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 -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, 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
185 =head2 Alphabetical Listing of Perl Functions
196 A file test, where X is one of the letters listed below. This unary
197 operator takes one argument, either a filename or a filehandle, and
198 tests the associated file to see if something is true about it. If the
199 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
200 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
201 the undefined value if the file doesn't exist. Despite the funny
202 names, precedence is the same as any other named unary operator, and
203 the argument may be parenthesized like any other unary operator. The
204 operator may be any of:
206 -r File is readable by effective uid/gid.
207 -w File is writable by effective uid/gid.
208 -x File is executable by effective uid/gid.
209 -o File is owned by effective uid.
211 -R File is readable by real uid/gid.
212 -W File is writable by real uid/gid.
213 -X File is executable by real uid/gid.
214 -O File is owned by real uid.
217 -z File has zero size.
218 -s File has non-zero size (returns size).
220 -f File is a plain file.
221 -d File is a directory.
222 -l File is a symbolic link.
223 -p File is a named pipe (FIFO).
225 -b File is a block special file.
226 -c File is a character special file.
227 -t Filehandle is opened to a tty.
229 -u File has setuid bit set.
230 -g File has setgid bit set.
231 -k File has sticky bit set.
233 -T File is a text file.
234 -B File is a binary file (opposite of -T).
236 -M Age of file in days when script started.
237 -A Same for access time.
238 -C Same for inode change time.
240 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
241 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
242 uids and gids of the user. There may be other reasons you can't actually
243 read, write or execute the file. Also note that, for the superuser,
244 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
245 1 if any execute bit is set in the mode. Scripts run by the superuser may
246 thus need to do a stat() in order to determine the actual mode of the
247 file, or temporarily set the uid to something else.
253 next unless -f $_; # ignore specials
257 Note that C<-s/a/b/> does not do a negated substitution. Saying
258 C<-exp($foo)> still works as expected, however--only single letters
259 following a minus are interpreted as file tests.
261 The C<-T> and C<-B> switches work as follows. The first block or so of the
262 file is examined for odd characters such as strange control codes or
263 characters with the high bit set. If too many odd characters (>30%)
264 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
265 containing null in the first block is considered a binary file. If C<-T>
266 or C<-B> is used on a filehandle, the current stdio buffer is examined
267 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
268 file, or a file at EOF when testing a filehandle. Because you have to
269 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
270 against the file first, as in C<next unless -f $file && -T $file>.
272 If any of the file tests (or either the stat() or lstat() operators) are given the
273 special filehandle consisting of a solitary underline, then the stat
274 structure of the previous file test (or stat operator) is used, saving
275 a system call. (This doesn't work with C<-t>, and you need to remember
276 that lstat() and C<-l> will leave values in the stat structure for the
277 symbolic link, not the real file.) Example:
279 print "Can do.\n" if -r $a || -w _ || -x _;
282 print "Readable\n" if -r _;
283 print "Writable\n" if -w _;
284 print "Executable\n" if -x _;
285 print "Setuid\n" if -u _;
286 print "Setgid\n" if -g _;
287 print "Sticky\n" if -k _;
288 print "Text\n" if -T _;
289 print "Binary\n" if -B _;
293 Returns the absolute value of its argument.
295 =item accept NEWSOCKET,GENERICSOCKET
297 Accepts an incoming socket connect, just as the accept(2) system call
298 does. Returns the packed address if it succeeded, FALSE otherwise.
299 See example in L<perlipc/"Sockets: Client/Server Communication">.
303 Arranges to have a SIGALRM delivered to this process after the
304 specified number of seconds have elapsed. (On some machines,
305 unfortunately, the elapsed time may be up to one second less than you
306 specified because of how seconds are counted.) Only one timer may be
307 counting at once. Each call disables the previous timer, and an
308 argument of 0 may be supplied to cancel the previous timer without
309 starting a new one. The returned value is the amount of time remaining
310 on the previous timer.
312 For delays of finer granularity than one second, you may use Perl's
313 syscall() interface to access setitimer(2) if your system supports it,
314 or else see L</select()> below. It is not advised to intermix alarm()
319 Returns the arctangent of Y/X in the range -PI to PI.
321 =item bind SOCKET,NAME
323 Binds a network address to a socket, just as the bind system call
324 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
325 packed address of the appropriate type for the socket. See the examples in
326 L<perlipc/"Sockets: Client/Server Communication">.
328 =item binmode FILEHANDLE
330 Arranges for the file to be read or written in "binary" mode in operating
331 systems that distinguish between binary and text files. Files that are
332 not in binary mode have CR LF sequences translated to LF on input and LF
333 translated to CR LF on output. Binmode has no effect under Unix; in DOS
334 and similarly archaic systems, it may be imperative--otherwise your
335 DOS-damaged C library may mangle your file. The key distinction between
336 systems that need binmode and those that don't is their text file
337 formats. Systems like Unix and Plan9 that delimit lines with a single
338 character, and that encode that character in C as '\n', do not need
339 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
340 is taken as the name of the filehandle.
342 =item bless REF,CLASSNAME
346 This function tells the referenced object (passed as REF) that it is now
347 an object in the CLASSNAME package--or the current package if no CLASSNAME
348 is specified, which is often the case. It returns the reference for
349 convenience, since a bless() is often the last thing in a constructor.
350 Always use the two-argument version if the function doing the blessing
351 might be inherited by a derived class. See L<perlobj> for more about the
352 blessing (and blessings) of objects.
358 Returns the context of the current subroutine call. In a scalar context,
359 returns TRUE if there is a caller, that is, if we're in a subroutine or
360 eval() or require(), and FALSE otherwise. In a list context, returns
362 ($package, $filename, $line) = caller;
364 With EXPR, it returns some extra information that the debugger uses to
365 print a stack trace. The value of EXPR indicates how many call frames
366 to go back before the current one.
368 ($package, $filename, $line,
369 $subroutine, $hasargs, $wantargs) = caller($i);
371 Furthermore, when called from within the DB package, caller returns more
372 detailed information: it sets the list variable @DB::args to be the
373 arguments with which that subroutine was invoked.
377 Changes the working directory to EXPR, if possible. If EXPR is
378 omitted, changes to home directory. Returns TRUE upon success, FALSE
379 otherwise. See example under die().
383 Changes the permissions of a list of files. The first element of the
384 list must be the numerical mode, which should probably be an octal
385 number. Returns the number of files successfully changed.
387 $cnt = chmod 0755, 'foo', 'bar';
388 chmod 0755, @executables;
396 This is a slightly safer version of chop (see below). It removes any
397 line ending that corresponds to the current value of C<$/> (also known as
398 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
399 of characters removed. It's often used to remove the newline from the
400 end of an input record when you're worried that the final record may be
401 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
402 trailing newlines from the string. If VARIABLE is omitted, it chomps
406 chomp; # avoid \n on last field
411 You can actually chomp anything that's an lvalue, including an assignment:
414 chomp($answer = <STDIN>);
416 If you chomp a list, each element is chomped, and the total number of
417 characters removed is returned.
425 Chops off the last character of a string and returns the character
426 chopped. It's used primarily to remove the newline from the end of an
427 input record, but is much more efficient than C<s/\n//> because it neither
428 scans nor copies the string. If VARIABLE is omitted, chops $_.
432 chop; # avoid \n on last field
437 You can actually chop anything that's an lvalue, including an assignment:
440 chop($answer = <STDIN>);
442 If you chop a list, each element is chopped. Only the value of the
443 last chop is returned.
445 Note that chop returns the last character. To return all but the last
446 character, use C<substr($string, 0, -1)>.
450 Changes the owner (and group) of a list of files. The first two
451 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
452 Returns the number of files successfully changed.
454 $cnt = chown $uid, $gid, 'foo', 'bar';
455 chown $uid, $gid, @filenames;
457 Here's an example that looks up non-numeric uids in the passwd file:
460 chop($user = <STDIN>);
462 chop($pattern = <STDIN>);
464 ($login,$pass,$uid,$gid) = getpwnam($user)
465 or die "$user not in passwd file";
467 @ary = <${pattern}>; # expand filenames
468 chown $uid, $gid, @ary;
470 On most systems, you are not allowed to change the ownership of the
471 file unless you're the superuser, although you should be able to change
472 the group to any of your secondary groups. On insecure systems, these
473 restrictions may be relaxed, but this is not a portable assumption.
477 Returns the character represented by that NUMBER in the character set.
478 For example, C<chr(65)> is "A" in ASCII.
480 =item chroot FILENAME
482 This function works as the system call by the same name: it makes the
483 named directory the new root directory for all further pathnames that
484 begin with a "/" by your process and all of its children. (It doesn't
485 change your current working directory is unaffected.) For security
486 reasons, this call is restricted to the superuser. If FILENAME is
487 omitted, does chroot to $_.
489 =item close FILEHANDLE
491 Closes the file or pipe associated with the file handle, returning TRUE
492 only if stdio successfully flushes buffers and closes the system file
493 descriptor. You don't have to close FILEHANDLE if you are immediately
494 going to do another open() on it, since open() will close it for you. (See
495 open().) However, an explicit close on an input file resets the line
496 counter ($.), while the implicit close done by open() does not. Also,
497 closing a pipe will wait for the process executing on the pipe to
498 complete, in case you want to look at the output of the pipe
499 afterwards. Closing a pipe explicitly also puts the status value of
500 the command into C<$?>. Example:
502 open(OUTPUT, '|sort >foo'); # pipe to sort
503 ... # print stuff to output
504 close OUTPUT; # wait for sort to finish
505 open(INPUT, 'foo'); # get sort's results
507 FILEHANDLE may be an expression whose value gives the real filehandle name.
509 =item closedir DIRHANDLE
511 Closes a directory opened by opendir().
513 =item connect SOCKET,NAME
515 Attempts to connect to a remote socket, just as the connect system call
516 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
517 packed address of the appropriate type for the socket. See the examples in
518 L<perlipc/"Sockets: Client/Server Communication">.
522 Actually a flow control statement rather than a function. If there is a
523 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
524 C<foreach>), it is always executed just before the conditional is about to
525 be evaluated again, just like the third part of a C<for> loop in C. Thus
526 it can be used to increment a loop variable, even when the loop has been
527 continued via the C<next> statement (which is similar to the C C<continue>
532 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
535 =item crypt PLAINTEXT,SALT
537 Encrypts a string exactly like the crypt(3) function in the C library
538 (assuming that you actually have a version there that has not been
539 extirpated as a potential munition). This can prove useful for checking
540 the password file for lousy passwords, amongst other things. Only the
541 guys wearing white hats should do this.
543 Here's an example that makes sure that whoever runs this program knows
546 $pwd = (getpwuid($<))[1];
547 $salt = substr($pwd, 0, 2);
551 chop($word = <STDIN>);
555 if (crypt($word, $salt) ne $pwd) {
561 Of course, typing in your own password to whoever asks you
564 =item dbmclose ASSOC_ARRAY
566 [This function has been superseded by the untie() function.]
