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 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
689 Returns the character represented by that NUMBER in the character set.
690 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
691 chr(0x263a) is a Unicode smiley face (but only within the scope of
692 a C<use utf8>). For the reverse, use L</ord>.
693 See L<utf8> for more about Unicode.
695 If NUMBER is omitted, uses C<$_>.
697 =item chroot FILENAME
701 This function works like the system call by the same name: it makes the
702 named directory the new root directory for all further pathnames that
703 begin with a C</> by your process and all its children. (It doesn't
704 change your current working directory, which is unaffected.) For security
705 reasons, this call is restricted to the superuser. If FILENAME is
706 omitted, does a C<chroot> to C<$_>.
708 =item close FILEHANDLE
712 Closes the file or pipe associated with the file handle, returning true
713 only if stdio successfully flushes buffers and closes the system file
714 descriptor. Closes the currently selected filehandle if the argument
717 You don't have to close FILEHANDLE if you are immediately going to do
718 another C<open> on it, because C<open> will close it for you. (See
719 C<open>.) However, an explicit C<close> on an input file resets the line
720 counter (C<$.>), while the implicit close done by C<open> does not.
722 If the file handle came from a piped open C<close> will additionally
723 return false if one of the other system calls involved fails or if the
724 program exits with non-zero status. (If the only problem was that the
725 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
726 also waits for the process executing on the pipe to complete, in case you
727 want to look at the output of the pipe afterwards, and
728 implicitly puts the exit status value of that command into C<$?>.
730 Prematurely closing the read end of a pipe (i.e. before the process
731 writing to it at the other end has closed it) will result in a
732 SIGPIPE being delivered to the writer. If the other end can't
733 handle that, be sure to read all the data before closing the pipe.
737 open(OUTPUT, '|sort >foo') # pipe to sort
738 or die "Can't start sort: $!";
739 #... # print stuff to output
740 close OUTPUT # wait for sort to finish
741 or warn $! ? "Error closing sort pipe: $!"
742 : "Exit status $? from sort";
743 open(INPUT, 'foo') # get sort's results
744 or die "Can't open 'foo' for input: $!";
746 FILEHANDLE may be an expression whose value can be used as an indirect
747 filehandle, usually the real filehandle name.
749 =item closedir DIRHANDLE
751 Closes a directory opened by C<opendir> and returns the success of that
754 DIRHANDLE may be an expression whose value can be used as an indirect
755 dirhandle, usually the real dirhandle name.
757 =item connect SOCKET,NAME
759 Attempts to connect to a remote socket, just as the connect system call
760 does. Returns true if it succeeded, false otherwise. NAME should be a
761 packed address of the appropriate type for the socket. See the examples in
762 L<perlipc/"Sockets: Client/Server Communication">.
766 Actually a flow control statement rather than a function. If there is a
767 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
768 C<foreach>), it is always executed just before the conditional is about to
769 be evaluated again, just like the third part of a C<for> loop in C. Thus
770 it can be used to increment a loop variable, even when the loop has been
771 continued via the C<next> statement (which is similar to the C C<continue>
774 C<last>, C<next>, or C<redo> may appear within a C<continue>
775 block. C<last> and C<redo> will behave as if they had been executed within
776 the main block. So will C<next>, but since it will execute a C<continue>
777 block, it may be more entertaining.
780 ### redo always comes here
783 ### next always comes here
785 # then back the top to re-check EXPR
787 ### last always comes here
789 Omitting the C<continue> section is semantically equivalent to using an
790 empty one, logically enough. In that case, C<next> goes directly back
791 to check the condition at the top of the loop.
795 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
796 takes cosine of C<$_>.
798 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
799 function, or use this relation:
801 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
803 =item crypt PLAINTEXT,SALT
805 Encrypts a string exactly like the crypt(3) function in the C library
806 (assuming that you actually have a version there that has not been
807 extirpated as a potential munition). This can prove useful for checking
808 the password file for lousy passwords, amongst other things. Only the
809 guys wearing white hats should do this.
811 Note that C<crypt> is intended to be a one-way function, much like breaking
812 eggs to make an omelette. There is no (known) corresponding decrypt
813 function. As a result, this function isn't all that useful for
814 cryptography. (For that, see your nearby CPAN mirror.)
816 When verifying an existing encrypted string you should use the encrypted
817 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
818 allows your code to work with the standard C<crypt> and with more
819 exotic implementations. When choosing a new salt create a random two
820 character string whose characters come from the set C<[./0-9A-Za-z]>
821 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
823 Here's an example that makes sure that whoever runs this program knows
826 $pwd = (getpwuid($<))[1];
830 chomp($word = <STDIN>);
834 if (crypt($word, $pwd) ne $pwd) {
840 Of course, typing in your own password to whoever asks you
843 The L<crypt> function is unsuitable for encrypting large quantities
844 of data, not least of all because you can't get the information
845 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
846 on your favorite CPAN mirror for a slew of potentially useful
851 [This function has been largely superseded by the C<untie> function.]
853 Breaks the binding between a DBM file and a hash.
855 =item dbmopen HASH,DBNAME,MASK
857 [This function has been largely superseded by the C<tie> function.]
859 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
860 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
861 argument is I<not> a filehandle, even though it looks like one). DBNAME
862 is the name of the database (without the F<.dir> or F<.pag> extension if
863 any). If the database does not exist, it is created with protection
864 specified by MASK (as modified by the C<umask>). If your system supports
865 only the older DBM functions, you may perform only one C<dbmopen> in your
866 program. In older versions of Perl, if your system had neither DBM nor
867 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
870 If you don't have write access to the DBM file, you can only read hash
871 variables, not set them. If you want to test whether you can write,
872 either use file tests or try setting a dummy hash entry inside an C<eval>,
873 which will trap the error.
875 Note that functions such as C<keys> and C<values> may return huge lists
876 when used on large DBM files. You may prefer to use the C<each>
877 function to iterate over large DBM files. Example:
879 # print out history file offsets
880 dbmopen(%HIST,'/usr/lib/news/history',0666);
881 while (($key,$val) = each %HIST) {
882 print $key, ' = ', unpack('L',$val), "\n";
886 See also L<AnyDBM_File> for a more general description of the pros and
887 cons of the various dbm approaches, as well as L<DB_File> for a particularly
890 You can control which DBM library you use by loading that library
891 before you call dbmopen():
894 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
895 or die "Can't open netscape history file: $!";
901 Returns a Boolean value telling whether EXPR has a value other than
902 the undefined value C<undef>. If EXPR is not present, C<$_> will be
905 Many operations return C<undef> to indicate failure, end of file,
906 system error, uninitialized variable, and other exceptional
907 conditions. This function allows you to distinguish C<undef> from
908 other values. (A simple Boolean test will not distinguish among
909 C<undef>, zero, the empty string, and C<"0">, which are all equally
910 false.) Note that since C<undef> is a valid scalar, its presence
911 doesn't I<necessarily> indicate an exceptional condition: C<pop>
912 returns C<undef> when its argument is an empty array, I<or> when the
913 element to return happens to be C<undef>.
915 You may also use C<defined(&func)> to check whether subroutine C<&func>
916 has ever been defined. The return value is unaffected by any forward
917 declarations of C<&foo>.
919 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
920 used to report whether memory for that aggregate has ever been
921 allocated. This behavior may disappear in future versions of Perl.
922 You should instead use a simple test for size:
924 if (@an_array) { print "has array elements\n" }
925 if (%a_hash) { print "has hash members\n" }
927 When used on a hash element, it tells you whether the value is defined,
928 not whether the key exists in the hash. Use L</exists> for the latter
933 print if defined $switch{'D'};
934 print "$val\n" while defined($val = pop(@ary));
935 die "Can't readlink $sym: $!"
936 unless defined($value = readlink $sym);
937 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
938 $debugging = 0 unless defined $debugging;
940 Note: Many folks tend to overuse C<defined>, and then are surprised to
941 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
942 defined values. For example, if you say
946 The pattern match succeeds, and C<$1> is defined, despite the fact that it
947 matched "nothing". But it didn't really match nothing--rather, it
948 matched something that happened to be zero characters long. This is all
949 very above-board and honest. When a function returns an undefined value,
950 it's an admission that it couldn't give you an honest answer. So you
951 should use C<defined> only when you're questioning the integrity of what
952 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
955 See also L</undef>, L</exists>, L</ref>.
959 Given an expression that specifies a hash element, array element, hash slice,
960 or array slice, deletes the specified element(s) from the hash or array.
961 In the case of an array, if the array elements happen to be at the end,
962 the size of the array will shrink to the highest element that tests
963 true for exists() (or 0 if no such element exists).
965 Returns each element so deleted or the undefined value if there was no such
966 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
967 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
968 from a C<tie>d hash or array may not necessarily return anything.
970 Deleting an array element effectively returns that position of the array
971 to its initial, uninitialized state. Subsequently testing for the same
972 element with exists() will return false. Note that deleting array
973 elements in the middle of an array will not shift the index of the ones
974 after them down--use splice() for that. See L</exists>.
976 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
978 foreach $key (keys %HASH) {
982 foreach $index (0 .. $#ARRAY) {
983 delete $ARRAY[$index];
988 delete @HASH{keys %HASH};
990 delete @ARRAY[0 .. $#ARRAY];
992 But both of these are slower than just assigning the empty list
993 or undefining %HASH or @ARRAY:
995 %HASH = (); # completely empty %HASH
996 undef %HASH; # forget %HASH ever existed
998 @ARRAY = (); # completely empty @ARRAY
999 undef @ARRAY; # forget @ARRAY ever existed
1001 Note that the EXPR can be arbitrarily complicated as long as the final
1002 operation is a hash element, array element, hash slice, or array slice
1005 delete $ref->[$x][$y]{$key};
1006 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1008 delete $ref->[$x][$y][$index];
1009 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1013 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1014 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1015 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1016 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1017 an C<eval(),> the error message is stuffed into C<$@> and the
1018 C<eval> is terminated with the undefined value. This makes
1019 C<die> the way to raise an exception.
1021 Equivalent examples:
1023 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1024 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1026 If the value of EXPR does not end in a newline, the current script line
1027 number and input line number (if any) are also printed, and a newline
1028 is supplied. Note that the "input line number" (also known as "chunk")
1029 is subject to whatever notion of "line" happens to be currently in
1030 effect, and is also available as the special variable C<$.>.
1031 See L<perlvar/"$/"> and L<perlvar/"$.">.
1033 Hint: sometimes appending C<", stopped"> to your message
1034 will cause it to make better sense when the string C<"at foo line 123"> is
1035 appended. Suppose you are running script "canasta".
1037 die "/etc/games is no good";
1038 die "/etc/games is no good, stopped";
1040 produce, respectively
1042 /etc/games is no good at canasta line 123.
1043 /etc/games is no good, stopped at canasta line 123.
1045 See also exit(), warn(), and the Carp module.
1047 If LIST is empty and C<$@> already contains a value (typically from a
1048 previous eval) that value is reused after appending C<"\t...propagated">.
1049 This is useful for propagating exceptions:
1052 die unless $@ =~ /Expected exception/;
1054 If C<$@> is empty then the string C<"Died"> is used.
1056 die() can also be called with a reference argument. If this happens to be
1057 trapped within an eval(), $@ contains the reference. This behavior permits
1058 a more elaborate exception handling implementation using objects that
1059 maintain arbitrary state about the nature of the exception. Such a scheme
1060 is sometimes preferable to matching particular string values of $@ using
1061 regular expressions. Here's an example:
1063 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1065 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1066 # handle Some::Module::Exception
1069 # handle all other possible exceptions
1073 Because perl will stringify uncaught exception messages before displaying
1074 them, you may want to overload stringification operations on such custom
1075 exception objects. See L<overload> for details about that.
1077 You can arrange for a callback to be run just before the C<die>
1078 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1079 handler will be called with the error text and can change the error
1080 message, if it sees fit, by calling C<die> again. See
1081 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1082 L<"eval BLOCK"> for some examples. Although this feature was meant
1083 to be run only right before your program was to exit, this is not
1084 currently the case--the C<$SIG{__DIE__}> hook is currently called
1085 even inside eval()ed blocks/strings! If one wants the hook to do
1086 nothing in such situations, put
1090 as the first line of the handler (see L<perlvar/$^S>). Because
1091 this promotes strange action at a distance, this counterintuitive
1092 behavior may be fixed in a future release.
1096 Not really a function. Returns the value of the last command in the
1097 sequence of commands indicated by BLOCK. When modified by a loop
1098 modifier, executes the BLOCK once before testing the loop condition.
1099 (On other statements the loop modifiers test the conditional first.)
1101 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1102 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1103 See L<perlsyn> for alternative strategies.
1105 =item do SUBROUTINE(LIST)
1107 A deprecated form of subroutine call. See L<perlsub>.
1111 Uses the value of EXPR as a filename and executes the contents of the
1112 file as a Perl script. Its primary use is to include subroutines
1113 from a Perl subroutine library.
1119 scalar eval `cat stat.pl`;
1121 except that it's more efficient and concise, keeps track of the current
1122 filename for error messages, searches the @INC libraries, and updates
1123 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1124 variables. It also differs in that code evaluated with C<do FILENAME>
1125 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1126 same, however, in that it does reparse the file every time you call it,
1127 so you probably don't want to do this inside a loop.
1129 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1130 error. If C<do> can read the file but cannot compile it, it
1131 returns undef and sets an error message in C<$@>. If the file is
1132 successfully compiled, C<do> returns the value of the last expression
1135 Note that inclusion of library modules is better done with the
1136 C<use> and C<require> operators, which also do automatic error checking
1137 and raise an exception if there's a problem.
1139 You might like to use C<do> to read in a program configuration
1140 file. Manual error checking can be done this way:
1142 # read in config files: system first, then user
1143 for $file ("/share/prog/defaults.rc",
1144 "$ENV{HOME}/.someprogrc")
1146 unless ($return = do $file) {
1147 warn "couldn't parse $file: $@" if $@;
1148 warn "couldn't do $file: $!" unless defined $return;
1149 warn "couldn't run $file" unless $return;
1157 This function causes an immediate core dump. See also the B<-u>
1158 command-line switch in L<perlrun>, which does the same thing.
1159 Primarily this is so that you can use the B<undump> program (not
1160 supplied) to turn your core dump into an executable binary after
1161 having initialized all your variables at the beginning of the
1162 program. When the new binary is executed it will begin by executing
1163 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1164 Think of it as a goto with an intervening core dump and reincarnation.
1165 If C<LABEL> is omitted, restarts the program from the top.
1167 B<WARNING>: Any files opened at the time of the dump will I<not>
1168 be open any more when the program is reincarnated, with possible
1169 resulting confusion on the part of Perl.
1171 This function is now largely obsolete, partly because it's very
1172 hard to convert a core file into an executable, and because the
1173 real compiler backends for generating portable bytecode and compilable
1174 C code have superseded it.
1176 If you're looking to use L<dump> to speed up your program, consider
1177 generating bytecode or native C code as described in L<perlcc>. If
1178 you're just trying to accelerate a CGI script, consider using the
1179 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1180 You might also consider autoloading or selfloading, which at least
1181 make your program I<appear> to run faster.
1185 When called in list context, returns a 2-element list consisting of the
1186 key and value for the next element of a hash, so that you can iterate over
1187 it. When called in scalar context, returns the key for only the "next"
1188 element in the hash.
1190 Entries are returned in an apparently random order. The actual random
1191 order is subject to change in future versions of perl, but it is guaranteed
1192 to be in the same order as either the C<keys> or C<values> function
1193 would produce on the same (unmodified) hash.
1195 When the hash is entirely read, a null array is returned in list context
1196 (which when assigned produces a false (C<0>) value), and C<undef> in
1197 scalar context. The next call to C<each> after that will start iterating
1198 again. There is a single iterator for each hash, shared by all C<each>,
1199 C<keys>, and C<values> function calls in the program; it can be reset by
1200 reading all the elements from the hash, or by evaluating C<keys HASH> or
1201 C<values HASH>. If you add or delete elements of a hash while you're
1202 iterating over it, you may get entries skipped or duplicated, so don't.
1204 The following prints out your environment like the printenv(1) program,
1205 only in a different order:
1207 while (($key,$value) = each %ENV) {
1208 print "$key=$value\n";
1211 See also C<keys>, C<values> and C<sort>.
1213 =item eof FILEHANDLE
1219 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1220 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1221 gives the real filehandle. (Note that this function actually
1222 reads a character and then C<ungetc>s it, so isn't very useful in an
1223 interactive context.) Do not read from a terminal file (or call
1224 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1225 as terminals may lose the end-of-file condition if you do.
1227 An C<eof> without an argument uses the last file read. Using C<eof()>
1228 with empty parentheses is very different. It refers to the pseudo file
1229 formed from the files listed on the command line and accessed via the
1230 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1231 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1232 used will cause C<@ARGV> to be examined to determine if input is
1235 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1236 detect the end of each file, C<eof()> will only detect the end of the
1237 last file. Examples:
1239 # reset line numbering on each input file
1241 next if /^\s*#/; # skip comments
1244 close ARGV if eof; # Not eof()!
1247 # insert dashes just before last line of last file
1249 if (eof()) { # check for end of current file
1250 print "--------------\n";
1251 close(ARGV); # close or last; is needed if we
1252 # are reading from the terminal
1257 Practical hint: you almost never need to use C<eof> in Perl, because the
1258 input operators typically return C<undef> when they run out of data, or if
1265 In the first form, the return value of EXPR is parsed and executed as if it
1266 were a little Perl program. The value of the expression (which is itself
1267 determined within scalar context) is first parsed, and if there weren't any
1268 errors, executed in the lexical context of the current Perl program, so
1269 that any variable settings or subroutine and format definitions remain
1270 afterwards. Note that the value is parsed every time the eval executes.
1271 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1272 delay parsing and subsequent execution of the text of EXPR until run time.
1274 In the second form, the code within the BLOCK is parsed only once--at the
1275 same time the code surrounding the eval itself was parsed--and executed
1276 within the context of the current Perl program. This form is typically
1277 used to trap exceptions more efficiently than the first (see below), while
1278 also providing the benefit of checking the code within BLOCK at compile
1281 The final semicolon, if any, may be omitted from the value of EXPR or within
1284 In both forms, the value returned is the value of the last expression
1285 evaluated inside the mini-program; a return statement may be also used, just
1286 as with subroutines. The expression providing the return value is evaluated
1287 in void, scalar, or list context, depending on the context of the eval itself.
1288 See L</wantarray> for more on how the evaluation context can be determined.
1290 If there is a syntax error or runtime error, or a C<die> statement is
1291 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1292 error message. If there was no error, C<$@> is guaranteed to be a null
1293 string. Beware that using C<eval> neither silences perl from printing
1294 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1295 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1296 L</warn> and L<perlvar>.
