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 only
18 ever be 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 parens.) If you use the parens, 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+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
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 non-abortive 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:
61 I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in a scalar context. Some operators return the
67 length of the list that would have been returned in a list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
73 =head2 Perl Functions by Category
75 Here are Perl's functions (including things that look like
76 functions, like some of the keywords and named operators)
77 arranged by category. Some functions appear in more
82 =item Functions for SCALARs or strings
84 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86 sprintf, substr, tr///, uc, ucfirst, y///
88 =item Regular expressions and pattern matching
90 m//, pos, quotemeta, s///, split, study
92 =item Numeric functions
94 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
97 =item Functions for real @ARRAYs
99 pop, push, shift, splice, unshift
101 =item Functions for list data
103 grep, join, map, qw/STRING/, reverse, sort, unpack
105 =item Functions for real %HASHes
107 delete, each, exists, keys, values
109 =item Input and output functions
111 binmode, close, closedir, dbmclose, dbmopen, die, eof,
112 fileno, flock, format, getc, print, printf, read, readdir,
113 rewinddir, seek, seekdir, select, syscall, sysread,
114 syswrite, tell, telldir, truncate, warn, write
116 =item Functions for fixed length data or records
118 pack, read, syscall, sysread, syswrite, unpack, vec
120 =item Functions for filehandles, files, or directories
122 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
123 lstat, mkdir, open, opendir, readlink, rename, rmdir,
124 stat, symlink, umask, unlink, utime
126 =item Keywords related to the control flow of your perl program
128 caller, continue, die, do, dump, eval, exit, goto, last,
129 next, redo, return, sub, wantarray
131 =item Keywords related to scoping
133 caller, import, local, my, package, use
135 =item Miscellaneous functions
137 defined, dump, eval, formline, local, my, reset, scalar,
140 =item Functions for processes and process groups
142 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
146 =item Keywords related to perl modules
148 do, import, no, package, require, use
150 =item Keywords related to classes and object-orientedness
152 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
154 =item Low-level socket functions
156 accept, bind, connect, getpeername, getsockname,
157 getsockopt, listen, recv, send, setsockopt, shutdown,
160 =item System V interprocess communication functions
162 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163 shmctl, shmget, shmread, shmwrite
165 =item Fetching user and group info
167 endgrent, endhostent, endnetent, endpwent, getgrent,
168 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169 getpwuid, setgrent, setpwent
171 =item Fetching network info
173 endprotoent, endservent, gethostbyaddr, gethostbyname,
174 gethostent, getnetbyaddr, getnetbyname, getnetent,
175 getprotobyname, getprotobynumber, getprotoent,
176 getservbyname, getservbyport, getservent, sethostent,
177 setnetent, setprotoent, setservent
179 =item Time-related functions
181 gmtime, localtime, time, times
183 =item Functions new in perl5
185 abs, bless, chomp, chr, exists, formline, glob, import, lc,
186 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
187 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
189 * - C<sub> was a keyword in perl4, but in perl5 it is an
190 operator which can be used in expressions.
192 =item Functions obsoleted in perl5
199 =head2 Alphabetical Listing of Perl Functions
210 A file test, where X is one of the letters listed below. This unary
211 operator takes one argument, either a filename or a filehandle, and
212 tests the associated file to see if something is true about it. If the
213 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215 the undefined value if the file doesn't exist. Despite the funny
216 names, precedence is the same as any other named unary operator, and
217 the argument may be parenthesized like any other unary operator. The
218 operator may be any of:
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
254 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256 uids and gids of the user. There may be other reasons you can't actually
257 read, write or execute the file. Also note that, for the superuser,
258 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
259 1 if any execute bit is set in the mode. Scripts run by the superuser may
260 thus need to do a stat() in order to determine the actual mode of the
261 file, or temporarily set the uid to something else.
267 next unless -f $_; # ignore specials
271 Note that C<-s/a/b/> does not do a negated substitution. Saying
272 C<-exp($foo)> still works as expected, however--only single letters
273 following a minus are interpreted as file tests.
275 The C<-T> and C<-B> switches work as follows. The first block or so of the
276 file is examined for odd characters such as strange control codes or
277 characters with the high bit set. If too many odd characters (E<gt>30%)
278 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279 containing null in the first block is considered a binary file. If C<-T>
280 or C<-B> is used on a filehandle, the current stdio buffer is examined
281 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
282 file, or a file at EOF when testing a filehandle. Because you have to
283 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284 against the file first, as in C<next unless -f $file && -T $file>.
286 If any of the file tests (or either the stat() or lstat() operators) are given the
287 special filehandle consisting of a solitary underline, then the stat
288 structure of the previous file test (or stat operator) is used, saving
289 a system call. (This doesn't work with C<-t>, and you need to remember
290 that lstat() and C<-l> will leave values in the stat structure for the
291 symbolic link, not the real file.) Example:
293 print "Can do.\n" if -r $a || -w _ || -x _;
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
309 Returns the absolute value of its argument.
310 If VALUE is omitted, uses $_.
312 =item accept NEWSOCKET,GENERICSOCKET
314 Accepts an incoming socket connect, just as the accept(2) system call
315 does. Returns the packed address if it succeeded, FALSE otherwise.
316 See example in L<perlipc/"Sockets: Client/Server Communication">.
322 Arranges to have a SIGALRM delivered to this process after the
323 specified number of seconds have elapsed. If SECONDS is not specified,
324 the value stored in $_ is used. (On some machines,
325 unfortunately, the elapsed time may be up to one second less than you
326 specified because of how seconds are counted.) Only one timer may be
327 counting at once. Each call disables the previous timer, and an
328 argument of 0 may be supplied to cancel the previous timer without
329 starting a new one. The returned value is the amount of time remaining
330 on the previous timer.
332 For delays of finer granularity than one second, you may use Perl's
333 syscall() interface to access setitimer(2) if your system supports it,
334 or else see L</select()> below. It is not advised to intermix alarm()
337 If you want to use alarm() to time out a system call you need to use an
338 eval/die pair. You can't rely on the alarm causing the system call to
339 fail with $! set to EINTR because Perl sets up signal handlers to
340 restart system calls on some systems. Using eval/die always works.
343 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
344 $nread = sysread SOCKET, $buffer, $size;
346 die if $@ && $@ ne "alarm\n"; # propagate errors
356 Returns the arctangent of Y/X in the range -PI to PI.
358 =item bind SOCKET,NAME
360 Binds a network address to a socket, just as the bind system call
361 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
362 packed address of the appropriate type for the socket. See the examples in
363 L<perlipc/"Sockets: Client/Server Communication">.
365 =item binmode FILEHANDLE
367 Arranges for the file to be read or written in "binary" mode in operating
368 systems that distinguish between binary and text files. Files that are
369 not in binary mode have CR LF sequences translated to LF on input and LF
370 translated to CR LF on output. Binmode has no effect under Unix; in DOS
371 and similarly archaic systems, it may be imperative--otherwise your
372 DOS-damaged C library may mangle your file. The key distinction between
373 systems that need binmode and those that don't is their text file
374 formats. Systems like Unix and Plan9 that delimit lines with a single
375 character, and that encode that character in C as '\n', do not need
376 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
377 is taken as the name of the filehandle.
379 =item bless REF,CLASSNAME
383 This function tells the referenced object (passed as REF) that it is now
384 an object in the CLASSNAME package--or the current package if no CLASSNAME
385 is specified, which is often the case. It returns the reference for
386 convenience, since a bless() is often the last thing in a constructor.
387 Always use the two-argument version if the function doing the blessing
388 might be inherited by a derived class. See L<perlobj> for more about the
389 blessing (and blessings) of objects.
395 Returns the context of the current subroutine call. In a scalar context,
396 returns TRUE if there is a caller, that is, if we're in a subroutine or
397 eval() or require(), and FALSE otherwise. In a list context, returns
399 ($package, $filename, $line) = caller;
401 With EXPR, it returns some extra information that the debugger uses to
402 print a stack trace. The value of EXPR indicates how many call frames
403 to go back before the current one.
405 ($package, $filename, $line,
406 $subroutine, $hasargs, $wantargs) = caller($i);
408 Furthermore, when called from within the DB package, caller returns more
409 detailed information: it sets the list variable @DB::args to be the
410 arguments with which that subroutine was invoked.
414 Changes the working directory to EXPR, if possible. If EXPR is
415 omitted, changes to home directory. Returns TRUE upon success, FALSE
416 otherwise. See example under die().
420 Changes the permissions of a list of files. The first element of the
421 list must be the numerical mode, which should probably be an octal
422 number. Returns the number of files successfully changed.
424 $cnt = chmod 0755, 'foo', 'bar';
425 chmod 0755, @executables;
433 This is a slightly safer version of chop (see below). It removes any
434 line ending that corresponds to the current value of C<$/> (also known as
435 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
436 of characters removed. It's often used to remove the newline from the
437 end of an input record when you're worried that the final record may be
438 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
439 trailing newlines from the string. If VARIABLE is omitted, it chomps
443 chomp; # avoid \n on last field
448 You can actually chomp anything that's an lvalue, including an assignment:
451 chomp($answer = <STDIN>);
453 If you chomp a list, each element is chomped, and the total number of
454 characters removed is returned.
462 Chops off the last character of a string and returns the character
463 chopped. It's used primarily to remove the newline from the end of an
464 input record, but is much more efficient than C<s/\n//> because it neither
465 scans nor copies the string. If VARIABLE is omitted, chops $_.
469 chop; # avoid \n on last field
474 You can actually chop anything that's an lvalue, including an assignment:
477 chop($answer = <STDIN>);
479 If you chop a list, each element is chopped. Only the value of the
480 last chop is returned.
482 Note that chop returns the last character. To return all but the last
483 character, use C<substr($string, 0, -1)>.
487 Changes the owner (and group) of a list of files. The first two
488 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
489 Returns the number of files successfully changed.
491 $cnt = chown $uid, $gid, 'foo', 'bar';
492 chown $uid, $gid, @filenames;
494 Here's an example that looks up non-numeric uids in the passwd file:
497 chop($user = <STDIN>);
499 chop($pattern = <STDIN>);
501 ($login,$pass,$uid,$gid) = getpwnam($user)
502 or die "$user not in passwd file";
504 @ary = <${pattern}>; # expand filenames
505 chown $uid, $gid, @ary;
507 On most systems, you are not allowed to change the ownership of the
508 file unless you're the superuser, although you should be able to change
509 the group to any of your secondary groups. On insecure systems, these
510 restrictions may be relaxed, but this is not a portable assumption.
516 Returns the character represented by that NUMBER in the character set.
517 For example, C<chr(65)> is "A" in ASCII.
519 If NUMBER is omitted, uses $_.
521 =item chroot FILENAME
525 This function works as the system call by the same name: it makes the
526 named directory the new root directory for all further pathnames that
527 begin with a "/" by your process and all of its children. (It doesn't
528 change your current working directory is unaffected.) For security
529 reasons, this call is restricted to the superuser. If FILENAME is
530 omitted, does chroot to $_.
