4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar and list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one list argument.) For instance, splice() has three scalar
20 arguments followed by a list.
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 For functions that can be used in either a scalar or list context,
52 nonabortive failure is generally indicated in a scalar context by
53 returning the undefined value, and in a list context by returning the
56 Remember the following rule:
60 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in a scalar context. Some operators return the
66 length of the list that would have been returned in a list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 =head2 Perl Functions by Category
74 Here are Perl's functions (including things that look like
75 functions, like some of the keywords and named operators)
76 arranged by category. Some functions appear in more
81 =item Functions for SCALARs or strings
83 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
84 C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
85 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///
87 =item Regular expressions and pattern matching
89 C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
91 =item Numeric functions
93 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
94 C<sin>, C<sqrt>, C<srand>
96 =item Functions for real @ARRAYs
98 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
100 =item Functions for list data
102 C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
104 =item Functions for real %HASHes
106 C<delete>, C<each>, C<exists>, C<keys>, C<values>
108 =item Input and output functions
110 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
111 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
112 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
113 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
116 =item Functions for fixed length data or records
118 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
120 =item Functions for filehandles, files, or directories
122 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
123 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
124 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
126 =item Keywords related to the control flow of your perl program
128 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
129 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
131 =item Keywords related to scoping
133 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
135 =item Miscellaneous functions
137 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
138 C<scalar>, C<undef>, C<wantarray>
140 =item Functions for processes and process groups
142 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
143 C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
144 C<times>, C<wait>, C<waitpid>
146 =item Keywords related to perl modules
148 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
150 =item Keywords related to classes and object-orientedness
152 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
155 =item Low-level socket functions
157 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
158 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
159 C<socket>, C<socketpair>
161 =item System V interprocess communication functions
163 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
164 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
166 =item Fetching user and group info
168 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
169 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
170 C<getpwuid>, C<setgrent>, C<setpwent>
172 =item Fetching network info
174 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
175 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
176 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
177 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
178 C<setnetent>, C<setprotoent>, C<setservent>
180 =item Time-related functions
182 C<gmtime>, C<localtime>, C<time>, C<times>
184 =item Functions new in perl5
186 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
187 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
188 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
189 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
191 * - C<sub> was a keyword in perl4, but in perl5 it is an
192 operator which can be used in expressions.
194 =item Functions obsoleted in perl5
196 C<dbmclose>, C<dbmopen>
200 =head2 Alphabetical Listing of Perl Functions
204 =item I<-X> FILEHANDLE
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 nonzero 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() 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
287 the 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 _;
309 Returns the absolute value of its argument.
310 If VALUE is omitted, uses $_.
312 =item accept NEWSOCKET,GENERICSOCKET
314 Accepts an incoming socket connect, just as the accept(2) system call
315 does. Returns the packed address if it succeeded, FALSE otherwise.
316 See example in L<perlipc/"Sockets: Client/Server Communication">.
322 Arranges to have a SIGALRM delivered to this process after the
323 specified number of seconds have elapsed. If SECONDS is not specified,
324 the value stored in $_ is used. (On some machines,
325 unfortunately, the elapsed time may be up to one second less than you
326 specified because of how seconds are counted.) Only one timer may be
327 counting at once. Each call disables the previous timer, and an
328 argument of 0 may be supplied to cancel the previous timer without
329 starting a new one. The returned value is the amount of time remaining
330 on the previous timer.
332 For delays of finer granularity than one second, you may use Perl's
333 syscall() interface to access setitimer(2) if your system supports it,
334 or else see L</select()>. It is usually a mistake to intermix alarm()
337 If you want to use alarm() to time out a system call you need to use an
338 eval/die pair. You can't rely on the alarm causing the system call to
339 fail with $! set to EINTR because Perl sets up signal handlers to
340 restart system calls on some systems. Using eval/die always works.
343 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
345 $nread = sysread SOCKET, $buffer, $size;
348 die if $@ && $@ ne "alarm\n"; # propagate errors
358 Returns the arctangent of Y/X in the range -PI to PI.
360 For the tangent operation, you may use the POSIX::tan()
361 function, or use the familiar relation:
363 sub tan { sin($_[0]) / cos($_[0]) }
365 =item bind SOCKET,NAME
367 Binds a network address to a socket, just as the bind system call
368 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
369 packed address of the appropriate type for the socket. See the examples in
370 L<perlipc/"Sockets: Client/Server Communication">.
372 =item binmode FILEHANDLE
374 Arranges for the file to be read or written in "binary" mode in operating
375 systems that distinguish between binary and text files. Files that are
376 not in binary mode have CR LF sequences translated to LF on input and LF
377 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
378 and similarly archaic systems, it may be imperative--otherwise your
379 MS-DOS-damaged C library may mangle your file. The key distinction between
380 systems that need binmode and those that don't is their text file
381 formats. Systems like Unix and Plan9 that delimit lines with a single
382 character, and that encode that character in C as '\n', do not need
383 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
384 is taken as the name of the filehandle.
386 =item bless REF,CLASSNAME
390 This function tells the thingy referenced by REF that it is now
391 an object in the CLASSNAME package--or the current package if no CLASSNAME
392 is specified, which is often the case. It returns the reference for
393 convenience, because a bless() is often the last thing in a constructor.
394 Always use the two-argument version if the function doing the blessing
395 might be inherited by a derived class. See L<perlobj> for more about the
396 blessing (and blessings) of objects.
402 Returns the context of the current subroutine call. In a scalar context,
403 returns the caller's package name if there is a caller, that is, if
404 we're in a subroutine or eval() or require(), and the undefined value
405 otherwise. In a list context, returns
407 ($package, $filename, $line) = caller;
409 With EXPR, it returns some extra information that the debugger uses to
410 print a stack trace. The value of EXPR indicates how many call frames
411 to go back before the current one.
413 ($package, $filename, $line, $subroutine,
414 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
416 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
417 call, but an C<eval>. In such a case additional elements $evaltext and
418 $is_require are set: $is_require is true if the frame is created by a
419 C<require> or C<use> statement, $evaltext contains the text of the
420 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
421 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
422 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
425 Furthermore, when called from within the DB package, caller returns more
426 detailed information: it sets the list variable @DB::args to be the
427 arguments with which the subroutine was invoked.
431 Changes the working directory to EXPR, if possible. If EXPR is
432 omitted, changes to home directory. Returns TRUE upon success, FALSE
433 otherwise. See example under die().
437 Changes the permissions of a list of files. The first element of the
438 list must be the numerical mode, which should probably be an octal
439 number, and which definitely should I<not> a string of octal digits:
440 C<0644> is okay, C<'0644'> is not. Returns the number of files
441 successfully changed. See also L</oct>, if all you have is a string.
443 $cnt = chmod 0755, 'foo', 'bar';
444 chmod 0755, @executables;
445 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
446 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
447 $mode = 0644; chmod $mode, 'foo'; # this is best
455 This is a slightly safer version of L</chop>. It removes any
456 line ending that corresponds to the current value of C<$/> (also known as
457 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
458 number of characters removed from all its arguments. It's often used to
459 remove the newline from the end of an input record when you're worried
460 that the final record may be missing its newline. When in paragraph mode
461 (C<$/ = "">), it removes all trailing newlines from the string. If
462 VARIABLE is omitted, it chomps $_. Example:
465 chomp; # avoid \n on last field
470 You can actually chomp anything that's an lvalue, including an assignment:
473 chomp($answer = <STDIN>);
475 If you chomp a list, each element is chomped, and the total number of
476 characters removed is returned.
484 Chops off the last character of a string and returns the character
485 chopped. It's used primarily to remove the newline from the end of an
486 input record, but is much more efficient than C<s/\n//> because it neither
487 scans nor copies the string. If VARIABLE is omitted, chops $_.
491 chop; # avoid \n on last field
496 You can actually chop anything that's an lvalue, including an assignment:
499 chop($answer = <STDIN>);
501 If you chop a list, each element is chopped. Only the value of the
502 last chop is returned.
504 Note that chop returns the last character. To return all but the last
505 character, use C<substr($string, 0, -1)>.
509 Changes the owner (and group) of a list of files. The first two
510 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
511 Returns the number of files successfully changed.
513 $cnt = chown $uid, $gid, 'foo', 'bar';
514 chown $uid, $gid, @filenames;
516 Here's an example that looks up nonnumeric uids in the passwd file:
519 chop($user = <STDIN>);
521 chop($pattern = <STDIN>);
523 ($login,$pass,$uid,$gid) = getpwnam($user)
524 or die "$user not in passwd file";
526 @ary = <${pattern}>; # expand filenames
527 chown $uid, $gid, @ary;
529 On most systems, you are not allowed to change the ownership of the
530 file unless you're the superuser, although you should be able to change
531 the group to any of your secondary groups. On insecure systems, these
532 restrictions may be relaxed, but this is not a portable assumption.
538 Returns the character represented by that NUMBER in the character set.
539 For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
541 If NUMBER is omitted, uses $_.
543 =item chroot FILENAME
547 This function works as the system call by the same name: it makes the
548 named directory the new root directory for all further pathnames that
549 begin with a "/" by your process and all of its children. (It doesn't
550 change your current working directory, which is unaffected.) For security
551 reasons, this call is restricted to the superuser. If FILENAME is
552 omitted, does chroot to $_.
554 =item close FILEHANDLE
556 Closes the file or pipe associated with the file handle, returning TRUE
557 only if stdio successfully flushes buffers and closes the system file
560 You don't have to close FILEHANDLE if you are immediately going to do
561 another open() on it, because open() will close it for you. (See
562 open().) However, an explicit close on an input file resets the line
563 counter ($.), while the implicit close done by open() does not.
565 If the file handle came from a piped open C<close> will additionally
566 return FALSE if one of the other system calls involved fails or if the
567 program exits with non-zero status. (If the only problem was that the
568 program exited non-zero $! will be set to 0.) Also, closing a pipe will
569 wait for the process executing on the pipe to complete, in case you
570 want to look at the output of the pipe afterwards. Closing a pipe
571 explicitly also puts the exit status value of the command into C<$?>.
574 open(OUTPUT, '|sort >foo') # pipe to sort
575 or die "Can't start sort: $!";
576 ... # print stuff to output
577 close OUTPUT # wait for sort to finish
578 or warn $! ? "Error closing sort pipe: $!"
579 : "Exit status $? from sort";
580 open(INPUT, 'foo') # get sort's results
581 or die "Can't open 'foo' for input: $!";
583 FILEHANDLE may be an expression whose value gives the real filehandle name.
585 =item closedir DIRHANDLE
587 Closes a directory opened by opendir().
589 =item connect SOCKET,NAME
591 Attempts to connect to a remote socket, just as the connect system call
592 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
593 packed address of the appropriate type for the socket. See the examples in
594 L<perlipc/"Sockets: Client/Server Communication">.
598 Actually a flow control statement rather than a function. If there is a
599 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
600 C<foreach>), it is always executed just before the conditional is about to
601 be evaluated again, just like the third part of a C<for> loop in C. Thus
602 it can be used to increment a loop variable, even when the loop has been
603 continued via the C<next> statement (which is similar to the C C<continue>
608 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
611 For the inverse cosine operation, you may use the POSIX::acos()
612 function, or use this relation:
614 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
616 =item crypt PLAINTEXT,SALT
618 Encrypts a string exactly like the crypt(3) function in the C library
619 (assuming that you actually have a version there that has not been
620 extirpated as a potential munition). This can prove useful for checking
621 the password file for lousy passwords, amongst other things. Only the
622 guys wearing white hats should do this.
624 Note that crypt is intended to be a one-way function, much like breaking
625 eggs to make an omelette. There is no (known) corresponding decrypt
626 function. As a result, this function isn't all that useful for
627 cryptography. (For that, see your nearby CPAN mirror.)
629 Here's an example that makes sure that whoever runs this program knows
632 $pwd = (getpwuid($<))[1];
633 $salt = substr($pwd, 0, 2);
637 chop($word = <STDIN>);
641 if (crypt($word, $salt) ne $pwd) {
647 Of course, typing in your own password to whoever asks you
652 [This function has been superseded by the untie() function.]
654 Breaks the binding between a DBM file and a hash.
656 =item dbmopen HASH,DBNAME,MODE
658 [This function has been superseded by the tie() function.]
660 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
661 hash. HASH is the name of the hash. (Unlike normal open, the first
662 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
663 is the name of the database (without the F<.dir> or F<.pag> extension if
664 any). If the database does not exist, it is created with protection
665 specified by MODE (as modified by the umask()). If your system supports
666 only the older DBM functions, you may perform only one dbmopen() in your
667 program. In older versions of Perl, if your system had neither DBM nor
668 ndbm, calling dbmopen() produced a fatal error; it now falls back to
671 If you don't have write access to the DBM file, you can only read hash
672 variables, not set them. If you want to test whether you can write,
673 either use file tests or try setting a dummy hash entry inside an eval(),
674 which will trap the error.
676 Note that functions such as keys() and values() may return huge array
677 values when used on large DBM files. You may prefer to use the each()
678 function to iterate over large DBM files. Example:
680 # print out history file offsets
681 dbmopen(%HIST,'/usr/lib/news/history',0666);
682 while (($key,$val) = each %HIST) {
683 print $key, ' = ', unpack('L',$val), "\n";
687 See also L<AnyDBM_File> for a more general description of the pros and
688 cons of the various dbm approaches, as well as L<DB_File> for a particularly
695 Returns a Boolean value telling whether EXPR has a value other than
696 the undefined value C<undef>. If EXPR is not present, C<$_> will be
699 Many operations return C<undef> to indicate failure, end of file,
700 system error, uninitialized variable, and other exceptional
701 conditions. This function allows you to distinguish C<undef> from
702 other values. (A simple Boolean test will not distinguish among
703 C<undef>, zero, the empty string, and "0", which are all equally
704 false.) Note that since C<undef> is a valid scalar, its presence
705 doesn't I<necessarily> indicate an exceptional condition: pop()
706 returns C<undef> when its argument is an empty array, I<or> when the
707 element to return happens to be C<undef>.
