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 C<chown(2)>, C<fork(2)>, C<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 C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<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>, C<qr//>
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 C<$_>, 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 C<1>, and C<-x> and C<-X> return
268 C<1> if any execute bit is set in the mode. Scripts run by the superuser may
269 thus need to do a C<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 C<stat()> or C<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 $a || -w _ || -x _;
305 print "Readable\n" if -r _;
306 print "Writable\n" if -w _;
307 print "Executable\n" if -x _;
308 print "Setuid\n" if -u _;
309 print "Setgid\n" if -g _;
310 print "Sticky\n" if -k _;
311 print "Text\n" if -T _;
312 print "Binary\n" if -B _;
318 Returns the absolute value of its argument.
319 If VALUE is omitted, uses C<$_>.
321 =item accept NEWSOCKET,GENERICSOCKET
323 Accepts an incoming socket connect, just as the C<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 C<$_> is used. (On some machines,
334 unfortunately, the elapsed time may be up to one second less than you
335 specified because of how seconds are counted.) Only one timer may be
336 counting at once. Each call disables the previous timer, and an
337 argument of C<0> may be supplied to cancel the previous timer without
338 starting a new one. The returned value is the amount of time remaining
339 on the previous timer.
341 For delays of finer granularity than one second, you may use Perl's
342 C<syscall()> interface to access C<setitimer(2)> if your system supports it,
343 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
344 and C<sleep()> calls.
346 If you want to use C<alarm()> to time out a system call you need to use an
347 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
348 fail with C<$!> set to EINTR because Perl sets up signal handlers to
349 restart system calls on some systems. Using C<eval>/C<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 $@ eq "alarm\n"; # propagate unexpected errors
368 Returns the arctangent of Y/X in the range -PI to PI.
370 For the tangent operation, you may use the C<POSIX::tan()>
371 function, or use the familiar relation:
373 sub tan { sin($_[0]) / cos($_[0]) }
375 =item bind SOCKET,NAME
377 Binds a network address to a socket, just as the bind system call
378 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
379 packed address of the appropriate type for the socket. See the examples in
380 L<perlipc/"Sockets: Client/Server Communication">.
382 =item binmode FILEHANDLE
384 Arranges for the file to be read or written in "binary" mode in operating
385 systems that distinguish between binary and text files. Files that are
386 not in binary mode have CR LF sequences translated to LF on input and LF
387 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
388 and similarly archaic systems, it may be imperative--otherwise your
389 MS-DOS-damaged C library may mangle your file. The key distinction between
390 systems that need C<binmode> and those that don't is their text file
391 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
392 character, and that encode that character in C as C<"\n">, do not need
393 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
394 is taken as the name of the filehandle.
396 =item bless REF,CLASSNAME
400 This function tells the thingy referenced by REF that it is now
401 an object in the CLASSNAME package--or the current package if no CLASSNAME
402 is specified, which is often the case. It returns the reference for
403 convenience, because a C<bless()> is often the last thing in a constructor.
404 Always use the two-argument version if the function doing the blessing
405 might be inherited by a derived class. See L<perltoot> and L<perlobj>
406 for more about the blessing (and blessings) of objects.
412 Returns the context of the current subroutine call. In scalar context,
413 returns the caller's package name if there is a caller, that is, if
414 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
415 otherwise. In list context, returns
417 ($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 C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
427 call, but an C<eval>. In such a case additional elements C<$evaltext> and
428 C<$is_require> are set: C<$is_require> is true if the frame is created by a
429 C<require> or C<use> statement, C<$evaltext> contains the text of the
430 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
431 C<$filename> is C<"(eval)">, but C<$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 C<@DB::args> to be the
437 arguments with which the subroutine was invoked.
439 Be aware that the optimizer might have optimized call frames away before
440 C<caller> had a chance to get the information. That means that C<caller(N)>
441 might not return information about the call frame you expect it do, for
442 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
443 previous time C<caller()> was called.
447 Changes the working directory to EXPR, if possible. If EXPR is
448 omitted, changes to home directory. Returns TRUE upon success, FALSE
449 otherwise. See example under C<die()>.
453 Changes the permissions of a list of files. The first element of the
454 list must be the numerical mode, which should probably be an octal
455 number, and which definitely should I<not> a string of octal digits:
456 C<0644> is okay, C<'0644'> is not. Returns the number of files
457 successfully changed. See also L</oct>, if all you have is a string.
459 $cnt = chmod 0755, 'foo', 'bar';
460 chmod 0755, @executables;
461 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
462 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
463 $mode = 0644; chmod $mode, 'foo'; # this is best
471 This is a slightly safer version of L</chop>. It removes any
472 line ending that corresponds to the current value of C<$/> (also known as
473 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
474 number of characters removed from all its arguments. It's often used to
475 remove the newline from the end of an input record when you're worried
476 that the final record may be missing its newline. When in paragraph mode
477 (C<$/ = "">), it removes all trailing newlines from the string. If
478 VARIABLE is omitted, it chomps C<$_>. Example:
481 chomp; # avoid \n on last field
486 You can actually chomp anything that's an lvalue, including an assignment:
489 chomp($answer = <STDIN>);
491 If you chomp a list, each element is chomped, and the total number of
492 characters removed is returned.
500 Chops off the last character of a string and returns the character
501 chopped. It's used primarily to remove the newline from the end of an
502 input record, but is much more efficient than C<s/\n//> because it neither
503 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
507 chop; # avoid \n on last field
512 You can actually chop anything that's an lvalue, including an assignment:
515 chop($answer = <STDIN>);
517 If you chop a list, each element is chopped. Only the value of the
518 last C<chop> is returned.
520 Note that C<chop> returns the last character. To return all but the last
521 character, use C<substr($string, 0, -1)>.
525 Changes the owner (and group) of a list of files. The first two
526 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
527 Returns the number of files successfully changed.
529 $cnt = chown $uid, $gid, 'foo', 'bar';
530 chown $uid, $gid, @filenames;
532 Here's an example that looks up nonnumeric uids in the passwd file:
535 chop($user = <STDIN>);
537 chop($pattern = <STDIN>);
539 ($login,$pass,$uid,$gid) = getpwnam($user)
540 or die "$user not in passwd file";
542 @ary = glob($pattern); # expand filenames
543 chown $uid, $gid, @ary;
545 On most systems, you are not allowed to change the ownership of the
546 file unless you're the superuser, although you should be able to change
547 the group to any of your secondary groups. On insecure systems, these
548 restrictions may be relaxed, but this is not a portable assumption.
554 Returns the character represented by that NUMBER in the character set.
555 For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>.
557 If NUMBER is omitted, uses C<$_>.
559 =item chroot FILENAME
563 This function works like the system call by the same name: it makes the
564 named directory the new root directory for all further pathnames that
565 begin with a C<"/"> by your process and all its children. (It doesn't
566 change your current working directory, which is unaffected.) For security
567 reasons, this call is restricted to the superuser. If FILENAME is
568 omitted, does a C<chroot> to C<$_>.
570 =item close FILEHANDLE
574 Closes the file or pipe associated with the file handle, returning TRUE
575 only if stdio successfully flushes buffers and closes the system file
576 descriptor. Closes the currently selected filehandle if the argument
579 You don't have to close FILEHANDLE if you are immediately going to do
580 another C<open()> on it, because C<open()> will close it for you. (See
581 C<open()>.) However, an explicit C<close> on an input file resets the line
582 counter (C<$.>), while the implicit close done by C<open()> does not.
584 If the file handle came from a piped open C<close> will additionally
585 return FALSE if one of the other system calls involved fails or if the
586 program exits with non-zero status. (If the only problem was that the
587 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
588 waits for the process executing on the pipe to complete, in case you
589 want to look at the output of the pipe afterwards. Closing a pipe
590 explicitly also puts the exit status value of the command into C<$?>.
594 open(OUTPUT, '|sort >foo') # pipe to sort
595 or die "Can't start sort: $!";
596 #... # print stuff to output
597 close OUTPUT # wait for sort to finish
598 or warn $! ? "Error closing sort pipe: $!"
599 : "Exit status $? from sort";
600 open(INPUT, 'foo') # get sort's results
601 or die "Can't open 'foo' for input: $!";
603 FILEHANDLE may be an expression whose value can be used as an indirect
604 filehandle, usually the real filehandle name.
606 =item closedir DIRHANDLE
608 Closes a directory opened by C<opendir()> and returns the success of that
611 DIRHANDLE may be an expression whose value can be used as an indirect
612 dirhandle, usually the real dirhandle name.
614 =item connect SOCKET,NAME
616 Attempts to connect to a remote socket, just as the connect system call
617 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
618 packed address of the appropriate type for the socket. See the examples in
619 L<perlipc/"Sockets: Client/Server Communication">.
623 Actually a flow control statement rather than a function. If there is a
624 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
625 C<foreach>), it is always executed just before the conditional is about to
626 be evaluated again, just like the third part of a C<for> loop in C. Thus
627 it can be used to increment a loop variable, even when the loop has been
628 continued via the C<next> statement (which is similar to the C C<continue>
631 C<last>, C<next>, or C<redo> may appear within a C<continue>
632 block. C<last> and C<redo> will behave as if they had been executed within
633 the main block. So will C<next>, but since it will execute a C<continue>
634 block, it may be more entertaining.
637 ### redo always comes here
640 ### next always comes here
642 # then back the top to re-check EXPR
644 ### last always comes here
646 Omitting the C<continue> section is semantically equivalent to using an
647 empty one, logically enough. In that case, C<next> goes directly back
648 to check the condition at the top of the loop.
652 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
653 takes cosine of C<$_>.
655 For the inverse cosine operation, you may use the C<POSIX::acos()>
656 function, or use this relation:
658 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
660 =item crypt PLAINTEXT,SALT
662 Encrypts a string exactly like the C<crypt(3)> function in the C library
663 (assuming that you actually have a version there that has not been
664 extirpated as a potential munition). This can prove useful for checking
665 the password file for lousy passwords, amongst other things. Only the
666 guys wearing white hats should do this.
668 Note that C<crypt()> is intended to be a one-way function, much like breaking
669 eggs to make an omelette. There is no (known) corresponding decrypt
670 function. As a result, this function isn't all that useful for
671 cryptography. (For that, see your nearby CPAN mirror.)
673 Here's an example that makes sure that whoever runs this program knows
676 $pwd = (getpwuid($<))[1];
677 $salt = substr($pwd, 0, 2);
681 chop($word = <STDIN>);
685 if (crypt($word, $salt) ne $pwd) {
691 Of course, typing in your own password to whoever asks you
696 [This function has been superseded by the C<untie()> function.]
698 Breaks the binding between a DBM file and a hash.
700 =item dbmopen HASH,DBNAME,MODE
702 [This function has been superseded by the C<tie()> function.]
704 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
705 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
706 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
707 is the name of the database (without the F<.dir> or F<.pag> extension if
708 any). If the database does not exist, it is created with protection
709 specified by MODE (as modified by the C<umask()>). If your system supports
710 only the older DBM functions, you may perform only one C<dbmopen()> in your
711 program. In older versions of Perl, if your system had neither DBM nor
712 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
715 If you don't have write access to the DBM file, you can only read hash
716 variables, not set them. If you want to test whether you can write,
717 either use file tests or try setting a dummy hash entry inside an C<eval()>,
718 which will trap the error.
720 Note that functions such as C<keys()> and C<values()> may return huge lists
721 when used on large DBM files. You may prefer to use the C<each()>
722 function to iterate over large DBM files. Example:
724 # print out history file offsets
725 dbmopen(%HIST,'/usr/lib/news/history',0666);
726 while (($key,$val) = each %HIST) {
727 print $key, ' = ', unpack('L',$val), "\n";
731 See also L<AnyDBM_File> for a more general description of the pros and
732 cons of the various dbm approaches, as well as L<DB_File> for a particularly
739 Returns a Boolean value telling whether EXPR has a value other than
740 the undefined value C<undef>. If EXPR is not present, C<$_> will be
743 Many operations return C<undef> to indicate failure, end of file,
744 system error, uninitialized variable, and other exceptional
745 conditions. This function allows you to distinguish C<undef> from
746 other values. (A simple Boolean test will not distinguish among
747 C<undef>, zero, the empty string, and C<"0">, which are all equally
748 false.) Note that since C<undef> is a valid scalar, its presence
749 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
750 returns C<undef> when its argument is an empty array, I<or> when the
751 element to return happens to be C<undef>.
753 You may also use C<defined()> to check whether a subroutine exists, by
754 saying C<defined &func> without parentheses. On the other hand, use
755 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
756 produce intuitive results, and should probably be avoided.
758 When used on a hash element, it tells you whether the value is defined,
759 not whether the key exists in the hash. Use L</exists> for the latter
764 print if defined $switch{'D'};
765 print "$val\n" while defined($val = pop(@ary));
766 die "Can't readlink $sym: $!"
767 unless defined($value = readlink $sym);
768 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
769 $debugging = 0 unless defined $debugging;
771 Note: Many folks tend to overuse C<defined()>, and then are surprised to
772 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
773 defined values. For example, if you say
777 The pattern match succeeds, and C<$1> is defined, despite the fact that it
778 matched "nothing". But it didn't really match nothing--rather, it
779 matched something that happened to be C<0> characters long. This is all
780 very above-board and honest. When a function returns an undefined value,
781 it's an admission that it couldn't give you an honest answer. So you
782 should use C<defined()> only when you're questioning the integrity of what
783 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
786 Currently, using C<defined()> on an entire array or hash reports whether
787 memory for that aggregate has ever been allocated. So an array you set
788 to the empty list appears undefined initially, and one that once was full
789 and that you then set to the empty list still appears defined. You
790 should instead use a simple test for size:
792 if (@an_array) { print "has array elements\n" }
793 if (%a_hash) { print "has hash members\n" }
795 Using C<undef()> on these, however, does clear their memory and then report
796 them as not defined anymore, but you shouldn't do that unless you don't
797 plan to use them again, because it saves time when you load them up
798 again to have memory already ready to be filled. The normal way to
799 free up space used by an aggregate is to assign the empty list.
801 This counterintuitive behavior of C<defined()> on aggregates may be
802 changed, fixed, or broken in a future release of Perl.
804 See also L</undef>, L</exists>, L</ref>.
808 Deletes the specified key(s) and their associated values from a hash.
809 For each key, returns the deleted value associated with that key, or
810 the undefined value if there was no such key. Deleting from C<$ENV{}>
811 modifies the environment. Deleting from a hash tied to a DBM file
812 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
813 doesn't necessarily return anything.)
