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 An 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<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 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>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
379 On systems that support a close-on-exec flag on files, the flag will
380 be set for the newly opened file descriptor, as determined by the
381 value of $^F. See L<perlvar/$^F>.
387 Arranges to have a SIGALRM delivered to this process after the
388 specified number of seconds have elapsed. If SECONDS is not specified,
389 the value stored in C<$_> is used. (On some machines,
390 unfortunately, the elapsed time may be up to one second less than you
391 specified because of how seconds are counted.) Only one timer may be
392 counting at once. Each call disables the previous timer, and an
393 argument of C<0> may be supplied to cancel the previous timer without
394 starting a new one. The returned value is the amount of time remaining
395 on the previous timer.
397 For delays of finer granularity than one second, you may use Perl's
398 four-argument version of select() leaving the first three arguments
399 undefined, or you might be able to use the C<syscall> interface to
400 access setitimer(2) if your system supports it. The Time::HiRes module
401 from CPAN may also prove useful.
403 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404 (C<sleep> may be internally implemented in your system with C<alarm>)
406 If you want to use C<alarm> to time out a system call you need to use an
407 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
408 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
409 restart system calls on some systems. Using C<eval>/C<die> always works,
410 modulo the caveats given in L<perlipc/"Signals">.
413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
415 $nread = sysread SOCKET, $buffer, $size;
419 die unless $@ eq "alarm\n"; # propagate unexpected errors
428 Returns the arctangent of Y/X in the range -PI to PI.
430 For the tangent operation, you may use the C<Math::Trig::tan>
431 function, or use the familiar relation:
433 sub tan { sin($_[0]) / cos($_[0]) }
435 =item bind SOCKET,NAME
437 Binds a network address to a socket, just as the bind system call
438 does. Returns true if it succeeded, false otherwise. NAME should be a
439 packed address of the appropriate type for the socket. See the examples in
440 L<perlipc/"Sockets: Client/Server Communication">.
442 =item binmode FILEHANDLE, DISCIPLINE
444 =item binmode FILEHANDLE
446 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447 on systems where the run-time libraries distinguish between binary and
448 text files. If FILEHANDLE is an expression, the value is taken as the
449 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451 omitted, it defaults to C<":raw">.
453 binmode() should be called after open() but before any I/O is done on
456 On many systems binmode() currently has no effect, but in future, it
457 will be extended to support user-defined input and output disciplines.
458 On some systems binmode() is necessary when you're not working with a
459 text file. For the sake of portability it is a good idea to always use
460 it when appropriate, and to never use it when it isn't appropriate.
462 In other words: Regardless of platform, use binmode() on binary
463 files, and do not use binmode() on text files.
465 The C<open> pragma can be used to establish default disciplines.
468 The operating system, device drivers, C libraries, and Perl run-time
469 system all work together to let the programmer treat a single
470 character (C<\n>) as the line terminator, irrespective of the external
471 representation. On many operating systems, the native text file
472 representation matches the internal representation, but on some
473 platforms the external representation of C<\n> is made up of more than
476 Mac OS and all variants of Unix use a single character to end each line
477 in the external representation of text (even though that single
478 character is not necessarily the same across these platforms).
479 Consequently binmode() has no effect on these operating systems. In
480 other systems like VMS, MS-DOS and the various flavors of MS-Windows
481 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482 files are the two characters C<\cM\cJ>. That means that, if you don't
483 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484 converted to C<\n> on input, and any C<\n> in your program will be
485 converted back to C<\cM\cJ> on output. This is what you want for text
486 files, but it can be disastrous for binary files.
488 Another consequence of using binmode() (on some systems) is that
489 special end-of-file markers will be seen as part of the data stream.
490 For systems from the Microsoft family this means that if your binary
491 data contains C<\cZ>, the I/O subsystem will regard it as the end of
492 the file, unless you use binmode().
494 binmode() is not only important for readline() and print() operations,
495 but also when using read(), seek(), sysread(), syswrite() and tell()
496 (see L<perlport> for more details). See the C<$/> and C<$\> variables
497 in L<perlvar> for how to manually set your input and output
498 line-termination sequences.
500 =item bless REF,CLASSNAME
504 This function tells the thingy referenced by REF that it is now an object
505 in the CLASSNAME package. If CLASSNAME is omitted, the current package
506 is used. Because a C<bless> is often the last thing in a constructor,
507 it returns the reference for convenience. Always use the two-argument
508 version if the function doing the blessing might be inherited by a
509 derived class. See L<perltoot> and L<perlobj> for more about the blessing
510 (and blessings) of objects.
512 Consider always blessing objects in CLASSNAMEs that are mixed case.
513 Namespaces with all lowercase names are considered reserved for
514 Perl pragmata. Builtin types have all uppercase names, so to prevent
515 confusion, you may wish to avoid such package names as well. Make sure
516 that CLASSNAME is a true value.
518 See L<perlmod/"Perl Modules">.
524 Returns the context of the current subroutine call. In scalar context,
525 returns the caller's package name if there is a caller, that is, if
526 we're in a subroutine or C<eval> or C<require>, and the undefined value
527 otherwise. In list context, returns
529 ($package, $filename, $line) = caller;
531 With EXPR, it returns some extra information that the debugger uses to
532 print a stack trace. The value of EXPR indicates how many call frames
533 to go back before the current one.
535 ($package, $filename, $line, $subroutine, $hasargs,
536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
538 Here $subroutine may be C<(eval)> if the frame is not a subroutine
539 call, but an C<eval>. In such a case additional elements $evaltext and
540 C<$is_require> are set: C<$is_require> is true if the frame is created by a
541 C<require> or C<use> statement, $evaltext contains the text of the
542 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
543 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
544 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
545 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
546 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
547 was compiled with. The C<$hints> and C<$bitmask> values are subject to
548 change between versions of Perl, and are not meant for external use.
550 Furthermore, when called from within the DB package, caller returns more
551 detailed information: it sets the list variable C<@DB::args> to be the
552 arguments with which the subroutine was invoked.
554 Be aware that the optimizer might have optimized call frames away before
555 C<caller> had a chance to get the information. That means that C<caller(N)>
556 might not return information about the call frame you expect it do, for
557 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
558 previous time C<caller> was called.
562 Changes the working directory to EXPR, if possible. If EXPR is omitted,
563 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
564 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
565 set, C<chdir> does nothing. It returns true upon success, false
566 otherwise. See the example under C<die>.
570 Changes the permissions of a list of files. The first element of the
571 list must be the numerical mode, which should probably be an octal
572 number, and which definitely should I<not> a string of octal digits:
573 C<0644> is okay, C<'0644'> is not. Returns the number of files
574 successfully changed. See also L</oct>, if all you have is a string.
576 $cnt = chmod 0755, 'foo', 'bar';
577 chmod 0755, @executables;
578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
581 $mode = 0644; chmod $mode, 'foo'; # this is best
583 You can also import the symbolic C<S_I*> constants from the Fcntl
588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
589 # This is identical to the chmod 0755 of the above example.
597 This safer version of L</chop> removes any trailing string
598 that corresponds to the current value of C<$/> (also known as
599 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
600 number of characters removed from all its arguments. It's often used to
601 remove the newline from the end of an input record when you're worried
602 that the final record may be missing its newline. When in paragraph
603 mode (C<$/ = "">), it removes all trailing newlines from the string.
604 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
605 a reference to an integer or the like, see L<perlvar>) chomp() won't
607 If VARIABLE is omitted, it chomps C<$_>. Example:
610 chomp; # avoid \n on last field
615 You can actually chomp anything that's an lvalue, including an assignment:
618 chomp($answer = <STDIN>);
620 If you chomp a list, each element is chomped, and the total number of
621 characters removed is returned.
629 Chops off the last character of a string and returns the character
630 chopped. It's used primarily to remove the newline from the end of an
631 input record, but is much more efficient than C<s/\n//> because it neither
632 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
636 chop; # avoid \n on last field
641 You can actually chop anything that's an lvalue, including an assignment:
644 chop($answer = <STDIN>);
646 If you chop a list, each element is chopped. Only the value of the
647 last C<chop> is returned.
649 Note that C<chop> returns the last character. To return all but the last
650 character, use C<substr($string, 0, -1)>.
654 Changes the owner (and group) of a list of files. The first two
655 elements of the list must be the I<numeric> uid and gid, in that
656 order. A value of -1 in either position is interpreted by most
657 systems to leave that value unchanged. Returns the number of files
658 successfully changed.
660 $cnt = chown $uid, $gid, 'foo', 'bar';
661 chown $uid, $gid, @filenames;
663 Here's an example that looks up nonnumeric uids in the passwd file:
666 chomp($user = <STDIN>);
668 chomp($pattern = <STDIN>);
670 ($login,$pass,$uid,$gid) = getpwnam($user)
671 or die "$user not in passwd file";
673 @ary = glob($pattern); # expand filenames
674 chown $uid, $gid, @ary;
676 On most systems, you are not allowed to change the ownership of the
677 file unless you're the superuser, although you should be able to change
678 the group to any of your secondary groups. On insecure systems, these
679 restrictions may be relaxed, but this is not a portable assumption.
680 On POSIX systems, you can detect this condition this way:
682 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
683 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
689 Returns the character represented by that NUMBER in the character set.
690 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
691 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
692 characters higher than 127 are encoded in Unicode; if you don't want
693 this, temporarily C<use bytes> or use C<pack("C*",...)>
695 For the reverse, use L</ord>.
696 See L<utf8> for more about Unicode.
698 If NUMBER is omitted, uses C<$_>.
700 =item chroot FILENAME
704 This function works like the system call by the same name: it makes the
705 named directory the new root directory for all further pathnames that
706 begin with a C</> by your process and all its children. (It doesn't
707 change your current working directory, which is unaffected.) For security
708 reasons, this call is restricted to the superuser. If FILENAME is
709 omitted, does a C<chroot> to C<$_>.
711 =item close FILEHANDLE
715 Closes the file or pipe associated with the file handle, returning true
716 only if stdio successfully flushes buffers and closes the system file
717 descriptor. Closes the currently selected filehandle if the argument
720 You don't have to close FILEHANDLE if you are immediately going to do
721 another C<open> on it, because C<open> will close it for you. (See
722 C<open>.) However, an explicit C<close> on an input file resets the line
723 counter (C<$.>), while the implicit close done by C<open> does not.
725 If the file handle came from a piped open C<close> will additionally
726 return false if one of the other system calls involved fails or if the
727 program exits with non-zero status. (If the only problem was that the
728 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
729 also waits for the process executing on the pipe to complete, in case you
730 want to look at the output of the pipe afterwards, and
731 implicitly puts the exit status value of that command into C<$?>.
733 Prematurely closing the read end of a pipe (i.e. before the process
734 writing to it at the other end has closed it) will result in a
735 SIGPIPE being delivered to the writer. If the other end can't
736 handle that, be sure to read all the data before closing the pipe.
740 open(OUTPUT, '|sort >foo') # pipe to sort
741 or die "Can't start sort: $!";
742 #... # print stuff to output
743 close OUTPUT # wait for sort to finish
744 or warn $! ? "Error closing sort pipe: $!"
745 : "Exit status $? from sort";
746 open(INPUT, 'foo') # get sort's results
747 or die "Can't open 'foo' for input: $!";
749 FILEHANDLE may be an expression whose value can be used as an indirect
750 filehandle, usually the real filehandle name.
752 =item closedir DIRHANDLE
754 Closes a directory opened by C<opendir> and returns the success of that
757 DIRHANDLE may be an expression whose value can be used as an indirect
758 dirhandle, usually the real dirhandle name.
760 =item connect SOCKET,NAME
762 Attempts to connect to a remote socket, just as the connect system call
763 does. Returns true if it succeeded, false otherwise. NAME should be a
764 packed address of the appropriate type for the socket. See the examples in
765 L<perlipc/"Sockets: Client/Server Communication">.
769 Actually a flow control statement rather than a function. If there is a
770 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
771 C<foreach>), it is always executed just before the conditional is about to
772 be evaluated again, just like the third part of a C<for> loop in C. Thus
773 it can be used to increment a loop variable, even when the loop has been
774 continued via the C<next> statement (which is similar to the C C<continue>
777 C<last>, C<next>, or C<redo> may appear within a C<continue>
778 block. C<last> and C<redo> will behave as if they had been executed within
779 the main block. So will C<next>, but since it will execute a C<continue>
780 block, it may be more entertaining.
783 ### redo always comes here
786 ### next always comes here
788 # then back the top to re-check EXPR
790 ### last always comes here
792 Omitting the C<continue> section is semantically equivalent to using an
793 empty one, logically enough. In that case, C<next> goes directly back
794 to check the condition at the top of the loop.
798 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
799 takes cosine of C<$_>.
801 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
802 function, or use this relation:
804 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
806 =item crypt PLAINTEXT,SALT
808 Encrypts a string exactly like the crypt(3) function in the C library
809 (assuming that you actually have a version there that has not been
810 extirpated as a potential munition). This can prove useful for checking
811 the password file for lousy passwords, amongst other things. Only the
812 guys wearing white hats should do this.
814 Note that C<crypt> is intended to be a one-way function, much like breaking
815 eggs to make an omelette. There is no (known) corresponding decrypt
816 function. As a result, this function isn't all that useful for
817 cryptography. (For that, see your nearby CPAN mirror.)
819 When verifying an existing encrypted string you should use the encrypted
820 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
821 allows your code to work with the standard C<crypt> and with more
822 exotic implementations. When choosing a new salt create a random two
823 character string whose characters come from the set C<[./0-9A-Za-z]>
824 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
826 Here's an example that makes sure that whoever runs this program knows
829 $pwd = (getpwuid($<))[1];
833 chomp($word = <STDIN>);
837 if (crypt($word, $pwd) ne $pwd) {
843 Of course, typing in your own password to whoever asks you
846 The L<crypt> function is unsuitable for encrypting large quantities
847 of data, not least of all because you can't get the information
848 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
849 on your favorite CPAN mirror for a slew of potentially useful
854 [This function has been largely superseded by the C<untie> function.]
856 Breaks the binding between a DBM file and a hash.
858 =item dbmopen HASH,DBNAME,MASK
860 [This function has been largely superseded by the C<tie> function.]
862 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
863 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
864 argument is I<not> a filehandle, even though it looks like one). DBNAME
865 is the name of the database (without the F<.dir> or F<.pag> extension if
866 any). If the database does not exist, it is created with protection
867 specified by MASK (as modified by the C<umask>). If your system supports
868 only the older DBM functions, you may perform only one C<dbmopen> in your
869 program. In older versions of Perl, if your system had neither DBM nor
870 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
873 If you don't have write access to the DBM file, you can only read hash
874 variables, not set them. If you want to test whether you can write,
875 either use file tests or try setting a dummy hash entry inside an C<eval>,
876 which will trap the error.
878 Note that functions such as C<keys> and C<values> may return huge lists
879 when used on large DBM files. You may prefer to use the C<each>
880 function to iterate over large DBM files. Example:
882 # print out history file offsets
883 dbmopen(%HIST,'/usr/lib/news/history',0666);
884 while (($key,$val) = each %HIST) {
885 print $key, ' = ', unpack('L',$val), "\n";
889 See also L<AnyDBM_File> for a more general description of the pros and
890 cons of the various dbm approaches, as well as L<DB_File> for a particularly
893 You can control which DBM library you use by loading that library
894 before you call dbmopen():
897 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
898 or die "Can't open netscape history file: $!";
904 Returns a Boolean value telling whether EXPR has a value other than
905 the undefined value C<undef>. If EXPR is not present, C<$_> will be
908 Many operations return C<undef> to indicate failure, end of file,
909 system error, uninitialized variable, and other exceptional
910 conditions. This function allows you to distinguish C<undef> from
911 other values. (A simple Boolean test will not distinguish among
912 C<undef>, zero, the empty string, and C<"0">, which are all equally
913 false.) Note that since C<undef> is a valid scalar, its presence
914 doesn't I<necessarily> indicate an exceptional condition: C<pop>
915 returns C<undef> when its argument is an empty array, I<or> when the
916 element to return happens to be C<undef>.
918 You may also use C<defined(&func)> to check whether subroutine C<&func>
919 has ever been defined. The return value is unaffected by any forward
920 declarations of C<&foo>.
922 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
923 used to report whether memory for that aggregate has ever been
924 allocated. This behavior may disappear in future versions of Perl.
925 You should instead use a simple test for size:
927 if (@an_array) { print "has array elements\n" }
928 if (%a_hash) { print "has hash members\n" }
930 When used on a hash element, it tells you whether the value is defined,
931 not whether the key exists in the hash. Use L</exists> for the latter
936 print if defined $switch{'D'};
937 print "$val\n" while defined($val = pop(@ary));
938 die "Can't readlink $sym: $!"
939 unless defined($value = readlink $sym);
940 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
941 $debugging = 0 unless defined $debugging;
943 Note: Many folks tend to overuse C<defined>, and then are surprised to
944 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
945 defined values. For example, if you say
949 The pattern match succeeds, and C<$1> is defined, despite the fact that it
950 matched "nothing". But it didn't really match nothing--rather, it
951 matched something that happened to be zero characters long. This is all
952 very above-board and honest. When a function returns an undefined value,
953 it's an admission that it couldn't give you an honest answer. So you
954 should use C<defined> only when you're questioning the integrity of what
955 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
958 See also L</undef>, L</exists>, L</ref>.
962 Given an expression that specifies a hash element, array element, hash slice,
963 or array slice, deletes the specified element(s) from the hash or array.
964 In the case of an array, if the array elements happen to be at the end,
965 the size of the array will shrink to the highest element that tests
966 true for exists() (or 0 if no such element exists).
968 Returns each element so deleted or the undefined value if there was no such
969 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
970 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
971 from a C<tie>d hash or array may not necessarily return anything.
973 Deleting an array element effectively returns that position of the array
974 to its initial, uninitialized state. Subsequently testing for the same
975 element with exists() will return false. Note that deleting array
976 elements in the middle of an array will not shift the index of the ones
977 after them down--use splice() for that. See L</exists>.
979 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
981 foreach $key (keys %HASH) {
985 foreach $index (0 .. $#ARRAY) {
986 delete $ARRAY[$index];
991 delete @HASH{keys %HASH};
993 delete @ARRAY[0 .. $#ARRAY];
995 But both of these are slower than just assigning the empty list
996 or undefining %HASH or @ARRAY:
998 %HASH = (); # completely empty %HASH
999 undef %HASH; # forget %HASH ever existed
1001 @ARRAY = (); # completely empty @ARRAY
1002 undef @ARRAY; # forget @ARRAY ever existed
1004 Note that the EXPR can be arbitrarily complicated as long as the final
1005 operation is a hash element, array element, hash slice, or array slice
1008 delete $ref->[$x][$y]{$key};
1009 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1011 delete $ref->[$x][$y][$index];
1012 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1016 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1017 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1018 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1019 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1020 an C<eval(),> the error message is stuffed into C<$@> and the
1021 C<eval> is terminated with the undefined value. This makes
1022 C<die> the way to raise an exception.
1024 Equivalent examples:
1026 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1027 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1029 If the value of EXPR does not end in a newline, the current script line
1030 number and input line number (if any) are also printed, and a newline
1031 is supplied. Note that the "input line number" (also known as "chunk")
1032 is subject to whatever notion of "line" happens to be currently in
1033 effect, and is also available as the special variable C<$.>.
1034 See L<perlvar/"$/"> and L<perlvar/"$.">.
1036 Hint: sometimes appending C<", stopped"> to your message
1037 will cause it to make better sense when the string C<"at foo line 123"> is
1038 appended. Suppose you are running script "canasta".
1040 die "/etc/games is no good";
1041 die "/etc/games is no good, stopped";
1043 produce, respectively
1045 /etc/games is no good at canasta line 123.
1046 /etc/games is no good, stopped at canasta line 123.
