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 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 rule:
59 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
63 Each operator and function decides which sort of value it would be most
64 appropriate to return in a scalar context. Some operators return the
65 length of the list that would have been returned in a list context. Some
66 operators return the first value in the list. Some operators return the
67 last value in the list. Some operators return a count of successful
68 operations. In general, they do what you want, unless you want
71 =head2 Perl Functions by Category
73 Here are Perl's functions (including things that look like
74 functions, like some of the keywords and named operators)
75 arranged by category. Some functions appear in more
80 =item Functions for SCALARs or strings
82 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84 sprintf, substr, tr///, uc, ucfirst, y///
86 =item Regular expressions and pattern matching
88 m//, pos, quotemeta, s///, split, study
90 =item Numeric functions
92 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95 =item Functions for real @ARRAYs
97 pop, push, shift, splice, unshift
99 =item Functions for list data
101 grep, join, map, qw/STRING/, reverse, sort, unpack
103 =item Functions for real %HASHes
105 delete, each, exists, keys, values
107 =item Input and output functions
109 binmode, close, closedir, dbmclose, dbmopen, die, eof,
110 fileno, flock, format, getc, print, printf, read, readdir,
111 rewinddir, seek, seekdir, select, syscall, sysread, sysseek,
112 syswrite, tell, telldir, truncate, warn, write
114 =item Functions for fixed length data or records
116 pack, read, syscall, sysread, syswrite, unpack, vec
118 =item Functions for filehandles, files, or directories
120 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
121 lstat, mkdir, open, opendir, readlink, rename, rmdir,
122 stat, symlink, umask, unlink, utime
124 =item Keywords related to the control flow of your perl program
126 caller, continue, die, do, dump, eval, exit, goto, last,
127 next, redo, return, sub, wantarray
129 =item Keywords related to scoping
131 caller, import, local, my, package, use
133 =item Miscellaneous functions
135 defined, dump, eval, formline, local, my, reset, scalar,
138 =item Functions for processes and process groups
140 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144 =item Keywords related to perl modules
146 do, import, no, package, require, use
148 =item Keywords related to classes and object-orientedness
150 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
152 =item Low-level socket functions
154 accept, bind, connect, getpeername, getsockname,
155 getsockopt, listen, recv, send, setsockopt, shutdown,
158 =item System V interprocess communication functions
160 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161 shmctl, shmget, shmread, shmwrite
163 =item Fetching user and group info
165 endgrent, endhostent, endnetent, endpwent, getgrent,
166 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167 getpwuid, setgrent, setpwent
169 =item Fetching network info
171 endprotoent, endservent, gethostbyaddr, gethostbyname,
172 gethostent, getnetbyaddr, getnetbyname, getnetent,
173 getprotobyname, getprotobynumber, getprotoent,
174 getservbyname, getservbyport, getservent, sethostent,
175 setnetent, setprotoent, setservent
177 =item Time-related functions
179 gmtime, localtime, time, times
181 =item Functions new in perl5
183 abs, bless, chomp, chr, exists, formline, glob, import, lc,
184 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
187 * - C<sub> was a keyword in perl4, but in perl5 it is an
188 operator which can be used in expressions.
190 =item Functions obsoleted in perl5
196 =head2 Alphabetical Listing of Perl Functions
206 A file test, where X is one of the letters listed below. This unary
207 operator takes one argument, either a filename or a filehandle, and
208 tests the associated file to see if something is true about it. If the
209 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
210 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
211 the undefined value if the file doesn't exist. Despite the funny
212 names, precedence is the same as any other named unary operator, and
213 the argument may be parenthesized like any other unary operator. The
214 operator may be any of:
216 -r File is readable by effective uid/gid.
217 -w File is writable by effective uid/gid.
218 -x File is executable by effective uid/gid.
219 -o File is owned by effective uid.
221 -R File is readable by real uid/gid.
222 -W File is writable by real uid/gid.
223 -X File is executable by real uid/gid.
224 -O File is owned by real uid.
227 -z File has zero size.
228 -s File has nonzero size (returns size).
230 -f File is a plain file.
231 -d File is a directory.
232 -l File is a symbolic link.
233 -p File is a named pipe (FIFO).
235 -b File is a block special file.
236 -c File is a character special file.
237 -t Filehandle is opened to a tty.
239 -u File has setuid bit set.
240 -g File has setgid bit set.
241 -k File has sticky bit set.
243 -T File is a text file.
244 -B File is a binary file (opposite of -T).
246 -M Age of file in days when script started.
247 -A Same for access time.
248 -C Same for inode change time.
250 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
251 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
252 uids and gids of the user. There may be other reasons you can't actually
253 read, write or execute the file. Also note that, for the superuser,
254 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
255 1 if any execute bit is set in the mode. Scripts run by the superuser may
256 thus need to do a stat() to determine the actual mode of the
257 file, or temporarily set the uid to something else.
263 next unless -f $_; # ignore specials
267 Note that C<-s/a/b/> does not do a negated substitution. Saying
268 C<-exp($foo)> still works as expected, however--only single letters
269 following a minus are interpreted as file tests.
271 The C<-T> and C<-B> switches work as follows. The first block or so of the
272 file is examined for odd characters such as strange control codes or
273 characters with the high bit set. If too many odd characters (E<gt>30%)
274 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
275 containing null in the first block is considered a binary file. If C<-T>
276 or C<-B> is used on a filehandle, the current stdio buffer is examined
277 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
278 file, or a file at EOF when testing a filehandle. Because you have to
279 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
280 against the file first, as in C<next unless -f $file && -T $file>.
282 If any of the file tests (or either the stat() or lstat() operators) are given
283 the special filehandle consisting of a solitary underline, then the stat
284 structure of the previous file test (or stat operator) is used, saving
285 a system call. (This doesn't work with C<-t>, and you need to remember
286 that lstat() and C<-l> will leave values in the stat structure for the
287 symbolic link, not the real file.) Example:
289 print "Can do.\n" if -r $a || -w _ || -x _;
292 print "Readable\n" if -r _;
293 print "Writable\n" if -w _;
294 print "Executable\n" if -x _;
295 print "Setuid\n" if -u _;
296 print "Setgid\n" if -g _;
297 print "Sticky\n" if -k _;
298 print "Text\n" if -T _;
299 print "Binary\n" if -B _;
305 Returns the absolute value of its argument.
306 If VALUE is omitted, uses $_.
308 =item accept NEWSOCKET,GENERICSOCKET
310 Accepts an incoming socket connect, just as the accept(2) system call
311 does. Returns the packed address if it succeeded, FALSE otherwise.
312 See example in L<perlipc/"Sockets: Client/Server Communication">.
318 Arranges to have a SIGALRM delivered to this process after the
319 specified number of seconds have elapsed. If SECONDS is not specified,
320 the value stored in $_ is used. (On some machines,
321 unfortunately, the elapsed time may be up to one second less than you
322 specified because of how seconds are counted.) Only one timer may be
323 counting at once. Each call disables the previous timer, and an
324 argument of 0 may be supplied to cancel the previous timer without
325 starting a new one. The returned value is the amount of time remaining
326 on the previous timer.
328 For delays of finer granularity than one second, you may use Perl's
329 syscall() interface to access setitimer(2) if your system supports it,
330 or else see L</select()>. It is usually a mistake to intermix alarm()
333 If you want to use alarm() to time out a system call you need to use an
334 eval/die pair. You can't rely on the alarm causing the system call to
335 fail with $! set to EINTR because Perl sets up signal handlers to
336 restart system calls on some systems. Using eval/die always works.
339 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
341 $nread = sysread SOCKET, $buffer, $size;
344 die if $@ && $@ ne "alarm\n"; # propagate errors
354 Returns the arctangent of Y/X in the range -PI to PI.
356 For the tangent operation, you may use the POSIX::tan()
357 function, or use the familiar relation:
359 sub tan { sin($_[0]) / cos($_[0]) }
361 =item bind SOCKET,NAME
363 Binds a network address to a socket, just as the bind system call
364 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
365 packed address of the appropriate type for the socket. See the examples in
366 L<perlipc/"Sockets: Client/Server Communication">.
368 =item binmode FILEHANDLE
370 Arranges for the file to be read or written in "binary" mode in operating
371 systems that distinguish between binary and text files. Files that are
372 not in binary mode have CR LF sequences translated to LF on input and LF
373 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
374 and similarly archaic systems, it may be imperative--otherwise your
375 MS-DOS-damaged C library may mangle your file. The key distinction between
376 systems that need binmode and those that don't is their text file
377 formats. Systems like Unix and Plan9 that delimit lines with a single
378 character, and that encode that character in C as '\n', do not need
379 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
380 is taken as the name of the filehandle.
382 =item bless REF,CLASSNAME
386 This function tells the thingy referenced by REF that it is now
387 an object in the CLASSNAME package--or the current package if no CLASSNAME
388 is specified, which is often the case. It returns the reference for
389 convenience, because a bless() is often the last thing in a constructor.
390 Always use the two-argument version if the function doing the blessing
391 might be inherited by a derived class. See L<perlobj> for more about the
392 blessing (and blessings) of objects.
398 Returns the context of the current subroutine call. In a scalar context,
399 returns the caller's package name if there is a caller, that is, if
400 we're in a subroutine or eval() or require(), and the undefined value
401 otherwise. In a list context, returns
403 ($package, $filename, $line) = caller;
405 With EXPR, it returns some extra information that the debugger uses to
406 print a stack trace. The value of EXPR indicates how many call frames
407 to go back before the current one.
409 ($package, $filename, $line, $subroutine,
410 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
412 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
413 call, but an C<eval>. In such a case additional elements $evaltext and
414 $is_require are set: $is_require is true if the frame is created by a
415 C<require> or C<use> statement, $evaltext contains the text of the
416 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
417 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
418 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
421 Furthermore, when called from within the DB package, caller returns more
422 detailed information: it sets the list variable @DB::args to be the
423 arguments with which the subroutine was invoked.
427 Changes the working directory to EXPR, if possible. If EXPR is
428 omitted, changes to home directory. Returns TRUE upon success, FALSE
429 otherwise. See example under die().
433 Changes the permissions of a list of files. The first element of the
434 list must be the numerical mode, which should probably be an octal
435 number, and which definitely should I<not> a string of octal digits:
436 C<0644> is okay, C<'0644'> is not. Returns the number of files
437 successfully changed. See also L</oct>, if all you have is a string.
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
441 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
442 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
443 $mode = 0644; chmod $mode, 'foo'; # this is best
451 This is a slightly safer version of chop (see below). It removes any
452 line ending that corresponds to the current value of C<$/> (also known as
453 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
454 number of characters removed from all its arguments. It's often used to
455 remove the newline from the end of an input record when you're worried
456 that the final record may be missing its newline. When in paragraph mode
457 (C<$/ = "">), it removes all trailing newlines from the string. If
458 VARIABLE is omitted, it chomps $_. Example:
461 chomp; # avoid \n on last field
466 You can actually chomp anything that's an lvalue, including an assignment:
469 chomp($answer = <STDIN>);
471 If you chomp a list, each element is chomped, and the total number of
472 characters removed is returned.
480 Chops off the last character of a string and returns the character
481 chopped. It's used primarily to remove the newline from the end of an
482 input record, but is much more efficient than C<s/\n//> because it neither
483 scans nor copies the string. If VARIABLE is omitted, chops $_.
487 chop; # avoid \n on last field
492 You can actually chop anything that's an lvalue, including an assignment:
495 chop($answer = <STDIN>);
497 If you chop a list, each element is chopped. Only the value of the
498 last chop is returned.
500 Note that chop returns the last character. To return all but the last
501 character, use C<substr($string, 0, -1)>.
505 Changes the owner (and group) of a list of files. The first two
506 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
507 Returns the number of files successfully changed.
509 $cnt = chown $uid, $gid, 'foo', 'bar';
510 chown $uid, $gid, @filenames;
512 Here's an example that looks up nonnumeric uids in the passwd file:
515 chop($user = <STDIN>);
517 chop($pattern = <STDIN>);
519 ($login,$pass,$uid,$gid) = getpwnam($user)
520 or die "$user not in passwd file";
522 @ary = <${pattern}>; # expand filenames
523 chown $uid, $gid, @ary;
525 On most systems, you are not allowed to change the ownership of the
526 file unless you're the superuser, although you should be able to change
527 the group to any of your secondary groups. On insecure systems, these
528 restrictions may be relaxed, but this is not a portable assumption.
534 Returns the character represented by that NUMBER in the character set.
535 For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
537 If NUMBER is omitted, uses $_.
539 =item chroot FILENAME
543 This function works as the system call by the same name: it makes the
544 named directory the new root directory for all further pathnames that
545 begin with a "/" by your process and all of its children. (It doesn't
546 change your current working directory, which is unaffected.) For security
547 reasons, this call is restricted to the superuser. If FILENAME is
548 omitted, does chroot to $_.
550 =item close FILEHANDLE
552 Closes the file or pipe associated with the file handle, returning TRUE
553 only if stdio successfully flushes buffers and closes the system file
554 descriptor. If the file handle came from a piped open C<close> will
555 additionally return FALSE if one of the other system calls involved
556 fails or if the program exits with non-zero status. (If the problem was
557 that the program exited non-zero $! will be set to 0.)
