3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<sockatmark>, C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of wallclock seconds have elapsed. If SECONDS is not
390 specified, the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less or more
392 than you specified because of how seconds are counted, and process
393 scheduling may delay the delivery of the signal even further.)
395 Only one timer may be counting at once. Each call disables the
396 previous timer, and an argument of C<0> may be supplied to cancel the
397 previous timer without starting a new one. The returned value is the
398 amount of time remaining on the previous timer.
400 For delays of finer granularity than one second, you may use Perl's
401 four-argument version of select() leaving the first three arguments
402 undefined, or you might be able to use the C<syscall> interface to
403 access setitimer(2) if your system supports it. The Time::HiRes
404 module (from CPAN, and starting from Perl 5.8 part of the standard
405 distribution) may also prove useful.
407 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
408 (C<sleep> may be internally implemented in your system with C<alarm>)
410 If you want to use C<alarm> to time out a system call you need to use an
411 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
412 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
413 restart system calls on some systems. Using C<eval>/C<die> always works,
414 modulo the caveats given in L<perlipc/"Signals">.
417 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
419 $nread = sysread SOCKET, $buffer, $size;
423 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 Returns the arctangent of Y/X in the range -PI to PI.
434 For the tangent operation, you may use the C<Math::Trig::tan>
435 function, or use the familiar relation:
437 sub tan { sin($_[0]) / cos($_[0]) }
439 =item bind SOCKET,NAME
441 Binds a network address to a socket, just as the bind system call
442 does. Returns true if it succeeded, false otherwise. NAME should be a
443 packed address of the appropriate type for the socket. See the examples in
444 L<perlipc/"Sockets: Client/Server Communication">.
446 =item binmode FILEHANDLE, DISCIPLINE
448 =item binmode FILEHANDLE
450 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
451 on systems where the run-time libraries distinguish between binary and
452 text files. If FILEHANDLE is an expression, the value is taken as the
453 name of the filehandle. DISCIPLINE can be either of C<:raw> for
454 binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
455 omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
456 failure. The C<:raw> are C<:clrf>, and any other directives of the
457 form C<:...>, are called I/O I<disciplines>.
459 The C<open> pragma can be used to establish default I/O disciplines.
462 In general, binmode() should be called after open() but before any I/O
463 is done on the filehandle. Calling binmode() will flush any possibly
464 pending buffered input or output data on the handle. The only
465 exception to this is the C<:encoding> discipline that changes
466 the default character encoding of the handle, see L<open>.
467 The C<:encoding> discipline sometimes needs to be called in
468 mid-stream, and it doesn't flush the stream.
470 On some systems binmode() is necessary when you're not working with a
471 text file. For the sake of portability it is a good idea to always use
472 it when appropriate, and to never use it when it isn't appropriate.
474 In other words: Regardless of platform, use binmode() on binary
475 files, and do not use binmode() on text files.
477 The operating system, device drivers, C libraries, and Perl run-time
478 system all work together to let the programmer treat a single
479 character (C<\n>) as the line terminator, irrespective of the external
480 representation. On many operating systems, the native text file
481 representation matches the internal representation, but on some
482 platforms the external representation of C<\n> is made up of more than
485 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
486 character to end each line in the external representation of text (even
487 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
488 on Unix and most VMS files). Consequently binmode() has no effect on
489 these operating systems. In other systems like OS/2, DOS and the various
490 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
491 what's stored in text files are the two characters C<\cM\cJ>. That means
492 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
493 disk will be converted to C<\n> on input, and any C<\n> in your program
494 will be converted back to C<\cM\cJ> on output. This is what you want for
495 text files, but it can be disastrous for binary files.
497 Another consequence of using binmode() (on some systems) is that
498 special end-of-file markers will be seen as part of the data stream.
499 For systems from the Microsoft family this means that if your binary
500 data contains C<\cZ>, the I/O subsystem will regard it as the end of
501 the file, unless you use binmode().
503 binmode() is not only important for readline() and print() operations,
504 but also when using read(), seek(), sysread(), syswrite() and tell()
505 (see L<perlport> for more details). See the C<$/> and C<$\> variables
506 in L<perlvar> for how to manually set your input and output
507 line-termination sequences.
509 =item bless REF,CLASSNAME
513 This function tells the thingy referenced by REF that it is now an object
514 in the CLASSNAME package. If CLASSNAME is omitted, the current package
515 is used. Because a C<bless> is often the last thing in a constructor,
516 it returns the reference for convenience. Always use the two-argument
517 version if the function doing the blessing might be inherited by a
518 derived class. See L<perltoot> and L<perlobj> for more about the blessing
519 (and blessings) of objects.
521 Consider always blessing objects in CLASSNAMEs that are mixed case.
522 Namespaces with all lowercase names are considered reserved for
523 Perl pragmata. Builtin types have all uppercase names, so to prevent
524 confusion, you may wish to avoid such package names as well. Make sure
525 that CLASSNAME is a true value.
527 See L<perlmod/"Perl Modules">.
533 Returns the context of the current subroutine call. In scalar context,
534 returns the caller's package name if there is a caller, that is, if
535 we're in a subroutine or C<eval> or C<require>, and the undefined value
536 otherwise. In list context, returns
538 ($package, $filename, $line) = caller;
540 With EXPR, it returns some extra information that the debugger uses to
541 print a stack trace. The value of EXPR indicates how many call frames
542 to go back before the current one.
544 ($package, $filename, $line, $subroutine, $hasargs,
545 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
547 Here $subroutine may be C<(eval)> if the frame is not a subroutine
548 call, but an C<eval>. In such a case additional elements $evaltext and
549 C<$is_require> are set: C<$is_require> is true if the frame is created by a
550 C<require> or C<use> statement, $evaltext contains the text of the
551 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
552 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
553 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
554 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
555 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
556 was compiled with. The C<$hints> and C<$bitmask> values are subject to
557 change between versions of Perl, and are not meant for external use.
559 Furthermore, when called from within the DB package, caller returns more
560 detailed information: it sets the list variable C<@DB::args> to be the
561 arguments with which the subroutine was invoked.
563 Be aware that the optimizer might have optimized call frames away before
564 C<caller> had a chance to get the information. That means that C<caller(N)>
565 might not return information about the call frame you expect it do, for
566 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
567 previous time C<caller> was called.
571 Changes the working directory to EXPR, if possible. If EXPR is omitted,
572 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
573 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
574 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
575 neither is set, C<chdir> does nothing. It returns true upon success,
576 false otherwise. See the example under C<die>.
580 Changes the permissions of a list of files. The first element of the
581 list must be the numerical mode, which should probably be an octal
582 number, and which definitely should I<not> a string of octal digits:
583 C<0644> is okay, C<'0644'> is not. Returns the number of files
584 successfully changed. See also L</oct>, if all you have is a string.
586 $cnt = chmod 0755, 'foo', 'bar';
587 chmod 0755, @executables;
588 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
590 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
591 $mode = 0644; chmod $mode, 'foo'; # this is best
593 You can also import the symbolic C<S_I*> constants from the Fcntl
598 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
599 # This is identical to the chmod 0755 of the above example.
607 This safer version of L</chop> removes any trailing string
608 that corresponds to the current value of C<$/> (also known as
609 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
610 number of characters removed from all its arguments. It's often used to
611 remove the newline from the end of an input record when you're worried
612 that the final record may be missing its newline. When in paragraph
613 mode (C<$/ = "">), it removes all trailing newlines from the string.
614 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
615 a reference to an integer or the like, see L<perlvar>) chomp() won't
617 If VARIABLE is omitted, it chomps C<$_>. Example:
620 chomp; # avoid \n on last field
625 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
627 You can actually chomp anything that's an lvalue, including an assignment:
630 chomp($answer = <STDIN>);
632 If you chomp a list, each element is chomped, and the total number of
633 characters removed is returned.
641 Chops off the last character of a string and returns the character
642 chopped. It is much more efficient than C<s/.$//s> because it neither
643 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
644 If VARIABLE is a hash, it chops the hash's values, but not its keys.
646 You can actually chop anything that's an lvalue, including an assignment.
648 If you chop a list, each element is chopped. Only the value of the
649 last C<chop> is returned.
651 Note that C<chop> returns the last character. To return all but the last
652 character, use C<substr($string, 0, -1)>.
656 Changes the owner (and group) of a list of files. The first two
657 elements of the list must be the I<numeric> uid and gid, in that
658 order. A value of -1 in either position is interpreted by most
659 systems to leave that value unchanged. Returns the number of files
660 successfully changed.
662 $cnt = chown $uid, $gid, 'foo', 'bar';
663 chown $uid, $gid, @filenames;
665 Here's an example that looks up nonnumeric uids in the passwd file:
668 chomp($user = <STDIN>);
670 chomp($pattern = <STDIN>);
672 ($login,$pass,$uid,$gid) = getpwnam($user)
673 or die "$user not in passwd file";
675 @ary = glob($pattern); # expand filenames
676 chown $uid, $gid, @ary;
678 On most systems, you are not allowed to change the ownership of the
679 file unless you're the superuser, although you should be able to change
680 the group to any of your secondary groups. On insecure systems, these
681 restrictions may be relaxed, but this is not a portable assumption.
682 On POSIX systems, you can detect this condition this way:
684 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
685 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
691 Returns the character represented by that NUMBER in the character set.
692 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
693 chr(0x263a) is a Unicode smiley face. Note that characters from 127
694 to 255 (inclusive) are by default not encoded in Unicode for backward
695 compatibility reasons (but see L<encoding>).
697 For the reverse, use L</ord>.
698 See L<perlunicode> and L<encoding> for more about Unicode.
700 If NUMBER is omitted, uses C<$_>.
702 =item chroot FILENAME
706 This function works like the system call by the same name: it makes the
707 named directory the new root directory for all further pathnames that
708 begin with a C</> by your process and all its children. (It doesn't
709 change your current working directory, which is unaffected.) For security
710 reasons, this call is restricted to the superuser. If FILENAME is
711 omitted, does a C<chroot> to C<$_>.
713 =item close FILEHANDLE
717 Closes the file or pipe associated with the file handle, returning
718 true only if IO buffers are successfully flushed and closes the system
719 file descriptor. Closes the currently selected filehandle if the
722 You don't have to close FILEHANDLE if you are immediately going to do
723 another C<open> on it, because C<open> will close it for you. (See
724 C<open>.) However, an explicit C<close> on an input file resets the line
725 counter (C<$.>), while the implicit close done by C<open> does not.
727 If the file handle came from a piped open C<close> will additionally
728 return false if one of the other system calls involved fails or if the
729 program exits with non-zero status. (If the only problem was that the
730 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
731 also waits for the process executing on the pipe to complete, in case you
732 want to look at the output of the pipe afterwards, and
733 implicitly puts the exit status value of that command into C<$?>.
735 Prematurely closing the read end of a pipe (i.e. before the process
736 writing to it at the other end has closed it) will result in a
737 SIGPIPE being delivered to the writer. If the other end can't
738 handle that, be sure to read all the data before closing the pipe.
742 open(OUTPUT, '|sort >foo') # pipe to sort
743 or die "Can't start sort: $!";
744 #... # print stuff to output
745 close OUTPUT # wait for sort to finish
746 or warn $! ? "Error closing sort pipe: $!"
747 : "Exit status $? from sort";
748 open(INPUT, 'foo') # get sort's results
749 or die "Can't open 'foo' for input: $!";
751 FILEHANDLE may be an expression whose value can be used as an indirect
752 filehandle, usually the real filehandle name.
754 =item closedir DIRHANDLE
756 Closes a directory opened by C<opendir> and returns the success of that
759 DIRHANDLE may be an expression whose value can be used as an indirect
760 dirhandle, usually the real dirhandle name.
762 =item connect SOCKET,NAME
764 Attempts to connect to a remote socket, just as the connect system call
765 does. Returns true if it succeeded, false otherwise. NAME should be a
766 packed address of the appropriate type for the socket. See the examples in
767 L<perlipc/"Sockets: Client/Server Communication">.
771 Actually a flow control statement rather than a function. If there is a
772 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
773 C<foreach>), it is always executed just before the conditional is about to
774 be evaluated again, just like the third part of a C<for> loop in C. Thus
775 it can be used to increment a loop variable, even when the loop has been
776 continued via the C<next> statement (which is similar to the C C<continue>
779 C<last>, C<next>, or C<redo> may appear within a C<continue>
780 block. C<last> and C<redo> will behave as if they had been executed within
781 the main block. So will C<next>, but since it will execute a C<continue>
782 block, it may be more entertaining.
785 ### redo always comes here
788 ### next always comes here
790 # then back the top to re-check EXPR
792 ### last always comes here
794 Omitting the C<continue> section is semantically equivalent to using an
795 empty one, logically enough. In that case, C<next> goes directly back
796 to check the condition at the top of the loop.
802 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
803 takes cosine of C<$_>.
805 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
806 function, or use this relation:
808 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
810 =item crypt PLAINTEXT,SALT
812 Encrypts a string exactly like the crypt(3) function in the C library
813 (assuming that you actually have a version there that has not been
814 extirpated as a potential munition). This can prove useful for checking
815 the password file for lousy passwords, amongst other things. Only the
816 guys wearing white hats should do this.
818 Note that C<crypt> is intended to be a one-way function, much like
819 breaking eggs to make an omelette. There is no (known) corresponding
820 decrypt function (in other words, the crypt() is a one-way hash
821 function). As a result, this function isn't all that useful for
822 cryptography. (For that, see your nearby CPAN mirror.)
824 When verifying an existing encrypted string you should use the
825 encrypted text as the salt (like C<crypt($plain, $crypted) eq
826 $crypted>). This allows your code to work with the standard C<crypt>
827 and with more exotic implementations. In other words, do not assume
828 anything about the returned string itself, or how many bytes in
829 the encrypted string matter.
831 Traditionally the result is a string of 13 bytes: two first bytes of
832 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
833 the first eight bytes of the encrypted string mattered, but
834 alternative hashing schemes (like MD5), higher level security schemes
835 (like C2), and implementations on non-UNIX platforms may produce
838 When choosing a new salt create a random two character string whose
839 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
840 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
842 Here's an example that makes sure that whoever runs this program knows
845 $pwd = (getpwuid($<))[1];
849 chomp($word = <STDIN>);
853 if (crypt($word, $pwd) ne $pwd) {
859 Of course, typing in your own password to whoever asks you
862 The L<crypt> function is unsuitable for encrypting large quantities
863 of data, not least of all because you can't get the information
864 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
865 on your favorite CPAN mirror for a slew of potentially useful
868 If using crypt() on a Unicode string (which potentially has
869 characters with codepoints above 255), Perl tries to make sense of
870 the situation by using only the low eight bits of the characters when
875 [This function has been largely superseded by the C<untie> function.]
877 Breaks the binding between a DBM file and a hash.
879 =item dbmopen HASH,DBNAME,MASK
881 [This function has been largely superseded by the C<tie> function.]
883 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
884 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
885 argument is I<not> a filehandle, even though it looks like one). DBNAME
886 is the name of the database (without the F<.dir> or F<.pag> extension if
887 any). If the database does not exist, it is created with protection
888 specified by MASK (as modified by the C<umask>). If your system supports
889 only the older DBM functions, you may perform only one C<dbmopen> in your
890 program. In older versions of Perl, if your system had neither DBM nor
891 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
894 If you don't have write access to the DBM file, you can only read hash
895 variables, not set them. If you want to test whether you can write,
896 either use file tests or try setting a dummy hash entry inside an C<eval>,
897 which will trap the error.
899 Note that functions such as C<keys> and C<values> may return huge lists
900 when used on large DBM files. You may prefer to use the C<each>
901 function to iterate over large DBM files. Example:
903 # print out history file offsets
904 dbmopen(%HIST,'/usr/lib/news/history',0666);
905 while (($key,$val) = each %HIST) {
906 print $key, ' = ', unpack('L',$val), "\n";
910 See also L<AnyDBM_File> for a more general description of the pros and
911 cons of the various dbm approaches, as well as L<DB_File> for a particularly
914 You can control which DBM library you use by loading that library
915 before you call dbmopen():
918 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
919 or die "Can't open netscape history file: $!";
925 Returns a Boolean value telling whether EXPR has a value other than
926 the undefined value C<undef>. If EXPR is not present, C<$_> will be
929 Many operations return C<undef> to indicate failure, end of file,
930 system error, uninitialized variable, and other exceptional
931 conditions. This function allows you to distinguish C<undef> from
932 other values. (A simple Boolean test will not distinguish among
933 C<undef>, zero, the empty string, and C<"0">, which are all equally
934 false.) Note that since C<undef> is a valid scalar, its presence
935 doesn't I<necessarily> indicate an exceptional condition: C<pop>
936 returns C<undef> when its argument is an empty array, I<or> when the
937 element to return happens to be C<undef>.
939 You may also use C<defined(&func)> to check whether subroutine C<&func>
940 has ever been defined. The return value is unaffected by any forward
941 declarations of C<&foo>. Note that a subroutine which is not defined
942 may still be callable: its package may have an C<AUTOLOAD> method that
943 makes it spring into existence the first time that it is called -- see
946 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
947 used to report whether memory for that aggregate has ever been
948 allocated. This behavior may disappear in future versions of Perl.
949 You should instead use a simple test for size:
951 if (@an_array) { print "has array elements\n" }
952 if (%a_hash) { print "has hash members\n" }
954 When used on a hash element, it tells you whether the value is defined,
955 not whether the key exists in the hash. Use L</exists> for the latter
960 print if defined $switch{'D'};
961 print "$val\n" while defined($val = pop(@ary));
962 die "Can't readlink $sym: $!"
963 unless defined($value = readlink $sym);
964 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
965 $debugging = 0 unless defined $debugging;
967 Note: Many folks tend to overuse C<defined>, and then are surprised to
968 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
969 defined values. For example, if you say
973 The pattern match succeeds, and C<$1> is defined, despite the fact that it
974 matched "nothing". But it didn't really match nothing--rather, it
975 matched something that happened to be zero characters long. This is all
976 very above-board and honest. When a function returns an undefined value,
977 it's an admission that it couldn't give you an honest answer. So you
978 should use C<defined> only when you're questioning the integrity of what
979 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
982 See also L</undef>, L</exists>, L</ref>.
986 Given an expression that specifies a hash element, array element, hash slice,
987 or array slice, deletes the specified element(s) from the hash or array.
988 In the case of an array, if the array elements happen to be at the end,
989 the size of the array will shrink to the highest element that tests
990 true for exists() (or 0 if no such element exists).
992 Returns each element so deleted or the undefined value if there was no such
993 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
994 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
995 from a C<tie>d hash or array may not necessarily return anything.
997 Deleting an array element effectively returns that position of the array
998 to its initial, uninitialized state. Subsequently testing for the same
999 element with exists() will return false. Note that deleting array
1000 elements in the middle of an array will not shift the index of the ones
1001 after them down--use splice() for that. See L</exists>.
1003 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1005 foreach $key (keys %HASH) {
1009 foreach $index (0 .. $#ARRAY) {
1010 delete $ARRAY[$index];
1015 delete @HASH{keys %HASH};
1017 delete @ARRAY[0 .. $#ARRAY];
1019 But both of these are slower than just assigning the empty list
1020 or undefining %HASH or @ARRAY:
1022 %HASH = (); # completely empty %HASH
1023 undef %HASH; # forget %HASH ever existed
1025 @ARRAY = (); # completely empty @ARRAY
1026 undef @ARRAY; # forget @ARRAY ever existed
1028 Note that the EXPR can be arbitrarily complicated as long as the final
1029 operation is a hash element, array element, hash slice, or array slice
1032 delete $ref->[$x][$y]{$key};
1033 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1035 delete $ref->[$x][$y][$index];
1036 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1040 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1041 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1042 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1043 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1044 an C<eval(),> the error message is stuffed into C<$@> and the
1045 C<eval> is terminated with the undefined value. This makes
1046 C<die> the way to raise an exception.
1048 Equivalent examples:
1050 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1051 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1053 If the value of EXPR does not end in a newline, the current script line
1054 number and input line number (if any) are also printed, and a newline
1055 is supplied. Note that the "input line number" (also known as "chunk")
1056 is subject to whatever notion of "line" happens to be currently in
1057 effect, and is also available as the special variable C<$.>.
1058 See L<perlvar/"$/"> and L<perlvar/"$.">.
1060 Hint: sometimes appending C<", stopped"> to your message
1061 will cause it to make better sense when the string C<"at foo line 123"> is
1062 appended. Suppose you are running script "canasta".
1064 die "/etc/games is no good";
1065 die "/etc/games is no good, stopped";
1067 produce, respectively
1069 /etc/games is no good at canasta line 123.
1070 /etc/games is no good, stopped at canasta line 123.
1072 See also exit(), warn(), and the Carp module.
1074 If LIST is empty and C<$@> already contains a value (typically from a
1075 previous eval) that value is reused after appending C<"\t...propagated">.
1076 This is useful for propagating exceptions:
1079 die unless $@ =~ /Expected exception/;
1081 If LIST is empty and C<$@> contains an object reference that has a
1082 C<PROPAGATE> method, that method will be called with additional file
1083 and line number parameters. The return value replaces the value in
1084 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1087 If C<$@> is empty then the string C<"Died"> is used.
1089 die() can also be called with a reference argument. If this happens to be
1090 trapped within an eval(), $@ contains the reference. This behavior permits
1091 a more elaborate exception handling implementation using objects that
1092 maintain arbitrary state about the nature of the exception. Such a scheme
1093 is sometimes preferable to matching particular string values of $@ using
1094 regular expressions. Here's an example:
1096 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1098 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1099 # handle Some::Module::Exception
1102 # handle all other possible exceptions
1106 Because perl will stringify uncaught exception messages before displaying
1107 them, you may want to overload stringification operations on such custom
1108 exception objects. See L<overload> for details about that.