709 You may also use defined() to check whether a subroutine exists. On
710 the other hand, use of defined() upon aggregates (hashes and arrays)
711 is not guaranteed to produce intuitive results, and should probably be
714 When used on a hash element, it tells you whether the value is defined,
715 not whether the key exists in the hash. Use L</exists> for the latter
720 print if defined $switch{'D'};
721 print "$val\n" while defined($val = pop(@ary));
722 die "Can't readlink $sym: $!"
723 unless defined($value = readlink $sym);
724 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
725 $debugging = 0 unless defined $debugging;
727 Note: Many folks tend to overuse defined(), and then are surprised to
728 discover that the number 0 and "" (the zero-length string) are, in fact,
729 defined values. For example, if you say
733 the pattern match succeeds, and $1 is defined, despite the fact that it
734 matched "nothing". But it didn't really match nothing--rather, it
735 matched something that happened to be 0 characters long. This is all
736 very above-board and honest. When a function returns an undefined value,
737 it's an admission that it couldn't give you an honest answer. So you
738 should use defined() only when you're questioning the integrity of what
739 you're trying to do. At other times, a simple comparison to 0 or "" is
742 Currently, using defined() on an entire array or hash reports whether
743 memory for that aggregate has ever been allocated. So an array you set
744 to the empty list appears undefined initially, and one that once was full
745 and that you then set to the empty list still appears defined. You
746 should instead use a simple test for size:
748 if (@an_array) { print "has array elements\n" }
749 if (%a_hash) { print "has hash members\n" }
751 Using undef() on these, however, does clear their memory and then report
752 them as not defined anymore, but you shoudln't do that unless you don't
753 plan to use them again, because it saves time when you load them up
754 again to have memory already ready to be filled.
756 This counterintuitive behaviour of defined() on aggregates may be
757 changed, fixed, or broken in a future release of Perl.
759 See also L</undef>, L</exists>, L</ref>.
763 Deletes the specified key(s) and their associated values from a hash.
764 For each key, returns the deleted value associated with that key, or
765 the undefined value if there was no such key. Deleting from C<$ENV{}>
766 modifies the environment. Deleting from a hash tied to a DBM file
767 deletes the entry from the DBM file. (But deleting from a tie()d hash
768 doesn't necessarily return anything.)
770 The following deletes all the values of a hash:
772 foreach $key (keys %HASH) {
778 delete @HASH{keys %HASH}
780 (But both of these are slower than the undef() command.) Note that the
781 EXPR can be arbitrarily complicated as long as the final operation is a
782 hash element lookup or hash slice:
784 delete $ref->[$x][$y]{$key};
785 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
789 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
790 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
791 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
792 is 0, exits with 255. Inside an eval(), the error message is stuffed into
793 C<$@>, and the eval() is terminated with the undefined value; this makes
794 die() the way to raise an exception.
798 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
799 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
801 If the value of EXPR does not end in a newline, the current script line
802 number and input line number (if any) are also printed, and a newline
803 is supplied. Hint: sometimes appending ", stopped" to your message
804 will cause it to make better sense when the string "at foo line 123" is
805 appended. Suppose you are running script "canasta".
807 die "/etc/games is no good";
808 die "/etc/games is no good, stopped";
810 produce, respectively
812 /etc/games is no good at canasta line 123.
813 /etc/games is no good, stopped at canasta line 123.
815 See also exit() and warn().
817 If LIST is empty and $@ already contains a value (typically from a
818 previous eval) that value is reused after appending "\t...propagated".
819 This is useful for propagating exceptions:
822 die unless $@ =~ /Expected exception/;
824 If $@ is empty then the string "Died" is used.
826 You can arrange for a callback to be called just before the die() does
827 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
828 will be called with the error text and can change the error message, if
829 it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
830 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
832 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
833 blocks/strings. If one wants the hook to do nothing in such
838 as the first line of the handler (see L<perlvar/$^S>).
842 Not really a function. Returns the value of the last command in the
843 sequence of commands indicated by BLOCK. When modified by a loop
844 modifier, executes the BLOCK once before testing the loop condition.
845 (On other statements the loop modifiers test the conditional first.)
847 =item do SUBROUTINE(LIST)
849 A deprecated form of subroutine call. See L<perlsub>.
853 Uses the value of EXPR as a filename and executes the contents of the
854 file as a Perl script. Its primary use is to include subroutines
855 from a Perl subroutine library.
861 scalar eval `cat stat.pl`;
863 except that it's more efficient, more concise, keeps track of the
864 current filename for error messages, and searches all the B<-I>
865 libraries if the file isn't in the current directory (see also the @INC
866 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
867 reparse the file every time you call it, so you probably don't want to
868 do this inside a loop.
870 Note that inclusion of library modules is better done with the
871 use() and require() operators, which also do error checking
872 and raise an exception if there's a problem.
876 This causes an immediate core dump. Primarily this is so that you can
877 use the B<undump> program to turn your core dump into an executable binary
878 after having initialized all your variables at the beginning of the
879 program. When the new binary is executed it will begin by executing a
880 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
881 it as a goto with an intervening core dump and reincarnation. If LABEL
882 is omitted, restarts the program from the top. WARNING: any files
883 opened at the time of the dump will NOT be open any more when the
884 program is reincarnated, with possible resulting confusion on the part
885 of Perl. See also B<-u> option in L<perlrun>.
902 dump QUICKSTART if $ARGV[0] eq '-d';
909 When called in a list context, returns a 2-element array consisting of the
910 key and value for the next element of a hash, so that you can iterate over
911 it. When called in a scalar context, returns the key for only the next
912 element in the hash. (Note: Keys may be "0" or "", which are logically
913 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
916 Entries are returned in an apparently random order. When the hash is
917 entirely read, a null array is returned in list context (which when
918 assigned produces a FALSE (0) value), and C<undef> is returned in a
919 scalar context. The next call to each() after that will start iterating
920 again. There is a single iterator for each hash, shared by all each(),
921 keys(), and values() function calls in the program; it can be reset by
922 reading all the elements from the hash, or by evaluating C<keys HASH> or
923 C<values HASH>. If you add or delete elements of a hash while you're
924 iterating over it, you may get entries skipped or duplicated, so don't.
926 The following prints out your environment like the printenv(1) program,
927 only in a different order:
929 while (($key,$value) = each %ENV) {
930 print "$key=$value\n";
933 See also keys() and values().
941 Returns 1 if the next read on FILEHANDLE will return end of file, or if
942 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
943 gives the real filehandle name. (Note that this function actually
944 reads a character and then ungetc()s it, so it is not very useful in an
945 interactive context.) Do not read from a terminal file (or call
946 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
947 as terminals may lose the end-of-file condition if you do.
949 An C<eof> without an argument uses the last file read as argument.
950 Empty parentheses () may be used to indicate the pseudo file formed of
951 the files listed on the command line, i.e., C<eof()> is reasonable to
952 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
953 last file. Use C<eof(ARGV)> or eof without the parentheses to test
954 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
956 # reset line numbering on each input file
959 close(ARGV) if (eof); # Not eof().
962 # insert dashes just before last line of last file
965 print "--------------\n";
966 close(ARGV); # close or break; is needed if we
967 # are reading from the terminal
972 Practical hint: you almost never need to use C<eof> in Perl, because the
973 input operators return undef when they run out of data.
979 In the first form, the return value of EXPR is parsed and executed as if it
980 were a little Perl program. The value of the expression (which is itself
981 determined within a scalar context) is first parsed, and if there are no
982 errors, executed in the context of the current Perl program, so that any
983 variable settings or subroutine and format definitions remain afterwards.
984 Note that the value is parsed every time the eval executes. If EXPR is
985 omitted, evaluates C<$_>. This form is typically used to delay parsing
986 and subsequent execution of the text of EXPR until run time.
988 In the second form, the code within the BLOCK is parsed only once--at the
989 same time the code surrounding the eval itself was parsed--and executed
990 within the context of the current Perl program. This form is typically
991 used to trap exceptions more efficiently than the first (see below), while
992 also providing the benefit of checking the code within BLOCK at compile
995 The final semicolon, if any, may be omitted from the value of EXPR or within
998 In both forms, the value returned is the value of the last expression
999 evaluated inside the mini-program, or a return statement may be used, just
1000 as with subroutines. The expression providing the return value is evaluated
1001 in void, scalar or array context, depending on the context of the eval itself.
1002 See L</wantarray> for more on how the evaluation context can be determined.
1004 If there is a syntax error or runtime error, or a die() statement is
1005 executed, an undefined value is returned by eval(), and C<$@> is set to the
1006 error message. If there was no error, C<$@> is guaranteed to be a null
1007 string. Beware that using eval() neither silences perl from printing
1008 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1009 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1010 L</warn> and L<perlvar>.
1012 Note that, because eval() traps otherwise-fatal errors, it is useful for
1013 determining whether a particular feature (such as socket() or symlink())
1014 is implemented. It is also Perl's exception trapping mechanism, where
1015 the die operator is used to raise exceptions.
1017 If the code to be executed doesn't vary, you may use the eval-BLOCK
1018 form to trap run-time errors without incurring the penalty of
1019 recompiling each time. The error, if any, is still returned in C<$@>.
1022 # make divide-by-zero nonfatal
1023 eval { $answer = $a / $b; }; warn $@ if $@;
1025 # same thing, but less efficient
1026 eval '$answer = $a / $b'; warn $@ if $@;
1028 # a compile-time error
1032 eval '$answer ='; # sets $@
1034 When using the eval{} form as an exception trap in libraries, you may
1035 wish not to trigger any C<__DIE__> hooks that user code may have
1036 installed. You can use the C<local $SIG{__DIE__}> construct for this
1037 purpose, as shown in this example:
1039 # a very private exception trap for divide-by-zero
1040 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1042 This is especially significant, given that C<__DIE__> hooks can call
1043 die() again, which has the effect of changing their error messages:
1045 # __DIE__ hooks may modify error messages
1047 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1048 eval { die "foo lives here" };
1049 print $@ if $@; # prints "bar lives here"
1052 With an eval(), you should be especially careful to remember what's
1053 being looked at when:
1059 eval { $x }; # CASE 4
1061 eval "\$$x++" # CASE 5
1064 Cases 1 and 2 above behave identically: they run the code contained in
1065 the variable $x. (Although case 2 has misleading double quotes making
1066 the reader wonder what else might be happening (nothing is).) Cases 3
1067 and 4 likewise behave in the same way: they run the code '$x', which
1068 does nothing but return the value of C<$x>. (Case 4 is preferred for
1069 purely visual reasons, but it also has the advantage of compiling at
1070 compile-time instead of at run-time.) Case 5 is a place where
1071 normally you I<WOULD> like to use double quotes, except that in this
1072 particular situation, you can just use symbolic references instead, as
1077 The exec() function executes a system command I<AND NEVER RETURNS> -
1078 use system() instead of exec() if you want it to return. It fails and
1079 returns FALSE only if the command does not exist I<and> it is executed
1080 directly instead of via your system's command shell (see below).
1082 If there is more than one argument in LIST, or if LIST is an array with
1083 more than one value, calls execvp(3) with the arguments in LIST. If
1084 there is only one scalar argument, the argument is checked for shell
1085 metacharacters, and if there are any, the entire argument is passed to
1086 the system's command shell for parsing (this is C</bin/sh -c> on Unix
1087 platforms, but varies on other platforms). If there are no shell
1088 metacharacters in the argument, it is split into words and passed
1089 directly to execvp(), which is more efficient. Note: exec() and
1090 system() do not flush your output buffer, so you may need to set C<$|>
1091 to avoid lost output. Examples:
1093 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1094 exec "sort $outfile | uniq";
1096 If you don't really want to execute the first argument, but want to lie
1097 to the program you are executing about its own name, you can specify
1098 the program you actually want to run as an "indirect object" (without a
1099 comma) in front of the LIST. (This always forces interpretation of the
1100 LIST as a multivalued list, even if there is only a single scalar in
1103 $shell = '/bin/csh';
1104 exec $shell '-sh'; # pretend it's a login shell
1108 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1110 When the arguments get executed via the system shell, results will
1111 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1116 Returns TRUE if the specified hash key exists in its hash array, even
1117 if the corresponding value is undefined.
1119 print "Exists\n" if exists $array{$key};
1120 print "Defined\n" if defined $array{$key};
1121 print "True\n" if $array{$key};
1123 A hash element can be TRUE only if it's defined, and defined if
1124 it exists, but the reverse doesn't necessarily hold true.
1126 Note that the EXPR can be arbitrarily complicated as long as the final
1127 operation is a hash key lookup:
1129 if (exists $ref->[$x][$y]{$key}) { ... }
1133 Evaluates EXPR and exits immediately with that value. (Actually, it
1134 calls any defined C<END> routines first, but the C<END> routines may not
1135 abort the exit. Likewise any object destructors that need to be called
1136 are called before exit.) Example:
1139 exit 0 if $ans =~ /^[Xx]/;
1141 See also die(). If EXPR is omitted, exits with 0 status. The only
1142 universally portable values for EXPR are 0 for success and 1 for error;
1143 all other values are subject to unpredictable interpretation depending
1144 on the environment in which the Perl program is running.
1146 You shouldn't use exit() to abort a subroutine if there's any chance that
1147 someone might want to trap whatever error happened. Use die() instead,
1148 which can be trapped by an eval().
1154 Returns I<e> (the natural logarithm base) to the power of EXPR.
1155 If EXPR is omitted, gives C<exp($_)>.
1157 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1159 Implements the fcntl(2) function. You'll probably have to say
1163 first to get the correct function definitions. Argument processing and
1164 value return works just like ioctl() below. Note that fcntl() will produce
1165 a fatal error if used on a machine that doesn't implement fcntl(2).
1169 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1171 =item fileno FILEHANDLE
1173 Returns the file descriptor for a filehandle. This is useful for
1174 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1175 value is taken as the name of the filehandle.
1177 =item flock FILEHANDLE,OPERATION
1179 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1180 success, FALSE on failure. Produces a fatal error if used on a machine
1181 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1182 is Perl's portable file locking interface, although it locks only entire
1185 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1186 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1187 you can use the symbolic names if import them from the Fcntl module,
1188 either individually, or as a group using the ':flock' tag. LOCK_SH
1189 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1190 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1191 LOCK_EX then flock() will return immediately rather than blocking
1192 waiting for the lock (check the return status to see if you got it).
