3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+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:
59 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
63 Each operator and function decides which sort of value it would be most
64 appropriate to return in a scalar context. Some operators return the
65 length of the list that would have been returned in a list context. Some
66 operators return the first value in the list. Some operators return the
67 last value in the list. Some operators return a count of successful
68 operations. In general, they do what you want, unless you want
71 =head2 Perl Functions by Category
73 Here are Perl's functions (including things that look like
74 functions, like some of the keywords and named operators)
75 arranged by category. Some functions appear in more
80 =item Functions for SCALARs or strings
82 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84 sprintf, substr, tr///, uc, ucfirst, y///
86 =item Regular expressions and pattern matching
88 m//, pos, quotemeta, s///, split, study
90 =item Numeric functions
92 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95 =item Functions for real @ARRAYs
97 pop, push, shift, splice, unshift
99 =item Functions for list data
101 grep, join, map, qw/STRING/, reverse, sort, unpack
103 =item Functions for real %HASHes
105 delete, each, exists, keys, values
107 =item Input and output functions
109 binmode, close, closedir, dbmclose, dbmopen, die, eof,
110 fileno, flock, format, getc, print, printf, read, readdir,
111 rewinddir, seek, seekdir, select, syscall, sysread,
112 syswrite, tell, telldir, truncate, warn, write
114 =item Functions for fixed length data or records
116 pack, read, syscall, sysread, syswrite, unpack, vec
118 =item Functions for filehandles, files, or directories
120 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
121 lstat, mkdir, open, opendir, readlink, rename, rmdir,
122 stat, symlink, umask, unlink, utime
124 =item Keywords related to the control flow of your perl program
126 caller, continue, die, do, dump, eval, exit, goto, last,
127 next, redo, return, sub, wantarray
129 =item Keywords related to scoping
131 caller, import, local, my, package, use
133 =item Miscellaneous functions
135 defined, dump, eval, formline, local, my, reset, scalar,
138 =item Functions for processes and process groups
140 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144 =item Keywords related to perl modules
146 do, import, no, package, require, use
148 =item Keywords related to classes and object-orientedness
150 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
152 =item Low-level socket functions
154 accept, bind, connect, getpeername, getsockname,
155 getsockopt, listen, recv, send, setsockopt, shutdown,
158 =item System V interprocess communication functions
160 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161 shmctl, shmget, shmread, shmwrite
163 =item Fetching user and group info
165 endgrent, endhostent, endnetent, endpwent, getgrent,
166 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167 getpwuid, setgrent, setpwent
169 =item Fetching network info
171 endprotoent, endservent, gethostbyaddr, gethostbyname,
172 gethostent, getnetbyaddr, getnetbyname, getnetent,
173 getprotobyname, getprotobynumber, getprotoent,
174 getservbyname, getservbyport, getservent, sethostent,
175 setnetent, setprotoent, setservent
177 =item Time-related functions
179 gmtime, localtime, time, times
181 =item Functions new in perl5
183 abs, bless, chomp, chr, exists, formline, glob, import, lc,
184 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
187 * - C<sub> was a keyword in perl4, but in perl5 it is an
188 operator which can be used in expressions.
190 =item Functions obsoleted in perl5
197 =head2 Alphabetical Listing of Perl Functions
208 A file test, where X is one of the letters listed below. This unary
209 operator takes one argument, either a filename or a filehandle, and
210 tests the associated file to see if something is true about it. If the
211 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
212 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
213 the undefined value if the file doesn't exist. Despite the funny
214 names, precedence is the same as any other named unary operator, and
215 the argument may be parenthesized like any other unary operator. The
216 operator may be any of:
218 -r File is readable by effective uid/gid.
219 -w File is writable by effective uid/gid.
220 -x File is executable by effective uid/gid.
221 -o File is owned by effective uid.
223 -R File is readable by real uid/gid.
224 -W File is writable by real uid/gid.
225 -X File is executable by real uid/gid.
226 -O File is owned by real uid.
229 -z File has zero size.
230 -s File has non-zero size (returns size).
232 -f File is a plain file.
233 -d File is a directory.
234 -l File is a symbolic link.
235 -p File is a named pipe (FIFO).
237 -b File is a block special file.
238 -c File is a character special file.
239 -t Filehandle is opened to a tty.
241 -u File has setuid bit set.
242 -g File has setgid bit set.
243 -k File has sticky bit set.
245 -T File is a text file.
246 -B File is a binary file (opposite of -T).
248 -M Age of file in days when script started.
249 -A Same for access time.
250 -C Same for inode change time.
252 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
253 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
254 uids and gids of the user. There may be other reasons you can't actually
255 read, write or execute the file. Also note that, for the superuser,
256 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
257 1 if any execute bit is set in the mode. Scripts run by the superuser may
258 thus need to do a stat() to determine the actual mode of the
259 file, or temporarily set the uid to something else.
265 next unless -f $_; # ignore specials
269 Note that C<-s/a/b/> does not do a negated substitution. Saying
270 C<-exp($foo)> still works as expected, however--only single letters
271 following a minus are interpreted as file tests.
273 The C<-T> and C<-B> switches work as follows. The first block or so of the
274 file is examined for odd characters such as strange control codes or
275 characters with the high bit set. If too many odd characters (E<gt>30%)
276 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
277 containing null in the first block is considered a binary file. If C<-T>
278 or C<-B> is used on a filehandle, the current stdio buffer is examined
279 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
280 file, or a file at EOF when testing a filehandle. Because you have to
281 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
282 against the file first, as in C<next unless -f $file && -T $file>.
284 If any of the file tests (or either the stat() or lstat() operators) are given the
285 special filehandle consisting of a solitary underline, then the stat
286 structure of the previous file test (or stat operator) is used, saving
287 a system call. (This doesn't work with C<-t>, and you need to remember
288 that lstat() and C<-l> will leave values in the stat structure for the
289 symbolic link, not the real file.) Example:
291 print "Can do.\n" if -r $a || -w _ || -x _;
294 print "Readable\n" if -r _;
295 print "Writable\n" if -w _;
296 print "Executable\n" if -x _;
297 print "Setuid\n" if -u _;
298 print "Setgid\n" if -g _;
299 print "Sticky\n" if -k _;
300 print "Text\n" if -T _;
301 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
308 If VALUE is omitted, uses $_.
310 =item accept NEWSOCKET,GENERICSOCKET
312 Accepts an incoming socket connect, just as the accept(2) system call
313 does. Returns the packed address if it succeeded, FALSE otherwise.
314 See example in L<perlipc/"Sockets: Client/Server Communication">.
320 Arranges to have a SIGALRM delivered to this process after the
321 specified number of seconds have elapsed. If SECONDS is not specified,
322 the value stored in $_ is used. (On some machines,
323 unfortunately, the elapsed time may be up to one second less than you
324 specified because of how seconds are counted.) Only one timer may be
325 counting at once. Each call disables the previous timer, and an
326 argument of 0 may be supplied to cancel the previous timer without
327 starting a new one. The returned value is the amount of time remaining
328 on the previous timer.
330 For delays of finer granularity than one second, you may use Perl's
331 syscall() interface to access setitimer(2) if your system supports it,
332 or else see L</select()> below. It is not advised to intermix alarm()
335 If you want to use alarm() to time out a system call you need to use an
336 eval/die pair. You can't rely on the alarm causing the system call to
337 fail with $! set to EINTR because Perl sets up signal handlers to
338 restart system calls on some systems. Using eval/die always works.
341 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
343 $nread = sysread SOCKET, $buffer, $size;
346 die if $@ && $@ ne "alarm\n"; # propagate errors
356 Returns the arctangent of Y/X in the range -PI to PI.
358 =item bind SOCKET,NAME
360 Binds a network address to a socket, just as the bind system call
361 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
362 packed address of the appropriate type for the socket. See the examples in
363 L<perlipc/"Sockets: Client/Server Communication">.
365 =item binmode FILEHANDLE
367 Arranges for the file to be read or written in "binary" mode in operating
368 systems that distinguish between binary and text files. Files that are
369 not in binary mode have CR LF sequences translated to LF on input and LF
370 translated to CR LF on output. Binmode has no effect under Unix; in DOS
371 and similarly archaic systems, it may be imperative--otherwise your
372 DOS-damaged C library may mangle your file. The key distinction between
373 systems that need binmode and those that don't is their text file
374 formats. Systems like Unix and Plan9 that delimit lines with a single
375 character, and that encode that character in C as '\n', do not need
376 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
377 is taken as the name of the filehandle.
379 =item bless REF,CLASSNAME
383 This function tells the referenced object (passed as REF) that it is now
384 an object in the CLASSNAME package--or the current package if no CLASSNAME
385 is specified, which is often the case. It returns the reference for
386 convenience, because a bless() is often the last thing in a constructor.
387 Always use the two-argument version if the function doing the blessing
388 might be inherited by a derived class. See L<perlobj> for more about the
389 blessing (and blessings) of objects.
395 Returns the context of the current subroutine call. In a scalar context,
396 returns TRUE if there is a caller, that is, if we're in a subroutine or
397 eval() or require(), and FALSE otherwise. In a list context, returns
399 ($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 $_.
532 =item close FILEHANDLE
534 Closes the file or pipe associated with the file handle, returning TRUE
535 only if stdio successfully flushes buffers and closes the system file
536 descriptor. You don't have to close FILEHANDLE if you are immediately
537 going to do another open() on it, because open() will close it for you. (See
538 open().) However, an explicit close on an input file resets the line
539 counter ($.), 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 $pwd) {
604 Of course, typing in your own password to whomever asks you
607 =item dbmclose ASSOC_ARRAY
609 [This function has been superseded by the untie() function.]
611 Breaks the binding between a DBM file and an associative array.
613 =item dbmopen ASSOC,DBNAME,MODE
615 [This function has been superseded by the tie() function.]
617 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
618 associative array. ASSOC is the name of the associative array. (Unlike
619 normal open, the first argument is I<NOT> a filehandle, even though it
620 looks like one). DBNAME is the name of the database (without the F<.dir>
621 or F<.pag> extension if any). If the database does not exist, it is
622 created with protection specified by MODE (as modified by the umask()).
623 If your system supports only the older DBM functions, you may perform only
624 one dbmopen() in your program. In older versions of Perl, if your system
625 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
626 falls back to sdbm(3).
628 If you don't have write access to the DBM file, you can only read
629 associative array variables, not set them. If you want to test whether
630 you can write, either use file tests or try setting a dummy array entry
631 inside an eval(), which will trap the error.
633 Note that functions such as keys() and values() may return huge array
634 values when used on large DBM files. You may prefer to use the each()
635 function to iterate over large DBM files. Example:
637 # print out history file offsets
638 dbmopen(%HIST,'/usr/lib/news/history',0666);
639 while (($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 $1 is defined, despite the fact that it
684 matched "nothing". But it didn't really match nothing--rather, it
685 matched something that happened to be 0 characters long. This is all
686 very above-board and honest. When a function returns an undefined value,
687 it's an admission that it couldn't give you an honest answer. So
688 you should use defined() only when you're questioning the integrity
689 of what you're trying to do. At other times, a simple comparison to
690 0 or "" is what you want.
694 Deletes the specified key(s) and their associated values from a hash
695 array. For each key, returns the deleted value associated with that key,
696 or the undefined value if there was no such key. Deleting from C<$ENV{}>
697 modifies the environment. Deleting from an array tied to a DBM file
698 deletes the entry from the DBM file. (But deleting from a tie()d hash
699 doesn't necessarily return anything.)
