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 $1 is defined, despite the fact that it
684 matched "nothing". But it didn't really match nothing--rather, it
685 matched something that happened to be 0 characters long. This is all
686 very above-board and honest. When a function returns an undefined value,
687 it's an admission that it couldn't give you an honest answer. So
688 you should use defined() only when you're questioning the integrity
689 of what you're trying to do. At other times, a simple comparison to
690 0 or "" is what you want.
692 Another surprise is that using defined() on an entire array or
693 hash reports whether memory for that aggregate has ever been
694 allocated. So an array you set to the empty list appears undefined
695 initially, and one that once was full and that you then set to
696 the empty list still appears defined. You should instead use a
697 simple test for size:
699 if (@an_array) { print "has array elements\n" }
700 if (%a_hash) { print "has hash members\n" }
702 Using undef() on these, however, does clear their memory and then report
703 them as not defined anymore, but you shoudln't do that unless you don't
704 plan to use them again, because it saves time when you load them up
705 again to have memory already ready to be filled.
707 This counter-intuitive behaviour of defined() on aggregates may be
708 changed, fixed, or broken in a future release of Perl.
712 Deletes the specified key(s) and their associated values from a hash
713 array. For each key, returns the deleted value associated with that key,
714 or the undefined value if there was no such key. Deleting from C<$ENV{}>
715 modifies the environment. Deleting from an array tied to a DBM file
716 deletes the entry from the DBM file. (But deleting from a tie()d hash
717 doesn't necessarily return anything.)
719 The following deletes all the values of an associative array:
721 foreach $key (keys %HASH) {
727 delete @HASH{keys %HASH}
729 (But both of these are slower than the undef() command.) Note that the
730 EXPR can be arbitrarily complicated as long as the final operation is a
731 hash element lookup or hash slice:
733 delete $ref->[$x][$y]{$key};
734 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
738 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
739 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
740 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
741 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
742 and the eval() is terminated with the undefined value; this makes die()
743 the way to raise an exception.
747 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
748 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
750 If the value of EXPR does not end in a newline, the current script line
751 number and input line number (if any) are also printed, and a newline
752 is supplied. Hint: sometimes appending ", stopped" to your message
753 will cause it to make better sense when the string "at foo line 123" is
754 appended. Suppose you are running script "canasta".
756 die "/etc/games is no good";
757 die "/etc/games is no good, stopped";
759 produce, respectively
761 /etc/games is no good at canasta line 123.
762 /etc/games is no good, stopped at canasta line 123.
764 See also exit() and warn().
768 Not really a function. Returns the value of the last command in the
769 sequence of commands indicated by BLOCK. When modified by a loop
770 modifier, executes the BLOCK once before testing the loop condition.
771 (On other statements the loop modifiers test the conditional first.)
773 =item do SUBROUTINE(LIST)
775 A deprecated form of subroutine call. See L<perlsub>.
779 Uses the value of EXPR as a filename and executes the contents of the
780 file as a Perl script. Its primary use is to include subroutines
781 from a Perl subroutine library.
789 except that it's more efficient, more concise, keeps track of the
790 current filename for error messages, and searches all the B<-I>
791 libraries if the file isn't in the current directory (see also the @INC
792 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
793 re-parse the file every time you call it, so you probably don't want to
794 do this inside a loop.
796 Note that inclusion of library modules is better done with the
797 use() and require() operators, which also do error checking
798 and raise an exception if there's a problem.
802 This causes an immediate core dump. Primarily this is so that you can
803 use the B<undump> program to turn your core dump into an executable binary
804 after having initialized all your variables at the beginning of the
805 program. When the new binary is executed it will begin by executing a
806 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
807 it as a goto with an intervening core dump and reincarnation. If LABEL
808 is omitted, restarts the program from the top. WARNING: any files
809 opened at the time of the dump will NOT be open any more when the
810 program is reincarnated, with possible resulting confusion on the part
811 of Perl. See also B<-u> option in L<perlrun>.
828 dump QUICKSTART if $ARGV[0] eq '-d';
833 =item each ASSOC_ARRAY
835 When called in a list context, returns a 2-element array consisting
836 of the key and value for the next element of an associative array,
837 so that you can iterate over it. When called in a scalar context,
838 returns the key for only the next element in the associative array.
839 Entries are returned in an apparently random order. When the array is
840 entirely read, a null array is returned in list context (which when
841 assigned produces a FALSE (0) value), and C<undef> is returned in a
842 scalar context. The next call to each() after that will start
843 iterating again. The iterator can be reset only by reading all the
844 elements from the array. You should not add elements to an array while
845 you're iterating over it. There is a single iterator for each
846 associative array, shared by all each(), keys(), and values() function
847 calls in the program. The following prints out your environment like
848 the printenv(1) program, only in a different order:
850 while (($key,$value) = each %ENV) {
851 print "$key=$value\n";
854 See also keys() and values().
862 Returns 1 if the next read on FILEHANDLE will return end of file, or if
863 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
864 gives the real filehandle name. (Note that this function actually
865 reads a character and then ungetc()s it, so it is not very useful in an
866 interactive context.) Do not read from a terminal file (or call
867 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
868 as terminals may lose the end-of-file condition if you do.
870 An C<eof> without an argument uses the last file read as argument.
871 Empty parentheses () may be used to indicate
872 the pseudo file formed of the files listed on the command line, i.e.,
873 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
874 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
875 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
877 # reset line numbering on each input file
880 close(ARGV) if (eof); # Not eof().
883 # insert dashes just before last line of last file
886 print "--------------\n";
887 close(ARGV); # close or break; is needed if we
888 # are reading from the terminal
893 Practical hint: you almost never need to use C<eof> in Perl, because the
894 input operators return undef when they run out of data.
900 EXPR is parsed and executed as if it were a little Perl program. It
901 is executed in the context of the current Perl program, so that any
902 variable settings or subroutine and format definitions remain afterwards.
903 The value returned is the value of the last expression evaluated, or a
904 return statement may be used, just as with subroutines. The last
905 expression is evaluated in scalar or array context, depending on the
908 If there is a syntax error or runtime error, or a die() statement is
909 executed, an undefined value is returned by eval(), and C<$@> is set to the
910 error message. If there was no error, C<$@> is guaranteed to be a null
911 string. If EXPR is omitted, evaluates $_. The final semicolon, if
912 any, may be omitted from the expression.
914 Note that, because eval() traps otherwise-fatal errors, it is useful for
915 determining whether a particular feature (such as socket() or symlink())
916 is implemented. It is also Perl's exception trapping mechanism, where
917 the die operator is used to raise exceptions.
919 If the code to be executed doesn't vary, you may use the eval-BLOCK
920 form to trap run-time errors without incurring the penalty of
921 recompiling each time. The error, if any, is still returned in C<$@>.
924 # make divide-by-zero non-fatal
925 eval { $answer = $a / $b; }; warn $@ if $@;
927 # same thing, but less efficient
928 eval '$answer = $a / $b'; warn $@ if $@;
930 # a compile-time error
934 eval '$answer ='; # sets $@
936 With an eval(), you should be especially careful to remember what's
937 being looked at when:
943 eval { $x }; # CASE 4
945 eval "\$$x++" # CASE 5
948 Cases 1 and 2 above behave identically: they run the code contained in the
949 variable $x. (Although case 2 has misleading double quotes making the
950 reader wonder what else might be happening (nothing is).) Cases 3 and 4
951 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
952 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
953 is a place where normally you I<WOULD> like to use double quotes, except
954 that in that particular situation, you can just use symbolic references
955 instead, as in case 6.
959 The exec() function executes a system command I<AND NEVER RETURNS>,
960 unless the command does not exist and is executed directly instead of
961 via C</bin/sh -c> (see below). Use system() instead of exec() if you
964 If there is more than one argument in LIST, or if LIST is an array with
965 more than one value, calls execvp(3) with the arguments in LIST. If
966 there is only one scalar argument, the argument is checked for shell
967 metacharacters. If there are any, the entire argument is passed to
968 C</bin/sh -c> for parsing. If there are none, the argument is split
969 into words and passed directly to execvp(), which is more efficient.
970 Note: exec() and system() do not flush your output buffer, so you may
971 need to set C<$|> to avoid lost output. Examples:
973 exec '/bin/echo', 'Your arguments are: ', @ARGV;
974 exec "sort $outfile | uniq";
976 If you don't really want to execute the first argument, but want to lie
977 to the program you are executing about its own name, you can specify
978 the program you actually want to run as an "indirect object" (without a
979 comma) in front of the LIST. (This always forces interpretation of the
980 LIST as a multi-valued list, even if there is only a single scalar in
984 exec $shell '-sh'; # pretend it's a login shell
988 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
992 Returns TRUE if the specified hash key exists in its hash array, even
993 if the corresponding value is undefined.
995 print "Exists\n" if exists $array{$key};
996 print "Defined\n" if defined $array{$key};
997 print "True\n" if $array{$key};
999 A hash element can be TRUE only if it's defined, and defined if
1000 it exists, but the reverse doesn't necessarily hold true.
1002 Note that the EXPR can be arbitrarily complicated as long as the final
1003 operation is a hash key lookup:
1005 if (exists $ref->[$x][$y]{$key}) { ... }
1009 Evaluates EXPR and exits immediately with that value. (Actually, it
1010 calls any defined C<END> routines first, but the C<END> routines may not
1011 abort the exit. Likewise any object destructors that need to be called
1012 are called before exit.) Example:
1015 exit 0 if $ans =~ /^[Xx]/;
1017 See also die(). If EXPR is omitted, exits with 0 status.
1023 Returns I<e> (the natural logarithm base) to the power of EXPR.
1024 If EXPR is omitted, gives C<exp($_)>.
1026 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1028 Implements the fcntl(2) function. You'll probably have to say
1032 first to get the correct function definitions. Argument processing and
1033 value return works just like ioctl() below. Note that fcntl() will produce
1034 a fatal error if used on a machine that doesn't implement fcntl(2).
1038 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1040 =item fileno FILEHANDLE
1042 Returns the file descriptor for a filehandle. This is useful for
1043 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1044 value is taken as the name of the filehandle.
1046 =item flock FILEHANDLE,OPERATION
1048 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1049 success, FALSE on failure. Will produce a fatal error if used on a
1050 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1051 flock() is Perl's portable file locking interface, although it will lock
1052 only entire files, not records.