1194 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1195 before (un)locking it.
1197 Note that the emulation built with lockf(3) doesn't provide shared
1198 locks, and it requires that FILEHANDLE be open with write intent. These
1199 are the semantics that lockf(3) implements. Most (all?) systems
1200 implement lockf(3) in terms of fcntl(2) locking, though, so the
1201 differing semantics shouldn't bite too many people.
1203 Note also that some versions of flock() cannot lock things over the
1204 network; you would need to use the more system-specific fcntl() for
1205 that. If you like you can force Perl to ignore your system's flock(2)
1206 function, and so provide its own fcntl(2)-based emulation, by passing
1207 the switch C<-Ud_flock> to the F<Configure> program when you configure
1210 Here's a mailbox appender for BSD systems.
1212 use Fcntl ':flock'; # import LOCK_* constants
1215 flock(MBOX,LOCK_EX);
1216 # and, in case someone appended
1217 # while we were waiting...
1222 flock(MBOX,LOCK_UN);
1225 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1226 or die "Can't open mailbox: $!";
1229 print MBOX $msg,"\n\n";
1232 See also L<DB_File> for other flock() examples.
1236 Does a fork(2) system call. Returns the child pid to the parent process
1237 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1238 Note: unflushed buffers remain unflushed in both processes, which means
1239 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1240 method of IO::Handle to avoid duplicate output.
1242 If you fork() without ever waiting on your children, you will accumulate
1245 $SIG{CHLD} = sub { wait };
1247 There's also the double-fork trick (error checking on
1248 fork() returns omitted);
1250 unless ($pid = fork) {
1252 exec "what you really wanna do";
1255 ## (some_perl_code_here)
1262 See also L<perlipc> for more examples of forking and reaping
1265 Note that if your forked child inherits system file descriptors like
1266 STDIN and STDOUT that are actually connected by a pipe or socket, even
1267 if you exit, the remote server (such as, say, httpd or rsh) won't think
1268 you're done. You should reopen those to /dev/null if it's any issue.
1272 Declare a picture format with use by the write() function. For
1276 Test: @<<<<<<<< @||||| @>>>>>
1277 $str, $%, '$' . int($num)
1281 $num = $cost/$quantity;
1285 See L<perlform> for many details and examples.
1288 =item formline PICTURE,LIST
1290 This is an internal function used by C<format>s, though you may call it
1291 too. It formats (see L<perlform>) a list of values according to the
1292 contents of PICTURE, placing the output into the format output
1293 accumulator, C<$^A> (or $ACCUMULATOR in English).
1294 Eventually, when a write() is done, the contents of
1295 C<$^A> are written to some filehandle, but you could also read C<$^A>
1296 yourself and then set C<$^A> back to "". Note that a format typically
1297 does one formline() per line of form, but the formline() function itself
1298 doesn't care how many newlines are embedded in the PICTURE. This means
1299 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1300 You may therefore need to use multiple formlines to implement a single
1301 record format, just like the format compiler.
1303 Be careful if you put double quotes around the picture, because an "C<@>"
1304 character may be taken to mean the beginning of an array name.
1305 formline() always returns TRUE. See L<perlform> for other examples.
1307 =item getc FILEHANDLE
1311 Returns the next character from the input file attached to FILEHANDLE,
1312 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1313 This is not particularly efficient. It cannot be used to get unbuffered
1314 single-characters, however. For that, try something more like:
1317 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1320 system "stty", '-icanon', 'eol', "\001";
1326 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1329 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1333 Determination of whether $BSD_STYLE should be set
1334 is left as an exercise to the reader.
1336 The POSIX::getattr() function can do this more portably on systems
1337 alleging POSIX compliance.
1338 See also the C<Term::ReadKey> module from your nearest CPAN site;
1339 details on CPAN can be found on L<perlmod/CPAN>.
1343 Returns the current login from F</etc/utmp>, if any. If null, use
1346 $login = getlogin || getpwuid($<) || "Kilroy";
1348 Do not consider getlogin() for authentication: it is not as
1349 secure as getpwuid().
1351 =item getpeername SOCKET
1353 Returns the packed sockaddr address of other end of the SOCKET connection.
1356 $hersockaddr = getpeername(SOCK);
1357 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1358 $herhostname = gethostbyaddr($iaddr, AF_INET);
1359 $herstraddr = inet_ntoa($iaddr);
1363 Returns the current process group for the specified PID. Use
1364 a PID of 0 to get the current process group for the
1365 current process. Will raise an exception if used on a machine that
1366 doesn't implement getpgrp(2). If PID is omitted, returns process
1367 group of current process. Note that the POSIX version of getpgrp()
1368 does not accept a PID argument, so only PID==0 is truly portable.
1372 Returns the process id of the parent process.
1374 =item getpriority WHICH,WHO
1376 Returns the current priority for a process, a process group, or a user.
1377 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1378 machine that doesn't implement getpriority(2).
1384 =item gethostbyname NAME
1386 =item getnetbyname NAME
1388 =item getprotobyname NAME
1394 =item getservbyname NAME,PROTO
1396 =item gethostbyaddr ADDR,ADDRTYPE
1398 =item getnetbyaddr ADDR,ADDRTYPE
1400 =item getprotobynumber NUMBER
1402 =item getservbyport PORT,PROTO
1420 =item sethostent STAYOPEN
1422 =item setnetent STAYOPEN
1424 =item setprotoent STAYOPEN
1426 =item setservent STAYOPEN
1440 These routines perform the same functions as their counterparts in the
1441 system library. Within a list context, the return values from the
1442 various get routines are as follows:
1444 ($name,$passwd,$uid,$gid,
1445 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1446 ($name,$passwd,$gid,$members) = getgr*
1447 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1448 ($name,$aliases,$addrtype,$net) = getnet*
1449 ($name,$aliases,$proto) = getproto*
1450 ($name,$aliases,$port,$proto) = getserv*
1452 (If the entry doesn't exist you get a null list.)
1454 Within a scalar context, you get the name, unless the function was a
1455 lookup by name, in which case you get the other thing, whatever it is.
1456 (If the entry doesn't exist you get the undefined value.) For example:
1466 In I<getpw*()> the fields $quota, $comment, and $expire are special
1467 cases in the sense that in many systems they are unsupported. If the
1468 $quota is unsupported, it is an empty scalar. If it is supported, it
1469 usually encodes the disk quota. If the $comment field is unsupported,
1470 it is an empty scalar. If it is supported it usually encodes some
1471 administrative comment about the user. In some systems the $quota
1472 field may be $change or $age, fields that have to do with password
1473 aging. In some systems the $comment field may be $class. The $expire
1474 field, if present, encodes the expiration period of the account or the
1475 password. For the availability and the exact meaning of these fields
1476 in your system, please consult your getpwnam(3) documentation and your
1477 <pwd.h> file. You can also find out from within Perl which meaning
1478 your $quota and $comment fields have and whether you have the $expire
1479 field by using the Config module and the values d_pwquota, d_pwage,
1480 d_pwchange, d_pwcomment, and d_pwexpire.
1482 The $members value returned by I<getgr*()> is a space separated list of
1483 the login names of the members of the group.
1485 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1486 C, it will be returned to you via C<$?> if the function call fails. The
1487 @addrs value returned by a successful call is a list of the raw
1488 addresses returned by the corresponding system library call. In the
1489 Internet domain, each address is four bytes long and you can unpack it
1490 by saying something like:
1492 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1494 =item getsockname SOCKET
1496 Returns the packed sockaddr address of this end of the SOCKET connection.
1499 $mysockaddr = getsockname(SOCK);
1500 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1502 =item getsockopt SOCKET,LEVEL,OPTNAME
1504 Returns the socket option requested, or undefined if there is an error.
1510 Returns the value of EXPR with filename expansions such as a shell would
1511 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1512 operator, but you can use it directly. If EXPR is omitted, $_ is used.
1513 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1514 L<perlop/"I/O Operators">.
1518 Converts a time as returned by the time function to a 9-element array
1519 with the time localized for the standard Greenwich time zone.
1520 Typically used as follows:
1523 ($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 with sunday as day 0. Also, $year is the number of
1529 years since 1900, I<not> simply the last two digits of the year.
1531 If EXPR is omitted, does C<gmtime(time())>.
1533 In a scalar context, returns the ctime(3) value:
1535 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1537 Also see the timegm() function provided by the Time::Local module,
1538 and the strftime(3) function available via the POSIX module.
1546 The goto-LABEL form finds the statement labeled with LABEL and resumes
1547 execution there. It may not be used to go into any construct that
1548 requires initialization, such as a subroutine or a foreach loop. It
1549 also can't be used to go into a construct that is optimized away,
1550 or to get out of a block or subroutine given to sort().
1551 It can be used to go almost anywhere else within the dynamic scope,
1552 including out of subroutines, but it's usually better to use some other
1553 construct such as last or die. The author of Perl has never felt the
1554 need to use this form of goto (in Perl, that is--C is another matter).
1556 The goto-EXPR form expects a label name, whose scope will be resolved
1557 dynamically. This allows for computed gotos per FORTRAN, but isn't
1558 necessarily recommended if you're optimizing for maintainability:
1560 goto ("FOO", "BAR", "GLARCH")[$i];
1562 The goto-&NAME form is highly magical, and substitutes a call to the
1563 named subroutine for the currently running subroutine. This is used by
1564 AUTOLOAD subroutines that wish to load another subroutine and then
1565 pretend that the other subroutine had been called in the first place
1566 (except that any modifications to @_ in the current subroutine are
1567 propagated to the other subroutine.) After the goto, not even caller()
1568 will be able to tell that this routine was called first.
1570 =item grep BLOCK LIST
1572 =item grep EXPR,LIST
1574 This is similar in spirit to, but not the same as, grep(1)
1575 and its relatives. In particular, it is not limited to using
1576 regular expressions.
1578 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1579 $_ to each element) and returns the list value consisting of those
1580 elements for which the expression evaluated to TRUE. In a scalar
1581 context, returns the number of times the expression was TRUE.
1583 @foo = grep(!/^#/, @bar); # weed out comments
1587 @foo = grep {!/^#/} @bar; # weed out comments
1589 Note that, because $_ is a reference into the list value, it can be used
1590 to modify the elements of the array. While this is useful and
1591 supported, it can cause bizarre results if the LIST is not a named
1592 array. Similarly, grep returns aliases into the original list,
1593 much like the way that L<Foreach Loops>'s index variable aliases the list
1594 elements. That is, modifying an element of a list returned by grep
1595 (for example, in a C<foreach>, C<map> or another C<grep>)
1596 actually modifies the element in the original list.
1598 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1604 Interprets EXPR as a hex string and returns the corresponding
1605 value. (To convert strings that might start with either 0 or 0x
1606 see L</oct>.) If EXPR is omitted, uses $_.
1608 print hex '0xAf'; # prints '175'
1609 print hex 'aF'; # same
1613 There is no builtin import() function. It is merely an ordinary
1614 method (subroutine) defined (or inherited) by modules that wish to export
1615 names to another module. The use() function calls the import() method
1616 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1618 =item index STR,SUBSTR,POSITION
1620 =item index STR,SUBSTR
1622 Returns the position of the first occurrence of SUBSTR in STR at or after
1623 POSITION. If POSITION is omitted, starts searching from the beginning of
1624 the string. The return value is based at 0 (or whatever you've set the C<$[>
1625 variable to--but don't do that). If the substring is not found, returns
1626 one less than the base, ordinarily -1.
1632 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1634 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1636 Implements the ioctl(2) function. You'll probably have to say
1638 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1640 first to get the correct function definitions. If F<ioctl.ph> doesn't
1641 exist or doesn't have the correct definitions you'll have to roll your
1642 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1643 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1644 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1645 written depending on the FUNCTION--a pointer to the string value of SCALAR
1646 will be passed as the third argument of the actual ioctl call. (If SCALAR
1647 has no string value but does have a numeric value, that value will be
1648 passed rather than a pointer to the string value. To guarantee this to be
1649 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1650 functions are useful for manipulating the values of structures used by
1651 ioctl(). The following example sets the erase character to DEL.
1655 die "NO TIOCGETP" if $@ || !$getp;
1656 $sgttyb_t = "ccccs"; # 4 chars and a short
1657 if (ioctl(STDIN,$getp,$sgttyb)) {
1658 @ary = unpack($sgttyb_t,$sgttyb);
1660 $sgttyb = pack($sgttyb_t,@ary);
1661 ioctl(STDIN,&TIOCSETP,$sgttyb)
1662 || die "Can't ioctl: $!";
1665 The return value of ioctl (and fcntl) is as follows:
1667 if OS returns: then Perl returns:
1669 0 string "0 but true"
1670 anything else that number
1672 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1673 still easily determine the actual value returned by the operating
1676 ($retval = ioctl(...)) || ($retval = -1);
1677 printf "System returned %d\n", $retval;
1679 =item join EXPR,LIST
1681 Joins the separate strings of LIST into a single string with
1682 fields separated by the value of EXPR, and returns the string.
1685 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1687 See L<perlfunc/split>.
1691 Returns a normal array consisting of all the keys of the named hash. (In
1692 a scalar context, returns the number of keys.) The keys are returned in
1693 an apparently random order, but it is the same order as either the
1694 values() or each() function produces (given that the hash has not been
1695 modified). As a side effect, it resets HASH's iterator.
1697 Here is yet another way to print your environment:
1700 @values = values %ENV;
1701 while ($#keys >= 0) {
1702 print pop(@keys), '=', pop(@values), "\n";
1705 or how about sorted by key:
1707 foreach $key (sort(keys %ENV)) {
1708 print $key, '=', $ENV{$key}, "\n";
1711 To sort an array by value, you'll need to use a C<sort> function.
1712 Here's a descending numeric sort of a hash by its values:
1714 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1715 printf "%4d %s\n", $hash{$key}, $key;
1718 As an lvalue C<keys> allows you to increase the number of hash buckets
1719 allocated for the given hash. This can gain you a measure of efficiency if
1720 you know the hash is going to get big. (This is similar to pre-extending
1721 an array by assigning a larger number to $#array.) If you say
1725 then C<%hash> will have at least 200 buckets allocated for it. These
1726 buckets will be retained even if you do C<%hash = ()>, use C<undef
1727 %hash> if you want to free the storage while C<%hash> is still in scope.
