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<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm>
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> 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> to check whether a subroutine exists, by
885 saying C<defined &func> without parentheses. On the other hand, use
886 of C<defined> upon aggregates (hashes and arrays) is not guaranteed to
887 produce intuitive results, and should probably be avoided.
889 When used on a hash element, it tells you whether the value is defined,
890 not whether the key exists in the hash. Use L</exists> for the latter
895 print if defined $switch{'D'};
896 print "$val\n" while defined($val = pop(@ary));
897 die "Can't readlink $sym: $!"
898 unless defined($value = readlink $sym);
899 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
900 $debugging = 0 unless defined $debugging;
902 Note: Many folks tend to overuse C<defined>, and then are surprised to
903 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
904 defined values. For example, if you say
908 The pattern match succeeds, and C<$1> is defined, despite the fact that it
909 matched "nothing". But it didn't really match nothing--rather, it
910 matched something that happened to be zero characters long. This is all
911 very above-board and honest. When a function returns an undefined value,
912 it's an admission that it couldn't give you an honest answer. So you
913 should use C<defined> only when you're questioning the integrity of what
914 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
917 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
918 used to report whether memory for that aggregate has ever been
919 allocated. This behavior may disappear in future versions of Perl.
920 You should instead use a simple test for size:
922 if (@an_array) { print "has array elements\n" }
923 if (%a_hash) { print "has hash members\n" }
925 See also L</undef>, L</exists>, L</ref>.
929 Deletes the specified key(s) and their associated values from a hash.
930 For each key, returns the deleted value associated with that key, or
931 the undefined value if there was no such key. Deleting from C<$ENV{}>
932 modifies the environment. Deleting from a hash tied to a DBM file
933 deletes the entry from the DBM file. (But deleting from a C<tie>d hash
934 doesn't necessarily return anything.)
936 The following deletes all the values of a hash:
938 foreach $key (keys %HASH) {
944 delete @HASH{keys %HASH}
946 But both of these are slower than just assigning the empty list
949 %hash = (); # completely empty %hash
950 undef %hash; # forget %hash every existed
952 Note that the EXPR can be arbitrarily complicated as long as the final
953 operation is a hash element lookup or hash slice:
955 delete $ref->[$x][$y]{$key};
956 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
960 Outside an C<eval>, prints the value of LIST to C<STDERR> and
961 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
962 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
963 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
964 an C<eval(),> the error message is stuffed into C<$@> and the
965 C<eval> is terminated with the undefined value. This makes
966 C<die> the way to raise an exception.
970 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
971 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
973 If the value of EXPR does not end in a newline, the current script line
974 number and input line number (if any) are also printed, and a newline
975 is supplied. Note that the "input line number" (also known as "chunk")
976 is subject to whatever notion of "line" happens to be currently in
977 effect, and is also available as the special variable C<$.>.
978 See L<perlvar/"$/"> and L<perlvar/"$.">.
980 Hint: sometimes appending C<", stopped"> to your message
981 will cause it to make better sense when the string C<"at foo line 123"> is
982 appended. Suppose you are running script "canasta".
984 die "/etc/games is no good";
985 die "/etc/games is no good, stopped";
987 produce, respectively
989 /etc/games is no good at canasta line 123.
990 /etc/games is no good, stopped at canasta line 123.
992 See also exit(), warn(), and the Carp module.
994 If LIST is empty and C<$@> already contains a value (typically from a
995 previous eval) that value is reused after appending C<"\t...propagated">.
996 This is useful for propagating exceptions:
999 die unless $@ =~ /Expected exception/;
1001 If C<$@> is empty then the string C<"Died"> is used.
1003 die() can also be called with a reference argument. If this happens to be
1004 trapped within an eval(), $@ contains the reference. This behavior permits
1005 a more elaborate exception handling implementation using objects that
1006 maintain arbitary state about the nature of the exception. Such a scheme
1007 is sometimes preferable to matching particular string values of $@ using
1008 regular expressions. Here's an example:
1010 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1012 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1013 # handle Some::Module::Exception
1016 # handle all other possible exceptions
1020 Because perl will stringify uncaught exception messages before displaying
1021 them, you may want to overload stringification operations on such custom
1022 exception objects. See L<overload> for details about that.
1024 You can arrange for a callback to be run just before the C<die>
1025 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1026 handler will be called with the error text and can change the error
1027 message, if it sees fit, by calling C<die> again. See
1028 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1029 L<"eval BLOCK"> for some examples. Although this feature was meant
1030 to be run only right before your program was to exit, this is not
1031 currently the case--the C<$SIG{__DIE__}> hook is currently called
1032 even inside eval()ed blocks/strings! If one wants the hook to do
1033 nothing in such situations, put
1037 as the first line of the handler (see L<perlvar/$^S>). Because
1038 this promotes strange action at a distance, this counterintuitive
1039 behavior may be fixed in a future release.
1043 Not really a function. Returns the value of the last command in the
1044 sequence of commands indicated by BLOCK. When modified by a loop
1045 modifier, executes the BLOCK once before testing the loop condition.
1046 (On other statements the loop modifiers test the conditional first.)
1048 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1049 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1050 See L<perlsyn> for alternative strategies.
1052 =item do SUBROUTINE(LIST)
1054 A deprecated form of subroutine call. See L<perlsub>.
1058 Uses the value of EXPR as a filename and executes the contents of the
1059 file as a Perl script. Its primary use is to include subroutines
1060 from a Perl subroutine library.
1066 scalar eval `cat stat.pl`;
1068 except that it's more efficient and concise, keeps track of the current
1069 filename for error messages, searches the @INC libraries, and updates
1070 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1071 variables. It also differs in that code evaluated with C<do FILENAME>
1072 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1073 same, however, in that it does reparse the file every time you call it,
1074 so you probably don't want to do this inside a loop.
1076 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1077 error. If C<do> can read the file but cannot compile it, it
1078 returns undef and sets an error message in C<$@>. If the file is
1079 successfully compiled, C<do> returns the value of the last expression
1082 Note that inclusion of library modules is better done with the
1083 C<use> and C<require> operators, which also do automatic error checking
1084 and raise an exception if there's a problem.
1086 You might like to use C<do> to read in a program configuration
1087 file. Manual error checking can be done this way:
1089 # read in config files: system first, then user
1090 for $file ("/share/prog/defaults.rc",
1091 "$ENV{HOME}/.someprogrc")
1093 unless ($return = do $file) {
1094 warn "couldn't parse $file: $@" if $@;
1095 warn "couldn't do $file: $!" unless defined $return;
1096 warn "couldn't run $file" unless $return;
1104 This function causes an immediate core dump. See also the B<-u>
1105 command-line switch in L<perlrun>, which does the same thing.
1106 Primarily this is so that you can use the B<undump> program (not
1107 supplied) to turn your core dump into an executable binary after
1108 having initialized all your variables at the beginning of the
1109 program. When the new binary is executed it will begin by executing
1110 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1111 Think of it as a goto with an intervening core dump and reincarnation.
1112 If C<LABEL> is omitted, restarts the program from the top.
1114 B<WARNING>: Any files opened at the time of the dump will I<not>
1115 be open any more when the program is reincarnated, with possible
1116 resulting confusion on the part of Perl.
1118 This function is now largely obsolete, partly because it's very
1119 hard to convert a core file into an executable, and because the
1120 real compiler backends for generating portable bytecode and compilable
1121 C code have superseded it.
1123 If you're looking to use L<dump> to speed up your program, consider
1124 generating bytecode or native C code as described in L<perlcc>. If
1125 you're just trying to accelerate a CGI script, consider using the
1126 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1127 You might also consider autoloading or selfloading, which at least
1128 make your program I<appear> to run faster.
1132 When called in list context, returns a 2-element list consisting of the
1133 key and value for the next element of a hash, so that you can iterate over
1134 it. When called in scalar context, returns the key for only the "next"
1135 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1136 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1139 Entries are returned in an apparently random order. The actual random
1140 order is subject to change in future versions of perl, but it is guaranteed
1141 to be in the same order as either the C<keys> or C<values> function
1142 would produce on the same (unmodified) hash.
1144 When the hash is entirely read, a null array is returned in list context
1145 (which when assigned produces a false (C<0>) value), and C<undef> in
1146 scalar context. The next call to C<each> after that will start iterating
1147 again. There is a single iterator for each hash, shared by all C<each>,
1148 C<keys>, and C<values> function calls in the program; it can be reset by
1149 reading all the elements from the hash, or by evaluating C<keys HASH> or
1150 C<values HASH>. If you add or delete elements of a hash while you're
1151 iterating over it, you may get entries skipped or duplicated, so don't.
1153 The following prints out your environment like the printenv(1) program,
1154 only in a different order:
1156 while (($key,$value) = each %ENV) {
1157 print "$key=$value\n";
1160 See also C<keys>, C<values> and C<sort>.
1162 =item eof FILEHANDLE
1168 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1169 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1170 gives the real filehandle. (Note that this function actually
1171 reads a character and then C<ungetc>s it, so isn't very useful in an
1172 interactive context.) Do not read from a terminal file (or call
1173 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1174 as terminals may lose the end-of-file condition if you do.
1176 An C<eof> without an argument uses the last file read as argument.
1177 Using C<eof()> with empty parentheses is very different. It indicates
1178 the pseudo file formed of the files listed on the command line,
1179 i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
1180 loop to detect the end of only the last file. Use C<eof(ARGV)> or
1181 C<eof> without the parentheses to test I<each> file in a while
1182 (E<lt>E<gt>) loop. Examples:
1184 # reset line numbering on each input file
1186 next if /^\s*#/; # skip comments
1189 close ARGV if eof; # Not eof()!
1192 # insert dashes just before last line of last file
1194 if (eof()) { # check for end of current file
1195 print "--------------\n";
1196 close(ARGV); # close or last; is needed if we
1197 # are reading from the terminal
1202 Practical hint: you almost never need to use C<eof> in Perl, because the
1203 input operators return false values when they run out of data, or if there
1210 In the first form, the return value of EXPR is parsed and executed as if it
1211 were a little Perl program. The value of the expression (which is itself
1212 determined within scalar context) is first parsed, and if there weren't any
1213 errors, executed in the context of the current Perl program, so that any
1214 variable settings or subroutine and format definitions remain afterwards.
1215 Note that the value is parsed every time the eval executes. If EXPR is
1216 omitted, evaluates C<$_>. This form is typically used to delay parsing
1217 and subsequent execution of the text of EXPR until run time.
1219 In the second form, the code within the BLOCK is parsed only once--at the
1220 same time the code surrounding the eval itself was parsed--and executed
1221 within the context of the current Perl program. This form is typically
1222 used to trap exceptions more efficiently than the first (see below), while
1223 also providing the benefit of checking the code within BLOCK at compile
1226 The final semicolon, if any, may be omitted from the value of EXPR or within
1229 In both forms, the value returned is the value of the last expression
1230 evaluated inside the mini-program; a return statement may be also used, just
1231 as with subroutines. The expression providing the return value is evaluated
1232 in void, scalar, or list context, depending on the context of the eval itself.
1233 See L</wantarray> for more on how the evaluation context can be determined.
1235 If there is a syntax error or runtime error, or a C<die> statement is
1236 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1237 error message. If there was no error, C<$@> is guaranteed to be a null
1238 string. Beware that using C<eval> neither silences perl from printing
1239 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1240 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1241 L</warn> and L<perlvar>.
1243 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1244 determining whether a particular feature (such as C<socket> or C<symlink>)
1245 is implemented. It is also Perl's exception trapping mechanism, where
1246 the die operator is used to raise exceptions.
1248 If the code to be executed doesn't vary, you may use the eval-BLOCK
1249 form to trap run-time errors without incurring the penalty of
1250 recompiling each time. The error, if any, is still returned in C<$@>.
1253 # make divide-by-zero nonfatal
1254 eval { $answer = $a / $b; }; warn $@ if $@;
1256 # same thing, but less efficient
1257 eval '$answer = $a / $b'; warn $@ if $@;
1259 # a compile-time error
1260 eval { $answer = }; # WRONG
1263 eval '$answer ='; # sets $@
1265 Due to the current arguably broken state of C<__DIE__> hooks, when using
1266 the C<eval{}> form as an exception trap in libraries, you may wish not
1267 to trigger any C<__DIE__> hooks that user code may have installed.
1268 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1269 as shown in this example:
1271 # a very private exception trap for divide-by-zero
1272 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1275 This is especially significant, given that C<__DIE__> hooks can call
1276 C<die> again, which has the effect of changing their error messages:
1278 # __DIE__ hooks may modify error messages
1280 local $SIG{'__DIE__'} =
1281 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1282 eval { die "foo lives here" };
1283 print $@ if $@; # prints "bar lives here"
1286 Because this promotes action at a distance, this counterintuitive behavior
1287 may be fixed in a future release.
1289 With an C<eval>, you should be especially careful to remember what's
1290 being looked at when:
1296 eval { $x }; # CASE 4
1298 eval "\$$x++"; # CASE 5
1301 Cases 1 and 2 above behave identically: they run the code contained in
1302 the variable $x. (Although case 2 has misleading double quotes making
1303 the reader wonder what else might be happening (nothing is).) Cases 3
1304 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1305 does nothing but return the value of $x. (Case 4 is preferred for
1306 purely visual reasons, but it also has the advantage of compiling at
1307 compile-time instead of at run-time.) Case 5 is a place where
1308 normally you I<would> like to use double quotes, except that in this
1309 particular situation, you can just use symbolic references instead, as
1312 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1313 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1317 =item exec PROGRAM LIST
1319 The C<exec> function executes a system command I<and never returns>--
1320 use C<system> instead of C<exec> if you want it to return. It fails and
1321 returns false only if the command does not exist I<and> it is executed
1322 directly instead of via your system's command shell (see below).
1324 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1325 warns you if there is a following statement which isn't C<die>, C<warn>,
1326 or C<exit> (if C<-w> is set - but you always do that). If you
1327 I<really> want to follow an C<exec> with some other statement, you
1328 can use one of these styles to avoid the warning:
1330 exec ('foo') or print STDERR "couldn't exec foo: $!";
1331 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1333 If there is more than one argument in LIST, or if LIST is an array
1334 with more than one value, calls execvp(3) with the arguments in LIST.
1335 If there is only one scalar argument or an array with one element in it,
1336 the argument is checked for shell metacharacters, and if there are any,
1337 the entire argument is passed to the system's command shell for parsing
1338 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1339 If there are no shell metacharacters in the argument, it is split into
1340 words and passed directly to C<execvp>, which is more efficient.
1343 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1344 exec "sort $outfile | uniq";
1346 If you don't really want to execute the first argument, but want to lie
1347 to the program you are executing about its own name, you can specify
1348 the program you actually want to run as an "indirect object" (without a
1349 comma) in front of the LIST. (This always forces interpretation of the
1350 LIST as a multivalued list, even if there is only a single scalar in
1353 $shell = '/bin/csh';
1354 exec $shell '-sh'; # pretend it's a login shell
1358 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1360 When the arguments get executed via the system shell, results will
1361 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1364 Using an indirect object with C<exec> or C<system> is also more
1365 secure. This usage (which also works fine with system()) forces
1366 interpretation of the arguments as a multivalued list, even if the
1367 list had just one argument. That way you're safe from the shell
1368 expanding wildcards or splitting up words with whitespace in them.