1048 See also exit(), warn(), and the Carp module.
1050 If LIST is empty and C<$@> already contains a value (typically from a
1051 previous eval) that value is reused after appending C<"\t...propagated">.
1052 This is useful for propagating exceptions:
1055 die unless $@ =~ /Expected exception/;
1057 If C<$@> is empty then the string C<"Died"> is used.
1059 die() can also be called with a reference argument. If this happens to be
1060 trapped within an eval(), $@ contains the reference. This behavior permits
1061 a more elaborate exception handling implementation using objects that
1062 maintain arbitrary state about the nature of the exception. Such a scheme
1063 is sometimes preferable to matching particular string values of $@ using
1064 regular expressions. Here's an example:
1066 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1068 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1069 # handle Some::Module::Exception
1072 # handle all other possible exceptions
1076 Because perl will stringify uncaught exception messages before displaying
1077 them, you may want to overload stringification operations on such custom
1078 exception objects. See L<overload> for details about that.
1080 You can arrange for a callback to be run just before the C<die>
1081 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1082 handler will be called with the error text and can change the error
1083 message, if it sees fit, by calling C<die> again. See
1084 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1085 L<"eval BLOCK"> for some examples. Although this feature was meant
1086 to be run only right before your program was to exit, this is not
1087 currently the case--the C<$SIG{__DIE__}> hook is currently called
1088 even inside eval()ed blocks/strings! If one wants the hook to do
1089 nothing in such situations, put
1093 as the first line of the handler (see L<perlvar/$^S>). Because
1094 this promotes strange action at a distance, this counterintuitive
1095 behavior may be fixed in a future release.
1099 Not really a function. Returns the value of the last command in the
1100 sequence of commands indicated by BLOCK. When modified by a loop
1101 modifier, executes the BLOCK once before testing the loop condition.
1102 (On other statements the loop modifiers test the conditional first.)
1104 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1105 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1106 See L<perlsyn> for alternative strategies.
1108 =item do SUBROUTINE(LIST)
1110 A deprecated form of subroutine call. See L<perlsub>.
1114 Uses the value of EXPR as a filename and executes the contents of the
1115 file as a Perl script. Its primary use is to include subroutines
1116 from a Perl subroutine library.
1122 scalar eval `cat stat.pl`;
1124 except that it's more efficient and concise, keeps track of the current
1125 filename for error messages, searches the @INC libraries, and updates
1126 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1127 variables. It also differs in that code evaluated with C<do FILENAME>
1128 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1129 same, however, in that it does reparse the file every time you call it,
1130 so you probably don't want to do this inside a loop.
1132 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1133 error. If C<do> can read the file but cannot compile it, it
1134 returns undef and sets an error message in C<$@>. If the file is
1135 successfully compiled, C<do> returns the value of the last expression
1138 Note that inclusion of library modules is better done with the
1139 C<use> and C<require> operators, which also do automatic error checking
1140 and raise an exception if there's a problem.
1142 You might like to use C<do> to read in a program configuration
1143 file. Manual error checking can be done this way:
1145 # read in config files: system first, then user
1146 for $file ("/share/prog/defaults.rc",
1147 "$ENV{HOME}/.someprogrc")
1149 unless ($return = do $file) {
1150 warn "couldn't parse $file: $@" if $@;
1151 warn "couldn't do $file: $!" unless defined $return;
1152 warn "couldn't run $file" unless $return;
1160 This function causes an immediate core dump. See also the B<-u>
1161 command-line switch in L<perlrun>, which does the same thing.
1162 Primarily this is so that you can use the B<undump> program (not
1163 supplied) to turn your core dump into an executable binary after
1164 having initialized all your variables at the beginning of the
1165 program. When the new binary is executed it will begin by executing
1166 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1167 Think of it as a goto with an intervening core dump and reincarnation.
1168 If C<LABEL> is omitted, restarts the program from the top.
1170 B<WARNING>: Any files opened at the time of the dump will I<not>
1171 be open any more when the program is reincarnated, with possible
1172 resulting confusion on the part of Perl.
1174 This function is now largely obsolete, partly because it's very
1175 hard to convert a core file into an executable, and because the
1176 real compiler backends for generating portable bytecode and compilable
1177 C code have superseded it.
1179 If you're looking to use L<dump> to speed up your program, consider
1180 generating bytecode or native C code as described in L<perlcc>. If
1181 you're just trying to accelerate a CGI script, consider using the
1182 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1183 You might also consider autoloading or selfloading, which at least
1184 make your program I<appear> to run faster.
1188 When called in list context, returns a 2-element list consisting of the
1189 key and value for the next element of a hash, so that you can iterate over
1190 it. When called in scalar context, returns the key for only the "next"
1191 element in the hash.
1193 Entries are returned in an apparently random order. The actual random
1194 order is subject to change in future versions of perl, but it is guaranteed
1195 to be in the same order as either the C<keys> or C<values> function
1196 would produce on the same (unmodified) hash.
1198 When the hash is entirely read, a null array is returned in list context
1199 (which when assigned produces a false (C<0>) value), and C<undef> in
1200 scalar context. The next call to C<each> after that will start iterating
1201 again. There is a single iterator for each hash, shared by all C<each>,
1202 C<keys>, and C<values> function calls in the program; it can be reset by
1203 reading all the elements from the hash, or by evaluating C<keys HASH> or
1204 C<values HASH>. If you add or delete elements of a hash while you're
1205 iterating over it, you may get entries skipped or duplicated, so don't.
1207 The following prints out your environment like the printenv(1) program,
1208 only in a different order:
1210 while (($key,$value) = each %ENV) {
1211 print "$key=$value\n";
1214 See also C<keys>, C<values> and C<sort>.
1216 =item eof FILEHANDLE
1222 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1223 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1224 gives the real filehandle. (Note that this function actually
1225 reads a character and then C<ungetc>s it, so isn't very useful in an
1226 interactive context.) Do not read from a terminal file (or call
1227 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1228 as terminals may lose the end-of-file condition if you do.
1230 An C<eof> without an argument uses the last file read. Using C<eof()>
1231 with empty parentheses is very different. It refers to the pseudo file
1232 formed from the files listed on the command line and accessed via the
1233 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1234 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1235 used will cause C<@ARGV> to be examined to determine if input is
1238 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1239 detect the end of each file, C<eof()> will only detect the end of the
1240 last file. Examples:
1242 # reset line numbering on each input file
1244 next if /^\s*#/; # skip comments
1247 close ARGV if eof; # Not eof()!
1250 # insert dashes just before last line of last file
1252 if (eof()) { # check for end of current file
1253 print "--------------\n";
1254 close(ARGV); # close or last; is needed if we
1255 # are reading from the terminal
1260 Practical hint: you almost never need to use C<eof> in Perl, because the
1261 input operators typically return C<undef> when they run out of data, or if
1268 In the first form, the return value of EXPR is parsed and executed as if it
1269 were a little Perl program. The value of the expression (which is itself
1270 determined within scalar context) is first parsed, and if there weren't any
1271 errors, executed in the lexical context of the current Perl program, so
1272 that any variable settings or subroutine and format definitions remain
1273 afterwards. Note that the value is parsed every time the eval executes.
1274 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1275 delay parsing and subsequent execution of the text of EXPR until run time.
1277 In the second form, the code within the BLOCK is parsed only once--at the
1278 same time the code surrounding the eval itself was parsed--and executed
1279 within the context of the current Perl program. This form is typically
1280 used to trap exceptions more efficiently than the first (see below), while
1281 also providing the benefit of checking the code within BLOCK at compile
1284 The final semicolon, if any, may be omitted from the value of EXPR or within
1287 In both forms, the value returned is the value of the last expression
1288 evaluated inside the mini-program; a return statement may be also used, just
1289 as with subroutines. The expression providing the return value is evaluated
1290 in void, scalar, or list context, depending on the context of the eval itself.
1291 See L</wantarray> for more on how the evaluation context can be determined.
1293 If there is a syntax error or runtime error, or a C<die> statement is
1294 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1295 error message. If there was no error, C<$@> is guaranteed to be a null
1296 string. Beware that using C<eval> neither silences perl from printing
1297 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1298 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1299 L</warn> and L<perlvar>.
1301 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1302 determining whether a particular feature (such as C<socket> or C<symlink>)
1303 is implemented. It is also Perl's exception trapping mechanism, where
1304 the die operator is used to raise exceptions.
1306 If the code to be executed doesn't vary, you may use the eval-BLOCK
1307 form to trap run-time errors without incurring the penalty of
1308 recompiling each time. The error, if any, is still returned in C<$@>.
1311 # make divide-by-zero nonfatal
1312 eval { $answer = $a / $b; }; warn $@ if $@;
1314 # same thing, but less efficient
1315 eval '$answer = $a / $b'; warn $@ if $@;
1317 # a compile-time error
1318 eval { $answer = }; # WRONG
1321 eval '$answer ='; # sets $@
1323 Due to the current arguably broken state of C<__DIE__> hooks, when using
1324 the C<eval{}> form as an exception trap in libraries, you may wish not
1325 to trigger any C<__DIE__> hooks that user code may have installed.
1326 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1327 as shown in this example:
1329 # a very private exception trap for divide-by-zero
1330 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1333 This is especially significant, given that C<__DIE__> hooks can call
1334 C<die> again, which has the effect of changing their error messages:
1336 # __DIE__ hooks may modify error messages
1338 local $SIG{'__DIE__'} =
1339 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1340 eval { die "foo lives here" };
1341 print $@ if $@; # prints "bar lives here"
1344 Because this promotes action at a distance, this counterintuitive behavior
1345 may be fixed in a future release.
1347 With an C<eval>, you should be especially careful to remember what's
1348 being looked at when:
1354 eval { $x }; # CASE 4
1356 eval "\$$x++"; # CASE 5
1359 Cases 1 and 2 above behave identically: they run the code contained in
1360 the variable $x. (Although case 2 has misleading double quotes making
1361 the reader wonder what else might be happening (nothing is).) Cases 3
1362 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1363 does nothing but return the value of $x. (Case 4 is preferred for
1364 purely visual reasons, but it also has the advantage of compiling at
1365 compile-time instead of at run-time.) Case 5 is a place where
1366 normally you I<would> like to use double quotes, except that in this
1367 particular situation, you can just use symbolic references instead, as
1370 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1371 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1375 =item exec PROGRAM LIST
1377 The C<exec> function executes a system command I<and never returns>--
1378 use C<system> instead of C<exec> if you want it to return. It fails and
1379 returns false only if the command does not exist I<and> it is executed
1380 directly instead of via your system's command shell (see below).
1382 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1383 warns you if there is a following statement which isn't C<die>, C<warn>,
1384 or C<exit> (if C<-w> is set - but you always do that). If you
1385 I<really> want to follow an C<exec> with some other statement, you
1386 can use one of these styles to avoid the warning:
1388 exec ('foo') or print STDERR "couldn't exec foo: $!";
1389 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1391 If there is more than one argument in LIST, or if LIST is an array
1392 with more than one value, calls execvp(3) with the arguments in LIST.
1393 If there is only one scalar argument or an array with one element in it,
1394 the argument is checked for shell metacharacters, and if there are any,
1395 the entire argument is passed to the system's command shell for parsing
1396 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1397 If there are no shell metacharacters in the argument, it is split into
1398 words and passed directly to C<execvp>, which is more efficient.
1401 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1402 exec "sort $outfile | uniq";
1404 If you don't really want to execute the first argument, but want to lie
1405 to the program you are executing about its own name, you can specify
1406 the program you actually want to run as an "indirect object" (without a
1407 comma) in front of the LIST. (This always forces interpretation of the
1408 LIST as a multivalued list, even if there is only a single scalar in
1411 $shell = '/bin/csh';
1412 exec $shell '-sh'; # pretend it's a login shell
1416 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1418 When the arguments get executed via the system shell, results will
1419 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1422 Using an indirect object with C<exec> or C<system> is also more
1423 secure. This usage (which also works fine with system()) forces
1424 interpretation of the arguments as a multivalued list, even if the
1425 list had just one argument. That way you're safe from the shell
1426 expanding wildcards or splitting up words with whitespace in them.
1428 @args = ( "echo surprise" );
1430 exec @args; # subject to shell escapes
1432 exec { $args[0] } @args; # safe even with one-arg list
1434 The first version, the one without the indirect object, ran the I<echo>
1435 program, passing it C<"surprise"> an argument. The second version
1436 didn't--it tried to run a program literally called I<"echo surprise">,
1437 didn't find it, and set C<$?> to a non-zero value indicating failure.
1439 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1440 output before the exec, but this may not be supported on some platforms
1441 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1442 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1443 open handles in order to avoid lost output.
1445 Note that C<exec> will not call your C<END> blocks, nor will it call
1446 any C<DESTROY> methods in your objects.
1450 Given an expression that specifies a hash element or array element,
1451 returns true if the specified element in the hash or array has ever
1452 been initialized, even if the corresponding value is undefined. The
1453 element is not autovivified if it doesn't exist.
1455 print "Exists\n" if exists $hash{$key};
1456 print "Defined\n" if defined $hash{$key};
1457 print "True\n" if $hash{$key};
1459 print "Exists\n" if exists $array[$index];
1460 print "Defined\n" if defined $array[$index];
1461 print "True\n" if $array[$index];
1463 A hash or array element can be true only if it's defined, and defined if
1464 it exists, but the reverse doesn't necessarily hold true.
1466 Given an expression that specifies the name of a subroutine,
1467 returns true if the specified subroutine has ever been declared, even
1468 if it is undefined. Mentioning a subroutine name for exists or defined
1469 does not count as declaring it.
1471 print "Exists\n" if exists &subroutine;
1472 print "Defined\n" if defined &subroutine;
1474 Note that the EXPR can be arbitrarily complicated as long as the final
1475 operation is a hash or array key lookup or subroutine name:
1477 if (exists $ref->{A}->{B}->{$key}) { }
1478 if (exists $hash{A}{B}{$key}) { }
1480 if (exists $ref->{A}->{B}->[$ix]) { }
1481 if (exists $hash{A}{B}[$ix]) { }
1483 if (exists &{$ref->{A}{B}{$key}}) { }
1485 Although the deepest nested array or hash will not spring into existence
1486 just because its existence was tested, any intervening ones will.
1487 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1488 into existence due to the existence test for the $key element above.
1489 This happens anywhere the arrow operator is used, including even:
1492 if (exists $ref->{"Some key"}) { }
1493 print $ref; # prints HASH(0x80d3d5c)
1495 This surprising autovivification in what does not at first--or even
1496 second--glance appear to be an lvalue context may be fixed in a future
1499 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1500 on how exists() acts when used on a pseudo-hash.
1502 Use of a subroutine call, rather than a subroutine name, as an argument
1503 to exists() is an error.
1506 exists &sub(); # Error
1510 Evaluates EXPR and exits immediately with that value. Example:
1513 exit 0 if $ans =~ /^[Xx]/;
1515 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1516 universally recognized values for EXPR are C<0> for success and C<1>
1517 for error; other values are subject to interpretation depending on the
1518 environment in which the Perl program is running. For example, exiting
1519 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1520 the mailer to return the item undelivered, but that's not true everywhere.
1522 Don't use C<exit> to abort a subroutine if there's any chance that
1523 someone might want to trap whatever error happened. Use C<die> instead,
1524 which can be trapped by an C<eval>.
1526 The exit() function does not always exit immediately. It calls any
1527 defined C<END> routines first, but these C<END> routines may not
1528 themselves abort the exit. Likewise any object destructors that need to
1529 be called are called before the real exit. If this is a problem, you
1530 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1531 See L<perlmod> for details.
1537 Returns I<e> (the natural logarithm base) to the power of EXPR.
1538 If EXPR is omitted, gives C<exp($_)>.
1540 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1542 Implements the fcntl(2) function. You'll probably have to say
1546 first to get the correct constant definitions. Argument processing and
1547 value return works just like C<ioctl> below.
1551 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1552 or die "can't fcntl F_GETFL: $!";
1554 You don't have to check for C<defined> on the return from C<fnctl>.
1555 Like C<ioctl>, it maps a C<0> return from the system call into
1556 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1557 in numeric context. It is also exempt from the normal B<-w> warnings
1558 on improper numeric conversions.
1560 Note that C<fcntl> will produce a fatal error if used on a machine that
1561 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1562 manpage to learn what functions are available on your system.
1564 =item fileno FILEHANDLE
1566 Returns the file descriptor for a filehandle, or undefined if the
1567 filehandle is not open. This is mainly useful for constructing
1568 bitmaps for C<select> and low-level POSIX tty-handling operations.
1569 If FILEHANDLE is an expression, the value is taken as an indirect
1570 filehandle, generally its name.
1572 You can use this to find out whether two handles refer to the
1573 same underlying descriptor:
1575 if (fileno(THIS) == fileno(THAT)) {
1576 print "THIS and THAT are dups\n";
1579 =item flock FILEHANDLE,OPERATION
1581 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1582 for success, false on failure. Produces a fatal error if used on a
1583 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1584 C<flock> is Perl's portable file locking interface, although it locks
1585 only entire files, not records.
1587 Two potentially non-obvious but traditional C<flock> semantics are
1588 that it waits indefinitely until the lock is granted, and that its locks
1589 B<merely advisory>. Such discretionary locks are more flexible, but offer
1590 fewer guarantees. This means that files locked with C<flock> may be
1591 modified by programs that do not also use C<flock>. See L<perlport>,
1592 your port's specific documentation, or your system-specific local manpages
1593 for details. It's best to assume traditional behavior if you're writing
1594 portable programs. (But if you're not, you should as always feel perfectly
1595 free to write for your own system's idiosyncrasies (sometimes called
1596 "features"). Slavish adherence to portability concerns shouldn't get
1597 in the way of your getting your job done.)
1599 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1600 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1601 you can use the symbolic names if you import them from the Fcntl module,
1602 either individually, or as a group using the ':flock' tag. LOCK_SH
1603 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1604 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1605 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1606 waiting for the lock (check the return status to see if you got it).
1608 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1609 before locking or unlocking it.
1611 Note that the emulation built with lockf(3) doesn't provide shared
1612 locks, and it requires that FILEHANDLE be open with write intent. These
1613 are the semantics that lockf(3) implements. Most if not all systems
1614 implement lockf(3) in terms of fcntl(2) locking, though, so the
1615 differing semantics shouldn't bite too many people.
1617 Note also that some versions of C<flock> cannot lock things over the
1618 network; you would need to use the more system-specific C<fcntl> for
1619 that. If you like you can force Perl to ignore your system's flock(2)
1620 function, and so provide its own fcntl(2)-based emulation, by passing
1621 the switch C<-Ud_flock> to the F<Configure> program when you configure
1624 Here's a mailbox appender for BSD systems.
1626 use Fcntl ':flock'; # import LOCK_* constants
1629 flock(MBOX,LOCK_EX);
1630 # and, in case someone appended
1631 # while we were waiting...
1636 flock(MBOX,LOCK_UN);
1639 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1640 or die "Can't open mailbox: $!";
1643 print MBOX $msg,"\n\n";
1646 On systems that support a real flock(), locks are inherited across fork()
1647 calls, whereas those that must resort to the more capricious fcntl()
1648 function lose the locks, making it harder to write servers.
1650 See also L<DB_File> for other flock() examples.
1654 Does a fork(2) system call to create a new process running the
1655 same program at the same point. It returns the child pid to the
1656 parent process, C<0> to the child process, or C<undef> if the fork is
1657 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1658 are shared, while everything else is copied. On most systems supporting
1659 fork(), great care has gone into making it extremely efficient (for
1660 example, using copy-on-write technology on data pages), making it the
1661 dominant paradigm for multitasking over the last few decades.
1663 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1664 output before forking the child process, but this may not be supported
1665 on some platforms (see L<perlport>). To be safe, you may need to set
1666 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1667 C<IO::Handle> on any open handles in order to avoid duplicate output.