532 =item close FILEHANDLE
534 Closes the file or pipe associated with the file handle, returning TRUE
535 only if stdio successfully flushes buffers and closes the system file
536 descriptor. You don't have to close FILEHANDLE if you are immediately
537 going to do another open() on it, since open() will close it for you. (See
538 open().) However, an explicit close on an input file resets the line
539 counter ($.), while the implicit close done by open() does not. Also,
540 closing a pipe will wait for the process executing on the pipe to
541 complete, in case you want to look at the output of the pipe
542 afterwards. Closing a pipe explicitly also puts the status value of
543 the command into C<$?>. Example:
545 open(OUTPUT, '|sort >foo'); # pipe to sort
546 ... # print stuff to output
547 close OUTPUT; # wait for sort to finish
548 open(INPUT, 'foo'); # get sort's results
550 FILEHANDLE may be an expression whose value gives the real filehandle name.
552 =item closedir DIRHANDLE
554 Closes a directory opened by opendir().
556 =item connect SOCKET,NAME
558 Attempts to connect to a remote socket, just as the connect system call
559 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
560 packed address of the appropriate type for the socket. See the examples in
561 L<perlipc/"Sockets: Client/Server Communication">.
565 Actually a flow control statement rather than a function. If there is a
566 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
567 C<foreach>), it is always executed just before the conditional is about to
568 be evaluated again, just like the third part of a C<for> loop in C. Thus
569 it can be used to increment a loop variable, even when the loop has been
570 continued via the C<next> statement (which is similar to the C C<continue>
575 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
578 =item crypt PLAINTEXT,SALT
580 Encrypts a string exactly like the crypt(3) function in the C library
581 (assuming that you actually have a version there that has not been
582 extirpated as a potential munition). This can prove useful for checking
583 the password file for lousy passwords, amongst other things. Only the
584 guys wearing white hats should do this.
586 Here's an example that makes sure that whoever runs this program knows
589 $pwd = (getpwuid($<))[1];
590 $salt = substr($pwd, 0, 2);
594 chop($word = <STDIN>);
598 if (crypt($word, $salt) ne $pwd) {
604 Of course, typing in your own password to whoever asks you
607 =item dbmclose ASSOC_ARRAY
609 [This function has been superseded by the untie() function.]
611 Breaks the binding between a DBM file and an associative array.
613 =item dbmopen ASSOC,DBNAME,MODE
615 [This function has been superseded by the tie() function.]
617 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
618 associative array. ASSOC is the name of the associative array. (Unlike
619 normal open, the first argument is I<NOT> a filehandle, even though it
620 looks like one). DBNAME is the name of the database (without the F<.dir>
621 or F<.pag> extension if any). If the database does not exist, it is
622 created with protection specified by MODE (as modified by the umask()).
623 If your system only supports the older DBM functions, you may perform only
624 one dbmopen() in your program. In older versions of Perl, if your system
625 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
626 falls back to sdbm(3).
628 If you don't have write access to the DBM file, you can only read
629 associative array variables, not set them. If you want to test whether
630 you can write, either use file tests or try setting a dummy array entry
631 inside an eval(), which will trap the error.
633 Note that functions such as keys() and values() may return huge array
634 values when used on large DBM files. You may prefer to use the each()
635 function to iterate over large DBM files. Example:
637 # print out history file offsets
638 dbmopen(%HIST,'/usr/lib/news/history',0666);
639 while (($key,$val) = each %HIST) {
640 print $key, ' = ', unpack('L',$val), "\n";
644 See also L<AnyDBM_File> for a more general description of the pros and
645 cons of the various dbm approaches, as well as L<DB_File> for a particularly
652 Returns a boolean value saying whether EXPR has a real value
653 or not. If EXPR is not present, $_ will be checked. Many operations
654 return the undefined value under exceptional conditions, such as end of
655 file, uninitialized variable, system error and such. This function
656 allows you to distinguish between an undefined
657 null scalar and a defined null scalar with operations that might return
658 a real null string, such as referencing elements of an array. You may
659 also check to see if arrays or subroutines exist. Use of defined on
660 predefined variables is not guaranteed to produce intuitive results.
662 When used on a hash array element, it tells you whether the value
663 is defined, not whether the key exists in the hash. Use exists() for that.
667 print if defined $switch{'D'};
668 print "$val\n" while defined($val = pop(@ary));
669 die "Can't readlink $sym: $!"
670 unless defined($value = readlink $sym);
671 eval '@foo = ()' if defined(@foo);
672 die "No XYZ package defined" unless defined %_XYZ;
673 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
677 Note: many folks tend to overuse defined(), and then are surprised to
678 discover that the number 0 and the null string are, in fact, defined
679 concepts. For example, if you say
683 the pattern match succeeds, and $1 is defined, despite the fact that it
684 matched "nothing". But it didn't really match nothing--rather, it
685 matched something that happened to be 0 characters long. This is all
686 very above-board and honest. When a function returns an undefined value,
687 it's an admission that it couldn't give you an honest answer. So
688 you should only use defined() when you're questioning the integrity
689 of what you're trying to do. At other times, a simple comparison to
690 0 or "" is what you want.
694 Deletes the specified value from its hash array. Returns the deleted
695 value, or the undefined value if nothing was deleted. Deleting from
696 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
697 file deletes the entry from the DBM file. (But deleting from a tie()d
698 hash doesn't necessarily return anything.)
700 The following deletes all the values of an associative array:
702 foreach $key (keys %ARRAY) {
706 (But it would be faster to use the undef() command.) Note that the
707 EXPR can be arbitrarily complicated as long as the final operation is
710 delete $ref->[$x][$y]{$key};
714 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
715 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
716 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
717 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
718 and the eval() is terminated with the undefined value; this makes die()
719 the way to raise an exception.
723 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
724 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
726 If the value of EXPR does not end in a newline, the current script line
727 number and input line number (if any) are also printed, and a newline
728 is supplied. Hint: sometimes appending ", stopped" to your message
729 will cause it to make better sense when the string "at foo line 123" is
730 appended. Suppose you are running script "canasta".
732 die "/etc/games is no good";
733 die "/etc/games is no good, stopped";
735 produce, respectively
737 /etc/games is no good at canasta line 123.
738 /etc/games is no good, stopped at canasta line 123.
740 See also exit() and warn().
744 Not really a function. Returns the value of the last command in the
745 sequence of commands indicated by BLOCK. When modified by a loop
746 modifier, executes the BLOCK once before testing the loop condition.
747 (On other statements the loop modifiers test the conditional first.)
749 =item do SUBROUTINE(LIST)
751 A deprecated form of subroutine call. See L<perlsub>.
755 Uses the value of EXPR as a filename and executes the contents of the
756 file as a Perl script. Its primary use is to include subroutines
757 from a Perl subroutine library.
765 except that it's more efficient, more concise, keeps track of the
766 current filename for error messages, and searches all the B<-I>
767 libraries if the file isn't in the current directory (see also the @INC
768 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
769 reparse the file every time you call it, so you probably don't want to
770 do this inside a loop.
772 Note that inclusion of library modules is better done with the
773 use() and require() operators, which also do error checking
774 and raise an exception if there's a problem.
778 This causes an immediate core dump. Primarily this is so that you can
779 use the B<undump> program to turn your core dump into an executable binary
780 after having initialized all your variables at the beginning of the
781 program. When the new binary is executed it will begin by executing a
782 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
783 it as a goto with an intervening core dump and reincarnation. If LABEL
784 is omitted, restarts the program from the top. WARNING: any files
785 opened at the time of the dump will NOT be open any more when the
786 program is reincarnated, with possible resulting confusion on the part
787 of Perl. See also B<-u> option in L<perlrun>.
804 dump QUICKSTART if $ARGV[0] eq '-d';
809 =item each ASSOC_ARRAY
811 When called in a list context, returns a 2-element array consisting
812 of the key and value for the next element of an associative array,
813 so that you can iterate over it. When called in a scalar context,
814 returns the key only for the next element in the associative array.
815 Entries are returned in an apparently random order. When the array is
816 entirely read, a null array is returned in list context (which when
817 assigned produces a FALSE (0) value), and C<undef> is returned in a
818 scalar context. The next call to each() after that will start
819 iterating again. The iterator can be reset only by reading all the
820 elements from the array. You should not add elements to an array while
821 you're iterating over it. There is a single iterator for each
822 associative array, shared by all each(), keys() and values() function
823 calls in the program. The following prints out your environment like
824 the printenv(1) program, only in a different order:
826 while (($key,$value) = each %ENV) {
827 print "$key=$value\n";
830 See also keys() and values().
838 Returns 1 if the next read on FILEHANDLE will return end of file, or if
839 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
840 gives the real filehandle name. (Note that this function actually
841 reads a character and then ungetc()s it, so it is not very useful in an
842 interactive context.) Do not read from a terminal file (or call
843 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
844 as terminals may lose the end-of-file condition if you do.
846 An C<eof> without an argument uses the last file read as argument.
847 Empty parentheses () may be used to indicate
848 the pseudofile formed of the files listed on the command line, i.e.
849 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
850 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
851 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
853 # reset line numbering on each input file
856 close(ARGV) if (eof); # Not eof().
859 # insert dashes just before last line of last file
862 print "--------------\n";
863 close(ARGV); # close or break; is needed if we
864 # are reading from the terminal
869 Practical hint: you almost never need to use C<eof> in Perl, because the
870 input operators return undef when they run out of data.
876 EXPR is parsed and executed as if it were a little Perl program. It
877 is executed in the context of the current Perl program, so that any
878 variable settings, subroutine or format definitions remain afterwards.
879 The value returned is the value of the last expression evaluated, or a
880 return statement may be used, just as with subroutines. The last
881 expression is evaluated in scalar or array context, depending on the
884 If there is a syntax error or runtime error, or a die() statement is
885 executed, an undefined value is returned by eval(), and C<$@> is set to the
886 error message. If there was no error, C<$@> is guaranteed to be a null
887 string. If EXPR is omitted, evaluates $_. The final semicolon, if
888 any, may be omitted from the expression.
890 Note that, since eval() traps otherwise-fatal errors, it is useful for
891 determining whether a particular feature (such as socket() or symlink())
892 is implemented. It is also Perl's exception trapping mechanism, where
893 the die operator is used to raise exceptions.
895 If the code to be executed doesn't vary, you may use the eval-BLOCK
896 form to trap run-time errors without incurring the penalty of
897 recompiling each time. The error, if any, is still returned in C<$@>.
900 # make divide-by-zero non-fatal
901 eval { $answer = $a / $b; }; warn $@ if $@;
903 # same thing, but less efficient
904 eval '$answer = $a / $b'; warn $@ if $@;
906 # a compile-time error
910 eval '$answer ='; # sets $@
912 With an eval(), you should be especially careful to remember what's
913 being looked at when:
919 eval { $x }; # CASE 4
921 eval "\$$x++" # CASE 5
924 Cases 1 and 2 above behave identically: they run the code contained in the
925 variable $x. (Although case 2 has misleading double quotes making the
926 reader wonder what else might be happening (nothing is).) Cases 3 and 4
927 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
928 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
929 is a place where normally you I<WOULD> like to use double quotes, except
930 that in that particular situation, you can just use symbolic references
931 instead, as in case 6.
935 The exec() function executes a system command I<AND NEVER RETURNS>,
936 unless the command does not exist and is executed directly instead of
937 via C</bin/sh -c> (see below). Use system() instead of exec() if you
940 If there is more than one argument in LIST, or if LIST is an array with
941 more than one value, calls execvp(3) with the arguments in LIST. If
942 there is only one scalar argument, the argument is checked for shell
943 metacharacters. If there are any, the entire argument is passed to
944 C</bin/sh -c> for parsing. If there are none, the argument is split
945 into words and passed directly to execvp(), which is more efficient.
946 Note: exec() and system() do not flush your output buffer, so you may
947 need to set C<$|> to avoid lost output. Examples:
949 exec '/bin/echo', 'Your arguments are: ', @ARGV;
950 exec "sort $outfile | uniq";
952 If you don't really want to execute the first argument, but want to lie
953 to the program you are executing about its own name, you can specify
954 the program you actually want to run as an "indirect object" (without a
955 comma) in front of the LIST. (This always forces interpretation of the
956 LIST as a multi-valued list, even if there is only a single scalar in
960 exec $shell '-sh'; # pretend it's a login shell
964 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
968 Returns TRUE if the specified hash key exists in its hash array, even
969 if the corresponding value is undefined.
971 print "Exists\n" if exists $array{$key};
972 print "Defined\n" if defined $array{$key};
973 print "True\n" if $array{$key};
975 A hash element can only be TRUE if it's defined, and defined if
976 it exists, but the reverse doesn't necessarily hold true.
