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 and 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.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
96 =item Regular expressions and pattern matching
98 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
136 =item Keywords related to the control flow of your perl program
138 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
139 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
141 =item Keywords related to scoping
143 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
145 =item Miscellaneous functions
147 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
148 C<scalar>, C<undef>, C<wantarray>
150 =item Functions for processes and process groups
152 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
153 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
154 C<times>, C<wait>, C<waitpid>
156 =item Keywords related to perl modules
158 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
160 =item Keywords related to classes and object-orientedness
162 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
165 =item Low-level socket functions
167 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
168 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
169 C<socket>, C<socketpair>
171 =item System V interprocess communication functions
173 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
174 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
176 =item Fetching user and group info
178 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
179 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
180 C<getpwuid>, C<setgrent>, C<setpwent>
182 =item Fetching network info
184 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
185 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
186 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
187 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
188 C<setnetent>, C<setprotoent>, C<setservent>
190 =item Time-related functions
192 C<gmtime>, C<localtime>, C<time>, C<times>
194 =item Functions new in perl5
196 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
197 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
198 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
199 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
201 * - C<sub> was a keyword in perl4, but in perl5 it is an
202 operator, which can be used in expressions.
204 =item Functions obsoleted in perl5
206 C<dbmclose>, C<dbmopen>
212 Perl was born in UNIX and therefore it can access all the common UNIX
213 system calls. In non-UNIX environments the functionality of many
214 UNIX system calls may not be available or the details of the available
215 functionality may be slightly different. The Perl functions affected
218 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
219 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
220 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
221 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
222 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
223 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
224 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
225 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
226 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
227 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
228 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
229 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
230 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
231 C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
232 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
235 For more information about the portability of these functions, see
236 L<perlport> and other available platform-specific documentation.
238 =head2 Alphabetical Listing of Perl Functions
242 =item I<-X> FILEHANDLE
248 A file test, where X is one of the letters listed below. This unary
249 operator takes one argument, either a filename or a filehandle, and
250 tests the associated file to see if something is true about it. If the
251 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
252 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
253 the undefined value if the file doesn't exist. Despite the funny
254 names, precedence is the same as any other named unary operator, and
255 the argument may be parenthesized like any other unary operator. The
256 operator may be any of:
257 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>
258 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
260 -r File is readable by effective uid/gid.
261 -w File is writable by effective uid/gid.
262 -x File is executable by effective uid/gid.
263 -o File is owned by effective uid.
265 -R File is readable by real uid/gid.
266 -W File is writable by real uid/gid.
267 -X File is executable by real uid/gid.
268 -O File is owned by real uid.
271 -z File has zero size.
272 -s File has nonzero size (returns size).
274 -f File is a plain file.
275 -d File is a directory.
276 -l File is a symbolic link.
277 -p File is a named pipe (FIFO), or Filehandle is a pipe.
279 -b File is a block special file.
280 -c File is a character special file.
281 -t Filehandle is opened to a tty.
283 -u File has setuid bit set.
284 -g File has setgid bit set.
285 -k File has sticky bit set.
287 -T File is a text file.
288 -B File is a binary file (opposite of -T).
290 -M Age of file in days when script started.
291 -A Same for access time.
292 -C Same for inode change time.
298 next unless -f $_; # ignore specials
302 The interpretation of the file permission operators C<-r>, C<-R>,
303 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
304 of the file and the uids and gids of the user. There may be other
305 reasons you can't actually read, write, or execute the file. Such
306 reasons may be for example network filesystem access controls, ACLs
307 (access control lists), read-only filesystems, and unrecognized
310 Also note that, for the superuser on the local filesystems, C<-r>,
311 C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
312 if any execute bit is set in the mode. Scripts run by the superuser
313 may thus need to do a stat() to determine the actual mode of the file,
314 or temporarily set the uid to something else.
316 If you are using ACLs, there is a pragma called C<filetest> that may
317 produce more accurate results than the bare stat() mode bits.
319 When under the C<use filetest 'access'> the above-mentioned filetests
320 will test whether the permission can (not) be granted using the
321 access() family of system calls. Also note that the C<-x> and C<-X> may
322 under this pragma return true even if there are no execute permission
323 bits set (nor any extra execute permission ACLs). This strangeness is
324 due to the underlying system calls' definitions. Read the
325 documentation for the C<filetest> pragma for more information.
327 Note that C<-s/a/b/> does not do a negated substitution. Saying
328 C<-exp($foo)> still works as expected, however--only single letters
329 following a minus are interpreted as file tests.
331 The C<-T> and C<-B> switches work as follows. The first block or so of the
332 file is examined for odd characters such as strange control codes or
333 characters with the high bit set. If too many strange characters (E<gt>30%)
334 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
335 containing null in the first block is considered a binary file. If C<-T>
336 or C<-B> is used on a filehandle, the current stdio buffer is examined
337 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
338 file, or a file at EOF when testing a filehandle. Because you have to
339 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
340 against the file first, as in C<next unless -f $file && -T $file>.
342 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
343 the special filehandle consisting of a solitary underline, then the stat
344 structure of the previous file test (or stat operator) is used, saving
345 a system call. (This doesn't work with C<-t>, and you need to remember
346 that lstat() and C<-l> will leave values in the stat structure for the
347 symbolic link, not the real file.) Example:
349 print "Can do.\n" if -r $a || -w _ || -x _;
352 print "Readable\n" if -r _;
353 print "Writable\n" if -w _;
354 print "Executable\n" if -x _;
355 print "Setuid\n" if -u _;
356 print "Setgid\n" if -g _;
357 print "Sticky\n" if -k _;
358 print "Text\n" if -T _;
359 print "Binary\n" if -B _;
365 Returns the absolute value of its argument.
366 If VALUE is omitted, uses C<$_>.
368 =item accept NEWSOCKET,GENERICSOCKET
370 Accepts an incoming socket connect, just as the accept(2) system call
371 does. Returns the packed address if it succeeded, FALSE otherwise.
372 See example in L<perlipc/"Sockets: Client/Server Communication">.
378 Arranges to have a SIGALRM delivered to this process after the
379 specified number of seconds have elapsed. If SECONDS is not specified,
380 the value stored in C<$_> is used. (On some machines,
381 unfortunately, the elapsed time may be up to one second less than you
382 specified because of how seconds are counted.) Only one timer may be
383 counting at once. Each call disables the previous timer, and an
384 argument of C<0> may be supplied to cancel the previous timer without
385 starting a new one. The returned value is the amount of time remaining
386 on the previous timer.
388 For delays of finer granularity than one second, you may use Perl's
389 C<syscall()> interface to access setitimer(2) if your system supports it,
390 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
391 and C<sleep()> calls.
393 If you want to use C<alarm()> to time out a system call you need to use an
394 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
395 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
396 restart system calls on some systems. Using C<eval()>/C<die()> always works,
397 modulo the caveats given in L<perlipc/"Signals">.
400 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
402 $nread = sysread SOCKET, $buffer, $size;
406 die unless $@ eq "alarm\n"; # propagate unexpected errors
415 Returns the arctangent of Y/X in the range -PI to PI.
417 For the tangent operation, you may use the C<POSIX::tan()>
418 function, or use the familiar relation:
420 sub tan { sin($_[0]) / cos($_[0]) }
422 =item bind SOCKET,NAME
424 Binds a network address to a socket, just as the bind system call
425 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
426 packed address of the appropriate type for the socket. See the examples in
427 L<perlipc/"Sockets: Client/Server Communication">.
429 =item binmode FILEHANDLE
431 Arranges for the file to be read or written in "binary" mode in operating
432 systems that distinguish between binary and text files. Files that are
433 not in binary mode have CR LF sequences translated to LF on input and LF
434 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
435 and similarly archaic systems, it may be imperative--otherwise your
436 MS-DOS-damaged C library may mangle your file. The key distinction between
437 systems that need C<binmode()> and those that don't is their text file
438 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
439 character, and that encode that character in C as C<"\n">, do not need
440 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
441 is taken as the name of the filehandle.
443 =item bless REF,CLASSNAME
447 This function tells the thingy referenced by REF that it is now
448 an object in the CLASSNAME package--or the current package if no CLASSNAME
449 is specified, which is often the case. It returns the reference for
450 convenience, because a C<bless()> is often the last thing in a constructor.
451 Always use the two-argument version if the function doing the blessing
452 might be inherited by a derived class. See L<perltoot> and L<perlobj>
453 for more about the blessing (and blessings) of objects.
455 Consider always blessing objects in CLASSNAMEs that are mixed case.
456 Namespaces with all lowercase names are considered reserved for Perl
457 pragmata. Builtin types have all uppercase names, so to prevent confusion,
458 it is best to avoid such package names as well.
460 See L<perlmod/"Perl Modules">.
466 Returns the context of the current subroutine call. In scalar context,
467 returns the caller's package name if there is a caller, that is, if
468 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
469 otherwise. In list context, returns
471 ($package, $filename, $line) = caller;
473 With EXPR, it returns some extra information that the debugger uses to
474 print a stack trace. The value of EXPR indicates how many call frames
475 to go back before the current one.
477 ($package, $filename, $line, $subroutine,
478 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
480 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
481 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
482 C<$is_require> are set: C<$is_require> is true if the frame is created by a
483 C<require> or C<use> statement, C<$evaltext> contains the text of the
484 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
485 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
486 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
489 Furthermore, when called from within the DB package, caller returns more
490 detailed information: it sets the list variable C<@DB::args> to be the
491 arguments with which the subroutine was invoked.
493 Be aware that the optimizer might have optimized call frames away before
494 C<caller()> had a chance to get the information. That means that C<caller(N)>
495 might not return information about the call frame you expect it do, for
496 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
497 previous time C<caller()> was called.
501 Changes the working directory to EXPR, if possible. If EXPR is
502 omitted, changes to home directory. Returns TRUE upon success, FALSE
503 otherwise. See example under C<die()>.
507 Changes the permissions of a list of files. The first element of the
508 list must be the numerical mode, which should probably be an octal
509 number, and which definitely should I<not> a string of octal digits:
510 C<0644> is okay, C<'0644'> is not. Returns the number of files
511 successfully changed. See also L</oct>, if all you have is a string.
513 $cnt = chmod 0755, 'foo', 'bar';
514 chmod 0755, @executables;
515 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
517 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
518 $mode = 0644; chmod $mode, 'foo'; # this is best
526 This is a slightly safer version of L</chop>. It removes any
527 line ending that corresponds to the current value of C<$/> (also known as
528 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
529 number of characters removed from all its arguments. It's often used to
530 remove the newline from the end of an input record when you're worried
531 that the final record may be missing its newline. When in paragraph mode
532 (C<$/ = "">), it removes all trailing newlines from the string. If
533 VARIABLE is omitted, it chomps C<$_>. Example:
536 chomp; # avoid \n on last field
541 You can actually chomp anything that's an lvalue, including an assignment:
544 chomp($answer = <STDIN>);
546 If you chomp a list, each element is chomped, and the total number of
547 characters removed is returned.
555 Chops off the last character of a string and returns the character
556 chopped. It's used primarily to remove the newline from the end of an
557 input record, but is much more efficient than C<s/\n//> because it neither
558 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
562 chop; # avoid \n on last field
567 You can actually chop anything that's an lvalue, including an assignment:
570 chop($answer = <STDIN>);
572 If you chop a list, each element is chopped. Only the value of the
573 last C<chop()> is returned.
575 Note that C<chop()> returns the last character. To return all but the last
576 character, use C<substr($string, 0, -1)>.
580 Changes the owner (and group) of a list of files. The first two
581 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
582 Returns the number of files successfully changed.
584 $cnt = chown $uid, $gid, 'foo', 'bar';
585 chown $uid, $gid, @filenames;
587 Here's an example that looks up nonnumeric uids in the passwd file:
590 chop($user = <STDIN>);
592 chop($pattern = <STDIN>);
594 ($login,$pass,$uid,$gid) = getpwnam($user)
595 or die "$user not in passwd file";
597 @ary = glob($pattern); # expand filenames
598 chown $uid, $gid, @ary;
600 On most systems, you are not allowed to change the ownership of the
601 file unless you're the superuser, although you should be able to change
602 the group to any of your secondary groups. On insecure systems, these
603 restrictions may be relaxed, but this is not a portable assumption.
609 Returns the character represented by that NUMBER in the character set.
610 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
611 chr(0x263a) is a Unicode smiley face (but only within the scope of a
612 C<use utf8>). For the reverse, use L</ord>.
614 If NUMBER is omitted, uses C<$_>.
616 =item chroot FILENAME
620 This function works like the system call by the same name: it makes the
621 named directory the new root directory for all further pathnames that
622 begin with a C<"/"> by your process and all its children. (It doesn't
623 change your current working directory, which is unaffected.) For security
624 reasons, this call is restricted to the superuser. If FILENAME is
625 omitted, does a C<chroot()> to C<$_>.
627 =item close FILEHANDLE
631 Closes the file or pipe associated with the file handle, returning TRUE
632 only if stdio successfully flushes buffers and closes the system file
633 descriptor. Closes the currently selected filehandle if the argument
636 You don't have to close FILEHANDLE if you are immediately going to do
637 another C<open()> on it, because C<open()> will close it for you. (See
638 C<open()>.) However, an explicit C<close()> on an input file resets the line
639 counter (C<$.>), while the implicit close done by C<open()> does not.
641 If the file handle came from a piped open C<close()> will additionally
642 return FALSE if one of the other system calls involved fails or if the
643 program exits with non-zero status. (If the only problem was that the
644 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
645 waits for the process executing on the pipe to complete, in case you
646 want to look at the output of the pipe afterwards. Closing a pipe
647 explicitly also puts the exit status value of the command into C<$?>.
651 open(OUTPUT, '|sort >foo') # pipe to sort
652 or die "Can't start sort: $!";
653 #... # print stuff to output
654 close OUTPUT # wait for sort to finish
655 or warn $! ? "Error closing sort pipe: $!"
656 : "Exit status $? from sort";
657 open(INPUT, 'foo') # get sort's results
658 or die "Can't open 'foo' for input: $!";
660 FILEHANDLE may be an expression whose value can be used as an indirect
661 filehandle, usually the real filehandle name.
663 =item closedir DIRHANDLE
665 Closes a directory opened by C<opendir()> and returns the success of that
668 DIRHANDLE may be an expression whose value can be used as an indirect
669 dirhandle, usually the real dirhandle name.
671 =item connect SOCKET,NAME
673 Attempts to connect to a remote socket, just as the connect system call
674 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
675 packed address of the appropriate type for the socket. See the examples in
676 L<perlipc/"Sockets: Client/Server Communication">.
680 Actually a flow control statement rather than a function. If there is a
681 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
682 C<foreach>), it is always executed just before the conditional is about to
683 be evaluated again, just like the third part of a C<for> loop in C. Thus
684 it can be used to increment a loop variable, even when the loop has been
685 continued via the C<next> statement (which is similar to the C C<continue>
688 C<last>, C<next>, or C<redo> may appear within a C<continue>
689 block. C<last> and C<redo> will behave as if they had been executed within
690 the main block. So will C<next>, but since it will execute a C<continue>
691 block, it may be more entertaining.
694 ### redo always comes here
697 ### next always comes here
699 # then back the top to re-check EXPR
701 ### last always comes here
703 Omitting the C<continue> section is semantically equivalent to using an
704 empty one, logically enough. In that case, C<next> goes directly back
705 to check the condition at the top of the loop.
709 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
710 takes cosine of C<$_>.
712 For the inverse cosine operation, you may use the C<POSIX::acos()>
713 function, or use this relation:
715 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
717 =item crypt PLAINTEXT,SALT
719 Encrypts a string exactly like the crypt(3) function in the C library
720 (assuming that you actually have a version there that has not been
721 extirpated as a potential munition). This can prove useful for checking
722 the password file for lousy passwords, amongst other things. Only the
723 guys wearing white hats should do this.
725 Note that C<crypt()> is intended to be a one-way function, much like breaking
726 eggs to make an omelette. There is no (known) corresponding decrypt
727 function. As a result, this function isn't all that useful for
728 cryptography. (For that, see your nearby CPAN mirror.)
