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,
516 $hasargs, $wantarray, $evaltext, $is_require) = caller($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>)
527 Furthermore, when called from within the DB package, caller returns more
528 detailed information: it sets the list variable C<@DB::args> to be the
529 arguments with which the subroutine was invoked.
531 Be aware that the optimizer might have optimized call frames away before
532 C<caller> had a chance to get the information. That means that C<caller(N)>
533 might not return information about the call frame you expect it do, for
534 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
535 previous time C<caller> was called.
539 Changes the working directory to EXPR, if possible. If EXPR is omitted,
540 changes to the user's home directory. Returns true upon success,
541 false otherwise. See the example under C<die>.
545 Changes the permissions of a list of files. The first element of the
546 list must be the numerical mode, which should probably be an octal
547 number, and which definitely should I<not> a string of octal digits:
548 C<0644> is okay, C<'0644'> is not. Returns the number of files
549 successfully changed. See also L</oct>, if all you have is a string.
551 $cnt = chmod 0755, 'foo', 'bar';
552 chmod 0755, @executables;
553 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
555 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
556 $mode = 0644; chmod $mode, 'foo'; # this is best
564 This safer version of L</chop> removes any trailing string
565 that corresponds to the current value of C<$/> (also known as
566 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
567 number of characters removed from all its arguments. It's often used to
568 remove the newline from the end of an input record when you're worried
569 that the final record may be missing its newline. When in paragraph
570 mode (C<$/ = "">), it removes all trailing newlines from the string.
571 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
572 a reference to an integer or the like, see L<perlvar>) chomp() won't
574 If VARIABLE is omitted, it chomps C<$_>. Example:
577 chomp; # avoid \n on last field
582 You can actually chomp anything that's an lvalue, including an assignment:
585 chomp($answer = <STDIN>);
587 If you chomp a list, each element is chomped, and the total number of
588 characters removed is returned.
596 Chops off the last character of a string and returns the character
597 chopped. It's used primarily to remove the newline from the end of an
598 input record, but is much more efficient than C<s/\n//> because it neither
599 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
603 chop; # avoid \n on last field
608 You can actually chop anything that's an lvalue, including an assignment:
611 chop($answer = <STDIN>);
613 If you chop a list, each element is chopped. Only the value of the
614 last C<chop> is returned.
616 Note that C<chop> returns the last character. To return all but the last
617 character, use C<substr($string, 0, -1)>.
621 Changes the owner (and group) of a list of files. The first two
622 elements of the list must be the I<numeric> uid and gid, in that
623 order. A value of -1 in either position is interpreted by most
624 systems to leave that value unchanged. Returns the number of files
625 successfully changed.
627 $cnt = chown $uid, $gid, 'foo', 'bar';
628 chown $uid, $gid, @filenames;
630 Here's an example that looks up nonnumeric uids in the passwd file:
633 chomp($user = <STDIN>);
635 chomp($pattern = <STDIN>);
637 ($login,$pass,$uid,$gid) = getpwnam($user)
638 or die "$user not in passwd file";
640 @ary = glob($pattern); # expand filenames
641 chown $uid, $gid, @ary;
643 On most systems, you are not allowed to change the ownership of the
644 file unless you're the superuser, although you should be able to change
645 the group to any of your secondary groups. On insecure systems, these
646 restrictions may be relaxed, but this is not a portable assumption.
647 On POSIX systems, you can detect this condition this way:
649 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
650 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
656 Returns the character represented by that NUMBER in the character set.
657 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
658 chr(0x263a) is a Unicode smiley face (but only within the scope of
659 a C<use utf8>). For the reverse, use L</ord>.
660 See L<utf8> for more about Unicode.
662 If NUMBER is omitted, uses C<$_>.
664 =item chroot FILENAME
668 This function works like the system call by the same name: it makes the
669 named directory the new root directory for all further pathnames that
670 begin with a C<"/"> by your process and all its children. (It doesn't
671 change your current working directory, which is unaffected.) For security
672 reasons, this call is restricted to the superuser. If FILENAME is
673 omitted, does a C<chroot> to C<$_>.
675 =item close FILEHANDLE
679 Closes the file or pipe associated with the file handle, returning true
680 only if stdio successfully flushes buffers and closes the system file
681 descriptor. Closes the currently selected filehandle if the argument
684 You don't have to close FILEHANDLE if you are immediately going to do
685 another C<open> on it, because C<open> will close it for you. (See
686 C<open>.) However, an explicit C<close> on an input file resets the line
687 counter (C<$.>), while the implicit close done by C<open> does not.
689 If the file handle came from a piped open C<close> will additionally
690 return false if one of the other system calls involved fails or if the
691 program exits with non-zero status. (If the only problem was that the
692 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
693 also waits for the process executing on the pipe to complete, in case you
694 want to look at the output of the pipe afterwards, and
695 implicitly puts the exit status value of that command into C<$?>.
699 open(OUTPUT, '|sort >foo') # pipe to sort
700 or die "Can't start sort: $!";
701 #... # print stuff to output
702 close OUTPUT # wait for sort to finish
703 or warn $! ? "Error closing sort pipe: $!"
704 : "Exit status $? from sort";
705 open(INPUT, 'foo') # get sort's results
706 or die "Can't open 'foo' for input: $!";
708 FILEHANDLE may be an expression whose value can be used as an indirect
709 filehandle, usually the real filehandle name.
711 =item closedir DIRHANDLE
713 Closes a directory opened by C<opendir> and returns the success of that
716 DIRHANDLE may be an expression whose value can be used as an indirect
717 dirhandle, usually the real dirhandle name.
719 =item connect SOCKET,NAME
721 Attempts to connect to a remote socket, just as the connect system call
722 does. Returns true if it succeeded, false otherwise. NAME should be a
723 packed address of the appropriate type for the socket. See the examples in
724 L<perlipc/"Sockets: Client/Server Communication">.
728 Actually a flow control statement rather than a function. If there is a
729 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
730 C<foreach>), it is always executed just before the conditional is about to
731 be evaluated again, just like the third part of a C<for> loop in C. Thus
732 it can be used to increment a loop variable, even when the loop has been
733 continued via the C<next> statement (which is similar to the C C<continue>
736 C<last>, C<next>, or C<redo> may appear within a C<continue>
737 block. C<last> and C<redo> will behave as if they had been executed within
738 the main block. So will C<next>, but since it will execute a C<continue>
739 block, it may be more entertaining.
742 ### redo always comes here
745 ### next always comes here
747 # then back the top to re-check EXPR
749 ### last always comes here
751 Omitting the C<continue> section is semantically equivalent to using an
752 empty one, logically enough. In that case, C<next> goes directly back
753 to check the condition at the top of the loop.
757 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
758 takes cosine of C<$_>.
760 For the inverse cosine operation, you may use the C<POSIX::acos()>
761 function, or use this relation:
763 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
765 =item crypt PLAINTEXT,SALT
767 Encrypts a string exactly like the crypt(3) function in the C library
768 (assuming that you actually have a version there that has not been
769 extirpated as a potential munition). This can prove useful for checking
770 the password file for lousy passwords, amongst other things. Only the
771 guys wearing white hats should do this.
773 Note that C<crypt> is intended to be a one-way function, much like breaking
774 eggs to make an omelette. There is no (known) corresponding decrypt
775 function. As a result, this function isn't all that useful for
776 cryptography. (For that, see your nearby CPAN mirror.)
778 When verifying an existing encrypted string you should use the encrypted
779 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
780 allows your code to work with the standard C<crypt> and with more
781 exotic implementations. When choosing a new salt create a random two
782 character string whose characters come from the set C<[./0-9A-Za-z]>
783 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
785 Here's an example that makes sure that whoever runs this program knows
788 $pwd = (getpwuid($<))[1];
792 chomp($word = <STDIN>);
796 if (crypt($word, $pwd) ne $pwd) {
802 Of course, typing in your own password to whoever asks you
805 The L<crypt> function is unsuitable for encrypting large quantities
806 of data, not least of all because you can't get the information
807 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
808 on your favorite CPAN mirror for a slew of potentially useful
813 [This function has been largely superseded by the C<untie> function.]
815 Breaks the binding between a DBM file and a hash.
817 =item dbmopen HASH,DBNAME,MASK
819 [This function has been largely superseded by the C<tie> function.]
821 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
822 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
823 argument is I<not> a filehandle, even though it looks like one). DBNAME
824 is the name of the database (without the F<.dir> or F<.pag> extension if
825 any). If the database does not exist, it is created with protection
826 specified by MASK (as modified by the C<umask>). If your system supports
827 only the older DBM functions, you may perform only one C<dbmopen> in your
828 program. In older versions of Perl, if your system had neither DBM nor
829 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
832 If you don't have write access to the DBM file, you can only read hash
833 variables, not set them. If you want to test whether you can write,
834 either use file tests or try setting a dummy hash entry inside an C<eval>,
835 which will trap the error.
837 Note that functions such as C<keys> and C<values> may return huge lists
838 when used on large DBM files. You may prefer to use the C<each>
839 function to iterate over large DBM files. Example:
841 # print out history file offsets
842 dbmopen(%HIST,'/usr/lib/news/history',0666);
843 while (($key,$val) = each %HIST) {
844 print $key, ' = ', unpack('L',$val), "\n";
848 See also L<AnyDBM_File> for a more general description of the pros and
849 cons of the various dbm approaches, as well as L<DB_File> for a particularly
852 You can control which DBM library you use by loading that library
853 before you call dbmopen():
856 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
857 or die "Can't open netscape history file: $!";
863 Returns a Boolean value telling whether EXPR has a value other than
864 the undefined value C<undef>. If EXPR is not present, C<$_> will be
867 Many operations return C<undef> to indicate failure, end of file,
868 system error, uninitialized variable, and other exceptional
869 conditions. This function allows you to distinguish C<undef> from
870 other values. (A simple Boolean test will not distinguish among
871 C<undef>, zero, the empty string, and C<"0">, which are all equally
872 false.) Note that since C<undef> is a valid scalar, its presence
873 doesn't I<necessarily> indicate an exceptional condition: C<pop>
874 returns C<undef> when its argument is an empty array, I<or> when the
875 element to return happens to be C<undef>.
877 You may also use C<defined> to check whether a subroutine exists, by
878 saying C<defined &func> without parentheses. On the other hand, use
879 of C<defined> upon aggregates (hashes and arrays) is not guaranteed to
880 produce intuitive results, and should probably be avoided.
882 When used on a hash element, it tells you whether the value is defined,
883 not whether the key exists in the hash. Use L</exists> for the latter
888 print if defined $switch{'D'};
889 print "$val\n" while defined($val = pop(@ary));
890 die "Can't readlink $sym: $!"
891 unless defined($value = readlink $sym);
892 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
893 $debugging = 0 unless defined $debugging;
895 Note: Many folks tend to overuse C<defined>, and then are surprised to
896 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
897 defined values. For example, if you say
901 The pattern match succeeds, and C<$1> is defined, despite the fact that it
902 matched "nothing". But it didn't really match nothing--rather, it
903 matched something that happened to be zero characters long. This is all
904 very above-board and honest. When a function returns an undefined value,
905 it's an admission that it couldn't give you an honest answer. So you
906 should use C<defined> only when you're questioning the integrity of what
907 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
910 Currently, using C<defined> on an entire array or hash reports whether
911 memory for that aggregate has ever been allocated. So an array you set
912 to the empty list appears undefined initially, and one that once was full
913 and that you then set to the empty list still appears defined. You
914 should instead use a simple test for size:
916 if (@an_array) { print "has array elements\n" }
917 if (%a_hash) { print "has hash members\n" }
919 Using C<undef> on these, however, does clear their memory and then report
920 them as not defined anymore, but you shouldn't do that unless you don't
921 plan to use them again, because it saves time when you load them up
922 again to have memory already ready to be filled. The normal way to
923 free up space used by an aggregate is to assign the empty list.
925 This counterintuitive behavior of C<defined> on aggregates may be
926 changed, fixed, or broken in a future release of Perl.
928 See also L</undef>, L</exists>, L</ref>.
932 Deletes the specified key(s) and their associated values from a hash.
933 For each key, returns the deleted value associated with that key, or
934 the undefined value if there was no such key. Deleting from C<$ENV{}>
935 modifies the environment. Deleting from a hash tied to a DBM file
936 deletes the entry from the DBM file. (But deleting from a C<tie>d hash
937 doesn't necessarily return anything.)
939 The following deletes all the values of a hash:
941 foreach $key (keys %HASH) {
947 delete @HASH{keys %HASH}
949 But both of these are slower than just assigning the empty list
952 %hash = (); # completely empty %hash
953 undef %hash; # forget %hash every existed
955 Note that the EXPR can be arbitrarily complicated as long as the final
956 operation is a hash element lookup or hash slice:
958 delete $ref->[$x][$y]{$key};
959 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
963 Outside an C<eval>, prints the value of LIST to C<STDERR> and
964 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
965 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
966 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
967 an C<eval(),> the error message is stuffed into C<$@> and the
968 C<eval> is terminated with the undefined value. This makes
969 C<die> the way to raise an exception.
973 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
974 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
976 If the value of EXPR does not end in a newline, the current script line
977 number and input line number (if any) are also printed, and a newline
978 is supplied. Note that the "input line number" (also known as "chunk")
979 is subject to whatever notion of "line" happens to be currently in
980 effect, and is also available as the special variable C<$.>.
981 See L<perlvar/"$/"> and L<perlvar/"$.">.
983 Hint: sometimes appending C<", stopped"> to your message
984 will cause it to make better sense when the string C<"at foo line 123"> is
985 appended. Suppose you are running script "canasta".
987 die "/etc/games is no good";
988 die "/etc/games is no good, stopped";
990 produce, respectively
992 /etc/games is no good at canasta line 123.
993 /etc/games is no good, stopped at canasta line 123.
995 See also exit(), warn(), and the Carp module.
997 If LIST is empty and C<$@> already contains a value (typically from a
998 previous eval) that value is reused after appending C<"\t...propagated">.
999 This is useful for propagating exceptions:
1002 die unless $@ =~ /Expected exception/;
1004 If C<$@> is empty then the string C<"Died"> is used.
1006 die() can also be called with a reference argument. If this happens to be
1007 trapped within an eval(), $@ contains the reference. This behavior permits
1008 a more elaborate exception handling implementation using objects that
1009 maintain arbitary state about the nature of the exception. Such a scheme
1010 is sometimes preferable to matching particular string values of $@ using
1011 regular expressions. Here's an example:
1013 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1015 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1016 # handle Some::Module::Exception
1019 # handle all other possible exceptions
1023 Because perl will stringify uncaught exception messages before displaying
1024 them, you may want to overload stringification operations on such custom
1025 exception objects. See L<overload> for details about that.
1027 You can arrange for a callback to be run just before the C<die>
1028 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1029 handler will be called with the error text and can change the error
1030 message, if it sees fit, by calling C<die> again. See
1031 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1032 L<"eval BLOCK"> for some examples. Although this feature was meant
1033 to be run only right before your program was to exit, this is not
1034 currently the case--the C<$SIG{__DIE__}> hook is currently called
1035 even inside eval()ed blocks/strings! If one wants the hook to do
1036 nothing in such situations, put
1040 as the first line of the handler (see L<perlvar/$^S>). Because
1041 this promotes strange action at a distance, this counterintuitive
1042 behavior may be fixed in a future release.
1046 Not really a function. Returns the value of the last command in the
1047 sequence of commands indicated by BLOCK. When modified by a loop
1048 modifier, executes the BLOCK once before testing the loop condition.
1049 (On other statements the loop modifiers test the conditional first.)
1051 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1052 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1053 See L<perlsyn> for alternative strategies.
1055 =item do SUBROUTINE(LIST)
1057 A deprecated form of subroutine call. See L<perlsub>.
1061 Uses the value of EXPR as a filename and executes the contents of the
1062 file as a Perl script. Its primary use is to include subroutines
1063 from a Perl subroutine library.
1069 scalar eval `cat stat.pl`;
1071 except that it's more efficient and concise, keeps track of the current
1072 filename for error messages, searches the @INC libraries, and updates
1073 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1074 variables. It also differs in that code evaluated with C<do FILENAME>
1075 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1076 same, however, in that it does reparse the file every time you call it,
1077 so you probably don't want to do this inside a loop.
1079 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1080 error. If C<do> can read the file but cannot compile it, it
1081 returns undef and sets an error message in C<$@>. If the file is
1082 successfully compiled, C<do> returns the value of the last expression
1085 Note that inclusion of library modules is better done with the
1086 C<use> and C<require> operators, which also do automatic error checking
1087 and raise an exception if there's a problem.