815 The following deletes all the values of a hash:
817 foreach $key (keys %HASH) {
823 delete @HASH{keys %HASH}
825 (But both of these are slower than just assigning the empty list, or
826 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
827 long as the final operation is a hash element lookup or hash slice:
829 delete $ref->[$x][$y]{$key};
830 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
834 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
835 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
836 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
837 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
838 C<$@> and the C<eval()> is terminated with the undefined value. This makes
839 C<die()> the way to raise an exception.
843 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
844 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
846 If the value of EXPR does not end in a newline, the current script line
847 number and input line number (if any) are also printed, and a newline
848 is supplied. Hint: sometimes appending C<", stopped"> to your message
849 will cause it to make better sense when the string C<"at foo line 123"> is
850 appended. Suppose you are running script "canasta".
852 die "/etc/games is no good";
853 die "/etc/games is no good, stopped";
855 produce, respectively
857 /etc/games is no good at canasta line 123.
858 /etc/games is no good, stopped at canasta line 123.
860 See also C<exit()> and C<warn()>.
862 If LIST is empty and C<$@> already contains a value (typically from a
863 previous eval) that value is reused after appending C<"\t...propagated">.
864 This is useful for propagating exceptions:
867 die unless $@ =~ /Expected exception/;
869 If C<$@> is empty then the string C<"Died"> is used.
871 You can arrange for a callback to be run just before the C<die()> does
872 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
873 will be called with the error text and can change the error message, if
874 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
875 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
877 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
878 blocks/strings. If one wants the hook to do nothing in such
883 as the first line of the handler (see L<perlvar/$^S>).
887 Not really a function. Returns the value of the last command in the
888 sequence of commands indicated by BLOCK. When modified by a loop
889 modifier, executes the BLOCK once before testing the loop condition.
890 (On other statements the loop modifiers test the conditional first.)
892 =item do SUBROUTINE(LIST)
894 A deprecated form of subroutine call. See L<perlsub>.
898 Uses the value of EXPR as a filename and executes the contents of the
899 file as a Perl script. Its primary use is to include subroutines
900 from a Perl subroutine library.
906 scalar eval `cat stat.pl`;
908 except that it's more efficient and concise, keeps track of the
909 current filename for error messages, and searches all the B<-I>
910 libraries if the file isn't in the current directory (see also the @INC
911 array in L<perlvar/Predefined Names>). It is also different in how
912 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
913 scope like C<eval STRING> does. It's the same, however, in that it does
914 reparse the file every time you call it, so you probably don't want to
915 do this inside a loop.
917 Note that inclusion of library modules is better done with the
918 C<use()> and C<require()> operators, which also do automatic error checking
919 and raise an exception if there's a problem.
921 You might like to use C<do> to read in a program configuration
922 file. Manual error checking can be done this way:
924 # read in config files: system first, then user
925 for $file ('/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") {
926 unless ($return = do $file) {
927 warn "couldn't parse $file: $@" if $@;
928 warn "couldn't do $file: $!" unless defined $return;
929 warn "couldn't run $file" unless $return;
935 This causes an immediate core dump. Primarily this is so that you can
936 use the B<undump> program to turn your core dump into an executable binary
937 after having initialized all your variables at the beginning of the
938 program. When the new binary is executed it will begin by executing a
939 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
940 it as a goto with an intervening core dump and reincarnation. If LABEL
941 is omitted, restarts the program from the top. WARNING: Any files
942 opened at the time of the dump will NOT be open any more when the
943 program is reincarnated, with possible resulting confusion on the part
944 of Perl. See also B<-u> option in L<perlrun>.
961 dump QUICKSTART if $ARGV[0] eq '-d';
966 This operator is largely obsolete, partly because it's very hard to
967 convert a core file into an executable, and because the real perl-to-C
968 compiler has superseded it.
972 When called in list context, returns a 2-element list consisting of the
973 key and value for the next element of a hash, so that you can iterate over
974 it. When called in scalar context, returns the key for only the "next"
975 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
976 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
979 Entries are returned in an apparently random order. When the hash is
980 entirely read, a null array is returned in list context (which when
981 assigned produces a FALSE (C<0>) value), and C<undef> in
982 scalar context. The next call to C<each()> after that will start iterating
983 again. There is a single iterator for each hash, shared by all C<each()>,
984 C<keys()>, and C<values()> function calls in the program; it can be reset by
985 reading all the elements from the hash, or by evaluating C<keys HASH> or
986 C<values HASH>. If you add or delete elements of a hash while you're
987 iterating over it, you may get entries skipped or duplicated, so don't.
989 The following prints out your environment like the C<printenv(1)> program,
990 only in a different order:
992 while (($key,$value) = each %ENV) {
993 print "$key=$value\n";
996 See also C<keys()> and C<values()>.
1004 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1005 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1006 gives the real filehandle. (Note that this function actually
1007 reads a character and then C<ungetc()>s it, so isn't very useful in an
1008 interactive context.) Do not read from a terminal file (or call
1009 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1010 as terminals may lose the end-of-file condition if you do.
1012 An C<eof> without an argument uses the last file read as argument.
1013 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1014 the files listed on the command line, i.e., C<eof()> is reasonable to
1015 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1016 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1017 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1019 # reset line numbering on each input file
1021 next if /^\s*#/; # skip comments
1024 close ARGV if eof; # Not eof()!
1027 # insert dashes just before last line of last file
1029 if (eof()) { # check for end of current file
1030 print "--------------\n";
1031 close(ARGV); # close or break; is needed if we
1032 # are reading from the terminal
1037 Practical hint: you almost never need to use C<eof> in Perl, because the
1038 input operators return false values when they run out of data, or if there
1045 In the first form, the return value of EXPR is parsed and executed as if it
1046 were a little Perl program. The value of the expression (which is itself
1047 determined within scalar context) is first parsed, and if there weren't any
1048 errors, executed in the context of the current Perl program, so that any
1049 variable settings or subroutine and format definitions remain afterwards.
1050 Note that the value is parsed every time the eval executes. If EXPR is
1051 omitted, evaluates C<$_>. This form is typically used to delay parsing
1052 and subsequent execution of the text of EXPR until run time.
1054 In the second form, the code within the BLOCK is parsed only once--at the
1055 same time the code surrounding the eval itself was parsed--and executed
1056 within the context of the current Perl program. This form is typically
1057 used to trap exceptions more efficiently than the first (see below), while
1058 also providing the benefit of checking the code within BLOCK at compile
1061 The final semicolon, if any, may be omitted from the value of EXPR or within
1064 In both forms, the value returned is the value of the last expression
1065 evaluated inside the mini-program; a return statement may be also used, just
1066 as with subroutines. The expression providing the return value is evaluated
1067 in void, scalar, or list context, depending on the context of the eval itself.
1068 See L</wantarray> for more on how the evaluation context can be determined.
1070 If there is a syntax error or runtime error, or a C<die()> statement is
1071 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1072 error message. If there was no error, C<$@> is guaranteed to be a null
1073 string. Beware that using C<eval()> neither silences perl from printing
1074 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1075 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1076 L</warn> and L<perlvar>.
1078 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1079 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1080 is implemented. It is also Perl's exception trapping mechanism, where
1081 the die operator is used to raise exceptions.
1083 If the code to be executed doesn't vary, you may use the eval-BLOCK
1084 form to trap run-time errors without incurring the penalty of
1085 recompiling each time. The error, if any, is still returned in C<$@>.
1088 # make divide-by-zero nonfatal
1089 eval { $answer = $a / $b; }; warn $@ if $@;
1091 # same thing, but less efficient
1092 eval '$answer = $a / $b'; warn $@ if $@;
1094 # a compile-time error
1095 eval { $answer = }; # WRONG
1098 eval '$answer ='; # sets $@
1100 When using the C<eval{}> form as an exception trap in libraries, you may
1101 wish not to trigger any C<__DIE__> hooks that user code may have
1102 installed. You can use the C<local $SIG{__DIE__}> construct for this
1103 purpose, as shown in this example:
1105 # a very private exception trap for divide-by-zero
1106 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1108 This is especially significant, given that C<__DIE__> hooks can call
1109 C<die()> again, which has the effect of changing their error messages:
1111 # __DIE__ hooks may modify error messages
1113 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1114 eval { die "foo lives here" };
1115 print $@ if $@; # prints "bar lives here"
1118 With an C<eval()>, you should be especially careful to remember what's
1119 being looked at when:
1125 eval { $x }; # CASE 4
1127 eval "\$$x++"; # CASE 5
1130 Cases 1 and 2 above behave identically: they run the code contained in
1131 the variable C<$x>. (Although case 2 has misleading double quotes making
1132 the reader wonder what else might be happening (nothing is).) Cases 3
1133 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1134 does nothing but return the value of C<$x>. (Case 4 is preferred for
1135 purely visual reasons, but it also has the advantage of compiling at
1136 compile-time instead of at run-time.) Case 5 is a place where
1137 normally you I<WOULD> like to use double quotes, except that in this
1138 particular situation, you can just use symbolic references instead, as
1143 =item exec PROGRAM LIST
1145 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1146 use C<system()> instead of C<exec()> if you want it to return. It fails and
1147 returns FALSE only if the command does not exist I<and> it is executed
1148 directly instead of via your system's command shell (see below).
1150 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1151 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1152 or C<exit()> (if C<-w> is set - but you always do that). If you
1153 I<really> want to follow an C<exec()> with some other statement, you
1154 can use one of these styles to avoid the warning:
1156 exec ('foo') or print STDERR "couldn't exec foo: $!";
1157 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1159 If there is more than one argument in LIST, or if LIST is an array
1160 with more than one value, calls C<execvp(3)> with the arguments in LIST.
1161 If there is only one scalar argument or an array with one element in it,
1162 the argument is checked for shell metacharacters, and if there are any,
1163 the entire argument is passed to the system's command shell for parsing
1164 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1165 If there are no shell metacharacters in the argument, it is split into
1166 words and passed directly to C<execvp()>, which is more efficient. Note:
1167 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1168 set C<$|> to avoid lost output. Examples:
1170 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1171 exec "sort $outfile | uniq";
1173 If you don't really want to execute the first argument, but want to lie
1174 to the program you are executing about its own name, you can specify
1175 the program you actually want to run as an "indirect object" (without a
1176 comma) in front of the LIST. (This always forces interpretation of the
1177 LIST as a multivalued list, even if there is only a single scalar in
1180 $shell = '/bin/csh';
1181 exec $shell '-sh'; # pretend it's a login shell
1185 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1187 When the arguments get executed via the system shell, results will
1188 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1191 Using an indirect object with C<exec> or C<system> is also more secure.
1192 This usage forces interpretation of the arguments as a multivalued list,
1193 even if the list had just one argument. That way you're safe from the
1194 shell expanding wildcards or splitting up words with whitespace in them.
1196 @args = ( "echo surprise" );
1198 system @args; # subject to shell escapes if @args == 1
1199 system { $args[0] } @args; # safe even with one-arg list
1201 The first version, the one without the indirect object, ran the I<echo>
1202 program, passing it C<"surprise"> an argument. The second version
1203 didn't--it tried to run a program literally called I<"echo surprise">,
1204 didn't find it, and set C<$?> to a non-zero value indicating failure.
1206 Note that C<exec> will not call your C<END> blocks, nor will it call
1207 any C<DESTROY> methods in your objects.
1211 Returns TRUE if the specified hash key exists in its hash array, even
1212 if the corresponding value is undefined.
1214 print "Exists\n" if exists $array{$key};
1215 print "Defined\n" if defined $array{$key};
1216 print "True\n" if $array{$key};
1218 A hash element can be TRUE only if it's defined, and defined if
1219 it exists, but the reverse doesn't necessarily hold true.
1221 Note that the EXPR can be arbitrarily complicated as long as the final
1222 operation is a hash key lookup:
1224 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1226 Although the last element will not spring into existence just because its
1227 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1228 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1229 test for a $key element. This autovivification may be fixed in a later
1234 Evaluates EXPR and exits immediately with that value. (Actually, it
1235 calls any defined C<END> routines first, but the C<END> routines may not
1236 abort the exit. Likewise any object destructors that need to be called
1237 are called before exit.) Example:
1240 exit 0 if $ans =~ /^[Xx]/;
1242 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1243 universally portable values for EXPR are C<0> for success and C<1> for error;
1244 all other values are subject to unpredictable interpretation depending
1245 on the environment in which the Perl program is running.
1247 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1248 someone might want to trap whatever error happened. Use C<die()> instead,
1249 which can be trapped by an C<eval()>.
1251 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1257 Returns I<e> (the natural logarithm base) to the power of EXPR.
1258 If EXPR is omitted, gives C<exp($_)>.
1260 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1262 Implements the C<fcntl(2)> function. You'll probably have to say
1266 first to get the correct constant definitions. Argument processing and
1267 value return works just like C<ioctl()> below.
1271 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1272 or die "can't fcntl F_GETFL: $!";
1274 You don't have to check for C<defined> on the return from
1275 C<fnctl>. Like C<ioctl>, it maps a C<0> return from the system
1276 call into "C<0> but true" in Perl. This string is true in
1277 boolean context and C<0> in numeric context. It is also
1278 exempt from the normal B<-w> warnings on improper numeric
1281 Note that C<fcntl()> will produce a fatal error if used on a machine that
1282 doesn't implement C<fcntl(2)>.
1284 =item fileno FILEHANDLE
1286 Returns the file descriptor for a filehandle. This is useful for
1287 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1288 operations. If FILEHANDLE is an expression, the value is taken as
1289 an indirect filehandle, generally its name.
1291 You can use this to find out whether two handles refer to the
1292 same underlying descriptor:
1294 if (fileno(THIS) == fileno(THAT)) {
1295 print "THIS and THAT are dups\n";
1298 =item flock FILEHANDLE,OPERATION
1300 Calls C<flock(2)>, or an emulation of it, on FILEHANDLE. Returns TRUE for
1301 success, FALSE on failure. Produces a fatal error if used on a machine
1302 that doesn't implement C<flock(2)>, C<fcntl(2)> locking, or C<lockf(3)>. C<flock()>
1303 is Perl's portable file locking interface, although it locks only entire
1306 On many platforms (including most versions or clones of Unix), locks
1307 established by C<flock()> are B<merely advisory>. Such discretionary locks
1308 are more flexible, but offer fewer guarantees. This means that files
1309 locked with C<flock()> may be modified by programs that do not also use
1310 C<flock()>. Windows NT and OS/2 are among the platforms which
1311 enforce mandatory locking. See your local documentation for details.
1313 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1314 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1315 you can use the symbolic names if import them from the Fcntl module,
1316 either individually, or as a group using the ':flock' tag. LOCK_SH
1317 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1318 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1319 LOCK_EX then C<flock()> will return immediately rather than blocking
1320 waiting for the lock (check the return status to see if you got it).
1322 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1323 before (un)locking it.
1325 Note that the emulation built with C<lockf(3)> doesn't provide shared
1326 locks, and it requires that FILEHANDLE be open with write intent. These
1327 are the semantics that C<lockf(3)> implements. Most (all?) systems
1328 implement C<lockf(3)> in terms of C<fcntl(2)> locking, though, so the
1329 differing semantics shouldn't bite too many people.