1110 You can arrange for a callback to be run just before the C<die>
1111 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1112 handler will be called with the error text and can change the error
1113 message, if it sees fit, by calling C<die> again. See
1114 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1115 L<"eval BLOCK"> for some examples. Although this feature was meant
1116 to be run only right before your program was to exit, this is not
1117 currently the case--the C<$SIG{__DIE__}> hook is currently called
1118 even inside eval()ed blocks/strings! If one wants the hook to do
1119 nothing in such situations, put
1123 as the first line of the handler (see L<perlvar/$^S>). Because
1124 this promotes strange action at a distance, this counterintuitive
1125 behavior may be fixed in a future release.
1129 Not really a function. Returns the value of the last command in the
1130 sequence of commands indicated by BLOCK. When modified by a loop
1131 modifier, executes the BLOCK once before testing the loop condition.
1132 (On other statements the loop modifiers test the conditional first.)
1134 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1135 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1136 See L<perlsyn> for alternative strategies.
1138 =item do SUBROUTINE(LIST)
1140 A deprecated form of subroutine call. See L<perlsub>.
1144 Uses the value of EXPR as a filename and executes the contents of the
1145 file as a Perl script. Its primary use is to include subroutines
1146 from a Perl subroutine library.
1154 except that it's more efficient and concise, keeps track of the current
1155 filename for error messages, searches the @INC libraries, and updates
1156 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1157 variables. It also differs in that code evaluated with C<do FILENAME>
1158 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1159 same, however, in that it does reparse the file every time you call it,
1160 so you probably don't want to do this inside a loop.
1162 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1163 error. If C<do> can read the file but cannot compile it, it
1164 returns undef and sets an error message in C<$@>. If the file is
1165 successfully compiled, C<do> returns the value of the last expression
1168 Note that inclusion of library modules is better done with the
1169 C<use> and C<require> operators, which also do automatic error checking
1170 and raise an exception if there's a problem.
1172 You might like to use C<do> to read in a program configuration
1173 file. Manual error checking can be done this way:
1175 # read in config files: system first, then user
1176 for $file ("/share/prog/defaults.rc",
1177 "$ENV{HOME}/.someprogrc")
1179 unless ($return = do $file) {
1180 warn "couldn't parse $file: $@" if $@;
1181 warn "couldn't do $file: $!" unless defined $return;
1182 warn "couldn't run $file" unless $return;
1190 This function causes an immediate core dump. See also the B<-u>
1191 command-line switch in L<perlrun>, which does the same thing.
1192 Primarily this is so that you can use the B<undump> program (not
1193 supplied) to turn your core dump into an executable binary after
1194 having initialized all your variables at the beginning of the
1195 program. When the new binary is executed it will begin by executing
1196 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1197 Think of it as a goto with an intervening core dump and reincarnation.
1198 If C<LABEL> is omitted, restarts the program from the top.
1200 B<WARNING>: Any files opened at the time of the dump will I<not>
1201 be open any more when the program is reincarnated, with possible
1202 resulting confusion on the part of Perl.
1204 This function is now largely obsolete, partly because it's very
1205 hard to convert a core file into an executable, and because the
1206 real compiler backends for generating portable bytecode and compilable
1207 C code have superseded it.
1209 If you're looking to use L<dump> to speed up your program, consider
1210 generating bytecode or native C code as described in L<perlcc>. If
1211 you're just trying to accelerate a CGI script, consider using the
1212 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1213 You might also consider autoloading or selfloading, which at least
1214 make your program I<appear> to run faster.
1218 When called in list context, returns a 2-element list consisting of the
1219 key and value for the next element of a hash, so that you can iterate over
1220 it. When called in scalar context, returns only the key for the next
1221 element in the hash.
1223 Entries are returned in an apparently random order. The actual random
1224 order is subject to change in future versions of perl, but it is guaranteed
1225 to be in the same order as either the C<keys> or C<values> function
1226 would produce on the same (unmodified) hash.
1228 When the hash is entirely read, a null array is returned in list context
1229 (which when assigned produces a false (C<0>) value), and C<undef> in
1230 scalar context. The next call to C<each> after that will start iterating
1231 again. There is a single iterator for each hash, shared by all C<each>,
1232 C<keys>, and C<values> function calls in the program; it can be reset by
1233 reading all the elements from the hash, or by evaluating C<keys HASH> or
1234 C<values HASH>. If you add or delete elements of a hash while you're
1235 iterating over it, you may get entries skipped or duplicated, so
1236 don't. Exception: It is always safe to delete the item most recently
1237 returned by C<each()>, which means that the following code will work:
1239 while (($key, $value) = each %hash) {
1241 delete $hash{$key}; # This is safe
1244 The following prints out your environment like the printenv(1) program,
1245 only in a different order:
1247 while (($key,$value) = each %ENV) {
1248 print "$key=$value\n";
1251 See also C<keys>, C<values> and C<sort>.
1253 =item eof FILEHANDLE
1259 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1260 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1261 gives the real filehandle. (Note that this function actually
1262 reads a character and then C<ungetc>s it, so isn't very useful in an
1263 interactive context.) Do not read from a terminal file (or call
1264 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1265 as terminals may lose the end-of-file condition if you do.
1267 An C<eof> without an argument uses the last file read. Using C<eof()>
1268 with empty parentheses is very different. It refers to the pseudo file
1269 formed from the files listed on the command line and accessed via the
1270 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1271 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1272 used will cause C<@ARGV> to be examined to determine if input is
1275 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1276 detect the end of each file, C<eof()> will only detect the end of the
1277 last file. Examples:
1279 # reset line numbering on each input file
1281 next if /^\s*#/; # skip comments
1284 close ARGV if eof; # Not eof()!
1287 # insert dashes just before last line of last file
1289 if (eof()) { # check for end of current file
1290 print "--------------\n";
1291 close(ARGV); # close or last; is needed if we
1292 # are reading from the terminal
1297 Practical hint: you almost never need to use C<eof> in Perl, because the
1298 input operators typically return C<undef> when they run out of data, or if
1305 In the first form, the return value of EXPR is parsed and executed as if it
1306 were a little Perl program. The value of the expression (which is itself
1307 determined within scalar context) is first parsed, and if there weren't any
1308 errors, executed in the lexical context of the current Perl program, so
1309 that any variable settings or subroutine and format definitions remain
1310 afterwards. Note that the value is parsed every time the eval executes.
1311 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1312 delay parsing and subsequent execution of the text of EXPR until run time.
1314 In the second form, the code within the BLOCK is parsed only once--at the
1315 same time the code surrounding the eval itself was parsed--and executed
1316 within the context of the current Perl program. This form is typically
1317 used to trap exceptions more efficiently than the first (see below), while
1318 also providing the benefit of checking the code within BLOCK at compile
1321 The final semicolon, if any, may be omitted from the value of EXPR or within
1324 In both forms, the value returned is the value of the last expression
1325 evaluated inside the mini-program; a return statement may be also used, just
1326 as with subroutines. The expression providing the return value is evaluated
1327 in void, scalar, or list context, depending on the context of the eval itself.
1328 See L</wantarray> for more on how the evaluation context can be determined.
1330 If there is a syntax error or runtime error, or a C<die> statement is
1331 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1332 error message. If there was no error, C<$@> is guaranteed to be a null
1333 string. Beware that using C<eval> neither silences perl from printing
1334 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1335 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1336 L</warn> and L<perlvar>.
1338 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1339 determining whether a particular feature (such as C<socket> or C<symlink>)
1340 is implemented. It is also Perl's exception trapping mechanism, where
1341 the die operator is used to raise exceptions.
1343 If the code to be executed doesn't vary, you may use the eval-BLOCK
1344 form to trap run-time errors without incurring the penalty of
1345 recompiling each time. The error, if any, is still returned in C<$@>.
1348 # make divide-by-zero nonfatal
1349 eval { $answer = $a / $b; }; warn $@ if $@;
1351 # same thing, but less efficient
1352 eval '$answer = $a / $b'; warn $@ if $@;
1354 # a compile-time error
1355 eval { $answer = }; # WRONG
1358 eval '$answer ='; # sets $@
1360 Due to the current arguably broken state of C<__DIE__> hooks, when using
1361 the C<eval{}> form as an exception trap in libraries, you may wish not
1362 to trigger any C<__DIE__> hooks that user code may have installed.
1363 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1364 as shown in this example:
1366 # a very private exception trap for divide-by-zero
1367 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1370 This is especially significant, given that C<__DIE__> hooks can call
1371 C<die> again, which has the effect of changing their error messages:
1373 # __DIE__ hooks may modify error messages
1375 local $SIG{'__DIE__'} =
1376 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1377 eval { die "foo lives here" };
1378 print $@ if $@; # prints "bar lives here"
1381 Because this promotes action at a distance, this counterintuitive behavior
1382 may be fixed in a future release.
1384 With an C<eval>, you should be especially careful to remember what's
1385 being looked at when:
1391 eval { $x }; # CASE 4
1393 eval "\$$x++"; # CASE 5
1396 Cases 1 and 2 above behave identically: they run the code contained in
1397 the variable $x. (Although case 2 has misleading double quotes making
1398 the reader wonder what else might be happening (nothing is).) Cases 3
1399 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1400 does nothing but return the value of $x. (Case 4 is preferred for
1401 purely visual reasons, but it also has the advantage of compiling at
1402 compile-time instead of at run-time.) Case 5 is a place where
1403 normally you I<would> like to use double quotes, except that in this
1404 particular situation, you can just use symbolic references instead, as
1407 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1408 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1412 =item exec PROGRAM LIST
1414 The C<exec> function executes a system command I<and never returns>--
1415 use C<system> instead of C<exec> if you want it to return. It fails and
1416 returns false only if the command does not exist I<and> it is executed
1417 directly instead of via your system's command shell (see below).
1419 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1420 warns you if there is a following statement which isn't C<die>, C<warn>,
1421 or C<exit> (if C<-w> is set - but you always do that). If you
1422 I<really> want to follow an C<exec> with some other statement, you
1423 can use one of these styles to avoid the warning:
1425 exec ('foo') or print STDERR "couldn't exec foo: $!";
1426 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1428 If there is more than one argument in LIST, or if LIST is an array
1429 with more than one value, calls execvp(3) with the arguments in LIST.
1430 If there is only one scalar argument or an array with one element in it,
1431 the argument is checked for shell metacharacters, and if there are any,
1432 the entire argument is passed to the system's command shell for parsing
1433 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1434 If there are no shell metacharacters in the argument, it is split into
1435 words and passed directly to C<execvp>, which is more efficient.
1438 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1439 exec "sort $outfile | uniq";
1441 If you don't really want to execute the first argument, but want to lie
1442 to the program you are executing about its own name, you can specify
1443 the program you actually want to run as an "indirect object" (without a
1444 comma) in front of the LIST. (This always forces interpretation of the
1445 LIST as a multivalued list, even if there is only a single scalar in
1448 $shell = '/bin/csh';
1449 exec $shell '-sh'; # pretend it's a login shell
1453 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1455 When the arguments get executed via the system shell, results will
1456 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1459 Using an indirect object with C<exec> or C<system> is also more
1460 secure. This usage (which also works fine with system()) forces
1461 interpretation of the arguments as a multivalued list, even if the
1462 list had just one argument. That way you're safe from the shell
1463 expanding wildcards or splitting up words with whitespace in them.
1465 @args = ( "echo surprise" );
1467 exec @args; # subject to shell escapes
1469 exec { $args[0] } @args; # safe even with one-arg list
1471 The first version, the one without the indirect object, ran the I<echo>
1472 program, passing it C<"surprise"> an argument. The second version
1473 didn't--it tried to run a program literally called I<"echo surprise">,
1474 didn't find it, and set C<$?> to a non-zero value indicating failure.
1476 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1477 output before the exec, but this may not be supported on some platforms
1478 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1479 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1480 open handles in order to avoid lost output.
1482 Note that C<exec> will not call your C<END> blocks, nor will it call
1483 any C<DESTROY> methods in your objects.
1487 Given an expression that specifies a hash element or array element,
1488 returns true if the specified element in the hash or array has ever
1489 been initialized, even if the corresponding value is undefined. The
1490 element is not autovivified if it doesn't exist.
1492 print "Exists\n" if exists $hash{$key};
1493 print "Defined\n" if defined $hash{$key};
1494 print "True\n" if $hash{$key};
1496 print "Exists\n" if exists $array[$index];
1497 print "Defined\n" if defined $array[$index];
1498 print "True\n" if $array[$index];
1500 A hash or array element can be true only if it's defined, and defined if
1501 it exists, but the reverse doesn't necessarily hold true.
1503 Given an expression that specifies the name of a subroutine,
1504 returns true if the specified subroutine has ever been declared, even
1505 if it is undefined. Mentioning a subroutine name for exists or defined
1506 does not count as declaring it. Note that a subroutine which does not
1507 exist may still be callable: its package may have an C<AUTOLOAD>
1508 method that makes it spring into existence the first time that it is
1509 called -- see L<perlsub>.
1511 print "Exists\n" if exists &subroutine;
1512 print "Defined\n" if defined &subroutine;
1514 Note that the EXPR can be arbitrarily complicated as long as the final
1515 operation is a hash or array key lookup or subroutine name:
1517 if (exists $ref->{A}->{B}->{$key}) { }
1518 if (exists $hash{A}{B}{$key}) { }
1520 if (exists $ref->{A}->{B}->[$ix]) { }
1521 if (exists $hash{A}{B}[$ix]) { }
1523 if (exists &{$ref->{A}{B}{$key}}) { }
1525 Although the deepest nested array or hash will not spring into existence
1526 just because its existence was tested, any intervening ones will.
1527 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1528 into existence due to the existence test for the $key element above.
1529 This happens anywhere the arrow operator is used, including even:
1532 if (exists $ref->{"Some key"}) { }
1533 print $ref; # prints HASH(0x80d3d5c)
1535 This surprising autovivification in what does not at first--or even
1536 second--glance appear to be an lvalue context may be fixed in a future
1539 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1540 on how exists() acts when used on a pseudo-hash.
1542 Use of a subroutine call, rather than a subroutine name, as an argument
1543 to exists() is an error.
1546 exists &sub(); # Error
1550 Evaluates EXPR and exits immediately with that value. Example:
1553 exit 0 if $ans =~ /^[Xx]/;
1555 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1556 universally recognized values for EXPR are C<0> for success and C<1>
1557 for error; other values are subject to interpretation depending on the
1558 environment in which the Perl program is running. For example, exiting
1559 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1560 the mailer to return the item undelivered, but that's not true everywhere.
1562 Don't use C<exit> to abort a subroutine if there's any chance that
1563 someone might want to trap whatever error happened. Use C<die> instead,
1564 which can be trapped by an C<eval>.
1566 The exit() function does not always exit immediately. It calls any
1567 defined C<END> routines first, but these C<END> routines may not
1568 themselves abort the exit. Likewise any object destructors that need to
1569 be called are called before the real exit. If this is a problem, you
1570 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1571 See L<perlmod> for details.
1577 Returns I<e> (the natural logarithm base) to the power of EXPR.
1578 If EXPR is omitted, gives C<exp($_)>.
1580 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1582 Implements the fcntl(2) function. You'll probably have to say
1586 first to get the correct constant definitions. Argument processing and
1587 value return works just like C<ioctl> below.
1591 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1592 or die "can't fcntl F_GETFL: $!";
1594 You don't have to check for C<defined> on the return from C<fnctl>.
1595 Like C<ioctl>, it maps a C<0> return from the system call into
1596 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1597 in numeric context. It is also exempt from the normal B<-w> warnings
1598 on improper numeric conversions.
1600 Note that C<fcntl> will produce a fatal error if used on a machine that
1601 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1602 manpage to learn what functions are available on your system.
1604 =item fileno FILEHANDLE
1606 Returns the file descriptor for a filehandle, or undefined if the
1607 filehandle is not open. This is mainly useful for constructing
1608 bitmaps for C<select> and low-level POSIX tty-handling operations.
1609 If FILEHANDLE is an expression, the value is taken as an indirect
1610 filehandle, generally its name.
1612 You can use this to find out whether two handles refer to the
1613 same underlying descriptor:
1615 if (fileno(THIS) == fileno(THAT)) {
1616 print "THIS and THAT are dups\n";
1619 (Filehandles connected to memory objects via new features of C<open> may
1620 return undefined even though they are open.)
1623 =item flock FILEHANDLE,OPERATION
1625 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1626 for success, false on failure. Produces a fatal error if used on a
1627 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1628 C<flock> is Perl's portable file locking interface, although it locks
1629 only entire files, not records.
1631 Two potentially non-obvious but traditional C<flock> semantics are
1632 that it waits indefinitely until the lock is granted, and that its locks
1633 B<merely advisory>. Such discretionary locks are more flexible, but offer
1634 fewer guarantees. This means that files locked with C<flock> may be
1635 modified by programs that do not also use C<flock>. See L<perlport>,
1636 your port's specific documentation, or your system-specific local manpages
1637 for details. It's best to assume traditional behavior if you're writing
1638 portable programs. (But if you're not, you should as always feel perfectly
1639 free to write for your own system's idiosyncrasies (sometimes called
1640 "features"). Slavish adherence to portability concerns shouldn't get
1641 in the way of your getting your job done.)
1643 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1644 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1645 you can use the symbolic names if you import them from the Fcntl module,
1646 either individually, or as a group using the ':flock' tag. LOCK_SH
1647 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1648 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1649 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1650 waiting for the lock (check the return status to see if you got it).
1652 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1653 before locking or unlocking it.
1655 Note that the emulation built with lockf(3) doesn't provide shared
1656 locks, and it requires that FILEHANDLE be open with write intent. These
1657 are the semantics that lockf(3) implements. Most if not all systems
1658 implement lockf(3) in terms of fcntl(2) locking, though, so the
1659 differing semantics shouldn't bite too many people.
1661 Note also that some versions of C<flock> cannot lock things over the
1662 network; you would need to use the more system-specific C<fcntl> for
1663 that. If you like you can force Perl to ignore your system's flock(2)
1664 function, and so provide its own fcntl(2)-based emulation, by passing
1665 the switch C<-Ud_flock> to the F<Configure> program when you configure
1668 Here's a mailbox appender for BSD systems.
1670 use Fcntl ':flock'; # import LOCK_* constants
1673 flock(MBOX,LOCK_EX);
1674 # and, in case someone appended
1675 # while we were waiting...
1680 flock(MBOX,LOCK_UN);
1683 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1684 or die "Can't open mailbox: $!";
1687 print MBOX $msg,"\n\n";
1690 On systems that support a real flock(), locks are inherited across fork()
1691 calls, whereas those that must resort to the more capricious fcntl()
1692 function lose the locks, making it harder to write servers.
1694 See also L<DB_File> for other flock() examples.
1698 Does a fork(2) system call to create a new process running the
1699 same program at the same point. It returns the child pid to the
1700 parent process, C<0> to the child process, or C<undef> if the fork is
1701 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1702 are shared, while everything else is copied. On most systems supporting
1703 fork(), great care has gone into making it extremely efficient (for
1704 example, using copy-on-write technology on data pages), making it the
1705 dominant paradigm for multitasking over the last few decades.
1707 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1708 output before forking the child process, but this may not be supported
1709 on some platforms (see L<perlport>). To be safe, you may need to set
1710 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1711 C<IO::Handle> on any open handles in order to avoid duplicate output.
1713 If you C<fork> without ever waiting on your children, you will
1714 accumulate zombies. On some systems, you can avoid this by setting
1715 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1716 forking and reaping moribund children.
1718 Note that if your forked child inherits system file descriptors like
1719 STDIN and STDOUT that are actually connected by a pipe or socket, even
1720 if you exit, then the remote server (such as, say, a CGI script or a
1721 backgrounded job launched from a remote shell) won't think you're done.
1722 You should reopen those to F</dev/null> if it's any issue.
1726 Declare a picture format for use by the C<write> function. For
1730 Test: @<<<<<<<< @||||| @>>>>>
1731 $str, $%, '$' . int($num)
1735 $num = $cost/$quantity;
1739 See L<perlform> for many details and examples.
1741 =item formline PICTURE,LIST
1743 This is an internal function used by C<format>s, though you may call it,
1744 too. It formats (see L<perlform>) a list of values according to the
1745 contents of PICTURE, placing the output into the format output
1746 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1747 Eventually, when a C<write> is done, the contents of
1748 C<$^A> are written to some filehandle, but you could also read C<$^A>
1749 yourself and then set C<$^A> back to C<"">. Note that a format typically
1750 does one C<formline> per line of form, but the C<formline> function itself
1751 doesn't care how many newlines are embedded in the PICTURE. This means
1752 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1753 You may therefore need to use multiple formlines to implement a single
1754 record format, just like the format compiler.
1756 Be careful if you put double quotes around the picture, because an C<@>
1757 character may be taken to mean the beginning of an array name.
1758 C<formline> always returns true. See L<perlform> for other examples.
1760 =item getc FILEHANDLE
1764 Returns the next character from the input file attached to FILEHANDLE,
1765 or the undefined value at end of file, or if there was an error.
1766 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1767 efficient. However, it cannot be used by itself to fetch single
1768 characters without waiting for the user to hit enter. For that, try
1769 something more like:
1772 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1775 system "stty", '-icanon', 'eol', "\001";
1781 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1784 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1788 Determination of whether $BSD_STYLE should be set
1789 is left as an exercise to the reader.
1791 The C<POSIX::getattr> function can do this more portably on
1792 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1793 module from your nearest CPAN site; details on CPAN can be found on
1798 Implements the C library function of the same name, which on most
1799 systems returns the current login from F</etc/utmp>, if any. If null,
1802 $login = getlogin || getpwuid($<) || "Kilroy";
1804 Do not consider C<getlogin> for authentication: it is not as
1805 secure as C<getpwuid>.