978 Note that the EXPR can be arbitrarily complicated as long as the final
979 operation is a hash key lookup:
981 if (exists $ref->[$x][$y]{$key}) { ... }
985 Evaluates EXPR and exits immediately with that value. (Actually, it
986 calls any defined C<END> routines first, but the C<END> routines may not
987 abort the exit. Likewise any object destructors that need to be called
988 are called before exit.) Example:
991 exit 0 if $ans =~ /^[Xx]/;
993 See also die(). If EXPR is omitted, exits with 0 status.
999 Returns I<e> (the natural logarithm base) to the power of EXPR.
1000 If EXPR is omitted, gives C<exp($_)>.
1002 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1004 Implements the fcntl(2) function. You'll probably have to say
1008 first to get the correct function definitions. Argument processing and
1009 value return works just like ioctl() below. Note that fcntl() will produce
1010 a fatal error if used on a machine that doesn't implement fcntl(2).
1014 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1016 =item fileno FILEHANDLE
1018 Returns the file descriptor for a filehandle. This is useful for
1019 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1020 value is taken as the name of the filehandle.
1022 =item flock FILEHANDLE,OPERATION
1024 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
1025 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
1026 fatal error if used on a machine that doesn't implement either flock(2) or
1027 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
1028 is missing from your system. This makes flock() the portable file locking
1029 strategy, although it will only lock entire files, not records. Note also
1030 that some versions of flock() cannot lock things over the network; you
1031 would need to use the more system-specific fcntl() for that.
1033 Here's a mailbox appender for BSD systems.
1041 flock(MBOX,$LOCK_EX);
1042 # and, in case someone appended
1043 # while we were waiting...
1048 flock(MBOX,$LOCK_UN);
1051 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1052 or die "Can't open mailbox: $!";
1055 print MBOX $msg,"\n\n";
1058 See also L<DB_File> for other flock() examples.
1062 Does a fork(2) system call. Returns the child pid to the parent process
1063 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1064 Note: unflushed buffers remain unflushed in both processes, which means
1065 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1066 autoflush() FileHandle method to avoid duplicate output.
1068 If you fork() without ever waiting on your children, you will accumulate
1071 $SIG{CHLD} = sub { wait };
1073 There's also the double-fork trick (error checking on
1074 fork() returns omitted);
1076 unless ($pid = fork) {
1078 exec "what you really wanna do";
1081 ## (some_perl_code_here)
1088 See also L<perlipc> for more examples of forking and reaping
1093 Declare a picture format with use by the write() function. For
1097 Test: @<<<<<<<< @||||| @>>>>>
1098 $str, $%, '$' . int($num)
1102 $num = $cost/$quantity;
1106 See L<perlform> for many details and examples.
1109 =item formline PICTURE, LIST
1111 This is an internal function used by C<format>s, though you may call it
1112 too. It formats (see L<perlform>) a list of values according to the
1113 contents of PICTURE, placing the output into the format output
1114 accumulator, C<$^A> (or $ACCUMULATOR in English).
1115 Eventually, when a write() is done, the contents of
1116 C<$^A> are written to some filehandle, but you could also read C<$^A>
1117 yourself and then set C<$^A> back to "". Note that a format typically
1118 does one formline() per line of form, but the formline() function itself
1119 doesn't care how many newlines are embedded in the PICTURE. This means
1120 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1121 You may therefore need to use multiple formlines to implement a single
1122 record format, just like the format compiler.
1124 Be careful if you put double quotes around the picture, since an "C<@>"
1125 character may be taken to mean the beginning of an array name.
1126 formline() always returns TRUE. See L<perlform> for other examples.
1128 =item getc FILEHANDLE
1132 Returns the next character from the input file attached to FILEHANDLE,
1133 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1134 This is not particularly efficient. It cannot be used to get unbuffered
1135 single-characters, however. For that, try something more like:
1138 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1141 system "stty", '-icanon', 'eol', "\001";
1147 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1150 system "stty", 'icanon', 'eol', '^@'; # ascii null
1154 Determination of whether to whether $BSD_STYLE should be set
1155 is left as an exercise to the reader.
1157 See also the C<Term::ReadKey> module from your nearest CPAN site;
1158 details on CPAN can be found on L<perlmod/CPAN>
1162 Returns the current login from F</etc/utmp>, if any. If null, use
1165 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1167 Do not consider getlogin() for authentication: it is not as
1168 secure as getpwuid().
1170 =item getpeername SOCKET
1172 Returns the packed sockaddr address of other end of the SOCKET connection.
1175 $hersockaddr = getpeername(SOCK);
1176 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1177 $herhostname = gethostbyaddr($iaddr, AF_INET);
1178 $herstraddr = inet_ntoa($iaddr);
1182 Returns the current process group for the specified PID. Use
1183 a PID of 0 to get the current process group for the
1184 current process. Will raise an exception if used on a machine that
1185 doesn't implement getpgrp(2). If PID is omitted, returns process
1186 group of current process. Note that the POSIX version of getpgrp()
1187 does not accept a PID argument, so only PID==0 is truly portable.
1191 Returns the process id of the parent process.
1193 =item getpriority WHICH,WHO
1195 Returns the current priority for a process, a process group, or a user.
1196 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1197 machine that doesn't implement getpriority(2).
1203 =item gethostbyname NAME
1205 =item getnetbyname NAME
1207 =item getprotobyname NAME
1213 =item getservbyname NAME,PROTO
1215 =item gethostbyaddr ADDR,ADDRTYPE
1217 =item getnetbyaddr ADDR,ADDRTYPE
1219 =item getprotobynumber NUMBER
1221 =item getservbyport PORT,PROTO
1239 =item sethostent STAYOPEN
1241 =item setnetent STAYOPEN
1243 =item setprotoent STAYOPEN
1245 =item setservent STAYOPEN
1259 These routines perform the same functions as their counterparts in the
1260 system library. Within a list context, the return values from the
1261 various get routines are as follows:
1263 ($name,$passwd,$uid,$gid,
1264 $quota,$comment,$gcos,$dir,$shell) = getpw*
1265 ($name,$passwd,$gid,$members) = getgr*
1266 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1267 ($name,$aliases,$addrtype,$net) = getnet*
1268 ($name,$aliases,$proto) = getproto*
1269 ($name,$aliases,$port,$proto) = getserv*
1271 (If the entry doesn't exist you get a null list.)
1273 Within a scalar context, you get the name, unless the function was a
1274 lookup by name, in which case you get the other thing, whatever it is.
1275 (If the entry doesn't exist you get the undefined value.) For example:
1285 The $members value returned by I<getgr*()> is a space separated list of
1286 the login names of the members of the group.
1288 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1289 C, it will be returned to you via C<$?> if the function call fails. The
1290 @addrs value returned by a successful call is a list of the raw
1291 addresses returned by the corresponding system library call. In the
1292 Internet domain, each address is four bytes long and you can unpack it
1293 by saying something like:
1295 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1297 =item getsockname SOCKET
1299 Returns the packed sockaddr address of this end of the SOCKET connection.
1302 $mysockaddr = getsockname(SOCK);
1303 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1305 =item getsockopt SOCKET,LEVEL,OPTNAME
1307 Returns the socket option requested, or undefined if there is an error.
1311 Returns the value of EXPR with filename expansions such as a shell
1312 would do. This is the internal function implementing the E<lt>*.*E<gt>
1313 operator, except it's easier to use.
1317 Converts a time as returned by the time function to a 9-element array
1318 with the time localized for the standard Greenwich timezone.
1319 Typically used as follows:
1322 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1325 All array elements are numeric, and come straight out of a struct tm.
1326 In particular this means that $mon has the range 0..11 and $wday has
1327 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1335 The goto-LABEL form finds the statement labeled with LABEL and resumes
1336 execution there. It may not be used to go into any construct that
1337 requires initialization, such as a subroutine or a foreach loop. It
1338 also can't be used to go into a construct that is optimized away. It
1339 can be used to go almost anywhere else within the dynamic scope,
1340 including out of subroutines, but it's usually better to use some other
1341 construct such as last or die. The author of Perl has never felt the
1342 need to use this form of goto (in Perl, that is--C is another matter).
1344 The goto-EXPR form expects a label name, whose scope will be resolved
1345 dynamically. This allows for computed gotos per FORTRAN, but isn't
1346 necessarily recommended if you're optimizing for maintainability:
1348 goto ("FOO", "BAR", "GLARCH")[$i];
1350 The goto-&NAME form is highly magical, and substitutes a call to the
1351 named subroutine for the currently running subroutine. This is used by
1352 AUTOLOAD subroutines that wish to load another subroutine and then
1353 pretend that the other subroutine had been called in the first place
1354 (except that any modifications to @_ in the current subroutine are
1355 propagated to the other subroutine.) After the goto, not even caller()
1356 will be able to tell that this routine was called first.
1358 =item grep BLOCK LIST
1360 =item grep EXPR,LIST
1362 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1363 $_ to each element) and returns the list value consisting of those
1364 elements for which the expression evaluated to TRUE. In a scalar
1365 context, returns the number of times the expression was TRUE.
1367 @foo = grep(!/^#/, @bar); # weed out comments
1371 @foo = grep {!/^#/} @bar; # weed out comments
1373 Note that, since $_ is a reference into the list value, it can be used
1374 to modify the elements of the array. While this is useful and
1375 supported, it can cause bizarre results if the LIST is not a named
1382 Interprets EXPR as a hex string and returns the corresponding decimal
1383 value. (To convert strings that might start with 0 or 0x see
1384 oct().) If EXPR is omitted, uses $_.
1388 There is no built-in import() function. It is merely an ordinary
1389 method (subroutine) defined (or inherited) by modules that wish to export
1390 names to another module. The use() function calls the import() method
1391 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1393 =item index STR,SUBSTR,POSITION
1395 =item index STR,SUBSTR
1397 Returns the position of the first occurrence of SUBSTR in STR at or after
1398 POSITION. If POSITION is omitted, starts searching from the beginning of
1399 the string. The return value is based at 0 (or whatever you've set the C<$[>
1400 variable to--but don't do that). If the substring is not found, returns
1401 one less than the base, ordinarily -1.
1407 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1409 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1411 Implements the ioctl(2) function. You'll probably have to say
1413 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1415 first to get the correct function definitions. If F<ioctl.ph> doesn't
1416 exist or doesn't have the correct definitions you'll have to roll your
1417 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1418 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1419 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1420 written depending on the FUNCTION--a pointer to the string value of SCALAR
1421 will be passed as the third argument of the actual ioctl call. (If SCALAR
1422 has no string value but does have a numeric value, that value will be
1423 passed rather than a pointer to the string value. To guarantee this to be
1424 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1425 functions are useful for manipulating the values of structures used by
1426 ioctl(). The following example sets the erase character to DEL.
1430 die "NO TIOCGETP" if $@ || !$getp;
1431 $sgttyb_t = "ccccs"; # 4 chars and a short
1432 if (ioctl(STDIN,$getp,$sgttyb)) {
1433 @ary = unpack($sgttyb_t,$sgttyb);
1435 $sgttyb = pack($sgttyb_t,@ary);
1436 ioctl(STDIN,&TIOCSETP,$sgttyb)
1437 || die "Can't ioctl: $!";
1440 The return value of ioctl (and fcntl) is as follows:
1442 if OS returns: then Perl returns:
1444 0 string "0 but true"
1445 anything else that number
1447 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1448 still easily determine the actual value returned by the operating
1451 ($retval = ioctl(...)) || ($retval = -1);
1452 printf "System returned %d\n", $retval;
1454 =item join EXPR,LIST
1456 Joins the separate strings of LIST or ARRAY into a single string with
1457 fields separated by the value of EXPR, and returns the string.
1460 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1462 See L<perlfunc/split>.
