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<umask>,
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<sockatmark>, 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<sockatmark>, 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 Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
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.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of wallclock seconds have elapsed. If SECONDS is not
390 specified, the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less or more
392 than you specified because of how seconds are counted, and process
393 scheduling may delay the delivery of the signal even further.)
395 Only one timer may be counting at once. Each call disables the
396 previous timer, and an argument of C<0> may be supplied to cancel the
397 previous timer without starting a new one. The returned value is the
398 amount of time remaining on the previous timer.
400 For delays of finer granularity than one second, you may use Perl's
401 four-argument version of select() leaving the first three arguments
402 undefined, or you might be able to use the C<syscall> interface to
403 access setitimer(2) if your system supports it. The Time::HiRes
404 module (from CPAN, and starting from Perl 5.8 part of the standard
405 distribution) may also prove useful.
407 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
408 (C<sleep> may be internally implemented in your system with C<alarm>)
410 If you want to use C<alarm> to time out a system call you need to use an
411 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
412 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
413 restart system calls on some systems. Using C<eval>/C<die> always works,
414 modulo the caveats given in L<perlipc/"Signals">.
417 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
419 $nread = sysread SOCKET, $buffer, $size;
423 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 Returns the arctangent of Y/X in the range -PI to PI.
434 For the tangent operation, you may use the C<Math::Trig::tan>
435 function, or use the familiar relation:
437 sub tan { sin($_[0]) / cos($_[0]) }
439 =item bind SOCKET,NAME
441 Binds a network address to a socket, just as the bind system call
442 does. Returns true if it succeeded, false otherwise. NAME should be a
443 packed address of the appropriate type for the socket. See the examples in
444 L<perlipc/"Sockets: Client/Server Communication">.
446 =item binmode FILEHANDLE, DISCIPLINE
448 =item binmode FILEHANDLE
450 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
451 on systems where the run-time libraries distinguish between binary and
452 text files. If FILEHANDLE is an expression, the value is taken as the
453 name of the filehandle. DISCIPLINE can be either of C<:raw> for
454 binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
455 omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
456 failure. The C<:raw> are C<:clrf>, and any other directives of the
457 form C<:...>, are called I/O I<disciplines>.
459 The C<open> pragma can be used to establish default I/O disciplines.
462 In general, binmode() should be called after open() but before any I/O
463 is done on the filehandle. Calling binmode() will flush any possibly
464 pending buffered input or output data on the handle. The only
465 exception to this is the C<:encoding> discipline that changes
466 the default character encoding of the handle, see L<open>.
467 The C<:encoding> discipline sometimes needs to be called in
468 mid-stream, and it doesn't flush the stream.
470 On some systems binmode() is necessary when you're not working with a
471 text file. For the sake of portability it is a good idea to always use
472 it when appropriate, and to never use it when it isn't appropriate.
474 In other words: Regardless of platform, use binmode() on binary
475 files, and do not use binmode() on text files.
477 The operating system, device drivers, C libraries, and Perl run-time
478 system all work together to let the programmer treat a single
479 character (C<\n>) as the line terminator, irrespective of the external
480 representation. On many operating systems, the native text file
481 representation matches the internal representation, but on some
482 platforms the external representation of C<\n> is made up of more than
485 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
486 character to end each line in the external representation of text (even
487 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
488 on Unix and most VMS files). Consequently binmode() has no effect on
489 these operating systems. In other systems like OS/2, DOS and the various
490 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
491 what's stored in text files are the two characters C<\cM\cJ>. That means
492 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
493 disk will be converted to C<\n> on input, and any C<\n> in your program
494 will be converted back to C<\cM\cJ> on output. This is what you want for
495 text files, but it can be disastrous for binary files.
497 Another consequence of using binmode() (on some systems) is that
498 special end-of-file markers will be seen as part of the data stream.
499 For systems from the Microsoft family this means that if your binary
500 data contains C<\cZ>, the I/O subsystem will regard it as the end of
501 the file, unless you use binmode().
503 binmode() is not only important for readline() and print() operations,
504 but also when using read(), seek(), sysread(), syswrite() and tell()
505 (see L<perlport> for more details). See the C<$/> and C<$\> variables
506 in L<perlvar> for how to manually set your input and output
507 line-termination sequences.
509 =item bless REF,CLASSNAME
513 This function tells the thingy referenced by REF that it is now an object
514 in the CLASSNAME package. If CLASSNAME is omitted, the current package
515 is used. Because a C<bless> is often the last thing in a constructor,
516 it returns the reference for convenience. Always use the two-argument
517 version if the function doing the blessing might be inherited by a
518 derived class. See L<perltoot> and L<perlobj> for more about the blessing
519 (and blessings) of objects.
521 Consider always blessing objects in CLASSNAMEs that are mixed case.
522 Namespaces with all lowercase names are considered reserved for
523 Perl pragmata. Builtin types have all uppercase names, so to prevent
524 confusion, you may wish to avoid such package names as well. Make sure
525 that CLASSNAME is a true value.
527 See L<perlmod/"Perl Modules">.
533 Returns the context of the current subroutine call. In scalar context,
534 returns the caller's package name if there is a caller, that is, if
535 we're in a subroutine or C<eval> or C<require>, and the undefined value
536 otherwise. In list context, returns
538 ($package, $filename, $line) = caller;
540 With EXPR, it returns some extra information that the debugger uses to
541 print a stack trace. The value of EXPR indicates how many call frames
542 to go back before the current one.
544 ($package, $filename, $line, $subroutine, $hasargs,
545 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
547 Here $subroutine may be C<(eval)> if the frame is not a subroutine
548 call, but an C<eval>. In such a case additional elements $evaltext and
549 C<$is_require> are set: C<$is_require> is true if the frame is created by a
550 C<require> or C<use> statement, $evaltext contains the text of the
551 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
552 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
553 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
554 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
555 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
556 was compiled with. The C<$hints> and C<$bitmask> values are subject to
557 change between versions of Perl, and are not meant for external use.
559 Furthermore, when called from within the DB package, caller returns more
560 detailed information: it sets the list variable C<@DB::args> to be the
561 arguments with which the subroutine was invoked.
563 Be aware that the optimizer might have optimized call frames away before
564 C<caller> had a chance to get the information. That means that C<caller(N)>
565 might not return information about the call frame you expect it do, for
566 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
567 previous time C<caller> was called.
571 Changes the working directory to EXPR, if possible. If EXPR is omitted,
572 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
573 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
574 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
575 neither is set, C<chdir> does nothing. It returns true upon success,
576 false otherwise. See the example under C<die>.
580 Changes the permissions of a list of files. The first element of the
581 list must be the numerical mode, which should probably be an octal
582 number, and which definitely should I<not> a string of octal digits:
583 C<0644> is okay, C<'0644'> is not. Returns the number of files
584 successfully changed. See also L</oct>, if all you have is a string.
586 $cnt = chmod 0755, 'foo', 'bar';
587 chmod 0755, @executables;
588 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
590 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
591 $mode = 0644; chmod $mode, 'foo'; # this is best
593 You can also import the symbolic C<S_I*> constants from the Fcntl
598 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
599 # This is identical to the chmod 0755 of the above example.
607 This safer version of L</chop> removes any trailing string
608 that corresponds to the current value of C<$/> (also known as
609 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
610 number of characters removed from all its arguments. It's often used to
611 remove the newline from the end of an input record when you're worried
612 that the final record may be missing its newline. When in paragraph
613 mode (C<$/ = "">), it removes all trailing newlines from the string.
614 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
615 a reference to an integer or the like, see L<perlvar>) chomp() won't
617 If VARIABLE is omitted, it chomps C<$_>. Example:
620 chomp; # avoid \n on last field
625 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
627 You can actually chomp anything that's an lvalue, including an assignment:
630 chomp($answer = <STDIN>);
632 If you chomp a list, each element is chomped, and the total number of
633 characters removed is returned.
641 Chops off the last character of a string and returns the character
642 chopped. It is much more efficient than C<s/.$//s> because it neither
643 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
644 If VARIABLE is a hash, it chops the hash's values, but not its keys.
646 You can actually chop anything that's an lvalue, including an assignment.
648 If you chop a list, each element is chopped. Only the value of the
649 last C<chop> is returned.
651 Note that C<chop> returns the last character. To return all but the last
652 character, use C<substr($string, 0, -1)>.
656 Changes the owner (and group) of a list of files. The first two
657 elements of the list must be the I<numeric> uid and gid, in that
658 order. A value of -1 in either position is interpreted by most
659 systems to leave that value unchanged. Returns the number of files
660 successfully changed.
662 $cnt = chown $uid, $gid, 'foo', 'bar';
663 chown $uid, $gid, @filenames;
665 Here's an example that looks up nonnumeric uids in the passwd file:
668 chomp($user = <STDIN>);
670 chomp($pattern = <STDIN>);
672 ($login,$pass,$uid,$gid) = getpwnam($user)
673 or die "$user not in passwd file";
675 @ary = glob($pattern); # expand filenames
676 chown $uid, $gid, @ary;
678 On most systems, you are not allowed to change the ownership of the
679 file unless you're the superuser, although you should be able to change
680 the group to any of your secondary groups. On insecure systems, these
681 restrictions may be relaxed, but this is not a portable assumption.
682 On POSIX systems, you can detect this condition this way:
684 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
685 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
691 Returns the character represented by that NUMBER in the character set.
692 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
693 chr(0x263a) is a Unicode smiley face. Note that characters from 127
694 to 255 (inclusive) are by default not encoded in Unicode for backward
695 compatibility reasons (but see L<encoding>).
697 For the reverse, use L</ord>.
698 See L<perlunicode> and L<encoding> for more about Unicode.
700 If NUMBER is omitted, uses C<$_>.
702 =item chroot FILENAME
706 This function works like the system call by the same name: it makes the
707 named directory the new root directory for all further pathnames that
708 begin with a C</> by your process and all its children. (It doesn't
709 change your current working directory, which is unaffected.) For security
710 reasons, this call is restricted to the superuser. If FILENAME is
711 omitted, does a C<chroot> to C<$_>.
713 =item close FILEHANDLE
717 Closes the file or pipe associated with the file handle, returning
718 true only if IO buffers are successfully flushed and closes the system
719 file descriptor. Closes the currently selected filehandle if the
722 You don't have to close FILEHANDLE if you are immediately going to do
723 another C<open> on it, because C<open> will close it for you. (See
724 C<open>.) However, an explicit C<close> on an input file resets the line
725 counter (C<$.>), while the implicit close done by C<open> does not.
727 If the file handle came from a piped open C<close> will additionally
728 return false if one of the other system calls involved fails or if the
729 program exits with non-zero status. (If the only problem was that the
730 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
731 also waits for the process executing on the pipe to complete, in case you
732 want to look at the output of the pipe afterwards, and
733 implicitly puts the exit status value of that command into C<$?>.
735 Prematurely closing the read end of a pipe (i.e. before the process
736 writing to it at the other end has closed it) will result in a
737 SIGPIPE being delivered to the writer. If the other end can't
738 handle that, be sure to read all the data before closing the pipe.
742 open(OUTPUT, '|sort >foo') # pipe to sort
743 or die "Can't start sort: $!";
744 #... # print stuff to output
745 close OUTPUT # wait for sort to finish
746 or warn $! ? "Error closing sort pipe: $!"
747 : "Exit status $? from sort";
748 open(INPUT, 'foo') # get sort's results
749 or die "Can't open 'foo' for input: $!";
751 FILEHANDLE may be an expression whose value can be used as an indirect
752 filehandle, usually the real filehandle name.
754 =item closedir DIRHANDLE
756 Closes a directory opened by C<opendir> and returns the success of that
759 DIRHANDLE may be an expression whose value can be used as an indirect
760 dirhandle, usually the real dirhandle name.
762 =item connect SOCKET,NAME
764 Attempts to connect to a remote socket, just as the connect system call
765 does. Returns true if it succeeded, false otherwise. NAME should be a
766 packed address of the appropriate type for the socket. See the examples in
767 L<perlipc/"Sockets: Client/Server Communication">.
771 Actually a flow control statement rather than a function. If there is a
772 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
773 C<foreach>), it is always executed just before the conditional is about to
774 be evaluated again, just like the third part of a C<for> loop in C. Thus
775 it can be used to increment a loop variable, even when the loop has been
776 continued via the C<next> statement (which is similar to the C C<continue>
779 C<last>, C<next>, or C<redo> may appear within a C<continue>
780 block. C<last> and C<redo> will behave as if they had been executed within
781 the main block. So will C<next>, but since it will execute a C<continue>
782 block, it may be more entertaining.
785 ### redo always comes here
788 ### next always comes here
790 # then back the top to re-check EXPR
792 ### last always comes here
794 Omitting the C<continue> section is semantically equivalent to using an
795 empty one, logically enough. In that case, C<next> goes directly back
796 to check the condition at the top of the loop.
802 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
803 takes cosine of C<$_>.
805 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
806 function, or use this relation:
808 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
810 =item crypt PLAINTEXT,SALT
812 Encrypts a string exactly like the crypt(3) function in the C library
813 (assuming that you actually have a version there that has not been
814 extirpated as a potential munition). This can prove useful for checking
815 the password file for lousy passwords, amongst other things. Only the
816 guys wearing white hats should do this.
818 Note that C<crypt> is intended to be a one-way function, much like
819 breaking eggs to make an omelette. There is no (known) corresponding
820 decrypt function (in other words, the crypt() is a one-way hash
821 function). As a result, this function isn't all that useful for
822 cryptography. (For that, see your nearby CPAN mirror.)
824 When verifying an existing encrypted string you should use the
825 encrypted text as the salt (like C<crypt($plain, $crypted) eq
826 $crypted>). This allows your code to work with the standard C<crypt>
827 and with more exotic implementations. In other words, do not assume
828 anything about the returned string itself, or how many bytes in
829 the encrypted string matter.
831 Traditionally the result is a string of 13 bytes: two first bytes of
832 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
833 the first eight bytes of the encrypted string mattered, but
834 alternative hashing schemes (like MD5), higher level security schemes
835 (like C2), and implementations on non-UNIX platforms may produce
838 When choosing a new salt create a random two character string whose
839 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
840 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
842 Here's an example that makes sure that whoever runs this program knows
845 $pwd = (getpwuid($<))[1];
849 chomp($word = <STDIN>);
853 if (crypt($word, $pwd) ne $pwd) {
859 Of course, typing in your own password to whoever asks you
862 The L<crypt> function is unsuitable for encrypting large quantities
863 of data, not least of all because you can't get the information
864 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
865 on your favorite CPAN mirror for a slew of potentially useful
868 If using crypt() on a Unicode string (which potentially has
869 characters with codepoints above 255), Perl tries to make sense of
870 the situation by using only the low eight bits of the characters when
875 [This function has been largely superseded by the C<untie> function.]
877 Breaks the binding between a DBM file and a hash.
879 =item dbmopen HASH,DBNAME,MASK
881 [This function has been largely superseded by the C<tie> function.]
883 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
884 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
885 argument is I<not> a filehandle, even though it looks like one). DBNAME
886 is the name of the database (without the F<.dir> or F<.pag> extension if
887 any). If the database does not exist, it is created with protection
888 specified by MASK (as modified by the C<umask>). If your system supports
889 only the older DBM functions, you may perform only one C<dbmopen> in your
890 program. In older versions of Perl, if your system had neither DBM nor
891 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
894 If you don't have write access to the DBM file, you can only read hash
895 variables, not set them. If you want to test whether you can write,
896 either use file tests or try setting a dummy hash entry inside an C<eval>,
897 which will trap the error.
899 Note that functions such as C<keys> and C<values> may return huge lists
900 when used on large DBM files. You may prefer to use the C<each>
901 function to iterate over large DBM files. Example:
903 # print out history file offsets
904 dbmopen(%HIST,'/usr/lib/news/history',0666);
905 while (($key,$val) = each %HIST) {
906 print $key, ' = ', unpack('L',$val), "\n";
910 See also L<AnyDBM_File> for a more general description of the pros and
911 cons of the various dbm approaches, as well as L<DB_File> for a particularly
914 You can control which DBM library you use by loading that library
915 before you call dbmopen():
918 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
919 or die "Can't open netscape history file: $!";
925 Returns a Boolean value telling whether EXPR has a value other than
926 the undefined value C<undef>. If EXPR is not present, C<$_> will be
929 Many operations return C<undef> to indicate failure, end of file,
930 system error, uninitialized variable, and other exceptional
931 conditions. This function allows you to distinguish C<undef> from
932 other values. (A simple Boolean test will not distinguish among
933 C<undef>, zero, the empty string, and C<"0">, which are all equally
934 false.) Note that since C<undef> is a valid scalar, its presence
935 doesn't I<necessarily> indicate an exceptional condition: C<pop>
936 returns C<undef> when its argument is an empty array, I<or> when the
937 element to return happens to be C<undef>.
939 You may also use C<defined(&func)> to check whether subroutine C<&func>
940 has ever been defined. The return value is unaffected by any forward
941 declarations of C<&foo>. Note that a subroutine which is not defined
942 may still be callable: its package may have an C<AUTOLOAD> method that
943 makes it spring into existence the first time that it is called -- see
946 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
947 used to report whether memory for that aggregate has ever been
948 allocated. This behavior may disappear in future versions of Perl.
949 You should instead use a simple test for size:
951 if (@an_array) { print "has array elements\n" }
952 if (%a_hash) { print "has hash members\n" }
954 When used on a hash element, it tells you whether the value is defined,
955 not whether the key exists in the hash. Use L</exists> for the latter
960 print if defined $switch{'D'};
961 print "$val\n" while defined($val = pop(@ary));
962 die "Can't readlink $sym: $!"
963 unless defined($value = readlink $sym);
964 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
965 $debugging = 0 unless defined $debugging;
967 Note: Many folks tend to overuse C<defined>, and then are surprised to
968 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
969 defined values. For example, if you say
973 The pattern match succeeds, and C<$1> is defined, despite the fact that it
974 matched "nothing". But it didn't really match nothing--rather, it
975 matched something that happened to be zero characters long. This is all
976 very above-board and honest. When a function returns an undefined value,
977 it's an admission that it couldn't give you an honest answer. So you
978 should use C<defined> only when you're questioning the integrity of what
979 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
982 See also L</undef>, L</exists>, L</ref>.
986 Given an expression that specifies a hash element, array element, hash slice,
987 or array slice, deletes the specified element(s) from the hash or array.
988 In the case of an array, if the array elements happen to be at the end,
989 the size of the array will shrink to the highest element that tests
990 true for exists() (or 0 if no such element exists).
992 Returns each element so deleted or the undefined value if there was no such
993 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
994 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
995 from a C<tie>d hash or array may not necessarily return anything.
997 Deleting an array element effectively returns that position of the array
998 to its initial, uninitialized state. Subsequently testing for the same
999 element with exists() will return false. Note that deleting array
1000 elements in the middle of an array will not shift the index of the ones
1001 after them down--use splice() for that. See L</exists>.
1003 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1005 foreach $key (keys %HASH) {
1009 foreach $index (0 .. $#ARRAY) {
1010 delete $ARRAY[$index];
1015 delete @HASH{keys %HASH};
1017 delete @ARRAY[0 .. $#ARRAY];
1019 But both of these are slower than just assigning the empty list
1020 or undefining %HASH or @ARRAY:
1022 %HASH = (); # completely empty %HASH
1023 undef %HASH; # forget %HASH ever existed
1025 @ARRAY = (); # completely empty @ARRAY
1026 undef @ARRAY; # forget @ARRAY ever existed
1028 Note that the EXPR can be arbitrarily complicated as long as the final
1029 operation is a hash element, array element, hash slice, or array slice
1032 delete $ref->[$x][$y]{$key};
1033 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1035 delete $ref->[$x][$y][$index];
1036 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1040 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1041 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1042 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1043 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1044 an C<eval(),> the error message is stuffed into C<$@> and the
1045 C<eval> is terminated with the undefined value. This makes
1046 C<die> the way to raise an exception.
1048 Equivalent examples:
1050 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1051 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1053 If the value of EXPR does not end in a newline, the current script line
1054 number and input line number (if any) are also printed, and a newline
1055 is supplied. Note that the "input line number" (also known as "chunk")
1056 is subject to whatever notion of "line" happens to be currently in
1057 effect, and is also available as the special variable C<$.>.
1058 See L<perlvar/"$/"> and L<perlvar/"$.">.
1060 Hint: sometimes appending C<", stopped"> to your message
1061 will cause it to make better sense when the string C<"at foo line 123"> is
1062 appended. Suppose you are running script "canasta".
1064 die "/etc/games is no good";
1065 die "/etc/games is no good, stopped";
1067 produce, respectively
1069 /etc/games is no good at canasta line 123.
1070 /etc/games is no good, stopped at canasta line 123.
1072 See also exit(), warn(), and the Carp module.
1074 If LIST is empty and C<$@> already contains a value (typically from a
1075 previous eval) that value is reused after appending C<"\t...propagated">.
1076 This is useful for propagating exceptions:
1079 die unless $@ =~ /Expected exception/;
1081 If LIST is empty and C<$@> contains an object reference that has a
1082 C<PROPAGATE> method, that method will be called with additional file
1083 and line number parameters. The return value replaces the value in
1084 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1087 If C<$@> is empty then the string C<"Died"> is used.
1089 die() can also be called with a reference argument. If this happens to be
1090 trapped within an eval(), $@ contains the reference. This behavior permits
1091 a more elaborate exception handling implementation using objects that
1092 maintain arbitrary state about the nature of the exception. Such a scheme
1093 is sometimes preferable to matching particular string values of $@ using
1094 regular expressions. Here's an example:
1096 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1098 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1099 # handle Some::Module::Exception
1102 # handle all other possible exceptions
1106 Because perl will stringify uncaught exception messages before displaying
1107 them, you may want to overload stringification operations on such custom
1108 exception objects. See L<overload> for details about that.
