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 omitted or specified as C<:raw> the filehandle is made
459 suitable for passing binary data. This includes turning off possible CRLF
460 translation and marking it as bytes (as opposed to Unicode characters).
461 Note that as desipite what may be implied in I<"Programming Perl">
462 (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
463 - other disciplines which would affect binary nature of the stream are
464 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
465 PERLIO environment variable.
467 On some systems (in general, DOS and Windows-based systems) binmode()
468 is necessary when you're not working with a text file. For the sake
469 of portability it is a good idea to always use it when appropriate,
470 and to never use it when it isn't appropriate.
472 In other words: regardless of platform, use binmode() on binary files
473 (like for example images).
475 If DISCIPLINE is present it is a single string, but may contain
476 multiple directives. The directives alter the behaviour of the
477 file handle. When DISCIPLINE is present using binmode on text
480 To mark FILEHANDLE as UTF-8, use C<:utf8>.
482 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
483 form C<:...>, are called I/O I<disciplines>. The normal implementation
484 of disciplines in Perl 5.8 and later is in terms of I<layers>. See
485 L<PerlIO>. (There is typically a one-to-one correspondence between
486 layers and disiplines.) The C<open> pragma can be used to establish
487 default I/O disciplines. See L<open>.
489 In general, binmode() should be called after open() but before any I/O
490 is done on the filehandle. Calling binmode() will normally flush any
491 pending buffered output data (and perhaps pending input data) on the
492 handle. An exception to this is the C<:encoding> discipline that
493 changes the default character encoding of the handle, see L<open>.
494 The C<:encoding> discipline sometimes needs to be called in
495 mid-stream, and it doesn't flush the stream.
497 The operating system, device drivers, C libraries, and Perl run-time
498 system all work together to let the programmer treat a single
499 character (C<\n>) as the line terminator, irrespective of the external
500 representation. On many operating systems, the native text file
501 representation matches the internal representation, but on some
502 platforms the external representation of C<\n> is made up of more than
505 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
506 character to end each line in the external representation of text (even
507 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
508 on Unix and most VMS files). In other systems like OS/2, DOS and the
509 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
510 but what's stored in text files are the two characters C<\cM\cJ>. That
511 means that, if you don't use binmode() on these systems, C<\cM\cJ>
512 sequences on disk will be converted to C<\n> on input, and any C<\n> in
513 your program will be converted back to C<\cM\cJ> on output. This is what
514 you want for text files, but it can be disastrous for binary files.
516 Another consequence of using binmode() (on some systems) is that
517 special end-of-file markers will be seen as part of the data stream.
518 For systems from the Microsoft family this means that if your binary
519 data contains C<\cZ>, the I/O subsystem will regard it as the end of
520 the file, unless you use binmode().
522 binmode() is not only important for readline() and print() operations,
523 but also when using read(), seek(), sysread(), syswrite() and tell()
524 (see L<perlport> for more details). See the C<$/> and C<$\> variables
525 in L<perlvar> for how to manually set your input and output
526 line-termination sequences.
528 =item bless REF,CLASSNAME
532 This function tells the thingy referenced by REF that it is now an object
533 in the CLASSNAME package. If CLASSNAME is omitted, the current package
534 is used. Because a C<bless> is often the last thing in a constructor,
535 it returns the reference for convenience. Always use the two-argument
536 version if the function doing the blessing might be inherited by a
537 derived class. See L<perltoot> and L<perlobj> for more about the blessing
538 (and blessings) of objects.
540 Consider always blessing objects in CLASSNAMEs that are mixed case.
541 Namespaces with all lowercase names are considered reserved for
542 Perl pragmata. Builtin types have all uppercase names, so to prevent
543 confusion, you may wish to avoid such package names as well. Make sure
544 that CLASSNAME is a true value.
546 See L<perlmod/"Perl Modules">.
552 Returns the context of the current subroutine call. In scalar context,
553 returns the caller's package name if there is a caller, that is, if
554 we're in a subroutine or C<eval> or C<require>, and the undefined value
555 otherwise. In list context, returns
557 ($package, $filename, $line) = caller;
559 With EXPR, it returns some extra information that the debugger uses to
560 print a stack trace. The value of EXPR indicates how many call frames
561 to go back before the current one.
563 ($package, $filename, $line, $subroutine, $hasargs,
564 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
566 Here $subroutine may be C<(eval)> if the frame is not a subroutine
567 call, but an C<eval>. In such a case additional elements $evaltext and
568 C<$is_require> are set: C<$is_require> is true if the frame is created by a
569 C<require> or C<use> statement, $evaltext contains the text of the
570 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
571 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
572 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
573 frame.) $subroutine may also be C<(unknown)> if this particular
574 subroutine happens to have been deleted from the symbol table.
575 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
576 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
577 compiled with. The C<$hints> and C<$bitmask> values are subject to change
578 between versions of Perl, and are not meant for external use.
580 Furthermore, when called from within the DB package, caller returns more
581 detailed information: it sets the list variable C<@DB::args> to be the
582 arguments with which the subroutine was invoked.
584 Be aware that the optimizer might have optimized call frames away before
585 C<caller> had a chance to get the information. That means that C<caller(N)>
586 might not return information about the call frame you expect it do, for
587 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
588 previous time C<caller> was called.
592 Changes the working directory to EXPR, if possible. If EXPR is omitted,
593 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
594 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
595 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
596 neither is set, C<chdir> does nothing. It returns true upon success,
597 false otherwise. See the example under C<die>.
601 Changes the permissions of a list of files. The first element of the
602 list must be the numerical mode, which should probably be an octal
603 number, and which definitely should I<not> a string of octal digits:
604 C<0644> is okay, C<'0644'> is not. Returns the number of files
605 successfully changed. See also L</oct>, if all you have is a string.
607 $cnt = chmod 0755, 'foo', 'bar';
608 chmod 0755, @executables;
609 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
611 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
612 $mode = 0644; chmod $mode, 'foo'; # this is best
614 You can also import the symbolic C<S_I*> constants from the Fcntl
619 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
620 # This is identical to the chmod 0755 of the above example.
628 This safer version of L</chop> removes any trailing string
629 that corresponds to the current value of C<$/> (also known as
630 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
631 number of characters removed from all its arguments. It's often used to
632 remove the newline from the end of an input record when you're worried
633 that the final record may be missing its newline. When in paragraph
634 mode (C<$/ = "">), it removes all trailing newlines from the string.
635 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
636 a reference to an integer or the like, see L<perlvar>) chomp() won't
638 If VARIABLE is omitted, it chomps C<$_>. Example:
641 chomp; # avoid \n on last field
646 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
648 You can actually chomp anything that's an lvalue, including an assignment:
651 chomp($answer = <STDIN>);
653 If you chomp a list, each element is chomped, and the total number of
654 characters removed is returned.
662 Chops off the last character of a string and returns the character
663 chopped. It is much more efficient than C<s/.$//s> because it neither
664 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
665 If VARIABLE is a hash, it chops the hash's values, but not its keys.
667 You can actually chop anything that's an lvalue, including an assignment.
669 If you chop a list, each element is chopped. Only the value of the
670 last C<chop> is returned.
672 Note that C<chop> returns the last character. To return all but the last
673 character, use C<substr($string, 0, -1)>.
677 Changes the owner (and group) of a list of files. The first two
678 elements of the list must be the I<numeric> uid and gid, in that
679 order. A value of -1 in either position is interpreted by most
680 systems to leave that value unchanged. Returns the number of files
681 successfully changed.
683 $cnt = chown $uid, $gid, 'foo', 'bar';
684 chown $uid, $gid, @filenames;
686 Here's an example that looks up nonnumeric uids in the passwd file:
689 chomp($user = <STDIN>);
691 chomp($pattern = <STDIN>);
693 ($login,$pass,$uid,$gid) = getpwnam($user)
694 or die "$user not in passwd file";
696 @ary = glob($pattern); # expand filenames
697 chown $uid, $gid, @ary;
699 On most systems, you are not allowed to change the ownership of the
700 file unless you're the superuser, although you should be able to change
701 the group to any of your secondary groups. On insecure systems, these
702 restrictions may be relaxed, but this is not a portable assumption.
703 On POSIX systems, you can detect this condition this way:
705 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
706 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
712 Returns the character represented by that NUMBER in the character set.
713 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
714 chr(0x263a) is a Unicode smiley face. Note that characters from 127
715 to 255 (inclusive) are by default not encoded in Unicode for backward
716 compatibility reasons (but see L<encoding>).
718 For the reverse, use L</ord>.
719 See L<perlunicode> and L<encoding> for more about Unicode.
721 If NUMBER is omitted, uses C<$_>.
723 =item chroot FILENAME
727 This function works like the system call by the same name: it makes the
728 named directory the new root directory for all further pathnames that
729 begin with a C</> by your process and all its children. (It doesn't
730 change your current working directory, which is unaffected.) For security
731 reasons, this call is restricted to the superuser. If FILENAME is
732 omitted, does a C<chroot> to C<$_>.
734 =item close FILEHANDLE
738 Closes the file or pipe associated with the file handle, returning
739 true only if IO buffers are successfully flushed and closes the system
740 file descriptor. Closes the currently selected filehandle if the
743 You don't have to close FILEHANDLE if you are immediately going to do
744 another C<open> on it, because C<open> will close it for you. (See
745 C<open>.) However, an explicit C<close> on an input file resets the line
746 counter (C<$.>), while the implicit close done by C<open> does not.
748 If the file handle came from a piped open C<close> will additionally
749 return false if one of the other system calls involved fails or if the
750 program exits with non-zero status. (If the only problem was that the
751 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
752 also waits for the process executing on the pipe to complete, in case you
753 want to look at the output of the pipe afterwards, and
754 implicitly puts the exit status value of that command into C<$?>.
756 Prematurely closing the read end of a pipe (i.e. before the process
757 writing to it at the other end has closed it) will result in a
758 SIGPIPE being delivered to the writer. If the other end can't
759 handle that, be sure to read all the data before closing the pipe.
763 open(OUTPUT, '|sort >foo') # pipe to sort
764 or die "Can't start sort: $!";
765 #... # print stuff to output
766 close OUTPUT # wait for sort to finish
767 or warn $! ? "Error closing sort pipe: $!"
768 : "Exit status $? from sort";
769 open(INPUT, 'foo') # get sort's results
770 or die "Can't open 'foo' for input: $!";
772 FILEHANDLE may be an expression whose value can be used as an indirect
773 filehandle, usually the real filehandle name.
775 =item closedir DIRHANDLE
777 Closes a directory opened by C<opendir> and returns the success of that
780 DIRHANDLE may be an expression whose value can be used as an indirect
781 dirhandle, usually the real dirhandle name.
783 =item connect SOCKET,NAME
785 Attempts to connect to a remote socket, just as the connect system call
786 does. Returns true if it succeeded, false otherwise. NAME should be a
787 packed address of the appropriate type for the socket. See the examples in
788 L<perlipc/"Sockets: Client/Server Communication">.
792 Actually a flow control statement rather than a function. If there is a
793 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
794 C<foreach>), it is always executed just before the conditional is about to
795 be evaluated again, just like the third part of a C<for> loop in C. Thus
796 it can be used to increment a loop variable, even when the loop has been
797 continued via the C<next> statement (which is similar to the C C<continue>
800 C<last>, C<next>, or C<redo> may appear within a C<continue>
801 block. C<last> and C<redo> will behave as if they had been executed within
802 the main block. So will C<next>, but since it will execute a C<continue>
803 block, it may be more entertaining.
806 ### redo always comes here
809 ### next always comes here
811 # then back the top to re-check EXPR
813 ### last always comes here
815 Omitting the C<continue> section is semantically equivalent to using an
816 empty one, logically enough. In that case, C<next> goes directly back
817 to check the condition at the top of the loop.
823 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
824 takes cosine of C<$_>.
826 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
827 function, or use this relation:
829 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
831 =item crypt PLAINTEXT,SALT
833 Encrypts a string exactly like the crypt(3) function in the C library
834 (assuming that you actually have a version there that has not been
835 extirpated as a potential munition). This can prove useful for checking
836 the password file for lousy passwords, amongst other things. Only the
837 guys wearing white hats should do this.
839 Note that C<crypt> is intended to be a one-way function, much like
840 breaking eggs to make an omelette. There is no (known) corresponding
841 decrypt function (in other words, the crypt() is a one-way hash
842 function). As a result, this function isn't all that useful for
843 cryptography. (For that, see your nearby CPAN mirror.)
845 When verifying an existing encrypted string you should use the
846 encrypted text as the salt (like C<crypt($plain, $crypted) eq
847 $crypted>). This allows your code to work with the standard C<crypt>
848 and with more exotic implementations. In other words, do not assume
849 anything about the returned string itself, or how many bytes in
850 the encrypted string matter.
852 Traditionally the result is a string of 13 bytes: two first bytes of
853 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
854 the first eight bytes of the encrypted string mattered, but
855 alternative hashing schemes (like MD5), higher level security schemes
856 (like C2), and implementations on non-UNIX platforms may produce
859 When choosing a new salt create a random two character string whose
860 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
861 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
863 Here's an example that makes sure that whoever runs this program knows
866 $pwd = (getpwuid($<))[1];
870 chomp($word = <STDIN>);
874 if (crypt($word, $pwd) ne $pwd) {
880 Of course, typing in your own password to whoever asks you
883 The L<crypt> function is unsuitable for encrypting large quantities
884 of data, not least of all because you can't get the information
885 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
886 on your favorite CPAN mirror for a slew of potentially useful
889 If using crypt() on a Unicode string (which I<potentially> has
890 characters with codepoints above 255), Perl tries to make sense
891 of the situation by trying to downgrade (a copy of the string)
892 the string back to an eight-bit byte string before calling crypt()
893 (on that copy). If that works, good. If not, crypt() dies with
894 C<Wide character in crypt>.
898 [This function has been largely superseded by the C<untie> function.]
900 Breaks the binding between a DBM file and a hash.
902 =item dbmopen HASH,DBNAME,MASK
904 [This function has been largely superseded by the C<tie> function.]
906 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
907 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
908 argument is I<not> a filehandle, even though it looks like one). DBNAME
909 is the name of the database (without the F<.dir> or F<.pag> extension if
910 any). If the database does not exist, it is created with protection
911 specified by MASK (as modified by the C<umask>). If your system supports
912 only the older DBM functions, you may perform only one C<dbmopen> in your
913 program. In older versions of Perl, if your system had neither DBM nor
914 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
917 If you don't have write access to the DBM file, you can only read hash
918 variables, not set them. If you want to test whether you can write,
919 either use file tests or try setting a dummy hash entry inside an C<eval>,
920 which will trap the error.
922 Note that functions such as C<keys> and C<values> may return huge lists
923 when used on large DBM files. You may prefer to use the C<each>
924 function to iterate over large DBM files. Example:
926 # print out history file offsets
927 dbmopen(%HIST,'/usr/lib/news/history',0666);
928 while (($key,$val) = each %HIST) {
929 print $key, ' = ', unpack('L',$val), "\n";
933 See also L<AnyDBM_File> for a more general description of the pros and
934 cons of the various dbm approaches, as well as L<DB_File> for a particularly
937 You can control which DBM library you use by loading that library
938 before you call dbmopen():
941 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
942 or die "Can't open netscape history file: $!";
948 Returns a Boolean value telling whether EXPR has a value other than
949 the undefined value C<undef>. If EXPR is not present, C<$_> will be
952 Many operations return C<undef> to indicate failure, end of file,
953 system error, uninitialized variable, and other exceptional
954 conditions. This function allows you to distinguish C<undef> from
955 other values. (A simple Boolean test will not distinguish among
956 C<undef>, zero, the empty string, and C<"0">, which are all equally
957 false.) Note that since C<undef> is a valid scalar, its presence
958 doesn't I<necessarily> indicate an exceptional condition: C<pop>
959 returns C<undef> when its argument is an empty array, I<or> when the
960 element to return happens to be C<undef>.
962 You may also use C<defined(&func)> to check whether subroutine C<&func>
963 has ever been defined. The return value is unaffected by any forward
964 declarations of C<&foo>. Note that a subroutine which is not defined
965 may still be callable: its package may have an C<AUTOLOAD> method that
966 makes it spring into existence the first time that it is called -- see
969 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
970 used to report whether memory for that aggregate has ever been
971 allocated. This behavior may disappear in future versions of Perl.
972 You should instead use a simple test for size:
974 if (@an_array) { print "has array elements\n" }
975 if (%a_hash) { print "has hash members\n" }
977 When used on a hash element, it tells you whether the value is defined,
978 not whether the key exists in the hash. Use L</exists> for the latter
983 print if defined $switch{'D'};
984 print "$val\n" while defined($val = pop(@ary));
985 die "Can't readlink $sym: $!"
986 unless defined($value = readlink $sym);
987 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
988 $debugging = 0 unless defined $debugging;
990 Note: Many folks tend to overuse C<defined>, and then are surprised to
991 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
992 defined values. For example, if you say
996 The pattern match succeeds, and C<$1> is defined, despite the fact that it
997 matched "nothing". But it didn't really match nothing--rather, it
998 matched something that happened to be zero characters long. This is all
999 very above-board and honest. When a function returns an undefined value,
1000 it's an admission that it couldn't give you an honest answer. So you
1001 should use C<defined> only when you're questioning the integrity of what
1002 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1005 See also L</undef>, L</exists>, L</ref>.
1009 Given an expression that specifies a hash element, array element, hash slice,
1010 or array slice, deletes the specified element(s) from the hash or array.
1011 In the case of an array, if the array elements happen to be at the end,
1012 the size of the array will shrink to the highest element that tests
1013 true for exists() (or 0 if no such element exists).
1015 Returns each element so deleted or the undefined value if there was no such
1016 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1017 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1018 from a C<tie>d hash or array may not necessarily return anything.
1020 Deleting an array element effectively returns that position of the array
1021 to its initial, uninitialized state. Subsequently testing for the same
1022 element with exists() will return false. Note that deleting array
1023 elements in the middle of an array will not shift the index of the ones
1024 after them down--use splice() for that. See L</exists>.
1026 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1028 foreach $key (keys %HASH) {
1032 foreach $index (0 .. $#ARRAY) {
1033 delete $ARRAY[$index];
1038 delete @HASH{keys %HASH};
1040 delete @ARRAY[0 .. $#ARRAY];
1042 But both of these are slower than just assigning the empty list
1043 or undefining %HASH or @ARRAY:
1045 %HASH = (); # completely empty %HASH
1046 undef %HASH; # forget %HASH ever existed
1048 @ARRAY = (); # completely empty @ARRAY
1049 undef @ARRAY; # forget @ARRAY ever existed
1051 Note that the EXPR can be arbitrarily complicated as long as the final
1052 operation is a hash element, array element, hash slice, or array slice
1055 delete $ref->[$x][$y]{$key};
1056 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1058 delete $ref->[$x][$y][$index];
1059 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1063 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1064 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1065 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1066 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1067 an C<eval(),> the error message is stuffed into C<$@> and the
1068 C<eval> is terminated with the undefined value. This makes
1069 C<die> the way to raise an exception.
1071 Equivalent examples:
1073 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1074 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1076 If the last element of LIST does not end in a newline, the current
1077 script line number and input line number (if any) are also printed,
1078 and a newline is supplied. Note that the "input line number" (also
1079 known as "chunk") is subject to whatever notion of "line" happens to
1080 be currently in effect, and is also available as the special variable
1081 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1083 Hint: sometimes appending C<", stopped"> to your message will cause it
1084 to make better sense when the string C<"at foo line 123"> is appended.
1085 Suppose you are running script "canasta".
1087 die "/etc/games is no good";
1088 die "/etc/games is no good, stopped";
1090 produce, respectively
1092 /etc/games is no good at canasta line 123.
1093 /etc/games is no good, stopped at canasta line 123.
1095 See also exit(), warn(), and the Carp module.
1097 If LIST is empty and C<$@> already contains a value (typically from a
1098 previous eval) that value is reused after appending C<"\t...propagated">.
1099 This is useful for propagating exceptions:
1102 die unless $@ =~ /Expected exception/;
1104 If LIST is empty and C<$@> contains an object reference that has a
1105 C<PROPAGATE> method, that method will be called with additional file
1106 and line number parameters. The return value replaces the value in
1107 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1110 If C<$@> is empty then the string C<"Died"> is used.
1112 die() can also be called with a reference argument. If this happens to be
1113 trapped within an eval(), $@ contains the reference. This behavior permits
1114 a more elaborate exception handling implementation using objects that
1115 maintain arbitrary state about the nature of the exception. Such a scheme
1116 is sometimes preferable to matching particular string values of $@ using
1117 regular expressions. Here's an example:
1119 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1121 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1122 # handle Some::Module::Exception
1125 # handle all other possible exceptions
1129 Because perl will stringify uncaught exception messages before displaying
1130 them, you may want to overload stringification operations on such custom
1131 exception objects. See L<overload> for details about that.
1133 You can arrange for a callback to be run just before the C<die>
1134 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1135 handler will be called with the error text and can change the error
1136 message, if it sees fit, by calling C<die> again. See
1137 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1138 L<"eval BLOCK"> for some examples. Although this feature was meant
1139 to be run only right before your program was to exit, this is not
1140 currently the case--the C<$SIG{__DIE__}> hook is currently called
1141 even inside eval()ed blocks/strings! If one wants the hook to do
1142 nothing in such situations, put
1146 as the first line of the handler (see L<perlvar/$^S>). Because
1147 this promotes strange action at a distance, this counterintuitive
1148 behavior may be fixed in a future release.
1152 Not really a function. Returns the value of the last command in the
1153 sequence of commands indicated by BLOCK. When modified by a loop
1154 modifier, executes the BLOCK once before testing the loop condition.
1155 (On other statements the loop modifiers test the conditional first.)
1157 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1158 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1159 See L<perlsyn> for alternative strategies.
