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:
228 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
229 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
231 -r File is readable by effective uid/gid.
232 -w File is writable by effective uid/gid.
233 -x File is executable by effective uid/gid.
234 -o File is owned by effective uid.
236 -R File is readable by real uid/gid.
237 -W File is writable by real uid/gid.
238 -X File is executable by real uid/gid.
239 -O File is owned by real uid.
242 -z File has zero size.
243 -s File has nonzero size (returns size).
245 -f File is a plain file.
246 -d File is a directory.
247 -l File is a symbolic link.
248 -p File is a named pipe (FIFO), or Filehandle is a pipe.
250 -b File is a block special file.
251 -c File is a character special file.
252 -t Filehandle is opened to a tty.
254 -u File has setuid bit set.
255 -g File has setgid bit set.
256 -k File has sticky bit set.
258 -T File is a text file.
259 -B File is a binary file (opposite of -T).
261 -M Age of file in days when script started.
262 -A Same for access time.
263 -C Same for inode change time.
265 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
266 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
267 uids and gids of the user. There may be other reasons you can't actually
268 read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
269 C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return
270 C<1> if any execute bit is set in the mode. Scripts run by the superuser may
271 thus need to do a C<stat()> to determine the actual mode of the
272 file, or temporarily set the uid to something else.
278 next unless -f $_; # ignore specials
282 Note that C<-s/a/b/> does not do a negated substitution. Saying
283 C<-exp($foo)> still works as expected, however--only single letters
284 following a minus are interpreted as file tests.
286 The C<-T> and C<-B> switches work as follows. The first block or so of the
287 file is examined for odd characters such as strange control codes or
288 characters with the high bit set. If too many strange characters (E<gt>30%)
289 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
290 containing null in the first block is considered a binary file. If C<-T>
291 or C<-B> is used on a filehandle, the current stdio buffer is examined
292 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
293 file, or a file at EOF when testing a filehandle. Because you have to
294 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
295 against the file first, as in C<next unless -f $file && -T $file>.
297 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
298 the special filehandle consisting of a solitary underline, then the stat
299 structure of the previous file test (or stat operator) is used, saving
300 a system call. (This doesn't work with C<-t>, and you need to remember
301 that lstat() and C<-l> will leave values in the stat structure for the
302 symbolic link, not the real file.) Example:
304 print "Can do.\n" if -r $a || -w _ || -x _;
307 print "Readable\n" if -r _;
308 print "Writable\n" if -w _;
309 print "Executable\n" if -x _;
310 print "Setuid\n" if -u _;
311 print "Setgid\n" if -g _;
312 print "Sticky\n" if -k _;
313 print "Text\n" if -T _;
314 print "Binary\n" if -B _;
320 Returns the absolute value of its argument.
321 If VALUE is omitted, uses C<$_>.
323 =item accept NEWSOCKET,GENERICSOCKET
325 Accepts an incoming socket connect, just as the accept(2) system call
326 does. Returns the packed address if it succeeded, FALSE otherwise.
327 See example in L<perlipc/"Sockets: Client/Server Communication">.
333 Arranges to have a SIGALRM delivered to this process after the
334 specified number of seconds have elapsed. If SECONDS is not specified,
335 the value stored in C<$_> is used. (On some machines,
336 unfortunately, the elapsed time may be up to one second less than you
337 specified because of how seconds are counted.) Only one timer may be
338 counting at once. Each call disables the previous timer, and an
339 argument of C<0> may be supplied to cancel the previous timer without
340 starting a new one. The returned value is the amount of time remaining
341 on the previous timer.
343 For delays of finer granularity than one second, you may use Perl's
344 C<syscall()> interface to access setitimer(2) if your system supports it,
345 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
346 and C<sleep()> calls.
348 If you want to use C<alarm()> to time out a system call you need to use an
349 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
350 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
351 restart system calls on some systems. Using C<eval()>/C<die()> always works,
352 modulo the caveats given in L<perlipc/"Signals">.
355 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
357 $nread = sysread SOCKET, $buffer, $size;
361 die unless $@ eq "alarm\n"; # propagate unexpected errors
370 Returns the arctangent of Y/X in the range -PI to PI.
372 For the tangent operation, you may use the C<POSIX::tan()>
373 function, or use the familiar relation:
375 sub tan { sin($_[0]) / cos($_[0]) }
377 =item bind SOCKET,NAME
379 Binds a network address to a socket, just as the bind system call
380 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
381 packed address of the appropriate type for the socket. See the examples in
382 L<perlipc/"Sockets: Client/Server Communication">.
384 =item binmode FILEHANDLE
386 Arranges for the file to be read or written in "binary" mode in operating
387 systems that distinguish between binary and text files. Files that are
388 not in binary mode have CR LF sequences translated to LF on input and LF
389 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
390 and similarly archaic systems, it may be imperative--otherwise your
391 MS-DOS-damaged C library may mangle your file. The key distinction between
392 systems that need C<binmode()> and those that don't is their text file
393 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
394 character, and that encode that character in C as C<"\n">, do not need
395 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
396 is taken as the name of the filehandle.
398 =item bless REF,CLASSNAME
402 This function tells the thingy referenced by REF that it is now
403 an object in the CLASSNAME package--or the current package if no CLASSNAME
404 is specified, which is often the case. It returns the reference for
405 convenience, because a C<bless()> is often the last thing in a constructor.
406 Always use the two-argument version if the function doing the blessing
407 might be inherited by a derived class. See L<perltoot> and L<perlobj>
408 for more about the blessing (and blessings) of objects.
414 Returns the context of the current subroutine call. In scalar context,
415 returns the caller's package name if there is a caller, that is, if
416 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
417 otherwise. In list context, returns
419 ($package, $filename, $line) = caller;
421 With EXPR, it returns some extra information that the debugger uses to
422 print a stack trace. The value of EXPR indicates how many call frames
423 to go back before the current one.
425 ($package, $filename, $line, $subroutine,
426 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
428 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
429 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
430 C<$is_require> are set: C<$is_require> is true if the frame is created by a
431 C<require> or C<use> statement, C<$evaltext> contains the text of the
432 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
433 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
434 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
437 Furthermore, when called from within the DB package, caller returns more
438 detailed information: it sets the list variable C<@DB::args> to be the
439 arguments with which the subroutine was invoked.
441 Be aware that the optimizer might have optimized call frames away before
442 C<caller()> had a chance to get the information. That means that C<caller(N)>
443 might not return information about the call frame you expect it do, for
444 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
445 previous time C<caller()> was called.
449 Changes the working directory to EXPR, if possible. If EXPR is
450 omitted, changes to home directory. Returns TRUE upon success, FALSE
451 otherwise. See example under C<die()>.
455 Changes the permissions of a list of files. The first element of the
456 list must be the numerical mode, which should probably be an octal
457 number, and which definitely should I<not> a string of octal digits:
458 C<0644> is okay, C<'0644'> is not. Returns the number of files
459 successfully changed. See also L</oct>, if all you have is a string.
461 $cnt = chmod 0755, 'foo', 'bar';
462 chmod 0755, @executables;
463 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
465 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
466 $mode = 0644; chmod $mode, 'foo'; # this is best
474 This is a slightly safer version of L</chop>. It removes any
475 line ending that corresponds to the current value of C<$/> (also known as
476 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
477 number of characters removed from all its arguments. It's often used to
478 remove the newline from the end of an input record when you're worried
479 that the final record may be missing its newline. When in paragraph mode
480 (C<$/ = "">), it removes all trailing newlines from the string. If
481 VARIABLE is omitted, it chomps C<$_>. Example:
484 chomp; # avoid \n on last field
489 You can actually chomp anything that's an lvalue, including an assignment:
492 chomp($answer = <STDIN>);
494 If you chomp a list, each element is chomped, and the total number of
495 characters removed is returned.
503 Chops off the last character of a string and returns the character
504 chopped. It's used primarily to remove the newline from the end of an
505 input record, but is much more efficient than C<s/\n//> because it neither
506 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
510 chop; # avoid \n on last field
515 You can actually chop anything that's an lvalue, including an assignment:
518 chop($answer = <STDIN>);
520 If you chop a list, each element is chopped. Only the value of the
521 last C<chop()> is returned.
523 Note that C<chop()> returns the last character. To return all but the last
524 character, use C<substr($string, 0, -1)>.
528 Changes the owner (and group) of a list of files. The first two
529 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
530 Returns the number of files successfully changed.
532 $cnt = chown $uid, $gid, 'foo', 'bar';
533 chown $uid, $gid, @filenames;
535 Here's an example that looks up nonnumeric uids in the passwd file:
538 chop($user = <STDIN>);
540 chop($pattern = <STDIN>);
542 ($login,$pass,$uid,$gid) = getpwnam($user)
543 or die "$user not in passwd file";
545 @ary = glob($pattern); # expand filenames
546 chown $uid, $gid, @ary;
548 On most systems, you are not allowed to change the ownership of the
549 file unless you're the superuser, although you should be able to change
550 the group to any of your secondary groups. On insecure systems, these
551 restrictions may be relaxed, but this is not a portable assumption.
557 Returns the character represented by that NUMBER in the character set.
558 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
559 chr(0x263a) is a Unicode smiley face (but only within the scope of a
560 C<use utf8>). For the reverse, use L</ord>.
562 If NUMBER is omitted, uses C<$_>.
564 =item chroot FILENAME
568 This function works like the system call by the same name: it makes the
569 named directory the new root directory for all further pathnames that
570 begin with a C<"/"> by your process and all its children. (It doesn't
571 change your current working directory, which is unaffected.) For security
572 reasons, this call is restricted to the superuser. If FILENAME is
573 omitted, does a C<chroot()> to C<$_>.
575 =item close FILEHANDLE
579 Closes the file or pipe associated with the file handle, returning TRUE
580 only if stdio successfully flushes buffers and closes the system file
581 descriptor. Closes the currently selected filehandle if the argument
584 You don't have to close FILEHANDLE if you are immediately going to do
585 another C<open()> on it, because C<open()> will close it for you. (See
586 C<open()>.) However, an explicit C<close()> on an input file resets the line
587 counter (C<$.>), while the implicit close done by C<open()> does not.
589 If the file handle came from a piped open C<close()> will additionally
590 return FALSE if one of the other system calls involved fails or if the
591 program exits with non-zero status. (If the only problem was that the
592 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
593 waits for the process executing on the pipe to complete, in case you
594 want to look at the output of the pipe afterwards. Closing a pipe
595 explicitly also puts the exit status value of the command into C<$?>.
599 open(OUTPUT, '|sort >foo') # pipe to sort
600 or die "Can't start sort: $!";
601 #... # print stuff to output
602 close OUTPUT # wait for sort to finish
603 or warn $! ? "Error closing sort pipe: $!"
604 : "Exit status $? from sort";
605 open(INPUT, 'foo') # get sort's results
606 or die "Can't open 'foo' for input: $!";
608 FILEHANDLE may be an expression whose value can be used as an indirect
609 filehandle, usually the real filehandle name.
611 =item closedir DIRHANDLE
613 Closes a directory opened by C<opendir()> and returns the success of that
616 DIRHANDLE may be an expression whose value can be used as an indirect
617 dirhandle, usually the real dirhandle name.
619 =item connect SOCKET,NAME
621 Attempts to connect to a remote socket, just as the connect system call
622 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
623 packed address of the appropriate type for the socket. See the examples in
624 L<perlipc/"Sockets: Client/Server Communication">.
628 Actually a flow control statement rather than a function. If there is a
629 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
630 C<foreach>), it is always executed just before the conditional is about to
631 be evaluated again, just like the third part of a C<for> loop in C. Thus
632 it can be used to increment a loop variable, even when the loop has been
633 continued via the C<next> statement (which is similar to the C C<continue>
636 C<last>, C<next>, or C<redo> may appear within a C<continue>
637 block. C<last> and C<redo> will behave as if they had been executed within
638 the main block. So will C<next>, but since it will execute a C<continue>
639 block, it may be more entertaining.
642 ### redo always comes here
645 ### next always comes here
647 # then back the top to re-check EXPR
649 ### last always comes here
651 Omitting the C<continue> section is semantically equivalent to using an
652 empty one, logically enough. In that case, C<next> goes directly back
653 to check the condition at the top of the loop.
657 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
658 takes cosine of C<$_>.
660 For the inverse cosine operation, you may use the C<POSIX::acos()>
661 function, or use this relation:
663 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
665 =item crypt PLAINTEXT,SALT
667 Encrypts a string exactly like the crypt(3) function in the C library
668 (assuming that you actually have a version there that has not been
669 extirpated as a potential munition). This can prove useful for checking
670 the password file for lousy passwords, amongst other things. Only the
671 guys wearing white hats should do this.
673 Note that C<crypt()> is intended to be a one-way function, much like breaking
674 eggs to make an omelette. There is no (known) corresponding decrypt
675 function. As a result, this function isn't all that useful for
676 cryptography. (For that, see your nearby CPAN mirror.)
678 Here's an example that makes sure that whoever runs this program knows
681 $pwd = (getpwuid($<))[1];
682 $salt = substr($pwd, 0, 2);
686 chop($word = <STDIN>);
690 if (crypt($word, $salt) ne $pwd) {
696 Of course, typing in your own password to whoever asks you
701 [This function has been superseded by the C<untie()> function.]
703 Breaks the binding between a DBM file and a hash.
705 =item dbmopen HASH,DBNAME,MODE
707 [This function has been superseded by the C<tie()> function.]
709 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
710 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
711 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
712 is the name of the database (without the F<.dir> or F<.pag> extension if
713 any). If the database does not exist, it is created with protection
714 specified by MODE (as modified by the C<umask()>). If your system supports
715 only the older DBM functions, you may perform only one C<dbmopen()> in your
716 program. In older versions of Perl, if your system had neither DBM nor
717 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
720 If you don't have write access to the DBM file, you can only read hash
721 variables, not set them. If you want to test whether you can write,
722 either use file tests or try setting a dummy hash entry inside an C<eval()>,
723 which will trap the error.
725 Note that functions such as C<keys()> and C<values()> may return huge lists
726 when used on large DBM files. You may prefer to use the C<each()>
727 function to iterate over large DBM files. Example:
729 # print out history file offsets
730 dbmopen(%HIST,'/usr/lib/news/history',0666);
731 while (($key,$val) = each %HIST) {
732 print $key, ' = ', unpack('L',$val), "\n";
736 See also L<AnyDBM_File> for a more general description of the pros and
737 cons of the various dbm approaches, as well as L<DB_File> for a particularly
744 Returns a Boolean value telling whether EXPR has a value other than
745 the undefined value C<undef>. If EXPR is not present, C<$_> will be
748 Many operations return C<undef> to indicate failure, end of file,
749 system error, uninitialized variable, and other exceptional
750 conditions. This function allows you to distinguish C<undef> from
751 other values. (A simple Boolean test will not distinguish among
752 C<undef>, zero, the empty string, and C<"0">, which are all equally
753 false.) Note that since C<undef> is a valid scalar, its presence
754 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
755 returns C<undef> when its argument is an empty array, I<or> when the
756 element to return happens to be C<undef>.