568 Breaks the binding between a DBM file and an associative array.
570 =item dbmopen ASSOC,DBNAME,MODE
572 [This function has been superseded by the tie() function.]
574 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
575 associative array. ASSOC is the name of the associative array. (Unlike
576 normal open, the first argument is I<NOT> a filehandle, even though it
577 looks like one). DBNAME is the name of the database (without the F<.dir>
578 or F<.pag> extension if any). If the database does not exist, it is
579 created with protection specified by MODE (as modified by the umask()).
580 If your system only supports the older DBM functions, you may perform only
581 one dbmopen() in your program. In older versions of Perl, if your system
582 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
583 falls back to sdbm(3).
585 If you don't have write access to the DBM file, you can only read
586 associative array variables, not set them. If you want to test whether
587 you can write, either use file tests or try setting a dummy array entry
588 inside an eval(), which will trap the error.
590 Note that functions such as keys() and values() may return huge array
591 values when used on large DBM files. You may prefer to use the each()
592 function to iterate over large DBM files. Example:
594 # print out history file offsets
595 dbmopen(%HIST,'/usr/lib/news/history',0666);
596 while (($key,$val) = each %HIST) {
597 print $key, ' = ', unpack('L',$val), "\n";
601 See also L<AnyDBM_File> for a more general description of the pros and
602 cons of the various dbm apparoches, as well as L<DB_File> for a particularly
607 Returns a boolean value saying whether EXPR has a real value
608 or not. Many operations return the undefined value under exceptional
609 conditions, such as end of file, uninitialized variable, system error
610 and such. This function allows you to distinguish between an undefined
611 null scalar and a defined null scalar with operations that might return
612 a real null string, such as referencing elements of an array. You may
613 also check to see if arrays or subroutines exist. Use of defined on
614 predefined variables is not guaranteed to produce intuitive results.
616 When used on a hash array element, it tells you whether the value
617 is defined, not whether the key exists in the hash. Use exists() for that.
621 print if defined $switch{'D'};
622 print "$val\n" while defined($val = pop(@ary));
623 die "Can't readlink $sym: $!"
624 unless defined($value = readlink $sym);
625 eval '@foo = ()' if defined(@foo);
626 die "No XYZ package defined" unless defined %_XYZ;
627 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
633 Deletes the specified value from its hash array. Returns the deleted
634 value, or the undefined value if nothing was deleted. Deleting from
635 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
636 file deletes the entry from the DBM file. (But deleting from a tie()d
637 hash doesn't necessarily return anything.)
639 The following deletes all the values of an associative array:
641 foreach $key (keys %ARRAY) {
645 (But it would be faster to use the undef() command.) Note that the
646 EXPR can be arbitrarily complicated as long as the final operation is
649 delete $ref->[$x][$y]{$key};
653 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
654 the current value of $! (errno). If $! is 0, exits with the value of
655 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
656 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
657 and the eval() is terminated with the undefined value; this makes die()
658 the way to raise an exception.
662 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
663 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
665 If the value of EXPR does not end in a newline, the current script line
666 number and input line number (if any) are also printed, and a newline
667 is supplied. Hint: sometimes appending ", stopped" to your message
668 will cause it to make better sense when the string "at foo line 123" is
669 appended. Suppose you are running script "canasta".
671 die "/etc/games is no good";
672 die "/etc/games is no good, stopped";
674 produce, respectively
676 /etc/games is no good at canasta line 123.
677 /etc/games is no good, stopped at canasta line 123.
679 See also exit() and warn().
683 Not really a function. Returns the value of the last command in the
684 sequence of commands indicated by BLOCK. When modified by a loop
685 modifier, executes the BLOCK once before testing the loop condition.
686 (On other statements the loop modifiers test the conditional first.)
688 =item do SUBROUTINE(LIST)
690 A deprecated form of subroutine call. See L<perlsub>.
694 Uses the value of EXPR as a filename and executes the contents of the
695 file as a Perl script. Its primary use is to include subroutines
696 from a Perl subroutine library.
704 except that it's more efficient, more concise, keeps track of the
705 current filename for error messages, and searches all the B<-I>
706 libraries if the file isn't in the current directory (see also the @INC
707 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
708 reparse the file every time you call it, so you probably don't want to
709 do this inside a loop.
711 Note that inclusion of library modules is better done with the
712 use() and require() operators, which also do error checking
713 and raise an exception if there's a problem.
717 This causes an immediate core dump. Primarily this is so that you can
718 use the B<undump> program to turn your core dump into an executable binary
719 after having initialized all your variables at the beginning of the
720 program. When the new binary is executed it will begin by executing a
721 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
722 it as a goto with an intervening core dump and reincarnation. If LABEL
723 is omitted, restarts the program from the top. WARNING: any files
724 opened at the time of the dump will NOT be open any more when the
725 program is reincarnated, with possible resulting confusion on the part
726 of Perl. See also B<-u> option in L<perlrun>.
743 dump QUICKSTART if $ARGV[0] eq '-d';
748 =item each ASSOC_ARRAY
750 Returns a 2-element array consisting of the key and value for the next
751 value of an associative array, so that you can iterate over it.
752 Entries are returned in an apparently random order. When the array is
753 entirely read, a null array is returned (which when assigned produces a
754 FALSE (0) value). The next call to each() after that will start
755 iterating again. The iterator can be reset only by reading all the
756 elements from the array. You should not add elements to an array while
757 you're iterating over it. There is a single iterator for each
758 associative array, shared by all each(), keys() and values() function
759 calls in the program. The following prints out your environment like
760 the printenv(1) program, only in a different order:
762 while (($key,$value) = each %ENV) {
763 print "$key=$value\n";
766 See also keys() and values().
774 Returns 1 if the next read on FILEHANDLE will return end of file, or if
775 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
776 gives the real filehandle name. (Note that this function actually
777 reads a character and then ungetc()s it, so it is not very useful in an
778 interactive context.) Do not read from a terminal file (or call
779 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
780 as terminals may lose the end-of-file condition if you do.
782 An C<eof> without an argument uses the last file read as argument.
783 Empty parentheses () may be used to indicate
784 the pseudofile formed of the files listed on the command line, i.e.
785 C<eof()> is reasonable to use inside a while (<>) loop to detect the end
786 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
787 test I<EACH> file in a while (<>) loop. Examples:
789 # reset line numbering on each input file
792 close(ARGV) if (eof); # Not eof().
795 # insert dashes just before last line of last file
798 print "--------------\n";
799 close(ARGV); # close or break; is needed if we
800 # are reading from the terminal
805 Practical hint: you almost never need to use C<eof> in Perl, because the
806 input operators return undef when they run out of data. Testing C<eof>
812 EXPR is parsed and executed as if it were a little Perl program. It
813 is executed in the context of the current Perl program, so that any
814 variable settings, subroutine or format definitions remain afterwards.
815 The value returned is the value of the last expression evaluated, or a
816 return statement may be used, just as with subroutines.
818 If there is a syntax error or runtime error, or a die() statement is
819 executed, an undefined value is returned by eval(), and C<$@> is set to the
820 error message. If there was no error, C<$@> is guaranteed to be a null
821 string. If EXPR is omitted, evaluates $_. The final semicolon, if
822 any, may be omitted from the expression.
824 Note that, since eval() traps otherwise-fatal errors, it is useful for
825 determining whether a particular feature (such as socket() or symlink())
826 is implemented. It is also Perl's exception trapping mechanism, where
827 the die operator is used to raise exceptions.
829 If the code to be executed doesn't vary, you may use the eval-BLOCK
830 form to trap run-time errors without incurring the penalty of
831 recompiling each time. The error, if any, is still returned in C<$@>.
834 # make divide-by-zero non-fatal
835 eval { $answer = $a / $b; }; warn $@ if $@;
837 # same thing, but less efficient
838 eval '$answer = $a / $b'; warn $@ if $@;
840 # a compile-time error
844 eval '$answer ='; # sets $@
846 With an eval(), you should be especially careful to remember what's
847 being looked at when:
853 eval { $x }; # CASE 4
855 eval "\$$x++" # CASE 5
858 Cases 1 and 2 above behave identically: they run the code contained in the
859 variable $x. (Although case 2 has misleading double quotes making the
860 reader wonder what else might be happening (nothing is).) Cases 3 and 4
861 likewise behave in the same way: they run the code <$x>, which does
862 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
863 is a place where normally you I<WOULD> like to use double quotes, except
864 that in that particular situation, you can just use symbolic references
865 instead, as in case 6.
869 The exec() function executes a system command I<AND NEVER RETURNS>. Use
870 the system() function if you want it to return.
872 If there is more than one argument in LIST, or if LIST is an array with
873 more than one value, calls execvp(3) with the arguments in LIST. If
874 there is only one scalar argument, the argument is checked for shell
875 metacharacters. If there are any, the entire argument is passed to
876 C</bin/sh -c> for parsing. If there are none, the argument is split
877 into words and passed directly to execvp(), which is more efficient.
878 Note: exec() (and system(0) do not flush your output buffer, so you may
879 need to set C<$|> to avoid lost output. Examples:
881 exec '/bin/echo', 'Your arguments are: ', @ARGV;
882 exec "sort $outfile | uniq";
884 If you don't really want to execute the first argument, but want to lie
885 to the program you are executing about its own name, you can specify
886 the program you actually want to run as an "indirect object" (without a
887 comma) in front of the LIST. (This always forces interpretation of the
888 LIST as a multi-valued list, even if there is only a single scalar in
892 exec $shell '-sh'; # pretend it's a login shell
896 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
900 Returns TRUE if the specified hash key exists in its hash array, even
901 if the corresponding value is undefined.
903 print "Exists\n" if exists $array{$key};
904 print "Defined\n" if defined $array{$key};
905 print "True\n" if $array{$key};
907 A hash element can only be TRUE if it's defined, and defined if
908 it exists, but the reverse doesn't necessarily hold true.
910 Note that the EXPR can be arbitrarily complicated as long as the final
911 operation is a hash key lookup:
913 if (exists $ref->[$x][$y]{$key}) { ... }
917 Evaluates EXPR and exits immediately with that value. (Actually, it
918 calls any defined C<END> routines first, but the C<END> routines may not
919 abort the exit. Likewise any object destructors that need to be called
920 are called before exit.) Example:
923 exit 0 if $ans =~ /^[Xx]/;
925 See also die(). If EXPR is omitted, exits with 0 status.
929 Returns I<e> (the natural logarithm base) to the power of EXPR.
930 If EXPR is omitted, gives C<exp($_)>.
932 =item fcntl FILEHANDLE,FUNCTION,SCALAR
934 Implements the fcntl(2) function. You'll probably have to say
938 first to get the correct function definitions. Argument processing and
939 value return works just like ioctl() below. Note that fcntl() will produce
940 a fatal error if used on a machine that doesn't implement fcntl(2).