1161 =item do SUBROUTINE(LIST)
1163 A deprecated form of subroutine call. See L<perlsub>.
1167 Uses the value of EXPR as a filename and executes the contents of the
1168 file as a Perl script. Its primary use is to include subroutines
1169 from a Perl subroutine library.
1177 except that it's more efficient and concise, keeps track of the current
1178 filename for error messages, searches the @INC libraries, and updates
1179 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1180 variables. It also differs in that code evaluated with C<do FILENAME>
1181 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1182 same, however, in that it does reparse the file every time you call it,
1183 so you probably don't want to do this inside a loop.
1185 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1186 error. If C<do> can read the file but cannot compile it, it
1187 returns undef and sets an error message in C<$@>. If the file is
1188 successfully compiled, C<do> returns the value of the last expression
1191 Note that inclusion of library modules is better done with the
1192 C<use> and C<require> operators, which also do automatic error checking
1193 and raise an exception if there's a problem.
1195 You might like to use C<do> to read in a program configuration
1196 file. Manual error checking can be done this way:
1198 # read in config files: system first, then user
1199 for $file ("/share/prog/defaults.rc",
1200 "$ENV{HOME}/.someprogrc")
1202 unless ($return = do $file) {
1203 warn "couldn't parse $file: $@" if $@;
1204 warn "couldn't do $file: $!" unless defined $return;
1205 warn "couldn't run $file" unless $return;
1213 This function causes an immediate core dump. See also the B<-u>
1214 command-line switch in L<perlrun>, which does the same thing.
1215 Primarily this is so that you can use the B<undump> program (not
1216 supplied) to turn your core dump into an executable binary after
1217 having initialized all your variables at the beginning of the
1218 program. When the new binary is executed it will begin by executing
1219 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1220 Think of it as a goto with an intervening core dump and reincarnation.
1221 If C<LABEL> is omitted, restarts the program from the top.
1223 B<WARNING>: Any files opened at the time of the dump will I<not>
1224 be open any more when the program is reincarnated, with possible
1225 resulting confusion on the part of Perl.
1227 This function is now largely obsolete, partly because it's very
1228 hard to convert a core file into an executable, and because the
1229 real compiler backends for generating portable bytecode and compilable
1230 C code have superseded it. That's why you should now invoke it as
1231 C<CORE::dump()>, if you don't want to be warned against a possible
1234 If you're looking to use L<dump> to speed up your program, consider
1235 generating bytecode or native C code as described in L<perlcc>. If
1236 you're just trying to accelerate a CGI script, consider using the
1237 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1238 You might also consider autoloading or selfloading, which at least
1239 make your program I<appear> to run faster.
1243 When called in list context, returns a 2-element list consisting of the
1244 key and value for the next element of a hash, so that you can iterate over
1245 it. When called in scalar context, returns only the key for the next
1246 element in the hash.
1248 Entries are returned in an apparently random order. The actual random
1249 order is subject to change in future versions of perl, but it is guaranteed
1250 to be in the same order as either the C<keys> or C<values> function
1251 would produce on the same (unmodified) hash.
1253 When the hash is entirely read, a null array is returned in list context
1254 (which when assigned produces a false (C<0>) value), and C<undef> in
1255 scalar context. The next call to C<each> after that will start iterating
1256 again. There is a single iterator for each hash, shared by all C<each>,
1257 C<keys>, and C<values> function calls in the program; it can be reset by
1258 reading all the elements from the hash, or by evaluating C<keys HASH> or
1259 C<values HASH>. If you add or delete elements of a hash while you're
1260 iterating over it, you may get entries skipped or duplicated, so
1261 don't. Exception: It is always safe to delete the item most recently
1262 returned by C<each()>, which means that the following code will work:
1264 while (($key, $value) = each %hash) {
1266 delete $hash{$key}; # This is safe
1269 The following prints out your environment like the printenv(1) program,
1270 only in a different order:
1272 while (($key,$value) = each %ENV) {
1273 print "$key=$value\n";
1276 See also C<keys>, C<values> and C<sort>.
1278 =item eof FILEHANDLE
1284 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1285 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1286 gives the real filehandle. (Note that this function actually
1287 reads a character and then C<ungetc>s it, so isn't very useful in an
1288 interactive context.) Do not read from a terminal file (or call
1289 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1290 as terminals may lose the end-of-file condition if you do.
1292 An C<eof> without an argument uses the last file read. Using C<eof()>
1293 with empty parentheses is very different. It refers to the pseudo file
1294 formed from the files listed on the command line and accessed via the
1295 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1296 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1297 used will cause C<@ARGV> to be examined to determine if input is
1298 available. Similarly, an C<eof()> after C<< <> >> has returned
1299 end-of-file will assume you are processing another C<@ARGV> list,
1300 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1301 see L<perlop/"I/O Operators">.
1303 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1304 detect the end of each file, C<eof()> will only detect the end of the
1305 last file. Examples:
1307 # reset line numbering on each input file
1309 next if /^\s*#/; # skip comments
1312 close ARGV if eof; # Not eof()!
1315 # insert dashes just before last line of last file
1317 if (eof()) { # check for end of current file
1318 print "--------------\n";
1319 close(ARGV); # close or last; is needed if we
1320 # are reading from the terminal
1325 Practical hint: you almost never need to use C<eof> in Perl, because the
1326 input operators typically return C<undef> when they run out of data, or if
1333 In the first form, the return value of EXPR is parsed and executed as if it
1334 were a little Perl program. The value of the expression (which is itself
1335 determined within scalar context) is first parsed, and if there weren't any
1336 errors, executed in the lexical context of the current Perl program, so
1337 that any variable settings or subroutine and format definitions remain
1338 afterwards. Note that the value is parsed every time the eval executes.
1339 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1340 delay parsing and subsequent execution of the text of EXPR until run time.
1342 In the second form, the code within the BLOCK is parsed only once--at the
1343 same time the code surrounding the eval itself was parsed--and executed
1344 within the context of the current Perl program. This form is typically
1345 used to trap exceptions more efficiently than the first (see below), while
1346 also providing the benefit of checking the code within BLOCK at compile
1349 The final semicolon, if any, may be omitted from the value of EXPR or within
1352 In both forms, the value returned is the value of the last expression
1353 evaluated inside the mini-program; a return statement may be also used, just
1354 as with subroutines. The expression providing the return value is evaluated
1355 in void, scalar, or list context, depending on the context of the eval itself.
1356 See L</wantarray> for more on how the evaluation context can be determined.
1358 If there is a syntax error or runtime error, or a C<die> statement is
1359 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1360 error message. If there was no error, C<$@> is guaranteed to be a null
1361 string. Beware that using C<eval> neither silences perl from printing
1362 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1363 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1364 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1365 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1367 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1368 determining whether a particular feature (such as C<socket> or C<symlink>)
1369 is implemented. It is also Perl's exception trapping mechanism, where
1370 the die operator is used to raise exceptions.
1372 If the code to be executed doesn't vary, you may use the eval-BLOCK
1373 form to trap run-time errors without incurring the penalty of
1374 recompiling each time. The error, if any, is still returned in C<$@>.
1377 # make divide-by-zero nonfatal
1378 eval { $answer = $a / $b; }; warn $@ if $@;
1380 # same thing, but less efficient
1381 eval '$answer = $a / $b'; warn $@ if $@;
1383 # a compile-time error
1384 eval { $answer = }; # WRONG
1387 eval '$answer ='; # sets $@
1389 Due to the current arguably broken state of C<__DIE__> hooks, when using
1390 the C<eval{}> form as an exception trap in libraries, you may wish not
1391 to trigger any C<__DIE__> hooks that user code may have installed.
1392 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1393 as shown in this example:
1395 # a very private exception trap for divide-by-zero
1396 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1399 This is especially significant, given that C<__DIE__> hooks can call
1400 C<die> again, which has the effect of changing their error messages:
1402 # __DIE__ hooks may modify error messages
1404 local $SIG{'__DIE__'} =
1405 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1406 eval { die "foo lives here" };
1407 print $@ if $@; # prints "bar lives here"
1410 Because this promotes action at a distance, this counterintuitive behavior
1411 may be fixed in a future release.
1413 With an C<eval>, you should be especially careful to remember what's
1414 being looked at when:
1420 eval { $x }; # CASE 4
1422 eval "\$$x++"; # CASE 5
1425 Cases 1 and 2 above behave identically: they run the code contained in
1426 the variable $x. (Although case 2 has misleading double quotes making
1427 the reader wonder what else might be happening (nothing is).) Cases 3
1428 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1429 does nothing but return the value of $x. (Case 4 is preferred for
1430 purely visual reasons, but it also has the advantage of compiling at
1431 compile-time instead of at run-time.) Case 5 is a place where
1432 normally you I<would> like to use double quotes, except that in this
1433 particular situation, you can just use symbolic references instead, as
1436 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1437 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1441 =item exec PROGRAM LIST
1443 The C<exec> function executes a system command I<and never returns>--
1444 use C<system> instead of C<exec> if you want it to return. It fails and
1445 returns false only if the command does not exist I<and> it is executed
1446 directly instead of via your system's command shell (see below).
1448 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1449 warns you if there is a following statement which isn't C<die>, C<warn>,
1450 or C<exit> (if C<-w> is set - but you always do that). If you
1451 I<really> want to follow an C<exec> with some other statement, you
1452 can use one of these styles to avoid the warning:
1454 exec ('foo') or print STDERR "couldn't exec foo: $!";
1455 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1457 If there is more than one argument in LIST, or if LIST is an array
1458 with more than one value, calls execvp(3) with the arguments in LIST.
1459 If there is only one scalar argument or an array with one element in it,
1460 the argument is checked for shell metacharacters, and if there are any,
1461 the entire argument is passed to the system's command shell for parsing
1462 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1463 If there are no shell metacharacters in the argument, it is split into
1464 words and passed directly to C<execvp>, which is more efficient.
1467 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1468 exec "sort $outfile | uniq";
1470 If you don't really want to execute the first argument, but want to lie
1471 to the program you are executing about its own name, you can specify
1472 the program you actually want to run as an "indirect object" (without a
1473 comma) in front of the LIST. (This always forces interpretation of the
1474 LIST as a multivalued list, even if there is only a single scalar in
1477 $shell = '/bin/csh';
1478 exec $shell '-sh'; # pretend it's a login shell
1482 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1484 When the arguments get executed via the system shell, results will
1485 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1488 Using an indirect object with C<exec> or C<system> is also more
1489 secure. This usage (which also works fine with system()) forces
1490 interpretation of the arguments as a multivalued list, even if the
1491 list had just one argument. That way you're safe from the shell
1492 expanding wildcards or splitting up words with whitespace in them.
1494 @args = ( "echo surprise" );
1496 exec @args; # subject to shell escapes
1498 exec { $args[0] } @args; # safe even with one-arg list
1500 The first version, the one without the indirect object, ran the I<echo>
1501 program, passing it C<"surprise"> an argument. The second version
1502 didn't--it tried to run a program literally called I<"echo surprise">,
1503 didn't find it, and set C<$?> to a non-zero value indicating failure.
1505 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1506 output before the exec, but this may not be supported on some platforms
1507 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1508 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1509 open handles in order to avoid lost output.
1511 Note that C<exec> will not call your C<END> blocks, nor will it call
1512 any C<DESTROY> methods in your objects.
1516 Given an expression that specifies a hash element or array element,
1517 returns true if the specified element in the hash or array has ever
1518 been initialized, even if the corresponding value is undefined. The
1519 element is not autovivified if it doesn't exist.
1521 print "Exists\n" if exists $hash{$key};
1522 print "Defined\n" if defined $hash{$key};
1523 print "True\n" if $hash{$key};
1525 print "Exists\n" if exists $array[$index];
1526 print "Defined\n" if defined $array[$index];
1527 print "True\n" if $array[$index];
1529 A hash or array element can be true only if it's defined, and defined if
1530 it exists, but the reverse doesn't necessarily hold true.
1532 Given an expression that specifies the name of a subroutine,
1533 returns true if the specified subroutine has ever been declared, even
1534 if it is undefined. Mentioning a subroutine name for exists or defined
1535 does not count as declaring it. Note that a subroutine which does not
1536 exist may still be callable: its package may have an C<AUTOLOAD>
1537 method that makes it spring into existence the first time that it is
1538 called -- see L<perlsub>.
1540 print "Exists\n" if exists &subroutine;
1541 print "Defined\n" if defined &subroutine;
1543 Note that the EXPR can be arbitrarily complicated as long as the final
1544 operation is a hash or array key lookup or subroutine name:
1546 if (exists $ref->{A}->{B}->{$key}) { }
1547 if (exists $hash{A}{B}{$key}) { }
1549 if (exists $ref->{A}->{B}->[$ix]) { }
1550 if (exists $hash{A}{B}[$ix]) { }
1552 if (exists &{$ref->{A}{B}{$key}}) { }
1554 Although the deepest nested array or hash will not spring into existence
1555 just because its existence was tested, any intervening ones will.
1556 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1557 into existence due to the existence test for the $key element above.
1558 This happens anywhere the arrow operator is used, including even:
1561 if (exists $ref->{"Some key"}) { }
1562 print $ref; # prints HASH(0x80d3d5c)
1564 This surprising autovivification in what does not at first--or even
1565 second--glance appear to be an lvalue context may be fixed in a future
1568 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1569 on how exists() acts when used on a pseudo-hash.
1571 Use of a subroutine call, rather than a subroutine name, as an argument
1572 to exists() is an error.
1575 exists &sub(); # Error
1579 Evaluates EXPR and exits immediately with that value. Example:
1582 exit 0 if $ans =~ /^[Xx]/;
1584 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1585 universally recognized values for EXPR are C<0> for success and C<1>
1586 for error; other values are subject to interpretation depending on the
1587 environment in which the Perl program is running. For example, exiting
1588 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1589 the mailer to return the item undelivered, but that's not true everywhere.
1591 Don't use C<exit> to abort a subroutine if there's any chance that
1592 someone might want to trap whatever error happened. Use C<die> instead,
1593 which can be trapped by an C<eval>.
1595 The exit() function does not always exit immediately. It calls any
1596 defined C<END> routines first, but these C<END> routines may not
1597 themselves abort the exit. Likewise any object destructors that need to
1598 be called are called before the real exit. If this is a problem, you
1599 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1600 See L<perlmod> for details.
1606 Returns I<e> (the natural logarithm base) to the power of EXPR.
1607 If EXPR is omitted, gives C<exp($_)>.
1609 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1611 Implements the fcntl(2) function. You'll probably have to say
1615 first to get the correct constant definitions. Argument processing and
1616 value return works just like C<ioctl> below.
1620 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1621 or die "can't fcntl F_GETFL: $!";
1623 You don't have to check for C<defined> on the return from C<fnctl>.
1624 Like C<ioctl>, it maps a C<0> return from the system call into
1625 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1626 in numeric context. It is also exempt from the normal B<-w> warnings
1627 on improper numeric conversions.
1629 Note that C<fcntl> will produce a fatal error if used on a machine that
1630 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1631 manpage to learn what functions are available on your system.
1633 =item fileno FILEHANDLE
1635 Returns the file descriptor for a filehandle, or undefined if the
1636 filehandle is not open. This is mainly useful for constructing
1637 bitmaps for C<select> and low-level POSIX tty-handling operations.
1638 If FILEHANDLE is an expression, the value is taken as an indirect
1639 filehandle, generally its name.
1641 You can use this to find out whether two handles refer to the
1642 same underlying descriptor:
1644 if (fileno(THIS) == fileno(THAT)) {
1645 print "THIS and THAT are dups\n";
1648 (Filehandles connected to memory objects via new features of C<open> may
1649 return undefined even though they are open.)
1652 =item flock FILEHANDLE,OPERATION
1654 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1655 for success, false on failure. Produces a fatal error if used on a
1656 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1657 C<flock> is Perl's portable file locking interface, although it locks
1658 only entire files, not records.
1660 Two potentially non-obvious but traditional C<flock> semantics are
1661 that it waits indefinitely until the lock is granted, and that its locks
1662 B<merely advisory>. Such discretionary locks are more flexible, but offer
1663 fewer guarantees. This means that files locked with C<flock> may be
1664 modified by programs that do not also use C<flock>. See L<perlport>,
1665 your port's specific documentation, or your system-specific local manpages
1666 for details. It's best to assume traditional behavior if you're writing
1667 portable programs. (But if you're not, you should as always feel perfectly
1668 free to write for your own system's idiosyncrasies (sometimes called
1669 "features"). Slavish adherence to portability concerns shouldn't get
1670 in the way of your getting your job done.)
1672 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1673 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1674 you can use the symbolic names if you import them from the Fcntl module,
1675 either individually, or as a group using the ':flock' tag. LOCK_SH
1676 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1677 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1678 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1679 waiting for the lock (check the return status to see if you got it).
1681 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1682 before locking or unlocking it.
1684 Note that the emulation built with lockf(3) doesn't provide shared
1685 locks, and it requires that FILEHANDLE be open with write intent. These
1686 are the semantics that lockf(3) implements. Most if not all systems
1687 implement lockf(3) in terms of fcntl(2) locking, though, so the
1688 differing semantics shouldn't bite too many people.
1690 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1691 be open with read intent to use LOCK_SH and requires that it be open
1692 with write intent to use LOCK_EX.
1694 Note also that some versions of C<flock> cannot lock things over the
1695 network; you would need to use the more system-specific C<fcntl> for
1696 that. If you like you can force Perl to ignore your system's flock(2)
1697 function, and so provide its own fcntl(2)-based emulation, by passing
1698 the switch C<-Ud_flock> to the F<Configure> program when you configure
1701 Here's a mailbox appender for BSD systems.
1703 use Fcntl ':flock'; # import LOCK_* constants
1706 flock(MBOX,LOCK_EX);
1707 # and, in case someone appended
1708 # while we were waiting...
1713 flock(MBOX,LOCK_UN);
1716 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1717 or die "Can't open mailbox: $!";
1720 print MBOX $msg,"\n\n";
1723 On systems that support a real flock(), locks are inherited across fork()
1724 calls, whereas those that must resort to the more capricious fcntl()
1725 function lose the locks, making it harder to write servers.
1727 See also L<DB_File> for other flock() examples.
1731 Does a fork(2) system call to create a new process running the
1732 same program at the same point. It returns the child pid to the
1733 parent process, C<0> to the child process, or C<undef> if the fork is
1734 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1735 are shared, while everything else is copied. On most systems supporting
1736 fork(), great care has gone into making it extremely efficient (for
1737 example, using copy-on-write technology on data pages), making it the
1738 dominant paradigm for multitasking over the last few decades.
1740 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1741 output before forking the child process, but this may not be supported
1742 on some platforms (see L<perlport>). To be safe, you may need to set
1743 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1744 C<IO::Handle> on any open handles in order to avoid duplicate output.
1746 If you C<fork> without ever waiting on your children, you will
1747 accumulate zombies. On some systems, you can avoid this by setting
1748 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1749 forking and reaping moribund children.
1751 Note that if your forked child inherits system file descriptors like
1752 STDIN and STDOUT that are actually connected by a pipe or socket, even
1753 if you exit, then the remote server (such as, say, a CGI script or a
1754 backgrounded job launched from a remote shell) won't think you're done.
1755 You should reopen those to F</dev/null> if it's any issue.
1759 Declare a picture format for use by the C<write> function. For
1763 Test: @<<<<<<<< @||||| @>>>>>
1764 $str, $%, '$' . int($num)
1768 $num = $cost/$quantity;
1772 See L<perlform> for many details and examples.
1774 =item formline PICTURE,LIST
1776 This is an internal function used by C<format>s, though you may call it,
1777 too. It formats (see L<perlform>) a list of values according to the
1778 contents of PICTURE, placing the output into the format output
1779 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1780 Eventually, when a C<write> is done, the contents of
1781 C<$^A> are written to some filehandle, but you could also read C<$^A>
1782 yourself and then set C<$^A> back to C<"">. Note that a format typically
1783 does one C<formline> per line of form, but the C<formline> function itself
1784 doesn't care how many newlines are embedded in the PICTURE. This means
1785 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1786 You may therefore need to use multiple formlines to implement a single
1787 record format, just like the format compiler.
1789 Be careful if you put double quotes around the picture, because an C<@>
1790 character may be taken to mean the beginning of an array name.
1791 C<formline> always returns true. See L<perlform> for other examples.
1793 =item getc FILEHANDLE
1797 Returns the next character from the input file attached to FILEHANDLE,
1798 or the undefined value at end of file, or if there was an error.
1799 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1800 efficient. However, it cannot be used by itself to fetch single
1801 characters without waiting for the user to hit enter. For that, try
1802 something more like:
1805 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1808 system "stty", '-icanon', 'eol', "\001";
1814 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1817 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1821 Determination of whether $BSD_STYLE should be set
1822 is left as an exercise to the reader.
1824 The C<POSIX::getattr> function can do this more portably on
1825 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1826 module from your nearest CPAN site; details on CPAN can be found on
1831 Implements the C library function of the same name, which on most
1832 systems returns the current login from F</etc/utmp>, if any. If null,
1835 $login = getlogin || getpwuid($<) || "Kilroy";
1837 Do not consider C<getlogin> for authentication: it is not as
1838 secure as C<getpwuid>.
1840 =item getpeername SOCKET
1842 Returns the packed sockaddr address of other end of the SOCKET connection.
1845 $hersockaddr = getpeername(SOCK);
1846 ($port, $iaddr) = sockaddr_in($hersockaddr);
1847 $herhostname = gethostbyaddr($iaddr, AF_INET);
1848 $herstraddr = inet_ntoa($iaddr);
1852 Returns the current process group for the specified PID. Use
1853 a PID of C<0> to get the current process group for the
1854 current process. Will raise an exception if used on a machine that
1855 doesn't implement getpgrp(2). If PID is omitted, returns process
1856 group of current process. Note that the POSIX version of C<getpgrp>
1857 does not accept a PID argument, so only C<PID==0> is truly portable.
1861 Returns the process id of the parent process.
1863 =item getpriority WHICH,WHO
1865 Returns the current priority for a process, a process group, or a user.
1866 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1867 machine that doesn't implement getpriority(2).