1298 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1299 determining whether a particular feature (such as C<socket> or C<symlink>)
1300 is implemented. It is also Perl's exception trapping mechanism, where
1301 the die operator is used to raise exceptions.
1303 If the code to be executed doesn't vary, you may use the eval-BLOCK
1304 form to trap run-time errors without incurring the penalty of
1305 recompiling each time. The error, if any, is still returned in C<$@>.
1308 # make divide-by-zero nonfatal
1309 eval { $answer = $a / $b; }; warn $@ if $@;
1311 # same thing, but less efficient
1312 eval '$answer = $a / $b'; warn $@ if $@;
1314 # a compile-time error
1315 eval { $answer = }; # WRONG
1318 eval '$answer ='; # sets $@
1320 Due to the current arguably broken state of C<__DIE__> hooks, when using
1321 the C<eval{}> form as an exception trap in libraries, you may wish not
1322 to trigger any C<__DIE__> hooks that user code may have installed.
1323 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1324 as shown in this example:
1326 # a very private exception trap for divide-by-zero
1327 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1330 This is especially significant, given that C<__DIE__> hooks can call
1331 C<die> again, which has the effect of changing their error messages:
1333 # __DIE__ hooks may modify error messages
1335 local $SIG{'__DIE__'} =
1336 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1337 eval { die "foo lives here" };
1338 print $@ if $@; # prints "bar lives here"
1341 Because this promotes action at a distance, this counterintuitive behavior
1342 may be fixed in a future release.
1344 With an C<eval>, you should be especially careful to remember what's
1345 being looked at when:
1351 eval { $x }; # CASE 4
1353 eval "\$$x++"; # CASE 5
1356 Cases 1 and 2 above behave identically: they run the code contained in
1357 the variable $x. (Although case 2 has misleading double quotes making
1358 the reader wonder what else might be happening (nothing is).) Cases 3
1359 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1360 does nothing but return the value of $x. (Case 4 is preferred for
1361 purely visual reasons, but it also has the advantage of compiling at
1362 compile-time instead of at run-time.) Case 5 is a place where
1363 normally you I<would> like to use double quotes, except that in this
1364 particular situation, you can just use symbolic references instead, as
1367 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1368 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1372 =item exec PROGRAM LIST
1374 The C<exec> function executes a system command I<and never returns>--
1375 use C<system> instead of C<exec> if you want it to return. It fails and
1376 returns false only if the command does not exist I<and> it is executed
1377 directly instead of via your system's command shell (see below).
1379 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1380 warns you if there is a following statement which isn't C<die>, C<warn>,
1381 or C<exit> (if C<-w> is set - but you always do that). If you
1382 I<really> want to follow an C<exec> with some other statement, you
1383 can use one of these styles to avoid the warning:
1385 exec ('foo') or print STDERR "couldn't exec foo: $!";
1386 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1388 If there is more than one argument in LIST, or if LIST is an array
1389 with more than one value, calls execvp(3) with the arguments in LIST.
1390 If there is only one scalar argument or an array with one element in it,
1391 the argument is checked for shell metacharacters, and if there are any,
1392 the entire argument is passed to the system's command shell for parsing
1393 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1394 If there are no shell metacharacters in the argument, it is split into
1395 words and passed directly to C<execvp>, which is more efficient.
1398 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1399 exec "sort $outfile | uniq";
1401 If you don't really want to execute the first argument, but want to lie
1402 to the program you are executing about its own name, you can specify
1403 the program you actually want to run as an "indirect object" (without a
1404 comma) in front of the LIST. (This always forces interpretation of the
1405 LIST as a multivalued list, even if there is only a single scalar in
1408 $shell = '/bin/csh';
1409 exec $shell '-sh'; # pretend it's a login shell
1413 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1415 When the arguments get executed via the system shell, results will
1416 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1419 Using an indirect object with C<exec> or C<system> is also more
1420 secure. This usage (which also works fine with system()) forces
1421 interpretation of the arguments as a multivalued list, even if the
1422 list had just one argument. That way you're safe from the shell
1423 expanding wildcards or splitting up words with whitespace in them.
1425 @args = ( "echo surprise" );
1427 exec @args; # subject to shell escapes
1429 exec { $args[0] } @args; # safe even with one-arg list
1431 The first version, the one without the indirect object, ran the I<echo>
1432 program, passing it C<"surprise"> an argument. The second version
1433 didn't--it tried to run a program literally called I<"echo surprise">,
1434 didn't find it, and set C<$?> to a non-zero value indicating failure.
1436 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1437 output before the exec, but this may not be supported on some platforms
1438 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1439 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1440 open handles in order to avoid lost output.
1442 Note that C<exec> will not call your C<END> blocks, nor will it call
1443 any C<DESTROY> methods in your objects.
1447 Given an expression that specifies a hash element or array element,
1448 returns true if the specified element in the hash or array has ever
1449 been initialized, even if the corresponding value is undefined. The
1450 element is not autovivified if it doesn't exist.
1452 print "Exists\n" if exists $hash{$key};
1453 print "Defined\n" if defined $hash{$key};
1454 print "True\n" if $hash{$key};
1456 print "Exists\n" if exists $array[$index];
1457 print "Defined\n" if defined $array[$index];
1458 print "True\n" if $array[$index];
1460 A hash or array element can be true only if it's defined, and defined if
1461 it exists, but the reverse doesn't necessarily hold true.
1463 Given an expression that specifies the name of a subroutine,
1464 returns true if the specified subroutine has ever been declared, even
1465 if it is undefined. Mentioning a subroutine name for exists or defined
1466 does not count as declaring it.
1468 print "Exists\n" if exists &subroutine;
1469 print "Defined\n" if defined &subroutine;
1471 Note that the EXPR can be arbitrarily complicated as long as the final
1472 operation is a hash or array key lookup or subroutine name:
1474 if (exists $ref->{A}->{B}->{$key}) { }
1475 if (exists $hash{A}{B}{$key}) { }
1477 if (exists $ref->{A}->{B}->[$ix]) { }
1478 if (exists $hash{A}{B}[$ix]) { }
1480 if (exists &{$ref->{A}{B}{$key}}) { }
1482 Although the deepest nested array or hash will not spring into existence
1483 just because its existence was tested, any intervening ones will.
1484 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1485 into existence due to the existence test for the $key element above.
1486 This happens anywhere the arrow operator is used, including even:
1489 if (exists $ref->{"Some key"}) { }
1490 print $ref; # prints HASH(0x80d3d5c)
1492 This surprising autovivification in what does not at first--or even
1493 second--glance appear to be an lvalue context may be fixed in a future
1496 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1497 on how exists() acts when used on a pseudo-hash.
1499 Use of a subroutine call, rather than a subroutine name, as an argument
1500 to exists() is an error.
1503 exists &sub(); # Error
1507 Evaluates EXPR and exits immediately with that value. Example:
1510 exit 0 if $ans =~ /^[Xx]/;
1512 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1513 universally recognized values for EXPR are C<0> for success and C<1>
1514 for error; other values are subject to interpretation depending on the
1515 environment in which the Perl program is running. For example, exiting
1516 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1517 the mailer to return the item undelivered, but that's not true everywhere.
1519 Don't use C<exit> to abort a subroutine if there's any chance that
1520 someone might want to trap whatever error happened. Use C<die> instead,
1521 which can be trapped by an C<eval>.
1523 The exit() function does not always exit immediately. It calls any
1524 defined C<END> routines first, but these C<END> routines may not
1525 themselves abort the exit. Likewise any object destructors that need to
1526 be called are called before the real exit. If this is a problem, you
1527 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1528 See L<perlmod> for details.
1534 Returns I<e> (the natural logarithm base) to the power of EXPR.
1535 If EXPR is omitted, gives C<exp($_)>.
1537 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1539 Implements the fcntl(2) function. You'll probably have to say
1543 first to get the correct constant definitions. Argument processing and
1544 value return works just like C<ioctl> below.
1548 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1549 or die "can't fcntl F_GETFL: $!";
1551 You don't have to check for C<defined> on the return from C<fnctl>.
1552 Like C<ioctl>, it maps a C<0> return from the system call into
1553 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1554 in numeric context. It is also exempt from the normal B<-w> warnings
1555 on improper numeric conversions.
1557 Note that C<fcntl> will produce a fatal error if used on a machine that
1558 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1559 manpage to learn what functions are available on your system.
1561 =item fileno FILEHANDLE
1563 Returns the file descriptor for a filehandle, or undefined if the
1564 filehandle is not open. This is mainly useful for constructing
1565 bitmaps for C<select> and low-level POSIX tty-handling operations.
1566 If FILEHANDLE is an expression, the value is taken as an indirect
1567 filehandle, generally its name.
1569 You can use this to find out whether two handles refer to the
1570 same underlying descriptor:
1572 if (fileno(THIS) == fileno(THAT)) {
1573 print "THIS and THAT are dups\n";
1576 =item flock FILEHANDLE,OPERATION
1578 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1579 for success, false on failure. Produces a fatal error if used on a
1580 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1581 C<flock> is Perl's portable file locking interface, although it locks
1582 only entire files, not records.
1584 Two potentially non-obvious but traditional C<flock> semantics are
1585 that it waits indefinitely until the lock is granted, and that its locks
1586 B<merely advisory>. Such discretionary locks are more flexible, but offer
1587 fewer guarantees. This means that files locked with C<flock> may be
1588 modified by programs that do not also use C<flock>. See L<perlport>,
1589 your port's specific documentation, or your system-specific local manpages
1590 for details. It's best to assume traditional behavior if you're writing
1591 portable programs. (But if you're not, you should as always feel perfectly
1592 free to write for your own system's idiosyncrasies (sometimes called
1593 "features"). Slavish adherence to portability concerns shouldn't get
1594 in the way of your getting your job done.)
1596 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1597 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1598 you can use the symbolic names if you import them from the Fcntl module,
1599 either individually, or as a group using the ':flock' tag. LOCK_SH
1600 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1601 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1602 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1603 waiting for the lock (check the return status to see if you got it).
1605 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1606 before locking or unlocking it.
1608 Note that the emulation built with lockf(3) doesn't provide shared
1609 locks, and it requires that FILEHANDLE be open with write intent. These
1610 are the semantics that lockf(3) implements. Most if not all systems
1611 implement lockf(3) in terms of fcntl(2) locking, though, so the
1612 differing semantics shouldn't bite too many people.
1614 Note also that some versions of C<flock> cannot lock things over the
1615 network; you would need to use the more system-specific C<fcntl> for
1616 that. If you like you can force Perl to ignore your system's flock(2)
1617 function, and so provide its own fcntl(2)-based emulation, by passing
1618 the switch C<-Ud_flock> to the F<Configure> program when you configure
1621 Here's a mailbox appender for BSD systems.
1623 use Fcntl ':flock'; # import LOCK_* constants
1626 flock(MBOX,LOCK_EX);
1627 # and, in case someone appended
1628 # while we were waiting...
1633 flock(MBOX,LOCK_UN);
1636 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1637 or die "Can't open mailbox: $!";
1640 print MBOX $msg,"\n\n";
1643 On systems that support a real flock(), locks are inherited across fork()
1644 calls, whereas those that must resort to the more capricious fcntl()
1645 function lose the locks, making it harder to write servers.
1647 See also L<DB_File> for other flock() examples.
1651 Does a fork(2) system call to create a new process running the
1652 same program at the same point. It returns the child pid to the
1653 parent process, C<0> to the child process, or C<undef> if the fork is
1654 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1655 are shared, while everything else is copied. On most systems supporting
1656 fork(), great care has gone into making it extremely efficient (for
1657 example, using copy-on-write technology on data pages), making it the
1658 dominant paradigm for multitasking over the last few decades.
1660 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1661 output before forking the child process, but this may not be supported
1662 on some platforms (see L<perlport>). To be safe, you may need to set
1663 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1664 C<IO::Handle> on any open handles in order to avoid duplicate output.
1666 If you C<fork> without ever waiting on your children, you will
1667 accumulate zombies. On some systems, you can avoid this by setting
1668 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1669 forking and reaping moribund children.
1671 Note that if your forked child inherits system file descriptors like
1672 STDIN and STDOUT that are actually connected by a pipe or socket, even
1673 if you exit, then the remote server (such as, say, a CGI script or a
1674 backgrounded job launched from a remote shell) won't think you're done.
1675 You should reopen those to F</dev/null> if it's any issue.
1679 Declare a picture format for use by the C<write> function. For
1683 Test: @<<<<<<<< @||||| @>>>>>
1684 $str, $%, '$' . int($num)
1688 $num = $cost/$quantity;
1692 See L<perlform> for many details and examples.
1694 =item formline PICTURE,LIST
1696 This is an internal function used by C<format>s, though you may call it,
1697 too. It formats (see L<perlform>) a list of values according to the
1698 contents of PICTURE, placing the output into the format output
1699 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1700 Eventually, when a C<write> is done, the contents of
1701 C<$^A> are written to some filehandle, but you could also read C<$^A>
1702 yourself and then set C<$^A> back to C<"">. Note that a format typically
1703 does one C<formline> per line of form, but the C<formline> function itself
1704 doesn't care how many newlines are embedded in the PICTURE. This means
1705 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1706 You may therefore need to use multiple formlines to implement a single
1707 record format, just like the format compiler.
1709 Be careful if you put double quotes around the picture, because an C<@>
1710 character may be taken to mean the beginning of an array name.
1711 C<formline> always returns true. See L<perlform> for other examples.
1713 =item getc FILEHANDLE
1717 Returns the next character from the input file attached to FILEHANDLE,
1718 or the undefined value at end of file, or if there was an error.
1719 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1720 efficient. However, it cannot be used by itself to fetch single
1721 characters without waiting for the user to hit enter. For that, try
1722 something more like:
1725 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1728 system "stty", '-icanon', 'eol', "\001";
1734 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1737 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1741 Determination of whether $BSD_STYLE should be set
1742 is left as an exercise to the reader.
1744 The C<POSIX::getattr> function can do this more portably on
1745 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1746 module from your nearest CPAN site; details on CPAN can be found on
1751 Implements the C library function of the same name, which on most
1752 systems returns the current login from F</etc/utmp>, if any. If null,
1755 $login = getlogin || getpwuid($<) || "Kilroy";
1757 Do not consider C<getlogin> for authentication: it is not as
1758 secure as C<getpwuid>.
1760 =item getpeername SOCKET
1762 Returns the packed sockaddr address of other end of the SOCKET connection.
1765 $hersockaddr = getpeername(SOCK);
1766 ($port, $iaddr) = sockaddr_in($hersockaddr);
1767 $herhostname = gethostbyaddr($iaddr, AF_INET);
1768 $herstraddr = inet_ntoa($iaddr);
1772 Returns the current process group for the specified PID. Use
1773 a PID of C<0> to get the current process group for the
1774 current process. Will raise an exception if used on a machine that
1775 doesn't implement getpgrp(2). If PID is omitted, returns process
1776 group of current process. Note that the POSIX version of C<getpgrp>
1777 does not accept a PID argument, so only C<PID==0> is truly portable.
1781 Returns the process id of the parent process.
1783 =item getpriority WHICH,WHO
1785 Returns the current priority for a process, a process group, or a user.
1786 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1787 machine that doesn't implement getpriority(2).
1793 =item gethostbyname NAME
1795 =item getnetbyname NAME
1797 =item getprotobyname NAME
1803 =item getservbyname NAME,PROTO
1805 =item gethostbyaddr ADDR,ADDRTYPE
1807 =item getnetbyaddr ADDR,ADDRTYPE
1809 =item getprotobynumber NUMBER
1811 =item getservbyport PORT,PROTO
1829 =item sethostent STAYOPEN
1831 =item setnetent STAYOPEN
1833 =item setprotoent STAYOPEN
1835 =item setservent STAYOPEN
1849 These routines perform the same functions as their counterparts in the
1850 system library. In list context, the return values from the
1851 various get routines are as follows:
1853 ($name,$passwd,$uid,$gid,
1854 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1855 ($name,$passwd,$gid,$members) = getgr*
1856 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1857 ($name,$aliases,$addrtype,$net) = getnet*
1858 ($name,$aliases,$proto) = getproto*
1859 ($name,$aliases,$port,$proto) = getserv*
1861 (If the entry doesn't exist you get a null list.)
1863 The exact meaning of the $gcos field varies but it usually contains
1864 the real name of the user (as opposed to the login name) and other
1865 information pertaining to the user. Beware, however, that in many
1866 system users are able to change this information and therefore it
1867 cannot be trusted and therefore the $gcos is tainted (see
1868 L<perlsec>). The $passwd and $shell, user's encrypted password and
1869 login shell, are also tainted, because of the same reason.
1871 In scalar context, you get the name, unless the function was a
1872 lookup by name, in which case you get the other thing, whatever it is.
1873 (If the entry doesn't exist you get the undefined value.) For example:
1875 $uid = getpwnam($name);
1876 $name = getpwuid($num);
1878 $gid = getgrnam($name);
1879 $name = getgrgid($num;
1883 In I<getpw*()> the fields $quota, $comment, and $expire are special
1884 cases in the sense that in many systems they are unsupported. If the
1885 $quota is unsupported, it is an empty scalar. If it is supported, it
1886 usually encodes the disk quota. If the $comment field is unsupported,
1887 it is an empty scalar. If it is supported it usually encodes some
1888 administrative comment about the user. In some systems the $quota
1889 field may be $change or $age, fields that have to do with password
1890 aging. In some systems the $comment field may be $class. The $expire
1891 field, if present, encodes the expiration period of the account or the
1892 password. For the availability and the exact meaning of these fields
1893 in your system, please consult your getpwnam(3) documentation and your
1894 F<pwd.h> file. You can also find out from within Perl what your
1895 $quota and $comment fields mean and whether you have the $expire field
1896 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1897 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1898 files are only supported if your vendor has implemented them in the
1899 intuitive fashion that calling the regular C library routines gets the
1900 shadow versions if you're running under privilege or if there exists
1901 the shadow(3) functions as found in System V ( this includes Solaris
1902 and Linux.) Those systems which implement a proprietary shadow password
1903 facility are unlikely to be supported.
1905 The $members value returned by I<getgr*()> is a space separated list of
1906 the login names of the members of the group.
