<N>,
3439 C<v>, and C<V>, their byte endianness and size are known.
3440 See also L<perlport>.
3444 Real numbers (floats and doubles) are in the native machine format only;
3445 due to the multiplicity of floating formats around, and the lack of a
3446 standard "network" representation, no facility for interchange has been
3447 made. This means that packed floating point data written on one machine
3448 may not be readable on another - even if both use IEEE floating point
3449 arithmetic (as the endian-ness of the memory representation is not part
3450 of the IEEE spec). See also L<perlport>.
3452 Note that Perl uses doubles internally for all numeric calculation, and
3453 converting from double into float and thence back to double again will
3454 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3459 If the pattern begins with a C<U>, the resulting string will be treated
3460 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3461 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3462 characters. If you don't want this to happen, you can begin your pattern
3463 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3464 string, and then follow this with a C<U*> somewhere in your pattern.
3468 You must yourself do any alignment or padding by inserting for example
3469 enough C<'x'>es while packing. There is no way to pack() and unpack()
3470 could know where the bytes are going to or coming from. Therefore
3471 C<pack> (and C<unpack>) handle their output and input as flat
3476 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3477 take a repeat count, both as postfix, and via the C</> template
3482 C<x> and C<X> accept C<!> modifier. In this case they act as
3483 alignment commands: they jump forward/back to the closest position
3484 aligned at a multiple of C<count> bytes. For example, to pack() or
3485 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3486 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3487 aligned on the double's size.
3489 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3490 both result in no-ops.
3494 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3498 If TEMPLATE requires more arguments to pack() than actually given, pack()
3499 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3500 to pack() than actually given, extra arguments are ignored.
3506 $foo = pack("CCCC",65,66,67,68);
3508 $foo = pack("C4",65,66,67,68);
3510 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3511 # same thing with Unicode circled letters
3513 $foo = pack("ccxxcc",65,66,67,68);
3516 # note: the above examples featuring "C" and "c" are true
3517 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3518 # and UTF-8. In EBCDIC the first example would be
3519 # $foo = pack("CCCC",193,194,195,196);
3521 $foo = pack("s2",1,2);
3522 # "\1\0\2\0" on little-endian
3523 # "\0\1\0\2" on big-endian
3525 $foo = pack("a4","abcd","x","y","z");
3528 $foo = pack("aaaa","abcd","x","y","z");
3531 $foo = pack("a14","abcdefg");
3532 # "abcdefg\0\0\0\0\0\0\0"
3534 $foo = pack("i9pl", gmtime);
3535 # a real struct tm (on my system anyway)
3537 $utmp_template = "Z8 Z8 Z16 L";
3538 $utmp = pack($utmp_template, @utmp1);
3539 # a struct utmp (BSDish)
3541 @utmp2 = unpack($utmp_template, $utmp);
3542 # "@utmp1" eq "@utmp2"
3545 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3548 $foo = pack('sx2l', 12, 34);
3549 # short 12, two zero bytes padding, long 34
3550 $bar = pack('s@4l', 12, 34);
3551 # short 12, zero fill to position 4, long 34
3554 The same template may generally also be used in unpack().
3556 =item package NAMESPACE
3560 Declares the compilation unit as being in the given namespace. The scope
3561 of the package declaration is from the declaration itself through the end
3562 of the enclosing block, file, or eval (the same as the C<my> operator).
3563 All further unqualified dynamic identifiers will be in this namespace.
3564 A package statement affects only dynamic variables--including those
3565 you've used C<local> on--but I<not> lexical variables, which are created
3566 with C<my>. Typically it would be the first declaration in a file to
3567 be included by the C<require> or C<use> operator. You can switch into a
3568 package in more than one place; it merely influences which symbol table
3569 is used by the compiler for the rest of that block. You can refer to
3570 variables and filehandles in other packages by prefixing the identifier
3571 with the package name and a double colon: C<$Package::Variable>.
3572 If the package name is null, the C<main> package as assumed. That is,
3573 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3574 still seen in older code).
3576 If NAMESPACE is omitted, then there is no current package, and all
3577 identifiers must be fully qualified or lexicals. However, you are
3578 strongly advised not to make use of this feature. Its use can cause
3579 unexpected behaviour, even crashing some versions of Perl. It is
3580 deprecated, and will be removed from a future release.
3582 See L<perlmod/"Packages"> for more information about packages, modules,
3583 and classes. See L<perlsub> for other scoping issues.
3585 =item pipe READHANDLE,WRITEHANDLE
3587 Opens a pair of connected pipes like the corresponding system call.
3588 Note that if you set up a loop of piped processes, deadlock can occur
3589 unless you are very careful. In addition, note that Perl's pipes use
3590 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3591 after each command, depending on the application.
3593 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3594 for examples of such things.
3596 On systems that support a close-on-exec flag on files, the flag will be set
3597 for the newly opened file descriptors as determined by the value of $^F.
3604 Pops and returns the last value of the array, shortening the array by
3605 one element. Has an effect similar to
3609 If there are no elements in the array, returns the undefined value
3610 (although this may happen at other times as well). If ARRAY is
3611 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3612 array in subroutines, just like C<shift>.
3618 Returns the offset of where the last C<m//g> search left off for the variable
3619 in question (C<$_> is used when the variable is not specified). May be
3620 modified to change that offset. Such modification will also influence
3621 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3624 =item print FILEHANDLE LIST
3630 Prints a string or a list of strings. Returns true if successful.
3631 FILEHANDLE may be a scalar variable name, in which case the variable
3632 contains the name of or a reference to the filehandle, thus introducing
3633 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3634 the next token is a term, it may be misinterpreted as an operator
3635 unless you interpose a C<+> or put parentheses around the arguments.)
3636 If FILEHANDLE is omitted, prints by default to standard output (or
3637 to the last selected output channel--see L</select>). If LIST is
3638 also omitted, prints C<$_> to the currently selected output channel.
3639 To set the default output channel to something other than STDOUT
3640 use the select operation. The current value of C<$,> (if any) is
3641 printed between each LIST item. The current value of C<$\> (if
3642 any) is printed after the entire LIST has been printed. Because
3643 print takes a LIST, anything in the LIST is evaluated in list
3644 context, and any subroutine that you call will have one or more of
3645 its expressions evaluated in list context. Also be careful not to
3646 follow the print keyword with a left parenthesis unless you want
3647 the corresponding right parenthesis to terminate the arguments to
3648 the print--interpose a C<+> or put parentheses around all the
3651 Note that if you're storing FILEHANDLES in an array or other expression,
3652 you will have to use a block returning its value instead:
3654 print { $files[$i] } "stuff\n";
3655 print { $OK ? STDOUT : STDERR } "stuff\n";
3657 =item printf FILEHANDLE FORMAT, LIST
3659 =item printf FORMAT, LIST
3661 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3662 (the output record separator) is not appended. The first argument
3663 of the list will be interpreted as the C<printf> format. See C<sprintf>
3664 for an explanation of the format argument. If C<use locale> is in effect,
3665 the character used for the decimal point in formatted real numbers is
3666 affected by the LC_NUMERIC locale. See L<perllocale>.
3668 Don't fall into the trap of using a C<printf> when a simple
3669 C<print> would do. The C<print> is more efficient and less
3672 =item prototype FUNCTION
3674 Returns the prototype of a function as a string (or C<undef> if the
3675 function has no prototype). FUNCTION is a reference to, or the name of,
3676 the function whose prototype you want to retrieve.
3678 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3679 name for Perl builtin. If the builtin is not I<overridable> (such as
3680 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3681 C<system>) returns C<undef> because the builtin does not really behave
3682 like a Perl function. Otherwise, the string describing the equivalent
3683 prototype is returned.
3685 =item push ARRAY,LIST
3687 Treats ARRAY as a stack, and pushes the values of LIST
3688 onto the end of ARRAY. The length of ARRAY increases by the length of
3689 LIST. Has the same effect as
3692 $ARRAY[++$#ARRAY] = $value;
3695 but is more efficient. Returns the new number of elements in the array.
3707 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3709 =item quotemeta EXPR
3713 Returns the value of EXPR with all non-"word"
3714 characters backslashed. (That is, all characters not matching
3715 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3716 returned string, regardless of any locale settings.)
3717 This is the internal function implementing
3718 the C<\Q> escape in double-quoted strings.
3720 If EXPR is omitted, uses C<$_>.
3726 Returns a random fractional number greater than or equal to C<0> and less
3727 than the value of EXPR. (EXPR should be positive.) If EXPR is
3728 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3729 unless C<srand> has already been called. See also C<srand>.
3731 Apply C<int()> to the value returned by C<rand()> if you want random
3732 integers instead of random fractional numbers. For example,
3736 returns a random integer between C<0> and C<9>, inclusive.
3738 (Note: If your rand function consistently returns numbers that are too
3739 large or too small, then your version of Perl was probably compiled
3740 with the wrong number of RANDBITS.)
3742 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3744 =item read FILEHANDLE,SCALAR,LENGTH
3746 Attempts to read LENGTH I<characters> of data into variable SCALAR
3747 from the specified FILEHANDLE. Returns the number of characters
3748 actually read, C<0> at end of file, or undef if there was an error.
3749 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3750 needs growing, the new bytes will be zero bytes. An OFFSET may be
3751 specified to place the read data into some other place in SCALAR than
3752 the beginning. The call is actually implemented in terms of either
3753 Perl's or system's fread() call. To get a true read(2) system call,
3756 Note the I<characters>: depending on the status of the filehandle,
3757 either (8-bit) bytes or characters are read. By default all
3758 filehandles operate on bytes, but for example if the filehandle has
3759 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3760 pragma, L<open>), the I/O will operate on characters, not bytes.
3762 =item readdir DIRHANDLE
3764 Returns the next directory entry for a directory opened by C<opendir>.
3765 If used in list context, returns all the rest of the entries in the
3766 directory. If there are no more entries, returns an undefined value in
3767 scalar context or a null list in list context.
3769 If you're planning to filetest the return values out of a C<readdir>, you'd
3770 better prepend the directory in question. Otherwise, because we didn't
3771 C<chdir> there, it would have been testing the wrong file.
3773 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3774 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3779 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3780 context, each call reads and returns the next line, until end-of-file is
3781 reached, whereupon the subsequent call returns undef. In list context,
3782 reads until end-of-file is reached and returns a list of lines. Note that
3783 the notion of "line" used here is however you may have defined it
3784 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3786 When C<$/> is set to C<undef>, when readline() is in scalar
3787 context (i.e. file slurp mode), and when an empty file is read, it
3788 returns C<''> the first time, followed by C<undef> subsequently.
