3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can only
18 ever be one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parens.) If you use the parens, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 non-abortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
61 I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in a scalar context. Some operators return the
67 length of the list that would have been returned in a list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
73 =head2 Perl Functions by Category
75 Here are Perl's functions (including things that look like
76 functions, like some of the keywords and named operators)
77 arranged by category. Some functions appear in more
82 =item Functions for SCALARs or strings
84 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86 sprintf, substr, tr///, uc, ucfirst, y///
88 =item Regular expressions and pattern matching
90 m//, pos, quotemeta, s///, split, study
92 =item Numeric functions
94 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
97 =item Functions for real @ARRAYs
99 pop, push, shift, splice, unshift
101 =item Functions for list data
103 grep, join, map, qw/STRING/, reverse, sort, unpack
105 =item Functions for real %HASHes
107 delete, each, exists, keys, values
109 =item Input and output functions
111 binmode, close, closedir, dbmclose, dbmopen, die, eof,
112 fileno, flock, format, getc, print, printf, read, readdir,
113 rewinddir, seek, seekdir, select, syscall, sysread,
114 syswrite, tell, telldir, truncate, warn, write
116 =item Functions for fixed length data or records
118 pack, read, syscall, sysread, syswrite, unpack, vec
120 =item Functions for filehandles, files, or directories
122 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
123 lstat, mkdir, open, opendir, readlink, rename, rmdir,
124 stat, symlink, umask, unlink, utime
126 =item Keywords related to the control flow of your perl program
128 caller, continue, die, do, dump, eval, exit, goto, last,
129 next, redo, return, sub, wantarray
131 =item Keywords related to scoping
133 caller, import, local, my, package, use
135 =item Miscellaneous functions
137 defined, dump, eval, formline, local, my, reset, scalar,
140 =item Functions for processes and process groups
142 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
146 =item Keywords related to perl modules
148 do, import, no, package, require, use
150 =item Keywords related to classes and object-orientedness
152 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
154 =item Low-level socket functions
156 accept, bind, connect, getpeername, getsockname,
157 getsockopt, listen, recv, send, setsockopt, shutdown,
160 =item System V interprocess communication functions
162 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163 shmctl, shmget, shmread, shmwrite
165 =item Fetching user and group info
167 endgrent, endhostent, endnetent, endpwent, getgrent,
168 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169 getpwuid, setgrent, setpwent
171 =item Fetching network info
173 endprotoent, endservent, gethostbyaddr, gethostbyname,
174 gethostent, getnetbyaddr, getnetbyname, getnetent,
175 getprotobyname, getprotobynumber, getprotoent,
176 getservbyname, getservbyport, getservent, sethostent,
177 setnetent, setprotoent, setservent
179 =item Time-related functions
181 gmtime, localtime, time, times
183 =item Functions new in perl5
185 abs, bless, chomp, chr, exists, formline, glob, import, lc,
186 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
187 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
189 * - C<sub> was a keyword in perl4, but in perl5 it is an
190 operator which can be used in expressions.
192 =item Functions obsoleted in perl5
199 =head2 Alphabetical Listing of Perl Functions
210 A file test, where X is one of the letters listed below. This unary
211 operator takes one argument, either a filename or a filehandle, and
212 tests the associated file to see if something is true about it. If the
213 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215 the undefined value if the file doesn't exist. Despite the funny
216 names, precedence is the same as any other named unary operator, and
217 the argument may be parenthesized like any other unary operator. The
218 operator may be any of:
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
254 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256 uids and gids of the user. There may be other reasons you can't actually
257 read, write or execute the file. Also note that, for the superuser,
258 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
259 1 if any execute bit is set in the mode. Scripts run by the superuser may
260 thus need to do a stat() in order to determine the actual mode of the
261 file, or temporarily set the uid to something else.
267 next unless -f $_; # ignore specials
271 Note that C<-s/a/b/> does not do a negated substitution. Saying
272 C<-exp($foo)> still works as expected, however--only single letters
273 following a minus are interpreted as file tests.
275 The C<-T> and C<-B> switches work as follows. The first block or so of the
276 file is examined for odd characters such as strange control codes or
277 characters with the high bit set. If too many odd characters (E<gt>30%)
278 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279 containing null in the first block is considered a binary file. If C<-T>
280 or C<-B> is used on a filehandle, the current stdio buffer is examined
281 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
282 file, or a file at EOF when testing a filehandle. Because you have to
283 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284 against the file first, as in C<next unless -f $file && -T $file>.
286 If any of the file tests (or either the stat() or lstat() operators) are given the
287 special filehandle consisting of a solitary underline, then the stat
288 structure of the previous file test (or stat operator) is used, saving
289 a system call. (This doesn't work with C<-t>, and you need to remember
290 that lstat() and C<-l> will leave values in the stat structure for the
291 symbolic link, not the real file.) Example:
293 print "Can do.\n" if -r $a || -w _ || -x _;
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
309 =item accept NEWSOCKET,GENERICSOCKET
311 Accepts an incoming socket connect, just as the accept(2) system call
312 does. Returns the packed address if it succeeded, FALSE otherwise.
313 See example in L<perlipc/"Sockets: Client/Server Communication">.
317 Arranges to have a SIGALRM delivered to this process after the
318 specified number of seconds have elapsed. (On some machines,
319 unfortunately, the elapsed time may be up to one second less than you
320 specified because of how seconds are counted.) Only one timer may be
321 counting at once. Each call disables the previous timer, and an
322 argument of 0 may be supplied to cancel the previous timer without
323 starting a new one. The returned value is the amount of time remaining
324 on the previous timer.
326 For delays of finer granularity than one second, you may use Perl's
327 syscall() interface to access setitimer(2) if your system supports it,
328 or else see L</select()> below. It is not advised to intermix alarm()
333 Returns the arctangent of Y/X in the range -PI to PI.
335 =item bind SOCKET,NAME
337 Binds a network address to a socket, just as the bind system call
338 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
339 packed address of the appropriate type for the socket. See the examples in
340 L<perlipc/"Sockets: Client/Server Communication">.
342 =item binmode FILEHANDLE
344 Arranges for the file to be read or written in "binary" mode in operating
345 systems that distinguish between binary and text files. Files that are
346 not in binary mode have CR LF sequences translated to LF on input and LF
347 translated to CR LF on output. Binmode has no effect under Unix; in DOS
348 and similarly archaic systems, it may be imperative--otherwise your
349 DOS-damaged C library may mangle your file. The key distinction between
350 systems that need binmode and those that don't is their text file
351 formats. Systems like Unix and Plan9 that delimit lines with a single
352 character, and that encode that character in C as '\n', do not need
353 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
354 is taken as the name of the filehandle.
356 =item bless REF,CLASSNAME
360 This function tells the referenced object (passed as REF) that it is now
361 an object in the CLASSNAME package--or the current package if no CLASSNAME
362 is specified, which is often the case. It returns the reference for
363 convenience, since a bless() is often the last thing in a constructor.
364 Always use the two-argument version if the function doing the blessing
365 might be inherited by a derived class. See L<perlobj> for more about the
366 blessing (and blessings) of objects.
372 Returns the context of the current subroutine call. In a scalar context,
373 returns TRUE if there is a caller, that is, if we're in a subroutine or
374 eval() or require(), and FALSE otherwise. In a list context, returns
376 ($package, $filename, $line) = caller;
378 With EXPR, it returns some extra information that the debugger uses to
379 print a stack trace. The value of EXPR indicates how many call frames
380 to go back before the current one.
382 ($package, $filename, $line,
383 $subroutine, $hasargs, $wantargs) = caller($i);
385 Furthermore, when called from within the DB package, caller returns more
386 detailed information: it sets the list variable @DB::args to be the
387 arguments with which that subroutine was invoked.
391 Changes the working directory to EXPR, if possible. If EXPR is
392 omitted, changes to home directory. Returns TRUE upon success, FALSE
393 otherwise. See example under die().
397 Changes the permissions of a list of files. The first element of the
398 list must be the numerical mode, which should probably be an octal
399 number. Returns the number of files successfully changed.
401 $cnt = chmod 0755, 'foo', 'bar';
402 chmod 0755, @executables;
410 This is a slightly safer version of chop (see below). It removes any
411 line ending that corresponds to the current value of C<$/> (also known as
412 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
413 of characters removed. It's often used to remove the newline from the
414 end of an input record when you're worried that the final record may be
415 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
416 trailing newlines from the string. If VARIABLE is omitted, it chomps
420 chomp; # avoid \n on last field
425 You can actually chomp anything that's an lvalue, including an assignment:
428 chomp($answer = <STDIN>);
430 If you chomp a list, each element is chomped, and the total number of
431 characters removed is returned.
439 Chops off the last character of a string and returns the character
440 chopped. It's used primarily to remove the newline from the end of an
441 input record, but is much more efficient than C<s/\n//> because it neither
442 scans nor copies the string. If VARIABLE is omitted, chops $_.
446 chop; # avoid \n on last field
451 You can actually chop anything that's an lvalue, including an assignment:
454 chop($answer = <STDIN>);
456 If you chop a list, each element is chopped. Only the value of the
457 last chop is returned.
459 Note that chop returns the last character. To return all but the last
460 character, use C<substr($string, 0, -1)>.
464 Changes the owner (and group) of a list of files. The first two
465 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
466 Returns the number of files successfully changed.
468 $cnt = chown $uid, $gid, 'foo', 'bar';
469 chown $uid, $gid, @filenames;
471 Here's an example that looks up non-numeric uids in the passwd file:
474 chop($user = <STDIN>);
476 chop($pattern = <STDIN>);
478 ($login,$pass,$uid,$gid) = getpwnam($user)
479 or die "$user not in passwd file";
481 @ary = <${pattern}>; # expand filenames
482 chown $uid, $gid, @ary;
484 On most systems, you are not allowed to change the ownership of the
485 file unless you're the superuser, although you should be able to change
486 the group to any of your secondary groups. On insecure systems, these
487 restrictions may be relaxed, but this is not a portable assumption.
491 Returns the character represented by that NUMBER in the character set.
492 For example, C<chr(65)> is "A" in ASCII.
494 =item chroot FILENAME
496 This function works as the system call by the same name: it makes the
497 named directory the new root directory for all further pathnames that
498 begin with a "/" by your process and all of its children. (It doesn't
499 change your current working directory is unaffected.) For security
500 reasons, this call is restricted to the superuser. If FILENAME is
501 omitted, does chroot to $_.
503 =item close FILEHANDLE
505 Closes the file or pipe associated with the file handle, returning TRUE
506 only if stdio successfully flushes buffers and closes the system file
507 descriptor. You don't have to close FILEHANDLE if you are immediately
508 going to do another open() on it, since open() will close it for you. (See
509 open().) However, an explicit close on an input file resets the line
510 counter ($.), while the implicit close done by open() does not. Also,
511 closing a pipe will wait for the process executing on the pipe to
512 complete, in case you want to look at the output of the pipe
513 afterwards. Closing a pipe explicitly also puts the status value of
514 the command into C<$?>. Example:
516 open(OUTPUT, '|sort >foo'); # pipe to sort
517 ... # print stuff to output
518 close OUTPUT; # wait for sort to finish
519 open(INPUT, 'foo'); # get sort's results
521 FILEHANDLE may be an expression whose value gives the real filehandle name.
523 =item closedir DIRHANDLE
525 Closes a directory opened by opendir().
527 =item connect SOCKET,NAME
529 Attempts to connect to a remote socket, just as the connect system call
530 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
531 packed address of the appropriate type for the socket. See the examples in
532 L<perlipc/"Sockets: Client/Server Communication">.
536 Actually a flow control statement rather than a function. If there is a
537 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
538 C<foreach>), it is always executed just before the conditional is about to
539 be evaluated again, just like the third part of a C<for> loop in C. Thus
540 it can be used to increment a loop variable, even when the loop has been
541 continued via the C<next> statement (which is similar to the C C<continue>
546 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
549 =item crypt PLAINTEXT,SALT
551 Encrypts a string exactly like the crypt(3) function in the C library
552 (assuming that you actually have a version there that has not been
553 extirpated as a potential munition). This can prove useful for checking
554 the password file for lousy passwords, amongst other things. Only the
555 guys wearing white hats should do this.