558 You don't have to close FILEHANDLE if you are immediately
559 going to do another open() on it, because open() will close it for you. (See
560 open().) However, an explicit close on an input file resets the line
561 counter ($.), while the implicit close done by open() does not. Also,
562 closing a pipe will wait for the process executing on the pipe to
563 complete, in case you want to look at the output of the pipe
564 afterwards. Closing a pipe explicitly also puts the status value of
565 the command into C<$?>. Example:
567 open(OUTPUT, '|sort >foo'); # pipe to sort
568 ... # print stuff to output
569 close OUTPUT; # wait for sort to finish
570 open(INPUT, 'foo'); # get sort's results
572 FILEHANDLE may be an expression whose value gives the real filehandle name.
574 =item closedir DIRHANDLE
576 Closes a directory opened by opendir().
578 =item connect SOCKET,NAME
580 Attempts to connect to a remote socket, just as the connect system call
581 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
582 packed address of the appropriate type for the socket. See the examples in
583 L<perlipc/"Sockets: Client/Server Communication">.
587 Actually a flow control statement rather than a function. If there is a
588 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
589 C<foreach>), it is always executed just before the conditional is about to
590 be evaluated again, just like the third part of a C<for> loop in C. Thus
591 it can be used to increment a loop variable, even when the loop has been
592 continued via the C<next> statement (which is similar to the C C<continue>
597 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
600 For the inverse cosine operation, you may use the POSIX::acos()
601 function, or use this relation:
603 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
605 =item crypt PLAINTEXT,SALT
607 Encrypts a string exactly like the crypt(3) function in the C library
608 (assuming that you actually have a version there that has not been
609 extirpated as a potential munition). This can prove useful for checking
610 the password file for lousy passwords, amongst other things. Only the
611 guys wearing white hats should do this.
613 Note that crypt is intended to be a one-way function, much like breaking
614 eggs to make an omelette. There is no (known) corresponding decrypt
615 function. As a result, this function isn't all that useful for
616 cryptography. (For that, see your nearby CPAN mirror.)
618 Here's an example that makes sure that whoever runs this program knows
621 $pwd = (getpwuid($<))[1];
622 $salt = substr($pwd, 0, 2);
626 chop($word = <STDIN>);
630 if (crypt($word, $salt) ne $pwd) {
636 Of course, typing in your own password to whomever asks you
641 [This function has been superseded by the untie() function.]
643 Breaks the binding between a DBM file and a hash.
645 =item dbmopen HASH,DBNAME,MODE
647 [This function has been superseded by the tie() function.]
649 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
650 hash. HASH is the name of the hash. (Unlike normal open, the first
651 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
652 is the name of the database (without the F<.dir> or F<.pag> extension if
653 any). If the database does not exist, it is created with protection
654 specified by MODE (as modified by the umask()). If your system supports
655 only the older DBM functions, you may perform only one dbmopen() in your
656 program. In older versions of Perl, if your system had neither DBM nor
657 ndbm, calling dbmopen() produced a fatal error; it now falls back to
660 If you don't have write access to the DBM file, you can only read hash
661 variables, not set them. If you want to test whether you can write,
662 either use file tests or try setting a dummy hash entry inside an eval(),
663 which will trap the error.
665 Note that functions such as keys() and values() may return huge array
666 values when used on large DBM files. You may prefer to use the each()
667 function to iterate over large DBM files. Example:
669 # print out history file offsets
670 dbmopen(%HIST,'/usr/lib/news/history',0666);
671 while (($key,$val) = each %HIST) {
672 print $key, ' = ', unpack('L',$val), "\n";
676 See also L<AnyDBM_File> for a more general description of the pros and
677 cons of the various dbm approaches, as well as L<DB_File> for a particularly
684 Returns a Boolean value telling whether EXPR has a value other than
685 the undefined value C<undef>. If EXPR is not present, C<$_> will be
688 Many operations return C<undef> to indicate failure, end of file,
689 system error, uninitialized variable, and other exceptional
690 conditions. This function allows you to distinguish C<undef> from
691 other values. (A simple Boolean test will not distinguish among
692 C<undef>, zero, the empty string, and "0", which are all equally
693 false.) Note that since C<undef> is a valid scalar, its presence
694 doesn't I<necessarily> indicate an exceptional condition: pop()
695 returns C<undef> when its argument is an empty array, I<or> when the
696 element to return happens to be C<undef>.
698 You may also use defined() to check whether a subroutine exists. On
699 the other hand, use of defined() upon aggregates (hashes and arrays)
700 is not guaranteed to produce intuitive results, and should probably be
703 When used on a hash element, it tells you whether the value is defined,
704 not whether the key exists in the hash. Use L</exists> for the latter
709 print if defined $switch{'D'};
710 print "$val\n" while defined($val = pop(@ary));
711 die "Can't readlink $sym: $!"
712 unless defined($value = readlink $sym);
713 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
714 $debugging = 0 unless defined $debugging;
716 Note: Many folks tend to overuse defined(), and then are surprised to
717 discover that the number 0 and "" (the zero-length string) are, in fact,
718 defined values. For example, if you say
722 the pattern match succeeds, and $1 is defined, despite the fact that it
723 matched "nothing". But it didn't really match nothing--rather, it
724 matched something that happened to be 0 characters long. This is all
725 very above-board and honest. When a function returns an undefined value,
726 it's an admission that it couldn't give you an honest answer. So you
727 should use defined() only when you're questioning the integrity of what
728 you're trying to do. At other times, a simple comparison to 0 or "" is
731 Currently, using defined() on an entire array or hash reports whether
732 memory for that aggregate has ever been allocated. So an array you set
733 to the empty list appears undefined initially, and one that once was full
734 and that you then set to the empty list still appears defined. You
735 should instead use a simple test for size:
737 if (@an_array) { print "has array elements\n" }
738 if (%a_hash) { print "has hash members\n" }
740 Using undef() on these, however, does clear their memory and then report
741 them as not defined anymore, but you shoudln't do that unless you don't
742 plan to use them again, because it saves time when you load them up
743 again to have memory already ready to be filled.
745 This counterintuitive behaviour of defined() on aggregates may be
746 changed, fixed, or broken in a future release of Perl.
748 See also L</undef>, L</exists>, L</ref>.
752 Deletes the specified key(s) and their associated values from a hash.
753 For each key, returns the deleted value associated with that key, or
754 the undefined value if there was no such key. Deleting from C<$ENV{}>
755 modifies the environment. Deleting from a hash tied to a DBM file
756 deletes the entry from the DBM file. (But deleting from a tie()d hash
757 doesn't necessarily return anything.)
759 The following deletes all the values of a hash:
761 foreach $key (keys %HASH) {
767 delete @HASH{keys %HASH}
769 (But both of these are slower than the undef() command.) Note that the
770 EXPR can be arbitrarily complicated as long as the final operation is a
771 hash element lookup or hash slice:
773 delete $ref->[$x][$y]{$key};
774 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
778 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
779 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
780 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
781 is 0, exits with 255. Inside an eval(), the error message is stuffed into
782 C<$@>, and the eval() is terminated with the undefined value; this makes
783 die() the way to raise an exception.
787 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
788 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
790 If the value of EXPR does not end in a newline, the current script line
791 number and input line number (if any) are also printed, and a newline
792 is supplied. Hint: sometimes appending ", stopped" to your message
793 will cause it to make better sense when the string "at foo line 123" is
794 appended. Suppose you are running script "canasta".
796 die "/etc/games is no good";
797 die "/etc/games is no good, stopped";
799 produce, respectively
801 /etc/games is no good at canasta line 123.
802 /etc/games is no good, stopped at canasta line 123.
804 See also exit() and warn().
806 You can arrange for a callback to be called just before the die() does
807 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
808 will be called with the error text and can change the error message, if
809 it sees fit, by calling die() again. See L<perlvar> for details on
810 setting C<%SIG> entries, and eval() for some examples.
814 Not really a function. Returns the value of the last command in the
815 sequence of commands indicated by BLOCK. When modified by a loop
816 modifier, executes the BLOCK once before testing the loop condition.
817 (On other statements the loop modifiers test the conditional first.)
819 =item do SUBROUTINE(LIST)
821 A deprecated form of subroutine call. See L<perlsub>.
825 Uses the value of EXPR as a filename and executes the contents of the
826 file as a Perl script. Its primary use is to include subroutines
827 from a Perl subroutine library.
835 except that it's more efficient, more concise, keeps track of the
836 current filename for error messages, and searches all the B<-I>
837 libraries if the file isn't in the current directory (see also the @INC
838 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
839 reparse the file every time you call it, so you probably don't want to
840 do this inside a loop.
842 Note that inclusion of library modules is better done with the
843 use() and require() operators, which also do error checking
844 and raise an exception if there's a problem.
848 This causes an immediate core dump. Primarily this is so that you can
849 use the B<undump> program to turn your core dump into an executable binary
850 after having initialized all your variables at the beginning of the
851 program. When the new binary is executed it will begin by executing a
852 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
853 it as a goto with an intervening core dump and reincarnation. If LABEL
854 is omitted, restarts the program from the top. WARNING: any files
855 opened at the time of the dump will NOT be open any more when the
856 program is reincarnated, with possible resulting confusion on the part
857 of Perl. See also B<-u> option in L<perlrun>.
874 dump QUICKSTART if $ARGV[0] eq '-d';
881 When called in a list context, returns a 2-element array consisting of the
882 key and value for the next element of a hash, so that you can iterate over
883 it. When called in a scalar context, returns the key for only the next
884 element in the hash. (Note: Keys may be "0" or "", which are logically
885 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
888 Entries are returned in an apparently random order. When the hash is
889 entirely read, a null array is returned in list context (which when
890 assigned produces a FALSE (0) value), and C<undef> is returned in a
891 scalar context. The next call to each() after that will start iterating
892 again. There is a single iterator for each hash, shared by all each(),
893 keys(), and values() function calls in the program; it can be reset by
894 reading all the elements from the hash, or by evaluating C<keys HASH> or
895 C<values HASH>. If you add or delete elements of a hash while you're
896 iterating over it, you may get entries skipped or duplicated, so don't.
898 The following prints out your environment like the printenv(1) program,
899 only in a different order:
901 while (($key,$value) = each %ENV) {
902 print "$key=$value\n";
905 See also keys() and values().
913 Returns 1 if the next read on FILEHANDLE will return end of file, or if
914 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
915 gives the real filehandle name. (Note that this function actually
916 reads a character and then ungetc()s it, so it is not very useful in an
917 interactive context.) Do not read from a terminal file (or call
918 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
919 as terminals may lose the end-of-file condition if you do.
921 An C<eof> without an argument uses the last file read as argument.
922 Empty parentheses () may be used to indicate the pseudo file formed of
923 the files listed on the command line, i.e., C<eof()> is reasonable to
924 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
925 last file. Use C<eof(ARGV)> or eof without the parentheses to test
926 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
928 # reset line numbering on each input file
931 close(ARGV) if (eof); # Not eof().
934 # insert dashes just before last line of last file
937 print "--------------\n";
938 close(ARGV); # close or break; is needed if we
939 # are reading from the terminal
944 Practical hint: you almost never need to use C<eof> in Perl, because the
945 input operators return undef when they run out of data.
951 EXPR is parsed and executed as if it were a little Perl program. It
952 is executed in the context of the current Perl program, so that any
953 variable settings or subroutine and format definitions remain afterwards.
954 The value returned is the value of the last expression evaluated, or a
955 return statement may be used, just as with subroutines. The last
956 expression is evaluated in scalar or array context, depending on the
959 If there is a syntax error or runtime error, or a die() statement is
960 executed, an undefined value is returned by eval(), and C<$@> is set to the
961 error message. If there was no error, C<$@> is guaranteed to be a null
962 string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
963 any, may be omitted from the expression. Beware that using eval()
964 neither silences perl from printing warnings to STDERR, nor does it
965 stuff the text of warning messages into C<$@>. To do either of those,
966 you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
968 Note that, because eval() traps otherwise-fatal errors, it is useful for
969 determining whether a particular feature (such as socket() or symlink())
970 is implemented. It is also Perl's exception trapping mechanism, where
971 the die operator is used to raise exceptions.
973 If the code to be executed doesn't vary, you may use the eval-BLOCK
974 form to trap run-time errors without incurring the penalty of
975 recompiling each time. The error, if any, is still returned in C<$@>.
978 # make divide-by-zero nonfatal
979 eval { $answer = $a / $b; }; warn $@ if $@;
981 # same thing, but less efficient
982 eval '$answer = $a / $b'; warn $@ if $@;
984 # a compile-time error
988 eval '$answer ='; # sets $@
990 When using the eval{} form as an exception trap in libraries, you may
991 wish not to trigger any C<__DIE__> hooks that user code may have
992 installed. You can use the C<local $SIG{__DIE__}> construct for this
993 purpose, as shown in this example:
995 # a very private exception trap for divide-by-zero
996 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
998 This is especially significant, given that C<__DIE__> hooks can call
999 die() again, which has the effect of changing their error messages:
1001 # __DIE__ hooks may modify error messages
1003 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1004 eval { die "foo foofs here" };
1005 print $@ if $@; # prints "bar barfs here"
1008 With an eval(), you should be especially careful to remember what's
1009 being looked at when:
1015 eval { $x }; # CASE 4
1017 eval "\$$x++" # CASE 5
1020 Cases 1 and 2 above behave identically: they run the code contained in
1021 the variable $x. (Although case 2 has misleading double quotes making
1022 the reader wonder what else might be happening (nothing is).) Cases 3
1023 and 4 likewise behave in the same way: they run the code '$x', which
1024 does nothing but return the value of C<$x>. (Case 4 is preferred for
1025 purely visual reasons, but it also has the advantage of compiling at
1026 compile-time instead of at run-time.) Case 5 is a place where
1027 normally you I<WOULD> like to use double quotes, except that in this
1028 particular situation, you can just use symbolic references instead, as
1033 The exec() function executes a system command I<AND NEVER RETURNS>,
1034 unless the command does not exist and is executed directly instead of
1035 via C</bin/sh -c> (see below). Use system() instead of exec() if you
1038 If there is more than one argument in LIST, or if LIST is an array with
1039 more than one value, calls execvp(3) with the arguments in LIST. If
1040 there is only one scalar argument, the argument is checked for shell
1041 metacharacters. If there are any, the entire argument is passed to
1042 C</bin/sh -c> for parsing. If there are none, the argument is split
1043 into words and passed directly to execvp(), which is more efficient.