1807 =item getpeername SOCKET
1809 Returns the packed sockaddr address of other end of the SOCKET connection.
1812 $hersockaddr = getpeername(SOCK);
1813 ($port, $iaddr) = sockaddr_in($hersockaddr);
1814 $herhostname = gethostbyaddr($iaddr, AF_INET);
1815 $herstraddr = inet_ntoa($iaddr);
1819 Returns the current process group for the specified PID. Use
1820 a PID of C<0> to get the current process group for the
1821 current process. Will raise an exception if used on a machine that
1822 doesn't implement getpgrp(2). If PID is omitted, returns process
1823 group of current process. Note that the POSIX version of C<getpgrp>
1824 does not accept a PID argument, so only C<PID==0> is truly portable.
1828 Returns the process id of the parent process.
1830 =item getpriority WHICH,WHO
1832 Returns the current priority for a process, a process group, or a user.
1833 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1834 machine that doesn't implement getpriority(2).
1840 =item gethostbyname NAME
1842 =item getnetbyname NAME
1844 =item getprotobyname NAME
1850 =item getservbyname NAME,PROTO
1852 =item gethostbyaddr ADDR,ADDRTYPE
1854 =item getnetbyaddr ADDR,ADDRTYPE
1856 =item getprotobynumber NUMBER
1858 =item getservbyport PORT,PROTO
1876 =item sethostent STAYOPEN
1878 =item setnetent STAYOPEN
1880 =item setprotoent STAYOPEN
1882 =item setservent STAYOPEN
1896 These routines perform the same functions as their counterparts in the
1897 system library. In list context, the return values from the
1898 various get routines are as follows:
1900 ($name,$passwd,$uid,$gid,
1901 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1902 ($name,$passwd,$gid,$members) = getgr*
1903 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1904 ($name,$aliases,$addrtype,$net) = getnet*
1905 ($name,$aliases,$proto) = getproto*
1906 ($name,$aliases,$port,$proto) = getserv*
1908 (If the entry doesn't exist you get a null list.)
1910 The exact meaning of the $gcos field varies but it usually contains
1911 the real name of the user (as opposed to the login name) and other
1912 information pertaining to the user. Beware, however, that in many
1913 system users are able to change this information and therefore it
1914 cannot be trusted and therefore the $gcos is tainted (see
1915 L<perlsec>). The $passwd and $shell, user's encrypted password and
1916 login shell, are also tainted, because of the same reason.
1918 In scalar context, you get the name, unless the function was a
1919 lookup by name, in which case you get the other thing, whatever it is.
1920 (If the entry doesn't exist you get the undefined value.) For example:
1922 $uid = getpwnam($name);
1923 $name = getpwuid($num);
1925 $gid = getgrnam($name);
1926 $name = getgrgid($num;
1930 In I<getpw*()> the fields $quota, $comment, and $expire are special
1931 cases in the sense that in many systems they are unsupported. If the
1932 $quota is unsupported, it is an empty scalar. If it is supported, it
1933 usually encodes the disk quota. If the $comment field is unsupported,
1934 it is an empty scalar. If it is supported it usually encodes some
1935 administrative comment about the user. In some systems the $quota
1936 field may be $change or $age, fields that have to do with password
1937 aging. In some systems the $comment field may be $class. The $expire
1938 field, if present, encodes the expiration period of the account or the
1939 password. For the availability and the exact meaning of these fields
1940 in your system, please consult your getpwnam(3) documentation and your
1941 F<pwd.h> file. You can also find out from within Perl what your
1942 $quota and $comment fields mean and whether you have the $expire field
1943 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1944 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1945 files are only supported if your vendor has implemented them in the
1946 intuitive fashion that calling the regular C library routines gets the
1947 shadow versions if you're running under privilege or if there exists
1948 the shadow(3) functions as found in System V ( this includes Solaris
1949 and Linux.) Those systems which implement a proprietary shadow password
1950 facility are unlikely to be supported.
1952 The $members value returned by I<getgr*()> is a space separated list of
1953 the login names of the members of the group.
1955 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1956 C, it will be returned to you via C<$?> if the function call fails. The
1957 C<@addrs> value returned by a successful call is a list of the raw
1958 addresses returned by the corresponding system library call. In the
1959 Internet domain, each address is four bytes long and you can unpack it
1960 by saying something like:
1962 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1964 The Socket library makes this slightly easier:
1967 $iaddr = inet_aton("127.1"); # or whatever address
1968 $name = gethostbyaddr($iaddr, AF_INET);
1970 # or going the other way
1971 $straddr = inet_ntoa($iaddr);
1973 If you get tired of remembering which element of the return list
1974 contains which return value, by-name interfaces are provided
1975 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1976 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1977 and C<User::grent>. These override the normal built-ins, supplying
1978 versions that return objects with the appropriate names
1979 for each field. For example:
1983 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1985 Even though it looks like they're the same method calls (uid),
1986 they aren't, because a C<File::stat> object is different from
1987 a C<User::pwent> object.
1989 =item getsockname SOCKET
1991 Returns the packed sockaddr address of this end of the SOCKET connection,
1992 in case you don't know the address because you have several different
1993 IPs that the connection might have come in on.
1996 $mysockaddr = getsockname(SOCK);
1997 ($port, $myaddr) = sockaddr_in($mysockaddr);
1998 printf "Connect to %s [%s]\n",
1999 scalar gethostbyaddr($myaddr, AF_INET),
2002 =item getsockopt SOCKET,LEVEL,OPTNAME
2004 Returns the socket option requested, or undef if there is an error.
2010 Returns the value of EXPR with filename expansions such as the
2011 standard Unix shell F</bin/csh> would do. This is the internal function
2012 implementing the C<< <*.c> >> operator, but you can use it directly.
2013 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2014 discussed in more detail in L<perlop/"I/O Operators">.
2016 Beginning with v5.6.0, this operator is implemented using the standard
2017 C<File::Glob> extension. See L<File::Glob> for details.
2021 Converts a time as returned by the time function to an 8-element list
2022 with the time localized for the standard Greenwich time zone.
2023 Typically used as follows:
2026 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2029 All list elements are numeric, and come straight out of the C `struct
2030 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2031 specified time. $mday is the day of the month, and $mon is the month
2032 itself, in the range C<0..11> with 0 indicating January and 11
2033 indicating December. $year is the number of years since 1900. That
2034 is, $year is C<123> in year 2023. $wday is the day of the week, with
2035 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2036 the year, in the range C<0..364> (or C<0..365> in leap years.)
2038 Note that the $year element is I<not> simply the last two digits of
2039 the year. If you assume it is, then you create non-Y2K-compliant
2040 programs--and you wouldn't want to do that, would you?
2042 The proper way to get a complete 4-digit year is simply:
2046 And to get the last two digits of the year (e.g., '01' in 2001) do:
2048 $year = sprintf("%02d", $year % 100);
2050 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2052 In scalar context, C<gmtime()> returns the ctime(3) value:
2054 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2056 Also see the C<timegm> function provided by the C<Time::Local> module,
2057 and the strftime(3) function available via the POSIX module.
2059 This scalar value is B<not> locale dependent (see L<perllocale>), but
2060 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2061 strftime(3) and mktime(3) functions available via the POSIX module. To
2062 get somewhat similar but locale dependent date strings, set up your
2063 locale environment variables appropriately (please see L<perllocale>)
2064 and try for example:
2066 use POSIX qw(strftime);
2067 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2069 Note that the C<%a> and C<%b> escapes, which represent the short forms
2070 of the day of the week and the month of the year, may not necessarily
2071 be three characters wide in all locales.
2079 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2080 execution there. It may not be used to go into any construct that
2081 requires initialization, such as a subroutine or a C<foreach> loop. It
2082 also can't be used to go into a construct that is optimized away,
2083 or to get out of a block or subroutine given to C<sort>.
2084 It can be used to go almost anywhere else within the dynamic scope,
2085 including out of subroutines, but it's usually better to use some other
2086 construct such as C<last> or C<die>. The author of Perl has never felt the
2087 need to use this form of C<goto> (in Perl, that is--C is another matter).
2088 (The difference being that C does not offer named loops combined with
2089 loop control. Perl does, and this replaces most structured uses of C<goto>
2090 in other languages.)
2092 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2093 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2094 necessarily recommended if you're optimizing for maintainability:
2096 goto ("FOO", "BAR", "GLARCH")[$i];
2098 The C<goto-&NAME> form is quite different from the other forms of
2099 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2100 doesn't have the stigma associated with other gotos. Instead, it
2101 exits the current subroutine (losing any changes set by local()) and
2102 immediately calls in its place the named subroutine using the current
2103 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2104 load another subroutine and then pretend that the other subroutine had
2105 been called in the first place (except that any modifications to C<@_>
2106 in the current subroutine are propagated to the other subroutine.)
2107 After the C<goto>, not even C<caller> will be able to tell that this
2108 routine was called first.
2110 NAME needn't be the name of a subroutine; it can be a scalar variable
2111 containing a code reference, or a block which evaluates to a code
2114 =item grep BLOCK LIST
2116 =item grep EXPR,LIST
2118 This is similar in spirit to, but not the same as, grep(1) and its
2119 relatives. In particular, it is not limited to using regular expressions.
2121 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2122 C<$_> to each element) and returns the list value consisting of those
2123 elements for which the expression evaluated to true. In scalar
2124 context, returns the number of times the expression was true.
2126 @foo = grep(!/^#/, @bar); # weed out comments
2130 @foo = grep {!/^#/} @bar; # weed out comments
2132 Note that C<$_> is an alias to the list value, so it can be used to
2133 modify the elements of the LIST. While this is useful and supported,
2134 it can cause bizarre results if the elements of LIST are not variables.
2135 Similarly, grep returns aliases into the original list, much as a for
2136 loop's index variable aliases the list elements. That is, modifying an
2137 element of a list returned by grep (for example, in a C<foreach>, C<map>
2138 or another C<grep>) actually modifies the element in the original list.
2139 This is usually something to be avoided when writing clear code.
2141 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2147 Interprets EXPR as a hex string and returns the corresponding value.
2148 (To convert strings that might start with either 0, 0x, or 0b, see
2149 L</oct>.) If EXPR is omitted, uses C<$_>.
2151 print hex '0xAf'; # prints '175'
2152 print hex 'aF'; # same
2154 Hex strings may only represent integers. Strings that would cause
2155 integer overflow trigger a warning. Leading whitespace is not stripped,
2160 There is no builtin C<import> function. It is just an ordinary
2161 method (subroutine) defined (or inherited) by modules that wish to export
2162 names to another module. The C<use> function calls the C<import> method
2163 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2165 =item index STR,SUBSTR,POSITION
2167 =item index STR,SUBSTR
2169 The index function searches for one string within another, but without
2170 the wildcard-like behavior of a full regular-expression pattern match.
2171 It returns the position of the first occurrence of SUBSTR in STR at
2172 or after POSITION. If POSITION is omitted, starts searching from the
2173 beginning of the string. The return value is based at C<0> (or whatever
2174 you've set the C<$[> variable to--but don't do that). If the substring
2175 is not found, returns one less than the base, ordinarily C<-1>.
2181 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2182 You should not use this function for rounding: one because it truncates
2183 towards C<0>, and two because machine representations of floating point
2184 numbers can sometimes produce counterintuitive results. For example,
2185 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2186 because it's really more like -268.99999999999994315658 instead. Usually,
2187 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2188 functions will serve you better than will int().
2190 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2192 Implements the ioctl(2) function. You'll probably first have to say
2194 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2196 to get the correct function definitions. If F<ioctl.ph> doesn't
2197 exist or doesn't have the correct definitions you'll have to roll your
2198 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2199 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2200 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2201 written depending on the FUNCTION--a pointer to the string value of SCALAR
2202 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2203 has no string value but does have a numeric value, that value will be
2204 passed rather than a pointer to the string value. To guarantee this to be
2205 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2206 functions may be needed to manipulate the values of structures used by
2209 The return value of C<ioctl> (and C<fcntl>) is as follows:
2211 if OS returns: then Perl returns:
2213 0 string "0 but true"
2214 anything else that number
2216 Thus Perl returns true on success and false on failure, yet you can
2217 still easily determine the actual value returned by the operating
2220 $retval = ioctl(...) || -1;
2221 printf "System returned %d\n", $retval;
2223 The special string "C<0> but true" is exempt from B<-w> complaints
2224 about improper numeric conversions.
2226 Here's an example of setting a filehandle named C<REMOTE> to be
2227 non-blocking at the system level. You'll have to negotiate C<$|>
2228 on your own, though.
2230 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2232 $flags = fcntl(REMOTE, F_GETFL, 0)
2233 or die "Can't get flags for the socket: $!\n";
2235 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2236 or die "Can't set flags for the socket: $!\n";
2238 =item join EXPR,LIST
2240 Joins the separate strings of LIST into a single string with fields
2241 separated by the value of EXPR, and returns that new string. Example:
2243 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2245 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2246 first argument. Compare L</split>.
2250 Returns a list consisting of all the keys of the named hash. (In
2251 scalar context, returns the number of keys.) The keys are returned in
2252 an apparently random order. The actual random order is subject to
2253 change in future versions of perl, but it is guaranteed to be the same
2254 order as either the C<values> or C<each> function produces (given
2255 that the hash has not been modified). As a side effect, it resets
2258 Here is yet another way to print your environment:
2261 @values = values %ENV;
2263 print pop(@keys), '=', pop(@values), "\n";
2266 or how about sorted by key:
2268 foreach $key (sort(keys %ENV)) {
2269 print $key, '=', $ENV{$key}, "\n";
2272 The returned values are copies of the original keys in the hash, so
2273 modifying them will not affect the original hash. Compare L</values>.
2275 To sort a hash by value, you'll need to use a C<sort> function.
2276 Here's a descending numeric sort of a hash by its values:
2278 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2279 printf "%4d %s\n", $hash{$key}, $key;
2282 As an lvalue C<keys> allows you to increase the number of hash buckets
2283 allocated for the given hash. This can gain you a measure of efficiency if
2284 you know the hash is going to get big. (This is similar to pre-extending
2285 an array by assigning a larger number to $#array.) If you say
2289 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2290 in fact, since it rounds up to the next power of two. These
2291 buckets will be retained even if you do C<%hash = ()>, use C<undef
2292 %hash> if you want to free the storage while C<%hash> is still in scope.
2293 You can't shrink the number of buckets allocated for the hash using
2294 C<keys> in this way (but you needn't worry about doing this by accident,
2295 as trying has no effect).
2297 See also C<each>, C<values> and C<sort>.
2299 =item kill SIGNAL, LIST
2301 Sends a signal to a list of processes. Returns the number of
2302 processes successfully signaled (which is not necessarily the
2303 same as the number actually killed).
2305 $cnt = kill 1, $child1, $child2;
2308 If SIGNAL is zero, no signal is sent to the process. This is a
2309 useful way to check that the process is alive and hasn't changed
2310 its UID. See L<perlport> for notes on the portability of this
2313 Unlike in the shell, if SIGNAL is negative, it kills
2314 process groups instead of processes. (On System V, a negative I<PROCESS>
2315 number will also kill process groups, but that's not portable.) That
2316 means you usually want to use positive not negative signals. You may also
2317 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2323 The C<last> command is like the C<break> statement in C (as used in
2324 loops); it immediately exits the loop in question. If the LABEL is
2325 omitted, the command refers to the innermost enclosing loop. The
2326 C<continue> block, if any, is not executed:
2328 LINE: while (<STDIN>) {
2329 last LINE if /^$/; # exit when done with header
2333 C<last> cannot be used to exit a block which returns a value such as
2334 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2335 a grep() or map() operation.
2337 Note that a block by itself is semantically identical to a loop
2338 that executes once. Thus C<last> can be used to effect an early
2339 exit out of such a block.
2341 See also L</continue> for an illustration of how C<last>, C<next>, and
2348 Returns a lowercased version of EXPR. This is the internal function
2349 implementing the C<\L> escape in double-quoted strings. Respects
2350 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2351 and L<perlunicode> for more details about locale and Unicode support.
2353 If EXPR is omitted, uses C<$_>.
2359 Returns the value of EXPR with the first character lowercased. This
2360 is the internal function implementing the C<\l> escape in
2361 double-quoted strings. Respects current LC_CTYPE locale if C<use
2362 locale> in force. See L<perllocale> and L<perlunicode> for more
2363 details about locale and Unicode support.
2365 If EXPR is omitted, uses C<$_>.
2371 Returns the length in characters of the value of EXPR. If EXPR is
2372 omitted, returns length of C<$_>. Note that this cannot be used on
2373 an entire array or hash to find out how many elements these have.
2374 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2376 =item link OLDFILE,NEWFILE
2378 Creates a new filename linked to the old filename. Returns true for
2379 success, false otherwise.
2381 =item listen SOCKET,QUEUESIZE
2383 Does the same thing that the listen system call does. Returns true if
2384 it succeeded, false otherwise. See the example in
2385 L<perlipc/"Sockets: Client/Server Communication">.
2389 You really probably want to be using C<my> instead, because C<local> isn't
2390 what most people think of as "local". See
2391 L<perlsub/"Private Variables via my()"> for details.
2393 A local modifies the listed variables to be local to the enclosing
2394 block, file, or eval. If more than one value is listed, the list must
2395 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2396 for details, including issues with tied arrays and hashes.
2398 =item localtime EXPR
2400 Converts a time as returned by the time function to a 9-element list
2401 with the time analyzed for the local time zone. Typically used as
2405 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2408 All list elements are numeric, and come straight out of the C `struct
2409 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2410 specified time. $mday is the day of the month, and $mon is the month
2411 itself, in the range C<0..11> with 0 indicating January and 11
2412 indicating December. $year is the number of years since 1900. That
2413 is, $year is C<123> in year 2023. $wday is the day of the week, with
2414 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2415 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2416 is true if the specified time occurs during daylight savings time,
2419 Note that the $year element is I<not> simply the last two digits of
2420 the year. If you assume it is, then you create non-Y2K-compliant
2421 programs--and you wouldn't want to do that, would you?
2423 The proper way to get a complete 4-digit year is simply:
2427 And to get the last two digits of the year (e.g., '01' in 2001) do:
2429 $year = sprintf("%02d", $year % 100);
2431 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2433 In scalar context, C<localtime()> returns the ctime(3) value:
2435 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2437 This scalar value is B<not> locale dependent, see L<perllocale>, but
2438 instead a Perl builtin. Also see the C<Time::Local> module
2439 (to convert the second, minutes, hours, ... back to seconds since the
2440 stroke of midnight the 1st of January 1970, the value returned by
2441 time()), and the strftime(3) and mktime(3) functions available via the
2442 POSIX module. To get somewhat similar but locale dependent date
2443 strings, set up your locale environment variables appropriately
2444 (please see L<perllocale>) and try for example:
2446 use POSIX qw(strftime);
2447 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2449 Note that the C<%a> and C<%b>, the short forms of the day of the week
2450 and the month of the year, may not necessarily be three characters wide.
2454 This function places an advisory lock on a variable, subroutine,
2455 or referenced object contained in I<THING> until the lock goes out
2456 of scope. This is a built-in function only if your version of Perl
2457 was built with threading enabled, and if you've said C<use Thread>.
2458 Otherwise a user-defined function by this name will be called.
2465 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2466 returns log of C<$_>. To get the log of another base, use basic algebra:
2467 The base-N log of a number is equal to the natural log of that number
2468 divided by the natural log of N. For example:
2472 return log($n)/log(10);
2475 See also L</exp> for the inverse operation.
2481 Does the same thing as the C<stat> function (including setting the
2482 special C<_> filehandle) but stats a symbolic link instead of the file
2483 the symbolic link points to. If symbolic links are unimplemented on
2484 your system, a normal C<stat> is done.
2486 If EXPR is omitted, stats C<$_>.
2490 The match operator. See L<perlop>.
2492 =item map BLOCK LIST
2496 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2497 C<$_> to each element) and returns the list value composed of the
2498 results of each such evaluation. In scalar context, returns the
2499 total number of elements so generated. Evaluates BLOCK or EXPR in
2500 list context, so each element of LIST may produce zero, one, or
2501 more elements in the returned value.
2503 @chars = map(chr, @nums);
2505 translates a list of numbers to the corresponding characters. And
2507 %hash = map { getkey($_) => $_ } @array;
2509 is just a funny way to write
2512 foreach $_ (@array) {
2513 $hash{getkey($_)} = $_;
2516 Note that C<$_> is an alias to the list value, so it can be used to
2517 modify the elements of the LIST. While this is useful and supported,
2518 it can cause bizarre results if the elements of LIST are not variables.
2519 Using a regular C<foreach> loop for this purpose would be clearer in
2520 most cases. See also L</grep> for an array composed of those items of
2521 the original list for which the BLOCK or EXPR evaluates to true.
2523 C<{> starts both hash references and blocks, so C<map { ...> could be either
2524 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2525 ahead for the closing C<}> it has to take a guess at which its dealing with
2526 based what it finds just after the C<{>. Usually it gets it right, but if it
2527 doesn't it won't realize something is wrong until it gets to the C<}> and
2528 encounters the missing (or unexpected) comma. The syntax error will be
2529 reported close to the C<}> but you'll need to change something near the C<{>
2530 such as using a unary C<+> to give perl some help:
2532 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2533 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2534 %hash = map { ("\L$_", 1) } @array # this also works
2535 %hash = map { lc($_), 1 } @array # as does this.
2536 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2538 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2540 or to force an anon hash constructor use C<+{>
2542 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2544 and you get list of anonymous hashes each with only 1 entry.
2546 =item mkdir FILENAME,MASK
2548 =item mkdir FILENAME
2550 Creates the directory specified by FILENAME, with permissions
2551 specified by MASK (as modified by C<umask>). If it succeeds it
2552 returns true, otherwise it returns false and sets C<$!> (errno).
2553 If omitted, MASK defaults to 0777.
2555 In general, it is better to create directories with permissive MASK,
2556 and let the user modify that with their C<umask>, than it is to supply
2557 a restrictive MASK and give the user no way to be more permissive.