557 Here's an example that makes sure that whoever runs this program knows
560 $pwd = (getpwuid($<))[1];
561 $salt = substr($pwd, 0, 2);
565 chop($word = <STDIN>);
569 if (crypt($word, $salt) ne $pwd) {
575 Of course, typing in your own password to whoever asks you
578 =item dbmclose ASSOC_ARRAY
580 [This function has been superseded by the untie() function.]
582 Breaks the binding between a DBM file and an associative array.
584 =item dbmopen ASSOC,DBNAME,MODE
586 [This function has been superseded by the tie() function.]
588 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
589 associative array. ASSOC is the name of the associative array. (Unlike
590 normal open, the first argument is I<NOT> a filehandle, even though it
591 looks like one). DBNAME is the name of the database (without the F<.dir>
592 or F<.pag> extension if any). If the database does not exist, it is
593 created with protection specified by MODE (as modified by the umask()).
594 If your system only supports the older DBM functions, you may perform only
595 one dbmopen() in your program. In older versions of Perl, if your system
596 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
597 falls back to sdbm(3).
599 If you don't have write access to the DBM file, you can only read
600 associative array variables, not set them. If you want to test whether
601 you can write, either use file tests or try setting a dummy array entry
602 inside an eval(), which will trap the error.
604 Note that functions such as keys() and values() may return huge array
605 values when used on large DBM files. You may prefer to use the each()
606 function to iterate over large DBM files. Example:
608 # print out history file offsets
609 dbmopen(%HIST,'/usr/lib/news/history',0666);
610 while (($key,$val) = each %HIST) {
611 print $key, ' = ', unpack('L',$val), "\n";
615 See also L<AnyDBM_File> for a more general description of the pros and
616 cons of the various dbm approaches, as well as L<DB_File> for a particularly
621 Returns a boolean value saying whether EXPR has a real value
622 or not. Many operations return the undefined value under exceptional
623 conditions, such as end of file, uninitialized variable, system error
624 and such. This function allows you to distinguish between an undefined
625 null scalar and a defined null scalar with operations that might return
626 a real null string, such as referencing elements of an array. You may
627 also check to see if arrays or subroutines exist. Use of defined on
628 predefined variables is not guaranteed to produce intuitive results.
630 When used on a hash array element, it tells you whether the value
631 is defined, not whether the key exists in the hash. Use exists() for that.
635 print if defined $switch{'D'};
636 print "$val\n" while defined($val = pop(@ary));
637 die "Can't readlink $sym: $!"
638 unless defined($value = readlink $sym);
639 eval '@foo = ()' if defined(@foo);
640 die "No XYZ package defined" unless defined %_XYZ;
641 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
645 Note: many folks tend to overuse defined(), and then are surprised to
646 discover that the number 0 and the null string are, in fact, defined
647 concepts. For example, if you say
651 the pattern match succeeds, and $1 is defined, despite the fact that it
652 matched "nothing". But it didn't really match nothing--rather, it
653 matched something that happened to be 0 characters long. This is all
654 very above-board and honest. When a function returns an undefined value,
655 it's an admission that it couldn't give you an honest answer. So
656 you should only use defined() when you're questioning the integrity
657 of what you're trying to do. At other times, a simple comparison to
658 0 or "" is what you want.
662 Deletes the specified value from its hash array. Returns the deleted
663 value, or the undefined value if nothing was deleted. Deleting from
664 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
665 file deletes the entry from the DBM file. (But deleting from a tie()d
666 hash doesn't necessarily return anything.)
668 The following deletes all the values of an associative array:
670 foreach $key (keys %ARRAY) {
674 (But it would be faster to use the undef() command.) Note that the
675 EXPR can be arbitrarily complicated as long as the final operation is
678 delete $ref->[$x][$y]{$key};
682 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
683 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
684 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
685 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
686 and the eval() is terminated with the undefined value; this makes die()
687 the way to raise an exception.
691 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
692 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
694 If the value of EXPR does not end in a newline, the current script line
695 number and input line number (if any) are also printed, and a newline
696 is supplied. Hint: sometimes appending ", stopped" to your message
697 will cause it to make better sense when the string "at foo line 123" is
698 appended. Suppose you are running script "canasta".
700 die "/etc/games is no good";
701 die "/etc/games is no good, stopped";
703 produce, respectively
705 /etc/games is no good at canasta line 123.
706 /etc/games is no good, stopped at canasta line 123.
708 See also exit() and warn().
712 Not really a function. Returns the value of the last command in the
713 sequence of commands indicated by BLOCK. When modified by a loop
714 modifier, executes the BLOCK once before testing the loop condition.
715 (On other statements the loop modifiers test the conditional first.)
717 =item do SUBROUTINE(LIST)
719 A deprecated form of subroutine call. See L<perlsub>.
723 Uses the value of EXPR as a filename and executes the contents of the
724 file as a Perl script. Its primary use is to include subroutines
725 from a Perl subroutine library.
733 except that it's more efficient, more concise, keeps track of the
734 current filename for error messages, and searches all the B<-I>
735 libraries if the file isn't in the current directory (see also the @INC
736 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
737 reparse the file every time you call it, so you probably don't want to
738 do this inside a loop.
740 Note that inclusion of library modules is better done with the
741 use() and require() operators, which also do error checking
742 and raise an exception if there's a problem.
746 This causes an immediate core dump. Primarily this is so that you can
747 use the B<undump> program to turn your core dump into an executable binary
748 after having initialized all your variables at the beginning of the
749 program. When the new binary is executed it will begin by executing a
750 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
751 it as a goto with an intervening core dump and reincarnation. If LABEL
752 is omitted, restarts the program from the top. WARNING: any files
753 opened at the time of the dump will NOT be open any more when the
754 program is reincarnated, with possible resulting confusion on the part
755 of Perl. See also B<-u> option in L<perlrun>.
772 dump QUICKSTART if $ARGV[0] eq '-d';
777 =item each ASSOC_ARRAY
779 When called in a list context, returns a 2-element array consisting
780 of the key and value for the next element of an associative array,
781 so that you can iterate over it. When called in a scalar context,
782 returns the key only for the next element in the associative array.
783 Entries are returned in an apparently random order. When the array is
784 entirely read, a null array is returned in list context (which when
785 assigned produces a FALSE (0) value), and C<undef> is returned in a
786 scalar context. The next call to each() after that will start
787 iterating again. The iterator can be reset only by reading all the
788 elements from the array. You should not add elements to an array while
789 you're iterating over it. There is a single iterator for each
790 associative array, shared by all each(), keys() and values() function
791 calls in the program. The following prints out your environment like
792 the printenv(1) program, only in a different order:
794 while (($key,$value) = each %ENV) {
795 print "$key=$value\n";
798 See also keys() and values().
806 Returns 1 if the next read on FILEHANDLE will return end of file, or if
807 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
808 gives the real filehandle name. (Note that this function actually
809 reads a character and then ungetc()s it, so it is not very useful in an
810 interactive context.) Do not read from a terminal file (or call
811 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
812 as terminals may lose the end-of-file condition if you do.
814 An C<eof> without an argument uses the last file read as argument.
815 Empty parentheses () may be used to indicate
816 the pseudofile formed of the files listed on the command line, i.e.
817 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
818 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
819 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
821 # reset line numbering on each input file
824 close(ARGV) if (eof); # Not eof().
827 # insert dashes just before last line of last file
830 print "--------------\n";
831 close(ARGV); # close or break; is needed if we
832 # are reading from the terminal
837 Practical hint: you almost never need to use C<eof> in Perl, because the
838 input operators return undef when they run out of data.
844 EXPR is parsed and executed as if it were a little Perl program. It
845 is executed in the context of the current Perl program, so that any
846 variable settings, subroutine or format definitions remain afterwards.
847 The value returned is the value of the last expression evaluated, or a
848 return statement may be used, just as with subroutines.
850 If there is a syntax error or runtime error, or a die() statement is
851 executed, an undefined value is returned by eval(), and C<$@> is set to the
852 error message. If there was no error, C<$@> is guaranteed to be a null
853 string. If EXPR is omitted, evaluates $_. The final semicolon, if
854 any, may be omitted from the expression.
856 Note that, since eval() traps otherwise-fatal errors, it is useful for
857 determining whether a particular feature (such as socket() or symlink())
858 is implemented. It is also Perl's exception trapping mechanism, where
859 the die operator is used to raise exceptions.
861 If the code to be executed doesn't vary, you may use the eval-BLOCK
862 form to trap run-time errors without incurring the penalty of
863 recompiling each time. The error, if any, is still returned in C<$@>.
866 # make divide-by-zero non-fatal
867 eval { $answer = $a / $b; }; warn $@ if $@;
869 # same thing, but less efficient
870 eval '$answer = $a / $b'; warn $@ if $@;
872 # a compile-time error
876 eval '$answer ='; # sets $@
878 With an eval(), you should be especially careful to remember what's
879 being looked at when:
885 eval { $x }; # CASE 4
887 eval "\$$x++" # CASE 5
890 Cases 1 and 2 above behave identically: they run the code contained in the
891 variable $x. (Although case 2 has misleading double quotes making the
892 reader wonder what else might be happening (nothing is).) Cases 3 and 4
893 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
894 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
895 is a place where normally you I<WOULD> like to use double quotes, except
896 that in that particular situation, you can just use symbolic references
897 instead, as in case 6.
901 The exec() function executes a system command I<AND NEVER RETURNS>. Use
902 the system() function if you want it to return.
904 If there is more than one argument in LIST, or if LIST is an array with
905 more than one value, calls execvp(3) with the arguments in LIST. If
906 there is only one scalar argument, the argument is checked for shell
907 metacharacters. If there are any, the entire argument is passed to
908 C</bin/sh -c> for parsing. If there are none, the argument is split
909 into words and passed directly to execvp(), which is more efficient.
910 Note: exec() and system() do not flush your output buffer, so you may
911 need to set C<$|> to avoid lost output. Examples:
913 exec '/bin/echo', 'Your arguments are: ', @ARGV;
914 exec "sort $outfile | uniq";
916 If you don't really want to execute the first argument, but want to lie
917 to the program you are executing about its own name, you can specify
918 the program you actually want to run as an "indirect object" (without a
919 comma) in front of the LIST. (This always forces interpretation of the
920 LIST as a multi-valued list, even if there is only a single scalar in
924 exec $shell '-sh'; # pretend it's a login shell
928 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
932 Returns TRUE if the specified hash key exists in its hash array, even
933 if the corresponding value is undefined.
935 print "Exists\n" if exists $array{$key};
936 print "Defined\n" if defined $array{$key};
937 print "True\n" if $array{$key};
939 A hash element can only be TRUE if it's defined, and defined if
940 it exists, but the reverse doesn't necessarily hold true.