1669 If you C<fork> without ever waiting on your children, you will
1670 accumulate zombies. On some systems, you can avoid this by setting
1671 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1672 forking and reaping moribund children.
1674 Note that if your forked child inherits system file descriptors like
1675 STDIN and STDOUT that are actually connected by a pipe or socket, even
1676 if you exit, then the remote server (such as, say, a CGI script or a
1677 backgrounded job launched from a remote shell) won't think you're done.
1678 You should reopen those to F</dev/null> if it's any issue.
1682 Declare a picture format for use by the C<write> function. For
1686 Test: @<<<<<<<< @||||| @>>>>>
1687 $str, $%, '$' . int($num)
1691 $num = $cost/$quantity;
1695 See L<perlform> for many details and examples.
1697 =item formline PICTURE,LIST
1699 This is an internal function used by C<format>s, though you may call it,
1700 too. It formats (see L<perlform>) a list of values according to the
1701 contents of PICTURE, placing the output into the format output
1702 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1703 Eventually, when a C<write> is done, the contents of
1704 C<$^A> are written to some filehandle, but you could also read C<$^A>
1705 yourself and then set C<$^A> back to C<"">. Note that a format typically
1706 does one C<formline> per line of form, but the C<formline> function itself
1707 doesn't care how many newlines are embedded in the PICTURE. This means
1708 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1709 You may therefore need to use multiple formlines to implement a single
1710 record format, just like the format compiler.
1712 Be careful if you put double quotes around the picture, because an C<@>
1713 character may be taken to mean the beginning of an array name.
1714 C<formline> always returns true. See L<perlform> for other examples.
1716 =item getc FILEHANDLE
1720 Returns the next character from the input file attached to FILEHANDLE,
1721 or the undefined value at end of file, or if there was an error.
1722 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1723 efficient. However, it cannot be used by itself to fetch single
1724 characters without waiting for the user to hit enter. For that, try
1725 something more like:
1728 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1731 system "stty", '-icanon', 'eol', "\001";
1737 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1740 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1744 Determination of whether $BSD_STYLE should be set
1745 is left as an exercise to the reader.
1747 The C<POSIX::getattr> function can do this more portably on
1748 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1749 module from your nearest CPAN site; details on CPAN can be found on
1754 Implements the C library function of the same name, which on most
1755 systems returns the current login from F</etc/utmp>, if any. If null,
1758 $login = getlogin || getpwuid($<) || "Kilroy";
1760 Do not consider C<getlogin> for authentication: it is not as
1761 secure as C<getpwuid>.
1763 =item getpeername SOCKET
1765 Returns the packed sockaddr address of other end of the SOCKET connection.
1768 $hersockaddr = getpeername(SOCK);
1769 ($port, $iaddr) = sockaddr_in($hersockaddr);
1770 $herhostname = gethostbyaddr($iaddr, AF_INET);
1771 $herstraddr = inet_ntoa($iaddr);
1775 Returns the current process group for the specified PID. Use
1776 a PID of C<0> to get the current process group for the
1777 current process. Will raise an exception if used on a machine that
1778 doesn't implement getpgrp(2). If PID is omitted, returns process
1779 group of current process. Note that the POSIX version of C<getpgrp>
1780 does not accept a PID argument, so only C<PID==0> is truly portable.
1784 Returns the process id of the parent process.
1786 =item getpriority WHICH,WHO
1788 Returns the current priority for a process, a process group, or a user.
1789 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1790 machine that doesn't implement getpriority(2).
1796 =item gethostbyname NAME
1798 =item getnetbyname NAME
1800 =item getprotobyname NAME
1806 =item getservbyname NAME,PROTO
1808 =item gethostbyaddr ADDR,ADDRTYPE
1810 =item getnetbyaddr ADDR,ADDRTYPE
1812 =item getprotobynumber NUMBER
1814 =item getservbyport PORT,PROTO
1832 =item sethostent STAYOPEN
1834 =item setnetent STAYOPEN
1836 =item setprotoent STAYOPEN
1838 =item setservent STAYOPEN
1852 These routines perform the same functions as their counterparts in the
1853 system library. In list context, the return values from the
1854 various get routines are as follows:
1856 ($name,$passwd,$uid,$gid,
1857 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1858 ($name,$passwd,$gid,$members) = getgr*
1859 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1860 ($name,$aliases,$addrtype,$net) = getnet*
1861 ($name,$aliases,$proto) = getproto*
1862 ($name,$aliases,$port,$proto) = getserv*
1864 (If the entry doesn't exist you get a null list.)
1866 The exact meaning of the $gcos field varies but it usually contains
1867 the real name of the user (as opposed to the login name) and other
1868 information pertaining to the user. Beware, however, that in many
1869 system users are able to change this information and therefore it
1870 cannot be trusted and therefore the $gcos is tainted (see
1871 L<perlsec>). The $passwd and $shell, user's encrypted password and
1872 login shell, are also tainted, because of the same reason.
1874 In scalar context, you get the name, unless the function was a
1875 lookup by name, in which case you get the other thing, whatever it is.
1876 (If the entry doesn't exist you get the undefined value.) For example:
1878 $uid = getpwnam($name);
1879 $name = getpwuid($num);
1881 $gid = getgrnam($name);
1882 $name = getgrgid($num;
1886 In I<getpw*()> the fields $quota, $comment, and $expire are special
1887 cases in the sense that in many systems they are unsupported. If the
1888 $quota is unsupported, it is an empty scalar. If it is supported, it
1889 usually encodes the disk quota. If the $comment field is unsupported,
1890 it is an empty scalar. If it is supported it usually encodes some
1891 administrative comment about the user. In some systems the $quota
1892 field may be $change or $age, fields that have to do with password
1893 aging. In some systems the $comment field may be $class. The $expire
1894 field, if present, encodes the expiration period of the account or the
1895 password. For the availability and the exact meaning of these fields
1896 in your system, please consult your getpwnam(3) documentation and your
1897 F<pwd.h> file. You can also find out from within Perl what your
1898 $quota and $comment fields mean and whether you have the $expire field
1899 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1900 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1901 files are only supported if your vendor has implemented them in the
1902 intuitive fashion that calling the regular C library routines gets the
1903 shadow versions if you're running under privilege or if there exists
1904 the shadow(3) functions as found in System V ( this includes Solaris
1905 and Linux.) Those systems which implement a proprietary shadow password
1906 facility are unlikely to be supported.
1908 The $members value returned by I<getgr*()> is a space separated list of
1909 the login names of the members of the group.
1911 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1912 C, it will be returned to you via C<$?> if the function call fails. The
1913 C<@addrs> value returned by a successful call is a list of the raw
1914 addresses returned by the corresponding system library call. In the
1915 Internet domain, each address is four bytes long and you can unpack it
1916 by saying something like:
1918 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1920 The Socket library makes this slightly easier:
1923 $iaddr = inet_aton("127.1"); # or whatever address
1924 $name = gethostbyaddr($iaddr, AF_INET);
1926 # or going the other way
1927 $straddr = inet_ntoa($iaddr);
1929 If you get tired of remembering which element of the return list
1930 contains which return value, by-name interfaces are provided
1931 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1932 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1933 and C<User::grent>. These override the normal built-ins, supplying
1934 versions that return objects with the appropriate names
1935 for each field. For example:
1939 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1941 Even though it looks like they're the same method calls (uid),
1942 they aren't, because a C<File::stat> object is different from
1943 a C<User::pwent> object.
1945 =item getsockname SOCKET
1947 Returns the packed sockaddr address of this end of the SOCKET connection,
1948 in case you don't know the address because you have several different
1949 IPs that the connection might have come in on.
1952 $mysockaddr = getsockname(SOCK);
1953 ($port, $myaddr) = sockaddr_in($mysockaddr);
1954 printf "Connect to %s [%s]\n",
1955 scalar gethostbyaddr($myaddr, AF_INET),
1958 =item getsockopt SOCKET,LEVEL,OPTNAME
1960 Returns the socket option requested, or undef if there is an error.
1966 Returns the value of EXPR with filename expansions such as the
1967 standard Unix shell F</bin/csh> would do. This is the internal function
1968 implementing the C<< <*.c> >> operator, but you can use it directly.
1969 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1970 discussed in more detail in L<perlop/"I/O Operators">.
1972 Beginning with v5.6.0, this operator is implemented using the standard
1973 C<File::Glob> extension. See L<File::Glob> for details.
1977 Converts a time as returned by the time function to a 8-element list
1978 with the time localized for the standard Greenwich time zone.
1979 Typically used as follows:
1982 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1985 All list elements are numeric, and come straight out of the C `struct
1986 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1987 specified time. $mday is the day of the month, and $mon is the month
1988 itself, in the range C<0..11> with 0 indicating January and 11
1989 indicating December. $year is the number of years since 1900. That
1990 is, $year is C<123> in year 2023. $wday is the day of the week, with
1991 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1992 the year, in the range C<0..364> (or C<0..365> in leap years.)
1994 Note that the $year element is I<not> simply the last two digits of
1995 the year. If you assume it is, then you create non-Y2K-compliant
1996 programs--and you wouldn't want to do that, would you?
1998 The proper way to get a complete 4-digit year is simply:
2002 And to get the last two digits of the year (e.g., '01' in 2001) do:
2004 $year = sprintf("%02d", $year % 100);
2006 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2008 In scalar context, C<gmtime()> returns the ctime(3) value:
2010 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2012 Also see the C<timegm> function provided by the C<Time::Local> module,
2013 and the strftime(3) function available via the POSIX module.
2015 This scalar value is B<not> locale dependent (see L<perllocale>), but
2016 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2017 strftime(3) and mktime(3) functions available via the POSIX module. To
2018 get somewhat similar but locale dependent date strings, set up your
2019 locale environment variables appropriately (please see L<perllocale>)
2020 and try for example:
2022 use POSIX qw(strftime);
2023 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2025 Note that the C<%a> and C<%b> escapes, which represent the short forms
2026 of the day of the week and the month of the year, may not necessarily
2027 be three characters wide in all locales.
2035 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2036 execution there. It may not be used to go into any construct that
2037 requires initialization, such as a subroutine or a C<foreach> loop. It
2038 also can't be used to go into a construct that is optimized away,
2039 or to get out of a block or subroutine given to C<sort>.
2040 It can be used to go almost anywhere else within the dynamic scope,
2041 including out of subroutines, but it's usually better to use some other
2042 construct such as C<last> or C<die>. The author of Perl has never felt the
2043 need to use this form of C<goto> (in Perl, that is--C is another matter).
2045 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2046 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2047 necessarily recommended if you're optimizing for maintainability:
2049 goto ("FOO", "BAR", "GLARCH")[$i];
2051 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2052 In fact, it isn't a goto in the normal sense at all, and doesn't have
2053 the stigma associated with other gotos. Instead, it
2054 substitutes a call to the named subroutine for the currently running
2055 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2056 another subroutine and then pretend that the other subroutine had been
2057 called in the first place (except that any modifications to C<@_>
2058 in the current subroutine are propagated to the other subroutine.)
2059 After the C<goto>, not even C<caller> will be able to tell that this
2060 routine was called first.
2062 NAME needn't be the name of a subroutine; it can be a scalar variable
2063 containing a code reference, or a block which evaluates to a code
2066 =item grep BLOCK LIST
2068 =item grep EXPR,LIST
2070 This is similar in spirit to, but not the same as, grep(1) and its
2071 relatives. In particular, it is not limited to using regular expressions.
2073 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2074 C<$_> to each element) and returns the list value consisting of those
2075 elements for which the expression evaluated to true. In scalar
2076 context, returns the number of times the expression was true.
2078 @foo = grep(!/^#/, @bar); # weed out comments
2082 @foo = grep {!/^#/} @bar; # weed out comments
2084 Note that C<$_> is an alias to the list value, so it can be used to
2085 modify the elements of the LIST. While this is useful and supported,
2086 it can cause bizarre results if the elements of LIST are not variables.
2087 Similarly, grep returns aliases into the original list, much as a for
2088 loop's index variable aliases the list elements. That is, modifying an
2089 element of a list returned by grep (for example, in a C<foreach>, C<map>
2090 or another C<grep>) actually modifies the element in the original list.
2091 This is usually something to be avoided when writing clear code.
2093 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2099 Interprets EXPR as a hex string and returns the corresponding value.
2100 (To convert strings that might start with either 0, 0x, or 0b, see
2101 L</oct>.) If EXPR is omitted, uses C<$_>.
2103 print hex '0xAf'; # prints '175'
2104 print hex 'aF'; # same
2106 Hex strings may only represent integers. Strings that would cause
2107 integer overflow trigger a warning.
2111 There is no builtin C<import> function. It is just an ordinary
2112 method (subroutine) defined (or inherited) by modules that wish to export
2113 names to another module. The C<use> function calls the C<import> method
2114 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2116 =item index STR,SUBSTR,POSITION
2118 =item index STR,SUBSTR
2120 The index function searches for one string within another, but without
2121 the wildcard-like behavior of a full regular-expression pattern match.
2122 It returns the position of the first occurrence of SUBSTR in STR at
2123 or after POSITION. If POSITION is omitted, starts searching from the
2124 beginning of the string. The return value is based at C<0> (or whatever
2125 you've set the C<$[> variable to--but don't do that). If the substring
2126 is not found, returns one less than the base, ordinarily C<-1>.
2132 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2133 You should not use this function for rounding: one because it truncates
2134 towards C<0>, and two because machine representations of floating point
2135 numbers can sometimes produce counterintuitive results. For example,
2136 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2137 because it's really more like -268.99999999999994315658 instead. Usually,
2138 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2139 functions will serve you better than will int().
2141 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2143 Implements the ioctl(2) function. You'll probably first have to say
2145 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2147 to get the correct function definitions. If F<ioctl.ph> doesn't
2148 exist or doesn't have the correct definitions you'll have to roll your
2149 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2150 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2151 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2152 written depending on the FUNCTION--a pointer to the string value of SCALAR
2153 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2154 has no string value but does have a numeric value, that value will be
2155 passed rather than a pointer to the string value. To guarantee this to be
2156 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2157 functions may be needed to manipulate the values of structures used by
2160 The return value of C<ioctl> (and C<fcntl>) is as follows:
2162 if OS returns: then Perl returns:
2164 0 string "0 but true"
2165 anything else that number
2167 Thus Perl returns true on success and false on failure, yet you can
2168 still easily determine the actual value returned by the operating
2171 $retval = ioctl(...) || -1;
2172 printf "System returned %d\n", $retval;
2174 The special string "C<0> but true" is exempt from B<-w> complaints
2175 about improper numeric conversions.
2177 Here's an example of setting a filehandle named C<REMOTE> to be
2178 non-blocking at the system level. You'll have to negotiate C<$|>
2179 on your own, though.
2181 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2183 $flags = fcntl(REMOTE, F_GETFL, 0)
2184 or die "Can't get flags for the socket: $!\n";
2186 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2187 or die "Can't set flags for the socket: $!\n";
2189 =item join EXPR,LIST
2191 Joins the separate strings of LIST into a single string with fields
2192 separated by the value of EXPR, and returns that new string. Example:
2194 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2196 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2197 first argument. Compare L</split>.
2201 Returns a list consisting of all the keys of the named hash. (In
2202 scalar context, returns the number of keys.) The keys are returned in
2203 an apparently random order. The actual random order is subject to
2204 change in future versions of perl, but it is guaranteed to be the same
2205 order as either the C<values> or C<each> function produces (given
2206 that the hash has not been modified). As a side effect, it resets
2209 Here is yet another way to print your environment:
2212 @values = values %ENV;
2214 print pop(@keys), '=', pop(@values), "\n";
2217 or how about sorted by key:
2219 foreach $key (sort(keys %ENV)) {
2220 print $key, '=', $ENV{$key}, "\n";
2223 The returned values are copies of the original keys in the hash, so
2224 modifying them will not affect the original hash. Compare L</values>.
2226 To sort a hash by value, you'll need to use a C<sort> function.
2227 Here's a descending numeric sort of a hash by its values:
2229 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2230 printf "%4d %s\n", $hash{$key}, $key;
2233 As an lvalue C<keys> allows you to increase the number of hash buckets
2234 allocated for the given hash. This can gain you a measure of efficiency if
2235 you know the hash is going to get big. (This is similar to pre-extending
2236 an array by assigning a larger number to $#array.) If you say
2240 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2241 in fact, since it rounds up to the next power of two. These
2242 buckets will be retained even if you do C<%hash = ()>, use C<undef
2243 %hash> if you want to free the storage while C<%hash> is still in scope.
2244 You can't shrink the number of buckets allocated for the hash using
2245 C<keys> in this way (but you needn't worry about doing this by accident,
2246 as trying has no effect).
2248 See also C<each>, C<values> and C<sort>.
2250 =item kill SIGNAL, LIST
2252 Sends a signal to a list of processes. Returns the number of
2253 processes successfully signaled (which is not necessarily the
2254 same as the number actually killed).
2256 $cnt = kill 1, $child1, $child2;
2259 If SIGNAL is zero, no signal is sent to the process. This is a
2260 useful way to check that the process is alive and hasn't changed
2261 its UID. See L<perlport> for notes on the portability of this
2264 Unlike in the shell, if SIGNAL is negative, it kills
2265 process groups instead of processes. (On System V, a negative I<PROCESS>
2266 number will also kill process groups, but that's not portable.) That
2267 means you usually want to use positive not negative signals. You may also
2268 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2274 The C<last> command is like the C<break> statement in C (as used in
2275 loops); it immediately exits the loop in question. If the LABEL is
2276 omitted, the command refers to the innermost enclosing loop. The
2277 C<continue> block, if any, is not executed:
2279 LINE: while (<STDIN>) {
2280 last LINE if /^$/; # exit when done with header
2284 C<last> cannot be used to exit a block which returns a value such as
2285 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2286 a grep() or map() operation.
2288 Note that a block by itself is semantically identical to a loop
2289 that executes once. Thus C<last> can be used to effect an early
2290 exit out of such a block.
2292 See also L</continue> for an illustration of how C<last>, C<next>, and
2299 Returns an lowercased version of EXPR. This is the internal function
2300 implementing the C<\L> escape in double-quoted strings.
2301 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2304 If EXPR is omitted, uses C<$_>.
2310 Returns the value of EXPR with the first character lowercased. This is
2311 the internal function implementing the C<\l> escape in double-quoted strings.
2312 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2314 If EXPR is omitted, uses C<$_>.
2320 Returns the length in characters of the value of EXPR. If EXPR is
2321 omitted, returns length of C<$_>. Note that this cannot be used on
2322 an entire array or hash to find out how many elements these have.
2323 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2325 =item link OLDFILE,NEWFILE
2327 Creates a new filename linked to the old filename. Returns true for
2328 success, false otherwise.
2330 =item listen SOCKET,QUEUESIZE
2332 Does the same thing that the listen system call does. Returns true if
2333 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2337 You really probably want to be using C<my> instead, because C<local> isn't
2338 what most people think of as "local". See L<perlsub/"Private Variables
2339 via my()"> for details.
2341 A local modifies the listed variables to be local to the enclosing
2342 block, file, or eval. If more than one value is listed, the list must
2343 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2344 for details, including issues with tied arrays and hashes.
2346 =item localtime EXPR
2348 Converts a time as returned by the time function to a 9-element list
2349 with the time analyzed for the local time zone. Typically used as
2353 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2356 All list elements are numeric, and come straight out of the C `struct
2357 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2358 specified time. $mday is the day of the month, and $mon is the month
2359 itself, in the range C<0..11> with 0 indicating January and 11
2360 indicating December. $year is the number of years since 1900. That
2361 is, $year is C<123> in year 2023. $wday is the day of the week, with
2362 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2363 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2364 is true if the specified time occurs during daylight savings time,
2367 Note that the $year element is I<not> simply the last two digits of
2368 the year. If you assume it is, then you create non-Y2K-compliant
2369 programs--and you wouldn't want to do that, would you?