3790 This is the internal function implementing the C<< <EXPR> >>
3791 operator, but you can use it directly. The C<< <EXPR> >>
3792 operator is discussed in more detail in L<perlop/"I/O Operators">.
3795 $line = readline(*STDIN); # same thing
3801 Returns the value of a symbolic link, if symbolic links are
3802 implemented. If not, gives a fatal error. If there is some system
3803 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3804 omitted, uses C<$_>.
3808 EXPR is executed as a system command.
3809 The collected standard output of the command is returned.
3810 In scalar context, it comes back as a single (potentially
3811 multi-line) string. In list context, returns a list of lines
3812 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3813 This is the internal function implementing the C<qx/EXPR/>
3814 operator, but you can use it directly. The C<qx/EXPR/>
3815 operator is discussed in more detail in L<perlop/"I/O Operators">.
3817 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3819 Receives a message on a socket. Attempts to receive LENGTH characters
3820 of data into variable SCALAR from the specified SOCKET filehandle.
3821 SCALAR will be grown or shrunk to the length actually read. Takes the
3822 same flags as the system call of the same name. Returns the address
3823 of the sender if SOCKET's protocol supports this; returns an empty
3824 string otherwise. If there's an error, returns the undefined value.
3825 This call is actually implemented in terms of recvfrom(2) system call.
3826 See L<perlipc/"UDP: Message Passing"> for examples.
3828 Note the I<characters>: depending on the status of the socket, either
3829 (8-bit) bytes or characters are received. By default all sockets
3830 operate on bytes, but for example if the socket has been changed using
3831 binmode() to operate with the C<:utf8> discipline (see the C<open>
3832 pragma, L<open>), the I/O will operate on characters, not bytes.
3838 The C<redo> command restarts the loop block without evaluating the
3839 conditional again. The C<continue> block, if any, is not executed. If
3840 the LABEL is omitted, the command refers to the innermost enclosing
3841 loop. This command is normally used by programs that want to lie to
3842 themselves about what was just input:
3844 # a simpleminded Pascal comment stripper
3845 # (warning: assumes no { or } in strings)
3846 LINE: while (<STDIN>) {
3847 while (s|({.*}.*){.*}|$1 |) {}
3852 if (/}/) { # end of comment?
3861 C<redo> cannot be used to retry a block which returns a value such as
3862 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3863 a grep() or map() operation.
3865 Note that a block by itself is semantically identical to a loop
3866 that executes once. Thus C<redo> inside such a block will effectively
3867 turn it into a looping construct.
3869 See also L</continue> for an illustration of how C<last>, C<next>, and
3876 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3877 is not specified, C<$_> will be used. The value returned depends on the
3878 type of thing the reference is a reference to.
3879 Builtin types include:
3889 If the referenced object has been blessed into a package, then that package
3890 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3892 if (ref($r) eq "HASH") {
3893 print "r is a reference to a hash.\n";
3896 print "r is not a reference at all.\n";
3898 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3899 print "r is a reference to something that isa hash.\n";
3902 See also L<perlref>.
3904 =item rename OLDNAME,NEWNAME
3906 Changes the name of a file; an existing file NEWNAME will be
3907 clobbered. Returns true for success, false otherwise.
3909 Behavior of this function varies wildly depending on your system
3910 implementation. For example, it will usually not work across file system
3911 boundaries, even though the system I<mv> command sometimes compensates
3912 for this. Other restrictions include whether it works on directories,
3913 open files, or pre-existing files. Check L<perlport> and either the
3914 rename(2) manpage or equivalent system documentation for details.
3916 =item require VERSION
3922 Demands a version of Perl specified by VERSION, or demands some semantics
3923 specified by EXPR or by C<$_> if EXPR is not supplied.
3925 VERSION may be either a numeric argument such as 5.006, which will be
3926 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3927 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3928 VERSION is greater than the version of the current Perl interpreter.
3929 Compare with L</use>, which can do a similar check at compile time.
3931 Specifying VERSION as a literal of the form v5.6.1 should generally be
3932 avoided, because it leads to misleading error messages under earlier
3933 versions of Perl which do not support this syntax. The equivalent numeric
3934 version should be used instead.
3936 require v5.6.1; # run time version check
3937 require 5.6.1; # ditto
3938 require 5.006_001; # ditto; preferred for backwards compatibility
3940 Otherwise, demands that a library file be included if it hasn't already
3941 been included. The file is included via the do-FILE mechanism, which is
3942 essentially just a variety of C<eval>. Has semantics similar to the following
3947 return 1 if $INC{$filename};
3948 my($realfilename,$result);
3950 foreach $prefix (@INC) {
3951 $realfilename = "$prefix/$filename";
3952 if (-f $realfilename) {
3953 $INC{$filename} = $realfilename;
3954 $result = do $realfilename;
3958 die "Can't find $filename in \@INC";
3960 delete $INC{$filename} if $@ || !$result;
3962 die "$filename did not return true value" unless $result;
3966 Note that the file will not be included twice under the same specified
3967 name. The file must return true as the last statement to indicate
3968 successful execution of any initialization code, so it's customary to
3969 end such a file with C<1;> unless you're sure it'll return true
3970 otherwise. But it's better just to put the C<1;>, in case you add more
3973 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3974 replaces "F<::>" with "F</>" in the filename for you,
3975 to make it easy to load standard modules. This form of loading of
3976 modules does not risk altering your namespace.
3978 In other words, if you try this:
3980 require Foo::Bar; # a splendid bareword
3982 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3983 directories specified in the C<@INC> array.
3985 But if you try this:
3987 $class = 'Foo::Bar';
3988 require $class; # $class is not a bareword
3990 require "Foo::Bar"; # not a bareword because of the ""
3992 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3993 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3995 eval "require $class";
3997 You can also insert hooks into the import facility, by putting directly
3998 Perl code into the @INC array. There are three forms of hooks: subroutine
3999 references, array references and blessed objects.
4001 Subroutine references are the simplest case. When the inclusion system
4002 walks through @INC and encounters a subroutine, this subroutine gets
4003 called with two parameters, the first being a reference to itself, and the
4004 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4005 subroutine should return C<undef> or a filehandle, from which the file to
4006 include will be read. If C<undef> is returned, C<require> will look at
4007 the remaining elements of @INC.
4009 If the hook is an array reference, its first element must be a subroutine
4010 reference. This subroutine is called as above, but the first parameter is
4011 the array reference. This enables to pass indirectly some arguments to
4014 In other words, you can write:
4016 push @INC, \&my_sub;
4018 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4024 push @INC, [ \&my_sub, $x, $y, ... ];
4026 my ($arrayref, $filename) = @_;
4027 # Retrieve $x, $y, ...
4028 my @parameters = @$arrayref[1..$#$arrayref];
4032 If the hook is an object, it must provide an INC method, that will be
4033 called as above, the first parameter being the object itself. (Note that
4034 you must fully qualify the sub's name, as it is always forced into package
4035 C<main>.) Here is a typical code layout:
4041 my ($self, $filename) = @_;
4045 # In the main program
4046 push @INC, new Foo(...);
4048 Note that these hooks are also permitted to set the %INC entry
4049 corresponding to the files they have loaded. See L<perlvar/%INC>.
4051 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4057 Generally used in a C<continue> block at the end of a loop to clear
4058 variables and reset C<??> searches so that they work again. The
4059 expression is interpreted as a list of single characters (hyphens
4060 allowed for ranges). All variables and arrays beginning with one of
4061 those letters are reset to their pristine state. If the expression is
4062 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4063 only variables or searches in the current package. Always returns
4066 reset 'X'; # reset all X variables
4067 reset 'a-z'; # reset lower case variables
4068 reset; # just reset ?one-time? searches
4070 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4071 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4072 variables--lexical variables are unaffected, but they clean themselves
4073 up on scope exit anyway, so you'll probably want to use them instead.
4080 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4081 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4082 context, depending on how the return value will be used, and the context
4083 may vary from one execution to the next (see C<wantarray>). If no EXPR
4084 is given, returns an empty list in list context, the undefined value in
4085 scalar context, and (of course) nothing at all in a void context.
4087 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4088 or do FILE will automatically return the value of the last expression
4093 In list context, returns a list value consisting of the elements
4094 of LIST in the opposite order. In scalar context, concatenates the
4095 elements of LIST and returns a string value with all characters
4096 in the opposite order.
4098 print reverse <>; # line tac, last line first
4100 undef $/; # for efficiency of <>
4101 print scalar reverse <>; # character tac, last line tsrif
4103 This operator is also handy for inverting a hash, although there are some
4104 caveats. If a value is duplicated in the original hash, only one of those
4105 can be represented as a key in the inverted hash. Also, this has to
4106 unwind one hash and build a whole new one, which may take some time
4107 on a large hash, such as from a DBM file.
4109 %by_name = reverse %by_address; # Invert the hash
4111 =item rewinddir DIRHANDLE
4113 Sets the current position to the beginning of the directory for the
4114 C<readdir> routine on DIRHANDLE.
4116 =item rindex STR,SUBSTR,POSITION
4118 =item rindex STR,SUBSTR
4120 Works just like index() except that it returns the position of the LAST
4121 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4122 last occurrence at or before that position.
4124 =item rmdir FILENAME
4128 Deletes the directory specified by FILENAME if that directory is empty. If it
4129 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4130 FILENAME is omitted, uses C<$_>.
4134 The substitution operator. See L<perlop>.
4138 Forces EXPR to be interpreted in scalar context and returns the value
4141 @counts = ( scalar @a, scalar @b, scalar @c );
4143 There is no equivalent operator to force an expression to
4144 be interpolated in list context because in practice, this is never
4145 needed. If you really wanted to do so, however, you could use
4146 the construction C<@{[ (some expression) ]}>, but usually a simple
4147 C<(some expression)> suffices.
4149 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4150 parenthesized list, this behaves as a scalar comma expression, evaluating
4151 all but the last element in void context and returning the final element
4152 evaluated in scalar context. This is seldom what you want.
4154 The following single statement:
4156 print uc(scalar(&foo,$bar)),$baz;
4158 is the moral equivalent of these two:
4161 print(uc($bar),$baz);
4163 See L<perlop> for more details on unary operators and the comma operator.
4165 =item seek FILEHANDLE,POSITION,WHENCE
4167 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4168 FILEHANDLE may be an expression whose value gives the name of the
4169 filehandle. The values for WHENCE are C<0> to set the new position
4170 I<in bytes> to POSITION, C<1> to set it to the current position plus
4171 POSITION, and C<2> to set it to EOF plus POSITION (typically
4172 negative). For WHENCE you may use the constants C<SEEK_SET>,
4173 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4174 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4177 Note the I<in bytes>: even if the filehandle has been set to
4178 operate on characters (for example by using the C<:utf8> open
4179 discipline), tell() will return byte offsets, not character offsets
4180 (because implementing that would render seek() and tell() rather slow).