1044 Note: exec() and system() do not flush your output buffer, so you may
1045 need to set C<$|> to avoid lost output. Examples:
1047 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1048 exec "sort $outfile | uniq";
1050 If you don't really want to execute the first argument, but want to lie
1051 to the program you are executing about its own name, you can specify
1052 the program you actually want to run as an "indirect object" (without a
1053 comma) in front of the LIST. (This always forces interpretation of the
1054 LIST as a multivalued list, even if there is only a single scalar in
1057 $shell = '/bin/csh';
1058 exec $shell '-sh'; # pretend it's a login shell
1062 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1066 Returns TRUE if the specified hash key exists in its hash array, even
1067 if the corresponding value is undefined.
1069 print "Exists\n" if exists $array{$key};
1070 print "Defined\n" if defined $array{$key};
1071 print "True\n" if $array{$key};
1073 A hash element can be TRUE only if it's defined, and defined if
1074 it exists, but the reverse doesn't necessarily hold true.
1076 Note that the EXPR can be arbitrarily complicated as long as the final
1077 operation is a hash key lookup:
1079 if (exists $ref->[$x][$y]{$key}) { ... }
1083 Evaluates EXPR and exits immediately with that value. (Actually, it
1084 calls any defined C<END> routines first, but the C<END> routines may not
1085 abort the exit. Likewise any object destructors that need to be called
1086 are called before exit.) Example:
1089 exit 0 if $ans =~ /^[Xx]/;
1091 See also die(). If EXPR is omitted, exits with 0 status. The only
1092 universally portable values for EXPR are 0 for success and 1 for error;
1093 all other values are subject to unpredictable interpretation depending
1094 on the environment in which the Perl program is running.
1096 You shouldn't use exit() to abort a subroutine if there's any chance that
1097 someone might want to trap whatever error happened. Use die() instead,
1098 which can be trapped by an eval().
1104 Returns I<e> (the natural logarithm base) to the power of EXPR.
1105 If EXPR is omitted, gives C<exp($_)>.
1107 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1109 Implements the fcntl(2) function. You'll probably have to say
1113 first to get the correct function definitions. Argument processing and
1114 value return works just like ioctl() below. Note that fcntl() will produce
1115 a fatal error if used on a machine that doesn't implement fcntl(2).
1119 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1121 =item fileno FILEHANDLE
1123 Returns the file descriptor for a filehandle. This is useful for
1124 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1125 value is taken as the name of the filehandle.
1127 =item flock FILEHANDLE,OPERATION
1129 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1130 success, FALSE on failure. Produces a fatal error if used on a machine
1131 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1132 is Perl's portable file locking interface, although it locks only entire
1135 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1136 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1137 you can use the symbolic names if import them from the Fcntl module,
1138 either individually, or as a group using the ':flock' tag. LOCK_SH
1139 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1140 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1141 LOCK_EX then flock() will return immediately rather than blocking
1142 waiting for the lock (check the return status to see if you got it).
1144 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1145 before (un)locking it.
1147 Note that the emulation built with lockf(3) doesn't provide shared
1148 locks, and it requires that FILEHANDLE be open with write intent. These
1149 are the semantics that lockf(3) implements. Most (all?) systems
1150 implement lockf(3) in terms of fcntl(2) locking, though, so the
1151 differing semantics shouldn't bite too many people.
1153 Note also that some versions of flock() cannot lock things over the
1154 network; you would need to use the more system-specific fcntl() for
1155 that. If you like you can force Perl to ignore your system's flock(2)
1156 function, and so provide its own fcntl(2)-based emulation, by passing
1157 the switch C<-Ud_flock> to the F<Configure> program when you configure
1160 Here's a mailbox appender for BSD systems.
1162 use Fcntl ':flock'; # import LOCK_* constants
1165 flock(MBOX,LOCK_EX);
1166 # and, in case someone appended
1167 # while we were waiting...
1172 flock(MBOX,LOCK_UN);
1175 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1176 or die "Can't open mailbox: $!";
1179 print MBOX $msg,"\n\n";
1182 See also L<DB_File> for other flock() examples.
1186 Does a fork(2) system call. Returns the child pid to the parent process
1187 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1188 Note: unflushed buffers remain unflushed in both processes, which means
1189 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1190 method of IO::Handle to avoid duplicate output.
1192 If you fork() without ever waiting on your children, you will accumulate
1195 $SIG{CHLD} = sub { wait };
1197 There's also the double-fork trick (error checking on
1198 fork() returns omitted);
1200 unless ($pid = fork) {
1202 exec "what you really wanna do";
1205 ## (some_perl_code_here)
1212 See also L<perlipc> for more examples of forking and reaping
1215 Note that if your forked child inherits system file descriptors like
1216 STDIN and STDOUT that are actually connected by a pipe or socket, even
1217 if you exit, the remote server (such as, say, httpd or rsh) won't think
1218 you're done. You should reopen those to /dev/null if it's any issue.
1222 Declare a picture format with use by the write() function. For
1226 Test: @<<<<<<<< @||||| @>>>>>
1227 $str, $%, '$' . int($num)
1231 $num = $cost/$quantity;
1235 See L<perlform> for many details and examples.
1238 =item formline PICTURE,LIST
1240 This is an internal function used by C<format>s, though you may call it
1241 too. It formats (see L<perlform>) a list of values according to the
1242 contents of PICTURE, placing the output into the format output
1243 accumulator, C<$^A> (or $ACCUMULATOR in English).
1244 Eventually, when a write() is done, the contents of
1245 C<$^A> are written to some filehandle, but you could also read C<$^A>
1246 yourself and then set C<$^A> back to "". Note that a format typically
1247 does one formline() per line of form, but the formline() function itself
1248 doesn't care how many newlines are embedded in the PICTURE. This means
1249 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1250 You may therefore need to use multiple formlines to implement a single
1251 record format, just like the format compiler.
1253 Be careful if you put double quotes around the picture, because an "C<@>"
1254 character may be taken to mean the beginning of an array name.
1255 formline() always returns TRUE. See L<perlform> for other examples.
1257 =item getc FILEHANDLE
1261 Returns the next character from the input file attached to FILEHANDLE,
1262 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1263 This is not particularly efficient. It cannot be used to get unbuffered
1264 single-characters, however. For that, try something more like:
1267 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1270 system "stty", '-icanon', 'eol', "\001";
1276 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1279 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1283 Determination of whether $BSD_STYLE should be set
1284 is left as an exercise to the reader.
1286 The POSIX::getattr() function can do this more portably on systems
1287 alleging POSIX compliance.
1288 See also the C<Term::ReadKey> module from your nearest CPAN site;
1289 details on CPAN can be found on L<perlmod/CPAN>.
1293 Returns the current login from F</etc/utmp>, if any. If null, use
1296 $login = getlogin || getpwuid($<) || "Kilroy";
1298 Do not consider getlogin() for authentication: it is not as
1299 secure as getpwuid().
1301 =item getpeername SOCKET
1303 Returns the packed sockaddr address of other end of the SOCKET connection.
1306 $hersockaddr = getpeername(SOCK);
1307 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1308 $herhostname = gethostbyaddr($iaddr, AF_INET);
1309 $herstraddr = inet_ntoa($iaddr);
1313 Returns the current process group for the specified PID. Use
1314 a PID of 0 to get the current process group for the
1315 current process. Will raise an exception if used on a machine that
1316 doesn't implement getpgrp(2). If PID is omitted, returns process
1317 group of current process. Note that the POSIX version of getpgrp()
1318 does not accept a PID argument, so only PID==0 is truly portable.
1322 Returns the process id of the parent process.
1324 =item getpriority WHICH,WHO
1326 Returns the current priority for a process, a process group, or a user.
1327 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1328 machine that doesn't implement getpriority(2).
1334 =item gethostbyname NAME
1336 =item getnetbyname NAME
1338 =item getprotobyname NAME
1344 =item getservbyname NAME,PROTO
1346 =item gethostbyaddr ADDR,ADDRTYPE
1348 =item getnetbyaddr ADDR,ADDRTYPE
1350 =item getprotobynumber NUMBER
1352 =item getservbyport PORT,PROTO
1370 =item sethostent STAYOPEN
1372 =item setnetent STAYOPEN
1374 =item setprotoent STAYOPEN
1376 =item setservent STAYOPEN
1390 These routines perform the same functions as their counterparts in the
1391 system library. Within a list context, the return values from the
1392 various get routines are as follows:
1394 ($name,$passwd,$uid,$gid,
1395 $quota,$comment,$gcos,$dir,$shell) = getpw*
1396 ($name,$passwd,$gid,$members) = getgr*
1397 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1398 ($name,$aliases,$addrtype,$net) = getnet*
1399 ($name,$aliases,$proto) = getproto*
1400 ($name,$aliases,$port,$proto) = getserv*
1402 (If the entry doesn't exist you get a null list.)
1404 Within a scalar context, you get the name, unless the function was a
1405 lookup by name, in which case you get the other thing, whatever it is.
1406 (If the entry doesn't exist you get the undefined value.) For example:
1416 The $members value returned by I<getgr*()> is a space separated list of
1417 the login names of the members of the group.
1419 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1420 C, it will be returned to you via C<$?> if the function call fails. The
1421 @addrs value returned by a successful call is a list of the raw
1422 addresses returned by the corresponding system library call. In the
1423 Internet domain, each address is four bytes long and you can unpack it
1424 by saying something like:
1426 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1428 =item getsockname SOCKET
1430 Returns the packed sockaddr address of this end of the SOCKET connection.
1433 $mysockaddr = getsockname(SOCK);
1434 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1436 =item getsockopt SOCKET,LEVEL,OPTNAME
1438 Returns the socket option requested, or undefined if there is an error.
1444 Returns the value of EXPR with filename expansions such as a shell would
1445 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1446 operator, but you can use it directly. If EXPR is omitted, $_ is used.
1447 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1448 L<perlop/"I/O Operators">.
1452 Converts a time as returned by the time function to a 9-element array
1453 with the time localized for the standard Greenwich time zone.
1454 Typically used as follows:
1457 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1460 All array elements are numeric, and come straight out of a struct tm.
1461 In particular this means that $mon has the range 0..11 and $wday has
1462 the range 0..6 with sunday as day 0. Also, $year is the number of
1463 years since 1900, I<not> simply the last two digits of the year.
1465 If EXPR is omitted, does C<gmtime(time())>.
1467 In a scalar context, returns the ctime(3) value:
1469 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1471 Also see the timegm() function provided by the Time::Local module,
1472 and the strftime(3) function available via the POSIX module.
1480 The goto-LABEL form finds the statement labeled with LABEL and resumes
1481 execution there. It may not be used to go into any construct that
1482 requires initialization, such as a subroutine or a foreach loop. It
1483 also can't be used to go into a construct that is optimized away,
1484 or to get out of a block or subroutine given to sort().
1485 It can be used to go almost anywhere else within the dynamic scope,
1486 including out of subroutines, but it's usually better to use some other
1487 construct such as last or die. The author of Perl has never felt the
1488 need to use this form of goto (in Perl, that is--C is another matter).
1490 The goto-EXPR form expects a label name, whose scope will be resolved
1491 dynamically. This allows for computed gotos per FORTRAN, but isn't
1492 necessarily recommended if you're optimizing for maintainability:
1494 goto ("FOO", "BAR", "GLARCH")[$i];
1496 The goto-&NAME form is highly magical, and substitutes a call to the
1497 named subroutine for the currently running subroutine. This is used by
1498 AUTOLOAD subroutines that wish to load another subroutine and then
1499 pretend that the other subroutine had been called in the first place
1500 (except that any modifications to @_ in the current subroutine are
1501 propagated to the other subroutine.) After the goto, not even caller()
1502 will be able to tell that this routine was called first.
1504 =item grep BLOCK LIST
1506 =item grep EXPR,LIST
1508 This is similar in spirit to, but not the same as, grep(1)
1509 and its relatives. In particular, it is not limited to using
1510 regular expressions.
1512 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1513 $_ to each element) and returns the list value consisting of those
1514 elements for which the expression evaluated to TRUE. In a scalar
1515 context, returns the number of times the expression was TRUE.
1517 @foo = grep(!/^#/, @bar); # weed out comments
1521 @foo = grep {!/^#/} @bar; # weed out comments
1523 Note that, because $_ is a reference into the list value, it can be used
1524 to modify the elements of the array. While this is useful and
1525 supported, it can cause bizarre results if the LIST is not a named
1526 array. Similarly, grep returns aliases into the original list,
1527 much like the way that L<Foreach Loops>'s index variable aliases the list
1528 elements. That is, modifying an element of a list returned by grep
1529 actually modifies the element in the original list.
1535 Interprets EXPR as a hex string and returns the corresponding
1536 value. (To convert strings that might start with either 0 or 0x
1537 see L</oct>.) If EXPR is omitted, uses $_.
1539 print hex '0xAf'; # prints '175'
1540 print hex 'aF'; # same
1544 There is no builtin import() function. It is merely an ordinary
1545 method (subroutine) defined (or inherited) by modules that wish to export
1546 names to another module. The use() function calls the import() method
1547 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1549 =item index STR,SUBSTR,POSITION
1551 =item index STR,SUBSTR
1553 Returns the position of the first occurrence of SUBSTR in STR at or after
1554 POSITION. If POSITION is omitted, starts searching from the beginning of
1555 the string. The return value is based at 0 (or whatever you've set the C<$[>
1556 variable to--but don't do that). If the substring is not found, returns
1557 one less than the base, ordinarily -1.
1563 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1565 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1567 Implements the ioctl(2) function. You'll probably have to say
1569 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1571 first to get the correct function definitions. If F<ioctl.ph> doesn't
1572 exist or doesn't have the correct definitions you'll have to roll your
1573 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1574 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1575 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1576 written depending on the FUNCTION--a pointer to the string value of SCALAR
1577 will be passed as the third argument of the actual ioctl call. (If SCALAR
1578 has no string value but does have a numeric value, that value will be
1579 passed rather than a pointer to the string value. To guarantee this to be
1580 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1581 functions are useful for manipulating the values of structures used by
1582 ioctl(). The following example sets the erase character to DEL.
