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 I<potentially> has
869 characters with codepoints above 255), Perl tries to make sense
870 of the situation by trying to downgrade (a copy of the string)
871 the string back to an eight-bit byte string before calling crypt()
872 (on that copy). If that works, good. If not, crypt() dies with
873 C<Wide character in crypt>.
877 [This function has been largely superseded by the C<untie> function.]
879 Breaks the binding between a DBM file and a hash.
881 =item dbmopen HASH,DBNAME,MASK
883 [This function has been largely superseded by the C<tie> function.]
885 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
886 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
887 argument is I<not> a filehandle, even though it looks like one). DBNAME
888 is the name of the database (without the F<.dir> or F<.pag> extension if
889 any). If the database does not exist, it is created with protection
890 specified by MASK (as modified by the C<umask>). If your system supports
891 only the older DBM functions, you may perform only one C<dbmopen> in your
892 program. In older versions of Perl, if your system had neither DBM nor
893 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
896 If you don't have write access to the DBM file, you can only read hash
897 variables, not set them. If you want to test whether you can write,
898 either use file tests or try setting a dummy hash entry inside an C<eval>,
899 which will trap the error.
901 Note that functions such as C<keys> and C<values> may return huge lists
902 when used on large DBM files. You may prefer to use the C<each>
903 function to iterate over large DBM files. Example:
905 # print out history file offsets
906 dbmopen(%HIST,'/usr/lib/news/history',0666);
907 while (($key,$val) = each %HIST) {
908 print $key, ' = ', unpack('L',$val), "\n";
912 See also L<AnyDBM_File> for a more general description of the pros and
913 cons of the various dbm approaches, as well as L<DB_File> for a particularly
916 You can control which DBM library you use by loading that library
917 before you call dbmopen():
920 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
921 or die "Can't open netscape history file: $!";
927 Returns a Boolean value telling whether EXPR has a value other than
928 the undefined value C<undef>. If EXPR is not present, C<$_> will be
931 Many operations return C<undef> to indicate failure, end of file,
932 system error, uninitialized variable, and other exceptional
933 conditions. This function allows you to distinguish C<undef> from
934 other values. (A simple Boolean test will not distinguish among
935 C<undef>, zero, the empty string, and C<"0">, which are all equally
936 false.) Note that since C<undef> is a valid scalar, its presence
937 doesn't I<necessarily> indicate an exceptional condition: C<pop>
938 returns C<undef> when its argument is an empty array, I<or> when the
939 element to return happens to be C<undef>.
941 You may also use C<defined(&func)> to check whether subroutine C<&func>
942 has ever been defined. The return value is unaffected by any forward
943 declarations of C<&foo>. Note that a subroutine which is not defined
944 may still be callable: its package may have an C<AUTOLOAD> method that
945 makes it spring into existence the first time that it is called -- see
948 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
949 used to report whether memory for that aggregate has ever been
950 allocated. This behavior may disappear in future versions of Perl.
951 You should instead use a simple test for size:
953 if (@an_array) { print "has array elements\n" }
954 if (%a_hash) { print "has hash members\n" }
956 When used on a hash element, it tells you whether the value is defined,
957 not whether the key exists in the hash. Use L</exists> for the latter
962 print if defined $switch{'D'};
963 print "$val\n" while defined($val = pop(@ary));
964 die "Can't readlink $sym: $!"
965 unless defined($value = readlink $sym);
966 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
967 $debugging = 0 unless defined $debugging;
969 Note: Many folks tend to overuse C<defined>, and then are surprised to
970 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
971 defined values. For example, if you say
975 The pattern match succeeds, and C<$1> is defined, despite the fact that it
976 matched "nothing". But it didn't really match nothing--rather, it
977 matched something that happened to be zero characters long. This is all
978 very above-board and honest. When a function returns an undefined value,
979 it's an admission that it couldn't give you an honest answer. So you
980 should use C<defined> only when you're questioning the integrity of what
981 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
984 See also L</undef>, L</exists>, L</ref>.
988 Given an expression that specifies a hash element, array element, hash slice,
989 or array slice, deletes the specified element(s) from the hash or array.
990 In the case of an array, if the array elements happen to be at the end,
991 the size of the array will shrink to the highest element that tests
992 true for exists() (or 0 if no such element exists).
994 Returns each element so deleted or the undefined value if there was no such
995 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
996 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
997 from a C<tie>d hash or array may not necessarily return anything.
999 Deleting an array element effectively returns that position of the array
1000 to its initial, uninitialized state. Subsequently testing for the same
1001 element with exists() will return false. Note that deleting array
1002 elements in the middle of an array will not shift the index of the ones
1003 after them down--use splice() for that. See L</exists>.
1005 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1007 foreach $key (keys %HASH) {
1011 foreach $index (0 .. $#ARRAY) {
1012 delete $ARRAY[$index];
1017 delete @HASH{keys %HASH};
1019 delete @ARRAY[0 .. $#ARRAY];
1021 But both of these are slower than just assigning the empty list
1022 or undefining %HASH or @ARRAY:
1024 %HASH = (); # completely empty %HASH
1025 undef %HASH; # forget %HASH ever existed
1027 @ARRAY = (); # completely empty @ARRAY
1028 undef @ARRAY; # forget @ARRAY ever existed
1030 Note that the EXPR can be arbitrarily complicated as long as the final
1031 operation is a hash element, array element, hash slice, or array slice
1034 delete $ref->[$x][$y]{$key};
1035 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1037 delete $ref->[$x][$y][$index];
1038 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1042 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1043 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1044 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1045 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1046 an C<eval(),> the error message is stuffed into C<$@> and the
1047 C<eval> is terminated with the undefined value. This makes
1048 C<die> the way to raise an exception.
1050 Equivalent examples:
1052 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1053 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1055 If the value of EXPR does not end in a newline, the current script line
1056 number and input line number (if any) are also printed, and a newline
1057 is supplied. Note that the "input line number" (also known as "chunk")
1058 is subject to whatever notion of "line" happens to be currently in
1059 effect, and is also available as the special variable C<$.>.
1060 See L<perlvar/"$/"> and L<perlvar/"$.">.
1062 Hint: sometimes appending C<", stopped"> to your message
1063 will cause it to make better sense when the string C<"at foo line 123"> is
1064 appended. Suppose you are running script "canasta".
1066 die "/etc/games is no good";
1067 die "/etc/games is no good, stopped";
1069 produce, respectively
1071 /etc/games is no good at canasta line 123.
1072 /etc/games is no good, stopped at canasta line 123.
1074 See also exit(), warn(), and the Carp module.
1076 If LIST is empty and C<$@> already contains a value (typically from a
1077 previous eval) that value is reused after appending C<"\t...propagated">.
1078 This is useful for propagating exceptions:
1081 die unless $@ =~ /Expected exception/;
1083 If LIST is empty and C<$@> contains an object reference that has a
1084 C<PROPAGATE> method, that method will be called with additional file
1085 and line number parameters. The return value replaces the value in
1086 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1089 If C<$@> is empty then the string C<"Died"> is used.
1091 die() can also be called with a reference argument. If this happens to be
1092 trapped within an eval(), $@ contains the reference. This behavior permits
1093 a more elaborate exception handling implementation using objects that
1094 maintain arbitrary state about the nature of the exception. Such a scheme
1095 is sometimes preferable to matching particular string values of $@ using
1096 regular expressions. Here's an example:
1098 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1100 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1101 # handle Some::Module::Exception
1104 # handle all other possible exceptions
1108 Because perl will stringify uncaught exception messages before displaying
1109 them, you may want to overload stringification operations on such custom
1110 exception objects. See L<overload> for details about that.
1112 You can arrange for a callback to be run just before the C<die>
1113 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1114 handler will be called with the error text and can change the error
1115 message, if it sees fit, by calling C<die> again. See
1116 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1117 L<"eval BLOCK"> for some examples. Although this feature was meant
1118 to be run only right before your program was to exit, this is not
1119 currently the case--the C<$SIG{__DIE__}> hook is currently called
1120 even inside eval()ed blocks/strings! If one wants the hook to do
1121 nothing in such situations, put
1125 as the first line of the handler (see L<perlvar/$^S>). Because
1126 this promotes strange action at a distance, this counterintuitive
1127 behavior may be fixed in a future release.
1131 Not really a function. Returns the value of the last command in the
1132 sequence of commands indicated by BLOCK. When modified by a loop
1133 modifier, executes the BLOCK once before testing the loop condition.
1134 (On other statements the loop modifiers test the conditional first.)
1136 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1137 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1138 See L<perlsyn> for alternative strategies.
1140 =item do SUBROUTINE(LIST)
1142 A deprecated form of subroutine call. See L<perlsub>.
1146 Uses the value of EXPR as a filename and executes the contents of the
1147 file as a Perl script. Its primary use is to include subroutines
1148 from a Perl subroutine library.
1156 except that it's more efficient and concise, keeps track of the current
1157 filename for error messages, searches the @INC libraries, and updates
1158 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1159 variables. It also differs in that code evaluated with C<do FILENAME>
1160 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1161 same, however, in that it does reparse the file every time you call it,
1162 so you probably don't want to do this inside a loop.
1164 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1165 error. If C<do> can read the file but cannot compile it, it
1166 returns undef and sets an error message in C<$@>. If the file is
1167 successfully compiled, C<do> returns the value of the last expression
1170 Note that inclusion of library modules is better done with the
1171 C<use> and C<require> operators, which also do automatic error checking
1172 and raise an exception if there's a problem.
1174 You might like to use C<do> to read in a program configuration
1175 file. Manual error checking can be done this way:
1177 # read in config files: system first, then user
1178 for $file ("/share/prog/defaults.rc",
1179 "$ENV{HOME}/.someprogrc")
1181 unless ($return = do $file) {
1182 warn "couldn't parse $file: $@" if $@;
1183 warn "couldn't do $file: $!" unless defined $return;
1184 warn "couldn't run $file" unless $return;
1192 This function causes an immediate core dump. See also the B<-u>
1193 command-line switch in L<perlrun>, which does the same thing.
1194 Primarily this is so that you can use the B<undump> program (not
1195 supplied) to turn your core dump into an executable binary after
1196 having initialized all your variables at the beginning of the
1197 program. When the new binary is executed it will begin by executing
1198 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1199 Think of it as a goto with an intervening core dump and reincarnation.
1200 If C<LABEL> is omitted, restarts the program from the top.
1202 B<WARNING>: Any files opened at the time of the dump will I<not>
1203 be open any more when the program is reincarnated, with possible
1204 resulting confusion on the part of Perl.
1206 This function is now largely obsolete, partly because it's very
1207 hard to convert a core file into an executable, and because the
1208 real compiler backends for generating portable bytecode and compilable
1209 C code have superseded it. That's why you should now invoke it as
1210 C<CORE::dump()>, if you don't want to be warned against a possible
1213 If you're looking to use L<dump> to speed up your program, consider
1214 generating bytecode or native C code as described in L<perlcc>. If
1215 you're just trying to accelerate a CGI script, consider using the
1216 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1217 You might also consider autoloading or selfloading, which at least
1218 make your program I<appear> to run faster.
1222 When called in list context, returns a 2-element list consisting of the
1223 key and value for the next element of a hash, so that you can iterate over
1224 it. When called in scalar context, returns only the key for the next
1225 element in the hash.
1227 Entries are returned in an apparently random order. The actual random
1228 order is subject to change in future versions of perl, but it is guaranteed
1229 to be in the same order as either the C<keys> or C<values> function
1230 would produce on the same (unmodified) hash.
1232 When the hash is entirely read, a null array is returned in list context
1233 (which when assigned produces a false (C<0>) value), and C<undef> in
1234 scalar context. The next call to C<each> after that will start iterating
1235 again. There is a single iterator for each hash, shared by all C<each>,
1236 C<keys>, and C<values> function calls in the program; it can be reset by
1237 reading all the elements from the hash, or by evaluating C<keys HASH> or
1238 C<values HASH>. If you add or delete elements of a hash while you're
1239 iterating over it, you may get entries skipped or duplicated, so
1240 don't. Exception: It is always safe to delete the item most recently
1241 returned by C<each()>, which means that the following code will work:
1243 while (($key, $value) = each %hash) {
1245 delete $hash{$key}; # This is safe
1248 The following prints out your environment like the printenv(1) program,
1249 only in a different order:
1251 while (($key,$value) = each %ENV) {
1252 print "$key=$value\n";
1255 See also C<keys>, C<values> and C<sort>.
1257 =item eof FILEHANDLE
1263 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1264 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1265 gives the real filehandle. (Note that this function actually
1266 reads a character and then C<ungetc>s it, so isn't very useful in an
1267 interactive context.) Do not read from a terminal file (or call
1268 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1269 as terminals may lose the end-of-file condition if you do.
1271 An C<eof> without an argument uses the last file read. Using C<eof()>
1272 with empty parentheses is very different. It refers to the pseudo file
1273 formed from the files listed on the command line and accessed via the
1274 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1275 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1276 used will cause C<@ARGV> to be examined to determine if input is
1279 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1280 detect the end of each file, C<eof()> will only detect the end of the
1281 last file. Examples:
1283 # reset line numbering on each input file
1285 next if /^\s*#/; # skip comments
1288 close ARGV if eof; # Not eof()!
1291 # insert dashes just before last line of last file
1293 if (eof()) { # check for end of current file
1294 print "--------------\n";
1295 close(ARGV); # close or last; is needed if we
1296 # are reading from the terminal
1301 Practical hint: you almost never need to use C<eof> in Perl, because the
1302 input operators typically return C<undef> when they run out of data, or if
1309 In the first form, the return value of EXPR is parsed and executed as if it
1310 were a little Perl program. The value of the expression (which is itself
1311 determined within scalar context) is first parsed, and if there weren't any
1312 errors, executed in the lexical context of the current Perl program, so
1313 that any variable settings or subroutine and format definitions remain
1314 afterwards. Note that the value is parsed every time the eval executes.
1315 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1316 delay parsing and subsequent execution of the text of EXPR until run time.
1318 In the second form, the code within the BLOCK is parsed only once--at the
1319 same time the code surrounding the eval itself was parsed--and executed
1320 within the context of the current Perl program. This form is typically
1321 used to trap exceptions more efficiently than the first (see below), while
1322 also providing the benefit of checking the code within BLOCK at compile
1325 The final semicolon, if any, may be omitted from the value of EXPR or within
1328 In both forms, the value returned is the value of the last expression
1329 evaluated inside the mini-program; a return statement may be also used, just
1330 as with subroutines. The expression providing the return value is evaluated
1331 in void, scalar, or list context, depending on the context of the eval itself.
1332 See L</wantarray> for more on how the evaluation context can be determined.
1334 If there is a syntax error or runtime error, or a C<die> statement is
1335 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1336 error message. If there was no error, C<$@> is guaranteed to be a null
1337 string. Beware that using C<eval> neither silences perl from printing
1338 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1339 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1340 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1341 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1343 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1344 determining whether a particular feature (such as C<socket> or C<symlink>)
1345 is implemented. It is also Perl's exception trapping mechanism, where
1346 the die operator is used to raise exceptions.
1348 If the code to be executed doesn't vary, you may use the eval-BLOCK
1349 form to trap run-time errors without incurring the penalty of
1350 recompiling each time. The error, if any, is still returned in C<$@>.
1353 # make divide-by-zero nonfatal
1354 eval { $answer = $a / $b; }; warn $@ if $@;
1356 # same thing, but less efficient
1357 eval '$answer = $a / $b'; warn $@ if $@;
1359 # a compile-time error
1360 eval { $answer = }; # WRONG
1363 eval '$answer ='; # sets $@
1365 Due to the current arguably broken state of C<__DIE__> hooks, when using
1366 the C<eval{}> form as an exception trap in libraries, you may wish not
1367 to trigger any C<__DIE__> hooks that user code may have installed.
1368 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1369 as shown in this example:
1371 # a very private exception trap for divide-by-zero
1372 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1375 This is especially significant, given that C<__DIE__> hooks can call
1376 C<die> again, which has the effect of changing their error messages:
1378 # __DIE__ hooks may modify error messages
1380 local $SIG{'__DIE__'} =
1381 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1382 eval { die "foo lives here" };
1383 print $@ if $@; # prints "bar lives here"
1386 Because this promotes action at a distance, this counterintuitive behavior
1387 may be fixed in a future release.
1389 With an C<eval>, you should be especially careful to remember what's
1390 being looked at when:
1396 eval { $x }; # CASE 4
1398 eval "\$$x++"; # CASE 5
1401 Cases 1 and 2 above behave identically: they run the code contained in
1402 the variable $x. (Although case 2 has misleading double quotes making
1403 the reader wonder what else might be happening (nothing is).) Cases 3
1404 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1405 does nothing but return the value of $x. (Case 4 is preferred for
1406 purely visual reasons, but it also has the advantage of compiling at
1407 compile-time instead of at run-time.) Case 5 is a place where
1408 normally you I<would> like to use double quotes, except that in this
1409 particular situation, you can just use symbolic references instead, as
1412 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1413 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1417 =item exec PROGRAM LIST
1419 The C<exec> function executes a system command I<and never returns>--
1420 use C<system> instead of C<exec> if you want it to return. It fails and
1421 returns false only if the command does not exist I<and> it is executed
1422 directly instead of via your system's command shell (see below).
1424 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1425 warns you if there is a following statement which isn't C<die>, C<warn>,
1426 or C<exit> (if C<-w> is set - but you always do that). If you
1427 I<really> want to follow an C<exec> with some other statement, you
1428 can use one of these styles to avoid the warning:
1430 exec ('foo') or print STDERR "couldn't exec foo: $!";
1431 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1433 If there is more than one argument in LIST, or if LIST is an array
1434 with more than one value, calls execvp(3) with the arguments in LIST.
1435 If there is only one scalar argument or an array with one element in it,
1436 the argument is checked for shell metacharacters, and if there are any,
1437 the entire argument is passed to the system's command shell for parsing
1438 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1439 If there are no shell metacharacters in the argument, it is split into
1440 words and passed directly to C<execvp>, which is more efficient.
1443 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1444 exec "sort $outfile | uniq";
1446 If you don't really want to execute the first argument, but want to lie
1447 to the program you are executing about its own name, you can specify
1448 the program you actually want to run as an "indirect object" (without a
1449 comma) in front of the LIST. (This always forces interpretation of the
1450 LIST as a multivalued list, even if there is only a single scalar in
1453 $shell = '/bin/csh';
1454 exec $shell '-sh'; # pretend it's a login shell
1458 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1460 When the arguments get executed via the system shell, results will
1461 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1464 Using an indirect object with C<exec> or C<system> is also more
1465 secure. This usage (which also works fine with system()) forces
1466 interpretation of the arguments as a multivalued list, even if the
1467 list had just one argument. That way you're safe from the shell
1468 expanding wildcards or splitting up words with whitespace in them.
1470 @args = ( "echo surprise" );
1472 exec @args; # subject to shell escapes
1474 exec { $args[0] } @args; # safe even with one-arg list
1476 The first version, the one without the indirect object, ran the I<echo>
1477 program, passing it C<"surprise"> an argument. The second version
1478 didn't--it tried to run a program literally called I<"echo surprise">,
1479 didn't find it, and set C<$?> to a non-zero value indicating failure.
1481 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1482 output before the exec, but this may not be supported on some platforms
1483 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1484 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1485 open handles in order to avoid lost output.
1487 Note that C<exec> will not call your C<END> blocks, nor will it call
1488 any C<DESTROY> methods in your objects.
1492 Given an expression that specifies a hash element or array element,
1493 returns true if the specified element in the hash or array has ever
1494 been initialized, even if the corresponding value is undefined. The
1495 element is not autovivified if it doesn't exist.
1497 print "Exists\n" if exists $hash{$key};
1498 print "Defined\n" if defined $hash{$key};
1499 print "True\n" if $hash{$key};
1501 print "Exists\n" if exists $array[$index];
1502 print "Defined\n" if defined $array[$index];
1503 print "True\n" if $array[$index];
1505 A hash or array element can be true only if it's defined, and defined if
1506 it exists, but the reverse doesn't necessarily hold true.