1110 You can arrange for a callback to be run just before the C<die>
1111 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1112 handler will be called with the error text and can change the error
1113 message, if it sees fit, by calling C<die> again. See
1114 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1115 L<"eval BLOCK"> for some examples. Although this feature was meant
1116 to be run only right before your program was to exit, this is not
1117 currently the case--the C<$SIG{__DIE__}> hook is currently called
1118 even inside eval()ed blocks/strings! If one wants the hook to do
1119 nothing in such situations, put
1123 as the first line of the handler (see L<perlvar/$^S>). Because
1124 this promotes strange action at a distance, this counterintuitive
1125 behavior may be fixed in a future release.
1129 Not really a function. Returns the value of the last command in the
1130 sequence of commands indicated by BLOCK. When modified by a loop
1131 modifier, executes the BLOCK once before testing the loop condition.
1132 (On other statements the loop modifiers test the conditional first.)
1134 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1135 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1136 See L<perlsyn> for alternative strategies.
1138 =item do SUBROUTINE(LIST)
1140 A deprecated form of subroutine call. See L<perlsub>.
1144 Uses the value of EXPR as a filename and executes the contents of the
1145 file as a Perl script. Its primary use is to include subroutines
1146 from a Perl subroutine library.
1154 except that it's more efficient and concise, keeps track of the current
1155 filename for error messages, searches the @INC libraries, and updates
1156 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1157 variables. It also differs in that code evaluated with C<do FILENAME>
1158 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1159 same, however, in that it does reparse the file every time you call it,
1160 so you probably don't want to do this inside a loop.
1162 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1163 error. If C<do> can read the file but cannot compile it, it
1164 returns undef and sets an error message in C<$@>. If the file is
1165 successfully compiled, C<do> returns the value of the last expression
1168 Note that inclusion of library modules is better done with the
1169 C<use> and C<require> operators, which also do automatic error checking
1170 and raise an exception if there's a problem.
1172 You might like to use C<do> to read in a program configuration
1173 file. Manual error checking can be done this way:
1175 # read in config files: system first, then user
1176 for $file ("/share/prog/defaults.rc",
1177 "$ENV{HOME}/.someprogrc")
1179 unless ($return = do $file) {
1180 warn "couldn't parse $file: $@" if $@;
1181 warn "couldn't do $file: $!" unless defined $return;
1182 warn "couldn't run $file" unless $return;
1190 This function causes an immediate core dump. See also the B<-u>
1191 command-line switch in L<perlrun>, which does the same thing.
1192 Primarily this is so that you can use the B<undump> program (not
1193 supplied) to turn your core dump into an executable binary after
1194 having initialized all your variables at the beginning of the
1195 program. When the new binary is executed it will begin by executing
1196 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1197 Think of it as a goto with an intervening core dump and reincarnation.
1198 If C<LABEL> is omitted, restarts the program from the top.
1200 B<WARNING>: Any files opened at the time of the dump will I<not>
1201 be open any more when the program is reincarnated, with possible
1202 resulting confusion on the part of Perl.
1204 This function is now largely obsolete, partly because it's very
1205 hard to convert a core file into an executable, and because the
1206 real compiler backends for generating portable bytecode and compilable
1207 C code have superseded it.
1209 If you're looking to use L<dump> to speed up your program, consider
1210 generating bytecode or native C code as described in L<perlcc>. If
1211 you're just trying to accelerate a CGI script, consider using the
1212 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1213 You might also consider autoloading or selfloading, which at least
1214 make your program I<appear> to run faster.
1218 When called in list context, returns a 2-element list consisting of the
1219 key and value for the next element of a hash, so that you can iterate over
1220 it. When called in scalar context, returns only the key for the next
1221 element in the hash.
1223 Entries are returned in an apparently random order. The actual random
1224 order is subject to change in future versions of perl, but it is guaranteed
1225 to be in the same order as either the C<keys> or C<values> function
1226 would produce on the same (unmodified) hash.
1228 When the hash is entirely read, a null array is returned in list context
1229 (which when assigned produces a false (C<0>) value), and C<undef> in
1230 scalar context. The next call to C<each> after that will start iterating
1231 again. There is a single iterator for each hash, shared by all C<each>,
1232 C<keys>, and C<values> function calls in the program; it can be reset by
1233 reading all the elements from the hash, or by evaluating C<keys HASH> or
1234 C<values HASH>. If you add or delete elements of a hash while you're
1235 iterating over it, you may get entries skipped or duplicated, so
1236 don't. Exception: It is always safe to delete the item most recently
1237 returned by C<each()>, which means that the following code will work:
1239 while (($key, $value) = each %hash) {
1241 delete $hash{$key}; # This is safe
1244 The following prints out your environment like the printenv(1) program,
1245 only in a different order:
1247 while (($key,$value) = each %ENV) {
1248 print "$key=$value\n";
1251 See also C<keys>, C<values> and C<sort>.
1253 =item eof FILEHANDLE
1259 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1260 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1261 gives the real filehandle. (Note that this function actually
1262 reads a character and then C<ungetc>s it, so isn't very useful in an
1263 interactive context.) Do not read from a terminal file (or call
1264 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1265 as terminals may lose the end-of-file condition if you do.
1267 An C<eof> without an argument uses the last file read. Using C<eof()>
1268 with empty parentheses is very different. It refers to the pseudo file
1269 formed from the files listed on the command line and accessed via the
1270 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1271 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1272 used will cause C<@ARGV> to be examined to determine if input is
1275 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1276 detect the end of each file, C<eof()> will only detect the end of the
1277 last file. Examples:
1279 # reset line numbering on each input file
1281 next if /^\s*#/; # skip comments
1284 close ARGV if eof; # Not eof()!
1287 # insert dashes just before last line of last file
1289 if (eof()) { # check for end of current file
1290 print "--------------\n";
1291 close(ARGV); # close or last; is needed if we
1292 # are reading from the terminal
1297 Practical hint: you almost never need to use C<eof> in Perl, because the
1298 input operators typically return C<undef> when they run out of data, or if
1305 In the first form, the return value of EXPR is parsed and executed as if it
1306 were a little Perl program. The value of the expression (which is itself
1307 determined within scalar context) is first parsed, and if there weren't any
1308 errors, executed in the lexical context of the current Perl program, so
1309 that any variable settings or subroutine and format definitions remain
1310 afterwards. Note that the value is parsed every time the eval executes.
1311 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1312 delay parsing and subsequent execution of the text of EXPR until run time.
1314 In the second form, the code within the BLOCK is parsed only once--at the
1315 same time the code surrounding the eval itself was parsed--and executed
1316 within the context of the current Perl program. This form is typically
1317 used to trap exceptions more efficiently than the first (see below), while
1318 also providing the benefit of checking the code within BLOCK at compile
1321 The final semicolon, if any, may be omitted from the value of EXPR or within
1324 In both forms, the value returned is the value of the last expression
1325 evaluated inside the mini-program; a return statement may be also used, just
1326 as with subroutines. The expression providing the return value is evaluated
1327 in void, scalar, or list context, depending on the context of the eval itself.
1328 See L</wantarray> for more on how the evaluation context can be determined.
1330 If there is a syntax error or runtime error, or a C<die> statement is
1331 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1332 error message. If there was no error, C<$@> is guaranteed to be a null
1333 string. Beware that using C<eval> neither silences perl from printing
1334 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1335 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1336 L</warn> and L<perlvar>.
1338 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1339 determining whether a particular feature (such as C<socket> or C<symlink>)
1340 is implemented. It is also Perl's exception trapping mechanism, where
1341 the die operator is used to raise exceptions.
1343 If the code to be executed doesn't vary, you may use the eval-BLOCK
1344 form to trap run-time errors without incurring the penalty of
1345 recompiling each time. The error, if any, is still returned in C<$@>.
1348 # make divide-by-zero nonfatal
1349 eval { $answer = $a / $b; }; warn $@ if $@;
1351 # same thing, but less efficient
1352 eval '$answer = $a / $b'; warn $@ if $@;
1354 # a compile-time error
1355 eval { $answer = }; # WRONG
1358 eval '$answer ='; # sets $@
1360 Due to the current arguably broken state of C<__DIE__> hooks, when using
1361 the C<eval{}> form as an exception trap in libraries, you may wish not
1362 to trigger any C<__DIE__> hooks that user code may have installed.
1363 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1364 as shown in this example:
1366 # a very private exception trap for divide-by-zero
1367 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1370 This is especially significant, given that C<__DIE__> hooks can call
1371 C<die> again, which has the effect of changing their error messages:
1373 # __DIE__ hooks may modify error messages
1375 local $SIG{'__DIE__'} =
1376 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1377 eval { die "foo lives here" };
1378 print $@ if $@; # prints "bar lives here"
1381 Because this promotes action at a distance, this counterintuitive behavior
1382 may be fixed in a future release.
1384 With an C<eval>, you should be especially careful to remember what's
1385 being looked at when:
1391 eval { $x }; # CASE 4
1393 eval "\$$x++"; # CASE 5
1396 Cases 1 and 2 above behave identically: they run the code contained in
1397 the variable $x. (Although case 2 has misleading double quotes making
1398 the reader wonder what else might be happening (nothing is).) Cases 3
1399 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1400 does nothing but return the value of $x. (Case 4 is preferred for
1401 purely visual reasons, but it also has the advantage of compiling at
1402 compile-time instead of at run-time.) Case 5 is a place where
1403 normally you I<would> like to use double quotes, except that in this
1404 particular situation, you can just use symbolic references instead, as
1407 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1408 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1412 =item exec PROGRAM LIST
1414 The C<exec> function executes a system command I<and never returns>--
1415 use C<system> instead of C<exec> if you want it to return. It fails and
1416 returns false only if the command does not exist I<and> it is executed
1417 directly instead of via your system's command shell (see below).
1419 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1420 warns you if there is a following statement which isn't C<die>, C<warn>,
1421 or C<exit> (if C<-w> is set - but you always do that). If you
1422 I<really> want to follow an C<exec> with some other statement, you
1423 can use one of these styles to avoid the warning:
1425 exec ('foo') or print STDERR "couldn't exec foo: $!";
1426 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1428 If there is more than one argument in LIST, or if LIST is an array
1429 with more than one value, calls execvp(3) with the arguments in LIST.
1430 If there is only one scalar argument or an array with one element in it,
1431 the argument is checked for shell metacharacters, and if there are any,
1432 the entire argument is passed to the system's command shell for parsing
1433 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1434 If there are no shell metacharacters in the argument, it is split into
1435 words and passed directly to C<execvp>, which is more efficient.
1438 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1439 exec "sort $outfile | uniq";
1441 If you don't really want to execute the first argument, but want to lie
1442 to the program you are executing about its own name, you can specify
1443 the program you actually want to run as an "indirect object" (without a
1444 comma) in front of the LIST. (This always forces interpretation of the
1445 LIST as a multivalued list, even if there is only a single scalar in
1448 $shell = '/bin/csh';
1449 exec $shell '-sh'; # pretend it's a login shell
1453 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1455 When the arguments get executed via the system shell, results will
1456 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1459 Using an indirect object with C<exec> or C<system> is also more
1460 secure. This usage (which also works fine with system()) forces
1461 interpretation of the arguments as a multivalued list, even if the
1462 list had just one argument. That way you're safe from the shell
1463 expanding wildcards or splitting up words with whitespace in them.
1465 @args = ( "echo surprise" );
1467 exec @args; # subject to shell escapes
1469 exec { $args[0] } @args; # safe even with one-arg list
1471 The first version, the one without the indirect object, ran the I<echo>
1472 program, passing it C<"surprise"> an argument. The second version
1473 didn't--it tried to run a program literally called I<"echo surprise">,
1474 didn't find it, and set C<$?> to a non-zero value indicating failure.
1476 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1477 output before the exec, but this may not be supported on some platforms
1478 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1479 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1480 open handles in order to avoid lost output.
1482 Note that C<exec> will not call your C<END> blocks, nor will it call
1483 any C<DESTROY> methods in your objects.
1487 Given an expression that specifies a hash element or array element,
1488 returns true if the specified element in the hash or array has ever
1489 been initialized, even if the corresponding value is undefined. The
1490 element is not autovivified if it doesn't exist.
1492 print "Exists\n" if exists $hash{$key};
1493 print "Defined\n" if defined $hash{$key};
1494 print "True\n" if $hash{$key};
1496 print "Exists\n" if exists $array[$index];
1497 print "Defined\n" if defined $array[$index];
1498 print "True\n" if $array[$index];
1500 A hash or array element can be true only if it's defined, and defined if
1501 it exists, but the reverse doesn't necessarily hold true.
1503 Given an expression that specifies the name of a subroutine,
1504 returns true if the specified subroutine has ever been declared, even
1505 if it is undefined. Mentioning a subroutine name for exists or defined
1506 does not count as declaring it. Note that a subroutine which does not
1507 exist may still be callable: its package may have an C<AUTOLOAD>
1508 method that makes it spring into existence the first time that it is
1509 called -- see L<perlsub>.
1511 print "Exists\n" if exists &subroutine;
1512 print "Defined\n" if defined &subroutine;
1514 Note that the EXPR can be arbitrarily complicated as long as the final
1515 operation is a hash or array key lookup or subroutine name:
1517 if (exists $ref->{A}->{B}->{$key}) { }
1518 if (exists $hash{A}{B}{$key}) { }
1520 if (exists $ref->{A}->{B}->[$ix]) { }
1521 if (exists $hash{A}{B}[$ix]) { }
1523 if (exists &{$ref->{A}{B}{$key}}) { }
1525 Although the deepest nested array or hash will not spring into existence
1526 just because its existence was tested, any intervening ones will.
1527 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1528 into existence due to the existence test for the $key element above.
1529 This happens anywhere the arrow operator is used, including even:
1532 if (exists $ref->{"Some key"}) { }
1533 print $ref; # prints HASH(0x80d3d5c)
1535 This surprising autovivification in what does not at first--or even
1536 second--glance appear to be an lvalue context may be fixed in a future
1539 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1540 on how exists() acts when used on a pseudo-hash.
1542 Use of a subroutine call, rather than a subroutine name, as an argument
1543 to exists() is an error.
1546 exists &sub(); # Error
1550 Evaluates EXPR and exits immediately with that value. Example:
1553 exit 0 if $ans =~ /^[Xx]/;
1555 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1556 universally recognized values for EXPR are C<0> for success and C<1>
1557 for error; other values are subject to interpretation depending on the
1558 environment in which the Perl program is running. For example, exiting
1559 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1560 the mailer to return the item undelivered, but that's not true everywhere.
1562 Don't use C<exit> to abort a subroutine if there's any chance that
1563 someone might want to trap whatever error happened. Use C<die> instead,
1564 which can be trapped by an C<eval>.
1566 The exit() function does not always exit immediately. It calls any
1567 defined C<END> routines first, but these C<END> routines may not
1568 themselves abort the exit. Likewise any object destructors that need to
1569 be called are called before the real exit. If this is a problem, you
1570 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1571 See L<perlmod> for details.
1577 Returns I<e> (the natural logarithm base) to the power of EXPR.
1578 If EXPR is omitted, gives C<exp($_)>.
1580 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1582 Implements the fcntl(2) function. You'll probably have to say
1586 first to get the correct constant definitions. Argument processing and
1587 value return works just like C<ioctl> below.
1591 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1592 or die "can't fcntl F_GETFL: $!";
1594 You don't have to check for C<defined> on the return from C<fnctl>.
1595 Like C<ioctl>, it maps a C<0> return from the system call into
1596 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1597 in numeric context. It is also exempt from the normal B<-w> warnings
1598 on improper numeric conversions.
1600 Note that C<fcntl> will produce a fatal error if used on a machine that
1601 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1602 manpage to learn what functions are available on your system.
1604 =item fileno FILEHANDLE
1606 Returns the file descriptor for a filehandle, or undefined if the
1607 filehandle is not open. This is mainly useful for constructing
1608 bitmaps for C<select> and low-level POSIX tty-handling operations.
1609 If FILEHANDLE is an expression, the value is taken as an indirect
1610 filehandle, generally its name.
1612 You can use this to find out whether two handles refer to the
1613 same underlying descriptor:
1615 if (fileno(THIS) == fileno(THAT)) {
1616 print "THIS and THAT are dups\n";
1619 (Filehandles connected to memory objects via new features of C<open> may
1620 return undefined even though they are open.)
1623 =item flock FILEHANDLE,OPERATION
1625 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1626 for success, false on failure. Produces a fatal error if used on a
1627 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1628 C<flock> is Perl's portable file locking interface, although it locks
1629 only entire files, not records.
1631 Two potentially non-obvious but traditional C<flock> semantics are
1632 that it waits indefinitely until the lock is granted, and that its locks
1633 B<merely advisory>. Such discretionary locks are more flexible, but offer
1634 fewer guarantees. This means that files locked with C<flock> may be
1635 modified by programs that do not also use C<flock>. See L<perlport>,
1636 your port's specific documentation, or your system-specific local manpages
1637 for details. It's best to assume traditional behavior if you're writing
1638 portable programs. (But if you're not, you should as always feel perfectly
1639 free to write for your own system's idiosyncrasies (sometimes called
1640 "features"). Slavish adherence to portability concerns shouldn't get
1641 in the way of your getting your job done.)
1643 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1644 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1645 you can use the symbolic names if you import them from the Fcntl module,
1646 either individually, or as a group using the ':flock' tag. LOCK_SH
1647 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1648 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1649 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1650 waiting for the lock (check the return status to see if you got it).
1652 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1653 before locking or unlocking it.
1655 Note that the emulation built with lockf(3) doesn't provide shared
1656 locks, and it requires that FILEHANDLE be open with write intent. These
1657 are the semantics that lockf(3) implements. Most if not all systems
1658 implement lockf(3) in terms of fcntl(2) locking, though, so the
1659 differing semantics shouldn't bite too many people.
1661 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1662 be open with read intent to use LOCK_SH and requires that it be open
1663 with write intent to use LOCK_EX.
1665 Note also that some versions of C<flock> cannot lock things over the
1666 network; you would need to use the more system-specific C<fcntl> for
1667 that. If you like you can force Perl to ignore your system's flock(2)
1668 function, and so provide its own fcntl(2)-based emulation, by passing
1669 the switch C<-Ud_flock> to the F<Configure> program when you configure
1672 Here's a mailbox appender for BSD systems.
1674 use Fcntl ':flock'; # import LOCK_* constants
1677 flock(MBOX,LOCK_EX);
1678 # and, in case someone appended
1679 # while we were waiting...
1684 flock(MBOX,LOCK_UN);
1687 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1688 or die "Can't open mailbox: $!";
1691 print MBOX $msg,"\n\n";
1694 On systems that support a real flock(), locks are inherited across fork()
1695 calls, whereas those that must resort to the more capricious fcntl()
1696 function lose the locks, making it harder to write servers.
1698 See also L<DB_File> for other flock() examples.
1702 Does a fork(2) system call to create a new process running the
1703 same program at the same point. It returns the child pid to the
1704 parent process, C<0> to the child process, or C<undef> if the fork is
1705 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1706 are shared, while everything else is copied. On most systems supporting
1707 fork(), great care has gone into making it extremely efficient (for
1708 example, using copy-on-write technology on data pages), making it the
1709 dominant paradigm for multitasking over the last few decades.
1711 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1712 output before forking the child process, but this may not be supported
1713 on some platforms (see L<perlport>). To be safe, you may need to set
1714 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1715 C<IO::Handle> on any open handles in order to avoid duplicate output.
1717 If you C<fork> without ever waiting on your children, you will
1718 accumulate zombies. On some systems, you can avoid this by setting
1719 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1720 forking and reaping moribund children.
1722 Note that if your forked child inherits system file descriptors like
1723 STDIN and STDOUT that are actually connected by a pipe or socket, even
1724 if you exit, then the remote server (such as, say, a CGI script or a
1725 backgrounded job launched from a remote shell) won't think you're done.
1726 You should reopen those to F</dev/null> if it's any issue.
1730 Declare a picture format for use by the C<write> function. For
1734 Test: @<<<<<<<< @||||| @>>>>>
1735 $str, $%, '$' . int($num)
1739 $num = $cost/$quantity;
1743 See L<perlform> for many details and examples.
1745 =item formline PICTURE,LIST
1747 This is an internal function used by C<format>s, though you may call it,
1748 too. It formats (see L<perlform>) a list of values according to the
1749 contents of PICTURE, placing the output into the format output
1750 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1751 Eventually, when a C<write> is done, the contents of
1752 C<$^A> are written to some filehandle, but you could also read C<$^A>
1753 yourself and then set C<$^A> back to C<"">. Note that a format typically
1754 does one C<formline> per line of form, but the C<formline> function itself
1755 doesn't care how many newlines are embedded in the PICTURE. This means
1756 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1757 You may therefore need to use multiple formlines to implement a single
1758 record format, just like the format compiler.
1760 Be careful if you put double quotes around the picture, because an C<@>
1761 character may be taken to mean the beginning of an array name.
1762 C<formline> always returns true. See L<perlform> for other examples.
1764 =item getc FILEHANDLE
1768 Returns the next character from the input file attached to FILEHANDLE,
1769 or the undefined value at end of file, or if there was an error.
1770 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1771 efficient. However, it cannot be used by itself to fetch single
1772 characters without waiting for the user to hit enter. For that, try
1773 something more like:
1776 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1779 system "stty", '-icanon', 'eol', "\001";
1785 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1788 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1792 Determination of whether $BSD_STYLE should be set
1793 is left as an exercise to the reader.
1795 The C<POSIX::getattr> function can do this more portably on
1796 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1797 module from your nearest CPAN site; details on CPAN can be found on
1802 Implements the C library function of the same name, which on most
1803 systems returns the current login from F</etc/utmp>, if any. If null,
1806 $login = getlogin || getpwuid($<) || "Kilroy";
1808 Do not consider C<getlogin> for authentication: it is not as
1809 secure as C<getpwuid>.