730 When verifying an existing encrypted string you should use the encrypted
731 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
732 allows your code to work with the standard C<crypt()> and with more
733 exotic implementations. When choosing a new salt create a random two
734 character string whose characters come from the set C<[./0-9A-Za-z]>
735 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
737 Here's an example that makes sure that whoever runs this program knows
740 $pwd = (getpwuid($<))[1];
744 chomp($word = <STDIN>);
748 if (crypt($word, $pwd) ne $pwd) {
754 Of course, typing in your own password to whoever asks you
759 [This function has been superseded by the C<untie()> function.]
761 Breaks the binding between a DBM file and a hash.
763 =item dbmopen HASH,DBNAME,MODE
765 [This function has been superseded by the C<tie()> function.]
767 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
768 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
769 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
770 is the name of the database (without the F<.dir> or F<.pag> extension if
771 any). If the database does not exist, it is created with protection
772 specified by MODE (as modified by the C<umask()>). If your system supports
773 only the older DBM functions, you may perform only one C<dbmopen()> in your
774 program. In older versions of Perl, if your system had neither DBM nor
775 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
778 If you don't have write access to the DBM file, you can only read hash
779 variables, not set them. If you want to test whether you can write,
780 either use file tests or try setting a dummy hash entry inside an C<eval()>,
781 which will trap the error.
783 Note that functions such as C<keys()> and C<values()> may return huge lists
784 when used on large DBM files. You may prefer to use the C<each()>
785 function to iterate over large DBM files. Example:
787 # print out history file offsets
788 dbmopen(%HIST,'/usr/lib/news/history',0666);
789 while (($key,$val) = each %HIST) {
790 print $key, ' = ', unpack('L',$val), "\n";
794 See also L<AnyDBM_File> for a more general description of the pros and
795 cons of the various dbm approaches, as well as L<DB_File> for a particularly
802 Returns a Boolean value telling whether EXPR has a value other than
803 the undefined value C<undef>. If EXPR is not present, C<$_> will be
806 Many operations return C<undef> to indicate failure, end of file,
807 system error, uninitialized variable, and other exceptional
808 conditions. This function allows you to distinguish C<undef> from
809 other values. (A simple Boolean test will not distinguish among
810 C<undef>, zero, the empty string, and C<"0">, which are all equally
811 false.) Note that since C<undef> is a valid scalar, its presence
812 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
813 returns C<undef> when its argument is an empty array, I<or> when the
814 element to return happens to be C<undef>.
816 You may also use C<defined()> to check whether a subroutine exists, by
817 saying C<defined &func> without parentheses. On the other hand, use
818 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
819 produce intuitive results, and should probably be avoided.
821 When used on a hash element, it tells you whether the value is defined,
822 not whether the key exists in the hash. Use L</exists> for the latter
827 print if defined $switch{'D'};
828 print "$val\n" while defined($val = pop(@ary));
829 die "Can't readlink $sym: $!"
830 unless defined($value = readlink $sym);
831 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
832 $debugging = 0 unless defined $debugging;
834 Note: Many folks tend to overuse C<defined()>, and then are surprised to
835 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
836 defined values. For example, if you say
840 The pattern match succeeds, and C<$1> is defined, despite the fact that it
841 matched "nothing". But it didn't really match nothing--rather, it
842 matched something that happened to be C<0> characters long. This is all
843 very above-board and honest. When a function returns an undefined value,
844 it's an admission that it couldn't give you an honest answer. So you
845 should use C<defined()> only when you're questioning the integrity of what
846 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
849 Currently, using C<defined()> on an entire array or hash reports whether
850 memory for that aggregate has ever been allocated. So an array you set
851 to the empty list appears undefined initially, and one that once was full
852 and that you then set to the empty list still appears defined. You
853 should instead use a simple test for size:
855 if (@an_array) { print "has array elements\n" }
856 if (%a_hash) { print "has hash members\n" }
858 Using C<undef()> on these, however, does clear their memory and then report
859 them as not defined anymore, but you shouldn't do that unless you don't
860 plan to use them again, because it saves time when you load them up
861 again to have memory already ready to be filled. The normal way to
862 free up space used by an aggregate is to assign the empty list.
864 This counterintuitive behavior of C<defined()> on aggregates may be
865 changed, fixed, or broken in a future release of Perl.
867 See also L</undef>, L</exists>, L</ref>.
871 Deletes the specified key(s) and their associated values from a hash.
872 For each key, returns the deleted value associated with that key, or
873 the undefined value if there was no such key. Deleting from C<$ENV{}>
874 modifies the environment. Deleting from a hash tied to a DBM file
875 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
876 doesn't necessarily return anything.)
878 The following deletes all the values of a hash:
880 foreach $key (keys %HASH) {
886 delete @HASH{keys %HASH}
888 (But both of these are slower than just assigning the empty list, or
889 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
890 long as the final operation is a hash element lookup or hash slice:
892 delete $ref->[$x][$y]{$key};
893 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
897 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
898 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
899 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
900 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
901 C<$@> and the C<eval()> is terminated with the undefined value. This makes
902 C<die()> the way to raise an exception.
906 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
907 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
909 If the value of EXPR does not end in a newline, the current script line
910 number and input line number (if any) are also printed, and a newline
911 is supplied. Note that the "input line number" (also known as "chunk")
912 is subject to whatever notion of "line" happens to be currently in
913 effect, and is also available as the special variable C<$.>.
914 See L<perlvar/"$/"> and L<perlvar/"$.">.
916 Hint: sometimes appending C<", stopped"> to your message
917 will cause it to make better sense when the string C<"at foo line 123"> is
918 appended. Suppose you are running script "canasta".
920 die "/etc/games is no good";
921 die "/etc/games is no good, stopped";
923 produce, respectively
925 /etc/games is no good at canasta line 123.
926 /etc/games is no good, stopped at canasta line 123.
928 See also C<exit()> and C<warn()>.
930 If LIST is empty and C<$@> already contains a value (typically from a
931 previous eval) that value is reused after appending C<"\t...propagated">.
932 This is useful for propagating exceptions:
935 die unless $@ =~ /Expected exception/;
937 If C<$@> is empty then the string C<"Died"> is used.
939 You can arrange for a callback to be run just before the C<die()> does
940 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
941 will be called with the error text and can change the error message, if
942 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
943 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
945 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
946 blocks/strings. If one wants the hook to do nothing in such
951 as the first line of the handler (see L<perlvar/$^S>).
955 Not really a function. Returns the value of the last command in the
956 sequence of commands indicated by BLOCK. When modified by a loop
957 modifier, executes the BLOCK once before testing the loop condition.
958 (On other statements the loop modifiers test the conditional first.)
960 C<do BLOCK> does I<not> count as a loop, so the loop control statements
961 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
963 =item do SUBROUTINE(LIST)
965 A deprecated form of subroutine call. See L<perlsub>.
969 Uses the value of EXPR as a filename and executes the contents of the
970 file as a Perl script. Its primary use is to include subroutines
971 from a Perl subroutine library.
977 scalar eval `cat stat.pl`;
979 except that it's more efficient and concise, keeps track of the
980 current filename for error messages, and searches all the B<-I>
981 libraries if the file isn't in the current directory (see also the @INC
982 array in L<perlvar/Predefined Names>). It is also different in how
983 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
984 scope like C<eval STRING> does. It's the same, however, in that it does
985 reparse the file every time you call it, so you probably don't want to
986 do this inside a loop.
988 If C<do> cannot read the file, it returns undef and sets C<$!> to the
989 error. If C<do> can read the file but cannot compile it, it
990 returns undef and sets an error message in C<$@>. If the file is
991 successfully compiled, C<do> returns the value of the last expression
994 Note that inclusion of library modules is better done with the
995 C<use()> and C<require()> operators, which also do automatic error checking
996 and raise an exception if there's a problem.
998 You might like to use C<do> to read in a program configuration
999 file. Manual error checking can be done this way:
1001 # read in config files: system first, then user
1002 for $file ("/share/prog/defaults.rc",
1003 "$ENV{HOME}/.someprogrc") {
1004 unless ($return = do $file) {
1005 warn "couldn't parse $file: $@" if $@;
1006 warn "couldn't do $file: $!" unless defined $return;
1007 warn "couldn't run $file" unless $return;
1015 This causes an immediate core dump. Primarily this is so that you can
1016 use the B<undump> program to turn your core dump into an executable binary
1017 after having initialized all your variables at the beginning of the
1018 program. When the new binary is executed it will begin by executing a
1019 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1020 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1021 is omitted, restarts the program from the top. WARNING: Any files
1022 opened at the time of the dump will NOT be open any more when the
1023 program is reincarnated, with possible resulting confusion on the part
1024 of Perl. See also B<-u> option in L<perlrun>.
1029 require 'getopt.pl';
1041 dump QUICKSTART if $ARGV[0] eq '-d';
1046 This operator is largely obsolete, partly because it's very hard to
1047 convert a core file into an executable, and because the real perl-to-C
1048 compiler has superseded it.
1052 When called in list context, returns a 2-element list consisting of the
1053 key and value for the next element of a hash, so that you can iterate over
1054 it. When called in scalar context, returns the key for only the "next"
1055 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1056 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1059 Entries are returned in an apparently random order. The actual random
1060 order is subject to change in future versions of perl, but it is guaranteed
1061 to be in the same order as either the C<keys()> or C<values()> function
1062 would produce on the same (unmodified) hash.
1064 When the hash is entirely read, a null array is returned in list context
1065 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1066 scalar context. The next call to C<each()> after that will start iterating
1067 again. There is a single iterator for each hash, shared by all C<each()>,
1068 C<keys()>, and C<values()> function calls in the program; it can be reset by
1069 reading all the elements from the hash, or by evaluating C<keys HASH> or
1070 C<values HASH>. If you add or delete elements of a hash while you're
1071 iterating over it, you may get entries skipped or duplicated, so don't.
1073 The following prints out your environment like the printenv(1) program,
1074 only in a different order:
1076 while (($key,$value) = each %ENV) {
1077 print "$key=$value\n";
1080 See also C<keys()>, C<values()> and C<sort()>.
1082 =item eof FILEHANDLE
1088 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1089 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1090 gives the real filehandle. (Note that this function actually
1091 reads a character and then C<ungetc()>s it, so isn't very useful in an
1092 interactive context.) Do not read from a terminal file (or call
1093 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1094 as terminals may lose the end-of-file condition if you do.
1096 An C<eof> without an argument uses the last file read as argument.
1097 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1098 the files listed on the command line, i.e., C<eof()> is reasonable to
1099 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1100 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1101 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1103 # reset line numbering on each input file
1105 next if /^\s*#/; # skip comments
1108 close ARGV if eof; # Not eof()!
1111 # insert dashes just before last line of last file
1113 if (eof()) { # check for end of current file
1114 print "--------------\n";
1115 close(ARGV); # close or break; is needed if we
1116 # are reading from the terminal
1121 Practical hint: you almost never need to use C<eof> in Perl, because the
1122 input operators return false values when they run out of data, or if there
1129 In the first form, the return value of EXPR is parsed and executed as if it
1130 were a little Perl program. The value of the expression (which is itself
1131 determined within scalar context) is first parsed, and if there weren't any
1132 errors, executed in the context of the current Perl program, so that any
1133 variable settings or subroutine and format definitions remain afterwards.
1134 Note that the value is parsed every time the eval executes. If EXPR is
1135 omitted, evaluates C<$_>. This form is typically used to delay parsing
1136 and subsequent execution of the text of EXPR until run time.
1138 In the second form, the code within the BLOCK is parsed only once--at the
1139 same time the code surrounding the eval itself was parsed--and executed
1140 within the context of the current Perl program. This form is typically
1141 used to trap exceptions more efficiently than the first (see below), while
1142 also providing the benefit of checking the code within BLOCK at compile
1145 The final semicolon, if any, may be omitted from the value of EXPR or within
1148 In both forms, the value returned is the value of the last expression
1149 evaluated inside the mini-program; a return statement may be also used, just
1150 as with subroutines. The expression providing the return value is evaluated
1151 in void, scalar, or list context, depending on the context of the eval itself.
1152 See L</wantarray> for more on how the evaluation context can be determined.
1154 If there is a syntax error or runtime error, or a C<die()> statement is
1155 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1156 error message. If there was no error, C<$@> is guaranteed to be a null
1157 string. Beware that using C<eval()> neither silences perl from printing
1158 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1159 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1160 L</warn> and L<perlvar>.
1162 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1163 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1164 is implemented. It is also Perl's exception trapping mechanism, where
1165 the die operator is used to raise exceptions.
1167 If the code to be executed doesn't vary, you may use the eval-BLOCK
1168 form to trap run-time errors without incurring the penalty of
1169 recompiling each time. The error, if any, is still returned in C<$@>.
1172 # make divide-by-zero nonfatal
1173 eval { $answer = $a / $b; }; warn $@ if $@;
1175 # same thing, but less efficient
1176 eval '$answer = $a / $b'; warn $@ if $@;
1178 # a compile-time error
1179 eval { $answer = }; # WRONG
1182 eval '$answer ='; # sets $@
1184 When using the C<eval{}> form as an exception trap in libraries, you may
1185 wish not to trigger any C<__DIE__> hooks that user code may have
1186 installed. You can use the C<local $SIG{__DIE__}> construct for this
1187 purpose, as shown in this example:
1189 # a very private exception trap for divide-by-zero
1190 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1193 This is especially significant, given that C<__DIE__> hooks can call
1194 C<die()> again, which has the effect of changing their error messages:
1196 # __DIE__ hooks may modify error messages
1198 local $SIG{'__DIE__'} =
1199 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1200 eval { die "foo lives here" };
1201 print $@ if $@; # prints "bar lives here"
1204 With an C<eval()>, you should be especially careful to remember what's
1205 being looked at when:
1211 eval { $x }; # CASE 4
1213 eval "\$$x++"; # CASE 5
1216 Cases 1 and 2 above behave identically: they run the code contained in
1217 the variable C<$x>. (Although case 2 has misleading double quotes making
1218 the reader wonder what else might be happening (nothing is).) Cases 3
1219 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1220 does nothing but return the value of C<$x>. (Case 4 is preferred for
1221 purely visual reasons, but it also has the advantage of compiling at
1222 compile-time instead of at run-time.) Case 5 is a place where
1223 normally you I<WOULD> like to use double quotes, except that in this
1224 particular situation, you can just use symbolic references instead, as
1227 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1228 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1233 =item exec PROGRAM LIST
1235 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1236 use C<system()> instead of C<exec()> if you want it to return. It fails and
1237 returns FALSE only if the command does not exist I<and> it is executed
1238 directly instead of via your system's command shell (see below).
1240 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1241 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1242 or C<exit()> (if C<-w> is set - but you always do that). If you
1243 I<really> want to follow an C<exec()> with some other statement, you
1244 can use one of these styles to avoid the warning:
1246 exec ('foo') or print STDERR "couldn't exec foo: $!";
1247 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1249 If there is more than one argument in LIST, or if LIST is an array
1250 with more than one value, calls execvp(3) with the arguments in LIST.
1251 If there is only one scalar argument or an array with one element in it,
1252 the argument is checked for shell metacharacters, and if there are any,
1253 the entire argument is passed to the system's command shell for parsing
1254 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1255 If there are no shell metacharacters in the argument, it is split into
1256 words and passed directly to C<execvp()>, which is more efficient. Note:
1257 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1258 set C<$|> to avoid lost output. Examples:
1260 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1261 exec "sort $outfile | uniq";
1263 If you don't really want to execute the first argument, but want to lie
1264 to the program you are executing about its own name, you can specify
1265 the program you actually want to run as an "indirect object" (without a
1266 comma) in front of the LIST. (This always forces interpretation of the
1267 LIST as a multivalued list, even if there is only a single scalar in
1270 $shell = '/bin/csh';
1271 exec $shell '-sh'; # pretend it's a login shell
1275 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1277 When the arguments get executed via the system shell, results will
1278 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1281 Using an indirect object with C<exec()> or C<system()> is also more secure.
1282 This usage forces interpretation of the arguments as a multivalued list,
1283 even if the list had just one argument. That way you're safe from the
1284 shell expanding wildcards or splitting up words with whitespace in them.