1370 @args = ( "echo surprise" );
1372 exec @args; # subject to shell escapes
1374 exec { $args[0] } @args; # safe even with one-arg list
1376 The first version, the one without the indirect object, ran the I<echo>
1377 program, passing it C<"surprise"> an argument. The second version
1378 didn't--it tried to run a program literally called I<"echo surprise">,
1379 didn't find it, and set C<$?> to a non-zero value indicating failure.
1381 Note that C<exec> will not call your C<END> blocks, nor will it call
1382 any C<DESTROY> methods in your objects.
1386 Returns true if the specified hash key exists in its hash, even
1387 if the corresponding value is undefined.
1389 print "Exists\n" if exists $array{$key};
1390 print "Defined\n" if defined $array{$key};
1391 print "True\n" if $array{$key};
1393 A hash element can be true only if it's defined, and defined if
1394 it exists, but the reverse doesn't necessarily hold true.
1396 Note that the EXPR can be arbitrarily complicated as long as the final
1397 operation is a hash key lookup:
1399 if (exists $ref->{A}->{B}->{$key}) { }
1400 if (exists $hash{A}{B}{$key}) { }
1402 Although the last element will not spring into existence just because
1403 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1404 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1405 existence test for a $key element. This happens anywhere the arrow
1406 operator is used, including even
1409 if (exists $ref->{"Some key"}) { }
1410 print $ref; # prints HASH(0x80d3d5c)
1412 This surprising autovivification in what does not at first--or even
1413 second--glance appear to be an lvalue context may be fixed in a future
1418 Evaluates EXPR and exits immediately with that value. Example:
1421 exit 0 if $ans =~ /^[Xx]/;
1423 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1424 universally recognized values for EXPR are C<0> for success and C<1>
1425 for error; other values are subject to interpretation depending on the
1426 environment in which the Perl program is running. For example, exiting
1427 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1428 the mailer to return the item undelivered, but that's not true everywhere.
1430 Don't use C<exit> to abort a subroutine if there's any chance that
1431 someone might want to trap whatever error happened. Use C<die> instead,
1432 which can be trapped by an C<eval>.
1434 The exit() function does not always exit immediately. It calls any
1435 defined C<END> routines first, but these C<END> routines may not
1436 themselves abort the exit. Likewise any object destructors that need to
1437 be called are called before the real exit. If this is a problem, you
1438 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1439 See L<perlsub> for details.
1445 Returns I<e> (the natural logarithm base) to the power of EXPR.
1446 If EXPR is omitted, gives C<exp($_)>.
1448 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1450 Implements the fcntl(2) function. You'll probably have to say
1454 first to get the correct constant definitions. Argument processing and
1455 value return works just like C<ioctl> below.
1459 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1460 or die "can't fcntl F_GETFL: $!";
1462 You don't have to check for C<defined> on the return from C<fnctl>.
1463 Like C<ioctl>, it maps a C<0> return from the system call into C<"0
1464 but true"> in Perl. This string is true in boolean context and C<0>
1465 in numeric context. It is also exempt from the normal B<-w> warnings
1466 on improper numeric conversions.
1468 Note that C<fcntl> will produce a fatal error if used on a machine that
1469 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1470 manpage to learn what functions are available on your system.
1472 =item fileno FILEHANDLE
1474 Returns the file descriptor for a filehandle, or undefined if the
1475 filehandle is not open. This is mainly useful for constructing
1476 bitmaps for C<select> and low-level POSIX tty-handling operations.
1477 If FILEHANDLE is an expression, the value is taken as an indirect
1478 filehandle, generally its name.
1480 You can use this to find out whether two handles refer to the
1481 same underlying descriptor:
1483 if (fileno(THIS) == fileno(THAT)) {
1484 print "THIS and THAT are dups\n";
1487 =item flock FILEHANDLE,OPERATION
1489 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1490 for success, false on failure. Produces a fatal error if used on a
1491 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1492 C<flock> is Perl's portable file locking interface, although it locks
1493 only entire files, not records.
1495 Two potentially non-obvious but traditional C<flock> semantics are
1496 that it waits indefinitely until the lock is granted, and that its locks
1497 B<merely advisory>. Such discretionary locks are more flexible, but offer
1498 fewer guarantees. This means that files locked with C<flock> may be
1499 modified by programs that do not also use C<flock>. See L<perlport>,
1500 your port's specific documentation, or your system-specific local manpages
1501 for details. It's best to assume traditional behavior if you're writing
1502 portable programs. (But if you're not, you should as always feel perfectly
1503 free to write for your own system's idiosyncrasies (sometimes called
1504 "features"). Slavish adherence to portability concerns shouldn't get
1505 in the way of your getting your job done.)
1507 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1508 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1509 you can use the symbolic names if import them from the Fcntl module,
1510 either individually, or as a group using the ':flock' tag. LOCK_SH
1511 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1512 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1513 LOCK_EX then C<flock> will return immediately rather than blocking
1514 waiting for the lock (check the return status to see if you got it).
1516 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1517 before locking or unlocking it.
1519 Note that the emulation built with lockf(3) doesn't provide shared
1520 locks, and it requires that FILEHANDLE be open with write intent. These
1521 are the semantics that lockf(3) implements. Most if not all systems
1522 implement lockf(3) in terms of fcntl(2) locking, though, so the
1523 differing semantics shouldn't bite too many people.
1525 Note also that some versions of C<flock> cannot lock things over the
1526 network; you would need to use the more system-specific C<fcntl> for
1527 that. If you like you can force Perl to ignore your system's flock(2)
1528 function, and so provide its own fcntl(2)-based emulation, by passing
1529 the switch C<-Ud_flock> to the F<Configure> program when you configure
1532 Here's a mailbox appender for BSD systems.
1534 use Fcntl ':flock'; # import LOCK_* constants
1537 flock(MBOX,LOCK_EX);
1538 # and, in case someone appended
1539 # while we were waiting...
1544 flock(MBOX,LOCK_UN);
1547 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1548 or die "Can't open mailbox: $!";
1551 print MBOX $msg,"\n\n";
1554 On systems that support a real flock(), locks are inherited across fork()
1555 calls, whereas those that must resort to the more capricious fcntl()
1556 function lose the locks, making it harder to write servers.
1558 See also L<DB_File> for other flock() examples.
1562 Does a fork(2) system call to create a new process running the
1563 same program at the same point. It returns the child pid to the
1564 parent process, C<0> to the child process, or C<undef> if the fork is
1565 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1566 are shared, while everything else is copied. On most systems supporting
1567 fork(), great care has gone into making it extremely efficient (for
1568 example, using copy-on-write technology on data pages), making it the
1569 dominant paradigm for multitasking over the last few decades.
1571 All files opened for output are flushed before forking the child process.
1573 If you C<fork> without ever waiting on your children, you will
1574 accumulate zombies. On some systems, you can avoid this by setting
1575 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1576 forking and reaping moribund children.
1578 Note that if your forked child inherits system file descriptors like
1579 STDIN and STDOUT that are actually connected by a pipe or socket, even
1580 if you exit, then the remote server (such as, say, a CGI script or a
1581 backgrounded job launched from a remote shell) won't think you're done.
1582 You should reopen those to F</dev/null> if it's any issue.
1586 Declare a picture format for use by the C<write> function. For
1590 Test: @<<<<<<<< @||||| @>>>>>
1591 $str, $%, '$' . int($num)
1595 $num = $cost/$quantity;
1599 See L<perlform> for many details and examples.
1601 =item formline PICTURE,LIST
1603 This is an internal function used by C<format>s, though you may call it,
1604 too. It formats (see L<perlform>) a list of values according to the
1605 contents of PICTURE, placing the output into the format output
1606 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1607 Eventually, when a C<write> is done, the contents of
1608 C<$^A> are written to some filehandle, but you could also read C<$^A>
1609 yourself and then set C<$^A> back to C<"">. Note that a format typically
1610 does one C<formline> per line of form, but the C<formline> function itself
1611 doesn't care how many newlines are embedded in the PICTURE. This means
1612 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1613 You may therefore need to use multiple formlines to implement a single
1614 record format, just like the format compiler.
1616 Be careful if you put double quotes around the picture, because an C<@>
1617 character may be taken to mean the beginning of an array name.
1618 C<formline> always returns true. See L<perlform> for other examples.
1620 =item getc FILEHANDLE
1624 Returns the next character from the input file attached to FILEHANDLE,
1625 or the undefined value at end of file, or if there was an error.
1626 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1627 efficient. However, it cannot be used by itself to fetch single
1628 characters without waiting for the user to hit enter. For that, try
1629 something more like:
1632 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1635 system "stty", '-icanon', 'eol', "\001";
1641 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1644 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1648 Determination of whether $BSD_STYLE should be set
1649 is left as an exercise to the reader.
1651 The C<POSIX::getattr> function can do this more portably on
1652 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1653 module from your nearest CPAN site; details on CPAN can be found on
1658 Implements the C library function of the same name, which on most
1659 systems returns the current login from F</etc/utmp>, if any. If null,
1662 $login = getlogin || getpwuid($<) || "Kilroy";
1664 Do not consider C<getlogin> for authentication: it is not as
1665 secure as C<getpwuid>.
1667 =item getpeername SOCKET
1669 Returns the packed sockaddr address of other end of the SOCKET connection.
1672 $hersockaddr = getpeername(SOCK);
1673 ($port, $iaddr) = sockaddr_in($hersockaddr);
1674 $herhostname = gethostbyaddr($iaddr, AF_INET);
1675 $herstraddr = inet_ntoa($iaddr);
1679 Returns the current process group for the specified PID. Use
1680 a PID of C<0> to get the current process group for the
1681 current process. Will raise an exception if used on a machine that
1682 doesn't implement getpgrp(2). If PID is omitted, returns process
1683 group of current process. Note that the POSIX version of C<getpgrp>
1684 does not accept a PID argument, so only C<PID==0> is truly portable.
1688 Returns the process id of the parent process.
1690 =item getpriority WHICH,WHO
1692 Returns the current priority for a process, a process group, or a user.
1693 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1694 machine that doesn't implement getpriority(2).
1700 =item gethostbyname NAME
1702 =item getnetbyname NAME
1704 =item getprotobyname NAME
1710 =item getservbyname NAME,PROTO
1712 =item gethostbyaddr ADDR,ADDRTYPE
1714 =item getnetbyaddr ADDR,ADDRTYPE
1716 =item getprotobynumber NUMBER
1718 =item getservbyport PORT,PROTO
1736 =item sethostent STAYOPEN
1738 =item setnetent STAYOPEN
1740 =item setprotoent STAYOPEN
1742 =item setservent STAYOPEN
1756 These routines perform the same functions as their counterparts in the
1757 system library. In list context, the return values from the
1758 various get routines are as follows:
1760 ($name,$passwd,$uid,$gid,
1761 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1762 ($name,$passwd,$gid,$members) = getgr*
1763 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1764 ($name,$aliases,$addrtype,$net) = getnet*
1765 ($name,$aliases,$proto) = getproto*
1766 ($name,$aliases,$port,$proto) = getserv*
1768 (If the entry doesn't exist you get a null list.)
1770 In scalar context, you get the name, unless the function was a
1771 lookup by name, in which case you get the other thing, whatever it is.
1772 (If the entry doesn't exist you get the undefined value.) For example:
1774 $uid = getpwnam($name);
1775 $name = getpwuid($num);
1777 $gid = getgrnam($name);
1778 $name = getgrgid($num;
1782 In I<getpw*()> the fields $quota, $comment, and $expire are
1783 special cases in the sense that in many systems they are unsupported.
1784 If the $quota is unsupported, it is an empty scalar. If it is
1785 supported, it usually encodes the disk quota. If the $comment
1786 field is unsupported, it is an empty scalar. If it is supported it
1787 usually encodes some administrative comment about the user. In some
1788 systems the $quota field may be $change or $age, fields that have
1789 to do with password aging. In some systems the $comment field may
1790 be $class. The $expire field, if present, encodes the expiration
1791 period of the account or the password. For the availability and the
1792 exact meaning of these fields in your system, please consult your
1793 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1794 out from within Perl what your $quota and $comment fields mean
1795 and whether you have the $expire field by using the C<Config> module
1796 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1797 and C<d_pwexpire>. Shadow password files are only supported if your
1798 vendor has implemented them in the intuitive fashion that calling the
1799 regular C library routines gets the shadow versions if you're running
1800 under privilege. Those that incorrectly implement a separate library
1801 call are not supported.
1803 The $members value returned by I<getgr*()> is a space separated list of
1804 the login names of the members of the group.
1806 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1807 C, it will be returned to you via C<$?> if the function call fails. The
1808 C<@addrs> value returned by a successful call is a list of the raw
1809 addresses returned by the corresponding system library call. In the
1810 Internet domain, each address is four bytes long and you can unpack it
1811 by saying something like:
1813 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1815 The Socket library makes this slightly easier:
1818 $iaddr = inet_aton("127.1"); # or whatever address
1819 $name = gethostbyaddr($iaddr, AF_INET);
1821 # or going the other way
1822 $straddr = inet_ntoa($iaddr);
1824 If you get tired of remembering which element of the return list
1825 contains which return value, by-name interfaces are provided
1826 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1827 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1828 and C<User::grent>. These override the normal built-ins, supplying
1829 versions that return objects with the appropriate names
1830 for each field. For example:
1834 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1836 Even though it looks like they're the same method calls (uid),
1837 they aren't, because a C<File::stat> object is different from
1838 a C<User::pwent> object.
1840 =item getsockname SOCKET
1842 Returns the packed sockaddr address of this end of the SOCKET connection,
1843 in case you don't know the address because you have several different
1844 IPs that the connection might have come in on.
1847 $mysockaddr = getsockname(SOCK);
1848 ($port, $myaddr) = sockaddr_in($mysockaddr);
1849 printf "Connect to %s [%s]\n",
1850 scalar gethostbyaddr($myaddr, AF_INET),
1853 =item getsockopt SOCKET,LEVEL,OPTNAME
1855 Returns the socket option requested, or undef if there is an error.
1861 Returns the value of EXPR with filename expansions such as the
1862 standard Unix shell F</bin/csh> would do. This is the internal function
1863 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1864 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1865 discussed in more detail in L<perlop/"I/O Operators">.
1869 Converts a time as returned by the time function to a 9-element list
1870 with the time localized for the standard Greenwich time zone.
1871 Typically used as follows:
1874 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1877 All list elements are numeric, and come straight out of a struct tm.
1878 In particular this means that $mon has the range C<0..11> and $wday
1879 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1880 number of years since 1900, that is, $year is C<123> in year 2023,
1881 I<not> simply the last two digits of the year. If you assume it is,
1882 then you create non-Y2K-compliant programs--and you wouldn't want to do
1885 If EXPR is omitted, does C<gmtime(time())>.
1887 In scalar context, returns the ctime(3) value:
1889 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1891 Also see the C<timegm> function provided by the C<Time::Local> module,
1892 and the strftime(3) function available via the POSIX module.