942 Note that the EXPR can be arbitrarily complicated as long as the final
943 operation is a hash key lookup:
945 if (exists $ref->[$x][$y]{$key}) { ... }
949 Evaluates EXPR and exits immediately with that value. (Actually, it
950 calls any defined C<END> routines first, but the C<END> routines may not
951 abort the exit. Likewise any object destructors that need to be called
952 are called before exit.) Example:
955 exit 0 if $ans =~ /^[Xx]/;
957 See also die(). If EXPR is omitted, exits with 0 status.
961 Returns I<e> (the natural logarithm base) to the power of EXPR.
962 If EXPR is omitted, gives C<exp($_)>.
964 =item fcntl FILEHANDLE,FUNCTION,SCALAR
966 Implements the fcntl(2) function. You'll probably have to say
970 first to get the correct function definitions. Argument processing and
971 value return works just like ioctl() below. Note that fcntl() will produce
972 a fatal error if used on a machine that doesn't implement fcntl(2).
976 fcntl($filehandle, F_GETLK, $packed_return_buffer);
978 =item fileno FILEHANDLE
980 Returns the file descriptor for a filehandle. This is useful for
981 constructing bitmaps for select(). If FILEHANDLE is an expression, the
982 value is taken as the name of the filehandle.
984 =item flock FILEHANDLE,OPERATION
986 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
987 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
988 fatal error if used on a machine that doesn't implement either flock(2) or
989 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
990 is missing from your system. This makes flock() the portable file locking
991 strategy, although it will only lock entire files, not records. Note also
992 that some versions of flock() cannot lock things over the network; you
993 would need to use the more system-specific fcntl() for that.
995 Here's a mailbox appender for BSD systems.
1003 flock(MBOX,$LOCK_EX);
1004 # and, in case someone appended
1005 # while we were waiting...
1010 flock(MBOX,$LOCK_UN);
1013 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1014 or die "Can't open mailbox: $!";
1017 print MBOX $msg,"\n\n";
1020 See also L<DB_File> for other flock() examples.
1024 Does a fork(2) system call. Returns the child pid to the parent process
1025 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1026 Note: unflushed buffers remain unflushed in both processes, which means
1027 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1028 autoflush() FileHandle method to avoid duplicate output.
1030 If you fork() without ever waiting on your children, you will accumulate
1033 $SIG{CHLD} = sub { wait };
1035 There's also the double-fork trick (error checking on
1036 fork() returns omitted);
1038 unless ($pid = fork) {
1040 exec "what you really wanna do";
1043 ## (some_perl_code_here)
1050 See also L<perlipc> for more examples of forking and reaping
1055 Declare a picture format with use by the write() function. For
1059 Test: @<<<<<<<< @||||| @>>>>>
1060 $str, $%, '$' . int($num)
1064 $num = $cost/$quantity;
1068 See L<perlform> for many details and examples.
1071 =item formline PICTURE, LIST
1073 This is an internal function used by C<format>s, though you may call it
1074 too. It formats (see L<perlform>) a list of values according to the
1075 contents of PICTURE, placing the output into the format output
1076 accumulator, C<$^A> (or $ACCUMULATOR in English).
1077 Eventually, when a write() is done, the contents of
1078 C<$^A> are written to some filehandle, but you could also read C<$^A>
1079 yourself and then set C<$^A> back to "". Note that a format typically
1080 does one formline() per line of form, but the formline() function itself
1081 doesn't care how many newlines are embedded in the PICTURE. This means
1082 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1083 You may therefore need to use multiple formlines to implement a single
1084 record format, just like the format compiler.
1086 Be careful if you put double quotes around the picture, since an "C<@>"
1087 character may be taken to mean the beginning of an array name.
1088 formline() always returns TRUE. See L<perlform> for other examples.
1090 =item getc FILEHANDLE
1094 Returns the next character from the input file attached to FILEHANDLE,
1095 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1096 This is not particularly efficient. It cannot be used to get unbuffered
1097 single-characters, however. For that, try something more like:
1100 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1103 system "stty", '-icanon', 'eol', "\001";
1109 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1112 system "stty", 'icanon', 'eol', '^@'; # ascii null
1116 Determination of whether to whether $BSD_STYLE should be set
1117 is left as an exercise to the reader.
1119 See also the C<Term::ReadKey> module from your nearest CPAN site;
1120 details on CPAN can be found on L<perlmod/CPAN>
1124 Returns the current login from F</etc/utmp>, if any. If null, use
1127 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1129 Do not consider getlogin() for authentication: it is not as
1130 secure as getpwuid().
1132 =item getpeername SOCKET
1134 Returns the packed sockaddr address of other end of the SOCKET connection.
1137 $hersockaddr = getpeername(SOCK);
1138 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1139 $herhostname = gethostbyaddr($iaddr, AF_INET);
1140 $herstraddr = inet_ntoa($iaddr);
1144 Returns the current process group for the specified PID. Use
1145 a PID of 0 to get the current process group for the
1146 current process. Will raise an exception if used on a machine that
1147 doesn't implement getpgrp(2). If PID is omitted, returns process
1148 group of current process. Note that the POSIX version of getpgrp()
1149 does not accept a PID argument, so only PID==0 is truly portable.
1153 Returns the process id of the parent process.
1155 =item getpriority WHICH,WHO
1157 Returns the current priority for a process, a process group, or a user.
1158 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1159 machine that doesn't implement getpriority(2).
1165 =item gethostbyname NAME
1167 =item getnetbyname NAME
1169 =item getprotobyname NAME
1175 =item getservbyname NAME,PROTO
1177 =item gethostbyaddr ADDR,ADDRTYPE
1179 =item getnetbyaddr ADDR,ADDRTYPE
1181 =item getprotobynumber NUMBER
1183 =item getservbyport PORT,PROTO
1201 =item sethostent STAYOPEN
1203 =item setnetent STAYOPEN
1205 =item setprotoent STAYOPEN
1207 =item setservent STAYOPEN
1221 These routines perform the same functions as their counterparts in the
1222 system library. Within a list context, the return values from the
1223 various get routines are as follows:
1225 ($name,$passwd,$uid,$gid,
1226 $quota,$comment,$gcos,$dir,$shell) = getpw*
1227 ($name,$passwd,$gid,$members) = getgr*
1228 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1229 ($name,$aliases,$addrtype,$net) = getnet*
1230 ($name,$aliases,$proto) = getproto*
1231 ($name,$aliases,$port,$proto) = getserv*
1233 (If the entry doesn't exist you get a null list.)
1235 Within a scalar context, you get the name, unless the function was a
1236 lookup by name, in which case you get the other thing, whatever it is.
1237 (If the entry doesn't exist you get the undefined value.) For example:
1247 The $members value returned by I<getgr*()> is a space separated list of
1248 the login names of the members of the group.
1250 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1251 C, it will be returned to you via C<$?> if the function call fails. The
1252 @addrs value returned by a successful call is a list of the raw
1253 addresses returned by the corresponding system library call. In the
1254 Internet domain, each address is four bytes long and you can unpack it
1255 by saying something like:
1257 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1259 =item getsockname SOCKET
1261 Returns the packed sockaddr address of this end of the SOCKET connection.
1264 $mysockaddr = getsockname(SOCK);
1265 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1267 =item getsockopt SOCKET,LEVEL,OPTNAME
1269 Returns the socket option requested, or undefined if there is an error.
1273 Returns the value of EXPR with filename expansions such as a shell
1274 would do. This is the internal function implementing the E<lt>*.*E<gt>
1275 operator, except it's easier to use.
1279 Converts a time as returned by the time function to a 9-element array
1280 with the time localized for the standard Greenwich timezone.
1281 Typically used as follows:
1284 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1287 All array elements are numeric, and come straight out of a struct tm.
1288 In particular this means that $mon has the range 0..11 and $wday has
1289 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1297 The goto-LABEL form finds the statement labeled with LABEL and resumes
1298 execution there. It may not be used to go into any construct that
1299 requires initialization, such as a subroutine or a foreach loop. It
1300 also can't be used to go into a construct that is optimized away. It
1301 can be used to go almost anywhere else within the dynamic scope,
1302 including out of subroutines, but it's usually better to use some other
1303 construct such as last or die. The author of Perl has never felt the
1304 need to use this form of goto (in Perl, that is--C is another matter).
1306 The goto-EXPR form expects a label name, whose scope will be resolved
1307 dynamically. This allows for computed gotos per FORTRAN, but isn't
1308 necessarily recommended if you're optimizing for maintainability:
1310 goto ("FOO", "BAR", "GLARCH")[$i];
1312 The goto-&NAME form is highly magical, and substitutes a call to the
1313 named subroutine for the currently running subroutine. This is used by
1314 AUTOLOAD subroutines that wish to load another subroutine and then
1315 pretend that the other subroutine had been called in the first place
1316 (except that any modifications to @_ in the current subroutine are
1317 propagated to the other subroutine.) After the goto, not even caller()
1318 will be able to tell that this routine was called first.
1320 =item grep BLOCK LIST
1322 =item grep EXPR,LIST
1324 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1325 $_ to each element) and returns the list value consisting of those
1326 elements for which the expression evaluated to TRUE. In a scalar
1327 context, returns the number of times the expression was TRUE.
1329 @foo = grep(!/^#/, @bar); # weed out comments
1333 @foo = grep {!/^#/} @bar; # weed out comments
1335 Note that, since $_ is a reference into the list value, it can be used
1336 to modify the elements of the array. While this is useful and
1337 supported, it can cause bizarre results if the LIST is not a named
1342 Interprets EXPR as a hex string and returns the corresponding decimal
1343 value. (To convert strings that might start with 0 or 0x see
1344 oct().) If EXPR is omitted, uses $_.
1348 There is no built-in import() function. It is merely an ordinary
1349 method (subroutine) defined (or inherited) by modules that wish to export
1350 names to another module. The use() function calls the import() method
1351 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1353 =item index STR,SUBSTR,POSITION
1355 =item index STR,SUBSTR
1357 Returns the position of the first occurrence of SUBSTR in STR at or after
1358 POSITION. If POSITION is omitted, starts searching from the beginning of
1359 the string. The return value is based at 0 (or whatever you've set the C<$[>
1360 variable to--but don't do that). If the substring is not found, returns
1361 one less than the base, ordinarily -1.
1365 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1367 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1369 Implements the ioctl(2) function. You'll probably have to say
1371 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1373 first to get the correct function definitions. If F<ioctl.ph> doesn't
1374 exist or doesn't have the correct definitions you'll have to roll your
1375 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1376 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1377 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1378 written depending on the FUNCTION--a pointer to the string value of SCALAR
1379 will be passed as the third argument of the actual ioctl call. (If SCALAR
1380 has no string value but does have a numeric value, that value will be
1381 passed rather than a pointer to the string value. To guarantee this to be
1382 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1383 functions are useful for manipulating the values of structures used by
1384 ioctl(). The following example sets the erase character to DEL.
1388 die "NO TIOCGETP" if $@ || !$getp;
1389 $sgttyb_t = "ccccs"; # 4 chars and a short
1390 if (ioctl(STDIN,$getp,$sgttyb)) {
1391 @ary = unpack($sgttyb_t,$sgttyb);
1393 $sgttyb = pack($sgttyb_t,@ary);
1394 ioctl(STDIN,&TIOCSETP,$sgttyb)
1395 || die "Can't ioctl: $!";
1398 The return value of ioctl (and fcntl) is as follows:
1400 if OS returns: then Perl returns:
1402 0 string "0 but true"
1403 anything else that number
1405 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1406 still easily determine the actual value returned by the operating
1409 ($retval = ioctl(...)) || ($retval = -1);
1410 printf "System returned %d\n", $retval;
1412 =item join EXPR,LIST
1414 Joins the separate strings of LIST or ARRAY into a single string with
1415 fields separated by the value of EXPR, and returns the string.
