3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return 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), or Filehandle is a pipe.
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 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<$_> is used. (On some machines,
334 unfortunately, the elapsed time may be up to one second less than you
335 specified because of how seconds are counted.) Only one timer may be
336 counting at once. Each call disables the previous timer, and an
337 argument of C<0> may be supplied to cancel the previous timer without
338 starting a new one. The returned value is the amount of time remaining
339 on the previous timer.
341 For delays of finer granularity than one second, you may use Perl's
342 C<syscall()> interface to access setitimer(2) if your system supports it,
343 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
344 and C<sleep()> calls.
346 If you want to use C<alarm()> to time out a system call you need to use an
347 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
348 fail with C<$!> set to C<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
463 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
464 $mode = 0644; chmod $mode, 'foo'; # this is best
472 This is a slightly safer version of L</chop>. It removes any
473 line ending that corresponds to the current value of C<$/> (also known as
474 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
475 number of characters removed from all its arguments. It's often used to
476 remove the newline from the end of an input record when you're worried
477 that the final record may be missing its newline. When in paragraph mode
478 (C<$/ = "">), it removes all trailing newlines from the string. If
479 VARIABLE is omitted, it chomps C<$_>. Example:
482 chomp; # avoid \n on last field
487 You can actually chomp anything that's an lvalue, including an assignment:
490 chomp($answer = <STDIN>);
492 If you chomp a list, each element is chomped, and the total number of
493 characters removed is returned.
501 Chops off the last character of a string and returns the character
502 chopped. It's used primarily to remove the newline from the end of an
503 input record, but is much more efficient than C<s/\n//> because it neither
504 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
508 chop; # avoid \n on last field
513 You can actually chop anything that's an lvalue, including an assignment:
516 chop($answer = <STDIN>);
518 If you chop a list, each element is chopped. Only the value of the
519 last C<chop()> is returned.
521 Note that C<chop()> returns the last character. To return all but the last
522 character, use C<substr($string, 0, -1)>.
526 Changes the owner (and group) of a list of files. The first two
527 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
528 Returns the number of files successfully changed.
530 $cnt = chown $uid, $gid, 'foo', 'bar';
531 chown $uid, $gid, @filenames;
533 Here's an example that looks up nonnumeric uids in the passwd file:
536 chop($user = <STDIN>);
538 chop($pattern = <STDIN>);
540 ($login,$pass,$uid,$gid) = getpwnam($user)
541 or die "$user not in passwd file";
543 @ary = glob($pattern); # expand filenames
544 chown $uid, $gid, @ary;
546 On most systems, you are not allowed to change the ownership of the
547 file unless you're the superuser, although you should be able to change
548 the group to any of your secondary groups. On insecure systems, these
549 restrictions may be relaxed, but this is not a portable assumption.
555 Returns the character represented by that NUMBER in the character set.
556 For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>.
558 If NUMBER is omitted, uses C<$_>.
560 =item chroot FILENAME
564 This function works like the system call by the same name: it makes the
565 named directory the new root directory for all further pathnames that
566 begin with a C<"/"> by your process and all its children. (It doesn't
567 change your current working directory, which is unaffected.) For security
568 reasons, this call is restricted to the superuser. If FILENAME is
569 omitted, does a C<chroot()> to C<$_>.
571 =item close FILEHANDLE
575 Closes the file or pipe associated with the file handle, returning TRUE
576 only if stdio successfully flushes buffers and closes the system file
577 descriptor. Closes the currently selected filehandle if the argument
580 You don't have to close FILEHANDLE if you are immediately going to do
581 another C<open()> on it, because C<open()> will close it for you. (See
582 C<open()>.) However, an explicit C<close()> on an input file resets the line
583 counter (C<$.>), while the implicit close done by C<open()> does not.
585 If the file handle came from a piped open C<close()> will additionally
586 return FALSE if one of the other system calls involved fails or if the
587 program exits with non-zero status. (If the only problem was that the
588 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
589 waits for the process executing on the pipe to complete, in case you
590 want to look at the output of the pipe afterwards. Closing a pipe
591 explicitly also puts the exit status value of the command into C<$?>.
595 open(OUTPUT, '|sort >foo') # pipe to sort
596 or die "Can't start sort: $!";
597 #... # print stuff to output
598 close OUTPUT # wait for sort to finish
599 or warn $! ? "Error closing sort pipe: $!"
600 : "Exit status $? from sort";
601 open(INPUT, 'foo') # get sort's results
602 or die "Can't open 'foo' for input: $!";
604 FILEHANDLE may be an expression whose value can be used as an indirect
605 filehandle, usually the real filehandle name.
607 =item closedir DIRHANDLE
609 Closes a directory opened by C<opendir()> and returns the success of that
612 DIRHANDLE may be an expression whose value can be used as an indirect
613 dirhandle, usually the real dirhandle name.
615 =item connect SOCKET,NAME
617 Attempts to connect to a remote socket, just as the connect system call
618 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
619 packed address of the appropriate type for the socket. See the examples in
620 L<perlipc/"Sockets: Client/Server Communication">.
624 Actually a flow control statement rather than a function. If there is a
625 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
626 C<foreach>), it is always executed just before the conditional is about to
627 be evaluated again, just like the third part of a C<for> loop in C. Thus
628 it can be used to increment a loop variable, even when the loop has been
629 continued via the C<next> statement (which is similar to the C C<continue>
632 C<last>, C<next>, or C<redo> may appear within a C<continue>
633 block. C<last> and C<redo> will behave as if they had been executed within
634 the main block. So will C<next>, but since it will execute a C<continue>
635 block, it may be more entertaining.
638 ### redo always comes here
641 ### next always comes here
643 # then back the top to re-check EXPR
645 ### last always comes here
647 Omitting the C<continue> section is semantically equivalent to using an
648 empty one, logically enough. In that case, C<next> goes directly back
649 to check the condition at the top of the loop.
653 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
654 takes cosine of C<$_>.
656 For the inverse cosine operation, you may use the C<POSIX::acos()>
657 function, or use this relation:
659 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
661 =item crypt PLAINTEXT,SALT
663 Encrypts a string exactly like the crypt(3) function in the C library
664 (assuming that you actually have a version there that has not been
665 extirpated as a potential munition). This can prove useful for checking
666 the password file for lousy passwords, amongst other things. Only the
667 guys wearing white hats should do this.
669 Note that C<crypt()> is intended to be a one-way function, much like breaking
670 eggs to make an omelette. There is no (known) corresponding decrypt
671 function. As a result, this function isn't all that useful for
672 cryptography. (For that, see your nearby CPAN mirror.)
674 Here's an example that makes sure that whoever runs this program knows
677 $pwd = (getpwuid($<))[1];
678 $salt = substr($pwd, 0, 2);
682 chop($word = <STDIN>);
686 if (crypt($word, $salt) ne $pwd) {
692 Of course, typing in your own password to whoever asks you
697 [This function has been superseded by the C<untie()> function.]
699 Breaks the binding between a DBM file and a hash.
701 =item dbmopen HASH,DBNAME,MODE
703 [This function has been superseded by the C<tie()> function.]
705 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
706 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
707 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
708 is the name of the database (without the F<.dir> or F<.pag> extension if
709 any). If the database does not exist, it is created with protection
710 specified by MODE (as modified by the C<umask()>). If your system supports
711 only the older DBM functions, you may perform only one C<dbmopen()> in your
712 program. In older versions of Perl, if your system had neither DBM nor
713 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
716 If you don't have write access to the DBM file, you can only read hash
717 variables, not set them. If you want to test whether you can write,
718 either use file tests or try setting a dummy hash entry inside an C<eval()>,
719 which will trap the error.
721 Note that functions such as C<keys()> and C<values()> may return huge lists
722 when used on large DBM files. You may prefer to use the C<each()>
723 function to iterate over large DBM files. Example:
725 # print out history file offsets
726 dbmopen(%HIST,'/usr/lib/news/history',0666);
727 while (($key,$val) = each %HIST) {
728 print $key, ' = ', unpack('L',$val), "\n";
732 See also L<AnyDBM_File> for a more general description of the pros and
733 cons of the various dbm approaches, as well as L<DB_File> for a particularly
740 Returns a Boolean value telling whether EXPR has a value other than
741 the undefined value C<undef>. If EXPR is not present, C<$_> will be
744 Many operations return C<undef> to indicate failure, end of file,
745 system error, uninitialized variable, and other exceptional
746 conditions. This function allows you to distinguish C<undef> from
747 other values. (A simple Boolean test will not distinguish among
748 C<undef>, zero, the empty string, and C<"0">, which are all equally
749 false.) Note that since C<undef> is a valid scalar, its presence
750 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
751 returns C<undef> when its argument is an empty array, I<or> when the
752 element to return happens to be C<undef>.
754 You may also use C<defined()> to check whether a subroutine exists, by
755 saying C<defined &func> without parentheses. On the other hand, use
756 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
757 produce intuitive results, and should probably be avoided.
759 When used on a hash element, it tells you whether the value is defined,
760 not whether the key exists in the hash. Use L</exists> for the latter
765 print if defined $switch{'D'};
766 print "$val\n" while defined($val = pop(@ary));
767 die "Can't readlink $sym: $!"
768 unless defined($value = readlink $sym);
769 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
770 $debugging = 0 unless defined $debugging;
772 Note: Many folks tend to overuse C<defined()>, and then are surprised to
773 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
774 defined values. For example, if you say
778 The pattern match succeeds, and C<$1> is defined, despite the fact that it
779 matched "nothing". But it didn't really match nothing--rather, it
780 matched something that happened to be C<0> characters long. This is all
781 very above-board and honest. When a function returns an undefined value,
782 it's an admission that it couldn't give you an honest answer. So you
783 should use C<defined()> only when you're questioning the integrity of what
784 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
787 Currently, using C<defined()> on an entire array or hash reports whether
788 memory for that aggregate has ever been allocated. So an array you set
789 to the empty list appears undefined initially, and one that once was full
790 and that you then set to the empty list still appears defined. You
791 should instead use a simple test for size:
793 if (@an_array) { print "has array elements\n" }
794 if (%a_hash) { print "has hash members\n" }
796 Using C<undef()> on these, however, does clear their memory and then report
797 them as not defined anymore, but you shouldn't do that unless you don't
798 plan to use them again, because it saves time when you load them up
799 again to have memory already ready to be filled. The normal way to
800 free up space used by an aggregate is to assign the empty list.
802 This counterintuitive behavior of C<defined()> on aggregates may be
803 changed, fixed, or broken in a future release of Perl.
805 See also L</undef>, L</exists>, L</ref>.
809 Deletes the specified key(s) and their associated values from a hash.
810 For each key, returns the deleted value associated with that key, or
811 the undefined value if there was no such key. Deleting from C<$ENV{}>
812 modifies the environment. Deleting from a hash tied to a DBM file
813 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
814 doesn't necessarily return anything.)
816 The following deletes all the values of a hash:
818 foreach $key (keys %HASH) {
824 delete @HASH{keys %HASH}
826 (But both of these are slower than just assigning the empty list, or
827 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
828 long as the final operation is a hash element lookup or hash slice:
830 delete $ref->[$x][$y]{$key};
831 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
835 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
836 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
837 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
838 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
839 C<$@> and the C<eval()> is terminated with the undefined value. This makes
840 C<die()> the way to raise an exception.
844 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
845 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
847 If the value of EXPR does not end in a newline, the current script line
848 number and input line number (if any) are also printed, and a newline
849 is supplied. Hint: sometimes appending C<", stopped"> to your message
850 will cause it to make better sense when the string C<"at foo line 123"> is
851 appended. Suppose you are running script "canasta".
853 die "/etc/games is no good";
854 die "/etc/games is no good, stopped";
856 produce, respectively
858 /etc/games is no good at canasta line 123.
859 /etc/games is no good, stopped at canasta line 123.
861 See also C<exit()> and C<warn()>.
863 If LIST is empty and C<$@> already contains a value (typically from a
864 previous eval) that value is reused after appending C<"\t...propagated">.
865 This is useful for propagating exceptions:
868 die unless $@ =~ /Expected exception/;
870 If C<$@> is empty then the string C<"Died"> is used.
872 You can arrange for a callback to be run just before the C<die()> does
873 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
874 will be called with the error text and can change the error message, if
875 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
876 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
878 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
879 blocks/strings. If one wants the hook to do nothing in such
884 as the first line of the handler (see L<perlvar/$^S>).
888 Not really a function. Returns the value of the last command in the
889 sequence of commands indicated by BLOCK. When modified by a loop
890 modifier, executes the BLOCK once before testing the loop condition.
891 (On other statements the loop modifiers test the conditional first.)
893 =item do SUBROUTINE(LIST)
895 A deprecated form of subroutine call. See L<perlsub>.
899 Uses the value of EXPR as a filename and executes the contents of the
900 file as a Perl script. Its primary use is to include subroutines
901 from a Perl subroutine library.
907 scalar eval `cat stat.pl`;
909 except that it's more efficient and concise, keeps track of the
910 current filename for error messages, and searches all the B<-I>
911 libraries if the file isn't in the current directory (see also the @INC
912 array in L<perlvar/Predefined Names>). It is also different in how
913 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
914 scope like C<eval STRING> does. It's the same, however, in that it does
915 reparse the file every time you call it, so you probably don't want to
916 do this inside a loop.
918 Note that inclusion of library modules is better done with the
919 C<use()> and C<require()> operators, which also do automatic error checking
920 and raise an exception if there's a problem.
922 You might like to use C<do> to read in a program configuration
923 file. Manual error checking can be done this way:
925 # read in config files: system first, then user
926 for $file ("/share/prog/defaults.rc",
927 "$ENV{HOME}/.someprogrc") {
928 unless ($return = do $file) {
929 warn "couldn't parse $file: $@" if $@;
930 warn "couldn't do $file: $!" unless defined $return;
931 warn "couldn't run $file" unless $return;
937 This causes an immediate core dump. Primarily this is so that you can
938 use the B<undump> program to turn your core dump into an executable binary
939 after having initialized all your variables at the beginning of the
940 program. When the new binary is executed it will begin by executing a
941 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
942 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
943 is omitted, restarts the program from the top. WARNING: Any files
944 opened at the time of the dump will NOT be open any more when the
945 program is reincarnated, with possible resulting confusion on the part
946 of Perl. See also B<-u> option in L<perlrun>.