758 You may also use C<defined()> to check whether a subroutine exists, by
759 saying C<defined &func> without parentheses. On the other hand, use
760 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
761 produce intuitive results, and should probably be avoided.
763 When used on a hash element, it tells you whether the value is defined,
764 not whether the key exists in the hash. Use L</exists> for the latter
769 print if defined $switch{'D'};
770 print "$val\n" while defined($val = pop(@ary));
771 die "Can't readlink $sym: $!"
772 unless defined($value = readlink $sym);
773 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
774 $debugging = 0 unless defined $debugging;
776 Note: Many folks tend to overuse C<defined()>, and then are surprised to
777 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
778 defined values. For example, if you say
782 The pattern match succeeds, and C<$1> is defined, despite the fact that it
783 matched "nothing". But it didn't really match nothing--rather, it
784 matched something that happened to be C<0> characters long. This is all
785 very above-board and honest. When a function returns an undefined value,
786 it's an admission that it couldn't give you an honest answer. So you
787 should use C<defined()> only when you're questioning the integrity of what
788 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
791 Currently, using C<defined()> on an entire array or hash reports whether
792 memory for that aggregate has ever been allocated. So an array you set
793 to the empty list appears undefined initially, and one that once was full
794 and that you then set to the empty list still appears defined. You
795 should instead use a simple test for size:
797 if (@an_array) { print "has array elements\n" }
798 if (%a_hash) { print "has hash members\n" }
800 Using C<undef()> on these, however, does clear their memory and then report
801 them as not defined anymore, but you shouldn't do that unless you don't
802 plan to use them again, because it saves time when you load them up
803 again to have memory already ready to be filled. The normal way to
804 free up space used by an aggregate is to assign the empty list.
806 This counterintuitive behavior of C<defined()> on aggregates may be
807 changed, fixed, or broken in a future release of Perl.
809 See also L</undef>, L</exists>, L</ref>.
813 Deletes the specified key(s) and their associated values from a hash.
814 For each key, returns the deleted value associated with that key, or
815 the undefined value if there was no such key. Deleting from C<$ENV{}>
816 modifies the environment. Deleting from a hash tied to a DBM file
817 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
818 doesn't necessarily return anything.)
820 The following deletes all the values of a hash:
822 foreach $key (keys %HASH) {
828 delete @HASH{keys %HASH}
830 (But both of these are slower than just assigning the empty list, or
831 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
832 long as the final operation is a hash element lookup or hash slice:
834 delete $ref->[$x][$y]{$key};
835 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
839 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
840 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
841 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
842 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
843 C<$@> and the C<eval()> is terminated with the undefined value. This makes
844 C<die()> the way to raise an exception.
848 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
849 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
851 If the value of EXPR does not end in a newline, the current script line
852 number and input line number (if any) are also printed, and a newline
853 is supplied. Hint: sometimes appending C<", stopped"> to your message
854 will cause it to make better sense when the string C<"at foo line 123"> is
855 appended. Suppose you are running script "canasta".
857 die "/etc/games is no good";
858 die "/etc/games is no good, stopped";
860 produce, respectively
862 /etc/games is no good at canasta line 123.
863 /etc/games is no good, stopped at canasta line 123.
865 See also C<exit()> and C<warn()>.
867 If LIST is empty and C<$@> already contains a value (typically from a
868 previous eval) that value is reused after appending C<"\t...propagated">.
869 This is useful for propagating exceptions:
872 die unless $@ =~ /Expected exception/;
874 If C<$@> is empty then the string C<"Died"> is used.
876 You can arrange for a callback to be run just before the C<die()> does
877 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
878 will be called with the error text and can change the error message, if
879 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
880 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
882 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
883 blocks/strings. If one wants the hook to do nothing in such
888 as the first line of the handler (see L<perlvar/$^S>).
892 Not really a function. Returns the value of the last command in the
893 sequence of commands indicated by BLOCK. When modified by a loop
894 modifier, executes the BLOCK once before testing the loop condition.
895 (On other statements the loop modifiers test the conditional first.)
897 =item do SUBROUTINE(LIST)
899 A deprecated form of subroutine call. See L<perlsub>.
903 Uses the value of EXPR as a filename and executes the contents of the
904 file as a Perl script. Its primary use is to include subroutines
905 from a Perl subroutine library.
911 scalar eval `cat stat.pl`;
913 except that it's more efficient and concise, keeps track of the
914 current filename for error messages, and searches all the B<-I>
915 libraries if the file isn't in the current directory (see also the @INC
916 array in L<perlvar/Predefined Names>). It is also different in how
917 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
918 scope like C<eval STRING> does. It's the same, however, in that it does
919 reparse the file every time you call it, so you probably don't want to
920 do this inside a loop.
922 If C<do> cannot read the file, it returns undef and sets C<$!> to the
923 error. If C<do> can read the file but cannot compile it, it
924 returns undef and sets an error message in C<$@>. If the file is
925 successfully compiled, C<do> returns the value of the last expression
928 Note that inclusion of library modules is better done with the
929 C<use()> and C<require()> operators, which also do automatic error checking
930 and raise an exception if there's a problem.
932 You might like to use C<do> to read in a program configuration
933 file. Manual error checking can be done this way:
935 # read in config files: system first, then user
936 for $file ("/share/prog/defaults.rc",
937 "$ENV{HOME}/.someprogrc") {
938 unless ($return = do $file) {
939 warn "couldn't parse $file: $@" if $@;
940 warn "couldn't do $file: $!" unless defined $return;
941 warn "couldn't run $file" unless $return;
947 This causes an immediate core dump. Primarily this is so that you can
948 use the B<undump> program to turn your core dump into an executable binary
949 after having initialized all your variables at the beginning of the
950 program. When the new binary is executed it will begin by executing a
951 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
952 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
953 is omitted, restarts the program from the top. WARNING: Any files
954 opened at the time of the dump will NOT be open any more when the
955 program is reincarnated, with possible resulting confusion on the part
956 of Perl. See also B<-u> option in L<perlrun>.
973 dump QUICKSTART if $ARGV[0] eq '-d';
978 This operator is largely obsolete, partly because it's very hard to
979 convert a core file into an executable, and because the real perl-to-C
980 compiler has superseded it.
984 When called in list context, returns a 2-element list consisting of the
985 key and value for the next element of a hash, so that you can iterate over
986 it. When called in scalar context, returns the key for only the "next"
987 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
988 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
991 Entries are returned in an apparently random order. When the hash is
992 entirely read, a null array is returned in list context (which when
993 assigned produces a FALSE (C<0>) value), and C<undef> in
994 scalar context. The next call to C<each()> after that will start iterating
995 again. There is a single iterator for each hash, shared by all C<each()>,
996 C<keys()>, and C<values()> function calls in the program; it can be reset by
997 reading all the elements from the hash, or by evaluating C<keys HASH> or
998 C<values HASH>. If you add or delete elements of a hash while you're
999 iterating over it, you may get entries skipped or duplicated, so don't.
1001 The following prints out your environment like the printenv(1) program,
1002 only in a different order:
1004 while (($key,$value) = each %ENV) {
1005 print "$key=$value\n";
1008 See also C<keys()> and C<values()>.
1010 =item eof FILEHANDLE
1016 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1017 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1018 gives the real filehandle. (Note that this function actually
1019 reads a character and then C<ungetc()>s it, so isn't very useful in an
1020 interactive context.) Do not read from a terminal file (or call
1021 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1022 as terminals may lose the end-of-file condition if you do.
1024 An C<eof> without an argument uses the last file read as argument.
1025 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1026 the files listed on the command line, i.e., C<eof()> is reasonable to
1027 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1028 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1029 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1031 # reset line numbering on each input file
1033 next if /^\s*#/; # skip comments
1036 close ARGV if eof; # Not eof()!
1039 # insert dashes just before last line of last file
1041 if (eof()) { # check for end of current file
1042 print "--------------\n";
1043 close(ARGV); # close or break; is needed if we
1044 # are reading from the terminal
1049 Practical hint: you almost never need to use C<eof> in Perl, because the
1050 input operators return false values when they run out of data, or if there
1057 In the first form, the return value of EXPR is parsed and executed as if it
1058 were a little Perl program. The value of the expression (which is itself
1059 determined within scalar context) is first parsed, and if there weren't any
1060 errors, executed in the context of the current Perl program, so that any
1061 variable settings or subroutine and format definitions remain afterwards.
1062 Note that the value is parsed every time the eval executes. If EXPR is
1063 omitted, evaluates C<$_>. This form is typically used to delay parsing
1064 and subsequent execution of the text of EXPR until run time.
1066 In the second form, the code within the BLOCK is parsed only once--at the
1067 same time the code surrounding the eval itself was parsed--and executed
1068 within the context of the current Perl program. This form is typically
1069 used to trap exceptions more efficiently than the first (see below), while
1070 also providing the benefit of checking the code within BLOCK at compile
1073 The final semicolon, if any, may be omitted from the value of EXPR or within
1076 In both forms, the value returned is the value of the last expression
1077 evaluated inside the mini-program; a return statement may be also used, just
1078 as with subroutines. The expression providing the return value is evaluated
1079 in void, scalar, or list context, depending on the context of the eval itself.
1080 See L</wantarray> for more on how the evaluation context can be determined.
1082 If there is a syntax error or runtime error, or a C<die()> statement is
1083 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1084 error message. If there was no error, C<$@> is guaranteed to be a null
1085 string. Beware that using C<eval()> neither silences perl from printing
1086 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1087 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1088 L</warn> and L<perlvar>.
1090 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1091 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1092 is implemented. It is also Perl's exception trapping mechanism, where
1093 the die operator is used to raise exceptions.
1095 If the code to be executed doesn't vary, you may use the eval-BLOCK
1096 form to trap run-time errors without incurring the penalty of
1097 recompiling each time. The error, if any, is still returned in C<$@>.
1100 # make divide-by-zero nonfatal
1101 eval { $answer = $a / $b; }; warn $@ if $@;
1103 # same thing, but less efficient
1104 eval '$answer = $a / $b'; warn $@ if $@;
1106 # a compile-time error
1107 eval { $answer = }; # WRONG
1110 eval '$answer ='; # sets $@
1112 When using the C<eval{}> form as an exception trap in libraries, you may
1113 wish not to trigger any C<__DIE__> hooks that user code may have
1114 installed. You can use the C<local $SIG{__DIE__}> construct for this
1115 purpose, as shown in this example:
1117 # a very private exception trap for divide-by-zero
1118 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1121 This is especially significant, given that C<__DIE__> hooks can call
1122 C<die()> again, which has the effect of changing their error messages:
1124 # __DIE__ hooks may modify error messages
1126 local $SIG{'__DIE__'} =
1127 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1128 eval { die "foo lives here" };
1129 print $@ if $@; # prints "bar lives here"
1132 With an C<eval()>, you should be especially careful to remember what's
1133 being looked at when:
1139 eval { $x }; # CASE 4
1141 eval "\$$x++"; # CASE 5
1144 Cases 1 and 2 above behave identically: they run the code contained in
1145 the variable C<$x>. (Although case 2 has misleading double quotes making
1146 the reader wonder what else might be happening (nothing is).) Cases 3
1147 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1148 does nothing but return the value of C<$x>. (Case 4 is preferred for
1149 purely visual reasons, but it also has the advantage of compiling at
1150 compile-time instead of at run-time.) Case 5 is a place where
1151 normally you I<WOULD> like to use double quotes, except that in this
1152 particular situation, you can just use symbolic references instead, as
1157 =item exec PROGRAM LIST
1159 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1160 use C<system()> instead of C<exec()> if you want it to return. It fails and
1161 returns FALSE only if the command does not exist I<and> it is executed
1162 directly instead of via your system's command shell (see below).
1164 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1165 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1166 or C<exit()> (if C<-w> is set - but you always do that). If you
1167 I<really> want to follow an C<exec()> with some other statement, you
1168 can use one of these styles to avoid the warning:
1170 exec ('foo') or print STDERR "couldn't exec foo: $!";
1171 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1173 If there is more than one argument in LIST, or if LIST is an array
1174 with more than one value, calls execvp(3) with the arguments in LIST.
1175 If there is only one scalar argument or an array with one element in it,
1176 the argument is checked for shell metacharacters, and if there are any,
1177 the entire argument is passed to the system's command shell for parsing
1178 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1179 If there are no shell metacharacters in the argument, it is split into
1180 words and passed directly to C<execvp()>, which is more efficient. Note:
1181 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1182 set C<$|> to avoid lost output. Examples:
1184 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1185 exec "sort $outfile | uniq";
1187 If you don't really want to execute the first argument, but want to lie
1188 to the program you are executing about its own name, you can specify
1189 the program you actually want to run as an "indirect object" (without a
1190 comma) in front of the LIST. (This always forces interpretation of the
1191 LIST as a multivalued list, even if there is only a single scalar in
1194 $shell = '/bin/csh';
1195 exec $shell '-sh'; # pretend it's a login shell
1199 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1201 When the arguments get executed via the system shell, results will
1202 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1205 Using an indirect object with C<exec()> or C<system()> is also more secure.
1206 This usage forces interpretation of the arguments as a multivalued list,
1207 even if the list had just one argument. That way you're safe from the
1208 shell expanding wildcards or splitting up words with whitespace in them.
1210 @args = ( "echo surprise" );
1212 system @args; # subject to shell escapes
1214 system { $args[0] } @args; # safe even with one-arg list
1216 The first version, the one without the indirect object, ran the I<echo>
1217 program, passing it C<"surprise"> an argument. The second version
1218 didn't--it tried to run a program literally called I<"echo surprise">,
1219 didn't find it, and set C<$?> to a non-zero value indicating failure.
1221 Note that C<exec()> will not call your C<END> blocks, nor will it call
1222 any C<DESTROY> methods in your objects.
1226 Returns TRUE if the specified hash key exists in its hash array, even
1227 if the corresponding value is undefined.
1229 print "Exists\n" if exists $array{$key};
1230 print "Defined\n" if defined $array{$key};
1231 print "True\n" if $array{$key};
1233 A hash element can be TRUE only if it's defined, and defined if
1234 it exists, but the reverse doesn't necessarily hold true.
1236 Note that the EXPR can be arbitrarily complicated as long as the final
1237 operation is a hash key lookup:
1239 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1241 Although the last element will not spring into existence just because its
1242 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1243 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1244 test for a $key element. This autovivification may be fixed in a later
1249 Evaluates EXPR and exits immediately with that value. (Actually, it
1250 calls any defined C<END> routines first, but the C<END> routines may not
1251 abort the exit. Likewise any object destructors that need to be called
1252 are called before exit.) Example:
1255 exit 0 if $ans =~ /^[Xx]/;
1257 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1258 universally portable values for EXPR are C<0> for success and C<1> for error;
1259 all other values are subject to unpredictable interpretation depending
1260 on the environment in which the Perl program is running.
1262 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1263 someone might want to trap whatever error happened. Use C<die()> instead,
1264 which can be trapped by an C<eval()>.
1266 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1272 Returns I<e> (the natural logarithm base) to the power of EXPR.
1273 If EXPR is omitted, gives C<exp($_)>.
1275 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1277 Implements the fcntl(2) function. You'll probably have to say
1281 first to get the correct constant definitions. Argument processing and
1282 value return works just like C<ioctl()> below.