1873 =item gethostbyname NAME
1875 =item getnetbyname NAME
1877 =item getprotobyname NAME
1883 =item getservbyname NAME,PROTO
1885 =item gethostbyaddr ADDR,ADDRTYPE
1887 =item getnetbyaddr ADDR,ADDRTYPE
1889 =item getprotobynumber NUMBER
1891 =item getservbyport PORT,PROTO
1909 =item sethostent STAYOPEN
1911 =item setnetent STAYOPEN
1913 =item setprotoent STAYOPEN
1915 =item setservent STAYOPEN
1929 These routines perform the same functions as their counterparts in the
1930 system library. In list context, the return values from the
1931 various get routines are as follows:
1933 ($name,$passwd,$uid,$gid,
1934 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1935 ($name,$passwd,$gid,$members) = getgr*
1936 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1937 ($name,$aliases,$addrtype,$net) = getnet*
1938 ($name,$aliases,$proto) = getproto*
1939 ($name,$aliases,$port,$proto) = getserv*
1941 (If the entry doesn't exist you get a null list.)
1943 The exact meaning of the $gcos field varies but it usually contains
1944 the real name of the user (as opposed to the login name) and other
1945 information pertaining to the user. Beware, however, that in many
1946 system users are able to change this information and therefore it
1947 cannot be trusted and therefore the $gcos is tainted (see
1948 L<perlsec>). The $passwd and $shell, user's encrypted password and
1949 login shell, are also tainted, because of the same reason.
1951 In scalar context, you get the name, unless the function was a
1952 lookup by name, in which case you get the other thing, whatever it is.
1953 (If the entry doesn't exist you get the undefined value.) For example:
1955 $uid = getpwnam($name);
1956 $name = getpwuid($num);
1958 $gid = getgrnam($name);
1959 $name = getgrgid($num;
1963 In I<getpw*()> the fields $quota, $comment, and $expire are special
1964 cases in the sense that in many systems they are unsupported. If the
1965 $quota is unsupported, it is an empty scalar. If it is supported, it
1966 usually encodes the disk quota. If the $comment field is unsupported,
1967 it is an empty scalar. If it is supported it usually encodes some
1968 administrative comment about the user. In some systems the $quota
1969 field may be $change or $age, fields that have to do with password
1970 aging. In some systems the $comment field may be $class. The $expire
1971 field, if present, encodes the expiration period of the account or the
1972 password. For the availability and the exact meaning of these fields
1973 in your system, please consult your getpwnam(3) documentation and your
1974 F<pwd.h> file. You can also find out from within Perl what your
1975 $quota and $comment fields mean and whether you have the $expire field
1976 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1977 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1978 files are only supported if your vendor has implemented them in the
1979 intuitive fashion that calling the regular C library routines gets the
1980 shadow versions if you're running under privilege or if there exists
1981 the shadow(3) functions as found in System V ( this includes Solaris
1982 and Linux.) Those systems which implement a proprietary shadow password
1983 facility are unlikely to be supported.
1985 The $members value returned by I<getgr*()> is a space separated list of
1986 the login names of the members of the group.
1988 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1989 C, it will be returned to you via C<$?> if the function call fails. The
1990 C<@addrs> value returned by a successful call is a list of the raw
1991 addresses returned by the corresponding system library call. In the
1992 Internet domain, each address is four bytes long and you can unpack it
1993 by saying something like:
1995 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1997 The Socket library makes this slightly easier:
2000 $iaddr = inet_aton("127.1"); # or whatever address
2001 $name = gethostbyaddr($iaddr, AF_INET);
2003 # or going the other way
2004 $straddr = inet_ntoa($iaddr);
2006 If you get tired of remembering which element of the return list
2007 contains which return value, by-name interfaces are provided
2008 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2009 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2010 and C<User::grent>. These override the normal built-ins, supplying
2011 versions that return objects with the appropriate names
2012 for each field. For example:
2016 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2018 Even though it looks like they're the same method calls (uid),
2019 they aren't, because a C<File::stat> object is different from
2020 a C<User::pwent> object.
2022 =item getsockname SOCKET
2024 Returns the packed sockaddr address of this end of the SOCKET connection,
2025 in case you don't know the address because you have several different
2026 IPs that the connection might have come in on.
2029 $mysockaddr = getsockname(SOCK);
2030 ($port, $myaddr) = sockaddr_in($mysockaddr);
2031 printf "Connect to %s [%s]\n",
2032 scalar gethostbyaddr($myaddr, AF_INET),
2035 =item getsockopt SOCKET,LEVEL,OPTNAME
2037 Returns the socket option requested, or undef if there is an error.
2043 In list context, returns a (possibly empty) list of filename expansions on
2044 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2045 scalar context, glob iterates through such filename expansions, returning
2046 undef when the list is exhausted. This is the internal function
2047 implementing the C<< <*.c> >> operator, but you can use it directly. If
2048 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2049 more detail in L<perlop/"I/O Operators">.
2051 Beginning with v5.6.0, this operator is implemented using the standard
2052 C<File::Glob> extension. See L<File::Glob> for details.
2056 Converts a time as returned by the time function to an 8-element list
2057 with the time localized for the standard Greenwich time zone.
2058 Typically used as follows:
2061 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2064 All list elements are numeric, and come straight out of the C `struct
2065 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2066 specified time. $mday is the day of the month, and $mon is the month
2067 itself, in the range C<0..11> with 0 indicating January and 11
2068 indicating December. $year is the number of years since 1900. That
2069 is, $year is C<123> in year 2023. $wday is the day of the week, with
2070 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2071 the year, in the range C<0..364> (or C<0..365> in leap years.)
2073 Note that the $year element is I<not> simply the last two digits of
2074 the year. If you assume it is, then you create non-Y2K-compliant
2075 programs--and you wouldn't want to do that, would you?
2077 The proper way to get a complete 4-digit year is simply:
2081 And to get the last two digits of the year (e.g., '01' in 2001) do:
2083 $year = sprintf("%02d", $year % 100);
2085 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2087 In scalar context, C<gmtime()> returns the ctime(3) value:
2089 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2091 Also see the C<timegm> function provided by the C<Time::Local> module,
2092 and the strftime(3) function available via the POSIX module.
2094 This scalar value is B<not> locale dependent (see L<perllocale>), but
2095 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2096 strftime(3) and mktime(3) functions available via the POSIX module. To
2097 get somewhat similar but locale dependent date strings, set up your
2098 locale environment variables appropriately (please see L<perllocale>)
2099 and try for example:
2101 use POSIX qw(strftime);
2102 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2104 Note that the C<%a> and C<%b> escapes, which represent the short forms
2105 of the day of the week and the month of the year, may not necessarily
2106 be three characters wide in all locales.
2114 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2115 execution there. It may not be used to go into any construct that
2116 requires initialization, such as a subroutine or a C<foreach> loop. It
2117 also can't be used to go into a construct that is optimized away,
2118 or to get out of a block or subroutine given to C<sort>.
2119 It can be used to go almost anywhere else within the dynamic scope,
2120 including out of subroutines, but it's usually better to use some other
2121 construct such as C<last> or C<die>. The author of Perl has never felt the
2122 need to use this form of C<goto> (in Perl, that is--C is another matter).
2123 (The difference being that C does not offer named loops combined with
2124 loop control. Perl does, and this replaces most structured uses of C<goto>
2125 in other languages.)
2127 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2128 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2129 necessarily recommended if you're optimizing for maintainability:
2131 goto ("FOO", "BAR", "GLARCH")[$i];
2133 The C<goto-&NAME> form is quite different from the other forms of
2134 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2135 doesn't have the stigma associated with other gotos. Instead, it
2136 exits the current subroutine (losing any changes set by local()) and
2137 immediately calls in its place the named subroutine using the current
2138 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2139 load another subroutine and then pretend that the other subroutine had
2140 been called in the first place (except that any modifications to C<@_>
2141 in the current subroutine are propagated to the other subroutine.)
2142 After the C<goto>, not even C<caller> will be able to tell that this
2143 routine was called first.
2145 NAME needn't be the name of a subroutine; it can be a scalar variable
2146 containing a code reference, or a block which evaluates to a code
2149 =item grep BLOCK LIST
2151 =item grep EXPR,LIST
2153 This is similar in spirit to, but not the same as, grep(1) and its
2154 relatives. In particular, it is not limited to using regular expressions.
2156 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2157 C<$_> to each element) and returns the list value consisting of those
2158 elements for which the expression evaluated to true. In scalar
2159 context, returns the number of times the expression was true.
2161 @foo = grep(!/^#/, @bar); # weed out comments
2165 @foo = grep {!/^#/} @bar; # weed out comments
2167 Note that C<$_> is an alias to the list value, so it can be used to
2168 modify the elements of the LIST. While this is useful and supported,
2169 it can cause bizarre results if the elements of LIST are not variables.
2170 Similarly, grep returns aliases into the original list, much as a for
2171 loop's index variable aliases the list elements. That is, modifying an
2172 element of a list returned by grep (for example, in a C<foreach>, C<map>
2173 or another C<grep>) actually modifies the element in the original list.
2174 This is usually something to be avoided when writing clear code.
2176 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2182 Interprets EXPR as a hex string and returns the corresponding value.
2183 (To convert strings that might start with either 0, 0x, or 0b, see
2184 L</oct>.) If EXPR is omitted, uses C<$_>.
2186 print hex '0xAf'; # prints '175'
2187 print hex 'aF'; # same
2189 Hex strings may only represent integers. Strings that would cause
2190 integer overflow trigger a warning. Leading whitespace is not stripped,
2195 There is no builtin C<import> function. It is just an ordinary
2196 method (subroutine) defined (or inherited) by modules that wish to export
2197 names to another module. The C<use> function calls the C<import> method
2198 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2200 =item index STR,SUBSTR,POSITION
2202 =item index STR,SUBSTR
2204 The index function searches for one string within another, but without
2205 the wildcard-like behavior of a full regular-expression pattern match.
2206 It returns the position of the first occurrence of SUBSTR in STR at
2207 or after POSITION. If POSITION is omitted, starts searching from the
2208 beginning of the string. The return value is based at C<0> (or whatever
2209 you've set the C<$[> variable to--but don't do that). If the substring
2210 is not found, returns one less than the base, ordinarily C<-1>.
2216 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2217 You should not use this function for rounding: one because it truncates
2218 towards C<0>, and two because machine representations of floating point
2219 numbers can sometimes produce counterintuitive results. For example,
2220 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2221 because it's really more like -268.99999999999994315658 instead. Usually,
2222 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2223 functions will serve you better than will int().
2225 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2227 Implements the ioctl(2) function. You'll probably first have to say
2229 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2231 to get the correct function definitions. If F<ioctl.ph> doesn't
2232 exist or doesn't have the correct definitions you'll have to roll your
2233 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2234 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2235 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2236 written depending on the FUNCTION--a pointer to the string value of SCALAR
2237 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2238 has no string value but does have a numeric value, that value will be
2239 passed rather than a pointer to the string value. To guarantee this to be
2240 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2241 functions may be needed to manipulate the values of structures used by
2244 The return value of C<ioctl> (and C<fcntl>) is as follows:
2246 if OS returns: then Perl returns:
2248 0 string "0 but true"
2249 anything else that number
2251 Thus Perl returns true on success and false on failure, yet you can
2252 still easily determine the actual value returned by the operating
2255 $retval = ioctl(...) || -1;
2256 printf "System returned %d\n", $retval;
2258 The special string "C<0> but true" is exempt from B<-w> complaints
2259 about improper numeric conversions.
2261 Here's an example of setting a filehandle named C<REMOTE> to be
2262 non-blocking at the system level. You'll have to negotiate C<$|>
2263 on your own, though.
2265 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2267 $flags = fcntl(REMOTE, F_GETFL, 0)
2268 or die "Can't get flags for the socket: $!\n";
2270 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2271 or die "Can't set flags for the socket: $!\n";
2273 =item join EXPR,LIST
2275 Joins the separate strings of LIST into a single string with fields
2276 separated by the value of EXPR, and returns that new string. Example:
2278 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2280 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2281 first argument. Compare L</split>.
2285 Returns a list consisting of all the keys of the named hash. (In
2286 scalar context, returns the number of keys.) The keys are returned in
2287 an apparently random order. The actual random order is subject to
2288 change in future versions of perl, but it is guaranteed to be the same
2289 order as either the C<values> or C<each> function produces (given
2290 that the hash has not been modified). As a side effect, it resets
2293 Here is yet another way to print your environment:
2296 @values = values %ENV;
2298 print pop(@keys), '=', pop(@values), "\n";
2301 or how about sorted by key:
2303 foreach $key (sort(keys %ENV)) {
2304 print $key, '=', $ENV{$key}, "\n";
2307 The returned values are copies of the original keys in the hash, so
2308 modifying them will not affect the original hash. Compare L</values>.
2310 To sort a hash by value, you'll need to use a C<sort> function.
2311 Here's a descending numeric sort of a hash by its values:
2313 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2314 printf "%4d %s\n", $hash{$key}, $key;
2317 As an lvalue C<keys> allows you to increase the number of hash buckets
2318 allocated for the given hash. This can gain you a measure of efficiency if
2319 you know the hash is going to get big. (This is similar to pre-extending
2320 an array by assigning a larger number to $#array.) If you say
2324 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2325 in fact, since it rounds up to the next power of two. These
2326 buckets will be retained even if you do C<%hash = ()>, use C<undef
2327 %hash> if you want to free the storage while C<%hash> is still in scope.
2328 You can't shrink the number of buckets allocated for the hash using
2329 C<keys> in this way (but you needn't worry about doing this by accident,
2330 as trying has no effect).
2332 See also C<each>, C<values> and C<sort>.
2334 =item kill SIGNAL, LIST
2336 Sends a signal to a list of processes. Returns the number of
2337 processes successfully signaled (which is not necessarily the
2338 same as the number actually killed).
2340 $cnt = kill 1, $child1, $child2;
2343 If SIGNAL is zero, no signal is sent to the process. This is a
2344 useful way to check that the process is alive and hasn't changed
2345 its UID. See L<perlport> for notes on the portability of this
2348 Unlike in the shell, if SIGNAL is negative, it kills
2349 process groups instead of processes. (On System V, a negative I<PROCESS>
2350 number will also kill process groups, but that's not portable.) That
2351 means you usually want to use positive not negative signals. You may also
2352 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2358 The C<last> command is like the C<break> statement in C (as used in
2359 loops); it immediately exits the loop in question. If the LABEL is
2360 omitted, the command refers to the innermost enclosing loop. The
2361 C<continue> block, if any, is not executed:
2363 LINE: while (<STDIN>) {
2364 last LINE if /^$/; # exit when done with header
2368 C<last> cannot be used to exit a block which returns a value such as
2369 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2370 a grep() or map() operation.
2372 Note that a block by itself is semantically identical to a loop
2373 that executes once. Thus C<last> can be used to effect an early
2374 exit out of such a block.
2376 See also L</continue> for an illustration of how C<last>, C<next>, and
2383 Returns a lowercased version of EXPR. This is the internal function
2384 implementing the C<\L> escape in double-quoted strings. Respects
2385 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2386 and L<perlunicode> for more details about locale and Unicode support.
2388 If EXPR is omitted, uses C<$_>.
2394 Returns the value of EXPR with the first character lowercased. This
2395 is the internal function implementing the C<\l> escape in
2396 double-quoted strings. Respects current LC_CTYPE locale if C<use
2397 locale> in force. See L<perllocale> and L<perlunicode> for more
2398 details about locale and Unicode support.
2400 If EXPR is omitted, uses C<$_>.
2406 Returns the length in characters of the value of EXPR. If EXPR is
2407 omitted, returns length of C<$_>. Note that this cannot be used on
2408 an entire array or hash to find out how many elements these have.
2409 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2411 =item link OLDFILE,NEWFILE
2413 Creates a new filename linked to the old filename. Returns true for
2414 success, false otherwise.
2416 =item listen SOCKET,QUEUESIZE
2418 Does the same thing that the listen system call does. Returns true if
2419 it succeeded, false otherwise. See the example in
2420 L<perlipc/"Sockets: Client/Server Communication">.
2424 You really probably want to be using C<my> instead, because C<local> isn't
2425 what most people think of as "local". See
2426 L<perlsub/"Private Variables via my()"> for details.
2428 A local modifies the listed variables to be local to the enclosing
2429 block, file, or eval. If more than one value is listed, the list must
2430 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2431 for details, including issues with tied arrays and hashes.
2433 =item localtime EXPR
2435 Converts a time as returned by the time function to a 9-element list
2436 with the time analyzed for the local time zone. Typically used as
2440 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2443 All list elements are numeric, and come straight out of the C `struct
2444 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2445 specified time. $mday is the day of the month, and $mon is the month
2446 itself, in the range C<0..11> with 0 indicating January and 11
2447 indicating December. $year is the number of years since 1900. That
2448 is, $year is C<123> in year 2023. $wday is the day of the week, with
2449 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2450 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2451 is true if the specified time occurs during daylight savings time,
2454 Note that the $year element is I<not> simply the last two digits of
2455 the year. If you assume it is, then you create non-Y2K-compliant
2456 programs--and you wouldn't want to do that, would you?
2458 The proper way to get a complete 4-digit year is simply:
2462 And to get the last two digits of the year (e.g., '01' in 2001) do:
2464 $year = sprintf("%02d", $year % 100);
2466 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2468 In scalar context, C<localtime()> returns the ctime(3) value:
2470 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2472 This scalar value is B<not> locale dependent, see L<perllocale>, but
2473 instead a Perl builtin. Also see the C<Time::Local> module
2474 (to convert the second, minutes, hours, ... back to seconds since the
2475 stroke of midnight the 1st of January 1970, the value returned by
2476 time()), and the strftime(3) and mktime(3) functions available via the
2477 POSIX module. To get somewhat similar but locale dependent date
2478 strings, set up your locale environment variables appropriately
2479 (please see L<perllocale>) and try for example:
2481 use POSIX qw(strftime);
2482 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2484 Note that the C<%a> and C<%b>, the short forms of the day of the week
2485 and the month of the year, may not necessarily be three characters wide.
2489 This function places an advisory lock on a shared variable, or referenced
2490 object contained in I<THING> until the lock goes out of scope.
2492 lock() is a "weak keyword" : this means that if you've defined a function
2493 by this name (before any calls to it), that function will be called
2494 instead. (However, if you've said C<use threads>, lock() is always a
2495 keyword.) See L<threads>.
2501 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2502 returns log of C<$_>. To get the log of another base, use basic algebra:
2503 The base-N log of a number is equal to the natural log of that number
2504 divided by the natural log of N. For example:
2508 return log($n)/log(10);
2511 See also L</exp> for the inverse operation.
2517 Does the same thing as the C<stat> function (including setting the
2518 special C<_> filehandle) but stats a symbolic link instead of the file
2519 the symbolic link points to. If symbolic links are unimplemented on
2520 your system, a normal C<stat> is done.
2522 If EXPR is omitted, stats C<$_>.
2526 The match operator. See L<perlop>.
2528 =item map BLOCK LIST
2532 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2533 C<$_> to each element) and returns the list value composed of the
2534 results of each such evaluation. In scalar context, returns the
2535 total number of elements so generated. Evaluates BLOCK or EXPR in
2536 list context, so each element of LIST may produce zero, one, or
2537 more elements in the returned value.
2539 @chars = map(chr, @nums);
2541 translates a list of numbers to the corresponding characters. And
2543 %hash = map { getkey($_) => $_ } @array;
2545 is just a funny way to write
2548 foreach $_ (@array) {
2549 $hash{getkey($_)} = $_;
2552 Note that C<$_> is an alias to the list value, so it can be used to
2553 modify the elements of the LIST. While this is useful and supported,
2554 it can cause bizarre results if the elements of LIST are not variables.
2555 Using a regular C<foreach> loop for this purpose would be clearer in
2556 most cases. See also L</grep> for an array composed of those items of
2557 the original list for which the BLOCK or EXPR evaluates to true.
2559 C<{> starts both hash references and blocks, so C<map { ...> could be either
2560 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2561 ahead for the closing C<}> it has to take a guess at which its dealing with
2562 based what it finds just after the C<{>. Usually it gets it right, but if it
2563 doesn't it won't realize something is wrong until it gets to the C<}> and
2564 encounters the missing (or unexpected) comma. The syntax error will be
2565 reported close to the C<}> but you'll need to change something near the C<{>
2566 such as using a unary C<+> to give perl some help:
2568 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2569 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2570 %hash = map { ("\L$_", 1) } @array # this also works
2571 %hash = map { lc($_), 1 } @array # as does this.
2572 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2574 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2576 or to force an anon hash constructor use C<+{>
2578 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2580 and you get list of anonymous hashes each with only 1 entry.
2582 =item mkdir FILENAME,MASK
2584 =item mkdir FILENAME
2586 Creates the directory specified by FILENAME, with permissions
2587 specified by MASK (as modified by C<umask>). If it succeeds it
2588 returns true, otherwise it returns false and sets C<$!> (errno).
2589 If omitted, MASK defaults to 0777.
2591 In general, it is better to create directories with permissive MASK,
2592 and let the user modify that with their C<umask>, than it is to supply
2593 a restrictive MASK and give the user no way to be more permissive.
2594 The exceptions to this rule are when the file or directory should be
2595 kept private (mail files, for instance). The perlfunc(1) entry on
2596 C<umask> discusses the choice of MASK in more detail.
2598 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2599 number of trailing slashes. Some operating and filesystems do not get
2600 this right, so Perl automatically removes all trailing slashes to keep
2603 =item msgctl ID,CMD,ARG
2605 Calls the System V IPC function msgctl(2). You'll probably have to say
2609 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2610 then ARG must be a variable which will hold the returned C<msqid_ds>
2611 structure. Returns like C<ioctl>: the undefined value for error,
2612 C<"0 but true"> for zero, or the actual return value otherwise. See also
2613 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2615 =item msgget KEY,FLAGS
2617 Calls the System V IPC function msgget(2). Returns the message queue
2618 id, or the undefined value if there is an error. See also
2619 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2621 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2623 Calls the System V IPC function msgrcv to receive a message from
2624 message queue ID into variable VAR with a maximum message size of
2625 SIZE. Note that when a message is received, the message type as a
2626 native long integer will be the first thing in VAR, followed by the
2627 actual message. This packing may be opened with C<unpack("l! a*")>.
