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 or 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 such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
84 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 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>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (E<gt>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, FALSE otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
383 Arranges to have a SIGALRM delivered to this process after the
384 specified number of seconds have elapsed. If SECONDS is not specified,
385 the value stored in C<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-arugment version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall()> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm()>
400 and C<sleep()> calls.
402 If you want to use C<alarm()> to time out a system call you need to use an
403 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
405 restart system calls on some systems. Using C<eval()>/C<die()> always works,
406 modulo the caveats given in L<perlipc/"Signals">.
409 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
411 $nread = sysread SOCKET, $buffer, $size;
415 die unless $@ eq "alarm\n"; # propagate unexpected errors
424 Returns the arctangent of Y/X in the range -PI to PI.
426 For the tangent operation, you may use the C<POSIX::tan()>
427 function, or use the familiar relation:
429 sub tan { sin($_[0]) / cos($_[0]) }
431 =item bind SOCKET,NAME
433 Binds a network address to a socket, just as the bind system call
434 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
435 packed address of the appropriate type for the socket. See the examples in
436 L<perlipc/"Sockets: Client/Server Communication">.
438 =item binmode FILEHANDLE
440 Arranges for the file to be read or written in "binary" mode in operating
441 systems that distinguish between binary and text files. Files that
442 are not in binary mode have CR LF sequences translated to LF on input
443 and LF translated to CR LF on output. Binmode has no effect under
444 many sytems, but in MS-DOS and similarly archaic systems, it may be
445 imperative--otherwise your MS-DOS-damaged C library may mangle your file.
446 The key distinction between systems that need C<binmode()> and those
447 that don't is their text file formats. Systems like Unix, MacOS, and
448 Plan9 that delimit lines with a single character, and that encode that
449 character in C as C<"\n">, do not need C<binmode()>. The rest may need it.
450 If FILEHANDLE is an expression, the value is taken as the name of the
453 If the system does care about it, using it when you shouldn't is just as
454 perilous as failing to use it when you should. Fortunately for most of
455 us, you can't go wrong using binmode() on systems that don't care about
458 =item bless REF,CLASSNAME
462 This function tells the thingy referenced by REF that it is now an object
463 in the CLASSNAME package. If CLASSNAME is omitted, the current package
464 is used. Because a C<bless()> is often the last thing in a constructor.
465 it returns the reference for convenience. Always use the two-argument
466 version if the function doing the blessing might be inherited by a
467 derived class. See L<perltoot> and L<perlobj> for more about the blessing
468 (and blessings) of objects.
470 Consider always blessing objects in CLASSNAMEs that are mixed case.
471 Namespaces with all lowercase names are considered reserved for
472 Perl pragmata. Builtin types have all uppercase names, so to prevent
473 confusion, you may wish to avoid such package names as well. Make sure
474 that CLASSNAME is a true value.
476 See L<perlmod/"Perl Modules">.
482 Returns the context of the current subroutine call. In scalar context,
483 returns the caller's package name if there is a caller, that is, if
484 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
485 otherwise. In list context, returns
487 ($package, $filename, $line) = caller;
489 With EXPR, it returns some extra information that the debugger uses to
490 print a stack trace. The value of EXPR indicates how many call frames
491 to go back before the current one.
493 ($package, $filename, $line, $subroutine,
494 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
496 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
497 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
498 C<$is_require> are set: C<$is_require> is true if the frame is created by a
499 C<require> or C<use> statement, C<$evaltext> contains the text of the
500 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
501 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
502 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
505 Furthermore, when called from within the DB package, caller returns more
506 detailed information: it sets the list variable C<@DB::args> to be the
507 arguments with which the subroutine was invoked.
509 Be aware that the optimizer might have optimized call frames away before
510 C<caller()> had a chance to get the information. That means that C<caller(N)>
511 might not return information about the call frame you expect it do, for
512 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
513 previous time C<caller()> was called.
517 Changes the working directory to EXPR, if possible. If EXPR is omitted,
518 changes to the user's home directory. Returns TRUE upon success,
519 FALSE otherwise. See the example under C<die()>.
523 Changes the permissions of a list of files. The first element of the
524 list must be the numerical mode, which should probably be an octal
525 number, and which definitely should I<not> a string of octal digits:
526 C<0644> is okay, C<'0644'> is not. Returns the number of files
527 successfully changed. See also L</oct>, if all you have is a string.
529 $cnt = chmod 0755, 'foo', 'bar';
530 chmod 0755, @executables;
531 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
533 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
534 $mode = 0644; chmod $mode, 'foo'; # this is best
542 This safer version of L</chop> removes any trailing string
543 that corresponds to the current value of C<$/> (also known as
544 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
545 number of characters removed from all its arguments. It's often used to
546 remove the newline from the end of an input record when you're worried
547 that the final record may be missing its newline. When in paragraph
548 mode (C<$/ = "">), it removes all trailing newlines from the string.
549 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
550 a reference to an integer or the like, see L<perlvar>) chomp() won't
551 remove anything. If VARIABLE is omitted, it chomps C<$_>. Example:
554 chomp; # avoid \n on last field
559 You can actually chomp anything that's an lvalue, including an assignment:
562 chomp($answer = <STDIN>);
564 If you chomp a list, each element is chomped, and the total number of
565 characters removed is returned.
573 Chops off the last character of a string and returns the character
574 chopped. It's used primarily to remove the newline from the end of an
575 input record, but is much more efficient than C<s/\n//> because it neither
576 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
580 chop; # avoid \n on last field
585 You can actually chop anything that's an lvalue, including an assignment:
588 chop($answer = <STDIN>);
590 If you chop a list, each element is chopped. Only the value of the
591 last C<chop()> is returned.
593 Note that C<chop()> returns the last character. To return all but the last
594 character, use C<substr($string, 0, -1)>.
598 Changes the owner (and group) of a list of files. The first two
599 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
600 Returns the number of files successfully changed.
602 $cnt = chown $uid, $gid, 'foo', 'bar';
603 chown $uid, $gid, @filenames;
605 Here's an example that looks up nonnumeric uids in the passwd file:
608 chop($user = <STDIN>);
610 chop($pattern = <STDIN>);
612 ($login,$pass,$uid,$gid) = getpwnam($user)
613 or die "$user not in passwd file";
615 @ary = glob($pattern); # expand filenames
616 chown $uid, $gid, @ary;
618 On most systems, you are not allowed to change the ownership of the
619 file unless you're the superuser, although you should be able to change
620 the group to any of your secondary groups. On insecure systems, these
621 restrictions may be relaxed, but this is not a portable assumption.
627 Returns the character represented by that NUMBER in the character set.
628 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
629 chr(0x263a) is a Unicode smiley face (but only within the scope of
630 a C<use utf8>). For the reverse, use L</ord>.
631 See L<utf8> for more about Unicode.
633 If NUMBER is omitted, uses C<$_>.
635 =item chroot FILENAME
639 This function works like the system call by the same name: it makes the
640 named directory the new root directory for all further pathnames that
641 begin with a C<"/"> by your process and all its children. (It doesn't
642 change your current working directory, which is unaffected.) For security
643 reasons, this call is restricted to the superuser. If FILENAME is
644 omitted, does a C<chroot()> to C<$_>.
646 =item close FILEHANDLE
650 Closes the file or pipe associated with the file handle, returning TRUE
651 only if stdio successfully flushes buffers and closes the system file
652 descriptor. Closes the currently selected filehandle if the argument
655 You don't have to close FILEHANDLE if you are immediately going to do
656 another C<open()> on it, because C<open()> will close it for you. (See
657 C<open()>.) However, an explicit C<close()> on an input file resets the line
658 counter (C<$.>), while the implicit close done by C<open()> does not.
660 If the file handle came from a piped open C<close()> will additionally
661 return FALSE if one of the other system calls involved fails or if the
662 program exits with non-zero status. (If the only problem was that the
663 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
664 also waits for the process executing on the pipe to complete, in case you
665 want to look at the output of the pipe afterwards, and
666 implicitly puts the exit status value of that command into C<$?>.
670 open(OUTPUT, '|sort >foo') # pipe to sort
671 or die "Can't start sort: $!";
672 #... # print stuff to output
673 close OUTPUT # wait for sort to finish
674 or warn $! ? "Error closing sort pipe: $!"
675 : "Exit status $? from sort";
676 open(INPUT, 'foo') # get sort's results
677 or die "Can't open 'foo' for input: $!";
679 FILEHANDLE may be an expression whose value can be used as an indirect
680 filehandle, usually the real filehandle name.
682 =item closedir DIRHANDLE
684 Closes a directory opened by C<opendir()> and returns the success of that
687 DIRHANDLE may be an expression whose value can be used as an indirect
688 dirhandle, usually the real dirhandle name.
690 =item connect SOCKET,NAME
692 Attempts to connect to a remote socket, just as the connect system call
693 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
694 packed address of the appropriate type for the socket. See the examples in
695 L<perlipc/"Sockets: Client/Server Communication">.
699 Actually a flow control statement rather than a function. If there is a
700 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
701 C<foreach>), it is always executed just before the conditional is about to
702 be evaluated again, just like the third part of a C<for> loop in C. Thus
703 it can be used to increment a loop variable, even when the loop has been
704 continued via the C<next> statement (which is similar to the C C<continue>
707 C<last>, C<next>, or C<redo> may appear within a C<continue>
708 block. C<last> and C<redo> will behave as if they had been executed within
709 the main block. So will C<next>, but since it will execute a C<continue>
710 block, it may be more entertaining.
713 ### redo always comes here
716 ### next always comes here
718 # then back the top to re-check EXPR
720 ### last always comes here
722 Omitting the C<continue> section is semantically equivalent to using an
723 empty one, logically enough. In that case, C<next> goes directly back
724 to check the condition at the top of the loop.
728 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
729 takes cosine of C<$_>.
731 For the inverse cosine operation, you may use the C<POSIX::acos()>
732 function, or use this relation:
734 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
736 =item crypt PLAINTEXT,SALT
738 Encrypts a string exactly like the crypt(3) function in the C library
739 (assuming that you actually have a version there that has not been
740 extirpated as a potential munition). This can prove useful for checking
741 the password file for lousy passwords, amongst other things. Only the
742 guys wearing white hats should do this.
744 Note that C<crypt()> is intended to be a one-way function, much like breaking
745 eggs to make an omelette. There is no (known) corresponding decrypt
746 function. As a result, this function isn't all that useful for
747 cryptography. (For that, see your nearby CPAN mirror.)
749 When verifying an existing encrypted string you should use the encrypted
750 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
751 allows your code to work with the standard C<crypt()> and with more
752 exotic implementations. When choosing a new salt create a random two
753 character string whose characters come from the set C<[./0-9A-Za-z]>
754 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
756 Here's an example that makes sure that whoever runs this program knows
759 $pwd = (getpwuid($<))[1];
763 chomp($word = <STDIN>);
767 if (crypt($word, $pwd) ne $pwd) {
773 Of course, typing in your own password to whoever asks you
778 [This function has been largely superseded by the C<untie()> function.]
780 Breaks the binding between a DBM file and a hash.
782 =item dbmopen HASH,DBNAME,MODE
784 [This function has been largely superseded by the C<tie()> function.]
786 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
787 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
788 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
789 is the name of the database (without the F<.dir> or F<.pag> extension if
790 any). If the database does not exist, it is created with protection
791 specified by MODE (as modified by the C<umask()>). If your system supports
792 only the older DBM functions, you may perform only one C<dbmopen()> in your
793 program. In older versions of Perl, if your system had neither DBM nor
794 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
797 If you don't have write access to the DBM file, you can only read hash
798 variables, not set them. If you want to test whether you can write,
799 either use file tests or try setting a dummy hash entry inside an C<eval()>,
800 which will trap the error.
802 Note that functions such as C<keys()> and C<values()> may return huge lists
803 when used on large DBM files. You may prefer to use the C<each()>
804 function to iterate over large DBM files. Example:
806 # print out history file offsets
807 dbmopen(%HIST,'/usr/lib/news/history',0666);
808 while (($key,$val) = each %HIST) {
809 print $key, ' = ', unpack('L',$val), "\n";
813 See also L<AnyDBM_File> for a more general description of the pros and
814 cons of the various dbm approaches, as well as L<DB_File> for a particularly
817 You can control which DBM library you use by loading that library
818 before you call dbmopen():
821 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
822 or die "Can't open netscape history file: $!";
828 Returns a Boolean value telling whether EXPR has a value other than
829 the undefined value C<undef>. If EXPR is not present, C<$_> will be
832 Many operations return C<undef> to indicate failure, end of file,
833 system error, uninitialized variable, and other exceptional
834 conditions. This function allows you to distinguish C<undef> from
835 other values. (A simple Boolean test will not distinguish among
836 C<undef>, zero, the empty string, and C<"0">, which are all equally
837 false.) Note that since C<undef> is a valid scalar, its presence
838 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
839 returns C<undef> when its argument is an empty array, I<or> when the
840 element to return happens to be C<undef>.
842 You may also use C<defined()> to check whether a subroutine exists, by
843 saying C<defined &func> without parentheses. On the other hand, use
844 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
845 produce intuitive results, and should probably be avoided.
847 When used on a hash element, it tells you whether the value is defined,
848 not whether the key exists in the hash. Use L</exists> for the latter
853 print if defined $switch{'D'};
854 print "$val\n" while defined($val = pop(@ary));
855 die "Can't readlink $sym: $!"
856 unless defined($value = readlink $sym);
857 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
858 $debugging = 0 unless defined $debugging;
860 Note: Many folks tend to overuse C<defined()>, and then are surprised to
861 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
862 defined values. For example, if you say
866 The pattern match succeeds, and C<$1> is defined, despite the fact that it
867 matched "nothing". But it didn't really match nothing--rather, it
868 matched something that happened to be zero characters long. This is all
869 very above-board and honest. When a function returns an undefined value,
870 it's an admission that it couldn't give you an honest answer. So you
871 should use C<defined()> only when you're questioning the integrity of what
872 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
875 Currently, using C<defined()> on an entire array or hash reports whether
876 memory for that aggregate has ever been allocated. So an array you set
877 to the empty list appears undefined initially, and one that once was full
878 and that you then set to the empty list still appears defined. You
879 should instead use a simple test for size:
881 if (@an_array) { print "has array elements\n" }
882 if (%a_hash) { print "has hash members\n" }
884 Using C<undef()> on these, however, does clear their memory and then report
885 them as not defined anymore, but you shouldn't do that unless you don't
886 plan to use them again, because it saves time when you load them up
887 again to have memory already ready to be filled. The normal way to
888 free up space used by an aggregate is to assign the empty list.
890 This counterintuitive behavior of C<defined()> on aggregates may be
891 changed, fixed, or broken in a future release of Perl.
893 See also L</undef>, L</exists>, L</ref>.
897 Deletes the specified key(s) and their associated values from a hash.
898 For each key, returns the deleted value associated with that key, or
899 the undefined value if there was no such key. Deleting from C<$ENV{}>
900 modifies the environment. Deleting from a hash tied to a DBM file
901 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
902 doesn't necessarily return anything.)
904 The following deletes all the values of a hash:
906 foreach $key (keys %HASH) {
912 delete @HASH{keys %HASH}
914 But both of these are slower than just assigning the empty list
917 %hash = (); # completely empty %hash
918 undef %hash; # forget %hash every existed
920 Note that the EXPR can be arbitrarily complicated as long as the final
921 operation is a hash element lookup or hash slice:
923 delete $ref->[$x][$y]{$key};
924 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
928 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
929 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
930 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
931 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
932 C<$@> and the C<eval()> is terminated with the undefined value. This makes
933 C<die()> the way to raise an exception.
937 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
938 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
940 If the value of EXPR does not end in a newline, the current script line
941 number and input line number (if any) are also printed, and a newline
942 is supplied. Note that the "input line number" (also known as "chunk")
943 is subject to whatever notion of "line" happens to be currently in
944 effect, and is also available as the special variable C<$.>.
945 See L<perlvar/"$/"> and L<perlvar/"$.">.
947 Hint: sometimes appending C<", stopped"> to your message
948 will cause it to make better sense when the string C<"at foo line 123"> is
949 appended. Suppose you are running script "canasta".
951 die "/etc/games is no good";
952 die "/etc/games is no good, stopped";
954 produce, respectively
956 /etc/games is no good at canasta line 123.
957 /etc/games is no good, stopped at canasta line 123.
959 See also exit(), warn(), and the Carp module.
961 If LIST is empty and C<$@> already contains a value (typically from a
962 previous eval) that value is reused after appending C<"\t...propagated">.