1054 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1055 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1056 you can use the symbolic names if you pull them in with an explicit
1057 request to the Fcntl module. The names can be requested as a group with
1058 the :flock tag (or they can be requested individually, of course).
1059 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1060 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1061 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1062 blocking waiting for the lock (check the return status to see if you got
1065 Note that the emulation built with lockf(3) doesn't provide shared
1066 locks, and it requires that FILEHANDLE be open with write intent. These
1067 are the semantics that lockf(3) implements. Most (all?) systems
1068 implement lockf(3) in terms of fcntl(2) locking, though, so the
1069 differing semantics shouldn't bite too many people.
1071 Note also that some versions of flock() cannot lock things over the
1072 network; you would need to use the more system-specific fcntl() for
1073 that. If you like you can force Perl to ignore your system's flock(2)
1074 function, and so provide its own fcntl(2)-based emulation, by passing
1075 the switch C<-Ud_flock> to the F<Configure> program when you configure
1078 Here's a mailbox appender for BSD systems.
1080 use Fcntl ':flock'; # import LOCK_* constants
1083 flock(MBOX,LOCK_EX);
1084 # and, in case someone appended
1085 # while we were waiting...
1090 flock(MBOX,LOCK_UN);
1093 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1094 or die "Can't open mailbox: $!";
1097 print MBOX $msg,"\n\n";
1100 See also L<DB_File> for other flock() examples.
1104 Does a fork(2) system call. Returns the child pid to the parent process
1105 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1106 Note: unflushed buffers remain unflushed in both processes, which means
1107 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1108 autoflush() FileHandle method to avoid duplicate output.
1110 If you fork() without ever waiting on your children, you will accumulate
1113 $SIG{CHLD} = sub { wait };
1115 There's also the double-fork trick (error checking on
1116 fork() returns omitted);
1118 unless ($pid = fork) {
1120 exec "what you really wanna do";
1123 ## (some_perl_code_here)
1130 See also L<perlipc> for more examples of forking and reaping
1135 Declare a picture format with use by the write() function. For
1139 Test: @<<<<<<<< @||||| @>>>>>
1140 $str, $%, '$' . int($num)
1144 $num = $cost/$quantity;
1148 See L<perlform> for many details and examples.
1151 =item formline PICTURE, LIST
1153 This is an internal function used by C<format>s, though you may call it
1154 too. It formats (see L<perlform>) a list of values according to the
1155 contents of PICTURE, placing the output into the format output
1156 accumulator, C<$^A> (or $ACCUMULATOR in English).
1157 Eventually, when a write() is done, the contents of
1158 C<$^A> are written to some filehandle, but you could also read C<$^A>
1159 yourself and then set C<$^A> back to "". Note that a format typically
1160 does one formline() per line of form, but the formline() function itself
1161 doesn't care how many newlines are embedded in the PICTURE. This means
1162 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1163 You may therefore need to use multiple formlines to implement a single
1164 record format, just like the format compiler.
1166 Be careful if you put double quotes around the picture, because an "C<@>"
1167 character may be taken to mean the beginning of an array name.
1168 formline() always returns TRUE. See L<perlform> for other examples.
1170 =item getc FILEHANDLE
1174 Returns the next character from the input file attached to FILEHANDLE,
1175 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1176 This is not particularly efficient. It cannot be used to get unbuffered
1177 single-characters, however. For that, try something more like:
1180 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1183 system "stty", '-icanon', 'eol', "\001";
1189 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1192 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1196 Determination of whether to whether $BSD_STYLE should be set
1197 is left as an exercise to the reader.
1199 See also the C<Term::ReadKey> module from your nearest CPAN site;
1200 details on CPAN can be found on L<perlmod/CPAN>
1204 Returns the current login from F</etc/utmp>, if any. If null, use
1207 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1209 Do not consider getlogin() for authentication: it is not as
1210 secure as getpwuid().
1212 =item getpeername SOCKET
1214 Returns the packed sockaddr address of other end of the SOCKET connection.
1217 $hersockaddr = getpeername(SOCK);
1218 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1219 $herhostname = gethostbyaddr($iaddr, AF_INET);
1220 $herstraddr = inet_ntoa($iaddr);
1224 Returns the current process group for the specified PID. Use
1225 a PID of 0 to get the current process group for the
1226 current process. Will raise an exception if used on a machine that
1227 doesn't implement getpgrp(2). If PID is omitted, returns process
1228 group of current process. Note that the POSIX version of getpgrp()
1229 does not accept a PID argument, so only PID==0 is truly portable.
1233 Returns the process id of the parent process.
1235 =item getpriority WHICH,WHO
1237 Returns the current priority for a process, a process group, or a user.
1238 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1239 machine that doesn't implement getpriority(2).
1245 =item gethostbyname NAME
1247 =item getnetbyname NAME
1249 =item getprotobyname NAME
1255 =item getservbyname NAME,PROTO
1257 =item gethostbyaddr ADDR,ADDRTYPE
1259 =item getnetbyaddr ADDR,ADDRTYPE
1261 =item getprotobynumber NUMBER
1263 =item getservbyport PORT,PROTO
1281 =item sethostent STAYOPEN
1283 =item setnetent STAYOPEN
1285 =item setprotoent STAYOPEN
1287 =item setservent STAYOPEN
1301 These routines perform the same functions as their counterparts in the
1302 system library. Within a list context, the return values from the
1303 various get routines are as follows:
1305 ($name,$passwd,$uid,$gid,
1306 $quota,$comment,$gcos,$dir,$shell) = getpw*
1307 ($name,$passwd,$gid,$members) = getgr*
1308 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1309 ($name,$aliases,$addrtype,$net) = getnet*
1310 ($name,$aliases,$proto) = getproto*
1311 ($name,$aliases,$port,$proto) = getserv*
1313 (If the entry doesn't exist you get a null list.)
1315 Within a scalar context, you get the name, unless the function was a
1316 lookup by name, in which case you get the other thing, whatever it is.
1317 (If the entry doesn't exist you get the undefined value.) For example:
1327 The $members value returned by I<getgr*()> is a space separated list of
1328 the login names of the members of the group.
1330 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1331 C, it will be returned to you via C<$?> if the function call fails. The
1332 @addrs value returned by a successful call is a list of the raw
1333 addresses returned by the corresponding system library call. In the
1334 Internet domain, each address is four bytes long and you can unpack it
1335 by saying something like:
1337 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1339 =item getsockname SOCKET
1341 Returns the packed sockaddr address of this end of the SOCKET connection.
1344 $mysockaddr = getsockname(SOCK);
1345 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1347 =item getsockopt SOCKET,LEVEL,OPTNAME
1349 Returns the socket option requested, or undefined if there is an error.
1353 Returns the value of EXPR with filename expansions such as a shell
1354 would do. This is the internal function implementing the E<lt>*.*E<gt>
1355 operator, except it's easier to use.
1359 Converts a time as returned by the time function to a 9-element array
1360 with the time localized for the standard Greenwich time zone.
1361 Typically used as follows:
1364 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1367 All array elements are numeric, and come straight out of a struct tm.
1368 In particular this means that $mon has the range 0..11 and $wday has
1369 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1377 The goto-LABEL form finds the statement labeled with LABEL and resumes
1378 execution there. It may not be used to go into any construct that
1379 requires initialization, such as a subroutine or a foreach loop. It
1380 also can't be used to go into a construct that is optimized away. It
1381 can be used to go almost anywhere else within the dynamic scope,
1382 including out of subroutines, but it's usually better to use some other
1383 construct such as last or die. The author of Perl has never felt the
1384 need to use this form of goto (in Perl, that is--C is another matter).
1386 The goto-EXPR form expects a label name, whose scope will be resolved
1387 dynamically. This allows for computed gotos per FORTRAN, but isn't
1388 necessarily recommended if you're optimizing for maintainability:
1390 goto ("FOO", "BAR", "GLARCH")[$i];
1392 The goto-&NAME form is highly magical, and substitutes a call to the
1393 named subroutine for the currently running subroutine. This is used by
1394 AUTOLOAD subroutines that wish to load another subroutine and then
1395 pretend that the other subroutine had been called in the first place
1396 (except that any modifications to @_ in the current subroutine are
1397 propagated to the other subroutine.) After the goto, not even caller()
1398 will be able to tell that this routine was called first.
1400 =item grep BLOCK LIST
1402 =item grep EXPR,LIST
1404 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1405 $_ to each element) and returns the list value consisting of those
1406 elements for which the expression evaluated to TRUE. In a scalar
1407 context, returns the number of times the expression was TRUE.
1409 @foo = grep(!/^#/, @bar); # weed out comments
1413 @foo = grep {!/^#/} @bar; # weed out comments
1415 Note that, because $_ is a reference into the list value, it can be used
1416 to modify the elements of the array. While this is useful and
1417 supported, it can cause bizarre results if the LIST is not a named
1424 Interprets EXPR as a hex string and returns the corresponding decimal
1425 value. (To convert strings that might start with 0 or 0x see
1426 oct().) If EXPR is omitted, uses $_.
1430 There is no built-in import() function. It is merely an ordinary
1431 method (subroutine) defined (or inherited) by modules that wish to export
1432 names to another module. The use() function calls the import() method
1433 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1435 =item index STR,SUBSTR,POSITION
1437 =item index STR,SUBSTR
1439 Returns the position of the first occurrence of SUBSTR in STR at or after
1440 POSITION. If POSITION is omitted, starts searching from the beginning of
1441 the string. The return value is based at 0 (or whatever you've set the C<$[>
1442 variable to--but don't do that). If the substring is not found, returns
1443 one less than the base, ordinarily -1.
1449 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1451 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1453 Implements the ioctl(2) function. You'll probably have to say
1455 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1457 first to get the correct function definitions. If F<ioctl.ph> doesn't
1458 exist or doesn't have the correct definitions you'll have to roll your
1459 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1460 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1461 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1462 written depending on the FUNCTION--a pointer to the string value of SCALAR
1463 will be passed as the third argument of the actual ioctl call. (If SCALAR
1464 has no string value but does have a numeric value, that value will be
1465 passed rather than a pointer to the string value. To guarantee this to be
1466 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1467 functions are useful for manipulating the values of structures used by
1468 ioctl(). The following example sets the erase character to DEL.