2558 The exceptions to this rule are when the file or directory should be
2559 kept private (mail files, for instance). The perlfunc(1) entry on
2560 C<umask> discusses the choice of MASK in more detail.
2562 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2563 number of trailing slashes. Some operating and filesystems do not get
2564 this right, so Perl automatically removes all trailing slashes to keep
2567 =item msgctl ID,CMD,ARG
2569 Calls the System V IPC function msgctl(2). You'll probably have to say
2573 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2574 then ARG must be a variable which will hold the returned C<msqid_ds>
2575 structure. Returns like C<ioctl>: the undefined value for error,
2576 C<"0 but true"> for zero, or the actual return value otherwise. See also
2577 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2579 =item msgget KEY,FLAGS
2581 Calls the System V IPC function msgget(2). Returns the message queue
2582 id, or the undefined value if there is an error. See also
2583 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2585 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2587 Calls the System V IPC function msgrcv to receive a message from
2588 message queue ID into variable VAR with a maximum message size of
2589 SIZE. Note that when a message is received, the message type as a
2590 native long integer will be the first thing in VAR, followed by the
2591 actual message. This packing may be opened with C<unpack("l! a*")>.
2592 Taints the variable. Returns true if successful, or false if there is
2593 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2594 C<IPC::SysV::Msg> documentation.
2596 =item msgsnd ID,MSG,FLAGS
2598 Calls the System V IPC function msgsnd to send the message MSG to the
2599 message queue ID. MSG must begin with the native long integer message
2600 type, and be followed by the length of the actual message, and finally
2601 the message itself. This kind of packing can be achieved with
2602 C<pack("l! a*", $type, $message)>. Returns true if successful,
2603 or false if there is an error. See also C<IPC::SysV>
2604 and C<IPC::SysV::Msg> documentation.
2608 =item my EXPR : ATTRIBUTES
2610 A C<my> declares the listed variables to be local (lexically) to the
2611 enclosing block, file, or C<eval>. If
2612 more than one value is listed, the list must be placed in parentheses. See
2613 L<perlsub/"Private Variables via my()"> for details.
2619 The C<next> command is like the C<continue> statement in C; it starts
2620 the next iteration of the loop:
2622 LINE: while (<STDIN>) {
2623 next LINE if /^#/; # discard comments
2627 Note that if there were a C<continue> block on the above, it would get
2628 executed even on discarded lines. If the LABEL is omitted, the command
2629 refers to the innermost enclosing loop.
2631 C<next> cannot be used to exit a block which returns a value such as
2632 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2633 a grep() or map() operation.
2635 Note that a block by itself is semantically identical to a loop
2636 that executes once. Thus C<next> will exit such a block early.
2638 See also L</continue> for an illustration of how C<last>, C<next>, and
2641 =item no Module LIST
2643 See the L</use> function, which C<no> is the opposite of.
2649 Interprets EXPR as an octal string and returns the corresponding
2650 value. (If EXPR happens to start off with C<0x>, interprets it as a
2651 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2652 binary string. Leading whitespace is ignored in all three cases.)
2653 The following will handle decimal, binary, octal, and hex in the standard
2656 $val = oct($val) if $val =~ /^0/;
2658 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2659 in octal), use sprintf() or printf():
2661 $perms = (stat("filename"))[2] & 07777;
2662 $oct_perms = sprintf "%lo", $perms;
2664 The oct() function is commonly used when a string such as C<644> needs
2665 to be converted into a file mode, for example. (Although perl will
2666 automatically convert strings into numbers as needed, this automatic
2667 conversion assumes base 10.)
2669 =item open FILEHANDLE,EXPR
2671 =item open FILEHANDLE,MODE,EXPR
2673 =item open FILEHANDLE,MODE,EXPR,LIST
2675 =item open FILEHANDLE,MODE,REFERENCE
2677 =item open FILEHANDLE
2679 Opens the file whose filename is given by EXPR, and associates it with
2682 (The following is a comprehensive reference to open(): for a gentler
2683 introduction you may consider L<perlopentut>.)
2685 If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2686 assigned a reference to a new anonymous filehandle, otherwise if
2687 FILEHANDLE is an expression, its value is used as the name of the real
2688 filehandle wanted. (This is considered a symbolic reference, so C<use
2689 strict 'refs'> should I<not> be in effect.)
2691 If EXPR is omitted, the scalar variable of the same name as the
2692 FILEHANDLE contains the filename. (Note that lexical variables--those
2693 declared with C<my>--will not work for this purpose; so if you're
2694 using C<my>, specify EXPR in your call to open.)
2696 If three or more arguments are specified then the mode of opening and
2697 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2698 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2699 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2700 the file is opened for appending, again being created if necessary.
2702 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2703 indicate that you want both read and write access to the file; thus
2704 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2705 '+>' >> mode would clobber the file first. You can't usually use
2706 either read-write mode for updating textfiles, since they have
2707 variable length records. See the B<-i> switch in L<perlrun> for a
2708 better approach. The file is created with permissions of C<0666>
2709 modified by the process' C<umask> value.
2711 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2712 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2714 In the 2-arguments (and 1-argument) form of the call the mode and
2715 filename should be concatenated (in this order), possibly separated by
2716 spaces. It is possible to omit the mode in these forms if the mode is
2719 If the filename begins with C<'|'>, the filename is interpreted as a
2720 command to which output is to be piped, and if the filename ends with a
2721 C<'|'>, the filename is interpreted as a command which pipes output to
2722 us. See L<perlipc/"Using open() for IPC">
2723 for more examples of this. (You are not allowed to C<open> to a command
2724 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2725 and L<perlipc/"Bidirectional Communication with Another Process">
2728 For three or more arguments if MODE is C<'|-'>, the filename is
2729 interpreted as a command to which output is to be piped, and if MODE
2730 is C<'-|'>, the filename is interpreted as a command which pipes
2731 output to us. In the 2-arguments (and 1-argument) form one should
2732 replace dash (C<'-'>) with the command.
2733 See L<perlipc/"Using open() for IPC"> for more examples of this.
2734 (You are not allowed to C<open> to a command that pipes both in I<and>
2735 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2736 L<perlipc/"Bidirectional Communication"> for alternatives.)
2738 In the three-or-more argument form of pipe opens, if LIST is specified
2739 (extra arguments after the command name) then LIST becomes arguments
2740 to the command invoked if the platform supports it. The meaning of
2741 C<open> with more than three arguments for non-pipe modes is not yet
2742 specified. Experimental "layers" may give extra LIST arguments
2745 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2746 and opening C<< '>-' >> opens STDOUT.
2748 You may use the three-argument form of open to specify
2749 I<I/O disciplines> that affect how the input and output
2750 are processed: see L</binmode> and L<open>. For example
2752 open(FH, "<:utf8", "file")
2754 will open the UTF-8 encoded file containing Unicode characters,
2755 see L<perluniintro>.
2757 Open returns nonzero upon success, the undefined value otherwise. If
2758 the C<open> involved a pipe, the return value happens to be the pid of
2761 If you're running Perl on a system that distinguishes between text
2762 files and binary files, then you should check out L</binmode> for tips
2763 for dealing with this. The key distinction between systems that need
2764 C<binmode> and those that don't is their text file formats. Systems
2765 like Unix, MacOS, and Plan9, which delimit lines with a single
2766 character, and which encode that character in C as C<"\n">, do not
2767 need C<binmode>. The rest need it.
2769 In the three argument form MODE may also contain a list of IO "layers"
2770 (see L<open> and L<PerlIO> for more details) to be applied to the
2771 handle. This can be used to achieve the effect of C<binmode> as well
2772 as more complex behaviours.
2774 When opening a file, it's usually a bad idea to continue normal execution
2775 if the request failed, so C<open> is frequently used in connection with
2776 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2777 where you want to make a nicely formatted error message (but there are
2778 modules that can help with that problem)) you should always check
2779 the return value from opening a file. The infrequent exception is when
2780 working with an unopened filehandle is actually what you want to do.
2782 As a special case the 3 arg form with a read/write mode and the third
2783 argument being C<undef>:
2785 open(TMP, "+>", undef) or die ...
2787 opens a filehandle to an anonymous temporary file.
2789 File handles can be opened to "in memory" files held in Perl scalars via:
2791 open($fh,'>', \$variable) || ..
2796 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2797 while (<ARTICLE>) {...
2799 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2800 # if the open fails, output is discarded
2802 open(DBASE, '+<', 'dbase.mine') # open for update
2803 or die "Can't open 'dbase.mine' for update: $!";
2805 open(DBASE, '+<dbase.mine') # ditto
2806 or die "Can't open 'dbase.mine' for update: $!";
2808 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2809 or die "Can't start caesar: $!";
2811 open(ARTICLE, "caesar <$article |") # ditto
2812 or die "Can't start caesar: $!";
2814 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2815 or die "Can't start sort: $!";
2818 open(MEMORY,'>', \$var)
2819 or die "Can't open memory file: $!";
2820 print MEMORY "foo!\n"; # output will end up in $var
2822 # process argument list of files along with any includes
2824 foreach $file (@ARGV) {
2825 process($file, 'fh00');
2829 my($filename, $input) = @_;
2830 $input++; # this is a string increment
2831 unless (open($input, $filename)) {
2832 print STDERR "Can't open $filename: $!\n";
2837 while (<$input>) { # note use of indirection
2838 if (/^#include "(.*)"/) {
2839 process($1, $input);
2846 You may also, in the Bourne shell tradition, specify an EXPR beginning
2847 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2848 name of a filehandle (or file descriptor, if numeric) to be
2849 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2850 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2851 mode you specify should match the mode of the original filehandle.
2852 (Duping a filehandle does not take into account any existing contents of
2853 IO buffers.) If you use the 3 arg form then you can pass either a number,
2854 the name of a filehandle or the normal "reference to a glob".
2856 Here is a script that saves, redirects, and restores STDOUT and
2860 open(my $oldout, ">&", \*STDOUT);
2861 open(OLDERR, ">&STDERR");
2863 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2864 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2866 select(STDERR); $| = 1; # make unbuffered
2867 select(STDOUT); $| = 1; # make unbuffered
2869 print STDOUT "stdout 1\n"; # this works for
2870 print STDERR "stderr 1\n"; # subprocesses too
2875 open(STDOUT, ">&OLDOUT");
2876 open(STDERR, ">&OLDERR");
2878 print STDOUT "stdout 2\n";
2879 print STDERR "stderr 2\n";
2881 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2882 do an equivalent of C's C<fdopen> of that file descriptor; this is
2883 more parsimonious of file descriptors. For example:
2885 open(FILEHANDLE, "<&=$fd")
2889 open(FILEHANDLE, "<&=", $fd)
2891 Note that if Perl is using the standard C libraries' fdopen() then on
2892 many UNIX systems, fdopen() is known to fail when file descriptors
2893 exceed a certain value, typically 255. If you need more file
2894 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2896 You can see whether Perl has been compiled with PerlIO or not by
2897 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2898 is C<define>, you have PerlIO, otherwise you don't.
2900 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2901 with 2-arguments (or 1-argument) form of open(), then
2902 there is an implicit fork done, and the return value of open is the pid
2903 of the child within the parent process, and C<0> within the child
2904 process. (Use C<defined($pid)> to determine whether the open was successful.)
2905 The filehandle behaves normally for the parent, but i/o to that
2906 filehandle is piped from/to the STDOUT/STDIN of the child process.
2907 In the child process the filehandle isn't opened--i/o happens from/to
2908 the new STDOUT or STDIN. Typically this is used like the normal
2909 piped open when you want to exercise more control over just how the
2910 pipe command gets executed, such as when you are running setuid, and
2911 don't want to have to scan shell commands for metacharacters.
2912 The following triples are more or less equivalent:
2914 open(FOO, "|tr '[a-z]' '[A-Z]'");
2915 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2916 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2917 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2919 open(FOO, "cat -n '$file'|");
2920 open(FOO, '-|', "cat -n '$file'");
2921 open(FOO, '-|') || exec 'cat', '-n', $file;
2922 open(FOO, '-|', "cat", '-n', $file);
2924 The last example in each block shows the pipe as "list form", which is
2925 not yet supported on all platforms.
2927 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2929 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2930 output before any operation that may do a fork, but this may not be
2931 supported on some platforms (see L<perlport>). To be safe, you may need
2932 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2933 of C<IO::Handle> on any open handles.
2935 On systems that support a close-on-exec flag on files, the flag will
2936 be set for the newly opened file descriptor as determined by the value
2937 of $^F. See L<perlvar/$^F>.
2939 Closing any piped filehandle causes the parent process to wait for the
2940 child to finish, and returns the status value in C<$?>.
2942 The filename passed to 2-argument (or 1-argument) form of open() will
2943 have leading and trailing whitespace deleted, and the normal
2944 redirection characters honored. This property, known as "magic open",
2945 can often be used to good effect. A user could specify a filename of
2946 F<"rsh cat file |">, or you could change certain filenames as needed:
2948 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2949 open(FH, $filename) or die "Can't open $filename: $!";
2951 Use 3-argument form to open a file with arbitrary weird characters in it,
2953 open(FOO, '<', $file);
2955 otherwise it's necessary to protect any leading and trailing whitespace:
2957 $file =~ s#^(\s)#./$1#;
2958 open(FOO, "< $file\0");
2960 (this may not work on some bizarre filesystems). One should
2961 conscientiously choose between the I<magic> and 3-arguments form
2966 will allow the user to specify an argument of the form C<"rsh cat file |">,
2967 but will not work on a filename which happens to have a trailing space, while
2969 open IN, '<', $ARGV[0];
2971 will have exactly the opposite restrictions.
2973 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2974 should use the C<sysopen> function, which involves no such magic (but
2975 may use subtly different filemodes than Perl open(), which is mapped
2976 to C fopen()). This is
2977 another way to protect your filenames from interpretation. For example:
2980 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2981 or die "sysopen $path: $!";
2982 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2983 print HANDLE "stuff $$\n";
2985 print "File contains: ", <HANDLE>;
2987 Using the constructor from the C<IO::Handle> package (or one of its
2988 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2989 filehandles that have the scope of whatever variables hold references to
2990 them, and automatically close whenever and however you leave that scope:
2994 sub read_myfile_munged {
2996 my $handle = new IO::File;
2997 open($handle, "myfile") or die "myfile: $!";
2999 or return (); # Automatically closed here.
3000 mung $first or die "mung failed"; # Or here.
3001 return $first, <$handle> if $ALL; # Or here.
3005 See L</seek> for some details about mixing reading and writing.
3007 =item opendir DIRHANDLE,EXPR
3009 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3010 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3011 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3017 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3018 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3021 For the reverse, see L</chr>.
3022 See L<perlunicode> and L<encoding> for more about Unicode.
3026 =item our EXPR : ATTRIBUTES
3028 An C<our> declares the listed variables to be valid globals within
3029 the enclosing block, file, or C<eval>. That is, it has the same
3030 scoping rules as a "my" declaration, but does not create a local
3031 variable. If more than one value is listed, the list must be placed
3032 in parentheses. The C<our> declaration has no semantic effect unless
3033 "use strict vars" is in effect, in which case it lets you use the
3034 declared global variable without qualifying it with a package name.
3035 (But only within the lexical scope of the C<our> declaration. In this
3036 it differs from "use vars", which is package scoped.)
3038 An C<our> declaration declares a global variable that will be visible
3039 across its entire lexical scope, even across package boundaries. The
3040 package in which the variable is entered is determined at the point
3041 of the declaration, not at the point of use. This means the following
3045 our $bar; # declares $Foo::bar for rest of lexical scope
3049 print $bar; # prints 20
3051 Multiple C<our> declarations in the same lexical scope are allowed
3052 if they are in different packages. If they happened to be in the same
3053 package, Perl will emit warnings if you have asked for them.
3057 our $bar; # declares $Foo::bar for rest of lexical scope
3061 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3062 print $bar; # prints 30
3064 our $bar; # emits warning
3066 An C<our> declaration may also have a list of attributes associated
3067 with it. B<WARNING>: This is an experimental feature that may be
3068 changed or removed in future releases of Perl. It should not be
3071 The only currently recognized attribute is C<unique> which indicates
3072 that a single copy of the global is to be used by all interpreters
3073 should the program happen to be running in a multi-interpreter
3074 environment. (The default behaviour would be for each interpreter to
3075 have its own copy of the global.) In such an environment, this
3076 attribute also has the effect of making the global readonly.
3079 our @EXPORT : unique = qw(foo);
3080 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3081 our $VERSION : unique = "1.00";
3083 Multi-interpreter environments can come to being either through the
3084 fork() emulation on Windows platforms, or by embedding perl in a
3085 multi-threaded application. The C<unique> attribute does nothing in
3086 all other environments.
3088 =item pack TEMPLATE,LIST
3090 Takes a LIST of values and converts it into a string using the rules
3091 given by the TEMPLATE. The resulting string is the concatenation of
3092 the converted values. Typically, each converted value looks
3093 like its machine-level representation. For example, on 32-bit machines
3094 a converted integer may be represented by a sequence of 4 bytes.
3097 sequence of characters that give the order and type of values, as
3100 a A string with arbitrary binary data, will be null padded.
3101 A A text (ASCII) string, will be space padded.
3102 Z A null terminated (ASCIZ) string, will be null padded.
3104 b A bit string (ascending bit order inside each byte, like vec()).
3105 B A bit string (descending bit order inside each byte).
3106 h A hex string (low nybble first).
3107 H A hex string (high nybble first).
3109 c A signed char value.
3110 C An unsigned char value. Only does bytes. See U for Unicode.
3112 s A signed short value.
3113 S An unsigned short value.
3114 (This 'short' is _exactly_ 16 bits, which may differ from
3115 what a local C compiler calls 'short'. If you want
3116 native-length shorts, use the '!' suffix.)
3118 i A signed integer value.
3119 I An unsigned integer value.
3120 (This 'integer' is _at_least_ 32 bits wide. Its exact
3121 size depends on what a local C compiler calls 'int',
3122 and may even be larger than the 'long' described in
3125 l A signed long value.
3126 L An unsigned long value.
3127 (This 'long' is _exactly_ 32 bits, which may differ from
3128 what a local C compiler calls 'long'. If you want
3129 native-length longs, use the '!' suffix.)
3131 n An unsigned short in "network" (big-endian) order.
3132 N An unsigned long in "network" (big-endian) order.
3133 v An unsigned short in "VAX" (little-endian) order.
3134 V An unsigned long in "VAX" (little-endian) order.
3135 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3136 _exactly_ 32 bits, respectively.)
3138 q A signed quad (64-bit) value.
3139 Q An unsigned quad value.
3140 (Quads are available only if your system supports 64-bit
3141 integer values _and_ if Perl has been compiled to support those.
3142 Causes a fatal error otherwise.)
3144 f A single-precision float in the native format.
3145 d A double-precision float in the native format.
3147 p A pointer to a null-terminated string.
3148 P A pointer to a structure (fixed-length string).
3150 u A uuencoded string.
3151 U A Unicode character number. Encodes to UTF-8 internally
3152 (or UTF-EBCDIC in EBCDIC platforms).
3154 w A BER compressed integer. Its bytes represent an unsigned
3155 integer in base 128, most significant digit first, with as
3156 few digits as possible. Bit eight (the high bit) is set
3157 on each byte except the last.
3161 @ Null fill to absolute position.
3163 The following rules apply:
3169 Each letter may optionally be followed by a number giving a repeat
3170 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3171 C<H>, and C<P> the pack function will gobble up that many values from
3172 the LIST. A C<*> for the repeat count means to use however many items are
3173 left, except for C<@>, C<x>, C<X>, where it is equivalent
3174 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3177 When used with C<Z>, C<*> results in the addition of a trailing null
3178 byte (so the packed result will be one longer than the byte C<length>
3181 The repeat count for C<u> is interpreted as the maximal number of bytes
3182 to encode per line of output, with 0 and 1 replaced by 45.
3186 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3187 string of length count, padding with nulls or spaces as necessary. When
3188 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3189 after the first null, and C<a> returns data verbatim. When packing,
3190 C<a>, and C<Z> are equivalent.
3192 If the value-to-pack is too long, it is truncated. If too long and an
3193 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3194 by a null byte. Thus C<Z> always packs a trailing null byte under
3199 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3200 Each byte of the input field of pack() generates 1 bit of the result.
3201 Each result bit is based on the least-significant bit of the corresponding
3202 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3203 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3205 Starting from the beginning of the input string of pack(), each 8-tuple
3206 of bytes is converted to 1 byte of output. With format C<b>
3207 the first byte of the 8-tuple determines the least-significant bit of a
3208 byte, and with format C<B> it determines the most-significant bit of
3211 If the length of the input string is not exactly divisible by 8, the
3212 remainder is packed as if the input string were padded by null bytes
3213 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3215 If the input string of pack() is longer than needed, extra bytes are ignored.
3216 A C<*> for the repeat count of pack() means to use all the bytes of
3217 the input field. On unpack()ing the bits are converted to a string
3218 of C<"0">s and C<"1">s.
3222 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3223 representable as hexadecimal digits, 0-9a-f) long.
3225 Each byte of the input field of pack() generates 4 bits of the result.
3226 For non-alphabetical bytes the result is based on the 4 least-significant
3227 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3228 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3229 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3230 is compatible with the usual hexadecimal digits, so that C<"a"> and
3231 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3232 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3234 Starting from the beginning of the input string of pack(), each pair
3235 of bytes is converted to 1 byte of output. With format C<h> the
3236 first byte of the pair determines the least-significant nybble of the
3237 output byte, and with format C<H> it determines the most-significant
3240 If the length of the input string is not even, it behaves as if padded
3241 by a null byte at the end. Similarly, during unpack()ing the "extra"
3242 nybbles are ignored.