1089 You might like to use C<do> to read in a program configuration
1090 file. Manual error checking can be done this way:
1092 # read in config files: system first, then user
1093 for $file ("/share/prog/defaults.rc",
1094 "$ENV{HOME}/.someprogrc")
1096 unless ($return = do $file) {
1097 warn "couldn't parse $file: $@" if $@;
1098 warn "couldn't do $file: $!" unless defined $return;
1099 warn "couldn't run $file" unless $return;
1107 This function causes an immediate core dump. See also the B<-u>
1108 command-line switch in L<perlrun>, which does the same thing.
1109 Primarily this is so that you can use the B<undump> program (not
1110 supplied) to turn your core dump into an executable binary after
1111 having initialized all your variables at the beginning of the
1112 program. When the new binary is executed it will begin by executing
1113 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1114 Think of it as a goto with an intervening core dump and reincarnation.
1115 If C<LABEL> is omitted, restarts the program from the top.
1117 B<WARNING>: Any files opened at the time of the dump will I<not>
1118 be open any more when the program is reincarnated, with possible
1119 resulting confusion on the part of Perl.
1121 This function is now largely obsolete, partly because it's very
1122 hard to convert a core file into an executable, and because the
1123 real compiler backends for generating portable bytecode and compilable
1124 C code have superseded it.
1126 If you're looking to use L<dump> to speed up your program, consider
1127 generating bytecode or native C code as described in L<perlcc>. If
1128 you're just trying to accelerate a CGI script, consider using the
1129 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1130 You might also consider autoloading or selfloading, which at least
1131 make your program I<appear> to run faster.
1135 When called in list context, returns a 2-element list consisting of the
1136 key and value for the next element of a hash, so that you can iterate over
1137 it. When called in scalar context, returns the key for only the "next"
1138 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1139 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1142 Entries are returned in an apparently random order. The actual random
1143 order is subject to change in future versions of perl, but it is guaranteed
1144 to be in the same order as either the C<keys> or C<values> function
1145 would produce on the same (unmodified) hash.
1147 When the hash is entirely read, a null array is returned in list context
1148 (which when assigned produces a false (C<0>) value), and C<undef> in
1149 scalar context. The next call to C<each> after that will start iterating
1150 again. There is a single iterator for each hash, shared by all C<each>,
1151 C<keys>, and C<values> function calls in the program; it can be reset by
1152 reading all the elements from the hash, or by evaluating C<keys HASH> or
1153 C<values HASH>. If you add or delete elements of a hash while you're
1154 iterating over it, you may get entries skipped or duplicated, so don't.
1156 The following prints out your environment like the printenv(1) program,
1157 only in a different order:
1159 while (($key,$value) = each %ENV) {
1160 print "$key=$value\n";
1163 See also C<keys>, C<values> and C<sort>.
1165 =item eof FILEHANDLE
1171 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1172 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1173 gives the real filehandle. (Note that this function actually
1174 reads a character and then C<ungetc>s it, so isn't very useful in an
1175 interactive context.) Do not read from a terminal file (or call
1176 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1177 as terminals may lose the end-of-file condition if you do.
1179 An C<eof> without an argument uses the last file read as argument.
1180 Using C<eof()> with empty parentheses is very different. It indicates
1181 the pseudo file formed of the files listed on the command line,
1182 i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
1183 loop to detect the end of only the last file. Use C<eof(ARGV)> or
1184 C<eof> without the parentheses to test I<each> file in a while
1185 (E<lt>E<gt>) loop. Examples:
1187 # reset line numbering on each input file
1189 next if /^\s*#/; # skip comments
1192 close ARGV if eof; # Not eof()!
1195 # insert dashes just before last line of last file
1197 if (eof()) { # check for end of current file
1198 print "--------------\n";
1199 close(ARGV); # close or last; is needed if we
1200 # are reading from the terminal
1205 Practical hint: you almost never need to use C<eof> in Perl, because the
1206 input operators return false values when they run out of data, or if there
1213 In the first form, the return value of EXPR is parsed and executed as if it
1214 were a little Perl program. The value of the expression (which is itself
1215 determined within scalar context) is first parsed, and if there weren't any
1216 errors, executed in the context of the current Perl program, so that any
1217 variable settings or subroutine and format definitions remain afterwards.
1218 Note that the value is parsed every time the eval executes. If EXPR is
1219 omitted, evaluates C<$_>. This form is typically used to delay parsing
1220 and subsequent execution of the text of EXPR until run time.
1222 In the second form, the code within the BLOCK is parsed only once--at the
1223 same time the code surrounding the eval itself was parsed--and executed
1224 within the context of the current Perl program. This form is typically
1225 used to trap exceptions more efficiently than the first (see below), while
1226 also providing the benefit of checking the code within BLOCK at compile
1229 The final semicolon, if any, may be omitted from the value of EXPR or within
1232 In both forms, the value returned is the value of the last expression
1233 evaluated inside the mini-program; a return statement may be also used, just
1234 as with subroutines. The expression providing the return value is evaluated
1235 in void, scalar, or list context, depending on the context of the eval itself.
1236 See L</wantarray> for more on how the evaluation context can be determined.
1238 If there is a syntax error or runtime error, or a C<die> statement is
1239 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1240 error message. If there was no error, C<$@> is guaranteed to be a null
1241 string. Beware that using C<eval> neither silences perl from printing
1242 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1243 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1244 L</warn> and L<perlvar>.
1246 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1247 determining whether a particular feature (such as C<socket> or C<symlink>)
1248 is implemented. It is also Perl's exception trapping mechanism, where
1249 the die operator is used to raise exceptions.
1251 If the code to be executed doesn't vary, you may use the eval-BLOCK
1252 form to trap run-time errors without incurring the penalty of
1253 recompiling each time. The error, if any, is still returned in C<$@>.
1256 # make divide-by-zero nonfatal
1257 eval { $answer = $a / $b; }; warn $@ if $@;
1259 # same thing, but less efficient
1260 eval '$answer = $a / $b'; warn $@ if $@;
1262 # a compile-time error
1263 eval { $answer = }; # WRONG
1266 eval '$answer ='; # sets $@
1268 Due to the current arguably broken state of C<__DIE__> hooks, when using
1269 the C<eval{}> form as an exception trap in libraries, you may wish not
1270 to trigger any C<__DIE__> hooks that user code may have installed.
1271 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1272 as shown in this example:
1274 # a very private exception trap for divide-by-zero
1275 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1278 This is especially significant, given that C<__DIE__> hooks can call
1279 C<die> again, which has the effect of changing their error messages:
1281 # __DIE__ hooks may modify error messages
1283 local $SIG{'__DIE__'} =
1284 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1285 eval { die "foo lives here" };
1286 print $@ if $@; # prints "bar lives here"
1289 Because this promotes action at a distance, this counterintuitive behavior
1290 may be fixed in a future release.
1292 With an C<eval>, you should be especially careful to remember what's
1293 being looked at when:
1299 eval { $x }; # CASE 4
1301 eval "\$$x++"; # CASE 5
1304 Cases 1 and 2 above behave identically: they run the code contained in
1305 the variable $x. (Although case 2 has misleading double quotes making
1306 the reader wonder what else might be happening (nothing is).) Cases 3
1307 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1308 does nothing but return the value of $x. (Case 4 is preferred for
1309 purely visual reasons, but it also has the advantage of compiling at
1310 compile-time instead of at run-time.) Case 5 is a place where
1311 normally you I<would> like to use double quotes, except that in this
1312 particular situation, you can just use symbolic references instead, as
1315 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1316 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1320 =item exec PROGRAM LIST
1322 The C<exec> function executes a system command I<and never returns>--
1323 use C<system> instead of C<exec> if you want it to return. It fails and
1324 returns false only if the command does not exist I<and> it is executed
1325 directly instead of via your system's command shell (see below).
1327 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1328 warns you if there is a following statement which isn't C<die>, C<warn>,
1329 or C<exit> (if C<-w> is set - but you always do that). If you
1330 I<really> want to follow an C<exec> with some other statement, you
1331 can use one of these styles to avoid the warning:
1333 exec ('foo') or print STDERR "couldn't exec foo: $!";
1334 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1336 If there is more than one argument in LIST, or if LIST is an array
1337 with more than one value, calls execvp(3) with the arguments in LIST.
1338 If there is only one scalar argument or an array with one element in it,
1339 the argument is checked for shell metacharacters, and if there are any,
1340 the entire argument is passed to the system's command shell for parsing
1341 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1342 If there are no shell metacharacters in the argument, it is split into
1343 words and passed directly to C<execvp>, which is more efficient.
1346 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1347 exec "sort $outfile | uniq";
1349 If you don't really want to execute the first argument, but want to lie
1350 to the program you are executing about its own name, you can specify
1351 the program you actually want to run as an "indirect object" (without a
1352 comma) in front of the LIST. (This always forces interpretation of the
1353 LIST as a multivalued list, even if there is only a single scalar in
1356 $shell = '/bin/csh';
1357 exec $shell '-sh'; # pretend it's a login shell
1361 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1363 When the arguments get executed via the system shell, results will
1364 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1367 Using an indirect object with C<exec> or C<system> is also more
1368 secure. This usage (which also works fine with system()) forces
1369 interpretation of the arguments as a multivalued list, even if the
1370 list had just one argument. That way you're safe from the shell
1371 expanding wildcards or splitting up words with whitespace in them.
1373 @args = ( "echo surprise" );
1375 exec @args; # subject to shell escapes
1377 exec { $args[0] } @args; # safe even with one-arg list
1379 The first version, the one without the indirect object, ran the I<echo>
1380 program, passing it C<"surprise"> an argument. The second version
1381 didn't--it tried to run a program literally called I<"echo surprise">,
1382 didn't find it, and set C<$?> to a non-zero value indicating failure.
1384 Note that C<exec> will not call your C<END> blocks, nor will it call
1385 any C<DESTROY> methods in your objects.
1389 Returns true if the specified hash key exists in its hash, even
1390 if the corresponding value is undefined.
1392 print "Exists\n" if exists $array{$key};
1393 print "Defined\n" if defined $array{$key};
1394 print "True\n" if $array{$key};
1396 A hash element can be true only if it's defined, and defined if
1397 it exists, but the reverse doesn't necessarily hold true.
1399 Note that the EXPR can be arbitrarily complicated as long as the final
1400 operation is a hash key lookup:
1402 if (exists $ref->{A}->{B}->{$key}) { }
1403 if (exists $hash{A}{B}{$key}) { }
1405 Although the last element will not spring into existence just because
1406 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1407 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1408 existence test for a $key element. This happens anywhere the arrow
1409 operator is used, including even
1412 if (exists $ref->{"Some key"}) { }
1413 print $ref; # prints HASH(0x80d3d5c)
1415 This surprising autovivification in what does not at first--or even
1416 second--glance appear to be an lvalue context may be fixed in a future
1421 Evaluates EXPR and exits immediately with that value. Example:
1424 exit 0 if $ans =~ /^[Xx]/;
1426 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1427 universally recognized values for EXPR are C<0> for success and C<1>
1428 for error; other values are subject to interpretation depending on the
1429 environment in which the Perl program is running. For example, exiting
1430 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1431 the mailer to return the item undelivered, but that's not true everywhere.
1433 Don't use C<exit> to abort a subroutine if there's any chance that
1434 someone might want to trap whatever error happened. Use C<die> instead,
1435 which can be trapped by an C<eval>.
1437 The exit() function does not always exit immediately. It calls any
1438 defined C<END> routines first, but these C<END> routines may not
1439 themselves abort the exit. Likewise any object destructors that need to
1440 be called are called before the real exit. If this is a problem, you
1441 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1442 See L<perlsub> for details.
1448 Returns I<e> (the natural logarithm base) to the power of EXPR.
1449 If EXPR is omitted, gives C<exp($_)>.
1451 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1453 Implements the fcntl(2) function. You'll probably have to say
1457 first to get the correct constant definitions. Argument processing and
1458 value return works just like C<ioctl> below.
1462 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1463 or die "can't fcntl F_GETFL: $!";
1465 You don't have to check for C<defined> on the return from C<fnctl>.
1466 Like C<ioctl>, it maps a C<0> return from the system call into C<"0
1467 but true"> in Perl. This string is true in boolean context and C<0>
1468 in numeric context. It is also exempt from the normal B<-w> warnings
1469 on improper numeric conversions.
1471 Note that C<fcntl> will produce a fatal error if used on a machine that
1472 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1473 manpage to learn what functions are available on your system.
1475 =item fileno FILEHANDLE
1477 Returns the file descriptor for a filehandle, or undefined if the
1478 filehandle is not open. This is mainly useful for constructing
1479 bitmaps for C<select> and low-level POSIX tty-handling operations.
1480 If FILEHANDLE is an expression, the value is taken as an indirect
1481 filehandle, generally its name.
1483 You can use this to find out whether two handles refer to the
1484 same underlying descriptor:
1486 if (fileno(THIS) == fileno(THAT)) {
1487 print "THIS and THAT are dups\n";
1490 =item flock FILEHANDLE,OPERATION
1492 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1493 for success, false on failure. Produces a fatal error if used on a
1494 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1495 C<flock> is Perl's portable file locking interface, although it locks
1496 only entire files, not records.
1498 Two potentially non-obvious but traditional C<flock> semantics are
1499 that it waits indefinitely until the lock is granted, and that its locks
1500 B<merely advisory>. Such discretionary locks are more flexible, but offer
1501 fewer guarantees. This means that files locked with C<flock> may be
1502 modified by programs that do not also use C<flock>. See L<perlport>,
1503 your port's specific documentation, or your system-specific local manpages
1504 for details. It's best to assume traditional behavior if you're writing
1505 portable programs. (But if you're not, you should as always feel perfectly
1506 free to write for your own system's idiosyncrasies (sometimes called
1507 "features"). Slavish adherence to portability concerns shouldn't get
1508 in the way of your getting your job done.)
1510 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1511 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1512 you can use the symbolic names if import them from the Fcntl module,
1513 either individually, or as a group using the ':flock' tag. LOCK_SH
1514 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1515 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1516 LOCK_EX then C<flock> will return immediately rather than blocking
1517 waiting for the lock (check the return status to see if you got it).
1519 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1520 before locking or unlocking it.
1522 Note that the emulation built with lockf(3) doesn't provide shared
1523 locks, and it requires that FILEHANDLE be open with write intent. These
1524 are the semantics that lockf(3) implements. Most if not all systems
1525 implement lockf(3) in terms of fcntl(2) locking, though, so the
1526 differing semantics shouldn't bite too many people.
1528 Note also that some versions of C<flock> cannot lock things over the
1529 network; you would need to use the more system-specific C<fcntl> for
1530 that. If you like you can force Perl to ignore your system's flock(2)
1531 function, and so provide its own fcntl(2)-based emulation, by passing
1532 the switch C<-Ud_flock> to the F<Configure> program when you configure
1535 Here's a mailbox appender for BSD systems.
1537 use Fcntl ':flock'; # import LOCK_* constants
1540 flock(MBOX,LOCK_EX);
1541 # and, in case someone appended
1542 # while we were waiting...
1547 flock(MBOX,LOCK_UN);
1550 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1551 or die "Can't open mailbox: $!";
1554 print MBOX $msg,"\n\n";
1557 On systems that support a real flock(), locks are inherited across fork()
1558 calls, whereas those that must resort to the more capricious fcntl()
1559 function lose the locks, making it harder to write servers.
1561 See also L<DB_File> for other flock() examples.
1565 Does a fork(2) system call to create a new process running the
1566 same program at the same point. It returns the child pid to the
1567 parent process, C<0> to the child process, or C<undef> if the fork is
1568 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1569 are shared, while everything else is copied. On most systems supporting
1570 fork(), great care has gone into making it extremely efficient (for
1571 example, using copy-on-write technology on data pages), making it the
1572 dominant paradigm for multitasking over the last few decades.
1574 All files opened for output are flushed before forking the child process.
1576 If you C<fork> without ever waiting on your children, you will
1577 accumulate zombies. On some systems, you can avoid this by setting
1578 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1579 forking and reaping moribund children.
1581 Note that if your forked child inherits system file descriptors like
1582 STDIN and STDOUT that are actually connected by a pipe or socket, even
1583 if you exit, then the remote server (such as, say, a CGI script or a
1584 backgrounded job launched from a remote shell) won't think you're done.