1508 Given an expression that specifies the name of a subroutine,
1509 returns true if the specified subroutine has ever been declared, even
1510 if it is undefined. Mentioning a subroutine name for exists or defined
1511 does not count as declaring it. Note that a subroutine which does not
1512 exist may still be callable: its package may have an C<AUTOLOAD>
1513 method that makes it spring into existence the first time that it is
1514 called -- see L<perlsub>.
1516 print "Exists\n" if exists &subroutine;
1517 print "Defined\n" if defined &subroutine;
1519 Note that the EXPR can be arbitrarily complicated as long as the final
1520 operation is a hash or array key lookup or subroutine name:
1522 if (exists $ref->{A}->{B}->{$key}) { }
1523 if (exists $hash{A}{B}{$key}) { }
1525 if (exists $ref->{A}->{B}->[$ix]) { }
1526 if (exists $hash{A}{B}[$ix]) { }
1528 if (exists &{$ref->{A}{B}{$key}}) { }
1530 Although the deepest nested array or hash will not spring into existence
1531 just because its existence was tested, any intervening ones will.
1532 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1533 into existence due to the existence test for the $key element above.
1534 This happens anywhere the arrow operator is used, including even:
1537 if (exists $ref->{"Some key"}) { }
1538 print $ref; # prints HASH(0x80d3d5c)
1540 This surprising autovivification in what does not at first--or even
1541 second--glance appear to be an lvalue context may be fixed in a future
1544 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1545 on how exists() acts when used on a pseudo-hash.
1547 Use of a subroutine call, rather than a subroutine name, as an argument
1548 to exists() is an error.
1551 exists &sub(); # Error
1555 Evaluates EXPR and exits immediately with that value. Example:
1558 exit 0 if $ans =~ /^[Xx]/;
1560 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1561 universally recognized values for EXPR are C<0> for success and C<1>
1562 for error; other values are subject to interpretation depending on the
1563 environment in which the Perl program is running. For example, exiting
1564 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1565 the mailer to return the item undelivered, but that's not true everywhere.
1567 Don't use C<exit> to abort a subroutine if there's any chance that
1568 someone might want to trap whatever error happened. Use C<die> instead,
1569 which can be trapped by an C<eval>.
1571 The exit() function does not always exit immediately. It calls any
1572 defined C<END> routines first, but these C<END> routines may not
1573 themselves abort the exit. Likewise any object destructors that need to
1574 be called are called before the real exit. If this is a problem, you
1575 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1576 See L<perlmod> for details.
1582 Returns I<e> (the natural logarithm base) to the power of EXPR.
1583 If EXPR is omitted, gives C<exp($_)>.
1585 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1587 Implements the fcntl(2) function. You'll probably have to say
1591 first to get the correct constant definitions. Argument processing and
1592 value return works just like C<ioctl> below.
1596 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1597 or die "can't fcntl F_GETFL: $!";
1599 You don't have to check for C<defined> on the return from C<fnctl>.
1600 Like C<ioctl>, it maps a C<0> return from the system call into
1601 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1602 in numeric context. It is also exempt from the normal B<-w> warnings
1603 on improper numeric conversions.
1605 Note that C<fcntl> will produce a fatal error if used on a machine that
1606 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1607 manpage to learn what functions are available on your system.
1609 =item fileno FILEHANDLE
1611 Returns the file descriptor for a filehandle, or undefined if the
1612 filehandle is not open. This is mainly useful for constructing
1613 bitmaps for C<select> and low-level POSIX tty-handling operations.
1614 If FILEHANDLE is an expression, the value is taken as an indirect
1615 filehandle, generally its name.
1617 You can use this to find out whether two handles refer to the
1618 same underlying descriptor:
1620 if (fileno(THIS) == fileno(THAT)) {
1621 print "THIS and THAT are dups\n";
1624 (Filehandles connected to memory objects via new features of C<open> may
1625 return undefined even though they are open.)
1628 =item flock FILEHANDLE,OPERATION
1630 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1631 for success, false on failure. Produces a fatal error if used on a
1632 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1633 C<flock> is Perl's portable file locking interface, although it locks
1634 only entire files, not records.
1636 Two potentially non-obvious but traditional C<flock> semantics are
1637 that it waits indefinitely until the lock is granted, and that its locks
1638 B<merely advisory>. Such discretionary locks are more flexible, but offer
1639 fewer guarantees. This means that files locked with C<flock> may be
1640 modified by programs that do not also use C<flock>. See L<perlport>,
1641 your port's specific documentation, or your system-specific local manpages
1642 for details. It's best to assume traditional behavior if you're writing
1643 portable programs. (But if you're not, you should as always feel perfectly
1644 free to write for your own system's idiosyncrasies (sometimes called
1645 "features"). Slavish adherence to portability concerns shouldn't get
1646 in the way of your getting your job done.)
1648 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1649 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1650 you can use the symbolic names if you import them from the Fcntl module,
1651 either individually, or as a group using the ':flock' tag. LOCK_SH
1652 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1653 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1654 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1655 waiting for the lock (check the return status to see if you got it).
1657 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1658 before locking or unlocking it.
1660 Note that the emulation built with lockf(3) doesn't provide shared
1661 locks, and it requires that FILEHANDLE be open with write intent. These
1662 are the semantics that lockf(3) implements. Most if not all systems
1663 implement lockf(3) in terms of fcntl(2) locking, though, so the
1664 differing semantics shouldn't bite too many people.
1666 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1667 be open with read intent to use LOCK_SH and requires that it be open
1668 with write intent to use LOCK_EX.
1670 Note also that some versions of C<flock> cannot lock things over the
1671 network; you would need to use the more system-specific C<fcntl> for
1672 that. If you like you can force Perl to ignore your system's flock(2)
1673 function, and so provide its own fcntl(2)-based emulation, by passing
1674 the switch C<-Ud_flock> to the F<Configure> program when you configure
1677 Here's a mailbox appender for BSD systems.
1679 use Fcntl ':flock'; # import LOCK_* constants
1682 flock(MBOX,LOCK_EX);
1683 # and, in case someone appended
1684 # while we were waiting...
1689 flock(MBOX,LOCK_UN);
1692 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1693 or die "Can't open mailbox: $!";
1696 print MBOX $msg,"\n\n";
1699 On systems that support a real flock(), locks are inherited across fork()
1700 calls, whereas those that must resort to the more capricious fcntl()
1701 function lose the locks, making it harder to write servers.
1703 See also L<DB_File> for other flock() examples.
1707 Does a fork(2) system call to create a new process running the
1708 same program at the same point. It returns the child pid to the
1709 parent process, C<0> to the child process, or C<undef> if the fork is
1710 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1711 are shared, while everything else is copied. On most systems supporting
1712 fork(), great care has gone into making it extremely efficient (for
1713 example, using copy-on-write technology on data pages), making it the
1714 dominant paradigm for multitasking over the last few decades.
1716 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1717 output before forking the child process, but this may not be supported
1718 on some platforms (see L<perlport>). To be safe, you may need to set
1719 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1720 C<IO::Handle> on any open handles in order to avoid duplicate output.
1722 If you C<fork> without ever waiting on your children, you will
1723 accumulate zombies. On some systems, you can avoid this by setting
1724 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1725 forking and reaping moribund children.
1727 Note that if your forked child inherits system file descriptors like
1728 STDIN and STDOUT that are actually connected by a pipe or socket, even
1729 if you exit, then the remote server (such as, say, a CGI script or a
1730 backgrounded job launched from a remote shell) won't think you're done.
1731 You should reopen those to F</dev/null> if it's any issue.
1735 Declare a picture format for use by the C<write> function. For
1739 Test: @<<<<<<<< @||||| @>>>>>
1740 $str, $%, '$' . int($num)
1744 $num = $cost/$quantity;
1748 See L<perlform> for many details and examples.
1750 =item formline PICTURE,LIST
1752 This is an internal function used by C<format>s, though you may call it,
1753 too. It formats (see L<perlform>) a list of values according to the
1754 contents of PICTURE, placing the output into the format output
1755 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1756 Eventually, when a C<write> is done, the contents of
1757 C<$^A> are written to some filehandle, but you could also read C<$^A>
1758 yourself and then set C<$^A> back to C<"">. Note that a format typically
1759 does one C<formline> per line of form, but the C<formline> function itself
1760 doesn't care how many newlines are embedded in the PICTURE. This means
1761 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1762 You may therefore need to use multiple formlines to implement a single
1763 record format, just like the format compiler.
1765 Be careful if you put double quotes around the picture, because an C<@>
1766 character may be taken to mean the beginning of an array name.
1767 C<formline> always returns true. See L<perlform> for other examples.
1769 =item getc FILEHANDLE
1773 Returns the next character from the input file attached to FILEHANDLE,
1774 or the undefined value at end of file, or if there was an error.
1775 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1776 efficient. However, it cannot be used by itself to fetch single
1777 characters without waiting for the user to hit enter. For that, try
1778 something more like:
1781 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1784 system "stty", '-icanon', 'eol', "\001";
1790 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1793 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1797 Determination of whether $BSD_STYLE should be set
1798 is left as an exercise to the reader.
1800 The C<POSIX::getattr> function can do this more portably on
1801 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1802 module from your nearest CPAN site; details on CPAN can be found on
1807 Implements the C library function of the same name, which on most
1808 systems returns the current login from F</etc/utmp>, if any. If null,
1811 $login = getlogin || getpwuid($<) || "Kilroy";
1813 Do not consider C<getlogin> for authentication: it is not as
1814 secure as C<getpwuid>.
1816 =item getpeername SOCKET
1818 Returns the packed sockaddr address of other end of the SOCKET connection.
1821 $hersockaddr = getpeername(SOCK);
1822 ($port, $iaddr) = sockaddr_in($hersockaddr);
1823 $herhostname = gethostbyaddr($iaddr, AF_INET);
1824 $herstraddr = inet_ntoa($iaddr);
1828 Returns the current process group for the specified PID. Use
1829 a PID of C<0> to get the current process group for the
1830 current process. Will raise an exception if used on a machine that
1831 doesn't implement getpgrp(2). If PID is omitted, returns process
1832 group of current process. Note that the POSIX version of C<getpgrp>
1833 does not accept a PID argument, so only C<PID==0> is truly portable.
1837 Returns the process id of the parent process.
1839 =item getpriority WHICH,WHO
1841 Returns the current priority for a process, a process group, or a user.
1842 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1843 machine that doesn't implement getpriority(2).
1849 =item gethostbyname NAME
1851 =item getnetbyname NAME
1853 =item getprotobyname NAME
1859 =item getservbyname NAME,PROTO
1861 =item gethostbyaddr ADDR,ADDRTYPE
1863 =item getnetbyaddr ADDR,ADDRTYPE
1865 =item getprotobynumber NUMBER
1867 =item getservbyport PORT,PROTO
1885 =item sethostent STAYOPEN
1887 =item setnetent STAYOPEN
1889 =item setprotoent STAYOPEN
1891 =item setservent STAYOPEN
1905 These routines perform the same functions as their counterparts in the
1906 system library. In list context, the return values from the
1907 various get routines are as follows:
1909 ($name,$passwd,$uid,$gid,
1910 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1911 ($name,$passwd,$gid,$members) = getgr*
1912 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1913 ($name,$aliases,$addrtype,$net) = getnet*
1914 ($name,$aliases,$proto) = getproto*
1915 ($name,$aliases,$port,$proto) = getserv*
1917 (If the entry doesn't exist you get a null list.)
1919 The exact meaning of the $gcos field varies but it usually contains
1920 the real name of the user (as opposed to the login name) and other
1921 information pertaining to the user. Beware, however, that in many
1922 system users are able to change this information and therefore it
1923 cannot be trusted and therefore the $gcos is tainted (see
1924 L<perlsec>). The $passwd and $shell, user's encrypted password and
1925 login shell, are also tainted, because of the same reason.
1927 In scalar context, you get the name, unless the function was a
1928 lookup by name, in which case you get the other thing, whatever it is.
1929 (If the entry doesn't exist you get the undefined value.) For example:
1931 $uid = getpwnam($name);
1932 $name = getpwuid($num);
1934 $gid = getgrnam($name);
1935 $name = getgrgid($num;
1939 In I<getpw*()> the fields $quota, $comment, and $expire are special
1940 cases in the sense that in many systems they are unsupported. If the
1941 $quota is unsupported, it is an empty scalar. If it is supported, it
1942 usually encodes the disk quota. If the $comment field is unsupported,
1943 it is an empty scalar. If it is supported it usually encodes some
1944 administrative comment about the user. In some systems the $quota
1945 field may be $change or $age, fields that have to do with password
1946 aging. In some systems the $comment field may be $class. The $expire
1947 field, if present, encodes the expiration period of the account or the
1948 password. For the availability and the exact meaning of these fields
1949 in your system, please consult your getpwnam(3) documentation and your
1950 F<pwd.h> file. You can also find out from within Perl what your
1951 $quota and $comment fields mean and whether you have the $expire field
1952 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1953 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1954 files are only supported if your vendor has implemented them in the
1955 intuitive fashion that calling the regular C library routines gets the
1956 shadow versions if you're running under privilege or if there exists
1957 the shadow(3) functions as found in System V ( this includes Solaris
1958 and Linux.) Those systems which implement a proprietary shadow password
1959 facility are unlikely to be supported.
1961 The $members value returned by I<getgr*()> is a space separated list of
1962 the login names of the members of the group.
1964 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1965 C, it will be returned to you via C<$?> if the function call fails. The
1966 C<@addrs> value returned by a successful call is a list of the raw
1967 addresses returned by the corresponding system library call. In the
1968 Internet domain, each address is four bytes long and you can unpack it
1969 by saying something like:
1971 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1973 The Socket library makes this slightly easier:
1976 $iaddr = inet_aton("127.1"); # or whatever address
1977 $name = gethostbyaddr($iaddr, AF_INET);
1979 # or going the other way
1980 $straddr = inet_ntoa($iaddr);
1982 If you get tired of remembering which element of the return list
1983 contains which return value, by-name interfaces are provided
1984 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1985 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1986 and C<User::grent>. These override the normal built-ins, supplying
1987 versions that return objects with the appropriate names
1988 for each field. For example:
1992 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1994 Even though it looks like they're the same method calls (uid),
1995 they aren't, because a C<File::stat> object is different from
1996 a C<User::pwent> object.
1998 =item getsockname SOCKET
2000 Returns the packed sockaddr address of this end of the SOCKET connection,
2001 in case you don't know the address because you have several different
2002 IPs that the connection might have come in on.
2005 $mysockaddr = getsockname(SOCK);
2006 ($port, $myaddr) = sockaddr_in($mysockaddr);
2007 printf "Connect to %s [%s]\n",
2008 scalar gethostbyaddr($myaddr, AF_INET),
2011 =item getsockopt SOCKET,LEVEL,OPTNAME
2013 Returns the socket option requested, or undef if there is an error.
2019 Returns the value of EXPR with filename expansions such as the
2020 standard Unix shell F</bin/csh> would do. This is the internal function
2021 implementing the C<< <*.c> >> operator, but you can use it directly.
2022 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2023 discussed in more detail in L<perlop/"I/O Operators">.
2025 Beginning with v5.6.0, this operator is implemented using the standard
2026 C<File::Glob> extension. See L<File::Glob> for details.
2030 Converts a time as returned by the time function to an 8-element list
2031 with the time localized for the standard Greenwich time zone.
2032 Typically used as follows:
2035 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2038 All list elements are numeric, and come straight out of the C `struct
2039 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2040 specified time. $mday is the day of the month, and $mon is the month
2041 itself, in the range C<0..11> with 0 indicating January and 11
2042 indicating December. $year is the number of years since 1900. That
2043 is, $year is C<123> in year 2023. $wday is the day of the week, with
2044 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2045 the year, in the range C<0..364> (or C<0..365> in leap years.)
2047 Note that the $year element is I<not> simply the last two digits of
2048 the year. If you assume it is, then you create non-Y2K-compliant
2049 programs--and you wouldn't want to do that, would you?
2051 The proper way to get a complete 4-digit year is simply:
2055 And to get the last two digits of the year (e.g., '01' in 2001) do:
2057 $year = sprintf("%02d", $year % 100);
2059 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2061 In scalar context, C<gmtime()> returns the ctime(3) value:
2063 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2065 Also see the C<timegm> function provided by the C<Time::Local> module,
2066 and the strftime(3) function available via the POSIX module.
2068 This scalar value is B<not> locale dependent (see L<perllocale>), but
2069 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2070 strftime(3) and mktime(3) functions available via the POSIX module. To
2071 get somewhat similar but locale dependent date strings, set up your
2072 locale environment variables appropriately (please see L<perllocale>)
2073 and try for example:
2075 use POSIX qw(strftime);
2076 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2078 Note that the C<%a> and C<%b> escapes, which represent the short forms
2079 of the day of the week and the month of the year, may not necessarily
2080 be three characters wide in all locales.
2088 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2089 execution there. It may not be used to go into any construct that
2090 requires initialization, such as a subroutine or a C<foreach> loop. It
2091 also can't be used to go into a construct that is optimized away,
2092 or to get out of a block or subroutine given to C<sort>.
2093 It can be used to go almost anywhere else within the dynamic scope,
2094 including out of subroutines, but it's usually better to use some other
2095 construct such as C<last> or C<die>. The author of Perl has never felt the
2096 need to use this form of C<goto> (in Perl, that is--C is another matter).
2097 (The difference being that C does not offer named loops combined with
2098 loop control. Perl does, and this replaces most structured uses of C<goto>
2099 in other languages.)
2101 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2102 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2103 necessarily recommended if you're optimizing for maintainability:
2105 goto ("FOO", "BAR", "GLARCH")[$i];
2107 The C<goto-&NAME> form is quite different from the other forms of
2108 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2109 doesn't have the stigma associated with other gotos. Instead, it
2110 exits the current subroutine (losing any changes set by local()) and
2111 immediately calls in its place the named subroutine using the current
2112 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2113 load another subroutine and then pretend that the other subroutine had
2114 been called in the first place (except that any modifications to C<@_>
2115 in the current subroutine are propagated to the other subroutine.)
2116 After the C<goto>, not even C<caller> will be able to tell that this
2117 routine was called first.
2119 NAME needn't be the name of a subroutine; it can be a scalar variable
2120 containing a code reference, or a block which evaluates to a code
2123 =item grep BLOCK LIST
2125 =item grep EXPR,LIST
2127 This is similar in spirit to, but not the same as, grep(1) and its
2128 relatives. In particular, it is not limited to using regular expressions.
2130 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2131 C<$_> to each element) and returns the list value consisting of those
2132 elements for which the expression evaluated to true. In scalar
2133 context, returns the number of times the expression was true.
2135 @foo = grep(!/^#/, @bar); # weed out comments
2139 @foo = grep {!/^#/} @bar; # weed out comments
2141 Note that C<$_> is an alias to the list value, so it can be used to
2142 modify the elements of the LIST. While this is useful and supported,
2143 it can cause bizarre results if the elements of LIST are not variables.
2144 Similarly, grep returns aliases into the original list, much as a for
2145 loop's index variable aliases the list elements. That is, modifying an
2146 element of a list returned by grep (for example, in a C<foreach>, C<map>
2147 or another C<grep>) actually modifies the element in the original list.
2148 This is usually something to be avoided when writing clear code.
2150 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2156 Interprets EXPR as a hex string and returns the corresponding value.
2157 (To convert strings that might start with either 0, 0x, or 0b, see
2158 L</oct>.) If EXPR is omitted, uses C<$_>.
2160 print hex '0xAf'; # prints '175'
2161 print hex 'aF'; # same
2163 Hex strings may only represent integers. Strings that would cause
2164 integer overflow trigger a warning. Leading whitespace is not stripped,
2169 There is no builtin C<import> function. It is just an ordinary
2170 method (subroutine) defined (or inherited) by modules that wish to export
2171 names to another module. The C<use> function calls the C<import> method
2172 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2174 =item index STR,SUBSTR,POSITION
2176 =item index STR,SUBSTR
2178 The index function searches for one string within another, but without
2179 the wildcard-like behavior of a full regular-expression pattern match.