1464 =item keys ASSOC_ARRAY
1466 Returns a normal array consisting of all the keys of the named
1467 associative array. (In a scalar context, returns the number of keys.)
1468 The keys are returned in an apparently random order, but it is the same
1469 order as either the values() or each() function produces (given that
1470 the associative array has not been modified). Here is yet another way
1471 to print your environment:
1474 @values = values %ENV;
1475 while ($#keys >= 0) {
1476 print pop(@keys), '=', pop(@values), "\n";
1479 or how about sorted by key:
1481 foreach $key (sort(keys %ENV)) {
1482 print $key, '=', $ENV{$key}, "\n";
1485 To sort an array by value, you'll need to use a C<sort{}>
1486 function. Here's a descending numeric sort of a hash by its values:
1488 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1489 printf "%4d %s\n", $hash{$key}, $key;
1492 As an lvalue C<keys> allows you to increase the number of hash buckets
1493 allocated for the given associative array. This can gain you a measure
1494 of efficiency if you know the hash is going to get big. (This is
1495 similar to pre-extending an array by assigning a larger number to
1496 $#array.) If you say
1500 then C<%hash> will have at least 200 buckets allocated for it. These
1501 buckets will be retained even if you do C<%hash = ()>, use C<undef
1502 %hash> if you want to free the storage while C<%hash> is still in scope.
1503 You can't shrink the number of buckets allocated for the hash using
1504 C<keys> in this way (but you needn't worry about doing this by accident,
1505 as trying has no effect).
1509 Sends a signal to a list of processes. The first element of
1510 the list must be the signal to send. Returns the number of
1511 processes successfully signaled.
1513 $cnt = kill 1, $child1, $child2;
1516 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1517 process groups instead of processes. (On System V, a negative I<PROCESS>
1518 number will also kill process groups, but that's not portable.) That
1519 means you usually want to use positive not negative signals. You may also
1520 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1526 The C<last> command is like the C<break> statement in C (as used in
1527 loops); it immediately exits the loop in question. If the LABEL is
1528 omitted, the command refers to the innermost enclosing loop. The
1529 C<continue> block, if any, is not executed:
1531 LINE: while (<STDIN>) {
1532 last LINE if /^$/; # exit when done with header
1540 Returns an lowercased version of EXPR. This is the internal function
1541 implementing the \L escape in double-quoted strings.
1542 Should respect any POSIX setlocale() settings.
1544 If EXPR is omitted, uses $_.
1550 Returns the value of EXPR with the first character lowercased. This is
1551 the internal function implementing the \l escape in double-quoted strings.
1552 Should respect any POSIX setlocale() settings.
1554 If EXPR is omitted, uses $_.
1560 Returns the length in characters of the value of EXPR. If EXPR is
1561 omitted, returns length of $_.
1563 =item link OLDFILE,NEWFILE
1565 Creates a new filename linked to the old filename. Returns 1 for
1566 success, 0 otherwise.
1568 =item listen SOCKET,QUEUESIZE
1570 Does the same thing that the listen system call does. Returns TRUE if
1571 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1575 A local modifies the listed variables to be local to the enclosing block,
1576 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1577 list must be placed in parens. See L<perlsub/"Temporary Values via
1578 local()"> for details.
1580 But you really probably want to be using my() instead, because local() isn't
1581 what most people think of as "local"). See L<perlsub/"Private Variables
1582 via my()"> for details.
1584 =item localtime EXPR
1586 Converts a time as returned by the time function to a 9-element array
1587 with the time analyzed for the local timezone. Typically used as
1590 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1593 All array elements are numeric, and come straight out of a struct tm.
1594 In particular this means that $mon has the range 0..11 and $wday has
1595 the range 0..6. If EXPR is omitted, does localtime(time).
1597 In a scalar context, prints out the ctime(3) value:
1599 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1601 Also see the F<timelocal.pl> library, and the strftime(3) function available
1602 via the POSIX module.
1608 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1611 =item lstat FILEHANDLE
1617 Does the same thing as the stat() function, but stats a symbolic link
1618 instead of the file the symbolic link points to. If symbolic links are
1619 unimplemented on your system, a normal stat() is done.
1621 If EXPR is omitted, stats $_.
1625 The match operator. See L<perlop>.
1627 =item map BLOCK LIST
1631 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1632 element) and returns the list value composed of the results of each such
1633 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1634 may produce zero, one, or more elements in the returned value.
1636 @chars = map(chr, @nums);
1638 translates a list of numbers to the corresponding characters. And
1640 %hash = map { getkey($_) => $_ } @array;
1642 is just a funny way to write
1645 foreach $_ (@array) {
1646 $hash{getkey($_)} = $_;
1649 =item mkdir FILENAME,MODE
1651 Creates the directory specified by FILENAME, with permissions specified
1652 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1653 it returns 0 and sets C<$!> (errno).
1655 =item msgctl ID,CMD,ARG
1657 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1658 must be a variable which will hold the returned msqid_ds structure.
1659 Returns like ioctl: the undefined value for error, "0 but true" for
1660 zero, or the actual return value otherwise.
1662 =item msgget KEY,FLAGS
1664 Calls the System V IPC function msgget(2). Returns the message queue id,
1665 or the undefined value if there is an error.
1667 =item msgsnd ID,MSG,FLAGS
1669 Calls the System V IPC function msgsnd to send the message MSG to the
1670 message queue ID. MSG must begin with the long integer message type,
1671 which may be created with C<pack("l", $type)>. Returns TRUE if
1672 successful, or FALSE if there is an error.
1674 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1676 Calls the System V IPC function msgrcv to receive a message from
1677 message queue ID into variable VAR with a maximum message size of
1678 SIZE. Note that if a message is received, the message type will be the
1679 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1680 of the message type. Returns TRUE if successful, or FALSE if there is
1685 A "my" declares the listed variables to be local (lexically) to the
1686 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1687 more than one value is listed, the list must be placed in parens. See
1688 L<perlsub/"Private Variables via my()"> for details.
1694 The C<next> command is like the C<continue> statement in C; it starts
1695 the next iteration of the loop:
1697 LINE: while (<STDIN>) {
1698 next LINE if /^#/; # discard comments
1702 Note that if there were a C<continue> block on the above, it would get
1703 executed even on discarded lines. If the LABEL is omitted, the command
1704 refers to the innermost enclosing loop.
1706 =item no Module LIST
1708 See the "use" function, which "no" is the opposite of.
1714 Interprets EXPR as an octal string and returns the corresponding
1715 decimal value. (If EXPR happens to start off with 0x, interprets it as
1716 a hex string instead.) The following will handle decimal, octal, and
1717 hex in the standard Perl or C notation:
1719 $val = oct($val) if $val =~ /^0/;
1721 If EXPR is omitted, uses $_.
1723 =item open FILEHANDLE,EXPR
1725 =item open FILEHANDLE
1727 Opens the file whose filename is given by EXPR, and associates it with
1728 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1729 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1730 the same name as the FILEHANDLE contains the filename. If the filename
1731 begins with "E<lt>" or nothing, the file is opened for input. If the filename
1732 begins with "E<gt>", the file is opened for output. If the filename begins
1733 with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1734 front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1735 access to the file; thus '+E<lt>' is usually preferred for read/write
1736 updates--the '+E<gt>' mode would clobber the file first. These correspond to
1737 the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1739 If the filename begins with "|", the filename is interpreted
1740 as a command to which output is to be piped, and if the filename ends with
1741 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1742 for more examples of this. as command which pipes input to us. (You may
1743 not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
1744 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1746 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1747 non-zero upon success, the undefined value otherwise. If the open
1748 involved a pipe, the return value happens to be the pid of the
1751 If you're unfortunate enough to be running Perl on a system that
1752 distinguishes between text files and binary files (modern operating
1753 systems don't care), then you should check out L</binmode> for tips for
1754 dealing with this. The key distinction between systems that need binmode
1755 and those that don't is their text file formats. Systems like Unix and
1756 Plan9 that delimit lines with a single character, and that encode that
1757 character in C as '\n', do not need C<binmode>. The rest need it.
1762 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1763 while (<ARTICLE>) {...
1765 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1767 open(DBASE, '+<dbase.mine'); # open for update
1769 open(ARTICLE, "caesar <$article |"); # decrypt article
1771 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1773 # process argument list of files along with any includes
1775 foreach $file (@ARGV) {
1776 process($file, 'fh00');
1780 local($filename, $input) = @_;
1781 $input++; # this is a string increment
1782 unless (open($input, $filename)) {
1783 print STDERR "Can't open $filename: $!\n";
1787 while (<$input>) { # note use of indirection
1788 if (/^#include "(.*)"/) {
1789 process($1, $input);
1796 You may also, in the Bourne shell tradition, specify an EXPR beginning
1797 with "E<gt>&", in which case the rest of the string is interpreted as the
1798 name of a filehandle (or file descriptor, if numeric) which is to be
1799 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1800 +E<gt>E<gt> and +E<lt>. The
1801 mode you specify should match the mode of the original filehandle.
1802 (Duping a filehandle does not take into account any existing contents of
1804 Here is a script that saves, redirects, and restores STDOUT and
1808 open(SAVEOUT, ">&STDOUT");
1809 open(SAVEERR, ">&STDERR");
1811 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1812 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1814 select(STDERR); $| = 1; # make unbuffered
1815 select(STDOUT); $| = 1; # make unbuffered
1817 print STDOUT "stdout 1\n"; # this works for
1818 print STDERR "stderr 1\n"; # subprocesses too
1823 open(STDOUT, ">&SAVEOUT");
1824 open(STDERR, ">&SAVEERR");
1826 print STDOUT "stdout 2\n";
1827 print STDERR "stderr 2\n";
1830 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1831 equivalent of C's fdopen() of that file descriptor; this is more
1832 parsimonious of file descriptors. For example:
1834 open(FILEHANDLE, "<&=$fd")
1836 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1837 there is an implicit fork done, and the return value of open is the pid
1838 of the child within the parent process, and 0 within the child
1839 process. (Use C<defined($pid)> to determine whether the open was successful.)
1840 The filehandle behaves normally for the parent, but i/o to that
1841 filehandle is piped from/to the STDOUT/STDIN of the child process.
1842 In the child process the filehandle isn't opened--i/o happens from/to
1843 the new STDOUT or STDIN. Typically this is used like the normal
1844 piped open when you want to exercise more control over just how the
1845 pipe command gets executed, such as when you are running setuid, and
1846 don't want to have to scan shell commands for metacharacters.
1847 The following pairs are more or less equivalent:
1849 open(FOO, "|tr '[a-z]' '[A-Z]'");
1850 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1852 open(FOO, "cat -n '$file'|");
1853 open(FOO, "-|") || exec 'cat', '-n', $file;
1855 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1857 Explicitly closing any piped filehandle causes the parent process to
1858 wait for the child to finish, and returns the status value in C<$?>.
1859 Note: on any operation which may do a fork, unflushed buffers remain
1860 unflushed in both processes, which means you may need to set C<$|> to
1861 avoid duplicate output.
1863 Using the FileHandle constructor from the FileHandle package,
1864 you can generate anonymous filehandles which have the scope of whatever
1865 variables hold references to them, and automatically close whenever
1866 and however you leave that scope:
1870 sub read_myfile_munged {
1872 my $handle = new FileHandle;
1873 open($handle, "myfile") or die "myfile: $!";
1875 or return (); # Automatically closed here.
1876 mung $first or die "mung failed"; # Or here.
1877 return $first, <$handle> if $ALL; # Or here.