701 The following deletes all the values of an associative array:
703 foreach $key (keys %HASH) {
709 delete @HASH{keys %HASH}
711 (But both of these are slower than the undef() command.) Note that the
712 EXPR can be arbitrarily complicated as long as the final operation is a
713 hash element lookup or hash slice:
715 delete $ref->[$x][$y]{$key};
716 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
720 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
721 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
722 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
723 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
724 and the eval() is terminated with the undefined value; this makes die()
725 the way to raise an exception.
729 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
730 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
732 If the value of EXPR does not end in a newline, the current script line
733 number and input line number (if any) are also printed, and a newline
734 is supplied. Hint: sometimes appending ", stopped" to your message
735 will cause it to make better sense when the string "at foo line 123" is
736 appended. Suppose you are running script "canasta".
738 die "/etc/games is no good";
739 die "/etc/games is no good, stopped";
741 produce, respectively
743 /etc/games is no good at canasta line 123.
744 /etc/games is no good, stopped at canasta line 123.
746 See also exit() and warn().
750 Not really a function. Returns the value of the last command in the
751 sequence of commands indicated by BLOCK. When modified by a loop
752 modifier, executes the BLOCK once before testing the loop condition.
753 (On other statements the loop modifiers test the conditional first.)
755 =item do SUBROUTINE(LIST)
757 A deprecated form of subroutine call. See L<perlsub>.
761 Uses the value of EXPR as a filename and executes the contents of the
762 file as a Perl script. Its primary use is to include subroutines
763 from a Perl subroutine library.
771 except that it's more efficient, more concise, keeps track of the
772 current filename for error messages, and searches all the B<-I>
773 libraries if the file isn't in the current directory (see also the @INC
774 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
775 re-parse the file every time you call it, so you probably don't want to
776 do this inside a loop.
778 Note that inclusion of library modules is better done with the
779 use() and require() operators, which also do error checking
780 and raise an exception if there's a problem.
784 This causes an immediate core dump. Primarily this is so that you can
785 use the B<undump> program to turn your core dump into an executable binary
786 after having initialized all your variables at the beginning of the
787 program. When the new binary is executed it will begin by executing a
788 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
789 it as a goto with an intervening core dump and reincarnation. If LABEL
790 is omitted, restarts the program from the top. WARNING: any files
791 opened at the time of the dump will NOT be open any more when the
792 program is reincarnated, with possible resulting confusion on the part
793 of Perl. See also B<-u> option in L<perlrun>.
810 dump QUICKSTART if $ARGV[0] eq '-d';
815 =item each ASSOC_ARRAY
817 When called in a list context, returns a 2-element array consisting
818 of the key and value for the next element of an associative array,
819 so that you can iterate over it. When called in a scalar context,
820 returns the key for only the next element in the associative array.
821 Entries are returned in an apparently random order. When the array is
822 entirely read, a null array is returned in list context (which when
823 assigned produces a FALSE (0) value), and C<undef> is returned in a
824 scalar context. The next call to each() after that will start
825 iterating again. The iterator can be reset only by reading all the
826 elements from the array. You should not add elements to an array while
827 you're iterating over it. There is a single iterator for each
828 associative array, shared by all each(), keys(), and values() function
829 calls in the program. The following prints out your environment like
830 the printenv(1) program, only in a different order:
832 while (($key,$value) = each %ENV) {
833 print "$key=$value\n";
836 See also keys() and values().
844 Returns 1 if the next read on FILEHANDLE will return end of file, or if
845 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
846 gives the real filehandle name. (Note that this function actually
847 reads a character and then ungetc()s it, so it is not very useful in an
848 interactive context.) Do not read from a terminal file (or call
849 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
850 as terminals may lose the end-of-file condition if you do.
852 An C<eof> without an argument uses the last file read as argument.
853 Empty parentheses () may be used to indicate
854 the pseudo file formed of the files listed on the command line, i.e.,
855 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
856 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
857 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
859 # reset line numbering on each input file
862 close(ARGV) if (eof); # Not eof().
865 # insert dashes just before last line of last file
868 print "--------------\n";
869 close(ARGV); # close or break; is needed if we
870 # are reading from the terminal
875 Practical hint: you almost never need to use C<eof> in Perl, because the
876 input operators return undef when they run out of data.
882 EXPR is parsed and executed as if it were a little Perl program. It
883 is executed in the context of the current Perl program, so that any
884 variable settings or subroutine and format definitions remain afterwards.
885 The value returned is the value of the last expression evaluated, or a
886 return statement may be used, just as with subroutines. The last
887 expression is evaluated in scalar or array context, depending on the
890 If there is a syntax error or runtime error, or a die() statement is
891 executed, an undefined value is returned by eval(), and C<$@> is set to the
892 error message. If there was no error, C<$@> is guaranteed to be a null
893 string. If EXPR is omitted, evaluates $_. The final semicolon, if
894 any, may be omitted from the expression.
896 Note that, because eval() traps otherwise-fatal errors, it is useful for
897 determining whether a particular feature (such as socket() or symlink())
898 is implemented. It is also Perl's exception trapping mechanism, where
899 the die operator is used to raise exceptions.
901 If the code to be executed doesn't vary, you may use the eval-BLOCK
902 form to trap run-time errors without incurring the penalty of
903 recompiling each time. The error, if any, is still returned in C<$@>.
906 # make divide-by-zero non-fatal
907 eval { $answer = $a / $b; }; warn $@ if $@;
909 # same thing, but less efficient
910 eval '$answer = $a / $b'; warn $@ if $@;
912 # a compile-time error
916 eval '$answer ='; # sets $@
918 With an eval(), you should be especially careful to remember what's
919 being looked at when:
925 eval { $x }; # CASE 4
927 eval "\$$x++" # CASE 5
930 Cases 1 and 2 above behave identically: they run the code contained in the
931 variable $x. (Although case 2 has misleading double quotes making the
932 reader wonder what else might be happening (nothing is).) Cases 3 and 4
933 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
934 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
935 is a place where normally you I<WOULD> like to use double quotes, except
936 that in that particular situation, you can just use symbolic references
937 instead, as in case 6.
941 The exec() function executes a system command I<AND NEVER RETURNS>,
942 unless the command does not exist and is executed directly instead of
943 via C</bin/sh -c> (see below). Use system() instead of exec() if you
946 If there is more than one argument in LIST, or if LIST is an array with
947 more than one value, calls execvp(3) with the arguments in LIST. If
948 there is only one scalar argument, the argument is checked for shell
949 metacharacters. If there are any, the entire argument is passed to
950 C</bin/sh -c> for parsing. If there are none, the argument is split
951 into words and passed directly to execvp(), which is more efficient.
952 Note: exec() and system() do not flush your output buffer, so you may
953 need to set C<$|> to avoid lost output. Examples:
955 exec '/bin/echo', 'Your arguments are: ', @ARGV;
956 exec "sort $outfile | uniq";
958 If you don't really want to execute the first argument, but want to lie
959 to the program you are executing about its own name, you can specify
960 the program you actually want to run as an "indirect object" (without a
961 comma) in front of the LIST. (This always forces interpretation of the
962 LIST as a multi-valued list, even if there is only a single scalar in
966 exec $shell '-sh'; # pretend it's a login shell
970 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
974 Returns TRUE if the specified hash key exists in its hash array, even
975 if the corresponding value is undefined.
977 print "Exists\n" if exists $array{$key};
978 print "Defined\n" if defined $array{$key};
979 print "True\n" if $array{$key};
981 A hash element can be TRUE only if it's defined, and defined if
982 it exists, but the reverse doesn't necessarily hold true.
984 Note that the EXPR can be arbitrarily complicated as long as the final
985 operation is a hash key lookup:
987 if (exists $ref->[$x][$y]{$key}) { ... }
991 Evaluates EXPR and exits immediately with that value. (Actually, it
992 calls any defined C<END> routines first, but the C<END> routines may not
993 abort the exit. Likewise any object destructors that need to be called
994 are called before exit.) Example:
997 exit 0 if $ans =~ /^[Xx]/;
999 See also die(). If EXPR is omitted, exits with 0 status.
1005 Returns I<e> (the natural logarithm base) to the power of EXPR.
1006 If EXPR is omitted, gives C<exp($_)>.
1008 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1010 Implements the fcntl(2) function. You'll probably have to say
1014 first to get the correct function definitions. Argument processing and
1015 value return works just like ioctl() below. Note that fcntl() will produce
1016 a fatal error if used on a machine that doesn't implement fcntl(2).
1020 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1022 =item fileno FILEHANDLE
1024 Returns the file descriptor for a filehandle. This is useful for
1025 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1026 value is taken as the name of the filehandle.
1028 =item flock FILEHANDLE,OPERATION
1030 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1031 success, FALSE on failure. Will produce a fatal error if used on a
1032 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1033 flock() is Perl's portable file locking interface, although it will lock
1034 only entire files, not records.
1036 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1037 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1038 you can use the symbolic names if you pull them in with an explicit
1039 request to the Fcntl module. The names can be requested as a group with
1040 the :flock tag (or they can be requested individually, of course).
1041 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1042 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1043 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1044 blocking waiting for the lock (check the return status to see if you got
1047 Note that the emulation built with lockf(3) doesn't provide shared
1048 locks, and it requires that FILEHANDLE be open with write intent. These
1049 are the semantics that lockf(3) implements. Most (all?) systems
1050 implement lockf(3) in terms of fcntl(2) locking, though, so the
1051 differing semantics shouldn't bite too many people.
1053 Note also that some versions of flock() cannot lock things over the
1054 network; you would need to use the more system-specific fcntl() for
1055 that. If you like you can force Perl to ignore your system's flock(2)
1056 function, and so provide its own fcntl(2)-based emulation, by passing
1057 the switch C<-Ud_flock> to the F<Configure> program when you configure
1060 Here's a mailbox appender for BSD systems.
1062 use Fcntl ':flock'; # import LOCK_* constants
1065 flock(MBOX,LOCK_EX);
1066 # and, in case someone appended
1067 # while we were waiting...
1072 flock(MBOX,LOCK_UN);
1075 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1076 or die "Can't open mailbox: $!";
1079 print MBOX $msg,"\n\n";
1082 See also L<DB_File> for other flock() examples.
1086 Does a fork(2) system call. Returns the child pid to the parent process
1087 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1088 Note: unflushed buffers remain unflushed in both processes, which means
1089 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1090 autoflush() FileHandle method to avoid duplicate output.
1092 If you fork() without ever waiting on your children, you will accumulate
1095 $SIG{CHLD} = sub { wait };
1097 There's also the double-fork trick (error checking on
1098 fork() returns omitted);
1100 unless ($pid = fork) {
1102 exec "what you really wanna do";
1105 ## (some_perl_code_here)
1112 See also L<perlipc> for more examples of forking and reaping
1117 Declare a picture format with use by the write() function. For
1121 Test: @<<<<<<<< @||||| @>>>>>
1122 $str, $%, '$' . int($num)
1126 $num = $cost/$quantity;
1130 See L<perlform> for many details and examples.
1133 =item formline PICTURE, LIST
1135 This is an internal function used by C<format>s, though you may call it
1136 too. It formats (see L<perlform>) a list of values according to the
1137 contents of PICTURE, placing the output into the format output
1138 accumulator, C<$^A> (or $ACCUMULATOR in English).