1586 die "NO TIOCGETP" if $@ || !$getp;
1587 $sgttyb_t = "ccccs"; # 4 chars and a short
1588 if (ioctl(STDIN,$getp,$sgttyb)) {
1589 @ary = unpack($sgttyb_t,$sgttyb);
1591 $sgttyb = pack($sgttyb_t,@ary);
1592 ioctl(STDIN,&TIOCSETP,$sgttyb)
1593 || die "Can't ioctl: $!";
1596 The return value of ioctl (and fcntl) is as follows:
1598 if OS returns: then Perl returns:
1600 0 string "0 but true"
1601 anything else that number
1603 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1604 still easily determine the actual value returned by the operating
1607 ($retval = ioctl(...)) || ($retval = -1);
1608 printf "System returned %d\n", $retval;
1610 =item join EXPR,LIST
1612 Joins the separate strings of LIST into a single string with
1613 fields separated by the value of EXPR, and returns the string.
1616 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1618 See L<perlfunc/split>.
1622 Returns a normal array consisting of all the keys of the named hash. (In
1623 a scalar context, returns the number of keys.) The keys are returned in
1624 an apparently random order, but it is the same order as either the
1625 values() or each() function produces (given that the hash has not been
1626 modified). As a side effect, it resets HASH's iterator.
1628 Here is yet another way to print your environment:
1631 @values = values %ENV;
1632 while ($#keys >= 0) {
1633 print pop(@keys), '=', pop(@values), "\n";
1636 or how about sorted by key:
1638 foreach $key (sort(keys %ENV)) {
1639 print $key, '=', $ENV{$key}, "\n";
1642 To sort an array by value, you'll need to use a C<sort> function.
1643 Here's a descending numeric sort of a hash by its values:
1645 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1646 printf "%4d %s\n", $hash{$key}, $key;
1649 As an lvalue C<keys> allows you to increase the number of hash buckets
1650 allocated for the given hash. This can gain you a measure of efficiency if
1651 you know the hash is going to get big. (This is similar to pre-extending
1652 an array by assigning a larger number to $#array.) If you say
1656 then C<%hash> will have at least 200 buckets allocated for it. These
1657 buckets will be retained even if you do C<%hash = ()>, use C<undef
1658 %hash> if you want to free the storage while C<%hash> is still in scope.
1659 You can't shrink the number of buckets allocated for the hash using
1660 C<keys> in this way (but you needn't worry about doing this by accident,
1661 as trying has no effect).
1665 Sends a signal to a list of processes. The first element of
1666 the list must be the signal to send. Returns the number of
1667 processes successfully signaled.
1669 $cnt = kill 1, $child1, $child2;
1672 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1673 process groups instead of processes. (On System V, a negative I<PROCESS>
1674 number will also kill process groups, but that's not portable.) That
1675 means you usually want to use positive not negative signals. You may also
1676 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1682 The C<last> command is like the C<break> statement in C (as used in
1683 loops); it immediately exits the loop in question. If the LABEL is
1684 omitted, the command refers to the innermost enclosing loop. The
1685 C<continue> block, if any, is not executed:
1687 LINE: while (<STDIN>) {
1688 last LINE if /^$/; # exit when done with header
1696 Returns an lowercased version of EXPR. This is the internal function
1697 implementing the \L escape in double-quoted strings.
1698 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1700 If EXPR is omitted, uses $_.
1706 Returns the value of EXPR with the first character lowercased. This is
1707 the internal function implementing the \l escape in double-quoted strings.
1708 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1710 If EXPR is omitted, uses $_.
1716 Returns the length in characters of the value of EXPR. If EXPR is
1717 omitted, returns length of $_.
1719 =item link OLDFILE,NEWFILE
1721 Creates a new filename linked to the old filename. Returns 1 for
1722 success, 0 otherwise.
1724 =item listen SOCKET,QUEUESIZE
1726 Does the same thing that the listen system call does. Returns TRUE if
1727 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1731 A local modifies the listed variables to be local to the enclosing block,
1732 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1733 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1734 local()"> for details.
1736 But you really probably want to be using my() instead, because local() isn't
1737 what most people think of as "local"). See L<perlsub/"Private Variables
1738 via my()"> for details.
1740 =item localtime EXPR
1742 Converts a time as returned by the time function to a 9-element array
1743 with the time analyzed for the local time zone. Typically used as
1747 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1750 All array elements are numeric, and come straight out of a struct tm.
1751 In particular this means that $mon has the range 0..11 and $wday has
1752 the range 0..6 with sunday as day 0. Also, $year is the number of
1753 years since 1900, that is, $year is 123 in year 2023.
1755 If EXPR is omitted, uses the current time (C<localtime(time)>).
1757 In a scalar context, returns the ctime(3) value:
1759 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1761 Also see the Time::Local module, and the strftime(3) and mktime(3)
1762 function available via the POSIX module.
1768 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1771 =item lstat FILEHANDLE
1777 Does the same thing as the stat() function, but stats a symbolic link
1778 instead of the file the symbolic link points to. If symbolic links are
1779 unimplemented on your system, a normal stat() is done.
1781 If EXPR is omitted, stats $_.
1785 The match operator. See L<perlop>.
1787 =item map BLOCK LIST
1791 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1792 element) and returns the list value composed of the results of each such
1793 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1794 may produce zero, one, or more elements in the returned value.
1796 @chars = map(chr, @nums);
1798 translates a list of numbers to the corresponding characters. And
1800 %hash = map { getkey($_) => $_ } @array;
1802 is just a funny way to write
1805 foreach $_ (@array) {
1806 $hash{getkey($_)} = $_;
1809 =item mkdir FILENAME,MODE
1811 Creates the directory specified by FILENAME, with permissions specified
1812 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1813 it returns 0 and sets C<$!> (errno).
1815 =item msgctl ID,CMD,ARG
1817 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1818 must be a variable which will hold the returned msqid_ds structure.
1819 Returns like ioctl: the undefined value for error, "0 but true" for
1820 zero, or the actual return value otherwise.
1822 =item msgget KEY,FLAGS
1824 Calls the System V IPC function msgget(2). Returns the message queue id,
1825 or the undefined value if there is an error.
1827 =item msgsnd ID,MSG,FLAGS
1829 Calls the System V IPC function msgsnd to send the message MSG to the
1830 message queue ID. MSG must begin with the long integer message type,
1831 which may be created with C<pack("l", $type)>. Returns TRUE if
1832 successful, or FALSE if there is an error.
1834 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1836 Calls the System V IPC function msgrcv to receive a message from
1837 message queue ID into variable VAR with a maximum message size of
1838 SIZE. Note that if a message is received, the message type will be the
1839 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1840 of the message type. Returns TRUE if successful, or FALSE if there is
1845 A "my" declares the listed variables to be local (lexically) to the
1846 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1847 more than one value is listed, the list must be placed in parentheses. See
1848 L<perlsub/"Private Variables via my()"> for details.
1854 The C<next> command is like the C<continue> statement in C; it starts
1855 the next iteration of the loop:
1857 LINE: while (<STDIN>) {
1858 next LINE if /^#/; # discard comments
1862 Note that if there were a C<continue> block on the above, it would get
1863 executed even on discarded lines. If the LABEL is omitted, the command
1864 refers to the innermost enclosing loop.
1866 =item no Module LIST
1868 See the "use" function, which "no" is the opposite of.
1874 Interprets EXPR as an octal string and returns the corresponding
1875 value. (If EXPR happens to start off with 0x, interprets it as
1876 a hex string instead.) The following will handle decimal, octal, and
1877 hex in the standard Perl or C notation:
1879 $val = oct($val) if $val =~ /^0/;
1881 If EXPR is omitted, uses $_. This function is commonly used when
1882 a string such as "644" needs to be converted into a file mode, for
1883 example. (Although perl will automatically convert strings into
1884 numbers as needed, this automatic conversion assumes base 10.)
1886 =item open FILEHANDLE,EXPR
1888 =item open FILEHANDLE
1890 Opens the file whose filename is given by EXPR, and associates it with
1891 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1892 name of the real filehandle wanted. If EXPR is omitted, the scalar
1893 variable of the same name as the FILEHANDLE contains the filename.
1894 (Note that lexical variables--those declared with C<my>--will not work
1895 for this purpose; so if you're using C<my>, specify EXPR in your call
1898 If the filename begins with '<' or nothing, the file is opened for input.
1899 If the filename begins with '>', the file is truncated and opened for
1900 output. If the filename begins with '>>', the file is opened for
1901 appending. You can put a '+' in front of the '>' or '<' to indicate that
1902 you want both read and write access to the file; thus '+<' is almost
1903 always preferred for read/write updates--the '+>' mode would clobber the
1904 file first. The prefix and the filename may be separated with spaces.
1905 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1906 'w+', 'a', and 'a+'.
1908 If the filename begins with "|", the filename is interpreted as a command
1909 to which output is to be piped, and if the filename ends with a "|", the
1910 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1911 examples of this. as command which pipes input to us. (You may not have
1912 a raw open() to a command that pipes both in I<and> out, but see
1913 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1916 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1917 nonzero upon success, the undefined value otherwise. If the open
1918 involved a pipe, the return value happens to be the pid of the
1921 If you're unfortunate enough to be running Perl on a system that
1922 distinguishes between text files and binary files (modern operating
1923 systems don't care), then you should check out L</binmode> for tips for
1924 dealing with this. The key distinction between systems that need binmode
1925 and those that don't is their text file formats. Systems like Unix and
1926 Plan9 that delimit lines with a single character, and that encode that
1927 character in C as '\n', do not need C<binmode>. The rest need it.
1932 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1933 while (<ARTICLE>) {...
1935 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1937 open(DBASE, '+<dbase.mine'); # open for update
1939 open(ARTICLE, "caesar <$article |"); # decrypt article
1941 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1943 # process argument list of files along with any includes
1945 foreach $file (@ARGV) {
1946 process($file, 'fh00');
1950 local($filename, $input) = @_;
1951 $input++; # this is a string increment
1952 unless (open($input, $filename)) {
1953 print STDERR "Can't open $filename: $!\n";
1957 while (<$input>) { # note use of indirection
1958 if (/^#include "(.*)"/) {
1959 process($1, $input);
1966 You may also, in the Bourne shell tradition, specify an EXPR beginning
1967 with "E<gt>&", in which case the rest of the string is interpreted as the
1968 name of a filehandle (or file descriptor, if numeric) which is to be
1969 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1970 +E<gt>E<gt>, and +E<lt>. The
1971 mode you specify should match the mode of the original filehandle.
1972 (Duping a filehandle does not take into account any existing contents of
1974 Here is a script that saves, redirects, and restores STDOUT and
1978 open(SAVEOUT, ">&STDOUT");
1979 open(SAVEERR, ">&STDERR");
1981 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1982 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1984 select(STDERR); $| = 1; # make unbuffered
1985 select(STDOUT); $| = 1; # make unbuffered
1987 print STDOUT "stdout 1\n"; # this works for
1988 print STDERR "stderr 1\n"; # subprocesses too
1993 open(STDOUT, ">&SAVEOUT");
1994 open(STDERR, ">&SAVEERR");
1996 print STDOUT "stdout 2\n";
1997 print STDERR "stderr 2\n";
2000 If you specify "E<lt>&=N", where N is a number, then Perl will do an
2001 equivalent of C's fdopen() of that file descriptor; this is more
2002 parsimonious of file descriptors. For example:
2004 open(FILEHANDLE, "<&=$fd")
2006 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
2007 there is an implicit fork done, and the return value of open is the pid
2008 of the child within the parent process, and 0 within the child
2009 process. (Use C<defined($pid)> to determine whether the open was successful.)
2010 The filehandle behaves normally for the parent, but i/o to that
2011 filehandle is piped from/to the STDOUT/STDIN of the child process.
2012 In the child process the filehandle isn't opened--i/o happens from/to
2013 the new STDOUT or STDIN. Typically this is used like the normal
2014 piped open when you want to exercise more control over just how the
2015 pipe command gets executed, such as when you are running setuid, and
2016 don't want to have to scan shell commands for metacharacters.
2017 The following pairs are more or less equivalent:
2019 open(FOO, "|tr '[a-z]' '[A-Z]'");
2020 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2022 open(FOO, "cat -n '$file'|");
2023 open(FOO, "-|") || exec 'cat', '-n', $file;
2025 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2027 NOTE: On any operation which may do a fork, unflushed buffers remain
2028 unflushed in both processes, which means you may need to set C<$|> to
2029 avoid duplicate output.
2031 Closing any piped filehandle causes the parent process to wait for the
2032 child to finish, and returns the status value in C<$?>.
2034 Using the constructor from the IO::Handle package (or one of its
2035 subclasses, such as IO::File or IO::Socket),
2036 you can generate anonymous filehandles which have the scope of whatever
2037 variables hold references to them, and automatically close whenever
2038 and however you leave that scope:
2042 sub read_myfile_munged {
2044 my $handle = new IO::File;
2045 open($handle, "myfile") or die "myfile: $!";
2047 or return (); # Automatically closed here.
2048 mung $first or die "mung failed"; # Or here.
2049 return $first, <$handle> if $ALL; # Or here.
2053 The filename that is passed to open will have leading and trailing
2054 whitespace deleted. To open a file with arbitrary weird
2055 characters in it, it's necessary to protect any leading and trailing
2058 $file =~ s#^(\s)#./$1#;
2059 open(FOO, "< $file\0");
2061 If you want a "real" C open() (see L<open(2)> on your system), then
2062 you should use the sysopen() function. This is another way to
2063 protect your filenames from interpretation. For example:
2066 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2067 or die "sysopen $path: $!";
2068 HANDLE->autoflush(1);
2069 HANDLE->print("stuff $$\n");
2071 print "File contains: ", <HANDLE>;
2073 See L</seek()> for some details about mixing reading and writing.
2075 =item opendir DIRHANDLE,EXPR
2077 Opens a directory named EXPR for processing by readdir(), telldir(),
2078 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2079 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2085 Returns the numeric ascii value of the first character of EXPR. If
2086 EXPR is omitted, uses $_. For the reverse, see L</chr>.
2088 =item pack TEMPLATE,LIST
2090 Takes an array or list of values and packs it into a binary structure,
2091 returning the string containing the structure. The TEMPLATE is a
2092 sequence of characters that give the order and type of values, as
2095 A An ascii string, will be space padded.
2096 a An ascii string, will be null padded.