1286 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1287 or die "can't fcntl F_GETFL: $!";
1289 You don't have to check for C<defined()> on the return from
1290 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1291 call into "C<0> but true" in Perl. This string is true in
1292 boolean context and C<0> in numeric context. It is also
1293 exempt from the normal B<-w> warnings on improper numeric
1296 Note that C<fcntl()> will produce a fatal error if used on a machine that
1297 doesn't implement fcntl(2).
1299 =item fileno FILEHANDLE
1301 Returns the file descriptor for a filehandle. This is useful for
1302 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1303 operations. If FILEHANDLE is an expression, the value is taken as
1304 an indirect filehandle, generally its name.
1306 You can use this to find out whether two handles refer to the
1307 same underlying descriptor:
1309 if (fileno(THIS) == fileno(THAT)) {
1310 print "THIS and THAT are dups\n";
1313 =item flock FILEHANDLE,OPERATION
1315 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1316 success, FALSE on failure. Produces a fatal error if used on a machine
1317 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1318 is Perl's portable file locking interface, although it locks only entire
1321 On many platforms (including most versions or clones of Unix), locks
1322 established by C<flock()> are B<merely advisory>. Such discretionary locks
1323 are more flexible, but offer fewer guarantees. This means that files
1324 locked with C<flock()> may be modified by programs that do not also use
1325 C<flock()>. Windows NT and OS/2 are among the platforms which
1326 enforce mandatory locking. See your local documentation for details.
1328 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1329 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1330 you can use the symbolic names if import them from the Fcntl module,
1331 either individually, or as a group using the ':flock' tag. LOCK_SH
1332 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1333 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1334 LOCK_EX then C<flock()> will return immediately rather than blocking
1335 waiting for the lock (check the return status to see if you got it).
1337 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1338 before (un)locking it.
1340 Note that the emulation built with lockf(3) doesn't provide shared
1341 locks, and it requires that FILEHANDLE be open with write intent. These
1342 are the semantics that lockf(3) implements. Most (all?) systems
1343 implement lockf(3) in terms of fcntl(2) locking, though, so the
1344 differing semantics shouldn't bite too many people.
1346 Note also that some versions of C<flock()> cannot lock things over the
1347 network; you would need to use the more system-specific C<fcntl()> for
1348 that. If you like you can force Perl to ignore your system's flock(2)
1349 function, and so provide its own fcntl(2)-based emulation, by passing
1350 the switch C<-Ud_flock> to the F<Configure> program when you configure
1353 Here's a mailbox appender for BSD systems.
1355 use Fcntl ':flock'; # import LOCK_* constants
1358 flock(MBOX,LOCK_EX);
1359 # and, in case someone appended
1360 # while we were waiting...
1365 flock(MBOX,LOCK_UN);
1368 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1369 or die "Can't open mailbox: $!";
1372 print MBOX $msg,"\n\n";
1375 See also L<DB_File> for other flock() examples.
1379 Does a fork(2) system call. Returns the child pid to the parent process,
1380 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1382 Note: unflushed buffers remain unflushed in both processes, which means
1383 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1384 method of C<IO::Handle> to avoid duplicate output.
1386 If you C<fork()> without ever waiting on your children, you will accumulate
1389 $SIG{CHLD} = sub { wait };
1391 There's also the double-fork trick (error checking on
1392 C<fork()> returns omitted);
1394 unless ($pid = fork) {
1396 exec "what you really wanna do";
1399 ## (some_perl_code_here)
1406 See also L<perlipc> for more examples of forking and reaping
1409 Note that if your forked child inherits system file descriptors like
1410 STDIN and STDOUT that are actually connected by a pipe or socket, even
1411 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1412 you're done. You should reopen those to F</dev/null> if it's any issue.
1416 Declare a picture format for use by the C<write()> function. For
1420 Test: @<<<<<<<< @||||| @>>>>>
1421 $str, $%, '$' . int($num)
1425 $num = $cost/$quantity;
1429 See L<perlform> for many details and examples.
1431 =item formline PICTURE,LIST
1433 This is an internal function used by C<format>s, though you may call it,
1434 too. It formats (see L<perlform>) a list of values according to the
1435 contents of PICTURE, placing the output into the format output
1436 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1437 Eventually, when a C<write()> is done, the contents of
1438 C<$^A> are written to some filehandle, but you could also read C<$^A>
1439 yourself and then set C<$^A> back to C<"">. Note that a format typically
1440 does one C<formline()> per line of form, but the C<formline()> function itself
1441 doesn't care how many newlines are embedded in the PICTURE. This means
1442 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1443 You may therefore need to use multiple formlines to implement a single
1444 record format, just like the format compiler.
1446 Be careful if you put double quotes around the picture, because an "C<@>"
1447 character may be taken to mean the beginning of an array name.
1448 C<formline()> always returns TRUE. See L<perlform> for other examples.
1450 =item getc FILEHANDLE
1454 Returns the next character from the input file attached to FILEHANDLE,
1455 or the undefined value at end of file, or if there was an error. If
1456 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1457 efficient. It cannot be used to get unbuffered single-characters,
1458 however. For that, try something more like:
1461 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1464 system "stty", '-icanon', 'eol', "\001";
1470 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1473 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1477 Determination of whether $BSD_STYLE should be set
1478 is left as an exercise to the reader.
1480 The C<POSIX::getattr()> function can do this more portably on systems
1481 purporting POSIX compliance.
1482 See also the C<Term::ReadKey> module from your nearest CPAN site;
1483 details on CPAN can be found on L<perlmod/CPAN>.
1487 Implements the C library function of the same name, which on most
1488 systems returns the current login from F</etc/utmp>, if any. If null,
1491 $login = getlogin || getpwuid($<) || "Kilroy";
1493 Do not consider C<getlogin()> for authentication: it is not as
1494 secure as C<getpwuid()>.
1496 =item getpeername SOCKET
1498 Returns the packed sockaddr address of other end of the SOCKET connection.
1501 $hersockaddr = getpeername(SOCK);
1502 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1503 $herhostname = gethostbyaddr($iaddr, AF_INET);
1504 $herstraddr = inet_ntoa($iaddr);
1508 Returns the current process group for the specified PID. Use
1509 a PID of C<0> to get the current process group for the
1510 current process. Will raise an exception if used on a machine that
1511 doesn't implement getpgrp(2). If PID is omitted, returns process
1512 group of current process. Note that the POSIX version of C<getpgrp()>
1513 does not accept a PID argument, so only C<PID==0> is truly portable.
1517 Returns the process id of the parent process.
1519 =item getpriority WHICH,WHO
1521 Returns the current priority for a process, a process group, or a user.
1522 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1523 machine that doesn't implement getpriority(2).
1529 =item gethostbyname NAME
1531 =item getnetbyname NAME
1533 =item getprotobyname NAME
1539 =item getservbyname NAME,PROTO
1541 =item gethostbyaddr ADDR,ADDRTYPE
1543 =item getnetbyaddr ADDR,ADDRTYPE
1545 =item getprotobynumber NUMBER
1547 =item getservbyport PORT,PROTO
1565 =item sethostent STAYOPEN
1567 =item setnetent STAYOPEN
1569 =item setprotoent STAYOPEN
1571 =item setservent STAYOPEN
1585 These routines perform the same functions as their counterparts in the
1586 system library. In list context, the return values from the
1587 various get routines are as follows:
1589 ($name,$passwd,$uid,$gid,
1590 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1591 ($name,$passwd,$gid,$members) = getgr*
1592 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1593 ($name,$aliases,$addrtype,$net) = getnet*
1594 ($name,$aliases,$proto) = getproto*
1595 ($name,$aliases,$port,$proto) = getserv*
1597 (If the entry doesn't exist you get a null list.)
1599 In scalar context, you get the name, unless the function was a
1600 lookup by name, in which case you get the other thing, whatever it is.
1601 (If the entry doesn't exist you get the undefined value.) For example:
1603 $uid = getpwnam($name);
1604 $name = getpwuid($num);
1606 $gid = getgrnam($name);
1607 $name = getgrgid($num;
1611 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1612 cases in the sense that in many systems they are unsupported. If the
1613 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1614 usually encodes the disk quota. If the C<$comment> field is unsupported,
1615 it is an empty scalar. If it is supported it usually encodes some
1616 administrative comment about the user. In some systems the $quota
1617 field may be C<$change> or C<$age>, fields that have to do with password
1618 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1619 field, if present, encodes the expiration period of the account or the
1620 password. For the availability and the exact meaning of these fields
1621 in your system, please consult your getpwnam(3) documentation and your
1622 F<pwd.h> file. You can also find out from within Perl which meaning
1623 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1624 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1625 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1627 The C<$members> value returned by I<getgr*()> is a space separated list of
1628 the login names of the members of the group.
1630 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1631 C, it will be returned to you via C<$?> if the function call fails. The
1632 C<@addrs> value returned by a successful call is a list of the raw
1633 addresses returned by the corresponding system library call. In the
1634 Internet domain, each address is four bytes long and you can unpack it
1635 by saying something like:
1637 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1639 If you get tired of remembering which element of the return list contains
1640 which return value, by-name interfaces are also provided in modules:
1641 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1642 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1643 normal built-in, replacing them with versions that return objects with
1644 the appropriate names for each field. For example:
1648 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1650 Even though it looks like they're the same method calls (uid),
1651 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1653 =item getsockname SOCKET
1655 Returns the packed sockaddr address of this end of the SOCKET connection.
1658 $mysockaddr = getsockname(SOCK);
1659 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1661 =item getsockopt SOCKET,LEVEL,OPTNAME
1663 Returns the socket option requested, or undef if there is an error.
1669 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1670 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1671 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1672 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1673 L<perlop/"I/O Operators">.
1677 Converts a time as returned by the time function to a 9-element array
1678 with the time localized for the standard Greenwich time zone.
1679 Typically used as follows:
1682 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1685 All array elements are numeric, and come straight out of a struct tm.
1686 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1687 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1688 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1690 If EXPR is omitted, does C<gmtime(time())>.
1692 In scalar context, returns the ctime(3) value:
1694 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1696 Also see the C<timegm()> function provided by the C<Time::Local> module,
1697 and the strftime(3) function available via the POSIX module.
1699 This scalar value is B<not> locale dependent, see L<perllocale>, but
1700 instead a Perl builtin. Also see the C<Time::Local> module, and the
1701 strftime(3) and mktime(3) function available via the POSIX module. To
1702 get somewhat similar but locale dependent date strings, set up your
1703 locale environment variables appropriately (please see L<perllocale>)
1704 and try for example:
1706 use POSIX qw(strftime);
1707 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1709 Note that the C<%a> and C<%b>, the short forms of the day of the week
1710 and the month of the year, may not necessarily be three characters wide.
1718 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1719 execution there. It may not be used to go into any construct that
1720 requires initialization, such as a subroutine or a C<foreach> loop. It
1721 also can't be used to go into a construct that is optimized away,
1722 or to get out of a block or subroutine given to C<sort()>.
1723 It can be used to go almost anywhere else within the dynamic scope,
1724 including out of subroutines, but it's usually better to use some other
1725 construct such as C<last> or C<die()>. The author of Perl has never felt the
1726 need to use this form of C<goto> (in Perl, that is--C is another matter).
1728 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1729 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1730 necessarily recommended if you're optimizing for maintainability:
1732 goto ("FOO", "BAR", "GLARCH")[$i];
1734 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1735 named subroutine for the currently running subroutine. This is used by
1736 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1737 pretend that the other subroutine had been called in the first place
1738 (except that any modifications to C<@_> in the current subroutine are
1739 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1740 will be able to tell that this routine was called first.
1742 =item grep BLOCK LIST
1744 =item grep EXPR,LIST
1746 This is similar in spirit to, but not the same as, grep(1)
1747 and its relatives. In particular, it is not limited to using
1748 regular expressions.
1750 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1751 C<$_> to each element) and returns the list value consisting of those
1752 elements for which the expression evaluated to TRUE. In a scalar
1753 context, returns the number of times the expression was TRUE.
1755 @foo = grep(!/^#/, @bar); # weed out comments
1759 @foo = grep {!/^#/} @bar; # weed out comments
1761 Note that, because C<$_> is a reference into the list value, it can be used
1762 to modify the elements of the array. While this is useful and
1763 supported, it can cause bizarre results if the LIST is not a named
1764 array. Similarly, grep returns aliases into the original list,
1765 much like the way that a for loop's index variable aliases the list
1766 elements. That is, modifying an element of a list returned by grep
1767 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1768 actually modifies the element in the original list.
1770 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1776 Interprets EXPR as a hex string and returns the corresponding
1777 value. (To convert strings that might start with either 0 or 0x
1778 see L</oct>.) If EXPR is omitted, uses C<$_>.
1780 print hex '0xAf'; # prints '175'
1781 print hex 'aF'; # same
1785 There is no builtin C<import()> function. It is just an ordinary
1786 method (subroutine) defined (or inherited) by modules that wish to export
1787 names to another module. The C<use()> function calls the C<import()> method
1788 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1790 =item index STR,SUBSTR,POSITION
1792 =item index STR,SUBSTR
1794 Returns the position of the first occurrence of SUBSTR in STR at or after
1795 POSITION. If POSITION is omitted, starts searching from the beginning of
1796 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1797 variable to--but don't do that). If the substring is not found, returns
1798 one less than the base, ordinarily C<-1>.
1804 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1805 You should not use this for rounding, because it truncates
1806 towards C<0>, and because machine representations of floating point
1807 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1808 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1810 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1812 Implements the ioctl(2) function. You'll probably have to say
1814 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1816 first to get the correct function definitions. If F<ioctl.ph> doesn't
1817 exist or doesn't have the correct definitions you'll have to roll your
1818 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1819 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1820 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1821 written depending on the FUNCTION--a pointer to the string value of SCALAR
1822 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1823 has no string value but does have a numeric value, that value will be
1824 passed rather than a pointer to the string value. To guarantee this to be
1825 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1826 functions are useful for manipulating the values of structures used by
1827 C<ioctl()>. The following example sets the erase character to DEL.
1831 die "NO TIOCGETP" if $@ || !$getp;
1832 $sgttyb_t = "ccccs"; # 4 chars and a short
1833 if (ioctl(STDIN,$getp,$sgttyb)) {
1834 @ary = unpack($sgttyb_t,$sgttyb);
1836 $sgttyb = pack($sgttyb_t,@ary);
1837 ioctl(STDIN,&TIOCSETP,$sgttyb)
1838 || die "Can't ioctl: $!";
1841 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1843 if OS returns: then Perl returns:
1845 0 string "0 but true"
1846 anything else that number
1848 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1849 still easily determine the actual value returned by the operating
1852 ($retval = ioctl(...)) || ($retval = -1);
1853 printf "System returned %d\n", $retval;
1855 The special string "C<0> but true" is excempt from B<-w> complaints
1856 about improper numeric conversions.
1858 =item join EXPR,LIST
1860 Joins the separate strings of LIST into a single string with
1861 fields separated by the value of EXPR, and returns the string.