1472 die "NO TIOCGETP" if $@ || !$getp;
1473 $sgttyb_t = "ccccs"; # 4 chars and a short
1474 if (ioctl(STDIN,$getp,$sgttyb)) {
1475 @ary = unpack($sgttyb_t,$sgttyb);
1477 $sgttyb = pack($sgttyb_t,@ary);
1478 ioctl(STDIN,&TIOCSETP,$sgttyb)
1479 || die "Can't ioctl: $!";
1482 The return value of ioctl (and fcntl) is as follows:
1484 if OS returns: then Perl returns:
1486 0 string "0 but true"
1487 anything else that number
1489 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1490 still easily determine the actual value returned by the operating
1493 ($retval = ioctl(...)) || ($retval = -1);
1494 printf "System returned %d\n", $retval;
1496 =item join EXPR,LIST
1498 Joins the separate strings of LIST or ARRAY into a single string with
1499 fields separated by the value of EXPR, and returns the string.
1502 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1504 See L<perlfunc/split>.
1506 =item keys ASSOC_ARRAY
1508 Returns a normal array consisting of all the keys of the named
1509 associative array. (In a scalar context, returns the number of keys.)
1510 The keys are returned in an apparently random order, but it is the same
1511 order as either the values() or each() function produces (given that
1512 the associative array has not been modified). Here is yet another way
1513 to print your environment:
1516 @values = values %ENV;
1517 while ($#keys >= 0) {
1518 print pop(@keys), '=', pop(@values), "\n";
1521 or how about sorted by key:
1523 foreach $key (sort(keys %ENV)) {
1524 print $key, '=', $ENV{$key}, "\n";
1527 To sort an array by value, you'll need to use a C<sort{}>
1528 function. Here's a descending numeric sort of a hash by its values:
1530 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1531 printf "%4d %s\n", $hash{$key}, $key;
1534 As an lvalue C<keys> allows you to increase the number of hash buckets
1535 allocated for the given associative array. This can gain you a measure
1536 of efficiency if you know the hash is going to get big. (This is
1537 similar to pre-extending an array by assigning a larger number to
1538 $#array.) If you say
1542 then C<%hash> will have at least 200 buckets allocated for it. These
1543 buckets will be retained even if you do C<%hash = ()>, use C<undef
1544 %hash> if you want to free the storage while C<%hash> is still in scope.
1545 You can't shrink the number of buckets allocated for the hash using
1546 C<keys> in this way (but you needn't worry about doing this by accident,
1547 as trying has no effect).
1551 Sends a signal to a list of processes. The first element of
1552 the list must be the signal to send. Returns the number of
1553 processes successfully signaled.
1555 $cnt = kill 1, $child1, $child2;
1558 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1559 process groups instead of processes. (On System V, a negative I<PROCESS>
1560 number will also kill process groups, but that's not portable.) That
1561 means you usually want to use positive not negative signals. You may also
1562 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1568 The C<last> command is like the C<break> statement in C (as used in
1569 loops); it immediately exits the loop in question. If the LABEL is
1570 omitted, the command refers to the innermost enclosing loop. The
1571 C<continue> block, if any, is not executed:
1573 LINE: while (<STDIN>) {
1574 last LINE if /^$/; # exit when done with header
1582 Returns an lowercased version of EXPR. This is the internal function
1583 implementing the \L escape in double-quoted strings.
1584 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1586 If EXPR is omitted, uses $_.
1592 Returns the value of EXPR with the first character lowercased. This is
1593 the internal function implementing the \l escape in double-quoted strings.
1594 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1596 If EXPR is omitted, uses $_.
1602 Returns the length in characters of the value of EXPR. If EXPR is
1603 omitted, returns length of $_.
1605 =item link OLDFILE,NEWFILE
1607 Creates a new filename linked to the old filename. Returns 1 for
1608 success, 0 otherwise.
1610 =item listen SOCKET,QUEUESIZE
1612 Does the same thing that the listen system call does. Returns TRUE if
1613 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1617 A local modifies the listed variables to be local to the enclosing block,
1618 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1619 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1620 local()"> for details.
1622 But you really probably want to be using my() instead, because local() isn't
1623 what most people think of as "local"). See L<perlsub/"Private Variables
1624 via my()"> for details.
1626 =item localtime EXPR
1628 Converts a time as returned by the time function to a 9-element array
1629 with the time analyzed for the local time zone. Typically used as
1632 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1635 All array elements are numeric, and come straight out of a struct tm.
1636 In particular this means that $mon has the range 0..11 and $wday has
1637 the range 0..6. If EXPR is omitted, does localtime(time).
1639 In a scalar context, prints out the ctime(3) value:
1641 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1643 Also see the F<timelocal.pl> library, and the strftime(3) function available
1644 via the POSIX module.
1650 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1653 =item lstat FILEHANDLE
1659 Does the same thing as the stat() function, but stats a symbolic link
1660 instead of the file the symbolic link points to. If symbolic links are
1661 unimplemented on your system, a normal stat() is done.
1663 If EXPR is omitted, stats $_.
1667 The match operator. See L<perlop>.
1669 =item map BLOCK LIST
1673 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1674 element) and returns the list value composed of the results of each such
1675 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1676 may produce zero, one, or more elements in the returned value.
1678 @chars = map(chr, @nums);
1680 translates a list of numbers to the corresponding characters. And
1682 %hash = map { getkey($_) => $_ } @array;
1684 is just a funny way to write
1687 foreach $_ (@array) {
1688 $hash{getkey($_)} = $_;
1691 =item mkdir FILENAME,MODE
1693 Creates the directory specified by FILENAME, with permissions specified
1694 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1695 it returns 0 and sets C<$!> (errno).
1697 =item msgctl ID,CMD,ARG
1699 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1700 must be a variable which will hold the returned msqid_ds structure.
1701 Returns like ioctl: the undefined value for error, "0 but true" for
1702 zero, or the actual return value otherwise.
1704 =item msgget KEY,FLAGS
1706 Calls the System V IPC function msgget(2). Returns the message queue id,
1707 or the undefined value if there is an error.
1709 =item msgsnd ID,MSG,FLAGS
1711 Calls the System V IPC function msgsnd to send the message MSG to the
1712 message queue ID. MSG must begin with the long integer message type,
1713 which may be created with C<pack("l", $type)>. Returns TRUE if
1714 successful, or FALSE if there is an error.
1716 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1718 Calls the System V IPC function msgrcv to receive a message from
1719 message queue ID into variable VAR with a maximum message size of
1720 SIZE. Note that if a message is received, the message type will be the
1721 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1722 of the message type. Returns TRUE if successful, or FALSE if there is
1727 A "my" declares the listed variables to be local (lexically) to the
1728 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1729 more than one value is listed, the list must be placed in parentheses. See
1730 L<perlsub/"Private Variables via my()"> for details.
1736 The C<next> command is like the C<continue> statement in C; it starts
1737 the next iteration of the loop:
1739 LINE: while (<STDIN>) {
1740 next LINE if /^#/; # discard comments
1744 Note that if there were a C<continue> block on the above, it would get
1745 executed even on discarded lines. If the LABEL is omitted, the command
1746 refers to the innermost enclosing loop.
1748 =item no Module LIST
1750 See the "use" function, which "no" is the opposite of.
1756 Interprets EXPR as an octal string and returns the corresponding
1757 decimal value. (If EXPR happens to start off with 0x, interprets it as
1758 a hex string instead.) The following will handle decimal, octal, and
1759 hex in the standard Perl or C notation:
1761 $val = oct($val) if $val =~ /^0/;
1763 If EXPR is omitted, uses $_.
1765 =item open FILEHANDLE,EXPR
1767 =item open FILEHANDLE
1769 Opens the file whose filename is given by EXPR, and associates it with
1770 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1771 name of the real filehandle wanted. If EXPR is omitted, the scalar
1772 variable of the same name as the FILEHANDLE contains the filename.
1773 (Note that lexical variables--those declared with C<my>--will not work
1774 for this purpose; so if you're using C<my>, specify EXPR in your call
1777 If the filename begins with '<' or nothing, the file is opened for input.
1778 If the filename begins with '>', the file is truncated and opened for
1779 output. If the filename begins with '>>', the file is opened for
1780 appending. You can put a '+' in front of the '>' or '<' to indicate that
1781 you want both read and write access to the file; thus '+<' is almost
1782 always preferred for read/write updates--the '+>' mode would clobber the
1783 file first. The prefix and the filename may be separated with spaces.
1784 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1785 'w+', 'a', and 'a+'.
1787 If the filename begins with "|", the filename is interpreted as a command
1788 to which output is to be piped, and if the filename ends with a "|", the
1789 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1790 examples of this. as command which pipes input to us. (You may not have
1791 a raw open() to a command that pipes both in I<and> out, but see
1792 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1795 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1796 non-zero upon success, the undefined value otherwise. If the open
1797 involved a pipe, the return value happens to be the pid of the
1800 If you're unfortunate enough to be running Perl on a system that
1801 distinguishes between text files and binary files (modern operating
1802 systems don't care), then you should check out L</binmode> for tips for
1803 dealing with this. The key distinction between systems that need binmode
1804 and those that don't is their text file formats. Systems like Unix and
1805 Plan9 that delimit lines with a single character, and that encode that
1806 character in C as '\n', do not need C<binmode>. The rest need it.
1811 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1812 while (<ARTICLE>) {...
1814 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1816 open(DBASE, '+<dbase.mine'); # open for update
1818 open(ARTICLE, "caesar <$article |"); # decrypt article
1820 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1822 # process argument list of files along with any includes
1824 foreach $file (@ARGV) {
1825 process($file, 'fh00');
1829 local($filename, $input) = @_;
1830 $input++; # this is a string increment
1831 unless (open($input, $filename)) {
1832 print STDERR "Can't open $filename: $!\n";
1836 while (<$input>) { # note use of indirection
1837 if (/^#include "(.*)"/) {
1838 process($1, $input);
1845 You may also, in the Bourne shell tradition, specify an EXPR beginning
1846 with "E<gt>&", in which case the rest of the string is interpreted as the
1847 name of a filehandle (or file descriptor, if numeric) which is to be
1848 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1849 +E<gt>E<gt>, and +E<lt>. The
1850 mode you specify should match the mode of the original filehandle.
