3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return -1 on failure. Exceptions to this rule are wait(),
78 waitpid(), and syscall(). System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///
96 =item Regular expressions and pattern matching
98 C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
133 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
135 =item Keywords related to the control flow of your perl program
137 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
138 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
140 =item Keywords related to scoping
142 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
144 =item Miscellaneous functions
146 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
147 C<scalar>, C<undef>, C<wantarray>
149 =item Functions for processes and process groups
151 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
152 C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
153 C<times>, C<wait>, C<waitpid>
155 =item Keywords related to perl modules
157 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
159 =item Keywords related to classes and object-orientedness
161 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
164 =item Low-level socket functions
166 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
167 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
168 C<socket>, C<socketpair>
170 =item System V interprocess communication functions
172 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
173 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
175 =item Fetching user and group info
177 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
178 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
179 C<getpwuid>, C<setgrent>, C<setpwent>
181 =item Fetching network info
183 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
184 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
185 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
186 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
187 C<setnetent>, C<setprotoent>, C<setservent>
189 =item Time-related functions
191 C<gmtime>, C<localtime>, C<time>, C<times>
193 =item Functions new in perl5
195 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
200 * - C<sub> was a keyword in perl4, but in perl5 it is an
201 operator, which can be used in expressions.
203 =item Functions obsoleted in perl5
205 C<dbmclose>, C<dbmopen>
209 =head2 Alphabetical Listing of Perl Functions
213 =item I<-X> FILEHANDLE
219 A file test, where X is one of the letters listed below. This unary
220 operator takes one argument, either a filename or a filehandle, and
221 tests the associated file to see if something is true about it. If the
222 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
223 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
224 the undefined value if the file doesn't exist. Despite the funny
225 names, precedence is the same as any other named unary operator, and
226 the argument may be parenthesized like any other unary operator. The
227 operator may be any of:
229 -r File is readable by effective uid/gid.
230 -w File is writable by effective uid/gid.
231 -x File is executable by effective uid/gid.
232 -o File is owned by effective uid.
234 -R File is readable by real uid/gid.
235 -W File is writable by real uid/gid.
236 -X File is executable by real uid/gid.
237 -O File is owned by real uid.
240 -z File has zero size.
241 -s File has nonzero size (returns size).
243 -f File is a plain file.
244 -d File is a directory.
245 -l File is a symbolic link.
246 -p File is a named pipe (FIFO).
248 -b File is a block special file.
249 -c File is a character special file.
250 -t Filehandle is opened to a tty.
252 -u File has setuid bit set.
253 -g File has setgid bit set.
254 -k File has sticky bit set.
256 -T File is a text file.
257 -B File is a binary file (opposite of -T).
259 -M Age of file in days when script started.
260 -A Same for access time.
261 -C Same for inode change time.
263 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
264 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
265 uids and gids of the user. There may be other reasons you can't actually
266 read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
267 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
268 1 if any execute bit is set in the mode. Scripts run by the superuser may
269 thus need to do a stat() to determine the actual mode of the
270 file, or temporarily set the uid to something else.
276 next unless -f $_; # ignore specials
280 Note that C<-s/a/b/> does not do a negated substitution. Saying
281 C<-exp($foo)> still works as expected, however--only single letters
282 following a minus are interpreted as file tests.
284 The C<-T> and C<-B> switches work as follows. The first block or so of the
285 file is examined for odd characters such as strange control codes or
286 characters with the high bit set. If too many strange characters (E<gt>30%)
287 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
288 containing null in the first block is considered a binary file. If C<-T>
289 or C<-B> is used on a filehandle, the current stdio buffer is examined
290 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
291 file, or a file at EOF when testing a filehandle. Because you have to
292 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
293 against the file first, as in C<next unless -f $file && -T $file>.
295 If any of the file tests (or either the stat() or lstat() operators) are given
296 the special filehandle consisting of a solitary underline, then the stat
297 structure of the previous file test (or stat operator) is used, saving
298 a system call. (This doesn't work with C<-t>, and you need to remember
299 that lstat() and C<-l> will leave values in the stat structure for the
300 symbolic link, not the real file.) Example:
302 print "Can do.\n" if -r $a || -w _ || -x _;
305 print "Readable\n" if -r _;
306 print "Writable\n" if -w _;
307 print "Executable\n" if -x _;
308 print "Setuid\n" if -u _;
309 print "Setgid\n" if -g _;
310 print "Sticky\n" if -k _;
311 print "Text\n" if -T _;
312 print "Binary\n" if -B _;
318 Returns the absolute value of its argument.
319 If VALUE is omitted, uses $_.
321 =item accept NEWSOCKET,GENERICSOCKET
323 Accepts an incoming socket connect, just as the accept(2) system call
324 does. Returns the packed address if it succeeded, FALSE otherwise.
325 See example in L<perlipc/"Sockets: Client/Server Communication">.
331 Arranges to have a SIGALRM delivered to this process after the
332 specified number of seconds have elapsed. If SECONDS is not specified,
333 the value stored in $_ is used. (On some machines,
334 unfortunately, the elapsed time may be up to one second less than you
335 specified because of how seconds are counted.) Only one timer may be
336 counting at once. Each call disables the previous timer, and an
337 argument of 0 may be supplied to cancel the previous timer without
338 starting a new one. The returned value is the amount of time remaining
339 on the previous timer.
341 For delays of finer granularity than one second, you may use Perl's
342 syscall() interface to access setitimer(2) if your system supports it,
343 or else see L</select()>. It is usually a mistake to intermix alarm()
346 If you want to use alarm() to time out a system call you need to use an
347 eval/die pair. You can't rely on the alarm causing the system call to
348 fail with C<$!> set to EINTR because Perl sets up signal handlers to
349 restart system calls on some systems. Using eval/die always works,
350 modulo the caveats given in L<perlipc/"Signals">.
353 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
355 $nread = sysread SOCKET, $buffer, $size;
359 die unless $@ eq "alarm\n"; # propagate unexpected errors
368 Returns the arctangent of Y/X in the range -PI to PI.
370 For the tangent operation, you may use the POSIX::tan()
371 function, or use the familiar relation:
373 sub tan { sin($_[0]) / cos($_[0]) }
375 =item bind SOCKET,NAME
377 Binds a network address to a socket, just as the bind system call
378 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
379 packed address of the appropriate type for the socket. See the examples in
380 L<perlipc/"Sockets: Client/Server Communication">.
382 =item binmode FILEHANDLE
384 Arranges for the file to be read or written in "binary" mode in operating
385 systems that distinguish between binary and text files. Files that are
386 not in binary mode have CR LF sequences translated to LF on input and LF
387 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
388 and similarly archaic systems, it may be imperative--otherwise your
389 MS-DOS-damaged C library may mangle your file. The key distinction between
390 systems that need binmode and those that don't is their text file
391 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
392 character, and that encode that character in C as '\n', do not need
393 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
394 is taken as the name of the filehandle.
396 =item bless REF,CLASSNAME
400 This function tells the thingy referenced by REF that it is now
401 an object in the CLASSNAME package--or the current package if no CLASSNAME
402 is specified, which is often the case. It returns the reference for
403 convenience, because a bless() is often the last thing in a constructor.
404 Always use the two-argument version if the function doing the blessing
405 might be inherited by a derived class. See L<perltoot> and L<perlobj>
406 for more about the blessing (and blessings) of objects.
412 Returns the context of the current subroutine call. In scalar context,
413 returns the caller's package name if there is a caller, that is, if
414 we're in a subroutine or eval() or require(), and the undefined value
415 otherwise. In list context, returns
417 ($package, $filename, $line) = caller;
419 With EXPR, it returns some extra information that the debugger uses to
420 print a stack trace. The value of EXPR indicates how many call frames
421 to go back before the current one.
423 ($package, $filename, $line, $subroutine,
424 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
426 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
427 call, but an C<eval>. In such a case additional elements $evaltext and
428 $is_require are set: $is_require is true if the frame is created by a
429 C<require> or C<use> statement, $evaltext contains the text of the
430 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
431 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
432 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
435 Furthermore, when called from within the DB package, caller returns more
436 detailed information: it sets the list variable @DB::args to be the
437 arguments with which the subroutine was invoked.
441 Changes the working directory to EXPR, if possible. If EXPR is
442 omitted, changes to home directory. Returns TRUE upon success, FALSE
443 otherwise. See example under die().
447 Changes the permissions of a list of files. The first element of the
448 list must be the numerical mode, which should probably be an octal
449 number, and which definitely should I<not> a string of octal digits:
450 C<0644> is okay, C<'0644'> is not. Returns the number of files
451 successfully changed. See also L</oct>, if all you have is a string.
453 $cnt = chmod 0755, 'foo', 'bar';
454 chmod 0755, @executables;
455 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
456 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
457 $mode = 0644; chmod $mode, 'foo'; # this is best
465 This is a slightly safer version of L</chop>. It removes any
466 line ending that corresponds to the current value of C<$/> (also known as
467 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
468 number of characters removed from all its arguments. It's often used to
469 remove the newline from the end of an input record when you're worried
470 that the final record may be missing its newline. When in paragraph mode
471 (C<$/ = "">), it removes all trailing newlines from the string. If
472 VARIABLE is omitted, it chomps $_. Example:
475 chomp; # avoid \n on last field
480 You can actually chomp anything that's an lvalue, including an assignment:
483 chomp($answer = <STDIN>);
485 If you chomp a list, each element is chomped, and the total number of
486 characters removed is returned.
494 Chops off the last character of a string and returns the character
495 chopped. It's used primarily to remove the newline from the end of an
496 input record, but is much more efficient than C<s/\n//> because it neither
497 scans nor copies the string. If VARIABLE is omitted, chops $_.
501 chop; # avoid \n on last field
506 You can actually chop anything that's an lvalue, including an assignment:
509 chop($answer = <STDIN>);
511 If you chop a list, each element is chopped. Only the value of the
512 last chop is returned.
514 Note that chop returns the last character. To return all but the last
515 character, use C<substr($string, 0, -1)>.
519 Changes the owner (and group) of a list of files. The first two
520 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
521 Returns the number of files successfully changed.
523 $cnt = chown $uid, $gid, 'foo', 'bar';
524 chown $uid, $gid, @filenames;
526 Here's an example that looks up nonnumeric uids in the passwd file:
529 chop($user = <STDIN>);
531 chop($pattern = <STDIN>);
533 ($login,$pass,$uid,$gid) = getpwnam($user)
534 or die "$user not in passwd file";
536 @ary = glob($pattern); # expand filenames
537 chown $uid, $gid, @ary;
539 On most systems, you are not allowed to change the ownership of the
540 file unless you're the superuser, although you should be able to change
541 the group to any of your secondary groups. On insecure systems, these
542 restrictions may be relaxed, but this is not a portable assumption.
548 Returns the character represented by that NUMBER in the character set.
549 For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
551 If NUMBER is omitted, uses $_.
553 =item chroot FILENAME
557 This function works like the system call by the same name: it makes the
558 named directory the new root directory for all further pathnames that
559 begin with a "/" by your process and all its children. (It doesn't
560 change your current working directory, which is unaffected.) For security
561 reasons, this call is restricted to the superuser. If FILENAME is
562 omitted, does a chroot to $_.
564 =item close FILEHANDLE
568 Closes the file or pipe associated with the file handle, returning TRUE
569 only if stdio successfully flushes buffers and closes the system file
570 descriptor. Closes the currently selected filehandle if the argument
573 You don't have to close FILEHANDLE if you are immediately going to do
574 another open() on it, because open() will close it for you. (See
575 open().) However, an explicit close on an input file resets the line
576 counter ($.), while the implicit close done by open() does not.
578 If the file handle came from a piped open C<close> will additionally
579 return FALSE if one of the other system calls involved fails or if the
580 program exits with non-zero status. (If the only problem was that the
581 program exited non-zero $! will be set to 0.) Also, closing a pipe
582 waits for the process executing on the pipe to complete, in case you
583 want to look at the output of the pipe afterwards. Closing a pipe
584 explicitly also puts the exit status value of the command into C<$?>.
588 open(OUTPUT, '|sort >foo') # pipe to sort
589 or die "Can't start sort: $!";
590 #... # print stuff to output
591 close OUTPUT # wait for sort to finish
592 or warn $! ? "Error closing sort pipe: $!"
593 : "Exit status $? from sort";
594 open(INPUT, 'foo') # get sort's results
595 or die "Can't open 'foo' for input: $!";
597 FILEHANDLE may be an expression whose value can be used as an indirect
598 filehandle, usually the real filehandle name.
600 =item closedir DIRHANDLE
602 Closes a directory opened by opendir() and returns the success of that
605 DIRHANDLE may be an expression whose value can be used as an indirect
606 dirhandle, usually the real dirhandle name.
608 =item connect SOCKET,NAME
610 Attempts to connect to a remote socket, just as the connect system call
611 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
612 packed address of the appropriate type for the socket. See the examples in
613 L<perlipc/"Sockets: Client/Server Communication">.
617 Actually a flow control statement rather than a function. If there is a
618 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
619 C<foreach>), it is always executed just before the conditional is about to
620 be evaluated again, just like the third part of a C<for> loop in C. Thus
621 it can be used to increment a loop variable, even when the loop has been
622 continued via the C<next> statement (which is similar to the C C<continue>
625 C<last>, C<next>, or C<redo> may appear within a C<continue>
626 block. C<last> and C<redo> will behave as if they had been executed within
627 the main block. So will C<next>, but since it will execute a C<continue>
628 block, it may be more entertaining.
631 ### redo always comes here
634 ### next always comes here
636 # then back the top to re-check EXPR
638 ### last always comes here
640 Omitting the C<continue> section is semantically equivalent to using an
641 empty one, logically enough. In that case, C<next> goes directly back
642 to check the condition at the top of the loop.
646 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
649 For the inverse cosine operation, you may use the POSIX::acos()
650 function, or use this relation:
652 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
654 =item crypt PLAINTEXT,SALT
656 Encrypts a string exactly like the crypt(3) function in the C library
657 (assuming that you actually have a version there that has not been
658 extirpated as a potential munition). This can prove useful for checking
659 the password file for lousy passwords, amongst other things. Only the
660 guys wearing white hats should do this.
662 Note that crypt is intended to be a one-way function, much like breaking
663 eggs to make an omelette. There is no (known) corresponding decrypt
664 function. As a result, this function isn't all that useful for
665 cryptography. (For that, see your nearby CPAN mirror.)
667 Here's an example that makes sure that whoever runs this program knows
670 $pwd = (getpwuid($<))[1];
671 $salt = substr($pwd, 0, 2);
675 chop($word = <STDIN>);
679 if (crypt($word, $salt) ne $pwd) {
685 Of course, typing in your own password to whoever asks you
690 [This function has been superseded by the untie() function.]
692 Breaks the binding between a DBM file and a hash.
694 =item dbmopen HASH,DBNAME,MODE
696 [This function has been superseded by the tie() function.]
698 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
699 hash. HASH is the name of the hash. (Unlike normal open, the first
700 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
701 is the name of the database (without the F<.dir> or F<.pag> extension if
702 any). If the database does not exist, it is created with protection
703 specified by MODE (as modified by the umask()). If your system supports
704 only the older DBM functions, you may perform only one dbmopen() in your
705 program. In older versions of Perl, if your system had neither DBM nor
706 ndbm, calling dbmopen() produced a fatal error; it now falls back to
709 If you don't have write access to the DBM file, you can only read hash
710 variables, not set them. If you want to test whether you can write,
711 either use file tests or try setting a dummy hash entry inside an eval(),
712 which will trap the error.