944 fcntl($filehandle, F_GETLK, $packed_return_buffer);
946 =item fileno FILEHANDLE
948 Returns the file descriptor for a filehandle. This is useful for
949 constructing bitmaps for select(). If FILEHANDLE is an expression, the
950 value is taken as the name of the filehandle.
952 =item flock FILEHANDLE,OPERATION
954 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
955 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
956 fatal error if used on a machine that doesn't implement either flock(2) or
957 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
958 is missing from your system. This makes flock() the portable file locking
959 strategy, although it will only lock entire files, not records. Note also
960 that some versions of flock() cannot lock things over the network; you
961 would need to use the more system-specific fcntl() for that.
963 Here's a mailbox appender for BSD systems.
971 flock(MBOX,$LOCK_EX);
972 # and, in case someone appended
973 # while we were waiting...
978 flock(MBOX,$LOCK_UN);
981 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
982 or die "Can't open mailbox: $!";
985 print MBOX $msg,"\n\n";
988 See also L<DB_File> for other flock() examples.
992 Does a fork(2) system call. Returns the child pid to the parent process
993 and 0 to the child process, or C<undef> if the fork is unsuccessful.
994 Note: unflushed buffers remain unflushed in both processes, which means
995 you may need to set C<$|> ($AUTOFLUSH in English) or call the
996 autoflush() FileHandle method to avoid duplicate output.
998 If you fork() without ever waiting on your children, you will accumulate
1001 $SIG{CHLD} = sub { wait };
1003 There's also the double-fork trick (error checking on
1004 fork() returns omitted);
1006 unless ($pid = fork) {
1008 exec "what you really wanna do";
1011 ## (some_perl_code_here)
1018 See also L<perlipc> for more examples of forking and reaping
1023 Declare a picture format with use by the write() function. For
1027 Test: @<<<<<<<< @||||| @>>>>>
1028 $str, $%, '$' . int($num)
1032 $num = $cost/$quantiy;
1036 See L<perlform> for many details and examples.
1039 =item formline PICTURE, LIST
1041 This is an internal function used by C<format>s, though you may call it
1042 too. It formats (see L<perlform>) a list of values according to the
1043 contents of PICTURE, placing the output into the format output
1044 accumulator, C<$^A> (or $ACCUMULATOR in English).
1045 Eventually, when a write() is done, the contents of
1046 C<$^A> are written to some filehandle, but you could also read C<$^A>
1047 yourself and then set C<$^A> back to "". Note that a format typically
1048 does one formline() per line of form, but the formline() function itself
1049 doesn't care how many newlines are embedded in the PICTURE. This means
1050 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1051 You may therefore need to use multiple formlines to implement a single
1052 record format, just like the format compiler.
1054 Be careful if you put double quotes around the picture, since an "C<@>"
1055 character may be taken to mean the beginning of an array name.
1056 formline() always returns TRUE. See L<perlform> for other examples.
1058 =item getc FILEHANDLE
1062 Returns the next character from the input file attached to FILEHANDLE,
1063 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1064 This is not particularly efficient. It cannot be used to get unbuffered
1065 single-characters, however. For that, try something more like:
1068 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1071 system "stty", '-icanon', 'eol', "\001";
1077 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1080 system "stty", 'icanon', 'eol', '^@'; # ascii null
1084 Determination of whether to whether $BSD_STYLE should be set
1085 is left as an exercise to the reader.
1087 See also the C<Term::ReadKey> module from your nearest CPAN site;
1088 details on CPAN can be found on L<perlmod/CPAN>
1092 Returns the current login from F</etc/utmp>, if any. If null, use
1095 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1097 Do not consider getlogin() for authorentication: it is not as
1098 secure as getpwuid().
1100 =item getpeername SOCKET
1102 Returns the packed sockaddr address of other end of the SOCKET connection.
1105 $hersockaddr = getpeername(SOCK);
1106 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1107 $herhostname = gethostbyaddr($iaddr, AF_INET);
1108 $herstraddr = inet_ntoa($iaddr);
1112 Returns the current process group for the specified PID, 0 for the
1113 current process. Will raise an exception if used on a machine that
1114 doesn't implement getpgrp(2). If PID is omitted, returns process
1115 group of current process.
1119 Returns the process id of the parent process.
1121 =item getpriority WHICH,WHO
1123 Returns the current priority for a process, a process group, or a user.
1124 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1125 machine that doesn't implement getpriority(2).
1131 =item gethostbyname NAME
1133 =item getnetbyname NAME
1135 =item getprotobyname NAME
1141 =item getservbyname NAME,PROTO
1143 =item gethostbyaddr ADDR,ADDRTYPE
1145 =item getnetbyaddr ADDR,ADDRTYPE
1147 =item getprotobynumber NUMBER
1149 =item getservbyport PORT,PROTO
1167 =item sethostent STAYOPEN
1169 =item setnetent STAYOPEN
1171 =item setprotoent STAYOPEN
1173 =item setservent STAYOPEN
1187 These routines perform the same functions as their counterparts in the
1188 system library. Within a list context, the return values from the
1189 various get routines are as follows:
1191 ($name,$passwd,$uid,$gid,
1192 $quota,$comment,$gcos,$dir,$shell) = getpw*
1193 ($name,$passwd,$gid,$members) = getgr*
1194 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1195 ($name,$aliases,$addrtype,$net) = getnet*
1196 ($name,$aliases,$proto) = getproto*
1197 ($name,$aliases,$port,$proto) = getserv*
1199 (If the entry doesn't exist you get a null list.)
1201 Within a scalar context, you get the name, unless the function was a
1202 lookup by name, in which case you get the other thing, whatever it is.
1203 (If the entry doesn't exist you get the undefined value.) For example:
1213 The $members value returned by I<getgr*()> is a space separated list of
1214 the login names of the members of the group.
1216 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1217 C, it will be returned to you via C<$?> if the function call fails. The
1218 @addrs value returned by a successful call is a list of the raw
1219 addresses returned by the corresponding system library call. In the
1220 Internet domain, each address is four bytes long and you can unpack it
1221 by saying something like:
1223 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1225 =item getsockname SOCKET
1227 Returns the packed sockaddr address of this end of the SOCKET connection.
1230 $mysockaddr = getsockname(SOCK);
1231 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1233 =item getsockopt SOCKET,LEVEL,OPTNAME
1235 Returns the socket option requested, or undefined if there is an error.
1239 Returns the value of EXPR with filename expansions such as a shell
1240 would do. This is the internal function implementing the <*.*>
1241 operator, except it's easier to use.
1245 Converts a time as returned by the time function to a 9-element array
1246 with the time localized for the standard Greenwich timezone.
1247 Typically used as follows:
1250 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1253 All array elements are numeric, and come straight out of a struct tm.
1254 In particular this means that $mon has the range 0..11 and $wday has
1255 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1263 The goto-LABEL form finds the statement labeled with LABEL and resumes
1264 execution there. It may not be used to go into any construct that
1265 requires initialization, such as a subroutine or a foreach loop. It
1266 also can't be used to go into a construct that is optimized away. It
1267 can be used to go almost anywhere else within the dynamic scope,
1268 including out of subroutines, but it's usually better to use some other
1269 construct such as last or die. The author of Perl has never felt the
1270 need to use this form of goto (in Perl, that is--C is another matter).
1272 The goto-EXPR form expects a label name, whose scope will be resolved
1273 dynamically. This allows for computed gotos per FORTRAN, but isn't
1274 necessarily recommended if you're optimizing for maintainability:
1276 goto ("FOO", "BAR", "GLARCH")[$i];
1278 The goto-&NAME form is highly magical, and substitutes a call to the
1279 named subroutine for the currently running subroutine. This is used by
1280 AUTOLOAD subroutines that wish to load another subroutine and then
1281 pretend that the other subroutine had been called in the first place
1282 (except that any modifications to @_ in the current subroutine are
1283 propagated to the other subroutine.) After the goto, not even caller()
1284 will be able to tell that this routine was called first.
1286 =item grep BLOCK LIST
1288 =item grep EXPR,LIST
1290 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1291 $_ to each element) and returns the list value consisting of those
1292 elements for which the expression evaluated to TRUE. In a scalar
1293 context, returns the number of times the expression was TRUE.
1295 @foo = grep(!/^#/, @bar); # weed out comments
1299 @foo = grep {!/^#/} @bar; # weed out comments
1301 Note that, since $_ is a reference into the list value, it can be used
1302 to modify the elements of the array. While this is useful and
1303 supported, it can cause bizarre results if the LIST is not a named
1308 Interprets EXPR as a hex string and returns the corresponding decimal
1309 value. (To convert strings that might start with 0 or 0x see
1310 oct().) If EXPR is omitted, uses $_.
1314 There is no built-in import() function. It is merely an ordinary
1315 method (subroutine) defined (or inherited) by modules that wish to export
1316 names to another module. The use() function calls the import() method
1317 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1319 =item index STR,SUBSTR,POSITION
1321 =item index STR,SUBSTR
1323 Returns the position of the first occurrence of SUBSTR in STR at or after
1324 POSITION. If POSITION is omitted, starts searching from the beginning of
1325 the string. The return value is based at 0 (or whatever you've set the $[
1326 variable to--but don't do that). If the substring is not found, returns
1327 one less than the base, ordinarily -1.
1331 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1333 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1335 Implements the ioctl(2) function. You'll probably have to say
1337 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1339 first to get the correct function definitions. If F<ioctl.ph> doesn't
1340 exist or doesn't have the correct definitions you'll have to roll your
1341 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1342 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1343 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1344 written depending on the FUNCTION--a pointer to the string value of SCALAR
1345 will be passed as the third argument of the actual ioctl call. (If SCALAR
1346 has no string value but does have a numeric value, that value will be
1347 passed rather than a pointer to the string value. To guarantee this to be
1348 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1349 functions are useful for manipulating the values of structures used by
1350 ioctl(). The following example sets the erase character to DEL.
1354 die "NO TIOCGETP" if $@ || !$getp;
1355 $sgttyb_t = "ccccs"; # 4 chars and a short
1356 if (ioctl(STDIN,$getp,$sgttyb)) {
1357 @ary = unpack($sgttyb_t,$sgttyb);
1359 $sgttyb = pack($sgttyb_t,@ary);
1360 ioctl(STDIN,&TIOCSETP,$sgttyb)
1361 || die "Can't ioctl: $!";
1364 The return value of ioctl (and fcntl) is as follows:
1366 if OS returns: then Perl returns:
1368 0 string "0 but true"
1369 anything else that number
1371 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1372 still easily determine the actual value returned by the operating
1375 ($retval = ioctl(...)) || ($retval = -1);
1376 printf "System returned %d\n", $retval;
1378 =item join EXPR,LIST
1380 Joins the separate strings of LIST or ARRAY into a single string with
1381 fields separated by the value of EXPR, and returns the string.
1384 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1386 See L<perlfunc/split>.