963 This is useful for propagating exceptions:
966 die unless $@ =~ /Expected exception/;
968 If C<$@> is empty then the string C<"Died"> is used.
970 die() can also be called with a reference argument. If this happens to be
971 trapped within an eval(), $@ contains the reference. This behavior permits
972 a more elaborate exception handling implementation using objects that
973 maintain arbitary state about the nature of the exception. Such a scheme
974 is sometimes preferable to matching particular string values of $@ using
975 regular expressions. Here's an example:
977 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
979 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
980 # handle Some::Module::Exception
983 # handle all other possible exceptions
987 Since perl will stringify uncaught exception messages before displaying
988 them, you may want to overload stringification operations on such custom
989 exception objects. See L<overload> for details about that.
991 You can arrange for a callback to be run just before the C<die()> does
992 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
993 will be called with the error text and can change the error message, if
994 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
995 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
997 Note that the C<$SIG{__DIE__}> hook is currently called even inside
998 eval()ed blocks/strings! If one wants the hook to do nothing in such
1003 as the first line of the handler (see L<perlvar/$^S>). Because this
1004 promotes action at a distance, this counterintuitive behavior may be fixed
1005 in a future release.
1009 Not really a function. Returns the value of the last command in the
1010 sequence of commands indicated by BLOCK. When modified by a loop
1011 modifier, executes the BLOCK once before testing the loop condition.
1012 (On other statements the loop modifiers test the conditional first.)
1014 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1015 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1016 See L<perlsyn> for alternative strategies.
1018 =item do SUBROUTINE(LIST)
1020 A deprecated form of subroutine call. See L<perlsub>.
1024 Uses the value of EXPR as a filename and executes the contents of the
1025 file as a Perl script. Its primary use is to include subroutines
1026 from a Perl subroutine library.
1032 scalar eval `cat stat.pl`;
1034 except that it's more efficient and concise, keeps track of the current
1035 filename for error messages, searches the @INC libraries, and updates
1036 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1037 variables. It also differs in that code evaluated with C<do FILENAME>
1038 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1039 same, however, in that it does reparse the file every time you call it,
1040 so you probably don't want to do this inside a loop.
1042 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1043 error. If C<do> can read the file but cannot compile it, it
1044 returns undef and sets an error message in C<$@>. If the file is
1045 successfully compiled, C<do> returns the value of the last expression
1048 Note that inclusion of library modules is better done with the
1049 C<use()> and C<require()> operators, which also do automatic error checking
1050 and raise an exception if there's a problem.
1052 You might like to use C<do> to read in a program configuration
1053 file. Manual error checking can be done this way:
1055 # read in config files: system first, then user
1056 for $file ("/share/prog/defaults.rc",
1057 "$ENV{HOME}/.someprogrc")
1059 unless ($return = do $file) {
1060 warn "couldn't parse $file: $@" if $@;
1061 warn "couldn't do $file: $!" unless defined $return;
1062 warn "couldn't run $file" unless $return;
1070 This causes an immediate core dump. Primarily this is so that you can
1071 use the B<undump> program to turn your core dump into an executable binary
1072 after having initialized all your variables at the beginning of the
1073 program. When the new binary is executed it will begin by executing a
1074 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1075 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1076 is omitted, restarts the program from the top. WARNING: Any files
1077 opened at the time of the dump will NOT be open any more when the
1078 program is reincarnated, with possible resulting confusion on the part
1079 of Perl. See also B<-u> option in L<perlrun>.
1084 require 'getopt.pl';
1096 dump QUICKSTART if $ARGV[0] eq '-d';
1101 This operator is largely obsolete, partly because it's very hard to
1102 convert a core file into an executable, and because the real perl-to-C
1103 compiler has superseded it.
1107 When called in list context, returns a 2-element list consisting of the
1108 key and value for the next element of a hash, so that you can iterate over
1109 it. When called in scalar context, returns the key for only the "next"
1110 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1111 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1114 Entries are returned in an apparently random order. The actual random
1115 order is subject to change in future versions of perl, but it is guaranteed
1116 to be in the same order as either the C<keys()> or C<values()> function
1117 would produce on the same (unmodified) hash.
1119 When the hash is entirely read, a null array is returned in list context
1120 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1121 scalar context. The next call to C<each()> after that will start iterating
1122 again. There is a single iterator for each hash, shared by all C<each()>,
1123 C<keys()>, and C<values()> function calls in the program; it can be reset by
1124 reading all the elements from the hash, or by evaluating C<keys HASH> or
1125 C<values HASH>. If you add or delete elements of a hash while you're
1126 iterating over it, you may get entries skipped or duplicated, so don't.
1128 The following prints out your environment like the printenv(1) program,
1129 only in a different order:
1131 while (($key,$value) = each %ENV) {
1132 print "$key=$value\n";
1135 See also C<keys()>, C<values()> and C<sort()>.
1137 =item eof FILEHANDLE
1143 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1144 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1145 gives the real filehandle. (Note that this function actually
1146 reads a character and then C<ungetc()>s it, so isn't very useful in an
1147 interactive context.) Do not read from a terminal file (or call
1148 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1149 as terminals may lose the end-of-file condition if you do.
1151 An C<eof> without an argument uses the last file read as argument.
1152 Using C<eof()> with empty parentheses is very different. It indicates
1153 the pseudo file formed of the files listed on the command line, i.e.,
1154 C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)> loop to
1155 detect the end of only the last file. Use C<eof(ARGV)> or eof without the
1156 parentheses to test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1158 # reset line numbering on each input file
1160 next if /^\s*#/; # skip comments
1163 close ARGV if eof; # Not eof()!
1166 # insert dashes just before last line of last file
1168 if (eof()) { # check for end of current file
1169 print "--------------\n";
1170 close(ARGV); # close or last; is needed if we
1171 # are reading from the terminal
1176 Practical hint: you almost never need to use C<eof> in Perl, because the
1177 input operators return false values when they run out of data, or if there
1184 In the first form, the return value of EXPR is parsed and executed as if it
1185 were a little Perl program. The value of the expression (which is itself
1186 determined within scalar context) is first parsed, and if there weren't any
1187 errors, executed in the context of the current Perl program, so that any
1188 variable settings or subroutine and format definitions remain afterwards.
1189 Note that the value is parsed every time the eval executes. If EXPR is
1190 omitted, evaluates C<$_>. This form is typically used to delay parsing
1191 and subsequent execution of the text of EXPR until run time.
1193 In the second form, the code within the BLOCK is parsed only once--at the
1194 same time the code surrounding the eval itself was parsed--and executed
1195 within the context of the current Perl program. This form is typically
1196 used to trap exceptions more efficiently than the first (see below), while
1197 also providing the benefit of checking the code within BLOCK at compile
1200 The final semicolon, if any, may be omitted from the value of EXPR or within
1203 In both forms, the value returned is the value of the last expression
1204 evaluated inside the mini-program; a return statement may be also used, just
1205 as with subroutines. The expression providing the return value is evaluated
1206 in void, scalar, or list context, depending on the context of the eval itself.
1207 See L</wantarray> for more on how the evaluation context can be determined.
1209 If there is a syntax error or runtime error, or a C<die()> statement is
1210 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1211 error message. If there was no error, C<$@> is guaranteed to be a null
1212 string. Beware that using C<eval()> neither silences perl from printing
1213 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1214 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1215 L</warn> and L<perlvar>.
1217 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1218 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1219 is implemented. It is also Perl's exception trapping mechanism, where
1220 the die operator is used to raise exceptions.
1222 If the code to be executed doesn't vary, you may use the eval-BLOCK
1223 form to trap run-time errors without incurring the penalty of
1224 recompiling each time. The error, if any, is still returned in C<$@>.
1227 # make divide-by-zero nonfatal
1228 eval { $answer = $a / $b; }; warn $@ if $@;
1230 # same thing, but less efficient
1231 eval '$answer = $a / $b'; warn $@ if $@;
1233 # a compile-time error
1234 eval { $answer = }; # WRONG
1237 eval '$answer ='; # sets $@
1239 Due to the current arguably broken state of C<__DIE__> hooks, when using
1240 the C<eval{}> form as an exception trap in libraries, you may wish not
1241 to trigger any C<__DIE__> hooks that user code may have installed.
1242 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1243 as shown in this example:
1245 # a very private exception trap for divide-by-zero
1246 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1249 This is especially significant, given that C<__DIE__> hooks can call
1250 C<die()> again, which has the effect of changing their error messages:
1252 # __DIE__ hooks may modify error messages
1254 local $SIG{'__DIE__'} =
1255 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1256 eval { die "foo lives here" };
1257 print $@ if $@; # prints "bar lives here"
1260 Because this promotes action at a distance, this counterintuive behavior
1261 may be fixed in a future release.
1263 With an C<eval()>, you should be especially careful to remember what's
1264 being looked at when:
1270 eval { $x }; # CASE 4
1272 eval "\$$x++"; # CASE 5
1275 Cases 1 and 2 above behave identically: they run the code contained in
1276 the variable C<$x>. (Although case 2 has misleading double quotes making
1277 the reader wonder what else might be happening (nothing is).) Cases 3
1278 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1279 does nothing but return the value of C<$x>. (Case 4 is preferred for
1280 purely visual reasons, but it also has the advantage of compiling at
1281 compile-time instead of at run-time.) Case 5 is a place where
1282 normally you I<WOULD> like to use double quotes, except that in this
1283 particular situation, you can just use symbolic references instead, as
1286 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1287 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1291 =item exec PROGRAM LIST
1293 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1294 use C<system()> instead of C<exec()> if you want it to return. It fails and
1295 returns FALSE only if the command does not exist I<and> it is executed
1296 directly instead of via your system's command shell (see below).
1298 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1299 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1300 or C<exit()> (if C<-w> is set - but you always do that). If you
1301 I<really> want to follow an C<exec()> with some other statement, you
1302 can use one of these styles to avoid the warning:
1304 exec ('foo') or print STDERR "couldn't exec foo: $!";
1305 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1307 If there is more than one argument in LIST, or if LIST is an array
1308 with more than one value, calls execvp(3) with the arguments in LIST.
1309 If there is only one scalar argument or an array with one element in it,
1310 the argument is checked for shell metacharacters, and if there are any,
1311 the entire argument is passed to the system's command shell for parsing
1312 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1313 If there are no shell metacharacters in the argument, it is split into
1314 words and passed directly to C<execvp()>, which is more efficient.
1316 All files opened for output are flushed before attempting the exec().
1318 If you don't really want to execute the first argument, but want to lie
1319 to the program you are executing about its own name, you can specify
1320 the program you actually want to run as an "indirect object" (without a
1321 comma) in front of the LIST. (This always forces interpretation of the
1322 LIST as a multivalued list, even if there is only a single scalar in
1325 $shell = '/bin/csh';
1326 exec $shell '-sh'; # pretend it's a login shell
1330 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1332 When the arguments get executed via the system shell, results will
1333 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1336 Using an indirect object with C<exec()> or C<system()> is also more secure.
1337 This usage forces interpretation of the arguments as a multivalued list,
1338 even if the list had just one argument. That way you're safe from the
1339 shell expanding wildcards or splitting up words with whitespace in them.
1341 @args = ( "echo surprise" );
1343 exec @args; # subject to shell escapes
1345 exec { $args[0] } @args; # safe even with one-arg list
1347 The first version, the one without the indirect object, ran the I<echo>
1348 program, passing it C<"surprise"> an argument. The second version
1349 didn't--it tried to run a program literally called I<"echo surprise">,
1350 didn't find it, and set C<$?> to a non-zero value indicating failure.
1352 Note that C<exec()> will not call your C<END> blocks, nor will it call
1353 any C<DESTROY> methods in your objects.
1357 Returns TRUE if the specified hash key exists in its hash array, even
1358 if the corresponding value is undefined.
1360 print "Exists\n" if exists $array{$key};
1361 print "Defined\n" if defined $array{$key};
1362 print "True\n" if $array{$key};
1364 A hash element can be TRUE only if it's defined, and defined if
1365 it exists, but the reverse doesn't necessarily hold true.
1367 Note that the EXPR can be arbitrarily complicated as long as the final
1368 operation is a hash key lookup:
1370 if (exists $ref->{A}->{B}->{$key}) { }
1371 if (exists $hash{A}{B}{$key}) { }
1373 Although the last element will not spring into existence just because
1374 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1375 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1376 existence test for a $key element. This happens anywhere the arrow
1377 operator is used, including even
1380 if (exists $ref->{"Some key"}) { }
1381 print $ref; # prints HASH(0x80d3d5c)
1383 This surprising autovivification in what does not at first--or even
1384 second--glance appear to be an lvalue context may be fixed in a future
1389 Evaluates EXPR and exits immediately with that value. Example:
1392 exit 0 if $ans =~ /^[Xx]/;
1394 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1395 universally recognized values for EXPR are C<0> for success and C<1>
1396 for error; other values are subject to interpretation depending on the
1397 environment in which the Perl program is running. For example, exiting
1398 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1399 the mailer to return the item undelivered, but that's not true everywhere.
1401 Don't use C<exit()> to abort a subroutine if there's any chance that
1402 someone might want to trap whatever error happened. Use C<die()> instead,
1403 which can be trapped by an C<eval()>.
1405 The exit() function does not always exit immediately. It calls any
1406 defined C<END> routines first, but these C<END> routines may not
1407 themselves abort the exit. Likewise any object destructors that need to
1408 be called are called before the real exit. If this is a problem, you
1409 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1410 See L<perlsub> for details.
1416 Returns I<e> (the natural logarithm base) to the power of EXPR.
1417 If EXPR is omitted, gives C<exp($_)>.
1419 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1421 Implements the fcntl(2) function. You'll probably have to say
1425 first to get the correct constant definitions. Argument processing and
1426 value return works just like C<ioctl()> below.
1430 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1431 or die "can't fcntl F_GETFL: $!";
1433 You don't have to check for C<defined()> on the return from C<fnctl()>.
1434 Like C<ioctl()>, it maps a C<0> return from the system call into "C<0>
1435 but true" in Perl. This string is true in boolean context and C<0>
1436 in numeric context. It is also exempt from the normal B<-w> warnings
1437 on improper numeric conversions.
1439 Note that C<fcntl()> will produce a fatal error if used on a machine that
1440 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1441 manpage to learn what functions are available on your system.
1443 =item fileno FILEHANDLE
1445 Returns the file descriptor for a filehandle, or undefined if the
1446 filehandle is not open. This is mainly useful for constructing
1447 bitmaps for C<select()> and low-level POSIX tty-handling operations.
1448 If FILEHANDLE is an expression, the value is taken as an indirect
1449 filehandle, generally its name.
1451 You can use this to find out whether two handles refer to the
1452 same underlying descriptor:
1454 if (fileno(THIS) == fileno(THAT)) {
1455 print "THIS and THAT are dups\n";
1458 =item flock FILEHANDLE,OPERATION
1460 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE
1461 for success, FALSE on failure. Produces a fatal error if used on a
1462 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1463 C<flock()> is Perl's portable file locking interface, although it locks
1464 only entire files, not records.
1466 Two potentially non-obvious but traditional C<flock> semantics are
1467 that it waits indefinitely until the lock is granted, and that its locks
1468 B<merely advisory>. Such discretionary locks are more flexible, but offer
1469 fewer guarantees. This means that files locked with C<flock()> may be
1470 modified by programs that do not also use C<flock()>. See L<perlport>,
1471 your port's specific documentation, or your system-specific local manpages
1472 for details. It's best to assume traditional behavior if you're writing
1473 portable programs. (But if you're not, you should as always feel perfectly
1474 free to write for your own system's idiosyncrasies (sometimes called
1475 "features"). Slavish adherence to portability concerns shouldn't get
1476 in the way of your getting your job done.)
1478 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1479 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1480 you can use the symbolic names if import them from the Fcntl module,
1481 either individually, or as a group using the ':flock' tag. LOCK_SH
1482 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1483 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1484 LOCK_EX then C<flock()> will return immediately rather than blocking
1485 waiting for the lock (check the return status to see if you got it).
1487 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1488 before locking or unlocking it.
1490 Note that the emulation built with lockf(3) doesn't provide shared
1491 locks, and it requires that FILEHANDLE be open with write intent. These
1492 are the semantics that lockf(3) implements. Most if not all systems
1493 implement lockf(3) in terms of fcntl(2) locking, though, so the
1494 differing semantics shouldn't bite too many people.