1418 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1420 See L<perlfunc/split>.
1422 =item keys ASSOC_ARRAY
1424 Returns a normal array consisting of all the keys of the named
1425 associative array. (In a scalar context, returns the number of keys.)
1426 The keys are returned in an apparently random order, but it is the same
1427 order as either the values() or each() function produces (given that
1428 the associative array has not been modified). Here is yet another way
1429 to print your environment:
1432 @values = values %ENV;
1433 while ($#keys >= 0) {
1434 print pop(@keys), '=', pop(@values), "\n";
1437 or how about sorted by key:
1439 foreach $key (sort(keys %ENV)) {
1440 print $key, '=', $ENV{$key}, "\n";
1443 To sort an array by value, you'll need to use a C<sort{}>
1444 function. Here's a descending numeric sort of a hash by its values:
1446 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1447 printf "%4d %s\n", $hash{$key}, $key;
1452 Sends a signal to a list of processes. The first element of
1453 the list must be the signal to send. Returns the number of
1454 processes successfully signaled.
1456 $cnt = kill 1, $child1, $child2;
1459 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1460 process groups instead of processes. (On System V, a negative I<PROCESS>
1461 number will also kill process groups, but that's not portable.) That
1462 means you usually want to use positive not negative signals. You may also
1463 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1469 The C<last> command is like the C<break> statement in C (as used in
1470 loops); it immediately exits the loop in question. If the LABEL is
1471 omitted, the command refers to the innermost enclosing loop. The
1472 C<continue> block, if any, is not executed:
1474 LINE: while (<STDIN>) {
1475 last LINE if /^$/; # exit when done with header
1481 Returns an lowercased version of EXPR. This is the internal function
1482 implementing the \L escape in double-quoted strings.
1483 Should respect any POSIX setlocale() settings.
1487 Returns the value of EXPR with the first character lowercased. This is
1488 the internal function implementing the \l escape in double-quoted strings.
1489 Should respect any POSIX setlocale() settings.
1493 Returns the length in characters of the value of EXPR. If EXPR is
1494 omitted, returns length of $_.
1496 =item link OLDFILE,NEWFILE
1498 Creates a new filename linked to the old filename. Returns 1 for
1499 success, 0 otherwise.
1501 =item listen SOCKET,QUEUESIZE
1503 Does the same thing that the listen system call does. Returns TRUE if
1504 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1508 A local modifies the listed variables to be local to the enclosing block,
1509 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1510 list must be placed in parens. See L<perlsub/"Temporary Values via
1511 local()"> for details.
1513 But you really probably want to be using my() instead, because local() isn't
1514 what most people think of as "local"). See L<perlsub/"Private Variables
1515 via my()"> for details.
1517 =item localtime EXPR
1519 Converts a time as returned by the time function to a 9-element array
1520 with the time analyzed for the local timezone. Typically used as
1523 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1526 All array elements are numeric, and come straight out of a struct tm.
1527 In particular this means that $mon has the range 0..11 and $wday has
1528 the range 0..6. If EXPR is omitted, does localtime(time).
1530 In a scalar context, prints out the ctime(3) value:
1532 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1534 Also see the F<timelocal.pl> library, and the strftime(3) function available
1535 via the POSIX module.
1539 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1542 =item lstat FILEHANDLE
1546 Does the same thing as the stat() function, but stats a symbolic link
1547 instead of the file the symbolic link points to. If symbolic links are
1548 unimplemented on your system, a normal stat() is done.
1552 The match operator. See L<perlop>.
1554 =item map BLOCK LIST
1558 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1559 element) and returns the list value composed of the results of each such
1560 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1561 may produce zero, one, or more elements in the returned value.
1563 @chars = map(chr, @nums);
1565 translates a list of numbers to the corresponding characters. And
1567 %hash = map { getkey($_) => $_ } @array;
1569 is just a funny way to write
1572 foreach $_ (@array) {
1573 $hash{getkey($_)} = $_;
1576 =item mkdir FILENAME,MODE
1578 Creates the directory specified by FILENAME, with permissions specified
1579 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1580 it returns 0 and sets C<$!> (errno).
1582 =item msgctl ID,CMD,ARG
1584 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1585 must be a variable which will hold the returned msqid_ds structure.
1586 Returns like ioctl: the undefined value for error, "0 but true" for
1587 zero, or the actual return value otherwise.
1589 =item msgget KEY,FLAGS
1591 Calls the System V IPC function msgget(2). Returns the message queue id,
1592 or the undefined value if there is an error.
1594 =item msgsnd ID,MSG,FLAGS
1596 Calls the System V IPC function msgsnd to send the message MSG to the
1597 message queue ID. MSG must begin with the long integer message type,
1598 which may be created with C<pack("l", $type)>. Returns TRUE if
1599 successful, or FALSE if there is an error.
1601 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1603 Calls the System V IPC function msgrcv to receive a message from
1604 message queue ID into variable VAR with a maximum message size of
1605 SIZE. Note that if a message is received, the message type will be the
1606 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1607 of the message type. Returns TRUE if successful, or FALSE if there is
1612 A "my" declares the listed variables to be local (lexically) to the
1613 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1614 more than one value is listed, the list must be placed in parens. See
1615 L<perlsub/"Private Variables via my()"> for details.
1621 The C<next> command is like the C<continue> statement in C; it starts
1622 the next iteration of the loop:
1624 LINE: while (<STDIN>) {
1625 next LINE if /^#/; # discard comments
1629 Note that if there were a C<continue> block on the above, it would get
1630 executed even on discarded lines. If the LABEL is omitted, the command
1631 refers to the innermost enclosing loop.
1633 =item no Module LIST
1635 See the "use" function, which "no" is the opposite of.
1639 Interprets EXPR as an octal string and returns the corresponding
1640 decimal value. (If EXPR happens to start off with 0x, interprets it as
1641 a hex string instead.) The following will handle decimal, octal, and
1642 hex in the standard Perl or C notation:
1644 $val = oct($val) if $val =~ /^0/;
1646 If EXPR is omitted, uses $_.
1648 =item open FILEHANDLE,EXPR
1650 =item open FILEHANDLE
1652 Opens the file whose filename is given by EXPR, and associates it with
1653 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1654 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1655 the same name as the FILEHANDLE contains the filename. If the filename
1656 begins with "E<lt>" or nothing, the file is opened for input. If the filename
1657 begins with "E<gt>", the file is opened for output. If the filename begins
1658 with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1659 front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1660 access to the file; thus '+E<lt>' is usually preferred for read/write
1661 updates--the '+E<gt>' mode would clobber the file first. These correspond to
1662 the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1664 If the filename begins with "|", the filename is interpreted
1665 as a command to which output is to be piped, and if the filename ends with
1666 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1667 for more examples of this. as command which pipes input to us. (You may
1668 not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
1669 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1671 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1672 non-zero upon success, the undefined value otherwise. If the open
1673 involved a pipe, the return value happens to be the pid of the
1676 If you're unfortunate enough to be running Perl on a system that
1677 distinguishes between text files and binary files (modern operating
1678 systems don't care), then you should check out L</binmode> for tips for
1679 dealing with this. The key distinction between systems that need binmode
1680 and those that don't is their text file formats. Systems like Unix and
1681 Plan9 that delimit lines with a single character, and that encode that
1682 character in C as '\n', do not need C<binmode>. The rest need it.
1687 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1688 while (<ARTICLE>) {...
1690 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1692 open(DBASE, '+<dbase.mine'); # open for update
1694 open(ARTICLE, "caesar <$article |"); # decrypt article
1696 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1698 # process argument list of files along with any includes
1700 foreach $file (@ARGV) {
1701 process($file, 'fh00');
1705 local($filename, $input) = @_;
1706 $input++; # this is a string increment
1707 unless (open($input, $filename)) {
1708 print STDERR "Can't open $filename: $!\n";
1712 while (<$input>) { # note use of indirection
1713 if (/^#include "(.*)"/) {
1714 process($1, $input);
1721 You may also, in the Bourne shell tradition, specify an EXPR beginning
1722 with "E<gt>&", in which case the rest of the string is interpreted as the
1723 name of a filehandle (or file descriptor, if numeric) which is to be
1724 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1725 +E<gt>E<gt> and +E<lt>. The
1726 mode you specify should match the mode of the original filehandle.
1727 (Duping a filehandle does not take into account any existing contents of
1729 Here is a script that saves, redirects, and restores STDOUT and
1733 open(SAVEOUT, ">&STDOUT");
1734 open(SAVEERR, ">&STDERR");
1736 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1737 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1739 select(STDERR); $| = 1; # make unbuffered
1740 select(STDOUT); $| = 1; # make unbuffered
1742 print STDOUT "stdout 1\n"; # this works for
1743 print STDERR "stderr 1\n"; # subprocesses too
1748 open(STDOUT, ">&SAVEOUT");
1749 open(STDERR, ">&SAVEERR");
1751 print STDOUT "stdout 2\n";
1752 print STDERR "stderr 2\n";
1755 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1756 equivalent of C's fdopen() of that file descriptor; this is more
1757 parsimonious of file descriptors. For example:
1759 open(FILEHANDLE, "<&=$fd")
1761 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1762 there is an implicit fork done, and the return value of open is the pid
1763 of the child within the parent process, and 0 within the child
1764 process. (Use C<defined($pid)> to determine whether the open was successful.)
1765 The filehandle behaves normally for the parent, but i/o to that
1766 filehandle is piped from/to the STDOUT/STDIN of the child process.
1767 In the child process the filehandle isn't opened--i/o happens from/to
1768 the new STDOUT or STDIN. Typically this is used like the normal
1769 piped open when you want to exercise more control over just how the
1770 pipe command gets executed, such as when you are running setuid, and
1771 don't want to have to scan shell commands for metacharacters.
1772 The following pairs are more or less equivalent:
1774 open(FOO, "|tr '[a-z]' '[A-Z]'");
1775 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1777 open(FOO, "cat -n '$file'|");
1778 open(FOO, "-|") || exec 'cat', '-n', $file;
1780 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1782 Explicitly closing any piped filehandle causes the parent process to
1783 wait for the child to finish, and returns the status value in C<$?>.
1784 Note: on any operation which may do a fork, unflushed buffers remain
1785 unflushed in both processes, which means you may need to set C<$|> to
1786 avoid duplicate output.
1788 Using the FileHandle constructor from the FileHandle package,
1789 you can generate anonymous filehandles which have the scope of whatever
1790 variables hold references to them, and automatically close whenever
1791 and however you leave that scope:
1795 sub read_myfile_munged {
1797 my $handle = new FileHandle;
1798 open($handle, "myfile") or die "myfile: $!";
1800 or return (); # Automatically closed here.
1801 mung $first or die "mung failed"; # Or here.
1802 return $first, <$handle> if $ALL; # Or here.