2628 Taints the variable. Returns true if successful, or false if there is
2629 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2630 C<IPC::SysV::Msg> documentation.
2632 =item msgsnd ID,MSG,FLAGS
2634 Calls the System V IPC function msgsnd to send the message MSG to the
2635 message queue ID. MSG must begin with the native long integer message
2636 type, and be followed by the length of the actual message, and finally
2637 the message itself. This kind of packing can be achieved with
2638 C<pack("l! a*", $type, $message)>. Returns true if successful,
2639 or false if there is an error. See also C<IPC::SysV>
2640 and C<IPC::SysV::Msg> documentation.
2646 =item my EXPR : ATTRS
2648 =item my TYPE EXPR : ATTRS
2650 A C<my> declares the listed variables to be local (lexically) to the
2651 enclosing block, file, or C<eval>. If more than one value is listed,
2652 the list must be placed in parentheses.
2654 The exact semantics and interface of TYPE and ATTRS are still
2655 evolving. TYPE is currently bound to the use of C<fields> pragma,
2656 and attributes are handled using the C<attributes> pragma, or starting
2657 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2658 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2659 L<attributes>, and L<Attribute::Handlers>.
2665 The C<next> command is like the C<continue> statement in C; it starts
2666 the next iteration of the loop:
2668 LINE: while (<STDIN>) {
2669 next LINE if /^#/; # discard comments
2673 Note that if there were a C<continue> block on the above, it would get
2674 executed even on discarded lines. If the LABEL is omitted, the command
2675 refers to the innermost enclosing loop.
2677 C<next> cannot be used to exit a block which returns a value such as
2678 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2679 a grep() or map() operation.
2681 Note that a block by itself is semantically identical to a loop
2682 that executes once. Thus C<next> will exit such a block early.
2684 See also L</continue> for an illustration of how C<last>, C<next>, and
2687 =item no Module VERSION LIST
2689 =item no Module VERSION
2691 =item no Module LIST
2695 See the L</use> function, which C<no> is the opposite of.
2701 Interprets EXPR as an octal string and returns the corresponding
2702 value. (If EXPR happens to start off with C<0x>, interprets it as a
2703 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2704 binary string. Leading whitespace is ignored in all three cases.)
2705 The following will handle decimal, binary, octal, and hex in the standard
2708 $val = oct($val) if $val =~ /^0/;
2710 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2711 in octal), use sprintf() or printf():
2713 $perms = (stat("filename"))[2] & 07777;
2714 $oct_perms = sprintf "%lo", $perms;
2716 The oct() function is commonly used when a string such as C<644> needs
2717 to be converted into a file mode, for example. (Although perl will
2718 automatically convert strings into numbers as needed, this automatic
2719 conversion assumes base 10.)
2721 =item open FILEHANDLE,EXPR
2723 =item open FILEHANDLE,MODE,EXPR
2725 =item open FILEHANDLE,MODE,EXPR,LIST
2727 =item open FILEHANDLE,MODE,REFERENCE
2729 =item open FILEHANDLE
2731 Opens the file whose filename is given by EXPR, and associates it with
2734 (The following is a comprehensive reference to open(): for a gentler
2735 introduction you may consider L<perlopentut>.)
2737 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2738 assigned a reference to a new anonymous filehandle, otherwise if
2739 FILEHANDLE is an expression, its value is used as the name of the real
2740 filehandle wanted. (This is considered a symbolic reference, so C<use
2741 strict 'refs'> should I<not> be in effect.)
2743 If EXPR is omitted, the scalar variable of the same name as the
2744 FILEHANDLE contains the filename. (Note that lexical variables--those
2745 declared with C<my>--will not work for this purpose; so if you're
2746 using C<my>, specify EXPR in your call to open.)
2748 If three or more arguments are specified then the mode of opening and
2749 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2750 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2751 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2752 the file is opened for appending, again being created if necessary.
2754 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2755 indicate that you want both read and write access to the file; thus
2756 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2757 '+>' >> mode would clobber the file first. You can't usually use
2758 either read-write mode for updating textfiles, since they have
2759 variable length records. See the B<-i> switch in L<perlrun> for a
2760 better approach. The file is created with permissions of C<0666>
2761 modified by the process' C<umask> value.
2763 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2764 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2766 In the 2-arguments (and 1-argument) form of the call the mode and
2767 filename should be concatenated (in this order), possibly separated by
2768 spaces. It is possible to omit the mode in these forms if the mode is
2771 If the filename begins with C<'|'>, the filename is interpreted as a
2772 command to which output is to be piped, and if the filename ends with a
2773 C<'|'>, the filename is interpreted as a command which pipes output to
2774 us. See L<perlipc/"Using open() for IPC">
2775 for more examples of this. (You are not allowed to C<open> to a command
2776 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2777 and L<perlipc/"Bidirectional Communication with Another Process">
2780 For three or more arguments if MODE is C<'|-'>, the filename is
2781 interpreted as a command to which output is to be piped, and if MODE
2782 is C<'-|'>, the filename is interpreted as a command which pipes
2783 output to us. In the 2-arguments (and 1-argument) form one should
2784 replace dash (C<'-'>) with the command.
2785 See L<perlipc/"Using open() for IPC"> for more examples of this.
2786 (You are not allowed to C<open> to a command that pipes both in I<and>
2787 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2788 L<perlipc/"Bidirectional Communication"> for alternatives.)
2790 In the three-or-more argument form of pipe opens, if LIST is specified
2791 (extra arguments after the command name) then LIST becomes arguments
2792 to the command invoked if the platform supports it. The meaning of
2793 C<open> with more than three arguments for non-pipe modes is not yet
2794 specified. Experimental "layers" may give extra LIST arguments
2797 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2798 and opening C<< '>-' >> opens STDOUT.
2800 You may use the three-argument form of open to specify
2801 I<I/O disciplines> or IO "layers" to be applied to the handle that affect how the input and output
2802 are processed: (see L<open> and L<PerlIO> for more details).
2805 open(FH, "<:utf8", "file")
2807 will open the UTF-8 encoded file containing Unicode characters,
2808 see L<perluniintro>. (Note that if disciplines are specified in the
2809 three-arg form then default disciplines set by the C<open> pragma are
2812 Open returns nonzero upon success, the undefined value otherwise. If
2813 the C<open> involved a pipe, the return value happens to be the pid of
2816 If you're running Perl on a system that distinguishes between text
2817 files and binary files, then you should check out L</binmode> for tips
2818 for dealing with this. The key distinction between systems that need
2819 C<binmode> and those that don't is their text file formats. Systems
2820 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2821 character, and which encode that character in C as C<"\n">, do not
2822 need C<binmode>. The rest need it.
2824 When opening a file, it's usually a bad idea to continue normal execution
2825 if the request failed, so C<open> is frequently used in connection with
2826 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2827 where you want to make a nicely formatted error message (but there are
2828 modules that can help with that problem)) you should always check
2829 the return value from opening a file. The infrequent exception is when
2830 working with an unopened filehandle is actually what you want to do.
2832 As a special case the 3 arg form with a read/write mode and the third
2833 argument being C<undef>:
2835 open(TMP, "+>", undef) or die ...
2837 opens a filehandle to an anonymous temporary file.
2839 File handles can be opened to "in memory" files held in Perl scalars via:
2841 open($fh, '>', \$variable) || ..
2843 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2844 file, you have to close it first:
2847 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2852 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2853 while (<ARTICLE>) {...
2855 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2856 # if the open fails, output is discarded
2858 open(DBASE, '+<', 'dbase.mine') # open for update
2859 or die "Can't open 'dbase.mine' for update: $!";
2861 open(DBASE, '+<dbase.mine') # ditto
2862 or die "Can't open 'dbase.mine' for update: $!";
2864 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2865 or die "Can't start caesar: $!";
2867 open(ARTICLE, "caesar <$article |") # ditto
2868 or die "Can't start caesar: $!";
2870 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2871 or die "Can't start sort: $!";
2874 open(MEMORY,'>', \$var)
2875 or die "Can't open memory file: $!";
2876 print MEMORY "foo!\n"; # output will end up in $var
2878 # process argument list of files along with any includes
2880 foreach $file (@ARGV) {
2881 process($file, 'fh00');
2885 my($filename, $input) = @_;
2886 $input++; # this is a string increment
2887 unless (open($input, $filename)) {
2888 print STDERR "Can't open $filename: $!\n";
2893 while (<$input>) { # note use of indirection
2894 if (/^#include "(.*)"/) {
2895 process($1, $input);
2902 You may also, in the Bourne shell tradition, specify an EXPR beginning
2903 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2904 name of a filehandle (or file descriptor, if numeric) to be
2905 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2906 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2907 mode you specify should match the mode of the original filehandle.
2908 (Duping a filehandle does not take into account any existing contents of
2909 IO buffers.) If you use the 3 arg form then you can pass either a number,
2910 the name of a filehandle or the normal "reference to a glob".
2912 Here is a script that saves, redirects, and restores C<STDOUT> and
2913 C<STDERR> using various methods:
2916 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2917 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2919 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2920 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2922 select STDERR; $| = 1; # make unbuffered
2923 select STDOUT; $| = 1; # make unbuffered
2925 print STDOUT "stdout 1\n"; # this works for
2926 print STDERR "stderr 1\n"; # subprocesses too
2931 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2932 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2934 print STDOUT "stdout 2\n";
2935 print STDERR "stderr 2\n";
2937 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2938 do an equivalent of C's C<fdopen> of that file descriptor; this is
2939 more parsimonious of file descriptors. For example:
2941 open(FILEHANDLE, "<&=$fd")
2945 open(FILEHANDLE, "<&=", $fd)
2947 Note that if Perl is using the standard C libraries' fdopen() then on
2948 many UNIX systems, fdopen() is known to fail when file descriptors
2949 exceed a certain value, typically 255. If you need more file
2950 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2952 You can see whether Perl has been compiled with PerlIO or not by
2953 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2954 is C<define>, you have PerlIO, otherwise you don't.
2956 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2957 with 2-arguments (or 1-argument) form of open(), then
2958 there is an implicit fork done, and the return value of open is the pid
2959 of the child within the parent process, and C<0> within the child
2960 process. (Use C<defined($pid)> to determine whether the open was successful.)
2961 The filehandle behaves normally for the parent, but i/o to that
2962 filehandle is piped from/to the STDOUT/STDIN of the child process.
2963 In the child process the filehandle isn't opened--i/o happens from/to
2964 the new STDOUT or STDIN. Typically this is used like the normal
2965 piped open when you want to exercise more control over just how the
2966 pipe command gets executed, such as when you are running setuid, and
2967 don't want to have to scan shell commands for metacharacters.
2968 The following triples are more or less equivalent:
2970 open(FOO, "|tr '[a-z]' '[A-Z]'");
2971 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2972 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2973 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2975 open(FOO, "cat -n '$file'|");
2976 open(FOO, '-|', "cat -n '$file'");
2977 open(FOO, '-|') || exec 'cat', '-n', $file;
2978 open(FOO, '-|', "cat", '-n', $file);
2980 The last example in each block shows the pipe as "list form", which is
2981 not yet supported on all platforms. A good rule of thumb is that if
2982 your platform has true C<fork()> (in other words, if your platform is
2983 UNIX) you can use the list form.
2985 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2987 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2988 output before any operation that may do a fork, but this may not be
2989 supported on some platforms (see L<perlport>). To be safe, you may need
2990 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2991 of C<IO::Handle> on any open handles.
2993 On systems that support a close-on-exec flag on files, the flag will
2994 be set for the newly opened file descriptor as determined by the value
2995 of $^F. See L<perlvar/$^F>.
2997 Closing any piped filehandle causes the parent process to wait for the
2998 child to finish, and returns the status value in C<$?>.
3000 The filename passed to 2-argument (or 1-argument) form of open() will
3001 have leading and trailing whitespace deleted, and the normal
3002 redirection characters honored. This property, known as "magic open",
3003 can often be used to good effect. A user could specify a filename of
3004 F<"rsh cat file |">, or you could change certain filenames as needed:
3006 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3007 open(FH, $filename) or die "Can't open $filename: $!";
3009 Use 3-argument form to open a file with arbitrary weird characters in it,
3011 open(FOO, '<', $file);
3013 otherwise it's necessary to protect any leading and trailing whitespace:
3015 $file =~ s#^(\s)#./$1#;
3016 open(FOO, "< $file\0");
3018 (this may not work on some bizarre filesystems). One should
3019 conscientiously choose between the I<magic> and 3-arguments form
3024 will allow the user to specify an argument of the form C<"rsh cat file |">,
3025 but will not work on a filename which happens to have a trailing space, while
3027 open IN, '<', $ARGV[0];
3029 will have exactly the opposite restrictions.
3031 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3032 should use the C<sysopen> function, which involves no such magic (but
3033 may use subtly different filemodes than Perl open(), which is mapped
3034 to C fopen()). This is
3035 another way to protect your filenames from interpretation. For example:
3038 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3039 or die "sysopen $path: $!";
3040 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3041 print HANDLE "stuff $$\n";
3043 print "File contains: ", <HANDLE>;
3045 Using the constructor from the C<IO::Handle> package (or one of its
3046 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3047 filehandles that have the scope of whatever variables hold references to
3048 them, and automatically close whenever and however you leave that scope:
3052 sub read_myfile_munged {
3054 my $handle = new IO::File;
3055 open($handle, "myfile") or die "myfile: $!";
3057 or return (); # Automatically closed here.
3058 mung $first or die "mung failed"; # Or here.
3059 return $first, <$handle> if $ALL; # Or here.
3063 See L</seek> for some details about mixing reading and writing.
3065 =item opendir DIRHANDLE,EXPR
3067 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3068 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3069 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3075 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3076 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3079 For the reverse, see L</chr>.
3080 See L<perlunicode> and L<encoding> for more about Unicode.
3086 =item our EXPR : ATTRS
3088 =item our TYPE EXPR : ATTRS
3090 An C<our> declares the listed variables to be valid globals within
3091 the enclosing block, file, or C<eval>. That is, it has the same
3092 scoping rules as a "my" declaration, but does not create a local
3093 variable. If more than one value is listed, the list must be placed
3094 in parentheses. The C<our> declaration has no semantic effect unless
3095 "use strict vars" is in effect, in which case it lets you use the
3096 declared global variable without qualifying it with a package name.
3097 (But only within the lexical scope of the C<our> declaration. In this
3098 it differs from "use vars", which is package scoped.)
3100 An C<our> declaration declares a global variable that will be visible
3101 across its entire lexical scope, even across package boundaries. The
3102 package in which the variable is entered is determined at the point
3103 of the declaration, not at the point of use. This means the following
3107 our $bar; # declares $Foo::bar for rest of lexical scope
3111 print $bar; # prints 20
3113 Multiple C<our> declarations in the same lexical scope are allowed
3114 if they are in different packages. If they happened to be in the same
3115 package, Perl will emit warnings if you have asked for them.
3119 our $bar; # declares $Foo::bar for rest of lexical scope
3123 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3124 print $bar; # prints 30
3126 our $bar; # emits warning
3128 An C<our> declaration may also have a list of attributes associated
3131 The exact semantics and interface of TYPE and ATTRS are still
3132 evolving. TYPE is currently bound to the use of C<fields> pragma,
3133 and attributes are handled using the C<attributes> pragma, or starting
3134 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3135 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3136 L<attributes>, and L<Attribute::Handlers>.
3138 The only currently recognized C<our()> attribute is C<unique> which
3139 indicates that a single copy of the global is to be used by all
3140 interpreters should the program happen to be running in a
3141 multi-interpreter environment. (The default behaviour would be for
3142 each interpreter to have its own copy of the global.) Examples:
3144 our @EXPORT : unique = qw(foo);
3145 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3146 our $VERSION : unique = "1.00";
3148 Note that this attribute also has the effect of making the global
3149 readonly when the first new interpreter is cloned (for example,
3150 when the first new thread is created).
3152 Multi-interpreter environments can come to being either through the
3153 fork() emulation on Windows platforms, or by embedding perl in a
3154 multi-threaded application. The C<unique> attribute does nothing in
3155 all other environments.
3157 =item pack TEMPLATE,LIST
3159 Takes a LIST of values and converts it into a string using the rules
3160 given by the TEMPLATE. The resulting string is the concatenation of
3161 the converted values. Typically, each converted value looks
3162 like its machine-level representation. For example, on 32-bit machines
3163 a converted integer may be represented by a sequence of 4 bytes.
3165 The TEMPLATE is a sequence of characters that give the order and type
3166 of values, as follows:
3168 a A string with arbitrary binary data, will be null padded.
3169 A A text (ASCII) string, will be space padded.
3170 Z A null terminated (ASCIZ) string, will be null padded.
3172 b A bit string (ascending bit order inside each byte, like vec()).
3173 B A bit string (descending bit order inside each byte).
3174 h A hex string (low nybble first).
3175 H A hex string (high nybble first).
3177 c A signed char value.
3178 C An unsigned char value. Only does bytes. See U for Unicode.
3180 s A signed short value.
3181 S An unsigned short value.
3182 (This 'short' is _exactly_ 16 bits, which may differ from
3183 what a local C compiler calls 'short'. If you want
3184 native-length shorts, use the '!' suffix.)
3186 i A signed integer value.
3187 I An unsigned integer value.
3188 (This 'integer' is _at_least_ 32 bits wide. Its exact
3189 size depends on what a local C compiler calls 'int',
3190 and may even be larger than the 'long' described in
3193 l A signed long value.
3194 L An unsigned long value.
3195 (This 'long' is _exactly_ 32 bits, which may differ from
3196 what a local C compiler calls 'long'. If you want
3197 native-length longs, use the '!' suffix.)
3199 n An unsigned short in "network" (big-endian) order.
3200 N An unsigned long in "network" (big-endian) order.
3201 v An unsigned short in "VAX" (little-endian) order.
3202 V An unsigned long in "VAX" (little-endian) order.
3203 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3204 _exactly_ 32 bits, respectively.)
3206 q A signed quad (64-bit) value.
3207 Q An unsigned quad value.
3208 (Quads are available only if your system supports 64-bit
3209 integer values _and_ if Perl has been compiled to support those.
3210 Causes a fatal error otherwise.)
3212 j A signed integer value (a Perl internal integer, IV).
3213 J An unsigned integer value (a Perl internal unsigned integer, UV).
3215 f A single-precision float in the native format.
3216 d A double-precision float in the native format.
3218 F A floating point value in the native native format
3219 (a Perl internal floating point value, NV).
3220 D A long double-precision float in the native format.
3221 (Long doubles are available only if your system supports long
3222 double values _and_ if Perl has been compiled to support those.
3223 Causes a fatal error otherwise.)
3225 p A pointer to a null-terminated string.
3226 P A pointer to a structure (fixed-length string).
3228 u A uuencoded string.
3229 U A Unicode character number. Encodes to UTF-8 internally
3230 (or UTF-EBCDIC in EBCDIC platforms).
3232 w A BER compressed integer. Its bytes represent an unsigned
3233 integer in base 128, most significant digit first, with as
3234 few digits as possible. Bit eight (the high bit) is set
3235 on each byte except the last.
3239 @ Null fill to absolute position.
3240 ( Start of a ()-group.
3242 The following rules apply:
3248 Each letter may optionally be followed by a number giving a repeat
3249 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3250 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3251 many values from the LIST. A C<*> for the repeat count means to use
3252 however many items are left, except for C<@>, C<x>, C<X>, where it is
3253 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3254 is the same). A numeric repeat count may optionally be enclosed in
3255 brackets, as in C<pack 'C[80]', @arr>.
3257 One can replace the numeric repeat count by a template enclosed in brackets;
3258 then the packed length of this template in bytes is used as a count.
3259 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3260 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3261 If the template in brackets contains alignment commands (such as C<x![d]>),
3262 its packed length is calculated as if the start of the template has the maximal
3265 When used with C<Z>, C<*> results in the addition of a trailing null
3266 byte (so the packed result will be one longer than the byte C<length>
3269 The repeat count for C<u> is interpreted as the maximal number of bytes
3270 to encode per line of output, with 0 and 1 replaced by 45.
3274 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3275 string of length count, padding with nulls or spaces as necessary. When
3276 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3277 after the first null, and C<a> returns data verbatim. When packing,
3278 C<a>, and C<Z> are equivalent.
3280 If the value-to-pack is too long, it is truncated. If too long and an
3281 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3282 by a null byte. Thus C<Z> always packs a trailing null byte under
3287 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3288 Each byte of the input field of pack() generates 1 bit of the result.
3289 Each result bit is based on the least-significant bit of the corresponding
3290 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3291 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3293 Starting from the beginning of the input string of pack(), each 8-tuple
3294 of bytes is converted to 1 byte of output. With format C<b>
3295 the first byte of the 8-tuple determines the least-significant bit of a
3296 byte, and with format C<B> it determines the most-significant bit of
3299 If the length of the input string is not exactly divisible by 8, the
3300 remainder is packed as if the input string were padded by null bytes
3301 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3303 If the input string of pack() is longer than needed, extra bytes are ignored.
3304 A C<*> for the repeat count of pack() means to use all the bytes of
3305 the input field. On unpack()ing the bits are converted to a string
3306 of C<"0">s and C<"1">s.
3310 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3311 representable as hexadecimal digits, 0-9a-f) long.