1851 (Duping a filehandle does not take into account any existing contents of
1853 Here is a script that saves, redirects, and restores STDOUT and
1857 open(SAVEOUT, ">&STDOUT");
1858 open(SAVEERR, ">&STDERR");
1860 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1861 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1863 select(STDERR); $| = 1; # make unbuffered
1864 select(STDOUT); $| = 1; # make unbuffered
1866 print STDOUT "stdout 1\n"; # this works for
1867 print STDERR "stderr 1\n"; # subprocesses too
1872 open(STDOUT, ">&SAVEOUT");
1873 open(STDERR, ">&SAVEERR");
1875 print STDOUT "stdout 2\n";
1876 print STDERR "stderr 2\n";
1879 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1880 equivalent of C's fdopen() of that file descriptor; this is more
1881 parsimonious of file descriptors. For example:
1883 open(FILEHANDLE, "<&=$fd")
1885 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1886 there is an implicit fork done, and the return value of open is the pid
1887 of the child within the parent process, and 0 within the child
1888 process. (Use C<defined($pid)> to determine whether the open was successful.)
1889 The filehandle behaves normally for the parent, but i/o to that
1890 filehandle is piped from/to the STDOUT/STDIN of the child process.
1891 In the child process the filehandle isn't opened--i/o happens from/to
1892 the new STDOUT or STDIN. Typically this is used like the normal
1893 piped open when you want to exercise more control over just how the
1894 pipe command gets executed, such as when you are running setuid, and
1895 don't want to have to scan shell commands for metacharacters.
1896 The following pairs are more or less equivalent:
1898 open(FOO, "|tr '[a-z]' '[A-Z]'");
1899 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1901 open(FOO, "cat -n '$file'|");
1902 open(FOO, "-|") || exec 'cat', '-n', $file;
1904 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1906 Explicitly closing any piped filehandle causes the parent process to
1907 wait for the child to finish, and returns the status value in C<$?>.
1908 Note: on any operation which may do a fork, unflushed buffers remain
1909 unflushed in both processes, which means you may need to set C<$|> to
1910 avoid duplicate output.
1912 Using the constructor from the IO::Handle package (or one of its
1913 subclasses, such as IO::File or IO::Socket),
1914 you can generate anonymous filehandles which have the scope of whatever
1915 variables hold references to them, and automatically close whenever
1916 and however you leave that scope:
1920 sub read_myfile_munged {
1922 my $handle = new IO::File;
1923 open($handle, "myfile") or die "myfile: $!";
1925 or return (); # Automatically closed here.
1926 mung $first or die "mung failed"; # Or here.
1927 return $first, <$handle> if $ALL; # Or here.
1931 The filename that is passed to open will have leading and trailing
1932 whitespace deleted. To open a file with arbitrary weird
1933 characters in it, it's necessary to protect any leading and trailing
1936 $file =~ s#^(\s)#./$1#;
1937 open(FOO, "< $file\0");
1939 If you want a "real" C open() (see L<open(2)> on your system), then
1940 you should use the sysopen() function. This is another way to
1941 protect your filenames from interpretation. For example:
1944 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1945 or die "sysopen $path: $!";
1946 HANDLE->autoflush(1);
1947 HANDLE->print("stuff $$\n");
1949 print "File contains: ", <HANDLE>;
1951 See L</seek()> for some details about mixing reading and writing.
1953 =item opendir DIRHANDLE,EXPR
1955 Opens a directory named EXPR for processing by readdir(), telldir(),
1956 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
1957 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1963 Returns the numeric ascii value of the first character of EXPR. If
1964 EXPR is omitted, uses $_.
1966 =item pack TEMPLATE,LIST
1968 Takes an array or list of values and packs it into a binary structure,
1969 returning the string containing the structure. The TEMPLATE is a
1970 sequence of characters that give the order and type of values, as
1973 A An ascii string, will be space padded.
1974 a An ascii string, will be null padded.
1975 b A bit string (ascending bit order, like vec()).
1976 B A bit string (descending bit order).
1977 h A hex string (low nybble first).
1978 H A hex string (high nybble first).
1980 c A signed char value.
1981 C An unsigned char value.
1982 s A signed short value.
1983 S An unsigned short value.
1984 i A signed integer value.
1985 I An unsigned integer value.
1986 l A signed long value.
1987 L An unsigned long value.
1989 n A short in "network" order.
1990 N A long in "network" order.
1991 v A short in "VAX" (little-endian) order.
1992 V A long in "VAX" (little-endian) order.
1994 f A single-precision float in the native format.
1995 d A double-precision float in the native format.
1997 p A pointer to a null-terminated string.
1998 P A pointer to a structure (fixed-length string).
2000 u A uuencoded string.
2002 w A BER compressed integer. Bytes give an unsigned integer base
2003 128, most significant digit first, with as few digits as
2004 possible, and with the bit 8 of each byte except the last set
2009 @ Null fill to absolute position.
2011 Each letter may optionally be followed by a number which gives a repeat
2012 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2013 pack function will gobble up that many values from the LIST. A * for the
2014 repeat count means to use however many items are left. The "a" and "A"
2015 types gobble just one value, but pack it as a string of length count,
2016 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2017 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2018 fields pack a string that many bits long. The "h" and "H" fields pack a
2019 string that many nybbles long. The "P" packs a pointer to a structure of
2020 the size indicated by the length. Real numbers (floats and doubles) are
2021 in the native machine format only; due to the multiplicity of floating
2022 formats around, and the lack of a standard "network" representation, no
2023 facility for interchange has been made. This means that packed floating
2024 point data written on one machine may not be readable on another - even if
2025 both use IEEE floating point arithmetic (as the endian-ness of the memory
2026 representation is not part of the IEEE spec). Note that Perl uses doubles
2027 internally for all numeric calculation, and converting from double into
2028 float and thence back to double again will lose precision (i.e.,
2029 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2033 $foo = pack("cccc",65,66,67,68);
2035 $foo = pack("c4",65,66,67,68);
2038 $foo = pack("ccxxcc",65,66,67,68);
2041 $foo = pack("s2",1,2);
2042 # "\1\0\2\0" on little-endian
2043 # "\0\1\0\2" on big-endian
2045 $foo = pack("a4","abcd","x","y","z");
2048 $foo = pack("aaaa","abcd","x","y","z");
2051 $foo = pack("a14","abcdefg");
2052 # "abcdefg\0\0\0\0\0\0\0"
2054 $foo = pack("i9pl", gmtime);
2055 # a real struct tm (on my system anyway)
2058 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2061 The same template may generally also be used in the unpack function.
2063 =item package NAMESPACE
2065 Declares the compilation unit as being in the given namespace. The scope
2066 of the package declaration is from the declaration itself through the end of
2067 the enclosing block (the same scope as the local() operator). All further
2068 unqualified dynamic identifiers will be in this namespace. A package
2069 statement affects only dynamic variables--including those you've used
2070 local() on--but I<not> lexical variables created with my(). Typically it
2071 would be the first declaration in a file to be included by the C<require>
2072 or C<use> operator. You can switch into a package in more than one place;
2073 it influences merely which symbol table is used by the compiler for the
2074 rest of that block. You can refer to variables and filehandles in other
2075 packages by prefixing the identifier with the package name and a double
2076 colon: C<$Package::Variable>. If the package name is null, the C<main>
2077 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2079 See L<perlmod/"Packages"> for more information about packages, modules,
2080 and classes. See L<perlsub> for other scoping issues.
2082 =item pipe READHANDLE,WRITEHANDLE
2084 Opens a pair of connected pipes like the corresponding system call.
2085 Note that if you set up a loop of piped processes, deadlock can occur
2086 unless you are very careful. In addition, note that Perl's pipes use
2087 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2088 after each command, depending on the application.
2090 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2091 for examples of such things.
2095 Pops and returns the last value of the array, shortening the array by
2096 1. Has a similar effect to
2098 $tmp = $ARRAY[$#ARRAY--];
2100 If there are no elements in the array, returns the undefined value.
2101 If ARRAY is omitted, pops the
2102 @ARGV array in the main program, and the @_ array in subroutines, just
2109 Returns the offset of where the last C<m//g> search left off for the variable
2110 is in question ($_ is used when the variable is not specified). May be
2111 modified to change that offset.
2113 =item print FILEHANDLE LIST
2119 Prints a string or a comma-separated list of strings. Returns TRUE
2120 if successful. FILEHANDLE may be a scalar variable name, in which case
2121 the variable contains the name of or a reference to the filehandle, thus introducing one
2122 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2123 token is a term, it may be misinterpreted as an operator unless you
2124 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2125 omitted, prints by default to standard output (or to the last selected
2126 output channel--see L</select>). If LIST is also omitted, prints $_ to
2127 STDOUT. To set the default output channel to something other than
2128 STDOUT use the select operation. Note that, because print takes a
2129 LIST, anything in the LIST is evaluated in a list context, and any
2130 subroutine that you call will have one or more of its expressions
2131 evaluated in a list context. Also be careful not to follow the print
2132 keyword with a left parenthesis unless you want the corresponding right
2133 parenthesis to terminate the arguments to the print--interpose a + or
2134 put parentheses around all the arguments.
2136 Note that if you're storing FILEHANDLES in an array or other expression,
2137 you will have to use a block returning its value instead:
2139 print { $files[$i] } "stuff\n";
2140 print { $OK ? STDOUT : STDERR } "stuff\n";
2142 =item printf FILEHANDLE FORMAT, LIST
2144 =item printf FORMAT, LIST
2146 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2147 of the list will be interpreted as the printf format. If C<use locale> is
2148 in effect, the character used for the decimal point in formatted real numbers
2149 is affected by the LC_NUMERIC locale. See L<perllocale>.
2151 =item prototype FUNCTION
2153 Returns the prototype of a function as a string (or C<undef> if the
2154 function has no prototype). FUNCTION is a reference to, or the name of,
2155 the function whose prototype you want to retrieve.
2157 =item push ARRAY,LIST
2159 Treats ARRAY as a stack, and pushes the values of LIST
2160 onto the end of ARRAY. The length of ARRAY increases by the length of
2161 LIST. Has the same effect as
2164 $ARRAY[++$#ARRAY] = $value;
2167 but is more efficient. Returns the new number of elements in the array.