1139 Eventually, when a write() is done, the contents of
1140 C<$^A> are written to some filehandle, but you could also read C<$^A>
1141 yourself and then set C<$^A> back to "". Note that a format typically
1142 does one formline() per line of form, but the formline() function itself
1143 doesn't care how many newlines are embedded in the PICTURE. This means
1144 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1145 You may therefore need to use multiple formlines to implement a single
1146 record format, just like the format compiler.
1148 Be careful if you put double quotes around the picture, because an "C<@>"
1149 character may be taken to mean the beginning of an array name.
1150 formline() always returns TRUE. See L<perlform> for other examples.
1152 =item getc FILEHANDLE
1156 Returns the next character from the input file attached to FILEHANDLE,
1157 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1158 This is not particularly efficient. It cannot be used to get unbuffered
1159 single-characters, however. For that, try something more like:
1162 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1165 system "stty", '-icanon', 'eol', "\001";
1171 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1174 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1178 Determination of whether to whether $BSD_STYLE should be set
1179 is left as an exercise to the reader.
1181 See also the C<Term::ReadKey> module from your nearest CPAN site;
1182 details on CPAN can be found on L<perlmod/CPAN>
1186 Returns the current login from F</etc/utmp>, if any. If null, use
1189 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1191 Do not consider getlogin() for authentication: it is not as
1192 secure as getpwuid().
1194 =item getpeername SOCKET
1196 Returns the packed sockaddr address of other end of the SOCKET connection.
1199 $hersockaddr = getpeername(SOCK);
1200 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1201 $herhostname = gethostbyaddr($iaddr, AF_INET);
1202 $herstraddr = inet_ntoa($iaddr);
1206 Returns the current process group for the specified PID. Use
1207 a PID of 0 to get the current process group for the
1208 current process. Will raise an exception if used on a machine that
1209 doesn't implement getpgrp(2). If PID is omitted, returns process
1210 group of current process. Note that the POSIX version of getpgrp()
1211 does not accept a PID argument, so only PID==0 is truly portable.
1215 Returns the process id of the parent process.
1217 =item getpriority WHICH,WHO
1219 Returns the current priority for a process, a process group, or a user.
1220 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1221 machine that doesn't implement getpriority(2).
1227 =item gethostbyname NAME
1229 =item getnetbyname NAME
1231 =item getprotobyname NAME
1237 =item getservbyname NAME,PROTO
1239 =item gethostbyaddr ADDR,ADDRTYPE
1241 =item getnetbyaddr ADDR,ADDRTYPE
1243 =item getprotobynumber NUMBER
1245 =item getservbyport PORT,PROTO
1263 =item sethostent STAYOPEN
1265 =item setnetent STAYOPEN
1267 =item setprotoent STAYOPEN
1269 =item setservent STAYOPEN
1283 These routines perform the same functions as their counterparts in the
1284 system library. Within a list context, the return values from the
1285 various get routines are as follows:
1287 ($name,$passwd,$uid,$gid,
1288 $quota,$comment,$gcos,$dir,$shell) = getpw*
1289 ($name,$passwd,$gid,$members) = getgr*
1290 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1291 ($name,$aliases,$addrtype,$net) = getnet*
1292 ($name,$aliases,$proto) = getproto*
1293 ($name,$aliases,$port,$proto) = getserv*
1295 (If the entry doesn't exist you get a null list.)
1297 Within a scalar context, you get the name, unless the function was a
1298 lookup by name, in which case you get the other thing, whatever it is.
1299 (If the entry doesn't exist you get the undefined value.) For example:
1309 The $members value returned by I<getgr*()> is a space separated list of
1310 the login names of the members of the group.
1312 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1313 C, it will be returned to you via C<$?> if the function call fails. The
1314 @addrs value returned by a successful call is a list of the raw
1315 addresses returned by the corresponding system library call. In the
1316 Internet domain, each address is four bytes long and you can unpack it
1317 by saying something like:
1319 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1321 =item getsockname SOCKET
1323 Returns the packed sockaddr address of this end of the SOCKET connection.
1326 $mysockaddr = getsockname(SOCK);
1327 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1329 =item getsockopt SOCKET,LEVEL,OPTNAME
1331 Returns the socket option requested, or undefined if there is an error.
1335 Returns the value of EXPR with filename expansions such as a shell
1336 would do. This is the internal function implementing the E<lt>*.*E<gt>
1337 operator, except it's easier to use.
1341 Converts a time as returned by the time function to a 9-element array
1342 with the time localized for the standard Greenwich time zone.
1343 Typically used as follows:
1346 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1349 All array elements are numeric, and come straight out of a struct tm.
1350 In particular this means that $mon has the range 0..11 and $wday has
1351 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1359 The goto-LABEL form finds the statement labeled with LABEL and resumes
1360 execution there. It may not be used to go into any construct that
1361 requires initialization, such as a subroutine or a foreach loop. It
1362 also can't be used to go into a construct that is optimized away. It
1363 can be used to go almost anywhere else within the dynamic scope,
1364 including out of subroutines, but it's usually better to use some other
1365 construct such as last or die. The author of Perl has never felt the
1366 need to use this form of goto (in Perl, that is--C is another matter).
1368 The goto-EXPR form expects a label name, whose scope will be resolved
1369 dynamically. This allows for computed gotos per FORTRAN, but isn't
1370 necessarily recommended if you're optimizing for maintainability:
1372 goto ("FOO", "BAR", "GLARCH")[$i];
1374 The goto-&NAME form is highly magical, and substitutes a call to the
1375 named subroutine for the currently running subroutine. This is used by
1376 AUTOLOAD subroutines that wish to load another subroutine and then
1377 pretend that the other subroutine had been called in the first place
1378 (except that any modifications to @_ in the current subroutine are
1379 propagated to the other subroutine.) After the goto, not even caller()
1380 will be able to tell that this routine was called first.
1382 =item grep BLOCK LIST
1384 =item grep EXPR,LIST
1386 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1387 $_ to each element) and returns the list value consisting of those
1388 elements for which the expression evaluated to TRUE. In a scalar
1389 context, returns the number of times the expression was TRUE.
1391 @foo = grep(!/^#/, @bar); # weed out comments
1395 @foo = grep {!/^#/} @bar; # weed out comments
1397 Note that, because $_ is a reference into the list value, it can be used
1398 to modify the elements of the array. While this is useful and
1399 supported, it can cause bizarre results if the LIST is not a named
1406 Interprets EXPR as a hex string and returns the corresponding decimal
1407 value. (To convert strings that might start with 0 or 0x see
1408 oct().) If EXPR is omitted, uses $_.
1412 There is no built-in import() function. It is merely an ordinary
1413 method (subroutine) defined (or inherited) by modules that wish to export
1414 names to another module. The use() function calls the import() method
1415 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1417 =item index STR,SUBSTR,POSITION
1419 =item index STR,SUBSTR
1421 Returns the position of the first occurrence of SUBSTR in STR at or after
1422 POSITION. If POSITION is omitted, starts searching from the beginning of
1423 the string. The return value is based at 0 (or whatever you've set the C<$[>
1424 variable to--but don't do that). If the substring is not found, returns
1425 one less than the base, ordinarily -1.
1431 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1433 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1435 Implements the ioctl(2) function. You'll probably have to say
1437 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1439 first to get the correct function definitions. If F<ioctl.ph> doesn't
1440 exist or doesn't have the correct definitions you'll have to roll your
1441 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1442 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1443 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1444 written depending on the FUNCTION--a pointer to the string value of SCALAR
1445 will be passed as the third argument of the actual ioctl call. (If SCALAR
1446 has no string value but does have a numeric value, that value will be
1447 passed rather than a pointer to the string value. To guarantee this to be
1448 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1449 functions are useful for manipulating the values of structures used by
1450 ioctl(). The following example sets the erase character to DEL.
1454 die "NO TIOCGETP" if $@ || !$getp;
1455 $sgttyb_t = "ccccs"; # 4 chars and a short
1456 if (ioctl(STDIN,$getp,$sgttyb)) {
1457 @ary = unpack($sgttyb_t,$sgttyb);
1459 $sgttyb = pack($sgttyb_t,@ary);
1460 ioctl(STDIN,&TIOCSETP,$sgttyb)
1461 || die "Can't ioctl: $!";
1464 The return value of ioctl (and fcntl) is as follows:
1466 if OS returns: then Perl returns:
1468 0 string "0 but true"
1469 anything else that number
1471 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1472 still easily determine the actual value returned by the operating
1475 ($retval = ioctl(...)) || ($retval = -1);
1476 printf "System returned %d\n", $retval;
1478 =item join EXPR,LIST
1480 Joins the separate strings of LIST or ARRAY into a single string with
1481 fields separated by the value of EXPR, and returns the string.
1484 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1486 See L<perlfunc/split>.
1488 =item keys ASSOC_ARRAY
1490 Returns a normal array consisting of all the keys of the named
1491 associative array. (In a scalar context, returns the number of keys.)
1492 The keys are returned in an apparently random order, but it is the same
1493 order as either the values() or each() function produces (given that
1494 the associative array has not been modified). Here is yet another way
1495 to print your environment:
1498 @values = values %ENV;
1499 while ($#keys >= 0) {
1500 print pop(@keys), '=', pop(@values), "\n";
1503 or how about sorted by key:
1505 foreach $key (sort(keys %ENV)) {
1506 print $key, '=', $ENV{$key}, "\n";
1509 To sort an array by value, you'll need to use a C<sort{}>
1510 function. Here's a descending numeric sort of a hash by its values:
1512 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1513 printf "%4d %s\n", $hash{$key}, $key;
1516 As an lvalue C<keys> allows you to increase the number of hash buckets
1517 allocated for the given associative array. This can gain you a measure
1518 of efficiency if you know the hash is going to get big. (This is
1519 similar to pre-extending an array by assigning a larger number to
1520 $#array.) If you say
1524 then C<%hash> will have at least 200 buckets allocated for it. These
1525 buckets will be retained even if you do C<%hash = ()>, use C<undef
1526 %hash> if you want to free the storage while C<%hash> is still in scope.
1527 You can't shrink the number of buckets allocated for the hash using
1528 C<keys> in this way (but you needn't worry about doing this by accident,
1529 as trying has no effect).
1533 Sends a signal to a list of processes. The first element of
1534 the list must be the signal to send. Returns the number of
1535 processes successfully signaled.
1537 $cnt = kill 1, $child1, $child2;
1540 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1541 process groups instead of processes. (On System V, a negative I<PROCESS>
1542 number will also kill process groups, but that's not portable.) That
1543 means you usually want to use positive not negative signals. You may also
1544 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1550 The C<last> command is like the C<break> statement in C (as used in
1551 loops); it immediately exits the loop in question. If the LABEL is
1552 omitted, the command refers to the innermost enclosing loop. The
1553 C<continue> block, if any, is not executed:
1555 LINE: while (<STDIN>) {
1556 last LINE if /^$/; # exit when done with header
1564 Returns an lowercased version of EXPR. This is the internal function
1565 implementing the \L escape in double-quoted strings.
1566 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1568 If EXPR is omitted, uses $_.
1574 Returns the value of EXPR with the first character lowercased. This is
1575 the internal function implementing the \l escape in double-quoted strings.
1576 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1578 If EXPR is omitted, uses $_.
1584 Returns the length in characters of the value of EXPR. If EXPR is
1585 omitted, returns length of $_.
1587 =item link OLDFILE,NEWFILE
1589 Creates a new filename linked to the old filename. Returns 1 for
1590 success, 0 otherwise.