2097 b A bit string (ascending bit order, like vec()).
2098 B A bit string (descending bit order).
2099 h A hex string (low nybble first).
2100 H A hex string (high nybble first).
2102 c A signed char value.
2103 C An unsigned char value.
2105 s A signed short value.
2106 S An unsigned short value.
2107 (This 'short' is _exactly_ 16 bits, which may differ from
2108 what a local C compiler calls 'short'.)
2110 i A signed integer value.
2111 I An unsigned integer value.
2112 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2113 depends on what a local C compiler calls 'int', and may
2114 even be larger than the 'long' described in the next item.)
2116 l A signed long value.
2117 L An unsigned long value.
2118 (This 'long' is _exactly_ 32 bits, which may differ from
2119 what a local C compiler calls 'long'.)
2121 n A short in "network" (big-endian) order.
2122 N A long in "network" (big-endian) order.
2123 v A short in "VAX" (little-endian) order.
2124 V A long in "VAX" (little-endian) order.
2125 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2126 _exactly_ 32 bits, respectively.)
2128 f A single-precision float in the native format.
2129 d A double-precision float in the native format.
2131 p A pointer to a null-terminated string.
2132 P A pointer to a structure (fixed-length string).
2134 u A uuencoded string.
2136 w A BER compressed integer. Its bytes represent an unsigned
2137 integer in base 128, most significant digit first, with as few
2138 digits as possible. Bit eight (the high bit) is set on each
2139 byte except the last.
2143 @ Null fill to absolute position.
2145 Each letter may optionally be followed by a number which gives a repeat
2146 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2147 pack function will gobble up that many values from the LIST. A * for the
2148 repeat count means to use however many items are left. The "a" and "A"
2149 types gobble just one value, but pack it as a string of length count,
2150 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2151 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2152 fields pack a string that many bits long. The "h" and "H" fields pack a
2153 string that many nybbles long. The "P" packs a pointer to a structure of
2154 the size indicated by the length. Real numbers (floats and doubles) are
2155 in the native machine format only; due to the multiplicity of floating
2156 formats around, and the lack of a standard "network" representation, no
2157 facility for interchange has been made. This means that packed floating
2158 point data written on one machine may not be readable on another - even if
2159 both use IEEE floating point arithmetic (as the endian-ness of the memory
2160 representation is not part of the IEEE spec). Note that Perl uses doubles
2161 internally for all numeric calculation, and converting from double into
2162 float and thence back to double again will lose precision (i.e.,
2163 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2167 $foo = pack("cccc",65,66,67,68);
2169 $foo = pack("c4",65,66,67,68);
2172 $foo = pack("ccxxcc",65,66,67,68);
2175 $foo = pack("s2",1,2);
2176 # "\1\0\2\0" on little-endian
2177 # "\0\1\0\2" on big-endian
2179 $foo = pack("a4","abcd","x","y","z");
2182 $foo = pack("aaaa","abcd","x","y","z");
2185 $foo = pack("a14","abcdefg");
2186 # "abcdefg\0\0\0\0\0\0\0"
2188 $foo = pack("i9pl", gmtime);
2189 # a real struct tm (on my system anyway)
2192 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2195 The same template may generally also be used in the unpack function.
2197 =item package NAMESPACE
2199 Declares the compilation unit as being in the given namespace. The scope
2200 of the package declaration is from the declaration itself through the end of
2201 the enclosing block (the same scope as the local() operator). All further
2202 unqualified dynamic identifiers will be in this namespace. A package
2203 statement affects only dynamic variables--including those you've used
2204 local() on--but I<not> lexical variables created with my(). Typically it
2205 would be the first declaration in a file to be included by the C<require>
2206 or C<use> operator. You can switch into a package in more than one place;
2207 it influences merely which symbol table is used by the compiler for the
2208 rest of that block. You can refer to variables and filehandles in other
2209 packages by prefixing the identifier with the package name and a double
2210 colon: C<$Package::Variable>. If the package name is null, the C<main>
2211 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2213 See L<perlmod/"Packages"> for more information about packages, modules,
2214 and classes. See L<perlsub> for other scoping issues.
2216 =item pipe READHANDLE,WRITEHANDLE
2218 Opens a pair of connected pipes like the corresponding system call.
2219 Note that if you set up a loop of piped processes, deadlock can occur
2220 unless you are very careful. In addition, note that Perl's pipes use
2221 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2222 after each command, depending on the application.
2224 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2225 for examples of such things.
2231 Pops and returns the last value of the array, shortening the array by
2232 1. Has a similar effect to
2234 $tmp = $ARRAY[$#ARRAY--];
2236 If there are no elements in the array, returns the undefined value.
2237 If ARRAY is omitted, pops the
2238 @ARGV array in the main program, and the @_ array in subroutines, just
2245 Returns the offset of where the last C<m//g> search left off for the variable
2246 is in question ($_ is used when the variable is not specified). May be
2247 modified to change that offset. Such modification will also influence
2248 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2251 =item print FILEHANDLE LIST
2257 Prints a string or a comma-separated list of strings. Returns TRUE
2258 if successful. FILEHANDLE may be a scalar variable name, in which case
2259 the variable contains the name of or a reference to the filehandle, thus introducing one
2260 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2261 token is a term, it may be misinterpreted as an operator unless you
2262 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2263 omitted, prints by default to standard output (or to the last selected
2264 output channel--see L</select>). If LIST is also omitted, prints $_ to
2265 STDOUT. To set the default output channel to something other than
2266 STDOUT use the select operation. Note that, because print takes a
2267 LIST, anything in the LIST is evaluated in a list context, and any
2268 subroutine that you call will have one or more of its expressions
2269 evaluated in a list context. Also be careful not to follow the print
2270 keyword with a left parenthesis unless you want the corresponding right
2271 parenthesis to terminate the arguments to the print--interpose a + or
2272 put parentheses around all the arguments.
2274 Note that if you're storing FILEHANDLES in an array or other expression,
2275 you will have to use a block returning its value instead:
2277 print { $files[$i] } "stuff\n";
2278 print { $OK ? STDOUT : STDERR } "stuff\n";
2280 =item printf FILEHANDLE FORMAT, LIST
2282 =item printf FORMAT, LIST
2284 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2285 of the list will be interpreted as the printf format. If C<use locale> is
2286 in effect, the character used for the decimal point in formatted real numbers
2287 is affected by the LC_NUMERIC locale. See L<perllocale>.
2289 Don't fall into the trap of using a printf() when a simple
2290 print() would do. The print() is more efficient, and less
2293 =item prototype FUNCTION
2295 Returns the prototype of a function as a string (or C<undef> if the
2296 function has no prototype). FUNCTION is a reference to, or the name of,
2297 the function whose prototype you want to retrieve.
2299 =item push ARRAY,LIST
2301 Treats ARRAY as a stack, and pushes the values of LIST
2302 onto the end of ARRAY. The length of ARRAY increases by the length of
2303 LIST. Has the same effect as
2306 $ARRAY[++$#ARRAY] = $value;
2309 but is more efficient. Returns the new number of elements in the array.
2319 Generalized quotes. See L<perlop>.
2321 =item quotemeta EXPR
2325 Returns the value of EXPR with all non-alphanumeric
2326 characters backslashed. (That is, all characters not matching
2327 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2328 returned string, regardless of any locale settings.)
2329 This is the internal function implementing
2330 the \Q escape in double-quoted strings.
2332 If EXPR is omitted, uses $_.
2338 Returns a random fractional number between 0 and the value of EXPR.
2339 (EXPR should be positive.) If EXPR is omitted, returns a value between
2340 0 and 1. Automatically calls srand() unless srand() has already been
2341 called. See also srand().
2343 (Note: If your rand function consistently returns numbers that are too
2344 large or too small, then your version of Perl was probably compiled
2345 with the wrong number of RANDBITS.)
2347 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2349 =item read FILEHANDLE,SCALAR,LENGTH
2351 Attempts to read LENGTH bytes of data into variable SCALAR from the
2352 specified FILEHANDLE. Returns the number of bytes actually read, or
2353 undef if there was an error. SCALAR will be grown or shrunk to the
2354 length actually read. An OFFSET may be specified to place the read
2355 data at some other place than the beginning of the string. This call
2356 is actually implemented in terms of stdio's fread call. To get a true
2357 read system call, see sysread().
2359 =item readdir DIRHANDLE
2361 Returns the next directory entry for a directory opened by opendir().
2362 If used in a list context, returns all the rest of the entries in the
2363 directory. If there are no more entries, returns an undefined value in
2364 a scalar context or a null list in a list context.
2366 If you're planning to filetest the return values out of a readdir(), you'd
2367 better prepend the directory in question. Otherwise, because we didn't
2368 chdir() there, it would have been testing the wrong file.
2370 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2371 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2378 Returns the value of a symbolic link, if symbolic links are
2379 implemented. If not, gives a fatal error. If there is some system
2380 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2383 =item recv SOCKET,SCALAR,LEN,FLAGS
2385 Receives a message on a socket. Attempts to receive LENGTH bytes of
2386 data into variable SCALAR from the specified SOCKET filehandle.
2387 Actually does a C recvfrom(), so that it can returns the address of the
2388 sender. Returns the undefined value if there's an error. SCALAR will
2389 be grown or shrunk to the length actually read. Takes the same flags
2390 as the system call of the same name.
2391 See L<perlipc/"UDP: Message Passing"> for examples.
2397 The C<redo> command restarts the loop block without evaluating the
2398 conditional again. The C<continue> block, if any, is not executed. If
2399 the LABEL is omitted, the command refers to the innermost enclosing
2400 loop. This command is normally used by programs that want to lie to
2401 themselves about what was just input:
2403 # a simpleminded Pascal comment stripper
2404 # (warning: assumes no { or } in strings)
2405 LINE: while (<STDIN>) {
2406 while (s|({.*}.*){.*}|$1 |) {}
2411 if (/}/) { # end of comment?
2424 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2425 is not specified, $_ will be used. The value returned depends on the
2426 type of thing the reference is a reference to.
2427 Builtin types include:
2436 If the referenced object has been blessed into a package, then that package
2437 name is returned instead. You can think of ref() as a typeof() operator.
2439 if (ref($r) eq "HASH") {
2440 print "r is a reference to a hash.\n";
2443 print "r is not a reference at all.\n";
2446 See also L<perlref>.
2448 =item rename OLDNAME,NEWNAME
2450 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2451 not work across file system boundaries.
2457 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2458 supplied. If EXPR is numeric, demands that the current version of Perl
2459 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2461 Otherwise, demands that a library file be included if it hasn't already
2462 been included. The file is included via the do-FILE mechanism, which is
2463 essentially just a variety of eval(). Has semantics similar to the following
2467 local($filename) = @_;
2468 return 1 if $INC{$filename};
2469 local($realfilename,$result);
2471 foreach $prefix (@INC) {
2472 $realfilename = "$prefix/$filename";
2473 if (-f $realfilename) {
2474 $result = do $realfilename;
2478 die "Can't find $filename in \@INC";
2481 die "$filename did not return true value" unless $result;
2482 $INC{$filename} = $realfilename;
2486 Note that the file will not be included twice under the same specified
2487 name. The file must return TRUE as the last statement to indicate
2488 successful execution of any initialization code, so it's customary to
2489 end such a file with "1;" unless you're sure it'll return TRUE
2490 otherwise. But it's better just to put the "C<1;>", in case you add more
2493 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2494 replaces "F<::>" with "F</>" in the filename for you,
2495 to make it easy to load standard modules. This form of loading of
2496 modules does not risk altering your namespace.
2498 For a yet-more-powerful import facility, see L</use> and
2505 Generally used in a C<continue> block at the end of a loop to clear
2506 variables and reset ?? searches so that they work again. The
2507 expression is interpreted as a list of single characters (hyphens
2508 allowed for ranges). All variables and arrays beginning with one of
2509 those letters are reset to their pristine state. If the expression is
2510 omitted, one-match searches (?pattern?) are reset to match again. Resets
2511 only variables or searches in the current package. Always returns
2514 reset 'X'; # reset all X variables
2515 reset 'a-z'; # reset lower case variables
2516 reset; # just reset ?? searches
2518 Resetting "A-Z" is not recommended because you'll wipe out your
2519 ARGV and ENV arrays. Resets only package variables--lexical variables
2520 are unaffected, but they clean themselves up on scope exit anyway,
2521 so you'll probably want to use them instead. See L</my>.
2527 Returns from a subroutine, eval(), or do FILE with the value of the
2528 given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2529 context, depending on how the return value will be used, and the context
2530 may vary from one execution to the next (see wantarray()). If no EXPR
2531 is given, returns an empty list in a list context, an undefined value in
2532 a scalar context, or nothing in a void context.
2534 (Note that in the absence of a return, a subroutine, eval, or do FILE
2535 will automatically return the value of the last expression evaluated.)
2539 In a list context, returns a list value consisting of the elements
2540 of LIST in the opposite order. In a scalar context, concatenates the
2541 elements of LIST, and returns a string value consisting of those bytes,
2542 but in the opposite order.
2544 print reverse <>; # line tac, last line first
2546 undef $/; # for efficiency of <>
2547 print scalar reverse <>; # byte tac, last line tsrif
2549 This operator is also handy for inverting a hash, although there are some
2550 caveats. If a value is duplicated in the original hash, only one of those
2551 can be represented as a key in the inverted hash. Also, this has to
2552 unwind one hash and build a whole new one, which may take some time
2555 %by_name = reverse %by_address; # Invert the hash
2557 =item rewinddir DIRHANDLE
2559 Sets the current position to the beginning of the directory for the
2560 readdir() routine on DIRHANDLE.