2177 Generalized quotes. See L<perlop>.
2179 =item quotemeta EXPR
2183 Returns the value of EXPR with with all non-alphanumeric
2184 characters backslashed. (That is, all characters not matching
2185 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2186 returned string, regardless of any locale settings.)
2187 This is the internal function implementing
2188 the \Q escape in double-quoted strings.
2190 If EXPR is omitted, uses $_.
2196 Returns a random fractional number between 0 and the value of EXPR.
2197 (EXPR should be positive.) If EXPR is omitted, returns a value between
2198 0 and 1. This function produces repeatable sequences unless srand()
2199 is invoked. See also srand().
2201 (Note: if your rand function consistently returns numbers that are too
2202 large or too small, then your version of Perl was probably compiled
2203 with the wrong number of RANDBITS. As a workaround, you can usually
2204 multiply EXPR by the correct power of 2 to get the range you want.
2205 This will make your script unportable, however. It's better to recompile
2208 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2210 =item read FILEHANDLE,SCALAR,LENGTH
2212 Attempts to read LENGTH bytes of data into variable SCALAR from the
2213 specified FILEHANDLE. Returns the number of bytes actually read, or
2214 undef if there was an error. SCALAR will be grown or shrunk to the
2215 length actually read. An OFFSET may be specified to place the read
2216 data at some other place than the beginning of the string. This call
2217 is actually implemented in terms of stdio's fread call. To get a true
2218 read system call, see sysread().
2220 =item readdir DIRHANDLE
2222 Returns the next directory entry for a directory opened by opendir().
2223 If used in a list context, returns all the rest of the entries in the
2224 directory. If there are no more entries, returns an undefined value in
2225 a scalar context or a null list in a list context.
2227 If you're planning to filetest the return values out of a readdir(), you'd
2228 better prepend the directory in question. Otherwise, because we didn't
2229 chdir() there, it would have been testing the wrong file.
2231 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2232 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2239 Returns the value of a symbolic link, if symbolic links are
2240 implemented. If not, gives a fatal error. If there is some system
2241 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2244 =item recv SOCKET,SCALAR,LEN,FLAGS
2246 Receives a message on a socket. Attempts to receive LENGTH bytes of
2247 data into variable SCALAR from the specified SOCKET filehandle.
2248 Actually does a C recvfrom(), so that it can returns the address of the
2249 sender. Returns the undefined value if there's an error. SCALAR will
2250 be grown or shrunk to the length actually read. Takes the same flags
2251 as the system call of the same name.
2252 See L<perlipc/"UDP: Message Passing"> for examples.
2258 The C<redo> command restarts the loop block without evaluating the
2259 conditional again. The C<continue> block, if any, is not executed. If
2260 the LABEL is omitted, the command refers to the innermost enclosing
2261 loop. This command is normally used by programs that want to lie to
2262 themselves about what was just input:
2264 # a simpleminded Pascal comment stripper
2265 # (warning: assumes no { or } in strings)
2266 LINE: while (<STDIN>) {
2267 while (s|({.*}.*){.*}|$1 |) {}
2272 if (/}/) { # end of comment?
2285 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2286 is not specified, $_ will be used. The value returned depends on the
2287 type of thing the reference is a reference to.
2288 Builtin types include:
2297 If the referenced object has been blessed into a package, then that package
2298 name is returned instead. You can think of ref() as a typeof() operator.
2300 if (ref($r) eq "HASH") {
2301 print "r is a reference to an associative array.\n";
2304 print "r is not a reference at all.\n";
2307 See also L<perlref>.
2309 =item rename OLDNAME,NEWNAME
2311 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2312 not work across file system boundaries.
2318 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2319 supplied. If EXPR is numeric, demands that the current version of Perl
2320 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2322 Otherwise, demands that a library file be included if it hasn't already
2323 been included. The file is included via the do-FILE mechanism, which is
2324 essentially just a variety of eval(). Has semantics similar to the following
2328 local($filename) = @_;
2329 return 1 if $INC{$filename};
2330 local($realfilename,$result);
2332 foreach $prefix (@INC) {
2333 $realfilename = "$prefix/$filename";
2334 if (-f $realfilename) {
2335 $result = do $realfilename;
2339 die "Can't find $filename in \@INC";
2342 die "$filename did not return true value" unless $result;
2343 $INC{$filename} = $realfilename;
2347 Note that the file will not be included twice under the same specified
2348 name. The file must return TRUE as the last statement to indicate
2349 successful execution of any initialization code, so it's customary to
2350 end such a file with "1;" unless you're sure it'll return TRUE
2351 otherwise. But it's better just to put the "C<1;>", in case you add more
2354 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2355 replaces "F<::>" with "F</>" in the filename for you,
2356 to make it easy to load standard modules. This form of loading of
2357 modules does not risk altering your namespace.
2359 For a yet-more-powerful import facility, see L</use> and
2366 Generally used in a C<continue> block at the end of a loop to clear
2367 variables and reset ?? searches so that they work again. The
2368 expression is interpreted as a list of single characters (hyphens
2369 allowed for ranges). All variables and arrays beginning with one of
2370 those letters are reset to their pristine state. If the expression is
2371 omitted, one-match searches (?pattern?) are reset to match again. Resets
2372 only variables or searches in the current package. Always returns
2375 reset 'X'; # reset all X variables
2376 reset 'a-z'; # reset lower case variables
2377 reset; # just reset ?? searches
2379 Resetting "A-Z" is not recommended because you'll wipe out your
2380 ARGV and ENV arrays. Resets only package variables--lexical variables
2381 are unaffected, but they clean themselves up on scope exit anyway,
2382 so you'll probably want to use them instead. See L</my>.
2386 Returns from a subroutine or eval with the value specified. (Note that
2387 in the absence of a return a subroutine or eval() will automatically
2388 return the value of the last expression evaluated.)
2392 In a list context, returns a list value consisting of the elements
2393 of LIST in the opposite order. In a scalar context, returns a string
2394 value consisting of the bytes of the first element of LIST in the
2397 print reverse <>; # line tac
2400 print scalar reverse scalar <>; # byte tac
2402 =item rewinddir DIRHANDLE
2404 Sets the current position to the beginning of the directory for the
2405 readdir() routine on DIRHANDLE.
2407 =item rindex STR,SUBSTR,POSITION
2409 =item rindex STR,SUBSTR
2411 Works just like index except that it returns the position of the LAST
2412 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2413 last occurrence at or before that position.
2415 =item rmdir FILENAME
2419 Deletes the directory specified by FILENAME if it is empty. If it
2420 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2421 FILENAME is omitted, uses $_.
2425 The substitution operator. See L<perlop>.
2429 Forces EXPR to be interpreted in a scalar context and returns the value
2432 @counts = ( scalar @a, scalar @b, scalar @c );
2434 There is no equivalent operator to force an expression to
2435 be interpolated in a list context because it's in practice never
2436 needed. If you really wanted to do so, however, you could use
2437 the construction C<@{[ (some expression) ]}>, but usually a simple
2438 C<(some expression)> suffices.
2440 =item seek FILEHANDLE,POSITION,WHENCE
2442 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2443 call of stdio. FILEHANDLE may be an expression whose value gives the name
2444 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2445 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2446 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2447 this from POSIX module. Returns 1 upon success, 0 otherwise.
2449 On some systems you have to do a seek whenever you switch between reading
2450 and writing. Amongst other things, this may have the effect of calling
2451 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2456 This is also useful for applications emulating C<tail -f>. Once you hit
2457 EOF on your read, and then sleep for a while, you might have to stick in a
2458 seek() to reset things. First the simple trick listed above to clear the
2459 filepointer. The seek() doesn't change the current position, but it
2460 I<does> clear the end-of-file condition on the handle, so that the next
2461 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2463 If that doesn't work (some stdios are particularly cantankerous), then
2464 you may need something more like this:
2467 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2468 # search for some stuff and put it into files
2470 sleep($for_a_while);
2471 seek(FILE, $curpos, 0);
2474 =item seekdir DIRHANDLE,POS
2476 Sets the current position for the readdir() routine on DIRHANDLE. POS
2477 must be a value returned by telldir(). Has the same caveats about
2478 possible directory compaction as the corresponding system library
2481 =item select FILEHANDLE
2485 Returns the currently selected filehandle. Sets the current default
2486 filehandle for output, if FILEHANDLE is supplied. This has two
2487 effects: first, a C<write> or a C<print> without a filehandle will
2488 default to this FILEHANDLE. Second, references to variables related to
2489 output will refer to this output channel. For example, if you have to
2490 set the top of form format for more than one output channel, you might
2498 FILEHANDLE may be an expression whose value gives the name of the
2499 actual filehandle. Thus:
2501 $oldfh = select(STDERR); $| = 1; select($oldfh);
2503 Some programmers may prefer to think of filehandles as objects with
2504 methods, preferring to write the last example as:
2507 STDERR->autoflush(1);
2509 =item select RBITS,WBITS,EBITS,TIMEOUT
2511 This calls the select(2) system call with the bit masks specified, which
2512 can be constructed using fileno() and vec(), along these lines:
2514 $rin = $win = $ein = '';
2515 vec($rin,fileno(STDIN),1) = 1;
2516 vec($win,fileno(STDOUT),1) = 1;
2519 If you want to select on many filehandles you might wish to write a
2523 local(@fhlist) = split(' ',$_[0]);
2526 vec($bits,fileno($_),1) = 1;
2530 $rin = fhbits('STDIN TTY SOCK');
2534 ($nfound,$timeleft) =
2535 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2537 or to block until something becomes ready just do this
2539 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2541 Most systems do not bother to return anything useful in $timeleft, so
2542 calling select() in a scalar context just returns $nfound.
2544 Any of the bit masks can also be undef. The timeout, if specified, is
2545 in seconds, which may be fractional. Note: not all implementations are
2546 capable of returning the $timeleft. If not, they always return
2547 $timeleft equal to the supplied $timeout.
2549 You can effect a sleep of 250 milliseconds this way:
2551 select(undef, undef, undef, 0.25);
2553 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2554 with select(). You have to use sysread() instead.
