3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can only
18 ever be one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parens.) If you use the parens, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 non-abortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
61 I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in a scalar context. Some operators return the
67 length of the list that would have been returned in a list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
73 =head2 Perl Functions by Category
75 Here are Perl's functions (including things that look like
76 functions, like some of the keywords and named operators)
77 arranged by category. Some functions appear in more
82 =item Functions for SCALARs or strings
84 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86 sprintf, substr, tr///, uc, ucfirst, y///
88 =item Regular expressions and pattern matching
90 m//, pos, quotemeta, s///, split, study
92 =item Numeric functions
94 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
97 =item Functions for real @ARRAYs
99 pop, push, shift, splice, unshift
101 =item Functions for list data
103 grep, join, map, qw/STRING/, reverse, sort, unpack
105 =item Functions for real %HASHes
107 delete, each, exists, keys, values
109 =item Input and output functions
111 binmode, close, closedir, dbmclose, dbmopen, die, eof,
112 fileno, flock, format, getc, print, printf, read, readdir,
113 rewinddir, seek, seekdir, select, syscall, sysread,
114 syswrite, tell, telldir, truncate, warn, write
116 =item Functions for fixed length data or records
118 pack, read, syscall, sysread, syswrite, unpack, vec
120 =item Functions for filehandles, files, or directories
122 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
123 lstat, mkdir, open, opendir, readlink, rename, rmdir,
124 stat, symlink, umask, unlink, utime
126 =item Keywords related to the control flow of your perl program
128 caller, continue, die, do, dump, eval, exit, goto, last,
129 next, redo, return, sub, wantarray
131 =item Keywords related to scoping
133 caller, import, local, my, package, use
135 =item Miscellaneous functions
137 defined, dump, eval, formline, local, my, reset, scalar,
140 =item Functions for processes and process groups
142 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
146 =item Keywords related to perl modules
148 do, import, no, package, require, use
150 =item Keywords related to classes and object-orientedness
152 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
154 =item Low-level socket functions
156 accept, bind, connect, getpeername, getsockname,
157 getsockopt, listen, recv, send, setsockopt, shutdown,
160 =item System V interprocess communication functions
162 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163 shmctl, shmget, shmread, shmwrite
165 =item Fetching user and group info
167 endgrent, endhostent, endnetent, endpwent, getgrent,
168 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169 getpwuid, setgrent, setpwent
171 =item Fetching network info
173 endprotoent, endservent, gethostbyaddr, gethostbyname,
174 gethostent, getnetbyaddr, getnetbyname, getnetent,
175 getprotobyname, getprotobynumber, getprotoent,
176 getservbyname, getservbyport, getservent, sethostent,
177 setnetent, setprotoent, setservent
179 =item Time-related functions
181 gmtime, localtime, time, times
183 =item Functions new in perl5
185 abs, bless, chomp, chr, exists, formline, glob, import, lc,
186 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
187 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
189 * - C<sub> was a keyword in perl4, but in perl5 it is an
190 operator which can be used in expressions.
192 =item Functions obsoleted in perl5
199 =head2 Alphabetical Listing of Perl Functions
210 A file test, where X is one of the letters listed below. This unary
211 operator takes one argument, either a filename or a filehandle, and
212 tests the associated file to see if something is true about it. If the
213 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215 the undefined value if the file doesn't exist. Despite the funny
216 names, precedence is the same as any other named unary operator, and
217 the argument may be parenthesized like any other unary operator. The
218 operator may be any of:
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
254 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256 uids and gids of the user. There may be other reasons you can't actually
257 read, write or execute the file. Also note that, for the superuser,
258 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
259 1 if any execute bit is set in the mode. Scripts run by the superuser may
260 thus need to do a stat() in order to determine the actual mode of the
261 file, or temporarily set the uid to something else.
267 next unless -f $_; # ignore specials
271 Note that C<-s/a/b/> does not do a negated substitution. Saying
272 C<-exp($foo)> still works as expected, however--only single letters
273 following a minus are interpreted as file tests.
275 The C<-T> and C<-B> switches work as follows. The first block or so of the
276 file is examined for odd characters such as strange control codes or
277 characters with the high bit set. If too many odd characters (E<gt>30%)
278 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279 containing null in the first block is considered a binary file. If C<-T>
280 or C<-B> is used on a filehandle, the current stdio buffer is examined
281 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
282 file, or a file at EOF when testing a filehandle. Because you have to
283 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284 against the file first, as in C<next unless -f $file && -T $file>.
286 If any of the file tests (or either the stat() or lstat() operators) are given the
287 special filehandle consisting of a solitary underline, then the stat
288 structure of the previous file test (or stat operator) is used, saving
289 a system call. (This doesn't work with C<-t>, and you need to remember
290 that lstat() and C<-l> will leave values in the stat structure for the
291 symbolic link, not the real file.) Example:
293 print "Can do.\n" if -r $a || -w _ || -x _;
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
309 =item accept NEWSOCKET,GENERICSOCKET
311 Accepts an incoming socket connect, just as the accept(2) system call
312 does. Returns the packed address if it succeeded, FALSE otherwise.
313 See example in L<perlipc/"Sockets: Client/Server Communication">.
317 Arranges to have a SIGALRM delivered to this process after the
318 specified number of seconds have elapsed. (On some machines,
319 unfortunately, the elapsed time may be up to one second less than you
320 specified because of how seconds are counted.) Only one timer may be
321 counting at once. Each call disables the previous timer, and an
322 argument of 0 may be supplied to cancel the previous timer without
323 starting a new one. The returned value is the amount of time remaining
324 on the previous timer.
326 For delays of finer granularity than one second, you may use Perl's
327 syscall() interface to access setitimer(2) if your system supports it,
328 or else see L</select()> below. It is not advised to intermix alarm()
333 Returns the arctangent of Y/X in the range -PI to PI.
335 =item bind SOCKET,NAME
337 Binds a network address to a socket, just as the bind system call
338 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
339 packed address of the appropriate type for the socket. See the examples in
340 L<perlipc/"Sockets: Client/Server Communication">.
342 =item binmode FILEHANDLE
344 Arranges for the file to be read or written in "binary" mode in operating
345 systems that distinguish between binary and text files. Files that are
346 not in binary mode have CR LF sequences translated to LF on input and LF
347 translated to CR LF on output. Binmode has no effect under Unix; in DOS
348 and similarly archaic systems, it may be imperative--otherwise your
349 DOS-damaged C library may mangle your file. The key distinction between
350 systems that need binmode and those that don't is their text file
351 formats. Systems like Unix and Plan9 that delimit lines with a single
352 character, and that encode that character in C as '\n', do not need
353 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
354 is taken as the name of the filehandle.
356 =item bless REF,CLASSNAME
360 This function tells the referenced object (passed as REF) that it is now
361 an object in the CLASSNAME package--or the current package if no CLASSNAME
362 is specified, which is often the case. It returns the reference for
363 convenience, since a bless() is often the last thing in a constructor.
364 Always use the two-argument version if the function doing the blessing
365 might be inherited by a derived class. See L<perlobj> for more about the
366 blessing (and blessings) of objects.
372 Returns the context of the current subroutine call. In a scalar context,
373 returns TRUE if there is a caller, that is, if we're in a subroutine or
374 eval() or require(), and FALSE otherwise. In a list context, returns
376 ($package, $filename, $line) = caller;
378 With EXPR, it returns some extra information that the debugger uses to
379 print a stack trace. The value of EXPR indicates how many call frames
380 to go back before the current one.
382 ($package, $filename, $line,
383 $subroutine, $hasargs, $wantargs) = caller($i);
385 Furthermore, when called from within the DB package, caller returns more
386 detailed information: it sets the list variable @DB::args to be the
387 arguments with which that subroutine was invoked.
391 Changes the working directory to EXPR, if possible. If EXPR is
392 omitted, changes to home directory. Returns TRUE upon success, FALSE
393 otherwise. See example under die().
397 Changes the permissions of a list of files. The first element of the
398 list must be the numerical mode, which should probably be an octal
399 number. Returns the number of files successfully changed.
401 $cnt = chmod 0755, 'foo', 'bar';
402 chmod 0755, @executables;
410 This is a slightly safer version of chop (see below). It removes any
411 line ending that corresponds to the current value of C<$/> (also known as
412 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
413 of characters removed. It's often used to remove the newline from the
414 end of an input record when you're worried that the final record may be
415 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
416 trailing newlines from the string. If VARIABLE is omitted, it chomps
420 chomp; # avoid \n on last field
425 You can actually chomp anything that's an lvalue, including an assignment:
428 chomp($answer = <STDIN>);
430 If you chomp a list, each element is chomped, and the total number of
431 characters removed is returned.
439 Chops off the last character of a string and returns the character
440 chopped. It's used primarily to remove the newline from the end of an
441 input record, but is much more efficient than C<s/\n//> because it neither
442 scans nor copies the string. If VARIABLE is omitted, chops $_.
446 chop; # avoid \n on last field
451 You can actually chop anything that's an lvalue, including an assignment:
454 chop($answer = <STDIN>);
456 If you chop a list, each element is chopped. Only the value of the
457 last chop is returned.
459 Note that chop returns the last character. To return all but the last
460 character, use C<substr($string, 0, -1)>.
464 Changes the owner (and group) of a list of files. The first two
465 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
466 Returns the number of files successfully changed.
468 $cnt = chown $uid, $gid, 'foo', 'bar';
469 chown $uid, $gid, @filenames;
471 Here's an example that looks up non-numeric uids in the passwd file:
474 chop($user = <STDIN>);
476 chop($pattern = <STDIN>);
478 ($login,$pass,$uid,$gid) = getpwnam($user)
479 or die "$user not in passwd file";
481 @ary = <${pattern}>; # expand filenames
482 chown $uid, $gid, @ary;
484 On most systems, you are not allowed to change the ownership of the
485 file unless you're the superuser, although you should be able to change
486 the group to any of your secondary groups. On insecure systems, these
487 restrictions may be relaxed, but this is not a portable assumption.
491 Returns the character represented by that NUMBER in the character set.
492 For example, C<chr(65)> is "A" in ASCII.
494 =item chroot FILENAME
496 This function works as the system call by the same name: it makes the
497 named directory the new root directory for all further pathnames that
498 begin with a "/" by your process and all of its children. (It doesn't
499 change your current working directory is unaffected.) For security
500 reasons, this call is restricted to the superuser. If FILENAME is
501 omitted, does chroot to $_.
503 =item close FILEHANDLE
505 Closes the file or pipe associated with the file handle, returning TRUE
506 only if stdio successfully flushes buffers and closes the system file
507 descriptor. You don't have to close FILEHANDLE if you are immediately
508 going to do another open() on it, since open() will close it for you. (See
509 open().) However, an explicit close on an input file resets the line
510 counter ($.), while the implicit close done by open() does not. Also,
511 closing a pipe will wait for the process executing on the pipe to
512 complete, in case you want to look at the output of the pipe
513 afterwards. Closing a pipe explicitly also puts the status value of
514 the command into C<$?>. Example:
516 open(OUTPUT, '|sort >foo'); # pipe to sort
517 ... # print stuff to output
518 close OUTPUT; # wait for sort to finish
519 open(INPUT, 'foo'); # get sort's results
521 FILEHANDLE may be an expression whose value gives the real filehandle name.
523 =item closedir DIRHANDLE
525 Closes a directory opened by opendir().
527 =item connect SOCKET,NAME
529 Attempts to connect to a remote socket, just as the connect system call
530 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
531 packed address of the appropriate type for the socket. See the examples in
532 L<perlipc/"Sockets: Client/Server Communication">.
536 Actually a flow control statement rather than a function. If there is a
537 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
538 C<foreach>), it is always executed just before the conditional is about to
539 be evaluated again, just like the third part of a C<for> loop in C. Thus
540 it can be used to increment a loop variable, even when the loop has been
541 continued via the C<next> statement (which is similar to the C C<continue>
546 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
549 =item crypt PLAINTEXT,SALT
551 Encrypts a string exactly like the crypt(3) function in the C library
552 (assuming that you actually have a version there that has not been
553 extirpated as a potential munition). This can prove useful for checking
554 the password file for lousy passwords, amongst other things. Only the
555 guys wearing white hats should do this.