1585 You should reopen those to F</dev/null> if it's any issue.
1589 Declare a picture format for use by the C<write> function. For
1593 Test: @<<<<<<<< @||||| @>>>>>
1594 $str, $%, '$' . int($num)
1598 $num = $cost/$quantity;
1602 See L<perlform> for many details and examples.
1604 =item formline PICTURE,LIST
1606 This is an internal function used by C<format>s, though you may call it,
1607 too. It formats (see L<perlform>) a list of values according to the
1608 contents of PICTURE, placing the output into the format output
1609 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1610 Eventually, when a C<write> is done, the contents of
1611 C<$^A> are written to some filehandle, but you could also read C<$^A>
1612 yourself and then set C<$^A> back to C<"">. Note that a format typically
1613 does one C<formline> per line of form, but the C<formline> function itself
1614 doesn't care how many newlines are embedded in the PICTURE. This means
1615 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1616 You may therefore need to use multiple formlines to implement a single
1617 record format, just like the format compiler.
1619 Be careful if you put double quotes around the picture, because an C<@>
1620 character may be taken to mean the beginning of an array name.
1621 C<formline> always returns true. See L<perlform> for other examples.
1623 =item getc FILEHANDLE
1627 Returns the next character from the input file attached to FILEHANDLE,
1628 or the undefined value at end of file, or if there was an error.
1629 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1630 efficient. However, it cannot be used by itself to fetch single
1631 characters without waiting for the user to hit enter. For that, try
1632 something more like:
1635 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1638 system "stty", '-icanon', 'eol', "\001";
1644 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1647 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1651 Determination of whether $BSD_STYLE should be set
1652 is left as an exercise to the reader.
1654 The C<POSIX::getattr> function can do this more portably on
1655 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1656 module from your nearest CPAN site; details on CPAN can be found on
1661 Implements the C library function of the same name, which on most
1662 systems returns the current login from F</etc/utmp>, if any. If null,
1665 $login = getlogin || getpwuid($<) || "Kilroy";
1667 Do not consider C<getlogin> for authentication: it is not as
1668 secure as C<getpwuid>.
1670 =item getpeername SOCKET
1672 Returns the packed sockaddr address of other end of the SOCKET connection.
1675 $hersockaddr = getpeername(SOCK);
1676 ($port, $iaddr) = sockaddr_in($hersockaddr);
1677 $herhostname = gethostbyaddr($iaddr, AF_INET);
1678 $herstraddr = inet_ntoa($iaddr);
1682 Returns the current process group for the specified PID. Use
1683 a PID of C<0> to get the current process group for the
1684 current process. Will raise an exception if used on a machine that
1685 doesn't implement getpgrp(2). If PID is omitted, returns process
1686 group of current process. Note that the POSIX version of C<getpgrp>
1687 does not accept a PID argument, so only C<PID==0> is truly portable.
1691 Returns the process id of the parent process.
1693 =item getpriority WHICH,WHO
1695 Returns the current priority for a process, a process group, or a user.
1696 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1697 machine that doesn't implement getpriority(2).
1703 =item gethostbyname NAME
1705 =item getnetbyname NAME
1707 =item getprotobyname NAME
1713 =item getservbyname NAME,PROTO
1715 =item gethostbyaddr ADDR,ADDRTYPE
1717 =item getnetbyaddr ADDR,ADDRTYPE
1719 =item getprotobynumber NUMBER
1721 =item getservbyport PORT,PROTO
1739 =item sethostent STAYOPEN
1741 =item setnetent STAYOPEN
1743 =item setprotoent STAYOPEN
1745 =item setservent STAYOPEN
1759 These routines perform the same functions as their counterparts in the
1760 system library. In list context, the return values from the
1761 various get routines are as follows:
1763 ($name,$passwd,$uid,$gid,
1764 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1765 ($name,$passwd,$gid,$members) = getgr*
1766 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1767 ($name,$aliases,$addrtype,$net) = getnet*
1768 ($name,$aliases,$proto) = getproto*
1769 ($name,$aliases,$port,$proto) = getserv*
1771 (If the entry doesn't exist you get a null list.)
1773 In scalar context, you get the name, unless the function was a
1774 lookup by name, in which case you get the other thing, whatever it is.
1775 (If the entry doesn't exist you get the undefined value.) For example:
1777 $uid = getpwnam($name);
1778 $name = getpwuid($num);
1780 $gid = getgrnam($name);
1781 $name = getgrgid($num;
1785 In I<getpw*()> the fields $quota, $comment, and $expire are
1786 special cases in the sense that in many systems they are unsupported.
1787 If the $quota is unsupported, it is an empty scalar. If it is
1788 supported, it usually encodes the disk quota. If the $comment
1789 field is unsupported, it is an empty scalar. If it is supported it
1790 usually encodes some administrative comment about the user. In some
1791 systems the $quota field may be $change or $age, fields that have
1792 to do with password aging. In some systems the $comment field may
1793 be $class. The $expire field, if present, encodes the expiration
1794 period of the account or the password. For the availability and the
1795 exact meaning of these fields in your system, please consult your
1796 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1797 out from within Perl what your $quota and $comment fields mean
1798 and whether you have the $expire field by using the C<Config> module
1799 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1800 and C<d_pwexpire>. Shadow password files are only supported if your
1801 vendor has implemented them in the intuitive fashion that calling the
1802 regular C library routines gets the shadow versions if you're running
1803 under privilege. Those that incorrectly implement a separate library
1804 call are not supported.
1806 The $members value returned by I<getgr*()> is a space separated list of
1807 the login names of the members of the group.
1809 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1810 C, it will be returned to you via C<$?> if the function call fails. The
1811 C<@addrs> value returned by a successful call is a list of the raw
1812 addresses returned by the corresponding system library call. In the
1813 Internet domain, each address is four bytes long and you can unpack it
1814 by saying something like:
1816 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1818 The Socket library makes this slightly easier:
1821 $iaddr = inet_aton("127.1"); # or whatever address
1822 $name = gethostbyaddr($iaddr, AF_INET);
1824 # or going the other way
1825 $straddr = inet_ntoa($iaddr);
1827 If you get tired of remembering which element of the return list
1828 contains which return value, by-name interfaces are provided
1829 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1830 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1831 and C<User::grent>. These override the normal built-ins, supplying
1832 versions that return objects with the appropriate names
1833 for each field. For example:
1837 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1839 Even though it looks like they're the same method calls (uid),
1840 they aren't, because a C<File::stat> object is different from
1841 a C<User::pwent> object.
1843 =item getsockname SOCKET
1845 Returns the packed sockaddr address of this end of the SOCKET connection,
1846 in case you don't know the address because you have several different
1847 IPs that the connection might have come in on.
1850 $mysockaddr = getsockname(SOCK);
1851 ($port, $myaddr) = sockaddr_in($mysockaddr);
1852 printf "Connect to %s [%s]\n",
1853 scalar gethostbyaddr($myaddr, AF_INET),
1856 =item getsockopt SOCKET,LEVEL,OPTNAME
1858 Returns the socket option requested, or undef if there is an error.
1864 Returns the value of EXPR with filename expansions such as the
1865 standard Unix shell F</bin/csh> would do. This is the internal function
1866 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1867 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1868 discussed in more detail in L<perlop/"I/O Operators">.
1872 Converts a time as returned by the time function to a 9-element list
1873 with the time localized for the standard Greenwich time zone.
1874 Typically used as follows:
1877 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1880 All list elements are numeric, and come straight out of a struct tm.
1881 In particular this means that $mon has the range C<0..11> and $wday
1882 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1883 number of years since 1900, that is, $year is C<123> in year 2023,
1884 I<not> simply the last two digits of the year. If you assume it is,
1885 then you create non-Y2K-compliant programs--and you wouldn't want to do
1888 If EXPR is omitted, does C<gmtime(time())>.
1890 In scalar context, returns the ctime(3) value:
1892 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1894 Also see the C<timegm> function provided by the C<Time::Local> module,
1895 and the strftime(3) function available via the POSIX module.
1897 This scalar value is B<not> locale dependent (see L<perllocale>), but
1898 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1899 strftime(3) and mktime(3) functions available via the POSIX module. To
1900 get somewhat similar but locale dependent date strings, set up your
1901 locale environment variables appropriately (please see L<perllocale>)
1902 and try for example:
1904 use POSIX qw(strftime);
1905 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1907 Note that the C<%a> and C<%b> escapes, which represent the short forms
1908 of the day of the week and the month of the year, may not necessarily
1909 be three characters wide in all locales.
1917 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1918 execution there. It may not be used to go into any construct that
1919 requires initialization, such as a subroutine or a C<foreach> loop. It
1920 also can't be used to go into a construct that is optimized away,
1921 or to get out of a block or subroutine given to C<sort>.
1922 It can be used to go almost anywhere else within the dynamic scope,
1923 including out of subroutines, but it's usually better to use some other
1924 construct such as C<last> or C<die>. The author of Perl has never felt the
1925 need to use this form of C<goto> (in Perl, that is--C is another matter).
1927 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1928 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1929 necessarily recommended if you're optimizing for maintainability:
1931 goto ("FOO", "BAR", "GLARCH")[$i];
1933 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1934 named subroutine for the currently running subroutine. This is used by
1935 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1936 pretend that the other subroutine had been called in the first place
1937 (except that any modifications to C<@_> in the current subroutine are
1938 propagated to the other subroutine.) After the C<goto>, not even C<caller>
1939 will be able to tell that this routine was called first.
1941 =item grep BLOCK LIST
1943 =item grep EXPR,LIST
1945 This is similar in spirit to, but not the same as, grep(1) and its
1946 relatives. In particular, it is not limited to using regular expressions.
1948 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1949 C<$_> to each element) and returns the list value consisting of those
1950 elements for which the expression evaluated to true. In scalar
1951 context, returns the number of times the expression was true.
1953 @foo = grep(!/^#/, @bar); # weed out comments
1957 @foo = grep {!/^#/} @bar; # weed out comments
1959 Note that, because C<$_> is a reference into the list value, it can
1960 be used to modify the elements of the array. While this is useful and
1961 supported, it can cause bizarre results if the LIST is not a named array.
1962 Similarly, grep returns aliases into the original list, much as a for
1963 loop's index variable aliases the list elements. That is, modifying an
1964 element of a list returned by grep (for example, in a C<foreach>, C<map>
1965 or another C<grep>) actually modifies the element in the original list.
1966 This is usually something to be avoided when writing clear code.
1968 See also L</map> for a list composed of the results of the BLOCK or EXPR.
1974 Interprets EXPR as a hex string and returns the corresponding value.
1975 (To convert strings that might start with either 0, 0x, or 0b, see
1976 L</oct>.) If EXPR is omitted, uses C<$_>.
1978 print hex '0xAf'; # prints '175'
1979 print hex 'aF'; # same
1981 Hex strings may only represent integers. Strings that would cause
1982 integer overflow trigger a mandatory error message.
1986 There is no builtin C<import> function. It is just an ordinary
1987 method (subroutine) defined (or inherited) by modules that wish to export
1988 names to another module. The C<use> function calls the C<import> method
1989 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1991 =item index STR,SUBSTR,POSITION
1993 =item index STR,SUBSTR
1995 The index function searches for one string within another, but without
1996 the wildcard-like behavior of a full regular-expression pattern match.
1997 It returns the position of the first occurrence of SUBSTR in STR at
1998 or after POSITION. If POSITION is omitted, starts searching from the
1999 beginning of the string. The return value is based at C<0> (or whatever
2000 you've set the C<$[> variable to--but don't do that). If the substring
2001 is not found, returns one less than the base, ordinarily C<-1>.
2007 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2008 You should not use this function for rounding: one because it truncates
2009 towards C<0>, and two because machine representations of floating point
2010 numbers can sometimes produce counterintuitive results. For example,
2011 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2012 because it's really more like -268.99999999999994315658 instead. Usually,
2013 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2014 functions will serve you better than will int().
2016 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2018 Implements the ioctl(2) function. You'll probably first have to say
2020 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2022 to get the correct function definitions. If F<ioctl.ph> doesn't
2023 exist or doesn't have the correct definitions you'll have to roll your
2024 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2025 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2026 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2027 written depending on the FUNCTION--a pointer to the string value of SCALAR
2028 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2029 has no string value but does have a numeric value, that value will be
2030 passed rather than a pointer to the string value. To guarantee this to be
2031 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2032 functions may be needed to manipulate the values of structures used by
2035 The return value of C<ioctl> (and C<fcntl>) is as follows:
2037 if OS returns: then Perl returns:
2039 0 string "0 but true"
2040 anything else that number
2042 Thus Perl returns true on success and false on failure, yet you can
2043 still easily determine the actual value returned by the operating
2046 $retval = ioctl(...) || -1;
2047 printf "System returned %d\n", $retval;
2049 The special string "C<0> but true" is exempt from B<-w> complaints
2050 about improper numeric conversions.
2052 Here's an example of setting a filehandle named C<REMOTE> to be
2053 non-blocking at the system level. You'll have to negotiate C<$|>
2054 on your own, though.
2056 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2058 $flags = fcntl(REMOTE, F_GETFL, 0)
2059 or die "Can't get flags for the socket: $!\n";
2061 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2062 or die "Can't set flags for the socket: $!\n";
2064 =item join EXPR,LIST
2066 Joins the separate strings of LIST into a single string with fields
2067 separated by the value of EXPR, and returns that new string. Example:
2069 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2075 Returns a list consisting of all the keys of the named hash. (In
2076 scalar context, returns the number of keys.) The keys are returned in
2077 an apparently random order. The actual random order is subject to
2078 change in future versions of perl, but it is guaranteed to be the same
2079 order as either the C<values> or C<each> function produces (given
2080 that the hash has not been modified). As a side effect, it resets
2083 Here is yet another way to print your environment:
2086 @values = values %ENV;
2088 print pop(@keys), '=', pop(@values), "\n";
2091 or how about sorted by key:
2093 foreach $key (sort(keys %ENV)) {
2094 print $key, '=', $ENV{$key}, "\n";
2097 To sort a hash by value, you'll need to use a C<sort> function.
2098 Here's a descending numeric sort of a hash by its values:
2100 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2101 printf "%4d %s\n", $hash{$key}, $key;
2104 As an lvalue C<keys> allows you to increase the number of hash buckets
2105 allocated for the given hash. This can gain you a measure of efficiency if
2106 you know the hash is going to get big. (This is similar to pre-extending
2107 an array by assigning a larger number to $#array.) If you say
2111 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2112 in fact, since it rounds up to the next power of two. These
2113 buckets will be retained even if you do C<%hash = ()>, use C<undef
2114 %hash> if you want to free the storage while C<%hash> is still in scope.
2115 You can't shrink the number of buckets allocated for the hash using
2116 C<keys> in this way (but you needn't worry about doing this by accident,
2117 as trying has no effect).
2119 See also C<each>, C<values> and C<sort>.
2123 Sends a signal to a list of processes. The first element of
2124 the list must be the signal to send. Returns the number of
2125 processes successfully signaled.
2127 $cnt = kill 1, $child1, $child2;
2130 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2131 process groups instead of processes. (On System V, a negative I<PROCESS>
2132 number will also kill process groups, but that's not portable.) That
2133 means you usually want to use positive not negative signals. You may also
2134 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2140 The C<last> command is like the C<break> statement in C (as used in
2141 loops); it immediately exits the loop in question. If the LABEL is
2142 omitted, the command refers to the innermost enclosing loop. The
2143 C<continue> block, if any, is not executed:
2145 LINE: while (<STDIN>) {
2146 last LINE if /^$/; # exit when done with header
2150 C<last> cannot be used to exit a block which returns a value such as
2151 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2152 a grep() or map() operation.
2154 See also L</continue> for an illustration of how C<last>, C<next>, and
2161 Returns an lowercased version of EXPR. This is the internal function
2162 implementing the C<\L> escape in double-quoted strings.
2163 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2166 If EXPR is omitted, uses C<$_>.
2172 Returns the value of EXPR with the first character lowercased. This is
2173 the internal function implementing the C<\l> escape in double-quoted strings.
2174 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2176 If EXPR is omitted, uses C<$_>.
2182 Returns the length in characters of the value of EXPR. If EXPR is
2183 omitted, returns length of C<$_>. Note that this cannot be used on
2184 an entire array or hash to find out how many elements these have.
2185 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2187 =item link OLDFILE,NEWFILE
2189 Creates a new filename linked to the old filename. Returns true for
2190 success, false otherwise.