1728 You can't shrink the number of buckets allocated for the hash using
1729 C<keys> in this way (but you needn't worry about doing this by accident,
1730 as trying has no effect).
1734 Sends a signal to a list of processes. The first element of
1735 the list must be the signal to send. Returns the number of
1736 processes successfully signaled.
1738 $cnt = kill 1, $child1, $child2;
1741 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1742 process groups instead of processes. (On System V, a negative I<PROCESS>
1743 number will also kill process groups, but that's not portable.) That
1744 means you usually want to use positive not negative signals. You may also
1745 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1751 The C<last> command is like the C<break> statement in C (as used in
1752 loops); it immediately exits the loop in question. If the LABEL is
1753 omitted, the command refers to the innermost enclosing loop. The
1754 C<continue> block, if any, is not executed:
1756 LINE: while (<STDIN>) {
1757 last LINE if /^$/; # exit when done with header
1765 Returns an lowercased version of EXPR. This is the internal function
1766 implementing the \L escape in double-quoted strings.
1767 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1769 If EXPR is omitted, uses $_.
1775 Returns the value of EXPR with the first character lowercased. This is
1776 the internal function implementing the \l escape in double-quoted strings.
1777 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1779 If EXPR is omitted, uses $_.
1785 Returns the length in characters of the value of EXPR. If EXPR is
1786 omitted, returns length of $_.
1788 =item link OLDFILE,NEWFILE
1790 Creates a new filename linked to the old filename. Returns 1 for
1791 success, 0 otherwise.
1793 =item listen SOCKET,QUEUESIZE
1795 Does the same thing that the listen system call does. Returns TRUE if
1796 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1800 A local modifies the listed variables to be local to the enclosing block,
1801 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1802 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1803 local()"> for details, including issues with tied arrays and hashes.
1805 But you really probably want to be using my() instead, because local() isn't
1806 what most people think of as "local"). See L<perlsub/"Private Variables
1807 via my()"> for details.
1809 =item localtime EXPR
1811 Converts a time as returned by the time function to a 9-element array
1812 with the time analyzed for the local time zone. Typically used as
1816 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1819 All array elements are numeric, and come straight out of a struct tm.
1820 In particular this means that $mon has the range 0..11 and $wday has
1821 the range 0..6 with sunday as day 0. Also, $year is the number of
1822 years since 1900, that is, $year is 123 in year 2023.
1824 If EXPR is omitted, uses the current time (C<localtime(time)>).
1826 In a scalar context, returns the ctime(3) value:
1828 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1830 This scalar value is B<not> locale dependent, see L<perllocale>,
1831 but instead a Perl builtin.
1832 Also see the Time::Local module, and the strftime(3) and mktime(3)
1833 function available via the POSIX module.
1839 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1842 =item lstat FILEHANDLE
1848 Does the same thing as the stat() function, but stats a symbolic link
1849 instead of the file the symbolic link points to. If symbolic links are
1850 unimplemented on your system, a normal stat() is done.
1852 If EXPR is omitted, stats $_.
1856 The match operator. See L<perlop>.
1858 =item map BLOCK LIST
1862 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1863 element) and returns the list value composed of the results of each such
1864 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1865 may produce zero, one, or more elements in the returned value.
1867 @chars = map(chr, @nums);
1869 translates a list of numbers to the corresponding characters. And
1871 %hash = map { getkey($_) => $_ } @array;
1873 is just a funny way to write
1876 foreach $_ (@array) {
1877 $hash{getkey($_)} = $_;
1880 Note that, because $_ is a reference into the list value, it can be used
1881 to modify the elements of the array. While this is useful and
1882 supported, it can cause bizarre results if the LIST is not a named
1883 array. See also L</grep> for an array composed of those items of the
1884 original list for which the BLOCK or EXPR evaluates to true.
1886 =item mkdir FILENAME,MODE
1888 Creates the directory specified by FILENAME, with permissions specified
1889 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1890 it returns 0 and sets C<$!> (errno).
1892 =item msgctl ID,CMD,ARG
1894 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1895 must be a variable which will hold the returned msqid_ds structure.
1896 Returns like ioctl: the undefined value for error, "0 but true" for
1897 zero, or the actual return value otherwise.
1899 =item msgget KEY,FLAGS
1901 Calls the System V IPC function msgget(2). Returns the message queue id,
1902 or the undefined value if there is an error.
1904 =item msgsnd ID,MSG,FLAGS
1906 Calls the System V IPC function msgsnd to send the message MSG to the
1907 message queue ID. MSG must begin with the long integer message type,
1908 which may be created with C<pack("l", $type)>. Returns TRUE if
1909 successful, or FALSE if there is an error.
1911 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1913 Calls the System V IPC function msgrcv to receive a message from
1914 message queue ID into variable VAR with a maximum message size of
1915 SIZE. Note that if a message is received, the message type will be the
1916 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1917 of the message type. Returns TRUE if successful, or FALSE if there is
1922 A "my" declares the listed variables to be local (lexically) to the
1923 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1924 more than one value is listed, the list must be placed in parentheses. See
1925 L<perlsub/"Private Variables via my()"> for details.
1931 The C<next> command is like the C<continue> statement in C; it starts
1932 the next iteration of the loop:
1934 LINE: while (<STDIN>) {
1935 next LINE if /^#/; # discard comments
1939 Note that if there were a C<continue> block on the above, it would get
1940 executed even on discarded lines. If the LABEL is omitted, the command
1941 refers to the innermost enclosing loop.
1943 =item no Module LIST
1945 See the "use" function, which "no" is the opposite of.
1951 Interprets EXPR as an octal string and returns the corresponding
1952 value. (If EXPR happens to start off with 0x, interprets it as
1953 a hex string instead.) The following will handle decimal, octal, and
1954 hex in the standard Perl or C notation:
1956 $val = oct($val) if $val =~ /^0/;
1958 If EXPR is omitted, uses $_. This function is commonly used when
1959 a string such as "644" needs to be converted into a file mode, for
1960 example. (Although perl will automatically convert strings into
1961 numbers as needed, this automatic conversion assumes base 10.)
1963 =item open FILEHANDLE,EXPR
1965 =item open FILEHANDLE
1967 Opens the file whose filename is given by EXPR, and associates it with
1968 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1969 name of the real filehandle wanted. If EXPR is omitted, the scalar
1970 variable of the same name as the FILEHANDLE contains the filename.
1971 (Note that lexical variables--those declared with C<my>--will not work
1972 for this purpose; so if you're using C<my>, specify EXPR in your call
1975 If the filename begins with '<' or nothing, the file is opened for input.
1976 If the filename begins with '>', the file is truncated and opened for
1977 output. If the filename begins with '>>', the file is opened for
1978 appending. You can put a '+' in front of the '>' or '<' to indicate that
1979 you want both read and write access to the file; thus '+<' is almost
1980 always preferred for read/write updates--the '+>' mode would clobber the
1981 file first. The prefix and the filename may be separated with spaces.
1982 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1983 'w+', 'a', and 'a+'.
1985 If the filename begins with "|", the filename is interpreted as a command
1986 to which output is to be piped, and if the filename ends with a "|", the
1987 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1988 examples of this. as command which pipes input to us. (You may not have
1989 a raw open() to a command that pipes both in I<and> out, but see
1990 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1993 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1994 nonzero upon success, the undefined value otherwise. If the open
1995 involved a pipe, the return value happens to be the pid of the
1998 If you're unfortunate enough to be running Perl on a system that
1999 distinguishes between text files and binary files (modern operating
2000 systems don't care), then you should check out L</binmode> for tips for
2001 dealing with this. The key distinction between systems that need binmode
2002 and those that don't is their text file formats. Systems like Unix and
2003 Plan9 that delimit lines with a single character, and that encode that
2004 character in C as '\n', do not need C<binmode>. The rest need it.
2006 When opening a file, it's usually a bad idea to continue normal execution
2007 if the request failed, so C<open> is frequently used in connection with
2008 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2009 where you want to make a nicely formatted error message (but there are
2010 modules which can help with that problem)) you should always check
2011 the return value from opening a file. The infrequent exception is when
2012 working with an unopened filehandle is actually what you want to do.
2017 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2018 while (<ARTICLE>) {...
2020 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2021 # if the open fails, output is discarded
2023 open(DBASE, '+<dbase.mine') # open for update
2024 or die "Can't open 'dbase.mine' for update: $!";
2026 open(ARTICLE, "caesar <$article |") # decrypt article
2027 or die "Can't start caesar: $!";
2029 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2030 or die "Can't start sort: $!";
2032 # process argument list of files along with any includes
2034 foreach $file (@ARGV) {
2035 process($file, 'fh00');
2039 local($filename, $input) = @_;
2040 $input++; # this is a string increment
2041 unless (open($input, $filename)) {
2042 print STDERR "Can't open $filename: $!\n";
2046 while (<$input>) { # note use of indirection
2047 if (/^#include "(.*)"/) {
2048 process($1, $input);
2055 You may also, in the Bourne shell tradition, specify an EXPR beginning
2056 with "E<gt>&", in which case the rest of the string is interpreted as the
2057 name of a filehandle (or file descriptor, if numeric) which is to be
2058 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
2059 +E<gt>E<gt>, and +E<lt>. The
2060 mode you specify should match the mode of the original filehandle.
2061 (Duping a filehandle does not take into account any existing contents of
2063 Here is a script that saves, redirects, and restores STDOUT and
2067 open(SAVEOUT, ">&STDOUT");
2068 open(SAVEERR, ">&STDERR");
2070 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2071 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2073 select(STDERR); $| = 1; # make unbuffered
2074 select(STDOUT); $| = 1; # make unbuffered
2076 print STDOUT "stdout 1\n"; # this works for
2077 print STDERR "stderr 1\n"; # subprocesses too
2082 open(STDOUT, ">&SAVEOUT");
2083 open(STDERR, ">&SAVEERR");
2085 print STDOUT "stdout 2\n";
2086 print STDERR "stderr 2\n";
2089 If you specify "E<lt>&=N", where N is a number, then Perl will do an
2090 equivalent of C's fdopen() of that file descriptor; this is more
2091 parsimonious of file descriptors. For example:
2093 open(FILEHANDLE, "<&=$fd")
2095 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
2096 there is an implicit fork done, and the return value of open is the pid
2097 of the child within the parent process, and 0 within the child
2098 process. (Use C<defined($pid)> to determine whether the open was successful.)
2099 The filehandle behaves normally for the parent, but i/o to that
2100 filehandle is piped from/to the STDOUT/STDIN of the child process.
2101 In the child process the filehandle isn't opened--i/o happens from/to
2102 the new STDOUT or STDIN. Typically this is used like the normal
2103 piped open when you want to exercise more control over just how the
2104 pipe command gets executed, such as when you are running setuid, and
2105 don't want to have to scan shell commands for metacharacters.
2106 The following pairs are more or less equivalent:
2108 open(FOO, "|tr '[a-z]' '[A-Z]'");
2109 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2111 open(FOO, "cat -n '$file'|");
2112 open(FOO, "-|") || exec 'cat', '-n', $file;
2114 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2116 NOTE: On any operation which may do a fork, unflushed buffers remain
2117 unflushed in both processes, which means you may need to set C<$|> to
2118 avoid duplicate output.
2120 Closing any piped filehandle causes the parent process to wait for the
2121 child to finish, and returns the status value in C<$?>.
2123 Using the constructor from the IO::Handle package (or one of its
2124 subclasses, such as IO::File or IO::Socket),
2125 you can generate anonymous filehandles which have the scope of whatever
2126 variables hold references to them, and automatically close whenever
2127 and however you leave that scope:
2131 sub read_myfile_munged {
2133 my $handle = new IO::File;
2134 open($handle, "myfile") or die "myfile: $!";
2136 or return (); # Automatically closed here.
2137 mung $first or die "mung failed"; # Or here.
2138 return $first, <$handle> if $ALL; # Or here.
2142 The filename that is passed to open will have leading and trailing
2143 whitespace deleted. To open a file with arbitrary weird
2144 characters in it, it's necessary to protect any leading and trailing
2147 $file =~ s#^(\s)#./$1#;
2148 open(FOO, "< $file\0");
2150 If you want a "real" C open() (see L<open(2)> on your system), then
2151 you should use the sysopen() function. This is another way to
2152 protect your filenames from interpretation. For example:
2155 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2156 or die "sysopen $path: $!";
2157 HANDLE->autoflush(1);
2158 HANDLE->print("stuff $$\n");
2160 print "File contains: ", <HANDLE>;
2162 See L</seek()> for some details about mixing reading and writing.
2164 =item opendir DIRHANDLE,EXPR
2166 Opens a directory named EXPR for processing by readdir(), telldir(),
2167 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2168 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2174 Returns the numeric ascii value of the first character of EXPR. If
2175 EXPR is omitted, uses $_. For the reverse, see L</chr>.
2177 =item pack TEMPLATE,LIST
2179 Takes an array or list of values and packs it into a binary structure,
2180 returning the string containing the structure. The TEMPLATE is a
2181 sequence of characters that give the order and type of values, as
2184 A An ascii string, will be space padded.
2185 a An ascii string, will be null padded.
2186 b A bit string (ascending bit order, like vec()).
2187 B A bit string (descending bit order).
2188 h A hex string (low nybble first).
2189 H A hex string (high nybble first).
2191 c A signed char value.
2192 C An unsigned char value.
2194 s A signed short value.
2195 S An unsigned short value.
2196 (This 'short' is _exactly_ 16 bits, which may differ from
2197 what a local C compiler calls 'short'.)
2199 i A signed integer value.
2200 I An unsigned integer value.
2201 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2202 depends on what a local C compiler calls 'int', and may
2203 even be larger than the 'long' described in the next item.)
2205 l A signed long value.
2206 L An unsigned long value.
2207 (This 'long' is _exactly_ 32 bits, which may differ from
2208 what a local C compiler calls 'long'.)
2210 n A short in "network" (big-endian) order.
2211 N A long in "network" (big-endian) order.
2212 v A short in "VAX" (little-endian) order.