3244 If the input string of pack() is longer than needed, extra bytes are ignored.
3245 A C<*> for the repeat count of pack() means to use all the bytes of
3246 the input field. On unpack()ing the bits are converted to a string
3247 of hexadecimal digits.
3251 The C<p> type packs a pointer to a null-terminated string. You are
3252 responsible for ensuring the string is not a temporary value (which can
3253 potentially get deallocated before you get around to using the packed result).
3254 The C<P> type packs a pointer to a structure of the size indicated by the
3255 length. A NULL pointer is created if the corresponding value for C<p> or
3256 C<P> is C<undef>, similarly for unpack().
3260 The C</> template character allows packing and unpacking of strings where
3261 the packed structure contains a byte count followed by the string itself.
3262 You write I<length-item>C</>I<string-item>.
3264 The I<length-item> can be any C<pack> template letter,
3265 and describes how the length value is packed.
3266 The ones likely to be of most use are integer-packing ones like
3267 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3268 and C<N> (for Sun XDR).
3270 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3271 For C<unpack> the length of the string is obtained from the I<length-item>,
3272 but if you put in the '*' it will be ignored.
3274 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3275 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3276 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3278 The I<length-item> is not returned explicitly from C<unpack>.
3280 Adding a count to the I<length-item> letter is unlikely to do anything
3281 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3282 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3283 which Perl does not regard as legal in numeric strings.
3287 The integer types C<s>, C<S>, C<l>, and C<L> may be
3288 immediately followed by a C<!> suffix to signify native shorts or
3289 longs--as you can see from above for example a bare C<l> does mean
3290 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3291 may be larger. This is an issue mainly in 64-bit platforms. You can
3292 see whether using C<!> makes any difference by
3294 print length(pack("s")), " ", length(pack("s!")), "\n";
3295 print length(pack("l")), " ", length(pack("l!")), "\n";
3297 C<i!> and C<I!> also work but only because of completeness;
3298 they are identical to C<i> and C<I>.
3300 The actual sizes (in bytes) of native shorts, ints, longs, and long
3301 longs on the platform where Perl was built are also available via
3305 print $Config{shortsize}, "\n";
3306 print $Config{intsize}, "\n";
3307 print $Config{longsize}, "\n";
3308 print $Config{longlongsize}, "\n";
3310 (The C<$Config{longlongsize}> will be undefine if your system does
3311 not support long longs.)
3315 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3316 are inherently non-portable between processors and operating systems
3317 because they obey the native byteorder and endianness. For example a
3318 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3319 (arranged in and handled by the CPU registers) into bytes as
3321 0x12 0x34 0x56 0x78 # big-endian
3322 0x78 0x56 0x34 0x12 # little-endian
3324 Basically, the Intel and VAX CPUs are little-endian, while everybody
3325 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3326 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3327 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3330 The names `big-endian' and `little-endian' are comic references to
3331 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3332 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3333 the egg-eating habits of the Lilliputians.
3335 Some systems may have even weirder byte orders such as
3340 You can see your system's preference with
3342 print join(" ", map { sprintf "%#02x", $_ }
3343 unpack("C*",pack("L",0x12345678))), "\n";
3345 The byteorder on the platform where Perl was built is also available
3349 print $Config{byteorder}, "\n";
3351 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3352 and C<'87654321'> are big-endian.
3354 If you want portable packed integers use the formats C<n>, C<N>,
3355 C<v>, and C<V>, their byte endianness and size are known.
3356 See also L<perlport>.
3360 Real numbers (floats and doubles) are in the native machine format only;
3361 due to the multiplicity of floating formats around, and the lack of a
3362 standard "network" representation, no facility for interchange has been
3363 made. This means that packed floating point data written on one machine
3364 may not be readable on another - even if both use IEEE floating point
3365 arithmetic (as the endian-ness of the memory representation is not part
3366 of the IEEE spec). See also L<perlport>.
3368 Note that Perl uses doubles internally for all numeric calculation, and
3369 converting from double into float and thence back to double again will
3370 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3375 If the pattern begins with a C<U>, the resulting string will be treated
3376 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3377 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3378 characters. If you don't want this to happen, you can begin your pattern
3379 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3380 string, and then follow this with a C<U*> somewhere in your pattern.
3384 You must yourself do any alignment or padding by inserting for example
3385 enough C<'x'>es while packing. There is no way to pack() and unpack()
3386 could know where the bytes are going to or coming from. Therefore
3387 C<pack> (and C<unpack>) handle their output and input as flat
3392 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3396 If TEMPLATE requires more arguments to pack() than actually given, pack()
3397 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3398 to pack() than actually given, extra arguments are ignored.
3404 $foo = pack("CCCC",65,66,67,68);
3406 $foo = pack("C4",65,66,67,68);
3408 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3409 # same thing with Unicode circled letters
3411 $foo = pack("ccxxcc",65,66,67,68);
3414 # note: the above examples featuring "C" and "c" are true
3415 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3416 # and UTF-8. In EBCDIC the first example would be
3417 # $foo = pack("CCCC",193,194,195,196);
3419 $foo = pack("s2",1,2);
3420 # "\1\0\2\0" on little-endian
3421 # "\0\1\0\2" on big-endian
3423 $foo = pack("a4","abcd","x","y","z");
3426 $foo = pack("aaaa","abcd","x","y","z");
3429 $foo = pack("a14","abcdefg");
3430 # "abcdefg\0\0\0\0\0\0\0"
3432 $foo = pack("i9pl", gmtime);
3433 # a real struct tm (on my system anyway)
3435 $utmp_template = "Z8 Z8 Z16 L";
3436 $utmp = pack($utmp_template, @utmp1);
3437 # a struct utmp (BSDish)
3439 @utmp2 = unpack($utmp_template, $utmp);
3440 # "@utmp1" eq "@utmp2"
3443 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3446 $foo = pack('sx2l', 12, 34);
3447 # short 12, two zero bytes padding, long 34
3448 $bar = pack('s@4l', 12, 34);
3449 # short 12, zero fill to position 4, long 34
3452 The same template may generally also be used in unpack().
3454 =item package NAMESPACE
3458 Declares the compilation unit as being in the given namespace. The scope
3459 of the package declaration is from the declaration itself through the end
3460 of the enclosing block, file, or eval (the same as the C<my> operator).
3461 All further unqualified dynamic identifiers will be in this namespace.
3462 A package statement affects only dynamic variables--including those
3463 you've used C<local> on--but I<not> lexical variables, which are created
3464 with C<my>. Typically it would be the first declaration in a file to
3465 be included by the C<require> or C<use> operator. You can switch into a
3466 package in more than one place; it merely influences which symbol table
3467 is used by the compiler for the rest of that block. You can refer to
3468 variables and filehandles in other packages by prefixing the identifier
3469 with the package name and a double colon: C<$Package::Variable>.
3470 If the package name is null, the C<main> package as assumed. That is,
3471 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3472 still seen in older code).
3474 If NAMESPACE is omitted, then there is no current package, and all
3475 identifiers must be fully qualified or lexicals. However, you are
3476 strongly advised not to make use of this feature. Its use can cause
3477 unexpected behaviour, even crashing some versions of Perl. It is
3478 deprecated, and will be removed from a future release.
3480 See L<perlmod/"Packages"> for more information about packages, modules,
3481 and classes. See L<perlsub> for other scoping issues.
3483 =item pipe READHANDLE,WRITEHANDLE
3485 Opens a pair of connected pipes like the corresponding system call.
3486 Note that if you set up a loop of piped processes, deadlock can occur
3487 unless you are very careful. In addition, note that Perl's pipes use
3488 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3489 after each command, depending on the application.
3491 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3492 for examples of such things.
3494 On systems that support a close-on-exec flag on files, the flag will be set
3495 for the newly opened file descriptors as determined by the value of $^F.
3502 Pops and returns the last value of the array, shortening the array by
3503 one element. Has an effect similar to
3507 If there are no elements in the array, returns the undefined value
3508 (although this may happen at other times as well). If ARRAY is
3509 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3510 array in subroutines, just like C<shift>.
3516 Returns the offset of where the last C<m//g> search left off for the variable
3517 in question (C<$_> is used when the variable is not specified). May be
3518 modified to change that offset. Such modification will also influence
3519 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3522 =item print FILEHANDLE LIST
3528 Prints a string or a list of strings. Returns true if successful.
3529 FILEHANDLE may be a scalar variable name, in which case the variable
3530 contains the name of or a reference to the filehandle, thus introducing
3531 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3532 the next token is a term, it may be misinterpreted as an operator
3533 unless you interpose a C<+> or put parentheses around the arguments.)
3534 If FILEHANDLE is omitted, prints by default to standard output (or
3535 to the last selected output channel--see L</select>). If LIST is
3536 also omitted, prints C<$_> to the currently selected output channel.
3537 To set the default output channel to something other than STDOUT
3538 use the select operation. The current value of C<$,> (if any) is
3539 printed between each LIST item. The current value of C<$\> (if
3540 any) is printed after the entire LIST has been printed. Because
3541 print takes a LIST, anything in the LIST is evaluated in list
3542 context, and any subroutine that you call will have one or more of
3543 its expressions evaluated in list context. Also be careful not to
3544 follow the print keyword with a left parenthesis unless you want
3545 the corresponding right parenthesis to terminate the arguments to
3546 the print--interpose a C<+> or put parentheses around all the
3549 Note that if you're storing FILEHANDLES in an array or other expression,
3550 you will have to use a block returning its value instead:
3552 print { $files[$i] } "stuff\n";
3553 print { $OK ? STDOUT : STDERR } "stuff\n";
3555 =item printf FILEHANDLE FORMAT, LIST
3557 =item printf FORMAT, LIST
3559 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3560 (the output record separator) is not appended. The first argument
3561 of the list will be interpreted as the C<printf> format. See C<sprintf>
3562 for an explanation of the format argument. If C<use locale> is in effect,
3563 the character used for the decimal point in formatted real numbers is
3564 affected by the LC_NUMERIC locale. See L<perllocale>.
3566 Don't fall into the trap of using a C<printf> when a simple
3567 C<print> would do. The C<print> is more efficient and less
3570 =item prototype FUNCTION
3572 Returns the prototype of a function as a string (or C<undef> if the
3573 function has no prototype). FUNCTION is a reference to, or the name of,
3574 the function whose prototype you want to retrieve.
3576 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3577 name for Perl builtin. If the builtin is not I<overridable> (such as
3578 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3579 C<system>) returns C<undef> because the builtin does not really behave
3580 like a Perl function. Otherwise, the string describing the equivalent
3581 prototype is returned.
3583 =item push ARRAY,LIST
3585 Treats ARRAY as a stack, and pushes the values of LIST
3586 onto the end of ARRAY. The length of ARRAY increases by the length of
3587 LIST. Has the same effect as
3590 $ARRAY[++$#ARRAY] = $value;
3593 but is more efficient. Returns the new number of elements in the array.
3605 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3607 =item quotemeta EXPR
3611 Returns the value of EXPR with all non-"word"
3612 characters backslashed. (That is, all characters not matching
3613 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3614 returned string, regardless of any locale settings.)
3615 This is the internal function implementing
3616 the C<\Q> escape in double-quoted strings.
3618 If EXPR is omitted, uses C<$_>.
3624 Returns a random fractional number greater than or equal to C<0> and less
3625 than the value of EXPR. (EXPR should be positive.) If EXPR is
3626 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3627 unless C<srand> has already been called. See also C<srand>.
3629 Apply C<int()> to the value returned by C<rand()> if you want random
3630 integers instead of random fractional numbers. For example,
3634 returns a random integer between C<0> and C<9>, inclusive.
3636 (Note: If your rand function consistently returns numbers that are too
3637 large or too small, then your version of Perl was probably compiled
3638 with the wrong number of RANDBITS.)
3640 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3642 =item read FILEHANDLE,SCALAR,LENGTH
3644 Attempts to read LENGTH I<characters> of data into variable SCALAR
3645 from the specified FILEHANDLE. Returns the number of characters
3646 actually read, C<0> at end of file, or undef if there was an error.
3647 SCALAR will be grown or shrunk to the length actually read. If SCALAR
3648 needs growing, the new bytes will be zero bytes. An OFFSET may be
3649 specified to place the read data into some other place in SCALAR than
3650 the beginning. The call is actually implemented in terms of either
3651 Perl's or system's fread() call. To get a true read(2) system call,
3654 Note the I<characters>: depending on the status of the filehandle,
3655 either (8-bit) bytes or characters are read. By default all
3656 filehandles operate on bytes, but for example if the filehandle has
3657 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
3658 pragma, L<open>), the I/O will operate on characters, not bytes.
3660 =item readdir DIRHANDLE
3662 Returns the next directory entry for a directory opened by C<opendir>.
3663 If used in list context, returns all the rest of the entries in the
3664 directory. If there are no more entries, returns an undefined value in
3665 scalar context or a null list in list context.
3667 If you're planning to filetest the return values out of a C<readdir>, you'd
3668 better prepend the directory in question. Otherwise, because we didn't
3669 C<chdir> there, it would have been testing the wrong file.
3671 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3672 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3677 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3678 context, each call reads and returns the next line, until end-of-file is
3679 reached, whereupon the subsequent call returns undef. In list context,
3680 reads until end-of-file is reached and returns a list of lines. Note that
3681 the notion of "line" used here is however you may have defined it
3682 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3684 When C<$/> is set to C<undef>, when readline() is in scalar
3685 context (i.e. file slurp mode), and when an empty file is read, it
3686 returns C<''> the first time, followed by C<undef> subsequently.
3688 This is the internal function implementing the C<< <EXPR> >>
3689 operator, but you can use it directly. The C<< <EXPR> >>
3690 operator is discussed in more detail in L<perlop/"I/O Operators">.
3693 $line = readline(*STDIN); # same thing
3699 Returns the value of a symbolic link, if symbolic links are
3700 implemented. If not, gives a fatal error. If there is some system
3701 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3702 omitted, uses C<$_>.
3706 EXPR is executed as a system command.
3707 The collected standard output of the command is returned.
3708 In scalar context, it comes back as a single (potentially
3709 multi-line) string. In list context, returns a list of lines
3710 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3711 This is the internal function implementing the C<qx/EXPR/>
3712 operator, but you can use it directly. The C<qx/EXPR/>
3713 operator is discussed in more detail in L<perlop/"I/O Operators">.
3715 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3717 Receives a message on a socket. Attempts to receive LENGTH characters
3718 of data into variable SCALAR from the specified SOCKET filehandle.
3719 SCALAR will be grown or shrunk to the length actually read. Takes the
3720 same flags as the system call of the same name. Returns the address
3721 of the sender if SOCKET's protocol supports this; returns an empty
3722 string otherwise. If there's an error, returns the undefined value.
3723 This call is actually implemented in terms of recvfrom(2) system call.
3724 See L<perlipc/"UDP: Message Passing"> for examples.
3726 Note the I<characters>: depending on the status of the socket, either
3727 (8-bit) bytes or characters are received. By default all sockets
3728 operate on bytes, but for example if the socket has been changed using
3729 binmode() to operate with the C<:utf8> discipline (see the C<open>
3730 pragma, L<open>), the I/O will operate on characters, not bytes.
3736 The C<redo> command restarts the loop block without evaluating the
3737 conditional again. The C<continue> block, if any, is not executed. If
3738 the LABEL is omitted, the command refers to the innermost enclosing
3739 loop. This command is normally used by programs that want to lie to
3740 themselves about what was just input:
3742 # a simpleminded Pascal comment stripper
3743 # (warning: assumes no { or } in strings)
3744 LINE: while (<STDIN>) {
3745 while (s|({.*}.*){.*}|$1 |) {}
3750 if (/}/) { # end of comment?
3759 C<redo> cannot be used to retry a block which returns a value such as
3760 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3761 a grep() or map() operation.
3763 Note that a block by itself is semantically identical to a loop
3764 that executes once. Thus C<redo> inside such a block will effectively
3765 turn it into a looping construct.
3767 See also L</continue> for an illustration of how C<last>, C<next>, and
3774 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3775 is not specified, C<$_> will be used. The value returned depends on the
3776 type of thing the reference is a reference to.
3777 Builtin types include:
3787 If the referenced object has been blessed into a package, then that package
3788 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3790 if (ref($r) eq "HASH") {
3791 print "r is a reference to a hash.\n";
3794 print "r is not a reference at all.\n";
3796 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3797 print "r is a reference to something that isa hash.\n";
3800 See also L<perlref>.
3802 =item rename OLDNAME,NEWNAME
3804 Changes the name of a file; an existing file NEWNAME will be
3805 clobbered. Returns true for success, false otherwise.
3807 Behavior of this function varies wildly depending on your system
3808 implementation. For example, it will usually not work across file system
3809 boundaries, even though the system I<mv> command sometimes compensates
3810 for this. Other restrictions include whether it works on directories,
3811 open files, or pre-existing files. Check L<perlport> and either the
3812 rename(2) manpage or equivalent system documentation for details.
3814 =item require VERSION
3820 Demands a version of Perl specified by VERSION, or demands some semantics
3821 specified by EXPR or by C<$_> if EXPR is not supplied.
3823 VERSION may be either a numeric argument such as 5.006, which will be
3824 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3825 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3826 VERSION is greater than the version of the current Perl interpreter.
3827 Compare with L</use>, which can do a similar check at compile time.
3829 Specifying VERSION as a literal of the form v5.6.1 should generally be
3830 avoided, because it leads to misleading error messages under earlier
3831 versions of Perl which do not support this syntax. The equivalent numeric
3832 version should be used instead.
3834 require v5.6.1; # run time version check
3835 require 5.6.1; # ditto
3836 require 5.006_001; # ditto; preferred for backwards compatibility
3838 Otherwise, demands that a library file be included if it hasn't already
3839 been included. The file is included via the do-FILE mechanism, which is
3840 essentially just a variety of C<eval>. Has semantics similar to the following
3845 return 1 if $INC{$filename};
3846 my($realfilename,$result);
3848 foreach $prefix (@INC) {
3849 $realfilename = "$prefix/$filename";
3850 if (-f $realfilename) {
3851 $INC{$filename} = $realfilename;
3852 $result = do $realfilename;
3856 die "Can't find $filename in \@INC";
3858 delete $INC{$filename} if $@ || !$result;
3860 die "$filename did not return true value" unless $result;
3864 Note that the file will not be included twice under the same specified
3865 name. The file must return true as the last statement to indicate
3866 successful execution of any initialization code, so it's customary to
3867 end such a file with C<1;> unless you're sure it'll return true
3868 otherwise. But it's better just to put the C<1;>, in case you add more
3871 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3872 replaces "F<::>" with "F</>" in the filename for you,
3873 to make it easy to load standard modules. This form of loading of
3874 modules does not risk altering your namespace.
3876 In other words, if you try this:
3878 require Foo::Bar; # a splendid bareword
3880 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3881 directories specified in the C<@INC> array.
3883 But if you try this:
3885 $class = 'Foo::Bar';
3886 require $class; # $class is not a bareword
3888 require "Foo::Bar"; # not a bareword because of the ""
3890 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3891 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3893 eval "require $class";
3895 You can also insert hooks into the import facility, by putting directly
3896 Perl code into the @INC array. There are three forms of hooks: subroutine
3897 references, array references and blessed objects.
3899 Subroutine references are the simplest case. When the inclusion system
3900 walks through @INC and encounters a subroutine, this subroutine gets
3901 called with two parameters, the first being a reference to itself, and the
3902 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
3903 subroutine should return C<undef> or a filehandle, from which the file to
3904 include will be read. If C<undef> is returned, C<require> will look at
3905 the remaining elements of @INC.
3907 If the hook is an array reference, its first element must be a subroutine
3908 reference. This subroutine is called as above, but the first parameter is
3909 the array reference. This enables to pass indirectly some arguments to
3912 In other words, you can write:
3914 push @INC, \&my_sub;
3916 my ($coderef, $filename) = @_; # $coderef is \&my_sub
3922 push @INC, [ \&my_sub, $x, $y, ... ];
3924 my ($arrayref, $filename) = @_;
3925 # Retrieve $x, $y, ...
3926 my @parameters = @$arrayref[1..$#$arrayref];
3930 If the hook is an object, it must provide an INC method, that will be
3931 called as above, the first parameter being the object itself. (Note that
3932 you must fully qualify the sub's name, as it is always forced into package
3933 C<main>.) Here is a typical code layout:
3939 my ($self, $filename) = @_;
3943 # In the main program
3944 push @INC, new Foo(...);
3946 Note that these hooks are also permitted to set the %INC entry
3947 corresponding to the files they have loaded. See L<perlvar/%INC>.
3949 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3955 Generally used in a C<continue> block at the end of a loop to clear
3956 variables and reset C<??> searches so that they work again. The
3957 expression is interpreted as a list of single characters (hyphens
3958 allowed for ranges). All variables and arrays beginning with one of
3959 those letters are reset to their pristine state. If the expression is
3960 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3961 only variables or searches in the current package. Always returns
3964 reset 'X'; # reset all X variables
3965 reset 'a-z'; # reset lower case variables
3966 reset; # just reset ?one-time? searches
3968 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3969 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3970 variables--lexical variables are unaffected, but they clean themselves
3971 up on scope exit anyway, so you'll probably want to use them instead.
3978 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3979 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3980 context, depending on how the return value will be used, and the context
3981 may vary from one execution to the next (see C<wantarray>). If no EXPR
3982 is given, returns an empty list in list context, the undefined value in
3983 scalar context, and (of course) nothing at all in a void context.
3985 (Note that in the absence of an explicit C<return>, a subroutine, eval,
3986 or do FILE will automatically return the value of the last expression
3991 In list context, returns a list value consisting of the elements
3992 of LIST in the opposite order. In scalar context, concatenates the
3993 elements of LIST and returns a string value with all characters
3994 in the opposite order.