1286 @args = ( "echo surprise" );
1288 system @args; # subject to shell escapes
1290 system { $args[0] } @args; # safe even with one-arg list
1292 The first version, the one without the indirect object, ran the I<echo>
1293 program, passing it C<"surprise"> an argument. The second version
1294 didn't--it tried to run a program literally called I<"echo surprise">,
1295 didn't find it, and set C<$?> to a non-zero value indicating failure.
1297 Note that C<exec()> will not call your C<END> blocks, nor will it call
1298 any C<DESTROY> methods in your objects.
1302 Returns TRUE if the specified hash key exists in its hash array, even
1303 if the corresponding value is undefined.
1305 print "Exists\n" if exists $array{$key};
1306 print "Defined\n" if defined $array{$key};
1307 print "True\n" if $array{$key};
1309 A hash element can be TRUE only if it's defined, and defined if
1310 it exists, but the reverse doesn't necessarily hold true.
1312 Note that the EXPR can be arbitrarily complicated as long as the final
1313 operation is a hash key lookup:
1315 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1317 Although the last element will not spring into existence just because its
1318 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1319 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1320 test for a $key element. This autovivification may be fixed in a later
1325 Evaluates EXPR and exits immediately with that value. (Actually, it
1326 calls any defined C<END> routines first, but the C<END> routines may not
1327 abort the exit. Likewise any object destructors that need to be called
1328 are called before exit.) Example:
1331 exit 0 if $ans =~ /^[Xx]/;
1333 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1334 universally portable values for EXPR are C<0> for success and C<1> for error;
1335 all other values are subject to unpredictable interpretation depending
1336 on the environment in which the Perl program is running.
1338 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1339 someone might want to trap whatever error happened. Use C<die()> instead,
1340 which can be trapped by an C<eval()>.
1342 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1348 Returns I<e> (the natural logarithm base) to the power of EXPR.
1349 If EXPR is omitted, gives C<exp($_)>.
1351 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1353 Implements the fcntl(2) function. You'll probably have to say
1357 first to get the correct constant definitions. Argument processing and
1358 value return works just like C<ioctl()> below.
1362 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1363 or die "can't fcntl F_GETFL: $!";
1365 You don't have to check for C<defined()> on the return from
1366 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1367 call into "C<0> but true" in Perl. This string is true in
1368 boolean context and C<0> in numeric context. It is also
1369 exempt from the normal B<-w> warnings on improper numeric
1372 Note that C<fcntl()> will produce a fatal error if used on a machine that
1373 doesn't implement fcntl(2).
1375 =item fileno FILEHANDLE
1377 Returns the file descriptor for a filehandle. This is useful for
1378 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1379 operations. If FILEHANDLE is an expression, the value is taken as
1380 an indirect filehandle, generally its name.
1382 You can use this to find out whether two handles refer to the
1383 same underlying descriptor:
1385 if (fileno(THIS) == fileno(THAT)) {
1386 print "THIS and THAT are dups\n";
1389 =item flock FILEHANDLE,OPERATION
1391 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1392 success, FALSE on failure. Produces a fatal error if used on a machine
1393 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1394 is Perl's portable file locking interface, although it locks only entire
1397 On many platforms (including most versions or clones of Unix), locks
1398 established by C<flock()> are B<merely advisory>. Such discretionary locks
1399 are more flexible, but offer fewer guarantees. This means that files
1400 locked with C<flock()> may be modified by programs that do not also use
1401 C<flock()>. Windows NT and OS/2 are among the platforms which
1402 enforce mandatory locking. See your local documentation for details.
1404 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1405 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1406 you can use the symbolic names if import them from the Fcntl module,
1407 either individually, or as a group using the ':flock' tag. LOCK_SH
1408 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1409 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1410 LOCK_EX then C<flock()> will return immediately rather than blocking
1411 waiting for the lock (check the return status to see if you got it).
1413 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1414 before (un)locking it.
1416 Note that the emulation built with lockf(3) doesn't provide shared
1417 locks, and it requires that FILEHANDLE be open with write intent. These
1418 are the semantics that lockf(3) implements. Most (all?) systems
1419 implement lockf(3) in terms of fcntl(2) locking, though, so the
1420 differing semantics shouldn't bite too many people.
1422 Note also that some versions of C<flock()> cannot lock things over the
1423 network; you would need to use the more system-specific C<fcntl()> for
1424 that. If you like you can force Perl to ignore your system's flock(2)
1425 function, and so provide its own fcntl(2)-based emulation, by passing
1426 the switch C<-Ud_flock> to the F<Configure> program when you configure
1429 Here's a mailbox appender for BSD systems.
1431 use Fcntl ':flock'; # import LOCK_* constants
1434 flock(MBOX,LOCK_EX);
1435 # and, in case someone appended
1436 # while we were waiting...
1441 flock(MBOX,LOCK_UN);
1444 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1445 or die "Can't open mailbox: $!";
1448 print MBOX $msg,"\n\n";
1451 See also L<DB_File> for other flock() examples.
1455 Does a fork(2) system call. Returns the child pid to the parent process,
1456 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1458 Note: unflushed buffers remain unflushed in both processes, which means
1459 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1460 method of C<IO::Handle> to avoid duplicate output.
1462 If you C<fork()> without ever waiting on your children, you will accumulate
1465 $SIG{CHLD} = sub { wait };
1467 There's also the double-fork trick (error checking on
1468 C<fork()> returns omitted);
1470 unless ($pid = fork) {
1472 exec "what you really wanna do";
1475 ## (some_perl_code_here)
1482 See also L<perlipc> for more examples of forking and reaping
1485 Note that if your forked child inherits system file descriptors like
1486 STDIN and STDOUT that are actually connected by a pipe or socket, even
1487 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1488 you're done. You should reopen those to F</dev/null> if it's any issue.
1492 Declare a picture format for use by the C<write()> function. For
1496 Test: @<<<<<<<< @||||| @>>>>>
1497 $str, $%, '$' . int($num)
1501 $num = $cost/$quantity;
1505 See L<perlform> for many details and examples.
1507 =item formline PICTURE,LIST
1509 This is an internal function used by C<format>s, though you may call it,
1510 too. It formats (see L<perlform>) a list of values according to the
1511 contents of PICTURE, placing the output into the format output
1512 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1513 Eventually, when a C<write()> is done, the contents of
1514 C<$^A> are written to some filehandle, but you could also read C<$^A>
1515 yourself and then set C<$^A> back to C<"">. Note that a format typically
1516 does one C<formline()> per line of form, but the C<formline()> function itself
1517 doesn't care how many newlines are embedded in the PICTURE. This means
1518 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1519 You may therefore need to use multiple formlines to implement a single
1520 record format, just like the format compiler.
1522 Be careful if you put double quotes around the picture, because an "C<@>"
1523 character may be taken to mean the beginning of an array name.
1524 C<formline()> always returns TRUE. See L<perlform> for other examples.
1526 =item getc FILEHANDLE
1530 Returns the next character from the input file attached to FILEHANDLE,
1531 or the undefined value at end of file, or if there was an error. If
1532 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1533 efficient. It cannot be used to get unbuffered single-characters,
1534 however. For that, try something more like:
1537 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1540 system "stty", '-icanon', 'eol', "\001";
1546 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1549 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1553 Determination of whether $BSD_STYLE should be set
1554 is left as an exercise to the reader.
1556 The C<POSIX::getattr()> function can do this more portably on systems
1557 purporting POSIX compliance.
1558 See also the C<Term::ReadKey> module from your nearest CPAN site;
1559 details on CPAN can be found on L<perlmodlib/CPAN>.
1563 Implements the C library function of the same name, which on most
1564 systems returns the current login from F</etc/utmp>, if any. If null,
1567 $login = getlogin || getpwuid($<) || "Kilroy";
1569 Do not consider C<getlogin()> for authentication: it is not as
1570 secure as C<getpwuid()>.
1572 =item getpeername SOCKET
1574 Returns the packed sockaddr address of other end of the SOCKET connection.
1577 $hersockaddr = getpeername(SOCK);
1578 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1579 $herhostname = gethostbyaddr($iaddr, AF_INET);
1580 $herstraddr = inet_ntoa($iaddr);
1584 Returns the current process group for the specified PID. Use
1585 a PID of C<0> to get the current process group for the
1586 current process. Will raise an exception if used on a machine that
1587 doesn't implement getpgrp(2). If PID is omitted, returns process
1588 group of current process. Note that the POSIX version of C<getpgrp()>
1589 does not accept a PID argument, so only C<PID==0> is truly portable.
1593 Returns the process id of the parent process.
1595 =item getpriority WHICH,WHO
1597 Returns the current priority for a process, a process group, or a user.
1598 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1599 machine that doesn't implement getpriority(2).
1605 =item gethostbyname NAME
1607 =item getnetbyname NAME
1609 =item getprotobyname NAME
1615 =item getservbyname NAME,PROTO
1617 =item gethostbyaddr ADDR,ADDRTYPE
1619 =item getnetbyaddr ADDR,ADDRTYPE
1621 =item getprotobynumber NUMBER
1623 =item getservbyport PORT,PROTO
1641 =item sethostent STAYOPEN
1643 =item setnetent STAYOPEN
1645 =item setprotoent STAYOPEN
1647 =item setservent STAYOPEN
1661 These routines perform the same functions as their counterparts in the
1662 system library. In list context, the return values from the
1663 various get routines are as follows:
1665 ($name,$passwd,$uid,$gid,
1666 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1667 ($name,$passwd,$gid,$members) = getgr*
1668 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1669 ($name,$aliases,$addrtype,$net) = getnet*
1670 ($name,$aliases,$proto) = getproto*
1671 ($name,$aliases,$port,$proto) = getserv*
1673 (If the entry doesn't exist you get a null list.)
1675 In scalar context, you get the name, unless the function was a
1676 lookup by name, in which case you get the other thing, whatever it is.
1677 (If the entry doesn't exist you get the undefined value.) For example:
1679 $uid = getpwnam($name);
1680 $name = getpwuid($num);
1682 $gid = getgrnam($name);
1683 $name = getgrgid($num;
1687 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1688 cases in the sense that in many systems they are unsupported. If the
1689 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1690 usually encodes the disk quota. If the C<$comment> field is unsupported,
1691 it is an empty scalar. If it is supported it usually encodes some
1692 administrative comment about the user. In some systems the $quota
1693 field may be C<$change> or C<$age>, fields that have to do with password
1694 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1695 field, if present, encodes the expiration period of the account or the
1696 password. For the availability and the exact meaning of these fields
1697 in your system, please consult your getpwnam(3) documentation and your
1698 F<pwd.h> file. You can also find out from within Perl which meaning
1699 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1700 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1701 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1703 The C<$members> value returned by I<getgr*()> is a space separated list of
1704 the login names of the members of the group.
1706 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1707 C, it will be returned to you via C<$?> if the function call fails. The
1708 C<@addrs> value returned by a successful call is a list of the raw
1709 addresses returned by the corresponding system library call. In the
1710 Internet domain, each address is four bytes long and you can unpack it
1711 by saying something like:
1713 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1715 If you get tired of remembering which element of the return list contains
1716 which return value, by-name interfaces are also provided in modules:
1717 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1718 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1719 normal built-in, replacing them with versions that return objects with
1720 the appropriate names for each field. For example:
1724 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1726 Even though it looks like they're the same method calls (uid),
1727 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1729 =item getsockname SOCKET
1731 Returns the packed sockaddr address of this end of the SOCKET connection.
1734 $mysockaddr = getsockname(SOCK);
1735 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1737 =item getsockopt SOCKET,LEVEL,OPTNAME
1739 Returns the socket option requested, or undef if there is an error.
1745 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1746 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1747 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1748 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1749 L<perlop/"I/O Operators">.
1753 Converts a time as returned by the time function to a 9-element array
1754 with the time localized for the standard Greenwich time zone.
1755 Typically used as follows:
1758 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1761 All array elements are numeric, and come straight out of a struct tm.
1762 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1763 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1764 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1766 If EXPR is omitted, does C<gmtime(time())>.
1768 In scalar context, returns the ctime(3) value:
1770 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1772 Also see the C<timegm()> function provided by the C<Time::Local> module,
1773 and the strftime(3) function available via the POSIX module.
1775 This scalar value is B<not> locale dependent, see L<perllocale>, but
1776 instead a Perl builtin. Also see the C<Time::Local> module, and the
1777 strftime(3) and mktime(3) function available via the POSIX module. To
1778 get somewhat similar but locale dependent date strings, set up your
1779 locale environment variables appropriately (please see L<perllocale>)
1780 and try for example:
1782 use POSIX qw(strftime);
1783 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1785 Note that the C<%a> and C<%b>, the short forms of the day of the week
1786 and the month of the year, may not necessarily be three characters wide.
1794 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1795 execution there. It may not be used to go into any construct that
1796 requires initialization, such as a subroutine or a C<foreach> loop. It
1797 also can't be used to go into a construct that is optimized away,
1798 or to get out of a block or subroutine given to C<sort()>.
1799 It can be used to go almost anywhere else within the dynamic scope,
1800 including out of subroutines, but it's usually better to use some other
1801 construct such as C<last> or C<die()>. The author of Perl has never felt the
1802 need to use this form of C<goto> (in Perl, that is--C is another matter).
1804 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1805 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1806 necessarily recommended if you're optimizing for maintainability:
1808 goto ("FOO", "BAR", "GLARCH")[$i];
1810 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1811 named subroutine for the currently running subroutine. This is used by
1812 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1813 pretend that the other subroutine had been called in the first place
1814 (except that any modifications to C<@_> in the current subroutine are
1815 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1816 will be able to tell that this routine was called first.
1818 =item grep BLOCK LIST
1820 =item grep EXPR,LIST
1822 This is similar in spirit to, but not the same as, grep(1)
1823 and its relatives. In particular, it is not limited to using
1824 regular expressions.
1826 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1827 C<$_> to each element) and returns the list value consisting of those
1828 elements for which the expression evaluated to TRUE. In a scalar
1829 context, returns the number of times the expression was TRUE.
1831 @foo = grep(!/^#/, @bar); # weed out comments
1835 @foo = grep {!/^#/} @bar; # weed out comments
1837 Note that, because C<$_> is a reference into the list value, it can be used
1838 to modify the elements of the array. While this is useful and
1839 supported, it can cause bizarre results if the LIST is not a named
1840 array. Similarly, grep returns aliases into the original list,
1841 much like the way that a for loop's index variable aliases the list
1842 elements. That is, modifying an element of a list returned by grep
1843 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1844 actually modifies the element in the original list.
1846 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1852 Interprets EXPR as a hex string and returns the corresponding
1853 value. (To convert strings that might start with either 0 or 0x
1854 see L</oct>.) If EXPR is omitted, uses C<$_>.
1856 print hex '0xAf'; # prints '175'
1857 print hex 'aF'; # same
1861 There is no builtin C<import()> function. It is just an ordinary
1862 method (subroutine) defined (or inherited) by modules that wish to export
1863 names to another module. The C<use()> function calls the C<import()> method
1864 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1866 =item index STR,SUBSTR,POSITION
1868 =item index STR,SUBSTR
1870 Returns the position of the first occurrence of SUBSTR in STR at or after
1871 POSITION. If POSITION is omitted, starts searching from the beginning of
1872 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1873 variable to--but don't do that). If the substring is not found, returns
1874 one less than the base, ordinarily C<-1>.
1880 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1881 You should not use this for rounding, because it truncates
1882 towards C<0>, and because machine representations of floating point
1883 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1884 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1886 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1888 Implements the ioctl(2) function. You'll probably have to say
1890 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1892 first to get the correct function definitions. If F<ioctl.ph> doesn't
1893 exist or doesn't have the correct definitions you'll have to roll your
1894 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1895 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1896 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1897 written depending on the FUNCTION--a pointer to the string value of SCALAR
1898 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1899 has no string value but does have a numeric value, that value will be
1900 passed rather than a pointer to the string value. To guarantee this to be
1901 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1902 functions are useful for manipulating the values of structures used by
1903 C<ioctl()>. The following example sets the erase character to DEL.
1907 die "NO TIOCGETP" if $@ || !$getp;
1908 $sgttyb_t = "ccccs"; # 4 chars and a short
1909 if (ioctl(STDIN,$getp,$sgttyb)) {
1910 @ary = unpack($sgttyb_t,$sgttyb);
1912 $sgttyb = pack($sgttyb_t,@ary);
1913 ioctl(STDIN,&TIOCSETP,$sgttyb)
1914 || die "Can't ioctl: $!";
1917 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1919 if OS returns: then Perl returns:
1921 0 string "0 but true"
1922 anything else that number
1924 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1925 still easily determine the actual value returned by the operating
1928 ($retval = ioctl(...)) || ($retval = -1);
1929 printf "System returned %d\n", $retval;
1931 The special string "C<0> but true" is exempt from B<-w> complaints
1932 about improper numeric conversions.