1894 This scalar value is B<not> locale dependent (see L<perllocale>), but
1895 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1896 strftime(3) and mktime(3) functions available via the POSIX module. To
1897 get somewhat similar but locale dependent date strings, set up your
1898 locale environment variables appropriately (please see L<perllocale>)
1899 and try for example:
1901 use POSIX qw(strftime);
1902 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1904 Note that the C<%a> and C<%b> escapes, which represent the short forms
1905 of the day of the week and the month of the year, may not necessarily
1906 be three characters wide in all locales.
1914 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1915 execution there. It may not be used to go into any construct that
1916 requires initialization, such as a subroutine or a C<foreach> loop. It
1917 also can't be used to go into a construct that is optimized away,
1918 or to get out of a block or subroutine given to C<sort>.
1919 It can be used to go almost anywhere else within the dynamic scope,
1920 including out of subroutines, but it's usually better to use some other
1921 construct such as C<last> or C<die>. The author of Perl has never felt the
1922 need to use this form of C<goto> (in Perl, that is--C is another matter).
1924 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1925 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1926 necessarily recommended if you're optimizing for maintainability:
1928 goto ("FOO", "BAR", "GLARCH")[$i];
1930 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1931 named subroutine for the currently running subroutine. This is used by
1932 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1933 pretend that the other subroutine had been called in the first place
1934 (except that any modifications to C<@_> in the current subroutine are
1935 propagated to the other subroutine.) After the C<goto>, not even C<caller>
1936 will be able to tell that this routine was called first.
1938 =item grep BLOCK LIST
1940 =item grep EXPR,LIST
1942 This is similar in spirit to, but not the same as, grep(1) and its
1943 relatives. In particular, it is not limited to using regular expressions.
1945 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1946 C<$_> to each element) and returns the list value consisting of those
1947 elements for which the expression evaluated to true. In scalar
1948 context, returns the number of times the expression was true.
1950 @foo = grep(!/^#/, @bar); # weed out comments
1954 @foo = grep {!/^#/} @bar; # weed out comments
1956 Note that, because C<$_> is a reference into the list value, it can
1957 be used to modify the elements of the array. While this is useful and
1958 supported, it can cause bizarre results if the LIST is not a named array.
1959 Similarly, grep returns aliases into the original list, much as a for
1960 loop's index variable aliases the list elements. That is, modifying an
1961 element of a list returned by grep (for example, in a C<foreach>, C<map>
1962 or another C<grep>) actually modifies the element in the original list.
1963 This is usually something to be avoided when writing clear code.
1965 See also L</map> for a list composed of the results of the BLOCK or EXPR.
1971 Interprets EXPR as a hex string and returns the corresponding value.
1972 (To convert strings that might start with either 0, 0x, or 0b, see
1973 L</oct>.) If EXPR is omitted, uses C<$_>.
1975 print hex '0xAf'; # prints '175'
1976 print hex 'aF'; # same
1978 Hex strings may only represent integers. Strings that would cause
1979 integer overflow trigger a mandatory error message.
1983 There is no builtin C<import> function. It is just an ordinary
1984 method (subroutine) defined (or inherited) by modules that wish to export
1985 names to another module. The C<use> function calls the C<import> method
1986 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1988 =item index STR,SUBSTR,POSITION
1990 =item index STR,SUBSTR
1992 The index function searches for one string within another, but without
1993 the wildcard-like behavior of a full regular-expression pattern match.
1994 It returns the position of the first occurrence of SUBSTR in STR at
1995 or after POSITION. If POSITION is omitted, starts searching from the
1996 beginning of the string. The return value is based at C<0> (or whatever
1997 you've set the C<$[> variable to--but don't do that). If the substring
1998 is not found, returns one less than the base, ordinarily C<-1>.
2004 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2005 You should not use this function for rounding: one because it truncates
2006 towards C<0>, and two because machine representations of floating point
2007 numbers can sometimes produce counterintuitive results. For example,
2008 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2009 because it's really more like -268.99999999999994315658 instead. Usually,
2010 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2011 functions will serve you better than will int().
2013 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2015 Implements the ioctl(2) function. You'll probably first have to say
2017 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2019 to get the correct function definitions. If F<ioctl.ph> doesn't
2020 exist or doesn't have the correct definitions you'll have to roll your
2021 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2022 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2023 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2024 written depending on the FUNCTION--a pointer to the string value of SCALAR
2025 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2026 has no string value but does have a numeric value, that value will be
2027 passed rather than a pointer to the string value. To guarantee this to be
2028 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2029 functions may be needed to manipulate the values of structures used by
2032 The return value of C<ioctl> (and C<fcntl>) is as follows:
2034 if OS returns: then Perl returns:
2036 0 string "0 but true"
2037 anything else that number
2039 Thus Perl returns true on success and false on failure, yet you can
2040 still easily determine the actual value returned by the operating
2043 $retval = ioctl(...) || -1;
2044 printf "System returned %d\n", $retval;
2046 The special string "C<0> but true" is exempt from B<-w> complaints
2047 about improper numeric conversions.
2049 Here's an example of setting a filehandle named C<REMOTE> to be
2050 non-blocking at the system level. You'll have to negotiate C<$|>
2051 on your own, though.
2053 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2055 $flags = fcntl(REMOTE, F_GETFL, 0)
2056 or die "Can't get flags for the socket: $!\n";
2058 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2059 or die "Can't set flags for the socket: $!\n";
2061 =item join EXPR,LIST
2063 Joins the separate strings of LIST into a single string with fields
2064 separated by the value of EXPR, and returns that new string. Example:
2066 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2072 Returns a list consisting of all the keys of the named hash. (In
2073 scalar context, returns the number of keys.) The keys are returned in
2074 an apparently random order. The actual random order is subject to
2075 change in future versions of perl, but it is guaranteed to be the same
2076 order as either the C<values> or C<each> function produces (given
2077 that the hash has not been modified). As a side effect, it resets
2080 Here is yet another way to print your environment:
2083 @values = values %ENV;
2085 print pop(@keys), '=', pop(@values), "\n";
2088 or how about sorted by key:
2090 foreach $key (sort(keys %ENV)) {
2091 print $key, '=', $ENV{$key}, "\n";
2094 To sort a hash by value, you'll need to use a C<sort> function.
2095 Here's a descending numeric sort of a hash by its values:
2097 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2098 printf "%4d %s\n", $hash{$key}, $key;
2101 As an lvalue C<keys> allows you to increase the number of hash buckets
2102 allocated for the given hash. This can gain you a measure of efficiency if
2103 you know the hash is going to get big. (This is similar to pre-extending
2104 an array by assigning a larger number to $#array.) If you say
2108 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2109 in fact, since it rounds up to the next power of two. These
2110 buckets will be retained even if you do C<%hash = ()>, use C<undef
2111 %hash> if you want to free the storage while C<%hash> is still in scope.
2112 You can't shrink the number of buckets allocated for the hash using
2113 C<keys> in this way (but you needn't worry about doing this by accident,
2114 as trying has no effect).
2116 See also C<each>, C<values> and C<sort>.
2120 Sends a signal to a list of processes. The first element of
2121 the list must be the signal to send. Returns the number of
2122 processes successfully signaled.
2124 $cnt = kill 1, $child1, $child2;
2127 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2128 process groups instead of processes. (On System V, a negative I<PROCESS>
2129 number will also kill process groups, but that's not portable.) That
2130 means you usually want to use positive not negative signals. You may also
2131 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2137 The C<last> command is like the C<break> statement in C (as used in
2138 loops); it immediately exits the loop in question. If the LABEL is
2139 omitted, the command refers to the innermost enclosing loop. The
2140 C<continue> block, if any, is not executed:
2142 LINE: while (<STDIN>) {
2143 last LINE if /^$/; # exit when done with header
2147 C<last> cannot be used to exit a block which returns a value such as
2148 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2149 a grep() or map() operation.
2151 See also L</continue> for an illustration of how C<last>, C<next>, and
2158 Returns an lowercased version of EXPR. This is the internal function
2159 implementing the C<\L> escape in double-quoted strings.
2160 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2163 If EXPR is omitted, uses C<$_>.
2169 Returns the value of EXPR with the first character lowercased. This is
2170 the internal function implementing the C<\l> escape in double-quoted strings.
2171 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2173 If EXPR is omitted, uses C<$_>.
2179 Returns the length in characters of the value of EXPR. If EXPR is
2180 omitted, returns length of C<$_>. Note that this cannot be used on
2181 an entire array or hash to find out how many elements these have.
2182 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2184 =item link OLDFILE,NEWFILE
2186 Creates a new filename linked to the old filename. Returns true for
2187 success, false otherwise.
2189 =item listen SOCKET,QUEUESIZE
2191 Does the same thing that the listen system call does. Returns true if
2192 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2196 You really probably want to be using C<my> instead, because C<local> isn't
2197 what most people think of as "local". See L<perlsub/"Private Variables
2198 via my()"> for details.
2200 A local modifies the listed variables to be local to the enclosing
2201 block, file, or eval. If more than one value is listed, the list must
2202 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2203 for details, including issues with tied arrays and hashes.
2205 =item localtime EXPR
2207 Converts a time as returned by the time function to a 9-element list
2208 with the time analyzed for the local time zone. Typically used as
2212 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2215 All list elements are numeric, and come straight out of a struct tm.
2216 In particular this means that $mon has the range C<0..11> and $wday
2217 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2218 number of years since 1900, that is, $year is C<123> in year 2023,
2219 and I<not> simply the last two digits of the year. If you assume it is,
2220 then you create non-Y2K-compliant programs--and you wouldn't want to do
2223 If EXPR is omitted, uses the current time (C<localtime(time)>).
2225 In scalar context, returns the ctime(3) value:
2227 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2229 This scalar value is B<not> locale dependent, see L<perllocale>, but
2230 instead a Perl builtin. Also see the C<Time::Local> module, and the
2231 strftime(3) and mktime(3) function available via the POSIX module. To
2232 get somewhat similar but locale dependent date strings, set up your
2233 locale environment variables appropriately (please see L<perllocale>)
2234 and try for example:
2236 use POSIX qw(strftime);
2237 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2239 Note that the C<%a> and C<%b>, the short forms of the day of the week
2240 and the month of the year, may not necessarily be three characters wide.
2246 This function places an advisory lock on a variable, subroutine,
2247 or referenced object contained in I<THING> until the lock goes out
2248 of scope. This is a built-in function only if your version of Perl
2249 was built with threading enabled, and if you've said C<use Threads>.
2250 Otherwise a user-defined function by this name will be called. See
2257 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2258 returns log of C<$_>. To get the log of another base, use basic algebra:
2259 The base-N log of a number is equal to the natural log of that number
2260 divided by the natural log of N. For example:
2264 return log($n)/log(10);
2267 See also L</exp> for the inverse operation.
2269 =item lstat FILEHANDLE
2275 Does the same thing as the C<stat> function (including setting the
2276 special C<_> filehandle) but stats a symbolic link instead of the file
2277 the symbolic link points to. If symbolic links are unimplemented on
2278 your system, a normal C<stat> is done.
2280 If EXPR is omitted, stats C<$_>.
2284 The match operator. See L<perlop>.
2286 =item map BLOCK LIST
2290 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2291 C<$_> to each element) and returns the list value composed of the
2292 results of each such evaluation. In scalar context, returns the
2293 total number of elements so generated. Evaluates BLOCK or EXPR in
2294 list context, so each element of LIST may produce zero, one, or
2295 more elements in the returned value.
2297 @chars = map(chr, @nums);
2299 translates a list of numbers to the corresponding characters. And
2301 %hash = map { getkey($_) => $_ } @array;
2303 is just a funny way to write
2306 foreach $_ (@array) {
2307 $hash{getkey($_)} = $_;
2310 Note that, because C<$_> is a reference into the list value, it can
2311 be used to modify the elements of the array. While this is useful and
2312 supported, it can cause bizarre results if the LIST is not a named array.
2313 Using a regular C<foreach> loop for this purpose would be clearer in
2314 most cases. See also L</grep> for an array composed of those items of
2315 the original list for which the BLOCK or EXPR evaluates to true.
2317 =item mkdir FILENAME,MASK
2319 Creates the directory specified by FILENAME, with permissions
2320 specified by MASK (as modified by C<umask>). If it succeeds it
2321 returns true, otherwise it returns false and sets C<$!> (errno).
2323 In general, it is better to create directories with permissive MASK,
2324 and let the user modify that with their C<umask>, than it is to supply
2325 a restrictive MASK and give the user no way to be more permissive.
2326 The exceptions to this rule are when the file or directory should be
2327 kept private (mail files, for instance). The perlfunc(1) entry on
2328 C<umask> discusses the choice of MASK in more detail.
2330 =item msgctl ID,CMD,ARG
2332 Calls the System V IPC function msgctl(2). You'll probably have to say
2336 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2337 then ARG must be a variable which will hold the returned C<msqid_ds>
2338 structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
2339 true"> for zero, or the actual return value otherwise. See also
2340 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2342 =item msgget KEY,FLAGS
2344 Calls the System V IPC function msgget(2). Returns the message queue
2345 id, or the undefined value if there is an error. See also C<IPC::SysV>
2346 and C<IPC::Msg> documentation.
2348 =item msgsnd ID,MSG,FLAGS
2350 Calls the System V IPC function msgsnd to send the message MSG to the
2351 message queue ID. MSG must begin with the long integer message type,
2352 which may be created with C<pack("l", $type)>. Returns true if
2353 successful, or false if there is an error. See also C<IPC::SysV>
2354 and C<IPC::SysV::Msg> documentation.
2356 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2358 Calls the System V IPC function msgrcv to receive a message from
2359 message queue ID into variable VAR with a maximum message size of
2360 SIZE. Note that if a message is received, the message type will be
2361 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2362 size of the message type. Returns true if successful, or false if
2363 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2367 A C<my> declares the listed variables to be local (lexically) to the
2368 enclosing block, file, or C<eval>. If
2369 more than one value is listed, the list must be placed in parentheses. See
2370 L<perlsub/"Private Variables via my()"> for details.
2376 The C<next> command is like the C<continue> statement in C; it starts
2377 the next iteration of the loop:
2379 LINE: while (<STDIN>) {
2380 next LINE if /^#/; # discard comments
2384 Note that if there were a C<continue> block on the above, it would get
2385 executed even on discarded lines. If the LABEL is omitted, the command
2386 refers to the innermost enclosing loop.
2388 C<next> cannot be used to exit a block which returns a value such as
2389 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2390 a grep() or map() operation.
2392 See also L</continue> for an illustration of how C<last>, C<next>, and
2395 =item no Module LIST
2397 See the L</use> function, which C<no> is the opposite of.
2403 Interprets EXPR as an octal string and returns the corresponding
2404 value. (If EXPR happens to start off with C<0x>, interprets it as a
2405 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2406 binary string.) The following will handle decimal, binary, octal, and
2407 hex in the standard Perl or C notation:
2409 $val = oct($val) if $val =~ /^0/;
2411 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2412 in octal), use sprintf() or printf():
2414 $perms = (stat("filename"))[2] & 07777;
2415 $oct_perms = sprintf "%lo", $perms;
2417 The oct() function is commonly used when a string such as C<644> needs
2418 to be converted into a file mode, for example. (Although perl will
2419 automatically convert strings into numbers as needed, this automatic
2420 conversion assumes base 10.)