3996 print reverse <>; # line tac, last line first
3998 undef $/; # for efficiency of <>
3999 print scalar reverse <>; # character tac, last line tsrif
4001 This operator is also handy for inverting a hash, although there are some
4002 caveats. If a value is duplicated in the original hash, only one of those
4003 can be represented as a key in the inverted hash. Also, this has to
4004 unwind one hash and build a whole new one, which may take some time
4005 on a large hash, such as from a DBM file.
4007 %by_name = reverse %by_address; # Invert the hash
4009 =item rewinddir DIRHANDLE
4011 Sets the current position to the beginning of the directory for the
4012 C<readdir> routine on DIRHANDLE.
4014 =item rindex STR,SUBSTR,POSITION
4016 =item rindex STR,SUBSTR
4018 Works just like index() except that it returns the position of the LAST
4019 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4020 last occurrence at or before that position.
4022 =item rmdir FILENAME
4026 Deletes the directory specified by FILENAME if that directory is empty. If it
4027 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4028 FILENAME is omitted, uses C<$_>.
4032 The substitution operator. See L<perlop>.
4036 Forces EXPR to be interpreted in scalar context and returns the value
4039 @counts = ( scalar @a, scalar @b, scalar @c );
4041 There is no equivalent operator to force an expression to
4042 be interpolated in list context because in practice, this is never
4043 needed. If you really wanted to do so, however, you could use
4044 the construction C<@{[ (some expression) ]}>, but usually a simple
4045 C<(some expression)> suffices.
4047 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4048 parenthesized list, this behaves as a scalar comma expression, evaluating
4049 all but the last element in void context and returning the final element
4050 evaluated in scalar context. This is seldom what you want.
4052 The following single statement:
4054 print uc(scalar(&foo,$bar)),$baz;
4056 is the moral equivalent of these two:
4059 print(uc($bar),$baz);
4061 See L<perlop> for more details on unary operators and the comma operator.
4063 =item seek FILEHANDLE,POSITION,WHENCE
4065 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4066 FILEHANDLE may be an expression whose value gives the name of the
4067 filehandle. The values for WHENCE are C<0> to set the new position
4068 I<in bytes> to POSITION, C<1> to set it to the current position plus
4069 POSITION, and C<2> to set it to EOF plus POSITION (typically
4070 negative). For WHENCE you may use the constants C<SEEK_SET>,
4071 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4072 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4075 Note the I<in bytes>: even if the filehandle has been set to
4076 operate on characters (for example by using the C<:utf8> open
4077 discipline), tell() will return byte offsets, not character offsets
4078 (because implementing that would render seek() and tell() rather slow).
4080 If you want to position file for C<sysread> or C<syswrite>, don't use
4081 C<seek>--buffering makes its effect on the file's system position
4082 unpredictable and non-portable. Use C<sysseek> instead.
4084 Due to the rules and rigors of ANSI C, on some systems you have to do a
4085 seek whenever you switch between reading and writing. Amongst other
4086 things, this may have the effect of calling stdio's clearerr(3).
4087 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4091 This is also useful for applications emulating C<tail -f>. Once you hit
4092 EOF on your read, and then sleep for a while, you might have to stick in a
4093 seek() to reset things. The C<seek> doesn't change the current position,
4094 but it I<does> clear the end-of-file condition on the handle, so that the
4095 next C<< <FILE> >> makes Perl try again to read something. We hope.
4097 If that doesn't work (some IO implementations are particularly
4098 cantankerous), then you may need something more like this:
4101 for ($curpos = tell(FILE); $_ = <FILE>;
4102 $curpos = tell(FILE)) {
4103 # search for some stuff and put it into files
4105 sleep($for_a_while);
4106 seek(FILE, $curpos, 0);
4109 =item seekdir DIRHANDLE,POS
4111 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4112 must be a value returned by C<telldir>. Has the same caveats about
4113 possible directory compaction as the corresponding system library
4116 =item select FILEHANDLE
4120 Returns the currently selected filehandle. Sets the current default
4121 filehandle for output, if FILEHANDLE is supplied. This has two
4122 effects: first, a C<write> or a C<print> without a filehandle will
4123 default to this FILEHANDLE. Second, references to variables related to
4124 output will refer to this output channel. For example, if you have to
4125 set the top of form format for more than one output channel, you might
4133 FILEHANDLE may be an expression whose value gives the name of the
4134 actual filehandle. Thus:
4136 $oldfh = select(STDERR); $| = 1; select($oldfh);
4138 Some programmers may prefer to think of filehandles as objects with
4139 methods, preferring to write the last example as:
4142 STDERR->autoflush(1);
4144 =item select RBITS,WBITS,EBITS,TIMEOUT
4146 This calls the select(2) system call with the bit masks specified, which
4147 can be constructed using C<fileno> and C<vec>, along these lines:
4149 $rin = $win = $ein = '';
4150 vec($rin,fileno(STDIN),1) = 1;
4151 vec($win,fileno(STDOUT),1) = 1;
4154 If you want to select on many filehandles you might wish to write a
4158 my(@fhlist) = split(' ',$_[0]);
4161 vec($bits,fileno($_),1) = 1;
4165 $rin = fhbits('STDIN TTY SOCK');
4169 ($nfound,$timeleft) =
4170 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4172 or to block until something becomes ready just do this
4174 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4176 Most systems do not bother to return anything useful in $timeleft, so
4177 calling select() in scalar context just returns $nfound.
4179 Any of the bit masks can also be undef. The timeout, if specified, is
4180 in seconds, which may be fractional. Note: not all implementations are
4181 capable of returning the $timeleft. If not, they always return
4182 $timeleft equal to the supplied $timeout.
4184 You can effect a sleep of 250 milliseconds this way:
4186 select(undef, undef, undef, 0.25);
4188 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4189 or <FH>) with C<select>, except as permitted by POSIX, and even
4190 then only on POSIX systems. You have to use C<sysread> instead.
4192 =item semctl ID,SEMNUM,CMD,ARG
4194 Calls the System V IPC function C<semctl>. You'll probably have to say
4198 first to get the correct constant definitions. If CMD is IPC_STAT or
4199 GETALL, then ARG must be a variable which will hold the returned
4200 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4201 the undefined value for error, "C<0 but true>" for zero, or the actual
4202 return value otherwise. The ARG must consist of a vector of native
4203 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4204 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4207 =item semget KEY,NSEMS,FLAGS
4209 Calls the System V IPC function semget. Returns the semaphore id, or
4210 the undefined value if there is an error. See also
4211 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4214 =item semop KEY,OPSTRING
4216 Calls the System V IPC function semop to perform semaphore operations
4217 such as signalling and waiting. OPSTRING must be a packed array of
4218 semop structures. Each semop structure can be generated with
4219 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4220 operations is implied by the length of OPSTRING. Returns true if
4221 successful, or false if there is an error. As an example, the
4222 following code waits on semaphore $semnum of semaphore id $semid:
4224 $semop = pack("s!3", $semnum, -1, 0);
4225 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4227 To signal the semaphore, replace C<-1> with C<1>. See also
4228 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4231 =item send SOCKET,MSG,FLAGS,TO
4233 =item send SOCKET,MSG,FLAGS
4235 Sends a message on a socket. Attemps to send the scalar MSG to the
4236 SOCKET filehandle. Takes the same flags as the system call of the
4237 same name. On unconnected sockets you must specify a destination to
4238 send TO, in which case it does a C C<sendto>. Returns the number of
4239 characters sent, or the undefined value if there is an error. The C
4240 system call sendmsg(2) is currently unimplemented. See
4241 L<perlipc/"UDP: Message Passing"> for examples.
4243 Note the I<characters>: depending on the status of the socket, either
4244 (8-bit) bytes or characters are sent. By default all sockets operate
4245 on bytes, but for example if the socket has been changed using
4246 binmode() to operate with the C<:utf8> discipline (see L</open>, or
4247 the C<open> pragma, L<open>), the I/O will operate on characters, not
4250 =item setpgrp PID,PGRP
4252 Sets the current process group for the specified PID, C<0> for the current
4253 process. Will produce a fatal error if used on a machine that doesn't
4254 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4255 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4256 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4259 =item setpriority WHICH,WHO,PRIORITY
4261 Sets the current priority for a process, a process group, or a user.
4262 (See setpriority(2).) Will produce a fatal error if used on a machine
4263 that doesn't implement setpriority(2).
4265 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4267 Sets the socket option requested. Returns undefined if there is an
4268 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4275 Shifts the first value of the array off and returns it, shortening the
4276 array by 1 and moving everything down. If there are no elements in the
4277 array, returns the undefined value. If ARRAY is omitted, shifts the
4278 C<@_> array within the lexical scope of subroutines and formats, and the
4279 C<@ARGV> array at file scopes or within the lexical scopes established by
4280 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4283 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4284 same thing to the left end of an array that C<pop> and C<push> do to the
4287 =item shmctl ID,CMD,ARG
4289 Calls the System V IPC function shmctl. You'll probably have to say
4293 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4294 then ARG must be a variable which will hold the returned C<shmid_ds>
4295 structure. Returns like ioctl: the undefined value for error, "C<0> but
4296 true" for zero, or the actual return value otherwise.
4297 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4299 =item shmget KEY,SIZE,FLAGS
4301 Calls the System V IPC function shmget. Returns the shared memory
4302 segment id, or the undefined value if there is an error.
4303 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4305 =item shmread ID,VAR,POS,SIZE
4307 =item shmwrite ID,STRING,POS,SIZE
4309 Reads or writes the System V shared memory segment ID starting at
4310 position POS for size SIZE by attaching to it, copying in/out, and
4311 detaching from it. When reading, VAR must be a variable that will
4312 hold the data read. When writing, if STRING is too long, only SIZE
4313 bytes are used; if STRING is too short, nulls are written to fill out
4314 SIZE bytes. Return true if successful, or false if there is an error.
4315 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4316 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4318 =item shutdown SOCKET,HOW
4320 Shuts down a socket connection in the manner indicated by HOW, which
4321 has the same interpretation as in the system call of the same name.
4323 shutdown(SOCKET, 0); # I/we have stopped reading data
4324 shutdown(SOCKET, 1); # I/we have stopped writing data
4325 shutdown(SOCKET, 2); # I/we have stopped using this socket
4327 This is useful with sockets when you want to tell the other
4328 side you're done writing but not done reading, or vice versa.
4329 It's also a more insistent form of close because it also
4330 disables the file descriptor in any forked copies in other
4337 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4338 returns sine of C<$_>.
4340 For the inverse sine operation, you may use the C<Math::Trig::asin>
4341 function, or use this relation:
4343 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4349 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4350 May be interrupted if the process receives a signal such as C<SIGALRM>.
4351 Returns the number of seconds actually slept. You probably cannot
4352 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4355 On some older systems, it may sleep up to a full second less than what
4356 you requested, depending on how it counts seconds. Most modern systems
4357 always sleep the full amount. They may appear to sleep longer than that,
4358 however, because your process might not be scheduled right away in a
4359 busy multitasking system.
4361 For delays of finer granularity than one second, you may use Perl's
4362 C<syscall> interface to access setitimer(2) if your system supports
4363 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4364 and starting from Perl 5.8 part of the standard distribution) may also
4367 See also the POSIX module's C<pause> function.
4369 =item sockatmark SOCKET
4371 Returns true if the socket is positioned at the out-of-band mark
4372 (also known as the urgent data mark), false otherwise. Use right
4373 after reading from the socket.
4375 Not available directly, one has to import the function from
4376 the IO::Socket extension
4378 use IO::Socket 'sockatmark';
4380 Even this doesn't guarantee that sockatmark() really is available,
4381 though, because sockatmark() is a relatively recent addition to
4382 the family of socket functions. If it is unavailable, attempt to
4385 IO::Socket::atmark not implemented on this architecture ...
4387 See also L<IO::Socket>.
4389 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4391 Opens a socket of the specified kind and attaches it to filehandle
4392 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4393 the system call of the same name. You should C<use Socket> first
4394 to get the proper definitions imported. See the examples in
4395 L<perlipc/"Sockets: Client/Server Communication">.
4397 On systems that support a close-on-exec flag on files, the flag will
4398 be set for the newly opened file descriptor, as determined by the
4399 value of $^F. See L<perlvar/$^F>.
4401 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4403 Creates an unnamed pair of sockets in the specified domain, of the
4404 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4405 for the system call of the same name. If unimplemented, yields a fatal
4406 error. Returns true if successful.
4408 On systems that support a close-on-exec flag on files, the flag will
4409 be set for the newly opened file descriptors, as determined by the value
4410 of $^F. See L<perlvar/$^F>.
4412 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4413 to C<pipe(Rdr, Wtr)> is essentially:
4416 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4417 shutdown(Rdr, 1); # no more writing for reader
4418 shutdown(Wtr, 0); # no more reading for writer
4420 See L<perlipc> for an example of socketpair use.
4422 =item sort SUBNAME LIST
4424 =item sort BLOCK LIST
4428 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4429 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4430 specified, it gives the name of a subroutine that returns an integer
4431 less than, equal to, or greater than C<0>, depending on how the elements
4432 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4433 operators are extremely useful in such routines.) SUBNAME may be a
4434 scalar variable name (unsubscripted), in which case the value provides
4435 the name of (or a reference to) the actual subroutine to use. In place
4436 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4439 If the subroutine's prototype is C<($$)>, the elements to be compared
4440 are passed by reference in C<@_>, as for a normal subroutine. This is
4441 slower than unprototyped subroutines, where the elements to be
4442 compared are passed into the subroutine
4443 as the package global variables $a and $b (see example below). Note that
4444 in the latter case, it is usually counter-productive to declare $a and
4447 In either case, the subroutine may not be recursive. The values to be
4448 compared are always passed by reference, so don't modify them.
4450 You also cannot exit out of the sort block or subroutine using any of the
4451 loop control operators described in L<perlsyn> or with C<goto>.
4453 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4454 current collation locale. See L<perllocale>.
4456 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4457 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4458 preserves the input order of elements that compare equal. Although
4459 quicksort's run time is O(NlogN) when averaged over all arrays of
4460 length N, the time can be O(N**2), I<quadratic> behavior, for some
4461 inputs.) In 5.7, the quicksort implementation was replaced with
4462 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4463 But benchmarks indicated that for some inputs, on some platforms,
4464 the original quicksort was faster. 5.8 has a sort pragma for
4465 limited control of the sort. Its rather blunt control of the
4466 underlying algorithm may not persist into future perls, but the
4467 ability to characterize the input or output in implementation
4468 independent ways quite probably will. See L</use>.
4473 @articles = sort @files;
4475 # same thing, but with explicit sort routine
4476 @articles = sort {$a cmp $b} @files;
4478 # now case-insensitively
4479 @articles = sort {uc($a) cmp uc($b)} @files;
4481 # same thing in reversed order
4482 @articles = sort {$b cmp $a} @files;
4484 # sort numerically ascending
4485 @articles = sort {$a <=> $b} @files;
4487 # sort numerically descending
4488 @articles = sort {$b <=> $a} @files;
4490 # this sorts the %age hash by value instead of key
4491 # using an in-line function
4492 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4494 # sort using explicit subroutine name
4496 $age{$a} <=> $age{$b}; # presuming numeric
4498 @sortedclass = sort byage @class;
4500 sub backwards { $b cmp $a }
4501 @harry = qw(dog cat x Cain Abel);
4502 @george = qw(gone chased yz Punished Axed);
4504 # prints AbelCaincatdogx
4505 print sort backwards @harry;
4506 # prints xdogcatCainAbel
4507 print sort @george, 'to', @harry;
4508 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4510 # inefficiently sort by descending numeric compare using
4511 # the first integer after the first = sign, or the
4512 # whole record case-insensitively otherwise
4515 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4520 # same thing, but much more efficiently;
4521 # we'll build auxiliary indices instead
4525 push @nums, /=(\d+)/;
4530 $nums[$b] <=> $nums[$a]
4532 $caps[$a] cmp $caps[$b]
4536 # same thing, but without any temps
4537 @new = map { $_->[0] }
4538 sort { $b->[1] <=> $a->[1]
4541 } map { [$_, /=(\d+)/, uc($_)] } @old;
4543 # using a prototype allows you to use any comparison subroutine
4544 # as a sort subroutine (including other package's subroutines)
4546 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4549 @new = sort other::backwards @old;
4551 # guarantee stability, regardless of algorithm
4553 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4555 # force use of quicksort (not portable outside Perl 5.8)
4556 use sort '_quicksort'; # note discouraging _
4557 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4559 # similar to the previous example, but demand stability as well
4560 use sort qw( _mergesort stable );
4561 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4563 If you're using strict, you I<must not> declare $a
4564 and $b as lexicals. They are package globals. That means
4565 if you're in the C<main> package and type
4567 @articles = sort {$b <=> $a} @files;
4569 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4570 but if you're in the C<FooPack> package, it's the same as typing
4572 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4574 The comparison function is required to behave. If it returns
4575 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4576 sometimes saying the opposite, for example) the results are not
4579 =item splice ARRAY,OFFSET,LENGTH,LIST
4581 =item splice ARRAY,OFFSET,LENGTH
4583 =item splice ARRAY,OFFSET
4587 Removes the elements designated by OFFSET and LENGTH from an array, and
4588 replaces them with the elements of LIST, if any. In list context,
4589 returns the elements removed from the array. In scalar context,
4590 returns the last element removed, or C<undef> if no elements are
4591 removed. The array grows or shrinks as necessary.
4592 If OFFSET is negative then it starts that far from the end of the array.
4593 If LENGTH is omitted, removes everything from OFFSET onward.
4594 If LENGTH is negative, leaves that many elements off the end of the array.
4595 If both OFFSET and LENGTH are omitted, removes everything.
4597 The following equivalences hold (assuming C<$[ == 0>):
4599 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4600 pop(@a) splice(@a,-1)
4601 shift(@a) splice(@a,0,1)
4602 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4603 $a[$x] = $y splice(@a,$x,1,$y)
4605 Example, assuming array lengths are passed before arrays:
4607 sub aeq { # compare two list values
4608 my(@a) = splice(@_,0,shift);
4609 my(@b) = splice(@_,0,shift);
4610 return 0 unless @a == @b; # same len?
4612 return 0 if pop(@a) ne pop(@b);
4616 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4618 =item split /PATTERN/,EXPR,LIMIT
4620 =item split /PATTERN/,EXPR
4622 =item split /PATTERN/
4626 Splits a string into a list of strings and returns that list. By default,
4627 empty leading fields are preserved, and empty trailing ones are deleted.
4629 In scalar context, returns the number of fields found and splits into
4630 the C<@_> array. Use of split in scalar context is deprecated, however,
4631 because it clobbers your subroutine arguments.
4633 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4634 splits on whitespace (after skipping any leading whitespace). Anything
4635 matching PATTERN is taken to be a delimiter separating the fields. (Note
4636 that the delimiter may be longer than one character.)
4638 If LIMIT is specified and positive, it represents the maximum number
4639 of fields the EXPR will be split into, though the actual number of
4640 fields returned depends on the number of times PATTERN matches within
4641 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4642 stripped (which potential users of C<pop> would do well to remember).
4643 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4644 had been specified. Note that splitting an EXPR that evaluates to the
4645 empty string always returns the empty list, regardless of the LIMIT
4648 A pattern matching the null string (not to be confused with
4649 a null pattern C<//>, which is just one member of the set of patterns
4650 matching a null string) will split the value of EXPR into separate
4651 characters at each point it matches that way. For example:
4653 print join(':', split(/ */, 'hi there'));
4655 produces the output 'h:i:t:h:e:r:e'.
4657 Using the empty pattern C<//> specifically matches the null string, and is
4658 not be confused with the use of C<//> to mean "the last successful pattern
4661 Empty leading (or trailing) fields are produced when there positive width
4662 matches at the beginning (or end) of the string; a zero-width match at the
4663 beginning (or end) of the string does not produce an empty field. For
4666 print join(':', split(/(?=\w)/, 'hi there!'));
4668 produces the output 'h:i :t:h:e:r:e!'.
4670 The LIMIT parameter can be used to split a line partially
4672 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4674 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4675 one larger than the number of variables in the list, to avoid
4676 unnecessary work. For the list above LIMIT would have been 4 by
4677 default. In time critical applications it behooves you not to split
4678 into more fields than you really need.
4680 If the PATTERN contains parentheses, additional list elements are
4681 created from each matching substring in the delimiter.
4683 split(/([,-])/, "1-10,20", 3);
4685 produces the list value
4687 (1, '-', 10, ',', 20)
4689 If you had the entire header of a normal Unix email message in $header,
4690 you could split it up into fields and their values this way:
4692 $header =~ s/\n\s+/ /g; # fix continuation lines
4693 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4695 The pattern C</PATTERN/> may be replaced with an expression to specify
4696 patterns that vary at runtime. (To do runtime compilation only once,
4697 use C</$variable/o>.)
4699 As a special case, specifying a PATTERN of space (C<' '>) will split on
4700 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4701 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4702 will give you as many null initial fields as there are leading spaces.
4703 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4704 whitespace produces a null first field. A C<split> with no arguments
4705 really does a C<split(' ', $_)> internally.
4707 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4712 open(PASSWD, '/etc/passwd');
4715 ($login, $passwd, $uid, $gid,
4716 $gcos, $home, $shell) = split(/:/);
4720 As with regular pattern matching, any capturing parentheses that are not
4721 matched in a C<split()> will be set to C<undef> when returned:
4723 @fields = split /(A)|B/, "1A2B3";
4724 # @fields is (1, 'A', 2, undef, 3)
4726 =item sprintf FORMAT, LIST
4728 Returns a string formatted by the usual C<printf> conventions of the C
4729 library function C<sprintf>. See below for more details
4730 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4731 the general principles.