1331 Note also that some versions of C<flock()> cannot lock things over the
1332 network; you would need to use the more system-specific C<fcntl()> for
1333 that. If you like you can force Perl to ignore your system's C<flock(2)>
1334 function, and so provide its own C<fcntl(2)>-based emulation, by passing
1335 the switch C<-Ud_flock> to the F<Configure> program when you configure
1338 Here's a mailbox appender for BSD systems.
1340 use Fcntl ':flock'; # import LOCK_* constants
1343 flock(MBOX,LOCK_EX);
1344 # and, in case someone appended
1345 # while we were waiting...
1350 flock(MBOX,LOCK_UN);
1353 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1354 or die "Can't open mailbox: $!";
1357 print MBOX $msg,"\n\n";
1360 See also L<DB_File> for other flock() examples.
1364 Does a C<fork(2)> system call. Returns the child pid to the parent process,
1365 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1367 Note: unflushed buffers remain unflushed in both processes, which means
1368 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1369 method of C<IO::Handle> to avoid duplicate output.
1371 If you C<fork()> without ever waiting on your children, you will accumulate
1374 $SIG{CHLD} = sub { wait };
1376 There's also the double-fork trick (error checking on
1377 C<fork()> returns omitted);
1379 unless ($pid = fork) {
1381 exec "what you really wanna do";
1384 ## (some_perl_code_here)
1391 See also L<perlipc> for more examples of forking and reaping
1394 Note that if your forked child inherits system file descriptors like
1395 STDIN and STDOUT that are actually connected by a pipe or socket, even
1396 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1397 you're done. You should reopen those to F</dev/null> if it's any issue.
1401 Declare a picture format for use by the C<write()> function. For
1405 Test: @<<<<<<<< @||||| @>>>>>
1406 $str, $%, '$' . int($num)
1410 $num = $cost/$quantity;
1414 See L<perlform> for many details and examples.
1416 =item formline PICTURE,LIST
1418 This is an internal function used by C<format>s, though you may call it,
1419 too. It formats (see L<perlform>) a list of values according to the
1420 contents of PICTURE, placing the output into the format output
1421 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1422 Eventually, when a C<write()> is done, the contents of
1423 C<$^A> are written to some filehandle, but you could also read C<$^A>
1424 yourself and then set C<$^A> back to C<"">. Note that a format typically
1425 does one C<formline()> per line of form, but the C<formline()> function itself
1426 doesn't care how many newlines are embedded in the PICTURE. This means
1427 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1428 You may therefore need to use multiple formlines to implement a single
1429 record format, just like the format compiler.
1431 Be careful if you put double quotes around the picture, because an "C<@>"
1432 character may be taken to mean the beginning of an array name.
1433 C<formline()> always returns TRUE. See L<perlform> for other examples.
1435 =item getc FILEHANDLE
1439 Returns the next character from the input file attached to FILEHANDLE,
1440 or the undefined value at end of file, or if there was an error. If
1441 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1442 efficient. It cannot be used to get unbuffered single-characters,
1443 however. For that, try something more like:
1446 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1449 system "stty", '-icanon', 'eol', "\001";
1455 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1458 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1462 Determination of whether $BSD_STYLE should be set
1463 is left as an exercise to the reader.
1465 The C<POSIX::getattr()> function can do this more portably on systems
1466 purporting POSIX compliance.
1467 See also the C<Term::ReadKey> module from your nearest CPAN site;
1468 details on CPAN can be found on L<perlmod/CPAN>.
1472 Implements the C library function of the same name, which on most
1473 systems returns the current login from F</etc/utmp>, if any. If null,
1476 $login = getlogin || getpwuid($<) || "Kilroy";
1478 Do not consider C<getlogin()> for authentication: it is not as
1479 secure as C<getpwuid()>.
1481 =item getpeername SOCKET
1483 Returns the packed sockaddr address of other end of the SOCKET connection.
1486 $hersockaddr = getpeername(SOCK);
1487 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1488 $herhostname = gethostbyaddr($iaddr, AF_INET);
1489 $herstraddr = inet_ntoa($iaddr);
1493 Returns the current process group for the specified PID. Use
1494 a PID of C<0> to get the current process group for the
1495 current process. Will raise an exception if used on a machine that
1496 doesn't implement C<getpgrp(2)>. If PID is omitted, returns process
1497 group of current process. Note that the POSIX version of C<getpgrp()>
1498 does not accept a PID argument, so only C<PID==0> is truly portable.
1502 Returns the process id of the parent process.
1504 =item getpriority WHICH,WHO
1506 Returns the current priority for a process, a process group, or a user.
1507 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1508 machine that doesn't implement C<getpriority(2)>.
1514 =item gethostbyname NAME
1516 =item getnetbyname NAME
1518 =item getprotobyname NAME
1524 =item getservbyname NAME,PROTO
1526 =item gethostbyaddr ADDR,ADDRTYPE
1528 =item getnetbyaddr ADDR,ADDRTYPE
1530 =item getprotobynumber NUMBER
1532 =item getservbyport PORT,PROTO
1550 =item sethostent STAYOPEN
1552 =item setnetent STAYOPEN
1554 =item setprotoent STAYOPEN
1556 =item setservent STAYOPEN
1570 These routines perform the same functions as their counterparts in the
1571 system library. In list context, the return values from the
1572 various get routines are as follows:
1574 ($name,$passwd,$uid,$gid,
1575 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1576 ($name,$passwd,$gid,$members) = getgr*
1577 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1578 ($name,$aliases,$addrtype,$net) = getnet*
1579 ($name,$aliases,$proto) = getproto*
1580 ($name,$aliases,$port,$proto) = getserv*
1582 (If the entry doesn't exist you get a null list.)
1584 In scalar context, you get the name, unless the function was a
1585 lookup by name, in which case you get the other thing, whatever it is.
1586 (If the entry doesn't exist you get the undefined value.) For example:
1588 $uid = getpwnam($name);
1589 $name = getpwuid($num);
1591 $gid = getgrnam($name);
1592 $name = getgrgid($num;
1596 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1597 cases in the sense that in many systems they are unsupported. If the
1598 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1599 usually encodes the disk quota. If the C<$comment> field is unsupported,
1600 it is an empty scalar. If it is supported it usually encodes some
1601 administrative comment about the user. In some systems the $quota
1602 field may be C<$change> or C<$age>, fields that have to do with password
1603 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1604 field, if present, encodes the expiration period of the account or the
1605 password. For the availability and the exact meaning of these fields
1606 in your system, please consult your C<getpwnam(3)> documentation and your
1607 F<pwd.h> file. You can also find out from within Perl which meaning
1608 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1609 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1610 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1612 The C<$members> value returned by I<getgr*()> is a space separated list of
1613 the login names of the members of the group.
1615 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1616 C, it will be returned to you via C<$?> if the function call fails. The
1617 C<@addrs> value returned by a successful call is a list of the raw
1618 addresses returned by the corresponding system library call. In the
1619 Internet domain, each address is four bytes long and you can unpack it
1620 by saying something like:
1622 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1624 If you get tired of remembering which element of the return list contains
1625 which return value, by-name interfaces are also provided in modules:
1626 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1627 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1628 normal built-in, replacing them with versions that return objects with
1629 the appropriate names for each field. For example:
1633 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1635 Even though it looks like they're the same method calls (uid),
1636 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1638 =item getsockname SOCKET
1640 Returns the packed sockaddr address of this end of the SOCKET connection.
1643 $mysockaddr = getsockname(SOCK);
1644 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1646 =item getsockopt SOCKET,LEVEL,OPTNAME
1648 Returns the socket option requested, or undef if there is an error.
1654 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1655 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1656 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1657 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1658 L<perlop/"I/O Operators">.
1662 Converts a time as returned by the time function to a 9-element array
1663 with the time localized for the standard Greenwich time zone.
1664 Typically used as follows:
1667 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1670 All array elements are numeric, and come straight out of a struct tm.
1671 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1672 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1673 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1675 If EXPR is omitted, does C<gmtime(time())>.
1677 In scalar context, returns the C<ctime(3)> value:
1679 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1681 Also see the C<timegm()> function provided by the C<Time::Local> module,
1682 and the C<strftime(3)> function available via the POSIX module.
1684 This scalar value is B<not> locale dependent, see L<perllocale>, but
1685 instead a Perl builtin. Also see the C<Time::Local> module, and the
1686 C<strftime(3)> and C<mktime(3)> function available via the POSIX module. To
1687 get somewhat similar but locale dependent date strings, set up your
1688 locale environment variables appropriately (please see L<perllocale>)
1689 and try for example:
1691 use POSIX qw(strftime);
1692 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1694 Note that the C<%a> and C<%b>, the short forms of the day of the week
1695 and the month of the year, may not necessarily be three characters wide.
1703 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1704 execution there. It may not be used to go into any construct that
1705 requires initialization, such as a subroutine or a C<foreach> loop. It
1706 also can't be used to go into a construct that is optimized away,
1707 or to get out of a block or subroutine given to C<sort()>.
1708 It can be used to go almost anywhere else within the dynamic scope,
1709 including out of subroutines, but it's usually better to use some other
1710 construct such as C<last> or C<die>. The author of Perl has never felt the
1711 need to use this form of C<goto> (in Perl, that is--C is another matter).
1713 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1714 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1715 necessarily recommended if you're optimizing for maintainability:
1717 goto ("FOO", "BAR", "GLARCH")[$i];
1719 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1720 named subroutine for the currently running subroutine. This is used by
1721 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1722 pretend that the other subroutine had been called in the first place
1723 (except that any modifications to C<@_> in the current subroutine are
1724 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1725 will be able to tell that this routine was called first.
1727 =item grep BLOCK LIST
1729 =item grep EXPR,LIST
1731 This is similar in spirit to, but not the same as, C<grep(1)>
1732 and its relatives. In particular, it is not limited to using
1733 regular expressions.
1735 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1736 C<$_> to each element) and returns the list value consisting of those
1737 elements for which the expression evaluated to TRUE. In a scalar
1738 context, returns the number of times the expression was TRUE.
1740 @foo = grep(!/^#/, @bar); # weed out comments
1744 @foo = grep {!/^#/} @bar; # weed out comments
1746 Note that, because C<$_> is a reference into the list value, it can be used
1747 to modify the elements of the array. While this is useful and
1748 supported, it can cause bizarre results if the LIST is not a named
1749 array. Similarly, grep returns aliases into the original list,
1750 much like the way that a for loops's index variable aliases the list
1751 elements. That is, modifying an element of a list returned by grep
1752 (for example, in a C<foreach>, C<map> or another C<grep>)
1753 actually modifies the element in the original list.
1755 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1761 Interprets EXPR as a hex string and returns the corresponding
1762 value. (To convert strings that might start with either 0 or 0x
1763 see L</oct>.) If EXPR is omitted, uses C<$_>.
1765 print hex '0xAf'; # prints '175'
1766 print hex 'aF'; # same
1770 There is no builtin C<import()> function. It is just an ordinary
1771 method (subroutine) defined (or inherited) by modules that wish to export
1772 names to another module. The C<use()> function calls the C<import()> method
1773 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1775 =item index STR,SUBSTR,POSITION
1777 =item index STR,SUBSTR
1779 Returns the position of the first occurrence of SUBSTR in STR at or after
1780 POSITION. If POSITION is omitted, starts searching from the beginning of
1781 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1782 variable to--but don't do that). If the substring is not found, returns
1783 one less than the base, ordinarily C<-1>.
1789 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1790 You should not use this for rounding, because it truncates
1791 towards C<0>, and because machine representations of floating point
1792 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1793 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1795 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1797 Implements the C<ioctl(2)> function. You'll probably have to say
1799 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1801 first to get the correct function definitions. If F<ioctl.ph> doesn't
1802 exist or doesn't have the correct definitions you'll have to roll your
1803 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1804 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1805 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1806 written depending on the FUNCTION--a pointer to the string value of SCALAR
1807 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
1808 has no string value but does have a numeric value, that value will be
1809 passed rather than a pointer to the string value. To guarantee this to be
1810 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1811 functions are useful for manipulating the values of structures used by
1812 C<ioctl()>. The following example sets the erase character to DEL.
1816 die "NO TIOCGETP" if $@ || !$getp;
1817 $sgttyb_t = "ccccs"; # 4 chars and a short
1818 if (ioctl(STDIN,$getp,$sgttyb)) {
1819 @ary = unpack($sgttyb_t,$sgttyb);
1821 $sgttyb = pack($sgttyb_t,@ary);
1822 ioctl(STDIN,&TIOCSETP,$sgttyb)
1823 || die "Can't ioctl: $!";
1826 The return value of C<ioctl> (and C<fcntl>) is as follows:
1828 if OS returns: then Perl returns:
1830 0 string "0 but true"
1831 anything else that number
1833 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1834 still easily determine the actual value returned by the operating
1837 ($retval = ioctl(...)) || ($retval = -1);
1838 printf "System returned %d\n", $retval;
1840 The special string "C<0> but true" is excempt from B<-w> complaints
1841 about improper numeric conversions.
1843 =item join EXPR,LIST
1845 Joins the separate strings of LIST into a single string with
1846 fields separated by the value of EXPR, and returns the string.
1849 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1855 Returns a list consisting of all the keys of the named hash. (In a
1856 scalar context, returns the number of keys.) The keys are returned in
1857 an apparently random order, but it is the same order as either the
1858 C<values()> or C<each()> function produces (given that the hash has not been
1859 modified). As a side effect, it resets HASH's iterator.
1861 Here is yet another way to print your environment:
1864 @values = values %ENV;
1865 while ($#keys >= 0) {
1866 print pop(@keys), '=', pop(@values), "\n";
1869 or how about sorted by key:
1871 foreach $key (sort(keys %ENV)) {
1872 print $key, '=', $ENV{$key}, "\n";
1875 To sort an array by value, you'll need to use a C<sort> function.
1876 Here's a descending numeric sort of a hash by its values:
1878 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1879 printf "%4d %s\n", $hash{$key}, $key;
1882 As an lvalue C<keys> allows you to increase the number of hash buckets
1883 allocated for the given hash. This can gain you a measure of efficiency if
1884 you know the hash is going to get big. (This is similar to pre-extending
1885 an array by assigning a larger number to $#array.) If you say
1889 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1890 it rounds up to the next power of two. These
1891 buckets will be retained even if you do C<%hash = ()>, use C<undef
1892 %hash> if you want to free the storage while C<%hash> is still in scope.
1893 You can't shrink the number of buckets allocated for the hash using
1894 C<keys> in this way (but you needn't worry about doing this by accident,
1895 as trying has no effect).
1899 Sends a signal to a list of processes. The first element of
1900 the list must be the signal to send. Returns the number of
1901 processes successfully signaled.