2371 The proper way to get a complete 4-digit year is simply:
2375 And to get the last two digits of the year (e.g., '01' in 2001) do:
2377 $year = sprintf("%02d", $year % 100);
2379 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2381 In scalar context, C<localtime()> returns the ctime(3) value:
2383 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2385 This scalar value is B<not> locale dependent, see L<perllocale>, but
2386 instead a Perl builtin. Also see the C<Time::Local> module
2387 (to convert the second, minutes, hours, ... back to seconds since the
2388 stroke of midnight the 1st of January 1970, the value returned by
2389 time()), and the strftime(3) and mktime(3) functions available via the
2390 POSIX module. To get somewhat similar but locale dependent date
2391 strings, set up your locale environment variables appropriately
2392 (please see L<perllocale>) and try for example:
2394 use POSIX qw(strftime);
2395 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2397 Note that the C<%a> and C<%b>, the short forms of the day of the week
2398 and the month of the year, may not necessarily be three characters wide.
2404 This function places an advisory lock on a variable, subroutine,
2405 or referenced object contained in I<THING> until the lock goes out
2406 of scope. This is a built-in function only if your version of Perl
2407 was built with threading enabled, and if you've said C<use Threads>.
2408 Otherwise a user-defined function by this name will be called. See
2415 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2416 returns log of C<$_>. To get the log of another base, use basic algebra:
2417 The base-N log of a number is equal to the natural log of that number
2418 divided by the natural log of N. For example:
2422 return log($n)/log(10);
2425 See also L</exp> for the inverse operation.
2431 Does the same thing as the C<stat> function (including setting the
2432 special C<_> filehandle) but stats a symbolic link instead of the file
2433 the symbolic link points to. If symbolic links are unimplemented on
2434 your system, a normal C<stat> is done.
2436 If EXPR is omitted, stats C<$_>.
2440 The match operator. See L<perlop>.
2442 =item map BLOCK LIST
2446 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2447 C<$_> to each element) and returns the list value composed of the
2448 results of each such evaluation. In scalar context, returns the
2449 total number of elements so generated. Evaluates BLOCK or EXPR in
2450 list context, so each element of LIST may produce zero, one, or
2451 more elements in the returned value.
2453 @chars = map(chr, @nums);
2455 translates a list of numbers to the corresponding characters. And
2457 %hash = map { getkey($_) => $_ } @array;
2459 is just a funny way to write
2462 foreach $_ (@array) {
2463 $hash{getkey($_)} = $_;
2466 Note that C<$_> is an alias to the list value, so it can be used to
2467 modify the elements of the LIST. While this is useful and supported,
2468 it can cause bizarre results if the elements of LIST are not variables.
2469 Using a regular C<foreach> loop for this purpose would be clearer in
2470 most cases. See also L</grep> for an array composed of those items of
2471 the original list for which the BLOCK or EXPR evaluates to true.
2473 =item mkdir FILENAME,MASK
2475 =item mkdir FILENAME
2477 Creates the directory specified by FILENAME, with permissions
2478 specified by MASK (as modified by C<umask>). If it succeeds it
2479 returns true, otherwise it returns false and sets C<$!> (errno).
2480 If omitted, MASK defaults to 0777.
2482 In general, it is better to create directories with permissive MASK,
2483 and let the user modify that with their C<umask>, than it is to supply
2484 a restrictive MASK and give the user no way to be more permissive.
2485 The exceptions to this rule are when the file or directory should be
2486 kept private (mail files, for instance). The perlfunc(1) entry on
2487 C<umask> discusses the choice of MASK in more detail.
2489 =item msgctl ID,CMD,ARG
2491 Calls the System V IPC function msgctl(2). You'll probably have to say
2495 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2496 then ARG must be a variable which will hold the returned C<msqid_ds>
2497 structure. Returns like C<ioctl>: the undefined value for error,
2498 C<"0 but true"> for zero, or the actual return value otherwise. See also
2499 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2501 =item msgget KEY,FLAGS
2503 Calls the System V IPC function msgget(2). Returns the message queue
2504 id, or the undefined value if there is an error. See also
2505 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2507 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2509 Calls the System V IPC function msgrcv to receive a message from
2510 message queue ID into variable VAR with a maximum message size of
2511 SIZE. Note that when a message is received, the message type as a
2512 native long integer will be the first thing in VAR, followed by the
2513 actual message. This packing may be opened with C<unpack("l! a*")>.
2514 Taints the variable. Returns true if successful, or false if there is
2515 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2516 C<IPC::SysV::Msg> documentation.
2518 =item msgsnd ID,MSG,FLAGS
2520 Calls the System V IPC function msgsnd to send the message MSG to the
2521 message queue ID. MSG must begin with the native long integer message
2522 type, and be followed by the length of the actual message, and finally
2523 the message itself. This kind of packing can be achieved with
2524 C<pack("l! a*", $type, $message)>. Returns true if successful,
2525 or false if there is an error. See also C<IPC::SysV>
2526 and C<IPC::SysV::Msg> documentation.
2530 =item my EXPR : ATTRIBUTES
2532 A C<my> declares the listed variables to be local (lexically) to the
2533 enclosing block, file, or C<eval>. If
2534 more than one value is listed, the list must be placed in parentheses. See
2535 L<perlsub/"Private Variables via my()"> for details.
2541 The C<next> command is like the C<continue> statement in C; it starts
2542 the next iteration of the loop:
2544 LINE: while (<STDIN>) {
2545 next LINE if /^#/; # discard comments
2549 Note that if there were a C<continue> block on the above, it would get
2550 executed even on discarded lines. If the LABEL is omitted, the command
2551 refers to the innermost enclosing loop.
2553 C<next> cannot be used to exit a block which returns a value such as
2554 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2555 a grep() or map() operation.
2557 Note that a block by itself is semantically identical to a loop
2558 that executes once. Thus C<next> will exit such a block early.
2560 See also L</continue> for an illustration of how C<last>, C<next>, and
2563 =item no Module LIST
2565 See the L</use> function, which C<no> is the opposite of.
2571 Interprets EXPR as an octal string and returns the corresponding
2572 value. (If EXPR happens to start off with C<0x>, interprets it as a
2573 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2574 binary string.) The following will handle decimal, binary, octal, and
2575 hex in the standard Perl or C notation:
2577 $val = oct($val) if $val =~ /^0/;
2579 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2580 in octal), use sprintf() or printf():
2582 $perms = (stat("filename"))[2] & 07777;
2583 $oct_perms = sprintf "%lo", $perms;
2585 The oct() function is commonly used when a string such as C<644> needs
2586 to be converted into a file mode, for example. (Although perl will
2587 automatically convert strings into numbers as needed, this automatic
2588 conversion assumes base 10.)
2590 =item open FILEHANDLE,MODE,LIST
2592 =item open FILEHANDLE,EXPR
2594 =item open FILEHANDLE
2596 Opens the file whose filename is given by EXPR, and associates it with
2597 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2598 name of the real filehandle wanted. (This is considered a symbolic
2599 reference, so C<use strict 'refs'> should I<not> be in effect.)
2601 If EXPR is omitted, the scalar
2602 variable of the same name as the FILEHANDLE contains the filename.
2603 (Note that lexical variables--those declared with C<my>--will not work
2604 for this purpose; so if you're using C<my>, specify EXPR in your call
2605 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2608 If MODE is C<< '<' >> or nothing, the file is opened for input.
2609 If MODE is C<< '>' >>, the file is truncated and opened for
2610 output, being created if necessary. If MODE is C<<< '>>' >>>,
2611 the file is opened for appending, again being created if necessary.
2612 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2613 you want both read and write access to the file; thus C<< '+<' >> is almost
2614 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2615 file first. You can't usually use either read-write mode for updating
2616 textfiles, since they have variable length records. See the B<-i>
2617 switch in L<perlrun> for a better approach. The file is created with
2618 permissions of C<0666> modified by the process' C<umask> value.
2620 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2621 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2623 In the 2-arguments (and 1-argument) form of the call the mode and
2624 filename should be concatenated (in this order), possibly separated by
2625 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2627 If the filename begins with C<'|'>, the filename is interpreted as a
2628 command to which output is to be piped, and if the filename ends with a
2629 C<'|'>, the filename is interpreted as a command which pipes output to
2630 us. See L<perlipc/"Using open() for IPC">
2631 for more examples of this. (You are not allowed to C<open> to a command
2632 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2633 and L<perlipc/"Bidirectional Communication with Another Process">
2636 If MODE is C<'|-'>, the filename is interpreted as a
2637 command to which output is to be piped, and if MODE is
2638 C<'-|'>, the filename is interpreted as a command which pipes output to
2639 us. In the 2-arguments (and 1-argument) form one should replace dash
2640 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2641 for more examples of this. (You are not allowed to C<open> to a command
2642 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2643 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2645 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2646 and opening C<< '>-' >> opens STDOUT.
2649 nonzero upon success, the undefined value otherwise. If the C<open>
2650 involved a pipe, the return value happens to be the pid of the
2653 If you're unfortunate enough to be running Perl on a system that
2654 distinguishes between text files and binary files (modern operating
2655 systems don't care), then you should check out L</binmode> for tips for
2656 dealing with this. The key distinction between systems that need C<binmode>
2657 and those that don't is their text file formats. Systems like Unix, MacOS, and
2658 Plan9, which delimit lines with a single character, and which encode that
2659 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2661 When opening a file, it's usually a bad idea to continue normal execution
2662 if the request failed, so C<open> is frequently used in connection with
2663 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2664 where you want to make a nicely formatted error message (but there are
2665 modules that can help with that problem)) you should always check
2666 the return value from opening a file. The infrequent exception is when
2667 working with an unopened filehandle is actually what you want to do.
2672 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2673 while (<ARTICLE>) {...
2675 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2676 # if the open fails, output is discarded
2678 open(DBASE, '+<', 'dbase.mine') # open for update
2679 or die "Can't open 'dbase.mine' for update: $!";
2681 open(DBASE, '+<dbase.mine') # ditto
2682 or die "Can't open 'dbase.mine' for update: $!";
2684 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2685 or die "Can't start caesar: $!";
2687 open(ARTICLE, "caesar <$article |") # ditto
2688 or die "Can't start caesar: $!";
2690 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2691 or die "Can't start sort: $!";
2693 # process argument list of files along with any includes
2695 foreach $file (@ARGV) {
2696 process($file, 'fh00');
2700 my($filename, $input) = @_;
2701 $input++; # this is a string increment
2702 unless (open($input, $filename)) {
2703 print STDERR "Can't open $filename: $!\n";
2708 while (<$input>) { # note use of indirection
2709 if (/^#include "(.*)"/) {
2710 process($1, $input);
2717 You may also, in the Bourne shell tradition, specify an EXPR beginning
2718 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2719 name of a filehandle (or file descriptor, if numeric) to be
2720 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2721 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2722 mode you specify should match the mode of the original filehandle.
2723 (Duping a filehandle does not take into account any existing contents of
2724 stdio buffers.) Duping file handles is not yet supported for 3-argument
2727 Here is a script that saves, redirects, and restores STDOUT and
2731 open(OLDOUT, ">&STDOUT");
2732 open(OLDERR, ">&STDERR");
2734 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2735 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2737 select(STDERR); $| = 1; # make unbuffered
2738 select(STDOUT); $| = 1; # make unbuffered
2740 print STDOUT "stdout 1\n"; # this works for
2741 print STDERR "stderr 1\n"; # subprocesses too
2746 open(STDOUT, ">&OLDOUT");
2747 open(STDERR, ">&OLDERR");
2749 print STDOUT "stdout 2\n";
2750 print STDERR "stderr 2\n";
2752 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2753 equivalent of C's C<fdopen> of that file descriptor; this is more
2754 parsimonious of file descriptors. For example:
2756 open(FILEHANDLE, "<&=$fd")
2758 Note that this feature depends on the fdopen() C library function.
2759 On many UNIX systems, fdopen() is known to fail when file descriptors
2760 exceed a certain value, typically 255. If you need more file
2761 descriptors than that, consider rebuilding Perl to use the C<sfio>
2764 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2765 with 2-arguments (or 1-argument) form of open(), then
2766 there is an implicit fork done, and the return value of open is the pid
2767 of the child within the parent process, and C<0> within the child
2768 process. (Use C<defined($pid)> to determine whether the open was successful.)
2769 The filehandle behaves normally for the parent, but i/o to that
2770 filehandle is piped from/to the STDOUT/STDIN of the child process.
2771 In the child process the filehandle isn't opened--i/o happens from/to
2772 the new STDOUT or STDIN. Typically this is used like the normal
2773 piped open when you want to exercise more control over just how the
2774 pipe command gets executed, such as when you are running setuid, and
2775 don't want to have to scan shell commands for metacharacters.
2776 The following triples are more or less equivalent:
2778 open(FOO, "|tr '[a-z]' '[A-Z]'");
2779 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2780 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2782 open(FOO, "cat -n '$file'|");
2783 open(FOO, '-|', "cat -n '$file'");
2784 open(FOO, '-|') || exec 'cat', '-n', $file;
2786 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2788 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2789 output before any operation that may do a fork, but this may not be
2790 supported on some platforms (see L<perlport>). To be safe, you may need
2791 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2792 of C<IO::Handle> on any open handles.
2794 On systems that support a
2795 close-on-exec flag on files, the flag will be set for the newly opened
2796 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2798 Closing any piped filehandle causes the parent process to wait for the
2799 child to finish, and returns the status value in C<$?>.
2801 The filename passed to 2-argument (or 1-argument) form of open()
2802 will have leading and trailing
2803 whitespace deleted, and the normal redirection characters
2804 honored. This property, known as "magic open",
2805 can often be used to good effect. A user could specify a filename of
2806 F<"rsh cat file |">, or you could change certain filenames as needed:
2808 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2809 open(FH, $filename) or die "Can't open $filename: $!";
2811 Use 3-argument form to open a file with arbitrary weird characters in it,
2813 open(FOO, '<', $file);
2815 otherwise it's necessary to protect any leading and trailing whitespace:
2817 $file =~ s#^(\s)#./$1#;
2818 open(FOO, "< $file\0");
2820 (this may not work on some bizzare filesystems). One should
2821 conscientiously choose between the I<magic> and 3-arguments form
2826 will allow the user to specify an argument of the form C<"rsh cat file |">,
2827 but will not work on a filename which happens to have a trailing space, while
2829 open IN, '<', $ARGV[0];
2831 will have exactly the opposite restrictions.
2833 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2834 should use the C<sysopen> function, which involves no such magic (but
2835 may use subtly different filemodes than Perl open(), which is mapped
2836 to C fopen()). This is
2837 another way to protect your filenames from interpretation. For example:
2840 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2841 or die "sysopen $path: $!";
2842 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2843 print HANDLE "stuff $$\n");
2845 print "File contains: ", <HANDLE>;
2847 Using the constructor from the C<IO::Handle> package (or one of its
2848 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2849 filehandles that have the scope of whatever variables hold references to
2850 them, and automatically close whenever and however you leave that scope:
2854 sub read_myfile_munged {
2856 my $handle = new IO::File;
2857 open($handle, "myfile") or die "myfile: $!";
2859 or return (); # Automatically closed here.
2860 mung $first or die "mung failed"; # Or here.
2861 return $first, <$handle> if $ALL; # Or here.
2865 See L</seek> for some details about mixing reading and writing.
2867 =item opendir DIRHANDLE,EXPR
2869 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2870 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2871 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2877 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2878 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2879 See L<utf8> for more about Unicode.
2883 An C<our> declares the listed variables to be valid globals within
2884 the enclosing block, file, or C<eval>. That is, it has the same
2885 scoping rules as a "my" declaration, but does not create a local
2886 variable. If more than one value is listed, the list must be placed
2887 in parentheses. The C<our> declaration has no semantic effect unless
2888 "use strict vars" is in effect, in which case it lets you use the
2889 declared global variable without qualifying it with a package name.
2890 (But only within the lexical scope of the C<our> declaration. In this
2891 it differs from "use vars", which is package scoped.)
2893 An C<our> declaration declares a global variable that will be visible
2894 across its entire lexical scope, even across package boundaries. The
2895 package in which the variable is entered is determined at the point
2896 of the declaration, not at the point of use. This means the following
2900 our $bar; # declares $Foo::bar for rest of lexical scope
2904 print $bar; # prints 20
2906 Multiple C<our> declarations in the same lexical scope are allowed
2907 if they are in different packages. If they happened to be in the same
2908 package, Perl will emit warnings if you have asked for them.
2912 our $bar; # declares $Foo::bar for rest of lexical scope
2916 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2917 print $bar; # prints 30
2919 our $bar; # emits warning
2921 =item pack TEMPLATE,LIST
2923 Takes a LIST of values and converts it into a string using the rules
2924 given by the TEMPLATE. The resulting string is the concatenation of
2925 the converted values. Typically, each converted value looks
2926 like its machine-level representation. For example, on 32-bit machines
2927 a converted integer may be represented by a sequence of 4 bytes.
2930 sequence of characters that give the order and type of values, as
2933 a A string with arbitrary binary data, will be null padded.
2934 A An ASCII string, will be space padded.
2935 Z A null terminated (asciz) string, will be null padded.
2937 b A bit string (ascending bit order inside each byte, like vec()).
2938 B A bit string (descending bit order inside each byte).
2939 h A hex string (low nybble first).
2940 H A hex string (high nybble first).
2942 c A signed char value.
2943 C An unsigned char value. Only does bytes. See U for Unicode.
2945 s A signed short value.
2946 S An unsigned short value.
2947 (This 'short' is _exactly_ 16 bits, which may differ from
2948 what a local C compiler calls 'short'. If you want
2949 native-length shorts, use the '!' suffix.)
2951 i A signed integer value.
2952 I An unsigned integer value.
2953 (This 'integer' is _at_least_ 32 bits wide. Its exact
2954 size depends on what a local C compiler calls 'int',
2955 and may even be larger than the 'long' described in
2958 l A signed long value.
2959 L An unsigned long value.
2960 (This 'long' is _exactly_ 32 bits, which may differ from
2961 what a local C compiler calls 'long'. If you want
2962 native-length longs, use the '!' suffix.)
2964 n An unsigned short in "network" (big-endian) order.
2965 N An unsigned long in "network" (big-endian) order.
2966 v An unsigned short in "VAX" (little-endian) order.
2967 V An unsigned long in "VAX" (little-endian) order.
2968 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2969 _exactly_ 32 bits, respectively.)
2971 q A signed quad (64-bit) value.
2972 Q An unsigned quad value.
2973 (Quads are available only if your system supports 64-bit
2974 integer values _and_ if Perl has been compiled to support those.
2975 Causes a fatal error otherwise.)
2977 f A single-precision float in the native format.
2978 d A double-precision float in the native format.
2980 p A pointer to a null-terminated string.
2981 P A pointer to a structure (fixed-length string).
2983 u A uuencoded string.
2984 U A Unicode character number. Encodes to UTF-8 internally.
2985 Works even if C<use utf8> is not in effect.
2987 w A BER compressed integer. Its bytes represent an unsigned
2988 integer in base 128, most significant digit first, with as
2989 few digits as possible. Bit eight (the high bit) is set
2990 on each byte except the last.
2994 @ Null fill to absolute position.
2996 The following rules apply:
3002 Each letter may optionally be followed by a number giving a repeat
3003 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3004 C<H>, and C<P> the pack function will gobble up that many values from
3005 the LIST. A C<*> for the repeat count means to use however many items are
3006 left, except for C<@>, C<x>, C<X>, where it is equivalent
3007 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3010 When used with C<Z>, C<*> results in the addition of a trailing null
3011 byte (so the packed result will be one longer than the byte C<length>
3014 The repeat count for C<u> is interpreted as the maximal number of bytes
3015 to encode per line of output, with 0 and 1 replaced by 45.