1592 =item listen SOCKET,QUEUESIZE
1594 Does the same thing that the listen system call does. Returns TRUE if
1595 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1599 A local modifies the listed variables to be local to the enclosing block,
1600 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1601 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1602 local()"> for details.
1604 But you really probably want to be using my() instead, because local() isn't
1605 what most people think of as "local"). See L<perlsub/"Private Variables
1606 via my()"> for details.
1608 =item localtime EXPR
1610 Converts a time as returned by the time function to a 9-element array
1611 with the time analyzed for the local time zone. Typically used as
1614 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1617 All array elements are numeric, and come straight out of a struct tm.
1618 In particular this means that $mon has the range 0..11 and $wday has
1619 the range 0..6. If EXPR is omitted, does localtime(time).
1621 In a scalar context, prints out the ctime(3) value:
1623 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1625 Also see the F<timelocal.pl> library, and the strftime(3) function available
1626 via the POSIX module.
1632 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1635 =item lstat FILEHANDLE
1641 Does the same thing as the stat() function, but stats a symbolic link
1642 instead of the file the symbolic link points to. If symbolic links are
1643 unimplemented on your system, a normal stat() is done.
1645 If EXPR is omitted, stats $_.
1649 The match operator. See L<perlop>.
1651 =item map BLOCK LIST
1655 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1656 element) and returns the list value composed of the results of each such
1657 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1658 may produce zero, one, or more elements in the returned value.
1660 @chars = map(chr, @nums);
1662 translates a list of numbers to the corresponding characters. And
1664 %hash = map { getkey($_) => $_ } @array;
1666 is just a funny way to write
1669 foreach $_ (@array) {
1670 $hash{getkey($_)} = $_;
1673 =item mkdir FILENAME,MODE
1675 Creates the directory specified by FILENAME, with permissions specified
1676 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1677 it returns 0 and sets C<$!> (errno).
1679 =item msgctl ID,CMD,ARG
1681 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1682 must be a variable which will hold the returned msqid_ds structure.
1683 Returns like ioctl: the undefined value for error, "0 but true" for
1684 zero, or the actual return value otherwise.
1686 =item msgget KEY,FLAGS
1688 Calls the System V IPC function msgget(2). Returns the message queue id,
1689 or the undefined value if there is an error.
1691 =item msgsnd ID,MSG,FLAGS
1693 Calls the System V IPC function msgsnd to send the message MSG to the
1694 message queue ID. MSG must begin with the long integer message type,
1695 which may be created with C<pack("l", $type)>. Returns TRUE if
1696 successful, or FALSE if there is an error.
1698 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1700 Calls the System V IPC function msgrcv to receive a message from
1701 message queue ID into variable VAR with a maximum message size of
1702 SIZE. Note that if a message is received, the message type will be the
1703 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1704 of the message type. Returns TRUE if successful, or FALSE if there is
1709 A "my" declares the listed variables to be local (lexically) to the
1710 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1711 more than one value is listed, the list must be placed in parentheses. See
1712 L<perlsub/"Private Variables via my()"> for details.
1718 The C<next> command is like the C<continue> statement in C; it starts
1719 the next iteration of the loop:
1721 LINE: while (<STDIN>) {
1722 next LINE if /^#/; # discard comments
1726 Note that if there were a C<continue> block on the above, it would get
1727 executed even on discarded lines. If the LABEL is omitted, the command
1728 refers to the innermost enclosing loop.
1730 =item no Module LIST
1732 See the "use" function, which "no" is the opposite of.
1738 Interprets EXPR as an octal string and returns the corresponding
1739 decimal value. (If EXPR happens to start off with 0x, interprets it as
1740 a hex string instead.) The following will handle decimal, octal, and
1741 hex in the standard Perl or C notation:
1743 $val = oct($val) if $val =~ /^0/;
1745 If EXPR is omitted, uses $_.
1747 =item open FILEHANDLE,EXPR
1749 =item open FILEHANDLE
1751 Opens the file whose filename is given by EXPR, and associates it with
1752 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1753 name of the real filehandle wanted. If EXPR is omitted, the scalar
1754 variable of the same name as the FILEHANDLE contains the filename.
1755 (Note that lexical variables--those declared with C<my>--will not work
1756 for this purpose; so if you're using C<my>, specify EXPR in your call
1759 If the filename begins with '<' or nothing, the file is opened for input.
1760 If the filename begins with '>', the file is truncated and opened for
1761 output. If the filename begins with '>>', the file is opened for
1762 appending. You can put a '+' in front of the '>' or '<' to indicate that
1763 you want both read and write access to the file; thus '+<' is almost
1764 always preferred for read/write updates--the '+>' mode would clobber the
1765 file first. The prefix and the filename may be separated with spaces.
1766 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1767 'w+', 'a', and 'a+'.
1769 If the filename begins with "|", the filename is interpreted as a command
1770 to which output is to be piped, and if the filename ends with a "|", the
1771 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1772 examples of this. as command which pipes input to us. (You may not have
1773 a raw open() to a command that pipes both in I<and> out, but see
1774 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1777 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1778 non-zero upon success, the undefined value otherwise. If the open
1779 involved a pipe, the return value happens to be the pid of the
1782 If you're unfortunate enough to be running Perl on a system that
1783 distinguishes between text files and binary files (modern operating
1784 systems don't care), then you should check out L</binmode> for tips for
1785 dealing with this. The key distinction between systems that need binmode
1786 and those that don't is their text file formats. Systems like Unix and
1787 Plan9 that delimit lines with a single character, and that encode that
1788 character in C as '\n', do not need C<binmode>. The rest need it.
1793 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1794 while (<ARTICLE>) {...
1796 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1798 open(DBASE, '+<dbase.mine'); # open for update
1800 open(ARTICLE, "caesar <$article |"); # decrypt article
1802 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1804 # process argument list of files along with any includes
1806 foreach $file (@ARGV) {
1807 process($file, 'fh00');
1811 local($filename, $input) = @_;
1812 $input++; # this is a string increment
1813 unless (open($input, $filename)) {
1814 print STDERR "Can't open $filename: $!\n";
1818 while (<$input>) { # note use of indirection
1819 if (/^#include "(.*)"/) {
1820 process($1, $input);
1827 You may also, in the Bourne shell tradition, specify an EXPR beginning
1828 with "E<gt>&", in which case the rest of the string is interpreted as the
1829 name of a filehandle (or file descriptor, if numeric) which is to be
1830 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1831 +E<gt>E<gt>, and +E<lt>. The
1832 mode you specify should match the mode of the original filehandle.
1833 (Duping a filehandle does not take into account any existing contents of
1835 Here is a script that saves, redirects, and restores STDOUT and
1839 open(SAVEOUT, ">&STDOUT");
1840 open(SAVEERR, ">&STDERR");
1842 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1843 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1845 select(STDERR); $| = 1; # make unbuffered
1846 select(STDOUT); $| = 1; # make unbuffered
1848 print STDOUT "stdout 1\n"; # this works for
1849 print STDERR "stderr 1\n"; # subprocesses too
1854 open(STDOUT, ">&SAVEOUT");
1855 open(STDERR, ">&SAVEERR");
1857 print STDOUT "stdout 2\n";
1858 print STDERR "stderr 2\n";
1861 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1862 equivalent of C's fdopen() of that file descriptor; this is more
1863 parsimonious of file descriptors. For example:
1865 open(FILEHANDLE, "<&=$fd")
1867 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1868 there is an implicit fork done, and the return value of open is the pid
1869 of the child within the parent process, and 0 within the child
1870 process. (Use C<defined($pid)> to determine whether the open was successful.)
1871 The filehandle behaves normally for the parent, but i/o to that
1872 filehandle is piped from/to the STDOUT/STDIN of the child process.
1873 In the child process the filehandle isn't opened--i/o happens from/to
1874 the new STDOUT or STDIN. Typically this is used like the normal
1875 piped open when you want to exercise more control over just how the
1876 pipe command gets executed, such as when you are running setuid, and
1877 don't want to have to scan shell commands for metacharacters.
1878 The following pairs are more or less equivalent:
1880 open(FOO, "|tr '[a-z]' '[A-Z]'");
1881 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1883 open(FOO, "cat -n '$file'|");
1884 open(FOO, "-|") || exec 'cat', '-n', $file;
1886 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1888 Explicitly closing any piped filehandle causes the parent process to
1889 wait for the child to finish, and returns the status value in C<$?>.
1890 Note: on any operation which may do a fork, unflushed buffers remain
1891 unflushed in both processes, which means you may need to set C<$|> to
1892 avoid duplicate output.
1894 Using the constructor from the IO::Handle package (or one of its
1895 subclasses, such as IO::File or IO::Socket),
1896 you can generate anonymous filehandles which have the scope of whatever
1897 variables hold references to them, and automatically close whenever
1898 and however you leave that scope:
1902 sub read_myfile_munged {
1904 my $handle = new IO::File;
1905 open($handle, "myfile") or die "myfile: $!";
1907 or return (); # Automatically closed here.
1908 mung $first or die "mung failed"; # Or here.
1909 return $first, <$handle> if $ALL; # Or here.
1913 The filename that is passed to open will have leading and trailing
1914 whitespace deleted. To open a file with arbitrary weird
1915 characters in it, it's necessary to protect any leading and trailing
1918 $file =~ s#^(\s)#./$1#;
1919 open(FOO, "< $file\0");
1921 If you want a "real" C open() (see L<open(2)> on your system), then
1922 you should use the sysopen() function. This is another way to
1923 protect your filenames from interpretation. For example:
1926 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1927 or die "sysopen $path: $!";
1928 HANDLE->autoflush(1);
1929 HANDLE->print("stuff $$\n");
1931 print "File contains: ", <HANDLE>;
1933 See L</seek()> for some details about mixing reading and writing.
1935 =item opendir DIRHANDLE,EXPR
1937 Opens a directory named EXPR for processing by readdir(), telldir(),
1938 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
1939 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1945 Returns the numeric ascii value of the first character of EXPR. If
1946 EXPR is omitted, uses $_.
1948 =item pack TEMPLATE,LIST
1950 Takes an array or list of values and packs it into a binary structure,
1951 returning the string containing the structure. The TEMPLATE is a
1952 sequence of characters that give the order and type of values, as
1955 A An ascii string, will be space padded.
1956 a An ascii string, will be null padded.
1957 b A bit string (ascending bit order, like vec()).
1958 B A bit string (descending bit order).
1959 h A hex string (low nybble first).
1960 H A hex string (high nybble first).
1962 c A signed char value.
1963 C An unsigned char value.
1964 s A signed short value.
1965 S An unsigned short value.
1966 i A signed integer value.
1967 I An unsigned integer value.
1968 l A signed long value.
1969 L An unsigned long value.
1971 n A short in "network" order.
1972 N A long in "network" order.
1973 v A short in "VAX" (little-endian) order.
1974 V A long in "VAX" (little-endian) order.
1976 f A single-precision float in the native format.
1977 d A double-precision float in the native format.
1979 p A pointer to a null-terminated string.
1980 P A pointer to a structure (fixed-length string).
1982 u A uuencoded string.
1984 w A BER compressed integer. Bytes give an unsigned integer base
1985 128, most significant digit first, with as few digits as
1986 possible, and with the bit 8 of each byte except the last set
1991 @ Null fill to absolute position.