2192 =item listen SOCKET,QUEUESIZE
2194 Does the same thing that the listen system call does. Returns true if
2195 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2199 You really probably want to be using C<my> instead, because C<local> isn't
2200 what most people think of as "local". See L<perlsub/"Private Variables
2201 via my()"> for details.
2203 A local modifies the listed variables to be local to the enclosing
2204 block, file, or eval. If more than one value is listed, the list must
2205 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2206 for details, including issues with tied arrays and hashes.
2208 =item localtime EXPR
2210 Converts a time as returned by the time function to a 9-element list
2211 with the time analyzed for the local time zone. Typically used as
2215 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2218 All list elements are numeric, and come straight out of a struct tm.
2219 In particular this means that $mon has the range C<0..11> and $wday
2220 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2221 number of years since 1900, that is, $year is C<123> in year 2023,
2222 and I<not> simply the last two digits of the year. If you assume it is,
2223 then you create non-Y2K-compliant programs--and you wouldn't want to do
2226 If EXPR is omitted, uses the current time (C<localtime(time)>).
2228 In scalar context, returns the ctime(3) value:
2230 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2232 This scalar value is B<not> locale dependent, see L<perllocale>, but
2233 instead a Perl builtin. Also see the C<Time::Local> module, and the
2234 strftime(3) and mktime(3) function available via the POSIX module. To
2235 get somewhat similar but locale dependent date strings, set up your
2236 locale environment variables appropriately (please see L<perllocale>)
2237 and try for example:
2239 use POSIX qw(strftime);
2240 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2242 Note that the C<%a> and C<%b>, the short forms of the day of the week
2243 and the month of the year, may not necessarily be three characters wide.
2249 This function places an advisory lock on a variable, subroutine,
2250 or referenced object contained in I<THING> until the lock goes out
2251 of scope. This is a built-in function only if your version of Perl
2252 was built with threading enabled, and if you've said C<use Threads>.
2253 Otherwise a user-defined function by this name will be called. See
2260 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2261 returns log of C<$_>. To get the log of another base, use basic algebra:
2262 The base-N log of a number is equal to the natural log of that number
2263 divided by the natural log of N. For example:
2267 return log($n)/log(10);
2270 See also L</exp> for the inverse operation.
2272 =item lstat FILEHANDLE
2278 Does the same thing as the C<stat> function (including setting the
2279 special C<_> filehandle) but stats a symbolic link instead of the file
2280 the symbolic link points to. If symbolic links are unimplemented on
2281 your system, a normal C<stat> is done.
2283 If EXPR is omitted, stats C<$_>.
2287 The match operator. See L<perlop>.
2289 =item map BLOCK LIST
2293 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2294 C<$_> to each element) and returns the list value composed of the
2295 results of each such evaluation. In scalar context, returns the
2296 total number of elements so generated. Evaluates BLOCK or EXPR in
2297 list context, so each element of LIST may produce zero, one, or
2298 more elements in the returned value.
2300 @chars = map(chr, @nums);
2302 translates a list of numbers to the corresponding characters. And
2304 %hash = map { getkey($_) => $_ } @array;
2306 is just a funny way to write
2309 foreach $_ (@array) {
2310 $hash{getkey($_)} = $_;
2313 Note that, because C<$_> is a reference into the list value, it can
2314 be used to modify the elements of the array. While this is useful and
2315 supported, it can cause bizarre results if the LIST is not a named array.
2316 Using a regular C<foreach> loop for this purpose would be clearer in
2317 most cases. See also L</grep> for an array composed of those items of
2318 the original list for which the BLOCK or EXPR evaluates to true.
2320 =item mkdir FILENAME,MASK
2322 Creates the directory specified by FILENAME, with permissions
2323 specified by MASK (as modified by C<umask>). If it succeeds it
2324 returns true, otherwise it returns false and sets C<$!> (errno).
2326 In general, it is better to create directories with permissive MASK,
2327 and let the user modify that with their C<umask>, than it is to supply
2328 a restrictive MASK and give the user no way to be more permissive.
2329 The exceptions to this rule are when the file or directory should be
2330 kept private (mail files, for instance). The perlfunc(1) entry on
2331 C<umask> discusses the choice of MASK in more detail.
2333 =item msgctl ID,CMD,ARG
2335 Calls the System V IPC function msgctl(2). You'll probably have to say
2339 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2340 then ARG must be a variable which will hold the returned C<msqid_ds>
2341 structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
2342 true"> for zero, or the actual return value otherwise. See also
2343 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2345 =item msgget KEY,FLAGS
2347 Calls the System V IPC function msgget(2). Returns the message queue
2348 id, or the undefined value if there is an error. See also C<IPC::SysV>
2349 and C<IPC::Msg> documentation.
2351 =item msgsnd ID,MSG,FLAGS
2353 Calls the System V IPC function msgsnd to send the message MSG to the
2354 message queue ID. MSG must begin with the long integer message type,
2355 which may be created with C<pack("l", $type)>. Returns true if
2356 successful, or false if there is an error. See also C<IPC::SysV>
2357 and C<IPC::SysV::Msg> documentation.
2359 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2361 Calls the System V IPC function msgrcv to receive a message from
2362 message queue ID into variable VAR with a maximum message size of
2363 SIZE. Note that if a message is received, the message type will be
2364 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2365 size of the message type. Returns true if successful, or false if
2366 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2370 A C<my> declares the listed variables to be local (lexically) to the
2371 enclosing block, file, or C<eval>. If
2372 more than one value is listed, the list must be placed in parentheses. See
2373 L<perlsub/"Private Variables via my()"> for details.
2379 The C<next> command is like the C<continue> statement in C; it starts
2380 the next iteration of the loop:
2382 LINE: while (<STDIN>) {
2383 next LINE if /^#/; # discard comments
2387 Note that if there were a C<continue> block on the above, it would get
2388 executed even on discarded lines. If the LABEL is omitted, the command
2389 refers to the innermost enclosing loop.
2391 C<next> cannot be used to exit a block which returns a value such as
2392 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2393 a grep() or map() operation.
2395 See also L</continue> for an illustration of how C<last>, C<next>, and
2398 =item no Module LIST
2400 See the L</use> function, which C<no> is the opposite of.
2406 Interprets EXPR as an octal string and returns the corresponding
2407 value. (If EXPR happens to start off with C<0x>, interprets it as a
2408 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2409 binary string.) The following will handle decimal, binary, octal, and
2410 hex in the standard Perl or C notation:
2412 $val = oct($val) if $val =~ /^0/;
2414 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2415 in octal), use sprintf() or printf():
2417 $perms = (stat("filename"))[2] & 07777;
2418 $oct_perms = sprintf "%lo", $perms;
2420 The oct() function is commonly used when a string such as C<644> needs
2421 to be converted into a file mode, for example. (Although perl will
2422 automatically convert strings into numbers as needed, this automatic
2423 conversion assumes base 10.)
2425 =item open FILEHANDLE,EXPR
2427 =item open FILEHANDLE
2429 Opens the file whose filename is given by EXPR, and associates it with
2430 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2431 name of the real filehandle wanted. If EXPR is omitted, the scalar
2432 variable of the same name as the FILEHANDLE contains the filename.
2433 (Note that lexical variables--those declared with C<my>--will not work
2434 for this purpose; so if you're using C<my>, specify EXPR in your call
2435 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2438 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2439 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2440 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2441 the file is opened for appending, again being created if necessary.
2442 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2443 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2444 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2445 file first. You can't usually use either read-write mode for updating
2446 textfiles, since they have variable length records. See the B<-i>
2447 switch in L<perlrun> for a better approach. The file is created with
2448 permissions of C<0666> modified by the process' C<umask> value.
2450 The prefix and the filename may be separated with spaces.
2451 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2452 C<'w+'>, C<'a'>, and C<'a+'>.
2454 If the filename begins with C<'|'>, the filename is interpreted as a
2455 command to which output is to be piped, and if the filename ends with a
2456 C<'|'>, the filename is interpreted as a command which pipes output to
2457 us. See L<perlipc/"Using open() for IPC">
2458 for more examples of this. (You are not allowed to C<open> to a command
2459 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2460 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2462 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2463 nonzero upon success, the undefined value otherwise. If the C<open>
2464 involved a pipe, the return value happens to be the pid of the
2467 If you're unfortunate enough to be running Perl on a system that
2468 distinguishes between text files and binary files (modern operating
2469 systems don't care), then you should check out L</binmode> for tips for
2470 dealing with this. The key distinction between systems that need C<binmode>
2471 and those that don't is their text file formats. Systems like Unix, MacOS, and
2472 Plan9, which delimit lines with a single character, and which encode that
2473 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2475 When opening a file, it's usually a bad idea to continue normal execution
2476 if the request failed, so C<open> is frequently used in connection with
2477 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2478 where you want to make a nicely formatted error message (but there are
2479 modules that can help with that problem)) you should always check
2480 the return value from opening a file. The infrequent exception is when
2481 working with an unopened filehandle is actually what you want to do.
2486 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2487 while (<ARTICLE>) {...
2489 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2490 # if the open fails, output is discarded
2492 open(DBASE, '+<dbase.mine') # open for update
2493 or die "Can't open 'dbase.mine' for update: $!";
2495 open(ARTICLE, "caesar <$article |") # decrypt article
2496 or die "Can't start caesar: $!";
2498 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2499 or die "Can't start sort: $!";
2501 # process argument list of files along with any includes
2503 foreach $file (@ARGV) {
2504 process($file, 'fh00');
2508 my($filename, $input) = @_;
2509 $input++; # this is a string increment
2510 unless (open($input, $filename)) {
2511 print STDERR "Can't open $filename: $!\n";
2516 while (<$input>) { # note use of indirection
2517 if (/^#include "(.*)"/) {
2518 process($1, $input);
2525 You may also, in the Bourne shell tradition, specify an EXPR beginning
2526 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2527 name of a filehandle (or file descriptor, if numeric) to be
2528 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2529 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2530 mode you specify should match the mode of the original filehandle.
2531 (Duping a filehandle does not take into account any existing contents of
2533 Here is a script that saves, redirects, and restores STDOUT and
2537 open(OLDOUT, ">&STDOUT");
2538 open(OLDERR, ">&STDERR");
2540 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2541 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2543 select(STDERR); $| = 1; # make unbuffered
2544 select(STDOUT); $| = 1; # make unbuffered
2546 print STDOUT "stdout 1\n"; # this works for
2547 print STDERR "stderr 1\n"; # subprocesses too
2552 open(STDOUT, ">&OLDOUT");
2553 open(STDERR, ">&OLDERR");
2555 print STDOUT "stdout 2\n";
2556 print STDERR "stderr 2\n";
2558 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2559 equivalent of C's C<fdopen> of that file descriptor; this is more
2560 parsimonious of file descriptors. For example:
2562 open(FILEHANDLE, "<&=$fd")
2564 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2565 there is an implicit fork done, and the return value of open is the pid
2566 of the child within the parent process, and C<0> within the child
2567 process. (Use C<defined($pid)> to determine whether the open was successful.)
2568 The filehandle behaves normally for the parent, but i/o to that
2569 filehandle is piped from/to the STDOUT/STDIN of the child process.
2570 In the child process the filehandle isn't opened--i/o happens from/to
2571 the new STDOUT or STDIN. Typically this is used like the normal
2572 piped open when you want to exercise more control over just how the
2573 pipe command gets executed, such as when you are running setuid, and
2574 don't want to have to scan shell commands for metacharacters.
2575 The following pairs are more or less equivalent:
2577 open(FOO, "|tr '[a-z]' '[A-Z]'");
2578 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2580 open(FOO, "cat -n '$file'|");
2581 open(FOO, "-|") || exec 'cat', '-n', $file;
2583 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2585 NOTE: On any operation that may do a fork, all files opened for output
2586 are flushed before the fork is attempted. On systems that support a
2587 close-on-exec flag on files, the flag will be set for the newly opened
2588 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2590 Closing any piped filehandle causes the parent process to wait for the
2591 child to finish, and returns the status value in C<$?>.
2593 The filename passed to open will have leading and trailing
2594 whitespace deleted, and the normal redirection characters
2595 honored. This property, known as "magic open",
2596 can often be used to good effect. A user could specify a filename of
2597 F<"rsh cat file |">, or you could change certain filenames as needed:
2599 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2600 open(FH, $filename) or die "Can't open $filename: $!";
2602 However, to open a file with arbitrary weird characters in it, it's
2603 necessary to protect any leading and trailing whitespace:
2605 $file =~ s#^(\s)#./$1#;
2606 open(FOO, "< $file\0");
2608 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2609 should use the C<sysopen> function, which involves no such magic. This is
2610 another way to protect your filenames from interpretation. For example:
2613 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2614 or die "sysopen $path: $!";
2615 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2616 print HANDLE "stuff $$\n");
2618 print "File contains: ", <HANDLE>;
2620 Using the constructor from the C<IO::Handle> package (or one of its
2621 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2622 filehandles that have the scope of whatever variables hold references to
2623 them, and automatically close whenever and however you leave that scope:
2627 sub read_myfile_munged {
2629 my $handle = new IO::File;
2630 open($handle, "myfile") or die "myfile: $!";
2632 or return (); # Automatically closed here.
2633 mung $first or die "mung failed"; # Or here.
2634 return $first, <$handle> if $ALL; # Or here.
2638 See L</seek> for some details about mixing reading and writing.
2640 =item opendir DIRHANDLE,EXPR
2642 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2643 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2644 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2650 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2651 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2652 See L<utf8> for more about Unicode.
2654 =item pack TEMPLATE,LIST
2656 Takes a list of values and packs it into a binary structure,
2657 returning the string containing the structure. The TEMPLATE is a
2658 sequence of characters that give the order and type of values, as
2661 a A string with arbitrary binary data, will be null padded.
2662 A An ascii string, will be space padded.
2663 Z A null terminated (asciz) string, will be null padded.
2665 b A bit string (ascending bit order, like vec()).
2666 B A bit string (descending bit order).
2667 h A hex string (low nybble first).
2668 H A hex string (high nybble first).
2670 c A signed char value.
2671 C An unsigned char value. Only does bytes. See U for Unicode.
2673 s A signed short value.
2674 S An unsigned short value.
2675 (This 'short' is _exactly_ 16 bits, which may differ from
2676 what a local C compiler calls 'short'.)
2678 i A signed integer value.
2679 I An unsigned integer value.
2680 (This 'integer' is _at_least_ 32 bits wide. Its exact
2681 size depends on what a local C compiler calls 'int',
2682 and may even be larger than the 'long' described in
2685 l A signed long value.
2686 L An unsigned long value.
2687 (This 'long' is _exactly_ 32 bits, which may differ from
2688 what a local C compiler calls 'long'.)
2690 n A short in "network" (big-endian) order.
2691 N A long in "network" (big-endian) order.
2692 v A short in "VAX" (little-endian) order.
2693 V A long in "VAX" (little-endian) order.
2694 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2695 _exactly_ 32 bits, respectively.)
2697 q A signed quad (64-bit) value.
2698 Q An unsigned quad value.
2699 (Available only if your system supports 64-bit integer values
2700 _and_ if Perl has been compiled to support those.
2701 Causes a fatal error otherwise.)
2703 f A single-precision float in the native format.
2704 d A double-precision float in the native format.
2706 p A pointer to a null-terminated string.
2707 P A pointer to a structure (fixed-length string).
2709 u A uuencoded string.
2710 U A Unicode character number. Encodes to UTF-8 internally.
2711 Works even if C<use utf8> is not in effect.
2713 w A BER compressed integer. Its bytes represent an unsigned
2714 integer in base 128, most significant digit first, with as
2715 few digits as possible. Bit eight (the high bit) is set
2716 on each byte except the last.
2720 @ Null fill to absolute position.
2722 The following rules apply:
2728 Each letter may optionally be followed by a number giving a repeat
2729 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2730 C<"H">, and C<"P"> the pack function will gobble up that many values from
2731 the LIST. A C<*> for the repeat count means to use however many items are
2736 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2737 string of length count, padding with nulls or spaces as necessary. When
2738 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2739 after the first null, and C<"a"> returns data verbatim.
2743 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2747 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2751 The C<"p"> type packs a pointer to a null-terminated string. You are
2752 responsible for ensuring the string is not a temporary value (which can
2753 potentially get deallocated before you get around to using the packed result).