4182 If you want to position file for C<sysread> or C<syswrite>, don't use
4183 C<seek>--buffering makes its effect on the file's system position
4184 unpredictable and non-portable. Use C<sysseek> instead.
4186 Due to the rules and rigors of ANSI C, on some systems you have to do a
4187 seek whenever you switch between reading and writing. Amongst other
4188 things, this may have the effect of calling stdio's clearerr(3).
4189 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4193 This is also useful for applications emulating C<tail -f>. Once you hit
4194 EOF on your read, and then sleep for a while, you might have to stick in a
4195 seek() to reset things. The C<seek> doesn't change the current position,
4196 but it I<does> clear the end-of-file condition on the handle, so that the
4197 next C<< <FILE> >> makes Perl try again to read something. We hope.
4199 If that doesn't work (some IO implementations are particularly
4200 cantankerous), then you may need something more like this:
4203 for ($curpos = tell(FILE); $_ = <FILE>;
4204 $curpos = tell(FILE)) {
4205 # search for some stuff and put it into files
4207 sleep($for_a_while);
4208 seek(FILE, $curpos, 0);
4211 =item seekdir DIRHANDLE,POS
4213 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4214 must be a value returned by C<telldir>. Has the same caveats about
4215 possible directory compaction as the corresponding system library
4218 =item select FILEHANDLE
4222 Returns the currently selected filehandle. Sets the current default
4223 filehandle for output, if FILEHANDLE is supplied. This has two
4224 effects: first, a C<write> or a C<print> without a filehandle will
4225 default to this FILEHANDLE. Second, references to variables related to
4226 output will refer to this output channel. For example, if you have to
4227 set the top of form format for more than one output channel, you might
4235 FILEHANDLE may be an expression whose value gives the name of the
4236 actual filehandle. Thus:
4238 $oldfh = select(STDERR); $| = 1; select($oldfh);
4240 Some programmers may prefer to think of filehandles as objects with
4241 methods, preferring to write the last example as:
4244 STDERR->autoflush(1);
4246 =item select RBITS,WBITS,EBITS,TIMEOUT
4248 This calls the select(2) system call with the bit masks specified, which
4249 can be constructed using C<fileno> and C<vec>, along these lines:
4251 $rin = $win = $ein = '';
4252 vec($rin,fileno(STDIN),1) = 1;
4253 vec($win,fileno(STDOUT),1) = 1;
4256 If you want to select on many filehandles you might wish to write a
4260 my(@fhlist) = split(' ',$_[0]);
4263 vec($bits,fileno($_),1) = 1;
4267 $rin = fhbits('STDIN TTY SOCK');
4271 ($nfound,$timeleft) =
4272 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4274 or to block until something becomes ready just do this
4276 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4278 Most systems do not bother to return anything useful in $timeleft, so
4279 calling select() in scalar context just returns $nfound.
4281 Any of the bit masks can also be undef. The timeout, if specified, is
4282 in seconds, which may be fractional. Note: not all implementations are
4283 capable of returning the $timeleft. If not, they always return
4284 $timeleft equal to the supplied $timeout.
4286 You can effect a sleep of 250 milliseconds this way:
4288 select(undef, undef, undef, 0.25);
4290 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4291 is implementation-dependent.
4293 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4294 or <FH>) with C<select>, except as permitted by POSIX, and even
4295 then only on POSIX systems. You have to use C<sysread> instead.
4297 =item semctl ID,SEMNUM,CMD,ARG
4299 Calls the System V IPC function C<semctl>. You'll probably have to say
4303 first to get the correct constant definitions. If CMD is IPC_STAT or
4304 GETALL, then ARG must be a variable which will hold the returned
4305 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4306 the undefined value for error, "C<0 but true>" for zero, or the actual
4307 return value otherwise. The ARG must consist of a vector of native
4308 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4309 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4312 =item semget KEY,NSEMS,FLAGS
4314 Calls the System V IPC function semget. Returns the semaphore id, or
4315 the undefined value if there is an error. See also
4316 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4319 =item semop KEY,OPSTRING
4321 Calls the System V IPC function semop to perform semaphore operations
4322 such as signalling and waiting. OPSTRING must be a packed array of
4323 semop structures. Each semop structure can be generated with
4324 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4325 operations is implied by the length of OPSTRING. Returns true if
4326 successful, or false if there is an error. As an example, the
4327 following code waits on semaphore $semnum of semaphore id $semid:
4329 $semop = pack("s!3", $semnum, -1, 0);
4330 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4332 To signal the semaphore, replace C<-1> with C<1>. See also
4333 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4336 =item send SOCKET,MSG,FLAGS,TO
4338 =item send SOCKET,MSG,FLAGS
4340 Sends a message on a socket. Attempts to send the scalar MSG to the
4341 SOCKET filehandle. Takes the same flags as the system call of the
4342 same name. On unconnected sockets you must specify a destination to
4343 send TO, in which case it does a C C<sendto>. Returns the number of
4344 characters sent, or the undefined value if there is an error. The C
4345 system call sendmsg(2) is currently unimplemented. See
4346 L<perlipc/"UDP: Message Passing"> for examples.
4348 Note the I<characters>: depending on the status of the socket, either
4349 (8-bit) bytes or characters are sent. By default all sockets operate
4350 on bytes, but for example if the socket has been changed using
4351 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4352 the C<open> pragma, L<open>), the I/O will operate on characters, not
4355 =item setpgrp PID,PGRP
4357 Sets the current process group for the specified PID, C<0> for the current
4358 process. Will produce a fatal error if used on a machine that doesn't
4359 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4360 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4361 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4364 =item setpriority WHICH,WHO,PRIORITY
4366 Sets the current priority for a process, a process group, or a user.
4367 (See setpriority(2).) Will produce a fatal error if used on a machine
4368 that doesn't implement setpriority(2).
4370 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4372 Sets the socket option requested. Returns undefined if there is an
4373 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4380 Shifts the first value of the array off and returns it, shortening the
4381 array by 1 and moving everything down. If there are no elements in the
4382 array, returns the undefined value. If ARRAY is omitted, shifts the
4383 C<@_> array within the lexical scope of subroutines and formats, and the
4384 C<@ARGV> array at file scopes or within the lexical scopes established by
4385 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4388 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4389 same thing to the left end of an array that C<pop> and C<push> do to the
4392 =item shmctl ID,CMD,ARG
4394 Calls the System V IPC function shmctl. You'll probably have to say
4398 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4399 then ARG must be a variable which will hold the returned C<shmid_ds>
4400 structure. Returns like ioctl: the undefined value for error, "C<0> but
4401 true" for zero, or the actual return value otherwise.
4402 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4404 =item shmget KEY,SIZE,FLAGS
4406 Calls the System V IPC function shmget. Returns the shared memory
4407 segment id, or the undefined value if there is an error.
4408 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4410 =item shmread ID,VAR,POS,SIZE
4412 =item shmwrite ID,STRING,POS,SIZE
4414 Reads or writes the System V shared memory segment ID starting at
4415 position POS for size SIZE by attaching to it, copying in/out, and
4416 detaching from it. When reading, VAR must be a variable that will
4417 hold the data read. When writing, if STRING is too long, only SIZE
4418 bytes are used; if STRING is too short, nulls are written to fill out
4419 SIZE bytes. Return true if successful, or false if there is an error.
4420 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4421 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4423 =item shutdown SOCKET,HOW
4425 Shuts down a socket connection in the manner indicated by HOW, which
4426 has the same interpretation as in the system call of the same name.
4428 shutdown(SOCKET, 0); # I/we have stopped reading data
4429 shutdown(SOCKET, 1); # I/we have stopped writing data
4430 shutdown(SOCKET, 2); # I/we have stopped using this socket
4432 This is useful with sockets when you want to tell the other
4433 side you're done writing but not done reading, or vice versa.
4434 It's also a more insistent form of close because it also
4435 disables the file descriptor in any forked copies in other
4442 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4443 returns sine of C<$_>.
4445 For the inverse sine operation, you may use the C<Math::Trig::asin>
4446 function, or use this relation:
4448 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4454 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4455 May be interrupted if the process receives a signal such as C<SIGALRM>.
4456 Returns the number of seconds actually slept. You probably cannot
4457 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4460 On some older systems, it may sleep up to a full second less than what
4461 you requested, depending on how it counts seconds. Most modern systems
4462 always sleep the full amount. They may appear to sleep longer than that,
4463 however, because your process might not be scheduled right away in a
4464 busy multitasking system.
4466 For delays of finer granularity than one second, you may use Perl's
4467 C<syscall> interface to access setitimer(2) if your system supports
4468 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4469 and starting from Perl 5.8 part of the standard distribution) may also
4472 See also the POSIX module's C<pause> function.
4474 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4476 Opens a socket of the specified kind and attaches it to filehandle
4477 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4478 the system call of the same name. You should C<use Socket> first
4479 to get the proper definitions imported. See the examples in
4480 L<perlipc/"Sockets: Client/Server Communication">.
4482 On systems that support a close-on-exec flag on files, the flag will
4483 be set for the newly opened file descriptor, as determined by the
4484 value of $^F. See L<perlvar/$^F>.
4486 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4488 Creates an unnamed pair of sockets in the specified domain, of the
4489 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4490 for the system call of the same name. If unimplemented, yields a fatal
4491 error. Returns true if successful.
4493 On systems that support a close-on-exec flag on files, the flag will
4494 be set for the newly opened file descriptors, as determined by the value
4495 of $^F. See L<perlvar/$^F>.
4497 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4498 to C<pipe(Rdr, Wtr)> is essentially:
4501 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4502 shutdown(Rdr, 1); # no more writing for reader
4503 shutdown(Wtr, 0); # no more reading for writer
4505 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4506 emulate socketpair using IP sockets to localhost if your system implements
4507 sockets but not socketpair.
4509 =item sort SUBNAME LIST
4511 =item sort BLOCK LIST
4515 In list context, this sorts the LIST and returns the sorted list value.
4516 In scalar context, the behaviour of C<sort()> is undefined.
4518 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4519 order. If SUBNAME is specified, it gives the name of a subroutine
4520 that returns an integer less than, equal to, or greater than C<0>,
4521 depending on how the elements of the list are to be ordered. (The C<<
4522 <=> >> and C<cmp> operators are extremely useful in such routines.)
4523 SUBNAME may be a scalar variable name (unsubscripted), in which case
4524 the value provides the name of (or a reference to) the actual
4525 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4526 an anonymous, in-line sort subroutine.