557 Here's an example that makes sure that whoever runs this program knows
560 $pwd = (getpwuid($<))[1];
561 $salt = substr($pwd, 0, 2);
565 chop($word = <STDIN>);
569 if (crypt($word, $salt) ne $pwd) {
575 Of course, typing in your own password to whoever asks you
578 =item dbmclose ASSOC_ARRAY
580 [This function has been superseded by the untie() function.]
582 Breaks the binding between a DBM file and an associative array.
584 =item dbmopen ASSOC,DBNAME,MODE
586 [This function has been superseded by the tie() function.]
588 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
589 associative array. ASSOC is the name of the associative array. (Unlike
590 normal open, the first argument is I<NOT> a filehandle, even though it
591 looks like one). DBNAME is the name of the database (without the F<.dir>
592 or F<.pag> extension if any). If the database does not exist, it is
593 created with protection specified by MODE (as modified by the umask()).
594 If your system only supports the older DBM functions, you may perform only
595 one dbmopen() in your program. In older versions of Perl, if your system
596 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
597 falls back to sdbm(3).
599 If you don't have write access to the DBM file, you can only read
600 associative array variables, not set them. If you want to test whether
601 you can write, either use file tests or try setting a dummy array entry
602 inside an eval(), which will trap the error.
604 Note that functions such as keys() and values() may return huge array
605 values when used on large DBM files. You may prefer to use the each()
606 function to iterate over large DBM files. Example:
608 # print out history file offsets
609 dbmopen(%HIST,'/usr/lib/news/history',0666);
610 while (($key,$val) = each %HIST) {
611 print $key, ' = ', unpack('L',$val), "\n";
615 See also L<AnyDBM_File> for a more general description of the pros and
616 cons of the various dbm approaches, as well as L<DB_File> for a particularly
621 Returns a boolean value saying whether EXPR has a real value
622 or not. Many operations return the undefined value under exceptional
623 conditions, such as end of file, uninitialized variable, system error
624 and such. This function allows you to distinguish between an undefined
625 null scalar and a defined null scalar with operations that might return
626 a real null string, such as referencing elements of an array. You may
627 also check to see if arrays or subroutines exist. Use of defined on
628 predefined variables is not guaranteed to produce intuitive results.
630 When used on a hash array element, it tells you whether the value
631 is defined, not whether the key exists in the hash. Use exists() for that.
635 print if defined $switch{'D'};
636 print "$val\n" while defined($val = pop(@ary));
637 die "Can't readlink $sym: $!"
638 unless defined($value = readlink $sym);
639 eval '@foo = ()' if defined(@foo);
640 die "No XYZ package defined" unless defined %_XYZ;
641 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
645 Note: many folks tend to overuse defined(), and then are surprised to
646 discover that the number 0 and the null string are, in fact, defined
647 concepts. For example, if you say
651 the pattern match succeeds, and $1 is defined, despite the fact that it
652 matched "nothing". But it didn't really match nothing--rather, it
653 matched something that happened to be 0 characters long. This is all
654 very above-board and honest. When a function returns an undefined value,
655 it's an admission that it couldn't give you an honest answer. So
656 you should only use defined() when you're questioning the integrity
657 of what you're trying to do. At other times, a simple comparison to
658 0 or "" is what you want.
662 Deletes the specified value from its hash array. Returns the deleted
663 value, or the undefined value if nothing was deleted. Deleting from
664 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
665 file deletes the entry from the DBM file. (But deleting from a tie()d
666 hash doesn't necessarily return anything.)
668 The following deletes all the values of an associative array:
670 foreach $key (keys %ARRAY) {
674 (But it would be faster to use the undef() command.) Note that the
675 EXPR can be arbitrarily complicated as long as the final operation is
678 delete $ref->[$x][$y]{$key};
682 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
683 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
684 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
685 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
686 and the eval() is terminated with the undefined value; this makes die()
687 the way to raise an exception.
691 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
692 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
694 If the value of EXPR does not end in a newline, the current script line
695 number and input line number (if any) are also printed, and a newline
696 is supplied. Hint: sometimes appending ", stopped" to your message
697 will cause it to make better sense when the string "at foo line 123" is
698 appended. Suppose you are running script "canasta".
700 die "/etc/games is no good";
701 die "/etc/games is no good, stopped";
703 produce, respectively
705 /etc/games is no good at canasta line 123.
706 /etc/games is no good, stopped at canasta line 123.
708 See also exit() and warn().
712 Not really a function. Returns the value of the last command in the
713 sequence of commands indicated by BLOCK. When modified by a loop
714 modifier, executes the BLOCK once before testing the loop condition.
715 (On other statements the loop modifiers test the conditional first.)
717 =item do SUBROUTINE(LIST)
719 A deprecated form of subroutine call. See L<perlsub>.
723 Uses the value of EXPR as a filename and executes the contents of the
724 file as a Perl script. Its primary use is to include subroutines
725 from a Perl subroutine library.
733 except that it's more efficient, more concise, keeps track of the
734 current filename for error messages, and searches all the B<-I>
735 libraries if the file isn't in the current directory (see also the @INC
736 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
737 reparse the file every time you call it, so you probably don't want to
738 do this inside a loop.
740 Note that inclusion of library modules is better done with the
741 use() and require() operators, which also do error checking
742 and raise an exception if there's a problem.
746 This causes an immediate core dump. Primarily this is so that you can
747 use the B<undump> program to turn your core dump into an executable binary
748 after having initialized all your variables at the beginning of the
749 program. When the new binary is executed it will begin by executing a
750 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
751 it as a goto with an intervening core dump and reincarnation. If LABEL
752 is omitted, restarts the program from the top. WARNING: any files
753 opened at the time of the dump will NOT be open any more when the
754 program is reincarnated, with possible resulting confusion on the part
755 of Perl. See also B<-u> option in L<perlrun>.
772 dump QUICKSTART if $ARGV[0] eq '-d';
777 =item each ASSOC_ARRAY
779 When called in a list context, returns a 2-element array consisting
780 of the key and value for the next element of an associative array,
781 so that you can iterate over it. When called in a scalar context,
782 returns the key only for the next element in the associative array.
783 Entries are returned in an apparently random order. When the array is
784 entirely read, a null array is returned in list context (which when
785 assigned produces a FALSE (0) value), and C<undef> is returned in a
786 scalar context. The next call to each() after that will start
787 iterating again. The iterator can be reset only by reading all the
788 elements from the array. You should not add elements to an array while
789 you're iterating over it. There is a single iterator for each
790 associative array, shared by all each(), keys() and values() function
791 calls in the program. The following prints out your environment like
792 the printenv(1) program, only in a different order:
794 while (($key,$value) = each %ENV) {
795 print "$key=$value\n";
798 See also keys() and values().
806 Returns 1 if the next read on FILEHANDLE will return end of file, or if
807 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
808 gives the real filehandle name. (Note that this function actually
809 reads a character and then ungetc()s it, so it is not very useful in an
810 interactive context.) Do not read from a terminal file (or call
811 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
812 as terminals may lose the end-of-file condition if you do.
814 An C<eof> without an argument uses the last file read as argument.
815 Empty parentheses () may be used to indicate
816 the pseudofile formed of the files listed on the command line, i.e.
817 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
818 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
819 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
821 # reset line numbering on each input file
824 close(ARGV) if (eof); # Not eof().
827 # insert dashes just before last line of last file
830 print "--------------\n";
831 close(ARGV); # close or break; is needed if we
832 # are reading from the terminal
837 Practical hint: you almost never need to use C<eof> in Perl, because the
838 input operators return undef when they run out of data.
844 EXPR is parsed and executed as if it were a little Perl program. It
845 is executed in the context of the current Perl program, so that any
846 variable settings, subroutine or format definitions remain afterwards.
847 The value returned is the value of the last expression evaluated, or a
848 return statement may be used, just as with subroutines. The last
849 expression is evaluated in scalar or array context, depending on the
852 If there is a syntax error or runtime error, or a die() statement is
853 executed, an undefined value is returned by eval(), and C<$@> is set to the
854 error message. If there was no error, C<$@> is guaranteed to be a null
855 string. If EXPR is omitted, evaluates $_. The final semicolon, if
856 any, may be omitted from the expression.
858 Note that, since eval() traps otherwise-fatal errors, it is useful for
859 determining whether a particular feature (such as socket() or symlink())
860 is implemented. It is also Perl's exception trapping mechanism, where
861 the die operator is used to raise exceptions.
863 If the code to be executed doesn't vary, you may use the eval-BLOCK
864 form to trap run-time errors without incurring the penalty of
865 recompiling each time. The error, if any, is still returned in C<$@>.
868 # make divide-by-zero non-fatal
869 eval { $answer = $a / $b; }; warn $@ if $@;
871 # same thing, but less efficient
872 eval '$answer = $a / $b'; warn $@ if $@;
874 # a compile-time error
878 eval '$answer ='; # sets $@
880 With an eval(), you should be especially careful to remember what's
881 being looked at when:
887 eval { $x }; # CASE 4
889 eval "\$$x++" # CASE 5
892 Cases 1 and 2 above behave identically: they run the code contained in the
893 variable $x. (Although case 2 has misleading double quotes making the
894 reader wonder what else might be happening (nothing is).) Cases 3 and 4
895 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
896 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
897 is a place where normally you I<WOULD> like to use double quotes, except
898 that in that particular situation, you can just use symbolic references
899 instead, as in case 6.
903 The exec() function executes a system command I<AND NEVER RETURNS>,
904 unless the command does not exist and is executed directly instead of
905 via C</bin/sh -c> (see below). Use system() instead of exec() if you
908 If there is more than one argument in LIST, or if LIST is an array with
909 more than one value, calls execvp(3) with the arguments in LIST. If
910 there is only one scalar argument, the argument is checked for shell
911 metacharacters. If there are any, the entire argument is passed to
912 C</bin/sh -c> for parsing. If there are none, the argument is split
913 into words and passed directly to execvp(), which is more efficient.
914 Note: exec() and system() do not flush your output buffer, so you may
915 need to set C<$|> to avoid lost output. Examples:
917 exec '/bin/echo', 'Your arguments are: ', @ARGV;
918 exec "sort $outfile | uniq";
920 If you don't really want to execute the first argument, but want to lie
921 to the program you are executing about its own name, you can specify
922 the program you actually want to run as an "indirect object" (without a
923 comma) in front of the LIST. (This always forces interpretation of the
924 LIST as a multi-valued list, even if there is only a single scalar in
928 exec $shell '-sh'; # pretend it's a login shell
932 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
936 Returns TRUE if the specified hash key exists in its hash array, even
937 if the corresponding value is undefined.
939 print "Exists\n" if exists $array{$key};
940 print "Defined\n" if defined $array{$key};
941 print "True\n" if $array{$key};
943 A hash element can only be TRUE if it's defined, and defined if
944 it exists, but the reverse doesn't necessarily hold true.