1908 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1909 C, it will be returned to you via C<$?> if the function call fails. The
1910 C<@addrs> value returned by a successful call is a list of the raw
1911 addresses returned by the corresponding system library call. In the
1912 Internet domain, each address is four bytes long and you can unpack it
1913 by saying something like:
1915 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1917 The Socket library makes this slightly easier:
1920 $iaddr = inet_aton("127.1"); # or whatever address
1921 $name = gethostbyaddr($iaddr, AF_INET);
1923 # or going the other way
1924 $straddr = inet_ntoa($iaddr);
1926 If you get tired of remembering which element of the return list
1927 contains which return value, by-name interfaces are provided
1928 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1929 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1930 and C<User::grent>. These override the normal built-ins, supplying
1931 versions that return objects with the appropriate names
1932 for each field. For example:
1936 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1938 Even though it looks like they're the same method calls (uid),
1939 they aren't, because a C<File::stat> object is different from
1940 a C<User::pwent> object.
1942 =item getsockname SOCKET
1944 Returns the packed sockaddr address of this end of the SOCKET connection,
1945 in case you don't know the address because you have several different
1946 IPs that the connection might have come in on.
1949 $mysockaddr = getsockname(SOCK);
1950 ($port, $myaddr) = sockaddr_in($mysockaddr);
1951 printf "Connect to %s [%s]\n",
1952 scalar gethostbyaddr($myaddr, AF_INET),
1955 =item getsockopt SOCKET,LEVEL,OPTNAME
1957 Returns the socket option requested, or undef if there is an error.
1963 Returns the value of EXPR with filename expansions such as the
1964 standard Unix shell F</bin/csh> would do. This is the internal function
1965 implementing the C<< <*.c> >> operator, but you can use it directly.
1966 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1967 discussed in more detail in L<perlop/"I/O Operators">.
1969 Beginning with v5.6.0, this operator is implemented using the standard
1970 C<File::Glob> extension. See L<File::Glob> for details.
1974 Converts a time as returned by the time function to a 8-element list
1975 with the time localized for the standard Greenwich time zone.
1976 Typically used as follows:
1979 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1982 All list elements are numeric, and come straight out of the C `struct
1983 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1984 specified time. $mday is the day of the month, and $mon is the month
1985 itself, in the range C<0..11> with 0 indicating January and 11
1986 indicating December. $year is the number of years since 1900. That
1987 is, $year is C<123> in year 2023. $wday is the day of the week, with
1988 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1989 the year, in the range C<0..364> (or C<0..365> in leap years.)
1991 Note that the $year element is I<not> simply the last two digits of
1992 the year. If you assume it is, then you create non-Y2K-compliant
1993 programs--and you wouldn't want to do that, would you?
1995 The proper way to get a complete 4-digit year is simply:
1999 And to get the last two digits of the year (e.g., '01' in 2001) do:
2001 $year = sprintf("%02d", $year % 100);
2003 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2005 In scalar context, C<gmtime()> returns the ctime(3) value:
2007 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2009 Also see the C<timegm> function provided by the C<Time::Local> module,
2010 and the strftime(3) function available via the POSIX module.
2012 This scalar value is B<not> locale dependent (see L<perllocale>), but
2013 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2014 strftime(3) and mktime(3) functions available via the POSIX module. To
2015 get somewhat similar but locale dependent date strings, set up your
2016 locale environment variables appropriately (please see L<perllocale>)
2017 and try for example:
2019 use POSIX qw(strftime);
2020 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2022 Note that the C<%a> and C<%b> escapes, which represent the short forms
2023 of the day of the week and the month of the year, may not necessarily
2024 be three characters wide in all locales.
2032 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2033 execution there. It may not be used to go into any construct that
2034 requires initialization, such as a subroutine or a C<foreach> loop. It
2035 also can't be used to go into a construct that is optimized away,
2036 or to get out of a block or subroutine given to C<sort>.
2037 It can be used to go almost anywhere else within the dynamic scope,
2038 including out of subroutines, but it's usually better to use some other
2039 construct such as C<last> or C<die>. The author of Perl has never felt the
2040 need to use this form of C<goto> (in Perl, that is--C is another matter).
2042 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2043 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2044 necessarily recommended if you're optimizing for maintainability:
2046 goto ("FOO", "BAR", "GLARCH")[$i];
2048 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2049 In fact, it isn't a goto in the normal sense at all, and doesn't have
2050 the stigma associated with other gotos. Instead, it
2051 substitutes a call to the named subroutine for the currently running
2052 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2053 another subroutine and then pretend that the other subroutine had been
2054 called in the first place (except that any modifications to C<@_>
2055 in the current subroutine are propagated to the other subroutine.)
2056 After the C<goto>, not even C<caller> will be able to tell that this
2057 routine was called first.
2059 NAME needn't be the name of a subroutine; it can be a scalar variable
2060 containing a code reference, or a block which evaluates to a code
2063 =item grep BLOCK LIST
2065 =item grep EXPR,LIST
2067 This is similar in spirit to, but not the same as, grep(1) and its
2068 relatives. In particular, it is not limited to using regular expressions.
2070 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2071 C<$_> to each element) and returns the list value consisting of those
2072 elements for which the expression evaluated to true. In scalar
2073 context, returns the number of times the expression was true.
2075 @foo = grep(!/^#/, @bar); # weed out comments
2079 @foo = grep {!/^#/} @bar; # weed out comments
2081 Note that C<$_> is an alias to the list value, so it can be used to
2082 modify the elements of the LIST. While this is useful and supported,
2083 it can cause bizarre results if the elements of LIST are not variables.
2084 Similarly, grep returns aliases into the original list, much as a for
2085 loop's index variable aliases the list elements. That is, modifying an
2086 element of a list returned by grep (for example, in a C<foreach>, C<map>
2087 or another C<grep>) actually modifies the element in the original list.
2088 This is usually something to be avoided when writing clear code.
2090 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2096 Interprets EXPR as a hex string and returns the corresponding value.
2097 (To convert strings that might start with either 0, 0x, or 0b, see
2098 L</oct>.) If EXPR is omitted, uses C<$_>.
2100 print hex '0xAf'; # prints '175'
2101 print hex 'aF'; # same
2103 Hex strings may only represent integers. Strings that would cause
2104 integer overflow trigger a warning.
2108 There is no builtin C<import> function. It is just an ordinary
2109 method (subroutine) defined (or inherited) by modules that wish to export
2110 names to another module. The C<use> function calls the C<import> method
2111 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2113 =item index STR,SUBSTR,POSITION
2115 =item index STR,SUBSTR
2117 The index function searches for one string within another, but without
2118 the wildcard-like behavior of a full regular-expression pattern match.
2119 It returns the position of the first occurrence of SUBSTR in STR at
2120 or after POSITION. If POSITION is omitted, starts searching from the
2121 beginning of the string. The return value is based at C<0> (or whatever
2122 you've set the C<$[> variable to--but don't do that). If the substring
2123 is not found, returns one less than the base, ordinarily C<-1>.
2129 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2130 You should not use this function for rounding: one because it truncates
2131 towards C<0>, and two because machine representations of floating point
2132 numbers can sometimes produce counterintuitive results. For example,
2133 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2134 because it's really more like -268.99999999999994315658 instead. Usually,
2135 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2136 functions will serve you better than will int().
2138 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2140 Implements the ioctl(2) function. You'll probably first have to say
2142 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2144 to get the correct function definitions. If F<ioctl.ph> doesn't
2145 exist or doesn't have the correct definitions you'll have to roll your
2146 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2147 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2148 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2149 written depending on the FUNCTION--a pointer to the string value of SCALAR
2150 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2151 has no string value but does have a numeric value, that value will be
2152 passed rather than a pointer to the string value. To guarantee this to be
2153 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2154 functions may be needed to manipulate the values of structures used by
2157 The return value of C<ioctl> (and C<fcntl>) is as follows:
2159 if OS returns: then Perl returns:
2161 0 string "0 but true"
2162 anything else that number
2164 Thus Perl returns true on success and false on failure, yet you can
2165 still easily determine the actual value returned by the operating
2168 $retval = ioctl(...) || -1;
2169 printf "System returned %d\n", $retval;
2171 The special string "C<0> but true" is exempt from B<-w> complaints
2172 about improper numeric conversions.
2174 Here's an example of setting a filehandle named C<REMOTE> to be
2175 non-blocking at the system level. You'll have to negotiate C<$|>
2176 on your own, though.
2178 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2180 $flags = fcntl(REMOTE, F_GETFL, 0)
2181 or die "Can't get flags for the socket: $!\n";
2183 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2184 or die "Can't set flags for the socket: $!\n";
2186 =item join EXPR,LIST
2188 Joins the separate strings of LIST into a single string with fields
2189 separated by the value of EXPR, and returns that new string. Example:
2191 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2193 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2194 first argument. Compare L</split>.
2198 Returns a list consisting of all the keys of the named hash. (In
2199 scalar context, returns the number of keys.) The keys are returned in
2200 an apparently random order. The actual random order is subject to
2201 change in future versions of perl, but it is guaranteed to be the same
2202 order as either the C<values> or C<each> function produces (given
2203 that the hash has not been modified). As a side effect, it resets
2206 Here is yet another way to print your environment:
2209 @values = values %ENV;
2211 print pop(@keys), '=', pop(@values), "\n";
2214 or how about sorted by key:
2216 foreach $key (sort(keys %ENV)) {
2217 print $key, '=', $ENV{$key}, "\n";
2220 The returned values are copies of the original keys in the hash, so
2221 modifying them will not affect the original hash. Compare L</values>.
2223 To sort a hash by value, you'll need to use a C<sort> function.
2224 Here's a descending numeric sort of a hash by its values:
2226 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2227 printf "%4d %s\n", $hash{$key}, $key;
2230 As an lvalue C<keys> allows you to increase the number of hash buckets
2231 allocated for the given hash. This can gain you a measure of efficiency if
2232 you know the hash is going to get big. (This is similar to pre-extending
2233 an array by assigning a larger number to $#array.) If you say
2237 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2238 in fact, since it rounds up to the next power of two. These
2239 buckets will be retained even if you do C<%hash = ()>, use C<undef
2240 %hash> if you want to free the storage while C<%hash> is still in scope.
2241 You can't shrink the number of buckets allocated for the hash using
2242 C<keys> in this way (but you needn't worry about doing this by accident,
2243 as trying has no effect).
2245 See also C<each>, C<values> and C<sort>.
2247 =item kill SIGNAL, LIST
2249 Sends a signal to a list of processes. Returns the number of
2250 processes successfully signaled (which is not necessarily the
2251 same as the number actually killed).
2253 $cnt = kill 1, $child1, $child2;
2256 If SIGNAL is zero, no signal is sent to the process. This is a
2257 useful way to check that the process is alive and hasn't changed
2258 its UID. See L<perlport> for notes on the portability of this
2261 Unlike in the shell, if SIGNAL is negative, it kills
2262 process groups instead of processes. (On System V, a negative I<PROCESS>
2263 number will also kill process groups, but that's not portable.) That
2264 means you usually want to use positive not negative signals. You may also
2265 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2271 The C<last> command is like the C<break> statement in C (as used in
2272 loops); it immediately exits the loop in question. If the LABEL is
2273 omitted, the command refers to the innermost enclosing loop. The
2274 C<continue> block, if any, is not executed:
2276 LINE: while (<STDIN>) {
2277 last LINE if /^$/; # exit when done with header
2281 C<last> cannot be used to exit a block which returns a value such as
2282 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2283 a grep() or map() operation.
2285 Note that a block by itself is semantically identical to a loop
2286 that executes once. Thus C<last> can be used to effect an early
2287 exit out of such a block.
2289 See also L</continue> for an illustration of how C<last>, C<next>, and
2296 Returns an lowercased version of EXPR. This is the internal function
2297 implementing the C<\L> escape in double-quoted strings.
2298 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2301 If EXPR is omitted, uses C<$_>.
2307 Returns the value of EXPR with the first character lowercased. This is
2308 the internal function implementing the C<\l> escape in double-quoted strings.
2309 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2311 If EXPR is omitted, uses C<$_>.
2317 Returns the length in characters of the value of EXPR. If EXPR is
2318 omitted, returns length of C<$_>. Note that this cannot be used on
2319 an entire array or hash to find out how many elements these have.
2320 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2322 =item link OLDFILE,NEWFILE
2324 Creates a new filename linked to the old filename. Returns true for
2325 success, false otherwise.
2327 =item listen SOCKET,QUEUESIZE
2329 Does the same thing that the listen system call does. Returns true if
2330 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2334 You really probably want to be using C<my> instead, because C<local> isn't
2335 what most people think of as "local". See L<perlsub/"Private Variables
2336 via my()"> for details.
2338 A local modifies the listed variables to be local to the enclosing
2339 block, file, or eval. If more than one value is listed, the list must
2340 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2341 for details, including issues with tied arrays and hashes.
2343 =item localtime EXPR
2345 Converts a time as returned by the time function to a 9-element list
2346 with the time analyzed for the local time zone. Typically used as
2350 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2353 All list elements are numeric, and come straight out of the C `struct
2354 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2355 specified time. $mday is the day of the month, and $mon is the month
2356 itself, in the range C<0..11> with 0 indicating January and 11
2357 indicating December. $year is the number of years since 1900. That
2358 is, $year is C<123> in year 2023. $wday is the day of the week, with
2359 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2360 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2361 is true if the specified time occurs during daylight savings time,
2364 Note that the $year element is I<not> simply the last two digits of
2365 the year. If you assume it is, then you create non-Y2K-compliant
2366 programs--and you wouldn't want to do that, would you?
2368 The proper way to get a complete 4-digit year is simply:
2372 And to get the last two digits of the year (e.g., '01' in 2001) do:
2374 $year = sprintf("%02d", $year % 100);
2376 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2378 In scalar context, C<localtime()> returns the ctime(3) value:
2380 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2382 This scalar value is B<not> locale dependent, see L<perllocale>, but
2383 instead a Perl builtin. Also see the C<Time::Local> module
2384 (to convert the second, minutes, hours, ... back to seconds since the
2385 stroke of midnight the 1st of January 1970, the value returned by
2386 time()), and the strftime(3) and mktime(3) functions available via the
2387 POSIX module. To get somewhat similar but locale dependent date
2388 strings, set up your locale environment variables appropriately
2389 (please see L<perllocale>) and try for example:
2391 use POSIX qw(strftime);
2392 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2394 Note that the C<%a> and C<%b>, the short forms of the day of the week
2395 and the month of the year, may not necessarily be three characters wide.
2401 This function places an advisory lock on a variable, subroutine,
2402 or referenced object contained in I<THING> until the lock goes out
2403 of scope. This is a built-in function only if your version of Perl
2404 was built with threading enabled, and if you've said C<use Threads>.
2405 Otherwise a user-defined function by this name will be called. See
2412 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2413 returns log of C<$_>. To get the log of another base, use basic algebra:
2414 The base-N log of a number is equal to the natural log of that number
2415 divided by the natural log of N. For example:
2419 return log($n)/log(10);
2422 See also L</exp> for the inverse operation.
2424 =item lstat FILEHANDLE
2430 Does the same thing as the C<stat> function (including setting the
2431 special C<_> filehandle) but stats a symbolic link instead of the file
2432 the symbolic link points to. If symbolic links are unimplemented on
2433 your system, a normal C<stat> is done.
2435 If EXPR is omitted, stats C<$_>.
2439 The match operator. See L<perlop>.
2441 =item map BLOCK LIST
2445 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2446 C<$_> to each element) and returns the list value composed of the
2447 results of each such evaluation. In scalar context, returns the
2448 total number of elements so generated. Evaluates BLOCK or EXPR in
2449 list context, so each element of LIST may produce zero, one, or
2450 more elements in the returned value.
2452 @chars = map(chr, @nums);
2454 translates a list of numbers to the corresponding characters. And
2456 %hash = map { getkey($_) => $_ } @array;
2458 is just a funny way to write
2461 foreach $_ (@array) {
2462 $hash{getkey($_)} = $_;
2465 Note that C<$_> is an alias to the list value, so it can be used to
2466 modify the elements of the LIST. While this is useful and supported,
2467 it can cause bizarre results if the elements of LIST are not variables.
2468 Using a regular C<foreach> loop for this purpose would be clearer in
2469 most cases. See also L</grep> for an array composed of those items of
2470 the original list for which the BLOCK or EXPR evaluates to true.
2472 =item mkdir FILENAME,MASK
2474 =item mkdir FILENAME
2476 Creates the directory specified by FILENAME, with permissions
2477 specified by MASK (as modified by C<umask>). If it succeeds it
2478 returns true, otherwise it returns false and sets C<$!> (errno).
2479 If omitted, MASK defaults to 0777.
2481 In general, it is better to create directories with permissive MASK,
2482 and let the user modify that with their C<umask>, than it is to supply
2483 a restrictive MASK and give the user no way to be more permissive.
2484 The exceptions to this rule are when the file or directory should be
2485 kept private (mail files, for instance). The perlfunc(1) entry on
2486 C<umask> discusses the choice of MASK in more detail.
2488 =item msgctl ID,CMD,ARG
2490 Calls the System V IPC function msgctl(2). You'll probably have to say
2494 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2495 then ARG must be a variable which will hold the returned C<msqid_ds>
2496 structure. Returns like C<ioctl>: the undefined value for error,
2497 C<"0 but true"> for zero, or the actual return value otherwise. See also
2498 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2500 =item msgget KEY,FLAGS
2502 Calls the System V IPC function msgget(2). Returns the message queue
2503 id, or the undefined value if there is an error. See also
2504 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2506 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2508 Calls the System V IPC function msgrcv to receive a message from
2509 message queue ID into variable VAR with a maximum message size of
2510 SIZE. Note that when a message is received, the message type as a
2511 native long integer will be the first thing in VAR, followed by the
2512 actual message. This packing may be opened with C<unpack("l! a*")>.
2513 Taints the variable. Returns true if successful, or false if there is
2514 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2515 C<IPC::SysV::Msg> documentation.
2517 =item msgsnd ID,MSG,FLAGS
2519 Calls the System V IPC function msgsnd to send the message MSG to the
2520 message queue ID. MSG must begin with the native long integer message
2521 type, and be followed by the length of the actual message, and finally
2522 the message itself. This kind of packing can be achieved with
2523 C<pack("l! a*", $type, $message)>. Returns true if successful,
2524 or false if there is an error. See also C<IPC::SysV>
2525 and C<IPC::SysV::Msg> documentation.
2529 =item my EXPR : ATTRIBUTES
2531 A C<my> declares the listed variables to be local (lexically) to the
2532 enclosing block, file, or C<eval>. If
2533 more than one value is listed, the list must be placed in parentheses. See
2534 L<perlsub/"Private Variables via my()"> for details.
2540 The C<next> command is like the C<continue> statement in C; it starts
2541 the next iteration of the loop:
2543 LINE: while (<STDIN>) {
2544 next LINE if /^#/; # discard comments
2548 Note that if there were a C<continue> block on the above, it would get
2549 executed even on discarded lines. If the LABEL is omitted, the command
2550 refers to the innermost enclosing loop.
2552 C<next> cannot be used to exit a block which returns a value such as
2553 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2554 a grep() or map() operation.
