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<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, 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<prototype>, C<qx>,
204 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 (E<gt>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">.
383 Arranges to have a SIGALRM delivered to this process after the
384 specified number of seconds have elapsed. If SECONDS is not specified,
385 the value stored in C<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
400 (C<sleep> may be internally implemented in your system with C<alarm>)
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
405 restart system calls on some systems. Using C<eval>/C<die> always works,
406 modulo the caveats given in L<perlipc/"Signals">.
409 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
411 $nread = sysread SOCKET, $buffer, $size;
415 die unless $@ eq "alarm\n"; # propagate unexpected errors
424 Returns the arctangent of Y/X in the range -PI to PI.
426 For the tangent operation, you may use the C<POSIX::tan()>
427 function, or use the familiar relation:
429 sub tan { sin($_[0]) / cos($_[0]) }
431 =item bind SOCKET,NAME
433 Binds a network address to a socket, just as the bind system call
434 does. Returns true if it succeeded, false otherwise. NAME should be a
435 packed address of the appropriate type for the socket. See the examples in
436 L<perlipc/"Sockets: Client/Server Communication">.
438 =item binmode FILEHANDLE
440 Arranges for FILEHANDLE to be read or written in "binary" mode on
441 systems whose run-time libraries force the programmer to guess
442 between binary and text files. If FILEHANDLE is an expression, the
443 value is taken as the name of the filehandle. binmode() should be
444 called after the C<open> but before any I/O is done on the filehandle.
445 The only way to reset binary mode on a filehandle is to reopen the
448 The operating system, device drivers, C libraries, and Perl run-time
449 system all conspire to let the programmer conveniently treat a
450 simple, one-byte C<\n> as the line terminator, irrespective of its
451 external representation. On Unix and its brethren, the native file
452 representation exactly matches the internal representation, making
453 everyone's lives unbelievably simpler. Consequently, L<binmode>
454 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
455 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
456 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
457 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
458 it's only one byte, and the internal and external representations
461 In legacy systems like MS-DOS and its embellishments, your program
462 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
463 that's not what's physically stored on disk. What's worse, these
464 systems refuse to help you with this; it's up to you to remember
465 what to do. And you mustn't go applying binmode() with wild abandon,
466 either, because if your system does care about binmode(), then using
467 it when you shouldn't is just as perilous as failing to use it when
470 That means that on any version of Microsoft WinXX that you might
471 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
472 to be converted to C<\n> when read into your program, and causes
473 any C<\n> in your program to be converted back to C<\cM\cJ> on
474 output to disk. This sad discrepancy leads to no end of
475 problems in not just the readline operator, but also when using
476 seek(), tell(), and read() calls. See L<perlport> for other painful
477 details. See the C<$/> and C<$\> variables in L<perlvar> for how
478 to manually set your input and output line-termination sequences.
480 =item bless REF,CLASSNAME
484 This function tells the thingy referenced by REF that it is now an object
485 in the CLASSNAME package. If CLASSNAME is omitted, the current package
486 is used. Because a C<bless> is often the last thing in a constructor,
487 it returns the reference for convenience. Always use the two-argument
488 version if the function doing the blessing might be inherited by a
489 derived class. See L<perltoot> and L<perlobj> for more about the blessing
490 (and blessings) of objects.
492 Consider always blessing objects in CLASSNAMEs that are mixed case.
493 Namespaces with all lowercase names are considered reserved for
494 Perl pragmata. Builtin types have all uppercase names, so to prevent
495 confusion, you may wish to avoid such package names as well. Make sure
496 that CLASSNAME is a true value.
498 See L<perlmod/"Perl Modules">.
504 Returns the context of the current subroutine call. In scalar context,
505 returns the caller's package name if there is a caller, that is, if
506 we're in a subroutine or C<eval> or C<require>, and the undefined value
507 otherwise. In list context, returns
509 ($package, $filename, $line) = caller;
511 With EXPR, it returns some extra information that the debugger uses to
512 print a stack trace. The value of EXPR indicates how many call frames
513 to go back before the current one.
515 ($package, $filename, $line, $subroutine, $hasargs,
516 $wantarray, $evaltext, $is_require, $hints) = caller($i);
518 Here $subroutine may be C<(eval)> if the frame is not a subroutine
519 call, but an C<eval>. In such a case additional elements $evaltext and
520 C<$is_require> are set: C<$is_require> is true if the frame is created by a
521 C<require> or C<use> statement, $evaltext contains the text of the
522 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
523 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
524 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
525 frame. C<$hints> contains pragmatic hints that the caller was
526 compiled with. It currently only reflects the hint corresponding to
529 Furthermore, when called from within the DB package, caller returns more
530 detailed information: it sets the list variable C<@DB::args> to be the
531 arguments with which the subroutine was invoked.
533 Be aware that the optimizer might have optimized call frames away before
534 C<caller> had a chance to get the information. That means that C<caller(N)>
535 might not return information about the call frame you expect it do, for
536 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537 previous time C<caller> was called.
541 Changes the working directory to EXPR, if possible. If EXPR is omitted,
542 changes to the user's home directory. Returns true upon success,
543 false otherwise. See the example under C<die>.
547 Changes the permissions of a list of files. The first element of the
548 list must be the numerical mode, which should probably be an octal
549 number, and which definitely should I<not> a string of octal digits:
550 C<0644> is okay, C<'0644'> is not. Returns the number of files
551 successfully changed. See also L</oct>, if all you have is a string.
553 $cnt = chmod 0755, 'foo', 'bar';
554 chmod 0755, @executables;
555 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
557 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
558 $mode = 0644; chmod $mode, 'foo'; # this is best
566 This safer version of L</chop> removes any trailing string
567 that corresponds to the current value of C<$/> (also known as
568 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
569 number of characters removed from all its arguments. It's often used to
570 remove the newline from the end of an input record when you're worried
571 that the final record may be missing its newline. When in paragraph
572 mode (C<$/ = "">), it removes all trailing newlines from the string.
573 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
574 a reference to an integer or the like, see L<perlvar>) chomp() won't
576 If VARIABLE is omitted, it chomps C<$_>. Example:
579 chomp; # avoid \n on last field
584 You can actually chomp anything that's an lvalue, including an assignment:
587 chomp($answer = <STDIN>);
589 If you chomp a list, each element is chomped, and the total number of
590 characters removed is returned.
598 Chops off the last character of a string and returns the character
599 chopped. It's used primarily to remove the newline from the end of an
600 input record, but is much more efficient than C<s/\n//> because it neither
601 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
605 chop; # avoid \n on last field
610 You can actually chop anything that's an lvalue, including an assignment:
613 chop($answer = <STDIN>);
615 If you chop a list, each element is chopped. Only the value of the
616 last C<chop> is returned.
618 Note that C<chop> returns the last character. To return all but the last
619 character, use C<substr($string, 0, -1)>.
623 Changes the owner (and group) of a list of files. The first two
624 elements of the list must be the I<numeric> uid and gid, in that
625 order. A value of -1 in either position is interpreted by most
626 systems to leave that value unchanged. Returns the number of files
627 successfully changed.
629 $cnt = chown $uid, $gid, 'foo', 'bar';
630 chown $uid, $gid, @filenames;
632 Here's an example that looks up nonnumeric uids in the passwd file:
635 chomp($user = <STDIN>);
637 chomp($pattern = <STDIN>);
639 ($login,$pass,$uid,$gid) = getpwnam($user)
640 or die "$user not in passwd file";
642 @ary = glob($pattern); # expand filenames
643 chown $uid, $gid, @ary;
645 On most systems, you are not allowed to change the ownership of the
646 file unless you're the superuser, although you should be able to change
647 the group to any of your secondary groups. On insecure systems, these
648 restrictions may be relaxed, but this is not a portable assumption.
649 On POSIX systems, you can detect this condition this way:
651 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
652 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
658 Returns the character represented by that NUMBER in the character set.
659 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
660 chr(0x263a) is a Unicode smiley face (but only within the scope of
661 a C<use utf8>). For the reverse, use L</ord>.
662 See L<utf8> for more about Unicode.
664 If NUMBER is omitted, uses C<$_>.
666 =item chroot FILENAME
670 This function works like the system call by the same name: it makes the
671 named directory the new root directory for all further pathnames that
672 begin with a C</> by your process and all its children. (It doesn't
673 change your current working directory, which is unaffected.) For security
674 reasons, this call is restricted to the superuser. If FILENAME is
675 omitted, does a C<chroot> to C<$_>.
677 =item close FILEHANDLE
681 Closes the file or pipe associated with the file handle, returning true
682 only if stdio successfully flushes buffers and closes the system file
683 descriptor. Closes the currently selected filehandle if the argument
686 You don't have to close FILEHANDLE if you are immediately going to do
687 another C<open> on it, because C<open> will close it for you. (See
688 C<open>.) However, an explicit C<close> on an input file resets the line
689 counter (C<$.>), while the implicit close done by C<open> does not.
691 If the file handle came from a piped open C<close> will additionally
692 return false if one of the other system calls involved fails or if the
693 program exits with non-zero status. (If the only problem was that the
694 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
695 also waits for the process executing on the pipe to complete, in case you
696 want to look at the output of the pipe afterwards, and
697 implicitly puts the exit status value of that command into C<$?>.
699 Prematurely closing the read end of a pipe (i.e. before the process
700 writing to it at the other end has closed it) will result in a
701 SIGPIPE being delivered to the writer. If the other end can't
702 handle that, be sure to read all the data before closing the pipe.
706 open(OUTPUT, '|sort >foo') # pipe to sort
707 or die "Can't start sort: $!";
708 #... # print stuff to output
709 close OUTPUT # wait for sort to finish
710 or warn $! ? "Error closing sort pipe: $!"
711 : "Exit status $? from sort";
712 open(INPUT, 'foo') # get sort's results
713 or die "Can't open 'foo' for input: $!";
715 FILEHANDLE may be an expression whose value can be used as an indirect
716 filehandle, usually the real filehandle name.
718 =item closedir DIRHANDLE
720 Closes a directory opened by C<opendir> and returns the success of that
723 DIRHANDLE may be an expression whose value can be used as an indirect
724 dirhandle, usually the real dirhandle name.
726 =item connect SOCKET,NAME
728 Attempts to connect to a remote socket, just as the connect system call
729 does. Returns true if it succeeded, false otherwise. NAME should be a
730 packed address of the appropriate type for the socket. See the examples in
731 L<perlipc/"Sockets: Client/Server Communication">.
735 Actually a flow control statement rather than a function. If there is a
736 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
737 C<foreach>), it is always executed just before the conditional is about to
738 be evaluated again, just like the third part of a C<for> loop in C. Thus
739 it can be used to increment a loop variable, even when the loop has been
740 continued via the C<next> statement (which is similar to the C C<continue>
743 C<last>, C<next>, or C<redo> may appear within a C<continue>
744 block. C<last> and C<redo> will behave as if they had been executed within
745 the main block. So will C<next>, but since it will execute a C<continue>
746 block, it may be more entertaining.
749 ### redo always comes here
752 ### next always comes here
754 # then back the top to re-check EXPR
756 ### last always comes here
758 Omitting the C<continue> section is semantically equivalent to using an
759 empty one, logically enough. In that case, C<next> goes directly back
760 to check the condition at the top of the loop.
764 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
765 takes cosine of C<$_>.
767 For the inverse cosine operation, you may use the C<POSIX::acos()>
768 function, or use this relation:
770 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
772 =item crypt PLAINTEXT,SALT
774 Encrypts a string exactly like the crypt(3) function in the C library
775 (assuming that you actually have a version there that has not been
776 extirpated as a potential munition). This can prove useful for checking
777 the password file for lousy passwords, amongst other things. Only the
778 guys wearing white hats should do this.
780 Note that C<crypt> is intended to be a one-way function, much like breaking
781 eggs to make an omelette. There is no (known) corresponding decrypt
782 function. As a result, this function isn't all that useful for
783 cryptography. (For that, see your nearby CPAN mirror.)
785 When verifying an existing encrypted string you should use the encrypted
786 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
787 allows your code to work with the standard C<crypt> and with more
788 exotic implementations. When choosing a new salt create a random two
789 character string whose characters come from the set C<[./0-9A-Za-z]>
790 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
792 Here's an example that makes sure that whoever runs this program knows
795 $pwd = (getpwuid($<))[1];
799 chomp($word = <STDIN>);
803 if (crypt($word, $pwd) ne $pwd) {
809 Of course, typing in your own password to whoever asks you
812 The L<crypt> function is unsuitable for encrypting large quantities
813 of data, not least of all because you can't get the information
814 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
815 on your favorite CPAN mirror for a slew of potentially useful
820 [This function has been largely superseded by the C<untie> function.]
822 Breaks the binding between a DBM file and a hash.
824 =item dbmopen HASH,DBNAME,MASK
826 [This function has been largely superseded by the C<tie> function.]
828 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
829 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
830 argument is I<not> a filehandle, even though it looks like one). DBNAME
831 is the name of the database (without the F<.dir> or F<.pag> extension if
832 any). If the database does not exist, it is created with protection
833 specified by MASK (as modified by the C<umask>). If your system supports
834 only the older DBM functions, you may perform only one C<dbmopen> in your
835 program. In older versions of Perl, if your system had neither DBM nor
836 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
839 If you don't have write access to the DBM file, you can only read hash
840 variables, not set them. If you want to test whether you can write,
841 either use file tests or try setting a dummy hash entry inside an C<eval>,
842 which will trap the error.
844 Note that functions such as C<keys> and C<values> may return huge lists
845 when used on large DBM files. You may prefer to use the C<each>
846 function to iterate over large DBM files. Example:
848 # print out history file offsets
849 dbmopen(%HIST,'/usr/lib/news/history',0666);
850 while (($key,$val) = each %HIST) {
851 print $key, ' = ', unpack('L',$val), "\n";
855 See also L<AnyDBM_File> for a more general description of the pros and
856 cons of the various dbm approaches, as well as L<DB_File> for a particularly
859 You can control which DBM library you use by loading that library
860 before you call dbmopen():
863 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
864 or die "Can't open netscape history file: $!";
870 Returns a Boolean value telling whether EXPR has a value other than
871 the undefined value C<undef>. If EXPR is not present, C<$_> will be
874 Many operations return C<undef> to indicate failure, end of file,
875 system error, uninitialized variable, and other exceptional
876 conditions. This function allows you to distinguish C<undef> from
877 other values. (A simple Boolean test will not distinguish among
878 C<undef>, zero, the empty string, and C<"0">, which are all equally
879 false.) Note that since C<undef> is a valid scalar, its presence
880 doesn't I<necessarily> indicate an exceptional condition: C<pop>
881 returns C<undef> when its argument is an empty array, I<or> when the
882 element to return happens to be C<undef>.
884 You may also use C<defined(&func)> to check whether subroutine C<&func>
885 has ever been defined. The return value is unaffected by any forward
886 declarations of C<&foo>.
888 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
889 used to report whether memory for that aggregate has ever been
890 allocated. This behavior may disappear in future versions of Perl.
891 You should instead use a simple test for size:
893 if (@an_array) { print "has array elements\n" }
894 if (%a_hash) { print "has hash members\n" }
896 When used on a hash element, it tells you whether the value is defined,
897 not whether the key exists in the hash. Use L</exists> for the latter
902 print if defined $switch{'D'};
903 print "$val\n" while defined($val = pop(@ary));
904 die "Can't readlink $sym: $!"
905 unless defined($value = readlink $sym);
906 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
907 $debugging = 0 unless defined $debugging;
909 Note: Many folks tend to overuse C<defined>, and then are surprised to
910 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
911 defined values. For example, if you say
915 The pattern match succeeds, and C<$1> is defined, despite the fact that it
916 matched "nothing". But it didn't really match nothing--rather, it
917 matched something that happened to be zero characters long. This is all
918 very above-board and honest. When a function returns an undefined value,
919 it's an admission that it couldn't give you an honest answer. So you
920 should use C<defined> only when you're questioning the integrity of what
921 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
924 See also L</undef>, L</exists>, L</ref>.
928 Given an expression that specifies a hash element, array element, hash slice,
929 or array slice, deletes the specified element(s) from the hash or array.
930 If the array elements happen to be at the end of the array, the size
931 of the array will shrink by that number of elements.
933 Returns each element so deleted or the undefined value if there was no such
934 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
935 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
936 from a C<tie>d hash or array may not necessarily return anything.
938 Deleting an array element effectively returns that position of the array
939 to its initial, uninitialized state. Subsequently testing for the same
940 element with exists() will return false. See L</exists>.
942 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
944 foreach $key (keys %HASH) {
948 foreach $index (0 .. $#ARRAY) {
949 delete $ARRAY[$index];
954 delete @HASH{keys %HASH};
956 delete @ARRAY[0 .. $#ARRAY];
958 But both of these are slower than just assigning the empty list
959 or undefining %HASH or @ARRAY:
961 %HASH = (); # completely empty %HASH
962 undef %HASH; # forget %HASH ever existed
964 @ARRAY = (); # completely empty @ARRAY
965 undef @ARRAY; # forget @ARRAY ever existed
967 Note that the EXPR can be arbitrarily complicated as long as the final
968 operation is a hash element, array element, hash slice, or array slice
971 delete $ref->[$x][$y]{$key};
972 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
974 delete $ref->[$x][$y][$index];
975 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
979 Outside an C<eval>, prints the value of LIST to C<STDERR> and
980 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
981 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
982 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
983 an C<eval(),> the error message is stuffed into C<$@> and the
984 C<eval> is terminated with the undefined value. This makes
985 C<die> the way to raise an exception.
989 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
990 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
992 If the value of EXPR does not end in a newline, the current script line
993 number and input line number (if any) are also printed, and a newline
994 is supplied. Note that the "input line number" (also known as "chunk")
995 is subject to whatever notion of "line" happens to be currently in
996 effect, and is also available as the special variable C<$.>.
997 See L<perlvar/"$/"> and L<perlvar/"$.">.
999 Hint: sometimes appending C<", stopped"> to your message
1000 will cause it to make better sense when the string C<"at foo line 123"> is
1001 appended. Suppose you are running script "canasta".
1003 die "/etc/games is no good";
1004 die "/etc/games is no good, stopped";
1006 produce, respectively
1008 /etc/games is no good at canasta line 123.
1009 /etc/games is no good, stopped at canasta line 123.
1011 See also exit(), warn(), and the Carp module.
1013 If LIST is empty and C<$@> already contains a value (typically from a
1014 previous eval) that value is reused after appending C<"\t...propagated">.
1015 This is useful for propagating exceptions:
1018 die unless $@ =~ /Expected exception/;
1020 If C<$@> is empty then the string C<"Died"> is used.
1022 die() can also be called with a reference argument. If this happens to be
1023 trapped within an eval(), $@ contains the reference. This behavior permits
1024 a more elaborate exception handling implementation using objects that
1025 maintain arbitary state about the nature of the exception. Such a scheme
1026 is sometimes preferable to matching particular string values of $@ using
1027 regular expressions. Here's an example:
1029 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1031 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1032 # handle Some::Module::Exception
1035 # handle all other possible exceptions
1039 Because perl will stringify uncaught exception messages before displaying
1040 them, you may want to overload stringification operations on such custom
1041 exception objects. See L<overload> for details about that.