4528 If the subroutine's prototype is C<($$)>, the elements to be compared
4529 are passed by reference in C<@_>, as for a normal subroutine. This is
4530 slower than unprototyped subroutines, where the elements to be
4531 compared are passed into the subroutine
4532 as the package global variables $a and $b (see example below). Note that
4533 in the latter case, it is usually counter-productive to declare $a and
4536 In either case, the subroutine may not be recursive. The values to be
4537 compared are always passed by reference, so don't modify them.
4539 You also cannot exit out of the sort block or subroutine using any of the
4540 loop control operators described in L<perlsyn> or with C<goto>.
4542 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4543 current collation locale. See L<perllocale>.
4545 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4546 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4547 preserves the input order of elements that compare equal. Although
4548 quicksort's run time is O(NlogN) when averaged over all arrays of
4549 length N, the time can be O(N**2), I<quadratic> behavior, for some
4550 inputs.) In 5.7, the quicksort implementation was replaced with
4551 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4552 But benchmarks indicated that for some inputs, on some platforms,
4553 the original quicksort was faster. 5.8 has a sort pragma for
4554 limited control of the sort. Its rather blunt control of the
4555 underlying algorithm may not persist into future perls, but the
4556 ability to characterize the input or output in implementation
4557 independent ways quite probably will. See L</use>.
4562 @articles = sort @files;
4564 # same thing, but with explicit sort routine
4565 @articles = sort {$a cmp $b} @files;
4567 # now case-insensitively
4568 @articles = sort {uc($a) cmp uc($b)} @files;
4570 # same thing in reversed order
4571 @articles = sort {$b cmp $a} @files;
4573 # sort numerically ascending
4574 @articles = sort {$a <=> $b} @files;
4576 # sort numerically descending
4577 @articles = sort {$b <=> $a} @files;
4579 # this sorts the %age hash by value instead of key
4580 # using an in-line function
4581 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4583 # sort using explicit subroutine name
4585 $age{$a} <=> $age{$b}; # presuming numeric
4587 @sortedclass = sort byage @class;
4589 sub backwards { $b cmp $a }
4590 @harry = qw(dog cat x Cain Abel);
4591 @george = qw(gone chased yz Punished Axed);
4593 # prints AbelCaincatdogx
4594 print sort backwards @harry;
4595 # prints xdogcatCainAbel
4596 print sort @george, 'to', @harry;
4597 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4599 # inefficiently sort by descending numeric compare using
4600 # the first integer after the first = sign, or the
4601 # whole record case-insensitively otherwise
4604 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4609 # same thing, but much more efficiently;
4610 # we'll build auxiliary indices instead
4614 push @nums, /=(\d+)/;
4619 $nums[$b] <=> $nums[$a]
4621 $caps[$a] cmp $caps[$b]
4625 # same thing, but without any temps
4626 @new = map { $_->[0] }
4627 sort { $b->[1] <=> $a->[1]
4630 } map { [$_, /=(\d+)/, uc($_)] } @old;
4632 # using a prototype allows you to use any comparison subroutine
4633 # as a sort subroutine (including other package's subroutines)
4635 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4638 @new = sort other::backwards @old;
4640 # guarantee stability, regardless of algorithm
4642 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4644 # force use of mergesort (not portable outside Perl 5.8)
4645 use sort '_mergesort'; # note discouraging _
4646 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4648 If you're using strict, you I<must not> declare $a
4649 and $b as lexicals. They are package globals. That means
4650 if you're in the C<main> package and type
4652 @articles = sort {$b <=> $a} @files;
4654 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4655 but if you're in the C<FooPack> package, it's the same as typing
4657 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4659 The comparison function is required to behave. If it returns
4660 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4661 sometimes saying the opposite, for example) the results are not
4664 =item splice ARRAY,OFFSET,LENGTH,LIST
4666 =item splice ARRAY,OFFSET,LENGTH
4668 =item splice ARRAY,OFFSET
4672 Removes the elements designated by OFFSET and LENGTH from an array, and
4673 replaces them with the elements of LIST, if any. In list context,
4674 returns the elements removed from the array. In scalar context,
4675 returns the last element removed, or C<undef> if no elements are
4676 removed. The array grows or shrinks as necessary.
4677 If OFFSET is negative then it starts that far from the end of the array.
4678 If LENGTH is omitted, removes everything from OFFSET onward.
4679 If LENGTH is negative, removes the elements from OFFSET onward
4680 except for -LENGTH elements at the end of the array.
4681 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4682 past the end of the array, perl issues a warning, and splices at the
4685 The following equivalences hold (assuming C<$[ == 0>):
4687 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4688 pop(@a) splice(@a,-1)
4689 shift(@a) splice(@a,0,1)
4690 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4691 $a[$x] = $y splice(@a,$x,1,$y)
4693 Example, assuming array lengths are passed before arrays:
4695 sub aeq { # compare two list values
4696 my(@a) = splice(@_,0,shift);
4697 my(@b) = splice(@_,0,shift);
4698 return 0 unless @a == @b; # same len?
4700 return 0 if pop(@a) ne pop(@b);
4704 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4706 =item split /PATTERN/,EXPR,LIMIT
4708 =item split /PATTERN/,EXPR
4710 =item split /PATTERN/
4714 Splits a string into a list of strings and returns that list. By default,
4715 empty leading fields are preserved, and empty trailing ones are deleted.
4717 In scalar context, returns the number of fields found and splits into
4718 the C<@_> array. Use of split in scalar context is deprecated, however,
4719 because it clobbers your subroutine arguments.
4721 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4722 splits on whitespace (after skipping any leading whitespace). Anything
4723 matching PATTERN is taken to be a delimiter separating the fields. (Note
4724 that the delimiter may be longer than one character.)
4726 If LIMIT is specified and positive, it represents the maximum number
4727 of fields the EXPR will be split into, though the actual number of
4728 fields returned depends on the number of times PATTERN matches within
4729 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4730 stripped (which potential users of C<pop> would do well to remember).
4731 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4732 had been specified. Note that splitting an EXPR that evaluates to the
4733 empty string always returns the empty list, regardless of the LIMIT
4736 A pattern matching the null string (not to be confused with
4737 a null pattern C<//>, which is just one member of the set of patterns
4738 matching a null string) will split the value of EXPR into separate
4739 characters at each point it matches that way. For example:
4741 print join(':', split(/ */, 'hi there'));
4743 produces the output 'h:i:t:h:e:r:e'.
4745 Using the empty pattern C<//> specifically matches the null string, and is
4746 not be confused with the use of C<//> to mean "the last successful pattern
4749 Empty leading (or trailing) fields are produced when there are positive width
4750 matches at the beginning (or end) of the string; a zero-width match at the
4751 beginning (or end) of the string does not produce an empty field. For
4754 print join(':', split(/(?=\w)/, 'hi there!'));
4756 produces the output 'h:i :t:h:e:r:e!'.
4758 The LIMIT parameter can be used to split a line partially
4760 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4762 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4763 one larger than the number of variables in the list, to avoid
4764 unnecessary work. For the list above LIMIT would have been 4 by
4765 default. In time critical applications it behooves you not to split
4766 into more fields than you really need.
4768 If the PATTERN contains parentheses, additional list elements are
4769 created from each matching substring in the delimiter.
4771 split(/([,-])/, "1-10,20", 3);
4773 produces the list value
4775 (1, '-', 10, ',', 20)
4777 If you had the entire header of a normal Unix email message in $header,
4778 you could split it up into fields and their values this way:
4780 $header =~ s/\n\s+/ /g; # fix continuation lines
4781 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4783 The pattern C</PATTERN/> may be replaced with an expression to specify
4784 patterns that vary at runtime. (To do runtime compilation only once,
4785 use C</$variable/o>.)
4787 As a special case, specifying a PATTERN of space (C<' '>) will split on
4788 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4789 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4790 will give you as many null initial fields as there are leading spaces.
4791 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4792 whitespace produces a null first field. A C<split> with no arguments
4793 really does a C<split(' ', $_)> internally.
4795 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4800 open(PASSWD, '/etc/passwd');
4803 ($login, $passwd, $uid, $gid,
4804 $gcos, $home, $shell) = split(/:/);
4808 As with regular pattern matching, any capturing parentheses that are not
4809 matched in a C<split()> will be set to C<undef> when returned:
4811 @fields = split /(A)|B/, "1A2B3";
4812 # @fields is (1, 'A', 2, undef, 3)
4814 =item sprintf FORMAT, LIST
4816 Returns a string formatted by the usual C<printf> conventions of the C
4817 library function C<sprintf>. See below for more details
4818 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4819 the general principles.
4823 # Format number with up to 8 leading zeroes
4824 $result = sprintf("%08d", $number);
4826 # Round number to 3 digits after decimal point
4827 $rounded = sprintf("%.3f", $number);
4829 Perl does its own C<sprintf> formatting--it emulates the C
4830 function C<sprintf>, but it doesn't use it (except for floating-point
4831 numbers, and even then only the standard modifiers are allowed). As a
4832 result, any non-standard extensions in your local C<sprintf> are not
4833 available from Perl.
4835 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4836 pass it an array as your first argument. The array is given scalar context,
4837 and instead of using the 0th element of the array as the format, Perl will
4838 use the count of elements in the array as the format, which is almost never
4841 Perl's C<sprintf> permits the following universally-known conversions:
4844 %c a character with the given number
4846 %d a signed integer, in decimal
4847 %u an unsigned integer, in decimal
4848 %o an unsigned integer, in octal
4849 %x an unsigned integer, in hexadecimal
4850 %e a floating-point number, in scientific notation
4851 %f a floating-point number, in fixed decimal notation
4852 %g a floating-point number, in %e or %f notation
4854 In addition, Perl permits the following widely-supported conversions:
4856 %X like %x, but using upper-case letters
4857 %E like %e, but using an upper-case "E"
4858 %G like %g, but with an upper-case "E" (if applicable)
4859 %b an unsigned integer, in binary
4860 %p a pointer (outputs the Perl value's address in hexadecimal)
4861 %n special: *stores* the number of characters output so far
4862 into the next variable in the parameter list
4864 Finally, for backward (and we do mean "backward") compatibility, Perl
4865 permits these unnecessary but widely-supported conversions:
4868 %D a synonym for %ld
4869 %U a synonym for %lu
4870 %O a synonym for %lo
4873 Note that the number of exponent digits in the scientific notation by
4874 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4875 exponent less than 100 is system-dependent: it may be three or less
4876 (zero-padded as necessary). In other words, 1.23 times ten to the
4877 99th may be either "1.23e99" or "1.23e099".