1903 $cnt = kill 1, $child1, $child2;
1906 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1907 process groups instead of processes. (On System V, a negative I<PROCESS>
1908 number will also kill process groups, but that's not portable.) That
1909 means you usually want to use positive not negative signals. You may also
1910 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1916 The C<last> command is like the C<break> statement in C (as used in
1917 loops); it immediately exits the loop in question. If the LABEL is
1918 omitted, the command refers to the innermost enclosing loop. The
1919 C<continue> block, if any, is not executed:
1921 LINE: while (<STDIN>) {
1922 last LINE if /^$/; # exit when done with header
1926 See also L</continue> for an illustration of how C<last>, C<next>, and
1933 Returns an lowercased version of EXPR. This is the internal function
1934 implementing the C<\L> escape in double-quoted strings.
1935 Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
1937 If EXPR is omitted, uses C<$_>.
1943 Returns the value of EXPR with the first character lowercased. This is
1944 the internal function implementing the C<\l> escape in double-quoted strings.
1945 Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
1947 If EXPR is omitted, uses C<$_>.
1953 Returns the length in bytes of the value of EXPR. If EXPR is
1954 omitted, returns length of C<$_>.
1956 =item link OLDFILE,NEWFILE
1958 Creates a new filename linked to the old filename. Returns TRUE for
1959 success, FALSE otherwise.
1961 =item listen SOCKET,QUEUESIZE
1963 Does the same thing that the listen system call does. Returns TRUE if
1964 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1968 A local modifies the listed variables to be local to the enclosing
1969 block, file, or eval. If more than one value is listed, the list must
1970 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1971 for details, including issues with tied arrays and hashes.
1973 You really probably want to be using C<my()> instead, because C<local()> isn't
1974 what most people think of as "local". See L<perlsub/"Private Variables
1975 via my()"> for details.
1977 =item localtime EXPR
1979 Converts a time as returned by the time function to a 9-element array
1980 with the time analyzed for the local time zone. Typically used as
1984 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1987 All array elements are numeric, and come straight out of a struct tm.
1988 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1989 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1990 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
1992 If EXPR is omitted, uses the current time (C<localtime(time)>).
1994 In scalar context, returns the C<ctime(3)> value:
1996 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1998 This scalar value is B<not> locale dependent, see L<perllocale>, but
1999 instead a Perl builtin. Also see the C<Time::Local> module, and the
2000 C<strftime(3)> and C<mktime(3)> function available via the POSIX module. To
2001 get somewhat similar but locale dependent date strings, set up your
2002 locale environment variables appropriately (please see L<perllocale>)
2003 and try for example:
2005 use POSIX qw(strftime);
2006 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2008 Note that the C<%a> and C<%b>, the short forms of the day of the week
2009 and the month of the year, may not necessarily be three characters wide.
2015 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2018 =item lstat FILEHANDLE
2024 Does the same thing as the C<stat()> function (including setting the
2025 special C<_> filehandle) but stats a symbolic link instead of the file
2026 the symbolic link points to. If symbolic links are unimplemented on
2027 your system, a normal C<stat()> is done.
2029 If EXPR is omitted, stats C<$_>.
2033 The match operator. See L<perlop>.
2035 =item map BLOCK LIST
2039 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2040 element) and returns the list value composed of the results of each such
2041 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2042 may produce zero, one, or more elements in the returned value.
2044 @chars = map(chr, @nums);
2046 translates a list of numbers to the corresponding characters. And
2048 %hash = map { getkey($_) => $_ } @array;
2050 is just a funny way to write
2053 foreach $_ (@array) {
2054 $hash{getkey($_)} = $_;
2057 Note that, because C<$_> is a reference into the list value, it can be used
2058 to modify the elements of the array. While this is useful and
2059 supported, it can cause bizarre results if the LIST is not a named
2060 array. See also L</grep> for an array composed of those items of the
2061 original list for which the BLOCK or EXPR evaluates to true.
2063 =item mkdir FILENAME,MODE
2065 Creates the directory specified by FILENAME, with permissions specified
2066 by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2067 it returns FALSE and sets C<$!> (errno).
2069 =item msgctl ID,CMD,ARG
2071 Calls the System V IPC function C<msgctl(2)>. You'll probably have to say
2075 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2076 then ARG must be a variable which will hold the returned C<msqid_ds>
2077 structure. Returns like C<ioctl>: the undefined value for error, "C<0> but
2078 true" for zero, or the actual return value otherwise. See also
2079 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2081 =item msgget KEY,FLAGS
2083 Calls the System V IPC function C<msgget(2)>. Returns the message queue
2084 id, or the undefined value if there is an error. See also C<IPC::SysV>
2085 and C<IPC::SysV::Msg> documentation.
2087 =item msgsnd ID,MSG,FLAGS
2089 Calls the System V IPC function msgsnd to send the message MSG to the
2090 message queue ID. MSG must begin with the long integer message type,
2091 which may be created with C<pack("l", $type)>. Returns TRUE if
2092 successful, or FALSE if there is an error. See also C<IPC::SysV>
2093 and C<IPC::SysV::Msg> documentation.
2095 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2097 Calls the System V IPC function msgrcv to receive a message from
2098 message queue ID into variable VAR with a maximum message size of
2099 SIZE. Note that if a message is received, the message type will be
2100 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2101 size of the message type. Returns TRUE if successful, or FALSE if
2102 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2106 A C<my> declares the listed variables to be local (lexically) to the
2107 enclosing block, file, or C<eval>. If
2108 more than one value is listed, the list must be placed in parentheses. See
2109 L<perlsub/"Private Variables via my()"> for details.
2115 The C<next> command is like the C<continue> statement in C; it starts
2116 the next iteration of the loop:
2118 LINE: while (<STDIN>) {
2119 next LINE if /^#/; # discard comments
2123 Note that if there were a C<continue> block on the above, it would get
2124 executed even on discarded lines. If the LABEL is omitted, the command
2125 refers to the innermost enclosing loop.
2127 See also L</continue> for an illustration of how C<last>, C<next>, and
2130 =item no Module LIST
2132 See the L</use> function, which C<no> is the opposite of.
2138 Interprets EXPR as an octal string and returns the corresponding
2139 value. (If EXPR happens to start off with C<0x>, interprets it as
2140 a hex string instead.) The following will handle decimal, octal, and
2141 hex in the standard Perl or C notation:
2143 $val = oct($val) if $val =~ /^0/;
2145 If EXPR is omitted, uses C<$_>. This function is commonly used when
2146 a string such as C<644> needs to be converted into a file mode, for
2147 example. (Although perl will automatically convert strings into
2148 numbers as needed, this automatic conversion assumes base 10.)
2150 =item open FILEHANDLE,EXPR
2152 =item open FILEHANDLE
2154 Opens the file whose filename is given by EXPR, and associates it with
2155 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2156 name of the real filehandle wanted. If EXPR is omitted, the scalar
2157 variable of the same name as the FILEHANDLE contains the filename.
2158 (Note that lexical variables--those declared with C<my>--will not work
2159 for this purpose; so if you're using C<my>, specify EXPR in your call
2162 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2163 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2164 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2165 the file is opened for appending, again being created if necessary.
2166 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2167 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2168 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2169 file first. You can't usually use either read-write mode for updating
2170 textfiles, since they have variable length records. See the B<-i>
2171 switch in L<perlrun> for a better approach.
2173 The prefix and the filename may be separated with spaces.
2174 These various prefixes correspond to the C<fopen(3)> modes of C<'r'>, C<'r+'>, C<'w'>,
2175 C<'w+'>, C<'a'>, and C<'a+'>.
2177 If the filename begins with C<'|'>, the filename is interpreted as a
2178 command to which output is to be piped, and if the filename ends with a
2179 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2180 for more examples of this. (You are not allowed to C<open()> to a command
2181 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2182 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2184 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2185 nonzero upon success, the undefined value otherwise. If the C<open>
2186 involved a pipe, the return value happens to be the pid of the
2189 If you're unfortunate enough to be running Perl on a system that
2190 distinguishes between text files and binary files (modern operating
2191 systems don't care), then you should check out L</binmode> for tips for
2192 dealing with this. The key distinction between systems that need C<binmode>
2193 and those that don't is their text file formats. Systems like Unix, MacOS, and
2194 Plan9, which delimit lines with a single character, and which encode that
2195 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2197 When opening a file, it's usually a bad idea to continue normal execution
2198 if the request failed, so C<open> is frequently used in connection with
2199 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2200 where you want to make a nicely formatted error message (but there are
2201 modules that can help with that problem)) you should always check
2202 the return value from opening a file. The infrequent exception is when
2203 working with an unopened filehandle is actually what you want to do.
2208 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2209 while (<ARTICLE>) {...
2211 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2212 # if the open fails, output is discarded
2214 open(DBASE, '+<dbase.mine') # open for update
2215 or die "Can't open 'dbase.mine' for update: $!";
2217 open(ARTICLE, "caesar <$article |") # decrypt article
2218 or die "Can't start caesar: $!";
2220 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2221 or die "Can't start sort: $!";
2223 # process argument list of files along with any includes
2225 foreach $file (@ARGV) {
2226 process($file, 'fh00');
2230 my($filename, $input) = @_;
2231 $input++; # this is a string increment
2232 unless (open($input, $filename)) {
2233 print STDERR "Can't open $filename: $!\n";
2238 while (<$input>) { # note use of indirection
2239 if (/^#include "(.*)"/) {
2240 process($1, $input);
2247 You may also, in the Bourne shell tradition, specify an EXPR beginning
2248 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2249 name of a filehandle (or file descriptor, if numeric) to be
2250 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2251 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2252 mode you specify should match the mode of the original filehandle.
2253 (Duping a filehandle does not take into account any existing contents of
2255 Here is a script that saves, redirects, and restores STDOUT and
2259 open(OLDOUT, ">&STDOUT");
2260 open(OLDERR, ">&STDERR");
2262 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2263 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2265 select(STDERR); $| = 1; # make unbuffered
2266 select(STDOUT); $| = 1; # make unbuffered
2268 print STDOUT "stdout 1\n"; # this works for
2269 print STDERR "stderr 1\n"; # subprocesses too
2274 open(STDOUT, ">&OLDOUT");
2275 open(STDERR, ">&OLDERR");
2277 print STDOUT "stdout 2\n";
2278 print STDERR "stderr 2\n";
2281 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2282 equivalent of C's C<fdopen()> of that file descriptor; this is more
2283 parsimonious of file descriptors. For example:
2285 open(FILEHANDLE, "<&=$fd")
2287 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2288 there is an implicit fork done, and the return value of open is the pid
2289 of the child within the parent process, and C<0> within the child
2290 process. (Use C<defined($pid)> to determine whether the open was successful.)
2291 The filehandle behaves normally for the parent, but i/o to that
2292 filehandle is piped from/to the STDOUT/STDIN of the child process.
2293 In the child process the filehandle isn't opened--i/o happens from/to
2294 the new STDOUT or STDIN. Typically this is used like the normal
2295 piped open when you want to exercise more control over just how the
2296 pipe command gets executed, such as when you are running setuid, and
2297 don't want to have to scan shell commands for metacharacters.
2298 The following pairs are more or less equivalent:
2300 open(FOO, "|tr '[a-z]' '[A-Z]'");
2301 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2303 open(FOO, "cat -n '$file'|");
2304 open(FOO, "-|") || exec 'cat', '-n', $file;
2306 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2308 NOTE: On any operation that may do a fork, any unflushed buffers remain
2309 unflushed in both processes, which means you may need to set C<$|> to
2310 avoid duplicate output.
2312 Closing any piped filehandle causes the parent process to wait for the
2313 child to finish, and returns the status value in C<$?>.
2315 The filename passed to open will have leading and trailing
2316 whitespace deleted, and the normal redirection chararacters
2317 honored. This property, known as "magic open",
2318 can often be used to good effect. A user could specify a filename of
2319 F<"rsh cat file |">, or you could change certain filenames as needed:
2321 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2322 open(FH, $filename) or die "Can't open $filename: $!";
2324 However, to open a file with arbitrary weird characters in it, it's
2325 necessary to protect any leading and trailing whitespace:
2327 $file =~ s#^(\s)#./$1#;
2328 open(FOO, "< $file\0");
2330 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2331 should use the C<sysopen()> function, which involves no such magic. This is
2332 another way to protect your filenames from interpretation. For example:
2335 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2336 or die "sysopen $path: $!";
2337 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2338 print HANDLE "stuff $$\n");
2340 print "File contains: ", <HANDLE>;
2342 Using the constructor from the C<IO::Handle> package (or one of its
2343 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2344 filehandles that have the scope of whatever variables hold references to
2345 them, and automatically close whenever and however you leave that scope:
2349 sub read_myfile_munged {
2351 my $handle = new IO::File;
2352 open($handle, "myfile") or die "myfile: $!";
2354 or return (); # Automatically closed here.
2355 mung $first or die "mung failed"; # Or here.
2356 return $first, <$handle> if $ALL; # Or here.
2360 See L</seek()> for some details about mixing reading and writing.
2362 =item opendir DIRHANDLE,EXPR
2364 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2365 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2366 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2372 Returns the numeric ascii value of the first character of EXPR. If
2373 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2375 =item pack TEMPLATE,LIST
2377 Takes an array or list of values and packs it into a binary structure,
2378 returning the string containing the structure. The TEMPLATE is a
2379 sequence of characters that give the order and type of values, as
2382 A An ascii string, will be space padded.
2383 a An ascii string, will be null padded.
2384 b A bit string (ascending bit order, like vec()).
2385 B A bit string (descending bit order).
2386 h A hex string (low nybble first).
2387 H A hex string (high nybble first).
2389 c A signed char value.
2390 C An unsigned char value.
2392 s A signed short value.
2393 S An unsigned short value.
2394 (This 'short' is _exactly_ 16 bits, which may differ from
2395 what a local C compiler calls 'short'.)
2397 i A signed integer value.
2398 I An unsigned integer value.
2399 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2400 depends on what a local C compiler calls 'int', and may
2401 even be larger than the 'long' described in the next item.)
2403 l A signed long value.
2404 L An unsigned long value.
2405 (This 'long' is _exactly_ 32 bits, which may differ from
2406 what a local C compiler calls 'long'.)
2408 n A short in "network" (big-endian) order.
2409 N A long in "network" (big-endian) order.
2410 v A short in "VAX" (little-endian) order.
2411 V A long in "VAX" (little-endian) order.
2412 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2413 _exactly_ 32 bits, respectively.)
2415 f A single-precision float in the native format.
2416 d A double-precision float in the native format.
2418 p A pointer to a null-terminated string.
2419 P A pointer to a structure (fixed-length string).
2421 u A uuencoded string.
2423 w A BER compressed integer. Its bytes represent an unsigned
2424 integer in base 128, most significant digit first, with as few
2425 digits as possible. Bit eight (the high bit) is set on each
2426 byte except the last.
2430 @ Null fill to absolute position.