2556 Note that a block by itself is semantically identical to a loop
2557 that executes once. Thus C<next> will exit such a block early.
2559 See also L</continue> for an illustration of how C<last>, C<next>, and
2562 =item no Module LIST
2564 See the L</use> function, which C<no> is the opposite of.
2570 Interprets EXPR as an octal string and returns the corresponding
2571 value. (If EXPR happens to start off with C<0x>, interprets it as a
2572 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2573 binary string.) The following will handle decimal, binary, octal, and
2574 hex in the standard Perl or C notation:
2576 $val = oct($val) if $val =~ /^0/;
2578 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2579 in octal), use sprintf() or printf():
2581 $perms = (stat("filename"))[2] & 07777;
2582 $oct_perms = sprintf "%lo", $perms;
2584 The oct() function is commonly used when a string such as C<644> needs
2585 to be converted into a file mode, for example. (Although perl will
2586 automatically convert strings into numbers as needed, this automatic
2587 conversion assumes base 10.)
2589 =item open FILEHANDLE,MODE,LIST
2591 =item open FILEHANDLE,EXPR
2593 =item open FILEHANDLE
2595 Opens the file whose filename is given by EXPR, and associates it with
2596 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2597 name of the real filehandle wanted. (This is considered a symbolic
2598 reference, so C<use strict 'refs'> should I<not> be in effect.)
2600 If EXPR is omitted, the scalar
2601 variable of the same name as the FILEHANDLE contains the filename.
2602 (Note that lexical variables--those declared with C<my>--will not work
2603 for this purpose; so if you're using C<my>, specify EXPR in your call
2604 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2607 If MODE is C<< '<' >> or nothing, the file is opened for input.
2608 If MODE is C<< '>' >>, the file is truncated and opened for
2609 output, being created if necessary. If MODE is C<<< '>>' >>>,
2610 the file is opened for appending, again being created if necessary.
2611 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2612 you want both read and write access to the file; thus C<< '+<' >> is almost
2613 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2614 file first. You can't usually use either read-write mode for updating
2615 textfiles, since they have variable length records. See the B<-i>
2616 switch in L<perlrun> for a better approach. The file is created with
2617 permissions of C<0666> modified by the process' C<umask> value.
2619 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2620 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2622 In the 2-arguments (and 1-argument) form of the call the mode and
2623 filename should be concatenated (in this order), possibly separated by
2624 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2626 If the filename begins with C<'|'>, the filename is interpreted as a
2627 command to which output is to be piped, and if the filename ends with a
2628 C<'|'>, the filename is interpreted as a command which pipes output to
2629 us. See L<perlipc/"Using open() for IPC">
2630 for more examples of this. (You are not allowed to C<open> to a command
2631 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2632 and L<perlipc/"Bidirectional Communication with Another Process">
2635 If MODE is C<'|-'>, the filename is interpreted as a
2636 command to which output is to be piped, and if MODE is
2637 C<'-|'>, the filename is interpreted as a command which pipes output to
2638 us. In the 2-arguments (and 1-argument) form one should replace dash
2639 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2640 for more examples of this. (You are not allowed to C<open> to a command
2641 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2642 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2644 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2645 and opening C<< '>-' >> opens STDOUT.
2648 nonzero upon success, the undefined value otherwise. If the C<open>
2649 involved a pipe, the return value happens to be the pid of the
2652 If you're unfortunate enough to be running Perl on a system that
2653 distinguishes between text files and binary files (modern operating
2654 systems don't care), then you should check out L</binmode> for tips for
2655 dealing with this. The key distinction between systems that need C<binmode>
2656 and those that don't is their text file formats. Systems like Unix, MacOS, and
2657 Plan9, which delimit lines with a single character, and which encode that
2658 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2660 When opening a file, it's usually a bad idea to continue normal execution
2661 if the request failed, so C<open> is frequently used in connection with
2662 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2663 where you want to make a nicely formatted error message (but there are
2664 modules that can help with that problem)) you should always check
2665 the return value from opening a file. The infrequent exception is when
2666 working with an unopened filehandle is actually what you want to do.
2671 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2672 while (<ARTICLE>) {...
2674 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2675 # if the open fails, output is discarded
2677 open(DBASE, '+<', 'dbase.mine') # open for update
2678 or die "Can't open 'dbase.mine' for update: $!";
2680 open(DBASE, '+<dbase.mine') # ditto
2681 or die "Can't open 'dbase.mine' for update: $!";
2683 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2684 or die "Can't start caesar: $!";
2686 open(ARTICLE, "caesar <$article |") # ditto
2687 or die "Can't start caesar: $!";
2689 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2690 or die "Can't start sort: $!";
2692 # process argument list of files along with any includes
2694 foreach $file (@ARGV) {
2695 process($file, 'fh00');
2699 my($filename, $input) = @_;
2700 $input++; # this is a string increment
2701 unless (open($input, $filename)) {
2702 print STDERR "Can't open $filename: $!\n";
2707 while (<$input>) { # note use of indirection
2708 if (/^#include "(.*)"/) {
2709 process($1, $input);
2716 You may also, in the Bourne shell tradition, specify an EXPR beginning
2717 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2718 name of a filehandle (or file descriptor, if numeric) to be
2719 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2720 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2721 mode you specify should match the mode of the original filehandle.
2722 (Duping a filehandle does not take into account any existing contents of
2723 stdio buffers.) Duping file handles is not yet supported for 3-argument
2726 Here is a script that saves, redirects, and restores STDOUT and
2730 open(OLDOUT, ">&STDOUT");
2731 open(OLDERR, ">&STDERR");
2733 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2734 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2736 select(STDERR); $| = 1; # make unbuffered
2737 select(STDOUT); $| = 1; # make unbuffered
2739 print STDOUT "stdout 1\n"; # this works for
2740 print STDERR "stderr 1\n"; # subprocesses too
2745 open(STDOUT, ">&OLDOUT");
2746 open(STDERR, ">&OLDERR");
2748 print STDOUT "stdout 2\n";
2749 print STDERR "stderr 2\n";
2751 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2752 equivalent of C's C<fdopen> of that file descriptor; this is more
2753 parsimonious of file descriptors. For example:
2755 open(FILEHANDLE, "<&=$fd")
2757 Note that this feature depends on the fdopen() C library function.
2758 On many UNIX systems, fdopen() is known to fail when file descriptors
2759 exceed a certain value, typically 255. If you need more file
2760 descriptors than that, consider rebuilding Perl to use the C<sfio>
2763 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2764 with 2-arguments (or 1-argument) form of open(), then
2765 there is an implicit fork done, and the return value of open is the pid
2766 of the child within the parent process, and C<0> within the child
2767 process. (Use C<defined($pid)> to determine whether the open was successful.)
2768 The filehandle behaves normally for the parent, but i/o to that
2769 filehandle is piped from/to the STDOUT/STDIN of the child process.
2770 In the child process the filehandle isn't opened--i/o happens from/to
2771 the new STDOUT or STDIN. Typically this is used like the normal
2772 piped open when you want to exercise more control over just how the
2773 pipe command gets executed, such as when you are running setuid, and
2774 don't want to have to scan shell commands for metacharacters.
2775 The following triples are more or less equivalent:
2777 open(FOO, "|tr '[a-z]' '[A-Z]'");
2778 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2779 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2781 open(FOO, "cat -n '$file'|");
2782 open(FOO, '-|', "cat -n '$file'");
2783 open(FOO, '-|') || exec 'cat', '-n', $file;
2785 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2787 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2788 output before any operation that may do a fork, but this may not be
2789 supported on some platforms (see L<perlport>). To be safe, you may need
2790 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2791 of C<IO::Handle> on any open handles.
2793 On systems that support a
2794 close-on-exec flag on files, the flag will be set for the newly opened
2795 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2797 Closing any piped filehandle causes the parent process to wait for the
2798 child to finish, and returns the status value in C<$?>.
2800 The filename passed to 2-argument (or 1-argument) form of open()
2801 will have leading and trailing
2802 whitespace deleted, and the normal redirection characters
2803 honored. This property, known as "magic open",
2804 can often be used to good effect. A user could specify a filename of
2805 F<"rsh cat file |">, or you could change certain filenames as needed:
2807 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2808 open(FH, $filename) or die "Can't open $filename: $!";
2810 Use 3-argument form to open a file with arbitrary weird characters in it,
2812 open(FOO, '<', $file);
2814 otherwise it's necessary to protect any leading and trailing whitespace:
2816 $file =~ s#^(\s)#./$1#;
2817 open(FOO, "< $file\0");
2819 (this may not work on some bizzare filesystems). One should
2820 conscientiously choose between the I<magic> and 3-arguments form
2825 will allow the user to specify an argument of the form C<"rsh cat file |">,
2826 but will not work on a filename which happens to have a trailing space, while
2828 open IN, '<', $ARGV[0];
2830 will have exactly the opposite restrictions.
2832 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2833 should use the C<sysopen> function, which involves no such magic (but
2834 may use subtly different filemodes than Perl open(), which is mapped
2835 to C fopen()). This is
2836 another way to protect your filenames from interpretation. For example:
2839 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2840 or die "sysopen $path: $!";
2841 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2842 print HANDLE "stuff $$\n");
2844 print "File contains: ", <HANDLE>;
2846 Using the constructor from the C<IO::Handle> package (or one of its
2847 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2848 filehandles that have the scope of whatever variables hold references to
2849 them, and automatically close whenever and however you leave that scope:
2853 sub read_myfile_munged {
2855 my $handle = new IO::File;
2856 open($handle, "myfile") or die "myfile: $!";
2858 or return (); # Automatically closed here.
2859 mung $first or die "mung failed"; # Or here.
2860 return $first, <$handle> if $ALL; # Or here.
2864 See L</seek> for some details about mixing reading and writing.
2866 =item opendir DIRHANDLE,EXPR
2868 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2869 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2870 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2876 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2877 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2878 See L<utf8> for more about Unicode.
2882 An C<our> declares the listed variables to be valid globals within
2883 the enclosing block, file, or C<eval>. That is, it has the same
2884 scoping rules as a "my" declaration, but does not create a local
2885 variable. If more than one value is listed, the list must be placed
2886 in parentheses. The C<our> declaration has no semantic effect unless
2887 "use strict vars" is in effect, in which case it lets you use the
2888 declared global variable without qualifying it with a package name.
2889 (But only within the lexical scope of the C<our> declaration. In this
2890 it differs from "use vars", which is package scoped.)
2892 An C<our> declaration declares a global variable that will be visible
2893 across its entire lexical scope, even across package boundaries. The
2894 package in which the variable is entered is determined at the point
2895 of the declaration, not at the point of use. This means the following
2899 our $bar; # declares $Foo::bar for rest of lexical scope
2903 print $bar; # prints 20
2905 Multiple C<our> declarations in the same lexical scope are allowed
2906 if they are in different packages. If they happened to be in the same
2907 package, Perl will emit warnings if you have asked for them.
2911 our $bar; # declares $Foo::bar for rest of lexical scope
2915 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2916 print $bar; # prints 30
2918 our $bar; # emits warning
2920 =item pack TEMPLATE,LIST
2922 Takes a LIST of values and converts it into a string using the rules
2923 given by the TEMPLATE. The resulting string is the concatenation of
2924 the converted values. Typically, each converted value looks
2925 like its machine-level representation. For example, on 32-bit machines
2926 a converted integer may be represented by a sequence of 4 bytes.
2929 sequence of characters that give the order and type of values, as
2932 a A string with arbitrary binary data, will be null padded.
2933 A An ASCII string, will be space padded.
2934 Z A null terminated (asciz) string, will be null padded.
2936 b A bit string (ascending bit order inside each byte, like vec()).
2937 B A bit string (descending bit order inside each byte).
2938 h A hex string (low nybble first).
2939 H A hex string (high nybble first).
2941 c A signed char value.
2942 C An unsigned char value. Only does bytes. See U for Unicode.
2944 s A signed short value.
2945 S An unsigned short value.
2946 (This 'short' is _exactly_ 16 bits, which may differ from
2947 what a local C compiler calls 'short'. If you want
2948 native-length shorts, use the '!' suffix.)
2950 i A signed integer value.
2951 I An unsigned integer value.
2952 (This 'integer' is _at_least_ 32 bits wide. Its exact
2953 size depends on what a local C compiler calls 'int',
2954 and may even be larger than the 'long' described in
2957 l A signed long value.
2958 L An unsigned long value.
2959 (This 'long' is _exactly_ 32 bits, which may differ from
2960 what a local C compiler calls 'long'. If you want
2961 native-length longs, use the '!' suffix.)
2963 n An unsigned short in "network" (big-endian) order.
2964 N An unsigned long in "network" (big-endian) order.
2965 v An unsigned short in "VAX" (little-endian) order.
2966 V An unsigned long in "VAX" (little-endian) order.
2967 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2968 _exactly_ 32 bits, respectively.)
2970 q A signed quad (64-bit) value.
2971 Q An unsigned quad value.
2972 (Quads are available only if your system supports 64-bit
2973 integer values _and_ if Perl has been compiled to support those.
2974 Causes a fatal error otherwise.)
2976 f A single-precision float in the native format.
2977 d A double-precision float in the native format.
2979 p A pointer to a null-terminated string.
2980 P A pointer to a structure (fixed-length string).
2982 u A uuencoded string.
2983 U A Unicode character number. Encodes to UTF-8 internally.
2984 Works even if C<use utf8> is not in effect.
2986 w A BER compressed integer. Its bytes represent an unsigned
2987 integer in base 128, most significant digit first, with as
2988 few digits as possible. Bit eight (the high bit) is set
2989 on each byte except the last.
2993 @ Null fill to absolute position.
2995 The following rules apply:
3001 Each letter may optionally be followed by a number giving a repeat
3002 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3003 C<H>, and C<P> the pack function will gobble up that many values from
3004 the LIST. A C<*> for the repeat count means to use however many items are
3005 left, except for C<@>, C<x>, C<X>, where it is equivalent
3006 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3009 When used with C<Z>, C<*> results in the addition of a trailing null
3010 byte (so the packed result will be one longer than the byte C<length>
3013 The repeat count for C<u> is interpreted as the maximal number of bytes
3014 to encode per line of output, with 0 and 1 replaced by 45.
3018 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3019 string of length count, padding with nulls or spaces as necessary. When
3020 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3021 after the first null, and C<a> returns data verbatim. When packing,
3022 C<a>, and C<Z> are equivalent.
3024 If the value-to-pack is too long, it is truncated. If too long and an
3025 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3026 by a null byte. Thus C<Z> always packs a trailing null byte under
3031 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3032 Each byte of the input field of pack() generates 1 bit of the result.
3033 Each result bit is based on the least-significant bit of the corresponding
3034 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3035 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3037 Starting from the beginning of the input string of pack(), each 8-tuple
3038 of bytes is converted to 1 byte of output. With format C<b>
3039 the first byte of the 8-tuple determines the least-significant bit of a
3040 byte, and with format C<B> it determines the most-significant bit of
3043 If the length of the input string is not exactly divisible by 8, the
3044 remainder is packed as if the input string were padded by null bytes
3045 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3047 If the input string of pack() is longer than needed, extra bytes are ignored.
3048 A C<*> for the repeat count of pack() means to use all the bytes of
3049 the input field. On unpack()ing the bits are converted to a string
3050 of C<"0">s and C<"1">s.
3054 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3055 representable as hexadecimal digits, 0-9a-f) long.
3057 Each byte of the input field of pack() generates 4 bits of the result.
3058 For non-alphabetical bytes the result is based on the 4 least-significant
3059 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3060 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3061 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3062 is compatible with the usual hexadecimal digits, so that C<"a"> and
3063 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3064 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3066 Starting from the beginning of the input string of pack(), each pair
3067 of bytes is converted to 1 byte of output. With format C<h> the
3068 first byte of the pair determines the least-significant nybble of the
3069 output byte, and with format C<H> it determines the most-significant
3072 If the length of the input string is not even, it behaves as if padded
3073 by a null byte at the end. Similarly, during unpack()ing the "extra"
3074 nybbles are ignored.
3076 If the input string of pack() is longer than needed, extra bytes are ignored.
3077 A C<*> for the repeat count of pack() means to use all the bytes of
3078 the input field. On unpack()ing the bits are converted to a string
3079 of hexadecimal digits.
3083 The C<p> type packs a pointer to a null-terminated string. You are
3084 responsible for ensuring the string is not a temporary value (which can
3085 potentially get deallocated before you get around to using the packed result).
3086 The C<P> type packs a pointer to a structure of the size indicated by the
3087 length. A NULL pointer is created if the corresponding value for C<p> or
3088 C<P> is C<undef>, similarly for unpack().
3092 The C</> template character allows packing and unpacking of strings where
3093 the packed structure contains a byte count followed by the string itself.
3094 You write I<length-item>C</>I<string-item>.
3096 The I<length-item> can be any C<pack> template letter,
3097 and describes how the length value is packed.
3098 The ones likely to be of most use are integer-packing ones like
3099 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3100 and C<N> (for Sun XDR).
3102 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3103 For C<unpack> the length of the string is obtained from the I<length-item>,
3104 but if you put in the '*' it will be ignored.
3106 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3107 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3108 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3110 The I<length-item> is not returned explicitly from C<unpack>.
3112 Adding a count to the I<length-item> letter is unlikely to do anything
3113 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3114 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3115 which Perl does not regard as legal in numeric strings.
3119 The integer types C<s>, C<S>, C<l>, and C<L> may be
3120 immediately followed by a C<!> suffix to signify native shorts or
3121 longs--as you can see from above for example a bare C<l> does mean
3122 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3123 may be larger. This is an issue mainly in 64-bit platforms. You can
3124 see whether using C<!> makes any difference by
3126 print length(pack("s")), " ", length(pack("s!")), "\n";
3127 print length(pack("l")), " ", length(pack("l!")), "\n";
3129 C<i!> and C<I!> also work but only because of completeness;
3130 they are identical to C<i> and C<I>.
3132 The actual sizes (in bytes) of native shorts, ints, longs, and long
3133 longs on the platform where Perl was built are also available via
3137 print $Config{shortsize}, "\n";
3138 print $Config{intsize}, "\n";
3139 print $Config{longsize}, "\n";
3140 print $Config{longlongsize}, "\n";
3142 (The C<$Config{longlongsize}> will be undefine if your system does
3143 not support long longs.)
3147 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3148 are inherently non-portable between processors and operating systems
3149 because they obey the native byteorder and endianness. For example a
3150 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3151 (arranged in and handled by the CPU registers) into bytes as
3153 0x12 0x34 0x56 0x78 # little-endian
3154 0x78 0x56 0x34 0x12 # big-endian
3156 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3157 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3158 Power, and Cray are big-endian. MIPS can be either: Digital used it
3159 in little-endian mode; SGI uses it in big-endian mode.