2180 It returns the position of the first occurrence of SUBSTR in STR at
2181 or after POSITION. If POSITION is omitted, starts searching from the
2182 beginning of the string. The return value is based at C<0> (or whatever
2183 you've set the C<$[> variable to--but don't do that). If the substring
2184 is not found, returns one less than the base, ordinarily C<-1>.
2190 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2191 You should not use this function for rounding: one because it truncates
2192 towards C<0>, and two because machine representations of floating point
2193 numbers can sometimes produce counterintuitive results. For example,
2194 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2195 because it's really more like -268.99999999999994315658 instead. Usually,
2196 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2197 functions will serve you better than will int().
2199 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2201 Implements the ioctl(2) function. You'll probably first have to say
2203 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2205 to get the correct function definitions. If F<ioctl.ph> doesn't
2206 exist or doesn't have the correct definitions you'll have to roll your
2207 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2208 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2209 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2210 written depending on the FUNCTION--a pointer to the string value of SCALAR
2211 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2212 has no string value but does have a numeric value, that value will be
2213 passed rather than a pointer to the string value. To guarantee this to be
2214 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2215 functions may be needed to manipulate the values of structures used by
2218 The return value of C<ioctl> (and C<fcntl>) is as follows:
2220 if OS returns: then Perl returns:
2222 0 string "0 but true"
2223 anything else that number
2225 Thus Perl returns true on success and false on failure, yet you can
2226 still easily determine the actual value returned by the operating
2229 $retval = ioctl(...) || -1;
2230 printf "System returned %d\n", $retval;
2232 The special string "C<0> but true" is exempt from B<-w> complaints
2233 about improper numeric conversions.
2235 Here's an example of setting a filehandle named C<REMOTE> to be
2236 non-blocking at the system level. You'll have to negotiate C<$|>
2237 on your own, though.
2239 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2241 $flags = fcntl(REMOTE, F_GETFL, 0)
2242 or die "Can't get flags for the socket: $!\n";
2244 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2245 or die "Can't set flags for the socket: $!\n";
2247 =item join EXPR,LIST
2249 Joins the separate strings of LIST into a single string with fields
2250 separated by the value of EXPR, and returns that new string. Example:
2252 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2254 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2255 first argument. Compare L</split>.
2259 Returns a list consisting of all the keys of the named hash. (In
2260 scalar context, returns the number of keys.) The keys are returned in
2261 an apparently random order. The actual random order is subject to
2262 change in future versions of perl, but it is guaranteed to be the same
2263 order as either the C<values> or C<each> function produces (given
2264 that the hash has not been modified). As a side effect, it resets
2267 Here is yet another way to print your environment:
2270 @values = values %ENV;
2272 print pop(@keys), '=', pop(@values), "\n";
2275 or how about sorted by key:
2277 foreach $key (sort(keys %ENV)) {
2278 print $key, '=', $ENV{$key}, "\n";
2281 The returned values are copies of the original keys in the hash, so
2282 modifying them will not affect the original hash. Compare L</values>.
2284 To sort a hash by value, you'll need to use a C<sort> function.
2285 Here's a descending numeric sort of a hash by its values:
2287 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2288 printf "%4d %s\n", $hash{$key}, $key;
2291 As an lvalue C<keys> allows you to increase the number of hash buckets
2292 allocated for the given hash. This can gain you a measure of efficiency if
2293 you know the hash is going to get big. (This is similar to pre-extending
2294 an array by assigning a larger number to $#array.) If you say
2298 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2299 in fact, since it rounds up to the next power of two. These
2300 buckets will be retained even if you do C<%hash = ()>, use C<undef
2301 %hash> if you want to free the storage while C<%hash> is still in scope.
2302 You can't shrink the number of buckets allocated for the hash using
2303 C<keys> in this way (but you needn't worry about doing this by accident,
2304 as trying has no effect).
2306 See also C<each>, C<values> and C<sort>.
2308 =item kill SIGNAL, LIST
2310 Sends a signal to a list of processes. Returns the number of
2311 processes successfully signaled (which is not necessarily the
2312 same as the number actually killed).
2314 $cnt = kill 1, $child1, $child2;
2317 If SIGNAL is zero, no signal is sent to the process. This is a
2318 useful way to check that the process is alive and hasn't changed
2319 its UID. See L<perlport> for notes on the portability of this
2322 Unlike in the shell, if SIGNAL is negative, it kills
2323 process groups instead of processes. (On System V, a negative I<PROCESS>
2324 number will also kill process groups, but that's not portable.) That
2325 means you usually want to use positive not negative signals. You may also
2326 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2332 The C<last> command is like the C<break> statement in C (as used in
2333 loops); it immediately exits the loop in question. If the LABEL is
2334 omitted, the command refers to the innermost enclosing loop. The
2335 C<continue> block, if any, is not executed:
2337 LINE: while (<STDIN>) {
2338 last LINE if /^$/; # exit when done with header
2342 C<last> cannot be used to exit a block which returns a value such as
2343 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2344 a grep() or map() operation.
2346 Note that a block by itself is semantically identical to a loop
2347 that executes once. Thus C<last> can be used to effect an early
2348 exit out of such a block.
2350 See also L</continue> for an illustration of how C<last>, C<next>, and
2357 Returns a lowercased version of EXPR. This is the internal function
2358 implementing the C<\L> escape in double-quoted strings. Respects
2359 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2360 and L<perlunicode> for more details about locale and Unicode support.
2362 If EXPR is omitted, uses C<$_>.
2368 Returns the value of EXPR with the first character lowercased. This
2369 is the internal function implementing the C<\l> escape in
2370 double-quoted strings. Respects current LC_CTYPE locale if C<use
2371 locale> in force. See L<perllocale> and L<perlunicode> for more
2372 details about locale and Unicode support.
2374 If EXPR is omitted, uses C<$_>.
2380 Returns the length in characters of the value of EXPR. If EXPR is
2381 omitted, returns length of C<$_>. Note that this cannot be used on
2382 an entire array or hash to find out how many elements these have.
2383 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2385 =item link OLDFILE,NEWFILE
2387 Creates a new filename linked to the old filename. Returns true for
2388 success, false otherwise.
2390 =item listen SOCKET,QUEUESIZE
2392 Does the same thing that the listen system call does. Returns true if
2393 it succeeded, false otherwise. See the example in
2394 L<perlipc/"Sockets: Client/Server Communication">.
2398 You really probably want to be using C<my> instead, because C<local> isn't
2399 what most people think of as "local". See
2400 L<perlsub/"Private Variables via my()"> for details.
2402 A local modifies the listed variables to be local to the enclosing
2403 block, file, or eval. If more than one value is listed, the list must
2404 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2405 for details, including issues with tied arrays and hashes.
2407 =item localtime EXPR
2409 Converts a time as returned by the time function to a 9-element list
2410 with the time analyzed for the local time zone. Typically used as
2414 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2417 All list elements are numeric, and come straight out of the C `struct
2418 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2419 specified time. $mday is the day of the month, and $mon is the month
2420 itself, in the range C<0..11> with 0 indicating January and 11
2421 indicating December. $year is the number of years since 1900. That
2422 is, $year is C<123> in year 2023. $wday is the day of the week, with
2423 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2424 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2425 is true if the specified time occurs during daylight savings time,
2428 Note that the $year element is I<not> simply the last two digits of
2429 the year. If you assume it is, then you create non-Y2K-compliant
2430 programs--and you wouldn't want to do that, would you?
2432 The proper way to get a complete 4-digit year is simply:
2436 And to get the last two digits of the year (e.g., '01' in 2001) do:
2438 $year = sprintf("%02d", $year % 100);
2440 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2442 In scalar context, C<localtime()> returns the ctime(3) value:
2444 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2446 This scalar value is B<not> locale dependent, see L<perllocale>, but
2447 instead a Perl builtin. Also see the C<Time::Local> module
2448 (to convert the second, minutes, hours, ... back to seconds since the
2449 stroke of midnight the 1st of January 1970, the value returned by
2450 time()), and the strftime(3) and mktime(3) functions available via the
2451 POSIX module. To get somewhat similar but locale dependent date
2452 strings, set up your locale environment variables appropriately
2453 (please see L<perllocale>) and try for example:
2455 use POSIX qw(strftime);
2456 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2458 Note that the C<%a> and C<%b>, the short forms of the day of the week
2459 and the month of the year, may not necessarily be three characters wide.
2463 This function places an advisory lock on a variable, subroutine,
2464 or referenced object contained in I<THING> until the lock goes out
2465 of scope. This is a built-in function only if your version of Perl
2466 was built with threading enabled, and if you've said C<use Thread>.
2467 Otherwise a user-defined function by this name will be called.
2474 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2475 returns log of C<$_>. To get the log of another base, use basic algebra:
2476 The base-N log of a number is equal to the natural log of that number
2477 divided by the natural log of N. For example:
2481 return log($n)/log(10);
2484 See also L</exp> for the inverse operation.
2490 Does the same thing as the C<stat> function (including setting the
2491 special C<_> filehandle) but stats a symbolic link instead of the file
2492 the symbolic link points to. If symbolic links are unimplemented on
2493 your system, a normal C<stat> is done.
2495 If EXPR is omitted, stats C<$_>.
2499 The match operator. See L<perlop>.
2501 =item map BLOCK LIST
2505 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2506 C<$_> to each element) and returns the list value composed of the
2507 results of each such evaluation. In scalar context, returns the
2508 total number of elements so generated. Evaluates BLOCK or EXPR in
2509 list context, so each element of LIST may produce zero, one, or
2510 more elements in the returned value.
2512 @chars = map(chr, @nums);
2514 translates a list of numbers to the corresponding characters. And
2516 %hash = map { getkey($_) => $_ } @array;
2518 is just a funny way to write
2521 foreach $_ (@array) {
2522 $hash{getkey($_)} = $_;
2525 Note that C<$_> is an alias to the list value, so it can be used to
2526 modify the elements of the LIST. While this is useful and supported,
2527 it can cause bizarre results if the elements of LIST are not variables.
2528 Using a regular C<foreach> loop for this purpose would be clearer in
2529 most cases. See also L</grep> for an array composed of those items of
2530 the original list for which the BLOCK or EXPR evaluates to true.
2532 C<{> starts both hash references and blocks, so C<map { ...> could be either
2533 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2534 ahead for the closing C<}> it has to take a guess at which its dealing with
2535 based what it finds just after the C<{>. Usually it gets it right, but if it
2536 doesn't it won't realize something is wrong until it gets to the C<}> and
2537 encounters the missing (or unexpected) comma. The syntax error will be
2538 reported close to the C<}> but you'll need to change something near the C<{>
2539 such as using a unary C<+> to give perl some help:
2541 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2542 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2543 %hash = map { ("\L$_", 1) } @array # this also works
2544 %hash = map { lc($_), 1 } @array # as does this.
2545 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2547 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2549 or to force an anon hash constructor use C<+{>
2551 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2553 and you get list of anonymous hashes each with only 1 entry.
2555 =item mkdir FILENAME,MASK
2557 =item mkdir FILENAME
2559 Creates the directory specified by FILENAME, with permissions
2560 specified by MASK (as modified by C<umask>). If it succeeds it
2561 returns true, otherwise it returns false and sets C<$!> (errno).
2562 If omitted, MASK defaults to 0777.
2564 In general, it is better to create directories with permissive MASK,
2565 and let the user modify that with their C<umask>, than it is to supply
2566 a restrictive MASK and give the user no way to be more permissive.
2567 The exceptions to this rule are when the file or directory should be
2568 kept private (mail files, for instance). The perlfunc(1) entry on
2569 C<umask> discusses the choice of MASK in more detail.
2571 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2572 number of trailing slashes. Some operating and filesystems do not get
2573 this right, so Perl automatically removes all trailing slashes to keep
2576 =item msgctl ID,CMD,ARG
2578 Calls the System V IPC function msgctl(2). You'll probably have to say
2582 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2583 then ARG must be a variable which will hold the returned C<msqid_ds>
2584 structure. Returns like C<ioctl>: the undefined value for error,
2585 C<"0 but true"> for zero, or the actual return value otherwise. See also
2586 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2588 =item msgget KEY,FLAGS
2590 Calls the System V IPC function msgget(2). Returns the message queue
2591 id, or the undefined value if there is an error. See also
2592 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2594 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2596 Calls the System V IPC function msgrcv to receive a message from
2597 message queue ID into variable VAR with a maximum message size of
2598 SIZE. Note that when a message is received, the message type as a
2599 native long integer will be the first thing in VAR, followed by the
2600 actual message. This packing may be opened with C<unpack("l! a*")>.
2601 Taints the variable. Returns true if successful, or false if there is
2602 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2603 C<IPC::SysV::Msg> documentation.
2605 =item msgsnd ID,MSG,FLAGS
2607 Calls the System V IPC function msgsnd to send the message MSG to the
2608 message queue ID. MSG must begin with the native long integer message
2609 type, and be followed by the length of the actual message, and finally
2610 the message itself. This kind of packing can be achieved with
2611 C<pack("l! a*", $type, $message)>. Returns true if successful,
2612 or false if there is an error. See also C<IPC::SysV>
2613 and C<IPC::SysV::Msg> documentation.
2617 =item my EXPR : ATTRIBUTES
2619 A C<my> declares the listed variables to be local (lexically) to the
2620 enclosing block, file, or C<eval>. If
2621 more than one value is listed, the list must be placed in parentheses. See
2622 L<perlsub/"Private Variables via my()"> for details.
2628 The C<next> command is like the C<continue> statement in C; it starts
2629 the next iteration of the loop:
2631 LINE: while (<STDIN>) {
2632 next LINE if /^#/; # discard comments
2636 Note that if there were a C<continue> block on the above, it would get
2637 executed even on discarded lines. If the LABEL is omitted, the command
2638 refers to the innermost enclosing loop.
2640 C<next> cannot be used to exit a block which returns a value such as
2641 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2642 a grep() or map() operation.
2644 Note that a block by itself is semantically identical to a loop
2645 that executes once. Thus C<next> will exit such a block early.
2647 See also L</continue> for an illustration of how C<last>, C<next>, and
2650 =item no Module LIST
2652 See the L</use> function, which C<no> is the opposite of.
2658 Interprets EXPR as an octal string and returns the corresponding
2659 value. (If EXPR happens to start off with C<0x>, interprets it as a
2660 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2661 binary string. Leading whitespace is ignored in all three cases.)
2662 The following will handle decimal, binary, octal, and hex in the standard
2665 $val = oct($val) if $val =~ /^0/;
2667 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2668 in octal), use sprintf() or printf():
2670 $perms = (stat("filename"))[2] & 07777;
2671 $oct_perms = sprintf "%lo", $perms;
2673 The oct() function is commonly used when a string such as C<644> needs
2674 to be converted into a file mode, for example. (Although perl will
2675 automatically convert strings into numbers as needed, this automatic
2676 conversion assumes base 10.)
2678 =item open FILEHANDLE,EXPR
2680 =item open FILEHANDLE,MODE,EXPR
2682 =item open FILEHANDLE,MODE,EXPR,LIST
2684 =item open FILEHANDLE,MODE,REFERENCE
2686 =item open FILEHANDLE
2688 Opens the file whose filename is given by EXPR, and associates it with
2691 (The following is a comprehensive reference to open(): for a gentler
2692 introduction you may consider L<perlopentut>.)
2694 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2695 assigned a reference to a new anonymous filehandle, otherwise if
2696 FILEHANDLE is an expression, its value is used as the name of the real
2697 filehandle wanted. (This is considered a symbolic reference, so C<use
2698 strict 'refs'> should I<not> be in effect.)
2700 If EXPR is omitted, the scalar variable of the same name as the
2701 FILEHANDLE contains the filename. (Note that lexical variables--those
2702 declared with C<my>--will not work for this purpose; so if you're
2703 using C<my>, specify EXPR in your call to open.)
2705 If three or more arguments are specified then the mode of opening and
2706 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2707 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2708 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2709 the file is opened for appending, again being created if necessary.
2711 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2712 indicate that you want both read and write access to the file; thus
2713 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2714 '+>' >> mode would clobber the file first. You can't usually use
2715 either read-write mode for updating textfiles, since they have
2716 variable length records. See the B<-i> switch in L<perlrun> for a
2717 better approach. The file is created with permissions of C<0666>
2718 modified by the process' C<umask> value.
2720 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2721 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2723 In the 2-arguments (and 1-argument) form of the call the mode and
2724 filename should be concatenated (in this order), possibly separated by
2725 spaces. It is possible to omit the mode in these forms if the mode is
2728 If the filename begins with C<'|'>, the filename is interpreted as a
2729 command to which output is to be piped, and if the filename ends with a
2730 C<'|'>, the filename is interpreted as a command which pipes output to
2731 us. See L<perlipc/"Using open() for IPC">
2732 for more examples of this. (You are not allowed to C<open> to a command
2733 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2734 and L<perlipc/"Bidirectional Communication with Another Process">
2737 For three or more arguments if MODE is C<'|-'>, the filename is
2738 interpreted as a command to which output is to be piped, and if MODE
2739 is C<'-|'>, the filename is interpreted as a command which pipes
2740 output to us. In the 2-arguments (and 1-argument) form one should
2741 replace dash (C<'-'>) with the command.
2742 See L<perlipc/"Using open() for IPC"> for more examples of this.
2743 (You are not allowed to C<open> to a command that pipes both in I<and>
2744 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2745 L<perlipc/"Bidirectional Communication"> for alternatives.)
2747 In the three-or-more argument form of pipe opens, if LIST is specified
2748 (extra arguments after the command name) then LIST becomes arguments
2749 to the command invoked if the platform supports it. The meaning of
2750 C<open> with more than three arguments for non-pipe modes is not yet
2751 specified. Experimental "layers" may give extra LIST arguments
2754 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2755 and opening C<< '>-' >> opens STDOUT.
2757 You may use the three-argument form of open to specify
2758 I<I/O disciplines> that affect how the input and output
2759 are processed: see L</binmode> and L<open>. For example
2761 open(FH, "<:utf8", "file")
2763 will open the UTF-8 encoded file containing Unicode characters,
2764 see L<perluniintro>.
2766 Open returns nonzero upon success, the undefined value otherwise. If
2767 the C<open> involved a pipe, the return value happens to be the pid of
2770 If you're running Perl on a system that distinguishes between text
2771 files and binary files, then you should check out L</binmode> for tips
2772 for dealing with this. The key distinction between systems that need
2773 C<binmode> and those that don't is their text file formats. Systems
2774 like Unix, MacOS, and Plan9, which delimit lines with a single
2775 character, and which encode that character in C as C<"\n">, do not
2776 need C<binmode>. The rest need it.
2778 In the three argument form MODE may also contain a list of IO "layers"
2779 (see L<open> and L<PerlIO> for more details) to be applied to the
2780 handle. This can be used to achieve the effect of C<binmode> as well
2781 as more complex behaviours.
2783 When opening a file, it's usually a bad idea to continue normal execution
2784 if the request failed, so C<open> is frequently used in connection with
2785 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2786 where you want to make a nicely formatted error message (but there are
2787 modules that can help with that problem)) you should always check
2788 the return value from opening a file. The infrequent exception is when
2789 working with an unopened filehandle is actually what you want to do.
2791 As a special case the 3 arg form with a read/write mode and the third
2792 argument being C<undef>:
2794 open(TMP, "+>", undef) or die ...
2796 opens a filehandle to an anonymous temporary file.
2798 File handles can be opened to "in memory" files held in Perl scalars via:
2800 open($fh,'>', \$variable) || ..
2805 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2806 while (<ARTICLE>) {...