2432 Each letter may optionally be followed by a number giving a repeat
2433 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2434 pack function will gobble up that many values from the LIST. A C<*> for the
2435 repeat count means to use however many items are left. The C<"a"> and C<"A">
2436 types gobble just one value, but pack it as a string of length count,
2437 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2438 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2439 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2440 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2441 terminated string. You are responsible for ensuring the string is not a
2442 temporary value (which can potentially get deallocated before you get
2443 around to using the packed result). The C<"P"> packs a pointer to a structure
2444 of the size indicated by the length. A NULL pointer is created if the
2445 corresponding value for C<"p"> or C<"P"> is C<undef>.
2446 Real numbers (floats and doubles) are
2447 in the native machine format only; due to the multiplicity of floating
2448 formats around, and the lack of a standard "network" representation, no
2449 facility for interchange has been made. This means that packed floating
2450 point data written on one machine may not be readable on another - even if
2451 both use IEEE floating point arithmetic (as the endian-ness of the memory
2452 representation is not part of the IEEE spec). Note that Perl uses doubles
2453 internally for all numeric calculation, and converting from double into
2454 float and thence back to double again will lose precision (i.e.,
2455 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2459 $foo = pack("cccc",65,66,67,68);
2461 $foo = pack("c4",65,66,67,68);
2464 $foo = pack("ccxxcc",65,66,67,68);
2467 $foo = pack("s2",1,2);
2468 # "\1\0\2\0" on little-endian
2469 # "\0\1\0\2" on big-endian
2471 $foo = pack("a4","abcd","x","y","z");
2474 $foo = pack("aaaa","abcd","x","y","z");
2477 $foo = pack("a14","abcdefg");
2478 # "abcdefg\0\0\0\0\0\0\0"
2480 $foo = pack("i9pl", gmtime);
2481 # a real struct tm (on my system anyway)
2484 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2487 The same template may generally also be used in the unpack function.
2491 =item package NAMESPACE
2493 Declares the compilation unit as being in the given namespace. The scope
2494 of the package declaration is from the declaration itself through the end of
2495 the enclosing block (the same scope as the C<local()> operator). All further
2496 unqualified dynamic identifiers will be in this namespace. A package
2497 statement affects only dynamic variables--including those you've used
2498 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2499 would be the first declaration in a file to be included by the C<require>
2500 or C<use> operator. You can switch into a package in more than one place;
2501 it merely influences which symbol table is used by the compiler for the
2502 rest of that block. You can refer to variables and filehandles in other
2503 packages by prefixing the identifier with the package name and a double
2504 colon: C<$Package::Variable>. If the package name is null, the C<main>
2505 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2507 If NAMESPACE is omitted, then there is no current package, and all
2508 identifiers must be fully qualified or lexicals. This is stricter
2509 than C<use strict>, since it also extends to function names.
2511 See L<perlmod/"Packages"> for more information about packages, modules,
2512 and classes. See L<perlsub> for other scoping issues.
2514 =item pipe READHANDLE,WRITEHANDLE
2516 Opens a pair of connected pipes like the corresponding system call.
2517 Note that if you set up a loop of piped processes, deadlock can occur
2518 unless you are very careful. In addition, note that Perl's pipes use
2519 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2520 after each command, depending on the application.
2522 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2523 for examples of such things.
2529 Pops and returns the last value of the array, shortening the array by
2530 1. Has a similar effect to
2532 $tmp = $ARRAY[$#ARRAY--];
2534 If there are no elements in the array, returns the undefined value.
2535 If ARRAY is omitted, pops the
2536 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2543 Returns the offset of where the last C<m//g> search left off for the variable
2544 is in question (C<$_> is used when the variable is not specified). May be
2545 modified to change that offset. Such modification will also influence
2546 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2549 =item print FILEHANDLE LIST
2555 Prints a string or a comma-separated list of strings. Returns TRUE
2556 if successful. FILEHANDLE may be a scalar variable name, in which case
2557 the variable contains the name of or a reference to the filehandle, thus introducing one
2558 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2559 token is a term, it may be misinterpreted as an operator unless you
2560 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2561 omitted, prints by default to standard output (or to the last selected
2562 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2563 the currently selected output channel. To set the default output channel to something other than
2564 STDOUT use the select operation. Note that, because print takes a
2565 LIST, anything in the LIST is evaluated in list context, and any
2566 subroutine that you call will have one or more of its expressions
2567 evaluated in list context. Also be careful not to follow the print
2568 keyword with a left parenthesis unless you want the corresponding right
2569 parenthesis to terminate the arguments to the print--interpose a C<+> or
2570 put parentheses around all the arguments.
2572 Note that if you're storing FILEHANDLES in an array or other expression,
2573 you will have to use a block returning its value instead:
2575 print { $files[$i] } "stuff\n";
2576 print { $OK ? STDOUT : STDERR } "stuff\n";
2578 =item printf FILEHANDLE FORMAT, LIST
2580 =item printf FORMAT, LIST
2582 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2583 (the output record separator) is not appended. The first argument
2584 of the list will be interpreted as the C<printf> format. If C<use locale> is
2585 in effect, the character used for the decimal point in formatted real numbers
2586 is affected by the LC_NUMERIC locale. See L<perllocale>.
2588 Don't fall into the trap of using a C<printf()> when a simple
2589 C<print()> would do. The C<print()> is more efficient and less
2592 =item prototype FUNCTION
2594 Returns the prototype of a function as a string (or C<undef> if the
2595 function has no prototype). FUNCTION is a reference to, or the name of,
2596 the function whose prototype you want to retrieve.
2598 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2599 a name for Perl builtin. If builtin is not I<overridable> (such as
2600 C<qw>) or its arguments cannot be expressed by a prototype (such as
2601 C<system>) - in other words, the builtin does not behave like a Perl
2602 function - returns C<undef>. Otherwise, the string describing the
2603 equivalent prototype is returned.
2605 =item push ARRAY,LIST
2607 Treats ARRAY as a stack, and pushes the values of LIST
2608 onto the end of ARRAY. The length of ARRAY increases by the length of
2609 LIST. Has the same effect as
2612 $ARRAY[++$#ARRAY] = $value;
2615 but is more efficient. Returns the new number of elements in the array.
2627 Generalized quotes. See L<perlop>.
2629 =item quotemeta EXPR
2633 Returns the value of EXPR with all non-alphanumeric
2634 characters backslashed. (That is, all characters not matching
2635 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2636 returned string, regardless of any locale settings.)
2637 This is the internal function implementing
2638 the C<\Q> escape in double-quoted strings.
2640 If EXPR is omitted, uses C<$_>.
2646 Returns a random fractional number greater than or equal to C<0> and less
2647 than the value of EXPR. (EXPR should be positive.) If EXPR is
2648 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2649 C<srand()> has already been called. See also C<srand()>.
2651 (Note: If your rand function consistently returns numbers that are too
2652 large or too small, then your version of Perl was probably compiled
2653 with the wrong number of RANDBITS.)
2655 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2657 =item read FILEHANDLE,SCALAR,LENGTH
2659 Attempts to read LENGTH bytes of data into variable SCALAR from the
2660 specified FILEHANDLE. Returns the number of bytes actually read,
2661 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2662 or shrunk to the length actually read. An OFFSET may be specified to
2663 place the read data at some other place than the beginning of the
2664 string. This call is actually implemented in terms of stdio's C<fread(3)>
2665 call. To get a true C<read(2)> system call, see C<sysread()>.
2667 =item readdir DIRHANDLE
2669 Returns the next directory entry for a directory opened by C<opendir()>.
2670 If used in list context, returns all the rest of the entries in the
2671 directory. If there are no more entries, returns an undefined value in
2672 scalar context or a null list in list context.
2674 If you're planning to filetest the return values out of a C<readdir()>, you'd
2675 better prepend the directory in question. Otherwise, because we didn't
2676 C<chdir()> there, it would have been testing the wrong file.
2678 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2679 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2684 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2685 is read and returned. In list context, reads until end-of-file is
2686 reached and returns a list of lines (however you've defined lines
2687 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2688 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2689 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2690 operator is discussed in more detail in L<perlop/"I/O Operators">.
2693 $line = readline(*STDIN); # same thing
2699 Returns the value of a symbolic link, if symbolic links are
2700 implemented. If not, gives a fatal error. If there is some system
2701 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2702 omitted, uses C<$_>.
2706 EXPR is executed as a system command.
2707 The collected standard output of the command is returned.
2708 In scalar context, it comes back as a single (potentially
2709 multi-line) string. In list context, returns a list of lines
2710 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2711 This is the internal function implementing the C<qx/EXPR/>
2712 operator, but you can use it directly. The C<qx/EXPR/>
2713 operator is discussed in more detail in L<perlop/"I/O Operators">.
2715 =item recv SOCKET,SCALAR,LEN,FLAGS
2717 Receives a message on a socket. Attempts to receive LENGTH bytes of
2718 data into variable SCALAR from the specified SOCKET filehandle.
2719 Actually does a C C<recvfrom()>, so that it can return the address of the
2720 sender. Returns the undefined value if there's an error. SCALAR will
2721 be grown or shrunk to the length actually read. Takes the same flags
2722 as the system call of the same name.
2723 See L<perlipc/"UDP: Message Passing"> for examples.
2729 The C<redo> command restarts the loop block without evaluating the
2730 conditional again. The C<continue> block, if any, is not executed. If
2731 the LABEL is omitted, the command refers to the innermost enclosing
2732 loop. This command is normally used by programs that want to lie to
2733 themselves about what was just input:
2735 # a simpleminded Pascal comment stripper
2736 # (warning: assumes no { or } in strings)
2737 LINE: while (<STDIN>) {
2738 while (s|({.*}.*){.*}|$1 |) {}
2743 if (/}/) { # end of comment?
2752 See also L</continue> for an illustration of how C<last>, C<next>, and
2759 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2760 is not specified, C<$_> will be used. The value returned depends on the
2761 type of thing the reference is a reference to.
2762 Builtin types include:
2771 If the referenced object has been blessed into a package, then that package
2772 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2774 if (ref($r) eq "HASH") {
2775 print "r is a reference to a hash.\n";
2778 print "r is not a reference at all.\n";
2781 See also L<perlref>.
2783 =item rename OLDNAME,NEWNAME
2785 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2786 not work across file system boundaries.
2792 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2793 supplied. If EXPR is numeric, demands that the current version of Perl
2794 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2796 Otherwise, demands that a library file be included if it hasn't already
2797 been included. The file is included via the do-FILE mechanism, which is
2798 essentially just a variety of C<eval()>. Has semantics similar to the following
2803 return 1 if $INC{$filename};
2804 my($realfilename,$result);
2806 foreach $prefix (@INC) {
2807 $realfilename = "$prefix/$filename";
2808 if (-f $realfilename) {
2809 $result = do $realfilename;
2813 die "Can't find $filename in \@INC";
2816 die "$filename did not return true value" unless $result;
2817 $INC{$filename} = $realfilename;
2821 Note that the file will not be included twice under the same specified
2822 name. The file must return TRUE as the last statement to indicate
2823 successful execution of any initialization code, so it's customary to
2824 end such a file with "C<1;>" unless you're sure it'll return TRUE
2825 otherwise. But it's better just to put the "C<1;>", in case you add more
2828 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2829 replaces "F<::>" with "F</>" in the filename for you,
2830 to make it easy to load standard modules. This form of loading of
2831 modules does not risk altering your namespace.
2833 In other words, if you try this:
2835 require Foo::Bar; # a splendid bareword
2837 The require function will actually look for the "F<Foo/Bar.pm>" file in the
2838 directories specified in the C<@INC> array.
2840 But if you try this:
2842 $class = 'Foo::Bar';
2843 require $class; # $class is not a bareword
2845 require "Foo::Bar"; # not a bareword because of the ""
2847 The require function will look for the "F<Foo::Bar>" file in the @INC array and
2848 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
2850 eval "require $class";
2852 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2858 Generally used in a C<continue> block at the end of a loop to clear
2859 variables and reset C<??> searches so that they work again. The
2860 expression is interpreted as a list of single characters (hyphens
2861 allowed for ranges). All variables and arrays beginning with one of
2862 those letters are reset to their pristine state. If the expression is
2863 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
2864 only variables or searches in the current package. Always returns
2867 reset 'X'; # reset all X variables
2868 reset 'a-z'; # reset lower case variables
2869 reset; # just reset ?? searches
2871 Resetting C<"A-Z"> is not recommended because you'll wipe out your
2872 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
2873 are unaffected, but they clean themselves up on scope exit anyway,
2874 so you'll probably want to use them instead. See L</my>.
2880 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
2881 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
2882 context, depending on how the return value will be used, and the context
2883 may vary from one execution to the next (see C<wantarray()>). If no EXPR
2884 is given, returns an empty list in list context, an undefined value in
2885 scalar context, or nothing in a void context.
2887 (Note that in the absence of a return, a subroutine, eval, or do FILE
2888 will automatically return the value of the last expression evaluated.)
2892 In list context, returns a list value consisting of the elements
2893 of LIST in the opposite order. In scalar context, concatenates the
2894 elements of LIST, and returns a string value consisting of those bytes,
2895 but in the opposite order.
2897 print reverse <>; # line tac, last line first
2899 undef $/; # for efficiency of <>
2900 print scalar reverse <>; # byte tac, last line tsrif
2902 This operator is also handy for inverting a hash, although there are some
2903 caveats. If a value is duplicated in the original hash, only one of those
2904 can be represented as a key in the inverted hash. Also, this has to
2905 unwind one hash and build a whole new one, which may take some time
2908 %by_name = reverse %by_address; # Invert the hash
2910 =item rewinddir DIRHANDLE
2912 Sets the current position to the beginning of the directory for the
2913 C<readdir()> routine on DIRHANDLE.
2915 =item rindex STR,SUBSTR,POSITION
2917 =item rindex STR,SUBSTR
2919 Works just like index except that it returns the position of the LAST
2920 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2921 last occurrence at or before that position.
2923 =item rmdir FILENAME
2927 Deletes the directory specified by FILENAME if that directory is empty. If it
2928 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
2929 FILENAME is omitted, uses C<$_>.
2933 The substitution operator. See L<perlop>.
2937 Forces EXPR to be interpreted in scalar context and returns the value
2940 @counts = ( scalar @a, scalar @b, scalar @c );
2942 There is no equivalent operator to force an expression to
2943 be interpolated in list context because it's in practice never
2944 needed. If you really wanted to do so, however, you could use
2945 the construction C<@{[ (some expression) ]}>, but usually a simple
2946 C<(some expression)> suffices.
2948 =item seek FILEHANDLE,POSITION,WHENCE
2950 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio>.
2951 FILEHANDLE may be an expression whose value gives the name of the
2952 filehandle. The values for WHENCE are C<0> to set the new position to
2953 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
2954 set it to EOF plus POSITION (typically negative). For WHENCE you may
2955 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2956 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
2958 If you want to position file for C<sysread()> or C<syswrite()>, don't use
2959 C<seek()> -- buffering makes its effect on the file's system position
2960 unpredictable and non-portable. Use C<sysseek()> instead.