1496 Note also that some versions of C<flock()> cannot lock things over the
1497 network; you would need to use the more system-specific C<fcntl()> for
1498 that. If you like you can force Perl to ignore your system's flock(2)
1499 function, and so provide its own fcntl(2)-based emulation, by passing
1500 the switch C<-Ud_flock> to the F<Configure> program when you configure
1503 Here's a mailbox appender for BSD systems.
1505 use Fcntl ':flock'; # import LOCK_* constants
1508 flock(MBOX,LOCK_EX);
1509 # and, in case someone appended
1510 # while we were waiting...
1515 flock(MBOX,LOCK_UN);
1518 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1519 or die "Can't open mailbox: $!";
1522 print MBOX $msg,"\n\n";
1525 On systems that support a real flock(), locks are inherited across fork()
1526 calls, whereas those that must resort to the more capricious fcntl()
1527 function lose the locks, making it harder to write servers.
1529 See also L<DB_File> for other flock() examples.
1533 Does a fork(2) system call to create a new process running the
1534 same program at the same point. It returns the child pid to the
1535 parent process, C<0> to the child process, or C<undef> if the fork is
1536 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1537 are shared, while everything else is copied. On most systems supporting
1538 fork(), great care has gone into making it extremely efficient (for
1539 example, using copy-on-write technology on data pages), making it the
1540 dominant paradigm for multitasking over the last few decades.
1542 All files opened for output are flushed before forking the child process.
1544 If you C<fork()> without ever waiting on your children, you will
1545 accumulate zombies. On some systems, you can avoid this by setting
1546 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1547 forking and reaping moribund children.
1549 Note that if your forked child inherits system file descriptors like
1550 STDIN and STDOUT that are actually connected by a pipe or socket, even
1551 if you exit, then the remote server (such as, say, a CGI script or a
1552 backgrounded job launced from a remote shell) won't think you're done.
1553 You should reopen those to F</dev/null> if it's any issue.
1557 Declare a picture format for use by the C<write()> function. For
1561 Test: @<<<<<<<< @||||| @>>>>>
1562 $str, $%, '$' . int($num)
1566 $num = $cost/$quantity;
1570 See L<perlform> for many details and examples.
1572 =item formline PICTURE,LIST
1574 This is an internal function used by C<format>s, though you may call it,
1575 too. It formats (see L<perlform>) a list of values according to the
1576 contents of PICTURE, placing the output into the format output
1577 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1578 Eventually, when a C<write()> is done, the contents of
1579 C<$^A> are written to some filehandle, but you could also read C<$^A>
1580 yourself and then set C<$^A> back to C<"">. Note that a format typically
1581 does one C<formline()> per line of form, but the C<formline()> function itself
1582 doesn't care how many newlines are embedded in the PICTURE. This means
1583 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1584 You may therefore need to use multiple formlines to implement a single
1585 record format, just like the format compiler.
1587 Be careful if you put double quotes around the picture, because an "C<@>"
1588 character may be taken to mean the beginning of an array name.
1589 C<formline()> always returns TRUE. See L<perlform> for other examples.
1591 =item getc FILEHANDLE
1595 Returns the next character from the input file attached to FILEHANDLE,
1596 or the undefined value at end of file, or if there was an error.
1597 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1598 efficient. However, it cannot be used by itself to fetch single
1599 characters without waiting for the user to hit enter. For that, try
1600 something more like:
1603 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1606 system "stty", '-icanon', 'eol', "\001";
1612 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1615 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1619 Determination of whether $BSD_STYLE should be set
1620 is left as an exercise to the reader.
1622 The C<POSIX::getattr()> function can do this more portably on
1623 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1624 module from your nearest CPAN site; details on CPAN can be found on
1629 Implements the C library function of the same name, which on most
1630 systems returns the current login from F</etc/utmp>, if any. If null,
1633 $login = getlogin || getpwuid($<) || "Kilroy";
1635 Do not consider C<getlogin()> for authentication: it is not as
1636 secure as C<getpwuid()>.
1638 =item getpeername SOCKET
1640 Returns the packed sockaddr address of other end of the SOCKET connection.
1643 $hersockaddr = getpeername(SOCK);
1644 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1645 $herhostname = gethostbyaddr($iaddr, AF_INET);
1646 $herstraddr = inet_ntoa($iaddr);
1650 Returns the current process group for the specified PID. Use
1651 a PID of C<0> to get the current process group for the
1652 current process. Will raise an exception if used on a machine that
1653 doesn't implement getpgrp(2). If PID is omitted, returns process
1654 group of current process. Note that the POSIX version of C<getpgrp()>
1655 does not accept a PID argument, so only C<PID==0> is truly portable.
1659 Returns the process id of the parent process.
1661 =item getpriority WHICH,WHO
1663 Returns the current priority for a process, a process group, or a user.
1664 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1665 machine that doesn't implement getpriority(2).
1671 =item gethostbyname NAME
1673 =item getnetbyname NAME
1675 =item getprotobyname NAME
1681 =item getservbyname NAME,PROTO
1683 =item gethostbyaddr ADDR,ADDRTYPE
1685 =item getnetbyaddr ADDR,ADDRTYPE
1687 =item getprotobynumber NUMBER
1689 =item getservbyport PORT,PROTO
1707 =item sethostent STAYOPEN
1709 =item setnetent STAYOPEN
1711 =item setprotoent STAYOPEN
1713 =item setservent STAYOPEN
1727 These routines perform the same functions as their counterparts in the
1728 system library. In list context, the return values from the
1729 various get routines are as follows:
1731 ($name,$passwd,$uid,$gid,
1732 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1733 ($name,$passwd,$gid,$members) = getgr*
1734 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1735 ($name,$aliases,$addrtype,$net) = getnet*
1736 ($name,$aliases,$proto) = getproto*
1737 ($name,$aliases,$port,$proto) = getserv*
1739 (If the entry doesn't exist you get a null list.)
1741 In scalar context, you get the name, unless the function was a
1742 lookup by name, in which case you get the other thing, whatever it is.
1743 (If the entry doesn't exist you get the undefined value.) For example:
1745 $uid = getpwnam($name);
1746 $name = getpwuid($num);
1748 $gid = getgrnam($name);
1749 $name = getgrgid($num;
1753 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are
1754 special cases in the sense that in many systems they are unsupported.
1755 If the C<$quota> is unsupported, it is an empty scalar. If it is
1756 supported, it usually encodes the disk quota. If the C<$comment>
1757 field is unsupported, it is an empty scalar. If it is supported it
1758 usually encodes some administrative comment about the user. In some
1759 systems the $quota field may be C<$change> or C<$age>, fields that have
1760 to do with password aging. In some systems the C<$comment> field may
1761 be C<$class>. The C<$expire> field, if present, encodes the expiration
1762 period of the account or the password. For the availability and the
1763 exact meaning of these fields in your system, please consult your
1764 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1765 out from within Perl what your C<$quota> and C<$comment> fields mean
1766 and whether you have the C<$expire> field by using the C<Config> module
1767 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1768 and C<d_pwexpire>. Shadow password files are only supported if your
1769 vendor has implemented them in the intuitive fashion that calling the
1770 regular C library routines gets the shadow versions if you're running
1771 under privilege. Those that incorrectly implement a separate library
1772 call are not supported.
1774 The C<$members> value returned by I<getgr*()> is a space separated list of
1775 the login names of the members of the group.
1777 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1778 C, it will be returned to you via C<$?> if the function call fails. The
1779 C<@addrs> value returned by a successful call is a list of the raw
1780 addresses returned by the corresponding system library call. In the
1781 Internet domain, each address is four bytes long and you can unpack it
1782 by saying something like:
1784 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1786 The Socket library makes this slightly easier:
1789 $iaddr = inet_aton("127.1"); # or whatever address
1790 $name = gethostbyaddr($iaddr, AF_INET);
1792 # or going the other way
1793 $straddr = inet_ntoa($iaddr");
1795 If you get tired of remembering which element of the return list contains
1796 which return value, by-name interfaces are also provided in modules:
1797 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1798 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1799 normal built-in, replacing them with versions that return objects with
1800 the appropriate names for each field. For example:
1804 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1806 Even though it looks like they're the same method calls (uid),
1807 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1809 =item getsockname SOCKET
1811 Returns the packed sockaddr address of this end of the SOCKET connection.
1814 $mysockaddr = getsockname(SOCK);
1815 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1817 =item getsockopt SOCKET,LEVEL,OPTNAME
1819 Returns the socket option requested, or undef if there is an error.
1825 Returns the value of EXPR with filename expansions such as the
1826 standard Unix shell F</bin/csh> would do. This is the internal function
1827 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1828 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1829 discussed in more detail in L<perlop/"I/O Operators">.
1833 Converts a time as returned by the time function to a 9-element array
1834 with the time localized for the standard Greenwich time zone.
1835 Typically used as follows:
1838 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1841 All array elements are numeric, and come straight out of a struct tm.
1842 In particular this means that C<$mon> has the range C<0..11> and C<$wday>
1843 has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
1844 number of years since 1900, that is, C<$year> is C<123> in year 2023,
1845 I<not> simply the last two digits of the year. If you assume it is,
1846 then you create non-Y2K-compliant programs--and you wouldn't want to do
1849 If EXPR is omitted, does C<gmtime(time())>.
1851 In scalar context, returns the ctime(3) value:
1853 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1855 Also see the C<timegm()> function provided by the C<Time::Local> module,
1856 and the strftime(3) function available via the POSIX module.
1858 This scalar value is B<not> locale dependent (see L<perllocale>), but
1859 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1860 strftime(3) and mktime(3) functions available via the POSIX module. To
1861 get somewhat similar but locale dependent date strings, set up your
1862 locale environment variables appropriately (please see L<perllocale>)
1863 and try for example:
1865 use POSIX qw(strftime);
1866 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1868 Note that the C<%a> and C<%b> escapes, which represent the short forms
1869 of the day of the week and the month of the year, may not necessarily
1870 be three characters wide in all locales.
1878 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1879 execution there. It may not be used to go into any construct that
1880 requires initialization, such as a subroutine or a C<foreach> loop. It
1881 also can't be used to go into a construct that is optimized away,
1882 or to get out of a block or subroutine given to C<sort()>.
1883 It can be used to go almost anywhere else within the dynamic scope,
1884 including out of subroutines, but it's usually better to use some other
1885 construct such as C<last> or C<die()>. The author of Perl has never felt the
1886 need to use this form of C<goto> (in Perl, that is--C is another matter).
1888 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1889 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1890 necessarily recommended if you're optimizing for maintainability:
1892 goto ("FOO", "BAR", "GLARCH")[$i];
1894 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1895 named subroutine for the currently running subroutine. This is used by
1896 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1897 pretend that the other subroutine had been called in the first place
1898 (except that any modifications to C<@_> in the current subroutine are
1899 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1900 will be able to tell that this routine was called first.
1902 =item grep BLOCK LIST
1904 =item grep EXPR,LIST
1906 This is similar in spirit to, but not the same as, grep(1) and its
1907 relatives. In particular, it is not limited to using regular expressions.
1909 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1910 C<$_> to each element) and returns the list value consisting of those
1911 elements for which the expression evaluated to TRUE. In scalar
1912 context, returns the number of times the expression was TRUE.
1914 @foo = grep(!/^#/, @bar); # weed out comments
1918 @foo = grep {!/^#/} @bar; # weed out comments
1920 Note that, because C<$_> is a reference into the list value, it can
1921 be used to modify the elements of the array. While this is useful and
1922 supported, it can cause bizarre results if the LIST is not a named array.
1923 Similarly, grep returns aliases into the original list, much as a for
1924 loop's index variable aliases the list elements. That is, modifying an
1925 element of a list returned by grep (for example, in a C<foreach>, C<map()>
1926 or another C<grep()>) actually modifies the element in the original list.
1927 This is usually something to be avoided when writing clear code.
1929 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1935 Interprets EXPR as a hex string and returns the corresponding value.
1936 (To convert strings that might start with either 0, 0x, or 0b, see
1937 L</oct>.) If EXPR is omitted, uses C<$_>.
1939 print hex '0xAf'; # prints '175'
1940 print hex 'aF'; # same
1944 There is no builtin C<import()> function. It is just an ordinary
1945 method (subroutine) defined (or inherited) by modules that wish to export
1946 names to another module. The C<use()> function calls the C<import()> method
1947 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1949 =item index STR,SUBSTR,POSITION
1951 =item index STR,SUBSTR
1953 The index function searches for one string within another, but without
1954 the wildcard-like behavior of a full regular-expression pattern match.
1955 It returns the position of the first occurrence of SUBSTR in STR at
1956 or after POSITION. If POSITION is omitted, starts searching from the
1957 beginning of the string. The return value is based at C<0> (or whatever
1958 you've set the C<$[> variable to--but don't do that). If the substring
1959 is not found, returns one less than the base, ordinarily C<-1>.
1965 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1966 You should not use this function for rounding: one because it truncates
1967 towards C<0>, and two because machine representations of floating point
1968 numbers can sometimes produce counterintuitive results. For example,
1969 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
1970 because it's really more like -268.99999999999994315658 instead. Usually,
1971 the C<sprintf()>, C<printf()>, or the C<POSIX::floor> and C<POSIX::ceil>
1972 functions will serve you better than will int().
1974 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1976 Implements the ioctl(2) function. You'll probably first have to say
1978 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1980 to get the correct function definitions. If F<ioctl.ph> doesn't
1981 exist or doesn't have the correct definitions you'll have to roll your
1982 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1983 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1984 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1985 written depending on the FUNCTION--a pointer to the string value of SCALAR
1986 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1987 has no string value but does have a numeric value, that value will be
1988 passed rather than a pointer to the string value. To guarantee this to be
1989 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1990 functions are useful for manipulating the values of structures used by
1991 C<ioctl()>. The following example sets the erase character to DEL.
1995 die "NO TIOCGETP" if $@ || !$getp;
1996 $sgttyb_t = "ccccs"; # 4 chars and a short
1997 if (ioctl(STDIN,$getp,$sgttyb)) {
1998 @ary = unpack($sgttyb_t,$sgttyb);
2000 $sgttyb = pack($sgttyb_t,@ary);
2001 ioctl(STDIN,&TIOCSETP,$sgttyb)
2002 || die "Can't ioctl: $!";
2005 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
2007 if OS returns: then Perl returns:
2009 0 string "0 but true"
2010 anything else that number
2012 Thus Perl returns TRUE on success and FALSE on failure, yet you can
2013 still easily determine the actual value returned by the operating
2016 $retval = ioctl(...) || -1;
2017 printf "System returned %d\n", $retval;
2019 The special string "C<0> but true" is exempt from B<-w> complaints
2020 about improper numeric conversions.
2022 =item join EXPR,LIST
2024 Joins the separate strings of LIST into a single string with fields
2025 separated by the value of EXPR, and returns that new string. Example:
2027 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2033 Returns a list consisting of all the keys of the named hash. (In a
2034 scalar context, returns the number of keys.) The keys are returned in
2035 an apparently random order. The actual random order is subject to
2036 change in future versions of perl, but it is guaranteed to be the same
2037 order as either the C<values()> or C<each()> function produces (given
2038 that the hash has not been modified). As a side effect, it resets
2041 Here is yet another way to print your environment:
2044 @values = values %ENV;
2045 while ($#keys >= 0) {
2046 print pop(@keys), '=', pop(@values), "\n";
2049 or how about sorted by key:
2051 foreach $key (sort(keys %ENV)) {
2052 print $key, '=', $ENV{$key}, "\n";
2055 To sort a hash by value, you'll need to use a C<sort()> function.
2056 Here's a descending numeric sort of a hash by its values:
2058 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2059 printf "%4d %s\n", $hash{$key}, $key;
2062 As an lvalue C<keys()> allows you to increase the number of hash buckets
2063 allocated for the given hash. This can gain you a measure of efficiency if
2064 you know the hash is going to get big. (This is similar to pre-extending
2065 an array by assigning a larger number to $#array.) If you say
2069 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2070 in fact, since it rounds up to the next power of two. These
2071 buckets will be retained even if you do C<%hash = ()>, use C<undef
2072 %hash> if you want to free the storage while C<%hash> is still in scope.
2073 You can't shrink the number of buckets allocated for the hash using
2074 C<keys()> in this way (but you needn't worry about doing this by accident,
2075 as trying has no effect).
2077 See also C<each()>, C<values()> and C<sort()>.
2081 Sends a signal to a list of processes. The first element of
2082 the list must be the signal to send. Returns the number of
2083 processes successfully signaled.