2213 V A long in "VAX" (little-endian) order.
2214 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2215 _exactly_ 32 bits, respectively.)
2217 f A single-precision float in the native format.
2218 d A double-precision float in the native format.
2220 p A pointer to a null-terminated string.
2221 P A pointer to a structure (fixed-length string).
2223 u A uuencoded string.
2225 w A BER compressed integer. Its bytes represent an unsigned
2226 integer in base 128, most significant digit first, with as few
2227 digits as possible. Bit eight (the high bit) is set on each
2228 byte except the last.
2232 @ Null fill to absolute position.
2234 Each letter may optionally be followed by a number which gives a repeat
2235 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2236 pack function will gobble up that many values from the LIST. A * for the
2237 repeat count means to use however many items are left. The "a" and "A"
2238 types gobble just one value, but pack it as a string of length count,
2239 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2240 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2241 fields pack a string that many bits long. The "h" and "H" fields pack a
2242 string that many nybbles long. The "p" type packs a pointer to a null-
2243 terminated string. You are responsible for ensuring the string is not a
2244 temporary value (which can potentially get deallocated before you get
2245 around to using the packed result). The "P" packs a pointer to a structure
2246 of the size indicated by the length. A NULL pointer is created if the
2247 corresponding value for "p" or "P" is C<undef>.
2248 Real numbers (floats and doubles) are
2249 in the native machine format only; due to the multiplicity of floating
2250 formats around, and the lack of a standard "network" representation, no
2251 facility for interchange has been made. This means that packed floating
2252 point data written on one machine may not be readable on another - even if
2253 both use IEEE floating point arithmetic (as the endian-ness of the memory
2254 representation is not part of the IEEE spec). Note that Perl uses doubles
2255 internally for all numeric calculation, and converting from double into
2256 float and thence back to double again will lose precision (i.e.,
2257 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2261 $foo = pack("cccc",65,66,67,68);
2263 $foo = pack("c4",65,66,67,68);
2266 $foo = pack("ccxxcc",65,66,67,68);
2269 $foo = pack("s2",1,2);
2270 # "\1\0\2\0" on little-endian
2271 # "\0\1\0\2" on big-endian
2273 $foo = pack("a4","abcd","x","y","z");
2276 $foo = pack("aaaa","abcd","x","y","z");
2279 $foo = pack("a14","abcdefg");
2280 # "abcdefg\0\0\0\0\0\0\0"
2282 $foo = pack("i9pl", gmtime);
2283 # a real struct tm (on my system anyway)
2286 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2289 The same template may generally also be used in the unpack function.
2291 =item package NAMESPACE
2293 Declares the compilation unit as being in the given namespace. The scope
2294 of the package declaration is from the declaration itself through the end of
2295 the enclosing block (the same scope as the local() operator). All further
2296 unqualified dynamic identifiers will be in this namespace. A package
2297 statement affects only dynamic variables--including those you've used
2298 local() on--but I<not> lexical variables created with my(). Typically it
2299 would be the first declaration in a file to be included by the C<require>
2300 or C<use> operator. You can switch into a package in more than one place;
2301 it influences merely which symbol table is used by the compiler for the
2302 rest of that block. You can refer to variables and filehandles in other
2303 packages by prefixing the identifier with the package name and a double
2304 colon: C<$Package::Variable>. If the package name is null, the C<main>
2305 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2307 See L<perlmod/"Packages"> for more information about packages, modules,
2308 and classes. See L<perlsub> for other scoping issues.
2310 =item pipe READHANDLE,WRITEHANDLE
2312 Opens a pair of connected pipes like the corresponding system call.
2313 Note that if you set up a loop of piped processes, deadlock can occur
2314 unless you are very careful. In addition, note that Perl's pipes use
2315 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2316 after each command, depending on the application.
2318 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2319 for examples of such things.
2325 Pops and returns the last value of the array, shortening the array by
2326 1. Has a similar effect to
2328 $tmp = $ARRAY[$#ARRAY--];
2330 If there are no elements in the array, returns the undefined value.
2331 If ARRAY is omitted, pops the
2332 @ARGV array in the main program, and the @_ array in subroutines, just
2339 Returns the offset of where the last C<m//g> search left off for the variable
2340 is in question ($_ is used when the variable is not specified). May be
2341 modified to change that offset. Such modification will also influence
2342 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2345 =item print FILEHANDLE LIST
2351 Prints a string or a comma-separated list of strings. Returns TRUE
2352 if successful. FILEHANDLE may be a scalar variable name, in which case
2353 the variable contains the name of or a reference to the filehandle, thus introducing one
2354 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2355 token is a term, it may be misinterpreted as an operator unless you
2356 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2357 omitted, prints by default to standard output (or to the last selected
2358 output channel--see L</select>). If LIST is also omitted, prints $_ to
2359 STDOUT. To set the default output channel to something other than
2360 STDOUT use the select operation. Note that, because print takes a
2361 LIST, anything in the LIST is evaluated in a list context, and any
2362 subroutine that you call will have one or more of its expressions
2363 evaluated in a list context. Also be careful not to follow the print
2364 keyword with a left parenthesis unless you want the corresponding right
2365 parenthesis to terminate the arguments to the print--interpose a + or
2366 put parentheses around all the arguments.
2368 Note that if you're storing FILEHANDLES in an array or other expression,
2369 you will have to use a block returning its value instead:
2371 print { $files[$i] } "stuff\n";
2372 print { $OK ? STDOUT : STDERR } "stuff\n";
2374 =item printf FILEHANDLE FORMAT, LIST
2376 =item printf FORMAT, LIST
2378 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2379 of the list will be interpreted as the printf format. If C<use locale> is
2380 in effect, the character used for the decimal point in formatted real numbers
2381 is affected by the LC_NUMERIC locale. See L<perllocale>.
2383 Don't fall into the trap of using a printf() when a simple
2384 print() would do. The print() is more efficient, and less
2387 =item prototype FUNCTION
2389 Returns the prototype of a function as a string (or C<undef> if the
2390 function has no prototype). FUNCTION is a reference to, or the name of,
2391 the function whose prototype you want to retrieve.
2393 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2394 a name for Perl builtin. If builtin is not I<overridable> (such as
2395 C<qw>) or its arguments cannot be expressed by a prototype (such as
2396 C<system>) - in other words, the builtin does not behave like a Perl
2397 function - returns C<undef>. Otherwise, the string describing the
2398 equivalent prototype is returned.
2400 =item push ARRAY,LIST
2402 Treats ARRAY as a stack, and pushes the values of LIST
2403 onto the end of ARRAY. The length of ARRAY increases by the length of
2404 LIST. Has the same effect as
2407 $ARRAY[++$#ARRAY] = $value;
2410 but is more efficient. Returns the new number of elements in the array.
2420 Generalized quotes. See L<perlop>.
2422 =item quotemeta EXPR
2426 Returns the value of EXPR with all non-alphanumeric
2427 characters backslashed. (That is, all characters not matching
2428 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2429 returned string, regardless of any locale settings.)
2430 This is the internal function implementing
2431 the \Q escape in double-quoted strings.
2433 If EXPR is omitted, uses $_.
2439 Returns a random fractional number greater than or equal to 0 and less
2440 than the value of EXPR. (EXPR should be positive.) If EXPR is
2441 omitted, the value 1 is used. Automatically calls srand() unless
2442 srand() has already been called. See also srand().
2444 (Note: If your rand function consistently returns numbers that are too
2445 large or too small, then your version of Perl was probably compiled
2446 with the wrong number of RANDBITS.)
2448 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2450 =item read FILEHANDLE,SCALAR,LENGTH
2452 Attempts to read LENGTH bytes of data into variable SCALAR from the
2453 specified FILEHANDLE. Returns the number of bytes actually read, or
2454 undef if there was an error. SCALAR will be grown or shrunk to the
2455 length actually read. An OFFSET may be specified to place the read
2456 data at some other place than the beginning of the string. This call
2457 is actually implemented in terms of stdio's fread call. To get a true
2458 read system call, see sysread().
2460 =item readdir DIRHANDLE
2462 Returns the next directory entry for a directory opened by opendir().
2463 If used in a list context, returns all the rest of the entries in the
2464 directory. If there are no more entries, returns an undefined value in
2465 a scalar context or a null list in a list context.
2467 If you're planning to filetest the return values out of a readdir(), you'd
2468 better prepend the directory in question. Otherwise, because we didn't
2469 chdir() there, it would have been testing the wrong file.
2471 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2472 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2477 Reads from the file handle EXPR. In scalar context, a single line
2478 is read and returned. In list context, reads until end-of-file is
2479 reached and returns a list of lines (however you've defined lines
2480 with $/ or $INPUT_RECORD_SEPARATOR).
2481 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2482 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2483 operator is discussed in more detail in L<perlop/"I/O Operators">.
2489 Returns the value of a symbolic link, if symbolic links are
2490 implemented. If not, gives a fatal error. If there is some system
2491 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2496 EXPR is interpolated and then executed as a system command.
2497 The collected standard output of the command is returned.
2498 In scalar context, it comes back as a single (potentially
2499 multi-line) string. In list context, returns a list of lines
2500 (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2501 This is the internal function implementing the C<qx/EXPR/>
2502 operator, but you can use it directly. The C<qx/EXPR/>
2503 operator is discussed in more detail in L<perlop/"I/O Operators">.
2505 =item recv SOCKET,SCALAR,LEN,FLAGS
2507 Receives a message on a socket. Attempts to receive LENGTH bytes of
2508 data into variable SCALAR from the specified SOCKET filehandle.
2509 Actually does a C recvfrom(), so that it can returns the address of the
2510 sender. Returns the undefined value if there's an error. SCALAR will
2511 be grown or shrunk to the length actually read. Takes the same flags
2512 as the system call of the same name.
2513 See L<perlipc/"UDP: Message Passing"> for examples.
2519 The C<redo> command restarts the loop block without evaluating the
2520 conditional again. The C<continue> block, if any, is not executed. If
2521 the LABEL is omitted, the command refers to the innermost enclosing
2522 loop. This command is normally used by programs that want to lie to
2523 themselves about what was just input:
2525 # a simpleminded Pascal comment stripper
2526 # (warning: assumes no { or } in strings)
2527 LINE: while (<STDIN>) {
2528 while (s|({.*}.*){.*}|$1 |) {}
2533 if (/}/) { # end of comment?
2546 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2547 is not specified, $_ will be used. The value returned depends on the
2548 type of thing the reference is a reference to.
2549 Builtin types include:
2558 If the referenced object has been blessed into a package, then that package
2559 name is returned instead. You can think of ref() as a typeof() operator.
2561 if (ref($r) eq "HASH") {
2562 print "r is a reference to a hash.\n";
2565 print "r is not a reference at all.\n";
2568 See also L<perlref>.
2570 =item rename OLDNAME,NEWNAME
2572 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2573 not work across file system boundaries.
2579 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2580 supplied. If EXPR is numeric, demands that the current version of Perl
2581 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2583 Otherwise, demands that a library file be included if it hasn't already
2584 been included. The file is included via the do-FILE mechanism, which is
2585 essentially just a variety of eval(). Has semantics similar to the following
2589 local($filename) = @_;
2590 return 1 if $INC{$filename};
2591 local($realfilename,$result);
2593 foreach $prefix (@INC) {
2594 $realfilename = "$prefix/$filename";
2595 if (-f $realfilename) {
2596 $result = do $realfilename;
2600 die "Can't find $filename in \@INC";
2603 die "$filename did not return true value" unless $result;
2604 $INC{$filename} = $realfilename;
2608 Note that the file will not be included twice under the same specified
2609 name. The file must return TRUE as the last statement to indicate
2610 successful execution of any initialization code, so it's customary to
2611 end such a file with "1;" unless you're sure it'll return TRUE
2612 otherwise. But it's better just to put the "C<1;>", in case you add more
2615 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2616 replaces "F<::>" with "F</>" in the filename for you,
2617 to make it easy to load standard modules. This form of loading of
2618 modules does not risk altering your namespace.
2620 For a yet-more-powerful import facility, see L</use> and
2627 Generally used in a C<continue> block at the end of a loop to clear
2628 variables and reset ?? searches so that they work again. The
2629 expression is interpreted as a list of single characters (hyphens
2630 allowed for ranges). All variables and arrays beginning with one of
2631 those letters are reset to their pristine state. If the expression is
2632 omitted, one-match searches (?pattern?) are reset to match again. Resets
2633 only variables or searches in the current package. Always returns
2636 reset 'X'; # reset all X variables
2637 reset 'a-z'; # reset lower case variables
2638 reset; # just reset ?? searches
2640 Resetting "A-Z" is not recommended because you'll wipe out your
2641 ARGV and ENV arrays. Resets only package variables--lexical variables
2642 are unaffected, but they clean themselves up on scope exit anyway,
2643 so you'll probably want to use them instead. See L</my>.
2649 Returns from a subroutine, eval(), or do FILE with the value of the
2650 given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2651 context, depending on how the return value will be used, and the context
2652 may vary from one execution to the next (see wantarray()). If no EXPR
2653 is given, returns an empty list in a list context, an undefined value in
2654 a scalar context, or nothing in a void context.
2656 (Note that in the absence of a return, a subroutine, eval, or do FILE
2657 will automatically return the value of the last expression evaluated.)
2661 In a list context, returns a list value consisting of the elements
2662 of LIST in the opposite order. In a scalar context, concatenates the
2663 elements of LIST, and returns a string value consisting of those bytes,
2664 but in the opposite order.
2666 print reverse <>; # line tac, last line first
2668 undef $/; # for efficiency of <>
2669 print scalar reverse <>; # byte tac, last line tsrif
2671 This operator is also handy for inverting a hash, although there are some
2672 caveats. If a value is duplicated in the original hash, only one of those
2673 can be represented as a key in the inverted hash. Also, this has to
2674 unwind one hash and build a whole new one, which may take some time
2677 %by_name = reverse %by_address; # Invert the hash
2679 =item rewinddir DIRHANDLE
2681 Sets the current position to the beginning of the directory for the
2682 readdir() routine on DIRHANDLE.
2684 =item rindex STR,SUBSTR,POSITION
2686 =item rindex STR,SUBSTR
2688 Works just like index except that it returns the position of the LAST
2689 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2690 last occurrence at or before that position.