2962 On some systems you have to do a seek whenever you switch between reading
2963 and writing. Amongst other things, this may have the effect of calling
2964 stdio's C<clearerr(3)>. A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
2969 This is also useful for applications emulating C<tail -f>. Once you hit
2970 EOF on your read, and then sleep for a while, you might have to stick in a
2971 seek() to reset things. The C<seek()> doesn't change the current position,
2972 but it I<does> clear the end-of-file condition on the handle, so that the
2973 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2975 If that doesn't work (some stdios are particularly cantankerous), then
2976 you may need something more like this:
2979 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2980 # search for some stuff and put it into files
2982 sleep($for_a_while);
2983 seek(FILE, $curpos, 0);
2986 =item seekdir DIRHANDLE,POS
2988 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
2989 must be a value returned by C<telldir()>. Has the same caveats about
2990 possible directory compaction as the corresponding system library
2993 =item select FILEHANDLE
2997 Returns the currently selected filehandle. Sets the current default
2998 filehandle for output, if FILEHANDLE is supplied. This has two
2999 effects: first, a C<write> or a C<print> without a filehandle will
3000 default to this FILEHANDLE. Second, references to variables related to
3001 output will refer to this output channel. For example, if you have to
3002 set the top of form format for more than one output channel, you might
3010 FILEHANDLE may be an expression whose value gives the name of the
3011 actual filehandle. Thus:
3013 $oldfh = select(STDERR); $| = 1; select($oldfh);
3015 Some programmers may prefer to think of filehandles as objects with
3016 methods, preferring to write the last example as:
3019 STDERR->autoflush(1);
3021 =item select RBITS,WBITS,EBITS,TIMEOUT
3023 This calls the C<select(2)> system call with the bit masks specified, which
3024 can be constructed using C<fileno()> and C<vec()>, along these lines:
3026 $rin = $win = $ein = '';
3027 vec($rin,fileno(STDIN),1) = 1;
3028 vec($win,fileno(STDOUT),1) = 1;
3031 If you want to select on many filehandles you might wish to write a
3035 my(@fhlist) = split(' ',$_[0]);
3038 vec($bits,fileno($_),1) = 1;
3042 $rin = fhbits('STDIN TTY SOCK');
3046 ($nfound,$timeleft) =
3047 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3049 or to block until something becomes ready just do this
3051 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3053 Most systems do not bother to return anything useful in C<$timeleft>, so
3054 calling select() in scalar context just returns C<$nfound>.
3056 Any of the bit masks can also be undef. The timeout, if specified, is
3057 in seconds, which may be fractional. Note: not all implementations are
3058 capable of returning theC<$timeleft>. If not, they always return
3059 C<$timeleft> equal to the supplied C<$timeout>.
3061 You can effect a sleep of 250 milliseconds this way:
3063 select(undef, undef, undef, 0.25);
3065 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3066 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3067 then only on POSIX systems. You have to use C<sysread()> instead.
3069 =item semctl ID,SEMNUM,CMD,ARG
3071 Calls the System V IPC function C<semctl>. You'll probably have to say
3075 first to get the correct constant definitions. If CMD is IPC_STAT or
3076 GETALL, then ARG must be a variable which will hold the returned
3077 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3078 undefined value for error, "C<0> but true" for zero, or the actual return
3079 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3081 =item semget KEY,NSEMS,FLAGS
3083 Calls the System V IPC function semget. Returns the semaphore id, or
3084 the undefined value if there is an error. See also C<IPC::SysV> and
3085 C<IPC::SysV::Semaphore> documentation.
3087 =item semop KEY,OPSTRING
3089 Calls the System V IPC function semop to perform semaphore operations
3090 such as signaling and waiting. OPSTRING must be a packed array of
3091 semop structures. Each semop structure can be generated with
3092 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3093 operations is implied by the length of OPSTRING. Returns TRUE if
3094 successful, or FALSE if there is an error. As an example, the
3095 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3097 $semop = pack("sss", $semnum, -1, 0);
3098 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3100 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3101 and C<IPC::SysV::Semaphore> documentation.
3103 =item send SOCKET,MSG,FLAGS,TO
3105 =item send SOCKET,MSG,FLAGS
3107 Sends a message on a socket. Takes the same flags as the system call
3108 of the same name. On unconnected sockets you must specify a
3109 destination to send TO, in which case it does a C C<sendto()>. Returns
3110 the number of characters sent, or the undefined value if there is an
3112 See L<perlipc/"UDP: Message Passing"> for examples.
3114 =item setpgrp PID,PGRP
3116 Sets the current process group for the specified PID, C<0> for the current
3117 process. Will produce a fatal error if used on a machine that doesn't
3118 implement C<setpgrp(2)>. If the arguments are omitted, it defaults to
3119 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3120 arguments, so only setpgrp C<0,0> is portable.
3122 =item setpriority WHICH,WHO,PRIORITY
3124 Sets the current priority for a process, a process group, or a user.
3125 (See C<setpriority(2)>.) Will produce a fatal error if used on a machine
3126 that doesn't implement C<setpriority(2)>.
3128 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3130 Sets the socket option requested. Returns undefined if there is an
3131 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3138 Shifts the first value of the array off and returns it, shortening the
3139 array by 1 and moving everything down. If there are no elements in the
3140 array, returns the undefined value. If ARRAY is omitted, shifts the
3141 C<@_> array within the lexical scope of subroutines and formats, and the
3142 C<@ARGV> array at file scopes or within the lexical scopes established by
3143 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3144 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3145 same thing to the left end of an array that C<pop()> and C<push()> do to the
3148 =item shmctl ID,CMD,ARG
3150 Calls the System V IPC function shmctl. You'll probably have to say
3154 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3155 then ARG must be a variable which will hold the returned C<shmid_ds>
3156 structure. Returns like ioctl: the undefined value for error, "C<0> but
3157 true" for zero, or the actual return value otherwise.
3158 See also C<IPC::SysV> documentation.
3160 =item shmget KEY,SIZE,FLAGS
3162 Calls the System V IPC function shmget. Returns the shared memory
3163 segment id, or the undefined value if there is an error.
3164 See also C<IPC::SysV> documentation.
3166 =item shmread ID,VAR,POS,SIZE
3168 =item shmwrite ID,STRING,POS,SIZE
3170 Reads or writes the System V shared memory segment ID starting at
3171 position POS for size SIZE by attaching to it, copying in/out, and
3172 detaching from it. When reading, VAR must be a variable that will
3173 hold the data read. When writing, if STRING is too long, only SIZE
3174 bytes are used; if STRING is too short, nulls are written to fill out
3175 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3176 See also C<IPC::SysV> documentation.
3178 =item shutdown SOCKET,HOW
3180 Shuts down a socket connection in the manner indicated by HOW, which
3181 has the same interpretation as in the system call of the same name.
3183 shutdown(SOCKET, 0); # I/we have stopped reading data
3184 shutdown(SOCKET, 1); # I/we have stopped writing data
3185 shutdown(SOCKET, 2); # I/we have stopped using this socket
3187 This is useful with sockets when you want to tell the other
3188 side you're done writing but not done reading, or vice versa.
3189 It's also a more insistent form of close because it also
3190 disables the filedescriptor in any forked copies in other
3197 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3198 returns sine of C<$_>.
3200 For the inverse sine operation, you may use the C<POSIX::asin()>
3201 function, or use this relation:
3203 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3209 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3210 May be interrupted if the process receives a signal such as C<SIGALRM>.
3211 Returns the number of seconds actually slept. You probably cannot
3212 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3215 On some older systems, it may sleep up to a full second less than what
3216 you requested, depending on how it counts seconds. Most modern systems
3217 always sleep the full amount. They may appear to sleep longer than that,
3218 however, because your process might not be scheduled right away in a
3219 busy multitasking system.
3221 For delays of finer granularity than one second, you may use Perl's
3222 C<syscall()> interface to access C<setitimer(2)> if your system supports it,
3223 or else see L</select()> above.
3225 See also the POSIX module's C<sigpause()> function.
3227 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3229 Opens a socket of the specified kind and attaches it to filehandle
3230 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3231 system call of the same name. You should "C<use Socket;>" first to get
3232 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3234 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3236 Creates an unnamed pair of sockets in the specified domain, of the
3237 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3238 for the system call of the same name. If unimplemented, yields a fatal
3239 error. Returns TRUE if successful.
3241 Some systems defined C<pipe()> in terms of C<socketpair>, in which a call
3242 to C<pipe(Rdr, Wtr)> is essentially:
3245 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3246 shutdown(Rdr, 1); # no more writing for reader
3247 shutdown(Wtr, 0); # no more reading for writer
3249 See L<perlipc> for an example of socketpair use.
3251 =item sort SUBNAME LIST
3253 =item sort BLOCK LIST
3257 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3258 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3259 specified, it gives the name of a subroutine that returns an integer
3260 less than, equal to, or greater than C<0>, depending on how the elements
3261 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3262 operators are extremely useful in such routines.) SUBNAME may be a
3263 scalar variable name (unsubscripted), in which case the value provides
3264 the name of (or a reference to) the actual subroutine to use. In place
3265 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3268 In the interests of efficiency the normal calling code for subroutines is
3269 bypassed, with the following effects: the subroutine may not be a
3270 recursive subroutine, and the two elements to be compared are passed into
3271 the subroutine not via C<@_> but as the package global variables C<$a> and
3272 C<$b> (see example below). They are passed by reference, so don't
3273 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3275 You also cannot exit out of the sort block or subroutine using any of the
3276 loop control operators described in L<perlsyn> or with C<goto()>.
3278 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3279 current collation locale. See L<perllocale>.
3284 @articles = sort @files;
3286 # same thing, but with explicit sort routine
3287 @articles = sort {$a cmp $b} @files;
3289 # now case-insensitively
3290 @articles = sort {uc($a) cmp uc($b)} @files;
3292 # same thing in reversed order
3293 @articles = sort {$b cmp $a} @files;
3295 # sort numerically ascending
3296 @articles = sort {$a <=> $b} @files;
3298 # sort numerically descending
3299 @articles = sort {$b <=> $a} @files;
3301 # sort using explicit subroutine name
3303 $age{$a} <=> $age{$b}; # presuming numeric
3305 @sortedclass = sort byage @class;
3307 # this sorts the %age hash by value instead of key
3308 # using an in-line function
3309 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3311 sub backwards { $b cmp $a; }
3312 @harry = ('dog','cat','x','Cain','Abel');
3313 @george = ('gone','chased','yz','Punished','Axed');
3315 # prints AbelCaincatdogx
3316 print sort backwards @harry;
3317 # prints xdogcatCainAbel
3318 print sort @george, 'to', @harry;
3319 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3321 # inefficiently sort by descending numeric compare using
3322 # the first integer after the first = sign, or the
3323 # whole record case-insensitively otherwise
3326 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3331 # same thing, but much more efficiently;
3332 # we'll build auxiliary indices instead
3336 push @nums, /=(\d+)/;
3341 $nums[$b] <=> $nums[$a]
3343 $caps[$a] cmp $caps[$b]
3347 # same thing using a Schwartzian Transform (no temps)
3348 @new = map { $_->[0] }
3349 sort { $b->[1] <=> $a->[1]
3352 } map { [$_, /=(\d+)/, uc($_)] } @old;
3354 If you're using strict, you I<MUST NOT> declare C<$a>
3355 and C<$b> as lexicals. They are package globals. That means
3356 if you're in the C<main> package, it's
3358 @articles = sort {$main::b <=> $main::a} @files;
3362 @articles = sort {$::b <=> $::a} @files;
3364 but if you're in the C<FooPack> package, it's
3366 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3368 The comparison function is required to behave. If it returns
3369 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3370 sometimes saying the opposite, for example) the results are not
3373 =item splice ARRAY,OFFSET,LENGTH,LIST
3375 =item splice ARRAY,OFFSET,LENGTH
3377 =item splice ARRAY,OFFSET
3379 Removes the elements designated by OFFSET and LENGTH from an array, and
3380 replaces them with the elements of LIST, if any. In list context,
3381 returns the elements removed from the array. In scalar context,
3382 returns the last element removed, or C<undef> if no elements are
3383 removed. The array grows or shrinks as necessary.
3384 If OFFSET is negative then it start that far from the end of the array.
3385 If LENGTH is omitted, removes everything from OFFSET onward.
3386 If LENGTH is negative, leave that many elements off the end of the array.
3387 The following equivalences hold (assuming C<$[ == 0>):
3389 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3390 pop(@a) splice(@a,-1)
3391 shift(@a) splice(@a,0,1)
3392 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3393 $a[$x] = $y splice(@a,$x,1,$y)
3395 Example, assuming array lengths are passed before arrays:
3397 sub aeq { # compare two list values
3398 my(@a) = splice(@_,0,shift);
3399 my(@b) = splice(@_,0,shift);
3400 return 0 unless @a == @b; # same len?
3402 return 0 if pop(@a) ne pop(@b);
3406 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3408 =item split /PATTERN/,EXPR,LIMIT
3410 =item split /PATTERN/,EXPR
3412 =item split /PATTERN/
3416 Splits a string into an array of strings, and returns it. By default,
3417 empty leading fields are preserved, and empty trailing ones are deleted.
3419 If not in list context, returns the number of fields found and splits into
3420 the C<@_> array. (In list context, you can force the split into C<@_> by
3421 using C<??> as the pattern delimiters, but it still returns the list
3422 value.) The use of implicit split to C<@_> is deprecated, however, because
3423 it clobbers your subroutine arguments.
3425 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3426 splits on whitespace (after skipping any leading whitespace). Anything
3427 matching PATTERN is taken to be a delimiter separating the fields. (Note
3428 that the delimiter may be longer than one character.)
3430 If LIMIT is specified and positive, splits into no more than that
3431 many fields (though it may split into fewer). If LIMIT is unspecified
3432 or zero, trailing null fields are stripped (which potential users
3433 of C<pop()> would do well to remember). If LIMIT is negative, it is
3434 treated as if an arbitrarily large LIMIT had been specified.
3436 A pattern matching the null string (not to be confused with
3437 a null pattern C<//>, which is just one member of the set of patterns
3438 matching a null string) will split the value of EXPR into separate
3439 characters at each point it matches that way. For example:
3441 print join(':', split(/ */, 'hi there'));
3443 produces the output 'h:i:t:h:e:r:e'.
3445 The LIMIT parameter can be used to split a line partially
3447 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3449 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3450 one larger than the number of variables in the list, to avoid
3451 unnecessary work. For the list above LIMIT would have been 4 by
3452 default. In time critical applications it behooves you not to split
3453 into more fields than you really need.
3455 If the PATTERN contains parentheses, additional array elements are
3456 created from each matching substring in the delimiter.