4879 Perl permits the following universally-known flags between the C<%>
4880 and the conversion letter:
4882 space prefix positive number with a space
4883 + prefix positive number with a plus sign
4884 - left-justify within the field
4885 0 use zeros, not spaces, to right-justify
4886 # prefix non-zero octal with "0", non-zero hex with "0x"
4887 number minimum field width
4888 .number "precision": digits after decimal point for
4889 floating-point, max length for string, minimum length
4891 l interpret integer as C type "long" or "unsigned long"
4892 h interpret integer as C type "short" or "unsigned short"
4893 If no flags, interpret integer as C type "int" or "unsigned"
4895 Perl supports parameter ordering, in other words, fetching the
4896 parameters in some explicitly specified "random" ordering as opposed
4897 to the default implicit sequential ordering. The syntax is, instead
4898 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4899 where the I<digits> is the wanted index, from one upwards. For example:
4901 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4902 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4904 Note that using the reordering syntax does not interfere with the usual
4905 implicit sequential fetching of the parameters:
4907 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4908 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4909 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4910 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4911 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4913 There are also two Perl-specific flags:
4915 V interpret integer as Perl's standard integer type
4916 v interpret string as a vector of integers, output as
4917 numbers separated either by dots, or by an arbitrary
4918 string received from the argument list when the flag
4921 Where a number would appear in the flags, an asterisk (C<*>) may be
4922 used instead, in which case Perl uses the next item in the parameter
4923 list as the given number (that is, as the field width or precision).
4924 If a field width obtained through C<*> is negative, it has the same
4925 effect as the C<-> flag: left-justification.
4927 The C<v> flag is useful for displaying ordinal values of characters
4928 in arbitrary strings:
4930 printf "version is v%vd\n", $^V; # Perl's version
4931 printf "address is %*vX\n", ":", $addr; # IPv6 address
4932 printf "bits are %*vb\n", " ", $bits; # random bitstring
4934 If C<use locale> is in effect, the character used for the decimal
4935 point in formatted real numbers is affected by the LC_NUMERIC locale.
4938 If Perl understands "quads" (64-bit integers) (this requires
4939 either that the platform natively support quads or that Perl
4940 be specifically compiled to support quads), the characters
4944 print quads, and they may optionally be preceded by
4952 You can find out whether your Perl supports quads via L<Config>:
4955 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4958 If Perl understands "long doubles" (this requires that the platform
4959 support long doubles), the flags
4963 may optionally be preceded by
4971 You can find out whether your Perl supports long doubles via L<Config>:
4974 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4980 Return the square root of EXPR. If EXPR is omitted, returns square
4981 root of C<$_>. Only works on non-negative operands, unless you've
4982 loaded the standard Math::Complex module.
4985 print sqrt(-2); # prints 1.4142135623731i
4991 Sets the random number seed for the C<rand> operator.
4993 The point of the function is to "seed" the C<rand> function so that
4994 C<rand> can produce a different sequence each time you run your
4997 If srand() is not called explicitly, it is called implicitly at the
4998 first use of the C<rand> operator. However, this was not the case in
4999 versions of Perl before 5.004, so if your script will run under older
5000 Perl versions, it should call C<srand>.
5002 Most programs won't even call srand() at all, except those that
5003 need a cryptographically-strong starting point rather than the
5004 generally acceptable default, which is based on time of day,
5005 process ID, and memory allocation, or the F</dev/urandom> device,
5008 You can call srand($seed) with the same $seed to reproduce the
5009 I<same> sequence from rand(), but this is usually reserved for
5010 generating predictable results for testing or debugging.
5011 Otherwise, don't call srand() more than once in your program.
5013 Do B<not> call srand() (i.e. without an argument) more than once in
5014 a script. The internal state of the random number generator should
5015 contain more entropy than can be provided by any seed, so calling
5016 srand() again actually I<loses> randomness.
5018 Most implementations of C<srand> take an integer and will silently
5019 truncate decimal numbers. This means C<srand(42)> will usually
5020 produce the same results as C<srand(42.1)>. To be safe, always pass
5021 C<srand> an integer.
5023 In versions of Perl prior to 5.004 the default seed was just the
5024 current C<time>. This isn't a particularly good seed, so many old
5025 programs supply their own seed value (often C<time ^ $$> or C<time ^
5026 ($$ + ($$ << 15))>), but that isn't necessary any more.
5028 Note that you need something much more random than the default seed for
5029 cryptographic purposes. Checksumming the compressed output of one or more
5030 rapidly changing operating system status programs is the usual method. For
5033 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5035 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5038 Frequently called programs (like CGI scripts) that simply use
5042 for a seed can fall prey to the mathematical property that
5046 one-third of the time. So don't do that.
5048 =item stat FILEHANDLE
5054 Returns a 13-element list giving the status info for a file, either
5055 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5056 it stats C<$_>. Returns a null list if the stat fails. Typically used
5059 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5060 $atime,$mtime,$ctime,$blksize,$blocks)
5063 Not all fields are supported on all filesystem types. Here are the
5064 meaning of the fields:
5066 0 dev device number of filesystem
5068 2 mode file mode (type and permissions)
5069 3 nlink number of (hard) links to the file
5070 4 uid numeric user ID of file's owner
5071 5 gid numeric group ID of file's owner
5072 6 rdev the device identifier (special files only)
5073 7 size total size of file, in bytes
5074 8 atime last access time in seconds since the epoch
5075 9 mtime last modify time in seconds since the epoch
5076 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5077 11 blksize preferred block size for file system I/O
5078 12 blocks actual number of blocks allocated
5080 (The epoch was at 00:00 January 1, 1970 GMT.)
5082 If stat is passed the special filehandle consisting of an underline, no
5083 stat is done, but the current contents of the stat structure from the
5084 last stat or filetest are returned. Example:
5086 if (-x $file && (($d) = stat(_)) && $d < 0) {
5087 print "$file is executable NFS file\n";
5090 (This works on machines only for which the device number is negative
5093 Because the mode contains both the file type and its permissions, you
5094 should mask off the file type portion and (s)printf using a C<"%o">
5095 if you want to see the real permissions.
5097 $mode = (stat($filename))[2];
5098 printf "Permissions are %04o\n", $mode & 07777;
5100 In scalar context, C<stat> returns a boolean value indicating success
5101 or failure, and, if successful, sets the information associated with
5102 the special filehandle C<_>.
5104 The File::stat module provides a convenient, by-name access mechanism:
5107 $sb = stat($filename);
5108 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5109 $filename, $sb->size, $sb->mode & 07777,
5110 scalar localtime $sb->mtime;
5112 You can import symbolic mode constants (C<S_IF*>) and functions
5113 (C<S_IS*>) from the Fcntl module:
5117 $mode = (stat($filename))[2];
5119 $user_rwx = ($mode & S_IRWXU) >> 6;
5120 $group_read = ($mode & S_IRGRP) >> 3;
5121 $other_execute = $mode & S_IXOTH;
5123 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5125 $is_setuid = $mode & S_ISUID;
5126 $is_setgid = S_ISDIR($mode);
5128 You could write the last two using the C<-u> and C<-d> operators.
5129 The commonly available S_IF* constants are
5131 # Permissions: read, write, execute, for user, group, others.
5133 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5134 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5135 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5137 # Setuid/Setgid/Stickiness.
5139 S_ISUID S_ISGID S_ISVTX S_ISTXT
5141 # File types. Not necessarily all are available on your system.
5143 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5145 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5147 S_IREAD S_IWRITE S_IEXEC
5149 and the S_IF* functions are
5151 S_IFMODE($mode) the part of $mode containing the permission bits
5152 and the setuid/setgid/sticky bits
5154 S_IFMT($mode) the part of $mode containing the file type
5155 which can be bit-anded with e.g. S_IFREG
5156 or with the following functions
5158 # The operators -f, -d, -l, -b, -c, -p, and -s.
5160 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5161 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5163 # No direct -X operator counterpart, but for the first one
5164 # the -g operator is often equivalent. The ENFMT stands for
5165 # record flocking enforcement, a platform-dependent feature.
5167 S_ISENFMT($mode) S_ISWHT($mode)
5169 See your native chmod(2) and stat(2) documentation for more details
5170 about the S_* constants.
5176 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5177 doing many pattern matches on the string before it is next modified.
5178 This may or may not save time, depending on the nature and number of
5179 patterns you are searching on, and on the distribution of character
5180 frequencies in the string to be searched--you probably want to compare
5181 run times with and without it to see which runs faster. Those loops
5182 which scan for many short constant strings (including the constant
5183 parts of more complex patterns) will benefit most. You may have only
5184 one C<study> active at a time--if you study a different scalar the first
5185 is "unstudied". (The way C<study> works is this: a linked list of every
5186 character in the string to be searched is made, so we know, for
5187 example, where all the C<'k'> characters are. From each search string,
5188 the rarest character is selected, based on some static frequency tables
5189 constructed from some C programs and English text. Only those places
5190 that contain this "rarest" character are examined.)
5192 For example, here is a loop that inserts index producing entries
5193 before any line containing a certain pattern:
5197 print ".IX foo\n" if /\bfoo\b/;
5198 print ".IX bar\n" if /\bbar\b/;
5199 print ".IX blurfl\n" if /\bblurfl\b/;
5204 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5205 will be looked at, because C<f> is rarer than C<o>. In general, this is
5206 a big win except in pathological cases. The only question is whether
5207 it saves you more time than it took to build the linked list in the
5210 Note that if you have to look for strings that you don't know till
5211 runtime, you can build an entire loop as a string and C<eval> that to
5212 avoid recompiling all your patterns all the time. Together with
5213 undefining C<$/> to input entire files as one record, this can be very
5214 fast, often faster than specialized programs like fgrep(1). The following
5215 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5216 out the names of those files that contain a match:
5218 $search = 'while (<>) { study;';
5219 foreach $word (@words) {
5220 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5225 eval $search; # this screams
5226 $/ = "\n"; # put back to normal input delimiter
5227 foreach $file (sort keys(%seen)) {
5231 =item sub NAME BLOCK
5233 =item sub NAME (PROTO) BLOCK
5235 =item sub NAME : ATTRS BLOCK
5237 =item sub NAME (PROTO) : ATTRS BLOCK
5239 This is subroutine definition, not a real function I<per se>.
5240 Without a BLOCK it's just a forward declaration. Without a NAME,
5241 it's an anonymous function declaration, and does actually return
5242 a value: the CODE ref of the closure you just created.
5244 See L<perlsub> and L<perlref> for details about subroutines and
5245 references, and L<attributes> and L<Attribute::Handlers> for more
5246 information about attributes.