714 Note that functions such as keys() and values() may return huge lists
715 when used on large DBM files. You may prefer to use the each()
716 function to iterate over large DBM files. Example:
718 # print out history file offsets
719 dbmopen(%HIST,'/usr/lib/news/history',0666);
720 while (($key,$val) = each %HIST) {
721 print $key, ' = ', unpack('L',$val), "\n";
725 See also L<AnyDBM_File> for a more general description of the pros and
726 cons of the various dbm approaches, as well as L<DB_File> for a particularly
733 Returns a Boolean value telling whether EXPR has a value other than
734 the undefined value C<undef>. If EXPR is not present, C<$_> will be
737 Many operations return C<undef> to indicate failure, end of file,
738 system error, uninitialized variable, and other exceptional
739 conditions. This function allows you to distinguish C<undef> from
740 other values. (A simple Boolean test will not distinguish among
741 C<undef>, zero, the empty string, and "0", which are all equally
742 false.) Note that since C<undef> is a valid scalar, its presence
743 doesn't I<necessarily> indicate an exceptional condition: pop()
744 returns C<undef> when its argument is an empty array, I<or> when the
745 element to return happens to be C<undef>.
747 You may also use defined() to check whether a subroutine exists, by
748 saying C<defined &func> without parentheses. On the other hand, use
749 of defined() upon aggregates (hashes and arrays) is not guaranteed to
750 produce intuitive results, and should probably be avoided.
752 When used on a hash element, it tells you whether the value is defined,
753 not whether the key exists in the hash. Use L</exists> for the latter
758 print if defined $switch{'D'};
759 print "$val\n" while defined($val = pop(@ary));
760 die "Can't readlink $sym: $!"
761 unless defined($value = readlink $sym);
762 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
763 $debugging = 0 unless defined $debugging;
765 Note: Many folks tend to overuse defined(), and then are surprised to
766 discover that the number 0 and "" (the zero-length string) are, in fact,
767 defined values. For example, if you say
771 The pattern match succeeds, and $1 is defined, despite the fact that it
772 matched "nothing". But it didn't really match nothing--rather, it
773 matched something that happened to be 0 characters long. This is all
774 very above-board and honest. When a function returns an undefined value,
775 it's an admission that it couldn't give you an honest answer. So you
776 should use defined() only when you're questioning the integrity of what
777 you're trying to do. At other times, a simple comparison to 0 or "" is
780 Currently, using defined() on an entire array or hash reports whether
781 memory for that aggregate has ever been allocated. So an array you set
782 to the empty list appears undefined initially, and one that once was full
783 and that you then set to the empty list still appears defined. You
784 should instead use a simple test for size:
786 if (@an_array) { print "has array elements\n" }
787 if (%a_hash) { print "has hash members\n" }
789 Using undef() on these, however, does clear their memory and then report
790 them as not defined anymore, but you shouldn't do that unless you don't
791 plan to use them again, because it saves time when you load them up
792 again to have memory already ready to be filled. The normal way to
793 free up space used by an aggregate is to assign the empty list.
795 This counterintuitive behavior of defined() on aggregates may be
796 changed, fixed, or broken in a future release of Perl.
798 See also L</undef>, L</exists>, L</ref>.
802 Deletes the specified key(s) and their associated values from a hash.
803 For each key, returns the deleted value associated with that key, or
804 the undefined value if there was no such key. Deleting from C<$ENV{}>
805 modifies the environment. Deleting from a hash tied to a DBM file
806 deletes the entry from the DBM file. (But deleting from a tie()d hash
807 doesn't necessarily return anything.)
809 The following deletes all the values of a hash:
811 foreach $key (keys %HASH) {
817 delete @HASH{keys %HASH}
819 (But both of these are slower than just assigning the empty list, or
820 using undef().) Note that the EXPR can be arbitrarily complicated as
821 long as the final operation is a hash element lookup or hash slice:
823 delete $ref->[$x][$y]{$key};
824 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
828 Outside an eval(), prints the value of LIST to C<STDERR> and exits with
829 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
830 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
831 is 0, exits with 255. Inside an eval(), the error message is stuffed into
832 C<$@> and the eval() is terminated with the undefined value. This makes
833 die() the way to raise an exception.
837 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
838 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
840 If the value of EXPR does not end in a newline, the current script line
841 number and input line number (if any) are also printed, and a newline
842 is supplied. Hint: sometimes appending ", stopped" to your message
843 will cause it to make better sense when the string "at foo line 123" is
844 appended. Suppose you are running script "canasta".
846 die "/etc/games is no good";
847 die "/etc/games is no good, stopped";
849 produce, respectively
851 /etc/games is no good at canasta line 123.
852 /etc/games is no good, stopped at canasta line 123.
854 See also exit() and warn().
856 If LIST is empty and $@ already contains a value (typically from a
857 previous eval) that value is reused after appending "\t...propagated".
858 This is useful for propagating exceptions:
861 die unless $@ =~ /Expected exception/;
863 If $@ is empty then the string "Died" is used.
865 You can arrange for a callback to be run just before the die() does
866 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
867 will be called with the error text and can change the error message, if
868 it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
869 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
871 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
872 blocks/strings. If one wants the hook to do nothing in such
877 as the first line of the handler (see L<perlvar/$^S>).
881 Not really a function. Returns the value of the last command in the
882 sequence of commands indicated by BLOCK. When modified by a loop
883 modifier, executes the BLOCK once before testing the loop condition.
884 (On other statements the loop modifiers test the conditional first.)
886 =item do SUBROUTINE(LIST)
888 A deprecated form of subroutine call. See L<perlsub>.
892 Uses the value of EXPR as a filename and executes the contents of the
893 file as a Perl script. Its primary use is to include subroutines
894 from a Perl subroutine library.
900 scalar eval `cat stat.pl`;
902 except that it's more efficient and concise, keeps track of the
903 current filename for error messages, and searches all the B<-I>
904 libraries if the file isn't in the current directory (see also the @INC
905 array in L<perlvar/Predefined Names>). It is also different in how
906 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
907 scope like C<eval STRING> does. It's the same, however, in that it does
908 reparse the file every time you call it, so you probably don't want to
909 do this inside a loop.
911 Note that inclusion of library modules is better done with the
912 use() and require() operators, which also do automatic error checking
913 and raise an exception if there's a problem.
915 You might like to use C<do> to read in a program configuration
916 file. Manual error checking can be done this way:
918 # read in config files: system first, then user
919 for $file ('/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") {
920 unless ($return = do $file) {
921 warn "couldn't parse $file: $@" if $@;
922 warn "couldn't do $file: $!" unless defined $return;
923 warn "couldn't run $file" unless $return;
929 This causes an immediate core dump. Primarily this is so that you can
930 use the B<undump> program to turn your core dump into an executable binary
931 after having initialized all your variables at the beginning of the
932 program. When the new binary is executed it will begin by executing a
933 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
934 it as a goto with an intervening core dump and reincarnation. If LABEL
935 is omitted, restarts the program from the top. WARNING: Any files
936 opened at the time of the dump will NOT be open any more when the
937 program is reincarnated, with possible resulting confusion on the part
938 of Perl. See also B<-u> option in L<perlrun>.
955 dump QUICKSTART if $ARGV[0] eq '-d';
960 This operator is largely obsolete, partly because it's very hard to
961 convert a core file into an executable, and because the real perl-to-C
962 compiler has superseded it.
966 When called in list context, returns a 2-element list consisting of the
967 key and value for the next element of a hash, so that you can iterate over
968 it. When called in scalar context, returns the key for only the "next"
969 element in the hash. (Note: Keys may be "0" or "", which are logically
970 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
973 Entries are returned in an apparently random order. When the hash is
974 entirely read, a null array is returned in list context (which when
975 assigned produces a FALSE (0) value), and C<undef> in
976 scalar context. The next call to each() after that will start iterating
977 again. There is a single iterator for each hash, shared by all each(),
978 keys(), and values() function calls in the program; it can be reset by
979 reading all the elements from the hash, or by evaluating C<keys HASH> or
980 C<values HASH>. If you add or delete elements of a hash while you're
981 iterating over it, you may get entries skipped or duplicated, so don't.
983 The following prints out your environment like the printenv(1) program,
984 only in a different order:
986 while (($key,$value) = each %ENV) {
987 print "$key=$value\n";
990 See also keys() and values().
998 Returns 1 if the next read on FILEHANDLE will return end of file, or if
999 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1000 gives the real filehandle. (Note that this function actually
1001 reads a character and then ungetc()s it, so isn't very useful in an
1002 interactive context.) Do not read from a terminal file (or call
1003 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1004 as terminals may lose the end-of-file condition if you do.
1006 An C<eof> without an argument uses the last file read as argument.
1007 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1008 the files listed on the command line, i.e., C<eof()> is reasonable to
1009 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1010 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1011 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1013 # reset line numbering on each input file
1015 next if /^\s*#/; # skip comments
1018 close ARGV if eof; # Not eof()!
1021 # insert dashes just before last line of last file
1023 if (eof()) { # check for end of current file
1024 print "--------------\n";
1025 close(ARGV); # close or break; is needed if we
1026 # are reading from the terminal
1031 Practical hint: you almost never need to use C<eof> in Perl, because the
1032 input operators return false values when they run out of data, or if there
1039 In the first form, the return value of EXPR is parsed and executed as if it
1040 were a little Perl program. The value of the expression (which is itself
1041 determined within scalar context) is first parsed, and if there weren't any
1042 errors, executed in the context of the current Perl program, so that any
1043 variable settings or subroutine and format definitions remain afterwards.
1044 Note that the value is parsed every time the eval executes. If EXPR is
1045 omitted, evaluates C<$_>. This form is typically used to delay parsing
1046 and subsequent execution of the text of EXPR until run time.
1048 In the second form, the code within the BLOCK is parsed only once--at the
1049 same time the code surrounding the eval itself was parsed--and executed
1050 within the context of the current Perl program. This form is typically
1051 used to trap exceptions more efficiently than the first (see below), while
1052 also providing the benefit of checking the code within BLOCK at compile
1055 The final semicolon, if any, may be omitted from the value of EXPR or within
1058 In both forms, the value returned is the value of the last expression
1059 evaluated inside the mini-program; a return statement may be also used, just
1060 as with subroutines. The expression providing the return value is evaluated
1061 in void, scalar, or list context, depending on the context of the eval itself.
1062 See L</wantarray> for more on how the evaluation context can be determined.
1064 If there is a syntax error or runtime error, or a die() statement is
1065 executed, an undefined value is returned by eval(), and C<$@> is set to the
1066 error message. If there was no error, C<$@> is guaranteed to be a null
1067 string. Beware that using eval() neither silences perl from printing
1068 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1069 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1070 L</warn> and L<perlvar>.
1072 Note that, because eval() traps otherwise-fatal errors, it is useful for
1073 determining whether a particular feature (such as socket() or symlink())
1074 is implemented. It is also Perl's exception trapping mechanism, where
1075 the die operator is used to raise exceptions.
1077 If the code to be executed doesn't vary, you may use the eval-BLOCK
1078 form to trap run-time errors without incurring the penalty of
1079 recompiling each time. The error, if any, is still returned in C<$@>.
1082 # make divide-by-zero nonfatal
1083 eval { $answer = $a / $b; }; warn $@ if $@;
1085 # same thing, but less efficient
1086 eval '$answer = $a / $b'; warn $@ if $@;
1088 # a compile-time error
1089 eval { $answer = }; # WRONG
1092 eval '$answer ='; # sets $@
1094 When using the eval{} form as an exception trap in libraries, you may
1095 wish not to trigger any C<__DIE__> hooks that user code may have
1096 installed. You can use the C<local $SIG{__DIE__}> construct for this
1097 purpose, as shown in this example:
1099 # a very private exception trap for divide-by-zero
1100 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1102 This is especially significant, given that C<__DIE__> hooks can call
1103 die() again, which has the effect of changing their error messages:
1105 # __DIE__ hooks may modify error messages
1107 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1108 eval { die "foo lives here" };
1109 print $@ if $@; # prints "bar lives here"
1112 With an eval(), you should be especially careful to remember what's
1113 being looked at when:
1119 eval { $x }; # CASE 4
1121 eval "\$$x++"; # CASE 5
1124 Cases 1 and 2 above behave identically: they run the code contained in
1125 the variable $x. (Although case 2 has misleading double quotes making
1126 the reader wonder what else might be happening (nothing is).) Cases 3
1127 and 4 likewise behave in the same way: they run the code '$x', which
1128 does nothing but return the value of C<$x>. (Case 4 is preferred for
1129 purely visual reasons, but it also has the advantage of compiling at
1130 compile-time instead of at run-time.) Case 5 is a place where
1131 normally you I<WOULD> like to use double quotes, except that in this
1132 particular situation, you can just use symbolic references instead, as
1137 =item exec PROGRAM LIST
1139 The exec() function executes a system command I<AND NEVER RETURNS> -
1140 use system() instead of exec() if you want it to return. It fails and
1141 returns FALSE only if the command does not exist I<and> it is executed
1142 directly instead of via your system's command shell (see below).
1144 Since it's a common mistake to use system() instead of exec(), Perl
1145 warns you if there is a following statement which isn't die(), warn(),
1146 or exit() (if C<-w> is set - but you always do that). If you
1147 I<really> want to follow an exec() with some other statement, you
1148 can use one of these styles to avoid the warning:
1150 exec ('foo') or print STDERR "couldn't exec foo: $!";
1151 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1153 If there is more than one argument in LIST, or if LIST is an array
1154 with more than one value, calls execvp(3) with the arguments in LIST.
1155 If there is only one scalar argument or an array with one element in it,
1156 the argument is checked for shell metacharacters, and if there are any,
1157 the entire argument is passed to the system's command shell for parsing
1158 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1159 If there are no shell metacharacters in the argument, it is split into
1160 words and passed directly to execvp(), which is more efficient. Note:
1161 exec() and system() do not flush your output buffer, so you may need to
1162 set C<$|> to avoid lost output. Examples:
1164 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1165 exec "sort $outfile | uniq";
1167 If you don't really want to execute the first argument, but want to lie
1168 to the program you are executing about its own name, you can specify
1169 the program you actually want to run as an "indirect object" (without a
1170 comma) in front of the LIST. (This always forces interpretation of the
1171 LIST as a multivalued list, even if there is only a single scalar in
1174 $shell = '/bin/csh';
1175 exec $shell '-sh'; # pretend it's a login shell
1179 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1181 When the arguments get executed via the system shell, results will
1182 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1185 Using an indirect object with C<exec> or C<system> is also more secure.
1186 This usage forces interpretation of the arguments as a multivalued list,
1187 even if the list had just one argument. That way you're safe from the
1188 shell expanding wildcards or splitting up words with whitespace in them.
1190 @args = ( "echo surprise" );
1192 system @args; # subject to shell escapes if @args == 1
1193 system { $args[0] } @args; # safe even with one-arg list
1195 The first version, the one without the indirect object, ran the I<echo>
1196 program, passing it C<"surprise"> an argument. The second version
1197 didn't--it tried to run a program literally called I<"echo surprise">,
1198 didn't find it, and set C<$?> to a non-zero value indicating failure.
1202 Returns TRUE if the specified hash key exists in its hash array, even
1203 if the corresponding value is undefined.
1205 print "Exists\n" if exists $array{$key};
1206 print "Defined\n" if defined $array{$key};
1207 print "True\n" if $array{$key};
1209 A hash element can be TRUE only if it's defined, and defined if
1210 it exists, but the reverse doesn't necessarily hold true.