2754 The C<"P"> type packs a pointer to a structure of the size indicated by the
2755 length. A NULL pointer is created if the corresponding value for C<"p"> or
2760 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2761 immediately followed by a C<"!"> to signify native shorts or longs--as
2762 you can see from above for example a bare C<"l"> does mean exactly 32
2763 bits, the native C<long> (as seen by the local C compiler) may be
2764 larger. This is an issue mainly in 64-bit platforms. You can see
2765 whether using C<"!"> makes any difference by
2767 print length(pack("s")), " ", length(pack("s!")), "\n";
2768 print length(pack("l")), " ", length(pack("l!")), "\n";
2770 C<"i!"> and C<"I!"> also work but only because of completeness;
2771 they are identical to C<"i"> and C<"I">.
2773 The actual sizes (in bytes) of native shorts, ints, and longs on
2774 the platform where Perl was built are also available via L<Config>:
2776 The actual sizes (in bytes) of native shorts, ints, longs, and long
2777 longs on the platform where Perl was built are also available via
2781 print $Config{shortsize}, "\n";
2782 print $Config{intsize}, "\n";
2783 print $Config{longsize}, "\n";
2784 print $Config{longlongsize}, "\n";
2788 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2789 are inherently non-portable between processors and operating systems
2790 because they obey the native byteorder and endianness. For example a
2791 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
2792 (arranged in and handled by the CPU registers) into bytes as
2794 0x12 0x34 0x56 0x78 # little-endian
2795 0x78 0x56 0x34 0x12 # big-endian
2797 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2798 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2799 Power, and Cray are big-endian. MIPS can be either: Digital used it
2800 in little-endian mode; SGI uses it in big-endian mode.
2802 The names `big-endian' and `little-endian' are comic references to
2803 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2804 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2805 the egg-eating habits of the Lilliputians.
2807 Some systems may even have weird byte orders such as
2812 You can see your system's preference with
2814 print join(" ", map { sprintf "%#02x", $_ }
2815 unpack("C*",pack("L",0x12345678))), "\n";
2817 The byteorder on the platform where Perl was built is also available
2821 print $Config{byteorder}, "\n";
2823 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2824 and C<'87654321'> are big-endian.
2826 If you want portable packed integers use the formats C<"n">, C<"N">,
2827 C<"v">, and C<"V">, their byte endianness and size is known.
2831 Real numbers (floats and doubles) are in the native machine format only;
2832 due to the multiplicity of floating formats around, and the lack of a
2833 standard "network" representation, no facility for interchange has been
2834 made. This means that packed floating point data written on one machine
2835 may not be readable on another - even if both use IEEE floating point
2836 arithmetic (as the endian-ness of the memory representation is not part
2839 Note that Perl uses doubles internally for all numeric calculation, and
2840 converting from double into float and thence back to double again will
2841 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2848 $foo = pack("CCCC",65,66,67,68);
2850 $foo = pack("C4",65,66,67,68);
2852 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2853 # same thing with Unicode circled letters
2855 $foo = pack("ccxxcc",65,66,67,68);
2858 $foo = pack("s2",1,2);
2859 # "\1\0\2\0" on little-endian
2860 # "\0\1\0\2" on big-endian
2862 $foo = pack("a4","abcd","x","y","z");
2865 $foo = pack("aaaa","abcd","x","y","z");
2868 $foo = pack("a14","abcdefg");
2869 # "abcdefg\0\0\0\0\0\0\0"
2871 $foo = pack("i9pl", gmtime);
2872 # a real struct tm (on my system anyway)
2874 $utmp_template = "Z8 Z8 Z16 L";
2875 $utmp = pack($utmp_template, @utmp1);
2876 # a struct utmp (BSDish)
2878 @utmp2 = unpack($utmp_template, $utmp);
2879 # "@utmp1" eq "@utmp2"
2882 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2885 The same template may generally also be used in unpack().
2889 =item package NAMESPACE
2891 Declares the compilation unit as being in the given namespace. The scope
2892 of the package declaration is from the declaration itself through the end
2893 of the enclosing block, file, or eval (the same as the C<my> operator).
2894 All further unqualified dynamic identifiers will be in this namespace.
2895 A package statement affects only dynamic variables--including those
2896 you've used C<local> on--but I<not> lexical variables, which are created
2897 with C<my>. Typically it would be the first declaration in a file to
2898 be included by the C<require> or C<use> operator. You can switch into a
2899 package in more than one place; it merely influences which symbol table
2900 is used by the compiler for the rest of that block. You can refer to
2901 variables and filehandles in other packages by prefixing the identifier
2902 with the package name and a double colon: C<$Package::Variable>.
2903 If the package name is null, the C<main> package as assumed. That is,
2904 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2905 still seen in older code).
2907 If NAMESPACE is omitted, then there is no current package, and all
2908 identifiers must be fully qualified or lexicals. This is stricter
2909 than C<use strict>, since it also extends to function names.
2911 See L<perlmod/"Packages"> for more information about packages, modules,
2912 and classes. See L<perlsub> for other scoping issues.
2914 =item pipe READHANDLE,WRITEHANDLE
2916 Opens a pair of connected pipes like the corresponding system call.
2917 Note that if you set up a loop of piped processes, deadlock can occur
2918 unless you are very careful. In addition, note that Perl's pipes use
2919 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2920 after each command, depending on the application.
2922 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2923 for examples of such things.
2925 On systems that support a close-on-exec flag on files, the flag will be set
2926 for the newly opened file descriptors as determined by the value of $^F.
2933 Pops and returns the last value of the array, shortening the array by
2934 one element. Has an effect similar to
2938 If there are no elements in the array, returns the undefined value
2939 (although this may happen at other times as well). If ARRAY is
2940 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
2941 array in subroutines, just like C<shift>.
2947 Returns the offset of where the last C<m//g> search left off for the variable
2948 is in question (C<$_> is used when the variable is not specified). May be
2949 modified to change that offset. Such modification will also influence
2950 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2953 =item print FILEHANDLE LIST
2959 Prints a string or a list of strings. Returns true if successful.
2960 FILEHANDLE may be a scalar variable name, in which case the variable
2961 contains the name of or a reference to the filehandle, thus introducing
2962 one level of indirection. (NOTE: If FILEHANDLE is a variable and
2963 the next token is a term, it may be misinterpreted as an operator
2964 unless you interpose a C<+> or put parentheses around the arguments.)
2965 If FILEHANDLE is omitted, prints by default to standard output (or
2966 to the last selected output channel--see L</select>). If LIST is
2967 also omitted, prints C<$_> to the currently selected output channel.
2968 To set the default output channel to something other than STDOUT
2969 use the select operation. The current value of C<$,> (if any) is
2970 printed between each LIST item. The current value of C<$\> (if
2971 any) is printed after the entire LIST has been printed. Because
2972 print takes a LIST, anything in the LIST is evaluated in list
2973 context, and any subroutine that you call will have one or more of
2974 its expressions evaluated in list context. Also be careful not to
2975 follow the print keyword with a left parenthesis unless you want
2976 the corresponding right parenthesis to terminate the arguments to
2977 the print--interpose a C<+> or put parentheses around all the
2980 Note that if you're storing FILEHANDLES in an array or other expression,
2981 you will have to use a block returning its value instead:
2983 print { $files[$i] } "stuff\n";
2984 print { $OK ? STDOUT : STDERR } "stuff\n";
2986 =item printf FILEHANDLE FORMAT, LIST
2988 =item printf FORMAT, LIST
2990 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2991 (the output record separator) is not appended. The first argument
2992 of the list will be interpreted as the C<printf> format. If C<use locale> is
2993 in effect, the character used for the decimal point in formatted real numbers
2994 is affected by the LC_NUMERIC locale. See L<perllocale>.
2996 Don't fall into the trap of using a C<printf> when a simple
2997 C<print> would do. The C<print> is more efficient and less
3000 =item prototype FUNCTION
3002 Returns the prototype of a function as a string (or C<undef> if the
3003 function has no prototype). FUNCTION is a reference to, or the name of,
3004 the function whose prototype you want to retrieve.
3006 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3007 name for Perl builtin. If the builtin is not I<overridable> (such as
3008 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3009 C<system>) returns C<undef> because the builtin does not really behave
3010 like a Perl function. Otherwise, the string describing the equivalent
3011 prototype is returned.
3013 =item push ARRAY,LIST
3015 Treats ARRAY as a stack, and pushes the values of LIST
3016 onto the end of ARRAY. The length of ARRAY increases by the length of
3017 LIST. Has the same effect as
3020 $ARRAY[++$#ARRAY] = $value;
3023 but is more efficient. Returns the new number of elements in the array.
3035 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3037 =item quotemeta EXPR
3041 Returns the value of EXPR with all non-alphanumeric
3042 characters backslashed. (That is, all characters not matching
3043 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3044 returned string, regardless of any locale settings.)
3045 This is the internal function implementing
3046 the C<\Q> escape in double-quoted strings.
3048 If EXPR is omitted, uses C<$_>.
3054 Returns a random fractional number greater than or equal to C<0> and less
3055 than the value of EXPR. (EXPR should be positive.) If EXPR is
3056 omitted, the value C<1> is used. Automatically calls C<srand> unless
3057 C<srand> has already been called. See also C<srand>.
3059 (Note: If your rand function consistently returns numbers that are too
3060 large or too small, then your version of Perl was probably compiled
3061 with the wrong number of RANDBITS.)
3063 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3065 =item read FILEHANDLE,SCALAR,LENGTH
3067 Attempts to read LENGTH bytes of data into variable SCALAR from the
3068 specified FILEHANDLE. Returns the number of bytes actually read,
3069 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3070 or shrunk to the length actually read. An OFFSET may be specified to
3071 place the read data at some other place than the beginning of the
3072 string. This call is actually implemented in terms of stdio's fread(3)
3073 call. To get a true read(2) system call, see C<sysread>.
3075 =item readdir DIRHANDLE
3077 Returns the next directory entry for a directory opened by C<opendir>.
3078 If used in list context, returns all the rest of the entries in the
3079 directory. If there are no more entries, returns an undefined value in
3080 scalar context or a null list in list context.
3082 If you're planning to filetest the return values out of a C<readdir>, you'd
3083 better prepend the directory in question. Otherwise, because we didn't
3084 C<chdir> there, it would have been testing the wrong file.
3086 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3087 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3092 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3093 context, each call reads and returns the next line, until end-of-file is
3094 reached, whereupon the subsequent call returns undef. In list context,
3095 reads until end-of-file is reached and returns a list of lines. Note that
3096 the notion of "line" used here is however you may have defined it
3097 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3099 When C<$/> is set to C<undef>, when readline() is in scalar
3100 context (i.e. file slurp mode), and when an empty file is read, it
3101 returns C<''> the first time, followed by C<undef> subsequently.
3103 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3104 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3105 operator is discussed in more detail in L<perlop/"I/O Operators">.
3108 $line = readline(*STDIN); # same thing
3114 Returns the value of a symbolic link, if symbolic links are
3115 implemented. If not, gives a fatal error. If there is some system
3116 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3117 omitted, uses C<$_>.
3121 EXPR is executed as a system command.
3122 The collected standard output of the command is returned.
3123 In scalar context, it comes back as a single (potentially
3124 multi-line) string. In list context, returns a list of lines
3125 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3126 This is the internal function implementing the C<qx/EXPR/>
3127 operator, but you can use it directly. The C<qx/EXPR/>
3128 operator is discussed in more detail in L<perlop/"I/O Operators">.
3130 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3132 Receives a message on a socket. Attempts to receive LENGTH bytes of
3133 data into variable SCALAR from the specified SOCKET filehandle.
3134 Actually does a C C<recvfrom>, so that it can return the address of the
3135 sender. Returns the undefined value if there's an error. SCALAR will
3136 be grown or shrunk to the length actually read. Takes the same flags
3137 as the system call of the same name.
3138 See L<perlipc/"UDP: Message Passing"> for examples.
3144 The C<redo> command restarts the loop block without evaluating the
3145 conditional again. The C<continue> block, if any, is not executed. If
3146 the LABEL is omitted, the command refers to the innermost enclosing
3147 loop. This command is normally used by programs that want to lie to
3148 themselves about what was just input:
3150 # a simpleminded Pascal comment stripper
3151 # (warning: assumes no { or } in strings)
3152 LINE: while (<STDIN>) {
3153 while (s|({.*}.*){.*}|$1 |) {}
3158 if (/}/) { # end of comment?
3167 C<redo> cannot be used to retry a block which returns a value such as
3168 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3169 a grep() or map() operation.
3171 See also L</continue> for an illustration of how C<last>, C<next>, and
3178 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3179 is not specified, C<$_> will be used. The value returned depends on the
3180 type of thing the reference is a reference to.
3181 Builtin types include:
3191 If the referenced object has been blessed into a package, then that package
3192 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3194 if (ref($r) eq "HASH") {
3195 print "r is a reference to a hash.\n";
3198 print "r is not a reference at all.\n";
3200 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3201 print "r is a reference to something that isa hash.\n";
3204 See also L<perlref>.
3206 =item rename OLDNAME,NEWNAME
3208 Changes the name of a file; an existing file NEWNAME will be
3209 clobbered. Returns true for success, false otherwise.
3211 Behavior of this function varies wildly depending on your system
3212 implementation. For example, it will usually not work across file system
3213 boundaries, even though the system I<mv> command sometimes compensates
3214 for this. Other restrictions include whether it works on directories,
3215 open files, or pre-existing files. Check L<perlport> and either the
3216 rename(2) manpage or equivalent system documentation for details.
3222 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3223 supplied. If EXPR is numeric, demands that the current version of Perl
3224 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3226 Otherwise, demands that a library file be included if it hasn't already
3227 been included. The file is included via the do-FILE mechanism, which is
3228 essentially just a variety of C<eval>. Has semantics similar to the following
3233 return 1 if $INC{$filename};
3234 my($realfilename,$result);
3236 foreach $prefix (@INC) {
3237 $realfilename = "$prefix/$filename";
3238 if (-f $realfilename) {
3239 $result = do $realfilename;
3243 die "Can't find $filename in \@INC";
3246 die "$filename did not return true value" unless $result;
3247 $INC{$filename} = $realfilename;
3251 Note that the file will not be included twice under the same specified
3252 name. The file must return true as the last statement to indicate
3253 successful execution of any initialization code, so it's customary to
3254 end such a file with C<1;> unless you're sure it'll return true
3255 otherwise. But it's better just to put the C<1;>, in case you add more
3258 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3259 replaces "F<::>" with "F</>" in the filename for you,
3260 to make it easy to load standard modules. This form of loading of
3261 modules does not risk altering your namespace.
3263 In other words, if you try this:
3265 require Foo::Bar; # a splendid bareword
3267 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3268 directories specified in the C<@INC> array.
3270 But if you try this:
3272 $class = 'Foo::Bar';
3273 require $class; # $class is not a bareword
3275 require "Foo::Bar"; # not a bareword because of the ""
3277 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3278 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3280 eval "require $class";
3282 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3288 Generally used in a C<continue> block at the end of a loop to clear
3289 variables and reset C<??> searches so that they work again. The
3290 expression is interpreted as a list of single characters (hyphens
3291 allowed for ranges). All variables and arrays beginning with one of
3292 those letters are reset to their pristine state. If the expression is
3293 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3294 only variables or searches in the current package. Always returns
3297 reset 'X'; # reset all X variables
3298 reset 'a-z'; # reset lower case variables
3299 reset; # just reset ?one-time? searches
3301 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3302 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3303 variables--lexical variables are unaffected, but they clean themselves
3304 up on scope exit anyway, so you'll probably want to use them instead.
3311 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3312 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3313 context, depending on how the return value will be used, and the context
3314 may vary from one execution to the next (see C<wantarray>). If no EXPR
3315 is given, returns an empty list in list context, the undefined value in
3316 scalar context, and (of course) nothing at all in a void context.
3318 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3319 or do FILE will automatically return the value of the last expression
3324 In list context, returns a list value consisting of the elements
3325 of LIST in the opposite order. In scalar context, concatenates the
3326 elements of LIST and returns a string value with all characters
3327 in the opposite order.
3329 print reverse <>; # line tac, last line first
3331 undef $/; # for efficiency of <>
3332 print scalar reverse <>; # character tac, last line tsrif
3334 This operator is also handy for inverting a hash, although there are some
3335 caveats. If a value is duplicated in the original hash, only one of those
3336 can be represented as a key in the inverted hash. Also, this has to
3337 unwind one hash and build a whole new one, which may take some time
3338 on a large hash, such as from a DBM file.