3019 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3020 string of length count, padding with nulls or spaces as necessary. When
3021 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3022 after the first null, and C<a> returns data verbatim. When packing,
3023 C<a>, and C<Z> are equivalent.
3025 If the value-to-pack is too long, it is truncated. If too long and an
3026 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3027 by a null byte. Thus C<Z> always packs a trailing null byte under
3032 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3033 Each byte of the input field of pack() generates 1 bit of the result.
3034 Each result bit is based on the least-significant bit of the corresponding
3035 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3036 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3038 Starting from the beginning of the input string of pack(), each 8-tuple
3039 of bytes is converted to 1 byte of output. With format C<b>
3040 the first byte of the 8-tuple determines the least-significant bit of a
3041 byte, and with format C<B> it determines the most-significant bit of
3044 If the length of the input string is not exactly divisible by 8, the
3045 remainder is packed as if the input string were padded by null bytes
3046 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3048 If the input string of pack() is longer than needed, extra bytes are ignored.
3049 A C<*> for the repeat count of pack() means to use all the bytes of
3050 the input field. On unpack()ing the bits are converted to a string
3051 of C<"0">s and C<"1">s.
3055 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3056 representable as hexadecimal digits, 0-9a-f) long.
3058 Each byte of the input field of pack() generates 4 bits of the result.
3059 For non-alphabetical bytes the result is based on the 4 least-significant
3060 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3061 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3062 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3063 is compatible with the usual hexadecimal digits, so that C<"a"> and
3064 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3065 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3067 Starting from the beginning of the input string of pack(), each pair
3068 of bytes is converted to 1 byte of output. With format C<h> the
3069 first byte of the pair determines the least-significant nybble of the
3070 output byte, and with format C<H> it determines the most-significant
3073 If the length of the input string is not even, it behaves as if padded
3074 by a null byte at the end. Similarly, during unpack()ing the "extra"
3075 nybbles are ignored.
3077 If the input string of pack() is longer than needed, extra bytes are ignored.
3078 A C<*> for the repeat count of pack() means to use all the bytes of
3079 the input field. On unpack()ing the bits are converted to a string
3080 of hexadecimal digits.
3084 The C<p> type packs a pointer to a null-terminated string. You are
3085 responsible for ensuring the string is not a temporary value (which can
3086 potentially get deallocated before you get around to using the packed result).
3087 The C<P> type packs a pointer to a structure of the size indicated by the
3088 length. A NULL pointer is created if the corresponding value for C<p> or
3089 C<P> is C<undef>, similarly for unpack().
3093 The C</> template character allows packing and unpacking of strings where
3094 the packed structure contains a byte count followed by the string itself.
3095 You write I<length-item>C</>I<string-item>.
3097 The I<length-item> can be any C<pack> template letter,
3098 and describes how the length value is packed.
3099 The ones likely to be of most use are integer-packing ones like
3100 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3101 and C<N> (for Sun XDR).
3103 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3104 For C<unpack> the length of the string is obtained from the I<length-item>,
3105 but if you put in the '*' it will be ignored.
3107 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3108 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3109 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3111 The I<length-item> is not returned explicitly from C<unpack>.
3113 Adding a count to the I<length-item> letter is unlikely to do anything
3114 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3115 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3116 which Perl does not regard as legal in numeric strings.
3120 The integer types C<s>, C<S>, C<l>, and C<L> may be
3121 immediately followed by a C<!> suffix to signify native shorts or
3122 longs--as you can see from above for example a bare C<l> does mean
3123 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3124 may be larger. This is an issue mainly in 64-bit platforms. You can
3125 see whether using C<!> makes any difference by
3127 print length(pack("s")), " ", length(pack("s!")), "\n";
3128 print length(pack("l")), " ", length(pack("l!")), "\n";
3130 C<i!> and C<I!> also work but only because of completeness;
3131 they are identical to C<i> and C<I>.
3133 The actual sizes (in bytes) of native shorts, ints, longs, and long
3134 longs on the platform where Perl was built are also available via
3138 print $Config{shortsize}, "\n";
3139 print $Config{intsize}, "\n";
3140 print $Config{longsize}, "\n";
3141 print $Config{longlongsize}, "\n";
3143 (The C<$Config{longlongsize}> will be undefine if your system does
3144 not support long longs.)
3148 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3149 are inherently non-portable between processors and operating systems
3150 because they obey the native byteorder and endianness. For example a
3151 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3152 (arranged in and handled by the CPU registers) into bytes as
3154 0x12 0x34 0x56 0x78 # little-endian
3155 0x78 0x56 0x34 0x12 # big-endian
3157 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3158 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3159 Power, and Cray are big-endian. MIPS can be either: Digital used it
3160 in little-endian mode; SGI uses it in big-endian mode.
3162 The names `big-endian' and `little-endian' are comic references to
3163 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3164 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3165 the egg-eating habits of the Lilliputians.
3167 Some systems may have even weirder byte orders such as
3172 You can see your system's preference with
3174 print join(" ", map { sprintf "%#02x", $_ }
3175 unpack("C*",pack("L",0x12345678))), "\n";
3177 The byteorder on the platform where Perl was built is also available
3181 print $Config{byteorder}, "\n";
3183 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3184 and C<'87654321'> are big-endian.
3186 If you want portable packed integers use the formats C<n>, C<N>,
3187 C<v>, and C<V>, their byte endianness and size is known.
3188 See also L<perlport>.
3192 Real numbers (floats and doubles) are in the native machine format only;
3193 due to the multiplicity of floating formats around, and the lack of a
3194 standard "network" representation, no facility for interchange has been
3195 made. This means that packed floating point data written on one machine
3196 may not be readable on another - even if both use IEEE floating point
3197 arithmetic (as the endian-ness of the memory representation is not part
3198 of the IEEE spec). See also L<perlport>.
3200 Note that Perl uses doubles internally for all numeric calculation, and
3201 converting from double into float and thence back to double again will
3202 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3207 If the pattern begins with a C<U>, the resulting string will be treated
3208 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3209 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3210 characters. If you don't want this to happen, you can begin your pattern
3211 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3212 string, and then follow this with a C<U*> somewhere in your pattern.
3216 You must yourself do any alignment or padding by inserting for example
3217 enough C<'x'>es while packing. There is no way to pack() and unpack()
3218 could know where the bytes are going to or coming from. Therefore
3219 C<pack> (and C<unpack>) handle their output and input as flat
3224 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3228 If TEMPLATE requires more arguments to pack() than actually given, pack()
3229 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3230 to pack() than actually given, extra arguments are ignored.
3236 $foo = pack("CCCC",65,66,67,68);
3238 $foo = pack("C4",65,66,67,68);
3240 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3241 # same thing with Unicode circled letters
3243 $foo = pack("ccxxcc",65,66,67,68);
3246 # note: the above examples featuring "C" and "c" are true
3247 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3248 # and UTF-8. In EBCDIC the first example would be
3249 # $foo = pack("CCCC",193,194,195,196);
3251 $foo = pack("s2",1,2);
3252 # "\1\0\2\0" on little-endian
3253 # "\0\1\0\2" on big-endian
3255 $foo = pack("a4","abcd","x","y","z");
3258 $foo = pack("aaaa","abcd","x","y","z");
3261 $foo = pack("a14","abcdefg");
3262 # "abcdefg\0\0\0\0\0\0\0"
3264 $foo = pack("i9pl", gmtime);
3265 # a real struct tm (on my system anyway)
3267 $utmp_template = "Z8 Z8 Z16 L";
3268 $utmp = pack($utmp_template, @utmp1);
3269 # a struct utmp (BSDish)
3271 @utmp2 = unpack($utmp_template, $utmp);
3272 # "@utmp1" eq "@utmp2"
3275 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3278 $foo = pack('sx2l', 12, 34);
3279 # short 12, two zero bytes padding, long 34
3280 $bar = pack('s@4l', 12, 34);
3281 # short 12, zero fill to position 4, long 34
3284 The same template may generally also be used in unpack().
3288 =item package NAMESPACE
3290 Declares the compilation unit as being in the given namespace. The scope
3291 of the package declaration is from the declaration itself through the end
3292 of the enclosing block, file, or eval (the same as the C<my> operator).
3293 All further unqualified dynamic identifiers will be in this namespace.
3294 A package statement affects only dynamic variables--including those
3295 you've used C<local> on--but I<not> lexical variables, which are created
3296 with C<my>. Typically it would be the first declaration in a file to
3297 be included by the C<require> or C<use> operator. You can switch into a
3298 package in more than one place; it merely influences which symbol table
3299 is used by the compiler for the rest of that block. You can refer to
3300 variables and filehandles in other packages by prefixing the identifier
3301 with the package name and a double colon: C<$Package::Variable>.
3302 If the package name is null, the C<main> package as assumed. That is,
3303 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3304 still seen in older code).
3306 If NAMESPACE is omitted, then there is no current package, and all
3307 identifiers must be fully qualified or lexicals. This is stricter
3308 than C<use strict>, since it also extends to function names.
3310 See L<perlmod/"Packages"> for more information about packages, modules,
3311 and classes. See L<perlsub> for other scoping issues.
3313 =item pipe READHANDLE,WRITEHANDLE
3315 Opens a pair of connected pipes like the corresponding system call.
3316 Note that if you set up a loop of piped processes, deadlock can occur
3317 unless you are very careful. In addition, note that Perl's pipes use
3318 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3319 after each command, depending on the application.
3321 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3322 for examples of such things.
3324 On systems that support a close-on-exec flag on files, the flag will be set
3325 for the newly opened file descriptors as determined by the value of $^F.
3332 Pops and returns the last value of the array, shortening the array by
3333 one element. Has an effect similar to
3337 If there are no elements in the array, returns the undefined value
3338 (although this may happen at other times as well). If ARRAY is
3339 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3340 array in subroutines, just like C<shift>.
3346 Returns the offset of where the last C<m//g> search left off for the variable
3347 is in question (C<$_> is used when the variable is not specified). May be
3348 modified to change that offset. Such modification will also influence
3349 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3352 =item print FILEHANDLE LIST
3358 Prints a string or a list of strings. Returns true if successful.
3359 FILEHANDLE may be a scalar variable name, in which case the variable
3360 contains the name of or a reference to the filehandle, thus introducing
3361 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3362 the next token is a term, it may be misinterpreted as an operator
3363 unless you interpose a C<+> or put parentheses around the arguments.)
3364 If FILEHANDLE is omitted, prints by default to standard output (or
3365 to the last selected output channel--see L</select>). If LIST is
3366 also omitted, prints C<$_> to the currently selected output channel.
3367 To set the default output channel to something other than STDOUT
3368 use the select operation. The current value of C<$,> (if any) is
3369 printed between each LIST item. The current value of C<$\> (if
3370 any) is printed after the entire LIST has been printed. Because
3371 print takes a LIST, anything in the LIST is evaluated in list
3372 context, and any subroutine that you call will have one or more of
3373 its expressions evaluated in list context. Also be careful not to
3374 follow the print keyword with a left parenthesis unless you want
3375 the corresponding right parenthesis to terminate the arguments to
3376 the print--interpose a C<+> or put parentheses around all the
3379 Note that if you're storing FILEHANDLES in an array or other expression,
3380 you will have to use a block returning its value instead:
3382 print { $files[$i] } "stuff\n";
3383 print { $OK ? STDOUT : STDERR } "stuff\n";
3385 =item printf FILEHANDLE FORMAT, LIST
3387 =item printf FORMAT, LIST
3389 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3390 (the output record separator) is not appended. The first argument
3391 of the list will be interpreted as the C<printf> format. If C<use locale> is
3392 in effect, the character used for the decimal point in formatted real numbers
3393 is affected by the LC_NUMERIC locale. See L<perllocale>.
3395 Don't fall into the trap of using a C<printf> when a simple
3396 C<print> would do. The C<print> is more efficient and less
3399 =item prototype FUNCTION
3401 Returns the prototype of a function as a string (or C<undef> if the
3402 function has no prototype). FUNCTION is a reference to, or the name of,
3403 the function whose prototype you want to retrieve.
3405 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3406 name for Perl builtin. If the builtin is not I<overridable> (such as
3407 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3408 C<system>) returns C<undef> because the builtin does not really behave
3409 like a Perl function. Otherwise, the string describing the equivalent
3410 prototype is returned.
3412 =item push ARRAY,LIST
3414 Treats ARRAY as a stack, and pushes the values of LIST
3415 onto the end of ARRAY. The length of ARRAY increases by the length of
3416 LIST. Has the same effect as
3419 $ARRAY[++$#ARRAY] = $value;
3422 but is more efficient. Returns the new number of elements in the array.
3434 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3436 =item quotemeta EXPR
3440 Returns the value of EXPR with all non-"word"
3441 characters backslashed. (That is, all characters not matching
3442 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3443 returned string, regardless of any locale settings.)
3444 This is the internal function implementing
3445 the C<\Q> escape in double-quoted strings.
3447 If EXPR is omitted, uses C<$_>.
3453 Returns a random fractional number greater than or equal to C<0> and less
3454 than the value of EXPR. (EXPR should be positive.) If EXPR is
3455 omitted, the value C<1> is used. Automatically calls C<srand> unless
3456 C<srand> has already been called. See also C<srand>.
3458 (Note: If your rand function consistently returns numbers that are too
3459 large or too small, then your version of Perl was probably compiled
3460 with the wrong number of RANDBITS.)
3462 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3464 =item read FILEHANDLE,SCALAR,LENGTH
3466 Attempts to read LENGTH bytes of data into variable SCALAR from the
3467 specified FILEHANDLE. Returns the number of bytes actually read,
3468 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3469 or shrunk to the length actually read. An OFFSET may be specified to
3470 place the read data at some other place than the beginning of the
3471 string. This call is actually implemented in terms of stdio's fread(3)
3472 call. To get a true read(2) system call, see C<sysread>.
3474 =item readdir DIRHANDLE
3476 Returns the next directory entry for a directory opened by C<opendir>.
3477 If used in list context, returns all the rest of the entries in the
3478 directory. If there are no more entries, returns an undefined value in
3479 scalar context or a null list in list context.
3481 If you're planning to filetest the return values out of a C<readdir>, you'd
3482 better prepend the directory in question. Otherwise, because we didn't
3483 C<chdir> there, it would have been testing the wrong file.
3485 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3486 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3491 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3492 context, each call reads and returns the next line, until end-of-file is
3493 reached, whereupon the subsequent call returns undef. In list context,
3494 reads until end-of-file is reached and returns a list of lines. Note that
3495 the notion of "line" used here is however you may have defined it
3496 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3498 When C<$/> is set to C<undef>, when readline() is in scalar
3499 context (i.e. file slurp mode), and when an empty file is read, it
3500 returns C<''> the first time, followed by C<undef> subsequently.
3502 This is the internal function implementing the C<< <EXPR> >>
3503 operator, but you can use it directly. The C<< <EXPR> >>
3504 operator is discussed in more detail in L<perlop/"I/O Operators">.
3507 $line = readline(*STDIN); # same thing
3513 Returns the value of a symbolic link, if symbolic links are
3514 implemented. If not, gives a fatal error. If there is some system
3515 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3516 omitted, uses C<$_>.
3520 EXPR is executed as a system command.
3521 The collected standard output of the command is returned.
3522 In scalar context, it comes back as a single (potentially
3523 multi-line) string. In list context, returns a list of lines
3524 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3525 This is the internal function implementing the C<qx/EXPR/>
3526 operator, but you can use it directly. The C<qx/EXPR/>
3527 operator is discussed in more detail in L<perlop/"I/O Operators">.
3529 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3531 Receives a message on a socket. Attempts to receive LENGTH bytes of
3532 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3533 will be grown or shrunk to the length actually read. Takes the same
3534 flags as the system call of the same name. Returns the address of the
3535 sender if SOCKET's protocol supports this; returns an empty string
3536 otherwise. If there's an error, returns the undefined value. This call
3537 is actually implemented in terms of recvfrom(2) system call. See
3538 L<perlipc/"UDP: Message Passing"> for examples.
3544 The C<redo> command restarts the loop block without evaluating the
3545 conditional again. The C<continue> block, if any, is not executed. If
3546 the LABEL is omitted, the command refers to the innermost enclosing
3547 loop. This command is normally used by programs that want to lie to
3548 themselves about what was just input:
3550 # a simpleminded Pascal comment stripper
3551 # (warning: assumes no { or } in strings)
3552 LINE: while (<STDIN>) {
3553 while (s|({.*}.*){.*}|$1 |) {}
3558 if (/}/) { # end of comment?
3567 C<redo> cannot be used to retry a block which returns a value such as
3568 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3569 a grep() or map() operation.
3571 Note that a block by itself is semantically identical to a loop
3572 that executes once. Thus C<redo> inside such a block will effectively
3573 turn it into a looping construct.
3575 See also L</continue> for an illustration of how C<last>, C<next>, and
3582 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3583 is not specified, C<$_> will be used. The value returned depends on the
3584 type of thing the reference is a reference to.
3585 Builtin types include:
3595 If the referenced object has been blessed into a package, then that package
3596 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3598 if (ref($r) eq "HASH") {
3599 print "r is a reference to a hash.\n";
3602 print "r is not a reference at all.\n";
3604 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3605 print "r is a reference to something that isa hash.\n";
3608 See also L<perlref>.
3610 =item rename OLDNAME,NEWNAME
3612 Changes the name of a file; an existing file NEWNAME will be
3613 clobbered. Returns true for success, false otherwise.
3615 Behavior of this function varies wildly depending on your system
3616 implementation. For example, it will usually not work across file system
3617 boundaries, even though the system I<mv> command sometimes compensates
3618 for this. Other restrictions include whether it works on directories,
3619 open files, or pre-existing files. Check L<perlport> and either the
3620 rename(2) manpage or equivalent system documentation for details.
3622 =item require VERSION
3628 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3631 If a VERSION is specified as a literal of the form v5.6.1,
3632 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3633 at least as recent as that version, at run time. (For compatibility
3634 with older versions of Perl, a numeric argument will also be interpreted
3635 as VERSION.) Compare with L</use>, which can do a similar check at
3638 require v5.6.1; # run time version check
3639 require 5.6.1; # ditto
3640 require 5.005_03; # float version allowed for compatibility
3642 Otherwise, demands that a library file be included if it hasn't already
3643 been included. The file is included via the do-FILE mechanism, which is
3644 essentially just a variety of C<eval>. Has semantics similar to the following
3649 return 1 if $INC{$filename};
3650 my($realfilename,$result);
3652 foreach $prefix (@INC) {
3653 $realfilename = "$prefix/$filename";
3654 if (-f $realfilename) {
3655 $INC{$filename} = $realfilename;
3656 $result = do $realfilename;
3660 die "Can't find $filename in \@INC";
3662 delete $INC{$filename} if $@ || !$result;
3664 die "$filename did not return true value" unless $result;
3668 Note that the file will not be included twice under the same specified
3669 name. The file must return true as the last statement to indicate
3670 successful execution of any initialization code, so it's customary to
3671 end such a file with C<1;> unless you're sure it'll return true
3672 otherwise. But it's better just to put the C<1;>, in case you add more
3675 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3676 replaces "F<::>" with "F</>" in the filename for you,
3677 to make it easy to load standard modules. This form of loading of
3678 modules does not risk altering your namespace.
3680 In other words, if you try this:
3682 require Foo::Bar; # a splendid bareword
3684 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3685 directories specified in the C<@INC> array.
3687 But if you try this:
3689 $class = 'Foo::Bar';
3690 require $class; # $class is not a bareword
3692 require "Foo::Bar"; # not a bareword because of the ""
3694 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3695 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3697 eval "require $class";
3699 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3705 Generally used in a C<continue> block at the end of a loop to clear
3706 variables and reset C<??> searches so that they work again. The
3707 expression is interpreted as a list of single characters (hyphens
3708 allowed for ranges). All variables and arrays beginning with one of
3709 those letters are reset to their pristine state. If the expression is
3710 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3711 only variables or searches in the current package. Always returns
3714 reset 'X'; # reset all X variables
3715 reset 'a-z'; # reset lower case variables
3716 reset; # just reset ?one-time? searches
3718 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3719 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3720 variables--lexical variables are unaffected, but they clean themselves
3721 up on scope exit anyway, so you'll probably want to use them instead.