2556 =item semctl ID,SEMNUM,CMD,ARG
2558 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2559 &GETALL, then ARG must be a variable which will hold the returned
2560 semid_ds structure or semaphore value array. Returns like ioctl: the
2561 undefined value for error, "0 but true" for zero, or the actual return
2564 =item semget KEY,NSEMS,FLAGS
2566 Calls the System V IPC function semget. Returns the semaphore id, or
2567 the undefined value if there is an error.
2569 =item semop KEY,OPSTRING
2571 Calls the System V IPC function semop to perform semaphore operations
2572 such as signaling and waiting. OPSTRING must be a packed array of
2573 semop structures. Each semop structure can be generated with
2574 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2575 operations is implied by the length of OPSTRING. Returns TRUE if
2576 successful, or FALSE if there is an error. As an example, the
2577 following code waits on semaphore $semnum of semaphore id $semid:
2579 $semop = pack("sss", $semnum, -1, 0);
2580 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2582 To signal the semaphore, replace "-1" with "1".
2584 =item send SOCKET,MSG,FLAGS,TO
2586 =item send SOCKET,MSG,FLAGS
2588 Sends a message on a socket. Takes the same flags as the system call
2589 of the same name. On unconnected sockets you must specify a
2590 destination to send TO, in which case it does a C sendto(). Returns
2591 the number of characters sent, or the undefined value if there is an
2593 See L<perlipc/"UDP: Message Passing"> for examples.
2595 =item setpgrp PID,PGRP
2597 Sets the current process group for the specified PID, 0 for the current
2598 process. Will produce a fatal error if used on a machine that doesn't
2599 implement setpgrp(2). If the arguments are omitted, it defaults to
2600 0,0. Note that the POSIX version of setpgrp() does not accept any
2601 arguments, so only setpgrp 0,0 is portable.
2603 =item setpriority WHICH,WHO,PRIORITY
2605 Sets the current priority for a process, a process group, or a user.
2606 (See setpriority(2).) Will produce a fatal error if used on a machine
2607 that doesn't implement setpriority(2).
2609 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2611 Sets the socket option requested. Returns undefined if there is an
2612 error. OPTVAL may be specified as undef if you don't want to pass an
2619 Shifts the first value of the array off and returns it, shortening the
2620 array by 1 and moving everything down. If there are no elements in the
2621 array, returns the undefined value. If ARRAY is omitted, shifts the
2622 @ARGV array in the main program, and the @_ array in subroutines.
2623 (This is determined lexically.) See also unshift(), push(), and pop().
2624 Shift() and unshift() do the same thing to the left end of an array
2625 that push() and pop() do to the right end.
2627 =item shmctl ID,CMD,ARG
2629 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2630 must be a variable which will hold the returned shmid_ds structure.
2631 Returns like ioctl: the undefined value for error, "0 but true" for
2632 zero, or the actual return value otherwise.
2634 =item shmget KEY,SIZE,FLAGS
2636 Calls the System V IPC function shmget. Returns the shared memory
2637 segment id, or the undefined value if there is an error.
2639 =item shmread ID,VAR,POS,SIZE
2641 =item shmwrite ID,STRING,POS,SIZE
2643 Reads or writes the System V shared memory segment ID starting at
2644 position POS for size SIZE by attaching to it, copying in/out, and
2645 detaching from it. When reading, VAR must be a variable which will
2646 hold the data read. When writing, if STRING is too long, only SIZE
2647 bytes are used; if STRING is too short, nulls are written to fill out
2648 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2650 =item shutdown SOCKET,HOW
2652 Shuts down a socket connection in the manner indicated by HOW, which
2653 has the same interpretation as in the system call of the same name.
2659 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2666 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2667 May be interrupted by sending the process a SIGALRM. Returns the
2668 number of seconds actually slept. You probably cannot mix alarm() and
2669 sleep() calls, because sleep() is often implemented using alarm().
2671 On some older systems, it may sleep up to a full second less than what
2672 you requested, depending on how it counts seconds. Most modern systems
2673 always sleep the full amount.
2675 For delays of finer granularity than one second, you may use Perl's
2676 syscall() interface to access setitimer(2) if your system supports it,
2677 or else see L</select()> below.
2679 See also the POSIX module's sigpause() function.
2681 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2683 Opens a socket of the specified kind and attaches it to filehandle
2684 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2685 system call of the same name. You should "use Socket;" first to get
2686 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2688 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2690 Creates an unnamed pair of sockets in the specified domain, of the
2691 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2692 for the system call of the same name. If unimplemented, yields a fatal
2693 error. Returns TRUE if successful.
2695 =item sort SUBNAME LIST
2697 =item sort BLOCK LIST
2701 Sorts the LIST and returns the sorted list value. Nonexistent values
2702 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2703 in standard string comparison order. If SUBNAME is specified, it
2704 gives the name of a subroutine that returns an integer less than, equal
2705 to, or greater than 0, depending on how the elements of the array are
2706 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2707 routines.) SUBNAME may be a scalar variable name, in which case the
2708 value provides the name of the subroutine to use. In place of a
2709 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2712 In the interests of efficiency the normal calling code for subroutines is
2713 bypassed, with the following effects: the subroutine may not be a
2714 recursive subroutine, and the two elements to be compared are passed into
2715 the subroutine not via @_ but as the package global variables $a and
2716 $b (see example below). They are passed by reference, so don't
2717 modify $a and $b. And don't try to declare them as lexicals either.
2719 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2720 current collation locale. See L<perllocale>.
2725 @articles = sort @files;
2727 # same thing, but with explicit sort routine
2728 @articles = sort {$a cmp $b} @files;
2730 # now case-insensitively
2731 @articles = sort { uc($a) cmp uc($b)} @files;
2733 # same thing in reversed order
2734 @articles = sort {$b cmp $a} @files;
2736 # sort numerically ascending
2737 @articles = sort {$a <=> $b} @files;
2739 # sort numerically descending
2740 @articles = sort {$b <=> $a} @files;
2742 # sort using explicit subroutine name
2744 $age{$a} <=> $age{$b}; # presuming integers
2746 @sortedclass = sort byage @class;
2748 # this sorts the %age associative arrays by value
2749 # instead of key using an in-line function
2750 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2752 sub backwards { $b cmp $a; }
2753 @harry = ('dog','cat','x','Cain','Abel');
2754 @george = ('gone','chased','yz','Punished','Axed');
2756 # prints AbelCaincatdogx
2757 print sort backwards @harry;
2758 # prints xdogcatCainAbel
2759 print sort @george, 'to', @harry;
2760 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2762 # inefficiently sort by descending numeric compare using
2763 # the first integer after the first = sign, or the
2764 # whole record case-insensitively otherwise
2767 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2772 # same thing, but much more efficiently;
2773 # we'll build auxiliary indices instead
2777 push @nums, /=(\d+)/;
2782 $nums[$b] <=> $nums[$a]
2784 $caps[$a] cmp $caps[$b]
2788 # same thing using a Schwartzian Transform (no temps)
2789 @new = map { $_->[0] }
2790 sort { $b->[1] <=> $a->[1]
2793 } map { [$_, /=(\d+)/, uc($_)] } @old;
2795 If you're using strict, you I<MUST NOT> declare $a
2796 and $b as lexicals. They are package globals. That means
2797 if you're in the C<main> package, it's
2799 @articles = sort {$main::b <=> $main::a} @files;
2803 @articles = sort {$::b <=> $::a} @files;
2805 but if you're in the C<FooPack> package, it's
2807 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2809 The comparison function is required to behave. If it returns
2810 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2811 sometimes saying the opposite, for example) the Perl interpreter will
2812 probably crash and dump core. This is entirely due to and dependent
2813 upon your system's qsort(3) library routine; this routine often avoids
2814 sanity checks in the interest of speed.
2816 =item splice ARRAY,OFFSET,LENGTH,LIST
2818 =item splice ARRAY,OFFSET,LENGTH
2820 =item splice ARRAY,OFFSET
2822 Removes the elements designated by OFFSET and LENGTH from an array, and
2823 replaces them with the elements of LIST, if any. Returns the elements
2824 removed from the array. The array grows or shrinks as necessary. If
2825 LENGTH is omitted, removes everything from OFFSET onward. The
2826 following equivalences hold (assuming C<$[ == 0>):
2828 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2829 pop(@a) splice(@a,-1)
2830 shift(@a) splice(@a,0,1)
2831 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2832 $a[$x] = $y splice(@a,$x,1,$y);
2834 Example, assuming array lengths are passed before arrays:
2836 sub aeq { # compare two list values
2837 local(@a) = splice(@_,0,shift);
2838 local(@b) = splice(@_,0,shift);
2839 return 0 unless @a == @b; # same len?
2841 return 0 if pop(@a) ne pop(@b);
2845 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2847 =item split /PATTERN/,EXPR,LIMIT
2849 =item split /PATTERN/,EXPR
2851 =item split /PATTERN/
2855 Splits a string into an array of strings, and returns it.
2857 If not in a list context, returns the number of fields found and splits into
2858 the @_ array. (In a list context, you can force the split into @_ by
2859 using C<??> as the pattern delimiters, but it still returns the array
2860 value.) The use of implicit split to @_ is deprecated, however.
2862 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2863 splits on whitespace (after skipping any leading whitespace). Anything
2864 matching PATTERN is taken to be a delimiter separating the fields. (Note
2865 that the delimiter may be longer than one character.) If LIMIT is
2866 specified and is not negative, splits into no more than that many fields
2867 (though it may split into fewer). If LIMIT is unspecified, trailing null
2868 fields are stripped (which potential users of pop() would do well to
2869 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2870 LIMIT had been specified.
2872 A pattern matching the null string (not to be confused with
2873 a null pattern C<//>, which is just one member of the set of patterns
2874 matching a null string) will split the value of EXPR into separate
2875 characters at each point it matches that way. For example:
2877 print join(':', split(/ */, 'hi there'));
2879 produces the output 'h:i:t:h:e:r:e'.
2881 The LIMIT parameter can be used to split a line partially
2883 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2885 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2886 one larger than the number of variables in the list, to avoid
2887 unnecessary work. For the list above LIMIT would have been 4 by
2888 default. In time critical applications it behooves you not to split
2889 into more fields than you really need.
2891 If the PATTERN contains parentheses, additional array elements are
2892 created from each matching substring in the delimiter.