1881 The filename that is passed to open will have leading and trailing
1882 whitespace deleted. In order to open a file with arbitrary weird
1883 characters in it, it's necessary to protect any leading and trailing
1886 $file =~ s#^(\s)#./$1#;
1887 open(FOO, "< $file\0");
1889 If you want a "real" C open() (see L<open(2)> on your system), then
1890 you should use the sysopen() function. This is another way to
1891 protect your filenames from interpretation. For example:
1894 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1895 or die "sysopen $path: $!";
1896 HANDLE->autoflush(1);
1897 HANDLE->print("stuff $$\n");
1899 print "File contains: ", <HANDLE>;
1901 See L</seek()> for some details about mixing reading and writing.
1903 =item opendir DIRHANDLE,EXPR
1905 Opens a directory named EXPR for processing by readdir(), telldir(),
1906 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1907 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1913 Returns the numeric ascii value of the first character of EXPR. If
1914 EXPR is omitted, uses $_.
1916 =item pack TEMPLATE,LIST
1918 Takes an array or list of values and packs it into a binary structure,
1919 returning the string containing the structure. The TEMPLATE is a
1920 sequence of characters that give the order and type of values, as
1923 A An ascii string, will be space padded.
1924 a An ascii string, will be null padded.
1925 b A bit string (ascending bit order, like vec()).
1926 B A bit string (descending bit order).
1927 h A hex string (low nybble first).
1928 H A hex string (high nybble first).
1930 c A signed char value.
1931 C An unsigned char value.
1932 s A signed short value.
1933 S An unsigned short value.
1934 i A signed integer value.
1935 I An unsigned integer value.
1936 l A signed long value.
1937 L An unsigned long value.
1939 n A short in "network" order.
1940 N A long in "network" order.
1941 v A short in "VAX" (little-endian) order.
1942 V A long in "VAX" (little-endian) order.
1944 f A single-precision float in the native format.
1945 d A double-precision float in the native format.
1947 p A pointer to a null-terminated string.
1948 P A pointer to a structure (fixed-length string).
1950 u A uuencoded string.
1952 w A BER compressed integer. Bytes give an unsigned integer base
1953 128, most significant digit first, with as few digits as
1954 possible, and with the bit 8 of each byte except the last set
1959 @ Null fill to absolute position.
1961 Each letter may optionally be followed by a number which gives a repeat
1962 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1963 pack function will gobble up that many values from the LIST. A * for the
1964 repeat count means to use however many items are left. The "a" and "A"
1965 types gobble just one value, but pack it as a string of length count,
1966 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1967 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1968 fields pack a string that many bits long. The "h" and "H" fields pack a
1969 string that many nybbles long. The "P" packs a pointer to a structure of
1970 the size indicated by the length. Real numbers (floats and doubles) are
1971 in the native machine format only; due to the multiplicity of floating
1972 formats around, and the lack of a standard "network" representation, no
1973 facility for interchange has been made. This means that packed floating
1974 point data written on one machine may not be readable on another - even if
1975 both use IEEE floating point arithmetic (as the endian-ness of the memory
1976 representation is not part of the IEEE spec). Note that Perl uses doubles
1977 internally for all numeric calculation, and converting from double into
1978 float and thence back to double again will lose precision (i.e.
1979 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1983 $foo = pack("cccc",65,66,67,68);
1985 $foo = pack("c4",65,66,67,68);
1988 $foo = pack("ccxxcc",65,66,67,68);
1991 $foo = pack("s2",1,2);
1992 # "\1\0\2\0" on little-endian
1993 # "\0\1\0\2" on big-endian
1995 $foo = pack("a4","abcd","x","y","z");
1998 $foo = pack("aaaa","abcd","x","y","z");
2001 $foo = pack("a14","abcdefg");
2002 # "abcdefg\0\0\0\0\0\0\0"
2004 $foo = pack("i9pl", gmtime);
2005 # a real struct tm (on my system anyway)
2008 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2011 The same template may generally also be used in the unpack function.
2013 =item package NAMESPACE
2015 Declares the compilation unit as being in the given namespace. The scope
2016 of the package declaration is from the declaration itself through the end of
2017 the enclosing block (the same scope as the local() operator). All further
2018 unqualified dynamic identifiers will be in this namespace. A package
2019 statement only affects dynamic variables--including those you've used
2020 local() on--but I<not> lexical variables created with my(). Typically it
2021 would be the first declaration in a file to be included by the C<require>
2022 or C<use> operator. You can switch into a package in more than one place;
2023 it merely influences which symbol table is used by the compiler for the
2024 rest of that block. You can refer to variables and filehandles in other
2025 packages by prefixing the identifier with the package name and a double
2026 colon: C<$Package::Variable>. If the package name is null, the C<main>
2027 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2029 See L<perlmod/"Packages"> for more information about packages, modules,
2030 and classes. See L<perlsub> for other scoping issues.
2032 =item pipe READHANDLE,WRITEHANDLE
2034 Opens a pair of connected pipes like the corresponding system call.
2035 Note that if you set up a loop of piped processes, deadlock can occur
2036 unless you are very careful. In addition, note that Perl's pipes use
2037 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2038 after each command, depending on the application.
2040 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
2041 for examples of such things.
2045 Pops and returns the last value of the array, shortening the array by
2046 1. Has a similar effect to
2048 $tmp = $ARRAY[$#ARRAY--];
2050 If there are no elements in the array, returns the undefined value.
2051 If ARRAY is omitted, pops the
2052 @ARGV array in the main program, and the @_ array in subroutines, just
2059 Returns the offset of where the last C<m//g> search left off for the variable
2060 is in question ($_ is used when the variable is not specified). May be
2061 modified to change that offset.
2063 =item print FILEHANDLE LIST
2069 Prints a string or a comma-separated list of strings. Returns TRUE
2070 if successful. FILEHANDLE may be a scalar variable name, in which case
2071 the variable contains the name of or a reference to the filehandle, thus introducing one
2072 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2073 token is a term, it may be misinterpreted as an operator unless you
2074 interpose a + or put parens around the arguments.) If FILEHANDLE is
2075 omitted, prints by default to standard output (or to the last selected
2076 output channel--see L</select>). If LIST is also omitted, prints $_ to
2077 STDOUT. To set the default output channel to something other than
2078 STDOUT use the select operation. Note that, because print takes a
2079 LIST, anything in the LIST is evaluated in a list context, and any
2080 subroutine that you call will have one or more of its expressions
2081 evaluated in a list context. Also be careful not to follow the print
2082 keyword with a left parenthesis unless you want the corresponding right
2083 parenthesis to terminate the arguments to the print--interpose a + or
2084 put parens around all the arguments.
2086 Note that if you're storing FILEHANDLES in an array or other expression,
2087 you will have to use a block returning its value instead:
2089 print { $files[$i] } "stuff\n";
2090 print { $OK ? STDOUT : STDERR } "stuff\n";
2092 =item printf FILEHANDLE LIST
2096 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2097 of the list will be interpreted as the printf format.
2099 =item prototype FUNCTION
2101 Returns the prototype of a function as a string (or C<undef> if the
2102 function has no prototype). FUNCTION is a reference to the the
2103 function whose prototype you want to retrieve.
2105 =item push ARRAY,LIST
2107 Treats ARRAY as a stack, and pushes the values of LIST
2108 onto the end of ARRAY. The length of ARRAY increases by the length of
2109 LIST. Has the same effect as
2112 $ARRAY[++$#ARRAY] = $value;
2115 but is more efficient. Returns the new number of elements in the array.
2125 Generalized quotes. See L<perlop>.
2127 =item quotemeta EXPR
2131 Returns the value of EXPR with with all regular expression
2132 metacharacters backslashed. This is the internal function implementing
2133 the \Q escape in double-quoted strings.
2135 If EXPR is omitted, uses $_.
2141 Returns a random fractional number between 0 and the value of EXPR.
2142 (EXPR should be positive.) If EXPR is omitted, returns a value between
2143 0 and 1. This function produces repeatable sequences unless srand()
2144 is invoked. See also srand().
2146 (Note: if your rand function consistently returns numbers that are too
2147 large or too small, then your version of Perl was probably compiled
2148 with the wrong number of RANDBITS. As a workaround, you can usually
2149 multiply EXPR by the correct power of 2 to get the range you want.
2150 This will make your script unportable, however. It's better to recompile
2153 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2155 =item read FILEHANDLE,SCALAR,LENGTH
2157 Attempts to read LENGTH bytes of data into variable SCALAR from the
2158 specified FILEHANDLE. Returns the number of bytes actually read, or
2159 undef if there was an error. SCALAR will be grown or shrunk to the
2160 length actually read. An OFFSET may be specified to place the read
2161 data at some other place than the beginning of the string. This call
2162 is actually implemented in terms of stdio's fread call. To get a true
2163 read system call, see sysread().
2165 =item readdir DIRHANDLE
2167 Returns the next directory entry for a directory opened by opendir().
2168 If used in a list context, returns all the rest of the entries in the
2169 directory. If there are no more entries, returns an undefined value in
2170 a scalar context or a null list in a list context.
2172 If you're planning to filetest the return values out of a readdir(), you'd
2173 better prepend the directory in question. Otherwise, since we didn't
2174 chdir() there, it would have been testing the wrong file.
2176 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2177 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2184 Returns the value of a symbolic link, if symbolic links are
2185 implemented. If not, gives a fatal error. If there is some system
2186 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2189 =item recv SOCKET,SCALAR,LEN,FLAGS
2191 Receives a message on a socket. Attempts to receive LENGTH bytes of
2192 data into variable SCALAR from the specified SOCKET filehandle.
2193 Actually does a C recvfrom(), so that it can returns the address of the
2194 sender. Returns the undefined value if there's an error. SCALAR will
2195 be grown or shrunk to the length actually read. Takes the same flags
2196 as the system call of the same name.
2197 See L<perlipc/"UDP: Message Passing"> for examples.
2203 The C<redo> command restarts the loop block without evaluating the
2204 conditional again. The C<continue> block, if any, is not executed. If
2205 the LABEL is omitted, the command refers to the innermost enclosing
2206 loop. This command is normally used by programs that want to lie to
2207 themselves about what was just input:
2209 # a simpleminded Pascal comment stripper
2210 # (warning: assumes no { or } in strings)
2211 LINE: while (<STDIN>) {
2212 while (s|({.*}.*){.*}|$1 |) {}
2217 if (/}/) { # end of comment?
2230 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2231 is not specified, $_ will be used. The value returned depends on the
2232 type of thing the reference is a reference to.
2233 Builtin types include:
2242 If the referenced object has been blessed into a package, then that package
2243 name is returned instead. You can think of ref() as a typeof() operator.
2245 if (ref($r) eq "HASH") {
2246 print "r is a reference to an associative array.\n";
2249 print "r is not a reference at all.\n";
2252 See also L<perlref>.
2254 =item rename OLDNAME,NEWNAME
2256 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2257 not work across filesystem boundaries.
2263 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2264 supplied. If EXPR is numeric, demands that the current version of Perl
2265 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2267 Otherwise, demands that a library file be included if it hasn't already
2268 been included. The file is included via the do-FILE mechanism, which is
2269 essentially just a variety of eval(). Has semantics similar to the following
2273 local($filename) = @_;
2274 return 1 if $INC{$filename};
2275 local($realfilename,$result);
2277 foreach $prefix (@INC) {
2278 $realfilename = "$prefix/$filename";
2279 if (-f $realfilename) {
2280 $result = do $realfilename;
2284 die "Can't find $filename in \@INC";
2287 die "$filename did not return true value" unless $result;
2288 $INC{$filename} = $realfilename;
2292 Note that the file will not be included twice under the same specified
2293 name. The file must return TRUE as the last statement to indicate
2294 successful execution of any initialization code, so it's customary to
2295 end such a file with "1;" unless you're sure it'll return TRUE
2296 otherwise. But it's better just to put the "C<1;>", in case you add more
2299 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2300 replaces "F<::>" with "F</>" in the filename for you,
2301 to make it easy to load standard modules. This form of loading of
2302 modules does not risk altering your namespace.