946 Note that the EXPR can be arbitrarily complicated as long as the final
947 operation is a hash key lookup:
949 if (exists $ref->[$x][$y]{$key}) { ... }
953 Evaluates EXPR and exits immediately with that value. (Actually, it
954 calls any defined C<END> routines first, but the C<END> routines may not
955 abort the exit. Likewise any object destructors that need to be called
956 are called before exit.) Example:
959 exit 0 if $ans =~ /^[Xx]/;
961 See also die(). If EXPR is omitted, exits with 0 status.
965 Returns I<e> (the natural logarithm base) to the power of EXPR.
966 If EXPR is omitted, gives C<exp($_)>.
968 =item fcntl FILEHANDLE,FUNCTION,SCALAR
970 Implements the fcntl(2) function. You'll probably have to say
974 first to get the correct function definitions. Argument processing and
975 value return works just like ioctl() below. Note that fcntl() will produce
976 a fatal error if used on a machine that doesn't implement fcntl(2).
980 fcntl($filehandle, F_GETLK, $packed_return_buffer);
982 =item fileno FILEHANDLE
984 Returns the file descriptor for a filehandle. This is useful for
985 constructing bitmaps for select(). If FILEHANDLE is an expression, the
986 value is taken as the name of the filehandle.
988 =item flock FILEHANDLE,OPERATION
990 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
991 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
992 fatal error if used on a machine that doesn't implement either flock(2) or
993 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
994 is missing from your system. This makes flock() the portable file locking
995 strategy, although it will only lock entire files, not records. Note also
996 that some versions of flock() cannot lock things over the network; you
997 would need to use the more system-specific fcntl() for that.
999 Here's a mailbox appender for BSD systems.
1007 flock(MBOX,$LOCK_EX);
1008 # and, in case someone appended
1009 # while we were waiting...
1014 flock(MBOX,$LOCK_UN);
1017 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1018 or die "Can't open mailbox: $!";
1021 print MBOX $msg,"\n\n";
1024 See also L<DB_File> for other flock() examples.
1028 Does a fork(2) system call. Returns the child pid to the parent process
1029 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1030 Note: unflushed buffers remain unflushed in both processes, which means
1031 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1032 autoflush() FileHandle method to avoid duplicate output.
1034 If you fork() without ever waiting on your children, you will accumulate
1037 $SIG{CHLD} = sub { wait };
1039 There's also the double-fork trick (error checking on
1040 fork() returns omitted);
1042 unless ($pid = fork) {
1044 exec "what you really wanna do";
1047 ## (some_perl_code_here)
1054 See also L<perlipc> for more examples of forking and reaping
1059 Declare a picture format with use by the write() function. For
1063 Test: @<<<<<<<< @||||| @>>>>>
1064 $str, $%, '$' . int($num)
1068 $num = $cost/$quantity;
1072 See L<perlform> for many details and examples.
1075 =item formline PICTURE, LIST
1077 This is an internal function used by C<format>s, though you may call it
1078 too. It formats (see L<perlform>) a list of values according to the
1079 contents of PICTURE, placing the output into the format output
1080 accumulator, C<$^A> (or $ACCUMULATOR in English).
1081 Eventually, when a write() is done, the contents of
1082 C<$^A> are written to some filehandle, but you could also read C<$^A>
1083 yourself and then set C<$^A> back to "". Note that a format typically
1084 does one formline() per line of form, but the formline() function itself
1085 doesn't care how many newlines are embedded in the PICTURE. This means
1086 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1087 You may therefore need to use multiple formlines to implement a single
1088 record format, just like the format compiler.
1090 Be careful if you put double quotes around the picture, since an "C<@>"
1091 character may be taken to mean the beginning of an array name.
1092 formline() always returns TRUE. See L<perlform> for other examples.
1094 =item getc FILEHANDLE
1098 Returns the next character from the input file attached to FILEHANDLE,
1099 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1100 This is not particularly efficient. It cannot be used to get unbuffered
1101 single-characters, however. For that, try something more like:
1104 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1107 system "stty", '-icanon', 'eol', "\001";
1113 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1116 system "stty", 'icanon', 'eol', '^@'; # ascii null
1120 Determination of whether to whether $BSD_STYLE should be set
1121 is left as an exercise to the reader.
1123 See also the C<Term::ReadKey> module from your nearest CPAN site;
1124 details on CPAN can be found on L<perlmod/CPAN>
1128 Returns the current login from F</etc/utmp>, if any. If null, use
1131 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1133 Do not consider getlogin() for authentication: it is not as
1134 secure as getpwuid().
1136 =item getpeername SOCKET
1138 Returns the packed sockaddr address of other end of the SOCKET connection.
1141 $hersockaddr = getpeername(SOCK);
1142 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1143 $herhostname = gethostbyaddr($iaddr, AF_INET);
1144 $herstraddr = inet_ntoa($iaddr);
1148 Returns the current process group for the specified PID. Use
1149 a PID of 0 to get the current process group for the
1150 current process. Will raise an exception if used on a machine that
1151 doesn't implement getpgrp(2). If PID is omitted, returns process
1152 group of current process. Note that the POSIX version of getpgrp()
1153 does not accept a PID argument, so only PID==0 is truly portable.
1157 Returns the process id of the parent process.
1159 =item getpriority WHICH,WHO
1161 Returns the current priority for a process, a process group, or a user.
1162 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1163 machine that doesn't implement getpriority(2).
1169 =item gethostbyname NAME
1171 =item getnetbyname NAME
1173 =item getprotobyname NAME
1179 =item getservbyname NAME,PROTO
1181 =item gethostbyaddr ADDR,ADDRTYPE
1183 =item getnetbyaddr ADDR,ADDRTYPE
1185 =item getprotobynumber NUMBER
1187 =item getservbyport PORT,PROTO
1205 =item sethostent STAYOPEN
1207 =item setnetent STAYOPEN
1209 =item setprotoent STAYOPEN
1211 =item setservent STAYOPEN
1225 These routines perform the same functions as their counterparts in the
1226 system library. Within a list context, the return values from the
1227 various get routines are as follows:
1229 ($name,$passwd,$uid,$gid,
1230 $quota,$comment,$gcos,$dir,$shell) = getpw*
1231 ($name,$passwd,$gid,$members) = getgr*
1232 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1233 ($name,$aliases,$addrtype,$net) = getnet*
1234 ($name,$aliases,$proto) = getproto*
1235 ($name,$aliases,$port,$proto) = getserv*
1237 (If the entry doesn't exist you get a null list.)
1239 Within a scalar context, you get the name, unless the function was a
1240 lookup by name, in which case you get the other thing, whatever it is.
1241 (If the entry doesn't exist you get the undefined value.) For example:
1251 The $members value returned by I<getgr*()> is a space separated list of
1252 the login names of the members of the group.
1254 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1255 C, it will be returned to you via C<$?> if the function call fails. The
1256 @addrs value returned by a successful call is a list of the raw
1257 addresses returned by the corresponding system library call. In the
1258 Internet domain, each address is four bytes long and you can unpack it
1259 by saying something like:
1261 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1263 =item getsockname SOCKET
1265 Returns the packed sockaddr address of this end of the SOCKET connection.
1268 $mysockaddr = getsockname(SOCK);
1269 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1271 =item getsockopt SOCKET,LEVEL,OPTNAME
1273 Returns the socket option requested, or undefined if there is an error.
1277 Returns the value of EXPR with filename expansions such as a shell
1278 would do. This is the internal function implementing the E<lt>*.*E<gt>
1279 operator, except it's easier to use.
1283 Converts a time as returned by the time function to a 9-element array
1284 with the time localized for the standard Greenwich timezone.
1285 Typically used as follows:
1288 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1291 All array elements are numeric, and come straight out of a struct tm.
1292 In particular this means that $mon has the range 0..11 and $wday has
1293 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1301 The goto-LABEL form finds the statement labeled with LABEL and resumes
1302 execution there. It may not be used to go into any construct that
1303 requires initialization, such as a subroutine or a foreach loop. It
1304 also can't be used to go into a construct that is optimized away. It
1305 can be used to go almost anywhere else within the dynamic scope,
1306 including out of subroutines, but it's usually better to use some other
1307 construct such as last or die. The author of Perl has never felt the
1308 need to use this form of goto (in Perl, that is--C is another matter).
1310 The goto-EXPR form expects a label name, whose scope will be resolved
1311 dynamically. This allows for computed gotos per FORTRAN, but isn't
1312 necessarily recommended if you're optimizing for maintainability:
1314 goto ("FOO", "BAR", "GLARCH")[$i];
1316 The goto-&NAME form is highly magical, and substitutes a call to the
1317 named subroutine for the currently running subroutine. This is used by
1318 AUTOLOAD subroutines that wish to load another subroutine and then
1319 pretend that the other subroutine had been called in the first place
1320 (except that any modifications to @_ in the current subroutine are
1321 propagated to the other subroutine.) After the goto, not even caller()
1322 will be able to tell that this routine was called first.
1324 =item grep BLOCK LIST
1326 =item grep EXPR,LIST
1328 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1329 $_ to each element) and returns the list value consisting of those
1330 elements for which the expression evaluated to TRUE. In a scalar
1331 context, returns the number of times the expression was TRUE.
1333 @foo = grep(!/^#/, @bar); # weed out comments
1337 @foo = grep {!/^#/} @bar; # weed out comments
1339 Note that, since $_ is a reference into the list value, it can be used
1340 to modify the elements of the array. While this is useful and
1341 supported, it can cause bizarre results if the LIST is not a named
1346 Interprets EXPR as a hex string and returns the corresponding decimal
1347 value. (To convert strings that might start with 0 or 0x see
1348 oct().) If EXPR is omitted, uses $_.
1352 There is no built-in import() function. It is merely an ordinary
1353 method (subroutine) defined (or inherited) by modules that wish to export
1354 names to another module. The use() function calls the import() method
1355 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1357 =item index STR,SUBSTR,POSITION
1359 =item index STR,SUBSTR
1361 Returns the position of the first occurrence of SUBSTR in STR at or after
1362 POSITION. If POSITION is omitted, starts searching from the beginning of
1363 the string. The return value is based at 0 (or whatever you've set the C<$[>
1364 variable to--but don't do that). If the substring is not found, returns
1365 one less than the base, ordinarily -1.
1369 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1371 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1373 Implements the ioctl(2) function. You'll probably have to say
1375 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1377 first to get the correct function definitions. If F<ioctl.ph> doesn't
1378 exist or doesn't have the correct definitions you'll have to roll your
1379 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1380 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1381 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1382 written depending on the FUNCTION--a pointer to the string value of SCALAR
1383 will be passed as the third argument of the actual ioctl call. (If SCALAR
1384 has no string value but does have a numeric value, that value will be
1385 passed rather than a pointer to the string value. To guarantee this to be
1386 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1387 functions are useful for manipulating the values of structures used by
1388 ioctl(). The following example sets the erase character to DEL.
1392 die "NO TIOCGETP" if $@ || !$getp;
1393 $sgttyb_t = "ccccs"; # 4 chars and a short
1394 if (ioctl(STDIN,$getp,$sgttyb)) {
1395 @ary = unpack($sgttyb_t,$sgttyb);
1397 $sgttyb = pack($sgttyb_t,@ary);
1398 ioctl(STDIN,&TIOCSETP,$sgttyb)
1399 || die "Can't ioctl: $!";
1402 The return value of ioctl (and fcntl) is as follows:
1404 if OS returns: then Perl returns:
1406 0 string "0 but true"
1407 anything else that number
1409 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1410 still easily determine the actual value returned by the operating
1413 ($retval = ioctl(...)) || ($retval = -1);
1414 printf "System returned %d\n", $retval;
1416 =item join EXPR,LIST
1418 Joins the separate strings of LIST or ARRAY into a single string with
1419 fields separated by the value of EXPR, and returns the string.
1422 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1424 See L<perlfunc/split>.
1426 =item keys ASSOC_ARRAY
1428 Returns a normal array consisting of all the keys of the named
1429 associative array. (In a scalar context, returns the number of keys.)