963 dump QUICKSTART if $ARGV[0] eq '-d';
968 This operator is largely obsolete, partly because it's very hard to
969 convert a core file into an executable, and because the real perl-to-C
970 compiler has superseded it.
974 When called in list context, returns a 2-element list consisting of the
975 key and value for the next element of a hash, so that you can iterate over
976 it. When called in scalar context, returns the key for only the "next"
977 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
978 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
981 Entries are returned in an apparently random order. When the hash is
982 entirely read, a null array is returned in list context (which when
983 assigned produces a FALSE (C<0>) value), and C<undef> in
984 scalar context. The next call to C<each()> after that will start iterating
985 again. There is a single iterator for each hash, shared by all C<each()>,
986 C<keys()>, and C<values()> function calls in the program; it can be reset by
987 reading all the elements from the hash, or by evaluating C<keys HASH> or
988 C<values HASH>. If you add or delete elements of a hash while you're
989 iterating over it, you may get entries skipped or duplicated, so don't.
991 The following prints out your environment like the printenv(1) program,
992 only in a different order:
994 while (($key,$value) = each %ENV) {
995 print "$key=$value\n";
998 See also C<keys()> and C<values()>.
1000 =item eof FILEHANDLE
1006 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1007 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1008 gives the real filehandle. (Note that this function actually
1009 reads a character and then C<ungetc()>s it, so isn't very useful in an
1010 interactive context.) Do not read from a terminal file (or call
1011 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1012 as terminals may lose the end-of-file condition if you do.
1014 An C<eof> without an argument uses the last file read as argument.
1015 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1016 the files listed on the command line, i.e., C<eof()> is reasonable to
1017 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1018 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1019 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1021 # reset line numbering on each input file
1023 next if /^\s*#/; # skip comments
1026 close ARGV if eof; # Not eof()!
1029 # insert dashes just before last line of last file
1031 if (eof()) { # check for end of current file
1032 print "--------------\n";
1033 close(ARGV); # close or break; is needed if we
1034 # are reading from the terminal
1039 Practical hint: you almost never need to use C<eof> in Perl, because the
1040 input operators return false values when they run out of data, or if there
1047 In the first form, the return value of EXPR is parsed and executed as if it
1048 were a little Perl program. The value of the expression (which is itself
1049 determined within scalar context) is first parsed, and if there weren't any
1050 errors, executed in the context of the current Perl program, so that any
1051 variable settings or subroutine and format definitions remain afterwards.
1052 Note that the value is parsed every time the eval executes. If EXPR is
1053 omitted, evaluates C<$_>. This form is typically used to delay parsing
1054 and subsequent execution of the text of EXPR until run time.
1056 In the second form, the code within the BLOCK is parsed only once--at the
1057 same time the code surrounding the eval itself was parsed--and executed
1058 within the context of the current Perl program. This form is typically
1059 used to trap exceptions more efficiently than the first (see below), while
1060 also providing the benefit of checking the code within BLOCK at compile
1063 The final semicolon, if any, may be omitted from the value of EXPR or within
1066 In both forms, the value returned is the value of the last expression
1067 evaluated inside the mini-program; a return statement may be also used, just
1068 as with subroutines. The expression providing the return value is evaluated
1069 in void, scalar, or list context, depending on the context of the eval itself.
1070 See L</wantarray> for more on how the evaluation context can be determined.
1072 If there is a syntax error or runtime error, or a C<die()> statement is
1073 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1074 error message. If there was no error, C<$@> is guaranteed to be a null
1075 string. Beware that using C<eval()> neither silences perl from printing
1076 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1077 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1078 L</warn> and L<perlvar>.
1080 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1081 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1082 is implemented. It is also Perl's exception trapping mechanism, where
1083 the die operator is used to raise exceptions.
1085 If the code to be executed doesn't vary, you may use the eval-BLOCK
1086 form to trap run-time errors without incurring the penalty of
1087 recompiling each time. The error, if any, is still returned in C<$@>.
1090 # make divide-by-zero nonfatal
1091 eval { $answer = $a / $b; }; warn $@ if $@;
1093 # same thing, but less efficient
1094 eval '$answer = $a / $b'; warn $@ if $@;
1096 # a compile-time error
1097 eval { $answer = }; # WRONG
1100 eval '$answer ='; # sets $@
1102 When using the C<eval{}> form as an exception trap in libraries, you may
1103 wish not to trigger any C<__DIE__> hooks that user code may have
1104 installed. You can use the C<local $SIG{__DIE__}> construct for this
1105 purpose, as shown in this example:
1107 # a very private exception trap for divide-by-zero
1108 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1111 This is especially significant, given that C<__DIE__> hooks can call
1112 C<die()> again, which has the effect of changing their error messages:
1114 # __DIE__ hooks may modify error messages
1116 local $SIG{'__DIE__'} =
1117 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1118 eval { die "foo lives here" };
1119 print $@ if $@; # prints "bar lives here"
1122 With an C<eval()>, you should be especially careful to remember what's
1123 being looked at when:
1129 eval { $x }; # CASE 4
1131 eval "\$$x++"; # CASE 5
1134 Cases 1 and 2 above behave identically: they run the code contained in
1135 the variable C<$x>. (Although case 2 has misleading double quotes making
1136 the reader wonder what else might be happening (nothing is).) Cases 3
1137 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1138 does nothing but return the value of C<$x>. (Case 4 is preferred for
1139 purely visual reasons, but it also has the advantage of compiling at
1140 compile-time instead of at run-time.) Case 5 is a place where
1141 normally you I<WOULD> like to use double quotes, except that in this
1142 particular situation, you can just use symbolic references instead, as
1147 =item exec PROGRAM LIST
1149 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1150 use C<system()> instead of C<exec()> if you want it to return. It fails and
1151 returns FALSE only if the command does not exist I<and> it is executed
1152 directly instead of via your system's command shell (see below).
1154 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1155 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1156 or C<exit()> (if C<-w> is set - but you always do that). If you
1157 I<really> want to follow an C<exec()> with some other statement, you
1158 can use one of these styles to avoid the warning:
1160 exec ('foo') or print STDERR "couldn't exec foo: $!";
1161 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1163 If there is more than one argument in LIST, or if LIST is an array
1164 with more than one value, calls execvp(3) with the arguments in LIST.
1165 If there is only one scalar argument or an array with one element in it,
1166 the argument is checked for shell metacharacters, and if there are any,
1167 the entire argument is passed to the system's command shell for parsing
1168 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1169 If there are no shell metacharacters in the argument, it is split into
1170 words and passed directly to C<execvp()>, which is more efficient. Note:
1171 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1172 set C<$|> to avoid lost output. Examples:
1174 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1175 exec "sort $outfile | uniq";
1177 If you don't really want to execute the first argument, but want to lie
1178 to the program you are executing about its own name, you can specify
1179 the program you actually want to run as an "indirect object" (without a
1180 comma) in front of the LIST. (This always forces interpretation of the
1181 LIST as a multivalued list, even if there is only a single scalar in
1184 $shell = '/bin/csh';
1185 exec $shell '-sh'; # pretend it's a login shell
1189 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1191 When the arguments get executed via the system shell, results will
1192 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1195 Using an indirect object with C<exec()> or C<system()> is also more secure.
1196 This usage forces interpretation of the arguments as a multivalued list,
1197 even if the list had just one argument. That way you're safe from the
1198 shell expanding wildcards or splitting up words with whitespace in them.
1200 @args = ( "echo surprise" );
1202 system @args; # subject to shell escapes
1204 system { $args[0] } @args; # safe even with one-arg list
1206 The first version, the one without the indirect object, ran the I<echo>
1207 program, passing it C<"surprise"> an argument. The second version
1208 didn't--it tried to run a program literally called I<"echo surprise">,
1209 didn't find it, and set C<$?> to a non-zero value indicating failure.
1211 Note that C<exec()> will not call your C<END> blocks, nor will it call
1212 any C<DESTROY> methods in your objects.
1216 Returns TRUE if the specified hash key exists in its hash array, even
1217 if the corresponding value is undefined.
1219 print "Exists\n" if exists $array{$key};
1220 print "Defined\n" if defined $array{$key};
1221 print "True\n" if $array{$key};
1223 A hash element can be TRUE only if it's defined, and defined if
1224 it exists, but the reverse doesn't necessarily hold true.
1226 Note that the EXPR can be arbitrarily complicated as long as the final
1227 operation is a hash key lookup:
1229 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1231 Although the last element will not spring into existence just because its
1232 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1233 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1234 test for a $key element. This autovivification may be fixed in a later
1239 Evaluates EXPR and exits immediately with that value. (Actually, it
1240 calls any defined C<END> routines first, but the C<END> routines may not
1241 abort the exit. Likewise any object destructors that need to be called
1242 are called before exit.) Example:
1245 exit 0 if $ans =~ /^[Xx]/;
1247 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1248 universally portable values for EXPR are C<0> for success and C<1> for error;
1249 all other values are subject to unpredictable interpretation depending
1250 on the environment in which the Perl program is running.
1252 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1253 someone might want to trap whatever error happened. Use C<die()> instead,
1254 which can be trapped by an C<eval()>.
1256 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1262 Returns I<e> (the natural logarithm base) to the power of EXPR.
1263 If EXPR is omitted, gives C<exp($_)>.
1265 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1267 Implements the fcntl(2) function. You'll probably have to say
1271 first to get the correct constant definitions. Argument processing and
1272 value return works just like C<ioctl()> below.
1276 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1277 or die "can't fcntl F_GETFL: $!";
1279 You don't have to check for C<defined()> on the return from
1280 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1281 call into "C<0> but true" in Perl. This string is true in
1282 boolean context and C<0> in numeric context. It is also
1283 exempt from the normal B<-w> warnings on improper numeric
1286 Note that C<fcntl()> will produce a fatal error if used on a machine that
1287 doesn't implement fcntl(2).
1289 =item fileno FILEHANDLE
1291 Returns the file descriptor for a filehandle. This is useful for
1292 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1293 operations. If FILEHANDLE is an expression, the value is taken as
1294 an indirect filehandle, generally its name.
1296 You can use this to find out whether two handles refer to the
1297 same underlying descriptor:
1299 if (fileno(THIS) == fileno(THAT)) {
1300 print "THIS and THAT are dups\n";
1303 =item flock FILEHANDLE,OPERATION
1305 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1306 success, FALSE on failure. Produces a fatal error if used on a machine
1307 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1308 is Perl's portable file locking interface, although it locks only entire
1311 On many platforms (including most versions or clones of Unix), locks
1312 established by C<flock()> are B<merely advisory>. Such discretionary locks
1313 are more flexible, but offer fewer guarantees. This means that files
1314 locked with C<flock()> may be modified by programs that do not also use
1315 C<flock()>. Windows NT and OS/2 are among the platforms which
1316 enforce mandatory locking. See your local documentation for details.
1318 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1319 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1320 you can use the symbolic names if import them from the Fcntl module,
1321 either individually, or as a group using the ':flock' tag. LOCK_SH
1322 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1323 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1324 LOCK_EX then C<flock()> will return immediately rather than blocking
1325 waiting for the lock (check the return status to see if you got it).
1327 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1328 before (un)locking it.
1330 Note that the emulation built with lockf(3) doesn't provide shared
1331 locks, and it requires that FILEHANDLE be open with write intent. These
1332 are the semantics that lockf(3) implements. Most (all?) systems
1333 implement lockf(3) in terms of fcntl(2) locking, though, so the
1334 differing semantics shouldn't bite too many people.
1336 Note also that some versions of C<flock()> cannot lock things over the
1337 network; you would need to use the more system-specific C<fcntl()> for
1338 that. If you like you can force Perl to ignore your system's flock(2)
1339 function, and so provide its own fcntl(2)-based emulation, by passing
1340 the switch C<-Ud_flock> to the F<Configure> program when you configure
1343 Here's a mailbox appender for BSD systems.
1345 use Fcntl ':flock'; # import LOCK_* constants
1348 flock(MBOX,LOCK_EX);
1349 # and, in case someone appended
1350 # while we were waiting...
1355 flock(MBOX,LOCK_UN);
1358 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1359 or die "Can't open mailbox: $!";
1362 print MBOX $msg,"\n\n";
1365 See also L<DB_File> for other flock() examples.
1369 Does a fork(2) system call. Returns the child pid to the parent process,
1370 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1372 Note: unflushed buffers remain unflushed in both processes, which means
1373 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1374 method of C<IO::Handle> to avoid duplicate output.
1376 If you C<fork()> without ever waiting on your children, you will accumulate
1379 $SIG{CHLD} = sub { wait };
1381 There's also the double-fork trick (error checking on
1382 C<fork()> returns omitted);
1384 unless ($pid = fork) {
1386 exec "what you really wanna do";
1389 ## (some_perl_code_here)
1396 See also L<perlipc> for more examples of forking and reaping
1399 Note that if your forked child inherits system file descriptors like
1400 STDIN and STDOUT that are actually connected by a pipe or socket, even
1401 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1402 you're done. You should reopen those to F</dev/null> if it's any issue.
1406 Declare a picture format for use by the C<write()> function. For
1410 Test: @<<<<<<<< @||||| @>>>>>
1411 $str, $%, '$' . int($num)
1415 $num = $cost/$quantity;
1419 See L<perlform> for many details and examples.
1421 =item formline PICTURE,LIST
1423 This is an internal function used by C<format>s, though you may call it,
1424 too. It formats (see L<perlform>) a list of values according to the
1425 contents of PICTURE, placing the output into the format output
1426 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1427 Eventually, when a C<write()> is done, the contents of
1428 C<$^A> are written to some filehandle, but you could also read C<$^A>
1429 yourself and then set C<$^A> back to C<"">. Note that a format typically
1430 does one C<formline()> per line of form, but the C<formline()> function itself
1431 doesn't care how many newlines are embedded in the PICTURE. This means
1432 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1433 You may therefore need to use multiple formlines to implement a single
1434 record format, just like the format compiler.
1436 Be careful if you put double quotes around the picture, because an "C<@>"
1437 character may be taken to mean the beginning of an array name.