1864 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1870 Returns a list consisting of all the keys of the named hash. (In a
1871 scalar context, returns the number of keys.) The keys are returned in
1872 an apparently random order, but it is the same order as either the
1873 C<values()> or C<each()> function produces (given that the hash has not been
1874 modified). As a side effect, it resets HASH's iterator.
1876 Here is yet another way to print your environment:
1879 @values = values %ENV;
1880 while ($#keys >= 0) {
1881 print pop(@keys), '=', pop(@values), "\n";
1884 or how about sorted by key:
1886 foreach $key (sort(keys %ENV)) {
1887 print $key, '=', $ENV{$key}, "\n";
1890 To sort an array by value, you'll need to use a C<sort()> function.
1891 Here's a descending numeric sort of a hash by its values:
1893 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1894 printf "%4d %s\n", $hash{$key}, $key;
1897 As an lvalue C<keys()> allows you to increase the number of hash buckets
1898 allocated for the given hash. This can gain you a measure of efficiency if
1899 you know the hash is going to get big. (This is similar to pre-extending
1900 an array by assigning a larger number to $#array.) If you say
1904 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1905 it rounds up to the next power of two. These
1906 buckets will be retained even if you do C<%hash = ()>, use C<undef
1907 %hash> if you want to free the storage while C<%hash> is still in scope.
1908 You can't shrink the number of buckets allocated for the hash using
1909 C<keys()> in this way (but you needn't worry about doing this by accident,
1910 as trying has no effect).
1914 Sends a signal to a list of processes. The first element of
1915 the list must be the signal to send. Returns the number of
1916 processes successfully signaled.
1918 $cnt = kill 1, $child1, $child2;
1921 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1922 process groups instead of processes. (On System V, a negative I<PROCESS>
1923 number will also kill process groups, but that's not portable.) That
1924 means you usually want to use positive not negative signals. You may also
1925 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1931 The C<last> command is like the C<break> statement in C (as used in
1932 loops); it immediately exits the loop in question. If the LABEL is
1933 omitted, the command refers to the innermost enclosing loop. The
1934 C<continue> block, if any, is not executed:
1936 LINE: while (<STDIN>) {
1937 last LINE if /^$/; # exit when done with header
1941 See also L</continue> for an illustration of how C<last>, C<next>, and
1948 Returns an lowercased version of EXPR. This is the internal function
1949 implementing the C<\L> escape in double-quoted strings.
1950 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1952 If EXPR is omitted, uses C<$_>.
1958 Returns the value of EXPR with the first character lowercased. This is
1959 the internal function implementing the C<\l> escape in double-quoted strings.
1960 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1962 If EXPR is omitted, uses C<$_>.
1968 Returns the length in characters of the value of EXPR. If EXPR is
1969 omitted, returns length of C<$_>.
1971 =item link OLDFILE,NEWFILE
1973 Creates a new filename linked to the old filename. Returns TRUE for
1974 success, FALSE otherwise.
1976 =item listen SOCKET,QUEUESIZE
1978 Does the same thing that the listen system call does. Returns TRUE if
1979 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1983 A local modifies the listed variables to be local to the enclosing
1984 block, file, or eval. If more than one value is listed, the list must
1985 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1986 for details, including issues with tied arrays and hashes.
1988 You really probably want to be using C<my()> instead, because C<local()> isn't
1989 what most people think of as "local". See L<perlsub/"Private Variables
1990 via my()"> for details.
1992 =item localtime EXPR
1994 Converts a time as returned by the time function to a 9-element array
1995 with the time analyzed for the local time zone. Typically used as
1999 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2002 All array elements are numeric, and come straight out of a struct tm.
2003 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2004 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2005 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2007 If EXPR is omitted, uses the current time (C<localtime(time)>).
2009 In scalar context, returns the ctime(3) value:
2011 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2013 This scalar value is B<not> locale dependent, see L<perllocale>, but
2014 instead a Perl builtin. Also see the C<Time::Local> module, and the
2015 strftime(3) and mktime(3) function available via the POSIX module. To
2016 get somewhat similar but locale dependent date strings, set up your
2017 locale environment variables appropriately (please see L<perllocale>)
2018 and try for example:
2020 use POSIX qw(strftime);
2021 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2023 Note that the C<%a> and C<%b>, the short forms of the day of the week
2024 and the month of the year, may not necessarily be three characters wide.
2030 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2033 =item lstat FILEHANDLE
2039 Does the same thing as the C<stat()> function (including setting the
2040 special C<_> filehandle) but stats a symbolic link instead of the file
2041 the symbolic link points to. If symbolic links are unimplemented on
2042 your system, a normal C<stat()> is done.
2044 If EXPR is omitted, stats C<$_>.
2048 The match operator. See L<perlop>.
2050 =item map BLOCK LIST
2054 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2055 element) and returns the list value composed of the results of each such
2056 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2057 may produce zero, one, or more elements in the returned value.
2059 @chars = map(chr, @nums);
2061 translates a list of numbers to the corresponding characters. And
2063 %hash = map { getkey($_) => $_ } @array;
2065 is just a funny way to write
2068 foreach $_ (@array) {
2069 $hash{getkey($_)} = $_;
2072 Note that, because C<$_> is a reference into the list value, it can be used
2073 to modify the elements of the array. While this is useful and
2074 supported, it can cause bizarre results if the LIST is not a named
2075 array. See also L</grep> for an array composed of those items of the
2076 original list for which the BLOCK or EXPR evaluates to true.
2078 =item mkdir FILENAME,MODE
2080 Creates the directory specified by FILENAME, with permissions specified
2081 by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2082 it returns FALSE and sets C<$!> (errno).
2084 =item msgctl ID,CMD,ARG
2086 Calls the System V IPC function msgctl(2). You'll probably have to say
2090 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2091 then ARG must be a variable which will hold the returned C<msqid_ds>
2092 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2093 true" for zero, or the actual return value otherwise. See also
2094 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2096 =item msgget KEY,FLAGS
2098 Calls the System V IPC function msgget(2). Returns the message queue
2099 id, or the undefined value if there is an error. See also C<IPC::SysV>
2100 and C<IPC::SysV::Msg> documentation.
2102 =item msgsnd ID,MSG,FLAGS
2104 Calls the System V IPC function msgsnd to send the message MSG to the
2105 message queue ID. MSG must begin with the long integer message type,
2106 which may be created with C<pack("l", $type)>. Returns TRUE if
2107 successful, or FALSE if there is an error. See also C<IPC::SysV>
2108 and C<IPC::SysV::Msg> documentation.
2110 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2112 Calls the System V IPC function msgrcv to receive a message from
2113 message queue ID into variable VAR with a maximum message size of
2114 SIZE. Note that if a message is received, the message type will be
2115 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2116 size of the message type. Returns TRUE if successful, or FALSE if
2117 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2121 A C<my()> declares the listed variables to be local (lexically) to the
2122 enclosing block, file, or C<eval()>. If
2123 more than one value is listed, the list must be placed in parentheses. See
2124 L<perlsub/"Private Variables via my()"> for details.
2130 The C<next> command is like the C<continue> statement in C; it starts
2131 the next iteration of the loop:
2133 LINE: while (<STDIN>) {
2134 next LINE if /^#/; # discard comments
2138 Note that if there were a C<continue> block on the above, it would get
2139 executed even on discarded lines. If the LABEL is omitted, the command
2140 refers to the innermost enclosing loop.
2142 See also L</continue> for an illustration of how C<last>, C<next>, and
2145 =item no Module LIST
2147 See the L</use> function, which C<no> is the opposite of.
2153 Interprets EXPR as an octal string and returns the corresponding
2154 value. (If EXPR happens to start off with C<0x>, interprets it as
2155 a hex string instead.) The following will handle decimal, octal, and
2156 hex in the standard Perl or C notation:
2158 $val = oct($val) if $val =~ /^0/;
2160 If EXPR is omitted, uses C<$_>. This function is commonly used when
2161 a string such as C<644> needs to be converted into a file mode, for
2162 example. (Although perl will automatically convert strings into
2163 numbers as needed, this automatic conversion assumes base 10.)
2165 =item open FILEHANDLE,EXPR
2167 =item open FILEHANDLE
2169 Opens the file whose filename is given by EXPR, and associates it with
2170 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2171 name of the real filehandle wanted. If EXPR is omitted, the scalar
2172 variable of the same name as the FILEHANDLE contains the filename.
2173 (Note that lexical variables--those declared with C<my()>--will not work
2174 for this purpose; so if you're using C<my()>, specify EXPR in your call
2177 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2178 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2179 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2180 the file is opened for appending, again being created if necessary.
2181 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2182 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2183 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2184 file first. You can't usually use either read-write mode for updating
2185 textfiles, since they have variable length records. See the B<-i>
2186 switch in L<perlrun> for a better approach.
2188 The prefix and the filename may be separated with spaces.
2189 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2190 C<'w+'>, C<'a'>, and C<'a+'>.
2192 If the filename begins with C<'|'>, the filename is interpreted as a
2193 command to which output is to be piped, and if the filename ends with a
2194 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2195 for more examples of this. (You are not allowed to C<open()> to a command
2196 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2197 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2199 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2200 nonzero upon success, the undefined value otherwise. If the C<open()>
2201 involved a pipe, the return value happens to be the pid of the
2204 If you're unfortunate enough to be running Perl on a system that
2205 distinguishes between text files and binary files (modern operating
2206 systems don't care), then you should check out L</binmode> for tips for
2207 dealing with this. The key distinction between systems that need C<binmode()>
2208 and those that don't is their text file formats. Systems like Unix, MacOS, and
2209 Plan9, which delimit lines with a single character, and which encode that
2210 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2212 When opening a file, it's usually a bad idea to continue normal execution
2213 if the request failed, so C<open()> is frequently used in connection with
2214 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2215 where you want to make a nicely formatted error message (but there are
2216 modules that can help with that problem)) you should always check
2217 the return value from opening a file. The infrequent exception is when
2218 working with an unopened filehandle is actually what you want to do.
2223 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2224 while (<ARTICLE>) {...
2226 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2227 # if the open fails, output is discarded
2229 open(DBASE, '+<dbase.mine') # open for update
2230 or die "Can't open 'dbase.mine' for update: $!";
2232 open(ARTICLE, "caesar <$article |") # decrypt article
2233 or die "Can't start caesar: $!";
2235 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2236 or die "Can't start sort: $!";
2238 # process argument list of files along with any includes
2240 foreach $file (@ARGV) {
2241 process($file, 'fh00');
2245 my($filename, $input) = @_;
2246 $input++; # this is a string increment
2247 unless (open($input, $filename)) {
2248 print STDERR "Can't open $filename: $!\n";
2253 while (<$input>) { # note use of indirection
2254 if (/^#include "(.*)"/) {
2255 process($1, $input);
2262 You may also, in the Bourne shell tradition, specify an EXPR beginning
2263 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2264 name of a filehandle (or file descriptor, if numeric) to be
2265 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2266 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2267 mode you specify should match the mode of the original filehandle.
2268 (Duping a filehandle does not take into account any existing contents of
2270 Here is a script that saves, redirects, and restores STDOUT and
2274 open(OLDOUT, ">&STDOUT");
2275 open(OLDERR, ">&STDERR");
2277 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2278 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2280 select(STDERR); $| = 1; # make unbuffered
2281 select(STDOUT); $| = 1; # make unbuffered
2283 print STDOUT "stdout 1\n"; # this works for
2284 print STDERR "stderr 1\n"; # subprocesses too
2289 open(STDOUT, ">&OLDOUT");
2290 open(STDERR, ">&OLDERR");
2292 print STDOUT "stdout 2\n";
2293 print STDERR "stderr 2\n";
2296 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2297 equivalent of C's C<fdopen()> of that file descriptor; this is more
2298 parsimonious of file descriptors. For example:
2300 open(FILEHANDLE, "<&=$fd")
2302 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2303 there is an implicit fork done, and the return value of open is the pid
2304 of the child within the parent process, and C<0> within the child
2305 process. (Use C<defined($pid)> to determine whether the open was successful.)
2306 The filehandle behaves normally for the parent, but i/o to that
2307 filehandle is piped from/to the STDOUT/STDIN of the child process.
2308 In the child process the filehandle isn't opened--i/o happens from/to
2309 the new STDOUT or STDIN. Typically this is used like the normal
2310 piped open when you want to exercise more control over just how the
2311 pipe command gets executed, such as when you are running setuid, and
2312 don't want to have to scan shell commands for metacharacters.
2313 The following pairs are more or less equivalent:
2315 open(FOO, "|tr '[a-z]' '[A-Z]'");
2316 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2318 open(FOO, "cat -n '$file'|");
2319 open(FOO, "-|") || exec 'cat', '-n', $file;
2321 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2323 NOTE: On any operation that may do a fork, any unflushed buffers remain
2324 unflushed in both processes, which means you may need to set C<$|> to
2325 avoid duplicate output.
2327 Closing any piped filehandle causes the parent process to wait for the
2328 child to finish, and returns the status value in C<$?>.
2330 The filename passed to open will have leading and trailing
2331 whitespace deleted, and the normal redirection characters
2332 honored. This property, known as "magic open",
2333 can often be used to good effect. A user could specify a filename of
2334 F<"rsh cat file |">, or you could change certain filenames as needed:
2336 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2337 open(FH, $filename) or die "Can't open $filename: $!";
2339 However, to open a file with arbitrary weird characters in it, it's
2340 necessary to protect any leading and trailing whitespace:
2342 $file =~ s#^(\s)#./$1#;
2343 open(FOO, "< $file\0");
2345 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2346 should use the C<sysopen()> function, which involves no such magic. This is
2347 another way to protect your filenames from interpretation. For example:
2350 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2351 or die "sysopen $path: $!";
2352 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2353 print HANDLE "stuff $$\n");
2355 print "File contains: ", <HANDLE>;
2357 Using the constructor from the C<IO::Handle> package (or one of its
2358 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2359 filehandles that have the scope of whatever variables hold references to
2360 them, and automatically close whenever and however you leave that scope:
2364 sub read_myfile_munged {
2366 my $handle = new IO::File;
2367 open($handle, "myfile") or die "myfile: $!";
2369 or return (); # Automatically closed here.
2370 mung $first or die "mung failed"; # Or here.
2371 return $first, <$handle> if $ALL; # Or here.
2375 See L</seek()> for some details about mixing reading and writing.
2377 =item opendir DIRHANDLE,EXPR
2379 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2380 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2381 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2387 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2388 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2390 =item pack TEMPLATE,LIST
2392 Takes an array or list of values and packs it into a binary structure,
2393 returning the string containing the structure. The TEMPLATE is a
2394 sequence of characters that give the order and type of values, as
2397 A An ascii string, will be space padded.
2398 a An ascii string, will be null padded.
2399 b A bit string (ascending bit order, like vec()).
2400 B A bit string (descending bit order).
2401 h A hex string (low nybble first).
2402 H A hex string (high nybble first).
2404 c A signed char value.
2405 C An unsigned char value. Only does bytes. See U for Unicode.
2407 s A signed short value.
2408 S An unsigned short value.
2409 (This 'short' is _exactly_ 16 bits, which may differ from
2410 what a local C compiler calls 'short'.)
2412 i A signed integer value.