1430 The keys are returned in an apparently random order, but it is the same
1431 order as either the values() or each() function produces (given that
1432 the associative array has not been modified). Here is yet another way
1433 to print your environment:
1436 @values = values %ENV;
1437 while ($#keys >= 0) {
1438 print pop(@keys), '=', pop(@values), "\n";
1441 or how about sorted by key:
1443 foreach $key (sort(keys %ENV)) {
1444 print $key, '=', $ENV{$key}, "\n";
1447 To sort an array by value, you'll need to use a C<sort{}>
1448 function. Here's a descending numeric sort of a hash by its values:
1450 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1451 printf "%4d %s\n", $hash{$key}, $key;
1454 As an lvalue C<keys> allows you to increase the number of hash buckets
1455 allocated for the given associative array. This can gain you a measure
1456 of efficiency if you know the hash is going to get big. (This is
1457 similar to pre-extending an array by assigning a larger number to
1458 $#array.) If you say
1462 then C<%hash> will have at least 200 buckets allocated for it. These
1463 buckets will be retained even if you do C<%hash = ()>, use C<undef
1464 %hash> if you want to free the storage while C<%hash> is still in scope.
1465 You can't shrink the number of buckets allocated for the hash using
1466 C<keys> in this way (but you needn't worry about doing this by accident,
1467 as trying has no effect).
1471 Sends a signal to a list of processes. The first element of
1472 the list must be the signal to send. Returns the number of
1473 processes successfully signaled.
1475 $cnt = kill 1, $child1, $child2;
1478 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1479 process groups instead of processes. (On System V, a negative I<PROCESS>
1480 number will also kill process groups, but that's not portable.) That
1481 means you usually want to use positive not negative signals. You may also
1482 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1488 The C<last> command is like the C<break> statement in C (as used in
1489 loops); it immediately exits the loop in question. If the LABEL is
1490 omitted, the command refers to the innermost enclosing loop. The
1491 C<continue> block, if any, is not executed:
1493 LINE: while (<STDIN>) {
1494 last LINE if /^$/; # exit when done with header
1500 Returns an lowercased version of EXPR. This is the internal function
1501 implementing the \L escape in double-quoted strings.
1502 Should respect any POSIX setlocale() settings.
1506 Returns the value of EXPR with the first character lowercased. This is
1507 the internal function implementing the \l escape in double-quoted strings.
1508 Should respect any POSIX setlocale() settings.
1512 Returns the length in characters of the value of EXPR. If EXPR is
1513 omitted, returns length of $_.
1515 =item link OLDFILE,NEWFILE
1517 Creates a new filename linked to the old filename. Returns 1 for
1518 success, 0 otherwise.
1520 =item listen SOCKET,QUEUESIZE
1522 Does the same thing that the listen system call does. Returns TRUE if
1523 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1527 A local modifies the listed variables to be local to the enclosing block,
1528 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1529 list must be placed in parens. See L<perlsub/"Temporary Values via
1530 local()"> for details.
1532 But you really probably want to be using my() instead, because local() isn't
1533 what most people think of as "local"). See L<perlsub/"Private Variables
1534 via my()"> for details.
1536 =item localtime EXPR
1538 Converts a time as returned by the time function to a 9-element array
1539 with the time analyzed for the local timezone. Typically used as
1542 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1545 All array elements are numeric, and come straight out of a struct tm.
1546 In particular this means that $mon has the range 0..11 and $wday has
1547 the range 0..6. If EXPR is omitted, does localtime(time).
1549 In a scalar context, prints out the ctime(3) value:
1551 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1553 Also see the F<timelocal.pl> library, and the strftime(3) function available
1554 via the POSIX module.
1558 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1561 =item lstat FILEHANDLE
1565 Does the same thing as the stat() function, but stats a symbolic link
1566 instead of the file the symbolic link points to. If symbolic links are
1567 unimplemented on your system, a normal stat() is done.
1571 The match operator. See L<perlop>.
1573 =item map BLOCK LIST
1577 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1578 element) and returns the list value composed of the results of each such
1579 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1580 may produce zero, one, or more elements in the returned value.
1582 @chars = map(chr, @nums);
1584 translates a list of numbers to the corresponding characters. And
1586 %hash = map { getkey($_) => $_ } @array;
1588 is just a funny way to write
1591 foreach $_ (@array) {
1592 $hash{getkey($_)} = $_;
1595 =item mkdir FILENAME,MODE
1597 Creates the directory specified by FILENAME, with permissions specified
1598 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1599 it returns 0 and sets C<$!> (errno).
1601 =item msgctl ID,CMD,ARG
1603 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1604 must be a variable which will hold the returned msqid_ds structure.
1605 Returns like ioctl: the undefined value for error, "0 but true" for
1606 zero, or the actual return value otherwise.
1608 =item msgget KEY,FLAGS
1610 Calls the System V IPC function msgget(2). Returns the message queue id,
1611 or the undefined value if there is an error.
1613 =item msgsnd ID,MSG,FLAGS
1615 Calls the System V IPC function msgsnd to send the message MSG to the
1616 message queue ID. MSG must begin with the long integer message type,
1617 which may be created with C<pack("l", $type)>. Returns TRUE if
1618 successful, or FALSE if there is an error.
1620 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1622 Calls the System V IPC function msgrcv to receive a message from
1623 message queue ID into variable VAR with a maximum message size of
1624 SIZE. Note that if a message is received, the message type will be the
1625 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1626 of the message type. Returns TRUE if successful, or FALSE if there is
1631 A "my" declares the listed variables to be local (lexically) to the
1632 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1633 more than one value is listed, the list must be placed in parens. See
1634 L<perlsub/"Private Variables via my()"> for details.
1640 The C<next> command is like the C<continue> statement in C; it starts
1641 the next iteration of the loop:
1643 LINE: while (<STDIN>) {
1644 next LINE if /^#/; # discard comments
1648 Note that if there were a C<continue> block on the above, it would get
1649 executed even on discarded lines. If the LABEL is omitted, the command
1650 refers to the innermost enclosing loop.
1652 =item no Module LIST
1654 See the "use" function, which "no" is the opposite of.
1658 Interprets EXPR as an octal string and returns the corresponding
1659 decimal value. (If EXPR happens to start off with 0x, interprets it as
1660 a hex string instead.) The following will handle decimal, octal, and
1661 hex in the standard Perl or C notation:
1663 $val = oct($val) if $val =~ /^0/;
1665 If EXPR is omitted, uses $_.
1667 =item open FILEHANDLE,EXPR
1669 =item open FILEHANDLE
1671 Opens the file whose filename is given by EXPR, and associates it with
1672 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1673 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1674 the same name as the FILEHANDLE contains the filename. If the filename
1675 begins with "E<lt>" or nothing, the file is opened for input. If the filename
1676 begins with "E<gt>", the file is opened for output. If the filename begins
1677 with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1678 front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1679 access to the file; thus '+E<lt>' is usually preferred for read/write
1680 updates--the '+E<gt>' mode would clobber the file first. These correspond to
1681 the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1683 If the filename begins with "|", the filename is interpreted
1684 as a command to which output is to be piped, and if the filename ends with
1685 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1686 for more examples of this. as command which pipes input to us. (You may
1687 not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
1688 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1690 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1691 non-zero upon success, the undefined value otherwise. If the open
1692 involved a pipe, the return value happens to be the pid of the
1695 If you're unfortunate enough to be running Perl on a system that
1696 distinguishes between text files and binary files (modern operating
1697 systems don't care), then you should check out L</binmode> for tips for
1698 dealing with this. The key distinction between systems that need binmode
1699 and those that don't is their text file formats. Systems like Unix and
1700 Plan9 that delimit lines with a single character, and that encode that
1701 character in C as '\n', do not need C<binmode>. The rest need it.
1706 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1707 while (<ARTICLE>) {...
1709 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1711 open(DBASE, '+<dbase.mine'); # open for update
1713 open(ARTICLE, "caesar <$article |"); # decrypt article
1715 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1717 # process argument list of files along with any includes
1719 foreach $file (@ARGV) {
1720 process($file, 'fh00');
1724 local($filename, $input) = @_;
1725 $input++; # this is a string increment
1726 unless (open($input, $filename)) {
1727 print STDERR "Can't open $filename: $!\n";
1731 while (<$input>) { # note use of indirection
1732 if (/^#include "(.*)"/) {
1733 process($1, $input);
1740 You may also, in the Bourne shell tradition, specify an EXPR beginning
1741 with "E<gt>&", in which case the rest of the string is interpreted as the
1742 name of a filehandle (or file descriptor, if numeric) which is to be
1743 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1744 +E<gt>E<gt> and +E<lt>. The
1745 mode you specify should match the mode of the original filehandle.
1746 (Duping a filehandle does not take into account any existing contents of
1748 Here is a script that saves, redirects, and restores STDOUT and
1752 open(SAVEOUT, ">&STDOUT");
1753 open(SAVEERR, ">&STDERR");
1755 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1756 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1758 select(STDERR); $| = 1; # make unbuffered
1759 select(STDOUT); $| = 1; # make unbuffered
1761 print STDOUT "stdout 1\n"; # this works for
1762 print STDERR "stderr 1\n"; # subprocesses too
1767 open(STDOUT, ">&SAVEOUT");
1768 open(STDERR, ">&SAVEERR");
1770 print STDOUT "stdout 2\n";
1771 print STDERR "stderr 2\n";
1774 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1775 equivalent of C's fdopen() of that file descriptor; this is more
1776 parsimonious of file descriptors. For example:
1778 open(FILEHANDLE, "<&=$fd")
1780 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1781 there is an implicit fork done, and the return value of open is the pid
1782 of the child within the parent process, and 0 within the child
1783 process. (Use C<defined($pid)> to determine whether the open was successful.)
1784 The filehandle behaves normally for the parent, but i/o to that
1785 filehandle is piped from/to the STDOUT/STDIN of the child process.
1786 In the child process the filehandle isn't opened--i/o happens from/to
1787 the new STDOUT or STDIN. Typically this is used like the normal
1788 piped open when you want to exercise more control over just how the
1789 pipe command gets executed, such as when you are running setuid, and
1790 don't want to have to scan shell commands for metacharacters.
1791 The following pairs are more or less equivalent:
1793 open(FOO, "|tr '[a-z]' '[A-Z]'");
1794 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1796 open(FOO, "cat -n '$file'|");
1797 open(FOO, "-|") || exec 'cat', '-n', $file;
1799 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1801 Explicitly closing any piped filehandle causes the parent process to
1802 wait for the child to finish, and returns the status value in C<$?>.
1803 Note: on any operation which may do a fork, unflushed buffers remain
1804 unflushed in both processes, which means you may need to set C<$|> to
1805 avoid duplicate output.
1807 Using the FileHandle constructor from the FileHandle package,
1808 you can generate anonymous filehandles which have the scope of whatever
1809 variables hold references to them, and automatically close whenever
1810 and however you leave that scope:
1814 sub read_myfile_munged {
1816 my $handle = new FileHandle;
1817 open($handle, "myfile") or die "myfile: $!";
1819 or return (); # Automatically closed here.
1820 mung $first or die "mung failed"; # Or here.
1821 return $first, <$handle> if $ALL; # Or here.