1438 C<formline()> always returns TRUE. See L<perlform> for other examples.
1440 =item getc FILEHANDLE
1444 Returns the next character from the input file attached to FILEHANDLE,
1445 or the undefined value at end of file, or if there was an error. If
1446 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1447 efficient. It cannot be used to get unbuffered single-characters,
1448 however. For that, try something more like:
1451 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1454 system "stty", '-icanon', 'eol', "\001";
1460 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1463 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1467 Determination of whether $BSD_STYLE should be set
1468 is left as an exercise to the reader.
1470 The C<POSIX::getattr()> function can do this more portably on systems
1471 purporting POSIX compliance.
1472 See also the C<Term::ReadKey> module from your nearest CPAN site;
1473 details on CPAN can be found on L<perlmod/CPAN>.
1477 Implements the C library function of the same name, which on most
1478 systems returns the current login from F</etc/utmp>, if any. If null,
1481 $login = getlogin || getpwuid($<) || "Kilroy";
1483 Do not consider C<getlogin()> for authentication: it is not as
1484 secure as C<getpwuid()>.
1486 =item getpeername SOCKET
1488 Returns the packed sockaddr address of other end of the SOCKET connection.
1491 $hersockaddr = getpeername(SOCK);
1492 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1493 $herhostname = gethostbyaddr($iaddr, AF_INET);
1494 $herstraddr = inet_ntoa($iaddr);
1498 Returns the current process group for the specified PID. Use
1499 a PID of C<0> to get the current process group for the
1500 current process. Will raise an exception if used on a machine that
1501 doesn't implement getpgrp(2). If PID is omitted, returns process
1502 group of current process. Note that the POSIX version of C<getpgrp()>
1503 does not accept a PID argument, so only C<PID==0> is truly portable.
1507 Returns the process id of the parent process.
1509 =item getpriority WHICH,WHO
1511 Returns the current priority for a process, a process group, or a user.
1512 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1513 machine that doesn't implement getpriority(2).
1519 =item gethostbyname NAME
1521 =item getnetbyname NAME
1523 =item getprotobyname NAME
1529 =item getservbyname NAME,PROTO
1531 =item gethostbyaddr ADDR,ADDRTYPE
1533 =item getnetbyaddr ADDR,ADDRTYPE
1535 =item getprotobynumber NUMBER
1537 =item getservbyport PORT,PROTO
1555 =item sethostent STAYOPEN
1557 =item setnetent STAYOPEN
1559 =item setprotoent STAYOPEN
1561 =item setservent STAYOPEN
1575 These routines perform the same functions as their counterparts in the
1576 system library. In list context, the return values from the
1577 various get routines are as follows:
1579 ($name,$passwd,$uid,$gid,
1580 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1581 ($name,$passwd,$gid,$members) = getgr*
1582 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1583 ($name,$aliases,$addrtype,$net) = getnet*
1584 ($name,$aliases,$proto) = getproto*
1585 ($name,$aliases,$port,$proto) = getserv*
1587 (If the entry doesn't exist you get a null list.)
1589 In scalar context, you get the name, unless the function was a
1590 lookup by name, in which case you get the other thing, whatever it is.
1591 (If the entry doesn't exist you get the undefined value.) For example:
1593 $uid = getpwnam($name);
1594 $name = getpwuid($num);
1596 $gid = getgrnam($name);
1597 $name = getgrgid($num;
1601 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1602 cases in the sense that in many systems they are unsupported. If the
1603 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1604 usually encodes the disk quota. If the C<$comment> field is unsupported,
1605 it is an empty scalar. If it is supported it usually encodes some
1606 administrative comment about the user. In some systems the $quota
1607 field may be C<$change> or C<$age>, fields that have to do with password
1608 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1609 field, if present, encodes the expiration period of the account or the
1610 password. For the availability and the exact meaning of these fields
1611 in your system, please consult your getpwnam(3) documentation and your
1612 F<pwd.h> file. You can also find out from within Perl which meaning
1613 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1614 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1615 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1617 The C<$members> value returned by I<getgr*()> is a space separated list of
1618 the login names of the members of the group.
1620 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1621 C, it will be returned to you via C<$?> if the function call fails. The
1622 C<@addrs> value returned by a successful call is a list of the raw
1623 addresses returned by the corresponding system library call. In the
1624 Internet domain, each address is four bytes long and you can unpack it
1625 by saying something like:
1627 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1629 If you get tired of remembering which element of the return list contains
1630 which return value, by-name interfaces are also provided in modules:
1631 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1632 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1633 normal built-in, replacing them with versions that return objects with
1634 the appropriate names for each field. For example:
1638 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1640 Even though it looks like they're the same method calls (uid),
1641 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1643 =item getsockname SOCKET
1645 Returns the packed sockaddr address of this end of the SOCKET connection.
1648 $mysockaddr = getsockname(SOCK);
1649 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1651 =item getsockopt SOCKET,LEVEL,OPTNAME
1653 Returns the socket option requested, or undef if there is an error.
1659 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1660 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1661 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1662 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1663 L<perlop/"I/O Operators">.
1667 Converts a time as returned by the time function to a 9-element array
1668 with the time localized for the standard Greenwich time zone.
1669 Typically used as follows:
1672 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1675 All array elements are numeric, and come straight out of a struct tm.
1676 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1677 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1678 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1680 If EXPR is omitted, does C<gmtime(time())>.
1682 In scalar context, returns the ctime(3) value:
1684 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1686 Also see the C<timegm()> function provided by the C<Time::Local> module,
1687 and the strftime(3) function available via the POSIX module.
1689 This scalar value is B<not> locale dependent, see L<perllocale>, but
1690 instead a Perl builtin. Also see the C<Time::Local> module, and the
1691 strftime(3) and mktime(3) function available via the POSIX module. To
1692 get somewhat similar but locale dependent date strings, set up your
1693 locale environment variables appropriately (please see L<perllocale>)
1694 and try for example:
1696 use POSIX qw(strftime);
1697 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1699 Note that the C<%a> and C<%b>, the short forms of the day of the week
1700 and the month of the year, may not necessarily be three characters wide.
1708 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1709 execution there. It may not be used to go into any construct that
1710 requires initialization, such as a subroutine or a C<foreach> loop. It
1711 also can't be used to go into a construct that is optimized away,
1712 or to get out of a block or subroutine given to C<sort()>.
1713 It can be used to go almost anywhere else within the dynamic scope,
1714 including out of subroutines, but it's usually better to use some other
1715 construct such as C<last> or C<die()>. The author of Perl has never felt the
1716 need to use this form of C<goto> (in Perl, that is--C is another matter).
1718 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1719 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1720 necessarily recommended if you're optimizing for maintainability:
1722 goto ("FOO", "BAR", "GLARCH")[$i];
1724 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1725 named subroutine for the currently running subroutine. This is used by
1726 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1727 pretend that the other subroutine had been called in the first place
1728 (except that any modifications to C<@_> in the current subroutine are
1729 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1730 will be able to tell that this routine was called first.
1732 =item grep BLOCK LIST
1734 =item grep EXPR,LIST
1736 This is similar in spirit to, but not the same as, grep(1)
1737 and its relatives. In particular, it is not limited to using
1738 regular expressions.
1740 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1741 C<$_> to each element) and returns the list value consisting of those
1742 elements for which the expression evaluated to TRUE. In a scalar
1743 context, returns the number of times the expression was TRUE.
1745 @foo = grep(!/^#/, @bar); # weed out comments
1749 @foo = grep {!/^#/} @bar; # weed out comments
1751 Note that, because C<$_> is a reference into the list value, it can be used
1752 to modify the elements of the array. While this is useful and
1753 supported, it can cause bizarre results if the LIST is not a named
1754 array. Similarly, grep returns aliases into the original list,
1755 much like the way that a for loop's index variable aliases the list
1756 elements. That is, modifying an element of a list returned by grep
1757 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1758 actually modifies the element in the original list.
1760 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1766 Interprets EXPR as a hex string and returns the corresponding
1767 value. (To convert strings that might start with either 0 or 0x
1768 see L</oct>.) If EXPR is omitted, uses C<$_>.
1770 print hex '0xAf'; # prints '175'
1771 print hex 'aF'; # same
1775 There is no builtin C<import()> function. It is just an ordinary
1776 method (subroutine) defined (or inherited) by modules that wish to export
1777 names to another module. The C<use()> function calls the C<import()> method
1778 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1780 =item index STR,SUBSTR,POSITION
1782 =item index STR,SUBSTR
1784 Returns the position of the first occurrence of SUBSTR in STR at or after
1785 POSITION. If POSITION is omitted, starts searching from the beginning of
1786 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1787 variable to--but don't do that). If the substring is not found, returns
1788 one less than the base, ordinarily C<-1>.
1794 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1795 You should not use this for rounding, because it truncates
1796 towards C<0>, and because machine representations of floating point
1797 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1798 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1800 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1802 Implements the ioctl(2) function. You'll probably have to say
1804 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1806 first to get the correct function definitions. If F<ioctl.ph> doesn't
1807 exist or doesn't have the correct definitions you'll have to roll your
1808 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1809 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1810 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1811 written depending on the FUNCTION--a pointer to the string value of SCALAR
1812 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1813 has no string value but does have a numeric value, that value will be
1814 passed rather than a pointer to the string value. To guarantee this to be
1815 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1816 functions are useful for manipulating the values of structures used by
1817 C<ioctl()>. The following example sets the erase character to DEL.
1821 die "NO TIOCGETP" if $@ || !$getp;
1822 $sgttyb_t = "ccccs"; # 4 chars and a short
1823 if (ioctl(STDIN,$getp,$sgttyb)) {
1824 @ary = unpack($sgttyb_t,$sgttyb);
1826 $sgttyb = pack($sgttyb_t,@ary);
1827 ioctl(STDIN,&TIOCSETP,$sgttyb)
1828 || die "Can't ioctl: $!";
1831 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1833 if OS returns: then Perl returns:
1835 0 string "0 but true"
1836 anything else that number
1838 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1839 still easily determine the actual value returned by the operating
1842 ($retval = ioctl(...)) || ($retval = -1);
1843 printf "System returned %d\n", $retval;
1845 The special string "C<0> but true" is excempt from B<-w> complaints
1846 about improper numeric conversions.
1848 =item join EXPR,LIST
1850 Joins the separate strings of LIST into a single string with
1851 fields separated by the value of EXPR, and returns the string.
1854 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1860 Returns a list consisting of all the keys of the named hash. (In a
1861 scalar context, returns the number of keys.) The keys are returned in
1862 an apparently random order, but it is the same order as either the
1863 C<values()> or C<each()> function produces (given that the hash has not been
1864 modified). As a side effect, it resets HASH's iterator.
1866 Here is yet another way to print your environment:
1869 @values = values %ENV;
1870 while ($#keys >= 0) {
1871 print pop(@keys), '=', pop(@values), "\n";
1874 or how about sorted by key:
1876 foreach $key (sort(keys %ENV)) {
1877 print $key, '=', $ENV{$key}, "\n";
1880 To sort an array by value, you'll need to use a C<sort()> function.
1881 Here's a descending numeric sort of a hash by its values:
1883 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1884 printf "%4d %s\n", $hash{$key}, $key;
1887 As an lvalue C<keys()> allows you to increase the number of hash buckets
1888 allocated for the given hash. This can gain you a measure of efficiency if
1889 you know the hash is going to get big. (This is similar to pre-extending
1890 an array by assigning a larger number to $#array.) If you say
1894 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1895 it rounds up to the next power of two. These
1896 buckets will be retained even if you do C<%hash = ()>, use C<undef
1897 %hash> if you want to free the storage while C<%hash> is still in scope.
1898 You can't shrink the number of buckets allocated for the hash using
1899 C<keys()> in this way (but you needn't worry about doing this by accident,
1900 as trying has no effect).
1904 Sends a signal to a list of processes. The first element of
1905 the list must be the signal to send. Returns the number of
1906 processes successfully signaled.
1908 $cnt = kill 1, $child1, $child2;
1911 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1912 process groups instead of processes. (On System V, a negative I<PROCESS>
1913 number will also kill process groups, but that's not portable.) That
1914 means you usually want to use positive not negative signals. You may also
1915 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1921 The C<last> command is like the C<break> statement in C (as used in
1922 loops); it immediately exits the loop in question. If the LABEL is
1923 omitted, the command refers to the innermost enclosing loop. The
1924 C<continue> block, if any, is not executed:
1926 LINE: while (<STDIN>) {
1927 last LINE if /^$/; # exit when done with header
1931 See also L</continue> for an illustration of how C<last>, C<next>, and
1938 Returns an lowercased version of EXPR. This is the internal function
1939 implementing the C<\L> escape in double-quoted strings.
1940 Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
1942 If EXPR is omitted, uses C<$_>.
1948 Returns the value of EXPR with the first character lowercased. This is
1949 the internal function implementing the C<\l> escape in double-quoted strings.
1950 Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
1952 If EXPR is omitted, uses C<$_>.
1958 Returns the length in bytes of the value of EXPR. If EXPR is
1959 omitted, returns length of C<$_>.
1961 =item link OLDFILE,NEWFILE
1963 Creates a new filename linked to the old filename. Returns TRUE for
1964 success, FALSE otherwise.
1966 =item listen SOCKET,QUEUESIZE
1968 Does the same thing that the listen system call does. Returns TRUE if
1969 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1973 A local modifies the listed variables to be local to the enclosing
1974 block, file, or eval. If more than one value is listed, the list must
1975 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1976 for details, including issues with tied arrays and hashes.
1978 You really probably want to be using C<my()> instead, because C<local()> isn't
1979 what most people think of as "local". See L<perlsub/"Private Variables
1980 via my()"> for details.
1982 =item localtime EXPR
1984 Converts a time as returned by the time function to a 9-element array
1985 with the time analyzed for the local time zone. Typically used as
1989 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1992 All array elements are numeric, and come straight out of a struct tm.
1993 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1994 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1995 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
1997 If EXPR is omitted, uses the current time (C<localtime(time)>).
1999 In scalar context, returns the ctime(3) value:
2001 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2003 This scalar value is B<not> locale dependent, see L<perllocale>, but
2004 instead a Perl builtin. Also see the C<Time::Local> module, and the
2005 strftime(3) and mktime(3) function available via the POSIX module. To
2006 get somewhat similar but locale dependent date strings, set up your
2007 locale environment variables appropriately (please see L<perllocale>)
2008 and try for example:
2010 use POSIX qw(strftime);
2011 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2013 Note that the C<%a> and C<%b>, the short forms of the day of the week
2014 and the month of the year, may not necessarily be three characters wide.