1043 You can arrange for a callback to be run just before the C<die>
1044 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1045 handler will be called with the error text and can change the error
1046 message, if it sees fit, by calling C<die> again. See
1047 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1048 L<"eval BLOCK"> for some examples. Although this feature was meant
1049 to be run only right before your program was to exit, this is not
1050 currently the case--the C<$SIG{__DIE__}> hook is currently called
1051 even inside eval()ed blocks/strings! If one wants the hook to do
1052 nothing in such situations, put
1056 as the first line of the handler (see L<perlvar/$^S>). Because
1057 this promotes strange action at a distance, this counterintuitive
1058 behavior may be fixed in a future release.
1062 Not really a function. Returns the value of the last command in the
1063 sequence of commands indicated by BLOCK. When modified by a loop
1064 modifier, executes the BLOCK once before testing the loop condition.
1065 (On other statements the loop modifiers test the conditional first.)
1067 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1068 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1069 See L<perlsyn> for alternative strategies.
1071 =item do SUBROUTINE(LIST)
1073 A deprecated form of subroutine call. See L<perlsub>.
1077 Uses the value of EXPR as a filename and executes the contents of the
1078 file as a Perl script. Its primary use is to include subroutines
1079 from a Perl subroutine library.
1085 scalar eval `cat stat.pl`;
1087 except that it's more efficient and concise, keeps track of the current
1088 filename for error messages, searches the @INC libraries, and updates
1089 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1090 variables. It also differs in that code evaluated with C<do FILENAME>
1091 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1092 same, however, in that it does reparse the file every time you call it,
1093 so you probably don't want to do this inside a loop.
1095 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1096 error. If C<do> can read the file but cannot compile it, it
1097 returns undef and sets an error message in C<$@>. If the file is
1098 successfully compiled, C<do> returns the value of the last expression
1101 Note that inclusion of library modules is better done with the
1102 C<use> and C<require> operators, which also do automatic error checking
1103 and raise an exception if there's a problem.
1105 You might like to use C<do> to read in a program configuration
1106 file. Manual error checking can be done this way:
1108 # read in config files: system first, then user
1109 for $file ("/share/prog/defaults.rc",
1110 "$ENV{HOME}/.someprogrc")
1112 unless ($return = do $file) {
1113 warn "couldn't parse $file: $@" if $@;
1114 warn "couldn't do $file: $!" unless defined $return;
1115 warn "couldn't run $file" unless $return;
1123 This function causes an immediate core dump. See also the B<-u>
1124 command-line switch in L<perlrun>, which does the same thing.
1125 Primarily this is so that you can use the B<undump> program (not
1126 supplied) to turn your core dump into an executable binary after
1127 having initialized all your variables at the beginning of the
1128 program. When the new binary is executed it will begin by executing
1129 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1130 Think of it as a goto with an intervening core dump and reincarnation.
1131 If C<LABEL> is omitted, restarts the program from the top.
1133 B<WARNING>: Any files opened at the time of the dump will I<not>
1134 be open any more when the program is reincarnated, with possible
1135 resulting confusion on the part of Perl.
1137 This function is now largely obsolete, partly because it's very
1138 hard to convert a core file into an executable, and because the
1139 real compiler backends for generating portable bytecode and compilable
1140 C code have superseded it.
1142 If you're looking to use L<dump> to speed up your program, consider
1143 generating bytecode or native C code as described in L<perlcc>. If
1144 you're just trying to accelerate a CGI script, consider using the
1145 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1146 You might also consider autoloading or selfloading, which at least
1147 make your program I<appear> to run faster.
1151 When called in list context, returns a 2-element list consisting of the
1152 key and value for the next element of a hash, so that you can iterate over
1153 it. When called in scalar context, returns the key for only the "next"
1154 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1155 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1158 Entries are returned in an apparently random order. The actual random
1159 order is subject to change in future versions of perl, but it is guaranteed
1160 to be in the same order as either the C<keys> or C<values> function
1161 would produce on the same (unmodified) hash.
1163 When the hash is entirely read, a null array is returned in list context
1164 (which when assigned produces a false (C<0>) value), and C<undef> in
1165 scalar context. The next call to C<each> after that will start iterating
1166 again. There is a single iterator for each hash, shared by all C<each>,
1167 C<keys>, and C<values> function calls in the program; it can be reset by
1168 reading all the elements from the hash, or by evaluating C<keys HASH> or
1169 C<values HASH>. If you add or delete elements of a hash while you're
1170 iterating over it, you may get entries skipped or duplicated, so don't.
1172 The following prints out your environment like the printenv(1) program,
1173 only in a different order:
1175 while (($key,$value) = each %ENV) {
1176 print "$key=$value\n";
1179 See also C<keys>, C<values> and C<sort>.
1181 =item eof FILEHANDLE
1187 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1188 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1189 gives the real filehandle. (Note that this function actually
1190 reads a character and then C<ungetc>s it, so isn't very useful in an
1191 interactive context.) Do not read from a terminal file (or call
1192 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1193 as terminals may lose the end-of-file condition if you do.
1195 An C<eof> without an argument uses the last file read. Using C<eof()>
1196 with empty parentheses is very different. It refers to the pseudo file
1197 formed from the files listed on the command line and accessed via the
1198 C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1199 as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1200 used will cause C<@ARGV> to be examined to determine if input is
1203 In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1204 detect the end of each file, C<eof()> will only detect the end of the
1205 last file. Examples:
1207 # reset line numbering on each input file
1209 next if /^\s*#/; # skip comments
1212 close ARGV if eof; # Not eof()!
1215 # insert dashes just before last line of last file
1217 if (eof()) { # check for end of current file
1218 print "--------------\n";
1219 close(ARGV); # close or last; is needed if we
1220 # are reading from the terminal
1225 Practical hint: you almost never need to use C<eof> in Perl, because the
1226 input operators typically return C<undef> when they run out of data, or if
1233 In the first form, the return value of EXPR is parsed and executed as if it
1234 were a little Perl program. The value of the expression (which is itself
1235 determined within scalar context) is first parsed, and if there weren't any
1236 errors, executed in the context of the current Perl program, so that any
1237 variable settings or subroutine and format definitions remain afterwards.
1238 Note that the value is parsed every time the eval executes. If EXPR is
1239 omitted, evaluates C<$_>. This form is typically used to delay parsing
1240 and subsequent execution of the text of EXPR until run time.
1242 In the second form, the code within the BLOCK is parsed only once--at the
1243 same time the code surrounding the eval itself was parsed--and executed
1244 within the context of the current Perl program. This form is typically
1245 used to trap exceptions more efficiently than the first (see below), while
1246 also providing the benefit of checking the code within BLOCK at compile
1249 The final semicolon, if any, may be omitted from the value of EXPR or within
1252 In both forms, the value returned is the value of the last expression
1253 evaluated inside the mini-program; a return statement may be also used, just
1254 as with subroutines. The expression providing the return value is evaluated
1255 in void, scalar, or list context, depending on the context of the eval itself.
1256 See L</wantarray> for more on how the evaluation context can be determined.
1258 If there is a syntax error or runtime error, or a C<die> statement is
1259 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1260 error message. If there was no error, C<$@> is guaranteed to be a null
1261 string. Beware that using C<eval> neither silences perl from printing
1262 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1263 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1264 L</warn> and L<perlvar>.
1266 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1267 determining whether a particular feature (such as C<socket> or C<symlink>)
1268 is implemented. It is also Perl's exception trapping mechanism, where
1269 the die operator is used to raise exceptions.
1271 If the code to be executed doesn't vary, you may use the eval-BLOCK
1272 form to trap run-time errors without incurring the penalty of
1273 recompiling each time. The error, if any, is still returned in C<$@>.
1276 # make divide-by-zero nonfatal
1277 eval { $answer = $a / $b; }; warn $@ if $@;
1279 # same thing, but less efficient
1280 eval '$answer = $a / $b'; warn $@ if $@;
1282 # a compile-time error
1283 eval { $answer = }; # WRONG
1286 eval '$answer ='; # sets $@
1288 Due to the current arguably broken state of C<__DIE__> hooks, when using
1289 the C<eval{}> form as an exception trap in libraries, you may wish not
1290 to trigger any C<__DIE__> hooks that user code may have installed.
1291 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1292 as shown in this example:
1294 # a very private exception trap for divide-by-zero
1295 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1298 This is especially significant, given that C<__DIE__> hooks can call
1299 C<die> again, which has the effect of changing their error messages:
1301 # __DIE__ hooks may modify error messages
1303 local $SIG{'__DIE__'} =
1304 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1305 eval { die "foo lives here" };
1306 print $@ if $@; # prints "bar lives here"
1309 Because this promotes action at a distance, this counterintuitive behavior
1310 may be fixed in a future release.
1312 With an C<eval>, you should be especially careful to remember what's
1313 being looked at when:
1319 eval { $x }; # CASE 4
1321 eval "\$$x++"; # CASE 5
1324 Cases 1 and 2 above behave identically: they run the code contained in
1325 the variable $x. (Although case 2 has misleading double quotes making
1326 the reader wonder what else might be happening (nothing is).) Cases 3
1327 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1328 does nothing but return the value of $x. (Case 4 is preferred for
1329 purely visual reasons, but it also has the advantage of compiling at
1330 compile-time instead of at run-time.) Case 5 is a place where
1331 normally you I<would> like to use double quotes, except that in this
1332 particular situation, you can just use symbolic references instead, as
1335 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1336 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1340 =item exec PROGRAM LIST
1342 The C<exec> function executes a system command I<and never returns>--
1343 use C<system> instead of C<exec> if you want it to return. It fails and
1344 returns false only if the command does not exist I<and> it is executed
1345 directly instead of via your system's command shell (see below).
1347 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1348 warns you if there is a following statement which isn't C<die>, C<warn>,
1349 or C<exit> (if C<-w> is set - but you always do that). If you
1350 I<really> want to follow an C<exec> with some other statement, you
1351 can use one of these styles to avoid the warning:
1353 exec ('foo') or print STDERR "couldn't exec foo: $!";
1354 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1356 If there is more than one argument in LIST, or if LIST is an array
1357 with more than one value, calls execvp(3) with the arguments in LIST.
1358 If there is only one scalar argument or an array with one element in it,
1359 the argument is checked for shell metacharacters, and if there are any,
1360 the entire argument is passed to the system's command shell for parsing
1361 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1362 If there are no shell metacharacters in the argument, it is split into
1363 words and passed directly to C<execvp>, which is more efficient.
1366 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1367 exec "sort $outfile | uniq";
1369 If you don't really want to execute the first argument, but want to lie
1370 to the program you are executing about its own name, you can specify
1371 the program you actually want to run as an "indirect object" (without a
1372 comma) in front of the LIST. (This always forces interpretation of the
1373 LIST as a multivalued list, even if there is only a single scalar in
1376 $shell = '/bin/csh';
1377 exec $shell '-sh'; # pretend it's a login shell
1381 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1383 When the arguments get executed via the system shell, results will
1384 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1387 Using an indirect object with C<exec> or C<system> is also more
1388 secure. This usage (which also works fine with system()) forces
1389 interpretation of the arguments as a multivalued list, even if the
1390 list had just one argument. That way you're safe from the shell
1391 expanding wildcards or splitting up words with whitespace in them.
1393 @args = ( "echo surprise" );
1395 exec @args; # subject to shell escapes
1397 exec { $args[0] } @args; # safe even with one-arg list
1399 The first version, the one without the indirect object, ran the I<echo>
1400 program, passing it C<"surprise"> an argument. The second version
1401 didn't--it tried to run a program literally called I<"echo surprise">,
1402 didn't find it, and set C<$?> to a non-zero value indicating failure.
1404 Note that C<exec> will not call your C<END> blocks, nor will it call
1405 any C<DESTROY> methods in your objects.
1409 Given an expression that specifies a hash element or array element,
1410 returns true if the specified element in the hash or array has ever
1411 been initialized, even if the corresponding value is undefined. The
1412 element is not autovivified if it doesn't exist.
1414 print "Exists\n" if exists $hash{$key};
1415 print "Defined\n" if defined $hash{$key};
1416 print "True\n" if $hash{$key};
1418 print "Exists\n" if exists $array[$index];
1419 print "Defined\n" if defined $array[$index];
1420 print "True\n" if $array[$index];
1422 A hash or array element can be true only if it's defined, and defined if
1423 it exists, but the reverse doesn't necessarily hold true.
1425 Given an expression that specifies the name of a subroutine,
1426 returns true if the specified subroutine has ever been declared, even
1427 if it is undefined. Mentioning a subroutine name for exists or defined
1428 does not count as declaring it.
1430 print "Exists\n" if exists &subroutine;
1431 print "Defined\n" if defined &subroutine;
1433 Note that the EXPR can be arbitrarily complicated as long as the final
1434 operation is a hash or array key lookup or subroutine name:
1436 if (exists $ref->{A}->{B}->{$key}) { }
1437 if (exists $hash{A}{B}{$key}) { }
1439 if (exists $ref->{A}->{B}->[$ix]) { }
1440 if (exists $hash{A}{B}[$ix]) { }
1442 if (exists &{$ref->{A}{B}{$key}}) { }
1444 Although the deepest nested array or hash will not spring into existence
1445 just because its existence was tested, any intervening ones will.
1446 Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1447 into existence due to the existence test for the $key element above.
1448 This happens anywhere the arrow operator is used, including even:
1451 if (exists $ref->{"Some key"}) { }
1452 print $ref; # prints HASH(0x80d3d5c)
1454 This surprising autovivification in what does not at first--or even
1455 second--glance appear to be an lvalue context may be fixed in a future
1458 See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1459 used on a pseudo-hash.
1461 Use of a subroutine call, rather than a subroutine name, as an argument
1462 to exists() is an error.
1465 exists &sub(); # Error
1469 Evaluates EXPR and exits immediately with that value. Example:
1472 exit 0 if $ans =~ /^[Xx]/;
1474 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1475 universally recognized values for EXPR are C<0> for success and C<1>
1476 for error; other values are subject to interpretation depending on the
1477 environment in which the Perl program is running. For example, exiting
1478 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1479 the mailer to return the item undelivered, but that's not true everywhere.
1481 Don't use C<exit> to abort a subroutine if there's any chance that
1482 someone might want to trap whatever error happened. Use C<die> instead,
1483 which can be trapped by an C<eval>.
1485 The exit() function does not always exit immediately. It calls any
1486 defined C<END> routines first, but these C<END> routines may not
1487 themselves abort the exit. Likewise any object destructors that need to
1488 be called are called before the real exit. If this is a problem, you
1489 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1490 See L<perlmod> for details.
1496 Returns I<e> (the natural logarithm base) to the power of EXPR.
1497 If EXPR is omitted, gives C<exp($_)>.
1499 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1501 Implements the fcntl(2) function. You'll probably have to say
1505 first to get the correct constant definitions. Argument processing and
1506 value return works just like C<ioctl> below.
1510 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1511 or die "can't fcntl F_GETFL: $!";
1513 You don't have to check for C<defined> on the return from C<fnctl>.
1514 Like C<ioctl>, it maps a C<0> return from the system call into
1515 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1516 in numeric context. It is also exempt from the normal B<-w> warnings
1517 on improper numeric conversions.
1519 Note that C<fcntl> will produce a fatal error if used on a machine that
1520 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1521 manpage to learn what functions are available on your system.
1523 =item fileno FILEHANDLE
1525 Returns the file descriptor for a filehandle, or undefined if the
1526 filehandle is not open. This is mainly useful for constructing
1527 bitmaps for C<select> and low-level POSIX tty-handling operations.
1528 If FILEHANDLE is an expression, the value is taken as an indirect
1529 filehandle, generally its name.
1531 You can use this to find out whether two handles refer to the
1532 same underlying descriptor:
1534 if (fileno(THIS) == fileno(THAT)) {
1535 print "THIS and THAT are dups\n";
1538 =item flock FILEHANDLE,OPERATION
1540 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1541 for success, false on failure. Produces a fatal error if used on a
1542 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1543 C<flock> is Perl's portable file locking interface, although it locks
1544 only entire files, not records.
1546 Two potentially non-obvious but traditional C<flock> semantics are
1547 that it waits indefinitely until the lock is granted, and that its locks
1548 B<merely advisory>. Such discretionary locks are more flexible, but offer
1549 fewer guarantees. This means that files locked with C<flock> may be
1550 modified by programs that do not also use C<flock>. See L<perlport>,
1551 your port's specific documentation, or your system-specific local manpages
1552 for details. It's best to assume traditional behavior if you're writing
1553 portable programs. (But if you're not, you should as always feel perfectly
1554 free to write for your own system's idiosyncrasies (sometimes called
1555 "features"). Slavish adherence to portability concerns shouldn't get
1556 in the way of your getting your job done.)
1558 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1559 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1560 you can use the symbolic names if import them from the Fcntl module,
1561 either individually, or as a group using the ':flock' tag. LOCK_SH
1562 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1563 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1564 LOCK_EX then C<flock> will return immediately rather than blocking
1565 waiting for the lock (check the return status to see if you got it).
1567 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1568 before locking or unlocking it.
1570 Note that the emulation built with lockf(3) doesn't provide shared
1571 locks, and it requires that FILEHANDLE be open with write intent. These
1572 are the semantics that lockf(3) implements. Most if not all systems
1573 implement lockf(3) in terms of fcntl(2) locking, though, so the
1574 differing semantics shouldn't bite too many people.
1576 Note also that some versions of C<flock> cannot lock things over the
1577 network; you would need to use the more system-specific C<fcntl> for
1578 that. If you like you can force Perl to ignore your system's flock(2)
1579 function, and so provide its own fcntl(2)-based emulation, by passing
1580 the switch C<-Ud_flock> to the F<Configure> program when you configure
1583 Here's a mailbox appender for BSD systems.
1585 use Fcntl ':flock'; # import LOCK_* constants
1588 flock(MBOX,LOCK_EX);
1589 # and, in case someone appended
1590 # while we were waiting...
1595 flock(MBOX,LOCK_UN);
1598 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1599 or die "Can't open mailbox: $!";
1602 print MBOX $msg,"\n\n";
1605 On systems that support a real flock(), locks are inherited across fork()
1606 calls, whereas those that must resort to the more capricious fcntl()
1607 function lose the locks, making it harder to write servers.
1609 See also L<DB_File> for other flock() examples.
1613 Does a fork(2) system call to create a new process running the
1614 same program at the same point. It returns the child pid to the
1615 parent process, C<0> to the child process, or C<undef> if the fork is
1616 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1617 are shared, while everything else is copied. On most systems supporting
1618 fork(), great care has gone into making it extremely efficient (for
1619 example, using copy-on-write technology on data pages), making it the
1620 dominant paradigm for multitasking over the last few decades.