1811 =item getpeername SOCKET
1813 Returns the packed sockaddr address of other end of the SOCKET connection.
1816 $hersockaddr = getpeername(SOCK);
1817 ($port, $iaddr) = sockaddr_in($hersockaddr);
1818 $herhostname = gethostbyaddr($iaddr, AF_INET);
1819 $herstraddr = inet_ntoa($iaddr);
1823 Returns the current process group for the specified PID. Use
1824 a PID of C<0> to get the current process group for the
1825 current process. Will raise an exception if used on a machine that
1826 doesn't implement getpgrp(2). If PID is omitted, returns process
1827 group of current process. Note that the POSIX version of C<getpgrp>
1828 does not accept a PID argument, so only C<PID==0> is truly portable.
1832 Returns the process id of the parent process.
1834 =item getpriority WHICH,WHO
1836 Returns the current priority for a process, a process group, or a user.
1837 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1838 machine that doesn't implement getpriority(2).
1844 =item gethostbyname NAME
1846 =item getnetbyname NAME
1848 =item getprotobyname NAME
1854 =item getservbyname NAME,PROTO
1856 =item gethostbyaddr ADDR,ADDRTYPE
1858 =item getnetbyaddr ADDR,ADDRTYPE
1860 =item getprotobynumber NUMBER
1862 =item getservbyport PORT,PROTO
1880 =item sethostent STAYOPEN
1882 =item setnetent STAYOPEN
1884 =item setprotoent STAYOPEN
1886 =item setservent STAYOPEN
1900 These routines perform the same functions as their counterparts in the
1901 system library. In list context, the return values from the
1902 various get routines are as follows:
1904 ($name,$passwd,$uid,$gid,
1905 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1906 ($name,$passwd,$gid,$members) = getgr*
1907 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1908 ($name,$aliases,$addrtype,$net) = getnet*
1909 ($name,$aliases,$proto) = getproto*
1910 ($name,$aliases,$port,$proto) = getserv*
1912 (If the entry doesn't exist you get a null list.)
1914 The exact meaning of the $gcos field varies but it usually contains
1915 the real name of the user (as opposed to the login name) and other
1916 information pertaining to the user. Beware, however, that in many
1917 system users are able to change this information and therefore it
1918 cannot be trusted and therefore the $gcos is tainted (see
1919 L<perlsec>). The $passwd and $shell, user's encrypted password and
1920 login shell, are also tainted, because of the same reason.
1922 In scalar context, you get the name, unless the function was a
1923 lookup by name, in which case you get the other thing, whatever it is.
1924 (If the entry doesn't exist you get the undefined value.) For example:
1926 $uid = getpwnam($name);
1927 $name = getpwuid($num);
1929 $gid = getgrnam($name);
1930 $name = getgrgid($num;
1934 In I<getpw*()> the fields $quota, $comment, and $expire are special
1935 cases in the sense that in many systems they are unsupported. If the
1936 $quota is unsupported, it is an empty scalar. If it is supported, it
1937 usually encodes the disk quota. If the $comment field is unsupported,
1938 it is an empty scalar. If it is supported it usually encodes some
1939 administrative comment about the user. In some systems the $quota
1940 field may be $change or $age, fields that have to do with password
1941 aging. In some systems the $comment field may be $class. The $expire
1942 field, if present, encodes the expiration period of the account or the
1943 password. For the availability and the exact meaning of these fields
1944 in your system, please consult your getpwnam(3) documentation and your
1945 F<pwd.h> file. You can also find out from within Perl what your
1946 $quota and $comment fields mean and whether you have the $expire field
1947 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1948 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1949 files are only supported if your vendor has implemented them in the
1950 intuitive fashion that calling the regular C library routines gets the
1951 shadow versions if you're running under privilege or if there exists
1952 the shadow(3) functions as found in System V ( this includes Solaris
1953 and Linux.) Those systems which implement a proprietary shadow password
1954 facility are unlikely to be supported.
1956 The $members value returned by I<getgr*()> is a space separated list of
1957 the login names of the members of the group.
1959 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1960 C, it will be returned to you via C<$?> if the function call fails. The
1961 C<@addrs> value returned by a successful call is a list of the raw
1962 addresses returned by the corresponding system library call. In the
1963 Internet domain, each address is four bytes long and you can unpack it
1964 by saying something like:
1966 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1968 The Socket library makes this slightly easier:
1971 $iaddr = inet_aton("127.1"); # or whatever address
1972 $name = gethostbyaddr($iaddr, AF_INET);
1974 # or going the other way
1975 $straddr = inet_ntoa($iaddr);
1977 If you get tired of remembering which element of the return list
1978 contains which return value, by-name interfaces are provided
1979 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1980 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1981 and C<User::grent>. These override the normal built-ins, supplying
1982 versions that return objects with the appropriate names
1983 for each field. For example:
1987 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1989 Even though it looks like they're the same method calls (uid),
1990 they aren't, because a C<File::stat> object is different from
1991 a C<User::pwent> object.
1993 =item getsockname SOCKET
1995 Returns the packed sockaddr address of this end of the SOCKET connection,
1996 in case you don't know the address because you have several different
1997 IPs that the connection might have come in on.
2000 $mysockaddr = getsockname(SOCK);
2001 ($port, $myaddr) = sockaddr_in($mysockaddr);
2002 printf "Connect to %s [%s]\n",
2003 scalar gethostbyaddr($myaddr, AF_INET),
2006 =item getsockopt SOCKET,LEVEL,OPTNAME
2008 Returns the socket option requested, or undef if there is an error.
2014 Returns the value of EXPR with filename expansions such as the
2015 standard Unix shell F</bin/csh> would do. This is the internal function
2016 implementing the C<< <*.c> >> operator, but you can use it directly.
2017 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2018 discussed in more detail in L<perlop/"I/O Operators">.
2020 Beginning with v5.6.0, this operator is implemented using the standard
2021 C<File::Glob> extension. See L<File::Glob> for details.
2025 Converts a time as returned by the time function to an 8-element list
2026 with the time localized for the standard Greenwich time zone.
2027 Typically used as follows:
2030 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2033 All list elements are numeric, and come straight out of the C `struct
2034 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2035 specified time. $mday is the day of the month, and $mon is the month
2036 itself, in the range C<0..11> with 0 indicating January and 11
2037 indicating December. $year is the number of years since 1900. That
2038 is, $year is C<123> in year 2023. $wday is the day of the week, with
2039 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2040 the year, in the range C<0..364> (or C<0..365> in leap years.)
2042 Note that the $year element is I<not> simply the last two digits of
2043 the year. If you assume it is, then you create non-Y2K-compliant
2044 programs--and you wouldn't want to do that, would you?
2046 The proper way to get a complete 4-digit year is simply:
2050 And to get the last two digits of the year (e.g., '01' in 2001) do:
2052 $year = sprintf("%02d", $year % 100);
2054 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2056 In scalar context, C<gmtime()> returns the ctime(3) value:
2058 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2060 Also see the C<timegm> function provided by the C<Time::Local> module,
2061 and the strftime(3) function available via the POSIX module.
2063 This scalar value is B<not> locale dependent (see L<perllocale>), but
2064 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2065 strftime(3) and mktime(3) functions available via the POSIX module. To
2066 get somewhat similar but locale dependent date strings, set up your
2067 locale environment variables appropriately (please see L<perllocale>)
2068 and try for example:
2070 use POSIX qw(strftime);
2071 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2073 Note that the C<%a> and C<%b> escapes, which represent the short forms
2074 of the day of the week and the month of the year, may not necessarily
2075 be three characters wide in all locales.
2083 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2084 execution there. It may not be used to go into any construct that
2085 requires initialization, such as a subroutine or a C<foreach> loop. It
2086 also can't be used to go into a construct that is optimized away,
2087 or to get out of a block or subroutine given to C<sort>.
2088 It can be used to go almost anywhere else within the dynamic scope,
2089 including out of subroutines, but it's usually better to use some other
2090 construct such as C<last> or C<die>. The author of Perl has never felt the
2091 need to use this form of C<goto> (in Perl, that is--C is another matter).
2092 (The difference being that C does not offer named loops combined with
2093 loop control. Perl does, and this replaces most structured uses of C<goto>
2094 in other languages.)
2096 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2097 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2098 necessarily recommended if you're optimizing for maintainability:
2100 goto ("FOO", "BAR", "GLARCH")[$i];
2102 The C<goto-&NAME> form is quite different from the other forms of
2103 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2104 doesn't have the stigma associated with other gotos. Instead, it
2105 exits the current subroutine (losing any changes set by local()) and
2106 immediately calls in its place the named subroutine using the current
2107 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2108 load another subroutine and then pretend that the other subroutine had
2109 been called in the first place (except that any modifications to C<@_>
2110 in the current subroutine are propagated to the other subroutine.)
2111 After the C<goto>, not even C<caller> will be able to tell that this
2112 routine was called first.
2114 NAME needn't be the name of a subroutine; it can be a scalar variable
2115 containing a code reference, or a block which evaluates to a code
2118 =item grep BLOCK LIST
2120 =item grep EXPR,LIST
2122 This is similar in spirit to, but not the same as, grep(1) and its
2123 relatives. In particular, it is not limited to using regular expressions.
2125 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2126 C<$_> to each element) and returns the list value consisting of those
2127 elements for which the expression evaluated to true. In scalar
2128 context, returns the number of times the expression was true.
2130 @foo = grep(!/^#/, @bar); # weed out comments
2134 @foo = grep {!/^#/} @bar; # weed out comments
2136 Note that C<$_> is an alias to the list value, so it can be used to
2137 modify the elements of the LIST. While this is useful and supported,
2138 it can cause bizarre results if the elements of LIST are not variables.
2139 Similarly, grep returns aliases into the original list, much as a for
2140 loop's index variable aliases the list elements. That is, modifying an
2141 element of a list returned by grep (for example, in a C<foreach>, C<map>
2142 or another C<grep>) actually modifies the element in the original list.
2143 This is usually something to be avoided when writing clear code.
2145 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2151 Interprets EXPR as a hex string and returns the corresponding value.
2152 (To convert strings that might start with either 0, 0x, or 0b, see
2153 L</oct>.) If EXPR is omitted, uses C<$_>.
2155 print hex '0xAf'; # prints '175'
2156 print hex 'aF'; # same
2158 Hex strings may only represent integers. Strings that would cause
2159 integer overflow trigger a warning. Leading whitespace is not stripped,
2164 There is no builtin C<import> function. It is just an ordinary
2165 method (subroutine) defined (or inherited) by modules that wish to export
2166 names to another module. The C<use> function calls the C<import> method
2167 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2169 =item index STR,SUBSTR,POSITION
2171 =item index STR,SUBSTR
2173 The index function searches for one string within another, but without
2174 the wildcard-like behavior of a full regular-expression pattern match.
2175 It returns the position of the first occurrence of SUBSTR in STR at
2176 or after POSITION. If POSITION is omitted, starts searching from the
2177 beginning of the string. The return value is based at C<0> (or whatever
2178 you've set the C<$[> variable to--but don't do that). If the substring
2179 is not found, returns one less than the base, ordinarily C<-1>.
2185 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2186 You should not use this function for rounding: one because it truncates
2187 towards C<0>, and two because machine representations of floating point
2188 numbers can sometimes produce counterintuitive results. For example,
2189 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2190 because it's really more like -268.99999999999994315658 instead. Usually,
2191 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2192 functions will serve you better than will int().
2194 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2196 Implements the ioctl(2) function. You'll probably first have to say
2198 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2200 to get the correct function definitions. If F<ioctl.ph> doesn't
2201 exist or doesn't have the correct definitions you'll have to roll your
2202 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2203 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2204 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2205 written depending on the FUNCTION--a pointer to the string value of SCALAR
2206 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2207 has no string value but does have a numeric value, that value will be
2208 passed rather than a pointer to the string value. To guarantee this to be
2209 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2210 functions may be needed to manipulate the values of structures used by
2213 The return value of C<ioctl> (and C<fcntl>) is as follows:
2215 if OS returns: then Perl returns:
2217 0 string "0 but true"
2218 anything else that number
2220 Thus Perl returns true on success and false on failure, yet you can
2221 still easily determine the actual value returned by the operating
2224 $retval = ioctl(...) || -1;
2225 printf "System returned %d\n", $retval;
2227 The special string "C<0> but true" is exempt from B<-w> complaints
2228 about improper numeric conversions.
2230 Here's an example of setting a filehandle named C<REMOTE> to be
2231 non-blocking at the system level. You'll have to negotiate C<$|>
2232 on your own, though.
2234 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2236 $flags = fcntl(REMOTE, F_GETFL, 0)
2237 or die "Can't get flags for the socket: $!\n";
2239 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2240 or die "Can't set flags for the socket: $!\n";
2242 =item join EXPR,LIST
2244 Joins the separate strings of LIST into a single string with fields
2245 separated by the value of EXPR, and returns that new string. Example:
2247 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2249 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2250 first argument. Compare L</split>.
2254 Returns a list consisting of all the keys of the named hash. (In
2255 scalar context, returns the number of keys.) The keys are returned in
2256 an apparently random order. The actual random order is subject to
2257 change in future versions of perl, but it is guaranteed to be the same
2258 order as either the C<values> or C<each> function produces (given
2259 that the hash has not been modified). As a side effect, it resets
2262 Here is yet another way to print your environment:
2265 @values = values %ENV;
2267 print pop(@keys), '=', pop(@values), "\n";
2270 or how about sorted by key:
2272 foreach $key (sort(keys %ENV)) {
2273 print $key, '=', $ENV{$key}, "\n";
2276 The returned values are copies of the original keys in the hash, so
2277 modifying them will not affect the original hash. Compare L</values>.
2279 To sort a hash by value, you'll need to use a C<sort> function.
2280 Here's a descending numeric sort of a hash by its values:
2282 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2283 printf "%4d %s\n", $hash{$key}, $key;
2286 As an lvalue C<keys> allows you to increase the number of hash buckets
2287 allocated for the given hash. This can gain you a measure of efficiency if
2288 you know the hash is going to get big. (This is similar to pre-extending
2289 an array by assigning a larger number to $#array.) If you say
2293 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2294 in fact, since it rounds up to the next power of two. These
2295 buckets will be retained even if you do C<%hash = ()>, use C<undef
2296 %hash> if you want to free the storage while C<%hash> is still in scope.
2297 You can't shrink the number of buckets allocated for the hash using
2298 C<keys> in this way (but you needn't worry about doing this by accident,
2299 as trying has no effect).
2301 See also C<each>, C<values> and C<sort>.
2303 =item kill SIGNAL, LIST
2305 Sends a signal to a list of processes. Returns the number of
2306 processes successfully signaled (which is not necessarily the
2307 same as the number actually killed).
2309 $cnt = kill 1, $child1, $child2;
2312 If SIGNAL is zero, no signal is sent to the process. This is a
2313 useful way to check that the process is alive and hasn't changed
2314 its UID. See L<perlport> for notes on the portability of this
2317 Unlike in the shell, if SIGNAL is negative, it kills
2318 process groups instead of processes. (On System V, a negative I<PROCESS>
2319 number will also kill process groups, but that's not portable.) That
2320 means you usually want to use positive not negative signals. You may also
2321 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2327 The C<last> command is like the C<break> statement in C (as used in
2328 loops); it immediately exits the loop in question. If the LABEL is
2329 omitted, the command refers to the innermost enclosing loop. The
2330 C<continue> block, if any, is not executed:
2332 LINE: while (<STDIN>) {
2333 last LINE if /^$/; # exit when done with header
2337 C<last> cannot be used to exit a block which returns a value such as
2338 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2339 a grep() or map() operation.
2341 Note that a block by itself is semantically identical to a loop
2342 that executes once. Thus C<last> can be used to effect an early
2343 exit out of such a block.
2345 See also L</continue> for an illustration of how C<last>, C<next>, and
2352 Returns a lowercased version of EXPR. This is the internal function
2353 implementing the C<\L> escape in double-quoted strings. Respects
2354 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2355 and L<perlunicode> for more details about locale and Unicode support.
2357 If EXPR is omitted, uses C<$_>.
2363 Returns the value of EXPR with the first character lowercased. This
2364 is the internal function implementing the C<\l> escape in
2365 double-quoted strings. Respects current LC_CTYPE locale if C<use
2366 locale> in force. See L<perllocale> and L<perlunicode> for more
2367 details about locale and Unicode support.
2369 If EXPR is omitted, uses C<$_>.
2375 Returns the length in characters of the value of EXPR. If EXPR is
2376 omitted, returns length of C<$_>. Note that this cannot be used on
2377 an entire array or hash to find out how many elements these have.
2378 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2380 =item link OLDFILE,NEWFILE
2382 Creates a new filename linked to the old filename. Returns true for
2383 success, false otherwise.
2385 =item listen SOCKET,QUEUESIZE
2387 Does the same thing that the listen system call does. Returns true if
2388 it succeeded, false otherwise. See the example in
2389 L<perlipc/"Sockets: Client/Server Communication">.
2393 You really probably want to be using C<my> instead, because C<local> isn't
2394 what most people think of as "local". See
2395 L<perlsub/"Private Variables via my()"> for details.
2397 A local modifies the listed variables to be local to the enclosing
2398 block, file, or eval. If more than one value is listed, the list must
2399 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2400 for details, including issues with tied arrays and hashes.
2402 =item localtime EXPR
2404 Converts a time as returned by the time function to a 9-element list
2405 with the time analyzed for the local time zone. Typically used as
2409 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2412 All list elements are numeric, and come straight out of the C `struct
2413 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2414 specified time. $mday is the day of the month, and $mon is the month
2415 itself, in the range C<0..11> with 0 indicating January and 11
2416 indicating December. $year is the number of years since 1900. That
2417 is, $year is C<123> in year 2023. $wday is the day of the week, with
2418 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2419 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2420 is true if the specified time occurs during daylight savings time,
2423 Note that the $year element is I<not> simply the last two digits of
2424 the year. If you assume it is, then you create non-Y2K-compliant
2425 programs--and you wouldn't want to do that, would you?
2427 The proper way to get a complete 4-digit year is simply:
2431 And to get the last two digits of the year (e.g., '01' in 2001) do:
2433 $year = sprintf("%02d", $year % 100);
2435 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2437 In scalar context, C<localtime()> returns the ctime(3) value:
2439 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2441 This scalar value is B<not> locale dependent, see L<perllocale>, but
2442 instead a Perl builtin. Also see the C<Time::Local> module
2443 (to convert the second, minutes, hours, ... back to seconds since the
2444 stroke of midnight the 1st of January 1970, the value returned by
2445 time()), and the strftime(3) and mktime(3) functions available via the
2446 POSIX module. To get somewhat similar but locale dependent date
2447 strings, set up your locale environment variables appropriately
2448 (please see L<perllocale>) and try for example:
2450 use POSIX qw(strftime);
2451 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2453 Note that the C<%a> and C<%b>, the short forms of the day of the week
2454 and the month of the year, may not necessarily be three characters wide.
2458 This function places an advisory lock on a variable, subroutine,
2459 or referenced object contained in I<THING> until the lock goes out
2460 of scope. This is a built-in function only if your version of Perl
2461 was built with threading enabled, and if you've said C<use Thread>.
2462 Otherwise a user-defined function by this name will be called.
2469 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2470 returns log of C<$_>. To get the log of another base, use basic algebra:
2471 The base-N log of a number is equal to the natural log of that number
2472 divided by the natural log of N. For example:
2476 return log($n)/log(10);
2479 See also L</exp> for the inverse operation.
2485 Does the same thing as the C<stat> function (including setting the
2486 special C<_> filehandle) but stats a symbolic link instead of the file
2487 the symbolic link points to. If symbolic links are unimplemented on
2488 your system, a normal C<stat> is done.
2490 If EXPR is omitted, stats C<$_>.
2494 The match operator. See L<perlop>.
2496 =item map BLOCK LIST
2500 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2501 C<$_> to each element) and returns the list value composed of the
2502 results of each such evaluation. In scalar context, returns the
2503 total number of elements so generated. Evaluates BLOCK or EXPR in
2504 list context, so each element of LIST may produce zero, one, or
2505 more elements in the returned value.
2507 @chars = map(chr, @nums);
2509 translates a list of numbers to the corresponding characters. And
2511 %hash = map { getkey($_) => $_ } @array;
2513 is just a funny way to write
2516 foreach $_ (@array) {
2517 $hash{getkey($_)} = $_;
2520 Note that C<$_> is an alias to the list value, so it can be used to
2521 modify the elements of the LIST. While this is useful and supported,
2522 it can cause bizarre results if the elements of LIST are not variables.
2523 Using a regular C<foreach> loop for this purpose would be clearer in
2524 most cases. See also L</grep> for an array composed of those items of
2525 the original list for which the BLOCK or EXPR evaluates to true.
2527 C<{> starts both hash references and blocks, so C<map { ...> could be either
2528 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2529 ahead for the closing C<}> it has to take a guess at which its dealing with
2530 based what it finds just after the C<{>. Usually it gets it right, but if it
2531 doesn't it won't realize something is wrong until it gets to the C<}> and
2532 encounters the missing (or unexpected) comma. The syntax error will be
2533 reported close to the C<}> but you'll need to change something near the C<{>
2534 such as using a unary C<+> to give perl some help:
2536 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2537 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2538 %hash = map { ("\L$_", 1) } @array # this also works
2539 %hash = map { lc($_), 1 } @array # as does this.
2540 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2542 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2544 or to force an anon hash constructor use C<+{>
2546 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2548 and you get list of anonymous hashes each with only 1 entry.
2550 =item mkdir FILENAME,MASK
2552 =item mkdir FILENAME
2554 Creates the directory specified by FILENAME, with permissions
2555 specified by MASK (as modified by C<umask>). If it succeeds it
2556 returns true, otherwise it returns false and sets C<$!> (errno).