2422 =item open FILEHANDLE,EXPR
2424 =item open FILEHANDLE
2426 Opens the file whose filename is given by EXPR, and associates it with
2427 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2428 name of the real filehandle wanted. If EXPR is omitted, the scalar
2429 variable of the same name as the FILEHANDLE contains the filename.
2430 (Note that lexical variables--those declared with C<my>--will not work
2431 for this purpose; so if you're using C<my>, specify EXPR in your call
2432 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2435 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2436 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2437 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2438 the file is opened for appending, again being created if necessary.
2439 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2440 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2441 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2442 file first. You can't usually use either read-write mode for updating
2443 textfiles, since they have variable length records. See the B<-i>
2444 switch in L<perlrun> for a better approach. The file is created with
2445 permissions of C<0666> modified by the process' C<umask> value.
2447 The prefix and the filename may be separated with spaces.
2448 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2449 C<'w+'>, C<'a'>, and C<'a+'>.
2451 If the filename begins with C<'|'>, the filename is interpreted as a
2452 command to which output is to be piped, and if the filename ends with a
2453 C<'|'>, the filename is interpreted as a command which pipes output to
2454 us. See L<perlipc/"Using open() for IPC">
2455 for more examples of this. (You are not allowed to C<open> to a command
2456 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2457 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2459 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2460 nonzero upon success, the undefined value otherwise. If the C<open>
2461 involved a pipe, the return value happens to be the pid of the
2464 If you're unfortunate enough to be running Perl on a system that
2465 distinguishes between text files and binary files (modern operating
2466 systems don't care), then you should check out L</binmode> for tips for
2467 dealing with this. The key distinction between systems that need C<binmode>
2468 and those that don't is their text file formats. Systems like Unix, MacOS, and
2469 Plan9, which delimit lines with a single character, and which encode that
2470 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2472 When opening a file, it's usually a bad idea to continue normal execution
2473 if the request failed, so C<open> is frequently used in connection with
2474 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2475 where you want to make a nicely formatted error message (but there are
2476 modules that can help with that problem)) you should always check
2477 the return value from opening a file. The infrequent exception is when
2478 working with an unopened filehandle is actually what you want to do.
2483 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2484 while (<ARTICLE>) {...
2486 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2487 # if the open fails, output is discarded
2489 open(DBASE, '+<dbase.mine') # open for update
2490 or die "Can't open 'dbase.mine' for update: $!";
2492 open(ARTICLE, "caesar <$article |") # decrypt article
2493 or die "Can't start caesar: $!";
2495 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2496 or die "Can't start sort: $!";
2498 # process argument list of files along with any includes
2500 foreach $file (@ARGV) {
2501 process($file, 'fh00');
2505 my($filename, $input) = @_;
2506 $input++; # this is a string increment
2507 unless (open($input, $filename)) {
2508 print STDERR "Can't open $filename: $!\n";
2513 while (<$input>) { # note use of indirection
2514 if (/^#include "(.*)"/) {
2515 process($1, $input);
2522 You may also, in the Bourne shell tradition, specify an EXPR beginning
2523 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2524 name of a filehandle (or file descriptor, if numeric) to be
2525 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2526 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2527 mode you specify should match the mode of the original filehandle.
2528 (Duping a filehandle does not take into account any existing contents of
2530 Here is a script that saves, redirects, and restores STDOUT and
2534 open(OLDOUT, ">&STDOUT");
2535 open(OLDERR, ">&STDERR");
2537 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2538 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2540 select(STDERR); $| = 1; # make unbuffered
2541 select(STDOUT); $| = 1; # make unbuffered
2543 print STDOUT "stdout 1\n"; # this works for
2544 print STDERR "stderr 1\n"; # subprocesses too
2549 open(STDOUT, ">&OLDOUT");
2550 open(STDERR, ">&OLDERR");
2552 print STDOUT "stdout 2\n";
2553 print STDERR "stderr 2\n";
2555 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2556 equivalent of C's C<fdopen> of that file descriptor; this is more
2557 parsimonious of file descriptors. For example:
2559 open(FILEHANDLE, "<&=$fd")
2561 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2562 there is an implicit fork done, and the return value of open is the pid
2563 of the child within the parent process, and C<0> within the child
2564 process. (Use C<defined($pid)> to determine whether the open was successful.)
2565 The filehandle behaves normally for the parent, but i/o to that
2566 filehandle is piped from/to the STDOUT/STDIN of the child process.
2567 In the child process the filehandle isn't opened--i/o happens from/to
2568 the new STDOUT or STDIN. Typically this is used like the normal
2569 piped open when you want to exercise more control over just how the
2570 pipe command gets executed, such as when you are running setuid, and
2571 don't want to have to scan shell commands for metacharacters.
2572 The following pairs are more or less equivalent:
2574 open(FOO, "|tr '[a-z]' '[A-Z]'");
2575 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2577 open(FOO, "cat -n '$file'|");
2578 open(FOO, "-|") || exec 'cat', '-n', $file;
2580 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2582 NOTE: On any operation that may do a fork, all files opened for output
2583 are flushed before the fork is attempted. On systems that support a
2584 close-on-exec flag on files, the flag will be set for the newly opened
2585 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2587 Closing any piped filehandle causes the parent process to wait for the
2588 child to finish, and returns the status value in C<$?>.
2590 The filename passed to open will have leading and trailing
2591 whitespace deleted, and the normal redirection characters
2592 honored. This property, known as "magic open",
2593 can often be used to good effect. A user could specify a filename of
2594 F<"rsh cat file |">, or you could change certain filenames as needed:
2596 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2597 open(FH, $filename) or die "Can't open $filename: $!";
2599 However, to open a file with arbitrary weird characters in it, it's
2600 necessary to protect any leading and trailing whitespace:
2602 $file =~ s#^(\s)#./$1#;
2603 open(FOO, "< $file\0");
2605 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2606 should use the C<sysopen> function, which involves no such magic. This is
2607 another way to protect your filenames from interpretation. For example:
2610 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2611 or die "sysopen $path: $!";
2612 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2613 print HANDLE "stuff $$\n");
2615 print "File contains: ", <HANDLE>;
2617 Using the constructor from the C<IO::Handle> package (or one of its
2618 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2619 filehandles that have the scope of whatever variables hold references to
2620 them, and automatically close whenever and however you leave that scope:
2624 sub read_myfile_munged {
2626 my $handle = new IO::File;
2627 open($handle, "myfile") or die "myfile: $!";
2629 or return (); # Automatically closed here.
2630 mung $first or die "mung failed"; # Or here.
2631 return $first, <$handle> if $ALL; # Or here.
2635 See L</seek> for some details about mixing reading and writing.
2637 =item opendir DIRHANDLE,EXPR
2639 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2640 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2641 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2647 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2648 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2649 See L<utf8> for more about Unicode.
2651 =item pack TEMPLATE,LIST
2653 Takes a list of values and packs it into a binary structure,
2654 returning the string containing the structure. The TEMPLATE is a
2655 sequence of characters that give the order and type of values, as
2658 a A string with arbitrary binary data, will be null padded.
2659 A An ascii string, will be space padded.
2660 Z A null terminated (asciz) string, will be null padded.
2662 b A bit string (ascending bit order, like vec()).
2663 B A bit string (descending bit order).
2664 h A hex string (low nybble first).
2665 H A hex string (high nybble first).
2667 c A signed char value.
2668 C An unsigned char value. Only does bytes. See U for Unicode.
2670 s A signed short value.
2671 S An unsigned short value.
2672 (This 'short' is _exactly_ 16 bits, which may differ from
2673 what a local C compiler calls 'short'.)
2675 i A signed integer value.
2676 I An unsigned integer value.
2677 (This 'integer' is _at_least_ 32 bits wide. Its exact
2678 size depends on what a local C compiler calls 'int',
2679 and may even be larger than the 'long' described in
2682 l A signed long value.
2683 L An unsigned long value.
2684 (This 'long' is _exactly_ 32 bits, which may differ from
2685 what a local C compiler calls 'long'.)
2687 n A short in "network" (big-endian) order.
2688 N A long in "network" (big-endian) order.
2689 v A short in "VAX" (little-endian) order.
2690 V A long in "VAX" (little-endian) order.
2691 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2692 _exactly_ 32 bits, respectively.)
2694 q A signed quad (64-bit) value.
2695 Q An unsigned quad value.
2696 (Available only if your system supports 64-bit integer values
2697 _and_ if Perl has been compiled to support those.
2698 Causes a fatal error otherwise.)
2700 f A single-precision float in the native format.
2701 d A double-precision float in the native format.
2703 p A pointer to a null-terminated string.
2704 P A pointer to a structure (fixed-length string).
2706 u A uuencoded string.
2707 U A Unicode character number. Encodes to UTF-8 internally.
2708 Works even if C<use utf8> is not in effect.
2710 w A BER compressed integer. Its bytes represent an unsigned
2711 integer in base 128, most significant digit first, with as
2712 few digits as possible. Bit eight (the high bit) is set
2713 on each byte except the last.
2717 @ Null fill to absolute position.
2719 The following rules apply:
2725 Each letter may optionally be followed by a number giving a repeat
2726 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2727 C<"H">, and C<"P"> the pack function will gobble up that many values from
2728 the LIST. A C<*> for the repeat count means to use however many items are
2733 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2734 string of length count, padding with nulls or spaces as necessary. When
2735 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2736 after the first null, and C<"a"> returns data verbatim.
2740 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2744 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2748 The C<"p"> type packs a pointer to a null-terminated string. You are
2749 responsible for ensuring the string is not a temporary value (which can
2750 potentially get deallocated before you get around to using the packed result).
2751 The C<"P"> type packs a pointer to a structure of the size indicated by the
2752 length. A NULL pointer is created if the corresponding value for C<"p"> or
2757 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2758 immediately followed by a C<"!"> to signify native shorts or longs--as
2759 you can see from above for example a bare C<"l"> does mean exactly 32
2760 bits, the native C<long> (as seen by the local C compiler) may be
2761 larger. This is an issue mainly in 64-bit platforms. You can see
2762 whether using C<"!"> makes any difference by
2764 print length(pack("s")), " ", length(pack("s!")), "\n";
2765 print length(pack("l")), " ", length(pack("l!")), "\n";
2767 C<"i!"> and C<"I!"> also work but only because of completeness;
2768 they are identical to C<"i"> and C<"I">.
2770 The actual sizes (in bytes) of native shorts, ints, and longs on
2771 the platform where Perl was built are also available via L<Config>:
2773 The actual sizes (in bytes) of native shorts, ints, longs, and long
2774 longs on the platform where Perl was built are also available via
2778 print $Config{shortsize}, "\n";
2779 print $Config{intsize}, "\n";
2780 print $Config{longsize}, "\n";
2781 print $Config{longlongsize}, "\n";
2785 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2786 are inherently non-portable between processors and operating systems
2787 because they obey the native byteorder and endianness. For example a
2788 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
2789 (arranged in and handled by the CPU registers) into bytes as
2791 0x12 0x34 0x56 0x78 # little-endian
2792 0x78 0x56 0x34 0x12 # big-endian
2794 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2795 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2796 Power, and Cray are big-endian. MIPS can be either: Digital used it
2797 in little-endian mode; SGI uses it in big-endian mode.
2799 The names `big-endian' and `little-endian' are comic references to
2800 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2801 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2802 the egg-eating habits of the Lilliputians.
2804 Some systems may even have weird byte orders such as
2809 You can see your system's preference with
2811 print join(" ", map { sprintf "%#02x", $_ }
2812 unpack("C*",pack("L",0x12345678))), "\n";
2814 The byteorder on the platform where Perl was built is also available
2818 print $Config{byteorder}, "\n";
2820 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2821 and C<'87654321'> are big-endian.
2823 If you want portable packed integers use the formats C<"n">, C<"N">,
2824 C<"v">, and C<"V">, their byte endianness and size is known.
2828 Real numbers (floats and doubles) are in the native machine format only;
2829 due to the multiplicity of floating formats around, and the lack of a
2830 standard "network" representation, no facility for interchange has been
2831 made. This means that packed floating point data written on one machine
2832 may not be readable on another - even if both use IEEE floating point
2833 arithmetic (as the endian-ness of the memory representation is not part
2836 Note that Perl uses doubles internally for all numeric calculation, and
2837 converting from double into float and thence back to double again will
2838 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2845 $foo = pack("CCCC",65,66,67,68);
2847 $foo = pack("C4",65,66,67,68);
2849 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2850 # same thing with Unicode circled letters
2852 $foo = pack("ccxxcc",65,66,67,68);
2855 $foo = pack("s2",1,2);
2856 # "\1\0\2\0" on little-endian
2857 # "\0\1\0\2" on big-endian
2859 $foo = pack("a4","abcd","x","y","z");
2862 $foo = pack("aaaa","abcd","x","y","z");
2865 $foo = pack("a14","abcdefg");
2866 # "abcdefg\0\0\0\0\0\0\0"
2868 $foo = pack("i9pl", gmtime);
2869 # a real struct tm (on my system anyway)
2871 $utmp_template = "Z8 Z8 Z16 L";
2872 $utmp = pack($utmp_template, @utmp1);
2873 # a struct utmp (BSDish)
2875 @utmp2 = unpack($utmp_template, $utmp);
2876 # "@utmp1" eq "@utmp2"
2879 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2882 The same template may generally also be used in unpack().
2886 =item package NAMESPACE
2888 Declares the compilation unit as being in the given namespace. The scope
2889 of the package declaration is from the declaration itself through the end
2890 of the enclosing block, file, or eval (the same as the C<my> operator).
2891 All further unqualified dynamic identifiers will be in this namespace.
2892 A package statement affects only dynamic variables--including those
2893 you've used C<local> on--but I<not> lexical variables, which are created
2894 with C<my>. Typically it would be the first declaration in a file to
2895 be included by the C<require> or C<use> operator. You can switch into a
2896 package in more than one place; it merely influences which symbol table
2897 is used by the compiler for the rest of that block. You can refer to
2898 variables and filehandles in other packages by prefixing the identifier
2899 with the package name and a double colon: C<$Package::Variable>.
2900 If the package name is null, the C<main> package as assumed. That is,
2901 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2902 still seen in older code).
2904 If NAMESPACE is omitted, then there is no current package, and all
2905 identifiers must be fully qualified or lexicals. This is stricter
2906 than C<use strict>, since it also extends to function names.
2908 See L<perlmod/"Packages"> for more information about packages, modules,
2909 and classes. See L<perlsub> for other scoping issues.
2911 =item pipe READHANDLE,WRITEHANDLE
2913 Opens a pair of connected pipes like the corresponding system call.
2914 Note that if you set up a loop of piped processes, deadlock can occur
2915 unless you are very careful. In addition, note that Perl's pipes use
2916 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2917 after each command, depending on the application.
2919 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2920 for examples of such things.
2922 On systems that support a close-on-exec flag on files, the flag will be set
2923 for the newly opened file descriptors as determined by the value of $^F.