1388 =item keys ASSOC_ARRAY
1390 Returns a normal array consisting of all the keys of the named
1391 associative array. (In a scalar context, returns the number of keys.)
1392 The keys are returned in an apparently random order, but it is the same
1393 order as either the values() or each() function produces (given that
1394 the associative array has not been modified). Here is yet another way
1395 to print your environment:
1398 @values = values %ENV;
1399 while ($#keys >= 0) {
1400 print pop(@keys), '=', pop(@values), "\n";
1403 or how about sorted by key:
1405 foreach $key (sort(keys %ENV)) {
1406 print $key, '=', $ENV{$key}, "\n";
1409 To sort an array by value, you'll need to use a C<sort{}>
1410 function. Here's a descending numeric sort of a hash by its values:
1412 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1413 printf "%4d %s\n", $hash{$key}, $key;
1418 Sends a signal to a list of processes. The first element of
1419 the list must be the signal to send. Returns the number of
1420 processes successfully signaled.
1422 $cnt = kill 1, $child1, $child2;
1425 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1426 process groups instead of processes. (On System V, a negative I<PROCESS>
1427 number will also kill process groups, but that's not portable.) That
1428 means you usually want to use positive not negative signals. You may also
1429 use a signal name in quotes. See the L<perlipc/"Signals"> man page for details.
1435 The C<last> command is like the C<break> statement in C (as used in
1436 loops); it immediately exits the loop in question. If the LABEL is
1437 omitted, the command refers to the innermost enclosing loop. The
1438 C<continue> block, if any, is not executed:
1440 LINE: while (<STDIN>) {
1441 last LINE if /^$/; # exit when done with header
1447 Returns an lowercased version of EXPR. This is the internal function
1448 implementing the \L escape in double-quoted strings.
1449 Should respect any POSIX setlocale() settings.
1453 Returns the value of EXPR with the first character lowercased. This is
1454 the internal function implementing the \l escape in double-quoted strings.
1455 Should respect any POSIX setlocale() settings.
1459 Returns the length in characters of the value of EXPR. If EXPR is
1460 omitted, returns length of $_.
1462 =item link OLDFILE,NEWFILE
1464 Creates a new filename linked to the old filename. Returns 1 for
1465 success, 0 otherwise.
1467 =item listen SOCKET,QUEUESIZE
1469 Does the same thing that the listen system call does. Returns TRUE if
1470 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1474 A local modifies the listed variables to be local to the enclosing block,
1475 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1476 list must be placed in parens. See L<perlsub/"Temporary Values via
1477 local()"> for details.
1479 But you really probably want to be using my() instead, because local() isn't
1480 what most people think of as "local"). See L<perlsub/"Private Variables
1481 via my()"> for details.
1483 =item localtime EXPR
1485 Converts a time as returned by the time function to a 9-element array
1486 with the time analyzed for the local timezone. Typically used as
1489 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1492 All array elements are numeric, and come straight out of a struct tm.
1493 In particular this means that $mon has the range 0..11 and $wday has
1494 the range 0..6. If EXPR is omitted, does localtime(time).
1496 In a scalar context, prints out the ctime(3) value:
1498 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1500 See also L<perlmod/timelocal> and the strftime(3) function available
1501 via the POSIX modulie.
1505 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1508 =item lstat FILEHANDLE
1512 Does the same thing as the stat() function, but stats a symbolic link
1513 instead of the file the symbolic link points to. If symbolic links are
1514 unimplemented on your system, a normal stat() is done.
1518 The match operator. See L<perlop>.
1520 =item map BLOCK LIST
1524 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1525 element) and returns the list value composed of the results of each such
1526 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1527 may produce zero, one, or more elements in the returned value.
1529 @chars = map(chr, @nums);
1531 translates a list of numbers to the corresponding characters. And
1533 %hash = map { getkey($_) => $_ } @array;
1535 is just a funny way to write
1538 foreach $_ (@array) {
1539 $hash{getkey($_)} = $_;
1542 =item mkdir FILENAME,MODE
1544 Creates the directory specified by FILENAME, with permissions specified
1545 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1546 it returns 0 and sets $! (errno).
1548 =item msgctl ID,CMD,ARG
1550 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1551 must be a variable which will hold the returned msqid_ds structure.
1552 Returns like ioctl: the undefined value for error, "0 but true" for
1553 zero, or the actual return value otherwise.
1555 =item msgget KEY,FLAGS
1557 Calls the System V IPC function msgget(2). Returns the message queue id,
1558 or the undefined value if there is an error.
1560 =item msgsnd ID,MSG,FLAGS
1562 Calls the System V IPC function msgsnd to send the message MSG to the
1563 message queue ID. MSG must begin with the long integer message type,
1564 which may be created with C<pack("L", $type)>. Returns TRUE if
1565 successful, or FALSE if there is an error.
1567 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1569 Calls the System V IPC function msgrcv to receive a message from
1570 message queue ID into variable VAR with a maximum message size of
1571 SIZE. Note that if a message is received, the message type will be the
1572 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1573 of the message type. Returns TRUE if successful, or FALSE if there is
1578 A "my" declares the listed variables to be local (lexically) to the
1579 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1580 more than one value is listed, the list must be placed in parens. See
1581 L<perlsub/"Private Variables via my()"> for details.
1587 The C<next> command is like the C<continue> statement in C; it starts
1588 the next iteration of the loop:
1590 LINE: while (<STDIN>) {
1591 next LINE if /^#/; # discard comments
1595 Note that if there were a C<continue> block on the above, it would get
1596 executed even on discarded lines. If the LABEL is omitted, the command
1597 refers to the innermost enclosing loop.
1599 =item no Module LIST
1601 See the "use" function, which "no" is the opposite of.
1605 Interprets EXPR as an octal string and returns the corresponding
1606 decimal value. (If EXPR happens to start off with 0x, interprets it as
1607 a hex string instead.) The following will handle decimal, octal, and
1608 hex in the standard Perl or C notation:
1610 $val = oct($val) if $val =~ /^0/;
1612 If EXPR is omitted, uses $_.
1614 =item open FILEHANDLE,EXPR
1616 =item open FILEHANDLE
1618 Opens the file whose filename is given by EXPR, and associates it with
1619 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1620 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1621 the same name as the FILEHANDLE contains the filename. If the filename
1622 begins with "<" or nothing, the file is opened for input. If the filename
1623 begins with ">", the file is opened for output. If the filename begins
1624 with ">>", the file is opened for appending. You can put a '+' in front
1625 of the '>' or '<' to indicate that you want both read and write access to
1626 the file; thus '+<' is usually preferred for read/write updates--the '+>'
1627 mode would clobber the file first. These correspond to the fopen(3) modes
1628 of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1630 If the filename begins with "|", the filename is interpreted
1631 as a command to which output is to be piped, and if the filename ends with
1632 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1633 for more examples of this. as command which pipes input to us. (You may
1634 not have a raw open() to a command that pipes both in I<and> out, but see See L<open2>,
1635 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1637 Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
1638 non-zero upon success, the undefined value otherwise. If the open
1639 involved a pipe, the return value happens to be the pid of the
1642 If you're unfortunate enough to be running Perl on a system that
1643 distinguishes between text files and binary files (modern operating
1644 systems don't care), then you should check out L</binmode> for tips for
1645 dealing with this. The key distinction between systems that need binmode
1646 and those that don't is their text file formats. Systems like Unix and
1647 Plan9 that delimit lines with a single character, and that encode that
1648 character in C as '\n', do not need C<binmode>. The rest need it.
1654 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1655 while (<ARTICLE>) {...
1657 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1659 open(DBASE, '+<dbase.mine'); # open for update
1661 open(ARTICLE, "caesar <$article |"); # decrypt article
1663 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1665 # process argument list of files along with any includes
1667 foreach $file (@ARGV) {
1668 process($file, 'fh00');
1672 local($filename, $input) = @_;
1673 $input++; # this is a string increment
1674 unless (open($input, $filename)) {
1675 print STDERR "Can't open $filename: $!\n";
1679 while (<$input>) { # note use of indirection
1680 if (/^#include "(.*)"/) {
1681 process($1, $input);
1688 You may also, in the Bourne shell tradition, specify an EXPR beginning
1689 with ">&", in which case the rest of the string is interpreted as the
1690 name of a filehandle (or file descriptor, if numeric) which is to be
1691 duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
1692 mode you specify should match the mode of the original filehandle.
1693 (Duping a filehandle does not take into acount any existing contents of
1695 Here is a script that saves, redirects, and restores STDOUT and
1699 open(SAVEOUT, ">&STDOUT");
1700 open(SAVEERR, ">&STDERR");
1702 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1703 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1705 select(STDERR); $| = 1; # make unbuffered
1706 select(STDOUT); $| = 1; # make unbuffered
1708 print STDOUT "stdout 1\n"; # this works for
1709 print STDERR "stderr 1\n"; # subprocesses too
1714 open(STDOUT, ">&SAVEOUT");
1715 open(STDERR, ">&SAVEERR");
1717 print STDOUT "stdout 2\n";
1718 print STDERR "stderr 2\n";
1721 If you specify "<&=N", where N is a number, then Perl will do an
1722 equivalent of C's fdopen() of that file descriptor; this is more
1723 parsimonious of file descriptors. For example:
1725 open(FILEHANDLE, "<&=$fd")
1727 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1728 there is an implicit fork done, and the return value of open is the pid
1729 of the child within the parent process, and 0 within the child
1730 process. (Use defined($pid) to determine whether the open was successful.)
1731 The filehandle behaves normally for the parent, but i/o to that
1732 filehandle is piped from/to the STDOUT/STDIN of the child process.
1733 In the child process the filehandle isn't opened--i/o happens from/to
1734 the new STDOUT or STDIN. Typically this is used like the normal
1735 piped open when you want to exercise more control over just how the
1736 pipe command gets executed, such as when you are running setuid, and
1737 don't want to have to scan shell commands for metacharacters.
1738 The following pairs are more or less equivalent:
1740 open(FOO, "|tr '[a-z]' '[A-Z]'");
1741 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1743 open(FOO, "cat -n '$file'|");
1744 open(FOO, "-|") || exec 'cat', '-n', $file;
1746 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1748 Explicitly closing any piped filehandle causes the parent process to
1749 wait for the child to finish, and returns the status value in $?.
1750 Note: on any operation which may do a fork, unflushed buffers remain
1751 unflushed in both processes, which means you may need to set $| to
1752 avoid duplicate output.