1622 All files opened for output are flushed before forking the child process.
1624 If you C<fork> without ever waiting on your children, you will
1625 accumulate zombies. On some systems, you can avoid this by setting
1626 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1627 forking and reaping moribund children.
1629 Note that if your forked child inherits system file descriptors like
1630 STDIN and STDOUT that are actually connected by a pipe or socket, even
1631 if you exit, then the remote server (such as, say, a CGI script or a
1632 backgrounded job launched from a remote shell) won't think you're done.
1633 You should reopen those to F</dev/null> if it's any issue.
1637 Declare a picture format for use by the C<write> function. For
1641 Test: @<<<<<<<< @||||| @>>>>>
1642 $str, $%, '$' . int($num)
1646 $num = $cost/$quantity;
1650 See L<perlform> for many details and examples.
1652 =item formline PICTURE,LIST
1654 This is an internal function used by C<format>s, though you may call it,
1655 too. It formats (see L<perlform>) a list of values according to the
1656 contents of PICTURE, placing the output into the format output
1657 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1658 Eventually, when a C<write> is done, the contents of
1659 C<$^A> are written to some filehandle, but you could also read C<$^A>
1660 yourself and then set C<$^A> back to C<"">. Note that a format typically
1661 does one C<formline> per line of form, but the C<formline> function itself
1662 doesn't care how many newlines are embedded in the PICTURE. This means
1663 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1664 You may therefore need to use multiple formlines to implement a single
1665 record format, just like the format compiler.
1667 Be careful if you put double quotes around the picture, because an C<@>
1668 character may be taken to mean the beginning of an array name.
1669 C<formline> always returns true. See L<perlform> for other examples.
1671 =item getc FILEHANDLE
1675 Returns the next character from the input file attached to FILEHANDLE,
1676 or the undefined value at end of file, or if there was an error.
1677 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1678 efficient. However, it cannot be used by itself to fetch single
1679 characters without waiting for the user to hit enter. For that, try
1680 something more like:
1683 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1686 system "stty", '-icanon', 'eol', "\001";
1692 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1695 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1699 Determination of whether $BSD_STYLE should be set
1700 is left as an exercise to the reader.
1702 The C<POSIX::getattr> function can do this more portably on
1703 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1704 module from your nearest CPAN site; details on CPAN can be found on
1709 Implements the C library function of the same name, which on most
1710 systems returns the current login from F</etc/utmp>, if any. If null,
1713 $login = getlogin || getpwuid($<) || "Kilroy";
1715 Do not consider C<getlogin> for authentication: it is not as
1716 secure as C<getpwuid>.
1718 =item getpeername SOCKET
1720 Returns the packed sockaddr address of other end of the SOCKET connection.
1723 $hersockaddr = getpeername(SOCK);
1724 ($port, $iaddr) = sockaddr_in($hersockaddr);
1725 $herhostname = gethostbyaddr($iaddr, AF_INET);
1726 $herstraddr = inet_ntoa($iaddr);
1730 Returns the current process group for the specified PID. Use
1731 a PID of C<0> to get the current process group for the
1732 current process. Will raise an exception if used on a machine that
1733 doesn't implement getpgrp(2). If PID is omitted, returns process
1734 group of current process. Note that the POSIX version of C<getpgrp>
1735 does not accept a PID argument, so only C<PID==0> is truly portable.
1739 Returns the process id of the parent process.
1741 =item getpriority WHICH,WHO
1743 Returns the current priority for a process, a process group, or a user.
1744 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1745 machine that doesn't implement getpriority(2).
1751 =item gethostbyname NAME
1753 =item getnetbyname NAME
1755 =item getprotobyname NAME
1761 =item getservbyname NAME,PROTO
1763 =item gethostbyaddr ADDR,ADDRTYPE
1765 =item getnetbyaddr ADDR,ADDRTYPE
1767 =item getprotobynumber NUMBER
1769 =item getservbyport PORT,PROTO
1787 =item sethostent STAYOPEN
1789 =item setnetent STAYOPEN
1791 =item setprotoent STAYOPEN
1793 =item setservent STAYOPEN
1807 These routines perform the same functions as their counterparts in the
1808 system library. In list context, the return values from the
1809 various get routines are as follows:
1811 ($name,$passwd,$uid,$gid,
1812 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1813 ($name,$passwd,$gid,$members) = getgr*
1814 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1815 ($name,$aliases,$addrtype,$net) = getnet*
1816 ($name,$aliases,$proto) = getproto*
1817 ($name,$aliases,$port,$proto) = getserv*
1819 (If the entry doesn't exist you get a null list.)
1821 In scalar context, you get the name, unless the function was a
1822 lookup by name, in which case you get the other thing, whatever it is.
1823 (If the entry doesn't exist you get the undefined value.) For example:
1825 $uid = getpwnam($name);
1826 $name = getpwuid($num);
1828 $gid = getgrnam($name);
1829 $name = getgrgid($num;
1833 In I<getpw*()> the fields $quota, $comment, and $expire are
1834 special cases in the sense that in many systems they are unsupported.
1835 If the $quota is unsupported, it is an empty scalar. If it is
1836 supported, it usually encodes the disk quota. If the $comment
1837 field is unsupported, it is an empty scalar. If it is supported it
1838 usually encodes some administrative comment about the user. In some
1839 systems the $quota field may be $change or $age, fields that have
1840 to do with password aging. In some systems the $comment field may
1841 be $class. The $expire field, if present, encodes the expiration
1842 period of the account or the password. For the availability and the
1843 exact meaning of these fields in your system, please consult your
1844 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1845 out from within Perl what your $quota and $comment fields mean
1846 and whether you have the $expire field by using the C<Config> module
1847 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1848 and C<d_pwexpire>. Shadow password files are only supported if your
1849 vendor has implemented them in the intuitive fashion that calling the
1850 regular C library routines gets the shadow versions if you're running
1851 under privilege. Those that incorrectly implement a separate library
1852 call are not supported.
1854 The $members value returned by I<getgr*()> is a space separated list of
1855 the login names of the members of the group.
1857 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1858 C, it will be returned to you via C<$?> if the function call fails. The
1859 C<@addrs> value returned by a successful call is a list of the raw
1860 addresses returned by the corresponding system library call. In the
1861 Internet domain, each address is four bytes long and you can unpack it
1862 by saying something like:
1864 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1866 The Socket library makes this slightly easier:
1869 $iaddr = inet_aton("127.1"); # or whatever address
1870 $name = gethostbyaddr($iaddr, AF_INET);
1872 # or going the other way
1873 $straddr = inet_ntoa($iaddr);
1875 If you get tired of remembering which element of the return list
1876 contains which return value, by-name interfaces are provided
1877 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1878 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1879 and C<User::grent>. These override the normal built-ins, supplying
1880 versions that return objects with the appropriate names
1881 for each field. For example:
1885 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1887 Even though it looks like they're the same method calls (uid),
1888 they aren't, because a C<File::stat> object is different from
1889 a C<User::pwent> object.
1891 =item getsockname SOCKET
1893 Returns the packed sockaddr address of this end of the SOCKET connection,
1894 in case you don't know the address because you have several different
1895 IPs that the connection might have come in on.
1898 $mysockaddr = getsockname(SOCK);
1899 ($port, $myaddr) = sockaddr_in($mysockaddr);
1900 printf "Connect to %s [%s]\n",
1901 scalar gethostbyaddr($myaddr, AF_INET),
1904 =item getsockopt SOCKET,LEVEL,OPTNAME
1906 Returns the socket option requested, or undef if there is an error.
1912 Returns the value of EXPR with filename expansions such as the
1913 standard Unix shell F</bin/csh> would do. This is the internal function
1914 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1915 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1916 discussed in more detail in L<perlop/"I/O Operators">.
1920 Converts a time as returned by the time function to a 9-element list
1921 with the time localized for the standard Greenwich time zone.
1922 Typically used as follows:
1925 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1928 All list elements are numeric, and come straight out of a struct tm.
1929 In particular this means that $mon has the range C<0..11> and $wday
1930 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1931 number of years since 1900, that is, $year is C<123> in year 2023,
1932 I<not> simply the last two digits of the year. If you assume it is,
1933 then you create non-Y2K-compliant programs--and you wouldn't want to do
1936 The proper way to get a complete 4-digit year is simply:
1940 And to get the last two digits of the year (e.g., '01' in 2001) do:
1942 $year = sprintf("%02d", $year % 100);
1944 If EXPR is omitted, does C<gmtime(time())>.
1946 In scalar context, returns the ctime(3) value:
1948 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1950 Also see the C<timegm> function provided by the C<Time::Local> module,
1951 and the strftime(3) function available via the POSIX module.
1953 This scalar value is B<not> locale dependent (see L<perllocale>), but
1954 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1955 strftime(3) and mktime(3) functions available via the POSIX module. To
1956 get somewhat similar but locale dependent date strings, set up your
1957 locale environment variables appropriately (please see L<perllocale>)
1958 and try for example:
1960 use POSIX qw(strftime);
1961 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1963 Note that the C<%a> and C<%b> escapes, which represent the short forms
1964 of the day of the week and the month of the year, may not necessarily
1965 be three characters wide in all locales.
1973 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1974 execution there. It may not be used to go into any construct that
1975 requires initialization, such as a subroutine or a C<foreach> loop. It
1976 also can't be used to go into a construct that is optimized away,
1977 or to get out of a block or subroutine given to C<sort>.
1978 It can be used to go almost anywhere else within the dynamic scope,
1979 including out of subroutines, but it's usually better to use some other
1980 construct such as C<last> or C<die>. The author of Perl has never felt the
1981 need to use this form of C<goto> (in Perl, that is--C is another matter).
1983 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1984 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1985 necessarily recommended if you're optimizing for maintainability:
1987 goto ("FOO", "BAR", "GLARCH")[$i];
1989 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
1990 In fact, it isn't a goto in the normal sense at all, and doesn't have
1991 the stigma associated with other gotos. Instead, it
1992 substitutes a call to the named subroutine for the currently running
1993 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
1994 another subroutine and then pretend that the other subroutine had been
1995 called in the first place (except that any modifications to C<@_>
1996 in the current subroutine are propagated to the other subroutine.)
1997 After the C<goto>, not even C<caller> will be able to tell that this
1998 routine was called first.
2000 NAME needn't be the name of a subroutine; it can be a scalar variable
2001 containing a code reference, or a block which evaluates to a code
2004 =item grep BLOCK LIST
2006 =item grep EXPR,LIST
2008 This is similar in spirit to, but not the same as, grep(1) and its
2009 relatives. In particular, it is not limited to using regular expressions.
2011 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2012 C<$_> to each element) and returns the list value consisting of those
2013 elements for which the expression evaluated to true. In scalar
2014 context, returns the number of times the expression was true.
2016 @foo = grep(!/^#/, @bar); # weed out comments
2020 @foo = grep {!/^#/} @bar; # weed out comments
2022 Note that, because C<$_> is a reference into the list value, it can
2023 be used to modify the elements of the array. While this is useful and
2024 supported, it can cause bizarre results if the LIST is not a named array.
2025 Similarly, grep returns aliases into the original list, much as a for
2026 loop's index variable aliases the list elements. That is, modifying an
2027 element of a list returned by grep (for example, in a C<foreach>, C<map>
2028 or another C<grep>) actually modifies the element in the original list.
2029 This is usually something to be avoided when writing clear code.
2031 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2037 Interprets EXPR as a hex string and returns the corresponding value.
2038 (To convert strings that might start with either 0, 0x, or 0b, see
2039 L</oct>.) If EXPR is omitted, uses C<$_>.
2041 print hex '0xAf'; # prints '175'
2042 print hex 'aF'; # same
2044 Hex strings may only represent integers. Strings that would cause
2045 integer overflow trigger a warning.
2049 There is no builtin C<import> function. It is just an ordinary
2050 method (subroutine) defined (or inherited) by modules that wish to export
2051 names to another module. The C<use> function calls the C<import> method
2052 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2054 =item index STR,SUBSTR,POSITION
2056 =item index STR,SUBSTR
2058 The index function searches for one string within another, but without
2059 the wildcard-like behavior of a full regular-expression pattern match.
2060 It returns the position of the first occurrence of SUBSTR in STR at
2061 or after POSITION. If POSITION is omitted, starts searching from the
2062 beginning of the string. The return value is based at C<0> (or whatever
2063 you've set the C<$[> variable to--but don't do that). If the substring
2064 is not found, returns one less than the base, ordinarily C<-1>.
2070 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2071 You should not use this function for rounding: one because it truncates
2072 towards C<0>, and two because machine representations of floating point
2073 numbers can sometimes produce counterintuitive results. For example,
2074 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2075 because it's really more like -268.99999999999994315658 instead. Usually,
2076 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2077 functions will serve you better than will int().
2079 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2081 Implements the ioctl(2) function. You'll probably first have to say
2083 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2085 to get the correct function definitions. If F<ioctl.ph> doesn't
2086 exist or doesn't have the correct definitions you'll have to roll your
2087 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2088 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2089 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2090 written depending on the FUNCTION--a pointer to the string value of SCALAR
2091 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2092 has no string value but does have a numeric value, that value will be
2093 passed rather than a pointer to the string value. To guarantee this to be
2094 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2095 functions may be needed to manipulate the values of structures used by
2098 The return value of C<ioctl> (and C<fcntl>) is as follows:
2100 if OS returns: then Perl returns:
2102 0 string "0 but true"
2103 anything else that number
2105 Thus Perl returns true on success and false on failure, yet you can
2106 still easily determine the actual value returned by the operating
2109 $retval = ioctl(...) || -1;
2110 printf "System returned %d\n", $retval;
2112 The special string "C<0> but true" is exempt from B<-w> complaints
2113 about improper numeric conversions.
2115 Here's an example of setting a filehandle named C<REMOTE> to be
2116 non-blocking at the system level. You'll have to negotiate C<$|>
2117 on your own, though.
2119 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2121 $flags = fcntl(REMOTE, F_GETFL, 0)
2122 or die "Can't get flags for the socket: $!\n";
2124 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2125 or die "Can't set flags for the socket: $!\n";
2127 =item join EXPR,LIST
2129 Joins the separate strings of LIST into a single string with fields
2130 separated by the value of EXPR, and returns that new string. Example:
2132 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2134 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2135 first argument. Compare L</split>.
2139 Returns a list consisting of all the keys of the named hash. (In
2140 scalar context, returns the number of keys.) The keys are returned in
2141 an apparently random order. The actual random order is subject to
2142 change in future versions of perl, but it is guaranteed to be the same
2143 order as either the C<values> or C<each> function produces (given
2144 that the hash has not been modified). As a side effect, it resets
2147 Here is yet another way to print your environment:
2150 @values = values %ENV;
2152 print pop(@keys), '=', pop(@values), "\n";
2155 or how about sorted by key:
2157 foreach $key (sort(keys %ENV)) {
2158 print $key, '=', $ENV{$key}, "\n";
2161 To sort a hash by value, you'll need to use a C<sort> function.
2162 Here's a descending numeric sort of a hash by its values:
2164 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2165 printf "%4d %s\n", $hash{$key}, $key;
2168 As an lvalue C<keys> allows you to increase the number of hash buckets
2169 allocated for the given hash. This can gain you a measure of efficiency if
2170 you know the hash is going to get big. (This is similar to pre-extending
2171 an array by assigning a larger number to $#array.) If you say
2175 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2176 in fact, since it rounds up to the next power of two. These
2177 buckets will be retained even if you do C<%hash = ()>, use C<undef
2178 %hash> if you want to free the storage while C<%hash> is still in scope.
2179 You can't shrink the number of buckets allocated for the hash using
2180 C<keys> in this way (but you needn't worry about doing this by accident,
2181 as trying has no effect).
2183 See also C<each>, C<values> and C<sort>.
2185 =item kill SIGNAL, LIST
2187 Sends a signal to a list of processes. Returns the number of
2188 processes successfully signaled (which is not necessarily the
2189 same as the number actually killed).
2191 $cnt = kill 1, $child1, $child2;
2194 If SIGNAL is zero, no signal is sent to the process. This is a
2195 useful way to check that the process is alive and hasn't changed
2196 its UID. See L<perlport> for notes on the portability of this
2199 Unlike in the shell, if SIGNAL is negative, it kills
2200 process groups instead of processes. (On System V, a negative I<PROCESS>
2201 number will also kill process groups, but that's not portable.) That
2202 means you usually want to use positive not negative signals. You may also
2203 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2209 The C<last> command is like the C<break> statement in C (as used in
2210 loops); it immediately exits the loop in question. If the LABEL is
2211 omitted, the command refers to the innermost enclosing loop. The
2212 C<continue> block, if any, is not executed:
2214 LINE: while (<STDIN>) {
2215 last LINE if /^$/; # exit when done with header
2219 C<last> cannot be used to exit a block which returns a value such as
2220 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2221 a grep() or map() operation.
2223 Note that a block by itself is semantically identical to a loop
2224 that executes once. Thus C<last> can be used to effect an early
2225 exit out of such a block.
2227 See also L</continue> for an illustration of how C<last>, C<next>, and
2234 Returns an lowercased version of EXPR. This is the internal function
2235 implementing the C<\L> escape in double-quoted strings.
2236 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2239 If EXPR is omitted, uses C<$_>.
2245 Returns the value of EXPR with the first character lowercased. This is
2246 the internal function implementing the C<\l> escape in double-quoted strings.
2247 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2249 If EXPR is omitted, uses C<$_>.
2255 Returns the length in characters of the value of EXPR. If EXPR is
2256 omitted, returns length of C<$_>. Note that this cannot be used on
2257 an entire array or hash to find out how many elements these have.
2258 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2260 =item link OLDFILE,NEWFILE
2262 Creates a new filename linked to the old filename. Returns true for
2263 success, false otherwise.
2265 =item listen SOCKET,QUEUESIZE
2267 Does the same thing that the listen system call does. Returns true if
2268 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2272 You really probably want to be using C<my> instead, because C<local> isn't
2273 what most people think of as "local". See L<perlsub/"Private Variables
2274 via my()"> for details.
2276 A local modifies the listed variables to be local to the enclosing
2277 block, file, or eval. If more than one value is listed, the list must
2278 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2279 for details, including issues with tied arrays and hashes.
2281 =item localtime EXPR
2283 Converts a time as returned by the time function to a 9-element list
2284 with the time analyzed for the local time zone. Typically used as
2288 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2291 All list elements are numeric, and come straight out of a struct tm.