5248 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5250 =item substr EXPR,OFFSET,LENGTH
5252 =item substr EXPR,OFFSET
5254 Extracts a substring out of EXPR and returns it. First character is at
5255 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5256 If OFFSET is negative (or more precisely, less than C<$[>), starts
5257 that far from the end of the string. If LENGTH is omitted, returns
5258 everything to the end of the string. If LENGTH is negative, leaves that
5259 many characters off the end of the string.
5261 You can use the substr() function as an lvalue, in which case EXPR
5262 must itself be an lvalue. If you assign something shorter than LENGTH,
5263 the string will shrink, and if you assign something longer than LENGTH,
5264 the string will grow to accommodate it. To keep the string the same
5265 length you may need to pad or chop your value using C<sprintf>.
5267 If OFFSET and LENGTH specify a substring that is partly outside the
5268 string, only the part within the string is returned. If the substring
5269 is beyond either end of the string, substr() returns the undefined
5270 value and produces a warning. When used as an lvalue, specifying a
5271 substring that is entirely outside the string is a fatal error.
5272 Here's an example showing the behavior for boundary cases:
5275 substr($name, 4) = 'dy'; # $name is now 'freddy'
5276 my $null = substr $name, 6, 2; # returns '' (no warning)
5277 my $oops = substr $name, 7; # returns undef, with warning
5278 substr($name, 7) = 'gap'; # fatal error
5280 An alternative to using substr() as an lvalue is to specify the
5281 replacement string as the 4th argument. This allows you to replace
5282 parts of the EXPR and return what was there before in one operation,
5283 just as you can with splice().
5285 =item symlink OLDFILE,NEWFILE
5287 Creates a new filename symbolically linked to the old filename.
5288 Returns C<1> for success, C<0> otherwise. On systems that don't support
5289 symbolic links, produces a fatal error at run time. To check for that,
5292 $symlink_exists = eval { symlink("",""); 1 };
5296 Calls the system call specified as the first element of the list,
5297 passing the remaining elements as arguments to the system call. If
5298 unimplemented, produces a fatal error. The arguments are interpreted
5299 as follows: if a given argument is numeric, the argument is passed as
5300 an int. If not, the pointer to the string value is passed. You are
5301 responsible to make sure a string is pre-extended long enough to
5302 receive any result that might be written into a string. You can't use a
5303 string literal (or other read-only string) as an argument to C<syscall>
5304 because Perl has to assume that any string pointer might be written
5306 integer arguments are not literals and have never been interpreted in a
5307 numeric context, you may need to add C<0> to them to force them to look
5308 like numbers. This emulates the C<syswrite> function (or vice versa):
5310 require 'syscall.ph'; # may need to run h2ph
5312 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5314 Note that Perl supports passing of up to only 14 arguments to your system call,
5315 which in practice should usually suffice.
5317 Syscall returns whatever value returned by the system call it calls.
5318 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5319 Note that some system calls can legitimately return C<-1>. The proper
5320 way to handle such calls is to assign C<$!=0;> before the call and
5321 check the value of C<$!> if syscall returns C<-1>.
5323 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5324 number of the read end of the pipe it creates. There is no way
5325 to retrieve the file number of the other end. You can avoid this
5326 problem by using C<pipe> instead.
5328 =item sysopen FILEHANDLE,FILENAME,MODE
5330 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5332 Opens the file whose filename is given by FILENAME, and associates it
5333 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5334 the name of the real filehandle wanted. This function calls the
5335 underlying operating system's C<open> function with the parameters
5336 FILENAME, MODE, PERMS.
5338 The possible values and flag bits of the MODE parameter are
5339 system-dependent; they are available via the standard module C<Fcntl>.
5340 See the documentation of your operating system's C<open> to see which
5341 values and flag bits are available. You may combine several flags
5342 using the C<|>-operator.
5344 Some of the most common values are C<O_RDONLY> for opening the file in
5345 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5346 and C<O_RDWR> for opening the file in read-write mode, and.
5348 For historical reasons, some values work on almost every system
5349 supported by perl: zero means read-only, one means write-only, and two
5350 means read/write. We know that these values do I<not> work under
5351 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5352 use them in new code.
5354 If the file named by FILENAME does not exist and the C<open> call creates
5355 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5356 PERMS specifies the permissions of the newly created file. If you omit
5357 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5358 These permission values need to be in octal, and are modified by your
5359 process's current C<umask>.
5361 In many systems the C<O_EXCL> flag is available for opening files in
5362 exclusive mode. This is B<not> locking: exclusiveness means here that
5363 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5366 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5368 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5369 that takes away the user's option to have a more permissive umask.
5370 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5373 Note that C<sysopen> depends on the fdopen() C library function.
5374 On many UNIX systems, fdopen() is known to fail when file descriptors
5375 exceed a certain value, typically 255. If you need more file
5376 descriptors than that, consider rebuilding Perl to use the C<sfio>
5377 library, or perhaps using the POSIX::open() function.
5379 See L<perlopentut> for a kinder, gentler explanation of opening files.
5381 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5383 =item sysread FILEHANDLE,SCALAR,LENGTH
5385 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5386 the specified FILEHANDLE, using the system call read(2). It bypasses
5387 buffered IO, so mixing this with other kinds of reads, C<print>,
5388 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5389 stdio usually buffers data. Returns the number of characters actually
5390 read, C<0> at end of file, or undef if there was an error. SCALAR
5391 will be grown or shrunk so that the last byte actually read is the
5392 last byte of the scalar after the read.
5394 Note the I<characters>: depending on the status of the filehandle,
5395 either (8-bit) bytes or characters are read. By default all
5396 filehandles operate on bytes, but for example if the filehandle has
5397 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5398 pragma, L<open>), the I/O will operate on characters, not bytes.
5400 An OFFSET may be specified to place the read data at some place in the
5401 string other than the beginning. A negative OFFSET specifies
5402 placement at that many characters counting backwards from the end of
5403 the string. A positive OFFSET greater than the length of SCALAR
5404 results in the string being padded to the required size with C<"\0">
5405 bytes before the result of the read is appended.
5407 There is no syseof() function, which is ok, since eof() doesn't work
5408 very well on device files (like ttys) anyway. Use sysread() and check
5409 for a return value for 0 to decide whether you're done.
5411 =item sysseek FILEHANDLE,POSITION,WHENCE
5413 Sets FILEHANDLE's system position I<in bytes> using the system call
5414 lseek(2). FILEHANDLE may be an expression whose value gives the name
5415 of the filehandle. The values for WHENCE are C<0> to set the new
5416 position to POSITION, C<1> to set the it to the current position plus
5417 POSITION, and C<2> to set it to EOF plus POSITION (typically
5420 Note the I<in bytes>: even if the filehandle has been set to operate
5421 on characters (for example by using the C<:utf8> discipline), tell()
5422 will return byte offsets, not character offsets (because implementing
5423 that would render sysseek() very slow).
5425 sysseek() bypasses normal buffered io, so mixing this with reads (other
5426 than C<sysread>, for example >< or read()) C<print>, C<write>,
5427 C<seek>, C<tell>, or C<eof> may cause confusion.
5429 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5430 and C<SEEK_END> (start of the file, current position, end of the file)
5431 from the Fcntl module. Use of the constants is also more portable
5432 than relying on 0, 1, and 2. For example to define a "systell" function:
5434 use Fnctl 'SEEK_CUR';
5435 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5437 Returns the new position, or the undefined value on failure. A position
5438 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5439 true on success and false on failure, yet you can still easily determine
5444 =item system PROGRAM LIST
5446 Does exactly the same thing as C<exec LIST>, except that a fork is
5447 done first, and the parent process waits for the child process to
5448 complete. Note that argument processing varies depending on the
5449 number of arguments. If there is more than one argument in LIST,
5450 or if LIST is an array with more than one value, starts the program
5451 given by the first element of the list with arguments given by the
5452 rest of the list. If there is only one scalar argument, the argument
5453 is checked for shell metacharacters, and if there are any, the
5454 entire argument is passed to the system's command shell for parsing
5455 (this is C</bin/sh -c> on Unix platforms, but varies on other
5456 platforms). If there are no shell metacharacters in the argument,
5457 it is split into words and passed directly to C<execvp>, which is
5460 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5461 output before any operation that may do a fork, but this may not be
5462 supported on some platforms (see L<perlport>). To be safe, you may need
5463 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5464 of C<IO::Handle> on any open handles.
5466 The return value is the exit status of the program as returned by the
5467 C<wait> call. To get the actual exit value shift right by eight (see below).
5468 See also L</exec>. This is I<not> what you want to use to capture
5469 the output from a command, for that you should use merely backticks or
5470 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5471 indicates a failure to start the program (inspect $! for the reason).
5473 Like C<exec>, C<system> allows you to lie to a program about its name if
5474 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5476 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5477 killing the program they're running doesn't actually interrupt
5480 @args = ("command", "arg1", "arg2");
5482 or die "system @args failed: $?"
5484 You can check all the failure possibilities by inspecting
5487 $exit_value = $? >> 8;
5488 $signal_num = $? & 127;
5489 $dumped_core = $? & 128;
5491 or more portably by using the W*() calls of the POSIX extension;
5492 see L<perlport> for more information.
5494 When the arguments get executed via the system shell, results
5495 and return codes will be subject to its quirks and capabilities.
5496 See L<perlop/"`STRING`"> and L</exec> for details.
5498 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5500 =item syswrite FILEHANDLE,SCALAR,LENGTH
5502 =item syswrite FILEHANDLE,SCALAR
5504 Attempts to write LENGTH characters of data from variable SCALAR to
5505 the specified FILEHANDLE, using the system call write(2). If LENGTH
5506 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5507 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5508 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5509 buffers data. Returns the number of characters actually written, or
5510 C<undef> if there was an error. If the LENGTH is greater than the
5511 available data in the SCALAR after the OFFSET, only as much data as is
5512 available will be written.
5514 An OFFSET may be specified to write the data from some part of the
5515 string other than the beginning. A negative OFFSET specifies writing
5516 that many characters counting backwards from the end of the string.
5517 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5519 Note the I<characters>: depending on the status of the filehandle,
5520 either (8-bit) bytes or characters are written. By default all
5521 filehandles operate on bytes, but for example if the filehandle has
5522 been opened with the C<:utf8> discipline (see L</open>, and the open
5523 pragma, L<open>), the I/O will operate on characters, not bytes.
5525 =item tell FILEHANDLE
5529 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5530 error. FILEHANDLE may be an expression whose value gives the name of
5531 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5534 Note the I<in bytes>: even if the filehandle has been set to
5535 operate on characters (for example by using the C<:utf8> open
5536 discipline), tell() will return byte offsets, not character offsets
5537 (because that would render seek() and tell() rather slow).