1934 =item join EXPR,LIST
1936 Joins the separate strings of LIST into a single string with
1937 fields separated by the value of EXPR, and returns the string.
1940 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1946 Returns a list consisting of all the keys of the named hash. (In a
1947 scalar context, returns the number of keys.) The keys are returned in
1948 an apparently random order. The actual random order is subject to
1949 change in future versions of perl, but it is guaranteed to be the same
1950 order as either the C<values()> or C<each()> function produces (given
1951 that the hash has not been modified). As a side effect, it resets
1954 Here is yet another way to print your environment:
1957 @values = values %ENV;
1958 while ($#keys >= 0) {
1959 print pop(@keys), '=', pop(@values), "\n";
1962 or how about sorted by key:
1964 foreach $key (sort(keys %ENV)) {
1965 print $key, '=', $ENV{$key}, "\n";
1968 To sort a hash by value, you'll need to use a C<sort()> function.
1969 Here's a descending numeric sort of a hash by its values:
1971 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1972 printf "%4d %s\n", $hash{$key}, $key;
1975 As an lvalue C<keys()> allows you to increase the number of hash buckets
1976 allocated for the given hash. This can gain you a measure of efficiency if
1977 you know the hash is going to get big. (This is similar to pre-extending
1978 an array by assigning a larger number to $#array.) If you say
1982 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1983 in fact, since it rounds up to the next power of two. These
1984 buckets will be retained even if you do C<%hash = ()>, use C<undef
1985 %hash> if you want to free the storage while C<%hash> is still in scope.
1986 You can't shrink the number of buckets allocated for the hash using
1987 C<keys()> in this way (but you needn't worry about doing this by accident,
1988 as trying has no effect).
1990 See also C<each()>, C<values()> and C<sort()>.
1994 Sends a signal to a list of processes. The first element of
1995 the list must be the signal to send. Returns the number of
1996 processes successfully signaled.
1998 $cnt = kill 1, $child1, $child2;
2001 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2002 process groups instead of processes. (On System V, a negative I<PROCESS>
2003 number will also kill process groups, but that's not portable.) That
2004 means you usually want to use positive not negative signals. You may also
2005 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2011 The C<last> command is like the C<break> statement in C (as used in
2012 loops); it immediately exits the loop in question. If the LABEL is
2013 omitted, the command refers to the innermost enclosing loop. The
2014 C<continue> block, if any, is not executed:
2016 LINE: while (<STDIN>) {
2017 last LINE if /^$/; # exit when done with header
2021 C<last> cannot be used to exit a block which returns a value such as
2022 C<eval {}>, C<sub {}> or C<do {}>.
2024 See also L</continue> for an illustration of how C<last>, C<next>, and
2031 Returns an lowercased version of EXPR. This is the internal function
2032 implementing the C<\L> escape in double-quoted strings.
2033 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2035 If EXPR is omitted, uses C<$_>.
2041 Returns the value of EXPR with the first character lowercased. This is
2042 the internal function implementing the C<\l> escape in double-quoted strings.
2043 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2045 If EXPR is omitted, uses C<$_>.
2051 Returns the length in characters of the value of EXPR. If EXPR is
2052 omitted, returns length of C<$_>.
2054 =item link OLDFILE,NEWFILE
2056 Creates a new filename linked to the old filename. Returns TRUE for
2057 success, FALSE otherwise.
2059 =item listen SOCKET,QUEUESIZE
2061 Does the same thing that the listen system call does. Returns TRUE if
2062 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2066 A local modifies the listed variables to be local to the enclosing
2067 block, file, or eval. If more than one value is listed, the list must
2068 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2069 for details, including issues with tied arrays and hashes.
2071 You really probably want to be using C<my()> instead, because C<local()> isn't
2072 what most people think of as "local". See L<perlsub/"Private Variables
2073 via my()"> for details.
2075 =item localtime EXPR
2077 Converts a time as returned by the time function to a 9-element array
2078 with the time analyzed for the local time zone. Typically used as
2082 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2085 All array elements are numeric, and come straight out of a struct tm.
2086 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2087 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2088 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2090 If EXPR is omitted, uses the current time (C<localtime(time)>).
2092 In scalar context, returns the ctime(3) value:
2094 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2096 This scalar value is B<not> locale dependent, see L<perllocale>, but
2097 instead a Perl builtin. Also see the C<Time::Local> module, and the
2098 strftime(3) and mktime(3) function available via the POSIX module. To
2099 get somewhat similar but locale dependent date strings, set up your
2100 locale environment variables appropriately (please see L<perllocale>)
2101 and try for example:
2103 use POSIX qw(strftime);
2104 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2106 Note that the C<%a> and C<%b>, the short forms of the day of the week
2107 and the month of the year, may not necessarily be three characters wide.
2113 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2116 =item lstat FILEHANDLE
2122 Does the same thing as the C<stat()> function (including setting the
2123 special C<_> filehandle) but stats a symbolic link instead of the file
2124 the symbolic link points to. If symbolic links are unimplemented on
2125 your system, a normal C<stat()> is done.
2127 If EXPR is omitted, stats C<$_>.
2131 The match operator. See L<perlop>.
2133 =item map BLOCK LIST
2137 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2138 element) and returns the list value composed of the results of each such
2139 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2140 may produce zero, one, or more elements in the returned value.
2142 @chars = map(chr, @nums);
2144 translates a list of numbers to the corresponding characters. And
2146 %hash = map { getkey($_) => $_ } @array;
2148 is just a funny way to write
2151 foreach $_ (@array) {
2152 $hash{getkey($_)} = $_;
2155 Note that, because C<$_> is a reference into the list value, it can be used
2156 to modify the elements of the array. While this is useful and
2157 supported, it can cause bizarre results if the LIST is not a named
2158 array. See also L</grep> for an array composed of those items of the
2159 original list for which the BLOCK or EXPR evaluates to true.
2161 =item mkdir FILENAME,MODE
2163 Creates the directory specified by FILENAME, with permissions
2164 specified by MODE (as modified by C<umask>). If it succeeds it
2165 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2167 In general, it is better to create directories with permissive MODEs,
2168 and let the user modify that with their C<umask>, than it is to supply
2169 a restrictive MODE and give the user no way to be more permissive.
2170 The exceptions to this rule are when the file or directory should be
2171 kept private (mail files, for instance). The perlfunc(1) entry on
2172 C<umask> discusses the choice of MODE in more detail.
2174 =item msgctl ID,CMD,ARG
2176 Calls the System V IPC function msgctl(2). You'll probably have to say
2180 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2181 then ARG must be a variable which will hold the returned C<msqid_ds>
2182 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2183 true" for zero, or the actual return value otherwise. See also
2184 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2186 =item msgget KEY,FLAGS
2188 Calls the System V IPC function msgget(2). Returns the message queue
2189 id, or the undefined value if there is an error. See also C<IPC::SysV>
2190 and C<IPC::SysV::Msg> documentation.
2192 =item msgsnd ID,MSG,FLAGS
2194 Calls the System V IPC function msgsnd to send the message MSG to the
2195 message queue ID. MSG must begin with the long integer message type,
2196 which may be created with C<pack("l", $type)>. Returns TRUE if
2197 successful, or FALSE if there is an error. See also C<IPC::SysV>
2198 and C<IPC::SysV::Msg> documentation.
2200 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2202 Calls the System V IPC function msgrcv to receive a message from
2203 message queue ID into variable VAR with a maximum message size of
2204 SIZE. Note that if a message is received, the message type will be
2205 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2206 size of the message type. Returns TRUE if successful, or FALSE if
2207 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2211 A C<my()> declares the listed variables to be local (lexically) to the
2212 enclosing block, file, or C<eval()>. If
2213 more than one value is listed, the list must be placed in parentheses. See
2214 L<perlsub/"Private Variables via my()"> for details.
2220 The C<next> command is like the C<continue> statement in C; it starts
2221 the next iteration of the loop:
2223 LINE: while (<STDIN>) {
2224 next LINE if /^#/; # discard comments
2228 Note that if there were a C<continue> block on the above, it would get
2229 executed even on discarded lines. If the LABEL is omitted, the command
2230 refers to the innermost enclosing loop.
2232 C<next> cannot be used to exit a block which returns a value such as
2233 C<eval {}>, C<sub {}> or C<do {}>.
2235 See also L</continue> for an illustration of how C<last>, C<next>, and
2238 =item no Module LIST
2240 See the L</use> function, which C<no> is the opposite of.
2246 Interprets EXPR as an octal string and returns the corresponding
2247 value. (If EXPR happens to start off with C<0x>, interprets it as a
2248 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2249 binary string.) The following will handle decimal, binary, octal, and
2250 hex in the standard Perl or C notation:
2252 $val = oct($val) if $val =~ /^0/;
2254 If EXPR is omitted, uses C<$_>. This function is commonly used when
2255 a string such as C<644> needs to be converted into a file mode, for
2256 example. (Although perl will automatically convert strings into
2257 numbers as needed, this automatic conversion assumes base 10.)
2259 =item open FILEHANDLE,EXPR
2261 =item open FILEHANDLE
2263 Opens the file whose filename is given by EXPR, and associates it with
2264 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2265 name of the real filehandle wanted. If EXPR is omitted, the scalar
2266 variable of the same name as the FILEHANDLE contains the filename.
2267 (Note that lexical variables--those declared with C<my()>--will not work
2268 for this purpose; so if you're using C<my()>, specify EXPR in your call
2271 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2272 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2273 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2274 the file is opened for appending, again being created if necessary.
2275 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2276 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2277 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2278 file first. You can't usually use either read-write mode for updating
2279 textfiles, since they have variable length records. See the B<-i>
2280 switch in L<perlrun> for a better approach. The file is created with
2281 permissions of C<0666> modified by the process' C<umask> value.
2283 The prefix and the filename may be separated with spaces.
2284 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2285 C<'w+'>, C<'a'>, and C<'a+'>.
2287 If the filename begins with C<'|'>, the filename is interpreted as a
2288 command to which output is to be piped, and if the filename ends with a
2289 C<'|'>, the filename is interpreted as a command which pipes output to
2290 us. See L<perlipc/"Using open() for IPC">
2291 for more examples of this. (You are not allowed to C<open()> to a command
2292 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2293 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2295 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2296 nonzero upon success, the undefined value otherwise. If the C<open()>
2297 involved a pipe, the return value happens to be the pid of the
2300 If you're unfortunate enough to be running Perl on a system that
2301 distinguishes between text files and binary files (modern operating
2302 systems don't care), then you should check out L</binmode> for tips for
2303 dealing with this. The key distinction between systems that need C<binmode()>
2304 and those that don't is their text file formats. Systems like Unix, MacOS, and
2305 Plan9, which delimit lines with a single character, and which encode that
2306 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2308 When opening a file, it's usually a bad idea to continue normal execution
2309 if the request failed, so C<open()> is frequently used in connection with
2310 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2311 where you want to make a nicely formatted error message (but there are
2312 modules that can help with that problem)) you should always check
2313 the return value from opening a file. The infrequent exception is when
2314 working with an unopened filehandle is actually what you want to do.
2319 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2320 while (<ARTICLE>) {...
2322 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2323 # if the open fails, output is discarded
2325 open(DBASE, '+<dbase.mine') # open for update
2326 or die "Can't open 'dbase.mine' for update: $!";
2328 open(ARTICLE, "caesar <$article |") # decrypt article
2329 or die "Can't start caesar: $!";
2331 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2332 or die "Can't start sort: $!";
2334 # process argument list of files along with any includes
2336 foreach $file (@ARGV) {
2337 process($file, 'fh00');
2341 my($filename, $input) = @_;
2342 $input++; # this is a string increment
2343 unless (open($input, $filename)) {
2344 print STDERR "Can't open $filename: $!\n";
2349 while (<$input>) { # note use of indirection
2350 if (/^#include "(.*)"/) {
2351 process($1, $input);
2358 You may also, in the Bourne shell tradition, specify an EXPR beginning
2359 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2360 name of a filehandle (or file descriptor, if numeric) to be
2361 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2362 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2363 mode you specify should match the mode of the original filehandle.
2364 (Duping a filehandle does not take into account any existing contents of
2366 Here is a script that saves, redirects, and restores STDOUT and
2370 open(OLDOUT, ">&STDOUT");
2371 open(OLDERR, ">&STDERR");
2373 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2374 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2376 select(STDERR); $| = 1; # make unbuffered
2377 select(STDOUT); $| = 1; # make unbuffered
2379 print STDOUT "stdout 1\n"; # this works for
2380 print STDERR "stderr 1\n"; # subprocesses too
2385 open(STDOUT, ">&OLDOUT");
2386 open(STDERR, ">&OLDERR");
2388 print STDOUT "stdout 2\n";
2389 print STDERR "stderr 2\n";
2392 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2393 equivalent of C's C<fdopen()> of that file descriptor; this is more
2394 parsimonious of file descriptors. For example:
2396 open(FILEHANDLE, "<&=$fd")
2398 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2399 there is an implicit fork done, and the return value of open is the pid
2400 of the child within the parent process, and C<0> within the child
2401 process. (Use C<defined($pid)> to determine whether the open was successful.)
2402 The filehandle behaves normally for the parent, but i/o to that
2403 filehandle is piped from/to the STDOUT/STDIN of the child process.
2404 In the child process the filehandle isn't opened--i/o happens from/to
2405 the new STDOUT or STDIN. Typically this is used like the normal
2406 piped open when you want to exercise more control over just how the
2407 pipe command gets executed, such as when you are running setuid, and
2408 don't want to have to scan shell commands for metacharacters.
2409 The following pairs are more or less equivalent:
2411 open(FOO, "|tr '[a-z]' '[A-Z]'");
2412 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2414 open(FOO, "cat -n '$file'|");
2415 open(FOO, "-|") || exec 'cat', '-n', $file;
2417 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2419 NOTE: On any operation that may do a fork, any unflushed buffers remain
2420 unflushed in both processes, which means you may need to set C<$|> to
2421 avoid duplicate output. On systems that support a close-on-exec flag on
2422 files, the flag will be set for the newly opened file descriptor as
2423 determined by the value of $^F. See L<perlvar/$^F>.
2425 Closing any piped filehandle causes the parent process to wait for the
2426 child to finish, and returns the status value in C<$?>.
2428 The filename passed to open will have leading and trailing
2429 whitespace deleted, and the normal redirection characters
2430 honored. This property, known as "magic open",
2431 can often be used to good effect. A user could specify a filename of
2432 F<"rsh cat file |">, or you could change certain filenames as needed:
2434 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2435 open(FH, $filename) or die "Can't open $filename: $!";
2437 However, to open a file with arbitrary weird characters in it, it's
2438 necessary to protect any leading and trailing whitespace:
2440 $file =~ s#^(\s)#./$1#;
2441 open(FOO, "< $file\0");
2443 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2444 should use the C<sysopen()> function, which involves no such magic. This is
2445 another way to protect your filenames from interpretation. For example:
2448 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2449 or die "sysopen $path: $!";
2450 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2451 print HANDLE "stuff $$\n");
2453 print "File contains: ", <HANDLE>;
2455 Using the constructor from the C<IO::Handle> package (or one of its
2456 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2457 filehandles that have the scope of whatever variables hold references to
2458 them, and automatically close whenever and however you leave that scope:
2462 sub read_myfile_munged {
2464 my $handle = new IO::File;
2465 open($handle, "myfile") or die "myfile: $!";
2467 or return (); # Automatically closed here.
2468 mung $first or die "mung failed"; # Or here.
2469 return $first, <$handle> if $ALL; # Or here.