2930 Pops and returns the last value of the array, shortening the array by
2931 one element. Has an effect similar to
2935 If there are no elements in the array, returns the undefined value
2936 (although this may happen at other times as well). If ARRAY is
2937 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
2938 array in subroutines, just like C<shift>.
2944 Returns the offset of where the last C<m//g> search left off for the variable
2945 is in question (C<$_> is used when the variable is not specified). May be
2946 modified to change that offset. Such modification will also influence
2947 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2950 =item print FILEHANDLE LIST
2956 Prints a string or a list of strings. Returns true if successful.
2957 FILEHANDLE may be a scalar variable name, in which case the variable
2958 contains the name of or a reference to the filehandle, thus introducing
2959 one level of indirection. (NOTE: If FILEHANDLE is a variable and
2960 the next token is a term, it may be misinterpreted as an operator
2961 unless you interpose a C<+> or put parentheses around the arguments.)
2962 If FILEHANDLE is omitted, prints by default to standard output (or
2963 to the last selected output channel--see L</select>). If LIST is
2964 also omitted, prints C<$_> to the currently selected output channel.
2965 To set the default output channel to something other than STDOUT
2966 use the select operation. The current value of C<$,> (if any) is
2967 printed between each LIST item. The current value of C<$\> (if
2968 any) is printed after the entire LIST has been printed. Because
2969 print takes a LIST, anything in the LIST is evaluated in list
2970 context, and any subroutine that you call will have one or more of
2971 its expressions evaluated in list context. Also be careful not to
2972 follow the print keyword with a left parenthesis unless you want
2973 the corresponding right parenthesis to terminate the arguments to
2974 the print--interpose a C<+> or put parentheses around all the
2977 Note that if you're storing FILEHANDLES in an array or other expression,
2978 you will have to use a block returning its value instead:
2980 print { $files[$i] } "stuff\n";
2981 print { $OK ? STDOUT : STDERR } "stuff\n";
2983 =item printf FILEHANDLE FORMAT, LIST
2985 =item printf FORMAT, LIST
2987 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2988 (the output record separator) is not appended. The first argument
2989 of the list will be interpreted as the C<printf> format. If C<use locale> is
2990 in effect, the character used for the decimal point in formatted real numbers
2991 is affected by the LC_NUMERIC locale. See L<perllocale>.
2993 Don't fall into the trap of using a C<printf> when a simple
2994 C<print> would do. The C<print> is more efficient and less
2997 =item prototype FUNCTION
2999 Returns the prototype of a function as a string (or C<undef> if the
3000 function has no prototype). FUNCTION is a reference to, or the name of,
3001 the function whose prototype you want to retrieve.
3003 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3004 name for Perl builtin. If the builtin is not I<overridable> (such as
3005 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3006 C<system>) returns C<undef> because the builtin does not really behave
3007 like a Perl function. Otherwise, the string describing the equivalent
3008 prototype is returned.
3010 =item push ARRAY,LIST
3012 Treats ARRAY as a stack, and pushes the values of LIST
3013 onto the end of ARRAY. The length of ARRAY increases by the length of
3014 LIST. Has the same effect as
3017 $ARRAY[++$#ARRAY] = $value;
3020 but is more efficient. Returns the new number of elements in the array.
3032 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3034 =item quotemeta EXPR
3038 Returns the value of EXPR with all non-alphanumeric
3039 characters backslashed. (That is, all characters not matching
3040 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3041 returned string, regardless of any locale settings.)
3042 This is the internal function implementing
3043 the C<\Q> escape in double-quoted strings.
3045 If EXPR is omitted, uses C<$_>.
3051 Returns a random fractional number greater than or equal to C<0> and less
3052 than the value of EXPR. (EXPR should be positive.) If EXPR is
3053 omitted, the value C<1> is used. Automatically calls C<srand> unless
3054 C<srand> has already been called. See also C<srand>.
3056 (Note: If your rand function consistently returns numbers that are too
3057 large or too small, then your version of Perl was probably compiled
3058 with the wrong number of RANDBITS.)
3060 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3062 =item read FILEHANDLE,SCALAR,LENGTH
3064 Attempts to read LENGTH bytes of data into variable SCALAR from the
3065 specified FILEHANDLE. Returns the number of bytes actually read,
3066 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3067 or shrunk to the length actually read. An OFFSET may be specified to
3068 place the read data at some other place than the beginning of the
3069 string. This call is actually implemented in terms of stdio's fread(3)
3070 call. To get a true read(2) system call, see C<sysread>.
3072 =item readdir DIRHANDLE
3074 Returns the next directory entry for a directory opened by C<opendir>.
3075 If used in list context, returns all the rest of the entries in the
3076 directory. If there are no more entries, returns an undefined value in
3077 scalar context or a null list in list context.
3079 If you're planning to filetest the return values out of a C<readdir>, you'd
3080 better prepend the directory in question. Otherwise, because we didn't
3081 C<chdir> there, it would have been testing the wrong file.
3083 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3084 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3089 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3090 context, each call reads and returns the next line, until end-of-file is
3091 reached, whereupon the subsequent call returns undef. In list context,
3092 reads until end-of-file is reached and returns a list of lines. Note that
3093 the notion of "line" used here is however you may have defined it
3094 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3096 When C<$/> is set to C<undef>, when readline() is in scalar
3097 context (i.e. file slurp mode), and when an empty file is read, it
3098 returns C<''> the first time, followed by C<undef> subsequently.
3100 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3101 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3102 operator is discussed in more detail in L<perlop/"I/O Operators">.
3105 $line = readline(*STDIN); # same thing
3111 Returns the value of a symbolic link, if symbolic links are
3112 implemented. If not, gives a fatal error. If there is some system
3113 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3114 omitted, uses C<$_>.
3118 EXPR is executed as a system command.
3119 The collected standard output of the command is returned.
3120 In scalar context, it comes back as a single (potentially
3121 multi-line) string. In list context, returns a list of lines
3122 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3123 This is the internal function implementing the C<qx/EXPR/>
3124 operator, but you can use it directly. The C<qx/EXPR/>
3125 operator is discussed in more detail in L<perlop/"I/O Operators">.
3127 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3129 Receives a message on a socket. Attempts to receive LENGTH bytes of
3130 data into variable SCALAR from the specified SOCKET filehandle.
3131 Actually does a C C<recvfrom>, so that it can return the address of the
3132 sender. Returns the undefined value if there's an error. SCALAR will
3133 be grown or shrunk to the length actually read. Takes the same flags
3134 as the system call of the same name.
3135 See L<perlipc/"UDP: Message Passing"> for examples.
3141 The C<redo> command restarts the loop block without evaluating the
3142 conditional again. The C<continue> block, if any, is not executed. If
3143 the LABEL is omitted, the command refers to the innermost enclosing
3144 loop. This command is normally used by programs that want to lie to
3145 themselves about what was just input:
3147 # a simpleminded Pascal comment stripper
3148 # (warning: assumes no { or } in strings)
3149 LINE: while (<STDIN>) {
3150 while (s|({.*}.*){.*}|$1 |) {}
3155 if (/}/) { # end of comment?
3164 C<redo> cannot be used to retry a block which returns a value such as
3165 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3166 a grep() or map() operation.
3168 See also L</continue> for an illustration of how C<last>, C<next>, and
3175 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3176 is not specified, C<$_> will be used. The value returned depends on the
3177 type of thing the reference is a reference to.
3178 Builtin types include:
3188 If the referenced object has been blessed into a package, then that package
3189 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3191 if (ref($r) eq "HASH") {
3192 print "r is a reference to a hash.\n";
3195 print "r is not a reference at all.\n";
3197 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3198 print "r is a reference to something that isa hash.\n";
3201 See also L<perlref>.
3203 =item rename OLDNAME,NEWNAME
3205 Changes the name of a file; an existing file NEWNAME will be
3206 clobbered. Returns true for success, false otherwise.
3208 Behavior of this function varies wildly depending on your system
3209 implementation. For example, it will usually not work across file system
3210 boundaries, even though the system I<mv> command sometimes compensates
3211 for this. Other restrictions include whether it works on directories,
3212 open files, or pre-existing files. Check L<perlport> and either the
3213 rename(2) manpage or equivalent system documentation for details.
3219 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3220 supplied. If EXPR is numeric, demands that the current version of Perl
3221 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3223 Otherwise, demands that a library file be included if it hasn't already
3224 been included. The file is included via the do-FILE mechanism, which is
3225 essentially just a variety of C<eval>. Has semantics similar to the following
3230 return 1 if $INC{$filename};
3231 my($realfilename,$result);
3233 foreach $prefix (@INC) {
3234 $realfilename = "$prefix/$filename";
3235 if (-f $realfilename) {
3236 $result = do $realfilename;
3240 die "Can't find $filename in \@INC";
3243 die "$filename did not return true value" unless $result;
3244 $INC{$filename} = $realfilename;
3248 Note that the file will not be included twice under the same specified
3249 name. The file must return true as the last statement to indicate
3250 successful execution of any initialization code, so it's customary to
3251 end such a file with C<1;> unless you're sure it'll return true
3252 otherwise. But it's better just to put the C<1;>, in case you add more
3255 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3256 replaces "F<::>" with "F</>" in the filename for you,
3257 to make it easy to load standard modules. This form of loading of
3258 modules does not risk altering your namespace.
3260 In other words, if you try this:
3262 require Foo::Bar; # a splendid bareword
3264 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3265 directories specified in the C<@INC> array.
3267 But if you try this:
3269 $class = 'Foo::Bar';
3270 require $class; # $class is not a bareword
3272 require "Foo::Bar"; # not a bareword because of the ""
3274 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3275 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3277 eval "require $class";
3279 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3285 Generally used in a C<continue> block at the end of a loop to clear
3286 variables and reset C<??> searches so that they work again. The
3287 expression is interpreted as a list of single characters (hyphens
3288 allowed for ranges). All variables and arrays beginning with one of
3289 those letters are reset to their pristine state. If the expression is
3290 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3291 only variables or searches in the current package. Always returns
3294 reset 'X'; # reset all X variables
3295 reset 'a-z'; # reset lower case variables
3296 reset; # just reset ?one-time? searches
3298 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3299 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3300 variables--lexical variables are unaffected, but they clean themselves
3301 up on scope exit anyway, so you'll probably want to use them instead.
3308 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3309 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3310 context, depending on how the return value will be used, and the context
3311 may vary from one execution to the next (see C<wantarray>). If no EXPR
3312 is given, returns an empty list in list context, the undefined value in
3313 scalar context, and (of course) nothing at all in a void context.
3315 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3316 or do FILE will automatically return the value of the last expression
3321 In list context, returns a list value consisting of the elements
3322 of LIST in the opposite order. In scalar context, concatenates the
3323 elements of LIST and returns a string value with all characters
3324 in the opposite order.
3326 print reverse <>; # line tac, last line first
3328 undef $/; # for efficiency of <>
3329 print scalar reverse <>; # character tac, last line tsrif
3331 This operator is also handy for inverting a hash, although there are some
3332 caveats. If a value is duplicated in the original hash, only one of those
3333 can be represented as a key in the inverted hash. Also, this has to
3334 unwind one hash and build a whole new one, which may take some time
3335 on a large hash, such as from a DBM file.
3337 %by_name = reverse %by_address; # Invert the hash
3339 =item rewinddir DIRHANDLE
3341 Sets the current position to the beginning of the directory for the
3342 C<readdir> routine on DIRHANDLE.
3344 =item rindex STR,SUBSTR,POSITION
3346 =item rindex STR,SUBSTR
3348 Works just like index() except that it returns the position of the LAST
3349 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3350 last occurrence at or before that position.
3352 =item rmdir FILENAME
3356 Deletes the directory specified by FILENAME if that directory is empty. If it
3357 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3358 FILENAME is omitted, uses C<$_>.
3362 The substitution operator. See L<perlop>.
3366 Forces EXPR to be interpreted in scalar context and returns the value
3369 @counts = ( scalar @a, scalar @b, scalar @c );
3371 There is no equivalent operator to force an expression to
3372 be interpolated in list context because in practice, this is never
3373 needed. If you really wanted to do so, however, you could use
3374 the construction C<@{[ (some expression) ]}>, but usually a simple
3375 C<(some expression)> suffices.
3377 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3378 parenthesized list, this behaves as a scalar comma expression, evaluating
3379 all but the last element in void context and returning the final element
3380 evaluated in scalar context. This is seldom what you want.
3382 The following single statement:
3384 print uc(scalar(&foo,$bar)),$baz;
3386 is the moral equivalent of these two:
3389 print(uc($bar),$baz);
3391 See L<perlop> for more details on unary operators and the comma operator.
3393 =item seek FILEHANDLE,POSITION,WHENCE
3395 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3396 FILEHANDLE may be an expression whose value gives the name of the
3397 filehandle. The values for WHENCE are C<0> to set the new position to
3398 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3399 set it to EOF plus POSITION (typically negative). For WHENCE you may
3400 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3401 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3403 If you want to position file for C<sysread> or C<syswrite>, don't use
3404 C<seek>--buffering makes its effect on the file's system position
3405 unpredictable and non-portable. Use C<sysseek> instead.
3407 Due to the rules and rigors of ANSI C, on some systems you have to do a
3408 seek whenever you switch between reading and writing. Amongst other
3409 things, this may have the effect of calling stdio's clearerr(3).
3410 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3414 This is also useful for applications emulating C<tail -f>. Once you hit
3415 EOF on your read, and then sleep for a while, you might have to stick in a
3416 seek() to reset things. The C<seek> doesn't change the current position,
3417 but it I<does> clear the end-of-file condition on the handle, so that the
3418 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3420 If that doesn't work (some stdios are particularly cantankerous), then
3421 you may need something more like this:
3424 for ($curpos = tell(FILE); $_ = <FILE>;
3425 $curpos = tell(FILE)) {
3426 # search for some stuff and put it into files
3428 sleep($for_a_while);
3429 seek(FILE, $curpos, 0);
3432 =item seekdir DIRHANDLE,POS
3434 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3435 must be a value returned by C<telldir>. Has the same caveats about
3436 possible directory compaction as the corresponding system library
3439 =item select FILEHANDLE
3443 Returns the currently selected filehandle. Sets the current default
3444 filehandle for output, if FILEHANDLE is supplied. This has two
3445 effects: first, a C<write> or a C<print> without a filehandle will
3446 default to this FILEHANDLE. Second, references to variables related to
3447 output will refer to this output channel. For example, if you have to
3448 set the top of form format for more than one output channel, you might
3456 FILEHANDLE may be an expression whose value gives the name of the
3457 actual filehandle. Thus:
3459 $oldfh = select(STDERR); $| = 1; select($oldfh);
3461 Some programmers may prefer to think of filehandles as objects with
3462 methods, preferring to write the last example as:
3465 STDERR->autoflush(1);
3467 =item select RBITS,WBITS,EBITS,TIMEOUT
3469 This calls the select(2) system call with the bit masks specified, which
3470 can be constructed using C<fileno> and C<vec>, along these lines:
3472 $rin = $win = $ein = '';
3473 vec($rin,fileno(STDIN),1) = 1;
3474 vec($win,fileno(STDOUT),1) = 1;
3477 If you want to select on many filehandles you might wish to write a
3481 my(@fhlist) = split(' ',$_[0]);
3484 vec($bits,fileno($_),1) = 1;
3488 $rin = fhbits('STDIN TTY SOCK');
3492 ($nfound,$timeleft) =
3493 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3495 or to block until something becomes ready just do this
3497 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3499 Most systems do not bother to return anything useful in $timeleft, so
3500 calling select() in scalar context just returns $nfound.