2020 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2023 =item lstat FILEHANDLE
2029 Does the same thing as the C<stat()> function (including setting the
2030 special C<_> filehandle) but stats a symbolic link instead of the file
2031 the symbolic link points to. If symbolic links are unimplemented on
2032 your system, a normal C<stat()> is done.
2034 If EXPR is omitted, stats C<$_>.
2038 The match operator. See L<perlop>.
2040 =item map BLOCK LIST
2044 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2045 element) and returns the list value composed of the results of each such
2046 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2047 may produce zero, one, or more elements in the returned value.
2049 @chars = map(chr, @nums);
2051 translates a list of numbers to the corresponding characters. And
2053 %hash = map { getkey($_) => $_ } @array;
2055 is just a funny way to write
2058 foreach $_ (@array) {
2059 $hash{getkey($_)} = $_;
2062 Note that, because C<$_> is a reference into the list value, it can be used
2063 to modify the elements of the array. While this is useful and
2064 supported, it can cause bizarre results if the LIST is not a named
2065 array. See also L</grep> for an array composed of those items of the
2066 original list for which the BLOCK or EXPR evaluates to true.
2068 =item mkdir FILENAME,MODE
2070 Creates the directory specified by FILENAME, with permissions specified
2071 by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2072 it returns FALSE and sets C<$!> (errno).
2074 =item msgctl ID,CMD,ARG
2076 Calls the System V IPC function msgctl(2). You'll probably have to say
2080 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2081 then ARG must be a variable which will hold the returned C<msqid_ds>
2082 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2083 true" for zero, or the actual return value otherwise. See also
2084 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2086 =item msgget KEY,FLAGS
2088 Calls the System V IPC function msgget(2). Returns the message queue
2089 id, or the undefined value if there is an error. See also C<IPC::SysV>
2090 and C<IPC::SysV::Msg> documentation.
2092 =item msgsnd ID,MSG,FLAGS
2094 Calls the System V IPC function msgsnd to send the message MSG to the
2095 message queue ID. MSG must begin with the long integer message type,
2096 which may be created with C<pack("l", $type)>. Returns TRUE if
2097 successful, or FALSE if there is an error. See also C<IPC::SysV>
2098 and C<IPC::SysV::Msg> documentation.
2100 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2102 Calls the System V IPC function msgrcv to receive a message from
2103 message queue ID into variable VAR with a maximum message size of
2104 SIZE. Note that if a message is received, the message type will be
2105 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2106 size of the message type. Returns TRUE if successful, or FALSE if
2107 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2111 A C<my()> declares the listed variables to be local (lexically) to the
2112 enclosing block, file, or C<eval()>. If
2113 more than one value is listed, the list must be placed in parentheses. See
2114 L<perlsub/"Private Variables via my()"> for details.
2120 The C<next> command is like the C<continue> statement in C; it starts
2121 the next iteration of the loop:
2123 LINE: while (<STDIN>) {
2124 next LINE if /^#/; # discard comments
2128 Note that if there were a C<continue> block on the above, it would get
2129 executed even on discarded lines. If the LABEL is omitted, the command
2130 refers to the innermost enclosing loop.
2132 See also L</continue> for an illustration of how C<last>, C<next>, and
2135 =item no Module LIST
2137 See the L</use> function, which C<no> is the opposite of.
2143 Interprets EXPR as an octal string and returns the corresponding
2144 value. (If EXPR happens to start off with C<0x>, interprets it as
2145 a hex string instead.) The following will handle decimal, octal, and
2146 hex in the standard Perl or C notation:
2148 $val = oct($val) if $val =~ /^0/;
2150 If EXPR is omitted, uses C<$_>. This function is commonly used when
2151 a string such as C<644> needs to be converted into a file mode, for
2152 example. (Although perl will automatically convert strings into
2153 numbers as needed, this automatic conversion assumes base 10.)
2155 =item open FILEHANDLE,EXPR
2157 =item open FILEHANDLE
2159 Opens the file whose filename is given by EXPR, and associates it with
2160 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2161 name of the real filehandle wanted. If EXPR is omitted, the scalar
2162 variable of the same name as the FILEHANDLE contains the filename.
2163 (Note that lexical variables--those declared with C<my()>--will not work
2164 for this purpose; so if you're using C<my()>, specify EXPR in your call
2167 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2168 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2169 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2170 the file is opened for appending, again being created if necessary.
2171 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2172 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2173 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2174 file first. You can't usually use either read-write mode for updating
2175 textfiles, since they have variable length records. See the B<-i>
2176 switch in L<perlrun> for a better approach.
2178 The prefix and the filename may be separated with spaces.
2179 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2180 C<'w+'>, C<'a'>, and C<'a+'>.
2182 If the filename begins with C<'|'>, the filename is interpreted as a
2183 command to which output is to be piped, and if the filename ends with a
2184 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2185 for more examples of this. (You are not allowed to C<open()> to a command
2186 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2187 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2189 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2190 nonzero upon success, the undefined value otherwise. If the C<open()>
2191 involved a pipe, the return value happens to be the pid of the
2194 If you're unfortunate enough to be running Perl on a system that
2195 distinguishes between text files and binary files (modern operating
2196 systems don't care), then you should check out L</binmode> for tips for
2197 dealing with this. The key distinction between systems that need C<binmode()>
2198 and those that don't is their text file formats. Systems like Unix, MacOS, and
2199 Plan9, which delimit lines with a single character, and which encode that
2200 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2202 When opening a file, it's usually a bad idea to continue normal execution
2203 if the request failed, so C<open()> is frequently used in connection with
2204 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2205 where you want to make a nicely formatted error message (but there are
2206 modules that can help with that problem)) you should always check
2207 the return value from opening a file. The infrequent exception is when
2208 working with an unopened filehandle is actually what you want to do.
2213 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2214 while (<ARTICLE>) {...
2216 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2217 # if the open fails, output is discarded
2219 open(DBASE, '+<dbase.mine') # open for update
2220 or die "Can't open 'dbase.mine' for update: $!";
2222 open(ARTICLE, "caesar <$article |") # decrypt article
2223 or die "Can't start caesar: $!";
2225 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2226 or die "Can't start sort: $!";
2228 # process argument list of files along with any includes
2230 foreach $file (@ARGV) {
2231 process($file, 'fh00');
2235 my($filename, $input) = @_;
2236 $input++; # this is a string increment
2237 unless (open($input, $filename)) {
2238 print STDERR "Can't open $filename: $!\n";
2243 while (<$input>) { # note use of indirection
2244 if (/^#include "(.*)"/) {
2245 process($1, $input);
2252 You may also, in the Bourne shell tradition, specify an EXPR beginning
2253 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2254 name of a filehandle (or file descriptor, if numeric) to be
2255 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2256 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2257 mode you specify should match the mode of the original filehandle.
2258 (Duping a filehandle does not take into account any existing contents of
2260 Here is a script that saves, redirects, and restores STDOUT and
2264 open(OLDOUT, ">&STDOUT");
2265 open(OLDERR, ">&STDERR");
2267 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2268 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2270 select(STDERR); $| = 1; # make unbuffered
2271 select(STDOUT); $| = 1; # make unbuffered
2273 print STDOUT "stdout 1\n"; # this works for
2274 print STDERR "stderr 1\n"; # subprocesses too
2279 open(STDOUT, ">&OLDOUT");
2280 open(STDERR, ">&OLDERR");
2282 print STDOUT "stdout 2\n";
2283 print STDERR "stderr 2\n";
2286 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2287 equivalent of C's C<fdopen()> of that file descriptor; this is more
2288 parsimonious of file descriptors. For example:
2290 open(FILEHANDLE, "<&=$fd")
2292 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2293 there is an implicit fork done, and the return value of open is the pid
2294 of the child within the parent process, and C<0> within the child
2295 process. (Use C<defined($pid)> to determine whether the open was successful.)
2296 The filehandle behaves normally for the parent, but i/o to that
2297 filehandle is piped from/to the STDOUT/STDIN of the child process.
2298 In the child process the filehandle isn't opened--i/o happens from/to
2299 the new STDOUT or STDIN. Typically this is used like the normal
2300 piped open when you want to exercise more control over just how the
2301 pipe command gets executed, such as when you are running setuid, and
2302 don't want to have to scan shell commands for metacharacters.
2303 The following pairs are more or less equivalent:
2305 open(FOO, "|tr '[a-z]' '[A-Z]'");
2306 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2308 open(FOO, "cat -n '$file'|");
2309 open(FOO, "-|") || exec 'cat', '-n', $file;
2311 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2313 NOTE: On any operation that may do a fork, any unflushed buffers remain
2314 unflushed in both processes, which means you may need to set C<$|> to
2315 avoid duplicate output.
2317 Closing any piped filehandle causes the parent process to wait for the
2318 child to finish, and returns the status value in C<$?>.
2320 The filename passed to open will have leading and trailing
2321 whitespace deleted, and the normal redirection characters
2322 honored. This property, known as "magic open",
2323 can often be used to good effect. A user could specify a filename of
2324 F<"rsh cat file |">, or you could change certain filenames as needed:
2326 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2327 open(FH, $filename) or die "Can't open $filename: $!";
2329 However, to open a file with arbitrary weird characters in it, it's
2330 necessary to protect any leading and trailing whitespace:
2332 $file =~ s#^(\s)#./$1#;
2333 open(FOO, "< $file\0");
2335 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2336 should use the C<sysopen()> function, which involves no such magic. This is
2337 another way to protect your filenames from interpretation. For example:
2340 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2341 or die "sysopen $path: $!";
2342 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2343 print HANDLE "stuff $$\n");
2345 print "File contains: ", <HANDLE>;
2347 Using the constructor from the C<IO::Handle> package (or one of its
2348 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2349 filehandles that have the scope of whatever variables hold references to
2350 them, and automatically close whenever and however you leave that scope:
2354 sub read_myfile_munged {
2356 my $handle = new IO::File;
2357 open($handle, "myfile") or die "myfile: $!";
2359 or return (); # Automatically closed here.
2360 mung $first or die "mung failed"; # Or here.
2361 return $first, <$handle> if $ALL; # Or here.
2365 See L</seek()> for some details about mixing reading and writing.
2367 =item opendir DIRHANDLE,EXPR
2369 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2370 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2371 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2377 Returns the numeric ascii value of the first character of EXPR. If
2378 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2380 =item pack TEMPLATE,LIST
2382 Takes an array or list of values and packs it into a binary structure,
2383 returning the string containing the structure. The TEMPLATE is a
2384 sequence of characters that give the order and type of values, as
2387 A An ascii string, will be space padded.
2388 a An ascii string, will be null padded.
2389 b A bit string (ascending bit order, like vec()).
2390 B A bit string (descending bit order).
2391 h A hex string (low nybble first).
2392 H A hex string (high nybble first).
2394 c A signed char value.
2395 C An unsigned char value.
2397 s A signed short value.
2398 S An unsigned short value.
2399 (This 'short' is _exactly_ 16 bits, which may differ from
2400 what a local C compiler calls 'short'.)
2402 i A signed integer value.
2403 I An unsigned integer value.
2404 (This 'integer' is _at_least_ 32 bits wide. Its exact
2405 size depends on what a local C compiler calls 'int',
2406 and may even be larger than the 'long' described in
2409 l A signed long value.
2410 L An unsigned long value.
2411 (This 'long' is _exactly_ 32 bits, which may differ from
2412 what a local C compiler calls 'long'.)
2414 n A short in "network" (big-endian) order.
2415 N A long in "network" (big-endian) order.
2416 v A short in "VAX" (little-endian) order.
2417 V A long in "VAX" (little-endian) order.
2418 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2419 _exactly_ 32 bits, respectively.)
2421 f A single-precision float in the native format.
2422 d A double-precision float in the native format.
2424 p A pointer to a null-terminated string.
2425 P A pointer to a structure (fixed-length string).
2427 u A uuencoded string.
2429 w A BER compressed integer. Its bytes represent an unsigned
2430 integer in base 128, most significant digit first, with as
2431 few digits as possible. Bit eight (the high bit) is set
2432 on each byte except the last.
2436 @ Null fill to absolute position.
2438 Each letter may optionally be followed by a number giving a repeat
2439 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2440 pack function will gobble up that many values from the LIST. A C<*> for the
2441 repeat count means to use however many items are left. The C<"a"> and C<"A">
2442 types gobble just one value, but pack it as a string of length count,
2443 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2444 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2445 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2446 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2447 terminated string. You are responsible for ensuring the string is not a
2448 temporary value (which can potentially get deallocated before you get
2449 around to using the packed result). The C<"P"> packs a pointer to a structure
2450 of the size indicated by the length. A NULL pointer is created if the
2451 corresponding value for C<"p"> or C<"P"> is C<undef>.
2452 Real numbers (floats and doubles) are
2453 in the native machine format only; due to the multiplicity of floating
2454 formats around, and the lack of a standard "network" representation, no
2455 facility for interchange has been made. This means that packed floating
2456 point data written on one machine may not be readable on another - even if
2457 both use IEEE floating point arithmetic (as the endian-ness of the memory
2458 representation is not part of the IEEE spec). Note that Perl uses doubles
2459 internally for all numeric calculation, and converting from double into
2460 float and thence back to double again will lose precision (i.e.,
2461 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2465 $foo = pack("cccc",65,66,67,68);
2467 $foo = pack("c4",65,66,67,68);
2470 $foo = pack("ccxxcc",65,66,67,68);
2473 $foo = pack("s2",1,2);
2474 # "\1\0\2\0" on little-endian
2475 # "\0\1\0\2" on big-endian
2477 $foo = pack("a4","abcd","x","y","z");
2480 $foo = pack("aaaa","abcd","x","y","z");
2483 $foo = pack("a14","abcdefg");
2484 # "abcdefg\0\0\0\0\0\0\0"
2486 $foo = pack("i9pl", gmtime);
2487 # a real struct tm (on my system anyway)
2490 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2493 The same template may generally also be used in the unpack function.