3458 split(/([,-])/, "1-10,20", 3);
3460 produces the list value
3462 (1, '-', 10, ',', 20)
3464 If you had the entire header of a normal Unix email message in C<$header>,
3465 you could split it up into fields and their values this way:
3467 $header =~ s/\n\s+/ /g; # fix continuation lines
3468 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3470 The pattern C</PATTERN/> may be replaced with an expression to specify
3471 patterns that vary at runtime. (To do runtime compilation only once,
3472 use C</$variable/o>.)
3474 As a special case, specifying a PATTERN of space (C<' '>) will split on
3475 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
3476 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3477 will give you as many null initial fields as there are leading spaces.
3478 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
3479 whitespace produces a null first field. A C<split> with no arguments
3480 really does a C<split(' ', $_)> internally.
3484 open(PASSWD, '/etc/passwd');
3486 ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/);
3490 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3491 L</chomp>, and L</join>.)
3493 =item sprintf FORMAT, LIST
3495 Returns a string formatted by the usual C<printf> conventions of the
3496 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3497 on your system for an explanation of the general principles.
3499 Perl does its own C<sprintf()> formatting -- it emulates the C
3500 function C<sprintf()>, but it doesn't use it (except for floating-point
3501 numbers, and even then only the standard modifiers are allowed). As a
3502 result, any non-standard extensions in your local C<sprintf()> are not
3503 available from Perl.
3505 Perl's C<sprintf()> permits the following universally-known conversions:
3508 %c a character with the given number
3510 %d a signed integer, in decimal
3511 %u an unsigned integer, in decimal
3512 %o an unsigned integer, in octal
3513 %x an unsigned integer, in hexadecimal
3514 %e a floating-point number, in scientific notation
3515 %f a floating-point number, in fixed decimal notation
3516 %g a floating-point number, in %e or %f notation
3518 In addition, Perl permits the following widely-supported conversions:
3520 %X like %x, but using upper-case letters
3521 %E like %e, but using an upper-case "E"
3522 %G like %g, but with an upper-case "E" (if applicable)
3523 %p a pointer (outputs the Perl value's address in hexadecimal)
3524 %n special: *stores* the number of characters output so far
3525 into the next variable in the parameter list
3527 Finally, for backward (and we do mean "backward") compatibility, Perl
3528 permits these unnecessary but widely-supported conversions:
3531 %D a synonym for %ld
3532 %U a synonym for %lu
3533 %O a synonym for %lo
3536 Perl permits the following universally-known flags between the C<%>
3537 and the conversion letter:
3539 space prefix positive number with a space
3540 + prefix positive number with a plus sign
3541 - left-justify within the field
3542 0 use zeros, not spaces, to right-justify
3543 # prefix non-zero octal with "0", non-zero hex with "0x"
3544 number minimum field width
3545 .number "precision": digits after decimal point for floating-point,
3546 max length for string, minimum length for integer
3547 l interpret integer as C type "long" or "unsigned long"
3548 h interpret integer as C type "short" or "unsigned short"
3550 There is also one Perl-specific flag:
3552 V interpret integer as Perl's standard integer type
3554 Where a number would appear in the flags, an asterisk ("C<*>") may be
3555 used instead, in which case Perl uses the next item in the parameter
3556 list as the given number (that is, as the field width or precision).
3557 If a field width obtained through "C<*>" is negative, it has the same
3558 effect as the "C<->" flag: left-justification.
3560 If C<use locale> is in effect, the character used for the decimal
3561 point in formatted real numbers is affected by the LC_NUMERIC locale.
3568 Return the square root of EXPR. If EXPR is omitted, returns square
3575 Sets the random number seed for the C<rand> operator. If EXPR is
3576 omitted, uses a semi-random value based on the current time and process
3577 ID, among other things. In versions of Perl prior to 5.004 the default
3578 seed was just the current C<time()>. This isn't a particularly good seed,
3579 so many old programs supply their own seed value (often C<time ^ $$> or
3580 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3582 In fact, it's usually not necessary to call C<srand()> at all, because if
3583 it is not called explicitly, it is called implicitly at the first use of
3584 the C<rand> operator. However, this was not the case in version of Perl
3585 before 5.004, so if your script will run under older Perl versions, it
3586 should call C<srand()>.
3588 Note that you need something much more random than the default seed for
3589 cryptographic purposes. Checksumming the compressed output of one or more
3590 rapidly changing operating system status programs is the usual method. For
3593 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3595 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3598 Do I<not> call C<srand()> multiple times in your program unless you know
3599 exactly what you're doing and why you're doing it. The point of the
3600 function is to "seed" the C<rand()> function so that C<rand()> can produce
3601 a different sequence each time you run your program. Just do it once at the
3602 top of your program, or you I<won't> get random numbers out of C<rand()>!
3604 Frequently called programs (like CGI scripts) that simply use
3608 for a seed can fall prey to the mathematical property that
3612 one-third of the time. So don't do that.
3614 =item stat FILEHANDLE
3620 Returns a 13-element list giving the status info for a file, either
3621 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3622 it stats C<$_>. Returns a null list if the stat fails. Typically used
3625 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3626 $atime,$mtime,$ctime,$blksize,$blocks)
3629 Not all fields are supported on all filesystem types. Here are the
3630 meaning of the fields:
3632 0 dev device number of filesystem
3634 2 mode file mode (type and permissions)
3635 3 nlink number of (hard) links to the file
3636 4 uid numeric user ID of file's owner
3637 5 gid numeric group ID of file's owner
3638 6 rdev the device identifier (special files only)
3639 7 size total size of file, in bytes
3640 8 atime last access time since the epoch
3641 9 mtime last modify time since the epoch
3642 10 ctime inode change time (NOT creation time!) since the epoch
3643 11 blksize preferred block size for file system I/O
3644 12 blocks actual number of blocks allocated
3646 (The epoch was at 00:00 January 1, 1970 GMT.)
3648 If stat is passed the special filehandle consisting of an underline, no
3649 stat is done, but the current contents of the stat structure from the
3650 last stat or filetest are returned. Example:
3652 if (-x $file && (($d) = stat(_)) && $d < 0) {
3653 print "$file is executable NFS file\n";
3656 (This works on machines only for which the device number is negative under NFS.)
3658 In scalar context, C<stat> returns a boolean value indicating success
3659 or failure, and, if successful, sets the information associated with
3660 the special filehandle C<_>.
3666 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3667 doing many pattern matches on the string before it is next modified.
3668 This may or may not save time, depending on the nature and number of
3669 patterns you are searching on, and on the distribution of character
3670 frequencies in the string to be searched -- you probably want to compare
3671 run times with and without it to see which runs faster. Those loops
3672 which scan for many short constant strings (including the constant
3673 parts of more complex patterns) will benefit most. You may have only
3674 one C<study> active at a time -- if you study a different scalar the first
3675 is "unstudied". (The way C<study> works is this: a linked list of every
3676 character in the string to be searched is made, so we know, for
3677 example, where all the C<'k'> characters are. From each search string,
3678 the rarest character is selected, based on some static frequency tables
3679 constructed from some C programs and English text. Only those places
3680 that contain this "rarest" character are examined.)
3682 For example, here is a loop that inserts index producing entries
3683 before any line containing a certain pattern:
3687 print ".IX foo\n" if /\bfoo\b/;
3688 print ".IX bar\n" if /\bbar\b/;
3689 print ".IX blurfl\n" if /\bblurfl\b/;
3694 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3695 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3696 a big win except in pathological cases. The only question is whether
3697 it saves you more time than it took to build the linked list in the
3700 Note that if you have to look for strings that you don't know till
3701 runtime, you can build an entire loop as a string and C<eval> that to
3702 avoid recompiling all your patterns all the time. Together with
3703 undefining C<$/> to input entire files as one record, this can be very
3704 fast, often faster than specialized programs like C<fgrep(1)>. The following
3705 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3706 out the names of those files that contain a match:
3708 $search = 'while (<>) { study;';
3709 foreach $word (@words) {
3710 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3715 eval $search; # this screams
3716 $/ = "\n"; # put back to normal input delimiter
3717 foreach $file (sort keys(%seen)) {
3725 =item sub NAME BLOCK
3727 This is subroutine definition, not a real function I<per se>. With just a
3728 NAME (and possibly prototypes), it's just a forward declaration. Without
3729 a NAME, it's an anonymous function declaration, and does actually return a
3730 value: the CODE ref of the closure you just created. See L<perlsub> and
3731 L<perlref> for details.
3733 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3735 =item substr EXPR,OFFSET,LEN
3737 =item substr EXPR,OFFSET
3739 Extracts a substring out of EXPR and returns it. First character is at
3740 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3741 If OFFSET is negative (or more precisely, less than C<$[>), starts
3742 that far from the end of the string. If LEN is omitted, returns
3743 everything to the end of the string. If LEN is negative, leaves that
3744 many characters off the end of the string.
3746 If you specify a substring that is partly outside the string, the part
3747 within the string is returned. If the substring is totally outside
3748 the string a warning is produced.
3750 You can use the C<substr()> function
3751 as an lvalue, in which case EXPR must be an lvalue. If you assign
3752 something shorter than LEN, the string will shrink, and if you assign
3753 something longer than LEN, the string will grow to accommodate it. To
3754 keep the string the same length you may need to pad or chop your value
3757 An alternative to using C<substr()> as an lvalue is to specify the
3758 replacement string as the 4th argument. This allows you to replace
3759 parts of the EXPR and return what was there before in one operation.
3761 =item symlink OLDFILE,NEWFILE
3763 Creates a new filename symbolically linked to the old filename.
3764 Returns C<1> for success, C<0> otherwise. On systems that don't support
3765 symbolic links, produces a fatal error at run time. To check for that,
3768 $symlink_exists = eval { symlink("",""); 1 };
3772 Calls the system call specified as the first element of the list,
3773 passing the remaining elements as arguments to the system call. If
3774 unimplemented, produces a fatal error. The arguments are interpreted
3775 as follows: if a given argument is numeric, the argument is passed as
3776 an int. If not, the pointer to the string value is passed. You are
3777 responsible to make sure a string is pre-extended long enough to
3778 receive any result that might be written into a string. You can't use a
3779 string literal (or other read-only string) as an argument to C<syscall()>
3780 because Perl has to assume that any string pointer might be written
3782 integer arguments are not literals and have never been interpreted in a
3783 numeric context, you may need to add C<0> to them to force them to look
3784 like numbers. This emulates the C<syswrite()> function (or vice versa):
3786 require 'syscall.ph'; # may need to run h2ph
3788 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3790 Note that Perl supports passing of up to only 14 arguments to your system call,
3791 which in practice should usually suffice.
3793 Syscall returns whatever value returned by the system call it calls.
3794 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
3795 Note that some system calls can legitimately return C<-1>. The proper
3796 way to handle such calls is to assign C<$!=0;> before the call and
3797 check the value of C<$!> if syscall returns C<-1>.
3799 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3800 number of the read end of the pipe it creates. There is no way
3801 to retrieve the file number of the other end. You can avoid this
3802 problem by using C<pipe> instead.
3804 =item sysopen FILEHANDLE,FILENAME,MODE
3806 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3808 Opens the file whose filename is given by FILENAME, and associates it
3809 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3810 the name of the real filehandle wanted. This function calls the
3811 underlying operating system's C<open> function with the parameters
3812 FILENAME, MODE, PERMS.
3814 The possible values and flag bits of the MODE parameter are
3815 system-dependent; they are available via the standard module C<Fcntl>.
3816 However, for historical reasons, some values are universal: zero means
3817 read-only, one means write-only, and two means read/write.
3819 If the file named by FILENAME does not exist and the C<open> call creates
3820 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3821 PERMS specifies the permissions of the newly created file. If you omit
3822 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
3823 These permission values need to be in octal, and are modified by your
3824 process's current C<umask>. The C<umask> value is a number representing
3825 disabled permissions bits--if your C<umask> were C<027> (group can't write;
3826 others can't read, write, or execute), then passing C<sysopen> C<0666> would
3827 create a file with mode C<0640> (C<0666 &~ 027> is C<0640>).
3829 If you find this C<umask> talk confusing, here's some advice: supply a
3830 creation mode of C<0666> for regular files and one of C<0777> for directories
3831 (in C<mkdir>) and executable files. This gives users the freedom of
3832 choice: if they want protected files, they might choose process umasks
3833 of C<022>, C<027>, or even the particularly antisocial mask of C<077>. Programs
3834 should rarely if ever make policy decisions better left to the user.
3835 The exception to this is when writing files that should be kept private:
3836 mail files, web browser cookies, I<.rhosts> files, and so on. In short,
3837 seldom if ever use C<0644> as argument to C<sysopen> because that takes
3838 away the user's option to have a more permissive umask. Better to omit it.
3840 The C<IO::File> module provides a more object-oriented approach, if you're
3841 into that kind of thing.
3843 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3845 =item sysread FILEHANDLE,SCALAR,LENGTH
3847 Attempts to read LENGTH bytes of data into variable SCALAR from the
3848 specified FILEHANDLE, using the system call C<read(2)>. It bypasses
3849 stdio, so mixing this with other kinds of reads, C<print()>, C<write()>,
3850 C<seek()>, or C<tell()> can cause confusion because stdio usually buffers
3851 data. Returns the number of bytes actually read, C<0> at end of file,
3852 or undef if there was an error. SCALAR will be grown or shrunk so that
3853 the last byte actually read is the last byte of the scalar after the read.
3855 An OFFSET may be specified to place the read data at some place in the
3856 string other than the beginning. A negative OFFSET specifies
3857 placement at that many bytes counting backwards from the end of the
3858 string. A positive OFFSET greater than the length of SCALAR results
3859 in the string being padded to the required size with C<"\0"> bytes before
3860 the result of the read is appended.
3862 =item sysseek FILEHANDLE,POSITION,WHENCE
3864 Sets FILEHANDLE's system position using the system call C<lseek(2)>. It
3865 bypasses stdio, so mixing this with reads (other than C<sysread()>),
3866 C<print()>, C<write()>, C<seek()>, or C<tell()> may cause confusion. FILEHANDLE may
3867 be an expression whose value gives the name of the filehandle. The
3868 values for WHENCE are C<0> to set the new position to POSITION, C<1> to set
3869 the it to the current position plus POSITION, and C<2> to set it to EOF
3870 plus POSITION (typically negative). For WHENCE, you may use the
3871 constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable>
3872 or the POSIX module.
3874 Returns the new position, or the undefined value on failure. A position
3875 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
3876 TRUE on success and FALSE on failure, yet you can still easily determine
3881 =item system PROGRAM LIST
3883 Does exactly the same thing as "C<exec LIST>" except that a fork is done
3884 first, and the parent process waits for the child process to complete.
3885 Note that argument processing varies depending on the number of
3886 arguments. If there is more than one argument in LIST, or if LIST is
3887 an array with more than one value, starts the program given by the
3888 first element of the list with arguments given by the rest of the list.