2292 In particular this means that $mon has the range C<0..11> and $wday
2293 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2294 number of years since 1900, that is, $year is C<123> in year 2023,
2295 and I<not> simply the last two digits of the year. If you assume it is,
2296 then you create non-Y2K-compliant programs--and you wouldn't want to do
2299 The proper way to get a complete 4-digit year is simply:
2303 And to get the last two digits of the year (e.g., '01' in 2001) do:
2305 $year = sprintf("%02d", $year % 100);
2307 If EXPR is omitted, uses the current time (C<localtime(time)>).
2309 In scalar context, returns the ctime(3) value:
2311 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2313 This scalar value is B<not> locale dependent, see L<perllocale>, but
2314 instead a Perl builtin. Also see the C<Time::Local> module
2315 (to convert the second, minutes, hours, ... back to seconds since the
2316 stroke of midnight the 1st of January 1970, the value returned by
2317 time()), and the strftime(3) and mktime(3) function available via the
2318 POSIX module. To get somewhat similar but locale dependent date
2319 strings, set up your locale environment variables appropriately
2320 (please see L<perllocale>) and try for example:
2322 use POSIX qw(strftime);
2323 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2325 Note that the C<%a> and C<%b>, the short forms of the day of the week
2326 and the month of the year, may not necessarily be three characters wide.
2332 This function places an advisory lock on a variable, subroutine,
2333 or referenced object contained in I<THING> until the lock goes out
2334 of scope. This is a built-in function only if your version of Perl
2335 was built with threading enabled, and if you've said C<use Threads>.
2336 Otherwise a user-defined function by this name will be called. See
2343 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2344 returns log of C<$_>. To get the log of another base, use basic algebra:
2345 The base-N log of a number is equal to the natural log of that number
2346 divided by the natural log of N. For example:
2350 return log($n)/log(10);
2353 See also L</exp> for the inverse operation.
2355 =item lstat FILEHANDLE
2361 Does the same thing as the C<stat> function (including setting the
2362 special C<_> filehandle) but stats a symbolic link instead of the file
2363 the symbolic link points to. If symbolic links are unimplemented on
2364 your system, a normal C<stat> is done.
2366 If EXPR is omitted, stats C<$_>.
2370 The match operator. See L<perlop>.
2372 =item map BLOCK LIST
2376 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2377 C<$_> to each element) and returns the list value composed of the
2378 results of each such evaluation. In scalar context, returns the
2379 total number of elements so generated. Evaluates BLOCK or EXPR in
2380 list context, so each element of LIST may produce zero, one, or
2381 more elements in the returned value.
2383 @chars = map(chr, @nums);
2385 translates a list of numbers to the corresponding characters. And
2387 %hash = map { getkey($_) => $_ } @array;
2389 is just a funny way to write
2392 foreach $_ (@array) {
2393 $hash{getkey($_)} = $_;
2396 Note that, because C<$_> is a reference into the list value, it can
2397 be used to modify the elements of the array. While this is useful and
2398 supported, it can cause bizarre results if the LIST is not a named array.
2399 Using a regular C<foreach> loop for this purpose would be clearer in
2400 most cases. See also L</grep> for an array composed of those items of
2401 the original list for which the BLOCK or EXPR evaluates to true.
2403 =item mkdir FILENAME,MASK
2405 Creates the directory specified by FILENAME, with permissions
2406 specified by MASK (as modified by C<umask>). If it succeeds it
2407 returns true, otherwise it returns false and sets C<$!> (errno).
2409 In general, it is better to create directories with permissive MASK,
2410 and let the user modify that with their C<umask>, than it is to supply
2411 a restrictive MASK and give the user no way to be more permissive.
2412 The exceptions to this rule are when the file or directory should be
2413 kept private (mail files, for instance). The perlfunc(1) entry on
2414 C<umask> discusses the choice of MASK in more detail.
2416 =item msgctl ID,CMD,ARG
2418 Calls the System V IPC function msgctl(2). You'll probably have to say
2422 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2423 then ARG must be a variable which will hold the returned C<msqid_ds>
2424 structure. Returns like C<ioctl>: the undefined value for error,
2425 C<"0 but true"> for zero, or the actual return value otherwise. See also
2426 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2428 =item msgget KEY,FLAGS
2430 Calls the System V IPC function msgget(2). Returns the message queue
2431 id, or the undefined value if there is an error. See also C<IPC::SysV>
2432 and C<IPC::Msg> documentation.
2434 =item msgsnd ID,MSG,FLAGS
2436 Calls the System V IPC function msgsnd to send the message MSG to the
2437 message queue ID. MSG must begin with the long integer message type,
2438 which may be created with C<pack("l", $type)>. Returns true if
2439 successful, or false if there is an error. See also C<IPC::SysV>
2440 and C<IPC::SysV::Msg> documentation.
2442 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2444 Calls the System V IPC function msgrcv to receive a message from
2445 message queue ID into variable VAR with a maximum message size of
2446 SIZE. Note that if a message is received, the message type will be
2447 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2448 size of the message type. Returns true if successful, or false if
2449 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2453 =item my EXPR : ATTRIBUTES
2455 A C<my> declares the listed variables to be local (lexically) to the
2456 enclosing block, file, or C<eval>. If
2457 more than one value is listed, the list must be placed in parentheses. See
2458 L<perlsub/"Private Variables via my()"> for details.
2464 The C<next> command is like the C<continue> statement in C; it starts
2465 the next iteration of the loop:
2467 LINE: while (<STDIN>) {
2468 next LINE if /^#/; # discard comments
2472 Note that if there were a C<continue> block on the above, it would get
2473 executed even on discarded lines. If the LABEL is omitted, the command
2474 refers to the innermost enclosing loop.
2476 C<next> cannot be used to exit a block which returns a value such as
2477 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2478 a grep() or map() operation.
2480 Note that a block by itself is semantically identical to a loop
2481 that executes once. Thus C<next> will exit such a block early.
2483 See also L</continue> for an illustration of how C<last>, C<next>, and
2486 =item no Module LIST
2488 See the L</use> function, which C<no> is the opposite of.
2494 Interprets EXPR as an octal string and returns the corresponding
2495 value. (If EXPR happens to start off with C<0x>, interprets it as a
2496 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2497 binary string.) The following will handle decimal, binary, octal, and
2498 hex in the standard Perl or C notation:
2500 $val = oct($val) if $val =~ /^0/;
2502 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2503 in octal), use sprintf() or printf():
2505 $perms = (stat("filename"))[2] & 07777;
2506 $oct_perms = sprintf "%lo", $perms;
2508 The oct() function is commonly used when a string such as C<644> needs
2509 to be converted into a file mode, for example. (Although perl will
2510 automatically convert strings into numbers as needed, this automatic
2511 conversion assumes base 10.)
2513 =item open FILEHANDLE,MODE,EXPR
2515 =item open FILEHANDLE,EXPR
2517 =item open FILEHANDLE
2519 Opens the file whose filename is given by EXPR, and associates it with
2520 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2521 name of the real filehandle wanted. If EXPR is omitted, the scalar
2522 variable of the same name as the FILEHANDLE contains the filename.
2523 (Note that lexical variables--those declared with C<my>--will not work
2524 for this purpose; so if you're using C<my>, specify EXPR in your call
2525 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2528 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2529 If MODE is C<'E<gt>'>, the file is truncated and opened for
2530 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2531 the file is opened for appending, again being created if necessary.
2532 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2533 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2534 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2535 file first. You can't usually use either read-write mode for updating
2536 textfiles, since they have variable length records. See the B<-i>
2537 switch in L<perlrun> for a better approach. The file is created with
2538 permissions of C<0666> modified by the process' C<umask> value.
2540 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2541 C<'w+'>, C<'a'>, and C<'a+'>.
2543 In the 2-arguments (and 1-argument) form of the call the mode and
2544 filename should be concatenated (in this order), possibly separated by
2545 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2547 If the filename begins with C<'|'>, the filename is interpreted as a
2548 command to which output is to be piped, and if the filename ends with a
2549 C<'|'>, the filename is interpreted as a command which pipes output to
2550 us. See L<perlipc/"Using open() for IPC">
2551 for more examples of this. (You are not allowed to C<open> to a command
2552 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2553 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2555 If MODE is C<'|-'>, the filename is interpreted as a
2556 command to which output is to be piped, and if MODE is
2557 C<'-|'>, the filename is interpreted as a command which pipes output to
2558 us. In the 2-arguments (and 1-argument) form one should replace dash
2559 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2560 for more examples of this. (You are not allowed to C<open> to a command
2561 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2562 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2564 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2565 and opening C<'E<gt>-'> opens STDOUT.
2568 nonzero upon success, the undefined value otherwise. If the C<open>
2569 involved a pipe, the return value happens to be the pid of the
2572 If you're unfortunate enough to be running Perl on a system that
2573 distinguishes between text files and binary files (modern operating
2574 systems don't care), then you should check out L</binmode> for tips for
2575 dealing with this. The key distinction between systems that need C<binmode>
2576 and those that don't is their text file formats. Systems like Unix, MacOS, and
2577 Plan9, which delimit lines with a single character, and which encode that
2578 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2580 When opening a file, it's usually a bad idea to continue normal execution
2581 if the request failed, so C<open> is frequently used in connection with
2582 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2583 where you want to make a nicely formatted error message (but there are
2584 modules that can help with that problem)) you should always check
2585 the return value from opening a file. The infrequent exception is when
2586 working with an unopened filehandle is actually what you want to do.
2591 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2592 while (<ARTICLE>) {...
2594 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2595 # if the open fails, output is discarded
2597 open(DBASE, '+<', 'dbase.mine') # open for update
2598 or die "Can't open 'dbase.mine' for update: $!";
2600 open(DBASE, '+<dbase.mine') # ditto
2601 or die "Can't open 'dbase.mine' for update: $!";
2603 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2604 or die "Can't start caesar: $!";
2606 open(ARTICLE, "caesar <$article |") # ditto
2607 or die "Can't start caesar: $!";
2609 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2610 or die "Can't start sort: $!";
2612 # process argument list of files along with any includes
2614 foreach $file (@ARGV) {
2615 process($file, 'fh00');
2619 my($filename, $input) = @_;
2620 $input++; # this is a string increment
2621 unless (open($input, $filename)) {
2622 print STDERR "Can't open $filename: $!\n";
2627 while (<$input>) { # note use of indirection
2628 if (/^#include "(.*)"/) {
2629 process($1, $input);
2636 You may also, in the Bourne shell tradition, specify an EXPR beginning
2637 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2638 name of a filehandle (or file descriptor, if numeric) to be
2639 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2640 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2641 mode you specify should match the mode of the original filehandle.
2642 (Duping a filehandle does not take into account any existing contents of
2643 stdio buffers.) Duping file handles is not yet supported for 3-argument
2646 Here is a script that saves, redirects, and restores STDOUT and
2650 open(OLDOUT, ">&STDOUT");
2651 open(OLDERR, ">&STDERR");
2653 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2654 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2656 select(STDERR); $| = 1; # make unbuffered
2657 select(STDOUT); $| = 1; # make unbuffered
2659 print STDOUT "stdout 1\n"; # this works for
2660 print STDERR "stderr 1\n"; # subprocesses too
2665 open(STDOUT, ">&OLDOUT");
2666 open(STDERR, ">&OLDERR");
2668 print STDOUT "stdout 2\n";
2669 print STDERR "stderr 2\n";
2671 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2672 equivalent of C's C<fdopen> of that file descriptor; this is more
2673 parsimonious of file descriptors. For example:
2675 open(FILEHANDLE, "<&=$fd")
2677 Note that this feature depends on the fdopen() C library function.
2678 On many UNIX systems, fdopen() is known to fail when file descriptors
2679 exceed a certain value, typically 255. If you need more file
2680 descriptors than that, consider rebuilding Perl to use the C<sfio>
2683 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2684 with 2-arguments (or 1-argument) form of open(), then
2685 there is an implicit fork done, and the return value of open is the pid
2686 of the child within the parent process, and C<0> within the child
2687 process. (Use C<defined($pid)> to determine whether the open was successful.)
2688 The filehandle behaves normally for the parent, but i/o to that
2689 filehandle is piped from/to the STDOUT/STDIN of the child process.
2690 In the child process the filehandle isn't opened--i/o happens from/to
2691 the new STDOUT or STDIN. Typically this is used like the normal
2692 piped open when you want to exercise more control over just how the
2693 pipe command gets executed, such as when you are running setuid, and
2694 don't want to have to scan shell commands for metacharacters.
2695 The following triples are more or less equivalent:
2697 open(FOO, "|tr '[a-z]' '[A-Z]'");
2698 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2699 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2701 open(FOO, "cat -n '$file'|");
2702 open(FOO, '-|', "cat -n '$file'");
2703 open(FOO, '-|') || exec 'cat', '-n', $file;
2705 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2707 NOTE: On any operation that may do a fork, all files opened for output
2708 are flushed before the fork is attempted. On systems that support a
2709 close-on-exec flag on files, the flag will be set for the newly opened
2710 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2712 Closing any piped filehandle causes the parent process to wait for the
2713 child to finish, and returns the status value in C<$?>.
2715 The filename passed to 2-argument (or 1-argument) form of open()
2716 will have leading and trailing
2717 whitespace deleted, and the normal redirection characters
2718 honored. This property, known as "magic open",
2719 can often be used to good effect. A user could specify a filename of
2720 F<"rsh cat file |">, or you could change certain filenames as needed:
2722 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2723 open(FH, $filename) or die "Can't open $filename: $!";
2725 Use 3-argument form to open a file with arbitrary weird characters in it,
2727 open(FOO, '<', $file);
2729 otherwise it's necessary to protect any leading and trailing whitespace:
2731 $file =~ s#^(\s)#./$1#;
2732 open(FOO, "< $file\0");
2734 (this may not work on some bizzare filesystems). One should
2735 conscientiously choose between the the I<magic> and 3-arguments form
2740 will allow the user to specify an argument of the form C<"rsh cat file |">,
2741 but will not work on a filename which happens to have a trailing space, while
2743 open IN, '<', $ARGV[0];
2745 will have exactly the opposite restrictions.
2747 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2748 should use the C<sysopen> function, which involves no such magic (but
2749 may use subtly different filemodes than Perl open(), which is mapped
2750 to C fopen()). This is
2751 another way to protect your filenames from interpretation. For example:
2754 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2755 or die "sysopen $path: $!";
2756 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2757 print HANDLE "stuff $$\n");
2759 print "File contains: ", <HANDLE>;
2761 Using the constructor from the C<IO::Handle> package (or one of its
2762 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2763 filehandles that have the scope of whatever variables hold references to
2764 them, and automatically close whenever and however you leave that scope:
2768 sub read_myfile_munged {
2770 my $handle = new IO::File;
2771 open($handle, "myfile") or die "myfile: $!";
2773 or return (); # Automatically closed here.
2774 mung $first or die "mung failed"; # Or here.
2775 return $first, <$handle> if $ALL; # Or here.
2779 See L</seek> for some details about mixing reading and writing.
2781 =item opendir DIRHANDLE,EXPR
2783 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2784 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2785 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2791 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2792 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2793 See L<utf8> for more about Unicode.
2797 An C<our> declares the listed variables to be valid globals within
2798 the enclosing block, file, or C<eval>. That is, it has the same
2799 scoping rules as a "my" declaration, but does not create a local
2800 variable. If more than one value is listed, the list must be placed
2801 in parentheses. The C<our> declaration has no semantic effect unless
2802 "use strict vars" is in effect, in which case it lets you use the
2803 declared global variable without qualifying it with a package name.
2804 (But only within the lexical scope of the C<our> declaration. In this
2805 it differs from "use vars", which is package scoped.)
2807 An C<our> declaration declares a global variable that will be visible
2808 across its entire lexical scope, even across package boundaries. The
2809 package in which the variable is entered is determined at the point
2810 of the declaration, not at the point of use. This means the following
2814 our $bar; # declares $Foo::bar for rest of lexical scope
2818 print $bar; # prints 20
2820 Multiple C<our> declarations in the same lexical scope are allowed
2821 if they are in different packages. If they happened to be in the same
2822 package, Perl will emit warnings if you have asked for them.
2826 our $bar; # declares $Foo::bar for rest of lexical scope
2830 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2831 print $bar; # prints 30
2833 our $bar; # emits warning
2835 =item pack TEMPLATE,LIST
2837 Takes a LIST of values and converts it into a string using the rules
2838 given by the TEMPLATE. The resulting string is the concatenation of
2839 the converted values. Typically, each converted value looks
2840 like its machine-level representation. For example, on 32-bit machines
2841 a converted integer may be represented by a sequence of 4 bytes.
2844 sequence of characters that give the order and type of values, as
2847 a A string with arbitrary binary data, will be null padded.
2848 A An ascii string, will be space padded.
2849 Z A null terminated (asciz) string, will be null padded.
2851 b A bit string (ascending bit order inside each byte, like vec()).
2852 B A bit string (descending bit order inside each byte).
2853 h A hex string (low nybble first).
2854 H A hex string (high nybble first).
2856 c A signed char value.
2857 C An unsigned char value. Only does bytes. See U for Unicode.
2859 s A signed short value.
2860 S An unsigned short value.
2861 (This 'short' is _exactly_ 16 bits, which may differ from
2862 what a local C compiler calls 'short'. If you want
2863 native-length shorts, use the '!' suffix.)
2865 i A signed integer value.
2866 I An unsigned integer value.
2867 (This 'integer' is _at_least_ 32 bits wide. Its exact
2868 size depends on what a local C compiler calls 'int',
2869 and may even be larger than the 'long' described in
2872 l A signed long value.
2873 L An unsigned long value.
2874 (This 'long' is _exactly_ 32 bits, which may differ from
2875 what a local C compiler calls 'long'. If you want
2876 native-length longs, use the '!' suffix.)
2878 n An unsigned short in "network" (big-endian) order.
2879 N An unsigned long in "network" (big-endian) order.
2880 v An unsigned short in "VAX" (little-endian) order.
2881 V An unsigned long in "VAX" (little-endian) order.
2882 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2883 _exactly_ 32 bits, respectively.)
2885 q A signed quad (64-bit) value.
2886 Q An unsigned quad value.
2887 (Quads are available only if your system supports 64-bit
2888 integer values _and_ if Perl has been compiled to support those.
2889 Causes a fatal error otherwise.)
2891 f A single-precision float in the native format.
2892 d A double-precision float in the native format.
2894 p A pointer to a null-terminated string.
2895 P A pointer to a structure (fixed-length string).
2897 u A uuencoded string.
2898 U A Unicode character number. Encodes to UTF-8 internally.
2899 Works even if C<use utf8> is not in effect.
2901 w A BER compressed integer. Its bytes represent an unsigned
2902 integer in base 128, most significant digit first, with as
2903 few digits as possible. Bit eight (the high bit) is set
2904 on each byte except the last.
2908 @ Null fill to absolute position.
2910 The following rules apply:
2916 Each letter may optionally be followed by a number giving a repeat
2917 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2918 C<H>, and C<P> the pack function will gobble up that many values from
2919 the LIST. A C<*> for the repeat count means to use however many items are
2920 left, except for C<@>, C<x>, C<X>, where it is equivalent
2921 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2924 When used with C<Z>, C<*> results in the addition of a trailing null
2925 byte (so the packed result will be one longer than the byte C<length>
2928 The repeat count for C<u> is interpreted as the maximal number of bytes
2929 to encode per line of output, with 0 and 1 replaced by 45.