1212 Note that the EXPR can be arbitrarily complicated as long as the final
1213 operation is a hash key lookup:
1215 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1217 Although the last element will not spring into existence just because its
1218 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1219 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1220 test for a $key element. This autovivification may be fixed in a later
1225 Evaluates EXPR and exits immediately with that value. (Actually, it
1226 calls any defined C<END> routines first, but the C<END> routines may not
1227 abort the exit. Likewise any object destructors that need to be called
1228 are called before exit.) Example:
1231 exit 0 if $ans =~ /^[Xx]/;
1233 See also die(). If EXPR is omitted, exits with 0 status. The only
1234 universally portable values for EXPR are 0 for success and 1 for error;
1235 all other values are subject to unpredictable interpretation depending
1236 on the environment in which the Perl program is running.
1238 You shouldn't use exit() to abort a subroutine if there's any chance that
1239 someone might want to trap whatever error happened. Use die() instead,
1240 which can be trapped by an eval().
1242 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1248 Returns I<e> (the natural logarithm base) to the power of EXPR.
1249 If EXPR is omitted, gives C<exp($_)>.
1251 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1253 Implements the fcntl(2) function. You'll probably have to say
1257 first to get the correct function definitions. Argument processing and
1258 value return works just like ioctl() below.
1262 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1263 or die "can't fcntl F_GETFL: $!";
1265 You don't have to check for C<defined> on the return from
1266 fnctl. Like ioctl, it maps a 0 return from the system
1267 call into "0 but true" in Perl. This string is true in
1268 boolean context and 0 in numeric context. It is also
1269 exempt from the normal B<-w> warnings on improper numeric
1272 Note that fcntl() will produce a fatal error if used on a machine that
1273 doesn't implement fcntl(2).
1275 =item fileno FILEHANDLE
1277 Returns the file descriptor for a filehandle. This is useful for
1278 constructing bitmaps for select() and low-level POSIX tty-handling
1279 operations. If FILEHANDLE is an expression, the value is taken as
1280 an indirect filehandle, generally its name.
1282 You can use this to find out whether two handles refer to the
1283 same underlying descriptor:
1285 if (fileno(THIS) == fileno(THAT)) {
1286 print "THIS and THAT are dups\n";
1289 =item flock FILEHANDLE,OPERATION
1291 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1292 success, FALSE on failure. Produces a fatal error if used on a machine
1293 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1294 is Perl's portable file locking interface, although it locks only entire
1297 On many platforms (including most versions or clones of Unix), locks
1298 established by flock() are B<merely advisory>. Such discretionary locks
1299 are more flexible, but offer fewer guarantees. This means that files
1300 locked with flock() may be modified by programs that do not also use
1301 flock(). Windows NT and OS/2 are among the platforms which
1302 enforce mandatory locking. See your local documentation for details.
1304 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1305 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1306 you can use the symbolic names if import them from the Fcntl module,
1307 either individually, or as a group using the ':flock' tag. LOCK_SH
1308 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1309 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1310 LOCK_EX then flock() will return immediately rather than blocking
1311 waiting for the lock (check the return status to see if you got it).
1313 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1314 before (un)locking it.
1316 Note that the emulation built with lockf(3) doesn't provide shared
1317 locks, and it requires that FILEHANDLE be open with write intent. These
1318 are the semantics that lockf(3) implements. Most (all?) systems
1319 implement lockf(3) in terms of fcntl(2) locking, though, so the
1320 differing semantics shouldn't bite too many people.
1322 Note also that some versions of flock() cannot lock things over the
1323 network; you would need to use the more system-specific fcntl() for
1324 that. If you like you can force Perl to ignore your system's flock(2)
1325 function, and so provide its own fcntl(2)-based emulation, by passing
1326 the switch C<-Ud_flock> to the F<Configure> program when you configure
1329 Here's a mailbox appender for BSD systems.
1331 use Fcntl ':flock'; # import LOCK_* constants
1334 flock(MBOX,LOCK_EX);
1335 # and, in case someone appended
1336 # while we were waiting...
1341 flock(MBOX,LOCK_UN);
1344 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1345 or die "Can't open mailbox: $!";
1348 print MBOX $msg,"\n\n";
1351 See also L<DB_File> for other flock() examples.
1355 Does a fork(2) system call. Returns the child pid to the parent process,
1356 0 to the child process, or C<undef> if the fork is unsuccessful.
1358 Note: unflushed buffers remain unflushed in both processes, which means
1359 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1360 method of IO::Handle to avoid duplicate output.
1362 If you fork() without ever waiting on your children, you will accumulate
1365 $SIG{CHLD} = sub { wait };
1367 There's also the double-fork trick (error checking on
1368 fork() returns omitted);
1370 unless ($pid = fork) {
1372 exec "what you really wanna do";
1375 ## (some_perl_code_here)
1382 See also L<perlipc> for more examples of forking and reaping
1385 Note that if your forked child inherits system file descriptors like
1386 STDIN and STDOUT that are actually connected by a pipe or socket, even
1387 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1388 you're done. You should reopen those to /dev/null if it's any issue.
1392 Declare a picture format for use by the write() function. For
1396 Test: @<<<<<<<< @||||| @>>>>>
1397 $str, $%, '$' . int($num)
1401 $num = $cost/$quantity;
1405 See L<perlform> for many details and examples.
1407 =item formline PICTURE,LIST
1409 This is an internal function used by C<format>s, though you may call it,
1410 too. It formats (see L<perlform>) a list of values according to the
1411 contents of PICTURE, placing the output into the format output
1412 accumulator, C<$^A> (or $ACCUMULATOR in English).
1413 Eventually, when a write() is done, the contents of
1414 C<$^A> are written to some filehandle, but you could also read C<$^A>
1415 yourself and then set C<$^A> back to "". Note that a format typically
1416 does one formline() per line of form, but the formline() function itself
1417 doesn't care how many newlines are embedded in the PICTURE. This means
1418 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1419 You may therefore need to use multiple formlines to implement a single
1420 record format, just like the format compiler.
1422 Be careful if you put double quotes around the picture, because an "C<@>"
1423 character may be taken to mean the beginning of an array name.
1424 formline() always returns TRUE. See L<perlform> for other examples.
1426 =item getc FILEHANDLE
1430 Returns the next character from the input file attached to FILEHANDLE,
1431 or the undefined value at end of file, or if there was an error. If
1432 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1433 efficient. It cannot be used to get unbuffered single-characters,
1434 however. For that, try something more like:
1437 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1440 system "stty", '-icanon', 'eol', "\001";
1446 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1449 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1453 Determination of whether $BSD_STYLE should be set
1454 is left as an exercise to the reader.
1456 The POSIX::getattr() function can do this more portably on systems
1457 purporting POSIX compliance.
1458 See also the C<Term::ReadKey> module from your nearest CPAN site;
1459 details on CPAN can be found on L<perlmod/CPAN>.
1463 Implements the C library function of the same name, which on most
1464 systems returns the current login from F</etc/utmp>, if any. If null,
1467 $login = getlogin || getpwuid($<) || "Kilroy";
1469 Do not consider getlogin() for authentication: it is not as
1470 secure as getpwuid().
1472 =item getpeername SOCKET
1474 Returns the packed sockaddr address of other end of the SOCKET connection.
1477 $hersockaddr = getpeername(SOCK);
1478 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1479 $herhostname = gethostbyaddr($iaddr, AF_INET);
1480 $herstraddr = inet_ntoa($iaddr);
1484 Returns the current process group for the specified PID. Use
1485 a PID of 0 to get the current process group for the
1486 current process. Will raise an exception if used on a machine that
1487 doesn't implement getpgrp(2). If PID is omitted, returns process
1488 group of current process. Note that the POSIX version of getpgrp()
1489 does not accept a PID argument, so only PID==0 is truly portable.
1493 Returns the process id of the parent process.
1495 =item getpriority WHICH,WHO
1497 Returns the current priority for a process, a process group, or a user.
1498 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1499 machine that doesn't implement getpriority(2).
1505 =item gethostbyname NAME
1507 =item getnetbyname NAME
1509 =item getprotobyname NAME
1515 =item getservbyname NAME,PROTO
1517 =item gethostbyaddr ADDR,ADDRTYPE
1519 =item getnetbyaddr ADDR,ADDRTYPE
1521 =item getprotobynumber NUMBER
1523 =item getservbyport PORT,PROTO
1541 =item sethostent STAYOPEN
1543 =item setnetent STAYOPEN
1545 =item setprotoent STAYOPEN
1547 =item setservent STAYOPEN
1561 These routines perform the same functions as their counterparts in the
1562 system library. In list context, the return values from the
1563 various get routines are as follows:
1565 ($name,$passwd,$uid,$gid,
1566 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1567 ($name,$passwd,$gid,$members) = getgr*
1568 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1569 ($name,$aliases,$addrtype,$net) = getnet*
1570 ($name,$aliases,$proto) = getproto*
1571 ($name,$aliases,$port,$proto) = getserv*
1573 (If the entry doesn't exist you get a null list.)
1575 In scalar context, you get the name, unless the function was a
1576 lookup by name, in which case you get the other thing, whatever it is.
1577 (If the entry doesn't exist you get the undefined value.) For example:
1579 $uid = getpwnam($name);
1580 $name = getpwuid($num);
1582 $gid = getgrnam($name);
1583 $name = getgrgid($num;
1587 In I<getpw*()> the fields $quota, $comment, and $expire are special
1588 cases in the sense that in many systems they are unsupported. If the
1589 $quota is unsupported, it is an empty scalar. If it is supported, it
1590 usually encodes the disk quota. If the $comment field is unsupported,
1591 it is an empty scalar. If it is supported it usually encodes some
1592 administrative comment about the user. In some systems the $quota
1593 field may be $change or $age, fields that have to do with password
1594 aging. In some systems the $comment field may be $class. The $expire
1595 field, if present, encodes the expiration period of the account or the
1596 password. For the availability and the exact meaning of these fields
1597 in your system, please consult your getpwnam(3) documentation and your
1598 <pwd.h> file. You can also find out from within Perl which meaning
1599 your $quota and $comment fields have and whether you have the $expire
1600 field by using the Config module and the values d_pwquota, d_pwage,
1601 d_pwchange, d_pwcomment, and d_pwexpire.
1603 The $members value returned by I<getgr*()> is a space separated list of
1604 the login names of the members of the group.
1606 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1607 C, it will be returned to you via C<$?> if the function call fails. The
1608 @addrs value returned by a successful call is a list of the raw
1609 addresses returned by the corresponding system library call. In the
1610 Internet domain, each address is four bytes long and you can unpack it
1611 by saying something like:
1613 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1615 If you get tired of remembering which element of the return list contains
1616 which return value, by-name interfaces are also provided in modules:
1617 File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent,
1618 Time::gmtime, Time::localtime, and User::grent. These override the
1619 normal built-in, replacing them with versions that return objects with
1620 the appropriate names for each field. For example:
1624 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1626 Even though it looks like they're the same method calls (uid),
1627 they aren't, because a File::stat object is different from a User::pwent object.
1629 =item getsockname SOCKET
1631 Returns the packed sockaddr address of this end of the SOCKET connection.
1634 $mysockaddr = getsockname(SOCK);
1635 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1637 =item getsockopt SOCKET,LEVEL,OPTNAME
1639 Returns the socket option requested, or undef if there is an error.
1645 Returns the value of EXPR with filename expansions such as the standard Unix shell /bin/sh would
1646 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1647 operator, but you can use it directly. If EXPR is omitted, $_ is used.
1648 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1649 L<perlop/"I/O Operators">.
1653 Converts a time as returned by the time function to a 9-element array
1654 with the time localized for the standard Greenwich time zone.
1655 Typically used as follows:
1658 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1661 All array elements are numeric, and come straight out of a struct tm.
1662 In particular this means that $mon has the range 0..11 and $wday has
1663 the range 0..6 with sunday as day 0. Also, $year is the number of
1664 years since 1900, I<not> simply the last two digits of the year.
1666 If EXPR is omitted, does C<gmtime(time())>.
1668 In scalar context, returns the ctime(3) value:
1670 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1672 Also see the timegm() function provided by the Time::Local module,
1673 and the strftime(3) function available via the POSIX module.
1681 The goto-LABEL form finds the statement labeled with LABEL and resumes
1682 execution there. It may not be used to go into any construct that
1683 requires initialization, such as a subroutine or a foreach loop. It
1684 also can't be used to go into a construct that is optimized away,
1685 or to get out of a block or subroutine given to sort().
1686 It can be used to go almost anywhere else within the dynamic scope,
1687 including out of subroutines, but it's usually better to use some other
1688 construct such as last or die. The author of Perl has never felt the
1689 need to use this form of goto (in Perl, that is--C is another matter).
1691 The goto-EXPR form expects a label name, whose scope will be resolved
1692 dynamically. This allows for computed gotos per FORTRAN, but isn't
1693 necessarily recommended if you're optimizing for maintainability:
1695 goto ("FOO", "BAR", "GLARCH")[$i];
1697 The goto-&NAME form is highly magical, and substitutes a call to the
1698 named subroutine for the currently running subroutine. This is used by
1699 AUTOLOAD subroutines that wish to load another subroutine and then
1700 pretend that the other subroutine had been called in the first place
1701 (except that any modifications to @_ in the current subroutine are
1702 propagated to the other subroutine.) After the goto, not even caller()
1703 will be able to tell that this routine was called first.
1705 =item grep BLOCK LIST
1707 =item grep EXPR,LIST
1709 This is similar in spirit to, but not the same as, grep(1)
1710 and its relatives. In particular, it is not limited to using
1711 regular expressions.
1713 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1714 $_ to each element) and returns the list value consisting of those
1715 elements for which the expression evaluated to TRUE. In a scalar
1716 context, returns the number of times the expression was TRUE.
1718 @foo = grep(!/^#/, @bar); # weed out comments
1722 @foo = grep {!/^#/} @bar; # weed out comments
1724 Note that, because $_ is a reference into the list value, it can be used
1725 to modify the elements of the array. While this is useful and
1726 supported, it can cause bizarre results if the LIST is not a named
1727 array. Similarly, grep returns aliases into the original list,
1728 much like the way that a for loops's index variable aliases the list
1729 elements. That is, modifying an element of a list returned by grep
1730 (for example, in a C<foreach>, C<map> or another C<grep>)
1731 actually modifies the element in the original list.
1733 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1739 Interprets EXPR as a hex string and returns the corresponding
1740 value. (To convert strings that might start with either 0 or 0x
1741 see L</oct>.) If EXPR is omitted, uses $_.
1743 print hex '0xAf'; # prints '175'
1744 print hex 'aF'; # same
1748 There is no builtin import() function. It is just an ordinary
1749 method (subroutine) defined (or inherited) by modules that wish to export
1750 names to another module. The use() function calls the import() method
1751 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1753 =item index STR,SUBSTR,POSITION
1755 =item index STR,SUBSTR
1757 Returns the position of the first occurrence of SUBSTR in STR at or after
1758 POSITION. If POSITION is omitted, starts searching from the beginning of
1759 the string. The return value is based at 0 (or whatever you've set the C<$[>
1760 variable to--but don't do that). If the substring is not found, returns
1761 one less than the base, ordinarily -1.
1767 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1768 You should not use this for rounding, because it truncates
1769 towards 0, and because machine representations of floating point
1770 numbers can sometimes produce counterintuitive results. Usually sprintf() or printf(),
1771 or the POSIX::floor or POSIX::ceil functions, would serve you better.
1773 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1775 Implements the ioctl(2) function. You'll probably have to say
1777 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1779 first to get the correct function definitions. If F<ioctl.ph> doesn't
1780 exist or doesn't have the correct definitions you'll have to roll your
1781 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1782 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1783 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1784 written depending on the FUNCTION--a pointer to the string value of SCALAR
1785 will be passed as the third argument of the actual ioctl call. (If SCALAR
1786 has no string value but does have a numeric value, that value will be
1787 passed rather than a pointer to the string value. To guarantee this to be
1788 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1789 functions are useful for manipulating the values of structures used by
1790 ioctl(). The following example sets the erase character to DEL.