1825 The filename that is passed to open will have leading and trailing
1826 whitespace deleted. In order to open a file with arbitrary weird
1827 characters in it, it's necessary to protect any leading and trailing
1830 $file =~ s#^(\s)#./$1#;
1831 open(FOO, "< $file\0");
1833 If you want a "real" C open() (see L<open(2)> on your system), then
1834 you should use the sysopen() function. This is another way to
1835 protect your filenames from interpretation. For example:
1838 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1839 or die "sysopen $path: $!";
1840 HANDLE->autoflush(1);
1841 HANDLE->print("stuff $$\n");
1843 print "File contains: ", <HANDLE>;
1845 See L</seek()> for some details about mixing reading and writing.
1847 =item opendir DIRHANDLE,EXPR
1849 Opens a directory named EXPR for processing by readdir(), telldir(),
1850 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1851 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1855 Returns the numeric ascii value of the first character of EXPR. If
1856 EXPR is omitted, uses $_.
1858 =item pack TEMPLATE,LIST
1860 Takes an array or list of values and packs it into a binary structure,
1861 returning the string containing the structure. The TEMPLATE is a
1862 sequence of characters that give the order and type of values, as
1865 A An ascii string, will be space padded.
1866 a An ascii string, will be null padded.
1867 b A bit string (ascending bit order, like vec()).
1868 B A bit string (descending bit order).
1869 h A hex string (low nybble first).
1870 H A hex string (high nybble first).
1872 c A signed char value.
1873 C An unsigned char value.
1874 s A signed short value.
1875 S An unsigned short value.
1876 i A signed integer value.
1877 I An unsigned integer value.
1878 l A signed long value.
1879 L An unsigned long value.
1881 n A short in "network" order.
1882 N A long in "network" order.
1883 v A short in "VAX" (little-endian) order.
1884 V A long in "VAX" (little-endian) order.
1886 f A single-precision float in the native format.
1887 d A double-precision float in the native format.
1889 p A pointer to a null-terminated string.
1890 P A pointer to a structure (fixed-length string).
1892 u A uuencoded string.
1894 w A BER compressed integer. Bytes give an unsigned integer base
1895 128, most significant digit first, with as few digits as
1896 possible, and with the bit 8 of each byte except the last set
1901 @ Null fill to absolute position.
1903 Each letter may optionally be followed by a number which gives a repeat
1904 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1905 pack function will gobble up that many values from the LIST. A * for the
1906 repeat count means to use however many items are left. The "a" and "A"
1907 types gobble just one value, but pack it as a string of length count,
1908 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1909 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1910 fields pack a string that many bits long. The "h" and "H" fields pack a
1911 string that many nybbles long. The "P" packs a pointer to a structure of
1912 the size indicated by the length. Real numbers (floats and doubles) are
1913 in the native machine format only; due to the multiplicity of floating
1914 formats around, and the lack of a standard "network" representation, no
1915 facility for interchange has been made. This means that packed floating
1916 point data written on one machine may not be readable on another - even if
1917 both use IEEE floating point arithmetic (as the endian-ness of the memory
1918 representation is not part of the IEEE spec). Note that Perl uses doubles
1919 internally for all numeric calculation, and converting from double into
1920 float and thence back to double again will lose precision (i.e.
1921 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1925 $foo = pack("cccc",65,66,67,68);
1927 $foo = pack("c4",65,66,67,68);
1930 $foo = pack("ccxxcc",65,66,67,68);
1933 $foo = pack("s2",1,2);
1934 # "\1\0\2\0" on little-endian
1935 # "\0\1\0\2" on big-endian
1937 $foo = pack("a4","abcd","x","y","z");
1940 $foo = pack("aaaa","abcd","x","y","z");
1943 $foo = pack("a14","abcdefg");
1944 # "abcdefg\0\0\0\0\0\0\0"
1946 $foo = pack("i9pl", gmtime);
1947 # a real struct tm (on my system anyway)
1950 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1953 The same template may generally also be used in the unpack function.
1955 =item package NAMESPACE
1957 Declares the compilation unit as being in the given namespace. The scope
1958 of the package declaration is from the declaration itself through the end of
1959 the enclosing block (the same scope as the local() operator). All further
1960 unqualified dynamic identifiers will be in this namespace. A package
1961 statement only affects dynamic variables--including those you've used
1962 local() on--but I<not> lexical variables created with my(). Typically it
1963 would be the first declaration in a file to be included by the C<require>
1964 or C<use> operator. You can switch into a package in more than one place;
1965 it merely influences which symbol table is used by the compiler for the
1966 rest of that block. You can refer to variables and filehandles in other
1967 packages by prefixing the identifier with the package name and a double
1968 colon: C<$Package::Variable>. If the package name is null, the C<main>
1969 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1971 See L<perlmod/"Packages"> for more information about packages, modules,
1972 and classes. See L<perlsub> for other scoping issues.
1974 =item pipe READHANDLE,WRITEHANDLE
1976 Opens a pair of connected pipes like the corresponding system call.
1977 Note that if you set up a loop of piped processes, deadlock can occur
1978 unless you are very careful. In addition, note that Perl's pipes use
1979 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
1980 after each command, depending on the application.
1982 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1983 for examples of such things.
1987 Pops and returns the last value of the array, shortening the array by
1988 1. Has a similar effect to
1990 $tmp = $ARRAY[$#ARRAY--];
1992 If there are no elements in the array, returns the undefined value.
1993 If ARRAY is omitted, pops the
1994 @ARGV array in the main program, and the @_ array in subroutines, just
1999 Returns the offset of where the last C<m//g> search left off for the variable
2000 in question. May be modified to change that offset.
2002 =item print FILEHANDLE LIST
2008 Prints a string or a comma-separated list of strings. Returns TRUE
2009 if successful. FILEHANDLE may be a scalar variable name, in which case
2010 the variable contains the name of or a reference to the filehandle, thus introducing one
2011 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2012 token is a term, it may be misinterpreted as an operator unless you
2013 interpose a + or put parens around the arguments.) If FILEHANDLE is
2014 omitted, prints by default to standard output (or to the last selected
2015 output channel--see L</select>). If LIST is also omitted, prints $_ to
2016 STDOUT. To set the default output channel to something other than
2017 STDOUT use the select operation. Note that, because print takes a
2018 LIST, anything in the LIST is evaluated in a list context, and any
2019 subroutine that you call will have one or more of its expressions
2020 evaluated in a list context. Also be careful not to follow the print
2021 keyword with a left parenthesis unless you want the corresponding right
2022 parenthesis to terminate the arguments to the print--interpose a + or
2023 put parens around all the arguments.
2025 Note that if you're storing FILEHANDLES in an array or other expression,
2026 you will have to use a block returning its value instead:
2028 print { $files[$i] } "stuff\n";
2029 print { $OK ? STDOUT : STDERR } "stuff\n";
2031 =item printf FILEHANDLE LIST
2035 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2036 of the list will be interpreted as the printf format.
2038 =item prototype FUNCTION
2040 Returns the prototype of a function as a string (or C<undef> if the
2041 function has no prototype). FUNCTION is a reference to the the
2042 function whose prototype you want to retrieve.
2044 =item push ARRAY,LIST
2046 Treats ARRAY as a stack, and pushes the values of LIST
2047 onto the end of ARRAY. The length of ARRAY increases by the length of
2048 LIST. Has the same effect as
2051 $ARRAY[++$#ARRAY] = $value;
2054 but is more efficient. Returns the new number of elements in the array.
2064 Generalized quotes. See L<perlop>.
2066 =item quotemeta EXPR
2068 Returns the value of EXPR with with all regular expression
2069 metacharacters backslashed. This is the internal function implementing
2070 the \Q escape in double-quoted strings.
2076 Returns a random fractional number between 0 and the value of EXPR.
2077 (EXPR should be positive.) If EXPR is omitted, returns a value between
2078 0 and 1. This function produces repeatable sequences unless srand()
2079 is invoked. See also srand().
2081 (Note: if your rand function consistently returns numbers that are too
2082 large or too small, then your version of Perl was probably compiled
2083 with the wrong number of RANDBITS. As a workaround, you can usually
2084 multiply EXPR by the correct power of 2 to get the range you want.
2085 This will make your script unportable, however. It's better to recompile
2088 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2090 =item read FILEHANDLE,SCALAR,LENGTH
2092 Attempts to read LENGTH bytes of data into variable SCALAR from the
2093 specified FILEHANDLE. Returns the number of bytes actually read, or
2094 undef if there was an error. SCALAR will be grown or shrunk to the
2095 length actually read. An OFFSET may be specified to place the read
2096 data at some other place than the beginning of the string. This call
2097 is actually implemented in terms of stdio's fread call. To get a true
2098 read system call, see sysread().
2100 =item readdir DIRHANDLE
2102 Returns the next directory entry for a directory opened by opendir().
2103 If used in a list context, returns all the rest of the entries in the
2104 directory. If there are no more entries, returns an undefined value in
2105 a scalar context or a null list in a list context.
2107 If you're planning to filetest the return values out of a readdir(), you'd
2108 better prepend the directory in question. Otherwise, since we didn't
2109 chdir() there, it would have been testing the wrong file.
2111 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2112 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2117 Returns the value of a symbolic link, if symbolic links are
2118 implemented. If not, gives a fatal error. If there is some system
2119 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2122 =item recv SOCKET,SCALAR,LEN,FLAGS
2124 Receives a message on a socket. Attempts to receive LENGTH bytes of
2125 data into variable SCALAR from the specified SOCKET filehandle.
2126 Actually does a C recvfrom(), so that it can returns the address of the
2127 sender. Returns the undefined value if there's an error. SCALAR will
2128 be grown or shrunk to the length actually read. Takes the same flags
2129 as the system call of the same name.
2130 See L<perlipc/"UDP: Message Passing"> for examples.
2136 The C<redo> command restarts the loop block without evaluating the
2137 conditional again. The C<continue> block, if any, is not executed. If
2138 the LABEL is omitted, the command refers to the innermost enclosing
2139 loop. This command is normally used by programs that want to lie to
2140 themselves about what was just input:
2142 # a simpleminded Pascal comment stripper
2143 # (warning: assumes no { or } in strings)
2144 LINE: while (<STDIN>) {
2145 while (s|({.*}.*){.*}|$1 |) {}
2150 if (/}/) { # end of comment?
2161 Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2162 returned depends on the type of thing the reference is a reference to.
2163 Builtin types include:
2172 If the referenced object has been blessed into a package, then that package
2173 name is returned instead. You can think of ref() as a typeof() operator.
2175 if (ref($r) eq "HASH") {
2176 print "r is a reference to an associative array.\n";
2179 print "r is not a reference at all.\n";
2182 See also L<perlref>.
2184 =item rename OLDNAME,NEWNAME
2186 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2187 not work across filesystem boundaries.
2193 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2194 supplied. If EXPR is numeric, demands that the current version of Perl
2195 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2197 Otherwise, demands that a library file be included if it hasn't already
2198 been included. The file is included via the do-FILE mechanism, which is
2199 essentially just a variety of eval(). Has semantics similar to the following
2203 local($filename) = @_;
2204 return 1 if $INC{$filename};
2205 local($realfilename,$result);
2207 foreach $prefix (@INC) {
2208 $realfilename = "$prefix/$filename";
2209 if (-f $realfilename) {
2210 $result = do $realfilename;
2214 die "Can't find $filename in \@INC";
2217 die "$filename did not return true value" unless $result;
2218 $INC{$filename} = $realfilename;
2222 Note that the file will not be included twice under the same specified
2223 name. The file must return TRUE as the last statement to indicate
2224 successful execution of any initialization code, so it's customary to
2225 end such a file with "1;" unless you're sure it'll return TRUE
2226 otherwise. But it's better just to put the "C<1;>", in case you add more
2229 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2230 replaces "F<::>" with "F</>" in the filename for you,
2231 to make it easy to load standard modules. This form of loading of
2232 modules does not risk altering your namespace.