2557 If omitted, MASK defaults to 0777.
2559 In general, it is better to create directories with permissive MASK,
2560 and let the user modify that with their C<umask>, than it is to supply
2561 a restrictive MASK and give the user no way to be more permissive.
2562 The exceptions to this rule are when the file or directory should be
2563 kept private (mail files, for instance). The perlfunc(1) entry on
2564 C<umask> discusses the choice of MASK in more detail.
2566 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2567 number of trailing slashes. Some operating and filesystems do not get
2568 this right, so Perl automatically removes all trailing slashes to keep
2571 =item msgctl ID,CMD,ARG
2573 Calls the System V IPC function msgctl(2). You'll probably have to say
2577 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2578 then ARG must be a variable which will hold the returned C<msqid_ds>
2579 structure. Returns like C<ioctl>: the undefined value for error,
2580 C<"0 but true"> for zero, or the actual return value otherwise. See also
2581 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2583 =item msgget KEY,FLAGS
2585 Calls the System V IPC function msgget(2). Returns the message queue
2586 id, or the undefined value if there is an error. See also
2587 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2589 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2591 Calls the System V IPC function msgrcv to receive a message from
2592 message queue ID into variable VAR with a maximum message size of
2593 SIZE. Note that when a message is received, the message type as a
2594 native long integer will be the first thing in VAR, followed by the
2595 actual message. This packing may be opened with C<unpack("l! a*")>.
2596 Taints the variable. Returns true if successful, or false if there is
2597 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2598 C<IPC::SysV::Msg> documentation.
2600 =item msgsnd ID,MSG,FLAGS
2602 Calls the System V IPC function msgsnd to send the message MSG to the
2603 message queue ID. MSG must begin with the native long integer message
2604 type, and be followed by the length of the actual message, and finally
2605 the message itself. This kind of packing can be achieved with
2606 C<pack("l! a*", $type, $message)>. Returns true if successful,
2607 or false if there is an error. See also C<IPC::SysV>
2608 and C<IPC::SysV::Msg> documentation.
2612 =item my EXPR : ATTRIBUTES
2614 A C<my> declares the listed variables to be local (lexically) to the
2615 enclosing block, file, or C<eval>. If
2616 more than one value is listed, the list must be placed in parentheses. See
2617 L<perlsub/"Private Variables via my()"> for details.
2623 The C<next> command is like the C<continue> statement in C; it starts
2624 the next iteration of the loop:
2626 LINE: while (<STDIN>) {
2627 next LINE if /^#/; # discard comments
2631 Note that if there were a C<continue> block on the above, it would get
2632 executed even on discarded lines. If the LABEL is omitted, the command
2633 refers to the innermost enclosing loop.
2635 C<next> cannot be used to exit a block which returns a value such as
2636 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2637 a grep() or map() operation.
2639 Note that a block by itself is semantically identical to a loop
2640 that executes once. Thus C<next> will exit such a block early.
2642 See also L</continue> for an illustration of how C<last>, C<next>, and
2645 =item no Module LIST
2647 See the L</use> function, which C<no> is the opposite of.
2653 Interprets EXPR as an octal string and returns the corresponding
2654 value. (If EXPR happens to start off with C<0x>, interprets it as a
2655 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2656 binary string. Leading whitespace is ignored in all three cases.)
2657 The following will handle decimal, binary, octal, and hex in the standard
2660 $val = oct($val) if $val =~ /^0/;
2662 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2663 in octal), use sprintf() or printf():
2665 $perms = (stat("filename"))[2] & 07777;
2666 $oct_perms = sprintf "%lo", $perms;
2668 The oct() function is commonly used when a string such as C<644> needs
2669 to be converted into a file mode, for example. (Although perl will
2670 automatically convert strings into numbers as needed, this automatic
2671 conversion assumes base 10.)
2673 =item open FILEHANDLE,EXPR
2675 =item open FILEHANDLE,MODE,EXPR
2677 =item open FILEHANDLE,MODE,EXPR,LIST
2679 =item open FILEHANDLE,MODE,REFERENCE
2681 =item open FILEHANDLE
2683 Opens the file whose filename is given by EXPR, and associates it with
2686 (The following is a comprehensive reference to open(): for a gentler
2687 introduction you may consider L<perlopentut>.)
2689 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2690 assigned a reference to a new anonymous filehandle, otherwise if
2691 FILEHANDLE is an expression, its value is used as the name of the real
2692 filehandle wanted. (This is considered a symbolic reference, so C<use
2693 strict 'refs'> should I<not> be in effect.)
2695 If EXPR is omitted, the scalar variable of the same name as the
2696 FILEHANDLE contains the filename. (Note that lexical variables--those
2697 declared with C<my>--will not work for this purpose; so if you're
2698 using C<my>, specify EXPR in your call to open.)
2700 If three or more arguments are specified then the mode of opening and
2701 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2702 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2703 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2704 the file is opened for appending, again being created if necessary.
2706 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2707 indicate that you want both read and write access to the file; thus
2708 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2709 '+>' >> mode would clobber the file first. You can't usually use
2710 either read-write mode for updating textfiles, since they have
2711 variable length records. See the B<-i> switch in L<perlrun> for a
2712 better approach. The file is created with permissions of C<0666>
2713 modified by the process' C<umask> value.
2715 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2716 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2718 In the 2-arguments (and 1-argument) form of the call the mode and
2719 filename should be concatenated (in this order), possibly separated by
2720 spaces. It is possible to omit the mode in these forms if the mode is
2723 If the filename begins with C<'|'>, the filename is interpreted as a
2724 command to which output is to be piped, and if the filename ends with a
2725 C<'|'>, the filename is interpreted as a command which pipes output to
2726 us. See L<perlipc/"Using open() for IPC">
2727 for more examples of this. (You are not allowed to C<open> to a command
2728 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2729 and L<perlipc/"Bidirectional Communication with Another Process">
2732 For three or more arguments if MODE is C<'|-'>, the filename is
2733 interpreted as a command to which output is to be piped, and if MODE
2734 is C<'-|'>, the filename is interpreted as a command which pipes
2735 output to us. In the 2-arguments (and 1-argument) form one should
2736 replace dash (C<'-'>) with the command.
2737 See L<perlipc/"Using open() for IPC"> for more examples of this.
2738 (You are not allowed to C<open> to a command that pipes both in I<and>
2739 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2740 L<perlipc/"Bidirectional Communication"> for alternatives.)
2742 In the three-or-more argument form of pipe opens, if LIST is specified
2743 (extra arguments after the command name) then LIST becomes arguments
2744 to the command invoked if the platform supports it. The meaning of
2745 C<open> with more than three arguments for non-pipe modes is not yet
2746 specified. Experimental "layers" may give extra LIST arguments
2749 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2750 and opening C<< '>-' >> opens STDOUT.
2752 You may use the three-argument form of open to specify
2753 I<I/O disciplines> that affect how the input and output
2754 are processed: see L</binmode> and L<open>. For example
2756 open(FH, "<:utf8", "file")
2758 will open the UTF-8 encoded file containing Unicode characters,
2759 see L<perluniintro>.
2761 Open returns nonzero upon success, the undefined value otherwise. If
2762 the C<open> involved a pipe, the return value happens to be the pid of
2765 If you're running Perl on a system that distinguishes between text
2766 files and binary files, then you should check out L</binmode> for tips
2767 for dealing with this. The key distinction between systems that need
2768 C<binmode> and those that don't is their text file formats. Systems
2769 like Unix, MacOS, and Plan9, which delimit lines with a single
2770 character, and which encode that character in C as C<"\n">, do not
2771 need C<binmode>. The rest need it.
2773 In the three argument form MODE may also contain a list of IO "layers"
2774 (see L<open> and L<PerlIO> for more details) to be applied to the
2775 handle. This can be used to achieve the effect of C<binmode> as well
2776 as more complex behaviours.
2778 When opening a file, it's usually a bad idea to continue normal execution
2779 if the request failed, so C<open> is frequently used in connection with
2780 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2781 where you want to make a nicely formatted error message (but there are
2782 modules that can help with that problem)) you should always check
2783 the return value from opening a file. The infrequent exception is when
2784 working with an unopened filehandle is actually what you want to do.
2786 As a special case the 3 arg form with a read/write mode and the third
2787 argument being C<undef>:
2789 open(TMP, "+>", undef) or die ...
2791 opens a filehandle to an anonymous temporary file.
2793 File handles can be opened to "in memory" files held in Perl scalars via:
2795 open($fh,'>', \$variable) || ..
2800 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2801 while (<ARTICLE>) {...
2803 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2804 # if the open fails, output is discarded
2806 open(DBASE, '+<', 'dbase.mine') # open for update
2807 or die "Can't open 'dbase.mine' for update: $!";
2809 open(DBASE, '+<dbase.mine') # ditto
2810 or die "Can't open 'dbase.mine' for update: $!";
2812 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2813 or die "Can't start caesar: $!";
2815 open(ARTICLE, "caesar <$article |") # ditto
2816 or die "Can't start caesar: $!";
2818 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2819 or die "Can't start sort: $!";
2822 open(MEMORY,'>', \$var)
2823 or die "Can't open memory file: $!";
2824 print MEMORY "foo!\n"; # output will end up in $var
2826 # process argument list of files along with any includes
2828 foreach $file (@ARGV) {
2829 process($file, 'fh00');
2833 my($filename, $input) = @_;
2834 $input++; # this is a string increment
2835 unless (open($input, $filename)) {
2836 print STDERR "Can't open $filename: $!\n";
2841 while (<$input>) { # note use of indirection
2842 if (/^#include "(.*)"/) {
2843 process($1, $input);
2850 You may also, in the Bourne shell tradition, specify an EXPR beginning
2851 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2852 name of a filehandle (or file descriptor, if numeric) to be
2853 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2854 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2855 mode you specify should match the mode of the original filehandle.
2856 (Duping a filehandle does not take into account any existing contents of
2857 IO buffers.) If you use the 3 arg form then you can pass either a number,
2858 the name of a filehandle or the normal "reference to a glob".
2860 Here is a script that saves, redirects, and restores STDOUT and
2864 open(my $oldout, ">&", \*STDOUT);
2865 open(OLDERR, ">&STDERR");
2867 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2868 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2870 select(STDERR); $| = 1; # make unbuffered
2871 select(STDOUT); $| = 1; # make unbuffered
2873 print STDOUT "stdout 1\n"; # this works for
2874 print STDERR "stderr 1\n"; # subprocesses too
2879 open(STDOUT, ">&OLDOUT");
2880 open(STDERR, ">&OLDERR");
2882 print STDOUT "stdout 2\n";
2883 print STDERR "stderr 2\n";
2885 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2886 do an equivalent of C's C<fdopen> of that file descriptor; this is
2887 more parsimonious of file descriptors. For example:
2889 open(FILEHANDLE, "<&=$fd")
2893 open(FILEHANDLE, "<&=", $fd)
2895 Note that if Perl is using the standard C libraries' fdopen() then on
2896 many UNIX systems, fdopen() is known to fail when file descriptors
2897 exceed a certain value, typically 255. If you need more file
2898 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2900 You can see whether Perl has been compiled with PerlIO or not by
2901 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2902 is C<define>, you have PerlIO, otherwise you don't.
2904 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2905 with 2-arguments (or 1-argument) form of open(), then
2906 there is an implicit fork done, and the return value of open is the pid
2907 of the child within the parent process, and C<0> within the child
2908 process. (Use C<defined($pid)> to determine whether the open was successful.)
2909 The filehandle behaves normally for the parent, but i/o to that
2910 filehandle is piped from/to the STDOUT/STDIN of the child process.
2911 In the child process the filehandle isn't opened--i/o happens from/to
2912 the new STDOUT or STDIN. Typically this is used like the normal
2913 piped open when you want to exercise more control over just how the
2914 pipe command gets executed, such as when you are running setuid, and
2915 don't want to have to scan shell commands for metacharacters.
2916 The following triples are more or less equivalent:
2918 open(FOO, "|tr '[a-z]' '[A-Z]'");
2919 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2920 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2921 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2923 open(FOO, "cat -n '$file'|");
2924 open(FOO, '-|', "cat -n '$file'");
2925 open(FOO, '-|') || exec 'cat', '-n', $file;
2926 open(FOO, '-|', "cat", '-n', $file);
2928 The last example in each block shows the pipe as "list form", which is
2929 not yet supported on all platforms.
2931 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2933 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2934 output before any operation that may do a fork, but this may not be
2935 supported on some platforms (see L<perlport>). To be safe, you may need
2936 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2937 of C<IO::Handle> on any open handles.
2939 On systems that support a close-on-exec flag on files, the flag will
2940 be set for the newly opened file descriptor as determined by the value
2941 of $^F. See L<perlvar/$^F>.
2943 Closing any piped filehandle causes the parent process to wait for the
2944 child to finish, and returns the status value in C<$?>.
2946 The filename passed to 2-argument (or 1-argument) form of open() will
2947 have leading and trailing whitespace deleted, and the normal
2948 redirection characters honored. This property, known as "magic open",
2949 can often be used to good effect. A user could specify a filename of
2950 F<"rsh cat file |">, or you could change certain filenames as needed:
2952 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2953 open(FH, $filename) or die "Can't open $filename: $!";
2955 Use 3-argument form to open a file with arbitrary weird characters in it,
2957 open(FOO, '<', $file);
2959 otherwise it's necessary to protect any leading and trailing whitespace:
2961 $file =~ s#^(\s)#./$1#;
2962 open(FOO, "< $file\0");
2964 (this may not work on some bizarre filesystems). One should
2965 conscientiously choose between the I<magic> and 3-arguments form
2970 will allow the user to specify an argument of the form C<"rsh cat file |">,
2971 but will not work on a filename which happens to have a trailing space, while
2973 open IN, '<', $ARGV[0];
2975 will have exactly the opposite restrictions.
2977 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2978 should use the C<sysopen> function, which involves no such magic (but
2979 may use subtly different filemodes than Perl open(), which is mapped
2980 to C fopen()). This is
2981 another way to protect your filenames from interpretation. For example:
2984 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2985 or die "sysopen $path: $!";
2986 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2987 print HANDLE "stuff $$\n";
2989 print "File contains: ", <HANDLE>;
2991 Using the constructor from the C<IO::Handle> package (or one of its
2992 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2993 filehandles that have the scope of whatever variables hold references to
2994 them, and automatically close whenever and however you leave that scope:
2998 sub read_myfile_munged {
3000 my $handle = new IO::File;
3001 open($handle, "myfile") or die "myfile: $!";
3003 or return (); # Automatically closed here.
3004 mung $first or die "mung failed"; # Or here.
3005 return $first, <$handle> if $ALL; # Or here.
3009 See L</seek> for some details about mixing reading and writing.
3011 =item opendir DIRHANDLE,EXPR
3013 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3014 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3015 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3021 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3022 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3025 For the reverse, see L</chr>.
3026 See L<perlunicode> and L<encoding> for more about Unicode.
3030 =item our EXPR : ATTRIBUTES
3032 An C<our> declares the listed variables to be valid globals within
3033 the enclosing block, file, or C<eval>. That is, it has the same
3034 scoping rules as a "my" declaration, but does not create a local
3035 variable. If more than one value is listed, the list must be placed
3036 in parentheses. The C<our> declaration has no semantic effect unless
3037 "use strict vars" is in effect, in which case it lets you use the
3038 declared global variable without qualifying it with a package name.
3039 (But only within the lexical scope of the C<our> declaration. In this
3040 it differs from "use vars", which is package scoped.)
3042 An C<our> declaration declares a global variable that will be visible
3043 across its entire lexical scope, even across package boundaries. The
3044 package in which the variable is entered is determined at the point
3045 of the declaration, not at the point of use. This means the following
3049 our $bar; # declares $Foo::bar for rest of lexical scope
3053 print $bar; # prints 20
3055 Multiple C<our> declarations in the same lexical scope are allowed
3056 if they are in different packages. If they happened to be in the same
3057 package, Perl will emit warnings if you have asked for them.
3061 our $bar; # declares $Foo::bar for rest of lexical scope
3065 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3066 print $bar; # prints 30
3068 our $bar; # emits warning
3070 An C<our> declaration may also have a list of attributes associated
3071 with it. B<WARNING>: This is an experimental feature that may be
3072 changed or removed in future releases of Perl. It should not be
3075 The only currently recognized attribute is C<unique> which indicates
3076 that a single copy of the global is to be used by all interpreters
3077 should the program happen to be running in a multi-interpreter
3078 environment. (The default behaviour would be for each interpreter to
3079 have its own copy of the global.) In such an environment, this
3080 attribute also has the effect of making the global readonly.
3083 our @EXPORT : unique = qw(foo);
3084 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3085 our $VERSION : unique = "1.00";
3087 Multi-interpreter environments can come to being either through the
3088 fork() emulation on Windows platforms, or by embedding perl in a
3089 multi-threaded application. The C<unique> attribute does nothing in
3090 all other environments.
3092 =item pack TEMPLATE,LIST
3094 Takes a LIST of values and converts it into a string using the rules
3095 given by the TEMPLATE. The resulting string is the concatenation of
3096 the converted values. Typically, each converted value looks
3097 like its machine-level representation. For example, on 32-bit machines
3098 a converted integer may be represented by a sequence of 4 bytes.
3101 sequence of characters that give the order and type of values, as
3104 a A string with arbitrary binary data, will be null padded.
3105 A A text (ASCII) string, will be space padded.
3106 Z A null terminated (ASCIZ) string, will be null padded.
3108 b A bit string (ascending bit order inside each byte, like vec()).
3109 B A bit string (descending bit order inside each byte).
3110 h A hex string (low nybble first).
3111 H A hex string (high nybble first).
3113 c A signed char value.
3114 C An unsigned char value. Only does bytes. See U for Unicode.
3116 s A signed short value.
3117 S An unsigned short value.
3118 (This 'short' is _exactly_ 16 bits, which may differ from
3119 what a local C compiler calls 'short'. If you want
3120 native-length shorts, use the '!' suffix.)
3122 i A signed integer value.
3123 I An unsigned integer value.
3124 (This 'integer' is _at_least_ 32 bits wide. Its exact
3125 size depends on what a local C compiler calls 'int',
3126 and may even be larger than the 'long' described in
3129 l A signed long value.
3130 L An unsigned long value.
3131 (This 'long' is _exactly_ 32 bits, which may differ from
3132 what a local C compiler calls 'long'. If you want
3133 native-length longs, use the '!' suffix.)
3135 n An unsigned short in "network" (big-endian) order.
3136 N An unsigned long in "network" (big-endian) order.
3137 v An unsigned short in "VAX" (little-endian) order.
3138 V An unsigned long in "VAX" (little-endian) order.
3139 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3140 _exactly_ 32 bits, respectively.)
3142 q A signed quad (64-bit) value.
3143 Q An unsigned quad value.
3144 (Quads are available only if your system supports 64-bit
3145 integer values _and_ if Perl has been compiled to support those.
3146 Causes a fatal error otherwise.)
3148 f A single-precision float in the native format.
3149 d A double-precision float in the native format.
3151 p A pointer to a null-terminated string.
3152 P A pointer to a structure (fixed-length string).
3154 u A uuencoded string.
3155 U A Unicode character number. Encodes to UTF-8 internally
3156 (or UTF-EBCDIC in EBCDIC platforms).
3158 w A BER compressed integer. Its bytes represent an unsigned
3159 integer in base 128, most significant digit first, with as
3160 few digits as possible. Bit eight (the high bit) is set
3161 on each byte except the last.
3165 @ Null fill to absolute position.
3167 The following rules apply:
3173 Each letter may optionally be followed by a number giving a repeat
3174 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3175 C<H>, and C<P> the pack function will gobble up that many values from
3176 the LIST. A C<*> for the repeat count means to use however many items are
3177 left, except for C<@>, C<x>, C<X>, where it is equivalent
3178 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3181 When used with C<Z>, C<*> results in the addition of a trailing null
3182 byte (so the packed result will be one longer than the byte C<length>
3185 The repeat count for C<u> is interpreted as the maximal number of bytes
3186 to encode per line of output, with 0 and 1 replaced by 45.
3190 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3191 string of length count, padding with nulls or spaces as necessary. When
3192 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3193 after the first null, and C<a> returns data verbatim. When packing,
3194 C<a>, and C<Z> are equivalent.
3196 If the value-to-pack is too long, it is truncated. If too long and an
3197 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3198 by a null byte. Thus C<Z> always packs a trailing null byte under
3203 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3204 Each byte of the input field of pack() generates 1 bit of the result.
3205 Each result bit is based on the least-significant bit of the corresponding
3206 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3207 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3209 Starting from the beginning of the input string of pack(), each 8-tuple
3210 of bytes is converted to 1 byte of output. With format C<b>
3211 the first byte of the 8-tuple determines the least-significant bit of a
3212 byte, and with format C<B> it determines the most-significant bit of
3215 If the length of the input string is not exactly divisible by 8, the
3216 remainder is packed as if the input string were padded by null bytes
3217 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3219 If the input string of pack() is longer than needed, extra bytes are ignored.
3220 A C<*> for the repeat count of pack() means to use all the bytes of
3221 the input field. On unpack()ing the bits are converted to a string
3222 of C<"0">s and C<"1">s.
3226 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3227 representable as hexadecimal digits, 0-9a-f) long.
3229 Each byte of the input field of pack() generates 4 bits of the result.
3230 For non-alphabetical bytes the result is based on the 4 least-significant
3231 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3232 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3233 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3234 is compatible with the usual hexadecimal digits, so that C<"a"> and
3235 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3236 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3238 Starting from the beginning of the input string of pack(), each pair
3239 of bytes is converted to 1 byte of output. With format C<h> the
3240 first byte of the pair determines the least-significant nybble of the
3241 output byte, and with format C<H> it determines the most-significant
3244 If the length of the input string is not even, it behaves as if padded
3245 by a null byte at the end. Similarly, during unpack()ing the "extra"
3246 nybbles are ignored.