3161 The names `big-endian' and `little-endian' are comic references to
3162 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3163 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3164 the egg-eating habits of the Lilliputians.
3166 Some systems may have even weirder byte orders such as
3171 You can see your system's preference with
3173 print join(" ", map { sprintf "%#02x", $_ }
3174 unpack("C*",pack("L",0x12345678))), "\n";
3176 The byteorder on the platform where Perl was built is also available
3180 print $Config{byteorder}, "\n";
3182 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3183 and C<'87654321'> are big-endian.
3185 If you want portable packed integers use the formats C<n>, C<N>,
3186 C<v>, and C<V>, their byte endianness and size is known.
3187 See also L<perlport>.
3191 Real numbers (floats and doubles) are in the native machine format only;
3192 due to the multiplicity of floating formats around, and the lack of a
3193 standard "network" representation, no facility for interchange has been
3194 made. This means that packed floating point data written on one machine
3195 may not be readable on another - even if both use IEEE floating point
3196 arithmetic (as the endian-ness of the memory representation is not part
3197 of the IEEE spec). See also L<perlport>.
3199 Note that Perl uses doubles internally for all numeric calculation, and
3200 converting from double into float and thence back to double again will
3201 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3206 If the pattern begins with a C<U>, the resulting string will be treated
3207 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3208 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3209 characters. If you don't want this to happen, you can begin your pattern
3210 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3211 string, and then follow this with a C<U*> somewhere in your pattern.
3215 You must yourself do any alignment or padding by inserting for example
3216 enough C<'x'>es while packing. There is no way to pack() and unpack()
3217 could know where the bytes are going to or coming from. Therefore
3218 C<pack> (and C<unpack>) handle their output and input as flat
3223 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3227 If TEMPLATE requires more arguments to pack() than actually given, pack()
3228 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3229 to pack() than actually given, extra arguments are ignored.
3235 $foo = pack("CCCC",65,66,67,68);
3237 $foo = pack("C4",65,66,67,68);
3239 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3240 # same thing with Unicode circled letters
3242 $foo = pack("ccxxcc",65,66,67,68);
3245 # note: the above examples featuring "C" and "c" are true
3246 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3247 # and UTF-8. In EBCDIC the first example would be
3248 # $foo = pack("CCCC",193,194,195,196);
3250 $foo = pack("s2",1,2);
3251 # "\1\0\2\0" on little-endian
3252 # "\0\1\0\2" on big-endian
3254 $foo = pack("a4","abcd","x","y","z");
3257 $foo = pack("aaaa","abcd","x","y","z");
3260 $foo = pack("a14","abcdefg");
3261 # "abcdefg\0\0\0\0\0\0\0"
3263 $foo = pack("i9pl", gmtime);
3264 # a real struct tm (on my system anyway)
3266 $utmp_template = "Z8 Z8 Z16 L";
3267 $utmp = pack($utmp_template, @utmp1);
3268 # a struct utmp (BSDish)
3270 @utmp2 = unpack($utmp_template, $utmp);
3271 # "@utmp1" eq "@utmp2"
3274 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3277 $foo = pack('sx2l', 12, 34);
3278 # short 12, two zero bytes padding, long 34
3279 $bar = pack('s@4l', 12, 34);
3280 # short 12, zero fill to position 4, long 34
3283 The same template may generally also be used in unpack().
3287 =item package NAMESPACE
3289 Declares the compilation unit as being in the given namespace. The scope
3290 of the package declaration is from the declaration itself through the end
3291 of the enclosing block, file, or eval (the same as the C<my> operator).
3292 All further unqualified dynamic identifiers will be in this namespace.
3293 A package statement affects only dynamic variables--including those
3294 you've used C<local> on--but I<not> lexical variables, which are created
3295 with C<my>. Typically it would be the first declaration in a file to
3296 be included by the C<require> or C<use> operator. You can switch into a
3297 package in more than one place; it merely influences which symbol table
3298 is used by the compiler for the rest of that block. You can refer to
3299 variables and filehandles in other packages by prefixing the identifier
3300 with the package name and a double colon: C<$Package::Variable>.
3301 If the package name is null, the C<main> package as assumed. That is,
3302 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3303 still seen in older code).
3305 If NAMESPACE is omitted, then there is no current package, and all
3306 identifiers must be fully qualified or lexicals. This is stricter
3307 than C<use strict>, since it also extends to function names.
3309 See L<perlmod/"Packages"> for more information about packages, modules,
3310 and classes. See L<perlsub> for other scoping issues.
3312 =item pipe READHANDLE,WRITEHANDLE
3314 Opens a pair of connected pipes like the corresponding system call.
3315 Note that if you set up a loop of piped processes, deadlock can occur
3316 unless you are very careful. In addition, note that Perl's pipes use
3317 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3318 after each command, depending on the application.
3320 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3321 for examples of such things.
3323 On systems that support a close-on-exec flag on files, the flag will be set
3324 for the newly opened file descriptors as determined by the value of $^F.
3331 Pops and returns the last value of the array, shortening the array by
3332 one element. Has an effect similar to
3336 If there are no elements in the array, returns the undefined value
3337 (although this may happen at other times as well). If ARRAY is
3338 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3339 array in subroutines, just like C<shift>.
3345 Returns the offset of where the last C<m//g> search left off for the variable
3346 is in question (C<$_> is used when the variable is not specified). May be
3347 modified to change that offset. Such modification will also influence
3348 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3351 =item print FILEHANDLE LIST
3357 Prints a string or a list of strings. Returns true if successful.
3358 FILEHANDLE may be a scalar variable name, in which case the variable
3359 contains the name of or a reference to the filehandle, thus introducing
3360 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3361 the next token is a term, it may be misinterpreted as an operator
3362 unless you interpose a C<+> or put parentheses around the arguments.)
3363 If FILEHANDLE is omitted, prints by default to standard output (or
3364 to the last selected output channel--see L</select>). If LIST is
3365 also omitted, prints C<$_> to the currently selected output channel.
3366 To set the default output channel to something other than STDOUT
3367 use the select operation. The current value of C<$,> (if any) is
3368 printed between each LIST item. The current value of C<$\> (if
3369 any) is printed after the entire LIST has been printed. Because
3370 print takes a LIST, anything in the LIST is evaluated in list
3371 context, and any subroutine that you call will have one or more of
3372 its expressions evaluated in list context. Also be careful not to
3373 follow the print keyword with a left parenthesis unless you want
3374 the corresponding right parenthesis to terminate the arguments to
3375 the print--interpose a C<+> or put parentheses around all the
3378 Note that if you're storing FILEHANDLES in an array or other expression,
3379 you will have to use a block returning its value instead:
3381 print { $files[$i] } "stuff\n";
3382 print { $OK ? STDOUT : STDERR } "stuff\n";
3384 =item printf FILEHANDLE FORMAT, LIST
3386 =item printf FORMAT, LIST
3388 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3389 (the output record separator) is not appended. The first argument
3390 of the list will be interpreted as the C<printf> format. If C<use locale> is
3391 in effect, the character used for the decimal point in formatted real numbers
3392 is affected by the LC_NUMERIC locale. See L<perllocale>.
3394 Don't fall into the trap of using a C<printf> when a simple
3395 C<print> would do. The C<print> is more efficient and less
3398 =item prototype FUNCTION
3400 Returns the prototype of a function as a string (or C<undef> if the
3401 function has no prototype). FUNCTION is a reference to, or the name of,
3402 the function whose prototype you want to retrieve.
3404 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3405 name for Perl builtin. If the builtin is not I<overridable> (such as
3406 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3407 C<system>) returns C<undef> because the builtin does not really behave
3408 like a Perl function. Otherwise, the string describing the equivalent
3409 prototype is returned.
3411 =item push ARRAY,LIST
3413 Treats ARRAY as a stack, and pushes the values of LIST
3414 onto the end of ARRAY. The length of ARRAY increases by the length of
3415 LIST. Has the same effect as
3418 $ARRAY[++$#ARRAY] = $value;
3421 but is more efficient. Returns the new number of elements in the array.
3433 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3435 =item quotemeta EXPR
3439 Returns the value of EXPR with all non-"word"
3440 characters backslashed. (That is, all characters not matching
3441 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3442 returned string, regardless of any locale settings.)
3443 This is the internal function implementing
3444 the C<\Q> escape in double-quoted strings.
3446 If EXPR is omitted, uses C<$_>.
3452 Returns a random fractional number greater than or equal to C<0> and less
3453 than the value of EXPR. (EXPR should be positive.) If EXPR is
3454 omitted, the value C<1> is used. Automatically calls C<srand> unless
3455 C<srand> has already been called. See also C<srand>.
3457 (Note: If your rand function consistently returns numbers that are too
3458 large or too small, then your version of Perl was probably compiled
3459 with the wrong number of RANDBITS.)
3461 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3463 =item read FILEHANDLE,SCALAR,LENGTH
3465 Attempts to read LENGTH bytes of data into variable SCALAR from the
3466 specified FILEHANDLE. Returns the number of bytes actually read,
3467 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3468 or shrunk to the length actually read. An OFFSET may be specified to
3469 place the read data at some other place than the beginning of the
3470 string. This call is actually implemented in terms of stdio's fread(3)
3471 call. To get a true read(2) system call, see C<sysread>.
3473 =item readdir DIRHANDLE
3475 Returns the next directory entry for a directory opened by C<opendir>.
3476 If used in list context, returns all the rest of the entries in the
3477 directory. If there are no more entries, returns an undefined value in
3478 scalar context or a null list in list context.
3480 If you're planning to filetest the return values out of a C<readdir>, you'd
3481 better prepend the directory in question. Otherwise, because we didn't
3482 C<chdir> there, it would have been testing the wrong file.
3484 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3485 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3490 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3491 context, each call reads and returns the next line, until end-of-file is
3492 reached, whereupon the subsequent call returns undef. In list context,
3493 reads until end-of-file is reached and returns a list of lines. Note that
3494 the notion of "line" used here is however you may have defined it
3495 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3497 When C<$/> is set to C<undef>, when readline() is in scalar
3498 context (i.e. file slurp mode), and when an empty file is read, it
3499 returns C<''> the first time, followed by C<undef> subsequently.
3501 This is the internal function implementing the C<< <EXPR> >>
3502 operator, but you can use it directly. The C<< <EXPR> >>
3503 operator is discussed in more detail in L<perlop/"I/O Operators">.
3506 $line = readline(*STDIN); # same thing
3512 Returns the value of a symbolic link, if symbolic links are
3513 implemented. If not, gives a fatal error. If there is some system
3514 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3515 omitted, uses C<$_>.
3519 EXPR is executed as a system command.
3520 The collected standard output of the command is returned.
3521 In scalar context, it comes back as a single (potentially
3522 multi-line) string. In list context, returns a list of lines
3523 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3524 This is the internal function implementing the C<qx/EXPR/>
3525 operator, but you can use it directly. The C<qx/EXPR/>
3526 operator is discussed in more detail in L<perlop/"I/O Operators">.
3528 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3530 Receives a message on a socket. Attempts to receive LENGTH bytes of
3531 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3532 will be grown or shrunk to the length actually read. Takes the same
3533 flags as the system call of the same name. Returns the address of the
3534 sender if SOCKET's protocol supports this; returns an empty string
3535 otherwise. If there's an error, returns the undefined value. This call
3536 is actually implemented in terms of recvfrom(2) system call. See
3537 L<perlipc/"UDP: Message Passing"> for examples.
3543 The C<redo> command restarts the loop block without evaluating the
3544 conditional again. The C<continue> block, if any, is not executed. If
3545 the LABEL is omitted, the command refers to the innermost enclosing
3546 loop. This command is normally used by programs that want to lie to
3547 themselves about what was just input:
3549 # a simpleminded Pascal comment stripper
3550 # (warning: assumes no { or } in strings)
3551 LINE: while (<STDIN>) {
3552 while (s|({.*}.*){.*}|$1 |) {}
3557 if (/}/) { # end of comment?
3566 C<redo> cannot be used to retry a block which returns a value such as
3567 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3568 a grep() or map() operation.
3570 Note that a block by itself is semantically identical to a loop
3571 that executes once. Thus C<redo> inside such a block will effectively
3572 turn it into a looping construct.
3574 See also L</continue> for an illustration of how C<last>, C<next>, and
3581 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3582 is not specified, C<$_> will be used. The value returned depends on the
3583 type of thing the reference is a reference to.
3584 Builtin types include:
3594 If the referenced object has been blessed into a package, then that package
3595 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3597 if (ref($r) eq "HASH") {
3598 print "r is a reference to a hash.\n";
3601 print "r is not a reference at all.\n";
3603 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3604 print "r is a reference to something that isa hash.\n";
3607 See also L<perlref>.
3609 =item rename OLDNAME,NEWNAME
3611 Changes the name of a file; an existing file NEWNAME will be
3612 clobbered. Returns true for success, false otherwise.
3614 Behavior of this function varies wildly depending on your system
3615 implementation. For example, it will usually not work across file system
3616 boundaries, even though the system I<mv> command sometimes compensates
3617 for this. Other restrictions include whether it works on directories,
3618 open files, or pre-existing files. Check L<perlport> and either the
3619 rename(2) manpage or equivalent system documentation for details.
3621 =item require VERSION
3627 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3630 If a VERSION is specified as a literal of the form v5.6.1,
3631 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3632 at least as recent as that version, at run time. (For compatibility
3633 with older versions of Perl, a numeric argument will also be interpreted
3634 as VERSION.) Compare with L</use>, which can do a similar check at
3637 require v5.6.1; # run time version check
3638 require 5.6.1; # ditto
3639 require 5.005_03; # float version allowed for compatibility
3641 Otherwise, demands that a library file be included if it hasn't already
3642 been included. The file is included via the do-FILE mechanism, which is
3643 essentially just a variety of C<eval>. Has semantics similar to the following
3648 return 1 if $INC{$filename};
3649 my($realfilename,$result);
3651 foreach $prefix (@INC) {
3652 $realfilename = "$prefix/$filename";
3653 if (-f $realfilename) {
3654 $INC{$filename} = $realfilename;
3655 $result = do $realfilename;
3659 die "Can't find $filename in \@INC";
3661 delete $INC{$filename} if $@ || !$result;
3663 die "$filename did not return true value" unless $result;
3667 Note that the file will not be included twice under the same specified
3668 name. The file must return true as the last statement to indicate
3669 successful execution of any initialization code, so it's customary to
3670 end such a file with C<1;> unless you're sure it'll return true
3671 otherwise. But it's better just to put the C<1;>, in case you add more
3674 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3675 replaces "F<::>" with "F</>" in the filename for you,
3676 to make it easy to load standard modules. This form of loading of
3677 modules does not risk altering your namespace.
3679 In other words, if you try this:
3681 require Foo::Bar; # a splendid bareword
3683 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3684 directories specified in the C<@INC> array.
3686 But if you try this:
3688 $class = 'Foo::Bar';
3689 require $class; # $class is not a bareword
3691 require "Foo::Bar"; # not a bareword because of the ""
3693 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3694 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3696 eval "require $class";
3698 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3704 Generally used in a C<continue> block at the end of a loop to clear
3705 variables and reset C<??> searches so that they work again. The
3706 expression is interpreted as a list of single characters (hyphens
3707 allowed for ranges). All variables and arrays beginning with one of
3708 those letters are reset to their pristine state. If the expression is
3709 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3710 only variables or searches in the current package. Always returns
3713 reset 'X'; # reset all X variables
3714 reset 'a-z'; # reset lower case variables
3715 reset; # just reset ?one-time? searches
3717 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3718 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3719 variables--lexical variables are unaffected, but they clean themselves
3720 up on scope exit anyway, so you'll probably want to use them instead.
3727 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3728 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3729 context, depending on how the return value will be used, and the context
3730 may vary from one execution to the next (see C<wantarray>). If no EXPR
3731 is given, returns an empty list in list context, the undefined value in
3732 scalar context, and (of course) nothing at all in a void context.
3734 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3735 or do FILE will automatically return the value of the last expression
3740 In list context, returns a list value consisting of the elements
3741 of LIST in the opposite order. In scalar context, concatenates the
3742 elements of LIST and returns a string value with all characters
3743 in the opposite order.
3745 print reverse <>; # line tac, last line first
3747 undef $/; # for efficiency of <>
3748 print scalar reverse <>; # character tac, last line tsrif
3750 This operator is also handy for inverting a hash, although there are some
3751 caveats. If a value is duplicated in the original hash, only one of those
3752 can be represented as a key in the inverted hash. Also, this has to
3753 unwind one hash and build a whole new one, which may take some time
3754 on a large hash, such as from a DBM file.
3756 %by_name = reverse %by_address; # Invert the hash
3758 =item rewinddir DIRHANDLE
3760 Sets the current position to the beginning of the directory for the
3761 C<readdir> routine on DIRHANDLE.
3763 =item rindex STR,SUBSTR,POSITION
3765 =item rindex STR,SUBSTR
3767 Works just like index() except that it returns the position of the LAST
3768 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3769 last occurrence at or before that position.
3771 =item rmdir FILENAME
3775 Deletes the directory specified by FILENAME if that directory is empty. If it
3776 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3777 FILENAME is omitted, uses C<$_>.
3781 The substitution operator. See L<perlop>.
3785 Forces EXPR to be interpreted in scalar context and returns the value
3788 @counts = ( scalar @a, scalar @b, scalar @c );
3790 There is no equivalent operator to force an expression to
3791 be interpolated in list context because in practice, this is never
3792 needed. If you really wanted to do so, however, you could use
3793 the construction C<@{[ (some expression) ]}>, but usually a simple
3794 C<(some expression)> suffices.
3796 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3797 parenthesized list, this behaves as a scalar comma expression, evaluating
3798 all but the last element in void context and returning the final element
3799 evaluated in scalar context. This is seldom what you want.
3801 The following single statement:
3803 print uc(scalar(&foo,$bar)),$baz;
3805 is the moral equivalent of these two:
3808 print(uc($bar),$baz);
3810 See L<perlop> for more details on unary operators and the comma operator.
3812 =item seek FILEHANDLE,POSITION,WHENCE
3814 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3815 FILEHANDLE may be an expression whose value gives the name of the
3816 filehandle. The values for WHENCE are C<0> to set the new position to
3817 POSITION, C<1> to set it to the current position plus POSITION, and
3818 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3819 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3820 (start of the file, current position, end of the file) from the Fcntl
3821 module. Returns C<1> upon success, C<0> otherwise.
3823 If you want to position file for C<sysread> or C<syswrite>, don't use
3824 C<seek>--buffering makes its effect on the file's system position
3825 unpredictable and non-portable. Use C<sysseek> instead.