5539 The return value of tell() for the standard streams like the STDIN
5540 depends on the operating system: it may return -1 or something else.
5541 tell() on pipes, fifos, and sockets usually returns -1.
5543 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5545 Do not use tell() on a filehandle that has been opened using
5546 sysopen(), use sysseek() for that as described above. Why? Because
5547 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5548 buffered filehandles. sysseek() make sense only on the first kind,
5549 tell() only makes sense on the second kind.
5551 =item telldir DIRHANDLE
5553 Returns the current position of the C<readdir> routines on DIRHANDLE.
5554 Value may be given to C<seekdir> to access a particular location in a
5555 directory. Has the same caveats about possible directory compaction as
5556 the corresponding system library routine.
5558 =item tie VARIABLE,CLASSNAME,LIST
5560 This function binds a variable to a package class that will provide the
5561 implementation for the variable. VARIABLE is the name of the variable
5562 to be enchanted. CLASSNAME is the name of a class implementing objects
5563 of correct type. Any additional arguments are passed to the C<new>
5564 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5565 or C<TIEHASH>). Typically these are arguments such as might be passed
5566 to the C<dbm_open()> function of C. The object returned by the C<new>
5567 method is also returned by the C<tie> function, which would be useful
5568 if you want to access other methods in CLASSNAME.
5570 Note that functions such as C<keys> and C<values> may return huge lists
5571 when used on large objects, like DBM files. You may prefer to use the
5572 C<each> function to iterate over such. Example:
5574 # print out history file offsets
5576 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5577 while (($key,$val) = each %HIST) {
5578 print $key, ' = ', unpack('L',$val), "\n";
5582 A class implementing a hash should have the following methods:
5584 TIEHASH classname, LIST
5586 STORE this, key, value
5591 NEXTKEY this, lastkey
5595 A class implementing an ordinary array should have the following methods:
5597 TIEARRAY classname, LIST
5599 STORE this, key, value
5601 STORESIZE this, count
5607 SPLICE this, offset, length, LIST
5612 A class implementing a file handle should have the following methods:
5614 TIEHANDLE classname, LIST
5615 READ this, scalar, length, offset
5618 WRITE this, scalar, length, offset
5620 PRINTF this, format, LIST
5624 SEEK this, position, whence
5626 OPEN this, mode, LIST
5631 A class implementing a scalar should have the following methods:
5633 TIESCALAR classname, LIST
5639 Not all methods indicated above need be implemented. See L<perltie>,
5640 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5642 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5643 for you--you need to do that explicitly yourself. See L<DB_File>
5644 or the F<Config> module for interesting C<tie> implementations.
5646 For further details see L<perltie>, L<"tied VARIABLE">.
5650 Returns a reference to the object underlying VARIABLE (the same value
5651 that was originally returned by the C<tie> call that bound the variable
5652 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5657 Returns the number of non-leap seconds since whatever time the system
5658 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5659 and 00:00:00 UTC, January 1, 1970 for most other systems).
5660 Suitable for feeding to C<gmtime> and C<localtime>.
5662 For measuring time in better granularity than one second,
5663 you may use either the Time::HiRes module from CPAN, or
5664 if you have gettimeofday(2), you may be able to use the
5665 C<syscall> interface of Perl, see L<perlfaq8> for details.
5669 Returns a four-element list giving the user and system times, in
5670 seconds, for this process and the children of this process.
5672 ($user,$system,$cuser,$csystem) = times;
5674 In scalar context, C<times> returns C<$user>.
5678 The transliteration operator. Same as C<y///>. See L<perlop>.
5680 =item truncate FILEHANDLE,LENGTH
5682 =item truncate EXPR,LENGTH
5684 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5685 specified length. Produces a fatal error if truncate isn't implemented
5686 on your system. Returns true if successful, the undefined value
5689 The behavior is undefined if LENGTH is greater than the length of the
5696 Returns an uppercased version of EXPR. This is the internal function
5697 implementing the C<\U> escape in double-quoted strings. Respects
5698 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5699 and L<perlunicode> for more details about locale and Unicode support.
5700 It does not attempt to do titlecase mapping on initial letters. See
5701 C<ucfirst> for that.
5703 If EXPR is omitted, uses C<$_>.
5709 Returns the value of EXPR with the first character in uppercase
5710 (titlecase in Unicode). This is the internal function implementing
5711 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5712 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5713 for more details about locale and Unicode support.
5715 If EXPR is omitted, uses C<$_>.
5721 Sets the umask for the process to EXPR and returns the previous value.
5722 If EXPR is omitted, merely returns the current umask.
5724 The Unix permission C<rwxr-x---> is represented as three sets of three
5725 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5726 and isn't one of the digits). The C<umask> value is such a number
5727 representing disabled permissions bits. The permission (or "mode")
5728 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5729 even if you tell C<sysopen> to create a file with permissions C<0777>,
5730 if your umask is C<0022> then the file will actually be created with
5731 permissions C<0755>. If your C<umask> were C<0027> (group can't
5732 write; others can't read, write, or execute), then passing
5733 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5736 Here's some advice: supply a creation mode of C<0666> for regular
5737 files (in C<sysopen>) and one of C<0777> for directories (in
5738 C<mkdir>) and executable files. This gives users the freedom of
5739 choice: if they want protected files, they might choose process umasks
5740 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5741 Programs should rarely if ever make policy decisions better left to
5742 the user. The exception to this is when writing files that should be
5743 kept private: mail files, web browser cookies, I<.rhosts> files, and
5746 If umask(2) is not implemented on your system and you are trying to
5747 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5748 fatal error at run time. If umask(2) is not implemented and you are
5749 not trying to restrict access for yourself, returns C<undef>.
5751 Remember that a umask is a number, usually given in octal; it is I<not> a
5752 string of octal digits. See also L</oct>, if all you have is a string.
5758 Undefines the value of EXPR, which must be an lvalue. Use only on a
5759 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5760 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5761 will probably not do what you expect on most predefined variables or
5762 DBM list values, so don't do that; see L<delete>.) Always returns the
5763 undefined value. You can omit the EXPR, in which case nothing is
5764 undefined, but you still get an undefined value that you could, for
5765 instance, return from a subroutine, assign to a variable or pass as a
5766 parameter. Examples:
5769 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5773 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5774 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5775 select undef, undef, undef, 0.25;
5776 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5778 Note that this is a unary operator, not a list operator.
5784 Deletes a list of files. Returns the number of files successfully
5787 $cnt = unlink 'a', 'b', 'c';
5791 Note: C<unlink> will not delete directories unless you are superuser and
5792 the B<-U> flag is supplied to Perl. Even if these conditions are
5793 met, be warned that unlinking a directory can inflict damage on your
5794 filesystem. Use C<rmdir> instead.
5796 If LIST is omitted, uses C<$_>.
5798 =item unpack TEMPLATE,EXPR
5800 C<unpack> does the reverse of C<pack>: it takes a string
5801 and expands it out into a list of values.
5802 (In scalar context, it returns merely the first value produced.)
5804 The string is broken into chunks described by the TEMPLATE. Each chunk
5805 is converted separately to a value. Typically, either the string is a result
5806 of C<pack>, or the bytes of the string represent a C structure of some
5809 The TEMPLATE has the same format as in the C<pack> function.
5810 Here's a subroutine that does substring:
5813 my($what,$where,$howmuch) = @_;
5814 unpack("x$where a$howmuch", $what);
5819 sub ordinal { unpack("c",$_[0]); } # same as ord()
5821 In addition to fields allowed in pack(), you may prefix a field with
5822 a %<number> to indicate that
5823 you want a <number>-bit checksum of the items instead of the items
5824 themselves. Default is a 16-bit checksum. Checksum is calculated by
5825 summing numeric values of expanded values (for string fields the sum of
5826 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5828 For example, the following
5829 computes the same number as the System V sum program:
5833 unpack("%32C*",<>) % 65535;
5836 The following efficiently counts the number of set bits in a bit vector:
5838 $setbits = unpack("%32b*", $selectmask);
5840 The C<p> and C<P> formats should be used with care. Since Perl
5841 has no way of checking whether the value passed to C<unpack()>
5842 corresponds to a valid memory location, passing a pointer value that's
5843 not known to be valid is likely to have disastrous consequences.
5845 If the repeat count of a field is larger than what the remainder of
5846 the input string allows, repeat count is decreased. If the input string
5847 is longer than one described by the TEMPLATE, the rest is ignored.
5849 See L</pack> for more examples and notes.
5851 =item untie VARIABLE
5853 Breaks the binding between a variable and a package. (See C<tie>.)
5855 =item unshift ARRAY,LIST
5857 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5858 depending on how you look at it. Prepends list to the front of the
5859 array, and returns the new number of elements in the array.
5861 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5863 Note the LIST is prepended whole, not one element at a time, so the
5864 prepended elements stay in the same order. Use C<reverse> to do the
5867 =item use Module VERSION LIST
5869 =item use Module VERSION
5871 =item use Module LIST
5877 Imports some semantics into the current package from the named module,
5878 generally by aliasing certain subroutine or variable names into your
5879 package. It is exactly equivalent to
5881 BEGIN { require Module; import Module LIST; }
5883 except that Module I<must> be a bareword.
5885 VERSION may be either a numeric argument such as 5.006, which will be
5886 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5887 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5888 greater than the version of the current Perl interpreter; Perl will not
5889 attempt to parse the rest of the file. Compare with L</require>, which can
5890 do a similar check at run time.
5892 Specifying VERSION as a literal of the form v5.6.1 should generally be
5893 avoided, because it leads to misleading error messages under earlier
5894 versions of Perl which do not support this syntax. The equivalent numeric
5895 version should be used instead.
5897 use v5.6.1; # compile time version check
5899 use 5.006_001; # ditto; preferred for backwards compatibility
5901 This is often useful if you need to check the current Perl version before
5902 C<use>ing library modules that have changed in incompatible ways from
5903 older versions of Perl. (We try not to do this more than we have to.)
5905 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5906 C<require> makes sure the module is loaded into memory if it hasn't been
5907 yet. The C<import> is not a builtin--it's just an ordinary static method
5908 call into the C<Module> package to tell the module to import the list of
5909 features back into the current package. The module can implement its
5910 C<import> method any way it likes, though most modules just choose to
5911 derive their C<import> method via inheritance from the C<Exporter> class that
5912 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5913 method can be found then the call is skipped.
5915 If you do not want to call the package's C<import> method (for instance,
5916 to stop your namespace from being altered), explicitly supply the empty list:
5920 That is exactly equivalent to
5922 BEGIN { require Module }
5924 If the VERSION argument is present between Module and LIST, then the
5925 C<use> will call the VERSION method in class Module with the given
5926 version as an argument. The default VERSION method, inherited from
5927 the UNIVERSAL class, croaks if the given version is larger than the
5928 value of the variable C<$Module::VERSION>.