2413 I An unsigned integer value.
2414 (This 'integer' is _at_least_ 32 bits wide. Its exact
2415 size depends on what a local C compiler calls 'int',
2416 and may even be larger than the 'long' described in
2419 l A signed long value.
2420 L An unsigned long value.
2421 (This 'long' is _exactly_ 32 bits, which may differ from
2422 what a local C compiler calls 'long'.)
2424 n A short in "network" (big-endian) order.
2425 N A long in "network" (big-endian) order.
2426 v A short in "VAX" (little-endian) order.
2427 V A long in "VAX" (little-endian) order.
2428 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2429 _exactly_ 32 bits, respectively.)
2431 f A single-precision float in the native format.
2432 d A double-precision float in the native format.
2434 p A pointer to a null-terminated string.
2435 P A pointer to a structure (fixed-length string).
2437 u A uuencoded string.
2438 U A Unicode character number. Encodes to UTF-8 internally.
2439 Works even if C<use utf8> is not in effect.
2441 w A BER compressed integer. Its bytes represent an unsigned
2442 integer in base 128, most significant digit first, with as
2443 few digits as possible. Bit eight (the high bit) is set
2444 on each byte except the last.
2448 @ Null fill to absolute position.
2450 Each letter may optionally be followed by a number giving a repeat
2451 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2452 pack function will gobble up that many values from the LIST. A C<*> for the
2453 repeat count means to use however many items are left. The C<"a"> and C<"A">
2454 types gobble just one value, but pack it as a string of length count,
2455 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2456 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2457 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2458 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2459 terminated string. You are responsible for ensuring the string is not a
2460 temporary value (which can potentially get deallocated before you get
2461 around to using the packed result). The C<"P"> packs a pointer to a structure
2462 of the size indicated by the length. A NULL pointer is created if the
2463 corresponding value for C<"p"> or C<"P"> is C<undef>.
2464 Real numbers (floats and doubles) are
2465 in the native machine format only; due to the multiplicity of floating
2466 formats around, and the lack of a standard "network" representation, no
2467 facility for interchange has been made. This means that packed floating
2468 point data written on one machine may not be readable on another - even if
2469 both use IEEE floating point arithmetic (as the endian-ness of the memory
2470 representation is not part of the IEEE spec). Note that Perl uses doubles
2471 internally for all numeric calculation, and converting from double into
2472 float and thence back to double again will lose precision (i.e.,
2473 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2477 $foo = pack("CCCC",65,66,67,68);
2479 $foo = pack("C4",65,66,67,68);
2481 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2482 # same thing with Unicode circled letters
2484 $foo = pack("ccxxcc",65,66,67,68);
2487 $foo = pack("s2",1,2);
2488 # "\1\0\2\0" on little-endian
2489 # "\0\1\0\2" on big-endian
2491 $foo = pack("a4","abcd","x","y","z");
2494 $foo = pack("aaaa","abcd","x","y","z");
2497 $foo = pack("a14","abcdefg");
2498 # "abcdefg\0\0\0\0\0\0\0"
2500 $foo = pack("i9pl", gmtime);
2501 # a real struct tm (on my system anyway)
2504 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2507 The same template may generally also be used in the unpack function.
2511 =item package NAMESPACE
2513 Declares the compilation unit as being in the given namespace. The scope
2514 of the package declaration is from the declaration itself through the end of
2515 the enclosing block (the same scope as the C<local()> operator). All further
2516 unqualified dynamic identifiers will be in this namespace. A package
2517 statement affects only dynamic variables--including those you've used
2518 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2519 would be the first declaration in a file to be included by the C<require>
2520 or C<use> operator. You can switch into a package in more than one place;
2521 it merely influences which symbol table is used by the compiler for the
2522 rest of that block. You can refer to variables and filehandles in other
2523 packages by prefixing the identifier with the package name and a double
2524 colon: C<$Package::Variable>. If the package name is null, the C<main>
2525 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2527 If NAMESPACE is omitted, then there is no current package, and all
2528 identifiers must be fully qualified or lexicals. This is stricter
2529 than C<use strict>, since it also extends to function names.
2531 See L<perlmod/"Packages"> for more information about packages, modules,
2532 and classes. See L<perlsub> for other scoping issues.
2534 =item pipe READHANDLE,WRITEHANDLE
2536 Opens a pair of connected pipes like the corresponding system call.
2537 Note that if you set up a loop of piped processes, deadlock can occur
2538 unless you are very careful. In addition, note that Perl's pipes use
2539 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2540 after each command, depending on the application.
2542 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2543 for examples of such things.
2549 Pops and returns the last value of the array, shortening the array by
2550 1. Has a similar effect to
2552 $tmp = $ARRAY[$#ARRAY--];
2554 If there are no elements in the array, returns the undefined value.
2555 If ARRAY is omitted, pops the
2556 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2563 Returns the offset of where the last C<m//g> search left off for the variable
2564 is in question (C<$_> is used when the variable is not specified). May be
2565 modified to change that offset. Such modification will also influence
2566 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2569 =item print FILEHANDLE LIST
2575 Prints a string or a comma-separated list of strings. Returns TRUE
2576 if successful. FILEHANDLE may be a scalar variable name, in which case
2577 the variable contains the name of or a reference to the filehandle, thus introducing one
2578 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2579 token is a term, it may be misinterpreted as an operator unless you
2580 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2581 omitted, prints by default to standard output (or to the last selected
2582 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2583 the currently selected output channel. To set the default output channel to something other than
2584 STDOUT use the select operation. Note that, because print takes a
2585 LIST, anything in the LIST is evaluated in list context, and any
2586 subroutine that you call will have one or more of its expressions
2587 evaluated in list context. Also be careful not to follow the print
2588 keyword with a left parenthesis unless you want the corresponding right
2589 parenthesis to terminate the arguments to the print--interpose a C<+> or
2590 put parentheses around all the arguments.
2592 Note that if you're storing FILEHANDLES in an array or other expression,
2593 you will have to use a block returning its value instead:
2595 print { $files[$i] } "stuff\n";
2596 print { $OK ? STDOUT : STDERR } "stuff\n";
2598 =item printf FILEHANDLE FORMAT, LIST
2600 =item printf FORMAT, LIST
2602 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2603 (the output record separator) is not appended. The first argument
2604 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2605 in effect, the character used for the decimal point in formatted real numbers
2606 is affected by the LC_NUMERIC locale. See L<perllocale>.
2608 Don't fall into the trap of using a C<printf()> when a simple
2609 C<print()> would do. The C<print()> is more efficient and less
2612 =item prototype FUNCTION
2614 Returns the prototype of a function as a string (or C<undef> if the
2615 function has no prototype). FUNCTION is a reference to, or the name of,
2616 the function whose prototype you want to retrieve.
2618 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2619 a name for Perl builtin. If builtin is not I<overridable> (such as
2620 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2621 C<system()>) - in other words, the builtin does not behave like a Perl
2622 function - returns C<undef>. Otherwise, the string describing the
2623 equivalent prototype is returned.
2625 =item push ARRAY,LIST
2627 Treats ARRAY as a stack, and pushes the values of LIST
2628 onto the end of ARRAY. The length of ARRAY increases by the length of
2629 LIST. Has the same effect as
2632 $ARRAY[++$#ARRAY] = $value;
2635 but is more efficient. Returns the new number of elements in the array.
2647 Generalized quotes. See L<perlop>.
2649 =item quotemeta EXPR
2653 Returns the value of EXPR with all non-alphanumeric
2654 characters backslashed. (That is, all characters not matching
2655 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2656 returned string, regardless of any locale settings.)
2657 This is the internal function implementing
2658 the C<\Q> escape in double-quoted strings.
2660 If EXPR is omitted, uses C<$_>.
2666 Returns a random fractional number greater than or equal to C<0> and less
2667 than the value of EXPR. (EXPR should be positive.) If EXPR is
2668 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2669 C<srand()> has already been called. See also C<srand()>.
2671 (Note: If your rand function consistently returns numbers that are too
2672 large or too small, then your version of Perl was probably compiled
2673 with the wrong number of RANDBITS.)
2675 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2677 =item read FILEHANDLE,SCALAR,LENGTH
2679 Attempts to read LENGTH bytes of data into variable SCALAR from the
2680 specified FILEHANDLE. Returns the number of bytes actually read,
2681 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2682 or shrunk to the length actually read. An OFFSET may be specified to
2683 place the read data at some other place than the beginning of the
2684 string. This call is actually implemented in terms of stdio's fread(3)
2685 call. To get a true read(2) system call, see C<sysread()>.
2687 =item readdir DIRHANDLE
2689 Returns the next directory entry for a directory opened by C<opendir()>.
2690 If used in list context, returns all the rest of the entries in the
2691 directory. If there are no more entries, returns an undefined value in
2692 scalar context or a null list in list context.
2694 If you're planning to filetest the return values out of a C<readdir()>, you'd
2695 better prepend the directory in question. Otherwise, because we didn't
2696 C<chdir()> there, it would have been testing the wrong file.
2698 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2699 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2704 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2705 is read and returned. In list context, reads until end-of-file is
2706 reached and returns a list of lines (however you've defined lines
2707 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2708 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2709 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2710 operator is discussed in more detail in L<perlop/"I/O Operators">.
2713 $line = readline(*STDIN); # same thing
2719 Returns the value of a symbolic link, if symbolic links are
2720 implemented. If not, gives a fatal error. If there is some system
2721 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2722 omitted, uses C<$_>.
2726 EXPR is executed as a system command.
2727 The collected standard output of the command is returned.
2728 In scalar context, it comes back as a single (potentially
2729 multi-line) string. In list context, returns a list of lines
2730 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2731 This is the internal function implementing the C<qx/EXPR/>
2732 operator, but you can use it directly. The C<qx/EXPR/>
2733 operator is discussed in more detail in L<perlop/"I/O Operators">.
2735 =item recv SOCKET,SCALAR,LEN,FLAGS
2737 Receives a message on a socket. Attempts to receive LENGTH bytes of
2738 data into variable SCALAR from the specified SOCKET filehandle.
2739 Actually does a C C<recvfrom()>, so that it can return the address of the
2740 sender. Returns the undefined value if there's an error. SCALAR will
2741 be grown or shrunk to the length actually read. Takes the same flags
2742 as the system call of the same name.
2743 See L<perlipc/"UDP: Message Passing"> for examples.
2749 The C<redo> command restarts the loop block without evaluating the
2750 conditional again. The C<continue> block, if any, is not executed. If
2751 the LABEL is omitted, the command refers to the innermost enclosing
2752 loop. This command is normally used by programs that want to lie to
2753 themselves about what was just input:
2755 # a simpleminded Pascal comment stripper
2756 # (warning: assumes no { or } in strings)
2757 LINE: while (<STDIN>) {
2758 while (s|({.*}.*){.*}|$1 |) {}
2763 if (/}/) { # end of comment?
2772 See also L</continue> for an illustration of how C<last>, C<next>, and
2779 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2780 is not specified, C<$_> will be used. The value returned depends on the
2781 type of thing the reference is a reference to.
2782 Builtin types include:
2791 If the referenced object has been blessed into a package, then that package
2792 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2794 if (ref($r) eq "HASH") {
2795 print "r is a reference to a hash.\n";
2798 print "r is not a reference at all.\n";
2801 See also L<perlref>.
2803 =item rename OLDNAME,NEWNAME
2805 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2806 not work across file system boundaries.
2812 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2813 supplied. If EXPR is numeric, demands that the current version of Perl
2814 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2816 Otherwise, demands that a library file be included if it hasn't already
2817 been included. The file is included via the do-FILE mechanism, which is
2818 essentially just a variety of C<eval()>. Has semantics similar to the following
2823 return 1 if $INC{$filename};
2824 my($realfilename,$result);
2826 foreach $prefix (@INC) {
2827 $realfilename = "$prefix/$filename";
2828 if (-f $realfilename) {
2829 $result = do $realfilename;
2833 die "Can't find $filename in \@INC";
2836 die "$filename did not return true value" unless $result;
2837 $INC{$filename} = $realfilename;
2841 Note that the file will not be included twice under the same specified
2842 name. The file must return TRUE as the last statement to indicate
2843 successful execution of any initialization code, so it's customary to
2844 end such a file with "C<1;>" unless you're sure it'll return TRUE
2845 otherwise. But it's better just to put the "C<1;>", in case you add more
2848 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2849 replaces "F<::>" with "F</>" in the filename for you,
2850 to make it easy to load standard modules. This form of loading of
2851 modules does not risk altering your namespace.
2853 In other words, if you try this:
2855 require Foo::Bar; # a splendid bareword
2857 The require function will actually look for the "F<Foo/Bar.pm>" file in the
2858 directories specified in the C<@INC> array.
2860 But if you try this:
2862 $class = 'Foo::Bar';
2863 require $class; # $class is not a bareword
2865 require "Foo::Bar"; # not a bareword because of the ""
2867 The require function will look for the "F<Foo::Bar>" file in the @INC array and
2868 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
2870 eval "require $class";
2872 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2878 Generally used in a C<continue> block at the end of a loop to clear
2879 variables and reset C<??> searches so that they work again. The
2880 expression is interpreted as a list of single characters (hyphens
2881 allowed for ranges). All variables and arrays beginning with one of
2882 those letters are reset to their pristine state. If the expression is
2883 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
2884 only variables or searches in the current package. Always returns
2887 reset 'X'; # reset all X variables
2888 reset 'a-z'; # reset lower case variables
2889 reset; # just reset ?? searches
2891 Resetting C<"A-Z"> is not recommended because you'll wipe out your
2892 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
2893 are unaffected, but they clean themselves up on scope exit anyway,
2894 so you'll probably want to use them instead. See L</my>.
2900 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
2901 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
2902 context, depending on how the return value will be used, and the context
2903 may vary from one execution to the next (see C<wantarray()>). If no EXPR
2904 is given, returns an empty list in list context, an undefined value in
2905 scalar context, or nothing in a void context.
2907 (Note that in the absence of a return, a subroutine, eval, or do FILE
2908 will automatically return the value of the last expression evaluated.)
2912 In list context, returns a list value consisting of the elements
2913 of LIST in the opposite order. In scalar context, concatenates the
2914 elements of LIST, and returns a string value with all the characters
2915 in the opposite order.
2917 print reverse <>; # line tac, last line first
2919 undef $/; # for efficiency of <>
2920 print scalar reverse <>; # character tac, last line tsrif
2922 This operator is also handy for inverting a hash, although there are some
2923 caveats. If a value is duplicated in the original hash, only one of those
2924 can be represented as a key in the inverted hash. Also, this has to
2925 unwind one hash and build a whole new one, which may take some time
2928 %by_name = reverse %by_address; # Invert the hash
2930 =item rewinddir DIRHANDLE
2932 Sets the current position to the beginning of the directory for the
2933 C<readdir()> routine on DIRHANDLE.
2935 =item rindex STR,SUBSTR,POSITION
2937 =item rindex STR,SUBSTR
2939 Works just like index except that it returns the position of the LAST
2940 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2941 last occurrence at or before that position.