3827 Due to the rules and rigors of ANSI C, on some systems you have to do a
3828 seek whenever you switch between reading and writing. Amongst other
3829 things, this may have the effect of calling stdio's clearerr(3).
3830 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3834 This is also useful for applications emulating C<tail -f>. Once you hit
3835 EOF on your read, and then sleep for a while, you might have to stick in a
3836 seek() to reset things. The C<seek> doesn't change the current position,
3837 but it I<does> clear the end-of-file condition on the handle, so that the
3838 next C<< <FILE> >> makes Perl try again to read something. We hope.
3840 If that doesn't work (some stdios are particularly cantankerous), then
3841 you may need something more like this:
3844 for ($curpos = tell(FILE); $_ = <FILE>;
3845 $curpos = tell(FILE)) {
3846 # search for some stuff and put it into files
3848 sleep($for_a_while);
3849 seek(FILE, $curpos, 0);
3852 =item seekdir DIRHANDLE,POS
3854 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3855 must be a value returned by C<telldir>. Has the same caveats about
3856 possible directory compaction as the corresponding system library
3859 =item select FILEHANDLE
3863 Returns the currently selected filehandle. Sets the current default
3864 filehandle for output, if FILEHANDLE is supplied. This has two
3865 effects: first, a C<write> or a C<print> without a filehandle will
3866 default to this FILEHANDLE. Second, references to variables related to
3867 output will refer to this output channel. For example, if you have to
3868 set the top of form format for more than one output channel, you might
3876 FILEHANDLE may be an expression whose value gives the name of the
3877 actual filehandle. Thus:
3879 $oldfh = select(STDERR); $| = 1; select($oldfh);
3881 Some programmers may prefer to think of filehandles as objects with
3882 methods, preferring to write the last example as:
3885 STDERR->autoflush(1);
3887 =item select RBITS,WBITS,EBITS,TIMEOUT
3889 This calls the select(2) system call with the bit masks specified, which
3890 can be constructed using C<fileno> and C<vec>, along these lines:
3892 $rin = $win = $ein = '';
3893 vec($rin,fileno(STDIN),1) = 1;
3894 vec($win,fileno(STDOUT),1) = 1;
3897 If you want to select on many filehandles you might wish to write a
3901 my(@fhlist) = split(' ',$_[0]);
3904 vec($bits,fileno($_),1) = 1;
3908 $rin = fhbits('STDIN TTY SOCK');
3912 ($nfound,$timeleft) =
3913 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3915 or to block until something becomes ready just do this
3917 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3919 Most systems do not bother to return anything useful in $timeleft, so
3920 calling select() in scalar context just returns $nfound.
3922 Any of the bit masks can also be undef. The timeout, if specified, is
3923 in seconds, which may be fractional. Note: not all implementations are
3924 capable of returning the$timeleft. If not, they always return
3925 $timeleft equal to the supplied $timeout.
3927 You can effect a sleep of 250 milliseconds this way:
3929 select(undef, undef, undef, 0.25);
3931 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3932 or <FH>) with C<select>, except as permitted by POSIX, and even
3933 then only on POSIX systems. You have to use C<sysread> instead.
3935 =item semctl ID,SEMNUM,CMD,ARG
3937 Calls the System V IPC function C<semctl>. You'll probably have to say
3941 first to get the correct constant definitions. If CMD is IPC_STAT or
3942 GETALL, then ARG must be a variable which will hold the returned
3943 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3944 the undefined value for error, "C<0 but true>" for zero, or the actual
3945 return value otherwise. The ARG must consist of a vector of native
3946 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3947 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
3950 =item semget KEY,NSEMS,FLAGS
3952 Calls the System V IPC function semget. Returns the semaphore id, or
3953 the undefined value if there is an error. See also
3954 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
3957 =item semop KEY,OPSTRING
3959 Calls the System V IPC function semop to perform semaphore operations
3960 such as signaling and waiting. OPSTRING must be a packed array of
3961 semop structures. Each semop structure can be generated with
3962 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3963 operations is implied by the length of OPSTRING. Returns true if
3964 successful, or false if there is an error. As an example, the
3965 following code waits on semaphore $semnum of semaphore id $semid:
3967 $semop = pack("sss", $semnum, -1, 0);
3968 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3970 To signal the semaphore, replace C<-1> with C<1>. See also
3971 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
3974 =item send SOCKET,MSG,FLAGS,TO
3976 =item send SOCKET,MSG,FLAGS
3978 Sends a message on a socket. Takes the same flags as the system call
3979 of the same name. On unconnected sockets you must specify a
3980 destination to send TO, in which case it does a C C<sendto>. Returns
3981 the number of characters sent, or the undefined value if there is an
3982 error. The C system call sendmsg(2) is currently unimplemented.
3983 See L<perlipc/"UDP: Message Passing"> for examples.
3985 =item setpgrp PID,PGRP
3987 Sets the current process group for the specified PID, C<0> for the current
3988 process. Will produce a fatal error if used on a machine that doesn't
3989 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3990 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3991 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3994 =item setpriority WHICH,WHO,PRIORITY
3996 Sets the current priority for a process, a process group, or a user.
3997 (See setpriority(2).) Will produce a fatal error if used on a machine
3998 that doesn't implement setpriority(2).
4000 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4002 Sets the socket option requested. Returns undefined if there is an
4003 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4010 Shifts the first value of the array off and returns it, shortening the
4011 array by 1 and moving everything down. If there are no elements in the
4012 array, returns the undefined value. If ARRAY is omitted, shifts the
4013 C<@_> array within the lexical scope of subroutines and formats, and the
4014 C<@ARGV> array at file scopes or within the lexical scopes established by
4015 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4018 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4019 same thing to the left end of an array that C<pop> and C<push> do to the
4022 =item shmctl ID,CMD,ARG
4024 Calls the System V IPC function shmctl. You'll probably have to say
4028 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4029 then ARG must be a variable which will hold the returned C<shmid_ds>
4030 structure. Returns like ioctl: the undefined value for error, "C<0> but
4031 true" for zero, or the actual return value otherwise.
4032 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4034 =item shmget KEY,SIZE,FLAGS
4036 Calls the System V IPC function shmget. Returns the shared memory
4037 segment id, or the undefined value if there is an error.
4038 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4040 =item shmread ID,VAR,POS,SIZE
4042 =item shmwrite ID,STRING,POS,SIZE
4044 Reads or writes the System V shared memory segment ID starting at
4045 position POS for size SIZE by attaching to it, copying in/out, and
4046 detaching from it. When reading, VAR must be a variable that will
4047 hold the data read. When writing, if STRING is too long, only SIZE
4048 bytes are used; if STRING is too short, nulls are written to fill out
4049 SIZE bytes. Return true if successful, or false if there is an error.
4050 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4051 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4053 =item shutdown SOCKET,HOW
4055 Shuts down a socket connection in the manner indicated by HOW, which
4056 has the same interpretation as in the system call of the same name.
4058 shutdown(SOCKET, 0); # I/we have stopped reading data
4059 shutdown(SOCKET, 1); # I/we have stopped writing data
4060 shutdown(SOCKET, 2); # I/we have stopped using this socket
4062 This is useful with sockets when you want to tell the other
4063 side you're done writing but not done reading, or vice versa.
4064 It's also a more insistent form of close because it also
4065 disables the file descriptor in any forked copies in other
4072 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4073 returns sine of C<$_>.
4075 For the inverse sine operation, you may use the C<Math::Trig::asin>
4076 function, or use this relation:
4078 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4084 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4085 May be interrupted if the process receives a signal such as C<SIGALRM>.
4086 Returns the number of seconds actually slept. You probably cannot
4087 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4090 On some older systems, it may sleep up to a full second less than what
4091 you requested, depending on how it counts seconds. Most modern systems
4092 always sleep the full amount. They may appear to sleep longer than that,
4093 however, because your process might not be scheduled right away in a
4094 busy multitasking system.
4096 For delays of finer granularity than one second, you may use Perl's
4097 C<syscall> interface to access setitimer(2) if your system supports
4098 it, or else see L</select> above. The Time::HiRes module from CPAN
4101 See also the POSIX module's C<sigpause> function.
4103 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4105 Opens a socket of the specified kind and attaches it to filehandle
4106 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4107 the system call of the same name. You should C<use Socket> first
4108 to get the proper definitions imported. See the examples in
4109 L<perlipc/"Sockets: Client/Server Communication">.
4111 On systems that support a close-on-exec flag on files, the flag will
4112 be set for the newly opened file descriptor, as determined by the
4113 value of $^F. See L<perlvar/$^F>.
4115 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4117 Creates an unnamed pair of sockets in the specified domain, of the
4118 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4119 for the system call of the same name. If unimplemented, yields a fatal
4120 error. Returns true if successful.
4122 On systems that support a close-on-exec flag on files, the flag will
4123 be set for the newly opened file descriptors, as determined by the value
4124 of $^F. See L<perlvar/$^F>.
4126 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4127 to C<pipe(Rdr, Wtr)> is essentially:
4130 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4131 shutdown(Rdr, 1); # no more writing for reader
4132 shutdown(Wtr, 0); # no more reading for writer
4134 See L<perlipc> for an example of socketpair use.
4136 =item sort SUBNAME LIST
4138 =item sort BLOCK LIST
4142 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4143 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4144 specified, it gives the name of a subroutine that returns an integer
4145 less than, equal to, or greater than C<0>, depending on how the elements
4146 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4147 operators are extremely useful in such routines.) SUBNAME may be a
4148 scalar variable name (unsubscripted), in which case the value provides
4149 the name of (or a reference to) the actual subroutine to use. In place
4150 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4153 If the subroutine's prototype is C<($$)>, the elements to be compared
4154 are passed by reference in C<@_>, as for a normal subroutine. This is
4155 slower than unprototyped subroutines, where the elements to be
4156 compared are passed into the subroutine
4157 as the package global variables $a and $b (see example below). Note that
4158 in the latter case, it is usually counter-productive to declare $a and
4161 In either case, the subroutine may not be recursive. The values to be
4162 compared are always passed by reference, so don't modify them.
4164 You also cannot exit out of the sort block or subroutine using any of the
4165 loop control operators described in L<perlsyn> or with C<goto>.
4167 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4168 current collation locale. See L<perllocale>.
4173 @articles = sort @files;
4175 # same thing, but with explicit sort routine
4176 @articles = sort {$a cmp $b} @files;
4178 # now case-insensitively
4179 @articles = sort {uc($a) cmp uc($b)} @files;
4181 # same thing in reversed order
4182 @articles = sort {$b cmp $a} @files;
4184 # sort numerically ascending
4185 @articles = sort {$a <=> $b} @files;
4187 # sort numerically descending
4188 @articles = sort {$b <=> $a} @files;
4190 # this sorts the %age hash by value instead of key
4191 # using an in-line function
4192 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4194 # sort using explicit subroutine name
4196 $age{$a} <=> $age{$b}; # presuming numeric
4198 @sortedclass = sort byage @class;
4200 sub backwards { $b cmp $a }
4201 @harry = qw(dog cat x Cain Abel);
4202 @george = qw(gone chased yz Punished Axed);
4204 # prints AbelCaincatdogx
4205 print sort backwards @harry;
4206 # prints xdogcatCainAbel
4207 print sort @george, 'to', @harry;
4208 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4210 # inefficiently sort by descending numeric compare using
4211 # the first integer after the first = sign, or the
4212 # whole record case-insensitively otherwise
4215 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4220 # same thing, but much more efficiently;
4221 # we'll build auxiliary indices instead
4225 push @nums, /=(\d+)/;
4230 $nums[$b] <=> $nums[$a]
4232 $caps[$a] cmp $caps[$b]
4236 # same thing, but without any temps
4237 @new = map { $_->[0] }
4238 sort { $b->[1] <=> $a->[1]
4241 } map { [$_, /=(\d+)/, uc($_)] } @old;
4243 # using a prototype allows you to use any comparison subroutine
4244 # as a sort subroutine (including other package's subroutines)
4246 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4249 @new = sort other::backwards @old;
4251 If you're using strict, you I<must not> declare $a
4252 and $b as lexicals. They are package globals. That means
4253 if you're in the C<main> package and type
4255 @articles = sort {$b <=> $a} @files;
4257 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4258 but if you're in the C<FooPack> package, it's the same as typing
4260 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4262 The comparison function is required to behave. If it returns
4263 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4264 sometimes saying the opposite, for example) the results are not
4267 =item splice ARRAY,OFFSET,LENGTH,LIST
4269 =item splice ARRAY,OFFSET,LENGTH
4271 =item splice ARRAY,OFFSET
4275 Removes the elements designated by OFFSET and LENGTH from an array, and
4276 replaces them with the elements of LIST, if any. In list context,
4277 returns the elements removed from the array. In scalar context,
4278 returns the last element removed, or C<undef> if no elements are
4279 removed. The array grows or shrinks as necessary.
4280 If OFFSET is negative then it starts that far from the end of the array.
4281 If LENGTH is omitted, removes everything from OFFSET onward.
4282 If LENGTH is negative, leaves that many elements off the end of the array.
4283 If both OFFSET and LENGTH are omitted, removes everything.
4285 The following equivalences hold (assuming C<$[ == 0>):
4287 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4288 pop(@a) splice(@a,-1)
4289 shift(@a) splice(@a,0,1)
4290 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4291 $a[$x] = $y splice(@a,$x,1,$y)
4293 Example, assuming array lengths are passed before arrays:
4295 sub aeq { # compare two list values
4296 my(@a) = splice(@_,0,shift);
4297 my(@b) = splice(@_,0,shift);
4298 return 0 unless @a == @b; # same len?
4300 return 0 if pop(@a) ne pop(@b);
4304 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4306 =item split /PATTERN/,EXPR,LIMIT
4308 =item split /PATTERN/,EXPR
4310 =item split /PATTERN/
4314 Splits a string into a list of strings and returns that list. By default,
4315 empty leading fields are preserved, and empty trailing ones are deleted.
4317 In scalar context, returns the number of fields found and splits into
4318 the C<@_> array. Use of split in scalar context is deprecated, however,
4319 because it clobbers your subroutine arguments.
4321 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4322 splits on whitespace (after skipping any leading whitespace). Anything
4323 matching PATTERN is taken to be a delimiter separating the fields. (Note
4324 that the delimiter may be longer than one character.)
4326 If LIMIT is specified and positive, splits into no more than that
4327 many fields (though it may split into fewer). If LIMIT is unspecified
4328 or zero, trailing null fields are stripped (which potential users
4329 of C<pop> would do well to remember). If LIMIT is negative, it is
4330 treated as if an arbitrarily large LIMIT had been specified.
4332 A pattern matching the null string (not to be confused with
4333 a null pattern C<//>, which is just one member of the set of patterns
4334 matching a null string) will split the value of EXPR into separate
4335 characters at each point it matches that way. For example:
4337 print join(':', split(/ */, 'hi there'));
4339 produces the output 'h:i:t:h:e:r:e'.
4341 The LIMIT parameter can be used to split a line partially
4343 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4345 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4346 one larger than the number of variables in the list, to avoid
4347 unnecessary work. For the list above LIMIT would have been 4 by
4348 default. In time critical applications it behooves you not to split
4349 into more fields than you really need.
4351 If the PATTERN contains parentheses, additional list elements are
4352 created from each matching substring in the delimiter.
4354 split(/([,-])/, "1-10,20", 3);
4356 produces the list value
4358 (1, '-', 10, ',', 20)
4360 If you had the entire header of a normal Unix email message in $header,
4361 you could split it up into fields and their values this way:
4363 $header =~ s/\n\s+/ /g; # fix continuation lines
4364 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4366 The pattern C</PATTERN/> may be replaced with an expression to specify
4367 patterns that vary at runtime. (To do runtime compilation only once,
4368 use C</$variable/o>.)
4370 As a special case, specifying a PATTERN of space (C<' '>) will split on
4371 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4372 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4373 will give you as many null initial fields as there are leading spaces.
4374 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4375 whitespace produces a null first field. A C<split> with no arguments
4376 really does a C<split(' ', $_)> internally.
4378 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4383 open(PASSWD, '/etc/passwd');
4385 ($login, $passwd, $uid, $gid,
4386 $gcos, $home, $shell) = split(/:/);
4390 (Note that $shell above will still have a newline on it. See L</chop>,
4391 L</chomp>, and L</join>.)
4393 =item sprintf FORMAT, LIST
4395 Returns a string formatted by the usual C<printf> conventions of the C
4396 library function C<sprintf>. See below for more details
4397 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4398 the general principles.
4402 # Format number with up to 8 leading zeroes
4403 $result = sprintf("%08d", $number);
4405 # Round number to 3 digits after decimal point
4406 $rounded = sprintf("%.3f", $number);
4408 Perl does its own C<sprintf> formatting--it emulates the C
4409 function C<sprintf>, but it doesn't use it (except for floating-point
4410 numbers, and even then only the standard modifiers are allowed). As a
4411 result, any non-standard extensions in your local C<sprintf> are not
4412 available from Perl.
4414 Perl's C<sprintf> permits the following universally-known conversions:
4417 %c a character with the given number
4419 %d a signed integer, in decimal
4420 %u an unsigned integer, in decimal
4421 %o an unsigned integer, in octal
4422 %x an unsigned integer, in hexadecimal
4423 %e a floating-point number, in scientific notation
4424 %f a floating-point number, in fixed decimal notation
4425 %g a floating-point number, in %e or %f notation
4427 In addition, Perl permits the following widely-supported conversions:
4429 %X like %x, but using upper-case letters
4430 %E like %e, but using an upper-case "E"
4431 %G like %g, but with an upper-case "E" (if applicable)
4432 %b an unsigned integer, in binary
4433 %p a pointer (outputs the Perl value's address in hexadecimal)
4434 %n special: *stores* the number of characters output so far
4435 into the next variable in the parameter list
4437 Finally, for backward (and we do mean "backward") compatibility, Perl
4438 permits these unnecessary but widely-supported conversions:
4441 %D a synonym for %ld
4442 %U a synonym for %lu
4443 %O a synonym for %lo
4446 Perl permits the following universally-known flags between the C<%>
4447 and the conversion letter:
4449 space prefix positive number with a space
4450 + prefix positive number with a plus sign
4451 - left-justify within the field
4452 0 use zeros, not spaces, to right-justify
4453 # prefix non-zero octal with "0", non-zero hex with "0x"
4454 number minimum field width
4455 .number "precision": digits after decimal point for
4456 floating-point, max length for string, minimum length
4458 l interpret integer as C type "long" or "unsigned long"
4459 h interpret integer as C type "short" or "unsigned short"
4460 If no flags, interpret integer as C type "int" or "unsigned"
4462 There are also two Perl-specific flags:
4464 V interpret integer as Perl's standard integer type
4465 v interpret string as a vector of integers, output as
4466 numbers separated either by dots, or by an arbitrary
4467 string received from the argument list when the flag
4470 Where a number would appear in the flags, an asterisk (C<*>) may be
4471 used instead, in which case Perl uses the next item in the parameter
4472 list as the given number (that is, as the field width or precision).