2234 For a yet-more-powerful import facility, see L</use> and
2241 Generally used in a C<continue> block at the end of a loop to clear
2242 variables and reset ?? searches so that they work again. The
2243 expression is interpreted as a list of single characters (hyphens
2244 allowed for ranges). All variables and arrays beginning with one of
2245 those letters are reset to their pristine state. If the expression is
2246 omitted, one-match searches (?pattern?) are reset to match again. Only
2247 resets variables or searches in the current package. Always returns
2250 reset 'X'; # reset all X variables
2251 reset 'a-z'; # reset lower case variables
2252 reset; # just reset ?? searches
2254 Resetting "A-Z" is not recommended since you'll wipe out your
2255 ARGV and ENV arrays. Only resets package variables--lexical variables
2256 are unaffected, but they clean themselves up on scope exit anyway,
2257 so you'll probably want to use them instead. See L</my>.
2261 Returns from a subroutine or eval with the value specified. (Note that
2262 in the absence of a return a subroutine or eval() will automatically
2263 return the value of the last expression evaluated.)
2267 In a list context, returns a list value consisting of the elements
2268 of LIST in the opposite order. In a scalar context, returns a string
2269 value consisting of the bytes of the first element of LIST in the
2272 print reverse <>; # line tac
2275 print scalar reverse scalar <>; # byte tac
2277 =item rewinddir DIRHANDLE
2279 Sets the current position to the beginning of the directory for the
2280 readdir() routine on DIRHANDLE.
2282 =item rindex STR,SUBSTR,POSITION
2284 =item rindex STR,SUBSTR
2286 Works just like index except that it returns the position of the LAST
2287 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2288 last occurrence at or before that position.
2290 =item rmdir FILENAME
2292 Deletes the directory specified by FILENAME if it is empty. If it
2293 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2294 FILENAME is omitted, uses $_.
2298 The substitution operator. See L<perlop>.
2302 Forces EXPR to be interpreted in a scalar context and returns the value
2305 @counts = ( scalar @a, scalar @b, scalar @c );
2307 There is no equivalent operator to force an expression to
2308 be interpolated in a list context because it's in practice never
2309 needed. If you really wanted to do so, however, you could use
2310 the construction C<@{[ (some expression) ]}>, but usually a simple
2311 C<(some expression)> suffices.
2313 =item seek FILEHANDLE,POSITION,WHENCE
2315 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2316 call of stdio. FILEHANDLE may be an expression whose value gives the name
2317 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2318 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2319 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2320 this from POSIX module. Returns 1 upon success, 0 otherwise.
2322 On some systems you have to do a seek whenever you switch between reading
2323 and writing. Amongst other things, this may have the effect of calling
2324 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2329 This is also useful for applications emulating C<tail -f>. Once you hit
2330 EOF on your read, and then sleep for a while, you might have to stick in a
2331 seek() to reset things. First the simple trick listed above to clear the
2332 filepointer. The seek() doesn't change the current position, but it
2333 I<does> clear the end-of-file condition on the handle, so that the next
2334 C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
2336 If that doesn't work (some stdios are particularly cantankerous), then
2337 you may need something more like this:
2340 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2341 # search for some stuff and put it into files
2343 sleep($for_a_while);
2344 seek(FILE, $curpos, 0);
2347 =item seekdir DIRHANDLE,POS
2349 Sets the current position for the readdir() routine on DIRHANDLE. POS
2350 must be a value returned by telldir(). Has the same caveats about
2351 possible directory compaction as the corresponding system library
2354 =item select FILEHANDLE
2358 Returns the currently selected filehandle. Sets the current default
2359 filehandle for output, if FILEHANDLE is supplied. This has two
2360 effects: first, a C<write> or a C<print> without a filehandle will
2361 default to this FILEHANDLE. Second, references to variables related to
2362 output will refer to this output channel. For example, if you have to
2363 set the top of form format for more than one output channel, you might
2371 FILEHANDLE may be an expression whose value gives the name of the
2372 actual filehandle. Thus:
2374 $oldfh = select(STDERR); $| = 1; select($oldfh);
2376 Some programmers may prefer to think of filehandles as objects with
2377 methods, preferring to write the last example as:
2380 STDERR->autoflush(1);
2382 =item select RBITS,WBITS,EBITS,TIMEOUT
2384 This calls the select(2) system call with the bitmasks specified, which
2385 can be constructed using fileno() and vec(), along these lines:
2387 $rin = $win = $ein = '';
2388 vec($rin,fileno(STDIN),1) = 1;
2389 vec($win,fileno(STDOUT),1) = 1;
2392 If you want to select on many filehandles you might wish to write a
2396 local(@fhlist) = split(' ',$_[0]);
2399 vec($bits,fileno($_),1) = 1;
2403 $rin = fhbits('STDIN TTY SOCK');
2407 ($nfound,$timeleft) =
2408 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2410 or to block until something becomes ready just do this
2412 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2414 Most systems do not both to return anything useful in $timeleft, so
2415 calling select() in a scalar context just returns $nfound.
2417 Any of the bitmasks can also be undef. The timeout, if specified, is
2418 in seconds, which may be fractional. Note: not all implementations are
2419 capable of returning the $timeleft. If not, they always return
2420 $timeleft equal to the supplied $timeout.
2422 You can effect a 250-millisecond sleep this way:
2424 select(undef, undef, undef, 0.25);
2426 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2427 with select(). You have to use sysread() instead.
2429 =item semctl ID,SEMNUM,CMD,ARG
2431 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2432 &GETALL, then ARG must be a variable which will hold the returned
2433 semid_ds structure or semaphore value array. Returns like ioctl: the
2434 undefined value for error, "0 but true" for zero, or the actual return
2437 =item semget KEY,NSEMS,FLAGS
2439 Calls the System V IPC function semget. Returns the semaphore id, or
2440 the undefined value if there is an error.
2442 =item semop KEY,OPSTRING
2444 Calls the System V IPC function semop to perform semaphore operations
2445 such as signaling and waiting. OPSTRING must be a packed array of
2446 semop structures. Each semop structure can be generated with
2447 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2448 operations is implied by the length of OPSTRING. Returns TRUE if
2449 successful, or FALSE if there is an error. As an example, the
2450 following code waits on semaphore $semnum of semaphore id $semid:
2452 $semop = pack("sss", $semnum, -1, 0);
2453 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2455 To signal the semaphore, replace "-1" with "1".
2457 =item send SOCKET,MSG,FLAGS,TO
2459 =item send SOCKET,MSG,FLAGS
2461 Sends a message on a socket. Takes the same flags as the system call
2462 of the same name. On unconnected sockets you must specify a
2463 destination to send TO, in which case it does a C sendto(). Returns
2464 the number of characters sent, or the undefined value if there is an
2466 See L<perlipc/"UDP: Message Passing"> for examples.
2468 =item setpgrp PID,PGRP
2470 Sets the current process group for the specified PID, 0 for the current
2471 process. Will produce a fatal error if used on a machine that doesn't
2472 implement setpgrp(2). If the arguments are ommitted, it defaults to
2473 0,0. Note that the POSIX version of setpgrp() does not accept any
2474 arguments, so only setpgrp 0,0 is portable.
2476 =item setpriority WHICH,WHO,PRIORITY
2478 Sets the current priority for a process, a process group, or a user.
2479 (See setpriority(2).) Will produce a fatal error if used on a machine
2480 that doesn't implement setpriority(2).
2482 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2484 Sets the socket option requested. Returns undefined if there is an
2485 error. OPTVAL may be specified as undef if you don't want to pass an
2492 Shifts the first value of the array off and returns it, shortening the
2493 array by 1 and moving everything down. If there are no elements in the
2494 array, returns the undefined value. If ARRAY is omitted, shifts the
2495 @ARGV array in the main program, and the @_ array in subroutines.
2496 (This is determined lexically.) See also unshift(), push(), and pop().
2497 Shift() and unshift() do the same thing to the left end of an array
2498 that push() and pop() do to the right end.
2500 =item shmctl ID,CMD,ARG
2502 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2503 must be a variable which will hold the returned shmid_ds structure.
2504 Returns like ioctl: the undefined value for error, "0 but true" for
2505 zero, or the actual return value otherwise.
2507 =item shmget KEY,SIZE,FLAGS
2509 Calls the System V IPC function shmget. Returns the shared memory
2510 segment id, or the undefined value if there is an error.
2512 =item shmread ID,VAR,POS,SIZE
2514 =item shmwrite ID,STRING,POS,SIZE
2516 Reads or writes the System V shared memory segment ID starting at
2517 position POS for size SIZE by attaching to it, copying in/out, and
2518 detaching from it. When reading, VAR must be a variable which will
2519 hold the data read. When writing, if STRING is too long, only SIZE
2520 bytes are used; if STRING is too short, nulls are written to fill out
2521 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2523 =item shutdown SOCKET,HOW
2525 Shuts down a socket connection in the manner indicated by HOW, which
2526 has the same interpretation as in the system call of the same name.
2530 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2537 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2538 May be interrupted by sending the process a SIGALRM. Returns the
2539 number of seconds actually slept. You probably cannot mix alarm() and
2540 sleep() calls, since sleep() is often implemented using alarm().
2542 On some older systems, it may sleep up to a full second less than what
2543 you requested, depending on how it counts seconds. Most modern systems
2544 always sleep the full amount.
2546 For delays of finer granularity than one second, you may use Perl's
2547 syscall() interface to access setitimer(2) if your system supports it,
2548 or else see L</select()> below.
2550 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2552 Opens a socket of the specified kind and attaches it to filehandle
2553 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2554 system call of the same name. You should "use Socket;" first to get
2555 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2557 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2559 Creates an unnamed pair of sockets in the specified domain, of the
2560 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2561 for the system call of the same name. If unimplemented, yields a fatal
2562 error. Returns TRUE if successful.
2564 =item sort SUBNAME LIST
2566 =item sort BLOCK LIST
2570 Sorts the LIST and returns the sorted list value. Nonexistent values
2571 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2572 in standard string comparison order. If SUBNAME is specified, it
2573 gives the name of a subroutine that returns an integer less than, equal
2574 to, or greater than 0, depending on how the elements of the array are
2575 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2576 routines.) SUBNAME may be a scalar variable name, in which case the
2577 value provides the name of the subroutine to use. In place of a
2578 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2581 In the interests of efficiency the normal calling code for subroutines is
2582 bypassed, with the following effects: the subroutine may not be a
2583 recursive subroutine, and the two elements to be compared are passed into
2584 the subroutine not via @_ but as the package global variables $a and
2585 $b (see example below). They are passed by reference, so don't
2586 modify $a and $b. And don't try to declare them as lexicals either.
2591 @articles = sort @files;
2593 # same thing, but with explicit sort routine
2594 @articles = sort {$a cmp $b} @files;
2596 # now case-insensitively
2597 @articles = sort { uc($a) cmp uc($b)} @files;
2599 # same thing in reversed order
2600 @articles = sort {$b cmp $a} @files;
2602 # sort numerically ascending
2603 @articles = sort {$a <=> $b} @files;
2605 # sort numerically descending
2606 @articles = sort {$b <=> $a} @files;
2608 # sort using explicit subroutine name
2610 $age{$a} <=> $age{$b}; # presuming integers
2612 @sortedclass = sort byage @class;
2614 # this sorts the %age associative arrays by value
2615 # instead of key using an inline function
2616 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2618 sub backwards { $b cmp $a; }
2619 @harry = ('dog','cat','x','Cain','Abel');
2620 @george = ('gone','chased','yz','Punished','Axed');
2622 # prints AbelCaincatdogx
2623 print sort backwards @harry;
2624 # prints xdogcatCainAbel
2625 print sort @george, 'to', @harry;
2626 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2628 # inefficiently sort by descending numeric compare using
2629 # the first integer after the first = sign, or the
2630 # whole record case-insensitively otherwise
2633 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2638 # same thing, but much more efficiently;
2639 # we'll build auxiliary indices instead
2643 push @nums, /=(\d+)/;
2648 $nums[$b] <=> $nums[$a]
2650 $caps[$a] cmp $caps[$b]
2654 # same thing using a Schwartzian Transform (no temps)
2655 @new = map { $_->[0] }
2656 sort { $b->[1] <=> $a->[1]
2659 } map { [$_, /=(\d+)/, uc($_)] } @old;
2661 If you're using strict, you I<MUST NOT> declare $a
2662 and $b as lexicals. They are package globals. That means
2663 if you're in the C<main> package, it's
2665 @articles = sort {$main::b <=> $main::a} @files;
2669 @articles = sort {$::b <=> $::a} @files;
2671 but if you're in the C<FooPack> package, it's
2673 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2675 The comparison function is required to behave. If it returns
2676 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2677 sometimes saying the opposite, for example) the Perl interpreter will
2678 probably crash and dump core. This is entirely due to and dependent
2679 upon your system's qsort(3) library routine; this routine often avoids
2680 sanity checks in the interest of speed.