1993 Each letter may optionally be followed by a number which gives a repeat
1994 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
1995 pack function will gobble up that many values from the LIST. A * for the
1996 repeat count means to use however many items are left. The "a" and "A"
1997 types gobble just one value, but pack it as a string of length count,
1998 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1999 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2000 fields pack a string that many bits long. The "h" and "H" fields pack a
2001 string that many nybbles long. The "P" packs a pointer to a structure of
2002 the size indicated by the length. Real numbers (floats and doubles) are
2003 in the native machine format only; due to the multiplicity of floating
2004 formats around, and the lack of a standard "network" representation, no
2005 facility for interchange has been made. This means that packed floating
2006 point data written on one machine may not be readable on another - even if
2007 both use IEEE floating point arithmetic (as the endian-ness of the memory
2008 representation is not part of the IEEE spec). Note that Perl uses doubles
2009 internally for all numeric calculation, and converting from double into
2010 float and thence back to double again will lose precision (i.e.,
2011 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2015 $foo = pack("cccc",65,66,67,68);
2017 $foo = pack("c4",65,66,67,68);
2020 $foo = pack("ccxxcc",65,66,67,68);
2023 $foo = pack("s2",1,2);
2024 # "\1\0\2\0" on little-endian
2025 # "\0\1\0\2" on big-endian
2027 $foo = pack("a4","abcd","x","y","z");
2030 $foo = pack("aaaa","abcd","x","y","z");
2033 $foo = pack("a14","abcdefg");
2034 # "abcdefg\0\0\0\0\0\0\0"
2036 $foo = pack("i9pl", gmtime);
2037 # a real struct tm (on my system anyway)
2040 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2043 The same template may generally also be used in the unpack function.
2045 =item package NAMESPACE
2047 Declares the compilation unit as being in the given namespace. The scope
2048 of the package declaration is from the declaration itself through the end of
2049 the enclosing block (the same scope as the local() operator). All further
2050 unqualified dynamic identifiers will be in this namespace. A package
2051 statement affects only dynamic variables--including those you've used
2052 local() on--but I<not> lexical variables created with my(). Typically it
2053 would be the first declaration in a file to be included by the C<require>
2054 or C<use> operator. You can switch into a package in more than one place;
2055 it influences merely which symbol table is used by the compiler for the
2056 rest of that block. You can refer to variables and filehandles in other
2057 packages by prefixing the identifier with the package name and a double
2058 colon: C<$Package::Variable>. If the package name is null, the C<main>
2059 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2061 See L<perlmod/"Packages"> for more information about packages, modules,
2062 and classes. See L<perlsub> for other scoping issues.
2064 =item pipe READHANDLE,WRITEHANDLE
2066 Opens a pair of connected pipes like the corresponding system call.
2067 Note that if you set up a loop of piped processes, deadlock can occur
2068 unless you are very careful. In addition, note that Perl's pipes use
2069 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2070 after each command, depending on the application.
2072 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2073 for examples of such things.
2077 Pops and returns the last value of the array, shortening the array by
2078 1. Has a similar effect to
2080 $tmp = $ARRAY[$#ARRAY--];
2082 If there are no elements in the array, returns the undefined value.
2083 If ARRAY is omitted, pops the
2084 @ARGV array in the main program, and the @_ array in subroutines, just
2091 Returns the offset of where the last C<m//g> search left off for the variable
2092 is in question ($_ is used when the variable is not specified). May be
2093 modified to change that offset.
2095 =item print FILEHANDLE LIST
2101 Prints a string or a comma-separated list of strings. Returns TRUE
2102 if successful. FILEHANDLE may be a scalar variable name, in which case
2103 the variable contains the name of or a reference to the filehandle, thus introducing one
2104 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2105 token is a term, it may be misinterpreted as an operator unless you
2106 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2107 omitted, prints by default to standard output (or to the last selected
2108 output channel--see L</select>). If LIST is also omitted, prints $_ to
2109 STDOUT. To set the default output channel to something other than
2110 STDOUT use the select operation. Note that, because print takes a
2111 LIST, anything in the LIST is evaluated in a list context, and any
2112 subroutine that you call will have one or more of its expressions
2113 evaluated in a list context. Also be careful not to follow the print
2114 keyword with a left parenthesis unless you want the corresponding right
2115 parenthesis to terminate the arguments to the print--interpose a + or
2116 put parentheses around all the arguments.
2118 Note that if you're storing FILEHANDLES in an array or other expression,
2119 you will have to use a block returning its value instead:
2121 print { $files[$i] } "stuff\n";
2122 print { $OK ? STDOUT : STDERR } "stuff\n";
2124 =item printf FILEHANDLE FORMAT, LIST
2126 =item printf FORMAT, LIST
2128 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2129 of the list will be interpreted as the printf format. If C<use locale> is
2130 in effect, the character used for the decimal point in formatted real numbers
2131 is affected by the LC_NUMERIC locale. See L<perllocale>.
2133 =item prototype FUNCTION
2135 Returns the prototype of a function as a string (or C<undef> if the
2136 function has no prototype). FUNCTION is a reference to, or the name of,
2137 the function whose prototype you want to retrieve.
2139 =item push ARRAY,LIST
2141 Treats ARRAY as a stack, and pushes the values of LIST
2142 onto the end of ARRAY. The length of ARRAY increases by the length of
2143 LIST. Has the same effect as
2146 $ARRAY[++$#ARRAY] = $value;
2149 but is more efficient. Returns the new number of elements in the array.
2159 Generalized quotes. See L<perlop>.
2161 =item quotemeta EXPR
2165 Returns the value of EXPR with with all non-alphanumeric
2166 characters backslashed. (That is, all characters not matching
2167 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2168 returned string, regardless of any locale settings.)
2169 This is the internal function implementing
2170 the \Q escape in double-quoted strings.
2172 If EXPR is omitted, uses $_.
2178 Returns a random fractional number between 0 and the value of EXPR.
2179 (EXPR should be positive.) If EXPR is omitted, returns a value between
2180 0 and 1. This function produces repeatable sequences unless srand()
2181 is invoked. See also srand().
2183 (Note: if your rand function consistently returns numbers that are too
2184 large or too small, then your version of Perl was probably compiled
2185 with the wrong number of RANDBITS. As a workaround, you can usually
2186 multiply EXPR by the correct power of 2 to get the range you want.
2187 This will make your script unportable, however. It's better to recompile
2190 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2192 =item read FILEHANDLE,SCALAR,LENGTH
2194 Attempts to read LENGTH bytes of data into variable SCALAR from the
2195 specified FILEHANDLE. Returns the number of bytes actually read, or
2196 undef if there was an error. SCALAR will be grown or shrunk to the
2197 length actually read. An OFFSET may be specified to place the read
2198 data at some other place than the beginning of the string. This call
2199 is actually implemented in terms of stdio's fread call. To get a true
2200 read system call, see sysread().
2202 =item readdir DIRHANDLE
2204 Returns the next directory entry for a directory opened by opendir().
2205 If used in a list context, returns all the rest of the entries in the
2206 directory. If there are no more entries, returns an undefined value in
2207 a scalar context or a null list in a list context.
2209 If you're planning to filetest the return values out of a readdir(), you'd
2210 better prepend the directory in question. Otherwise, because we didn't
2211 chdir() there, it would have been testing the wrong file.
2213 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2214 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2221 Returns the value of a symbolic link, if symbolic links are
2222 implemented. If not, gives a fatal error. If there is some system
2223 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2226 =item recv SOCKET,SCALAR,LEN,FLAGS
2228 Receives a message on a socket. Attempts to receive LENGTH bytes of
2229 data into variable SCALAR from the specified SOCKET filehandle.
2230 Actually does a C recvfrom(), so that it can returns the address of the
2231 sender. Returns the undefined value if there's an error. SCALAR will
2232 be grown or shrunk to the length actually read. Takes the same flags
2233 as the system call of the same name.
2234 See L<perlipc/"UDP: Message Passing"> for examples.
2240 The C<redo> command restarts the loop block without evaluating the
2241 conditional again. The C<continue> block, if any, is not executed. If
2242 the LABEL is omitted, the command refers to the innermost enclosing
2243 loop. This command is normally used by programs that want to lie to
2244 themselves about what was just input:
2246 # a simpleminded Pascal comment stripper
2247 # (warning: assumes no { or } in strings)
2248 LINE: while (<STDIN>) {
2249 while (s|({.*}.*){.*}|$1 |) {}
2254 if (/}/) { # end of comment?
2267 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2268 is not specified, $_ will be used. The value returned depends on the
2269 type of thing the reference is a reference to.
2270 Builtin types include:
2279 If the referenced object has been blessed into a package, then that package
2280 name is returned instead. You can think of ref() as a typeof() operator.
2282 if (ref($r) eq "HASH") {
2283 print "r is a reference to an associative array.\n";
2286 print "r is not a reference at all.\n";
2289 See also L<perlref>.
2291 =item rename OLDNAME,NEWNAME
2293 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2294 not work across file system boundaries.
2300 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2301 supplied. If EXPR is numeric, demands that the current version of Perl
2302 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2304 Otherwise, demands that a library file be included if it hasn't already
2305 been included. The file is included via the do-FILE mechanism, which is
2306 essentially just a variety of eval(). Has semantics similar to the following
2310 local($filename) = @_;
2311 return 1 if $INC{$filename};
2312 local($realfilename,$result);
2314 foreach $prefix (@INC) {
2315 $realfilename = "$prefix/$filename";
2316 if (-f $realfilename) {
2317 $result = do $realfilename;
2321 die "Can't find $filename in \@INC";
2324 die "$filename did not return true value" unless $result;
2325 $INC{$filename} = $realfilename;
2329 Note that the file will not be included twice under the same specified
2330 name. The file must return TRUE as the last statement to indicate
2331 successful execution of any initialization code, so it's customary to
2332 end such a file with "1;" unless you're sure it'll return TRUE
2333 otherwise. But it's better just to put the "C<1;>", in case you add more
2336 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2337 replaces "F<::>" with "F</>" in the filename for you,
2338 to make it easy to load standard modules. This form of loading of
2339 modules does not risk altering your namespace.
2341 For a yet-more-powerful import facility, see L</use> and
2348 Generally used in a C<continue> block at the end of a loop to clear
2349 variables and reset ?? searches so that they work again. The
2350 expression is interpreted as a list of single characters (hyphens
2351 allowed for ranges). All variables and arrays beginning with one of
2352 those letters are reset to their pristine state. If the expression is
2353 omitted, one-match searches (?pattern?) are reset to match again. Resets
2354 only variables or searches in the current package. Always returns
2357 reset 'X'; # reset all X variables
2358 reset 'a-z'; # reset lower case variables
2359 reset; # just reset ?? searches
2361 Resetting "A-Z" is not recommended because you'll wipe out your
2362 ARGV and ENV arrays. Resets only package variables--lexical variables
2363 are unaffected, but they clean themselves up on scope exit anyway,
2364 so you'll probably want to use them instead. See L</my>.
2368 Returns from a subroutine or eval with the value specified. (Note that
2369 in the absence of a return a subroutine or eval() will automatically
2370 return the value of the last expression evaluated.)
2374 In a list context, returns a list value consisting of the elements
2375 of LIST in the opposite order. In a scalar context, returns a string
2376 value consisting of the bytes of the first element of LIST in the
2379 print reverse <>; # line tac
2382 print scalar reverse scalar <>; # byte tac
2384 =item rewinddir DIRHANDLE
2386 Sets the current position to the beginning of the directory for the
2387 readdir() routine on DIRHANDLE.
2389 =item rindex STR,SUBSTR,POSITION
2391 =item rindex STR,SUBSTR
2393 Works just like index except that it returns the position of the LAST
2394 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2395 last occurrence at or before that position.