2933 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
2934 string of length count, padding with nulls or spaces as necessary. When
2935 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2936 after the first null, and C<a> returns data verbatim. When packing,
2937 C<a>, and C<Z> are equivalent.
2939 If the value-to-pack is too long, it is truncated. If too long and an
2940 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2941 by a null byte. Thus C<Z> always packs a trailing null byte under
2946 Likewise, the C<b> and C<B> fields pack a string that many bits long.
2947 Each byte of the input field of pack() generates 1 bit of the result.
2948 Each result bit is based on the least-significant bit of the corresponding
2949 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2950 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2952 Starting from the beginning of the input string of pack(), each 8-tuple
2953 of bytes is converted to 1 byte of output. With format C<b>
2954 the first byte of the 8-tuple determines the least-significant bit of a
2955 byte, and with format C<B> it determines the most-significant bit of
2958 If the length of the input string is not exactly divisible by 8, the
2959 remainder is packed as if the input string were padded by null bytes
2960 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2962 If the input string of pack() is longer than needed, extra bytes are ignored.
2963 A C<*> for the repeat count of pack() means to use all the bytes of
2964 the input field. On unpack()ing the bits are converted to a string
2965 of C<"0">s and C<"1">s.
2969 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
2970 representable as hexadecimal digits, 0-9a-f) long.
2972 Each byte of the input field of pack() generates 4 bits of the result.
2973 For non-alphabetical bytes the result is based on the 4 least-significant
2974 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2975 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2976 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2977 is compatible with the usual hexadecimal digits, so that C<"a"> and
2978 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2979 C<"g".."z"> and C<"G".."Z"> is not well-defined.
2981 Starting from the beginning of the input string of pack(), each pair
2982 of bytes is converted to 1 byte of output. With format C<h> the
2983 first byte of the pair determines the least-significant nybble of the
2984 output byte, and with format C<H> it determines the most-significant
2987 If the length of the input string is not even, it behaves as if padded
2988 by a null byte at the end. Similarly, during unpack()ing the "extra"
2989 nybbles are ignored.
2991 If the input string of pack() is longer than needed, extra bytes are ignored.
2992 A C<*> for the repeat count of pack() means to use all the bytes of
2993 the input field. On unpack()ing the bits are converted to a string
2994 of hexadecimal digits.
2998 The C<p> type packs a pointer to a null-terminated string. You are
2999 responsible for ensuring the string is not a temporary value (which can
3000 potentially get deallocated before you get around to using the packed result).
3001 The C<P> type packs a pointer to a structure of the size indicated by the
3002 length. A NULL pointer is created if the corresponding value for C<p> or
3003 C<P> is C<undef>, similarly for unpack().
3007 The C</> template character allows packing and unpacking of strings where
3008 the packed structure contains a byte count followed by the string itself.
3009 You write I<length-item>C</>I<string-item>.
3011 The I<length-item> can be any C<pack> template letter,
3012 and describes how the length value is packed.
3013 The ones likely to be of most use are integer-packing ones like
3014 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3015 and C<N> (for Sun XDR).
3017 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3018 For C<unpack> the length of the string is obtained from the I<length-item>,
3019 but if you put in the '*' it will be ignored.
3021 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3022 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3023 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3025 The I<length-item> is not returned explicitly from C<unpack>.
3027 Adding a count to the I<length-item> letter is unlikely to do anything
3028 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3029 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3030 which Perl does not regard as legal in numeric strings.
3034 The integer types C<s>, C<S>, C<l>, and C<L> may be
3035 immediately followed by a C<!> suffix to signify native shorts or
3036 longs--as you can see from above for example a bare C<l> does mean
3037 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3038 may be larger. This is an issue mainly in 64-bit platforms. You can
3039 see whether using C<!> makes any difference by
3041 print length(pack("s")), " ", length(pack("s!")), "\n";
3042 print length(pack("l")), " ", length(pack("l!")), "\n";
3044 C<i!> and C<I!> also work but only because of completeness;
3045 they are identical to C<i> and C<I>.
3047 The actual sizes (in bytes) of native shorts, ints, longs, and long
3048 longs on the platform where Perl was built are also available via
3052 print $Config{shortsize}, "\n";
3053 print $Config{intsize}, "\n";
3054 print $Config{longsize}, "\n";
3055 print $Config{longlongsize}, "\n";
3057 (The C<$Config{longlongsize}> will be undefine if your system does
3058 not support long longs.)
3062 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3063 are inherently non-portable between processors and operating systems
3064 because they obey the native byteorder and endianness. For example a
3065 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3066 (arranged in and handled by the CPU registers) into bytes as
3068 0x12 0x34 0x56 0x78 # little-endian
3069 0x78 0x56 0x34 0x12 # big-endian
3071 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3072 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3073 Power, and Cray are big-endian. MIPS can be either: Digital used it
3074 in little-endian mode; SGI uses it in big-endian mode.
3076 The names `big-endian' and `little-endian' are comic references to
3077 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3078 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3079 the egg-eating habits of the Lilliputians.
3081 Some systems may have even weirder byte orders such as
3086 You can see your system's preference with
3088 print join(" ", map { sprintf "%#02x", $_ }
3089 unpack("C*",pack("L",0x12345678))), "\n";
3091 The byteorder on the platform where Perl was built is also available
3095 print $Config{byteorder}, "\n";
3097 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3098 and C<'87654321'> are big-endian.
3100 If you want portable packed integers use the formats C<n>, C<N>,
3101 C<v>, and C<V>, their byte endianness and size is known.
3102 See also L<perlport>.
3106 Real numbers (floats and doubles) are in the native machine format only;
3107 due to the multiplicity of floating formats around, and the lack of a
3108 standard "network" representation, no facility for interchange has been
3109 made. This means that packed floating point data written on one machine
3110 may not be readable on another - even if both use IEEE floating point
3111 arithmetic (as the endian-ness of the memory representation is not part
3112 of the IEEE spec). See also L<perlport>.
3114 Note that Perl uses doubles internally for all numeric calculation, and
3115 converting from double into float and thence back to double again will
3116 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3121 You must yourself do any alignment or padding by inserting for example
3122 enough C<'x'>es while packing. There is no way to pack() and unpack()
3123 could know where the bytes are going to or coming from. Therefore
3124 C<pack> (and C<unpack>) handle their output and input as flat
3129 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3133 If TEMPLATE requires more arguments to pack() than actually given, pack()
3134 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3135 to pack() than actually given, extra arguments are ignored.
3141 $foo = pack("CCCC",65,66,67,68);
3143 $foo = pack("C4",65,66,67,68);
3145 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3146 # same thing with Unicode circled letters
3148 $foo = pack("ccxxcc",65,66,67,68);
3151 # note: the above examples featuring "C" and "c" are true
3152 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3153 # and UTF-8. In EBCDIC the first example would be
3154 # $foo = pack("CCCC",193,194,195,196);
3156 $foo = pack("s2",1,2);
3157 # "\1\0\2\0" on little-endian
3158 # "\0\1\0\2" on big-endian
3160 $foo = pack("a4","abcd","x","y","z");
3163 $foo = pack("aaaa","abcd","x","y","z");
3166 $foo = pack("a14","abcdefg");
3167 # "abcdefg\0\0\0\0\0\0\0"
3169 $foo = pack("i9pl", gmtime);
3170 # a real struct tm (on my system anyway)
3172 $utmp_template = "Z8 Z8 Z16 L";
3173 $utmp = pack($utmp_template, @utmp1);
3174 # a struct utmp (BSDish)
3176 @utmp2 = unpack($utmp_template, $utmp);
3177 # "@utmp1" eq "@utmp2"
3180 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3183 $foo = pack('sx2l', 12, 34);
3184 # short 12, two zero bytes padding, long 34
3185 $bar = pack('s@4l', 12, 34);
3186 # short 12, zero fill to position 4, long 34
3189 The same template may generally also be used in unpack().
3193 =item package NAMESPACE
3195 Declares the compilation unit as being in the given namespace. The scope
3196 of the package declaration is from the declaration itself through the end
3197 of the enclosing block, file, or eval (the same as the C<my> operator).
3198 All further unqualified dynamic identifiers will be in this namespace.
3199 A package statement affects only dynamic variables--including those
3200 you've used C<local> on--but I<not> lexical variables, which are created
3201 with C<my>. Typically it would be the first declaration in a file to
3202 be included by the C<require> or C<use> operator. You can switch into a
3203 package in more than one place; it merely influences which symbol table
3204 is used by the compiler for the rest of that block. You can refer to
3205 variables and filehandles in other packages by prefixing the identifier
3206 with the package name and a double colon: C<$Package::Variable>.
3207 If the package name is null, the C<main> package as assumed. That is,
3208 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3209 still seen in older code).
3211 If NAMESPACE is omitted, then there is no current package, and all
3212 identifiers must be fully qualified or lexicals. This is stricter
3213 than C<use strict>, since it also extends to function names.
3215 See L<perlmod/"Packages"> for more information about packages, modules,
3216 and classes. See L<perlsub> for other scoping issues.
3218 =item pipe READHANDLE,WRITEHANDLE
3220 Opens a pair of connected pipes like the corresponding system call.
3221 Note that if you set up a loop of piped processes, deadlock can occur
3222 unless you are very careful. In addition, note that Perl's pipes use
3223 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3224 after each command, depending on the application.
3226 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3227 for examples of such things.
3229 On systems that support a close-on-exec flag on files, the flag will be set
3230 for the newly opened file descriptors as determined by the value of $^F.
3237 Pops and returns the last value of the array, shortening the array by
3238 one element. Has an effect similar to
3242 If there are no elements in the array, returns the undefined value
3243 (although this may happen at other times as well). If ARRAY is
3244 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3245 array in subroutines, just like C<shift>.
3251 Returns the offset of where the last C<m//g> search left off for the variable
3252 is in question (C<$_> is used when the variable is not specified). May be
3253 modified to change that offset. Such modification will also influence
3254 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3257 =item print FILEHANDLE LIST
3263 Prints a string or a list of strings. Returns true if successful.
3264 FILEHANDLE may be a scalar variable name, in which case the variable
3265 contains the name of or a reference to the filehandle, thus introducing
3266 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3267 the next token is a term, it may be misinterpreted as an operator
3268 unless you interpose a C<+> or put parentheses around the arguments.)
3269 If FILEHANDLE is omitted, prints by default to standard output (or
3270 to the last selected output channel--see L</select>). If LIST is
3271 also omitted, prints C<$_> to the currently selected output channel.
3272 To set the default output channel to something other than STDOUT
3273 use the select operation. The current value of C<$,> (if any) is
3274 printed between each LIST item. The current value of C<$\> (if
3275 any) is printed after the entire LIST has been printed. Because
3276 print takes a LIST, anything in the LIST is evaluated in list
3277 context, and any subroutine that you call will have one or more of
3278 its expressions evaluated in list context. Also be careful not to
3279 follow the print keyword with a left parenthesis unless you want
3280 the corresponding right parenthesis to terminate the arguments to
3281 the print--interpose a C<+> or put parentheses around all the
3284 Note that if you're storing FILEHANDLES in an array or other expression,
3285 you will have to use a block returning its value instead:
3287 print { $files[$i] } "stuff\n";
3288 print { $OK ? STDOUT : STDERR } "stuff\n";
3290 =item printf FILEHANDLE FORMAT, LIST
3292 =item printf FORMAT, LIST
3294 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3295 (the output record separator) is not appended. The first argument
3296 of the list will be interpreted as the C<printf> format. If C<use locale> is
3297 in effect, the character used for the decimal point in formatted real numbers
3298 is affected by the LC_NUMERIC locale. See L<perllocale>.
3300 Don't fall into the trap of using a C<printf> when a simple
3301 C<print> would do. The C<print> is more efficient and less
3304 =item prototype FUNCTION
3306 Returns the prototype of a function as a string (or C<undef> if the
3307 function has no prototype). FUNCTION is a reference to, or the name of,
3308 the function whose prototype you want to retrieve.
3310 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3311 name for Perl builtin. If the builtin is not I<overridable> (such as
3312 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3313 C<system>) returns C<undef> because the builtin does not really behave
3314 like a Perl function. Otherwise, the string describing the equivalent
3315 prototype is returned.
3317 =item push ARRAY,LIST
3319 Treats ARRAY as a stack, and pushes the values of LIST
3320 onto the end of ARRAY. The length of ARRAY increases by the length of
3321 LIST. Has the same effect as
3324 $ARRAY[++$#ARRAY] = $value;
3327 but is more efficient. Returns the new number of elements in the array.
3339 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3341 =item quotemeta EXPR
3345 Returns the value of EXPR with all non-alphanumeric
3346 characters backslashed. (That is, all characters not matching
3347 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3348 returned string, regardless of any locale settings.)
3349 This is the internal function implementing
3350 the C<\Q> escape in double-quoted strings.
3352 If EXPR is omitted, uses C<$_>.
3358 Returns a random fractional number greater than or equal to C<0> and less
3359 than the value of EXPR. (EXPR should be positive.) If EXPR is
3360 omitted, the value C<1> is used. Automatically calls C<srand> unless
3361 C<srand> has already been called. See also C<srand>.
3363 (Note: If your rand function consistently returns numbers that are too
3364 large or too small, then your version of Perl was probably compiled
3365 with the wrong number of RANDBITS.)
3367 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3369 =item read FILEHANDLE,SCALAR,LENGTH
3371 Attempts to read LENGTH bytes of data into variable SCALAR from the
3372 specified FILEHANDLE. Returns the number of bytes actually read,
3373 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3374 or shrunk to the length actually read. An OFFSET may be specified to
3375 place the read data at some other place than the beginning of the
3376 string. This call is actually implemented in terms of stdio's fread(3)
3377 call. To get a true read(2) system call, see C<sysread>.
3379 =item readdir DIRHANDLE
3381 Returns the next directory entry for a directory opened by C<opendir>.
3382 If used in list context, returns all the rest of the entries in the
3383 directory. If there are no more entries, returns an undefined value in
3384 scalar context or a null list in list context.
3386 If you're planning to filetest the return values out of a C<readdir>, you'd
3387 better prepend the directory in question. Otherwise, because we didn't
3388 C<chdir> there, it would have been testing the wrong file.
3390 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3391 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3396 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3397 context, each call reads and returns the next line, until end-of-file is
3398 reached, whereupon the subsequent call returns undef. In list context,
3399 reads until end-of-file is reached and returns a list of lines. Note that
3400 the notion of "line" used here is however you may have defined it
3401 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3403 When C<$/> is set to C<undef>, when readline() is in scalar
3404 context (i.e. file slurp mode), and when an empty file is read, it
3405 returns C<''> the first time, followed by C<undef> subsequently.
3407 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3408 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3409 operator is discussed in more detail in L<perlop/"I/O Operators">.
3412 $line = readline(*STDIN); # same thing
3418 Returns the value of a symbolic link, if symbolic links are
3419 implemented. If not, gives a fatal error. If there is some system
3420 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3421 omitted, uses C<$_>.
3425 EXPR is executed as a system command.
3426 The collected standard output of the command is returned.
3427 In scalar context, it comes back as a single (potentially
3428 multi-line) string. In list context, returns a list of lines
3429 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3430 This is the internal function implementing the C<qx/EXPR/>
3431 operator, but you can use it directly. The C<qx/EXPR/>
3432 operator is discussed in more detail in L<perlop/"I/O Operators">.
3434 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3436 Receives a message on a socket. Attempts to receive LENGTH bytes of
3437 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3438 will be grown or shrunk to the length actually read. Takes the same
3439 flags as the system call of the same name. Returns the address of the
3440 sender if SOCKET's protocol supports this; returns an empty string
3441 otherwise. If there's an error, returns the undefined value. This call
3442 is actually implemented in terms of recvfrom(2) system call. See
3443 L<perlipc/"UDP: Message Passing"> for examples.
3449 The C<redo> command restarts the loop block without evaluating the
3450 conditional again. The C<continue> block, if any, is not executed. If
3451 the LABEL is omitted, the command refers to the innermost enclosing
3452 loop. This command is normally used by programs that want to lie to
3453 themselves about what was just input:
3455 # a simpleminded Pascal comment stripper
3456 # (warning: assumes no { or } in strings)
3457 LINE: while (<STDIN>) {
3458 while (s|({.*}.*){.*}|$1 |) {}
3463 if (/}/) { # end of comment?
3472 C<redo> cannot be used to retry a block which returns a value such as
3473 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3474 a grep() or map() operation.
3476 Note that a block by itself is semantically identical to a loop
3477 that executes once. Thus C<redo> inside such a block will effectively
3478 turn it into a looping construct.
3480 See also L</continue> for an illustration of how C<last>, C<next>, and
3487 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3488 is not specified, C<$_> will be used. The value returned depends on the
3489 type of thing the reference is a reference to.
3490 Builtin types include:
3500 If the referenced object has been blessed into a package, then that package
3501 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3503 if (ref($r) eq "HASH") {
3504 print "r is a reference to a hash.\n";
3507 print "r is not a reference at all.\n";
3509 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3510 print "r is a reference to something that isa hash.\n";
3513 See also L<perlref>.
3515 =item rename OLDNAME,NEWNAME
3517 Changes the name of a file; an existing file NEWNAME will be
3518 clobbered. Returns true for success, false otherwise.
3520 Behavior of this function varies wildly depending on your system
3521 implementation. For example, it will usually not work across file system
3522 boundaries, even though the system I<mv> command sometimes compensates
3523 for this. Other restrictions include whether it works on directories,
3524 open files, or pre-existing files. Check L<perlport> and either the
3525 rename(2) manpage or equivalent system documentation for details.
3531 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3532 supplied. If EXPR is numeric, demands that the current version of Perl
3533 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3535 Otherwise, demands that a library file be included if it hasn't already
3536 been included. The file is included via the do-FILE mechanism, which is
3537 essentially just a variety of C<eval>. Has semantics similar to the following
3542 return 1 if $INC{$filename};
3543 my($realfilename,$result);
3545 foreach $prefix (@INC) {
3546 $realfilename = "$prefix/$filename";
3547 if (-f $realfilename) {
3548 $INC{$filename} = $realfilename;
3549 $result = do $realfilename;
3553 die "Can't find $filename in \@INC";
3555 delete $INC{$filename} if $@ || !$result;
3557 die "$filename did not return true value" unless $result;
3561 Note that the file will not be included twice under the same specified
3562 name. The file must return true as the last statement to indicate
3563 successful execution of any initialization code, so it's customary to
3564 end such a file with C<1;> unless you're sure it'll return true
3565 otherwise. But it's better just to put the C<1;>, in case you add more
3568 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3569 replaces "F<::>" with "F</>" in the filename for you,
3570 to make it easy to load standard modules. This form of loading of
3571 modules does not risk altering your namespace.
3573 In other words, if you try this:
3575 require Foo::Bar; # a splendid bareword
3577 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3578 directories specified in the C<@INC> array.