3313 Each byte of the input field of pack() generates 4 bits of the result.
3314 For non-alphabetical bytes the result is based on the 4 least-significant
3315 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3316 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3317 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3318 is compatible with the usual hexadecimal digits, so that C<"a"> and
3319 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3320 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3322 Starting from the beginning of the input string of pack(), each pair
3323 of bytes is converted to 1 byte of output. With format C<h> the
3324 first byte of the pair determines the least-significant nybble of the
3325 output byte, and with format C<H> it determines the most-significant
3328 If the length of the input string is not even, it behaves as if padded
3329 by a null byte at the end. Similarly, during unpack()ing the "extra"
3330 nybbles are ignored.
3332 If the input string of pack() is longer than needed, extra bytes are ignored.
3333 A C<*> for the repeat count of pack() means to use all the bytes of
3334 the input field. On unpack()ing the bits are converted to a string
3335 of hexadecimal digits.
3339 The C<p> type packs a pointer to a null-terminated string. You are
3340 responsible for ensuring the string is not a temporary value (which can
3341 potentially get deallocated before you get around to using the packed result).
3342 The C<P> type packs a pointer to a structure of the size indicated by the
3343 length. A NULL pointer is created if the corresponding value for C<p> or
3344 C<P> is C<undef>, similarly for unpack().
3348 The C</> template character allows packing and unpacking of strings where
3349 the packed structure contains a byte count followed by the string itself.
3350 You write I<length-item>C</>I<string-item>.
3352 The I<length-item> can be any C<pack> template letter, and describes
3353 how the length value is packed. The ones likely to be of most use are
3354 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3355 SNMP) and C<N> (for Sun XDR).
3357 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3358 For C<unpack> the length of the string is obtained from the I<length-item>,
3359 but if you put in the '*' it will be ignored.
3361 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3362 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3363 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3365 The I<length-item> is not returned explicitly from C<unpack>.
3367 Adding a count to the I<length-item> letter is unlikely to do anything
3368 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3369 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3370 which Perl does not regard as legal in numeric strings.
3374 The integer types C<s>, C<S>, C<l>, and C<L> may be
3375 immediately followed by a C<!> suffix to signify native shorts or
3376 longs--as you can see from above for example a bare C<l> does mean
3377 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3378 may be larger. This is an issue mainly in 64-bit platforms. You can
3379 see whether using C<!> makes any difference by
3381 print length(pack("s")), " ", length(pack("s!")), "\n";
3382 print length(pack("l")), " ", length(pack("l!")), "\n";
3384 C<i!> and C<I!> also work but only because of completeness;
3385 they are identical to C<i> and C<I>.
3387 The actual sizes (in bytes) of native shorts, ints, longs, and long
3388 longs on the platform where Perl was built are also available via
3392 print $Config{shortsize}, "\n";
3393 print $Config{intsize}, "\n";
3394 print $Config{longsize}, "\n";
3395 print $Config{longlongsize}, "\n";
3397 (The C<$Config{longlongsize}> will be undefine if your system does
3398 not support long longs.)
3402 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3403 are inherently non-portable between processors and operating systems
3404 because they obey the native byteorder and endianness. For example a
3405 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3406 (arranged in and handled by the CPU registers) into bytes as
3408 0x12 0x34 0x56 0x78 # big-endian
3409 0x78 0x56 0x34 0x12 # little-endian
3411 Basically, the Intel and VAX CPUs are little-endian, while everybody
3412 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3413 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3414 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3417 The names `big-endian' and `little-endian' are comic references to
3418 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3419 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3420 the egg-eating habits of the Lilliputians.
3422 Some systems may have even weirder byte orders such as
3427 You can see your system's preference with
3429 print join(" ", map { sprintf "%#02x", $_ }
3430 unpack("C*",pack("L",0x12345678))), "\n";
3432 The byteorder on the platform where Perl was built is also available
3436 print $Config{byteorder}, "\n";
3438 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3439 and C<'87654321'> are big-endian.
3441 If you want portable packed integers use the formats C<n>, C<N>,
3442 C<v>, and C<V>, their byte endianness and size are known.
3443 See also L<perlport>.
3447 Real numbers (floats and doubles) are in the native machine format only;
3448 due to the multiplicity of floating formats around, and the lack of a
3449 standard "network" representation, no facility for interchange has been
3450 made. This means that packed floating point data written on one machine
3451 may not be readable on another - even if both use IEEE floating point
3452 arithmetic (as the endian-ness of the memory representation is not part
3453 of the IEEE spec). See also L<perlport>.
3455 Note that Perl uses doubles internally for all numeric calculation, and
3456 converting from double into float and thence back to double again will
3457 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3462 If the pattern begins with a C<U>, the resulting string will be treated
3463 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3464 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3465 characters. If you don't want this to happen, you can begin your pattern
3466 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3467 string, and then follow this with a C<U*> somewhere in your pattern.
3471 You must yourself do any alignment or padding by inserting for example
3472 enough C<'x'>es while packing. There is no way to pack() and unpack()
3473 could know where the bytes are going to or coming from. Therefore
3474 C<pack> (and C<unpack>) handle their output and input as flat
3479 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3480 take a repeat count, both as postfix, and via the C</> template
3485 C<x> and C<X> accept C<!> modifier. In this case they act as
3486 alignment commands: they jump forward/back to the closest position
3487 aligned at a multiple of C<count> bytes. For example, to pack() or
3488 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3489 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3490 aligned on the double's size.
3492 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3493 both result in no-ops.
3497 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3501 If TEMPLATE requires more arguments to pack() than actually given, pack()
3502 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3503 to pack() than actually given, extra arguments are ignored.
3509 $foo = pack("CCCC",65,66,67,68);
3511 $foo = pack("C4",65,66,67,68);
3513 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3514 # same thing with Unicode circled letters
3516 $foo = pack("ccxxcc",65,66,67,68);
3519 # note: the above examples featuring "C" and "c" are true
3520 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3521 # and UTF-8. In EBCDIC the first example would be
3522 # $foo = pack("CCCC",193,194,195,196);
3524 $foo = pack("s2",1,2);
3525 # "\1\0\2\0" on little-endian
3526 # "\0\1\0\2" on big-endian
3528 $foo = pack("a4","abcd","x","y","z");
3531 $foo = pack("aaaa","abcd","x","y","z");
3534 $foo = pack("a14","abcdefg");
3535 # "abcdefg\0\0\0\0\0\0\0"
3537 $foo = pack("i9pl", gmtime);
3538 # a real struct tm (on my system anyway)
3540 $utmp_template = "Z8 Z8 Z16 L";
3541 $utmp = pack($utmp_template, @utmp1);
3542 # a struct utmp (BSDish)
3544 @utmp2 = unpack($utmp_template, $utmp);
3545 # "@utmp1" eq "@utmp2"
3548 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3551 $foo = pack('sx2l', 12, 34);
3552 # short 12, two zero bytes padding, long 34
3553 $bar = pack('s@4l', 12, 34);
3554 # short 12, zero fill to position 4, long 34
3557 The same template may generally also be used in unpack().
3559 =item package NAMESPACE
3563 Declares the compilation unit as being in the given namespace. The scope
3564 of the package declaration is from the declaration itself through the end
3565 of the enclosing block, file, or eval (the same as the C<my> operator).
3566 All further unqualified dynamic identifiers will be in this namespace.
3567 A package statement affects only dynamic variables--including those
3568 you've used C<local> on--but I<not> lexical variables, which are created
3569 with C<my>. Typically it would be the first declaration in a file to
3570 be included by the C<require> or C<use> operator. You can switch into a
3571 package in more than one place; it merely influences which symbol table
3572 is used by the compiler for the rest of that block. You can refer to
3573 variables and filehandles in other packages by prefixing the identifier
3574 with the package name and a double colon: C<$Package::Variable>.
3575 If the package name is null, the C<main> package as assumed. That is,
3576 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3577 still seen in older code).
3579 If NAMESPACE is omitted, then there is no current package, and all
3580 identifiers must be fully qualified or lexicals. However, you are
3581 strongly advised not to make use of this feature. Its use can cause
3582 unexpected behaviour, even crashing some versions of Perl. It is
3583 deprecated, and will be removed from a future release.
3585 See L<perlmod/"Packages"> for more information about packages, modules,
3586 and classes. See L<perlsub> for other scoping issues.
3588 =item pipe READHANDLE,WRITEHANDLE
3590 Opens a pair of connected pipes like the corresponding system call.
3591 Note that if you set up a loop of piped processes, deadlock can occur
3592 unless you are very careful. In addition, note that Perl's pipes use
3593 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3594 after each command, depending on the application.
3596 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3597 for examples of such things.
3599 On systems that support a close-on-exec flag on files, the flag will be set
3600 for the newly opened file descriptors as determined by the value of $^F.
3607 Pops and returns the last value of the array, shortening the array by
3608 one element. Has an effect similar to
3612 If there are no elements in the array, returns the undefined value
3613 (although this may happen at other times as well). If ARRAY is
3614 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3615 array in subroutines, just like C<shift>.
3621 Returns the offset of where the last C<m//g> search left off for the variable
3622 in question (C<$_> is used when the variable is not specified). May be
3623 modified to change that offset. Such modification will also influence
3624 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3627 =item print FILEHANDLE LIST
3633 Prints a string or a list of strings. Returns true if successful.
3634 FILEHANDLE may be a scalar variable name, in which case the variable
3635 contains the name of or a reference to the filehandle, thus introducing
3636 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3637 the next token is a term, it may be misinterpreted as an operator
3638 unless you interpose a C<+> or put parentheses around the arguments.)
3639 If FILEHANDLE is omitted, prints by default to standard output (or
3640 to the last selected output channel--see L</select>). If LIST is
3641 also omitted, prints C<$_> to the currently selected output channel.
3642 To set the default output channel to something other than STDOUT
3643 use the select operation. The current value of C<$,> (if any) is
3644 printed between each LIST item. The current value of C<$\> (if
3645 any) is printed after the entire LIST has been printed. Because
3646 print takes a LIST, anything in the LIST is evaluated in list
3647 context, and any subroutine that you call will have one or more of
3648 its expressions evaluated in list context. Also be careful not to
3649 follow the print keyword with a left parenthesis unless you want
3650 the corresponding right parenthesis to terminate the arguments to
3651 the print--interpose a C<+> or put parentheses around all the
3654 Note that if you're storing FILEHANDLES in an array or other expression,
3655 you will have to use a block returning its value instead:
3657 print { $files[$i] } "stuff\n";
3658 print { $OK ? STDOUT : STDERR } "stuff\n";
3660 =item printf FILEHANDLE FORMAT, LIST
3662 =item printf FORMAT, LIST
3664 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3665 (the output record separator) is not appended. The first argument
3666 of the list will be interpreted as the C<printf> format. See C<sprintf>
3667 for an explanation of the format argument. If C<use locale> is in effect,
3668 the character used for the decimal point in formatted real numbers is
3669 affected by the LC_NUMERIC locale. See L<perllocale>.
3671 Don't fall into the trap of using a C<printf> when a simple
3672 C<print> would do. The C<print> is more efficient and less
3675 =item prototype FUNCTION
3677 Returns the prototype of a function as a string (or C<undef> if the
3678 function has no prototype). FUNCTION is a reference to, or the name of,
3679 the function whose prototype you want to retrieve.
3681 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3682 name for Perl builtin. If the builtin is not I<overridable> (such as
3683 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3684 C<system>) returns C<undef> because the builtin does not really behave
3685 like a Perl function. Otherwise, the string describing the equivalent
3686 prototype is returned.
3688 =item push ARRAY,LIST
3690 Treats ARRAY as a stack, and pushes the values of LIST
3691 onto the end of ARRAY. The length of ARRAY increases by the length of
3692 LIST. Has the same effect as
3695 $ARRAY[++$#ARRAY] = $value;
3698 but is more efficient. Returns the new number of elements in the array.
3710 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3712 =item quotemeta EXPR
3716 Returns the value of EXPR with all non-"word"
3717 characters backslashed. (That is, all characters not matching
3718 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3719 returned string, regardless of any locale settings.)
3720 This is the internal function implementing
3721 the C<\Q> escape in double-quoted strings.
3723 If EXPR is omitted, uses C<$_>.
3729 Returns a random fractional number greater than or equal to C<0> and less
3730 than the value of EXPR. (EXPR should be positive.) If EXPR is
3731 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3732 unless C<srand> has already been called. See also C<srand>.
3734 Apply C<int()> to the value returned by C<rand()> if you want random
3735 integers instead of random fractional numbers. For example,
3739 returns a random integer between C<0> and C<9>, inclusive.
3741 (Note: If your rand function consistently returns numbers that are too
3742 large or too small, then your version of Perl was probably compiled
3743 with the wrong number of RANDBITS.)
3745 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3747 =item read FILEHANDLE,SCALAR,LENGTH
3749 Attempts to read LENGTH I<characters> of data into variable SCALAR
3750 from the specified FILEHANDLE. Returns the number of characters
3751 actually read, C<0> at end of file, or undef if there was an error.
3752 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3753 needs growing, the new bytes will be zero bytes. An OFFSET may be
3754 specified to place the read data into some other place in SCALAR than
3755 the beginning. The call is actually implemented in terms of either
3756 Perl's or system's fread() call. To get a true read(2) system call,
3759 Note the I<characters>: depending on the status of the filehandle,
3760 either (8-bit) bytes or characters are read. By default all
3761 filehandles operate on bytes, but for example if the filehandle has
3762 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3763 pragma, L<open>), the I/O will operate on characters, not bytes.
3765 =item readdir DIRHANDLE
3767 Returns the next directory entry for a directory opened by C<opendir>.
3768 If used in list context, returns all the rest of the entries in the
3769 directory. If there are no more entries, returns an undefined value in
3770 scalar context or a null list in list context.
3772 If you're planning to filetest the return values out of a C<readdir>, you'd
3773 better prepend the directory in question. Otherwise, because we didn't
3774 C<chdir> there, it would have been testing the wrong file.
3776 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3777 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3782 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3783 context, each call reads and returns the next line, until end-of-file is
3784 reached, whereupon the subsequent call returns undef. In list context,
3785 reads until end-of-file is reached and returns a list of lines. Note that
3786 the notion of "line" used here is however you may have defined it
3787 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3789 When C<$/> is set to C<undef>, when readline() is in scalar
3790 context (i.e. file slurp mode), and when an empty file is read, it
3791 returns C<''> the first time, followed by C<undef> subsequently.
3793 This is the internal function implementing the C<< <EXPR> >>
3794 operator, but you can use it directly. The C<< <EXPR> >>
3795 operator is discussed in more detail in L<perlop/"I/O Operators">.
3798 $line = readline(*STDIN); # same thing
3804 Returns the value of a symbolic link, if symbolic links are
3805 implemented. If not, gives a fatal error. If there is some system
3806 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3807 omitted, uses C<$_>.
3811 EXPR is executed as a system command.
3812 The collected standard output of the command is returned.
3813 In scalar context, it comes back as a single (potentially
3814 multi-line) string. In list context, returns a list of lines
3815 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3816 This is the internal function implementing the C<qx/EXPR/>
3817 operator, but you can use it directly. The C<qx/EXPR/>
3818 operator is discussed in more detail in L<perlop/"I/O Operators">.
3820 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3822 Receives a message on a socket. Attempts to receive LENGTH characters
3823 of data into variable SCALAR from the specified SOCKET filehandle.
3824 SCALAR will be grown or shrunk to the length actually read. Takes the
3825 same flags as the system call of the same name. Returns the address
3826 of the sender if SOCKET's protocol supports this; returns an empty
3827 string otherwise. If there's an error, returns the undefined value.
3828 This call is actually implemented in terms of recvfrom(2) system call.
3829 See L<perlipc/"UDP: Message Passing"> for examples.
3831 Note the I<characters>: depending on the status of the socket, either
3832 (8-bit) bytes or characters are received. By default all sockets
3833 operate on bytes, but for example if the socket has been changed using
3834 binmode() to operate with the C<:utf8> discipline (see the C<open>
3835 pragma, L<open>), the I/O will operate on characters, not bytes.
3841 The C<redo> command restarts the loop block without evaluating the
3842 conditional again. The C<continue> block, if any, is not executed. If
3843 the LABEL is omitted, the command refers to the innermost enclosing
3844 loop. This command is normally used by programs that want to lie to
3845 themselves about what was just input:
3847 # a simpleminded Pascal comment stripper
3848 # (warning: assumes no { or } in strings)
3849 LINE: while (<STDIN>) {
3850 while (s|({.*}.*){.*}|$1 |) {}
3855 if (/}/) { # end of comment?
3864 C<redo> cannot be used to retry a block which returns a value such as
3865 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3866 a grep() or map() operation.
3868 Note that a block by itself is semantically identical to a loop
3869 that executes once. Thus C<redo> inside such a block will effectively
3870 turn it into a looping construct.
3872 See also L</continue> for an illustration of how C<last>, C<next>, and
3879 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3880 is not specified, C<$_> will be used. The value returned depends on the
3881 type of thing the reference is a reference to.
3882 Builtin types include:
3892 If the referenced object has been blessed into a package, then that package
3893 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3895 if (ref($r) eq "HASH") {
3896 print "r is a reference to a hash.\n";
3899 print "r is not a reference at all.\n";
3901 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3902 print "r is a reference to something that isa hash.\n";
3905 See also L<perlref>.
3907 =item rename OLDNAME,NEWNAME
3909 Changes the name of a file; an existing file NEWNAME will be
3910 clobbered. Returns true for success, false otherwise.
3912 Behavior of this function varies wildly depending on your system
3913 implementation. For example, it will usually not work across file system
3914 boundaries, even though the system I<mv> command sometimes compensates
3915 for this. Other restrictions include whether it works on directories,
3916 open files, or pre-existing files. Check L<perlport> and either the
3917 rename(2) manpage or equivalent system documentation for details.
3919 =item require VERSION
3925 Demands a version of Perl specified by VERSION, or demands some semantics
3926 specified by EXPR or by C<$_> if EXPR is not supplied.
3928 VERSION may be either a numeric argument such as 5.006, which will be
3929 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3930 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3931 VERSION is greater than the version of the current Perl interpreter.
3932 Compare with L</use>, which can do a similar check at compile time.
3934 Specifying VERSION as a literal of the form v5.6.1 should generally be
3935 avoided, because it leads to misleading error messages under earlier
3936 versions of Perl which do not support this syntax. The equivalent numeric
3937 version should be used instead.
3939 require v5.6.1; # run time version check
3940 require 5.6.1; # ditto
3941 require 5.006_001; # ditto; preferred for backwards compatibility
3943 Otherwise, demands that a library file be included if it hasn't already
3944 been included. The file is included via the do-FILE mechanism, which is
3945 essentially just a variety of C<eval>. Has semantics similar to the following
3950 return 1 if $INC{$filename};
3951 my($realfilename,$result);
3953 foreach $prefix (@INC) {
3954 $realfilename = "$prefix/$filename";
3955 if (-f $realfilename) {
3956 $INC{$filename} = $realfilename;
3957 $result = do $realfilename;
3961 die "Can't find $filename in \@INC";
3963 delete $INC{$filename} if $@ || !$result;
3965 die "$filename did not return true value" unless $result;
3969 Note that the file will not be included twice under the same specified
3970 name. The file must return true as the last statement to indicate
3971 successful execution of any initialization code, so it's customary to
3972 end such a file with C<1;> unless you're sure it'll return true
3973 otherwise. But it's better just to put the C<1;>, in case you add more
3976 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3977 replaces "F<::>" with "F</>" in the filename for you,
3978 to make it easy to load standard modules. This form of loading of
3979 modules does not risk altering your namespace.
3981 In other words, if you try this:
3983 require Foo::Bar; # a splendid bareword
3985 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3986 directories specified in the C<@INC> array.
3988 But if you try this:
3990 $class = 'Foo::Bar';
3991 require $class; # $class is not a bareword
3993 require "Foo::Bar"; # not a bareword because of the ""
3995 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3996 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3998 eval "require $class";
4000 You can also insert hooks into the import facility, by putting directly
4001 Perl code into the @INC array. There are three forms of hooks: subroutine
4002 references, array references and blessed objects.
4004 Subroutine references are the simplest case. When the inclusion system
4005 walks through @INC and encounters a subroutine, this subroutine gets
4006 called with two parameters, the first being a reference to itself, and the
4007 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4008 subroutine should return C<undef> or a filehandle, from which the file to
4009 include will be read. If C<undef> is returned, C<require> will look at
4010 the remaining elements of @INC.
4012 If the hook is an array reference, its first element must be a subroutine
4013 reference. This subroutine is called as above, but the first parameter is
4014 the array reference. This enables to pass indirectly some arguments to
4017 In other words, you can write:
4019 push @INC, \&my_sub;
4021 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4027 push @INC, [ \&my_sub, $x, $y, ... ];
4029 my ($arrayref, $filename) = @_;
4030 # Retrieve $x, $y, ...
4031 my @parameters = @$arrayref[1..$#$arrayref];
4035 If the hook is an object, it must provide an INC method, that will be
4036 called as above, the first parameter being the object itself. (Note that
4037 you must fully qualify the sub's name, as it is always forced into package
4038 C<main>.) Here is a typical code layout:
4044 my ($self, $filename) = @_;
4048 # In the main program
4049 push @INC, new Foo(...);
4051 Note that these hooks are also permitted to set the %INC entry
4052 corresponding to the files they have loaded. See L<perlvar/%INC>.
4054 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4060 Generally used in a C<continue> block at the end of a loop to clear
4061 variables and reset C<??> searches so that they work again. The
4062 expression is interpreted as a list of single characters (hyphens
4063 allowed for ranges). All variables and arrays beginning with one of
4064 those letters are reset to their pristine state. If the expression is
4065 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4066 only variables or searches in the current package. Always returns
4069 reset 'X'; # reset all X variables
4070 reset 'a-z'; # reset lower case variables
4071 reset; # just reset ?one-time? searches
4073 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4074 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4075 variables--lexical variables are unaffected, but they clean themselves
4076 up on scope exit anyway, so you'll probably want to use them instead.