2562 =item rindex STR,SUBSTR,POSITION
2564 =item rindex STR,SUBSTR
2566 Works just like index except that it returns the position of the LAST
2567 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2568 last occurrence at or before that position.
2570 =item rmdir FILENAME
2574 Deletes the directory specified by FILENAME if it is empty. If it
2575 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2576 FILENAME is omitted, uses $_.
2580 The substitution operator. See L<perlop>.
2584 Forces EXPR to be interpreted in a scalar context and returns the value
2587 @counts = ( scalar @a, scalar @b, scalar @c );
2589 There is no equivalent operator to force an expression to
2590 be interpolated in a list context because it's in practice never
2591 needed. If you really wanted to do so, however, you could use
2592 the construction C<@{[ (some expression) ]}>, but usually a simple
2593 C<(some expression)> suffices.
2595 =item seek FILEHANDLE,POSITION,WHENCE
2597 Sets FILEHANDLE's position, just like the fseek() call of stdio.
2598 FILEHANDLE may be an expression whose value gives the name of the
2599 filehandle. The values for WHENCE are 0 to set the new position to
2600 POSITION, 1 to set it to the current position plus POSITION, and 2 to
2601 set it to EOF plus POSITION (typically negative). For WHENCE you may
2602 use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2603 IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2605 If you want to position file for sysread() or syswrite(), don't use
2606 seek() -- buffering makes its effect on the file's system position
2607 unpredictable and non-portable. Use sysseek() instead.
2609 On some systems you have to do a seek whenever you switch between reading
2610 and writing. Amongst other things, this may have the effect of calling
2611 stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2616 This is also useful for applications emulating C<tail -f>. Once you hit
2617 EOF on your read, and then sleep for a while, you might have to stick in a
2618 seek() to reset things. The seek() doesn't change the current position,
2619 but it I<does> clear the end-of-file condition on the handle, so that the
2620 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2622 If that doesn't work (some stdios are particularly cantankerous), then
2623 you may need something more like this:
2626 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2627 # search for some stuff and put it into files
2629 sleep($for_a_while);
2630 seek(FILE, $curpos, 0);
2633 =item seekdir DIRHANDLE,POS
2635 Sets the current position for the readdir() routine on DIRHANDLE. POS
2636 must be a value returned by telldir(). Has the same caveats about
2637 possible directory compaction as the corresponding system library
2640 =item select FILEHANDLE
2644 Returns the currently selected filehandle. Sets the current default
2645 filehandle for output, if FILEHANDLE is supplied. This has two
2646 effects: first, a C<write> or a C<print> without a filehandle will
2647 default to this FILEHANDLE. Second, references to variables related to
2648 output will refer to this output channel. For example, if you have to
2649 set the top of form format for more than one output channel, you might
2657 FILEHANDLE may be an expression whose value gives the name of the
2658 actual filehandle. Thus:
2660 $oldfh = select(STDERR); $| = 1; select($oldfh);
2662 Some programmers may prefer to think of filehandles as objects with
2663 methods, preferring to write the last example as:
2666 STDERR->autoflush(1);
2668 =item select RBITS,WBITS,EBITS,TIMEOUT
2670 This calls the select(2) system call with the bit masks specified, which
2671 can be constructed using fileno() and vec(), along these lines:
2673 $rin = $win = $ein = '';
2674 vec($rin,fileno(STDIN),1) = 1;
2675 vec($win,fileno(STDOUT),1) = 1;
2678 If you want to select on many filehandles you might wish to write a
2682 local(@fhlist) = split(' ',$_[0]);
2685 vec($bits,fileno($_),1) = 1;
2689 $rin = fhbits('STDIN TTY SOCK');
2693 ($nfound,$timeleft) =
2694 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2696 or to block until something becomes ready just do this
2698 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2700 Most systems do not bother to return anything useful in $timeleft, so
2701 calling select() in a scalar context just returns $nfound.
2703 Any of the bit masks can also be undef. The timeout, if specified, is
2704 in seconds, which may be fractional. Note: not all implementations are
2705 capable of returning the $timeleft. If not, they always return
2706 $timeleft equal to the supplied $timeout.
2708 You can effect a sleep of 250 milliseconds this way:
2710 select(undef, undef, undef, 0.25);
2712 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2713 with select(). You have to use sysread() instead.
2715 =item semctl ID,SEMNUM,CMD,ARG
2717 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2718 &GETALL, then ARG must be a variable which will hold the returned
2719 semid_ds structure or semaphore value array. Returns like ioctl: the
2720 undefined value for error, "0 but true" for zero, or the actual return
2723 =item semget KEY,NSEMS,FLAGS
2725 Calls the System V IPC function semget. Returns the semaphore id, or
2726 the undefined value if there is an error.
2728 =item semop KEY,OPSTRING
2730 Calls the System V IPC function semop to perform semaphore operations
2731 such as signaling and waiting. OPSTRING must be a packed array of
2732 semop structures. Each semop structure can be generated with
2733 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2734 operations is implied by the length of OPSTRING. Returns TRUE if
2735 successful, or FALSE if there is an error. As an example, the
2736 following code waits on semaphore $semnum of semaphore id $semid:
2738 $semop = pack("sss", $semnum, -1, 0);
2739 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2741 To signal the semaphore, replace "-1" with "1".
2743 =item send SOCKET,MSG,FLAGS,TO
2745 =item send SOCKET,MSG,FLAGS
2747 Sends a message on a socket. Takes the same flags as the system call
2748 of the same name. On unconnected sockets you must specify a
2749 destination to send TO, in which case it does a C sendto(). Returns
2750 the number of characters sent, or the undefined value if there is an
2752 See L<perlipc/"UDP: Message Passing"> for examples.
2754 =item setpgrp PID,PGRP
2756 Sets the current process group for the specified PID, 0 for the current
2757 process. Will produce a fatal error if used on a machine that doesn't
2758 implement setpgrp(2). If the arguments are omitted, it defaults to
2759 0,0. Note that the POSIX version of setpgrp() does not accept any
2760 arguments, so only setpgrp 0,0 is portable.
2762 =item setpriority WHICH,WHO,PRIORITY
2764 Sets the current priority for a process, a process group, or a user.
2765 (See setpriority(2).) Will produce a fatal error if used on a machine
2766 that doesn't implement setpriority(2).
2768 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2770 Sets the socket option requested. Returns undefined if there is an
2771 error. OPTVAL may be specified as undef if you don't want to pass an
2778 Shifts the first value of the array off and returns it, shortening the
2779 array by 1 and moving everything down. If there are no elements in the
2780 array, returns the undefined value. If ARRAY is omitted, shifts the
2781 @ARGV array in the main program, and the @_ array in subroutines.
2782 (This is determined lexically.) See also unshift(), push(), and pop().
2783 Shift() and unshift() do the same thing to the left end of an array
2784 that pop() and push() do to the right end.
2786 =item shmctl ID,CMD,ARG
2788 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2789 must be a variable which will hold the returned shmid_ds structure.
2790 Returns like ioctl: the undefined value for error, "0 but true" for
2791 zero, or the actual return value otherwise.
2793 =item shmget KEY,SIZE,FLAGS
2795 Calls the System V IPC function shmget. Returns the shared memory
2796 segment id, or the undefined value if there is an error.
2798 =item shmread ID,VAR,POS,SIZE
2800 =item shmwrite ID,STRING,POS,SIZE
2802 Reads or writes the System V shared memory segment ID starting at
2803 position POS for size SIZE by attaching to it, copying in/out, and
2804 detaching from it. When reading, VAR must be a variable which will
2805 hold the data read. When writing, if STRING is too long, only SIZE
2806 bytes are used; if STRING is too short, nulls are written to fill out
2807 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2809 =item shutdown SOCKET,HOW
2811 Shuts down a socket connection in the manner indicated by HOW, which
2812 has the same interpretation as in the system call of the same name.
2818 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2821 For the inverse sine operation, you may use the POSIX::asin()
2822 function, or use this relation:
2824 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2830 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2831 May be interrupted by sending the process a SIGALRM. Returns the
2832 number of seconds actually slept. You probably cannot mix alarm() and
2833 sleep() calls, because sleep() is often implemented using alarm().
2835 On some older systems, it may sleep up to a full second less than what
2836 you requested, depending on how it counts seconds. Most modern systems
2837 always sleep the full amount.
2839 For delays of finer granularity than one second, you may use Perl's
2840 syscall() interface to access setitimer(2) if your system supports it,
2841 or else see L</select()> below.
2843 See also the POSIX module's sigpause() function.
2845 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2847 Opens a socket of the specified kind and attaches it to filehandle
2848 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2849 system call of the same name. You should "use Socket;" first to get
2850 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2852 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2854 Creates an unnamed pair of sockets in the specified domain, of the
2855 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2856 for the system call of the same name. If unimplemented, yields a fatal
2857 error. Returns TRUE if successful.
2859 =item sort SUBNAME LIST
2861 =item sort BLOCK LIST
2865 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2866 is omitted, sorts in standard string comparison order. If SUBNAME is
2867 specified, it gives the name of a subroutine that returns an integer
2868 less than, equal to, or greater than 0, depending on how the elements
2869 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2870 operators are extremely useful in such routines.) SUBNAME may be a
2871 scalar variable name, in which case the value provides the name of the
2872 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2873 an anonymous, in-line sort subroutine.
2875 In the interests of efficiency the normal calling code for subroutines is
2876 bypassed, with the following effects: the subroutine may not be a
2877 recursive subroutine, and the two elements to be compared are passed into
2878 the subroutine not via @_ but as the package global variables $a and
2879 $b (see example below). They are passed by reference, so don't
2880 modify $a and $b. And don't try to declare them as lexicals either.
2882 You also cannot exit out of the sort block or subroutine using any of the
2883 loop control operators described in L<perlsyn> or with goto().
2885 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2886 current collation locale. See L<perllocale>.
2891 @articles = sort @files;
2893 # same thing, but with explicit sort routine
2894 @articles = sort {$a cmp $b} @files;
2896 # now case-insensitively
2897 @articles = sort {uc($a) cmp uc($b)} @files;
2899 # same thing in reversed order
2900 @articles = sort {$b cmp $a} @files;
2902 # sort numerically ascending
2903 @articles = sort {$a <=> $b} @files;
2905 # sort numerically descending
2906 @articles = sort {$b <=> $a} @files;
2908 # sort using explicit subroutine name
2910 $age{$a} <=> $age{$b}; # presuming numeric
2912 @sortedclass = sort byage @class;
2914 # this sorts the %age hash by value instead of key
2915 # using an in-line function
2916 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2918 sub backwards { $b cmp $a; }
2919 @harry = ('dog','cat','x','Cain','Abel');
2920 @george = ('gone','chased','yz','Punished','Axed');
2922 # prints AbelCaincatdogx
2923 print sort backwards @harry;
2924 # prints xdogcatCainAbel
2925 print sort @george, 'to', @harry;
2926 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2928 # inefficiently sort by descending numeric compare using
2929 # the first integer after the first = sign, or the
2930 # whole record case-insensitively otherwise
2933 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2938 # same thing, but much more efficiently;
2939 # we'll build auxiliary indices instead
2943 push @nums, /=(\d+)/;
2948 $nums[$b] <=> $nums[$a]
2950 $caps[$a] cmp $caps[$b]
2954 # same thing using a Schwartzian Transform (no temps)
2955 @new = map { $_->[0] }
2956 sort { $b->[1] <=> $a->[1]
2959 } map { [$_, /=(\d+)/, uc($_)] } @old;
2961 If you're using strict, you I<MUST NOT> declare $a
2962 and $b as lexicals. They are package globals. That means
2963 if you're in the C<main> package, it's
2965 @articles = sort {$main::b <=> $main::a} @files;
2969 @articles = sort {$::b <=> $::a} @files;
2971 but if you're in the C<FooPack> package, it's
2973 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2975 The comparison function is required to behave. If it returns
2976 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2977 sometimes saying the opposite, for example) the Perl interpreter will
2978 probably crash and dump core. This is entirely due to and dependent
2979 upon your system's qsort(3) library routine; this routine often avoids
2980 sanity checks in the interest of speed.
2982 =item splice ARRAY,OFFSET,LENGTH,LIST
2984 =item splice ARRAY,OFFSET,LENGTH
2986 =item splice ARRAY,OFFSET
2988 Removes the elements designated by OFFSET and LENGTH from an array, and
2989 replaces them with the elements of LIST, if any. Returns the elements
2990 removed from the array. The array grows or shrinks as necessary. If
2991 LENGTH is omitted, removes everything from OFFSET onward. The
2992 following equivalences hold (assuming C<$[ == 0>):
2994 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2995 pop(@a) splice(@a,-1)
2996 shift(@a) splice(@a,0,1)
2997 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2998 $a[$x] = $y splice(@a,$x,1,$y);
3000 Example, assuming array lengths are passed before arrays:
3002 sub aeq { # compare two list values
3003 local(@a) = splice(@_,0,shift);
3004 local(@b) = splice(@_,0,shift);
3005 return 0 unless @a == @b; # same len?
3007 return 0 if pop(@a) ne pop(@b);
3011 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3013 =item split /PATTERN/,EXPR,LIMIT
3015 =item split /PATTERN/,EXPR
3017 =item split /PATTERN/
3021 Splits a string into an array of strings, and returns it.
3023 If not in a list context, returns the number of fields found and splits into
3024 the @_ array. (In a list context, you can force the split into @_ by
3025 using C<??> as the pattern delimiters, but it still returns the array
3026 value.) The use of implicit split to @_ is deprecated, however.