2304 For a yet-more-powerful import facility, see L</use> and
2311 Generally used in a C<continue> block at the end of a loop to clear
2312 variables and reset ?? searches so that they work again. The
2313 expression is interpreted as a list of single characters (hyphens
2314 allowed for ranges). All variables and arrays beginning with one of
2315 those letters are reset to their pristine state. If the expression is
2316 omitted, one-match searches (?pattern?) are reset to match again. Only
2317 resets variables or searches in the current package. Always returns
2320 reset 'X'; # reset all X variables
2321 reset 'a-z'; # reset lower case variables
2322 reset; # just reset ?? searches
2324 Resetting "A-Z" is not recommended since you'll wipe out your
2325 ARGV and ENV arrays. Only resets package variables--lexical variables
2326 are unaffected, but they clean themselves up on scope exit anyway,
2327 so you'll probably want to use them instead. See L</my>.
2331 Returns from a subroutine or eval with the value specified. (Note that
2332 in the absence of a return a subroutine or eval() will automatically
2333 return the value of the last expression evaluated.)
2337 In a list context, returns a list value consisting of the elements
2338 of LIST in the opposite order. In a scalar context, returns a string
2339 value consisting of the bytes of the first element of LIST in the
2342 print reverse <>; # line tac
2345 print scalar reverse scalar <>; # byte tac
2347 =item rewinddir DIRHANDLE
2349 Sets the current position to the beginning of the directory for the
2350 readdir() routine on DIRHANDLE.
2352 =item rindex STR,SUBSTR,POSITION
2354 =item rindex STR,SUBSTR
2356 Works just like index except that it returns the position of the LAST
2357 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2358 last occurrence at or before that position.
2360 =item rmdir FILENAME
2364 Deletes the directory specified by FILENAME if it is empty. If it
2365 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2366 FILENAME is omitted, uses $_.
2370 The substitution operator. See L<perlop>.
2374 Forces EXPR to be interpreted in a scalar context and returns the value
2377 @counts = ( scalar @a, scalar @b, scalar @c );
2379 There is no equivalent operator to force an expression to
2380 be interpolated in a list context because it's in practice never
2381 needed. If you really wanted to do so, however, you could use
2382 the construction C<@{[ (some expression) ]}>, but usually a simple
2383 C<(some expression)> suffices.
2385 =item seek FILEHANDLE,POSITION,WHENCE
2387 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2388 call of stdio. FILEHANDLE may be an expression whose value gives the name
2389 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2390 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2391 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2392 this from POSIX module. Returns 1 upon success, 0 otherwise.
2394 On some systems you have to do a seek whenever you switch between reading
2395 and writing. Amongst other things, this may have the effect of calling
2396 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2401 This is also useful for applications emulating C<tail -f>. Once you hit
2402 EOF on your read, and then sleep for a while, you might have to stick in a
2403 seek() to reset things. First the simple trick listed above to clear the
2404 filepointer. The seek() doesn't change the current position, but it
2405 I<does> clear the end-of-file condition on the handle, so that the next
2406 C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
2408 If that doesn't work (some stdios are particularly cantankerous), then
2409 you may need something more like this:
2412 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2413 # search for some stuff and put it into files
2415 sleep($for_a_while);
2416 seek(FILE, $curpos, 0);
2419 =item seekdir DIRHANDLE,POS
2421 Sets the current position for the readdir() routine on DIRHANDLE. POS
2422 must be a value returned by telldir(). Has the same caveats about
2423 possible directory compaction as the corresponding system library
2426 =item select FILEHANDLE
2430 Returns the currently selected filehandle. Sets the current default
2431 filehandle for output, if FILEHANDLE is supplied. This has two
2432 effects: first, a C<write> or a C<print> without a filehandle will
2433 default to this FILEHANDLE. Second, references to variables related to
2434 output will refer to this output channel. For example, if you have to
2435 set the top of form format for more than one output channel, you might
2443 FILEHANDLE may be an expression whose value gives the name of the
2444 actual filehandle. Thus:
2446 $oldfh = select(STDERR); $| = 1; select($oldfh);
2448 Some programmers may prefer to think of filehandles as objects with
2449 methods, preferring to write the last example as:
2452 STDERR->autoflush(1);
2454 =item select RBITS,WBITS,EBITS,TIMEOUT
2456 This calls the select(2) system call with the bitmasks specified, which
2457 can be constructed using fileno() and vec(), along these lines:
2459 $rin = $win = $ein = '';
2460 vec($rin,fileno(STDIN),1) = 1;
2461 vec($win,fileno(STDOUT),1) = 1;
2464 If you want to select on many filehandles you might wish to write a
2468 local(@fhlist) = split(' ',$_[0]);
2471 vec($bits,fileno($_),1) = 1;
2475 $rin = fhbits('STDIN TTY SOCK');
2479 ($nfound,$timeleft) =
2480 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2482 or to block until something becomes ready just do this
2484 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2486 Most systems do not both to return anything useful in $timeleft, so
2487 calling select() in a scalar context just returns $nfound.
2489 Any of the bitmasks can also be undef. The timeout, if specified, is
2490 in seconds, which may be fractional. Note: not all implementations are
2491 capable of returning the $timeleft. If not, they always return
2492 $timeleft equal to the supplied $timeout.
2494 You can effect a sleep of 250 milliseconds this way:
2496 select(undef, undef, undef, 0.25);
2498 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2499 with select(). You have to use sysread() instead.
2501 =item semctl ID,SEMNUM,CMD,ARG
2503 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2504 &GETALL, then ARG must be a variable which will hold the returned
2505 semid_ds structure or semaphore value array. Returns like ioctl: the
2506 undefined value for error, "0 but true" for zero, or the actual return
2509 =item semget KEY,NSEMS,FLAGS
2511 Calls the System V IPC function semget. Returns the semaphore id, or
2512 the undefined value if there is an error.
2514 =item semop KEY,OPSTRING
2516 Calls the System V IPC function semop to perform semaphore operations
2517 such as signaling and waiting. OPSTRING must be a packed array of
2518 semop structures. Each semop structure can be generated with
2519 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2520 operations is implied by the length of OPSTRING. Returns TRUE if
2521 successful, or FALSE if there is an error. As an example, the
2522 following code waits on semaphore $semnum of semaphore id $semid:
2524 $semop = pack("sss", $semnum, -1, 0);
2525 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2527 To signal the semaphore, replace "-1" with "1".
2529 =item send SOCKET,MSG,FLAGS,TO
2531 =item send SOCKET,MSG,FLAGS
2533 Sends a message on a socket. Takes the same flags as the system call
2534 of the same name. On unconnected sockets you must specify a
2535 destination to send TO, in which case it does a C sendto(). Returns
2536 the number of characters sent, or the undefined value if there is an
2538 See L<perlipc/"UDP: Message Passing"> for examples.
2540 =item setpgrp PID,PGRP
2542 Sets the current process group for the specified PID, 0 for the current
2543 process. Will produce a fatal error if used on a machine that doesn't
2544 implement setpgrp(2). If the arguments are ommitted, it defaults to
2545 0,0. Note that the POSIX version of setpgrp() does not accept any
2546 arguments, so only setpgrp 0,0 is portable.
2548 =item setpriority WHICH,WHO,PRIORITY
2550 Sets the current priority for a process, a process group, or a user.
2551 (See setpriority(2).) Will produce a fatal error if used on a machine
2552 that doesn't implement setpriority(2).
2554 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2556 Sets the socket option requested. Returns undefined if there is an
2557 error. OPTVAL may be specified as undef if you don't want to pass an
2564 Shifts the first value of the array off and returns it, shortening the
2565 array by 1 and moving everything down. If there are no elements in the
2566 array, returns the undefined value. If ARRAY is omitted, shifts the
2567 @ARGV array in the main program, and the @_ array in subroutines.
2568 (This is determined lexically.) See also unshift(), push(), and pop().
2569 Shift() and unshift() do the same thing to the left end of an array
2570 that push() and pop() do to the right end.
2572 =item shmctl ID,CMD,ARG
2574 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2575 must be a variable which will hold the returned shmid_ds structure.
2576 Returns like ioctl: the undefined value for error, "0 but true" for
2577 zero, or the actual return value otherwise.
2579 =item shmget KEY,SIZE,FLAGS
2581 Calls the System V IPC function shmget. Returns the shared memory
2582 segment id, or the undefined value if there is an error.
2584 =item shmread ID,VAR,POS,SIZE
2586 =item shmwrite ID,STRING,POS,SIZE
2588 Reads or writes the System V shared memory segment ID starting at
2589 position POS for size SIZE by attaching to it, copying in/out, and
2590 detaching from it. When reading, VAR must be a variable which will
2591 hold the data read. When writing, if STRING is too long, only SIZE
2592 bytes are used; if STRING is too short, nulls are written to fill out
2593 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2595 =item shutdown SOCKET,HOW
2597 Shuts down a socket connection in the manner indicated by HOW, which
2598 has the same interpretation as in the system call of the same name.
2604 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2611 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2612 May be interrupted by sending the process a SIGALRM. Returns the
2613 number of seconds actually slept. You probably cannot mix alarm() and
2614 sleep() calls, since sleep() is often implemented using alarm().
2616 On some older systems, it may sleep up to a full second less than what
2617 you requested, depending on how it counts seconds. Most modern systems
2618 always sleep the full amount.
2620 For delays of finer granularity than one second, you may use Perl's
2621 syscall() interface to access setitimer(2) if your system supports it,
2622 or else see L</select()> below.
2624 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2626 Opens a socket of the specified kind and attaches it to filehandle
2627 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2628 system call of the same name. You should "use Socket;" first to get
2629 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2631 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2633 Creates an unnamed pair of sockets in the specified domain, of the
2634 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2635 for the system call of the same name. If unimplemented, yields a fatal
2636 error. Returns TRUE if successful.
2638 =item sort SUBNAME LIST
2640 =item sort BLOCK LIST
2644 Sorts the LIST and returns the sorted list value. Nonexistent values
2645 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2646 in standard string comparison order. If SUBNAME is specified, it
2647 gives the name of a subroutine that returns an integer less than, equal
2648 to, or greater than 0, depending on how the elements of the array are
2649 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2650 routines.) SUBNAME may be a scalar variable name, in which case the
2651 value provides the name of the subroutine to use. In place of a
2652 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2655 In the interests of efficiency the normal calling code for subroutines is
2656 bypassed, with the following effects: the subroutine may not be a
2657 recursive subroutine, and the two elements to be compared are passed into
2658 the subroutine not via @_ but as the package global variables $a and
2659 $b (see example below). They are passed by reference, so don't
2660 modify $a and $b. And don't try to declare them as lexicals either.
2665 @articles = sort @files;
2667 # same thing, but with explicit sort routine
2668 @articles = sort {$a cmp $b} @files;
2670 # now case-insensitively
2671 @articles = sort { uc($a) cmp uc($b)} @files;
2673 # same thing in reversed order
2674 @articles = sort {$b cmp $a} @files;
2676 # sort numerically ascending
2677 @articles = sort {$a <=> $b} @files;
2679 # sort numerically descending
2680 @articles = sort {$b <=> $a} @files;
2682 # sort using explicit subroutine name
2684 $age{$a} <=> $age{$b}; # presuming integers
2686 @sortedclass = sort byage @class;
2688 # this sorts the %age associative arrays by value
2689 # instead of key using an inline function
2690 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2692 sub backwards { $b cmp $a; }
2693 @harry = ('dog','cat','x','Cain','Abel');
2694 @george = ('gone','chased','yz','Punished','Axed');
2696 # prints AbelCaincatdogx
2697 print sort backwards @harry;
2698 # prints xdogcatCainAbel
2699 print sort @george, 'to', @harry;
2700 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2702 # inefficiently sort by descending numeric compare using
2703 # the first integer after the first = sign, or the
2704 # whole record case-insensitively otherwise
2707 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2712 # same thing, but much more efficiently;
2713 # we'll build auxiliary indices instead
2717 push @nums, /=(\d+)/;
2722 $nums[$b] <=> $nums[$a]
2724 $caps[$a] cmp $caps[$b]
2728 # same thing using a Schwartzian Transform (no temps)
2729 @new = map { $_->[0] }
2730 sort { $b->[1] <=> $a->[1]
2733 } map { [$_, /=(\d+)/, uc($_)] } @old;
2735 If you're using strict, you I<MUST NOT> declare $a
2736 and $b as lexicals. They are package globals. That means
2737 if you're in the C<main> package, it's
2739 @articles = sort {$main::b <=> $main::a} @files;
2743 @articles = sort {$::b <=> $::a} @files;
2745 but if you're in the C<FooPack> package, it's
2747 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2749 The comparison function is required to behave. If it returns
2750 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2751 sometimes saying the opposite, for example) the Perl interpreter will
2752 probably crash and dump core. This is entirely due to and dependent
2753 upon your system's qsort(3) library routine; this routine often avoids
2754 sanity checks in the interest of speed.