1794 die "NO TIOCGETP" if $@ || !$getp;
1795 $sgttyb_t = "ccccs"; # 4 chars and a short
1796 if (ioctl(STDIN,$getp,$sgttyb)) {
1797 @ary = unpack($sgttyb_t,$sgttyb);
1799 $sgttyb = pack($sgttyb_t,@ary);
1800 ioctl(STDIN,&TIOCSETP,$sgttyb)
1801 || die "Can't ioctl: $!";
1804 The return value of ioctl (and fcntl) is as follows:
1806 if OS returns: then Perl returns:
1808 0 string "0 but true"
1809 anything else that number
1811 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1812 still easily determine the actual value returned by the operating
1815 ($retval = ioctl(...)) || ($retval = -1);
1816 printf "System returned %d\n", $retval;
1818 The special string "0 but true" is excempt from B<-w> complaints
1819 about improper numeric conversions.
1821 =item join EXPR,LIST
1823 Joins the separate strings of LIST into a single string with
1824 fields separated by the value of EXPR, and returns the string.
1827 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1829 See L<perlfunc/split>.
1833 Returns a list consisting of all the keys of the named hash. (In a
1834 scalar context, returns the number of keys.) The keys are returned in
1835 an apparently random order, but it is the same order as either the
1836 values() or each() function produces (given that the hash has not been
1837 modified). As a side effect, it resets HASH's iterator.
1839 Here is yet another way to print your environment:
1842 @values = values %ENV;
1843 while ($#keys >= 0) {
1844 print pop(@keys), '=', pop(@values), "\n";
1847 or how about sorted by key:
1849 foreach $key (sort(keys %ENV)) {
1850 print $key, '=', $ENV{$key}, "\n";
1853 To sort an array by value, you'll need to use a C<sort> function.
1854 Here's a descending numeric sort of a hash by its values:
1856 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1857 printf "%4d %s\n", $hash{$key}, $key;
1860 As an lvalue C<keys> allows you to increase the number of hash buckets
1861 allocated for the given hash. This can gain you a measure of efficiency if
1862 you know the hash is going to get big. (This is similar to pre-extending
1863 an array by assigning a larger number to $#array.) If you say
1867 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1868 it rounds up to the next power of two. These
1869 buckets will be retained even if you do C<%hash = ()>, use C<undef
1870 %hash> if you want to free the storage while C<%hash> is still in scope.
1871 You can't shrink the number of buckets allocated for the hash using
1872 C<keys> in this way (but you needn't worry about doing this by accident,
1873 as trying has no effect).
1877 Sends a signal to a list of processes. The first element of
1878 the list must be the signal to send. Returns the number of
1879 processes successfully signaled.
1881 $cnt = kill 1, $child1, $child2;
1884 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1885 process groups instead of processes. (On System V, a negative I<PROCESS>
1886 number will also kill process groups, but that's not portable.) That
1887 means you usually want to use positive not negative signals. You may also
1888 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1894 The C<last> command is like the C<break> statement in C (as used in
1895 loops); it immediately exits the loop in question. If the LABEL is
1896 omitted, the command refers to the innermost enclosing loop. The
1897 C<continue> block, if any, is not executed:
1899 LINE: while (<STDIN>) {
1900 last LINE if /^$/; # exit when done with header
1904 See also L</continue> for an illustration of how C<last>, C<next>, and
1911 Returns an lowercased version of EXPR. This is the internal function
1912 implementing the \L escape in double-quoted strings.
1913 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1915 If EXPR is omitted, uses $_.
1921 Returns the value of EXPR with the first character lowercased. This is
1922 the internal function implementing the \l escape in double-quoted strings.
1923 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1925 If EXPR is omitted, uses $_.
1931 Returns the length in bytes of the value of EXPR. If EXPR is
1932 omitted, returns length of $_.
1934 =item link OLDFILE,NEWFILE
1936 Creates a new filename linked to the old filename. Returns TRUE for
1937 success, FALSE otherwise.
1939 =item listen SOCKET,QUEUESIZE
1941 Does the same thing that the listen system call does. Returns TRUE if
1942 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1946 A local modifies the listed variables to be local to the enclosing
1947 block, file, or eval. If more than one value is listed, the list must
1948 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1949 for details, including issues with tied arrays and hashes.
1951 You really probably want to be using my() instead, because local() isn't
1952 what most people think of as "local". See L<perlsub/"Private Variables
1953 via my()"> for details.
1955 =item localtime EXPR
1957 Converts a time as returned by the time function to a 9-element array
1958 with the time analyzed for the local time zone. Typically used as
1962 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1965 All array elements are numeric, and come straight out of a struct tm.
1966 In particular this means that $mon has the range 0..11 and $wday has
1967 the range 0..6 with sunday as day 0. Also, $year is the number of
1968 years since 1900, that is, $year is 123 in year 2023.
1970 If EXPR is omitted, uses the current time (C<localtime(time)>).
1972 In scalar context, returns the ctime(3) value:
1974 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1976 This scalar value is B<not> locale dependent, see L<perllocale>, but
1977 instead a Perl builtin. Also see the Time::Local module, and the
1978 strftime(3) and mktime(3) function available via the POSIX module. To
1979 get somewhat similar but locale dependent date strings, set up your
1980 locale environment variables appropriately (please see L<perllocale>)
1981 and try for example:
1983 use POSIX qw(strftime);
1984 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1986 Note that the C<%a> and C<%b>, the short forms of the day of the week
1987 and the month of the year, may not necessarily be three characters wide.
1993 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1996 =item lstat FILEHANDLE
2002 Does the same thing as the stat() function (including setting the
2003 special C<_> filehandle) but stats a symbolic link instead of the file
2004 the symbolic link points to. If symbolic links are unimplemented on
2005 your system, a normal stat() is done.
2007 If EXPR is omitted, stats $_.
2011 The match operator. See L<perlop>.
2013 =item map BLOCK LIST
2017 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
2018 element) and returns the list value composed of the results of each such
2019 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2020 may produce zero, one, or more elements in the returned value.
2022 @chars = map(chr, @nums);
2024 translates a list of numbers to the corresponding characters. And
2026 %hash = map { getkey($_) => $_ } @array;
2028 is just a funny way to write
2031 foreach $_ (@array) {
2032 $hash{getkey($_)} = $_;
2035 Note that, because $_ is a reference into the list value, it can be used
2036 to modify the elements of the array. While this is useful and
2037 supported, it can cause bizarre results if the LIST is not a named
2038 array. See also L</grep> for an array composed of those items of the
2039 original list for which the BLOCK or EXPR evaluates to true.
2041 =item mkdir FILENAME,MODE
2043 Creates the directory specified by FILENAME, with permissions specified
2044 by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2045 it returns FALSE and sets C<$!> (errno).
2047 =item msgctl ID,CMD,ARG
2049 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
2050 must be a variable that will hold the returned msqid_ds structure.
2051 Returns like ioctl: the undefined value for error, "0 but true" for
2052 zero, or the actual return value otherwise.
2054 =item msgget KEY,FLAGS
2056 Calls the System V IPC function msgget(2). Returns the message queue id,
2057 or the undefined value if there is an error.
2059 =item msgsnd ID,MSG,FLAGS
2061 Calls the System V IPC function msgsnd to send the message MSG to the
2062 message queue ID. MSG must begin with the long integer message type,
2063 which may be created with C<pack("l", $type)>. Returns TRUE if
2064 successful, or FALSE if there is an error.
2066 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2068 Calls the System V IPC function msgrcv to receive a message from
2069 message queue ID into variable VAR with a maximum message size of
2070 SIZE. Note that if a message is received, the message type will be the
2071 first thing in VAR, and the maximum length of VAR is SIZE plus the size
2072 of the message type. Returns TRUE if successful, or FALSE if there is
2077 A "my" declares the listed variables to be local (lexically) to the
2078 enclosing block, file, or C<eval>. If
2079 more than one value is listed, the list must be placed in parentheses. See
2080 L<perlsub/"Private Variables via my()"> for details.
2086 The C<next> command is like the C<continue> statement in C; it starts
2087 the next iteration of the loop:
2089 LINE: while (<STDIN>) {
2090 next LINE if /^#/; # discard comments
2094 Note that if there were a C<continue> block on the above, it would get
2095 executed even on discarded lines. If the LABEL is omitted, the command
2096 refers to the innermost enclosing loop.
2098 See also L</continue> for an illustration of how C<last>, C<next>, and
2101 =item no Module LIST
2103 See the "use" function, which "no" is the opposite of.
2109 Interprets EXPR as an octal string and returns the corresponding
2110 value. (If EXPR happens to start off with 0x, interprets it as
2111 a hex string instead.) The following will handle decimal, octal, and
2112 hex in the standard Perl or C notation:
2114 $val = oct($val) if $val =~ /^0/;
2116 If EXPR is omitted, uses $_. This function is commonly used when
2117 a string such as "644" needs to be converted into a file mode, for
2118 example. (Although perl will automatically convert strings into
2119 numbers as needed, this automatic conversion assumes base 10.)
2121 =item open FILEHANDLE,EXPR
2123 =item open FILEHANDLE
2125 Opens the file whose filename is given by EXPR, and associates it with
2126 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2127 name of the real filehandle wanted. If EXPR is omitted, the scalar
2128 variable of the same name as the FILEHANDLE contains the filename.
2129 (Note that lexical variables--those declared with C<my>--will not work
2130 for this purpose; so if you're using C<my>, specify EXPR in your call
2133 If the filename begins with '<' or nothing, the file is opened for input.
2134 If the filename begins with '>', the file is truncated and opened for
2135 output, being created if necessary. If the filename begins with '>>',
2136 the file is opened for appending, again being created if necessary.
2137 You can put a '+' in front of the '>' or '<' to indicate that
2138 you want both read and write access to the file; thus '+<' is almost
2139 always preferred for read/write updates--the '+>' mode would clobber the
2140 file first. You can't usually use either read-write mode for updating
2141 textfiles, since they have variable length records. See the B<-i>
2142 switch in L<perlrun> for a better approach.
2144 The prefix and the filename may be separated with spaces.
2145 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
2146 'w+', 'a', and 'a+'.
2148 If the filename begins with "|", the filename is interpreted as a
2149 command to which output is to be piped, and if the filename ends with a
2150 "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
2151 for more examples of this. (You are not allowed to open() to a command
2152 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2153 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2155 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
2156 nonzero upon success, the undefined value otherwise. If the open
2157 involved a pipe, the return value happens to be the pid of the
2160 If you're unfortunate enough to be running Perl on a system that
2161 distinguishes between text files and binary files (modern operating
2162 systems don't care), then you should check out L</binmode> for tips for
2163 dealing with this. The key distinction between systems that need binmode
2164 and those that don't is their text file formats. Systems like Unix, MacOS, and
2165 Plan9, which delimit lines with a single character, and which encode that
2166 character in C as '\n', do not need C<binmode>. The rest need it.
2168 When opening a file, it's usually a bad idea to continue normal execution
2169 if the request failed, so C<open> is frequently used in connection with
2170 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2171 where you want to make a nicely formatted error message (but there are
2172 modules that can help with that problem)) you should always check
2173 the return value from opening a file. The infrequent exception is when
2174 working with an unopened filehandle is actually what you want to do.
2179 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2180 while (<ARTICLE>) {...
2182 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2183 # if the open fails, output is discarded
2185 open(DBASE, '+<dbase.mine') # open for update
2186 or die "Can't open 'dbase.mine' for update: $!";
2188 open(ARTICLE, "caesar <$article |") # decrypt article
2189 or die "Can't start caesar: $!";
2191 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2192 or die "Can't start sort: $!";
2194 # process argument list of files along with any includes
2196 foreach $file (@ARGV) {
2197 process($file, 'fh00');
2201 my($filename, $input) = @_;
2202 $input++; # this is a string increment
2203 unless (open($input, $filename)) {
2204 print STDERR "Can't open $filename: $!\n";
2209 while (<$input>) { # note use of indirection
2210 if (/^#include "(.*)"/) {
2211 process($1, $input);
2218 You may also, in the Bourne shell tradition, specify an EXPR beginning
2219 with "E<gt>&", in which case the rest of the string is interpreted as the
2220 name of a filehandle (or file descriptor, if numeric) to be
2221 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
2222 +E<gt>E<gt>, and +E<lt>. The
2223 mode you specify should match the mode of the original filehandle.
2224 (Duping a filehandle does not take into account any existing contents of
2226 Here is a script that saves, redirects, and restores STDOUT and
2230 open(OLDOUT, ">&STDOUT");
2231 open(OLDERR, ">&STDERR");
2233 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2234 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2236 select(STDERR); $| = 1; # make unbuffered
2237 select(STDOUT); $| = 1; # make unbuffered
2239 print STDOUT "stdout 1\n"; # this works for
2240 print STDERR "stderr 1\n"; # subprocesses too
2245 open(STDOUT, ">&OLDOUT");
2246 open(STDERR, ">&OLDERR");
2248 print STDOUT "stdout 2\n";
2249 print STDERR "stderr 2\n";
2252 If you specify "E<lt>&=N", where N is a number, then Perl will do an
2253 equivalent of C's fdopen() of that file descriptor; this is more
2254 parsimonious of file descriptors. For example:
2256 open(FILEHANDLE, "<&=$fd")
2258 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
2259 there is an implicit fork done, and the return value of open is the pid
2260 of the child within the parent process, and 0 within the child
2261 process. (Use C<defined($pid)> to determine whether the open was successful.)
2262 The filehandle behaves normally for the parent, but i/o to that
2263 filehandle is piped from/to the STDOUT/STDIN of the child process.
2264 In the child process the filehandle isn't opened--i/o happens from/to
2265 the new STDOUT or STDIN. Typically this is used like the normal
2266 piped open when you want to exercise more control over just how the
2267 pipe command gets executed, such as when you are running setuid, and
2268 don't want to have to scan shell commands for metacharacters.
2269 The following pairs are more or less equivalent:
2271 open(FOO, "|tr '[a-z]' '[A-Z]'");
2272 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2274 open(FOO, "cat -n '$file'|");
2275 open(FOO, "-|") || exec 'cat', '-n', $file;
2277 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2279 NOTE: On any operation that may do a fork, any unflushed buffers remain
2280 unflushed in both processes, which means you may need to set C<$|> to
2281 avoid duplicate output.
2283 Closing any piped filehandle causes the parent process to wait for the
2284 child to finish, and returns the status value in C<$?>.
2286 The filename passed to open will have leading and trailing
2287 whitespace deleted, and the normal redirection chararacters
2288 honored. This property, known as "magic open",
2289 can often be used to good effect. A user could specify a filename of
2290 "rsh cat file |", or you could change certain filenames as needed:
2292 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2293 open(FH, $filename) or die "Can't open $filename: $!";
2295 However, to open a file with arbitrary weird characters in it, it's
2296 necessary to protect any leading and trailing whitespace:
2298 $file =~ s#^(\s)#./$1#;
2299 open(FOO, "< $file\0");
2301 If you want a "real" C open() (see L<open(2)> on your system), then you
2302 should use the sysopen() function, which involves no such magic. This is
2303 another way to protect your filenames from interpretation. For example:
2306 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2307 or die "sysopen $path: $!";
2308 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2309 print HANDLE "stuff $$\n");
2311 print "File contains: ", <HANDLE>;
2313 Using the constructor from the IO::Handle package (or one of its
2314 subclasses, such as IO::File or IO::Socket), you can generate anonymous
2315 filehandles that have the scope of whatever variables hold references to
2316 them, and automatically close whenever and however you leave that scope:
2320 sub read_myfile_munged {
2322 my $handle = new IO::File;
2323 open($handle, "myfile") or die "myfile: $!";
2325 or return (); # Automatically closed here.