3028 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
3029 splits on whitespace (after skipping any leading whitespace). Anything
3030 matching PATTERN is taken to be a delimiter separating the fields. (Note
3031 that the delimiter may be longer than one character.) If LIMIT is
3032 specified and is not negative, splits into no more than that many fields
3033 (though it may split into fewer). If LIMIT is unspecified, trailing null
3034 fields are stripped (which potential users of pop() would do well to
3035 remember). If LIMIT is negative, it is treated as if an arbitrarily large
3036 LIMIT had been specified.
3038 A pattern matching the null string (not to be confused with
3039 a null pattern C<//>, which is just one member of the set of patterns
3040 matching a null string) will split the value of EXPR into separate
3041 characters at each point it matches that way. For example:
3043 print join(':', split(/ */, 'hi there'));
3045 produces the output 'h:i:t:h:e:r:e'.
3047 The LIMIT parameter can be used to split a line partially
3049 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3051 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3052 one larger than the number of variables in the list, to avoid
3053 unnecessary work. For the list above LIMIT would have been 4 by
3054 default. In time critical applications it behooves you not to split
3055 into more fields than you really need.
3057 If the PATTERN contains parentheses, additional array elements are
3058 created from each matching substring in the delimiter.
3060 split(/([,-])/, "1-10,20", 3);
3062 produces the list value
3064 (1, '-', 10, ',', 20)
3066 If you had the entire header of a normal Unix email message in $header,
3067 you could split it up into fields and their values this way:
3069 $header =~ s/\n\s+/ /g; # fix continuation lines
3070 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3072 The pattern C</PATTERN/> may be replaced with an expression to specify
3073 patterns that vary at runtime. (To do runtime compilation only once,
3074 use C</$variable/o>.)
3076 As a special case, specifying a PATTERN of space (C<' '>) will split on
3077 white space just as split with no arguments does. Thus, split(' ') can
3078 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3079 will give you as many null initial fields as there are leading spaces.
3080 A split on /\s+/ is like a split(' ') except that any leading
3081 whitespace produces a null first field. A split with no arguments
3082 really does a C<split(' ', $_)> internally.
3086 open(passwd, '/etc/passwd');
3088 ($login, $passwd, $uid, $gid, $gcos,
3089 $home, $shell) = split(/:/);
3093 (Note that $shell above will still have a newline on it. See L</chop>,
3094 L</chomp>, and L</join>.)
3096 =item sprintf FORMAT, LIST
3098 Returns a string formatted by the usual printf conventions of the
3099 C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3100 on your system for an explanation of the general principles.
3102 Perl does all of its own sprintf() formatting -- it emulates the C
3103 function sprintf(), but it doesn't use it (except for floating-point
3104 numbers, and even then only the standard modifiers are allowed). As a
3105 result, any non-standard extensions in your local sprintf() are not
3106 available from Perl.
3108 Perl's sprintf() permits the following universally-known conversions:
3111 %c a character with the given number
3113 %d a signed integer, in decimal
3114 %u an unsigned integer, in decimal
3115 %o an unsigned integer, in octal
3116 %x an unsigned integer, in hexadecimal
3117 %e a floating-point number, in scientific notation
3118 %f a floating-point number, in fixed decimal notation
3119 %g a floating-point number, in %e or %f notation
3121 In addition, Perl permits the following widely-supported conversions:
3123 %X like %x, but using upper-case letters
3124 %E like %e, but using an upper-case "E"
3125 %G like %g, but with an upper-case "E" (if applicable)
3126 %p a pointer (outputs the Perl value's address in hexadecimal)
3127 %n special: *stores* the number of characters output so far
3128 into the next variable in the parameter list
3130 Finally, for backward (and we do mean "backward") compatibility, Perl
3131 permits these unnecessary but widely-supported conversions:
3134 %D a synonym for %ld
3135 %U a synonym for %lu
3136 %O a synonym for %lo
3139 Perl permits the following universally-known flags between the C<%>
3140 and the conversion letter:
3142 space prefix positive number with a space
3143 + prefix positive number with a plus sign
3144 - left-justify within the field
3145 0 use zeros, not spaces, to right-justify
3146 # prefix octal with "0", hex with "0x"
3147 number minimum field width
3148 .number "precision": digits after decimal point for floating-point,
3149 max length for string, minimum length for integer
3150 l interpret integer as C type "long" or "unsigned long"
3151 h interpret integer as C type "short" or "unsigned short"
3153 There is also one Perl-specific flag:
3155 V interpret integer as Perl's standard integer type
3157 Where a number would appear in the flags, an asterisk ("*") may be
3158 used instead, in which case Perl uses the next item in the parameter
3159 list as the given number (that is, as the field width or precision).
3160 If a field width obtained through "*" is negative, it has the same
3161 effect as the '-' flag: left-justification.
3163 If C<use locale> is in effect, the character used for the decimal
3164 point in formatted real numbers is affected by the LC_NUMERIC locale.
3171 Return the square root of EXPR. If EXPR is omitted, returns square
3178 Sets the random number seed for the C<rand> operator. If EXPR is
3179 omitted, uses a semi-random value based on the current time and process
3180 ID, among other things. In versions of Perl prior to 5.004 the default
3181 seed was just the current time(). This isn't a particularly good seed,
3182 so many old programs supply their own seed value (often C<time ^ $$> or
3183 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3185 In fact, it's usually not necessary to call srand() at all, because if
3186 it is not called explicitly, it is called implicitly at the first use of
3187 the C<rand> operator. However, this was not the case in version of Perl
3188 before 5.004, so if your script will run under older Perl versions, it
3189 should call srand().
3191 Note that you need something much more random than the default seed for
3192 cryptographic purposes. Checksumming the compressed output of one or more
3193 rapidly changing operating system status programs is the usual method. For
3196 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3198 If you're particularly concerned with this, see the Math::TrulyRandom
3201 Do I<not> call srand() multiple times in your program unless you know
3202 exactly what you're doing and why you're doing it. The point of the
3203 function is to "seed" the rand() function so that rand() can produce
3204 a different sequence each time you run your program. Just do it once at the
3205 top of your program, or you I<won't> get random numbers out of rand()!
3207 Frequently called programs (like CGI scripts) that simply use
3211 for a seed can fall prey to the mathematical property that
3215 one-third of the time. So don't do that.
3217 =item stat FILEHANDLE
3223 Returns a 13-element array giving the status info for a file, either the
3224 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3225 stats $_. Returns a null list if the stat fails. Typically used as
3229 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3230 $atime,$mtime,$ctime,$blksize,$blocks)
3233 Not all fields are supported on all filesystem types. Here are the
3234 meaning of the fields:
3236 0 dev device number of filesystem
3238 2 mode file mode (type and permissions)
3239 3 nlink number of (hard) links to the file
3240 4 uid numeric user ID of file's owner
3241 5 gid numeric group ID of file's owner
3242 6 rdev the device identifier (special files only)
3243 7 size total size of file, in bytes
3244 8 atime last access time since the epoch
3245 9 mtime last modify time since the epoch
3246 10 ctime inode change time (NOT creation time!) since the epoch
3247 11 blksize preferred block size for file system I/O
3248 12 blocks actual number of blocks allocated
3250 (The epoch was at 00:00 January 1, 1970 GMT.)
3252 If stat is passed the special filehandle consisting of an underline, no
3253 stat is done, but the current contents of the stat structure from the
3254 last stat or filetest are returned. Example:
3256 if (-x $file && (($d) = stat(_)) && $d < 0) {
3257 print "$file is executable NFS file\n";
3260 (This works on machines only for which the device number is negative under NFS.)
3266 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3267 doing many pattern matches on the string before it is next modified.
3268 This may or may not save time, depending on the nature and number of
3269 patterns you are searching on, and on the distribution of character
3270 frequencies in the string to be searched -- you probably want to compare
3271 run times with and without it to see which runs faster. Those loops
3272 which scan for many short constant strings (including the constant
3273 parts of more complex patterns) will benefit most. You may have only
3274 one study active at a time -- if you study a different scalar the first
3275 is "unstudied". (The way study works is this: a linked list of every
3276 character in the string to be searched is made, so we know, for
3277 example, where all the 'k' characters are. From each search string,
3278 the rarest character is selected, based on some static frequency tables
3279 constructed from some C programs and English text. Only those places
3280 that contain this "rarest" character are examined.)
3282 For example, here is a loop which inserts index producing entries
3283 before any line containing a certain pattern:
3287 print ".IX foo\n" if /\bfoo\b/;
3288 print ".IX bar\n" if /\bbar\b/;
3289 print ".IX blurfl\n" if /\bblurfl\b/;
3294 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3295 will be looked at, because "f" is rarer than "o". In general, this is
3296 a big win except in pathological cases. The only question is whether
3297 it saves you more time than it took to build the linked list in the
3300 Note that if you have to look for strings that you don't know till
3301 runtime, you can build an entire loop as a string and eval that to
3302 avoid recompiling all your patterns all the time. Together with
3303 undefining $/ to input entire files as one record, this can be very
3304 fast, often faster than specialized programs like fgrep(1). The following
3305 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3306 out the names of those files that contain a match:
3308 $search = 'while (<>) { study;';
3309 foreach $word (@words) {
3310 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3315 eval $search; # this screams
3316 $/ = "\n"; # put back to normal input delimiter
3317 foreach $file (sort keys(%seen)) {
3325 =item sub NAME BLOCK
3327 This is subroutine definition, not a real function I<per se>. With just a
3328 NAME (and possibly prototypes), it's just a forward declaration. Without
3329 a NAME, it's an anonymous function declaration, and does actually return a
3330 value: the CODE ref of the closure you just created. See L<perlsub> and
3331 L<perlref> for details.
3333 =item substr EXPR,OFFSET,LEN
3335 =item substr EXPR,OFFSET
3337 Extracts a substring out of EXPR and returns it. First character is at
3338 offset 0, or whatever you've set C<$[> to (but don't do that).
3339 If OFFSET is negative, starts
3340 that far from the end of the string. If LEN is omitted, returns
3341 everything to the end of the string. If LEN is negative, leaves that
3342 many characters off the end of the string.
3344 You can use the substr() function
3345 as an lvalue, in which case EXPR must be an lvalue. If you assign
3346 something shorter than LEN, the string will shrink, and if you assign
3347 something longer than LEN, the string will grow to accommodate it. To
3348 keep the string the same length you may need to pad or chop your value
3351 =item symlink OLDFILE,NEWFILE
3353 Creates a new filename symbolically linked to the old filename.
3354 Returns 1 for success, 0 otherwise. On systems that don't support
3355 symbolic links, produces a fatal error at run time. To check for that,
3358 $symlink_exists = (eval {symlink("","")};, $@ eq '');
3362 Calls the system call specified as the first element of the list,
3363 passing the remaining elements as arguments to the system call. If
3364 unimplemented, produces a fatal error. The arguments are interpreted
3365 as follows: if a given argument is numeric, the argument is passed as
3366 an int. If not, the pointer to the string value is passed. You are
3367 responsible to make sure a string is pre-extended long enough to
3368 receive any result that might be written into a string. If your
3369 integer arguments are not literals and have never been interpreted in a
3370 numeric context, you may need to add 0 to them to force them to look
3373 require 'syscall.ph'; # may need to run h2ph
3374 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3376 Note that Perl supports passing of up to only 14 arguments to your system call,
3377 which in practice should usually suffice.
3379 =item sysopen FILEHANDLE,FILENAME,MODE
3381 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3383 Opens the file whose filename is given by FILENAME, and associates it
3384 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3385 the name of the real filehandle wanted. This function calls the
3386 underlying operating system's C<open> function with the parameters
3387 FILENAME, MODE, PERMS.
3389 The possible values and flag bits of the MODE parameter are
3390 system-dependent; they are available via the standard module C<Fcntl>.
3391 However, for historical reasons, some values are universal: zero means
3392 read-only, one means write-only, and two means read/write.
3394 If the file named by FILENAME does not exist and the C<open> call
3395 creates it (typically because MODE includes the O_CREAT flag), then
3396 the value of PERMS specifies the permissions of the newly created
3397 file. If PERMS is omitted, the default value is 0666, which allows
3398 read and write for all. This default is reasonable: see C<umask>.
3400 The IO::File module provides a more object-oriented approach, if you're
3401 into that kind of thing.
3403 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3405 =item sysread FILEHANDLE,SCALAR,LENGTH
3407 Attempts to read LENGTH bytes of data into variable SCALAR from the
3408 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
3409 so mixing this with other kinds of reads, print(), write(), seek(), or
3410 tell() can cause confusion. Returns the number of bytes actually read,
3411 or undef if there was an error. SCALAR will be grown or shrunk so that
3412 the last byte actually read is the last byte of the scalar after the
3415 An OFFSET may be specified to place the read data at some place in the
3416 string other than the beginning. A negative OFFSET specifies
3417 placement at that many bytes counting backwards from the end of the
3418 string. A positive OFFSET greater than the length of SCALAR results
3419 in the string being padded to the required size with "\0" bytes before
3420 the result of the read is appended.
3422 =item sysseek FILEHANDLE,POSITION,WHENCE
3424 Sets FILEHANDLE's system position using the system call lseek(2). It
3425 bypasses stdio, so mixing this with reads (other than sysread()),
3426 print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3427 be an expression whose value gives the name of the filehandle. The
3428 values for WHENCE are 0 to set the new position to POSITION, 1 to set
3429 the it to the current position plus POSITION, and 2 to set it to EOF
3430 plus POSITION (typically negative). For WHENCE, you may use the
3431 constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3432 or the POSIX module.
3434 Returns the new position, or the undefined value on failure. A position
3435 of zero is returned as the string "0 but true"; thus sysseek() returns
3436 TRUE on success and FALSE on failure, yet you can still easily determine
3441 Does exactly the same thing as "exec LIST" except that a fork is done
3442 first, and the parent process waits for the child process to complete.
3443 Note that argument processing varies depending on the number of
3444 arguments. The return value is the exit status of the program as
3445 returned by the wait() call. To get the actual exit value divide by
3446 256. See also L</exec>. This is I<NOT> what you want to use to capture
3447 the output from a command, for that you should use merely backticks or
3448 qx//, as described in L<perlop/"`STRING`">.