2808 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2809 # if the open fails, output is discarded
2811 open(DBASE, '+<', 'dbase.mine') # open for update
2812 or die "Can't open 'dbase.mine' for update: $!";
2814 open(DBASE, '+<dbase.mine') # ditto
2815 or die "Can't open 'dbase.mine' for update: $!";
2817 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2818 or die "Can't start caesar: $!";
2820 open(ARTICLE, "caesar <$article |") # ditto
2821 or die "Can't start caesar: $!";
2823 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2824 or die "Can't start sort: $!";
2827 open(MEMORY,'>', \$var)
2828 or die "Can't open memory file: $!";
2829 print MEMORY "foo!\n"; # output will end up in $var
2831 # process argument list of files along with any includes
2833 foreach $file (@ARGV) {
2834 process($file, 'fh00');
2838 my($filename, $input) = @_;
2839 $input++; # this is a string increment
2840 unless (open($input, $filename)) {
2841 print STDERR "Can't open $filename: $!\n";
2846 while (<$input>) { # note use of indirection
2847 if (/^#include "(.*)"/) {
2848 process($1, $input);
2855 You may also, in the Bourne shell tradition, specify an EXPR beginning
2856 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2857 name of a filehandle (or file descriptor, if numeric) to be
2858 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2859 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2860 mode you specify should match the mode of the original filehandle.
2861 (Duping a filehandle does not take into account any existing contents of
2862 IO buffers.) If you use the 3 arg form then you can pass either a number,
2863 the name of a filehandle or the normal "reference to a glob".
2865 Here is a script that saves, redirects, and restores STDOUT and
2869 open(my $oldout, ">&", \*STDOUT);
2870 open(OLDERR, ">&STDERR");
2872 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2873 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2875 select(STDERR); $| = 1; # make unbuffered
2876 select(STDOUT); $| = 1; # make unbuffered
2878 print STDOUT "stdout 1\n"; # this works for
2879 print STDERR "stderr 1\n"; # subprocesses too
2884 open(STDOUT, ">&OLDOUT");
2885 open(STDERR, ">&OLDERR");
2887 print STDOUT "stdout 2\n";
2888 print STDERR "stderr 2\n";
2890 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2891 do an equivalent of C's C<fdopen> of that file descriptor; this is
2892 more parsimonious of file descriptors. For example:
2894 open(FILEHANDLE, "<&=$fd")
2898 open(FILEHANDLE, "<&=", $fd)
2900 Note that if Perl is using the standard C libraries' fdopen() then on
2901 many UNIX systems, fdopen() is known to fail when file descriptors
2902 exceed a certain value, typically 255. If you need more file
2903 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2905 You can see whether Perl has been compiled with PerlIO or not by
2906 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2907 is C<define>, you have PerlIO, otherwise you don't.
2909 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2910 with 2-arguments (or 1-argument) form of open(), then
2911 there is an implicit fork done, and the return value of open is the pid
2912 of the child within the parent process, and C<0> within the child
2913 process. (Use C<defined($pid)> to determine whether the open was successful.)
2914 The filehandle behaves normally for the parent, but i/o to that
2915 filehandle is piped from/to the STDOUT/STDIN of the child process.
2916 In the child process the filehandle isn't opened--i/o happens from/to
2917 the new STDOUT or STDIN. Typically this is used like the normal
2918 piped open when you want to exercise more control over just how the
2919 pipe command gets executed, such as when you are running setuid, and
2920 don't want to have to scan shell commands for metacharacters.
2921 The following triples are more or less equivalent:
2923 open(FOO, "|tr '[a-z]' '[A-Z]'");
2924 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2925 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2926 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2928 open(FOO, "cat -n '$file'|");
2929 open(FOO, '-|', "cat -n '$file'");
2930 open(FOO, '-|') || exec 'cat', '-n', $file;
2931 open(FOO, '-|', "cat", '-n', $file);
2933 The last example in each block shows the pipe as "list form", which is
2934 not yet supported on all platforms.
2936 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2938 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2939 output before any operation that may do a fork, but this may not be
2940 supported on some platforms (see L<perlport>). To be safe, you may need
2941 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2942 of C<IO::Handle> on any open handles.
2944 On systems that support a close-on-exec flag on files, the flag will
2945 be set for the newly opened file descriptor as determined by the value
2946 of $^F. See L<perlvar/$^F>.
2948 Closing any piped filehandle causes the parent process to wait for the
2949 child to finish, and returns the status value in C<$?>.
2951 The filename passed to 2-argument (or 1-argument) form of open() will
2952 have leading and trailing whitespace deleted, and the normal
2953 redirection characters honored. This property, known as "magic open",
2954 can often be used to good effect. A user could specify a filename of
2955 F<"rsh cat file |">, or you could change certain filenames as needed:
2957 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2958 open(FH, $filename) or die "Can't open $filename: $!";
2960 Use 3-argument form to open a file with arbitrary weird characters in it,
2962 open(FOO, '<', $file);
2964 otherwise it's necessary to protect any leading and trailing whitespace:
2966 $file =~ s#^(\s)#./$1#;
2967 open(FOO, "< $file\0");
2969 (this may not work on some bizarre filesystems). One should
2970 conscientiously choose between the I<magic> and 3-arguments form
2975 will allow the user to specify an argument of the form C<"rsh cat file |">,
2976 but will not work on a filename which happens to have a trailing space, while
2978 open IN, '<', $ARGV[0];
2980 will have exactly the opposite restrictions.
2982 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2983 should use the C<sysopen> function, which involves no such magic (but
2984 may use subtly different filemodes than Perl open(), which is mapped
2985 to C fopen()). This is
2986 another way to protect your filenames from interpretation. For example:
2989 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2990 or die "sysopen $path: $!";
2991 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2992 print HANDLE "stuff $$\n";
2994 print "File contains: ", <HANDLE>;
2996 Using the constructor from the C<IO::Handle> package (or one of its
2997 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2998 filehandles that have the scope of whatever variables hold references to
2999 them, and automatically close whenever and however you leave that scope:
3003 sub read_myfile_munged {
3005 my $handle = new IO::File;
3006 open($handle, "myfile") or die "myfile: $!";
3008 or return (); # Automatically closed here.
3009 mung $first or die "mung failed"; # Or here.
3010 return $first, <$handle> if $ALL; # Or here.
3014 See L</seek> for some details about mixing reading and writing.
3016 =item opendir DIRHANDLE,EXPR
3018 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3019 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3020 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3026 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3027 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3030 For the reverse, see L</chr>.
3031 See L<perlunicode> and L<encoding> for more about Unicode.
3035 =item our EXPR : ATTRIBUTES
3037 An C<our> declares the listed variables to be valid globals within
3038 the enclosing block, file, or C<eval>. That is, it has the same
3039 scoping rules as a "my" declaration, but does not create a local
3040 variable. If more than one value is listed, the list must be placed
3041 in parentheses. The C<our> declaration has no semantic effect unless
3042 "use strict vars" is in effect, in which case it lets you use the
3043 declared global variable without qualifying it with a package name.
3044 (But only within the lexical scope of the C<our> declaration. In this
3045 it differs from "use vars", which is package scoped.)
3047 An C<our> declaration declares a global variable that will be visible
3048 across its entire lexical scope, even across package boundaries. The
3049 package in which the variable is entered is determined at the point
3050 of the declaration, not at the point of use. This means the following
3054 our $bar; # declares $Foo::bar for rest of lexical scope
3058 print $bar; # prints 20
3060 Multiple C<our> declarations in the same lexical scope are allowed
3061 if they are in different packages. If they happened to be in the same
3062 package, Perl will emit warnings if you have asked for them.
3066 our $bar; # declares $Foo::bar for rest of lexical scope
3070 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3071 print $bar; # prints 30
3073 our $bar; # emits warning
3075 An C<our> declaration may also have a list of attributes associated
3076 with it. B<WARNING>: This is an experimental feature that may be
3077 changed or removed in future releases of Perl. It should not be
3080 The only currently recognized attribute is C<unique> which indicates
3081 that a single copy of the global is to be used by all interpreters
3082 should the program happen to be running in a multi-interpreter
3083 environment. (The default behaviour would be for each interpreter to
3084 have its own copy of the global.) In such an environment, this
3085 attribute also has the effect of making the global readonly.
3088 our @EXPORT : unique = qw(foo);
3089 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3090 our $VERSION : unique = "1.00";
3092 Multi-interpreter environments can come to being either through the
3093 fork() emulation on Windows platforms, or by embedding perl in a
3094 multi-threaded application. The C<unique> attribute does nothing in
3095 all other environments.
3097 =item pack TEMPLATE,LIST
3099 Takes a LIST of values and converts it into a string using the rules
3100 given by the TEMPLATE. The resulting string is the concatenation of
3101 the converted values. Typically, each converted value looks
3102 like its machine-level representation. For example, on 32-bit machines
3103 a converted integer may be represented by a sequence of 4 bytes.
3106 sequence of characters that give the order and type of values, as
3109 a A string with arbitrary binary data, will be null padded.
3110 A A text (ASCII) string, will be space padded.
3111 Z A null terminated (ASCIZ) string, will be null padded.
3113 b A bit string (ascending bit order inside each byte, like vec()).
3114 B A bit string (descending bit order inside each byte).
3115 h A hex string (low nybble first).
3116 H A hex string (high nybble first).
3118 c A signed char value.
3119 C An unsigned char value. Only does bytes. See U for Unicode.
3121 s A signed short value.
3122 S An unsigned short value.
3123 (This 'short' is _exactly_ 16 bits, which may differ from
3124 what a local C compiler calls 'short'. If you want
3125 native-length shorts, use the '!' suffix.)
3127 i A signed integer value.
3128 I An unsigned integer value.
3129 (This 'integer' is _at_least_ 32 bits wide. Its exact
3130 size depends on what a local C compiler calls 'int',
3131 and may even be larger than the 'long' described in
3134 l A signed long value.
3135 L An unsigned long value.
3136 (This 'long' is _exactly_ 32 bits, which may differ from
3137 what a local C compiler calls 'long'. If you want
3138 native-length longs, use the '!' suffix.)
3140 n An unsigned short in "network" (big-endian) order.
3141 N An unsigned long in "network" (big-endian) order.
3142 v An unsigned short in "VAX" (little-endian) order.
3143 V An unsigned long in "VAX" (little-endian) order.
3144 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3145 _exactly_ 32 bits, respectively.)
3147 q A signed quad (64-bit) value.
3148 Q An unsigned quad value.
3149 (Quads are available only if your system supports 64-bit
3150 integer values _and_ if Perl has been compiled to support those.
3151 Causes a fatal error otherwise.)
3153 f A single-precision float in the native format.
3154 d A double-precision float in the native format.
3156 p A pointer to a null-terminated string.
3157 P A pointer to a structure (fixed-length string).
3159 u A uuencoded string.
3160 U A Unicode character number. Encodes to UTF-8 internally
3161 (or UTF-EBCDIC in EBCDIC platforms).
3163 w A BER compressed integer. Its bytes represent an unsigned
3164 integer in base 128, most significant digit first, with as
3165 few digits as possible. Bit eight (the high bit) is set
3166 on each byte except the last.
3170 @ Null fill to absolute position.
3172 The following rules apply:
3178 Each letter may optionally be followed by a number giving a repeat
3179 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3180 C<H>, and C<P> the pack function will gobble up that many values from
3181 the LIST. A C<*> for the repeat count means to use however many items are
3182 left, except for C<@>, C<x>, C<X>, where it is equivalent
3183 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3186 When used with C<Z>, C<*> results in the addition of a trailing null
3187 byte (so the packed result will be one longer than the byte C<length>
3190 The repeat count for C<u> is interpreted as the maximal number of bytes
3191 to encode per line of output, with 0 and 1 replaced by 45.
3195 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3196 string of length count, padding with nulls or spaces as necessary. When
3197 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3198 after the first null, and C<a> returns data verbatim. When packing,
3199 C<a>, and C<Z> are equivalent.
3201 If the value-to-pack is too long, it is truncated. If too long and an
3202 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3203 by a null byte. Thus C<Z> always packs a trailing null byte under
3208 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3209 Each byte of the input field of pack() generates 1 bit of the result.
3210 Each result bit is based on the least-significant bit of the corresponding
3211 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3212 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3214 Starting from the beginning of the input string of pack(), each 8-tuple
3215 of bytes is converted to 1 byte of output. With format C<b>
3216 the first byte of the 8-tuple determines the least-significant bit of a
3217 byte, and with format C<B> it determines the most-significant bit of
3220 If the length of the input string is not exactly divisible by 8, the
3221 remainder is packed as if the input string were padded by null bytes
3222 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3224 If the input string of pack() is longer than needed, extra bytes are ignored.
3225 A C<*> for the repeat count of pack() means to use all the bytes of
3226 the input field. On unpack()ing the bits are converted to a string
3227 of C<"0">s and C<"1">s.
3231 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3232 representable as hexadecimal digits, 0-9a-f) long.
3234 Each byte of the input field of pack() generates 4 bits of the result.
3235 For non-alphabetical bytes the result is based on the 4 least-significant
3236 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3237 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3238 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3239 is compatible with the usual hexadecimal digits, so that C<"a"> and
3240 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3241 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3243 Starting from the beginning of the input string of pack(), each pair
3244 of bytes is converted to 1 byte of output. With format C<h> the
3245 first byte of the pair determines the least-significant nybble of the
3246 output byte, and with format C<H> it determines the most-significant
3249 If the length of the input string is not even, it behaves as if padded
3250 by a null byte at the end. Similarly, during unpack()ing the "extra"
3251 nybbles are ignored.
3253 If the input string of pack() is longer than needed, extra bytes are ignored.
3254 A C<*> for the repeat count of pack() means to use all the bytes of
3255 the input field. On unpack()ing the bits are converted to a string
3256 of hexadecimal digits.
3260 The C<p> type packs a pointer to a null-terminated string. You are
3261 responsible for ensuring the string is not a temporary value (which can
3262 potentially get deallocated before you get around to using the packed result).
3263 The C<P> type packs a pointer to a structure of the size indicated by the
3264 length. A NULL pointer is created if the corresponding value for C<p> or
3265 C<P> is C<undef>, similarly for unpack().
3269 The C</> template character allows packing and unpacking of strings where
3270 the packed structure contains a byte count followed by the string itself.
3271 You write I<length-item>C</>I<string-item>.
3273 The I<length-item> can be any C<pack> template letter,
3274 and describes how the length value is packed.
3275 The ones likely to be of most use are integer-packing ones like
3276 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3277 and C<N> (for Sun XDR).
3279 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3280 For C<unpack> the length of the string is obtained from the I<length-item>,
3281 but if you put in the '*' it will be ignored.
3283 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3284 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3285 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3287 The I<length-item> is not returned explicitly from C<unpack>.
3289 Adding a count to the I<length-item> letter is unlikely to do anything
3290 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3291 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3292 which Perl does not regard as legal in numeric strings.
3296 The integer types C<s>, C<S>, C<l>, and C<L> may be
3297 immediately followed by a C<!> suffix to signify native shorts or
3298 longs--as you can see from above for example a bare C<l> does mean
3299 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3300 may be larger. This is an issue mainly in 64-bit platforms. You can
3301 see whether using C<!> makes any difference by
3303 print length(pack("s")), " ", length(pack("s!")), "\n";
3304 print length(pack("l")), " ", length(pack("l!")), "\n";
3306 C<i!> and C<I!> also work but only because of completeness;
3307 they are identical to C<i> and C<I>.
3309 The actual sizes (in bytes) of native shorts, ints, longs, and long
3310 longs on the platform where Perl was built are also available via
3314 print $Config{shortsize}, "\n";
3315 print $Config{intsize}, "\n";
3316 print $Config{longsize}, "\n";
3317 print $Config{longlongsize}, "\n";
3319 (The C<$Config{longlongsize}> will be undefine if your system does
3320 not support long longs.)
3324 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3325 are inherently non-portable between processors and operating systems
3326 because they obey the native byteorder and endianness. For example a
3327 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3328 (arranged in and handled by the CPU registers) into bytes as
3330 0x12 0x34 0x56 0x78 # big-endian
3331 0x78 0x56 0x34 0x12 # little-endian
3333 Basically, the Intel and VAX CPUs are little-endian, while everybody
3334 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3335 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3336 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3339 The names `big-endian' and `little-endian' are comic references to
3340 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3341 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3342 the egg-eating habits of the Lilliputians.
3344 Some systems may have even weirder byte orders such as
3349 You can see your system's preference with
3351 print join(" ", map { sprintf "%#02x", $_ }
3352 unpack("C*",pack("L",0x12345678))), "\n";
3354 The byteorder on the platform where Perl was built is also available
3358 print $Config{byteorder}, "\n";
3360 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3361 and C<'87654321'> are big-endian.
3363 If you want portable packed integers use the formats C<n>, C<N>,
3364 C<v>, and C<V>, their byte endianness and size are known.
3365 See also L<perlport>.
3369 Real numbers (floats and doubles) are in the native machine format only;
3370 due to the multiplicity of floating formats around, and the lack of a
3371 standard "network" representation, no facility for interchange has been
3372 made. This means that packed floating point data written on one machine
3373 may not be readable on another - even if both use IEEE floating point
3374 arithmetic (as the endian-ness of the memory representation is not part
3375 of the IEEE spec). See also L<perlport>.
3377 Note that Perl uses doubles internally for all numeric calculation, and
3378 converting from double into float and thence back to double again will
3379 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3384 If the pattern begins with a C<U>, the resulting string will be treated
3385 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3386 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3387 characters. If you don't want this to happen, you can begin your pattern
3388 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3389 string, and then follow this with a C<U*> somewhere in your pattern.
3393 You must yourself do any alignment or padding by inserting for example
3394 enough C<'x'>es while packing. There is no way to pack() and unpack()
3395 could know where the bytes are going to or coming from. Therefore
3396 C<pack> (and C<unpack>) handle their output and input as flat
3401 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3405 If TEMPLATE requires more arguments to pack() than actually given, pack()
3406 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3407 to pack() than actually given, extra arguments are ignored.
3413 $foo = pack("CCCC",65,66,67,68);
3415 $foo = pack("C4",65,66,67,68);
3417 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3418 # same thing with Unicode circled letters
3420 $foo = pack("ccxxcc",65,66,67,68);
3423 # note: the above examples featuring "C" and "c" are true
3424 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3425 # and UTF-8. In EBCDIC the first example would be
3426 # $foo = pack("CCCC",193,194,195,196);
3428 $foo = pack("s2",1,2);
3429 # "\1\0\2\0" on little-endian
3430 # "\0\1\0\2" on big-endian
3432 $foo = pack("a4","abcd","x","y","z");
3435 $foo = pack("aaaa","abcd","x","y","z");
3438 $foo = pack("a14","abcdefg");
3439 # "abcdefg\0\0\0\0\0\0\0"
3441 $foo = pack("i9pl", gmtime);
3442 # a real struct tm (on my system anyway)
3444 $utmp_template = "Z8 Z8 Z16 L";
3445 $utmp = pack($utmp_template, @utmp1);
3446 # a struct utmp (BSDish)
3448 @utmp2 = unpack($utmp_template, $utmp);
3449 # "@utmp1" eq "@utmp2"
3452 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3455 $foo = pack('sx2l', 12, 34);
3456 # short 12, two zero bytes padding, long 34
3457 $bar = pack('s@4l', 12, 34);
3458 # short 12, zero fill to position 4, long 34
3461 The same template may generally also be used in unpack().
3463 =item package NAMESPACE
3467 Declares the compilation unit as being in the given namespace. The scope
3468 of the package declaration is from the declaration itself through the end
3469 of the enclosing block, file, or eval (the same as the C<my> operator).
3470 All further unqualified dynamic identifiers will be in this namespace.
3471 A package statement affects only dynamic variables--including those
3472 you've used C<local> on--but I<not> lexical variables, which are created
3473 with C<my>. Typically it would be the first declaration in a file to
3474 be included by the C<require> or C<use> operator. You can switch into a
3475 package in more than one place; it merely influences which symbol table
3476 is used by the compiler for the rest of that block. You can refer to
3477 variables and filehandles in other packages by prefixing the identifier
3478 with the package name and a double colon: C<$Package::Variable>.
3479 If the package name is null, the C<main> package as assumed. That is,
3480 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3481 still seen in older code).
3483 If NAMESPACE is omitted, then there is no current package, and all
3484 identifiers must be fully qualified or lexicals. However, you are
3485 strongly advised not to make use of this feature. Its use can cause
3486 unexpected behaviour, even crashing some versions of Perl. It is
3487 deprecated, and will be removed from a future release.