2085 $cnt = kill 1, $child1, $child2;
2088 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2089 process groups instead of processes. (On System V, a negative I<PROCESS>
2090 number will also kill process groups, but that's not portable.) That
2091 means you usually want to use positive not negative signals. You may also
2092 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2098 The C<last> command is like the C<break> statement in C (as used in
2099 loops); it immediately exits the loop in question. If the LABEL is
2100 omitted, the command refers to the innermost enclosing loop. The
2101 C<continue> block, if any, is not executed:
2103 LINE: while (<STDIN>) {
2104 last LINE if /^$/; # exit when done with header
2108 C<last> cannot be used to exit a block which returns a value such as
2109 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2110 a grep() or map() operation.
2112 See also L</continue> for an illustration of how C<last>, C<next>, and
2119 Returns an lowercased version of EXPR. This is the internal function
2120 implementing the C<\L> escape in double-quoted strings.
2121 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2123 If EXPR is omitted, uses C<$_>.
2129 Returns the value of EXPR with the first character lowercased. This is
2130 the internal function implementing the C<\l> escape in double-quoted strings.
2131 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2133 If EXPR is omitted, uses C<$_>.
2139 Returns the length in characters of the value of EXPR. If EXPR is
2140 omitted, returns length of C<$_>. Note that this cannot be used on
2141 an entire array or hash to find out how many elements these have.
2142 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2144 =item link OLDFILE,NEWFILE
2146 Creates a new filename linked to the old filename. Returns TRUE for
2147 success, FALSE otherwise.
2149 =item listen SOCKET,QUEUESIZE
2151 Does the same thing that the listen system call does. Returns TRUE if
2152 it succeeded, FALSE otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2156 You really probably want to be using C<my()> instead, because C<local()> isn't
2157 what most people think of as "local". See L<perlsub/"Private Variables
2158 via my()"> for details.
2160 A local modifies the listed variables to be local to the enclosing
2161 block, file, or eval. If more than one value is listed, the list must
2162 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2163 for details, including issues with tied arrays and hashes.
2165 =item localtime EXPR
2167 Converts a time as returned by the time function to a 9-element array
2168 with the time analyzed for the local time zone. Typically used as
2172 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2175 All array elements are numeric, and come straight out of a struct tm.
2176 In particular this means that C<$mon> has the range C<0..11> and C<$wday>
2177 has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
2178 number of years since 1900, that is, C<$year> is C<123> in year 2023,
2179 and I<not> simply the last two digits of the year. If you assume it is,
2180 then you create non-Y2K-compliant programs--and you wouldn't want to do
2183 If EXPR is omitted, uses the current time (C<localtime(time)>).
2185 In scalar context, returns the ctime(3) value:
2187 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2189 This scalar value is B<not> locale dependent, see L<perllocale>, but
2190 instead a Perl builtin. Also see the C<Time::Local> module, and the
2191 strftime(3) and mktime(3) function available via the POSIX module. To
2192 get somewhat similar but locale dependent date strings, set up your
2193 locale environment variables appropriately (please see L<perllocale>)
2194 and try for example:
2196 use POSIX qw(strftime);
2197 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2199 Note that the C<%a> and C<%b>, the short forms of the day of the week
2200 and the month of the year, may not necessarily be three characters wide.
2206 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2207 returns log of C<$_>. To get the log of another base, use basic algebra:
2208 The base-N log of a number is is equal to the natural log of that number
2209 divided by the natural log of N. For example:
2213 return log($n)/log(10);
2216 See also L</exp> for the inverse operation.
2218 =item lstat FILEHANDLE
2224 Does the same thing as the C<stat()> function (including setting the
2225 special C<_> filehandle) but stats a symbolic link instead of the file
2226 the symbolic link points to. If symbolic links are unimplemented on
2227 your system, a normal C<stat()> is done.
2229 If EXPR is omitted, stats C<$_>.
2233 The match operator. See L<perlop>.
2235 =item map BLOCK LIST
2239 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2240 element) and returns the list value composed of the results of each such
2241 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2242 may produce zero, one, or more elements in the returned value.
2244 In scalar context, returns the total number of elements so generated.
2246 @chars = map(chr, @nums);
2248 translates a list of numbers to the corresponding characters. And
2250 %hash = map { getkey($_) => $_ } @array;
2252 is just a funny way to write
2255 foreach $_ (@array) {
2256 $hash{getkey($_)} = $_;
2259 Note that, because C<$_> is a reference into the list value, it can
2260 be used to modify the elements of the array. While this is useful and
2261 supported, it can cause bizarre results if the LIST is not a named array.
2262 Using a regular C<foreach> loop for this purpose would be clearer in
2263 most cases. See also L</grep> for an array composed of those items of
2264 the original list for which the BLOCK or EXPR evaluates to true.
2266 =item mkdir FILENAME,MODE
2268 Creates the directory specified by FILENAME, with permissions
2269 specified by MODE (as modified by C<umask>). If it succeeds it
2270 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2272 In general, it is better to create directories with permissive MODEs,
2273 and let the user modify that with their C<umask>, than it is to supply
2274 a restrictive MODE and give the user no way to be more permissive.
2275 The exceptions to this rule are when the file or directory should be
2276 kept private (mail files, for instance). The perlfunc(1) entry on
2277 C<umask> discusses the choice of MODE in more detail.
2279 =item msgctl ID,CMD,ARG
2281 Calls the System V IPC function msgctl(2). You'll probably have to say
2285 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2286 then ARG must be a variable which will hold the returned C<msqid_ds>
2287 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2288 true" for zero, or the actual return value otherwise. See also
2289 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2291 =item msgget KEY,FLAGS
2293 Calls the System V IPC function msgget(2). Returns the message queue
2294 id, or the undefined value if there is an error. See also C<IPC::SysV>
2295 and C<IPC::SysV::Msg> documentation.
2297 =item msgsnd ID,MSG,FLAGS
2299 Calls the System V IPC function msgsnd to send the message MSG to the
2300 message queue ID. MSG must begin with the long integer message type,
2301 which may be created with C<pack("l", $type)>. Returns TRUE if
2302 successful, or FALSE if there is an error. See also C<IPC::SysV>
2303 and C<IPC::SysV::Msg> documentation.
2305 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2307 Calls the System V IPC function msgrcv to receive a message from
2308 message queue ID into variable VAR with a maximum message size of
2309 SIZE. Note that if a message is received, the message type will be
2310 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2311 size of the message type. Returns TRUE if successful, or FALSE if
2312 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2316 A C<my()> declares the listed variables to be local (lexically) to the
2317 enclosing block, file, or C<eval()>. If
2318 more than one value is listed, the list must be placed in parentheses. See
2319 L<perlsub/"Private Variables via my()"> for details.
2325 The C<next> command is like the C<continue> statement in C; it starts
2326 the next iteration of the loop:
2328 LINE: while (<STDIN>) {
2329 next LINE if /^#/; # discard comments
2333 Note that if there were a C<continue> block on the above, it would get
2334 executed even on discarded lines. If the LABEL is omitted, the command
2335 refers to the innermost enclosing loop.
2337 C<next> cannot be used to exit a block which returns a value such as
2338 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2339 a grep() or map() operation.
2341 See also L</continue> for an illustration of how C<last>, C<next>, and
2344 =item no Module LIST
2346 See the L</use> function, which C<no> is the opposite of.
2352 Interprets EXPR as an octal string and returns the corresponding
2353 value. (If EXPR happens to start off with C<0x>, interprets it as a
2354 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2355 binary string.) The following will handle decimal, binary, octal, and
2356 hex in the standard Perl or C notation:
2358 $val = oct($val) if $val =~ /^0/;
2360 If EXPR is omitted, uses C<$_>. This function is commonly used when
2361 a string such as C<644> needs to be converted into a file mode, for
2362 example. (Although perl will automatically convert strings into
2363 numbers as needed, this automatic conversion assumes base 10.)
2365 =item open FILEHANDLE,EXPR
2367 =item open FILEHANDLE
2369 Opens the file whose filename is given by EXPR, and associates it with
2370 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2371 name of the real filehandle wanted. If EXPR is omitted, the scalar
2372 variable of the same name as the FILEHANDLE contains the filename.
2373 (Note that lexical variables--those declared with C<my()>--will not work
2374 for this purpose; so if you're using C<my()>, specify EXPR in your call
2375 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2378 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2379 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2380 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2381 the file is opened for appending, again being created if necessary.
2382 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2383 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2384 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2385 file first. You can't usually use either read-write mode for updating
2386 textfiles, since they have variable length records. See the B<-i>
2387 switch in L<perlrun> for a better approach. The file is created with
2388 permissions of C<0666> modified by the process' C<umask> value.
2390 The prefix and the filename may be separated with spaces.
2391 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2392 C<'w+'>, C<'a'>, and C<'a+'>.
2394 If the filename begins with C<'|'>, the filename is interpreted as a
2395 command to which output is to be piped, and if the filename ends with a
2396 C<'|'>, the filename is interpreted as a command which pipes output to
2397 us. See L<perlipc/"Using open() for IPC">
2398 for more examples of this. (You are not allowed to C<open()> to a command
2399 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2400 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2402 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2403 nonzero upon success, the undefined value otherwise. If the C<open()>
2404 involved a pipe, the return value happens to be the pid of the
2407 If you're unfortunate enough to be running Perl on a system that
2408 distinguishes between text files and binary files (modern operating
2409 systems don't care), then you should check out L</binmode> for tips for
2410 dealing with this. The key distinction between systems that need C<binmode()>
2411 and those that don't is their text file formats. Systems like Unix, MacOS, and
2412 Plan9, which delimit lines with a single character, and which encode that
2413 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2415 When opening a file, it's usually a bad idea to continue normal execution
2416 if the request failed, so C<open()> is frequently used in connection with
2417 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2418 where you want to make a nicely formatted error message (but there are
2419 modules that can help with that problem)) you should always check
2420 the return value from opening a file. The infrequent exception is when
2421 working with an unopened filehandle is actually what you want to do.
2426 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2427 while (<ARTICLE>) {...
2429 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2430 # if the open fails, output is discarded
2432 open(DBASE, '+<dbase.mine') # open for update
2433 or die "Can't open 'dbase.mine' for update: $!";
2435 open(ARTICLE, "caesar <$article |") # decrypt article
2436 or die "Can't start caesar: $!";
2438 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2439 or die "Can't start sort: $!";
2441 # process argument list of files along with any includes
2443 foreach $file (@ARGV) {
2444 process($file, 'fh00');
2448 my($filename, $input) = @_;
2449 $input++; # this is a string increment
2450 unless (open($input, $filename)) {
2451 print STDERR "Can't open $filename: $!\n";
2456 while (<$input>) { # note use of indirection
2457 if (/^#include "(.*)"/) {
2458 process($1, $input);
2465 You may also, in the Bourne shell tradition, specify an EXPR beginning
2466 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2467 name of a filehandle (or file descriptor, if numeric) to be
2468 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2469 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2470 mode you specify should match the mode of the original filehandle.
2471 (Duping a filehandle does not take into account any existing contents of
2473 Here is a script that saves, redirects, and restores STDOUT and
2477 open(OLDOUT, ">&STDOUT");
2478 open(OLDERR, ">&STDERR");
2480 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2481 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2483 select(STDERR); $| = 1; # make unbuffered
2484 select(STDOUT); $| = 1; # make unbuffered
2486 print STDOUT "stdout 1\n"; # this works for
2487 print STDERR "stderr 1\n"; # subprocesses too
2492 open(STDOUT, ">&OLDOUT");
2493 open(STDERR, ">&OLDERR");
2495 print STDOUT "stdout 2\n";
2496 print STDERR "stderr 2\n";
2498 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2499 equivalent of C's C<fdopen()> of that file descriptor; this is more
2500 parsimonious of file descriptors. For example:
2502 open(FILEHANDLE, "<&=$fd")
2504 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2505 there is an implicit fork done, and the return value of open is the pid
2506 of the child within the parent process, and C<0> within the child
2507 process. (Use C<defined($pid)> to determine whether the open was successful.)
2508 The filehandle behaves normally for the parent, but i/o to that
2509 filehandle is piped from/to the STDOUT/STDIN of the child process.
2510 In the child process the filehandle isn't opened--i/o happens from/to
2511 the new STDOUT or STDIN. Typically this is used like the normal
2512 piped open when you want to exercise more control over just how the
2513 pipe command gets executed, such as when you are running setuid, and
2514 don't want to have to scan shell commands for metacharacters.
2515 The following pairs are more or less equivalent:
2517 open(FOO, "|tr '[a-z]' '[A-Z]'");
2518 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2520 open(FOO, "cat -n '$file'|");
2521 open(FOO, "-|") || exec 'cat', '-n', $file;
2523 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2525 NOTE: On any operation that may do a fork, all files opened for output
2526 are flushed before the fork is attempted. On systems that support a
2527 close-on-exec flag on files, the flag will be set for the newly opened
2528 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2530 Closing any piped filehandle causes the parent process to wait for the
2531 child to finish, and returns the status value in C<$?>.
2533 The filename passed to open will have leading and trailing
2534 whitespace deleted, and the normal redirection characters
2535 honored. This property, known as "magic open",
2536 can often be used to good effect. A user could specify a filename of
2537 F<"rsh cat file |">, or you could change certain filenames as needed:
2539 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2540 open(FH, $filename) or die "Can't open $filename: $!";
2542 However, to open a file with arbitrary weird characters in it, it's
2543 necessary to protect any leading and trailing whitespace:
2545 $file =~ s#^(\s)#./$1#;
2546 open(FOO, "< $file\0");
2548 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2549 should use the C<sysopen()> function, which involves no such magic. This is
2550 another way to protect your filenames from interpretation. For example:
2553 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2554 or die "sysopen $path: $!";
2555 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2556 print HANDLE "stuff $$\n");
2558 print "File contains: ", <HANDLE>;
2560 Using the constructor from the C<IO::Handle> package (or one of its
2561 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2562 filehandles that have the scope of whatever variables hold references to
2563 them, and automatically close whenever and however you leave that scope:
2567 sub read_myfile_munged {
2569 my $handle = new IO::File;
2570 open($handle, "myfile") or die "myfile: $!";
2572 or return (); # Automatically closed here.
2573 mung $first or die "mung failed"; # Or here.
2574 return $first, <$handle> if $ALL; # Or here.
2578 See L</seek> for some details about mixing reading and writing.
2580 =item opendir DIRHANDLE,EXPR
2582 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2583 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2584 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2590 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2591 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2592 See L<utf8> for more about Unicode.
2594 =item pack TEMPLATE,LIST
2596 Takes an array or list of values and packs it into a binary structure,
2597 returning the string containing the structure. The TEMPLATE is a
2598 sequence of characters that give the order and type of values, as
2601 a A string with arbitrary binary data, will be null padded.
2602 A An ascii string, will be space padded.
2603 Z A null terminated (asciz) string, will be null padded.
2605 b A bit string (ascending bit order, like vec()).
2606 B A bit string (descending bit order).
2607 h A hex string (low nybble first).
2608 H A hex string (high nybble first).
2610 c A signed char value.
2611 C An unsigned char value. Only does bytes. See U for Unicode.
2613 s A signed short value.
2614 S An unsigned short value.
2615 (This 'short' is _exactly_ 16 bits, which may differ from
2616 what a local C compiler calls 'short'.)
2618 i A signed integer value.
2619 I An unsigned integer value.
2620 (This 'integer' is _at least_ 32 bits wide. Its exact
2621 size depends on what a local C compiler calls 'int',
2622 and may even be larger than the 'long' described in
2625 l A signed long value.
2626 L An unsigned long value.
2627 (This 'long' is _exactly_ 32 bits, which may differ from
2628 what a local C compiler calls 'long'.)
2630 n A short in "network" (big-endian) order.
2631 N A long in "network" (big-endian) order.
2632 v A short in "VAX" (little-endian) order.
2633 V A long in "VAX" (little-endian) order.
2634 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2635 _exactly_ 32 bits, respectively.)
2637 q A signed quad (64-bit) value.
2638 Q An unsigned quad value.
2639 (Available only if your system supports 64-bit integer values
2640 _and_ if Perl has been compiled to support those.
2641 Causes a fatal error otherwise.)
2643 f A single-precision float in the native format.
2644 d A double-precision float in the native format.
2646 p A pointer to a null-terminated string.
2647 P A pointer to a structure (fixed-length string).
2649 u A uuencoded string.