2473 See L</seek> for some details about mixing reading and writing.
2475 =item opendir DIRHANDLE,EXPR
2477 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2478 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2479 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2485 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2486 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2488 =item pack TEMPLATE,LIST
2490 Takes an array or list of values and packs it into a binary structure,
2491 returning the string containing the structure. The TEMPLATE is a
2492 sequence of characters that give the order and type of values, as
2495 a A string with arbitrary binary data, will be null padded.
2496 A An ascii string, will be space padded.
2497 Z A null terminated (asciz) string, will be null padded.
2499 b A bit string (ascending bit order, like vec()).
2500 B A bit string (descending bit order).
2501 h A hex string (low nybble first).
2502 H A hex string (high nybble first).
2504 c A signed char value.
2505 C An unsigned char value. Only does bytes. See U for Unicode.
2507 s A signed short value.
2508 S An unsigned short value.
2509 (This 'short' is _exactly_ 16 bits, which may differ from
2510 what a local C compiler calls 'short'.)
2512 i A signed integer value.
2513 I An unsigned integer value.
2514 (This 'integer' is _at_least_ 32 bits wide. Its exact
2515 size depends on what a local C compiler calls 'int',
2516 and may even be larger than the 'long' described in
2519 l A signed long value.
2520 L An unsigned long value.
2521 (This 'long' is _exactly_ 32 bits, which may differ from
2522 what a local C compiler calls 'long'.)
2524 n A short in "network" (big-endian) order.
2525 N A long in "network" (big-endian) order.
2526 v A short in "VAX" (little-endian) order.
2527 V A long in "VAX" (little-endian) order.
2528 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2529 _exactly_ 32 bits, respectively.)
2531 q A signed quad (64-bit) value.
2532 Q An unsigned quad value.
2533 (Available only if your system supports 64-bit integer values
2534 _and_ if Perl has been compiled to support those.
2535 Causes a fatal error otherwise.)
2537 f A single-precision float in the native format.
2538 d A double-precision float in the native format.
2540 p A pointer to a null-terminated string.
2541 P A pointer to a structure (fixed-length string).
2543 u A uuencoded string.
2544 U A Unicode character number. Encodes to UTF-8 internally.
2545 Works even if C<use utf8> is not in effect.
2547 w A BER compressed integer. Its bytes represent an unsigned
2548 integer in base 128, most significant digit first, with as
2549 few digits as possible. Bit eight (the high bit) is set
2550 on each byte except the last.
2554 @ Null fill to absolute position.
2556 The following rules apply:
2562 Each letter may optionally be followed by a number giving a repeat
2563 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2564 C<"H">, and C<"P"> the pack function will gobble up that many values from
2565 the LIST. A C<*> for the repeat count means to use however many items are
2570 The C<"a">, C<"A"> and C<"Z"> types gobble just one value, but pack it as a
2571 string of length count, padding with nulls or spaces as necessary. When
2572 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2573 after the first null, and C<"a"> returns data verbatim.
2577 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2581 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2585 The C<"p"> type packs a pointer to a null-terminated string. You are
2586 responsible for ensuring the string is not a temporary value (which can
2587 potentially get deallocated before you get around to using the packed result).
2588 The C<"P"> type packs a pointer to a structure of the size indicated by the
2589 length. A NULL pointer is created if the corresponding value for C<"p"> or
2594 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2595 immediately followed by a C<"_"> to signify native shorts or longs--as
2596 you can see from above for example a bare C<"l"> does mean exactly 32
2597 bits, the native C<long> (as seen by the local C compiler) may be
2598 larger. This is an issue mainly in 64-bit platforms. You can see
2599 whether using C<"_"> makes any difference by
2601 print length(pack("s")), " ", length(pack("s_")), "\n";
2602 print length(pack("l")), " ", length(pack("l_")), "\n";
2604 C<"i_"> and C<"I_"> also work but only because of completeness;
2605 they are identical to C<"i"> and C<"I">.
2607 The actual sizes (in bytes) of native shorts, ints, and longs on
2608 the platform where Perl was built are available via L<Config>:
2611 print $Config{shortsize}, "\n";
2612 print $Config{intsize}, "\n";
2613 print $Config{longsize}, "\n";
2617 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2618 are inherently non-portable between processors and operating systems
2619 because they obey the native byteorder and endianness. For example a
2620 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
2621 (arranged in and handled by the CPU registers) into bytes as
2623 0x12 0x34 0x56 0x78 # big-endian
2624 0x78 0x56 0x34 0x12 # little-endian
2626 The names `big-endian' and `little-endian' are joking references to
2627 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2628 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2629 the egg-eating habits of the lilliputs.
2631 Some systems may even have weird byte orders such as
2636 You can see your system's preference with
2638 print join(" ", map { sprintf "%#02x", $_ }
2639 unpack("C*",pack("L",0x12345678))), "\n";
2641 The actual byteorder on the platform where Perl was built is available
2645 print $Config{byteorder}, "\n";
2647 If you want portable integers use the formats C<"n">, C<"N">, C<"v">, and
2648 "V", their byte endianness and size is known.
2652 Real numbers (floats and doubles) are in the native machine format only;
2653 due to the multiplicity of floating formats around, and the lack of a
2654 standard "network" representation, no facility for interchange has been
2655 made. This means that packed floating point data written on one machine
2656 may not be readable on another - even if both use IEEE floating point
2657 arithmetic (as the endian-ness of the memory representation is not part
2660 Note that Perl uses doubles internally for all numeric calculation, and
2661 converting from double into float and thence back to double again will
2662 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2669 $foo = pack("CCCC",65,66,67,68);
2671 $foo = pack("C4",65,66,67,68);
2673 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2674 # same thing with Unicode circled letters
2676 $foo = pack("ccxxcc",65,66,67,68);
2679 $foo = pack("s2",1,2);
2680 # "\1\0\2\0" on little-endian
2681 # "\0\1\0\2" on big-endian
2683 $foo = pack("a4","abcd","x","y","z");
2686 $foo = pack("aaaa","abcd","x","y","z");
2689 $foo = pack("a14","abcdefg");
2690 # "abcdefg\0\0\0\0\0\0\0"
2692 $foo = pack("i9pl", gmtime);
2693 # a real struct tm (on my system anyway)
2695 $utmp_template = "Z8 Z8 Z16 L";
2696 $utmp = pack($utmp_template, @utmp1);
2697 # a struct utmp (BSDish)
2699 @utmp2 = unpack($utmp_template, $utmp);
2700 # "@utmp1" eq "@utmp2"
2703 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2706 The same template may generally also be used in unpack().
2710 =item package NAMESPACE
2712 Declares the compilation unit as being in the given namespace. The scope
2713 of the package declaration is from the declaration itself through the end of
2714 the enclosing block (the same scope as the C<local()> operator). All further
2715 unqualified dynamic identifiers will be in this namespace. A package
2716 statement affects only dynamic variables--including those you've used
2717 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2718 would be the first declaration in a file to be included by the C<require>
2719 or C<use> operator. You can switch into a package in more than one place;
2720 it merely influences which symbol table is used by the compiler for the
2721 rest of that block. You can refer to variables and filehandles in other
2722 packages by prefixing the identifier with the package name and a double
2723 colon: C<$Package::Variable>. If the package name is null, the C<main>
2724 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2726 If NAMESPACE is omitted, then there is no current package, and all
2727 identifiers must be fully qualified or lexicals. This is stricter
2728 than C<use strict>, since it also extends to function names.
2730 See L<perlmod/"Packages"> for more information about packages, modules,
2731 and classes. See L<perlsub> for other scoping issues.
2733 =item pipe READHANDLE,WRITEHANDLE
2735 Opens a pair of connected pipes like the corresponding system call.
2736 Note that if you set up a loop of piped processes, deadlock can occur
2737 unless you are very careful. In addition, note that Perl's pipes use
2738 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2739 after each command, depending on the application.
2741 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2742 for examples of such things.
2744 On systems that support a close-on-exec flag on files, the flag will be set
2745 for the newly opened file descriptors as determined by the value of $^F.
2752 Pops and returns the last value of the array, shortening the array by
2753 1. Has a similar effect to
2755 $tmp = $ARRAY[$#ARRAY--];
2757 If there are no elements in the array, returns the undefined value.
2758 If ARRAY is omitted, pops the
2759 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2766 Returns the offset of where the last C<m//g> search left off for the variable
2767 is in question (C<$_> is used when the variable is not specified). May be
2768 modified to change that offset. Such modification will also influence
2769 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2772 =item print FILEHANDLE LIST
2778 Prints a string or a comma-separated list of strings. Returns TRUE
2779 if successful. FILEHANDLE may be a scalar variable name, in which case
2780 the variable contains the name of or a reference to the filehandle, thus introducing one
2781 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2782 token is a term, it may be misinterpreted as an operator unless you
2783 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2784 omitted, prints by default to standard output (or to the last selected
2785 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2786 the currently selected output channel. To set the default output channel to something other than
2787 STDOUT use the select operation. Note that, because print takes a
2788 LIST, anything in the LIST is evaluated in list context, and any
2789 subroutine that you call will have one or more of its expressions
2790 evaluated in list context. Also be careful not to follow the print
2791 keyword with a left parenthesis unless you want the corresponding right
2792 parenthesis to terminate the arguments to the print--interpose a C<+> or
2793 put parentheses around all the arguments.
2795 Note that if you're storing FILEHANDLES in an array or other expression,
2796 you will have to use a block returning its value instead:
2798 print { $files[$i] } "stuff\n";
2799 print { $OK ? STDOUT : STDERR } "stuff\n";
2801 =item printf FILEHANDLE FORMAT, LIST
2803 =item printf FORMAT, LIST
2805 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2806 (the output record separator) is not appended. The first argument
2807 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2808 in effect, the character used for the decimal point in formatted real numbers
2809 is affected by the LC_NUMERIC locale. See L<perllocale>.
2811 Don't fall into the trap of using a C<printf()> when a simple
2812 C<print()> would do. The C<print()> is more efficient and less
2815 =item prototype FUNCTION
2817 Returns the prototype of a function as a string (or C<undef> if the
2818 function has no prototype). FUNCTION is a reference to, or the name of,
2819 the function whose prototype you want to retrieve.
2821 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2822 a name for Perl builtin. If builtin is not I<overridable> (such as
2823 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2824 C<system()>) - in other words, the builtin does not behave like a Perl
2825 function - returns C<undef>. Otherwise, the string describing the
2826 equivalent prototype is returned.
2828 =item push ARRAY,LIST
2830 Treats ARRAY as a stack, and pushes the values of LIST
2831 onto the end of ARRAY. The length of ARRAY increases by the length of
2832 LIST. Has the same effect as
2835 $ARRAY[++$#ARRAY] = $value;
2838 but is more efficient. Returns the new number of elements in the array.
2850 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2852 =item quotemeta EXPR
2856 Returns the value of EXPR with all non-alphanumeric
2857 characters backslashed. (That is, all characters not matching
2858 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2859 returned string, regardless of any locale settings.)
2860 This is the internal function implementing
2861 the C<\Q> escape in double-quoted strings.
2863 If EXPR is omitted, uses C<$_>.
2869 Returns a random fractional number greater than or equal to C<0> and less
2870 than the value of EXPR. (EXPR should be positive.) If EXPR is
2871 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2872 C<srand()> has already been called. See also C<srand()>.
2874 (Note: If your rand function consistently returns numbers that are too
2875 large or too small, then your version of Perl was probably compiled
2876 with the wrong number of RANDBITS.)
2878 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2880 =item read FILEHANDLE,SCALAR,LENGTH
2882 Attempts to read LENGTH bytes of data into variable SCALAR from the
2883 specified FILEHANDLE. Returns the number of bytes actually read,
2884 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2885 or shrunk to the length actually read. An OFFSET may be specified to
2886 place the read data at some other place than the beginning of the
2887 string. This call is actually implemented in terms of stdio's fread(3)
2888 call. To get a true read(2) system call, see C<sysread()>.
2890 =item readdir DIRHANDLE
2892 Returns the next directory entry for a directory opened by C<opendir()>.
2893 If used in list context, returns all the rest of the entries in the
2894 directory. If there are no more entries, returns an undefined value in
2895 scalar context or a null list in list context.
2897 If you're planning to filetest the return values out of a C<readdir()>, you'd
2898 better prepend the directory in question. Otherwise, because we didn't
2899 C<chdir()> there, it would have been testing the wrong file.
2901 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2902 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2907 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
2908 context, each call reads and returns the next line, until end-of-file is
2909 reached, whereupon the subsequent call returns undef. In list context,
2910 reads until end-of-file is reached and returns a list of lines. Note that
2911 the notion of "line" used here is however you may have defined it
2912 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
2914 When C<$/> is set to C<undef> and when readline() is in a scalar
2915 context (i.e. file slurp mode), it returns C<''> the first time,
2916 followed by C<undef> subsequently.
2918 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2919 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2920 operator is discussed in more detail in L<perlop/"I/O Operators">.
2923 $line = readline(*STDIN); # same thing
2929 Returns the value of a symbolic link, if symbolic links are
2930 implemented. If not, gives a fatal error. If there is some system
2931 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2932 omitted, uses C<$_>.
2936 EXPR is executed as a system command.
2937 The collected standard output of the command is returned.
2938 In scalar context, it comes back as a single (potentially
2939 multi-line) string. In list context, returns a list of lines
2940 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2941 This is the internal function implementing the C<qx/EXPR/>
2942 operator, but you can use it directly. The C<qx/EXPR/>
2943 operator is discussed in more detail in L<perlop/"I/O Operators">.
2945 =item recv SOCKET,SCALAR,LEN,FLAGS
2947 Receives a message on a socket. Attempts to receive LENGTH bytes of
2948 data into variable SCALAR from the specified SOCKET filehandle.
2949 Actually does a C C<recvfrom()>, so that it can return the address of the
2950 sender. Returns the undefined value if there's an error. SCALAR will
2951 be grown or shrunk to the length actually read. Takes the same flags
2952 as the system call of the same name.
2953 See L<perlipc/"UDP: Message Passing"> for examples.
2959 The C<redo> command restarts the loop block without evaluating the
2960 conditional again. The C<continue> block, if any, is not executed. If
2961 the LABEL is omitted, the command refers to the innermost enclosing
2962 loop. This command is normally used by programs that want to lie to
2963 themselves about what was just input:
2965 # a simpleminded Pascal comment stripper
2966 # (warning: assumes no { or } in strings)
2967 LINE: while (<STDIN>) {
2968 while (s|({.*}.*){.*}|$1 |) {}
2973 if (/}/) { # end of comment?
2982 C<redo> cannot be used to retry a block which returns a value such as
2983 C<eval {}>, C<sub {}> or C<do {}>.
2985 See also L</continue> for an illustration of how C<last>, C<next>, and
2992 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2993 is not specified, C<$_> will be used. The value returned depends on the
2994 type of thing the reference is a reference to.
2995 Builtin types include:
3004 If the referenced object has been blessed into a package, then that package
3005 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
3007 if (ref($r) eq "HASH") {
3008 print "r is a reference to a hash.\n";
3011 print "r is not a reference at all.\n";
3014 See also L<perlref>.
3016 =item rename OLDNAME,NEWNAME
3018 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
3019 not work across file system boundaries.
3025 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3026 supplied. If EXPR is numeric, demands that the current version of Perl
3027 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3029 Otherwise, demands that a library file be included if it hasn't already
3030 been included. The file is included via the do-FILE mechanism, which is
3031 essentially just a variety of C<eval()>. Has semantics similar to the following
3036 return 1 if $INC{$filename};
3037 my($realfilename,$result);
3039 foreach $prefix (@INC) {
3040 $realfilename = "$prefix/$filename";
3041 if (-f $realfilename) {
3042 $result = do $realfilename;
3046 die "Can't find $filename in \@INC";
3049 die "$filename did not return true value" unless $result;
3050 $INC{$filename} = $realfilename;
3054 Note that the file will not be included twice under the same specified
3055 name. The file must return TRUE as the last statement to indicate
3056 successful execution of any initialization code, so it's customary to
3057 end such a file with "C<1;>" unless you're sure it'll return TRUE
3058 otherwise. But it's better just to put the "C<1;>", in case you add more
3061 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3062 replaces "F<::>" with "F</>" in the filename for you,
3063 to make it easy to load standard modules. This form of loading of
3064 modules does not risk altering your namespace.