2497 =item package NAMESPACE
2499 Declares the compilation unit as being in the given namespace. The scope
2500 of the package declaration is from the declaration itself through the end of
2501 the enclosing block (the same scope as the C<local()> operator). All further
2502 unqualified dynamic identifiers will be in this namespace. A package
2503 statement affects only dynamic variables--including those you've used
2504 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2505 would be the first declaration in a file to be included by the C<require>
2506 or C<use> operator. You can switch into a package in more than one place;
2507 it merely influences which symbol table is used by the compiler for the
2508 rest of that block. You can refer to variables and filehandles in other
2509 packages by prefixing the identifier with the package name and a double
2510 colon: C<$Package::Variable>. If the package name is null, the C<main>
2511 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2513 If NAMESPACE is omitted, then there is no current package, and all
2514 identifiers must be fully qualified or lexicals. This is stricter
2515 than C<use strict>, since it also extends to function names.
2517 See L<perlmod/"Packages"> for more information about packages, modules,
2518 and classes. See L<perlsub> for other scoping issues.
2520 =item pipe READHANDLE,WRITEHANDLE
2522 Opens a pair of connected pipes like the corresponding system call.
2523 Note that if you set up a loop of piped processes, deadlock can occur
2524 unless you are very careful. In addition, note that Perl's pipes use
2525 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2526 after each command, depending on the application.
2528 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2529 for examples of such things.
2535 Pops and returns the last value of the array, shortening the array by
2536 1. Has a similar effect to
2538 $tmp = $ARRAY[$#ARRAY--];
2540 If there are no elements in the array, returns the undefined value.
2541 If ARRAY is omitted, pops the
2542 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2549 Returns the offset of where the last C<m//g> search left off for the variable
2550 is in question (C<$_> is used when the variable is not specified). May be
2551 modified to change that offset. Such modification will also influence
2552 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2555 =item print FILEHANDLE LIST
2561 Prints a string or a comma-separated list of strings. Returns TRUE
2562 if successful. FILEHANDLE may be a scalar variable name, in which case
2563 the variable contains the name of or a reference to the filehandle, thus introducing one
2564 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2565 token is a term, it may be misinterpreted as an operator unless you
2566 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2567 omitted, prints by default to standard output (or to the last selected
2568 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2569 the currently selected output channel. To set the default output channel to something other than
2570 STDOUT use the select operation. Note that, because print takes a
2571 LIST, anything in the LIST is evaluated in list context, and any
2572 subroutine that you call will have one or more of its expressions
2573 evaluated in list context. Also be careful not to follow the print
2574 keyword with a left parenthesis unless you want the corresponding right
2575 parenthesis to terminate the arguments to the print--interpose a C<+> or
2576 put parentheses around all the arguments.
2578 Note that if you're storing FILEHANDLES in an array or other expression,
2579 you will have to use a block returning its value instead:
2581 print { $files[$i] } "stuff\n";
2582 print { $OK ? STDOUT : STDERR } "stuff\n";
2584 =item printf FILEHANDLE FORMAT, LIST
2586 =item printf FORMAT, LIST
2588 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2589 (the output record separator) is not appended. The first argument
2590 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2591 in effect, the character used for the decimal point in formatted real numbers
2592 is affected by the LC_NUMERIC locale. See L<perllocale>.
2594 Don't fall into the trap of using a C<printf()> when a simple
2595 C<print()> would do. The C<print()> is more efficient and less
2598 =item prototype FUNCTION
2600 Returns the prototype of a function as a string (or C<undef> if the
2601 function has no prototype). FUNCTION is a reference to, or the name of,
2602 the function whose prototype you want to retrieve.
2604 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2605 a name for Perl builtin. If builtin is not I<overridable> (such as
2606 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2607 C<system()>) - in other words, the builtin does not behave like a Perl
2608 function - returns C<undef>. Otherwise, the string describing the
2609 equivalent prototype is returned.
2611 =item push ARRAY,LIST
2613 Treats ARRAY as a stack, and pushes the values of LIST
2614 onto the end of ARRAY. The length of ARRAY increases by the length of
2615 LIST. Has the same effect as
2618 $ARRAY[++$#ARRAY] = $value;
2621 but is more efficient. Returns the new number of elements in the array.
2633 Generalized quotes. See L<perlop>.
2635 =item quotemeta EXPR
2639 Returns the value of EXPR with all non-alphanumeric
2640 characters backslashed. (That is, all characters not matching
2641 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2642 returned string, regardless of any locale settings.)
2643 This is the internal function implementing
2644 the C<\Q> escape in double-quoted strings.
2646 If EXPR is omitted, uses C<$_>.
2652 Returns a random fractional number greater than or equal to C<0> and less
2653 than the value of EXPR. (EXPR should be positive.) If EXPR is
2654 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2655 C<srand()> has already been called. See also C<srand()>.
2657 (Note: If your rand function consistently returns numbers that are too
2658 large or too small, then your version of Perl was probably compiled
2659 with the wrong number of RANDBITS.)
2661 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2663 =item read FILEHANDLE,SCALAR,LENGTH
2665 Attempts to read LENGTH bytes of data into variable SCALAR from the
2666 specified FILEHANDLE. Returns the number of bytes actually read,
2667 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2668 or shrunk to the length actually read. An OFFSET may be specified to
2669 place the read data at some other place than the beginning of the
2670 string. This call is actually implemented in terms of stdio's fread(3)
2671 call. To get a true read(2) system call, see C<sysread()>.
2673 =item readdir DIRHANDLE
2675 Returns the next directory entry for a directory opened by C<opendir()>.
2676 If used in list context, returns all the rest of the entries in the
2677 directory. If there are no more entries, returns an undefined value in
2678 scalar context or a null list in list context.
2680 If you're planning to filetest the return values out of a C<readdir()>, you'd
2681 better prepend the directory in question. Otherwise, because we didn't
2682 C<chdir()> there, it would have been testing the wrong file.
2684 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2685 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2690 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2691 is read and returned. In list context, reads until end-of-file is
2692 reached and returns a list of lines (however you've defined lines
2693 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2694 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2695 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2696 operator is discussed in more detail in L<perlop/"I/O Operators">.
2699 $line = readline(*STDIN); # same thing
2705 Returns the value of a symbolic link, if symbolic links are
2706 implemented. If not, gives a fatal error. If there is some system
2707 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2708 omitted, uses C<$_>.
2712 EXPR is executed as a system command.
2713 The collected standard output of the command is returned.
2714 In scalar context, it comes back as a single (potentially
2715 multi-line) string. In list context, returns a list of lines
2716 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2717 This is the internal function implementing the C<qx/EXPR/>
2718 operator, but you can use it directly. The C<qx/EXPR/>
2719 operator is discussed in more detail in L<perlop/"I/O Operators">.
2721 =item recv SOCKET,SCALAR,LEN,FLAGS
2723 Receives a message on a socket. Attempts to receive LENGTH bytes of
2724 data into variable SCALAR from the specified SOCKET filehandle.
2725 Actually does a C C<recvfrom()>, so that it can return the address of the
2726 sender. Returns the undefined value if there's an error. SCALAR will
2727 be grown or shrunk to the length actually read. Takes the same flags
2728 as the system call of the same name.
2729 See L<perlipc/"UDP: Message Passing"> for examples.
2735 The C<redo> command restarts the loop block without evaluating the
2736 conditional again. The C<continue> block, if any, is not executed. If
2737 the LABEL is omitted, the command refers to the innermost enclosing
2738 loop. This command is normally used by programs that want to lie to
2739 themselves about what was just input:
2741 # a simpleminded Pascal comment stripper
2742 # (warning: assumes no { or } in strings)
2743 LINE: while (<STDIN>) {
2744 while (s|({.*}.*){.*}|$1 |) {}
2749 if (/}/) { # end of comment?
2758 See also L</continue> for an illustration of how C<last>, C<next>, and
2765 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2766 is not specified, C<$_> will be used. The value returned depends on the
2767 type of thing the reference is a reference to.
2768 Builtin types include:
2777 If the referenced object has been blessed into a package, then that package
2778 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2780 if (ref($r) eq "HASH") {
2781 print "r is a reference to a hash.\n";
2784 print "r is not a reference at all.\n";
2787 See also L<perlref>.
2789 =item rename OLDNAME,NEWNAME
2791 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2792 not work across file system boundaries.
2798 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2799 supplied. If EXPR is numeric, demands that the current version of Perl
2800 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2802 Otherwise, demands that a library file be included if it hasn't already
2803 been included. The file is included via the do-FILE mechanism, which is
2804 essentially just a variety of C<eval()>. Has semantics similar to the following
2809 return 1 if $INC{$filename};
2810 my($realfilename,$result);
2812 foreach $prefix (@INC) {
2813 $realfilename = "$prefix/$filename";
2814 if (-f $realfilename) {
2815 $result = do $realfilename;
2819 die "Can't find $filename in \@INC";
2822 die "$filename did not return true value" unless $result;
2823 $INC{$filename} = $realfilename;
2827 Note that the file will not be included twice under the same specified
2828 name. The file must return TRUE as the last statement to indicate
2829 successful execution of any initialization code, so it's customary to
2830 end such a file with "C<1;>" unless you're sure it'll return TRUE
2831 otherwise. But it's better just to put the "C<1;>", in case you add more
2834 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2835 replaces "F<::>" with "F</>" in the filename for you,
2836 to make it easy to load standard modules. This form of loading of
2837 modules does not risk altering your namespace.
2839 In other words, if you try this:
2841 require Foo::Bar; # a splendid bareword
2843 The require function will actually look for the "F<Foo/Bar.pm>" file in the
2844 directories specified in the C<@INC> array.
2846 But if you try this:
2848 $class = 'Foo::Bar';
2849 require $class; # $class is not a bareword
2851 require "Foo::Bar"; # not a bareword because of the ""
2853 The require function will look for the "F<Foo::Bar>" file in the @INC array and
2854 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
2856 eval "require $class";
2858 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2864 Generally used in a C<continue> block at the end of a loop to clear
2865 variables and reset C<??> searches so that they work again. The
2866 expression is interpreted as a list of single characters (hyphens
2867 allowed for ranges). All variables and arrays beginning with one of
2868 those letters are reset to their pristine state. If the expression is
2869 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
2870 only variables or searches in the current package. Always returns
2873 reset 'X'; # reset all X variables
2874 reset 'a-z'; # reset lower case variables
2875 reset; # just reset ?? searches
2877 Resetting C<"A-Z"> is not recommended because you'll wipe out your
2878 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
2879 are unaffected, but they clean themselves up on scope exit anyway,
2880 so you'll probably want to use them instead. See L</my>.
2886 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
2887 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
2888 context, depending on how the return value will be used, and the context
2889 may vary from one execution to the next (see C<wantarray()>). If no EXPR
2890 is given, returns an empty list in list context, an undefined value in
2891 scalar context, or nothing in a void context.
2893 (Note that in the absence of a return, a subroutine, eval, or do FILE
2894 will automatically return the value of the last expression evaluated.)
2898 In list context, returns a list value consisting of the elements
2899 of LIST in the opposite order. In scalar context, concatenates the
2900 elements of LIST, and returns a string value consisting of those bytes,
2901 but in the opposite order.
2903 print reverse <>; # line tac, last line first
2905 undef $/; # for efficiency of <>
2906 print scalar reverse <>; # byte tac, last line tsrif
2908 This operator is also handy for inverting a hash, although there are some
2909 caveats. If a value is duplicated in the original hash, only one of those
2910 can be represented as a key in the inverted hash. Also, this has to
2911 unwind one hash and build a whole new one, which may take some time
2914 %by_name = reverse %by_address; # Invert the hash
2916 =item rewinddir DIRHANDLE
2918 Sets the current position to the beginning of the directory for the
2919 C<readdir()> routine on DIRHANDLE.
2921 =item rindex STR,SUBSTR,POSITION
2923 =item rindex STR,SUBSTR
2925 Works just like index except that it returns the position of the LAST
2926 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2927 last occurrence at or before that position.
2929 =item rmdir FILENAME
2933 Deletes the directory specified by FILENAME if that directory is empty. If it
2934 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
2935 FILENAME is omitted, uses C<$_>.
2939 The substitution operator. See L<perlop>.
2943 Forces EXPR to be interpreted in scalar context and returns the value
2946 @counts = ( scalar @a, scalar @b, scalar @c );
2948 There is no equivalent operator to force an expression to
2949 be interpolated in list context because it's in practice never
2950 needed. If you really wanted to do so, however, you could use
2951 the construction C<@{[ (some expression) ]}>, but usually a simple
2952 C<(some expression)> suffices.
2954 =item seek FILEHANDLE,POSITION,WHENCE
2956 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
2957 FILEHANDLE may be an expression whose value gives the name of the
2958 filehandle. The values for WHENCE are C<0> to set the new position to
2959 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
2960 set it to EOF plus POSITION (typically negative). For WHENCE you may
2961 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2962 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
2964 If you want to position file for C<sysread()> or C<syswrite()>, don't use
2965 C<seek()> -- buffering makes its effect on the file's system position
2966 unpredictable and non-portable. Use C<sysseek()> instead.
2968 On some systems you have to do a seek whenever you switch between reading
2969 and writing. Amongst other things, this may have the effect of calling
2970 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
2975 This is also useful for applications emulating C<tail -f>. Once you hit
2976 EOF on your read, and then sleep for a while, you might have to stick in a
2977 seek() to reset things. The C<seek()> doesn't change the current position,
2978 but it I<does> clear the end-of-file condition on the handle, so that the
2979 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2981 If that doesn't work (some stdios are particularly cantankerous), then
2982 you may need something more like this:
2985 for ($curpos = tell(FILE); $_ = <FILE>;
2986 $curpos = tell(FILE)) {
2987 # search for some stuff and put it into files
2989 sleep($for_a_while);
2990 seek(FILE, $curpos, 0);
2993 =item seekdir DIRHANDLE,POS
2995 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
2996 must be a value returned by C<telldir()>. Has the same caveats about
2997 possible directory compaction as the corresponding system library
3000 =item select FILEHANDLE
3004 Returns the currently selected filehandle. Sets the current default
3005 filehandle for output, if FILEHANDLE is supplied. This has two
3006 effects: first, a C<write()> or a C<print()> without a filehandle will
3007 default to this FILEHANDLE. Second, references to variables related to
3008 output will refer to this output channel. For example, if you have to
3009 set the top of form format for more than one output channel, you might
3017 FILEHANDLE may be an expression whose value gives the name of the
3018 actual filehandle. Thus:
3020 $oldfh = select(STDERR); $| = 1; select($oldfh);
3022 Some programmers may prefer to think of filehandles as objects with
3023 methods, preferring to write the last example as:
3026 STDERR->autoflush(1);
3028 =item select RBITS,WBITS,EBITS,TIMEOUT
3030 This calls the select(2) system call with the bit masks specified, which
3031 can be constructed using C<fileno()> and C<vec()>, along these lines:
3033 $rin = $win = $ein = '';
3034 vec($rin,fileno(STDIN),1) = 1;
3035 vec($win,fileno(STDOUT),1) = 1;
3038 If you want to select on many filehandles you might wish to write a
3042 my(@fhlist) = split(' ',$_[0]);
3045 vec($bits,fileno($_),1) = 1;
3049 $rin = fhbits('STDIN TTY SOCK');
3053 ($nfound,$timeleft) =
3054 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3056 or to block until something becomes ready just do this
3058 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3060 Most systems do not bother to return anything useful in C<$timeleft>, so
3061 calling select() in scalar context just returns C<$nfound>.