4473 If a field width obtained through C<*> is negative, it has the same
4474 effect as the C<-> flag: left-justification.
4476 The C<v> flag is useful for displaying ordinal values of characters
4477 in arbitrary strings:
4479 printf "version is v%vd\n", $^V; # Perl's version
4480 printf "address is %*vX\n", ":", $addr; # IPv6 address
4481 printf "bits are %*vb\n", " ", $bits; # random bitstring
4483 If C<use locale> is in effect, the character used for the decimal
4484 point in formatted real numbers is affected by the LC_NUMERIC locale.
4487 If Perl understands "quads" (64-bit integers) (this requires
4488 either that the platform natively support quads or that Perl
4489 be specifically compiled to support quads), the characters
4493 print quads, and they may optionally be preceded by
4501 You can find out whether your Perl supports quads via L<Config>:
4504 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4507 If Perl understands "long doubles" (this requires that the platform
4508 support long doubles), the flags
4512 may optionally be preceded by
4520 You can find out whether your Perl supports long doubles via L<Config>:
4523 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4529 Return the square root of EXPR. If EXPR is omitted, returns square
4530 root of C<$_>. Only works on non-negative operands, unless you've
4531 loaded the standard Math::Complex module.
4534 print sqrt(-2); # prints 1.4142135623731i
4540 Sets the random number seed for the C<rand> operator. If EXPR is
4541 omitted, uses a semi-random value supplied by the kernel (if it supports
4542 the F</dev/urandom> device) or based on the current time and process
4543 ID, among other things. In versions of Perl prior to 5.004 the default
4544 seed was just the current C<time>. This isn't a particularly good seed,
4545 so many old programs supply their own seed value (often C<time ^ $$> or
4546 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4548 In fact, it's usually not necessary to call C<srand> at all, because if
4549 it is not called explicitly, it is called implicitly at the first use of
4550 the C<rand> operator. However, this was not the case in version of Perl
4551 before 5.004, so if your script will run under older Perl versions, it
4552 should call C<srand>.
4554 Note that you need something much more random than the default seed for
4555 cryptographic purposes. Checksumming the compressed output of one or more
4556 rapidly changing operating system status programs is the usual method. For
4559 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4561 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4564 Do I<not> call C<srand> multiple times in your program unless you know
4565 exactly what you're doing and why you're doing it. The point of the
4566 function is to "seed" the C<rand> function so that C<rand> can produce
4567 a different sequence each time you run your program. Just do it once at the
4568 top of your program, or you I<won't> get random numbers out of C<rand>!
4570 Frequently called programs (like CGI scripts) that simply use
4574 for a seed can fall prey to the mathematical property that
4578 one-third of the time. So don't do that.
4580 =item stat FILEHANDLE
4586 Returns a 13-element list giving the status info for a file, either
4587 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4588 it stats C<$_>. Returns a null list if the stat fails. Typically used
4591 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4592 $atime,$mtime,$ctime,$blksize,$blocks)
4595 Not all fields are supported on all filesystem types. Here are the
4596 meaning of the fields:
4598 0 dev device number of filesystem
4600 2 mode file mode (type and permissions)
4601 3 nlink number of (hard) links to the file
4602 4 uid numeric user ID of file's owner
4603 5 gid numeric group ID of file's owner
4604 6 rdev the device identifier (special files only)
4605 7 size total size of file, in bytes
4606 8 atime last access time in seconds since the epoch
4607 9 mtime last modify time in seconds since the epoch
4608 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4609 11 blksize preferred block size for file system I/O
4610 12 blocks actual number of blocks allocated
4612 (The epoch was at 00:00 January 1, 1970 GMT.)
4614 If stat is passed the special filehandle consisting of an underline, no
4615 stat is done, but the current contents of the stat structure from the
4616 last stat or filetest are returned. Example:
4618 if (-x $file && (($d) = stat(_)) && $d < 0) {
4619 print "$file is executable NFS file\n";
4622 (This works on machines only for which the device number is negative
4625 Because the mode contains both the file type and its permissions, you
4626 should mask off the file type portion and (s)printf using a C<"%o">
4627 if you want to see the real permissions.
4629 $mode = (stat($filename))[2];
4630 printf "Permissions are %04o\n", $mode & 07777;
4632 In scalar context, C<stat> returns a boolean value indicating success
4633 or failure, and, if successful, sets the information associated with
4634 the special filehandle C<_>.
4636 The File::stat module provides a convenient, by-name access mechanism:
4639 $sb = stat($filename);
4640 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4641 $filename, $sb->size, $sb->mode & 07777,
4642 scalar localtime $sb->mtime;
4644 You can import symbolic mode constants (C<S_IF*>) and functions
4645 (C<S_IS*>) from the Fcntl module:
4649 $mode = (stat($filename))[2];
4651 $user_rwx = ($mode & S_IRWXU) >> 6;
4652 $group_read = ($mode & S_IRGRP) >> 3;
4653 $other_execute = $mode & S_IXOTH;
4655 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4657 $is_setuid = $mode & S_ISUID;
4658 $is_setgid = S_ISDIR($mode);
4660 You could write the last two using the C<-u> and C<-d> operators.
4661 The commonly available S_IF* constants are
4663 # Permissions: read, write, execute, for user, group, others.
4665 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4666 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4667 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4669 # Setuid/Setgid/Stickiness.
4671 S_ISUID S_ISGID S_ISVTX S_ISTXT
4673 # File types. Not necessarily all are available on your system.
4675 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4677 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4679 S_IREAD S_IWRITE S_IEXEC
4681 and the S_IF* functions are
4683 S_IFMODE($mode) the part of $mode containing the permission bits
4684 and the setuid/setgid/sticky bits
4686 S_IFMT($mode) the part of $mode containing the file type
4687 which can be bit-anded with e.g. S_IFREG
4688 or with the following functions
4690 # The operators -f, -d, -l, -b, -c, -p, and -s.
4692 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4693 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4695 # No direct -X operator counterpart, but for the first one
4696 # the -g operator is often equivalent. The ENFMT stands for
4697 # record flocking enforcement, a platform-dependent feature.
4699 S_ISENFMT($mode) S_ISWHT($mode)
4701 See your native chmod(2) and stat(2) documentation for more details
4702 about the S_* constants.
4708 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4709 doing many pattern matches on the string before it is next modified.
4710 This may or may not save time, depending on the nature and number of
4711 patterns you are searching on, and on the distribution of character
4712 frequencies in the string to be searched--you probably want to compare
4713 run times with and without it to see which runs faster. Those loops
4714 which scan for many short constant strings (including the constant
4715 parts of more complex patterns) will benefit most. You may have only
4716 one C<study> active at a time--if you study a different scalar the first
4717 is "unstudied". (The way C<study> works is this: a linked list of every
4718 character in the string to be searched is made, so we know, for
4719 example, where all the C<'k'> characters are. From each search string,
4720 the rarest character is selected, based on some static frequency tables
4721 constructed from some C programs and English text. Only those places
4722 that contain this "rarest" character are examined.)
4724 For example, here is a loop that inserts index producing entries
4725 before any line containing a certain pattern:
4729 print ".IX foo\n" if /\bfoo\b/;
4730 print ".IX bar\n" if /\bbar\b/;
4731 print ".IX blurfl\n" if /\bblurfl\b/;
4736 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4737 will be looked at, because C<f> is rarer than C<o>. In general, this is
4738 a big win except in pathological cases. The only question is whether
4739 it saves you more time than it took to build the linked list in the
4742 Note that if you have to look for strings that you don't know till
4743 runtime, you can build an entire loop as a string and C<eval> that to
4744 avoid recompiling all your patterns all the time. Together with
4745 undefining C<$/> to input entire files as one record, this can be very
4746 fast, often faster than specialized programs like fgrep(1). The following
4747 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4748 out the names of those files that contain a match:
4750 $search = 'while (<>) { study;';
4751 foreach $word (@words) {
4752 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4757 eval $search; # this screams
4758 $/ = "\n"; # put back to normal input delimiter
4759 foreach $file (sort keys(%seen)) {
4767 =item sub NAME BLOCK
4769 This is subroutine definition, not a real function I<per se>. With just a
4770 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4771 Without a NAME, it's an anonymous function declaration, and does actually
4772 return a value: the CODE ref of the closure you just created. See L<perlsub>
4773 and L<perlref> for details.
4775 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4777 =item substr EXPR,OFFSET,LENGTH
4779 =item substr EXPR,OFFSET
4781 Extracts a substring out of EXPR and returns it. First character is at
4782 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4783 If OFFSET is negative (or more precisely, less than C<$[>), starts
4784 that far from the end of the string. If LENGTH is omitted, returns
4785 everything to the end of the string. If LENGTH is negative, leaves that
4786 many characters off the end of the string.
4788 You can use the substr() function as an lvalue, in which case EXPR
4789 must itself be an lvalue. If you assign something shorter than LENGTH,
4790 the string will shrink, and if you assign something longer than LENGTH,
4791 the string will grow to accommodate it. To keep the string the same
4792 length you may need to pad or chop your value using C<sprintf>.
4794 If OFFSET and LENGTH specify a substring that is partly outside the
4795 string, only the part within the string is returned. If the substring
4796 is beyond either end of the string, substr() returns the undefined
4797 value and produces a warning. When used as an lvalue, specifying a
4798 substring that is entirely outside the string is a fatal error.
4799 Here's an example showing the behavior for boundary cases:
4802 substr($name, 4) = 'dy'; # $name is now 'freddy'
4803 my $null = substr $name, 6, 2; # returns '' (no warning)
4804 my $oops = substr $name, 7; # returns undef, with warning
4805 substr($name, 7) = 'gap'; # fatal error
4807 An alternative to using substr() as an lvalue is to specify the
4808 replacement string as the 4th argument. This allows you to replace
4809 parts of the EXPR and return what was there before in one operation,
4810 just as you can with splice().
4812 =item symlink OLDFILE,NEWFILE
4814 Creates a new filename symbolically linked to the old filename.
4815 Returns C<1> for success, C<0> otherwise. On systems that don't support
4816 symbolic links, produces a fatal error at run time. To check for that,
4819 $symlink_exists = eval { symlink("",""); 1 };
4823 Calls the system call specified as the first element of the list,
4824 passing the remaining elements as arguments to the system call. If
4825 unimplemented, produces a fatal error. The arguments are interpreted
4826 as follows: if a given argument is numeric, the argument is passed as
4827 an int. If not, the pointer to the string value is passed. You are
4828 responsible to make sure a string is pre-extended long enough to
4829 receive any result that might be written into a string. You can't use a
4830 string literal (or other read-only string) as an argument to C<syscall>
4831 because Perl has to assume that any string pointer might be written
4833 integer arguments are not literals and have never been interpreted in a
4834 numeric context, you may need to add C<0> to them to force them to look
4835 like numbers. This emulates the C<syswrite> function (or vice versa):
4837 require 'syscall.ph'; # may need to run h2ph
4839 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4841 Note that Perl supports passing of up to only 14 arguments to your system call,
4842 which in practice should usually suffice.
4844 Syscall returns whatever value returned by the system call it calls.
4845 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4846 Note that some system calls can legitimately return C<-1>. The proper
4847 way to handle such calls is to assign C<$!=0;> before the call and
4848 check the value of C<$!> if syscall returns C<-1>.
4850 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4851 number of the read end of the pipe it creates. There is no way
4852 to retrieve the file number of the other end. You can avoid this
4853 problem by using C<pipe> instead.
4855 =item sysopen FILEHANDLE,FILENAME,MODE
4857 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4859 Opens the file whose filename is given by FILENAME, and associates it
4860 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4861 the name of the real filehandle wanted. This function calls the
4862 underlying operating system's C<open> function with the parameters
4863 FILENAME, MODE, PERMS.
4865 The possible values and flag bits of the MODE parameter are
4866 system-dependent; they are available via the standard module C<Fcntl>.
4867 See the documentation of your operating system's C<open> to see which
4868 values and flag bits are available. You may combine several flags
4869 using the C<|>-operator.
4871 Some of the most common values are C<O_RDONLY> for opening the file in
4872 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4873 and C<O_RDWR> for opening the file in read-write mode, and.
4875 For historical reasons, some values work on almost every system
4876 supported by perl: zero means read-only, one means write-only, and two
4877 means read/write. We know that these values do I<not> work under
4878 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4879 use them in new code.
4881 If the file named by FILENAME does not exist and the C<open> call creates
4882 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4883 PERMS specifies the permissions of the newly created file. If you omit
4884 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4885 These permission values need to be in octal, and are modified by your
4886 process's current C<umask>.
4888 In many systems the C<O_EXCL> flag is available for opening files in
4889 exclusive mode. This is B<not> locking: exclusiveness means here that
4890 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4893 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4895 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4896 that takes away the user's option to have a more permissive umask.
4897 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4900 Note that C<sysopen> depends on the fdopen() C library function.
4901 On many UNIX systems, fdopen() is known to fail when file descriptors
4902 exceed a certain value, typically 255. If you need more file
4903 descriptors than that, consider rebuilding Perl to use the C<sfio>
4904 library, or perhaps using the POSIX::open() function.
4906 See L<perlopentut> for a kinder, gentler explanation of opening files.
4908 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4910 =item sysread FILEHANDLE,SCALAR,LENGTH
4912 Attempts to read LENGTH bytes of data into variable SCALAR from the
4913 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4914 so mixing this with other kinds of reads, C<print>, C<write>,
4915 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4916 usually buffers data. Returns the number of bytes actually read, C<0>
4917 at end of file, or undef if there was an error. SCALAR will be grown or
4918 shrunk so that the last byte actually read is the last byte of the
4919 scalar after the read.
4921 An OFFSET may be specified to place the read data at some place in the
4922 string other than the beginning. A negative OFFSET specifies
4923 placement at that many bytes counting backwards from the end of the
4924 string. A positive OFFSET greater than the length of SCALAR results
4925 in the string being padded to the required size with C<"\0"> bytes before
4926 the result of the read is appended.
4928 There is no syseof() function, which is ok, since eof() doesn't work
4929 very well on device files (like ttys) anyway. Use sysread() and check
4930 for a return value for 0 to decide whether you're done.
4932 =item sysseek FILEHANDLE,POSITION,WHENCE
4934 Sets FILEHANDLE's system position using the system call lseek(2). It
4935 bypasses stdio, so mixing this with reads (other than C<sysread>),
4936 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4937 FILEHANDLE may be an expression whose value gives the name of the
4938 filehandle. The values for WHENCE are C<0> to set the new position to
4939 POSITION, C<1> to set the it to the current position plus POSITION,
4940 and C<2> to set it to EOF plus POSITION (typically negative). For
4941 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4942 C<SEEK_END> (start of the file, current position, end of the file)
4943 from the Fcntl module.
4945 Returns the new position, or the undefined value on failure. A position
4946 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4947 true on success and false on failure, yet you can still easily determine
4952 =item system PROGRAM LIST
4954 Does exactly the same thing as C<exec LIST>, except that a fork is
4955 done first, and the parent process waits for the child process to
4956 complete. Note that argument processing varies depending on the
4957 number of arguments. If there is more than one argument in LIST,
4958 or if LIST is an array with more than one value, starts the program
4959 given by the first element of the list with arguments given by the
4960 rest of the list. If there is only one scalar argument, the argument
4961 is checked for shell metacharacters, and if there are any, the
4962 entire argument is passed to the system's command shell for parsing
4963 (this is C</bin/sh -c> on Unix platforms, but varies on other
4964 platforms). If there are no shell metacharacters in the argument,
4965 it is split into words and passed directly to C<execvp>, which is
4968 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4969 output before any operation that may do a fork, but this may not be
4970 supported on some platforms (see L<perlport>). To be safe, you may need
4971 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4972 of C<IO::Handle> on any open handles.
4974 The return value is the exit status of the program as
4975 returned by the C<wait> call. To get the actual exit value divide by
4976 256. See also L</exec>. This is I<not> what you want to use to capture
4977 the output from a command, for that you should use merely backticks or
4978 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4979 indicates a failure to start the program (inspect $! for the reason).
4981 Like C<exec>, C<system> allows you to lie to a program about its name if
4982 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4984 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4985 program they're running doesn't actually interrupt your program.
4987 @args = ("command", "arg1", "arg2");
4989 or die "system @args failed: $?"
4991 You can check all the failure possibilities by inspecting
4994 $exit_value = $? >> 8;
4995 $signal_num = $? & 127;
4996 $dumped_core = $? & 128;
4998 When the arguments get executed via the system shell, results
4999 and return codes will be subject to its quirks and capabilities.
5000 See L<perlop/"`STRING`"> and L</exec> for details.
5002 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5004 =item syswrite FILEHANDLE,SCALAR,LENGTH
5006 =item syswrite FILEHANDLE,SCALAR
5008 Attempts to write LENGTH bytes of data from variable SCALAR to the
5009 specified FILEHANDLE, using the system call write(2). If LENGTH
5010 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5011 this with reads (other than C<sysread())>, C<print>, C<write>,
5012 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5013 usually buffers data. Returns the number of bytes actually written,
5014 or C<undef> if there was an error. If the LENGTH is greater than
5015 the available data in the SCALAR after the OFFSET, only as much
5016 data as is available will be written.
5018 An OFFSET may be specified to write the data from some part of the
5019 string other than the beginning. A negative OFFSET specifies writing
5020 that many bytes counting backwards from the end of the string. In the
5021 case the SCALAR is empty you can use OFFSET but only zero offset.
5023 =item tell FILEHANDLE
5027 Returns the current position for FILEHANDLE. FILEHANDLE may be an
5028 expression whose value gives the name of the actual filehandle. If
5029 FILEHANDLE is omitted, assumes the file last read.
5031 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5033 =item telldir DIRHANDLE
5035 Returns the current position of the C<readdir> routines on DIRHANDLE.
5036 Value may be given to C<seekdir> to access a particular location in a
5037 directory. Has the same caveats about possible directory compaction as
5038 the corresponding system library routine.
5040 =item tie VARIABLE,CLASSNAME,LIST
5042 This function binds a variable to a package class that will provide the
5043 implementation for the variable. VARIABLE is the name of the variable
5044 to be enchanted. CLASSNAME is the name of a class implementing objects
5045 of correct type. Any additional arguments are passed to the C<new>
5046 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5047 or C<TIEHASH>). Typically these are arguments such as might be passed
5048 to the C<dbm_open()> function of C. The object returned by the C<new>
5049 method is also returned by the C<tie> function, which would be useful
5050 if you want to access other methods in CLASSNAME.