3502 Any of the bit masks can also be undef. The timeout, if specified, is
3503 in seconds, which may be fractional. Note: not all implementations are
3504 capable of returning the$timeleft. If not, they always return
3505 $timeleft equal to the supplied $timeout.
3507 You can effect a sleep of 250 milliseconds this way:
3509 select(undef, undef, undef, 0.25);
3511 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3512 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3513 then only on POSIX systems. You have to use C<sysread> instead.
3515 =item semctl ID,SEMNUM,CMD,ARG
3517 Calls the System V IPC function C<semctl>. You'll probably have to say
3521 first to get the correct constant definitions. If CMD is IPC_STAT or
3522 GETALL, then ARG must be a variable which will hold the returned
3523 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3524 undefined value for error, "C<0 but true>" for zero, or the actual return
3525 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3527 =item semget KEY,NSEMS,FLAGS
3529 Calls the System V IPC function semget. Returns the semaphore id, or
3530 the undefined value if there is an error. See also C<IPC::SysV> and
3531 C<IPC::SysV::Semaphore> documentation.
3533 =item semop KEY,OPSTRING
3535 Calls the System V IPC function semop to perform semaphore operations
3536 such as signaling and waiting. OPSTRING must be a packed array of
3537 semop structures. Each semop structure can be generated with
3538 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3539 operations is implied by the length of OPSTRING. Returns true if
3540 successful, or false if there is an error. As an example, the
3541 following code waits on semaphore $semnum of semaphore id $semid:
3543 $semop = pack("sss", $semnum, -1, 0);
3544 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3546 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3547 and C<IPC::SysV::Semaphore> documentation.
3549 =item send SOCKET,MSG,FLAGS,TO
3551 =item send SOCKET,MSG,FLAGS
3553 Sends a message on a socket. Takes the same flags as the system call
3554 of the same name. On unconnected sockets you must specify a
3555 destination to send TO, in which case it does a C C<sendto>. Returns
3556 the number of characters sent, or the undefined value if there is an
3557 error. The C system call sendmsg(2) is currently unimplemented.
3558 See L<perlipc/"UDP: Message Passing"> for examples.
3560 =item setpgrp PID,PGRP
3562 Sets the current process group for the specified PID, C<0> for the current
3563 process. Will produce a fatal error if used on a machine that doesn't
3564 implement setpgrp(2). If the arguments are omitted, it defaults to
3565 C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
3566 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3568 =item setpriority WHICH,WHO,PRIORITY
3570 Sets the current priority for a process, a process group, or a user.
3571 (See setpriority(2).) Will produce a fatal error if used on a machine
3572 that doesn't implement setpriority(2).
3574 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3576 Sets the socket option requested. Returns undefined if there is an
3577 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3584 Shifts the first value of the array off and returns it, shortening the
3585 array by 1 and moving everything down. If there are no elements in the
3586 array, returns the undefined value. If ARRAY is omitted, shifts the
3587 C<@_> array within the lexical scope of subroutines and formats, and the
3588 C<@ARGV> array at file scopes or within the lexical scopes established by
3589 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3590 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3591 same thing to the left end of an array that C<pop> and C<push> do to the
3594 =item shmctl ID,CMD,ARG
3596 Calls the System V IPC function shmctl. You'll probably have to say
3600 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3601 then ARG must be a variable which will hold the returned C<shmid_ds>
3602 structure. Returns like ioctl: the undefined value for error, "C<0> but
3603 true" for zero, or the actual return value otherwise.
3604 See also C<IPC::SysV> documentation.
3606 =item shmget KEY,SIZE,FLAGS
3608 Calls the System V IPC function shmget. Returns the shared memory
3609 segment id, or the undefined value if there is an error.
3610 See also C<IPC::SysV> documentation.
3612 =item shmread ID,VAR,POS,SIZE
3614 =item shmwrite ID,STRING,POS,SIZE
3616 Reads or writes the System V shared memory segment ID starting at
3617 position POS for size SIZE by attaching to it, copying in/out, and
3618 detaching from it. When reading, VAR must be a variable that will
3619 hold the data read. When writing, if STRING is too long, only SIZE
3620 bytes are used; if STRING is too short, nulls are written to fill out
3621 SIZE bytes. Return true if successful, or false if there is an error.
3622 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3625 =item shutdown SOCKET,HOW
3627 Shuts down a socket connection in the manner indicated by HOW, which
3628 has the same interpretation as in the system call of the same name.
3630 shutdown(SOCKET, 0); # I/we have stopped reading data
3631 shutdown(SOCKET, 1); # I/we have stopped writing data
3632 shutdown(SOCKET, 2); # I/we have stopped using this socket
3634 This is useful with sockets when you want to tell the other
3635 side you're done writing but not done reading, or vice versa.
3636 It's also a more insistent form of close because it also
3637 disables the file descriptor in any forked copies in other
3644 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3645 returns sine of C<$_>.
3647 For the inverse sine operation, you may use the C<POSIX::asin>
3648 function, or use this relation:
3650 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3656 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3657 May be interrupted if the process receives a signal such as C<SIGALRM>.
3658 Returns the number of seconds actually slept. You probably cannot
3659 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3662 On some older systems, it may sleep up to a full second less than what
3663 you requested, depending on how it counts seconds. Most modern systems
3664 always sleep the full amount. They may appear to sleep longer than that,
3665 however, because your process might not be scheduled right away in a
3666 busy multitasking system.
3668 For delays of finer granularity than one second, you may use Perl's
3669 C<syscall> interface to access setitimer(2) if your system supports it,
3670 or else see L</select> above.
3672 See also the POSIX module's C<sigpause> function.
3674 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3676 Opens a socket of the specified kind and attaches it to filehandle
3677 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3678 the system call of the same name. You should C<use Socket> first
3679 to get the proper definitions imported. See the examples in
3680 L<perlipc/"Sockets: Client/Server Communication">.
3682 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3684 Creates an unnamed pair of sockets in the specified domain, of the
3685 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3686 for the system call of the same name. If unimplemented, yields a fatal
3687 error. Returns true if successful.
3689 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
3690 to C<pipe(Rdr, Wtr)> is essentially:
3693 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3694 shutdown(Rdr, 1); # no more writing for reader
3695 shutdown(Wtr, 0); # no more reading for writer
3697 See L<perlipc> for an example of socketpair use.
3699 =item sort SUBNAME LIST
3701 =item sort BLOCK LIST
3705 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3706 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3707 specified, it gives the name of a subroutine that returns an integer
3708 less than, equal to, or greater than C<0>, depending on how the elements
3709 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3710 operators are extremely useful in such routines.) SUBNAME may be a
3711 scalar variable name (unsubscripted), in which case the value provides
3712 the name of (or a reference to) the actual subroutine to use. In place
3713 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3716 In the interests of efficiency the normal calling code for subroutines is
3717 bypassed, with the following effects: the subroutine may not be a
3718 recursive subroutine, and the two elements to be compared are passed into
3719 the subroutine not via C<@_> but as the package global variables $a and
3720 $b (see example below). They are passed by reference, so don't
3721 modify $a and $b. And don't try to declare them as lexicals either.
3723 You also cannot exit out of the sort block or subroutine using any of the
3724 loop control operators described in L<perlsyn> or with C<goto>.
3726 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3727 current collation locale. See L<perllocale>.
3732 @articles = sort @files;
3734 # same thing, but with explicit sort routine
3735 @articles = sort {$a cmp $b} @files;
3737 # now case-insensitively
3738 @articles = sort {uc($a) cmp uc($b)} @files;
3740 # same thing in reversed order
3741 @articles = sort {$b cmp $a} @files;
3743 # sort numerically ascending
3744 @articles = sort {$a <=> $b} @files;
3746 # sort numerically descending
3747 @articles = sort {$b <=> $a} @files;
3749 # this sorts the %age hash by value instead of key
3750 # using an in-line function
3751 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3753 # sort using explicit subroutine name
3755 $age{$a} <=> $age{$b}; # presuming numeric
3757 @sortedclass = sort byage @class;
3759 sub backwards { $b cmp $a }
3760 @harry = qw(dog cat x Cain Abel);
3761 @george = qw(gone chased yz Punished Axed);
3763 # prints AbelCaincatdogx
3764 print sort backwards @harry;
3765 # prints xdogcatCainAbel
3766 print sort @george, 'to', @harry;
3767 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3769 # inefficiently sort by descending numeric compare using
3770 # the first integer after the first = sign, or the
3771 # whole record case-insensitively otherwise
3774 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3779 # same thing, but much more efficiently;
3780 # we'll build auxiliary indices instead
3784 push @nums, /=(\d+)/;
3789 $nums[$b] <=> $nums[$a]
3791 $caps[$a] cmp $caps[$b]
3795 # same thing, but without any temps
3796 @new = map { $_->[0] }
3797 sort { $b->[1] <=> $a->[1]
3800 } map { [$_, /=(\d+)/, uc($_)] } @old;
3802 If you're using strict, you I<must not> declare $a
3803 and $b as lexicals. They are package globals. That means
3804 if you're in the C<main> package, it's
3806 @articles = sort {$main::b <=> $main::a} @files;
3810 @articles = sort {$::b <=> $::a} @files;
3812 but if you're in the C<FooPack> package, it's
3814 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3816 The comparison function is required to behave. If it returns
3817 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3818 sometimes saying the opposite, for example) the results are not
3821 =item splice ARRAY,OFFSET,LENGTH,LIST
3823 =item splice ARRAY,OFFSET,LENGTH
3825 =item splice ARRAY,OFFSET
3827 Removes the elements designated by OFFSET and LENGTH from an array, and
3828 replaces them with the elements of LIST, if any. In list context,
3829 returns the elements removed from the array. In scalar context,
3830 returns the last element removed, or C<undef> if no elements are
3831 removed. The array grows or shrinks as necessary.
3832 If OFFSET is negative then it starts that far from the end of the array.
3833 If LENGTH is omitted, removes everything from OFFSET onward.
3834 If LENGTH is negative, leave that many elements off the end of the array.
3835 The following equivalences hold (assuming C<$[ == 0>):
3837 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3838 pop(@a) splice(@a,-1)
3839 shift(@a) splice(@a,0,1)
3840 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3841 $a[$x] = $y splice(@a,$x,1,$y)
3843 Example, assuming array lengths are passed before arrays:
3845 sub aeq { # compare two list values
3846 my(@a) = splice(@_,0,shift);
3847 my(@b) = splice(@_,0,shift);
3848 return 0 unless @a == @b; # same len?
3850 return 0 if pop(@a) ne pop(@b);
3854 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3856 =item split /PATTERN/,EXPR,LIMIT
3858 =item split /PATTERN/,EXPR
3860 =item split /PATTERN/
3864 Splits a string into a list of strings and returns that list. By default,
3865 empty leading fields are preserved, and empty trailing ones are deleted.
3867 If not in list context, returns the number of fields found and splits into
3868 the C<@_> array. (In list context, you can force the split into C<@_> by
3869 using C<??> as the pattern delimiters, but it still returns the list
3870 value.) The use of implicit split to C<@_> is deprecated, however, because
3871 it clobbers your subroutine arguments.
3873 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3874 splits on whitespace (after skipping any leading whitespace). Anything
3875 matching PATTERN is taken to be a delimiter separating the fields. (Note
3876 that the delimiter may be longer than one character.)
3878 If LIMIT is specified and positive, splits into no more than that
3879 many fields (though it may split into fewer). If LIMIT is unspecified
3880 or zero, trailing null fields are stripped (which potential users
3881 of C<pop> would do well to remember). If LIMIT is negative, it is
3882 treated as if an arbitrarily large LIMIT had been specified.
3884 A pattern matching the null string (not to be confused with
3885 a null pattern C<//>, which is just one member of the set of patterns
3886 matching a null string) will split the value of EXPR into separate
3887 characters at each point it matches that way. For example:
3889 print join(':', split(/ */, 'hi there'));
3891 produces the output 'h:i:t:h:e:r:e'.
3893 The LIMIT parameter can be used to split a line partially
3895 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3897 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3898 one larger than the number of variables in the list, to avoid
3899 unnecessary work. For the list above LIMIT would have been 4 by
3900 default. In time critical applications it behooves you not to split
3901 into more fields than you really need.
3903 If the PATTERN contains parentheses, additional list elements are
3904 created from each matching substring in the delimiter.
3906 split(/([,-])/, "1-10,20", 3);
3908 produces the list value
3910 (1, '-', 10, ',', 20)
3912 If you had the entire header of a normal Unix email message in $header,
3913 you could split it up into fields and their values this way:
3915 $header =~ s/\n\s+/ /g; # fix continuation lines
3916 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3918 The pattern C</PATTERN/> may be replaced with an expression to specify
3919 patterns that vary at runtime. (To do runtime compilation only once,
3920 use C</$variable/o>.)
3922 As a special case, specifying a PATTERN of space (C<' '>) will split on
3923 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
3924 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3925 will give you as many null initial fields as there are leading spaces.
3926 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
3927 whitespace produces a null first field. A C<split> with no arguments
3928 really does a C<split(' ', $_)> internally.
3932 open(PASSWD, '/etc/passwd');
3934 ($login, $passwd, $uid, $gid,
3935 $gcos, $home, $shell) = split(/:/);
3939 (Note that $shell above will still have a newline on it. See L</chop>,
3940 L</chomp>, and L</join>.)
3942 =item sprintf FORMAT, LIST
3944 Returns a string formatted by the usual C<printf> conventions of the
3945 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
3946 on your system for an explanation of the general principles.
3948 Perl does its own C<sprintf> formatting--it emulates the C
3949 function C<sprintf>, but it doesn't use it (except for floating-point
3950 numbers, and even then only the standard modifiers are allowed). As a
3951 result, any non-standard extensions in your local C<sprintf> are not
3952 available from Perl.
3954 Perl's C<sprintf> permits the following universally-known conversions:
3957 %c a character with the given number
3959 %d a signed integer, in decimal
3960 %u an unsigned integer, in decimal
3961 %o an unsigned integer, in octal
3962 %x an unsigned integer, in hexadecimal
3963 %e a floating-point number, in scientific notation
3964 %f a floating-point number, in fixed decimal notation
3965 %g a floating-point number, in %e or %f notation
3967 In addition, Perl permits the following widely-supported conversions:
3969 %X like %x, but using upper-case letters
3970 %E like %e, but using an upper-case "E"
3971 %G like %g, but with an upper-case "E" (if applicable)
3972 %b an unsigned integer, in binary
3973 %p a pointer (outputs the Perl value's address in hexadecimal)
3974 %n special: *stores* the number of characters output so far
3975 into the next variable in the parameter list
3977 Finally, for backward (and we do mean "backward") compatibility, Perl
3978 permits these unnecessary but widely-supported conversions:
3981 %D a synonym for %ld
3982 %U a synonym for %lu
3983 %O a synonym for %lo
3986 Perl permits the following universally-known flags between the C<%>
3987 and the conversion letter:
3989 space prefix positive number with a space
3990 + prefix positive number with a plus sign
3991 - left-justify within the field
3992 0 use zeros, not spaces, to right-justify
3993 # prefix non-zero octal with "0", non-zero hex with "0x"
3994 number minimum field width
3995 .number "precision": digits after decimal point for
3996 floating-point, max length for string, minimum length
3998 l interpret integer as C type "long" or "unsigned long"
3999 h interpret integer as C type "short" or "unsigned short"
4001 There is also one Perl-specific flag:
4003 V interpret integer as Perl's standard integer type
4005 Where a number would appear in the flags, an asterisk (C<*>) may be
4006 used instead, in which case Perl uses the next item in the parameter
4007 list as the given number (that is, as the field width or precision).