3063 Any of the bit masks can also be undef. The timeout, if specified, is
3064 in seconds, which may be fractional. Note: not all implementations are
3065 capable of returning theC<$timeleft>. If not, they always return
3066 C<$timeleft> equal to the supplied C<$timeout>.
3068 You can effect a sleep of 250 milliseconds this way:
3070 select(undef, undef, undef, 0.25);
3072 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3073 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3074 then only on POSIX systems. You have to use C<sysread()> instead.
3076 =item semctl ID,SEMNUM,CMD,ARG
3078 Calls the System V IPC function C<semctl()>. You'll probably have to say
3082 first to get the correct constant definitions. If CMD is IPC_STAT or
3083 GETALL, then ARG must be a variable which will hold the returned
3084 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3085 undefined value for error, "C<0> but true" for zero, or the actual return
3086 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3088 =item semget KEY,NSEMS,FLAGS
3090 Calls the System V IPC function semget. Returns the semaphore id, or
3091 the undefined value if there is an error. See also C<IPC::SysV> and
3092 C<IPC::SysV::Semaphore> documentation.
3094 =item semop KEY,OPSTRING
3096 Calls the System V IPC function semop to perform semaphore operations
3097 such as signaling and waiting. OPSTRING must be a packed array of
3098 semop structures. Each semop structure can be generated with
3099 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3100 operations is implied by the length of OPSTRING. Returns TRUE if
3101 successful, or FALSE if there is an error. As an example, the
3102 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3104 $semop = pack("sss", $semnum, -1, 0);
3105 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3107 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3108 and C<IPC::SysV::Semaphore> documentation.
3110 =item send SOCKET,MSG,FLAGS,TO
3112 =item send SOCKET,MSG,FLAGS
3114 Sends a message on a socket. Takes the same flags as the system call
3115 of the same name. On unconnected sockets you must specify a
3116 destination to send TO, in which case it does a C C<sendto()>. Returns
3117 the number of characters sent, or the undefined value if there is an
3119 See L<perlipc/"UDP: Message Passing"> for examples.
3121 =item setpgrp PID,PGRP
3123 Sets the current process group for the specified PID, C<0> for the current
3124 process. Will produce a fatal error if used on a machine that doesn't
3125 implement setpgrp(2). If the arguments are omitted, it defaults to
3126 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3127 arguments, so only setpgrp C<0,0> is portable.
3129 =item setpriority WHICH,WHO,PRIORITY
3131 Sets the current priority for a process, a process group, or a user.
3132 (See setpriority(2).) Will produce a fatal error if used on a machine
3133 that doesn't implement setpriority(2).
3135 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3137 Sets the socket option requested. Returns undefined if there is an
3138 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3145 Shifts the first value of the array off and returns it, shortening the
3146 array by 1 and moving everything down. If there are no elements in the
3147 array, returns the undefined value. If ARRAY is omitted, shifts the
3148 C<@_> array within the lexical scope of subroutines and formats, and the
3149 C<@ARGV> array at file scopes or within the lexical scopes established by
3150 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3151 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3152 same thing to the left end of an array that C<pop()> and C<push()> do to the
3155 =item shmctl ID,CMD,ARG
3157 Calls the System V IPC function shmctl. You'll probably have to say
3161 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3162 then ARG must be a variable which will hold the returned C<shmid_ds>
3163 structure. Returns like ioctl: the undefined value for error, "C<0> but
3164 true" for zero, or the actual return value otherwise.
3165 See also C<IPC::SysV> documentation.
3167 =item shmget KEY,SIZE,FLAGS
3169 Calls the System V IPC function shmget. Returns the shared memory
3170 segment id, or the undefined value if there is an error.
3171 See also C<IPC::SysV> documentation.
3173 =item shmread ID,VAR,POS,SIZE
3175 =item shmwrite ID,STRING,POS,SIZE
3177 Reads or writes the System V shared memory segment ID starting at
3178 position POS for size SIZE by attaching to it, copying in/out, and
3179 detaching from it. When reading, VAR must be a variable that will
3180 hold the data read. When writing, if STRING is too long, only SIZE
3181 bytes are used; if STRING is too short, nulls are written to fill out
3182 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3183 See also C<IPC::SysV> documentation.
3185 =item shutdown SOCKET,HOW
3187 Shuts down a socket connection in the manner indicated by HOW, which
3188 has the same interpretation as in the system call of the same name.
3190 shutdown(SOCKET, 0); # I/we have stopped reading data
3191 shutdown(SOCKET, 1); # I/we have stopped writing data
3192 shutdown(SOCKET, 2); # I/we have stopped using this socket
3194 This is useful with sockets when you want to tell the other
3195 side you're done writing but not done reading, or vice versa.
3196 It's also a more insistent form of close because it also
3197 disables the filedescriptor in any forked copies in other
3204 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3205 returns sine of C<$_>.
3207 For the inverse sine operation, you may use the C<POSIX::asin()>
3208 function, or use this relation:
3210 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3216 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3217 May be interrupted if the process receives a signal such as C<SIGALRM>.
3218 Returns the number of seconds actually slept. You probably cannot
3219 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3222 On some older systems, it may sleep up to a full second less than what
3223 you requested, depending on how it counts seconds. Most modern systems
3224 always sleep the full amount. They may appear to sleep longer than that,
3225 however, because your process might not be scheduled right away in a
3226 busy multitasking system.
3228 For delays of finer granularity than one second, you may use Perl's
3229 C<syscall()> interface to access setitimer(2) if your system supports it,
3230 or else see L</select()> above.
3232 See also the POSIX module's C<sigpause()> function.
3234 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3236 Opens a socket of the specified kind and attaches it to filehandle
3237 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3238 system call of the same name. You should "C<use Socket;>" first to get
3239 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3241 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3243 Creates an unnamed pair of sockets in the specified domain, of the
3244 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3245 for the system call of the same name. If unimplemented, yields a fatal
3246 error. Returns TRUE if successful.
3248 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3249 to C<pipe(Rdr, Wtr)> is essentially:
3252 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3253 shutdown(Rdr, 1); # no more writing for reader
3254 shutdown(Wtr, 0); # no more reading for writer
3256 See L<perlipc> for an example of socketpair use.
3258 =item sort SUBNAME LIST
3260 =item sort BLOCK LIST
3264 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3265 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3266 specified, it gives the name of a subroutine that returns an integer
3267 less than, equal to, or greater than C<0>, depending on how the elements
3268 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3269 operators are extremely useful in such routines.) SUBNAME may be a
3270 scalar variable name (unsubscripted), in which case the value provides
3271 the name of (or a reference to) the actual subroutine to use. In place
3272 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3275 In the interests of efficiency the normal calling code for subroutines is
3276 bypassed, with the following effects: the subroutine may not be a
3277 recursive subroutine, and the two elements to be compared are passed into
3278 the subroutine not via C<@_> but as the package global variables C<$a> and
3279 C<$b> (see example below). They are passed by reference, so don't
3280 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3282 You also cannot exit out of the sort block or subroutine using any of the
3283 loop control operators described in L<perlsyn> or with C<goto()>.
3285 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3286 current collation locale. See L<perllocale>.
3291 @articles = sort @files;
3293 # same thing, but with explicit sort routine
3294 @articles = sort {$a cmp $b} @files;
3296 # now case-insensitively
3297 @articles = sort {uc($a) cmp uc($b)} @files;
3299 # same thing in reversed order
3300 @articles = sort {$b cmp $a} @files;
3302 # sort numerically ascending
3303 @articles = sort {$a <=> $b} @files;
3305 # sort numerically descending
3306 @articles = sort {$b <=> $a} @files;
3308 # sort using explicit subroutine name
3310 $age{$a} <=> $age{$b}; # presuming numeric
3312 @sortedclass = sort byage @class;
3314 # this sorts the %age hash by value instead of key
3315 # using an in-line function
3316 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3318 sub backwards { $b cmp $a; }
3319 @harry = ('dog','cat','x','Cain','Abel');
3320 @george = ('gone','chased','yz','Punished','Axed');
3322 # prints AbelCaincatdogx
3323 print sort backwards @harry;
3324 # prints xdogcatCainAbel
3325 print sort @george, 'to', @harry;
3326 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3328 # inefficiently sort by descending numeric compare using
3329 # the first integer after the first = sign, or the
3330 # whole record case-insensitively otherwise
3333 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3338 # same thing, but much more efficiently;
3339 # we'll build auxiliary indices instead
3343 push @nums, /=(\d+)/;
3348 $nums[$b] <=> $nums[$a]
3350 $caps[$a] cmp $caps[$b]
3354 # same thing using a Schwartzian Transform (no temps)
3355 @new = map { $_->[0] }
3356 sort { $b->[1] <=> $a->[1]
3359 } map { [$_, /=(\d+)/, uc($_)] } @old;
3361 If you're using strict, you I<MUST NOT> declare C<$a>
3362 and C<$b> as lexicals. They are package globals. That means
3363 if you're in the C<main> package, it's
3365 @articles = sort {$main::b <=> $main::a} @files;
3369 @articles = sort {$::b <=> $::a} @files;
3371 but if you're in the C<FooPack> package, it's
3373 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3375 The comparison function is required to behave. If it returns
3376 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3377 sometimes saying the opposite, for example) the results are not
3380 =item splice ARRAY,OFFSET,LENGTH,LIST
3382 =item splice ARRAY,OFFSET,LENGTH
3384 =item splice ARRAY,OFFSET
3386 Removes the elements designated by OFFSET and LENGTH from an array, and
3387 replaces them with the elements of LIST, if any. In list context,
3388 returns the elements removed from the array. In scalar context,
3389 returns the last element removed, or C<undef> if no elements are
3390 removed. The array grows or shrinks as necessary.
3391 If OFFSET is negative then it start that far from the end of the array.
3392 If LENGTH is omitted, removes everything from OFFSET onward.
3393 If LENGTH is negative, leave that many elements off the end of the array.
3394 The following equivalences hold (assuming C<$[ == 0>):
3396 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3397 pop(@a) splice(@a,-1)
3398 shift(@a) splice(@a,0,1)
3399 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3400 $a[$x] = $y splice(@a,$x,1,$y)
3402 Example, assuming array lengths are passed before arrays:
3404 sub aeq { # compare two list values
3405 my(@a) = splice(@_,0,shift);
3406 my(@b) = splice(@_,0,shift);
3407 return 0 unless @a == @b; # same len?
3409 return 0 if pop(@a) ne pop(@b);
3413 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3415 =item split /PATTERN/,EXPR,LIMIT
3417 =item split /PATTERN/,EXPR
3419 =item split /PATTERN/
3423 Splits a string into an array of strings, and returns it. By default,
3424 empty leading fields are preserved, and empty trailing ones are deleted.
3426 If not in list context, returns the number of fields found and splits into
3427 the C<@_> array. (In list context, you can force the split into C<@_> by
3428 using C<??> as the pattern delimiters, but it still returns the list
3429 value.) The use of implicit split to C<@_> is deprecated, however, because
3430 it clobbers your subroutine arguments.
3432 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3433 splits on whitespace (after skipping any leading whitespace). Anything
3434 matching PATTERN is taken to be a delimiter separating the fields. (Note
3435 that the delimiter may be longer than one character.)
3437 If LIMIT is specified and positive, splits into no more than that
3438 many fields (though it may split into fewer). If LIMIT is unspecified
3439 or zero, trailing null fields are stripped (which potential users
3440 of C<pop()> would do well to remember). If LIMIT is negative, it is
3441 treated as if an arbitrarily large LIMIT had been specified.
3443 A pattern matching the null string (not to be confused with
3444 a null pattern C<//>, which is just one member of the set of patterns
3445 matching a null string) will split the value of EXPR into separate
3446 characters at each point it matches that way. For example:
3448 print join(':', split(/ */, 'hi there'));
3450 produces the output 'h:i:t:h:e:r:e'.
3452 The LIMIT parameter can be used to split a line partially
3454 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3456 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3457 one larger than the number of variables in the list, to avoid
3458 unnecessary work. For the list above LIMIT would have been 4 by
3459 default. In time critical applications it behooves you not to split
3460 into more fields than you really need.
3462 If the PATTERN contains parentheses, additional array elements are
3463 created from each matching substring in the delimiter.
3465 split(/([,-])/, "1-10,20", 3);
3467 produces the list value
3469 (1, '-', 10, ',', 20)
3471 If you had the entire header of a normal Unix email message in C<$header>,
3472 you could split it up into fields and their values this way:
3474 $header =~ s/\n\s+/ /g; # fix continuation lines
3475 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3477 The pattern C</PATTERN/> may be replaced with an expression to specify
3478 patterns that vary at runtime. (To do runtime compilation only once,
3479 use C</$variable/o>.)
3481 As a special case, specifying a PATTERN of space (C<' '>) will split on
3482 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3483 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3484 will give you as many null initial fields as there are leading spaces.
3485 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3486 whitespace produces a null first field. A C<split()> with no arguments
3487 really does a C<split(' ', $_)> internally.
3491 open(PASSWD, '/etc/passwd');
3493 ($login, $passwd, $uid, $gid,
3494 $gcos, $home, $shell) = split(/:/);
3498 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3499 L</chomp>, and L</join>.)
3501 =item sprintf FORMAT, LIST
3503 Returns a string formatted by the usual C<printf()> conventions of the
3504 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3505 on your system for an explanation of the general principles.