4735 # Format number with up to 8 leading zeroes
4736 $result = sprintf("%08d", $number);
4738 # Round number to 3 digits after decimal point
4739 $rounded = sprintf("%.3f", $number);
4741 Perl does its own C<sprintf> formatting--it emulates the C
4742 function C<sprintf>, but it doesn't use it (except for floating-point
4743 numbers, and even then only the standard modifiers are allowed). As a
4744 result, any non-standard extensions in your local C<sprintf> are not
4745 available from Perl.
4747 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4748 pass it an array as your first argument. The array is given scalar context,
4749 and instead of using the 0th element of the array as the format, Perl will
4750 use the count of elements in the array as the format, which is almost never
4753 Perl's C<sprintf> permits the following universally-known conversions:
4756 %c a character with the given number
4758 %d a signed integer, in decimal
4759 %u an unsigned integer, in decimal
4760 %o an unsigned integer, in octal
4761 %x an unsigned integer, in hexadecimal
4762 %e a floating-point number, in scientific notation
4763 %f a floating-point number, in fixed decimal notation
4764 %g a floating-point number, in %e or %f notation
4766 In addition, Perl permits the following widely-supported conversions:
4768 %X like %x, but using upper-case letters
4769 %E like %e, but using an upper-case "E"
4770 %G like %g, but with an upper-case "E" (if applicable)
4771 %b an unsigned integer, in binary
4772 %p a pointer (outputs the Perl value's address in hexadecimal)
4773 %n special: *stores* the number of characters output so far
4774 into the next variable in the parameter list
4776 Finally, for backward (and we do mean "backward") compatibility, Perl
4777 permits these unnecessary but widely-supported conversions:
4780 %D a synonym for %ld
4781 %U a synonym for %lu
4782 %O a synonym for %lo
4785 Note that the number of exponent digits in the scientific notation by
4786 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4787 exponent less than 100 is system-dependent: it may be three or less
4788 (zero-padded as necessary). In other words, 1.23 times ten to the
4789 99th may be either "1.23e99" or "1.23e099".
4791 Perl permits the following universally-known flags between the C<%>
4792 and the conversion letter:
4794 space prefix positive number with a space
4795 + prefix positive number with a plus sign
4796 - left-justify within the field
4797 0 use zeros, not spaces, to right-justify
4798 # prefix non-zero octal with "0", non-zero hex with "0x"
4799 number minimum field width
4800 .number "precision": digits after decimal point for
4801 floating-point, max length for string, minimum length
4803 l interpret integer as C type "long" or "unsigned long"
4804 h interpret integer as C type "short" or "unsigned short"
4805 If no flags, interpret integer as C type "int" or "unsigned"
4807 Perl supports parameter ordering, in other words, fetching the
4808 parameters in some explicitly specified "random" ordering as opposed
4809 to the default implicit sequential ordering. The syntax is, instead
4810 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4811 where the I<digits> is the wanted index, from one upwards. For example:
4813 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4814 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4816 Note that using the reordering syntax does not interfere with the usual
4817 implicit sequential fetching of the parameters:
4819 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4820 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4821 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4822 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4823 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4825 There are also two Perl-specific flags:
4827 V interpret integer as Perl's standard integer type
4828 v interpret string as a vector of integers, output as
4829 numbers separated either by dots, or by an arbitrary
4830 string received from the argument list when the flag
4833 Where a number would appear in the flags, an asterisk (C<*>) may be
4834 used instead, in which case Perl uses the next item in the parameter
4835 list as the given number (that is, as the field width or precision).
4836 If a field width obtained through C<*> is negative, it has the same
4837 effect as the C<-> flag: left-justification.
4839 The C<v> flag is useful for displaying ordinal values of characters
4840 in arbitrary strings:
4842 printf "version is v%vd\n", $^V; # Perl's version
4843 printf "address is %*vX\n", ":", $addr; # IPv6 address
4844 printf "bits are %*vb\n", " ", $bits; # random bitstring
4846 If C<use locale> is in effect, the character used for the decimal
4847 point in formatted real numbers is affected by the LC_NUMERIC locale.
4850 If Perl understands "quads" (64-bit integers) (this requires
4851 either that the platform natively support quads or that Perl
4852 be specifically compiled to support quads), the characters
4856 print quads, and they may optionally be preceded by
4864 You can find out whether your Perl supports quads via L<Config>:
4867 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4870 If Perl understands "long doubles" (this requires that the platform
4871 support long doubles), the flags
4875 may optionally be preceded by
4883 You can find out whether your Perl supports long doubles via L<Config>:
4886 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4892 Return the square root of EXPR. If EXPR is omitted, returns square
4893 root of C<$_>. Only works on non-negative operands, unless you've
4894 loaded the standard Math::Complex module.
4897 print sqrt(-2); # prints 1.4142135623731i
4903 Sets the random number seed for the C<rand> operator.
4905 The point of the function is to "seed" the C<rand> function so that
4906 C<rand> can produce a different sequence each time you run your
4909 If srand() is not called explicitly, it is called implicitly at the
4910 first use of the C<rand> operator. However, this was not the case in
4911 versions of Perl before 5.004, so if your script will run under older
4912 Perl versions, it should call C<srand>.
4914 Most programs won't even call srand() at all, except those that
4915 need a cryptographically-strong starting point rather than the
4916 generally acceptable default, which is based on time of day,
4917 process ID, and memory allocation, or the F</dev/urandom> device,
4920 You can call srand($seed) with the same $seed to reproduce the
4921 I<same> sequence from rand(), but this is usually reserved for
4922 generating predictable results for testing or debugging.
4923 Otherwise, don't call srand() more than once in your program.
4925 Do B<not> call srand() (i.e. without an argument) more than once in
4926 a script. The internal state of the random number generator should
4927 contain more entropy than can be provided by any seed, so calling
4928 srand() again actually I<loses> randomness.
4930 Most implementations of C<srand> take an integer and will silently
4931 truncate decimal numbers. This means C<srand(42)> will usually
4932 produce the same results as C<srand(42.1)>. To be safe, always pass
4933 C<srand> an integer.
4935 In versions of Perl prior to 5.004 the default seed was just the
4936 current C<time>. This isn't a particularly good seed, so many old
4937 programs supply their own seed value (often C<time ^ $$> or C<time ^
4938 ($$ + ($$ << 15))>), but that isn't necessary any more.
4940 Note that you need something much more random than the default seed for
4941 cryptographic purposes. Checksumming the compressed output of one or more
4942 rapidly changing operating system status programs is the usual method. For
4945 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4947 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4950 Frequently called programs (like CGI scripts) that simply use
4954 for a seed can fall prey to the mathematical property that
4958 one-third of the time. So don't do that.
4960 =item stat FILEHANDLE
4966 Returns a 13-element list giving the status info for a file, either
4967 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4968 it stats C<$_>. Returns a null list if the stat fails. Typically used
4971 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4972 $atime,$mtime,$ctime,$blksize,$blocks)
4975 Not all fields are supported on all filesystem types. Here are the
4976 meaning of the fields:
4978 0 dev device number of filesystem
4980 2 mode file mode (type and permissions)
4981 3 nlink number of (hard) links to the file
4982 4 uid numeric user ID of file's owner
4983 5 gid numeric group ID of file's owner
4984 6 rdev the device identifier (special files only)
4985 7 size total size of file, in bytes
4986 8 atime last access time in seconds since the epoch
4987 9 mtime last modify time in seconds since the epoch
4988 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4989 11 blksize preferred block size for file system I/O
4990 12 blocks actual number of blocks allocated
4992 (The epoch was at 00:00 January 1, 1970 GMT.)
4994 If stat is passed the special filehandle consisting of an underline, no
4995 stat is done, but the current contents of the stat structure from the
4996 last stat or filetest are returned. Example:
4998 if (-x $file && (($d) = stat(_)) && $d < 0) {
4999 print "$file is executable NFS file\n";
5002 (This works on machines only for which the device number is negative
5005 Because the mode contains both the file type and its permissions, you
5006 should mask off the file type portion and (s)printf using a C<"%o">
5007 if you want to see the real permissions.
5009 $mode = (stat($filename))[2];
5010 printf "Permissions are %04o\n", $mode & 07777;
5012 In scalar context, C<stat> returns a boolean value indicating success
5013 or failure, and, if successful, sets the information associated with
5014 the special filehandle C<_>.
5016 The File::stat module provides a convenient, by-name access mechanism:
5019 $sb = stat($filename);
5020 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5021 $filename, $sb->size, $sb->mode & 07777,
5022 scalar localtime $sb->mtime;
5024 You can import symbolic mode constants (C<S_IF*>) and functions
5025 (C<S_IS*>) from the Fcntl module:
5029 $mode = (stat($filename))[2];
5031 $user_rwx = ($mode & S_IRWXU) >> 6;
5032 $group_read = ($mode & S_IRGRP) >> 3;
5033 $other_execute = $mode & S_IXOTH;
5035 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
5037 $is_setuid = $mode & S_ISUID;
5038 $is_setgid = S_ISDIR($mode);
5040 You could write the last two using the C<-u> and C<-d> operators.
5041 The commonly available S_IF* constants are
5043 # Permissions: read, write, execute, for user, group, others.
5045 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5046 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5047 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5049 # Setuid/Setgid/Stickiness.
5051 S_ISUID S_ISGID S_ISVTX S_ISTXT
5053 # File types. Not necessarily all are available on your system.
5055 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5057 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5059 S_IREAD S_IWRITE S_IEXEC
5061 and the S_IF* functions are
5063 S_IFMODE($mode) the part of $mode containing the permission bits
5064 and the setuid/setgid/sticky bits
5066 S_IFMT($mode) the part of $mode containing the file type
5067 which can be bit-anded with e.g. S_IFREG
5068 or with the following functions
5070 # The operators -f, -d, -l, -b, -c, -p, and -s.
5072 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5073 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5075 # No direct -X operator counterpart, but for the first one
5076 # the -g operator is often equivalent. The ENFMT stands for
5077 # record flocking enforcement, a platform-dependent feature.
5079 S_ISENFMT($mode) S_ISWHT($mode)
5081 See your native chmod(2) and stat(2) documentation for more details
5082 about the S_* constants.
5088 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5089 doing many pattern matches on the string before it is next modified.
5090 This may or may not save time, depending on the nature and number of
5091 patterns you are searching on, and on the distribution of character
5092 frequencies in the string to be searched--you probably want to compare
5093 run times with and without it to see which runs faster. Those loops
5094 which scan for many short constant strings (including the constant
5095 parts of more complex patterns) will benefit most. You may have only
5096 one C<study> active at a time--if you study a different scalar the first
5097 is "unstudied". (The way C<study> works is this: a linked list of every
5098 character in the string to be searched is made, so we know, for
5099 example, where all the C<'k'> characters are. From each search string,
5100 the rarest character is selected, based on some static frequency tables
5101 constructed from some C programs and English text. Only those places
5102 that contain this "rarest" character are examined.)
5104 For example, here is a loop that inserts index producing entries
5105 before any line containing a certain pattern:
5109 print ".IX foo\n" if /\bfoo\b/;
5110 print ".IX bar\n" if /\bbar\b/;
5111 print ".IX blurfl\n" if /\bblurfl\b/;
5116 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5117 will be looked at, because C<f> is rarer than C<o>. In general, this is
5118 a big win except in pathological cases. The only question is whether
5119 it saves you more time than it took to build the linked list in the
5122 Note that if you have to look for strings that you don't know till
5123 runtime, you can build an entire loop as a string and C<eval> that to
5124 avoid recompiling all your patterns all the time. Together with
5125 undefining C<$/> to input entire files as one record, this can be very
5126 fast, often faster than specialized programs like fgrep(1). The following
5127 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5128 out the names of those files that contain a match:
5130 $search = 'while (<>) { study;';
5131 foreach $word (@words) {
5132 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5137 eval $search; # this screams
5138 $/ = "\n"; # put back to normal input delimiter
5139 foreach $file (sort keys(%seen)) {
5147 =item sub NAME BLOCK
5149 This is subroutine definition, not a real function I<per se>. With just a
5150 NAME (and possibly prototypes or attributes), it's just a forward declaration.
5151 Without a NAME, it's an anonymous function declaration, and does actually
5152 return a value: the CODE ref of the closure you just created. See L<perlsub>
5153 and L<perlref> for details.
5155 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5157 =item substr EXPR,OFFSET,LENGTH
5159 =item substr EXPR,OFFSET
5161 Extracts a substring out of EXPR and returns it. First character is at
5162 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5163 If OFFSET is negative (or more precisely, less than C<$[>), starts
5164 that far from the end of the string. If LENGTH is omitted, returns
5165 everything to the end of the string. If LENGTH is negative, leaves that
5166 many characters off the end of the string.
5168 You can use the substr() function as an lvalue, in which case EXPR
5169 must itself be an lvalue. If you assign something shorter than LENGTH,
5170 the string will shrink, and if you assign something longer than LENGTH,
5171 the string will grow to accommodate it. To keep the string the same
5172 length you may need to pad or chop your value using C<sprintf>.
5174 If OFFSET and LENGTH specify a substring that is partly outside the
5175 string, only the part within the string is returned. If the substring
5176 is beyond either end of the string, substr() returns the undefined
5177 value and produces a warning. When used as an lvalue, specifying a
5178 substring that is entirely outside the string is a fatal error.
5179 Here's an example showing the behavior for boundary cases:
5182 substr($name, 4) = 'dy'; # $name is now 'freddy'
5183 my $null = substr $name, 6, 2; # returns '' (no warning)
5184 my $oops = substr $name, 7; # returns undef, with warning
5185 substr($name, 7) = 'gap'; # fatal error
5187 An alternative to using substr() as an lvalue is to specify the
5188 replacement string as the 4th argument. This allows you to replace
5189 parts of the EXPR and return what was there before in one operation,
5190 just as you can with splice().
5192 =item symlink OLDFILE,NEWFILE
5194 Creates a new filename symbolically linked to the old filename.
5195 Returns C<1> for success, C<0> otherwise. On systems that don't support
5196 symbolic links, produces a fatal error at run time. To check for that,
5199 $symlink_exists = eval { symlink("",""); 1 };
5203 Calls the system call specified as the first element of the list,
5204 passing the remaining elements as arguments to the system call. If
5205 unimplemented, produces a fatal error. The arguments are interpreted
5206 as follows: if a given argument is numeric, the argument is passed as
5207 an int. If not, the pointer to the string value is passed. You are
5208 responsible to make sure a string is pre-extended long enough to
5209 receive any result that might be written into a string. You can't use a
5210 string literal (or other read-only string) as an argument to C<syscall>
5211 because Perl has to assume that any string pointer might be written
5213 integer arguments are not literals and have never been interpreted in a
5214 numeric context, you may need to add C<0> to them to force them to look
5215 like numbers. This emulates the C<syswrite> function (or vice versa):
5217 require 'syscall.ph'; # may need to run h2ph
5219 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5221 Note that Perl supports passing of up to only 14 arguments to your system call,
5222 which in practice should usually suffice.
5224 Syscall returns whatever value returned by the system call it calls.
5225 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5226 Note that some system calls can legitimately return C<-1>. The proper
5227 way to handle such calls is to assign C<$!=0;> before the call and
5228 check the value of C<$!> if syscall returns C<-1>.
5230 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5231 number of the read end of the pipe it creates. There is no way
5232 to retrieve the file number of the other end. You can avoid this
5233 problem by using C<pipe> instead.
5235 =item sysopen FILEHANDLE,FILENAME,MODE
5237 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5239 Opens the file whose filename is given by FILENAME, and associates it
5240 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5241 the name of the real filehandle wanted. This function calls the
5242 underlying operating system's C<open> function with the parameters
5243 FILENAME, MODE, PERMS.
5245 The possible values and flag bits of the MODE parameter are
5246 system-dependent; they are available via the standard module C<Fcntl>.
5247 See the documentation of your operating system's C<open> to see which
5248 values and flag bits are available. You may combine several flags
5249 using the C<|>-operator.
5251 Some of the most common values are C<O_RDONLY> for opening the file in
5252 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5253 and C<O_RDWR> for opening the file in read-write mode, and.
5255 For historical reasons, some values work on almost every system
5256 supported by perl: zero means read-only, one means write-only, and two
5257 means read/write. We know that these values do I<not> work under
5258 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5259 use them in new code.
5261 If the file named by FILENAME does not exist and the C<open> call creates
5262 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5263 PERMS specifies the permissions of the newly created file. If you omit
5264 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5265 These permission values need to be in octal, and are modified by your
5266 process's current C<umask>.
5268 In many systems the C<O_EXCL> flag is available for opening files in
5269 exclusive mode. This is B<not> locking: exclusiveness means here that
5270 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5273 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5275 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5276 that takes away the user's option to have a more permissive umask.
5277 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5280 Note that C<sysopen> depends on the fdopen() C library function.
5281 On many UNIX systems, fdopen() is known to fail when file descriptors
5282 exceed a certain value, typically 255. If you need more file
5283 descriptors than that, consider rebuilding Perl to use the C<sfio>
5284 library, or perhaps using the POSIX::open() function.
5286 See L<perlopentut> for a kinder, gentler explanation of opening files.
5288 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5290 =item sysread FILEHANDLE,SCALAR,LENGTH
5292 Attempts to read LENGTH I<characters> of data into variable SCALAR from
5293 the specified FILEHANDLE, using the system call read(2). It bypasses
5294 buffered IO, so mixing this with other kinds of reads, C<print>,
5295 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because
5296 stdio usually buffers data. Returns the number of characters actually
5297 read, C<0> at end of file, or undef if there was an error. SCALAR
5298 will be grown or shrunk so that the last byte actually read is the
5299 last byte of the scalar after the read.
5301 Note the I<characters>: depending on the status of the filehandle,
5302 either (8-bit) bytes or characters are read. By default all
5303 filehandles operate on bytes, but for example if the filehandle has
5304 been opened with the C<:utf8> discipline (see L</open>, and the C<open>
5305 pragma, L<open>), the I/O will operate on characters, not bytes.
5307 An OFFSET may be specified to place the read data at some place in the
5308 string other than the beginning. A negative OFFSET specifies
5309 placement at that many characters counting backwards from the end of
5310 the string. A positive OFFSET greater than the length of SCALAR
5311 results in the string being padded to the required size with C<"\0">
5312 bytes before the result of the read is appended.
5314 There is no syseof() function, which is ok, since eof() doesn't work
5315 very well on device files (like ttys) anyway. Use sysread() and check
5316 for a return value for 0 to decide whether you're done.
5318 =item sysseek FILEHANDLE,POSITION,WHENCE
5320 Sets FILEHANDLE's system position I<in bytes> using the system call
5321 lseek(2). FILEHANDLE may be an expression whose value gives the name
5322 of the filehandle. The values for WHENCE are C<0> to set the new
5323 position to POSITION, C<1> to set the it to the current position plus
5324 POSITION, and C<2> to set it to EOF plus POSITION (typically
5327 Note the I<in bytes>: even if the filehandle has been set to operate
5328 on characters (for example by using the C<:utf8> discipline), tell()
5329 will return byte offsets, not character offsets (because implementing
5330 that would render sysseek() very slow).
5332 sysseek() bypasses normal buffered io, so mixing this with reads (other
5333 than C<sysread>, for example >< or read()) C<print>, C<write>,
5334 C<seek>, C<tell>, or C<eof> may cause confusion.
5336 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5337 and C<SEEK_END> (start of the file, current position, end of the file)
5338 from the Fcntl module. Use of the constants is also more portable
5339 than relying on 0, 1, and 2. For example to define a "systell" function:
5341 use Fnctl 'SEEK_CUR';
5342 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5344 Returns the new position, or the undefined value on failure. A position
5345 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5346 true on success and false on failure, yet you can still easily determine
5351 =item system PROGRAM LIST
5353 Does exactly the same thing as C<exec LIST>, except that a fork is
5354 done first, and the parent process waits for the child process to
5355 complete. Note that argument processing varies depending on the
5356 number of arguments. If there is more than one argument in LIST,
5357 or if LIST is an array with more than one value, starts the program
5358 given by the first element of the list with arguments given by the
5359 rest of the list. If there is only one scalar argument, the argument
5360 is checked for shell metacharacters, and if there are any, the
5361 entire argument is passed to the system's command shell for parsing
5362 (this is C</bin/sh -c> on Unix platforms, but varies on other
5363 platforms). If there are no shell metacharacters in the argument,
5364 it is split into words and passed directly to C<execvp>, which is
5367 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5368 output before any operation that may do a fork, but this may not be
5369 supported on some platforms (see L<perlport>). To be safe, you may need
5370 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5371 of C<IO::Handle> on any open handles.
5373 The return value is the exit status of the program as
5374 returned by the C<wait> call. To get the actual exit value divide by
5375 256. See also L</exec>. This is I<not> what you want to use to capture
5376 the output from a command, for that you should use merely backticks or
5377 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5378 indicates a failure to start the program (inspect $! for the reason).
5380 Like C<exec>, C<system> allows you to lie to a program about its name if
5381 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5383 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5384 program they're running doesn't actually interrupt your program.
5386 @args = ("command", "arg1", "arg2");
5388 or die "system @args failed: $?"
5390 You can check all the failure possibilities by inspecting
5393 $exit_value = $? >> 8;
5394 $signal_num = $? & 127;
5395 $dumped_core = $? & 128;
5397 When the arguments get executed via the system shell, results
5398 and return codes will be subject to its quirks and capabilities.
5399 See L<perlop/"`STRING`"> and L</exec> for details.
5401 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5403 =item syswrite FILEHANDLE,SCALAR,LENGTH
5405 =item syswrite FILEHANDLE,SCALAR
5407 Attempts to write LENGTH characters of data from variable SCALAR to
5408 the specified FILEHANDLE, using the system call write(2). If LENGTH
5409 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5410 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5411 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5412 buffers data. Returns the number of characters actually written, or
5413 C<undef> if there was an error. If the LENGTH is greater than the
5414 available data in the SCALAR after the OFFSET, only as much data as is
5415 available will be written.