1754 The filename that is passed to open will have leading and trailing
1755 whitespace deleted. In order to open a file with arbitrary weird
1756 characters in it, it's necessary to protect any leading and trailing
1759 $file =~ s#^(\s)#./$1#;
1760 open(FOO, "< $file\0");
1762 If you want a "real" C open() (see L<open(2)) on your system, then
1763 you should probably use the POSIX::open() function as found in the L<POSIX>
1764 documents. For example:
1767 use POSIX qw(:fcntl_h);
1768 $fd = POSIX::open($path, O_RDWR|O_CREAT|O_EXCL, 0700);
1769 die "POSIX::open $path: $!" unless defined $fd;
1770 $fh = FileHandle->new_from_fd($fd, $amode) || die "fdopen: $!";
1772 $fh->print("stuff $$\n");
1773 seek($fh, 0, SEEK_SET);
1774 print "File contains: ", <$fh>;
1776 See L</seek()> for some details about mixing reading and writing.
1778 =item opendir DIRHANDLE,EXPR
1780 Opens a directory named EXPR for processing by readdir(), telldir(),
1781 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1782 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1786 Returns the numeric ascii value of the first character of EXPR. If
1787 EXPR is omitted, uses $_.
1789 =item pack TEMPLATE,LIST
1791 Takes an array or list of values and packs it into a binary structure,
1792 returning the string containing the structure. The TEMPLATE is a
1793 sequence of characters that give the order and type of values, as
1796 A An ascii string, will be space padded.
1797 a An ascii string, will be null padded.
1798 b A bit string (ascending bit order, like vec()).
1799 B A bit string (descending bit order).
1800 h A hex string (low nybble first).
1801 H A hex string (high nybble first).
1803 c A signed char value.
1804 C An unsigned char value.
1805 s A signed short value.
1806 S An unsigned short value.
1807 i A signed integer value.
1808 I An unsigned integer value.
1809 l A signed long value.
1810 L An unsigned long value.
1812 n A short in "network" order.
1813 N A long in "network" order.
1814 v A short in "VAX" (little-endian) order.
1815 V A long in "VAX" (little-endian) order.
1817 f A single-precision float in the native format.
1818 d A double-precision float in the native format.
1820 p A pointer to a null-terminated string.
1821 P A pointer to a structure (fixed-length string).
1823 u A uuencoded string.
1827 @ Null fill to absolute position.
1829 Each letter may optionally be followed by a number which gives a repeat
1830 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1831 pack function will gobble up that many values from the LIST. A * for the
1832 repeat count means to use however many items are left. The "a" and "A"
1833 types gobble just one value, but pack it as a string of length count,
1834 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1835 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1836 fields pack a string that many bits long. The "h" and "H" fields pack a
1837 string that many nybbles long. The "P" packs a pointer to a structure of
1838 the size indicated by the length. Real numbers (floats and doubles) are
1839 in the native machine format only; due to the multiplicity of floating
1840 formats around, and the lack of a standard "network" representation, no
1841 facility for interchange has been made. This means that packed floating
1842 point data written on one machine may not be readable on another - even if
1843 both use IEEE floating point arithmetic (as the endian-ness of the memory
1844 representation is not part of the IEEE spec). Note that Perl uses doubles
1845 internally for all numeric calculation, and converting from double into
1846 float and thence back to double again will lose precision (i.e.
1847 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1851 $foo = pack("cccc",65,66,67,68);
1853 $foo = pack("c4",65,66,67,68);
1856 $foo = pack("ccxxcc",65,66,67,68);
1859 $foo = pack("s2",1,2);
1860 # "\1\0\2\0" on little-endian
1861 # "\0\1\0\2" on big-endian
1863 $foo = pack("a4","abcd","x","y","z");
1866 $foo = pack("aaaa","abcd","x","y","z");
1869 $foo = pack("a14","abcdefg");
1870 # "abcdefg\0\0\0\0\0\0\0"
1872 $foo = pack("i9pl", gmtime);
1873 # a real struct tm (on my system anyway)
1876 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1879 The same template may generally also be used in the unpack function.
1881 =item package NAMESPACE
1883 Declares the compilation unit as being in the given namespace. The scope
1884 of the package declaration is from the declaration itself through the end of
1885 the enclosing block (the same scope as the local() operator). All further
1886 unqualified dynamic identifiers will be in this namespace. A package
1887 statement only affects dynamic variables--including those you've used
1888 local() on--but I<not> lexical variables created with my(). Typically it
1889 would be the first declaration in a file to be included by the C<require>
1890 or C<use> operator. You can switch into a package in more than one place;
1891 it merely influences which symbol table is used by the compiler for the
1892 rest of that block. You can refer to variables and filehandles in other
1893 packages by prefixing the identifier with the package name and a double
1894 colon: C<$Package::Variable>. If the package name is null, the C<main>
1895 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1897 See L<perlmod/"Packages"> for more information about packages, modules,
1898 and classes. See L<perlsub> for other scoping issues.
1900 =item pipe READHANDLE,WRITEHANDLE
1902 Opens a pair of connected pipes like the corresponding system call.
1903 Note that if you set up a loop of piped processes, deadlock can occur
1904 unless you are very careful. In addition, note that Perl's pipes use
1905 stdio buffering, so you may need to set $| to flush your WRITEHANDLE
1906 after each command, depending on the application.
1908 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1909 for examples of such things.
1913 Pops and returns the last value of the array, shortening the array by
1914 1. Has a similar effect to
1916 $tmp = $ARRAY[$#ARRAY--];
1918 If there are no elements in the array, returns the undefined value.
1919 If ARRAY is omitted, pops the
1920 @ARGV array in the main program, and the @_ array in subroutines, just
1925 Returns the offset of where the last C<m//g> search left off for the variable
1926 in question. May be modified to change that offset.
1928 =item print FILEHANDLE LIST
1934 Prints a string or a comma-separated list of strings. Returns TRUE
1935 if successful. FILEHANDLE may be a scalar variable name, in which case
1936 the variable contains the name of or a reference to the filehandle, thus introducing one
1937 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1938 token is a term, it may be misinterpreted as an operator unless you
1939 interpose a + or put parens around the arguments.) If FILEHANDLE is
1940 omitted, prints by default to standard output (or to the last selected
1941 output channel--see select()). If LIST is also omitted, prints $_ to
1942 STDOUT. To set the default output channel to something other than
1943 STDOUT use the select operation. Note that, because print takes a
1944 LIST, anything in the LIST is evaluated in a list context, and any
1945 subroutine that you call will have one or more of its expressions
1946 evaluated in a list context. Also be careful not to follow the print
1947 keyword with a left parenthesis unless you want the corresponding right
1948 parenthesis to terminate the arguments to the print--interpose a + or
1949 put parens around all the arguments.
1951 Note that if you're storing FILEHANDLES in an array or other expression,
1952 you will have to use a block returning its value instead
1954 print { $files[$i] } "stuff\n";
1955 print { $OK ? STDOUT : STDERR } "stuff\n";
1957 =item printf FILEHANDLE LIST
1961 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
1962 of the list will be interpreted as the printf format.
1964 =item push ARRAY,LIST
1966 Treats ARRAY as a stack, and pushes the values of LIST
1967 onto the end of ARRAY. The length of ARRAY increases by the length of
1968 LIST. Has the same effect as
1971 $ARRAY[++$#ARRAY] = $value;
1974 but is more efficient. Returns the new number of elements in the array.
1984 Generalized quotes. See L<perlop>.
1986 =item quotemeta EXPR
1988 Returns the value of EXPR with with all regular expression
1989 metacharacters backslashed. This is the internal function implementing
1990 the \Q escape in double-quoted strings.
1996 Returns a random fractional number between 0 and the value of EXPR.
1997 (EXPR should be positive.) If EXPR is omitted, returns a value between
1998 0 and 1. This function produces repeatable sequences unless srand()
1999 is invoked. See also srand().
2001 (Note: if your rand function consistently returns numbers that are too
2002 large or too small, then your version of Perl was probably compiled
2003 with the wrong number of RANDBITS. As a workaround, you can usually
2004 multiply EXPR by the correct power of 2 to get the range you want.
2005 This will make your script unportable, however. It's better to recompile
2008 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2010 =item read FILEHANDLE,SCALAR,LENGTH
2012 Attempts to read LENGTH bytes of data into variable SCALAR from the
2013 specified FILEHANDLE. Returns the number of bytes actually read, or
2014 undef if there was an error. SCALAR will be grown or shrunk to the
2015 length actually read. An OFFSET may be specified to place the read
2016 data at some other place than the beginning of the string. This call
2017 is actually implemented in terms of stdio's fread call. To get a true
2018 read system call, see sysread().
2020 =item readdir DIRHANDLE
2022 Returns the next directory entry for a directory opened by opendir().
2023 If used in a list context, returns all the rest of the entries in the
2024 directory. If there are no more entries, returns an undefined value in
2025 a scalar context or a null list in a list context.
2027 If you're planning to filetest the return values out of a readdir(), you'd
2028 better prepend the directory in question. Otherwise, since we didn't
2029 chdir() there, it would have been testing the wrong file.
2031 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2032 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2037 Returns the value of a symbolic link, if symbolic links are
2038 implemented. If not, gives a fatal error. If there is some system
2039 error, returns the undefined value and sets $! (errno). If EXPR is
2042 =item recv SOCKET,SCALAR,LEN,FLAGS
2044 Receives a message on a socket. Attempts to receive LENGTH bytes of
2045 data into variable SCALAR from the specified SOCKET filehandle.
2046 Actually does a C recvfrom(), so that it can returns the address of the
2047 sender. Returns the undefined value if there's an error. SCALAR will
2048 be grown or shrunk to the length actually read. Takes the same flags
2049 as the system call of the same name.
2050 See L<perlipc/"UDP: Message Passing"> for examples.
2056 The C<redo> command restarts the loop block without evaluating the
2057 conditional again. The C<continue> block, if any, is not executed. If
2058 the LABEL is omitted, the command refers to the innermost enclosing
2059 loop. This command is normally used by programs that want to lie to
2060 themselves about what was just input:
2062 # a simpleminded Pascal comment stripper
2063 # (warning: assumes no { or } in strings)
2064 LINE: while (<STDIN>) {
2065 while (s|({.*}.*){.*}|$1 |) {}
2070 if (/}/) { # end of comment?
2081 Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2082 returned depends on the type of thing the reference is a reference to.
2083 Builtin types include:
2092 If the referenced object has been blessed into a package, then that package
2093 name is returned instead. You can think of ref() as a typeof() operator.
2095 if (ref($r) eq "HASH") {
2096 print "r is a reference to an associative array.\n";
2099 print "r is not a reference at all.\n";
2102 See also L<perlref>.
2104 =item rename OLDNAME,NEWNAME
2106 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2107 not work across filesystem boundaries.
2113 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2114 supplied. If EXPR is numeric, demands that the current version of Perl
2115 ($] or $PERL_VERSION) be equal or greater than EXPR.