3450 Because system() and backticks block SIGINT and SIGQUIT, killing the
3451 program they're running doesn't actually interrupt your program.
3453 @args = ("command", "arg1", "arg2");
3455 or die "system @args failed: $?"
3457 Here's a more elaborate example of analysing the return value from
3458 system() on a Unix system to check for all possibilities, including for
3459 signals and core dumps.
3461 $rc = 0xffff & system @args;
3462 printf "system(%s) returned %#04x: ", "@args", $rc;
3464 print "ran with normal exit\n";
3466 elsif ($rc == 0xff00) {
3467 print "command failed: $!\n";
3469 elsif ($rc > 0x80) {
3471 print "ran with non-zero exit status $rc\n";
3477 print "core dump from ";
3479 print "signal $rc\n"
3483 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3485 =item syswrite FILEHANDLE,SCALAR,LENGTH
3487 Attempts to write LENGTH bytes of data from variable SCALAR to the
3488 specified FILEHANDLE, using the system call write(2). It bypasses
3489 stdio, so mixing this with reads (other than sysread()), print(),
3490 write(), seek(), or tell() may cause confusion. Returns the number of
3491 bytes actually written, or undef if there was an error. If the length
3492 is greater than the available data, only as much data as is available
3495 An OFFSET may be specified to write the data from some part of the
3496 string other than the beginning. A negative OFFSET specifies writing
3497 that many bytes counting backwards from the end of the string.
3499 =item tell FILEHANDLE
3503 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3504 expression whose value gives the name of the actual filehandle. If
3505 FILEHANDLE is omitted, assumes the file last read.
3507 =item telldir DIRHANDLE
3509 Returns the current position of the readdir() routines on DIRHANDLE.
3510 Value may be given to seekdir() to access a particular location in a
3511 directory. Has the same caveats about possible directory compaction as
3512 the corresponding system library routine.
3514 =item tie VARIABLE,CLASSNAME,LIST
3516 This function binds a variable to a package class that will provide the
3517 implementation for the variable. VARIABLE is the name of the variable
3518 to be enchanted. CLASSNAME is the name of a class implementing objects
3519 of correct type. Any additional arguments are passed to the "new"
3520 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3521 Typically these are arguments such as might be passed to the dbm_open()
3522 function of C. The object returned by the "new" method is also
3523 returned by the tie() function, which would be useful if you want to
3524 access other methods in CLASSNAME.
3526 Note that functions such as keys() and values() may return huge array
3527 values when used on large objects, like DBM files. You may prefer to
3528 use the each() function to iterate over such. Example:
3530 # print out history file offsets
3532 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3533 while (($key,$val) = each %HIST) {
3534 print $key, ' = ', unpack('L',$val), "\n";
3538 A class implementing a hash should have the following methods:
3540 TIEHASH classname, LIST
3543 STORE this, key, value
3547 NEXTKEY this, lastkey
3549 A class implementing an ordinary array should have the following methods:
3551 TIEARRAY classname, LIST
3554 STORE this, key, value
3557 A class implementing a scalar should have the following methods:
3559 TIESCALAR classname, LIST
3564 Unlike dbmopen(), the tie() function will not use or require a module
3565 for you--you need to do that explicitly yourself. See L<DB_File>
3566 or the F<Config> module for interesting tie() implementations.
3570 Returns a reference to the object underlying VARIABLE (the same value
3571 that was originally returned by the tie() call which bound the variable
3572 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3577 Returns the number of non-leap seconds since whatever time the system
3578 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3579 and 00:00:00 UTC, January 1, 1970 for most other systems).
3580 Suitable for feeding to gmtime() and localtime().
3584 Returns a four-element array giving the user and system times, in
3585 seconds, for this process and the children of this process.
3587 ($user,$system,$cuser,$csystem) = times;
3591 The translation operator. Same as y///. See L<perlop>.
3593 =item truncate FILEHANDLE,LENGTH
3595 =item truncate EXPR,LENGTH
3597 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3598 specified length. Produces a fatal error if truncate isn't implemented
3605 Returns an uppercased version of EXPR. This is the internal function
3606 implementing the \U escape in double-quoted strings.
3607 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3609 If EXPR is omitted, uses $_.
3615 Returns the value of EXPR with the first character uppercased. This is
3616 the internal function implementing the \u escape in double-quoted strings.
3617 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3619 If EXPR is omitted, uses $_.
3625 Sets the umask for the process to EXPR and returns the previous value.
3626 If EXPR is omitted, merely returns the current umask. Remember that a
3627 umask is a number, usually given in octal; it is I<not> a string of octal
3628 digits. See also L</oct>, if all you have is a string.
3634 Undefines the value of EXPR, which must be an lvalue. Use only on a
3635 scalar value, an entire array, an entire hash, or a subroutine name (using
3636 "&"). (Using undef() will probably not do what you expect on most
3637 predefined variables or DBM list values, so don't do that.) Always
3638 returns the undefined value. You can omit the EXPR, in which case
3639 nothing is undefined, but you still get an undefined value that you
3640 could, for instance, return from a subroutine, assign to a variable or
3641 pass as a parameter. Examples:
3644 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
3648 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
3649 select undef, undef, undef, 0.25;
3650 ($a, $b, undef, $c) = &foo; # Ignore third value returned
3656 Deletes a list of files. Returns the number of files successfully
3659 $cnt = unlink 'a', 'b', 'c';
3663 Note: unlink will not delete directories unless you are superuser and
3664 the B<-U> flag is supplied to Perl. Even if these conditions are
3665 met, be warned that unlinking a directory can inflict damage on your
3666 filesystem. Use rmdir instead.
3668 If LIST is omitted, uses $_.
3670 =item unpack TEMPLATE,EXPR
3672 Unpack does the reverse of pack: it takes a string representing a
3673 structure and expands it out into a list value, returning the array
3674 value. (In a scalar context, it returns merely the first value
3675 produced.) The TEMPLATE has the same format as in the pack function.
3676 Here's a subroutine that does substring:
3679 local($what,$where,$howmuch) = @_;
3680 unpack("x$where a$howmuch", $what);
3685 sub ordinal { unpack("c",$_[0]); } # same as ord()
3687 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3688 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3689 themselves. Default is a 16-bit checksum. For example, the following
3690 computes the same number as the System V sum program:
3693 $checksum += unpack("%16C*", $_);
3697 The following efficiently counts the number of set bits in a bit vector:
3699 $setbits = unpack("%32b*", $selectmask);
3701 =item untie VARIABLE
3703 Breaks the binding between a variable and a package. (See tie().)
3705 =item unshift ARRAY,LIST
3707 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3708 depending on how you look at it. Prepends list to the front of the
3709 array, and returns the new number of elements in the array.
3711 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3713 Note the LIST is prepended whole, not one element at a time, so the
3714 prepended elements stay in the same order. Use reverse to do the
3717 =item use Module LIST
3721 =item use Module VERSION LIST
3725 Imports some semantics into the current package from the named module,
3726 generally by aliasing certain subroutine or variable names into your
3727 package. It is exactly equivalent to
3729 BEGIN { require Module; import Module LIST; }
3731 except that Module I<must> be a bareword.
3733 If the first argument to C<use> is a number, it is treated as a version
3734 number instead of a module name. If the version of the Perl interpreter
3735 is less than VERSION, then an error message is printed and Perl exits
3736 immediately. This is often useful if you need to check the current
3737 Perl version before C<use>ing library modules which have changed in
3738 incompatible ways from older versions of Perl. (We try not to do
3739 this more than we have to.)
3741 The BEGIN forces the require and import to happen at compile time. The
3742 require makes sure the module is loaded into memory if it hasn't been
3743 yet. The import is not a builtin--it's just an ordinary static method
3744 call into the "Module" package to tell the module to import the list of
3745 features back into the current package. The module can implement its
3746 import method any way it likes, though most modules just choose to
3747 derive their import method via inheritance from the Exporter class that
3748 is defined in the Exporter module. See L<Exporter>. If no import
3749 method can be found then the error is currently silently ignored. This
3750 may change to a fatal error in a future version.
3752 If you don't want your namespace altered, explicitly supply an empty list:
3756 That is exactly equivalent to
3758 BEGIN { require Module; }
3760 If the VERSION argument is present between Module and LIST, then the
3761 C<use> will call the VERSION method in class Module with the given
3762 version as an argument. The default VERSION method, inherited from
3763 the Universal class, croaks if the given version is larger than the
3764 value of the variable $Module::VERSION. (Note that there is not a
3765 comma after VERSION!)
3767 Because this is a wide-open interface, pragmas (compiler directives)
3768 are also implemented this way. Currently implemented pragmas are:
3772 use sigtrap qw(SEGV BUS);
3773 use strict qw(subs vars refs);
3774 use subs qw(afunc blurfl);
3776 These pseudo-modules import semantics into the current block scope, unlike
3777 ordinary modules, which import symbols into the current package (which are
3778 effective through the end of the file).
3780 There's a corresponding "no" command that unimports meanings imported
3781 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3786 If no unimport method can be found the call fails with a fatal error.
3788 See L<perlmod> for a list of standard modules and pragmas.
3792 Changes the access and modification times on each file of a list of
3793 files. The first two elements of the list must be the NUMERICAL access
3794 and modification times, in that order. Returns the number of files
3795 successfully changed. The inode modification time of each file is set
3796 to the current time. Example of a "touch" command:
3800 utime $now, $now, @ARGV;
3804 Returns a normal array consisting of all the values of the named hash.
3805 (In a scalar context, returns the number of values.) The values are
3806 returned in an apparently random order, but it is the same order as either
3807 the keys() or each() function would produce on the same hash. As a side
3808 effect, it resets HASH's iterator. See also keys(), each(), and sort().
3810 =item vec EXPR,OFFSET,BITS
3812 Treats the string in EXPR as a vector of unsigned integers, and
3813 returns the value of the bit field specified by OFFSET. BITS specifies
3814 the number of bits that are reserved for each entry in the bit
3815 vector. This must be a power of two from 1 to 32. vec() may also be
3816 assigned to, in which case parentheses are needed to give the expression
3817 the correct precedence as in
3819 vec($image, $max_x * $x + $y, 8) = 3;
3821 Vectors created with vec() can also be manipulated with the logical
3822 operators |, &, and ^, which will assume a bit vector operation is
3823 desired when both operands are strings.
3825 To transform a bit vector into a string or array of 0's and 1's, use these:
3827 $bits = unpack("b*", $vector);
3828 @bits = split(//, unpack("b*", $vector));
3830 If you know the exact length in bits, it can be used in place of the *.
3834 Waits for a child process to terminate and returns the pid of the
3835 deceased process, or -1 if there are no child processes. The status is
3838 =item waitpid PID,FLAGS
3840 Waits for a particular child process to terminate and returns the pid
3841 of the deceased process, or -1 if there is no such child process. The
3842 status is returned in C<$?>. If you say
3844 use POSIX ":sys_wait_h";
3846 waitpid(-1,&WNOHANG);
3848 then you can do a non-blocking wait for any process. Non-blocking wait
3849 is available on machines supporting either the waitpid(2) or
3850 wait4(2) system calls. However, waiting for a particular pid with
3851 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3852 by remembering the status values of processes that have exited but have
3853 not been harvested by the Perl script yet.)
3857 Returns TRUE if the context of the currently executing subroutine is
3858 looking for a list value. Returns FALSE if the context is looking
3859 for a scalar. Returns the undefined value if the context is looking
3860 for no value (void context).
3862 return unless defined wantarray; # don't bother doing more
3863 my @a = complex_calculation();
3864 return wantarray ? @a : "@a";
3868 Produces a message on STDERR just like die(), but doesn't exit or throw
3871 No message is printed if there is a C<$SIG{__WARN__}> handler
3872 installed. It is the handler's responsibility to deal with the message
3873 as it sees fit (like, for instance, converting it into a die()). Most
3874 handlers must therefore make arrangements to actually display the
3875 warnings that they are not prepared to deal with, by calling warn()
3876 again in the handler. Note that this is quite safe and will not
3877 produce an endless loop, since C<__WARN__> hooks are not called from
3880 You will find this behavior is slightly different from that of
3881 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
3882 instead call die() again to change it).
3884 Using a C<__WARN__> handler provides a powerful way to silence all
3885 warnings (even the so-called mandatory ones). An example:
3887 # wipe out *all* compile-time warnings
3888 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
3890 my $foo = 20; # no warning about duplicate my $foo,
3891 # but hey, you asked for it!
3892 # no compile-time or run-time warnings before here
3895 # run-time warnings enabled after here
3896 warn "\$foo is alive and $foo!"; # does show up
3898 See L<perlvar> for details on setting C<%SIG> entries, and for more
3901 =item write FILEHANDLE
3907 Writes a formatted record (possibly multi-line) to the specified file,
3908 using the format associated with that file. By default the format for
3909 a file is the one having the same name as the filehandle, but the
3910 format for the current output channel (see the select() function) may be set
3911 explicitly by assigning the name of the format to the C<$~> variable.
3913 Top of form processing is handled automatically: if there is
3914 insufficient room on the current page for the formatted record, the
3915 page is advanced by writing a form feed, a special top-of-page format
3916 is used to format the new page header, and then the record is written.
3917 By default the top-of-page format is the name of the filehandle with
3918 "_TOP" appended, but it may be dynamically set to the format of your
3919 choice by assigning the name to the C<$^> variable while the filehandle is
3920 selected. The number of lines remaining on the current page is in
3921 variable C<$->, which can be set to 0 to force a new page.
3923 If FILEHANDLE is unspecified, output goes to the current default output
3924 channel, which starts out as STDOUT but may be changed by the
3925 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3926 is evaluated and the resulting string is used to look up the name of
3927 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3929 Note that write is I<NOT> the opposite of read. Unfortunately.
3933 The translation operator. Same as tr///. See L<perlop>.
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