4083 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4084 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4085 context, depending on how the return value will be used, and the context
4086 may vary from one execution to the next (see C<wantarray>). If no EXPR
4087 is given, returns an empty list in list context, the undefined value in
4088 scalar context, and (of course) nothing at all in a void context.
4090 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4091 or do FILE will automatically return the value of the last expression
4096 In list context, returns a list value consisting of the elements
4097 of LIST in the opposite order. In scalar context, concatenates the
4098 elements of LIST and returns a string value with all characters
4099 in the opposite order.
4101 print reverse <>; # line tac, last line first
4103 undef $/; # for efficiency of <>
4104 print scalar reverse <>; # character tac, last line tsrif
4106 This operator is also handy for inverting a hash, although there are some
4107 caveats. If a value is duplicated in the original hash, only one of those
4108 can be represented as a key in the inverted hash. Also, this has to
4109 unwind one hash and build a whole new one, which may take some time
4110 on a large hash, such as from a DBM file.
4112 %by_name = reverse %by_address; # Invert the hash
4114 =item rewinddir DIRHANDLE
4116 Sets the current position to the beginning of the directory for the
4117 C<readdir> routine on DIRHANDLE.
4119 =item rindex STR,SUBSTR,POSITION
4121 =item rindex STR,SUBSTR
4123 Works just like index() except that it returns the position of the LAST
4124 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4125 last occurrence at or before that position.
4127 =item rmdir FILENAME
4131 Deletes the directory specified by FILENAME if that directory is empty. If it
4132 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4133 FILENAME is omitted, uses C<$_>.
4137 The substitution operator. See L<perlop>.
4141 Forces EXPR to be interpreted in scalar context and returns the value
4144 @counts = ( scalar @a, scalar @b, scalar @c );
4146 There is no equivalent operator to force an expression to
4147 be interpolated in list context because in practice, this is never
4148 needed. If you really wanted to do so, however, you could use
4149 the construction C<@{[ (some expression) ]}>, but usually a simple
4150 C<(some expression)> suffices.
4152 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4153 parenthesized list, this behaves as a scalar comma expression, evaluating
4154 all but the last element in void context and returning the final element
4155 evaluated in scalar context. This is seldom what you want.
4157 The following single statement:
4159 print uc(scalar(&foo,$bar)),$baz;
4161 is the moral equivalent of these two:
4164 print(uc($bar),$baz);
4166 See L<perlop> for more details on unary operators and the comma operator.
4168 =item seek FILEHANDLE,POSITION,WHENCE
4170 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4171 FILEHANDLE may be an expression whose value gives the name of the
4172 filehandle. The values for WHENCE are C<0> to set the new position
4173 I<in bytes> to POSITION, C<1> to set it to the current position plus
4174 POSITION, and C<2> to set it to EOF plus POSITION (typically
4175 negative). For WHENCE you may use the constants C<SEEK_SET>,
4176 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4177 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4180 Note the I<in bytes>: even if the filehandle has been set to
4181 operate on characters (for example by using the C<:utf8> open
4182 discipline), tell() will return byte offsets, not character offsets
4183 (because implementing that would render seek() and tell() rather slow).
4185 If you want to position file for C<sysread> or C<syswrite>, don't use
4186 C<seek>--buffering makes its effect on the file's system position
4187 unpredictable and non-portable. Use C<sysseek> instead.
4189 Due to the rules and rigors of ANSI C, on some systems you have to do a
4190 seek whenever you switch between reading and writing. Amongst other
4191 things, this may have the effect of calling stdio's clearerr(3).
4192 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4196 This is also useful for applications emulating C<tail -f>. Once you hit
4197 EOF on your read, and then sleep for a while, you might have to stick in a
4198 seek() to reset things. The C<seek> doesn't change the current position,
4199 but it I<does> clear the end-of-file condition on the handle, so that the
4200 next C<< <FILE> >> makes Perl try again to read something. We hope.
4202 If that doesn't work (some IO implementations are particularly
4203 cantankerous), then you may need something more like this:
4206 for ($curpos = tell(FILE); $_ = <FILE>;
4207 $curpos = tell(FILE)) {
4208 # search for some stuff and put it into files
4210 sleep($for_a_while);
4211 seek(FILE, $curpos, 0);
4214 =item seekdir DIRHANDLE,POS
4216 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4217 must be a value returned by C<telldir>. Has the same caveats about
4218 possible directory compaction as the corresponding system library
4221 =item select FILEHANDLE
4225 Returns the currently selected filehandle. Sets the current default
4226 filehandle for output, if FILEHANDLE is supplied. This has two
4227 effects: first, a C<write> or a C<print> without a filehandle will
4228 default to this FILEHANDLE. Second, references to variables related to
4229 output will refer to this output channel. For example, if you have to
4230 set the top of form format for more than one output channel, you might
4238 FILEHANDLE may be an expression whose value gives the name of the
4239 actual filehandle. Thus:
4241 $oldfh = select(STDERR); $| = 1; select($oldfh);
4243 Some programmers may prefer to think of filehandles as objects with
4244 methods, preferring to write the last example as:
4247 STDERR->autoflush(1);
4249 =item select RBITS,WBITS,EBITS,TIMEOUT
4251 This calls the select(2) system call with the bit masks specified, which
4252 can be constructed using C<fileno> and C<vec>, along these lines:
4254 $rin = $win = $ein = '';
4255 vec($rin,fileno(STDIN),1) = 1;
4256 vec($win,fileno(STDOUT),1) = 1;
4259 If you want to select on many filehandles you might wish to write a
4263 my(@fhlist) = split(' ',$_[0]);
4266 vec($bits,fileno($_),1) = 1;
4270 $rin = fhbits('STDIN TTY SOCK');
4274 ($nfound,$timeleft) =
4275 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4277 or to block until something becomes ready just do this
4279 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4281 Most systems do not bother to return anything useful in $timeleft, so
4282 calling select() in scalar context just returns $nfound.
4284 Any of the bit masks can also be undef. The timeout, if specified, is
4285 in seconds, which may be fractional. Note: not all implementations are
4286 capable of returning the $timeleft. If not, they always return
4287 $timeleft equal to the supplied $timeout.
4289 You can effect a sleep of 250 milliseconds this way:
4291 select(undef, undef, undef, 0.25);
4293 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4294 is implementation-dependent.
4296 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4297 or <FH>) with C<select>, except as permitted by POSIX, and even
4298 then only on POSIX systems. You have to use C<sysread> instead.
4300 =item semctl ID,SEMNUM,CMD,ARG
4302 Calls the System V IPC function C<semctl>. You'll probably have to say
4306 first to get the correct constant definitions. If CMD is IPC_STAT or
4307 GETALL, then ARG must be a variable which will hold the returned
4308 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4309 the undefined value for error, "C<0 but true>" for zero, or the actual
4310 return value otherwise. The ARG must consist of a vector of native
4311 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4312 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4315 =item semget KEY,NSEMS,FLAGS
4317 Calls the System V IPC function semget. Returns the semaphore id, or
4318 the undefined value if there is an error. See also
4319 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4322 =item semop KEY,OPSTRING
4324 Calls the System V IPC function semop to perform semaphore operations
4325 such as signalling and waiting. OPSTRING must be a packed array of
4326 semop structures. Each semop structure can be generated with
4327 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4328 operations is implied by the length of OPSTRING. Returns true if
4329 successful, or false if there is an error. As an example, the
4330 following code waits on semaphore $semnum of semaphore id $semid:
4332 $semop = pack("s!3", $semnum, -1, 0);
4333 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4335 To signal the semaphore, replace C<-1> with C<1>. See also
4336 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4339 =item send SOCKET,MSG,FLAGS,TO
4341 =item send SOCKET,MSG,FLAGS
4343 Sends a message on a socket. Attempts to send the scalar MSG to the
4344 SOCKET filehandle. Takes the same flags as the system call of the
4345 same name. On unconnected sockets you must specify a destination to
4346 send TO, in which case it does a C C<sendto>. Returns the number of
4347 characters sent, or the undefined value if there is an error. The C
4348 system call sendmsg(2) is currently unimplemented. See
4349 L<perlipc/"UDP: Message Passing"> for examples.
4351 Note the I<characters>: depending on the status of the socket, either
4352 (8-bit) bytes or characters are sent. By default all sockets operate
4353 on bytes, but for example if the socket has been changed using
4354 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4355 the C<open> pragma, L<open>), the I/O will operate on characters, not
4358 =item setpgrp PID,PGRP
4360 Sets the current process group for the specified PID, C<0> for the current
4361 process. Will produce a fatal error if used on a machine that doesn't
4362 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4363 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4364 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4367 =item setpriority WHICH,WHO,PRIORITY
4369 Sets the current priority for a process, a process group, or a user.
4370 (See setpriority(2).) Will produce a fatal error if used on a machine
4371 that doesn't implement setpriority(2).
4373 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4375 Sets the socket option requested. Returns undefined if there is an
4376 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4383 Shifts the first value of the array off and returns it, shortening the
4384 array by 1 and moving everything down. If there are no elements in the
4385 array, returns the undefined value. If ARRAY is omitted, shifts the
4386 C<@_> array within the lexical scope of subroutines and formats, and the
4387 C<@ARGV> array at file scopes or within the lexical scopes established by
4388 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4391 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4392 same thing to the left end of an array that C<pop> and C<push> do to the
4395 =item shmctl ID,CMD,ARG
4397 Calls the System V IPC function shmctl. You'll probably have to say
4401 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4402 then ARG must be a variable which will hold the returned C<shmid_ds>
4403 structure. Returns like ioctl: the undefined value for error, "C<0> but
4404 true" for zero, or the actual return value otherwise.
4405 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4407 =item shmget KEY,SIZE,FLAGS
4409 Calls the System V IPC function shmget. Returns the shared memory
4410 segment id, or the undefined value if there is an error.
4411 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4413 =item shmread ID,VAR,POS,SIZE
4415 =item shmwrite ID,STRING,POS,SIZE
4417 Reads or writes the System V shared memory segment ID starting at
4418 position POS for size SIZE by attaching to it, copying in/out, and
4419 detaching from it. When reading, VAR must be a variable that will
4420 hold the data read. When writing, if STRING is too long, only SIZE
4421 bytes are used; if STRING is too short, nulls are written to fill out
4422 SIZE bytes. Return true if successful, or false if there is an error.
4423 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4424 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4426 =item shutdown SOCKET,HOW
4428 Shuts down a socket connection in the manner indicated by HOW, which
4429 has the same interpretation as in the system call of the same name.
4431 shutdown(SOCKET, 0); # I/we have stopped reading data
4432 shutdown(SOCKET, 1); # I/we have stopped writing data
4433 shutdown(SOCKET, 2); # I/we have stopped using this socket
4435 This is useful with sockets when you want to tell the other
4436 side you're done writing but not done reading, or vice versa.
4437 It's also a more insistent form of close because it also
4438 disables the file descriptor in any forked copies in other
4445 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4446 returns sine of C<$_>.
4448 For the inverse sine operation, you may use the C<Math::Trig::asin>
4449 function, or use this relation:
4451 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4457 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4458 May be interrupted if the process receives a signal such as C<SIGALRM>.
4459 Returns the number of seconds actually slept. You probably cannot
4460 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4463 On some older systems, it may sleep up to a full second less than what
4464 you requested, depending on how it counts seconds. Most modern systems
4465 always sleep the full amount. They may appear to sleep longer than that,
4466 however, because your process might not be scheduled right away in a
4467 busy multitasking system.
4469 For delays of finer granularity than one second, you may use Perl's
4470 C<syscall> interface to access setitimer(2) if your system supports
4471 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4472 and starting from Perl 5.8 part of the standard distribution) may also
4475 See also the POSIX module's C<pause> function.
4477 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4479 Opens a socket of the specified kind and attaches it to filehandle
4480 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4481 the system call of the same name. You should C<use Socket> first
4482 to get the proper definitions imported. See the examples in
4483 L<perlipc/"Sockets: Client/Server Communication">.
4485 On systems that support a close-on-exec flag on files, the flag will
4486 be set for the newly opened file descriptor, as determined by the
4487 value of $^F. See L<perlvar/$^F>.
4489 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4491 Creates an unnamed pair of sockets in the specified domain, of the
4492 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4493 for the system call of the same name. If unimplemented, yields a fatal
4494 error. Returns true if successful.
4496 On systems that support a close-on-exec flag on files, the flag will
4497 be set for the newly opened file descriptors, as determined by the value
4498 of $^F. See L<perlvar/$^F>.
4500 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4501 to C<pipe(Rdr, Wtr)> is essentially:
4504 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4505 shutdown(Rdr, 1); # no more writing for reader
4506 shutdown(Wtr, 0); # no more reading for writer
4508 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4509 emulate socketpair using IP sockets to localhost if your system implements
4510 sockets but not socketpair.
4512 =item sort SUBNAME LIST
4514 =item sort BLOCK LIST
4518 In list context, this sorts the LIST and returns the sorted list value.
4519 In scalar context, the behaviour of C<sort()> is undefined.
4521 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4522 order. If SUBNAME is specified, it gives the name of a subroutine
4523 that returns an integer less than, equal to, or greater than C<0>,
4524 depending on how the elements of the list are to be ordered. (The C<<
4525 <=> >> and C<cmp> operators are extremely useful in such routines.)
4526 SUBNAME may be a scalar variable name (unsubscripted), in which case
4527 the value provides the name of (or a reference to) the actual
4528 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4529 an anonymous, in-line sort subroutine.
4531 If the subroutine's prototype is C<($$)>, the elements to be compared
4532 are passed by reference in C<@_>, as for a normal subroutine. This is
4533 slower than unprototyped subroutines, where the elements to be
4534 compared are passed into the subroutine
4535 as the package global variables $a and $b (see example below). Note that
4536 in the latter case, it is usually counter-productive to declare $a and
4539 In either case, the subroutine may not be recursive. The values to be
4540 compared are always passed by reference, so don't modify them.
4542 You also cannot exit out of the sort block or subroutine using any of the
4543 loop control operators described in L<perlsyn> or with C<goto>.
4545 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4546 current collation locale. See L<perllocale>.
4548 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4549 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4550 preserves the input order of elements that compare equal. Although
4551 quicksort's run time is O(NlogN) when averaged over all arrays of
4552 length N, the time can be O(N**2), I<quadratic> behavior, for some
4553 inputs.) In 5.7, the quicksort implementation was replaced with
4554 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4555 But benchmarks indicated that for some inputs, on some platforms,
4556 the original quicksort was faster. 5.8 has a sort pragma for
4557 limited control of the sort. Its rather blunt control of the
4558 underlying algorithm may not persist into future perls, but the
4559 ability to characterize the input or output in implementation
4560 independent ways quite probably will. See L</use>.
4565 @articles = sort @files;
4567 # same thing, but with explicit sort routine
4568 @articles = sort {$a cmp $b} @files;
4570 # now case-insensitively
4571 @articles = sort {uc($a) cmp uc($b)} @files;
4573 # same thing in reversed order
4574 @articles = sort {$b cmp $a} @files;
4576 # sort numerically ascending
4577 @articles = sort {$a <=> $b} @files;
4579 # sort numerically descending
4580 @articles = sort {$b <=> $a} @files;
4582 # this sorts the %age hash by value instead of key
4583 # using an in-line function
4584 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4586 # sort using explicit subroutine name
4588 $age{$a} <=> $age{$b}; # presuming numeric
4590 @sortedclass = sort byage @class;
4592 sub backwards { $b cmp $a }
4593 @harry = qw(dog cat x Cain Abel);
4594 @george = qw(gone chased yz Punished Axed);
4596 # prints AbelCaincatdogx
4597 print sort backwards @harry;
4598 # prints xdogcatCainAbel
4599 print sort @george, 'to', @harry;
4600 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4602 # inefficiently sort by descending numeric compare using
4603 # the first integer after the first = sign, or the
4604 # whole record case-insensitively otherwise
4607 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4612 # same thing, but much more efficiently;
4613 # we'll build auxiliary indices instead
4617 push @nums, /=(\d+)/;
4622 $nums[$b] <=> $nums[$a]
4624 $caps[$a] cmp $caps[$b]
4628 # same thing, but without any temps
4629 @new = map { $_->[0] }
4630 sort { $b->[1] <=> $a->[1]
4633 } map { [$_, /=(\d+)/, uc($_)] } @old;
4635 # using a prototype allows you to use any comparison subroutine
4636 # as a sort subroutine (including other package's subroutines)
4638 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4641 @new = sort other::backwards @old;
4643 # guarantee stability, regardless of algorithm
4645 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4647 # force use of mergesort (not portable outside Perl 5.8)
4648 use sort '_mergesort'; # note discouraging _
4649 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4651 If you're using strict, you I<must not> declare $a
4652 and $b as lexicals. They are package globals. That means
4653 if you're in the C<main> package and type
4655 @articles = sort {$b <=> $a} @files;
4657 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4658 but if you're in the C<FooPack> package, it's the same as typing
4660 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4662 The comparison function is required to behave. If it returns
4663 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4664 sometimes saying the opposite, for example) the results are not
4667 =item splice ARRAY,OFFSET,LENGTH,LIST
4669 =item splice ARRAY,OFFSET,LENGTH
4671 =item splice ARRAY,OFFSET
4675 Removes the elements designated by OFFSET and LENGTH from an array, and
4676 replaces them with the elements of LIST, if any. In list context,
4677 returns the elements removed from the array. In scalar context,
4678 returns the last element removed, or C<undef> if no elements are
4679 removed. The array grows or shrinks as necessary.
4680 If OFFSET is negative then it starts that far from the end of the array.
4681 If LENGTH is omitted, removes everything from OFFSET onward.
4682 If LENGTH is negative, removes the elements from OFFSET onward
4683 except for -LENGTH elements at the end of the array.
4684 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4685 past the end of the array, perl issues a warning, and splices at the
4688 The following equivalences hold (assuming C<$[ == 0>):
4690 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4691 pop(@a) splice(@a,-1)
4692 shift(@a) splice(@a,0,1)
4693 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4694 $a[$x] = $y splice(@a,$x,1,$y)
4696 Example, assuming array lengths are passed before arrays:
4698 sub aeq { # compare two list values
4699 my(@a) = splice(@_,0,shift);
4700 my(@b) = splice(@_,0,shift);
4701 return 0 unless @a == @b; # same len?
4703 return 0 if pop(@a) ne pop(@b);
4707 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4709 =item split /PATTERN/,EXPR,LIMIT
4711 =item split /PATTERN/,EXPR
4713 =item split /PATTERN/
4717 Splits a string into a list of strings and returns that list. By default,
4718 empty leading fields are preserved, and empty trailing ones are deleted.
4720 In scalar context, returns the number of fields found and splits into
4721 the C<@_> array. Use of split in scalar context is deprecated, however,
4722 because it clobbers your subroutine arguments.
4724 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4725 splits on whitespace (after skipping any leading whitespace). Anything
4726 matching PATTERN is taken to be a delimiter separating the fields. (Note
4727 that the delimiter may be longer than one character.)
4729 If LIMIT is specified and positive, it represents the maximum number
4730 of fields the EXPR will be split into, though the actual number of
4731 fields returned depends on the number of times PATTERN matches within
4732 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4733 stripped (which potential users of C<pop> would do well to remember).
4734 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4735 had been specified. Note that splitting an EXPR that evaluates to the
4736 empty string always returns the empty list, regardless of the LIMIT
4739 A pattern matching the null string (not to be confused with
4740 a null pattern C<//>, which is just one member of the set of patterns
4741 matching a null string) will split the value of EXPR into separate
4742 characters at each point it matches that way. For example:
4744 print join(':', split(/ */, 'hi there'));
4746 produces the output 'h:i:t:h:e:r:e'.
4748 Using the empty pattern C<//> specifically matches the null string, and is
4749 not be confused with the use of C<//> to mean "the last successful pattern
4752 Empty leading (or trailing) fields are produced when there are positive width
4753 matches at the beginning (or end) of the string; a zero-width match at the
4754 beginning (or end) of the string does not produce an empty field. For
4757 print join(':', split(/(?=\w)/, 'hi there!'));
4759 produces the output 'h:i :t:h:e:r:e!'.
4761 The LIMIT parameter can be used to split a line partially
4763 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4765 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4766 one larger than the number of variables in the list, to avoid
4767 unnecessary work. For the list above LIMIT would have been 4 by
4768 default. In time critical applications it behooves you not to split
4769 into more fields than you really need.
4771 If the PATTERN contains parentheses, additional list elements are
4772 created from each matching substring in the delimiter.
4774 split(/([,-])/, "1-10,20", 3);
4776 produces the list value
4778 (1, '-', 10, ',', 20)
4780 If you had the entire header of a normal Unix email message in $header,
4781 you could split it up into fields and their values this way:
4783 $header =~ s/\n\s+/ /g; # fix continuation lines
4784 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4786 The pattern C</PATTERN/> may be replaced with an expression to specify
4787 patterns that vary at runtime. (To do runtime compilation only once,
4788 use C</$variable/o>.)
4790 As a special case, specifying a PATTERN of space (C<' '>) will split on
4791 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4792 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4793 will give you as many null initial fields as there are leading spaces.
4794 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4795 whitespace produces a null first field. A C<split> with no arguments
4796 really does a C<split(' ', $_)> internally.
4798 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4803 open(PASSWD, '/etc/passwd');
4806 ($login, $passwd, $uid, $gid,
4807 $gcos, $home, $shell) = split(/:/);
4811 As with regular pattern matching, any capturing parentheses that are not
4812 matched in a C<split()> will be set to C<undef> when returned:
4814 @fields = split /(A)|B/, "1A2B3";
4815 # @fields is (1, 'A', 2, undef, 3)
4817 =item sprintf FORMAT, LIST
4819 Returns a string formatted by the usual C<printf> conventions of the C
4820 library function C<sprintf>. See below for more details
4821 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4822 the general principles.