2756 =item splice ARRAY,OFFSET,LENGTH,LIST
2758 =item splice ARRAY,OFFSET,LENGTH
2760 =item splice ARRAY,OFFSET
2762 Removes the elements designated by OFFSET and LENGTH from an array, and
2763 replaces them with the elements of LIST, if any. Returns the elements
2764 removed from the array. The array grows or shrinks as necessary. If
2765 LENGTH is omitted, removes everything from OFFSET onward. The
2766 following equivalencies hold (assuming C<$[ == 0>):
2768 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2769 pop(@a) splice(@a,-1)
2770 shift(@a) splice(@a,0,1)
2771 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2772 $a[$x] = $y splice(@a,$x,1,$y);
2774 Example, assuming array lengths are passed before arrays:
2776 sub aeq { # compare two list values
2777 local(@a) = splice(@_,0,shift);
2778 local(@b) = splice(@_,0,shift);
2779 return 0 unless @a == @b; # same len?
2781 return 0 if pop(@a) ne pop(@b);
2785 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2787 =item split /PATTERN/,EXPR,LIMIT
2789 =item split /PATTERN/,EXPR
2791 =item split /PATTERN/
2795 Splits a string into an array of strings, and returns it.
2797 If not in a list context, returns the number of fields found and splits into
2798 the @_ array. (In a list context, you can force the split into @_ by
2799 using C<??> as the pattern delimiters, but it still returns the array
2800 value.) The use of implicit split to @_ is deprecated, however.
2802 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2803 splits on whitespace (after skipping any leading whitespace). Anything
2804 matching PATTERN is taken to be a delimiter separating the fields. (Note
2805 that the delimiter may be longer than one character.) If LIMIT is
2806 specified and is not negative, splits into no more than that many fields
2807 (though it may split into fewer). If LIMIT is unspecified, trailing null
2808 fields are stripped (which potential users of pop() would do well to
2809 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2810 LIMIT had been specified.
2812 A pattern matching the null string (not to be confused with
2813 a null pattern C<//>, which is just one member of the set of patterns
2814 matching a null string) will split the value of EXPR into separate
2815 characters at each point it matches that way. For example:
2817 print join(':', split(/ */, 'hi there'));
2819 produces the output 'h:i:t:h:e:r:e'.
2821 The LIMIT parameter can be used to partially split a line
2823 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2825 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2826 one larger than the number of variables in the list, to avoid
2827 unnecessary work. For the list above LIMIT would have been 4 by
2828 default. In time critical applications it behooves you not to split
2829 into more fields than you really need.
2831 If the PATTERN contains parentheses, additional array elements are
2832 created from each matching substring in the delimiter.
2834 split(/([,-])/, "1-10,20", 3);
2836 produces the list value
2838 (1, '-', 10, ',', 20)
2840 If you had the entire header of a normal Unix email message in $header,
2841 you could split it up into fields and their values this way:
2843 $header =~ s/\n\s+/ /g; # fix continuation lines
2844 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2846 The pattern C</PATTERN/> may be replaced with an expression to specify
2847 patterns that vary at runtime. (To do runtime compilation only once,
2848 use C</$variable/o>.)
2850 As a special case, specifying a PATTERN of space (C<' '>) will split on
2851 white space just as split with no arguments does. Thus, split(' ') can
2852 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2853 will give you as many null initial fields as there are leading spaces.
2854 A split on /\s+/ is like a split(' ') except that any leading
2855 whitespace produces a null first field. A split with no arguments
2856 really does a C<split(' ', $_)> internally.
2860 open(passwd, '/etc/passwd');
2862 ($login, $passwd, $uid, $gid, $gcos,
2863 $home, $shell) = split(/:/);
2867 (Note that $shell above will still have a newline on it. See L</chop>,
2868 L</chomp>, and L</join>.)
2870 =item sprintf FORMAT,LIST
2872 Returns a string formatted by the usual printf conventions of the C
2873 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2874 (The * character for an indirectly specified length is not
2875 supported, but you can get the same effect by interpolating a variable
2876 into the pattern.) Some C libraries' implementations of sprintf() can
2877 dump core when fed ludicrous arguments.
2883 Return the square root of EXPR. If EXPR is omitted, returns square
2888 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2889 uses a semirandom value based on the current time and process ID, among
2890 other things. Of course, you'd need something much more random than that for
2891 cryptographic purposes, since it's easy to guess the current time.
2892 Checksumming the compressed output of rapidly changing operating system
2893 status programs is the usual method. Examples are posted regularly to
2894 the comp.security.unix newsgroup.
2896 =item stat FILEHANDLE
2902 Returns a 13-element array giving the status info for a file, either the
2903 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
2904 stats $_. Returns a null list if the stat fails. Typically used as
2908 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2909 $atime,$mtime,$ctime,$blksize,$blocks)
2912 Not all fields are supported on all filesystem types. Here are the
2913 meaning of the fields:
2915 dev device number of filesystem
2917 mode file mode (type and permissions)
2918 nlink number of (hard) links to the file
2919 uid numeric user ID of file's owner
2920 gid numer group ID of file's owner
2921 rdev the device identifier (special files only)
2922 size total size of file, in bytes
2923 atime last access time since the epoch
2924 mtime last modify time since the epoch
2925 ctime inode change time (NOT creation type!) since the epoch
2926 blksize preferred blocksize for file system I/O
2927 blocks actual number of blocks allocated
2929 (The epoch was at 00:00 January 1, 1970 GMT.)
2931 If stat is passed the special filehandle consisting of an underline, no
2932 stat is done, but the current contents of the stat structure from the
2933 last stat or filetest are returned. Example:
2935 if (-x $file && (($d) = stat(_)) && $d < 0) {
2936 print "$file is executable NFS file\n";
2939 (This only works on machines for which the device number is negative under NFS.)
2945 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2946 doing many pattern matches on the string before it is next modified.
2947 This may or may not save time, depending on the nature and number of
2948 patterns you are searching on, and on the distribution of character
2949 frequencies in the string to be searched--you probably want to compare
2950 runtimes with and without it to see which runs faster. Those loops
2951 which scan for many short constant strings (including the constant
2952 parts of more complex patterns) will benefit most. You may have only
2953 one study active at a time--if you study a different scalar the first
2954 is "unstudied". (The way study works is this: a linked list of every
2955 character in the string to be searched is made, so we know, for
2956 example, where all the 'k' characters are. From each search string,
2957 the rarest character is selected, based on some static frequency tables
2958 constructed from some C programs and English text. Only those places
2959 that contain this "rarest" character are examined.)
2961 For example, here is a loop which inserts index producing entries
2962 before any line containing a certain pattern:
2966 print ".IX foo\n" if /\bfoo\b/;
2967 print ".IX bar\n" if /\bbar\b/;
2968 print ".IX blurfl\n" if /\bblurfl\b/;
2973 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2974 will be looked at, because "f" is rarer than "o". In general, this is
2975 a big win except in pathological cases. The only question is whether
2976 it saves you more time than it took to build the linked list in the
2979 Note that if you have to look for strings that you don't know till
2980 runtime, you can build an entire loop as a string and eval that to
2981 avoid recompiling all your patterns all the time. Together with
2982 undefining $/ to input entire files as one record, this can be very
2983 fast, often faster than specialized programs like fgrep(1). The following
2984 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
2985 out the names of those files that contain a match:
2987 $search = 'while (<>) { study;';
2988 foreach $word (@words) {
2989 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2994 eval $search; # this screams
2995 $/ = "\n"; # put back to normal input delim
2996 foreach $file (sort keys(%seen)) {
3004 =item sub NAME BLOCK
3006 This is subroutine definition, not a real function I<per se>. With just a
3007 NAME (and possibly prototypes), it's just a forward declaration. Without
3008 a NAME, it's an anonymous function declaration, and does actually return a
3009 value: the CODE ref of the closure you just created. See L<perlsub> and
3010 L<perlref> for details.
3012 =item substr EXPR,OFFSET,LEN
3014 =item substr EXPR,OFFSET
3016 Extracts a substring out of EXPR and returns it. First character is at
3017 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3018 that far from the end of the string. If LEN is omitted, returns
3019 everything to the end of the string. If LEN is negative, leaves that
3020 many characters off the end of the string.
3022 You can use the substr() function
3023 as an lvalue, in which case EXPR must be an lvalue. If you assign
3024 something shorter than LEN, the string will shrink, and if you assign
3025 something longer than LEN, the string will grow to accommodate it. To
3026 keep the string the same length you may need to pad or chop your value
3029 =item symlink OLDFILE,NEWFILE
3031 Creates a new filename symbolically linked to the old filename.
3032 Returns 1 for success, 0 otherwise. On systems that don't support
3033 symbolic links, produces a fatal error at run time. To check for that,
3036 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3040 Calls the system call specified as the first element of the list,
3041 passing the remaining elements as arguments to the system call. If
3042 unimplemented, produces a fatal error. The arguments are interpreted
3043 as follows: if a given argument is numeric, the argument is passed as
3044 an int. If not, the pointer to the string value is passed. You are
3045 responsible to make sure a string is pre-extended long enough to
3046 receive any result that might be written into a string. If your
3047 integer arguments are not literals and have never been interpreted in a
3048 numeric context, you may need to add 0 to them to force them to look
3051 require 'syscall.ph'; # may need to run h2ph
3052 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3054 Note that Perl only supports passing of up to 14 arguments to your system call,
3055 which in practice should usually suffice.
3057 =item sysopen FILEHANDLE,FILENAME,MODE
3059 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3061 Opens the file whose filename is given by FILENAME, and associates it
3062 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3063 the name of the real filehandle wanted. This function calls the
3064 underlying operating system's C<open> function with the parameters
3065 FILENAME, MODE, PERMS.
3067 The possible values and flag bits of the MODE parameter are
3068 system-dependent; they are available via the standard module C<Fcntl>.
3069 However, for historical reasons, some values are universal: zero means
3070 read-only, one means write-only, and two means read/write.
3072 If the file named by FILENAME does not exist and the C<open> call
3073 creates it (typically because MODE includes the O_CREAT flag), then
3074 the value of PERMS specifies the permissions of the newly created
3075 file. If PERMS is omitted, the default value is 0666, which allows
3076 read and write for all. This default is reasonable: see C<umask>.
3078 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3080 =item sysread FILEHANDLE,SCALAR,LENGTH
3082 Attempts to read LENGTH bytes of data into variable SCALAR from the
3083 specified FILEHANDLE, using the system call read(2). It bypasses
3084 stdio, so mixing this with other kinds of reads may cause confusion.
3085 Returns the number of bytes actually read, or undef if there was an
3086 error. SCALAR will be grown or shrunk so that the last byte actually
3087 read is the last byte of the scalar after the read.
3089 An OFFSET may be specified to place the read data at some place in the
3090 string other than the beginning. A negative OFFSET specifies
3091 placement at that many bytes counting backwards from the end of the
3092 string. A positive OFFSET greater than the length of SCALAR results
3093 in the string being padded to the required size with "\0" bytes before
3094 the result of the read is appended.