5930 Again, there is a distinction between omitting LIST (C<import> called
5931 with no arguments) and an explicit empty LIST C<()> (C<import> not
5932 called). Note that there is no comma after VERSION!
5934 Because this is a wide-open interface, pragmas (compiler directives)
5935 are also implemented this way. Currently implemented pragmas are:
5940 use sigtrap qw(SEGV BUS);
5941 use strict qw(subs vars refs);
5942 use subs qw(afunc blurfl);
5943 use warnings qw(all);
5944 use sort qw(stable _quicksort _mergesort);
5946 Some of these pseudo-modules import semantics into the current
5947 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5948 which import symbols into the current package (which are effective
5949 through the end of the file).
5951 There's a corresponding C<no> command that unimports meanings imported
5952 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5958 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5959 for the C<-M> and C<-m> command-line options to perl that give C<use>
5960 functionality from the command-line.
5964 Changes the access and modification times on each file of a list of
5965 files. The first two elements of the list must be the NUMERICAL access
5966 and modification times, in that order. Returns the number of files
5967 successfully changed. The inode change time of each file is set
5968 to the current time. This code has the same effect as the C<touch>
5969 command if the files already exist:
5973 utime $now, $now, @ARGV;
5975 If the first two elements of the list are C<undef>, then the utime(2)
5976 function in the C library will be called with a null second argument.
5977 On most systems, this will set the file's access and modification
5978 times to the current time. (i.e. equivalent to the example above.)
5980 utime undef, undef, @ARGV;
5984 Returns a list consisting of all the values of the named hash. (In a
5985 scalar context, returns the number of values.) The values are
5986 returned in an apparently random order. The actual random order is
5987 subject to change in future versions of perl, but it is guaranteed to
5988 be the same order as either the C<keys> or C<each> function would
5989 produce on the same (unmodified) hash.
5991 Note that the values are not copied, which means modifying them will
5992 modify the contents of the hash:
5994 for (values %hash) { s/foo/bar/g } # modifies %hash values
5995 for (@hash{keys %hash}) { s/foo/bar/g } # same
5997 As a side effect, calling values() resets the HASH's internal iterator.
5998 See also C<keys>, C<each>, and C<sort>.
6000 =item vec EXPR,OFFSET,BITS
6002 Treats the string in EXPR as a bit vector made up of elements of
6003 width BITS, and returns the value of the element specified by OFFSET
6004 as an unsigned integer. BITS therefore specifies the number of bits
6005 that are reserved for each element in the bit vector. This must
6006 be a power of two from 1 to 32 (or 64, if your platform supports
6009 If BITS is 8, "elements" coincide with bytes of the input string.
6011 If BITS is 16 or more, bytes of the input string are grouped into chunks
6012 of size BITS/8, and each group is converted to a number as with
6013 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6014 for BITS==64). See L<"pack"> for details.
6016 If bits is 4 or less, the string is broken into bytes, then the bits
6017 of each byte are broken into 8/BITS groups. Bits of a byte are
6018 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6019 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6020 breaking the single input byte C<chr(0x36)> into two groups gives a list
6021 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6023 C<vec> may also be assigned to, in which case parentheses are needed
6024 to give the expression the correct precedence as in
6026 vec($image, $max_x * $x + $y, 8) = 3;
6028 If the selected element is outside the string, the value 0 is returned.
6029 If an element off the end of the string is written to, Perl will first
6030 extend the string with sufficiently many zero bytes. It is an error
6031 to try to write off the beginning of the string (i.e. negative OFFSET).
6033 The string should not contain any character with the value > 255 (which
6034 can only happen if you're using UTF8 encoding). If it does, it will be
6035 treated as something which is not UTF8 encoded. When the C<vec> was
6036 assigned to, other parts of your program will also no longer consider the
6037 string to be UTF8 encoded. In other words, if you do have such characters
6038 in your string, vec() will operate on the actual byte string, and not the
6039 conceptual character string.
6041 Strings created with C<vec> can also be manipulated with the logical
6042 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6043 vector operation is desired when both operands are strings.
6044 See L<perlop/"Bitwise String Operators">.
6046 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6047 The comments show the string after each step. Note that this code works
6048 in the same way on big-endian or little-endian machines.
6051 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6053 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6054 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6056 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6057 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6058 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6059 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6060 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6061 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6063 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6064 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6065 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6068 To transform a bit vector into a string or list of 0's and 1's, use these:
6070 $bits = unpack("b*", $vector);
6071 @bits = split(//, unpack("b*", $vector));
6073 If you know the exact length in bits, it can be used in place of the C<*>.
6075 Here is an example to illustrate how the bits actually fall in place:
6081 unpack("V",$_) 01234567890123456789012345678901
6082 ------------------------------------------------------------------
6087 for ($shift=0; $shift < $width; ++$shift) {
6088 for ($off=0; $off < 32/$width; ++$off) {
6089 $str = pack("B*", "0"x32);
6090 $bits = (1<<$shift);
6091 vec($str, $off, $width) = $bits;
6092 $res = unpack("b*",$str);
6093 $val = unpack("V", $str);
6100 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6101 $off, $width, $bits, $val, $res
6105 Regardless of the machine architecture on which it is run, the above
6106 example should print the following table:
6109 unpack("V",$_) 01234567890123456789012345678901
6110 ------------------------------------------------------------------
6111 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6112 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6113 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6114 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6115 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6116 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6117 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6118 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6119 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6120 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6121 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6122 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6123 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6124 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6125 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6126 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6127 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6128 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6129 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6130 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6131 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6132 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6133 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6134 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6135 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6136 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6137 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6138 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6139 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6140 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6141 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6142 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6143 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6144 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6145 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6146 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6147 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6148 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6149 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6150 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6151 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6152 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6153 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6154 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6155 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6156 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6157 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6158 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6159 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6160 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6161 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6162 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6163 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6164 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6165 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6166 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6167 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6168 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6169 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6170 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6171 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6172 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6173 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6174 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6175 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6176 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6177 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6178 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6179 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6180 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6181 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6182 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6183 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6184 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6185 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6186 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6187 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6188 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6189 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6190 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6191 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6192 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6193 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6194 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6195 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6196 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6197 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6198 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6199 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6200 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6201 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6202 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6203 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6204 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6205 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6206 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6207 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6208 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6209 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6210 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6211 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6212 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6213 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6214 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6215 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6216 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6217 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6218 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6219 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6220 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6221 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6222 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6223 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6224 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6225 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6226 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6227 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6228 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6229 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6230 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6231 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6232 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6233 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6234 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6235 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6236 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6237 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6238 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6242 Behaves like the wait(2) system call on your system: it waits for a child
6243 process to terminate and returns the pid of the deceased process, or
6244 C<-1> if there are no child processes. The status is returned in C<$?>.
6245 Note that a return value of C<-1> could mean that child processes are
6246 being automatically reaped, as described in L<perlipc>.
6248 =item waitpid PID,FLAGS
6250 Waits for a particular child process to terminate and returns the pid of
6251 the deceased process, or C<-1> if there is no such child process. On some
6252 systems, a value of 0 indicates that there are processes still running.
6253 The status is returned in C<$?>. If you say
6255 use POSIX ":sys_wait_h";
6258 $kid = waitpid(-1, WNOHANG);
6261 then you can do a non-blocking wait for all pending zombie processes.
6262 Non-blocking wait is available on machines supporting either the
6263 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6264 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6265 system call by remembering the status values of processes that have
6266 exited but have not been harvested by the Perl script yet.)
6268 Note that on some systems, a return value of C<-1> could mean that child
6269 processes are being automatically reaped. See L<perlipc> for details,
6270 and for other examples.
6274 Returns true if the context of the currently executing subroutine is
6275 looking for a list value. Returns false if the context is looking
6276 for a scalar. Returns the undefined value if the context is looking
6277 for no value (void context).
6279 return unless defined wantarray; # don't bother doing more
6280 my @a = complex_calculation();
6281 return wantarray ? @a : "@a";
6283 This function should have been named wantlist() instead.
6287 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6290 If LIST is empty and C<$@> already contains a value (typically from a
6291 previous eval) that value is used after appending C<"\t...caught">
6292 to C<$@>. This is useful for staying almost, but not entirely similar to
6295 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6297 No message is printed if there is a C<$SIG{__WARN__}> handler
6298 installed. It is the handler's responsibility to deal with the message
6299 as it sees fit (like, for instance, converting it into a C<die>). Most
6300 handlers must therefore make arrangements to actually display the
6301 warnings that they are not prepared to deal with, by calling C<warn>
6302 again in the handler. Note that this is quite safe and will not
6303 produce an endless loop, since C<__WARN__> hooks are not called from
6306 You will find this behavior is slightly different from that of
6307 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6308 instead call C<die> again to change it).
6310 Using a C<__WARN__> handler provides a powerful way to silence all
6311 warnings (even the so-called mandatory ones). An example:
6313 # wipe out *all* compile-time warnings
6314 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6316 my $foo = 20; # no warning about duplicate my $foo,
6317 # but hey, you asked for it!
6318 # no compile-time or run-time warnings before here
6321 # run-time warnings enabled after here
6322 warn "\$foo is alive and $foo!"; # does show up
6324 See L<perlvar> for details on setting C<%SIG> entries, and for more
6325 examples. See the Carp module for other kinds of warnings using its
6326 carp() and cluck() functions.
6328 =item write FILEHANDLE
6334 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6335 using the format associated with that file. By default the format for
6336 a file is the one having the same name as the filehandle, but the
6337 format for the current output channel (see the C<select> function) may be set
6338 explicitly by assigning the name of the format to the C<$~> variable.
6340 Top of form processing is handled automatically: if there is
6341 insufficient room on the current page for the formatted record, the
6342 page is advanced by writing a form feed, a special top-of-page format
6343 is used to format the new page header, and then the record is written.
6344 By default the top-of-page format is the name of the filehandle with
6345 "_TOP" appended, but it may be dynamically set to the format of your
6346 choice by assigning the name to the C<$^> variable while the filehandle is
6347 selected. The number of lines remaining on the current page is in
6348 variable C<$->, which can be set to C<0> to force a new page.
6350 If FILEHANDLE is unspecified, output goes to the current default output
6351 channel, which starts out as STDOUT but may be changed by the
6352 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6353 is evaluated and the resulting string is used to look up the name of
6354 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6356 Note that write is I<not> the opposite of C<read>. Unfortunately.
6360 The transliteration operator. Same as C<tr///>. See L<perlop>.