3728 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3729 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3730 context, depending on how the return value will be used, and the context
3731 may vary from one execution to the next (see C<wantarray>). If no EXPR
3732 is given, returns an empty list in list context, the undefined value in
3733 scalar context, and (of course) nothing at all in a void context.
3735 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3736 or do FILE will automatically return the value of the last expression
3741 In list context, returns a list value consisting of the elements
3742 of LIST in the opposite order. In scalar context, concatenates the
3743 elements of LIST and returns a string value with all characters
3744 in the opposite order.
3746 print reverse <>; # line tac, last line first
3748 undef $/; # for efficiency of <>
3749 print scalar reverse <>; # character tac, last line tsrif
3751 This operator is also handy for inverting a hash, although there are some
3752 caveats. If a value is duplicated in the original hash, only one of those
3753 can be represented as a key in the inverted hash. Also, this has to
3754 unwind one hash and build a whole new one, which may take some time
3755 on a large hash, such as from a DBM file.
3757 %by_name = reverse %by_address; # Invert the hash
3759 =item rewinddir DIRHANDLE
3761 Sets the current position to the beginning of the directory for the
3762 C<readdir> routine on DIRHANDLE.
3764 =item rindex STR,SUBSTR,POSITION
3766 =item rindex STR,SUBSTR
3768 Works just like index() except that it returns the position of the LAST
3769 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3770 last occurrence at or before that position.
3772 =item rmdir FILENAME
3776 Deletes the directory specified by FILENAME if that directory is empty. If it
3777 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3778 FILENAME is omitted, uses C<$_>.
3782 The substitution operator. See L<perlop>.
3786 Forces EXPR to be interpreted in scalar context and returns the value
3789 @counts = ( scalar @a, scalar @b, scalar @c );
3791 There is no equivalent operator to force an expression to
3792 be interpolated in list context because in practice, this is never
3793 needed. If you really wanted to do so, however, you could use
3794 the construction C<@{[ (some expression) ]}>, but usually a simple
3795 C<(some expression)> suffices.
3797 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3798 parenthesized list, this behaves as a scalar comma expression, evaluating
3799 all but the last element in void context and returning the final element
3800 evaluated in scalar context. This is seldom what you want.
3802 The following single statement:
3804 print uc(scalar(&foo,$bar)),$baz;
3806 is the moral equivalent of these two:
3809 print(uc($bar),$baz);
3811 See L<perlop> for more details on unary operators and the comma operator.
3813 =item seek FILEHANDLE,POSITION,WHENCE
3815 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3816 FILEHANDLE may be an expression whose value gives the name of the
3817 filehandle. The values for WHENCE are C<0> to set the new position to
3818 POSITION, C<1> to set it to the current position plus POSITION, and
3819 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3820 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3821 (start of the file, current position, end of the file) from the Fcntl
3822 module. Returns C<1> upon success, C<0> otherwise.
3824 If you want to position file for C<sysread> or C<syswrite>, don't use
3825 C<seek>--buffering makes its effect on the file's system position
3826 unpredictable and non-portable. Use C<sysseek> instead.
3828 Due to the rules and rigors of ANSI C, on some systems you have to do a
3829 seek whenever you switch between reading and writing. Amongst other
3830 things, this may have the effect of calling stdio's clearerr(3).
3831 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3835 This is also useful for applications emulating C<tail -f>. Once you hit
3836 EOF on your read, and then sleep for a while, you might have to stick in a
3837 seek() to reset things. The C<seek> doesn't change the current position,
3838 but it I<does> clear the end-of-file condition on the handle, so that the
3839 next C<< <FILE> >> makes Perl try again to read something. We hope.
3841 If that doesn't work (some stdios are particularly cantankerous), then
3842 you may need something more like this:
3845 for ($curpos = tell(FILE); $_ = <FILE>;
3846 $curpos = tell(FILE)) {
3847 # search for some stuff and put it into files
3849 sleep($for_a_while);
3850 seek(FILE, $curpos, 0);
3853 =item seekdir DIRHANDLE,POS
3855 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3856 must be a value returned by C<telldir>. Has the same caveats about
3857 possible directory compaction as the corresponding system library
3860 =item select FILEHANDLE
3864 Returns the currently selected filehandle. Sets the current default
3865 filehandle for output, if FILEHANDLE is supplied. This has two
3866 effects: first, a C<write> or a C<print> without a filehandle will
3867 default to this FILEHANDLE. Second, references to variables related to
3868 output will refer to this output channel. For example, if you have to
3869 set the top of form format for more than one output channel, you might
3877 FILEHANDLE may be an expression whose value gives the name of the
3878 actual filehandle. Thus:
3880 $oldfh = select(STDERR); $| = 1; select($oldfh);
3882 Some programmers may prefer to think of filehandles as objects with
3883 methods, preferring to write the last example as:
3886 STDERR->autoflush(1);
3888 =item select RBITS,WBITS,EBITS,TIMEOUT
3890 This calls the select(2) system call with the bit masks specified, which
3891 can be constructed using C<fileno> and C<vec>, along these lines:
3893 $rin = $win = $ein = '';
3894 vec($rin,fileno(STDIN),1) = 1;
3895 vec($win,fileno(STDOUT),1) = 1;
3898 If you want to select on many filehandles you might wish to write a
3902 my(@fhlist) = split(' ',$_[0]);
3905 vec($bits,fileno($_),1) = 1;
3909 $rin = fhbits('STDIN TTY SOCK');
3913 ($nfound,$timeleft) =
3914 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3916 or to block until something becomes ready just do this
3918 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3920 Most systems do not bother to return anything useful in $timeleft, so
3921 calling select() in scalar context just returns $nfound.
3923 Any of the bit masks can also be undef. The timeout, if specified, is
3924 in seconds, which may be fractional. Note: not all implementations are
3925 capable of returning the$timeleft. If not, they always return
3926 $timeleft equal to the supplied $timeout.
3928 You can effect a sleep of 250 milliseconds this way:
3930 select(undef, undef, undef, 0.25);
3932 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3933 or <FH>) with C<select>, except as permitted by POSIX, and even
3934 then only on POSIX systems. You have to use C<sysread> instead.
3936 =item semctl ID,SEMNUM,CMD,ARG
3938 Calls the System V IPC function C<semctl>. You'll probably have to say
3942 first to get the correct constant definitions. If CMD is IPC_STAT or
3943 GETALL, then ARG must be a variable which will hold the returned
3944 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3945 the undefined value for error, "C<0 but true>" for zero, or the actual
3946 return value otherwise. The ARG must consist of a vector of native
3947 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3948 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
3951 =item semget KEY,NSEMS,FLAGS
3953 Calls the System V IPC function semget. Returns the semaphore id, or
3954 the undefined value if there is an error. See also
3955 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
3958 =item semop KEY,OPSTRING
3960 Calls the System V IPC function semop to perform semaphore operations
3961 such as signaling and waiting. OPSTRING must be a packed array of
3962 semop structures. Each semop structure can be generated with
3963 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3964 operations is implied by the length of OPSTRING. Returns true if
3965 successful, or false if there is an error. As an example, the
3966 following code waits on semaphore $semnum of semaphore id $semid:
3968 $semop = pack("sss", $semnum, -1, 0);
3969 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3971 To signal the semaphore, replace C<-1> with C<1>. See also
3972 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
3975 =item send SOCKET,MSG,FLAGS,TO
3977 =item send SOCKET,MSG,FLAGS
3979 Sends a message on a socket. Takes the same flags as the system call
3980 of the same name. On unconnected sockets you must specify a
3981 destination to send TO, in which case it does a C C<sendto>. Returns
3982 the number of characters sent, or the undefined value if there is an
3983 error. The C system call sendmsg(2) is currently unimplemented.
3984 See L<perlipc/"UDP: Message Passing"> for examples.
3986 =item setpgrp PID,PGRP
3988 Sets the current process group for the specified PID, C<0> for the current
3989 process. Will produce a fatal error if used on a machine that doesn't
3990 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3991 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3992 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3995 =item setpriority WHICH,WHO,PRIORITY
3997 Sets the current priority for a process, a process group, or a user.
3998 (See setpriority(2).) Will produce a fatal error if used on a machine
3999 that doesn't implement setpriority(2).
4001 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4003 Sets the socket option requested. Returns undefined if there is an
4004 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4011 Shifts the first value of the array off and returns it, shortening the
4012 array by 1 and moving everything down. If there are no elements in the
4013 array, returns the undefined value. If ARRAY is omitted, shifts the
4014 C<@_> array within the lexical scope of subroutines and formats, and the
4015 C<@ARGV> array at file scopes or within the lexical scopes established by
4016 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4019 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4020 same thing to the left end of an array that C<pop> and C<push> do to the
4023 =item shmctl ID,CMD,ARG
4025 Calls the System V IPC function shmctl. You'll probably have to say
4029 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4030 then ARG must be a variable which will hold the returned C<shmid_ds>
4031 structure. Returns like ioctl: the undefined value for error, "C<0> but
4032 true" for zero, or the actual return value otherwise.
4033 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4035 =item shmget KEY,SIZE,FLAGS
4037 Calls the System V IPC function shmget. Returns the shared memory
4038 segment id, or the undefined value if there is an error.
4039 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4041 =item shmread ID,VAR,POS,SIZE
4043 =item shmwrite ID,STRING,POS,SIZE
4045 Reads or writes the System V shared memory segment ID starting at
4046 position POS for size SIZE by attaching to it, copying in/out, and
4047 detaching from it. When reading, VAR must be a variable that will
4048 hold the data read. When writing, if STRING is too long, only SIZE
4049 bytes are used; if STRING is too short, nulls are written to fill out
4050 SIZE bytes. Return true if successful, or false if there is an error.
4051 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4052 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4054 =item shutdown SOCKET,HOW
4056 Shuts down a socket connection in the manner indicated by HOW, which
4057 has the same interpretation as in the system call of the same name.
4059 shutdown(SOCKET, 0); # I/we have stopped reading data
4060 shutdown(SOCKET, 1); # I/we have stopped writing data
4061 shutdown(SOCKET, 2); # I/we have stopped using this socket
4063 This is useful with sockets when you want to tell the other
4064 side you're done writing but not done reading, or vice versa.
4065 It's also a more insistent form of close because it also
4066 disables the file descriptor in any forked copies in other
4073 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4074 returns sine of C<$_>.
4076 For the inverse sine operation, you may use the C<Math::Trig::asin>
4077 function, or use this relation:
4079 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4085 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4086 May be interrupted if the process receives a signal such as C<SIGALRM>.
4087 Returns the number of seconds actually slept. You probably cannot
4088 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4091 On some older systems, it may sleep up to a full second less than what
4092 you requested, depending on how it counts seconds. Most modern systems
4093 always sleep the full amount. They may appear to sleep longer than that,
4094 however, because your process might not be scheduled right away in a
4095 busy multitasking system.
4097 For delays of finer granularity than one second, you may use Perl's
4098 C<syscall> interface to access setitimer(2) if your system supports
4099 it, or else see L</select> above. The Time::HiRes module from CPAN
4102 See also the POSIX module's C<sigpause> function.
4104 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4106 Opens a socket of the specified kind and attaches it to filehandle
4107 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4108 the system call of the same name. You should C<use Socket> first
4109 to get the proper definitions imported. See the examples in
4110 L<perlipc/"Sockets: Client/Server Communication">.
4112 On systems that support a close-on-exec flag on files, the flag will
4113 be set for the newly opened file descriptor, as determined by the
4114 value of $^F. See L<perlvar/$^F>.
4116 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4118 Creates an unnamed pair of sockets in the specified domain, of the
4119 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4120 for the system call of the same name. If unimplemented, yields a fatal
4121 error. Returns true if successful.
4123 On systems that support a close-on-exec flag on files, the flag will
4124 be set for the newly opened file descriptors, as determined by the value
4125 of $^F. See L<perlvar/$^F>.
4127 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4128 to C<pipe(Rdr, Wtr)> is essentially:
4131 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4132 shutdown(Rdr, 1); # no more writing for reader
4133 shutdown(Wtr, 0); # no more reading for writer
4135 See L<perlipc> for an example of socketpair use.
4137 =item sort SUBNAME LIST
4139 =item sort BLOCK LIST
4143 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4144 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4145 specified, it gives the name of a subroutine that returns an integer
4146 less than, equal to, or greater than C<0>, depending on how the elements
4147 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4148 operators are extremely useful in such routines.) SUBNAME may be a
4149 scalar variable name (unsubscripted), in which case the value provides
4150 the name of (or a reference to) the actual subroutine to use. In place
4151 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4154 If the subroutine's prototype is C<($$)>, the elements to be compared
4155 are passed by reference in C<@_>, as for a normal subroutine. This is
4156 slower than unprototyped subroutines, where the elements to be
4157 compared are passed into the subroutine
4158 as the package global variables $a and $b (see example below). Note that
4159 in the latter case, it is usually counter-productive to declare $a and
4162 In either case, the subroutine may not be recursive. The values to be
4163 compared are always passed by reference, so don't modify them.
4165 You also cannot exit out of the sort block or subroutine using any of the
4166 loop control operators described in L<perlsyn> or with C<goto>.
4168 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4169 current collation locale. See L<perllocale>.
4174 @articles = sort @files;
4176 # same thing, but with explicit sort routine
4177 @articles = sort {$a cmp $b} @files;
4179 # now case-insensitively
4180 @articles = sort {uc($a) cmp uc($b)} @files;
4182 # same thing in reversed order
4183 @articles = sort {$b cmp $a} @files;
4185 # sort numerically ascending
4186 @articles = sort {$a <=> $b} @files;
4188 # sort numerically descending
4189 @articles = sort {$b <=> $a} @files;
4191 # this sorts the %age hash by value instead of key
4192 # using an in-line function
4193 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4195 # sort using explicit subroutine name
4197 $age{$a} <=> $age{$b}; # presuming numeric
4199 @sortedclass = sort byage @class;
4201 sub backwards { $b cmp $a }
4202 @harry = qw(dog cat x Cain Abel);
4203 @george = qw(gone chased yz Punished Axed);
4205 # prints AbelCaincatdogx
4206 print sort backwards @harry;
4207 # prints xdogcatCainAbel
4208 print sort @george, 'to', @harry;
4209 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4211 # inefficiently sort by descending numeric compare using
4212 # the first integer after the first = sign, or the
4213 # whole record case-insensitively otherwise
4216 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4221 # same thing, but much more efficiently;
4222 # we'll build auxiliary indices instead
4226 push @nums, /=(\d+)/;
4231 $nums[$b] <=> $nums[$a]
4233 $caps[$a] cmp $caps[$b]
4237 # same thing, but without any temps
4238 @new = map { $_->[0] }
4239 sort { $b->[1] <=> $a->[1]
4242 } map { [$_, /=(\d+)/, uc($_)] } @old;
4244 # using a prototype allows you to use any comparison subroutine
4245 # as a sort subroutine (including other package's subroutines)
4247 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4250 @new = sort other::backwards @old;
4252 If you're using strict, you I<must not> declare $a
4253 and $b as lexicals. They are package globals. That means
4254 if you're in the C<main> package and type
4256 @articles = sort {$b <=> $a} @files;
4258 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4259 but if you're in the C<FooPack> package, it's the same as typing
4261 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4263 The comparison function is required to behave. If it returns
4264 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4265 sometimes saying the opposite, for example) the results are not
4268 =item splice ARRAY,OFFSET,LENGTH,LIST
4270 =item splice ARRAY,OFFSET,LENGTH
4272 =item splice ARRAY,OFFSET
4276 Removes the elements designated by OFFSET and LENGTH from an array, and
4277 replaces them with the elements of LIST, if any. In list context,
4278 returns the elements removed from the array. In scalar context,
4279 returns the last element removed, or C<undef> if no elements are
4280 removed. The array grows or shrinks as necessary.
4281 If OFFSET is negative then it starts that far from the end of the array.
4282 If LENGTH is omitted, removes everything from OFFSET onward.
4283 If LENGTH is negative, leaves that many elements off the end of the array.
4284 If both OFFSET and LENGTH are omitted, removes everything.
4286 The following equivalences hold (assuming C<$[ == 0>):
4288 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4289 pop(@a) splice(@a,-1)
4290 shift(@a) splice(@a,0,1)
4291 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4292 $a[$x] = $y splice(@a,$x,1,$y)
4294 Example, assuming array lengths are passed before arrays:
4296 sub aeq { # compare two list values
4297 my(@a) = splice(@_,0,shift);
4298 my(@b) = splice(@_,0,shift);
4299 return 0 unless @a == @b; # same len?
4301 return 0 if pop(@a) ne pop(@b);
4305 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4307 =item split /PATTERN/,EXPR,LIMIT
4309 =item split /PATTERN/,EXPR
4311 =item split /PATTERN/
4315 Splits a string into a list of strings and returns that list. By default,
4316 empty leading fields are preserved, and empty trailing ones are deleted.
4318 In scalar context, returns the number of fields found and splits into
4319 the C<@_> array. Use of split in scalar context is deprecated, however,
4320 because it clobbers your subroutine arguments.
4322 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4323 splits on whitespace (after skipping any leading whitespace). Anything
4324 matching PATTERN is taken to be a delimiter separating the fields. (Note
4325 that the delimiter may be longer than one character.)
4327 If LIMIT is specified and positive, splits into no more than that
4328 many fields (though it may split into fewer). If LIMIT is unspecified
4329 or zero, trailing null fields are stripped (which potential users
4330 of C<pop> would do well to remember). If LIMIT is negative, it is
4331 treated as if an arbitrarily large LIMIT had been specified.
4333 A pattern matching the null string (not to be confused with
4334 a null pattern C<//>, which is just one member of the set of patterns
4335 matching a null string) will split the value of EXPR into separate
4336 characters at each point it matches that way. For example:
4338 print join(':', split(/ */, 'hi there'));
4340 produces the output 'h:i:t:h:e:r:e'.
4342 The LIMIT parameter can be used to split a line partially
4344 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4346 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4347 one larger than the number of variables in the list, to avoid
4348 unnecessary work. For the list above LIMIT would have been 4 by
4349 default. In time critical applications it behooves you not to split
4350 into more fields than you really need.
4352 If the PATTERN contains parentheses, additional list elements are
4353 created from each matching substring in the delimiter.
4355 split(/([,-])/, "1-10,20", 3);
4357 produces the list value
4359 (1, '-', 10, ',', 20)
4361 If you had the entire header of a normal Unix email message in $header,
4362 you could split it up into fields and their values this way:
4364 $header =~ s/\n\s+/ /g; # fix continuation lines
4365 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4367 The pattern C</PATTERN/> may be replaced with an expression to specify
4368 patterns that vary at runtime. (To do runtime compilation only once,
4369 use C</$variable/o>.)
4371 As a special case, specifying a PATTERN of space (C<' '>) will split on
4372 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4373 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4374 will give you as many null initial fields as there are leading spaces.
4375 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4376 whitespace produces a null first field. A C<split> with no arguments
4377 really does a C<split(' ', $_)> internally.
4379 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4384 open(PASSWD, '/etc/passwd');
4386 ($login, $passwd, $uid, $gid,
4387 $gcos, $home, $shell) = split(/:/);
4391 (Note that $shell above will still have a newline on it. See L</chop>,
4392 L</chomp>, and L</join>.)
4394 =item sprintf FORMAT, LIST
4396 Returns a string formatted by the usual C<printf> conventions of the C
4397 library function C<sprintf>. See below for more details
4398 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4399 the general principles.