4826 # Format number with up to 8 leading zeroes
4827 $result = sprintf("%08d", $number);
4829 # Round number to 3 digits after decimal point
4830 $rounded = sprintf("%.3f", $number);
4832 Perl does its own C<sprintf> formatting--it emulates the C
4833 function C<sprintf>, but it doesn't use it (except for floating-point
4834 numbers, and even then only the standard modifiers are allowed). As a
4835 result, any non-standard extensions in your local C<sprintf> are not
4836 available from Perl.
4838 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4839 pass it an array as your first argument. The array is given scalar context,
4840 and instead of using the 0th element of the array as the format, Perl will
4841 use the count of elements in the array as the format, which is almost never
4844 Perl's C<sprintf> permits the following universally-known conversions:
4847 %c a character with the given number
4849 %d a signed integer, in decimal
4850 %u an unsigned integer, in decimal
4851 %o an unsigned integer, in octal
4852 %x an unsigned integer, in hexadecimal
4853 %e a floating-point number, in scientific notation
4854 %f a floating-point number, in fixed decimal notation
4855 %g a floating-point number, in %e or %f notation
4857 In addition, Perl permits the following widely-supported conversions:
4859 %X like %x, but using upper-case letters
4860 %E like %e, but using an upper-case "E"
4861 %G like %g, but with an upper-case "E" (if applicable)
4862 %b an unsigned integer, in binary
4863 %p a pointer (outputs the Perl value's address in hexadecimal)
4864 %n special: *stores* the number of characters output so far
4865 into the next variable in the parameter list
4867 Finally, for backward (and we do mean "backward") compatibility, Perl
4868 permits these unnecessary but widely-supported conversions:
4871 %D a synonym for %ld
4872 %U a synonym for %lu
4873 %O a synonym for %lo
4876 Note that the number of exponent digits in the scientific notation by
4877 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4878 exponent less than 100 is system-dependent: it may be three or less
4879 (zero-padded as necessary). In other words, 1.23 times ten to the
4880 99th may be either "1.23e99" or "1.23e099".
4882 Perl permits the following universally-known flags between the C<%>
4883 and the conversion letter:
4885 space prefix positive number with a space
4886 + prefix positive number with a plus sign
4887 - left-justify within the field
4888 0 use zeros, not spaces, to right-justify
4889 # prefix non-zero octal with "0", non-zero hex with "0x"
4890 number minimum field width
4891 .number "precision": digits after decimal point for
4892 floating-point, max length for string, minimum length
4894 l interpret integer as C type "long" or "unsigned long"
4895 h interpret integer as C type "short" or "unsigned short"
4896 If no flags, interpret integer as C type "int" or "unsigned"
4898 Perl supports parameter ordering, in other words, fetching the
4899 parameters in some explicitly specified "random" ordering as opposed
4900 to the default implicit sequential ordering. The syntax is, instead
4901 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4902 where the I<digits> is the wanted index, from one upwards. For example:
4904 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4905 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4907 Note that using the reordering syntax does not interfere with the usual
4908 implicit sequential fetching of the parameters:
4910 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4911 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4912 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4913 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4914 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4916 There are also two Perl-specific flags:
4918 V interpret integer as Perl's standard integer type
4919 v interpret string as a vector of integers, output as
4920 numbers separated either by dots, or by an arbitrary
4921 string received from the argument list when the flag
4924 Where a number would appear in the flags, an asterisk (C<*>) may be
4925 used instead, in which case Perl uses the next item in the parameter
4926 list as the given number (that is, as the field width or precision).
4927 If a field width obtained through C<*> is negative, it has the same
4928 effect as the C<-> flag: left-justification.
4930 The C<v> flag is useful for displaying ordinal values of characters
4931 in arbitrary strings:
4933 printf "version is v%vd\n", $^V; # Perl's version
4934 printf "address is %*vX\n", ":", $addr; # IPv6 address
4935 printf "bits are %*vb\n", " ", $bits; # random bitstring
4937 If C<use locale> is in effect, the character used for the decimal
4938 point in formatted real numbers is affected by the LC_NUMERIC locale.
4941 If Perl understands "quads" (64-bit integers) (this requires
4942 either that the platform natively support quads or that Perl
4943 be specifically compiled to support quads), the characters
4947 print quads, and they may optionally be preceded by
4955 You can find out whether your Perl supports quads via L<Config>:
4958 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4961 If Perl understands "long doubles" (this requires that the platform
4962 support long doubles), the flags
4966 may optionally be preceded by
4974 You can find out whether your Perl supports long doubles via L<Config>:
4977 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4983 Return the square root of EXPR. If EXPR is omitted, returns square
4984 root of C<$_>. Only works on non-negative operands, unless you've
4985 loaded the standard Math::Complex module.
4988 print sqrt(-2); # prints 1.4142135623731i
4994 Sets the random number seed for the C<rand> operator.
4996 The point of the function is to "seed" the C<rand> function so that
4997 C<rand> can produce a different sequence each time you run your
5000 If srand() is not called explicitly, it is called implicitly at the
5001 first use of the C<rand> operator. However, this was not the case in
5002 versions of Perl before 5.004, so if your script will run under older
5003 Perl versions, it should call C<srand>.
5005 Most programs won't even call srand() at all, except those that
5006 need a cryptographically-strong starting point rather than the
5007 generally acceptable default, which is based on time of day,
5008 process ID, and memory allocation, or the F</dev/urandom> device,
5011 You can call srand($seed) with the same $seed to reproduce the
5012 I<same> sequence from rand(), but this is usually reserved for
5013 generating predictable results for testing or debugging.
5014 Otherwise, don't call srand() more than once in your program.
5016 Do B<not> call srand() (i.e. without an argument) more than once in
5017 a script. The internal state of the random number generator should
5018 contain more entropy than can be provided by any seed, so calling
5019 srand() again actually I<loses> randomness.
5021 Most implementations of C<srand> take an integer and will silently
5022 truncate decimal numbers. This means C<srand(42)> will usually
5023 produce the same results as C<srand(42.1)>. To be safe, always pass
5024 C<srand> an integer.
5026 In versions of Perl prior to 5.004 the default seed was just the
5027 current C<time>. This isn't a particularly good seed, so many old
5028 programs supply their own seed value (often C<time ^ $$> or C<time ^
5029 ($$ + ($$ << 15))>), but that isn't necessary any more.
5031 Note that you need something much more random than the default seed for
5032 cryptographic purposes. Checksumming the compressed output of one or more
5033 rapidly changing operating system status programs is the usual method. For
5036 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5038 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5041 Frequently called programs (like CGI scripts) that simply use
5045 for a seed can fall prey to the mathematical property that
5049 one-third of the time. So don't do that.
5051 =item stat FILEHANDLE
5057 Returns a 13-element list giving the status info for a file, either
5058 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5059 it stats C<$_>. Returns a null list if the stat fails. Typically used
5062 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5063 $atime,$mtime,$ctime,$blksize,$blocks)
5066 Not all fields are supported on all filesystem types. Here are the
5067 meaning of the fields:
5069 0 dev device number of filesystem
5071 2 mode file mode (type and permissions)
5072 3 nlink number of (hard) links to the file
5073 4 uid numeric user ID of file's owner
5074 5 gid numeric group ID of file's owner
5075 6 rdev the device identifier (special files only)
5076 7 size total size of file, in bytes
5077 8 atime last access time in seconds since the epoch
5078 9 mtime last modify time in seconds since the epoch
5079 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5080 11 blksize preferred block size for file system I/O
5081 12 blocks actual number of blocks allocated
5083 (The epoch was at 00:00 January 1, 1970 GMT.)
5085 If stat is passed the special filehandle consisting of an underline, no
5086 stat is done, but the current contents of the stat structure from the
5087 last stat or filetest are returned. Example:
5089 if (-x $file && (($d) = stat(_)) && $d < 0) {
5090 print "$file is executable NFS file\n";
5093 (This works on machines only for which the device number is negative
5096 Because the mode contains both the file type and its permissions, you
5097 should mask off the file type portion and (s)printf using a C<"%o">
5098 if you want to see the real permissions.
5100 $mode = (stat($filename))[2];
5101 printf "Permissions are %04o\n", $mode & 07777;
5103 In scalar context, C<stat> returns a boolean value indicating success
5104 or failure, and, if successful, sets the information associated with
5105 the special filehandle C<_>.
5107 The File::stat module provides a convenient, by-name access mechanism:
5110 $sb = stat($filename);
5111 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5112 $filename, $sb->size, $sb->mode & 07777,
5113 scalar localtime $sb->mtime;
5115 You can import symbolic mode constants (C<S_IF*>) and functions
5116 (C<S_IS*>) from the Fcntl module:
5120 $mode = (stat($filename))[2];
5122 $user_rwx = ($mode & S_IRWXU) >> 6;
5123 $group_read = ($mode & S_IRGRP) >> 3;
5124 $other_execute = $mode & S_IXOTH;
5126 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5128 $is_setuid = $mode & S_ISUID;
5129 $is_setgid = S_ISDIR($mode);
5131 You could write the last two using the C<-u> and C<-d> operators.
5132 The commonly available S_IF* constants are
5134 # Permissions: read, write, execute, for user, group, others.
5136 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5137 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5138 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5140 # Setuid/Setgid/Stickiness.
5142 S_ISUID S_ISGID S_ISVTX S_ISTXT
5144 # File types. Not necessarily all are available on your system.
5146 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5148 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5150 S_IREAD S_IWRITE S_IEXEC
5152 and the S_IF* functions are
5154 S_IFMODE($mode) the part of $mode containing the permission bits
5155 and the setuid/setgid/sticky bits
5157 S_IFMT($mode) the part of $mode containing the file type
5158 which can be bit-anded with e.g. S_IFREG
5159 or with the following functions
5161 # The operators -f, -d, -l, -b, -c, -p, and -s.
5163 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5164 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5166 # No direct -X operator counterpart, but for the first one
5167 # the -g operator is often equivalent. The ENFMT stands for
5168 # record flocking enforcement, a platform-dependent feature.
5170 S_ISENFMT($mode) S_ISWHT($mode)
5172 See your native chmod(2) and stat(2) documentation for more details
5173 about the S_* constants.
5179 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5180 doing many pattern matches on the string before it is next modified.
5181 This may or may not save time, depending on the nature and number of
5182 patterns you are searching on, and on the distribution of character
5183 frequencies in the string to be searched--you probably want to compare
5184 run times with and without it to see which runs faster. Those loops
5185 which scan for many short constant strings (including the constant
5186 parts of more complex patterns) will benefit most. You may have only
5187 one C<study> active at a time--if you study a different scalar the first
5188 is "unstudied". (The way C<study> works is this: a linked list of every
5189 character in the string to be searched is made, so we know, for
5190 example, where all the C<'k'> characters are. From each search string,
5191 the rarest character is selected, based on some static frequency tables
5192 constructed from some C programs and English text. Only those places
5193 that contain this "rarest" character are examined.)
5195 For example, here is a loop that inserts index producing entries
5196 before any line containing a certain pattern:
5200 print ".IX foo\n" if /\bfoo\b/;
5201 print ".IX bar\n" if /\bbar\b/;
5202 print ".IX blurfl\n" if /\bblurfl\b/;
5207 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5208 will be looked at, because C<f> is rarer than C<o>. In general, this is
5209 a big win except in pathological cases. The only question is whether
5210 it saves you more time than it took to build the linked list in the
5213 Note that if you have to look for strings that you don't know till
5214 runtime, you can build an entire loop as a string and C<eval> that to
5215 avoid recompiling all your patterns all the time. Together with
5216 undefining C<$/> to input entire files as one record, this can be very
5217 fast, often faster than specialized programs like fgrep(1). The following
5218 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5219 out the names of those files that contain a match:
5221 $search = 'while (<>) { study;';
5222 foreach $word (@words) {
5223 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5228 eval $search; # this screams
5229 $/ = "\n"; # put back to normal input delimiter
5230 foreach $file (sort keys(%seen)) {
5234 =item sub NAME BLOCK
5236 =item sub NAME (PROTO) BLOCK
5238 =item sub NAME : ATTRS BLOCK
5240 =item sub NAME (PROTO) : ATTRS BLOCK
5242 This is subroutine definition, not a real function I<per se>.
5243 Without a BLOCK it's just a forward declaration. Without a NAME,
5244 it's an anonymous function declaration, and does actually return
5245 a value: the CODE ref of the closure you just created.
5247 See L<perlsub> and L<perlref> for details about subroutines and
5248 references, and L<attributes> and L<Attribute::Handlers> for more
5249 information about attributes.
5251 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5253 =item substr EXPR,OFFSET,LENGTH
5255 =item substr EXPR,OFFSET
5257 Extracts a substring out of EXPR and returns it. First character is at
5258 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5259 If OFFSET is negative (or more precisely, less than C<$[>), starts
5260 that far from the end of the string. If LENGTH is omitted, returns
5261 everything to the end of the string. If LENGTH is negative, leaves that
5262 many characters off the end of the string.
5264 You can use the substr() function as an lvalue, in which case EXPR
5265 must itself be an lvalue. If you assign something shorter than LENGTH,
5266 the string will shrink, and if you assign something longer than LENGTH,
5267 the string will grow to accommodate it. To keep the string the same
5268 length you may need to pad or chop your value using C<sprintf>.
5270 If OFFSET and LENGTH specify a substring that is partly outside the
5271 string, only the part within the string is returned. If the substring
5272 is beyond either end of the string, substr() returns the undefined
5273 value and produces a warning. When used as an lvalue, specifying a
5274 substring that is entirely outside the string is a fatal error.
5275 Here's an example showing the behavior for boundary cases:
5278 substr($name, 4) = 'dy'; # $name is now 'freddy'
5279 my $null = substr $name, 6, 2; # returns '' (no warning)
5280 my $oops = substr $name, 7; # returns undef, with warning
5281 substr($name, 7) = 'gap'; # fatal error
5283 An alternative to using substr() as an lvalue is to specify the
5284 replacement string as the 4th argument. This allows you to replace
5285 parts of the EXPR and return what was there before in one operation,
5286 just as you can with splice().
5288 =item symlink OLDFILE,NEWFILE
5290 Creates a new filename symbolically linked to the old filename.
5291 Returns C<1> for success, C<0> otherwise. On systems that don't support
5292 symbolic links, produces a fatal error at run time. To check for that,
5295 $symlink_exists = eval { symlink("",""); 1 };
5299 Calls the system call specified as the first element of the list,
5300 passing the remaining elements as arguments to the system call. If
5301 unimplemented, produces a fatal error. The arguments are interpreted
5302 as follows: if a given argument is numeric, the argument is passed as
5303 an int. If not, the pointer to the string value is passed. You are
5304 responsible to make sure a string is pre-extended long enough to
5305 receive any result that might be written into a string. You can't use a
5306 string literal (or other read-only string) as an argument to C<syscall>
5307 because Perl has to assume that any string pointer might be written
5309 integer arguments are not literals and have never been interpreted in a
5310 numeric context, you may need to add C<0> to them to force them to look
5311 like numbers. This emulates the C<syswrite> function (or vice versa):
5313 require 'syscall.ph'; # may need to run h2ph
5315 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5317 Note that Perl supports passing of up to only 14 arguments to your system call,
5318 which in practice should usually suffice.
5320 Syscall returns whatever value returned by the system call it calls.
5321 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5322 Note that some system calls can legitimately return C<-1>. The proper
5323 way to handle such calls is to assign C<$!=0;> before the call and
5324 check the value of C<$!> if syscall returns C<-1>.
5326 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5327 number of the read end of the pipe it creates. There is no way
5328 to retrieve the file number of the other end. You can avoid this
5329 problem by using C<pipe> instead.
5331 =item sysopen FILEHANDLE,FILENAME,MODE
5333 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5335 Opens the file whose filename is given by FILENAME, and associates it
5336 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5337 the name of the real filehandle wanted. This function calls the
5338 underlying operating system's C<open> function with the parameters
5339 FILENAME, MODE, PERMS.
5341 The possible values and flag bits of the MODE parameter are
5342 system-dependent; they are available via the standard module C<Fcntl>.
5343 See the documentation of your operating system's C<open> to see which
5344 values and flag bits are available. You may combine several flags
5345 using the C<|>-operator.
5347 Some of the most common values are C<O_RDONLY> for opening the file in
5348 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5349 and C<O_RDWR> for opening the file in read-write mode, and.
5351 For historical reasons, some values work on almost every system
5352 supported by perl: zero means read-only, one means write-only, and two
5353 means read/write. We know that these values do I<not> work under
5354 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5355 use them in new code.
5357 If the file named by FILENAME does not exist and the C<open> call creates
5358 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5359 PERMS specifies the permissions of the newly created file. If you omit
5360 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5361 These permission values need to be in octal, and are modified by your
5362 process's current C<umask>.
5364 In many systems the C<O_EXCL> flag is available for opening files in
5365 exclusive mode. This is B<not> locking: exclusiveness means here that
5366 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5369 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5371 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5372 that takes away the user's option to have a more permissive umask.
5373 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5376 Note that C<sysopen> depends on the fdopen() C library function.
5377 On many UNIX systems, fdopen() is known to fail when file descriptors
5378 exceed a certain value, typically 255. If you need more file
5379 descriptors than that, consider rebuilding Perl to use the C<sfio>
5380 library, or perhaps using the POSIX::open() function.
5382 See L<perlopentut> for a kinder, gentler explanation of opening files.
5384 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5386 =item sysread FILEHANDLE,SCALAR,LENGTH
5388 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5389 the specified FILEHANDLE, using the system call read(2). It bypasses
5390 buffered IO, so mixing this with other kinds of reads, C<print>,
5391 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5392 stdio usually buffers data. Returns the number of characters actually
5393 read, C<0> at end of file, or undef if there was an error. SCALAR
5394 will be grown or shrunk so that the last byte actually read is the
5395 last byte of the scalar after the read.
5397 Note the I<characters>: depending on the status of the filehandle,
5398 either (8-bit) bytes or characters are read. By default all
5399 filehandles operate on bytes, but for example if the filehandle has
5400 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5401 pragma, L<open>), the I/O will operate on characters, not bytes.
5403 An OFFSET may be specified to place the read data at some place in the
5404 string other than the beginning. A negative OFFSET specifies
5405 placement at that many characters counting backwards from the end of
5406 the string. A positive OFFSET greater than the length of SCALAR
5407 results in the string being padded to the required size with C<"\0">
5408 bytes before the result of the read is appended.
5410 There is no syseof() function, which is ok, since eof() doesn't work
5411 very well on device files (like ttys) anyway. Use sysread() and check
5412 for a return value for 0 to decide whether you're done.
5414 =item sysseek FILEHANDLE,POSITION,WHENCE
5416 Sets FILEHANDLE's system position I<in bytes> using the system call
5417 lseek(2). FILEHANDLE may be an expression whose value gives the name
5418 of the filehandle. The values for WHENCE are C<0> to set the new
5419 position to POSITION, C<1> to set the it to the current position plus
5420 POSITION, and C<2> to set it to EOF plus POSITION (typically
5423 Note the I<in bytes>: even if the filehandle has been set to operate
5424 on characters (for example by using the C<:utf8> discipline), tell()
5425 will return byte offsets, not character offsets (because implementing
5426 that would render sysseek() very slow).
5428 sysseek() bypasses normal buffered io, so mixing this with reads (other
5429 than C<sysread>, for example >< or read()) C<print>, C<write>,
5430 C<seek>, C<tell>, or C<eof> may cause confusion.
5432 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5433 and C<SEEK_END> (start of the file, current position, end of the file)
5434 from the Fcntl module. Use of the constants is also more portable
5435 than relying on 0, 1, and 2. For example to define a "systell" function:
5437 use Fnctl 'SEEK_CUR';
5438 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5440 Returns the new position, or the undefined value on failure. A position
5441 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5442 true on success and false on failure, yet you can still easily determine
5447 =item system PROGRAM LIST
5449 Does exactly the same thing as C<exec LIST>, except that a fork is
5450 done first, and the parent process waits for the child process to
5451 complete. Note that argument processing varies depending on the
5452 number of arguments. If there is more than one argument in LIST,
5453 or if LIST is an array with more than one value, starts the program
5454 given by the first element of the list with arguments given by the
5455 rest of the list. If there is only one scalar argument, the argument
5456 is checked for shell metacharacters, and if there are any, the
5457 entire argument is passed to the system's command shell for parsing
5458 (this is C</bin/sh -c> on Unix platforms, but varies on other
5459 platforms). If there are no shell metacharacters in the argument,
5460 it is split into words and passed directly to C<execvp>, which is
5463 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5464 output before any operation that may do a fork, but this may not be
5465 supported on some platforms (see L<perlport>). To be safe, you may need
5466 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5467 of C<IO::Handle> on any open handles.
5469 The return value is the exit status of the program as returned by the
5470 C<wait> call. To get the actual exit value shift right by eight (see below).
5471 See also L</exec>. This is I<not> what you want to use to capture
5472 the output from a command, for that you should use merely backticks or
5473 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5474 indicates a failure to start the program (inspect $! for the reason).
5476 Like C<exec>, C<system> allows you to lie to a program about its name if
5477 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5479 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5480 killing the program they're running doesn't actually interrupt
5483 @args = ("command", "arg1", "arg2");
5485 or die "system @args failed: $?"
5487 You can check all the failure possibilities by inspecting
5490 $exit_value = $? >> 8;
5491 $signal_num = $? & 127;
5492 $dumped_core = $? & 128;
5494 or more portably by using the W*() calls of the POSIX extension;
5495 see L<perlport> for more information.
5497 When the arguments get executed via the system shell, results
5498 and return codes will be subject to its quirks and capabilities.
5499 See L<perlop/"`STRING`"> and L</exec> for details.
5501 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5503 =item syswrite FILEHANDLE,SCALAR,LENGTH
5505 =item syswrite FILEHANDLE,SCALAR
5507 Attempts to write LENGTH characters of data from variable SCALAR to
5508 the specified FILEHANDLE, using the system call write(2). If LENGTH
5509 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5510 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5511 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5512 buffers data. Returns the number of characters actually written, or
5513 C<undef> if there was an error. If the LENGTH is greater than the
5514 available data in the SCALAR after the OFFSET, only as much data as is
5515 available will be written.