1806 The filename that is passed to open will have leading and trailing
1807 whitespace deleted. In order to open a file with arbitrary weird
1808 characters in it, it's necessary to protect any leading and trailing
1811 $file =~ s#^(\s)#./$1#;
1812 open(FOO, "< $file\0");
1814 If you want a "real" C open() (see L<open(2)> on your system), then
1815 you should use the sysopen() function. This is another way to
1816 protect your filenames from interpretation. For example:
1819 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1820 or die "sysopen $path: $!";
1821 HANDLE->autoflush(1);
1822 HANDLE->print("stuff $$\n");
1824 print "File contains: ", <HANDLE>;
1826 See L</seek()> for some details about mixing reading and writing.
1828 =item opendir DIRHANDLE,EXPR
1830 Opens a directory named EXPR for processing by readdir(), telldir(),
1831 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1832 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1836 Returns the numeric ascii value of the first character of EXPR. If
1837 EXPR is omitted, uses $_.
1839 =item pack TEMPLATE,LIST
1841 Takes an array or list of values and packs it into a binary structure,
1842 returning the string containing the structure. The TEMPLATE is a
1843 sequence of characters that give the order and type of values, as
1846 A An ascii string, will be space padded.
1847 a An ascii string, will be null padded.
1848 b A bit string (ascending bit order, like vec()).
1849 B A bit string (descending bit order).
1850 h A hex string (low nybble first).
1851 H A hex string (high nybble first).
1853 c A signed char value.
1854 C An unsigned char value.
1855 s A signed short value.
1856 S An unsigned short value.
1857 i A signed integer value.
1858 I An unsigned integer value.
1859 l A signed long value.
1860 L An unsigned long value.
1862 n A short in "network" order.
1863 N A long in "network" order.
1864 v A short in "VAX" (little-endian) order.
1865 V A long in "VAX" (little-endian) order.
1867 f A single-precision float in the native format.
1868 d A double-precision float in the native format.
1870 p A pointer to a null-terminated string.
1871 P A pointer to a structure (fixed-length string).
1873 u A uuencoded string.
1875 w A BER compressed integer. Bytes give an unsigned integer base
1876 128, most significant digit first, with as few digits as
1877 possible, and with the bit 8 of each byte except the last set
1882 @ Null fill to absolute position.
1884 Each letter may optionally be followed by a number which gives a repeat
1885 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1886 pack function will gobble up that many values from the LIST. A * for the
1887 repeat count means to use however many items are left. The "a" and "A"
1888 types gobble just one value, but pack it as a string of length count,
1889 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1890 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1891 fields pack a string that many bits long. The "h" and "H" fields pack a
1892 string that many nybbles long. The "P" packs a pointer to a structure of
1893 the size indicated by the length. Real numbers (floats and doubles) are
1894 in the native machine format only; due to the multiplicity of floating
1895 formats around, and the lack of a standard "network" representation, no
1896 facility for interchange has been made. This means that packed floating
1897 point data written on one machine may not be readable on another - even if
1898 both use IEEE floating point arithmetic (as the endian-ness of the memory
1899 representation is not part of the IEEE spec). Note that Perl uses doubles
1900 internally for all numeric calculation, and converting from double into
1901 float and thence back to double again will lose precision (i.e.
1902 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1906 $foo = pack("cccc",65,66,67,68);
1908 $foo = pack("c4",65,66,67,68);
1911 $foo = pack("ccxxcc",65,66,67,68);
1914 $foo = pack("s2",1,2);
1915 # "\1\0\2\0" on little-endian
1916 # "\0\1\0\2" on big-endian
1918 $foo = pack("a4","abcd","x","y","z");
1921 $foo = pack("aaaa","abcd","x","y","z");
1924 $foo = pack("a14","abcdefg");
1925 # "abcdefg\0\0\0\0\0\0\0"
1927 $foo = pack("i9pl", gmtime);
1928 # a real struct tm (on my system anyway)
1931 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1934 The same template may generally also be used in the unpack function.
1936 =item package NAMESPACE
1938 Declares the compilation unit as being in the given namespace. The scope
1939 of the package declaration is from the declaration itself through the end of
1940 the enclosing block (the same scope as the local() operator). All further
1941 unqualified dynamic identifiers will be in this namespace. A package
1942 statement only affects dynamic variables--including those you've used
1943 local() on--but I<not> lexical variables created with my(). Typically it
1944 would be the first declaration in a file to be included by the C<require>
1945 or C<use> operator. You can switch into a package in more than one place;
1946 it merely influences which symbol table is used by the compiler for the
1947 rest of that block. You can refer to variables and filehandles in other
1948 packages by prefixing the identifier with the package name and a double
1949 colon: C<$Package::Variable>. If the package name is null, the C<main>
1950 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1952 See L<perlmod/"Packages"> for more information about packages, modules,
1953 and classes. See L<perlsub> for other scoping issues.
1955 =item pipe READHANDLE,WRITEHANDLE
1957 Opens a pair of connected pipes like the corresponding system call.
1958 Note that if you set up a loop of piped processes, deadlock can occur
1959 unless you are very careful. In addition, note that Perl's pipes use
1960 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
1961 after each command, depending on the application.
1963 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1964 for examples of such things.
1968 Pops and returns the last value of the array, shortening the array by
1969 1. Has a similar effect to
1971 $tmp = $ARRAY[$#ARRAY--];
1973 If there are no elements in the array, returns the undefined value.
1974 If ARRAY is omitted, pops the
1975 @ARGV array in the main program, and the @_ array in subroutines, just
1980 Returns the offset of where the last C<m//g> search left off for the variable
1981 in question. May be modified to change that offset.
1983 =item print FILEHANDLE LIST
1989 Prints a string or a comma-separated list of strings. Returns TRUE
1990 if successful. FILEHANDLE may be a scalar variable name, in which case
1991 the variable contains the name of or a reference to the filehandle, thus introducing one
1992 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1993 token is a term, it may be misinterpreted as an operator unless you
1994 interpose a + or put parens around the arguments.) If FILEHANDLE is
1995 omitted, prints by default to standard output (or to the last selected
1996 output channel--see L</select>). If LIST is also omitted, prints $_ to
1997 STDOUT. To set the default output channel to something other than
1998 STDOUT use the select operation. Note that, because print takes a
1999 LIST, anything in the LIST is evaluated in a list context, and any
2000 subroutine that you call will have one or more of its expressions
2001 evaluated in a list context. Also be careful not to follow the print
2002 keyword with a left parenthesis unless you want the corresponding right
2003 parenthesis to terminate the arguments to the print--interpose a + or
2004 put parens around all the arguments.
2006 Note that if you're storing FILEHANDLES in an array or other expression,
2007 you will have to use a block returning its value instead:
2009 print { $files[$i] } "stuff\n";
2010 print { $OK ? STDOUT : STDERR } "stuff\n";
2012 =item printf FILEHANDLE LIST
2016 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2017 of the list will be interpreted as the printf format.
2019 =item prototype FUNCTION
2021 Returns the prototype of a function as a string (or C<undef> if the
2022 function has no prototype). FUNCTION is a reference to the the
2023 function whose prototype you want to retrieve.
2025 =item push ARRAY,LIST
2027 Treats ARRAY as a stack, and pushes the values of LIST
2028 onto the end of ARRAY. The length of ARRAY increases by the length of
2029 LIST. Has the same effect as
2032 $ARRAY[++$#ARRAY] = $value;
2035 but is more efficient. Returns the new number of elements in the array.
2045 Generalized quotes. See L<perlop>.
2047 =item quotemeta EXPR
2049 Returns the value of EXPR with with all regular expression
2050 metacharacters backslashed. This is the internal function implementing
2051 the \Q escape in double-quoted strings.
2057 Returns a random fractional number between 0 and the value of EXPR.
2058 (EXPR should be positive.) If EXPR is omitted, returns a value between
2059 0 and 1. This function produces repeatable sequences unless srand()
2060 is invoked. See also srand().
2062 (Note: if your rand function consistently returns numbers that are too
2063 large or too small, then your version of Perl was probably compiled
2064 with the wrong number of RANDBITS. As a workaround, you can usually
2065 multiply EXPR by the correct power of 2 to get the range you want.
2066 This will make your script unportable, however. It's better to recompile
2069 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2071 =item read FILEHANDLE,SCALAR,LENGTH
2073 Attempts to read LENGTH bytes of data into variable SCALAR from the
2074 specified FILEHANDLE. Returns the number of bytes actually read, or
2075 undef if there was an error. SCALAR will be grown or shrunk to the
2076 length actually read. An OFFSET may be specified to place the read
2077 data at some other place than the beginning of the string. This call
2078 is actually implemented in terms of stdio's fread call. To get a true
2079 read system call, see sysread().
2081 =item readdir DIRHANDLE
2083 Returns the next directory entry for a directory opened by opendir().
2084 If used in a list context, returns all the rest of the entries in the
2085 directory. If there are no more entries, returns an undefined value in
2086 a scalar context or a null list in a list context.
2088 If you're planning to filetest the return values out of a readdir(), you'd
2089 better prepend the directory in question. Otherwise, since we didn't
2090 chdir() there, it would have been testing the wrong file.
2092 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2093 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2098 Returns the value of a symbolic link, if symbolic links are
2099 implemented. If not, gives a fatal error. If there is some system
2100 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2103 =item recv SOCKET,SCALAR,LEN,FLAGS
2105 Receives a message on a socket. Attempts to receive LENGTH bytes of
2106 data into variable SCALAR from the specified SOCKET filehandle.
2107 Actually does a C recvfrom(), so that it can returns the address of the
2108 sender. Returns the undefined value if there's an error. SCALAR will
2109 be grown or shrunk to the length actually read. Takes the same flags
2110 as the system call of the same name.
2111 See L<perlipc/"UDP: Message Passing"> for examples.
2117 The C<redo> command restarts the loop block without evaluating the
2118 conditional again. The C<continue> block, if any, is not executed. If
2119 the LABEL is omitted, the command refers to the innermost enclosing
2120 loop. This command is normally used by programs that want to lie to
2121 themselves about what was just input:
2123 # a simpleminded Pascal comment stripper
2124 # (warning: assumes no { or } in strings)
2125 LINE: while (<STDIN>) {
2126 while (s|({.*}.*){.*}|$1 |) {}
2131 if (/}/) { # end of comment?
2142 Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2143 returned depends on the type of thing the reference is a reference to.
2144 Builtin types include:
2153 If the referenced object has been blessed into a package, then that package
2154 name is returned instead. You can think of ref() as a typeof() operator.
2156 if (ref($r) eq "HASH") {
2157 print "r is a reference to an associative array.\n";
2160 print "r is not a reference at all.\n";
2163 See also L<perlref>.
2165 =item rename OLDNAME,NEWNAME
2167 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2168 not work across filesystem boundaries.
2174 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2175 supplied. If EXPR is numeric, demands that the current version of Perl
2176 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2178 Otherwise, demands that a library file be included if it hasn't already
2179 been included. The file is included via the do-FILE mechanism, which is
2180 essentially just a variety of eval(). Has semantics similar to the following
2184 local($filename) = @_;
2185 return 1 if $INC{$filename};
2186 local($realfilename,$result);
2188 foreach $prefix (@INC) {
2189 $realfilename = "$prefix/$filename";
2190 if (-f $realfilename) {
2191 $result = do $realfilename;
2195 die "Can't find $filename in \@INC";
2198 die "$filename did not return true value" unless $result;
2199 $INC{$filename} = $realfilename;
2203 Note that the file will not be included twice under the same specified
2204 name. The file must return TRUE as the last statement to indicate
2205 successful execution of any initialization code, so it's customary to
2206 end such a file with "1;" unless you're sure it'll return TRUE
2207 otherwise. But it's better just to put the "C<1;>", in case you add more
2210 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2211 replaces "F<::>" with "F</>" in the filename for you,
2212 to make it easy to load standard modules. This form of loading of
2213 modules does not risk altering your namespace.