2117 Otherwise, demands that a library file be included if it hasn't already
2118 been included. The file is included via the do-FILE mechanism, which is
2119 essentially just a variety of eval(). Has semantics similar to the following
2123 local($filename) = @_;
2124 return 1 if $INC{$filename};
2125 local($realfilename,$result);
2127 foreach $prefix (@INC) {
2128 $realfilename = "$prefix/$filename";
2129 if (-f $realfilename) {
2130 $result = do $realfilename;
2134 die "Can't find $filename in \@INC";
2137 die "$filename did not return true value" unless $result;
2138 $INC{$filename} = $realfilename;
2142 Note that the file will not be included twice under the same specified
2143 name. The file must return TRUE as the last statement to indicate
2144 successful execution of any initialization code, so it's customary to
2145 end such a file with "1;" unless you're sure it'll return TRUE
2146 otherwise. But it's better just to put the "C<1;>", in case you add more
2149 If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
2150 to make it easy to load standard modules. This form of loading of
2151 modules does not risk altering your namespace.
2153 For a yet-more-powerful import facility, see the L</use()> and
2160 Generally used in a C<continue> block at the end of a loop to clear
2161 variables and reset ?? searches so that they work again. The
2162 expression is interpreted as a list of single characters (hyphens
2163 allowed for ranges). All variables and arrays beginning with one of
2164 those letters are reset to their pristine state. If the expression is
2165 omitted, one-match searches (?pattern?) are reset to match again. Only
2166 resets variables or searches in the current package. Always returns
2169 reset 'X'; # reset all X variables
2170 reset 'a-z'; # reset lower case variables
2171 reset; # just reset ?? searches
2173 Resetting "A-Z" is not recommended since you'll wipe out your
2174 ARGV and ENV arrays. Only resets package variables--lexical variables
2175 are unaffected, but they clean themselves up on scope exit anyway,
2176 so anymore you probably want to use them instead. See L</my>.
2180 Returns from a subroutine or eval with the value specified. (Note that
2181 in the absence of a return a subroutine or eval() will automatically
2182 return the value of the last expression evaluated.)
2186 In a list context, returns a list value consisting of the elements
2187 of LIST in the opposite order. In a scalar context, returns a string
2188 value consisting of the bytes of the first element of LIST in the
2191 print reverse <>; # line tac
2194 print scalar reverse scalar <>; # byte tac
2196 =item rewinddir DIRHANDLE
2198 Sets the current position to the beginning of the directory for the
2199 readdir() routine on DIRHANDLE.
2201 =item rindex STR,SUBSTR,POSITION
2203 =item rindex STR,SUBSTR
2205 Works just like index except that it returns the position of the LAST
2206 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2207 last occurrence at or before that position.
2209 =item rmdir FILENAME
2211 Deletes the directory specified by FILENAME if it is empty. If it
2212 succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
2213 FILENAME is omitted, uses $_.
2217 The substitution operator. See L<perlop>.
2221 Forces EXPR to be interpreted in a scalar context and returns the value
2224 @counts = ( scalar @a, scalar @b, scalar @c );
2226 There is no equivalent operator to force an expression to
2227 be interpolated in a list context because it's in practice never
2228 needed. If you really wanted to do so, however, you could use
2229 the construction C<@{[ (some expression) ]}>, but usually a simple
2230 C<(some expression)> suffices.
2232 =item seek FILEHANDLE,POSITION,WHENCE
2234 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2235 call of stdio. FILEHANDLE may be an expression whose value gives the name
2236 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2237 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2238 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2239 this from POSIX module. Returns 1 upon success, 0 otherwise.
2241 On some systems you have to do a seek whenever you switch between reading
2242 and writing. Amongst other things, this may have the effect of calling
2243 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2248 This is also useful for applications emulating C<tail -f>. Once you hit
2249 EOF on your read, and then sleep for a while, you might have to stick in a
2250 seek() to reset things. First the simple trick listed above to clear the
2251 filepointer. The seek() doesn't change the current position, but it
2252 I<does> clear the end-of-file condition on the handle, so that the next
2253 C<E<lt>FILE<E<gt>> makes Perl try again to read something. Hopefully.
2255 If that doesn't work (some stdios are particularly cantankerous), then
2256 you may need something more like this:
2259 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2260 # search for some stuff and put it into files
2262 sleep($for_a_while);
2263 seek(FILE, $curpos, 0);
2266 =item seekdir DIRHANDLE,POS
2268 Sets the current position for the readdir() routine on DIRHANDLE. POS
2269 must be a value returned by telldir(). Has the same caveats about
2270 possible directory compaction as the corresponding system library
2273 =item select FILEHANDLE
2277 Returns the currently selected filehandle. Sets the current default
2278 filehandle for output, if FILEHANDLE is supplied. This has two
2279 effects: first, a C<write> or a C<print> without a filehandle will
2280 default to this FILEHANDLE. Second, references to variables related to
2281 output will refer to this output channel. For example, if you have to
2282 set the top of form format for more than one output channel, you might
2290 FILEHANDLE may be an expression whose value gives the name of the
2291 actual filehandle. Thus:
2293 $oldfh = select(STDERR); $| = 1; select($oldfh);
2295 Some programmers may prefer to think of filehandles as objects with
2296 methods, preferring to write the last example as:
2299 STDERR->autoflush(1);
2301 =item select RBITS,WBITS,EBITS,TIMEOUT
2303 This calls the select(2) system call with the bitmasks specified, which
2304 can be constructed using fileno() and vec(), along these lines:
2306 $rin = $win = $ein = '';
2307 vec($rin,fileno(STDIN),1) = 1;
2308 vec($win,fileno(STDOUT),1) = 1;
2311 If you want to select on many filehandles you might wish to write a
2315 local(@fhlist) = split(' ',$_[0]);
2318 vec($bits,fileno($_),1) = 1;
2322 $rin = fhbits('STDIN TTY SOCK');
2326 ($nfound,$timeleft) =
2327 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2329 or to block until something becomes ready:
2331 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2333 Any of the bitmasks can also be undef. The timeout, if specified, is
2334 in seconds, which may be fractional. Note: not all implementations are
2335 capable of returning the $timeleft. If not, they always return
2336 $timeleft equal to the supplied $timeout.
2338 You can effect a 250-microsecond sleep this way:
2340 select(undef, undef, undef, 0.25);
2342 B<WARNING>: Do not attempt to mix buffered I/O (like read() or <FH>)
2343 with select(). You have to use sysread() instead.
2345 =item semctl ID,SEMNUM,CMD,ARG
2347 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2348 &GETALL, then ARG must be a variable which will hold the returned
2349 semid_ds structure or semaphore value array. Returns like ioctl: the
2350 undefined value for error, "0 but true" for zero, or the actual return
2353 =item semget KEY,NSEMS,FLAGS
2355 Calls the System V IPC function semget. Returns the semaphore id, or
2356 the undefined value if there is an error.
2358 =item semop KEY,OPSTRING
2360 Calls the System V IPC function semop to perform semaphore operations
2361 such as signaling and waiting. OPSTRING must be a packed array of
2362 semop structures. Each semop structure can be generated with
2363 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2364 operations is implied by the length of OPSTRING. Returns TRUE if
2365 successful, or FALSE if there is an error. As an example, the
2366 following code waits on semaphore $semnum of semaphore id $semid:
2368 $semop = pack("sss", $semnum, -1, 0);
2369 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2371 To signal the semaphore, replace "-1" with "1".
2373 =item send SOCKET,MSG,FLAGS,TO
2375 =item send SOCKET,MSG,FLAGS
2377 Sends a message on a socket. Takes the same flags as the system call
2378 of the same name. On unconnected sockets you must specify a
2379 destination to send TO, in which case it does a C sendto(). Returns
2380 the number of characters sent, or the undefined value if there is an
2382 See L<perlipc/"UDP: Message Passing"> for examples.
2384 =item setpgrp PID,PGRP
2386 Sets the current process group for the specified PID, 0 for the current
2387 process. Will produce a fatal error if used on a machine that doesn't
2388 implement setpgrp(2).
2390 =item setpriority WHICH,WHO,PRIORITY
2392 Sets the current priority for a process, a process group, or a user.
2393 (See setpriority(2).) Will produce a fatal error if used on a machine
2394 that doesn't implement setpriority(2).
2396 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2398 Sets the socket option requested. Returns undefined if there is an
2399 error. OPTVAL may be specified as undef if you don't want to pass an
2406 Shifts the first value of the array off and returns it, shortening the
2407 array by 1 and moving everything down. If there are no elements in the
2408 array, returns the undefined value. If ARRAY is omitted, shifts the
2409 @ARGV array in the main program, and the @_ array in subroutines.
2410 (This is determined lexically.) See also unshift(), push(), and pop().
2411 Shift() and unshift() do the same thing to the left end of an array
2412 that push() and pop() do to the right end.
2414 =item shmctl ID,CMD,ARG
2416 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2417 must be a variable which will hold the returned shmid_ds structure.
2418 Returns like ioctl: the undefined value for error, "0 but true" for
2419 zero, or the actual return value otherwise.
2421 =item shmget KEY,SIZE,FLAGS
2423 Calls the System V IPC function shmget. Returns the shared memory
2424 segment id, or the undefined value if there is an error.
2426 =item shmread ID,VAR,POS,SIZE
2428 =item shmwrite ID,STRING,POS,SIZE
2430 Reads or writes the System V shared memory segment ID starting at
2431 position POS for size SIZE by attaching to it, copying in/out, and
2432 detaching from it. When reading, VAR must be a variable which will
2433 hold the data read. When writing, if STRING is too long, only SIZE
2434 bytes are used; if STRING is too short, nulls are written to fill out
2435 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2437 =item shutdown SOCKET,HOW
2439 Shuts down a socket connection in the manner indicated by HOW, which
2440 has the same interpretation as in the system call of the same name.
2444 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2451 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2452 May be interrupted by sending the process a SIGALRM. Returns the
2453 number of seconds actually slept. You probably cannot mix alarm() and
2454 sleep() calls, since sleep() is often implemented using alarm().
2456 On some older systems, it may sleep up to a full second less than what
2457 you requested, depending on how it counts seconds. Most modern systems
2458 always sleep the full amount.
2460 For delays of finer granularity than one second, you may use Perl's
2461 syscall() interface to access setitimer(2) if your system supports it,
2462 or else see L</select()> below.
2464 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2466 Opens a socket of the specified kind and attaches it to filehandle
2467 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2468 system call of the same name. You should "use Socket;" first to get
2469 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2471 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2473 Creates an unnamed pair of sockets in the specified domain, of the
2474 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2475 for the system call of the same name. If unimplemented, yields a fatal
2476 error. Returns TRUE if successful.
2478 =item sort SUBNAME LIST
2480 =item sort BLOCK LIST
2484 Sorts the LIST and returns the sorted list value. Nonexistent values
2485 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2486 in standard string comparison order. If SUBNAME is specified, it
2487 gives the name of a subroutine that returns an integer less than, equal
2488 to, or greater than 0, depending on how the elements of the array are
2489 to be ordered. (The <=> and cmp operators are extremely useful in such
2490 routines.) SUBNAME may be a scalar variable name, in which case the
2491 value provides the name of the subroutine to use. In place of a
2492 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2495 In the interests of efficiency the normal calling code for subroutines is
2496 bypassed, with the following effects: the subroutine may not be a
2497 recursive subroutine, and the two elements to be compared are passed into
2498 the subroutine not via @_ but as the package global variables $a and
2499 $b (see example below). They are passed by reference, so don't
2500 modify $a and $b. And don't try to declare them as lexicals either.