2682 =item splice ARRAY,OFFSET,LENGTH,LIST
2684 =item splice ARRAY,OFFSET,LENGTH
2686 =item splice ARRAY,OFFSET
2688 Removes the elements designated by OFFSET and LENGTH from an array, and
2689 replaces them with the elements of LIST, if any. Returns the elements
2690 removed from the array. The array grows or shrinks as necessary. If
2691 LENGTH is omitted, removes everything from OFFSET onward. The
2692 following equivalencies hold (assuming C<$[ == 0>):
2694 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2695 pop(@a) splice(@a,-1)
2696 shift(@a) splice(@a,0,1)
2697 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2698 $a[$x] = $y splice(@a,$x,1,$y);
2700 Example, assuming array lengths are passed before arrays:
2702 sub aeq { # compare two list values
2703 local(@a) = splice(@_,0,shift);
2704 local(@b) = splice(@_,0,shift);
2705 return 0 unless @a == @b; # same len?
2707 return 0 if pop(@a) ne pop(@b);
2711 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2713 =item split /PATTERN/,EXPR,LIMIT
2715 =item split /PATTERN/,EXPR
2717 =item split /PATTERN/
2721 Splits a string into an array of strings, and returns it.
2723 If not in a list context, returns the number of fields found and splits into
2724 the @_ array. (In a list context, you can force the split into @_ by
2725 using C<??> as the pattern delimiters, but it still returns the array
2726 value.) The use of implicit split to @_ is deprecated, however.
2728 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2729 splits on whitespace (after skipping any leading whitespace). Anything
2730 matching PATTERN is taken to be a delimiter separating the fields. (Note
2731 that the delimiter may be longer than one character.) If LIMIT is
2732 specified and is not negative, splits into no more than that many fields
2733 (though it may split into fewer). If LIMIT is unspecified, trailing null
2734 fields are stripped (which potential users of pop() would do well to
2735 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2736 LIMIT had been specified.
2738 A pattern matching the null string (not to be confused with
2739 a null pattern C<//>, which is just one member of the set of patterns
2740 matching a null string) will split the value of EXPR into separate
2741 characters at each point it matches that way. For example:
2743 print join(':', split(/ */, 'hi there'));
2745 produces the output 'h:i:t:h:e:r:e'.
2747 The LIMIT parameter can be used to partially split a line
2749 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2751 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2752 one larger than the number of variables in the list, to avoid
2753 unnecessary work. For the list above LIMIT would have been 4 by
2754 default. In time critical applications it behooves you not to split
2755 into more fields than you really need.
2757 If the PATTERN contains parentheses, additional array elements are
2758 created from each matching substring in the delimiter.
2760 split(/([,-])/, "1-10,20", 3);
2762 produces the list value
2764 (1, '-', 10, ',', 20)
2766 If you had the entire header of a normal Unix email message in $header,
2767 you could split it up into fields and their values this way:
2769 $header =~ s/\n\s+/ /g; # fix continuation lines
2770 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2772 The pattern C</PATTERN/> may be replaced with an expression to specify
2773 patterns that vary at runtime. (To do runtime compilation only once,
2774 use C</$variable/o>.)
2776 As a special case, specifying a PATTERN of space (C<' '>) will split on
2777 white space just as split with no arguments does. Thus, split(' ') can
2778 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2779 will give you as many null initial fields as there are leading spaces.
2780 A split on /\s+/ is like a split(' ') except that any leading
2781 whitespace produces a null first field. A split with no arguments
2782 really does a C<split(' ', $_)> internally.
2786 open(passwd, '/etc/passwd');
2788 ($login, $passwd, $uid, $gid, $gcos,
2789 $home, $shell) = split(/:/);
2793 (Note that $shell above will still have a newline on it. See L</chop>,
2794 L</chomp>, and L</join>.)
2796 =item sprintf FORMAT,LIST
2798 Returns a string formatted by the usual printf conventions of the C
2799 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2800 (The * character for an indirectly specified length is not
2801 supported, but you can get the same effect by interpolating a variable
2802 into the pattern.) Some C libraries' implementations of sprintf() can
2803 dump core when fed ludicrous arguments.
2807 Return the square root of EXPR. If EXPR is omitted, returns square
2812 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2813 uses a semirandom value based on the current time and process ID, among
2814 other things. Of course, you'd need something much more random than that for
2815 cryptographic purposes, since it's easy to guess the current time.
2816 Checksumming the compressed output of rapidly changing operating system
2817 status programs is the usual method. Examples are posted regularly to
2818 the comp.security.unix newsgroup.
2820 =item stat FILEHANDLE
2824 Returns a 13-element array giving the status info for a file, either the
2825 file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2826 the stat fails. Typically used as follows:
2828 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2829 $atime,$mtime,$ctime,$blksize,$blocks)
2832 Not all fields are supported on all filesystem types. Here are the
2833 meaning of the fields:
2835 dev device number of filesystem
2837 mode file mode (type and permissions)
2838 nlink number of (hard) links to the file
2839 uid numeric user ID of file's owner
2840 gid numer group ID of file's owner
2841 rdev the device identifier (special files only)
2842 size total size of file, in bytes
2843 atime last access time since the epoch
2844 mtime last modify time since the epoch
2845 ctime inode change time (NOT creation type!) since the epoch
2846 blksize preferred blocksize for file system I/O
2847 blocks actual number of blocks allocated
2849 (The epoch was at 00:00 January 1, 1970 GMT.)
2851 If stat is passed the special filehandle consisting of an underline, no
2852 stat is done, but the current contents of the stat structure from the
2853 last stat or filetest are returned. Example:
2855 if (-x $file && (($d) = stat(_)) && $d < 0) {
2856 print "$file is executable NFS file\n";
2859 (This only works on machines for which the device number is negative under NFS.)
2865 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2866 doing many pattern matches on the string before it is next modified.
2867 This may or may not save time, depending on the nature and number of
2868 patterns you are searching on, and on the distribution of character
2869 frequencies in the string to be searched--you probably want to compare
2870 runtimes with and without it to see which runs faster. Those loops
2871 which scan for many short constant strings (including the constant
2872 parts of more complex patterns) will benefit most. You may have only
2873 one study active at a time--if you study a different scalar the first
2874 is "unstudied". (The way study works is this: a linked list of every
2875 character in the string to be searched is made, so we know, for
2876 example, where all the 'k' characters are. From each search string,
2877 the rarest character is selected, based on some static frequency tables
2878 constructed from some C programs and English text. Only those places
2879 that contain this "rarest" character are examined.)
2881 For example, here is a loop which inserts index producing entries
2882 before any line containing a certain pattern:
2886 print ".IX foo\n" if /\bfoo\b/;
2887 print ".IX bar\n" if /\bbar\b/;
2888 print ".IX blurfl\n" if /\bblurfl\b/;
2893 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2894 will be looked at, because "f" is rarer than "o". In general, this is
2895 a big win except in pathological cases. The only question is whether
2896 it saves you more time than it took to build the linked list in the
2899 Note that if you have to look for strings that you don't know till
2900 runtime, you can build an entire loop as a string and eval that to
2901 avoid recompiling all your patterns all the time. Together with
2902 undefining $/ to input entire files as one record, this can be very
2903 fast, often faster than specialized programs like fgrep(1). The following
2904 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
2905 out the names of those files that contain a match:
2907 $search = 'while (<>) { study;';
2908 foreach $word (@words) {
2909 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2914 eval $search; # this screams
2915 $/ = "\n"; # put back to normal input delim
2916 foreach $file (sort keys(%seen)) {
2924 =item sub NAME BLOCK
2926 This is subroutine definition, not a real function I<per se>. With just a
2927 NAME (and possibly prototypes), it's just a forward declaration. Without
2928 a NAME, it's an anonymous function declaration, and does actually return a
2929 value: the CODE ref of the closure you just created. See L<perlsub> and
2930 L<perlref> for details.
2932 =item substr EXPR,OFFSET,LEN
2934 =item substr EXPR,OFFSET
2936 Extracts a substring out of EXPR and returns it. First character is at
2937 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2938 that far from the end of the string. If LEN is omitted, returns
2939 everything to the end of the string. If LEN is negative, leaves that
2940 many characters off the end of the string.
2942 You can use the substr() function
2943 as an lvalue, in which case EXPR must be an lvalue. If you assign
2944 something shorter than LEN, the string will shrink, and if you assign
2945 something longer than LEN, the string will grow to accommodate it. To
2946 keep the string the same length you may need to pad or chop your value
2949 =item symlink OLDFILE,NEWFILE
2951 Creates a new filename symbolically linked to the old filename.
2952 Returns 1 for success, 0 otherwise. On systems that don't support
2953 symbolic links, produces a fatal error at run time. To check for that,
2956 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2960 Calls the system call specified as the first element of the list,
2961 passing the remaining elements as arguments to the system call. If
2962 unimplemented, produces a fatal error. The arguments are interpreted
2963 as follows: if a given argument is numeric, the argument is passed as
2964 an int. If not, the pointer to the string value is passed. You are
2965 responsible to make sure a string is pre-extended long enough to
2966 receive any result that might be written into a string. If your
2967 integer arguments are not literals and have never been interpreted in a
2968 numeric context, you may need to add 0 to them to force them to look
2971 require 'syscall.ph'; # may need to run h2ph
2972 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2974 Note that Perl only supports passing of up to 14 arguments to your system call,
2975 which in practice should usually suffice.
2977 =item sysopen FILEHANDLE,FILENAME,MODE
2979 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
2981 Opens the file whose filename is given by FILENAME, and associates it
2982 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
2983 the name of the real filehandle wanted. This function calls the
2984 underlying operating system's C<open> function with the parameters
2985 FILENAME, MODE, PERMS.
2987 The possible values and flag bits of the MODE parameter are
2988 system-dependent; they are available via the standard module C<Fcntl>.
2989 However, for historical reasons, some values are universal: zero means
2990 read-only, one means write-only, and two means read/write.
2992 If the file named by FILENAME does not exist and the C<open> call
2993 creates it (typically because MODE includes the O_CREAT flag), then
2994 the value of PERMS specifies the permissions of the newly created
2995 file. If PERMS is omitted, the default value is 0666, which allows
2996 read and write for all. This default is reasonable: see C<umask>.
2998 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3000 =item sysread FILEHANDLE,SCALAR,LENGTH
3002 Attempts to read LENGTH bytes of data into variable SCALAR from the
3003 specified FILEHANDLE, using the system call read(2). It bypasses
3004 stdio, so mixing this with other kinds of reads may cause confusion.
3005 Returns the number of bytes actually read, or undef if there was an
3006 error. SCALAR will be grown or shrunk to the length actually read.