3340 %by_name = reverse %by_address; # Invert the hash
3342 =item rewinddir DIRHANDLE
3344 Sets the current position to the beginning of the directory for the
3345 C<readdir> routine on DIRHANDLE.
3347 =item rindex STR,SUBSTR,POSITION
3349 =item rindex STR,SUBSTR
3351 Works just like index() except that it returns the position of the LAST
3352 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3353 last occurrence at or before that position.
3355 =item rmdir FILENAME
3359 Deletes the directory specified by FILENAME if that directory is empty. If it
3360 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3361 FILENAME is omitted, uses C<$_>.
3365 The substitution operator. See L<perlop>.
3369 Forces EXPR to be interpreted in scalar context and returns the value
3372 @counts = ( scalar @a, scalar @b, scalar @c );
3374 There is no equivalent operator to force an expression to
3375 be interpolated in list context because in practice, this is never
3376 needed. If you really wanted to do so, however, you could use
3377 the construction C<@{[ (some expression) ]}>, but usually a simple
3378 C<(some expression)> suffices.
3380 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3381 parenthesized list, this behaves as a scalar comma expression, evaluating
3382 all but the last element in void context and returning the final element
3383 evaluated in scalar context. This is seldom what you want.
3385 The following single statement:
3387 print uc(scalar(&foo,$bar)),$baz;
3389 is the moral equivalent of these two:
3392 print(uc($bar),$baz);
3394 See L<perlop> for more details on unary operators and the comma operator.
3396 =item seek FILEHANDLE,POSITION,WHENCE
3398 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3399 FILEHANDLE may be an expression whose value gives the name of the
3400 filehandle. The values for WHENCE are C<0> to set the new position to
3401 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3402 set it to EOF plus POSITION (typically negative). For WHENCE you may
3403 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3404 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3406 If you want to position file for C<sysread> or C<syswrite>, don't use
3407 C<seek>--buffering makes its effect on the file's system position
3408 unpredictable and non-portable. Use C<sysseek> instead.
3410 Due to the rules and rigors of ANSI C, on some systems you have to do a
3411 seek whenever you switch between reading and writing. Amongst other
3412 things, this may have the effect of calling stdio's clearerr(3).
3413 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3417 This is also useful for applications emulating C<tail -f>. Once you hit
3418 EOF on your read, and then sleep for a while, you might have to stick in a
3419 seek() to reset things. The C<seek> doesn't change the current position,
3420 but it I<does> clear the end-of-file condition on the handle, so that the
3421 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3423 If that doesn't work (some stdios are particularly cantankerous), then
3424 you may need something more like this:
3427 for ($curpos = tell(FILE); $_ = <FILE>;
3428 $curpos = tell(FILE)) {
3429 # search for some stuff and put it into files
3431 sleep($for_a_while);
3432 seek(FILE, $curpos, 0);
3435 =item seekdir DIRHANDLE,POS
3437 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3438 must be a value returned by C<telldir>. Has the same caveats about
3439 possible directory compaction as the corresponding system library
3442 =item select FILEHANDLE
3446 Returns the currently selected filehandle. Sets the current default
3447 filehandle for output, if FILEHANDLE is supplied. This has two
3448 effects: first, a C<write> or a C<print> without a filehandle will
3449 default to this FILEHANDLE. Second, references to variables related to
3450 output will refer to this output channel. For example, if you have to
3451 set the top of form format for more than one output channel, you might
3459 FILEHANDLE may be an expression whose value gives the name of the
3460 actual filehandle. Thus:
3462 $oldfh = select(STDERR); $| = 1; select($oldfh);
3464 Some programmers may prefer to think of filehandles as objects with
3465 methods, preferring to write the last example as:
3468 STDERR->autoflush(1);
3470 =item select RBITS,WBITS,EBITS,TIMEOUT
3472 This calls the select(2) system call with the bit masks specified, which
3473 can be constructed using C<fileno> and C<vec>, along these lines:
3475 $rin = $win = $ein = '';
3476 vec($rin,fileno(STDIN),1) = 1;
3477 vec($win,fileno(STDOUT),1) = 1;
3480 If you want to select on many filehandles you might wish to write a
3484 my(@fhlist) = split(' ',$_[0]);
3487 vec($bits,fileno($_),1) = 1;
3491 $rin = fhbits('STDIN TTY SOCK');
3495 ($nfound,$timeleft) =
3496 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3498 or to block until something becomes ready just do this
3500 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3502 Most systems do not bother to return anything useful in $timeleft, so
3503 calling select() in scalar context just returns $nfound.
3505 Any of the bit masks can also be undef. The timeout, if specified, is
3506 in seconds, which may be fractional. Note: not all implementations are
3507 capable of returning the$timeleft. If not, they always return
3508 $timeleft equal to the supplied $timeout.
3510 You can effect a sleep of 250 milliseconds this way:
3512 select(undef, undef, undef, 0.25);
3514 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3515 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3516 then only on POSIX systems. You have to use C<sysread> instead.
3518 =item semctl ID,SEMNUM,CMD,ARG
3520 Calls the System V IPC function C<semctl>. You'll probably have to say
3524 first to get the correct constant definitions. If CMD is IPC_STAT or
3525 GETALL, then ARG must be a variable which will hold the returned
3526 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3527 undefined value for error, "C<0 but true>" for zero, or the actual return
3528 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3530 =item semget KEY,NSEMS,FLAGS
3532 Calls the System V IPC function semget. Returns the semaphore id, or
3533 the undefined value if there is an error. See also C<IPC::SysV> and
3534 C<IPC::SysV::Semaphore> documentation.
3536 =item semop KEY,OPSTRING
3538 Calls the System V IPC function semop to perform semaphore operations
3539 such as signaling and waiting. OPSTRING must be a packed array of
3540 semop structures. Each semop structure can be generated with
3541 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3542 operations is implied by the length of OPSTRING. Returns true if
3543 successful, or false if there is an error. As an example, the
3544 following code waits on semaphore $semnum of semaphore id $semid:
3546 $semop = pack("sss", $semnum, -1, 0);
3547 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3549 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3550 and C<IPC::SysV::Semaphore> documentation.
3552 =item send SOCKET,MSG,FLAGS,TO
3554 =item send SOCKET,MSG,FLAGS
3556 Sends a message on a socket. Takes the same flags as the system call
3557 of the same name. On unconnected sockets you must specify a
3558 destination to send TO, in which case it does a C C<sendto>. Returns
3559 the number of characters sent, or the undefined value if there is an
3560 error. The C system call sendmsg(2) is currently unimplemented.
3561 See L<perlipc/"UDP: Message Passing"> for examples.
3563 =item setpgrp PID,PGRP
3565 Sets the current process group for the specified PID, C<0> for the current
3566 process. Will produce a fatal error if used on a machine that doesn't
3567 implement setpgrp(2). If the arguments are omitted, it defaults to
3568 C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
3569 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3571 =item setpriority WHICH,WHO,PRIORITY
3573 Sets the current priority for a process, a process group, or a user.
3574 (See setpriority(2).) Will produce a fatal error if used on a machine
3575 that doesn't implement setpriority(2).
3577 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3579 Sets the socket option requested. Returns undefined if there is an
3580 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3587 Shifts the first value of the array off and returns it, shortening the
3588 array by 1 and moving everything down. If there are no elements in the
3589 array, returns the undefined value. If ARRAY is omitted, shifts the
3590 C<@_> array within the lexical scope of subroutines and formats, and the
3591 C<@ARGV> array at file scopes or within the lexical scopes established by
3592 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3593 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3594 same thing to the left end of an array that C<pop> and C<push> do to the
3597 =item shmctl ID,CMD,ARG
3599 Calls the System V IPC function shmctl. You'll probably have to say
3603 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3604 then ARG must be a variable which will hold the returned C<shmid_ds>
3605 structure. Returns like ioctl: the undefined value for error, "C<0> but
3606 true" for zero, or the actual return value otherwise.
3607 See also C<IPC::SysV> documentation.
3609 =item shmget KEY,SIZE,FLAGS
3611 Calls the System V IPC function shmget. Returns the shared memory
3612 segment id, or the undefined value if there is an error.
3613 See also C<IPC::SysV> documentation.
3615 =item shmread ID,VAR,POS,SIZE
3617 =item shmwrite ID,STRING,POS,SIZE
3619 Reads or writes the System V shared memory segment ID starting at
3620 position POS for size SIZE by attaching to it, copying in/out, and
3621 detaching from it. When reading, VAR must be a variable that will
3622 hold the data read. When writing, if STRING is too long, only SIZE
3623 bytes are used; if STRING is too short, nulls are written to fill out
3624 SIZE bytes. Return true if successful, or false if there is an error.
3625 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3628 =item shutdown SOCKET,HOW
3630 Shuts down a socket connection in the manner indicated by HOW, which
3631 has the same interpretation as in the system call of the same name.
3633 shutdown(SOCKET, 0); # I/we have stopped reading data
3634 shutdown(SOCKET, 1); # I/we have stopped writing data
3635 shutdown(SOCKET, 2); # I/we have stopped using this socket
3637 This is useful with sockets when you want to tell the other
3638 side you're done writing but not done reading, or vice versa.
3639 It's also a more insistent form of close because it also
3640 disables the file descriptor in any forked copies in other
3647 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3648 returns sine of C<$_>.
3650 For the inverse sine operation, you may use the C<POSIX::asin>
3651 function, or use this relation:
3653 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3659 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3660 May be interrupted if the process receives a signal such as C<SIGALRM>.
3661 Returns the number of seconds actually slept. You probably cannot
3662 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3665 On some older systems, it may sleep up to a full second less than what
3666 you requested, depending on how it counts seconds. Most modern systems
3667 always sleep the full amount. They may appear to sleep longer than that,
3668 however, because your process might not be scheduled right away in a
3669 busy multitasking system.
3671 For delays of finer granularity than one second, you may use Perl's
3672 C<syscall> interface to access setitimer(2) if your system supports it,
3673 or else see L</select> above.
3675 See also the POSIX module's C<sigpause> function.
3677 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3679 Opens a socket of the specified kind and attaches it to filehandle
3680 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3681 the system call of the same name. You should C<use Socket> first
3682 to get the proper definitions imported. See the examples in
3683 L<perlipc/"Sockets: Client/Server Communication">.
3685 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3687 Creates an unnamed pair of sockets in the specified domain, of the
3688 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3689 for the system call of the same name. If unimplemented, yields a fatal
3690 error. Returns true if successful.
3692 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
3693 to C<pipe(Rdr, Wtr)> is essentially:
3696 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3697 shutdown(Rdr, 1); # no more writing for reader
3698 shutdown(Wtr, 0); # no more reading for writer
3700 See L<perlipc> for an example of socketpair use.
3702 =item sort SUBNAME LIST
3704 =item sort BLOCK LIST
3708 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3709 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3710 specified, it gives the name of a subroutine that returns an integer
3711 less than, equal to, or greater than C<0>, depending on how the elements
3712 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3713 operators are extremely useful in such routines.) SUBNAME may be a
3714 scalar variable name (unsubscripted), in which case the value provides
3715 the name of (or a reference to) the actual subroutine to use. In place
3716 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3719 In the interests of efficiency the normal calling code for subroutines is
3720 bypassed, with the following effects: the subroutine may not be a
3721 recursive subroutine, and the two elements to be compared are passed into
3722 the subroutine not via C<@_> but as the package global variables $a and
3723 $b (see example below). They are passed by reference, so don't
3724 modify $a and $b. And don't try to declare them as lexicals either.
3726 You also cannot exit out of the sort block or subroutine using any of the
3727 loop control operators described in L<perlsyn> or with C<goto>.
3729 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3730 current collation locale. See L<perllocale>.
3735 @articles = sort @files;
3737 # same thing, but with explicit sort routine
3738 @articles = sort {$a cmp $b} @files;
3740 # now case-insensitively
3741 @articles = sort {uc($a) cmp uc($b)} @files;
3743 # same thing in reversed order
3744 @articles = sort {$b cmp $a} @files;
3746 # sort numerically ascending
3747 @articles = sort {$a <=> $b} @files;
3749 # sort numerically descending
3750 @articles = sort {$b <=> $a} @files;
3752 # this sorts the %age hash by value instead of key
3753 # using an in-line function
3754 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3756 # sort using explicit subroutine name
3758 $age{$a} <=> $age{$b}; # presuming numeric
3760 @sortedclass = sort byage @class;
3762 sub backwards { $b cmp $a }
3763 @harry = qw(dog cat x Cain Abel);
3764 @george = qw(gone chased yz Punished Axed);
3766 # prints AbelCaincatdogx
3767 print sort backwards @harry;
3768 # prints xdogcatCainAbel
3769 print sort @george, 'to', @harry;
3770 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3772 # inefficiently sort by descending numeric compare using
3773 # the first integer after the first = sign, or the
3774 # whole record case-insensitively otherwise
3777 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3782 # same thing, but much more efficiently;
3783 # we'll build auxiliary indices instead
3787 push @nums, /=(\d+)/;
3792 $nums[$b] <=> $nums[$a]
3794 $caps[$a] cmp $caps[$b]
3798 # same thing, but without any temps
3799 @new = map { $_->[0] }
3800 sort { $b->[1] <=> $a->[1]
3803 } map { [$_, /=(\d+)/, uc($_)] } @old;
3805 If you're using strict, you I<must not> declare $a
3806 and $b as lexicals. They are package globals. That means
3807 if you're in the C<main> package, it's
3809 @articles = sort {$main::b <=> $main::a} @files;
3813 @articles = sort {$::b <=> $::a} @files;
3815 but if you're in the C<FooPack> package, it's
3817 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3819 The comparison function is required to behave. If it returns
3820 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3821 sometimes saying the opposite, for example) the results are not
3824 =item splice ARRAY,OFFSET,LENGTH,LIST
3826 =item splice ARRAY,OFFSET,LENGTH
3828 =item splice ARRAY,OFFSET
3830 Removes the elements designated by OFFSET and LENGTH from an array, and
3831 replaces them with the elements of LIST, if any. In list context,
3832 returns the elements removed from the array. In scalar context,
3833 returns the last element removed, or C<undef> if no elements are
3834 removed. The array grows or shrinks as necessary.
3835 If OFFSET is negative then it starts that far from the end of the array.
3836 If LENGTH is omitted, removes everything from OFFSET onward.
3837 If LENGTH is negative, leave that many elements off the end of the array.
3838 The following equivalences hold (assuming C<$[ == 0>):
3840 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3841 pop(@a) splice(@a,-1)
3842 shift(@a) splice(@a,0,1)
3843 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3844 $a[$x] = $y splice(@a,$x,1,$y)
3846 Example, assuming array lengths are passed before arrays:
3848 sub aeq { # compare two list values
3849 my(@a) = splice(@_,0,shift);
3850 my(@b) = splice(@_,0,shift);
3851 return 0 unless @a == @b; # same len?
3853 return 0 if pop(@a) ne pop(@b);
3857 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3859 =item split /PATTERN/,EXPR,LIMIT
3861 =item split /PATTERN/,EXPR
3863 =item split /PATTERN/
3867 Splits a string into a list of strings and returns that list. By default,
3868 empty leading fields are preserved, and empty trailing ones are deleted.
3870 If not in list context, returns the number of fields found and splits into
3871 the C<@_> array. (In list context, you can force the split into C<@_> by
3872 using C<??> as the pattern delimiters, but it still returns the list
3873 value.) The use of implicit split to C<@_> is deprecated, however, because
3874 it clobbers your subroutine arguments.
3876 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3877 splits on whitespace (after skipping any leading whitespace). Anything
3878 matching PATTERN is taken to be a delimiter separating the fields. (Note
3879 that the delimiter may be longer than one character.)
3881 If LIMIT is specified and positive, splits into no more than that
3882 many fields (though it may split into fewer). If LIMIT is unspecified
3883 or zero, trailing null fields are stripped (which potential users
3884 of C<pop> would do well to remember). If LIMIT is negative, it is
3885 treated as if an arbitrarily large LIMIT had been specified.
3887 A pattern matching the null string (not to be confused with
3888 a null pattern C<//>, which is just one member of the set of patterns
3889 matching a null string) will split the value of EXPR into separate
3890 characters at each point it matches that way. For example:
3892 print join(':', split(/ */, 'hi there'));
3894 produces the output 'h:i:t:h:e:r:e'.
3896 The LIMIT parameter can be used to split a line partially
3898 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3900 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3901 one larger than the number of variables in the list, to avoid
3902 unnecessary work. For the list above LIMIT would have been 4 by
3903 default. In time critical applications it behooves you not to split
3904 into more fields than you really need.