5417 An OFFSET may be specified to write the data from some part of the
5418 string other than the beginning. A negative OFFSET specifies writing
5419 that many characters counting backwards from the end of the string.
5420 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5422 Note the I<characters>: depending on the status of the filehandle,
5423 either (8-bit) bytes or characters are written. By default all
5424 filehandles operate on bytes, but for example if the filehandle has
5425 been opened with the C<:utf8> discipline (see L</open>, and the open
5426 pragma, L<open>), the I/O will operate on characters, not bytes.
5428 =item tell FILEHANDLE
5432 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5433 error. FILEHANDLE may be an expression whose value gives the name of
5434 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5437 Note the I<in bytes>: even if the filehandle has been set to
5438 operate on characters (for example by using the C<:utf8> open
5439 discipline), tell() will return byte offsets, not character offsets
5440 (because that would render seek() and tell() rather slow).
5442 The return value of tell() for the standard streams like the STDIN
5443 depends on the operating system: it may return -1 or something else.
5444 tell() on pipes, fifos, and sockets usually returns -1.
5446 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5448 Do not use tell() on a filehandle that has been opened using
5449 sysopen(), use sysseek() for that as described above. Why? Because
5450 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5451 buffered filehandles. sysseek() make sense only on the first kind,
5452 tell() only makes sense on the second kind.
5454 =item telldir DIRHANDLE
5456 Returns the current position of the C<readdir> routines on DIRHANDLE.
5457 Value may be given to C<seekdir> to access a particular location in a
5458 directory. Has the same caveats about possible directory compaction as
5459 the corresponding system library routine.
5461 =item tie VARIABLE,CLASSNAME,LIST
5463 This function binds a variable to a package class that will provide the
5464 implementation for the variable. VARIABLE is the name of the variable
5465 to be enchanted. CLASSNAME is the name of a class implementing objects
5466 of correct type. Any additional arguments are passed to the C<new>
5467 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5468 or C<TIEHASH>). Typically these are arguments such as might be passed
5469 to the C<dbm_open()> function of C. The object returned by the C<new>
5470 method is also returned by the C<tie> function, which would be useful
5471 if you want to access other methods in CLASSNAME.
5473 Note that functions such as C<keys> and C<values> may return huge lists
5474 when used on large objects, like DBM files. You may prefer to use the
5475 C<each> function to iterate over such. Example:
5477 # print out history file offsets
5479 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5480 while (($key,$val) = each %HIST) {
5481 print $key, ' = ', unpack('L',$val), "\n";
5485 A class implementing a hash should have the following methods:
5487 TIEHASH classname, LIST
5489 STORE this, key, value
5494 NEXTKEY this, lastkey
5498 A class implementing an ordinary array should have the following methods:
5500 TIEARRAY classname, LIST
5502 STORE this, key, value
5504 STORESIZE this, count
5510 SPLICE this, offset, length, LIST
5515 A class implementing a file handle should have the following methods:
5517 TIEHANDLE classname, LIST
5518 READ this, scalar, length, offset
5521 WRITE this, scalar, length, offset
5523 PRINTF this, format, LIST
5527 SEEK this, position, whence
5529 OPEN this, mode, LIST
5534 A class implementing a scalar should have the following methods:
5536 TIESCALAR classname, LIST
5542 Not all methods indicated above need be implemented. See L<perltie>,
5543 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5545 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5546 for you--you need to do that explicitly yourself. See L<DB_File>
5547 or the F<Config> module for interesting C<tie> implementations.
5549 For further details see L<perltie>, L<"tied VARIABLE">.
5553 Returns a reference to the object underlying VARIABLE (the same value
5554 that was originally returned by the C<tie> call that bound the variable
5555 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5560 Returns the number of non-leap seconds since whatever time the system
5561 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5562 and 00:00:00 UTC, January 1, 1970 for most other systems).
5563 Suitable for feeding to C<gmtime> and C<localtime>.
5565 For measuring time in better granularity than one second,
5566 you may use either the Time::HiRes module from CPAN, or
5567 if you have gettimeofday(2), you may be able to use the
5568 C<syscall> interface of Perl, see L<perlfaq8> for details.
5572 Returns a four-element list giving the user and system times, in
5573 seconds, for this process and the children of this process.
5575 ($user,$system,$cuser,$csystem) = times;
5577 In scalar context, C<times> returns C<$user>.
5581 The transliteration operator. Same as C<y///>. See L<perlop>.
5583 =item truncate FILEHANDLE,LENGTH
5585 =item truncate EXPR,LENGTH
5587 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5588 specified length. Produces a fatal error if truncate isn't implemented
5589 on your system. Returns true if successful, the undefined value
5596 Returns an uppercased version of EXPR. This is the internal function
5597 implementing the C<\U> escape in double-quoted strings. Respects
5598 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5599 and L<perlunicode> for more details about locale and Unicode support.
5600 It does not attempt to do titlecase mapping on initial letters. See
5601 C<ucfirst> for that.
5603 If EXPR is omitted, uses C<$_>.
5609 Returns the value of EXPR with the first character in uppercase
5610 (titlecase in Unicode). This is the internal function implementing
5611 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5612 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5613 for more details about locale and Unicode support.
5615 If EXPR is omitted, uses C<$_>.
5621 Sets the umask for the process to EXPR and returns the previous value.
5622 If EXPR is omitted, merely returns the current umask.
5624 The Unix permission C<rwxr-x---> is represented as three sets of three
5625 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5626 and isn't one of the digits). The C<umask> value is such a number
5627 representing disabled permissions bits. The permission (or "mode")
5628 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5629 even if you tell C<sysopen> to create a file with permissions C<0777>,
5630 if your umask is C<0022> then the file will actually be created with
5631 permissions C<0755>. If your C<umask> were C<0027> (group can't
5632 write; others can't read, write, or execute), then passing
5633 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5636 Here's some advice: supply a creation mode of C<0666> for regular
5637 files (in C<sysopen>) and one of C<0777> for directories (in
5638 C<mkdir>) and executable files. This gives users the freedom of
5639 choice: if they want protected files, they might choose process umasks
5640 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5641 Programs should rarely if ever make policy decisions better left to
5642 the user. The exception to this is when writing files that should be
5643 kept private: mail files, web browser cookies, I<.rhosts> files, and
5646 If umask(2) is not implemented on your system and you are trying to
5647 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5648 fatal error at run time. If umask(2) is not implemented and you are
5649 not trying to restrict access for yourself, returns C<undef>.
5651 Remember that a umask is a number, usually given in octal; it is I<not> a
5652 string of octal digits. See also L</oct>, if all you have is a string.
5658 Undefines the value of EXPR, which must be an lvalue. Use only on a
5659 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5660 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5661 will probably not do what you expect on most predefined variables or
5662 DBM list values, so don't do that; see L<delete>.) Always returns the
5663 undefined value. You can omit the EXPR, in which case nothing is
5664 undefined, but you still get an undefined value that you could, for
5665 instance, return from a subroutine, assign to a variable or pass as a
5666 parameter. Examples:
5669 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5673 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5674 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5675 select undef, undef, undef, 0.25;
5676 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5678 Note that this is a unary operator, not a list operator.
5684 Deletes a list of files. Returns the number of files successfully
5687 $cnt = unlink 'a', 'b', 'c';
5691 Note: C<unlink> will not delete directories unless you are superuser and
5692 the B<-U> flag is supplied to Perl. Even if these conditions are
5693 met, be warned that unlinking a directory can inflict damage on your
5694 filesystem. Use C<rmdir> instead.
5696 If LIST is omitted, uses C<$_>.
5698 =item unpack TEMPLATE,EXPR
5700 C<unpack> does the reverse of C<pack>: it takes a string
5701 and expands it out into a list of values.
5702 (In scalar context, it returns merely the first value produced.)
5704 The string is broken into chunks described by the TEMPLATE. Each chunk
5705 is converted separately to a value. Typically, either the string is a result
5706 of C<pack>, or the bytes of the string represent a C structure of some
5709 The TEMPLATE has the same format as in the C<pack> function.
5710 Here's a subroutine that does substring:
5713 my($what,$where,$howmuch) = @_;
5714 unpack("x$where a$howmuch", $what);
5719 sub ordinal { unpack("c",$_[0]); } # same as ord()
5721 In addition to fields allowed in pack(), you may prefix a field with
5722 a %<number> to indicate that
5723 you want a <number>-bit checksum of the items instead of the items
5724 themselves. Default is a 16-bit checksum. Checksum is calculated by
5725 summing numeric values of expanded values (for string fields the sum of
5726 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5728 For example, the following
5729 computes the same number as the System V sum program:
5733 unpack("%32C*",<>) % 65535;
5736 The following efficiently counts the number of set bits in a bit vector:
5738 $setbits = unpack("%32b*", $selectmask);
5740 The C<p> and C<P> formats should be used with care. Since Perl
5741 has no way of checking whether the value passed to C<unpack()>
5742 corresponds to a valid memory location, passing a pointer value that's
5743 not known to be valid is likely to have disastrous consequences.
5745 If the repeat count of a field is larger than what the remainder of
5746 the input string allows, repeat count is decreased. If the input string
5747 is longer than one described by the TEMPLATE, the rest is ignored.
5749 See L</pack> for more examples and notes.
5751 =item untie VARIABLE
5753 Breaks the binding between a variable and a package. (See C<tie>.)
5755 =item unshift ARRAY,LIST
5757 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5758 depending on how you look at it. Prepends list to the front of the
5759 array, and returns the new number of elements in the array.
5761 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
5763 Note the LIST is prepended whole, not one element at a time, so the
5764 prepended elements stay in the same order. Use C<reverse> to do the
5767 =item use Module VERSION LIST
5769 =item use Module VERSION
5771 =item use Module LIST
5777 Imports some semantics into the current package from the named module,
5778 generally by aliasing certain subroutine or variable names into your
5779 package. It is exactly equivalent to
5781 BEGIN { require Module; import Module LIST; }
5783 except that Module I<must> be a bareword.
5785 VERSION may be either a numeric argument such as 5.006, which will be
5786 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5787 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
5788 greater than the version of the current Perl interpreter; Perl will not
5789 attempt to parse the rest of the file. Compare with L</require>, which can
5790 do a similar check at run time.
5792 Specifying VERSION as a literal of the form v5.6.1 should generally be
5793 avoided, because it leads to misleading error messages under earlier
5794 versions of Perl which do not support this syntax. The equivalent numeric
5795 version should be used instead.
5797 use v5.6.1; # compile time version check
5799 use 5.006_001; # ditto; preferred for backwards compatibility
5801 This is often useful if you need to check the current Perl version before
5802 C<use>ing library modules that have changed in incompatible ways from
5803 older versions of Perl. (We try not to do this more than we have to.)
5805 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5806 C<require> makes sure the module is loaded into memory if it hasn't been
5807 yet. The C<import> is not a builtin--it's just an ordinary static method
5808 call into the C<Module> package to tell the module to import the list of
5809 features back into the current package. The module can implement its
5810 C<import> method any way it likes, though most modules just choose to
5811 derive their C<import> method via inheritance from the C<Exporter> class that
5812 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5813 method can be found then the call is skipped.
5815 If you do not want to call the package's C<import> method (for instance,
5816 to stop your namespace from being altered), explicitly supply the empty list:
5820 That is exactly equivalent to
5822 BEGIN { require Module }
5824 If the VERSION argument is present between Module and LIST, then the
5825 C<use> will call the VERSION method in class Module with the given
5826 version as an argument. The default VERSION method, inherited from
5827 the UNIVERSAL class, croaks if the given version is larger than the
5828 value of the variable C<$Module::VERSION>.
5830 Again, there is a distinction between omitting LIST (C<import> called
5831 with no arguments) and an explicit empty LIST C<()> (C<import> not
5832 called). Note that there is no comma after VERSION!
5834 Because this is a wide-open interface, pragmas (compiler directives)
5835 are also implemented this way. Currently implemented pragmas are:
5840 use sigtrap qw(SEGV BUS);
5841 use strict qw(subs vars refs);
5842 use subs qw(afunc blurfl);
5843 use warnings qw(all);
5844 use sort qw(stable _quicksort _mergesort);
5846 Some of these pseudo-modules import semantics into the current
5847 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5848 which import symbols into the current package (which are effective
5849 through the end of the file).
5851 There's a corresponding C<no> command that unimports meanings imported
5852 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5858 If no C<unimport> method can be found the call fails with a fatal error.
5860 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5861 for the C<-M> and C<-m> command-line options to perl that give C<use>
5862 functionality from the command-line.
5866 Changes the access and modification times on each file of a list of
5867 files. The first two elements of the list must be the NUMERICAL access
5868 and modification times, in that order. Returns the number of files
5869 successfully changed. The inode change time of each file is set
5870 to the current time. This code has the same effect as the C<touch>
5871 command if the files already exist:
5875 utime $now, $now, @ARGV;
5877 If the first two elements of the list are C<undef>, then the utime(2)
5878 function in the C library will be called with a null second argument.
5879 On most systems, this will set the file's access and modification
5880 times to the current time. (i.e. equivalent to the example above.)
5882 utime undef, undef, @ARGV;
5886 Returns a list consisting of all the values of the named hash. (In a
5887 scalar context, returns the number of values.) The values are
5888 returned in an apparently random order. The actual random order is
5889 subject to change in future versions of perl, but it is guaranteed to
5890 be the same order as either the C<keys> or C<each> function would
5891 produce on the same (unmodified) hash.
5893 Note that the values are not copied, which means modifying them will
5894 modify the contents of the hash:
5896 for (values %hash) { s/foo/bar/g } # modifies %hash values
5897 for (@hash{keys %hash}) { s/foo/bar/g } # same
5899 As a side effect, calling values() resets the HASH's internal iterator.
5900 See also C<keys>, C<each>, and C<sort>.
5902 =item vec EXPR,OFFSET,BITS
5904 Treats the string in EXPR as a bit vector made up of elements of
5905 width BITS, and returns the value of the element specified by OFFSET
5906 as an unsigned integer. BITS therefore specifies the number of bits
5907 that are reserved for each element in the bit vector. This must
5908 be a power of two from 1 to 32 (or 64, if your platform supports
5911 If BITS is 8, "elements" coincide with bytes of the input string.
5913 If BITS is 16 or more, bytes of the input string are grouped into chunks
5914 of size BITS/8, and each group is converted to a number as with
5915 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5916 for BITS==64). See L<"pack"> for details.
5918 If bits is 4 or less, the string is broken into bytes, then the bits
5919 of each byte are broken into 8/BITS groups. Bits of a byte are
5920 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5921 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5922 breaking the single input byte C<chr(0x36)> into two groups gives a list
5923 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5925 C<vec> may also be assigned to, in which case parentheses are needed
5926 to give the expression the correct precedence as in
5928 vec($image, $max_x * $x + $y, 8) = 3;
5930 If the selected element is outside the string, the value 0 is returned.
5931 If an element off the end of the string is written to, Perl will first
5932 extend the string with sufficiently many zero bytes. It is an error
5933 to try to write off the beginning of the string (i.e. negative OFFSET).
5935 The string should not contain any character with the value > 255 (which
5936 can only happen if you're using UTF8 encoding). If it does, it will be
5937 treated as something which is not UTF8 encoded. When the C<vec> was
5938 assigned to, other parts of your program will also no longer consider the
5939 string to be UTF8 encoded. In other words, if you do have such characters
5940 in your string, vec() will operate on the actual byte string, and not the
5941 conceptual character string.
5943 Strings created with C<vec> can also be manipulated with the logical
5944 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5945 vector operation is desired when both operands are strings.
5946 See L<perlop/"Bitwise String Operators">.
5948 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5949 The comments show the string after each step. Note that this code works
5950 in the same way on big-endian or little-endian machines.
5953 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5955 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5956 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5958 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5959 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5960 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5961 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5962 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5963 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5965 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5966 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5967 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5970 To transform a bit vector into a string or list of 0's and 1's, use these:
5972 $bits = unpack("b*", $vector);
5973 @bits = split(//, unpack("b*", $vector));
5975 If you know the exact length in bits, it can be used in place of the C<*>.
5977 Here is an example to illustrate how the bits actually fall in place:
5983 unpack("V",$_) 01234567890123456789012345678901
5984 ------------------------------------------------------------------
5989 for ($shift=0; $shift < $width; ++$shift) {
5990 for ($off=0; $off < 32/$width; ++$off) {
5991 $str = pack("B*", "0"x32);
5992 $bits = (1<<$shift);
5993 vec($str, $off, $width) = $bits;
5994 $res = unpack("b*",$str);
5995 $val = unpack("V", $str);
6002 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6003 $off, $width, $bits, $val, $res
6007 Regardless of the machine architecture on which it is run, the above
6008 example should print the following table:
6011 unpack("V",$_) 01234567890123456789012345678901
6012 ------------------------------------------------------------------
6013 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6014 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6015 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6016 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6017 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6018 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6019 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6020 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6021 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6022 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6023 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6024 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6025 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6026 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6027 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6028 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6029 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6030 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6031 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6032 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6033 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6034 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6035 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6036 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6037 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6038 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6039 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6040 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6041 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6042 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6043 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6044 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6045 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6046 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6047 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6048 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6049 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6050 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6051 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6052 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6053 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6054 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6055 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6056 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6057 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6058 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6059 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6060 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6061 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6062 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6063 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6064 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6065 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6066 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6067 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6068 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6069 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6070 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6071 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6072 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6073 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6074 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6075 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6076 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6077 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6078 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6079 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6080 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6081 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6082 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6083 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6084 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6085 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6086 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6087 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6088 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6089 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6090 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6091 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6092 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6093 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6094 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6095 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6096 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6097 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6098 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6099 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6100 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6101 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6102 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6103 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6104 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6105 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6106 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6107 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6108 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6109 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6110 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6111 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6112 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6113 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6114 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6115 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6116 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6117 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6118 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6119 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6120 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6121 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6122 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6123 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6124 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6125 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6126 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6127 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6128 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6129 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6130 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6131 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6132 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6133 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6134 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6135 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6136 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6137 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6138 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6139 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6140 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6144 Behaves like the wait(2) system call on your system: it waits for a child
6145 process to terminate and returns the pid of the deceased process, or
6146 C<-1> if there are no child processes. The status is returned in C<$?>.
6147 Note that a return value of C<-1> could mean that child processes are
6148 being automatically reaped, as described in L<perlipc>.
6150 =item waitpid PID,FLAGS
6152 Waits for a particular child process to terminate and returns the pid of
6153 the deceased process, or C<-1> if there is no such child process. On some
6154 systems, a value of 0 indicates that there are processes still running.
6155 The status is returned in C<$?>. If you say
6157 use POSIX ":sys_wait_h";
6160 $kid = waitpid(-1, WNOHANG);
6163 then you can do a non-blocking wait for all pending zombie processes.
6164 Non-blocking wait is available on machines supporting either the
6165 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6166 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6167 system call by remembering the status values of processes that have
6168 exited but have not been harvested by the Perl script yet.)
6170 Note that on some systems, a return value of C<-1> could mean that child
6171 processes are being automatically reaped. See L<perlipc> for details,
6172 and for other examples.
6176 Returns true if the context of the currently executing subroutine is
6177 looking for a list value. Returns false if the context is looking
6178 for a scalar. Returns the undefined value if the context is looking
6179 for no value (void context).
6181 return unless defined wantarray; # don't bother doing more
6182 my @a = complex_calculation();
6183 return wantarray ? @a : "@a";
6185 This function should have been named wantlist() instead.
6189 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6192 If LIST is empty and C<$@> already contains a value (typically from a
6193 previous eval) that value is used after appending C<"\t...caught">
6194 to C<$@>. This is useful for staying almost, but not entirely similar to
6197 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6199 No message is printed if there is a C<$SIG{__WARN__}> handler
6200 installed. It is the handler's responsibility to deal with the message
6201 as it sees fit (like, for instance, converting it into a C<die>). Most
6202 handlers must therefore make arrangements to actually display the
6203 warnings that they are not prepared to deal with, by calling C<warn>
6204 again in the handler. Note that this is quite safe and will not
6205 produce an endless loop, since C<__WARN__> hooks are not called from
6208 You will find this behavior is slightly different from that of
6209 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6210 instead call C<die> again to change it).
6212 Using a C<__WARN__> handler provides a powerful way to silence all
6213 warnings (even the so-called mandatory ones). An example:
6215 # wipe out *all* compile-time warnings
6216 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6218 my $foo = 20; # no warning about duplicate my $foo,
6219 # but hey, you asked for it!
6220 # no compile-time or run-time warnings before here
6223 # run-time warnings enabled after here
6224 warn "\$foo is alive and $foo!"; # does show up
6226 See L<perlvar> for details on setting C<%SIG> entries, and for more
6227 examples. See the Carp module for other kinds of warnings using its
6228 carp() and cluck() functions.
6230 =item write FILEHANDLE
6236 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6237 using the format associated with that file. By default the format for
6238 a file is the one having the same name as the filehandle, but the
6239 format for the current output channel (see the C<select> function) may be set
6240 explicitly by assigning the name of the format to the C<$~> variable.
6242 Top of form processing is handled automatically: if there is
6243 insufficient room on the current page for the formatted record, the
6244 page is advanced by writing a form feed, a special top-of-page format
6245 is used to format the new page header, and then the record is written.
6246 By default the top-of-page format is the name of the filehandle with
6247 "_TOP" appended, but it may be dynamically set to the format of your
6248 choice by assigning the name to the C<$^> variable while the filehandle is
6249 selected. The number of lines remaining on the current page is in
6250 variable C<$->, which can be set to C<0> to force a new page.
6252 If FILEHANDLE is unspecified, output goes to the current default output
6253 channel, which starts out as STDOUT but may be changed by the
6254 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6255 is evaluated and the resulting string is used to look up the name of
6256 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6258 Note that write is I<not> the opposite of C<read>. Unfortunately.
6262 The transliteration operator. Same as C<tr///>. See L<perlop>.