2943 =item rmdir FILENAME
2947 Deletes the directory specified by FILENAME if that directory is empty. If it
2948 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
2949 FILENAME is omitted, uses C<$_>.
2953 The substitution operator. See L<perlop>.
2957 Forces EXPR to be interpreted in scalar context and returns the value
2960 @counts = ( scalar @a, scalar @b, scalar @c );
2962 There is no equivalent operator to force an expression to
2963 be interpolated in list context because it's in practice never
2964 needed. If you really wanted to do so, however, you could use
2965 the construction C<@{[ (some expression) ]}>, but usually a simple
2966 C<(some expression)> suffices.
2968 =item seek FILEHANDLE,POSITION,WHENCE
2970 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
2971 FILEHANDLE may be an expression whose value gives the name of the
2972 filehandle. The values for WHENCE are C<0> to set the new position to
2973 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
2974 set it to EOF plus POSITION (typically negative). For WHENCE you may
2975 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2976 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
2978 If you want to position file for C<sysread()> or C<syswrite()>, don't use
2979 C<seek()> -- buffering makes its effect on the file's system position
2980 unpredictable and non-portable. Use C<sysseek()> instead.
2982 On some systems you have to do a seek whenever you switch between reading
2983 and writing. Amongst other things, this may have the effect of calling
2984 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
2989 This is also useful for applications emulating C<tail -f>. Once you hit
2990 EOF on your read, and then sleep for a while, you might have to stick in a
2991 seek() to reset things. The C<seek()> doesn't change the current position,
2992 but it I<does> clear the end-of-file condition on the handle, so that the
2993 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2995 If that doesn't work (some stdios are particularly cantankerous), then
2996 you may need something more like this:
2999 for ($curpos = tell(FILE); $_ = <FILE>;
3000 $curpos = tell(FILE)) {
3001 # search for some stuff and put it into files
3003 sleep($for_a_while);
3004 seek(FILE, $curpos, 0);
3007 =item seekdir DIRHANDLE,POS
3009 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3010 must be a value returned by C<telldir()>. Has the same caveats about
3011 possible directory compaction as the corresponding system library
3014 =item select FILEHANDLE
3018 Returns the currently selected filehandle. Sets the current default
3019 filehandle for output, if FILEHANDLE is supplied. This has two
3020 effects: first, a C<write()> or a C<print()> without a filehandle will
3021 default to this FILEHANDLE. Second, references to variables related to
3022 output will refer to this output channel. For example, if you have to
3023 set the top of form format for more than one output channel, you might
3031 FILEHANDLE may be an expression whose value gives the name of the
3032 actual filehandle. Thus:
3034 $oldfh = select(STDERR); $| = 1; select($oldfh);
3036 Some programmers may prefer to think of filehandles as objects with
3037 methods, preferring to write the last example as:
3040 STDERR->autoflush(1);
3042 =item select RBITS,WBITS,EBITS,TIMEOUT
3044 This calls the select(2) system call with the bit masks specified, which
3045 can be constructed using C<fileno()> and C<vec()>, along these lines:
3047 $rin = $win = $ein = '';
3048 vec($rin,fileno(STDIN),1) = 1;
3049 vec($win,fileno(STDOUT),1) = 1;
3052 If you want to select on many filehandles you might wish to write a
3056 my(@fhlist) = split(' ',$_[0]);
3059 vec($bits,fileno($_),1) = 1;
3063 $rin = fhbits('STDIN TTY SOCK');
3067 ($nfound,$timeleft) =
3068 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3070 or to block until something becomes ready just do this
3072 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3074 Most systems do not bother to return anything useful in C<$timeleft>, so
3075 calling select() in scalar context just returns C<$nfound>.
3077 Any of the bit masks can also be undef. The timeout, if specified, is
3078 in seconds, which may be fractional. Note: not all implementations are
3079 capable of returning theC<$timeleft>. If not, they always return
3080 C<$timeleft> equal to the supplied C<$timeout>.
3082 You can effect a sleep of 250 milliseconds this way:
3084 select(undef, undef, undef, 0.25);
3086 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3087 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3088 then only on POSIX systems. You have to use C<sysread()> instead.
3090 =item semctl ID,SEMNUM,CMD,ARG
3092 Calls the System V IPC function C<semctl()>. You'll probably have to say
3096 first to get the correct constant definitions. If CMD is IPC_STAT or
3097 GETALL, then ARG must be a variable which will hold the returned
3098 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3099 undefined value for error, "C<0> but true" for zero, or the actual return
3100 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3102 =item semget KEY,NSEMS,FLAGS
3104 Calls the System V IPC function semget. Returns the semaphore id, or
3105 the undefined value if there is an error. See also C<IPC::SysV> and
3106 C<IPC::SysV::Semaphore> documentation.
3108 =item semop KEY,OPSTRING
3110 Calls the System V IPC function semop to perform semaphore operations
3111 such as signaling and waiting. OPSTRING must be a packed array of
3112 semop structures. Each semop structure can be generated with
3113 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3114 operations is implied by the length of OPSTRING. Returns TRUE if
3115 successful, or FALSE if there is an error. As an example, the
3116 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3118 $semop = pack("sss", $semnum, -1, 0);
3119 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3121 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3122 and C<IPC::SysV::Semaphore> documentation.
3124 =item send SOCKET,MSG,FLAGS,TO
3126 =item send SOCKET,MSG,FLAGS
3128 Sends a message on a socket. Takes the same flags as the system call
3129 of the same name. On unconnected sockets you must specify a
3130 destination to send TO, in which case it does a C C<sendto()>. Returns
3131 the number of characters sent, or the undefined value if there is an
3133 See L<perlipc/"UDP: Message Passing"> for examples.
3135 =item setpgrp PID,PGRP
3137 Sets the current process group for the specified PID, C<0> for the current
3138 process. Will produce a fatal error if used on a machine that doesn't
3139 implement setpgrp(2). If the arguments are omitted, it defaults to
3140 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3141 arguments, so only setpgrp C<0,0> is portable.
3143 =item setpriority WHICH,WHO,PRIORITY
3145 Sets the current priority for a process, a process group, or a user.
3146 (See setpriority(2).) Will produce a fatal error if used on a machine
3147 that doesn't implement setpriority(2).
3149 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3151 Sets the socket option requested. Returns undefined if there is an
3152 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3159 Shifts the first value of the array off and returns it, shortening the
3160 array by 1 and moving everything down. If there are no elements in the
3161 array, returns the undefined value. If ARRAY is omitted, shifts the
3162 C<@_> array within the lexical scope of subroutines and formats, and the
3163 C<@ARGV> array at file scopes or within the lexical scopes established by
3164 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3165 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3166 same thing to the left end of an array that C<pop()> and C<push()> do to the
3169 =item shmctl ID,CMD,ARG
3171 Calls the System V IPC function shmctl. You'll probably have to say
3175 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3176 then ARG must be a variable which will hold the returned C<shmid_ds>
3177 structure. Returns like ioctl: the undefined value for error, "C<0> but
3178 true" for zero, or the actual return value otherwise.
3179 See also C<IPC::SysV> documentation.
3181 =item shmget KEY,SIZE,FLAGS
3183 Calls the System V IPC function shmget. Returns the shared memory
3184 segment id, or the undefined value if there is an error.
3185 See also C<IPC::SysV> documentation.
3187 =item shmread ID,VAR,POS,SIZE
3189 =item shmwrite ID,STRING,POS,SIZE
3191 Reads or writes the System V shared memory segment ID starting at
3192 position POS for size SIZE by attaching to it, copying in/out, and
3193 detaching from it. When reading, VAR must be a variable that will
3194 hold the data read. When writing, if STRING is too long, only SIZE
3195 bytes are used; if STRING is too short, nulls are written to fill out
3196 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3197 See also C<IPC::SysV> documentation.
3199 =item shutdown SOCKET,HOW
3201 Shuts down a socket connection in the manner indicated by HOW, which
3202 has the same interpretation as in the system call of the same name.
3204 shutdown(SOCKET, 0); # I/we have stopped reading data
3205 shutdown(SOCKET, 1); # I/we have stopped writing data
3206 shutdown(SOCKET, 2); # I/we have stopped using this socket
3208 This is useful with sockets when you want to tell the other
3209 side you're done writing but not done reading, or vice versa.
3210 It's also a more insistent form of close because it also
3211 disables the filedescriptor in any forked copies in other
3218 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3219 returns sine of C<$_>.
3221 For the inverse sine operation, you may use the C<POSIX::asin()>
3222 function, or use this relation:
3224 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3230 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3231 May be interrupted if the process receives a signal such as C<SIGALRM>.
3232 Returns the number of seconds actually slept. You probably cannot
3233 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3236 On some older systems, it may sleep up to a full second less than what
3237 you requested, depending on how it counts seconds. Most modern systems
3238 always sleep the full amount. They may appear to sleep longer than that,
3239 however, because your process might not be scheduled right away in a
3240 busy multitasking system.
3242 For delays of finer granularity than one second, you may use Perl's
3243 C<syscall()> interface to access setitimer(2) if your system supports it,
3244 or else see L</select()> above.
3246 See also the POSIX module's C<sigpause()> function.
3248 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3250 Opens a socket of the specified kind and attaches it to filehandle
3251 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3252 system call of the same name. You should "C<use Socket;>" first to get
3253 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3255 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3257 Creates an unnamed pair of sockets in the specified domain, of the
3258 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3259 for the system call of the same name. If unimplemented, yields a fatal
3260 error. Returns TRUE if successful.
3262 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3263 to C<pipe(Rdr, Wtr)> is essentially:
3266 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3267 shutdown(Rdr, 1); # no more writing for reader
3268 shutdown(Wtr, 0); # no more reading for writer
3270 See L<perlipc> for an example of socketpair use.
3272 =item sort SUBNAME LIST
3274 =item sort BLOCK LIST
3278 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3279 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3280 specified, it gives the name of a subroutine that returns an integer
3281 less than, equal to, or greater than C<0>, depending on how the elements
3282 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3283 operators are extremely useful in such routines.) SUBNAME may be a
3284 scalar variable name (unsubscripted), in which case the value provides
3285 the name of (or a reference to) the actual subroutine to use. In place
3286 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3289 In the interests of efficiency the normal calling code for subroutines is
3290 bypassed, with the following effects: the subroutine may not be a
3291 recursive subroutine, and the two elements to be compared are passed into
3292 the subroutine not via C<@_> but as the package global variables C<$a> and
3293 C<$b> (see example below). They are passed by reference, so don't
3294 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3296 You also cannot exit out of the sort block or subroutine using any of the
3297 loop control operators described in L<perlsyn> or with C<goto()>.
3299 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3300 current collation locale. See L<perllocale>.
3305 @articles = sort @files;
3307 # same thing, but with explicit sort routine
3308 @articles = sort {$a cmp $b} @files;
3310 # now case-insensitively
3311 @articles = sort {uc($a) cmp uc($b)} @files;
3313 # same thing in reversed order
3314 @articles = sort {$b cmp $a} @files;
3316 # sort numerically ascending
3317 @articles = sort {$a <=> $b} @files;
3319 # sort numerically descending
3320 @articles = sort {$b <=> $a} @files;
3322 # sort using explicit subroutine name
3324 $age{$a} <=> $age{$b}; # presuming numeric
3326 @sortedclass = sort byage @class;
3328 # this sorts the %age hash by value instead of key
3329 # using an in-line function
3330 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3332 sub backwards { $b cmp $a; }
3333 @harry = ('dog','cat','x','Cain','Abel');
3334 @george = ('gone','chased','yz','Punished','Axed');
3336 # prints AbelCaincatdogx
3337 print sort backwards @harry;
3338 # prints xdogcatCainAbel
3339 print sort @george, 'to', @harry;
3340 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3342 # inefficiently sort by descending numeric compare using
3343 # the first integer after the first = sign, or the
3344 # whole record case-insensitively otherwise
3347 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3352 # same thing, but much more efficiently;
3353 # we'll build auxiliary indices instead
3357 push @nums, /=(\d+)/;
3362 $nums[$b] <=> $nums[$a]
3364 $caps[$a] cmp $caps[$b]
3368 # same thing using a Schwartzian Transform (no temps)
3369 @new = map { $_->[0] }
3370 sort { $b->[1] <=> $a->[1]
3373 } map { [$_, /=(\d+)/, uc($_)] } @old;
3375 If you're using strict, you I<MUST NOT> declare C<$a>
3376 and C<$b> as lexicals. They are package globals. That means
3377 if you're in the C<main> package, it's
3379 @articles = sort {$main::b <=> $main::a} @files;
3383 @articles = sort {$::b <=> $::a} @files;
3385 but if you're in the C<FooPack> package, it's
3387 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3389 The comparison function is required to behave. If it returns
3390 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3391 sometimes saying the opposite, for example) the results are not
3394 =item splice ARRAY,OFFSET,LENGTH,LIST
3396 =item splice ARRAY,OFFSET,LENGTH
3398 =item splice ARRAY,OFFSET
3400 Removes the elements designated by OFFSET and LENGTH from an array, and
3401 replaces them with the elements of LIST, if any. In list context,
3402 returns the elements removed from the array. In scalar context,
3403 returns the last element removed, or C<undef> if no elements are
3404 removed. The array grows or shrinks as necessary.
3405 If OFFSET is negative then it start that far from the end of the array.
3406 If LENGTH is omitted, removes everything from OFFSET onward.
3407 If LENGTH is negative, leave that many elements off the end of the array.
3408 The following equivalences hold (assuming C<$[ == 0>):
3410 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3411 pop(@a) splice(@a,-1)
3412 shift(@a) splice(@a,0,1)
3413 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3414 $a[$x] = $y splice(@a,$x,1,$y)
3416 Example, assuming array lengths are passed before arrays:
3418 sub aeq { # compare two list values
3419 my(@a) = splice(@_,0,shift);
3420 my(@b) = splice(@_,0,shift);
3421 return 0 unless @a == @b; # same len?
3423 return 0 if pop(@a) ne pop(@b);
3427 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3429 =item split /PATTERN/,EXPR,LIMIT
3431 =item split /PATTERN/,EXPR
3433 =item split /PATTERN/
3437 Splits a string into an array of strings, and returns it. By default,
3438 empty leading fields are preserved, and empty trailing ones are deleted.
3440 If not in list context, returns the number of fields found and splits into
3441 the C<@_> array. (In list context, you can force the split into C<@_> by
3442 using C<??> as the pattern delimiters, but it still returns the list
3443 value.) The use of implicit split to C<@_> is deprecated, however, because
3444 it clobbers your subroutine arguments.
3446 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3447 splits on whitespace (after skipping any leading whitespace). Anything
3448 matching PATTERN is taken to be a delimiter separating the fields. (Note
3449 that the delimiter may be longer than one character.)
3451 If LIMIT is specified and positive, splits into no more than that
3452 many fields (though it may split into fewer). If LIMIT is unspecified
3453 or zero, trailing null fields are stripped (which potential users
3454 of C<pop()> would do well to remember). If LIMIT is negative, it is
3455 treated as if an arbitrarily large LIMIT had been specified.
3457 A pattern matching the null string (not to be confused with
3458 a null pattern C<//>, which is just one member of the set of patterns
3459 matching a null string) will split the value of EXPR into separate
3460 characters at each point it matches that way. For example:
3462 print join(':', split(/ */, 'hi there'));
3464 produces the output 'h:i:t:h:e:r:e'.