2215 For a yet-more-powerful import facility, see L</use> and
2222 Generally used in a C<continue> block at the end of a loop to clear
2223 variables and reset ?? searches so that they work again. The
2224 expression is interpreted as a list of single characters (hyphens
2225 allowed for ranges). All variables and arrays beginning with one of
2226 those letters are reset to their pristine state. If the expression is
2227 omitted, one-match searches (?pattern?) are reset to match again. Only
2228 resets variables or searches in the current package. Always returns
2231 reset 'X'; # reset all X variables
2232 reset 'a-z'; # reset lower case variables
2233 reset; # just reset ?? searches
2235 Resetting "A-Z" is not recommended since you'll wipe out your
2236 ARGV and ENV arrays. Only resets package variables--lexical variables
2237 are unaffected, but they clean themselves up on scope exit anyway,
2238 so you'll probably want to use them instead. See L</my>.
2242 Returns from a subroutine or eval with the value specified. (Note that
2243 in the absence of a return a subroutine or eval() will automatically
2244 return the value of the last expression evaluated.)
2248 In a list context, returns a list value consisting of the elements
2249 of LIST in the opposite order. In a scalar context, returns a string
2250 value consisting of the bytes of the first element of LIST in the
2253 print reverse <>; # line tac
2256 print scalar reverse scalar <>; # byte tac
2258 =item rewinddir DIRHANDLE
2260 Sets the current position to the beginning of the directory for the
2261 readdir() routine on DIRHANDLE.
2263 =item rindex STR,SUBSTR,POSITION
2265 =item rindex STR,SUBSTR
2267 Works just like index except that it returns the position of the LAST
2268 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2269 last occurrence at or before that position.
2271 =item rmdir FILENAME
2273 Deletes the directory specified by FILENAME if it is empty. If it
2274 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2275 FILENAME is omitted, uses $_.
2279 The substitution operator. See L<perlop>.
2283 Forces EXPR to be interpreted in a scalar context and returns the value
2286 @counts = ( scalar @a, scalar @b, scalar @c );
2288 There is no equivalent operator to force an expression to
2289 be interpolated in a list context because it's in practice never
2290 needed. If you really wanted to do so, however, you could use
2291 the construction C<@{[ (some expression) ]}>, but usually a simple
2292 C<(some expression)> suffices.
2294 =item seek FILEHANDLE,POSITION,WHENCE
2296 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2297 call of stdio. FILEHANDLE may be an expression whose value gives the name
2298 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2299 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2300 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2301 this from POSIX module. Returns 1 upon success, 0 otherwise.
2303 On some systems you have to do a seek whenever you switch between reading
2304 and writing. Amongst other things, this may have the effect of calling
2305 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2310 This is also useful for applications emulating C<tail -f>. Once you hit
2311 EOF on your read, and then sleep for a while, you might have to stick in a
2312 seek() to reset things. First the simple trick listed above to clear the
2313 filepointer. The seek() doesn't change the current position, but it
2314 I<does> clear the end-of-file condition on the handle, so that the next
2315 C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
2317 If that doesn't work (some stdios are particularly cantankerous), then
2318 you may need something more like this:
2321 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2322 # search for some stuff and put it into files
2324 sleep($for_a_while);
2325 seek(FILE, $curpos, 0);
2328 =item seekdir DIRHANDLE,POS
2330 Sets the current position for the readdir() routine on DIRHANDLE. POS
2331 must be a value returned by telldir(). Has the same caveats about
2332 possible directory compaction as the corresponding system library
2335 =item select FILEHANDLE
2339 Returns the currently selected filehandle. Sets the current default
2340 filehandle for output, if FILEHANDLE is supplied. This has two
2341 effects: first, a C<write> or a C<print> without a filehandle will
2342 default to this FILEHANDLE. Second, references to variables related to
2343 output will refer to this output channel. For example, if you have to
2344 set the top of form format for more than one output channel, you might
2352 FILEHANDLE may be an expression whose value gives the name of the
2353 actual filehandle. Thus:
2355 $oldfh = select(STDERR); $| = 1; select($oldfh);
2357 Some programmers may prefer to think of filehandles as objects with
2358 methods, preferring to write the last example as:
2361 STDERR->autoflush(1);
2363 =item select RBITS,WBITS,EBITS,TIMEOUT
2365 This calls the select(2) system call with the bitmasks specified, which
2366 can be constructed using fileno() and vec(), along these lines:
2368 $rin = $win = $ein = '';
2369 vec($rin,fileno(STDIN),1) = 1;
2370 vec($win,fileno(STDOUT),1) = 1;
2373 If you want to select on many filehandles you might wish to write a
2377 local(@fhlist) = split(' ',$_[0]);
2380 vec($bits,fileno($_),1) = 1;
2384 $rin = fhbits('STDIN TTY SOCK');
2388 ($nfound,$timeleft) =
2389 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2391 or to block until something becomes ready just do this
2393 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2395 Most systems do not both to return anything useful in $timeleft, so
2396 calling select() in a scalar context just returns $nfound.
2398 Any of the bitmasks can also be undef. The timeout, if specified, is
2399 in seconds, which may be fractional. Note: not all implementations are
2400 capable of returning the $timeleft. If not, they always return
2401 $timeleft equal to the supplied $timeout.
2403 You can effect a 250-millisecond sleep this way:
2405 select(undef, undef, undef, 0.25);
2407 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2408 with select(). You have to use sysread() instead.
2410 =item semctl ID,SEMNUM,CMD,ARG
2412 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2413 &GETALL, then ARG must be a variable which will hold the returned
2414 semid_ds structure or semaphore value array. Returns like ioctl: the
2415 undefined value for error, "0 but true" for zero, or the actual return
2418 =item semget KEY,NSEMS,FLAGS
2420 Calls the System V IPC function semget. Returns the semaphore id, or
2421 the undefined value if there is an error.
2423 =item semop KEY,OPSTRING
2425 Calls the System V IPC function semop to perform semaphore operations
2426 such as signaling and waiting. OPSTRING must be a packed array of
2427 semop structures. Each semop structure can be generated with
2428 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2429 operations is implied by the length of OPSTRING. Returns TRUE if
2430 successful, or FALSE if there is an error. As an example, the
2431 following code waits on semaphore $semnum of semaphore id $semid:
2433 $semop = pack("sss", $semnum, -1, 0);
2434 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2436 To signal the semaphore, replace "-1" with "1".
2438 =item send SOCKET,MSG,FLAGS,TO
2440 =item send SOCKET,MSG,FLAGS
2442 Sends a message on a socket. Takes the same flags as the system call
2443 of the same name. On unconnected sockets you must specify a
2444 destination to send TO, in which case it does a C sendto(). Returns
2445 the number of characters sent, or the undefined value if there is an
2447 See L<perlipc/"UDP: Message Passing"> for examples.
2449 =item setpgrp PID,PGRP
2451 Sets the current process group for the specified PID, 0 for the current
2452 process. Will produce a fatal error if used on a machine that doesn't
2453 implement setpgrp(2). If the arguments are ommitted, it defaults to
2454 0,0. Note that the POSIX version of setpgrp() does not accept any
2455 arguments, so only setpgrp 0,0 is portable.
2457 =item setpriority WHICH,WHO,PRIORITY
2459 Sets the current priority for a process, a process group, or a user.
2460 (See setpriority(2).) Will produce a fatal error if used on a machine
2461 that doesn't implement setpriority(2).
2463 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2465 Sets the socket option requested. Returns undefined if there is an
2466 error. OPTVAL may be specified as undef if you don't want to pass an
2473 Shifts the first value of the array off and returns it, shortening the
2474 array by 1 and moving everything down. If there are no elements in the
2475 array, returns the undefined value. If ARRAY is omitted, shifts the
2476 @ARGV array in the main program, and the @_ array in subroutines.
2477 (This is determined lexically.) See also unshift(), push(), and pop().
2478 Shift() and unshift() do the same thing to the left end of an array
2479 that push() and pop() do to the right end.
2481 =item shmctl ID,CMD,ARG
2483 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2484 must be a variable which will hold the returned shmid_ds structure.
2485 Returns like ioctl: the undefined value for error, "0 but true" for
2486 zero, or the actual return value otherwise.
2488 =item shmget KEY,SIZE,FLAGS
2490 Calls the System V IPC function shmget. Returns the shared memory
2491 segment id, or the undefined value if there is an error.
2493 =item shmread ID,VAR,POS,SIZE
2495 =item shmwrite ID,STRING,POS,SIZE
2497 Reads or writes the System V shared memory segment ID starting at
2498 position POS for size SIZE by attaching to it, copying in/out, and
2499 detaching from it. When reading, VAR must be a variable which will
2500 hold the data read. When writing, if STRING is too long, only SIZE
2501 bytes are used; if STRING is too short, nulls are written to fill out
2502 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2504 =item shutdown SOCKET,HOW
2506 Shuts down a socket connection in the manner indicated by HOW, which
2507 has the same interpretation as in the system call of the same name.
2511 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2518 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2519 May be interrupted by sending the process a SIGALRM. Returns the
2520 number of seconds actually slept. You probably cannot mix alarm() and
2521 sleep() calls, since sleep() is often implemented using alarm().
2523 On some older systems, it may sleep up to a full second less than what
2524 you requested, depending on how it counts seconds. Most modern systems
2525 always sleep the full amount.
2527 For delays of finer granularity than one second, you may use Perl's
2528 syscall() interface to access setitimer(2) if your system supports it,
2529 or else see L</select()> below.
2531 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2533 Opens a socket of the specified kind and attaches it to filehandle
2534 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2535 system call of the same name. You should "use Socket;" first to get
2536 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2538 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2540 Creates an unnamed pair of sockets in the specified domain, of the
2541 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2542 for the system call of the same name. If unimplemented, yields a fatal
2543 error. Returns TRUE if successful.
2545 =item sort SUBNAME LIST
2547 =item sort BLOCK LIST
2551 Sorts the LIST and returns the sorted list value. Nonexistent values
2552 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2553 in standard string comparison order. If SUBNAME is specified, it
2554 gives the name of a subroutine that returns an integer less than, equal
2555 to, or greater than 0, depending on how the elements of the array are
2556 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2557 routines.) SUBNAME may be a scalar variable name, in which case the
2558 value provides the name of the subroutine to use. In place of a
2559 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2562 In the interests of efficiency the normal calling code for subroutines is
2563 bypassed, with the following effects: the subroutine may not be a
2564 recursive subroutine, and the two elements to be compared are passed into
2565 the subroutine not via @_ but as the package global variables $a and
2566 $b (see example below). They are passed by reference, so don't
2567 modify $a and $b. And don't try to declare them as lexicals either.