2894 split(/([,-])/, "1-10,20", 3);
2896 produces the list value
2898 (1, '-', 10, ',', 20)
2900 If you had the entire header of a normal Unix email message in $header,
2901 you could split it up into fields and their values this way:
2903 $header =~ s/\n\s+/ /g; # fix continuation lines
2904 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2906 The pattern C</PATTERN/> may be replaced with an expression to specify
2907 patterns that vary at runtime. (To do runtime compilation only once,
2908 use C</$variable/o>.)
2910 As a special case, specifying a PATTERN of space (C<' '>) will split on
2911 white space just as split with no arguments does. Thus, split(' ') can
2912 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2913 will give you as many null initial fields as there are leading spaces.
2914 A split on /\s+/ is like a split(' ') except that any leading
2915 whitespace produces a null first field. A split with no arguments
2916 really does a C<split(' ', $_)> internally.
2920 open(passwd, '/etc/passwd');
2922 ($login, $passwd, $uid, $gid, $gcos,
2923 $home, $shell) = split(/:/);
2927 (Note that $shell above will still have a newline on it. See L</chop>,
2928 L</chomp>, and L</join>.)
2930 =item sprintf FORMAT, LIST
2932 Returns a string formatted by the usual printf conventions of the C
2933 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2934 (The * character for an indirectly specified length is not
2935 supported, but you can get the same effect by interpolating a variable
2936 into the pattern.) If C<use locale> is
2937 in effect, the character used for the decimal point in formatted real numbers
2938 is affected by the LC_NUMERIC locale. See L<perllocale>.
2939 Some C libraries' implementations of sprintf() can
2940 dump core when fed ludicrous arguments.
2946 Return the square root of EXPR. If EXPR is omitted, returns square
2951 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2952 uses a semi-random value based on the current time and process ID, among
2953 other things. Of course, you'd need something much more random than that for
2954 cryptographic purposes, because it's easy to guess the current time.
2955 Checksumming the compressed output of rapidly changing operating system
2956 status programs is the usual method. Examples are posted regularly to
2957 the comp.security.unix newsgroup.
2959 =item stat FILEHANDLE
2965 Returns a 13-element array giving the status info for a file, either the
2966 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
2967 stats $_. Returns a null list if the stat fails. Typically used as
2971 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2972 $atime,$mtime,$ctime,$blksize,$blocks)
2975 Not all fields are supported on all filesystem types. Here are the
2976 meaning of the fields:
2978 dev device number of filesystem
2980 mode file mode (type and permissions)
2981 nlink number of (hard) links to the file
2982 uid numeric user ID of file's owner
2983 gid numeric group ID of file's owner
2984 rdev the device identifier (special files only)
2985 size total size of file, in bytes
2986 atime last access time since the epoch
2987 mtime last modify time since the epoch
2988 ctime inode change time (NOT creation type!) since the epoch
2989 blksize preferred block size for file system I/O
2990 blocks actual number of blocks allocated
2992 (The epoch was at 00:00 January 1, 1970 GMT.)
2994 If stat is passed the special filehandle consisting of an underline, no
2995 stat is done, but the current contents of the stat structure from the
2996 last stat or filetest are returned. Example:
2998 if (-x $file && (($d) = stat(_)) && $d < 0) {
2999 print "$file is executable NFS file\n";
3002 (This works on machines only for which the device number is negative under NFS.)
3008 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3009 doing many pattern matches on the string before it is next modified.
3010 This may or may not save time, depending on the nature and number of
3011 patterns you are searching on, and on the distribution of character
3012 frequencies in the string to be searched--you probably want to compare
3013 run times with and without it to see which runs faster. Those loops
3014 which scan for many short constant strings (including the constant
3015 parts of more complex patterns) will benefit most. You may have only
3016 one study active at a time--if you study a different scalar the first
3017 is "unstudied". (The way study works is this: a linked list of every
3018 character in the string to be searched is made, so we know, for
3019 example, where all the 'k' characters are. From each search string,
3020 the rarest character is selected, based on some static frequency tables
3021 constructed from some C programs and English text. Only those places
3022 that contain this "rarest" character are examined.)
3024 For example, here is a loop which inserts index producing entries
3025 before any line containing a certain pattern:
3029 print ".IX foo\n" if /\bfoo\b/;
3030 print ".IX bar\n" if /\bbar\b/;
3031 print ".IX blurfl\n" if /\bblurfl\b/;
3036 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3037 will be looked at, because "f" is rarer than "o". In general, this is
3038 a big win except in pathological cases. The only question is whether
3039 it saves you more time than it took to build the linked list in the
3042 Note that if you have to look for strings that you don't know till
3043 runtime, you can build an entire loop as a string and eval that to
3044 avoid recompiling all your patterns all the time. Together with
3045 undefining $/ to input entire files as one record, this can be very
3046 fast, often faster than specialized programs like fgrep(1). The following
3047 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3048 out the names of those files that contain a match:
3050 $search = 'while (<>) { study;';
3051 foreach $word (@words) {
3052 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3057 eval $search; # this screams
3058 $/ = "\n"; # put back to normal input delimiter
3059 foreach $file (sort keys(%seen)) {
3067 =item sub NAME BLOCK
3069 This is subroutine definition, not a real function I<per se>. With just a
3070 NAME (and possibly prototypes), it's just a forward declaration. Without
3071 a NAME, it's an anonymous function declaration, and does actually return a
3072 value: the CODE ref of the closure you just created. See L<perlsub> and
3073 L<perlref> for details.
3075 =item substr EXPR,OFFSET,LEN
3077 =item substr EXPR,OFFSET
3079 Extracts a substring out of EXPR and returns it. First character is at
3080 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3081 that far from the end of the string. If LEN is omitted, returns
3082 everything to the end of the string. If LEN is negative, leaves that
3083 many characters off the end of the string.
3085 You can use the substr() function
3086 as an lvalue, in which case EXPR must be an lvalue. If you assign
3087 something shorter than LEN, the string will shrink, and if you assign
3088 something longer than LEN, the string will grow to accommodate it. To
3089 keep the string the same length you may need to pad or chop your value
3092 =item symlink OLDFILE,NEWFILE
3094 Creates a new filename symbolically linked to the old filename.
3095 Returns 1 for success, 0 otherwise. On systems that don't support
3096 symbolic links, produces a fatal error at run time. To check for that,
3099 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3103 Calls the system call specified as the first element of the list,
3104 passing the remaining elements as arguments to the system call. If
3105 unimplemented, produces a fatal error. The arguments are interpreted
3106 as follows: if a given argument is numeric, the argument is passed as
3107 an int. If not, the pointer to the string value is passed. You are
3108 responsible to make sure a string is pre-extended long enough to
3109 receive any result that might be written into a string. If your
3110 integer arguments are not literals and have never been interpreted in a
3111 numeric context, you may need to add 0 to them to force them to look
3114 require 'syscall.ph'; # may need to run h2ph
3115 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3117 Note that Perl supports passing of up to only 14 arguments to your system call,
3118 which in practice should usually suffice.
3120 =item sysopen FILEHANDLE,FILENAME,MODE
3122 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3124 Opens the file whose filename is given by FILENAME, and associates it
3125 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3126 the name of the real filehandle wanted. This function calls the
3127 underlying operating system's C<open> function with the parameters
3128 FILENAME, MODE, PERMS.
3130 The possible values and flag bits of the MODE parameter are
3131 system-dependent; they are available via the standard module C<Fcntl>.
3132 However, for historical reasons, some values are universal: zero means
3133 read-only, one means write-only, and two means read/write.
3135 If the file named by FILENAME does not exist and the C<open> call
3136 creates it (typically because MODE includes the O_CREAT flag), then
3137 the value of PERMS specifies the permissions of the newly created
3138 file. If PERMS is omitted, the default value is 0666, which allows
3139 read and write for all. This default is reasonable: see C<umask>.
3141 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3143 =item sysread FILEHANDLE,SCALAR,LENGTH
3145 Attempts to read LENGTH bytes of data into variable SCALAR from the
3146 specified FILEHANDLE, using the system call read(2). It bypasses
3147 stdio, so mixing this with other kinds of reads may cause confusion.
3148 Returns the number of bytes actually read, or undef if there was an
3149 error. SCALAR will be grown or shrunk so that the last byte actually
3150 read is the last byte of the scalar after the read.
3152 An OFFSET may be specified to place the read data at some place in the
3153 string other than the beginning. A negative OFFSET specifies
3154 placement at that many bytes counting backwards from the end of the
3155 string. A positive OFFSET greater than the length of SCALAR results
3156 in the string being padded to the required size with "\0" bytes before
3157 the result of the read is appended.
3161 Does exactly the same thing as "exec LIST" except that a fork is done
3162 first, and the parent process waits for the child process to complete.
3163 Note that argument processing varies depending on the number of
3164 arguments. The return value is the exit status of the program as
3165 returned by the wait() call. To get the actual exit value divide by
3166 256. See also L</exec>. This is I<NOT> what you want to use to capture
3167 the output from a command, for that you should use merely back-ticks, as
3168 described in L<perlop/"`STRING`">.
3170 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3172 =item syswrite FILEHANDLE,SCALAR,LENGTH
3174 Attempts to write LENGTH bytes of data from variable SCALAR to the
3175 specified FILEHANDLE, using the system call write(2). It bypasses
3176 stdio, so mixing this with prints may cause confusion. Returns the
3177 number of bytes actually written, or undef if there was an error.
3178 If the length is greater than the available data, only as much data as
3179 is available will be written.
3181 An OFFSET may be specified to write the data from some part of the
3182 string other than the beginning. A negative OFFSET specifies writing
3183 from that many bytes counting backwards from the end of the string.
3185 =item tell FILEHANDLE
3189 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3190 expression whose value gives the name of the actual filehandle. If
3191 FILEHANDLE is omitted, assumes the file last read.
3193 =item telldir DIRHANDLE
3195 Returns the current position of the readdir() routines on DIRHANDLE.
3196 Value may be given to seekdir() to access a particular location in a
3197 directory. Has the same caveats about possible directory compaction as
3198 the corresponding system library routine.