2650 U A Unicode character number. Encodes to UTF-8 internally.
2651 Works even if C<use utf8> is not in effect.
2653 w A BER compressed integer. Its bytes represent an unsigned
2654 integer in base 128, most significant digit first, with as
2655 few digits as possible. Bit eight (the high bit) is set
2656 on each byte except the last.
2660 @ Null fill to absolute position.
2662 The following rules apply:
2668 Each letter may optionally be followed by a number giving a repeat
2669 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2670 C<"H">, and C<"P"> the pack function will gobble up that many values from
2671 the LIST. A C<*> for the repeat count means to use however many items are
2676 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2677 string of length count, padding with nulls or spaces as necessary. When
2678 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2679 after the first null, and C<"a"> returns data verbatim.
2683 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2687 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2691 The C<"p"> type packs a pointer to a null-terminated string. You are
2692 responsible for ensuring the string is not a temporary value (which can
2693 potentially get deallocated before you get around to using the packed result).
2694 The C<"P"> type packs a pointer to a structure of the size indicated by the
2695 length. A NULL pointer is created if the corresponding value for C<"p"> or
2700 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2701 immediately followed by a C<"!"> to signify native shorts or longs--as
2702 you can see from above for example a bare C<"l"> does mean exactly 32
2703 bits, the native C<long> (as seen by the local C compiler) may be
2704 larger. This is an issue mainly in 64-bit platforms. You can see
2705 whether using C<"!"> makes any difference by
2707 print length(pack("s")), " ", length(pack("s!")), "\n";
2708 print length(pack("l")), " ", length(pack("l!")), "\n";
2710 C<"i!"> and C<"I!"> also work but only because of completeness;
2711 they are identical to C<"i"> and C<"I">.
2713 The actual sizes (in bytes) of native shorts, ints, and longs on
2714 the platform where Perl was built are also available via L<Config>:
2717 print $Config{shortsize}, "\n";
2718 print $Config{intsize}, "\n";
2719 print $Config{longsize}, "\n";
2723 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2724 are inherently non-portable between processors and operating systems
2725 because they obey the native byteorder and endianness. For example a
2726 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
2727 (arranged in and handled by the CPU registers) into bytes as
2729 0x12 0x34 0x56 0x78 # little-endian
2730 0x78 0x56 0x34 0x12 # big-endian
2732 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2733 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2734 Power, and Cray are big-endian. MIPS can be either: Digital used it
2735 in little-endian mode, SGI uses it in big-endian mode.
2737 The names `big-endian' and `little-endian' are joking references to
2738 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2739 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2740 the egg-eating habits of the lilliputs.
2742 Some systems may even have weird byte orders such as
2747 You can see your system's preference with
2749 print join(" ", map { sprintf "%#02x", $_ }
2750 unpack("C*",pack("L",0x12345678))), "\n";
2752 The byteorder on the platform where Perl was built is also available
2756 print $Config{byteorder}, "\n";
2758 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2759 and C<'87654321'> are big-endian.
2761 If you want portable packed integers use the formats C<"n">, C<"N">,
2762 C<"v">, and C<"V">, their byte endianness and size is known.
2766 Real numbers (floats and doubles) are in the native machine format only;
2767 due to the multiplicity of floating formats around, and the lack of a
2768 standard "network" representation, no facility for interchange has been
2769 made. This means that packed floating point data written on one machine
2770 may not be readable on another - even if both use IEEE floating point
2771 arithmetic (as the endian-ness of the memory representation is not part
2774 Note that Perl uses doubles internally for all numeric calculation, and
2775 converting from double into float and thence back to double again will
2776 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2783 $foo = pack("CCCC",65,66,67,68);
2785 $foo = pack("C4",65,66,67,68);
2787 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2788 # same thing with Unicode circled letters
2790 $foo = pack("ccxxcc",65,66,67,68);
2793 $foo = pack("s2",1,2);
2794 # "\1\0\2\0" on little-endian
2795 # "\0\1\0\2" on big-endian
2797 $foo = pack("a4","abcd","x","y","z");
2800 $foo = pack("aaaa","abcd","x","y","z");
2803 $foo = pack("a14","abcdefg");
2804 # "abcdefg\0\0\0\0\0\0\0"
2806 $foo = pack("i9pl", gmtime);
2807 # a real struct tm (on my system anyway)
2809 $utmp_template = "Z8 Z8 Z16 L";
2810 $utmp = pack($utmp_template, @utmp1);
2811 # a struct utmp (BSDish)
2813 @utmp2 = unpack($utmp_template, $utmp);
2814 # "@utmp1" eq "@utmp2"
2817 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2820 The same template may generally also be used in unpack().
2824 =item package NAMESPACE
2826 Declares the compilation unit as being in the given namespace. The scope
2827 of the package declaration is from the declaration itself through the end
2828 of the enclosing block, file, or eval (the same as the C<my()> operator).
2829 All further unqualified dynamic identifiers will be in this namespace.
2830 A package statement affects only dynamic variables--including those
2831 you've used C<local()> on--but I<not> lexical variables, which are created
2832 with C<my()>. Typically it would be the first declaration in a file to
2833 be included by the C<require> or C<use> operator. You can switch into a
2834 package in more than one place; it merely influences which symbol table
2835 is used by the compiler for the rest of that block. You can refer to
2836 variables and filehandles in other packages by prefixing the identifier
2837 with the package name and a double colon: C<$Package::Variable>.
2838 If the package name is null, the C<main> package as assumed. That is,
2839 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2840 still seen in older code).
2842 If NAMESPACE is omitted, then there is no current package, and all
2843 identifiers must be fully qualified or lexicals. This is stricter
2844 than C<use strict>, since it also extends to function names.
2846 See L<perlmod/"Packages"> for more information about packages, modules,
2847 and classes. See L<perlsub> for other scoping issues.
2849 =item pipe READHANDLE,WRITEHANDLE
2851 Opens a pair of connected pipes like the corresponding system call.
2852 Note that if you set up a loop of piped processes, deadlock can occur
2853 unless you are very careful. In addition, note that Perl's pipes use
2854 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2855 after each command, depending on the application.
2857 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2858 for examples of such things.
2860 On systems that support a close-on-exec flag on files, the flag will be set
2861 for the newly opened file descriptors as determined by the value of $^F.
2868 Pops and returns the last value of the array, shortening the array by
2869 one element. Has a similar effect to
2871 $tmp = $ARRAY[$#ARRAY--];
2873 If there are no elements in the array, returns the undefined value.
2874 If ARRAY is omitted, pops the C<@ARGV> array in the main program, and
2875 the C<@_> array in subroutines, just like C<shift()>.
2881 Returns the offset of where the last C<m//g> search left off for the variable
2882 is in question (C<$_> is used when the variable is not specified). May be
2883 modified to change that offset. Such modification will also influence
2884 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2887 =item print FILEHANDLE LIST
2893 Prints a string or a comma-separated list of strings. Returns TRUE
2894 if successful. FILEHANDLE may be a scalar variable name, in which case
2895 the variable contains the name of or a reference to the filehandle, thus
2896 introducing one level of indirection. (NOTE: If FILEHANDLE is a variable
2897 and the next token is a term, it may be misinterpreted as an operator
2898 unless you interpose a C<+> or put parentheses around the arguments.)
2899 If FILEHANDLE is omitted, prints by default to standard output (or to the
2900 last selected output channel--see L</select>). If LIST is also omitted,
2901 prints C<$_> to the currently selected output channel. To set the default
2902 output channel to something other than STDOUT use the select operation.
2903 Note that, because print takes a LIST, anything in the LIST is evaluated
2904 in list context, and any subroutine that you call will have one or
2905 more of its expressions evaluated in list context. Also be careful
2906 not to follow the print keyword with a left parenthesis unless you want
2907 the corresponding right parenthesis to terminate the arguments to the
2908 print--interpose a C<+> or put parentheses around all the arguments.
2910 Note that if you're storing FILEHANDLES in an array or other expression,
2911 you will have to use a block returning its value instead:
2913 print { $files[$i] } "stuff\n";
2914 print { $OK ? STDOUT : STDERR } "stuff\n";
2916 =item printf FILEHANDLE FORMAT, LIST
2918 =item printf FORMAT, LIST
2920 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2921 (the output record separator) is not appended. The first argument
2922 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2923 in effect, the character used for the decimal point in formatted real numbers
2924 is affected by the LC_NUMERIC locale. See L<perllocale>.
2926 Don't fall into the trap of using a C<printf()> when a simple
2927 C<print()> would do. The C<print()> is more efficient and less
2930 =item prototype FUNCTION
2932 Returns the prototype of a function as a string (or C<undef> if the
2933 function has no prototype). FUNCTION is a reference to, or the name of,
2934 the function whose prototype you want to retrieve.
2936 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
2937 name for Perl builtin. If the builtin is not I<overridable> (such as
2938 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2939 C<system()>) returns C<undef> because the builtin does not really behave
2940 like a Perl function. Otherwise, the string describing the equivalent
2941 prototype is returned.
2943 =item push ARRAY,LIST
2945 Treats ARRAY as a stack, and pushes the values of LIST
2946 onto the end of ARRAY. The length of ARRAY increases by the length of
2947 LIST. Has the same effect as
2950 $ARRAY[++$#ARRAY] = $value;
2953 but is more efficient. Returns the new number of elements in the array.
2965 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2967 =item quotemeta EXPR
2971 Returns the value of EXPR with all non-alphanumeric
2972 characters backslashed. (That is, all characters not matching
2973 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2974 returned string, regardless of any locale settings.)
2975 This is the internal function implementing
2976 the C<\Q> escape in double-quoted strings.
2978 If EXPR is omitted, uses C<$_>.
2984 Returns a random fractional number greater than or equal to C<0> and less
2985 than the value of EXPR. (EXPR should be positive.) If EXPR is
2986 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2987 C<srand()> has already been called. See also C<srand()>.
2989 (Note: If your rand function consistently returns numbers that are too
2990 large or too small, then your version of Perl was probably compiled
2991 with the wrong number of RANDBITS.)
2993 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2995 =item read FILEHANDLE,SCALAR,LENGTH
2997 Attempts to read LENGTH bytes of data into variable SCALAR from the
2998 specified FILEHANDLE. Returns the number of bytes actually read,
2999 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3000 or shrunk to the length actually read. An OFFSET may be specified to
3001 place the read data at some other place than the beginning of the
3002 string. This call is actually implemented in terms of stdio's fread(3)
3003 call. To get a true read(2) system call, see C<sysread()>.
3005 =item readdir DIRHANDLE
3007 Returns the next directory entry for a directory opened by C<opendir()>.
3008 If used in list context, returns all the rest of the entries in the
3009 directory. If there are no more entries, returns an undefined value in
3010 scalar context or a null list in list context.
3012 If you're planning to filetest the return values out of a C<readdir()>, you'd
3013 better prepend the directory in question. Otherwise, because we didn't
3014 C<chdir()> there, it would have been testing the wrong file.
3016 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3017 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3022 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3023 context, each call reads and returns the next line, until end-of-file is
3024 reached, whereupon the subsequent call returns undef. In list context,
3025 reads until end-of-file is reached and returns a list of lines. Note that
3026 the notion of "line" used here is however you may have defined it
3027 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3029 When C<$/> is set to C<undef>, when readline() is in scalar
3030 context (i.e. file slurp mode), and when an empty file is read, it
3031 returns C<''> the first time, followed by C<undef> subsequently.
3033 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3034 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3035 operator is discussed in more detail in L<perlop/"I/O Operators">.
3038 $line = readline(*STDIN); # same thing
3044 Returns the value of a symbolic link, if symbolic links are
3045 implemented. If not, gives a fatal error. If there is some system
3046 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3047 omitted, uses C<$_>.
3051 EXPR is executed as a system command.
3052 The collected standard output of the command is returned.
3053 In scalar context, it comes back as a single (potentially
3054 multi-line) string. In list context, returns a list of lines
3055 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3056 This is the internal function implementing the C<qx/EXPR/>
3057 operator, but you can use it directly. The C<qx/EXPR/>
3058 operator is discussed in more detail in L<perlop/"I/O Operators">.
3060 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3062 Receives a message on a socket. Attempts to receive LENGTH bytes of
3063 data into variable SCALAR from the specified SOCKET filehandle.
3064 Actually does a C C<recvfrom()>, so that it can return the address of the
3065 sender. Returns the undefined value if there's an error. SCALAR will
3066 be grown or shrunk to the length actually read. Takes the same flags
3067 as the system call of the same name.
3068 See L<perlipc/"UDP: Message Passing"> for examples.
3074 The C<redo> command restarts the loop block without evaluating the
3075 conditional again. The C<continue> block, if any, is not executed. If
3076 the LABEL is omitted, the command refers to the innermost enclosing
3077 loop. This command is normally used by programs that want to lie to
3078 themselves about what was just input:
3080 # a simpleminded Pascal comment stripper
3081 # (warning: assumes no { or } in strings)
3082 LINE: while (<STDIN>) {
3083 while (s|({.*}.*){.*}|$1 |) {}
3088 if (/}/) { # end of comment?
3097 C<redo> cannot be used to retry a block which returns a value such as
3098 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3099 a grep() or map() operation.
3101 See also L</continue> for an illustration of how C<last>, C<next>, and
3108 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
3109 is not specified, C<$_> will be used. The value returned depends on the
3110 type of thing the reference is a reference to.
3111 Builtin types include:
3120 If the referenced object has been blessed into a package, then that package
3121 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
3123 if (ref($r) eq "HASH") {
3124 print "r is a reference to a hash.\n";
3127 print "r is not a reference at all.\n";
3129 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3130 print "r is a reference to something that isa hash.\n";
3133 See also L<perlref>.
3135 =item rename OLDNAME,NEWNAME
3137 Changes the name of a file. Returns C<1> for success, C<0> otherwise.
3138 Behavior of this function varies wildly depending on your system
3139 implementation. For example, it will usually not work across file system
3140 boundaries, even though the system I<mv> command sometimes compensates
3141 for this. Other restrictions include whether it works on directories,
3142 open files, or pre-existing files. Check L<perlport> and either the
3143 rename(2) manpage or equivalent system documentation for details.
3149 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3150 supplied. If EXPR is numeric, demands that the current version of Perl
3151 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3153 Otherwise, demands that a library file be included if it hasn't already
3154 been included. The file is included via the do-FILE mechanism, which is
3155 essentially just a variety of C<eval()>. Has semantics similar to the following
3160 return 1 if $INC{$filename};
3161 my($realfilename,$result);
3163 foreach $prefix (@INC) {
3164 $realfilename = "$prefix/$filename";
3165 if (-f $realfilename) {
3166 $result = do $realfilename;
3170 die "Can't find $filename in \@INC";
3173 die "$filename did not return true value" unless $result;
3174 $INC{$filename} = $realfilename;
3178 Note that the file will not be included twice under the same specified
3179 name. The file must return TRUE as the last statement to indicate
3180 successful execution of any initialization code, so it's customary to
3181 end such a file with "C<1;>" unless you're sure it'll return TRUE
3182 otherwise. But it's better just to put the "C<1;>", in case you add more
3185 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3186 replaces "F<::>" with "F</>" in the filename for you,
3187 to make it easy to load standard modules. This form of loading of
3188 modules does not risk altering your namespace.
3190 In other words, if you try this:
3192 require Foo::Bar; # a splendid bareword
3194 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3195 directories specified in the C<@INC> array.
3197 But if you try this:
3199 $class = 'Foo::Bar';
3200 require $class; # $class is not a bareword
3202 require "Foo::Bar"; # not a bareword because of the ""
3204 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3205 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3207 eval "require $class";
3209 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3215 Generally used in a C<continue> block at the end of a loop to clear
3216 variables and reset C<??> searches so that they work again. The
3217 expression is interpreted as a list of single characters (hyphens
3218 allowed for ranges). All variables and arrays beginning with one of
3219 those letters are reset to their pristine state. If the expression is
3220 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3221 only variables or searches in the current package. Always returns
3224 reset 'X'; # reset all X variables
3225 reset 'a-z'; # reset lower case variables
3226 reset; # just reset ?one-time? searches
3228 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3229 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3230 variables--lexical variables are unaffected, but they clean themselves
3231 up on scope exit anyway, so you'll probably want to use them instead.
3238 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3239 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3240 context, depending on how the return value will be used, and the context
3241 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3242 is given, returns an empty list in list context, the undefined value in
3243 scalar context, and (of course) nothing at all in a void context.
3245 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3246 or do FILE will automatically return the value of the last expression
3251 In list context, returns a list value consisting of the elements
3252 of LIST in the opposite order. In scalar context, concatenates the
3253 elements of LIST and returns a string value with all characters
3254 in the opposite order.