2692 =item rmdir FILENAME
2696 Deletes the directory specified by FILENAME if it is empty. If it
2697 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2698 FILENAME is omitted, uses $_.
2702 The substitution operator. See L<perlop>.
2706 Forces EXPR to be interpreted in a scalar context and returns the value
2709 @counts = ( scalar @a, scalar @b, scalar @c );
2711 There is no equivalent operator to force an expression to
2712 be interpolated in a list context because it's in practice never
2713 needed. If you really wanted to do so, however, you could use
2714 the construction C<@{[ (some expression) ]}>, but usually a simple
2715 C<(some expression)> suffices.
2717 =item seek FILEHANDLE,POSITION,WHENCE
2719 Sets FILEHANDLE's position, just like the fseek() call of stdio.
2720 FILEHANDLE may be an expression whose value gives the name of the
2721 filehandle. The values for WHENCE are 0 to set the new position to
2722 POSITION, 1 to set it to the current position plus POSITION, and 2 to
2723 set it to EOF plus POSITION (typically negative). For WHENCE you may
2724 use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2725 IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2727 If you want to position file for sysread() or syswrite(), don't use
2728 seek() -- buffering makes its effect on the file's system position
2729 unpredictable and non-portable. Use sysseek() instead.
2731 On some systems you have to do a seek whenever you switch between reading
2732 and writing. Amongst other things, this may have the effect of calling
2733 stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2738 This is also useful for applications emulating C<tail -f>. Once you hit
2739 EOF on your read, and then sleep for a while, you might have to stick in a
2740 seek() to reset things. The seek() doesn't change the current position,
2741 but it I<does> clear the end-of-file condition on the handle, so that the
2742 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2744 If that doesn't work (some stdios are particularly cantankerous), then
2745 you may need something more like this:
2748 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2749 # search for some stuff and put it into files
2751 sleep($for_a_while);
2752 seek(FILE, $curpos, 0);
2755 =item seekdir DIRHANDLE,POS
2757 Sets the current position for the readdir() routine on DIRHANDLE. POS
2758 must be a value returned by telldir(). Has the same caveats about
2759 possible directory compaction as the corresponding system library
2762 =item select FILEHANDLE
2766 Returns the currently selected filehandle. Sets the current default
2767 filehandle for output, if FILEHANDLE is supplied. This has two
2768 effects: first, a C<write> or a C<print> without a filehandle will
2769 default to this FILEHANDLE. Second, references to variables related to
2770 output will refer to this output channel. For example, if you have to
2771 set the top of form format for more than one output channel, you might
2779 FILEHANDLE may be an expression whose value gives the name of the
2780 actual filehandle. Thus:
2782 $oldfh = select(STDERR); $| = 1; select($oldfh);
2784 Some programmers may prefer to think of filehandles as objects with
2785 methods, preferring to write the last example as:
2788 STDERR->autoflush(1);
2790 =item select RBITS,WBITS,EBITS,TIMEOUT
2792 This calls the select(2) system call with the bit masks specified, which
2793 can be constructed using fileno() and vec(), along these lines:
2795 $rin = $win = $ein = '';
2796 vec($rin,fileno(STDIN),1) = 1;
2797 vec($win,fileno(STDOUT),1) = 1;
2800 If you want to select on many filehandles you might wish to write a
2804 local(@fhlist) = split(' ',$_[0]);
2807 vec($bits,fileno($_),1) = 1;
2811 $rin = fhbits('STDIN TTY SOCK');
2815 ($nfound,$timeleft) =
2816 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2818 or to block until something becomes ready just do this
2820 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2822 Most systems do not bother to return anything useful in $timeleft, so
2823 calling select() in a scalar context just returns $nfound.
2825 Any of the bit masks can also be undef. The timeout, if specified, is
2826 in seconds, which may be fractional. Note: not all implementations are
2827 capable of returning the $timeleft. If not, they always return
2828 $timeleft equal to the supplied $timeout.
2830 You can effect a sleep of 250 milliseconds this way:
2832 select(undef, undef, undef, 0.25);
2834 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2835 with select(). You have to use sysread() instead.
2837 =item semctl ID,SEMNUM,CMD,ARG
2839 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2840 &GETALL, then ARG must be a variable which will hold the returned
2841 semid_ds structure or semaphore value array. Returns like ioctl: the
2842 undefined value for error, "0 but true" for zero, or the actual return
2845 =item semget KEY,NSEMS,FLAGS
2847 Calls the System V IPC function semget. Returns the semaphore id, or
2848 the undefined value if there is an error.
2850 =item semop KEY,OPSTRING
2852 Calls the System V IPC function semop to perform semaphore operations
2853 such as signaling and waiting. OPSTRING must be a packed array of
2854 semop structures. Each semop structure can be generated with
2855 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2856 operations is implied by the length of OPSTRING. Returns TRUE if
2857 successful, or FALSE if there is an error. As an example, the
2858 following code waits on semaphore $semnum of semaphore id $semid:
2860 $semop = pack("sss", $semnum, -1, 0);
2861 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2863 To signal the semaphore, replace "-1" with "1".
2865 =item send SOCKET,MSG,FLAGS,TO
2867 =item send SOCKET,MSG,FLAGS
2869 Sends a message on a socket. Takes the same flags as the system call
2870 of the same name. On unconnected sockets you must specify a
2871 destination to send TO, in which case it does a C sendto(). Returns
2872 the number of characters sent, or the undefined value if there is an
2874 See L<perlipc/"UDP: Message Passing"> for examples.
2876 =item setpgrp PID,PGRP
2878 Sets the current process group for the specified PID, 0 for the current
2879 process. Will produce a fatal error if used on a machine that doesn't
2880 implement setpgrp(2). If the arguments are omitted, it defaults to
2881 0,0. Note that the POSIX version of setpgrp() does not accept any
2882 arguments, so only setpgrp 0,0 is portable.
2884 =item setpriority WHICH,WHO,PRIORITY
2886 Sets the current priority for a process, a process group, or a user.
2887 (See setpriority(2).) Will produce a fatal error if used on a machine
2888 that doesn't implement setpriority(2).
2890 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2892 Sets the socket option requested. Returns undefined if there is an
2893 error. OPTVAL may be specified as undef if you don't want to pass an
2900 Shifts the first value of the array off and returns it, shortening the
2901 array by 1 and moving everything down. If there are no elements in the
2902 array, returns the undefined value. If ARRAY is omitted, shifts the
2903 @_ array within the lexical scope of subroutines and formats, and the
2904 @ARGV array at file scopes or within the lexical scopes established by
2905 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2906 See also unshift(), push(), and pop(). Shift() and unshift() do the
2907 same thing to the left end of an array that pop() and push() do to the
2910 =item shmctl ID,CMD,ARG
2912 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2913 must be a variable which will hold the returned shmid_ds structure.
2914 Returns like ioctl: the undefined value for error, "0 but true" for
2915 zero, or the actual return value otherwise.
2917 =item shmget KEY,SIZE,FLAGS
2919 Calls the System V IPC function shmget. Returns the shared memory
2920 segment id, or the undefined value if there is an error.
2922 =item shmread ID,VAR,POS,SIZE
2924 =item shmwrite ID,STRING,POS,SIZE
2926 Reads or writes the System V shared memory segment ID starting at
2927 position POS for size SIZE by attaching to it, copying in/out, and
2928 detaching from it. When reading, VAR must be a variable which will
2929 hold the data read. When writing, if STRING is too long, only SIZE
2930 bytes are used; if STRING is too short, nulls are written to fill out
2931 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2933 =item shutdown SOCKET,HOW
2935 Shuts down a socket connection in the manner indicated by HOW, which
2936 has the same interpretation as in the system call of the same name.
2942 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2945 For the inverse sine operation, you may use the POSIX::asin()
2946 function, or use this relation:
2948 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2954 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2955 May be interrupted by sending the process a SIGALRM. Returns the
2956 number of seconds actually slept. You probably cannot mix alarm() and
2957 sleep() calls, because sleep() is often implemented using alarm().
2959 On some older systems, it may sleep up to a full second less than what
2960 you requested, depending on how it counts seconds. Most modern systems
2961 always sleep the full amount.
2963 For delays of finer granularity than one second, you may use Perl's
2964 syscall() interface to access setitimer(2) if your system supports it,
2965 or else see L</select()> below.
2967 See also the POSIX module's sigpause() function.
2969 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2971 Opens a socket of the specified kind and attaches it to filehandle
2972 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2973 system call of the same name. You should "use Socket;" first to get
2974 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2976 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2978 Creates an unnamed pair of sockets in the specified domain, of the
2979 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2980 for the system call of the same name. If unimplemented, yields a fatal
2981 error. Returns TRUE if successful.
2983 =item sort SUBNAME LIST
2985 =item sort BLOCK LIST
2989 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2990 is omitted, sorts in standard string comparison order. If SUBNAME is
2991 specified, it gives the name of a subroutine that returns an integer
2992 less than, equal to, or greater than 0, depending on how the elements
2993 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2994 operators are extremely useful in such routines.) SUBNAME may be a
2995 scalar variable name, in which case the value provides the name of the
2996 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2997 an anonymous, in-line sort subroutine.
2999 In the interests of efficiency the normal calling code for subroutines is
3000 bypassed, with the following effects: the subroutine may not be a
3001 recursive subroutine, and the two elements to be compared are passed into
3002 the subroutine not via @_ but as the package global variables $a and
3003 $b (see example below). They are passed by reference, so don't
3004 modify $a and $b. And don't try to declare them as lexicals either.
3006 You also cannot exit out of the sort block or subroutine using any of the
3007 loop control operators described in L<perlsyn> or with goto().
3009 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3010 current collation locale. See L<perllocale>.
3015 @articles = sort @files;
3017 # same thing, but with explicit sort routine
3018 @articles = sort {$a cmp $b} @files;
3020 # now case-insensitively
3021 @articles = sort {uc($a) cmp uc($b)} @files;
3023 # same thing in reversed order
3024 @articles = sort {$b cmp $a} @files;
3026 # sort numerically ascending
3027 @articles = sort {$a <=> $b} @files;
3029 # sort numerically descending
3030 @articles = sort {$b <=> $a} @files;
3032 # sort using explicit subroutine name
3034 $age{$a} <=> $age{$b}; # presuming numeric
3036 @sortedclass = sort byage @class;
3038 # this sorts the %age hash by value instead of key
3039 # using an in-line function
3040 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3042 sub backwards { $b cmp $a; }
3043 @harry = ('dog','cat','x','Cain','Abel');
3044 @george = ('gone','chased','yz','Punished','Axed');
3046 # prints AbelCaincatdogx
3047 print sort backwards @harry;
3048 # prints xdogcatCainAbel
3049 print sort @george, 'to', @harry;
3050 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3052 # inefficiently sort by descending numeric compare using
3053 # the first integer after the first = sign, or the
3054 # whole record case-insensitively otherwise
3057 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3062 # same thing, but much more efficiently;
3063 # we'll build auxiliary indices instead
3067 push @nums, /=(\d+)/;
3072 $nums[$b] <=> $nums[$a]
3074 $caps[$a] cmp $caps[$b]
3078 # same thing using a Schwartzian Transform (no temps)
3079 @new = map { $_->[0] }
3080 sort { $b->[1] <=> $a->[1]
3083 } map { [$_, /=(\d+)/, uc($_)] } @old;
3085 If you're using strict, you I<MUST NOT> declare $a
3086 and $b as lexicals. They are package globals. That means
3087 if you're in the C<main> package, it's
3089 @articles = sort {$main::b <=> $main::a} @files;
3093 @articles = sort {$::b <=> $::a} @files;
3095 but if you're in the C<FooPack> package, it's
3097 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3099 The comparison function is required to behave. If it returns
3100 inconsistent results (sometimes saying $x[1] is less than $x[2] and
3101 sometimes saying the opposite, for example) the Perl interpreter will
3102 probably crash and dump core. This is entirely due to and dependent
3103 upon your system's qsort(3) library routine; this routine often avoids
3104 sanity checks in the interest of speed.
3106 =item splice ARRAY,OFFSET,LENGTH,LIST
3108 =item splice ARRAY,OFFSET,LENGTH
3110 =item splice ARRAY,OFFSET
3112 Removes the elements designated by OFFSET and LENGTH from an array, and
3113 replaces them with the elements of LIST, if any. Returns the elements
3114 removed from the array. The array grows or shrinks as necessary. If
3115 LENGTH is omitted, removes everything from OFFSET onward. The
3116 following equivalences hold (assuming C<$[ == 0>):
3118 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3119 pop(@a) splice(@a,-1)
3120 shift(@a) splice(@a,0,1)
3121 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3122 $a[$x] = $y splice(@a,$x,1,$y);
3124 Example, assuming array lengths are passed before arrays:
3126 sub aeq { # compare two list values
3127 local(@a) = splice(@_,0,shift);
3128 local(@b) = splice(@_,0,shift);
3129 return 0 unless @a == @b; # same len?
3131 return 0 if pop(@a) ne pop(@b);
3135 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3137 =item split /PATTERN/,EXPR,LIMIT
3139 =item split /PATTERN/,EXPR
3141 =item split /PATTERN/
3145 Splits a string into an array of strings, and returns it.
3147 If not in a list context, returns the number of fields found and splits into
3148 the @_ array. (In a list context, you can force the split into @_ by
3149 using C<??> as the pattern delimiters, but it still returns the array
3150 value.) The use of implicit split to @_ is deprecated, however.
3152 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
3153 splits on whitespace (after skipping any leading whitespace). Anything
3154 matching PATTERN is taken to be a delimiter separating the fields. (Note
3155 that the delimiter may be longer than one character.)
3157 If LIMIT is specified and is not negative, splits into no more than
3158 that many fields (though it may split into fewer). If LIMIT is
3159 unspecified, trailing null fields are stripped (which potential users
3160 of pop() would do well to remember). If LIMIT is negative, it is
3161 treated as if an arbitrarily large LIMIT had been specified.
3163 A pattern matching the null string (not to be confused with
3164 a null pattern C<//>, which is just one member of the set of patterns
3165 matching a null string) will split the value of EXPR into separate
3166 characters at each point it matches that way. For example:
3168 print join(':', split(/ */, 'hi there'));
3170 produces the output 'h:i:t:h:e:r:e'.