3066 In other words, if you try this:
3068 require Foo::Bar; # a splendid bareword
3070 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3071 directories specified in the C<@INC> array.
3073 But if you try this:
3075 $class = 'Foo::Bar';
3076 require $class; # $class is not a bareword
3078 require "Foo::Bar"; # not a bareword because of the ""
3080 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3081 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3083 eval "require $class";
3085 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3091 Generally used in a C<continue> block at the end of a loop to clear
3092 variables and reset C<??> searches so that they work again. The
3093 expression is interpreted as a list of single characters (hyphens
3094 allowed for ranges). All variables and arrays beginning with one of
3095 those letters are reset to their pristine state. If the expression is
3096 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3097 only variables or searches in the current package. Always returns
3100 reset 'X'; # reset all X variables
3101 reset 'a-z'; # reset lower case variables
3102 reset; # just reset ?? searches
3104 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3105 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
3106 are unaffected, but they clean themselves up on scope exit anyway,
3107 so you'll probably want to use them instead. See L</my>.
3113 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3114 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3115 context, depending on how the return value will be used, and the context
3116 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3117 is given, returns an empty list in list context, an undefined value in
3118 scalar context, or nothing in a void context.
3120 (Note that in the absence of a return, a subroutine, eval, or do FILE
3121 will automatically return the value of the last expression evaluated.)
3125 In list context, returns a list value consisting of the elements
3126 of LIST in the opposite order. In scalar context, concatenates the
3127 elements of LIST, and returns a string value with all the characters
3128 in the opposite order.
3130 print reverse <>; # line tac, last line first
3132 undef $/; # for efficiency of <>
3133 print scalar reverse <>; # character tac, last line tsrif
3135 This operator is also handy for inverting a hash, although there are some
3136 caveats. If a value is duplicated in the original hash, only one of those
3137 can be represented as a key in the inverted hash. Also, this has to
3138 unwind one hash and build a whole new one, which may take some time
3141 %by_name = reverse %by_address; # Invert the hash
3143 =item rewinddir DIRHANDLE
3145 Sets the current position to the beginning of the directory for the
3146 C<readdir()> routine on DIRHANDLE.
3148 =item rindex STR,SUBSTR,POSITION
3150 =item rindex STR,SUBSTR
3152 Works just like index except that it returns the position of the LAST
3153 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3154 last occurrence at or before that position.
3156 =item rmdir FILENAME
3160 Deletes the directory specified by FILENAME if that directory is empty. If it
3161 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3162 FILENAME is omitted, uses C<$_>.
3166 The substitution operator. See L<perlop>.
3170 Forces EXPR to be interpreted in scalar context and returns the value
3173 @counts = ( scalar @a, scalar @b, scalar @c );
3175 There is no equivalent operator to force an expression to
3176 be interpolated in list context because it's in practice never
3177 needed. If you really wanted to do so, however, you could use
3178 the construction C<@{[ (some expression) ]}>, but usually a simple
3179 C<(some expression)> suffices.
3181 Though C<scalar> can be considered in general to be a unary operator,
3182 EXPR is also allowed to be a parenthesized list. The list in fact
3183 behaves as a scalar comma expression, evaluating all but the last
3184 element in void context and returning the final element evaluated in
3187 The following single statement:
3189 print uc(scalar(&foo,$bar)),$baz;
3191 is the moral equivalent of these two:
3194 print(uc($bar),$baz);
3196 See L<perlop> for more details on unary operators and the comma operator.
3198 =item seek FILEHANDLE,POSITION,WHENCE
3200 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3201 FILEHANDLE may be an expression whose value gives the name of the
3202 filehandle. The values for WHENCE are C<0> to set the new position to
3203 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3204 set it to EOF plus POSITION (typically negative). For WHENCE you may
3205 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3206 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3208 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3209 C<seek()> -- buffering makes its effect on the file's system position
3210 unpredictable and non-portable. Use C<sysseek()> instead.
3212 On some systems you have to do a seek whenever you switch between reading
3213 and writing. Amongst other things, this may have the effect of calling
3214 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3219 This is also useful for applications emulating C<tail -f>. Once you hit
3220 EOF on your read, and then sleep for a while, you might have to stick in a
3221 seek() to reset things. The C<seek()> doesn't change the current position,
3222 but it I<does> clear the end-of-file condition on the handle, so that the
3223 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3225 If that doesn't work (some stdios are particularly cantankerous), then
3226 you may need something more like this:
3229 for ($curpos = tell(FILE); $_ = <FILE>;
3230 $curpos = tell(FILE)) {
3231 # search for some stuff and put it into files
3233 sleep($for_a_while);
3234 seek(FILE, $curpos, 0);
3237 =item seekdir DIRHANDLE,POS
3239 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3240 must be a value returned by C<telldir()>. Has the same caveats about
3241 possible directory compaction as the corresponding system library
3244 =item select FILEHANDLE
3248 Returns the currently selected filehandle. Sets the current default
3249 filehandle for output, if FILEHANDLE is supplied. This has two
3250 effects: first, a C<write()> or a C<print()> without a filehandle will
3251 default to this FILEHANDLE. Second, references to variables related to
3252 output will refer to this output channel. For example, if you have to
3253 set the top of form format for more than one output channel, you might
3261 FILEHANDLE may be an expression whose value gives the name of the
3262 actual filehandle. Thus:
3264 $oldfh = select(STDERR); $| = 1; select($oldfh);
3266 Some programmers may prefer to think of filehandles as objects with
3267 methods, preferring to write the last example as:
3270 STDERR->autoflush(1);
3272 =item select RBITS,WBITS,EBITS,TIMEOUT
3274 This calls the select(2) system call with the bit masks specified, which
3275 can be constructed using C<fileno()> and C<vec()>, along these lines:
3277 $rin = $win = $ein = '';
3278 vec($rin,fileno(STDIN),1) = 1;
3279 vec($win,fileno(STDOUT),1) = 1;
3282 If you want to select on many filehandles you might wish to write a
3286 my(@fhlist) = split(' ',$_[0]);
3289 vec($bits,fileno($_),1) = 1;
3293 $rin = fhbits('STDIN TTY SOCK');
3297 ($nfound,$timeleft) =
3298 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3300 or to block until something becomes ready just do this
3302 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3304 Most systems do not bother to return anything useful in C<$timeleft>, so
3305 calling select() in scalar context just returns C<$nfound>.
3307 Any of the bit masks can also be undef. The timeout, if specified, is
3308 in seconds, which may be fractional. Note: not all implementations are
3309 capable of returning theC<$timeleft>. If not, they always return
3310 C<$timeleft> equal to the supplied C<$timeout>.
3312 You can effect a sleep of 250 milliseconds this way:
3314 select(undef, undef, undef, 0.25);
3316 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3317 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3318 then only on POSIX systems. You have to use C<sysread()> instead.
3320 =item semctl ID,SEMNUM,CMD,ARG
3322 Calls the System V IPC function C<semctl()>. You'll probably have to say
3326 first to get the correct constant definitions. If CMD is IPC_STAT or
3327 GETALL, then ARG must be a variable which will hold the returned
3328 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3329 undefined value for error, "C<0> but true" for zero, or the actual return
3330 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3332 =item semget KEY,NSEMS,FLAGS
3334 Calls the System V IPC function semget. Returns the semaphore id, or
3335 the undefined value if there is an error. See also C<IPC::SysV> and
3336 C<IPC::SysV::Semaphore> documentation.
3338 =item semop KEY,OPSTRING
3340 Calls the System V IPC function semop to perform semaphore operations
3341 such as signaling and waiting. OPSTRING must be a packed array of
3342 semop structures. Each semop structure can be generated with
3343 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3344 operations is implied by the length of OPSTRING. Returns TRUE if
3345 successful, or FALSE if there is an error. As an example, the
3346 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3348 $semop = pack("sss", $semnum, -1, 0);
3349 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3351 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3352 and C<IPC::SysV::Semaphore> documentation.
3354 =item send SOCKET,MSG,FLAGS,TO
3356 =item send SOCKET,MSG,FLAGS
3358 Sends a message on a socket. Takes the same flags as the system call
3359 of the same name. On unconnected sockets you must specify a
3360 destination to send TO, in which case it does a C C<sendto()>. Returns
3361 the number of characters sent, or the undefined value if there is an
3363 See L<perlipc/"UDP: Message Passing"> for examples.
3365 =item setpgrp PID,PGRP
3367 Sets the current process group for the specified PID, C<0> for the current
3368 process. Will produce a fatal error if used on a machine that doesn't
3369 implement setpgrp(2). If the arguments are omitted, it defaults to
3370 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3371 arguments, so only setpgrp C<0,0> is portable.
3373 =item setpriority WHICH,WHO,PRIORITY
3375 Sets the current priority for a process, a process group, or a user.
3376 (See setpriority(2).) Will produce a fatal error if used on a machine
3377 that doesn't implement setpriority(2).
3379 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3381 Sets the socket option requested. Returns undefined if there is an
3382 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3389 Shifts the first value of the array off and returns it, shortening the
3390 array by 1 and moving everything down. If there are no elements in the
3391 array, returns the undefined value. If ARRAY is omitted, shifts the
3392 C<@_> array within the lexical scope of subroutines and formats, and the
3393 C<@ARGV> array at file scopes or within the lexical scopes established by
3394 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3395 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3396 same thing to the left end of an array that C<pop()> and C<push()> do to the
3399 =item shmctl ID,CMD,ARG
3401 Calls the System V IPC function shmctl. You'll probably have to say
3405 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3406 then ARG must be a variable which will hold the returned C<shmid_ds>
3407 structure. Returns like ioctl: the undefined value for error, "C<0> but
3408 true" for zero, or the actual return value otherwise.
3409 See also C<IPC::SysV> documentation.
3411 =item shmget KEY,SIZE,FLAGS
3413 Calls the System V IPC function shmget. Returns the shared memory
3414 segment id, or the undefined value if there is an error.
3415 See also C<IPC::SysV> documentation.
3417 =item shmread ID,VAR,POS,SIZE
3419 =item shmwrite ID,STRING,POS,SIZE
3421 Reads or writes the System V shared memory segment ID starting at
3422 position POS for size SIZE by attaching to it, copying in/out, and
3423 detaching from it. When reading, VAR must be a variable that will
3424 hold the data read. When writing, if STRING is too long, only SIZE
3425 bytes are used; if STRING is too short, nulls are written to fill out
3426 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3427 See also C<IPC::SysV> documentation.
3429 =item shutdown SOCKET,HOW
3431 Shuts down a socket connection in the manner indicated by HOW, which
3432 has the same interpretation as in the system call of the same name.
3434 shutdown(SOCKET, 0); # I/we have stopped reading data
3435 shutdown(SOCKET, 1); # I/we have stopped writing data
3436 shutdown(SOCKET, 2); # I/we have stopped using this socket
3438 This is useful with sockets when you want to tell the other
3439 side you're done writing but not done reading, or vice versa.
3440 It's also a more insistent form of close because it also
3441 disables the filedescriptor in any forked copies in other
3448 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3449 returns sine of C<$_>.
3451 For the inverse sine operation, you may use the C<POSIX::asin()>
3452 function, or use this relation:
3454 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3460 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3461 May be interrupted if the process receives a signal such as C<SIGALRM>.
3462 Returns the number of seconds actually slept. You probably cannot
3463 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3466 On some older systems, it may sleep up to a full second less than what
3467 you requested, depending on how it counts seconds. Most modern systems
3468 always sleep the full amount. They may appear to sleep longer than that,
3469 however, because your process might not be scheduled right away in a
3470 busy multitasking system.
3472 For delays of finer granularity than one second, you may use Perl's
3473 C<syscall()> interface to access setitimer(2) if your system supports it,
3474 or else see L</select> above.
3476 See also the POSIX module's C<sigpause()> function.
3478 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3480 Opens a socket of the specified kind and attaches it to filehandle
3481 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3482 system call of the same name. You should "C<use Socket;>" first to get
3483 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3485 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3487 Creates an unnamed pair of sockets in the specified domain, of the
3488 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3489 for the system call of the same name. If unimplemented, yields a fatal
3490 error. Returns TRUE if successful.
3492 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3493 to C<pipe(Rdr, Wtr)> is essentially:
3496 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3497 shutdown(Rdr, 1); # no more writing for reader
3498 shutdown(Wtr, 0); # no more reading for writer
3500 See L<perlipc> for an example of socketpair use.
3502 =item sort SUBNAME LIST
3504 =item sort BLOCK LIST
3508 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3509 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3510 specified, it gives the name of a subroutine that returns an integer
3511 less than, equal to, or greater than C<0>, depending on how the elements
3512 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3513 operators are extremely useful in such routines.) SUBNAME may be a
3514 scalar variable name (unsubscripted), in which case the value provides
3515 the name of (or a reference to) the actual subroutine to use. In place
3516 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3519 In the interests of efficiency the normal calling code for subroutines is
3520 bypassed, with the following effects: the subroutine may not be a
3521 recursive subroutine, and the two elements to be compared are passed into
3522 the subroutine not via C<@_> but as the package global variables C<$a> and
3523 C<$b> (see example below). They are passed by reference, so don't
3524 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3526 You also cannot exit out of the sort block or subroutine using any of the
3527 loop control operators described in L<perlsyn> or with C<goto()>.
3529 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3530 current collation locale. See L<perllocale>.
3535 @articles = sort @files;
3537 # same thing, but with explicit sort routine
3538 @articles = sort {$a cmp $b} @files;
3540 # now case-insensitively
3541 @articles = sort {uc($a) cmp uc($b)} @files;
3543 # same thing in reversed order
3544 @articles = sort {$b cmp $a} @files;
3546 # sort numerically ascending
3547 @articles = sort {$a <=> $b} @files;
3549 # sort numerically descending
3550 @articles = sort {$b <=> $a} @files;
3552 # sort using explicit subroutine name
3554 $age{$a} <=> $age{$b}; # presuming numeric
3556 @sortedclass = sort byage @class;
3558 # this sorts the %age hash by value instead of key
3559 # using an in-line function
3560 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3562 sub backwards { $b cmp $a; }
3563 @harry = ('dog','cat','x','Cain','Abel');
3564 @george = ('gone','chased','yz','Punished','Axed');
3566 # prints AbelCaincatdogx
3567 print sort backwards @harry;
3568 # prints xdogcatCainAbel
3569 print sort @george, 'to', @harry;
3570 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3572 # inefficiently sort by descending numeric compare using
3573 # the first integer after the first = sign, or the
3574 # whole record case-insensitively otherwise
3577 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3582 # same thing, but much more efficiently;
3583 # we'll build auxiliary indices instead
3587 push @nums, /=(\d+)/;
3592 $nums[$b] <=> $nums[$a]
3594 $caps[$a] cmp $caps[$b]
3598 # same thing using a Schwartzian Transform (no temps)
3599 @new = map { $_->[0] }
3600 sort { $b->[1] <=> $a->[1]
3603 } map { [$_, /=(\d+)/, uc($_)] } @old;
3605 If you're using strict, you I<MUST NOT> declare C<$a>
3606 and C<$b> as lexicals. They are package globals. That means
3607 if you're in the C<main> package, it's
3609 @articles = sort {$main::b <=> $main::a} @files;
3613 @articles = sort {$::b <=> $::a} @files;
3615 but if you're in the C<FooPack> package, it's
3617 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3619 The comparison function is required to behave. If it returns
3620 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3621 sometimes saying the opposite, for example) the results are not
3624 =item splice ARRAY,OFFSET,LENGTH,LIST
3626 =item splice ARRAY,OFFSET,LENGTH
3628 =item splice ARRAY,OFFSET
3630 Removes the elements designated by OFFSET and LENGTH from an array, and
3631 replaces them with the elements of LIST, if any. In list context,
3632 returns the elements removed from the array. In scalar context,
3633 returns the last element removed, or C<undef> if no elements are
3634 removed. The array grows or shrinks as necessary.
3635 If OFFSET is negative then it start that far from the end of the array.
3636 If LENGTH is omitted, removes everything from OFFSET onward.
3637 If LENGTH is negative, leave that many elements off the end of the array.
3638 The following equivalences hold (assuming C<$[ == 0>):
3640 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3641 pop(@a) splice(@a,-1)
3642 shift(@a) splice(@a,0,1)
3643 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3644 $a[$x] = $y splice(@a,$x,1,$y)
3646 Example, assuming array lengths are passed before arrays:
3648 sub aeq { # compare two list values
3649 my(@a) = splice(@_,0,shift);
3650 my(@b) = splice(@_,0,shift);
3651 return 0 unless @a == @b; # same len?