2326 mung $first or die "mung failed"; # Or here.
2327 return $first, <$handle> if $ALL; # Or here.
2331 See L</seek()> for some details about mixing reading and writing.
2333 =item opendir DIRHANDLE,EXPR
2335 Opens a directory named EXPR for processing by readdir(), telldir(),
2336 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2337 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2343 Returns the numeric ascii value of the first character of EXPR. If
2344 EXPR is omitted, uses $_. For the reverse, see L</chr>.
2346 =item pack TEMPLATE,LIST
2348 Takes an array or list of values and packs it into a binary structure,
2349 returning the string containing the structure. The TEMPLATE is a
2350 sequence of characters that give the order and type of values, as
2353 A An ascii string, will be space padded.
2354 a An ascii string, will be null padded.
2355 b A bit string (ascending bit order, like vec()).
2356 B A bit string (descending bit order).
2357 h A hex string (low nybble first).
2358 H A hex string (high nybble first).
2360 c A signed char value.
2361 C An unsigned char value.
2363 s A signed short value.
2364 S An unsigned short value.
2365 (This 'short' is _exactly_ 16 bits, which may differ from
2366 what a local C compiler calls 'short'.)
2368 i A signed integer value.
2369 I An unsigned integer value.
2370 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2371 depends on what a local C compiler calls 'int', and may
2372 even be larger than the 'long' described in the next item.)
2374 l A signed long value.
2375 L An unsigned long value.
2376 (This 'long' is _exactly_ 32 bits, which may differ from
2377 what a local C compiler calls 'long'.)
2379 n A short in "network" (big-endian) order.
2380 N A long in "network" (big-endian) order.
2381 v A short in "VAX" (little-endian) order.
2382 V A long in "VAX" (little-endian) order.
2383 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2384 _exactly_ 32 bits, respectively.)
2386 f A single-precision float in the native format.
2387 d A double-precision float in the native format.
2389 p A pointer to a null-terminated string.
2390 P A pointer to a structure (fixed-length string).
2392 u A uuencoded string.
2394 w A BER compressed integer. Its bytes represent an unsigned
2395 integer in base 128, most significant digit first, with as few
2396 digits as possible. Bit eight (the high bit) is set on each
2397 byte except the last.
2401 @ Null fill to absolute position.
2403 Each letter may optionally be followed by a number giving a repeat
2404 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2405 pack function will gobble up that many values from the LIST. A * for the
2406 repeat count means to use however many items are left. The "a" and "A"
2407 types gobble just one value, but pack it as a string of length count,
2408 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2409 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2410 fields pack a string that many bits long. The "h" and "H" fields pack a
2411 string that many nybbles long. The "p" type packs a pointer to a null-
2412 terminated string. You are responsible for ensuring the string is not a
2413 temporary value (which can potentially get deallocated before you get
2414 around to using the packed result). The "P" packs a pointer to a structure
2415 of the size indicated by the length. A NULL pointer is created if the
2416 corresponding value for "p" or "P" is C<undef>.
2417 Real numbers (floats and doubles) are
2418 in the native machine format only; due to the multiplicity of floating
2419 formats around, and the lack of a standard "network" representation, no
2420 facility for interchange has been made. This means that packed floating
2421 point data written on one machine may not be readable on another - even if
2422 both use IEEE floating point arithmetic (as the endian-ness of the memory
2423 representation is not part of the IEEE spec). Note that Perl uses doubles
2424 internally for all numeric calculation, and converting from double into
2425 float and thence back to double again will lose precision (i.e.,
2426 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2430 $foo = pack("cccc",65,66,67,68);
2432 $foo = pack("c4",65,66,67,68);
2435 $foo = pack("ccxxcc",65,66,67,68);
2438 $foo = pack("s2",1,2);
2439 # "\1\0\2\0" on little-endian
2440 # "\0\1\0\2" on big-endian
2442 $foo = pack("a4","abcd","x","y","z");
2445 $foo = pack("aaaa","abcd","x","y","z");
2448 $foo = pack("a14","abcdefg");
2449 # "abcdefg\0\0\0\0\0\0\0"
2451 $foo = pack("i9pl", gmtime);
2452 # a real struct tm (on my system anyway)
2455 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2458 The same template may generally also be used in the unpack function.
2462 =item package NAMESPACE
2464 Declares the compilation unit as being in the given namespace. The scope
2465 of the package declaration is from the declaration itself through the end of
2466 the enclosing block (the same scope as the local() operator). All further
2467 unqualified dynamic identifiers will be in this namespace. A package
2468 statement affects only dynamic variables--including those you've used
2469 local() on--but I<not> lexical variables created with my(). Typically it
2470 would be the first declaration in a file to be included by the C<require>
2471 or C<use> operator. You can switch into a package in more than one place;
2472 it merely influences which symbol table is used by the compiler for the
2473 rest of that block. You can refer to variables and filehandles in other
2474 packages by prefixing the identifier with the package name and a double
2475 colon: C<$Package::Variable>. If the package name is null, the C<main>
2476 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2478 If NAMESPACE is omitted, then there is no current package, and all
2479 identifiers must be fully qualified or lexicals. This is stricter
2480 than C<use strict>, since it also extends to function names.
2482 See L<perlmod/"Packages"> for more information about packages, modules,
2483 and classes. See L<perlsub> for other scoping issues.
2485 =item pipe READHANDLE,WRITEHANDLE
2487 Opens a pair of connected pipes like the corresponding system call.
2488 Note that if you set up a loop of piped processes, deadlock can occur
2489 unless you are very careful. In addition, note that Perl's pipes use
2490 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2491 after each command, depending on the application.
2493 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2494 for examples of such things.
2500 Pops and returns the last value of the array, shortening the array by
2501 1. Has a similar effect to
2503 $tmp = $ARRAY[$#ARRAY--];
2505 If there are no elements in the array, returns the undefined value.
2506 If ARRAY is omitted, pops the
2507 @ARGV array in the main program, and the @_ array in subroutines, just
2514 Returns the offset of where the last C<m//g> search left off for the variable
2515 is in question ($_ is used when the variable is not specified). May be
2516 modified to change that offset. Such modification will also influence
2517 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2520 =item print FILEHANDLE LIST
2526 Prints a string or a comma-separated list of strings. Returns TRUE
2527 if successful. FILEHANDLE may be a scalar variable name, in which case
2528 the variable contains the name of or a reference to the filehandle, thus introducing one
2529 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2530 token is a term, it may be misinterpreted as an operator unless you
2531 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2532 omitted, prints by default to standard output (or to the last selected
2533 output channel--see L</select>). If LIST is also omitted, prints $_ to
2534 the currently selected output channel. To set the default output channel to something other than
2535 STDOUT use the select operation. Note that, because print takes a
2536 LIST, anything in the LIST is evaluated in list context, and any
2537 subroutine that you call will have one or more of its expressions
2538 evaluated in list context. Also be careful not to follow the print
2539 keyword with a left parenthesis unless you want the corresponding right
2540 parenthesis to terminate the arguments to the print--interpose a + or
2541 put parentheses around all the arguments.
2543 Note that if you're storing FILEHANDLES in an array or other expression,
2544 you will have to use a block returning its value instead:
2546 print { $files[$i] } "stuff\n";
2547 print { $OK ? STDOUT : STDERR } "stuff\n";
2549 =item printf FILEHANDLE FORMAT, LIST
2551 =item printf FORMAT, LIST
2553 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2554 (the output record separator) is not appended. The first argument
2555 of the list will be interpreted as the printf format. If C<use locale> is
2556 in effect, the character used for the decimal point in formatted real numbers
2557 is affected by the LC_NUMERIC locale. See L<perllocale>.
2559 Don't fall into the trap of using a printf() when a simple
2560 print() would do. The print() is more efficient and less
2563 =item prototype FUNCTION
2565 Returns the prototype of a function as a string (or C<undef> if the
2566 function has no prototype). FUNCTION is a reference to, or the name of,
2567 the function whose prototype you want to retrieve.
2569 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2570 a name for Perl builtin. If builtin is not I<overridable> (such as
2571 C<qw>) or its arguments cannot be expressed by a prototype (such as
2572 C<system>) - in other words, the builtin does not behave like a Perl
2573 function - returns C<undef>. Otherwise, the string describing the
2574 equivalent prototype is returned.
2576 =item push ARRAY,LIST
2578 Treats ARRAY as a stack, and pushes the values of LIST
2579 onto the end of ARRAY. The length of ARRAY increases by the length of
2580 LIST. Has the same effect as
2583 $ARRAY[++$#ARRAY] = $value;
2586 but is more efficient. Returns the new number of elements in the array.
2596 Generalized quotes. See L<perlop>.
2598 =item quotemeta EXPR
2602 Returns the value of EXPR with all non-alphanumeric
2603 characters backslashed. (That is, all characters not matching
2604 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2605 returned string, regardless of any locale settings.)
2606 This is the internal function implementing
2607 the \Q escape in double-quoted strings.
2609 If EXPR is omitted, uses $_.
2615 Returns a random fractional number greater than or equal to 0 and less
2616 than the value of EXPR. (EXPR should be positive.) If EXPR is
2617 omitted, the value 1 is used. Automatically calls srand() unless
2618 srand() has already been called. See also srand().
2620 (Note: If your rand function consistently returns numbers that are too
2621 large or too small, then your version of Perl was probably compiled
2622 with the wrong number of RANDBITS.)
2624 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2626 =item read FILEHANDLE,SCALAR,LENGTH
2628 Attempts to read LENGTH bytes of data into variable SCALAR from the
2629 specified FILEHANDLE. Returns the number of bytes actually read,
2630 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2631 or shrunk to the length actually read. An OFFSET may be specified to
2632 place the read data at some other place than the beginning of the
2633 string. This call is actually implemented in terms of stdio's fread(3)
2634 call. To get a true read(2) system call, see sysread().
2636 =item readdir DIRHANDLE
2638 Returns the next directory entry for a directory opened by opendir().
2639 If used in list context, returns all the rest of the entries in the
2640 directory. If there are no more entries, returns an undefined value in
2641 scalar context or a null list in list context.
2643 If you're planning to filetest the return values out of a readdir(), you'd
2644 better prepend the directory in question. Otherwise, because we didn't
2645 chdir() there, it would have been testing the wrong file.
2647 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2648 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2653 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2654 is read and returned. In list context, reads until end-of-file is
2655 reached and returns a list of lines (however you've defined lines
2656 with $/ or $INPUT_RECORD_SEPARATOR).
2657 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2658 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2659 operator is discussed in more detail in L<perlop/"I/O Operators">.
2662 $line = readline(*STDIN); # same thing
2668 Returns the value of a symbolic link, if symbolic links are
2669 implemented. If not, gives a fatal error. If there is some system
2670 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2675 EXPR is executed as a system command.
2676 The collected standard output of the command is returned.
2677 In scalar context, it comes back as a single (potentially
2678 multi-line) string. In list context, returns a list of lines
2679 (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2680 This is the internal function implementing the C<qx/EXPR/>
2681 operator, but you can use it directly. The C<qx/EXPR/>
2682 operator is discussed in more detail in L<perlop/"I/O Operators">.
2684 =item recv SOCKET,SCALAR,LEN,FLAGS
2686 Receives a message on a socket. Attempts to receive LENGTH bytes of
2687 data into variable SCALAR from the specified SOCKET filehandle.
2688 Actually does a C recvfrom(), so that it can return the address of the
2689 sender. Returns the undefined value if there's an error. SCALAR will
2690 be grown or shrunk to the length actually read. Takes the same flags
2691 as the system call of the same name.
2692 See L<perlipc/"UDP: Message Passing"> for examples.
2698 The C<redo> command restarts the loop block without evaluating the
2699 conditional again. The C<continue> block, if any, is not executed. If
2700 the LABEL is omitted, the command refers to the innermost enclosing
2701 loop. This command is normally used by programs that want to lie to
2702 themselves about what was just input:
2704 # a simpleminded Pascal comment stripper
2705 # (warning: assumes no { or } in strings)
2706 LINE: while (<STDIN>) {
2707 while (s|({.*}.*){.*}|$1 |) {}
2712 if (/}/) { # end of comment?
2721 See also L</continue> for an illustration of how C<last>, C<next>, and
2728 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2729 is not specified, $_ will be used. The value returned depends on the
2730 type of thing the reference is a reference to.
2731 Builtin types include:
2740 If the referenced object has been blessed into a package, then that package
2741 name is returned instead. You can think of ref() as a typeof() operator.
2743 if (ref($r) eq "HASH") {
2744 print "r is a reference to a hash.\n";
2747 print "r is not a reference at all.\n";
2750 See also L<perlref>.
2752 =item rename OLDNAME,NEWNAME
2754 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2755 not work across file system boundaries.
2761 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2762 supplied. If EXPR is numeric, demands that the current version of Perl
2763 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2765 Otherwise, demands that a library file be included if it hasn't already
2766 been included. The file is included via the do-FILE mechanism, which is
2767 essentially just a variety of eval(). Has semantics similar to the following
2772 return 1 if $INC{$filename};
2773 my($realfilename,$result);
2775 foreach $prefix (@INC) {
2776 $realfilename = "$prefix/$filename";
2777 if (-f $realfilename) {
2778 $result = do $realfilename;
2782 die "Can't find $filename in \@INC";
2785 die "$filename did not return true value" unless $result;
2786 $INC{$filename} = $realfilename;
2790 Note that the file will not be included twice under the same specified
2791 name. The file must return TRUE as the last statement to indicate
2792 successful execution of any initialization code, so it's customary to
2793 end such a file with "1;" unless you're sure it'll return TRUE
2794 otherwise. But it's better just to put the "C<1;>", in case you add more
2797 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2798 replaces "F<::>" with "F</>" in the filename for you,
2799 to make it easy to load standard modules. This form of loading of
2800 modules does not risk altering your namespace.
2802 In other words, if you try this:
2804 require Foo::Bar; # a splendid bareword
2806 The require function will actually look for the "Foo/Bar.pm" file in the
2807 directories specified in the @INC array.
2809 But if you try this:
2811 $class = 'Foo::Bar';
2812 require $class; # $class is not a bareword
2814 require "Foo::Bar"; # not a bareword because of the ""
2816 The require function will look for the "Foo::Bar" file in the @INC array and
2817 will complain about not finding "Foo::Bar" there. In this case you can do:
2819 eval "require $class";
2821 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2827 Generally used in a C<continue> block at the end of a loop to clear
2828 variables and reset ?? searches so that they work again. The
2829 expression is interpreted as a list of single characters (hyphens
2830 allowed for ranges). All variables and arrays beginning with one of
2831 those letters are reset to their pristine state. If the expression is
2832 omitted, one-match searches (?pattern?) are reset to match again. Resets
2833 only variables or searches in the current package. Always returns
2836 reset 'X'; # reset all X variables
2837 reset 'a-z'; # reset lower case variables
2838 reset; # just reset ?? searches
2840 Resetting "A-Z" is not recommended because you'll wipe out your
2841 ARGV and ENV arrays. Resets only package variables--lexical variables
2842 are unaffected, but they clean themselves up on scope exit anyway,
2843 so you'll probably want to use them instead. See L</my>.
2849 Returns from a subroutine, eval(), or C<do FILE> with the value
2850 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
2851 context, depending on how the return value will be used, and the context
2852 may vary from one execution to the next (see wantarray()). If no EXPR
2853 is given, returns an empty list in list context, an undefined value in
2854 scalar context, or nothing in a void context.
2856 (Note that in the absence of a return, a subroutine, eval, or do FILE
2857 will automatically return the value of the last expression evaluated.)
2861 In list context, returns a list value consisting of the elements
2862 of LIST in the opposite order. In scalar context, concatenates the
2863 elements of LIST, and returns a string value consisting of those bytes,
2864 but in the opposite order.