3489 See L<perlmod/"Packages"> for more information about packages, modules,
3490 and classes. See L<perlsub> for other scoping issues.
3492 =item pipe READHANDLE,WRITEHANDLE
3494 Opens a pair of connected pipes like the corresponding system call.
3495 Note that if you set up a loop of piped processes, deadlock can occur
3496 unless you are very careful. In addition, note that Perl's pipes use
3497 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3498 after each command, depending on the application.
3500 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3501 for examples of such things.
3503 On systems that support a close-on-exec flag on files, the flag will be set
3504 for the newly opened file descriptors as determined by the value of $^F.
3511 Pops and returns the last value of the array, shortening the array by
3512 one element. Has an effect similar to
3516 If there are no elements in the array, returns the undefined value
3517 (although this may happen at other times as well). If ARRAY is
3518 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3519 array in subroutines, just like C<shift>.
3525 Returns the offset of where the last C<m//g> search left off for the variable
3526 in question (C<$_> is used when the variable is not specified). May be
3527 modified to change that offset. Such modification will also influence
3528 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3531 =item print FILEHANDLE LIST
3537 Prints a string or a list of strings. Returns true if successful.
3538 FILEHANDLE may be a scalar variable name, in which case the variable
3539 contains the name of or a reference to the filehandle, thus introducing
3540 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3541 the next token is a term, it may be misinterpreted as an operator
3542 unless you interpose a C<+> or put parentheses around the arguments.)
3543 If FILEHANDLE is omitted, prints by default to standard output (or
3544 to the last selected output channel--see L</select>). If LIST is
3545 also omitted, prints C<$_> to the currently selected output channel.
3546 To set the default output channel to something other than STDOUT
3547 use the select operation. The current value of C<$,> (if any) is
3548 printed between each LIST item. The current value of C<$\> (if
3549 any) is printed after the entire LIST has been printed. Because
3550 print takes a LIST, anything in the LIST is evaluated in list
3551 context, and any subroutine that you call will have one or more of
3552 its expressions evaluated in list context. Also be careful not to
3553 follow the print keyword with a left parenthesis unless you want
3554 the corresponding right parenthesis to terminate the arguments to
3555 the print--interpose a C<+> or put parentheses around all the
3558 Note that if you're storing FILEHANDLES in an array or other expression,
3559 you will have to use a block returning its value instead:
3561 print { $files[$i] } "stuff\n";
3562 print { $OK ? STDOUT : STDERR } "stuff\n";
3564 =item printf FILEHANDLE FORMAT, LIST
3566 =item printf FORMAT, LIST
3568 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3569 (the output record separator) is not appended. The first argument
3570 of the list will be interpreted as the C<printf> format. See C<sprintf>
3571 for an explanation of the format argument. If C<use locale> is in effect,
3572 the character used for the decimal point in formatted real numbers is
3573 affected by the LC_NUMERIC locale. See L<perllocale>.
3575 Don't fall into the trap of using a C<printf> when a simple
3576 C<print> would do. The C<print> is more efficient and less
3579 =item prototype FUNCTION
3581 Returns the prototype of a function as a string (or C<undef> if the
3582 function has no prototype). FUNCTION is a reference to, or the name of,
3583 the function whose prototype you want to retrieve.
3585 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3586 name for Perl builtin. If the builtin is not I<overridable> (such as
3587 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3588 C<system>) returns C<undef> because the builtin does not really behave
3589 like a Perl function. Otherwise, the string describing the equivalent
3590 prototype is returned.
3592 =item push ARRAY,LIST
3594 Treats ARRAY as a stack, and pushes the values of LIST
3595 onto the end of ARRAY. The length of ARRAY increases by the length of
3596 LIST. Has the same effect as
3599 $ARRAY[++$#ARRAY] = $value;
3602 but is more efficient. Returns the new number of elements in the array.
3614 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3616 =item quotemeta EXPR
3620 Returns the value of EXPR with all non-"word"
3621 characters backslashed. (That is, all characters not matching
3622 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3623 returned string, regardless of any locale settings.)
3624 This is the internal function implementing
3625 the C<\Q> escape in double-quoted strings.
3627 If EXPR is omitted, uses C<$_>.
3633 Returns a random fractional number greater than or equal to C<0> and less
3634 than the value of EXPR. (EXPR should be positive.) If EXPR is
3635 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3636 unless C<srand> has already been called. See also C<srand>.
3638 Apply C<int()> to the value returned by C<rand()> if you want random
3639 integers instead of random fractional numbers. For example,
3643 returns a random integer between C<0> and C<9>, inclusive.
3645 (Note: If your rand function consistently returns numbers that are too
3646 large or too small, then your version of Perl was probably compiled
3647 with the wrong number of RANDBITS.)
3649 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3651 =item read FILEHANDLE,SCALAR,LENGTH
3653 Attempts to read LENGTH I<characters> of data into variable SCALAR
3654 from the specified FILEHANDLE. Returns the number of characters
3655 actually read, C<0> at end of file, or undef if there was an error.
3656 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3657 needs growing, the new bytes will be zero bytes. An OFFSET may be
3658 specified to place the read data into some other place in SCALAR than
3659 the beginning. The call is actually implemented in terms of either
3660 Perl's or system's fread() call. To get a true read(2) system call,
3663 Note the I<characters>: depending on the status of the filehandle,
3664 either (8-bit) bytes or characters are read. By default all
3665 filehandles operate on bytes, but for example if the filehandle has
3666 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3667 pragma, L<open>), the I/O will operate on characters, not bytes.
3669 =item readdir DIRHANDLE
3671 Returns the next directory entry for a directory opened by C<opendir>.
3672 If used in list context, returns all the rest of the entries in the
3673 directory. If there are no more entries, returns an undefined value in
3674 scalar context or a null list in list context.
3676 If you're planning to filetest the return values out of a C<readdir>, you'd
3677 better prepend the directory in question. Otherwise, because we didn't
3678 C<chdir> there, it would have been testing the wrong file.
3680 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3681 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3686 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3687 context, each call reads and returns the next line, until end-of-file is
3688 reached, whereupon the subsequent call returns undef. In list context,
3689 reads until end-of-file is reached and returns a list of lines. Note that
3690 the notion of "line" used here is however you may have defined it
3691 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3693 When C<$/> is set to C<undef>, when readline() is in scalar
3694 context (i.e. file slurp mode), and when an empty file is read, it
3695 returns C<''> the first time, followed by C<undef> subsequently.
3697 This is the internal function implementing the C<< <EXPR> >>
3698 operator, but you can use it directly. The C<< <EXPR> >>
3699 operator is discussed in more detail in L<perlop/"I/O Operators">.
3702 $line = readline(*STDIN); # same thing
3708 Returns the value of a symbolic link, if symbolic links are
3709 implemented. If not, gives a fatal error. If there is some system
3710 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3711 omitted, uses C<$_>.
3715 EXPR is executed as a system command.
3716 The collected standard output of the command is returned.
3717 In scalar context, it comes back as a single (potentially
3718 multi-line) string. In list context, returns a list of lines
3719 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3720 This is the internal function implementing the C<qx/EXPR/>
3721 operator, but you can use it directly. The C<qx/EXPR/>
3722 operator is discussed in more detail in L<perlop/"I/O Operators">.
3724 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3726 Receives a message on a socket. Attempts to receive LENGTH characters
3727 of data into variable SCALAR from the specified SOCKET filehandle.
3728 SCALAR will be grown or shrunk to the length actually read. Takes the
3729 same flags as the system call of the same name. Returns the address
3730 of the sender if SOCKET's protocol supports this; returns an empty
3731 string otherwise. If there's an error, returns the undefined value.
3732 This call is actually implemented in terms of recvfrom(2) system call.
3733 See L<perlipc/"UDP: Message Passing"> for examples.
3735 Note the I<characters>: depending on the status of the socket, either
3736 (8-bit) bytes or characters are received. By default all sockets
3737 operate on bytes, but for example if the socket has been changed using
3738 binmode() to operate with the C<:utf8> discipline (see the C<open>
3739 pragma, L<open>), the I/O will operate on characters, not bytes.
3745 The C<redo> command restarts the loop block without evaluating the
3746 conditional again. The C<continue> block, if any, is not executed. If
3747 the LABEL is omitted, the command refers to the innermost enclosing
3748 loop. This command is normally used by programs that want to lie to
3749 themselves about what was just input:
3751 # a simpleminded Pascal comment stripper
3752 # (warning: assumes no { or } in strings)
3753 LINE: while (<STDIN>) {
3754 while (s|({.*}.*){.*}|$1 |) {}
3759 if (/}/) { # end of comment?
3768 C<redo> cannot be used to retry a block which returns a value such as
3769 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3770 a grep() or map() operation.
3772 Note that a block by itself is semantically identical to a loop
3773 that executes once. Thus C<redo> inside such a block will effectively
3774 turn it into a looping construct.
3776 See also L</continue> for an illustration of how C<last>, C<next>, and
3783 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3784 is not specified, C<$_> will be used. The value returned depends on the
3785 type of thing the reference is a reference to.
3786 Builtin types include:
3796 If the referenced object has been blessed into a package, then that package
3797 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3799 if (ref($r) eq "HASH") {
3800 print "r is a reference to a hash.\n";
3803 print "r is not a reference at all.\n";
3805 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3806 print "r is a reference to something that isa hash.\n";
3809 See also L<perlref>.
3811 =item rename OLDNAME,NEWNAME
3813 Changes the name of a file; an existing file NEWNAME will be
3814 clobbered. Returns true for success, false otherwise.
3816 Behavior of this function varies wildly depending on your system
3817 implementation. For example, it will usually not work across file system
3818 boundaries, even though the system I<mv> command sometimes compensates
3819 for this. Other restrictions include whether it works on directories,
3820 open files, or pre-existing files. Check L<perlport> and either the
3821 rename(2) manpage or equivalent system documentation for details.
3823 =item require VERSION
3829 Demands a version of Perl specified by VERSION, or demands some semantics
3830 specified by EXPR or by C<$_> if EXPR is not supplied.
3832 VERSION may be either a numeric argument such as 5.006, which will be
3833 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3834 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3835 VERSION is greater than the version of the current Perl interpreter.
3836 Compare with L</use>, which can do a similar check at compile time.
3838 Specifying VERSION as a literal of the form v5.6.1 should generally be
3839 avoided, because it leads to misleading error messages under earlier
3840 versions of Perl which do not support this syntax. The equivalent numeric
3841 version should be used instead.
3843 require v5.6.1; # run time version check
3844 require 5.6.1; # ditto
3845 require 5.006_001; # ditto; preferred for backwards compatibility
3847 Otherwise, demands that a library file be included if it hasn't already
3848 been included. The file is included via the do-FILE mechanism, which is
3849 essentially just a variety of C<eval>. Has semantics similar to the following
3854 return 1 if $INC{$filename};
3855 my($realfilename,$result);
3857 foreach $prefix (@INC) {
3858 $realfilename = "$prefix/$filename";
3859 if (-f $realfilename) {
3860 $INC{$filename} = $realfilename;
3861 $result = do $realfilename;
3865 die "Can't find $filename in \@INC";
3867 delete $INC{$filename} if $@ || !$result;
3869 die "$filename did not return true value" unless $result;
3873 Note that the file will not be included twice under the same specified
3874 name. The file must return true as the last statement to indicate
3875 successful execution of any initialization code, so it's customary to
3876 end such a file with C<1;> unless you're sure it'll return true
3877 otherwise. But it's better just to put the C<1;>, in case you add more
3880 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3881 replaces "F<::>" with "F</>" in the filename for you,
3882 to make it easy to load standard modules. This form of loading of
3883 modules does not risk altering your namespace.
3885 In other words, if you try this:
3887 require Foo::Bar; # a splendid bareword
3889 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3890 directories specified in the C<@INC> array.
3892 But if you try this:
3894 $class = 'Foo::Bar';
3895 require $class; # $class is not a bareword
3897 require "Foo::Bar"; # not a bareword because of the ""
3899 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3900 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3902 eval "require $class";
3904 You can also insert hooks into the import facility, by putting directly
3905 Perl code into the @INC array. There are three forms of hooks: subroutine
3906 references, array references and blessed objects.
3908 Subroutine references are the simplest case. When the inclusion system
3909 walks through @INC and encounters a subroutine, this subroutine gets
3910 called with two parameters, the first being a reference to itself, and the
3911 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3912 subroutine should return C<undef> or a filehandle, from which the file to
3913 include will be read. If C<undef> is returned, C<require> will look at
3914 the remaining elements of @INC.
3916 If the hook is an array reference, its first element must be a subroutine
3917 reference. This subroutine is called as above, but the first parameter is
3918 the array reference. This enables to pass indirectly some arguments to
3921 In other words, you can write:
3923 push @INC, \&my_sub;
3925 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3931 push @INC, [ \&my_sub, $x, $y, ... ];
3933 my ($arrayref, $filename) = @_;
3934 # Retrieve $x, $y, ...
3935 my @parameters = @$arrayref[1..$#$arrayref];
3939 If the hook is an object, it must provide an INC method, that will be
3940 called as above, the first parameter being the object itself. (Note that
3941 you must fully qualify the sub's name, as it is always forced into package
3942 C<main>.) Here is a typical code layout:
3948 my ($self, $filename) = @_;
3952 # In the main program
3953 push @INC, new Foo(...);
3955 Note that these hooks are also permitted to set the %INC entry
3956 corresponding to the files they have loaded. See L<perlvar/%INC>.
3958 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3964 Generally used in a C<continue> block at the end of a loop to clear
3965 variables and reset C<??> searches so that they work again. The
3966 expression is interpreted as a list of single characters (hyphens
3967 allowed for ranges). All variables and arrays beginning with one of
3968 those letters are reset to their pristine state. If the expression is
3969 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3970 only variables or searches in the current package. Always returns
3973 reset 'X'; # reset all X variables
3974 reset 'a-z'; # reset lower case variables
3975 reset; # just reset ?one-time? searches
3977 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3978 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3979 variables--lexical variables are unaffected, but they clean themselves
3980 up on scope exit anyway, so you'll probably want to use them instead.
3987 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3988 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3989 context, depending on how the return value will be used, and the context
3990 may vary from one execution to the next (see C<wantarray>). If no EXPR
3991 is given, returns an empty list in list context, the undefined value in
3992 scalar context, and (of course) nothing at all in a void context.
3994 (Note that in the absence of an explicit C<return>, a subroutine, eval,
3995 or do FILE will automatically return the value of the last expression
4000 In list context, returns a list value consisting of the elements
4001 of LIST in the opposite order. In scalar context, concatenates the
4002 elements of LIST and returns a string value with all characters
4003 in the opposite order.
4005 print reverse <>; # line tac, last line first
4007 undef $/; # for efficiency of <>
4008 print scalar reverse <>; # character tac, last line tsrif
4010 This operator is also handy for inverting a hash, although there are some
4011 caveats. If a value is duplicated in the original hash, only one of those
4012 can be represented as a key in the inverted hash. Also, this has to
4013 unwind one hash and build a whole new one, which may take some time
4014 on a large hash, such as from a DBM file.
4016 %by_name = reverse %by_address; # Invert the hash
4018 =item rewinddir DIRHANDLE
4020 Sets the current position to the beginning of the directory for the
4021 C<readdir> routine on DIRHANDLE.
4023 =item rindex STR,SUBSTR,POSITION
4025 =item rindex STR,SUBSTR
4027 Works just like index() except that it returns the position of the LAST
4028 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4029 last occurrence at or before that position.
4031 =item rmdir FILENAME
4035 Deletes the directory specified by FILENAME if that directory is empty. If it
4036 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4037 FILENAME is omitted, uses C<$_>.
4041 The substitution operator. See L<perlop>.
4045 Forces EXPR to be interpreted in scalar context and returns the value
4048 @counts = ( scalar @a, scalar @b, scalar @c );
4050 There is no equivalent operator to force an expression to
4051 be interpolated in list context because in practice, this is never
4052 needed. If you really wanted to do so, however, you could use
4053 the construction C<@{[ (some expression) ]}>, but usually a simple
4054 C<(some expression)> suffices.
4056 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4057 parenthesized list, this behaves as a scalar comma expression, evaluating
4058 all but the last element in void context and returning the final element
4059 evaluated in scalar context. This is seldom what you want.
4061 The following single statement:
4063 print uc(scalar(&foo,$bar)),$baz;
4065 is the moral equivalent of these two:
4068 print(uc($bar),$baz);
4070 See L<perlop> for more details on unary operators and the comma operator.
4072 =item seek FILEHANDLE,POSITION,WHENCE
4074 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4075 FILEHANDLE may be an expression whose value gives the name of the
4076 filehandle. The values for WHENCE are C<0> to set the new position
4077 I<in bytes> to POSITION, C<1> to set it to the current position plus
4078 POSITION, and C<2> to set it to EOF plus POSITION (typically
4079 negative). For WHENCE you may use the constants C<SEEK_SET>,
4080 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4081 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4084 Note the I<in bytes>: even if the filehandle has been set to
4085 operate on characters (for example by using the C<:utf8> open
4086 discipline), tell() will return byte offsets, not character offsets
4087 (because implementing that would render seek() and tell() rather slow).
4089 If you want to position file for C<sysread> or C<syswrite>, don't use
4090 C<seek>--buffering makes its effect on the file's system position
4091 unpredictable and non-portable. Use C<sysseek> instead.
4093 Due to the rules and rigors of ANSI C, on some systems you have to do a
4094 seek whenever you switch between reading and writing. Amongst other
4095 things, this may have the effect of calling stdio's clearerr(3).
4096 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4100 This is also useful for applications emulating C<tail -f>. Once you hit
4101 EOF on your read, and then sleep for a while, you might have to stick in a
4102 seek() to reset things. The C<seek> doesn't change the current position,
4103 but it I<does> clear the end-of-file condition on the handle, so that the
4104 next C<< <FILE> >> makes Perl try again to read something. We hope.
4106 If that doesn't work (some IO implementations are particularly
4107 cantankerous), then you may need something more like this:
4110 for ($curpos = tell(FILE); $_ = <FILE>;
4111 $curpos = tell(FILE)) {
4112 # search for some stuff and put it into files
4114 sleep($for_a_while);
4115 seek(FILE, $curpos, 0);
4118 =item seekdir DIRHANDLE,POS
4120 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4121 must be a value returned by C<telldir>. Has the same caveats about
4122 possible directory compaction as the corresponding system library
4125 =item select FILEHANDLE
4129 Returns the currently selected filehandle. Sets the current default
4130 filehandle for output, if FILEHANDLE is supplied. This has two
4131 effects: first, a C<write> or a C<print> without a filehandle will
4132 default to this FILEHANDLE. Second, references to variables related to
4133 output will refer to this output channel. For example, if you have to
4134 set the top of form format for more than one output channel, you might
4142 FILEHANDLE may be an expression whose value gives the name of the
4143 actual filehandle. Thus:
4145 $oldfh = select(STDERR); $| = 1; select($oldfh);
4147 Some programmers may prefer to think of filehandles as objects with
4148 methods, preferring to write the last example as:
4151 STDERR->autoflush(1);
4153 =item select RBITS,WBITS,EBITS,TIMEOUT
4155 This calls the select(2) system call with the bit masks specified, which
4156 can be constructed using C<fileno> and C<vec>, along these lines:
4158 $rin = $win = $ein = '';
4159 vec($rin,fileno(STDIN),1) = 1;
4160 vec($win,fileno(STDOUT),1) = 1;
4163 If you want to select on many filehandles you might wish to write a
4167 my(@fhlist) = split(' ',$_[0]);
4170 vec($bits,fileno($_),1) = 1;
4174 $rin = fhbits('STDIN TTY SOCK');
4178 ($nfound,$timeleft) =
4179 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4181 or to block until something becomes ready just do this
4183 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4185 Most systems do not bother to return anything useful in $timeleft, so
4186 calling select() in scalar context just returns $nfound.
4188 Any of the bit masks can also be undef. The timeout, if specified, is
4189 in seconds, which may be fractional. Note: not all implementations are
4190 capable of returning the $timeleft. If not, they always return
4191 $timeleft equal to the supplied $timeout.