3507 Perl does its own C<sprintf()> formatting -- it emulates the C
3508 function C<sprintf()>, but it doesn't use it (except for floating-point
3509 numbers, and even then only the standard modifiers are allowed). As a
3510 result, any non-standard extensions in your local C<sprintf()> are not
3511 available from Perl.
3513 Perl's C<sprintf()> permits the following universally-known conversions:
3516 %c a character with the given number
3518 %d a signed integer, in decimal
3519 %u an unsigned integer, in decimal
3520 %o an unsigned integer, in octal
3521 %x an unsigned integer, in hexadecimal
3522 %e a floating-point number, in scientific notation
3523 %f a floating-point number, in fixed decimal notation
3524 %g a floating-point number, in %e or %f notation
3526 In addition, Perl permits the following widely-supported conversions:
3528 %X like %x, but using upper-case letters
3529 %E like %e, but using an upper-case "E"
3530 %G like %g, but with an upper-case "E" (if applicable)
3531 %p a pointer (outputs the Perl value's address in hexadecimal)
3532 %n special: *stores* the number of characters output so far
3533 into the next variable in the parameter list
3535 Finally, for backward (and we do mean "backward") compatibility, Perl
3536 permits these unnecessary but widely-supported conversions:
3539 %D a synonym for %ld
3540 %U a synonym for %lu
3541 %O a synonym for %lo
3544 Perl permits the following universally-known flags between the C<%>
3545 and the conversion letter:
3547 space prefix positive number with a space
3548 + prefix positive number with a plus sign
3549 - left-justify within the field
3550 0 use zeros, not spaces, to right-justify
3551 # prefix non-zero octal with "0", non-zero hex with "0x"
3552 number minimum field width
3553 .number "precision": digits after decimal point for
3554 floating-point, max length for string, minimum length
3556 l interpret integer as C type "long" or "unsigned long"
3557 h interpret integer as C type "short" or "unsigned short"
3559 There is also one Perl-specific flag:
3561 V interpret integer as Perl's standard integer type
3563 Where a number would appear in the flags, an asterisk ("C<*>") may be
3564 used instead, in which case Perl uses the next item in the parameter
3565 list as the given number (that is, as the field width or precision).
3566 If a field width obtained through "C<*>" is negative, it has the same
3567 effect as the "C<->" flag: left-justification.
3569 If C<use locale> is in effect, the character used for the decimal
3570 point in formatted real numbers is affected by the LC_NUMERIC locale.
3577 Return the square root of EXPR. If EXPR is omitted, returns square
3584 Sets the random number seed for the C<rand()> operator. If EXPR is
3585 omitted, uses a semi-random value based on the current time and process
3586 ID, among other things. In versions of Perl prior to 5.004 the default
3587 seed was just the current C<time()>. This isn't a particularly good seed,
3588 so many old programs supply their own seed value (often C<time ^ $$> or
3589 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3591 In fact, it's usually not necessary to call C<srand()> at all, because if
3592 it is not called explicitly, it is called implicitly at the first use of
3593 the C<rand()> operator. However, this was not the case in version of Perl
3594 before 5.004, so if your script will run under older Perl versions, it
3595 should call C<srand()>.
3597 Note that you need something much more random than the default seed for
3598 cryptographic purposes. Checksumming the compressed output of one or more
3599 rapidly changing operating system status programs is the usual method. For
3602 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3604 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3607 Do I<not> call C<srand()> multiple times in your program unless you know
3608 exactly what you're doing and why you're doing it. The point of the
3609 function is to "seed" the C<rand()> function so that C<rand()> can produce
3610 a different sequence each time you run your program. Just do it once at the
3611 top of your program, or you I<won't> get random numbers out of C<rand()>!
3613 Frequently called programs (like CGI scripts) that simply use
3617 for a seed can fall prey to the mathematical property that
3621 one-third of the time. So don't do that.
3623 =item stat FILEHANDLE
3629 Returns a 13-element list giving the status info for a file, either
3630 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3631 it stats C<$_>. Returns a null list if the stat fails. Typically used
3634 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3635 $atime,$mtime,$ctime,$blksize,$blocks)
3638 Not all fields are supported on all filesystem types. Here are the
3639 meaning of the fields:
3641 0 dev device number of filesystem
3643 2 mode file mode (type and permissions)
3644 3 nlink number of (hard) links to the file
3645 4 uid numeric user ID of file's owner
3646 5 gid numeric group ID of file's owner
3647 6 rdev the device identifier (special files only)
3648 7 size total size of file, in bytes
3649 8 atime last access time since the epoch
3650 9 mtime last modify time since the epoch
3651 10 ctime inode change time (NOT creation time!) since the epoch
3652 11 blksize preferred block size for file system I/O
3653 12 blocks actual number of blocks allocated
3655 (The epoch was at 00:00 January 1, 1970 GMT.)
3657 If stat is passed the special filehandle consisting of an underline, no
3658 stat is done, but the current contents of the stat structure from the
3659 last stat or filetest are returned. Example:
3661 if (-x $file && (($d) = stat(_)) && $d < 0) {
3662 print "$file is executable NFS file\n";
3665 (This works on machines only for which the device number is negative under NFS.)
3667 In scalar context, C<stat()> returns a boolean value indicating success
3668 or failure, and, if successful, sets the information associated with
3669 the special filehandle C<_>.
3675 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3676 doing many pattern matches on the string before it is next modified.
3677 This may or may not save time, depending on the nature and number of
3678 patterns you are searching on, and on the distribution of character
3679 frequencies in the string to be searched -- you probably want to compare
3680 run times with and without it to see which runs faster. Those loops
3681 which scan for many short constant strings (including the constant
3682 parts of more complex patterns) will benefit most. You may have only
3683 one C<study()> active at a time -- if you study a different scalar the first
3684 is "unstudied". (The way C<study()> works is this: a linked list of every
3685 character in the string to be searched is made, so we know, for
3686 example, where all the C<'k'> characters are. From each search string,
3687 the rarest character is selected, based on some static frequency tables
3688 constructed from some C programs and English text. Only those places
3689 that contain this "rarest" character are examined.)
3691 For example, here is a loop that inserts index producing entries
3692 before any line containing a certain pattern:
3696 print ".IX foo\n" if /\bfoo\b/;
3697 print ".IX bar\n" if /\bbar\b/;
3698 print ".IX blurfl\n" if /\bblurfl\b/;
3703 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3704 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3705 a big win except in pathological cases. The only question is whether
3706 it saves you more time than it took to build the linked list in the
3709 Note that if you have to look for strings that you don't know till
3710 runtime, you can build an entire loop as a string and C<eval()> that to
3711 avoid recompiling all your patterns all the time. Together with
3712 undefining C<$/> to input entire files as one record, this can be very
3713 fast, often faster than specialized programs like fgrep(1). The following
3714 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3715 out the names of those files that contain a match:
3717 $search = 'while (<>) { study;';
3718 foreach $word (@words) {
3719 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3724 eval $search; # this screams
3725 $/ = "\n"; # put back to normal input delimiter
3726 foreach $file (sort keys(%seen)) {
3734 =item sub NAME BLOCK
3736 This is subroutine definition, not a real function I<per se>. With just a
3737 NAME (and possibly prototypes), it's just a forward declaration. Without
3738 a NAME, it's an anonymous function declaration, and does actually return a
3739 value: the CODE ref of the closure you just created. See L<perlsub> and
3740 L<perlref> for details.
3742 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3744 =item substr EXPR,OFFSET,LEN
3746 =item substr EXPR,OFFSET
3748 Extracts a substring out of EXPR and returns it. First character is at
3749 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3750 If OFFSET is negative (or more precisely, less than C<$[>), starts
3751 that far from the end of the string. If LEN is omitted, returns
3752 everything to the end of the string. If LEN is negative, leaves that
3753 many characters off the end of the string.
3755 If you specify a substring that is partly outside the string, the part
3756 within the string is returned. If the substring is totally outside
3757 the string a warning is produced.
3759 You can use the C<substr()> function
3760 as an lvalue, in which case EXPR must be an lvalue. If you assign
3761 something shorter than LEN, the string will shrink, and if you assign
3762 something longer than LEN, the string will grow to accommodate it. To
3763 keep the string the same length you may need to pad or chop your value
3766 An alternative to using C<substr()> as an lvalue is to specify the
3767 replacement string as the 4th argument. This allows you to replace
3768 parts of the EXPR and return what was there before in one operation.
3770 =item symlink OLDFILE,NEWFILE
3772 Creates a new filename symbolically linked to the old filename.
3773 Returns C<1> for success, C<0> otherwise. On systems that don't support
3774 symbolic links, produces a fatal error at run time. To check for that,
3777 $symlink_exists = eval { symlink("",""); 1 };
3781 Calls the system call specified as the first element of the list,
3782 passing the remaining elements as arguments to the system call. If
3783 unimplemented, produces a fatal error. The arguments are interpreted
3784 as follows: if a given argument is numeric, the argument is passed as
3785 an int. If not, the pointer to the string value is passed. You are
3786 responsible to make sure a string is pre-extended long enough to
3787 receive any result that might be written into a string. You can't use a
3788 string literal (or other read-only string) as an argument to C<syscall()>
3789 because Perl has to assume that any string pointer might be written
3791 integer arguments are not literals and have never been interpreted in a
3792 numeric context, you may need to add C<0> to them to force them to look
3793 like numbers. This emulates the C<syswrite()> function (or vice versa):
3795 require 'syscall.ph'; # may need to run h2ph
3797 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3799 Note that Perl supports passing of up to only 14 arguments to your system call,
3800 which in practice should usually suffice.
3802 Syscall returns whatever value returned by the system call it calls.
3803 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3804 Note that some system calls can legitimately return C<-1>. The proper
3805 way to handle such calls is to assign C<$!=0;> before the call and
3806 check the value of C<$!> if syscall returns C<-1>.
3808 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3809 number of the read end of the pipe it creates. There is no way
3810 to retrieve the file number of the other end. You can avoid this
3811 problem by using C<pipe()> instead.
3813 =item sysopen FILEHANDLE,FILENAME,MODE
3815 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3817 Opens the file whose filename is given by FILENAME, and associates it
3818 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3819 the name of the real filehandle wanted. This function calls the
3820 underlying operating system's C<open()> function with the parameters
3821 FILENAME, MODE, PERMS.
3823 The possible values and flag bits of the MODE parameter are
3824 system-dependent; they are available via the standard module C<Fcntl>.
3825 However, for historical reasons, some values are universal: zero means
3826 read-only, one means write-only, and two means read/write.
3828 If the file named by FILENAME does not exist and the C<open()> call creates
3829 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3830 PERMS specifies the permissions of the newly created file. If you omit
3831 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
3832 These permission values need to be in octal, and are modified by your
3833 process's current C<umask>. The C<umask> value is a number representing
3834 disabled permissions bits--if your C<umask> were C<027> (group can't write;
3835 others can't read, write, or execute), then passing C<sysopen()> C<0666> would
3836 create a file with mode C<0640> (C<0666 &~ 027> is C<0640>).
3838 If you find this C<umask()> talk confusing, here's some advice: supply a
3839 creation mode of C<0666> for regular files and one of C<0777> for directories
3840 (in C<mkdir()>) and executable files. This gives users the freedom of
3841 choice: if they want protected files, they might choose process umasks
3842 of C<022>, C<027>, or even the particularly antisocial mask of C<077>. Programs
3843 should rarely if ever make policy decisions better left to the user.
3844 The exception to this is when writing files that should be kept private:
3845 mail files, web browser cookies, I<.rhosts> files, and so on. In short,
3846 seldom if ever use C<0644> as argument to C<sysopen()> because that takes
3847 away the user's option to have a more permissive umask. Better to omit it.
3849 The C<IO::File> module provides a more object-oriented approach, if you're
3850 into that kind of thing.
3852 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3854 =item sysread FILEHANDLE,SCALAR,LENGTH
3856 Attempts to read LENGTH bytes of data into variable SCALAR from the
3857 specified FILEHANDLE, using the system call read(2). It bypasses
3858 stdio, so mixing this with other kinds of reads, C<print()>, C<write()>,
3859 C<seek()>, or C<tell()> can cause confusion because stdio usually buffers
3860 data. Returns the number of bytes actually read, C<0> at end of file,
3861 or undef if there was an error. SCALAR will be grown or shrunk so that
3862 the last byte actually read is the last byte of the scalar after the read.
3864 An OFFSET may be specified to place the read data at some place in the
3865 string other than the beginning. A negative OFFSET specifies
3866 placement at that many bytes counting backwards from the end of the
3867 string. A positive OFFSET greater than the length of SCALAR results
3868 in the string being padded to the required size with C<"\0"> bytes before
3869 the result of the read is appended.
3871 =item sysseek FILEHANDLE,POSITION,WHENCE
3873 Sets FILEHANDLE's system position using the system call lseek(2). It
3874 bypasses stdio, so mixing this with reads (other than C<sysread()>),
3875 C<print()>, C<write()>, C<seek()>, or C<tell()> may cause confusion. FILEHANDLE may
3876 be an expression whose value gives the name of the filehandle. The
3877 values for WHENCE are C<0> to set the new position to POSITION, C<1> to set
3878 the it to the current position plus POSITION, and C<2> to set it to EOF
3879 plus POSITION (typically negative). For WHENCE, you may use the
3880 constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable>
3881 or the POSIX module.
3883 Returns the new position, or the undefined value on failure. A position
3884 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
3885 TRUE on success and FALSE on failure, yet you can still easily determine
3890 =item system PROGRAM LIST
3892 Does exactly the same thing as "C<exec LIST>" except that a fork is done
3893 first, and the parent process waits for the child process to complete.
3894 Note that argument processing varies depending on the number of
3895 arguments. If there is more than one argument in LIST, or if LIST is
3896 an array with more than one value, starts the program given by the
3897 first element of the list with arguments given by the rest of the list.
3898 If there is only one scalar argument, the argument is
3899 checked for shell metacharacters, and if there are any, the entire
3900 argument is passed to the system's command shell for parsing (this is
3901 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
3902 there are no shell metacharacters in the argument, it is split into
3903 words and passed directly to C<execvp()>, which is more efficient.