3580 But if you try this:
3582 $class = 'Foo::Bar';
3583 require $class; # $class is not a bareword
3585 require "Foo::Bar"; # not a bareword because of the ""
3587 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3588 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3590 eval "require $class";
3592 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3598 Generally used in a C<continue> block at the end of a loop to clear
3599 variables and reset C<??> searches so that they work again. The
3600 expression is interpreted as a list of single characters (hyphens
3601 allowed for ranges). All variables and arrays beginning with one of
3602 those letters are reset to their pristine state. If the expression is
3603 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3604 only variables or searches in the current package. Always returns
3607 reset 'X'; # reset all X variables
3608 reset 'a-z'; # reset lower case variables
3609 reset; # just reset ?one-time? searches
3611 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3612 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3613 variables--lexical variables are unaffected, but they clean themselves
3614 up on scope exit anyway, so you'll probably want to use them instead.
3621 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3622 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3623 context, depending on how the return value will be used, and the context
3624 may vary from one execution to the next (see C<wantarray>). If no EXPR
3625 is given, returns an empty list in list context, the undefined value in
3626 scalar context, and (of course) nothing at all in a void context.
3628 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3629 or do FILE will automatically return the value of the last expression
3634 In list context, returns a list value consisting of the elements
3635 of LIST in the opposite order. In scalar context, concatenates the
3636 elements of LIST and returns a string value with all characters
3637 in the opposite order.
3639 print reverse <>; # line tac, last line first
3641 undef $/; # for efficiency of <>
3642 print scalar reverse <>; # character tac, last line tsrif
3644 This operator is also handy for inverting a hash, although there are some
3645 caveats. If a value is duplicated in the original hash, only one of those
3646 can be represented as a key in the inverted hash. Also, this has to
3647 unwind one hash and build a whole new one, which may take some time
3648 on a large hash, such as from a DBM file.
3650 %by_name = reverse %by_address; # Invert the hash
3652 =item rewinddir DIRHANDLE
3654 Sets the current position to the beginning of the directory for the
3655 C<readdir> routine on DIRHANDLE.
3657 =item rindex STR,SUBSTR,POSITION
3659 =item rindex STR,SUBSTR
3661 Works just like index() except that it returns the position of the LAST
3662 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3663 last occurrence at or before that position.
3665 =item rmdir FILENAME
3669 Deletes the directory specified by FILENAME if that directory is empty. If it
3670 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3671 FILENAME is omitted, uses C<$_>.
3675 The substitution operator. See L<perlop>.
3679 Forces EXPR to be interpreted in scalar context and returns the value
3682 @counts = ( scalar @a, scalar @b, scalar @c );
3684 There is no equivalent operator to force an expression to
3685 be interpolated in list context because in practice, this is never
3686 needed. If you really wanted to do so, however, you could use
3687 the construction C<@{[ (some expression) ]}>, but usually a simple
3688 C<(some expression)> suffices.
3690 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3691 parenthesized list, this behaves as a scalar comma expression, evaluating
3692 all but the last element in void context and returning the final element
3693 evaluated in scalar context. This is seldom what you want.
3695 The following single statement:
3697 print uc(scalar(&foo,$bar)),$baz;
3699 is the moral equivalent of these two:
3702 print(uc($bar),$baz);
3704 See L<perlop> for more details on unary operators and the comma operator.
3706 =item seek FILEHANDLE,POSITION,WHENCE
3708 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3709 FILEHANDLE may be an expression whose value gives the name of the
3710 filehandle. The values for WHENCE are C<0> to set the new position to
3711 POSITION, C<1> to set it to the current position plus POSITION, and
3712 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3713 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3714 (start of the file, current position, end of the file) from any of the
3715 modules Fcntl, C<IO::Seekable>, or POSIX. Returns C<1> upon success,
3718 If you want to position file for C<sysread> or C<syswrite>, don't use
3719 C<seek>--buffering makes its effect on the file's system position
3720 unpredictable and non-portable. Use C<sysseek> instead.
3722 Due to the rules and rigors of ANSI C, on some systems you have to do a
3723 seek whenever you switch between reading and writing. Amongst other
3724 things, this may have the effect of calling stdio's clearerr(3).
3725 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3729 This is also useful for applications emulating C<tail -f>. Once you hit
3730 EOF on your read, and then sleep for a while, you might have to stick in a
3731 seek() to reset things. The C<seek> doesn't change the current position,
3732 but it I<does> clear the end-of-file condition on the handle, so that the
3733 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3735 If that doesn't work (some stdios are particularly cantankerous), then
3736 you may need something more like this:
3739 for ($curpos = tell(FILE); $_ = <FILE>;
3740 $curpos = tell(FILE)) {
3741 # search for some stuff and put it into files
3743 sleep($for_a_while);
3744 seek(FILE, $curpos, 0);
3747 =item seekdir DIRHANDLE,POS
3749 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3750 must be a value returned by C<telldir>. Has the same caveats about
3751 possible directory compaction as the corresponding system library
3754 =item select FILEHANDLE
3758 Returns the currently selected filehandle. Sets the current default
3759 filehandle for output, if FILEHANDLE is supplied. This has two
3760 effects: first, a C<write> or a C<print> without a filehandle will
3761 default to this FILEHANDLE. Second, references to variables related to
3762 output will refer to this output channel. For example, if you have to
3763 set the top of form format for more than one output channel, you might
3771 FILEHANDLE may be an expression whose value gives the name of the
3772 actual filehandle. Thus:
3774 $oldfh = select(STDERR); $| = 1; select($oldfh);
3776 Some programmers may prefer to think of filehandles as objects with
3777 methods, preferring to write the last example as:
3780 STDERR->autoflush(1);
3782 =item select RBITS,WBITS,EBITS,TIMEOUT
3784 This calls the select(2) system call with the bit masks specified, which
3785 can be constructed using C<fileno> and C<vec>, along these lines:
3787 $rin = $win = $ein = '';
3788 vec($rin,fileno(STDIN),1) = 1;
3789 vec($win,fileno(STDOUT),1) = 1;
3792 If you want to select on many filehandles you might wish to write a
3796 my(@fhlist) = split(' ',$_[0]);
3799 vec($bits,fileno($_),1) = 1;
3803 $rin = fhbits('STDIN TTY SOCK');
3807 ($nfound,$timeleft) =
3808 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3810 or to block until something becomes ready just do this
3812 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3814 Most systems do not bother to return anything useful in $timeleft, so
3815 calling select() in scalar context just returns $nfound.
3817 Any of the bit masks can also be undef. The timeout, if specified, is
3818 in seconds, which may be fractional. Note: not all implementations are
3819 capable of returning the$timeleft. If not, they always return
3820 $timeleft equal to the supplied $timeout.
3822 You can effect a sleep of 250 milliseconds this way:
3824 select(undef, undef, undef, 0.25);
3826 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3827 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3828 then only on POSIX systems. You have to use C<sysread> instead.
3830 =item semctl ID,SEMNUM,CMD,ARG
3832 Calls the System V IPC function C<semctl>. You'll probably have to say
3836 first to get the correct constant definitions. If CMD is IPC_STAT or
3837 GETALL, then ARG must be a variable which will hold the returned
3838 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3839 undefined value for error, "C<0 but true>" for zero, or the actual return
3840 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3842 =item semget KEY,NSEMS,FLAGS
3844 Calls the System V IPC function semget. Returns the semaphore id, or
3845 the undefined value if there is an error. See also C<IPC::SysV> and
3846 C<IPC::SysV::Semaphore> documentation.
3848 =item semop KEY,OPSTRING
3850 Calls the System V IPC function semop to perform semaphore operations
3851 such as signaling and waiting. OPSTRING must be a packed array of
3852 semop structures. Each semop structure can be generated with
3853 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3854 operations is implied by the length of OPSTRING. Returns true if
3855 successful, or false if there is an error. As an example, the
3856 following code waits on semaphore $semnum of semaphore id $semid:
3858 $semop = pack("sss", $semnum, -1, 0);
3859 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3861 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3862 and C<IPC::SysV::Semaphore> documentation.
3864 =item send SOCKET,MSG,FLAGS,TO
3866 =item send SOCKET,MSG,FLAGS
3868 Sends a message on a socket. Takes the same flags as the system call
3869 of the same name. On unconnected sockets you must specify a
3870 destination to send TO, in which case it does a C C<sendto>. Returns
3871 the number of characters sent, or the undefined value if there is an
3872 error. The C system call sendmsg(2) is currently unimplemented.
3873 See L<perlipc/"UDP: Message Passing"> for examples.
3875 =item setpgrp PID,PGRP
3877 Sets the current process group for the specified PID, C<0> for the current
3878 process. Will produce a fatal error if used on a machine that doesn't
3879 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3880 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3881 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3884 =item setpriority WHICH,WHO,PRIORITY
3886 Sets the current priority for a process, a process group, or a user.
3887 (See setpriority(2).) Will produce a fatal error if used on a machine
3888 that doesn't implement setpriority(2).
3890 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3892 Sets the socket option requested. Returns undefined if there is an
3893 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3900 Shifts the first value of the array off and returns it, shortening the
3901 array by 1 and moving everything down. If there are no elements in the
3902 array, returns the undefined value. If ARRAY is omitted, shifts the
3903 C<@_> array within the lexical scope of subroutines and formats, and the
3904 C<@ARGV> array at file scopes or within the lexical scopes established by
3905 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<STOP {}>, and C<END {}>
3908 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3909 same thing to the left end of an array that C<pop> and C<push> do to the
3912 =item shmctl ID,CMD,ARG
3914 Calls the System V IPC function shmctl. You'll probably have to say
3918 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3919 then ARG must be a variable which will hold the returned C<shmid_ds>
3920 structure. Returns like ioctl: the undefined value for error, "C<0> but
3921 true" for zero, or the actual return value otherwise.
3922 See also C<IPC::SysV> documentation.
3924 =item shmget KEY,SIZE,FLAGS
3926 Calls the System V IPC function shmget. Returns the shared memory
3927 segment id, or the undefined value if there is an error.
3928 See also C<IPC::SysV> documentation.
3930 =item shmread ID,VAR,POS,SIZE
3932 =item shmwrite ID,STRING,POS,SIZE
3934 Reads or writes the System V shared memory segment ID starting at
3935 position POS for size SIZE by attaching to it, copying in/out, and
3936 detaching from it. When reading, VAR must be a variable that will
3937 hold the data read. When writing, if STRING is too long, only SIZE
3938 bytes are used; if STRING is too short, nulls are written to fill out
3939 SIZE bytes. Return true if successful, or false if there is an error.
3940 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3943 =item shutdown SOCKET,HOW
3945 Shuts down a socket connection in the manner indicated by HOW, which
3946 has the same interpretation as in the system call of the same name.
3948 shutdown(SOCKET, 0); # I/we have stopped reading data
3949 shutdown(SOCKET, 1); # I/we have stopped writing data
3950 shutdown(SOCKET, 2); # I/we have stopped using this socket
3952 This is useful with sockets when you want to tell the other
3953 side you're done writing but not done reading, or vice versa.
3954 It's also a more insistent form of close because it also
3955 disables the file descriptor in any forked copies in other
3962 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3963 returns sine of C<$_>.
3965 For the inverse sine operation, you may use the C<POSIX::asin>
3966 function, or use this relation:
3968 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3974 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3975 May be interrupted if the process receives a signal such as C<SIGALRM>.
3976 Returns the number of seconds actually slept. You probably cannot
3977 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3980 On some older systems, it may sleep up to a full second less than what
3981 you requested, depending on how it counts seconds. Most modern systems
3982 always sleep the full amount. They may appear to sleep longer than that,
3983 however, because your process might not be scheduled right away in a
3984 busy multitasking system.
3986 For delays of finer granularity than one second, you may use Perl's
3987 C<syscall> interface to access setitimer(2) if your system supports
3988 it, or else see L</select> above. The Time::HiRes module from CPAN
3991 See also the POSIX module's C<sigpause> function.
3993 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3995 Opens a socket of the specified kind and attaches it to filehandle
3996 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3997 the system call of the same name. You should C<use Socket> first
3998 to get the proper definitions imported. See the examples in
3999 L<perlipc/"Sockets: Client/Server Communication">.
4001 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4003 Creates an unnamed pair of sockets in the specified domain, of the
4004 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4005 for the system call of the same name. If unimplemented, yields a fatal
4006 error. Returns true if successful.
4008 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4009 to C<pipe(Rdr, Wtr)> is essentially:
4012 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4013 shutdown(Rdr, 1); # no more writing for reader
4014 shutdown(Wtr, 0); # no more reading for writer
4016 See L<perlipc> for an example of socketpair use.
4018 =item sort SUBNAME LIST
4020 =item sort BLOCK LIST
4024 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4025 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4026 specified, it gives the name of a subroutine that returns an integer
4027 less than, equal to, or greater than C<0>, depending on how the elements
4028 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
4029 operators are extremely useful in such routines.) SUBNAME may be a
4030 scalar variable name (unsubscripted), in which case the value provides
4031 the name of (or a reference to) the actual subroutine to use. In place
4032 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4035 If the subroutine's prototype is C<($$)>, the elements to be compared
4036 are passed by reference in C<@_>, as for a normal subroutine. If not,
4037 the normal calling code for subroutines is bypassed in the interests of
4038 efficiency, and the elements to be compared are passed into the subroutine
4039 as the package global variables $a and $b (see example below). Note that
4040 in the latter case, it is usually counter-productive to declare $a and
4043 In either case, the subroutine may not be recursive. The values to be
4044 compared are always passed by reference, so don't modify them.
4046 You also cannot exit out of the sort block or subroutine using any of the
4047 loop control operators described in L<perlsyn> or with C<goto>.
4049 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4050 current collation locale. See L<perllocale>.
4055 @articles = sort @files;
4057 # same thing, but with explicit sort routine
4058 @articles = sort {$a cmp $b} @files;
4060 # now case-insensitively
4061 @articles = sort {uc($a) cmp uc($b)} @files;
4063 # same thing in reversed order
4064 @articles = sort {$b cmp $a} @files;
4066 # sort numerically ascending
4067 @articles = sort {$a <=> $b} @files;
4069 # sort numerically descending
4070 @articles = sort {$b <=> $a} @files;
4072 # this sorts the %age hash by value instead of key
4073 # using an in-line function
4074 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4076 # sort using explicit subroutine name
4078 $age{$a} <=> $age{$b}; # presuming numeric
4080 @sortedclass = sort byage @class;
4082 sub backwards { $b cmp $a }
4083 @harry = qw(dog cat x Cain Abel);
4084 @george = qw(gone chased yz Punished Axed);
4086 # prints AbelCaincatdogx
4087 print sort backwards @harry;
4088 # prints xdogcatCainAbel
4089 print sort @george, 'to', @harry;
4090 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4092 # inefficiently sort by descending numeric compare using
4093 # the first integer after the first = sign, or the
4094 # whole record case-insensitively otherwise
4097 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4102 # same thing, but much more efficiently;
4103 # we'll build auxiliary indices instead
4107 push @nums, /=(\d+)/;
4112 $nums[$b] <=> $nums[$a]
4114 $caps[$a] cmp $caps[$b]
4118 # same thing, but without any temps
4119 @new = map { $_->[0] }
4120 sort { $b->[1] <=> $a->[1]
4123 } map { [$_, /=(\d+)/, uc($_)] } @old;
4125 # using a prototype allows you to use any comparison subroutine
4126 # as a sort subroutine (including other package's subroutines)
4128 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4131 @new = sort other::backwards @old;
4133 If you're using strict, you I<must not> declare $a
4134 and $b as lexicals. They are package globals. That means
4135 if you're in the C<main> package, it's
4137 @articles = sort {$main::b <=> $main::a} @files;
4141 @articles = sort {$::b <=> $::a} @files;
4143 but if you're in the C<FooPack> package, it's
4145 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4147 The comparison function is required to behave. If it returns
4148 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4149 sometimes saying the opposite, for example) the results are not
4152 =item splice ARRAY,OFFSET,LENGTH,LIST
4154 =item splice ARRAY,OFFSET,LENGTH
4156 =item splice ARRAY,OFFSET
4158 Removes the elements designated by OFFSET and LENGTH from an array, and
4159 replaces them with the elements of LIST, if any. In list context,
4160 returns the elements removed from the array. In scalar context,
4161 returns the last element removed, or C<undef> if no elements are
4162 removed. The array grows or shrinks as necessary.
4163 If OFFSET is negative then it starts that far from the end of the array.
4164 If LENGTH is omitted, removes everything from OFFSET onward.
4165 If LENGTH is negative, leave that many elements off the end of the array.
4166 The following equivalences hold (assuming C<$[ == 0>):
4168 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4169 pop(@a) splice(@a,-1)
4170 shift(@a) splice(@a,0,1)
4171 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4172 $a[$x] = $y splice(@a,$x,1,$y)
4174 Example, assuming array lengths are passed before arrays:
4176 sub aeq { # compare two list values
4177 my(@a) = splice(@_,0,shift);
4178 my(@b) = splice(@_,0,shift);
4179 return 0 unless @a == @b; # same len?
4181 return 0 if pop(@a) ne pop(@b);
4185 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4187 =item split /PATTERN/,EXPR,LIMIT
4189 =item split /PATTERN/,EXPR
4191 =item split /PATTERN/
4195 Splits a string into a list of strings and returns that list. By default,
4196 empty leading fields are preserved, and empty trailing ones are deleted.
4198 If not in list context, returns the number of fields found and splits into
4199 the C<@_> array. (In list context, you can force the split into C<@_> by
4200 using C<??> as the pattern delimiters, but it still returns the list
4201 value.) The use of implicit split to C<@_> is deprecated, however, because
4202 it clobbers your subroutine arguments.
4204 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4205 splits on whitespace (after skipping any leading whitespace). Anything
4206 matching PATTERN is taken to be a delimiter separating the fields. (Note
4207 that the delimiter may be longer than one character.)
4209 If LIMIT is specified and positive, splits into no more than that
4210 many fields (though it may split into fewer). If LIMIT is unspecified
4211 or zero, trailing null fields are stripped (which potential users
4212 of C<pop> would do well to remember). If LIMIT is negative, it is
4213 treated as if an arbitrarily large LIMIT had been specified.
4215 A pattern matching the null string (not to be confused with
4216 a null pattern C<//>, which is just one member of the set of patterns
4217 matching a null string) will split the value of EXPR into separate
4218 characters at each point it matches that way. For example:
4220 print join(':', split(/ */, 'hi there'));
4222 produces the output 'h:i:t:h:e:r:e'.
4224 The LIMIT parameter can be used to split a line partially
4226 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4228 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4229 one larger than the number of variables in the list, to avoid
4230 unnecessary work. For the list above LIMIT would have been 4 by
4231 default. In time critical applications it behooves you not to split
4232 into more fields than you really need.
4234 If the PATTERN contains parentheses, additional list elements are
4235 created from each matching substring in the delimiter.