2397 =item rmdir FILENAME
2401 Deletes the directory specified by FILENAME if it is empty. If it
2402 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2403 FILENAME is omitted, uses $_.
2407 The substitution operator. See L<perlop>.
2411 Forces EXPR to be interpreted in a scalar context and returns the value
2414 @counts = ( scalar @a, scalar @b, scalar @c );
2416 There is no equivalent operator to force an expression to
2417 be interpolated in a list context because it's in practice never
2418 needed. If you really wanted to do so, however, you could use
2419 the construction C<@{[ (some expression) ]}>, but usually a simple
2420 C<(some expression)> suffices.
2422 =item seek FILEHANDLE,POSITION,WHENCE
2424 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2425 call of stdio. FILEHANDLE may be an expression whose value gives the name
2426 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2427 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2428 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2429 this from POSIX module. Returns 1 upon success, 0 otherwise.
2431 On some systems you have to do a seek whenever you switch between reading
2432 and writing. Amongst other things, this may have the effect of calling
2433 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2438 This is also useful for applications emulating C<tail -f>. Once you hit
2439 EOF on your read, and then sleep for a while, you might have to stick in a
2440 seek() to reset things. First the simple trick listed above to clear the
2441 filepointer. The seek() doesn't change the current position, but it
2442 I<does> clear the end-of-file condition on the handle, so that the next
2443 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2445 If that doesn't work (some stdios are particularly cantankerous), then
2446 you may need something more like this:
2449 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2450 # search for some stuff and put it into files
2452 sleep($for_a_while);
2453 seek(FILE, $curpos, 0);
2456 =item seekdir DIRHANDLE,POS
2458 Sets the current position for the readdir() routine on DIRHANDLE. POS
2459 must be a value returned by telldir(). Has the same caveats about
2460 possible directory compaction as the corresponding system library
2463 =item select FILEHANDLE
2467 Returns the currently selected filehandle. Sets the current default
2468 filehandle for output, if FILEHANDLE is supplied. This has two
2469 effects: first, a C<write> or a C<print> without a filehandle will
2470 default to this FILEHANDLE. Second, references to variables related to
2471 output will refer to this output channel. For example, if you have to
2472 set the top of form format for more than one output channel, you might
2480 FILEHANDLE may be an expression whose value gives the name of the
2481 actual filehandle. Thus:
2483 $oldfh = select(STDERR); $| = 1; select($oldfh);
2485 Some programmers may prefer to think of filehandles as objects with
2486 methods, preferring to write the last example as:
2489 STDERR->autoflush(1);
2491 =item select RBITS,WBITS,EBITS,TIMEOUT
2493 This calls the select(2) system call with the bit masks specified, which
2494 can be constructed using fileno() and vec(), along these lines:
2496 $rin = $win = $ein = '';
2497 vec($rin,fileno(STDIN),1) = 1;
2498 vec($win,fileno(STDOUT),1) = 1;
2501 If you want to select on many filehandles you might wish to write a
2505 local(@fhlist) = split(' ',$_[0]);
2508 vec($bits,fileno($_),1) = 1;
2512 $rin = fhbits('STDIN TTY SOCK');
2516 ($nfound,$timeleft) =
2517 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2519 or to block until something becomes ready just do this
2521 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2523 Most systems do not bother to return anything useful in $timeleft, so
2524 calling select() in a scalar context just returns $nfound.
2526 Any of the bit masks can also be undef. The timeout, if specified, is
2527 in seconds, which may be fractional. Note: not all implementations are
2528 capable of returning the $timeleft. If not, they always return
2529 $timeleft equal to the supplied $timeout.
2531 You can effect a sleep of 250 milliseconds this way:
2533 select(undef, undef, undef, 0.25);
2535 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2536 with select(). You have to use sysread() instead.
2538 =item semctl ID,SEMNUM,CMD,ARG
2540 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2541 &GETALL, then ARG must be a variable which will hold the returned
2542 semid_ds structure or semaphore value array. Returns like ioctl: the
2543 undefined value for error, "0 but true" for zero, or the actual return
2546 =item semget KEY,NSEMS,FLAGS
2548 Calls the System V IPC function semget. Returns the semaphore id, or
2549 the undefined value if there is an error.
2551 =item semop KEY,OPSTRING
2553 Calls the System V IPC function semop to perform semaphore operations
2554 such as signaling and waiting. OPSTRING must be a packed array of
2555 semop structures. Each semop structure can be generated with
2556 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2557 operations is implied by the length of OPSTRING. Returns TRUE if
2558 successful, or FALSE if there is an error. As an example, the
2559 following code waits on semaphore $semnum of semaphore id $semid:
2561 $semop = pack("sss", $semnum, -1, 0);
2562 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2564 To signal the semaphore, replace "-1" with "1".
2566 =item send SOCKET,MSG,FLAGS,TO
2568 =item send SOCKET,MSG,FLAGS
2570 Sends a message on a socket. Takes the same flags as the system call
2571 of the same name. On unconnected sockets you must specify a
2572 destination to send TO, in which case it does a C sendto(). Returns
2573 the number of characters sent, or the undefined value if there is an
2575 See L<perlipc/"UDP: Message Passing"> for examples.
2577 =item setpgrp PID,PGRP
2579 Sets the current process group for the specified PID, 0 for the current
2580 process. Will produce a fatal error if used on a machine that doesn't
2581 implement setpgrp(2). If the arguments are omitted, it defaults to
2582 0,0. Note that the POSIX version of setpgrp() does not accept any
2583 arguments, so only setpgrp 0,0 is portable.
2585 =item setpriority WHICH,WHO,PRIORITY
2587 Sets the current priority for a process, a process group, or a user.
2588 (See setpriority(2).) Will produce a fatal error if used on a machine
2589 that doesn't implement setpriority(2).
2591 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2593 Sets the socket option requested. Returns undefined if there is an
2594 error. OPTVAL may be specified as undef if you don't want to pass an
2601 Shifts the first value of the array off and returns it, shortening the
2602 array by 1 and moving everything down. If there are no elements in the
2603 array, returns the undefined value. If ARRAY is omitted, shifts the
2604 @ARGV array in the main program, and the @_ array in subroutines.
2605 (This is determined lexically.) See also unshift(), push(), and pop().
2606 Shift() and unshift() do the same thing to the left end of an array
2607 that push() and pop() do to the right end.
2609 =item shmctl ID,CMD,ARG
2611 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2612 must be a variable which will hold the returned shmid_ds structure.
2613 Returns like ioctl: the undefined value for error, "0 but true" for
2614 zero, or the actual return value otherwise.
2616 =item shmget KEY,SIZE,FLAGS
2618 Calls the System V IPC function shmget. Returns the shared memory
2619 segment id, or the undefined value if there is an error.
2621 =item shmread ID,VAR,POS,SIZE
2623 =item shmwrite ID,STRING,POS,SIZE
2625 Reads or writes the System V shared memory segment ID starting at
2626 position POS for size SIZE by attaching to it, copying in/out, and
2627 detaching from it. When reading, VAR must be a variable which will
2628 hold the data read. When writing, if STRING is too long, only SIZE
2629 bytes are used; if STRING is too short, nulls are written to fill out
2630 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2632 =item shutdown SOCKET,HOW
2634 Shuts down a socket connection in the manner indicated by HOW, which
2635 has the same interpretation as in the system call of the same name.
2641 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2648 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2649 May be interrupted by sending the process a SIGALRM. Returns the
2650 number of seconds actually slept. You probably cannot mix alarm() and
2651 sleep() calls, because sleep() is often implemented using alarm().
2653 On some older systems, it may sleep up to a full second less than what
2654 you requested, depending on how it counts seconds. Most modern systems
2655 always sleep the full amount.
2657 For delays of finer granularity than one second, you may use Perl's
2658 syscall() interface to access setitimer(2) if your system supports it,
2659 or else see L</select()> below.
2661 See also the POSIX module's sigpause() function.
2663 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2665 Opens a socket of the specified kind and attaches it to filehandle
2666 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2667 system call of the same name. You should "use Socket;" first to get
2668 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2670 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2672 Creates an unnamed pair of sockets in the specified domain, of the
2673 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2674 for the system call of the same name. If unimplemented, yields a fatal
2675 error. Returns TRUE if successful.
2677 =item sort SUBNAME LIST
2679 =item sort BLOCK LIST
2683 Sorts the LIST and returns the sorted list value. Nonexistent values
2684 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2685 in standard string comparison order. If SUBNAME is specified, it
2686 gives the name of a subroutine that returns an integer less than, equal
2687 to, or greater than 0, depending on how the elements of the array are
2688 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2689 routines.) SUBNAME may be a scalar variable name, in which case the
2690 value provides the name of the subroutine to use. In place of a
2691 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2694 In the interests of efficiency the normal calling code for subroutines is
2695 bypassed, with the following effects: the subroutine may not be a
2696 recursive subroutine, and the two elements to be compared are passed into
2697 the subroutine not via @_ but as the package global variables $a and
2698 $b (see example below). They are passed by reference, so don't
2699 modify $a and $b. And don't try to declare them as lexicals either.
2701 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2702 current collation locale. See L<perllocale>.
2707 @articles = sort @files;
2709 # same thing, but with explicit sort routine
2710 @articles = sort {$a cmp $b} @files;
2712 # now case-insensitively
2713 @articles = sort { uc($a) cmp uc($b)} @files;
2715 # same thing in reversed order
2716 @articles = sort {$b cmp $a} @files;
2718 # sort numerically ascending
2719 @articles = sort {$a <=> $b} @files;
2721 # sort numerically descending
2722 @articles = sort {$b <=> $a} @files;
2724 # sort using explicit subroutine name
2726 $age{$a} <=> $age{$b}; # presuming integers
2728 @sortedclass = sort byage @class;
2730 # this sorts the %age associative arrays by value
2731 # instead of key using an in-line function
2732 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2734 sub backwards { $b cmp $a; }
2735 @harry = ('dog','cat','x','Cain','Abel');
2736 @george = ('gone','chased','yz','Punished','Axed');
2738 # prints AbelCaincatdogx
2739 print sort backwards @harry;
2740 # prints xdogcatCainAbel
2741 print sort @george, 'to', @harry;
2742 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2744 # inefficiently sort by descending numeric compare using
2745 # the first integer after the first = sign, or the
2746 # whole record case-insensitively otherwise
2749 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2754 # same thing, but much more efficiently;
2755 # we'll build auxiliary indices instead
2759 push @nums, /=(\d+)/;
2764 $nums[$b] <=> $nums[$a]
2766 $caps[$a] cmp $caps[$b]
2770 # same thing using a Schwartzian Transform (no temps)
2771 @new = map { $_->[0] }
2772 sort { $b->[1] <=> $a->[1]
2775 } map { [$_, /=(\d+)/, uc($_)] } @old;
2777 If you're using strict, you I<MUST NOT> declare $a
2778 and $b as lexicals. They are package globals. That means
2779 if you're in the C<main> package, it's
2781 @articles = sort {$main::b <=> $main::a} @files;
2785 @articles = sort {$::b <=> $::a} @files;
2787 but if you're in the C<FooPack> package, it's
2789 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2791 The comparison function is required to behave. If it returns
2792 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2793 sometimes saying the opposite, for example) the Perl interpreter will
2794 probably crash and dump core. This is entirely due to and dependent
2795 upon your system's qsort(3) library routine; this routine often avoids
2796 sanity checks in the interest of speed.