2505 @articles = sort @files;
2507 # same thing, but with explicit sort routine
2508 @articles = sort {$a cmp $b} @files;
2510 # now case-insensitively
2511 @articles = sort { uc($a) cmp uc($b)} @files;
2513 # same thing in reversed order
2514 @articles = sort {$b cmp $a} @files;
2516 # sort numerically ascending
2517 @articles = sort {$a <=> $b} @files;
2519 # sort numerically descending
2520 @articles = sort {$b <=> $a} @files;
2522 # sort using explicit subroutine name
2524 $age{$a} <=> $age{$b}; # presuming integers
2526 @sortedclass = sort byage @class;
2528 sub backwards { $b cmp $a; }
2529 @harry = ('dog','cat','x','Cain','Abel');
2530 @george = ('gone','chased','yz','Punished','Axed');
2532 # prints AbelCaincatdogx
2533 print sort backwards @harry;
2534 # prints xdogcatCainAbel
2535 print sort @george, 'to', @harry;
2536 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2538 # inefficiently sort by descending numeric compare using
2539 # the first integer after the first = sign, or the
2540 # whole record case-insensitively otherwise
2543 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2548 # same thing, but much more efficiently;
2549 # we'll build auxiliary indices instead
2553 push @nums, /=(\d+)/;
2558 $nums[$b] <=> $nums[$a]
2560 $caps[$a] cmp $caps[$b]
2564 # same thing using a Schwartzian Transform (no temps)
2565 @new = map { $_->[0] }
2566 sort { $b->[1] <=> $a->[1]
2569 } map { [$_, /=(\d+)/, uc($_)] } @old;
2571 If you're and using strict, you I<MUST NOT> declare $a
2572 and $b as lexicals. They are package globals. That means
2573 if you're in the C<main> package, it's
2575 @articles = sort {$main::b <=> $main::a} @files;
2579 @articles = sort {$::b <=> $::a} @files;
2581 but if you're in the C<FooPack> package, it's
2583 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2585 =item splice ARRAY,OFFSET,LENGTH,LIST
2587 =item splice ARRAY,OFFSET,LENGTH
2589 =item splice ARRAY,OFFSET
2591 Removes the elements designated by OFFSET and LENGTH from an array, and
2592 replaces them with the elements of LIST, if any. Returns the elements
2593 removed from the array. The array grows or shrinks as necessary. If
2594 LENGTH is omitted, removes everything from OFFSET onward. The
2595 following equivalencies hold (assuming $[ == 0):
2597 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2598 pop(@a) splice(@a,-1)
2599 shift(@a) splice(@a,0,1)
2600 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2601 $a[$x] = $y splice(@a,$x,1,$y);
2603 Example, assuming array lengths are passed before arrays:
2605 sub aeq { # compare two list values
2606 local(@a) = splice(@_,0,shift);
2607 local(@b) = splice(@_,0,shift);
2608 return 0 unless @a == @b; # same len?
2610 return 0 if pop(@a) ne pop(@b);
2614 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2616 =item split /PATTERN/,EXPR,LIMIT
2618 =item split /PATTERN/,EXPR
2620 =item split /PATTERN/
2624 Splits a string into an array of strings, and returns it.
2626 If not in a list context, returns the number of fields found and splits into
2627 the @_ array. (In a list context, you can force the split into @_ by
2628 using C<??> as the pattern delimiters, but it still returns the array
2629 value.) The use of implicit split to @_ is deprecated, however.
2631 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2632 splits on whitespace (after skipping any leading whitespace). Anything
2633 matching PATTERN is taken to be a delimiter separating the fields. (Note
2634 that the delimiter may be longer than one character.) If LIMIT is
2635 specified and is not negative, splits into no more than that many fields
2636 (though it may split into fewer). If LIMIT is unspecified, trailing null
2637 fields are stripped (which potential users of pop() would do well to
2638 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2639 LIMIT had been specified.
2641 A pattern matching the null string (not to be confused with
2642 a null pattern C<//>, which is just one member of the set of patterns
2643 matching a null string) will split the value of EXPR into separate
2644 characters at each point it matches that way. For example:
2646 print join(':', split(/ */, 'hi there'));
2648 produces the output 'h:i:t:h:e:r:e'.
2650 The LIMIT parameter can be used to partially split a line
2652 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2654 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2655 one larger than the number of variables in the list, to avoid
2656 unnecessary work. For the list above LIMIT would have been 4 by
2657 default. In time critical applications it behooves you not to split
2658 into more fields than you really need.
2660 If the PATTERN contains parentheses, additional array elements are
2661 created from each matching substring in the delimiter.
2663 split(/([,-])/, "1-10,20");
2665 produces the list value
2667 (1, '-', 10, ',', 20)
2669 If you had the entire header of a normal Unix email message in $header,
2670 you could split it up into fields and their values this way:
2672 $header =~ s/\n\s+/ /g; # fix continuation lines
2673 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2675 The pattern C</PATTERN/> may be replaced with an expression to specify
2676 patterns that vary at runtime. (To do runtime compilation only once,
2677 use C</$variable/o>.)
2679 As a special case, specifying a PATTERN of space (C<' '>) will split on
2680 white space just as split with no arguments does. Thus, split(' ') can
2681 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2682 will give you as many null initial fields as there are leading spaces.
2683 A split on /\s+/ is like a split(' ') except that any leading
2684 whitespace produces a null first field. A split with no arguments
2685 really does a C<split(' ', $_)> internally.
2689 open(passwd, '/etc/passwd');
2691 ($login, $passwd, $uid, $gid, $gcos,
2692 $home, $shell) = split(/:/);
2696 (Note that $shell above will still have a newline on it. See L</chop>,
2697 L</chomp>, and L</join>.)
2699 =item sprintf FORMAT,LIST
2701 Returns a string formatted by the usual printf conventions of the C
2702 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2703 (The * character for an indirectly specified length is not
2704 supported, but you can get the same effect by interpolating a variable
2705 into the pattern.) Some C libraries' implementations of sprintf() can
2706 dump core when fed ludicrous arguments.
2710 Return the square root of EXPR. If EXPR is omitted, returns square
2715 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2716 does C<srand(time)>. Many folks use an explicit C<srand(time ^ $$)>
2717 instead. Of course, you'd need something much more random than that for
2718 cryptographic purposes, since it's easy to guess the current time.
2719 Checksumming the compressed output of rapidly changing operating system
2720 status programs is the usual method. Examples are posted regularly to
2721 the comp.security.unix newsgroup.
2723 =item stat FILEHANDLE
2727 Returns a 13-element array giving the status info for a file, either the
2728 file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2729 the stat fails. Typically used as follows:
2731 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2732 $atime,$mtime,$ctime,$blksize,$blocks)
2735 If stat is passed the special filehandle consisting of an underline, no
2736 stat is done, but the current contents of the stat structure from the
2737 last stat or filetest are returned. Example:
2739 if (-x $file && (($d) = stat(_)) && $d < 0) {
2740 print "$file is executable NFS file\n";
2743 (This only works on machines for which the device number is negative under NFS.)
2749 Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
2750 doing many pattern matches on the string before it is next modified.
2751 This may or may not save time, depending on the nature and number of
2752 patterns you are searching on, and on the distribution of character
2753 frequencies in the string to be searched--you probably want to compare
2754 runtimes with and without it to see which runs faster. Those loops
2755 which scan for many short constant strings (including the constant
2756 parts of more complex patterns) will benefit most. You may have only
2757 one study active at a time--if you study a different scalar the first
2758 is "unstudied". (The way study works is this: a linked list of every
2759 character in the string to be searched is made, so we know, for
2760 example, where all the 'k' characters are. From each search string,
2761 the rarest character is selected, based on some static frequency tables
2762 constructed from some C programs and English text. Only those places
2763 that contain this "rarest" character are examined.)
2765 For example, here is a loop which inserts index producing entries
2766 before any line containing a certain pattern:
2770 print ".IX foo\n" if /\bfoo\b/;
2771 print ".IX bar\n" if /\bbar\b/;
2772 print ".IX blurfl\n" if /\bblurfl\b/;
2777 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2778 will be looked at, because "f" is rarer than "o". In general, this is
2779 a big win except in pathological cases. The only question is whether
2780 it saves you more time than it took to build the linked list in the
2783 Note that if you have to look for strings that you don't know till
2784 runtime, you can build an entire loop as a string and eval that to
2785 avoid recompiling all your patterns all the time. Together with
2786 undefining $/ to input entire files as one record, this can be very
2787 fast, often faster than specialized programs like fgrep(1). The following
2788 scans a list of files (@files) for a list of words (@words), and prints
2789 out the names of those files that contain a match:
2791 $search = 'while (<>) { study;';
2792 foreach $word (@words) {
2793 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2798 eval $search; # this screams
2799 $/ = "\n"; # put back to normal input delim
2800 foreach $file (sort keys(%seen)) {
2808 =item sub NAME BLOCK
2810 This is subroutine definition, not a real function I<per se>. With just a
2811 NAME (and possibly prototypes), it's just a forward declaration. Without
2812 a NAME, it's an anonymous function declaration, and does actually return a
2813 value: the CODE ref of the closure you just created. See L<perlsub> and
2814 L<perlref> for details.
2816 =item substr EXPR,OFFSET,LEN
2818 =item substr EXPR,OFFSET
2820 Extracts a substring out of EXPR and returns it. First character is at
2821 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2822 that far from the end of the string. If LEN is omitted, returns
2823 everything to the end of the string. If LEN is negative, leaves that
2824 many characters off the end of the string.
2826 You can use the substr() function
2827 as an lvalue, in which case EXPR must be an lvalue. If you assign
2828 something shorter than LEN, the string will shrink, and if you assign
2829 something longer than LEN, the string will grow to accommodate it. To
2830 keep the string the same length you may need to pad or chop your value
2833 =item symlink OLDFILE,NEWFILE
2835 Creates a new filename symbolically linked to the old filename.
2836 Returns 1 for success, 0 otherwise. On systems that don't support
2837 symbolic links, produces a fatal error at run time. To check for that,
2840 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2844 Calls the system call specified as the first element of the list,
2845 passing the remaining elements as arguments to the system call. If
2846 unimplemented, produces a fatal error. The arguments are interpreted
2847 as follows: if a given argument is numeric, the argument is passed as
2848 an int. If not, the pointer to the string value is passed. You are
2849 responsible to make sure a string is pre-extended long enough to
2850 receive any result that might be written into a string. If your
2851 integer arguments are not literals and have never been interpreted in a
2852 numeric context, you may need to add 0 to them to force them to look
2855 require 'syscall.ph'; # may need to run h2ph
2856 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2858 Note that Perl only supports passing of up to 14 arguments to your system call,
2859 which in practice should usually suffice.