3248 If the input string of pack() is longer than needed, extra bytes are ignored.
3249 A C<*> for the repeat count of pack() means to use all the bytes of
3250 the input field. On unpack()ing the bits are converted to a string
3251 of hexadecimal digits.
3255 The C<p> type packs a pointer to a null-terminated string. You are
3256 responsible for ensuring the string is not a temporary value (which can
3257 potentially get deallocated before you get around to using the packed result).
3258 The C<P> type packs a pointer to a structure of the size indicated by the
3259 length. A NULL pointer is created if the corresponding value for C<p> or
3260 C<P> is C<undef>, similarly for unpack().
3264 The C</> template character allows packing and unpacking of strings where
3265 the packed structure contains a byte count followed by the string itself.
3266 You write I<length-item>C</>I<string-item>.
3268 The I<length-item> can be any C<pack> template letter,
3269 and describes how the length value is packed.
3270 The ones likely to be of most use are integer-packing ones like
3271 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3272 and C<N> (for Sun XDR).
3274 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3275 For C<unpack> the length of the string is obtained from the I<length-item>,
3276 but if you put in the '*' it will be ignored.
3278 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3279 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3280 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3282 The I<length-item> is not returned explicitly from C<unpack>.
3284 Adding a count to the I<length-item> letter is unlikely to do anything
3285 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3286 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3287 which Perl does not regard as legal in numeric strings.
3291 The integer types C<s>, C<S>, C<l>, and C<L> may be
3292 immediately followed by a C<!> suffix to signify native shorts or
3293 longs--as you can see from above for example a bare C<l> does mean
3294 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3295 may be larger. This is an issue mainly in 64-bit platforms. You can
3296 see whether using C<!> makes any difference by
3298 print length(pack("s")), " ", length(pack("s!")), "\n";
3299 print length(pack("l")), " ", length(pack("l!")), "\n";
3301 C<i!> and C<I!> also work but only because of completeness;
3302 they are identical to C<i> and C<I>.
3304 The actual sizes (in bytes) of native shorts, ints, longs, and long
3305 longs on the platform where Perl was built are also available via
3309 print $Config{shortsize}, "\n";
3310 print $Config{intsize}, "\n";
3311 print $Config{longsize}, "\n";
3312 print $Config{longlongsize}, "\n";
3314 (The C<$Config{longlongsize}> will be undefine if your system does
3315 not support long longs.)
3319 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3320 are inherently non-portable between processors and operating systems
3321 because they obey the native byteorder and endianness. For example a
3322 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3323 (arranged in and handled by the CPU registers) into bytes as
3325 0x12 0x34 0x56 0x78 # big-endian
3326 0x78 0x56 0x34 0x12 # little-endian
3328 Basically, the Intel and VAX CPUs are little-endian, while everybody
3329 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3330 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3331 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3334 The names `big-endian' and `little-endian' are comic references to
3335 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3336 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3337 the egg-eating habits of the Lilliputians.
3339 Some systems may have even weirder byte orders such as
3344 You can see your system's preference with
3346 print join(" ", map { sprintf "%#02x", $_ }
3347 unpack("C*",pack("L",0x12345678))), "\n";
3349 The byteorder on the platform where Perl was built is also available
3353 print $Config{byteorder}, "\n";
3355 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3356 and C<'87654321'> are big-endian.
3358 If you want portable packed integers use the formats C<n>, C<N>,
3359 C<v>, and C<V>, their byte endianness and size are known.
3360 See also L<perlport>.
3364 Real numbers (floats and doubles) are in the native machine format only;
3365 due to the multiplicity of floating formats around, and the lack of a
3366 standard "network" representation, no facility for interchange has been
3367 made. This means that packed floating point data written on one machine
3368 may not be readable on another - even if both use IEEE floating point
3369 arithmetic (as the endian-ness of the memory representation is not part
3370 of the IEEE spec). See also L<perlport>.
3372 Note that Perl uses doubles internally for all numeric calculation, and
3373 converting from double into float and thence back to double again will
3374 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3379 If the pattern begins with a C<U>, the resulting string will be treated
3380 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3381 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3382 characters. If you don't want this to happen, you can begin your pattern
3383 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3384 string, and then follow this with a C<U*> somewhere in your pattern.
3388 You must yourself do any alignment or padding by inserting for example
3389 enough C<'x'>es while packing. There is no way to pack() and unpack()
3390 could know where the bytes are going to or coming from. Therefore
3391 C<pack> (and C<unpack>) handle their output and input as flat
3396 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3400 If TEMPLATE requires more arguments to pack() than actually given, pack()
3401 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3402 to pack() than actually given, extra arguments are ignored.
3408 $foo = pack("CCCC",65,66,67,68);
3410 $foo = pack("C4",65,66,67,68);
3412 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3413 # same thing with Unicode circled letters
3415 $foo = pack("ccxxcc",65,66,67,68);
3418 # note: the above examples featuring "C" and "c" are true
3419 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3420 # and UTF-8. In EBCDIC the first example would be
3421 # $foo = pack("CCCC",193,194,195,196);
3423 $foo = pack("s2",1,2);
3424 # "\1\0\2\0" on little-endian
3425 # "\0\1\0\2" on big-endian
3427 $foo = pack("a4","abcd","x","y","z");
3430 $foo = pack("aaaa","abcd","x","y","z");
3433 $foo = pack("a14","abcdefg");
3434 # "abcdefg\0\0\0\0\0\0\0"
3436 $foo = pack("i9pl", gmtime);
3437 # a real struct tm (on my system anyway)
3439 $utmp_template = "Z8 Z8 Z16 L";
3440 $utmp = pack($utmp_template, @utmp1);
3441 # a struct utmp (BSDish)
3443 @utmp2 = unpack($utmp_template, $utmp);
3444 # "@utmp1" eq "@utmp2"
3447 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3450 $foo = pack('sx2l', 12, 34);
3451 # short 12, two zero bytes padding, long 34
3452 $bar = pack('s@4l', 12, 34);
3453 # short 12, zero fill to position 4, long 34
3456 The same template may generally also be used in unpack().
3458 =item package NAMESPACE
3462 Declares the compilation unit as being in the given namespace. The scope
3463 of the package declaration is from the declaration itself through the end
3464 of the enclosing block, file, or eval (the same as the C<my> operator).
3465 All further unqualified dynamic identifiers will be in this namespace.
3466 A package statement affects only dynamic variables--including those
3467 you've used C<local> on--but I<not> lexical variables, which are created
3468 with C<my>. Typically it would be the first declaration in a file to
3469 be included by the C<require> or C<use> operator. You can switch into a
3470 package in more than one place; it merely influences which symbol table
3471 is used by the compiler for the rest of that block. You can refer to
3472 variables and filehandles in other packages by prefixing the identifier
3473 with the package name and a double colon: C<$Package::Variable>.
3474 If the package name is null, the C<main> package as assumed. That is,
3475 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3476 still seen in older code).
3478 If NAMESPACE is omitted, then there is no current package, and all
3479 identifiers must be fully qualified or lexicals. However, you are
3480 strongly advised not to make use of this feature. Its use can cause
3481 unexpected behaviour, even crashing some versions of Perl. It is
3482 deprecated, and will be removed from a future release.
3484 See L<perlmod/"Packages"> for more information about packages, modules,
3485 and classes. See L<perlsub> for other scoping issues.
3487 =item pipe READHANDLE,WRITEHANDLE
3489 Opens a pair of connected pipes like the corresponding system call.
3490 Note that if you set up a loop of piped processes, deadlock can occur
3491 unless you are very careful. In addition, note that Perl's pipes use
3492 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3493 after each command, depending on the application.
3495 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3496 for examples of such things.
3498 On systems that support a close-on-exec flag on files, the flag will be set
3499 for the newly opened file descriptors as determined by the value of $^F.
3506 Pops and returns the last value of the array, shortening the array by
3507 one element. Has an effect similar to
3511 If there are no elements in the array, returns the undefined value
3512 (although this may happen at other times as well). If ARRAY is
3513 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3514 array in subroutines, just like C<shift>.
3520 Returns the offset of where the last C<m//g> search left off for the variable
3521 in question (C<$_> is used when the variable is not specified). May be
3522 modified to change that offset. Such modification will also influence
3523 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3526 =item print FILEHANDLE LIST
3532 Prints a string or a list of strings. Returns true if successful.
3533 FILEHANDLE may be a scalar variable name, in which case the variable
3534 contains the name of or a reference to the filehandle, thus introducing
3535 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3536 the next token is a term, it may be misinterpreted as an operator
3537 unless you interpose a C<+> or put parentheses around the arguments.)
3538 If FILEHANDLE is omitted, prints by default to standard output (or
3539 to the last selected output channel--see L</select>). If LIST is
3540 also omitted, prints C<$_> to the currently selected output channel.
3541 To set the default output channel to something other than STDOUT
3542 use the select operation. The current value of C<$,> (if any) is
3543 printed between each LIST item. The current value of C<$\> (if
3544 any) is printed after the entire LIST has been printed. Because
3545 print takes a LIST, anything in the LIST is evaluated in list
3546 context, and any subroutine that you call will have one or more of
3547 its expressions evaluated in list context. Also be careful not to
3548 follow the print keyword with a left parenthesis unless you want
3549 the corresponding right parenthesis to terminate the arguments to
3550 the print--interpose a C<+> or put parentheses around all the
3553 Note that if you're storing FILEHANDLES in an array or other expression,
3554 you will have to use a block returning its value instead:
3556 print { $files[$i] } "stuff\n";
3557 print { $OK ? STDOUT : STDERR } "stuff\n";
3559 =item printf FILEHANDLE FORMAT, LIST
3561 =item printf FORMAT, LIST
3563 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3564 (the output record separator) is not appended. The first argument
3565 of the list will be interpreted as the C<printf> format. See C<sprintf>
3566 for an explanation of the format argument. If C<use locale> is in effect,
3567 the character used for the decimal point in formatted real numbers is
3568 affected by the LC_NUMERIC locale. See L<perllocale>.
3570 Don't fall into the trap of using a C<printf> when a simple
3571 C<print> would do. The C<print> is more efficient and less
3574 =item prototype FUNCTION
3576 Returns the prototype of a function as a string (or C<undef> if the
3577 function has no prototype). FUNCTION is a reference to, or the name of,
3578 the function whose prototype you want to retrieve.
3580 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3581 name for Perl builtin. If the builtin is not I<overridable> (such as
3582 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3583 C<system>) returns C<undef> because the builtin does not really behave
3584 like a Perl function. Otherwise, the string describing the equivalent
3585 prototype is returned.
3587 =item push ARRAY,LIST
3589 Treats ARRAY as a stack, and pushes the values of LIST
3590 onto the end of ARRAY. The length of ARRAY increases by the length of
3591 LIST. Has the same effect as
3594 $ARRAY[++$#ARRAY] = $value;
3597 but is more efficient. Returns the new number of elements in the array.
3609 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3611 =item quotemeta EXPR
3615 Returns the value of EXPR with all non-"word"
3616 characters backslashed. (That is, all characters not matching
3617 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3618 returned string, regardless of any locale settings.)
3619 This is the internal function implementing
3620 the C<\Q> escape in double-quoted strings.
3622 If EXPR is omitted, uses C<$_>.
3628 Returns a random fractional number greater than or equal to C<0> and less
3629 than the value of EXPR. (EXPR should be positive.) If EXPR is
3630 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3631 unless C<srand> has already been called. See also C<srand>.
3633 Apply C<int()> to the value returned by C<rand()> if you want random
3634 integers instead of random fractional numbers. For example,
3638 returns a random integer between C<0> and C<9>, inclusive.
3640 (Note: If your rand function consistently returns numbers that are too
3641 large or too small, then your version of Perl was probably compiled
3642 with the wrong number of RANDBITS.)
3644 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3646 =item read FILEHANDLE,SCALAR,LENGTH
3648 Attempts to read LENGTH I<characters> of data into variable SCALAR
3649 from the specified FILEHANDLE. Returns the number of characters
3650 actually read, C<0> at end of file, or undef if there was an error.
3651 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3652 needs growing, the new bytes will be zero bytes. An OFFSET may be
3653 specified to place the read data into some other place in SCALAR than
3654 the beginning. The call is actually implemented in terms of either
3655 Perl's or system's fread() call. To get a true read(2) system call,
3658 Note the I<characters>: depending on the status of the filehandle,
3659 either (8-bit) bytes or characters are read. By default all
3660 filehandles operate on bytes, but for example if the filehandle has
3661 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3662 pragma, L<open>), the I/O will operate on characters, not bytes.
3664 =item readdir DIRHANDLE
3666 Returns the next directory entry for a directory opened by C<opendir>.
3667 If used in list context, returns all the rest of the entries in the
3668 directory. If there are no more entries, returns an undefined value in
3669 scalar context or a null list in list context.
3671 If you're planning to filetest the return values out of a C<readdir>, you'd
3672 better prepend the directory in question. Otherwise, because we didn't
3673 C<chdir> there, it would have been testing the wrong file.
3675 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3676 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3681 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3682 context, each call reads and returns the next line, until end-of-file is
3683 reached, whereupon the subsequent call returns undef. In list context,
3684 reads until end-of-file is reached and returns a list of lines. Note that
3685 the notion of "line" used here is however you may have defined it
3686 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3688 When C<$/> is set to C<undef>, when readline() is in scalar
3689 context (i.e. file slurp mode), and when an empty file is read, it
3690 returns C<''> the first time, followed by C<undef> subsequently.
3692 This is the internal function implementing the C<< <EXPR> >>
3693 operator, but you can use it directly. The C<< <EXPR> >>
3694 operator is discussed in more detail in L<perlop/"I/O Operators">.
3697 $line = readline(*STDIN); # same thing
3703 Returns the value of a symbolic link, if symbolic links are
3704 implemented. If not, gives a fatal error. If there is some system
3705 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3706 omitted, uses C<$_>.
3710 EXPR is executed as a system command.
3711 The collected standard output of the command is returned.
3712 In scalar context, it comes back as a single (potentially
3713 multi-line) string. In list context, returns a list of lines
3714 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3715 This is the internal function implementing the C<qx/EXPR/>
3716 operator, but you can use it directly. The C<qx/EXPR/>
3717 operator is discussed in more detail in L<perlop/"I/O Operators">.
3719 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3721 Receives a message on a socket. Attempts to receive LENGTH characters
3722 of data into variable SCALAR from the specified SOCKET filehandle.
3723 SCALAR will be grown or shrunk to the length actually read. Takes the
3724 same flags as the system call of the same name. Returns the address
3725 of the sender if SOCKET's protocol supports this; returns an empty
3726 string otherwise. If there's an error, returns the undefined value.
3727 This call is actually implemented in terms of recvfrom(2) system call.
3728 See L<perlipc/"UDP: Message Passing"> for examples.
3730 Note the I<characters>: depending on the status of the socket, either
3731 (8-bit) bytes or characters are received. By default all sockets
3732 operate on bytes, but for example if the socket has been changed using
3733 binmode() to operate with the C<:utf8> discipline (see the C<open>
3734 pragma, L<open>), the I/O will operate on characters, not bytes.
3740 The C<redo> command restarts the loop block without evaluating the
3741 conditional again. The C<continue> block, if any, is not executed. If
3742 the LABEL is omitted, the command refers to the innermost enclosing
3743 loop. This command is normally used by programs that want to lie to
3744 themselves about what was just input:
3746 # a simpleminded Pascal comment stripper
3747 # (warning: assumes no { or } in strings)
3748 LINE: while (<STDIN>) {
3749 while (s|({.*}.*){.*}|$1 |) {}
3754 if (/}/) { # end of comment?
3763 C<redo> cannot be used to retry a block which returns a value such as
3764 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3765 a grep() or map() operation.
3767 Note that a block by itself is semantically identical to a loop
3768 that executes once. Thus C<redo> inside such a block will effectively
3769 turn it into a looping construct.
3771 See also L</continue> for an illustration of how C<last>, C<next>, and
3778 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3779 is not specified, C<$_> will be used. The value returned depends on the
3780 type of thing the reference is a reference to.
3781 Builtin types include:
3791 If the referenced object has been blessed into a package, then that package
3792 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3794 if (ref($r) eq "HASH") {
3795 print "r is a reference to a hash.\n";
3798 print "r is not a reference at all.\n";
3800 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3801 print "r is a reference to something that isa hash.\n";
3804 See also L<perlref>.
3806 =item rename OLDNAME,NEWNAME
3808 Changes the name of a file; an existing file NEWNAME will be
3809 clobbered. Returns true for success, false otherwise.
3811 Behavior of this function varies wildly depending on your system
3812 implementation. For example, it will usually not work across file system
3813 boundaries, even though the system I<mv> command sometimes compensates
3814 for this. Other restrictions include whether it works on directories,
3815 open files, or pre-existing files. Check L<perlport> and either the
3816 rename(2) manpage or equivalent system documentation for details.
3818 =item require VERSION
3824 Demands a version of Perl specified by VERSION, or demands some semantics
3825 specified by EXPR or by C<$_> if EXPR is not supplied.
3827 VERSION may be either a numeric argument such as 5.006, which will be
3828 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3829 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3830 VERSION is greater than the version of the current Perl interpreter.
3831 Compare with L</use>, which can do a similar check at compile time.
3833 Specifying VERSION as a literal of the form v5.6.1 should generally be
3834 avoided, because it leads to misleading error messages under earlier
3835 versions of Perl which do not support this syntax. The equivalent numeric
3836 version should be used instead.
3838 require v5.6.1; # run time version check
3839 require 5.6.1; # ditto
3840 require 5.006_001; # ditto; preferred for backwards compatibility
3842 Otherwise, demands that a library file be included if it hasn't already
3843 been included. The file is included via the do-FILE mechanism, which is
3844 essentially just a variety of C<eval>. Has semantics similar to the following
3849 return 1 if $INC{$filename};
3850 my($realfilename,$result);
3852 foreach $prefix (@INC) {
3853 $realfilename = "$prefix/$filename";
3854 if (-f $realfilename) {
3855 $INC{$filename} = $realfilename;
3856 $result = do $realfilename;
3860 die "Can't find $filename in \@INC";
3862 delete $INC{$filename} if $@ || !$result;
3864 die "$filename did not return true value" unless $result;
3868 Note that the file will not be included twice under the same specified
3869 name. The file must return true as the last statement to indicate
3870 successful execution of any initialization code, so it's customary to
3871 end such a file with C<1;> unless you're sure it'll return true
3872 otherwise. But it's better just to put the C<1;>, in case you add more
3875 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3876 replaces "F<::>" with "F</>" in the filename for you,
3877 to make it easy to load standard modules. This form of loading of
3878 modules does not risk altering your namespace.
3880 In other words, if you try this:
3882 require Foo::Bar; # a splendid bareword
3884 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3885 directories specified in the C<@INC> array.
3887 But if you try this:
3889 $class = 'Foo::Bar';
3890 require $class; # $class is not a bareword
3892 require "Foo::Bar"; # not a bareword because of the ""
3894 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3895 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3897 eval "require $class";
3899 You can also insert hooks into the import facility, by putting directly
3900 Perl code into the @INC array. There are three forms of hooks: subroutine
3901 references, array references and blessed objects.
3903 Subroutine references are the simplest case. When the inclusion system
3904 walks through @INC and encounters a subroutine, this subroutine gets
3905 called with two parameters, the first being a reference to itself, and the
3906 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3907 subroutine should return C<undef> or a filehandle, from which the file to
3908 include will be read. If C<undef> is returned, C<require> will look at
3909 the remaining elements of @INC.
3911 If the hook is an array reference, its first element must be a subroutine
3912 reference. This subroutine is called as above, but the first parameter is
3913 the array reference. This enables to pass indirectly some arguments to
3916 In other words, you can write:
3918 push @INC, \&my_sub;
3920 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3926 push @INC, [ \&my_sub, $x, $y, ... ];
3928 my ($arrayref, $filename) = @_;
3929 # Retrieve $x, $y, ...
3930 my @parameters = @$arrayref[1..$#$arrayref];
3934 If the hook is an object, it must provide an INC method, that will be
3935 called as above, the first parameter being the object itself. (Note that
3936 you must fully qualify the sub's name, as it is always forced into package
3937 C<main>.) Here is a typical code layout:
3943 my ($self, $filename) = @_;
3947 # In the main program
3948 push @INC, new Foo(...);
3950 Note that these hooks are also permitted to set the %INC entry
3951 corresponding to the files they have loaded. See L<perlvar/%INC>.
3953 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3959 Generally used in a C<continue> block at the end of a loop to clear
3960 variables and reset C<??> searches so that they work again. The
3961 expression is interpreted as a list of single characters (hyphens
3962 allowed for ranges). All variables and arrays beginning with one of
3963 those letters are reset to their pristine state. If the expression is
3964 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3965 only variables or searches in the current package. Always returns
3968 reset 'X'; # reset all X variables
3969 reset 'a-z'; # reset lower case variables
3970 reset; # just reset ?one-time? searches
3972 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3973 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3974 variables--lexical variables are unaffected, but they clean themselves
3975 up on scope exit anyway, so you'll probably want to use them instead.
3982 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3983 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3984 context, depending on how the return value will be used, and the context
3985 may vary from one execution to the next (see C<wantarray>). If no EXPR
3986 is given, returns an empty list in list context, the undefined value in
3987 scalar context, and (of course) nothing at all in a void context.
3989 (Note that in the absence of an explicit C<return>, a subroutine, eval,
3990 or do FILE will automatically return the value of the last expression
3995 In list context, returns a list value consisting of the elements
3996 of LIST in the opposite order. In scalar context, concatenates the
3997 elements of LIST and returns a string value with all characters
3998 in the opposite order.