3098 Does exactly the same thing as "exec LIST" except that a fork is done
3099 first, and the parent process waits for the child process to complete.
3100 Note that argument processing varies depending on the number of
3101 arguments. The return value is the exit status of the program as
3102 returned by the wait() call. To get the actual exit value divide by
3103 256. See also L</exec>. This is I<NOT> what you want to use to capture
3104 the output from a command, for that you should merely use backticks, as
3105 described in L<perlop/"`STRING`">.
3107 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3109 =item syswrite FILEHANDLE,SCALAR,LENGTH
3111 Attempts to write LENGTH bytes of data from variable SCALAR to the
3112 specified FILEHANDLE, using the system call write(2). It bypasses
3113 stdio, so mixing this with prints may cause confusion. Returns the
3114 number of bytes actually written, or undef if there was an error.
3115 If the length is greater than the available data, only as much data as
3116 is available will be written.
3118 An OFFSET may be specified to write the data from some part of the
3119 string other than the beginning. A negative OFFSET specifies writing
3120 from that many bytes counting backwards from the end of the string.
3122 =item tell FILEHANDLE
3126 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3127 expression whose value gives the name of the actual filehandle. If
3128 FILEHANDLE is omitted, assumes the file last read.
3130 =item telldir DIRHANDLE
3132 Returns the current position of the readdir() routines on DIRHANDLE.
3133 Value may be given to seekdir() to access a particular location in a
3134 directory. Has the same caveats about possible directory compaction as
3135 the corresponding system library routine.
3137 =item tie VARIABLE,CLASSNAME,LIST
3139 This function binds a variable to a package class that will provide the
3140 implementation for the variable. VARIABLE is the name of the variable
3141 to be enchanted. CLASSNAME is the name of a class implementing objects
3142 of correct type. Any additional arguments are passed to the "new"
3143 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3144 Typically these are arguments such as might be passed to the dbm_open()
3145 function of C. The object returned by the "new" method is also
3146 returned by the tie() function, which would be useful if you want to
3147 access other methods in CLASSNAME.
3149 Note that functions such as keys() and values() may return huge array
3150 values when used on large objects, like DBM files. You may prefer to
3151 use the each() function to iterate over such. Example:
3153 # print out history file offsets
3155 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3156 while (($key,$val) = each %HIST) {
3157 print $key, ' = ', unpack('L',$val), "\n";
3161 A class implementing an associative array should have the following
3164 TIEHASH classname, LIST
3167 STORE this, key, value
3171 NEXTKEY this, lastkey
3173 A class implementing an ordinary array should have the following methods:
3175 TIEARRAY classname, LIST
3178 STORE this, key, value
3181 A class implementing a scalar should have the following methods:
3183 TIESCALAR classname, LIST
3188 Unlike dbmopen(), the tie() function will not use or require a module
3189 for you--you need to do that explicitly yourself. See L<DB_File>
3190 or the F<Config> module for interesting tie() implementations.
3194 Returns a reference to the object underlying VARIABLE (the same value
3195 that was originally returned by the tie() call which bound the variable
3196 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3201 Returns the number of non-leap seconds since whatever time the system
3202 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3203 and 00:00:00 UTC, January 1, 1970 for most other systems).
3204 Suitable for feeding to gmtime() and localtime().
3208 Returns a four-element array giving the user and system times, in
3209 seconds, for this process and the children of this process.
3211 ($user,$system,$cuser,$csystem) = times;
3215 The translation operator. See L<perlop>.
3217 =item truncate FILEHANDLE,LENGTH
3219 =item truncate EXPR,LENGTH
3221 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3222 specified length. Produces a fatal error if truncate isn't implemented
3229 Returns an uppercased version of EXPR. This is the internal function
3230 implementing the \U escape in double-quoted strings.
3231 Should respect any POSIX setlocale() settings.
3233 If EXPR is omitted, uses $_.
3239 Returns the value of EXPR with the first character uppercased. This is
3240 the internal function implementing the \u escape in double-quoted strings.
3241 Should respect any POSIX setlocale() settings.
3243 If EXPR is omitted, uses $_.
3249 Sets the umask for the process and returns the old one. If EXPR is
3250 omitted, merely returns current umask.
3256 Undefines the value of EXPR, which must be an lvalue. Use only on a
3257 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3258 will probably not do what you expect on most predefined variables or
3259 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3260 the EXPR, in which case nothing is undefined, but you still get an
3261 undefined value that you could, for instance, return from a
3262 subroutine. Examples:
3265 undef $bar{'blurfl'};
3269 return (wantarray ? () : undef) if $they_blew_it;
3275 Deletes a list of files. Returns the number of files successfully
3278 $cnt = unlink 'a', 'b', 'c';
3282 Note: unlink will not delete directories unless you are superuser and
3283 the B<-U> flag is supplied to Perl. Even if these conditions are
3284 met, be warned that unlinking a directory can inflict damage on your
3285 filesystem. Use rmdir instead.
3287 If LIST is omitted, uses $_.
3289 =item unpack TEMPLATE,EXPR
3291 Unpack does the reverse of pack: it takes a string representing a
3292 structure and expands it out into a list value, returning the array
3293 value. (In a scalar context, it merely returns the first value
3294 produced.) The TEMPLATE has the same format as in the pack function.
3295 Here's a subroutine that does substring:
3298 local($what,$where,$howmuch) = @_;
3299 unpack("x$where a$howmuch", $what);
3304 sub ordinal { unpack("c",$_[0]); } # same as ord()
3306 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3307 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3308 themselves. Default is a 16-bit checksum. For example, the following
3309 computes the same number as the System V sum program:
3312 $checksum += unpack("%16C*", $_);
3316 The following efficiently counts the number of set bits in a bit vector:
3318 $setbits = unpack("%32b*", $selectmask);
3320 =item untie VARIABLE
3322 Breaks the binding between a variable and a package. (See tie().)
3324 =item unshift ARRAY,LIST
3326 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3327 depending on how you look at it. Prepends list to the front of the
3328 array, and returns the new number of elements in the array.
3330 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3332 Note the LIST is prepended whole, not one element at a time, so the
3333 prepended elements stay in the same order. Use reverse to do the
3336 =item use Module LIST
3340 =item use Module VERSION LIST
3344 Imports some semantics into the current package from the named module,
3345 generally by aliasing certain subroutine or variable names into your
3346 package. It is exactly equivalent to
3348 BEGIN { require Module; import Module LIST; }
3350 except that Module I<must> be a bare word.
3352 If the first argument to C<use> is a number, it is treated as a version
3353 number instead of a module name. If the version of the Perl interpreter
3354 is less than VERSION, then an error message is printed and Perl exits
3355 immediately. This is often useful if you need to check the current
3356 Perl version before C<use>ing library modules which have changed in
3357 incompatible ways from older versions of Perl. (We try not to do
3358 this more than we have to.)
3360 The BEGIN forces the require and import to happen at compile time. The
3361 require makes sure the module is loaded into memory if it hasn't been
3362 yet. The import is not a builtin--it's just an ordinary static method
3363 call into the "Module" package to tell the module to import the list of
3364 features back into the current package. The module can implement its
3365 import method any way it likes, though most modules just choose to
3366 derive their import method via inheritance from the Exporter class that
3367 is defined in the Exporter module. See L<Exporter>. If no import
3368 method can be found then the error is currently silently ignored. This
3369 may change to a fatal error in a future version.
3371 If you don't want your namespace altered, explicitly supply an empty list:
3375 That is exactly equivalent to
3377 BEGIN { require Module; }
3379 If the VERSION argument is present between Module and LIST, then the
3380 C<use> will fail if the C<$VERSION> variable in package Module is
3383 Because this is a wide-open interface, pragmas (compiler directives)
3384 are also implemented this way. Currently implemented pragmas are:
3388 use sigtrap qw(SEGV BUS);
3389 use strict qw(subs vars refs);
3390 use subs qw(afunc blurfl);
3392 These pseudomodules import semantics into the current block scope, unlike
3393 ordinary modules, which import symbols into the current package (which are
3394 effective through the end of the file).
3396 There's a corresponding "no" command that unimports meanings imported
3397 by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
3402 If no unimport method can be found the call fails with a fatal error.
3404 See L<perlmod> for a list of standard modules and pragmas.
3408 Changes the access and modification times on each file of a list of
3409 files. The first two elements of the list must be the NUMERICAL access
3410 and modification times, in that order. Returns the number of files
3411 successfully changed. The inode modification time of each file is set
3412 to the current time. Example of a "touch" command:
3416 utime $now, $now, @ARGV;
3418 =item values ASSOC_ARRAY
3420 Returns a normal array consisting of all the values of the named
3421 associative array. (In a scalar context, returns the number of
3422 values.) The values are returned in an apparently random order, but it
3423 is the same order as either the keys() or each() function would produce
3424 on the same array. See also keys(), each(), and sort().
3426 =item vec EXPR,OFFSET,BITS
3428 Treats the string in EXPR as a vector of unsigned integers, and
3429 returns the value of the bitfield specified by OFFSET. BITS specifies
3430 the number of bits that are reserved for each entry in the bit
3431 vector. This must be a power of two from 1 to 32. vec() may also be
3432 assigned to, in which case parens are needed to give the expression
3433 the correct precedence as in
3435 vec($image, $max_x * $x + $y, 8) = 3;
3437 Vectors created with vec() can also be manipulated with the logical
3438 operators |, & and ^, which will assume a bit vector operation is
3439 desired when both operands are strings.
3441 To transform a bit vector into a string or array of 0's and 1's, use these:
3443 $bits = unpack("b*", $vector);
3444 @bits = split(//, unpack("b*", $vector));
3446 If you know the exact length in bits, it can be used in place of the *.
3450 Waits for a child process to terminate and returns the pid of the
3451 deceased process, or -1 if there are no child processes. The status is
3454 =item waitpid PID,FLAGS
3456 Waits for a particular child process to terminate and returns the pid
3457 of the deceased process, or -1 if there is no such child process. The
3458 status is returned in C<$?>. If you say
3460 use POSIX ":wait_h";
3462 waitpid(-1,&WNOHANG);
3464 then you can do a non-blocking wait for any process. Non-blocking wait
3465 is only available on machines supporting either the waitpid(2) or
3466 wait4(2) system calls. However, waiting for a particular pid with
3467 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3468 by remembering the status values of processes that have exited but have
3469 not been harvested by the Perl script yet.)
3473 Returns TRUE if the context of the currently executing subroutine is
3474 looking for a list value. Returns FALSE if the context is looking
3477 return wantarray ? () : undef;
3481 Produces a message on STDERR just like die(), but doesn't exit or
3484 =item write FILEHANDLE
3490 Writes a formatted record (possibly multi-line) to the specified file,
3491 using the format associated with that file. By default the format for
3492 a file is the one having the same name is the filehandle, but the
3493 format for the current output channel (see the select() function) may be set
3494 explicitly by assigning the name of the format to the C<$~> variable.
3496 Top of form processing is handled automatically: if there is
3497 insufficient room on the current page for the formatted record, the
3498 page is advanced by writing a form feed, a special top-of-page format
3499 is used to format the new page header, and then the record is written.
3500 By default the top-of-page format is the name of the filehandle with
3501 "_TOP" appended, but it may be dynamically set to the format of your
3502 choice by assigning the name to the C<$^> variable while the filehandle is
3503 selected. The number of lines remaining on the current page is in
3504 variable C<$->, which can be set to 0 to force a new page.
3506 If FILEHANDLE is unspecified, output goes to the current default output
3507 channel, which starts out as STDOUT but may be changed by the
3508 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3509 is evaluated and the resulting string is used to look up the name of
3510 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3512 Note that write is I<NOT> the opposite of read. Unfortunately.
3516 The translation operator. See L<perlop>.