3172 The LIMIT parameter can be used to split a line partially
3174 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3176 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3177 one larger than the number of variables in the list, to avoid
3178 unnecessary work. For the list above LIMIT would have been 4 by
3179 default. In time critical applications it behooves you not to split
3180 into more fields than you really need.
3182 If the PATTERN contains parentheses, additional array elements are
3183 created from each matching substring in the delimiter.
3185 split(/([,-])/, "1-10,20", 3);
3187 produces the list value
3189 (1, '-', 10, ',', 20)
3191 If you had the entire header of a normal Unix email message in $header,
3192 you could split it up into fields and their values this way:
3194 $header =~ s/\n\s+/ /g; # fix continuation lines
3195 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3197 The pattern C</PATTERN/> may be replaced with an expression to specify
3198 patterns that vary at runtime. (To do runtime compilation only once,
3199 use C</$variable/o>.)
3201 As a special case, specifying a PATTERN of space (C<' '>) will split on
3202 white space just as split with no arguments does. Thus, split(' ') can
3203 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3204 will give you as many null initial fields as there are leading spaces.
3205 A split on /\s+/ is like a split(' ') except that any leading
3206 whitespace produces a null first field. A split with no arguments
3207 really does a C<split(' ', $_)> internally.
3211 open(passwd, '/etc/passwd');
3213 ($login, $passwd, $uid, $gid, $gcos,
3214 $home, $shell) = split(/:/);
3218 (Note that $shell above will still have a newline on it. See L</chop>,
3219 L</chomp>, and L</join>.)
3221 =item sprintf FORMAT, LIST
3223 Returns a string formatted by the usual printf conventions of the
3224 C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3225 on your system for an explanation of the general principles.
3227 Perl does all of its own sprintf() formatting -- it emulates the C
3228 function sprintf(), but it doesn't use it (except for floating-point
3229 numbers, and even then only the standard modifiers are allowed). As a
3230 result, any non-standard extensions in your local sprintf() are not
3231 available from Perl.
3233 Perl's sprintf() permits the following universally-known conversions:
3236 %c a character with the given number
3238 %d a signed integer, in decimal
3239 %u an unsigned integer, in decimal
3240 %o an unsigned integer, in octal
3241 %x an unsigned integer, in hexadecimal
3242 %e a floating-point number, in scientific notation
3243 %f a floating-point number, in fixed decimal notation
3244 %g a floating-point number, in %e or %f notation
3246 In addition, Perl permits the following widely-supported conversions:
3248 %X like %x, but using upper-case letters
3249 %E like %e, but using an upper-case "E"
3250 %G like %g, but with an upper-case "E" (if applicable)
3251 %p a pointer (outputs the Perl value's address in hexadecimal)
3252 %n special: *stores* the number of characters output so far
3253 into the next variable in the parameter list
3255 Finally, for backward (and we do mean "backward") compatibility, Perl
3256 permits these unnecessary but widely-supported conversions:
3259 %D a synonym for %ld
3260 %U a synonym for %lu
3261 %O a synonym for %lo
3264 Perl permits the following universally-known flags between the C<%>
3265 and the conversion letter:
3267 space prefix positive number with a space
3268 + prefix positive number with a plus sign
3269 - left-justify within the field
3270 0 use zeros, not spaces, to right-justify
3271 # prefix octal with "0", hex with "0x"
3272 number minimum field width
3273 .number "precision": digits after decimal point for floating-point,
3274 max length for string, minimum length for integer
3275 l interpret integer as C type "long" or "unsigned long"
3276 h interpret integer as C type "short" or "unsigned short"
3278 There is also one Perl-specific flag:
3280 V interpret integer as Perl's standard integer type
3282 Where a number would appear in the flags, an asterisk ("*") may be
3283 used instead, in which case Perl uses the next item in the parameter
3284 list as the given number (that is, as the field width or precision).
3285 If a field width obtained through "*" is negative, it has the same
3286 effect as the '-' flag: left-justification.
3288 If C<use locale> is in effect, the character used for the decimal
3289 point in formatted real numbers is affected by the LC_NUMERIC locale.
3296 Return the square root of EXPR. If EXPR is omitted, returns square
3303 Sets the random number seed for the C<rand> operator. If EXPR is
3304 omitted, uses a semi-random value based on the current time and process
3305 ID, among other things. In versions of Perl prior to 5.004 the default
3306 seed was just the current time(). This isn't a particularly good seed,
3307 so many old programs supply their own seed value (often C<time ^ $$> or
3308 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3310 In fact, it's usually not necessary to call srand() at all, because if
3311 it is not called explicitly, it is called implicitly at the first use of
3312 the C<rand> operator. However, this was not the case in version of Perl
3313 before 5.004, so if your script will run under older Perl versions, it
3314 should call srand().
3316 Note that you need something much more random than the default seed for
3317 cryptographic purposes. Checksumming the compressed output of one or more
3318 rapidly changing operating system status programs is the usual method. For
3321 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3323 If you're particularly concerned with this, see the Math::TrulyRandom
3326 Do I<not> call srand() multiple times in your program unless you know
3327 exactly what you're doing and why you're doing it. The point of the
3328 function is to "seed" the rand() function so that rand() can produce
3329 a different sequence each time you run your program. Just do it once at the
3330 top of your program, or you I<won't> get random numbers out of rand()!
3332 Frequently called programs (like CGI scripts) that simply use
3336 for a seed can fall prey to the mathematical property that
3340 one-third of the time. So don't do that.
3342 =item stat FILEHANDLE
3348 Returns a 13-element array giving the status info for a file, either the
3349 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3350 stats $_. Returns a null list if the stat fails. Typically used as
3354 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3355 $atime,$mtime,$ctime,$blksize,$blocks)
3358 Not all fields are supported on all filesystem types. Here are the
3359 meaning of the fields:
3361 0 dev device number of filesystem
3363 2 mode file mode (type and permissions)
3364 3 nlink number of (hard) links to the file
3365 4 uid numeric user ID of file's owner
3366 5 gid numeric group ID of file's owner
3367 6 rdev the device identifier (special files only)
3368 7 size total size of file, in bytes
3369 8 atime last access time since the epoch
3370 9 mtime last modify time since the epoch
3371 10 ctime inode change time (NOT creation time!) since the epoch
3372 11 blksize preferred block size for file system I/O
3373 12 blocks actual number of blocks allocated
3375 (The epoch was at 00:00 January 1, 1970 GMT.)
3377 If stat is passed the special filehandle consisting of an underline, no
3378 stat is done, but the current contents of the stat structure from the
3379 last stat or filetest are returned. Example:
3381 if (-x $file && (($d) = stat(_)) && $d < 0) {
3382 print "$file is executable NFS file\n";
3385 (This works on machines only for which the device number is negative under NFS.)
3391 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3392 doing many pattern matches on the string before it is next modified.
3393 This may or may not save time, depending on the nature and number of
3394 patterns you are searching on, and on the distribution of character
3395 frequencies in the string to be searched -- you probably want to compare
3396 run times with and without it to see which runs faster. Those loops
3397 which scan for many short constant strings (including the constant
3398 parts of more complex patterns) will benefit most. You may have only
3399 one study active at a time -- if you study a different scalar the first
3400 is "unstudied". (The way study works is this: a linked list of every
3401 character in the string to be searched is made, so we know, for
3402 example, where all the 'k' characters are. From each search string,
3403 the rarest character is selected, based on some static frequency tables
3404 constructed from some C programs and English text. Only those places
3405 that contain this "rarest" character are examined.)
3407 For example, here is a loop which inserts index producing entries
3408 before any line containing a certain pattern:
3412 print ".IX foo\n" if /\bfoo\b/;
3413 print ".IX bar\n" if /\bbar\b/;
3414 print ".IX blurfl\n" if /\bblurfl\b/;
3419 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3420 will be looked at, because "f" is rarer than "o". In general, this is
3421 a big win except in pathological cases. The only question is whether
3422 it saves you more time than it took to build the linked list in the
3425 Note that if you have to look for strings that you don't know till
3426 runtime, you can build an entire loop as a string and eval that to
3427 avoid recompiling all your patterns all the time. Together with
3428 undefining $/ to input entire files as one record, this can be very
3429 fast, often faster than specialized programs like fgrep(1). The following
3430 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3431 out the names of those files that contain a match:
3433 $search = 'while (<>) { study;';
3434 foreach $word (@words) {
3435 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3440 eval $search; # this screams
3441 $/ = "\n"; # put back to normal input delimiter
3442 foreach $file (sort keys(%seen)) {
3450 =item sub NAME BLOCK
3452 This is subroutine definition, not a real function I<per se>. With just a
3453 NAME (and possibly prototypes), it's just a forward declaration. Without
3454 a NAME, it's an anonymous function declaration, and does actually return a
3455 value: the CODE ref of the closure you just created. See L<perlsub> and
3456 L<perlref> for details.
3458 =item substr EXPR,OFFSET,LEN
3460 =item substr EXPR,OFFSET
3462 Extracts a substring out of EXPR and returns it. First character is at
3463 offset 0, or whatever you've set C<$[> to (but don't do that).
3464 If OFFSET is negative (or more precisely, less than C<$[>), starts
3465 that far from the end of the string. If LEN is omitted, returns
3466 everything to the end of the string. If LEN is negative, leaves that
3467 many characters off the end of the string.
3469 If you specify a substring which is partly outside the string, the part
3470 within the string is returned. If the substring is totally outside
3471 the string a warning is produced.
3473 You can use the substr() function
3474 as an lvalue, in which case EXPR must be an lvalue. If you assign
3475 something shorter than LEN, the string will shrink, and if you assign
3476 something longer than LEN, the string will grow to accommodate it. To
3477 keep the string the same length you may need to pad or chop your value
3480 =item symlink OLDFILE,NEWFILE
3482 Creates a new filename symbolically linked to the old filename.
3483 Returns 1 for success, 0 otherwise. On systems that don't support
3484 symbolic links, produces a fatal error at run time. To check for that,
3487 $symlink_exists = (eval {symlink("","")};, $@ eq '');
3491 Calls the system call specified as the first element of the list,
3492 passing the remaining elements as arguments to the system call. If
3493 unimplemented, produces a fatal error. The arguments are interpreted
3494 as follows: if a given argument is numeric, the argument is passed as
3495 an int. If not, the pointer to the string value is passed. You are
3496 responsible to make sure a string is pre-extended long enough to
3497 receive any result that might be written into a string. If your
3498 integer arguments are not literals and have never been interpreted in a
3499 numeric context, you may need to add 0 to them to force them to look
3502 require 'syscall.ph'; # may need to run h2ph
3503 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3505 Note that Perl supports passing of up to only 14 arguments to your system call,
3506 which in practice should usually suffice.
3508 Syscall returns whatever value returned by the system call it calls.
3509 If the system call fails, syscall returns -1 and sets C<$!> (errno).
3510 Note that some system calls can legitimately return -1. The proper
3511 way to handle such calls is to assign C<$!=0;> before the call and
3512 check the value of <$!> if syscall returns -1.
3514 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3515 number of the read end of the pipe it creates. There is no way
3516 to retrieve the file number of the other end. You can avoid this
3517 problem by using C<pipe> instead.
3519 =item sysopen FILEHANDLE,FILENAME,MODE
3521 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3523 Opens the file whose filename is given by FILENAME, and associates it
3524 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3525 the name of the real filehandle wanted. This function calls the
3526 underlying operating system's C<open> function with the parameters
3527 FILENAME, MODE, PERMS.
3529 The possible values and flag bits of the MODE parameter are
3530 system-dependent; they are available via the standard module C<Fcntl>.
3531 However, for historical reasons, some values are universal: zero means
3532 read-only, one means write-only, and two means read/write.
3534 If the file named by FILENAME does not exist and the C<open> call
3535 creates it (typically because MODE includes the O_CREAT flag), then
3536 the value of PERMS specifies the permissions of the newly created
3537 file. If PERMS is omitted, the default value is 0666, which allows
3538 read and write for all. This default is reasonable: see C<umask>.
3540 The IO::File module provides a more object-oriented approach, if you're
3541 into that kind of thing.
3543 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3545 =item sysread FILEHANDLE,SCALAR,LENGTH
3547 Attempts to read LENGTH bytes of data into variable SCALAR from the
3548 specified FILEHANDLE, using the system call read(2). It bypasses
3549 stdio, so mixing this with other kinds of reads, print(), write(),
3550 seek(), or tell() can cause confusion because stdio usually buffers
3551 data. Returns the number of bytes actually read, or undef if there
3552 was an error. SCALAR will be grown or shrunk so that the last byte
3553 actually read is the last byte of the scalar after the read.
3555 An OFFSET may be specified to place the read data at some place in the
3556 string other than the beginning. A negative OFFSET specifies
3557 placement at that many bytes counting backwards from the end of the
3558 string. A positive OFFSET greater than the length of SCALAR results
3559 in the string being padded to the required size with "\0" bytes before
3560 the result of the read is appended.
3562 =item sysseek FILEHANDLE,POSITION,WHENCE
3564 Sets FILEHANDLE's system position using the system call lseek(2). It
3565 bypasses stdio, so mixing this with reads (other than sysread()),
3566 print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3567 be an expression whose value gives the name of the filehandle. The
3568 values for WHENCE are 0 to set the new position to POSITION, 1 to set
3569 the it to the current position plus POSITION, and 2 to set it to EOF
3570 plus POSITION (typically negative). For WHENCE, you may use the
3571 constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3572 or the POSIX module.
3574 Returns the new position, or the undefined value on failure. A position
3575 of zero is returned as the string "0 but true"; thus sysseek() returns
3576 TRUE on success and FALSE on failure, yet you can still easily determine
3581 Does exactly the same thing as "exec LIST" except that a fork is done
3582 first, and the parent process waits for the child process to complete.
3583 Note that argument processing varies depending on the number of
3584 arguments. The return value is the exit status of the program as
3585 returned by the wait() call. To get the actual exit value divide by
3586 256. See also L</exec>. This is I<NOT> what you want to use to capture
3587 the output from a command, for that you should use merely backticks or
3588 qx//, as described in L<perlop/"`STRING`">.