3256 print reverse <>; # line tac, last line first
3258 undef $/; # for efficiency of <>
3259 print scalar reverse <>; # character tac, last line tsrif
3261 This operator is also handy for inverting a hash, although there are some
3262 caveats. If a value is duplicated in the original hash, only one of those
3263 can be represented as a key in the inverted hash. Also, this has to
3264 unwind one hash and build a whole new one, which may take some time
3265 on a large hash, such as from a DBM file.
3267 %by_name = reverse %by_address; # Invert the hash
3269 =item rewinddir DIRHANDLE
3271 Sets the current position to the beginning of the directory for the
3272 C<readdir()> routine on DIRHANDLE.
3274 =item rindex STR,SUBSTR,POSITION
3276 =item rindex STR,SUBSTR
3278 Works just like index() except that it returns the position of the LAST
3279 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3280 last occurrence at or before that position.
3282 =item rmdir FILENAME
3286 Deletes the directory specified by FILENAME if that directory is empty. If it
3287 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3288 FILENAME is omitted, uses C<$_>.
3292 The substitution operator. See L<perlop>.
3296 Forces EXPR to be interpreted in scalar context and returns the value
3299 @counts = ( scalar @a, scalar @b, scalar @c );
3301 There is no equivalent operator to force an expression to
3302 be interpolated in list context because in practice, this is never
3303 needed. If you really wanted to do so, however, you could use
3304 the construction C<@{[ (some expression) ]}>, but usually a simple
3305 C<(some expression)> suffices.
3307 Wince C<scalar> is unary operator, if you accidentally use for EXPR a
3308 parenthesized list, this behaves as a scalar comma expression, evaluating
3309 all but the last element in void context and returning the final element
3310 evaluated in scalar context. This is seldom what you want.
3312 The following single statement:
3314 print uc(scalar(&foo,$bar)),$baz;
3316 is the moral equivalent of these two:
3319 print(uc($bar),$baz);
3321 See L<perlop> for more details on unary operators and the comma operator.
3323 =item seek FILEHANDLE,POSITION,WHENCE
3325 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3326 FILEHANDLE may be an expression whose value gives the name of the
3327 filehandle. The values for WHENCE are C<0> to set the new position to
3328 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3329 set it to EOF plus POSITION (typically negative). For WHENCE you may
3330 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3331 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3333 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3334 C<seek()> -- buffering makes its effect on the file's system position
3335 unpredictable and non-portable. Use C<sysseek()> instead.
3337 Due to the rules and rigors of ANSI C, on some systems you have to do a
3338 seek whenever you switch between reading and writing. Amongst other
3339 things, this may have the effect of calling stdio's clearerr(3).
3340 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3344 This is also useful for applications emulating C<tail -f>. Once you hit
3345 EOF on your read, and then sleep for a while, you might have to stick in a
3346 seek() to reset things. The C<seek()> doesn't change the current position,
3347 but it I<does> clear the end-of-file condition on the handle, so that the
3348 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3350 If that doesn't work (some stdios are particularly cantankerous), then
3351 you may need something more like this:
3354 for ($curpos = tell(FILE); $_ = <FILE>;
3355 $curpos = tell(FILE)) {
3356 # search for some stuff and put it into files
3358 sleep($for_a_while);
3359 seek(FILE, $curpos, 0);
3362 =item seekdir DIRHANDLE,POS
3364 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3365 must be a value returned by C<telldir()>. Has the same caveats about
3366 possible directory compaction as the corresponding system library
3369 =item select FILEHANDLE
3373 Returns the currently selected filehandle. Sets the current default
3374 filehandle for output, if FILEHANDLE is supplied. This has two
3375 effects: first, a C<write()> or a C<print()> without a filehandle will
3376 default to this FILEHANDLE. Second, references to variables related to
3377 output will refer to this output channel. For example, if you have to
3378 set the top of form format for more than one output channel, you might
3386 FILEHANDLE may be an expression whose value gives the name of the
3387 actual filehandle. Thus:
3389 $oldfh = select(STDERR); $| = 1; select($oldfh);
3391 Some programmers may prefer to think of filehandles as objects with
3392 methods, preferring to write the last example as:
3395 STDERR->autoflush(1);
3397 =item select RBITS,WBITS,EBITS,TIMEOUT
3399 This calls the select(2) system call with the bit masks specified, which
3400 can be constructed using C<fileno()> and C<vec()>, along these lines:
3402 $rin = $win = $ein = '';
3403 vec($rin,fileno(STDIN),1) = 1;
3404 vec($win,fileno(STDOUT),1) = 1;
3407 If you want to select on many filehandles you might wish to write a
3411 my(@fhlist) = split(' ',$_[0]);
3414 vec($bits,fileno($_),1) = 1;
3418 $rin = fhbits('STDIN TTY SOCK');
3422 ($nfound,$timeleft) =
3423 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3425 or to block until something becomes ready just do this
3427 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3429 Most systems do not bother to return anything useful in C<$timeleft>, so
3430 calling select() in scalar context just returns C<$nfound>.
3432 Any of the bit masks can also be undef. The timeout, if specified, is
3433 in seconds, which may be fractional. Note: not all implementations are
3434 capable of returning theC<$timeleft>. If not, they always return
3435 C<$timeleft> equal to the supplied C<$timeout>.
3437 You can effect a sleep of 250 milliseconds this way:
3439 select(undef, undef, undef, 0.25);
3441 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3442 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3443 then only on POSIX systems. You have to use C<sysread()> instead.
3445 =item semctl ID,SEMNUM,CMD,ARG
3447 Calls the System V IPC function C<semctl()>. You'll probably have to say
3451 first to get the correct constant definitions. If CMD is IPC_STAT or
3452 GETALL, then ARG must be a variable which will hold the returned
3453 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3454 undefined value for error, "C<0> but true" for zero, or the actual return
3455 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3457 =item semget KEY,NSEMS,FLAGS
3459 Calls the System V IPC function semget. Returns the semaphore id, or
3460 the undefined value if there is an error. See also C<IPC::SysV> and
3461 C<IPC::SysV::Semaphore> documentation.
3463 =item semop KEY,OPSTRING
3465 Calls the System V IPC function semop to perform semaphore operations
3466 such as signaling and waiting. OPSTRING must be a packed array of
3467 semop structures. Each semop structure can be generated with
3468 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3469 operations is implied by the length of OPSTRING. Returns TRUE if
3470 successful, or FALSE if there is an error. As an example, the
3471 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3473 $semop = pack("sss", $semnum, -1, 0);
3474 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3476 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3477 and C<IPC::SysV::Semaphore> documentation.
3479 =item send SOCKET,MSG,FLAGS,TO
3481 =item send SOCKET,MSG,FLAGS
3483 Sends a message on a socket. Takes the same flags as the system call
3484 of the same name. On unconnected sockets you must specify a
3485 destination to send TO, in which case it does a C C<sendto()>. Returns
3486 the number of characters sent, or the undefined value if there is an
3487 error. The C system call sendmsg(2) is currently unimplemented.
3488 See L<perlipc/"UDP: Message Passing"> for examples.
3490 =item setpgrp PID,PGRP
3492 Sets the current process group for the specified PID, C<0> for the current
3493 process. Will produce a fatal error if used on a machine that doesn't
3494 implement setpgrp(2). If the arguments are omitted, it defaults to
3495 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3496 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3498 =item setpriority WHICH,WHO,PRIORITY
3500 Sets the current priority for a process, a process group, or a user.
3501 (See setpriority(2).) Will produce a fatal error if used on a machine
3502 that doesn't implement setpriority(2).
3504 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3506 Sets the socket option requested. Returns undefined if there is an
3507 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3514 Shifts the first value of the array off and returns it, shortening the
3515 array by 1 and moving everything down. If there are no elements in the
3516 array, returns the undefined value. If ARRAY is omitted, shifts the
3517 C<@_> array within the lexical scope of subroutines and formats, and the
3518 C<@ARGV> array at file scopes or within the lexical scopes established by
3519 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3520 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3521 same thing to the left end of an array that C<pop()> and C<push()> do to the
3524 =item shmctl ID,CMD,ARG
3526 Calls the System V IPC function shmctl. You'll probably have to say
3530 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3531 then ARG must be a variable which will hold the returned C<shmid_ds>
3532 structure. Returns like ioctl: the undefined value for error, "C<0> but
3533 true" for zero, or the actual return value otherwise.
3534 See also C<IPC::SysV> documentation.
3536 =item shmget KEY,SIZE,FLAGS
3538 Calls the System V IPC function shmget. Returns the shared memory
3539 segment id, or the undefined value if there is an error.
3540 See also C<IPC::SysV> documentation.
3542 =item shmread ID,VAR,POS,SIZE
3544 =item shmwrite ID,STRING,POS,SIZE
3546 Reads or writes the System V shared memory segment ID starting at
3547 position POS for size SIZE by attaching to it, copying in/out, and
3548 detaching from it. When reading, VAR must be a variable that will
3549 hold the data read. When writing, if STRING is too long, only SIZE
3550 bytes are used; if STRING is too short, nulls are written to fill out
3551 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3552 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3555 =item shutdown SOCKET,HOW
3557 Shuts down a socket connection in the manner indicated by HOW, which
3558 has the same interpretation as in the system call of the same name.
3560 shutdown(SOCKET, 0); # I/we have stopped reading data
3561 shutdown(SOCKET, 1); # I/we have stopped writing data
3562 shutdown(SOCKET, 2); # I/we have stopped using this socket
3564 This is useful with sockets when you want to tell the other
3565 side you're done writing but not done reading, or vice versa.
3566 It's also a more insistent form of close because it also
3567 disables the filedescriptor in any forked copies in other
3574 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3575 returns sine of C<$_>.
3577 For the inverse sine operation, you may use the C<POSIX::asin()>
3578 function, or use this relation:
3580 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3586 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3587 May be interrupted if the process receives a signal such as C<SIGALRM>.
3588 Returns the number of seconds actually slept. You probably cannot
3589 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3592 On some older systems, it may sleep up to a full second less than what
3593 you requested, depending on how it counts seconds. Most modern systems
3594 always sleep the full amount. They may appear to sleep longer than that,
3595 however, because your process might not be scheduled right away in a
3596 busy multitasking system.
3598 For delays of finer granularity than one second, you may use Perl's
3599 C<syscall()> interface to access setitimer(2) if your system supports it,
3600 or else see L</select> above.
3602 See also the POSIX module's C<sigpause()> function.
3604 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3606 Opens a socket of the specified kind and attaches it to filehandle
3607 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3608 system call of the same name. You should "C<use Socket;>" first to get
3609 the proper definitions imported. See the examples in L<perlipc/"Sockets: Client/Server Communication">.
3611 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3613 Creates an unnamed pair of sockets in the specified domain, of the
3614 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3615 for the system call of the same name. If unimplemented, yields a fatal
3616 error. Returns TRUE if successful.
3618 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3619 to C<pipe(Rdr, Wtr)> is essentially:
3622 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3623 shutdown(Rdr, 1); # no more writing for reader
3624 shutdown(Wtr, 0); # no more reading for writer
3626 See L<perlipc> for an example of socketpair use.
3628 =item sort SUBNAME LIST
3630 =item sort BLOCK LIST
3634 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3635 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3636 specified, it gives the name of a subroutine that returns an integer
3637 less than, equal to, or greater than C<0>, depending on how the elements
3638 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3639 operators are extremely useful in such routines.) SUBNAME may be a
3640 scalar variable name (unsubscripted), in which case the value provides
3641 the name of (or a reference to) the actual subroutine to use. In place
3642 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3645 In the interests of efficiency the normal calling code for subroutines is
3646 bypassed, with the following effects: the subroutine may not be a
3647 recursive subroutine, and the two elements to be compared are passed into
3648 the subroutine not via C<@_> but as the package global variables C<$a> and
3649 C<$b> (see example below). They are passed by reference, so don't
3650 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3652 You also cannot exit out of the sort block or subroutine using any of the
3653 loop control operators described in L<perlsyn> or with C<goto()>.
3655 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3656 current collation locale. See L<perllocale>.
3661 @articles = sort @files;
3663 # same thing, but with explicit sort routine
3664 @articles = sort {$a cmp $b} @files;
3666 # now case-insensitively
3667 @articles = sort {uc($a) cmp uc($b)} @files;
3669 # same thing in reversed order
3670 @articles = sort {$b cmp $a} @files;
3672 # sort numerically ascending
3673 @articles = sort {$a <=> $b} @files;
3675 # sort numerically descending
3676 @articles = sort {$b <=> $a} @files;
3678 # sort using explicit subroutine name
3680 $age{$a} <=> $age{$b}; # presuming numeric
3682 @sortedclass = sort byage @class;
3684 # this sorts the %age hash by value instead of key
3685 # using an in-line function
3686 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3688 sub backwards { $b cmp $a; }
3689 @harry = ('dog','cat','x','Cain','Abel');
3690 @george = ('gone','chased','yz','Punished','Axed');
3692 # prints AbelCaincatdogx
3693 print sort backwards @harry;
3694 # prints xdogcatCainAbel
3695 print sort @george, 'to', @harry;
3696 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3698 # inefficiently sort by descending numeric compare using
3699 # the first integer after the first = sign, or the
3700 # whole record case-insensitively otherwise
3703 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3708 # same thing, but much more efficiently;
3709 # we'll build auxiliary indices instead
3713 push @nums, /=(\d+)/;
3718 $nums[$b] <=> $nums[$a]
3720 $caps[$a] cmp $caps[$b]
3724 # same thing using a Schwartzian Transform (no temps)
3725 @new = map { $_->[0] }
3726 sort { $b->[1] <=> $a->[1]
3729 } map { [$_, /=(\d+)/, uc($_)] } @old;
3731 If you're using strict, you I<MUST NOT> declare C<$a>
3732 and C<$b> as lexicals. They are package globals. That means
3733 if you're in the C<main> package, it's
3735 @articles = sort {$main::b <=> $main::a} @files;
3739 @articles = sort {$::b <=> $::a} @files;
3741 but if you're in the C<FooPack> package, it's
3743 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3745 The comparison function is required to behave. If it returns
3746 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3747 sometimes saying the opposite, for example) the results are not
3750 =item splice ARRAY,OFFSET,LENGTH,LIST
3752 =item splice ARRAY,OFFSET,LENGTH
3754 =item splice ARRAY,OFFSET
3756 Removes the elements designated by OFFSET and LENGTH from an array, and
3757 replaces them with the elements of LIST, if any. In list context,
3758 returns the elements removed from the array. In scalar context,
3759 returns the last element removed, or C<undef> if no elements are
3760 removed. The array grows or shrinks as necessary.
3761 If OFFSET is negative then it start that far from the end of the array.
3762 If LENGTH is omitted, removes everything from OFFSET onward.
3763 If LENGTH is negative, leave that many elements off the end of the array.
3764 The following equivalences hold (assuming C<$[ == 0>):
3766 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3767 pop(@a) splice(@a,-1)
3768 shift(@a) splice(@a,0,1)
3769 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3770 $a[$x] = $y splice(@a,$x,1,$y)
3772 Example, assuming array lengths are passed before arrays:
3774 sub aeq { # compare two list values
3775 my(@a) = splice(@_,0,shift);
3776 my(@b) = splice(@_,0,shift);
3777 return 0 unless @a == @b; # same len?
3779 return 0 if pop(@a) ne pop(@b);
3783 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3785 =item split /PATTERN/,EXPR,LIMIT
3787 =item split /PATTERN/,EXPR
3789 =item split /PATTERN/
3793 Splits a string into an array of strings, and returns it. By default,
3794 empty leading fields are preserved, and empty trailing ones are deleted.
3796 If not in list context, returns the number of fields found and splits into
3797 the C<@_> array. (In list context, you can force the split into C<@_> by
3798 using C<??> as the pattern delimiters, but it still returns the list
3799 value.) The use of implicit split to C<@_> is deprecated, however, because
3800 it clobbers your subroutine arguments.
3802 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3803 splits on whitespace (after skipping any leading whitespace). Anything
3804 matching PATTERN is taken to be a delimiter separating the fields. (Note
3805 that the delimiter may be longer than one character.)
3807 If LIMIT is specified and positive, splits into no more than that
3808 many fields (though it may split into fewer). If LIMIT is unspecified
3809 or zero, trailing null fields are stripped (which potential users
3810 of C<pop()> would do well to remember). If LIMIT is negative, it is
3811 treated as if an arbitrarily large LIMIT had been specified.