2866 print reverse <>; # line tac, last line first
2868 undef $/; # for efficiency of <>
2869 print scalar reverse <>; # byte tac, last line tsrif
2871 This operator is also handy for inverting a hash, although there are some
2872 caveats. If a value is duplicated in the original hash, only one of those
2873 can be represented as a key in the inverted hash. Also, this has to
2874 unwind one hash and build a whole new one, which may take some time
2877 %by_name = reverse %by_address; # Invert the hash
2879 =item rewinddir DIRHANDLE
2881 Sets the current position to the beginning of the directory for the
2882 readdir() routine on DIRHANDLE.
2884 =item rindex STR,SUBSTR,POSITION
2886 =item rindex STR,SUBSTR
2888 Works just like index except that it returns the position of the LAST
2889 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2890 last occurrence at or before that position.
2892 =item rmdir FILENAME
2896 Deletes the directory specified by FILENAME if that directory is empty. If it
2897 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
2898 FILENAME is omitted, uses $_.
2902 The substitution operator. See L<perlop>.
2906 Forces EXPR to be interpreted in scalar context and returns the value
2909 @counts = ( scalar @a, scalar @b, scalar @c );
2911 There is no equivalent operator to force an expression to
2912 be interpolated in list context because it's in practice never
2913 needed. If you really wanted to do so, however, you could use
2914 the construction C<@{[ (some expression) ]}>, but usually a simple
2915 C<(some expression)> suffices.
2917 =item seek FILEHANDLE,POSITION,WHENCE
2919 Sets FILEHANDLE's position, just like the fseek() call of stdio.
2920 FILEHANDLE may be an expression whose value gives the name of the
2921 filehandle. The values for WHENCE are 0 to set the new position to
2922 POSITION, 1 to set it to the current position plus POSITION, and 2 to
2923 set it to EOF plus POSITION (typically negative). For WHENCE you may
2924 use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2925 IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2927 If you want to position file for sysread() or syswrite(), don't use
2928 seek() -- buffering makes its effect on the file's system position
2929 unpredictable and non-portable. Use sysseek() instead.
2931 On some systems you have to do a seek whenever you switch between reading
2932 and writing. Amongst other things, this may have the effect of calling
2933 stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2938 This is also useful for applications emulating C<tail -f>. Once you hit
2939 EOF on your read, and then sleep for a while, you might have to stick in a
2940 seek() to reset things. The seek() doesn't change the current position,
2941 but it I<does> clear the end-of-file condition on the handle, so that the
2942 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2944 If that doesn't work (some stdios are particularly cantankerous), then
2945 you may need something more like this:
2948 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2949 # search for some stuff and put it into files
2951 sleep($for_a_while);
2952 seek(FILE, $curpos, 0);
2955 =item seekdir DIRHANDLE,POS
2957 Sets the current position for the readdir() routine on DIRHANDLE. POS
2958 must be a value returned by telldir(). Has the same caveats about
2959 possible directory compaction as the corresponding system library
2962 =item select FILEHANDLE
2966 Returns the currently selected filehandle. Sets the current default
2967 filehandle for output, if FILEHANDLE is supplied. This has two
2968 effects: first, a C<write> or a C<print> without a filehandle will
2969 default to this FILEHANDLE. Second, references to variables related to
2970 output will refer to this output channel. For example, if you have to
2971 set the top of form format for more than one output channel, you might
2979 FILEHANDLE may be an expression whose value gives the name of the
2980 actual filehandle. Thus:
2982 $oldfh = select(STDERR); $| = 1; select($oldfh);
2984 Some programmers may prefer to think of filehandles as objects with
2985 methods, preferring to write the last example as:
2988 STDERR->autoflush(1);
2990 =item select RBITS,WBITS,EBITS,TIMEOUT
2992 This calls the select(2) system call with the bit masks specified, which
2993 can be constructed using fileno() and vec(), along these lines:
2995 $rin = $win = $ein = '';
2996 vec($rin,fileno(STDIN),1) = 1;
2997 vec($win,fileno(STDOUT),1) = 1;
3000 If you want to select on many filehandles you might wish to write a
3004 my(@fhlist) = split(' ',$_[0]);
3007 vec($bits,fileno($_),1) = 1;
3011 $rin = fhbits('STDIN TTY SOCK');
3015 ($nfound,$timeleft) =
3016 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3018 or to block until something becomes ready just do this
3020 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3022 Most systems do not bother to return anything useful in $timeleft, so
3023 calling select() in scalar context just returns $nfound.
3025 Any of the bit masks can also be undef. The timeout, if specified, is
3026 in seconds, which may be fractional. Note: not all implementations are
3027 capable of returning the $timeleft. If not, they always return
3028 $timeleft equal to the supplied $timeout.
3030 You can effect a sleep of 250 milliseconds this way:
3032 select(undef, undef, undef, 0.25);
3034 B<WARNING>: One should not attempt to mix buffered I/O (like read()
3035 or E<lt>FHE<gt>) with select(), except as permitted by POSIX, and even
3036 then only on POSIX systems. You have to use sysread() instead.
3038 =item semctl ID,SEMNUM,CMD,ARG
3040 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
3041 &GETALL, then ARG must be a variable that will hold the returned
3042 semid_ds structure or semaphore value array. Returns like ioctl: the
3043 undefined value for error, "0 but true" for zero, or the actual return
3046 =item semget KEY,NSEMS,FLAGS
3048 Calls the System V IPC function semget. Returns the semaphore id, or
3049 the undefined value if there is an error.
3051 =item semop KEY,OPSTRING
3053 Calls the System V IPC function semop to perform semaphore operations
3054 such as signaling and waiting. OPSTRING must be a packed array of
3055 semop structures. Each semop structure can be generated with
3056 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3057 operations is implied by the length of OPSTRING. Returns TRUE if
3058 successful, or FALSE if there is an error. As an example, the
3059 following code waits on semaphore $semnum of semaphore id $semid:
3061 $semop = pack("sss", $semnum, -1, 0);
3062 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3064 To signal the semaphore, replace "-1" with "1".
3066 =item send SOCKET,MSG,FLAGS,TO
3068 =item send SOCKET,MSG,FLAGS
3070 Sends a message on a socket. Takes the same flags as the system call
3071 of the same name. On unconnected sockets you must specify a
3072 destination to send TO, in which case it does a C sendto(). Returns
3073 the number of characters sent, or the undefined value if there is an
3075 See L<perlipc/"UDP: Message Passing"> for examples.
3077 =item setpgrp PID,PGRP
3079 Sets the current process group for the specified PID, 0 for the current
3080 process. Will produce a fatal error if used on a machine that doesn't
3081 implement setpgrp(2). If the arguments are omitted, it defaults to
3082 0,0. Note that the POSIX version of setpgrp() does not accept any
3083 arguments, so only setpgrp 0,0 is portable.
3085 =item setpriority WHICH,WHO,PRIORITY
3087 Sets the current priority for a process, a process group, or a user.
3088 (See setpriority(2).) Will produce a fatal error if used on a machine
3089 that doesn't implement setpriority(2).
3091 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3093 Sets the socket option requested. Returns undefined if there is an
3094 error. OPTVAL may be specified as undef if you don't want to pass an
3101 Shifts the first value of the array off and returns it, shortening the
3102 array by 1 and moving everything down. If there are no elements in the
3103 array, returns the undefined value. If ARRAY is omitted, shifts the
3104 @_ array within the lexical scope of subroutines and formats, and the
3105 @ARGV array at file scopes or within the lexical scopes established by
3106 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3107 See also unshift(), push(), and pop(). Shift() and unshift() do the
3108 same thing to the left end of an array that pop() and push() do to the
3111 =item shmctl ID,CMD,ARG
3113 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
3114 must be a variable that will hold the returned shmid_ds structure.
3115 Returns like ioctl: the undefined value for error, "0 but true" for
3116 zero, or the actual return value otherwise.
3118 =item shmget KEY,SIZE,FLAGS
3120 Calls the System V IPC function shmget. Returns the shared memory
3121 segment id, or the undefined value if there is an error.
3123 =item shmread ID,VAR,POS,SIZE
3125 =item shmwrite ID,STRING,POS,SIZE
3127 Reads or writes the System V shared memory segment ID starting at
3128 position POS for size SIZE by attaching to it, copying in/out, and
3129 detaching from it. When reading, VAR must be a variable that will
3130 hold the data read. When writing, if STRING is too long, only SIZE
3131 bytes are used; if STRING is too short, nulls are written to fill out
3132 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3134 =item shutdown SOCKET,HOW
3136 Shuts down a socket connection in the manner indicated by HOW, which
3137 has the same interpretation as in the system call of the same name.
3139 shutdown(SOCKET, 0); # I/we have stopped reading data
3140 shutdown(SOCKET, 1); # I/we have stopped writing data
3141 shutdown(SOCKET, 2); # I/we have stopped using this socket
3143 This is useful with sockets when you want to tell the other
3144 side you're done writing but not done reading, or vice versa.
3145 It's also a more insistent form of close because it also
3146 disables the filedescriptor in any forked copies in other
3153 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3156 For the inverse sine operation, you may use the POSIX::asin()
3157 function, or use this relation:
3159 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3165 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3166 May be interrupted if the process receives a signal such as SIGALRM.
3167 Returns the number of seconds actually slept. You probably cannot
3168 mix alarm() and sleep() calls, because sleep() is often implemented
3171 On some older systems, it may sleep up to a full second less than what
3172 you requested, depending on how it counts seconds. Most modern systems
3173 always sleep the full amount. They may appear to sleep longer than that,
3174 however, because your process might not be scheduled right away in a
3175 busy multitasking system.
3177 For delays of finer granularity than one second, you may use Perl's
3178 syscall() interface to access setitimer(2) if your system supports it,
3179 or else see L</select()> above.
3181 See also the POSIX module's sigpause() function.
3183 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3185 Opens a socket of the specified kind and attaches it to filehandle
3186 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3187 system call of the same name. You should "use Socket;" first to get
3188 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3190 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3192 Creates an unnamed pair of sockets in the specified domain, of the
3193 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3194 for the system call of the same name. If unimplemented, yields a fatal
3195 error. Returns TRUE if successful.
3197 Some systems defined pipe() in terms of socketpair, in which a call
3198 to C<pipe(Rdr, Wtr)> is essentially:
3201 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3202 shutdown(Rdr, 1); # no more writing for reader
3203 shutdown(Wtr, 0); # no more reading for writer
3205 See L<perlipc> for an example of socketpair use.
3207 =item sort SUBNAME LIST
3209 =item sort BLOCK LIST
3213 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3214 is omitted, sorts in standard string comparison order. If SUBNAME is
3215 specified, it gives the name of a subroutine that returns an integer
3216 less than, equal to, or greater than 0, depending on how the elements
3217 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3218 operators are extremely useful in such routines.) SUBNAME may be a
3219 scalar variable name (unsubscripted), in which case the value provides
3220 the name of (or a reference to) the actual subroutine to use. In place
3221 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3224 In the interests of efficiency the normal calling code for subroutines is
3225 bypassed, with the following effects: the subroutine may not be a
3226 recursive subroutine, and the two elements to be compared are passed into
3227 the subroutine not via @_ but as the package global variables $a and
3228 $b (see example below). They are passed by reference, so don't
3229 modify $a and $b. And don't try to declare them as lexicals either.
3231 You also cannot exit out of the sort block or subroutine using any of the
3232 loop control operators described in L<perlsyn> or with goto().
3234 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3235 current collation locale. See L<perllocale>.
3240 @articles = sort @files;
3242 # same thing, but with explicit sort routine
3243 @articles = sort {$a cmp $b} @files;
3245 # now case-insensitively
3246 @articles = sort {uc($a) cmp uc($b)} @files;
3248 # same thing in reversed order
3249 @articles = sort {$b cmp $a} @files;
3251 # sort numerically ascending
3252 @articles = sort {$a <=> $b} @files;
3254 # sort numerically descending
3255 @articles = sort {$b <=> $a} @files;
3257 # sort using explicit subroutine name
3259 $age{$a} <=> $age{$b}; # presuming numeric
3261 @sortedclass = sort byage @class;
3263 # this sorts the %age hash by value instead of key
3264 # using an in-line function
3265 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3267 sub backwards { $b cmp $a; }
3268 @harry = ('dog','cat','x','Cain','Abel');
3269 @george = ('gone','chased','yz','Punished','Axed');
3271 # prints AbelCaincatdogx
3272 print sort backwards @harry;
3273 # prints xdogcatCainAbel
3274 print sort @george, 'to', @harry;
3275 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3277 # inefficiently sort by descending numeric compare using
3278 # the first integer after the first = sign, or the
3279 # whole record case-insensitively otherwise
3282 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3287 # same thing, but much more efficiently;
3288 # we'll build auxiliary indices instead
3292 push @nums, /=(\d+)/;
3297 $nums[$b] <=> $nums[$a]
3299 $caps[$a] cmp $caps[$b]
3303 # same thing using a Schwartzian Transform (no temps)
3304 @new = map { $_->[0] }
3305 sort { $b->[1] <=> $a->[1]
3308 } map { [$_, /=(\d+)/, uc($_)] } @old;
3310 If you're using strict, you I<MUST NOT> declare $a
3311 and $b as lexicals. They are package globals. That means
3312 if you're in the C<main> package, it's
3314 @articles = sort {$main::b <=> $main::a} @files;
3318 @articles = sort {$::b <=> $::a} @files;
3320 but if you're in the C<FooPack> package, it's
3322 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3324 The comparison function is required to behave. If it returns
3325 inconsistent results (sometimes saying $x[1] is less than $x[2] and
3326 sometimes saying the opposite, for example) the Perl interpreter will
3327 probably crash and dump core. This is entirely due to and dependent
3328 upon your system's qsort(3) library routine; this routine often avoids
3329 sanity checks in the interest of speed.
3331 =item splice ARRAY,OFFSET,LENGTH,LIST
3333 =item splice ARRAY,OFFSET,LENGTH
3335 =item splice ARRAY,OFFSET
3337 Removes the elements designated by OFFSET and LENGTH from an array, and
3338 replaces them with the elements of LIST, if any. In list context,
3339 returns the elements removed from the array. In scalar context,
3340 returns the last element removed, or C<undef> if no elements are
3341 removed. The array grows or shrinks as necessary.
3342 If OFFSET is negative then it start that far from the end of the array.
3343 If LENGTH is omitted, removes everything from OFFSET onward.
3344 If LENGTH is negative, leave that many elements off the end of the array.
3345 The following equivalences hold (assuming C<$[ == 0>):
3347 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3348 pop(@a) splice(@a,-1)
3349 shift(@a) splice(@a,0,1)
3350 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3351 $a[$x] = $y splice(@a,$x,1,$y)
3353 Example, assuming array lengths are passed before arrays:
3355 sub aeq { # compare two list values
3356 my(@a) = splice(@_,0,shift);
3357 my(@b) = splice(@_,0,shift);
3358 return 0 unless @a == @b; # same len?
3360 return 0 if pop(@a) ne pop(@b);
3364 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3366 =item split /PATTERN/,EXPR,LIMIT
3368 =item split /PATTERN/,EXPR
3370 =item split /PATTERN/
3374 Splits a string into an array of strings, and returns it. By default,
3375 empty leading fields are preserved, and empty trailing ones are deleted.
3377 If not in list context, returns the number of fields found and splits into
3378 the @_ array. (In list context, you can force the split into @_ by
3379 using C<??> as the pattern delimiters, but it still returns the list
3380 value.) The use of implicit split to @_ is deprecated, however, because
3381 it clobbers your subroutine arguments.