2798 =item splice ARRAY,OFFSET,LENGTH,LIST
2800 =item splice ARRAY,OFFSET,LENGTH
2802 =item splice ARRAY,OFFSET
2804 Removes the elements designated by OFFSET and LENGTH from an array, and
2805 replaces them with the elements of LIST, if any. Returns the elements
2806 removed from the array. The array grows or shrinks as necessary. If
2807 LENGTH is omitted, removes everything from OFFSET onward. The
2808 following equivalences hold (assuming C<$[ == 0>):
2810 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2811 pop(@a) splice(@a,-1)
2812 shift(@a) splice(@a,0,1)
2813 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2814 $a[$x] = $y splice(@a,$x,1,$y);
2816 Example, assuming array lengths are passed before arrays:
2818 sub aeq { # compare two list values
2819 local(@a) = splice(@_,0,shift);
2820 local(@b) = splice(@_,0,shift);
2821 return 0 unless @a == @b; # same len?
2823 return 0 if pop(@a) ne pop(@b);
2827 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2829 =item split /PATTERN/,EXPR,LIMIT
2831 =item split /PATTERN/,EXPR
2833 =item split /PATTERN/
2837 Splits a string into an array of strings, and returns it.
2839 If not in a list context, returns the number of fields found and splits into
2840 the @_ array. (In a list context, you can force the split into @_ by
2841 using C<??> as the pattern delimiters, but it still returns the array
2842 value.) The use of implicit split to @_ is deprecated, however.
2844 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2845 splits on whitespace (after skipping any leading whitespace). Anything
2846 matching PATTERN is taken to be a delimiter separating the fields. (Note
2847 that the delimiter may be longer than one character.) If LIMIT is
2848 specified and is not negative, splits into no more than that many fields
2849 (though it may split into fewer). If LIMIT is unspecified, trailing null
2850 fields are stripped (which potential users of pop() would do well to
2851 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2852 LIMIT had been specified.
2854 A pattern matching the null string (not to be confused with
2855 a null pattern C<//>, which is just one member of the set of patterns
2856 matching a null string) will split the value of EXPR into separate
2857 characters at each point it matches that way. For example:
2859 print join(':', split(/ */, 'hi there'));
2861 produces the output 'h:i:t:h:e:r:e'.
2863 The LIMIT parameter can be used to split a line partially
2865 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2867 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2868 one larger than the number of variables in the list, to avoid
2869 unnecessary work. For the list above LIMIT would have been 4 by
2870 default. In time critical applications it behooves you not to split
2871 into more fields than you really need.
2873 If the PATTERN contains parentheses, additional array elements are
2874 created from each matching substring in the delimiter.
2876 split(/([,-])/, "1-10,20", 3);
2878 produces the list value
2880 (1, '-', 10, ',', 20)
2882 If you had the entire header of a normal Unix email message in $header,
2883 you could split it up into fields and their values this way:
2885 $header =~ s/\n\s+/ /g; # fix continuation lines
2886 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2888 The pattern C</PATTERN/> may be replaced with an expression to specify
2889 patterns that vary at runtime. (To do runtime compilation only once,
2890 use C</$variable/o>.)
2892 As a special case, specifying a PATTERN of space (C<' '>) will split on
2893 white space just as split with no arguments does. Thus, split(' ') can
2894 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2895 will give you as many null initial fields as there are leading spaces.
2896 A split on /\s+/ is like a split(' ') except that any leading
2897 whitespace produces a null first field. A split with no arguments
2898 really does a C<split(' ', $_)> internally.
2902 open(passwd, '/etc/passwd');
2904 ($login, $passwd, $uid, $gid, $gcos,
2905 $home, $shell) = split(/:/);
2909 (Note that $shell above will still have a newline on it. See L</chop>,
2910 L</chomp>, and L</join>.)
2912 =item sprintf FORMAT, LIST
2914 Returns a string formatted by the usual printf conventions of the C
2915 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2916 (The * character for an indirectly specified length is not
2917 supported, but you can get the same effect by interpolating a variable
2918 into the pattern.) If C<use locale> is
2919 in effect, the character used for the decimal point in formatted real numbers
2920 is affected by the LC_NUMERIC locale. See L<perllocale>.
2921 Some C libraries' implementations of sprintf() can
2922 dump core when fed ludicrous arguments.
2928 Return the square root of EXPR. If EXPR is omitted, returns square
2933 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2934 uses a semi-random value based on the current time and process ID, among
2935 other things. Of course, you'd need something much more random than that for
2936 cryptographic purposes, because it's easy to guess the current time.
2937 Checksumming the compressed output of rapidly changing operating system
2938 status programs is the usual method. Examples are posted regularly to
2939 the comp.security.unix newsgroup.
2941 =item stat FILEHANDLE
2947 Returns a 13-element array giving the status info for a file, either the
2948 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
2949 stats $_. Returns a null list if the stat fails. Typically used as
2953 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2954 $atime,$mtime,$ctime,$blksize,$blocks)
2957 Not all fields are supported on all filesystem types. Here are the
2958 meaning of the fields:
2960 dev device number of filesystem
2962 mode file mode (type and permissions)
2963 nlink number of (hard) links to the file
2964 uid numeric user ID of file's owner
2965 gid numeric group ID of file's owner
2966 rdev the device identifier (special files only)
2967 size total size of file, in bytes
2968 atime last access time since the epoch
2969 mtime last modify time since the epoch
2970 ctime inode change time (NOT creation type!) since the epoch
2971 blksize preferred block size for file system I/O
2972 blocks actual number of blocks allocated
2974 (The epoch was at 00:00 January 1, 1970 GMT.)
2976 If stat is passed the special filehandle consisting of an underline, no
2977 stat is done, but the current contents of the stat structure from the
2978 last stat or filetest are returned. Example:
2980 if (-x $file && (($d) = stat(_)) && $d < 0) {
2981 print "$file is executable NFS file\n";
2984 (This works on machines only for which the device number is negative under NFS.)
2990 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2991 doing many pattern matches on the string before it is next modified.
2992 This may or may not save time, depending on the nature and number of
2993 patterns you are searching on, and on the distribution of character
2994 frequencies in the string to be searched--you probably want to compare
2995 run times with and without it to see which runs faster. Those loops
2996 which scan for many short constant strings (including the constant
2997 parts of more complex patterns) will benefit most. You may have only
2998 one study active at a time--if you study a different scalar the first
2999 is "unstudied". (The way study works is this: a linked list of every
3000 character in the string to be searched is made, so we know, for
3001 example, where all the 'k' characters are. From each search string,
3002 the rarest character is selected, based on some static frequency tables
3003 constructed from some C programs and English text. Only those places
3004 that contain this "rarest" character are examined.)
3006 For example, here is a loop which inserts index producing entries
3007 before any line containing a certain pattern:
3011 print ".IX foo\n" if /\bfoo\b/;
3012 print ".IX bar\n" if /\bbar\b/;
3013 print ".IX blurfl\n" if /\bblurfl\b/;
3018 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3019 will be looked at, because "f" is rarer than "o". In general, this is
3020 a big win except in pathological cases. The only question is whether
3021 it saves you more time than it took to build the linked list in the
3024 Note that if you have to look for strings that you don't know till
3025 runtime, you can build an entire loop as a string and eval that to
3026 avoid recompiling all your patterns all the time. Together with
3027 undefining $/ to input entire files as one record, this can be very
3028 fast, often faster than specialized programs like fgrep(1). The following
3029 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3030 out the names of those files that contain a match:
3032 $search = 'while (<>) { study;';
3033 foreach $word (@words) {
3034 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3039 eval $search; # this screams
3040 $/ = "\n"; # put back to normal input delimiter
3041 foreach $file (sort keys(%seen)) {
3049 =item sub NAME BLOCK
3051 This is subroutine definition, not a real function I<per se>. With just a
3052 NAME (and possibly prototypes), it's just a forward declaration. Without
3053 a NAME, it's an anonymous function declaration, and does actually return a
3054 value: the CODE ref of the closure you just created. See L<perlsub> and
3055 L<perlref> for details.
3057 =item substr EXPR,OFFSET,LEN
3059 =item substr EXPR,OFFSET
3061 Extracts a substring out of EXPR and returns it. First character is at
3062 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3063 that far from the end of the string. If LEN is omitted, returns
3064 everything to the end of the string. If LEN is negative, leaves that
3065 many characters off the end of the string.
3067 You can use the substr() function
3068 as an lvalue, in which case EXPR must be an lvalue. If you assign
3069 something shorter than LEN, the string will shrink, and if you assign
3070 something longer than LEN, the string will grow to accommodate it. To
3071 keep the string the same length you may need to pad or chop your value
3074 =item symlink OLDFILE,NEWFILE
3076 Creates a new filename symbolically linked to the old filename.
3077 Returns 1 for success, 0 otherwise. On systems that don't support
3078 symbolic links, produces a fatal error at run time. To check for that,
3081 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3085 Calls the system call specified as the first element of the list,
3086 passing the remaining elements as arguments to the system call. If
3087 unimplemented, produces a fatal error. The arguments are interpreted
3088 as follows: if a given argument is numeric, the argument is passed as
3089 an int. If not, the pointer to the string value is passed. You are
3090 responsible to make sure a string is pre-extended long enough to
3091 receive any result that might be written into a string. If your
3092 integer arguments are not literals and have never been interpreted in a
3093 numeric context, you may need to add 0 to them to force them to look
3096 require 'syscall.ph'; # may need to run h2ph
3097 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3099 Note that Perl supports passing of up to only 14 arguments to your system call,
3100 which in practice should usually suffice.
3102 =item sysopen FILEHANDLE,FILENAME,MODE
3104 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3106 Opens the file whose filename is given by FILENAME, and associates it
3107 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3108 the name of the real filehandle wanted. This function calls the
3109 underlying operating system's C<open> function with the parameters
3110 FILENAME, MODE, PERMS.
3112 The possible values and flag bits of the MODE parameter are
3113 system-dependent; they are available via the standard module C<Fcntl>.
3114 However, for historical reasons, some values are universal: zero means
3115 read-only, one means write-only, and two means read/write.
3117 If the file named by FILENAME does not exist and the C<open> call
3118 creates it (typically because MODE includes the O_CREAT flag), then
3119 the value of PERMS specifies the permissions of the newly created
3120 file. If PERMS is omitted, the default value is 0666, which allows
3121 read and write for all. This default is reasonable: see C<umask>.
3123 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3125 =item sysread FILEHANDLE,SCALAR,LENGTH
3127 Attempts to read LENGTH bytes of data into variable SCALAR from the
3128 specified FILEHANDLE, using the system call read(2). It bypasses
3129 stdio, so mixing this with other kinds of reads may cause confusion.
3130 Returns the number of bytes actually read, or undef if there was an
3131 error. SCALAR will be grown or shrunk so that the last byte actually
3132 read is the last byte of the scalar after the read.
3134 An OFFSET may be specified to place the read data at some place in the
3135 string other than the beginning. A negative OFFSET specifies
3136 placement at that many bytes counting backwards from the end of the
3137 string. A positive OFFSET greater than the length of SCALAR results
3138 in the string being padded to the required size with "\0" bytes before
3139 the result of the read is appended.
3143 Does exactly the same thing as "exec LIST" except that a fork is done
3144 first, and the parent process waits for the child process to complete.
3145 Note that argument processing varies depending on the number of
3146 arguments. The return value is the exit status of the program as
3147 returned by the wait() call. To get the actual exit value divide by
3148 256. See also L</exec>. This is I<NOT> what you want to use to capture
3149 the output from a command, for that you should use merely back-ticks, as
3150 described in L<perlop/"`STRING`">.