3653 return 0 if pop(@a) ne pop(@b);
3657 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3659 =item split /PATTERN/,EXPR,LIMIT
3661 =item split /PATTERN/,EXPR
3663 =item split /PATTERN/
3667 Splits a string into an array of strings, and returns it. By default,
3668 empty leading fields are preserved, and empty trailing ones are deleted.
3670 If not in list context, returns the number of fields found and splits into
3671 the C<@_> array. (In list context, you can force the split into C<@_> by
3672 using C<??> as the pattern delimiters, but it still returns the list
3673 value.) The use of implicit split to C<@_> is deprecated, however, because
3674 it clobbers your subroutine arguments.
3676 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3677 splits on whitespace (after skipping any leading whitespace). Anything
3678 matching PATTERN is taken to be a delimiter separating the fields. (Note
3679 that the delimiter may be longer than one character.)
3681 If LIMIT is specified and positive, splits into no more than that
3682 many fields (though it may split into fewer). If LIMIT is unspecified
3683 or zero, trailing null fields are stripped (which potential users
3684 of C<pop()> would do well to remember). If LIMIT is negative, it is
3685 treated as if an arbitrarily large LIMIT had been specified.
3687 A pattern matching the null string (not to be confused with
3688 a null pattern C<//>, which is just one member of the set of patterns
3689 matching a null string) will split the value of EXPR into separate
3690 characters at each point it matches that way. For example:
3692 print join(':', split(/ */, 'hi there'));
3694 produces the output 'h:i:t:h:e:r:e'.
3696 The LIMIT parameter can be used to split a line partially
3698 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3700 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3701 one larger than the number of variables in the list, to avoid
3702 unnecessary work. For the list above LIMIT would have been 4 by
3703 default. In time critical applications it behooves you not to split
3704 into more fields than you really need.
3706 If the PATTERN contains parentheses, additional array elements are
3707 created from each matching substring in the delimiter.
3709 split(/([,-])/, "1-10,20", 3);
3711 produces the list value
3713 (1, '-', 10, ',', 20)
3715 If you had the entire header of a normal Unix email message in C<$header>,
3716 you could split it up into fields and their values this way:
3718 $header =~ s/\n\s+/ /g; # fix continuation lines
3719 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3721 The pattern C</PATTERN/> may be replaced with an expression to specify
3722 patterns that vary at runtime. (To do runtime compilation only once,
3723 use C</$variable/o>.)
3725 As a special case, specifying a PATTERN of space (C<' '>) will split on
3726 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3727 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3728 will give you as many null initial fields as there are leading spaces.
3729 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3730 whitespace produces a null first field. A C<split()> with no arguments
3731 really does a C<split(' ', $_)> internally.
3735 open(PASSWD, '/etc/passwd');
3737 ($login, $passwd, $uid, $gid,
3738 $gcos, $home, $shell) = split(/:/);
3742 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3743 L</chomp>, and L</join>.)
3745 =item sprintf FORMAT, LIST
3747 Returns a string formatted by the usual C<printf()> conventions of the
3748 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3749 on your system for an explanation of the general principles.
3751 Perl does its own C<sprintf()> formatting -- it emulates the C
3752 function C<sprintf()>, but it doesn't use it (except for floating-point
3753 numbers, and even then only the standard modifiers are allowed). As a
3754 result, any non-standard extensions in your local C<sprintf()> are not
3755 available from Perl.
3757 Perl's C<sprintf()> permits the following universally-known conversions:
3760 %c a character with the given number
3762 %d a signed integer, in decimal
3763 %u an unsigned integer, in decimal
3764 %o an unsigned integer, in octal
3765 %x an unsigned integer, in hexadecimal
3766 %e a floating-point number, in scientific notation
3767 %f a floating-point number, in fixed decimal notation
3768 %g a floating-point number, in %e or %f notation
3770 In addition, Perl permits the following widely-supported conversions:
3772 %X like %x, but using upper-case letters
3773 %E like %e, but using an upper-case "E"
3774 %G like %g, but with an upper-case "E" (if applicable)
3775 %b an unsigned integer, in binary
3776 %p a pointer (outputs the Perl value's address in hexadecimal)
3777 %n special: *stores* the number of characters output so far
3778 into the next variable in the parameter list
3780 Finally, for backward (and we do mean "backward") compatibility, Perl
3781 permits these unnecessary but widely-supported conversions:
3784 %D a synonym for %ld
3785 %U a synonym for %lu
3786 %O a synonym for %lo
3789 Perl permits the following universally-known flags between the C<%>
3790 and the conversion letter:
3792 space prefix positive number with a space
3793 + prefix positive number with a plus sign
3794 - left-justify within the field
3795 0 use zeros, not spaces, to right-justify
3796 # prefix non-zero octal with "0", non-zero hex with "0x"
3797 number minimum field width
3798 .number "precision": digits after decimal point for
3799 floating-point, max length for string, minimum length
3801 l interpret integer as C type "long" or "unsigned long"
3802 h interpret integer as C type "short" or "unsigned short"
3804 There is also one Perl-specific flag:
3806 V interpret integer as Perl's standard integer type
3808 Where a number would appear in the flags, an asterisk ("C<*>") may be
3809 used instead, in which case Perl uses the next item in the parameter
3810 list as the given number (that is, as the field width or precision).
3811 If a field width obtained through "C<*>" is negative, it has the same
3812 effect as the "C<->" flag: left-justification.
3814 If C<use locale> is in effect, the character used for the decimal
3815 point in formatted real numbers is affected by the LC_NUMERIC locale.
3822 Return the square root of EXPR. If EXPR is omitted, returns square
3829 Sets the random number seed for the C<rand()> operator. If EXPR is
3830 omitted, uses a semi-random value supplied by the kernel (if it supports
3831 the F</dev/urandom> device) or based on the current time and process
3832 ID, among other things. In versions of Perl prior to 5.004 the default
3833 seed was just the current C<time()>. This isn't a particularly good seed,
3834 so many old programs supply their own seed value (often C<time ^ $$> or
3835 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3837 In fact, it's usually not necessary to call C<srand()> at all, because if
3838 it is not called explicitly, it is called implicitly at the first use of
3839 the C<rand()> operator. However, this was not the case in version of Perl
3840 before 5.004, so if your script will run under older Perl versions, it
3841 should call C<srand()>.
3843 Note that you need something much more random than the default seed for
3844 cryptographic purposes. Checksumming the compressed output of one or more
3845 rapidly changing operating system status programs is the usual method. For
3848 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3850 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3853 Do I<not> call C<srand()> multiple times in your program unless you know
3854 exactly what you're doing and why you're doing it. The point of the
3855 function is to "seed" the C<rand()> function so that C<rand()> can produce
3856 a different sequence each time you run your program. Just do it once at the
3857 top of your program, or you I<won't> get random numbers out of C<rand()>!
3859 Frequently called programs (like CGI scripts) that simply use
3863 for a seed can fall prey to the mathematical property that
3867 one-third of the time. So don't do that.
3869 =item stat FILEHANDLE
3875 Returns a 13-element list giving the status info for a file, either
3876 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3877 it stats C<$_>. Returns a null list if the stat fails. Typically used
3880 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3881 $atime,$mtime,$ctime,$blksize,$blocks)
3884 Not all fields are supported on all filesystem types. Here are the
3885 meaning of the fields:
3887 0 dev device number of filesystem
3889 2 mode file mode (type and permissions)
3890 3 nlink number of (hard) links to the file
3891 4 uid numeric user ID of file's owner
3892 5 gid numeric group ID of file's owner
3893 6 rdev the device identifier (special files only)
3894 7 size total size of file, in bytes
3895 8 atime last access time since the epoch
3896 9 mtime last modify time since the epoch
3897 10 ctime inode change time (NOT creation time!) since the epoch
3898 11 blksize preferred block size for file system I/O
3899 12 blocks actual number of blocks allocated
3901 (The epoch was at 00:00 January 1, 1970 GMT.)
3903 If stat is passed the special filehandle consisting of an underline, no
3904 stat is done, but the current contents of the stat structure from the
3905 last stat or filetest are returned. Example:
3907 if (-x $file && (($d) = stat(_)) && $d < 0) {
3908 print "$file is executable NFS file\n";
3911 (This works on machines only for which the device number is negative under NFS.)
3913 In scalar context, C<stat()> returns a boolean value indicating success
3914 or failure, and, if successful, sets the information associated with
3915 the special filehandle C<_>.
3921 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3922 doing many pattern matches on the string before it is next modified.
3923 This may or may not save time, depending on the nature and number of
3924 patterns you are searching on, and on the distribution of character
3925 frequencies in the string to be searched -- you probably want to compare
3926 run times with and without it to see which runs faster. Those loops
3927 which scan for many short constant strings (including the constant
3928 parts of more complex patterns) will benefit most. You may have only
3929 one C<study()> active at a time -- if you study a different scalar the first
3930 is "unstudied". (The way C<study()> works is this: a linked list of every
3931 character in the string to be searched is made, so we know, for
3932 example, where all the C<'k'> characters are. From each search string,
3933 the rarest character is selected, based on some static frequency tables
3934 constructed from some C programs and English text. Only those places
3935 that contain this "rarest" character are examined.)
3937 For example, here is a loop that inserts index producing entries
3938 before any line containing a certain pattern:
3942 print ".IX foo\n" if /\bfoo\b/;
3943 print ".IX bar\n" if /\bbar\b/;
3944 print ".IX blurfl\n" if /\bblurfl\b/;
3949 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3950 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3951 a big win except in pathological cases. The only question is whether
3952 it saves you more time than it took to build the linked list in the
3955 Note that if you have to look for strings that you don't know till
3956 runtime, you can build an entire loop as a string and C<eval()> that to
3957 avoid recompiling all your patterns all the time. Together with
3958 undefining C<$/> to input entire files as one record, this can be very
3959 fast, often faster than specialized programs like fgrep(1). The following
3960 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3961 out the names of those files that contain a match:
3963 $search = 'while (<>) { study;';
3964 foreach $word (@words) {
3965 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3970 eval $search; # this screams
3971 $/ = "\n"; # put back to normal input delimiter
3972 foreach $file (sort keys(%seen)) {
3980 =item sub NAME BLOCK
3982 This is subroutine definition, not a real function I<per se>. With just a
3983 NAME (and possibly prototypes), it's just a forward declaration. Without
3984 a NAME, it's an anonymous function declaration, and does actually return a
3985 value: the CODE ref of the closure you just created. See L<perlsub> and
3986 L<perlref> for details.
3988 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3990 =item substr EXPR,OFFSET,LEN
3992 =item substr EXPR,OFFSET
3994 Extracts a substring out of EXPR and returns it. First character is at
3995 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3996 If OFFSET is negative (or more precisely, less than C<$[>), starts
3997 that far from the end of the string. If LEN is omitted, returns
3998 everything to the end of the string. If LEN is negative, leaves that
3999 many characters off the end of the string.
4001 If you specify a substring that is partly outside the string, the part
4002 within the string is returned. If the substring is totally outside
4003 the string a warning is produced.
4005 You can use the C<substr()> function
4006 as an lvalue, in which case EXPR must be an lvalue. If you assign
4007 something shorter than LEN, the string will shrink, and if you assign
4008 something longer than LEN, the string will grow to accommodate it. To
4009 keep the string the same length you may need to pad or chop your value
4012 An alternative to using C<substr()> as an lvalue is to specify the
4013 replacement string as the 4th argument. This allows you to replace
4014 parts of the EXPR and return what was there before in one operation.
4016 =item symlink OLDFILE,NEWFILE
4018 Creates a new filename symbolically linked to the old filename.
4019 Returns C<1> for success, C<0> otherwise. On systems that don't support
4020 symbolic links, produces a fatal error at run time. To check for that,
4023 $symlink_exists = eval { symlink("",""); 1 };
4027 Calls the system call specified as the first element of the list,
4028 passing the remaining elements as arguments to the system call. If
4029 unimplemented, produces a fatal error. The arguments are interpreted
4030 as follows: if a given argument is numeric, the argument is passed as
4031 an int. If not, the pointer to the string value is passed. You are
4032 responsible to make sure a string is pre-extended long enough to
4033 receive any result that might be written into a string. You can't use a
4034 string literal (or other read-only string) as an argument to C<syscall()>
4035 because Perl has to assume that any string pointer might be written
4037 integer arguments are not literals and have never been interpreted in a
4038 numeric context, you may need to add C<0> to them to force them to look
4039 like numbers. This emulates the C<syswrite()> function (or vice versa):
4041 require 'syscall.ph'; # may need to run h2ph
4043 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4045 Note that Perl supports passing of up to only 14 arguments to your system call,
4046 which in practice should usually suffice.
4048 Syscall returns whatever value returned by the system call it calls.
4049 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
4050 Note that some system calls can legitimately return C<-1>. The proper
4051 way to handle such calls is to assign C<$!=0;> before the call and
4052 check the value of C<$!> if syscall returns C<-1>.
4054 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4055 number of the read end of the pipe it creates. There is no way
4056 to retrieve the file number of the other end. You can avoid this
4057 problem by using C<pipe()> instead.
4059 =item sysopen FILEHANDLE,FILENAME,MODE
4061 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4063 Opens the file whose filename is given by FILENAME, and associates it
4064 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4065 the name of the real filehandle wanted. This function calls the
4066 underlying operating system's C<open()> function with the parameters
4067 FILENAME, MODE, PERMS.
4069 The possible values and flag bits of the MODE parameter are
4070 system-dependent; they are available via the standard module C<Fcntl>.
4071 For historical reasons, some values work on almost every system
4072 supported by perl: zero means read-only, one means write-only, and two
4073 means read/write. We know that these values do I<not> work under
4074 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4075 use them in new code.
4077 If the file named by FILENAME does not exist and the C<open()> call creates
4078 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4079 PERMS specifies the permissions of the newly created file. If you omit
4080 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4081 These permission values need to be in octal, and are modified by your
4082 process's current C<umask>.
4084 Seldom if ever use C<0644> as argument to C<sysopen()> because that
4085 takes away the user's option to have a more permissive umask. Better
4086 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
4088 The C<IO::File> module provides a more object-oriented approach, if you're
4089 into that kind of thing.
4091 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4093 =item sysread FILEHANDLE,SCALAR,LENGTH
4095 Attempts to read LENGTH bytes of data into variable SCALAR from the
4096 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4097 so mixing this with other kinds of reads, C<print()>, C<write()>,
4098 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4099 usually buffers data. Returns the number of bytes actually read, C<0>
4100 at end of file, or undef if there was an error. SCALAR will be grown or
4101 shrunk so that the last byte actually read is the last byte of the
4102 scalar after the read.
4104 An OFFSET may be specified to place the read data at some place in the
4105 string other than the beginning. A negative OFFSET specifies
4106 placement at that many bytes counting backwards from the end of the
4107 string. A positive OFFSET greater than the length of SCALAR results
4108 in the string being padded to the required size with C<"\0"> bytes before
4109 the result of the read is appended.
4111 =item sysseek FILEHANDLE,POSITION,WHENCE
4113 Sets FILEHANDLE's system position using the system call lseek(2). It
4114 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4115 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4116 confusion. FILEHANDLE may be an expression whose value gives the name
4117 of the filehandle. The values for WHENCE are C<0> to set the new
4118 position to POSITION, C<1> to set the it to the current position plus
4119 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4120 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4121 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4123 Returns the new position, or the undefined value on failure. A position
4124 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4125 TRUE on success and FALSE on failure, yet you can still easily determine
4130 =item system PROGRAM LIST
4132 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4133 first, and the parent process waits for the child process to complete.
4134 Note that argument processing varies depending on the number of
4135 arguments. If there is more than one argument in LIST, or if LIST is
4136 an array with more than one value, starts the program given by the
4137 first element of the list with arguments given by the rest of the list.