3466 The LIMIT parameter can be used to split a line partially
3468 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3470 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3471 one larger than the number of variables in the list, to avoid
3472 unnecessary work. For the list above LIMIT would have been 4 by
3473 default. In time critical applications it behooves you not to split
3474 into more fields than you really need.
3476 If the PATTERN contains parentheses, additional array elements are
3477 created from each matching substring in the delimiter.
3479 split(/([,-])/, "1-10,20", 3);
3481 produces the list value
3483 (1, '-', 10, ',', 20)
3485 If you had the entire header of a normal Unix email message in C<$header>,
3486 you could split it up into fields and their values this way:
3488 $header =~ s/\n\s+/ /g; # fix continuation lines
3489 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3491 The pattern C</PATTERN/> may be replaced with an expression to specify
3492 patterns that vary at runtime. (To do runtime compilation only once,
3493 use C</$variable/o>.)
3495 As a special case, specifying a PATTERN of space (C<' '>) will split on
3496 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3497 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3498 will give you as many null initial fields as there are leading spaces.
3499 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3500 whitespace produces a null first field. A C<split()> with no arguments
3501 really does a C<split(' ', $_)> internally.
3505 open(PASSWD, '/etc/passwd');
3507 ($login, $passwd, $uid, $gid,
3508 $gcos, $home, $shell) = split(/:/);
3512 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3513 L</chomp>, and L</join>.)
3515 =item sprintf FORMAT, LIST
3517 Returns a string formatted by the usual C<printf()> conventions of the
3518 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3519 on your system for an explanation of the general principles.
3521 Perl does its own C<sprintf()> formatting -- it emulates the C
3522 function C<sprintf()>, but it doesn't use it (except for floating-point
3523 numbers, and even then only the standard modifiers are allowed). As a
3524 result, any non-standard extensions in your local C<sprintf()> are not
3525 available from Perl.
3527 Perl's C<sprintf()> permits the following universally-known conversions:
3530 %c a character with the given number
3532 %d a signed integer, in decimal
3533 %u an unsigned integer, in decimal
3534 %o an unsigned integer, in octal
3535 %x an unsigned integer, in hexadecimal
3536 %e a floating-point number, in scientific notation
3537 %f a floating-point number, in fixed decimal notation
3538 %g a floating-point number, in %e or %f notation
3540 In addition, Perl permits the following widely-supported conversions:
3542 %X like %x, but using upper-case letters
3543 %E like %e, but using an upper-case "E"
3544 %G like %g, but with an upper-case "E" (if applicable)
3545 %p a pointer (outputs the Perl value's address in hexadecimal)
3546 %n special: *stores* the number of characters output so far
3547 into the next variable in the parameter list
3549 Finally, for backward (and we do mean "backward") compatibility, Perl
3550 permits these unnecessary but widely-supported conversions:
3553 %D a synonym for %ld
3554 %U a synonym for %lu
3555 %O a synonym for %lo
3558 Perl permits the following universally-known flags between the C<%>
3559 and the conversion letter:
3561 space prefix positive number with a space
3562 + prefix positive number with a plus sign
3563 - left-justify within the field
3564 0 use zeros, not spaces, to right-justify
3565 # prefix non-zero octal with "0", non-zero hex with "0x"
3566 number minimum field width
3567 .number "precision": digits after decimal point for
3568 floating-point, max length for string, minimum length
3570 l interpret integer as C type "long" or "unsigned long"
3571 h interpret integer as C type "short" or "unsigned short"
3573 There is also one Perl-specific flag:
3575 V interpret integer as Perl's standard integer type
3577 Where a number would appear in the flags, an asterisk ("C<*>") may be
3578 used instead, in which case Perl uses the next item in the parameter
3579 list as the given number (that is, as the field width or precision).
3580 If a field width obtained through "C<*>" is negative, it has the same
3581 effect as the "C<->" flag: left-justification.
3583 If C<use locale> is in effect, the character used for the decimal
3584 point in formatted real numbers is affected by the LC_NUMERIC locale.
3591 Return the square root of EXPR. If EXPR is omitted, returns square
3598 Sets the random number seed for the C<rand()> operator. If EXPR is
3599 omitted, uses a semi-random value based on the current time and process
3600 ID, among other things. In versions of Perl prior to 5.004 the default
3601 seed was just the current C<time()>. This isn't a particularly good seed,
3602 so many old programs supply their own seed value (often C<time ^ $$> or
3603 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3605 In fact, it's usually not necessary to call C<srand()> at all, because if
3606 it is not called explicitly, it is called implicitly at the first use of
3607 the C<rand()> operator. However, this was not the case in version of Perl
3608 before 5.004, so if your script will run under older Perl versions, it
3609 should call C<srand()>.
3611 Note that you need something much more random than the default seed for
3612 cryptographic purposes. Checksumming the compressed output of one or more
3613 rapidly changing operating system status programs is the usual method. For
3616 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3618 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3621 Do I<not> call C<srand()> multiple times in your program unless you know
3622 exactly what you're doing and why you're doing it. The point of the
3623 function is to "seed" the C<rand()> function so that C<rand()> can produce
3624 a different sequence each time you run your program. Just do it once at the
3625 top of your program, or you I<won't> get random numbers out of C<rand()>!
3627 Frequently called programs (like CGI scripts) that simply use
3631 for a seed can fall prey to the mathematical property that
3635 one-third of the time. So don't do that.
3637 =item stat FILEHANDLE
3643 Returns a 13-element list giving the status info for a file, either
3644 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3645 it stats C<$_>. Returns a null list if the stat fails. Typically used
3648 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3649 $atime,$mtime,$ctime,$blksize,$blocks)
3652 Not all fields are supported on all filesystem types. Here are the
3653 meaning of the fields:
3655 0 dev device number of filesystem
3657 2 mode file mode (type and permissions)
3658 3 nlink number of (hard) links to the file
3659 4 uid numeric user ID of file's owner
3660 5 gid numeric group ID of file's owner
3661 6 rdev the device identifier (special files only)
3662 7 size total size of file, in bytes
3663 8 atime last access time since the epoch
3664 9 mtime last modify time since the epoch
3665 10 ctime inode change time (NOT creation time!) since the epoch
3666 11 blksize preferred block size for file system I/O
3667 12 blocks actual number of blocks allocated
3669 (The epoch was at 00:00 January 1, 1970 GMT.)
3671 If stat is passed the special filehandle consisting of an underline, no
3672 stat is done, but the current contents of the stat structure from the
3673 last stat or filetest are returned. Example:
3675 if (-x $file && (($d) = stat(_)) && $d < 0) {
3676 print "$file is executable NFS file\n";
3679 (This works on machines only for which the device number is negative under NFS.)
3681 In scalar context, C<stat()> returns a boolean value indicating success
3682 or failure, and, if successful, sets the information associated with
3683 the special filehandle C<_>.
3689 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3690 doing many pattern matches on the string before it is next modified.
3691 This may or may not save time, depending on the nature and number of
3692 patterns you are searching on, and on the distribution of character
3693 frequencies in the string to be searched -- you probably want to compare
3694 run times with and without it to see which runs faster. Those loops
3695 which scan for many short constant strings (including the constant
3696 parts of more complex patterns) will benefit most. You may have only
3697 one C<study()> active at a time -- if you study a different scalar the first
3698 is "unstudied". (The way C<study()> works is this: a linked list of every
3699 character in the string to be searched is made, so we know, for
3700 example, where all the C<'k'> characters are. From each search string,
3701 the rarest character is selected, based on some static frequency tables
3702 constructed from some C programs and English text. Only those places
3703 that contain this "rarest" character are examined.)
3705 For example, here is a loop that inserts index producing entries
3706 before any line containing a certain pattern:
3710 print ".IX foo\n" if /\bfoo\b/;
3711 print ".IX bar\n" if /\bbar\b/;
3712 print ".IX blurfl\n" if /\bblurfl\b/;
3717 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3718 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3719 a big win except in pathological cases. The only question is whether
3720 it saves you more time than it took to build the linked list in the
3723 Note that if you have to look for strings that you don't know till
3724 runtime, you can build an entire loop as a string and C<eval()> that to
3725 avoid recompiling all your patterns all the time. Together with
3726 undefining C<$/> to input entire files as one record, this can be very
3727 fast, often faster than specialized programs like fgrep(1). The following
3728 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3729 out the names of those files that contain a match:
3731 $search = 'while (<>) { study;';
3732 foreach $word (@words) {
3733 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3738 eval $search; # this screams
3739 $/ = "\n"; # put back to normal input delimiter
3740 foreach $file (sort keys(%seen)) {
3748 =item sub NAME BLOCK
3750 This is subroutine definition, not a real function I<per se>. With just a
3751 NAME (and possibly prototypes), it's just a forward declaration. Without
3752 a NAME, it's an anonymous function declaration, and does actually return a
3753 value: the CODE ref of the closure you just created. See L<perlsub> and
3754 L<perlref> for details.
3756 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3758 =item substr EXPR,OFFSET,LEN
3760 =item substr EXPR,OFFSET
3762 Extracts a substring out of EXPR and returns it. First character is at
3763 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3764 If OFFSET is negative (or more precisely, less than C<$[>), starts
3765 that far from the end of the string. If LEN is omitted, returns
3766 everything to the end of the string. If LEN is negative, leaves that
3767 many characters off the end of the string.
3769 If you specify a substring that is partly outside the string, the part
3770 within the string is returned. If the substring is totally outside
3771 the string a warning is produced.
3773 You can use the C<substr()> function
3774 as an lvalue, in which case EXPR must be an lvalue. If you assign
3775 something shorter than LEN, the string will shrink, and if you assign
3776 something longer than LEN, the string will grow to accommodate it. To
3777 keep the string the same length you may need to pad or chop your value
3780 An alternative to using C<substr()> as an lvalue is to specify the
3781 replacement string as the 4th argument. This allows you to replace
3782 parts of the EXPR and return what was there before in one operation.
3784 =item symlink OLDFILE,NEWFILE
3786 Creates a new filename symbolically linked to the old filename.
3787 Returns C<1> for success, C<0> otherwise. On systems that don't support
3788 symbolic links, produces a fatal error at run time. To check for that,
3791 $symlink_exists = eval { symlink("",""); 1 };
3795 Calls the system call specified as the first element of the list,
3796 passing the remaining elements as arguments to the system call. If
3797 unimplemented, produces a fatal error. The arguments are interpreted
3798 as follows: if a given argument is numeric, the argument is passed as
3799 an int. If not, the pointer to the string value is passed. You are
3800 responsible to make sure a string is pre-extended long enough to
3801 receive any result that might be written into a string. You can't use a
3802 string literal (or other read-only string) as an argument to C<syscall()>
3803 because Perl has to assume that any string pointer might be written
3805 integer arguments are not literals and have never been interpreted in a
3806 numeric context, you may need to add C<0> to them to force them to look
3807 like numbers. This emulates the C<syswrite()> function (or vice versa):
3809 require 'syscall.ph'; # may need to run h2ph
3811 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3813 Note that Perl supports passing of up to only 14 arguments to your system call,
3814 which in practice should usually suffice.
3816 Syscall returns whatever value returned by the system call it calls.
3817 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3818 Note that some system calls can legitimately return C<-1>. The proper
3819 way to handle such calls is to assign C<$!=0;> before the call and
3820 check the value of C<$!> if syscall returns C<-1>.
3822 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3823 number of the read end of the pipe it creates. There is no way
3824 to retrieve the file number of the other end. You can avoid this
3825 problem by using C<pipe()> instead.
3827 =item sysopen FILEHANDLE,FILENAME,MODE
3829 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3831 Opens the file whose filename is given by FILENAME, and associates it
3832 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3833 the name of the real filehandle wanted. This function calls the
3834 underlying operating system's C<open()> function with the parameters
3835 FILENAME, MODE, PERMS.
3837 The possible values and flag bits of the MODE parameter are
3838 system-dependent; they are available via the standard module C<Fcntl>.
3839 For historical reasons, some values work on almost every system
3840 supported by perl: zero means read-only, one means write-only, and two
3841 means read/write. We know that these values do I<not> work under
3842 OS/390 Unix and on the Macintosh; you probably don't want to use them
3845 If the file named by FILENAME does not exist and the C<open()> call creates
3846 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3847 PERMS specifies the permissions of the newly created file. If you omit
3848 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
3849 These permission values need to be in octal, and are modified by your
3850 process's current C<umask>. The C<umask> value is a number representing
3851 disabled permissions bits--if your C<umask> were C<027> (group can't write;
3852 others can't read, write, or execute), then passing C<sysopen()> C<0666> would
3853 create a file with mode C<0640> (C<0666 &~ 027> is C<0640>).
3855 If you find this C<umask()> talk confusing, here's some advice: supply a
3856 creation mode of C<0666> for regular files and one of C<0777> for directories
3857 (in C<mkdir()>) and executable files. This gives users the freedom of
3858 choice: if they want protected files, they might choose process umasks
3859 of C<022>, C<027>, or even the particularly antisocial mask of C<077>. Programs
3860 should rarely if ever make policy decisions better left to the user.
3861 The exception to this is when writing files that should be kept private:
3862 mail files, web browser cookies, I<.rhosts> files, and so on. In short,
3863 seldom if ever use C<0644> as argument to C<sysopen()> because that takes
3864 away the user's option to have a more permissive umask. Better to omit it.
3866 The C<IO::File> module provides a more object-oriented approach, if you're
3867 into that kind of thing.
3869 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3871 =item sysread FILEHANDLE,SCALAR,LENGTH
3873 Attempts to read LENGTH bytes of data into variable SCALAR from the
3874 specified FILEHANDLE, using the system call read(2). It bypasses
3875 stdio, so mixing this with other kinds of reads, C<print()>, C<write()>,
3876 C<seek()>, or C<tell()> can cause confusion because stdio usually buffers
3877 data. Returns the number of bytes actually read, C<0> at end of file,
3878 or undef if there was an error. SCALAR will be grown or shrunk so that
3879 the last byte actually read is the last byte of the scalar after the read.
3881 An OFFSET may be specified to place the read data at some place in the
3882 string other than the beginning. A negative OFFSET specifies
3883 placement at that many bytes counting backwards from the end of the
3884 string. A positive OFFSET greater than the length of SCALAR results
3885 in the string being padded to the required size with C<"\0"> bytes before
3886 the result of the read is appended.
3888 =item sysseek FILEHANDLE,POSITION,WHENCE
3890 Sets FILEHANDLE's system position using the system call lseek(2). It
3891 bypasses stdio, so mixing this with reads (other than C<sysread()>),
3892 C<print()>, C<write()>, C<seek()>, or C<tell()> may cause confusion. FILEHANDLE may
3893 be an expression whose value gives the name of the filehandle. The
3894 values for WHENCE are C<0> to set the new position to POSITION, C<1> to set
3895 the it to the current position plus POSITION, and C<2> to set it to EOF
3896 plus POSITION (typically negative). For WHENCE, you may use the
3897 constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable>
3898 or the POSIX module.
3900 Returns the new position, or the undefined value on failure. A position
3901 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
3902 TRUE on success and FALSE on failure, yet you can still easily determine
3907 =item system PROGRAM LIST
3909 Does exactly the same thing as "C<exec LIST>" except that a fork is done
3910 first, and the parent process waits for the child process to complete.