4193 You can effect a sleep of 250 milliseconds this way:
4195 select(undef, undef, undef, 0.25);
4197 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4198 or <FH>) with C<select>, except as permitted by POSIX, and even
4199 then only on POSIX systems. You have to use C<sysread> instead.
4201 =item semctl ID,SEMNUM,CMD,ARG
4203 Calls the System V IPC function C<semctl>. You'll probably have to say
4207 first to get the correct constant definitions. If CMD is IPC_STAT or
4208 GETALL, then ARG must be a variable which will hold the returned
4209 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4210 the undefined value for error, "C<0 but true>" for zero, or the actual
4211 return value otherwise. The ARG must consist of a vector of native
4212 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4213 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4216 =item semget KEY,NSEMS,FLAGS
4218 Calls the System V IPC function semget. Returns the semaphore id, or
4219 the undefined value if there is an error. See also
4220 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4223 =item semop KEY,OPSTRING
4225 Calls the System V IPC function semop to perform semaphore operations
4226 such as signalling and waiting. OPSTRING must be a packed array of
4227 semop structures. Each semop structure can be generated with
4228 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4229 operations is implied by the length of OPSTRING. Returns true if
4230 successful, or false if there is an error. As an example, the
4231 following code waits on semaphore $semnum of semaphore id $semid:
4233 $semop = pack("s!3", $semnum, -1, 0);
4234 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4236 To signal the semaphore, replace C<-1> with C<1>. See also
4237 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4240 =item send SOCKET,MSG,FLAGS,TO
4242 =item send SOCKET,MSG,FLAGS
4244 Sends a message on a socket. Attempts to send the scalar MSG to the
4245 SOCKET filehandle. Takes the same flags as the system call of the
4246 same name. On unconnected sockets you must specify a destination to
4247 send TO, in which case it does a C C<sendto>. Returns the number of
4248 characters sent, or the undefined value if there is an error. The C
4249 system call sendmsg(2) is currently unimplemented. See
4250 L<perlipc/"UDP: Message Passing"> for examples.
4252 Note the I<characters>: depending on the status of the socket, either
4253 (8-bit) bytes or characters are sent. By default all sockets operate
4254 on bytes, but for example if the socket has been changed using
4255 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4256 the C<open> pragma, L<open>), the I/O will operate on characters, not
4259 =item setpgrp PID,PGRP
4261 Sets the current process group for the specified PID, C<0> for the current
4262 process. Will produce a fatal error if used on a machine that doesn't
4263 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4264 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4265 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4268 =item setpriority WHICH,WHO,PRIORITY
4270 Sets the current priority for a process, a process group, or a user.
4271 (See setpriority(2).) Will produce a fatal error if used on a machine
4272 that doesn't implement setpriority(2).
4274 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4276 Sets the socket option requested. Returns undefined if there is an
4277 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4284 Shifts the first value of the array off and returns it, shortening the
4285 array by 1 and moving everything down. If there are no elements in the
4286 array, returns the undefined value. If ARRAY is omitted, shifts the
4287 C<@_> array within the lexical scope of subroutines and formats, and the
4288 C<@ARGV> array at file scopes or within the lexical scopes established by
4289 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4292 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4293 same thing to the left end of an array that C<pop> and C<push> do to the
4296 =item shmctl ID,CMD,ARG
4298 Calls the System V IPC function shmctl. You'll probably have to say
4302 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4303 then ARG must be a variable which will hold the returned C<shmid_ds>
4304 structure. Returns like ioctl: the undefined value for error, "C<0> but
4305 true" for zero, or the actual return value otherwise.
4306 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4308 =item shmget KEY,SIZE,FLAGS
4310 Calls the System V IPC function shmget. Returns the shared memory
4311 segment id, or the undefined value if there is an error.
4312 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4314 =item shmread ID,VAR,POS,SIZE
4316 =item shmwrite ID,STRING,POS,SIZE
4318 Reads or writes the System V shared memory segment ID starting at
4319 position POS for size SIZE by attaching to it, copying in/out, and
4320 detaching from it. When reading, VAR must be a variable that will
4321 hold the data read. When writing, if STRING is too long, only SIZE
4322 bytes are used; if STRING is too short, nulls are written to fill out
4323 SIZE bytes. Return true if successful, or false if there is an error.
4324 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4325 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4327 =item shutdown SOCKET,HOW
4329 Shuts down a socket connection in the manner indicated by HOW, which
4330 has the same interpretation as in the system call of the same name.
4332 shutdown(SOCKET, 0); # I/we have stopped reading data
4333 shutdown(SOCKET, 1); # I/we have stopped writing data
4334 shutdown(SOCKET, 2); # I/we have stopped using this socket
4336 This is useful with sockets when you want to tell the other
4337 side you're done writing but not done reading, or vice versa.
4338 It's also a more insistent form of close because it also
4339 disables the file descriptor in any forked copies in other
4346 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4347 returns sine of C<$_>.
4349 For the inverse sine operation, you may use the C<Math::Trig::asin>
4350 function, or use this relation:
4352 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4358 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4359 May be interrupted if the process receives a signal such as C<SIGALRM>.
4360 Returns the number of seconds actually slept. You probably cannot
4361 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4364 On some older systems, it may sleep up to a full second less than what
4365 you requested, depending on how it counts seconds. Most modern systems
4366 always sleep the full amount. They may appear to sleep longer than that,
4367 however, because your process might not be scheduled right away in a
4368 busy multitasking system.
4370 For delays of finer granularity than one second, you may use Perl's
4371 C<syscall> interface to access setitimer(2) if your system supports
4372 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4373 and starting from Perl 5.8 part of the standard distribution) may also
4376 See also the POSIX module's C<pause> function.
4378 =item sockatmark SOCKET
4380 Returns true if the socket is positioned at the out-of-band mark
4381 (also known as the urgent data mark), false otherwise. Use right
4382 after reading from the socket.
4384 Not available directly, one has to import the function from
4385 the IO::Socket extension
4387 use IO::Socket 'sockatmark';
4389 Even this doesn't guarantee that sockatmark() really is available,
4390 though, because sockatmark() is a relatively recent addition to
4391 the family of socket functions. If it is unavailable, attempt to
4394 IO::Socket::atmark not implemented on this architecture ...
4396 See also L<IO::Socket>.
4398 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4400 Opens a socket of the specified kind and attaches it to filehandle
4401 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4402 the system call of the same name. You should C<use Socket> first
4403 to get the proper definitions imported. See the examples in
4404 L<perlipc/"Sockets: Client/Server Communication">.
4406 On systems that support a close-on-exec flag on files, the flag will
4407 be set for the newly opened file descriptor, as determined by the
4408 value of $^F. See L<perlvar/$^F>.
4410 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4412 Creates an unnamed pair of sockets in the specified domain, of the
4413 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4414 for the system call of the same name. If unimplemented, yields a fatal
4415 error. Returns true if successful.
4417 On systems that support a close-on-exec flag on files, the flag will
4418 be set for the newly opened file descriptors, as determined by the value
4419 of $^F. See L<perlvar/$^F>.
4421 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4422 to C<pipe(Rdr, Wtr)> is essentially:
4425 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4426 shutdown(Rdr, 1); # no more writing for reader
4427 shutdown(Wtr, 0); # no more reading for writer
4429 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4430 emulate socketpair using IP sockets to localhost if your system implements
4431 sockets but not socketpair.
4433 =item sort SUBNAME LIST
4435 =item sort BLOCK LIST
4439 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4440 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4441 specified, it gives the name of a subroutine that returns an integer
4442 less than, equal to, or greater than C<0>, depending on how the elements
4443 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4444 operators are extremely useful in such routines.) SUBNAME may be a
4445 scalar variable name (unsubscripted), in which case the value provides
4446 the name of (or a reference to) the actual subroutine to use. In place
4447 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4450 If the subroutine's prototype is C<($$)>, the elements to be compared
4451 are passed by reference in C<@_>, as for a normal subroutine. This is
4452 slower than unprototyped subroutines, where the elements to be
4453 compared are passed into the subroutine
4454 as the package global variables $a and $b (see example below). Note that
4455 in the latter case, it is usually counter-productive to declare $a and
4458 In either case, the subroutine may not be recursive. The values to be
4459 compared are always passed by reference, so don't modify them.
4461 You also cannot exit out of the sort block or subroutine using any of the
4462 loop control operators described in L<perlsyn> or with C<goto>.
4464 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4465 current collation locale. See L<perllocale>.
4467 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4468 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4469 preserves the input order of elements that compare equal. Although
4470 quicksort's run time is O(NlogN) when averaged over all arrays of
4471 length N, the time can be O(N**2), I<quadratic> behavior, for some
4472 inputs.) In 5.7, the quicksort implementation was replaced with
4473 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4474 But benchmarks indicated that for some inputs, on some platforms,
4475 the original quicksort was faster. 5.8 has a sort pragma for
4476 limited control of the sort. Its rather blunt control of the
4477 underlying algorithm may not persist into future perls, but the
4478 ability to characterize the input or output in implementation
4479 independent ways quite probably will. See L</use>.
4484 @articles = sort @files;
4486 # same thing, but with explicit sort routine
4487 @articles = sort {$a cmp $b} @files;
4489 # now case-insensitively
4490 @articles = sort {uc($a) cmp uc($b)} @files;
4492 # same thing in reversed order
4493 @articles = sort {$b cmp $a} @files;
4495 # sort numerically ascending
4496 @articles = sort {$a <=> $b} @files;
4498 # sort numerically descending
4499 @articles = sort {$b <=> $a} @files;
4501 # this sorts the %age hash by value instead of key
4502 # using an in-line function
4503 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4505 # sort using explicit subroutine name
4507 $age{$a} <=> $age{$b}; # presuming numeric
4509 @sortedclass = sort byage @class;
4511 sub backwards { $b cmp $a }
4512 @harry = qw(dog cat x Cain Abel);
4513 @george = qw(gone chased yz Punished Axed);
4515 # prints AbelCaincatdogx
4516 print sort backwards @harry;
4517 # prints xdogcatCainAbel
4518 print sort @george, 'to', @harry;
4519 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4521 # inefficiently sort by descending numeric compare using
4522 # the first integer after the first = sign, or the
4523 # whole record case-insensitively otherwise
4526 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4531 # same thing, but much more efficiently;
4532 # we'll build auxiliary indices instead
4536 push @nums, /=(\d+)/;
4541 $nums[$b] <=> $nums[$a]
4543 $caps[$a] cmp $caps[$b]
4547 # same thing, but without any temps
4548 @new = map { $_->[0] }
4549 sort { $b->[1] <=> $a->[1]
4552 } map { [$_, /=(\d+)/, uc($_)] } @old;
4554 # using a prototype allows you to use any comparison subroutine
4555 # as a sort subroutine (including other package's subroutines)
4557 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4560 @new = sort other::backwards @old;
4562 # guarantee stability, regardless of algorithm
4564 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4566 # force use of quicksort (not portable outside Perl 5.8)
4567 use sort '_quicksort'; # note discouraging _
4568 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4570 # similar to the previous example, but demand stability as well
4571 use sort qw( _mergesort stable );
4572 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4574 If you're using strict, you I<must not> declare $a
4575 and $b as lexicals. They are package globals. That means
4576 if you're in the C<main> package and type
4578 @articles = sort {$b <=> $a} @files;
4580 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4581 but if you're in the C<FooPack> package, it's the same as typing
4583 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4585 The comparison function is required to behave. If it returns
4586 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4587 sometimes saying the opposite, for example) the results are not
4590 =item splice ARRAY,OFFSET,LENGTH,LIST
4592 =item splice ARRAY,OFFSET,LENGTH
4594 =item splice ARRAY,OFFSET
4598 Removes the elements designated by OFFSET and LENGTH from an array, and
4599 replaces them with the elements of LIST, if any. In list context,
4600 returns the elements removed from the array. In scalar context,
4601 returns the last element removed, or C<undef> if no elements are
4602 removed. The array grows or shrinks as necessary.
4603 If OFFSET is negative then it starts that far from the end of the array.
4604 If LENGTH is omitted, removes everything from OFFSET onward.
4605 If LENGTH is negative, leaves that many elements off the end of the array.
4606 If both OFFSET and LENGTH are omitted, removes everything.
4608 The following equivalences hold (assuming C<$[ == 0>):
4610 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4611 pop(@a) splice(@a,-1)
4612 shift(@a) splice(@a,0,1)
4613 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4614 $a[$x] = $y splice(@a,$x,1,$y)
4616 Example, assuming array lengths are passed before arrays:
4618 sub aeq { # compare two list values
4619 my(@a) = splice(@_,0,shift);
4620 my(@b) = splice(@_,0,shift);
4621 return 0 unless @a == @b; # same len?
4623 return 0 if pop(@a) ne pop(@b);
4627 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4629 =item split /PATTERN/,EXPR,LIMIT
4631 =item split /PATTERN/,EXPR
4633 =item split /PATTERN/
4637 Splits a string into a list of strings and returns that list. By default,
4638 empty leading fields are preserved, and empty trailing ones are deleted.
4640 In scalar context, returns the number of fields found and splits into
4641 the C<@_> array. Use of split in scalar context is deprecated, however,
4642 because it clobbers your subroutine arguments.
4644 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4645 splits on whitespace (after skipping any leading whitespace). Anything
4646 matching PATTERN is taken to be a delimiter separating the fields. (Note
4647 that the delimiter may be longer than one character.)
4649 If LIMIT is specified and positive, it represents the maximum number
4650 of fields the EXPR will be split into, though the actual number of
4651 fields returned depends on the number of times PATTERN matches within
4652 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4653 stripped (which potential users of C<pop> would do well to remember).
4654 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4655 had been specified. Note that splitting an EXPR that evaluates to the
4656 empty string always returns the empty list, regardless of the LIMIT
4659 A pattern matching the null string (not to be confused with
4660 a null pattern C<//>, which is just one member of the set of patterns
4661 matching a null string) will split the value of EXPR into separate
4662 characters at each point it matches that way. For example:
4664 print join(':', split(/ */, 'hi there'));
4666 produces the output 'h:i:t:h:e:r:e'.
4668 Using the empty pattern C<//> specifically matches the null string, and is
4669 not be confused with the use of C<//> to mean "the last successful pattern
4672 Empty leading (or trailing) fields are produced when there are positive width
4673 matches at the beginning (or end) of the string; a zero-width match at the
4674 beginning (or end) of the string does not produce an empty field. For
4677 print join(':', split(/(?=\w)/, 'hi there!'));
4679 produces the output 'h:i :t:h:e:r:e!'.
4681 The LIMIT parameter can be used to split a line partially
4683 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4685 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4686 one larger than the number of variables in the list, to avoid
4687 unnecessary work. For the list above LIMIT would have been 4 by
4688 default. In time critical applications it behooves you not to split
4689 into more fields than you really need.
4691 If the PATTERN contains parentheses, additional list elements are
4692 created from each matching substring in the delimiter.
4694 split(/([,-])/, "1-10,20", 3);
4696 produces the list value
4698 (1, '-', 10, ',', 20)
4700 If you had the entire header of a normal Unix email message in $header,
4701 you could split it up into fields and their values this way:
4703 $header =~ s/\n\s+/ /g; # fix continuation lines
4704 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4706 The pattern C</PATTERN/> may be replaced with an expression to specify
4707 patterns that vary at runtime. (To do runtime compilation only once,
4708 use C</$variable/o>.)
4710 As a special case, specifying a PATTERN of space (C<' '>) will split on
4711 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4712 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4713 will give you as many null initial fields as there are leading spaces.
4714 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4715 whitespace produces a null first field. A C<split> with no arguments
4716 really does a C<split(' ', $_)> internally.
4718 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4723 open(PASSWD, '/etc/passwd');
4726 ($login, $passwd, $uid, $gid,
4727 $gcos, $home, $shell) = split(/:/);
4731 As with regular pattern matching, any capturing parentheses that are not
4732 matched in a C<split()> will be set to C<undef> when returned:
4734 @fields = split /(A)|B/, "1A2B3";
4735 # @fields is (1, 'A', 2, undef, 3)
4737 =item sprintf FORMAT, LIST
4739 Returns a string formatted by the usual C<printf> conventions of the C
4740 library function C<sprintf>. See below for more details
4741 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4742 the general principles.
4746 # Format number with up to 8 leading zeroes
4747 $result = sprintf("%08d", $number);
4749 # Round number to 3 digits after decimal point
4750 $rounded = sprintf("%.3f", $number);
4752 Perl does its own C<sprintf> formatting--it emulates the C
4753 function C<sprintf>, but it doesn't use it (except for floating-point
4754 numbers, and even then only the standard modifiers are allowed). As a
4755 result, any non-standard extensions in your local C<sprintf> are not
4756 available from Perl.
4758 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4759 pass it an array as your first argument. The array is given scalar context,
4760 and instead of using the 0th element of the array as the format, Perl will
4761 use the count of elements in the array as the format, which is almost never
4764 Perl's C<sprintf> permits the following universally-known conversions:
4767 %c a character with the given number
4769 %d a signed integer, in decimal
4770 %u an unsigned integer, in decimal
4771 %o an unsigned integer, in octal
4772 %x an unsigned integer, in hexadecimal
4773 %e a floating-point number, in scientific notation
4774 %f a floating-point number, in fixed decimal notation
4775 %g a floating-point number, in %e or %f notation
4777 In addition, Perl permits the following widely-supported conversions:
4779 %X like %x, but using upper-case letters
4780 %E like %e, but using an upper-case "E"
4781 %G like %g, but with an upper-case "E" (if applicable)
4782 %b an unsigned integer, in binary
4783 %p a pointer (outputs the Perl value's address in hexadecimal)
4784 %n special: *stores* the number of characters output so far
4785 into the next variable in the parameter list
4787 Finally, for backward (and we do mean "backward") compatibility, Perl
4788 permits these unnecessary but widely-supported conversions:
4791 %D a synonym for %ld
4792 %U a synonym for %lu
4793 %O a synonym for %lo
4796 Note that the number of exponent digits in the scientific notation by
4797 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4798 exponent less than 100 is system-dependent: it may be three or less
4799 (zero-padded as necessary). In other words, 1.23 times ten to the
4800 99th may be either "1.23e99" or "1.23e099".
4802 Perl permits the following universally-known flags between the C<%>
4803 and the conversion letter:
4805 space prefix positive number with a space
4806 + prefix positive number with a plus sign
4807 - left-justify within the field
4808 0 use zeros, not spaces, to right-justify
4809 # prefix non-zero octal with "0", non-zero hex with "0x"
4810 number minimum field width
4811 .number "precision": digits after decimal point for
4812 floating-point, max length for string, minimum length
4814 l interpret integer as C type "long" or "unsigned long"
4815 h interpret integer as C type "short" or "unsigned short"
4816 If no flags, interpret integer as C type "int" or "unsigned"
4818 Perl supports parameter ordering, in other words, fetching the
4819 parameters in some explicitly specified "random" ordering as opposed
4820 to the default implicit sequential ordering. The syntax is, instead
4821 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4822 where the I<digits> is the wanted index, from one upwards. For example:
4824 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4825 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4827 Note that using the reordering syntax does not interfere with the usual
4828 implicit sequential fetching of the parameters:
4830 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4831 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4832 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4833 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4834 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4836 There are also two Perl-specific flags:
4838 V interpret integer as Perl's standard integer type
4839 v interpret string as a vector of integers, output as
4840 numbers separated either by dots, or by an arbitrary
4841 string received from the argument list when the flag
4844 Where a number would appear in the flags, an asterisk (C<*>) may be
4845 used instead, in which case Perl uses the next item in the parameter
4846 list as the given number (that is, as the field width or precision).
4847 If a field width obtained through C<*> is negative, it has the same
4848 effect as the C<-> flag: left-justification.
4850 The C<v> flag is useful for displaying ordinal values of characters
4851 in arbitrary strings:
4853 printf "version is v%vd\n", $^V; # Perl's version
4854 printf "address is %*vX\n", ":", $addr; # IPv6 address
4855 printf "bits are %*vb\n", " ", $bits; # random bitstring
4857 If C<use locale> is in effect, the character used for the decimal
4858 point in formatted real numbers is affected by the LC_NUMERIC locale.