3813 A pattern matching the null string (not to be confused with
3814 a null pattern C<//>, which is just one member of the set of patterns
3815 matching a null string) will split the value of EXPR into separate
3816 characters at each point it matches that way. For example:
3818 print join(':', split(/ */, 'hi there'));
3820 produces the output 'h:i:t:h:e:r:e'.
3822 The LIMIT parameter can be used to split a line partially
3824 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3826 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3827 one larger than the number of variables in the list, to avoid
3828 unnecessary work. For the list above LIMIT would have been 4 by
3829 default. In time critical applications it behooves you not to split
3830 into more fields than you really need.
3832 If the PATTERN contains parentheses, additional array elements are
3833 created from each matching substring in the delimiter.
3835 split(/([,-])/, "1-10,20", 3);
3837 produces the list value
3839 (1, '-', 10, ',', 20)
3841 If you had the entire header of a normal Unix email message in C<$header>,
3842 you could split it up into fields and their values this way:
3844 $header =~ s/\n\s+/ /g; # fix continuation lines
3845 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3847 The pattern C</PATTERN/> may be replaced with an expression to specify
3848 patterns that vary at runtime. (To do runtime compilation only once,
3849 use C</$variable/o>.)
3851 As a special case, specifying a PATTERN of space (C<' '>) will split on
3852 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3853 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3854 will give you as many null initial fields as there are leading spaces.
3855 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3856 whitespace produces a null first field. A C<split()> with no arguments
3857 really does a C<split(' ', $_)> internally.
3861 open(PASSWD, '/etc/passwd');
3863 ($login, $passwd, $uid, $gid,
3864 $gcos, $home, $shell) = split(/:/);
3868 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3869 L</chomp>, and L</join>.)
3871 =item sprintf FORMAT, LIST
3873 Returns a string formatted by the usual C<printf()> conventions of the
3874 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3875 on your system for an explanation of the general principles.
3877 Perl does its own C<sprintf()> formatting -- it emulates the C
3878 function C<sprintf()>, but it doesn't use it (except for floating-point
3879 numbers, and even then only the standard modifiers are allowed). As a
3880 result, any non-standard extensions in your local C<sprintf()> are not
3881 available from Perl.
3883 Perl's C<sprintf()> permits the following universally-known conversions:
3886 %c a character with the given number
3888 %d a signed integer, in decimal
3889 %u an unsigned integer, in decimal
3890 %o an unsigned integer, in octal
3891 %x an unsigned integer, in hexadecimal
3892 %e a floating-point number, in scientific notation
3893 %f a floating-point number, in fixed decimal notation
3894 %g a floating-point number, in %e or %f notation
3896 In addition, Perl permits the following widely-supported conversions:
3898 %X like %x, but using upper-case letters
3899 %E like %e, but using an upper-case "E"
3900 %G like %g, but with an upper-case "E" (if applicable)
3901 %b an unsigned integer, in binary
3902 %p a pointer (outputs the Perl value's address in hexadecimal)
3903 %n special: *stores* the number of characters output so far
3904 into the next variable in the parameter list
3906 Finally, for backward (and we do mean "backward") compatibility, Perl
3907 permits these unnecessary but widely-supported conversions:
3910 %D a synonym for %ld
3911 %U a synonym for %lu
3912 %O a synonym for %lo
3915 Perl permits the following universally-known flags between the C<%>
3916 and the conversion letter:
3918 space prefix positive number with a space
3919 + prefix positive number with a plus sign
3920 - left-justify within the field
3921 0 use zeros, not spaces, to right-justify
3922 # prefix non-zero octal with "0", non-zero hex with "0x"
3923 number minimum field width
3924 .number "precision": digits after decimal point for
3925 floating-point, max length for string, minimum length
3927 l interpret integer as C type "long" or "unsigned long"
3928 h interpret integer as C type "short" or "unsigned short"
3930 There is also one Perl-specific flag:
3932 V interpret integer as Perl's standard integer type
3934 Where a number would appear in the flags, an asterisk ("C<*>") may be
3935 used instead, in which case Perl uses the next item in the parameter
3936 list as the given number (that is, as the field width or precision).
3937 If a field width obtained through "C<*>" is negative, it has the same
3938 effect as the "C<->" flag: left-justification.
3940 If C<use locale> is in effect, the character used for the decimal
3941 point in formatted real numbers is affected by the LC_NUMERIC locale.
3948 Return the square root of EXPR. If EXPR is omitted, returns square
3949 root of C<$_>. Only works on non-negative operands, unless you've
3950 loaded the standard Math::Complex module.
3953 print sqrt(-2); # prints 1.4142135623731i
3959 Sets the random number seed for the C<rand()> operator. If EXPR is
3960 omitted, uses a semi-random value supplied by the kernel (if it supports
3961 the F</dev/urandom> device) or based on the current time and process
3962 ID, among other things. In versions of Perl prior to 5.004 the default
3963 seed was just the current C<time()>. This isn't a particularly good seed,
3964 so many old programs supply their own seed value (often C<time ^ $$> or
3965 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3967 In fact, it's usually not necessary to call C<srand()> at all, because if
3968 it is not called explicitly, it is called implicitly at the first use of
3969 the C<rand()> operator. However, this was not the case in version of Perl
3970 before 5.004, so if your script will run under older Perl versions, it
3971 should call C<srand()>.
3973 Note that you need something much more random than the default seed for
3974 cryptographic purposes. Checksumming the compressed output of one or more
3975 rapidly changing operating system status programs is the usual method. For
3978 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3980 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3983 Do I<not> call C<srand()> multiple times in your program unless you know
3984 exactly what you're doing and why you're doing it. The point of the
3985 function is to "seed" the C<rand()> function so that C<rand()> can produce
3986 a different sequence each time you run your program. Just do it once at the
3987 top of your program, or you I<won't> get random numbers out of C<rand()>!
3989 Frequently called programs (like CGI scripts) that simply use
3993 for a seed can fall prey to the mathematical property that
3997 one-third of the time. So don't do that.
3999 =item stat FILEHANDLE
4005 Returns a 13-element list giving the status info for a file, either
4006 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4007 it stats C<$_>. Returns a null list if the stat fails. Typically used
4010 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4011 $atime,$mtime,$ctime,$blksize,$blocks)
4014 Not all fields are supported on all filesystem types. Here are the
4015 meaning of the fields:
4017 0 dev device number of filesystem
4019 2 mode file mode (type and permissions)
4020 3 nlink number of (hard) links to the file
4021 4 uid numeric user ID of file's owner
4022 5 gid numeric group ID of file's owner
4023 6 rdev the device identifier (special files only)
4024 7 size total size of file, in bytes
4025 8 atime last access time since the epoch
4026 9 mtime last modify time since the epoch
4027 10 ctime inode change time (NOT creation time!) since the epoch
4028 11 blksize preferred block size for file system I/O
4029 12 blocks actual number of blocks allocated
4031 (The epoch was at 00:00 January 1, 1970 GMT.)
4033 If stat is passed the special filehandle consisting of an underline, no
4034 stat is done, but the current contents of the stat structure from the
4035 last stat or filetest are returned. Example:
4037 if (-x $file && (($d) = stat(_)) && $d < 0) {
4038 print "$file is executable NFS file\n";
4041 (This works on machines only for which the device number is negative under NFS.)
4043 Because the mode contains both the file type and its permissions, you
4044 should mask off the file type portion and (s)printf using a C<"%o">
4045 if you want to see the real permissions.
4047 $mode = (stat($filename))[2];
4048 printf "Permissions are %04o\n", $mode & 07777;
4051 In scalar context, C<stat()> returns a boolean value indicating success
4052 or failure, and, if successful, sets the information associated with
4053 the special filehandle C<_>.
4055 The File::stat module provides a convenient, by-name access mechanism:
4058 $sb = stat($filename);
4059 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4060 $filename, $sb->size, $sb->mode & 07777,
4061 scalar localtime $sb->mtime;
4067 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4068 doing many pattern matches on the string before it is next modified.
4069 This may or may not save time, depending on the nature and number of
4070 patterns you are searching on, and on the distribution of character
4071 frequencies in the string to be searched -- you probably want to compare
4072 run times with and without it to see which runs faster. Those loops
4073 which scan for many short constant strings (including the constant
4074 parts of more complex patterns) will benefit most. You may have only
4075 one C<study()> active at a time -- if you study a different scalar the first
4076 is "unstudied". (The way C<study()> works is this: a linked list of every
4077 character in the string to be searched is made, so we know, for
4078 example, where all the C<'k'> characters are. From each search string,
4079 the rarest character is selected, based on some static frequency tables
4080 constructed from some C programs and English text. Only those places
4081 that contain this "rarest" character are examined.)
4083 For example, here is a loop that inserts index producing entries
4084 before any line containing a certain pattern:
4088 print ".IX foo\n" if /\bfoo\b/;
4089 print ".IX bar\n" if /\bbar\b/;
4090 print ".IX blurfl\n" if /\bblurfl\b/;
4095 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4096 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4097 a big win except in pathological cases. The only question is whether
4098 it saves you more time than it took to build the linked list in the
4101 Note that if you have to look for strings that you don't know till
4102 runtime, you can build an entire loop as a string and C<eval()> that to
4103 avoid recompiling all your patterns all the time. Together with
4104 undefining C<$/> to input entire files as one record, this can be very
4105 fast, often faster than specialized programs like fgrep(1). The following
4106 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4107 out the names of those files that contain a match:
4109 $search = 'while (<>) { study;';
4110 foreach $word (@words) {
4111 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4116 eval $search; # this screams
4117 $/ = "\n"; # put back to normal input delimiter
4118 foreach $file (sort keys(%seen)) {
4126 =item sub NAME BLOCK
4128 This is subroutine definition, not a real function I<per se>. With just a
4129 NAME (and possibly prototypes), it's just a forward declaration. Without
4130 a NAME, it's an anonymous function declaration, and does actually return a
4131 value: the CODE ref of the closure you just created. See L<perlsub> and
4132 L<perlref> for details.
4134 =item substr EXPR,OFFSET,LEN,REPLACEMENT
4136 =item substr EXPR,OFFSET,LEN
4138 =item substr EXPR,OFFSET
4140 Extracts a substring out of EXPR and returns it. First character is at
4141 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4142 If OFFSET is negative (or more precisely, less than C<$[>), starts
4143 that far from the end of the string. If LEN is omitted, returns
4144 everything to the end of the string. If LEN is negative, leaves that
4145 many characters off the end of the string.
4147 If you specify a substring that is partly outside the string, the part
4148 within the string is returned. If the substring is totally outside
4149 the string a warning is produced.
4151 You can use the substr() function as an lvalue, in which case EXPR
4152 must itself be an lvalue. If you assign something shorter than LEN,
4153 the string will shrink, and if you assign something longer than LEN,
4154 the string will grow to accommodate it. To keep the string the same
4155 length you may need to pad or chop your value using C<sprintf()>.
4157 An alternative to using substr() as an lvalue is to specify the
4158 replacement string as the 4th argument. This allows you to replace
4159 parts of the EXPR and return what was there before in one operation,
4160 just as you can with splice().
4162 =item symlink OLDFILE,NEWFILE
4164 Creates a new filename symbolically linked to the old filename.
4165 Returns C<1> for success, C<0> otherwise. On systems that don't support
4166 symbolic links, produces a fatal error at run time. To check for that,
4169 $symlink_exists = eval { symlink("",""); 1 };
4173 Calls the system call specified as the first element of the list,
4174 passing the remaining elements as arguments to the system call. If
4175 unimplemented, produces a fatal error. The arguments are interpreted
4176 as follows: if a given argument is numeric, the argument is passed as
4177 an int. If not, the pointer to the string value is passed. You are
4178 responsible to make sure a string is pre-extended long enough to
4179 receive any result that might be written into a string. You can't use a
4180 string literal (or other read-only string) as an argument to C<syscall()>
4181 because Perl has to assume that any string pointer might be written
4183 integer arguments are not literals and have never been interpreted in a
4184 numeric context, you may need to add C<0> to them to force them to look
4185 like numbers. This emulates the C<syswrite()> function (or vice versa):
4187 require 'syscall.ph'; # may need to run h2ph
4189 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4191 Note that Perl supports passing of up to only 14 arguments to your system call,
4192 which in practice should usually suffice.
4194 Syscall returns whatever value returned by the system call it calls.
4195 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
4196 Note that some system calls can legitimately return C<-1>. The proper
4197 way to handle such calls is to assign C<$!=0;> before the call and
4198 check the value of C<$!> if syscall returns C<-1>.
4200 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4201 number of the read end of the pipe it creates. There is no way
4202 to retrieve the file number of the other end. You can avoid this
4203 problem by using C<pipe()> instead.
4205 =item sysopen FILEHANDLE,FILENAME,MODE
4207 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4209 Opens the file whose filename is given by FILENAME, and associates it
4210 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4211 the name of the real filehandle wanted. This function calls the
4212 underlying operating system's C<open()> function with the parameters
4213 FILENAME, MODE, PERMS.
4215 The possible values and flag bits of the MODE parameter are
4216 system-dependent; they are available via the standard module C<Fcntl>.
4217 For historical reasons, some values work on almost every system
4218 supported by perl: zero means read-only, one means write-only, and two
4219 means read/write. We know that these values do I<not> work under
4220 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4221 use them in new code.
4223 If the file named by FILENAME does not exist and the C<open()> call creates
4224 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4225 PERMS specifies the permissions of the newly created file. If you omit
4226 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4227 These permission values need to be in octal, and are modified by your
4228 process's current C<umask>.
4230 You should seldom if ever use C<0644> as argument to C<sysopen()>, because
4231 that takes away the user's option to have a more permissive umask.
4232 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4235 See L<perlopentut> for a kinder, gentler explanation of opening files.
4237 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4239 =item sysread FILEHANDLE,SCALAR,LENGTH
4241 Attempts to read LENGTH bytes of data into variable SCALAR from the
4242 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4243 so mixing this with other kinds of reads, C<print()>, C<write()>,
4244 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4245 usually buffers data. Returns the number of bytes actually read, C<0>
4246 at end of file, or undef if there was an error. SCALAR will be grown or
4247 shrunk so that the last byte actually read is the last byte of the
4248 scalar after the read.
4250 An OFFSET may be specified to place the read data at some place in the
4251 string other than the beginning. A negative OFFSET specifies
4252 placement at that many bytes counting backwards from the end of the
4253 string. A positive OFFSET greater than the length of SCALAR results
4254 in the string being padded to the required size with C<"\0"> bytes before
4255 the result of the read is appended.
4257 There is no syseof() function, which is ok, since eof() doesn't work
4258 very well on device files (like ttys) anyway. Use sysread() and check
4259 ofr a return value for 0 to decide whether you're done.
4261 =item sysseek FILEHANDLE,POSITION,WHENCE
4263 Sets FILEHANDLE's system position using the system call lseek(2). It
4264 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4265 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4266 confusion. FILEHANDLE may be an expression whose value gives the name
4267 of the filehandle. The values for WHENCE are C<0> to set the new
4268 position to POSITION, C<1> to set the it to the current position plus
4269 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4270 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4271 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4273 Returns the new position, or the undefined value on failure. A position
4274 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4275 TRUE on success and FALSE on failure, yet you can still easily determine
4280 =item system PROGRAM LIST
4282 Does exactly the same thing as "C<exec LIST>", except that a fork is done
4283 first, and the parent process waits for the child process to complete.
4284 Note that argument processing varies depending on the number of
4285 arguments. If there is more than one argument in LIST, or if LIST is
4286 an array with more than one value, starts the program given by the
4287 first element of the list with arguments given by the rest of the list.
4288 If there is only one scalar argument, the argument is
4289 checked for shell metacharacters, and if there are any, the entire
4290 argument is passed to the system's command shell for parsing (this is
4291 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4292 there are no shell metacharacters in the argument, it is split into
4293 words and passed directly to C<execvp()>, which is more efficient.
4295 The return value is the exit status of the program as
4296 returned by the C<wait()> call. To get the actual exit value divide by
4297 256. See also L</exec>. This is I<NOT> what you want to use to capture
4298 the output from a command, for that you should use merely backticks or
4299 C<qx//>, as described in L<perlop/"`STRING`">.
4301 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4302 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4304 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4305 program they're running doesn't actually interrupt your program.
4307 @args = ("command", "arg1", "arg2");
4309 or die "system @args failed: $?"