3383 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
3384 splits on whitespace (after skipping any leading whitespace). Anything
3385 matching PATTERN is taken to be a delimiter separating the fields. (Note
3386 that the delimiter may be longer than one character.)
3388 If LIMIT is specified and positive, splits into no more than that
3389 many fields (though it may split into fewer). If LIMIT is unspecified
3390 or zero, trailing null fields are stripped (which potential users
3391 of pop() would do well to remember). If LIMIT is negative, it is
3392 treated as if an arbitrarily large LIMIT had been specified.
3394 A pattern matching the null string (not to be confused with
3395 a null pattern C<//>, which is just one member of the set of patterns
3396 matching a null string) will split the value of EXPR into separate
3397 characters at each point it matches that way. For example:
3399 print join(':', split(/ */, 'hi there'));
3401 produces the output 'h:i:t:h:e:r:e'.
3403 The LIMIT parameter can be used to split a line partially
3405 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3407 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3408 one larger than the number of variables in the list, to avoid
3409 unnecessary work. For the list above LIMIT would have been 4 by
3410 default. In time critical applications it behooves you not to split
3411 into more fields than you really need.
3413 If the PATTERN contains parentheses, additional array elements are
3414 created from each matching substring in the delimiter.
3416 split(/([,-])/, "1-10,20", 3);
3418 produces the list value
3420 (1, '-', 10, ',', 20)
3422 If you had the entire header of a normal Unix email message in $header,
3423 you could split it up into fields and their values this way:
3425 $header =~ s/\n\s+/ /g; # fix continuation lines
3426 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3428 The pattern C</PATTERN/> may be replaced with an expression to specify
3429 patterns that vary at runtime. (To do runtime compilation only once,
3430 use C</$variable/o>.)
3432 As a special case, specifying a PATTERN of space (C<' '>) will split on
3433 white space just as split with no arguments does. Thus, split(' ') can
3434 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3435 will give you as many null initial fields as there are leading spaces.
3436 A split on /\s+/ is like a split(' ') except that any leading
3437 whitespace produces a null first field. A split with no arguments
3438 really does a C<split(' ', $_)> internally.
3442 open(PASSWD, '/etc/passwd');
3444 ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/);
3448 (Note that $shell above will still have a newline on it. See L</chop>,
3449 L</chomp>, and L</join>.)
3451 =item sprintf FORMAT, LIST
3453 Returns a string formatted by the usual printf conventions of the
3454 C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3455 on your system for an explanation of the general principles.
3457 Perl does its own sprintf() formatting -- it emulates the C
3458 function sprintf(), but it doesn't use it (except for floating-point
3459 numbers, and even then only the standard modifiers are allowed). As a
3460 result, any non-standard extensions in your local sprintf() are not
3461 available from Perl.
3463 Perl's sprintf() permits the following universally-known conversions:
3466 %c a character with the given number
3468 %d a signed integer, in decimal
3469 %u an unsigned integer, in decimal
3470 %o an unsigned integer, in octal
3471 %x an unsigned integer, in hexadecimal
3472 %e a floating-point number, in scientific notation
3473 %f a floating-point number, in fixed decimal notation
3474 %g a floating-point number, in %e or %f notation
3476 In addition, Perl permits the following widely-supported conversions:
3478 %X like %x, but using upper-case letters
3479 %E like %e, but using an upper-case "E"
3480 %G like %g, but with an upper-case "E" (if applicable)
3481 %p a pointer (outputs the Perl value's address in hexadecimal)
3482 %n special: *stores* the number of characters output so far
3483 into the next variable in the parameter list
3485 Finally, for backward (and we do mean "backward") compatibility, Perl
3486 permits these unnecessary but widely-supported conversions:
3489 %D a synonym for %ld
3490 %U a synonym for %lu
3491 %O a synonym for %lo
3494 Perl permits the following universally-known flags between the C<%>
3495 and the conversion letter:
3497 space prefix positive number with a space
3498 + prefix positive number with a plus sign
3499 - left-justify within the field
3500 0 use zeros, not spaces, to right-justify
3501 # prefix non-zero octal with "0", non-zero hex with "0x"
3502 number minimum field width
3503 .number "precision": digits after decimal point for floating-point,
3504 max length for string, minimum length for integer
3505 l interpret integer as C type "long" or "unsigned long"
3506 h interpret integer as C type "short" or "unsigned short"
3508 There is also one Perl-specific flag:
3510 V interpret integer as Perl's standard integer type
3512 Where a number would appear in the flags, an asterisk ("*") may be
3513 used instead, in which case Perl uses the next item in the parameter
3514 list as the given number (that is, as the field width or precision).
3515 If a field width obtained through "*" is negative, it has the same
3516 effect as the '-' flag: left-justification.
3518 If C<use locale> is in effect, the character used for the decimal
3519 point in formatted real numbers is affected by the LC_NUMERIC locale.
3526 Return the square root of EXPR. If EXPR is omitted, returns square
3533 Sets the random number seed for the C<rand> operator. If EXPR is
3534 omitted, uses a semi-random value based on the current time and process
3535 ID, among other things. In versions of Perl prior to 5.004 the default
3536 seed was just the current time(). This isn't a particularly good seed,
3537 so many old programs supply their own seed value (often C<time ^ $$> or
3538 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3540 In fact, it's usually not necessary to call srand() at all, because if
3541 it is not called explicitly, it is called implicitly at the first use of
3542 the C<rand> operator. However, this was not the case in version of Perl
3543 before 5.004, so if your script will run under older Perl versions, it
3544 should call srand().
3546 Note that you need something much more random than the default seed for
3547 cryptographic purposes. Checksumming the compressed output of one or more
3548 rapidly changing operating system status programs is the usual method. For
3551 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3553 If you're particularly concerned with this, see the Math::TrulyRandom
3556 Do I<not> call srand() multiple times in your program unless you know
3557 exactly what you're doing and why you're doing it. The point of the
3558 function is to "seed" the rand() function so that rand() can produce
3559 a different sequence each time you run your program. Just do it once at the
3560 top of your program, or you I<won't> get random numbers out of rand()!
3562 Frequently called programs (like CGI scripts) that simply use
3566 for a seed can fall prey to the mathematical property that
3570 one-third of the time. So don't do that.
3572 =item stat FILEHANDLE
3578 Returns a 13-element list giving the status info for a file, either
3579 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3580 it stats $_. Returns a null list if the stat fails. Typically used
3583 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3584 $atime,$mtime,$ctime,$blksize,$blocks)
3587 Not all fields are supported on all filesystem types. Here are the
3588 meaning of the fields:
3590 0 dev device number of filesystem
3592 2 mode file mode (type and permissions)
3593 3 nlink number of (hard) links to the file
3594 4 uid numeric user ID of file's owner
3595 5 gid numeric group ID of file's owner
3596 6 rdev the device identifier (special files only)
3597 7 size total size of file, in bytes
3598 8 atime last access time since the epoch
3599 9 mtime last modify time since the epoch
3600 10 ctime inode change time (NOT creation time!) since the epoch
3601 11 blksize preferred block size for file system I/O
3602 12 blocks actual number of blocks allocated
3604 (The epoch was at 00:00 January 1, 1970 GMT.)
3606 If stat is passed the special filehandle consisting of an underline, no
3607 stat is done, but the current contents of the stat structure from the
3608 last stat or filetest are returned. Example:
3610 if (-x $file && (($d) = stat(_)) && $d < 0) {
3611 print "$file is executable NFS file\n";
3614 (This works on machines only for which the device number is negative under NFS.)
3616 In scalar context, C<stat> returns a boolean value indicating success
3617 or failure, and, if successful, sets the information associated with
3618 the special filehandle C<_>.
3624 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3625 doing many pattern matches on the string before it is next modified.
3626 This may or may not save time, depending on the nature and number of
3627 patterns you are searching on, and on the distribution of character
3628 frequencies in the string to be searched -- you probably want to compare
3629 run times with and without it to see which runs faster. Those loops
3630 which scan for many short constant strings (including the constant
3631 parts of more complex patterns) will benefit most. You may have only
3632 one study active at a time -- if you study a different scalar the first
3633 is "unstudied". (The way study works is this: a linked list of every
3634 character in the string to be searched is made, so we know, for
3635 example, where all the 'k' characters are. From each search string,
3636 the rarest character is selected, based on some static frequency tables
3637 constructed from some C programs and English text. Only those places
3638 that contain this "rarest" character are examined.)
3640 For example, here is a loop that inserts index producing entries
3641 before any line containing a certain pattern:
3645 print ".IX foo\n" if /\bfoo\b/;
3646 print ".IX bar\n" if /\bbar\b/;
3647 print ".IX blurfl\n" if /\bblurfl\b/;
3652 In searching for C</\bfoo\b/>, only those locations in $_ that contain "f"
3653 will be looked at, because "f" is rarer than "o". In general, this is
3654 a big win except in pathological cases. The only question is whether
3655 it saves you more time than it took to build the linked list in the
3658 Note that if you have to look for strings that you don't know till
3659 runtime, you can build an entire loop as a string and eval that to
3660 avoid recompiling all your patterns all the time. Together with
3661 undefining $/ to input entire files as one record, this can be very
3662 fast, often faster than specialized programs like fgrep(1). The following
3663 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3664 out the names of those files that contain a match:
3666 $search = 'while (<>) { study;';
3667 foreach $word (@words) {
3668 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3673 eval $search; # this screams
3674 $/ = "\n"; # put back to normal input delimiter
3675 foreach $file (sort keys(%seen)) {
3683 =item sub NAME BLOCK
3685 This is subroutine definition, not a real function I<per se>. With just a
3686 NAME (and possibly prototypes), it's just a forward declaration. Without
3687 a NAME, it's an anonymous function declaration, and does actually return a
3688 value: the CODE ref of the closure you just created. See L<perlsub> and
3689 L<perlref> for details.
3691 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3693 =item substr EXPR,OFFSET,LEN
3695 =item substr EXPR,OFFSET
3697 Extracts a substring out of EXPR and returns it. First character is at
3698 offset 0, or whatever you've set C<$[> to (but don't do that).
3699 If OFFSET is negative (or more precisely, less than C<$[>), starts
3700 that far from the end of the string. If LEN is omitted, returns
3701 everything to the end of the string. If LEN is negative, leaves that
3702 many characters off the end of the string.
3704 If you specify a substring that is partly outside the string, the part
3705 within the string is returned. If the substring is totally outside
3706 the string a warning is produced.
3708 You can use the substr() function
3709 as an lvalue, in which case EXPR must be an lvalue. If you assign
3710 something shorter than LEN, the string will shrink, and if you assign
3711 something longer than LEN, the string will grow to accommodate it. To
3712 keep the string the same length you may need to pad or chop your value
3715 An alternative to using substr() as an lvalue is to specify the
3716 replacement string as the 4th argument. This allows you to replace
3717 parts of the EXPR and return what was there before in one operation.
3719 =item symlink OLDFILE,NEWFILE
3721 Creates a new filename symbolically linked to the old filename.
3722 Returns 1 for success, 0 otherwise. On systems that don't support
3723 symbolic links, produces a fatal error at run time. To check for that,
3726 $symlink_exists = eval { symlink("",""); 1 };
3730 Calls the system call specified as the first element of the list,
3731 passing the remaining elements as arguments to the system call. If
3732 unimplemented, produces a fatal error. The arguments are interpreted
3733 as follows: if a given argument is numeric, the argument is passed as
3734 an int. If not, the pointer to the string value is passed. You are
3735 responsible to make sure a string is pre-extended long enough to
3736 receive any result that might be written into a string. You can't use a
3737 string literal (or other read-only string) as an argument to syscall()
3738 because Perl has to assume that any string pointer might be written
3740 integer arguments are not literals and have never been interpreted in a
3741 numeric context, you may need to add 0 to them to force them to look
3742 like numbers. This emulates the syswrite() function (or vice versa):
3744 require 'syscall.ph'; # may need to run h2ph
3746 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3748 Note that Perl supports passing of up to only 14 arguments to your system call,
3749 which in practice should usually suffice.
3751 Syscall returns whatever value returned by the system call it calls.
3752 If the system call fails, syscall returns -1 and sets C<$!> (errno).
3753 Note that some system calls can legitimately return -1. The proper
3754 way to handle such calls is to assign C<$!=0;> before the call and
3755 check the value of C<$!> if syscall returns -1.
3757 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3758 number of the read end of the pipe it creates. There is no way
3759 to retrieve the file number of the other end. You can avoid this
3760 problem by using C<pipe> instead.
3762 =item sysopen FILEHANDLE,FILENAME,MODE
3764 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3766 Opens the file whose filename is given by FILENAME, and associates it
3767 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3768 the name of the real filehandle wanted. This function calls the
3769 underlying operating system's C<open> function with the parameters
3770 FILENAME, MODE, PERMS.
3772 The possible values and flag bits of the MODE parameter are
3773 system-dependent; they are available via the standard module C<Fcntl>.
3774 However, for historical reasons, some values are universal: zero means
3775 read-only, one means write-only, and two means read/write.
3777 If the file named by FILENAME does not exist and the C<open> call creates
3778 it (typically because MODE includes the O_CREAT flag), then the value of
3779 PERMS specifies the permissions of the newly created file. If you omit
3780 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
3781 These permission values need to be in octal, and are modified by your
3782 process's current C<umask>. The C<umask> value is a number representing
3783 disabled permissions bits--if your C<umask> were 027 (group can't write;
3784 others can't read, write, or execute), then passing C<sysopen> 0666 would
3785 create a file with mode 0640 (C<0666 &~ 027> is 0640).
3787 If you find this C<umask> talk confusing, here's some advice: supply a
3788 creation mode of 0666 for regular files and one of 0777 for directories
3789 (in C<mkdir>) and executable files. This gives users the freedom of
3790 choice: if they want protected files, they might choose process umasks
3791 of 022, 027, or even the particularly antisocial mask of 077. Programs
3792 should rarely if ever make policy decisions better left to the user.
3793 The exception to this is when writing files that should be kept private:
3794 mail files, web browser cookies, I<.rhosts> files, and so on. In short,
3795 seldom if ever use 0644 as argument to C<sysopen> because that takes
3796 away the user's option to have a more permissive umask. Better to omit it.
3798 The IO::File module provides a more object-oriented approach, if you're
3799 into that kind of thing.
3801 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3803 =item sysread FILEHANDLE,SCALAR,LENGTH
3805 Attempts to read LENGTH bytes of data into variable SCALAR from the
3806 specified FILEHANDLE, using the system call read(2). It bypasses
3807 stdio, so mixing this with other kinds of reads, print(), write(),
3808 seek(), or tell() can cause confusion because stdio usually buffers
3809 data. Returns the number of bytes actually read, C<0> at end of file,
3810 or undef if there was an error. SCALAR will be grown or shrunk so that
3811 the last byte actually read is the last byte of the scalar after the read.
3813 An OFFSET may be specified to place the read data at some place in the
3814 string other than the beginning. A negative OFFSET specifies
3815 placement at that many bytes counting backwards from the end of the
3816 string. A positive OFFSET greater than the length of SCALAR results
3817 in the string being padded to the required size with "\0" bytes before
3818 the result of the read is appended.
3820 =item sysseek FILEHANDLE,POSITION,WHENCE
3822 Sets FILEHANDLE's system position using the system call lseek(2). It
3823 bypasses stdio, so mixing this with reads (other than sysread()),
3824 print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3825 be an expression whose value gives the name of the filehandle. The
3826 values for WHENCE are 0 to set the new position to POSITION, 1 to set
3827 the it to the current position plus POSITION, and 2 to set it to EOF
3828 plus POSITION (typically negative). For WHENCE, you may use the
3829 constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3830 or the POSIX module.