4403 # Format number with up to 8 leading zeroes
4404 $result = sprintf("%08d", $number);
4406 # Round number to 3 digits after decimal point
4407 $rounded = sprintf("%.3f", $number);
4409 Perl does its own C<sprintf> formatting--it emulates the C
4410 function C<sprintf>, but it doesn't use it (except for floating-point
4411 numbers, and even then only the standard modifiers are allowed). As a
4412 result, any non-standard extensions in your local C<sprintf> are not
4413 available from Perl.
4415 Perl's C<sprintf> permits the following universally-known conversions:
4418 %c a character with the given number
4420 %d a signed integer, in decimal
4421 %u an unsigned integer, in decimal
4422 %o an unsigned integer, in octal
4423 %x an unsigned integer, in hexadecimal
4424 %e a floating-point number, in scientific notation
4425 %f a floating-point number, in fixed decimal notation
4426 %g a floating-point number, in %e or %f notation
4428 In addition, Perl permits the following widely-supported conversions:
4430 %X like %x, but using upper-case letters
4431 %E like %e, but using an upper-case "E"
4432 %G like %g, but with an upper-case "E" (if applicable)
4433 %b an unsigned integer, in binary
4434 %p a pointer (outputs the Perl value's address in hexadecimal)
4435 %n special: *stores* the number of characters output so far
4436 into the next variable in the parameter list
4438 Finally, for backward (and we do mean "backward") compatibility, Perl
4439 permits these unnecessary but widely-supported conversions:
4442 %D a synonym for %ld
4443 %U a synonym for %lu
4444 %O a synonym for %lo
4447 Perl permits the following universally-known flags between the C<%>
4448 and the conversion letter:
4450 space prefix positive number with a space
4451 + prefix positive number with a plus sign
4452 - left-justify within the field
4453 0 use zeros, not spaces, to right-justify
4454 # prefix non-zero octal with "0", non-zero hex with "0x"
4455 number minimum field width
4456 .number "precision": digits after decimal point for
4457 floating-point, max length for string, minimum length
4459 l interpret integer as C type "long" or "unsigned long"
4460 h interpret integer as C type "short" or "unsigned short"
4461 If no flags, interpret integer as C type "int" or "unsigned"
4463 There are also two Perl-specific flags:
4465 V interpret integer as Perl's standard integer type
4466 v interpret string as a vector of integers, output as
4467 numbers separated either by dots, or by an arbitrary
4468 string received from the argument list when the flag
4471 Where a number would appear in the flags, an asterisk (C<*>) may be
4472 used instead, in which case Perl uses the next item in the parameter
4473 list as the given number (that is, as the field width or precision).
4474 If a field width obtained through C<*> is negative, it has the same
4475 effect as the C<-> flag: left-justification.
4477 The C<v> flag is useful for displaying ordinal values of characters
4478 in arbitrary strings:
4480 printf "version is v%vd\n", $^V; # Perl's version
4481 printf "address is %*vX\n", ":", $addr; # IPv6 address
4482 printf "bits are %*vb\n", " ", $bits; # random bitstring
4484 If C<use locale> is in effect, the character used for the decimal
4485 point in formatted real numbers is affected by the LC_NUMERIC locale.
4488 If Perl understands "quads" (64-bit integers) (this requires
4489 either that the platform natively support quads or that Perl
4490 be specifically compiled to support quads), the characters
4494 print quads, and they may optionally be preceded by
4502 You can find out whether your Perl supports quads via L<Config>:
4505 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4508 If Perl understands "long doubles" (this requires that the platform
4509 support long doubles), the flags
4513 may optionally be preceded by
4521 You can find out whether your Perl supports long doubles via L<Config>:
4524 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4530 Return the square root of EXPR. If EXPR is omitted, returns square
4531 root of C<$_>. Only works on non-negative operands, unless you've
4532 loaded the standard Math::Complex module.
4535 print sqrt(-2); # prints 1.4142135623731i
4541 Sets the random number seed for the C<rand> operator. If EXPR is
4542 omitted, uses a semi-random value supplied by the kernel (if it supports
4543 the F</dev/urandom> device) or based on the current time and process
4544 ID, among other things. In versions of Perl prior to 5.004 the default
4545 seed was just the current C<time>. This isn't a particularly good seed,
4546 so many old programs supply their own seed value (often C<time ^ $$> or
4547 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4549 In fact, it's usually not necessary to call C<srand> at all, because if
4550 it is not called explicitly, it is called implicitly at the first use of
4551 the C<rand> operator. However, this was not the case in version of Perl
4552 before 5.004, so if your script will run under older Perl versions, it
4553 should call C<srand>.
4555 Note that you need something much more random than the default seed for
4556 cryptographic purposes. Checksumming the compressed output of one or more
4557 rapidly changing operating system status programs is the usual method. For
4560 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4562 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4565 Do I<not> call C<srand> multiple times in your program unless you know
4566 exactly what you're doing and why you're doing it. The point of the
4567 function is to "seed" the C<rand> function so that C<rand> can produce
4568 a different sequence each time you run your program. Just do it once at the
4569 top of your program, or you I<won't> get random numbers out of C<rand>!
4571 Frequently called programs (like CGI scripts) that simply use
4575 for a seed can fall prey to the mathematical property that
4579 one-third of the time. So don't do that.
4581 =item stat FILEHANDLE
4587 Returns a 13-element list giving the status info for a file, either
4588 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4589 it stats C<$_>. Returns a null list if the stat fails. Typically used
4592 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4593 $atime,$mtime,$ctime,$blksize,$blocks)
4596 Not all fields are supported on all filesystem types. Here are the
4597 meaning of the fields:
4599 0 dev device number of filesystem
4601 2 mode file mode (type and permissions)
4602 3 nlink number of (hard) links to the file
4603 4 uid numeric user ID of file's owner
4604 5 gid numeric group ID of file's owner
4605 6 rdev the device identifier (special files only)
4606 7 size total size of file, in bytes
4607 8 atime last access time in seconds since the epoch
4608 9 mtime last modify time in seconds since the epoch
4609 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4610 11 blksize preferred block size for file system I/O
4611 12 blocks actual number of blocks allocated
4613 (The epoch was at 00:00 January 1, 1970 GMT.)
4615 If stat is passed the special filehandle consisting of an underline, no
4616 stat is done, but the current contents of the stat structure from the
4617 last stat or filetest are returned. Example:
4619 if (-x $file && (($d) = stat(_)) && $d < 0) {
4620 print "$file is executable NFS file\n";
4623 (This works on machines only for which the device number is negative
4626 Because the mode contains both the file type and its permissions, you
4627 should mask off the file type portion and (s)printf using a C<"%o">
4628 if you want to see the real permissions.
4630 $mode = (stat($filename))[2];
4631 printf "Permissions are %04o\n", $mode & 07777;
4633 In scalar context, C<stat> returns a boolean value indicating success
4634 or failure, and, if successful, sets the information associated with
4635 the special filehandle C<_>.
4637 The File::stat module provides a convenient, by-name access mechanism:
4640 $sb = stat($filename);
4641 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4642 $filename, $sb->size, $sb->mode & 07777,
4643 scalar localtime $sb->mtime;
4645 You can import symbolic mode constants (C<S_IF*>) and functions
4646 (C<S_IS*>) from the Fcntl module:
4650 $mode = (stat($filename))[2];
4652 $user_rwx = ($mode & S_IRWXU) >> 6;
4653 $group_read = ($mode & S_IRGRP) >> 3;
4654 $other_execute = $mode & S_IXOTH;
4656 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4658 $is_setuid = $mode & S_ISUID;
4659 $is_setgid = S_ISDIR($mode);
4661 You could write the last two using the C<-u> and C<-d> operators.
4662 The commonly available S_IF* constants are
4664 # Permissions: read, write, execute, for user, group, others.
4666 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4667 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4668 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4670 # Setuid/Setgid/Stickiness.
4672 S_ISUID S_ISGID S_ISVTX S_ISTXT
4674 # File types. Not necessarily all are available on your system.
4676 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4678 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4680 S_IREAD S_IWRITE S_IEXEC
4682 and the S_IF* functions are
4684 S_IFMODE($mode) the part of $mode containing the permission bits
4685 and the setuid/setgid/sticky bits
4687 S_IFMT($mode) the part of $mode containing the file type
4688 which can be bit-anded with e.g. S_IFREG
4689 or with the following functions
4691 # The operators -f, -d, -l, -b, -c, -p, and -s.
4693 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4694 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4696 # No direct -X operator counterpart, but for the first one
4697 # the -g operator is often equivalent. The ENFMT stands for
4698 # record flocking enforcement, a platform-dependent feature.
4700 S_ISENFMT($mode) S_ISWHT($mode)
4702 See your native chmod(2) and stat(2) documentation for more details
4703 about the S_* constants.
4709 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4710 doing many pattern matches on the string before it is next modified.
4711 This may or may not save time, depending on the nature and number of
4712 patterns you are searching on, and on the distribution of character
4713 frequencies in the string to be searched--you probably want to compare
4714 run times with and without it to see which runs faster. Those loops
4715 which scan for many short constant strings (including the constant
4716 parts of more complex patterns) will benefit most. You may have only
4717 one C<study> active at a time--if you study a different scalar the first
4718 is "unstudied". (The way C<study> works is this: a linked list of every
4719 character in the string to be searched is made, so we know, for
4720 example, where all the C<'k'> characters are. From each search string,
4721 the rarest character is selected, based on some static frequency tables
4722 constructed from some C programs and English text. Only those places
4723 that contain this "rarest" character are examined.)
4725 For example, here is a loop that inserts index producing entries
4726 before any line containing a certain pattern:
4730 print ".IX foo\n" if /\bfoo\b/;
4731 print ".IX bar\n" if /\bbar\b/;
4732 print ".IX blurfl\n" if /\bblurfl\b/;
4737 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4738 will be looked at, because C<f> is rarer than C<o>. In general, this is
4739 a big win except in pathological cases. The only question is whether
4740 it saves you more time than it took to build the linked list in the
4743 Note that if you have to look for strings that you don't know till
4744 runtime, you can build an entire loop as a string and C<eval> that to
4745 avoid recompiling all your patterns all the time. Together with
4746 undefining C<$/> to input entire files as one record, this can be very
4747 fast, often faster than specialized programs like fgrep(1). The following
4748 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4749 out the names of those files that contain a match:
4751 $search = 'while (<>) { study;';
4752 foreach $word (@words) {
4753 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4758 eval $search; # this screams
4759 $/ = "\n"; # put back to normal input delimiter
4760 foreach $file (sort keys(%seen)) {
4768 =item sub NAME BLOCK
4770 This is subroutine definition, not a real function I<per se>. With just a
4771 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4772 Without a NAME, it's an anonymous function declaration, and does actually
4773 return a value: the CODE ref of the closure you just created. See L<perlsub>
4774 and L<perlref> for details.
4776 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4778 =item substr EXPR,OFFSET,LENGTH
4780 =item substr EXPR,OFFSET
4782 Extracts a substring out of EXPR and returns it. First character is at
4783 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4784 If OFFSET is negative (or more precisely, less than C<$[>), starts
4785 that far from the end of the string. If LENGTH is omitted, returns
4786 everything to the end of the string. If LENGTH is negative, leaves that
4787 many characters off the end of the string.
4789 You can use the substr() function as an lvalue, in which case EXPR
4790 must itself be an lvalue. If you assign something shorter than LENGTH,
4791 the string will shrink, and if you assign something longer than LENGTH,
4792 the string will grow to accommodate it. To keep the string the same
4793 length you may need to pad or chop your value using C<sprintf>.
4795 If OFFSET and LENGTH specify a substring that is partly outside the
4796 string, only the part within the string is returned. If the substring
4797 is beyond either end of the string, substr() returns the undefined
4798 value and produces a warning. When used as an lvalue, specifying a
4799 substring that is entirely outside the string is a fatal error.
4800 Here's an example showing the behavior for boundary cases:
4803 substr($name, 4) = 'dy'; # $name is now 'freddy'
4804 my $null = substr $name, 6, 2; # returns '' (no warning)
4805 my $oops = substr $name, 7; # returns undef, with warning
4806 substr($name, 7) = 'gap'; # fatal error
4808 An alternative to using substr() as an lvalue is to specify the
4809 replacement string as the 4th argument. This allows you to replace
4810 parts of the EXPR and return what was there before in one operation,
4811 just as you can with splice().
4813 =item symlink OLDFILE,NEWFILE
4815 Creates a new filename symbolically linked to the old filename.
4816 Returns C<1> for success, C<0> otherwise. On systems that don't support
4817 symbolic links, produces a fatal error at run time. To check for that,
4820 $symlink_exists = eval { symlink("",""); 1 };
4824 Calls the system call specified as the first element of the list,
4825 passing the remaining elements as arguments to the system call. If
4826 unimplemented, produces a fatal error. The arguments are interpreted
4827 as follows: if a given argument is numeric, the argument is passed as
4828 an int. If not, the pointer to the string value is passed. You are
4829 responsible to make sure a string is pre-extended long enough to
4830 receive any result that might be written into a string. You can't use a
4831 string literal (or other read-only string) as an argument to C<syscall>
4832 because Perl has to assume that any string pointer might be written
4834 integer arguments are not literals and have never been interpreted in a
4835 numeric context, you may need to add C<0> to them to force them to look
4836 like numbers. This emulates the C<syswrite> function (or vice versa):
4838 require 'syscall.ph'; # may need to run h2ph
4840 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4842 Note that Perl supports passing of up to only 14 arguments to your system call,
4843 which in practice should usually suffice.
4845 Syscall returns whatever value returned by the system call it calls.
4846 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4847 Note that some system calls can legitimately return C<-1>. The proper
4848 way to handle such calls is to assign C<$!=0;> before the call and
4849 check the value of C<$!> if syscall returns C<-1>.
4851 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4852 number of the read end of the pipe it creates. There is no way
4853 to retrieve the file number of the other end. You can avoid this
4854 problem by using C<pipe> instead.
4856 =item sysopen FILEHANDLE,FILENAME,MODE
4858 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4860 Opens the file whose filename is given by FILENAME, and associates it
4861 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4862 the name of the real filehandle wanted. This function calls the
4863 underlying operating system's C<open> function with the parameters
4864 FILENAME, MODE, PERMS.
4866 The possible values and flag bits of the MODE parameter are
4867 system-dependent; they are available via the standard module C<Fcntl>.
4868 See the documentation of your operating system's C<open> to see which
4869 values and flag bits are available. You may combine several flags
4870 using the C<|>-operator.
4872 Some of the most common values are C<O_RDONLY> for opening the file in
4873 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4874 and C<O_RDWR> for opening the file in read-write mode, and.
4876 For historical reasons, some values work on almost every system
4877 supported by perl: zero means read-only, one means write-only, and two
4878 means read/write. We know that these values do I<not> work under
4879 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4880 use them in new code.
4882 If the file named by FILENAME does not exist and the C<open> call creates
4883 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4884 PERMS specifies the permissions of the newly created file. If you omit
4885 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4886 These permission values need to be in octal, and are modified by your
4887 process's current C<umask>.
4889 In many systems the C<O_EXCL> flag is available for opening files in
4890 exclusive mode. This is B<not> locking: exclusiveness means here that
4891 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4894 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4896 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4897 that takes away the user's option to have a more permissive umask.
4898 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4901 Note that C<sysopen> depends on the fdopen() C library function.
4902 On many UNIX systems, fdopen() is known to fail when file descriptors
4903 exceed a certain value, typically 255. If you need more file
4904 descriptors than that, consider rebuilding Perl to use the C<sfio>
4905 library, or perhaps using the POSIX::open() function.
4907 See L<perlopentut> for a kinder, gentler explanation of opening files.
4909 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4911 =item sysread FILEHANDLE,SCALAR,LENGTH
4913 Attempts to read LENGTH bytes of data into variable SCALAR from the
4914 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4915 so mixing this with other kinds of reads, C<print>, C<write>,
4916 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4917 usually buffers data. Returns the number of bytes actually read, C<0>
4918 at end of file, or undef if there was an error. SCALAR will be grown or
4919 shrunk so that the last byte actually read is the last byte of the
4920 scalar after the read.
4922 An OFFSET may be specified to place the read data at some place in the
4923 string other than the beginning. A negative OFFSET specifies
4924 placement at that many bytes counting backwards from the end of the
4925 string. A positive OFFSET greater than the length of SCALAR results
4926 in the string being padded to the required size with C<"\0"> bytes before
4927 the result of the read is appended.
4929 There is no syseof() function, which is ok, since eof() doesn't work
4930 very well on device files (like ttys) anyway. Use sysread() and check
4931 for a return value for 0 to decide whether you're done.
4933 =item sysseek FILEHANDLE,POSITION,WHENCE
4935 Sets FILEHANDLE's system position using the system call lseek(2). It
4936 bypasses stdio, so mixing this with reads (other than C<sysread>),
4937 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4938 FILEHANDLE may be an expression whose value gives the name of the
4939 filehandle. The values for WHENCE are C<0> to set the new position to
4940 POSITION, C<1> to set the it to the current position plus POSITION,
4941 and C<2> to set it to EOF plus POSITION (typically negative). For
4942 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4943 C<SEEK_END> (start of the file, current position, end of the file)
4944 from the Fcntl module.
4946 Returns the new position, or the undefined value on failure. A position
4947 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4948 true on success and false on failure, yet you can still easily determine
4953 =item system PROGRAM LIST
4955 Does exactly the same thing as C<exec LIST>, except that a fork is
4956 done first, and the parent process waits for the child process to
4957 complete. Note that argument processing varies depending on the
4958 number of arguments. If there is more than one argument in LIST,
4959 or if LIST is an array with more than one value, starts the program
4960 given by the first element of the list with arguments given by the
4961 rest of the list. If there is only one scalar argument, the argument
4962 is checked for shell metacharacters, and if there are any, the
4963 entire argument is passed to the system's command shell for parsing
4964 (this is C</bin/sh -c> on Unix platforms, but varies on other
4965 platforms). If there are no shell metacharacters in the argument,
4966 it is split into words and passed directly to C<execvp>, which is
4969 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4970 output before any operation that may do a fork, but this may not be
4971 supported on some platforms (see L<perlport>). To be safe, you may need
4972 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4973 of C<IO::Handle> on any open handles.
4975 The return value is the exit status of the program as
4976 returned by the C<wait> call. To get the actual exit value divide by
4977 256. See also L</exec>. This is I<not> what you want to use to capture
4978 the output from a command, for that you should use merely backticks or
4979 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4980 indicates a failure to start the program (inspect $! for the reason).
4982 Like C<exec>, C<system> allows you to lie to a program about its name if
4983 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4985 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4986 program they're running doesn't actually interrupt your program.
4988 @args = ("command", "arg1", "arg2");
4990 or die "system @args failed: $?"
4992 You can check all the failure possibilities by inspecting
4995 $exit_value = $? >> 8;
4996 $signal_num = $? & 127;
4997 $dumped_core = $? & 128;
4999 When the arguments get executed via the system shell, results
5000 and return codes will be subject to its quirks and capabilities.
5001 See L<perlop/"`STRING`"> and L</exec> for details.
5003 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5005 =item syswrite FILEHANDLE,SCALAR,LENGTH
5007 =item syswrite FILEHANDLE,SCALAR
5009 Attempts to write LENGTH bytes of data from variable SCALAR to the
5010 specified FILEHANDLE, using the system call write(2). If LENGTH
5011 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5012 this with reads (other than C<sysread())>, C<print>, C<write>,
5013 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5014 usually buffers data. Returns the number of bytes actually written,
5015 or C<undef> if there was an error. If the LENGTH is greater than
5016 the available data in the SCALAR after the OFFSET, only as much
5017 data as is available will be written.
5019 An OFFSET may be specified to write the data from some part of the
5020 string other than the beginning. A negative OFFSET specifies writing
5021 that many bytes counting backwards from the end of the string. In the
5022 case the SCALAR is empty you can use OFFSET but only zero offset.