4000 print reverse <>; # line tac, last line first
4002 undef $/; # for efficiency of <>
4003 print scalar reverse <>; # character tac, last line tsrif
4005 This operator is also handy for inverting a hash, although there are some
4006 caveats. If a value is duplicated in the original hash, only one of those
4007 can be represented as a key in the inverted hash. Also, this has to
4008 unwind one hash and build a whole new one, which may take some time
4009 on a large hash, such as from a DBM file.
4011 %by_name = reverse %by_address; # Invert the hash
4013 =item rewinddir DIRHANDLE
4015 Sets the current position to the beginning of the directory for the
4016 C<readdir> routine on DIRHANDLE.
4018 =item rindex STR,SUBSTR,POSITION
4020 =item rindex STR,SUBSTR
4022 Works just like index() except that it returns the position of the LAST
4023 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4024 last occurrence at or before that position.
4026 =item rmdir FILENAME
4030 Deletes the directory specified by FILENAME if that directory is empty. If it
4031 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4032 FILENAME is omitted, uses C<$_>.
4036 The substitution operator. See L<perlop>.
4040 Forces EXPR to be interpreted in scalar context and returns the value
4043 @counts = ( scalar @a, scalar @b, scalar @c );
4045 There is no equivalent operator to force an expression to
4046 be interpolated in list context because in practice, this is never
4047 needed. If you really wanted to do so, however, you could use
4048 the construction C<@{[ (some expression) ]}>, but usually a simple
4049 C<(some expression)> suffices.
4051 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4052 parenthesized list, this behaves as a scalar comma expression, evaluating
4053 all but the last element in void context and returning the final element
4054 evaluated in scalar context. This is seldom what you want.
4056 The following single statement:
4058 print uc(scalar(&foo,$bar)),$baz;
4060 is the moral equivalent of these two:
4063 print(uc($bar),$baz);
4065 See L<perlop> for more details on unary operators and the comma operator.
4067 =item seek FILEHANDLE,POSITION,WHENCE
4069 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4070 FILEHANDLE may be an expression whose value gives the name of the
4071 filehandle. The values for WHENCE are C<0> to set the new position
4072 I<in bytes> to POSITION, C<1> to set it to the current position plus
4073 POSITION, and C<2> to set it to EOF plus POSITION (typically
4074 negative). For WHENCE you may use the constants C<SEEK_SET>,
4075 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4076 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4079 Note the I<in bytes>: even if the filehandle has been set to
4080 operate on characters (for example by using the C<:utf8> open
4081 discipline), tell() will return byte offsets, not character offsets
4082 (because implementing that would render seek() and tell() rather slow).
4084 If you want to position file for C<sysread> or C<syswrite>, don't use
4085 C<seek>--buffering makes its effect on the file's system position
4086 unpredictable and non-portable. Use C<sysseek> instead.
4088 Due to the rules and rigors of ANSI C, on some systems you have to do a
4089 seek whenever you switch between reading and writing. Amongst other
4090 things, this may have the effect of calling stdio's clearerr(3).
4091 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4095 This is also useful for applications emulating C<tail -f>. Once you hit
4096 EOF on your read, and then sleep for a while, you might have to stick in a
4097 seek() to reset things. The C<seek> doesn't change the current position,
4098 but it I<does> clear the end-of-file condition on the handle, so that the
4099 next C<< <FILE> >> makes Perl try again to read something. We hope.
4101 If that doesn't work (some IO implementations are particularly
4102 cantankerous), then you may need something more like this:
4105 for ($curpos = tell(FILE); $_ = <FILE>;
4106 $curpos = tell(FILE)) {
4107 # search for some stuff and put it into files
4109 sleep($for_a_while);
4110 seek(FILE, $curpos, 0);
4113 =item seekdir DIRHANDLE,POS
4115 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4116 must be a value returned by C<telldir>. Has the same caveats about
4117 possible directory compaction as the corresponding system library
4120 =item select FILEHANDLE
4124 Returns the currently selected filehandle. Sets the current default
4125 filehandle for output, if FILEHANDLE is supplied. This has two
4126 effects: first, a C<write> or a C<print> without a filehandle will
4127 default to this FILEHANDLE. Second, references to variables related to
4128 output will refer to this output channel. For example, if you have to
4129 set the top of form format for more than one output channel, you might
4137 FILEHANDLE may be an expression whose value gives the name of the
4138 actual filehandle. Thus:
4140 $oldfh = select(STDERR); $| = 1; select($oldfh);
4142 Some programmers may prefer to think of filehandles as objects with
4143 methods, preferring to write the last example as:
4146 STDERR->autoflush(1);
4148 =item select RBITS,WBITS,EBITS,TIMEOUT
4150 This calls the select(2) system call with the bit masks specified, which
4151 can be constructed using C<fileno> and C<vec>, along these lines:
4153 $rin = $win = $ein = '';
4154 vec($rin,fileno(STDIN),1) = 1;
4155 vec($win,fileno(STDOUT),1) = 1;
4158 If you want to select on many filehandles you might wish to write a
4162 my(@fhlist) = split(' ',$_[0]);
4165 vec($bits,fileno($_),1) = 1;
4169 $rin = fhbits('STDIN TTY SOCK');
4173 ($nfound,$timeleft) =
4174 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4176 or to block until something becomes ready just do this
4178 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4180 Most systems do not bother to return anything useful in $timeleft, so
4181 calling select() in scalar context just returns $nfound.
4183 Any of the bit masks can also be undef. The timeout, if specified, is
4184 in seconds, which may be fractional. Note: not all implementations are
4185 capable of returning the $timeleft. If not, they always return
4186 $timeleft equal to the supplied $timeout.
4188 You can effect a sleep of 250 milliseconds this way:
4190 select(undef, undef, undef, 0.25);
4192 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4193 or <FH>) with C<select>, except as permitted by POSIX, and even
4194 then only on POSIX systems. You have to use C<sysread> instead.
4196 =item semctl ID,SEMNUM,CMD,ARG
4198 Calls the System V IPC function C<semctl>. You'll probably have to say
4202 first to get the correct constant definitions. If CMD is IPC_STAT or
4203 GETALL, then ARG must be a variable which will hold the returned
4204 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4205 the undefined value for error, "C<0 but true>" for zero, or the actual
4206 return value otherwise. The ARG must consist of a vector of native
4207 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4208 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4211 =item semget KEY,NSEMS,FLAGS
4213 Calls the System V IPC function semget. Returns the semaphore id, or
4214 the undefined value if there is an error. See also
4215 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4218 =item semop KEY,OPSTRING
4220 Calls the System V IPC function semop to perform semaphore operations
4221 such as signalling and waiting. OPSTRING must be a packed array of
4222 semop structures. Each semop structure can be generated with
4223 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4224 operations is implied by the length of OPSTRING. Returns true if
4225 successful, or false if there is an error. As an example, the
4226 following code waits on semaphore $semnum of semaphore id $semid:
4228 $semop = pack("s!3", $semnum, -1, 0);
4229 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4231 To signal the semaphore, replace C<-1> with C<1>. See also
4232 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4235 =item send SOCKET,MSG,FLAGS,TO
4237 =item send SOCKET,MSG,FLAGS
4239 Sends a message on a socket. Attempts to send the scalar MSG to the
4240 SOCKET filehandle. Takes the same flags as the system call of the
4241 same name. On unconnected sockets you must specify a destination to
4242 send TO, in which case it does a C C<sendto>. Returns the number of
4243 characters sent, or the undefined value if there is an error. The C
4244 system call sendmsg(2) is currently unimplemented. See
4245 L<perlipc/"UDP: Message Passing"> for examples.
4247 Note the I<characters>: depending on the status of the socket, either
4248 (8-bit) bytes or characters are sent. By default all sockets operate
4249 on bytes, but for example if the socket has been changed using
4250 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4251 the C<open> pragma, L<open>), the I/O will operate on characters, not
4254 =item setpgrp PID,PGRP
4256 Sets the current process group for the specified PID, C<0> for the current
4257 process. Will produce a fatal error if used on a machine that doesn't
4258 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4259 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4260 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4263 =item setpriority WHICH,WHO,PRIORITY
4265 Sets the current priority for a process, a process group, or a user.
4266 (See setpriority(2).) Will produce a fatal error if used on a machine
4267 that doesn't implement setpriority(2).
4269 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4271 Sets the socket option requested. Returns undefined if there is an
4272 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4279 Shifts the first value of the array off and returns it, shortening the
4280 array by 1 and moving everything down. If there are no elements in the
4281 array, returns the undefined value. If ARRAY is omitted, shifts the
4282 C<@_> array within the lexical scope of subroutines and formats, and the
4283 C<@ARGV> array at file scopes or within the lexical scopes established by
4284 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4287 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4288 same thing to the left end of an array that C<pop> and C<push> do to the
4291 =item shmctl ID,CMD,ARG
4293 Calls the System V IPC function shmctl. You'll probably have to say
4297 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4298 then ARG must be a variable which will hold the returned C<shmid_ds>
4299 structure. Returns like ioctl: the undefined value for error, "C<0> but
4300 true" for zero, or the actual return value otherwise.
4301 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4303 =item shmget KEY,SIZE,FLAGS
4305 Calls the System V IPC function shmget. Returns the shared memory
4306 segment id, or the undefined value if there is an error.
4307 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4309 =item shmread ID,VAR,POS,SIZE
4311 =item shmwrite ID,STRING,POS,SIZE
4313 Reads or writes the System V shared memory segment ID starting at
4314 position POS for size SIZE by attaching to it, copying in/out, and
4315 detaching from it. When reading, VAR must be a variable that will
4316 hold the data read. When writing, if STRING is too long, only SIZE
4317 bytes are used; if STRING is too short, nulls are written to fill out
4318 SIZE bytes. Return true if successful, or false if there is an error.
4319 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4320 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4322 =item shutdown SOCKET,HOW
4324 Shuts down a socket connection in the manner indicated by HOW, which
4325 has the same interpretation as in the system call of the same name.
4327 shutdown(SOCKET, 0); # I/we have stopped reading data
4328 shutdown(SOCKET, 1); # I/we have stopped writing data
4329 shutdown(SOCKET, 2); # I/we have stopped using this socket
4331 This is useful with sockets when you want to tell the other
4332 side you're done writing but not done reading, or vice versa.
4333 It's also a more insistent form of close because it also
4334 disables the file descriptor in any forked copies in other
4341 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4342 returns sine of C<$_>.
4344 For the inverse sine operation, you may use the C<Math::Trig::asin>
4345 function, or use this relation:
4347 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4353 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4354 May be interrupted if the process receives a signal such as C<SIGALRM>.
4355 Returns the number of seconds actually slept. You probably cannot
4356 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4359 On some older systems, it may sleep up to a full second less than what
4360 you requested, depending on how it counts seconds. Most modern systems
4361 always sleep the full amount. They may appear to sleep longer than that,
4362 however, because your process might not be scheduled right away in a
4363 busy multitasking system.
4365 For delays of finer granularity than one second, you may use Perl's
4366 C<syscall> interface to access setitimer(2) if your system supports
4367 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4368 and starting from Perl 5.8 part of the standard distribution) may also
4371 See also the POSIX module's C<pause> function.
4373 =item sockatmark SOCKET
4375 Returns true if the socket is positioned at the out-of-band mark
4376 (also known as the urgent data mark), false otherwise. Use right
4377 after reading from the socket.
4379 Not available directly, one has to import the function from
4380 the IO::Socket extension
4382 use IO::Socket 'sockatmark';
4384 Even this doesn't guarantee that sockatmark() really is available,
4385 though, because sockatmark() is a relatively recent addition to
4386 the family of socket functions. If it is unavailable, attempt to
4389 IO::Socket::atmark not implemented on this architecture ...
4391 See also L<IO::Socket>.
4393 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4395 Opens a socket of the specified kind and attaches it to filehandle
4396 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4397 the system call of the same name. You should C<use Socket> first
4398 to get the proper definitions imported. See the examples in
4399 L<perlipc/"Sockets: Client/Server Communication">.
4401 On systems that support a close-on-exec flag on files, the flag will
4402 be set for the newly opened file descriptor, as determined by the
4403 value of $^F. See L<perlvar/$^F>.
4405 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4407 Creates an unnamed pair of sockets in the specified domain, of the
4408 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4409 for the system call of the same name. If unimplemented, yields a fatal
4410 error. Returns true if successful.
4412 On systems that support a close-on-exec flag on files, the flag will
4413 be set for the newly opened file descriptors, as determined by the value
4414 of $^F. See L<perlvar/$^F>.
4416 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4417 to C<pipe(Rdr, Wtr)> is essentially:
4420 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4421 shutdown(Rdr, 1); # no more writing for reader
4422 shutdown(Wtr, 0); # no more reading for writer
4424 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4425 emulate socketpair using IP sockets to localhost if your system implements
4426 sockets but not socketpair.
4428 =item sort SUBNAME LIST
4430 =item sort BLOCK LIST
4434 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4435 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4436 specified, it gives the name of a subroutine that returns an integer
4437 less than, equal to, or greater than C<0>, depending on how the elements
4438 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4439 operators are extremely useful in such routines.) SUBNAME may be a
4440 scalar variable name (unsubscripted), in which case the value provides
4441 the name of (or a reference to) the actual subroutine to use. In place
4442 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4445 If the subroutine's prototype is C<($$)>, the elements to be compared
4446 are passed by reference in C<@_>, as for a normal subroutine. This is
4447 slower than unprototyped subroutines, where the elements to be
4448 compared are passed into the subroutine
4449 as the package global variables $a and $b (see example below). Note that
4450 in the latter case, it is usually counter-productive to declare $a and
4453 In either case, the subroutine may not be recursive. The values to be
4454 compared are always passed by reference, so don't modify them.
4456 You also cannot exit out of the sort block or subroutine using any of the
4457 loop control operators described in L<perlsyn> or with C<goto>.
4459 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4460 current collation locale. See L<perllocale>.
4462 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4463 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4464 preserves the input order of elements that compare equal. Although
4465 quicksort's run time is O(NlogN) when averaged over all arrays of
4466 length N, the time can be O(N**2), I<quadratic> behavior, for some
4467 inputs.) In 5.7, the quicksort implementation was replaced with
4468 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4469 But benchmarks indicated that for some inputs, on some platforms,
4470 the original quicksort was faster. 5.8 has a sort pragma for
4471 limited control of the sort. Its rather blunt control of the
4472 underlying algorithm may not persist into future perls, but the
4473 ability to characterize the input or output in implementation
4474 independent ways quite probably will. See L</use>.
4479 @articles = sort @files;
4481 # same thing, but with explicit sort routine
4482 @articles = sort {$a cmp $b} @files;
4484 # now case-insensitively
4485 @articles = sort {uc($a) cmp uc($b)} @files;
4487 # same thing in reversed order
4488 @articles = sort {$b cmp $a} @files;
4490 # sort numerically ascending
4491 @articles = sort {$a <=> $b} @files;
4493 # sort numerically descending
4494 @articles = sort {$b <=> $a} @files;
4496 # this sorts the %age hash by value instead of key
4497 # using an in-line function
4498 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4500 # sort using explicit subroutine name
4502 $age{$a} <=> $age{$b}; # presuming numeric
4504 @sortedclass = sort byage @class;
4506 sub backwards { $b cmp $a }
4507 @harry = qw(dog cat x Cain Abel);
4508 @george = qw(gone chased yz Punished Axed);
4510 # prints AbelCaincatdogx
4511 print sort backwards @harry;
4512 # prints xdogcatCainAbel
4513 print sort @george, 'to', @harry;
4514 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4516 # inefficiently sort by descending numeric compare using
4517 # the first integer after the first = sign, or the
4518 # whole record case-insensitively otherwise
4521 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4526 # same thing, but much more efficiently;
4527 # we'll build auxiliary indices instead
4531 push @nums, /=(\d+)/;
4536 $nums[$b] <=> $nums[$a]
4538 $caps[$a] cmp $caps[$b]
4542 # same thing, but without any temps
4543 @new = map { $_->[0] }
4544 sort { $b->[1] <=> $a->[1]
4547 } map { [$_, /=(\d+)/, uc($_)] } @old;
4549 # using a prototype allows you to use any comparison subroutine
4550 # as a sort subroutine (including other package's subroutines)
4552 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4555 @new = sort other::backwards @old;
4557 # guarantee stability, regardless of algorithm
4559 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4561 # force use of quicksort (not portable outside Perl 5.8)
4562 use sort '_quicksort'; # note discouraging _
4563 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4565 # similar to the previous example, but demand stability as well
4566 use sort qw( _mergesort stable );
4567 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4569 If you're using strict, you I<must not> declare $a
4570 and $b as lexicals. They are package globals. That means
4571 if you're in the C<main> package and type
4573 @articles = sort {$b <=> $a} @files;
4575 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4576 but if you're in the C<FooPack> package, it's the same as typing
4578 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4580 The comparison function is required to behave. If it returns
4581 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4582 sometimes saying the opposite, for example) the results are not
4585 =item splice ARRAY,OFFSET,LENGTH,LIST
4587 =item splice ARRAY,OFFSET,LENGTH
4589 =item splice ARRAY,OFFSET
4593 Removes the elements designated by OFFSET and LENGTH from an array, and
4594 replaces them with the elements of LIST, if any. In list context,
4595 returns the elements removed from the array. In scalar context,
4596 returns the last element removed, or C<undef> if no elements are
4597 removed. The array grows or shrinks as necessary.
4598 If OFFSET is negative then it starts that far from the end of the array.
4599 If LENGTH is omitted, removes everything from OFFSET onward.
4600 If LENGTH is negative, leaves that many elements off the end of the array.
4601 If both OFFSET and LENGTH are omitted, removes everything.
4603 The following equivalences hold (assuming C<$[ == 0>):
4605 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4606 pop(@a) splice(@a,-1)
4607 shift(@a) splice(@a,0,1)
4608 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4609 $a[$x] = $y splice(@a,$x,1,$y)
4611 Example, assuming array lengths are passed before arrays:
4613 sub aeq { # compare two list values
4614 my(@a) = splice(@_,0,shift);
4615 my(@b) = splice(@_,0,shift);
4616 return 0 unless @a == @b; # same len?
4618 return 0 if pop(@a) ne pop(@b);
4622 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4624 =item split /PATTERN/,EXPR,LIMIT
4626 =item split /PATTERN/,EXPR
4628 =item split /PATTERN/
4632 Splits a string into a list of strings and returns that list. By default,
4633 empty leading fields are preserved, and empty trailing ones are deleted.
4635 In scalar context, returns the number of fields found and splits into
4636 the C<@_> array. Use of split in scalar context is deprecated, however,
4637 because it clobbers your subroutine arguments.
4639 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4640 splits on whitespace (after skipping any leading whitespace). Anything
4641 matching PATTERN is taken to be a delimiter separating the fields. (Note
4642 that the delimiter may be longer than one character.)
4644 If LIMIT is specified and positive, it represents the maximum number
4645 of fields the EXPR will be split into, though the actual number of
4646 fields returned depends on the number of times PATTERN matches within
4647 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4648 stripped (which potential users of C<pop> would do well to remember).
4649 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4650 had been specified. Note that splitting an EXPR that evaluates to the
4651 empty string always returns the empty list, regardless of the LIMIT
4654 A pattern matching the null string (not to be confused with
4655 a null pattern C<//>, which is just one member of the set of patterns
4656 matching a null string) will split the value of EXPR into separate
4657 characters at each point it matches that way. For example:
4659 print join(':', split(/ */, 'hi there'));
4661 produces the output 'h:i:t:h:e:r:e'.
4663 Using the empty pattern C<//> specifically matches the null string, and is
4664 not be confused with the use of C<//> to mean "the last successful pattern
4667 Empty leading (or trailing) fields are produced when there positive width
4668 matches at the beginning (or end) of the string; a zero-width match at the
4669 beginning (or end) of the string does not produce an empty field. For
4672 print join(':', split(/(?=\w)/, 'hi there!'));
4674 produces the output 'h:i :t:h:e:r:e!'.
4676 The LIMIT parameter can be used to split a line partially
4678 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4680 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4681 one larger than the number of variables in the list, to avoid
4682 unnecessary work. For the list above LIMIT would have been 4 by
4683 default. In time critical applications it behooves you not to split
4684 into more fields than you really need.
4686 If the PATTERN contains parentheses, additional list elements are
4687 created from each matching substring in the delimiter.
4689 split(/([,-])/, "1-10,20", 3);
4691 produces the list value
4693 (1, '-', 10, ',', 20)
4695 If you had the entire header of a normal Unix email message in $header,
4696 you could split it up into fields and their values this way:
4698 $header =~ s/\n\s+/ /g; # fix continuation lines
4699 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4701 The pattern C</PATTERN/> may be replaced with an expression to specify
4702 patterns that vary at runtime. (To do runtime compilation only once,
4703 use C</$variable/o>.)
4705 As a special case, specifying a PATTERN of space (C<' '>) will split on
4706 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4707 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4708 will give you as many null initial fields as there are leading spaces.
4709 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4710 whitespace produces a null first field. A C<split> with no arguments
4711 really does a C<split(' ', $_)> internally.
4713 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4718 open(PASSWD, '/etc/passwd');
4721 ($login, $passwd, $uid, $gid,
4722 $gcos, $home, $shell) = split(/:/);
4726 As with regular pattern matching, any capturing parentheses that are not
4727 matched in a C<split()> will be set to C<undef> when returned:
4729 @fields = split /(A)|B/, "1A2B3";
4730 # @fields is (1, 'A', 2, undef, 3)
4732 =item sprintf FORMAT, LIST
4734 Returns a string formatted by the usual C<printf> conventions of the C
4735 library function C<sprintf>. See below for more details
4736 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4737 the general principles.