3906 If the PATTERN contains parentheses, additional list elements are
3907 created from each matching substring in the delimiter.
3909 split(/([,-])/, "1-10,20", 3);
3911 produces the list value
3913 (1, '-', 10, ',', 20)
3915 If you had the entire header of a normal Unix email message in $header,
3916 you could split it up into fields and their values this way:
3918 $header =~ s/\n\s+/ /g; # fix continuation lines
3919 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3921 The pattern C</PATTERN/> may be replaced with an expression to specify
3922 patterns that vary at runtime. (To do runtime compilation only once,
3923 use C</$variable/o>.)
3925 As a special case, specifying a PATTERN of space (C<' '>) will split on
3926 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
3927 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3928 will give you as many null initial fields as there are leading spaces.
3929 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
3930 whitespace produces a null first field. A C<split> with no arguments
3931 really does a C<split(' ', $_)> internally.
3935 open(PASSWD, '/etc/passwd');
3937 ($login, $passwd, $uid, $gid,
3938 $gcos, $home, $shell) = split(/:/);
3942 (Note that $shell above will still have a newline on it. See L</chop>,
3943 L</chomp>, and L</join>.)
3945 =item sprintf FORMAT, LIST
3947 Returns a string formatted by the usual C<printf> conventions of the
3948 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
3949 on your system for an explanation of the general principles.
3951 Perl does its own C<sprintf> formatting--it emulates the C
3952 function C<sprintf>, but it doesn't use it (except for floating-point
3953 numbers, and even then only the standard modifiers are allowed). As a
3954 result, any non-standard extensions in your local C<sprintf> are not
3955 available from Perl.
3957 Perl's C<sprintf> permits the following universally-known conversions:
3960 %c a character with the given number
3962 %d a signed integer, in decimal
3963 %u an unsigned integer, in decimal
3964 %o an unsigned integer, in octal
3965 %x an unsigned integer, in hexadecimal
3966 %e a floating-point number, in scientific notation
3967 %f a floating-point number, in fixed decimal notation
3968 %g a floating-point number, in %e or %f notation
3970 In addition, Perl permits the following widely-supported conversions:
3972 %X like %x, but using upper-case letters
3973 %E like %e, but using an upper-case "E"
3974 %G like %g, but with an upper-case "E" (if applicable)
3975 %b an unsigned integer, in binary
3976 %p a pointer (outputs the Perl value's address in hexadecimal)
3977 %n special: *stores* the number of characters output so far
3978 into the next variable in the parameter list
3980 Finally, for backward (and we do mean "backward") compatibility, Perl
3981 permits these unnecessary but widely-supported conversions:
3984 %D a synonym for %ld
3985 %U a synonym for %lu
3986 %O a synonym for %lo
3989 Perl permits the following universally-known flags between the C<%>
3990 and the conversion letter:
3992 space prefix positive number with a space
3993 + prefix positive number with a plus sign
3994 - left-justify within the field
3995 0 use zeros, not spaces, to right-justify
3996 # prefix non-zero octal with "0", non-zero hex with "0x"
3997 number minimum field width
3998 .number "precision": digits after decimal point for
3999 floating-point, max length for string, minimum length
4001 l interpret integer as C type "long" or "unsigned long"
4002 h interpret integer as C type "short" or "unsigned short"
4004 There is also one Perl-specific flag:
4006 V interpret integer as Perl's standard integer type
4008 Where a number would appear in the flags, an asterisk (C<*>) may be
4009 used instead, in which case Perl uses the next item in the parameter
4010 list as the given number (that is, as the field width or precision).
4011 If a field width obtained through C<*> is negative, it has the same
4012 effect as the C<-> flag: left-justification.
4014 If C<use locale> is in effect, the character used for the decimal
4015 point in formatted real numbers is affected by the LC_NUMERIC locale.
4022 Return the square root of EXPR. If EXPR is omitted, returns square
4023 root of C<$_>. Only works on non-negative operands, unless you've
4024 loaded the standard Math::Complex module.
4027 print sqrt(-2); # prints 1.4142135623731i
4033 Sets the random number seed for the C<rand> operator. If EXPR is
4034 omitted, uses a semi-random value supplied by the kernel (if it supports
4035 the F</dev/urandom> device) or based on the current time and process
4036 ID, among other things. In versions of Perl prior to 5.004 the default
4037 seed was just the current C<time>. This isn't a particularly good seed,
4038 so many old programs supply their own seed value (often C<time ^ $$> or
4039 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4041 In fact, it's usually not necessary to call C<srand> at all, because if
4042 it is not called explicitly, it is called implicitly at the first use of
4043 the C<rand> operator. However, this was not the case in version of Perl
4044 before 5.004, so if your script will run under older Perl versions, it
4045 should call C<srand>.
4047 Note that you need something much more random than the default seed for
4048 cryptographic purposes. Checksumming the compressed output of one or more
4049 rapidly changing operating system status programs is the usual method. For
4052 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4054 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4057 Do I<not> call C<srand> multiple times in your program unless you know
4058 exactly what you're doing and why you're doing it. The point of the
4059 function is to "seed" the C<rand> function so that C<rand> can produce
4060 a different sequence each time you run your program. Just do it once at the
4061 top of your program, or you I<won't> get random numbers out of C<rand>!
4063 Frequently called programs (like CGI scripts) that simply use
4067 for a seed can fall prey to the mathematical property that
4071 one-third of the time. So don't do that.
4073 =item stat FILEHANDLE
4079 Returns a 13-element list giving the status info for a file, either
4080 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4081 it stats C<$_>. Returns a null list if the stat fails. Typically used
4084 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4085 $atime,$mtime,$ctime,$blksize,$blocks)
4088 Not all fields are supported on all filesystem types. Here are the
4089 meaning of the fields:
4091 0 dev device number of filesystem
4093 2 mode file mode (type and permissions)
4094 3 nlink number of (hard) links to the file
4095 4 uid numeric user ID of file's owner
4096 5 gid numeric group ID of file's owner
4097 6 rdev the device identifier (special files only)
4098 7 size total size of file, in bytes
4099 8 atime last access time since the epoch
4100 9 mtime last modify time since the epoch
4101 10 ctime inode change time (NOT creation time!) since the epoch
4102 11 blksize preferred block size for file system I/O
4103 12 blocks actual number of blocks allocated
4105 (The epoch was at 00:00 January 1, 1970 GMT.)
4107 If stat is passed the special filehandle consisting of an underline, no
4108 stat is done, but the current contents of the stat structure from the
4109 last stat or filetest are returned. Example:
4111 if (-x $file && (($d) = stat(_)) && $d < 0) {
4112 print "$file is executable NFS file\n";
4115 (This works on machines only for which the device number is negative under NFS.)
4117 Because the mode contains both the file type and its permissions, you
4118 should mask off the file type portion and (s)printf using a C<"%o">
4119 if you want to see the real permissions.
4121 $mode = (stat($filename))[2];
4122 printf "Permissions are %04o\n", $mode & 07777;
4124 In scalar context, C<stat> returns a boolean value indicating success
4125 or failure, and, if successful, sets the information associated with
4126 the special filehandle C<_>.
4128 The File::stat module provides a convenient, by-name access mechanism:
4131 $sb = stat($filename);
4132 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4133 $filename, $sb->size, $sb->mode & 07777,
4134 scalar localtime $sb->mtime;
4140 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4141 doing many pattern matches on the string before it is next modified.
4142 This may or may not save time, depending on the nature and number of
4143 patterns you are searching on, and on the distribution of character
4144 frequencies in the string to be searched--you probably want to compare
4145 run times with and without it to see which runs faster. Those loops
4146 which scan for many short constant strings (including the constant
4147 parts of more complex patterns) will benefit most. You may have only
4148 one C<study> active at a time--if you study a different scalar the first
4149 is "unstudied". (The way C<study> works is this: a linked list of every
4150 character in the string to be searched is made, so we know, for
4151 example, where all the C<'k'> characters are. From each search string,
4152 the rarest character is selected, based on some static frequency tables
4153 constructed from some C programs and English text. Only those places
4154 that contain this "rarest" character are examined.)
4156 For example, here is a loop that inserts index producing entries
4157 before any line containing a certain pattern:
4161 print ".IX foo\n" if /\bfoo\b/;
4162 print ".IX bar\n" if /\bbar\b/;
4163 print ".IX blurfl\n" if /\bblurfl\b/;
4168 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4169 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4170 a big win except in pathological cases. The only question is whether
4171 it saves you more time than it took to build the linked list in the
4174 Note that if you have to look for strings that you don't know till
4175 runtime, you can build an entire loop as a string and C<eval> that to
4176 avoid recompiling all your patterns all the time. Together with
4177 undefining C<$/> to input entire files as one record, this can be very
4178 fast, often faster than specialized programs like fgrep(1). The following
4179 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4180 out the names of those files that contain a match:
4182 $search = 'while (<>) { study;';
4183 foreach $word (@words) {
4184 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4189 eval $search; # this screams
4190 $/ = "\n"; # put back to normal input delimiter
4191 foreach $file (sort keys(%seen)) {
4199 =item sub NAME BLOCK
4201 This is subroutine definition, not a real function I<per se>. With just a
4202 NAME (and possibly prototypes), it's just a forward declaration. Without
4203 a NAME, it's an anonymous function declaration, and does actually return a
4204 value: the CODE ref of the closure you just created. See L<perlsub> and
4205 L<perlref> for details.
4207 =item substr EXPR,OFFSET,LEN,REPLACEMENT
4209 =item substr EXPR,OFFSET,LEN
4211 =item substr EXPR,OFFSET
4213 Extracts a substring out of EXPR and returns it. First character is at
4214 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4215 If OFFSET is negative (or more precisely, less than C<$[>), starts
4216 that far from the end of the string. If LEN is omitted, returns
4217 everything to the end of the string. If LEN is negative, leaves that
4218 many characters off the end of the string.
4220 If you specify a substring that is partly outside the string, the part
4221 within the string is returned. If the substring is totally outside
4222 the string a warning is produced.
4224 You can use the substr() function as an lvalue, in which case EXPR
4225 must itself be an lvalue. If you assign something shorter than LEN,
4226 the string will shrink, and if you assign something longer than LEN,
4227 the string will grow to accommodate it. To keep the string the same
4228 length you may need to pad or chop your value using C<sprintf>.
4230 An alternative to using substr() as an lvalue is to specify the
4231 replacement string as the 4th argument. This allows you to replace
4232 parts of the EXPR and return what was there before in one operation,
4233 just as you can with splice().
4235 =item symlink OLDFILE,NEWFILE
4237 Creates a new filename symbolically linked to the old filename.
4238 Returns C<1> for success, C<0> otherwise. On systems that don't support
4239 symbolic links, produces a fatal error at run time. To check for that,
4242 $symlink_exists = eval { symlink("",""); 1 };
4246 Calls the system call specified as the first element of the list,
4247 passing the remaining elements as arguments to the system call. If
4248 unimplemented, produces a fatal error. The arguments are interpreted
4249 as follows: if a given argument is numeric, the argument is passed as
4250 an int. If not, the pointer to the string value is passed. You are
4251 responsible to make sure a string is pre-extended long enough to
4252 receive any result that might be written into a string. You can't use a
4253 string literal (or other read-only string) as an argument to C<syscall>
4254 because Perl has to assume that any string pointer might be written
4256 integer arguments are not literals and have never been interpreted in a
4257 numeric context, you may need to add C<0> to them to force them to look
4258 like numbers. This emulates the C<syswrite> function (or vice versa):
4260 require 'syscall.ph'; # may need to run h2ph
4262 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4264 Note that Perl supports passing of up to only 14 arguments to your system call,
4265 which in practice should usually suffice.
4267 Syscall returns whatever value returned by the system call it calls.
4268 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4269 Note that some system calls can legitimately return C<-1>. The proper
4270 way to handle such calls is to assign C<$!=0;> before the call and
4271 check the value of C<$!> if syscall returns C<-1>.
4273 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4274 number of the read end of the pipe it creates. There is no way
4275 to retrieve the file number of the other end. You can avoid this
4276 problem by using C<pipe> instead.
4278 =item sysopen FILEHANDLE,FILENAME,MODE
4280 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4282 Opens the file whose filename is given by FILENAME, and associates it
4283 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4284 the name of the real filehandle wanted. This function calls the
4285 underlying operating system's C<open> function with the parameters
4286 FILENAME, MODE, PERMS.
4288 The possible values and flag bits of the MODE parameter are
4289 system-dependent; they are available via the standard module C<Fcntl>.
4290 For historical reasons, some values work on almost every system
4291 supported by perl: zero means read-only, one means write-only, and two
4292 means read/write. We know that these values do I<not> work under
4293 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4294 use them in new code.
4296 If the file named by FILENAME does not exist and the C<open> call creates
4297 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4298 PERMS specifies the permissions of the newly created file. If you omit
4299 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4300 These permission values need to be in octal, and are modified by your
4301 process's current C<umask>.
4303 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4304 that takes away the user's option to have a more permissive umask.
4305 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4308 See L<perlopentut> for a kinder, gentler explanation of opening files.
4310 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4312 =item sysread FILEHANDLE,SCALAR,LENGTH
4314 Attempts to read LENGTH bytes of data into variable SCALAR from the
4315 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4316 so mixing this with other kinds of reads, C<print>, C<write>,
4317 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4318 usually buffers data. Returns the number of bytes actually read, C<0>
4319 at end of file, or undef if there was an error. SCALAR will be grown or
4320 shrunk so that the last byte actually read is the last byte of the
4321 scalar after the read.
4323 An OFFSET may be specified to place the read data at some place in the
4324 string other than the beginning. A negative OFFSET specifies
4325 placement at that many bytes counting backwards from the end of the
4326 string. A positive OFFSET greater than the length of SCALAR results
4327 in the string being padded to the required size with C<"\0"> bytes before
4328 the result of the read is appended.
4330 There is no syseof() function, which is ok, since eof() doesn't work
4331 very well on device files (like ttys) anyway. Use sysread() and check
4332 for a return value for 0 to decide whether you're done.
4334 =item sysseek FILEHANDLE,POSITION,WHENCE
4336 Sets FILEHANDLE's system position using the system call lseek(2). It
4337 bypasses stdio, so mixing this with reads (other than C<sysread>),
4338 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause
4339 confusion. FILEHANDLE may be an expression whose value gives the name
4340 of the filehandle. The values for WHENCE are C<0> to set the new
4341 position to POSITION, C<1> to set the it to the current position plus
4342 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4343 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4344 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4346 Returns the new position, or the undefined value on failure. A position
4347 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4348 true on success and false on failure, yet you can still easily determine
4353 =item system PROGRAM LIST
4355 Does exactly the same thing as C<exec LIST>, except that a fork is
4356 done first, and the parent process waits for the child process to
4357 complete. Note that argument processing varies depending on the
4358 number of arguments. If there is more than one argument in LIST,
4359 or if LIST is an array with more than one value, starts the program
4360 given by the first element of the list with arguments given by the
4361 rest of the list. If there is only one scalar argument, the argument
4362 is checked for shell metacharacters, and if there are any, the
4363 entire argument is passed to the system's command shell for parsing
4364 (this is C</bin/sh -c> on Unix platforms, but varies on other
4365 platforms). If there are no shell metacharacters in the argument,
4366 it is split into words and passed directly to C<execvp>, which is
4369 All files opened for output are flushed before attempting the exec().
4371 The return value is the exit status of the program as
4372 returned by the C<wait> call. To get the actual exit value divide by
4373 256. See also L</exec>. This is I<not> what you want to use to capture
4374 the output from a command, for that you should use merely backticks or
4375 C<qx//>, as described in L<perlop/"`STRING`">.
4377 Like C<exec>, C<system> allows you to lie to a program about its name if
4378 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4380 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4381 program they're running doesn't actually interrupt your program.
4383 @args = ("command", "arg1", "arg2");
4385 or die "system @args failed: $?"
4387 You can check all the failure possibilities by inspecting
4390 $exit_value = $? >> 8;
4391 $signal_num = $? & 127;
4392 $dumped_core = $? & 128;
4394 When the arguments get executed via the system shell, results
4395 and return codes will be subject to its quirks and capabilities.