2572 @articles = sort @files;
2574 # same thing, but with explicit sort routine
2575 @articles = sort {$a cmp $b} @files;
2577 # now case-insensitively
2578 @articles = sort { uc($a) cmp uc($b)} @files;
2580 # same thing in reversed order
2581 @articles = sort {$b cmp $a} @files;
2583 # sort numerically ascending
2584 @articles = sort {$a <=> $b} @files;
2586 # sort numerically descending
2587 @articles = sort {$b <=> $a} @files;
2589 # sort using explicit subroutine name
2591 $age{$a} <=> $age{$b}; # presuming integers
2593 @sortedclass = sort byage @class;
2595 # this sorts the %age associative arrays by value
2596 # instead of key using an inline function
2597 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2599 sub backwards { $b cmp $a; }
2600 @harry = ('dog','cat','x','Cain','Abel');
2601 @george = ('gone','chased','yz','Punished','Axed');
2603 # prints AbelCaincatdogx
2604 print sort backwards @harry;
2605 # prints xdogcatCainAbel
2606 print sort @george, 'to', @harry;
2607 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2609 # inefficiently sort by descending numeric compare using
2610 # the first integer after the first = sign, or the
2611 # whole record case-insensitively otherwise
2614 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2619 # same thing, but much more efficiently;
2620 # we'll build auxiliary indices instead
2624 push @nums, /=(\d+)/;
2629 $nums[$b] <=> $nums[$a]
2631 $caps[$a] cmp $caps[$b]
2635 # same thing using a Schwartzian Transform (no temps)
2636 @new = map { $_->[0] }
2637 sort { $b->[1] <=> $a->[1]
2640 } map { [$_, /=(\d+)/, uc($_)] } @old;
2642 If you're using strict, you I<MUST NOT> declare $a
2643 and $b as lexicals. They are package globals. That means
2644 if you're in the C<main> package, it's
2646 @articles = sort {$main::b <=> $main::a} @files;
2650 @articles = sort {$::b <=> $::a} @files;
2652 but if you're in the C<FooPack> package, it's
2654 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2656 =item splice ARRAY,OFFSET,LENGTH,LIST
2658 =item splice ARRAY,OFFSET,LENGTH
2660 =item splice ARRAY,OFFSET
2662 Removes the elements designated by OFFSET and LENGTH from an array, and
2663 replaces them with the elements of LIST, if any. Returns the elements
2664 removed from the array. The array grows or shrinks as necessary. If
2665 LENGTH is omitted, removes everything from OFFSET onward. The
2666 following equivalencies hold (assuming C<$[ == 0>):
2668 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2669 pop(@a) splice(@a,-1)
2670 shift(@a) splice(@a,0,1)
2671 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2672 $a[$x] = $y splice(@a,$x,1,$y);
2674 Example, assuming array lengths are passed before arrays:
2676 sub aeq { # compare two list values
2677 local(@a) = splice(@_,0,shift);
2678 local(@b) = splice(@_,0,shift);
2679 return 0 unless @a == @b; # same len?
2681 return 0 if pop(@a) ne pop(@b);
2685 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2687 =item split /PATTERN/,EXPR,LIMIT
2689 =item split /PATTERN/,EXPR
2691 =item split /PATTERN/
2695 Splits a string into an array of strings, and returns it.
2697 If not in a list context, returns the number of fields found and splits into
2698 the @_ array. (In a list context, you can force the split into @_ by
2699 using C<??> as the pattern delimiters, but it still returns the array
2700 value.) The use of implicit split to @_ is deprecated, however.
2702 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2703 splits on whitespace (after skipping any leading whitespace). Anything
2704 matching PATTERN is taken to be a delimiter separating the fields. (Note
2705 that the delimiter may be longer than one character.) If LIMIT is
2706 specified and is not negative, splits into no more than that many fields
2707 (though it may split into fewer). If LIMIT is unspecified, trailing null
2708 fields are stripped (which potential users of pop() would do well to
2709 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2710 LIMIT had been specified.
2712 A pattern matching the null string (not to be confused with
2713 a null pattern C<//>, which is just one member of the set of patterns
2714 matching a null string) will split the value of EXPR into separate
2715 characters at each point it matches that way. For example:
2717 print join(':', split(/ */, 'hi there'));
2719 produces the output 'h:i:t:h:e:r:e'.
2721 The LIMIT parameter can be used to partially split a line
2723 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2725 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2726 one larger than the number of variables in the list, to avoid
2727 unnecessary work. For the list above LIMIT would have been 4 by
2728 default. In time critical applications it behooves you not to split
2729 into more fields than you really need.
2731 If the PATTERN contains parentheses, additional array elements are
2732 created from each matching substring in the delimiter.
2734 split(/([,-])/, "1-10,20", 3);
2736 produces the list value
2738 (1, '-', 10, ',', 20)
2740 If you had the entire header of a normal Unix email message in $header,
2741 you could split it up into fields and their values this way:
2743 $header =~ s/\n\s+/ /g; # fix continuation lines
2744 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2746 The pattern C</PATTERN/> may be replaced with an expression to specify
2747 patterns that vary at runtime. (To do runtime compilation only once,
2748 use C</$variable/o>.)
2750 As a special case, specifying a PATTERN of space (C<' '>) will split on
2751 white space just as split with no arguments does. Thus, split(' ') can
2752 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2753 will give you as many null initial fields as there are leading spaces.
2754 A split on /\s+/ is like a split(' ') except that any leading
2755 whitespace produces a null first field. A split with no arguments
2756 really does a C<split(' ', $_)> internally.
2760 open(passwd, '/etc/passwd');
2762 ($login, $passwd, $uid, $gid, $gcos,
2763 $home, $shell) = split(/:/);
2767 (Note that $shell above will still have a newline on it. See L</chop>,
2768 L</chomp>, and L</join>.)
2770 =item sprintf FORMAT,LIST
2772 Returns a string formatted by the usual printf conventions of the C
2773 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2774 (The * character for an indirectly specified length is not
2775 supported, but you can get the same effect by interpolating a variable
2776 into the pattern.) Some C libraries' implementations of sprintf() can
2777 dump core when fed ludicrous arguments.
2781 Return the square root of EXPR. If EXPR is omitted, returns square
2786 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2787 uses a semirandom value based on the current time and process ID, among
2788 other things. Of course, you'd need something much more random than that for
2789 cryptographic purposes, since it's easy to guess the current time.
2790 Checksumming the compressed output of rapidly changing operating system
2791 status programs is the usual method. Examples are posted regularly to
2792 the comp.security.unix newsgroup.
2794 =item stat FILEHANDLE
2798 Returns a 13-element array giving the status info for a file, either the
2799 file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2800 the stat fails. Typically used as follows:
2802 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2803 $atime,$mtime,$ctime,$blksize,$blocks)
2806 Not all fields are supported on all filesystem types. Here are the
2807 meaning of the fields:
2809 dev device number of filesystem
2811 mode file mode (type and permissions)
2812 nlink number of (hard) links to the file
2813 uid numeric user ID of file's owner
2814 gid numer group ID of file's owner
2815 rdev the device identifier (special files only)
2816 size total size of file, in bytes
2817 atime last access time since the epoch
2818 mtime last modify time since the epoch
2819 ctime inode change time (NOT creation type!) since the epoch
2820 blksize preferred blocksize for file system I/O
2821 blocks actual number of blocks allocated
2823 (The epoch was at 00:00 January 1, 1970 GMT.)
2825 If stat is passed the special filehandle consisting of an underline, no
2826 stat is done, but the current contents of the stat structure from the
2827 last stat or filetest are returned. Example:
2829 if (-x $file && (($d) = stat(_)) && $d < 0) {
2830 print "$file is executable NFS file\n";
2833 (This only works on machines for which the device number is negative under NFS.)
2839 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2840 doing many pattern matches on the string before it is next modified.
2841 This may or may not save time, depending on the nature and number of
2842 patterns you are searching on, and on the distribution of character
2843 frequencies in the string to be searched--you probably want to compare
2844 runtimes with and without it to see which runs faster. Those loops
2845 which scan for many short constant strings (including the constant
2846 parts of more complex patterns) will benefit most. You may have only
2847 one study active at a time--if you study a different scalar the first
2848 is "unstudied". (The way study works is this: a linked list of every
2849 character in the string to be searched is made, so we know, for
2850 example, where all the 'k' characters are. From each search string,
2851 the rarest character is selected, based on some static frequency tables
2852 constructed from some C programs and English text. Only those places
2853 that contain this "rarest" character are examined.)
2855 For example, here is a loop which inserts index producing entries
2856 before any line containing a certain pattern:
2860 print ".IX foo\n" if /\bfoo\b/;
2861 print ".IX bar\n" if /\bbar\b/;
2862 print ".IX blurfl\n" if /\bblurfl\b/;
2867 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2868 will be looked at, because "f" is rarer than "o". In general, this is
2869 a big win except in pathological cases. The only question is whether
2870 it saves you more time than it took to build the linked list in the
2873 Note that if you have to look for strings that you don't know till
2874 runtime, you can build an entire loop as a string and eval that to
2875 avoid recompiling all your patterns all the time. Together with
2876 undefining $/ to input entire files as one record, this can be very
2877 fast, often faster than specialized programs like fgrep(1). The following
2878 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
2879 out the names of those files that contain a match:
2881 $search = 'while (<>) { study;';
2882 foreach $word (@words) {
2883 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2888 eval $search; # this screams
2889 $/ = "\n"; # put back to normal input delim
2890 foreach $file (sort keys(%seen)) {
2898 =item sub NAME BLOCK
2900 This is subroutine definition, not a real function I<per se>. With just a
2901 NAME (and possibly prototypes), it's just a forward declaration. Without
2902 a NAME, it's an anonymous function declaration, and does actually return a
2903 value: the CODE ref of the closure you just created. See L<perlsub> and
2904 L<perlref> for details.
2906 =item substr EXPR,OFFSET,LEN
2908 =item substr EXPR,OFFSET
2910 Extracts a substring out of EXPR and returns it. First character is at
2911 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2912 that far from the end of the string. If LEN is omitted, returns
2913 everything to the end of the string. If LEN is negative, leaves that
2914 many characters off the end of the string.
2916 You can use the substr() function
2917 as an lvalue, in which case EXPR must be an lvalue. If you assign
2918 something shorter than LEN, the string will shrink, and if you assign
2919 something longer than LEN, the string will grow to accommodate it. To
2920 keep the string the same length you may need to pad or chop your value
2923 =item symlink OLDFILE,NEWFILE
2925 Creates a new filename symbolically linked to the old filename.
2926 Returns 1 for success, 0 otherwise. On systems that don't support
2927 symbolic links, produces a fatal error at run time. To check for that,
2930 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2934 Calls the system call specified as the first element of the list,
2935 passing the remaining elements as arguments to the system call. If
2936 unimplemented, produces a fatal error. The arguments are interpreted
2937 as follows: if a given argument is numeric, the argument is passed as
2938 an int. If not, the pointer to the string value is passed. You are
2939 responsible to make sure a string is pre-extended long enough to
2940 receive any result that might be written into a string. If your
2941 integer arguments are not literals and have never been interpreted in a
2942 numeric context, you may need to add 0 to them to force them to look
2945 require 'syscall.ph'; # may need to run h2ph
2946 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2948 Note that Perl only supports passing of up to 14 arguments to your system call,
2949 which in practice should usually suffice.
2951 =item sysopen FILEHANDLE,FILENAME,MODE
2953 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
2955 Opens the file whose filename is given by FILENAME, and associates it
2956 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
2957 the name of the real filehandle wanted. This function calls the
2958 underlying operating system's C<open> function with the parameters
2959 FILENAME, MODE, PERMS.
2961 The possible values and flag bits of the MODE parameter are
2962 system-dependent; they are available via the standard module C<Fcntl>.
2963 However, for historical reasons, some values are universal: zero means
2964 read-only, one means write-only, and two means read/write.
2966 If the file named by FILENAME does not exist and the C<open> call
2967 creates it (typically because MODE includes the O_CREAT flag), then
2968 the value of PERMS specifies the permissions of the newly created
2969 file. If PERMS is omitted, the default value is 0666, which allows
2970 read and write for all. This default is reasonable: see C<umask>.
2972 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2974 =item sysread FILEHANDLE,SCALAR,LENGTH
2976 Attempts to read LENGTH bytes of data into variable SCALAR from the
2977 specified FILEHANDLE, using the system call read(2). It bypasses
2978 stdio, so mixing this with other kinds of reads may cause confusion.