5052 Note that functions such as C<keys> and C<values> may return huge lists
5053 when used on large objects, like DBM files. You may prefer to use the
5054 C<each> function to iterate over such. Example:
5056 # print out history file offsets
5058 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5059 while (($key,$val) = each %HIST) {
5060 print $key, ' = ', unpack('L',$val), "\n";
5064 A class implementing a hash should have the following methods:
5066 TIEHASH classname, LIST
5068 STORE this, key, value
5073 NEXTKEY this, lastkey
5076 A class implementing an ordinary array should have the following methods:
5078 TIEARRAY classname, LIST
5080 STORE this, key, value
5082 STORESIZE this, count
5088 SPLICE this, offset, length, LIST
5092 A class implementing a file handle should have the following methods:
5094 TIEHANDLE classname, LIST
5095 READ this, scalar, length, offset
5098 WRITE this, scalar, length, offset
5100 PRINTF this, format, LIST
5104 A class implementing a scalar should have the following methods:
5106 TIESCALAR classname, LIST
5111 Not all methods indicated above need be implemented. See L<perltie>,
5112 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5114 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5115 for you--you need to do that explicitly yourself. See L<DB_File>
5116 or the F<Config> module for interesting C<tie> implementations.
5118 For further details see L<perltie>, L<"tied VARIABLE">.
5122 Returns a reference to the object underlying VARIABLE (the same value
5123 that was originally returned by the C<tie> call that bound the variable
5124 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5129 Returns the number of non-leap seconds since whatever time the system
5130 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5131 and 00:00:00 UTC, January 1, 1970 for most other systems).
5132 Suitable for feeding to C<gmtime> and C<localtime>.
5134 For measuring time in better granularity than one second,
5135 you may use either the Time::HiRes module from CPAN, or
5136 if you have gettimeofday(2), you may be able to use the
5137 C<syscall> interface of Perl, see L<perlfaq8> for details.
5141 Returns a four-element list giving the user and system times, in
5142 seconds, for this process and the children of this process.
5144 ($user,$system,$cuser,$csystem) = times;
5148 The transliteration operator. Same as C<y///>. See L<perlop>.
5150 =item truncate FILEHANDLE,LENGTH
5152 =item truncate EXPR,LENGTH
5154 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5155 specified length. Produces a fatal error if truncate isn't implemented
5156 on your system. Returns true if successful, the undefined value
5163 Returns an uppercased version of EXPR. This is the internal function
5164 implementing the C<\U> escape in double-quoted strings.
5165 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5166 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5167 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5169 If EXPR is omitted, uses C<$_>.
5175 Returns the value of EXPR with the first character
5176 in uppercase (titlecase in Unicode). This is
5177 the internal function implementing the C<\u> escape in double-quoted strings.
5178 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5181 If EXPR is omitted, uses C<$_>.
5187 Sets the umask for the process to EXPR and returns the previous value.
5188 If EXPR is omitted, merely returns the current umask.
5190 The Unix permission C<rwxr-x---> is represented as three sets of three
5191 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5192 and isn't one of the digits). The C<umask> value is such a number
5193 representing disabled permissions bits. The permission (or "mode")
5194 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5195 even if you tell C<sysopen> to create a file with permissions C<0777>,
5196 if your umask is C<0022> then the file will actually be created with
5197 permissions C<0755>. If your C<umask> were C<0027> (group can't
5198 write; others can't read, write, or execute), then passing
5199 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5202 Here's some advice: supply a creation mode of C<0666> for regular
5203 files (in C<sysopen>) and one of C<0777> for directories (in
5204 C<mkdir>) and executable files. This gives users the freedom of
5205 choice: if they want protected files, they might choose process umasks
5206 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5207 Programs should rarely if ever make policy decisions better left to
5208 the user. The exception to this is when writing files that should be
5209 kept private: mail files, web browser cookies, I<.rhosts> files, and
5212 If umask(2) is not implemented on your system and you are trying to
5213 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5214 fatal error at run time. If umask(2) is not implemented and you are
5215 not trying to restrict access for yourself, returns C<undef>.
5217 Remember that a umask is a number, usually given in octal; it is I<not> a
5218 string of octal digits. See also L</oct>, if all you have is a string.
5224 Undefines the value of EXPR, which must be an lvalue. Use only on a
5225 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5226 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5227 will probably not do what you expect on most predefined variables or
5228 DBM list values, so don't do that; see L<delete>.) Always returns the
5229 undefined value. You can omit the EXPR, in which case nothing is
5230 undefined, but you still get an undefined value that you could, for
5231 instance, return from a subroutine, assign to a variable or pass as a
5232 parameter. Examples:
5235 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5239 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5240 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5241 select undef, undef, undef, 0.25;
5242 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5244 Note that this is a unary operator, not a list operator.
5250 Deletes a list of files. Returns the number of files successfully
5253 $cnt = unlink 'a', 'b', 'c';
5257 Note: C<unlink> will not delete directories unless you are superuser and
5258 the B<-U> flag is supplied to Perl. Even if these conditions are
5259 met, be warned that unlinking a directory can inflict damage on your
5260 filesystem. Use C<rmdir> instead.
5262 If LIST is omitted, uses C<$_>.
5264 =item unpack TEMPLATE,EXPR
5266 C<unpack> does the reverse of C<pack>: it takes a string
5267 and expands it out into a list of values.
5268 (In scalar context, it returns merely the first value produced.)
5270 The string is broken into chunks described by the TEMPLATE. Each chunk
5271 is converted separately to a value. Typically, either the string is a result
5272 of C<pack>, or the bytes of the string represent a C structure of some
5275 The TEMPLATE has the same format as in the C<pack> function.
5276 Here's a subroutine that does substring:
5279 my($what,$where,$howmuch) = @_;
5280 unpack("x$where a$howmuch", $what);
5285 sub ordinal { unpack("c",$_[0]); } # same as ord()
5287 In addition to fields allowed in pack(), you may prefix a field with
5288 a %<number> to indicate that
5289 you want a <number>-bit checksum of the items instead of the items
5290 themselves. Default is a 16-bit checksum. Checksum is calculated by
5291 summing numeric values of expanded values (for string fields the sum of
5292 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5294 For example, the following
5295 computes the same number as the System V sum program:
5299 unpack("%32C*",<>) % 65535;
5302 The following efficiently counts the number of set bits in a bit vector:
5304 $setbits = unpack("%32b*", $selectmask);
5306 The C<p> and C<P> formats should be used with care. Since Perl
5307 has no way of checking whether the value passed to C<unpack()>
5308 corresponds to a valid memory location, passing a pointer value that's
5309 not known to be valid is likely to have disastrous consequences.
5311 If the repeat count of a field is larger than what the remainder of
5312 the input string allows, repeat count is decreased. If the input string
5313 is longer than one described by the TEMPLATE, the rest is ignored.
5315 See L</pack> for more examples and notes.
5317 =item untie VARIABLE
5319 Breaks the binding between a variable and a package. (See C<tie>.)
5321 =item unshift ARRAY,LIST
5323 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5324 depending on how you look at it. Prepends list to the front of the
5325 array, and returns the new number of elements in the array.
5327 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5329 Note the LIST is prepended whole, not one element at a time, so the
5330 prepended elements stay in the same order. Use C<reverse> to do the
5333 =item use Module VERSION LIST
5335 =item use Module VERSION
5337 =item use Module LIST
5343 Imports some semantics into the current package from the named module,
5344 generally by aliasing certain subroutine or variable names into your
5345 package. It is exactly equivalent to
5347 BEGIN { require Module; import Module LIST; }
5349 except that Module I<must> be a bareword.
5351 VERSION, which can be specified as a literal of the form v5.6.1, demands
5352 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5353 as recent as that version. (For compatibility with older versions of Perl,
5354 a numeric literal will also be interpreted as VERSION.) If the version
5355 of the running Perl interpreter is less than VERSION, then an error
5356 message is printed and Perl exits immediately without attempting to
5357 parse the rest of the file. Compare with L</require>, which can do a
5358 similar check at run time.
5360 use v5.6.1; # compile time version check
5362 use 5.005_03; # float version allowed for compatibility
5364 This is often useful if you need to check the current Perl version before
5365 C<use>ing library modules that have changed in incompatible ways from
5366 older versions of Perl. (We try not to do this more than we have to.)
5368 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5369 C<require> makes sure the module is loaded into memory if it hasn't been
5370 yet. The C<import> is not a builtin--it's just an ordinary static method
5371 call into the C<Module> package to tell the module to import the list of
5372 features back into the current package. The module can implement its
5373 C<import> method any way it likes, though most modules just choose to
5374 derive their C<import> method via inheritance from the C<Exporter> class that
5375 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5376 method can be found then the call is skipped.
5378 If you don't want your namespace altered, explicitly supply an empty list:
5382 That is exactly equivalent to
5384 BEGIN { require Module }
5386 If the VERSION argument is present between Module and LIST, then the
5387 C<use> will call the VERSION method in class Module with the given
5388 version as an argument. The default VERSION method, inherited from
5389 the UNIVERSAL class, croaks if the given version is larger than the
5390 value of the variable C<$Module::VERSION>.
5392 Again, there is a distinction between omitting LIST (C<import> called
5393 with no arguments) and an explicit empty LIST C<()> (C<import> not
5394 called). Note that there is no comma after VERSION!
5396 Because this is a wide-open interface, pragmas (compiler directives)
5397 are also implemented this way. Currently implemented pragmas are:
5401 use sigtrap qw(SEGV BUS);
5402 use strict qw(subs vars refs);
5403 use subs qw(afunc blurfl);
5404 use warnings qw(all);
5406 Some of these pseudo-modules import semantics into the current
5407 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5408 which import symbols into the current package (which are effective
5409 through the end of the file).
5411 There's a corresponding C<no> command that unimports meanings imported
5412 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5418 If no C<unimport> method can be found the call fails with a fatal error.
5420 See L<perlmod> for a list of standard modules and pragmas.
5424 Changes the access and modification times on each file of a list of
5425 files. The first two elements of the list must be the NUMERICAL access
5426 and modification times, in that order. Returns the number of files
5427 successfully changed. The inode change time of each file is set
5428 to the current time. This code has the same effect as the C<touch>
5429 command if the files already exist:
5433 utime $now, $now, @ARGV;
5437 Returns a list consisting of all the values of the named hash. (In a
5438 scalar context, returns the number of values.) The values are
5439 returned in an apparently random order. The actual random order is
5440 subject to change in future versions of perl, but it is guaranteed to
5441 be the same order as either the C<keys> or C<each> function would
5442 produce on the same (unmodified) hash.
5444 Note that the values are not copied, which means modifying them will
5445 modify the contents of the hash:
5447 for (values %hash) { s/foo/bar/g } # modifies %hash values
5448 for (@hash{keys %hash}) { s/foo/bar/g } # same
5450 As a side effect, calling values() resets the HASH's internal iterator.
5451 See also C<keys>, C<each>, and C<sort>.
5453 =item vec EXPR,OFFSET,BITS
5455 Treats the string in EXPR as a bit vector made up of elements of
5456 width BITS, and returns the value of the element specified by OFFSET
5457 as an unsigned integer. BITS therefore specifies the number of bits
5458 that are reserved for each element in the bit vector. This must
5459 be a power of two from 1 to 32 (or 64, if your platform supports
5462 If BITS is 8, "elements" coincide with bytes of the input string.
5464 If BITS is 16 or more, bytes of the input string are grouped into chunks
5465 of size BITS/8, and each group is converted to a number as with
5466 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5467 for BITS==64). See L<"pack"> for details.
5469 If bits is 4 or less, the string is broken into bytes, then the bits
5470 of each byte are broken into 8/BITS groups. Bits of a byte are
5471 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5472 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5473 breaking the single input byte C<chr(0x36)> into two groups gives a list
5474 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5476 C<vec> may also be assigned to, in which case parentheses are needed
5477 to give the expression the correct precedence as in
5479 vec($image, $max_x * $x + $y, 8) = 3;
5481 If the selected element is off the end of the string, the value 0 is
5482 returned. If an element off the end of the string is written to,
5483 Perl will first extend the string with sufficiently many zero bytes.
5485 Strings created with C<vec> can also be manipulated with the logical
5486 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5487 vector operation is desired when both operands are strings.
5488 See L<perlop/"Bitwise String Operators">.
5490 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5491 The comments show the string after each step. Note that this code works
5492 in the same way on big-endian or little-endian machines.
5495 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5497 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5498 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5500 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5501 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5502 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5503 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5504 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5505 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5507 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5508 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5509 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5512 To transform a bit vector into a string or list of 0's and 1's, use these:
5514 $bits = unpack("b*", $vector);
5515 @bits = split(//, unpack("b*", $vector));
5517 If you know the exact length in bits, it can be used in place of the C<*>.
5519 Here is an example to illustrate how the bits actually fall in place:
5525 unpack("V",$_) 01234567890123456789012345678901
5526 ------------------------------------------------------------------
5531 for ($shift=0; $shift < $width; ++$shift) {
5532 for ($off=0; $off < 32/$width; ++$off) {
5533 $str = pack("B*", "0"x32);
5534 $bits = (1<<$shift);
5535 vec($str, $off, $width) = $bits;
5536 $res = unpack("b*",$str);
5537 $val = unpack("V", $str);
5544 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5545 $off, $width, $bits, $val, $res
5549 Regardless of the machine architecture on which it is run, the above
5550 example should print the following table:
5553 unpack("V",$_) 01234567890123456789012345678901
5554 ------------------------------------------------------------------
5555 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5556 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5557 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5558 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5559 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5560 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5561 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5562 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5563 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5564 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5565 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5566 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5567 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5568 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5569 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5570 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5571 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5572 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5573 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5574 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5575 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5576 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5577 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5578 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5579 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5580 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5581 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5582 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5583 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5584 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5585 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5586 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5587 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5588 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5589 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5590 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5591 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5592 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5593 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5594 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5595 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5596 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5597 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5598 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5599 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5600 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5601 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5602 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5603 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5604 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5605 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5606 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5607 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5608 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5609 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5610 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5611 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5612 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5613 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5614 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5615 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5616 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5617 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5618 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5619 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5620 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5621 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5622 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5623 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5624 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5625 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5626 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5627 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5628 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5629 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5630 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5631 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5632 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5633 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5634 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5635 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5636 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5637 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5638 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5639 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5640 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5641 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5642 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5643 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5644 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5645 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5646 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5647 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5648 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5649 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5650 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5651 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5652 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5653 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5654 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5655 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5656 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5657 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5658 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5659 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5660 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5661 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5662 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5663 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5664 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5665 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5666 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5667 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5668 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5669 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5670 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5671 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5672 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5673 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5674 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5675 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5676 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5677 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5678 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5679 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5680 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5681 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5682 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5686 Behaves like the wait(2) system call on your system: it waits for a child
5687 process to terminate and returns the pid of the deceased process, or
5688 C<-1> if there are no child processes. The status is returned in C<$?>.
5689 Note that a return value of C<-1> could mean that child processes are
5690 being automatically reaped, as described in L<perlipc>.
5692 =item waitpid PID,FLAGS
5694 Waits for a particular child process to terminate and returns the pid of
5695 the deceased process, or C<-1> if there is no such child process. On some
5696 systems, a value of 0 indicates that there are processes still running.
5697 The status is returned in C<$?>. If you say
5699 use POSIX ":sys_wait_h";
5702 $kid = waitpid(-1,&WNOHANG);
5705 then you can do a non-blocking wait for all pending zombie processes.
5706 Non-blocking wait is available on machines supporting either the
5707 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5708 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5709 system call by remembering the status values of processes that have
5710 exited but have not been harvested by the Perl script yet.)
5712 Note that on some systems, a return value of C<-1> could mean that child
5713 processes are being automatically reaped. See L<perlipc> for details,
5714 and for other examples.
5718 Returns true if the context of the currently executing subroutine is
5719 looking for a list value. Returns false if the context is looking
5720 for a scalar. Returns the undefined value if the context is looking
5721 for no value (void context).
5723 return unless defined wantarray; # don't bother doing more
5724 my @a = complex_calculation();
5725 return wantarray ? @a : "@a";
5727 This function should have been named wantlist() instead.
5731 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5734 If LIST is empty and C<$@> already contains a value (typically from a
5735 previous eval) that value is used after appending C<"\t...caught">
5736 to C<$@>. This is useful for staying almost, but not entirely similar to
5739 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5741 No message is printed if there is a C<$SIG{__WARN__}> handler
5742 installed. It is the handler's responsibility to deal with the message
5743 as it sees fit (like, for instance, converting it into a C<die>). Most
5744 handlers must therefore make arrangements to actually display the
5745 warnings that they are not prepared to deal with, by calling C<warn>
5746 again in the handler. Note that this is quite safe and will not
5747 produce an endless loop, since C<__WARN__> hooks are not called from
5750 You will find this behavior is slightly different from that of
5751 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5752 instead call C<die> again to change it).
5754 Using a C<__WARN__> handler provides a powerful way to silence all
5755 warnings (even the so-called mandatory ones). An example:
5757 # wipe out *all* compile-time warnings
5758 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5760 my $foo = 20; # no warning about duplicate my $foo,
5761 # but hey, you asked for it!
5762 # no compile-time or run-time warnings before here
5765 # run-time warnings enabled after here
5766 warn "\$foo is alive and $foo!"; # does show up
5768 See L<perlvar> for details on setting C<%SIG> entries, and for more
5769 examples. See the Carp module for other kinds of warnings using its
5770 carp() and cluck() functions.
5772 =item write FILEHANDLE
5778 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5779 using the format associated with that file. By default the format for
5780 a file is the one having the same name as the filehandle, but the
5781 format for the current output channel (see the C<select> function) may be set
5782 explicitly by assigning the name of the format to the C<$~> variable.
5784 Top of form processing is handled automatically: if there is
5785 insufficient room on the current page for the formatted record, the
5786 page is advanced by writing a form feed, a special top-of-page format
5787 is used to format the new page header, and then the record is written.
5788 By default the top-of-page format is the name of the filehandle with
5789 "_TOP" appended, but it may be dynamically set to the format of your
5790 choice by assigning the name to the C<$^> variable while the filehandle is
5791 selected. The number of lines remaining on the current page is in
5792 variable C<$->, which can be set to C<0> to force a new page.
5794 If FILEHANDLE is unspecified, output goes to the current default output
5795 channel, which starts out as STDOUT but may be changed by the
5796 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5797 is evaluated and the resulting string is used to look up the name of
5798 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5800 Note that write is I<not> the opposite of C<read>. Unfortunately.
5804 The transliteration operator. Same as C<tr///>. See L<perlop>.