5024 =item tell FILEHANDLE
5028 Returns the current position for FILEHANDLE. FILEHANDLE may be an
5029 expression whose value gives the name of the actual filehandle. If
5030 FILEHANDLE is omitted, assumes the file last read.
5032 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5034 =item telldir DIRHANDLE
5036 Returns the current position of the C<readdir> routines on DIRHANDLE.
5037 Value may be given to C<seekdir> to access a particular location in a
5038 directory. Has the same caveats about possible directory compaction as
5039 the corresponding system library routine.
5041 =item tie VARIABLE,CLASSNAME,LIST
5043 This function binds a variable to a package class that will provide the
5044 implementation for the variable. VARIABLE is the name of the variable
5045 to be enchanted. CLASSNAME is the name of a class implementing objects
5046 of correct type. Any additional arguments are passed to the C<new>
5047 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5048 or C<TIEHASH>). Typically these are arguments such as might be passed
5049 to the C<dbm_open()> function of C. The object returned by the C<new>
5050 method is also returned by the C<tie> function, which would be useful
5051 if you want to access other methods in CLASSNAME.
5053 Note that functions such as C<keys> and C<values> may return huge lists
5054 when used on large objects, like DBM files. You may prefer to use the
5055 C<each> function to iterate over such. Example:
5057 # print out history file offsets
5059 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5060 while (($key,$val) = each %HIST) {
5061 print $key, ' = ', unpack('L',$val), "\n";
5065 A class implementing a hash should have the following methods:
5067 TIEHASH classname, LIST
5069 STORE this, key, value
5074 NEXTKEY this, lastkey
5077 A class implementing an ordinary array should have the following methods:
5079 TIEARRAY classname, LIST
5081 STORE this, key, value
5083 STORESIZE this, count
5089 SPLICE this, offset, length, LIST
5093 A class implementing a file handle should have the following methods:
5095 TIEHANDLE classname, LIST
5096 READ this, scalar, length, offset
5099 WRITE this, scalar, length, offset
5101 PRINTF this, format, LIST
5105 A class implementing a scalar should have the following methods:
5107 TIESCALAR classname, LIST
5112 Not all methods indicated above need be implemented. See L<perltie>,
5113 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5115 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5116 for you--you need to do that explicitly yourself. See L<DB_File>
5117 or the F<Config> module for interesting C<tie> implementations.
5119 For further details see L<perltie>, L<"tied VARIABLE">.
5123 Returns a reference to the object underlying VARIABLE (the same value
5124 that was originally returned by the C<tie> call that bound the variable
5125 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5130 Returns the number of non-leap seconds since whatever time the system
5131 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5132 and 00:00:00 UTC, January 1, 1970 for most other systems).
5133 Suitable for feeding to C<gmtime> and C<localtime>.
5135 For measuring time in better granularity than one second,
5136 you may use either the Time::HiRes module from CPAN, or
5137 if you have gettimeofday(2), you may be able to use the
5138 C<syscall> interface of Perl, see L<perlfaq8> for details.
5142 Returns a four-element list giving the user and system times, in
5143 seconds, for this process and the children of this process.
5145 ($user,$system,$cuser,$csystem) = times;
5149 The transliteration operator. Same as C<y///>. See L<perlop>.
5151 =item truncate FILEHANDLE,LENGTH
5153 =item truncate EXPR,LENGTH
5155 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5156 specified length. Produces a fatal error if truncate isn't implemented
5157 on your system. Returns true if successful, the undefined value
5164 Returns an uppercased version of EXPR. This is the internal function
5165 implementing the C<\U> escape in double-quoted strings.
5166 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5167 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5168 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5170 If EXPR is omitted, uses C<$_>.
5176 Returns the value of EXPR with the first character
5177 in uppercase (titlecase in Unicode). This is
5178 the internal function implementing the C<\u> escape in double-quoted strings.
5179 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5182 If EXPR is omitted, uses C<$_>.
5188 Sets the umask for the process to EXPR and returns the previous value.
5189 If EXPR is omitted, merely returns the current umask.
5191 The Unix permission C<rwxr-x---> is represented as three sets of three
5192 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5193 and isn't one of the digits). The C<umask> value is such a number
5194 representing disabled permissions bits. The permission (or "mode")
5195 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5196 even if you tell C<sysopen> to create a file with permissions C<0777>,
5197 if your umask is C<0022> then the file will actually be created with
5198 permissions C<0755>. If your C<umask> were C<0027> (group can't
5199 write; others can't read, write, or execute), then passing
5200 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5203 Here's some advice: supply a creation mode of C<0666> for regular
5204 files (in C<sysopen>) and one of C<0777> for directories (in
5205 C<mkdir>) and executable files. This gives users the freedom of
5206 choice: if they want protected files, they might choose process umasks
5207 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5208 Programs should rarely if ever make policy decisions better left to
5209 the user. The exception to this is when writing files that should be
5210 kept private: mail files, web browser cookies, I<.rhosts> files, and
5213 If umask(2) is not implemented on your system and you are trying to
5214 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5215 fatal error at run time. If umask(2) is not implemented and you are
5216 not trying to restrict access for yourself, returns C<undef>.
5218 Remember that a umask is a number, usually given in octal; it is I<not> a
5219 string of octal digits. See also L</oct>, if all you have is a string.
5225 Undefines the value of EXPR, which must be an lvalue. Use only on a
5226 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5227 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5228 will probably not do what you expect on most predefined variables or
5229 DBM list values, so don't do that; see L<delete>.) Always returns the
5230 undefined value. You can omit the EXPR, in which case nothing is
5231 undefined, but you still get an undefined value that you could, for
5232 instance, return from a subroutine, assign to a variable or pass as a
5233 parameter. Examples:
5236 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5240 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5241 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5242 select undef, undef, undef, 0.25;
5243 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5245 Note that this is a unary operator, not a list operator.
5251 Deletes a list of files. Returns the number of files successfully
5254 $cnt = unlink 'a', 'b', 'c';
5258 Note: C<unlink> will not delete directories unless you are superuser and
5259 the B<-U> flag is supplied to Perl. Even if these conditions are
5260 met, be warned that unlinking a directory can inflict damage on your
5261 filesystem. Use C<rmdir> instead.
5263 If LIST is omitted, uses C<$_>.
5265 =item unpack TEMPLATE,EXPR
5267 C<unpack> does the reverse of C<pack>: it takes a string
5268 and expands it out into a list of values.
5269 (In scalar context, it returns merely the first value produced.)
5271 The string is broken into chunks described by the TEMPLATE. Each chunk
5272 is converted separately to a value. Typically, either the string is a result
5273 of C<pack>, or the bytes of the string represent a C structure of some
5276 The TEMPLATE has the same format as in the C<pack> function.
5277 Here's a subroutine that does substring:
5280 my($what,$where,$howmuch) = @_;
5281 unpack("x$where a$howmuch", $what);
5286 sub ordinal { unpack("c",$_[0]); } # same as ord()
5288 In addition to fields allowed in pack(), you may prefix a field with
5289 a %<number> to indicate that
5290 you want a <number>-bit checksum of the items instead of the items
5291 themselves. Default is a 16-bit checksum. Checksum is calculated by
5292 summing numeric values of expanded values (for string fields the sum of
5293 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5295 For example, the following
5296 computes the same number as the System V sum program:
5300 unpack("%32C*",<>) % 65535;
5303 The following efficiently counts the number of set bits in a bit vector:
5305 $setbits = unpack("%32b*", $selectmask);
5307 The C<p> and C<P> formats should be used with care. Since Perl
5308 has no way of checking whether the value passed to C<unpack()>
5309 corresponds to a valid memory location, passing a pointer value that's
5310 not known to be valid is likely to have disastrous consequences.
5312 If the repeat count of a field is larger than what the remainder of
5313 the input string allows, repeat count is decreased. If the input string
5314 is longer than one described by the TEMPLATE, the rest is ignored.
5316 See L</pack> for more examples and notes.
5318 =item untie VARIABLE
5320 Breaks the binding between a variable and a package. (See C<tie>.)
5322 =item unshift ARRAY,LIST
5324 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5325 depending on how you look at it. Prepends list to the front of the
5326 array, and returns the new number of elements in the array.
5328 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5330 Note the LIST is prepended whole, not one element at a time, so the
5331 prepended elements stay in the same order. Use C<reverse> to do the
5334 =item use Module VERSION LIST
5336 =item use Module VERSION
5338 =item use Module LIST
5344 Imports some semantics into the current package from the named module,
5345 generally by aliasing certain subroutine or variable names into your
5346 package. It is exactly equivalent to
5348 BEGIN { require Module; import Module LIST; }
5350 except that Module I<must> be a bareword.
5352 VERSION, which can be specified as a literal of the form v5.6.1, demands
5353 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5354 as recent as that version. (For compatibility with older versions of Perl,
5355 a numeric literal will also be interpreted as VERSION.) If the version
5356 of the running Perl interpreter is less than VERSION, then an error
5357 message is printed and Perl exits immediately without attempting to
5358 parse the rest of the file. Compare with L</require>, which can do a
5359 similar check at run time.
5361 use v5.6.1; # compile time version check
5363 use 5.005_03; # float version allowed for compatibility
5365 This is often useful if you need to check the current Perl version before
5366 C<use>ing library modules that have changed in incompatible ways from
5367 older versions of Perl. (We try not to do this more than we have to.)
5369 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5370 C<require> makes sure the module is loaded into memory if it hasn't been
5371 yet. The C<import> is not a builtin--it's just an ordinary static method
5372 call into the C<Module> package to tell the module to import the list of
5373 features back into the current package. The module can implement its
5374 C<import> method any way it likes, though most modules just choose to
5375 derive their C<import> method via inheritance from the C<Exporter> class that
5376 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5377 method can be found then the call is skipped.
5379 If you do not want to call the package's C<import> method (for instance,
5380 to stop your namespace from being altered), explicitly supply the empty list:
5384 That is exactly equivalent to
5386 BEGIN { require Module }
5388 If the VERSION argument is present between Module and LIST, then the
5389 C<use> will call the VERSION method in class Module with the given
5390 version as an argument. The default VERSION method, inherited from
5391 the UNIVERSAL class, croaks if the given version is larger than the
5392 value of the variable C<$Module::VERSION>.
5394 Again, there is a distinction between omitting LIST (C<import> called
5395 with no arguments) and an explicit empty LIST C<()> (C<import> not
5396 called). Note that there is no comma after VERSION!
5398 Because this is a wide-open interface, pragmas (compiler directives)
5399 are also implemented this way. Currently implemented pragmas are:
5404 use sigtrap qw(SEGV BUS);
5405 use strict qw(subs vars refs);
5406 use subs qw(afunc blurfl);
5407 use warnings qw(all);
5409 Some of these pseudo-modules import semantics into the current
5410 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5411 which import symbols into the current package (which are effective
5412 through the end of the file).
5414 There's a corresponding C<no> command that unimports meanings imported
5415 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5421 If no C<unimport> method can be found the call fails with a fatal error.
5423 See L<perlmod> for a list of standard modules and pragmas. See L<perlrun>
5424 for the C<-M> and C<-m> command-line options to perl that give C<use>
5425 functionality from the command-line.
5429 Changes the access and modification times on each file of a list of
5430 files. The first two elements of the list must be the NUMERICAL access
5431 and modification times, in that order. Returns the number of files
5432 successfully changed. The inode change time of each file is set
5433 to the current time. This code has the same effect as the C<touch>
5434 command if the files already exist:
5438 utime $now, $now, @ARGV;
5442 Returns a list consisting of all the values of the named hash. (In a
5443 scalar context, returns the number of values.) The values are
5444 returned in an apparently random order. The actual random order is
5445 subject to change in future versions of perl, but it is guaranteed to
5446 be the same order as either the C<keys> or C<each> function would
5447 produce on the same (unmodified) hash.
5449 Note that the values are not copied, which means modifying them will
5450 modify the contents of the hash:
5452 for (values %hash) { s/foo/bar/g } # modifies %hash values
5453 for (@hash{keys %hash}) { s/foo/bar/g } # same
5455 As a side effect, calling values() resets the HASH's internal iterator.
5456 See also C<keys>, C<each>, and C<sort>.
5458 =item vec EXPR,OFFSET,BITS
5460 Treats the string in EXPR as a bit vector made up of elements of
5461 width BITS, and returns the value of the element specified by OFFSET
5462 as an unsigned integer. BITS therefore specifies the number of bits
5463 that are reserved for each element in the bit vector. This must
5464 be a power of two from 1 to 32 (or 64, if your platform supports
5467 If BITS is 8, "elements" coincide with bytes of the input string.
5469 If BITS is 16 or more, bytes of the input string are grouped into chunks
5470 of size BITS/8, and each group is converted to a number as with
5471 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5472 for BITS==64). See L<"pack"> for details.
5474 If bits is 4 or less, the string is broken into bytes, then the bits
5475 of each byte are broken into 8/BITS groups. Bits of a byte are
5476 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5477 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5478 breaking the single input byte C<chr(0x36)> into two groups gives a list
5479 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5481 C<vec> may also be assigned to, in which case parentheses are needed
5482 to give the expression the correct precedence as in
5484 vec($image, $max_x * $x + $y, 8) = 3;
5486 If the selected element is off the end of the string, the value 0 is
5487 returned. If an element off the end of the string is written to,
5488 Perl will first extend the string with sufficiently many zero bytes.
5490 Strings created with C<vec> can also be manipulated with the logical
5491 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5492 vector operation is desired when both operands are strings.
5493 See L<perlop/"Bitwise String Operators">.
5495 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5496 The comments show the string after each step. Note that this code works
5497 in the same way on big-endian or little-endian machines.
5500 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5502 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5503 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5505 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5506 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5507 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5508 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5509 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5510 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5512 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5513 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5514 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5517 To transform a bit vector into a string or list of 0's and 1's, use these:
5519 $bits = unpack("b*", $vector);
5520 @bits = split(//, unpack("b*", $vector));
5522 If you know the exact length in bits, it can be used in place of the C<*>.
5524 Here is an example to illustrate how the bits actually fall in place:
5530 unpack("V",$_) 01234567890123456789012345678901
5531 ------------------------------------------------------------------
5536 for ($shift=0; $shift < $width; ++$shift) {
5537 for ($off=0; $off < 32/$width; ++$off) {
5538 $str = pack("B*", "0"x32);
5539 $bits = (1<<$shift);
5540 vec($str, $off, $width) = $bits;
5541 $res = unpack("b*",$str);
5542 $val = unpack("V", $str);
5549 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5550 $off, $width, $bits, $val, $res
5554 Regardless of the machine architecture on which it is run, the above
5555 example should print the following table:
5558 unpack("V",$_) 01234567890123456789012345678901
5559 ------------------------------------------------------------------
5560 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5561 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5562 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5563 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5564 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5565 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5566 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5567 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5568 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5569 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5570 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5571 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5572 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5573 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5574 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5575 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5576 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5577 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5578 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5579 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5580 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5581 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5582 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5583 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5584 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5585 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5586 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5587 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5588 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5589 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5590 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5591 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5592 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5593 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5594 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5595 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5596 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5597 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5598 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5599 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5600 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5601 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5602 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5603 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5604 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5605 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5606 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5607 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5608 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5609 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5610 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5611 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5612 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5613 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5614 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5615 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5616 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5617 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5618 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5619 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5620 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5621 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5622 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5623 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5624 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5625 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5626 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5627 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5628 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5629 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5630 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5631 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5632 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5633 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5634 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5635 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5636 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5637 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5638 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5639 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5640 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5641 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5642 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5643 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5644 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5645 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5646 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5647 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5648 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5649 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5650 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5651 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5652 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5653 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5654 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5655 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5656 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5657 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5658 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5659 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5660 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5661 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5662 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5663 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5664 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5665 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5666 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5667 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5668 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5669 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5670 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5671 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5672 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5673 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5674 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5675 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5676 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5677 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5678 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5679 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5680 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5681 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5682 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5683 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5684 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5685 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5686 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5687 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5691 Behaves like the wait(2) system call on your system: it waits for a child
5692 process to terminate and returns the pid of the deceased process, or
5693 C<-1> if there are no child processes. The status is returned in C<$?>.
5694 Note that a return value of C<-1> could mean that child processes are
5695 being automatically reaped, as described in L<perlipc>.
5697 =item waitpid PID,FLAGS
5699 Waits for a particular child process to terminate and returns the pid of
5700 the deceased process, or C<-1> if there is no such child process. On some
5701 systems, a value of 0 indicates that there are processes still running.
5702 The status is returned in C<$?>. If you say
5704 use POSIX ":sys_wait_h";
5707 $kid = waitpid(-1,&WNOHANG);
5710 then you can do a non-blocking wait for all pending zombie processes.
5711 Non-blocking wait is available on machines supporting either the
5712 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5713 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5714 system call by remembering the status values of processes that have
5715 exited but have not been harvested by the Perl script yet.)
5717 Note that on some systems, a return value of C<-1> could mean that child
5718 processes are being automatically reaped. See L<perlipc> for details,
5719 and for other examples.
5723 Returns true if the context of the currently executing subroutine is
5724 looking for a list value. Returns false if the context is looking
5725 for a scalar. Returns the undefined value if the context is looking
5726 for no value (void context).
5728 return unless defined wantarray; # don't bother doing more
5729 my @a = complex_calculation();
5730 return wantarray ? @a : "@a";
5732 This function should have been named wantlist() instead.
5736 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5739 If LIST is empty and C<$@> already contains a value (typically from a
5740 previous eval) that value is used after appending C<"\t...caught">
5741 to C<$@>. This is useful for staying almost, but not entirely similar to
5744 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5746 No message is printed if there is a C<$SIG{__WARN__}> handler
5747 installed. It is the handler's responsibility to deal with the message
5748 as it sees fit (like, for instance, converting it into a C<die>). Most
5749 handlers must therefore make arrangements to actually display the
5750 warnings that they are not prepared to deal with, by calling C<warn>
5751 again in the handler. Note that this is quite safe and will not
5752 produce an endless loop, since C<__WARN__> hooks are not called from
5755 You will find this behavior is slightly different from that of
5756 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5757 instead call C<die> again to change it).
5759 Using a C<__WARN__> handler provides a powerful way to silence all
5760 warnings (even the so-called mandatory ones). An example:
5762 # wipe out *all* compile-time warnings
5763 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5765 my $foo = 20; # no warning about duplicate my $foo,
5766 # but hey, you asked for it!
5767 # no compile-time or run-time warnings before here
5770 # run-time warnings enabled after here
5771 warn "\$foo is alive and $foo!"; # does show up
5773 See L<perlvar> for details on setting C<%SIG> entries, and for more
5774 examples. See the Carp module for other kinds of warnings using its
5775 carp() and cluck() functions.
5777 =item write FILEHANDLE
5783 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5784 using the format associated with that file. By default the format for
5785 a file is the one having the same name as the filehandle, but the
5786 format for the current output channel (see the C<select> function) may be set
5787 explicitly by assigning the name of the format to the C<$~> variable.
5789 Top of form processing is handled automatically: if there is
5790 insufficient room on the current page for the formatted record, the
5791 page is advanced by writing a form feed, a special top-of-page format
5792 is used to format the new page header, and then the record is written.
5793 By default the top-of-page format is the name of the filehandle with
5794 "_TOP" appended, but it may be dynamically set to the format of your
5795 choice by assigning the name to the C<$^> variable while the filehandle is
5796 selected. The number of lines remaining on the current page is in
5797 variable C<$->, which can be set to C<0> to force a new page.
5799 If FILEHANDLE is unspecified, output goes to the current default output
5800 channel, which starts out as STDOUT but may be changed by the
5801 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5802 is evaluated and the resulting string is used to look up the name of
5803 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5805 Note that write is I<not> the opposite of C<read>. Unfortunately.
5809 The transliteration operator. Same as C<tr///>. See L<perlop>.