5517 An OFFSET may be specified to write the data from some part of the
5518 string other than the beginning. A negative OFFSET specifies writing
5519 that many characters counting backwards from the end of the string.
5520 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5522 Note the I<characters>: depending on the status of the filehandle,
5523 either (8-bit) bytes or characters are written. By default all
5524 filehandles operate on bytes, but for example if the filehandle has
5525 been opened with the C<:utf8> discipline (see L</open>, and the open
5526 pragma, L<open>), the I/O will operate on characters, not bytes.
5528 =item tell FILEHANDLE
5532 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5533 error. FILEHANDLE may be an expression whose value gives the name of
5534 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5537 Note the I<in bytes>: even if the filehandle has been set to
5538 operate on characters (for example by using the C<:utf8> open
5539 discipline), tell() will return byte offsets, not character offsets
5540 (because that would render seek() and tell() rather slow).
5542 The return value of tell() for the standard streams like the STDIN
5543 depends on the operating system: it may return -1 or something else.
5544 tell() on pipes, fifos, and sockets usually returns -1.
5546 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5548 Do not use tell() on a filehandle that has been opened using
5549 sysopen(), use sysseek() for that as described above. Why? Because
5550 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5551 buffered filehandles. sysseek() make sense only on the first kind,
5552 tell() only makes sense on the second kind.
5554 =item telldir DIRHANDLE
5556 Returns the current position of the C<readdir> routines on DIRHANDLE.
5557 Value may be given to C<seekdir> to access a particular location in a
5558 directory. Has the same caveats about possible directory compaction as
5559 the corresponding system library routine.
5561 =item tie VARIABLE,CLASSNAME,LIST
5563 This function binds a variable to a package class that will provide the
5564 implementation for the variable. VARIABLE is the name of the variable
5565 to be enchanted. CLASSNAME is the name of a class implementing objects
5566 of correct type. Any additional arguments are passed to the C<new>
5567 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5568 or C<TIEHASH>). Typically these are arguments such as might be passed
5569 to the C<dbm_open()> function of C. The object returned by the C<new>
5570 method is also returned by the C<tie> function, which would be useful
5571 if you want to access other methods in CLASSNAME.
5573 Note that functions such as C<keys> and C<values> may return huge lists
5574 when used on large objects, like DBM files. You may prefer to use the
5575 C<each> function to iterate over such. Example:
5577 # print out history file offsets
5579 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5580 while (($key,$val) = each %HIST) {
5581 print $key, ' = ', unpack('L',$val), "\n";
5585 A class implementing a hash should have the following methods:
5587 TIEHASH classname, LIST
5589 STORE this, key, value
5594 NEXTKEY this, lastkey
5598 A class implementing an ordinary array should have the following methods:
5600 TIEARRAY classname, LIST
5602 STORE this, key, value
5604 STORESIZE this, count
5610 SPLICE this, offset, length, LIST
5615 A class implementing a file handle should have the following methods:
5617 TIEHANDLE classname, LIST
5618 READ this, scalar, length, offset
5621 WRITE this, scalar, length, offset
5623 PRINTF this, format, LIST
5627 SEEK this, position, whence
5629 OPEN this, mode, LIST
5634 A class implementing a scalar should have the following methods:
5636 TIESCALAR classname, LIST
5642 Not all methods indicated above need be implemented. See L<perltie>,
5643 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5645 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5646 for you--you need to do that explicitly yourself. See L<DB_File>
5647 or the F<Config> module for interesting C<tie> implementations.
5649 For further details see L<perltie>, L<"tied VARIABLE">.
5653 Returns a reference to the object underlying VARIABLE (the same value
5654 that was originally returned by the C<tie> call that bound the variable
5655 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5660 Returns the number of non-leap seconds since whatever time the system
5661 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5662 and 00:00:00 UTC, January 1, 1970 for most other systems).
5663 Suitable for feeding to C<gmtime> and C<localtime>.
5665 For measuring time in better granularity than one second,
5666 you may use either the Time::HiRes module from CPAN, or
5667 if you have gettimeofday(2), you may be able to use the
5668 C<syscall> interface of Perl, see L<perlfaq8> for details.
5672 Returns a four-element list giving the user and system times, in
5673 seconds, for this process and the children of this process.
5675 ($user,$system,$cuser,$csystem) = times;
5677 In scalar context, C<times> returns C<$user>.
5681 The transliteration operator. Same as C<y///>. See L<perlop>.
5683 =item truncate FILEHANDLE,LENGTH
5685 =item truncate EXPR,LENGTH
5687 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5688 specified length. Produces a fatal error if truncate isn't implemented
5689 on your system. Returns true if successful, the undefined value
5692 The behavior is undefined if LENGTH is greater than the length of the
5699 Returns an uppercased version of EXPR. This is the internal function
5700 implementing the C<\U> escape in double-quoted strings. Respects
5701 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5702 and L<perlunicode> for more details about locale and Unicode support.
5703 It does not attempt to do titlecase mapping on initial letters. See
5704 C<ucfirst> for that.
5706 If EXPR is omitted, uses C<$_>.
5712 Returns the value of EXPR with the first character in uppercase
5713 (titlecase in Unicode). This is the internal function implementing
5714 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5715 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5716 for more details about locale and Unicode support.
5718 If EXPR is omitted, uses C<$_>.
5724 Sets the umask for the process to EXPR and returns the previous value.
5725 If EXPR is omitted, merely returns the current umask.
5727 The Unix permission C<rwxr-x---> is represented as three sets of three
5728 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5729 and isn't one of the digits). The C<umask> value is such a number
5730 representing disabled permissions bits. The permission (or "mode")
5731 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5732 even if you tell C<sysopen> to create a file with permissions C<0777>,
5733 if your umask is C<0022> then the file will actually be created with
5734 permissions C<0755>. If your C<umask> were C<0027> (group can't
5735 write; others can't read, write, or execute), then passing
5736 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5739 Here's some advice: supply a creation mode of C<0666> for regular
5740 files (in C<sysopen>) and one of C<0777> for directories (in
5741 C<mkdir>) and executable files. This gives users the freedom of
5742 choice: if they want protected files, they might choose process umasks
5743 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5744 Programs should rarely if ever make policy decisions better left to
5745 the user. The exception to this is when writing files that should be
5746 kept private: mail files, web browser cookies, I<.rhosts> files, and
5749 If umask(2) is not implemented on your system and you are trying to
5750 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5751 fatal error at run time. If umask(2) is not implemented and you are
5752 not trying to restrict access for yourself, returns C<undef>.
5754 Remember that a umask is a number, usually given in octal; it is I<not> a
5755 string of octal digits. See also L</oct>, if all you have is a string.
5761 Undefines the value of EXPR, which must be an lvalue. Use only on a
5762 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5763 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5764 will probably not do what you expect on most predefined variables or
5765 DBM list values, so don't do that; see L<delete>.) Always returns the
5766 undefined value. You can omit the EXPR, in which case nothing is
5767 undefined, but you still get an undefined value that you could, for
5768 instance, return from a subroutine, assign to a variable or pass as a
5769 parameter. Examples:
5772 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5776 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5777 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5778 select undef, undef, undef, 0.25;
5779 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5781 Note that this is a unary operator, not a list operator.
5787 Deletes a list of files. Returns the number of files successfully
5790 $cnt = unlink 'a', 'b', 'c';
5794 Note: C<unlink> will not delete directories unless you are superuser and
5795 the B<-U> flag is supplied to Perl. Even if these conditions are
5796 met, be warned that unlinking a directory can inflict damage on your
5797 filesystem. Use C<rmdir> instead.
5799 If LIST is omitted, uses C<$_>.
5801 =item unpack TEMPLATE,EXPR
5803 C<unpack> does the reverse of C<pack>: it takes a string
5804 and expands it out into a list of values.
5805 (In scalar context, it returns merely the first value produced.)
5807 The string is broken into chunks described by the TEMPLATE. Each chunk
5808 is converted separately to a value. Typically, either the string is a result
5809 of C<pack>, or the bytes of the string represent a C structure of some
5812 The TEMPLATE has the same format as in the C<pack> function.
5813 Here's a subroutine that does substring:
5816 my($what,$where,$howmuch) = @_;
5817 unpack("x$where a$howmuch", $what);
5822 sub ordinal { unpack("c",$_[0]); } # same as ord()
5824 In addition to fields allowed in pack(), you may prefix a field with
5825 a %<number> to indicate that
5826 you want a <number>-bit checksum of the items instead of the items
5827 themselves. Default is a 16-bit checksum. Checksum is calculated by
5828 summing numeric values of expanded values (for string fields the sum of
5829 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5831 For example, the following
5832 computes the same number as the System V sum program:
5836 unpack("%32C*",<>) % 65535;
5839 The following efficiently counts the number of set bits in a bit vector:
5841 $setbits = unpack("%32b*", $selectmask);
5843 The C<p> and C<P> formats should be used with care. Since Perl
5844 has no way of checking whether the value passed to C<unpack()>
5845 corresponds to a valid memory location, passing a pointer value that's
5846 not known to be valid is likely to have disastrous consequences.
5848 If the repeat count of a field is larger than what the remainder of
5849 the input string allows, repeat count is decreased. If the input string
5850 is longer than one described by the TEMPLATE, the rest is ignored.
5852 See L</pack> for more examples and notes.
5854 =item untie VARIABLE
5856 Breaks the binding between a variable and a package. (See C<tie>.)
5858 =item unshift ARRAY,LIST
5860 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5861 depending on how you look at it. Prepends list to the front of the
5862 array, and returns the new number of elements in the array.
5864 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5866 Note the LIST is prepended whole, not one element at a time, so the
5867 prepended elements stay in the same order. Use C<reverse> to do the
5870 =item use Module VERSION LIST
5872 =item use Module VERSION
5874 =item use Module LIST
5880 Imports some semantics into the current package from the named module,
5881 generally by aliasing certain subroutine or variable names into your
5882 package. It is exactly equivalent to
5884 BEGIN { require Module; import Module LIST; }
5886 except that Module I<must> be a bareword.
5888 VERSION may be either a numeric argument such as 5.006, which will be
5889 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5890 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5891 greater than the version of the current Perl interpreter; Perl will not
5892 attempt to parse the rest of the file. Compare with L</require>, which can
5893 do a similar check at run time.
5895 Specifying VERSION as a literal of the form v5.6.1 should generally be
5896 avoided, because it leads to misleading error messages under earlier
5897 versions of Perl which do not support this syntax. The equivalent numeric
5898 version should be used instead.
5900 use v5.6.1; # compile time version check
5902 use 5.006_001; # ditto; preferred for backwards compatibility
5904 This is often useful if you need to check the current Perl version before
5905 C<use>ing library modules that have changed in incompatible ways from
5906 older versions of Perl. (We try not to do this more than we have to.)
5908 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5909 C<require> makes sure the module is loaded into memory if it hasn't been
5910 yet. The C<import> is not a builtin--it's just an ordinary static method
5911 call into the C<Module> package to tell the module to import the list of
5912 features back into the current package. The module can implement its
5913 C<import> method any way it likes, though most modules just choose to
5914 derive their C<import> method via inheritance from the C<Exporter> class that
5915 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5916 method can be found then the call is skipped.
5918 If you do not want to call the package's C<import> method (for instance,
5919 to stop your namespace from being altered), explicitly supply the empty list:
5923 That is exactly equivalent to
5925 BEGIN { require Module }
5927 If the VERSION argument is present between Module and LIST, then the
5928 C<use> will call the VERSION method in class Module with the given
5929 version as an argument. The default VERSION method, inherited from
5930 the UNIVERSAL class, croaks if the given version is larger than the
5931 value of the variable C<$Module::VERSION>.
5933 Again, there is a distinction between omitting LIST (C<import> called
5934 with no arguments) and an explicit empty LIST C<()> (C<import> not
5935 called). Note that there is no comma after VERSION!
5937 Because this is a wide-open interface, pragmas (compiler directives)
5938 are also implemented this way. Currently implemented pragmas are:
5943 use sigtrap qw(SEGV BUS);
5944 use strict qw(subs vars refs);
5945 use subs qw(afunc blurfl);
5946 use warnings qw(all);
5947 use sort qw(stable _quicksort _mergesort);
5949 Some of these pseudo-modules import semantics into the current
5950 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5951 which import symbols into the current package (which are effective
5952 through the end of the file).
5954 There's a corresponding C<no> command that unimports meanings imported
5955 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5961 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5962 for the C<-M> and C<-m> command-line options to perl that give C<use>
5963 functionality from the command-line.
5967 Changes the access and modification times on each file of a list of
5968 files. The first two elements of the list must be the NUMERICAL access
5969 and modification times, in that order. Returns the number of files
5970 successfully changed. The inode change time of each file is set
5971 to the current time. This code has the same effect as the C<touch>
5972 command if the files already exist:
5976 utime $now, $now, @ARGV;
5978 If the first two elements of the list are C<undef>, then the utime(2)
5979 function in the C library will be called with a null second argument.
5980 On most systems, this will set the file's access and modification
5981 times to the current time. (i.e. equivalent to the example above.)
5983 utime undef, undef, @ARGV;
5987 Returns a list consisting of all the values of the named hash. (In a
5988 scalar context, returns the number of values.) The values are
5989 returned in an apparently random order. The actual random order is
5990 subject to change in future versions of perl, but it is guaranteed to
5991 be the same order as either the C<keys> or C<each> function would
5992 produce on the same (unmodified) hash.
5994 Note that the values are not copied, which means modifying them will
5995 modify the contents of the hash:
5997 for (values %hash) { s/foo/bar/g } # modifies %hash values
5998 for (@hash{keys %hash}) { s/foo/bar/g } # same
6000 As a side effect, calling values() resets the HASH's internal iterator.
6001 See also C<keys>, C<each>, and C<sort>.
6003 =item vec EXPR,OFFSET,BITS
6005 Treats the string in EXPR as a bit vector made up of elements of
6006 width BITS, and returns the value of the element specified by OFFSET
6007 as an unsigned integer. BITS therefore specifies the number of bits
6008 that are reserved for each element in the bit vector. This must
6009 be a power of two from 1 to 32 (or 64, if your platform supports
6012 If BITS is 8, "elements" coincide with bytes of the input string.
6014 If BITS is 16 or more, bytes of the input string are grouped into chunks
6015 of size BITS/8, and each group is converted to a number as with
6016 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6017 for BITS==64). See L<"pack"> for details.
6019 If bits is 4 or less, the string is broken into bytes, then the bits
6020 of each byte are broken into 8/BITS groups. Bits of a byte are
6021 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6022 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6023 breaking the single input byte C<chr(0x36)> into two groups gives a list
6024 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6026 C<vec> may also be assigned to, in which case parentheses are needed
6027 to give the expression the correct precedence as in
6029 vec($image, $max_x * $x + $y, 8) = 3;
6031 If the selected element is outside the string, the value 0 is returned.
6032 If an element off the end of the string is written to, Perl will first
6033 extend the string with sufficiently many zero bytes. It is an error
6034 to try to write off the beginning of the string (i.e. negative OFFSET).
6036 The string should not contain any character with the value > 255 (which
6037 can only happen if you're using UTF8 encoding). If it does, it will be
6038 treated as something which is not UTF8 encoded. When the C<vec> was
6039 assigned to, other parts of your program will also no longer consider the
6040 string to be UTF8 encoded. In other words, if you do have such characters
6041 in your string, vec() will operate on the actual byte string, and not the
6042 conceptual character string.
6044 Strings created with C<vec> can also be manipulated with the logical
6045 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6046 vector operation is desired when both operands are strings.
6047 See L<perlop/"Bitwise String Operators">.
6049 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6050 The comments show the string after each step. Note that this code works
6051 in the same way on big-endian or little-endian machines.
6054 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6056 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6057 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6059 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6060 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6061 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6062 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6063 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6064 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6066 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6067 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6068 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6071 To transform a bit vector into a string or list of 0's and 1's, use these:
6073 $bits = unpack("b*", $vector);
6074 @bits = split(//, unpack("b*", $vector));
6076 If you know the exact length in bits, it can be used in place of the C<*>.
6078 Here is an example to illustrate how the bits actually fall in place:
6084 unpack("V",$_) 01234567890123456789012345678901
6085 ------------------------------------------------------------------
6090 for ($shift=0; $shift < $width; ++$shift) {
6091 for ($off=0; $off < 32/$width; ++$off) {
6092 $str = pack("B*", "0"x32);
6093 $bits = (1<<$shift);
6094 vec($str, $off, $width) = $bits;
6095 $res = unpack("b*",$str);
6096 $val = unpack("V", $str);
6103 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6104 $off, $width, $bits, $val, $res
6108 Regardless of the machine architecture on which it is run, the above
6109 example should print the following table:
6112 unpack("V",$_) 01234567890123456789012345678901
6113 ------------------------------------------------------------------
6114 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6115 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6116 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6117 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6118 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6119 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6120 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6121 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6122 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6123 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6124 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6125 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6126 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6127 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6128 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6129 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6130 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6131 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6132 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6133 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6134 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6135 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6136 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6137 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6138 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6139 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6140 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6141 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6142 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6143 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6144 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6145 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6146 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6147 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6148 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6149 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6150 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6151 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6152 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6153 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6154 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6155 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6156 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6157 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6158 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6159 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6160 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6161 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6162 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6163 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6164 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6165 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6166 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6167 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6168 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6169 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6170 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6171 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6172 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6173 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6174 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6175 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6176 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6177 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6178 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6179 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6180 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6181 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6182 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6183 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6184 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6185 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6186 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6187 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6188 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6189 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6190 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6191 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6192 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6193 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6194 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6195 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6196 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6197 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6198 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6199 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6200 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6201 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6202 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6203 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6204 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6205 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6206 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6207 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6208 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6209 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6210 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6211 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6212 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6213 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6214 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6215 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6216 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6217 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6218 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6219 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6220 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6221 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6222 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6223 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6224 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6225 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6226 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6227 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6228 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6229 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6230 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6231 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6232 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6233 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6234 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6235 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6236 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6237 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6238 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6239 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6240 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6241 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6245 Behaves like the wait(2) system call on your system: it waits for a child
6246 process to terminate and returns the pid of the deceased process, or
6247 C<-1> if there are no child processes. The status is returned in C<$?>.
6248 Note that a return value of C<-1> could mean that child processes are
6249 being automatically reaped, as described in L<perlipc>.
6251 =item waitpid PID,FLAGS
6253 Waits for a particular child process to terminate and returns the pid of
6254 the deceased process, or C<-1> if there is no such child process. On some
6255 systems, a value of 0 indicates that there are processes still running.
6256 The status is returned in C<$?>. If you say
6258 use POSIX ":sys_wait_h";
6261 $kid = waitpid(-1, WNOHANG);
6264 then you can do a non-blocking wait for all pending zombie processes.
6265 Non-blocking wait is available on machines supporting either the
6266 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6267 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6268 system call by remembering the status values of processes that have
6269 exited but have not been harvested by the Perl script yet.)
6271 Note that on some systems, a return value of C<-1> could mean that child
6272 processes are being automatically reaped. See L<perlipc> for details,
6273 and for other examples.
6277 Returns true if the context of the currently executing subroutine is
6278 looking for a list value. Returns false if the context is looking
6279 for a scalar. Returns the undefined value if the context is looking
6280 for no value (void context).
6282 return unless defined wantarray; # don't bother doing more
6283 my @a = complex_calculation();
6284 return wantarray ? @a : "@a";
6286 This function should have been named wantlist() instead.
6290 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6293 If LIST is empty and C<$@> already contains a value (typically from a
6294 previous eval) that value is used after appending C<"\t...caught">
6295 to C<$@>. This is useful for staying almost, but not entirely similar to
6298 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6300 No message is printed if there is a C<$SIG{__WARN__}> handler
6301 installed. It is the handler's responsibility to deal with the message
6302 as it sees fit (like, for instance, converting it into a C<die>). Most
6303 handlers must therefore make arrangements to actually display the
6304 warnings that they are not prepared to deal with, by calling C<warn>
6305 again in the handler. Note that this is quite safe and will not
6306 produce an endless loop, since C<__WARN__> hooks are not called from
6309 You will find this behavior is slightly different from that of
6310 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6311 instead call C<die> again to change it).
6313 Using a C<__WARN__> handler provides a powerful way to silence all
6314 warnings (even the so-called mandatory ones). An example:
6316 # wipe out *all* compile-time warnings
6317 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6319 my $foo = 20; # no warning about duplicate my $foo,
6320 # but hey, you asked for it!
6321 # no compile-time or run-time warnings before here
6324 # run-time warnings enabled after here
6325 warn "\$foo is alive and $foo!"; # does show up
6327 See L<perlvar> for details on setting C<%SIG> entries, and for more
6328 examples. See the Carp module for other kinds of warnings using its
6329 carp() and cluck() functions.
6331 =item write FILEHANDLE
6337 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6338 using the format associated with that file. By default the format for
6339 a file is the one having the same name as the filehandle, but the
6340 format for the current output channel (see the C<select> function) may be set
6341 explicitly by assigning the name of the format to the C<$~> variable.
6343 Top of form processing is handled automatically: if there is
6344 insufficient room on the current page for the formatted record, the
6345 page is advanced by writing a form feed, a special top-of-page format
6346 is used to format the new page header, and then the record is written.
6347 By default the top-of-page format is the name of the filehandle with
6348 "_TOP" appended, but it may be dynamically set to the format of your
6349 choice by assigning the name to the C<$^> variable while the filehandle is
6350 selected. The number of lines remaining on the current page is in
6351 variable C<$->, which can be set to C<0> to force a new page.
6353 If FILEHANDLE is unspecified, output goes to the current default output
6354 channel, which starts out as STDOUT but may be changed by the
6355 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6356 is evaluated and the resulting string is used to look up the name of
6357 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6359 Note that write is I<not> the opposite of C<read>. Unfortunately.
6363 The transliteration operator. Same as C<tr///>. See L<perlop>.