3590 Because system() and backticks block SIGINT and SIGQUIT, killing the
3591 program they're running doesn't actually interrupt your program.
3593 @args = ("command", "arg1", "arg2");
3595 or die "system @args failed: $?"
3597 Here's a more elaborate example of analysing the return value from
3598 system() on a Unix system to check for all possibilities, including for
3599 signals and core dumps.
3601 $rc = 0xffff & system @args;
3602 printf "system(%s) returned %#04x: ", "@args", $rc;
3604 print "ran with normal exit\n";
3606 elsif ($rc == 0xff00) {
3607 print "command failed: $!\n";
3609 elsif ($rc > 0x80) {
3611 print "ran with non-zero exit status $rc\n";
3617 print "core dump from ";
3619 print "signal $rc\n"
3623 When the arguments get executed via the system shell, results will
3624 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
3627 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3629 =item syswrite FILEHANDLE,SCALAR,LENGTH
3631 Attempts to write LENGTH bytes of data from variable SCALAR to the
3632 specified FILEHANDLE, using the system call write(2). It bypasses
3633 stdio, so mixing this with reads (other than sysread()), print(),
3634 write(), seek(), or tell() may cause confusion because stdio usually
3635 buffers data. Returns the number of bytes actually written, or undef
3636 if there was an error. If the LENGTH is greater than the available
3637 data in the SCALAR after the OFFSET, only as much data as is available
3640 An OFFSET may be specified to write the data from some part of the
3641 string other than the beginning. A negative OFFSET specifies writing
3642 that many bytes counting backwards from the end of the string. In the
3643 case the SCALAR is empty you can use OFFSET but only zero offset.
3645 =item tell FILEHANDLE
3649 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3650 expression whose value gives the name of the actual filehandle. If
3651 FILEHANDLE is omitted, assumes the file last read.
3653 =item telldir DIRHANDLE
3655 Returns the current position of the readdir() routines on DIRHANDLE.
3656 Value may be given to seekdir() to access a particular location in a
3657 directory. Has the same caveats about possible directory compaction as
3658 the corresponding system library routine.
3660 =item tie VARIABLE,CLASSNAME,LIST
3662 This function binds a variable to a package class that will provide the
3663 implementation for the variable. VARIABLE is the name of the variable
3664 to be enchanted. CLASSNAME is the name of a class implementing objects
3665 of correct type. Any additional arguments are passed to the "new"
3666 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3667 Typically these are arguments such as might be passed to the dbm_open()
3668 function of C. The object returned by the "new" method is also
3669 returned by the tie() function, which would be useful if you want to
3670 access other methods in CLASSNAME.
3672 Note that functions such as keys() and values() may return huge array
3673 values when used on large objects, like DBM files. You may prefer to
3674 use the each() function to iterate over such. Example:
3676 # print out history file offsets
3678 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3679 while (($key,$val) = each %HIST) {
3680 print $key, ' = ', unpack('L',$val), "\n";
3684 A class implementing a hash should have the following methods:
3686 TIEHASH classname, LIST
3689 STORE this, key, value
3693 NEXTKEY this, lastkey
3695 A class implementing an ordinary array should have the following methods:
3697 TIEARRAY classname, LIST
3700 STORE this, key, value
3703 A class implementing a scalar should have the following methods:
3705 TIESCALAR classname, LIST
3710 Unlike dbmopen(), the tie() function will not use or require a module
3711 for you--you need to do that explicitly yourself. See L<DB_File>
3712 or the F<Config> module for interesting tie() implementations.
3714 For further details see L<perltie>, L<tied VARIABLE>.
3718 Returns a reference to the object underlying VARIABLE (the same value
3719 that was originally returned by the tie() call which bound the variable
3720 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3725 Returns the number of non-leap seconds since whatever time the system
3726 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3727 and 00:00:00 UTC, January 1, 1970 for most other systems).
3728 Suitable for feeding to gmtime() and localtime().
3732 Returns a four-element array giving the user and system times, in
3733 seconds, for this process and the children of this process.
3735 ($user,$system,$cuser,$csystem) = times;
3739 The transliteration operator. Same as y///. See L<perlop>.
3741 =item truncate FILEHANDLE,LENGTH
3743 =item truncate EXPR,LENGTH
3745 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3746 specified length. Produces a fatal error if truncate isn't implemented
3753 Returns an uppercased version of EXPR. This is the internal function
3754 implementing the \U escape in double-quoted strings.
3755 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3757 If EXPR is omitted, uses $_.
3763 Returns the value of EXPR with the first character uppercased. This is
3764 the internal function implementing the \u escape in double-quoted strings.
3765 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3767 If EXPR is omitted, uses $_.
3773 Sets the umask for the process to EXPR and returns the previous value.
3774 If EXPR is omitted, merely returns the current umask. Remember that a
3775 umask is a number, usually given in octal; it is I<not> a string of octal
3776 digits. See also L</oct>, if all you have is a string.
3782 Undefines the value of EXPR, which must be an lvalue. Use only on a
3783 scalar value, an entire array, an entire hash, or a subroutine name (using
3784 "&"). (Using undef() will probably not do what you expect on most
3785 predefined variables or DBM list values, so don't do that.) Always
3786 returns the undefined value. You can omit the EXPR, in which case
3787 nothing is undefined, but you still get an undefined value that you
3788 could, for instance, return from a subroutine, assign to a variable or
3789 pass as a parameter. Examples:
3792 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
3796 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
3797 select undef, undef, undef, 0.25;
3798 ($a, $b, undef, $c) = &foo; # Ignore third value returned
3804 Deletes a list of files. Returns the number of files successfully
3807 $cnt = unlink 'a', 'b', 'c';
3811 Note: unlink will not delete directories unless you are superuser and
3812 the B<-U> flag is supplied to Perl. Even if these conditions are
3813 met, be warned that unlinking a directory can inflict damage on your
3814 filesystem. Use rmdir instead.
3816 If LIST is omitted, uses $_.
3818 =item unpack TEMPLATE,EXPR
3820 Unpack does the reverse of pack: it takes a string representing a
3821 structure and expands it out into a list value, returning the array
3822 value. (In a scalar context, it returns merely the first value
3823 produced.) The TEMPLATE has the same format as in the pack function.
3824 Here's a subroutine that does substring:
3827 local($what,$where,$howmuch) = @_;
3828 unpack("x$where a$howmuch", $what);
3833 sub ordinal { unpack("c",$_[0]); } # same as ord()
3835 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3836 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3837 themselves. Default is a 16-bit checksum. For example, the following
3838 computes the same number as the System V sum program:
3841 $checksum += unpack("%16C*", $_);
3845 The following efficiently counts the number of set bits in a bit vector:
3847 $setbits = unpack("%32b*", $selectmask);
3849 =item untie VARIABLE
3851 Breaks the binding between a variable and a package. (See tie().)
3853 =item unshift ARRAY,LIST
3855 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3856 depending on how you look at it. Prepends list to the front of the
3857 array, and returns the new number of elements in the array.
3859 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3861 Note the LIST is prepended whole, not one element at a time, so the
3862 prepended elements stay in the same order. Use reverse to do the
3865 =item use Module LIST
3869 =item use Module VERSION LIST
3873 Imports some semantics into the current package from the named module,
3874 generally by aliasing certain subroutine or variable names into your
3875 package. It is exactly equivalent to
3877 BEGIN { require Module; import Module LIST; }
3879 except that Module I<must> be a bareword.
3881 If the first argument to C<use> is a number, it is treated as a version
3882 number instead of a module name. If the version of the Perl interpreter
3883 is less than VERSION, then an error message is printed and Perl exits
3884 immediately. This is often useful if you need to check the current
3885 Perl version before C<use>ing library modules which have changed in
3886 incompatible ways from older versions of Perl. (We try not to do
3887 this more than we have to.)
3889 The BEGIN forces the require and import to happen at compile time. The
3890 require makes sure the module is loaded into memory if it hasn't been
3891 yet. The import is not a builtin--it's just an ordinary static method
3892 call into the "Module" package to tell the module to import the list of
3893 features back into the current package. The module can implement its
3894 import method any way it likes, though most modules just choose to
3895 derive their import method via inheritance from the Exporter class that
3896 is defined in the Exporter module. See L<Exporter>. If no import
3897 method can be found then the error is currently silently ignored. This
3898 may change to a fatal error in a future version.
3900 If you don't want your namespace altered, explicitly supply an empty list:
3904 That is exactly equivalent to
3906 BEGIN { require Module; }
3908 If the VERSION argument is present between Module and LIST, then the
3909 C<use> will call the VERSION method in class Module with the given
3910 version as an argument. The default VERSION method, inherited from
3911 the Universal class, croaks if the given version is larger than the
3912 value of the variable $Module::VERSION. (Note that there is not a
3913 comma after VERSION!)
3915 Because this is a wide-open interface, pragmas (compiler directives)
3916 are also implemented this way. Currently implemented pragmas are:
3920 use sigtrap qw(SEGV BUS);
3921 use strict qw(subs vars refs);
3922 use subs qw(afunc blurfl);
3924 These pseudo-modules import semantics into the current block scope, unlike
3925 ordinary modules, which import symbols into the current package (which are
3926 effective through the end of the file).
3928 There's a corresponding "no" command that unimports meanings imported
3929 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3934 If no unimport method can be found the call fails with a fatal error.
3936 See L<perlmod> for a list of standard modules and pragmas.
3940 Changes the access and modification times on each file of a list of
3941 files. The first two elements of the list must be the NUMERICAL access
3942 and modification times, in that order. Returns the number of files
3943 successfully changed. The inode modification time of each file is set
3944 to the current time. Example of a "touch" command:
3948 utime $now, $now, @ARGV;
3952 Returns a normal array consisting of all the values of the named hash.
3953 (In a scalar context, returns the number of values.) The values are
3954 returned in an apparently random order, but it is the same order as either
3955 the keys() or each() function would produce on the same hash. As a side
3956 effect, it resets HASH's iterator. See also keys(), each(), and sort().
3958 =item vec EXPR,OFFSET,BITS
3960 Treats the string in EXPR as a vector of unsigned integers, and
3961 returns the value of the bit field specified by OFFSET. BITS specifies
3962 the number of bits that are reserved for each entry in the bit
3963 vector. This must be a power of two from 1 to 32. vec() may also be
3964 assigned to, in which case parentheses are needed to give the expression
3965 the correct precedence as in
3967 vec($image, $max_x * $x + $y, 8) = 3;
3969 Vectors created with vec() can also be manipulated with the logical
3970 operators |, &, and ^, which will assume a bit vector operation is
3971 desired when both operands are strings.
3973 To transform a bit vector into a string or array of 0's and 1's, use these:
3975 $bits = unpack("b*", $vector);
3976 @bits = split(//, unpack("b*", $vector));
3978 If you know the exact length in bits, it can be used in place of the *.
3982 Waits for a child process to terminate and returns the pid of the
3983 deceased process, or -1 if there are no child processes. The status is
3986 =item waitpid PID,FLAGS
3988 Waits for a particular child process to terminate and returns the pid
3989 of the deceased process, or -1 if there is no such child process. The
3990 status is returned in C<$?>. If you say
3992 use POSIX ":sys_wait_h";
3994 waitpid(-1,&WNOHANG);
3996 then you can do a non-blocking wait for any process. Non-blocking wait
3997 is available on machines supporting either the waitpid(2) or
3998 wait4(2) system calls. However, waiting for a particular pid with
3999 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
4000 by remembering the status values of processes that have exited but have
4001 not been harvested by the Perl script yet.)
4005 Returns TRUE if the context of the currently executing subroutine is
4006 looking for a list value. Returns FALSE if the context is looking
4007 for a scalar. Returns the undefined value if the context is looking
4008 for no value (void context).
4010 return unless defined wantarray; # don't bother doing more
4011 my @a = complex_calculation();
4012 return wantarray ? @a : "@a";
4016 Produces a message on STDERR just like die(), but doesn't exit or throw
4019 No message is printed if there is a C<$SIG{__WARN__}> handler
4020 installed. It is the handler's responsibility to deal with the message
4021 as it sees fit (like, for instance, converting it into a die()). Most
4022 handlers must therefore make arrangements to actually display the
4023 warnings that they are not prepared to deal with, by calling warn()
4024 again in the handler. Note that this is quite safe and will not
4025 produce an endless loop, since C<__WARN__> hooks are not called from
4028 You will find this behavior is slightly different from that of
4029 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4030 instead call die() again to change it).
4032 Using a C<__WARN__> handler provides a powerful way to silence all
4033 warnings (even the so-called mandatory ones). An example:
4035 # wipe out *all* compile-time warnings
4036 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4038 my $foo = 20; # no warning about duplicate my $foo,
4039 # but hey, you asked for it!
4040 # no compile-time or run-time warnings before here
4043 # run-time warnings enabled after here
4044 warn "\$foo is alive and $foo!"; # does show up
4046 See L<perlvar> for details on setting C<%SIG> entries, and for more
4049 =item write FILEHANDLE
4055 Writes a formatted record (possibly multi-line) to the specified file,
4056 using the format associated with that file. By default the format for
4057 a file is the one having the same name as the filehandle, but the
4058 format for the current output channel (see the select() function) may be set
4059 explicitly by assigning the name of the format to the C<$~> variable.
4061 Top of form processing is handled automatically: if there is
4062 insufficient room on the current page for the formatted record, the
4063 page is advanced by writing a form feed, a special top-of-page format
4064 is used to format the new page header, and then the record is written.
4065 By default the top-of-page format is the name of the filehandle with
4066 "_TOP" appended, but it may be dynamically set to the format of your
4067 choice by assigning the name to the C<$^> variable while the filehandle is
4068 selected. The number of lines remaining on the current page is in
4069 variable C<$->, which can be set to 0 to force a new page.
4071 If FILEHANDLE is unspecified, output goes to the current default output
4072 channel, which starts out as STDOUT but may be changed by the
4073 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4074 is evaluated and the resulting string is used to look up the name of
4075 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4077 Note that write is I<NOT> the opposite of read. Unfortunately.
4081 The transliteration operator. Same as tr///. See L<perlop>.
projects / p5sagit/p5-mst-13.2.git / blob