4741 # Format number with up to 8 leading zeroes
4742 $result = sprintf("%08d", $number);
4744 # Round number to 3 digits after decimal point
4745 $rounded = sprintf("%.3f", $number);
4747 Perl does its own C<sprintf> formatting--it emulates the C
4748 function C<sprintf>, but it doesn't use it (except for floating-point
4749 numbers, and even then only the standard modifiers are allowed). As a
4750 result, any non-standard extensions in your local C<sprintf> are not
4751 available from Perl.
4753 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4754 pass it an array as your first argument. The array is given scalar context,
4755 and instead of using the 0th element of the array as the format, Perl will
4756 use the count of elements in the array as the format, which is almost never
4759 Perl's C<sprintf> permits the following universally-known conversions:
4762 %c a character with the given number
4764 %d a signed integer, in decimal
4765 %u an unsigned integer, in decimal
4766 %o an unsigned integer, in octal
4767 %x an unsigned integer, in hexadecimal
4768 %e a floating-point number, in scientific notation
4769 %f a floating-point number, in fixed decimal notation
4770 %g a floating-point number, in %e or %f notation
4772 In addition, Perl permits the following widely-supported conversions:
4774 %X like %x, but using upper-case letters
4775 %E like %e, but using an upper-case "E"
4776 %G like %g, but with an upper-case "E" (if applicable)
4777 %b an unsigned integer, in binary
4778 %p a pointer (outputs the Perl value's address in hexadecimal)
4779 %n special: *stores* the number of characters output so far
4780 into the next variable in the parameter list
4782 Finally, for backward (and we do mean "backward") compatibility, Perl
4783 permits these unnecessary but widely-supported conversions:
4786 %D a synonym for %ld
4787 %U a synonym for %lu
4788 %O a synonym for %lo
4791 Note that the number of exponent digits in the scientific notation by
4792 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4793 exponent less than 100 is system-dependent: it may be three or less
4794 (zero-padded as necessary). In other words, 1.23 times ten to the
4795 99th may be either "1.23e99" or "1.23e099".
4797 Perl permits the following universally-known flags between the C<%>
4798 and the conversion letter:
4800 space prefix positive number with a space
4801 + prefix positive number with a plus sign
4802 - left-justify within the field
4803 0 use zeros, not spaces, to right-justify
4804 # prefix non-zero octal with "0", non-zero hex with "0x"
4805 number minimum field width
4806 .number "precision": digits after decimal point for
4807 floating-point, max length for string, minimum length
4809 l interpret integer as C type "long" or "unsigned long"
4810 h interpret integer as C type "short" or "unsigned short"
4811 If no flags, interpret integer as C type "int" or "unsigned"
4813 Perl supports parameter ordering, in other words, fetching the
4814 parameters in some explicitly specified "random" ordering as opposed
4815 to the default implicit sequential ordering. The syntax is, instead
4816 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4817 where the I<digits> is the wanted index, from one upwards. For example:
4819 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4820 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4822 Note that using the reordering syntax does not interfere with the usual
4823 implicit sequential fetching of the parameters:
4825 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4826 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4827 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4828 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4829 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4831 There are also two Perl-specific flags:
4833 V interpret integer as Perl's standard integer type
4834 v interpret string as a vector of integers, output as
4835 numbers separated either by dots, or by an arbitrary
4836 string received from the argument list when the flag
4839 Where a number would appear in the flags, an asterisk (C<*>) may be
4840 used instead, in which case Perl uses the next item in the parameter
4841 list as the given number (that is, as the field width or precision).
4842 If a field width obtained through C<*> is negative, it has the same
4843 effect as the C<-> flag: left-justification.
4845 The C<v> flag is useful for displaying ordinal values of characters
4846 in arbitrary strings:
4848 printf "version is v%vd\n", $^V; # Perl's version
4849 printf "address is %*vX\n", ":", $addr; # IPv6 address
4850 printf "bits are %*vb\n", " ", $bits; # random bitstring
4852 If C<use locale> is in effect, the character used for the decimal
4853 point in formatted real numbers is affected by the LC_NUMERIC locale.
4856 If Perl understands "quads" (64-bit integers) (this requires
4857 either that the platform natively support quads or that Perl
4858 be specifically compiled to support quads), the characters
4862 print quads, and they may optionally be preceded by
4870 You can find out whether your Perl supports quads via L<Config>:
4873 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4876 If Perl understands "long doubles" (this requires that the platform
4877 support long doubles), the flags
4881 may optionally be preceded by
4889 You can find out whether your Perl supports long doubles via L<Config>:
4892 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4898 Return the square root of EXPR. If EXPR is omitted, returns square
4899 root of C<$_>. Only works on non-negative operands, unless you've
4900 loaded the standard Math::Complex module.
4903 print sqrt(-2); # prints 1.4142135623731i
4909 Sets the random number seed for the C<rand> operator.
4911 The point of the function is to "seed" the C<rand> function so that
4912 C<rand> can produce a different sequence each time you run your
4915 If srand() is not called explicitly, it is called implicitly at the
4916 first use of the C<rand> operator. However, this was not the case in
4917 versions of Perl before 5.004, so if your script will run under older
4918 Perl versions, it should call C<srand>.
4920 Most programs won't even call srand() at all, except those that
4921 need a cryptographically-strong starting point rather than the
4922 generally acceptable default, which is based on time of day,
4923 process ID, and memory allocation, or the F</dev/urandom> device,
4926 You can call srand($seed) with the same $seed to reproduce the
4927 I<same> sequence from rand(), but this is usually reserved for
4928 generating predictable results for testing or debugging.
4929 Otherwise, don't call srand() more than once in your program.
4931 Do B<not> call srand() (i.e. without an argument) more than once in
4932 a script. The internal state of the random number generator should
4933 contain more entropy than can be provided by any seed, so calling
4934 srand() again actually I<loses> randomness.
4936 Most implementations of C<srand> take an integer and will silently
4937 truncate decimal numbers. This means C<srand(42)> will usually
4938 produce the same results as C<srand(42.1)>. To be safe, always pass
4939 C<srand> an integer.
4941 In versions of Perl prior to 5.004 the default seed was just the
4942 current C<time>. This isn't a particularly good seed, so many old
4943 programs supply their own seed value (often C<time ^ $$> or C<time ^
4944 ($$ + ($$ << 15))>), but that isn't necessary any more.
4946 Note that you need something much more random than the default seed for
4947 cryptographic purposes. Checksumming the compressed output of one or more
4948 rapidly changing operating system status programs is the usual method. For
4951 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4953 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4956 Frequently called programs (like CGI scripts) that simply use
4960 for a seed can fall prey to the mathematical property that
4964 one-third of the time. So don't do that.
4966 =item stat FILEHANDLE
4972 Returns a 13-element list giving the status info for a file, either
4973 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4974 it stats C<$_>. Returns a null list if the stat fails. Typically used
4977 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4978 $atime,$mtime,$ctime,$blksize,$blocks)
4981 Not all fields are supported on all filesystem types. Here are the
4982 meaning of the fields:
4984 0 dev device number of filesystem
4986 2 mode file mode (type and permissions)
4987 3 nlink number of (hard) links to the file
4988 4 uid numeric user ID of file's owner
4989 5 gid numeric group ID of file's owner
4990 6 rdev the device identifier (special files only)
4991 7 size total size of file, in bytes
4992 8 atime last access time in seconds since the epoch
4993 9 mtime last modify time in seconds since the epoch
4994 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4995 11 blksize preferred block size for file system I/O
4996 12 blocks actual number of blocks allocated
4998 (The epoch was at 00:00 January 1, 1970 GMT.)
5000 If stat is passed the special filehandle consisting of an underline, no
5001 stat is done, but the current contents of the stat structure from the
5002 last stat or filetest are returned. Example:
5004 if (-x $file && (($d) = stat(_)) && $d < 0) {
5005 print "$file is executable NFS file\n";
5008 (This works on machines only for which the device number is negative
5011 Because the mode contains both the file type and its permissions, you
5012 should mask off the file type portion and (s)printf using a C<"%o">
5013 if you want to see the real permissions.
5015 $mode = (stat($filename))[2];
5016 printf "Permissions are %04o\n", $mode & 07777;
5018 In scalar context, C<stat> returns a boolean value indicating success
5019 or failure, and, if successful, sets the information associated with
5020 the special filehandle C<_>.
5022 The File::stat module provides a convenient, by-name access mechanism:
5025 $sb = stat($filename);
5026 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5027 $filename, $sb->size, $sb->mode & 07777,
5028 scalar localtime $sb->mtime;
5030 You can import symbolic mode constants (C<S_IF*>) and functions
5031 (C<S_IS*>) from the Fcntl module:
5035 $mode = (stat($filename))[2];
5037 $user_rwx = ($mode & S_IRWXU) >> 6;
5038 $group_read = ($mode & S_IRGRP) >> 3;
5039 $other_execute = $mode & S_IXOTH;
5041 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5043 $is_setuid = $mode & S_ISUID;
5044 $is_setgid = S_ISDIR($mode);
5046 You could write the last two using the C<-u> and C<-d> operators.
5047 The commonly available S_IF* constants are
5049 # Permissions: read, write, execute, for user, group, others.
5051 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5052 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5053 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5055 # Setuid/Setgid/Stickiness.
5057 S_ISUID S_ISGID S_ISVTX S_ISTXT
5059 # File types. Not necessarily all are available on your system.
5061 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5063 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5065 S_IREAD S_IWRITE S_IEXEC
5067 and the S_IF* functions are
5069 S_IFMODE($mode) the part of $mode containing the permission bits
5070 and the setuid/setgid/sticky bits
5072 S_IFMT($mode) the part of $mode containing the file type
5073 which can be bit-anded with e.g. S_IFREG
5074 or with the following functions
5076 # The operators -f, -d, -l, -b, -c, -p, and -s.
5078 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5079 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5081 # No direct -X operator counterpart, but for the first one
5082 # the -g operator is often equivalent. The ENFMT stands for
5083 # record flocking enforcement, a platform-dependent feature.
5085 S_ISENFMT($mode) S_ISWHT($mode)
5087 See your native chmod(2) and stat(2) documentation for more details
5088 about the S_* constants.
5094 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5095 doing many pattern matches on the string before it is next modified.
5096 This may or may not save time, depending on the nature and number of
5097 patterns you are searching on, and on the distribution of character
5098 frequencies in the string to be searched--you probably want to compare
5099 run times with and without it to see which runs faster. Those loops
5100 which scan for many short constant strings (including the constant
5101 parts of more complex patterns) will benefit most. You may have only
5102 one C<study> active at a time--if you study a different scalar the first
5103 is "unstudied". (The way C<study> works is this: a linked list of every
5104 character in the string to be searched is made, so we know, for
5105 example, where all the C<'k'> characters are. From each search string,
5106 the rarest character is selected, based on some static frequency tables
5107 constructed from some C programs and English text. Only those places
5108 that contain this "rarest" character are examined.)
5110 For example, here is a loop that inserts index producing entries
5111 before any line containing a certain pattern:
5115 print ".IX foo\n" if /\bfoo\b/;
5116 print ".IX bar\n" if /\bbar\b/;
5117 print ".IX blurfl\n" if /\bblurfl\b/;
5122 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5123 will be looked at, because C<f> is rarer than C<o>. In general, this is
5124 a big win except in pathological cases. The only question is whether
5125 it saves you more time than it took to build the linked list in the
5128 Note that if you have to look for strings that you don't know till
5129 runtime, you can build an entire loop as a string and C<eval> that to
5130 avoid recompiling all your patterns all the time. Together with
5131 undefining C<$/> to input entire files as one record, this can be very
5132 fast, often faster than specialized programs like fgrep(1). The following
5133 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5134 out the names of those files that contain a match:
5136 $search = 'while (<>) { study;';
5137 foreach $word (@words) {
5138 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5143 eval $search; # this screams
5144 $/ = "\n"; # put back to normal input delimiter
5145 foreach $file (sort keys(%seen)) {
5153 =item sub NAME BLOCK
5155 This is subroutine definition, not a real function I<per se>. With just a
5156 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5157 Without a NAME, it's an anonymous function declaration, and does actually
5158 return a value: the CODE ref of the closure you just created. See L<perlsub>
5159 and L<perlref> for details.
5161 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5163 =item substr EXPR,OFFSET,LENGTH
5165 =item substr EXPR,OFFSET
5167 Extracts a substring out of EXPR and returns it. First character is at
5168 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5169 If OFFSET is negative (or more precisely, less than C<$[>), starts
5170 that far from the end of the string. If LENGTH is omitted, returns
5171 everything to the end of the string. If LENGTH is negative, leaves that
5172 many characters off the end of the string.
5174 You can use the substr() function as an lvalue, in which case EXPR
5175 must itself be an lvalue. If you assign something shorter than LENGTH,
5176 the string will shrink, and if you assign something longer than LENGTH,
5177 the string will grow to accommodate it. To keep the string the same
5178 length you may need to pad or chop your value using C<sprintf>.
5180 If OFFSET and LENGTH specify a substring that is partly outside the
5181 string, only the part within the string is returned. If the substring
5182 is beyond either end of the string, substr() returns the undefined
5183 value and produces a warning. When used as an lvalue, specifying a
5184 substring that is entirely outside the string is a fatal error.
5185 Here's an example showing the behavior for boundary cases:
5188 substr($name, 4) = 'dy'; # $name is now 'freddy'
5189 my $null = substr $name, 6, 2; # returns '' (no warning)
5190 my $oops = substr $name, 7; # returns undef, with warning
5191 substr($name, 7) = 'gap'; # fatal error
5193 An alternative to using substr() as an lvalue is to specify the
5194 replacement string as the 4th argument. This allows you to replace
5195 parts of the EXPR and return what was there before in one operation,
5196 just as you can with splice().
5198 =item symlink OLDFILE,NEWFILE
5200 Creates a new filename symbolically linked to the old filename.
5201 Returns C<1> for success, C<0> otherwise. On systems that don't support
5202 symbolic links, produces a fatal error at run time. To check for that,
5205 $symlink_exists = eval { symlink("",""); 1 };
5209 Calls the system call specified as the first element of the list,
5210 passing the remaining elements as arguments to the system call. If
5211 unimplemented, produces a fatal error. The arguments are interpreted
5212 as follows: if a given argument is numeric, the argument is passed as
5213 an int. If not, the pointer to the string value is passed. You are
5214 responsible to make sure a string is pre-extended long enough to
5215 receive any result that might be written into a string. You can't use a
5216 string literal (or other read-only string) as an argument to C<syscall>
5217 because Perl has to assume that any string pointer might be written
5219 integer arguments are not literals and have never been interpreted in a
5220 numeric context, you may need to add C<0> to them to force them to look
5221 like numbers. This emulates the C<syswrite> function (or vice versa):
5223 require 'syscall.ph'; # may need to run h2ph
5225 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5227 Note that Perl supports passing of up to only 14 arguments to your system call,
5228 which in practice should usually suffice.
5230 Syscall returns whatever value returned by the system call it calls.
5231 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5232 Note that some system calls can legitimately return C<-1>. The proper
5233 way to handle such calls is to assign C<$!=0;> before the call and
5234 check the value of C<$!> if syscall returns C<-1>.
5236 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5237 number of the read end of the pipe it creates. There is no way
5238 to retrieve the file number of the other end. You can avoid this
5239 problem by using C<pipe> instead.
5241 =item sysopen FILEHANDLE,FILENAME,MODE
5243 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5245 Opens the file whose filename is given by FILENAME, and associates it
5246 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5247 the name of the real filehandle wanted. This function calls the
5248 underlying operating system's C<open> function with the parameters
5249 FILENAME, MODE, PERMS.
5251 The possible values and flag bits of the MODE parameter are
5252 system-dependent; they are available via the standard module C<Fcntl>.
5253 See the documentation of your operating system's C<open> to see which
5254 values and flag bits are available. You may combine several flags
5255 using the C<|>-operator.
5257 Some of the most common values are C<O_RDONLY> for opening the file in
5258 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5259 and C<O_RDWR> for opening the file in read-write mode, and.
5261 For historical reasons, some values work on almost every system
5262 supported by perl: zero means read-only, one means write-only, and two
5263 means read/write. We know that these values do I<not> work under
5264 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5265 use them in new code.
5267 If the file named by FILENAME does not exist and the C<open> call creates
5268 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5269 PERMS specifies the permissions of the newly created file. If you omit
5270 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5271 These permission values need to be in octal, and are modified by your
5272 process's current C<umask>.
5274 In many systems the C<O_EXCL> flag is available for opening files in
5275 exclusive mode. This is B<not> locking: exclusiveness means here that
5276 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5279 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5281 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5282 that takes away the user's option to have a more permissive umask.
5283 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5286 Note that C<sysopen> depends on the fdopen() C library function.
5287 On many UNIX systems, fdopen() is known to fail when file descriptors
5288 exceed a certain value, typically 255. If you need more file
5289 descriptors than that, consider rebuilding Perl to use the C<sfio>
5290 library, or perhaps using the POSIX::open() function.
5292 See L<perlopentut> for a kinder, gentler explanation of opening files.
5294 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5296 =item sysread FILEHANDLE,SCALAR,LENGTH
5298 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5299 the specified FILEHANDLE, using the system call read(2). It bypasses
5300 buffered IO, so mixing this with other kinds of reads, C<print>,
5301 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5302 stdio usually buffers data. Returns the number of characters actually
5303 read, C<0> at end of file, or undef if there was an error. SCALAR
5304 will be grown or shrunk so that the last byte actually read is the
5305 last byte of the scalar after the read.
5307 Note the I<characters>: depending on the status of the filehandle,
5308 either (8-bit) bytes or characters are read. By default all
5309 filehandles operate on bytes, but for example if the filehandle has
5310 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5311 pragma, L<open>), the I/O will operate on characters, not bytes.
5313 An OFFSET may be specified to place the read data at some place in the
5314 string other than the beginning. A negative OFFSET specifies
5315 placement at that many characters counting backwards from the end of
5316 the string. A positive OFFSET greater than the length of SCALAR
5317 results in the string being padded to the required size with C<"\0">
5318 bytes before the result of the read is appended.
5320 There is no syseof() function, which is ok, since eof() doesn't work
5321 very well on device files (like ttys) anyway. Use sysread() and check
5322 for a return value for 0 to decide whether you're done.
5324 =item sysseek FILEHANDLE,POSITION,WHENCE
5326 Sets FILEHANDLE's system position I<in bytes> using the system call
5327 lseek(2). FILEHANDLE may be an expression whose value gives the name
5328 of the filehandle. The values for WHENCE are C<0> to set the new
5329 position to POSITION, C<1> to set the it to the current position plus
5330 POSITION, and C<2> to set it to EOF plus POSITION (typically
5333 Note the I<in bytes>: even if the filehandle has been set to operate
5334 on characters (for example by using the C<:utf8> discipline), tell()
5335 will return byte offsets, not character offsets (because implementing
5336 that would render sysseek() very slow).
5338 sysseek() bypasses normal buffered io, so mixing this with reads (other
5339 than C<sysread>, for example >< or read()) C<print>, C<write>,
5340 C<seek>, C<tell>, or C<eof> may cause confusion.
5342 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5343 and C<SEEK_END> (start of the file, current position, end of the file)
5344 from the Fcntl module. Use of the constants is also more portable
5345 than relying on 0, 1, and 2. For example to define a "systell" function:
5347 use Fnctl 'SEEK_CUR';
5348 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5350 Returns the new position, or the undefined value on failure. A position
5351 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5352 true on success and false on failure, yet you can still easily determine
5357 =item system PROGRAM LIST
5359 Does exactly the same thing as C<exec LIST>, except that a fork is
5360 done first, and the parent process waits for the child process to
5361 complete. Note that argument processing varies depending on the
5362 number of arguments. If there is more than one argument in LIST,
5363 or if LIST is an array with more than one value, starts the program
5364 given by the first element of the list with arguments given by the
5365 rest of the list. If there is only one scalar argument, the argument
5366 is checked for shell metacharacters, and if there are any, the
5367 entire argument is passed to the system's command shell for parsing
5368 (this is C</bin/sh -c> on Unix platforms, but varies on other
5369 platforms). If there are no shell metacharacters in the argument,
5370 it is split into words and passed directly to C<execvp>, which is
5373 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5374 output before any operation that may do a fork, but this may not be
5375 supported on some platforms (see L<perlport>). To be safe, you may need
5376 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5377 of C<IO::Handle> on any open handles.
5379 The return value is the exit status of the program as returned by the
5380 C<wait> call. To get the actual exit value shift right by eight (see below).
5381 See also L</exec>. This is I<not> what you want to use to capture
5382 the output from a command, for that you should use merely backticks or
5383 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5384 indicates a failure to start the program (inspect $! for the reason).
5386 Like C<exec>, C<system> allows you to lie to a program about its name if
5387 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5389 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5390 killing the program they're running doesn't actually interrupt
5393 @args = ("command", "arg1", "arg2");
5395 or die "system @args failed: $?"
5397 You can check all the failure possibilities by inspecting
5400 $exit_value = $? >> 8;
5401 $signal_num = $? & 127;
5402 $dumped_core = $? & 128;
5404 or more portably by using the W*() calls of the POSIX extension;
5405 see L<perlport> for more information.
5407 When the arguments get executed via the system shell, results
5408 and return codes will be subject to its quirks and capabilities.
5409 See L<perlop/"`STRING`"> and L</exec> for details.
5411 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5413 =item syswrite FILEHANDLE,SCALAR,LENGTH
5415 =item syswrite FILEHANDLE,SCALAR
5417 Attempts to write LENGTH characters of data from variable SCALAR to
5418 the specified FILEHANDLE, using the system call write(2). If LENGTH
5419 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5420 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5421 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5422 buffers data. Returns the number of characters actually written, or
5423 C<undef> if there was an error. If the LENGTH is greater than the
5424 available data in the SCALAR after the OFFSET, only as much data as is
5425 available will be written.