3007 An OFFSET may be specified to place the read data at some other
3008 place than the beginning of the string. A negative OFFSET counts
3009 backwards from the end of the string.
3013 Does exactly the same thing as "exec LIST" except that a fork is done
3014 first, and the parent process waits for the child process to complete.
3015 Note that argument processing varies depending on the number of
3016 arguments. The return value is the exit status of the program as
3017 returned by the wait() call. To get the actual exit value divide by
3018 256. See also L</exec>. This is I<NOT> what you want to use to capture
3019 the output from a command, for that you should merely use backticks, as
3020 described in L<perlop/"`STRING`">.
3022 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3024 =item syswrite FILEHANDLE,SCALAR,LENGTH
3026 Attempts to write LENGTH bytes of data from variable SCALAR to the
3027 specified FILEHANDLE, using the system call write(2). It bypasses
3028 stdio, so mixing this with prints may cause confusion. Returns the
3029 number of bytes actually written, or undef if there was an error.
3030 An OFFSET may be specified to get the write data from some other place
3031 than the beginning of the string. A negative OFFSET counts backwards
3032 from the end of the string.
3034 =item tell FILEHANDLE
3038 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3039 expression whose value gives the name of the actual filehandle. If
3040 FILEHANDLE is omitted, assumes the file last read.
3042 =item telldir DIRHANDLE
3044 Returns the current position of the readdir() routines on DIRHANDLE.
3045 Value may be given to seekdir() to access a particular location in a
3046 directory. Has the same caveats about possible directory compaction as
3047 the corresponding system library routine.
3049 =item tie VARIABLE,CLASSNAME,LIST
3051 This function binds a variable to a package class that will provide the
3052 implementation for the variable. VARIABLE is the name of the variable
3053 to be enchanted. CLASSNAME is the name of a class implementing objects
3054 of correct type. Any additional arguments are passed to the "new"
3055 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3056 Typically these are arguments such as might be passed to the dbm_open()
3057 function of C. The object returned by the "new" method is also
3058 returned by the tie() function, which would be useful if you want to
3059 access other methods in CLASSNAME.
3061 Note that functions such as keys() and values() may return huge array
3062 values when used on large objects, like DBM files. You may prefer to
3063 use the each() function to iterate over such. Example:
3065 # print out history file offsets
3067 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3068 while (($key,$val) = each %HIST) {
3069 print $key, ' = ', unpack('L',$val), "\n";
3073 A class implementing an associative array should have the following
3076 TIEHASH classname, LIST
3079 STORE this, key, value
3083 NEXTKEY this, lastkey
3085 A class implementing an ordinary array should have the following methods:
3087 TIEARRAY classname, LIST
3090 STORE this, key, value
3093 A class implementing a scalar should have the following methods:
3095 TIESCALAR classname, LIST
3100 Unlike dbmopen(), the tie() function will not use or require a module
3101 for you--you need to do that explicitly yourself. See L<DB_File>
3102 or the F<Config> module for interesting tie() implementations.
3106 Returns a reference to the object underlying VARIABLE (the same value
3107 that was originally returned by the tie() call which bound the variable
3108 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3113 Returns the number of non-leap seconds since whatever time the system
3114 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3115 and 00:00:00 UTC, January 1, 1970 for most other systems).
3116 Suitable for feeding to gmtime() and localtime().
3120 Returns a four-element array giving the user and system times, in
3121 seconds, for this process and the children of this process.
3123 ($user,$system,$cuser,$csystem) = times;
3127 The translation operator. See L<perlop>.
3129 =item truncate FILEHANDLE,LENGTH
3131 =item truncate EXPR,LENGTH
3133 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3134 specified length. Produces a fatal error if truncate isn't implemented
3139 Returns an uppercased version of EXPR. This is the internal function
3140 implementing the \U escape in double-quoted strings.
3141 Should respect any POSIX setlocale() settings.
3145 Returns the value of EXPR with the first character uppercased. This is
3146 the internal function implementing the \u escape in double-quoted strings.
3147 Should respect any POSIX setlocale() settings.
3153 Sets the umask for the process and returns the old one. If EXPR is
3154 omitted, merely returns current umask.
3160 Undefines the value of EXPR, which must be an lvalue. Use only on a
3161 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3162 will probably not do what you expect on most predefined variables or
3163 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3164 the EXPR, in which case nothing is undefined, but you still get an
3165 undefined value that you could, for instance, return from a
3166 subroutine. Examples:
3169 undef $bar{'blurfl'};
3173 return (wantarray ? () : undef) if $they_blew_it;
3177 Deletes a list of files. Returns the number of files successfully
3180 $cnt = unlink 'a', 'b', 'c';
3184 Note: unlink will not delete directories unless you are superuser and
3185 the B<-U> flag is supplied to Perl. Even if these conditions are
3186 met, be warned that unlinking a directory can inflict damage on your
3187 filesystem. Use rmdir instead.
3189 =item unpack TEMPLATE,EXPR
3191 Unpack does the reverse of pack: it takes a string representing a
3192 structure and expands it out into a list value, returning the array
3193 value. (In a scalar context, it merely returns the first value
3194 produced.) The TEMPLATE has the same format as in the pack function.
3195 Here's a subroutine that does substring:
3198 local($what,$where,$howmuch) = @_;
3199 unpack("x$where a$howmuch", $what);
3204 sub ordinal { unpack("c",$_[0]); } # same as ord()
3206 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3207 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3208 themselves. Default is a 16-bit checksum. For example, the following
3209 computes the same number as the System V sum program:
3212 $checksum += unpack("%16C*", $_);
3216 The following efficiently counts the number of set bits in a bit vector:
3218 $setbits = unpack("%32b*", $selectmask);
3220 =item untie VARIABLE
3222 Breaks the binding between a variable and a package. (See tie().)
3224 =item unshift ARRAY,LIST
3226 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3227 depending on how you look at it. Prepends list to the front of the
3228 array, and returns the new number of elements in the array.
3230 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3232 Note the LIST is prepended whole, not one element at a time, so the
3233 prepended elements stay in the same order. Use reverse to do the
3236 =item use Module LIST
3240 =item use Module VERSION LIST
3244 Imports some semantics into the current package from the named module,
3245 generally by aliasing certain subroutine or variable names into your
3246 package. It is exactly equivalent to
3248 BEGIN { require Module; import Module LIST; }
3250 except that Module I<must> be a bare word.
3252 If the first argument to C<use> is a number, it is treated as a version
3253 number instead of a module name. If the version of the Perl interpreter
3254 is less than VERSION, then an error message is printed and Perl exits
3255 immediately. This is often useful if you need to check the current
3256 Perl version before C<use>ing library modules which have changed in
3257 incompatible ways from older versions of Perl. (We try not to do
3258 this more than we have to.)
3260 The BEGIN forces the require and import to happen at compile time. The
3261 require makes sure the module is loaded into memory if it hasn't been
3262 yet. The import is not a builtin--it's just an ordinary static method
3263 call into the "Module" package to tell the module to import the list of
3264 features back into the current package. The module can implement its
3265 import method any way it likes, though most modules just choose to
3266 derive their import method via inheritance from the Exporter class that
3267 is defined in the Exporter module. See L<Exporter>. If no import
3268 method can be found then the error is currently silently ignored. This
3269 may change to a fatal error in a future version.
3271 If you don't want your namespace altered, explicitly supply an empty list:
3275 That is exactly equivalent to
3277 BEGIN { require Module; }
3279 If the VERSION argument is present between Module and LIST, then the
3280 C<use> will fail if the C<$VERSION> variable in package Module is
3283 Because this is a wide-open interface, pragmas (compiler directives)
3284 are also implemented this way. Currently implemented pragmas are:
3288 use sigtrap qw(SEGV BUS);
3289 use strict qw(subs vars refs);
3290 use subs qw(afunc blurfl);
3292 These pseudomodules import semantics into the current block scope, unlike
3293 ordinary modules, which import symbols into the current package (which are
3294 effective through the end of the file).
3296 There's a corresponding "no" command that unimports meanings imported
3297 by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
3302 If no unimport method can be found the call fails with a fatal error.
3304 See L<perlmod> for a list of standard modules and pragmas.
3308 Changes the access and modification times on each file of a list of
3309 files. The first two elements of the list must be the NUMERICAL access
3310 and modification times, in that order. Returns the number of files
3311 successfully changed. The inode modification time of each file is set
3312 to the current time. Example of a "touch" command:
3316 utime $now, $now, @ARGV;
3318 =item values ASSOC_ARRAY
3320 Returns a normal array consisting of all the values of the named
3321 associative array. (In a scalar context, returns the number of
3322 values.) The values are returned in an apparently random order, but it
3323 is the same order as either the keys() or each() function would produce
3324 on the same array. See also keys(), each(), and sort().
3326 =item vec EXPR,OFFSET,BITS
3328 Treats the string in EXPR as a vector of unsigned integers, and
3329 returns the value of the bitfield specified by OFFSET. BITS specifies
3330 the number of bits that are reserved for each entry in the bit
3331 vector. This must be a power of two from 1 to 32. vec() may also be
3332 assigned to, in which case parens are needed to give the expression
3333 the correct precedence as in
3335 vec($image, $max_x * $x + $y, 8) = 3;
3337 Vectors created with vec() can also be manipulated with the logical
3338 operators |, & and ^, which will assume a bit vector operation is
3339 desired when both operands are strings.
3341 To transform a bit vector into a string or array of 0's and 1's, use these:
3343 $bits = unpack("b*", $vector);
3344 @bits = split(//, unpack("b*", $vector));
3346 If you know the exact length in bits, it can be used in place of the *.
3350 Waits for a child process to terminate and returns the pid of the
3351 deceased process, or -1 if there are no child processes. The status is
3354 =item waitpid PID,FLAGS
3356 Waits for a particular child process to terminate and returns the pid
3357 of the deceased process, or -1 if there is no such child process. The
3358 status is returned in C<$?>. If you say
3360 use POSIX ":wait_h";
3362 waitpid(-1,&WNOHANG);
3364 then you can do a non-blocking wait for any process. Non-blocking wait
3365 is only available on machines supporting either the waitpid(2) or
3366 wait4(2) system calls. However, waiting for a particular pid with
3367 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3368 by remembering the status values of processes that have exited but have
3369 not been harvested by the Perl script yet.)
3373 Returns TRUE if the context of the currently executing subroutine is
3374 looking for a list value. Returns FALSE if the context is looking
3377 return wantarray ? () : undef;
3381 Produces a message on STDERR just like die(), but doesn't exit or
3384 =item write FILEHANDLE
3390 Writes a formatted record (possibly multi-line) to the specified file,
3391 using the format associated with that file. By default the format for
3392 a file is the one having the same name is the filehandle, but the
3393 format for the current output channel (see the select() function) may be set
3394 explicitly by assigning the name of the format to the C<$~> variable.
3396 Top of form processing is handled automatically: if there is
3397 insufficient room on the current page for the formatted record, the
3398 page is advanced by writing a form feed, a special top-of-page format
3399 is used to format the new page header, and then the record is written.
3400 By default the top-of-page format is the name of the filehandle with
3401 "_TOP" appended, but it may be dynamically set to the format of your
3402 choice by assigning the name to the C<$^> variable while the filehandle is
3403 selected. The number of lines remaining on the current page is in
3404 variable C<$->, which can be set to 0 to force a new page.
3406 If FILEHANDLE is unspecified, output goes to the current default output
3407 channel, which starts out as STDOUT but may be changed by the
3408 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3409 is evaluated and the resulting string is used to look up the name of
3410 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3412 Note that write is I<NOT> the opposite of read. Unfortunately.
3416 The translation operator. See L<perlop>.
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