2979 Returns the number of bytes actually read, or undef if there was an
2980 error. SCALAR will be grown or shrunk to the length actually read. An
2981 OFFSET may be specified to place the read data at some other place than
2982 the beginning of the string.
2986 Does exactly the same thing as "exec LIST" except that a fork is done
2987 first, and the parent process waits for the child process to complete.
2988 Note that argument processing varies depending on the number of
2989 arguments. The return value is the exit status of the program as
2990 returned by the wait() call. To get the actual exit value divide by
2991 256. See also L</exec>. This is I<NOT> what you want to use to capture
2992 the output from a command, for that you should merely use backticks, as
2993 described in L<perlop/"`STRING`">.
2995 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2997 =item syswrite FILEHANDLE,SCALAR,LENGTH
2999 Attempts to write LENGTH bytes of data from variable SCALAR to the
3000 specified FILEHANDLE, using the system call write(2). It bypasses
3001 stdio, so mixing this with prints may cause confusion. Returns the
3002 number of bytes actually written, or undef if there was an error. An
3003 OFFSET may be specified to get the write data from some other place than
3004 the beginning of the string.
3006 =item tell FILEHANDLE
3010 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3011 expression whose value gives the name of the actual filehandle. If
3012 FILEHANDLE is omitted, assumes the file last read.
3014 =item telldir DIRHANDLE
3016 Returns the current position of the readdir() routines on DIRHANDLE.
3017 Value may be given to seekdir() to access a particular location in a
3018 directory. Has the same caveats about possible directory compaction as
3019 the corresponding system library routine.
3021 =item tie VARIABLE,CLASSNAME,LIST
3023 This function binds a variable to a package class that will provide the
3024 implementation for the variable. VARIABLE is the name of the variable
3025 to be enchanted. CLASSNAME is the name of a class implementing objects
3026 of correct type. Any additional arguments are passed to the "new"
3027 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3028 Typically these are arguments such as might be passed to the dbm_open()
3029 function of C. The object returned by the "new" method is also
3030 returned by the tie() function, which would be useful if you want to
3031 access other methods in CLASSNAME.
3033 Note that functions such as keys() and values() may return huge array
3034 values when used on large objects, like DBM files. You may prefer to
3035 use the each() function to iterate over such. Example:
3037 # print out history file offsets
3039 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3040 while (($key,$val) = each %HIST) {
3041 print $key, ' = ', unpack('L',$val), "\n";
3045 A class implementing an associative array should have the following
3048 TIEHASH classname, LIST
3051 STORE this, key, value
3055 NEXTKEY this, lastkey
3057 A class implementing an ordinary array should have the following methods:
3059 TIEARRAY classname, LIST
3062 STORE this, key, value
3065 A class implementing a scalar should have the following methods:
3067 TIESCALAR classname, LIST
3072 Unlike dbmopen(), the tie() function will not use or require a module
3073 for you--you need to do that explicitly yourself. See L<DB_File>
3074 or the F<Config> module for interesting tie() implementations.
3078 Returns a reference to the object underlying VARIABLE (the same value
3079 that was originally returned by the tie() call which bound the variable
3080 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3085 Returns the number of non-leap seconds since whatever time the system
3086 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3087 and 00:00:00 UTC, January 1, 1970 for most other systems).
3088 Suitable for feeding to gmtime() and localtime().
3092 Returns a four-element array giving the user and system times, in
3093 seconds, for this process and the children of this process.
3095 ($user,$system,$cuser,$csystem) = times;
3099 The translation operator. See L<perlop>.
3101 =item truncate FILEHANDLE,LENGTH
3103 =item truncate EXPR,LENGTH
3105 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3106 specified length. Produces a fatal error if truncate isn't implemented
3111 Returns an uppercased version of EXPR. This is the internal function
3112 implementing the \U escape in double-quoted strings.
3113 Should respect any POSIX setlocale() settings.
3117 Returns the value of EXPR with the first character uppercased. This is
3118 the internal function implementing the \u escape in double-quoted strings.
3119 Should respect any POSIX setlocale() settings.
3125 Sets the umask for the process and returns the old one. If EXPR is
3126 omitted, merely returns current umask.
3132 Undefines the value of EXPR, which must be an lvalue. Use only on a
3133 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3134 will probably not do what you expect on most predefined variables or
3135 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3136 the EXPR, in which case nothing is undefined, but you still get an
3137 undefined value that you could, for instance, return from a
3138 subroutine. Examples:
3141 undef $bar{'blurfl'};
3145 return (wantarray ? () : undef) if $they_blew_it;
3149 Deletes a list of files. Returns the number of files successfully
3152 $cnt = unlink 'a', 'b', 'c';
3156 Note: unlink will not delete directories unless you are superuser and
3157 the B<-U> flag is supplied to Perl. Even if these conditions are
3158 met, be warned that unlinking a directory can inflict damage on your
3159 filesystem. Use rmdir instead.
3161 =item unpack TEMPLATE,EXPR
3163 Unpack does the reverse of pack: it takes a string representing a
3164 structure and expands it out into a list value, returning the array
3165 value. (In a scalar context, it merely returns the first value
3166 produced.) The TEMPLATE has the same format as in the pack function.
3167 Here's a subroutine that does substring:
3170 local($what,$where,$howmuch) = @_;
3171 unpack("x$where a$howmuch", $what);
3176 sub ordinal { unpack("c",$_[0]); } # same as ord()
3178 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3179 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3180 themselves. Default is a 16-bit checksum. For example, the following
3181 computes the same number as the System V sum program:
3184 $checksum += unpack("%16C*", $_);
3188 The following efficiently counts the number of set bits in a bit vector:
3190 $setbits = unpack("%32b*", $selectmask);
3192 =item untie VARIABLE
3194 Breaks the binding between a variable and a package. (See tie().)
3196 =item unshift ARRAY,LIST
3198 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3199 depending on how you look at it. Prepends list to the front of the
3200 array, and returns the new number of elements in the array.
3202 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3204 Note the LIST is prepended whole, not one element at a time, so the
3205 prepended elements stay in the same order. Use reverse to do the
3208 =item use Module LIST
3212 =item use Module VERSION LIST
3216 Imports some semantics into the current package from the named module,
3217 generally by aliasing certain subroutine or variable names into your
3218 package. It is exactly equivalent to
3220 BEGIN { require Module; import Module LIST; }
3222 except that Module I<must> be a bare word.
3224 If the first argument to C<use> is a number, it is treated as a version
3225 number instead of a module name. If the version of the Perl interpreter
3226 is less than VERSION, then an error message is printed and Perl exits
3227 immediately. This is often useful if you need to check the current
3228 Perl version before C<use>ing library modules which have changed in
3229 incompatible ways from older versions of Perl. (We try not to do
3230 this more than we have to.)
3232 The BEGIN forces the require and import to happen at compile time. The
3233 require makes sure the module is loaded into memory if it hasn't been
3234 yet. The import is not a builtin--it's just an ordinary static method
3235 call into the "Module" package to tell the module to import the list of
3236 features back into the current package. The module can implement its
3237 import method any way it likes, though most modules just choose to
3238 derive their import method via inheritance from the Exporter class that
3239 is defined in the Exporter module. See L<Exporter>.
3241 If you don't want your namespace altered, explicitly supply an empty list:
3245 That is exactly equivalent to
3247 BEGIN { require Module; }
3249 If the VERSION argument is present between Module and LIST, then the
3250 C<use> will fail if the C<$VERSION> variable in package Module is
3253 Because this is a wide-open interface, pragmas (compiler directives)
3254 are also implemented this way. Currently implemented pragmas are:
3258 use sigtrap qw(SEGV BUS);
3259 use strict qw(subs vars refs);
3260 use subs qw(afunc blurfl);
3262 These pseudomodules import semantics into the current block scope, unlike
3263 ordinary modules, which import symbols into the current package (which are
3264 effective through the end of the file).
3266 There's a corresponding "no" command that unimports meanings imported
3267 by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
3272 See L<perlmod> for a list of standard modules and pragmas.
3276 Changes the access and modification times on each file of a list of
3277 files. The first two elements of the list must be the NUMERICAL access
3278 and modification times, in that order. Returns the number of files
3279 successfully changed. The inode modification time of each file is set
3280 to the current time. Example of a "touch" command:
3284 utime $now, $now, @ARGV;
3286 =item values ASSOC_ARRAY
3288 Returns a normal array consisting of all the values of the named
3289 associative array. (In a scalar context, returns the number of
3290 values.) The values are returned in an apparently random order, but it
3291 is the same order as either the keys() or each() function would produce
3292 on the same array. See also keys(), each(), and sort().
3294 =item vec EXPR,OFFSET,BITS
3296 Treats the string in EXPR as a vector of unsigned integers, and
3297 returns the value of the bitfield specified by OFFSET. BITS specifies
3298 the number of bits that are reserved for each entry in the bit
3299 vector. This must be a power of two from 1 to 32. vec() may also be
3300 assigned to, in which case parens are needed to give the expression
3301 the correct precedence as in
3303 vec($image, $max_x * $x + $y, 8) = 3;
3305 Vectors created with vec() can also be manipulated with the logical
3306 operators |, & and ^, which will assume a bit vector operation is
3307 desired when both operands are strings.
3309 To transform a bit vector into a string or array of 0's and 1's, use these:
3311 $bits = unpack("b*", $vector);
3312 @bits = split(//, unpack("b*", $vector));
3314 If you know the exact length in bits, it can be used in place of the *.
3318 Waits for a child process to terminate and returns the pid of the
3319 deceased process, or -1 if there are no child processes. The status is
3322 =item waitpid PID,FLAGS
3324 Waits for a particular child process to terminate and returns the pid
3325 of the deceased process, or -1 if there is no such child process. The
3326 status is returned in C<$?>. If you say
3328 use POSIX ":wait_h";
3330 waitpid(-1,&WNOHANG);
3332 then you can do a non-blocking wait for any process. Non-blocking wait
3333 is only available on machines supporting either the waitpid(2) or
3334 wait4(2) system calls. However, waiting for a particular pid with
3335 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3336 by remembering the status values of processes that have exited but have
3337 not been harvested by the Perl script yet.)
3341 Returns TRUE if the context of the currently executing subroutine is
3342 looking for a list value. Returns FALSE if the context is looking
3345 return wantarray ? () : undef;
3349 Produces a message on STDERR just like die(), but doesn't exit or
3352 =item write FILEHANDLE
3358 Writes a formatted record (possibly multi-line) to the specified file,
3359 using the format associated with that file. By default the format for
3360 a file is the one having the same name is the filehandle, but the
3361 format for the current output channel (see the select() function) may be set
3362 explicitly by assigning the name of the format to the C<$~> variable.
3364 Top of form processing is handled automatically: if there is
3365 insufficient room on the current page for the formatted record, the
3366 page is advanced by writing a form feed, a special top-of-page format
3367 is used to format the new page header, and then the record is written.
3368 By default the top-of-page format is the name of the filehandle with
3369 "_TOP" appended, but it may be dynamically set to the format of your
3370 choice by assigning the name to the C<$^> variable while the filehandle is
3371 selected. The number of lines remaining on the current page is in
3372 variable C<$->, which can be set to 0 to force a new page.
3374 If FILEHANDLE is unspecified, output goes to the current default output
3375 channel, which starts out as STDOUT but may be changed by the
3376 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3377 is evaluated and the resulting string is used to look up the name of
3378 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3380 Note that write is I<NOT> the opposite of read. Unfortunately.
3384 The translation operator. See L<perlop>.
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