4237 split(/([,-])/, "1-10,20", 3);
4239 produces the list value
4241 (1, '-', 10, ',', 20)
4243 If you had the entire header of a normal Unix email message in $header,
4244 you could split it up into fields and their values this way:
4246 $header =~ s/\n\s+/ /g; # fix continuation lines
4247 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4249 The pattern C</PATTERN/> may be replaced with an expression to specify
4250 patterns that vary at runtime. (To do runtime compilation only once,
4251 use C</$variable/o>.)
4253 As a special case, specifying a PATTERN of space (C<' '>) will split on
4254 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4255 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4256 will give you as many null initial fields as there are leading spaces.
4257 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4258 whitespace produces a null first field. A C<split> with no arguments
4259 really does a C<split(' ', $_)> internally.
4263 open(PASSWD, '/etc/passwd');
4265 ($login, $passwd, $uid, $gid,
4266 $gcos, $home, $shell) = split(/:/);
4270 (Note that $shell above will still have a newline on it. See L</chop>,
4271 L</chomp>, and L</join>.)
4273 =item sprintf FORMAT, LIST
4275 Returns a string formatted by the usual C<printf> conventions of the
4276 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4277 on your system for an explanation of the general principles.
4279 Perl does its own C<sprintf> formatting--it emulates the C
4280 function C<sprintf>, but it doesn't use it (except for floating-point
4281 numbers, and even then only the standard modifiers are allowed). As a
4282 result, any non-standard extensions in your local C<sprintf> are not
4283 available from Perl.
4285 Perl's C<sprintf> permits the following universally-known conversions:
4288 %c a character with the given number
4290 %d a signed integer, in decimal
4291 %u an unsigned integer, in decimal
4292 %o an unsigned integer, in octal
4293 %x an unsigned integer, in hexadecimal
4294 %e a floating-point number, in scientific notation
4295 %f a floating-point number, in fixed decimal notation
4296 %g a floating-point number, in %e or %f notation
4298 In addition, Perl permits the following widely-supported conversions:
4300 %X like %x, but using upper-case letters
4301 %E like %e, but using an upper-case "E"
4302 %G like %g, but with an upper-case "E" (if applicable)
4303 %b an unsigned integer, in binary
4304 %p a pointer (outputs the Perl value's address in hexadecimal)
4305 %n special: *stores* the number of characters output so far
4306 into the next variable in the parameter list
4308 Finally, for backward (and we do mean "backward") compatibility, Perl
4309 permits these unnecessary but widely-supported conversions:
4312 %D a synonym for %ld
4313 %U a synonym for %lu
4314 %O a synonym for %lo
4317 Perl permits the following universally-known flags between the C<%>
4318 and the conversion letter:
4320 space prefix positive number with a space
4321 + prefix positive number with a plus sign
4322 - left-justify within the field
4323 0 use zeros, not spaces, to right-justify
4324 # prefix non-zero octal with "0", non-zero hex with "0x"
4325 number minimum field width
4326 .number "precision": digits after decimal point for
4327 floating-point, max length for string, minimum length
4329 l interpret integer as C type "long" or "unsigned long"
4330 h interpret integer as C type "short" or "unsigned short"
4331 If no flags, interpret integer as C type "int" or "unsigned"
4333 There is also one Perl-specific flag:
4335 V interpret integer as Perl's standard integer type
4337 Where a number would appear in the flags, an asterisk (C<*>) may be
4338 used instead, in which case Perl uses the next item in the parameter
4339 list as the given number (that is, as the field width or precision).
4340 If a field width obtained through C<*> is negative, it has the same
4341 effect as the C<-> flag: left-justification.
4343 If C<use locale> is in effect, the character used for the decimal
4344 point in formatted real numbers is affected by the LC_NUMERIC locale.
4347 If Perl understands "quads" (64-bit integers) (this requires
4348 either that the platform natively supports quads or that Perl
4349 has been specifically compiled to support quads), the characters
4353 print quads, and they may optionally be preceded by
4361 You can find out whether your Perl supports quads via L<Config>:
4364 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4367 If Perl understands "long doubles" (this requires that the platform
4368 supports long doubles), the flags
4372 may optionally be preceded by
4380 You can find out whether your Perl supports long doubles via L<Config>:
4383 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4389 Return the square root of EXPR. If EXPR is omitted, returns square
4390 root of C<$_>. Only works on non-negative operands, unless you've
4391 loaded the standard Math::Complex module.
4394 print sqrt(-2); # prints 1.4142135623731i
4400 Sets the random number seed for the C<rand> operator. If EXPR is
4401 omitted, uses a semi-random value supplied by the kernel (if it supports
4402 the F</dev/urandom> device) or based on the current time and process
4403 ID, among other things. In versions of Perl prior to 5.004 the default
4404 seed was just the current C<time>. This isn't a particularly good seed,
4405 so many old programs supply their own seed value (often C<time ^ $$> or
4406 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4408 In fact, it's usually not necessary to call C<srand> at all, because if
4409 it is not called explicitly, it is called implicitly at the first use of
4410 the C<rand> operator. However, this was not the case in version of Perl
4411 before 5.004, so if your script will run under older Perl versions, it
4412 should call C<srand>.
4414 Note that you need something much more random than the default seed for
4415 cryptographic purposes. Checksumming the compressed output of one or more
4416 rapidly changing operating system status programs is the usual method. For
4419 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4421 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4424 Do I<not> call C<srand> multiple times in your program unless you know
4425 exactly what you're doing and why you're doing it. The point of the
4426 function is to "seed" the C<rand> function so that C<rand> can produce
4427 a different sequence each time you run your program. Just do it once at the
4428 top of your program, or you I<won't> get random numbers out of C<rand>!
4430 Frequently called programs (like CGI scripts) that simply use
4434 for a seed can fall prey to the mathematical property that
4438 one-third of the time. So don't do that.
4440 =item stat FILEHANDLE
4446 Returns a 13-element list giving the status info for a file, either
4447 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4448 it stats C<$_>. Returns a null list if the stat fails. Typically used
4451 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4452 $atime,$mtime,$ctime,$blksize,$blocks)
4455 Not all fields are supported on all filesystem types. Here are the
4456 meaning of the fields:
4458 0 dev device number of filesystem
4460 2 mode file mode (type and permissions)
4461 3 nlink number of (hard) links to the file
4462 4 uid numeric user ID of file's owner
4463 5 gid numeric group ID of file's owner
4464 6 rdev the device identifier (special files only)
4465 7 size total size of file, in bytes
4466 8 atime last access time in seconds since the epoch
4467 9 mtime last modify time in seconds since the epoch
4468 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4469 11 blksize preferred block size for file system I/O
4470 12 blocks actual number of blocks allocated
4472 (The epoch was at 00:00 January 1, 1970 GMT.)
4474 If stat is passed the special filehandle consisting of an underline, no
4475 stat is done, but the current contents of the stat structure from the
4476 last stat or filetest are returned. Example:
4478 if (-x $file && (($d) = stat(_)) && $d < 0) {
4479 print "$file is executable NFS file\n";
4482 (This works on machines only for which the device number is negative under NFS.)
4484 Because the mode contains both the file type and its permissions, you
4485 should mask off the file type portion and (s)printf using a C<"%o">
4486 if you want to see the real permissions.
4488 $mode = (stat($filename))[2];
4489 printf "Permissions are %04o\n", $mode & 07777;
4491 In scalar context, C<stat> returns a boolean value indicating success
4492 or failure, and, if successful, sets the information associated with
4493 the special filehandle C<_>.
4495 The File::stat module provides a convenient, by-name access mechanism:
4498 $sb = stat($filename);
4499 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4500 $filename, $sb->size, $sb->mode & 07777,
4501 scalar localtime $sb->mtime;
4507 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4508 doing many pattern matches on the string before it is next modified.
4509 This may or may not save time, depending on the nature and number of
4510 patterns you are searching on, and on the distribution of character
4511 frequencies in the string to be searched--you probably want to compare
4512 run times with and without it to see which runs faster. Those loops
4513 which scan for many short constant strings (including the constant
4514 parts of more complex patterns) will benefit most. You may have only
4515 one C<study> active at a time--if you study a different scalar the first
4516 is "unstudied". (The way C<study> works is this: a linked list of every
4517 character in the string to be searched is made, so we know, for
4518 example, where all the C<'k'> characters are. From each search string,
4519 the rarest character is selected, based on some static frequency tables
4520 constructed from some C programs and English text. Only those places
4521 that contain this "rarest" character are examined.)
4523 For example, here is a loop that inserts index producing entries
4524 before any line containing a certain pattern:
4528 print ".IX foo\n" if /\bfoo\b/;
4529 print ".IX bar\n" if /\bbar\b/;
4530 print ".IX blurfl\n" if /\bblurfl\b/;
4535 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4536 will be looked at, because C<f> is rarer than C<o>. In general, this is
4537 a big win except in pathological cases. The only question is whether
4538 it saves you more time than it took to build the linked list in the
4541 Note that if you have to look for strings that you don't know till
4542 runtime, you can build an entire loop as a string and C<eval> that to
4543 avoid recompiling all your patterns all the time. Together with
4544 undefining C<$/> to input entire files as one record, this can be very
4545 fast, often faster than specialized programs like fgrep(1). The following
4546 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4547 out the names of those files that contain a match:
4549 $search = 'while (<>) { study;';
4550 foreach $word (@words) {
4551 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4556 eval $search; # this screams
4557 $/ = "\n"; # put back to normal input delimiter
4558 foreach $file (sort keys(%seen)) {
4566 =item sub NAME BLOCK
4568 This is subroutine definition, not a real function I<per se>. With just a
4569 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4570 Without a NAME, it's an anonymous function declaration, and does actually
4571 return a value: the CODE ref of the closure you just created. See L<perlsub>
4572 and L<perlref> for details.
4574 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4576 =item substr EXPR,OFFSET,LENGTH
4578 =item substr EXPR,OFFSET
4580 Extracts a substring out of EXPR and returns it. First character is at
4581 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4582 If OFFSET is negative (or more precisely, less than C<$[>), starts
4583 that far from the end of the string. If LENGTH is omitted, returns
4584 everything to the end of the string. If LENGTH is negative, leaves that
4585 many characters off the end of the string.
4587 You can use the substr() function as an lvalue, in which case EXPR
4588 must itself be an lvalue. If you assign something shorter than LENGTH,
4589 the string will shrink, and if you assign something longer than LENGTH,
4590 the string will grow to accommodate it. To keep the string the same
4591 length you may need to pad or chop your value using C<sprintf>.
4593 If OFFSET and LENGTH specify a substring that is partly outside the
4594 string, only the part within the string is returned. If the substring
4595 is beyond either end of the string, substr() returns the undefined
4596 value and produces a warning. When used as an lvalue, specifying a
4597 substring that is entirely outside the string is a fatal error.
4598 Here's an example showing the behavior for boundary cases:
4601 substr($name, 4) = 'dy'; # $name is now 'freddy'
4602 my $null = substr $name, 6, 2; # returns '' (no warning)
4603 my $oops = substr $name, 7; # returns undef, with warning
4604 substr($name, 7) = 'gap'; # fatal error
4606 An alternative to using substr() as an lvalue is to specify the
4607 replacement string as the 4th argument. This allows you to replace
4608 parts of the EXPR and return what was there before in one operation,
4609 just as you can with splice().
4611 =item symlink OLDFILE,NEWFILE
4613 Creates a new filename symbolically linked to the old filename.
4614 Returns C<1> for success, C<0> otherwise. On systems that don't support
4615 symbolic links, produces a fatal error at run time. To check for that,
4618 $symlink_exists = eval { symlink("",""); 1 };
4622 Calls the system call specified as the first element of the list,
4623 passing the remaining elements as arguments to the system call. If
4624 unimplemented, produces a fatal error. The arguments are interpreted
4625 as follows: if a given argument is numeric, the argument is passed as
4626 an int. If not, the pointer to the string value is passed. You are
4627 responsible to make sure a string is pre-extended long enough to
4628 receive any result that might be written into a string. You can't use a
4629 string literal (or other read-only string) as an argument to C<syscall>
4630 because Perl has to assume that any string pointer might be written
4632 integer arguments are not literals and have never been interpreted in a
4633 numeric context, you may need to add C<0> to them to force them to look
4634 like numbers. This emulates the C<syswrite> function (or vice versa):
4636 require 'syscall.ph'; # may need to run h2ph
4638 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4640 Note that Perl supports passing of up to only 14 arguments to your system call,
4641 which in practice should usually suffice.
4643 Syscall returns whatever value returned by the system call it calls.
4644 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4645 Note that some system calls can legitimately return C<-1>. The proper
4646 way to handle such calls is to assign C<$!=0;> before the call and
4647 check the value of C<$!> if syscall returns C<-1>.
4649 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4650 number of the read end of the pipe it creates. There is no way
4651 to retrieve the file number of the other end. You can avoid this
4652 problem by using C<pipe> instead.
4654 =item sysopen FILEHANDLE,FILENAME,MODE
4656 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4658 Opens the file whose filename is given by FILENAME, and associates it
4659 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4660 the name of the real filehandle wanted. This function calls the
4661 underlying operating system's C<open> function with the parameters
4662 FILENAME, MODE, PERMS.
4664 The possible values and flag bits of the MODE parameter are
4665 system-dependent; they are available via the standard module C<Fcntl>.
4666 See the documentation of your operating system's C<open> to see which
4667 values and flag bits are available. You may combine several flags
4668 using the C<|>-operator.
4670 Some of the most common values are C<O_RDONLY> for opening the file in
4671 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4672 and C<O_RDWR> for opening the file in read-write mode, and.
4674 For historical reasons, some values work on almost every system
4675 supported by perl: zero means read-only, one means write-only, and two
4676 means read/write. We know that these values do I<not> work under
4677 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4678 use them in new code.
4680 If the file named by FILENAME does not exist and the C<open> call creates
4681 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4682 PERMS specifies the permissions of the newly created file. If you omit
4683 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4684 These permission values need to be in octal, and are modified by your
4685 process's current C<umask>.
4687 In many systems the C<O_EXCL> flag is available for opening files in
4688 exclusive mode. This is B<not> locking: exclusiveness means here that
4689 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4692 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4694 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4695 that takes away the user's option to have a more permissive umask.
4696 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4699 Note that C<sysopen> depends on the fdopen() C library function.
4700 On many UNIX systems, fdopen() is known to fail when file descriptors
4701 exceed a certain value, typically 255. If you need more file
4702 descriptors than that, consider rebuilding Perl to use the C<sfio>
4703 library, or perhaps using the POSIX::open() function.
4705 See L<perlopentut> for a kinder, gentler explanation of opening files.
4707 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4709 =item sysread FILEHANDLE,SCALAR,LENGTH
4711 Attempts to read LENGTH bytes of data into variable SCALAR from the
4712 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4713 so mixing this with other kinds of reads, C<print>, C<write>,
4714 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4715 usually buffers data. Returns the number of bytes actually read, C<0>
4716 at end of file, or undef if there was an error. SCALAR will be grown or
4717 shrunk so that the last byte actually read is the last byte of the
4718 scalar after the read.
4720 An OFFSET may be specified to place the read data at some place in the
4721 string other than the beginning. A negative OFFSET specifies
4722 placement at that many bytes counting backwards from the end of the
4723 string. A positive OFFSET greater than the length of SCALAR results
4724 in the string being padded to the required size with C<"\0"> bytes before
4725 the result of the read is appended.
4727 There is no syseof() function, which is ok, since eof() doesn't work
4728 very well on device files (like ttys) anyway. Use sysread() and check
4729 for a return value for 0 to decide whether you're done.
4731 =item sysseek FILEHANDLE,POSITION,WHENCE
4733 Sets FILEHANDLE's system position using the system call lseek(2). It
4734 bypasses stdio, so mixing this with reads (other than C<sysread>),
4735 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4736 FILEHANDLE may be an expression whose value gives the name of the
4737 filehandle. The values for WHENCE are C<0> to set the new position to
4738 POSITION, C<1> to set the it to the current position plus POSITION,
4739 and C<2> to set it to EOF plus POSITION (typically negative). For
4740 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4741 C<SEEK_END> (start of the file, current position, end of the file)
4742 from any of the modules Fcntl, C<IO::Seekable>, or POSIX.
4744 Returns the new position, or the undefined value on failure. A position
4745 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4746 true on success and false on failure, yet you can still easily determine
4751 =item system PROGRAM LIST
4753 Does exactly the same thing as C<exec LIST>, except that a fork is
4754 done first, and the parent process waits for the child process to
4755 complete. Note that argument processing varies depending on the
4756 number of arguments. If there is more than one argument in LIST,
4757 or if LIST is an array with more than one value, starts the program
4758 given by the first element of the list with arguments given by the
4759 rest of the list. If there is only one scalar argument, the argument
4760 is checked for shell metacharacters, and if there are any, the
4761 entire argument is passed to the system's command shell for parsing
4762 (this is C</bin/sh -c> on Unix platforms, but varies on other
4763 platforms). If there are no shell metacharacters in the argument,
4764 it is split into words and passed directly to C<execvp>, which is
4767 All files opened for output are flushed before attempting the exec().
4769 The return value is the exit status of the program as
4770 returned by the C<wait> call. To get the actual exit value divide by
4771 256. See also L</exec>. This is I<not> what you want to use to capture
4772 the output from a command, for that you should use merely backticks or
4773 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4774 indicates a failure to start the program (inspect $! for the reason).
4776 Like C<exec>, C<system> allows you to lie to a program about its name if
4777 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4779 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4780 program they're running doesn't actually interrupt your program.
4782 @args = ("command", "arg1", "arg2");
4784 or die "system @args failed: $?"
4786 You can check all the failure possibilities by inspecting
4789 $exit_value = $? >> 8;
4790 $signal_num = $? & 127;
4791 $dumped_core = $? & 128;
4793 When the arguments get executed via the system shell, results
4794 and return codes will be subject to its quirks and capabilities.
4795 See L<perlop/"`STRING`"> and L</exec> for details.
4797 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4799 =item syswrite FILEHANDLE,SCALAR,LENGTH
4801 =item syswrite FILEHANDLE,SCALAR
4803 Attempts to write LENGTH bytes of data from variable SCALAR to the
4804 specified FILEHANDLE, using the system call write(2). If LENGTH
4805 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4806 this with reads (other than C<sysread())>, C<print>, C<write>,
4807 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4808 usually buffers data. Returns the number of bytes actually written,
4809 or C<undef> if there was an error. If the LENGTH is greater than
4810 the available data in the SCALAR after the OFFSET, only as much
4811 data as is available will be written.
4813 An OFFSET may be specified to write the data from some part of the
4814 string other than the beginning. A negative OFFSET specifies writing
4815 that many bytes counting backwards from the end of the string. In the
4816 case the SCALAR is empty you can use OFFSET but only zero offset.