4861 If Perl understands "quads" (64-bit integers) (this requires
4862 either that the platform natively support quads or that Perl
4863 be specifically compiled to support quads), the characters
4867 print quads, and they may optionally be preceded by
4875 You can find out whether your Perl supports quads via L<Config>:
4878 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4881 If Perl understands "long doubles" (this requires that the platform
4882 support long doubles), the flags
4886 may optionally be preceded by
4894 You can find out whether your Perl supports long doubles via L<Config>:
4897 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4903 Return the square root of EXPR. If EXPR is omitted, returns square
4904 root of C<$_>. Only works on non-negative operands, unless you've
4905 loaded the standard Math::Complex module.
4908 print sqrt(-2); # prints 1.4142135623731i
4914 Sets the random number seed for the C<rand> operator.
4916 The point of the function is to "seed" the C<rand> function so that
4917 C<rand> can produce a different sequence each time you run your
4920 If srand() is not called explicitly, it is called implicitly at the
4921 first use of the C<rand> operator. However, this was not the case in
4922 versions of Perl before 5.004, so if your script will run under older
4923 Perl versions, it should call C<srand>.
4925 Most programs won't even call srand() at all, except those that
4926 need a cryptographically-strong starting point rather than the
4927 generally acceptable default, which is based on time of day,
4928 process ID, and memory allocation, or the F</dev/urandom> device,
4931 You can call srand($seed) with the same $seed to reproduce the
4932 I<same> sequence from rand(), but this is usually reserved for
4933 generating predictable results for testing or debugging.
4934 Otherwise, don't call srand() more than once in your program.
4936 Do B<not> call srand() (i.e. without an argument) more than once in
4937 a script. The internal state of the random number generator should
4938 contain more entropy than can be provided by any seed, so calling
4939 srand() again actually I<loses> randomness.
4941 Most implementations of C<srand> take an integer and will silently
4942 truncate decimal numbers. This means C<srand(42)> will usually
4943 produce the same results as C<srand(42.1)>. To be safe, always pass
4944 C<srand> an integer.
4946 In versions of Perl prior to 5.004 the default seed was just the
4947 current C<time>. This isn't a particularly good seed, so many old
4948 programs supply their own seed value (often C<time ^ $$> or C<time ^
4949 ($$ + ($$ << 15))>), but that isn't necessary any more.
4951 Note that you need something much more random than the default seed for
4952 cryptographic purposes. Checksumming the compressed output of one or more
4953 rapidly changing operating system status programs is the usual method. For
4956 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4958 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4961 Frequently called programs (like CGI scripts) that simply use
4965 for a seed can fall prey to the mathematical property that
4969 one-third of the time. So don't do that.
4971 =item stat FILEHANDLE
4977 Returns a 13-element list giving the status info for a file, either
4978 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4979 it stats C<$_>. Returns a null list if the stat fails. Typically used
4982 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4983 $atime,$mtime,$ctime,$blksize,$blocks)
4986 Not all fields are supported on all filesystem types. Here are the
4987 meaning of the fields:
4989 0 dev device number of filesystem
4991 2 mode file mode (type and permissions)
4992 3 nlink number of (hard) links to the file
4993 4 uid numeric user ID of file's owner
4994 5 gid numeric group ID of file's owner
4995 6 rdev the device identifier (special files only)
4996 7 size total size of file, in bytes
4997 8 atime last access time in seconds since the epoch
4998 9 mtime last modify time in seconds since the epoch
4999 10 ctime inode change time (NOT creation time!) in seconds since the epoch
5000 11 blksize preferred block size for file system I/O
5001 12 blocks actual number of blocks allocated
5003 (The epoch was at 00:00 January 1, 1970 GMT.)
5005 If stat is passed the special filehandle consisting of an underline, no
5006 stat is done, but the current contents of the stat structure from the
5007 last stat or filetest are returned. Example:
5009 if (-x $file && (($d) = stat(_)) && $d < 0) {
5010 print "$file is executable NFS file\n";
5013 (This works on machines only for which the device number is negative
5016 Because the mode contains both the file type and its permissions, you
5017 should mask off the file type portion and (s)printf using a C<"%o">
5018 if you want to see the real permissions.
5020 $mode = (stat($filename))[2];
5021 printf "Permissions are %04o\n", $mode & 07777;
5023 In scalar context, C<stat> returns a boolean value indicating success
5024 or failure, and, if successful, sets the information associated with
5025 the special filehandle C<_>.
5027 The File::stat module provides a convenient, by-name access mechanism:
5030 $sb = stat($filename);
5031 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5032 $filename, $sb->size, $sb->mode & 07777,
5033 scalar localtime $sb->mtime;
5035 You can import symbolic mode constants (C<S_IF*>) and functions
5036 (C<S_IS*>) from the Fcntl module:
5040 $mode = (stat($filename))[2];
5042 $user_rwx = ($mode & S_IRWXU) >> 6;
5043 $group_read = ($mode & S_IRGRP) >> 3;
5044 $other_execute = $mode & S_IXOTH;
5046 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5048 $is_setuid = $mode & S_ISUID;
5049 $is_setgid = S_ISDIR($mode);
5051 You could write the last two using the C<-u> and C<-d> operators.
5052 The commonly available S_IF* constants are
5054 # Permissions: read, write, execute, for user, group, others.
5056 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5057 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5058 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5060 # Setuid/Setgid/Stickiness.
5062 S_ISUID S_ISGID S_ISVTX S_ISTXT
5064 # File types. Not necessarily all are available on your system.
5066 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5068 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5070 S_IREAD S_IWRITE S_IEXEC
5072 and the S_IF* functions are
5074 S_IFMODE($mode) the part of $mode containing the permission bits
5075 and the setuid/setgid/sticky bits
5077 S_IFMT($mode) the part of $mode containing the file type
5078 which can be bit-anded with e.g. S_IFREG
5079 or with the following functions
5081 # The operators -f, -d, -l, -b, -c, -p, and -s.
5083 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5084 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5086 # No direct -X operator counterpart, but for the first one
5087 # the -g operator is often equivalent. The ENFMT stands for
5088 # record flocking enforcement, a platform-dependent feature.
5090 S_ISENFMT($mode) S_ISWHT($mode)
5092 See your native chmod(2) and stat(2) documentation for more details
5093 about the S_* constants.
5099 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5100 doing many pattern matches on the string before it is next modified.
5101 This may or may not save time, depending on the nature and number of
5102 patterns you are searching on, and on the distribution of character
5103 frequencies in the string to be searched--you probably want to compare
5104 run times with and without it to see which runs faster. Those loops
5105 which scan for many short constant strings (including the constant
5106 parts of more complex patterns) will benefit most. You may have only
5107 one C<study> active at a time--if you study a different scalar the first
5108 is "unstudied". (The way C<study> works is this: a linked list of every
5109 character in the string to be searched is made, so we know, for
5110 example, where all the C<'k'> characters are. From each search string,
5111 the rarest character is selected, based on some static frequency tables
5112 constructed from some C programs and English text. Only those places
5113 that contain this "rarest" character are examined.)
5115 For example, here is a loop that inserts index producing entries
5116 before any line containing a certain pattern:
5120 print ".IX foo\n" if /\bfoo\b/;
5121 print ".IX bar\n" if /\bbar\b/;
5122 print ".IX blurfl\n" if /\bblurfl\b/;
5127 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5128 will be looked at, because C<f> is rarer than C<o>. In general, this is
5129 a big win except in pathological cases. The only question is whether
5130 it saves you more time than it took to build the linked list in the
5133 Note that if you have to look for strings that you don't know till
5134 runtime, you can build an entire loop as a string and C<eval> that to
5135 avoid recompiling all your patterns all the time. Together with
5136 undefining C<$/> to input entire files as one record, this can be very
5137 fast, often faster than specialized programs like fgrep(1). The following
5138 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5139 out the names of those files that contain a match:
5141 $search = 'while (<>) { study;';
5142 foreach $word (@words) {
5143 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5148 eval $search; # this screams
5149 $/ = "\n"; # put back to normal input delimiter
5150 foreach $file (sort keys(%seen)) {
5158 =item sub NAME BLOCK
5160 This is subroutine definition, not a real function I<per se>. With just a
5161 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5162 Without a NAME, it's an anonymous function declaration, and does actually
5163 return a value: the CODE ref of the closure you just created. See L<perlsub>
5164 and L<perlref> for details.
5166 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5168 =item substr EXPR,OFFSET,LENGTH
5170 =item substr EXPR,OFFSET
5172 Extracts a substring out of EXPR and returns it. First character is at
5173 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5174 If OFFSET is negative (or more precisely, less than C<$[>), starts
5175 that far from the end of the string. If LENGTH is omitted, returns
5176 everything to the end of the string. If LENGTH is negative, leaves that
5177 many characters off the end of the string.
5179 You can use the substr() function as an lvalue, in which case EXPR
5180 must itself be an lvalue. If you assign something shorter than LENGTH,
5181 the string will shrink, and if you assign something longer than LENGTH,
5182 the string will grow to accommodate it. To keep the string the same
5183 length you may need to pad or chop your value using C<sprintf>.
5185 If OFFSET and LENGTH specify a substring that is partly outside the
5186 string, only the part within the string is returned. If the substring
5187 is beyond either end of the string, substr() returns the undefined
5188 value and produces a warning. When used as an lvalue, specifying a
5189 substring that is entirely outside the string is a fatal error.
5190 Here's an example showing the behavior for boundary cases:
5193 substr($name, 4) = 'dy'; # $name is now 'freddy'
5194 my $null = substr $name, 6, 2; # returns '' (no warning)
5195 my $oops = substr $name, 7; # returns undef, with warning
5196 substr($name, 7) = 'gap'; # fatal error
5198 An alternative to using substr() as an lvalue is to specify the
5199 replacement string as the 4th argument. This allows you to replace
5200 parts of the EXPR and return what was there before in one operation,
5201 just as you can with splice().
5203 =item symlink OLDFILE,NEWFILE
5205 Creates a new filename symbolically linked to the old filename.
5206 Returns C<1> for success, C<0> otherwise. On systems that don't support
5207 symbolic links, produces a fatal error at run time. To check for that,
5210 $symlink_exists = eval { symlink("",""); 1 };
5214 Calls the system call specified as the first element of the list,
5215 passing the remaining elements as arguments to the system call. If
5216 unimplemented, produces a fatal error. The arguments are interpreted
5217 as follows: if a given argument is numeric, the argument is passed as
5218 an int. If not, the pointer to the string value is passed. You are
5219 responsible to make sure a string is pre-extended long enough to
5220 receive any result that might be written into a string. You can't use a
5221 string literal (or other read-only string) as an argument to C<syscall>
5222 because Perl has to assume that any string pointer might be written
5224 integer arguments are not literals and have never been interpreted in a
5225 numeric context, you may need to add C<0> to them to force them to look
5226 like numbers. This emulates the C<syswrite> function (or vice versa):
5228 require 'syscall.ph'; # may need to run h2ph
5230 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5232 Note that Perl supports passing of up to only 14 arguments to your system call,
5233 which in practice should usually suffice.
5235 Syscall returns whatever value returned by the system call it calls.
5236 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5237 Note that some system calls can legitimately return C<-1>. The proper
5238 way to handle such calls is to assign C<$!=0;> before the call and
5239 check the value of C<$!> if syscall returns C<-1>.
5241 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5242 number of the read end of the pipe it creates. There is no way
5243 to retrieve the file number of the other end. You can avoid this
5244 problem by using C<pipe> instead.
5246 =item sysopen FILEHANDLE,FILENAME,MODE
5248 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5250 Opens the file whose filename is given by FILENAME, and associates it
5251 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5252 the name of the real filehandle wanted. This function calls the
5253 underlying operating system's C<open> function with the parameters
5254 FILENAME, MODE, PERMS.
5256 The possible values and flag bits of the MODE parameter are
5257 system-dependent; they are available via the standard module C<Fcntl>.
5258 See the documentation of your operating system's C<open> to see which
5259 values and flag bits are available. You may combine several flags
5260 using the C<|>-operator.
5262 Some of the most common values are C<O_RDONLY> for opening the file in
5263 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5264 and C<O_RDWR> for opening the file in read-write mode, and.
5266 For historical reasons, some values work on almost every system
5267 supported by perl: zero means read-only, one means write-only, and two
5268 means read/write. We know that these values do I<not> work under
5269 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5270 use them in new code.
5272 If the file named by FILENAME does not exist and the C<open> call creates
5273 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5274 PERMS specifies the permissions of the newly created file. If you omit
5275 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5276 These permission values need to be in octal, and are modified by your
5277 process's current C<umask>.
5279 In many systems the C<O_EXCL> flag is available for opening files in
5280 exclusive mode. This is B<not> locking: exclusiveness means here that
5281 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5284 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5286 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5287 that takes away the user's option to have a more permissive umask.
5288 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5291 Note that C<sysopen> depends on the fdopen() C library function.
5292 On many UNIX systems, fdopen() is known to fail when file descriptors
5293 exceed a certain value, typically 255. If you need more file
5294 descriptors than that, consider rebuilding Perl to use the C<sfio>
5295 library, or perhaps using the POSIX::open() function.
5297 See L<perlopentut> for a kinder, gentler explanation of opening files.
5299 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5301 =item sysread FILEHANDLE,SCALAR,LENGTH
5303 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5304 the specified FILEHANDLE, using the system call read(2). It bypasses
5305 buffered IO, so mixing this with other kinds of reads, C<print>,
5306 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5307 stdio usually buffers data. Returns the number of characters actually
5308 read, C<0> at end of file, or undef if there was an error. SCALAR
5309 will be grown or shrunk so that the last byte actually read is the
5310 last byte of the scalar after the read.
5312 Note the I<characters>: depending on the status of the filehandle,
5313 either (8-bit) bytes or characters are read. By default all
5314 filehandles operate on bytes, but for example if the filehandle has
5315 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5316 pragma, L<open>), the I/O will operate on characters, not bytes.
5318 An OFFSET may be specified to place the read data at some place in the
5319 string other than the beginning. A negative OFFSET specifies
5320 placement at that many characters counting backwards from the end of
5321 the string. A positive OFFSET greater than the length of SCALAR
5322 results in the string being padded to the required size with C<"\0">
5323 bytes before the result of the read is appended.
5325 There is no syseof() function, which is ok, since eof() doesn't work
5326 very well on device files (like ttys) anyway. Use sysread() and check
5327 for a return value for 0 to decide whether you're done.
5329 =item sysseek FILEHANDLE,POSITION,WHENCE
5331 Sets FILEHANDLE's system position I<in bytes> using the system call
5332 lseek(2). FILEHANDLE may be an expression whose value gives the name
5333 of the filehandle. The values for WHENCE are C<0> to set the new
5334 position to POSITION, C<1> to set the it to the current position plus
5335 POSITION, and C<2> to set it to EOF plus POSITION (typically
5338 Note the I<in bytes>: even if the filehandle has been set to operate
5339 on characters (for example by using the C<:utf8> discipline), tell()
5340 will return byte offsets, not character offsets (because implementing
5341 that would render sysseek() very slow).
5343 sysseek() bypasses normal buffered io, so mixing this with reads (other
5344 than C<sysread>, for example >< or read()) C<print>, C<write>,
5345 C<seek>, C<tell>, or C<eof> may cause confusion.
5347 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5348 and C<SEEK_END> (start of the file, current position, end of the file)
5349 from the Fcntl module. Use of the constants is also more portable
5350 than relying on 0, 1, and 2. For example to define a "systell" function:
5352 use Fnctl 'SEEK_CUR';
5353 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5355 Returns the new position, or the undefined value on failure. A position
5356 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5357 true on success and false on failure, yet you can still easily determine
5362 =item system PROGRAM LIST
5364 Does exactly the same thing as C<exec LIST>, except that a fork is
5365 done first, and the parent process waits for the child process to
5366 complete. Note that argument processing varies depending on the
5367 number of arguments. If there is more than one argument in LIST,
5368 or if LIST is an array with more than one value, starts the program
5369 given by the first element of the list with arguments given by the
5370 rest of the list. If there is only one scalar argument, the argument
5371 is checked for shell metacharacters, and if there are any, the
5372 entire argument is passed to the system's command shell for parsing
5373 (this is C</bin/sh -c> on Unix platforms, but varies on other
5374 platforms). If there are no shell metacharacters in the argument,
5375 it is split into words and passed directly to C<execvp>, which is
5378 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5379 output before any operation that may do a fork, but this may not be
5380 supported on some platforms (see L<perlport>). To be safe, you may need
5381 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5382 of C<IO::Handle> on any open handles.
5384 The return value is the exit status of the program as returned by the
5385 C<wait> call. To get the actual exit value shift right by eight (see below).
5386 See also L</exec>. This is I<not> what you want to use to capture
5387 the output from a command, for that you should use merely backticks or
5388 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5389 indicates a failure to start the program (inspect $! for the reason).
5391 Like C<exec>, C<system> allows you to lie to a program about its name if
5392 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5394 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5395 killing the program they're running doesn't actually interrupt
5398 @args = ("command", "arg1", "arg2");
5400 or die "system @args failed: $?"
5402 You can check all the failure possibilities by inspecting
5405 $exit_value = $? >> 8;
5406 $signal_num = $? & 127;
5407 $dumped_core = $? & 128;
5409 or more portably by using the W*() calls of the POSIX extension;
5410 see L<perlport> for more information.
5412 When the arguments get executed via the system shell, results
5413 and return codes will be subject to its quirks and capabilities.
5414 See L<perlop/"`STRING`"> and L</exec> for details.
5416 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5418 =item syswrite FILEHANDLE,SCALAR,LENGTH
5420 =item syswrite FILEHANDLE,SCALAR
5422 Attempts to write LENGTH characters of data from variable SCALAR to
5423 the specified FILEHANDLE, using the system call write(2). If LENGTH
5424 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5425 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5426 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5427 buffers data. Returns the number of characters actually written, or
5428 C<undef> if there was an error. If the LENGTH is greater than the
5429 available data in the SCALAR after the OFFSET, only as much data as is
5430 available will be written.
5432 An OFFSET may be specified to write the data from some part of the
5433 string other than the beginning. A negative OFFSET specifies writing
5434 that many characters counting backwards from the end of the string.
5435 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5437 Note the I<characters>: depending on the status of the filehandle,
5438 either (8-bit) bytes or characters are written. By default all
5439 filehandles operate on bytes, but for example if the filehandle has
5440 been opened with the C<:utf8> discipline (see L</open>, and the open
5441 pragma, L<open>), the I/O will operate on characters, not bytes.
5443 =item tell FILEHANDLE
5447 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5448 error. FILEHANDLE may be an expression whose value gives the name of
5449 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5452 Note the I<in bytes>: even if the filehandle has been set to
5453 operate on characters (for example by using the C<:utf8> open
5454 discipline), tell() will return byte offsets, not character offsets
5455 (because that would render seek() and tell() rather slow).
5457 The return value of tell() for the standard streams like the STDIN
5458 depends on the operating system: it may return -1 or something else.
5459 tell() on pipes, fifos, and sockets usually returns -1.
5461 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5463 Do not use tell() on a filehandle that has been opened using
5464 sysopen(), use sysseek() for that as described above. Why? Because
5465 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5466 buffered filehandles. sysseek() make sense only on the first kind,
5467 tell() only makes sense on the second kind.
5469 =item telldir DIRHANDLE
5471 Returns the current position of the C<readdir> routines on DIRHANDLE.
5472 Value may be given to C<seekdir> to access a particular location in a
5473 directory. Has the same caveats about possible directory compaction as
5474 the corresponding system library routine.
5476 =item tie VARIABLE,CLASSNAME,LIST
5478 This function binds a variable to a package class that will provide the
5479 implementation for the variable. VARIABLE is the name of the variable
5480 to be enchanted. CLASSNAME is the name of a class implementing objects
5481 of correct type. Any additional arguments are passed to the C<new>
5482 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5483 or C<TIEHASH>). Typically these are arguments such as might be passed
5484 to the C<dbm_open()> function of C. The object returned by the C<new>
5485 method is also returned by the C<tie> function, which would be useful
5486 if you want to access other methods in CLASSNAME.