5427 An OFFSET may be specified to write the data from some part of the
5428 string other than the beginning. A negative OFFSET specifies writing
5429 that many characters counting backwards from the end of the string.
5430 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5432 Note the I<characters>: depending on the status of the filehandle,
5433 either (8-bit) bytes or characters are written. By default all
5434 filehandles operate on bytes, but for example if the filehandle has
5435 been opened with the C<:utf8> discipline (see L</open>, and the open
5436 pragma, L<open>), the I/O will operate on characters, not bytes.
5438 =item tell FILEHANDLE
5442 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5443 error. FILEHANDLE may be an expression whose value gives the name of
5444 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5447 Note the I<in bytes>: even if the filehandle has been set to
5448 operate on characters (for example by using the C<:utf8> open
5449 discipline), tell() will return byte offsets, not character offsets
5450 (because that would render seek() and tell() rather slow).
5452 The return value of tell() for the standard streams like the STDIN
5453 depends on the operating system: it may return -1 or something else.
5454 tell() on pipes, fifos, and sockets usually returns -1.
5456 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5458 Do not use tell() on a filehandle that has been opened using
5459 sysopen(), use sysseek() for that as described above. Why? Because
5460 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5461 buffered filehandles. sysseek() make sense only on the first kind,
5462 tell() only makes sense on the second kind.
5464 =item telldir DIRHANDLE
5466 Returns the current position of the C<readdir> routines on DIRHANDLE.
5467 Value may be given to C<seekdir> to access a particular location in a
5468 directory. Has the same caveats about possible directory compaction as
5469 the corresponding system library routine.
5471 =item tie VARIABLE,CLASSNAME,LIST
5473 This function binds a variable to a package class that will provide the
5474 implementation for the variable. VARIABLE is the name of the variable
5475 to be enchanted. CLASSNAME is the name of a class implementing objects
5476 of correct type. Any additional arguments are passed to the C<new>
5477 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5478 or C<TIEHASH>). Typically these are arguments such as might be passed
5479 to the C<dbm_open()> function of C. The object returned by the C<new>
5480 method is also returned by the C<tie> function, which would be useful
5481 if you want to access other methods in CLASSNAME.
5483 Note that functions such as C<keys> and C<values> may return huge lists
5484 when used on large objects, like DBM files. You may prefer to use the
5485 C<each> function to iterate over such. Example:
5487 # print out history file offsets
5489 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5490 while (($key,$val) = each %HIST) {
5491 print $key, ' = ', unpack('L',$val), "\n";
5495 A class implementing a hash should have the following methods:
5497 TIEHASH classname, LIST
5499 STORE this, key, value
5504 NEXTKEY this, lastkey
5508 A class implementing an ordinary array should have the following methods:
5510 TIEARRAY classname, LIST
5512 STORE this, key, value
5514 STORESIZE this, count
5520 SPLICE this, offset, length, LIST
5525 A class implementing a file handle should have the following methods:
5527 TIEHANDLE classname, LIST
5528 READ this, scalar, length, offset
5531 WRITE this, scalar, length, offset
5533 PRINTF this, format, LIST
5537 SEEK this, position, whence
5539 OPEN this, mode, LIST
5544 A class implementing a scalar should have the following methods:
5546 TIESCALAR classname, LIST
5552 Not all methods indicated above need be implemented. See L<perltie>,
5553 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5555 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5556 for you--you need to do that explicitly yourself. See L<DB_File>
5557 or the F<Config> module for interesting C<tie> implementations.
5559 For further details see L<perltie>, L<"tied VARIABLE">.
5563 Returns a reference to the object underlying VARIABLE (the same value
5564 that was originally returned by the C<tie> call that bound the variable
5565 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5570 Returns the number of non-leap seconds since whatever time the system
5571 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5572 and 00:00:00 UTC, January 1, 1970 for most other systems).
5573 Suitable for feeding to C<gmtime> and C<localtime>.
5575 For measuring time in better granularity than one second,
5576 you may use either the Time::HiRes module from CPAN, or
5577 if you have gettimeofday(2), you may be able to use the
5578 C<syscall> interface of Perl, see L<perlfaq8> for details.
5582 Returns a four-element list giving the user and system times, in
5583 seconds, for this process and the children of this process.
5585 ($user,$system,$cuser,$csystem) = times;
5587 In scalar context, C<times> returns C<$user>.
5591 The transliteration operator. Same as C<y///>. See L<perlop>.
5593 =item truncate FILEHANDLE,LENGTH
5595 =item truncate EXPR,LENGTH
5597 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5598 specified length. Produces a fatal error if truncate isn't implemented
5599 on your system. Returns true if successful, the undefined value
5606 Returns an uppercased version of EXPR. This is the internal function
5607 implementing the C<\U> escape in double-quoted strings. Respects
5608 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5609 and L<perlunicode> for more details about locale and Unicode support.
5610 It does not attempt to do titlecase mapping on initial letters. See
5611 C<ucfirst> for that.
5613 If EXPR is omitted, uses C<$_>.
5619 Returns the value of EXPR with the first character in uppercase
5620 (titlecase in Unicode). This is the internal function implementing
5621 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5622 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5623 for more details about locale and Unicode support.
5625 If EXPR is omitted, uses C<$_>.
5631 Sets the umask for the process to EXPR and returns the previous value.
5632 If EXPR is omitted, merely returns the current umask.
5634 The Unix permission C<rwxr-x---> is represented as three sets of three
5635 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5636 and isn't one of the digits). The C<umask> value is such a number
5637 representing disabled permissions bits. The permission (or "mode")
5638 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5639 even if you tell C<sysopen> to create a file with permissions C<0777>,
5640 if your umask is C<0022> then the file will actually be created with
5641 permissions C<0755>. If your C<umask> were C<0027> (group can't
5642 write; others can't read, write, or execute), then passing
5643 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5646 Here's some advice: supply a creation mode of C<0666> for regular
5647 files (in C<sysopen>) and one of C<0777> for directories (in
5648 C<mkdir>) and executable files. This gives users the freedom of
5649 choice: if they want protected files, they might choose process umasks
5650 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5651 Programs should rarely if ever make policy decisions better left to
5652 the user. The exception to this is when writing files that should be
5653 kept private: mail files, web browser cookies, I<.rhosts> files, and
5656 If umask(2) is not implemented on your system and you are trying to
5657 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5658 fatal error at run time. If umask(2) is not implemented and you are
5659 not trying to restrict access for yourself, returns C<undef>.
5661 Remember that a umask is a number, usually given in octal; it is I<not> a
5662 string of octal digits. See also L</oct>, if all you have is a string.
5668 Undefines the value of EXPR, which must be an lvalue. Use only on a
5669 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5670 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5671 will probably not do what you expect on most predefined variables or
5672 DBM list values, so don't do that; see L<delete>.) Always returns the
5673 undefined value. You can omit the EXPR, in which case nothing is
5674 undefined, but you still get an undefined value that you could, for
5675 instance, return from a subroutine, assign to a variable or pass as a
5676 parameter. Examples:
5679 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5683 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5684 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5685 select undef, undef, undef, 0.25;
5686 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5688 Note that this is a unary operator, not a list operator.
5694 Deletes a list of files. Returns the number of files successfully
5697 $cnt = unlink 'a', 'b', 'c';
5701 Note: C<unlink> will not delete directories unless you are superuser and
5702 the B<-U> flag is supplied to Perl. Even if these conditions are
5703 met, be warned that unlinking a directory can inflict damage on your
5704 filesystem. Use C<rmdir> instead.
5706 If LIST is omitted, uses C<$_>.
5708 =item unpack TEMPLATE,EXPR
5710 C<unpack> does the reverse of C<pack>: it takes a string
5711 and expands it out into a list of values.
5712 (In scalar context, it returns merely the first value produced.)
5714 The string is broken into chunks described by the TEMPLATE. Each chunk
5715 is converted separately to a value. Typically, either the string is a result
5716 of C<pack>, or the bytes of the string represent a C structure of some
5719 The TEMPLATE has the same format as in the C<pack> function.
5720 Here's a subroutine that does substring:
5723 my($what,$where,$howmuch) = @_;
5724 unpack("x$where a$howmuch", $what);
5729 sub ordinal { unpack("c",$_[0]); } # same as ord()
5731 In addition to fields allowed in pack(), you may prefix a field with
5732 a %<number> to indicate that
5733 you want a <number>-bit checksum of the items instead of the items
5734 themselves. Default is a 16-bit checksum. Checksum is calculated by
5735 summing numeric values of expanded values (for string fields the sum of
5736 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5738 For example, the following
5739 computes the same number as the System V sum program:
5743 unpack("%32C*",<>) % 65535;
5746 The following efficiently counts the number of set bits in a bit vector:
5748 $setbits = unpack("%32b*", $selectmask);
5750 The C<p> and C<P> formats should be used with care. Since Perl
5751 has no way of checking whether the value passed to C<unpack()>
5752 corresponds to a valid memory location, passing a pointer value that's
5753 not known to be valid is likely to have disastrous consequences.
5755 If the repeat count of a field is larger than what the remainder of
5756 the input string allows, repeat count is decreased. If the input string
5757 is longer than one described by the TEMPLATE, the rest is ignored.
5759 See L</pack> for more examples and notes.
5761 =item untie VARIABLE
5763 Breaks the binding between a variable and a package. (See C<tie>.)
5765 =item unshift ARRAY,LIST
5767 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5768 depending on how you look at it. Prepends list to the front of the
5769 array, and returns the new number of elements in the array.
5771 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5773 Note the LIST is prepended whole, not one element at a time, so the
5774 prepended elements stay in the same order. Use C<reverse> to do the
5777 =item use Module VERSION LIST
5779 =item use Module VERSION
5781 =item use Module LIST
5787 Imports some semantics into the current package from the named module,
5788 generally by aliasing certain subroutine or variable names into your
5789 package. It is exactly equivalent to
5791 BEGIN { require Module; import Module LIST; }
5793 except that Module I<must> be a bareword.
5795 VERSION may be either a numeric argument such as 5.006, which will be
5796 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5797 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5798 greater than the version of the current Perl interpreter; Perl will not
5799 attempt to parse the rest of the file. Compare with L</require>, which can
5800 do a similar check at run time.
5802 Specifying VERSION as a literal of the form v5.6.1 should generally be
5803 avoided, because it leads to misleading error messages under earlier
5804 versions of Perl which do not support this syntax. The equivalent numeric
5805 version should be used instead.
5807 use v5.6.1; # compile time version check
5809 use 5.006_001; # ditto; preferred for backwards compatibility
5811 This is often useful if you need to check the current Perl version before
5812 C<use>ing library modules that have changed in incompatible ways from
5813 older versions of Perl. (We try not to do this more than we have to.)
5815 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5816 C<require> makes sure the module is loaded into memory if it hasn't been
5817 yet. The C<import> is not a builtin--it's just an ordinary static method
5818 call into the C<Module> package to tell the module to import the list of
5819 features back into the current package. The module can implement its
5820 C<import> method any way it likes, though most modules just choose to
5821 derive their C<import> method via inheritance from the C<Exporter> class that
5822 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5823 method can be found then the call is skipped.
5825 If you do not want to call the package's C<import> method (for instance,
5826 to stop your namespace from being altered), explicitly supply the empty list:
5830 That is exactly equivalent to
5832 BEGIN { require Module }
5834 If the VERSION argument is present between Module and LIST, then the
5835 C<use> will call the VERSION method in class Module with the given
5836 version as an argument. The default VERSION method, inherited from
5837 the UNIVERSAL class, croaks if the given version is larger than the
5838 value of the variable C<$Module::VERSION>.
5840 Again, there is a distinction between omitting LIST (C<import> called
5841 with no arguments) and an explicit empty LIST C<()> (C<import> not
5842 called). Note that there is no comma after VERSION!
5844 Because this is a wide-open interface, pragmas (compiler directives)
5845 are also implemented this way. Currently implemented pragmas are:
5850 use sigtrap qw(SEGV BUS);
5851 use strict qw(subs vars refs);
5852 use subs qw(afunc blurfl);
5853 use warnings qw(all);
5854 use sort qw(stable _quicksort _mergesort);
5856 Some of these pseudo-modules import semantics into the current
5857 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5858 which import symbols into the current package (which are effective
5859 through the end of the file).
5861 There's a corresponding C<no> command that unimports meanings imported
5862 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5868 If no C<unimport> method can be found the call fails with a fatal error.
5870 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5871 for the C<-M> and C<-m> command-line options to perl that give C<use>
5872 functionality from the command-line.
5876 Changes the access and modification times on each file of a list of
5877 files. The first two elements of the list must be the NUMERICAL access
5878 and modification times, in that order. Returns the number of files
5879 successfully changed. The inode change time of each file is set
5880 to the current time. This code has the same effect as the C<touch>
5881 command if the files already exist:
5885 utime $now, $now, @ARGV;
5887 If the first two elements of the list are C<undef>, then the utime(2)
5888 function in the C library will be called with a null second argument.
5889 On most systems, this will set the file's access and modification
5890 times to the current time. (i.e. equivalent to the example above.)
5892 utime undef, undef, @ARGV;
5896 Returns a list consisting of all the values of the named hash. (In a
5897 scalar context, returns the number of values.) The values are
5898 returned in an apparently random order. The actual random order is
5899 subject to change in future versions of perl, but it is guaranteed to
5900 be the same order as either the C<keys> or C<each> function would
5901 produce on the same (unmodified) hash.
5903 Note that the values are not copied, which means modifying them will
5904 modify the contents of the hash:
5906 for (values %hash) { s/foo/bar/g } # modifies %hash values
5907 for (@hash{keys %hash}) { s/foo/bar/g } # same
5909 As a side effect, calling values() resets the HASH's internal iterator.
5910 See also C<keys>, C<each>, and C<sort>.
5912 =item vec EXPR,OFFSET,BITS
5914 Treats the string in EXPR as a bit vector made up of elements of
5915 width BITS, and returns the value of the element specified by OFFSET
5916 as an unsigned integer. BITS therefore specifies the number of bits
5917 that are reserved for each element in the bit vector. This must
5918 be a power of two from 1 to 32 (or 64, if your platform supports
5921 If BITS is 8, "elements" coincide with bytes of the input string.
5923 If BITS is 16 or more, bytes of the input string are grouped into chunks
5924 of size BITS/8, and each group is converted to a number as with
5925 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5926 for BITS==64). See L<"pack"> for details.
5928 If bits is 4 or less, the string is broken into bytes, then the bits
5929 of each byte are broken into 8/BITS groups. Bits of a byte are
5930 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5931 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5932 breaking the single input byte C<chr(0x36)> into two groups gives a list
5933 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5935 C<vec> may also be assigned to, in which case parentheses are needed
5936 to give the expression the correct precedence as in
5938 vec($image, $max_x * $x + $y, 8) = 3;
5940 If the selected element is outside the string, the value 0 is returned.
5941 If an element off the end of the string is written to, Perl will first
5942 extend the string with sufficiently many zero bytes. It is an error
5943 to try to write off the beginning of the string (i.e. negative OFFSET).
5945 The string should not contain any character with the value > 255 (which
5946 can only happen if you're using UTF8 encoding). If it does, it will be
5947 treated as something which is not UTF8 encoded. When the C<vec> was
5948 assigned to, other parts of your program will also no longer consider the
5949 string to be UTF8 encoded. In other words, if you do have such characters
5950 in your string, vec() will operate on the actual byte string, and not the
5951 conceptual character string.
5953 Strings created with C<vec> can also be manipulated with the logical
5954 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5955 vector operation is desired when both operands are strings.
5956 See L<perlop/"Bitwise String Operators">.
5958 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5959 The comments show the string after each step. Note that this code works
5960 in the same way on big-endian or little-endian machines.
5963 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5965 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5966 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5968 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5969 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5970 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5971 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5972 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5973 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5975 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5976 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5977 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5980 To transform a bit vector into a string or list of 0's and 1's, use these:
5982 $bits = unpack("b*", $vector);
5983 @bits = split(//, unpack("b*", $vector));
5985 If you know the exact length in bits, it can be used in place of the C<*>.
5987 Here is an example to illustrate how the bits actually fall in place:
5993 unpack("V",$_) 01234567890123456789012345678901
5994 ------------------------------------------------------------------
5999 for ($shift=0; $shift < $width; ++$shift) {
6000 for ($off=0; $off < 32/$width; ++$off) {
6001 $str = pack("B*", "0"x32);
6002 $bits = (1<<$shift);
6003 vec($str, $off, $width) = $bits;
6004 $res = unpack("b*",$str);
6005 $val = unpack("V", $str);
6012 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6013 $off, $width, $bits, $val, $res
6017 Regardless of the machine architecture on which it is run, the above
6018 example should print the following table:
6021 unpack("V",$_) 01234567890123456789012345678901
6022 ------------------------------------------------------------------
6023 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6024 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6025 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6026 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6027 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6028 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6029 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6030 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6031 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6032 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6033 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6034 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6035 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6036 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6037 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6038 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6039 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6040 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6041 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6042 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6043 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6044 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6045 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6046 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6047 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6048 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6049 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6050 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6051 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6052 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6053 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6054 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6055 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6056 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6057 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6058 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6059 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6060 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6061 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6062 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6063 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6064 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6065 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6066 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6067 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6068 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6069 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6070 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6071 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6072 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6073 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6074 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6075 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6076 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6077 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6078 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6079 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6080 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6081 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6082 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6083 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6084 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6085 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6086 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6087 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6088 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6089 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6090 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6091 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6092 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6093 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6094 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6095 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6096 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6097 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6098 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6099 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6100 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6101 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6102 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6103 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6104 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6105 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6106 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6107 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6108 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6109 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6110 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6111 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6112 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6113 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6114 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6115 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6116 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6117 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6118 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6119 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6120 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6121 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6122 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6123 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6124 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6125 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6126 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6127 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6128 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6129 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6130 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6131 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6132 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6133 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6134 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6135 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6136 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6137 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6138 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6139 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6140 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6141 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6142 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6143 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6144 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6145 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6146 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6147 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6148 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6149 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6150 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6154 Behaves like the wait(2) system call on your system: it waits for a child
6155 process to terminate and returns the pid of the deceased process, or
6156 C<-1> if there are no child processes. The status is returned in C<$?>.
6157 Note that a return value of C<-1> could mean that child processes are
6158 being automatically reaped, as described in L<perlipc>.
6160 =item waitpid PID,FLAGS
6162 Waits for a particular child process to terminate and returns the pid of
6163 the deceased process, or C<-1> if there is no such child process. On some
6164 systems, a value of 0 indicates that there are processes still running.
6165 The status is returned in C<$?>. If you say
6167 use POSIX ":sys_wait_h";
6170 $kid = waitpid(-1, WNOHANG);
6173 then you can do a non-blocking wait for all pending zombie processes.
6174 Non-blocking wait is available on machines supporting either the
6175 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6176 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6177 system call by remembering the status values of processes that have
6178 exited but have not been harvested by the Perl script yet.)
6180 Note that on some systems, a return value of C<-1> could mean that child
6181 processes are being automatically reaped. See L<perlipc> for details,
6182 and for other examples.
6186 Returns true if the context of the currently executing subroutine is
6187 looking for a list value. Returns false if the context is looking
6188 for a scalar. Returns the undefined value if the context is looking
6189 for no value (void context).
6191 return unless defined wantarray; # don't bother doing more
6192 my @a = complex_calculation();
6193 return wantarray ? @a : "@a";
6195 This function should have been named wantlist() instead.
6199 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6202 If LIST is empty and C<$@> already contains a value (typically from a
6203 previous eval) that value is used after appending C<"\t...caught">
6204 to C<$@>. This is useful for staying almost, but not entirely similar to
6207 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6209 No message is printed if there is a C<$SIG{__WARN__}> handler
6210 installed. It is the handler's responsibility to deal with the message
6211 as it sees fit (like, for instance, converting it into a C<die>). Most
6212 handlers must therefore make arrangements to actually display the
6213 warnings that they are not prepared to deal with, by calling C<warn>
6214 again in the handler. Note that this is quite safe and will not
6215 produce an endless loop, since C<__WARN__> hooks are not called from
6218 You will find this behavior is slightly different from that of
6219 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6220 instead call C<die> again to change it).
6222 Using a C<__WARN__> handler provides a powerful way to silence all
6223 warnings (even the so-called mandatory ones). An example:
6225 # wipe out *all* compile-time warnings
6226 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6228 my $foo = 20; # no warning about duplicate my $foo,
6229 # but hey, you asked for it!
6230 # no compile-time or run-time warnings before here
6233 # run-time warnings enabled after here
6234 warn "\$foo is alive and $foo!"; # does show up
6236 See L<perlvar> for details on setting C<%SIG> entries, and for more
6237 examples. See the Carp module for other kinds of warnings using its
6238 carp() and cluck() functions.
6240 =item write FILEHANDLE
6246 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6247 using the format associated with that file. By default the format for
6248 a file is the one having the same name as the filehandle, but the
6249 format for the current output channel (see the C<select> function) may be set
6250 explicitly by assigning the name of the format to the C<$~> variable.
6252 Top of form processing is handled automatically: if there is
6253 insufficient room on the current page for the formatted record, the
6254 page is advanced by writing a form feed, a special top-of-page format
6255 is used to format the new page header, and then the record is written.
6256 By default the top-of-page format is the name of the filehandle with
6257 "_TOP" appended, but it may be dynamically set to the format of your
6258 choice by assigning the name to the C<$^> variable while the filehandle is
6259 selected. The number of lines remaining on the current page is in
6260 variable C<$->, which can be set to C<0> to force a new page.
6262 If FILEHANDLE is unspecified, output goes to the current default output
6263 channel, which starts out as STDOUT but may be changed by the
6264 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6265 is evaluated and the resulting string is used to look up the name of
6266 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6268 Note that write is I<not> the opposite of C<read>. Unfortunately.
6272 The transliteration operator. Same as C<tr///>. See L<perlop>.