4138 If there is only one scalar argument, the argument is
4139 checked for shell metacharacters, and if there are any, the entire
4140 argument is passed to the system's command shell for parsing (this is
4141 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4142 there are no shell metacharacters in the argument, it is split into
4143 words and passed directly to C<execvp()>, which is more efficient.
4145 The return value is the exit status of the program as
4146 returned by the C<wait()> call. To get the actual exit value divide by
4147 256. See also L</exec>. This is I<NOT> what you want to use to capture
4148 the output from a command, for that you should use merely backticks or
4149 C<qx//>, as described in L<perlop/"`STRING`">.
4151 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4152 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4154 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4155 program they're running doesn't actually interrupt your program.
4157 @args = ("command", "arg1", "arg2");
4159 or die "system @args failed: $?"
4161 You can check all the failure possibilities by inspecting
4164 $exit_value = $? >> 8;
4165 $signal_num = $? & 127;
4166 $dumped_core = $? & 128;
4168 When the arguments get executed via the system shell, results
4169 and return codes will be subject to its quirks and capabilities.
4170 See L<perlop/"`STRING`"> and L</exec> for details.
4172 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4174 =item syswrite FILEHANDLE,SCALAR,LENGTH
4176 =item syswrite FILEHANDLE,SCALAR
4178 Attempts to write LENGTH bytes of data from variable SCALAR to the
4179 specified FILEHANDLE, using the system call write(2). If LENGTH is
4180 not specified, writes whole SCALAR. It bypasses
4181 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4182 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4183 because stdio usually buffers data. Returns the number of bytes
4184 actually written, or C<undef> if there was an error. If the LENGTH is
4185 greater than the available data in the SCALAR after the OFFSET, only as
4186 much data as is available will be written.
4188 An OFFSET may be specified to write the data from some part of the
4189 string other than the beginning. A negative OFFSET specifies writing
4190 that many bytes counting backwards from the end of the string. In the
4191 case the SCALAR is empty you can use OFFSET but only zero offset.
4193 =item tell FILEHANDLE
4197 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4198 expression whose value gives the name of the actual filehandle. If
4199 FILEHANDLE is omitted, assumes the file last read.
4201 =item telldir DIRHANDLE
4203 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4204 Value may be given to C<seekdir()> to access a particular location in a
4205 directory. Has the same caveats about possible directory compaction as
4206 the corresponding system library routine.
4208 =item tie VARIABLE,CLASSNAME,LIST
4210 This function binds a variable to a package class that will provide the
4211 implementation for the variable. VARIABLE is the name of the variable
4212 to be enchanted. CLASSNAME is the name of a class implementing objects
4213 of correct type. Any additional arguments are passed to the "C<new()>"
4214 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4215 or C<TIEHASH>). Typically these are arguments such as might be passed
4216 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4217 method is also returned by the C<tie()> function, which would be useful
4218 if you want to access other methods in CLASSNAME.
4220 Note that functions such as C<keys()> and C<values()> may return huge lists
4221 when used on large objects, like DBM files. You may prefer to use the
4222 C<each()> function to iterate over such. Example:
4224 # print out history file offsets
4226 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4227 while (($key,$val) = each %HIST) {
4228 print $key, ' = ', unpack('L',$val), "\n";
4232 A class implementing a hash should have the following methods:
4234 TIEHASH classname, LIST
4236 STORE this, key, value
4241 NEXTKEY this, lastkey
4244 A class implementing an ordinary array should have the following methods:
4246 TIEARRAY classname, LIST
4248 STORE this, key, value
4250 STORESIZE this, count
4256 SPLICE this, offset, length, LIST
4260 A class implementing a file handle should have the following methods:
4262 TIEHANDLE classname, LIST
4263 READ this, scalar, length, offset
4266 WRITE this, scalar, length, offset
4268 PRINTF this, format, LIST
4272 A class implementing a scalar should have the following methods:
4274 TIESCALAR classname, LIST
4279 Not all methods indicated above need be implemented. See L<perltie>,
4280 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>.
4282 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4283 for you--you need to do that explicitly yourself. See L<DB_File>
4284 or the F<Config> module for interesting C<tie()> implementations.
4286 For further details see L<perltie>, L<"tied VARIABLE">.
4290 Returns a reference to the object underlying VARIABLE (the same value
4291 that was originally returned by the C<tie()> call that bound the variable
4292 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4297 Returns the number of non-leap seconds since whatever time the system
4298 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4299 and 00:00:00 UTC, January 1, 1970 for most other systems).
4300 Suitable for feeding to C<gmtime()> and C<localtime()>.
4304 Returns a four-element list giving the user and system times, in
4305 seconds, for this process and the children of this process.
4307 ($user,$system,$cuser,$csystem) = times;
4311 The transliteration operator. Same as C<y///>. See L<perlop>.
4313 =item truncate FILEHANDLE,LENGTH
4315 =item truncate EXPR,LENGTH
4317 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4318 specified length. Produces a fatal error if truncate isn't implemented
4319 on your system. Returns TRUE if successful, the undefined value
4326 Returns an uppercased version of EXPR. This is the internal function
4327 implementing the C<\U> escape in double-quoted strings.
4328 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4329 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4330 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4332 If EXPR is omitted, uses C<$_>.
4338 Returns the value of EXPR with the first character
4339 in uppercase (titlecase in Unicode). This is
4340 the internal function implementing the C<\u> escape in double-quoted strings.
4341 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4343 If EXPR is omitted, uses C<$_>.
4349 Sets the umask for the process to EXPR and returns the previous value.
4350 If EXPR is omitted, merely returns the current umask.
4352 The Unix permission C<rwxr-x---> is represented as three sets of three
4353 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4354 and isn't one of the digits). The C<umask> value is such a number
4355 representing disabled permissions bits. The permission (or "mode")
4356 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4357 even if you tell C<sysopen> to create a file with permissions C<0777>,
4358 if your umask is C<0022> then the file will actually be created with
4359 permissions C<0755>. If your C<umask> were C<0027> (group can't
4360 write; others can't read, write, or execute), then passing
4361 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4364 Here's some advice: supply a creation mode of C<0666> for regular
4365 files (in C<sysopen()>) and one of C<0777> for directories (in
4366 C<mkdir()>) and executable files. This gives users the freedom of
4367 choice: if they want protected files, they might choose process umasks
4368 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4369 Programs should rarely if ever make policy decisions better left to
4370 the user. The exception to this is when writing files that should be
4371 kept private: mail files, web browser cookies, I<.rhosts> files, and
4374 If umask(2) is not implemented on your system and you are trying to
4375 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4376 fatal error at run time. If umask(2) is not implemented and you are
4377 not trying to restrict access for yourself, returns C<undef>.
4379 Remember that a umask is a number, usually given in octal; it is I<not> a
4380 string of octal digits. See also L</oct>, if all you have is a string.
4388 Undefines the value of EXPR, which must be an lvalue. Use only on a
4389 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4390 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4391 will probably not do what you expect on most predefined variables or
4392 DBM list values, so don't do that; see L<delete>.) Always returns the
4393 undefined value. You can omit the EXPR, in which case nothing is
4394 undefined, but you still get an undefined value that you could, for
4395 instance, return from a subroutine, assign to a variable or pass as a
4396 parameter. Examples:
4399 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4403 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4404 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4405 select undef, undef, undef, 0.25;
4406 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4408 Note that this is a unary operator, not a list operator.
4414 Deletes a list of files. Returns the number of files successfully
4417 $cnt = unlink 'a', 'b', 'c';
4421 Note: C<unlink()> will not delete directories unless you are superuser and
4422 the B<-U> flag is supplied to Perl. Even if these conditions are
4423 met, be warned that unlinking a directory can inflict damage on your
4424 filesystem. Use C<rmdir()> instead.
4426 If LIST is omitted, uses C<$_>.
4428 =item unpack TEMPLATE,EXPR
4430 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4431 structure and expands it out into a list value, returning the array
4432 value. (In scalar context, it returns merely the first value
4433 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4434 Here's a subroutine that does substring:
4437 my($what,$where,$howmuch) = @_;
4438 unpack("x$where a$howmuch", $what);
4443 sub ordinal { unpack("c",$_[0]); } # same as ord()
4445 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4446 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4447 themselves. Default is a 16-bit checksum. For example, the following
4448 computes the same number as the System V sum program:
4451 $checksum += unpack("%32C*", $_);
4455 The following efficiently counts the number of set bits in a bit vector:
4457 $setbits = unpack("%32b*", $selectmask);
4459 See L</pack> for more examples.
4461 =item untie VARIABLE
4463 Breaks the binding between a variable and a package. (See C<tie()>.)
4465 =item unshift ARRAY,LIST
4467 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4468 depending on how you look at it. Prepends list to the front of the
4469 array, and returns the new number of elements in the array.
4471 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4473 Note the LIST is prepended whole, not one element at a time, so the
4474 prepended elements stay in the same order. Use C<reverse()> to do the
4477 =item use Module LIST
4481 =item use Module VERSION LIST
4485 Imports some semantics into the current package from the named module,
4486 generally by aliasing certain subroutine or variable names into your
4487 package. It is exactly equivalent to
4489 BEGIN { require Module; import Module LIST; }
4491 except that Module I<must> be a bareword.
4493 If the first argument to C<use> is a number, it is treated as a version
4494 number instead of a module name. If the version of the Perl interpreter
4495 is less than VERSION, then an error message is printed and Perl exits
4496 immediately. This is often useful if you need to check the current
4497 Perl version before C<use>ing library modules that have changed in
4498 incompatible ways from older versions of Perl. (We try not to do
4499 this more than we have to.)
4501 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4502 C<require> makes sure the module is loaded into memory if it hasn't been
4503 yet. The C<import()> is not a builtin--it's just an ordinary static method
4504 call into the "C<Module>" package to tell the module to import the list of
4505 features back into the current package. The module can implement its
4506 C<import()> method any way it likes, though most modules just choose to
4507 derive their C<import()> method via inheritance from the C<Exporter> class that
4508 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4509 method can be found then the error is currently silently ignored. This
4510 may change to a fatal error in a future version.
4512 If you don't want your namespace altered, explicitly supply an empty list:
4516 That is exactly equivalent to
4518 BEGIN { require Module }
4520 If the VERSION argument is present between Module and LIST, then the
4521 C<use> will call the VERSION method in class Module with the given
4522 version as an argument. The default VERSION method, inherited from
4523 the Universal class, croaks if the given version is larger than the
4524 value of the variable C<$Module::VERSION>. (Note that there is not a
4525 comma after VERSION!)
4527 Because this is a wide-open interface, pragmas (compiler directives)
4528 are also implemented this way. Currently implemented pragmas are:
4532 use sigtrap qw(SEGV BUS);
4533 use strict qw(subs vars refs);
4534 use subs qw(afunc blurfl);
4536 Some of these these pseudo-modules import semantics into the current
4537 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4538 which import symbols into the current package (which are effective
4539 through the end of the file).
4541 There's a corresponding "C<no>" command that unimports meanings imported
4542 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4547 If no C<unimport()> method can be found the call fails with a fatal error.
4549 See L<perlmod> for a list of standard modules and pragmas.
4553 Changes the access and modification times on each file of a list of
4554 files. The first two elements of the list must be the NUMERICAL access
4555 and modification times, in that order. Returns the number of files
4556 successfully changed. The inode modification time of each file is set
4557 to the current time. This code has the same effect as the "C<touch>"
4558 command if the files already exist:
4562 utime $now, $now, @ARGV;
4566 Returns a list consisting of all the values of the named hash. (In a
4567 scalar context, returns the number of values.) The values are
4568 returned in an apparently random order. The actual random order is
4569 subject to change in future versions of perl, but it is guaranteed to
4570 be the same order as either the C<keys()> or C<each()> function would
4571 produce on the same (unmodified) hash.
4573 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4576 =item vec EXPR,OFFSET,BITS
4578 Treats the string in EXPR as a vector of unsigned integers, and
4579 returns the value of the bit field specified by OFFSET. BITS specifies
4580 the number of bits that are reserved for each entry in the bit
4581 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4582 assigned to, in which case parentheses are needed to give the expression
4583 the correct precedence as in
4585 vec($image, $max_x * $x + $y, 8) = 3;
4587 Vectors created with C<vec()> can also be manipulated with the logical
4588 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4589 desired when both operands are strings.
4591 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4592 The comments show the string after each step. Note that this code works
4593 in the same way on big-endian or little-endian machines.
4596 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4597 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4598 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4599 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4600 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4601 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4602 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4604 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4605 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4606 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4609 To transform a bit vector into a string or array of 0's and 1's, use these:
4611 $bits = unpack("b*", $vector);
4612 @bits = split(//, unpack("b*", $vector));
4614 If you know the exact length in bits, it can be used in place of the C<*>.
4618 Waits for a child process to terminate and returns the pid of the
4619 deceased process, or C<-1> if there are no child processes. The status is
4620 returned in C<$?>. Note that a return value of C<-1> could mean that
4621 child processes are being automatically reaped, as described in L<perlipc>.
4623 =item waitpid PID,FLAGS
4625 Waits for a particular child process to terminate and returns the pid
4626 of the deceased process, or C<-1> if there is no such child process. The
4627 status is returned in C<$?>. If you say
4629 use POSIX ":sys_wait_h";
4631 waitpid(-1,&WNOHANG);
4633 then you can do a non-blocking wait for any process. Non-blocking wait
4634 is available on machines supporting either the waitpid(2) or
4635 wait4(2) system calls. However, waiting for a particular pid with
4636 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4637 by remembering the status values of processes that have exited but have
4638 not been harvested by the Perl script yet.)
4640 Note that a return value of C<-1> could mean that child processes are being
4641 automatically reaped. See L<perlipc> for details, and for other examples.
4645 Returns TRUE if the context of the currently executing subroutine is
4646 looking for a list value. Returns FALSE if the context is looking
4647 for a scalar. Returns the undefined value if the context is looking
4648 for no value (void context).
4650 return unless defined wantarray; # don't bother doing more
4651 my @a = complex_calculation();
4652 return wantarray ? @a : "@a";
4656 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4659 If LIST is empty and C<$@> already contains a value (typically from a
4660 previous eval) that value is used after appending C<"\t...caught">
4661 to C<$@>. This is useful for staying almost, but not entirely similar to
4664 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4666 No message is printed if there is a C<$SIG{__WARN__}> handler
4667 installed. It is the handler's responsibility to deal with the message
4668 as it sees fit (like, for instance, converting it into a C<die()>). Most
4669 handlers must therefore make arrangements to actually display the
4670 warnings that they are not prepared to deal with, by calling C<warn()>
4671 again in the handler. Note that this is quite safe and will not
4672 produce an endless loop, since C<__WARN__> hooks are not called from
4675 You will find this behavior is slightly different from that of
4676 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4677 instead call C<die()> again to change it).
4679 Using a C<__WARN__> handler provides a powerful way to silence all
4680 warnings (even the so-called mandatory ones). An example:
4682 # wipe out *all* compile-time warnings
4683 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4685 my $foo = 20; # no warning about duplicate my $foo,
4686 # but hey, you asked for it!
4687 # no compile-time or run-time warnings before here
4690 # run-time warnings enabled after here
4691 warn "\$foo is alive and $foo!"; # does show up
4693 See L<perlvar> for details on setting C<%SIG> entries, and for more
4696 =item write FILEHANDLE
4702 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4703 using the format associated with that file. By default the format for
4704 a file is the one having the same name as the filehandle, but the
4705 format for the current output channel (see the C<select()> function) may be set
4706 explicitly by assigning the name of the format to the C<$~> variable.
4708 Top of form processing is handled automatically: if there is
4709 insufficient room on the current page for the formatted record, the
4710 page is advanced by writing a form feed, a special top-of-page format
4711 is used to format the new page header, and then the record is written.
4712 By default the top-of-page format is the name of the filehandle with
4713 "_TOP" appended, but it may be dynamically set to the format of your
4714 choice by assigning the name to the C<$^> variable while the filehandle is
4715 selected. The number of lines remaining on the current page is in
4716 variable C<$->, which can be set to C<0> to force a new page.
4718 If FILEHANDLE is unspecified, output goes to the current default output
4719 channel, which starts out as STDOUT but may be changed by the
4720 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4721 is evaluated and the resulting string is used to look up the name of
4722 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4724 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4728 The transliteration operator. Same as C<tr///>. See L<perlop>.