4311 You can check all the failure possibilities by inspecting
4314 $exit_value = $? >> 8;
4315 $signal_num = $? & 127;
4316 $dumped_core = $? & 128;
4318 When the arguments get executed via the system shell, results
4319 and return codes will be subject to its quirks and capabilities.
4320 See L<perlop/"`STRING`"> and L</exec> for details.
4322 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4324 =item syswrite FILEHANDLE,SCALAR,LENGTH
4326 =item syswrite FILEHANDLE,SCALAR
4328 Attempts to write LENGTH bytes of data from variable SCALAR to the
4329 specified FILEHANDLE, using the system call write(2). If LENGTH is
4330 not specified, writes whole SCALAR. It bypasses
4331 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4332 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4333 because stdio usually buffers data. Returns the number of bytes
4334 actually written, or C<undef> if there was an error. If the LENGTH is
4335 greater than the available data in the SCALAR after the OFFSET, only as
4336 much data as is available will be written.
4338 An OFFSET may be specified to write the data from some part of the
4339 string other than the beginning. A negative OFFSET specifies writing
4340 that many bytes counting backwards from the end of the string. In the
4341 case the SCALAR is empty you can use OFFSET but only zero offset.
4343 =item tell FILEHANDLE
4347 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4348 expression whose value gives the name of the actual filehandle. If
4349 FILEHANDLE is omitted, assumes the file last read.
4351 There is no C<systell()> function. Use C<sysseek(FH, 0, 1)> for that.
4353 =item telldir DIRHANDLE
4355 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4356 Value may be given to C<seekdir()> to access a particular location in a
4357 directory. Has the same caveats about possible directory compaction as
4358 the corresponding system library routine.
4360 =item tie VARIABLE,CLASSNAME,LIST
4362 This function binds a variable to a package class that will provide the
4363 implementation for the variable. VARIABLE is the name of the variable
4364 to be enchanted. CLASSNAME is the name of a class implementing objects
4365 of correct type. Any additional arguments are passed to the "C<new()>"
4366 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4367 or C<TIEHASH>). Typically these are arguments such as might be passed
4368 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4369 method is also returned by the C<tie()> function, which would be useful
4370 if you want to access other methods in CLASSNAME.
4372 Note that functions such as C<keys()> and C<values()> may return huge lists
4373 when used on large objects, like DBM files. You may prefer to use the
4374 C<each()> function to iterate over such. Example:
4376 # print out history file offsets
4378 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4379 while (($key,$val) = each %HIST) {
4380 print $key, ' = ', unpack('L',$val), "\n";
4384 A class implementing a hash should have the following methods:
4386 TIEHASH classname, LIST
4388 STORE this, key, value
4393 NEXTKEY this, lastkey
4396 A class implementing an ordinary array should have the following methods:
4398 TIEARRAY classname, LIST
4400 STORE this, key, value
4402 STORESIZE this, count
4408 SPLICE this, offset, length, LIST
4412 A class implementing a file handle should have the following methods:
4414 TIEHANDLE classname, LIST
4415 READ this, scalar, length, offset
4418 WRITE this, scalar, length, offset
4420 PRINTF this, format, LIST
4424 A class implementing a scalar should have the following methods:
4426 TIESCALAR classname, LIST
4431 Not all methods indicated above need be implemented. See L<perltie>,
4432 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4434 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4435 for you--you need to do that explicitly yourself. See L<DB_File>
4436 or the F<Config> module for interesting C<tie()> implementations.
4438 For further details see L<perltie>, L<"tied VARIABLE">.
4442 Returns a reference to the object underlying VARIABLE (the same value
4443 that was originally returned by the C<tie()> call that bound the variable
4444 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4449 Returns the number of non-leap seconds since whatever time the system
4450 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4451 and 00:00:00 UTC, January 1, 1970 for most other systems).
4452 Suitable for feeding to C<gmtime()> and C<localtime()>.
4456 Returns a four-element list giving the user and system times, in
4457 seconds, for this process and the children of this process.
4459 ($user,$system,$cuser,$csystem) = times;
4463 The transliteration operator. Same as C<y///>. See L<perlop>.
4465 =item truncate FILEHANDLE,LENGTH
4467 =item truncate EXPR,LENGTH
4469 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4470 specified length. Produces a fatal error if truncate isn't implemented
4471 on your system. Returns TRUE if successful, the undefined value
4478 Returns an uppercased version of EXPR. This is the internal function
4479 implementing the C<\U> escape in double-quoted strings.
4480 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4481 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4482 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4484 If EXPR is omitted, uses C<$_>.
4490 Returns the value of EXPR with the first character
4491 in uppercase (titlecase in Unicode). This is
4492 the internal function implementing the C<\u> escape in double-quoted strings.
4493 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4496 If EXPR is omitted, uses C<$_>.
4502 Sets the umask for the process to EXPR and returns the previous value.
4503 If EXPR is omitted, merely returns the current umask.
4505 The Unix permission C<rwxr-x---> is represented as three sets of three
4506 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4507 and isn't one of the digits). The C<umask> value is such a number
4508 representing disabled permissions bits. The permission (or "mode")
4509 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4510 even if you tell C<sysopen> to create a file with permissions C<0777>,
4511 if your umask is C<0022> then the file will actually be created with
4512 permissions C<0755>. If your C<umask> were C<0027> (group can't
4513 write; others can't read, write, or execute), then passing
4514 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4517 Here's some advice: supply a creation mode of C<0666> for regular
4518 files (in C<sysopen()>) and one of C<0777> for directories (in
4519 C<mkdir()>) and executable files. This gives users the freedom of
4520 choice: if they want protected files, they might choose process umasks
4521 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4522 Programs should rarely if ever make policy decisions better left to
4523 the user. The exception to this is when writing files that should be
4524 kept private: mail files, web browser cookies, I<.rhosts> files, and
4527 If umask(2) is not implemented on your system and you are trying to
4528 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4529 fatal error at run time. If umask(2) is not implemented and you are
4530 not trying to restrict access for yourself, returns C<undef>.
4532 Remember that a umask is a number, usually given in octal; it is I<not> a
4533 string of octal digits. See also L</oct>, if all you have is a string.
4539 Undefines the value of EXPR, which must be an lvalue. Use only on a
4540 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4541 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4542 will probably not do what you expect on most predefined variables or
4543 DBM list values, so don't do that; see L<delete>.) Always returns the
4544 undefined value. You can omit the EXPR, in which case nothing is
4545 undefined, but you still get an undefined value that you could, for
4546 instance, return from a subroutine, assign to a variable or pass as a
4547 parameter. Examples:
4550 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4554 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4555 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4556 select undef, undef, undef, 0.25;
4557 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4559 Note that this is a unary operator, not a list operator.
4565 Deletes a list of files. Returns the number of files successfully
4568 $cnt = unlink 'a', 'b', 'c';
4572 Note: C<unlink()> will not delete directories unless you are superuser and
4573 the B<-U> flag is supplied to Perl. Even if these conditions are
4574 met, be warned that unlinking a directory can inflict damage on your
4575 filesystem. Use C<rmdir()> instead.
4577 If LIST is omitted, uses C<$_>.
4579 =item unpack TEMPLATE,EXPR
4581 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4582 structure and expands it out into a list value, returning the array
4583 value. (In scalar context, it returns merely the first value
4584 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4585 Here's a subroutine that does substring:
4588 my($what,$where,$howmuch) = @_;
4589 unpack("x$where a$howmuch", $what);
4594 sub ordinal { unpack("c",$_[0]); } # same as ord()
4596 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4597 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4598 themselves. Default is a 16-bit checksum. For example, the following
4599 computes the same number as the System V sum program:
4602 $checksum += unpack("%32C*", $_);
4606 The following efficiently counts the number of set bits in a bit vector:
4608 $setbits = unpack("%32b*", $selectmask);
4610 The C<"p"> and C<"P"> formats should be used with care. Since Perl
4611 has no way of checking whether the value passed to C<unpack()>
4612 corresponds to a valid memory location, passing a pointer value that's
4613 not known to be valid is likely to have disastrous consequences.
4615 See L</pack> for more examples.
4617 =item untie VARIABLE
4619 Breaks the binding between a variable and a package. (See C<tie()>.)
4621 =item unshift ARRAY,LIST
4623 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4624 depending on how you look at it. Prepends list to the front of the
4625 array, and returns the new number of elements in the array.
4627 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4629 Note the LIST is prepended whole, not one element at a time, so the
4630 prepended elements stay in the same order. Use C<reverse()> to do the
4633 =item use Module LIST
4637 =item use Module VERSION LIST
4641 Imports some semantics into the current package from the named module,
4642 generally by aliasing certain subroutine or variable names into your
4643 package. It is exactly equivalent to
4645 BEGIN { require Module; import Module LIST; }
4647 except that Module I<must> be a bareword.
4649 If the first argument to C<use> is a number, it is treated as a version
4650 number instead of a module name. If the version of the Perl interpreter
4651 is less than VERSION, then an error message is printed and Perl exits
4652 immediately. This is often useful if you need to check the current
4653 Perl version before C<use>ing library modules that have changed in
4654 incompatible ways from older versions of Perl. (We try not to do
4655 this more than we have to.)
4657 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4658 C<require> makes sure the module is loaded into memory if it hasn't been
4659 yet. The C<import()> is not a builtin--it's just an ordinary static method
4660 call into the "C<Module>" package to tell the module to import the list of
4661 features back into the current package. The module can implement its
4662 C<import()> method any way it likes, though most modules just choose to
4663 derive their C<import()> method via inheritance from the C<Exporter> class that
4664 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4665 method can be found then the error is currently silently ignored. This
4666 may change to a fatal error in a future version.
4668 If you don't want your namespace altered, explicitly supply an empty list:
4672 That is exactly equivalent to
4674 BEGIN { require Module }
4676 If the VERSION argument is present between Module and LIST, then the
4677 C<use> will call the VERSION method in class Module with the given
4678 version as an argument. The default VERSION method, inherited from
4679 the Universal class, croaks if the given version is larger than the
4680 value of the variable C<$Module::VERSION>. (Note that there is not a
4681 comma after VERSION!)
4683 Because this is a wide-open interface, pragmas (compiler directives)
4684 are also implemented this way. Currently implemented pragmas are:
4688 use sigtrap qw(SEGV BUS);
4689 use strict qw(subs vars refs);
4690 use subs qw(afunc blurfl);
4692 Some of these these pseudo-modules import semantics into the current
4693 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4694 which import symbols into the current package (which are effective
4695 through the end of the file).
4697 There's a corresponding "C<no>" command that unimports meanings imported
4698 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4703 If no C<unimport()> method can be found the call fails with a fatal error.
4705 See L<perlmod> for a list of standard modules and pragmas.
4709 Changes the access and modification times on each file of a list of
4710 files. The first two elements of the list must be the NUMERICAL access
4711 and modification times, in that order. Returns the number of files
4712 successfully changed. The inode modification time of each file is set
4713 to the current time. This code has the same effect as the "C<touch>"
4714 command if the files already exist:
4718 utime $now, $now, @ARGV;
4722 Returns a list consisting of all the values of the named hash. (In a
4723 scalar context, returns the number of values.) The values are
4724 returned in an apparently random order. The actual random order is
4725 subject to change in future versions of perl, but it is guaranteed to
4726 be the same order as either the C<keys()> or C<each()> function would
4727 produce on the same (unmodified) hash.
4729 Note that you cannot modify the values of a hash this way, because the
4730 returned list is just a copy. You need to use a hash slice for that,
4731 since it's lvaluable in a way that values() is not.
4733 for (values %hash) { s/foo/bar/g } # FAILS!
4734 for (@hash{keys %hash}) { s/foo/bar/g } # ok
4736 As a side effect, calling values() resets the HASH's internal iterator.
4737 See also C<keys()>, C<each()>, and C<sort()>.
4739 =item vec EXPR,OFFSET,BITS
4741 Treats the string in EXPR as a vector of unsigned integers, and
4742 returns the value of the bit field specified by OFFSET. BITS specifies
4743 the number of bits that are reserved for each entry in the bit
4744 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4745 assigned to, in which case parentheses are needed to give the expression
4746 the correct precedence as in
4748 vec($image, $max_x * $x + $y, 8) = 3;
4750 Vectors created with C<vec()> can also be manipulated with the logical
4751 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4752 desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
4754 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4755 The comments show the string after each step. Note that this code works
4756 in the same way on big-endian or little-endian machines.
4759 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4760 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4761 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4762 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4763 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4764 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4765 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4767 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4768 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4769 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4772 To transform a bit vector into a string or array of 0's and 1's, use these:
4774 $bits = unpack("b*", $vector);
4775 @bits = split(//, unpack("b*", $vector));
4777 If you know the exact length in bits, it can be used in place of the C<*>.
4781 Behaves like the wait(2) system call on your system: it waits for a child
4782 process to terminate and returns the pid of the deceased process, or
4783 C<-1> if there are no child processes. The status is rketurned in C<$?>.
4784 Note that a return value of C<-1> could mean that child processes are
4785 being automatically reaped, as described in L<perlipc>.
4787 =item waitpid PID,FLAGS
4789 Waits for a particular child process to terminate and returns the pid of
4790 the deceased process, or C<-1> if there is no such child process. On some
4791 systems, a value of 0 indicates that there are processes still running.
4792 The status is returned in C<$?>. If you say
4794 use POSIX ":sys_wait_h";
4797 $kid = waitpid(-1,&WNOHANG);
4800 then you can do a non-blocking wait for all pending zombie processes.
4801 Non-blocking wait is available on machines supporting either the
4802 waitpid(2) or wait4(2) system calls. However, waiting for a particular
4803 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
4804 system call by remembering the status values of processes that have
4805 exited but have not been harvested by the Perl script yet.)
4807 Note that on some systems, a return value of C<-1> could mean that child
4808 processes are being automatically reaped. See L<perlipc> for details,
4809 and for other examples.
4813 Returns TRUE if the context of the currently executing subroutine is
4814 looking for a list value. Returns FALSE if the context is looking
4815 for a scalar. Returns the undefined value if the context is looking
4816 for no value (void context).
4818 return unless defined wantarray; # don't bother doing more
4819 my @a = complex_calculation();
4820 return wantarray ? @a : "@a";
4824 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4827 If LIST is empty and C<$@> already contains a value (typically from a
4828 previous eval) that value is used after appending C<"\t...caught">
4829 to C<$@>. This is useful for staying almost, but not entirely similar to
4832 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4834 No message is printed if there is a C<$SIG{__WARN__}> handler
4835 installed. It is the handler's responsibility to deal with the message
4836 as it sees fit (like, for instance, converting it into a C<die()>). Most
4837 handlers must therefore make arrangements to actually display the
4838 warnings that they are not prepared to deal with, by calling C<warn()>
4839 again in the handler. Note that this is quite safe and will not
4840 produce an endless loop, since C<__WARN__> hooks are not called from
4843 You will find this behavior is slightly different from that of
4844 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4845 instead call C<die()> again to change it).
4847 Using a C<__WARN__> handler provides a powerful way to silence all
4848 warnings (even the so-called mandatory ones). An example:
4850 # wipe out *all* compile-time warnings
4851 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4853 my $foo = 20; # no warning about duplicate my $foo,
4854 # but hey, you asked for it!
4855 # no compile-time or run-time warnings before here
4858 # run-time warnings enabled after here
4859 warn "\$foo is alive and $foo!"; # does show up
4861 See L<perlvar> for details on setting C<%SIG> entries, and for more
4862 examples. See the Carp module for other kinds of warnings using its
4863 carp() and cluck() functions.
4865 =item write FILEHANDLE
4871 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4872 using the format associated with that file. By default the format for
4873 a file is the one having the same name as the filehandle, but the
4874 format for the current output channel (see the C<select()> function) may be set
4875 explicitly by assigning the name of the format to the C<$~> variable.
4877 Top of form processing is handled automatically: if there is
4878 insufficient room on the current page for the formatted record, the
4879 page is advanced by writing a form feed, a special top-of-page format
4880 is used to format the new page header, and then the record is written.
4881 By default the top-of-page format is the name of the filehandle with
4882 "_TOP" appended, but it may be dynamically set to the format of your
4883 choice by assigning the name to the C<$^> variable while the filehandle is
4884 selected. The number of lines remaining on the current page is in
4885 variable C<$->, which can be set to C<0> to force a new page.
4887 If FILEHANDLE is unspecified, output goes to the current default output
4888 channel, which starts out as STDOUT but may be changed by the
4889 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4890 is evaluated and the resulting string is used to look up the name of
4891 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4893 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4897 The transliteration operator. Same as C<tr///>. See L<perlop>.