3889 If there is only one scalar argument, the argument is
3890 checked for shell metacharacters, and if there are any, the entire
3891 argument is passed to the system's command shell for parsing (this is
3892 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
3893 there are no shell metacharacters in the argument, it is split into
3894 words and passed directly to C<execvp()>, which is more efficient.
3896 The return value is the exit status of the program as
3897 returned by the C<wait()> call. To get the actual exit value divide by
3898 256. See also L</exec>. This is I<NOT> what you want to use to capture
3899 the output from a command, for that you should use merely backticks or
3900 C<qx//>, as described in L<perlop/"`STRING`">.
3902 Like C<exec()>, C<system()> allows you to lie to a program about its name if
3903 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
3905 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
3906 program they're running doesn't actually interrupt your program.
3908 @args = ("command", "arg1", "arg2");
3910 or die "system @args failed: $?"
3912 You can check all the failure possibilities by inspecting
3915 $exit_value = $? >> 8;
3916 $signal_num = $? & 127;
3917 $dumped_core = $? & 128;
3919 When the arguments get executed via the system shell, results
3920 and return codes will be subject to its quirks and capabilities.
3921 See L<perlop/"`STRING`"> and L</exec> for details.
3923 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3925 =item syswrite FILEHANDLE,SCALAR,LENGTH
3927 Attempts to write LENGTH bytes of data from variable SCALAR to the
3928 specified FILEHANDLE, using the system call C<write(2)>. It bypasses
3929 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
3930 C<write()>, C<seek()>, or C<tell()> may cause confusion because stdio usually
3931 buffers data. Returns the number of bytes actually written, or C<undef>
3932 if there was an error. If the LENGTH is greater than the available
3933 data in the SCALAR after the OFFSET, only as much data as is available
3936 An OFFSET may be specified to write the data from some part of the
3937 string other than the beginning. A negative OFFSET specifies writing
3938 that many bytes counting backwards from the end of the string. In the
3939 case the SCALAR is empty you can use OFFSET but only zero offset.
3941 =item tell FILEHANDLE
3945 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3946 expression whose value gives the name of the actual filehandle. If
3947 FILEHANDLE is omitted, assumes the file last read.
3949 =item telldir DIRHANDLE
3951 Returns the current position of the C<readdir()> routines on DIRHANDLE.
3952 Value may be given to C<seekdir()> to access a particular location in a
3953 directory. Has the same caveats about possible directory compaction as
3954 the corresponding system library routine.
3956 =item tie VARIABLE,CLASSNAME,LIST
3958 This function binds a variable to a package class that will provide the
3959 implementation for the variable. VARIABLE is the name of the variable
3960 to be enchanted. CLASSNAME is the name of a class implementing objects
3961 of correct type. Any additional arguments are passed to the "C<new>"
3962 method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
3963 Typically these are arguments such as might be passed to the C<dbm_open()>
3964 function of C. The object returned by the "C<new>" method is also
3965 returned by the C<tie()> function, which would be useful if you want to
3966 access other methods in CLASSNAME.
3968 Note that functions such as C<keys()> and C<values()> may return huge lists
3969 when used on large objects, like DBM files. You may prefer to use the
3970 C<each()> function to iterate over such. Example:
3972 # print out history file offsets
3974 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3975 while (($key,$val) = each %HIST) {
3976 print $key, ' = ', unpack('L',$val), "\n";
3980 A class implementing a hash should have the following methods:
3982 TIEHASH classname, LIST
3985 STORE this, key, value
3989 NEXTKEY this, lastkey
3991 A class implementing an ordinary array should have the following methods:
3993 TIEARRAY classname, LIST
3996 STORE this, key, value
3999 A class implementing a scalar should have the following methods:
4001 TIESCALAR classname, LIST
4006 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4007 for you--you need to do that explicitly yourself. See L<DB_File>
4008 or the F<Config> module for interesting C<tie()> implementations.
4010 For further details see L<perltie>, L<tied VARIABLE>.
4014 Returns a reference to the object underlying VARIABLE (the same value
4015 that was originally returned by the C<tie()> call that bound the variable
4016 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4021 Returns the number of non-leap seconds since whatever time the system
4022 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4023 and 00:00:00 UTC, January 1, 1970 for most other systems).
4024 Suitable for feeding to C<gmtime()> and C<localtime()>.
4028 Returns a four-element list giving the user and system times, in
4029 seconds, for this process and the children of this process.
4031 ($user,$system,$cuser,$csystem) = times;
4035 The transliteration operator. Same as C<y///>. See L<perlop>.
4037 =item truncate FILEHANDLE,LENGTH
4039 =item truncate EXPR,LENGTH
4041 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4042 specified length. Produces a fatal error if truncate isn't implemented
4043 on your system. Returns TRUE if successful, the undefined value
4050 Returns an uppercased version of EXPR. This is the internal function
4051 implementing the C<\U> escape in double-quoted strings.
4052 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4054 If EXPR is omitted, uses C<$_>.
4060 Returns the value of EXPR with the first character uppercased. This is
4061 the internal function implementing the C<\u> escape in double-quoted strings.
4062 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4064 If EXPR is omitted, uses C<$_>.
4070 Sets the umask for the process to EXPR and returns the previous value.
4071 If EXPR is omitted, merely returns the current umask. If C<umask(2)> is
4072 not implemented on your system, returns C<undef>. Remember that a
4073 umask is a number, usually given in octal; it is I<not> a string of octal
4074 digits. See also L</oct>, if all you have is a string.
4080 Undefines the value of EXPR, which must be an lvalue. Use only on a
4081 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4082 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4083 will probably not do what you expect on most predefined variables or
4084 DBM list values, so don't do that; see L<delete>.) Always returns the
4085 undefined value. You can omit the EXPR, in which case nothing is
4086 undefined, but you still get an undefined value that you could, for
4087 instance, return from a subroutine, assign to a variable or pass as a
4088 parameter. Examples:
4091 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4095 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4096 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4097 select undef, undef, undef, 0.25;
4098 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4100 Note that this is a unary operator, not a list operator.
4106 Deletes a list of files. Returns the number of files successfully
4109 $cnt = unlink 'a', 'b', 'c';
4113 Note: C<unlink> will not delete directories unless you are superuser and
4114 the B<-U> flag is supplied to Perl. Even if these conditions are
4115 met, be warned that unlinking a directory can inflict damage on your
4116 filesystem. Use C<rmdir> instead.
4118 If LIST is omitted, uses C<$_>.
4120 =item unpack TEMPLATE,EXPR
4122 C<Unpack> does the reverse of C<pack>: it takes a string representing a
4123 structure and expands it out into a list value, returning the array
4124 value. (In scalar context, it returns merely the first value
4125 produced.) The TEMPLATE has the same format as in the C<pack> function.
4126 Here's a subroutine that does substring:
4129 my($what,$where,$howmuch) = @_;
4130 unpack("x$where a$howmuch", $what);
4135 sub ordinal { unpack("c",$_[0]); } # same as ord()
4137 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4138 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4139 themselves. Default is a 16-bit checksum. For example, the following
4140 computes the same number as the System V sum program:
4143 $checksum += unpack("%16C*", $_);
4147 The following efficiently counts the number of set bits in a bit vector:
4149 $setbits = unpack("%32b*", $selectmask);
4151 =item untie VARIABLE
4153 Breaks the binding between a variable and a package. (See C<tie()>.)
4155 =item unshift ARRAY,LIST
4157 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4158 depending on how you look at it. Prepends list to the front of the
4159 array, and returns the new number of elements in the array.
4161 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4163 Note the LIST is prepended whole, not one element at a time, so the
4164 prepended elements stay in the same order. Use C<reverse> to do the
4167 =item use Module LIST
4171 =item use Module VERSION LIST
4175 Imports some semantics into the current package from the named module,
4176 generally by aliasing certain subroutine or variable names into your
4177 package. It is exactly equivalent to
4179 BEGIN { require Module; import Module LIST; }
4181 except that Module I<must> be a bareword.
4183 If the first argument to C<use> is a number, it is treated as a version
4184 number instead of a module name. If the version of the Perl interpreter
4185 is less than VERSION, then an error message is printed and Perl exits
4186 immediately. This is often useful if you need to check the current
4187 Perl version before C<use>ing library modules that have changed in
4188 incompatible ways from older versions of Perl. (We try not to do
4189 this more than we have to.)
4191 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
4192 C<require> makes sure the module is loaded into memory if it hasn't been
4193 yet. The C<import> is not a builtin--it's just an ordinary static method
4194 call into the "C<Module>" package to tell the module to import the list of
4195 features back into the current package. The module can implement its
4196 C<import> method any way it likes, though most modules just choose to
4197 derive their C<import> method via inheritance from the C<Exporter> class that
4198 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
4199 method can be found then the error is currently silently ignored. This
4200 may change to a fatal error in a future version.
4202 If you don't want your namespace altered, explicitly supply an empty list:
4206 That is exactly equivalent to
4208 BEGIN { require Module }
4210 If the VERSION argument is present between Module and LIST, then the
4211 C<use> will call the VERSION method in class Module with the given
4212 version as an argument. The default VERSION method, inherited from
4213 the Universal class, croaks if the given version is larger than the
4214 value of the variable C<$Module::VERSION>. (Note that there is not a
4215 comma after VERSION!)
4217 Because this is a wide-open interface, pragmas (compiler directives)
4218 are also implemented this way. Currently implemented pragmas are:
4222 use sigtrap qw(SEGV BUS);
4223 use strict qw(subs vars refs);
4224 use subs qw(afunc blurfl);
4226 Some of these these pseudo-modules import semantics into the current
4227 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4228 which import symbols into the current package (which are effective
4229 through the end of the file).
4231 There's a corresponding "C<no>" command that unimports meanings imported
4232 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
4237 If no C<unimport> method can be found the call fails with a fatal error.
4239 See L<perlmod> for a list of standard modules and pragmas.
4243 Changes the access and modification times on each file of a list of
4244 files. The first two elements of the list must be the NUMERICAL access
4245 and modification times, in that order. Returns the number of files
4246 successfully changed. The inode modification time of each file is set
4247 to the current time. This code has the same effect as the "C<touch>"
4248 command if the files already exist:
4252 utime $now, $now, @ARGV;
4256 Returns a list consisting of all the values of the named hash. (In a
4257 scalar context, returns the number of values.) The values are
4258 returned in an apparently random order, but it is the same order as
4259 either the C<keys()> or C<each()> function would produce on the same hash.
4260 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4263 =item vec EXPR,OFFSET,BITS
4265 Treats the string in EXPR as a vector of unsigned integers, and
4266 returns the value of the bit field specified by OFFSET. BITS specifies
4267 the number of bits that are reserved for each entry in the bit
4268 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4269 assigned to, in which case parentheses are needed to give the expression
4270 the correct precedence as in
4272 vec($image, $max_x * $x + $y, 8) = 3;
4274 Vectors created with C<vec()> can also be manipulated with the logical
4275 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4276 desired when both operands are strings.
4278 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4279 The comments show the string after each step. Note that this code works
4280 in the same way on big-endian or little-endian machines.
4283 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4284 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4285 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4286 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4287 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4288 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4289 vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72"
4290 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4291 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4292 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c"
4294 To transform a bit vector into a string or array of 0's and 1's, use these:
4296 $bits = unpack("b*", $vector);
4297 @bits = split(//, unpack("b*", $vector));
4299 If you know the exact length in bits, it can be used in place of the C<*>.
4303 Waits for a child process to terminate and returns the pid of the
4304 deceased process, or C<-1> if there are no child processes. The status is
4307 =item waitpid PID,FLAGS
4309 Waits for a particular child process to terminate and returns the pid
4310 of the deceased process, or C<-1> if there is no such child process. The
4311 status is returned in C<$?>. If you say
4313 use POSIX ":sys_wait_h";
4315 waitpid(-1,&WNOHANG);
4317 then you can do a non-blocking wait for any process. Non-blocking wait
4318 is available on machines supporting either the C<waitpid(2)> or
4319 C<wait4(2)> system calls. However, waiting for a particular pid with
4320 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4321 by remembering the status values of processes that have exited but have
4322 not been harvested by the Perl script yet.)
4324 See L<perlipc> for other examples.
4328 Returns TRUE if the context of the currently executing subroutine is
4329 looking for a list value. Returns FALSE if the context is looking
4330 for a scalar. Returns the undefined value if the context is looking
4331 for no value (void context).
4333 return unless defined wantarray; # don't bother doing more
4334 my @a = complex_calculation();
4335 return wantarray ? @a : "@a";
4339 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4342 If LIST is empty and C<$@> already contains a value (typically from a
4343 previous eval) that value is used after appending C<"\t...caught">
4344 to C<$@>. This is useful for staying almost, but not entirely similar to
4347 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4349 No message is printed if there is a C<$SIG{__WARN__}> handler
4350 installed. It is the handler's responsibility to deal with the message
4351 as it sees fit (like, for instance, converting it into a C<die()>). Most
4352 handlers must therefore make arrangements to actually display the
4353 warnings that they are not prepared to deal with, by calling C<warn()>
4354 again in the handler. Note that this is quite safe and will not
4355 produce an endless loop, since C<__WARN__> hooks are not called from
4358 You will find this behavior is slightly different from that of
4359 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4360 instead call C<die()> again to change it).
4362 Using a C<__WARN__> handler provides a powerful way to silence all
4363 warnings (even the so-called mandatory ones). An example:
4365 # wipe out *all* compile-time warnings
4366 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4368 my $foo = 20; # no warning about duplicate my $foo,
4369 # but hey, you asked for it!
4370 # no compile-time or run-time warnings before here
4373 # run-time warnings enabled after here
4374 warn "\$foo is alive and $foo!"; # does show up
4376 See L<perlvar> for details on setting C<%SIG> entries, and for more
4379 =item write FILEHANDLE
4385 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4386 using the format associated with that file. By default the format for
4387 a file is the one having the same name as the filehandle, but the
4388 format for the current output channel (see the C<select()> function) may be set
4389 explicitly by assigning the name of the format to the C<$~> variable.
4391 Top of form processing is handled automatically: if there is
4392 insufficient room on the current page for the formatted record, the
4393 page is advanced by writing a form feed, a special top-of-page format
4394 is used to format the new page header, and then the record is written.
4395 By default the top-of-page format is the name of the filehandle with
4396 "_TOP" appended, but it may be dynamically set to the format of your
4397 choice by assigning the name to the C<$^> variable while the filehandle is
4398 selected. The number of lines remaining on the current page is in
4399 variable C<$->, which can be set to C<0> to force a new page.
4401 If FILEHANDLE is unspecified, output goes to the current default output
4402 channel, which starts out as STDOUT but may be changed by the
4403 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4404 is evaluated and the resulting string is used to look up the name of
4405 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4407 Note that write is I<NOT> the opposite of C<read>. Unfortunately.
4411 The transliteration operator. Same as C<tr///>. See L<perlop>.