3832 Returns the new position, or the undefined value on failure. A position
3833 of zero is returned as the string "0 but true"; thus sysseek() returns
3834 TRUE on success and FALSE on failure, yet you can still easily determine
3839 =item system PROGRAM LIST
3841 Does exactly the same thing as "exec LIST" except that a fork is done
3842 first, and the parent process waits for the child process to complete.
3843 Note that argument processing varies depending on the number of
3844 arguments. If there is more than one argument in LIST, or if LIST is
3845 an array with more than one value, starts the program given by the
3846 first element of the list with arguments given by the rest of the list.
3847 If there is only one scalar argument, the argument is
3848 checked for shell metacharacters, and if there are any, the entire
3849 argument is passed to the system's command shell for parsing (this is
3850 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
3851 there are no shell metacharacters in the argument, it is split into
3852 words and passed directly to execvp(), which is more efficient.
3854 The return value is the exit status of the program as
3855 returned by the wait() call. To get the actual exit value divide by
3856 256. See also L</exec>. This is I<NOT> what you want to use to capture
3857 the output from a command, for that you should use merely backticks or
3858 qx//, as described in L<perlop/"`STRING`">.
3860 Like exec(), system() allows you to lie to a program about its name if
3861 you use the "system PROGRAM LIST" syntax. Again, see L</exec>.
3863 Because system() and backticks block SIGINT and SIGQUIT, killing the
3864 program they're running doesn't actually interrupt your program.
3866 @args = ("command", "arg1", "arg2");
3868 or die "system @args failed: $?"
3870 You can check all the failure possibilities by inspecting
3873 $exit_value = $? >> 8;
3874 $signal_num = $? & 127;
3875 $dumped_core = $? & 128;
3877 When the arguments get executed via the system shell, results
3878 and return codes will be subject to its quirks and capabilities.
3879 See L<perlop/"`STRING`"> and L</exec> for details.
3881 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3883 =item syswrite FILEHANDLE,SCALAR,LENGTH
3885 Attempts to write LENGTH bytes of data from variable SCALAR to the
3886 specified FILEHANDLE, using the system call write(2). It bypasses
3887 stdio, so mixing this with reads (other than sysread()), print(),
3888 write(), seek(), or tell() may cause confusion because stdio usually
3889 buffers data. Returns the number of bytes actually written, or undef
3890 if there was an error. If the LENGTH is greater than the available
3891 data in the SCALAR after the OFFSET, only as much data as is available
3894 An OFFSET may be specified to write the data from some part of the
3895 string other than the beginning. A negative OFFSET specifies writing
3896 that many bytes counting backwards from the end of the string. In the
3897 case the SCALAR is empty you can use OFFSET but only zero offset.
3899 =item tell FILEHANDLE
3903 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3904 expression whose value gives the name of the actual filehandle. If
3905 FILEHANDLE is omitted, assumes the file last read.
3907 =item telldir DIRHANDLE
3909 Returns the current position of the readdir() routines on DIRHANDLE.
3910 Value may be given to seekdir() to access a particular location in a
3911 directory. Has the same caveats about possible directory compaction as
3912 the corresponding system library routine.
3914 =item tie VARIABLE,CLASSNAME,LIST
3916 This function binds a variable to a package class that will provide the
3917 implementation for the variable. VARIABLE is the name of the variable
3918 to be enchanted. CLASSNAME is the name of a class implementing objects
3919 of correct type. Any additional arguments are passed to the "new"
3920 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3921 Typically these are arguments such as might be passed to the dbm_open()
3922 function of C. The object returned by the "new" method is also
3923 returned by the tie() function, which would be useful if you want to
3924 access other methods in CLASSNAME.
3926 Note that functions such as keys() and values() may return huge lists
3927 when used on large objects, like DBM files. You may prefer to use the
3928 each() function to iterate over such. Example:
3930 # print out history file offsets
3932 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3933 while (($key,$val) = each %HIST) {
3934 print $key, ' = ', unpack('L',$val), "\n";
3938 A class implementing a hash should have the following methods:
3940 TIEHASH classname, LIST
3943 STORE this, key, value
3947 NEXTKEY this, lastkey
3949 A class implementing an ordinary array should have the following methods:
3951 TIEARRAY classname, LIST
3954 STORE this, key, value
3957 A class implementing a scalar should have the following methods:
3959 TIESCALAR classname, LIST
3964 Unlike dbmopen(), the tie() function will not use or require a module
3965 for you--you need to do that explicitly yourself. See L<DB_File>
3966 or the F<Config> module for interesting tie() implementations.
3968 For further details see L<perltie>, L<tied VARIABLE>.
3972 Returns a reference to the object underlying VARIABLE (the same value
3973 that was originally returned by the tie() call that bound the variable
3974 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3979 Returns the number of non-leap seconds since whatever time the system
3980 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3981 and 00:00:00 UTC, January 1, 1970 for most other systems).
3982 Suitable for feeding to gmtime() and localtime().
3986 Returns a four-element list giving the user and system times, in
3987 seconds, for this process and the children of this process.
3989 ($user,$system,$cuser,$csystem) = times;
3993 The transliteration operator. Same as y///. See L<perlop>.
3995 =item truncate FILEHANDLE,LENGTH
3997 =item truncate EXPR,LENGTH
3999 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4000 specified length. Produces a fatal error if truncate isn't implemented
4001 on your system. Returns TRUE if successful, the undefined value
4008 Returns an uppercased version of EXPR. This is the internal function
4009 implementing the \U escape in double-quoted strings.
4010 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4012 If EXPR is omitted, uses $_.
4018 Returns the value of EXPR with the first character uppercased. This is
4019 the internal function implementing the \u escape in double-quoted strings.
4020 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4022 If EXPR is omitted, uses $_.
4028 Sets the umask for the process to EXPR and returns the previous value.
4029 If EXPR is omitted, merely returns the current umask. If umask(2) is
4030 not implemented on your system, returns C<undef>. Remember that a
4031 umask is a number, usually given in octal; it is I<not> a string of octal
4032 digits. See also L</oct>, if all you have is a string.
4038 Undefines the value of EXPR, which must be an lvalue. Use only on a
4039 scalar value, an array (using "@"), a hash (using "%"), a subroutine
4040 (using "&"), or a typeglob (using "*"). (Saying C<undef $hash{$key}>
4041 will probably not do what you expect on most predefined variables or
4042 DBM list values, so don't do that; see L<delete>.) Always returns the
4043 undefined value. You can omit the EXPR, in which case nothing is
4044 undefined, but you still get an undefined value that you could, for
4045 instance, return from a subroutine, assign to a variable or pass as a
4046 parameter. Examples:
4049 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4053 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4054 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4055 select undef, undef, undef, 0.25;
4056 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4058 Note that this is a unary operator, not a list operator.
4064 Deletes a list of files. Returns the number of files successfully
4067 $cnt = unlink 'a', 'b', 'c';
4071 Note: unlink will not delete directories unless you are superuser and
4072 the B<-U> flag is supplied to Perl. Even if these conditions are
4073 met, be warned that unlinking a directory can inflict damage on your
4074 filesystem. Use rmdir instead.
4076 If LIST is omitted, uses $_.
4078 =item unpack TEMPLATE,EXPR
4080 Unpack does the reverse of pack: it takes a string representing a
4081 structure and expands it out into a list value, returning the array
4082 value. (In scalar context, it returns merely the first value
4083 produced.) The TEMPLATE has the same format as in the pack function.
4084 Here's a subroutine that does substring:
4087 my($what,$where,$howmuch) = @_;
4088 unpack("x$where a$howmuch", $what);
4093 sub ordinal { unpack("c",$_[0]); } # same as ord()
4095 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4096 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4097 themselves. Default is a 16-bit checksum. For example, the following
4098 computes the same number as the System V sum program:
4101 $checksum += unpack("%16C*", $_);
4105 The following efficiently counts the number of set bits in a bit vector:
4107 $setbits = unpack("%32b*", $selectmask);
4109 =item untie VARIABLE
4111 Breaks the binding between a variable and a package. (See tie().)
4113 =item unshift ARRAY,LIST
4115 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4116 depending on how you look at it. Prepends list to the front of the
4117 array, and returns the new number of elements in the array.
4119 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4121 Note the LIST is prepended whole, not one element at a time, so the
4122 prepended elements stay in the same order. Use reverse to do the
4125 =item use Module LIST
4129 =item use Module VERSION LIST
4133 Imports some semantics into the current package from the named module,
4134 generally by aliasing certain subroutine or variable names into your
4135 package. It is exactly equivalent to
4137 BEGIN { require Module; import Module LIST; }
4139 except that Module I<must> be a bareword.
4141 If the first argument to C<use> is a number, it is treated as a version
4142 number instead of a module name. If the version of the Perl interpreter
4143 is less than VERSION, then an error message is printed and Perl exits
4144 immediately. This is often useful if you need to check the current
4145 Perl version before C<use>ing library modules that have changed in
4146 incompatible ways from older versions of Perl. (We try not to do
4147 this more than we have to.)
4149 The BEGIN forces the require and import to happen at compile time. The
4150 require makes sure the module is loaded into memory if it hasn't been
4151 yet. The import is not a builtin--it's just an ordinary static method
4152 call into the "Module" package to tell the module to import the list of
4153 features back into the current package. The module can implement its
4154 import method any way it likes, though most modules just choose to
4155 derive their import method via inheritance from the Exporter class that
4156 is defined in the Exporter module. See L<Exporter>. If no import
4157 method can be found then the error is currently silently ignored. This
4158 may change to a fatal error in a future version.
4160 If you don't want your namespace altered, explicitly supply an empty list:
4164 That is exactly equivalent to
4166 BEGIN { require Module }
4168 If the VERSION argument is present between Module and LIST, then the
4169 C<use> will call the VERSION method in class Module with the given
4170 version as an argument. The default VERSION method, inherited from
4171 the Universal class, croaks if the given version is larger than the
4172 value of the variable $Module::VERSION. (Note that there is not a
4173 comma after VERSION!)
4175 Because this is a wide-open interface, pragmas (compiler directives)
4176 are also implemented this way. Currently implemented pragmas are:
4180 use sigtrap qw(SEGV BUS);
4181 use strict qw(subs vars refs);
4182 use subs qw(afunc blurfl);
4184 Some of these these pseudo-modules import semantics into the current
4185 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4186 which import symbols into the current package (which are effective
4187 through the end of the file).
4189 There's a corresponding "no" command that unimports meanings imported
4190 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
4195 If no unimport method can be found the call fails with a fatal error.
4197 See L<perlmod> for a list of standard modules and pragmas.
4201 Changes the access and modification times on each file of a list of
4202 files. The first two elements of the list must be the NUMERICAL access
4203 and modification times, in that order. Returns the number of files
4204 successfully changed. The inode modification time of each file is set
4205 to the current time. This code has the same effect as the "touch"
4206 command if the files already exist:
4210 utime $now, $now, @ARGV;
4214 Returns a list consisting of all the values of the named hash. (In a
4215 scalar context, returns the number of values.) The values are
4216 returned in an apparently random order, but it is the same order as
4217 either the keys() or each() function would produce on the same hash.
4218 As a side effect, it resets HASH's iterator. See also keys(), each(),
4221 =item vec EXPR,OFFSET,BITS
4223 Treats the string in EXPR as a vector of unsigned integers, and
4224 returns the value of the bit field specified by OFFSET. BITS specifies
4225 the number of bits that are reserved for each entry in the bit
4226 vector. This must be a power of two from 1 to 32. vec() may also be
4227 assigned to, in which case parentheses are needed to give the expression
4228 the correct precedence as in
4230 vec($image, $max_x * $x + $y, 8) = 3;
4232 Vectors created with vec() can also be manipulated with the logical
4233 operators |, &, and ^, which will assume a bit vector operation is
4234 desired when both operands are strings.
4236 The following code will build up an ASCII string saying 'PerlPerlPerl'.
4237 The comments show the string after each step. Note that this code works
4238 in the same way on big-endian or little-endian machines.
4241 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4242 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4243 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4244 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4245 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4246 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4247 vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72"
4248 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4249 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4250 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c"
4252 To transform a bit vector into a string or array of 0's and 1's, use these:
4254 $bits = unpack("b*", $vector);
4255 @bits = split(//, unpack("b*", $vector));
4257 If you know the exact length in bits, it can be used in place of the *.
4261 Waits for a child process to terminate and returns the pid of the
4262 deceased process, or -1 if there are no child processes. The status is
4265 =item waitpid PID,FLAGS
4267 Waits for a particular child process to terminate and returns the pid
4268 of the deceased process, or -1 if there is no such child process. The
4269 status is returned in C<$?>. If you say
4271 use POSIX ":sys_wait_h";
4273 waitpid(-1,&WNOHANG);
4275 then you can do a non-blocking wait for any process. Non-blocking wait
4276 is available on machines supporting either the waitpid(2) or
4277 wait4(2) system calls. However, waiting for a particular pid with
4278 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
4279 by remembering the status values of processes that have exited but have
4280 not been harvested by the Perl script yet.)
4282 See L<perlipc> for other examples.
4286 Returns TRUE if the context of the currently executing subroutine is
4287 looking for a list value. Returns FALSE if the context is looking
4288 for a scalar. Returns the undefined value if the context is looking
4289 for no value (void context).
4291 return unless defined wantarray; # don't bother doing more
4292 my @a = complex_calculation();
4293 return wantarray ? @a : "@a";
4297 Produces a message on STDERR just like die(), but doesn't exit or throw
4300 If LIST is empty and $@ already contains a value (typically from a
4301 previous eval) that value is used after appending "\t...caught"
4302 to $@. This is useful for staying almost, but not entirely similar to
4305 If $@ is empty then the string "Warning: Something's wrong" is used.
4307 No message is printed if there is a C<$SIG{__WARN__}> handler
4308 installed. It is the handler's responsibility to deal with the message
4309 as it sees fit (like, for instance, converting it into a die()). Most
4310 handlers must therefore make arrangements to actually display the
4311 warnings that they are not prepared to deal with, by calling warn()
4312 again in the handler. Note that this is quite safe and will not
4313 produce an endless loop, since C<__WARN__> hooks are not called from
4316 You will find this behavior is slightly different from that of
4317 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4318 instead call die() again to change it).
4320 Using a C<__WARN__> handler provides a powerful way to silence all
4321 warnings (even the so-called mandatory ones). An example:
4323 # wipe out *all* compile-time warnings
4324 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4326 my $foo = 20; # no warning about duplicate my $foo,
4327 # but hey, you asked for it!
4328 # no compile-time or run-time warnings before here
4331 # run-time warnings enabled after here
4332 warn "\$foo is alive and $foo!"; # does show up
4334 See L<perlvar> for details on setting C<%SIG> entries, and for more
4337 =item write FILEHANDLE
4343 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4344 using the format associated with that file. By default the format for
4345 a file is the one having the same name as the filehandle, but the
4346 format for the current output channel (see the select() function) may be set
4347 explicitly by assigning the name of the format to the C<$~> variable.
4349 Top of form processing is handled automatically: if there is
4350 insufficient room on the current page for the formatted record, the
4351 page is advanced by writing a form feed, a special top-of-page format
4352 is used to format the new page header, and then the record is written.
4353 By default the top-of-page format is the name of the filehandle with
4354 "_TOP" appended, but it may be dynamically set to the format of your
4355 choice by assigning the name to the C<$^> variable while the filehandle is
4356 selected. The number of lines remaining on the current page is in
4357 variable C<$->, which can be set to 0 to force a new page.
4359 If FILEHANDLE is unspecified, output goes to the current default output
4360 channel, which starts out as STDOUT but may be changed by the
4361 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4362 is evaluated and the resulting string is used to look up the name of
4363 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4365 Note that write is I<NOT> the opposite of read. Unfortunately.
4369 The transliteration operator. Same as tr///. See L<perlop>.