4396 See L<perlop/"`STRING`"> and L</exec> for details.
4398 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4400 =item syswrite FILEHANDLE,SCALAR,LENGTH
4402 =item syswrite FILEHANDLE,SCALAR
4404 Attempts to write LENGTH bytes of data from variable SCALAR to the
4405 specified FILEHANDLE, using the system call write(2). If LENGTH
4406 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4407 this with reads (other than C<sysread())>, C<print>, C<write>,
4408 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4409 usually buffers data. Returns the number of bytes actually written,
4410 or C<undef> if there was an error. If the LENGTH is greater than
4411 the available data in the SCALAR after the OFFSET, only as much
4412 data as is available will be written.
4414 An OFFSET may be specified to write the data from some part of the
4415 string other than the beginning. A negative OFFSET specifies writing
4416 that many bytes counting backwards from the end of the string. In the
4417 case the SCALAR is empty you can use OFFSET but only zero offset.
4419 =item tell FILEHANDLE
4423 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4424 expression whose value gives the name of the actual filehandle. If
4425 FILEHANDLE is omitted, assumes the file last read.
4427 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4429 =item telldir DIRHANDLE
4431 Returns the current position of the C<readdir> routines on DIRHANDLE.
4432 Value may be given to C<seekdir> to access a particular location in a
4433 directory. Has the same caveats about possible directory compaction as
4434 the corresponding system library routine.
4436 =item tie VARIABLE,CLASSNAME,LIST
4438 This function binds a variable to a package class that will provide the
4439 implementation for the variable. VARIABLE is the name of the variable
4440 to be enchanted. CLASSNAME is the name of a class implementing objects
4441 of correct type. Any additional arguments are passed to the C<new>
4442 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4443 or C<TIEHASH>). Typically these are arguments such as might be passed
4444 to the C<dbm_open()> function of C. The object returned by the C<new>
4445 method is also returned by the C<tie> function, which would be useful
4446 if you want to access other methods in CLASSNAME.
4448 Note that functions such as C<keys> and C<values> may return huge lists
4449 when used on large objects, like DBM files. You may prefer to use the
4450 C<each> function to iterate over such. Example:
4452 # print out history file offsets
4454 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4455 while (($key,$val) = each %HIST) {
4456 print $key, ' = ', unpack('L',$val), "\n";
4460 A class implementing a hash should have the following methods:
4462 TIEHASH classname, LIST
4464 STORE this, key, value
4469 NEXTKEY this, lastkey
4472 A class implementing an ordinary array should have the following methods:
4474 TIEARRAY classname, LIST
4476 STORE this, key, value
4478 STORESIZE this, count
4484 SPLICE this, offset, length, LIST
4488 A class implementing a file handle should have the following methods:
4490 TIEHANDLE classname, LIST
4491 READ this, scalar, length, offset
4494 WRITE this, scalar, length, offset
4496 PRINTF this, format, LIST
4500 A class implementing a scalar should have the following methods:
4502 TIESCALAR classname, LIST
4507 Not all methods indicated above need be implemented. See L<perltie>,
4508 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4510 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4511 for you--you need to do that explicitly yourself. See L<DB_File>
4512 or the F<Config> module for interesting C<tie> implementations.
4514 For further details see L<perltie>, L<"tied VARIABLE">.
4518 Returns a reference to the object underlying VARIABLE (the same value
4519 that was originally returned by the C<tie> call that bound the variable
4520 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4525 Returns the number of non-leap seconds since whatever time the system
4526 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4527 and 00:00:00 UTC, January 1, 1970 for most other systems).
4528 Suitable for feeding to C<gmtime> and C<localtime>.
4532 Returns a four-element list giving the user and system times, in
4533 seconds, for this process and the children of this process.
4535 ($user,$system,$cuser,$csystem) = times;
4539 The transliteration operator. Same as C<y///>. See L<perlop>.
4541 =item truncate FILEHANDLE,LENGTH
4543 =item truncate EXPR,LENGTH
4545 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4546 specified length. Produces a fatal error if truncate isn't implemented
4547 on your system. Returns true if successful, the undefined value
4554 Returns an uppercased version of EXPR. This is the internal function
4555 implementing the C<\U> escape in double-quoted strings.
4556 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4557 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4558 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4560 If EXPR is omitted, uses C<$_>.
4566 Returns the value of EXPR with the first character
4567 in uppercase (titlecase in Unicode). This is
4568 the internal function implementing the C<\u> escape in double-quoted strings.
4569 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4572 If EXPR is omitted, uses C<$_>.
4578 Sets the umask for the process to EXPR and returns the previous value.
4579 If EXPR is omitted, merely returns the current umask.
4581 The Unix permission C<rwxr-x---> is represented as three sets of three
4582 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4583 and isn't one of the digits). The C<umask> value is such a number
4584 representing disabled permissions bits. The permission (or "mode")
4585 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4586 even if you tell C<sysopen> to create a file with permissions C<0777>,
4587 if your umask is C<0022> then the file will actually be created with
4588 permissions C<0755>. If your C<umask> were C<0027> (group can't
4589 write; others can't read, write, or execute), then passing
4590 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4593 Here's some advice: supply a creation mode of C<0666> for regular
4594 files (in C<sysopen>) and one of C<0777> for directories (in
4595 C<mkdir>) and executable files. This gives users the freedom of
4596 choice: if they want protected files, they might choose process umasks
4597 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4598 Programs should rarely if ever make policy decisions better left to
4599 the user. The exception to this is when writing files that should be
4600 kept private: mail files, web browser cookies, I<.rhosts> files, and
4603 If umask(2) is not implemented on your system and you are trying to
4604 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4605 fatal error at run time. If umask(2) is not implemented and you are
4606 not trying to restrict access for yourself, returns C<undef>.
4608 Remember that a umask is a number, usually given in octal; it is I<not> a
4609 string of octal digits. See also L</oct>, if all you have is a string.
4615 Undefines the value of EXPR, which must be an lvalue. Use only on a
4616 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
4617 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
4618 will probably not do what you expect on most predefined variables or
4619 DBM list values, so don't do that; see L<delete>.) Always returns the
4620 undefined value. You can omit the EXPR, in which case nothing is
4621 undefined, but you still get an undefined value that you could, for
4622 instance, return from a subroutine, assign to a variable or pass as a
4623 parameter. Examples:
4626 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4630 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4631 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4632 select undef, undef, undef, 0.25;
4633 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4635 Note that this is a unary operator, not a list operator.
4641 Deletes a list of files. Returns the number of files successfully
4644 $cnt = unlink 'a', 'b', 'c';
4648 Note: C<unlink> will not delete directories unless you are superuser and
4649 the B<-U> flag is supplied to Perl. Even if these conditions are
4650 met, be warned that unlinking a directory can inflict damage on your
4651 filesystem. Use C<rmdir> instead.
4653 If LIST is omitted, uses C<$_>.
4655 =item unpack TEMPLATE,EXPR
4657 C<unpack> does the reverse of C<pack>: it takes a string
4658 representing a structure and expands it out into a list of values.
4659 (In scalar context, it returns merely the first value produced.)
4660 The TEMPLATE has the same format as in the C<pack> function.
4661 Here's a subroutine that does substring:
4664 my($what,$where,$howmuch) = @_;
4665 unpack("x$where a$howmuch", $what);
4670 sub ordinal { unpack("c",$_[0]); } # same as ord()
4672 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4673 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4674 themselves. Default is a 16-bit checksum. For example, the following
4675 computes the same number as the System V sum program:
4679 unpack("%32C*",<>) % 65535;
4682 The following efficiently counts the number of set bits in a bit vector:
4684 $setbits = unpack("%32b*", $selectmask);
4686 The C<"p"> and C<"P"> formats should be used with care. Since Perl
4687 has no way of checking whether the value passed to C<unpack()>
4688 corresponds to a valid memory location, passing a pointer value that's
4689 not known to be valid is likely to have disastrous consequences.
4691 See L</pack> for more examples.
4693 =item untie VARIABLE
4695 Breaks the binding between a variable and a package. (See C<tie>.)
4697 =item unshift ARRAY,LIST
4699 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4700 depending on how you look at it. Prepends list to the front of the
4701 array, and returns the new number of elements in the array.
4703 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4705 Note the LIST is prepended whole, not one element at a time, so the
4706 prepended elements stay in the same order. Use C<reverse> to do the
4709 =item use Module LIST
4713 =item use Module VERSION LIST
4717 Imports some semantics into the current package from the named module,
4718 generally by aliasing certain subroutine or variable names into your
4719 package. It is exactly equivalent to
4721 BEGIN { require Module; import Module LIST; }
4723 except that Module I<must> be a bareword.
4725 If the first argument to C<use> is a number, it is treated as a version
4726 number instead of a module name. If the version of the Perl interpreter
4727 is less than VERSION, then an error message is printed and Perl exits
4728 immediately. This is often useful if you need to check the current
4729 Perl version before C<use>ing library modules that have changed in
4730 incompatible ways from older versions of Perl. (We try not to do
4731 this more than we have to.)
4733 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
4734 C<require> makes sure the module is loaded into memory if it hasn't been
4735 yet. The C<import> is not a builtin--it's just an ordinary static method
4736 call into the C<Module> package to tell the module to import the list of
4737 features back into the current package. The module can implement its
4738 C<import> method any way it likes, though most modules just choose to
4739 derive their C<import> method via inheritance from the C<Exporter> class that
4740 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
4741 method can be found then the error is currently silently ignored. This
4742 may change to a fatal error in a future version.
4744 If you don't want your namespace altered, explicitly supply an empty list:
4748 That is exactly equivalent to
4750 BEGIN { require Module }
4752 If the VERSION argument is present between Module and LIST, then the
4753 C<use> will call the VERSION method in class Module with the given
4754 version as an argument. The default VERSION method, inherited from
4755 the Universal class, croaks if the given version is larger than the
4756 value of the variable C<$Module::VERSION>. (Note that there is not a
4757 comma after VERSION!)
4759 Because this is a wide-open interface, pragmas (compiler directives)
4760 are also implemented this way. Currently implemented pragmas are:
4764 use sigtrap qw(SEGV BUS);
4765 use strict qw(subs vars refs);
4766 use subs qw(afunc blurfl);
4768 Some of these pseudo-modules import semantics into the current
4769 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4770 which import symbols into the current package (which are effective
4771 through the end of the file).
4773 There's a corresponding C<no> command that unimports meanings imported
4774 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
4779 If no C<unimport> method can be found the call fails with a fatal error.
4781 See L<perlmod> for a list of standard modules and pragmas.
4785 Changes the access and modification times on each file of a list of
4786 files. The first two elements of the list must be the NUMERICAL access
4787 and modification times, in that order. Returns the number of files
4788 successfully changed. The inode modification time of each file is set
4789 to the current time. This code has the same effect as the C<touch>
4790 command if the files already exist:
4794 utime $now, $now, @ARGV;
4798 Returns a list consisting of all the values of the named hash. (In a
4799 scalar context, returns the number of values.) The values are
4800 returned in an apparently random order. The actual random order is
4801 subject to change in future versions of perl, but it is guaranteed to
4802 be the same order as either the C<keys> or C<each> function would
4803 produce on the same (unmodified) hash.
4805 Note that you cannot modify the values of a hash this way, because the
4806 returned list is just a copy. You need to use a hash slice for that,
4807 since it's lvaluable in a way that values() is not.
4809 for (values %hash) { s/foo/bar/g } # FAILS!
4810 for (@hash{keys %hash}) { s/foo/bar/g } # ok
4812 As a side effect, calling values() resets the HASH's internal iterator.
4813 See also C<keys>, C<each>, and C<sort>.
4815 =item vec EXPR,OFFSET,BITS
4817 Treats the string in EXPR as a vector of unsigned integers, and
4818 returns the value of the bit field specified by OFFSET. BITS specifies
4819 the number of bits that are reserved for each entry in the bit
4820 vector. This must be a power of two from 1 to 32. C<vec> may also be
4821 assigned to, in which case parentheses are needed to give the expression
4822 the correct precedence as in
4824 vec($image, $max_x * $x + $y, 8) = 3;
4826 Vectors created with C<vec> can also be manipulated with the logical
4827 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4828 desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
4830 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4831 The comments show the string after each step. Note that this code works
4832 in the same way on big-endian or little-endian machines.
4835 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4836 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4837 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4838 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4839 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4840 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4841 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4843 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4844 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4845 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4848 To transform a bit vector into a string or list of 0's and 1's, use these:
4850 $bits = unpack("b*", $vector);
4851 @bits = split(//, unpack("b*", $vector));
4853 If you know the exact length in bits, it can be used in place of the C<*>.
4857 Behaves like the wait(2) system call on your system: it waits for a child
4858 process to terminate and returns the pid of the deceased process, or
4859 C<-1> if there are no child processes. The status is returned in C<$?>.
4860 Note that a return value of C<-1> could mean that child processes are
4861 being automatically reaped, as described in L<perlipc>.
4863 =item waitpid PID,FLAGS
4865 Waits for a particular child process to terminate and returns the pid of
4866 the deceased process, or C<-1> if there is no such child process. On some
4867 systems, a value of 0 indicates that there are processes still running.
4868 The status is returned in C<$?>. If you say
4870 use POSIX ":sys_wait_h";
4873 $kid = waitpid(-1,&WNOHANG);
4876 then you can do a non-blocking wait for all pending zombie processes.
4877 Non-blocking wait is available on machines supporting either the
4878 waitpid(2) or wait4(2) system calls. However, waiting for a particular
4879 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
4880 system call by remembering the status values of processes that have
4881 exited but have not been harvested by the Perl script yet.)
4883 Note that on some systems, a return value of C<-1> could mean that child
4884 processes are being automatically reaped. See L<perlipc> for details,
4885 and for other examples.
4889 Returns true if the context of the currently executing subroutine is
4890 looking for a list value. Returns false if the context is looking
4891 for a scalar. Returns the undefined value if the context is looking
4892 for no value (void context).
4894 return unless defined wantarray; # don't bother doing more
4895 my @a = complex_calculation();
4896 return wantarray ? @a : "@a";
4898 This function should have been named wantlist() instead.
4902 Produces a message on STDERR just like C<die>, but doesn't exit or throw
4905 If LIST is empty and C<$@> already contains a value (typically from a
4906 previous eval) that value is used after appending C<"\t...caught">
4907 to C<$@>. This is useful for staying almost, but not entirely similar to
4910 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4912 No message is printed if there is a C<$SIG{__WARN__}> handler
4913 installed. It is the handler's responsibility to deal with the message
4914 as it sees fit (like, for instance, converting it into a C<die>). Most
4915 handlers must therefore make arrangements to actually display the
4916 warnings that they are not prepared to deal with, by calling C<warn>
4917 again in the handler. Note that this is quite safe and will not
4918 produce an endless loop, since C<__WARN__> hooks are not called from
4921 You will find this behavior is slightly different from that of
4922 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4923 instead call C<die> again to change it).
4925 Using a C<__WARN__> handler provides a powerful way to silence all
4926 warnings (even the so-called mandatory ones). An example:
4928 # wipe out *all* compile-time warnings
4929 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4931 my $foo = 20; # no warning about duplicate my $foo,
4932 # but hey, you asked for it!
4933 # no compile-time or run-time warnings before here
4936 # run-time warnings enabled after here
4937 warn "\$foo is alive and $foo!"; # does show up
4939 See L<perlvar> for details on setting C<%SIG> entries, and for more
4940 examples. See the Carp module for other kinds of warnings using its
4941 carp() and cluck() functions.
4943 =item write FILEHANDLE
4949 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4950 using the format associated with that file. By default the format for
4951 a file is the one having the same name as the filehandle, but the
4952 format for the current output channel (see the C<select> function) may be set
4953 explicitly by assigning the name of the format to the C<$~> variable.
4955 Top of form processing is handled automatically: if there is
4956 insufficient room on the current page for the formatted record, the
4957 page is advanced by writing a form feed, a special top-of-page format
4958 is used to format the new page header, and then the record is written.
4959 By default the top-of-page format is the name of the filehandle with
4960 "_TOP" appended, but it may be dynamically set to the format of your
4961 choice by assigning the name to the C<$^> variable while the filehandle is
4962 selected. The number of lines remaining on the current page is in
4963 variable C<$->, which can be set to C<0> to force a new page.
4965 If FILEHANDLE is unspecified, output goes to the current default output
4966 channel, which starts out as STDOUT but may be changed by the
4967 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4968 is evaluated and the resulting string is used to look up the name of
4969 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4971 Note that write is I<not> the opposite of C<read>. Unfortunately.
4975 The transliteration operator. Same as C<tr///>. See L<perlop>.