3200 =item tie VARIABLE,CLASSNAME,LIST
3202 This function binds a variable to a package class that will provide the
3203 implementation for the variable. VARIABLE is the name of the variable
3204 to be enchanted. CLASSNAME is the name of a class implementing objects
3205 of correct type. Any additional arguments are passed to the "new"
3206 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3207 Typically these are arguments such as might be passed to the dbm_open()
3208 function of C. The object returned by the "new" method is also
3209 returned by the tie() function, which would be useful if you want to
3210 access other methods in CLASSNAME.
3212 Note that functions such as keys() and values() may return huge array
3213 values when used on large objects, like DBM files. You may prefer to
3214 use the each() function to iterate over such. Example:
3216 # print out history file offsets
3218 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3219 while (($key,$val) = each %HIST) {
3220 print $key, ' = ', unpack('L',$val), "\n";
3224 A class implementing an associative array should have the following
3227 TIEHASH classname, LIST
3230 STORE this, key, value
3234 NEXTKEY this, lastkey
3236 A class implementing an ordinary array should have the following methods:
3238 TIEARRAY classname, LIST
3241 STORE this, key, value
3244 A class implementing a scalar should have the following methods:
3246 TIESCALAR classname, LIST
3251 Unlike dbmopen(), the tie() function will not use or require a module
3252 for you--you need to do that explicitly yourself. See L<DB_File>
3253 or the F<Config> module for interesting tie() implementations.
3257 Returns a reference to the object underlying VARIABLE (the same value
3258 that was originally returned by the tie() call which bound the variable
3259 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3264 Returns the number of non-leap seconds since whatever time the system
3265 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3266 and 00:00:00 UTC, January 1, 1970 for most other systems).
3267 Suitable for feeding to gmtime() and localtime().
3271 Returns a four-element array giving the user and system times, in
3272 seconds, for this process and the children of this process.
3274 ($user,$system,$cuser,$csystem) = times;
3278 The translation operator. See L<perlop>.
3280 =item truncate FILEHANDLE,LENGTH
3282 =item truncate EXPR,LENGTH
3284 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3285 specified length. Produces a fatal error if truncate isn't implemented
3292 Returns an uppercased version of EXPR. This is the internal function
3293 implementing the \U escape in double-quoted strings.
3294 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3296 If EXPR is omitted, uses $_.
3302 Returns the value of EXPR with the first character uppercased. This is
3303 the internal function implementing the \u escape in double-quoted strings.
3304 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3306 If EXPR is omitted, uses $_.
3312 Sets the umask for the process and returns the old one. If EXPR is
3313 omitted, returns merely the current umask.
3319 Undefines the value of EXPR, which must be an lvalue. Use on only a
3320 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3321 will probably not do what you expect on most predefined variables or
3322 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3323 the EXPR, in which case nothing is undefined, but you still get an
3324 undefined value that you could, for instance, return from a
3325 subroutine. Examples:
3328 undef $bar{'blurfl'};
3332 return (wantarray ? () : undef) if $they_blew_it;
3338 Deletes a list of files. Returns the number of files successfully
3341 $cnt = unlink 'a', 'b', 'c';
3345 Note: unlink will not delete directories unless you are superuser and
3346 the B<-U> flag is supplied to Perl. Even if these conditions are
3347 met, be warned that unlinking a directory can inflict damage on your
3348 filesystem. Use rmdir instead.
3350 If LIST is omitted, uses $_.
3352 =item unpack TEMPLATE,EXPR
3354 Unpack does the reverse of pack: it takes a string representing a
3355 structure and expands it out into a list value, returning the array
3356 value. (In a scalar context, it returns merely the first value
3357 produced.) The TEMPLATE has the same format as in the pack function.
3358 Here's a subroutine that does substring:
3361 local($what,$where,$howmuch) = @_;
3362 unpack("x$where a$howmuch", $what);
3367 sub ordinal { unpack("c",$_[0]); } # same as ord()
3369 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3370 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3371 themselves. Default is a 16-bit checksum. For example, the following
3372 computes the same number as the System V sum program:
3375 $checksum += unpack("%16C*", $_);
3379 The following efficiently counts the number of set bits in a bit vector:
3381 $setbits = unpack("%32b*", $selectmask);
3383 =item untie VARIABLE
3385 Breaks the binding between a variable and a package. (See tie().)
3387 =item unshift ARRAY,LIST
3389 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3390 depending on how you look at it. Prepends list to the front of the
3391 array, and returns the new number of elements in the array.
3393 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3395 Note the LIST is prepended whole, not one element at a time, so the
3396 prepended elements stay in the same order. Use reverse to do the
3399 =item use Module LIST
3403 =item use Module VERSION LIST
3407 Imports some semantics into the current package from the named module,
3408 generally by aliasing certain subroutine or variable names into your
3409 package. It is exactly equivalent to
3411 BEGIN { require Module; import Module LIST; }
3413 except that Module I<must> be a bare word.
3415 If the first argument to C<use> is a number, it is treated as a version
3416 number instead of a module name. If the version of the Perl interpreter
3417 is less than VERSION, then an error message is printed and Perl exits
3418 immediately. This is often useful if you need to check the current
3419 Perl version before C<use>ing library modules which have changed in
3420 incompatible ways from older versions of Perl. (We try not to do
3421 this more than we have to.)
3423 The BEGIN forces the require and import to happen at compile time. The
3424 require makes sure the module is loaded into memory if it hasn't been
3425 yet. The import is not a builtin--it's just an ordinary static method
3426 call into the "Module" package to tell the module to import the list of
3427 features back into the current package. The module can implement its
3428 import method any way it likes, though most modules just choose to
3429 derive their import method via inheritance from the Exporter class that
3430 is defined in the Exporter module. See L<Exporter>. If no import
3431 method can be found then the error is currently silently ignored. This
3432 may change to a fatal error in a future version.
3434 If you don't want your namespace altered, explicitly supply an empty list:
3438 That is exactly equivalent to
3440 BEGIN { require Module; }
3442 If the VERSION argument is present between Module and LIST, then the
3443 C<use> will call the VERSION method in class Module with the given
3444 version as an argument. The default VERSION method, inherited from
3445 the Universal class, croaks if the given version is larger than the
3446 value of the variable $Module::VERSION. (Note that there is not a
3447 comma after VERSION!)
3449 Because this is a wide-open interface, pragmas (compiler directives)
3450 are also implemented this way. Currently implemented pragmas are:
3454 use sigtrap qw(SEGV BUS);
3455 use strict qw(subs vars refs);
3456 use subs qw(afunc blurfl);
3458 These pseudo-modules import semantics into the current block scope, unlike
3459 ordinary modules, which import symbols into the current package (which are
3460 effective through the end of the file).
3462 There's a corresponding "no" command that unimports meanings imported
3463 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3468 If no unimport method can be found the call fails with a fatal error.
3470 See L<perlmod> for a list of standard modules and pragmas.
3474 Changes the access and modification times on each file of a list of
3475 files. The first two elements of the list must be the NUMERICAL access
3476 and modification times, in that order. Returns the number of files
3477 successfully changed. The inode modification time of each file is set
3478 to the current time. Example of a "touch" command:
3482 utime $now, $now, @ARGV;
3484 =item values ASSOC_ARRAY
3486 Returns a normal array consisting of all the values of the named
3487 associative array. (In a scalar context, returns the number of
3488 values.) The values are returned in an apparently random order, but it
3489 is the same order as either the keys() or each() function would produce
3490 on the same array. See also keys(), each(), and sort().
3492 =item vec EXPR,OFFSET,BITS
3494 Treats the string in EXPR as a vector of unsigned integers, and
3495 returns the value of the bit field specified by OFFSET. BITS specifies
3496 the number of bits that are reserved for each entry in the bit
3497 vector. This must be a power of two from 1 to 32. vec() may also be
3498 assigned to, in which case parentheses are needed to give the expression
3499 the correct precedence as in
3501 vec($image, $max_x * $x + $y, 8) = 3;
3503 Vectors created with vec() can also be manipulated with the logical
3504 operators |, &, and ^, which will assume a bit vector operation is
3505 desired when both operands are strings.
3507 To transform a bit vector into a string or array of 0's and 1's, use these:
3509 $bits = unpack("b*", $vector);
3510 @bits = split(//, unpack("b*", $vector));
3512 If you know the exact length in bits, it can be used in place of the *.
3516 Waits for a child process to terminate and returns the pid of the
3517 deceased process, or -1 if there are no child processes. The status is
3520 =item waitpid PID,FLAGS
3522 Waits for a particular child process to terminate and returns the pid
3523 of the deceased process, or -1 if there is no such child process. The
3524 status is returned in C<$?>. If you say
3526 use POSIX ":sys_wait_h";
3528 waitpid(-1,&WNOHANG);
3530 then you can do a non-blocking wait for any process. Non-blocking wait
3531 is available on machines supporting either the waitpid(2) or
3532 wait4(2) system calls. However, waiting for a particular pid with
3533 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3534 by remembering the status values of processes that have exited but have
3535 not been harvested by the Perl script yet.)
3539 Returns TRUE if the context of the currently executing subroutine is
3540 looking for a list value. Returns FALSE if the context is looking
3543 return wantarray ? () : undef;
3547 Produces a message on STDERR just like die(), but doesn't exit or
3550 =item write FILEHANDLE
3556 Writes a formatted record (possibly multi-line) to the specified file,
3557 using the format associated with that file. By default the format for
3558 a file is the one having the same name is the filehandle, but the
3559 format for the current output channel (see the select() function) may be set
3560 explicitly by assigning the name of the format to the C<$~> variable.
3562 Top of form processing is handled automatically: if there is
3563 insufficient room on the current page for the formatted record, the
3564 page is advanced by writing a form feed, a special top-of-page format
3565 is used to format the new page header, and then the record is written.
3566 By default the top-of-page format is the name of the filehandle with
3567 "_TOP" appended, but it may be dynamically set to the format of your
3568 choice by assigning the name to the C<$^> variable while the filehandle is
3569 selected. The number of lines remaining on the current page is in
3570 variable C<$->, which can be set to 0 to force a new page.
3572 If FILEHANDLE is unspecified, output goes to the current default output
3573 channel, which starts out as STDOUT but may be changed by the
3574 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3575 is evaluated and the resulting string is used to look up the name of
3576 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3578 Note that write is I<NOT> the opposite of read. Unfortunately.
3582 The translation operator. See L<perlop>.