4818 =item tell FILEHANDLE
4822 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4823 expression whose value gives the name of the actual filehandle. If
4824 FILEHANDLE is omitted, assumes the file last read.
4826 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4828 =item telldir DIRHANDLE
4830 Returns the current position of the C<readdir> routines on DIRHANDLE.
4831 Value may be given to C<seekdir> to access a particular location in a
4832 directory. Has the same caveats about possible directory compaction as
4833 the corresponding system library routine.
4835 =item tie VARIABLE,CLASSNAME,LIST
4837 This function binds a variable to a package class that will provide the
4838 implementation for the variable. VARIABLE is the name of the variable
4839 to be enchanted. CLASSNAME is the name of a class implementing objects
4840 of correct type. Any additional arguments are passed to the C<new>
4841 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4842 or C<TIEHASH>). Typically these are arguments such as might be passed
4843 to the C<dbm_open()> function of C. The object returned by the C<new>
4844 method is also returned by the C<tie> function, which would be useful
4845 if you want to access other methods in CLASSNAME.
4847 Note that functions such as C<keys> and C<values> may return huge lists
4848 when used on large objects, like DBM files. You may prefer to use the
4849 C<each> function to iterate over such. Example:
4851 # print out history file offsets
4853 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4854 while (($key,$val) = each %HIST) {
4855 print $key, ' = ', unpack('L',$val), "\n";
4859 A class implementing a hash should have the following methods:
4861 TIEHASH classname, LIST
4863 STORE this, key, value
4868 NEXTKEY this, lastkey
4871 A class implementing an ordinary array should have the following methods:
4873 TIEARRAY classname, LIST
4875 STORE this, key, value
4877 STORESIZE this, count
4883 SPLICE this, offset, length, LIST
4887 A class implementing a file handle should have the following methods:
4889 TIEHANDLE classname, LIST
4890 READ this, scalar, length, offset
4893 WRITE this, scalar, length, offset
4895 PRINTF this, format, LIST
4899 A class implementing a scalar should have the following methods:
4901 TIESCALAR classname, LIST
4906 Not all methods indicated above need be implemented. See L<perltie>,
4907 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4909 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4910 for you--you need to do that explicitly yourself. See L<DB_File>
4911 or the F<Config> module for interesting C<tie> implementations.
4913 For further details see L<perltie>, L<"tied VARIABLE">.
4917 Returns a reference to the object underlying VARIABLE (the same value
4918 that was originally returned by the C<tie> call that bound the variable
4919 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4924 Returns the number of non-leap seconds since whatever time the system
4925 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4926 and 00:00:00 UTC, January 1, 1970 for most other systems).
4927 Suitable for feeding to C<gmtime> and C<localtime>.
4929 For measuring time in better granularity than one second,
4930 you may use either the Time::HiRes module from CPAN, or
4931 if you have gettimeofday(2), you may be able to use the
4932 C<syscall> interface of Perl, see L<perlfaq8> for details.
4936 Returns a four-element list giving the user and system times, in
4937 seconds, for this process and the children of this process.
4939 ($user,$system,$cuser,$csystem) = times;
4943 The transliteration operator. Same as C<y///>. See L<perlop>.
4945 =item truncate FILEHANDLE,LENGTH
4947 =item truncate EXPR,LENGTH
4949 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4950 specified length. Produces a fatal error if truncate isn't implemented
4951 on your system. Returns true if successful, the undefined value
4958 Returns an uppercased version of EXPR. This is the internal function
4959 implementing the C<\U> escape in double-quoted strings.
4960 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4961 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4962 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4964 If EXPR is omitted, uses C<$_>.
4970 Returns the value of EXPR with the first character
4971 in uppercase (titlecase in Unicode). This is
4972 the internal function implementing the C<\u> escape in double-quoted strings.
4973 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4976 If EXPR is omitted, uses C<$_>.
4982 Sets the umask for the process to EXPR and returns the previous value.
4983 If EXPR is omitted, merely returns the current umask.
4985 The Unix permission C<rwxr-x---> is represented as three sets of three
4986 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4987 and isn't one of the digits). The C<umask> value is such a number
4988 representing disabled permissions bits. The permission (or "mode")
4989 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4990 even if you tell C<sysopen> to create a file with permissions C<0777>,
4991 if your umask is C<0022> then the file will actually be created with
4992 permissions C<0755>. If your C<umask> were C<0027> (group can't
4993 write; others can't read, write, or execute), then passing
4994 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4997 Here's some advice: supply a creation mode of C<0666> for regular
4998 files (in C<sysopen>) and one of C<0777> for directories (in
4999 C<mkdir>) and executable files. This gives users the freedom of
5000 choice: if they want protected files, they might choose process umasks
5001 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5002 Programs should rarely if ever make policy decisions better left to
5003 the user. The exception to this is when writing files that should be
5004 kept private: mail files, web browser cookies, I<.rhosts> files, and
5007 If umask(2) is not implemented on your system and you are trying to
5008 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5009 fatal error at run time. If umask(2) is not implemented and you are
5010 not trying to restrict access for yourself, returns C<undef>.
5012 Remember that a umask is a number, usually given in octal; it is I<not> a
5013 string of octal digits. See also L</oct>, if all you have is a string.
5019 Undefines the value of EXPR, which must be an lvalue. Use only on a
5020 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5021 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5022 will probably not do what you expect on most predefined variables or
5023 DBM list values, so don't do that; see L<delete>.) Always returns the
5024 undefined value. You can omit the EXPR, in which case nothing is
5025 undefined, but you still get an undefined value that you could, for
5026 instance, return from a subroutine, assign to a variable or pass as a
5027 parameter. Examples:
5030 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5034 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5035 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5036 select undef, undef, undef, 0.25;
5037 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5039 Note that this is a unary operator, not a list operator.
5045 Deletes a list of files. Returns the number of files successfully
5048 $cnt = unlink 'a', 'b', 'c';
5052 Note: C<unlink> will not delete directories unless you are superuser and
5053 the B<-U> flag is supplied to Perl. Even if these conditions are
5054 met, be warned that unlinking a directory can inflict damage on your
5055 filesystem. Use C<rmdir> instead.
5057 If LIST is omitted, uses C<$_>.
5059 =item unpack TEMPLATE,EXPR
5061 C<unpack> does the reverse of C<pack>: it takes a string
5062 and expands it out into a list of values.
5063 (In scalar context, it returns merely the first value produced.)
5065 The string is broken into chunks described by the TEMPLATE. Each chunk
5066 is converted separately to a value. Typically, either the string is a result
5067 of C<pack>, or the bytes of the string represent a C structure of some
5070 The TEMPLATE has the same format as in the C<pack> function.
5071 Here's a subroutine that does substring:
5074 my($what,$where,$howmuch) = @_;
5075 unpack("x$where a$howmuch", $what);
5080 sub ordinal { unpack("c",$_[0]); } # same as ord()
5082 In addition to fields allowed in pack(), you may prefix a field with
5083 a %E<lt>numberE<gt> to indicate that
5084 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
5085 themselves. Default is a 16-bit checksum. Checksum is calculated by
5086 summing numeric values of expanded values (for string fields the sum of
5087 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5089 For example, the following
5090 computes the same number as the System V sum program:
5094 unpack("%32C*",<>) % 65535;
5097 The following efficiently counts the number of set bits in a bit vector:
5099 $setbits = unpack("%32b*", $selectmask);
5101 The C<p> and C<P> formats should be used with care. Since Perl
5102 has no way of checking whether the value passed to C<unpack()>
5103 corresponds to a valid memory location, passing a pointer value that's
5104 not known to be valid is likely to have disastrous consequences.
5106 If the repeat count of a field is larger than what the remainder of
5107 the input string allows, repeat count is decreased. If the input string
5108 is longer than one described by the TEMPLATE, the rest is ignored.
5110 See L</pack> for more examples and notes.
5112 =item untie VARIABLE
5114 Breaks the binding between a variable and a package. (See C<tie>.)
5116 =item unshift ARRAY,LIST
5118 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5119 depending on how you look at it. Prepends list to the front of the
5120 array, and returns the new number of elements in the array.
5122 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5124 Note the LIST is prepended whole, not one element at a time, so the
5125 prepended elements stay in the same order. Use C<reverse> to do the
5128 =item use Module LIST
5132 =item use Module VERSION LIST
5136 Imports some semantics into the current package from the named module,
5137 generally by aliasing certain subroutine or variable names into your
5138 package. It is exactly equivalent to
5140 BEGIN { require Module; import Module LIST; }
5142 except that Module I<must> be a bareword.
5144 If the first argument to C<use> is a number, it is treated as a version
5145 number instead of a module name. If the version of the Perl interpreter
5146 is less than VERSION, then an error message is printed and Perl exits
5147 immediately. This is often useful if you need to check the current
5148 Perl version before C<use>ing library modules that have changed in
5149 incompatible ways from older versions of Perl. (We try not to do
5150 this more than we have to.)
5152 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5153 C<require> makes sure the module is loaded into memory if it hasn't been
5154 yet. The C<import> is not a builtin--it's just an ordinary static method
5155 call into the C<Module> package to tell the module to import the list of
5156 features back into the current package. The module can implement its
5157 C<import> method any way it likes, though most modules just choose to
5158 derive their C<import> method via inheritance from the C<Exporter> class that
5159 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5160 method can be found then the error is currently silently ignored. This
5161 may change to a fatal error in a future version.
5163 If you don't want your namespace altered, explicitly supply an empty list:
5167 That is exactly equivalent to
5169 BEGIN { require Module }
5171 If the VERSION argument is present between Module and LIST, then the
5172 C<use> will call the VERSION method in class Module with the given
5173 version as an argument. The default VERSION method, inherited from
5174 the Universal class, croaks if the given version is larger than the
5175 value of the variable C<$Module::VERSION>. (Note that there is not a
5176 comma after VERSION!)
5178 Because this is a wide-open interface, pragmas (compiler directives)
5179 are also implemented this way. Currently implemented pragmas are:
5183 use sigtrap qw(SEGV BUS);
5184 use strict qw(subs vars refs);
5185 use subs qw(afunc blurfl);
5186 use warnings qw(all);
5188 Some of these pseudo-modules import semantics into the current
5189 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5190 which import symbols into the current package (which are effective
5191 through the end of the file).
5193 There's a corresponding C<no> command that unimports meanings imported
5194 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5200 If no C<unimport> method can be found the call fails with a fatal error.
5202 See L<perlmod> for a list of standard modules and pragmas.
5206 Changes the access and modification times on each file of a list of
5207 files. The first two elements of the list must be the NUMERICAL access
5208 and modification times, in that order. Returns the number of files
5209 successfully changed. The inode change time of each file is set
5210 to the current time. This code has the same effect as the C<touch>
5211 command if the files already exist:
5215 utime $now, $now, @ARGV;
5219 Returns a list consisting of all the values of the named hash. (In a
5220 scalar context, returns the number of values.) The values are
5221 returned in an apparently random order. The actual random order is
5222 subject to change in future versions of perl, but it is guaranteed to
5223 be the same order as either the C<keys> or C<each> function would
5224 produce on the same (unmodified) hash.
5226 Note that you cannot modify the values of a hash this way, because the
5227 returned list is just a copy. You need to use a hash slice for that,
5228 since it's lvaluable in a way that values() is not.
5230 for (values %hash) { s/foo/bar/g } # FAILS!
5231 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5233 As a side effect, calling values() resets the HASH's internal iterator.
5234 See also C<keys>, C<each>, and C<sort>.
5236 =item vec EXPR,OFFSET,BITS
5238 Treats the string in EXPR as a bit vector made up of elements of
5239 width BITS, and returns the value of the element specified by OFFSET
5240 as an unsigned integer. BITS therefore specifies the number of bits
5241 that are reserved for each element in the bit vector. This must
5242 be a power of two from 1 to 32 (or 64, if your platform supports
5245 If BITS is 8, "elements" coincide with bytes of the input string.
5247 If BITS is 16 or more, bytes of the input string are grouped into chunks
5248 of size BITS/8, and each group is converted to a number as with
5249 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5250 for BITS==64). See L<"pack"> for details.
5252 If bits is 4 or less, the string is broken into bytes, then the bits
5253 of each byte are broken into 8/BITS groups. Bits of a byte are
5254 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5255 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5256 breaking the single input byte C<chr(0x36)> into two groups gives a list
5257 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5259 C<vec> may also be assigned to, in which case parentheses are needed
5260 to give the expression the correct precedence as in
5262 vec($image, $max_x * $x + $y, 8) = 3;
5264 Vectors created with C<vec> can also be manipulated with the logical
5265 operators C<|>, C<&>, and C<^>, which will assume a bit vector
5266 operation is desired when both operands are strings.
5267 See L<perlop/"Bitwise String Operators">.
5269 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5270 The comments show the string after each step. Note that this code works
5271 in the same way on big-endian or little-endian machines.
5274 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5276 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5277 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5279 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5280 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5281 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5282 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5283 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5284 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5286 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5287 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5288 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5291 To transform a bit vector into a string or list of 0's and 1's, use these:
5293 $bits = unpack("b*", $vector);
5294 @bits = split(//, unpack("b*", $vector));
5296 If you know the exact length in bits, it can be used in place of the C<*>.
5298 Here is an example to illustrate how the bits actually fall in place:
5304 unpack("V",$_) 01234567890123456789012345678901
5305 ------------------------------------------------------------------
5310 for ($shift=0; $shift < $width; ++$shift) {
5311 for ($off=0; $off < 32/$width; ++$off) {
5312 $str = pack("B*", "0"x32);
5313 $bits = (1<<$shift);
5314 vec($str, $off, $width) = $bits;
5315 $res = unpack("b*",$str);
5316 $val = unpack("V", $str);
5323 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5324 $off, $width, $bits, $val, $res
5328 Regardless of the machine architecture on which it is run, the above
5329 example should print the following table:
5332 unpack("V",$_) 01234567890123456789012345678901
5333 ------------------------------------------------------------------
5334 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5335 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5336 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5337 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5338 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5339 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5340 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5341 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5342 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5343 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5344 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5345 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5346 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5347 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5348 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5349 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5350 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5351 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5352 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5353 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5354 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5355 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5356 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5357 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5358 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5359 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5360 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5361 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5362 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5363 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5364 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5365 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5366 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5367 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5368 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5369 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5370 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5371 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5372 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5373 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5374 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5375 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5376 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5377 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5378 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5379 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5380 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5381 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5382 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5383 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5384 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5385 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5386 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5387 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5388 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5389 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5390 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5391 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5392 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5393 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5394 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5395 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5396 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5397 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5398 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5399 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5400 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5401 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5402 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5403 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5404 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5405 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5406 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5407 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5408 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5409 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5410 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5411 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5412 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5413 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5414 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5415 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5416 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5417 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5418 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5419 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5420 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5421 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5422 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5423 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5424 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5425 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5426 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5427 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5428 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5429 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5430 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5431 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5432 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5433 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5434 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5435 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5436 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5437 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5438 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5439 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5440 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5441 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5442 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5443 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5444 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5445 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5446 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5447 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5448 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5449 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5450 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5451 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5452 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5453 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5454 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5455 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5456 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5457 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5458 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5459 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5460 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5461 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5465 Behaves like the wait(2) system call on your system: it waits for a child
5466 process to terminate and returns the pid of the deceased process, or
5467 C<-1> if there are no child processes. The status is returned in C<$?>.
5468 Note that a return value of C<-1> could mean that child processes are
5469 being automatically reaped, as described in L<perlipc>.
5471 =item waitpid PID,FLAGS
5473 Waits for a particular child process to terminate and returns the pid of
5474 the deceased process, or C<-1> if there is no such child process. On some
5475 systems, a value of 0 indicates that there are processes still running.
5476 The status is returned in C<$?>. If you say
5478 use POSIX ":sys_wait_h";
5481 $kid = waitpid(-1,&WNOHANG);
5484 then you can do a non-blocking wait for all pending zombie processes.
5485 Non-blocking wait is available on machines supporting either the
5486 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5487 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5488 system call by remembering the status values of processes that have
5489 exited but have not been harvested by the Perl script yet.)
5491 Note that on some systems, a return value of C<-1> could mean that child
5492 processes are being automatically reaped. See L<perlipc> for details,
5493 and for other examples.
5497 Returns true if the context of the currently executing subroutine is
5498 looking for a list value. Returns false if the context is looking
5499 for a scalar. Returns the undefined value if the context is looking
5500 for no value (void context).
5502 return unless defined wantarray; # don't bother doing more
5503 my @a = complex_calculation();
5504 return wantarray ? @a : "@a";
5506 This function should have been named wantlist() instead.
5510 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5513 If LIST is empty and C<$@> already contains a value (typically from a
5514 previous eval) that value is used after appending C<"\t...caught">
5515 to C<$@>. This is useful for staying almost, but not entirely similar to
5518 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5520 No message is printed if there is a C<$SIG{__WARN__}> handler
5521 installed. It is the handler's responsibility to deal with the message
5522 as it sees fit (like, for instance, converting it into a C<die>). Most
5523 handlers must therefore make arrangements to actually display the
5524 warnings that they are not prepared to deal with, by calling C<warn>
5525 again in the handler. Note that this is quite safe and will not
5526 produce an endless loop, since C<__WARN__> hooks are not called from
5529 You will find this behavior is slightly different from that of
5530 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5531 instead call C<die> again to change it).
5533 Using a C<__WARN__> handler provides a powerful way to silence all
5534 warnings (even the so-called mandatory ones). An example:
5536 # wipe out *all* compile-time warnings
5537 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5539 my $foo = 20; # no warning about duplicate my $foo,
5540 # but hey, you asked for it!
5541 # no compile-time or run-time warnings before here
5544 # run-time warnings enabled after here
5545 warn "\$foo is alive and $foo!"; # does show up
5547 See L<perlvar> for details on setting C<%SIG> entries, and for more
5548 examples. See the Carp module for other kinds of warnings using its
5549 carp() and cluck() functions.
5551 =item write FILEHANDLE
5557 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5558 using the format associated with that file. By default the format for
5559 a file is the one having the same name as the filehandle, but the
5560 format for the current output channel (see the C<select> function) may be set
5561 explicitly by assigning the name of the format to the C<$~> variable.
5563 Top of form processing is handled automatically: if there is
5564 insufficient room on the current page for the formatted record, the
5565 page is advanced by writing a form feed, a special top-of-page format
5566 is used to format the new page header, and then the record is written.
5567 By default the top-of-page format is the name of the filehandle with
5568 "_TOP" appended, but it may be dynamically set to the format of your
5569 choice by assigning the name to the C<$^> variable while the filehandle is
5570 selected. The number of lines remaining on the current page is in
5571 variable C<$->, which can be set to C<0> to force a new page.
5573 If FILEHANDLE is unspecified, output goes to the current default output
5574 channel, which starts out as STDOUT but may be changed by the
5575 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5576 is evaluated and the resulting string is used to look up the name of
5577 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5579 Note that write is I<not> the opposite of C<read>. Unfortunately.
5583 The transliteration operator. Same as C<tr///>. See L<perlop>.