3152 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3154 =item syswrite FILEHANDLE,SCALAR,LENGTH
3156 Attempts to write LENGTH bytes of data from variable SCALAR to the
3157 specified FILEHANDLE, using the system call write(2). It bypasses
3158 stdio, so mixing this with prints may cause confusion. Returns the
3159 number of bytes actually written, or undef if there was an error.
3160 If the length is greater than the available data, only as much data as
3161 is available will be written.
3163 An OFFSET may be specified to write the data from some part of the
3164 string other than the beginning. A negative OFFSET specifies writing
3165 from that many bytes counting backwards from the end of the string.
3167 =item tell FILEHANDLE
3171 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3172 expression whose value gives the name of the actual filehandle. If
3173 FILEHANDLE is omitted, assumes the file last read.
3175 =item telldir DIRHANDLE
3177 Returns the current position of the readdir() routines on DIRHANDLE.
3178 Value may be given to seekdir() to access a particular location in a
3179 directory. Has the same caveats about possible directory compaction as
3180 the corresponding system library routine.
3182 =item tie VARIABLE,CLASSNAME,LIST
3184 This function binds a variable to a package class that will provide the
3185 implementation for the variable. VARIABLE is the name of the variable
3186 to be enchanted. CLASSNAME is the name of a class implementing objects
3187 of correct type. Any additional arguments are passed to the "new"
3188 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3189 Typically these are arguments such as might be passed to the dbm_open()
3190 function of C. The object returned by the "new" method is also
3191 returned by the tie() function, which would be useful if you want to
3192 access other methods in CLASSNAME.
3194 Note that functions such as keys() and values() may return huge array
3195 values when used on large objects, like DBM files. You may prefer to
3196 use the each() function to iterate over such. Example:
3198 # print out history file offsets
3200 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3201 while (($key,$val) = each %HIST) {
3202 print $key, ' = ', unpack('L',$val), "\n";
3206 A class implementing an associative array should have the following
3209 TIEHASH classname, LIST
3212 STORE this, key, value
3216 NEXTKEY this, lastkey
3218 A class implementing an ordinary array should have the following methods:
3220 TIEARRAY classname, LIST
3223 STORE this, key, value
3226 A class implementing a scalar should have the following methods:
3228 TIESCALAR classname, LIST
3233 Unlike dbmopen(), the tie() function will not use or require a module
3234 for you--you need to do that explicitly yourself. See L<DB_File>
3235 or the F<Config> module for interesting tie() implementations.
3239 Returns a reference to the object underlying VARIABLE (the same value
3240 that was originally returned by the tie() call which bound the variable
3241 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3246 Returns the number of non-leap seconds since whatever time the system
3247 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3248 and 00:00:00 UTC, January 1, 1970 for most other systems).
3249 Suitable for feeding to gmtime() and localtime().
3253 Returns a four-element array giving the user and system times, in
3254 seconds, for this process and the children of this process.
3256 ($user,$system,$cuser,$csystem) = times;
3260 The translation operator. See L<perlop>.
3262 =item truncate FILEHANDLE,LENGTH
3264 =item truncate EXPR,LENGTH
3266 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3267 specified length. Produces a fatal error if truncate isn't implemented
3274 Returns an uppercased version of EXPR. This is the internal function
3275 implementing the \U escape in double-quoted strings.
3276 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3278 If EXPR is omitted, uses $_.
3284 Returns the value of EXPR with the first character uppercased. This is
3285 the internal function implementing the \u escape in double-quoted strings.
3286 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3288 If EXPR is omitted, uses $_.
3294 Sets the umask for the process and returns the old one. If EXPR is
3295 omitted, returns merely the current umask.
3301 Undefines the value of EXPR, which must be an lvalue. Use on only a
3302 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3303 will probably not do what you expect on most predefined variables or
3304 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3305 the EXPR, in which case nothing is undefined, but you still get an
3306 undefined value that you could, for instance, return from a
3307 subroutine. Examples:
3310 undef $bar{'blurfl'};
3314 return (wantarray ? () : undef) if $they_blew_it;
3320 Deletes a list of files. Returns the number of files successfully
3323 $cnt = unlink 'a', 'b', 'c';
3327 Note: unlink will not delete directories unless you are superuser and
3328 the B<-U> flag is supplied to Perl. Even if these conditions are
3329 met, be warned that unlinking a directory can inflict damage on your
3330 filesystem. Use rmdir instead.
3332 If LIST is omitted, uses $_.
3334 =item unpack TEMPLATE,EXPR
3336 Unpack does the reverse of pack: it takes a string representing a
3337 structure and expands it out into a list value, returning the array
3338 value. (In a scalar context, it returns merely the first value
3339 produced.) The TEMPLATE has the same format as in the pack function.
3340 Here's a subroutine that does substring:
3343 local($what,$where,$howmuch) = @_;
3344 unpack("x$where a$howmuch", $what);
3349 sub ordinal { unpack("c",$_[0]); } # same as ord()
3351 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3352 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3353 themselves. Default is a 16-bit checksum. For example, the following
3354 computes the same number as the System V sum program:
3357 $checksum += unpack("%16C*", $_);
3361 The following efficiently counts the number of set bits in a bit vector:
3363 $setbits = unpack("%32b*", $selectmask);
3365 =item untie VARIABLE
3367 Breaks the binding between a variable and a package. (See tie().)
3369 =item unshift ARRAY,LIST
3371 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3372 depending on how you look at it. Prepends list to the front of the
3373 array, and returns the new number of elements in the array.
3375 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3377 Note the LIST is prepended whole, not one element at a time, so the
3378 prepended elements stay in the same order. Use reverse to do the
3381 =item use Module LIST
3385 =item use Module VERSION LIST
3389 Imports some semantics into the current package from the named module,
3390 generally by aliasing certain subroutine or variable names into your
3391 package. It is exactly equivalent to
3393 BEGIN { require Module; import Module LIST; }
3395 except that Module I<must> be a bare word.
3397 If the first argument to C<use> is a number, it is treated as a version
3398 number instead of a module name. If the version of the Perl interpreter
3399 is less than VERSION, then an error message is printed and Perl exits
3400 immediately. This is often useful if you need to check the current
3401 Perl version before C<use>ing library modules which have changed in
3402 incompatible ways from older versions of Perl. (We try not to do
3403 this more than we have to.)
3405 The BEGIN forces the require and import to happen at compile time. The
3406 require makes sure the module is loaded into memory if it hasn't been
3407 yet. The import is not a builtin--it's just an ordinary static method
3408 call into the "Module" package to tell the module to import the list of
3409 features back into the current package. The module can implement its
3410 import method any way it likes, though most modules just choose to
3411 derive their import method via inheritance from the Exporter class that
3412 is defined in the Exporter module. See L<Exporter>. If no import
3413 method can be found then the error is currently silently ignored. This
3414 may change to a fatal error in a future version.
3416 If you don't want your namespace altered, explicitly supply an empty list:
3420 That is exactly equivalent to
3422 BEGIN { require Module; }
3424 If the VERSION argument is present between Module and LIST, then the
3425 C<use> will call the VERSION method in class Module with the given
3426 version as an argument. The default VERSION method, inherited from
3427 the Universal class, croaks if the given version is larger than the
3428 value of the variable $Module::VERSION. (Note that there is not a
3429 comma after VERSION!)
3431 Because this is a wide-open interface, pragmas (compiler directives)
3432 are also implemented this way. Currently implemented pragmas are:
3436 use sigtrap qw(SEGV BUS);
3437 use strict qw(subs vars refs);
3438 use subs qw(afunc blurfl);
3440 These pseudo-modules import semantics into the current block scope, unlike
3441 ordinary modules, which import symbols into the current package (which are
3442 effective through the end of the file).
3444 There's a corresponding "no" command that unimports meanings imported
3445 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3450 If no unimport method can be found the call fails with a fatal error.
3452 See L<perlmod> for a list of standard modules and pragmas.
3456 Changes the access and modification times on each file of a list of
3457 files. The first two elements of the list must be the NUMERICAL access
3458 and modification times, in that order. Returns the number of files
3459 successfully changed. The inode modification time of each file is set
3460 to the current time. Example of a "touch" command:
3464 utime $now, $now, @ARGV;
3466 =item values ASSOC_ARRAY
3468 Returns a normal array consisting of all the values of the named
3469 associative array. (In a scalar context, returns the number of
3470 values.) The values are returned in an apparently random order, but it
3471 is the same order as either the keys() or each() function would produce
3472 on the same array. See also keys(), each(), and sort().
3474 =item vec EXPR,OFFSET,BITS
3476 Treats the string in EXPR as a vector of unsigned integers, and
3477 returns the value of the bit field specified by OFFSET. BITS specifies
3478 the number of bits that are reserved for each entry in the bit
3479 vector. This must be a power of two from 1 to 32. vec() may also be
3480 assigned to, in which case parentheses are needed to give the expression
3481 the correct precedence as in
3483 vec($image, $max_x * $x + $y, 8) = 3;
3485 Vectors created with vec() can also be manipulated with the logical
3486 operators |, &, and ^, which will assume a bit vector operation is
3487 desired when both operands are strings.
3489 To transform a bit vector into a string or array of 0's and 1's, use these:
3491 $bits = unpack("b*", $vector);
3492 @bits = split(//, unpack("b*", $vector));
3494 If you know the exact length in bits, it can be used in place of the *.
3498 Waits for a child process to terminate and returns the pid of the
3499 deceased process, or -1 if there are no child processes. The status is
3502 =item waitpid PID,FLAGS
3504 Waits for a particular child process to terminate and returns the pid
3505 of the deceased process, or -1 if there is no such child process. The
3506 status is returned in C<$?>. If you say
3508 use POSIX ":sys_wait_h";
3510 waitpid(-1,&WNOHANG);
3512 then you can do a non-blocking wait for any process. Non-blocking wait
3513 is available on machines supporting either the waitpid(2) or
3514 wait4(2) system calls. However, waiting for a particular pid with
3515 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3516 by remembering the status values of processes that have exited but have
3517 not been harvested by the Perl script yet.)
3521 Returns TRUE if the context of the currently executing subroutine is
3522 looking for a list value. Returns FALSE if the context is looking
3525 return wantarray ? () : undef;
3529 Produces a message on STDERR just like die(), but doesn't exit or
3532 =item write FILEHANDLE
3538 Writes a formatted record (possibly multi-line) to the specified file,
3539 using the format associated with that file. By default the format for
3540 a file is the one having the same name is the filehandle, but the
3541 format for the current output channel (see the select() function) may be set
3542 explicitly by assigning the name of the format to the C<$~> variable.
3544 Top of form processing is handled automatically: if there is
3545 insufficient room on the current page for the formatted record, the
3546 page is advanced by writing a form feed, a special top-of-page format
3547 is used to format the new page header, and then the record is written.
3548 By default the top-of-page format is the name of the filehandle with
3549 "_TOP" appended, but it may be dynamically set to the format of your
3550 choice by assigning the name to the C<$^> variable while the filehandle is
3551 selected. The number of lines remaining on the current page is in
3552 variable C<$->, which can be set to 0 to force a new page.
3554 If FILEHANDLE is unspecified, output goes to the current default output
3555 channel, which starts out as STDOUT but may be changed by the
3556 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3557 is evaluated and the resulting string is used to look up the name of
3558 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3560 Note that write is I<NOT> the opposite of read. Unfortunately.
3564 The translation operator. See L<perlop>.