5488 Note that functions such as C<keys> and C<values> may return huge lists
5489 when used on large objects, like DBM files. You may prefer to use the
5490 C<each> function to iterate over such. Example:
5492 # print out history file offsets
5494 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5495 while (($key,$val) = each %HIST) {
5496 print $key, ' = ', unpack('L',$val), "\n";
5500 A class implementing a hash should have the following methods:
5502 TIEHASH classname, LIST
5504 STORE this, key, value
5509 NEXTKEY this, lastkey
5513 A class implementing an ordinary array should have the following methods:
5515 TIEARRAY classname, LIST
5517 STORE this, key, value
5519 STORESIZE this, count
5525 SPLICE this, offset, length, LIST
5530 A class implementing a file handle should have the following methods:
5532 TIEHANDLE classname, LIST
5533 READ this, scalar, length, offset
5536 WRITE this, scalar, length, offset
5538 PRINTF this, format, LIST
5542 SEEK this, position, whence
5544 OPEN this, mode, LIST
5549 A class implementing a scalar should have the following methods:
5551 TIESCALAR classname, LIST
5557 Not all methods indicated above need be implemented. See L<perltie>,
5558 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5560 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5561 for you--you need to do that explicitly yourself. See L<DB_File>
5562 or the F<Config> module for interesting C<tie> implementations.
5564 For further details see L<perltie>, L<"tied VARIABLE">.
5568 Returns a reference to the object underlying VARIABLE (the same value
5569 that was originally returned by the C<tie> call that bound the variable
5570 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5575 Returns the number of non-leap seconds since whatever time the system
5576 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5577 and 00:00:00 UTC, January 1, 1970 for most other systems).
5578 Suitable for feeding to C<gmtime> and C<localtime>.
5580 For measuring time in better granularity than one second,
5581 you may use either the Time::HiRes module from CPAN, or
5582 if you have gettimeofday(2), you may be able to use the
5583 C<syscall> interface of Perl, see L<perlfaq8> for details.
5587 Returns a four-element list giving the user and system times, in
5588 seconds, for this process and the children of this process.
5590 ($user,$system,$cuser,$csystem) = times;
5592 In scalar context, C<times> returns C<$user>.
5596 The transliteration operator. Same as C<y///>. See L<perlop>.
5598 =item truncate FILEHANDLE,LENGTH
5600 =item truncate EXPR,LENGTH
5602 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5603 specified length. Produces a fatal error if truncate isn't implemented
5604 on your system. Returns true if successful, the undefined value
5611 Returns an uppercased version of EXPR. This is the internal function
5612 implementing the C<\U> escape in double-quoted strings. Respects
5613 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5614 and L<perlunicode> for more details about locale and Unicode support.
5615 It does not attempt to do titlecase mapping on initial letters. See
5616 C<ucfirst> for that.
5618 If EXPR is omitted, uses C<$_>.
5624 Returns the value of EXPR with the first character in uppercase
5625 (titlecase in Unicode). This is the internal function implementing
5626 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5627 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5628 for more details about locale and Unicode support.
5630 If EXPR is omitted, uses C<$_>.
5636 Sets the umask for the process to EXPR and returns the previous value.
5637 If EXPR is omitted, merely returns the current umask.
5639 The Unix permission C<rwxr-x---> is represented as three sets of three
5640 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5641 and isn't one of the digits). The C<umask> value is such a number
5642 representing disabled permissions bits. The permission (or "mode")
5643 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5644 even if you tell C<sysopen> to create a file with permissions C<0777>,
5645 if your umask is C<0022> then the file will actually be created with
5646 permissions C<0755>. If your C<umask> were C<0027> (group can't
5647 write; others can't read, write, or execute), then passing
5648 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5651 Here's some advice: supply a creation mode of C<0666> for regular
5652 files (in C<sysopen>) and one of C<0777> for directories (in
5653 C<mkdir>) and executable files. This gives users the freedom of
5654 choice: if they want protected files, they might choose process umasks
5655 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5656 Programs should rarely if ever make policy decisions better left to
5657 the user. The exception to this is when writing files that should be
5658 kept private: mail files, web browser cookies, I<.rhosts> files, and
5661 If umask(2) is not implemented on your system and you are trying to
5662 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5663 fatal error at run time. If umask(2) is not implemented and you are
5664 not trying to restrict access for yourself, returns C<undef>.
5666 Remember that a umask is a number, usually given in octal; it is I<not> a
5667 string of octal digits. See also L</oct>, if all you have is a string.
5673 Undefines the value of EXPR, which must be an lvalue. Use only on a
5674 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5675 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5676 will probably not do what you expect on most predefined variables or
5677 DBM list values, so don't do that; see L<delete>.) Always returns the
5678 undefined value. You can omit the EXPR, in which case nothing is
5679 undefined, but you still get an undefined value that you could, for
5680 instance, return from a subroutine, assign to a variable or pass as a
5681 parameter. Examples:
5684 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5688 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5689 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5690 select undef, undef, undef, 0.25;
5691 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5693 Note that this is a unary operator, not a list operator.
5699 Deletes a list of files. Returns the number of files successfully
5702 $cnt = unlink 'a', 'b', 'c';
5706 Note: C<unlink> will not delete directories unless you are superuser and
5707 the B<-U> flag is supplied to Perl. Even if these conditions are
5708 met, be warned that unlinking a directory can inflict damage on your
5709 filesystem. Use C<rmdir> instead.
5711 If LIST is omitted, uses C<$_>.
5713 =item unpack TEMPLATE,EXPR
5715 C<unpack> does the reverse of C<pack>: it takes a string
5716 and expands it out into a list of values.
5717 (In scalar context, it returns merely the first value produced.)
5719 The string is broken into chunks described by the TEMPLATE. Each chunk
5720 is converted separately to a value. Typically, either the string is a result
5721 of C<pack>, or the bytes of the string represent a C structure of some
5724 The TEMPLATE has the same format as in the C<pack> function.
5725 Here's a subroutine that does substring:
5728 my($what,$where,$howmuch) = @_;
5729 unpack("x$where a$howmuch", $what);
5734 sub ordinal { unpack("c",$_[0]); } # same as ord()
5736 In addition to fields allowed in pack(), you may prefix a field with
5737 a %<number> to indicate that
5738 you want a <number>-bit checksum of the items instead of the items
5739 themselves. Default is a 16-bit checksum. Checksum is calculated by
5740 summing numeric values of expanded values (for string fields the sum of
5741 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5743 For example, the following
5744 computes the same number as the System V sum program:
5748 unpack("%32C*",<>) % 65535;
5751 The following efficiently counts the number of set bits in a bit vector:
5753 $setbits = unpack("%32b*", $selectmask);
5755 The C<p> and C<P> formats should be used with care. Since Perl
5756 has no way of checking whether the value passed to C<unpack()>
5757 corresponds to a valid memory location, passing a pointer value that's
5758 not known to be valid is likely to have disastrous consequences.
5760 If the repeat count of a field is larger than what the remainder of
5761 the input string allows, repeat count is decreased. If the input string
5762 is longer than one described by the TEMPLATE, the rest is ignored.
5764 See L</pack> for more examples and notes.
5766 =item untie VARIABLE
5768 Breaks the binding between a variable and a package. (See C<tie>.)
5770 =item unshift ARRAY,LIST
5772 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5773 depending on how you look at it. Prepends list to the front of the
5774 array, and returns the new number of elements in the array.
5776 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5778 Note the LIST is prepended whole, not one element at a time, so the
5779 prepended elements stay in the same order. Use C<reverse> to do the
5782 =item use Module VERSION LIST
5784 =item use Module VERSION
5786 =item use Module LIST
5792 Imports some semantics into the current package from the named module,
5793 generally by aliasing certain subroutine or variable names into your
5794 package. It is exactly equivalent to
5796 BEGIN { require Module; import Module LIST; }
5798 except that Module I<must> be a bareword.
5800 VERSION may be either a numeric argument such as 5.006, which will be
5801 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5802 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5803 greater than the version of the current Perl interpreter; Perl will not
5804 attempt to parse the rest of the file. Compare with L</require>, which can
5805 do a similar check at run time.
5807 Specifying VERSION as a literal of the form v5.6.1 should generally be
5808 avoided, because it leads to misleading error messages under earlier
5809 versions of Perl which do not support this syntax. The equivalent numeric
5810 version should be used instead.
5812 use v5.6.1; # compile time version check
5814 use 5.006_001; # ditto; preferred for backwards compatibility
5816 This is often useful if you need to check the current Perl version before
5817 C<use>ing library modules that have changed in incompatible ways from
5818 older versions of Perl. (We try not to do this more than we have to.)
5820 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5821 C<require> makes sure the module is loaded into memory if it hasn't been
5822 yet. The C<import> is not a builtin--it's just an ordinary static method
5823 call into the C<Module> package to tell the module to import the list of
5824 features back into the current package. The module can implement its
5825 C<import> method any way it likes, though most modules just choose to
5826 derive their C<import> method via inheritance from the C<Exporter> class that
5827 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5828 method can be found then the call is skipped.
5830 If you do not want to call the package's C<import> method (for instance,
5831 to stop your namespace from being altered), explicitly supply the empty list:
5835 That is exactly equivalent to
5837 BEGIN { require Module }
5839 If the VERSION argument is present between Module and LIST, then the
5840 C<use> will call the VERSION method in class Module with the given
5841 version as an argument. The default VERSION method, inherited from
5842 the UNIVERSAL class, croaks if the given version is larger than the
5843 value of the variable C<$Module::VERSION>.
5845 Again, there is a distinction between omitting LIST (C<import> called
5846 with no arguments) and an explicit empty LIST C<()> (C<import> not
5847 called). Note that there is no comma after VERSION!
5849 Because this is a wide-open interface, pragmas (compiler directives)
5850 are also implemented this way. Currently implemented pragmas are:
5855 use sigtrap qw(SEGV BUS);
5856 use strict qw(subs vars refs);
5857 use subs qw(afunc blurfl);
5858 use warnings qw(all);
5859 use sort qw(stable _quicksort _mergesort);
5861 Some of these pseudo-modules import semantics into the current
5862 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5863 which import symbols into the current package (which are effective
5864 through the end of the file).
5866 There's a corresponding C<no> command that unimports meanings imported
5867 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5873 If no C<unimport> method can be found the call fails with a fatal error.
5875 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5876 for the C<-M> and C<-m> command-line options to perl that give C<use>
5877 functionality from the command-line.
5881 Changes the access and modification times on each file of a list of
5882 files. The first two elements of the list must be the NUMERICAL access
5883 and modification times, in that order. Returns the number of files
5884 successfully changed. The inode change time of each file is set
5885 to the current time. This code has the same effect as the C<touch>
5886 command if the files already exist:
5890 utime $now, $now, @ARGV;
5892 If the first two elements of the list are C<undef>, then the utime(2)
5893 function in the C library will be called with a null second argument.
5894 On most systems, this will set the file's access and modification
5895 times to the current time. (i.e. equivalent to the example above.)
5897 utime undef, undef, @ARGV;
5901 Returns a list consisting of all the values of the named hash. (In a
5902 scalar context, returns the number of values.) The values are
5903 returned in an apparently random order. The actual random order is
5904 subject to change in future versions of perl, but it is guaranteed to
5905 be the same order as either the C<keys> or C<each> function would
5906 produce on the same (unmodified) hash.
5908 Note that the values are not copied, which means modifying them will
5909 modify the contents of the hash:
5911 for (values %hash) { s/foo/bar/g } # modifies %hash values
5912 for (@hash{keys %hash}) { s/foo/bar/g } # same
5914 As a side effect, calling values() resets the HASH's internal iterator.
5915 See also C<keys>, C<each>, and C<sort>.
5917 =item vec EXPR,OFFSET,BITS
5919 Treats the string in EXPR as a bit vector made up of elements of
5920 width BITS, and returns the value of the element specified by OFFSET
5921 as an unsigned integer. BITS therefore specifies the number of bits
5922 that are reserved for each element in the bit vector. This must
5923 be a power of two from 1 to 32 (or 64, if your platform supports
5926 If BITS is 8, "elements" coincide with bytes of the input string.
5928 If BITS is 16 or more, bytes of the input string are grouped into chunks
5929 of size BITS/8, and each group is converted to a number as with
5930 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5931 for BITS==64). See L<"pack"> for details.
5933 If bits is 4 or less, the string is broken into bytes, then the bits
5934 of each byte are broken into 8/BITS groups. Bits of a byte are
5935 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5936 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5937 breaking the single input byte C<chr(0x36)> into two groups gives a list
5938 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5940 C<vec> may also be assigned to, in which case parentheses are needed
5941 to give the expression the correct precedence as in
5943 vec($image, $max_x * $x + $y, 8) = 3;
5945 If the selected element is outside the string, the value 0 is returned.
5946 If an element off the end of the string is written to, Perl will first
5947 extend the string with sufficiently many zero bytes. It is an error
5948 to try to write off the beginning of the string (i.e. negative OFFSET).
5950 The string should not contain any character with the value > 255 (which
5951 can only happen if you're using UTF8 encoding). If it does, it will be
5952 treated as something which is not UTF8 encoded. When the C<vec> was
5953 assigned to, other parts of your program will also no longer consider the
5954 string to be UTF8 encoded. In other words, if you do have such characters
5955 in your string, vec() will operate on the actual byte string, and not the
5956 conceptual character string.
5958 Strings created with C<vec> can also be manipulated with the logical
5959 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5960 vector operation is desired when both operands are strings.
5961 See L<perlop/"Bitwise String Operators">.
5963 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5964 The comments show the string after each step. Note that this code works
5965 in the same way on big-endian or little-endian machines.
5968 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5970 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5971 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5973 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5974 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5975 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5976 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5977 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5978 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5980 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5981 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5982 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5985 To transform a bit vector into a string or list of 0's and 1's, use these:
5987 $bits = unpack("b*", $vector);
5988 @bits = split(//, unpack("b*", $vector));
5990 If you know the exact length in bits, it can be used in place of the C<*>.
5992 Here is an example to illustrate how the bits actually fall in place:
5998 unpack("V",$_) 01234567890123456789012345678901
5999 ------------------------------------------------------------------
6004 for ($shift=0; $shift < $width; ++$shift) {
6005 for ($off=0; $off < 32/$width; ++$off) {
6006 $str = pack("B*", "0"x32);
6007 $bits = (1<<$shift);
6008 vec($str, $off, $width) = $bits;
6009 $res = unpack("b*",$str);
6010 $val = unpack("V", $str);
6017 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6018 $off, $width, $bits, $val, $res
6022 Regardless of the machine architecture on which it is run, the above
6023 example should print the following table:
6026 unpack("V",$_) 01234567890123456789012345678901
6027 ------------------------------------------------------------------
6028 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6029 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6030 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6031 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6032 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6033 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6034 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6035 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6036 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6037 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6038 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6039 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6040 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6041 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6042 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6043 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6044 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6045 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6046 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6047 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6048 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6049 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6050 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6051 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6052 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6053 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6054 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6055 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6056 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6057 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6058 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6059 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6060 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6061 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6062 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6063 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6064 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6065 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6066 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6067 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6068 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6069 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6070 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6071 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6072 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6073 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6074 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6075 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6076 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6077 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6078 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6079 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6080 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6081 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6082 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6083 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6084 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6085 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6086 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6087 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6088 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6089 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6090 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6091 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6092 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6093 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6094 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6095 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6096 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6097 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6098 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6099 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6100 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6101 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6102 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6103 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6104 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6105 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6106 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6107 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6108 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6109 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6110 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6111 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6112 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6113 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6114 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6115 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6116 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6117 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6118 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6119 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6120 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6121 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6122 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6123 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6124 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6125 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6126 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6127 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6128 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6129 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6130 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6131 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6132 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6133 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6134 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6135 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6136 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6137 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6138 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6139 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6140 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6141 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6142 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6143 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6144 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6145 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6146 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6147 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6148 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6149 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6150 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6151 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6152 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6153 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6154 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6155 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6159 Behaves like the wait(2) system call on your system: it waits for a child
6160 process to terminate and returns the pid of the deceased process, or
6161 C<-1> if there are no child processes. The status is returned in C<$?>.
6162 Note that a return value of C<-1> could mean that child processes are
6163 being automatically reaped, as described in L<perlipc>.
6165 =item waitpid PID,FLAGS
6167 Waits for a particular child process to terminate and returns the pid of
6168 the deceased process, or C<-1> if there is no such child process. On some
6169 systems, a value of 0 indicates that there are processes still running.
6170 The status is returned in C<$?>. If you say
6172 use POSIX ":sys_wait_h";
6175 $kid = waitpid(-1, WNOHANG);
6178 then you can do a non-blocking wait for all pending zombie processes.
6179 Non-blocking wait is available on machines supporting either the
6180 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6181 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6182 system call by remembering the status values of processes that have
6183 exited but have not been harvested by the Perl script yet.)
6185 Note that on some systems, a return value of C<-1> could mean that child
6186 processes are being automatically reaped. See L<perlipc> for details,
6187 and for other examples.
6191 Returns true if the context of the currently executing subroutine is
6192 looking for a list value. Returns false if the context is looking
6193 for a scalar. Returns the undefined value if the context is looking
6194 for no value (void context).
6196 return unless defined wantarray; # don't bother doing more
6197 my @a = complex_calculation();
6198 return wantarray ? @a : "@a";
6200 This function should have been named wantlist() instead.
6204 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6207 If LIST is empty and C<$@> already contains a value (typically from a
6208 previous eval) that value is used after appending C<"\t...caught">
6209 to C<$@>. This is useful for staying almost, but not entirely similar to
6212 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6214 No message is printed if there is a C<$SIG{__WARN__}> handler
6215 installed. It is the handler's responsibility to deal with the message
6216 as it sees fit (like, for instance, converting it into a C<die>). Most
6217 handlers must therefore make arrangements to actually display the
6218 warnings that they are not prepared to deal with, by calling C<warn>
6219 again in the handler. Note that this is quite safe and will not
6220 produce an endless loop, since C<__WARN__> hooks are not called from
6223 You will find this behavior is slightly different from that of
6224 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6225 instead call C<die> again to change it).
6227 Using a C<__WARN__> handler provides a powerful way to silence all
6228 warnings (even the so-called mandatory ones). An example:
6230 # wipe out *all* compile-time warnings
6231 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6233 my $foo = 20; # no warning about duplicate my $foo,
6234 # but hey, you asked for it!
6235 # no compile-time or run-time warnings before here
6238 # run-time warnings enabled after here
6239 warn "\$foo is alive and $foo!"; # does show up
6241 See L<perlvar> for details on setting C<%SIG> entries, and for more
6242 examples. See the Carp module for other kinds of warnings using its
6243 carp() and cluck() functions.
6245 =item write FILEHANDLE
6251 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6252 using the format associated with that file. By default the format for
6253 a file is the one having the same name as the filehandle, but the
6254 format for the current output channel (see the C<select> function) may be set
6255 explicitly by assigning the name of the format to the C<$~> variable.
6257 Top of form processing is handled automatically: if there is
6258 insufficient room on the current page for the formatted record, the
6259 page is advanced by writing a form feed, a special top-of-page format
6260 is used to format the new page header, and then the record is written.
6261 By default the top-of-page format is the name of the filehandle with
6262 "_TOP" appended, but it may be dynamically set to the format of your
6263 choice by assigning the name to the C<$^> variable while the filehandle is
6264 selected. The number of lines remaining on the current page is in
6265 variable C<$->, which can be set to C<0> to force a new page.
6267 If FILEHANDLE is unspecified, output goes to the current default output
6268 channel, which starts out as STDOUT but may be changed by the
6269 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6270 is evaluated and the resulting string is used to look up the name of
6271 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6273 Note that write is I<not> the opposite of C<read>. Unfortunately.
6277 The transliteration operator. Same as C<tr///>. See L<perlop>.