2861 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2863 =item sysread FILEHANDLE,SCALAR,LENGTH
2865 Attempts to read LENGTH bytes of data into variable SCALAR from the
2866 specified FILEHANDLE, using the system call read(2). It bypasses
2867 stdio, so mixing this with other kinds of reads may cause confusion.
2868 Returns the number of bytes actually read, or undef if there was an
2869 error. SCALAR will be grown or shrunk to the length actually read. An
2870 OFFSET may be specified to place the read data at some other place than
2871 the beginning of the string.
2875 Does exactly the same thing as "exec LIST" except that a fork is done
2876 first, and the parent process waits for the child process to complete.
2877 Note that argument processing varies depending on the number of
2878 arguments. The return value is the exit status of the program as
2879 returned by the wait() call. To get the actual exit value divide by
2880 256. See also L</exec>. This is I<NOT> what you want to use to capture
2881 the output from a command, for that you should merely use backticks, as
2882 described in L<perlop/"`STRING`">.
2884 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2886 =item syswrite FILEHANDLE,SCALAR,LENGTH
2888 Attempts to write LENGTH bytes of data from variable SCALAR to the
2889 specified FILEHANDLE, using the system call write(2). It bypasses
2890 stdio, so mixing this with prints may cause confusion. Returns the
2891 number of bytes actually written, or undef if there was an error. An
2892 OFFSET may be specified to place the read data at some other place than
2893 the beginning of the string.
2895 =item tell FILEHANDLE
2899 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
2900 expression whose value gives the name of the actual filehandle. If
2901 FILEHANDLE is omitted, assumes the file last read.
2903 =item telldir DIRHANDLE
2905 Returns the current position of the readdir() routines on DIRHANDLE.
2906 Value may be given to seekdir() to access a particular location in a
2907 directory. Has the same caveats about possible directory compaction as
2908 the corresponding system library routine.
2910 =item tie VARIABLE,CLASSNAME,LIST
2912 This function binds a variable to a package class that will provide the
2913 implementation for the variable. VARIABLE is the name of the variable
2914 to be enchanted. CLASSNAME is the name of a class implementing objects
2915 of correct type. Any additional arguments are passed to the "new"
2916 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
2917 Typically these are arguments such as might be passed to the dbm_open()
2918 function of C. The object returned by the "new" method is also
2919 returned by the tie() function, which would be useful if you want to
2920 access other methods in CLASSNAME.
2922 Note that functions such as keys() and values() may return huge array
2923 values when used on large objects, like DBM files. You may prefer to
2924 use the each() function to iterate over such. Example:
2926 # print out history file offsets
2928 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
2929 while (($key,$val) = each %HIST) {
2930 print $key, ' = ', unpack('L',$val), "\n";
2934 A class implementing an associative array should have the following
2937 TIEHASH classname, LIST
2940 STORE this, key, value
2944 NEXTKEY this, lastkey
2946 A class implementing an ordinary array should have the following methods:
2948 TIEARRAY classname, LIST
2951 STORE this, key, value
2954 A class implementing a scalar should have the following methods:
2956 TIESCALAR classname, LIST
2961 Unlike dbmopen(), the tie() function will not use or require a module
2962 for you--you need to do that explicitly yourself. See L<DB_File>
2963 or the F<Config> module for interesting tie() implementations.
2967 Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
2968 1970. Suitable for feeding to gmtime() and localtime().
2972 Returns a four-element array giving the user and system times, in
2973 seconds, for this process and the children of this process.
2975 ($user,$system,$cuser,$csystem) = times;
2979 The translation operator. See L<perlop>.
2981 =item truncate FILEHANDLE,LENGTH
2983 =item truncate EXPR,LENGTH
2985 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
2986 specified length. Produces a fatal error if truncate isn't implemented
2991 Returns an uppercased version of EXPR. This is the internal function
2992 implementing the \U escape in double-quoted strings.
2993 Should respect any POSIX setlocale() settings.
2997 Returns the value of EXPR with the first character uppercased. This is
2998 the internal function implementing the \u escape in double-quoted strings.
2999 Should respect any POSIX setlocale() settings.
3005 Sets the umask for the process and returns the old one. If EXPR is
3006 omitted, merely returns current umask.
3012 Undefines the value of EXPR, which must be an lvalue. Use only on a
3013 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3014 will probably not do what you expect on most predefined variables or
3015 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3016 the EXPR, in which case nothing is undefined, but you still get an
3017 undefined value that you could, for instance, return from a
3018 subroutine. Examples:
3021 undef $bar{'blurfl'};
3025 return (wantarray ? () : undef) if $they_blew_it;
3029 Deletes a list of files. Returns the number of files successfully
3032 $cnt = unlink 'a', 'b', 'c';
3036 Note: unlink will not delete directories unless you are superuser and
3037 the B<-U> flag is supplied to Perl. Even if these conditions are
3038 met, be warned that unlinking a directory can inflict damage on your
3039 filesystem. Use rmdir instead.
3041 =item unpack TEMPLATE,EXPR
3043 Unpack does the reverse of pack: it takes a string representing a
3044 structure and expands it out into a list value, returning the array
3045 value. (In a scalar context, it merely returns the first value
3046 produced.) The TEMPLATE has the same format as in the pack function.
3047 Here's a subroutine that does substring:
3050 local($what,$where,$howmuch) = @_;
3051 unpack("x$where a$howmuch", $what);
3056 sub ordinal { unpack("c",$_[0]); } # same as ord()
3058 In addition, you may prefix a field with a %<number> to indicate that
3059 you want a <number>-bit checksum of the items instead of the items
3060 themselves. Default is a 16-bit checksum. For example, the following
3061 computes the same number as the System V sum program:
3064 $checksum += unpack("%16C*", $_);
3068 The following efficiently counts the number of set bits in a bit vector:
3070 $setbits = unpack("%32b*", $selectmask);
3072 =item untie VARIABLE
3074 Breaks the binding between a variable and a package. (See tie().)
3076 =item unshift ARRAY,LIST
3078 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3079 depending on how you look at it. Prepends list to the front of the
3080 array, and returns the new number of elements in the array.
3082 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3084 Note the LIST is prepended whole, not one element at a time, so the
3085 prepended elements stay in the same order. Use reverse to do the
3088 =item use Module LIST
3092 Imports some semantics into the current package from the named module,
3093 generally by aliasing certain subroutine or variable names into your
3094 package. It is exactly equivalent to
3096 BEGIN { require Module; import Module LIST; }
3098 The BEGIN forces the require and import to happen at compile time. The
3099 require makes sure the module is loaded into memory if it hasn't been
3100 yet. The import is not a builtin--it's just an ordinary static method
3101 call into the "Module" package to tell the module to import the list of
3102 features back into the current package. The module can implement its
3103 import method any way it likes, though most modules just choose to
3104 derive their import method via inheritance from the Exporter class that
3105 is defined in the Exporter module. See L<Exporter>.
3107 If you don't want your namespace altered, explicitly supply an empty list:
3111 That is exactly equivalent to
3113 BEGIN { require Module; }
3115 Because this is a wide-open interface, pragmas (compiler directives)
3116 are also implemented this way. Currently implemented pragmas are:
3120 use sigtrap qw(SEGV BUS);
3121 use strict qw(subs vars refs);
3122 use subs qw(afunc blurfl);
3124 These pseudomodules import semantics into the current block scope, unlike
3125 ordinary modules, which import symbols into the current package (which are
3126 effective through the end of the file).
3128 There's a corresponding "no" command that unimports meanings imported
3134 See L<perlmod> for a list of standard modules and pragmas.
3138 Changes the access and modification times on each file of a list of
3139 files. The first two elements of the list must be the NUMERICAL access
3140 and modification times, in that order. Returns the number of files
3141 successfully changed. The inode modification time of each file is set
3142 to the current time. Example of a "touch" command:
3146 utime $now, $now, @ARGV;
3148 =item values ASSOC_ARRAY
3150 Returns a normal array consisting of all the values of the named
3151 associative array. (In a scalar context, returns the number of
3152 values.) The values are returned in an apparently random order, but it
3153 is the same order as either the keys() or each() function would produce
3154 on the same array. See also keys() and each().
3156 =item vec EXPR,OFFSET,BITS
3158 Treats a string as a vector of unsigned integers, and returns the value
3159 of the bitfield specified. May also be assigned to. BITS must be a
3160 power of two from 1 to 32.
3162 Vectors created with vec() can also be manipulated with the logical
3163 operators |, & and ^, which will assume a bit vector operation is
3164 desired when both operands are strings.
3166 To transform a bit vector into a string or array of 0's and 1's, use these:
3168 $bits = unpack("b*", $vector);
3169 @bits = split(//, unpack("b*", $vector));
3171 If you know the exact length in bits, it can be used in place of the *.
3175 Waits for a child process to terminate and returns the pid of the
3176 deceased process, or -1 if there are no child processes. The status is
3179 =item waitpid PID,FLAGS
3181 Waits for a particular child process to terminate and returns the pid
3182 of the deceased process, or -1 if there is no such child process. The
3183 status is returned in $?. If you say
3187 waitpid(-1,&WNOHANG);
3189 then you can do a non-blocking wait for any process. Non-blocking wait
3190 is only available on machines supporting either the waitpid(2) or
3191 wait4(2) system calls. However, waiting for a particular pid with
3192 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3193 by remembering the status values of processes that have exited but have
3194 not been harvested by the Perl script yet.)
3198 Returns TRUE if the context of the currently executing subroutine is
3199 looking for a list value. Returns FALSE if the context is looking
3202 return wantarray ? () : undef;
3206 Produces a message on STDERR just like die(), but doesn't exit or
3209 =item write FILEHANDLE
3215 Writes a formatted record (possibly multi-line) to the specified file,
3216 using the format associated with that file. By default the format for
3217 a file is the one having the same name is the filehandle, but the
3218 format for the current output channel (see the select() function) may be set
3219 explicitly by assigning the name of the format to the $~ variable.
3221 Top of form processing is handled automatically: if there is
3222 insufficient room on the current page for the formatted record, the
3223 page is advanced by writing a form feed, a special top-of-page format
3224 is used to format the new page header, and then the record is written.
3225 By default the top-of-page format is the name of the filehandle with
3226 "_TOP" appended, but it may be dynamically set to the format of your
3227 choice by assigning the name to the $^ variable while the filehandle is
3228 selected. The number of lines remaining on the current page is in
3229 variable $-, which can be set to 0 to force a new page.
3231 If FILEHANDLE is unspecified, output goes to the current default output
3232 channel, which starts out as STDOUT but may be changed by the
3233 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3234 is evaluated and the resulting string is used to look up the name of
3235 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3237 Note that write is I<NOT> the opposite of read. Unfortunately.
3241 The translation operator. See L<perlop/tr///>.