3911 Note that argument processing varies depending on the number of
3912 arguments. If there is more than one argument in LIST, or if LIST is
3913 an array with more than one value, starts the program given by the
3914 first element of the list with arguments given by the rest of the list.
3915 If there is only one scalar argument, the argument is
3916 checked for shell metacharacters, and if there are any, the entire
3917 argument is passed to the system's command shell for parsing (this is
3918 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
3919 there are no shell metacharacters in the argument, it is split into
3920 words and passed directly to C<execvp()>, which is more efficient.
3922 The return value is the exit status of the program as
3923 returned by the C<wait()> call. To get the actual exit value divide by
3924 256. See also L</exec>. This is I<NOT> what you want to use to capture
3925 the output from a command, for that you should use merely backticks or
3926 C<qx//>, as described in L<perlop/"`STRING`">.
3928 Like C<exec()>, C<system()> allows you to lie to a program about its name if
3929 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
3931 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
3932 program they're running doesn't actually interrupt your program.
3934 @args = ("command", "arg1", "arg2");
3936 or die "system @args failed: $?"
3938 You can check all the failure possibilities by inspecting
3941 $exit_value = $? >> 8;
3942 $signal_num = $? & 127;
3943 $dumped_core = $? & 128;
3945 When the arguments get executed via the system shell, results
3946 and return codes will be subject to its quirks and capabilities.
3947 See L<perlop/"`STRING`"> and L</exec> for details.
3949 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3951 =item syswrite FILEHANDLE,SCALAR,LENGTH
3953 Attempts to write LENGTH bytes of data from variable SCALAR to the
3954 specified FILEHANDLE, using the system call write(2). It bypasses
3955 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
3956 C<write()>, C<seek()>, or C<tell()> may cause confusion because stdio usually
3957 buffers data. Returns the number of bytes actually written, or C<undef>
3958 if there was an error. If the LENGTH is greater than the available
3959 data in the SCALAR after the OFFSET, only as much data as is available
3962 An OFFSET may be specified to write the data from some part of the
3963 string other than the beginning. A negative OFFSET specifies writing
3964 that many bytes counting backwards from the end of the string. In the
3965 case the SCALAR is empty you can use OFFSET but only zero offset.
3967 =item tell FILEHANDLE
3971 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3972 expression whose value gives the name of the actual filehandle. If
3973 FILEHANDLE is omitted, assumes the file last read.
3975 =item telldir DIRHANDLE
3977 Returns the current position of the C<readdir()> routines on DIRHANDLE.
3978 Value may be given to C<seekdir()> to access a particular location in a
3979 directory. Has the same caveats about possible directory compaction as
3980 the corresponding system library routine.
3982 =item tie VARIABLE,CLASSNAME,LIST
3984 This function binds a variable to a package class that will provide the
3985 implementation for the variable. VARIABLE is the name of the variable
3986 to be enchanted. CLASSNAME is the name of a class implementing objects
3987 of correct type. Any additional arguments are passed to the "C<new()>"
3988 method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
3989 Typically these are arguments such as might be passed to the C<dbm_open()>
3990 function of C. The object returned by the "C<new()>" method is also
3991 returned by the C<tie()> function, which would be useful if you want to
3992 access other methods in CLASSNAME.
3994 Note that functions such as C<keys()> and C<values()> may return huge lists
3995 when used on large objects, like DBM files. You may prefer to use the
3996 C<each()> function to iterate over such. Example:
3998 # print out history file offsets
4000 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4001 while (($key,$val) = each %HIST) {
4002 print $key, ' = ', unpack('L',$val), "\n";
4006 A class implementing a hash should have the following methods:
4008 TIEHASH classname, LIST
4011 STORE this, key, value
4015 NEXTKEY this, lastkey
4017 A class implementing an ordinary array should have the following methods:
4019 TIEARRAY classname, LIST
4022 STORE this, key, value
4025 A class implementing a scalar should have the following methods:
4027 TIESCALAR classname, LIST
4032 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4033 for you--you need to do that explicitly yourself. See L<DB_File>
4034 or the F<Config> module for interesting C<tie()> implementations.
4036 For further details see L<perltie>, L<tied VARIABLE>.
4040 Returns a reference to the object underlying VARIABLE (the same value
4041 that was originally returned by the C<tie()> call that bound the variable
4042 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4047 Returns the number of non-leap seconds since whatever time the system
4048 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4049 and 00:00:00 UTC, January 1, 1970 for most other systems).
4050 Suitable for feeding to C<gmtime()> and C<localtime()>.
4054 Returns a four-element list giving the user and system times, in
4055 seconds, for this process and the children of this process.
4057 ($user,$system,$cuser,$csystem) = times;
4061 The transliteration operator. Same as C<y///>. See L<perlop>.
4063 =item truncate FILEHANDLE,LENGTH
4065 =item truncate EXPR,LENGTH
4067 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4068 specified length. Produces a fatal error if truncate isn't implemented
4069 on your system. Returns TRUE if successful, the undefined value
4076 Returns an uppercased version of EXPR. This is the internal function
4077 implementing the C<\U> escape in double-quoted strings.
4078 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4079 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4080 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4082 If EXPR is omitted, uses C<$_>.
4088 Returns the value of EXPR with the first character
4089 in uppercase (titlecase in Unicode). This is
4090 the internal function implementing the C<\u> escape in double-quoted strings.
4091 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4093 If EXPR is omitted, uses C<$_>.
4099 Sets the umask for the process to EXPR and returns the previous value.
4100 If EXPR is omitted, merely returns the current umask.
4102 If umask(2) is not implemented on your system and you are trying to
4103 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4104 fatal error at run time. If umask(2) is not implemented and you are
4105 not trying to restrict access for yourself, returns C<undef>.
4107 Remember that a umask is a number, usually given in octal; it is I<not> a
4108 string of octal digits. See also L</oct>, if all you have is a string.
4114 Undefines the value of EXPR, which must be an lvalue. Use only on a
4115 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4116 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4117 will probably not do what you expect on most predefined variables or
4118 DBM list values, so don't do that; see L<delete>.) Always returns the
4119 undefined value. You can omit the EXPR, in which case nothing is
4120 undefined, but you still get an undefined value that you could, for
4121 instance, return from a subroutine, assign to a variable or pass as a
4122 parameter. Examples:
4125 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4129 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4130 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4131 select undef, undef, undef, 0.25;
4132 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4134 Note that this is a unary operator, not a list operator.
4140 Deletes a list of files. Returns the number of files successfully
4143 $cnt = unlink 'a', 'b', 'c';
4147 Note: C<unlink()> will not delete directories unless you are superuser and
4148 the B<-U> flag is supplied to Perl. Even if these conditions are
4149 met, be warned that unlinking a directory can inflict damage on your
4150 filesystem. Use C<rmdir()> instead.
4152 If LIST is omitted, uses C<$_>.
4154 =item unpack TEMPLATE,EXPR
4156 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4157 structure and expands it out into a list value, returning the array
4158 value. (In scalar context, it returns merely the first value
4159 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4160 Here's a subroutine that does substring:
4163 my($what,$where,$howmuch) = @_;
4164 unpack("x$where a$howmuch", $what);
4169 sub ordinal { unpack("c",$_[0]); } # same as ord()
4171 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4172 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4173 themselves. Default is a 16-bit checksum. For example, the following
4174 computes the same number as the System V sum program:
4177 $checksum += unpack("%16C*", $_);
4181 The following efficiently counts the number of set bits in a bit vector:
4183 $setbits = unpack("%32b*", $selectmask);
4185 =item untie VARIABLE
4187 Breaks the binding between a variable and a package. (See C<tie()>.)
4189 =item unshift ARRAY,LIST
4191 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4192 depending on how you look at it. Prepends list to the front of the
4193 array, and returns the new number of elements in the array.
4195 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4197 Note the LIST is prepended whole, not one element at a time, so the
4198 prepended elements stay in the same order. Use C<reverse()> to do the
4201 =item use Module LIST
4205 =item use Module VERSION LIST
4209 Imports some semantics into the current package from the named module,
4210 generally by aliasing certain subroutine or variable names into your
4211 package. It is exactly equivalent to
4213 BEGIN { require Module; import Module LIST; }
4215 except that Module I<must> be a bareword.
4217 If the first argument to C<use> is a number, it is treated as a version
4218 number instead of a module name. If the version of the Perl interpreter
4219 is less than VERSION, then an error message is printed and Perl exits
4220 immediately. This is often useful if you need to check the current
4221 Perl version before C<use>ing library modules that have changed in
4222 incompatible ways from older versions of Perl. (We try not to do
4223 this more than we have to.)
4225 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4226 C<require> makes sure the module is loaded into memory if it hasn't been
4227 yet. The C<import()> is not a builtin--it's just an ordinary static method
4228 call into the "C<Module>" package to tell the module to import the list of
4229 features back into the current package. The module can implement its
4230 C<import()> method any way it likes, though most modules just choose to
4231 derive their C<import()> method via inheritance from the C<Exporter> class that
4232 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4233 method can be found then the error is currently silently ignored. This
4234 may change to a fatal error in a future version.
4236 If you don't want your namespace altered, explicitly supply an empty list:
4240 That is exactly equivalent to
4242 BEGIN { require Module }
4244 If the VERSION argument is present between Module and LIST, then the
4245 C<use> will call the VERSION method in class Module with the given
4246 version as an argument. The default VERSION method, inherited from
4247 the Universal class, croaks if the given version is larger than the
4248 value of the variable C<$Module::VERSION>. (Note that there is not a
4249 comma after VERSION!)
4251 Because this is a wide-open interface, pragmas (compiler directives)
4252 are also implemented this way. Currently implemented pragmas are:
4256 use sigtrap qw(SEGV BUS);
4257 use strict qw(subs vars refs);
4258 use subs qw(afunc blurfl);
4260 Some of these these pseudo-modules import semantics into the current
4261 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4262 which import symbols into the current package (which are effective
4263 through the end of the file).
4265 There's a corresponding "C<no>" command that unimports meanings imported
4266 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4271 If no C<unimport()> method can be found the call fails with a fatal error.
4273 See L<perlmod> for a list of standard modules and pragmas.
4277 Changes the access and modification times on each file of a list of
4278 files. The first two elements of the list must be the NUMERICAL access
4279 and modification times, in that order. Returns the number of files
4280 successfully changed. The inode modification time of each file is set
4281 to the current time. This code has the same effect as the "C<touch>"
4282 command if the files already exist:
4286 utime $now, $now, @ARGV;
4290 Returns a list consisting of all the values of the named hash. (In a
4291 scalar context, returns the number of values.) The values are
4292 returned in an apparently random order, but it is the same order as
4293 either the C<keys()> or C<each()> function would produce on the same hash.
4294 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4297 =item vec EXPR,OFFSET,BITS
4299 Treats the string in EXPR as a vector of unsigned integers, and
4300 returns the value of the bit field specified by OFFSET. BITS specifies
4301 the number of bits that are reserved for each entry in the bit
4302 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4303 assigned to, in which case parentheses are needed to give the expression
4304 the correct precedence as in
4306 vec($image, $max_x * $x + $y, 8) = 3;
4308 Vectors created with C<vec()> can also be manipulated with the logical
4309 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4310 desired when both operands are strings.
4312 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4313 The comments show the string after each step. Note that this code works
4314 in the same way on big-endian or little-endian machines.
4317 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4318 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4319 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4320 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4321 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4322 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4323 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4325 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4326 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4327 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4330 To transform a bit vector into a string or array of 0's and 1's, use these:
4332 $bits = unpack("b*", $vector);
4333 @bits = split(//, unpack("b*", $vector));
4335 If you know the exact length in bits, it can be used in place of the C<*>.
4339 Waits for a child process to terminate and returns the pid of the
4340 deceased process, or C<-1> if there are no child processes. The status is
4343 =item waitpid PID,FLAGS
4345 Waits for a particular child process to terminate and returns the pid
4346 of the deceased process, or C<-1> if there is no such child process. The
4347 status is returned in C<$?>. If you say
4349 use POSIX ":sys_wait_h";
4351 waitpid(-1,&WNOHANG);
4353 then you can do a non-blocking wait for any process. Non-blocking wait
4354 is available on machines supporting either the waitpid(2) or
4355 wait4(2) system calls. However, waiting for a particular pid with
4356 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4357 by remembering the status values of processes that have exited but have
4358 not been harvested by the Perl script yet.)
4360 See L<perlipc> for other examples.
4364 Returns TRUE if the context of the currently executing subroutine is
4365 looking for a list value. Returns FALSE if the context is looking
4366 for a scalar. Returns the undefined value if the context is looking
4367 for no value (void context).
4369 return unless defined wantarray; # don't bother doing more
4370 my @a = complex_calculation();
4371 return wantarray ? @a : "@a";
4375 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4378 If LIST is empty and C<$@> already contains a value (typically from a
4379 previous eval) that value is used after appending C<"\t...caught">
4380 to C<$@>. This is useful for staying almost, but not entirely similar to
4383 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4385 No message is printed if there is a C<$SIG{__WARN__}> handler
4386 installed. It is the handler's responsibility to deal with the message
4387 as it sees fit (like, for instance, converting it into a C<die()>). Most
4388 handlers must therefore make arrangements to actually display the
4389 warnings that they are not prepared to deal with, by calling C<warn()>
4390 again in the handler. Note that this is quite safe and will not
4391 produce an endless loop, since C<__WARN__> hooks are not called from
4394 You will find this behavior is slightly different from that of
4395 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4396 instead call C<die()> again to change it).
4398 Using a C<__WARN__> handler provides a powerful way to silence all
4399 warnings (even the so-called mandatory ones). An example:
4401 # wipe out *all* compile-time warnings
4402 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4404 my $foo = 20; # no warning about duplicate my $foo,
4405 # but hey, you asked for it!
4406 # no compile-time or run-time warnings before here
4409 # run-time warnings enabled after here
4410 warn "\$foo is alive and $foo!"; # does show up
4412 See L<perlvar> for details on setting C<%SIG> entries, and for more
4415 =item write FILEHANDLE
4421 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4422 using the format associated with that file. By default the format for
4423 a file is the one having the same name as the filehandle, but the
4424 format for the current output channel (see the C<select()> function) may be set
4425 explicitly by assigning the name of the format to the C<$~> variable.
4427 Top of form processing is handled automatically: if there is
4428 insufficient room on the current page for the formatted record, the
4429 page is advanced by writing a form feed, a special top-of-page format
4430 is used to format the new page header, and then the record is written.
4431 By default the top-of-page format is the name of the filehandle with
4432 "_TOP" appended, but it may be dynamically set to the format of your
4433 choice by assigning the name to the C<$^> variable while the filehandle is
4434 selected. The number of lines remaining on the current page is in
4435 variable C<$->, which can be set to C<0> to force a new page.
4437 If FILEHANDLE is unspecified, output goes to the current default output
4438 channel, which starts out as STDOUT but may be changed by the
4439 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4440 is evaluated and the resulting string is used to look up the name of
4441 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4443 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4447 The transliteration operator. Same as C<tr///>. See L<perlop>.