4008 If a field width obtained through C<*> is negative, it has the same
4009 effect as the C<-> flag: left-justification.
4011 If C<use locale> is in effect, the character used for the decimal
4012 point in formatted real numbers is affected by the LC_NUMERIC locale.
4019 Return the square root of EXPR. If EXPR is omitted, returns square
4020 root of C<$_>. Only works on non-negative operands, unless you've
4021 loaded the standard Math::Complex module.
4024 print sqrt(-2); # prints 1.4142135623731i
4030 Sets the random number seed for the C<rand> operator. If EXPR is
4031 omitted, uses a semi-random value supplied by the kernel (if it supports
4032 the F</dev/urandom> device) or based on the current time and process
4033 ID, among other things. In versions of Perl prior to 5.004 the default
4034 seed was just the current C<time>. This isn't a particularly good seed,
4035 so many old programs supply their own seed value (often C<time ^ $$> or
4036 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4038 In fact, it's usually not necessary to call C<srand> at all, because if
4039 it is not called explicitly, it is called implicitly at the first use of
4040 the C<rand> operator. However, this was not the case in version of Perl
4041 before 5.004, so if your script will run under older Perl versions, it
4042 should call C<srand>.
4044 Note that you need something much more random than the default seed for
4045 cryptographic purposes. Checksumming the compressed output of one or more
4046 rapidly changing operating system status programs is the usual method. For
4049 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4051 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4054 Do I<not> call C<srand> multiple times in your program unless you know
4055 exactly what you're doing and why you're doing it. The point of the
4056 function is to "seed" the C<rand> function so that C<rand> can produce
4057 a different sequence each time you run your program. Just do it once at the
4058 top of your program, or you I<won't> get random numbers out of C<rand>!
4060 Frequently called programs (like CGI scripts) that simply use
4064 for a seed can fall prey to the mathematical property that
4068 one-third of the time. So don't do that.
4070 =item stat FILEHANDLE
4076 Returns a 13-element list giving the status info for a file, either
4077 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4078 it stats C<$_>. Returns a null list if the stat fails. Typically used
4081 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4082 $atime,$mtime,$ctime,$blksize,$blocks)
4085 Not all fields are supported on all filesystem types. Here are the
4086 meaning of the fields:
4088 0 dev device number of filesystem
4090 2 mode file mode (type and permissions)
4091 3 nlink number of (hard) links to the file
4092 4 uid numeric user ID of file's owner
4093 5 gid numeric group ID of file's owner
4094 6 rdev the device identifier (special files only)
4095 7 size total size of file, in bytes
4096 8 atime last access time since the epoch
4097 9 mtime last modify time since the epoch
4098 10 ctime inode change time (NOT creation time!) since the epoch
4099 11 blksize preferred block size for file system I/O
4100 12 blocks actual number of blocks allocated
4102 (The epoch was at 00:00 January 1, 1970 GMT.)
4104 If stat is passed the special filehandle consisting of an underline, no
4105 stat is done, but the current contents of the stat structure from the
4106 last stat or filetest are returned. Example:
4108 if (-x $file && (($d) = stat(_)) && $d < 0) {
4109 print "$file is executable NFS file\n";
4112 (This works on machines only for which the device number is negative under NFS.)
4114 Because the mode contains both the file type and its permissions, you
4115 should mask off the file type portion and (s)printf using a C<"%o">
4116 if you want to see the real permissions.
4118 $mode = (stat($filename))[2];
4119 printf "Permissions are %04o\n", $mode & 07777;
4121 In scalar context, C<stat> returns a boolean value indicating success
4122 or failure, and, if successful, sets the information associated with
4123 the special filehandle C<_>.
4125 The File::stat module provides a convenient, by-name access mechanism:
4128 $sb = stat($filename);
4129 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4130 $filename, $sb->size, $sb->mode & 07777,
4131 scalar localtime $sb->mtime;
4137 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4138 doing many pattern matches on the string before it is next modified.
4139 This may or may not save time, depending on the nature and number of
4140 patterns you are searching on, and on the distribution of character
4141 frequencies in the string to be searched--you probably want to compare
4142 run times with and without it to see which runs faster. Those loops
4143 which scan for many short constant strings (including the constant
4144 parts of more complex patterns) will benefit most. You may have only
4145 one C<study> active at a time--if you study a different scalar the first
4146 is "unstudied". (The way C<study> works is this: a linked list of every
4147 character in the string to be searched is made, so we know, for
4148 example, where all the C<'k'> characters are. From each search string,
4149 the rarest character is selected, based on some static frequency tables
4150 constructed from some C programs and English text. Only those places
4151 that contain this "rarest" character are examined.)
4153 For example, here is a loop that inserts index producing entries
4154 before any line containing a certain pattern:
4158 print ".IX foo\n" if /\bfoo\b/;
4159 print ".IX bar\n" if /\bbar\b/;
4160 print ".IX blurfl\n" if /\bblurfl\b/;
4165 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4166 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4167 a big win except in pathological cases. The only question is whether
4168 it saves you more time than it took to build the linked list in the
4171 Note that if you have to look for strings that you don't know till
4172 runtime, you can build an entire loop as a string and C<eval> that to
4173 avoid recompiling all your patterns all the time. Together with
4174 undefining C<$/> to input entire files as one record, this can be very
4175 fast, often faster than specialized programs like fgrep(1). The following
4176 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4177 out the names of those files that contain a match:
4179 $search = 'while (<>) { study;';
4180 foreach $word (@words) {
4181 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4186 eval $search; # this screams
4187 $/ = "\n"; # put back to normal input delimiter
4188 foreach $file (sort keys(%seen)) {
4196 =item sub NAME BLOCK
4198 This is subroutine definition, not a real function I<per se>. With just a
4199 NAME (and possibly prototypes), it's just a forward declaration. Without
4200 a NAME, it's an anonymous function declaration, and does actually return a
4201 value: the CODE ref of the closure you just created. See L<perlsub> and
4202 L<perlref> for details.
4204 =item substr EXPR,OFFSET,LEN,REPLACEMENT
4206 =item substr EXPR,OFFSET,LEN
4208 =item substr EXPR,OFFSET
4210 Extracts a substring out of EXPR and returns it. First character is at
4211 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4212 If OFFSET is negative (or more precisely, less than C<$[>), starts
4213 that far from the end of the string. If LEN is omitted, returns
4214 everything to the end of the string. If LEN is negative, leaves that
4215 many characters off the end of the string.
4217 If you specify a substring that is partly outside the string, the part
4218 within the string is returned. If the substring is totally outside
4219 the string a warning is produced.
4221 You can use the substr() function as an lvalue, in which case EXPR
4222 must itself be an lvalue. If you assign something shorter than LEN,
4223 the string will shrink, and if you assign something longer than LEN,
4224 the string will grow to accommodate it. To keep the string the same
4225 length you may need to pad or chop your value using C<sprintf>.
4227 An alternative to using substr() as an lvalue is to specify the
4228 replacement string as the 4th argument. This allows you to replace
4229 parts of the EXPR and return what was there before in one operation,
4230 just as you can with splice().
4232 =item symlink OLDFILE,NEWFILE
4234 Creates a new filename symbolically linked to the old filename.
4235 Returns C<1> for success, C<0> otherwise. On systems that don't support
4236 symbolic links, produces a fatal error at run time. To check for that,
4239 $symlink_exists = eval { symlink("",""); 1 };
4243 Calls the system call specified as the first element of the list,
4244 passing the remaining elements as arguments to the system call. If
4245 unimplemented, produces a fatal error. The arguments are interpreted
4246 as follows: if a given argument is numeric, the argument is passed as
4247 an int. If not, the pointer to the string value is passed. You are
4248 responsible to make sure a string is pre-extended long enough to
4249 receive any result that might be written into a string. You can't use a
4250 string literal (or other read-only string) as an argument to C<syscall>
4251 because Perl has to assume that any string pointer might be written
4253 integer arguments are not literals and have never been interpreted in a
4254 numeric context, you may need to add C<0> to them to force them to look
4255 like numbers. This emulates the C<syswrite> function (or vice versa):
4257 require 'syscall.ph'; # may need to run h2ph
4259 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4261 Note that Perl supports passing of up to only 14 arguments to your system call,
4262 which in practice should usually suffice.
4264 Syscall returns whatever value returned by the system call it calls.
4265 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4266 Note that some system calls can legitimately return C<-1>. The proper
4267 way to handle such calls is to assign C<$!=0;> before the call and
4268 check the value of C<$!> if syscall returns C<-1>.
4270 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4271 number of the read end of the pipe it creates. There is no way
4272 to retrieve the file number of the other end. You can avoid this
4273 problem by using C<pipe> instead.
4275 =item sysopen FILEHANDLE,FILENAME,MODE
4277 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4279 Opens the file whose filename is given by FILENAME, and associates it
4280 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4281 the name of the real filehandle wanted. This function calls the
4282 underlying operating system's C<open> function with the parameters
4283 FILENAME, MODE, PERMS.
4285 The possible values and flag bits of the MODE parameter are
4286 system-dependent; they are available via the standard module C<Fcntl>.
4287 For historical reasons, some values work on almost every system
4288 supported by perl: zero means read-only, one means write-only, and two
4289 means read/write. We know that these values do I<not> work under
4290 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4291 use them in new code.
4293 If the file named by FILENAME does not exist and the C<open> call creates
4294 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4295 PERMS specifies the permissions of the newly created file. If you omit
4296 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4297 These permission values need to be in octal, and are modified by your
4298 process's current C<umask>.
4300 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4301 that takes away the user's option to have a more permissive umask.
4302 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4305 See L<perlopentut> for a kinder, gentler explanation of opening files.
4307 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4309 =item sysread FILEHANDLE,SCALAR,LENGTH
4311 Attempts to read LENGTH bytes of data into variable SCALAR from the
4312 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4313 so mixing this with other kinds of reads, C<print>, C<write>,
4314 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4315 usually buffers data. Returns the number of bytes actually read, C<0>
4316 at end of file, or undef if there was an error. SCALAR will be grown or
4317 shrunk so that the last byte actually read is the last byte of the
4318 scalar after the read.
4320 An OFFSET may be specified to place the read data at some place in the
4321 string other than the beginning. A negative OFFSET specifies
4322 placement at that many bytes counting backwards from the end of the
4323 string. A positive OFFSET greater than the length of SCALAR results
4324 in the string being padded to the required size with C<"\0"> bytes before
4325 the result of the read is appended.
4327 There is no syseof() function, which is ok, since eof() doesn't work
4328 very well on device files (like ttys) anyway. Use sysread() and check
4329 for a return value for 0 to decide whether you're done.
4331 =item sysseek FILEHANDLE,POSITION,WHENCE
4333 Sets FILEHANDLE's system position using the system call lseek(2). It
4334 bypasses stdio, so mixing this with reads (other than C<sysread>),
4335 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause
4336 confusion. FILEHANDLE may be an expression whose value gives the name
4337 of the filehandle. The values for WHENCE are C<0> to set the new
4338 position to POSITION, C<1> to set the it to the current position plus
4339 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4340 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4341 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4343 Returns the new position, or the undefined value on failure. A position
4344 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4345 true on success and false on failure, yet you can still easily determine
4350 =item system PROGRAM LIST
4352 Does exactly the same thing as C<exec LIST>, except that a fork is
4353 done first, and the parent process waits for the child process to
4354 complete. Note that argument processing varies depending on the
4355 number of arguments. If there is more than one argument in LIST,
4356 or if LIST is an array with more than one value, starts the program
4357 given by the first element of the list with arguments given by the
4358 rest of the list. If there is only one scalar argument, the argument
4359 is checked for shell metacharacters, and if there are any, the
4360 entire argument is passed to the system's command shell for parsing
4361 (this is C</bin/sh -c> on Unix platforms, but varies on other
4362 platforms). If there are no shell metacharacters in the argument,
4363 it is split into words and passed directly to C<execvp>, which is
4366 All files opened for output are flushed before attempting the exec().
4368 The return value is the exit status of the program as
4369 returned by the C<wait> call. To get the actual exit value divide by
4370 256. See also L</exec>. This is I<not> what you want to use to capture
4371 the output from a command, for that you should use merely backticks or
4372 C<qx//>, as described in L<perlop/"`STRING`">.
4374 Like C<exec>, C<system> allows you to lie to a program about its name if
4375 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4377 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4378 program they're running doesn't actually interrupt your program.
4380 @args = ("command", "arg1", "arg2");
4382 or die "system @args failed: $?"
4384 You can check all the failure possibilities by inspecting
4387 $exit_value = $? >> 8;
4388 $signal_num = $? & 127;
4389 $dumped_core = $? & 128;
4391 When the arguments get executed via the system shell, results
4392 and return codes will be subject to its quirks and capabilities.
4393 See L<perlop/"`STRING`"> and L</exec> for details.
4395 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4397 =item syswrite FILEHANDLE,SCALAR,LENGTH
4399 =item syswrite FILEHANDLE,SCALAR
4401 Attempts to write LENGTH bytes of data from variable SCALAR to the
4402 specified FILEHANDLE, using the system call write(2). If LENGTH
4403 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4404 this with reads (other than C<sysread())>, C<print>, C<write>,
4405 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4406 usually buffers data. Returns the number of bytes actually written,
4407 or C<undef> if there was an error. If the LENGTH is greater than
4408 the available data in the SCALAR after the OFFSET, only as much
4409 data as is available will be written.
4411 An OFFSET may be specified to write the data from some part of the
4412 string other than the beginning. A negative OFFSET specifies writing
4413 that many bytes counting backwards from the end of the string. In the
4414 case the SCALAR is empty you can use OFFSET but only zero offset.
4416 =item tell FILEHANDLE
4420 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4421 expression whose value gives the name of the actual filehandle. If
4422 FILEHANDLE is omitted, assumes the file last read.
4424 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4426 =item telldir DIRHANDLE
4428 Returns the current position of the C<readdir> routines on DIRHANDLE.
4429 Value may be given to C<seekdir> to access a particular location in a
4430 directory. Has the same caveats about possible directory compaction as
4431 the corresponding system library routine.