3905 The return value is the exit status of the program as
3906 returned by the C<wait()> call. To get the actual exit value divide by
3907 256. See also L</exec>. This is I<NOT> what you want to use to capture
3908 the output from a command, for that you should use merely backticks or
3909 C<qx//>, as described in L<perlop/"`STRING`">.
3911 Like C<exec()>, C<system()> allows you to lie to a program about its name if
3912 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
3914 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
3915 program they're running doesn't actually interrupt your program.
3917 @args = ("command", "arg1", "arg2");
3919 or die "system @args failed: $?"
3921 You can check all the failure possibilities by inspecting
3924 $exit_value = $? >> 8;
3925 $signal_num = $? & 127;
3926 $dumped_core = $? & 128;
3928 When the arguments get executed via the system shell, results
3929 and return codes will be subject to its quirks and capabilities.
3930 See L<perlop/"`STRING`"> and L</exec> for details.
3932 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3934 =item syswrite FILEHANDLE,SCALAR,LENGTH
3936 Attempts to write LENGTH bytes of data from variable SCALAR to the
3937 specified FILEHANDLE, using the system call write(2). It bypasses
3938 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
3939 C<write()>, C<seek()>, or C<tell()> may cause confusion because stdio usually
3940 buffers data. Returns the number of bytes actually written, or C<undef>
3941 if there was an error. If the LENGTH is greater than the available
3942 data in the SCALAR after the OFFSET, only as much data as is available
3945 An OFFSET may be specified to write the data from some part of the
3946 string other than the beginning. A negative OFFSET specifies writing
3947 that many bytes counting backwards from the end of the string. In the
3948 case the SCALAR is empty you can use OFFSET but only zero offset.
3950 =item tell FILEHANDLE
3954 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3955 expression whose value gives the name of the actual filehandle. If
3956 FILEHANDLE is omitted, assumes the file last read.
3958 =item telldir DIRHANDLE
3960 Returns the current position of the C<readdir()> routines on DIRHANDLE.
3961 Value may be given to C<seekdir()> to access a particular location in a
3962 directory. Has the same caveats about possible directory compaction as
3963 the corresponding system library routine.
3965 =item tie VARIABLE,CLASSNAME,LIST
3967 This function binds a variable to a package class that will provide the
3968 implementation for the variable. VARIABLE is the name of the variable
3969 to be enchanted. CLASSNAME is the name of a class implementing objects
3970 of correct type. Any additional arguments are passed to the "C<new()>"
3971 method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
3972 Typically these are arguments such as might be passed to the C<dbm_open()>
3973 function of C. The object returned by the "C<new()>" method is also
3974 returned by the C<tie()> function, which would be useful if you want to
3975 access other methods in CLASSNAME.
3977 Note that functions such as C<keys()> and C<values()> may return huge lists
3978 when used on large objects, like DBM files. You may prefer to use the
3979 C<each()> function to iterate over such. Example:
3981 # print out history file offsets
3983 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3984 while (($key,$val) = each %HIST) {
3985 print $key, ' = ', unpack('L',$val), "\n";
3989 A class implementing a hash should have the following methods:
3991 TIEHASH classname, LIST
3994 STORE this, key, value
3998 NEXTKEY this, lastkey
4000 A class implementing an ordinary array should have the following methods:
4002 TIEARRAY classname, LIST
4005 STORE this, key, value
4008 A class implementing a scalar should have the following methods:
4010 TIESCALAR classname, LIST
4015 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4016 for you--you need to do that explicitly yourself. See L<DB_File>
4017 or the F<Config> module for interesting C<tie()> implementations.
4019 For further details see L<perltie>, L<tied VARIABLE>.
4023 Returns a reference to the object underlying VARIABLE (the same value
4024 that was originally returned by the C<tie()> call that bound the variable
4025 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4030 Returns the number of non-leap seconds since whatever time the system
4031 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4032 and 00:00:00 UTC, January 1, 1970 for most other systems).
4033 Suitable for feeding to C<gmtime()> and C<localtime()>.
4037 Returns a four-element list giving the user and system times, in
4038 seconds, for this process and the children of this process.
4040 ($user,$system,$cuser,$csystem) = times;
4044 The transliteration operator. Same as C<y///>. See L<perlop>.
4046 =item truncate FILEHANDLE,LENGTH
4048 =item truncate EXPR,LENGTH
4050 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4051 specified length. Produces a fatal error if truncate isn't implemented
4052 on your system. Returns TRUE if successful, the undefined value
4059 Returns an uppercased version of EXPR. This is the internal function
4060 implementing the C<\U> escape in double-quoted strings.
4061 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4063 If EXPR is omitted, uses C<$_>.
4069 Returns the value of EXPR with the first character uppercased. This is
4070 the internal function implementing the C<\u> escape in double-quoted strings.
4071 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4073 If EXPR is omitted, uses C<$_>.
4079 Sets the umask for the process to EXPR and returns the previous value.
4080 If EXPR is omitted, merely returns the current umask.
4082 If umask(2) is not implemented on your system and you are trying to
4083 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4084 fatal error at run time. If umask(2) is not implemented and you are
4085 not trying to restrict access for yourself, returns C<undef>.
4087 Remember that a umask is a number, usually given in octal; it is I<not> a
4088 string of octal digits. See also L</oct>, if all you have is a string.
4094 Undefines the value of EXPR, which must be an lvalue. Use only on a
4095 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4096 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4097 will probably not do what you expect on most predefined variables or
4098 DBM list values, so don't do that; see L<delete>.) Always returns the
4099 undefined value. You can omit the EXPR, in which case nothing is
4100 undefined, but you still get an undefined value that you could, for
4101 instance, return from a subroutine, assign to a variable or pass as a
4102 parameter. Examples:
4105 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4109 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4110 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4111 select undef, undef, undef, 0.25;
4112 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4114 Note that this is a unary operator, not a list operator.
4120 Deletes a list of files. Returns the number of files successfully
4123 $cnt = unlink 'a', 'b', 'c';
4127 Note: C<unlink()> will not delete directories unless you are superuser and
4128 the B<-U> flag is supplied to Perl. Even if these conditions are
4129 met, be warned that unlinking a directory can inflict damage on your
4130 filesystem. Use C<rmdir()> instead.
4132 If LIST is omitted, uses C<$_>.
4134 =item unpack TEMPLATE,EXPR
4136 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4137 structure and expands it out into a list value, returning the array
4138 value. (In scalar context, it returns merely the first value
4139 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4140 Here's a subroutine that does substring:
4143 my($what,$where,$howmuch) = @_;
4144 unpack("x$where a$howmuch", $what);
4149 sub ordinal { unpack("c",$_[0]); } # same as ord()
4151 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4152 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4153 themselves. Default is a 16-bit checksum. For example, the following
4154 computes the same number as the System V sum program:
4157 $checksum += unpack("%16C*", $_);
4161 The following efficiently counts the number of set bits in a bit vector:
4163 $setbits = unpack("%32b*", $selectmask);
4165 =item untie VARIABLE
4167 Breaks the binding between a variable and a package. (See C<tie()>.)
4169 =item unshift ARRAY,LIST
4171 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4172 depending on how you look at it. Prepends list to the front of the
4173 array, and returns the new number of elements in the array.
4175 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4177 Note the LIST is prepended whole, not one element at a time, so the
4178 prepended elements stay in the same order. Use C<reverse()> to do the
4181 =item use Module LIST
4185 =item use Module VERSION LIST
4189 Imports some semantics into the current package from the named module,
4190 generally by aliasing certain subroutine or variable names into your
4191 package. It is exactly equivalent to
4193 BEGIN { require Module; import Module LIST; }
4195 except that Module I<must> be a bareword.
4197 If the first argument to C<use> is a number, it is treated as a version
4198 number instead of a module name. If the version of the Perl interpreter
4199 is less than VERSION, then an error message is printed and Perl exits
4200 immediately. This is often useful if you need to check the current
4201 Perl version before C<use>ing library modules that have changed in
4202 incompatible ways from older versions of Perl. (We try not to do
4203 this more than we have to.)
4205 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4206 C<require> makes sure the module is loaded into memory if it hasn't been
4207 yet. The C<import()> is not a builtin--it's just an ordinary static method
4208 call into the "C<Module>" package to tell the module to import the list of
4209 features back into the current package. The module can implement its
4210 C<import()> method any way it likes, though most modules just choose to
4211 derive their C<import()> method via inheritance from the C<Exporter> class that
4212 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4213 method can be found then the error is currently silently ignored. This
4214 may change to a fatal error in a future version.
4216 If you don't want your namespace altered, explicitly supply an empty list:
4220 That is exactly equivalent to
4222 BEGIN { require Module }
4224 If the VERSION argument is present between Module and LIST, then the
4225 C<use> will call the VERSION method in class Module with the given
4226 version as an argument. The default VERSION method, inherited from
4227 the Universal class, croaks if the given version is larger than the
4228 value of the variable C<$Module::VERSION>. (Note that there is not a
4229 comma after VERSION!)
4231 Because this is a wide-open interface, pragmas (compiler directives)
4232 are also implemented this way. Currently implemented pragmas are:
4236 use sigtrap qw(SEGV BUS);
4237 use strict qw(subs vars refs);
4238 use subs qw(afunc blurfl);
4240 Some of these these pseudo-modules import semantics into the current
4241 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4242 which import symbols into the current package (which are effective
4243 through the end of the file).
4245 There's a corresponding "C<no>" command that unimports meanings imported
4246 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4251 If no C<unimport()> method can be found the call fails with a fatal error.
4253 See L<perlmod> for a list of standard modules and pragmas.
4257 Changes the access and modification times on each file of a list of
4258 files. The first two elements of the list must be the NUMERICAL access
4259 and modification times, in that order. Returns the number of files
4260 successfully changed. The inode modification time of each file is set
4261 to the current time. This code has the same effect as the "C<touch>"
4262 command if the files already exist:
4266 utime $now, $now, @ARGV;
4270 Returns a list consisting of all the values of the named hash. (In a
4271 scalar context, returns the number of values.) The values are
4272 returned in an apparently random order, but it is the same order as
4273 either the C<keys()> or C<each()> function would produce on the same hash.
4274 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4277 =item vec EXPR,OFFSET,BITS
4279 Treats the string in EXPR as a vector of unsigned integers, and
4280 returns the value of the bit field specified by OFFSET. BITS specifies
4281 the number of bits that are reserved for each entry in the bit
4282 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4283 assigned to, in which case parentheses are needed to give the expression
4284 the correct precedence as in
4286 vec($image, $max_x * $x + $y, 8) = 3;
4288 Vectors created with C<vec()> can also be manipulated with the logical
4289 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4290 desired when both operands are strings.
4292 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4293 The comments show the string after each step. Note that this code works
4294 in the same way on big-endian or little-endian machines.
4297 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4298 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4299 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4300 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4301 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4302 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4303 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4305 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4306 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4307 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4310 To transform a bit vector into a string or array of 0's and 1's, use these:
4312 $bits = unpack("b*", $vector);
4313 @bits = split(//, unpack("b*", $vector));
4315 If you know the exact length in bits, it can be used in place of the C<*>.
4319 Waits for a child process to terminate and returns the pid of the
4320 deceased process, or C<-1> if there are no child processes. The status is
4323 =item waitpid PID,FLAGS
4325 Waits for a particular child process to terminate and returns the pid
4326 of the deceased process, or C<-1> if there is no such child process. The
4327 status is returned in C<$?>. If you say
4329 use POSIX ":sys_wait_h";
4331 waitpid(-1,&WNOHANG);
4333 then you can do a non-blocking wait for any process. Non-blocking wait
4334 is available on machines supporting either the waitpid(2) or
4335 wait4(2) system calls. However, waiting for a particular pid with
4336 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4337 by remembering the status values of processes that have exited but have
4338 not been harvested by the Perl script yet.)
4340 See L<perlipc> for other examples.
4344 Returns TRUE if the context of the currently executing subroutine is
4345 looking for a list value. Returns FALSE if the context is looking
4346 for a scalar. Returns the undefined value if the context is looking
4347 for no value (void context).
4349 return unless defined wantarray; # don't bother doing more
4350 my @a = complex_calculation();
4351 return wantarray ? @a : "@a";
4355 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4358 If LIST is empty and C<$@> already contains a value (typically from a
4359 previous eval) that value is used after appending C<"\t...caught">
4360 to C<$@>. This is useful for staying almost, but not entirely similar to
4363 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4365 No message is printed if there is a C<$SIG{__WARN__}> handler
4366 installed. It is the handler's responsibility to deal with the message
4367 as it sees fit (like, for instance, converting it into a C<die()>). Most
4368 handlers must therefore make arrangements to actually display the
4369 warnings that they are not prepared to deal with, by calling C<warn()>
4370 again in the handler. Note that this is quite safe and will not
4371 produce an endless loop, since C<__WARN__> hooks are not called from
4374 You will find this behavior is slightly different from that of
4375 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4376 instead call C<die()> again to change it).
4378 Using a C<__WARN__> handler provides a powerful way to silence all
4379 warnings (even the so-called mandatory ones). An example:
4381 # wipe out *all* compile-time warnings
4382 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4384 my $foo = 20; # no warning about duplicate my $foo,
4385 # but hey, you asked for it!
4386 # no compile-time or run-time warnings before here
4389 # run-time warnings enabled after here
4390 warn "\$foo is alive and $foo!"; # does show up
4392 See L<perlvar> for details on setting C<%SIG> entries, and for more
4395 =item write FILEHANDLE
4401 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4402 using the format associated with that file. By default the format for
4403 a file is the one having the same name as the filehandle, but the
4404 format for the current output channel (see the C<select()> function) may be set
4405 explicitly by assigning the name of the format to the C<$~> variable.
4407 Top of form processing is handled automatically: if there is
4408 insufficient room on the current page for the formatted record, the
4409 page is advanced by writing a form feed, a special top-of-page format
4410 is used to format the new page header, and then the record is written.
4411 By default the top-of-page format is the name of the filehandle with
4412 "_TOP" appended, but it may be dynamically set to the format of your
4413 choice by assigning the name to the C<$^> variable while the filehandle is
4414 selected. The number of lines remaining on the current page is in
4415 variable C<$->, which can be set to C<0> to force a new page.
4417 If FILEHANDLE is unspecified, output goes to the current default output
4418 channel, which starts out as STDOUT but may be changed by the
4419 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4420 is evaluated and the resulting string is used to look up the name of
4421 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4423 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4427 The transliteration operator. Same as C<tr///>. See L<perlop>.