4433 =item tie VARIABLE,CLASSNAME,LIST
4435 This function binds a variable to a package class that will provide the
4436 implementation for the variable. VARIABLE is the name of the variable
4437 to be enchanted. CLASSNAME is the name of a class implementing objects
4438 of correct type. Any additional arguments are passed to the C<new>
4439 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4440 or C<TIEHASH>). Typically these are arguments such as might be passed
4441 to the C<dbm_open()> function of C. The object returned by the C<new>
4442 method is also returned by the C<tie> function, which would be useful
4443 if you want to access other methods in CLASSNAME.
4445 Note that functions such as C<keys> and C<values> may return huge lists
4446 when used on large objects, like DBM files. You may prefer to use the
4447 C<each> function to iterate over such. Example:
4449 # print out history file offsets
4451 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4452 while (($key,$val) = each %HIST) {
4453 print $key, ' = ', unpack('L',$val), "\n";
4457 A class implementing a hash should have the following methods:
4459 TIEHASH classname, LIST
4461 STORE this, key, value
4466 NEXTKEY this, lastkey
4469 A class implementing an ordinary array should have the following methods:
4471 TIEARRAY classname, LIST
4473 STORE this, key, value
4475 STORESIZE this, count
4481 SPLICE this, offset, length, LIST
4485 A class implementing a file handle should have the following methods:
4487 TIEHANDLE classname, LIST
4488 READ this, scalar, length, offset
4491 WRITE this, scalar, length, offset
4493 PRINTF this, format, LIST
4497 A class implementing a scalar should have the following methods:
4499 TIESCALAR classname, LIST
4504 Not all methods indicated above need be implemented. See L<perltie>,
4505 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4507 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4508 for you--you need to do that explicitly yourself. See L<DB_File>
4509 or the F<Config> module for interesting C<tie> implementations.
4511 For further details see L<perltie>, L<"tied VARIABLE">.
4515 Returns a reference to the object underlying VARIABLE (the same value
4516 that was originally returned by the C<tie> call that bound the variable
4517 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4522 Returns the number of non-leap seconds since whatever time the system
4523 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4524 and 00:00:00 UTC, January 1, 1970 for most other systems).
4525 Suitable for feeding to C<gmtime> and C<localtime>.
4529 Returns a four-element list giving the user and system times, in
4530 seconds, for this process and the children of this process.
4532 ($user,$system,$cuser,$csystem) = times;
4536 The transliteration operator. Same as C<y///>. See L<perlop>.
4538 =item truncate FILEHANDLE,LENGTH
4540 =item truncate EXPR,LENGTH
4542 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4543 specified length. Produces a fatal error if truncate isn't implemented
4544 on your system. Returns true if successful, the undefined value
4551 Returns an uppercased version of EXPR. This is the internal function
4552 implementing the C<\U> escape in double-quoted strings.
4553 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4554 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4555 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4557 If EXPR is omitted, uses C<$_>.
4563 Returns the value of EXPR with the first character
4564 in uppercase (titlecase in Unicode). This is
4565 the internal function implementing the C<\u> escape in double-quoted strings.
4566 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4569 If EXPR is omitted, uses C<$_>.
4575 Sets the umask for the process to EXPR and returns the previous value.
4576 If EXPR is omitted, merely returns the current umask.
4578 The Unix permission C<rwxr-x---> is represented as three sets of three
4579 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4580 and isn't one of the digits). The C<umask> value is such a number
4581 representing disabled permissions bits. The permission (or "mode")
4582 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4583 even if you tell C<sysopen> to create a file with permissions C<0777>,
4584 if your umask is C<0022> then the file will actually be created with
4585 permissions C<0755>. If your C<umask> were C<0027> (group can't
4586 write; others can't read, write, or execute), then passing
4587 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4590 Here's some advice: supply a creation mode of C<0666> for regular
4591 files (in C<sysopen>) and one of C<0777> for directories (in
4592 C<mkdir>) and executable files. This gives users the freedom of
4593 choice: if they want protected files, they might choose process umasks
4594 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4595 Programs should rarely if ever make policy decisions better left to
4596 the user. The exception to this is when writing files that should be
4597 kept private: mail files, web browser cookies, I<.rhosts> files, and
4600 If umask(2) is not implemented on your system and you are trying to
4601 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4602 fatal error at run time. If umask(2) is not implemented and you are
4603 not trying to restrict access for yourself, returns C<undef>.
4605 Remember that a umask is a number, usually given in octal; it is I<not> a
4606 string of octal digits. See also L</oct>, if all you have is a string.
4612 Undefines the value of EXPR, which must be an lvalue. Use only on a
4613 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
4614 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
4615 will probably not do what you expect on most predefined variables or
4616 DBM list values, so don't do that; see L<delete>.) Always returns the
4617 undefined value. You can omit the EXPR, in which case nothing is
4618 undefined, but you still get an undefined value that you could, for
4619 instance, return from a subroutine, assign to a variable or pass as a
4620 parameter. Examples:
4623 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4627 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4628 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4629 select undef, undef, undef, 0.25;
4630 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4632 Note that this is a unary operator, not a list operator.
4638 Deletes a list of files. Returns the number of files successfully
4641 $cnt = unlink 'a', 'b', 'c';
4645 Note: C<unlink> will not delete directories unless you are superuser and
4646 the B<-U> flag is supplied to Perl. Even if these conditions are
4647 met, be warned that unlinking a directory can inflict damage on your
4648 filesystem. Use C<rmdir> instead.
4650 If LIST is omitted, uses C<$_>.
4652 =item unpack TEMPLATE,EXPR
4654 C<unpack> does the reverse of C<pack>: it takes a string
4655 representing a structure and expands it out into a list of values.
4656 (In scalar context, it returns merely the first value produced.)
4657 The TEMPLATE has the same format as in the C<pack> function.
4658 Here's a subroutine that does substring:
4661 my($what,$where,$howmuch) = @_;
4662 unpack("x$where a$howmuch", $what);
4667 sub ordinal { unpack("c",$_[0]); } # same as ord()
4669 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4670 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4671 themselves. Default is a 16-bit checksum. For example, the following
4672 computes the same number as the System V sum program:
4676 unpack("%32C*",<>) % 65535;
4679 The following efficiently counts the number of set bits in a bit vector:
4681 $setbits = unpack("%32b*", $selectmask);
4683 The C<"p"> and C<"P"> formats should be used with care. Since Perl
4684 has no way of checking whether the value passed to C<unpack()>
4685 corresponds to a valid memory location, passing a pointer value that's
4686 not known to be valid is likely to have disastrous consequences.
4688 See L</pack> for more examples.
4690 =item untie VARIABLE
4692 Breaks the binding between a variable and a package. (See C<tie>.)
4694 =item unshift ARRAY,LIST
4696 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4697 depending on how you look at it. Prepends list to the front of the
4698 array, and returns the new number of elements in the array.
4700 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4702 Note the LIST is prepended whole, not one element at a time, so the
4703 prepended elements stay in the same order. Use C<reverse> to do the
4706 =item use Module LIST
4710 =item use Module VERSION LIST
4714 Imports some semantics into the current package from the named module,
4715 generally by aliasing certain subroutine or variable names into your
4716 package. It is exactly equivalent to
4718 BEGIN { require Module; import Module LIST; }
4720 except that Module I<must> be a bareword.
4722 If the first argument to C<use> is a number, it is treated as a version
4723 number instead of a module name. If the version of the Perl interpreter
4724 is less than VERSION, then an error message is printed and Perl exits
4725 immediately. This is often useful if you need to check the current
4726 Perl version before C<use>ing library modules that have changed in
4727 incompatible ways from older versions of Perl. (We try not to do
4728 this more than we have to.)
4730 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
4731 C<require> makes sure the module is loaded into memory if it hasn't been
4732 yet. The C<import> is not a builtin--it's just an ordinary static method
4733 call into the C<Module> package to tell the module to import the list of
4734 features back into the current package. The module can implement its
4735 C<import> method any way it likes, though most modules just choose to
4736 derive their C<import> method via inheritance from the C<Exporter> class that
4737 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
4738 method can be found then the error is currently silently ignored. This
4739 may change to a fatal error in a future version.
4741 If you don't want your namespace altered, explicitly supply an empty list:
4745 That is exactly equivalent to
4747 BEGIN { require Module }
4749 If the VERSION argument is present between Module and LIST, then the
4750 C<use> will call the VERSION method in class Module with the given
4751 version as an argument. The default VERSION method, inherited from
4752 the Universal class, croaks if the given version is larger than the
4753 value of the variable C<$Module::VERSION>. (Note that there is not a
4754 comma after VERSION!)
4756 Because this is a wide-open interface, pragmas (compiler directives)
4757 are also implemented this way. Currently implemented pragmas are:
4761 use sigtrap qw(SEGV BUS);
4762 use strict qw(subs vars refs);
4763 use subs qw(afunc blurfl);
4765 Some of these pseudo-modules import semantics into the current
4766 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4767 which import symbols into the current package (which are effective
4768 through the end of the file).
4770 There's a corresponding C<no> command that unimports meanings imported
4771 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
4776 If no C<unimport> method can be found the call fails with a fatal error.
4778 See L<perlmod> for a list of standard modules and pragmas.
4782 Changes the access and modification times on each file of a list of
4783 files. The first two elements of the list must be the NUMERICAL access
4784 and modification times, in that order. Returns the number of files
4785 successfully changed. The inode modification time of each file is set
4786 to the current time. This code has the same effect as the C<touch>
4787 command if the files already exist:
4791 utime $now, $now, @ARGV;
4795 Returns a list consisting of all the values of the named hash. (In a
4796 scalar context, returns the number of values.) The values are
4797 returned in an apparently random order. The actual random order is
4798 subject to change in future versions of perl, but it is guaranteed to
4799 be the same order as either the C<keys> or C<each> function would
4800 produce on the same (unmodified) hash.
4802 Note that you cannot modify the values of a hash this way, because the
4803 returned list is just a copy. You need to use a hash slice for that,
4804 since it's lvaluable in a way that values() is not.
4806 for (values %hash) { s/foo/bar/g } # FAILS!
4807 for (@hash{keys %hash}) { s/foo/bar/g } # ok
4809 As a side effect, calling values() resets the HASH's internal iterator.
4810 See also C<keys>, C<each>, and C<sort>.
4812 =item vec EXPR,OFFSET,BITS
4814 Treats the string in EXPR as a vector of unsigned integers, and
4815 returns the value of the bit field specified by OFFSET. BITS specifies
4816 the number of bits that are reserved for each entry in the bit
4817 vector. This must be a power of two from 1 to 32. C<vec> may also be
4818 assigned to, in which case parentheses are needed to give the expression
4819 the correct precedence as in
4821 vec($image, $max_x * $x + $y, 8) = 3;
4823 Vectors created with C<vec> can also be manipulated with the logical
4824 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4825 desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
4827 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4828 The comments show the string after each step. Note that this code works
4829 in the same way on big-endian or little-endian machines.
4832 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4833 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4834 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4835 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4836 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4837 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4838 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4840 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4841 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4842 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4845 To transform a bit vector into a string or list of 0's and 1's, use these:
4847 $bits = unpack("b*", $vector);
4848 @bits = split(//, unpack("b*", $vector));
4850 If you know the exact length in bits, it can be used in place of the C<*>.
4854 Behaves like the wait(2) system call on your system: it waits for a child
4855 process to terminate and returns the pid of the deceased process, or
4856 C<-1> if there are no child processes. The status is returned in C<$?>.
4857 Note that a return value of C<-1> could mean that child processes are
4858 being automatically reaped, as described in L<perlipc>.
4860 =item waitpid PID,FLAGS
4862 Waits for a particular child process to terminate and returns the pid of
4863 the deceased process, or C<-1> if there is no such child process. On some
4864 systems, a value of 0 indicates that there are processes still running.
4865 The status is returned in C<$?>. If you say
4867 use POSIX ":sys_wait_h";
4870 $kid = waitpid(-1,&WNOHANG);
4873 then you can do a non-blocking wait for all pending zombie processes.
4874 Non-blocking wait is available on machines supporting either the
4875 waitpid(2) or wait4(2) system calls. However, waiting for a particular
4876 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
4877 system call by remembering the status values of processes that have
4878 exited but have not been harvested by the Perl script yet.)
4880 Note that on some systems, a return value of C<-1> could mean that child
4881 processes are being automatically reaped. See L<perlipc> for details,
4882 and for other examples.
4886 Returns true if the context of the currently executing subroutine is
4887 looking for a list value. Returns false if the context is looking
4888 for a scalar. Returns the undefined value if the context is looking
4889 for no value (void context).
4891 return unless defined wantarray; # don't bother doing more
4892 my @a = complex_calculation();
4893 return wantarray ? @a : "@a";
4895 This function should have been named wantlist() instead.
4899 Produces a message on STDERR just like C<die>, but doesn't exit or throw
4902 If LIST is empty and C<$@> already contains a value (typically from a
4903 previous eval) that value is used after appending C<"\t...caught">
4904 to C<$@>. This is useful for staying almost, but not entirely similar to
4907 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4909 No message is printed if there is a C<$SIG{__WARN__}> handler
4910 installed. It is the handler's responsibility to deal with the message
4911 as it sees fit (like, for instance, converting it into a C<die>). Most
4912 handlers must therefore make arrangements to actually display the
4913 warnings that they are not prepared to deal with, by calling C<warn>
4914 again in the handler. Note that this is quite safe and will not
4915 produce an endless loop, since C<__WARN__> hooks are not called from
4918 You will find this behavior is slightly different from that of
4919 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4920 instead call C<die> again to change it).
4922 Using a C<__WARN__> handler provides a powerful way to silence all
4923 warnings (even the so-called mandatory ones). An example:
4925 # wipe out *all* compile-time warnings
4926 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4928 my $foo = 20; # no warning about duplicate my $foo,
4929 # but hey, you asked for it!
4930 # no compile-time or run-time warnings before here
4933 # run-time warnings enabled after here
4934 warn "\$foo is alive and $foo!"; # does show up
4936 See L<perlvar> for details on setting C<%SIG> entries, and for more
4937 examples. See the Carp module for other kinds of warnings using its
4938 carp() and cluck() functions.
4940 =item write FILEHANDLE
4946 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4947 using the format associated with that file. By default the format for
4948 a file is the one having the same name as the filehandle, but the
4949 format for the current output channel (see the C<select> function) may be set
4950 explicitly by assigning the name of the format to the C<$~> variable.
4952 Top of form processing is handled automatically: if there is
4953 insufficient room on the current page for the formatted record, the
4954 page is advanced by writing a form feed, a special top-of-page format
4955 is used to format the new page header, and then the record is written.
4956 By default the top-of-page format is the name of the filehandle with
4957 "_TOP" appended, but it may be dynamically set to the format of your
4958 choice by assigning the name to the C<$^> variable while the filehandle is
4959 selected. The number of lines remaining on the current page is in
4960 variable C<$->, which can be set to C<0> to force a new page.
4962 If FILEHANDLE is unspecified, output goes to the current default output
4963 channel, which starts out as STDOUT but may be changed by the
4964 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4965 is evaluated and the resulting string is used to look up the name of
4966 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4968 Note that write is I<not> the opposite of C<read>. Unfortunately.
4972 The transliteration operator. Same as C<tr///>. See L<perlop>.